AN ABSTRACT OF THE THESIS OF David Allen Jenne for the degree of Master of Science in Geology presented on March 15,1978 Title:STRUCTURAL GEOLOGY AND METAMORPHIC PETROLOGY OF THE GOLD MOUNTAIN AREA SNOHOMISH COUNTY WASHINGTON d' Abstract approved: Signature redacted for privacy. Dr. Robert D. Lawrence The thesis area is located in the western foothills of the North Cascade Mountains immediately east of Darrington, Washington. Three major rock units are present and these include, from west to east, the Darrington Phyllite, a sedimentary m1ange unit, and the Shuksan Schist.Each of these units is bounded by major faults.In the present thesis only the Darrington Phyllite and the melange unit were studied in detail. The Darrington Phyllite consists of phyllitic metapelites with very local interbedded graywacke, conglomerate, and greenschist. The phyllites contain alternating layers of quartz-albite and musco- vite-graphite.In some rocks, this compositional layering is equiva- lent to sedimentary bedding.However, in most locations,it has resulted from metamorphic processes involving the transposition of bedding and metamorphic differentiation during mimetic recrystal- lization. Greenschist is present as tectonically emplaced blocks and as very local interbeds in the phyllite.Along the western margin of the thesis area, greenschist and meta-igneous rocks have been faulted into place.These rocks probably are part of the Jumbo Mountain Complex. The interbedded greenschists contain lawsonite that has been altered to chlorite.This suggests that blueschist.-facies meta- morphism was followed by a temperature increase and greenschist- facies metamorphism. The mlange unit consists of blocks of sedimentary, meta- igneous, and metamorphic rocks in tectonic contact with a sheared pelitic matrix. The sedimentary rock types include conglomerate, sandstone, shale,siltstone, and limestone.The meta-igneous rocks are generally mafic and have been metamorphosed to the greenschist or epidote-amphibolite facies.Low-grade metamorphism has affected the entire melange unit. The major rock units are separated from each other by major faults.The east-dipping Shuksan thrust fault lies below the Shuksan Schist and above the melange unit. A northeast-trending strike-slip fault parallels Decline Creek and is terminated bya major strike-slip fault along Dan Creek. The Dan Creek fault separates the melange unit from the Darrington Phyllite. A third strike-slip fault is present at the western end of Gold Mountain and extends south of the thesis area. Four deformation episodes were distinguished in the area. The earliest F1 deformation involved development of the earliest schistosity in the Darrington Phyllite and the Shuksan Schist.F2 and F3 deformation created crenulation and strain-slip cleavages in the Darrington Phyllite.Associated folds are present in all of the rock units, are coaxia', and have nearly horizontal axes.These folds were probably formed at the time of Shuksan thrust fault motion.Vertical F4 folds reflect later strike-slip faulting. Tertiary dikes, probably related to the Eocene Barlow Pass volcanics, trend north- south and dip steeply.Contact metamorphism, quartz-vein formation, and mineralization occurred at the northwest- ern end of Gold Mountain.These resulted from intrusion of a small stock into the Darrington Phyllite. The major rock types in the thesis area probably originated in an island arc environment. The Shuksan Schist consists of meta- morphosed island arc volcanic rocks.The meta-igneous rocks within the mlange unit are fragments of oceanic crust, probably froma back-arc basin.These rocks were uplifted and emplaced with the other mélange rocks during late Paleozoic subduction.The Darrington Phyllite consists of meta-sediments thatwere probably initially de- posited in a back-arc environment.Subduction ceased-prior to thrust faulting in the middle Cretaceous.Eocene strike-slip faulting brought the rocks into their present positions. Structural Geology and Metamorphic Petrology of the Gold Mountain Area, Snohomish County, Washington by David Allen Jenne A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed March 1978 Commencement June 1978 APPROVED: Signature redacted for privacy. Associate Professor of Geology in charge of major Signature redacted for privacy. Chairman of Deparent of Geology Signature redacted for privacy. Dean of Graduate School Date thesis is presented March 15, 1978 Typed by Opal Grossnicklaus for David Allen Jenne ACKNOWLEDGEMENT I express my appreciation to my wife,Susani, who helped me collect field data and assemble the manuscript and gave me much moral support.To Dr..obert D. Lawrence who suggested the study, provided much valuable aid, and critically