Transition from oceanic to continental lithosphere subduction in southern Tibet: Evidence from the Late Cretaceous–Early Oligocene (~ 91–30 Ma) intrusive rocks in the Chanang–Zedong area, southern Gangdese

Little is known about the detailed processes associated with the transition from oceanic to continental lithosphere subduction in the Gangdese Belt of southern Tibet (GBST). Here, we report zircon U–Pb age, major and trace element and Sr–Nd–Hf isotopic data for Late Cretaceous–Early Oligocene (~91–30Ma) intermediate-acid intrusive rocks in the Chanang–Zedong area immediately north of the Yarlung–Tsangpo suture zone. These rocks represent five magmatic episodes at ~91, ~77, ~62, ~48, and ~30Ma, respectively. The 91–48Ma rocks have slightly lower initial 87Sr/86Sr (0.7037 to 0.7047), and higher εNd(t) (+1.8 to +4.3) and εHf(t) (+3.5 to +14.7) values in comparison with those (0.7057 to 0.7062, −3.3 to −2.5 and +2.2 to +6.6) of the ~30Ma intrusive rocks. The ~91, ~62 and ~30Ma rocks are geochemically similar to slab-derived adakites. The ~91Ma Somka adakitic granodiorites were likely derived by partial melting of the subducting Neo-Tethyan oceanic crust with minor oceanic sediments, and the ~91Ma Somka dioritic rocks with a geochemical affinity of adakitic magnesian andesites likely resulted from interactions between adakitic magmas and overlying mantle wedge peridotite. The ~77Ma Luomu diorites were probably generated by partial melting of juvenile basaltic lower crust. The ~62Ma Naika and Zedong adakitic diorites and granodiorites were likely generated mainly by partial melting of thickened juvenile mafic lower crust but the source region of the Zedong adakitic rocks also contained enriched components corresponding to Indian continental crust. The ~48Ma Lamda granites were possibly generated by melting of a juvenile basaltic crust. The younger (~30Ma) Chongmuda adakitic quartz monzonites and minor granodiorites were most probably derived by partial melting of Early Oligocene northward-subducted Indian lower crust beneath the southern Lhasa Block. Taking into account the regional tectonic and magmatic data, we suggest that the Gangdese Belt of southern Tibet (GBST) underwent a tectonodynamic transition from oceanic subduction to continental subduction between 100 and 30Ma. It evolved through four stages: 100–65Ma roll-back of subducted Neo-Tethyan oceanic lithosphere; 65–60Ma initial collision between Indian and Asian continents; 60–40Ma breakoff of subducted Neo-Tethyan oceanic lithosphere; and ~30Ma northward subduction of the Indian continent.