AN ABSTRACT OF THE THESIS OF Brittain Eames Hill for the degree of Doctor of Philosophy in Geology presentedon October 17. 1991. Title: Petrogenesis of Compositionally Distinct Silicic Volcanoes in the Three Sisters Region of the Oregon Cascade Range: The Effects of Crustal Extension on the Development of Continental Arc Silicic Magmatism. Abstract Approved Signature redacted for privacy. tuwaru(1. iaylor - The Three Sisters region of the Oregon High Cascades has developed three compositionally and petrogenetically distinct silicic (i.e., Si02 58%)magma systems within the last 600 k.y. These silicic systems evolved from the same High Cascade niafic magma system and developed in thesame 20x30 km area of the arc, but did not interact. The Broken Top system (BT) evolved to 71% Si02 through a combination of plag+px+Fe-Ti oxides±ap (PPFA) fractionation and 20%-35% mixing of rhyolitic (74% Si02) crustal melts. In contrast, part of the Three Sisters system (3S) evolved to 66% SiO2 through PPFA fractionation alone, while other parts evolved to 66% SiO2 through PPFA fractionation coupled with 40% mixing of rhyolitic (72% Si02) crustal melts. The 3S system was intermittently active from s340 ka to 2 ka. The petrogenesis of interme- diate composition rocks at Middle Sister (<340 ka,>100 ka) was controlled by PPFA fractionation to s66% Si02. Rhyolite (72%-76% SiO2) was first erupted in the 3S system at100 ka, at the start of South Sister (SS) volcanismMajor and trace element abundances preclude derivation of 3S rhyolite through crystal fractionation, but are consistent with 20-30% dehydration melting of mafic amphibolite. The petrogenesis of intermediate composition rocks at SS was controlled by PPFA fractionation coupled with 30-40% rhyolitic magma mixing. However, the rhyolitic magma mixed into an essentially mafic system, which limited intermediate differentiation at SS to 66% SiO2. The BT system was active from600 ka to at least 200 ka. Major and trace element abun- dances preclude derivation of BT rhyolite (74% Si02) through crystal fractionation, butare consis- tent with 30% dehydration melting of older tonalitic intrusions. BT petrogenesis was controlled by PPFA fractionation accompanied by 10-20% mixing of rhyolitic magmas to 63% Si02, with 30% rhyolite mixing from 63% to 71% Si02. In contrast to the 3S system, differentiation proceeded beyond 66% SiO2 because rhyolitic magma was mixed into a more evolved (-60%-65% SiO2) system. The observed temporal and spatial variations in petrogenesis were not controlled by regional changes in tectonic setting, crustal thickness or crustal composition. However, small-scale changes in the magnitude of crustal extension occurred in this area, and are thought to have controlled petrogenesis by localizing mid-crustal mafic magmatism and thus crustal heat flow. C Copyright by Brittain E. Hill October 17, 1991 All Rights Reserved Petrogenesis of Compositionally Distinct Silicic Volcanoes in the Three Sisters Region of the Oregon Cascade Range: The Effects of Crustal Extension on the Development of Continental Arc Silicic Magmatism. by Brittain Eames Hill A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Completed October 17, 1991 Commencement June, 1992 ACKNOWLEDGEMENTS This study would not have been possible without the unfailing support and encouragement of two people: my wife, Ute, and my major professor, Edward M. Taylor. Without Ute's love and support throughout these last five years, I simply would not have been able to undertake or complete this project. Dr. Taylor's contributions to both this research and my education have been innurnera- ble, and I can't thank him enough for all that he has done for me. Anita Grunder and Roger Nielsen have also contributed much to this research, and I thank them for their patience and their insights. Anita Grunder, Cyrus Field, Roman Schmitt, and Herbert Huddleston were stalwart committee members, and I also thank them for their many improvements to both this dissertation and to my education. I must also thank the following additional people, who through their discussions, arguments and insights have helped me focus and refine various aspects of this research: David Blackwell, Larry Chitwood, Rich Conrey, Dave Draper, Bob Duncan, Cynthia Gardner, Scott Hughes, Norm MacLeod, Rob Negrini, Alan Niem, George Priest, Andrei Sarna-Wojcicki, Willie Scott, Dave Sherrod, Gary A. Smith, Don Swanson, and Jeff Tepper. The analytical work in this research would not have been possible without the technical support of the following people: Art Johnson, Roman Schmitt, Scott Hughes, Bob Walker, Terry Anderson, and Mike Conrady for INAA support at the O.S.U. Radiation Center; Bob Duncan, Lew Hogan, and Christine McBirney