DETAILED U-PB GEOCHRONOLOGY OF THE WOLF RIVER BATHOLITH, NORTHCENTRAL WISCONSIN: EVIDENCE FOR A SHORT-LIVED MAGMATIC EVENT CA. 1470 MA

The Mesoproterozoic, anorogenic Wolf River batholith (WRB) of Wisconsin is composed of nine distinct units. The nearby Wausau syenite complex includes the Nine Mile granite. High precision, single grain U-Pb zircon dating was done throughout the WRB and the syenite complex to determine (a) whether there is a resolvable age difference among units of the WRB and (b) whether the syenite complex is coeval with the WRB. U-Pb data from the WRB plot near concordia and yield ages ranging from 1468 ± 4 Ma to 1483 ± 3 Ma, with most falling in the interval 1470 to 1475 Ma. The oldest apparent age of 1483 Ma is probably due to inherited zircons. The other samples may be indistinguishable at 1470 ± 5 Ma, with slightly older ages due to small amounts of inheritance. This seems unlikely, however, because ages vary systematically with locality. The northeastern plutons (Peshtigo monzonite, Hager porphyry, Belongia granite, and High Falls granite) have zircon crystallization ages between 1468 and 1471 Ma, while the main batholith phases (Wolf River granite, Wiborgite porphyry, Red River granite, and Waupaca granite) have zircon crystallization ages that cluster at 1475 Ma. Thus, we believe that the NE lobe may be a few m.y. younger than the main part of the WRB. Crystallization ages for the Wausau syenite (1520 Ma) and the Nine Mile granite (1505 Ma) are significantly older than the Wolf River Batholith; the syenite complex may be a precursor to the larger WRB magmatism. Sm-Nd TDM ages for most WRB samples indicate formation from 1.8 to 1.9 Ga Proterozoic (Penokean) crust. One unit, which is adjacent to Archean gneiss, has a component of that older rock involved in its evolution. The two Wausau complex units appear to have some involvement of a juvenile Mesoproterozoic source.