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Title:
Petrographic and kinematic investigation of the volcaniclastic and plutonic rocks of the northern Alisitos arc, Baja California, Mexico
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Book
Language:
English
Creator:
Tutak, Fatin
Publisher:
University of South Florida
Place of Publication:
Tampa, Fla.
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Subjects

Subjects / Keywords:
Arc collision
Peninsular Ranges batholith
Ancestral Agua Blanca fault
Magmatic fabric
Strain partitioning
Dissertations, Academic -- Geology -- Masters -- USF   ( lcsh )
Genre:
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Abstract:
ABSTRACT: The Alisitos arc segment forms part of the western zone of Jura-Cretaceous Peninsular Ranges batholith of Baja California. It extends south from the ancestral Agua Blanca Fault to the state boundary between Baja Norte and Sur. The study area is located within a fold and thrust belt intruded by a number of plutons that were emplaced during and after the deformational event. The northern end of the Alisitos arc is characterized by subvertical tight to isoclinal folds and high-angle reverse faults that define a northwest trending, southwest vergent fold and thrust belt. The aABF defines the northern limit to the Alisitos arc segment. In this study we present the results of a petrographic study of igneous rocks in order to determine the relative timing and the distribution of deformation within the northern Alisitos arc segment. The study includes samples of the mylonitic shear zone of the aABF, and plutonic samples from intrusions proximal to the aABF emplaced later during regional deformation. These samples were investigated in order to characterize the distribution of the subsolidus strain in grain scale and the sense of shear during later phases of deformation in the northern Alisitos arc. The results are presented and discussed based on the mineralogical and textural observations from the Balbuena pluton, the Piedra Rodada pluton, and volcaniclastics that were deformed within the aABF. The Balbuena pluton, emplaced at ~ 108 Ma after the surrounding country rocks had already been folded, exhibits little if any evidence for subsolidus deformation. In contrast, the Piedra Rodada pluton, emplaced at ~ 105 Ma just to the southwest of the aABF, exhibits a strong magmatic fabric overprinted by a moderate subsolidus fabric to the southwest that grades into a strong subsolidus fabric with proximity to the fault. Kinematics observed from lineation parallel-foliation normal sections exhibit consistent top-to-the southwest sense of shear.
Thesis:
Thesis (M.S.)--University of South Florida, 2008.
Bibliography:
Includes bibliographical references.
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Mode of access: World Wide Web.
Statement of Responsibility:
by Fatin Tutak.
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Title from PDF of title page.
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Document formatted into pages; contains 161 pages.

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aleph - 001983537
oclc - 297178159
usfldc doi - E14-SFE0002342
usfldc handle - e14.2342
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Petrographic and kinematic investigation of the volcaniclastic and plutonic rocks of the northern Alisitos arc, Baja California, Mexico
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ABSTRACT: The Alisitos arc segment forms part of the western zone of Jura-Cretaceous Peninsular Ranges batholith of Baja California. It extends south from the ancestral Agua Blanca Fault to the state boundary between Baja Norte and Sur. The study area is located within a fold and thrust belt intruded by a number of plutons that were emplaced during and after the deformational event. The northern end of the Alisitos arc is characterized by subvertical tight to isoclinal folds and high-angle reverse faults that define a northwest trending, southwest vergent fold and thrust belt. The aABF defines the northern limit to the Alisitos arc segment. In this study we present the results of a petrographic study of igneous rocks in order to determine the relative timing and the distribution of deformation within the northern Alisitos arc segment. The study includes samples of the mylonitic shear zone of the aABF, and plutonic samples from intrusions proximal to the aABF emplaced later during regional deformation. These samples were investigated in order to characterize the distribution of the subsolidus strain in grain scale and the sense of shear during later phases of deformation in the northern Alisitos arc. The results are presented and discussed based on the mineralogical and textural observations from the Balbuena pluton, the Piedra Rodada pluton, and volcaniclastics that were deformed within the aABF. The Balbuena pluton, emplaced at ~ 108 Ma after the surrounding country rocks had already been folded, exhibits little if any evidence for subsolidus deformation. In contrast, the Piedra Rodada pluton, emplaced at ~ 105 Ma just to the southwest of the aABF, exhibits a strong magmatic fabric overprinted by a moderate subsolidus fabric to the southwest that grades into a strong subsolidus fabric with proximity to the fault. Kinematics observed from lineation parallel-foliation normal sections exhibit consistent top-to-the southwest sense of shear.
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Petrographic and Kinematic Inve stigation of the Volcaniclast ic and Plutonic Rocks of the Northern Alisitos Arc, Baja California, Mexico by Fatin Tutak A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Geology College of Arts and Sciences University of South Florida Major Professor: Paul H. Wetmore, Ph.D. Co-Major Professor: Jeffrey Ryan, Ph.D. Charles Connor, Ph.D. Date of Approval: February 19, 2008 Keywords: Arc collision, Peninsular Ranges batholith, ancestral Agua Blanca fault, magmatic fabric, strain partitioning Copyright 2008, Fatin Tutak

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Dedication Sukru’me...

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Acknowledgements I would like to thank my thesis committ ee for their input and comments: Dr. Paul H. Wetmore, Dr. Jeffrey Ryan and Dr. Char les Connor. I would like to thank Mikel Diez, Heather Lehto and Armando Saballos for helpin g me out and for their support. I would like to thank the Department of Geology, Univ ersity of South Florida for helping me. I would like to thank the Geological Society of America and the Ca naveral Mineral and Gem Society for funding and supporting my proj ect. I would like to thank to my husband; I know you deserve it the most.

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i Table of Contents List of Figures iii ABSTRACT xiv 1.Introduction 1 2. Geologic setting 6 2.1. Background geology of the study area 6 2.2. Local geology 8 2.2.1. Alisitos Arc 8 2.2.2. Piedra Rodada pluton 10 2.2.3. Balbuena Pluton 10 2.2.4. Fold and Thrust Belts 11 2.3. Previous work and field observations 12 3. Purpose 16 4. Identification of the Subsolidus Fabrics and Kinematic Analysis A Microstructural Study 18 5. Piedra Rodada Pluton and the aABF Mylonitic Ductile Shear Zone 26 5.1. Petrography 26 5.2. Subsolidus Deformation within the Piedra Rodada Pluton 28 5.3. Low subsolidus deformation (sample west) 29 5.4. Highest subsolidus deformation (sample central) 35 5.5. Moderate subsolidus deformation (sample central east) 42 5.6. Highest subsolidus deformation (sample southeastern) 49 5.7. aABF mylonitic shear zone A microsturctural study 58 5.8. North of the aABF 60 6. Balbuena Pluton 62

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ii 6.1. Microstructural observations 62 6.1.1. Quartz Diorite 64 6.1.2. Tonalite 65 6.1.3. Diorite 66 7. Summary 68 7.1. Piedra Rodada Pluton 68 7.2. Volcaniclastic samples of the aABF 69 7.3. Balbuena Pluton 70 8. Discussion 72 8.1. The Piedra Rodada pluton and the ancestral Agua Blanca Fault 72 8.2. Balbuena pluton 74 9. Conclusions 76 10. References 77 APPENDICES 85 Appendix A: Piedra Rodada pluton thin sections discriptions 86 Appendix B: Balbuena pluton thin sections descriptions 115 Appendix C: Volcaniclastic samples 151 Appendix D : Mylonitic rock samples 156

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iii List of Figures Figure 1. Geologic map of the northern Alisitos segment. 15 Figure 2. Photomicrograph of deformed quartz crystals with undulatory extinction. 19 Figure 3. Photomicrograph of subsolidus fabrics in Piedra Rodada pluton. 20 Figure 4. Photomicrograph from Pied ra Rodada pluton with dynamic recrystall ization features in quartz crystals. 20 Figure 5. Photomicrograph from Pied ra Rodada pluton shows sweeping undulator y extinction in biotite crystals 21 Figure 6. Photomicrograph from Piedra Rodada pluton, sigma clast fabric development. 21 Figure 7. Photomicrograph from Piedra R odada pluton displays the development of C surfaces of the shear zone. 22 Figure 8. Optical micrograph of sample TW-04 from Piedra Rodada pluton, mica fish development. 23 Figure 9. Optical micrograph of sample TW-06 from aABF, mica fish in ultramylonitic shear zone. 24 Figure 10. Photomicrograph of sample PHW 1/12/03 from aABF displays foliation. 25 Figure 11. Samples location map of the Piedra Rodada pluton. 28 Figure 12. Map of the collected samples that shows the distance from aABF. 29 Figure 13. Geologic map of the low subsolidus deformation samples. 30 Figure 14. Photomicrograph of sample TW-04 from Piedra Rodada pluton, general microstructure of the deformed rock. 31

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iv Figure 15. Photomicrograph of sample TW-04 from Piedra Rodada pluton, strongly deformed biotite forms the C surfaces of the microshearing. 31 Figure 16. Photomicrograph of sample TW -04 from Piedra Rodada pluton, plagioclase phenoc ryst with asymmetric biotite tail development. 32 Figure 17. Photomicrograph of sample PHW 7-01-1-F from Piedra Rodada pluton, biotite and plagiocl ase microstructures in the deformed rock sample. 33 Figure 18. Photomicrograph of sample 7-01-1-F from Piedra Rodada pluton, hornblende gain surrounded by plastically deformed quartz. 34 Figure 19. Photomicrograph of sample 701-1-F from Piedra Rodada pluton displays quartz ribbons are elongated due to subsolidus strain. 34 Figure 20. Geologic map for high subsolidus strain sample s locations. 37 Figure 21. Photomicrograph of sample 5/ 20/02 A from Piedra Rodada pluton shows Strong biotite foliation surrounding plag ioclase. 37 Figure 22. Photomicrograph of sample 5/ 20/02 A from Piedra Rodada pluton, microstructure of hi ghly deformed quartz crystals. 38 Figure 23. Optical micrograph of sample 1/ 17/04 B from Piedra Rodada pluton shows S-C surfaces of the microshear zone in plain pol ar light. 38 Figure 24. Optical micrograph of sample 1/ 17/04 B from Piedra Rodada pluton shows S-C surfaces of the microshear zone in cross polar light. 39 Figure 25. Optical micrograph of sample 1/ 17/04 B from Piedra Rodada pluton shows the C surfaces of the moderately defomed rock in PPL. 41 Figure 26. Optical micrograph of sample 1/ 17/04 B from Piedra Rodada pluton shows the C surfaces of the moderately defomed rock in CPL. 41 Figure 27. Map for moderate subsolidus strain samples locations. 43 Figure 28. Optical micrograph of sample 5/ 20/02 B from Piedra Rodada pluton displays foliati ons development by biotite. 44 Figure 29. Optical micrograph of sample PHW 5/20/02 Bfrom Piedra Rodada Pluton shows plastically de formed quartz and biotite crystsals. 44

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v Figure 30. Optical micrograph of sample PHW 5/20/02 B from Piedra Rodada Pluton shows hornblende su rrounded by fine-grained quartz crystals. 45 Figure 31. Optical micrograph of sample PHW 5/20/02 B from Piedra Rodada pluton shows neocrystal lized biotite crystals. 45 Figure 32. Optical micrograph of sample PHW 5/20/02 B from Piedra Rodada pluton displays elongated biotite due to subsoli dus strain. 46 Figure 33. Optical micrograph of sample PHW 1/17/04 D from Piedra Rodada pluton displays plastically deform ed biotite and undeformed plagioclase phenocryt in CPL. 47 Figure 34. Optical micrograph of sample PHW 1/17/04 D from Piedra Rodada pluton displays plastically deformed biotite and hornblende. 48 Figure 35. Optical micrograph of sample PHW 1/17/04 D from Piedra Rodada pluton displays plastically deform ed biotite forms the C surfaces. 48 Figure 36. Map for highly deformed samples from Piedra R odada pluton. 50 Figure 37. Optical micrograph of sample TW-03 from Piedra Rodada pluton shows dynamica lly recrystallized quartz. 51 Figure 38. Optical micrograph of sample TW-03 from Piedra Rodada pluton shows grain boundary migration in quartz crystals. 51 Figure 39. Optical micrograph of sample TW-03 from Piedra Rodada pluton shows biot ite folia that control the shape of quartz. 52 Figure 40. Optical micrograph of sample TW-03 from Piedra Rodada pluton shows biotite folia surrounded plagioclase phenocryst. 52 Figure 41. Optical micrograph of sample TW-03 from Piedra Rodada pluton shows sigma clast developmet in highly deformed rock. 53 Figure 42. Optical micrograph of sample TW-03 from Piedra Rodada pluton shows neocrystallized biot ite transformed into white mica. 53 Figure 43. Optical micrograph of sample TW-03 from Piedra Rodada pluton asymmetric tail developmen t of plagioclase phenocryst. 54 Figure 44. Optical micrograph of sample TW-03 from Piedra Rodada pluton displays S-C fa bics due to high strain. 54

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vi Figure 45. Optical micrograph of sample TW-01from Piedra Rodada Pluton shows cataclastic deformation features in quartz. 56 Figure 46. Optical micrograph of sample TW-01from Piedra Rodada Pluton shows microf ractured quartz filled with biotite. 56 Figure 47. Optical micrograph of sample TW-01 from Piedra Rodada pluton shows lenticular hornblende grain. 57 Figure 48. Optical micrograph of sample TW-01 from Piedra Rodada Pluton displays chess board texture in quartz. 57 Figure 49. Optical micrograph of sample TW-06 from aABF. 59 Figure 50. Optical micrograph of sample TW-06 from aABF displays strong mylonitic foliation. 59 Figure 51. Map for samples locations from nor th of the aABF. 61 Figure 52. Geologic map of samples collected from north of aABF. 63 Figure 53. Optical micrograph of sample PHW 5/20/02 from Piedra Rodada pluton shows poiki tic texture in hornblende grain. 87 Figure 54. Optical micrograph of sample PHW 5/20/02 from Piedra Rodada pluton displays undeformed plagioclase phenocry st. 87 Figure 55. Optical micrograph of sample PHW 5/20/02 from Piedra Rodada pluton shows strong biotie folia tion developed around plagioclase. 88 Figure 56. Optical micrograph of sample PHW 5/20/02 from Piedra Rodada pluton shows intracrystalline de formation features in quartz cr ystals. 88 Figure 57. Optical micrograph of sample PHW 5/20/02 from Piedra Rodada pluton shows hornblende crystals surrounded by bi otite folia. 89 Figure 58. Optical micrograph of sample PHW 5/20/02 from Piedra Rodada pluton shows as ymmetric biotite fish with well developed tail. 89 Figure 59. Optical micrograph of sample PHW 7-01-1-F from Piedra Rodada pluton defines the igneous texture in plagioclase phenocryst. 91 Figure 60. Optical micrograph of sample PHW 7-01-1-F from Piedra Rodada

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vii pluton shows hornblende which is surr ounded by fine-grained quartz crystals. 91 Figure 61. Optical micrograph of sample PHW 7-01-1-F from Piedra Rodada pluton shows simp le twinned hornblende with rounded inclusions of quartz crystals. 92 Figure 62. Optical micrograph of sample PHW 7-01-1-F from Piedra Rodada pluton showing that the spaces between plagioclase phenocrysts filled by dynamically recr ystallized quartz crystals. 92 Figure 63. Optical micrograph of sa mple 11704 B from Piedra Rodada pluton shows biotite foliation and th e development of C surfaces. 94 Figure 64. Optical micrograph of sa mple 11704 B from Piedra Rodada pluton shows the mosaic texture in quartz crystals. 94 Figure 65. Optical micrograph of sa mple 11704 B from Piedra Rodada pluton shows quartz crystals that are medium sized and moderately deformed. 95 Figure 66. Optical micrograph of sa mple 11704 Dfrom Piedra Rodada pluton shows plagioclase phenocry sts surrounded by biotite folia. 96 Figure 67. Optical micrograph of sample 5/20/02 B from Piedra Rodada pluton shows th e igneous texture in plagioclase phenocryst. 98 Figure 68. Optical micrograph of sample 5/20/02 B from Piedra Rodada pluton shows the lenticular hornblen de surrounded by quart. 98 Figure 69. Optical micrograph of sample 5/20/02 B from Piedra Rodada pluton shows plag ioclase with biotite foliation. 99 Figure 70. Optical micrograph of sample 11704 D from Piedra Rodada pluton shows the foliation development and intracrytalline defomation. 100 Figure 71. Optical micrograph of sample 11704 D from Piedra Rodada pluton shows the undulatory extinction in qu artz crystals. 101 Figure 72. Optical micrograph of sample TW-01 from Piedra Rodada pluton shows the fractured biotite crystals. 102 Figure 73. Optical micrograph of sample TW-01 from Piedra Rodada pluton shows the undeformed plagioclase phenocryst. 103

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viii Figure 74. Optical micrograph of sample TW-01 from Piedra Rodada pluton shows the minor alteration in plagiolcase. 103 Figure 75. Optical micrograph of sample TW-03 from Piedra Rodada pluton shows the spaces filled by necrystall ized biotite in plagioclas. 105 Figure 76. Optical micrograph of sample TW-03 from Piedra Rodada pluton shows the recrytallized quartz and neocrystallized biot ite crystals. 105 Figure 77. Optical micrograph of sample TW-03 from Piedra Rodada pluton displays the intr acrytalline deformation features in quartz. 106 Figure 78. Optical micrograph of sample TW-04 from Piedra Rodada pluton shows the microstructure of highly defomed rock. 107 Figure 79. Optical micrograph of sample TW-04 from Piedra Rodada pluton shows lenticular hornblende typical for s ubsolidus strain. 108 Figure 80. Optical micrograph of sample TW-04 from Piedra Rodada pluton shows the dynamically recrystallized quartz cr ystals. 108 Figure 81. Optical micrograph of sa mple 11704 A from Piedra Rodada pluton shows the magmatic flow without subsolidus strain. 110 Figure 82. Optical micrograph of sa mple 11704 A from Piedra Rodada pluton shows th e highly altered plagioclase phenocrysts in the undeformed rock. 110 Figure 83. Optical micrograph of samp le PHW 7-1-J from Arce pluton displays the magmatic flow within the pluton. 112 Figure 84. Optical micrograph of sa mple 7-1-J from Arce pluton shows the higly altered plagioclase phenocryst. 112 Figure 85. Optical micrograph of sample 1/8/ 03 from El Aleman pluton shows myrmekitic texture in plagioclase phenocryst. 114 Figure 86. Optical micrograph of samp le 1/8/03 from El Aleman pluton shows sigma clast development. 114 Figure 87. Optical micrograph of sample 6-9-1-E from Balbuena pluton shows the microstructure of the rock in PPL. 116

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ix Figure 88. Optical micrograph of sample 6-9-1-E from Balbuena pluton shows the fractures in plagioclase filled by quartz. 116 Figure 89. Optical micrograph of sample 6-9-1-E from Balbuena pluton indicating compositional change in the plagioclase phenoc ryst. 117 Figure 90. Optical micrograph of sample 6-9-1-E from Balbuena pluton shows intragranular microcrakcs in plagioclase phenocryst. 117 Figure 91. Optical micrograph of sample 6-9-1-E from Balbuena pluton shows zoned plagioclase phenocryst. 118 Figure 92. Optical micrograph of samp le GW-01 from Balbuena pluton shows the general microstructure of the rock. 119 Figure 93. Optical micrograph of samp le GW-01 from Balbuena pluton shows undulatory extinct ion in quartz crystals. 120 Figure 94. Optical micrograph of samp le GW-01 from Balbuena pluton shows the complex crystall ization history in plagioclase phenocryst. 120 Figure 95. Optical micrograph of samp le GW-01 from Balbuena pluton shows the microfr actured biotite crystal. 121 Figure 96. Optical micrograph of samp le GW-03 from Balbuena pluton shows the general microstructure of the rock. 122 Figure 97. Optical micrograph of samp le GW-03 from Balbuena pluton shows the fabric grains shapes in quartz crystals. 123 Figure 98. Optical micrograph of samp le GW-03 from Balbuena pluton shows evidence for magm atic flow in plagioclase phenocrysts. 123 Figure 99. Optical micrograph of samp le GW-03 from Balbuena pluton displays the in tragranular and transgranular microcracks in quartz crystals. 124 Figure 100. Optical micrograph of samp le GW-03 from Balbuena pluton shows the microfractured biotite crystal. 124 Figure 101. Optical micrograph of samp le GW-04 from Balbuena pluton shows the general texture of the rock. 126 Figure 102. Optical micrograph of samp le GW-04 from Balbuena pluton

