AN ABSTRACT OF THE DISSERTATION OF Mariek E. Schmidt for the degree of Doctor of Philosophy in Geology presented on November 11, 2005 Title: Deep Crustal and Mantle Inputs to North Sister Volcano, Oregon High Cascade Range Abstract approved: Signature redacted for privacy. Anita'Ii Grunder The Central Oregon High Cascade Range is an anomalously mafic segment of the Cascade Arc due to ongoing intra-arc extension, which allows most magmas to traverse the crust without stalling and evolving tomore evolved compositions. North Sister, a composite volcano in this arc segment, has produced a seemingly monotonous basaltic andesite (52.5-54.0% Si02) that is depleted in incompatible elements from 320 ka until 15 ka. This dissertation addresses the volcanic development of North Sister Volcano and deep crustal processes that produced its basaltic andesite. Stratigraphic variation signaling changes in eruptive style, unconformable contacts, 40Ar/39Ar dating, and geochemical correlations divide North Sister's eruptive history into four stages: (1) the early shield, (2) the subglacial stage, (3) the upper shield stage, and (4) the stratocone stage. Lastly, the north-striking, >1 1-km Matthieu Lakes Fissure (MLF) transected North Sister in three splays. North Sister's eruptive stages correspond to four compositional groups that record a general decrease in compatible elements such as Ni (112 to 35 ppm), while incompatible elements are constant or generally decrease (e.g. Ba 302 to 247 ppm) through time. Isotopic variations at North Sister are small, but systematic; Sr and Nd isotopic ratios becomemore mantle-like with time. Petrologic modeling places the generation of North Sister's basaltic andesite in the deep crust by a) deciphering upper crustal re-equilibration; b) mixinga primitive, low K tholeiite (LKT) with a silicic, Al-rich partial melt of 5-30 millionyear old crust; and c) high recharge rates by mantle-derived LKTs and interaction witha refractory crust. The Sr and Nd isotopic ratios of primitive basalts from the Cascade Range define four segments of the arc that reflect separate isotopic reservoirs and mantle melting regimes and correlate with major crustal domains. Thesesegments are 1) north, Mt. Meager to Glacier Peak; 2) Columbia, Mt. Rainierto Mt. Jefferson; 3) central, the North Sister to Medicine Lake, and 4) south, Mt. Shastato Mt. Lassen. The central segment has Basin and Range in the backarc and is the most restricted in bulk and isotopic composition. Deep Crustal and Mantle Inputs to North Sister Volcano, Oregon High Cascade Range by Mariek E. Schmidt A DISSERTATION submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Presented November 11, 2005 Commencement June 2006 ©Copyright by Mariek E. Schmidt November 11,2005 All Rights Reserved Doctor of Philosophy dissertation of Mariek E. Schmidt presented on November 11, 2005 APPROVED: Signature redacted for privacy. Major Professor, representing Geology Signature redacted br privacy. Chair of the Department of Geosciences Signature redacted for privacy. Dean of the a.I- 'ate School I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request Signature redacted for privacy. Mariek E. Schmidt, Author ACKNOWLEDGEMENTS I am grateful to everyone who helped with the project, including field assistants, lab gurus, and mentors. Additional acknowledgments and contributions are noted in each manuscript. I would first like to sincerely thankmy dissertation advisor and mentor, Dr. Anita Grunder, whose support and friendship has helpedme to succeed. Second, I thank my thesis committee members, Dr. Roger Nielsen, Dr. David Graham, Dr. Anne Trehu, and graduate school representative Dr. Margaret Burnett who have, through reviews and conversations, added significant understanding of North Sister Volcano. I deeply thank my life partner, Brian Scott who has always encouraged my dreams. I am also thankful of my parents, Pierre and Karen Schmidt who taughtme I am capable of anything I set my mind to. CONTRIBUTiON OF AUTHORS Dr. Anita L. Grunder has contributed greatlyto these manuscripts, designing the project and assisted in data analysis, writing, and revisions. MichaelC. Rowe contributed primitive basalt samples and major and trace element analyses fromthe Central Oregon Cascade Range. John C. Chesley provided Os isotopic analysesof North Sister, Little Brother, and regional primitive basaltsas well as in analyzing the data. TABLE OF CONTENTS Page INTRODUCTION i Mafic Volcanism in the Central Oregon CascadeArc .2 North Sister Volcano ...3 Purpose of This Work 6 THE EVOLUTION OF NORTH SISTER: A VOLCANOSHAPED BY EXTENSION AND ICE IN THE CENTRALOREGON CASCADE RANGE 8 Abstract 9 Introduction 9 Geologic Setting 11 Tectonic setting ii Petrologic setting 15 Glacial history 16 Analytical Techniques ..16 Field work- sampling and mapping 16 40ArI39Arage determination 16 Sample characterization 17 North Sister Volcano .18 Glaciers and glacial deposits around North Sister 19 Stage descriptions and field relations 20 Stage 1 Early shield .20 Stage 2 Glacial stage .25 Stage 3 Upper shield .26 Stage 4 Stratocone .27 Matthieu Lakes Fissure (MLF) .27 Other mafic vents and lavas ..35 TABLE OF CONTENTS (Continued) Page Little Brother 35 Northern glomerocrystic lava 35 Island Fissure 36 Holocene cinder cones .36 40Ar/39Ar dates . 