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A detrital zircon U-Pb and Lu-Hf isotopic study was carried out in the Middle Silurian to Late Carboniferous sedimentary strata of the northwestern Tarim Craton in order to understand accretionary processes in the southern part of the Central Asian Orogenic Belt. Detrital zircons from these strata yielded U-Pb ages clustering around 2.8-2.3 Ga, 2.0-1.7 Ga, 1.3-0.9 Ga, 880-600 Ma, and 500-400 Ma, with age populations and Hf isotopic signatures matching those of magmatic rocks in the Tarim Craton and the Central Tianshan Block. Abundant 500-400 Ma detrital zircons most likely reflect deposition in a retroarc foreland basin inboard of an Andean-type magmatic arc to the north, supporting the northern Tarim-Central Tianshan connection during early Paleozoic time. The absence of 380-310 Ma zircon population in the Carboniferous siliciclastic rocks suggests that the Central Tianshan Block may have been separated from the Tarim Craton in the Early Devonian, caused by the interarc/back-arc opening of the South Tianshan Ocean. We propose an accretionary orogenic model switching from advancing to retreating mode during Paleozoic time in the southwestern part of the Paleo-Asian Ocean. This transition most likely occurred coevally with the rifting of Southeast Asian blocks from the northeastern margin of Gondwana.

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 volcanicand 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

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