The Origin of Adakites in the Garibaldi Volcanic Complex, Southwestern British Columbia, Canada
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The Origin of Adakites in the Garibaldi Volcanic Complex, southwestern British Columbia, Canada A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements For the Degree of Master of Science In Geology University of Regina By Julie Anne Fillmore Regina, Saskatchewan November 2014 Copyright 2014: J.A. Fillmore UNIVERSITY OF REGINA FACULTY OF GRADUATE STUDIES AND RESEARCH SUPERVISORY AND EXAMINING COMMITTEE Julie Anne Fillmore, candidate for the degree of Master of Science in Geology, has presented a thesis titled, The Origin of Adakites in the Garibaldi Volcanic Complex, Southwestern British Columbia, Canada, in an oral examination held on August 22, 2014. The following committee members have found the thesis acceptable in form and content, and that the candidate demonstrated satisfactory knowledge of the subject material. External Examiner: Dr. Martin Beech, Campion College Supervisor: Dr. Ian M. Coulson, Department of Geology Committee Member: Dr. Tsilavo Raharimahefa, Department of Geology Chair of Defense: Dr. Josef Buttigieg, Department of Biology ii Abstract The Garibaldi Volcanic Complex (GVC) is located in southwestern British Columbia, Canada. It comprises two volcanic fields: the Garibaldi Lake Volcanic Field (GLVF) in the north and the Mount Garibaldi Volcanic Field (MGVF) in the south. Petrographical and geochemical studies on volcanic rocks collected from the GVC have determined that they exhibit adakitic characteristics; these intermediate rocks range from basaltic andesite to dacite represented mainly by lava flows, domes and minor pyroclastic material. All the lavas exhibit evidence of magma mixing, which include sieve textured crystals, dehydration reaction textures, differently sized phenocryst populations, xenocrysts and xenoliths. The geochemistry of the GVC magmas exhibit several adakitic indicators which # include high Sr/Y (> 40), Mg (~ 51), Al2O3 (> 15 wt. %), low K2O/Na2O (~ 0.3), low Yb (< 1.9 ppm) and fractionated rare earth element (REE) compositions (La/Yb(N)~ 10), which have not been identified in previous studies. Adakites are the product of subducted slab partial melts within the garnet stability field, subdivided into low silica adakites (LSA; < 60 wt. % SiO2) and high silica adakites (HSA; > 60 wt. % SiO2) reflecting differing source regions, where HSA are primary slab melts and LSA are partial melts of mantle wedge peridotite that has been previously modified by slab-derived magmas. Identification of adakitic rocks in tectonic regimes unrelated to subduction have led to the argument that the distinctive high Sr/Y and La/Yb ratios are not unique and cannot be used as an indicator of slab partial melting. Basalts in adakite suites are often enriched iii in high field strength elements (HFSE; Nb in particular) and are classified as niobium enriched basalts (NEB); NEB is argued to originate from mantle wedge peridotite that has been previously metasomatised by slab partial melts. Trace element modelling illustrates that the GVC adakites can be generated by partial melting of subducted ocean crust. Incompatible and compatible element ratios and relative crustal thickness beneath the GVC preclude AFC processes or high pressure partial melting or crystallisation of basalt as the source of the adakite signature in the GVC rocks and suggests that the GVC adakites likely result from slab partial melts. iv Acknowledgements I would like to acknowledge everyone who provided funding and support for this project. I thank my supervisor, Dr. Ian Coulson, who provided funding through his Natural Sciences and Engineering Research Council (Discovery Grant) and facilitated microprobe analysis of selected samples, as well as much support and consultation. The University of Regina provided financial support through the Faculty of Graduate Studies and Research scholarships, the Teaching Assistantship program and through the Department of Geology Teaching Assistantship program. I would especially like to thank Dr. Michael Clynne, former editor in chief of the Bulletin of Volcanology for extremely helpful comments and discussion on my paper published last year in the Bulletin and strongly influenced this study. Dr. William Minarik and Glenna Keating, who provided whole rock geochemical analysis of the samples studied at McGill University. Finally, I would like to thank my family and all my friends for their moral support and putting up with my frustrations. Most of all, I have to thank my wonderful and loving husband, without whom this project could not have been possible. v Table of Contents Abstract ii Acknowledgements iv Table of Contents v List of Figures viii List of Appendices x List of Acronyms xii 1. Introduction 1 1.1 Previous Work 2 1.2 Adakites in the GVC 6 2. Regional Geology 7 2.1 Eruptive History of the GVC 9 2.2 History and definition of adakites 12 2.3 Adakite – tonalite-trondhjemite-granodiorite (TTG) association 