DETRITAL FELDSPAR PB ISOTOPE PERSPECTIVES ON THE PROVENANCE OF THE MESOPROTEROZOIC HESS CANYON GROUP AND BELT SUPERGROUP, WESTERN USA

Pb isotope compositions of Earth materials vary based on differences in time-integrated U/Pb and Th/U ratios, and this isotope system has been used in studies to identify and map discrete crustal terranes. Feldspar represents the most important recorder of terrane Pb signatures because it contains significant trace levels of Pb but little to no U and Th, therefore retaining the initial common Pb isotope composition of the source magma. Consequently, there is significant potential in the use of detrital feldspar Pb isotope analysis to identify source terranes in provenance research but most studies have focused on detrital feldspar in modern sedimentary systems. Here, we explore the provenance implications of Pb-isotope ratios measured by LA-MC-ICP-MS in detrital feldspars in the Mesoproterozoic Hess Canyon Group and Belt Supergroup in the western US. These sequences are thought to have received material from non-North American sources (Australia and/or Antarctica being likely suspects), making these sequences key tests for reconstructing configurations of the Nuna supercontinent.

Detrital feldspars from the Hess Canyon Group have a relatively narrow range of Pb- isotope compositions that overlap with the Yavapai-Mazatzal province Pb evolution array defined in past studies and the detrital feldspars differ from Pb isotope signatures typically seen in Precambrian Australian sources. Therefore, the Hess Canyon detrital feldspars are likely derived from SW Laurentian sources (although we note that we cannot eliminate East Antarctica as a source due to a paucity of published Pb data for the relevant portion of that continent). Belt Supergroup detrital feldspars have variable Pb compositions with high ²⁰⁶Pb/²⁰⁴Pb values and relatively low ²⁰⁷Pb/²⁰⁴Pb values. These isotope compositions are best explained by Pb exchange during Mesozoic fluid flow or metamorphism associated with Cordilleran magmatism and thrusting. Independent evidence for multiple episodes of fluid flow in the Belt basin is also provided by post-depositional ages for low Th monazite ranging from Mesoproterozoic to Late Cretaceous.