Study On Petrogenesis Of Middle Triassic Granites In Chayouzhongqi, Inner Mongolia, The Middle Segment Of The Northern Margin Of The North China Craton

During the Early Mesozoic, extensive granitic magmatism occurred at the north margin of the North China Craton (NCC), forming a granite zone parallel to the boundary between the NCC and the Central Asian orogenic belt (CAOB). The researches in the petrogenesis of this granite zone could enriched our understanding of the tectonic evolution history of the north margin of the NCC. Many studies focusing on the eastern part of the granite zone have been carried out. However, the petrogenesis of the granites in the middle part of the north margin of the NCC is still in dispute. This thesis concerns the Wanlongchang-Danaobao(WD) pluton and the Baiyanaobao(BY) pluton, which are parts of the Dadongshan (DS) batholith in Chayouzhongqi, Inner Mongolia and located in the middle part of the northern margin of the NCC. Field investigations, zircon U-Pb chronology and major, trace elements and Sr-Nd-Hf isotopic geochemistry analyses were carried out to constructive petrogenesis mode and provide evidences to better understand evolution process of the continental crust of the north margin of the NCC during the Early Mesozoic.The LA-ICP-MS zircon U-Pb dating results show the WD pluton was emplaced at about 244-247Ma, and the BY pluton was emplaced at about 242-243Ma, thus, they were the products of the Middle Triassic magmatism. The SiO2 contents (65.9%～76.7%) in WD granites show more variety than those in BY granites (SiO2=70.6%～72.5%). These two plutons have high contents of alkali (K2O+Na2O=8.29%-9.46%), and belong to alkali-calcic weakly peraluminous I-type granites. They are all enriched in Rb, Th, K, Nd, Hf, Pb, LREE and depleted in Ba, Nb, Ta, Sr, P, Ti, HREE. The fractional crystallization of feldspar, apatite and zircon played a more important role in the formation of the WD granite than that in the BY granite. The WD granite with low SiO2 contents and the BYgranite are characterized by low Y andYb contents, but high Sr content,Sr/Y and (La/Yb)N ratios, which indicate that their parental magma most likely originated from the lower crust.The WD granite is characterized by low εNd(t) values(-9.9 to-12.2) and zircon εHf(t) values (-10.5 to-15.8), indicating that it was derived from melting of the ancient lower crust with some involvement of enriched lithospheric mantle materials. The BY granite displays lower εNd(t) values (-15.6 to-16.0) and zircon εHf(t) values (-18.4 to -24.4) relative to those of the WD granite, which suggests that it was derived mainly from the ancient lower crust. The WD and BY plutons are likely to form in syn-collisional to post-collisional/post-orogenic tectonic setting, suggesting that the final amalgamation of the CAOB and the north margin of the NCC had finished at the middle Triassic and then the north margin of the NCC turned into post-collisional/ post-orogenic extensional setting.