Origin of Early Cretaceous high-K calc-alkaline granitoids, western Tibet: implications for the evolution of the Tethys in NW China

The western Tibet Plateau comprises a series of crustal terranes that were successively accreted to the southern margin of Eurasia attending the transition of the Palaeo- to Neo-Tethys. We present the first detailed SHRIMP zircon U-Pb chronology, major and trace element, and Sr-Nd-Hf isotope geochemistry of three Cretaceous plutons (Kalaqigu, Aranbaotai, and Hongqilapu) from this area. SHRIMP zircon U-Pb dating shows that the three plutons were emplaced in the Early Cretaceous. The 114 Ma Kalaqigu pluton is composed of strongly peraluminous high-K calc-alkaline monzogranite and granodiorite. These rocks consist of plagioclase, K-feldspar, quartz, and biotite, and show variable Sr-Nd and Hf (in situ zircon) isotopic compositions [Sr-87/Sr-86 (T)=0.7098-0.7157, E-Nd (T)=-8.1 to -15.6, and E-Hf (T)=-10.2 to -19.8 (mean -14.8)]. Elemental and isotopic data suggest that the granitoids were generated by partial melting of metasedimentary basement in the normal to thickened lower-crust triggered by underplating or injection of hot mantle-derived magmas in a continental arc setting. The 110 Ma Aranbaotai pluton consists of strongly peraluminous high-K calc-alkaline two-mica monzogranites. These rocks are composed of K-feldspar, plagioclase, quartz, biotite, and muscovite, and also show variable Sr-Nd and Hf (in situ zircon) isotopic compositions [Sr-87/Sr-86 (T)=0.7075-0.7144, E-Nd (T)=-9.4 to -13.3, and E-Hf (T)=-5.4 to -16.4 (mean -10.0)]. Elemental and isotopic data suggest that the granites were formed by partial melting of subducted sediments at a depths of<40km to approximate to 40-50km in the collision (overthrust) zone during continental collision. The 102 Ma Hongqilapu pluton consists of metaluminous high-K calc-alkaline granodiorites. These rocks are composed of plagioclase+K-feldspar+quartz+amphibole+biotite +/- clinopyroxene, and show uniform Sr-Nd and Hf (in situ zircon) isotopic compositions [Sr-87/Sr-86 (T)=0.7078, E-Nd (T)=-7.6 to -8.0, and E-Hf (T)=-7.2 to -11.2 (mean -9.4)]. Elemental and isotopic data suggest that the granitoids were generated by crystal fractionation of basaltic magmas in a continental arc setting. Our new data suggest that at 114 Ma, the Karakorum terrane was still an active continental arc fuelled by the northword slab subduction of the Mesozoic Tethys; collision of the Kohistan terrane with the Karakorum terrane most likely occurred at 110 Ma, resulting in the closure of the Mesozoic Tethys; since 102 Ma, the southern Karakorum terrane hosted to a new continental arc coupled with the northward slab subduction of the Neo-Tethys.