A TALE OF TWO LATE PALEOZOIC ARCS: EVIDENCE FROM VOLCANIC ASH GEOCHEMISTRY IN THE MIDLAND BASIN, WEST TEXAS

The late Paleozoic assembly of Pangea was marked by closure of the Rheic ocean leading to diachronous Laurentia-Gondwana collision followed by subduction of Panthalassa beneath western Pangea. The timings and processes of the three related tectonic events are not well known, partially because of the poor preservation of the two volcanic arcs. Here we provide new insights on older Gondwanan and younger Pangea arcs by reporting new ages and geochemical signatures of late Paleozoic volcanic ashes in the Midland Basin of west Texas and use these to reconstruct the tectonic evolution of the surrounding region. Ten volcanic ash beds with thickness between 3 and 12 cm from the Lower Pennsylvanian Atokan Formation and lower Permian Wolfcamp B, Wolfcamp A and lower Spraberry units were used in this study. Major and trace element compositions of the ash samples were analyzed to characterize their protolith composition, and zircon U-Pb ages and εHf values were acquired by LA-ICPMS to constrain ash ages and sources. Spraberry ashes are calc-alkalic rhyolites and the Wolfcamp and Atoka samples are tholeiitic rhyolites and dacites, respectively. All the samples show continental arc signatures with low Rb_N/Ba_N (<3.0) and Zr/Nb (9.2-21.5). The REE patterns of the Wolfcamp and Atoka samples are both similar to that of average upper continental crust, while the Spraberry samples have weak negative Eu anomalies and HREE enrichment. The Atoka zircons have a tight age range with a peak at ~327 Ma and εHf values between +0.2 and +7.2. The Wolfcamp B ash contains very few Permian (296-282 Ma) grains, but many grains of 390 Ma -318 Ma with εHf values between +1.5 and +6.3. These signatures suggest that the ashes were largely recycled from a Middle Devonian – Early Pennsylvanian arc that likely was associated with subduction of the Rheic ocean beneath Gondwana. The Wolfcamp A and Spraberry grains are between 317 Ma and 273 Ma and have εHf values between -6.3 and +0.5. These signatures suggest an early Permian arc that incorporated older arc crust and was more evolved than the Middle Devonian – Early Pennsylvanian arc. The early Permian arc might be associated with subduction of the Panthalassic ocean beneath western Pangea. This study is the first to identify two distinct late Paleozoic arc systems that bracket continental collision to form Pangea.