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x shows the poikitic texture in biotite crystal. 126 Figure 103. Optical micrograph of samp le GW-04 from Balbuena pluton shows the aliged bi otite that surrounded by quartz crystals. 127 Figure 104. Optical micrograph of samp le GW-04 from Balbuena pluton shows bending twins in plagioclase phenocryst. 127 Figure 105. Optical micrograph of samp le GW-04 from Balbuena pluton shows the granular te xture and the attached plagio clase crystals. 128 Figure 106. Optical micrograph of samp le GW-06 from Balbuena pluton shows the oscillatory zoning in plagioclase. 129 Figure 107. Optical micrograph of samp le GW-06 from Balbuena pluton shows the general texture in these rock. 130 Figure 108. Optical micrograph of samp le GW-06 from Balbuena pluton shows the biotite crystals surrounding the plag ioclase. 130 Figure 109. Optical micrograph of samp le GW-06 from Balbuena pluton shows the spaces filled by biotite in plagioclase phenocryt. 131 Figure 110. Optical micrograph of samp le GW-06 from Balbuena pluton shows the igneous text ure with no evidence of subso lidus strain. 131 Figure 111. Optical micrograph of sample 6-9-1-B from Balbuena pluton shows the general texture in these rock sample. 133 Figure 112. Optical micrograph of sample 6-9-1-B from Balbuena pluton shows the micr ocracks in quartz crystal. 133 Figure 113. Optical micrograph of sample 6-9-1-B from Balbuena pluton shows the tartan twining in feldspar. 134 Figure 114. Optical micrograph of sample 6-9-1-B from Balbuena pluton shows the well devel oped albite twining in plag ioclase. 134 Figure 115. Optical micrograph of sample 6-9-1-B from Balbuena pluton shows the concentr ic zoning and fractures in plagioclase. 135 Figure 116. Optical micrograph of sample 6-9-1-F from Balbuena pluton shows the z oned plagioclase phenocryt. 136

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xi Figure 117. Optical micrograph of sample 6-9-1-F from Balbuena pluton shows the rimed and zoned plagioclase phenocryt. 137 Figure 118. Optical micrograph of sample 6-9-1-F from Balbuena pluton shows the reati on of plagioclase with the melt. 137 Figure 119. Optical micrograph of sample 6-9-1-F from Balbuena pluton shows the fine grained quartz crystals. 138 Figure 120. Optical micrograph of sample 6-9-1-F from Balbuena pluton shows the attachme nt of three plagioclase phenocrysts. 138 Figure 121. Optical micrograph of sample 6-9-1-F from Balbuena pluton shows the general microstructure of rock. 139 Figure 122. Optical micrograph of samp le GW-02 from Balbuena pluton shows the general microstructure of the rock. 140 Figure 123. Optical micrograph of samp le GW-02 from Balbuena pluton shows the undulat ory extinction in quartz crystal. 141 Figure 124. Optical micrograph of samp le GW-02 from Balbuena pluton shows clusters of crystals between plagioclase phenocrysts. 141 Figure 125. Optical micrograph of samp le GW-02 from Balbuena pluton shows the concentr ic zoning of plagioclase crystals. 142 Figure 126. Optical micrograph of samp le GW-02 from Balbuena pluton shows the fabrics grain shapes of quartz and plagioclase. 142 Figure 127. Optical micrograph of samp le GW-09 from Balbuena pluton shows the attached plagioclase crystals. 144 Figure 128. Optical micrograph of samp le GW-09 from Balbuena pluton shows the general texture of the rock. 144 Figure 129. Optical micrograph of samp le GW-09 from Balbuena pluton shows the undulat ory extinction in quartz crystals. 145 Figure 130. Optical micrograph of samp le GW-09 from Balbuena pluton shows the texture in zoned plagiolace. 145 Figure 131. Optical micrograph of samp le GW-05 from Balbuena pluton shows the general microstructure of the rock. 147

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xii Figure 132. Optical micrograph of samp le GW-05 from Balbuena pluton shows the zoned and the crystallized plagioclas e crystal. 147 Figure 133. Optical micrograph of samp le GW-05 from Balbuena pluton displays the we ll developed magmatic foliation. 148 Figure 134. Optical micrograph of samp le GW-10 from Balbuena pluton shows the zoned and rimed plagioclase phenocryt. 149 Figure 135. Optical micrograph of samp le GW-10 from Balbuena pluton shows the general texture of the rock. 150 Figure 136. Optical micrograph of samp le GW-10 from Balbuena pluton shows the attachment of plagioclase crystals. 150 Figure 137. Optical micrograph of volcan iclastic rock sample in PPL and CPL. 151 Figure 138. Optical micrograph of vol caniclastic rock sample shows the fine graine d quartz and feldspar phorphyroclast. 152 Figure139. Optical micrograph of vol caniclastic rock sample shows dynamically recr ystallized quartz crystals. 153 Figure 140. Optical micrograph of volcaniclast ic rock sample shows the micaceous mylonitic with plagioclase phorphyroclast. 153 Figure 141. Optical micrograph of volcan iclastic rock sample shows the pre deformational plagioclase. 154 Figure 142. Optical micrograph of volcan iclastic rock sample shows the pre-deformational plagioclase. 154 Figure 143. Two optical microgra phs of mylonitic foliation. 155 Figure 144. Optical micrograph of myloniric ro ck sample from aABF. 156 Figure 145. Optical micrograph of faulted mica fish. 156 Figure 146. Optical micrograph of conti nous foliation. 157 Figure 147. Optical micrograph shows feldspar grains in mylonite. 157

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xiii Figure 148. Optical micrograph shows the asymme tric tail of feldspar. 158 Figure 149. Optical micrograph shows the kine matics within the aABF. 159 Figure 150. Optical micrograph of the foliation plane. 160 Figure 151. Optical micrograph of the kinematics of the aABF. 160 Figure 152. Optical micrograph shows the boudina ge in plagioclase. 161

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xiv Petrographic and Kinematic Investigation of the Volcaniclastic and Plutonic rocks of the northern Alisitos Arc Segment, Baja California, Mexico Fatin Tutak ABSTRACT The Alisitos arc segment forms part of the western zone of Jura-Cretaceous Peninsular Ranges batholith of Baja California. It extends south from the ancestral Agua Blanca Fault to the state boundary between Baja Norte and Sur. The study area is located within a fold and thrust belt intruded by a num ber of plutons that were emplaced during and after the deformational event. The northern end of the Alisitos arc is characterized by subvertical tight to isoclinal folds and high-an gle reverse faults that define a northwest trending, southwest vergent fold and thrust be lt. The aABF defines the northern limit to the Alisitos arc segment. In th is study we present the resu lts of a petrographic study of igneous rocks in order to determine the relative timing and the distribution of deformation within the northe rn Alisitos arc segment. The study includes samples of the myloniti c shear zone of the aABF, and plutonic samples from intrusions proximal to the aABF emplaced la ter during regional

PAGE 18

xv deformation. These samples were investigated in order to characteri ze the distribution of the subsolidus strain in gr ain scale and the sense of shear during later phases of deformation in the northern Alisitos arc. The results are presented and discussed based on the mineralogical and textural observations from the Balbuena pluton, the Piedra Rodada pluton, and volcaniclastics that were deformed within the aABF. The Balbuena pluton, emplaced at ~ 108 Ma after the surrounding country rocks had already been fo lded, exhibits little if any evidence for subsolidus deformation. In contrast, the Pied ra Rodada pluton, emplaced at ~ 105 Ma just to the southwest of the aABF, exhibits a strong magmatic fabric overprinted by a moderate subsolidus fabric to the southwest that grades in to a strong subsolidus fabric with proximity to the fault. Kinematics obs erved from lineation pa rallel-foliation normal sections exhibit consistent topto-the southwest sense of shear.

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1 1. Introduction The study of a collided island arc in th e Peninsular Ranges Batholith (PRb) is considered important because it shows collis ion related deformati on which took place at amphibolite metamorphism facies. The Late Cretaceous Alisitos arc segment (Fig.1), located in the western Peninsular Ranges bath oliths, originated as an island arc. The basement stratigraphy of the Alisitos arc contains volcan iclastic and fossil-rich limestones that are intruded by numerous late Early Cretaceous plutonic bodies. Based on pluton geochronology and constraints from regi onal mapping, the collision of the Alisitos arc segment is inferred to have occurred between 115 Ma108 Ma (Johnson et al., 1999; Wetmore et al, 2005). When the Alisitos arc collided with th e southwestern margin of North America during the late Early Cretaceous (Todd et al ., 1988; Johnson et al., 1999a; Wetmore et al., 2002, 2003), the juxtaposition of the Alisitos ar c was accomplished through ductile shear along the ancestral Agua Blanca Fault (aABF) at the northern margin of the arc and the Main Mrtir Thrust on its eastern margin (J ohnson et al., 1999; Wetm ore et al, 2003). A broad (>15 km wide) fold and th rust belt (FTB) that parallels these two structures formed within the Alisitos arc du ring collision (Wetmore et al., 2005; Alsleben et al., in press ). As a result, the northern and eastern boundaries of the Alisitos arc are characterized by a

PAGE 20

2 strong and heterogeneous distribution of defo rmation that records a single episode of contraction. This makes it idea l for investigating processes of arc-continen t collision. Wetmore (2001) characterized the defo rmation in the FTB as exhibiting high ductile strain shortening near the fault z ones and plutons. The intense deformation is related to the collision a nd melt emplacement. Field observations in the Alisitos arc indicate that the intensity of deformation va ries from none in the west and southwest to isoclinal folds along ductile shear zones in th e east and northeast (Wetmore et al., 2002, 2003). Many workers use the microstructural ev idence to quantify deformation at the grain scale (Tullis et al., 1973; Vernon, 1975; White et al., 1978; Schmidt et al., 1980; Tullis and Yund, 1982; Urai et al., 1986; Kn ipe and Law, 1987; Hirth and Tullis, 1992). The plutonic rocks across the aABF are the starting points to unders tand the petrography and to constrain the deformational conditions experienced by these rocks following their emplacement. The aim of this study is to use microstructral data in conjunction with existing age data to constrain spatial variabil ity in the distribution of deformation across the aABF mylonitic ductile shear zone. This thesis concentrates on a ductile shear zone, the ancestral Agua Blanca fault (aABF), whic h is located approximately 80 km south of Ensenada, Mexico. The aABF was activ e between 115-105 Ma and the ductile deformation was preserved along a 100-200 m wide zone. When studying the microstructure of the deform ed rocks, it is necessary to identify the subsolidus fabric development. In th is study, standard techniques were used to determine the subsolidus fabric and the sens e of shear under the microscope (Passchier

PAGE 21

3 and Trouw 1996). Mica-fish, sigma-clasts and SC fabrics, can provide information about the direction of shear within rock bodies he lping to constrain overall kinematics of the system. In order to determine the kinematic s of the ancestral Aqua Blanca shear zone, field and microscopic observations were co mbined together. S-C fabrics, foliation and fibrous mineral growths were used as senseof-motion indicators to determine kinematics (Passchier and Trouw, 1996). In general, duct ile shear zones are ch aracterized by large strain accumulation relative to the surroundi ng structures (Ramsay and Graham, 1970; Ramsay, 1980), and the microstructures of s ubsolidus fabrics in deformed rocks are interpreted based on the identification of plas tic deformation at the grain scale. In our case, the relationships of 108 Ma Balbue na pluton and the younger 105 Ma Piedra Rodada pluton to the regional deformation c onstitute an important stage to understanding the tectonic development of the northern Alisitos arc as well as fold and thurst belts in general (Fig.1). Deformation within the 105 Ma Pied ra Rodada pluton, ~250 meters away, suggests that the deformation was distribute d over at least ~ 800 meters. The deformed and metamorphosed terrane allows us to study the subsolidus strain distribution at grain scale from aABF to at least ~ 7 km away from the shear zone, where Balbuena pluton was emplaced. The 108 Ma Balbuena pluton is exposed south of the younger 105 Ma Piedra Rodada pluton (Wetmore et al., 2005) (Fig.1), and it is composed of tonalite to quartz diorite which is surrounded by late Cr etaceous metavolcanic rocks of the northern Alisitos arc. At the outcrop scale, the crys tallized and cooled magmatic body shows no evidence for subsolidus deformation (Wetmore et al., 2005). Wetmor e et al. (2005) argue

PAGE 22

4 that the deformation associated with th e FTB most likely occu rred prior to pluton emplacement at ~ 108 Ma. Given the regional importance of the northern Alisitos arc segment and the relationships between plut ons and deformation, we have undertaken a detailed petrographic study to more clearly de lineate the relationships and to assess the distribution of strain through plutonic rock types across th e aABF in the Alisitos arc segment. Subsolidus fabric development with in the Piedra Rodada intrusive body defines the intensity of the deformation which increa ses to the eastern edges of the pluton and toward the aABF shear zone and shows th at deformation was not localized along the aABF shear zone. Igneous rocks are used in this study in order to understand the degree of the subsolidus deformation along the strike of the aABF in the Piedra Rodada pluton. Most of the detailed work was carried out on the strongly deformed rocks of the Piedra Rodada pluton that were intruded south of the aABF, micaceous quartzite mylonitic shear zone. Microstructures of the deformed Piedra Rodada pluton are interpreted by identification of subsolidus deformation conditions. I have studied the subsolidus fabrics in the plutonic rocks that are deformed under amphibolite facies conditions. Evidence that support subsoli dus fabrics include the intragranular deformation features in quart z and biotite crystals (Passc hier and Trouw 1996). Quartz grains show undulatory extinction and subgr ain development (mortar texture) that indicate the role of di slocation creep in the fabric shap e changes in quartz grains (Tullis and Yund, 1985; Groshong, 1988). We present data from field relations a nd petrographic analysis of two plutons in the northern Alisitos arc segment. Observati ons of subsolidus strain are completed

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5 through the study of thin sections of the plutonic rock sa mples that are intruded across the aABF and exhibit plastic deformation features (Fig.1). Since the fabrics within the Piedra Rodada pluton were modified by plastic deform ational processes, the subsolidus fabrics are used in this study to define the low st rain and high strain deformational conditions. The samples were classified into three sect ions; the first section includes the Piedra Rodada pluton which is emplaced close to th e aABF, the second section includes samples from the aABF shear zone, and the last se ction contains samples from the Balbuena pluton. Field observations combined with optic al observations indi cate significant subsolidus deformation over the younger 105 Ma Piedra Rodada pluton. The lack of the subsolidus strain within the 108 Ma Bal buena pluton, ~ 5km away from the aABF, indicates a strain shadow field within the ti me of emplacement. In addition to the above the kinematics of the shearing of the aABF within the 105 Ma Piedra Rodada pluton were deduced from field and optical studies. Petrographic analyses of ductile shear zone and plutonic rocks have been used to establish the intense of ductile deformation of the northern Alisitos arc segment at the grain scale.

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6 2. Geologic setting 2.1. Background geology of the study area The study area is located w ithin the Peninsular Ranges batholith (PRb) which is ~ 800 km long and extends from the Transver se Ranges in southern California through Baja California (Gastil et al., 1975; Kimbrough et al., 2002 ; Langenheim and Jachhens, 2003). The PRb is separated into two zones, the eastern PRb and the western PRb, which exhibit vastly different petr ologic characteristics and defo rmational histories (Gastil, 1975; Gromet and Silver, 1987; Silver and Ch appell, 1988; Gastil et al., 1990, Johnson et al., 1999). The western zone is dominated by gabbros/diorites and tonalites while the eastern zone is dominated by granodiorite-gr anite (Silver et al., 1979; Walawander et al., 1990). Based on geochemical and isotopic studies western intrusions are considered to be older than those of the eastern zone (Silver and Chappell, 1988). Variations across strike of the PRb have traditi onally been interpreted to indica te that the western zone of the PRb is underlain by oceanic crust that acc reted against the continental margin (i.e., the eastern PRb) during the Jurassic to middle Cretaceous (Gastil et al., 1975, 1981). The intrusions of the western PRb are t hought to have been emplaced at 3 to 7 km depth and between 125 and 105 Ma (Schmidt 2000). They were intruded into the volcanic rocks of the Santiago Peak Volcanics north of the Agua Blanca Fault (ABF); an

PAGE 25

7 active strike-slip fault that crosses the Baja Peninsular south of Ensenada, Mexico; and the volcanic rocks of the Alisitos Formation s outh of the ABF. Both sets of strata are Early Creataceous, though th e Alisitos are slig htly younger, and both are locally metamorphosed to lower greenschist facies (Silver et al., 1979; Silver and Chappell, 1988; Walawender et al., 1991). Gastil et al. (1975; 1981) identified many of the across strike variations to the PRb, as well as several north to south variati ons in the geology of the western PRb. Gastil and others divided the western zone into two arc segments, the Santiago Peak to the north and Alisitos arc segment to the south of the ABF. They argued that the western PRb as a whole originated as a fringing arc that coll apsed against the North America continental margin diachronously. Specifically, they suggest ed that Santiago Peak arc was reaccreted in the Aptian-Albian and Alisitos arc segm ent in the Albian. Gastil et al. (1981) characterized the Santiago Peak arc segment as being a subarial system, by contrast the Alisitos arc segment being ch aracterized by an abundance of marine sediments (e.g., limestones) and volcaniclastics supporting the interpretation that it was a submarine arc. They noted that variations along the wester n zone are most pronounced across the trace of the ABF. Wetmore et al. ( 2002; 2003; and 2005) noted severa l additional variations to the western zone of the PRb as well as the ex istence of an Early Cretaceous ductile shear zone, the aABF, which they argue represents a non-terminal suture between the Santiago Peak and Alisitos arc segments. The variations identified by Wetmore et al. (2002, 2003a) led to a tectonic model that differs from the model proposed by Gastil et al., (1981). Wetmore et al., (2002,

PAGE 26

8 2003a) stated that the western PR b did not evolve as a single tectonic element, and that arc segments north and south of the aABF did not share a common tectonic evolution until after their juxtaposition in the late Early Cretaceous. Wetmore et al. (2003) char acterized the structural re lationships between the two segments of the western zone of the PRb a nd identified the rocks of the North American continental margin. They demonstrate that the subareally deposited Santiago Peak volcanics rest atop of the Bedford Canyon Co mplex, a Triassic-Jurassic North America accreationary prism complex, across a non-conf ormity (Schroeder, 1976; Adams, 1979; Herzig, 1991; Sutherland et al., 2002). We tmore et al., (2002, 2003a) argue that the Bedford Canyon Complex were derived from th e North American margin, Santiago Peak arc was a continental margin arc. They show that the Alisitos arc is only in structural juxtaposition with the North American marg in, and the east and nor theast-dipping ductile shear zones represent th e non-terminal sutures. The Alisitos arc segment is interpreted to have been an island arc, exotic to North Amer ica and to the Santiago Peak arc prior to the collision in the Early Cretaceous (Johnson et al., 1999; Wetmore et al., 2003, 2005). 2.2. Local geology 2.2.1. Alisitos Arc The Alisitos Formation is the sole depos itional unit defining the Alisitos arc. It is characterized by deep to shallow marine and locally non-marine units (Allison, 1955; Gastil et al., 1975; Beggs, 1984). Additionally, th e Alisitos Formation includes a single, regionally-extensive limestone unit that can be traced from the Pacific coast southeast for more than 200 km. Volcanics of the Alisitos Formation range from basalts to rhyolites,