read the manuscript. To Dr. William H. Taubeneck for assistance in the petrographic work. To Dr. Cyrus W. Field, Dr. J. G. Johnson, and Dr. George Keller for critically reading the manuscript. To Oregon State University for providing a half-time teaching assistantship and for payment for thin sections used in the thesis.To the staff of the Darrington Ranger Station, Mt. Baker National Forest, for assistance in my field work during the summer of 1976. TABLE OF CONTENTS INTRODUCTION 1 Purpose of this Study 1 Equipment and Materials 2 Thesis Area 2 Regional Geology 4 Previous Work in the Area 7 DESCRIPTION OF ROCK UNITS 10 DARRINGTON PHYLLITE 11 Introduction 11 Meta-sedimentary Rocks 11 Field Description 11 Petrographic Description 17 Greenschist 22 Metamorphic Facies 24 Age and Correlation 27 Tectonically Emplaced Rocks 28 Introduction 28 Greenschists 29 Meta-igneous Rocks 30 Metamorphism and Emplacement 31 SHUKSAN SCHISTS 34 MELANGE UNIT 36 Introduction 36 Sedimentary Rock Types 37 Shales, Siltstones, and Pliyllites 37 Field Description 37 Petrographic Description 39 Sandstone 40 Field Description 40 Petrographic Description 41 Conglomerates 43 Volcanic Rocks 46 Marble 47 Depositional Environment 48 Age and Correlation 53 Meta-igneous Rocks 54 Field Description 54 Petrographic Description 56 Origin, Metamorphism, and Emplacement 58 Schistose Rocks 61 Introduction 61 Quartz-muscovite Schist 62 Blueschist 62 TERTIARY ROCKS 64 Dikes 64 Quartz Veins and Mineralization 65 Tufa Deposits 66 STRUCTURAL GEOLOGY 67 Introduction 67 General Description 67 Methods and Nomenclature Used 69 Darrington Phyllite 71 Introduction 71 Features Associated with Each Episode 73 Development of PhyUitic Schistosity and F1 Folding 73 Characteristics of Later Folding Episodes 95 Interaction of Episodes 101 Quartz Fabric Analysis 107 Structure of the Melange Unit 111 Introduction 111 Formation of the Melange 118 Later Folding of the Melange 119 Calcite Fabric Analysis 122 Relationship Between the M&lange Unit and the Darrington Phyllite 124 Faulting 127 Sequence of Metamorphism and Deformation 131 QUATERNARY GEOLOGY 137 GEOLOGIC HISTORY 139 SUMMARY OF SIGNIFICANT RESULTS 142 BIBLIOGRAPHY 144 APP ENDIC ES Appendix A 147 Appendix B 150 Appendix C 151 Appendix D 168 Appendix E 174 LIST OF ILLUSTRATIONS Figure Page Location of thesis area. 3 Regional geology. 5 Darrington Phyllite showing F2 or F3 folding. 13 Darrington Phyllite with original bedding. 13 Locations of greenschist, conglomerate, graywacke, and meta-igneous rocks. 16 Generalized pressure-temperature diagram. 26 Thin section showing bedding in phyllite from mélange unit. 44 Polished section of folded stretched-pebble conglomerate. 44 Classification of sandstones. 50 Qm - F - L + Qp diagram. 52 Meta-gabbro surrounded by pelitic matrix. 57 Brecciated mafic meta-igneous rock. 57 Major structures. 68 Data from 3 phyllite outcrops. 74 Location map of structural domains. 75 Domain I poles to S1. 76 Domain I poles to axial planes. 77 Domain I fold axes. 78 Domain II poles toS. 79 Figure Page Domain II poles to axial planes. 80 Domain II fold axes. 81 Domain II poles to S1. 82 Domain II poles to axial planes. 83 Domain III fold axes. 84 Domain IV poles to S1. 85 Domain IV poles to axial planes. 86 Domain IV fold axes. 87 Domain V poles to S1. 88 Domain V poles to axial planes. 89 Domain V fold axes. 90 Domain VI poles to S1. 91 Domain VI poles to axial planes. 92 Domain VI fold axes. 93 S1 foliation as compositional layering. 97 Severely contorted S1 foliation. 97 Well-developed phyllitic schistosity. 99 F2 fold with F3 crenulations. 99 Darrington Phyllite with folded quartz veinlets. 100 F4 fold-axis orientation. 104 Summary of Darrington Phyllite folding. 106 41. Qua.rtz c-axes in Darrington Phyllite. 109 Figure Page Poles to bedding planes in melange unit. 113 Poles to S1 foliation south of Decline Creek. 114 Poles to S1 foliation north of Decline Creek. 11 5 Fold axes in the melange unit. 116 Poles to axial planes of late-stage folds. 117 Melange phyllite showing crenulation cleavage. 120 Sheared and folded melange phyllite. 120 Stress orientations calculated from calcite. 123 Preferred orientations of 100 calcite c-axes in marble. 125 Alternative configurations of the Dan Creek fault. 129 Sequence of metamorphism and deformation. 135 Locations and trends of dikes. 136 LIST OF PLATES Plate Page Geology of the Gold Mountain Area. in pocket Cross Sections. in pocket LIST OF TABLES Table Pag Darrington Phyllite composition. 19 Sandstone composition. 42 Characteristics of conglomerates. 45 STRUCTURAL GEOLOGY AND METAMORPHIC PETROLOGY OF THE GOLD MOUNTAIN AREA, SNOHOMISH COUNTY, WASHINGTON INTRODUCTION Purpose of this Study The current study is one of