U-Pb-Hf isotope compositions of detrital zircons from Tongtian River sediments of northern-central Tibetan Plateau: Implications for the closure of the Jinshajiang Ocean

Abstract

The closure time of the Jinshajiang Ocean (a branch ocean of the Paleo-Tethys) has been widely debated due to the complex structure and strong deformation of the northern-central Tibetan Plateau. In this paper, the U-Pb-Hf isotope compositions of detrital zircons from Tongtian River sand sediments from the northern-central Tibetan Plateau were analyzed via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The UPb ages of the detrital zircons from Tongtian River sand sediments were distributed in six major groups: 2685–2346 Ma, 2035–1676 Ma, 1220–580 Ma, 544–407 Ma, 297–157 Ma, and 46–20 Ma. The ε_Hf(t) values (range from −30 to +19) of the detrital zircons varied within a wide range from negative to positive, indicating that the zircons were sourced from diverse host magmas. The 46–20 Ma zircons and their Hf isotope compositions reveal that the far-field effect of the Cenozoic India-Eurasia plate collision triggered reworking of the Neoproterozoic basement of the North Qiangtang terrane. The U-Pb-Hf isotope compositions of these detrital zircons indicate that the Tongtian River sand sediments are composed of a mixture of materials from the Bayan Har-Songpan Ganzi terrane and the North Qiangtang terrane, with a contribution ratio of 3:7. The UPb ages and Hf isotope compositions of the zircons from the Tongtian River sediments and regional igneous rocks suggest that the ε_Hf(t) values of the Permian zircons are predominantly positive while those of the Triassic, especially the Late Triassic, are negative. This marked shift most likely indicates that the closure of the Jinshajiang Ocean occurred by at least the Late Triassic.