15 2.4 Non-slab melt models for adakite genesis 18 3. Petrography 20 3.1 GLVF 20 3.1.1 Cheakamus Valley Basalts (CVB) 20 3.1.2 Helm Creek Basaltic Andesite (HCBA) 23 3.1.3 Desolation Valley Basaltic Andesite (DVBA) 27 3.1.4 Barrier andesite 29 3.1.5 Black Tusk 30 3.2 MGVF 33 3.2.1 Ring Creek andesite 33 vi 3.2.1.1 Proximal Ring Creek andesite 33 3.2.1.2 Distal Ring Creek andesite 35 3.2.2 Columnar Peak dacite 36 3.2.3 Paul Ridge andesite 37 4. Geochemistry 39 4.1 Analytical techniques 40 4.1.1 Whole Rock Geochemistry 40 4.1.2 Mineral Geochemistry 42 4.2 Results 43 4.2.1 Basalts and Basaltic Andesites 43 4.2.2 Adakites 48 5. Discussion 54 5.1 Adakite Geochemistry of the GVC 56 5.1.1 La and Cr 56 5.1.2 LSA versus HSA 57 5.1.3 Magma Mixing in the GVC 58 5.1.4 Interaction with Mantle Peridotite 70 5.1.5 Isotope Geochemistry 71 5.2 Possible models for Adakite Genesis 73 5.2.1 Partial melting of basaltic lower crust 73 5.2.2 High pressure fractionation/AFC of basaltic magma 77 5.2.3 Partial melting of subducted ocean crust 80 5.2.3.1 Association with niobium-enriched basalts (NEB) 84 5.2.3.2 Ni content in olivine 93 5.2.3.3 Regional tectonic regime for southwestern British Columbia 96 vii 5.3 Petrogenetic model for Adakite Genesis beneath the GVC 98 6. Conclusions 104 References 107 Appendix A – Whole Rock Geochemistry 127 Appendix B – Normative Mineralogy 130 Appendix C – Mineral Geochemistry 132 Appendix D – Trace element data for modelling of slab partial melts 142 viii List of Figures Fig. 1: Location Map of the GVC 3 Fig. 2: Geology of the GVC (modified from Green 1977) with sample locations 4 Fig. 3: Field Photographs 10 Fig. 4: Thin section photomicrographs of basalts and basaltic andesites 21 Fig. 5a, b: Mineral composition of mafic minerals (CVB and HCBA) 24 Fig. 5c, d: Mineral composition of mafic minerals (DVBA and PR) 25 Fig. 6a-f: Thin section photomicrographs of adakites (BF, BT, URC) 31 Fig. 6g-l: Thin section photomicrographs of adakites (LRC, CP, PR) 32 Fig. 7: Plot of total alkalis vs. silica 45 Fig. 8: Major element Harker diagrams 46 Fig. 9: Trace element Harker diagrams 47 Fig. 10: Primitive mantle normalised multi-element diagrams 49 Fig. 11: Chondrite normalised multi-element diagrams 50 Fig. 12: Adakite-normal arc rock differentiation plots (Sr/Y vs. Y, La/Yb vs. Yb) 53 Fig. 13: K/Rb vs. SiO2/MgO and Sr-K/Rb-[(SiO2/MgO)*100] plots 55 Fig. 14: LSA-HSA differentiation plots 59 Fig. 15: SEM photomicrographs of the GVC adakites 62 Fig. 16: Incompatible-compatible element ratio plots for the GVC adakites 68 Fig. 17: Adakitic indices plots for GVC and lower crustal melts 76 Fig. 18: Modelled slab partial melts compared to observed GVC adakite compositions 82 Fig. 19: Binary mixing plots for the CVB and HCBA 88 ix Fig. 20: Simple three component mixing model for the CVB 90 Fig. 21: Incompatible-compatible element ratio plots for the CVB and HCBA 91 Fig. 22: NiO vs. Fo content in GVC olivines 95 Fig. 23: Interpreted petrogenetic model for adakite genesis in the GVC 99 x List of Appendices Appendix A – Whole Rock Geochemistry 127 Table A1: Major and minor element composition of investigated 128 samples from the GVC Table A2: Trace and rare earth element composition of 129 investigated samples from the GVC Appendix B – Normative Mineralogy 130 Table B1: Normative mineralogy of representative samples 131 from the GVC Appendix C – Mineral Geochemistry 132 Table C1: Electron microprobe compositions of olivine from the 133 GVC Table C2: Electron microprobe compositions of clinopyroxene 136 from the GVC Table C3: Electron microprobe compositions of orthopyroxene 137 from the GVC Table C4: Electron microprobe compositions of plagioclase from 138 the GVC Table C5: Electron microprobe compositions of oxides from the 140 GVC Appendix D – Trace element modelling of slab partial melts 142 Table D1: Partition coefficients for elements used in slab partial 143 melt model Table D2: Modelled compositions of slab melts at various 143 melt fractions (F); residue=70% Cpx, 10% Gt, 20% Hbl Table D3: Modelled compositions of slab melts at various melt 143 fractions (F); residue=70% Cpx, 20% Gt, 9.5% Hbl, 0.5% Rut Table D4: Modelled compositions of slab melts at various melt 143 fractions (F); residue=70 % Cpx, 25 % Gt, 4.5 % Hbl, 0.5% Rut Table D5: Estimated starting composition of Juan de Fuca (JdF) 143 MORB xi Fig. D1: Failed slab partial melt models for various residual 144 compositions xii List of Acronyms ADR Andesite-dacite-rhyolite AFC Assimilation-fractional crystallisation BF Barrier flow BT Black Tusk Cay Mt. Cayley CP Columnar Peak dacite CVB Cheakamus Valley Basalt DVBA Desolation Valley Basaltic Andesite GLVF Garibaldi Lake Volcanic Field GVB Garibaldi Volcanic Belt GVC Garibaldi Volcanic Complex HCBA Helm Creek Basaltic Andesite HFSE High field strength element(s) HREE Heavy rare earth element(s) HSA High silica adakite LC Lower crust LILE Large ion lithophile element(s) LOI Loss on ignition LRC Lower Ring Creek andesite LREE Light rare earth element(s) LSA Low silica adakite MGVF Mt.