PAGE 27

9 but are dominated by basaltic and andesitic compositions in the study area (Wetmore et al., 2005). The Alisitos arc segment is characteri zed by deformation associated with both Main Martir thrust and the aABF. Deformati on is primarily associated with a southwestvergent FTB characterized by open folds to isoclinals south-west vergent folds and steeply NE-dipping thrust fau lts and ductile shear zones in cluding aABF (Johnson et al., 1990; Wetmore et al., 2005). The western boundar y of the Alisitos arc is characterized by low strain intensity/ductile shortening w ith lower greenschist facies metamorphism. However, rocks along the eastern boundary of the Alisitos arc segment are characterized by high strain intensity/ducti le shortening and lower amphi bolite facies metamorphism (Schmidt, 2000). In the Sierra San Pedro Mart ir area, crustal shortening in volcanic and sedimentary rocks is observed to the west of the Main Martir Thrust at kilometer scale (Gastil et al, 1975). Also a study by Johnson et al ., (1999) stated that the timing of crustal shortening in the Sierra San Pedro Martir area can be cons trained to between 115 and to 108 Ma. Several plutons have intruded during and after the deformation event. The Piedra Rodada pluton intruded close to the aABF shear zone and shows subsolidus fabric development along its northern margins. The plutons that intruded after the deformation event display magmatic fabrics that show no relationship to regional fabrics. The Balbuena pluton was intruded at ~ 108 Ma and records several magmatic pulses (Wetmore et al., 2002). The Balbuena pluton shows weak magmatic fabrics with the adjacent wall rocks strongly deflected away fr om regional trends. The lack of subsolidus

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10 fabrics in the Balbuena pluton was used to indicate that the deformation was concentrated along the aABF during and after the emplaceme nt at ~ 108 Ma (Wetmore et al., 2005). The Arce pluton is the northeastern-most intrusive body in the study area, and shows magmatic fabrics that are concordant with th e regional foliation (Wet more et al., 2005). The plutons are surrounded by coarse-grained volcaniclastic rocks. Fabrics in the metavolcanic unit indicate that these ro cks are highly deformed (Wetmore, 2002). 2.2.2. Piedra Rodada pluton Based on field relations, the shear is lo calized along the aABF and deforms the northern margin of the 105 Ma Piedra Rodada pluton (Fig.1). The northeastern half of the Piedra Rodada pluton was intruded clos e to the aABF at ~ 105 Ma and shows a strong subsolidus deformation fabric just ~ 250 meters away from the aABF shear zone (Wetmore et al., 2005; this study ). The interpretation of the de formational features within the Piedra Rodada pluton suggests that it was emplaced and deformed contemporaneously with shearing along the aABF. 2.2.3. Balbuena Pluton The Balbuena pluton was emplaced into th e northern Alisitos arc, south of the aABF at ~ 108 Ma (Wetmore et al., 2005). The pluton intruded into the Alisitos arc after most of the deformation associated with the formation of the fold and thrust belt at this location had already been completed. Emplacem ent of the Balbuena pluton involved the development of a kilometer wide structural au reole that includes deflections of host rock bedding and structures as much as 75 aw ay from regional tr ends. Based on field observations however, there is li ttle evidence of subsolidus strain within the intrusion

PAGE 29

11 suggesting that deformation had been lo calized along the aABF at the time of emplacement (Wetmore et al., 2005). The Balbuena pluton preserves a weak magmatic foliation that is discordant with the regiona l foliation/structure, as well as internal contacts between the various phases of th e intrusion. Structures in the host rocks surrounding the Balbuena pluton in clude a pronounced gradient in ductile strain proximal to the pluton margins (Wetmore et al., 2005). 2.2.4. Fold and Thrust Belts Main Martir Thrust (MMt) The Main Martir thrust coincides with the eastern limit of an ~ 17 km wide FTB. The regional structure of the northern and easte rn boundaries of the Alisitos arc is defined by southwest vergent fold and thrust belt (Johnson et al, 1999a). Johnson et al., (1999) studied the plutonic bodies which intruded th e fold and thrust belt through the area. Johnson et al., (1999b) defined the Main Martir Thrust as a non-terminal suture zone of the PRb. Johnson et al. (1999b, 2003) interpreted the magmatic fabrics in the plutons in the PRb to define the timing of the collis ion between the Alisitos oceanic arc and continental rocks from North America and esti mated the time to be ca. 110 Ma. However, the shortening in the continental margin con tinued from 132 Ma to 85 Ma (Schmidt and Paterson, 2002). Ancestral Agua Blanca fault (aABF) The aABF forms the northeastern limit to an ~ 17 km wide FTB and extends into the Alisitos arc. In 2005, Wetmore and co-workers charact erized the deformation in the

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12 FTB by high ductile strain shortening near the fault zones and plutons. The intense deformation is related to the collision of the Alisitos Arc with the North American margin. Field observations in the Alisitos arc indicate that the intensity of deformation increases from essentially none to the southw est to isoclinal folds along the ductile shear zones to the east and northeas t (Wetmore et al, 2003, 2005). 2.3. Previous work and field observations Wetmore (2003) mapped the study area a nd completed a deta iled stratigraphic study of Early Cretaceous Alisitos Formation in this region. The structural observations were done by Wetmore et al., (2002) acro ss the aABF in order to understand the deformation and the kinematics of the norther n Alisitos arc at a re gional scale. Wetmore et al., (2002) observed a number of faults between San Vicen te and the aABF. The faults from the northeast and southeast are the aABF, the El Tigre fault and El Ranchito fault. The aABF zone is a 100-200 m wide shear zone that is broadly distributed over 17 km in the northern Alisitos arc. The El Tigre fault is located southwest of the Balbuena pluton (Fig.1). The folds were mapped between San Vicente and the aABF by Wetmore et al. (2002), and show changes in tightness and di p based on their axial surfaces. The axial surfaces of all folds are subparallel at ~ 300 and the folds exhibit moderate hinge thickening. Between the northeastern margin of the San Vicente and El Ranchito fault upright and horizontal folds are observed. Betw een the El Ranchito and El Tigre faults, overturned tight to isoclinal fold is observe d. The overturned limb of the fold is cut out by the El Tigre fault. At least five overturned folds can be observed between El Tigre and the aABF. The axial surfaces of these folds st eepen to the northeast, while by contrast,

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13 the El Tigre fault steepens to the southwes t (Wetmore et al., 2002). North of the active ABF, upright and tight folds were observed between Aleman pluton and Agua Blanca pluton (Wetmore et al., 2002). The Northern Alisitos arc in western PRb, Baja California is an excellent location to study the Alisitos arc segm ent that juxtaposed by the aABF and MMt ductile shear zones. The plutons, emplaced across the aABF, have been studied by Wetmore (2002), who examined the deformation and regi onal metamorphism surrounding the plutons. Understanding the relationships of the 108 Ma Balbuena pluton and the younger 105 Ma Piedra Rodada pluton to regional deformation is an important step in understanding the tectonic development of the northern Alisitos ar c and the associated fold and thurst belt. Field observations show that ductile s hortening generally increases toward the aABF (Wetmore et al., 2005). The region surr ounding the El Ranchito fault, ~ 7 km southwest of the aABF, is also characterized by elevated strain inte nsities, and has an increase in percent ductile shortening. By c ontrast, the area surrounding the El Tigre fault is estimated to have low percentages of duc tile shortening at the outcrop scale. In the central part of the FTB, measured shorteni ng relating to the folding of the Alisitos arc segment adjacent to the Main Ma rtir thrust is as much as 60 % of ductile strain shortening (Wetmore et al., 2002). The Balbuena pluton is composed of th ree phases (Wetmore et al., 2001), a quartz dioritic middle phase and t onalitic inner and outer phase s. Wetmore (2003) provides a detailed geologic map of the Balbuena pluton and its surrounding host rocks.

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14 Additionally he reports U/ Pb zircon ages of 108.63.96 Ma and 107.73.4 Ma. Wetmore (2003) argues that emplacement of the Balbue na involved two stages of development. The early emplacement of the middle quartz di orite was accompanied the formation of a kilometer wide structural aureole, and include s large scale deflections of host rock panels by as much as 75 away from regional tre nds. The second phase of emplacement occurred during the emplacement of inner and outer t onalitic phases. Their emplacement involved large-scale stoping of the surrounding host ro cks and the earlier emplaced middle phase of the Balbuena pluton. Evidence for stoping by the inner and outer t onalite phases can be observed in the form of stoped blocks pres erved throughout the inne r and outer tonalite phases of the intrusion.

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15 Figure 1. Generalized geologic map of th e study area (After We tmore et al., 2003). Alisitos Arc Santiago Peak Arc Piedra Rodada Pluton Balbuena Pluton Ancestral Agua Blanca fault Active Agua Blanca fault Arce pluton El Aleman Pluton 0 5km N Overturned anticline Alisitos plutons Santiago Reverse fault 3120’N 11610’W

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16 3. Purpose The main goal of this study is to defi ne the petrographic variability within the Balbuena Pluton, subsolidus versus magmatic fabric development, and to understand the distribution of deformation within the pluton ic and volcanic rocks of the northern Alisitos arc segment. The study of subsolidus condi tion in the plutonic rocks is the key to understanding the deformation style that is asso ciated with the aABF in the Alisitos arc segment. Samples of the tonalitic rocks with different amounts of strain have been collected within the Piedra Rodada pluton across the aABF and the thin sections have been examined in order to constrain the dist ribution of deformation in the plutonic body during the shearing at the amphi bolite metamorphic grade. I studied the three domains within the A lisitos arc; the Piedra Rodada pluton (high-moderate strain), the aABF (high stra in mylonites) and the Balbuena pluton (low strain-undeformed). The plutonic bodies are treat ed separately in this study. All field and microscopic data are presented in Appendices A, B and C. We ha ve used petrographic evidence for subsolidus strain. A kinematic an alysis based on tails of feldspar, mica fish and quartz microstructures has been carried out to define the sense of shear on the Csurfaces in the plutonic and mylonitic rock samples.

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17 Samples used in this study included thos e collected by Wetmor e as part of his dissertation (Wetmore, 2003) and those collected during the field work completed in late fall of 2006. Figure 1 shows the sample locatio ns that were used in this study. All collected samples were oriented in the field such that kinematics determined from thin section observation could be reliably placed into field context. Thin section billets were cut in-house and sent off to be transformed in to large format (2X3 inch) sections. A total of 28 samples from the northern Alisitos arc se gment were analyzed in this study. For the microstructural analysis NIKON-Eclipse LV 100POL polarizing microscope was used. In this study I present the results of petrographic and kinematic analysis of the Northern Alisitos arc, Baja California, Mexic o. It is the main purpos e of this thesis to describe the mineralogical and textural ch anges that accompany regional metamorphism and deformation in the Northern Alisitos arc at thin section scale. The goals of this petrographic investigation are: a) to evaluate the petrographic variability within the Balbuena pluton, based on samples taken from eleven outcrops which were classified based on the mineralogic composition and minera l relations b) to evaluate the state of fabrics within the Balbuena, and distinguish magmatic versus subsolidus fabrics using samples were taken from ~1 to ~5 km distance away to the aABF shear zone, c) Samples were collected from the Piedra Rodada plut on in order to document the transition from the magmatic to strong subsolidus fabrics with in the Piedra Rodada Pluton, and to assess the kinematics of shear for the ro cks within the aABF shear zone.

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18 4. Identification of the Subsolidus Fabrics and Kinematic Analysis A Microstructural Study Various deformation mechanisms appear to have operated in the Piedra Rodada plutonic rocks. The dominant microstruc tures for quartz crystals are undulatory extinction with grain size reduction as is show in Figure 2, although intracrystalline microfractures are observed. Intracrystalline de formation features in quartz suggest that these crystals were formed by dynamic recrysta llization (Fig.3). Samples analyzed in this study demonstrated that quartz crystals ar e recrystallized, and have lobate grain boundaries, and grain boundary migration (f ig.4). Microstructures of deformation temperature ranges between 300 to 500 C. Figure 5 shows that near the plagioclase crystals, the quartz grains are smaller and have more irregular boundaries than those further away. Biotite grains are anhedral to acicular, and preserve plastic deformation features which recognized by sweeping undulat ory extinction as is shown in Figure 5. Biotite grains define the subsolidus foliation in all thin section samp les that is collected from the Piedra Rodada pluton, and the subso lidus foliation is mostly reflected around the plagioclase crystals as is shown in Figures 3 and 6. Hornblende grains occur in lenticular shapes, and irregular margins appear to have been caused by subsolidus deformation as shown in Figure 5. Figures 3 and 6 show that the plagioclase crystals have igneous textures suggesting magmatic flow with no evidence for subsolidus deformation, and the main deformation effects in plagioclase crys tals are minor microcracking as shown in

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19 Figure 6. The plagioclase grai ns display biotite folia development along their margins that forms a sigma clast (Fig.6), which was us ed in order to understand the kinematics of this study area. The S-C surfaces are shear sense indicator, and are often used to understand the intensity of deformation within the Piedra Rodada pluton. Shear related fabrics are preserved by elongated biotite a nd hornblende grains which define the C surfaces of the micro shearing, shown in Figure 7. Figure 2. Sample from Piedra Rodada pluton (thin section 5/20/02 A). Quartz crystals show grain boundary migration due to plas tic deformation. The grain boundaries are smoothly curved. Scale of view: 2 mm.

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20 Figure 3. Sample from Piedra Rodada pluton (thin section 5/20/02 A). Overgrowth twins in plagioclase surrounded by biot ite folia (B). The anhedral plagioclase crystal has a strong folia development on th e top. Scale of view: 1mm. Figure 4. Sample from Piedra Rodada plut on (TW-04). Amoeboid and interlobate quartz aggregates are due to dynamic crystallizat ion and grain boundary migration suggesting intracrystalline deformation. Scale of view: 2mm. B

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21 Figure 5. Sample from Piedra Rodada pluton (thin section TW-04). Undulose extinction in rotated biotite grain in the middle of the micrograph. Lenticular ho rnblende grains are abundant due to subsolidus defo rmation. Scale of view: 5mm. Figure 6. Sample from Piedra Rodada pluton (thin section 5/20/02 A) shows microcracking in plagioclas e. Plagioclase crystal is surrounded by biotite foliation. Biotite crystals have a nicely developed tail on the left side of the pi cture. Scale of view: 1mm.

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22 Figure 7. Sample from Piedra Rodada pl uton (thin section 1/17/04 B). Optical micrograph shows biotite folia development, and hornblende crystals which are drawn into folia. Scale of view: 2mm. In this study thin section samples were described with respect to the mineral composition, texture, subsolidus fabrics and ki nematics. The identified subsolidus fabrics were used to determine the di stribution of the strain with in the study area. All samples were reoriented in the field in order to analyze the kinematics within the aABF shear zone and the deformed plutonic rocks whic h are studied; however, the kinematic could not be seen in all samples. The optical mi crographs were taken for all thin section samples in order to demonstrate the intensity of the subsolidus deformation away from the aABF and to clearly define the kinematics of this study area. Thin section samples were used from Pi edra Rodada pluton and aABF shear zone, to study the kinematics of this area. Standard techniques were used to determine the sense H C

PAGE 41

23 of shear under the microscope (Passchier and Trouw 1996). Based on mica-fish, sigmaclasts and S-C fabrics, the direction of the movement is determined within the Piedra Rodada plutonsubsolidus fabrics. To determ ine the kinematics of the shear zone, field and microscopic observations were combined to gether. S-C fabrics, foliation and fibrous mineral growths were used as sense-of-m otion indicators to determine kinematics (Passchier and Trouw, 1996). Mica fish observed from collected sample within the aABF displays the direction of the shearing as shown in Figure 9 (a rrows show the direction of the movement). Crenulation cleavage S2 is common in the shear zones; in this study cleavage development is observed with in the aABF and shown in Figure 10. Microstructural observations also su ggest a microlithons development (S1 ) that appears between (S2 ) planes as shown in Figure 10. Figure 8. Sample from Piedra Rodada pl uton (thin section TW-04). Biotite grain leading into biotite folia, i ndication of top to the sout hwest (i.e., top of section dips to the northeast) movement. Scale of view 5mm.

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24 Figure 9. Sample from aABF shear zone (thi n section TW-06). Mica fish phorphyroclast in ultramylonitic shear zone Optical micrograph for samp le TW-06 from aABF shear zone. Scale of view: 7mm. SW NE

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25 Figure 10. Sample from the aABF (thin sec tion 1/12/03). The optical micrograph shows continuous foliation and crenulating s econdary foliation. Scale of view: 9mm S2 S1

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26 5. Piedra Rodada Pluton and the aABF Mylonitic Ductile Shear Zone The Piedra Rodada pluton is the northeas tern most intrusion entirely within the study area (Fig.11). The Piedra Rodada plut on intrudes the Alisitos Formation along the south side of the aABF approximately 3 km northeast of the Balbuena pluton. The Piedra Rodada pluton has been dated at 1053.4 Ma (Wetmore et al., 2005). The pluton is comprised of tonalite with small diorite and is characterized by magmatic fabrics within the southwestern half of the intrusion and increasingly str ong subsolidus fabrics in the northeastern half, proxima l to the aABF. The magmatic fabr ics, largely parallel the trace of the fault and the subsolidus fabrics in the northeastern por tion of the pluton. 5.1. Petrography The thin section analyses were made to establish percent proportion of minerals, grain size, mineral alignment, and deformatio nal microstructures. Mylonitic shear zone descriptions were made based on the percen tage of matrix compared to that of porphyroclasts using the tec hniques described by Schmid and Handy (1991). The dynamics of mylonite development were defined after White et al. (1980) and Tullis et al. (1990). The classifications of rock cl eavage were made based on Powell (1979).

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27 Based on the petrological data, the Pied ra Rodada pluton is a homogenous pluton, consisting of 65% plagioclase, 20% quartz, and % 15 mafic minerals. The plagioclase crystals are subhedral to anhe dral, with sizes ranges from 0.1 to 4mm, and they often exhibiting oscillatory zoning (cor e, 32 An%; rim, 43 An%). Quartz crystals are anhedral, and medium-fine grained, and their texture is irregular and show evid ence of subsolidus deformation such as undulatory extinction, a nd grain boundary migration, and grain size reduction as a result of dynamic recrystallizatio n. The recrystallized quartz grains are 0.08mm in diameter, and their shape is mosaic. Biotite is defined by pale brown to dark brown colored crystals which has anhedral shape, and has medium to fine grained crystals, being partly replaced by chlorite. In some of the thin sections, biot ite has undergone partial neocrytallization to well aligened new grain of biotite, has drawn into incipient folia, and up to 2mm. Hornblende occurs equant, pale green to da rk green, medium to coarse grained, and has small inclusions of Fe-oxides and rounded quartz crystals.

