Felsic magmas from Mt. Baker in the northern Cascade arc

111 Pages·2016·7.14 MB·English

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WWU Graduate School Collection WWU Graduate and Undergraduate Scholarship
2012
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Julie A. (Julie Angela) Gross
Western Washington University
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Gross, Julie A. (Julie Angela), "Felsic magmas from Mt. Baker in the northern Cascade arc: origin and role
in andesite production" (2012). WWU Graduate School Collection. 239.
https://cedar.wwu.edu/wwuet/239
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Felsic magmas from Mt. Baker
in the northern Cascade arc:
origin and role in andesite production

By
Julie Gross

Accepted in Partial Completion
Of the Requirements for the Degree
Master of Science

Kathleen L. Kitto, Dean of the Graduate School

ADVISORY COMMITTEE

Chair, Dr. Susan DeBari

Dr. Michael Clynne

Dr. Scott Linneman
MASTER’S THESIS

In presenting this thesis in partial fulfillment of the requirements for a master’s degree at
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Julie Gross
October 29, 2012
Felsic magmas from Mt. Baker
in the northern Cascade arc:
origin and role in andesite production

A Thesis
Presented to
The Faculty of
Western Washington University

In Partial Fulfillment
Of the Requirements for the Degree
Master of Science

By
Julie Gross
October 2012
Abstract
Dacitic magmas in volcanic arcs play a critical role in the growth and development of
felsic continental crust through mixing to form andesite, or to a lesser extent, by directly
adding new crustal material through fractionation of mantle derived basalts. Though dacitic
erupted lavas are scarce on Mt. Baker, this study discusses their importance in subsurface
processes such as mixing with more mafic magmas, and their potential to add directly to the
volume of continental crust. A comprehensive data set (including major, trace, and rare earth
element abundances, as well as petrography and mineral chemistry) reveals that the most Si-
rich, Mg-poor dacitic compositions analyzed in this study (dacite of Mazama Lake) can be
modeled as liquids derived by crystal fractionation from Mt. Baker high-Mg andesites.
These Si-rich compositions are in turn back-mixed with mafic magmas to produce more Si-
poor dacites (dacite of Cougar Divide) and andesites (andesite of Mazama Lake). The origin
of one enigmatic hornblende-bearing dacite unit (dacite of Nooksack Falls) is unconstrained.
None of the dacitic units have geochemical signatures that suggest an origin by melting of a
garnet-bearing source such as the subducting slab or the lower crust.
The dacite of Mazama Lake (plagioclase, clinopyroxene, orthopyroxene, Fe-Ti
oxides) represents a near end-member fractionated composition with only minor
contamination from xenocrystic material. Mineral populations commonly lack
disequilibrium textures, and exhibit normal zoning. Plagioclase and pyroxene chemistry
suggests the majority of the crystal population is original to the dacite of Mazama Lake.
Sparse resorbed olivine grains (<1% total crystal population) and weak reverse zoning in
some plagioclase and pyroxene grains indicates a minor addition of xenocrystic material.
The majority of the Mazama Lake compositions can be reproduced after 44% fractionation
iv
(55% remaining liquid) of a high-Mg andesite (the andesite of Glacier Creek), with
fractionating phases of 69% plagioclase, 16% orthopyroxene, 11% clinopyroxene, 3%
ilmenite, and 1% apatite. Excellent fits of major elements, most trace elements are provided
by this model.
The dacite of Cougar Divide (plagioclase, clinopyroxene, orthopyroxene, Fe-Ti
oxides, olivine) and the andesite of Mazama Lake (plagioclase, clinopyroxene,
orthopyroxene, Fe-Ti oxides, olivine) are more Si-poor, and exhibit evidence for magma
mixing. The Cougar Divide unit exhibits mingling textures in hand sample and both Si-poor
units exhibit mixing textures in thin section, such as calcic normal and sodic reverse zoned
plagioclase populations and pyroxene grains with abrupt Mg-rich rims. This suggests that
their primary geochemical characteristics come from mixing between more mafic and more
felsic magmas. The dacite of Mazama Lake can be used to reasonably reproduce
compositions observed in the mixed magmas. Mixing between the high-Mg andesite of
Glacier Creek and dacite of Mazama Lake can reproduce an average major and trace element
composition from the Cougar Divide unit in mixing proportions of ~60% andesite and ~40%
dacite. Major and trace element compositions from the andesite of Mazama Lake can be
reproduced by mixing ~30% the high-Mg basaltic andesite Tarn Plateau (a less fractionated
parent magma of the andesite of Glacier Creek) and ~70% Mazama Lake dacite.
The dacite of Nooksack Falls (plagioclase, hornblende, clinopyroxene,
orthopyroxene, Fe-Ti oxides) appears to represent a near-endmember composition, but
cannot be reproduced by fractional crystallization of any known parental composition at Mt.
Baker. A distinct set of minerals with compositions expected from a basaltic source (such as
calcic plagioclase grains, and Mg-rich clinopyroxene grains with high Cr concentrations)
v
suggests the dacite of Nooksack Falls acquired some xenocrystic material. However, removal
of this contamination does not permit a fractionation origin from known mafic compositions.
One possibility is that the dacite of Nooksack Falls was derived from more mafic magmas
that are not currently observed or erupted. These dacites are unlikely to be crustal melts given
their high H O contents. Ultimately, these hypotheses cannot be reconciled without isotopic
2
analysis.
The role of dacitic magmas at Mt. Baker is clear; (1) they have the potential to
directly contribute to the continental crust through fractionation, and (2) they have a role in
mixing, in which andesitic compositions (a common composition at arcs worldwide) are
formed.

