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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 Central Asian orogenic belt formed by accretion subsequent to the contraction of the paleo-Asian Ocean, and its southeastern segment terminated along the Solonker suture zone, amalgamating the Mongolian arcs and the North China craton by the end of the Early Triassic. Since typical regional collisional features are absent, its tectonic evolution remains speculative. An investigation into the variability of detrital zircon U-Pb ages and Hf ratios in Permian strata along a southeast-northwest transect from the Mongolian arcs to the North China craton reveals distinct differences: Northern basins carry a broad Mesoproterozoic to latest Precambrian age signature, and their sedimentary source terranes are of mixed juvenile to crustal magmatic origin. In contrast, southern basins contain detritus from the North China craton, and their sources are of dominantly crustal contaminated magmatic origin. Provenance analysis suggests that in the early Paleozoic (ca. 429 Ma), the paleo-Asian Ocean was consumed along the Uliastai arc and the North China craton, initiating the formation of the northern and southern accretionary orogens, respectively. By the end of the middle Carboniferous, the Mongolian arcs were consolidated after the accretion of the Uliastai arc. In the late Carboniferous (ca. 314 Ma), the Hegenshan back-arc basin opened, detaching the northern accretionary orogen. While subduction continued there, it may have temporarily ceased in the south after the accretion of a microcontinent (ca. 300 Ma). By the end of the Middle Permian (ca. 269 Ma), back-arc basin closure led to the formation and obduction of the suprasubduction zone Hegenshan ophiolite. Eventually, the paleo-Asian Ocean closed by double-sided subduction. Such accretionary wedge-wedge collision would not involve continental deep subduction, unlike other continental collisions. The early stages of the sequence likely resembled a Pacifi c-type scenario dominated by the subduction of oceanic lithosphere, including a Japanese Sea-type back-arc basin opening. The late stages involved the accretion of large-scale continental blocks, ultimately leading to fi nal ocean closure, which may be similar to the archipelago-type setting of present-day Southeast Asia.

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