NEW SHRIMP U-PB ZIRCON AGES FOR THE PALEOARCHEAN TO MESOARCHEAN ROCKS OF THE MINNESOTA RIVER VALLEY

Interest in Paleoarchean to early Mesoarchean crust in North America has been sparked by the recent identification of ca. 3.5-3.8 Ga rocks in the Assean Lake region of northern Manitoba. It has long been known that similarly ancient gneisses are exposed in the Minnesota River Valley and in northern Michigan, but the ages of these rocks has been poorly known because methods previously applied in the 1960s through late 1970s were inadequate to unravel the complexities of their thermotectonic history. Rocks exposed in the Minnesota River Valley include a complex of migmatitic granitic gneisses, schistose to gneissic amphibolite, metagabbro, and paragneisses. The best known units are the Morton gneiss, consisting of tonalitic to granodioritic migmatite with abundant amphibolitic enclaves; and the Montevideo gneiss, consisting of banded coarse to medium-grained granitic gneiss. Paragneiss, derived mostly from a greywacke protolith, occurs near Delhi and mafic gneisses, including metagabbro, hornblende-pyroxene gneiss, and amphibolite occur in the Granite Falls-Montevideo region. The complex of ancient gneisses is intruded by a major, somewhat younger, weakly deformed granite body, the 2.6 Ga Sacred Heart granite. New SHRIMP U-Pb ages of zircons yield the following ages (Ma): Montevideo gneiss, 3485 +/-10; tonalitic intrusion, 3385 +/-8; Morton gneiss, 3524 +/-9; tonalitic intrusion, 3370 +/-8; metamorphic overprint, 2601 +/-65; granite gneiss at Granite Falls, 3497 +/-9; metamorphic event 3300-3350; mafic intrusion 3141 +/-2; metamorphic overprint (rims) 2606 +/-4; biotite-garnet paragneiss, 2619 +/-20. Zircons from a feldspar-mica schist yielded well-defined age peaks at 3520, 3380, 3140, and 2600 Ma, showing detrital input from essentially all of the older rock units. Zircon age data from the tonalitic gneiss at Watersmeet Dome in northern Michigan indicate formation at ca. 3500 Ma. Thus, these rocks were formed in the late Paleoarchean and show a history of igneous activity and metamorphism in the Mesoarchean and Neoarchean. Because ancient crustal rocks occur on both the northern and southern margins of the ca. 2.7-2.8 Superior craton, further study may yield insights into the origin of the Superior craton and granite-greenstone belts in general.