Petrogenesis of Aoyougou High-Silica Adakite in the Western Sector of the North Qilian Mountain, NW China: Evidence for Decompression Melting of Oceanic Slab

A trondhjemitic body occurs in the Aoyougou area, the western part of the North Qilian Orogenic Belt. It resembles high-silica adakite that is geochemically characterized by high SiO2, Na2O (high Na/K), LILE, LREE, elevated Sr/Y (72~280), (La/Yb)N (15~28), positive Sr anomaly, low MgO, Ni, Cr, and depleted Nb, Ta, Ti. The zircon U-Pb SHRIMP analyses yield 438 ± 3 Ma, much younger than eclogite (460~490 Ma) from the North Qilian suture zone, suggesting that the formation of these adakitic granites is associated with the closure of the ancient Qilian Ocean. It exhibits ɛHf(t) value of 12.3 to 14.4 with young model Hf ages of 481 to 567 Ma. Considering the tectonic evolution of the North Qilian Orogen, this trondhjemitic pluton was most probably derived from slab melt induced by decompression melting of eclogite at the depth of ~ 60 km during exhumation and heating of the rising mantle after break-off of the continental lithosphere.