Geochronology, Geochemistry and Tectonic Settings of Granodiorite in Lalong Area, Namling, Tibet

The study on the tectono-magmatic mineralization of the Gangdese Indosinian is relatively deficient. This paper takes Lalong granodiorite masses located in the Gangdese Lunggar-Nyainqentanglha tectono-magmatic belt as the research object. The field geological surveying, petrology, LA-ICP-MS zircon U-Pb dating, geochemistry and Hf isotope data are systematically studied to discuss the petrogenesis and tectonic settings. It is found that the LA-ICP-MS zircon U-Pb ages show a weighted average age between 223.5±3.9 Ma and 225.4±4.9 Ma, indicating that they formed in Late Triassic epoch, and they are probably tectonic-magmatic products of late period of Indosinian. The rocks show low Ti(w(TiO₂)=0.46%-0.73%), rich in Si(w(SiO₂)=63.24%-67.94%), Al(w(Al₂O₃)=14.14%-15.85%), Na(w(Na₂O)=2.58%-3.04%) and K(w(K₂O)=3.06%-4.89%), belonging to the high-K calc-alkaline to shoshonite series and weakly peraluminous; enrichment in REE, and the ΣREE is 138.62×10^-6 to 255.53×10^-6. The rare earth element distribution pattern is the right tilt type of the light rare earth element enrichment, the (La/Yb)_N=8.35-17.90, with negative Eu anomaly (Eu/Eu^*=0.54-0.69) and without Ce anomaly (Ce/Ce^*=0.92-1.02). The rocks show depletion in Ba, Nb, Ta, Sr, Ti and P; enrichment in Rb, K, Th, U, Zr and Hf in the trace elements. The ε_Hf(t)=-6.4-2.5, coinciding with two-phase model age (t_DM2) of the crust of 980-1 479 Ma, and a same geologic age with the Nyainqentanglha Group. Lalong granodiorite masses belong to A-type granite. They are partially melted magmatic products induced by ancient lower crust material, and accompanied by a small amount of mantle material. They are formed by the local extension of the tectonic setting in the island arc environment formed by the ancient Tethys subduction.