Provenance and Tectonic Environment of Lower Devonian Apadalkan Formation in Southwest Tianshan

Luo Yanjun; Wang Bin; Wang Guocan; Chen Bo; Zhang Haidi; Ma Boyong

doi:10.3799/dqkx.2022.039

Abstract：The Late Paleozoic is the key period of ocean basin evolution in South Tianshan.By studying the Late Paleozoic sedimentary records,we can restore Late Paleozoic paleogeography and reveal tectonic evolution history of the South Tianshan.The Devonian strata are well preserved in the Biedieli area of Southwest Tianshan,as an important carrier for the study of the early Late Paleozoic tectonic evolution of the South Tianshan.In this paper,in order to reveal their source and geological significance,we study the geochemical characteristics,heavy mineral characteristics and geochronological data of sandstone in Lower Devonian Apadalkan Formation.The CIA and ICV of sandstone are 63.1 and 1.19,respectively,showing that the sandstone belongs to intermediate weathering and has experienced sorting and recycling.The heavy minerals are mainly zircon,apatite,rutile,etc.,and their sorting and roundness are poor.The ATi and ZTR index is 20.0-47.2 and 20-60,respectively,indicating the volcanic rocks in the source area of the Apadalkan Formation were abundant,and their transportation distance was relatively short.The geochemical provenance analysis diagram and heavy mineral characteristics indicate that the source rock is mainly felsic magmatic rocks,followed by recycled ancient sediment components.According to the LA-ICP-MS U-Pb ages of detrital zircon grains,the source rocks of Apadalkan Formation are derived from terrains with five different geological periods,mostly 407-486 Ma and 669-959 Ma,and subordinately 1 190-1 476 Ma,1 782-2 094 Ma and 2 300-3 660 Ma,indicating that source rocks are mostly derived from Early Paleozoic and Neoproterozoic,followed by Neoarchean,Paleoproterozoic and Mesoproterozoic.They are equivalent to magmatic event on the northern margin and the Precambrian basement of the Tarim.It also indicates that the materials of Apadalkan Formation are mainly derived from the northern margin of Tarim.The Tectonic discriminant diagrams show that the Apadalkan Formation is a product of an active continental margin environment.