The Structure and Petrology of Precambrian Metamorphic Rock Units, Northwestern Marathon County, Wisconsin
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THE STRUCTURE AND PETROLOGY OF PRECAMBRIAN METAMORPHIC ROCK UNITS, NORTHWESTERN MARATHON COUNTY, WISCONSIN A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY ELIZABETH ANN PALMER IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE DECElVlBER, 1980 i ABSTRACT Precambrian metamorphic units in northwestern Mar- athon County, Wisconsin, consist of an extrusive region composed of basaltic to felsic metavolcanics, and a gneissic region composed of granitic to dioritic gneisses. Both the metavolcanic and gneissic regions have been intruded by felsic plutonic rocks. The meta- volcanic region has also been intruded by mafic units. The contact between these two terranes is a cataclastic zone which trends approximately N75°E through Athens, Wisconsin. The zone itself appears to have gradational boundaries and may be anastamosing. Structural and petrologic investigations indicate two deformational phases in the area accompanied by varying degrees of recrystallization. The first def or- mational phase is represented by quartzofeldspathic and _mafic mineral assemblages diagnostic of amphibolite facies Relationships in the gneisses generally indicate textural disequilibrium although locally equilibrium textures are found. ·The presence of migmatites and complex folds is further evidence for regional metamorphism involving deformation at high temperatures and pressures. ii The first deformational phase is also represented by the metamorphism of volcanic rocks to greenschist facies and local deformation. Where the rocks are not highly deformed, primary volcanic textures and struc- tures are well preserved. The development of similar penetrative structural trends in both terranes is further evidence for the first deformational phase. The structural features suggests a strain history in the study area such that maximum elongation (extension) occurred parallel to lineation in the plane of foliation, while· shortening (flattening) occurred in a northwest-southeast direc- tion, perpendicular to foliation. Strain results indi- cate the rocks were shortened up to 40 percent of their original length. The second deformational phase is represented by an assortment of cataclastic rocks derived from both terranes, the local development of a linear fabric asso- ciated with cataclasis, slickenside surfaces, and the development of two major joint systems. The major joint system trends northwest-southeast, perpendicular to foliation, while a second system trends northeast- southwest, parallel with foliation. If these joint sets are tensional features, the stress field which caused their development would be consistent with the stress field which developed the foliation during the first deformational phase. iii The intrusion of tonalitic to granitic plutonic rocks into the gneissic and volcanic terranes probably occurred about 1840 m.y. ago. Pervasive cataclastic textures in the plutons suggests deformation was opera- tive after the emplacement cf the intrusions. The next event in the area was the emplacement and local recrystallization to greenschist facies metamor- phism of mafic rocks. These· units intrude the volcanic terrane and trend subparallel to the cataclastic zone. Following this, quartz veins were introduced into these and associated units. The quartz veins and their host rocks were then locally faulted. The petrologic and structural evidence in north- western Marathon County, Wisconsin, define a high-grade metamorphic complex which is lithologically distinct and tectonically separate from a lower-grade supra- structure. The contact between the two terranes is de- fined by a relatively thin boundary fault zone having a metamorphic gradient with complex shearing. iv TABLE OF CONTENTS ABSTRACT . • • • • • . • • • i TABLE OF CONTENTS •• . • • • • iv ILLUSTRATIONS •• . • • • . vi TABLES . • • • • • • • • . ·- .. • • • . x INTRODUCTION • • • • • • I 8 O· 8 · 8 a a • a 8 . 1 Geographic and Geologic Setting ·. • • • • • • 1 Regional Precambrian Geology. • • • • • • • • 1 Marathon County Geology .. • • • • • •. • • 8 Introduction • . • • • • • • • • • .. • • 8 Early (?) Precambrian Gneisses and Schists. • • • • • .. • • • • • • • • 8 Middle Precambrian • • • • • • • • • •. • 10 Late Precambrian • • • • • • • • • • • • 12 Structural Geology • • • • • • • • • • • 13 Cambrian Sandstones. • • • • • • • •. • • 16 Quaternary Geology • • • • • • • • •. • • 16 Previous Works. •. • • • • • • • • • • • • .. • 16 Statement of the Thesis Problem . • • • • • • 21 Methods-Field Work • • • • • • • • • • • • • 23 Methods-Laboratory Work. • • • • • • .. • 24 Acknowledgements. • • • • • • • • • . • • • • 27 PETROLOGY ••••• . ... • • . .. 