Granitic Magma Records Of Evolution From Permian Subduction And Accretion To Triassic Collision In The Southeastern Margin Of The Central Asian Orogenic Belt

The Central Asian Orogenic Belt(CAOB)is the largest Phanerozoic accretionary orogenic belt in the world,and its evolution is accompanied by the formation of a large amount of juvenile crust.The southeastern margin of the CAOB(the southern part of the Great Xing'an Range)is located in the central part of Inner Mongolia Autonomous Region,and the Solonker-Xar Moron suture zone records the final closure and disappearance of the Paleo-Asian Ocean and the collage of terranes on both sides.At the same time,the Permian-Triassic granitoids along the suture zone in the region record the magmatic evidence from subduction/accretion to collision in the southeastern margin of the CAOB.In this paper,the Middle Permian-Late Triassic granitoids in central Inner Mongolia,located in the southeastern margin of the Central Asian orogenic belt(the southern part of the Great Xing'an Range),are taken as the research objects.From the perspective of the evolution of magmatic properties with time,the tectono-magmatic evolution characteristics of the area from the subduction of the Early Permian to the(soft)collision of the Late Permian and then to the post orogenic extension of the Late Triassic are determined:(1)The granitoids from Middle Permian to late Triassic in different tectonic units on the north and south sides of the Solonker-Xar Moron suture zone have different geochemical and isotopic characteristics in this study.The whole rock?_Nd(t)value and zircon?_Hf(t)of these granitoids value gradually evolves from positive value to negative value(?_Nd(t):2.4?-19.5;?_Hf(t)value:11.6?-33.7).Li isotope shows that?⁷Li values range from+1.1‰to+6.6‰,with an average value of+3.37‰,slightly larger than the average value of the crust.These granitoids are formed in the transitory stage from subduction to collision,crust thickening stage caused by collision and post orogenic extension stage respectively,and recorded the tectono-magmatic evolution process from continental marginal arc to soft collision,and then to post collisional extension.(2)The magmatic record of soft collision is identified and characterized by thickened lower crust-derived high Sr/Y granitoids with a sub-linear distribution along the Solonker suture zone from the end of Permian to the Middle Triassic.In addition,the study on the temporal and spatial distribution characteristics of granitoids shows that in the Middle Permian,the granitoids formed along the Solonker-Xar Moron suture zone from west to east in the transition stage from subduction/accretion to collision and amalgamation of the CAOB,and their age peak gradually became younger(from about264 Ma to about 251 Ma),reflecting the“scissor-like”closure of Paleo-Asian ocean.(3)The temporal and spatial distribution,geochemistry and isotopic characteristics of the granitoids in the study area have periodic evolution characteristics.Integrated with previous studies,a three-stage tectonic model from the Permian to Triassic by accretion leading to collision on the southeastern margin of CAOB is proposed.(a)Early Permian(>ca.285 Ma):Juvenile magmatism on an active continental margin with double-sided subduction of the Paleo-Asian Ocean;(b)Middle Permian to Middle Triassic(ca.285–235 Ma):Magma source transition from juvenile to old crust induced by a tectonic switch from arc to“scissor-like”closure and subsequent intra-continental orogenic contraction;(c)Late Triassic(<ca.235 Ma):A-type and alkaline magmatism in response to post-collisional extension.