U–Pb geochronology of the Wolf River batholith, north-central Wisconsin: Evidence for successive magmatism between 1484 Ma and 1468 Ma

Single-grain zircon U–Pb age determinations were completed on individual phases of the Mesoproterozoic, anorogenic Wolf River batholith (WRB) of Wisconsin and the nearby Wausau syenite complex in order to (a) define the age more precisely and accurately, (b) determine whether there is a resolvable age difference within units of the WRB and (c) determine whether the Wausau syenite complex is coeval with any or all parts of the WRB. U–Pb results for many individual zircon grains from the WRB plot near concordia and yield ages ranging from 1468 ± 4 Ma to 1484 ± 2 Ma. Plutons of the northeastern lobe of the WRB (Peshtigo monzonite, Hager porphyry, Belongia granite, and High Falls granite) have a weighted average crystallization age of 1470.3 ± 1.2 Ma, which is younger than the main complex plutons (Wolf River granite, Wiborgite porphyry, Red River granite, and Waupaca granite), which yields a weighted average crystallization age of 1475.6 ± 1.7 Ma (excluding one sample of Wolf River granite from Hogarty which yields an age of 1484 ± 2 Ma). Two samples from the main complex also contain zircons with younger apparent ages of 1470–1471 Ma; these younger grains may reflect late stage alteration of some zircons coeval with emplacement of the northeastern lobe. The Wausau syenite and the Nine Mile granite of the nearby Wausau syenite complex (1522 Ma and 1506 Ma, respectively) are significantly older than the WRB and definitely not coeval or comagmatic. The syenite complex may be a precursor to the larger WRB magmatism. Sm–Nd TDM ages for most WRB samples indicate crust formation ages from 1.8 Ga to 1.9 Ga, compatible with formation of the magmas from late Paleoproterozoic (Penokean) crust. One unit, which locally intrudes Archean gneiss, has a small component of that older rock involved in its evolution. The two Wausau complex units have younger TDM ages and appear to have incorporated slightly greater amounts of juvenile constituents.