29 Introduction. • • •. • • • • • • • • • • • •. •. 29 Gneisses. • • • • • • • • • • • • • • • • • • 29 General Compositions and Textures. • • • 29 Mineralogy • • • • • • • • • • • • • • • JS Cataclastic Gneisses. • • • • • • • • • • • • 43 Meta-volcanic Rocks • • • • • • • • • • • • • 45 Mafic Volcanic • • • • • • • • • • 46 Intermediate Volcanic Rocks. • • • • • • 49 . Felsic Volcanic Rocks. • • • • • • • • • 50 Deformed Volcanic Rocks. • • • • • • • • 57 Meta-plutonic Rocks • • • • • • • • • • • • • 65 Introduction • • • • • • • . • • • • 65 Felsic Plutonic Rocks. • • • • • • • • • 65 Mafic Intrusive Rocks. • • • • • • • • • 71 v Interpretation of Petrology. 78 Gneissic Rocks. • • 78 Meta-volcanic Rocks 81 Meta-plutonic Rocks • 82 STRUCTURAL GEOLOGY. 83 Penetrative Structures • 8J Foliation and Schistosity • 83 Lineations. 85 Folds • 90 Nonpenetrative Structures. • 96 Shear Zones and Faults. • 96 Joints. • • • • 97 Strain Analysis. • 101 Analysis. • • 101 Conclusions • • 109 Problems. • 110 Deformation History. • 110 SUlV11VlARY AND CONCLUSIONS • •· 116 BIBLIOGRAPHY • • . .. 0 120 APPENDICES. • A-1 I Location of Samples. • A-1 II Operational Definitions. • e .. A-4 Metamorphic Rocks e .. A-4 Cataclastic Rocks • •· •· A-4 Igneous Rocks •· 0 •· A-8 vi LIST OF ILLUSTRATIONS Figures Page 1 Index map of Study Area, northwestern Marathon County, Wisconsin •••••• . 2 2 Map of northern Wisconsin showing distribution of Early and Middle Precambrian rocks. • • • • •. • • • . 4 3 Map of northern Wisconsin showing distribution of the various Middle Precambrian lithologies •••••• • • • e D 6 4 Geological map of Marathon County showing major lithologic units • . • • • 9 5 Marathon County Shear Zones. • •. • • . 14 6 Map showing approximate distribution of different till sheets in Marathon County • • .. • • . •. • • • • • • • • e· • • • 17 7 Geologic map of Marathon County showing location of study area • D e • a 8' II 22 8 Thesis map of northwestern Marathon County, Wisconsin • a • • • · 0 .. • • 25 9 Photomicrograph of a granitic gneiss . 32 10 Photomicrograph of a granitic gneiss • . 32 11 Photomicrograph of an amphibolite gneiss •• J4 12 Photomicrograph of an amphibolite gneiss •• 34 13 Photomicrograph of a recrystallized bimodal granodiorite gneiss ••••• . 36 14 Photomicrograph of a recrystallized bimodal granodiorite gneiss ••• . 36 15 Types of migmatite structures at Goodrich Dells in Taylor County. 37 vii ILLUSTRATIONS (continued) Figures Page 16 Photograph of a faulted amphibolite gneiss clast in migmatite •.•• • • • • 39 17 Photomicrograph of a cataclastic granitic gneiss ••••••••••••••• 44 18 Photograph of a metabasalt ••••••••• 47 19 Photograph of a lithic-crystal tuff unit . 52 20 Photomicrograph of a lithic-crystal tuff unit. .. 53 21 Photomicrograph of a lithic-crystal tuff unit. • •. •. •. • • • • • • • • • .. 53 22 Photomicrograph of a meta-quartz latite .•• 55 23 Photomicrograph of a rhyodaci te. • • •. • • • 58 24 Photomicrograph of a rhyodacite. ·- • Cl • 58 25 Photomicrograph of a felsite (?) volcanic unit. • • • • • • • •. • . • • 59 26 Photomicrograph of cataclasis along the contact between a granodiorite and rhyodaci te • • • • • • • •. • • •. "' • •. 0 0 D 61 27 Photomicrograph of a micro-faulted plagioclase phenocryst in a rhyodacite unit . 61 28 Photomicrograph of deformation of the volcanic groundmass in a rhyodacite unit • • 62 29 Photomicrograph of a cataclastic meta-rhyodacite ••••••••• C • 0 D 0 e 63 JO Photomicrograph of a porphyritic meta-dacite •••• ••••• . •· • • • • 63 31 Photomicrograph of a blastomylonite. • • • e 64 32 Photomicrograph of a mylonite gneiss • • • • 69 33 Photomicrograph of a mylonite gneiss • . 69 viii ILLUSTRATIONS (continued) Figures Page 34 Photomicrograph of a kinked muscovite grain •••.••• . 70 Photomicrograph of a garnetiferous myloni te gneiss. • • • • • • . • . • • • • 72 36 Photomicrograph of a garnetif erous mylonite gneiss ••••••••.•• . 72 37 Photograph of a massive meta-diabase . 74 38 Photomicrograph of a hornblende-actinolite schist . .. 74 39 Photomicrograph of a massive meta-diabase. 76 40 Poles to foliation planes from the gneissic terrane . • • • • • •. • . 84 41 Poles to foliations and schistosity from the metavolcanic-plutonic terrane . 84 42 Lineations from the gneissic terrane • . 86 43 Lineations from the metavolcanic terrane •• 86 44 Photograph of a boudinaged quartz vein • 88 45 Simplified map of structural relationships in central Wisconsin • • • • • •. • . • • • • 89 46 Photograph of folding in amphibolite gneisses and migmatites ••••• . • • 91 47 Photograph of ptygmatic fold • • • e a e D a 91 48 Plunge of fold axes from the gneissic terrane ••••••••• . 93 Photograph of local open folds in metavolcanics. • • • • • • • • • • • • 94 50 Photograph of small-scale open folds in intermediate metavolcanics