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The Petrology and Geochemistry of Archean Volcanics, Western

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The Petrology and Geochemistry of Archean Volcanics, Western THE PETROLOGY AND GEOCHEMISTRY OF ARCHEAN VOLCANICS, WESTERN VERMILION DISTRICT, NORTHEASTERN MTNNESOTA A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERISTY OF MINNESOTA FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DECEMBER 1977 TABLE OF CONTENTS i Page LIST OF TABLES ••••.•••••••.•••••••••••••••••••••••••• , ••••••• iv LIST 0}' FIGURES •••••••••.••••••••.. ••••••••••••••••••••••••••• v LIST OF APPENDICES ••••••••••••••••••••••••. ••••••••••••••••••• x DEDICATION ••••••••••••••••••••••••••••••••••••••••••••••••••• xi ACKNOWLEDGEMENTS ••••••••••••••••••••••••••••••••••••••••••••• xii ABS TRACT ••••••••••••••••••••••••••••• , ••••••••••••••••••••••• xiii INTRODUCTION ••••••••••••••••••••••••••••••••••• , ••••••••••••• THE VERMILION DISTRICT ••••••••••••••••••••••••••••••••••••••• 3 General Geology ..••••...•......•...••.........•...•...••• 3 Stratigraphy .....••..••......•.•...•....•.........••..•. 5 Structural Geology ..... .. .. •...• • ..........•.........•.. 8 Metamorphism. ......•....•.............•• • ..••..•......• 9 Geochronology ...•.....•...•....•........ ; .•..•.......... 10 Previous Petrologic and Geochemical Studies .•......•.•... 12 NATURE OF THE PRESENT STUDY •••••••••••••••••••••••••••••••••• 16 Specific Objectives ......••••..•..•..........•.........• 16 Methods of Study ..........•.•.....•....••....•.•.....•.. 18 GEOLOGY OF VERNILION DISTRICT VOLCANIC ROCKS ••••••••••••••••• 21 Ely Greenstone ......•••...•.........•..•. , .•...•........ 21 Lower Ely. Greens tone Member ............•.....•.......•.• 23 Pillowed and Massive Flows .............. ............. 27 Petrography ..•...•.•.•.....•.......•.....•.•.... 28 Volcaniclastic Rocks ..•.....•...•..•..•.......•.•... 29 Petrography ........• ·.· ................••....... •.• 31 Porphyries . .................................. $ ........... 32 Petrography .. .. ....••....•.......•...•... •• , ...... 32 Plagioclase - Quartz Porphyries ....•...•.... 32 Plagioclase - Hornblende Porphyries ........ 33 Geologic Environment ...•.............•...... .••.. 33 ii Page Upper Ely Greens tone Member .•.......•.•.•.............•. 38 Mafic Pillowed to Nassive and Diabases ..•.......•... 39 Petro.graphy •... ...•.•.• . .•... .•... .. ..•.•.....•• 41 Green Basal ts .•••.•....••..••.....•.......•. 41 Gray Porphyritic Basalts •........••.....•••. 44 Felsic Flows .......••••....••••..•...•.......•.• 48 Petrography ...•.•.....•.•..•............•.... 49 Vo lean iclas tic Rocks . •.• ....•••......•...•..•...• 52 Environment of Formation ..•.••••.•.•...........• 53 Lake Vermilion Formation ...••••.•...•••..........•.....• 55 Petrography •.....•.•...............•..••.•.......... 55 Newton Lake Formation ...•.........•......••... ·, • • . • . 56 Fels ic Member. • . • . • . • • • . • • • • . • • . • . • . • . 59 Mafic Volcanic Member. • • • . • • . • • • . • • . • . • • 60 Cedar Lake Area. • . • • • . • • • • • . • • • • . .. • . • . • • • • • . • 63 Other Localities and Basalt Types................... 82 Newton Lake Sills and Their Chilled Margins......... 