Southeast Asia is a complex array of granitoid belts, predominantly of Mesozoic age. An eastern belt, forming the eastern third of the Malay Peninsula and extending to the Indonesian islands of Bangka and Billiton, represents an epizonal Andean-type calc-alkaline volcano-plutonic arc which was active from the Permian to the Late Triassic. The plutonic suite ranges from gabbro, through hornblende-biotite granodiorite, to granitic, and is of mixed S- and I-type. The plutons are ringed by aureoles of andalusite-cordierite hornfels. The tin and tungsten deposits are related to the ilmenite-series members. The arc has remained isostatically stable since the Late Triassic, as reflected in the concordant Rb:Sr isotope dates.
A narrow central belt of Permian and Triassic granitoids and metamorphic complexes with local Cretaceous granites along its margins separates the eastern belt from the Main Range. The eastern margin of the Main Range is a serpentine-marked suture zone with local, strongly gneissic granites.
The Main Range batholith is the most remarkable Sn-granite in the world. It is predominantly of Late Triassic age but includes granites of Permian age. It is strictly of granitic composition and entirely of the S- or ilmenite-series. The main or eastern part is of Mesozonal emplacement into greenschist facies metasediments with no thermal overprint, and consists mainly of biotite granite which commonly contains megacrysts of maximum microcline. Muscovite results only from greisenization in hydrothermally affected areas. The Main Range belt imperceptibly grades westward through Penang, Langkawi, and peninsular Thailand to higher level plutons, which are unsheared and surrounded by andalusite and cordierite-bearing aureoles.
The north Thailand granites are predominantly Triassic and appear to be higher level than the eastern parts of the Main Range. They have some of the highest initial 87Sr/86Sr ratios of granites in the world, commonly in the range 0.722 to 0.734, and undoubtedly are entirely of upper crustal origin. Both the Main Range and northern Thai granites are devoid of contemporaneous volcanic associations, and they have been interpreted as collision-related, resulting from the closure of the central marginal basin in the Late Triassic. This represented the major orogenic event leading to the cratonization of Sundaland. Faulting, uplift, and hydrothermal circulation led to highly discordant Rb:Sr and K:Ar radiometric dates, which suggests that the region has been isostatically unstable since the Triassic; it continues to be characterized by hot springs. The granites of the western belt of Phuket and Burma are high level and characterized by good thermal aureoles, but they are not associated with obvious contemporaneous volcanic rocks. The granites and associated pegmatite emplacements were fault controlled.
The Triassic granites generally, and some of the Cretaceous granites which occur in diverse localities, are associated with tin, tungsten, and antimony deposits, representing one of the world’s greatest metallogenic provinces. These elements are thought to be recycled from the continental infrastructure of Sundaland.