THE ORIGIN AND CHEMICAL EVOLUTION OF PRECAMBRIAN BASEMENT UNITS IN NORTHERN UTAH AND SOUTHERN IDAHO

Geochemical, geochronologic and isotopic analyses from two basement terranes in northern Utah are underway to better understand the Archean to Paleoproterozoic assembly of western Laurentia. The Grouse Creek block (GC), locally exposed in northwestern Utah, contains Archean orthogneiss, schist, and amphibolite that were strongly overprinted during Jurassic to Neogene development of metamorphic core complexes in the Sevier hinterland. Samples of orthogneiss have been collected and zircon separates obtained for integrated U-Pb geochronologic and Hf isotopic analyses, including a search for older inherited cores. The Farmington Canyon Complex (FCC), locally exposed in thrust sheet of the Sevier belt in northern Utah, comprises a complex mixture of orthogneiss, paragneiss, amphibolite, and late-stage granites. Although prior work has established input of Archean materials and Paleoproterozoic metamorphism of the FCC, maximum depositional ages and sediment sources of paragneiss and associated meta-quartzite remain poorly understood, as well as relations to surrounding Archean blocks. Samples of orthogneiss, paragneiss, and amphibolite have been systematically collected from different parts of the FCC, based on geologic mapping, and have been prepared for U-Pb geochronologic and Hf isotopic analysis of igneous, metamorphic, and detrital zircon components. Major, minor, and trace element geochemistry of samples will also be determined. These data sets, along with comparison to existing data from the Archean Wyoming province, will constrain timing of crustal extraction and subsequent reworking by igneous and metamorphic processes, and improve understanding of how basement blocks were accreted within western Laurentia.