Initiation of the Wrangell arc: A record of tectonic changes in an arc-transform junction revealed by

217 Pages·2017·15.78 MB·English

Initiation of the Wrangell arc: A record of tectonic changes in an arc-transform junction revealed
by new geochemistry and geochronology of the ~29–18 ma Sonya Creek volcanic field, Alaska

by

Samuel Ethan Berkelhammer

B.S., University of Puget Sound, 2013

A THESIS

submitted in partial fulfillment of the requirements for the degree

MASTER OF SCIENCE

Department of Geology
College of Arts and Sciences

KANSAS STATE UNIVERSITY
Manhattan, Kansas

2017

Approved by:

Major Professor
Matthew Brueseke
Abstract
The Sonya Creek volcanic field (SCVF) contains the oldest in situ magmatic products in
the ~29 Ma–modern Wrangell arc (WA) in south-central Alaska. The WA is located within a
transition zone between Aleutian subduction to the west and dextral strike-slip tectonics along
the Queen Charlotte-Fairweather and Denali-Duke River fault systems to the east. WA
magmatism is due to the shallow subduction (11–16°) of the Yakutat microplate. New 40Ar/39Ar
and U-Pb geochronology of bedrock and modern river sediments shows that SCVF magmatism
occurred from ~29–18 Ma. Volcanic units are divided based on field mapping, physical
characteristics, geochronology, and new major and trace element geochemistry. A dacite dome
yields a ~29 Ma 40Ar/39Ar age and was followed by eruptions of basaltic-andesite to dacite lavas
and domes (~28–23 Ma Rocker Creek lavas and domes) that record hydrous, subduction-related,
calc-alkaline magmatism with an apparent adakite-like component. This was followed by a
westward shift to continued subduction-related magmatism without the adakite-like component
(e.g., mantle wedge melting), represented by ~23–21 Ma basaltic-andesite to dacite domes and
associated diorites (“intermediate domes”). These eruptions were followed by a westward shift in
volcanism to anhydrous, transitional, basaltic-andesite to rhyolite lavas of the ~23–18 Ma Sonya
Creek shield volcano (Cabin Creek lavas), including a rhyolite ignimbrite unit (~19 Ma Flat Top
tuff), recording the influence of local intra-arc extension. The end of SCVF activity was marked
by a southward shift in volcanism back to hydrous calc-alkaline lavas at ~22–19 Ma (Young
Creek rocks and Border Lavas). SCVF geochemical types are very similar to those from the <5
WA, and no alkaline lavas that characterize the ~18–10 Ma Yukon WA are present. Sr-Nd-Pb-Hf
radiogenic isotope data suggest the SCVF data were generated by contamination of a depleted
mantle wedge by ~0.2–4% subducted terrigenous sediment, agreeing with geologic evidence
from many places along the southern Alaskan margin. Our combined dataset reveals
geochemical and spatial transitions through the lifetime of the SCVF, which record changing
tectonic processes during the early evolution of the WA. The earliest SCVF phases suggest the
initiation of Yakutat microplate subduction. Early SCVF igneous rocks are also chemically
similar to hypabyssal intrusive rocks of similar ages that crop out to the west; together these
~29–20 Ma rocks imply that WA initiation occurred over a <100 km belt, ~50–60 km inboard
from the modern WA and current loci of arc magmatism that extends from Mt. Drum to Mt.
Churchill.
Table of Contents