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28 Figure 11. Geologic map of the Piedra R odada pluton includi ng sample locations. 5.2. Subsolidus Deformation within the Piedra Rodada Pluton To establish the spatial dist ribution of subsolidus strain associated with the aABF, we collected ten samples from Piedra Rodada Pluton south of the aABF as shown in Figure 11. For the sake of clarity, samples in this study were labeled in relation to their geographic position and distance from the aABF (fig.12). The first letter indicates the transect from which the sample is taken (W-f or the western, C-for the central, CE -for east central and E-for the eastern), correspondin g to the relative subs olidus conditions of tonalitic rocks. Piedra Rodada pluton N

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29 Figure 12. Samples collected from the Piedra Rodada pluton. 5.3. Low subsolidus deformation (sample west) Sample TW-04 was collected ~ 700m from aABF and shows high subsolidus strain rate. This rock consists of 18% quart z, 65% plagioclase, and 17% mafic minerals. In this sample, strain is concentrated in quartz and biotite wrapping around plagioclase phenocrysts. Plagioclase phenocrysts have microfractures, quartz and biotite filled fractures. Magmatic flow evidence observed in rectangular plagioclase crystals, which have concentric zoning and simple twinning f eatures, remain as an igneous texture. In this thin section sample, the plagioclase cr ystals have microfractures, and have weak Sample C Sample W Sample CE Sample E N

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30 fabric preferred orientation which is defined by elongated biotite grains. The elongated plagioclase phenocrysts are mantled by biotite as shown in Figure 14. Quartz crystals are dynamically recrystallized, and have suture d grain boundaries, and undulatory extinction. All features in quartz crysta ls are indicative of high gr ain boundary migration. Minor quartz subgrains development and undulatory extin ction in biotite grains are evidence of intracrytalline deformation as shown in Figur e 14. Biotite grains are anhedral, and have irregular boundary shapes, a nd undulatory extinction indica tes subsolidus deformation (Fig. 14 and Fig.15). Figure 16 shows the asy mmetric tails development in plagioclase crystal which indicates the se nse of shear. Hornblende gr ains are equant, and have poikilitic texture. Few lentic ular hornblende grains obs erved indicate subsolidus deformation. Figure 13. Geologic map shows sample loca tions for low subsolidus deformation. N Piedra Rodada pluton

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31 Figure 14. Sample from Piedra Rodada pluton (thin section TW-04). Plagioclase crystals have concentric zoning, igneous textures in plagioclase still remains. Plagioclase phenocryst do not have foliati on deflected around them. Undu lose extinction in rotated biotite grain in the middle of the microgr aph can be seen. Scale of view: 9mm. Figure 15. Sample from Piedra Rodada pluton (TW-04). Strongly deformed biotite on the top of the micrograph, shows undulose extinction and kink bands in general. Plagioclase crystals in the middle have deformation twins and irregular margins. Scale of view: 9mm.

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32 Figure 16. Sample from Piedra Rodada pluton (thin section TW-04). Plagioclase crystals have microcracks and microfractures as evidence for brittle deformation. Quartz aggregates are dynamically recrystallized. Plagioclase has asym metric biotite tail development. Scale of view: 9mm. Sample PHW 7-01-1-F was collected ~ 700m from aABF and shows moderate subsolidus strain. This sample consists of 20% quartz, 60% plagioclase, and 20% mafic minerals. The subsolidus deformational fabric in this thin section sample is relatively small and most of strain is accommodated by crystal plastic deformation of quartz and biotite. The subhedral shape of plagioclase phenocryst is largely preserved with an average size that ranges from 0.5 to 6 mm. Plagioclase phenocryst is zoned, microfractured, and highly altered, and surrounded by anhe dral quartz crystals which suggests magmatic flow. In this sample, plagioclase often exhi bit concentric and oscillatory zoning pattern (cor e, 38 An%; rim, 42 An%). The plagioclase phenocryst is

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33 aligned parallel to the biotite foliation, and mostly parallel to the hornblende grains. Quartz is anhedral, and elongated with irre gular margins. Figure 19 shows quartz ribbons indicating intracrystalline deformation, and these ribbons are elongated and may be formed due to grain boundary migration. Biotite is mostly anhedral, and pleochroic brownish to dark brownish, mostly grown between plagioclase phenoc rysts as coarse grained or rarely as foliated finegrained crystals as shown in figure 17. Boundaries between quartz and plagioclase are smoothly curved and quartz shows undulat ory extinction paralle l to plagioclase phenocryst margins (fig. 17). Hornblende occurs anhedral pale green to dark green grains with small rounded inclusions of quartz crystals. Simple twini ng is preserved in lenticular shaped hornblende that is surrounded by fine grained quartz crystals as shown in figure 18. Figure 17. Sample from Piedra Rodada pl uton (thin section 701-1-F). The optical micrograph shows albite and multiple twining in plagioclase crystals. The space between these two crystals filled with biotite (B). Elongated biotite fa brics are parallel to crystals margins. Scale of view: 5 mm. B

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34 Figure 18. Sample from Piedra Rodada pluton (thin section 7-01-1-F). Simple twining in euhedral hornblende (H) surr ounded by fine grained quartz cr ystals. Hornblende crystal has small inclusions of rounded plagiocl ase inclusions. Scale of view: 1mm. Figure 19. Sample from Piedra Rodada plut on (thin section 7-01-1F). Minor quartz (Q) ribbons observed. Scale of view: 1mm. H Q

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35 5.4. Highest subsolidus defo rmation (sample central) Samples 1/17/04 B and 5/20/02-A were collected ~ 285m from aABF and shows high subsolidus strain. These rocks consist of 13-15% quartz, 63-65% plagioclase, and 19-22% mafic minerals. The subsolidus deformati onal fabric in this thin section sample is relatively high and most of strain is acco mmodated by crystal plastic deformation of quartz and biotite which wra pping around plagioclase phenocry st. The subhedral shape of plagioclase phenocryst is largel y preserved with an average si ze that ranges from 0.2 to 4 mm. Plagioclase phenocryst is zoned, microf ractured, and highly altered, and surrounded by anhedral quartz crystals wh ich suggests magmatic flow. In this sample, plagioclase often exhibit concentric and oscillatory zoni ng pattern (core, 38 An%; rim, 48 An%). The plagioclase phenocryst is aligned parallel to the foliation formed by biotite, and mostly parallel to the hornblende grains. Around the ph enocryst of plagioclas e, the quartz grains are fine-grained and have more elongated and have irregular boundaries than those further away, and the foliation is mostly re flected around the phenocrysts of plagioclase, and biotite faces are parallel to plagioclase margins (Fig.21). Quartz is anhedral, and el ongated with irregular margin s, and uniform grain size (grain size reduction) suggest that they formed by dynamic recrystallization. The dynamic recrystallization in quartz switche s from grain boundary sliding to grain boundary migration close to the aA BF shear zone due to an increase in strain as shown in Figure 22. Boundaries between quartz and plag ioclase are sutured and quartz shows

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36 undulatory extinction and indica tive of high grain boundary migr ation as shown in figure 22. Biotite is mostly fine grained acicular, and pleochroic brownish to dark brownish, mostly aligned around plagioclase phenocrysts as foliated fine-grained crystals as is shown in figure 21. The strong development of bi otite folia can be clearly seen. C fabrics are well developed (Fig.23 and Fig.24) and ma rked by elongated biotit e. Biotite crystals are completely neocrystallized into fine grained crystals, and pervasive undulatory extinction microstructu ral features suggest subsolidus deformation. Spaces between plagioclase crystals are fill ed with elongated quartz crysta ls. Few lenticular hornblende grains indicate for subsolidus deformation. The subsolidus deformation clearly observed from the elongated phenocrysts of plagiocl ase that are surrounded by biotite folia and fine grained quartz aggregates The alignments of plagiocl ase and hornblende are both parallel to the biotite folia development (Fig.23 and Fig.24).

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37 Figure 20. Geologic map shows sample loca tions for high subsolidus deformation. Figure 21. Sample from Piedra Rodada pl uton (thin section 5/ 20/02 A) Rectangular plagioclase crystal is altered to sericite (S ). A strong biotite (B) foliation on the top and on the left of the plagioclase. Between bi otite foliation and plagioclase crystal, recystallized quartz grains are visible but not entirely elongated. Scale of view: 1mm. B S N Piedra Rodada pluton

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38 Figure 22. Sample from Piedra Rodada plut on (thin section 5/20/02 A). The Intra and intergranular microcracks ma rked by undulose extinction a nd the grains boundaries are irregular in quartz crystals (Q). Most cr acks are intragranular, a few intergranular microcracks cross several grain bounda ries. The arrow indicates for dynamic recrystallization. Scal e of view: 1mm. Figure 23. Sample from Piedra Rodada pl uton (thin section 1/17/04 B). The Optical micrograph show biotite folia development. Ho rnbende crystal is dr awn into folia. Scale of view: 2mm. Q C S

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39 Figure 24. Sample from Piedra Rodada plut on (thin section 1/17/04 B). S surfaces are well defined by biotite grains. Bi otite crystals are neocrysta lized and drawn into folia, C surfaces are defined by new biotite crystals. Scale of view: 5mm. Sample 1/17/04-C was collected ~ 48 0m from aABF and shows moderate subsolidus strain. These rock s consist of 20% quartz, 64% plagioclase, and 15% mafic minerals. The subsolidus deformational fabric in this thin section sample is relatively higher than the other samples collected fr om sample W, and most of strain is accommodated by crystal plastic deformation of quartz, and biotite which wrapped around plagioclase phenocryst and forms the C surfaces of the microshear zone. The subhedral shape of plagioclase phenocryst is largely preserve d with an average size that ranges from 1.5 to 3 mm. Plagioclase phenocry st is zoned, microf ractured, and highly altered, and surrounded by anhedr al quartz crystals which su ggests magmatic flow. In this sample, plagioclase often exhibit concen tric and oscillatory zoning pattern, ranging from An 32 to An 36. The plagioclase phe nocrysts are mostly surrounded by biotite foliation, and Biotite folia loca lly form tails around plagioclase crystals as shown in S C

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40 figure 26. In figure 26, the biotite folia surr ounding the plagioclase cr ystals shown with arrow suggest subsolidus deformation, and the plagioclase phenocryst shows asymmetrical clast tail which is deve loped by biotite. Around the phenocryst of plagioclase, the quartz grains are fine-grain ed, and elongated than those further away, and the foliation is mostly reflected around the phe nocrysts of plagioclase, and biotite faces are parallel to plagioclase margins (Fig.26). Quartz crystals are medium sized, and have straight and smoothly curved boundaries, and subgrains are elongated, and undulatory extinction features of subsolidus deformati on indicative of high grain boundary migration as shown in Figure 26. Biotite is mostly fine grained, and pleoch roic brownish to dark brownish, mostly aligned around plagioclase phenocrysts as foli ated fine-grained crystals as shown in figures 25 and 26. Biotite crystals are elongated, and show pervasive undulatory extinction microstructura l features suggest subsolidus defo rmation. In this thin section sample, the coarse biotite grai ns poikilitically encl ose rounded grains of quartz, and the included quartz crystals ha ve undulose extinctions and irregular margins. Figure 25 shows the elongated biotite which displays foliation and form C surfaces, however, biotite folia development is weaker than samples 1/17/04 B and 5/20/02-A at low strain rate, and no S surfaces developed in this th in section sample. The minor kink bands in biotite crystals indicate de formation temperatures of 300 C. Figure 25 show s the aligned hornblende crystals which grow parallel to the x-axis of the thin section, and these hornblende grains grow parallel to biotite folia development.

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41 Figure 25. Sample from Piedra Rodada pluton (thin section 1/17/04-C). Biotite foliation development can be seen in the optical micrograph (PPL). Scale of view: 10mm. Figure 26. Sample from Piedra Rodada pluton (thin section 1/17/04-C). Optical micrograph for the sample is taken in CPL. The discussion of the micrograph is in the comments section. Scale of view: 10mm. C

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42 5.5. Moderate subsolidus deform ation (sample central east) Sample 5/20/02-B was collected ~ 60 0m from aABF and shows moderate subsolidus strain. This rock consists of 20% quartz, 45% plagioclase, and 35% mafic minerals. The subsolidus deformational fabric in this thin section sample is relatively smaller than in the other samples collected from sample C, and most of strain is accommodated by crystal plastic deformation of quartz and biotite, which are wrapping around plagioclase and hornblende phenocry ts. The subhedral shape of plagioclase phenocryst is largely preserved with an av erage size that ranges from 0.4 to 3mm. Plagioclase phenocryst is zoned, broken, microfractured, and highly altered, being surrounded by anhedral quartz cr ystal. Plagioclase often exhibits concentric and oscillatory zoning patterns (c ore, 36 An%; rim, 40 An%). Quartz crystals are mostly elongated al ong the aligned biotite and fine grained anhedral, having smoothly curved boundaries The significant gr ain size reduction in quartz crystals is caused by different deformation mechan isms such as grain boundary migration and dynamic recrystallization. The bi otite is defined by pale brown to dark brown colored grains, and biotite is partly replaced by chlorite. The elongated biotite grains are neocrystallized, and tiled in this th in section sample. In addition white mica of pale green colors occurs around the pl agioclase and hornblende crystals. The neocrystallized biotite crysta ls are up to 0.1 mm and genera lly form C-surfaces of the foliation. Figures 28, 29 and 30 show the mosa ic textured and the dynamic recrystallized quartz grains. We can determine that evidence of subsolidus deforma tion is preserved in

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43 this thin section. Most of th e biotite has undergone neocryst alization and is aligned as shown in Figure 31. This figure also displays the hornblende crysta l that is surrounded by recrystallized quartz grains, a nd the initiation of biotite fo liation against the strong and fractured hornblende megacryst. The elongated biotite crystals preser ve a clear foliation, but the plagioclase crystals, which are surr ounded by biotite, are st ill rectangular in shape. Figure 32 shows the biotite crystal which is el ongated and displays sweeping undulatory extinction. Figure 27. Map for locations of the sample CE. N Piedra Rodada pluton

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44 Figure 28. Sample from Piedra Rodada pluton (thin section 5/20/02-B). Optical micrograph of the sample shows curved folia tion. Foliation develope d by biotite crystals and surrounds quartz and plagio clase grain margins. The foliation curves smoothly to become parallel to the lower e dge on the right of the micr ograph. The quartz crystals (Q) margins are parallel to the foliation, but not a ll quartz grains are el ongated. Scale of view: 5mm. Figure 29. Sample from Piedra Rodada pl uton (thin section 5/20/02-B). Submagmatic grain shape fabric. The biotite a nd quartz grains defi ne a grain fabric shape parallel to the top and to the bottom of this micrograph. R ecrystallized quartz grains are visible. Subgrains and undulatory extinction are clear evidence for deformation. Quartz crystals are elongate parallel to biotit e fabric. Scale of view: 5mm. Q

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45 Figure 30. Sample from Piedra Rodada plut on (thin section 5/20/02B). The lenticular shape in hornblende appears to have b een caused by solid state deformation. The recrytallized quartz aggregates are parallel to hornblende margins and they are elongated. Scale of view: 7mm. Figure 31. Sample from Piedra Rodada pluton (thin section 5/20/02-B) The initiation of biotite foliation against the strong and fractur ed hornblende magacryst. The hornblende crystal is surrounded by recrystallized quartz grains. Scale of view: 7mm.

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46 Figure 32. Sample from Piedra Rodada plut on (thin section 5/20/02-B). Biotite shows undulose extinction. This observation can be cons idered as magmatic to solid state flow. Scale of view: 7 mm. Sample 1/17/04-D was collected ~ 850m from aABF and shows low subsolidus strain. This rock consists of 20% quartz, 65% plagioclase, and 15% mafic minerals. The subsolidus deformational fabric is relatively small in this thin section sample. The subhedral shape of plagioclase phenocryst is largely preserve d with an average size that ranges from 0.6 to 2mm. Plagioclase phenocry st is zoned, and pa rtially altered, and microfractured, and surrounded by anhedral qu artz crystals whic h suggest magmatic flow. In this sample, plagioclase often exhi bit concentric and osci llatory zoning pattern, (core, 30 An%; rim, 33 An%). Quartz is anhe dral, and has smoothly curved and straight boundaries, and undulatory extinction ; all features being typical of subsolidus strain and indicative of low grain boundary migrati on. Grain size reduction observed in quartz suggests a transition from magmatic deformatio n to subsolidus deformation as shown in

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47 Figure 34. Biotite grains are anhedral and mostly broken, and show sweeping undulatory extinction as shown in Figures 33 and 35. Biotite crystals ar e usually surrounded by anhedral quartz indicative of magmatic flow. The biotite folia is weak but clearly observed in this thin section. Phenocrysts of plagioclase are elonga ted and parallel to weak biotite folia. The only deformation evid ence can be seen from the aligned biotite and hornblende crystals which suggest low temperature deformation. Figure 33 shows the phenocrysts of plagioclase which is elongate d parallel to biotite crystals (Fig.33). Hornblende crystals are anhe dral and rarely elongated. Figure 35 shows the simple twinned and fractured hornble nde grain that is surrounded by fine grained quartz crystals and grows parallel to the biotite foliation. Figure 33. Sample from Piedra Rodada pl uton (thin section 1/17/04-D). Plagioclase shows overgrowth twins. On the bottom ri ght, biotite is highly deformed. Elongated plagioclase phenocryst margins are parallel to biotite folia (B). Qu artz aggregates are medium sized and have interlobate and am oeboid grain shape. Scale of view: 4mm. B

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48 Figure 34. Sample from Piedra Rodada pl uton (thin section 1/17/04-D). The simple twinned and fractured hornble nde grain can be seen. On the top, rectangular plagioclase (P) still remain as igneous texture. Scale of view: 9mm. Figure 35. Sample from Piedra Rodada pluton (thin section 1/17/04-D). Quartz aggregates have undulose extinctions and grai n boundary migration. Bi otite grains show undulose extinction which is evidence for low temperature plastic deformation. Scale of view: 9mm. P

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49 5.6. Highest subsolidus deform ation (sample southeastern) Sample TW-03 was collected ~ 1.5km away from aABF and shows low subsolidus strain. This rock consists of 18% quartz, 65 % plagioclase, and 17% mafic minerals. The subsolidus deformational fabric is relatively high in this thin section sample. Based on microstructural observatio ns, sample TW-03 represents the most intensively deformed part of the Piedra Rodada pluton. The subhedral shape of plagioclase phenocryst is largely preserved with an average size that ranges from 0.1 to 2mm. Plagioclase phenocryst is anhedral zoned, microfractured, and completely surrounded by neocrystallized biot ite crystals as shown in Figure 39. In this sample, plagioclase often exhibit concen tric zoning pattern with An 38. Biotite crystals are neocry tallized and well aligned (Figs. 38, 39, 40, 41 and 42). In addition white mica occurs around the plag ioclase phenocryts as shown in Figure 42. The development of biotitie folia and biot ite fish usually a ppears around anhedral plagioclase crystals (Fig. 39 and 40). Quartz show intercrystalline deformation features, grain size reduction, grain boundary migration and polygonal quart z crystals as shown in Figures 37 and 40. Figure 41 displays the fine grained quartz crystals which surround the plagioclase, and are not elongated. Figures 38 and 39 show the plagioclase phenocrysts that are surrounded by aligned biotite crysta ls. They are not elonga ted and the igneous texture still remains; no deformation features can be observed in plagioclase crystals. S-C fabrics are well developed, implying for hi gh temperature subsolidus deformation. S

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50 surfaces are marked by elongated biotite crystals, and C surfaces are marked by neocrystallized biotite crysta ls (Fig.41 and Fig.42). The subs olidus deformational fabrics are more pronounced than in sample C, that wa s discussed in the prev ious section. Figure 44 shows the increased strain converted biotite to fine grained, and has elongated crystals which define the C surfaces of the microshear ing and the neocrystalli zed biotite crystals are drawn into incipient folia and defines th e S-surfaces of the microshearing. From the microshear structure development (S-C fabric s) and intense plastic deformation that is observed in both biotite and quartz grains, we can suggest that sample TW-03 was deformed at higher subsolidus temperature conditions than sample C (central crosssection). Figure 36. Location map of sample E. N Piedra Rodada pluton

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51 Figure 37. Microfractures in hornblende filled with quartz aggregates. Quartz is included in hornblende. The inclusion shape is rounded. Quartz subgrains are anhedral and have straight smoothly curved boundaries. Polygonal quartz crystals appear. Scale of view: 5mm. Figure 38. Grain boundary migrati on indicated by the arrow. Plagioclase crystals are totally surrounded by biotit e crystals. On the top of the plag ioclase, biotite crystals have strong shape preferred orientati on parallel to the folia tion. Quartz grain on the left (shown with an arrow) is controlled by biotite grains. Quartz boundar y is perpendicular to biotite. Scale of view: 2mm.

