Petrogenesis of the Eagle East Gabbro in Marquette County, MI, USA

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Petrogenesis of the Eagle East Gabbro in Marquette County, MI, USA, Is A Well-Researched Topic, It Is To Be Used As A Guide Or Framework For Your Research.

ABSTRACT

The Eagle East gabbroic intrusion in Michigan’s Upper Peninsula, is a subvertical pluglike intrusion found near the base of the Eagle East Ni-Cu-PGE deposit. U-Pb baddeleyite dating of the gabbro yields an age of 1103.4 ± 1.2 Ma, suggesting formation during the early stages of the Midcontinent Rift System (MRS). Drill core observations show that the intrusion cross-cuts the eastern most portion of the Eagle East ultramafic conduit. The internal portion of the intrusion is made up of two main rock types, gabbro and gabbro norite, which consist of plagioclase (50-60%), clinopyroxene (15-25%), orthopyroxene (0-15%), olivine (0-10%), and are generally sulfide-poor. Surrounding the gabbro is a 3-5-meter sleeve of melagabbro and feldspathic pyroxenites containing minor blebs of chalcopyrite and pyrrhotite. In addition, a peridotite-bearing unit containing semi-massive and massive textured sulfides is located within the gabbroic intrusion. Olivine compositions (Fo74) in this peridotite are similar the olivine hosted in the semi-massive sulfide-bearing units reported from the Eagle and Eagle East intrusions. Olivine in the gabbro is more enriched in iron (Fo24-30) than olivine grains from the peridotitic units from the Eagle and Eagle East intrusions. In addition, nickel concentrations in these iron-rich olivine are below the detection limit (100 ppm), indicating crystallization from a magma depleted in nickel. MELTS modelling shows that Eagle East gabbro could be explained by extensive fractional crystallization of a high-MgO picritic parent magma similar to the one that has been proposed for the Eagle and Eagle East intrusions.

TABLE OF CONTENTS

ACKNOWLEDMENTS ………………………………………………………………………………………………….. ii
LIST OF TABLES ……………………………………………………………………………………………………………v
LIST OF FIGURES ……………………………………………………………………………………………………….. vi
ABBREVIATIONS ……………………………………………………………………………………………………… viii
CHAPTER
I. INTRODUCTION ……………………………………………………………………………………………..1
The Eagle East Gabbroic Intrusion……………………………………………………1
Geologic Setting……………………………………………………………………….2
Gabbroic Rocks in the Midcontinent Rift ……………………………………………………………5
Magmatic Sulfide Deposits ……………………………………………………………………………….7
Research Questions ………………………………………………………………………………………….8
II. METHODS ……………………………………………………………………………………………………….9
Sample Collection ……………………………………………………………………………………………9
Electron Microprobe Analysis …………………………………………………………………………11
Whole Rock Geochemical Analysis …………………………………………………………………11
Age Dating Analysis…………………………………………………………………11
III. RESULTS ……………………………………………………………………………………………………….13
Geochronology ……………………………………………………………………………………………..13
Petrography ………………………………………………………………………………………………….14
Eagle East Gabbro ……………………………………………………………………………..14
Outer Margin …………………………………………………………………………………….16
‘Pocket’ Peridotite ……………………………………………………………………………..18
Whole Rock Geochemical Analysis ………………………………………………………………..19
Major and Trace Element Analysis………………………………………………………19
Electron Microprobe Analysis ………………………………………………………………………..24

Olivine……………………………………………………………………………………………..24
Pyroxene ………………………………………………………………………………………….25
Feldspars ………………………………………………………………………………………….26
IV. DISCUSSION …………………………………………………………………………………………………27
Magmatism in the Midcontinent Rift ………………………………………………………………..27
Petrological Comparison to the Eagle, Tamarack,
and the Bovine Igneous Complex …………………………………………………………………….30
Petrological Comparison to the Partridge River
and South Kawishiwi Intrusions ………………………………………………………………………32
Nature of the Outer Margin……………………………………………………………………………..36
Nature of the Pocket Peridotite ………………………………………………………………………..37
Parental Magmas ……………………………………………………………………………………………39
Crystallization Modelling ……………………………………………………………………………….40
Fractional Crystallization ……………………………………………………………………40
Equilibrium Crystallization …………………………………………………………………44
Polybaric Fractionation ………………………………………………………………………49
Origin of the Eagle East Gabbro ………………………………………………………………………50
Conclusions …………………………………………………………………………………………………..55
Future Work ………………………………………………………………………………………………….55
REFERENCES ………………………………………………………………………………………………………………56
APPENDICES
A. Sample Log ………………………………………………………………………………………………………..62
B. Electron Microprobe Analysis Data ………………………………………………………………………66
C. Whole Rock Geochemical Data ……………………………………………………………………………69
D. Geochronology Data ……………………………………………………………………………………………76

Additional information

Author

Kevin Rupp

No of Chapters

4

No of Pages

88

Reference

YES

Format

PDF

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