Final subduction-closure process of the Paleo-Asian Ocean in the middle segments of the southern Central Asian Orogenic Belt: Constraints from Permian volcanic rocks and late Triassic mafic dike swarms

Abstract

The final subduction-closure process of the Paleo-Asian Ocean (PAO) in the middle segments of the southern Central Asian Orogenic Belt (CAOB) is widely debated. Linear arc-related pyroclastic sedimentary rocks and subsequent mafic-alkaline intrusive rocks parallel to the orogenic belt provide valuable constraints on ocean closure. Linearly distributed volcanic samples from the northern margin of the Alxa Block have zircon UPb ages ranging from 272 to 271 Ma, displaying arc geochemical signatures and large ranges of variation (−14.4 to +6.4) of zircon Hf isotopic composition. The Honggueryulin mafic dikes in the Nuoergong–Langshan Belt (NLB) with zircon and apatite UPb dating of 232–231 Ma show a clear geochemical imprint of subduction components with enrichment of light rare earth elements and large-ion lithophile elements (e.g., K, Rb, and Ba) and depletion of high-field-strength elements (e.g., Nb, Ta). The whole-rock major and trace elements and Sr–Nd–Hf isotopic compositions suggest that the Late Triassic mafic dikes mainly originated from a lithospheric mantle that had been metasomatized by subduction-related fluids. Coupled with published data, our geochronological and geochemical data support an east–west-trending Late Triassic alkaline–mafic intrusive rock belt distributed along the northern margin of the Alxa Block–North China Craton, which was formed in a post-orogenic intraplate extensional setting. These pyroclastic sediments and mafic dikes in the NLB document the tectonic transition from the Permian subduction to the Late Triassic post-orogenic extension. Therefore, the final closure of the PAO in the middle segments of the southern CAOB is constrained to between the Early and Middle Triassic.