The Early Permian Bimodal Magmatism In Dongdashan Area,Duolun,Inner Mongolia

The research area is located in the east of Duolun County,Inner Mongolia,tectonically on the northern margin of the North China Plate,which is an important window to research the evolution process of the Paleo-Asian Ocean.This paper chosen the Dongdashan bimodal magmatites as the research object.We made detailed field geological investigation and systematically samples collecting and testing in this area after collected and analyzed comprehensively the early researched findings for it,hoping to fully understand its petrological,bulk rock geochemical,zircon U-Pb chronological and zircon Hf isotopic characteristics,and preliminarily confirm the types,series,genesis and tectonic setting of the Dongdashan bimodal magmatic rocks,and provide credible evidence for restricting the evolution process of the Paleo-Asian Ocean during the Early Permian.Finally the following understandings are obtained:1.Dongshan bimodal magmatites is composed of monzogranite and basalt.The Si O₂ content of monzogranite is between 65.87%and 67.17%,and that of basalt is between 48.83%and 50.64%,and there is an obvious gap between 50.64%and 65.87%.It exsits the typical characteristics of bimodal magmatic rocks.2.Zircon U-Pb dating shows that the zircon LA-ICP-MS U-Pb dating from monzogranite in Dongdashan area is 283±1.6Ma,and the zircon LA-ICP-MS U-Pb dating of basalt is 280±2.9Ma,indicating that the formation ages of the two kind of rocks are very close,and both of them are formed in the Early Permian.3.The bulk rock geochemical and zircon Hf isotopic characteristics indicate that the early Permian monzogranite and basalt in the Dongshan area exist the characteristics of typical bimodal magmatic rocks.The relatively similar trace elements,Hf isotopic characteristics and the complementarity of Ti and P elements indicate that they have the same magma source area and are the products of partial melting of the ancient basement of the North China Plate under the background of fast extension,which was caused by the upward migrating and gathering of the mantle magma formed from partial melting of mantle resulted in dewatering of subducted plate of the ancient Asian Ocean.The magma of monzogranite was formed early and didn't contain the mantle magma.Basalt magma is the mixings of mantle-derived magma with the magma from the melting of the same ancient basement of North China Plate containing the remained apatite and rutile after the monzogranite magma separated,under the continuous heating of mantle-derived magma.The Dongdashan bimodal magmatites may be formed in the subcontinental side of the active continental margin.4.During the Early Permian,the research area was still subjected to the continuous subduction of the Paleo-Asian Ocean,and is a part of the Early Permian active continental margin in the northern margin of the North China Plate,which including these areas:Damao Banner,Shangdu County and Xianghuang Banner locked in the west of the research area,Chaoyangdi and Wengniute Banner in Chifeng City locked in the east of the research and Sunite Left Banner,Xilinhot City and Linxi County locked in the north of the research.