Differentiated Mafic-Ultramafic Sills in the Archean Vermilion District
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DIFFERENTIATED !-'1..1"\.FIC-ULTRAMAFIC SILLS IN THE ARCHEAN VERMILION DISTRICT, NORTHEASTERN .MINNESOTA A THESIS SUBMITTED TO THE FACULTY OF THE SCEOCL OF THE OF 1'1INNESOTl:. BY SCI·JULZ IN PARTIAL FULF'ILU--1EN'l1 OF TEE REQun;.EMEN'i.'S FOR THE DEGREE OF :MAS'rER OF SCIENCE FEBRUARY 1, 1974 i . ABSTRACT Detailed mapping of part of the Newton Lake Forma- tion north of Ely, Minnesota has shown the presence of numerous sill-like mafic-ultramafic intrusions in the Archean volcanic sequence. Three types of intrusions have been found: layered, differentiated mafic-ultra- mafic sills ranging from 400 to 1500 feet thick and at least 4 miles long; gabbroic sills of variable thickness and e x tent; and ultrarnafic lenses ranging from 75 to 300 feet thick and up to .3000 feet long. The layered sills were studied in detail. They have a wel1-developed internal stratigraphy which consists of a lower chilled-margin, peridotite, pyroxenite, bron- zite gabbro, and gabbro units and an upper chilled-margin. Petrographic studies indicate that the units formed by gravitational accumulation of mineral grains, The ::;tr at- igrap.hic succession reflects the general order of crystal- ization: 1) chromite, 2) chromite· + olivine, 3) clino- pyroxene + orthopyroxene, 4) clinopyroxene + orthopyroxene + plagioclase, 5) clinopyroxene + plagioclase. Microprobe analyses of cumulus pyroxenes has shown crypti c variations exist between units. Th e gabbroic sills are generally similar to the upper gabbroic parts of the layered sills. The ultramafic lense s consist of a central peridotite unit surround by a com- plex chilled-margin. Textural and structural features of ii the u1tramafic lenses suggest intrusion of an olivine bearing liquid, with flowage differentiation forming . the peridoti te unit and rapid chilling of a crystal free liquid forming the chilled-margin. Di£ferentiation trends of whole rock samples and calculated liquid compositions of the layered intrus- ions are generally in close agreement and show a gen- eral iron but ·minimal alkali enrichment with differ" entiation. Chemical analyses of chilled-:-ma.rgins are low in Al2o3 {lO wt%) and high in MgO (11 wt%) with a high Ca0/Al2o3 ratio {0.85). The calculated bulk composition is distinctly ultrabasic in nature (MgO= 17 wt%). Mixing calculations utilizing least square methods sugg.est that the magma Wi:lS partially di ff er- entiated at the time of intrusion. The bulk tion for the layered sills i.s very similar to high::-EgO basalts from the Archean of Western Australia, An i .nital magma temperature for the layered intrus-:- ions of between 1200°C and 1100°C is inferred from element partitioning between coexisting mineral phases. Textural relations ana phase equilibria suggest that crysta1lization of both sills and lenses probably did not take place under pressures greater than about l kb. ( 2. 