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Spilite and Diabase, with Minor Chert, and (2) Piagogranite Hornblende Gabhro, Cum'ilate Gahbro An'3

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Spilite and Diabase, with Minor Chert, and (2) Piagogranite Hornblende Gabhro, Cum'ilate Gahbro An'3 AN ABSTRACT OF THE THESIS OF SCOTT GARRET VAIL for thedegree of DOCTOR OFPHLLOSOPHT in GEOLOGY presented on February 4, 1977 Title: GEOLOGY AND GEOCHEMISTRY OF THE OREGON MOUNTAIN AREA, SOUTHWESTERN ORECON AND NORTHERN CALIFORNIA Abstract approved: Signature redacted for privacy. The Oregon MDuntain area contains ai oihiohic asernb.ee ef rocks which can he divided iato two majorzones on thea ci' iit1 ology:(1) spilite and diabase, with minor chert, and (2) piagogranite hornblende gabhro, cum'ilate gahbro an'3. cnmulatt ultramaficrccs The two zones have a total estimated thickness of ahcut 4. 6 kmThe lower contact of the cumulatezone is gradational into the 3osephi.ne Peridotite, an extensive body of alpine-type harzburgite, Thetipper- most spilites are depositionally overlain by the Galice Forma ien (Jurassic, Kimmeridgi.an). Rocks in the diabase-spilitezone have unde rgone metamorphism similar to sea -floor burial metancrphirn. Metamorphic grade progresses from zeolite facies through chlorite, epidote and actinolite zones of the greenschist facies to amphibolito facie s.Prehnite -purnpel]yite facie s me arnerphi sm has been superimposed on the lowest temperature assemblages. probably owing to later burial by- the Galice Formation. The plutonic zone consists of a sequence of ultramaEic and rnafic cumulate rocks which culminates in a residue of granophyric plagiogranite.Hornbiende gabbro, in part cumulate and in part intrusive, occurs near the top of the sequence. The Josephine Peridotite consists of olivine-rich harzburgite and minor amounts of dunite, chromitite and pyroxenite, all with tectonite textures.It is essentially indtmguishable from other occurrences of alpine-type harzburgite. The Galice Formation is predominantly slaty shale.Lithic graywacke and pebbly cong].omerate are volumetrically minor corn-- ponents. The gray-wacke contains abundant auartz,roicanic fr.agmens with minor feldspar, mafic minerals and metamorphic fragments which indicate a mixed continental and volcanic arc provenance. Rocks of the Oregon Mountain area chemicall'r resemble other well known and well studied ophiolites. Major eler.cert variations in the diabase-spilite zone are interpretable in terms of sea water inter-- acton with mid-ocean ridge tholeiite.Increases in 3iO, Na20, H2O and FeC and decreases in CaO and MgO relative to mid-ocean ridge hasaits were. observed in most specimens. Lrs consistent changes occur in K20, A1Z03, TiO. and MnO. Titanium and zirconirn abundances suggest affinities to mid- ocean ridge basalts, but there is some evidence of mobilization of these elements. Rare earth element distributions in rocks from the di.abase- spilite zone appear to have been generally unchanged during ineta- morphism although modification of Ce abundance has evidently occur- red in one specimen. The distribution patterns are typicallfiat (La/Lu = 1- 1.5) with low abundances (ay. La = 15x chronditic abun- dance)indicating that basaltic rocks from the Oregon Mountain area have a.ffithties with mid-ocean ridge basalts and arc tholeiitcs. Theoretical models indicate that a 15 to 20 percent melt of mantle source rock with a REE abundances about 2 to 3 times those of chondrites is a reasonable mechanism to produce the observed REE distributions. The ophiolite exposed in the Oregon Mountain area apparently formed as oceanic crust in a marginal basin environment during Jurassic time prior to the Nevadan Orogeny. The marginal basin probably lay between the Rogue-Galice island arc and the western continental margin. Nevadan deformation telescoped the arc-marginal basin terrain and caused accretion of the Jurassic rocks onto the continental margin. Geology ard Geochemistry of the Oregon Mountain Area, Southwestern Oregon and Northern CaUfornia by Scott Garret Vail A. THE3iS submitted to Oregon State Univeri:y in partial fulfillment of the requirements for the degree of Doctor of PhilosopI'r Completed Febr;ary 4, i)77 Cnmencerr.Lar'.t June 1977 ACKNOWLEDGEMENTS I am indebted to many peouie for encouragement and assistance during the course of this investigation.Richard A. Weisner, Karleen E. Davis, C. Stewart Eldridge arid Robert Dorsett gave able assis- tance and companionship in the field. chnical assistance in analyt- ical procedures was given by Roberta L. Connard, Takaaki Fukuoka, Kathleen Heide, Kenneth F. Scheidegger, Roman A. Schmitt and Ronald G. SerAechai at Oregon State University, and Gary Cunningham at the University of Oregon.I am particularly thankful to Michael 0. Garcia at U.C.L.A. for the electron microprobe analyses included herein.Early drafts of the thesis were typed and photography was done