Syncollisional tholeiitic magmatism induced by asthenosphere upwelling owing to slab detachment at the southern margin of the Central Asian Orogenic Belt
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The mafic–ultramafic complexes containing magmatic sulphides at the southern margins of the Central Asian Orogenic Belt have been recently proposed to result from an Early Permian mantle plume. However, in this study we show that the plume model cannot account for the observed geological characteristics of the Huangshan–Jingerquan mafic–ultramafic belt in the Northern Tianshan. Low K2O contents and positive correlation between TiO2 and (Fe2O3)T/MgO of the mafic–ultramafic complexes of this belt demonstrate a tholeiitic affinity. Enrichment of large ion lithophile elements and depletion of high field strength elements (in particular Nb and Ta) relative to mid-ocean ridge basalt indicate a subduction-modified mantle source. Lead isotope values and compositions of chromite indicate a significant contribution from the melting of asthenosphere. The absence of Late Carboniferous strata in the Huangshan–Jingerquan belt and Early Permian exhumation of blueschist and eclogite along the Aqikkuduk suture at the southern boundary of the belt indicate that an arc–continent collision occurred in the Late Carboniferous to Early Permian. We propose that the detachment of oceanic lithosphere from continental lithosphere during the collision induced asthenospheric upwelling, which resulted in melting of both the asthenosphere and the overlying metasomatized mantle wedge, and the formation of the mafic–ultramafic complexes with ages of 270–285 Ma along the Huangshan–Jingerquan belt.