List of Figures .............................................................................................................................. viii
List of Tables ................................................................................................................................. xi
Acknowledgements ....................................................................................................................... xii
Dedication ..................................................................................................................................... xv
Chapter 1 - Introduction .................................................................................................................. 1
Objectives ................................................................................................................................... 1
Regional Tectonic Framework .................................................................................................... 1
Wrangell Arc Geology- Underlying Rocks and Wrangell Arc Volcanic Rocks .................... 5
Previous Geochemical Work ...................................................................................................... 8
Western Wrangell Arc Volcanism: Geochemical Trends ....................................................... 8
Eastern Wrangell Arc Volcanism: “Leaky” Strike-Slip ....................................................... 10
Central Wrangell Arc Volcanism: Frederika Formation ...................................................... 12
Age Progression of Wrangell Arc Volcanism....................................................................... 13
>20 Ma Magmatism From the Central Wrangell Arc ............................................................... 14
Sonya Creek Volcanic Field Study Area .................................................................................. 15
Structural Evidence for Local Extensional Faulting ............................................................. 17
Chapter 2 - Methods...................................................................................................................... 26
Field Methods ........................................................................................................................... 26
Analytical Methods ................................................................................................................... 26
Major, Trace, and Rare Earth Element Analysis .................................................................. 27
Sr-Nd-Pb-Hf Isotope Analysis .............................................................................................. 28
40Ar/39Ar Geochronology Analysis ....................................................................................... 30
iv
Petrography ............................................................................................................................... 32
Chapter 3 - Results ........................................................................................................................ 33
Field Relations, Physical Characteristics, and Geochronology ................................................ 33
Geochemistry Results ............................................................................................................... 37
Geochemical Classification................................................................................................... 37
Major Element Geochemistry ............................................................................................... 39
Trace Element Geochemistry ................................................................................................ 40
Rare Earth Element Geochemistry ........................................................................................ 40
Primitive Mantle Normalized Multi-Element Diagrams ...................................................... 42
Trace Element Ratios and Other Parameters: Sr/Y, (Sr/P)n, Ba/Nb, Eu/Eu* ....................... 43
Cabin Creek Chemostratigraphy ........................................................................................... 44
Sr-Nd-Pb-Hf Radiogenic Isotope Results ................................................................................. 45
Chapter 4 - Discussion .................................................................................................................. 80
Initiation of Wrangell Arc Magmatism .................................................................................... 80
Volcanic History of the Sonya Creek Volcanic Field (SCVF) ................................................. 81
Volcanic Architecture – Geochronology and Field Relations .............................................. 81
Geochemical Trends Through Time, and Implications For the Development of the Sonya
Creek Volcanic Field ............................................................................................................ 86
Rocker Creek Lavas and Domes ....................................................................................... 86
Source of Adakite-like Signature .................................................................................. 87
Silicic Lavas ...................................................................................................................... 92
Intermediate Domes .......................................................................................................... 94
Sonya Creek Shield Volcano ............................................................................................ 94
v
Cabin Creek Chemostratigraphy ................................................................................... 96
Flat Top ......................................................................................................................... 96
Silicic Plug .................................................................................................................... 97
Young Creek Rocks .......................................................................................................... 98
Border Lavas ..................................................................................................................... 99
Discussion of Radiogenic Isotope data ................................................................................... 100
Mixing Models .................................................................................................................... 101
Crustal Contamination of Primitive Magmas ................................................................. 102
Contamination of the Mantle Wedge by Subducted Sediment ....................................... 104
Geologic Evidence for Oligo-Miocene Sediment Delivery to the Trench ................. 105
Mixing Models Between Mantle and Subducted Sediment ........................................ 107
Problems with the Mixing Model and Adjustment of Pb Concentrations .................. 110
Comparisons to Other Wrangell Arc Volcanic Centers .......................................................... 113
Comparisons to Global Arc-Transform Settings .................................................................... 115
Tectonic Implications for Wrangell Arc Initiation ................................................................. 118
Temporal Connections to Regional Research ..................................................................... 119
Original Position and Subsequent Geographic Migration of the Wrangell Arc ................. 121
Final Thought .......................................................................................................................... 122
Key Remaining Questions/Future Work................................................................................. 123
Chapter 5 - Conclusions .............................................................................................................. 145
References ................................................................................................................................... 148
Appendix A - Sample Locations and Petrographic Descriptions ............................................... 165
Appendix B - Geochemical and Geochronologic Data ............................................................... 188
vi
Appendix C - 40Ar/39Ar Step-Heat Spectra ................................................................................. 196

