Genesis And Tectonic Setting Of Late Paleozoic Granites In The Northwestern Tianshan Mountains Of The Central Asian Orogenic Belt

The Central Asian Orogenic Belt(CAOB)is one of the most important sites for continental growth and reworking in the world and,therefore,has attracted extensive in recent years.However,controversies still exist surrounding some scientific issues related to the formation of the CAOB,such as mechanisms of growth and preservation of the juvenile crust and the subduction/closure history of oceanic basins.This master's thesis report petrographical features,LA-ICP-MS U-Pb ages,zircon Hf-O isotope,and whole-rock elemental and Sr-Nd isotopic results for five representative granitoid plutons from the Northwestern Tianshan,SW CAOB.The thesis aims to unveil the petrogenesis of these rocks and provide some crucial constraints for tectonic evolution and continental of the study areas.Based on LA-ICP-MS U-Pb analyzes carried in the present study,the Dunmazha pluton was emplaced in the Early Carboniferous(338±2Ma),the Qiao'erma(311±4 Ma),Sikeshu(307±2 Ma and 299±2Ma),and Jingheshantou plutons(305±2Ma,298±2Ma,293±2Ma)in the Late Carboniferous,and the Heishantou pluton in the Early Permian(282±2Ma,289±2Ma,and268±2Ma).In general,samples from the five studied granitoid plutons exhibit a K-,Al-and total alkali-rich characteristic.They belong to the calc-alkaline to high-K calc-alkaline and metaluminous to weakly peraluminous series.Regarding trace elemental patterns,all the studied samples are enriched in large-ion-lithophile elements,e.g.,K and Rb,and depleted in high-fieldstrength elements,e.g.,Nb,Ta,and Ti.In terms of the chondrite-normalized rare earth element patterns,the Heishantou samples are characterized by the strong enrichment of light rare earth elements(LREE)compared with heavy rare earth elements(HREE)and the absence of negative Eu anomalies.By contrast,samples from the other four plutons show relatively lower LREE/HREE ratios and variable-degree negative Eu anomalies.Given petrographic and wholerock geochemical features,granitoids from the five plutons are of I-type granite in genesis.Besides,due to high Sr/Y(64.74 to 215.00)and La/Yb(14.27 to 35.86)ratios,the Heishantou granites can be classified as adakitic rocks.Zircon Hf and whole-rock Nd isotopic results consistently suggest magma sources dominated by juvenile materials,as suggested by positive ?Hf(t)and ?Nd(t)values and young two-stage model ages.Some zircons from the Dunmazha pluton show low ?¹⁸O values(3.93 to 5.68),implying the substantial involvement of altered oceanic crustal materials in the magma source.The Jingheshantou samples yield high ?¹⁸O values(8.29 to 10.25),plausibly indicating the assimilation of supracrustal rocks.Compared with ones from other plutons,zircons of Heishantou pluton show ?¹⁸O values close to that of the depleted mantle,implying the contribution from the mantle-derived materials to the magma source.In summary,the parental magma of the Heishantou pluton might be derived from the partial melting of the thickened lower crust,whereas those of Jingheshantou,Sikeshu,Qiao'erma,and Dunmazha plutons from the partial melting of meta-igneous rocks within lower crust with a normal thickness.In combination with previous studies,this thesis proposes the Early Carboniferous trench retreat(³50 to ³10Ma)in the northern Yili Block,associated with the rollback of the subducted slab of the North Tianshan Ocean(NTO).Such retreating subduction continued until the NTO finally closed during Late Carboniferous times.The trench retreat triggered the supra-subduction extension,the development of the back-arc basin,and resulted in significant continental growth,as illustrated by the increasing involvement of juvenile components in the felsic magmatism.