Paleozoic magmatism and metamorphism in the Central Tianshan block revealed by U–Pb and Lu–Hf isotope studies of detrital zircons from the South Tianshan belt, NW China

Abstract

As a major Precambrian microcontinent in the southernmost Central Asian Orogenic Belt (CAOB), the Central Tianshan block (CTS) in the Chinese Tianshan is essential for understanding the final assembly of the southern CAOB. It experienced multistage Paleozoic magmatism and metamorphism, but the detailed processes are still controversial and far from being completely understood. This paper reports coupled U–Pb and Lu–Hf isotopic data of detrital zircons from late Paleozoic (meta-)sedimentary strata in the South Tianshan belt, which can provide new insight into deciphering the Paleozoic evolution of the eastern segment of the CTS block. Characterized by typical oscillatory zoning and high Th/U ratios (>0.2), detrital zircons in the Permian sedimentary samples yield dominant age populations at ca. 505–490Ma, 475–440Ma, 430–400Ma and 340–250Ma, pinpointing the development of four episodes of magmatism in the eastern CTS block. Particularly, Ordovician–Silurian (475–440Ma) zircons, possessing low negative εHf(t) values, predominate in sedimentary strata in and surrounding the CTS block, indicating that the 475–440Ma magmatic rocks probably constitute the main body of the CTS block. The origin of this (early Paleozoic) episode of magmatism was most likely related to the southward subduction of the Junggar Ocean beneath the CTS block. Carboniferous–Triassic (340–250Ma) zircons have dominantly positive εHf(t) values, probably derived from the post-collisional juvenile rocks in the CTS block. Combined with previous studies, our data suggest that the single source terrane for the sampled strata was the CTS block, which had been a topographic high providing substantial detritus to the surrounding areas at least since the Early Permian.In the metasedimentary sample, detrital zircons mostly show partially/fully recrystallized internal textures and low Th/U ratios (<0.2), probably sourced from the amphibolite- to granulite-facies metamorphosed rocks in the eastern CTS block. The youngest zircons suggest that the sampled metasedimentary strata were Carboniferous (341–305Ma) in age, rather than Neoproterozoic (or Early Sinian in Chinese literatures) as previously interpreted. Based on U–Pb–Hf isotopes and Ti-in-zircon thermometer of metamorphic zircons in this study, as well as previous investigations, we suggest that the eastern CTS block has experienced (a) a 400–370Ma collisional event, related to the closure of the eastern segment of the South Tianshan Ocean, and (b) a 360–340Ma extensional event, as a result of post-collisional mantle upwelling. Therefore, the final assembly of the Eastern Tianshan took place during Late Carboniferous time.