Origin of two types of rhyolites in the Tarim Large Igneous Province: Consequences of incubation and melting of a mantle plume

The Early Permian Tarim Large Igneous Province (LIP) in northwestern China contains a large area of silicic volcanics (~48,000km²) which are spatially and temporally associated with mafic-ultramafic rocks. In order to understand the behavior of crust above a mantle plume, selected rhyolitic samples are investigated in terms of U-Pb zircon dating, geochemical and isotopic analyses. The Tarim rhyolites have high A/CNK ratios (=molar Al₂O₃/CaO+Na₂O+K₂O), Fe^#, Ga/Al ratios, concentrations of high field strength elements (HFSEs) such as Zr and Nb, and rare earth elements (REEs), along with high zircon saturation temperatures (872-940°C), typical of aluminous A-type granitoids. Two contrasting rock types have been recognized. The low Nb-Ta type rhyolites are mainly associated with the first phase of the Tarim flood basalt magmatism at ~290Ma. They are characterized by negative Nb-Ta anomalies, low ε_Nd(t) and ε_Hf(t) values, and high ⁸⁷Sr/⁸⁶Sr(t) and δ¹⁸O_zircon values, consistent with a derivation from continental crustal source. The high Nb-Ta type rhyolites and their plutonic equivalents are associated with the second episode of Tarim magmatism (283-272Ma). They are characterized by small negative to positive Nb-Ta anomalies, oceanic island basalt (OIB)-like trace element ratios, low ⁸⁷Sr/⁸⁶Sr(t) and high ε_Nd(t) and ε_Hf(t) values. These high Nb-Ta rhyolites are best interpreted as hybrid products of crystal fractionation of mafic magmas, coupled with crustal assimilation.The temporal and compositional evolution of the Tarim rhyolites reflects various extents of thermal and mass exchange between mantle-derived basaltic magma and crustal material above a mantle plume. When the plume head rises to the base of the Tarim craton, it first melts enriched components in the lithospheric mantle (~. 290. Ma), part of which may have ponded near the crust-mantle boundary and induced crustal anatexis leading to the formation of the low Nb-Ta type rhyolites. At ~. 280. Ma, large magma chambers and plumbing systems were formed due to increasing magma supply rate during decompression melting of the mantle plume. This led to the formation of a mafic-ultramafic and felsic association of which the high Nb-Ta type rhyolites are a part.