Late Carboniferous collision between the Tarim and Kazakhstan–Yili terranes in the western segment of the South Tian Shan Orogen, Central Asia, and implications for the Northern Xinjiang, western China

The Tian Shan of Central Asia is located in the southwestern part of the Central Asian Orogenic Belt (CAOB, also known as the Central Asian Orogenic System or CAOS). Formation of the South Tian Shan Orogen is a diachronous, scissors-like process during the Paleozoic and its western segment in China–Kyrgyzstan contiguous regions is accepted as the site of the final collision zone between the Tarim craton to the south and the Kazakhstan–Yili terrane to the north in the Late Paleozoic. However, when the final collision occurred is still in hot debate. Particularly, an end-Permian to Triassic collisional model is recently proposed for the western segment of the South Tian Shan Orogen. This even leads to the speculation that the complicated accretion–collision processes in the Northern Xinjiang of western China, which involved the terrane amalgamation in the East and West Junggar and the collision between the Altai and Kazakhstan terranes and between the Yili–Central Tian Shan and Junggar terranes, were finally terminated during the end-Permian to mid-Triassic, rather than the Late Paleozoic as usually accepted. Obviously, the western segment of the South Tian Shan Orogen also presents the key issue associated with the termination time of accretion–collision processes in the Northern Xinjiang. A collisional model that is derived from the knowledge of the Himalayan Orogen is helpful for establishing a sequence of major tectonothermal events in the western segment of the South Tian Shan Orogen and constraining the time of collision between the Tarim craton and the Kazakhstan–Yili terrane. e western segment of the South Tian Shan Orogen, the end-Permian to Triassic collisional model is mainly based on Triassic zircon U–Pb ages of 234 to 226 Ma from the West Tian Shan eclogite and two suspected Late Permian radiolarian specimens Albaillella excelsa Ishiga, Kito and Imoto (?) from the Baleigong ophiolitic mélange. Actually, the poor preservation of the two radiolarian specimens and the lack of a ventral wing make their identifications difficult. Furthermore, the Baleigong ophiolitic mélange was intruded by one granite pluton with a zircon age of 273 Ma, and this provides geological evidence against the reliability of the Late Permian radiolarian specimens. Because the Triassic zircons contain no index mineral inclusions such as omphacite and coesite grown under high to ultrahigh pressure conditions, it is difficult to link their ages to high to ultrahigh pressure peak metamorphism. In addition, this model is not compatible with extensive Permian plutonism and molasse sedimentation and Triassic to Jurassic tectonomagmatic quiescence and continental deposits in the collisional zone and adjacent tectonic units. trast, new U–Pb ages of the zircon domains containing omphacite and phengite inclusions and Sm–Nd and rutile U–Pb ages of eclogite samples from the western segment of the South Tian Shan Orogen consistently indicate that high pressure peak metamorphism of subducted oceanic material occurred at ~ 319 Ma (the end of the Early Carboniferous). This and the youngest Early Carboniferous radiolarian and conodonts fossils from ophiolitic mélanges show that the collision must have taken place after the Early Carboniferous, whereas the oldest stitching granitic plutons in the collisional zone place an upper-age bound of ~ 300 Ma (the end of the Late Carboniferous) for the collision. These specify that the final collision in the western segment of the South Tian Shan took place in the Late Carboniferous rather than the end-Permian to Triassic. Noticeably, syn-collisional granitoids are rare, but Permian post-collisional plutonism and molasse sedimentation are widespread in the western segment of the South Tian Shan and adjacent tectonic units, and the oldest post-collisional plutons were nearly concurrent with low pressure, high temperature metamorphism in the south edge of the Kazakhstan–Yili terrane. All these suggest a significant geodynamic change at ~ 300 Ma, which may be caused by delamination of the thickened lithospheric root and asthenospheric upwelling. Such a process might have provided heat for low pressure, high temperature metamorphism and triggered partial melting of the lower crust and underlying lithosphere in the western segment of the South Tian Shan Orogen and adjacent tectonic units. The Late Carboniferous collisional model is also compatible with the Triassic to Jurassic tectonomagmatic quiescence and continental deposits in the western segment of the South Tian Shan Orogen and adjacent tectonic units. e South Tian Shan Orogen, the final collision in the western segment occurred in the Late Carboniferous, significantly younger than that in the eastern segment. In the Northern Xinjiang, the Late Carboniferous collision in the western segment of the South Tian Shan Orogen was nearly simultaneous with the final collision in the North Tian Shan collisional zone between the Yili–Central Tian Shan and Junggar terranes and in the Irtysh–Zaysan collisional zone between the Altai and Kazakhstan terranes, and these collisional events postdated the terrane amalgamation in the East and West Junggar. Therefore, the accretion–collision processes in the Northern Xinjiang were finally terminated during the Late Carboniferous rather than the end-Permian to mid-Triassic.