Petrogenesis And Tectonic Implications Of Qiukesu Granite Pluton And Its Enclaves In The Western Kunlun Orogen Belt, NW China

The western Kunlun orogen occupies a key position along the tectonic junction between the Pan-Asian and Tethyan domains, reflecting Proto-and Palaeo-Tethys subduction and terrane collision during early Palaeozoic to early Mesozoic time. Geologic study about this region was restricted by the harsh climate, steep topography and poor traffic conditions. We present the first detailed zircon U-Pb chronology, major and trace element, and Sr-Nd-O-Hf isotope geochemistry of the Qiukesu pluton and its microgranular enclaves from this multiple orogenic belt.SHRIMP zircon U-Pb dating shows that the Qiukesu pluton was emplaced in the early Silurian (ca.435Ma). It consists of weakly peraluminous high-K calc-alkaline monzogranite and syenogranite, with initial Sr/Sr ratios of0.7131-0.7229, εNd(T) of-4.1to-5.7,δ18O of8.0-10.8%, and εHf(T)(in situ zircon) of-4.9. Elemental and isotopic data suggest that the granites formed by partial melting of lower-crustal granulitized metasedimentary and metaigneous Precambrian basement triggered by under plating of coeval mantle-derived enclave-forming intermediate magmas. Fractional crystallization of these purely crustal melts may explain the more felsic end-member granitic rocks, whereas such crustal melts plus additional input from coeval enclave-forming intermediate magma could account for the less felsic granites. The enclaves are inter mediate (SiO257.6-62.2wt.%) with high K2O (1.8-3.6wt.%). They have initial87Sr/86Sr ratios of0.7132-0.7226, εNd(T) of-5.0to-6.0,ε18O of 6.9-9.9%o, and εHf(T) in situ zircon) of-8.1. We interpret the enclave magmas as having been derived by partial melting of subduction-modified mantle in the P-T transition zone between the spinel and spinel-garnet stability fields. This mantle-derived intermediate magma was injected into cooler felsic magma, cool down and crystallized as these discrete enclaves.Our new data suggest that subduction of the Proto-Tethyan oceanic crust was continuous to the early Silurian (ca.435Ma); the final closure of the Proto-Tethys occurred in the middle Silurian.