Permian tectonic switch of the southern Central Asian Orogenic Belt: Constraints from magmatism in the southern Alxa region, NW China

Late Paleozoic plutons are widely distributed in the Alxa region, southernmost Central Asian Orogenic Belt, and provided an important clue in constraint for the closing time of the Paleo-Asian Ocean (PAO). In this article, we present new zircon U-Pb ages and whole-rock geochemical data from the Permian Huoersen and Zongnaishan plutons in the southern Alxa region. The Huoersen gabbro (ca. 285 Ma) is enriched in large-ion lithophile elements and depleted in high-field strength elements, similar to the features of continental marginal arc. They were most likely generated by partial melting of depleted mantle that was modified by subduction metasomatic fluids. The Zongnaishan granites (ca. 267 Ma) show characteristics of I-type granites and were generated in a syn-collision setting. The Huoersen granites (ca. 259 Ma) are peralkaline and have positive Ɛ Nd(t) (+1.2 to +1.5) values, exhibiting A2-subtype granites affinities. They were formed by melting of lower crust in post-collision extension setting. Based on geodynamic mechanism, a three-stage evolution model is delineated: subduction, syn-collision to post-collisional extension for oceanic branch of the PAO during the Permian. In general, the rock assemblages indicate a tectonic switch from subduction to post-collision extension regimes and the final closure of the PAO.