Abstract
SHRIMP zircon U-Pb ages of 272.1±3.1Ma and 271.4±1.7Ma from two rhyolite samples, which come from Dashizai Formation rhyolites from Sunid Left Banner in the middle part of Inner Mongolia, respectively uniformly indicate that these rhyolites of the Dashizhai Formation erupted in the early Permian. Their initial Hf isotopic ratios range from +10.8 to +13.1. According to the Hf-Nd coupling relationship of terrestrial array, the authors obtained their corresponding conversion Nd values, which are very similar to Nd isotopic initial values of the coeval basalts exposed in the Dashizhai Formation and some other places, implying their common derivation from the depleted mantle. The rhyolites are characterized by high content of SiO2, but low content of CaO, Fe2O3?, and MgO. Most samples exhibit sub-alkaline and peraluminous characteristics, and show moderate fractionation REE patterns with pronounced negative Eu anomalies. In the spider diagram, they show enrichment in Cs, Rb, Th, U and Zr, and depletion in Ba, Sr, Nb, Ta, P and Ti. All these features reveal that these rhyolites evolved from calc-alkaline mafic magmas with enriched water by fractional crystallization of mineral phases, and were probably generated in the continental arc system.
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