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 (similar to 91-30 Ma) intermediate-acid intrusive rocks in the Chanang-Zedong area immediately north of the Yarlung-Tsangpo suture zone. These rocks represent five magmatic episodes at similar to 91, similar to 77, similar to 62, similar to 48, and similar to 30 Ma, respectively. The 91-48 Ma rocks have slightly lower initial Sr-87/Sr-86 (0.7037 to 0.7047), and higher epsilon(Nd)(t) (+1.8 to +4.3) and epsilon(Hf)(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 similar to 30 Ma intrusive rocks. The similar to 91, similar to 62 and similar to 30 Ma rocks are geochemically similar to slab-derived adakites. The similar to 91 Ma Somka adakitic granodiorites were likely derived by partial melting of the subducting Neo-Tethyan oceanic crust with minor oceanic sediments, and the similar to 91 Ma Somka dioritic rocks with a geochemical affinity of adakitic magnesian andesites likely resulted from interactions between adakitic magmas and overlying mantle wedge peridotite. The similar to 77 Ma Luomu diorites were probably generated by partial melting of juvenile basaltic lower crust. The similar to 62 Ma Naika and Zedong adaldtic 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 similar to 48 Ma Lamda granites were possibly generated by melting of a juvenile basaltic crust. The younger (similar to 30 Ma) Chongmuda adakitic quartz monzonites and minor granodiorites were most probably derived by partial melting of Early Oligocene north-ward-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 30 Ma. It evolved through four stages: 100-65 Ma roll-back of subducted Neo-Tethyan oceanic lithosphere; 65-60 Ma initial collision between Indian and Asian continents; 60-40 Ma breakoff of subducted Neo-Tethyan oceanic lithosphere; and similar to 30 Ma northward subduction of the Indian continent (C) 2014 Elsevier B.V. All rights reserved.