Tectono-magmatic rejuvenation of the Qaidam craton, northern Tibet

Abstract

Ascertaining the enigmatic characteristics of the crystalline basement of the Qaidam basin, the deepest intracontinental basin yet located in the highest orogenic plateau with the thickest continental crust, has broad implications for the Cenozoic basin evolution, in particular, and intracontinental tectonics of Asia, in general. We carried out a comprehensive investigation on the ages and geochemical-isotopic characteristics of the Qaidam and Suganhu basement rocks obtained from drill-cores. Petrological observations and U-Pb zircon dating (LA-ICP-MS) of granitoid samples from drill cores indicate that Early Neoproterozoic, Early Paleozoic and Late Paleozoic-Mesozoic intrusions are largely represented, suggesting that the Qaidam and Suganhu blocks were involved into three tectono-magmatic episodes. Similar whole-rock geochemistry and Sr–Nd isotopic characteristics between the Early Neoproterozoic granitoids beneath the Qaidam basin and the contemporaneous basement rocks in the surrounding orogenic belts confirm the existence of pervasive Early Neoproterozoic metamorphism and magmatism along the present-day northern Tibetan Plateau. By comparing the trace and rare earth elements patterns as well as the Sr-Nd compositions of the Ordovician arc-related granitoids and Silurian-Devonian post-collisional granitoids beneath the Qaidam and Suganhu basins with the corresponding granitoids from the surrounding orogenic belts, we assert that the Proto-Tethys subduction events that were recorded within the Eastern Kunlun, Altyn Tagh and the Qilian Shan orogenic belts, were different. They generated distinctive subduction-related arc magmas, whereas the closure of the oceanic basins resulted in the amalgamation of the orogenic belts inducing subsequent extensive post-collisional magmatism within the Qaidam block. The widespread Permian and Triassic granitoids beneath the northwestern/northern Qaidam basin mainly involved a lower crust or a crust-mantle source including Mesoproterozoic crust. They demonstrate that the influence of the Paleo-Tethys evolution penetrated the Qaidam block northwards, reaching to the northern edges of the block. We conclude that the Qaidam block is not a craton as typical as the rigid Tarim block, but pertains to a tectono-magmatic rejuvenated craton, which is not mechanically-stronger than its surrounding regions. However, trapped between the lithospheric left lateral strike-slip Altyn Tagh fault system to the northwest and the Qilian Shan transpressional system to the northeast, the Qaidam block forms a relatively stable domain that escaped from significant Cenozoic deformation.