Sumatran granitoids and their relationship to southeast Asian terranes

Three subparallel, N-S-trending granitoid provinces occur in Southeast Asia: the Eastern Granitoid Province of peninsular Malaysia; the Central Granitoid Province, extending from northwestern Thailand to the western part of peninsular Malaysia and to the "Tin Islands" (Indonesia); and the Western Granitoid Province, in western Thailand and Burma. It has been asserted that each of these granitoid provinces is restricted to one of the Southeast Asian terranes defined using Gondwanan Palaeozoic stratigraphic relations and faunal distributions. Very little is known about possible continuations of these terranes in Sumatra, and here we use the "granite basement terrane" concept and the geochemical and isotopic compositions of Sumatran granitoids and fragmental rocks to extend the Southeast Asian terrane boundaries into Sumatra and investigate the nature of the Sumatran lithosphere. Granitoids west of the Semangko fault and within the basement of the Quaternary volcanic arc have low initial 87Sr 86Sr values (< 0.706) and I-type characteristics, similar to those of the young arc volcanics, and may therefore represent young post-Gondwanan Sumatran arc lithosphere. Many of these granitoids predate the volcanic arc. Granitoids in eastern Sumatra, east of the Semangko fault, including those of the "Tin Islands", of Bukit Batu close to Palembang, the Hatapang pluton and granodiorites and fragmental volcanics of the Lake Toba area, and possibly Sijunjung pluton in central Sumatra, all share high 87Sr 86Sr values and other S-type isotopic and compositional similarities, and seem to be related to the Central Granitoid Province S-type granitoids of the sibumasu terrane (= SIam-Burma-MAlaysia-SUmatra; Metcalfe, 1984). These similarities suggest that the granitoids define a "granite basement terrane" which may form the basement of the sibumasu terrane. They also imply that the magmatism of the Toba caldera was derived from the same crustal source. Granitoids that are geochemically and isotopically similar to those of the Western Province are not yet known in Sumatra, and granitoids similar to those of the Eastern Province are rare. The distribution of Sumatran granitoids suggests that the Semangko fault and the Sunda Strait may mark the southwesternmost and southeasternmost limits of the sibumasu terrane, and that the boundary between the Central and the Eastern Granite Provinces may run through the "Tin Islands". © 1995.