Tectonic evolution of the Alxa region, NW China: from a Precambrian continental ribbon to a Paleozoic–early Triassic accretionary orogen

The Alxa tectonic belt is a key region for our understanding of the architecture and accretionary history of the southern Central Asian Orogenic Belt during the late Paleozoic–Triassic assembly of Pangaea. However, its origin and tectonic evolution have been debated for decades. Here, we use integrated geological, geochronological and isotopic data to uncover the complex history of the Alxa region from the Neoarchean to Triassic. The Precambrian proto-Alxa belt was a continental ribbon consisting of Archean–Paleoproterozoic amphibolite to granulite facies rocks and Meso- to Neoproterozoic gneiss–schist complexes, which underwent multiple tectonic cycles associated with the assembly of Nuna, Rodinia and Gondwana. From the Ordovician, subduction of the Palaeo-Asian Ocean generated calc-alkaline magmatic rocks with enriched Nd–Hf isotopic compositions in Andean-type continental arcs. Rollback of the subducting Palaeo-Asian Ocean in the Devonian induced regional extension, followed by opening of the Quagan Qulu back-arc basin in the late Carboniferous–early Permian, as evidenced by mantle-derived magmatism, shelf-type carbonate rocks and rifting-related volcanic–sedimentary sequences. The Permian archipelago scenario with multiple subduction zones was terminated by the Mid- to Late Triassic when the amalgamated intra-oceanic arcs collided with the Alxa continental arc. The accretion of the Alxa region illustrates the importance of continental ribbons in the construction of the Central Asian Orogenic Belt and sheds light on our understanding of the architecture of accretionary orogens.