Permian-Jurassic Magmatism And Its Tectonic Significance In Sumdo Area,Southern Tibet

Subduction of oceanic plate will result in the generation of arc magmatism.Arc magmatism recognition is the basis for reducting paleooceanic evolution.The Sumdo Paleo-Tethyan Ocean suture zone located in Lhasa terrane,represents the remnants of the Paleo-Tethyan Ocean.However,the subduction and closure process of the Sumdo Paleo-Tethyan Ocean remains unclear.This paper presents the results of a study of the Permian-Jurassic magmatic rocks in the Sumdo area.Based on new fieldwork,petrography,zircon U-Pb dating and Lu-Hf isotopes,whole rock geochemistry and Sr-Nd isotopes,in combination with published data,this paper discusses the petrogenesis and crust-mantle interaction of the Permian-Jurassic magmatic rocks,explores the relationship between multi-stage magmatism and evolution of the Tibetan Plateau,and ultimately reconstruct the process of tectonic-magmatic evolution.Based on the LA-ICP-MS zircon U-Pb dating of magmatic rocks from TangjiaSumdo area,four stages of arc magmatism were recognized: Middle Permian diorite yielded U-Pb age of 263 ± 3 Ma,Middle Triassic granite yielded U-Pb age of 238 ± 1 Ma,Late Triassic granodiorites yielded U-Pb age of 208~201 Ma,Early Jurassic granodiorites yielded U-Pb age of 201~194 Ma.Based on the previous geochronological study,the formation history of the Sumdo arc magmatism is divided into four stages: Middle Permian(ca.263 Ma),Middle Triassic(ca.238 Ma),early Late Triassic(237~221 Ma),and lately Late Triassic-Early Jurassic(213~190 Ma).The Middle Permian magmatic rocks are exposed in the Sumdo formation.The high Mg O content and Mg~# indicate the rocks are high-Mg diorite,and belong to Sanukitic high-Mg andesite.The whole rock geochemistry and Sr-Nd isotopes data of the Sanukitic rocks show that parental magma was mainly from depleted mantle source metasomatized by the subducted oceanic crust melt.Based on the data of eclogites and oceanic island rocks in the study area,it is suggested that the Sumdo Middle Permian magmatic rocks may form in an arc magmatic environment caused by the initial northward subduction of the Sumdo Paleo-Tethyan Ocean.The Middle Triassic pluton intruded in the deformation strata of the Sumdo formation.The petrographic and whole rock geochemical studies show that the rocks are peraluminous granites,mainly origin from the partial melting of mature crustal materials.Zircon Lu-Hf isotopes and whole rock Sr-Nd isotopes indicate that the magma source has contribution of mantle materials,the magma comes from the ancient lower crust which is metasomatized by the mantle material underplating.Combined with the structural deformation of the Sumdo formation and the metamorphic age of the Sumdo eclogites,it is suggests that the Middle Triassic magmatic rocks may formed in the ocean-contient transformation environment of the Sumdo Paleo-Tethyan Ocean.The early Late Triassic magmatic rocks emerged in Dabula area of central Lhasa terrane and Changguo area of Southern Lhasa terrane.The geochemical characteristics indicate that the Dabula plutons are strongly peraluminous granites and the Changguo volcanic rocks are arc volcanics.Based on these results and regional geological data,it is proposed that the Dabula plutons were formed by break-off of the Sumdo Paleo-Tethyan Ocean after the collision of central and southern Lhasa terrane,and Changguo volcanic arc formed as a result of early northward subduction of the Neo-Tethyan oceanic lithosphere.The lately Late Triassic-Early Jurassic magmatic rocks are widely exposed in Sumdo area.The studies of petrology and petrography indicate magmatic mixing.Whole rock geochemistry and Sr-Nd isotopes indicate that the magma originated from the partial melting of ancient lower crust materials,and accompanied by the addition of mantle-derived materials affected by the metasomatism of subducted plate fluids.Combined with the contemporaneous magmatism in the Lhasa terrane,it is considered that the lately Late Triassic-Early Jurassic arc magmatism was formed by the subduction of the Neo-Tethyan oceanic crust.Our results,together with existing data on metamorphic rocks,ophiolites,stratigraphy,and paleomagnetism,are used to develop a model of Sumdo Paleo-Tethyan Ocean's initial subduction in the Middle Permian and ocean-contient transformation in the Middle Triassic.The Sumdo area was affected by the initial subduction of the Neo-Tethyan Ocean in the early Late Triassic,and a large-scale magmatic arc was formed by the continuous subduction from the lately Late Triassic to the Early Jurassic.