Discussion on magmatism and tectonics in SE Asia

Abstract

Dr D. Stephenson drew attention to work recently completed by an IGS team on late-Tertiary and Quaternary volcanoes in Burma, which strongly suggests the presence of an eastward-dipping subduction zone beneath continental crust throughout Central Burma. Three distinct N-S trending volcanic lines may be recognized, both spatially and in petrographic and chemical trends. The volcanics in general are markedly potassic with both K 2 O/SiO 2 and (Na 2 O + K 2 O)/SiO 2 increasing from W to E. There thus exists a ‘classical’ progression from ‘calc-alkaline’ characteristics in the western line (Narcondam and Barren Islands) to ‘alkaline’ in the eastern line (along the Sagaing-Hninzee fault system). This evidence gives strong support to the model, proposed by Beckinsale et al., of a subduction origin for the Tertiary ‘western peralkaline granites’ of Burma and Thailand. It seems reasonable to suggest that this subduction zone is a northern continuation of the one presently active beneath Sumatra, where the younger volcanics have petrographic and geochemical similarities with those in Burma of comparable age. Complications do arise in this model owing to the presence of the NE-SW trending Andaman Sea spreading ridge between Burma and Sumatra. Transform faults which cut this ridge probably continue northwards into Burma as active, dextral, transcurrent faults, some of which have acted as channels for magmatic activity. D r H. G. R eading said that: Listening to several of the talks and the subsequent discussions, especially about the Andaman Sea, Sumatra, the Andes and the Hercynian fold belt, perhaps the solution to many problems may be reached if