U-PB GEOCHRONOLOGY OF PRE-BELT SUPERGROUP ROCKS IN THE CLEARWATER COMPLEX, IDAHO: IMPLICATIONS FOR PRECAMBRIAN BASEMENT PROVINCES AND STRATIGRAPHY OF THE NORTHERN ROCKIES

The Clearwater metamorphic core complex in northern Idaho is comprised of polymetamorphosed Precambrian rocks. While the presence of basement intrusive rocks in the Clearwater complex has long been known, only recent geochronological data have shed light on their ages and spatial extent. The ages of metasedimentary rocks in the Clearwater complex remain cryptic, however, as direct correlation with ~1450 Ma Belt Supergroup lithologies is nearly impossible due to the obliteration of sedimentary structures through multiple metamorphic events, and it is possible that these rocks are exposures of previously unknown basement. LA-ICPMS U-Pb detrital and igneous zircon geochronology, coupled with 1:24,000-scale mapping, have identified exposures of pre-Belt quartzite, as well as 1870 ± 9 Ma orthogneiss and surrounding basement rocks that were hitherto unknown. Probability density plots of zircon ages from the feldspar-poor quartzites show a dominant zircon population near ~1800 Ma, with smaller Neo- and Mesoarchean peaks, and thus suggest correlation with the pre-Belt Neihart Quartzite. The quartzites lie stratigraphically above feldspathic schist that was intruded by the 1870 Ma orthogneiss. The contact between the quartzite and the schist therefore represents the boundary between basement and an overlying sedimentary package. The orthogneiss is similar in age to other meta-igneous rocks related to arc magmatism along the southern margin of the Archean Medicine Hat block in the Great Falls Tectonic Zone. This previously unknown basement complex provides a new opportunity to study Proterozoic magmatism and metamorphism, as well as aid in supercontinent reconstructions. Further analysis of these rocks, including Hf-analyses of the zircons, will lead to a better understanding of Archean-Paleoproterozoic provinces of the northern Rockies as well as the evolution of the Belt Basin.