vi
Acknowledgements
I would like to recognize the numerous individuals and organizations whose time and
support helped make this project a success. First and foremost, I would like to extend a
heartfelt thank you to my advisor, Sue DeBari. Your guidance, encouragement, and
knowledge have helped me develop scientific and critical thinking skills I never knew I had.
In addition, I would like to thank my committee members Mike Clynne, and Scott Linneman,
whose thoughtful comments and many e-mails aided immensely in polishing this work.
A deep thanks to the organizations that made this research possible: The Jack
Kleinman Memorial Grant from Cascade Volcanic Observatory, the Graduate Student
Research Grant from the Geologic Society of America, a research grant from the Mount
Baker Volcanic Research Center, Western Washington University RSP Fund for the
Enhancement of Graduate Research, and funds from our own distinguished WWU Geology
department.
I would like to extend a warm thanks to George Mustoe for instilling in me all the
knowledge of sample preparation (of every kind) he has collected over the years. In addition,
George was miraculously available when the equipment seemed to be behaving the most
ornery. To Dave Tucker, who was a priceless asset in the field with regards to locating the
Nooksack Falls dacite. To Nikki Moore, and Steve Shaw, my predecessors who provided tips
and valuable advice for my thesis as well as for basic grad student existence. To my fellow
graduate students, in particular Rose Bloom and Angela Cota. Finally I would like to thank
my husband Michael, who supported me on all fronts, mentally, physically, and emotionally.
In spite of all the support around me, this would have been a wearisome and lonely journey
without you.
vii
Contents
Abstract .................................................................................................................................................................iv 
Acknowledgements ............................................................................................................................................. vii 
List of Tables .........................................................................................................................................................ix 
List of Figures ........................................................................................................................................................ x 
Introduction ............................................................................................................................................................ 1 
Geology and tectonic setting .................................................................................................................................. 3 
Methods .................................................................................................................................................................. 8 
Whole Rock Analysis ........................................................................................................................................ 8 
Electron Microprobe ........................................................................................................................................ 10 
Results .................................................................................................................................................................. 11 
Petrography and Mineral Chemistry ................................................................................................................ 11 
The dacite of Nooksack Falls ...................................................................................................................... 11 
The dacite of Cougar Divide ....................................................................................................................... 14 
The dacite of Mazama Lake ........................................................................................................................ 16 
Whole Rock Chemistry .................................................................................................................................... 18 
The dacite of Nooksack Falls ...................................................................................................................... 19 
The dacite of Cougar Divide ....................................................................................................................... 20 
The dacite of Mazama Lake ........................................................................................................................ 20 
Intensive Parameters ........................................................................................................................................ 22 
Discussion ............................................................................................................................................................ 23 
Origin of the dacites; slab or crust derived? .................................................................................................... 24 
Petrologic evidence for endmember vs. hybrid magmas ................................................................................. 25 
The dacite of Nooksack Falls ...................................................................................................................... 26 
The dacite of Mazama Lake ........................................................................................................................ 29 
The dacite of Cougar Divide and the andesite of Mazama Lake ................................................................ 30 
Genesis of the felsic endmembers ................................................................................................................... 31 
The dacite of Nooksack Falls ...................................................................................................................... 33 
The dacite of Mazama Lake ........................................................................................................................ 34 
The role of magma mixing .............................................................................................................................. 35 
A petrogenetic model for Mt. Baker ................................................................................................................ 38 
Conclusions .......................................................................................................................................................... 39 
References Cited ................................................................................................................................................... 41 
Appendix A .......................................................................................................................................................... 99 
Appendix B......................................................................................................................................................... 100
viii
List of Tables
Table 1. Summary of petrography and mineral chemistry ......................................................47
Table 2. Whole rock major and trace element data .................................................................48
Table 3. Plagioclase compositions ...........................................................................................51
Table 4. Hornblende compositions ..........................................................................................56
Table 5. Clinopyroxene compositions .....................................................................................57
Table 6. Orthopyroxene compositions .....................................................................................62
Table 7. Fe-Ti oxide compositions ..........................................................................................65
Table 8. Olivine compositions .................................................................................................67
Table 9. Fe-Ti oxide thermobarometry ....................................................................................68
Table 10. Pyroxene thermometry .............................................................................................68
Table 11. Summary of important mineral textures, zoning, and interpretations .....................69
Table 12. Partition coefficients and representative compositions for crystal fractionation
models ......................................................................................................................................71
Table 13. Least squares modeling results for the dacite of Nooksack Falls ............................72
Table 14. REE, and incompatible element results for the dacite of Nooksack Falls ...............72
Table 15. Least squares modeling results for the dacite of Mazama Lake ..............................73
Table 16. REE, and incompatible element results for the dacite of Mazama Lake .................73
Table 17. Mixing calculations for the dacite of Cougar Divide ..............................................74
Table 18. Mixing calculations for the andesite of Mazama Lake ............................................74
Table 19. Mixing calculations for the andesite of Mazama Lake ............................................74
ix