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52 Figure 39. Elongated plagioclase crystal ha s a nice biotite folia around it. Quartz aggregates are elongated and show regular gr ain boundary shape. Biotite folia control the shape of the quartz aggregat es. Scale of view: 4mm. Figure 40. Irregular grain boundaries in plag ioclase surrounded with biotite folia. Plagioclase has a patchy deformation twins. Quartz aggregates have irregular boundary shapes and show undulose extinc tion. Scale of view: 4mm.

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53 Figure 41. Biotite foliation deflected around simple twinned plagioclase crystal. Irregular margins in plagioclase are due to the grai n boundary migration. Quartz aggregates have amoeboid grain shape regionally in this thin section. Scale of view: 5mm. Figure 42. Plagioclase phenocryst has biotit e tail and foliation deflected around it. Plagioclase phenocryst has small rounded quartz inclusions. Mica deformed to fine grain aggregates on the top left (shown in circle). Scale of view: 5mm.

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54 Figure 43. Plagioclase (P) crystal in the middl e of the picture has developed a tail from biotite folia. Quartz aggregates have irregular grain boundary sh apes in general due to the grain boundary migration. Quartz grains show grain size reduction. Scale of view: 9mm. Figure 44. Biotite grains have fabric preferred orientation and show strong folia around plagioclase crystals. Scale of view: 9mm. P C S

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55 Sample TW-01 was collected ~ 1km away from aABF and shows low subsolidus strain. This rock consists of 19% quartz, 66% plagioclase, and 15% mafic minerals. The subsolidus deformational fabric is relatively high in this thin section sample. Based on the microstructural observations, the sample TW-03 represents the most intensively deformed part of the Piedra Rodada pl uton. The subhedral shape of plagioclase phenocryst is largely preserved with an average size that ranges from 0.4 to 3.75mm. Plagioclase phenocryst is subhedral, zoned, and microfractured. In this sample, plagioclase often exhibit con centric zoning patterns with An 33. Cataclastic deformation can be observed both in plagioclase and quart z crystals, as well as the micro-cracks and micro-fracture between and across the grains in the thin section as shown in Figure 45. Subsolidus deformation indicators obse rved mostly in quartz, which have undulatory extinction and grain boundary rotati on, suggest low subsolidus deformation. Quartz crystals are anhedral and medium size d, and microfractured. No interstitial quartz is observed in this thin section. Figure 46 s hows the micro-fractured quartz crystal that is filled by biotite. Biotite is mostly anhedral, and pleochroic brownish to dark brownish, mostly grown between plagioclas e phenocrysts as coarse graine d. No fine grained folia of biotite and/or hornblende are obs erved in this thin section sa mple. Aligned biotite crystals surrounded by anhedral non-deformed quartz ch aracterize magmatic flow. Both TW-01 and TW03 show grain boundary migration and grain boundary sliding that suggests subsolidus deformation. Hornblende occurs eq uant coarse grained. Fe w lenticular shaped grains are preserved due to weak subsoli dus strain. Figure 47 shows the lenticular hornblende grain that is surrounded by the acic ular biotite crystals. Minor chessboard

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56 texture in quartz observed in this thin sect ion indicates high subsolidus deformation as shown in Figure 48. Figure 45. Cataclastic deformation observed bot h in plagioclase a nd quartz crystals. Intragranular (1) and intergranular (2) ar e microfractures. Scale of view: 2mm. Figure 46. Microfractures in quart z are filled with crystalliz ed biotite. Intergranular micro-fracture can be seen in quartz crys tal which has irregular smoothly curved boundary shape. Scale of view: 2mm. 2 Q 1

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57 Figure 47. Lenticular hornblende crystals with irregular bound ary shape and have numerous amounts of inclusions. Inclusions of both plagioclase and Fe-oxides are rounded. The lenticular shape in hornblende is due to the solid state deformation. The lenticular hornblende did not draw n into folia. Scale of view: 2mm. Figure 48. Chessboard texture in quartz indicat es high subsolidus deformation. Scale of view 2mm.

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58 5.7. aABF mylonitic shear zone A microsturctural study Sample TW-06 collected within the aABF shear zone, the average grain size of the quartz has decreased due to grain size re duction. The thin section has ultramylonitic texture with no plagioclase porphyroclast. This thin section is composed of mica, chlorite, quartz and minor amount of feldspar and has a length of about 32.5 mm. Grain size reduction of quartz, mica, plagioclase, and chlorite crystals suggest dynamic recrystallization. In this sample shapes of minerals are usua lly euhedral with irregular boundaries. The dominant mineral in the aABF is quartz which is anhedral, and has grains show minor elongation. The undulose exti nction of the quartz is more intense than in samples from the margins of the shear zone. Figure 49 shows the increase in the percentage of small quartz grains, and elongate d quartz crystals mark the C surfaces of the microshear zone. In Figures 50, SA represents the mylonitic foliation, composed of fine grained feldspar and quartz, and SB is composed of larger grain sizes and dynamically recrystallized quartz grains and defined by the elongate grain shape alignment. The quartz and plagioclase size varies between 0.03 mm to 0.2 mm. Individual quartz grains are elongated with average size 0.01 mm. The oblique foliation of SA and elongated grain shape of SB indicate a sinist ral shear sense.

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59 Figure 49. Sample within the aABF shear zone. Figure 50. A strongly foliated mylonite shows bended quartz and mica cl asts in ultrafinegrained matrix and exhibits oblique foliati on. This is a micrograph from thin section TW06. Scale of view 5 mm. S C S(A) S(B)

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60 5.8. North of the aABF North of the aABF, three samples were an alyzed; sample 7/3/1-J, 1/17/04-A and 1/8/03 (Fig.52). The sample from El Aleman pluton (sample 1/8/03) represents magmatic fabric development that have not been aff ected by the aABF shear zone. Samples 7-3-1-J and 1/17/04-A were taken from Arce pluton a nd they show strong magmatic fabrics. For all sample, plagioclase phenocrysts remain in the igneous texture, and highly sericitized, and micro-fractured, and have c oncentric zoning-all features typical is of magmatic flow. Field observations suggest that the aABF is truncated by the Arce pluton. Based on the petrographic observations, samples from the Arce pluton do not have subsolidus fabric development and suggest that Arce pl uton intruded without being affected by the shear deformation event. All samples show, the alignment of elongated plagioclase and hornblende phenocrysts that formed without any evidence of plastic deformation, or elongation and grain size reduction of the qua rtz. The aligned plagioclase phenocrysts display oscillatory zoning patterns and they are highly altered to sericite. Biotite is mostly grown between plagioclase phenocry sts as coarse grained crystals; all features are typical of magmatic flow. Petrographic data for th ese samples are available in Appendix-D.

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61 Figure 51. Location of thin section sample s from north of the aABF shear zone. E l Aleman pluton Arce pluton Piedra Rodada pluton N

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62 6. Balbuena Pluton 6.1. Microstructural observations We collected samples in the Balbuena pl uton in order to define the mineralogic composition and textural characteristics of the pluton. Three samples from the inner phase, five samples from the middle phase, and three samples from the outer phase were collected from different outcrops within the pluton and Figure 53 shows the samples locations for this study. These samples were then analyzed in order to assess the relationship between the magmatic fabrics wi thin the Balbuena pluton to the deformed regional structure.

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63 Figure 52. Samples locations fr om the Balbuena pluton We used the magmatic plagioclase phenocry sts in the Balbuena pluton in order to understand the zoning and the melting patterns in the pluton. All samples had zoned plagioclase feldspar. The compositions of plagioclase are reported and included in Appendix-B as mole percent An and were determined by measurements of extinction angles according to the Michel Levy method th at uses the combined albite and Carlsbad twins in order of preference. Simple twin ing and oscillatory zoning in plagioclase N

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64 feldspar were mostly used to understand the igneous microstructure of the Balbuena pluton (Vernon, 1976, 1986a). Based on the petrogr aphic variations, the Balbuena pluton contains three different lithologies and each group is treated separately in this study: 6.1.1. Quartz Diorite Based on optical observations, samples GW-01, GW-02, GW-03, GW-04, and GW-06 have similar mineralogical compositions and mineral textures. Samples GW-02 and GW-03 have the same mineralogical and textural compositions suggesting that they have similar magmatic origin. All thin s ection samples have concentrically zoned plagioclase which reflects the overgrowth history and the rapi d change in the temperature of cooling. A very minor patc hy zoning observed in thin se ction GW-02 suggests a rapid increase in crystals or implies for magma mixing. The overgrowth textures of albite and multiple twining and zoned feldspars imply an earlier magmatic phase. Quartz dioritic samples contain broken K-feldsp ar and plagioclase feldspar crystals. Oscillatory zoning suggests repeated injection of mafic melt into the felsic magmatic chamber. From the observed igneous textures in plagioclase feld spar it can be suggested that the crystals changed in composition over time. Sample GW-06 has sharper oscillatory zoning patterns sugges ting greater magma interaction within the melt (Karsli et al., 2004) in the northeastern side of the pluton than in the northwestern side of the pluton. Th e abundance of oscillat ory zoned plagioclase phenocrysts in samples GW-01, GW-02, GW-06 s uggests that the repe ated injection of mafic melts was more likely for magma e xposed on the northern si de of the pluton.

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65 Sample GW-01 has a variety of zoning pa tterns which indicate a shift in the magmatic conditions at the time of the cr ystallization. Sample GW-01 contains more potassium feldspar (15%) than in the other samples. The percent proportion of potassium feldspar decreased to 5% in samples GW-02, GW03 and GW-04. In sample GW-04 the size of the feldspar crystals are smaller th an in the other samples. Zoned and rimmed plagioclase crystals are abundant in all samples. 6.1.2. Tonalite Samples 6-9-1-E, 6-9-1-F and GW-10 ha ve similar petrological compositions. Sample 6-9-1-F is more mafic than samples 6-9-1-E and GW-10. In all tonalite samples, plagioclase phenocrysts have rounded crystal sh apes, which may be related to be caused by dissolution at high temperat ures. Based on the petrograph ic observations, sample 6-91-E has more rounded plagioclase crystals than sample GW-10, and the plagioclase crystals have sharper boundaries in sample 6-9-1-F suggesting that the temperature was decreasing in the southernmo st part of the pluton. Microstructural observations reveal that these samples have undergone a complex crystallization and cooling history under di sequilibrium conditions. They contain more reactivated grains which imply new magmatic injection. The sizes of grains in sample GW-10 are larger than in thin sections 69-1-E and 6-9-1-F. The very fine-grained groundmass in sample GW-10 suggests that the cooling was rapid. In addition, few crystal attachments were observed which sugge sts an early crystallization stage when crystals were freely suspended in the melt and this implies that the magma was very fluid in the early stages of crys tallization (Hogan 1993). The plag ioclase phenocrysts that are

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66 associated with the early phase have simila r zoning and overgrowth textures. GW-10 has a reverse zoning pattern which suggests a chan ge in plagioclase crystal composition or possible magma mixing. In sample 6-9-1-F, th e groundmass is finer grained than in thin section of sample GW-10 and plagioclase cr ystals have smoother margins than thin section GW-10. This thin section contains tr ansgranuler fractures w ith an average length of 19 mm. The matrix contains subhedral pl agioclase crystals, rounded quartz crystals, and mica with lobate grain shapes. Sample 6-9-1-F shows high alteration in biotite crystals than in any other sample, and the porphoritic texture indicates a different cooling rate for this tonalitic rock. The rimmed plag ioclase phenocrysts in sample GW-10 may be in equilibrium condition with the melt at the time of cooling. Small inclusions of rounded plagioclase crystals in the rimmed plagiocl ase phenocrysts may be due to the mutual attachment of the crystals in the early magmatic phase. Th e complex magmatic evolution in these samples suggests that the felsic melt migrated to a structur ally higher portion in the pluton. 6.1.3. Diorite This diorite could be related to the earl ier stage of magmatic evolution. Samples GW-05 and GW-09 have similar mineralogical compositions. Sample GW-09 shows more patchy zoning in the pl agioclase phenocrysts than sample GW-05. GW-05 is taken from the northeastern margin of the Balbuena pluton and indicates that the crystallization of the minerals was slower than in sample GW-09, which was taken from the southern margin of the pluton. Dioritic rock samp les represent the most mafic phase in the Balbuena pluton. Samples GW-05 and GW-09 have more pronounced magmatic fabric

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67 shape, represented mostly by the alignment of euhedral plagioclase crystals. The shape of the plagioclase phenocrysts is rectangular with sharp margin s. Based on the petrographic observations we can suggest that the magm a had a very quick cooling event in the southernmostern part of the pluton.

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68 7. Summary 7.1. Piedra Rodada Pluton The Piedra Rodada Pluton shows a range of deformational styles. The intensity of deformation increases toward the aABF shear zone. The highest temperature and strain deformation is defined by mylonitized am phibolites within the aABF shear zone. The mylonitic rocks are characterized by both quartz and mica having highly embayed grain boundaries, patchy undulatory extinction, and dy namic recrystallization of quartz which are microstructural characteristics of disloc ation creep. Moderate deformation of the Piedra Rodada pluton observed at moderate distance from the aABF was accommodated by subsolidus fabric development. Quartz grains show undulose ex tinction, numerous subgrains, and grain boundary migration which is also considered to be typical of dislocation creep. Biotite crystals have de veloped C surfaces and plagioclase phenocrysts have formed sigma-clast microstructure. During the highest subsolidus temperature deformation, biotite formed S-C fabrics and quartz retained high grain boundary migration which is evident from undulos e extinction, sutured grain boundaries, and numerous subgrains. During the lowest te mperature subsolidus deformation, quartz

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69 grains show undulose extincti on and numerous subgrains, but biotite grains have sweeping extinctions and form only th e C surfaces of the shear bands. Farthest from the aABF are moderate stra in plutonic rocks that are deformed in at subsolidus condition. In these samples, strain is concentrated in quart z. Quartz grains are equant to elongate in shape, and have undulat ory extinction with l obate grain boundaries. The deformation is relatively smaller in sample TW-01 which was taken ~ 1km away from the aABF. However, sample TW-03 represents the most intensively deformed rock sample in the Piedra Rodada pluton. TW-03 ha s typical features of high subsolidus strain and indicative of high grain mobility. Quartz is completely recrystallized and turned into polygonal fine grained crystals, and has sm ooth boundaries. Biotite grains are mostly neocrystallized and define S-surfaces, while the elongated ones define the C-surfaces of the microshearing. The microshear preserved more strongly in this sec tion than in sample C. 7.2. Volcaniclastic sa mples of the aABF In general mylonitic rock samples consis t of mica, chlorite quartz, and some plagioclase. Some of the minerals show alte ration. The fine-grained matrix consists of quartz which occurs only in the matrix with plagioclase. The mylonitic rock samples show excellent S-C fabrics. The C fabrics ar e defined by chlorite and biotite crystals, while the S fabrics are defined by elongated adjacent quartz crysta ls. The S-C mylonites are used to determine the kinematics of th e ductile shear zone. Based on S-C shear

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70 indicators, the sense of shear is consistent top to the southwes t (i.e., top of section dips to the northeast). The deformation caused foldi ng of the matrix foliation. Microlithones and closely spaced foliation are observed. Th ese observations indicate two different deformation histories; a c ontinuous deformation and an increase of the deformation intensity. 7.3. Balbuena Pluton The Balbuena pluton mostly preserves igne ous texture, often has phenocryst, and consists of large-medium subhedral grains of twinned and zoned pl agioclase, hornblende, biotite and interstitial quar tz and rare K-feldspar. Th e oscillatory zoning patterns investigated for all samples, except GW-09, An (% mol) content, show small variations between the core and the rim of the crysta ls, suggesting the magma chemistry did not change during cr ystallization. Based on the petrographic observations, the Balbuena pluton is a homogenous intrusive body and contains three different stages. The first stage produced dioritic intrusions and was followed by quartz dior ite intrusions which contain subhedralanhedral plagioclase phenocry sts, equant hornblende crys tals and anhedral quartz crystals. Quartz crystals show minor microcraking and mortar texture. The dioritic rocks contain euhedral to subhedral plagioclase, equant hornblende crys tals, and minor quartz crystals. The dioritic rock samples show weak magmatic foliation from euhedral

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71 plagioclase and biotite grains. The samples of tonalitic rocks have subhedral plagioclase phenocrysts, equant hornblende and in terstitial quartz crystals.

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72 8. Discussion 8.1. The Piedra Rodada pluton and the ancestral Agua Blanca Fault Twelve thin sections from the Piedra R odada and fault rocks of the aABF were analyzed as part of this study. All exhibit some degree of subsolidus deformation, an observation that differs from those made during field inve stigations and sampling. An assessment of kinematics reveals a consistent shear sense throughout this part of the study area. Based on observations made in this investigation we suggest that sample TW03 represents the most intensively deformed part of the pluton and displays strong microshear bands. In the Piedra Rodada pluton, the main de formation observed in the west side of the pluton, and it is dominated by biotite fo liation along with plag ioclase phenocrysts. Quartz crystals are weakly deformed, exhi biting undulose extincti on and microcracking. On the eastern side of the Piedra Rodada, samples within ~700 m of the aABF exhibit strong subsolidus deformation. Foliation within these samples is very strong, defined by biotite and plagioclase, as we ll as quartz ribbons. Biotite crys tals in Piedra Rodada pluton mostly display sweeping undulatory extincti on or foliation defined by fine grained neocrystals. The observation of subsolidus st rain within the samp les collected along the

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73 southwestern side of the Piedra Rodada plut on is unexpected due to the reporting of only magmatic fabrics in this ar ea by Wetmore (2003) and Wetmore et al. (2005). However, while subsolidus strain is observed away from the aABF in the Piedra Rodada pluton, it is quite minor in comparison with the extreme fabrics and strain observed along the northeastern side. An assessment of various asymmetries obser ved in thin sections cut parallel to lineations and normal to foliatio ns (features observable at th e hand sample scale) suggest a consistent top to the so uthwest sense of shear. Am ong the kinematic indicators observered were mica-fish (Fig.9), sigma-cl asts (plagioclase phenocrysts; Figs.6, 8, 16, 26, 41), and S-C fabrics (Figs.23, 24, 43, 44). The observation of top to the southwest shear sense is consistent with kinematic interpretations made from field observations along the aABF and within the Pied ra Rodada pluton by Wetmore (2003). Based on mapping by Wetmore (2003), samples TW-01 and TW-03 were collected from the southeastern part of the Piedra Rodada pluton (Figure 1). However, based on their location relative to the trace of the aABF and the extreme degree of subsolidus strain observed within these sample s, it seems unlikely that the intrusive rocks from which the samples were collected are actually deformed by aABF shearing event. The samples, which were collected from distances in excess of a kilometer from the aABF, all exhibit strong subsolidus fabrics si milar, and, perhaps, more intense than the fabrics observed within the samples from even cl oser to the structure in the main part of the pluton to the northwest. If the samples we re deformed by aABF shearing, it is hard to conceive how the spatial distribution of strain (i.e. the thickness of the zone of intense

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74 strain) could vary so dramatically over such short distances (~2 km) along the strike of the fault in the same intrusive body. Based on the above microstructural variations we suggest that sample TW-03 was strong ly deformed by different fault zone. 8.2. Balbuena pluton The Balbuena Pluton was emplaced into the northern Alisitos arc at ~108 Ma after most of the deformation of the fold and th rust belt had been completed. The Balbuena pluton has a very minor but well formed st ructural aureole. From the petrographic descriptions, the pluton has little evidence fo r subsolidus deformation besides the minor undulose extinction and intragranular deforma tion in quartz that is observed in the analyzed samples. Most of the mafic (diorite) phase observe d in this study represents probably the parent or primary magma which forms the earliest or middle phase mapped by Wetmore et al. (2001; 2005). The quartz di orite samples could be a more evolved derivative of the diorite having formed by; a) having mixing with the younger-m iddle phase tonalites, or b) it could be simple AFC prior to the intrusion of the later tonalite melts. However, it is difficult to differentiate between these models based on the current volume of geochemical and petrolog ical data available. Based on the map and thin sections observati ons, fabrics have weak magmatic fabric development with no strong alignment of crystals, except samples GW-05 and 6-9-1-B. The strikes of magmatic fabrics are shown in Fi gure 1. It is clear from this that magmatic

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75 fabrics cross-cut the internal contacts w ithout deflection. Additionally, they do not parallel the trends of regiona l structures or inferred stre ss fields. Rather, they likely reflect late-stage magmatic flow or locally evolved stress fields (e.g., Paterson et al., 1998). The observation that the magm atic fabrics within the Ba lbuena pluton are both weak and unrelated to regional stru ctures or stress, even though it can be demonstrated that shear continued along the aABF after crystallizati on of the 105 Ma Piedra Rodada pluton suggests that the Balbuena pluton was somehow within a stress/strain shadow during latestage assembly of the pluton. Recent studies of magmatic fabrics have demonstrated that fabric formation, particularly in fine grained intrusions li ke the Balbuena pluton form very late in the magmatic evolution of intrusive bodies (Paterson et al., 1998). Additionally, even very small regional differentia l stresses seem to be recorded (Zak et al., 2007). If these interpretations are correct, then the observa tion that fabrics within the Balbuena pluton are wholly inconsistent from place to place throughout the body seems to suggest that coincident shear across the aABF <5 km to th e northeast, had little, if any effect on fabric formation in the Balbuena plut on. Strain associated with the fault, was strongly localized around the structure as suggested by Wetmore et al. (2005).

