藏南林周盆地林子宗火山岩的时代、成因及其地质意义: 锆石U-Pb年龄和Hf同位素证据

In this study a dacitic breccia and a rhyolitic ignimbrite were collected from the bottom layers of the Dianzhong and Nianbo Formations, respectively, in the Linzhou basin, the typical locality of the Linzizong volcanic successions, for zircon U-Pb dating and Hf isotope analysis. The results indicate that the former erupted at 62.5 ± 1.1Ma and the latter at 56.4 ± 1.2Ma, implying the Linzizong volcanism to have started right after the K/T boundary. Igneous zircons from the Dianzhong and Nianbo samples yield ε_Hf(T) values of + 9.0 ∼ + 0.5 and + 11.0 ∼ + 4.8, respectively, suggesting involvement of the depleted mantle source in the magma generation. These ε_Hf(T) values are corresponding to those of igneous zircons from the coeval Gangdese batholith, thus allowing us to infer a cognate origin related to partial melting of the mantle wedge and associated magma underplating/remelting. However, inherited zircons with Carboniferous ages in the Dianzhong sample have ε_Hf(T) values from - 0.6 to - 3.2 and Mesoproterozoic model ages, similar to those of inherited zircons from the Nyainqentanglha granitoids, indicating an involvement of ancient crustal materials from the Lhasa terrane in the generation of these volcanical rocks. Based on all above and regional geology, we interpret the Linzizong volcanic successions in the Lhasa terrane, southern Tibet as the products of northward subduction of the Neo-Tethyan oceanic slab and its subsequent breakoff at the early stage of the India-Asia collision.