Abstract: The Baiyingaolao Formation in the Honggeer area of central Inner Mongolia is mainly composed of rhyolites, with minor dacite and latite. The rhyolites are characterized by high SiO2, Al2O3, K2O, and low CaO, MgO. Their high Sr/Y, La/Yb, and low Y, Yb values show an adakitic geochemical affinity. The results of K2O/Na2O and A/CNK reveal that the rhyolites belong to a peraluminous highK calcalkalic type. The rocks are obviously enriched in LILEs such as K, Rb, Th, U, and depleted in HFSEs such as Nb, Ta, Ti and P with obvious negative anomalies. The total of REE content is relatively low, with enrichment in LREE and medium negative Eu anomaly. The SrNdPb isotopic compositions show positive εNd(t)(+088 to +266), and low tDM(t) (5883 to 7164 Ma). The above geochemical characteristics reveal that the rhyolites of the Baiyingaolao Formation in the Honggeer area belong to highK calcalkine and Ctype adakitic rocks, and they were the products of partial melting of the thickened continental crust. The rocks were formed in an intraplate extensional tectonic setting after postorogenic thickened lithospheric gravitational collapse at Early Cretaceous, which was related with the collision after closure of MongolOkhotsk Ocean. Garnet was the residual phase during the partial melting, and fractional crystallization with the removal of plagioclase, ilmenite and apatite played an important role in magma evolution.

Key words: Baiyingaolao Formation, rhyolite, Ctype adakitic rock, geochemisty, Honggeer area, central Inner Mongolia