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76 9. Conclusions Based on the petrographic investigation along the aABF and within the Piedra Rodada pluton, the top to the southwest shearing is consistent with kinematic interpretations made from field observations. Grain-scale observati ons suggest that the study area experienced a dramatic amount of shortening during the time of deformation. Field observations combined with optical observations suggest significant subsolidus deformation over the younger 105 Ma Piedra Rodada pluton. Thin section samples from Piedra Roda da pluton indicate that the amount of suboslidus strain is decreasing to the west. The lack of the subsolidus strain within the 108 Ma Balbuena pluton, ~ 5km away from th e aABF, indicates a strain shadow field during the time of emplacement. The petrogra phic observations indicate that the intensity of deformation decreases < 5 kilometers away from the aABF.

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77 10. References Adams, M. A., 1979, Stratigraphy and petrology of the Santiago Peak Volcanics East of Rancho Santa Fe, California [M.S. thes is]: San Diego Stat e University, 123 p. Ague, J. J., 1991, Evidence for major mass tr ansfer and volume strain during regional metamorphism of pelites: Geology, v. 19, p. 855-858. Allison, E. C., 1955, Middle Cretaceous Gast ropoda from Punta China, Baja California, Mexico: Journal of Paleontology, v. 29, p. 400-432. Allison, E. C., 1955, Middle Cretaceous Gast ropoda from Punta China, Baja California, Mexico: Journal of Paleontology, v. 29, p. 400-432. Alsleben, H., Wetmore, P. H., Schmidt, K. L., Paterson, S. R., and Melis, E. A., 2007, Complex deformation during arc-continent coll ision: Quantifying finite strain in the accreted Alisitos arc, Peninsular Ranges batholith, Baja California: submitted to Journal of Structrual Geology. Alvarez, W., Engelder, T. & Lowrie, W. 1976, Formation of spaced cleavage and folds in brittle limestone by dissolution. Geology, 4 698–701

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78 Beggs, J. M., 1984, Volcaniclastic rocks of the Alisitos Group, Baja California, Mexico, in Frizzell, V. A., ed., Geology of the Baja Califo rnia peninsula, Society of Economic Paleontologists and Mi neralogists, Pacific Secti on, annual meeting; p. 43-52. Burg, J.P. and Laurent, P. H., 1978, Strain anal ysis of a shear zone in a. granodiorite. Tectonophysics, 47, 14-42 DePaolo, D. J., 1981, A neodymium and stro ntium isotopic study of the Mesozoic calc-alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California: Journal of Geophysical Research, B, So lid Earth and Planets, v. 86, no. 11, p. 1047010488. Engelder, T. and Marshak, S., J., 1985, Di sjunctive cleavage formed at shallow depths in sedimentary rocks of Structural Geology, 7, 327-343 Gastil, R. G., Diamond, C. K., Knaack, C., Walawender, M. J., Marshal, M., Boyles,C., Chadwick, B., and Erskine, B., 1990, The problem of the magnetite/ilmenite boundary in southern and Baja California, in Anderson, J. L., ed., The Nature and Origin of Cordilleran Magmatism: Geological Society of America Memoir 174, p. 19-32. Gastil, R. G., Morgan, G. J., and Krummenacher, D., 1981, The tectonic history of peninsular California and adjacent Mexic o, in Ernst, W. G., ed., The geotectonic development of California: Englewood Cli ffs, New Jersey, Prentice-Hall, p. 284-306. Gastil, R. G., Phillips, R., and Allison, E., 1975, Reconnaissance geology of the State of Baja California: Geological Society of America Memoir 140, p. 170.

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79 Gromet, L. P., and Silver, L. T., 1987, REE variations across the Peninsular Ranges Batholith; implications for bat holithic petrogenesis and crusta l growth in magmatic arcs: Journal of Petrology, v. 28, no. 1, p. 75-125. Groshong, R. H., Jr. 1988. Low-temprature deformation mechanisms and their interpretation. Bull. Geol.Soc. Am. V. 100, p. 1329-1360. Herzig, C. T., 1991, Petrogenetic and tecton ic development of the Santiago Peak Volcanics, northern Santa Ana Mountains, Ca lifornia [Doctoral thesis]: University of California Riverside, 393 p. Hogan, J. P., 1993, Monomineralic glomeroc rysts: Textural evidence for mineral resorption during crystalliz ation of igneous rocks: Journal of Geology, v. 101, p. 531540. Johnson, S. E., Paterson, S. R., and Tate M. C., 1999b, Structure and emplacement history of a multiple-center, cone-sheet-b earing ring complex: The Zarza intrusive complex, Baja California, Mexico: Geologi cal Society of Amer ica Bulletin, v. 111, no. 4, p. 607-619. Johnson, S. E., Tate, M. C., and Fanni ng, C. M., 1999a, New geologic mapping and SHRIMP U-Pb zircon data in the Peninsular Ranges Bathol ith, Baja California, Mexico; evidence for a suture?: Geology, v. 27, no. 8, p. 743-746.

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80 Karsli, O., Aydin, F. and Sadiklar, M., 2004, Magma intraction recorded in plagioclase zoning in granitoid systems, Zi gana Granitoid, Eastern Pontides, Turkey: Turkish journal of science, v.13, p. 287-305 Mehnert, K. R. and Busch, W., 1981: The Pa sschier, C. W. and Trouw, R. A. J. 1996, Microtectonics. Springer-Verlag, Berlin Heidelberg New York. Paterson, S.R., K. Fowler Jr., K. Schmidt, A. Yoshinobu, and S. Yuan (1998), Interpreting magmatic fa bric patterns in plutons, Lithos, 44, 53-82. Powel, C. M. A. 1979. A morphological classification of rock cleavage. Tectonophysics, v. 58, p.21-34. Ramsay, J.G., 1980, The crack-seal mechan ism of rock deformation. Nature, 284, 135-139 Ramsay, J. G. & Graham, R. H. 1970 Strain variation in shear belts Can. J. Earth Sci. 7, 786-813 S. H. (eds) Shear Zones in Rocks, Special Issue of Journal of Structural Geology, 2 175–187. Schmidt, K. L., 2000, Investigations of arc processes: relationships among deformation magmatism, mountain building, a nd the role of crusta l anisotropy in the evolution of the Peninsular Ranges batholith, Ba ja California [Ph.D. thesis]: University of Southern California, 324 p.

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81 Schmidt, K. L., and Paterson, S. R., 2002, A doubly-vergent fan structure in the Peninsular Ranges batholith: Transpression or local complex flow around a continental margin buttress?: Tectonics, v. 21, no. 5, p. 1050, doi10.1029/2001TC001353. Silver, L. T., and Chappell, B. W., 1988, Th e Peninsular Ranges Batholith: an insight into the evolution of the Cordilleran ba tholiths of southwestern North America, Transactions of the Royal Society of Edinburgh, p. 105-121. Silver, L. T., and Chappell, B. W., 1988, Th e Peninsular Ranges Batholith: an insight into the evolution of the Cordilleran ba tholiths of southwestern North America, Transactions of the Royal Society of Edinburgh, p. 105-121. Silver, L. T., Taylor, H. P., Jr., a nd Chappell, B., 1979, So me petrological, geochemical and geochronological observations of the Peninsular Ranges Batholith near the international border of the U.S.A. and Mexico, in Abbott, P. L., and Todd, V. R., eds., Mesozoic crystalline rocks; Peninsular Ranges Batholith and pegmatites; Point Sal Ophiolite: Geological Society of Am erica Annual meeting Guidebook, p. 83-110. Todd, V. R., Erskine, B. G., and Morton, D. M., 1988, Metamorphic and tectonic evolution of the northern Penins ular Ranges Batholith, Southern California, in Ernst, W. G., ed., Rubey colloquium on Metamorphism and crustal evolution of the Western United States, Prentice-Hall, p. 894-937. Tullis, J., Dell’Angelo, L., and Yund, R., A ., 1990: Ductile shear zones from brittle precursors in feldspathic rocks: the role of dynamic recrysta llizaion. In Duba, A.,

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82 Durham, W., Handin, W. and Wa ng, H. (eds): The brittle-duc tile transiton. American Geophysical Union, Geophysical MonoMonograph, v.56, p. 67-82. Tullis, J. & Yund, R. A. 1985: Dynamic recrys tallization of feldsp ar: a mechanism for ductile shear zone formation. Geology, 13, 238–241 Vernon, R. H., 1986: K-feldspar megacr ysts in granites-phenocrysts, not porphyroblast. Earth Science Reviews, v. 23, p., 1-63. Vernon, R. H., 1976: Metamorphic Proce sses. London: Murby. New York: Wiley. Walawender, M. J., Gastil, R. G., Cl inkenbeard, W. V., McCormic, W. V., Eastman,B. G., Wernicke, M. S., Wardlaw, M. S., Gunn, S. H., and Smith, B. M., 1990, Origin and evolution of the zoned La Post a-type plutons, easter n Peninsular Ranges batholith, southern and Baja California, in A nderson, J. L., ed., The Nature and Origin of Cordilleran Magmatism, Geological So ciety of America Memoir 174 p. 1-18. Walawender, M. J., Girty, G. H., Lombar di, M. R., Kimbrough, D., Girty, M. S., andAnderson, C., 1991, A synthesis of recent work in the Peninsular Ranges batholith, in Walawender, M. J., and Hanan, B. B., eds., Ge ological Excursions in southern California and Mexico, p. 297-318. Wetmore, P. H., 2003, Investigation into th e tectonic significan ce of along strike variations of the Peninsular Ranges bat holith, southern and Baja California [PhD dissertation thesis]: University of Southern California, 199 p.

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83 Wetmore, P. H., Alsleben, H., Paterson, S. R., Ducea, M. N., Gehrels, G. E., and Valencia, V. A., 2005, Field trip to the north ern Alisitos arc segment: Ancestral Agua Blanca fault region, Field Conference Guidebook for the VII Internati onal Meeting of the Peninsular Geological So ciety: Ensenada, Baja California, MX, 39 p. Wetmore, P. H., Ducea, M. N., Gehrels, G. E., Schmidt, K. L., and Paterson, S. R.,2003b, Magmatic response to differential crus tal thickening; geochemical constraints on the tectonic evolution of the Alisitos Arc segment of Baja California, Mexico: Geological Society of America Annual Mee ting Abstracts with Programs, v. 35, no. 6, p. 114-115. Wetmore, P. H., Herzig, C., Alsleben, H ., Sutherland, M., Schmidt, K. L., Schultz, P.W., and Paterson, S. R., 2003a, Mesozoic tect onic evolution of th e Peninsular Ranges of southern and Baja Californi a, in Johnson, S. E., Paterson, S. R.,Fletcher, J. M., Girty, G. H., Kimbrough, D. L., and Martin-Bar ajas, A., eds.,Tectonic Evolution of Northwestern Mexico and the Southwestern Un ited States:Geological Society of America Special Paper 374, p. 93-116. Wetmore, P. H., Schmidt, K. L., Paterson, S. R., and Herzig, C., 2002, Tectonic implications for the along-strike variation of the Peninsular Range s Batholith, Southern and Baja California: Geology, v. 30, no. 3, p. 247-250. White, S. H., Burrows, S. E. Carreras, J. Shaw, N. D. & Humphreys, F. J. 1980. On mylonites in ductile shear zones. Journal of structural geolog, 2, 75-87.

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84 Zak, J., Paterson, S.R., Memeti, V., 2007, Four magmatic fabrics in the Tuolumne batholith, central Sierra Nevada California (USA): implications for interpreting fabric pattens in plutons and evolution of magma chambers in the upper crust: GSA Bulletin; January/February 2007; v. 119; no. 1/2; p. 000–000; doi: 10.1130/B25773.1.

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85 APPENDICES

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86 Appendix A: Piedra Rodada pluton thin sections discriptions PHW 5/20/02 Plagioclase feldspar: 65 %, 0.2-3.5mm, An48, defined by subhedral to anhedral grains, usually appear with al bite twining, and have minor al teration, and have inclusions of rounded quartz. The proportion of concentric zoning in the plagioclas e crystals in this thin section is about %80. Quartz: 13%, 0.1-1.5 mm, defined by anhedral crystals and irregular boundary shapes. Nearly all crystals e xhibit undulose extincti on in this thin sec tion. In additions, quartz crystals observed as chadacrysts in poikilitic biotite and these crystals are anhedral, and range in size from 0.01 to 0.4 mm. Biotite: 17 %, 0.05-0.25 mm, defined by light to dark brown crystals. The elongate books of biotite crystals occu r with aspect ratios of 3:1 up to 6:1. Hornblende: 5%, 0.15-3 mm, crystals occur anhedr al pale green to dark green crystals. In addition, poikiti c hornblende crystals occur w ith plagioclase and Fe-oxides inclusions.

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87 Figure 53. Hornblende crystal has simple twin with poikitic texture. Scale of view: 1mm. Figure 54. Rectangular plagioclas e crystallized along its margin s. Quartz crystals on the bottom and on the bottom of the plagioclase are elongated. On the right and the left of the plagioclase crystal, quartz cr ystals are still r ounded with smooth boundaries. Scale of view: 1mm.

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88 Figure 55. Strong biotite (B) foliation develope d on the top of the pl agioclase crystal. Scale of view: 1mm. Figure 56. The left margin of the crystals woul d be cause of intercrystalline deformation and dynamic recrystallization of quartz cr ystals. Plagioclase crystals have rounded inclusion of hornblende and it is crystallized along the ri ght margins. Scale of view: 1mm. B

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89 Figure 57. Lenticular hornblende with well de veloped biotite folia around it margins. The alignment of euhedral hornble nde is parallels the biotite gr ain fabrics shape. Scale of view: 1mm. Figure 58. Asymmetric biotite fish with well de veloped tail on the left and right side of the optical micrograph (CPL ).Scale of view: 1mm. Mica Fish H

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90 PHW 7-01-1-F Plagioclase feldspar: 60 %, 1.5-6mm, An38. Plagioclase crystals are defined by anhedral and subhedral shaped grains, and concentric zoni ng, with minor alteration to sericite. In addition, some plagioclase chadac rysts included within poikilitic hornblende crystals, and these crystals are anhe dral, and range in size from 0.15-0.35 mm. Quartz: 20%, 0.1-0.3 mm, defined by anhedral crystals and irregular boundary shapes, and they are interstitial between plagioclase, typically forming clusters of crystals between larger phenocrysts, and undulatory extinction. In additions, quartz crystals observed as chadacrysts in poiki litic biotite and these crystals are anhedral, and range in size from 0.1 to 0.55 mm. Biotite: 5-12 %, 0.4-1.0 mm, crystals occurs anhedral, and defined by pale brown to dark brown crystals crystals. Biotite cr ystals are mostly grown between plagioclase phenocrysts as coarse grained bi otite in this thin section. Hornblende: 20%, 0.25-1 mm, crystals occur anhedr al pale green to dark green crystals, and mostly show intragranular te xture; medium grained hornblende crystals filled the spaces between plagioclase grains; and th ey are mostly altered to chlorite in this thin section. In addition, poi kitic hornblende crystals occu r with plagioclase and Feoxides inclusions.

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91 Figure 59. The aligned plagioclase is su rrounded by anhedral, non deformed grains, anhedral quartz aggregates of quartz implies for magmatic flow. Scale of view: 1mm. Figure 60. Simple twining in euhedral hornble nde (H) surrounded by fine grained quartz crystals. Hornblende crystal has small inclus ions of rounded plagioclase inclusions. Scale of view: 1mm. B H

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92 Figure 61. Simple twin in hornblende (H) has rounded quartz and Fe-oxides inclusions in general. The microfractures are filled with quartz crys tals. Scale of view: 1mm. Figure 62. The spaces between plagioclase crysta ls filled with rounded quartz crystals. The plagioclase crystal on the left shows osc illatory zoning as a resu lt of magmatic flow. Scale of view: 5mm. H

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93 11704 B Plagioclase feldspar: 63%, 0.55-4mm, An38, crystals occur subhedral to rarely anhedral, concentrically zoned, and usually the long axes of the crystals are subparallel to x-axis of thin section + 15. In addition, some plagioclase observed as chadacrysts which included within poikilitic hornblende crystals, and these crystals are anhedral and range in size from 0.1-3 mm. Quartz: 17%, 0.15-0.3mm, crystals occur a nhedral, and has smoothly curved boundaries, and appears interstiti al between plagioclase which typically forming clusters of crystals between larger henocrysts, and displays undulatory extinction. Additional quartz crystals are observed as chadacrysts in poikilitic biotite, a nd these crystals are anhedral, and ranges in si ze from 0.025 to 0.3 mm, and lack any sign of undulose extinction or subgrain development. Biotite: 14%, 0.25-0.75mm, defined by plae tan to dark brown crystals, they are usually elongated with aspect ratios of 2:1 up to 7:1. In addition, few crystals are poikilitic containing chadacrysts of quartz. Approximately 20-35% of biotite crystals exhibit some alteration to white mica. Hornblende: 5%, 0.5-1.5mm, defined by equant and irregular margins, and simple twining, and light green to dark green cr ystals. Few crystals ar e poikilitic in this thin section with chadacrysts and de ep embayments of plagioclase.

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94 Figure 63. Biotite is forming nice folia on the left margin of the plagioclase crystal. Weak biotite folia have observe d in the thin section. The rect angular plagioclase crystals still remain as igneous microstructure. The alignment of subhedral plagioclase observed regionally. Broken hornblende suggest for magmatic flow. Scale of view: 2mm. Figure 64. Quartz crystals have mosaic textur e. Lenticular hornblende suggest for solid state deformation. Interlobate quartz filled the spaces between plagioclase crystals. Grain boundary migration in quartz is due to solid state deformation. Scale of view: 2mm

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95 Figure 65. Quartz aggregates in between biotite folia still rounded. Quartz subgrains are medium sized and have straight and smoot hly curved boundaries. Scale of view: 0.4mm 11704C Plagioclase feldspar: 64%, 1.5-3mm, An32, crystals occur subhedral, and show oscillatory zoning patterns, have minor alte ration. Additional plag ioclase observed as chadacrysts included within poikilitic hornbl ende crystals, and these crystals are anhedral, and range in size from 0.045-0.1 mm. Weak fabric prefe rred orientation in plagioclase crystals is observ ed in this thin section. Quartz: 20%, 0.15-1.5mm, crystals occur anhedral, and they are mildy recrytallised, and have irre gular boundaries, and usually appear interstitial between plagioclase, which is typically forming cluste rs of crystals between larger phenocrysts.

