CHARACTERIZATION OF ZIRCONS FROM BASEMENT COBBLES COLLECTED FROM THE NEOPROTEROZOIC POCATELLO FORMATION TO ASSESS A POTENTIAL ARCHIVE OF BASEMENT-ROCK THERMAL HISTORIES

Neoproterozoic sedimentary rocks that contain clastic materials shed from Laurentia’s interior, including the Yavapai and Mazatzal provinces, archive parts of the region’s tectonic history that are otherwise not available in the rock record. Large cobbles of crystalline rocks deposited in the diamictites of the Cryogenian Pocatello Formation (SE Idaho) are sourced from basement rocks that were unequivocally at Earth’s surface ca. 700 Ma, and therefore may record regional basement exhumation during this period in Earth’s history. Here, we assess the provenance of four cobbles, and their potential for recording the thermal history of their provenance region and/or the Pocatello Formation, using optical mineralogy, zircon geo-thermochronology, and visual assessment of the metamictization of zircon crystals. Cobbles were collected from diamictites of the Scout Mountain Member (SMM) of the Pocatello Formation in the hanging wall of the Sevier Paris-Putnam thrust. One granitic cobble collected from the southernmost exposure of the SMM has published U/Pb zircon ages of 2450-2700 Ma, which suggests a Wyoming Craton provenance. The zircon crystals from this sample are extremely opaque—a metamict texture associated with high radiation-damage dose. However, the effective uranium concentrations ([eU]) from the laser-ablation spots on these grains are ~140-480 ppm. This is relatively low [eU] for zircon and suggests this cobble has resided at temperatures <~300˚C, accumulating radiation damage, for >1 Gy since Neoarchean time and that Late Cretaceous heating in the Sevier thrust belt did not anneal this visible damage. In contrast, zircon crystals from gneissic cobbles collected near the SMM type section exhibit a range of metamict textures that suggest less radiation damage. Zircon crystals from two granitic cobbles are translucent with a range of light to dark brown coloring. A gneissic cobble yielded zircon crystals that are semi transparent, with a distinctive lavender (hyacinth) color. Ongoing zircon characterization, U/Pb geochronology, and U-Th/He thermochronology are assessing the relationship between visible zircon metamictization, measured ages, and the cobbles’ multiphase tectonic history.