Early Triassic roll-back of subducted Paleo-Tethys oceanic lithosphere; insights from A2-type silicic igneous rocks in the Pingxiang area, southwest China

A suite of subduction-related early Triassic igneous rocks crops out in the Pingxiang area of the Dian-Qiong suture (DQS) in southwest China; this suite represents an important geological record of subduction and closure of the Paleo-Tethys Ocean. In this study, we report geochronological, geochemical, and Nd-Hf isotope data for newly discovered rhyolites and biotite granites in the Pingxiang area. We use these data to constrain their emplacement ages, origins, and geodynamic implications. Zircon U-Pb dating indicates that the rhyolites and biotite granites were emplaced at 251-250 Ma and 249 Ma (early Triassic), respectively. The rhyolites and biotite granites have elevated FeOT/(FeOT + MgO) (0.78-0.89) and 10,000 × Ga/Al (2.83-4.11) ratios, with geochemical affinities to A2-type granites. These rocks are enriched in some large-ion lithophile elements (e.g., Rb, Th, and Ba) and depleted in high-field-strength elements (e.g., Nb, Ta, and Ti), indicating their formation in a subduction-related arc. The rhyolites and biotite granites have negative whole-rock εNd(t) (-11.5 to -9.7) and zircon εHf(t) (-14.5 to -6.2) values, suggesting that these magmas were derived from an ancient crust-dominated source. Geochemical and Nd isotope data reveal that the peraluminous A2-type rhyolites and biotite granites were derived by partial melting of felsic crustal rocks under low-pressure and high-temperature conditions. By integrating all the available data with the regional tectonic evolution of the southwestern Youjiang Basin and adjacent regions, we attribute the generation of the peraluminous A2-type rhyolites and biotite granites to the extensional setting that existed during oceanic subduction, which was induced by roll-back of the Paleo-Tethys oceanic lithosphere at 251-249 Ma. This study indicates that subduction-related magmatism related to Paleo-Tethys oceanic lithosphere was still active in the Early Triassic.