U–Pb age and Hf isotopic composition of detrital zircons from the modern river of the South Tianshan orogenic belt and their implications for Palaeozoic tectonic and crustal evolution

Abstract

The U-Pb and Lu-Hf isotopic research of detrital zircons was carried out in southern modern rivers of the South Tianshan (STS) orogenic belt to understand the accretionary processes and crustal evolution in this region. Detrital zircons from river sands yield two dominant age populations at 460-390 Ma and 310-260 Ma with several Precambrian age groups. Such age patterns imply that the significant magmatic events occurred during the Palaeozoic in the STS. Comparative studies on U-Pb age of the detrital zircons from northern and southern river sands in the STS belt provide constraints on the tectonic evolution of the STS belt and its surrounding blocks. Abundant 500-460 Ma detrital zircons in northern river sands probably reflect a bidirectional subduction of the Palaeozoic Central Tianshan Ocean under the Yili Block and the Central Tianshan Block. The 460-390 Ma zircon population in both northern and southern river sands likely implies a bidirectional subduction of the South Tianshan Ocean. A lack of the 380-320 Ma zircon grains in the southern river sands may indicates the northward subduction of the South Tianshan Ocean. A final collision between the Tarim Craton and the southwestern Central Asian Orogenic Belt possibly took place in the Late Carboniferous; the 310-260 Ma zircon population in both northern and southern river sands suggests syn-and post-collisional magmatism. Based on the Hf isotopic signatures in the detrital zircon and magmatic zircons, we propose new constraints on the Palaeozoic crustal evolution of the STS. The continent formation in the STS belt was characterised by reworking of the ancient crust in a compressional environment during the Ordovician (ca. 460-440 Ma). Abundant significant juvenile materials progressively added to the crust through time in an extensional tectonic regime since Late Ordovician to Devonian (ca. 440-390 Ma). The crustal formation is mainly dominated by reworking of the Precambrian basement rocks with limited contribution of juvenile components input into magma formation during Late Carboniferous to Early Permian (ca. 310-260 Ma).