Constraints on the timing of opening and closing of the Tianshan Paleozoic oceanic basin:Evidence from Sinian and Carboniferous volcanic rocks

Sinian and Early Cambrian volcano -sedimentary series is distributed in various Precambrian mi-croblocks in the Tianshan Mountains and their neighboring areas.Phosphatic rocks are commonly developed at the bottom of the Cambrian and two or three roughly comparable tillite beds are observed in the Sini-an.This suggests that the various microblocks may have been the component parts of a unifying large con-tinental block.The Sinian and Early Cambrian continental extension-continental rift volcanism were the precursor of the opening of the Tianshan Paleozoic oceanic basin.It implies that the Early Cambrian is the lower limit of the opening stage of the Tianshan Paleozoic oceanic basin.The coarse clastic rocks at the bottom of Lower Carboniferous Ma'anqiao Formation in the Baluntay microblock of the central Tianshan Mountains are the geologic record of the collisional orogen.It implies that the paleozoic ocean basin of Tianshan had closed at that time.InCarboniferous ,the Tianshan orogenic belt entered the post-orogenic rift-extensional stage,accompanied by extensive continental rift bimodal volcanism and granitic magmatism.The Early Carboniferous is the upper limit of the closing stage of the Tianshan Paleozoic oceanic basin.