the Case from the Jurassic Bonanza Arc section, Vancouver Island

125 Pages·2008·12.33 MB·English

The Role of Amphibole in the Evolution of Arc Magmas and Crust: the Case from the
Jurassic Bonanza Arc section, Vancouver Island, Canada

by

Jeffrey Paul Larocque
B.Sc., McGill University, 2005

A Thesis Submitted in Partial Fulfillment of the
Requirements for the Degree of

MASTER OF SCIENCE

In the School of Earth and Ocean Sciences

© Jeffrey Paul Larocque, 2008
University of Victoria

All rights reserved. This thesis may not be reproduced in whole or in part, by photocopy
or other means, without the permission of the author
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The Role of Amphibole in the Evolution of Arc Magmas and Crust: the Case from the
Jurassic Bonanza Arc section, Vancouver Island, Canada
by
Jeffrey Paul Larocque
B.Sc., McGill University, 2005

Supervisory Committee

Dr. Dante Canil, (School of Earth and Ocean Sciences)
Supervisor

Dr. Laurence Coogan, (School of Earth and Ocean Sciences)
Departmental Member

Dr. Kevin Telmer, (School of Earth and Ocean Sciences)
Departmental Member
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Supervisory Committee

Dr. Dante Canil, Supervisor (School of Earth and Ocean Sciences)
Dr. Laurence Coogan, Departmental Member (School of Earth and Ocean Sciences)
Dr. Kevin Telmer, Departmental Member (School of Earth and Ocean Sciences)

ABSTRACT
Exposed on Vancouver Island, British Columbia, the Jurassic Bonanza arc is believed to
represent the southerly continuation of the Talkeetna arc. Small bodies of mafic and
ultramafic cumulates within deeper plutonic levels of the arc constrain the fractionation
pathways leading from high-MgO basalt to andesite-dacite compositions. The removal of
amphibole from the most primitive non-cumulate compositions controls the compositions
of mafic plutons and volcanics until the onset of plagioclase crystallization. This removal
is accomplished by the intercumulus crystallization of large amphibole oikocrysts in
primitive olivine hornblendite cumulates. Experimental hornblende compositions that
crystallize from high-MgO basalts similar to primitive basalts from the Bonanza arc show
a good correlation between octahedral Al in hornblende and pressure, and provide a
means of estimating crystallization pressures during differentiation of primitive arc
basalt. Application of an empirical barometer derived from experimental amphibole data
(P = Al(6)/0.056 – 0.143; r2 = 0.923) to natural hornblendes from this study suggests that
crystallization of primitive basalts took place at 470-880 MPa. Two-pyroxene
thermometry gives a result of 1058 +/- 91 ºC for the only olivine hornblendite sample
with both pyroxenes. Lever rule calculations require the removal of 30-45 % hornblende
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from the most primitive basalt compositions to generate basaltic andesite, and a further
48% crystallization of hornblende gabbro to generate dacitic compositions. Hornblende
removal is more efficient at generating intermediate compositions than anhydrous
gabbroic fractionating assemblages, which require up to 70% crystallization to reach
basaltic andesite from similar starting compositions. There are no magmatic analogues to
bulk continental crust in the Bonanza arc; no amount of delamination of ultramafic
cumulates will push the bulk arc composition to high-Mg# andesite. Garnet removal
appears to be a key factor in producing bulk continental crust.
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TABLE OF CONTENTS

Supervisory Committee .………………………………………………...……………..p. ii
Abstract …………………………………………………………………..……………p. iii
Table of Contents ……………………………………………………………………...p. iv
List of Tables ………………………………………………………………….………p. vi
List of Figures …………………………………………………………………….…..p. vii
Acknowledgments …………………………………………………………………….p. ix
CHAPTER 1
Introduction ……………………………………………………………… …....p. 1
Thesis Overview …………………………………………………………….…p. 3
Methods ………………………………………………………………………..p. 4
CHAPTER 2
Introduction …………………………………………………………………….p. 6
Geologic Setting ………………………………………………………………p. 11
Petrography …………………………………………………………..……….p. 19
Geochemistry ………………………………………………………………....p. 23
Discussion
Amphibole and arc magma differentiation ………………………...…p. 34
Conditions of Bonanza arc differentiation ……………………………p. 39
Generation of andesite ………………………………………………..p. 45
Comparison with Talkeetna arc …………………………...………….p. 51
Implications for continental crust …………………………………….p. 54