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96 Subgrain development observed in this thin section without clear ly defined neograin boundaries in plain polarized li ght (ppl). Additional quartz crystals are observed as chadacrysts in poikilitic biotite, and these crys tals are all anhedral, and range in size from 0.05 to 0.1 mm. Biotite: 10%, 0.3-0.5mm, defined by subhedr al pale to dark brown colored crystals. The elongate books of biotite crystals have aspect ratios of 3:1 up to 7:1. Most of the biotite crystals are poikilitic containi ng chadacrysts of quartz. 5-15 % of biotite crystals show alteration to chlorite and white mica. Hornblende: 5%, 0.25-2mm, crystals occur anhedr al light green and dark green colored crystals. All hornblende crystals are poikilitic with chadacrysts and deep embayments of plagioclase. 35-50% of hornbl ende crystals are altered to chlorite. Figure 66. Plagioclase crystal is surrounded by biotite (B) folia. B

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97 PHW 5/20/02 B: Plagioclase feldspar: 45%, 0.4-3mm, An40, crystals occur subhedral to rarely anhedral broken fragments, and show minor alteration, and have microfractures. The fractures in between two plagioclase crystals are filled with quartz crystals. Additional plagioclase observed as chadacrysts included within poikilitic hornblende crystals, and these crystals are anhedral, and range in size from 0.08-0.4 mm. Quartz: 20%, 0.15-0.75mm, crystals are interstitial between plagioclase, typically forming clusters of crystals between larger phenocrysts. Nearly all crystals subgrain development without clearly de fined neograin boundaries in pl ain polarized light (ppl). Additional quartz crystals are observed as chadacrysts in poikilitic biotite, and these crystals are all anhedral, and ra nge in size from 0.5 to 0.1 mm. Biotite: 10%, 0.1-3.5mm, defined by subhedral to rarely light to dark brown colored crystals. The elongate books of biotite cr ystals have aspect ratios of 3:1 up to 5:1. Most crystals are poikilitic containing chadacr ysts of quartz. 10-15 % of biotite crystals show alteration to white mica. Hornblende: 25%, 0.05-2.5mm, defined by light gr een to dark green colored equant crystals. Hornblende cr ystals are poikilitic with chad acrysts and deep embayments of plagioclase. 10-25% of crystals show alteration to chlorite.

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98 Figure 67. The euhedral outline of the plagio clase megacryst has concentric zoning and rounded hornblende inclusion. The plagioclas e megacryst is recrystallized along its margins and the bending of plagioclase twin s is visible. The fabric shape and the intracrystalline deformation features can be interpreted as submagmatic fabric. Figure 68.The lenticular shape in hornblende appears to have been caused by solid state deformation. The recrytallized quartz aggregat es are parallel to hornblende margins and they are elongated.

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99 Figure 69. Plagioclase crystals have biot ote foliation parallel to its margins. 11704D Plagioclase feldspar: 65%, 0.6-2mm, An30, crystals occur anhedral to rarely subhedral crystals, and have concentric zoni ng pattern, show minor alteration to sericite. The contacts between two plagioclase crystals are filled with quartz crystals. Additional plagioclase observed as chadacrysts included within poikilitic hornblende crystals and these crystals are anhedral and range in size from 0.25-0.1 mm. Quartz: 20%, 0.3-0.75mm, crystals o ccur as equant crystals which are interstitial between plagioclase, and typically formi ng clusters of crystals between larger phenocrysts. The subgrains of quartz crysta ls are forming a new grain boundary which intersecting the plagioclase crys tals. The shapes of quartz cr ystals are rounded and have irregular boundaries in this thin section. Additional quartz crystals are observed as

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100 chadacrysts in poikilitic biotite and these crys tals are anhedral, and range in size from 0.05 to 0.5 mm. Biotite: 10%, 0.2-3.5mm, crystals occur subhedral to rarely anhedral light to deep red crystals. The elongate books of biotite crys tasl have aspect ratios of 3:1 up to 7:1. Most crystals are poikilitic containing chadacr ysts of quartz. 20-25% of biotite crystals exhibit some alteration to white mica. Hornblende: 5 %, 0.5-2mm, crystals occur anhedr al light green to dark brown colored crystals, and they ar e broken, and mostly have numerous small inclusions. All hornblende crystals are poikiliti c with chadacrysts and deep embayments of plagioclase. Figure 70. Biotite filled with quartz crystals. Quartz crysta ls are anhedral and medium sized. Scale of view: 9mm.

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101 Figure 71. Quartz aggregates have undulose extinctions a nd grain boundary migration. Biotite grains show undulose extinction evidence for low temperature plastic deformation. Scale of view: 9mm. TW-01 Plagioclase feldspar: 66%, 0.4-3.75mm, An33, the pl agioclase grain shapes are mostly anhedral to rarely subhedral with smoothly curved or plain boundary shapes, and show oscillatory zoning pattern, and have minor alteration. Additional plagioclase observed as chadacrysts included within poikili tic hornblende crystals, and these crystals are anhedral and range in size from 0.025-0.2 mm. Quartz: 19%, 0.3-2mm, crystals occur anhe dral, and show undulatory extinction, and have micro-crakcs and mi cro-fractures, and have in terbolate grain shape with irregular curved boundaries. Quartz crystals ar e interstitial between pl agioclase, typically

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102 forming clusters of crystals between larger phenocrysts. Additional quartz crystals are observed as chadacrysts in poikilitic hornble nde and biotite, and these crystals are all anhedral, range in size from 0.025 to 0.35 mm. Biotite: 8%, 0.2-0.75mm, defined by light to dark brown anhedral crystals, and show minor undulatory extinction. Most of bi otite crystals are poikilitic containing chadacrysts of quartz. % 10 of biotite crystals has some alteration to chlorite or another white mica. Hornblende: 5%, 0.5-0.15mm, crystals occur anhe dral light green to dark brown crystals. All crystals are poiki litic with chadacrysts and deep embayments of plagioclase. 30-35% of biotite crystals altered to chlorite. Figure 72. Fracture formed during separation an d has been filled with quartz. Quartz crystals are elongated. Scale of view: 2mm.

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103 Figure 73. Plagioclase phenocryst showing ma rked discontinuity between core with patchy zoning and rim with oscillato ry zoning. Scale of view: 5mm. Figure 74. Overgrowth twins in plagioclase w ith minor alteration to sericite. Scale of view: 2mm.

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104 TW-03 Plagioclase feldspar: 65%, 0.1-2mm, An38, crystals occur subhedral to rarely anhedral crystals, and most of the crystals exhibit oscillatory zoning with irregular boundaries, and altered, and the long axes of the crystals are subparallel to x-axis of thin section + 15. Additional plagioclase also observed as chadacrysts included within poikilitic hornblende crystals and these crystals are anhe dral and range in size from 0.035-0.2 mm. Quartz: 17%, 0.08-0.45mm, crystals occur anhedr al to slightly elongate crystals, and have sutured boundaries, and crystals are interstitial between plagioclase which is typically forming clusters of crystals betw een larger phenocrysts. Nearly all crystals exhibit undulatorye extincti on without clearly defined neograin boundaries in plain polarized light (ppl). Additional quartz crysta ls are observed as chadacrysts in poikilitic biotite, and these crystals are all anhedral, range in si ze from 0.05 to 0.2 mm, with no evidence of undulose extinction or subgrain development. Biotite: 10%, 0.1-2mm, crystals defined by acicular crystals, and pleocroic colored light to dark brown crystals. The el ongate books of biotite crystals have aspect ratios of 6:1. In addition, 10-25% of biotite crystals exhibit so me alteration to chlorite or another white mica. Hornblende: 7%, 0.5-2mm, crystals occur anhedr al, and defined by light green to dark green subequant crystals. Most of th e hornblende crystals are poikilitic with

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105 chadacrysts and deep embayments of plagio clase. 10-15% of hornbl ende crystals are altered to chlorite. Figure 75. Spaces between plagioclases filled with fine grained quartz and acicular biotite. Scale of view: 2mm. Figure 76. Most of the spaces are filled with quartz aggregates and acicular biotite. Quartz crystals in this optical microgra ph have smoothly curved boundary shapes, and the plagioclase megacryst on the left end of the picture show oscillatory zoning. Scale of view: 2mm.

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106 Figure 77. Elongated quartz subgrains show undulose extinction. Plagioclase megacrysts on the right have regular margins on the top. Scale of view: 5mm. TW-04 Plagioclase feldspar: 65%, 0.4-4mm, An33-44, crystals occur subhedral to rarely anhedral, and have concentric zoning patter n, and they are highly altered to sericite. Additional plagioclase crystals are observed as chadacrysts incl uded within poikilitic hornblende crystals and these crystals ar e anhedral and range in size from 0.075-0.15 mm. Quartz: 18%, 0.1-0.5mm, crystals occur anhe dral, and undulatory extinction, and they are interstitial between pl agioclase, which is typically forming clusters of crystals between larger phenocrysts. Additional quartz crystals are observed as chadacrysts in poikilitic biotite. These crystals are all anhedral, range in size from 0.075 to 0.125mm,

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107 and lack any sign of undulose extinction or subgr ain development. Quartz crystals have irregular grain boundary shapes in this thin section due to the gr ain boundary migration. Biotite: 10%, 0.1-2.5mm, crystals defined by subhedral to rarely light to dark brown colored crystals. The Elongate books of bi otite crystals have aspect ratios of 2:1 up to 5:1. Some of the biotit e crystals show minor kinki ng of the lattice undulatory extinction. Hornblende: 7%, 0.1-1mm, crystals defined by light green to dark green subequant crystals. All hornblende crystals are poikilitic with chadacrysts and deep embayments of plagioclase. Also 5-15% of hornblende crystals are altered to chlorite. Figure 78. Lenticular hornblende in the middl e of the picture is simple twined and surrounded by elongated plagioclase on the bottom and recryastallised quartz. On the top left, undulose extinction in biotite can be easily see n. Scale of view: 9mm.

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108 Figure 79. Lenticular hornblende has quartz, magnetite and plagioclase inclusions. Plagioclase inclusions are elongated and have simple twining. -Qua rtz and magnetite are rounded in shape. Scale of view: 2mm. Figure 80. Spaces between plagioclase pyhnocryst filled with dynamically recrystsallized quartz. Quartz aggregates are polygonal and interlobate with smoothly curved boundaries. Plagioclase phynocryst have simple twin and growth twins with irregular margins due to grain boundary migration. Scale of view: 2mm.

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109 11704A Plagioclase feldspar: 70%, 12.5mm, crystals occur anhedral, and they are oscillatory zoned, and highly altered, and fr actured. The fractures filled with quartz crystals. In addition, some pl agioclase observed as chadac rysts which included within poikilitic hornblende crystals and these crystals are anhe dral and range in size from 0.025-0.05 mm. Also, plagioclase crystals have inclusions of elongate d biotite crystals and rounded hornblende crystals. Quartz: 20%, 0.25-0.5mm, crystals occur a nhedral, and interstitial between plagioclase, which typically forming clusters of crystals between larger phenocrysts. Quartz crystals generally have microcr acks, and show undulatory extinction with irregular and smoothly curved boundaries. In addition, some quartz crystals are observed as chadacrysts in poikilitic biotite, and thes e crystals are anhedral, range in size from 0.025 to 0.5 mm, and they do not show of undul ose extinction or subgrain development. Biotite: 2%, 0.2-0.5mm, crystals occur subhedr al to rarely anhedral, and defined by light to dark brown crystals. Most of the biotite crystals are poi kilitic which containing chadacrysts of quartz. In addi tion, 50-80% of biotite crystals exhibit some alteration to chlorite. Hornblende: 5%, 0.15-1mm, defined by light gree n and brown equant crystals. All crystals are poikilitic in this thin section with chadacr ysts and deep embayments of

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110 plagioclase. In addition, hornblende crystals exhibit some degree (30-45%) alteration to chlorite. Figure 81. Optical micrograph shows the attachme nt of plagioclase crystals suggest for a magmatic flow. Scale of view: 2mm. Figure 82. Quartz are coarse grained, have sutured and st raight boundaries. Plagioclase crystals are highly altered and have micro fractures. Scale of view: 2mm.

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111 PHW 7-3-1-J Plagioclase feldspar: 25%, 0.4-2mm, An28, Orthoclase : 40%, crystals occur subhedral, and have irregular edge, and ha ve concentric zoning pattern, and highly altered to sericite. Additional plagioclas e observed as chadacrysts included within poikilitic hornblende cr ystals, and these crystals are a nhedral, and range in size from 0.10.3 mm. Quartz: 7%, 0.1-1.25mm, crystals occur anhe dral, and have smooth boundaries, and contain crack along their long axes. The cr acks observed between two quartz crystals which have is linear along these two quartz grai ns. Quartz crystals ar e interstitial between plagioclase, typically forming clusters of crystals between larger phenocrysts Amphibole: 25%, 0.1-3mm, defined by light gr een and brown colored equant crystals. All crystals are poiki litic with chadacrysts and deep embayments of plagioclase. Most of the crystals exhibit 7090% of alterati on to chlorite.

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112 Figure 83. Most of the fractures between plagioclase crysta ls are filled with quartz crystals. Scale of view: 5mm. Figure 84. Numerous small inclusions in pl agioclase observed. Alteration caused by fine inclusions of mica. Scale of view: 2mm.

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113 1/8/03 D: Tonalite Plagioclase feldspar: 45%, 0.5-3.00 mm, An29, crys tals occur subhedral, and have concnetric zoning pattern, and altered to sericite, and have microcracked. The microcracks of plagioclase grains filled with bitotite crystals. Potassium feldspar : 5%, occur anhedral, and the long axes are subparallel to xaxis of thin section + 15, and show myrm ekitic texture, and have microcracks. The microcracks are filled with bitotie crystals. Quartz: 25%, 1-3mm, crystals occur anhe dral, and show undulatory extinction, and have curved and irregular grain boundary shapes, and mosaic texture. Additional quartz crystals are observed as chadacrysts in poikilitic biotite, a nd these crystals are anhedral, and range in size from 0.05 to 0.2 mm. Biotite: 15%, 0.1-0.2mm, crystals occur acic ular light to dark brown colored crystals, and the elongated books have aspect ratios of 3:1 up to 6:1 Hornblende: 10%, 0.4-1mm, defined by light green to dark brown colored crystasls subsequent crystals. All hornblende crystals are poikilitic with chadacrysts and deep embayments of plagioclase. 50-60% of hornblende crystasl show alteration to chlorite.

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114 Figure 85. Optical micrograph of the myrmek ite(M), overgrowth texture ,replacing the margins of potassium feldspar (F). Scale of view: 2 mm. Figure 86. Biotite crystals show tail developm ent along the plagioclase crystal margins. Scale of view: 4mm. M F

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115 Appendix B: Balbuena pluton thin s ections descriptions 6-9-1-E Plagioclase feldspar: 34%, 0.4 to 3.25 mm, An38, groundmass: 31 %, 0.075mm, crystals are anhedral, and rounded, and have albite twining, and mi nor concentric zoning, and have minor alteration ro sericite. Add itional plagioclase observed as chadacrysts included within poikilitic hornblende crystals, and these crystals are anhedral and range in size from 0.1-0.3 mm. Quartz: 3%, 0.2-0.75 mm, groundmass: 26%, 0.00350 mm, crystals are anhedral, and fine-grained. Additional quartz crystals are observed as chadacrysts in poikilitic biotite, and these crystals are all anhedral, range in si ze from 0.05 to 0.2 mm, and show no evidence of undulose extincti on or subgrain development. Biotite: 4%, 0.1-0.5 mm, crystals are subhedral to rarely anhedral, and defined by light to dark brown colored grai ns. 15-20% of biotite crystals show alteration to chlorite. Hornblende: 2%, 0.1-1 mm, defined by plae green to dark green colored equant crystals. 45-60% of crystals have alteration chlorite.

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116 Figure 87.The optical micrographs are taken both in cpl and ppl. Figure 88. Plagioclase phenocryst. Quartz fille d the fractured plagioclase phenocryst, the size of rounded quartz crysta ls is 0.0035 mm. Plagioclase phenocryst has simple twin. Scale of view: 1mm.

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117 Figure 89. Reaction of the plagioclase phe nocryst along the margins indicates the compositional change. Scale of view: 1mm. Figure 90. Intragranuler microcra cks are avaible in simple tw ined plagioclase phenocryst. Scale of view: 1mm.

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118 Figure 91. Euhedral plagioclase is recrys tallized and has margin zoning. Zoned plagioclase has numerous inclusions of plagio clase crystals on its margins. The continued rimmed plagioclase is in equilibrium with the matrix. Scale of view: 5mm GW-01: Plagioclase feldspar: 22 %, 0.3 to 6 mm, An32-1, crys tals are anhedral, and have interlobate grain shape with irregular smoothly curved boundaries, and zoned, and have minor alteration. Plagioclase crystals are containing microcracks and microfractures across and between crystals which suggest for brittle deformation. The spaces between plagioclase grains are filled with newly recrystallized quartz subgrains. Alkali feldspar: 40 %, crystals are anhedral and have pericline twins.

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119 Quartz: 17%, 0.15-3 mm, crystals have micr ocracks across and between quartz and plagioclase, and they are anhedral, and have random shapes; medium grained interlobate quartz and fine gr ained amoeboid quartz crystals. Biotite: 11 %, 0.2-2.0 mm, crystals are anhedr al, and have small inclusions of rounded quartz and Fe-oxides, and defined by lig ht brown to dark brown colored grains. Approximately 15-20% of biotite crystals exhibit some alteration to chlorite Hornblende: 5 %, 0.3-3.00 mm, crystals are equant and defined by light green to dark green colors. Figure 92. Medium grained quartz, plagioclase, biotite crystals are available in general. Plagioclase crystals show undulose extinction. Bi otite grains have numerous inclusions of medium grained quartz. Grain boundaries with adjacent plagioclase and bioitite are straight. Biotite grains have strong pleochroism. Scale of view 5mm.

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120 Figure 93. Quartz crystals are vi sible in the middle of the pict ure. Quartz aggregates are filled the spaces between feldspar crystals Quartz shows undulose extinction. Quartz crystals are rounded and anhedral in shape. Scale of view 1mm. Figure 94. Plagioclase phenocryst has numer ous inclusions with irregular margins implies for complex crystallization history. Al so the phenocryst of plagioclase has twins and concentric zoning. Scale of view 1mm.

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121 Figure 95. Plagioclase crystal is enclosed in biotite and microfractured with albite twinning. Plagioclase crystal has irregular marg ins. Biotite is present along the fracture surface. Scale of view: 0.5mm. GW-03 : Plagioclase feldspar: 65%, 0.3-4.75 mm, An42, crysta ls are subhedral, and have amoeboid grain fabric shape, and show oscill atory extinction, and have minor alteration to sericite. The spaces in between plagio clase grains are filled with biotite. Potassium felpspar, 5%, crystals are broken, a nd have tartan twins. Quartz: 18%, 0.1-2.00 mm, crystals are anhedr al, and have smoothly curved, and show undulatory extinction. Additional quartz cr ystals are observed as chadacrysts in poikilitic biotite, and these crystals are all anhedral, a nd range in size from 0.1-0.5mm.

