Characteristics of Petrology,Geochemistry,Heavy Minerals and Isotope Chronology of Upper Carboniferous Detrital Rocks in the Middle Segment of South Tianshan and Constraints to the Provenance and Tectonic Evolution

We can restore Late Paleozoic paleogeography and reveal tectonic evolution history of South Tianshan, based on the study of Carboniferous sedimentary records,when was in Late Paleozoic that Tianshan tectonic zone happened to great changes.The sandstones of Upper Carboniferous Ayilihe Formation in the middle segment of South Tianshan are mostly composed of pebbly sandstone,litharenite,sublitharenite, arkose and ferruginous sandstone.The component of the sandstones includes mostly quartz 67.41%,next lithic fragment 21.8%,and a little feldspar 7.52%.The sandstones have average of 2.07for Q/F+L,and low textural and compositional maturity.The detrital rocks have average contents of SiO2 64.78%,Al2 O3 10.22%,TiO2 0.35%,TFe2 O3 4.23%,MgO 1.37%,CaO 7.16%,K2 O 2.13%,Na2 O 0.93%,and average 130.1×10-6 of∑REE,0.7ofδEu and 9.4of LaN /YbN.Heavy mineral assemblage of the sandstones includes mostly zircon,leucoxene,limonite and pyrite,next tourmaline,apatite,rutile,garnet,chromite and anatase.The source rocks of the Ayilihe Formation are mostly composed of acidic igneous rock,lowermedium metamorphism rocks and sedimentary rocks,and a little basic rock.The provenance of the sandstones had tectonic settings of mostly subduction complex,next arc orogen and collision orogen,based on the QFL discriminant diagrams.According to the REE,trace elements,and Th-Co-Zr/10,Th-Sc-Zr/ 10,Ti/Zr-La/Sc discriminant diagrams,the provenance of detrital rocks had the tectonic settings of continental island arc and active continental margin,and with some femic constituents compared with the source region of the upper crust.The chemical index of alteration corrected(CIAcorr.)of the detrital rocks has an average of 71.25(52.25~89.40),which indicates that provenance had mostly gone moderate chemical weathering.According to the U-Pb ages of LA-ICP-MS of 79detrital zircon grains,the source rocks of Upper Carboniferous detrital rocks came from terrains with two different geological times,most 404~542Ma,second 1797~2436Ma,which indicates that source rocks came mostly from Early Paleozoic and paleo-Proterozoic.They are equivalent to magma arc of Early Paleozoic subduction zone in the northern of South Tianshan,and southeastern Proterozoic basement of Kulukedage in the northern of Tarim.Hence,it also indicates that the collision time of South Tianshan Orogen may be limited in the end of Early Devonian to Middle Devonian,subduction and converge happened during Early Paleozoic,and large-scale uplifting and exhumation happened after Permian.