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Title: Volcanic Stratigraphy of the Deschutes Formation, Green Ridge to Fly Creek, North-Central Oregon

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Title: Volcanic Stratigraphy of the Deschutes Formation, Green Ridge to Fly Creek, North-Central Oregon AN ABSTRACT OF THE THESIS OF RichardM. Conrey forthe degree ofMaster of Science in Geology presented on March 6,1985 Title: Volcanic Stratigraphy of the Deschutes Formation, Green Ridge to Fly Creek, North-central Oregon Abs t r act ap proved _.. Edward M. About 225 lava flows and ash-flow tuffs of the Des- chutes Formation (DF) were mapped at Green Ridge. The units fill east-trending paleovalleys which "sky-out" westward and dipeast;the eastward dip of the units de- creases as they are tracedeastward. The source of the units was west of Green Ridge as evidenced by the increase in the number of ash-flow tuffs and lava flows inthe DF from east to west toward GreenRidge,and the presence of viscous siliciclavas and lag deposits inash-flow tuffs on the west flank of GreenRidge. The exposed DF section at Green Ridge is 1400 feetthick,and consists mainly of basaltic andesites with subordinate diktytaxiticbasalts, andesites,dacites,aphyric basaltic andesites and ande- sites rich in Fe andTi,ash-flowtuffs,and sediments. The upper 400 feet of the section is devoid ofash-flow tuffs anddacites. Nine paleomagnetic polarity intervals, five normal and fourreversed,are recognized in the sec- tion;tentative correlation of these intervals with the magnetic time scale suggests that the oldest DF unit exposed at Green Ridge is 6.5 Ma, and the youngest4.4 Ma. The DF units at the crest, and west of Green Ridge are cut by at least five N- toN13W-trending, down-to-the- west normal faults ina zoneabout five miles wide.. These faults were possibly active as much as 5 Ma; the develop- ing faults may have provided structural pathways for the ascent of the mafic lavas in the upper 400 feetof the section. The faulting which created the Green Ridge escarpment was probably finished by 3 Ma, and perhaps earlier. Thepresenceof dense, Fe- and Ti-rich lavas throughout the DF section implies that magmas hadrela- tively easyaccess tothe surface during DF time (7.6-4.4 Ma), whichsuggeststhat the E-W extensional tectonism that culminated in the formation of the Green Ridge faults was present throughout deposition of theDF. The major element chemistry of DF diktytaxitic basalts, basaltic andesites, and andesites appears to changeregularly with Ti02 content, as shown by decreasing CaO/FeO* (FeO* = totalFe expressed asFeO) with increas- ing Ti02, andby decreasingA1203 with increasing Ti02 at constant MgO content. The basalts and basaltic andesites with the lowest Ti02 contents also have the lowestalkali contents. These chemicaltrends appearto be caused by plagioclase, with subordinate olivine, fractionation, and are consistent with the fact that plagioclaseand olivine are virtually the only phenocryst mineralsin the mafic DF rocks. It is likely that the diktytaxitic basalt with the lowest Ti02 content isa parental magma. Fractionationmay accountfor the chemical variations within the mafic rock groups considered separately,but the progressionfrom basalt to andesiteappears to be the result of mixing of silicicand mafic magmas. Thechemis- try of the rocks, especially the CaO/Al203 ratios, is con- sistent with suchmixing, asis the petrography; opaque minerals are very late crystallizers in the basalts and basaltic andesites, and there is increasing evidencefor magmamixing in the series basalt through andesite. Such evidence includes multiple plagioclasepopulations in the same rock, both of which areresorbed, and resorbed pyroxenesand amphiboles. Three flows of plagioclasemegacryst-bearing basaltic andesite occur near the top of theDF section. The plagioclase megacrystsare up to 5 cm in length,commonly contain apatite inclusions, andresemble the plagioclase in anorthosite bodies. Ilmenite megacrysts are also pre- sent. The megacrysts and the presence ofhighly plagio- clase-fractionated aphyric lavas rich inFe and Ti in the DF suggest that anorthosite bodies arepresent beneath the north-central Oregon Cascade Range. Two xenoliths of partly meltedcordierite-sillima- nite-quartz granulite gneiss were foundin a DF ash-flow tuff. The xenoliths demonstrate thatgranulite-grade metamorphism and at least local partialmelting of the lower crust took place beneath thenorth-central Oregon Cascades. Volcanic Stratigraphy of the DeschutesFormation, Green Ridge to FlyCreek,North-central Oregon by Richard M. Conrey A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed March 6, 1985 CommencementJune 1985 APPROVED: Associate Professorof G-orogy e Schoo Date thesis is presented March 6, 1985 Typed by Jeanne W. Shermanfor Richard M. Conrey In memory of David Robert Conrey ACKNOWLEDGEMENTS I gratefully acknowledge