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122 Biotite: 10 %, 0.15-1 mm, crystals are a nhedral, and they are poikilitic, containing chadacrysts of quartz Hornblende: 6%, 0.1-1.75 mm, crystals are anhe dral, and defined by light green to dark green colored grains. Figure 96. Plagioclase grains are elongated a nd few oscillatory zoning can be seen in plagioclase. Plagioclase phenocrysts have albite multiple twins in general. Scale of view 5mm.

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123 Figure 97. Quartz grains are amoeboid and inte rlobate quartz grains have straight and smoothly curved boundaries. Concen trically zoned plagioclase cr ystals can be seen on the right. Scale of view 1mm. Figure 98. Euhedral plagioclase phenocrysts can be seen as evidence for magmatic flow. The igneous texture observed. The plagioclase phenocrysts are not parallel to biotite grain fabric shape. The plagioclase megacrys t on the left have multiple twins. Scale of view 5mm.

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124 Figure 99. Intragranuler and transgranular mi crocracks can be seen. Tartan twins in potassium feldspar. Sc ale of view 1mm. Figure 100. Biotite grain can be seen in the mi ddle of the optical micrograph. Plagioclase filled the fracture in the biotite grain. On the top, biotite fracture filled with rounded quartz. Scale of view: 5mm.

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125 GW-04 : Plagioclase feldspar: 65%, 0.3 to 3.00 mm, An30-34, crystals are mostly anhedral with irregular boundary shapes, and the spaces between plag ioclase crystals are filled with acicular biotite.Inclusions of biotite have seen in plagioclase crystals, and the inclusion shapes are acicular and anhedral. 85 % of crystals have concentric zoning, and 10 % of crystals have periclin e twinig, and 5 % of large crystals are zoned and rimmed. Almost all crystals have alteration to sericite. Quartz: 18 %, 0.1-2.5 mm, crystals are anhe dral coarse grained, and have intragranuler microcrakcs, and have undulator y extinction. Additional quartz crystals are observed as chadacrysts in poiki litic biotite, and these crysta ls are all anhedral, rounded in shape, range in size from 0.035 to 0.1 mm. Biotite: 13%, 0.5-2.00 mm, crystals are anhe dral, and defined by ten color to dark brown grains, and have irregular marg ins, and usually grown between plagioclase crystals Hornblende: 3 %, 0.15-0.75 mm, crystals defined by light green to dark green colored, and equant shapes.

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126 Figure 101. The optical micrograph of sample GW-04 shows fine grained quartz and plagioclase crystals. Almost all the quartz grains show undulatory extinction. Scale of view: 4mm. Figure 102. Poikitic biotite cr ystal is broken and filled with elongated and anhedral plagioclase crystals. Scale of view: 1mm.

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127 Figure 103. The alignment of biotite crystal su rround by anhedral quart z crystals suggest for magmatic flow. Quartz crystals have st raight and smoothly curved. Scale of view: 1mm. Figure 104. Euhedral outline of fractured plagio clase megacryst is crystallized along its margins and the bending of plagioclase twin s is visible. Plagioclase megacryst is surrounded by polygonal aggregates of plagiocl ase and quartz. Scale of view: 1mm.

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128 Figure 105. Optical micrograph suggests for gr anular texture Mutu al attachment of crystals in the melt observed locally. Synneusis suggest magm atic flow. Scale of view: 1mm. GW-06: Plagioclase feldspar: 60%, 0.3-2.5mm, An43, crystals are anhedral, and have oscillatory zonation, and show alteration to sericite. Potassium feldspar : 5%, 0.5-2.5mm, defined by broken fragments, and have anhedral shape. Quartz: 22%, 0.15-3mm, crystals are a nhedral, and show random shape orientation; the smoothly curved medium si zed quartz grains and sutured medium sized grained quartz crysta ls. Undulose extinction with sutured-smoothly curved boundaries are typical for quartz aggregates in this thin section. Additional quartz crystals are observed

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129 as chadacrysts in poikilitic biotite, and these crystals are anhedral, range in size from 0.05 to 0.1 mm. Biotite: 13 %, 0.4-1.5mm, crystals are anhe dral, and defined by light to dark brown color. Approximately 5% of biotite crystals exhibit some alteration to chlorite and muscovite. Hornblende: 5%, 0.45-1.5 mm, defined by lig ht green equant crystals. Horndlende crystals are poikili tic with chadacrysts and deep embayments of plagioclase, and 45% of crystals show alteration to chlorite. Figure 106. Plagioclase phenocryst show perfect oscillatory zoning. S cale of view: 1mm.

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130 Figure 107. Biotite (B) filled th e spaces between plagioclas e phenocryts. Scale of view: 5mm. Figure 108. Biotite crystals surrounding plag ioclase crystals. S cale of view: 1mm. B

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131 Figure 109. Plagioclase phenocrysts have multiple twining, biotite grain filled spaces in between feldspars. Scale of view: 1mm. Figure 110.The optical micrograph shows the sa mple in cpl. ignenous texture remains with no evidence for subsolidus deformation. Scale of view: 9mm.

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132 6-9-1-B : Plagioclase feldspar: 72%, 0.2 to 2.25 mm, An30, cr ystals are subhedral to rarely anhedral, and have oscillatory extinc tion, and minor alteration to sericite, and the spaces in between plagioclase phynocrysts are f illed with coarse and medium grain sized quartz and bended biotite. Subhedral plagio clase phenocrysts have green hornblende inclusions. Alkali feldspar : 5 %, defined by tartan twins. Quartz: 2 %, 0.2 to 0.5 mm, crystals are anhe dral, and have in terlobate grain shapes, and show undulose extinction. Quar tz subgrains with undul ose extinction have intragranuler microcrakcs. Biotite: 10 %, 0.4-2 mm, crystals are aubhedral to rarely anhedra, and defined by medium light to dark brown colored grains and have numerous small inclusions of plagioclase and cubic Fe-oxide s. The plagioclase inclusions shapes are anhedral with irregular margins. Hornblende: 15 %, 0.1-3.25 mm, crystals are mos tly equant, and light to dark green colored. All crystals are poikilitic with chadacrysts and deep embayments of plagioclase. Hornblende grai ns have numerous inclusi ons of elongated plagioclase crystals with irregu lar boundaries and rounde d quartz crystals.

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133 Figure 111. Alignment of plagioclase, hornble nde and biotite suggest for magmitc flow. The optical micrograph show inte rgranular texture. The sample is coarse grained and the plagioclase crystals have almo st euhedral alignment. Inters titial hornblende grains are anhedral. Subhedral plagioclase crystals have albite twins, more regular boundaries and overgrowth textures than thin se ction GW01. Scale of view: 5mm. Figure 112. Microcrakcs observed in quartz. Qu artz shows undulose extinction. Scale of view: 1mm.

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134 Figure 113. Alkali feldspar observed show tartan twining. Scale of view: 1mm. Figure 114.Plagioclase crystals have well developed albite twining regular grain boundaries. Scale of view: 1mm.

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135 Figure 115.Concentric zoning and fractured pl agioclase phenocrysts. Spaces between two phenocrysts filled with biotite. Scale of view: 1mm. 6-9-1-F: Plagioclase feldspar : 45-50 % present as phenoc ryst, 0.5 to 2.5 mm, An26-38, and 10-15 % present in the groundmass, 0.05mm, crystals are anhedral to rarely subhedral, have internal fabric s perpendicular to growth text ure, and 65 % of plagioclase phencrysts have simple and multiple twins, and 20 % of plagioclase phenicrysts have concentric zoning, and 9 % of plagioclase ph enocrysts have oscill atory zoning, and 6 % of plagioclase phenocrysts are zoned and rimmed. Quartz: 6 %, 0.2-1.75 mm, 20-25 % present in gr oundmass, crystals are anhedral, and have intragranuler and intergranuler microcracks, and show undulose extinction.

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136 Additional quartz crystals are observed as chadacrysts in poikilitic biotite, and these crystals are all anhedral, range in size from 0.1 to 0.4 mm. Hornblende: 8 %, 0.1-1.6 mm, crystals are mos tly anhedral, and defined by light green and brown colored grains. 50-70% of crys tals have alteration to biotite, and 5 % of the crystals have alteration to chlorite. Biotite: 5%, 0.5 mm, crystals are anhedral, and defined by light to dark brown colored grains. Approximately 5-10% of biotite crystals ha ve alteration mica. Figure 116. Plagioclase crystal is crystal lized along its margins. The elongated plagioclase enclave has been stretched during magmatic flow. The plagioclase phenocryst has multiple twins. Simple twining separated the crystal in to equal parts, and each part of the crystals show albite tw inning. Scale of view: 1mm.

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137 Figure 117. Plagioclase enclaves are oscillatory zoned and rimed. Scale of view: 5mm. Figure 118. Plagioclase phenocrysts show reactio n with the melt. Scale of view: 1mm.

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138 Figure 119. Fine grained quartz crystals and plagioclase crystals formed along the fracture during cooling even t. Scale of view: 1mm. Figure 120. The attachment of three phenocryst s of plagioclase crystals. Oscillatory zoning is available. Scale of view: 5mm.

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139 Figure 121. Optical micrographs of the thin s ection were taken in cpl. Scale of view 5mm. GW02 : Plagioclase feldspar: 67%, 0.5-3.5 mm, An44, crystals are subhedral to rarely anhedral, and have oscillatory zonation with patchy core. Additional plagioclase crystals are observed as chadacrysts in poikilitic hor nblende, and these crysta ls are all anhedral, size is about 0.3 mm.High alteration to sericite Potassium felpspar: 15 %, Crystals are defi ned by tartan twining Quartz: 17%, 0.01-2.5mm, crystals are anhe dral, and have smoothly curved boundaries, and show undulatory extinction.

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140 Biotite: 10%, 0.25-3mm, crystals are anhe dral, and defined by light brown to dark brown colored grains. Hornblende: 5%, 0.45-1.5 mm, crystals are equa nt, and have light green and brown color. Hornblende crystals ex hibit 5% of alteration to chlorite. Figure 122. Medium grained sized of plagioclase and biotite crystals are available. Quartz crystals have random grain size. Medium grai ned interlobate crystals and fined grained amoeboid crystals. Scale of view 5mm.

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141 Figure 123. Elongated plagioclase grains have albite-twining. Plagioclase crystals are usually zoned and have simple twinning. Quartz in the middle has chessboard structure. The thin section shows magmatic flow in general. Scale of view 5mm. Figure 124. Interstitial between plagioclase, typically forming clusters of crystals between plagioclase phenocryst s. Scale of view 1mm.

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142 Figure 125. Plagioclase crystal has con centric zoning. Scal e of view 5mm. Figure 126. Plagioclase phenocryst has regul ar margins and quartz subgrains are elongated along plagioclase margins. Quartz cr ystals have with straight and smoothly curved boundaries. Scale of view 1mm.

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143 GW-09 : Plagioclase feldspar : 27% present as a phenocryst s, 0.5-2.75, An (indeterminate) 35% in the groundmass, 0.3-0.6 mm, crystals ar e subhedral to rarely anhedral, and the resorbed plagioclase phenocrysts have ir regular boundary shapes. Few plagioclase phenocrysts are attached to each and they cr ystallized along their margins. In addition, plagioclase phenocrysts have internal fabric that parallel to growth twining. Additional plagioclase observed as chadacrysts included w ithin poikilitic biotit ee crystals, and these crystals are anhedral and range in size from 0.025-0.1 mm. Quartz: 3%, 0.2-3.5 mm, crystals are anhedral, and have intregranuler microcracks, undulatory extionction. Additional quartz observed as chadacrysts included within poikilitic biotitee crystals, and these cr ystals are anhedral a nd range in size from 0.05-0.3 mm. Biotite: 8%, 0.1-0.35 mm, crystals defined by light to dark brown colored anhedral grain shapes. Few crystals are poi kilitic containing chad acrysts of quartz Hornblende: 17%, 0.05-0.65 mm, crystals are mos tly anhedral with irregular margins, and defined by light tan color to dark green colored gr ains 10-15% of hornblende show alteration to chlorite. Groundmass: In fined grained groundmass, the la ths of elongated plagioclase and amoeboid quartz crystals observed.

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144 Figure 127. General view of sample GW-09. Scale of view: 5mm. Figure 128. The mutual attachment of the crystals. Scale of view 5mm.

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145 Figure 129. Biotite filled the sp aces between plagioclase and quartz. Scale of view: 5mm. Figure 130. Resorbed and zoned plag ioclase. Scale of view: 2.5 mm.

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146 GW-05 : Plagioclase feldspar: 70%, 0.7-1.9 mm, An15-30, crystals are subhedral to rarely anhedral, and have interlobate grain shapes and show concentric zoning, and they are altered to chlorite. Hornblende inclus ions observed in zoned plagioclase. Amphibole 25%, 0.15-1.5mm, defined by green to dark green colored equant crystals, and irregular and smoothly curved boundaries. Numerous of small rounded Fe oxides inclusions in hornblende observed Quartz: 2%, 0.25-1.25mm, crystals are anhe dral, and interstitial between plagioclase, typically forming clusters of cr ystals between larger phenocrysts, and show undulose extinction, and have amoe boid grain boundary shapes. Biotite: 10%, 0.3-1.75mm, occurs anhedral to rarely acicular, light brown to dark brown colored crystals.

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147 Figure 131. Aligned biotite surrounded by a nhedral quartz aggregates suggest for magmatic flow. Scale of view: 5mm. Figure 132. Plagioclase crystal is zoned and crystallized along its margins. Biotite adjacent to plagioclase has straight margins. Scale of view: 1mm.

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148 Figure 133. Fabric shape preferre d orientation in plagioclas e and hornblende observed as evidence for magmatic flow. Aligned plagio clase has irregular margins intergranular texture. Scale of view: 1mm. GW-10 : Plagioclase feldspar: 57% present as a phenocry sts, 0.35-5.5mm, An37 10% in the groundmass, 0.0075 mm, crystals are subhedral to rarely anhedral, and the grain boundaries are usually irregular, and attach ed to each other, and they are oscillatory zoned, and have minor alteration to chlo rite. Additional plag ioclase observed as chadacrysts included within poikilitic hornbl ende crystals, and these crystals are anhedral, and range in size from 0.05-0.1 mm. Quartz: 7%, Phenocryst: 0.2-0.35 mm, groundma ss: 20%, 0.045 mm, crystal are anhedral, and have intregranuler microcrack s, and show undulatory extinction. Additional

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149 quartz observed as chadacrysts included within poikilitic biotite crystals, and these crystals are anhedral, and range in size from 0.05-0.5 mm. Biotite: 2%, 0.05-0.5 mm, crystals are a nhedral, and have good cleavage development, and defined by light to dark brown colored crysta sl. Most of biotite crystals are poikilitic which are contai ning chadacrysts of quartz. Hornblende: 3%, 0.1-1.5 mm, crystals are anhedr al with irregular margins, and defined by light green to dark gr een equant crystals .Most of the crystals have 45-70% of alteration to chlorite. Groundmass: In fined grained groundmass, oscillat ory zoned plagioclase, and amoeboid quartz crystals observed. Few transgranular microfracutres have seen in thin section GW10. The average length for the microfractur es is about 12.5 mm. fractures show minor Fe oxidation. Figure 134. Plagioclase phenocryst is zone d and rimmed. Scale of view: 1mm.

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150 Figure 135. Completely zoned pl agioclase crystals show sy nneusis and the zoning around the whole aggregates formed afte r synneusis. Scale of view: 5mm. Figure 136. Plagioclase phenocryst showing a discontinuity between the core, patchy zoning in the centre, oscill atory zoning in the margin s. Scale of view: 5mm.

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151 Appendix C: Volcaniclastic samples Sample TW-02 is highly deformed and metamorphosed metavolcanic, taken ~ 1. 25 km away from the shear zone (Fig.88). The microstructure of the quartz grains have plane or smoothly curved boundaries, indica ting grain growth after recovery and recrystallization. The conditions of deforma tion are constrained by quartz. The dominant deformation mechanism in quartz is disloc ation creep and dynamic recystallization. Figure 137.The optical micrographs of sample TW02, ppl and cpl: enti rely recrystallized quartzite. Micas are few microns thick. This sample TW-05 is collected ~300 me ters away from the shear zone. The thin section has a mylonitic texture. Pro mylonite consists of slightly elongated, 2-6 mm large grains. The average size of quartz subgrains is 0.05 mm. This shape preferred orientation forms a strong foliation that is clear in the outcrop. The le ngth of elongated subgrains can

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152 reach 3 mm. The grain boundaries have an irregular, lobate morphology with euhedral grain boundary shape. Some of feldspar por phyroclasts show twining and have more rounded shape. Fractures are observed in feldspar porphyroclasts indicates for amphibolite grade metamorphic conditions bu t does not occur under low temperature deformation. Quartz grains show grai n size reduction. Feldspar porphyroclasts (P) enclosed in fine grained quartz chlorite rich matrix. Highly deformed (Q) quartz aggregates contain dynamically recrystallized grains with an av erage size of 0.3 mm. Elongated quartz aggregates define the foliation. In the optical photomicrograph, the foliation is oriented parallel to the x-plane of the thin section. Figure 138. The fine-grained matrix wrapped around the feldspar phorphyroclast, in the middle of the photomicrograph.Scale of view 9mm.

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153 Figure 139. Quartz grains are fine and dynamica lly recrystallized in micaeous quartzite matrix. Scale of view 1 mm. Figure 140. Micaceous mylonite wi th plagioclase feldspar po rphyroclast, mica and quartz that lies between the northeastern Piedra R oda pluton and the aABF shear zone. Feldspar porphyroclast is twinned with irregular bounda ries. The size of quartz grains are about 0.05 mm. Quartz grains are separated by mica bands. Scale of view 1mm. P

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154 Figure 141. Pre-deformational plagioclase pho rphyroclast has rounded inclusions and surrounded by external fabric s. Scale of view: 1mm. Figure 142. Pre-deformational plagioclase in mica-chlori te. Scale of view 1 mm. P

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155 Scale of view: 9mm. Figure 143. The two optical micrographs s how both mylonitic foliation and elongated quartz ribbons foliation. Scale of view: 1mm Q

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156 Appendix D : Mylonitic rock samples Figure 144. This photomicrograph shows the al ignment of quartz and micas. Not all quartz crystals are elongated. Th e irregular boundaries in quartz crystals are due to fabric shape control by biotite. Scale of view 1 mm. Figure 145. Faulted mica fish, the sense of sh ear is northeast side up. Scale of view: 1mm.

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157 Figure 146. TW06: Continuous foliation in micace ous quartzite ultramylonite. Scale of view: 5mm. Sample 11203-B Figure 147. Feldspar grain is reduced in size by brittle fracturing and reaction to form mica and quartz which are composing the matrix Fractures filled with recrystallized quartz aggregates. Feldspar is extended along the x-lineation plane. Scale of view: 1 mm.

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158 Figure 148. Plagioclase has asymmetric tails consist of feldspar and quartz grains. Plagioclase porphyroclast on the ri ght has internal fabrics parall el to the external fabrics foliation. Plagioclase phorphyroclast on the left side has irregular margins and contain internal fabrics not parallel to the foliati on plane suggest for a rotation. Scale of view: 1mm.

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159 Figure 149. Feldspar porphyroclast in the middl e of the micrograph is elongated along the foliation plane. Feldspar porphyroclast has as ymmetric tails consis t of plagioclase and quartz aggregates. Sc ale of view 5mm.

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160 Figure 150. Plagioclase phorphyroclast is paralle l to the foliation plan e but still rounded. The foliation is wrapped around the plagio clase crystal. Scale of view 1mm. Figure 151. Feldspar phorphyroclast indicates the sense of move ment. The view is to the northwest and the sense of shear is northeast side up.Scale of view: 1mm.

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161 Figure 152. Optical micrograph from sa mple 11203 shows boudinage plagioclase porphyroclast, the view is to the northwest a nd the sence of shear is northeast side up. Scale of view: 10mm.