vii
List of Figures
Figure 1.1: Map of southern Alaska and western Canada showing major volcanic and tectonic
features. ................................................................................................................................. 20
Figure 1.2: Simplified geologic map of the Sonya Creek volcanic field (SCVF), adapted from
Richter et al. (2000) .............................................................................................................. 22
Figure 1.3: Compiled geologic maps showing the inferred location and sense of motion of the
Beaver Creek fault in Alaska and Yukon Territory .............................................................. 24
Figure 3.1: Photographs from the Rocker Creek area ................................................................. 47
Figure 3.2: Photographs of the intermediate domes .................................................................... 48
Figure 3.3: Photographs of Cabin Creek lavas. ........................................................................... 49
Figure 3.4: Photographs of Flat Top, Young Creek area, and Border Lavas .............................. 50
Figure 3.5: Schematic geologic cross-sections of the SCVF ....................................................... 51
Figure 3.6: Photomicrographs of SCVF thin sections. ................................................................ 52
Figure 3.7: Photomicrographs of SCVF thin sections ................................................................. 53
Figure 3.8: Photomicrographs of SCVF thin sections ................................................................. 54
Figure 3.9: Photomicrographs of SCVF thin sections ................................................................. 55
Figure 3.10: Probability distribution diagram.............................................................................. 56
Figure 3.11: Stratigraphic section of North Cabin Creek ............................................................ 57
Figure 3.12: Stratigraphic section of South Cabin Creek ............................................................ 58
Figure 3.13: Total alkali silica and AFM discrimination diagrams ............................................. 60
Figure 3.14: FeO*/MgO vs. SiO2 and K2O andesite classification diagrams.............................. 61
Figure 3.15: Felsic classification diagrams.................................................................................. 62
Figure 3.16: Harker diagrams illustrating major element variations ........................................... 63
viii
Figure 3.17: Harker diagrams illustrating selected trace element variations ............................... 65
Figure 3.18: Chondrite-normalized rare earth element (REE) data for SCVF rocks .................. 67
Figure 3.19: Primitive mantle normalized multi-element variation diagrams of SCVF samples.68
Figure 3.20: Selected trace element ratios ................................................................................... 69
Figure 3.21: Chemo-stratigraphy of the North and South Cabin Creek sections ........................ 70
Figure 3.22: 87Sr/86Sr vs. εNd for eight SCVF samples ............................................................... 71
Figure 3.23: 207Pb/204Pb vs. 206Pb/204Pb for eight SCVF samples ................................................ 73
Figure 3.24: 208Pb/204Pb vs. 206Pb/204Pb for eight SCVF samples ................................................ 74
Figure 3.25: εHf vs. εNd for five SCVF samples ........................................................................ 75
Figure 3.26: 87Sr/86Sr vs. 206Pb/204Pb for eight SCVF samples .................................................... 76
Figure 4.1: Schematic illustration of the volcanic evolution of the SCVF through time .......... 126
Figure 4.2: Y vs. Rb discrimination diagram for all SCVF rocks, and Cr vs. SiO variations of
2
Rocker Creek lavas and domes ........................................................................................... 128
Figure 4.3: Additional trace element diagrams .......................................................................... 129
Figure 4.4: εNd vs. 87Sr/86Sr plot, showing results of crustal contamination binary mixing
calculations. ........................................................................................................................ 130
Figure 4.5: 207Pb/204Pb vs. 206Pb/204Pb plot, showing results of crustal contamination binary
mixing calculations ............................................................................................................. 131
Figure 4.6: εHf vs. εNd plot, showing results of crustal contamination binary mixing
calculations ......................................................................................................................... 132
Figure 4.7: εNd vs. 87Sr/86Sr plot, showing results of binary mixing calculations between
depleted mantle (M) and various end-members .................................................................. 133
ix
Figure 4.8: 207Pb/204Pb vs. 206Pb/204Pb plot, showing results of binary mixing calculations
between depleted mantle (M) and various end-members ................................................... 134
Figure 4.9: 208Pb/204Pb vs. 206Pb/204Pb plot, showing results of binary mixing calculations
between depleted mantle (M) and various end-members ................................................... 135
Figure 4.10: εHf vs. εNd plot, showing results of binary mixing calculations between depleted
mantle (M) and various end-members ................................................................................ 136
Figure 4.11: 87Sr/86Sr vs. 206Pb/204Pb plot, showing results of binary mixing calculations between
depleted mantle (M) and various end-members .................................................................. 137
Figure 4.12: 207Pb/204Pb vs. 206Pb/204Pb showing results of binary mixing model after the
adjustment of Pb concentrations in all end-members ......................................................... 138
Figure 4.13: 208Pb/204Pb vs. 206Pb/204Pb showing results of binary mixing model after the
adjustment of Pb concentrations in all end-members ......................................................... 139
Figure 4.14: 87Sr/86Sr vs. 206Pb/204Pb showing results of binary mixing model after the
adjustment of Pb concentrations in all end-members ......................................................... 140
Figure 4.15: εNd vs. 206Pb/204Pb showing results of binary mixing model after the adjustment of
Pb concentrations in all end-members ................................................................................ 141
Figure 4.16: εHf vs. 206Pb/204Pb showing results of binary mixing model after the adjustment of
Pb concentrations in all end-members ................................................................................ 142

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