Provenance of Late Permian Nb-Zr-REE-Ga enrichment in western Guizhou: Implications for the waning volcanism of Emeishan large igneous province

Abstract

Nb-Zr-REE-Ga-enriched beds are widely found in the Late Permian clastic sediments in southwestern China and are related to the Emeishan large igneous province (LIP). To determine the provenance of these beds, we analyzed their geochemical and mineralogical compositions, along with zircon U−Pb ages, Hf isotopes, and trace elements. Our data show that these polymetallic beds have high Al2O3 contents, Al2O3/TiO2 (e.g., mostly >8) and Th/Sc (e.g., mostly >0.6) ratios, low Fe2O3, TiO2 contents, and Ti/Y ratios (e.g., mostly <500). These beds exhibit strong negative Ti, Eu, and Ba anomalies, positive HFSEs (high field strength elements) and REEs (rare earth elements) anomalies in trace element patterns, and show geochemical affinities with the alkaline silicic rocks at the top of the Emeishan lavas. Zircons from the polymetallic beds yield U−Pb ages of 259.2–256.2 Ma and ɛHf(t) values of −10.1 − +13.4, which are consistent with the zircon data of the alkaline rhyolites. These results suggest that the mineralized beds dominantly derived from the waning Emeishan silicic volcanism, whereas the barren samples have mixed basaltic compositions. The primary rutile was associated with low-temperature zircon, and was replaced by Nb-rutile and columbite, which were likely to crystallize from an alkaline magma that experienced hydrothermal process. Additionally, the temporal decreases of zircon Eu/Eu* values and crystallization temperatures indicate that rare metals were enriched during the protracted alkaline magmatism. The zircons from the older samples show higher Th/Nb, U/Yb ratios and more negative ɛHf(t) values than those from the younger samples, suggesting that the eroded source rocks were generated by waning silicic volcanism with decreasing crustal assimilation.