Two Types of mafic rocks in southern Tibet: A mark of tectonic setting change from Neo-Tethyan oceanic crust subduction to Indian continental crust subduction

[Display omitted] •The mantle source for Eocene mafic rocks was the asthenospheric mantle with the recycled oceanic crust.•The mantle source for Miocene mafic rocks was the (asthenospheric or lithospheric) mantle with continental crust.•Evolution from oceanic to continental crust subduction in southern Tibet was recorded in the Cenozoic mafic rocks. We collated existing data for the Eocene Langshan mafic rocks (Eocene mafic rocks) and the Miocene potassic–ultrapotassic mafic rocks (Miocene mafic rocks) in southern Tibet to investigate the tectonic transition from Neo-Tethyan oceanic crust subduction to Indian continental crust subduction. The Eocene mafic rocks have high Na2O contents (K2O/Na2O = 0.03–0.2) and show OIB-like trace element patterns (e.g., positive Nb and Ta anomalies) and depleted radiogenic Sr–Nd isotope compositions (87Sr/86Sr of apatite = 0.7031, εNd(t) = +5.1 to +6.1). In contrast, the Miocene mafic rocks have high K2O contents (K2O/Na2O = 1.9–8.5) and exhibit arc-like trace element patterns (enrichment in LILEs and depletion in HFSEs) and enriched radiogenic Sr–Nd isotope compositions (87Sr/86Sr = 0.7115–0.7362, εNd(t) = −16 to −12.4). The mantle source for the Eocene mafic rocks was generated by reactions between asthenospheric mantle wedge and felsic melts from subducted Neo-Tethyan oceanic crust (outside the field of rutile stability). In contrast, the mantle source of the Miocene mafic rocks was generated by reactions between asthenospheric (or lithospheric) mantle wedge and felsic melts from subducted Indian continental crust. Taking into account the regional tectonic evolution, we propose that break-off of the Neo-Tethyan oceanic slab and roll-back and/or break-off of the Indian continental slab were the most likely geodynamic mechanisms that led to the production of the Eocene and Miocene mafic rocks, respectively. Therefore, the transition from the Eocene to Miocene mafic rocks in southern Tibet provides an opportunity to understand the tectonic transition from Neo-Tethyan oceanic to Indian continental crust subduction.