Petrogenesis Of Late Paleozoic Granites From Alatag Region Of The Central Tianshan,and Their Geological Significance

The Central Tianshan is located in the southwestern part of the Central Asian orogenic belt,it is a key location for the collision and convergence of various plates.The study of the genesis and tectonic evolution of late Paleozoic intrusive rocks in the middle section of Central Tianshan is helpful for the overall understanding the tectonic background evolution of the Central Tianshan.In this paper,we report whole-rock geochemistry,Sr-Nd isotopes,zircon oxygen isotopes and trace elements and zircon U-Pb chronology for the Late Paleozoic granitic rocks from the Alatag area of the Central Tianshan.The Alatag granites including gneissic granite and monzonitic granite,The U-Pb ages of zircons were obtained from two samples with 347.8±2 Ma and 279.3±3 Ma,respectively.In addition,a group of 309.4±3 Ma ages is also identified from the monzonitic granite,which reveals the late Paleozoic magmatism in the Alatag area.Considering the published geochronological records in this area,the paper thinks that there is widespread Late Paleozoic magmatic activity in the Central Tianshan,and the three groups of ages belong to the peak periods of different magmatic activities.Whole-rock geochemical analysis shows that both phases of granite have the characteristics of enrichment of large ion philophile elements Th,U and Rb,depletion of high field-strength elements Nb,Ta and Ti,high-K calc-alkaline,metaluminous to weak peraluminous affinities,combined with the characteristics of mineral assemblages,they belong to I-type granites.The oxygen isotope of the zircons in the early Permian monzonitic granite range?¹⁸O=6.55-7.34‰,which is characterized by the oxygen isotope composition of zircons of crustal origin;It's initial value of Sr isotope(⁸⁷Sr/⁸⁶Sr)_i is 0.7073,and?_Nd?t?value of-1.8,with?_Nd?t?model age of 1003 Ma,indicating the Magma was derived from partial melting of the Mesoproterozoic continental crust;The gneissic granite's initial value of Sr isotope(⁸⁷Sr/⁸⁶Sr)_i is 0.7098,and?_Nd?t?value of-1.8,with?_Nd?t?model age of 1190Ma,also indicating the Magma was derived from partial melting of the Mesoproterozoic continental crust.Based on the isotope data in the region,the magmatic source areas of the Early Carboniferous and Early Permian granites in Alatag,Central Tianshan are both crust sources.Integrating with the regional geological data,zircon trace elements and isotopes,we believe that the Early Carboniferous gneissic granite was formed in the continental arc background subducted northward in the South Tianshan ocean basin under the Central Tianshan block.The Early Permian monzonitic granite was formed by decompression and melting in a post collisional tectonic environment.