Implications from zircon-saturation temperatures and lithological assemblages for Early Permian thermal anomaly in northwest China

Despite considerable efforts, the Paleozoic tectonic setting (subduction vs. post-collision vs. mantle plume) in the Central Asian Orogenic Belt (CAOB) remains controversial. With aims of solving this issue, we examined zircon saturation temperatures of granitoids emplaced in different settings, and demonstrated that zircon saturation thermometry of felsic igneous rocks, especially when integrated with their lithology and geochemistry, can be a simple and powerful tool for tectonic discrimination. This method is then applied in northern Xinjiang where the Early Permian granitoids have higher saturation temperatures (> 800 °C) than the Silurian-Carboniferous granitoids (mainly < 800 °C). Moreover, the latter are mainly of I-type and are characterized by relatively narrow range of uniformly higher εNd. Consistent with high εHf in zircons from contemporaneous ultramafic-mafic complexes, they were most likely derived from subduction-related processes. In contrast, the high-temperature Early Permian granitoids and associated ultramafic-mafic rocks show a wide range in εNd and εHf in zircons, consistent with the involvement of a great variety of crustal and mantle materials in magma genesis. The thermal anomaly and diversity of Permian magmatic rocks in the CAOB can be ascribed to a mantle plume activity, in agreement with widespread contemporaneous flood basalts, and associated mafic dykes and ultramafic complex in Tarim. Therefore, the temporal variation in temperature and geochemistry of Paleozoic granitoids in northern Xinjiang mirrors a significant change in tectonic setting from a subduction-collision setting during Silurian-Carboniferous to an intra-plate setting since Early Permian.