ARCHEAN AND PROTEROZOIC TECTONOMAGMATIC ACTIVITY ALONG THE SOUTHERN MARGIN OF THE SUPERIOR PROVINCE IN NORTHWESTERN IOWA, UNITED-STATES

A Precambrian igneous body of ultramafic and mafic rocks, named the Ot ter Creek layered igneous complex, occurs within the basement of north western Iowa, United States. It is marked by a circular magnetic anoma ly, one of several that lie north and west of an inferred suture betwe en the Archean Superior Province and Early Proterozoic juvenile crust. Sm-Nd whole-rock analyses for several rock types from the Otter Creek complex yield an isochron age of 2890 +/- 90 Ma, with an epsilon(Nd)( t) of -0.9 +/- 2.4. A block of older banded iron formation, itself int ruded by lamprophyre dikes, is contained within the layered sequence. The iron formation-lamprophyre block has undergone high-temperature me tamorphism followed by a retrograde event. A quartz monzodiorite gneis s, with a U-Pb age of 2523 +/- 5 Ma, occurs near the layered complex, but the contact relations are not known. The layered series is overlai n by Proterozoic keratophyre with a U -Pb age of 1782 +/- 10 Ma. These felsic pyroclastic rocks are extremely depleted in K, Rb, Ba, and Cs. Our data are consistent with Archean greenstone-belt formation, inclu ding chemical sedimentation followed by mafic-ultramafic magmatism at approximately 2.9 Ga, followed by two later episodes of magmatism, one at approximately 2.5 Ga and the other at approximately 1.78 Ga. The O tter Creek complex is the first Archean greenstone reported south of t he Great Lakes Tectonic Zone (GLTZ); its 2,9 Ga age is older than thos e reported for the granite-greenstone rocks north of the GLTZ. The sou thern portion of the Superior Province thus appears to have formed lat er, and in a different tectonic environment, than the high-grade gneis ses of the Minnesota River Valley, but before the bulk of the granite- greenstone rocks exposed in northern Minnesota, Ontario, and eastern M anitoba.