PRECAMBRIAN BASEMENT ROCKS OF THE CLEARWATER METAMORPHIC CORE COMPLEX: A NEW PIERCING POINT ALONG THE WESTERN LAURENTIAN MARGIN

Interest in the geochronology of Eocene aged metamorphic core complexes in the northern Cordillera of the United States has spiked in the past decade. The driving forces behind many of these studies have been the discovery of previously unrecognized Precambrian basement rocks and the increased precision and accessibility of geochronology instrumentation. Until this study, only a handful of dates have been published for the Clearwater metamorphic core complex (CMCC) of northern Idaho. Now through remapping coupled with laser ablation zircon geochronology, new exposures of Neoarchean and Paleoprotoerozoic meta-igneous rocks have been identified directly adjacent to Mesoprotoerozoic meta-sedimentary rocks of the Belt-Purcell Supergroup. Ages of these meta-igneous rocks fall into two age ranges of ~2670 and ~1860 Ma. In comparison, surrounding meta-sedimentary rocks have depositional ages of ~1410 Ma and contain a strong non-North American (i.e. 1600 Ma) zircon component. The results given by the meta-sedimentary analysis are consistent with information for the Prichard Formation, which is thought to represent the base unit of the Belt-Purcell Supergroup. Although the exact nature of the contact between the meta-igneous basement rocks and the surrounding meta-sedimentary rocks has yet to be defined in detail, the results of this study are two fold: first, the transition from Neoarchean and Paleoprotoerozoic rocks to lithologies of the Mesoprotoerozoic of the Belt-Purcell Supergroup marks an uncommon exposure of the base of the Supergroup and; two, it creates a new piercing point along the western margin of the craton that can be used in the reconstruction of Rodinia.