7 h"1) and may have been even lower. Metamorphism of the lower gra.d.es of the greenschist facies has affected all rocks of the Newton Lake Forma- tion. The ultramafic rocks are serpentinized to varying degrees with olivine often the only altered phas e. ..L..L..L Serpentinization appears to have along microfractures which cut the peridotite. The mafic rocks typically have actinolite, sausseritized ioclase, chlorite, epidote, and rare sphene. Relict grains are common in both ultramafic and mafic units suggesting that metamorphic reactions were retarded perhaps by low H2o and co 2 availability and pressure, The layered sills and lenses of the Newton Lake Formation apparently formed penecontemporaneously with the surrounding basic volcanic rocks, The layered sills probably formed as high level synvolcanic intrusions,. some of which may have also acted as magma chambers for gabbroic liquids which formed sills and/or flows. The ultramafic lenses could be smaller injections of the same magma which formed the larger layered sills, iv TABLE OF CONTENTS Page ABSTRACT ...••.•..... , .•• . , , , . .. , , .. i TABLE OF CONTENTS .. , ... , ••••••• .. .. , ..... •.• iv LIST .OF ILLUSTRATIONS. • •.••.•.•. , . ' . , .. vi INTRODUCTION. • • . • • • . • • .•• . ' ... 1 Location ..••..•••. I • ' • • . .. .. "' .. ' . PE'J.'l.(OLOGY II ••••••• y ill ••• , •••••••••• ' , .. , .••.. , . 87 Formation of the Layered Sequence in the Differ- entiated Sills.,..... • ..••••••• ,. , .•••.•. 87 Peridotite.. ..••.••••••..•.••.•.•.•. .91 Pyroxenite.. • ., .•.•.•• ,,96 Bronzite Gabbro and Gabbro. ·, .• , •...•••. 100 Formation of the Chilled-Margins. • ••. .••• , . , 104 Chilled-Margins of the Ultramafic Lenses .••••. 109 PETROCHEMICAL ASPECTS OF Nm\1TON LAKE SILLS. • . .•. 110 Mineral for the Layered Sills •.•. , .. 110 ,... ..,., ... ... 110 Orthopyroxene..... ···•·i·••·•· ,113 li tlg i te ... , ... ... .. ... ••••. 116 Chromian • • • • !'. ' • , ••••••• ' • f , , • , 119 Amphibole ••. , .... .. ' ,. ' .. 120 Plagioclase ... ,. , . ... , .... , . .., •• 126 Major Element Geochemistry of the Layered SiJ.J.s ....... , ......................... ., ·. ·• ·• .130 v Cedar Lake Sill •••••. , ••.•••.•••• , .•. ,.,,,130 S i· 1 1ca.· , . , ......... ....... ! • ', . • , • , • , 137 Aluminum . ..... , " .•.. ,. ........ , ... , .•. , 13 7 Iron (Fe2o3 + FeO) .•. , .•.•.•.•.•••.•. 137 Magnesium .... , , •.•.•• , ,. •.•.•. #. •• • 138 Calcium ....... .,, .....•. , .•.... ,,., ... .,138 Sodium . , .... , ., , ... , .. f •• , ••••• , • ,. , • 13 9 Potassium . ... , ... , .. , , ..... , .. , .... !It • 13 9 Titanium . .... !' ••• , •••••• .,, ••••• , • , • , • • 13 9 Manganese ... f. !I,, .... , •••.• ••• 139 Phosphorus ..• , .. , .•••... , ..••.•.•••. ,140 Picketts Lake Sill Peridotite .•••..• ,., .•• 140 Differentiation Trends •.•• ,., .•.•.•. , .•••••• ,. ,141 Fe?t - MgO - Cao ..•• .•. •• 141 - FeOt - MgO Var1at1on ...•• , .•. ,., .•. ,142 Oxides Versus MgO ..•.... , .•...•.•••... ,.,.142 Successive Magmatic Liquid Compositions in a Di£ferentiated Layered Sill .. • .•....•.•. , ••. ,.,147 Bulk Compositions of the Layered Sills .•.•... ,.153 Comparison of the Bulk and Chilled--Margin Compo,... si tions w·i th other maf ic rock types .. , . , .•.•.•. 159 Order o .f Crystallization in the Layered Sills .. 167 Temoerature and Pressure of Formation of the I .. Sills based on Mineral Coi:Tlposi ti.on and Textural Relations ..•....•...••....•.•.•.•.