Origin Geological Significance Of Paleozoic Granite Plutons From Bayinbuluk-Bulusitai District In Southern Tianshan

Bayinbuluk-Bulusitai district stretches across the Central-Southern Tianshan tectonic belt, locating in the border of the Central and Southern belt, in which different kinds of Paleozoic magmatic intrusions are widely exposed, making this district an ideal place to investigate the Paleozoic tectonic evolution of Tianshan. LA-ICP-MS zircon U-Pb dating yielded forming ages of 464.5±3.5Ma and 427.9±5.5Ma for monzonitic granite and K-feldspar granite of the Bayinbuluk pluton in the north margin of the Southern Tianshan tectonic belt, forming ages of 445.5±2.9Ma and 430.9±2.4Ma for diorite and gabbro diabase vein of the Northern Bayinbuluk pluton, and a forming age of 435.9±3.3Ma for granodiorite of the Geluntanggushi pluton; yielded forming ages of 312.4±3.9Ma and 311.0±2.2Ma for quartz diorite and gabbro xenoliths of the Dabulusitai pluton in the south margin of the Central Tianshan tectonic belt and also a forming age of 302.4±3.2Ma for diorite of the Xiaobulusitai pluton. All of these crystallizing ages reveal that there are two phases important magmatic activities during the middle Ordovician-late Silurian and late Carboniferous in the north margin of Southern Tianshan and south margin of Central Tianshan. Amongst the studied samples, the granitoid and diorite of Bayinbuluk both show the characteristics of enrichment in LREE and poor HFSE and various degrees of depletion of Nb, Ta, Ti and P, displaying characteristics of high potassium calc-alkaline granite type I from active epicontinental arc. The Hf?t? values of middle Ordovician monzonitic granite range from -9.06 to -6.56 with T_DM^C between 1.58?1.71 Ga. The Hf?t? values of diorite varies from 10.01 to 12.61 with T_DM^C between 588?721 Ma. K-feldspar granite of late Silurian gains the Hf?t? values ranging from -5.38 to -2.25 with T_DM^C between 1.33?1.49 Ga. Gabbro diabase vein from the same time period obtains the Hf?t? values ranging from 7.97 to 11.6 with TDM between 630?748 Ma.The wide range of negative to positive Hf?t? and Mesoproterozoic -Neoproterozoic Hf continental crust model age and depleted mantle model age in the end of Neoproterozoic certify these granite are the products of partial melting of the Mesoproterozoic-Neoproterozoic continental crust material, while diorite formed from partial melting of juvenile continental crust, and mafic dikes are the outcome of magma from depleted mantle being modified by the subduction component, all of which consistently reflects that the study area is related to arc magmatic activity in oceanic subduction setting, indicating that the oceanic subduction began at least in the Ordovician, and lasted till the late Carboniferous.The Dabulusitai and Xiaobulusitai pluton in north margin of Central Tianshan are rich in LREE and poor in HFSE and have positive ?Hf?t? values and Hf continental crust model age in Neoproterozoic. Moreover, gabbro xenoliths of the pluton express more positive ?Hf?t? values and depleted mantle Hf model age in the end of Neoproterozoic, consistent with the feature of the basite from same period. All of these point out that plutons and the xenoliths are from partial melting of Neoproterozoic continental crust basement rocks and the product of modified depleted mantle magmatic activities, respectively. There also exists substantial differences in element geochemistry, with no sign of genetic evolution relationship, which indicates that is not the subduction arc magmatic diamictite. Combined with the existence of a number of undeformed granitoids with binary magma mixing genesis in the same period, these late Carboniferous granitoids represent a significant crust-mantle magmation in non-compressing environment of the post-collision stage.Systematic study of geochronology, petrogeochemistry and isotope-tracing of the granitoids from Bayinbuluk-Bulusitai district demonstrate that the oceanic subduction at early Paleozoic in Tianshan region might commence at the 452Ma ago in middle Ordovician, and continued to 429Ma in late Silurian. During late Carboniferous of the Late Paleozoic Tianshan Orogenic belt walked into the post-collision evolutionary stage featured by extension.