DEEP-TIME THERMOCHRONOLOGY REVEALS THE GREAT UNCONFORMITY FORMATION AND PALEOZOIC REHEATING OF PRECAMBRIAN BASEMENT ROCKS IN THE US UPPER MIDWEST

The Great Unconformity (GU) is a prominent erosive feature throughout Laurentia that separates Precambrian crystalline basement from overlying Paleozoic sedimentary strata. Recent work has shown the utility of deep-time thermochronology to understand the cooling (and by inference, exhumation) history of this important feature. We integrate biotite and K-feldspar ⁴⁰Ar/³⁹Ar, zircon (U-Th)/He (ZHe), and apatite fission track and (U-Th)/He data on outcrop locations of the GU surface in the US Upper Midwest cratonic platform. Our sample sites include the 1.47 Ga Wolf River Batholith (WRB) and the 1.6 Ga Baraboo Quartzite, WI, and the 2.7 Ga Compeau Creek Gneiss (CCG) from the Upper Peninsula of MI. A comparison of GU exhumation between these sites, within the Laurentian interior and at the margins could further illuminate the possible driving mechanism for GU formation. ZHe results from the WRB and the CCG reveal similar negative age-eU trends with ages ranging from ca. 900 Ma to 0 Ma. Both datasets define age-eU pediments at ~200 – 250 Ma, which roughly coincide with apatite He ages (n=7) of 150 – 275 Ma from both sites, suggesting a significant Phanerozoic thermal resetting event. In contrast, the Baraboo Quartzite zircons have a higher maximum He age of 1285 Ma and a prominent negative age-eU trend at lower eU concentrations. We utilized thermal history modeling with the zircon radiation damage and annealing model (Guenthner et al., 2013) in both a forward and inverse sense. Across all sample locations, thermal history models infer Paleozoic reheating of 100 - 150 ^oC. We attribute these resetting events to a combination of some Paleozoic sedimentary burial and possible basin-scale brine migration during the formation of the supercontinent Pangea. Thermal history models of the WRB (U-Th)/He data, combined with published K-feldspar ⁴⁰Ar/³⁹Ar data, suggest mid-late Neoproterozoic cooling, which may support the role of Snowball Earth glaciations in causing large-scale exhumation. Preliminary models of the Baraboo ZHe data hints at possible protracted low temperature residence from ca. 1.4 Ga to 0.5 Ga. However, further investigation is required as the model’s inferred Paleozoic resetting may have caused sensitivity loss in resolving its Precambrian time-temperature paths.