Geochronology, Hf Isotope Of Phanerozoic Granitoids In Baotou-erenhot Area And Its Tectonic Implications

Baotou-Erenhot area across the western Xing’an Mongolian orogenic belt and northern margin of North China Craton. In order to enhance the understanding of closure time of the Paleo-Asian Ocean,the genisis and tectonic setting based on the zircon U-Pb age, geochemistry and zircon Hf isotope study of the Xiaojinggou granite in the northern margin of North China Craton and the Erennaoer granite in the Xing’an Mongolian orogenic belt are researched.For enhancing the understanding of distribution of juvenile crust and old crust in the Baotou-Erenhot area,basing on the geochronological structure of Phanerozoic granitoids,zircon Hf isotope of the granitoids are discussed.The zircon U-Pb dating results indicate that the Xiaojinggou pluton in the northern margin of North China Craton was intruded at Permian(275±1 Ma). Xiaojinggou pluton is slightly peraluminous and belongs to the high-K calcalkaline series,showing geochemical features with an I-type affinity.Hf isotope characters indicates that the Xiaojinggou pluton is probably generated by partial melting of Lower Proterozoic crust,accompanied by the possible assimilation of mantle-derived magmas. The zircon U-Pb dating results indicate that the Weijing pluton(279±1 Ma) and Chagandele pluton(276±4 Ma) in the Xing’an Mongolian orogenic belt was intruded at Permian. Both of two pluton are slightly peraluminous and belong to the high-K calcalkaline series and Chagandele granite contains amphibole,showing petrological and geochemical features with an I-type affinity. Hf isotope characters indicates that the Weijing pluton is probably generated by partial melting of juvenile crust and the Chagandele pluton is probably generated by partial melting of old crust.Statistics of High-precision geochronological data indicates the intrusive epoch of Phanerozoic granitoids in the Baotou-Erenhot area can be divided into five major stages, namely Ordovician- Devonian(485~359 Ma), Carboniferous(359~299 Ma), Permian(299~252 Ma), Triassic(252~201 Ma), Jurassic- Cretaceous(201~114 Ma),and the Permian granitoids is the most widely distributed.Basing on the zircon Hf isotope characters of granitoids,the study area can be divided into south unit and north unit by Solonker- Xar Moron suture zone. The granitoids in the south unit are characteristized of negative εHf values and old model ages. The granitoids in the north unit are characteristized of positive εHf values and young model ages.In addition, Hf isotope characters of Chagandele granite shows old basement or micro continent may exist in north unit.The study area from south Mongolia to the northern margin of North China Craton is not a united tectonic setting in Permain. The granitoids in the South unit formed in the transition period from subduction to collision/post-collision,whereas the granitoids in the north unit formed in the post orogenic tectonic setting,and the Paleo-Asian Ocean should close before late Permian.