Lead isotope variability of fine-grained river sediments in Tibetan Plateau water catchments: implications for geochemical provinces and crustal evolution
The crustal structure of the Tibetan Plateau records the dynamic processes of several terranes that underwent disaggregation, aggregation, and amalgamation. The dynamic evolution of continental crusts is best understood from these three processes. However, the detailed geochemical province division of the Tibetan Plateau and the palaeo-tectonic affinity or evolution of terranes remains unclear. In this paper, the acid-insoluble fraction of fine-grained river sediments from catchments in the whole Tibetan Plateau was measured for lead isotopes and trace elements. This study aims to reveal lead isotopic characteristics, to delineate different lead isotope geochemical provinces, and to trace and uncover the tectonic affinities of various terranes in the Tibetan Plateau. Results show that by weak acid chemical treatment, the characteristics of the Pb isotopes of fine-grained river sediments can be utilized to represent and discriminate geochemical provinces. The characteristics of Pb isotopes enable the identification of different geochemical provinces and the palaeo-affinity of various tectonic units. Constrained by tectonic evolution, the Tibetan Plateau tectonic units can be divided into the following five Pb isotope geochemical provinces: 1) Qilian Terrane; 2) Northern Tibetan Plateau geochemical province, including Eastern Kunlun-Qaidam, Songpan-Ganzi and Eastern Qiangtang Terrane; 3) Northern Lhasa Terrane; 4) Southern Lhasa Terrane, and 5) Himalaya Terrane. In relation to the controversy concerning the palaeo-affinity of the Qilian and Songpan-Ganzi Terranes, the Pb isotopic compositions of fine-grained river sediments suggest that they were much more likely separated from the Yangtze Craton than from the North China Craton. The characteristics of Pb isotopes and trace elements of the Eastern Kunlun-Qaidam and Eastern Qiangtang Terrane show some similarities with the Songpan-Ganzi Terrane, which indicate that they also possibly originated from the disaggregation of the Yangtze Craton. The various partial melting and contamination of the old Himalaya continental crust from radiogenic Pb isotopes and Tethys mantle materials with low radiogenic Pb isotopes caused different Pb isotopes and trace elemental associations in the Southern and Northern Lhasa Terranes. These factors also led to the evolution of the two different geochemical provinces.