•. , .17 5 Temperature . ..................... '! ..... t • , • , • 175 Orthopyrox ene-Clinopy roxene .•.•. ,., .• 175 Olivine-Chromia.n Sp inel . ...... ...•.• 176 Ca/Ca+ Mg Ratios in Pyroxenes ..•.•.• 178 Eval ua ti on . .. ,. , .. , .. 11 •• , • , 11 " • .. ••• , 178 Press t1r e ........ ., .......•. ,. ......... , ...... 18 0 METAMORPHISM OF THE NE\tJTON LAKE SILLS ............... 18 2 General Preservation of Textures ..•. , .....•.... 182 Ul tr amaf.ic Rocks ...........•................... 18 3 11-.af ic Rocks . •..... ....••...•.. a , . ........ ., ••••••• • 18 9 PETROGENESIS J._ND GEOLOGIC HIS'rORY FOR NEWTON LAKE S I LL S .. , , . .. " . , . • . , . , . ! • • • , 191 Classification .....••..•.... ,., ...•.•. ,.,. ,. , .• 191 Origin of the Newton Lake Sills and Lenses ••.•• 193 Summary of the Geologic History of the Newton Lake Form.a tion . .•.... , ...•. , . , . , .......•.•.• • 19 6 ECONOiciIC GEOLOGY ......•...•.••.• ••• , •....•...• , .. 198 SUMMARY AND CONCLUSIONS ......•. .... ! •• , • , ••••••••• , •• 200 ... ............. ,. .....•• it" •• ,.. 11 ?',.,.,.,.,.,. 205 vi LIST OF ILLUSTRATIONS Plates Page I Geologic Map of a Portion of the Newton Lake Formati.on. ,, .......... , ............... , ..... Pocket II NW-SE Structural Section of a Portion of the Newton Lake Formation ..••••••.•. ,. 1 ., ••• , ••• Pocket Tables 1. Rock-type Classification .....•.•...•...•••..... 17 2. Alteration products for layered sill minerals .. 25 3. Modal compositions for chilled-margin zone •..•• 30 4. Modal compositions for peridotite unit .•.••.. ,.41 5. Modal compositions for pyroxenite unit .... , ...• 50 6. Modal compositions for bronzite gabbro unit.,.,59 7, Modal compositions for gabbro unit ........•.•• ,67 8. Chemical composition and structural formulas for analyzed oli vines ....•. , ... , .•..... , •.... 111 9. Chemical composition and structural formulas for analyzed pyroxenes .....•...•.....•...•.•.• 115 10. Chemical composition and structural formulas for analyzed chromian spinels ... 11. Chemical composition and st:ructural formulas for analyzed-primary amphiboles ...... , ..•.•••. 125 12. Chemical analyses from the Newton Lake layered s i )".ls . ...... , ......................... 1 ·: •••• 13 3 13. Reca.lculated chemical analy ses ....... , .....•.• J. 34 . 14. C.I.P.W. norms for chemical a11a·:?:yses • .•...•.••...•..••.. , • _ •.••.•• , ....... , 13 5 15. BuJ_k analyses for relevant igneous rock types.165 Figures 1. Locationmap and geologic setting, .•.. , ..•. , ••• 2 2a. General geologic map for the weste rn portion of the Vermilion District., .. , ...•. , .........•• 5 2b. General geologic map of the central portion of the Vermilion District ... , .•.......•..... , .. 6 3. Major zones and units for a typical layered sill ....•. , .•...•...•.•. , .. , •.. 18 4. Stratigraphic columns for some of the Newton Lake s i 11 s . ! • • • • • • • • , • • , !t , 1 • , • • • , • • • • • 2 O 5. Rhythmic layering· in a bronzi te , • , • 23 6, Lamination in peridotite .......... , ... , .•• , ••. ,23 7, Ideal chilled-margin sequence, .•...•••••...•••. 28 8. Photomicrograph: Unit A, chilled-margin.,,., ••. 32 9. Photomicrograph: amygdule, Unit B, chilled,... mar·g i r.t . •••.••• r • ., • fJ .....