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The lower Precambrian granulite-facies gneisses near Granite Falls and Montevideo have undergone four phases of folding, two periods of metamorphism, and both pretectonic and posttectonic intrusive events. Rare F1 folds occur only in thin quartzofeldspathic veins that are at a high angle to the pervasive S1 foliation and banding in the gneisses and that formed during the generation of S1. The F2 folding generated the most prominent structural features in the area—a large-scale, gently plunging antiform near Granite Falls and an inferred synform between Granite Falls and Montevideo. The F3 and F4 fold phases are not evident from mapping of the major F2 structures, but they are well represented by minor structures, and their relation to the F2 folding event is deduced from an analysis of the minor folds and mineral lineations in the gneisses. This analysis indicates a systematic variation in the axial orientations of the minor F3 and F4 folds with their position on the limbs of the major F2 structures.

Analysis of the structures in a 3,050-m.y.-old granitic unit intruding the Montevideo Gneiss suggests the S1 foliation and high-grade M1 metamorphism accompanying S1 were initiated prior to 3,050 m.y. ago. The timing of F2 folding is not well constrained, but F3 folding occurred after 3,050 m.y., and both events were accompanied by M1 metamorphism. Mineral ages of 2,650 m.y. have been interpreted as resulting from the M1 metamorphism and suggest an extreme duration from earlier than 3,050 m.y. to 2,650 m.y. for the high-grade metamorphism.

The F4 folding and intrusion of a set of tholeiitic diabase dikes followed M1 metamorphism but preceded the formation of narrow shear zones about 2,400 m.y. ago. Intrusion of hornblende andesite dikes and a small quartz monzonite pluton 1,800 to 1,850 m.y. ago was accompanied by low-grade metamorphism (M2) without deformation.

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