LATE PALEOZOIC TECTONIC REGIMES RECORDED IN TUFF ZIRCON GEOCHEMISTRY IN SOUTHWESTERN LAURENTIA

Late Paleozoic subduction of the Rheic ocean beneath Gondwana leading to Gondwana-Laurentia collision and the formation of a new arc on the western margin of Pangea have long been inferred, yet evidence for these changing subduction geometries is sparse and the timings of the two processes remain poorly constrained. Our zircon U-Pb ages and Hf isotope data collected from several thin Pennsylvanian and early Permian tuffs in subsurface of the Midland Basin in west Texas suggest two time-overlapping late Paleozoic arc systems, most likely related to the two tectonic processes. One arc may have lasted from at least ~327 Ma to 287 Ma with juvenile signature (ℇHf > 0) and was inferred to be a northern Gondwana arc formed by subduction of Rhiec ocean beneath Gondwana. The other arc lasted from ~314 Ma to at least 276 Ma with evolved signature (ℇHf< 0) and was inferred to be a western Pangea arc produced by subduction of the Panthalassa Ocean beneath western . Here we collect new zircon geochemical data from middle Permian tuffs in the Guadalupe Mountains in west Texas and Late Mississippian Stanley tuffs in Oklahoma to test the interpretation. These tuffs have been dated previously using zircon U-Pb LA-ICPMS or TIMS geochronology, and their ages fit in the lifespans of our proposed two arc systems. New zircon U-Pb dates were collected from these tuffs using a LA-ICPMS to screen young zircons that were derived directly from the sources. These grains will be analyzed for Hf isotopic values and REE concentrations to test the interpretation and characterize the arc signatures and magmatism. This study will contribute to the understanding of tectonic regimes during the assemblage of the supercontinent Pangea.