SP-05 Geology of Precambrian Rocks, Granite Falls-Montevideo Area, Southwestern Minnesota

Abstract

Precambrian rocks exposed in the Granite Falls-Montevideo area, within the Minnesota River valley, consist of interlayered metamorphic rocks that are intruded by mafic dikes and a small adamellite body. Lithologically the metamorphic rocks are granitic gneiss, hornblende- pyroxene gneiss, garnet-biotite gneiss, and a heterogeneous sequence of interlayered gneisses containing variable proportions of biotite, hornblende, pyroxene, feldspar, and quartz. The mafic dikes are tholeiitic diabase, hornblende andesite, and olivine diabase. Dynamothermal metamorphism approximately 2500-2700m.y. ago produced an inclined, cylindrical fold system that plunges approximately 15° N. 85 W. Most mineral assemblages resulting from the metamorphism belong to the granulite facies. Mineral assemblages characteristic of the amphibolite facies are interlayered with those of the granulite facies, and there is no indication of metamorphic zoning. Coexisting mineral assemblages indicate that there was an approach to chemical equilibrium and that there were no significant variations in physical conditions during metamorphism. Common retrograde metamorphic textures are "serpentine" veins in orthopyroxene, rims of cummingtonite on orthopyroxene, and rims of sea-green actinolite-hornblende on clinopyroxene and hornblende. Intrusion of tholeiitic diabase dikes along a northeast-trending fracture system occurred after the metamorphism and folding. A minimum age for the tholeiitic diabase is 2080 m.y. Cataclastic deformation, represented by narrow northwest- trending shear zones and by granulation, took place after intrusion of the tholeiitic diabase but before intrusion of the 1700-1800 m.y. old hornblende andesite dikes. The 1800 m.y. event is also represented by intrusion of a small adamellite body that was contemporaneous with a thermal event that resulted in the resetting of the biotite ages in the metamorphic rocks.