Zircon U-Pb geochronology and petrogenesis of rhyolites in Manketouebo Formation from the Kundu area in Jarud Basin, Inner Mongo-lia

Age and tectonic implications of the Late Jurassic rhyolites in Kundu area of Jarud Basin were studied by using zircon U-Pb dating, in situ Lu-Hf isotopic analysis, petrographic analysis and other geochemical methods. LA-ICP-MS zircon U-Pb dating results show that the rhyolites were formed in Late Jurassic, with their formation age being 151.2±1.2Ma. The geochemical study suggests that rhyolites are rich in silicon and alkali, but poor in calcium and magnesia. The REE values of them are between 110.38×10^-6 and 138.88×10^-6, displaying medium LREE-enriched and HREE-depleted REE patterns(La/Yb)_N=6.24~7.43, with weak negative Eu anomaly (δEu=0.72~0.98). The trace element geochemistry is characterized evidently by enrichment of Cs, Rb, Ba and LREE, strong depletion of Sr, P, Ti, and mediate depletion of Nb, Ta, with the ε_Hf(t) values varying from -10.1~4.9, corresponding to T_DM^C model ages of 1192~3639Ma, which shows that the rhyolitc magma originated mainly from the partial melting of Proterozoic and Archean crustal rocks, and suffered fractional crystallization. Based on the above result, in combination with previous studies of the contemporaneous magma-tectonic activities in Da Hinggan Mountains, the authors hold that the rhyolites in Manketouebo Formation were formed in an extensional setting related to Mongolia Okhotsk orogenesis