TECTONIC EVOLUTION FROM OCEANIC SUBDUCTION TO CONTINENT-ARC COLLISION OF THE JUNGGAR OCEAN: CONSTRAINTS FROM U-PB DATING AND HF ISOTOPES OF DETRITAL ZIRCONS FROM THE NORTH TIANSHAN BELT

The Junggar ocean, once situated between the Central Tianshan and Junggar blocks, was part of the Paleo-Asian Ocean whose opening, expansion and final closure led to the development of the Central Asian Orogenic Belt between Eastern Europe-Siberia and Tarim-North China cratons. However, the formation and evolution of the Junggar ocean has not been well constrained. This paper reports LA-ICPMS U-Pb dating results and Lu-Hf isotope analysis of detrital zircons from four sandstones exposed north of the Central Tianshan block, which provide important insights into understanding evolution of the Junggar ocean. Most detrital zircons have oscillatory zoning and high Th/U ratios (>0.2), typical of igneous origin. The Paleozoic magmatic zircons yield U-Pb age peaks of ca. 258 Ma, 294 Ma, 311 Ma, 327 Ma, 338 Ma, 375 Ma, 440-455 Ma, 474 Ma and 502 Ma and are interpreted to be derived from a long-lived volcanic arc system developed by southward subduction of the Junggar ocean and subsequently collisional/post-collisional magmatism. The comparatively minor Precambrian zircons cluster at ca. 550 Ma, 680-765 Ma, 890 Ma, 970-990 Ma, 1160-1250 Ma, 1495-1550 Ma, 1690-1750 Ma, 1840-1970 Ma, 2440-2500 Ma and 2615-2700 Ma, which are, as a whole, comparable with those from the adjacent Beishan block and the Kuluketage region of the Tarim block. Therefore, we suggest a synchronous Precambrian evolution for these three regions. Most of the 300-430 Ma zircons show positive ε_Hf(t) values, while the >430 Ma zircons unexceptionally possess low negative ε_Hf(t) values, suggesting that the Junggar oceanic plate retreated at ca. 430 Ma, probably due to opening of the South Tianshan ocean between the Central Tianshan and the Tarim blocks, forming the subsequent juvenile crust in the North Tianshan belt. Some <300 Ma zircons are characterized by low negative ε_Hf(t) values, implying that subduction of the Junggar ocean ceased at ca. 300 Ma, followed by subsequent continent-arc collision and post-collisional bimodal magmas. It deserves mentioning that the 1300-1500 Ma (detrital) zircons and definitely contemporaneous magmatism have only been reported in the Central Tianshan block, but neither in the Beishan nor the Tarim blocks. Therefore, more investigations are needed to constrain the origin and evolution of the Central Tianshan block.