the field and laboratory support provided by a generous geothermal research grant from the Oregon Department of Energy,arrangedwith the kind help of George Priest (DOGAMI) and Ed Taylor. I thank Ed Taylor for suggesting this project, and for his help and patience during its completion. Ed has taught me a great manythings over the years;I particu- larly valuethe senseof curiousity which he has imparted tome. I thank W. H. Taubeneck for his influence on my educationas ageologist; from him I learned to appreciate the science of petrography, and much besides, both geologic and non-geologic. I thank H.E. Enlows for teaching me the rudiments of petrology, and for a field trip to Denio that strongly influenced my decision to becomea field geologist. I have enjoyed working andrelaxing with my col- leagues incentral Oregon: Jere Jay, Glen Hayman, Gary Smith,DebbieCannon, Dave Thormahlen, Neil Bingert, Britt Hill,and especiallyTom Dill, Gene Yogodzinski, Dave Wendland,and AngelaMcDannel. Innumerable discussions, by no means alwaysgeologic, with Tom, "Yogi",Dave, and Angela have been mosteducational for me. I thank D. Wendlandand particularly A. K. (S.B.)McDannelfor their financialassistance. I owe my improvedphysical fitness during the final months ofthis project to my friendBart Dafoe. I am gratefulto Therese Belden for her help on countless occasions. I am especiallygrateful to my father, Fred L. Conrey, for his financial support during the writing of most of this thesis. To my mother,JeanneW. Sherman, who has invested many hours and done anoutstanding job in typing this thesis, I can only inadequately express my thanks here. TABLE OF CONTENTS Page INTRODUCTION Definition and Purpose LocationandAccess 2 Methods of Study 4 Previous and Concurrent Work 9 THE DESCHUTES FORMATION 9 THE GREEN RIDGE FAULT ZONE 14 Regional Geology 17 Andesite Petrogenesis 31 STRATIGRAPHY 42 The Old Volcano 42 Deschutes Formation 47 VOLCANIC STRATIGRAPHY 47 PALEOMAGNETIC STRATIGRAPHY Pliocene fault-related sediments 66 Squawback Ridge volcano 68 Late Pliocene(?) Metolius intracanyon lava 70 Quaternary vents and the "WizardFalls" basalt 74 Late Pleistocene airfall pumices 76 STRUCTURE 80 The Green Ridge Fault Zone 80 Regional Structural Relations 87 LAVA FLOWS OF THE DESCHUTES FORMATION 93 Introduction 93 Diktytaxitic Basalts 94 Porphyritic Olivine Basalts 119 Basaltic Andesites 121 I. "Squawback type" basaltic andesite 122 II. Glomeroporphyritic basaltic andesites 123 III. Feldspar-phyric basaltic andesite 125 IV. Fine-grained basaltic andesites 127 V. Olivine-bearing basaltic andesites 127 VI. "Fine-grained, mixed type" basaltic andesites 128 VII.Porphyritic, reverse-zoned,plagioclase 128 basaltic andesites SUMMARY OF BASALTIC ANDESITE PETROGRAPHY 130 CHEMISTRY AND PETROGENESIS 131 Andesites 145 A comment about trace elements 154 Aphyric, fine-grained, Fe- and Ti-rich basaltic 156 andesites and andesites 156 I. Description and petrogenesis II. Are the aphyric rocks related to anorthosite? 175 III. Tectonic and eruptive setting 176 Mixed Lavas and Dike 178 Dacites 206 Rhyodacite Hales' banded and WR-205 lavas 212 RC-717 lava 224 RC-585 lava 230 Megacryst-bearing Basaltic Andesites 231 DESCRIPTION AND POSSIBLE GENESIS 231 SIMILARITY TO ARCHAEAN MEGACRYST-BEARING LAVAS 240 ARE THERE ANORTHOSITE BODIES BENEATH THECASCADES? 241 ASH-FLOW TUFFS OF THE DESCHUTESFORMATION 245 Introduction 245 Fly Creek Tuff 247 Six Creek Tuff 260 Hoodoo Tuff 268 RC-402 Ash-flow Tuff 272 PLUTONICAND METAMORPHICXENOLITHS 274 I. Epizonal plutonic xenoliths 274 II. Propylitically altered xenoliths 279 III. Partly melted granite xenolith 280 IV.Partly melted granulite xenoliths 285 CONCLUSIONS 292 Petrogenetic summary 292 Geologic History 300 BIBLIOGRAPHY 304 APPENDIX 1. PETROGENETIC MODELING 329 Fractionation effects of common minerals 329 Multiple injection model of O'Hara(1977) 333 APPENDIX 2. UNIT DESCRIPTIONS 335 LIST OF FIGURES Figure Page 1. Index map showing location of the thesis area. 3 2. View of Green Ridge from theair,facing north. 6 3. Geologic map of Oregon. 18 4. Dikes and layering in the core of theold volcano. 43 5. Cut-and-fillrelationship on the westflankof Green Ridge. 49 6. Stratigraphicrelations on the westflankof Green Ridge 54 south of the "LookoutRidge." 7. Stratigraphicrelationson thewest flank of Green Ridge 55 north of the "LookoutRidge." 8. Stratigraphic relations on the east flank of Green Ridge 56 south of Six Creek. 9. Stratigraphic relations on the east flank of Green Ridge 57 between Six Creek and S. Fk. Spring Creek. 10. Stratigraphic relations on the east flank of GreenRidge 58 north of S. Fk. Spring Creek. 11.Paleomagnetic correlation chart. 65 12. Structuremap and cross-sections of the GreenRidge fault 82 zone. 13. Possible relationship between the Green Ridge fault zone 89 and the Brothers fault zone. 14. Textural detail
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