for support of the K-Ar dates; Roger Nielsen and Bill Gallahan for microprobe support; Peter Hooper, Rich Conrey, and Diane Johnson for help in obtaining XRF analyses at W.S.U. Financial support for the analytical work in this thesis was provided in part by facilities grants from Bob Duncan., Peter Hooper, Art Johnson, and the O.S.U. Microprobe committee. This research has also received partial financial support from the Geological Society of America (grant #4453-90), Sigma Xi (1988), a Shell graduate fellowship (1986-1988), Amoco and Chevron field research scholarships (1986, 1987), and the O.S.U. High Cascade Research Fund.I also thank UNOCAL Geothermal for releasing the six strontium isotopic analyses in this study. Finally, I'd like to thank Bev Taylor for her unquestioning friendship, Planters Punch, and for keeping a straight face during all of those geology talks.Relative doses of sanity have been provided by fellow travelers Bill Gallahan, John and Val Curless, Moot, Scott and Vivian Hughes- Golightly, Angela McDannel, the Northwest Cell of the Friends of the Pleistocene, Dan and Margaret Mumford, Gary Smith, Henry Weinhard's, Dave Wendland, the Friends of Igneous Rocks, Tom Horning, Amy Hoover, and Bandit the wonder dog.In Magma Veritas. TABLE OF CONTENTS INTRODUCTION 1 1.1 Overview of the Research Topic 1 1.2 Location and Methods 2 1.3 Summary of Previous Work 4 REGIONAL GEOLOGY OF THE CENTRAL HIGH CASCADES AREA 6 2.1 Crustal Composition 6 2.2 Deep Crustal Structure 8 2.3 Regional Tectonics 8 2.4 Regional Structure 10 2.5 A Brief History of the Oregon Cascades 12 GEOLOGY OF THE THREE SISTERS AREA 14 3.1 Introduction 14 3.2 Deschutes Formation 14 3.2.1 The Bull Springs Inlier of the Deschutes Formation 16 3.3 Tumalo Volcanic Center 20 3.3.1 Fyroclastic Deposits of the TVC 23 3.3.la Desert Spring Tuff 23 3.3.lb Tumalo Tuff 25 3.3.lc Lava Island Tuff 28 3.3.ld Pumice of Columbia Canyon 28 3.3.le Century Drive Tuff 29 3.3.lf Shevlin Park Tuff 29 3.4 Todd Lake Volcano 30 3.5 Tam MacArthur Rim Broken Top 30 3.6 The Three Sisters 30 3.7 Strontium Isotopes 31 MINERALOGY 33 4.1 Feldspar 33 4.2 Pyroxene 43 4.2.1 Clinopyroxene 43 4.2.2 Orthopyroxene 47 4.2.3 Pigeonite 50 4.3 Olivine 50 4.4 Iron-Titanium Oxides 52 4.5 Amphibole 56 4.6 Accessory Minerals 58 ESTIMATES OF INTENSIVE VARIABLES 63 5.1 Fe-Ti Oxide Temperatures and Oxygen Fugacities 63 5.2 2-Pyroxene Geothermometry 67 5.3 Combined Estimates of Temperature 69 5.4 Estimates of Pressure 69 5.5 Estimates of Water Content 74 5.5.1 Amphibole equilibria 74 5.5.2 Plagioclase equilibria 77 6. GEOCHEMISTRY 79 6.1 Introduction 79 6.2 Terminology 79 6.3 General Major Element Trends 79 MAFIC PETROGENESIS 88 7.1 Introduction 88 7.2 Crystal Fractionation 88 7.3 Open System Models 97 7.3.1 Crystal fractionation 98 7.3.2 Magma recharge and eruption 100 7.3.3 Assimilation 104 7.4 Summary of Mafic Petrogenesis 108 PETROGENESIS OF ANDESITES, DACITES AND RHYODACITES 112 8.1 Introduction 112 8.2 Three Sisters System 113 8.3 Broken Top Intermediate System 125 8.4 Summary of Intermediate Series Petrogenesis 137 PETROGENESIS OF RHYOLITES 138 9.1 Three Sisters Rhyolites 138 9.1.1 Crystal fractionation 143 9.1.2 Partial melting 147 9.1.3 Summary of Three Sisters rhyolite petrogenesis 150 9.2 Broken Top System Rhyolites 152 9.2.1 Crystal fractionation 152 9.2.2 Partial melting 155 9.2.3 Summary of Broken Top rhyolite petrogenesis 163 SUMMARY OF CENTRAL HIGH CASCADE PETROGENESIS 164 10.1 Petrogenesis of Volcanos in the Three Sisters area since 500 ka 164 10.2 The Occurrence of Rhyolite in the Oregon High Cascades 169 10.3 Tectonic Controls on Continental Arc Petrogenesis 172 10.4 A Paradigm of Crustal Stress and Igneous Petrogenesis 173 10.5 A Tectonomagmatic model of Three Sisters Area Petrogenesis 176 CONCLUSIONS 178 REFERENCES 181 APPENDIX Analytical Methods 199 APPENDIX Visual estimates of phenocryst abundances and additional petrographic information for studied rocks 202 APPENDIXC-i: Feldspar compositions determined through electron microprobe analysis 205 APPENDIX Composition of pyroxenes determined through electron microprobe analysis 209 APPENDIX Composition of amphiboles determined through electron microprobe analysis212 APPENDIX Composition of spinels determined through electron microprobe analysis 215 APPENDIX Composition of ilmenites determined through electron microprobe analysis 216 APPENDIXD-i: Compositions and locations of Quaternary rocks from the Three Sisters area217 APPENDIXD-2: Compositions and locations of Deschutes Formation rocks in the Three Sisters-Bend area 230 APPENDIX Compilation of mineral/melt distribution coefficients for calc-alkaline and mildly alkaline rocks 231 APPENDIX Published rare earth element phenocryst/matrix