GEON 14 OROGENESIS IN SOUTHERN LAURENTIA: A PERSPECTIVE FROM THE MIDCONTINENTAL BARABOO OROGENY (Invited Presentation)

In midcontinental Laurentia (the present–day southern Lake Superior region), geochronologic data demonstrate that emplacement of the 1476–1470 Ma Wolf River granitic batholith was accompanied by sedimentation and regional high–temperature/low–pressure (HT/LP) metamorphism and deformation. This tectonomagmatic event is herein referred to as the Baraboo orogeny, because of the definitive evidence for geon 14 folding and metamorphism provided by supermature, siliciclastic strata of the post-Mazatzal Baraboo Interval. In Waterloo metapelite of the Baraboo Interval, muscovite delineating a crenulation cleavage has a ⁴⁰Ar/³⁹Ar plateau age of 1465 ± 5 Ma; in the Baraboo Range, ⁴⁰Ar/³⁹Ar plateau ages for muscovite are 1493 ± 3 Ma to 1473 ± 3Ma in the Seeley slate, 1472 ± 3 Ma in quartzite breccia, 1476 ± 12 Ma in hydrothermal veins, and 1467 ± 11 Ma in metamorphosed paleosol. Also in the Baraboo Range, U–Th–total Pb dating of neoblastic overgrowths on detrital monazite gives an age of 1502 ± 30 Ma, and recrystallized hematite in folded metapelite gives a mean U/Th–He age of 1507 ± 153 Ma. Arkosic polymictic conglomerate, which is located at the northeastern margin of the Wolf River batholith, is intruded by 1470 Ma granite porphyry and contains detrital zircon with a minimum peak age of 1493 Ma. The Wolf River batholith itself consists of A-type, ferroan granite, in contrast to the I-type, magnesian granites that are typical of convergent margins. The major element, trace element, and Hf isotopic compositions of Wolf River granite indicate that the batholith was derived by partial melting of lower continental crust and emplaced in a within-plate tectonic setting. The Baraboo orogeny provides a midcontinental link between the Pinware orogeny to the northeast and the Picuris orogeny to the southwest, completing the extent of geon 14 orogenesis for 5000 km along the southern margin of Laurentia. This transcontinental orogen is unique among Precambrian orogenies for its great width (~1600 km), the predominance of A–type granites derived from continental crust, and the prevalence of regional HT/LP metamorphism associated with magma emplacement in the middle to upper crust.