THE GEON 14 WOLF RIVER TECTONOMAGMATIC EVENT: A MIDCONTINENTAL LINK BETWEEN THE PICURIS AND PINWARIAN OROGENIES

The 1476–1470 Ma Wolf River batholith is a prominent and important feature of the Precambrian basement in Wisconsin. New geochronologic data demonstrate that batholith emplacement was not an isolated, anorogenic igneous phenomenon, but instead represents the magmatic component of a major geon 14 tectonomagmatic event that was accompanied by sedimentation, regional deformation, and regional metamorphism. The arkosic, polymictic Baldwin conglomerate occurs at the northeastern margin of the batholith, where it lies unconformably on Penokean metavolcanic and metasedimentary rocks and is intruded by 1470 Ma Wolf River feldspar porphyry. The maximum depositional age (MDA) for the youngest statistically homogenous population of detrital zircons is 1458 ± 10 Ma (MSWD = 0.2; n = 13), demonstrating that deposition of the conglomerate occurred shortly after crystallization of the batholith. Geon 14 regional deformation and metamorphism is recorded by post–Mazatzal Baraboo Interval metasedimentary rocks (MDA = 1643 ± 11 Ma; MSWD = 1.05; n = 42). In the Baraboo Range, muscovite parallel to slatey cleavage in Seeley Slate yields ⁴⁰Ar/³⁹Ar plateau ages of 1473, 1483, 1493, and 1496 Ma (all at ± 3 Ma), and muscovite defining crenulation cleavage in Waterloo metapelite yields 1465 ± 7 Ma. In addition, muscovite ⁴⁰Ar/³⁹Ar ages are 1472 ± 3 Ma in breccia in the Baraboo Quartzite, 1480 ± 11 Ma in hydrothermal veins at the base of the quartzite, and 1469 ± 11 Ma in metamorphosed paleosol beneath the quartzite. U–Pb dating by electron microprobe of monazite in the Seeley Slate yields Penokean and older ages for detrital monazite grains and a date of 1502 ± 30 Ma for neoblastic monazite overgrowths. Folded metapelite layers in the Baraboo Quartzite contain tiny grains (50–100 mm) of recrystallized hematite, which yield a mean U/Th–He age of 1507 ± 153 Ma. These new geochronologic data demonstrate that emplacement of the Wolf River batholith was accompanied by sedimentation and regional deformation and metamorphism, representing a major geon 14 tectonomagmatic event that provides a midcontinental link between the northeastern Pinwarian orogeny and the southwestern Picuris orogeny, thus completing the transcontinental extent of geon 14 orogenesis for 5000 km parallel to the southern margin of Laurentia.