Browsing by Author "Wilson, Wendell Eugene"

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    Trace Element Geochemistry and Geochronology of Early Precambrian Granulite Facies Metamorphic Rocks near Granite Falls in the Minnesota River Valley
    (1976-12) Wilson, Wendell Eugene
    The Sr isotopic composition and the trace element contents of K, Rb, Sr and Ba have been measured for 27 whole-rock samples and 18 mineral separates taken from four rock units in the Minnesota River Valley near Granite Falls, Minnesota. In addition, 4 whole-rock samples and 1 mineral separate were analyzed from samples of amphibolite xenoliths from the Morton-Sacred Heart area southeast of Granite Falls. The isochron ages obtained from the Rb-Sr data are given below in AE (billions of years); ages determined for metamorphic events are given in brackets. The initial ratios (Ri) given refer to the whole-rock isochrons. Garnet-Biotite Gneiss: 3.54 ± .14 [1.81], Ri=.7008 ±.0009 Hornblende-Pyroxene Gneiss (Outer Unit): 3.31 ± .26 [1.78], Ri=.7011 +.0012 Hornblende-Pyroxene Gneiss (Inner Unit): Geochemical disequilibrium precludes dating but suggests an age greater than 3.8. Metagabbro of Himmelberg: 2.68 +.20 [l.80, 2.41], Ri=.7037 +.0001 Amphibolite xenoliths: 3.52 + .15, R.=.7094 +.0013 The trace element distribution patterns suggest the following interpretations. The hornblende-pyroxene gneiss (inner unit) is older than the Montevideo gneiss (which has been previously dated at 3.7 AE); the closest analog is probably a metamorphosed island arc basalt. The hornblende-pyroxene gneiss (outer unit) is probably analogous to a metamorphosed oceanic alkali basalt. The metagabbro is probably analogous to a metamorphosed ocean floor basalt or low-K tholeiite and the age of 2.68 AE is only a minimum. The garnet-biotite gneiss is probably analogous to a metamorphosed graywacke and the age of 3.54 AE is only a minimum. The amphibolite inclusions analyzed are probably analogous to metamorphosed graywackes and are older than the Morton gneiss, which has been previously dated as being 3.55 AE old. The rocks at Granite Falls therefore probably represent a very old (3.7 AE) layered sequence of basaltic rocks and graywackes which was intruded by the Montevideo gneiss 3.7 AE ago. The rock types are analogous to those of the less metamorphosed. Archean greenstone-granite complexes, and are compatible with formation in an ancient island arc tectonic environment. They are probably the remnants of the original continental nucleus of the North American Craton.