92 Significance of Textures in the Newton Lake Flows... 95 Geologic Environment.. • . • . • . • . • • . • . • . 107 MAJOR AND TR.ACE ELEHENT CHEMISTRY ....••...•..•............... 109 Introduction. • . • . • . • . • . • . • . • . • • . • 109 Chemical Effects of Alteration ••...••..•.•..........• ,.. 112 Vermilion Greenstones.......... .... ... ......... .. • . 114 Conclusion ........•...••••.••....•..••......... ·.•... 12.8 Classification. • . • . • . • . • . • 130· · Classification Based on Percent of Si0 . ..•.•....••. 132 2 The Method of Irvine and Baragar ...........•.•..•. • 132 The Jensen Cation Plot................................ 134 Conclusion. • . • • . • . • • . • . • . • • . • . • . • . • 140 Chemical Variation of the Vermilion District Volcanic Rocks. • . • . • . • • . • . • . • . 142 Introduction........................................ 142 Ely Greens tone. • . • . • . • . • . • . • . • • . 143 Lower Ely Greens tone Member...... .. ............. 143 Upper Ely Greens tone Member..................... 150 Ely Felsic Flows and Vermilion Porphyries.... • • 176 Lake Vermilion Formation Basalts.. • . • • • • . • • 180 Newton Lake Formation. • • . • . • • . • • . • . • • . 182 Fels ic Hemb er .. • . • . • . • . • . 182 Yafic Member...................................... 184 Rb-Sr Isotopic Systematics ....•. , ..•.. • .. .• 195 Conclusion......... ... ......... ............ 206 Comparison with Recent Mafic Rocks ..........•... 207 Komatiites.... .......... .. 207 iii f,age Classification and Chemical Characteristics. 211 Compositional Variations and Comparisons ..•• 213 High Iron Tholeiites and the Type A Basalts. 219 Conclusion . ............· .......... ......... 228 PETROLOGY .•••••.••.••.••...•.••.•..•- •.•••••...••.•••...••..•. 229 Vermilion Cale-Alkaline Volcanic Rocks .•....•••.••.....• 229 Con cl us ion .•••.••••.•••..••...••.•• • .•.••.•••.•..•.·• 235 Upper Ely Greens tone Member ..•.••.••• , .•.•..•...•••...•• 238 Newton Lake Formation ..•..•••.••..•- .••.•.- •••••.••.•.•.•. 262 Pyroxene Chemistry ..•••..•••..••.•.•.•.•.•..•....••. 262 Petrogenesis of the Newton Lake- Basalts •..•••.....•• 271 Type C Basal ts .••.•• -••- .•..•••••...•••.••.•.•...• 271 Type A Basal ts ....••••..•.••......•.•..••••..•.• 281 Conclusion ................................... ,.. o. C> 290 CONCLUSION •.••.•..••.•.....••..•..•.•..•.•.••.•.••••..••..••• 292 Geologic History of the Vermilion Greenstone Belt. .•.•.• 300 Discusssion •..••.••.•...••..•..••••...••..•••...•.•. 303 Tectonic Hodel .............••...•.••••.•••..•.•.••..•._ •• 307 BIBLIOGRAPHY •..................•.............•........•....•. 311 APPENDICES •• ••• _••••••••.••.••.••••••••.•••••••••••••••••••••• 329 Appendix 1 ..••.....••..........••....•........•.•....... 329 Appendix la ........................................ 330 Appendix 1b •....•...•.•..•.•..••.•.••...•.•••••...•• 331 Appendix 2. • . • . • • . • . • . • . • . • • . • . • 338 Appendix 3.. • . •. • . • . • . ... • . • . • . • . • 345 LIST OF TABLES. Table 1. Stratigraphic Sequence, Vermilion District ..•• 6 2, Important Rb - · Sr ages ....••.• ,•,• .••.• , .•. !,........ 11 3. Arth and Hanson conclusions for plutonic rocks ...•.•. l4 4. cooling rRte study, 15, quartz nonnative. basalt ..•.. , •.... , ••••• , •. , ••.••..