40Ar/39Ar THERMOCHRONOLOGY OF DRILL CORE SAMPLES FROM THE CENTRAL YAVAPAI PROVINCE, U.S. MIDCONTINENT

Much is known about the Precambrian U.S. tectonic evolution in regions where bedrock is exposed for direct observation (Rocky Mtns, Lake Superior region). However, Phanerozoic sedimentary cover overlying the central Yavapai Province between the Rockies and the Great Lakes, limits our knowledge of that important region. Ar-Ar thermochronology from the upper Great Lakes region and western U.S. has proven critical for assessing Proterozoic tectonometamorphic overprinting during and following growth and stabilization of southern Laurentia. Here we present the first results of Ar-Ar thermochronology on basement drill core rocks from easternmost Colorado, Nebraska, and southern South Dakota. In SD, a c. 1860 Ma biotite age is consistent with cooling after peak metamorphism related to Trans-Hudson/Penokean accretion. Most surprising is a preferred c. 2450 Ma hornblende age, which suggests the presence of Archean crust within the Proterozoic mobile belts of southern Laurentia. In southern NE, hornblende from a sheared tonalite yields a complicated age spectra with a total gas age of 1468 Ma and a preferred age of 1487 Ma. Interestingly, mica age results demonstrate that the geon 17 Yavapai Province rocks experienced the affects of younger, Proterozoic events largely unfelt throughout neighboring regions. Mica ages range between 1138 Ma and 1267 Ma across a >700 km swath of southern NE and eastern CO. Similarly young ages are reported from only a handful of the hundreds of samples dated in the Rockies and the upper Great Lakes region. Because Proterozoic crust in the southern Lake Superior were virtually unaffected thermally by 1100 Ma rifting, we consider it unlikely that our 1130-1260 Ma mica ages represent widespread partial resetting during continental rifting and magmatic underplating. In CO and NM, similarly young mica ages are interpreted as partially reset by Tertiary igneous activity or representing deeper, and therefore more slowly cooled crustal levels. However, both interpretations seem unlikely for our study area. We tentatively suggest that a regional thermal/fluidization event related to Grenville age deformation may be responsible for the young mica Ar-Ar ages. Our results demonstrate the utility of Ar-Ar thermochronology for delineating tectonometamorphic episodes in buried crystalline rock.