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Title:: Geochronological and Geochemical Characteristics of Triassic Aluminous Atype Granites in Wulanwutai Area of Inner Mongolia

Author(s):: WU Rong-ze; ZHANG Shu-dong; LAI Lin; Tianjin Institute of Geological Survey, Tianjin 300191, China

Keywords:: A-type granite; petrochemistry; zircon; U-Pb dating; Triassic; orogenic belt; tectonic background; Inner Mongolia

Abstract:: Based on 1∶50 000 regional geological surveys, aluminous A-type granite was identified in Wulanwutai area of Inner Mongolia. LA-ICP-MS zircon U-Pb ages and geochemical characteristics of porphyritic adamellite were studied, and the genesis and geotectonic setting characteristics in Early-Middle Triassic were discussed. The results show that the weighted average of single zircon U-Pb age of porphyritic adamellite in Wulanwutai area of Inner Mongolia is (231.84±0.99)Ma; it is a product of Indosinian magmatic activity; the rock is composed of medium fine and fine grained porphyritic adamellites; petrological and geochemical characteristic is high-K calc-alkaline series; major and trace elements have the characteristics of high silica and alkali, w(K₂O)/w(Na₂O) is more than 1, and A/CNK values are 1.01-1.10; Eu anomaly is obvious negative, Rb,Th, K and Pb are relatively rich, Sr and Ti are loss; Ba, Nb, Sr and Ti anomalies are obvious negative, Rb, Th, K and Pb anomalies are positive; w(Rb)/w(Sr) and w(Rb)/w(Ba) are high, and w(Sr)/w(Ba) is low. These characteristics indicate that the granite is magma emplacement formed by dehydration and partial melting of middle-lower crust, and is partial extension tectonic setting under the postcollisional compression environment; it marks the end of large-scale collision orogenic movement and the start of new intraplate tectonic movement and evolution.