Geochemical Characteristics And Petrogenesis Of Granitoid Rocks In The Atengtao Mountains Of The Yining Block

The three plutons(Taledesayi,Wulubujisayi and Kalesijun)are representative plutons in the Atengtao Mountains of the Yining Block.Detailed study about the geochemical characteristics and petrogenesis of the three plutons will provide important information for the granitic magmatism and tectonic background of the Yining Block.Thus,authors conducted detailed field and indoor geological survey on the three plutons,including section investigations,whole-rock chemical analysis,zircon U-Pb isotopic dating and Lu-Hf isotopic analysis.Meanwhile,we collected and analyzed the data of grantiods in the Chinese Western Tianshan(especially the Yining Block and Middle Tianshan Block).The results are below:1.The Taledesayi and Wulubujisayi plutons mainly consist of two types of intrusive bodies: syenogranite and granite porphyry.The Kalesijun pluton mainly consists of three types of intrusive bodies: monzonite,syenogranite and granodiorite.The above plutons were intruded by gabbro,diorite or quartz-diorite dikes.The contact relationships between the plutons and surrounding rocks are complex.Some intrusive bodies intruded into surrounding rocks,some were covered by the Carboniferous Strata and some contacted with the Carboniferous Strata by faults.2.LA-ICP-MS zircon U-Pb dating shows that the age of granite porphyry in Taledesayi pluton,syenogranite and gabbro in Wulubujisayi pluton and syenogranite and monzonite in Kalesijun pluton is 338 Ma,357Ma,345 Ma,344Ma and 356 Ma,respectively,ranging from Tournaisian to Visean.3.The felsic granites in the above plutons are characterized by high Si,alkali-rich,low Fe,Ti,Mg,Cr,Co,Ni and so on.They are metaluminous or peraluminous and contain more LREE than HREE.In addition,they are enriched in LILEs(such as Rb,Th and K)and depleted in HFSEs(such as Nb,Ti and Ta).Geochemical characteristics show that they are I-type granites with island arc marks.Notably,granites in Taledesayi pluton are enriched in Rb but depleted in Ba and strongly depleted in Sr,showing they underwent a high degree of differentiation.The gabbros in Wulubujisayi pluton contain more Nb contents than MROB(mid-ocean ridge basalts),showing they are Nb-enriched gabbros.4.The zircon ?Hf(t)of syenogranites and gabbros in Wulubujisayi pluton is 1.3?6.8,1.2?8.7 and TDM2=929?1283Ma,799?1273Ma,respectively.Based on the understanding of other geochemical characteristics in the grantiods and analysis of zircon Lu-Hf isotopic data of contemporary magmatic rocks(362?325Ma),we believed that the parental magma of the felsic granites could be derived from patial melting of Proterozoic basement of amphibolite facies with participation of mantle components and then underwent fractional crystallization with different degree.However,the Nb-enriched gabbros were formed by the partial melting of the enriched mantle wedge that had been metasomatized by subducted slab-derived fluids with the presence of asthenospheric materials.5.Combined with the geochemical characteristics of the Early Carboniferous special magmatic rocks in the Atengtao Mountains and adjacent areas such as the bubble rhyolites,high-Mg andesites,bimodal volcanic rocks,O-type adakites and Nb-enriched basalts,we believed that the three plutons should be formed in the back-arc basin of the South Tianshan Ocean and the Early Carboniferous intense magmatic activities could be resulted from slab roll-back of the South Tianshan Ocean.6.The invasion of granitoids in the Yining Block could be divided into five periods: Neoproterozoic(943Ma),Early Cambrian to Early Ordovician(524-470Ma),Late Ordovician to Early Devonian(446-398Ma),Late Devonian to Early Carboniferous(362-331Ma)and Late Carboniferous to Early Permian(320-277Ma).During 524-470 Ma,the Terskey Ocean subducted into Tarim Plate in the south and Yining Block in the north,respectively.During 446?331Ma,the South Tianshan Ocean subducted into Yining Block in the north.The South Tianshan Ocean came to an end in ?323Ma and then the Yining Block entered an intracontinental evolutionary stage in 320-277 Ma.