•.. ,., •.. 100 5. a) Summary of the classification of Ely Greenstone volcanic rocks •..•...•.•..••.•••.•..•.•.•............ 140a b) Summary of the classification of Newton Lake volcanic rocks ...•..........•.•••.•••.•.....•...... , • 1 40 b 6. Archean and recent calc - alkaline volcanic rocks •••• 149 7. Low Ti02 basalts, upper Ely Greenstone •......•.....•. j55 8. High TiOz basalts, upper Ely Greenstone •...........•. 155 9. Archean and recent tholeiitic basalts ..•...•....•..•. 166 10. Rb - Sr isotope data, Newton Lake mafic member ....••. 199 11. Representative pyroxene compositions from gray, high Ti0 basalt, upper Ely Greenstone ....•......•...••... 239 2 12. Mixing calculations for upper Ely Greenstone basalts.248 13. Calculated primary upper Ely Greenstone ..... 2.50 14. Representative pyroxene compositions from Newton:Lake Formation basalts, ., , .•. ••...•.•..• . •.............•. ·263 15. Calculated Newton Lake sill compositions ...•.......•. 275 16. Mixing C:'!lculations for Type C Newton Lake basalts .•. 276 17. High pressure mixing calculations, Newton Lake basal ts ...........•..• • ...... • •...... , •.........•.... 288 LIST OP FIGURES v Figure Page 1. Geologic map, western VE:rmilion district •.•.......... 4 2. Nap of exam]ned lithologic units and specific loca:.. · tions, weste:.:n Vermilion district. ....•....•....••.•. 7 3. Sampling corridors, this study ......•..•....•....••.. 17 4. Lower Ely Grecnstone, mapped sections ..••.•....•...•. 25 5. Photomicrograph uf plagioclase pheo.ocrysts, . porphy- . ritic andesite, lower Ely Greenstone ••.•. , ••........• 30 6. Tuff lov:er Ely Greenstone. ..•••• • ·•.••.•.••••• 36 7. Pillow of gray basalt, upper Ely Greenstone ..•...•.•. 36 8. Pho tomicro graph; typical upper Ely Greeas tone .••••••• 42 9. Photomicrogre!ph, ske.leta:L plagicclase microlites in upper Ely Green stoP..e be.sa1. t; ........... ............... 43 10. Photomicrograph, skeletal plagioc.lase microlites in upper Ely Greenstone basalt ....•.•..••.•••..• , ..•• , •• 43 11. Photomicrogr..5.pll of glcneroporphyTitic gray basalt, upper Ely G.t.e£ns to:Qe .•••• . •••••••••••••• . 46 12. Photomicrograph of gra;t upper Ely Greensto01e. 47 13. Photomicrograph of ·glomeroporphyritic pyroxene, gray · basalt, upper Ely Greenstcne ..•.•.•.•.••..•.•.••..... 47 14. Garnet amphibollt,:! in uppet.- E:i.y Greenstone .•....•.••. 50 15. showing inclusions in gan1et porphroblasts .............•....•.... ·•.• ·.. , ...• ....•. 50 16. Photomir.: rog!'.'aph· of horc.blende garnet laye::..-- in a:mp!-1il>oli·te . , . ............. .,, .. ,. .... .......... " .... 50 17. Phot:::;::r::.crograph !:ihc·.wfr.g puruict:!OUS tuff u;>per Ely .......... •..•..••.. .. •....••.... ......• 57 18. Geologic map of Cedar La.kc a"!. ea. •••••••••••••••••••••• 61 19. F .. ... r. .-..1 ·- · -- ·· in.. "'••' cw- .. .. on T... , ., • :. ·"'I · ... r .·.·...or ......., . ..... .. 64 20. in Ne;'ltcn ;:,ake FrJn:1at:L:'lJ. ••••••.••••• 6/f ' 21. l'hotomicrograph of from chilled margin of Cedar L;:;.ke "s-:f.11" .•••••••••••••••••••• • ••.••••••.• 69 22. Ph0tonic:-.:og::.ar11 of frorJ. lo:;-J=-r chilled m&:!:gin of Lake "sj 11" .. ... .... ............... ..•• , •••. 69 1 1 2J. Pho tc.micrograph of microgabbro,
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