Conclusions …………………………………………………………………..p. 54
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Chapter 3
Summary ……………………………………………………………...………p. 57
Future Work ………………………………………………………………..…p. 59
Bibliography ……………………………………………………………….………p. 60
Appendices
Table 1 …………………………………………………………………..……p. 68
Table 2 …………………………………………………………………….....p. 80
Table 3 ……………………………………………………………...………..p. 84
Table 4 ………………………………………………………………………p. 111
Table 5 …………………………………………………………...………….p. 112
Figure 20 …………………………………………………………………….p. 115
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List of Tables

Table 1. Whole rock geochemistry data for all analyzed samples from the current study.
Includes major and trace elements, and select radiogenic isotope ratios.
Table 2. Petrographic descriptions of samples collected for the current study. Modal
proportions are based on visual estimates.
Table 3. Mineral chemistry data from microprobe analyses. Data is reported as wt.%
oxide; nomenclature based on stoichiometric results. Were different from charge-balance,
methods and references are given.
Table 4. Results of geothermobarometry for select samples.
Table 5. Synthesis of sample data indicating which samples have petrographic, whole-
rock geochemical, and mineral chemistry data.
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List of Figures

Figure 1. Schematic subduction zone diagram………………………………………..p. 10
Figure 2. Schematic stratigraphic section for southern Vancouver Island ………...…p. 13
Figure 3. Location of field area ……………………………………………….……...p. 15
Figure 4. Geology map of field area ………………………………………………….p. 17
Figure 5. Field exposures of WCC and olivine cumulate in outcrop ………..….…….p. 18
Figure 6. Photomicrographs showing textures in cumulates ………………..….…….p. 20
Figure 7. Total alkali-silica plot for all Bonanza arc lithologies ……………………..p. 25
Figure 8. Histogram plot of the Si concentrations of Bonanza arc subunits …….……p. 26
Figure 9. Harker diagrams for all Bonanza arc lithologies ……………………...……p. 27
Figure 10. Chondrite-normalized trace element spider diagrams ………...………..…p. 28
Figure 11. Chondrite-normalized rare earth element plots …………………..……….p. 30
Figure 12. Trace element plots for Bonanza arc lithologies ………………….………p. 31
Figure 13. Harker diagrams illustrating the relationship between the WCC plutons, the
Bonanza volcanics, and hornblende oikocrysts in ultramafic cumulates ……….……p. 36
Figure 14. Mg# vs Al plots of whole rocks and hornblende compositions showing
relationship between hornblende oikocrysts and Bonanza array …………......………p. 37
Figure 15. Phase diagram for water-saturated basalt ………………………...….……p. 40
Figure 16. Experimental amphibole data …………………………………………..…p. 42
Figure 17. Permissible parental compositions based on Fe/Mg partitioning between
amphibole and basalt ……………………………………………………….…………p. 46
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Figure 18. Major element plots illustrating the mineralogic controls on the Bonanza arc
lithologies …………………………………………………………………….....…….p. 48
Figure 19. Relationship between primitive Bonanza arc volcanics, primary island arc
basalts from the literature, and potential fractionation assemblages …………...…….p. 53
Figure 20. Plotted sample locations for data collected during 2006 field mapping p. 115
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Acknowledgments

Firstly, I would like to thank Dante for all of his support during my time at UVic. He
gave me plenty of space to pursue my own ideas, but was always there when I needed
guidance. What more could I ask for. Secondly, thank you to Robyn, for pushing me
when it was needed. Thanks as well to Laurence, who suggested many useful papers, as
well as helping me through my analytical difficulties. Gary Pearson and Perry
Heatherington were instrumental in getting the project off the ground, providing funding
as well as local expertise – thank you both very much. Funding for field and analytical
work was also provided by Geoscience BC. I would also like to thank Holly Steenkamp
for her assistance in the field. I benefited greatly from working under the tutelage of
Graham Nixon at the BCGS. Thank you Graham for your input, and for showing me how
proper field work is done. Lastly, thanks to those people who made my windowless office
space a pleasant place: Dave, Abi, Jody, Jason, Zhihuan, Angela, Sussi, and Kimberly.