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Geochemical and geochronologic data are presented for the Archean rocks in the Granite Falls area of the Minnesota River Valley, southwestern Minnesota. The rocks form two major groups: mafic and felsic gneisses. The mafic rocks include layered hornblende-pyroxene-plagioclase and biotite-pyroxene-plagioclase types with variants of each, amphibolite, and metagabbro. The biotite-pyroxene gneiss and some of the hornblende-pyroxene gneiss are metasedimentary with graywacke precursors. The amphibolites are of tholeiitic and basaltic komatiite composition, both of igneous derivation. Younger (Proterozoic) diabase dikes approach the tholeiitic amphibolite in composition.

The felsic gneisses have a wide range of composition and include tonalite, granodiorite, adamellite, and pegmatitic granite, all of igneous derivation and of different ages. Tonalite, granodiorite, and lesser amounts of adamellite gneiss are interlayered on a large scale with the layered mafic gneiss, and the apparent concordant relationships may be, in part, the result of deformation, but may also be interpreted as (1) an original volcanogenic-sedimentary pile or (2) sill-like intrusions of felsic rocks in an older mafic basement complex. The older felsic and mafic gneisses were folded before the intrusion of younger granitic pegmatite and adamellite, and subsequently the region was folded in late Archean time.

U-Pb analyses of zircon give minimum ages of 3,230 m.y. for the granodiorite and related old gneiss, 3,050 m.y. for the adamellite gneiss, 2,600 m.y. for the late Archean high-grade metamorphic event, and 1,800 m.y. for the Proterozoic igneous activity. Rb-Sr whole-rock analyses indicate an age of 3,600 m.y. for the older metamorphic complex and 3,000 to 3,100 m.y. for the pegmatitic granite and adamellite gneisses.

Chemical and petrographic data reveal some of the interactions that occurred during the emplacement of the pegmatitic granite and adamellite and suggest that the magmas were emplaced approximately 3,050 m.y. ago. Mixed-rock and contaminated samples are not easily recognized in the field. These not only reflect interactions between country rock and the invading magmas, but also the high-grade metamorphism 2,600 m.y. ago and subsequent events that involved shearing and hydrothermal alteration.

Although there are marked differences in rock types, metamorphic grade, and structure in the Granite Falls and Morton areas, the geologic history of the migmatitic terranes is similar. In the Granite Falls area the paleosome is largely granodiorite gneiss with minor amounts of adamellite and amphibolite, whereas in the Morton area it is tonalite gneiss and amphibolite with minor amounts of granodiorite. In both areas the neosome is pegmatitic granite and adamellite gneiss. The thick sequences of layered mafic gneiss at Granite Falls are lacking in the Morton area. The younger adamellite-2 and the aplite dikes, approximately 2,600 m.y. old, of the Morton area have not been found in the Granite Falls area.

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