Zircon LA-ICPMS U-Pb dating and petrogenesis of rhyolites in Baiyingaolao Formation from the southern Manzhouli, Inner-Mongolia
满洲里南部白音高老组流纹岩锆石U-Pb定年及岩石成因

The volcanic rocks of Baiyingaolao Formation in the southern Manzhouli arc composed of rhyolites and a little amount of perlites and rhyolitic tufts. U-Pb age determinations of zircon by LA-ICPMS show the rhyolites were formed at Early Cretaceous, and their formation ages ranged from 141Ma to 139Ma. The geochemical study suggests that rhyolites are rich in silicon, alkalis, poor in calcium, magnesium and have high FeO~T/MgO ratio. All samples have similar characteristics of REE patterns which are characterized by relatively high total REE contents (∑REE = 103×10~(-6)～488×10~(-6) ), significant fractionation of HREE and LREE (La/Yb) = 4.12～30.94] and strong negative Eu anoalies (δEu=0.12～0.46). The trace element geochemistry are characterized evidently by enrichment of Rb, Th, U, K, strong depletion of Ba, Sr, P, Ti, mediate depletion of Nb, Ta and higher Ga/Al ratio. The geochemical characteristics of rhyolites show an affinity with the A-type granite. The initial ~(176) Hf/~(177) Hf ratios of the magmatic zircons from rhyolites vary in 0.282785～0.282970 and the corresponding ε_(Hf)(t) values are positive ranging from 3.78 to 9.98. The characteristics above indicate that the rhyolitic magma was origTinated from partial melting of basaltic lower crustal materials in the plagioclase-stable field. Combining with the regional researching results,we suggest that the rhyolites were formed in an intra-plate anorogenic extensional setting.