Tectonic evolution from subduction to arc-continent collision of the Junggar ocean: Constraints from U-Pb dating and Hf isotopes of detrital zircons from the North Tianshan belt, NW China
The Junggar ocean, once situated north (present coordinate) of the Yili and Central Tianshan blocks during early Paleozoic to late Carboniferous time, was a major southern branch of the Paleo–Asian Ocean, the opening, expansion, and final closure of which led to the development of the Central Asian Orogenic Belt between Eastern Europe–Siberia and Tarim–North China. However, the detailed evolution of the Junggar ocean has not been well constrained. This paper reports U-Pb and Lu-Hf isotopic data of detrital zircons from sandstones in the North Tianshan belt, which can provide new insights into understanding the Paleozoic development of the Junggar ocean. Most detrital zircons exhibit oscillatory zoning and high Th/U ratios, typical of igneous origin. The predominant Paleozoic zircons yield major age populations at ca. 294, 313–327, 338–375, 440–455, and 474–502 Ma and are interpreted to have been derived from the long-lived volcanic- and island-arc systems formed by the southward subduction of the Junggar ocean and subsequent collisional and postcollisional magmatism. The minor Precambrian zircons yield ages scattering at ca. 550, 680–765, 890, 970–990, 1160–1250, 1500, 1690–1750, 1840–1970, 2440–2500, and 2615–2700 Ma, which are nearly, but not fully, congruent with those from the adjacent Beishan and Kuluketage Precambrian terranes. Therefore, our results indicate that the Central Tianshan block was once part of the Tarim block during Precambrian time. Most of the 541–440 Ma zircons possess low negative eHf(t) values, while the <440–300 Ma zircons exhibit dominantly positive eHf(t) values, which can be linked to the Junggar oceanic slab rollback since ca. 440 Ma. This event, subsequently, gave rise to the opening of the South Tianshan back-arc basin/ocean between the Central Tianshan and Tarim blocks, exhumation of high-pressure granulites, and formation of a series of island arcs in the Junggar ocean. Combined with previous studies, we suggest that the Junggar ocean was probably closed at ca. 300 Ma in association with arc-continent collision, followed by postcollisional magmatism. It deserves mentioning that the ca. 0.5 and 1.4–1.5 Ga (detrital) zircons and contemporaneous magmatic rocks only occur in the Central Tianshan block, not in the Tarim block. Therefore, more detailed investigations are needed to better elucidate the origin and Precambrian evolution of the Central Tianshan block.