Northward Subduction and Late Carboniferous Soft Collision in the South Tianshan Orogenic Belt: Constraints from Detrital Zircon U-Pb-Hf-REE Geochemistry

The South Tianshan Orogenic Belt, as the southernmost part of the Central Asian Orogenic Belt (CAOB), records the final closure of the Paleo-Asian Ocean and the subsequent collision between the Tarim Craton and the Central Tianshan–Yili Block. However, the timing of this collision and the subduction polarity of the South Tianshan Ocean remain controversial. In this study, we present integrated detrital zircon U-Pb geochronology, Lu-Hf isotopes, and rare earth element (REE) geochemistry for Carboniferous to Permian sedimentary rocks from the northern Tarim Craton and the South Tianshan Orogenic Belt. Our results show that the detrital zircon age spectra are dominated by peaks at ~290 Ma, ~420 Ma, ~640–620 Ma, ~840–830 Ma, and ~1900–1800 Ma, with a notable scarcity of 380–310 Ma zircons. The εHf(t) values of Paleozoic zircons are predominantly negative, consistent with the magmatic records of the Tarim Craton but distinct from those of the Central Tianshan–Yili Block. These provenance signatures indicate that most of the Carboniferous–Permian sedimentary rocks were derived from the Tarim Craton, with only a minor mixed-source component (e.g., the Balikelike Formation silty limestone) involving the Central Tianshan–Yili Block. The absence of significant 380–310 Ma detrital zircons in the foreland strata, together with the passive margin affinity of the northern Tarim, supports northward subduction of the South Tianshan Ocean. A sharp decrease in detrital zircon rDz values and crystallization temperatures at ~310 Ma signals a tectonic transition from convergence to collision, constraining the final closure to the Late Carboniferous (~310 Ma). Furthermore, crustal thickness estimates based on zircon Eu anomalies remain stable at ~60 km across the subduction and collision stages, suggesting limited tectonic shortening and thus a soft collision.