A tectonic model for Cenozoic igneous activities in the eastern Indo¯Asian collision zone

Geochronologic dating and compilation of existing age data suggest that Cenozoic activities in the eastern Indo¯Asian collision zone of southeast China and Indochina occurred in two episodes, each with distinctive geochemical signatures, at 42¯24 Myr and 16¯0 Myr. The older rocks are localized along major strike¯slip faults such as the Red River fault system and erupted synchronously with transpression. The younger rocks are widely distributed in rift basins and are coeval with east¯west extension of Tibet and eastern Asia. Geochemical data suggest that the early igneous phase was generated by continental subduction while the late episode was caused by decompression melting of a metasomatically altered, depleted mantle. The magmatic gap between the two magmatic sequences represents an important geodynamic transition in the evolution of the eastern Indo¯Asian collision zone, from processes controlled mainly by crustal deformation to that largely dominated by mantle tectonics.