Provenance of the southern Junggar Basin in the Jurassic: Evidence from detrital zircon geochronology and depositional environments

• Provenance of the southern Junggar Basin in the Jurassic.
• Combine detrital zircon U-Pb ages with depositional environments.
• Confirm a late Jurassic volcanic activity in the Tian Shan.

The goal of this paper is to study the provenance of the southern Junggar Basin during the late Triassic to early Cretaceous, based on the detrital U-Pb geochronology, petrography and depositional environments. Eight sandstone samples from the Upper Triassic to Lower Cretaceous were collected for detrital zircon U-Pb dating. A total of 794 effective U-Pb ages was obtained and divided into four groups: 488–2537 Ma (basement zircons), 328–482 Ma (subduction-related magmatic zircons), 254–322 Ma (post-collisional magmatic zircons), and 135–250 Ma (syndepositional magmatic zircons). These ages relate to three stages of basin evolution. (1) From the early to middle Jurassic, Tian Shan experienced continued exhumation, accompanied by progressive southward expansion of the Junggar Basin, and a peneplain was formed by the time Xishanyao Formation was deposited. Organic-rich sediments formed in a delta environment were well-developed in the southern Junggar Basin, with source rocks gradually switching from post-collisional volcanic rocks (295–307 Ma with a peak age of 300 Ma) of the southern North Tian Shan to post-collisional volcanic rocks of the Central Tian Shan (280–320 Ma with a peak age of 316 Ma) and then to subduction-related island arc rocks (402–423 Ma with a peak age of 415 Ma) of the Central Tian Shan. (2) During deposition of the Toutunhe and Qigu Formations, large scale volcanic activities occurred along the North Tian Shan Fault. Source rocks at this time include syndepositional volcanic rocks (151–161 Ma), and post-collisional volcanic rocks (290–320 Ma) of the North Tian Shan. By the time of deposition of the Kalazha Formation, Tian Shan experienced rapid tectonic uplift, leading to rapid lake regression. Alluvial fans were well developed in the southern Junggar Basin with source rocks being the underlying sedimentary strata of the north margin of the North Tian Shan. (3) During the early Cretaceous, exhumation of the Tian Shan and lake transgression in the Junggar Basin happened again. Shallow lake sediments were developed in the southern Junggar Basin with source rocks being subduction-related volcanic rocks (339–419 Ma with a peak age of 415 Ma) of the Central Tian Shan and post-collisional volcanic rocks (254–305 Ma with a peak age of 298 Ma) of the North Tian Shan.