40AR/39AR K-FELDSPAR THERMOCHRONOLOGY RESULTS FROM THE SOUTHERN LAKE SUPERIOR REGION: REHEATING AND STABILIZATION OF PROTEROZOIC LITHOSPHERE RELATED TO LATE PROTEROZOIC MAGMATIC UNDERPLATING

Recent reconstructions of intermediate-temperature thermal histories of Archean portions of the southern Canadian Shield suggest some amount of prolonged mid-crustal residence followed by significant (>5 km) exhumation at or after ca. 1000 Ma interpreted to be caused by crustal thickening and isostatic uplift due to Midcontinent Rift System (MRS) magmatic underplating (McDannell et al., 2018). Conventional ⁴⁰Ar/³⁹Ar microcline plateau ages of ca. 1000 and 900 Ma in Wisconsin suggest the 1900-1700 Ma accreted Paleoproterozoic terranes also finally cooled below 250 C at and after 1000 Ma. Additionally, new K-feldspar multi-diffusion domain (MDD) inverse thermal history modeling results from central and southern Wisconsin yield complex spectra signifying either slow cooling or resetting beginning ca. 1100 Ma. Although the currently exposed levels of both Archean and Proterozoic crust of the southern Canadian Shield yield similar intermediate-temperature cooling ages and thermal histories, geologic evidence indicates cooling of the Proterozoic province rocks to be related primarily to reheating of already shallow upper-crustal levels, not to cooling via exhumation of mid-crustal levels. Such evidence includes geon 14 supracrustal sedimentary rocks intruded by the shallow level Wolf River batholith, and the presence of MRS dike swarms. At ca. 1.0 Ga, magmatic underplating of already stabilized Archean Superior Province lithosphere to the north caused it to be ‘destabilized’ and to undergo widespread exhumation. In contrast, relatively young Proterozoic continental lithosphere of southern Laurentia was extensively underplated by mafic magmatism, which may have ultimately contributed to its stabilization.