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
Himmelberg, Glen R..
SP-05 Geology of Precambrian Rocks, Granite Falls-Montevideo Area, Southwestern Minnesota.
Minnesota Geological Survey.
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