The Wulonggou metaluminous A2-type granites in the Eastern Kunlun Orogenic Belt, NW China: Rejuvenation of subduction-related felsic crust and implications for post-collision extension

The Wulonggou Pluton is located in Wulonggou area, eastern segment of the Eastern Kunlun Orogenic Belt, NW China, and consists of mainly alkali-feldspar granites covering an area of about 150 km2. Petrogenesis of these granitoids has been investigated through an integrated study of petrography, zircon UPb ages, whole-rock geochemistry, and HfNd isotopic compositions. UPb dating of magmatic zircons indicated these granites crystallized during 426-424 Ma in the middle Silurian. The granites display high SiO2 (75.26-77.55 wt%), K2O + Na2O (7.98-9.03 wt%), extremely low MgO (0.04-0.19 wt%), CaO (0.28-0.61 wt%), and TiO2 (0.05-0.09 wt%) contents showing metaluminous, calcic-alkali and ferroan features; enrichment in Rb and some HFSEs (Zr, U, Nb, Ta, and Y), depletion in Sr, Ba, P, and Ti, mostly right-inclined REE curve, flat HREE patterns, high 10,000 ∗ Ga/Al and intensively negative Eu anomalies, exhibiting an A2-type granite affinity with Y/Nb > 1.2 mostly. The primitive magma of these large quantities of granites was generated under a high temperature, low pressure, reduced and anhydrous environment indicating intense upwelling of asthenosphere. Combining with the positive uniform zircon εHf(t) values of −0.2 to +3.8 and decoupled εNd(t) values of −4.9 to −2.1 at t = 424 Ma, it can be concluded that subduction-related juvenile materials, probably calc-alkaline granitoids, are the source of these A-type granites. Geochemical studies of Wulonggou granites, spatial and temporal distributions of regional magmatism, metamorphism, and sedimentary records throughout the Eastern Kunlun Orogen Belt jointly indicate that the whole orogenic belt was in a typical post-collision extension setting and experienced an isostatic uplift during the middle Silurian triggered by delamination after the convergence of the northeastern margin of Gondwana.