39 Volume estimates 42 Compositional Characteristics 42 Dikes 44 Discussion 45 The persistence of North Sister Volcano 45 Volcano-ice interactions 46 Erosional features .46 Constructional glacial features .47 Correlation with global glaciations 52 North Sister's dikes 53 Comparison with other Cascade basaltic andesite volcanoes 54 Life spans of Cascade Volcanoes 55 Eruptive style and subglacial eruptions ...56 Dikes and plugs .57 Tectonism and Volcanism .58 The occurrence of basaltic andesite types .59 Conclusions ..60 Acknowledgements 61 TABLE OF CONTENTS (Continued) Page VARIATIONS IN SR AND ND ISOTOPES ALONG THE CASCADE ARC 62 Abstract .63 Introduction 63 Magma types 64 Tectonic controls along the Cascade Arc 67 Along arc occurrence of magma types 68 Variations in Sr and Nd isotopes ..69 Tectonic implications 74 The CAB component 74 Conclusions 75 Acknowledgements 76 EVIDENCE FOR RECHARGE AND INTERACTION WITH REFRACTORY DEEP CRUST TO PRODUCE BASALTIC ANDESITE BENEATH NORTH SISTER VOLCANO, CENTRAL OREGON CASCADE RANGE, USA 77 Abstract 78 Introduction 79 The Central Oregon Cascade Arc 80 Basaltic inputs to the Cascade Arc 83 The nature of arc Lower crust . 87 North Sister Volcano 88 TABLE OF CONTENTS (Continued) Page Analytical Techniques ..89 Results 93 Rock Description ..93 Major and trace element characterization 95 Early primitive North Sister ..98 Group 1 .98 Group 2ap 101 Group 2ae ..ioi Group 2b 102 Group 3-4 102 Dikes 103 MLF ..108 Little Brother 110 Sr, Nd, Pb, and 0 isotopes 110 Os isotopes 114 Discussion .115 Modeling North Sister and MLF intra- group trends 120 Processes modeled 120 Modeling groups 2ap and 2ae 124 Modeling group 3-4 . 128 Modeling the MLF 130 Mixing to model primitive North Sister 134 Modeling North Sister's variations through time .138 The EC-ERXAFC model 139 Input parameters 141 TABLE OF CONTENTS (Continued) Page Results of EC-ERAFC modeling 144 Summary and assessment of the model 146 Magma petrogenesis in the deep crust beneath 147 Estimated volumes 151 The age of the deep crust by Os isotopic constraints 153 Implications for magma genesis in the Oregon Cascades 157 The MLF and dike propagation 158 Periodicity of long-lived mafic composite volcanoes 159 Conclusions 161 Acknowledgements 162 SUMMARY AND CONCLUSIONS 163 Summary 163 Conclusions 168 BIBLIOGRAPHY .170 APPENDICES ..186 LIST OF FIGURES Figure Page 1-1 Time versus Si02 of erupted material in the Central OR High Cascades Range 4 2-1 Tectonic map of Oregon 10 2-2 Map showing distribution of volcanic vents and faults in Central OR High Cascades 13 2-3 Geologic Map of North Sister Volcano, Matthieu Lakes Fissure (MLF), and Little Brother 21 2-4 View of North Sister Volcano's north flank with eruptive stages 24 2-5 Major element variation diagrams for North Sister, Little Brother, andMLF 31 2-6 a) Photo of North Sister Volcano's NE ridge palagonitic tuff interfingered with lava; b) SEM image of palagonitic tuff 33 2-7 MLF's Si02 plotted against distance north south 37 2-8 Crystal content of MLF versus. Si02 38 2-9 Summary of field and chemical data including stratigraphic sections, 40Ar/39Ar dates, compositional groups, and 18O of seawater 50 3-1 a) Tectonic map of the Pacific Northwest; b) Histogram of 87Sr/86Sr for volcanic rocks along the Cascade Arc 66 3-2 '43NdJ'44Ndversus 87Sr/86Sr for main Cascade primitive basalt types a) low K tholeiite (LKT) and ocean island-like basalt (01- like); and b) calc-alkaline basalt (CAB) . 70 3-3 87Sr/86Srversus a) Sr and b) Sr/P for primitive arc basalts from the Cascade Range 72 4-1 Tectonic map of Oregon . 81 LIST OF FIGURES (Continued) Figure Page 4-2 Rare earth element abundance diagrams for a) primitive Cascade endmember basalt and b) North Sister-type and Mount Washington-type basaltic andesites 84 4-3 SiO and MgO variation diagrams for volcanic rocks from North Sister Volcano and MLF 96 4-4 Ni variation diagrams for volcanic rocks from North Sister Volcano and MLF 99 4-5 V variation diagrams for volcanic rocks from North Sister Volcano andMLF 104