The Volcanic Architecture And Tectonic Setting Of The Lianshishan In Eastern Tianshan,Aqishan,NW China

The Tianshan orogene is an importantly polymetallic ore deposit in China,A major debate related to the late Paleozoic large-scale magmatic thermal event in the Tianshan orogenic belt is centred on the origin and tectonic setting.Here we through the detailed field mapping,summarizes volcanoes apparatus rules and structure characteristics in the area,combined with Previous work,to discuss the origin and tectonic setting of igneous rock that in East Tianshan of Aqishan area.We obtained nine igneous rock samples in the Eastern Tianshan of Aqishan area,8 of them are volcanic rocks,the other sample is granite enclaves in the basalt.The data suggest that these eight sample are sub-alkaline volcanic rock series,including high-K calc-alkaline and calc-alkaline rocks.These are characterized by similar REE and trace elements,negative anomaly of Eu,LREE with limited fractionation relative to HREE,enrichment of LILE and depletion of HFSE.We analysed Zircon U-Pb isotope and Hf isotope composition for thses four samples.Zircon LA-ICPMS U-Pb dating gives the eruption age of andesite in 334±2Ma(MSWD=0.97)with ?Hf(t)of-6.6 to 8.1,dacite porphyry in 333±1Ma(MSWD=11.3)with ?Hf(t)of 3.4 to 6.7,andesite-porphyrite in 234±2Ma(MSWD=0.83)with ?Hf(t)of-31.7 to-3.9,The diagenetic age of granite enclaves was 323±2Ma(MSWD=0.34)with ?Hf(t)of 4.9 to 15.4.suggesting that this area includes two periods,namely Early Carboniferous to early stage of Late Carboniferous and Late Triassic.Based on Zircon Hf isotope composition and MgO of andesite in Early Carboni-ferous,the magma of andesite was formed by the interaction of the magma by dehydration of subducting plate and overlying mantle wedge.Andesite and dacite porphyry are characterized by similar REE and trace elements,and thus these are formed by magmatism with same period.Based on the comparison of granite enclaves and intrusive rock,granite enclaves was formed by magmatic process collision orogeny,suggesting that this area is collision orogeny in Late Carboniferous.The stress field is changed into post orogenic extension environment,while the ersion of granite-magma in this period is possible caused by decompression melting of newly formed lower crust.The granite-magma is constantly formed from partial melting of early formed crust and thus lead to the increases density of magma source,the unsteadiness of gravity could be delamination and magmatism in Late Triassic.