Tracing provenances of detrital zircons in the South Tianshan and northern Tarim Craton : constraints on Paleozoic-Mesozoic evolution of the southern Central Asian Orogenic Belt

Abstract

The South Tianshan (STS) Orogenic Belt represents a suture zone resulting from the collision of the Tarim Craton with the southern Central Asian Orogenic Belt (CAOB). Controversy has long surrounded the timing and process during the development and consumption of the intervening oceanic domain, namely the STS Ocean, and the resultant continental collision throughout Paleozoic-Mesozoic time. This project attempts to address these issues by integrating detrital zircon provenance analysis with preexisting geological and geochronological data. A further extension of this research suggests a spatial and temporal coupling between accretion-collision tectonics along the periphery of the Tarim and North China cratons and the assembly and dispersal of northeastern Gondwana.

Zircon U-Pb-Hf data indicate that Silurian-Early Devonian strata in the northwestern Tarim region have detrital zircon age spectra and εHf(t) values matching those of magmatic rocks in the Tarim Craton and Central Tianshan (CTS) Block. Abundant 500-400 Ma detrital zircons in these strata were probably shed from a magmatic arc built upon the once-united CTS Block and northern Tarim margin. The separation between them most likely occurred in the Early Devonian, as implied by the contrast between the absence of <400 Ma detrital zircons in Carboniferous strata in the northern Tarim region and intensive 400-300 Ma magmatism in the CTS Block. A new model highlights a mid-Paleozoic switch from advancing to retreating accretionary orogenesis along the southwestern Paleo-Asian Ocean, causing the back-arc opening of the STS Ocean.

Detrital zircon provenance analysis suggests a Paleozoic connection between the main body of the STS belt and the Tarim Craton, and a northward subduction of the STS oceanic lithosphere before the Tarim and CTS-Yili collision. The collision probably occurred in the late Carboniferous based on a synthesis of magmatic, metamorphic, and zircon εHf(t) data. However, the foreland basin in the STS and northern Tarim regions was starved of orogenic detritus throughout late Carboniferous-Permian time, implying insignificant surface uplifting and crustal shortening. This study proposes a plume-modified orogenic model to reconcile current data and observations. Significant surface uplift of the western Tianshan was most likely initiated at the mid-Triassic, according to detrital zircon U-Pb-Hf data of Mesozoic strata, regional stratigraphy and tectonothermal events. The uplifting possibly reflects an intracontinental orogeny related to the far-field effects of the Qiangtang-Tarim collision.

By combining zircon U-Pb-Hf data and geological evidence, this study shows that the mid-Paleozoic (~400 Ma) switching accretionary tectonics along the northern Tarim margin most likely also occurred near the northern margin of the North China Craton, and that relevant oceanic subduction in both regions commenced at ~500 Ma. The subduction initiation corresponded both to the ~500 Ma continental collision events along the southern margins of the two cratons, and to the final assembly of Gondwana; the mid-Paleozoic switching tectonics coincided with the opening of the Paleo-Tethys Ocean. This study correlates these events in an innovative way to link the accretionary processes along the southern CAOB to the Paleozoic evolution of Gondwana.