Association of Permian gabbro and granite in the Langshan, southern Central Asian Orogenic Belt: Age, origin, and tectonic implications

Abstract

The Permian magmatism in the Alxa region provides critical geological evidence to constrain the geodynamic processes in the southern Central Asian Orogenic Belt. The Shouji batholith is located in the southern Langshan, eastern Alxa region, and contains monzogranites, K-feldspar granites, and gabbros. New zircon UPb age data show that the Shouji batholith formed at 282–268 Ma. The Shouji granites have high (Na2O + K2O), Nb, and Zr contents, and high 10000∗Ga/Al, FeOT/(FeOT + MgO), Yb/Nb, and Yb/Ta ratios, similar to the values of A2-type granites. The Shouij granites exhibit enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE); depletion in Nb, Ti, and Sr; and enriched isotopic compositions. These Shouji granites were generated by partial melting of dehydrated tonalite–trondhjemite–granodiorite rocks of the Diebusige Complex in the lower crust. The Shouji gabbros are sodium-rich and low-K calc-alkaline rocks. They are enriched in LREE and LILE, depleted in high field strength elements, and have enriched isotopic compositions. These Shouji gabbros were generated by partial melting of a lithospheric mantle source metasomatized by melts derived from an ancient continental crust in an active continental margin. Roll-back of the Enger Us subducting slab accounts for generations of the Shouji A2-type granites and gabbros. The zircon UPb ages of Carboniferous–Permian magmatism show a younging trend toward the northwest in the southern Alxa region. Evidence from late Paleozoic magmatism and arc-derived sediments indicates that the southern Alxa switched from an active continental margin to an intra-oceanic arc during the early Carboniferous–Late Permian as a result of retreat of the subducting Enger Us slab.