by Janice M. Stevens.The final draft was typed by Lyndal'u Sikes.The illustrations were drafted by Barbara Priest. Discussions with many ieople helped formulate my ideas and direction for research; however responsibility for errors, of course, is mine.In partuiaI thank Drs. Hans G. Aye? Lallemant, Robert G. Coleman, Henry J. B. Dick, Cyrus VT. Field, Michael 0. Garcia, Clifford A.. Hopson, William P. Leean and Thomas P. Thayer.I am especially indebted to Len Ramp for suggesting an appropriate area for study. I have received much helt .1ro. the members of my thesis committee -J. Granvilie Johnson. Robert A. Lawrence and William H. Taubeneck.I am especiaUy grateful to my advisor E Julius Dasch for his encouragement and admirable patience during my work. I thank the Geological Society of America for three Penrose Research Grants, and the Oregon Department of Geology and Mineral Industries for providing a vehicle for the field portion of this investi- gation.Without the assistance of these two agencies I would have been unable to complete this project. Most importantly, I am grateful o my parents who have never faltered in their. faith in my azrbitions, encouragement and financial support. TABLE OFCONTENTS INTRODUCTION 1 Purpose I Location and Accessibility I Ophiolites and the Ophiolite Problem 3 PREVIOUS WORK 9 REGIONAL GEOLOGY 11 Introduction 11 Western Paleozoic and Triassi.c Belt ii Western. Jurassic Belt 11 Nevadan Orogeny 17 Coast Range Province .18 PETROGRAPHY OF THE MAFIC COMPLEX 20 General Statement 20 Contact Relations 20 Distribuior and Thickness 21 Diabase-Spilito Zone 21 Hornb.iende Gabbro and Quartz Diorite 32 Cumulate Rocks 36 Norite Dikes a'-id Rodingite 49 JOSEPHINE PERIDOTITE 52 Introduction 52 Harzhurgite and Dunite 52 Ciinopyroxene -Rich Lherzolite Veins S5 Orthoprroxenite Veins 56 Se rpentinization 56 C-ALICE FORMATION AND LATE JURASSIC DIKES 57 Galice Formation 57 Late Jurassic Dikes 59 STRUCTURAL GEOLOGY 61 General Statement 61 Early Structures 61 Nevadan Structures 62 Post-Orogenic Uplift 62 Age of the Mafic Complex 64 GEOCHEMISTRY 6 Introduction 65 Methods 65 Major Element Chemistry 68 TraceElement Geochemistr,r REE Models of Oregon Mountain Area Petrogenesis 103 DISCUSSION 113 Regional Relationships n9 BIBLIOGRAPHY 121 APPENDIX I: Location and Description of Analyzed Samples 136 APPENDIX Ii: Chemical Analyses 143 APPENDLX III, Parameters Used in Neutron Activation AnaLyses APPENDIX IV: Microprobe Analyses of Minerals From Mafic Complex Cumulate Rocks 156 APPENDIX V: Distribution Coefficients Ued in Trace Element Models 158 VITA 159 ILLUSTRATIONS Lre Page Sketch map of southwestern Oregon and Northern California showing the Oregon Mountain area and areas of pertinent previous investigations. 2 Comparison of the stratigraphic th. ckriess of igneous units from various ophiolite masses with the geophysical estimate of the oceanic crustal layers of Hess and Shor and Rait. 4 Generalized geologic map of the Kiamath Mountains. 12 Generalized geology of the northern part of the western Jurassic belt southwestern Oregon and northern California. 13 Stratigraphic sections of Permian arid Mesozoic rocks in the Kiamath Mountains. 15 Composite section through the mafic complex, Oregon Mountain area. 22 Basalt pillows in diabase-spilite zone; Shelly Creek. 24 JP 326, spilite. 26 JP 416, epidosite. 28 JP 366, diabase. 29 ii. JP 340, porphyritic quartz keratophyre. 31 12. JP 318, altered plagiogranite. 34 13. Distribution of minerals in the cumulate zone of the mafic complex. 38 Figure Pao'e C1inopyroxeneo1ivine cumulate s; north slope of Oregon Mountain. 39 Mine ra1graded layering in orthopyroxerte - olive cumulates, Whiskey Creek. 40 JP 178, clinopvroxene-orthopyroxene ortho- cumulate. 41 JP 322, norite adcumulate, 42 Part of the pyroxene quadrilateral showing analyzed ciinopyroxene s, or thopyroxene s, and olivines from Oregon Mountain area cumulates. 45 Branching rodingite dike and veinlets in cumulate harzburgite; Whiskey Creek, 51 Banded harzburgite cut by serpentine v- ns; Cook Trail, north of Bain Station. 54 FeO*/MgO vs major element oxides in rocks from the diabasespilite zone, Oregon Mountain area. 70 Si02 and Na70 vs H2O in diabase- spilite zone rocks, Oregon Mountain area. Na20 + K20 in rocks from the diabase-spilite zone, Oregon Mountain area. 73 A.FM comparison plot for diabase-spilite zone rocks, Oregon Mountain area. 74 AFM comparison plot for plutonic rocks, Oregon Mountain area. 81 26. Ti vs Zr discrimination plot of sea-floor basalts. 83 re ae Ti vs Zr discrimination plot of Oregon Mountain area diabase-spi1ite zone rocks. 66 Rare earth element patterns of island arc tholeiites, mid--ocean ridge tholeiites and caic-alkaline, shshonitic and aikalic rocks of island arcs. 88 Rare earth element patterns for rocks from the diabase-.spilite zone, Oregon Mountain area. 90 Rare earth element pattern for a sill (JP 391)
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