Tectonic settings for emplacement of Southeast Asian tin granites

Abstract

Most tin-bearing granitic rocks in southeast asia occur in one of three main belts: an eastern belt largely of Late Cretaceous to Early Triassic age, a central belt with abundant Late Triassic granites, and a western belt with widespread Cretaceous to Early Eocene plutons. From the Paleozoic to Early Triassic, four main stratigraphic-structural zones are recognised: the Shan State - Northwest Thailand - Peninsular Burma and Thailand - West Malaya (Zone 1); the Eastern foothills of Malaya (Zone 2); medial Malaya and Central Thailand (Zone 3)and Eastern Malaya and east central Thailand (Zone 4). Lithology and deformation of theses Zones indicates that the eastern belt of tin-bearing granites was emplaced in continental crust of Zone 4, above an eastward-dipping Benioff zone, subducton took place beneath an outer arc (Zone 2) to the west of a volcanic arc (Zone 3 & 4). Middle to Late Triassic closure of the ocean basin between Zone 1 and Zones 2 and 3 was accompanied by continental collission resulting in 'Indosinian Orogeny' with emplacement of the central belt syn-collision late orogenic granites in Zones 1 and 2. The western belt of granites was emplaced in Zone 1 above an eastward-dipping Benioff Zone approaching the surface beneath or west of the IndoBurman Ranges, prior to Late Cenozoic northward movement of western Burma with respect to the Shan States. The abundance of tin in Southeast Asia is probably directly related to the forfuitous emplacement within continental crust of the three adjacent belts of subduction and collision-related magmatic rocks; there is little evidence of either a pre-Carboniferous concentration of metal in the crust, or of remobilisation of metal in the older mineralised granites by the younger granites.