Chapter 5 Granites

Abstract

Knowledge of the granites of Sumatra has been gathered mainly as the result of systematic mapping programmes conducted with the aim of identifying mineral resources and providing a geological data base for more detailed studies. Mapping programmes were conducted principally by Dutch and Indonesian geologists prior to the second world war, mainly in southern Sumatra and the Tin Islands. In the 1970s a combined Indonesian Directorate of Mineral Resources (DMR)/British Geological Survey (BGS) project was set up to map the geology of Sumatra to the north of the Equator. On completion of this project in the mid-1980s geological and geochemical maps for the region were published at the scale of 1:250 000, together with descriptive sheet bulletins. Another useful compilation which may be refered to is the 1:2.5 million scale geological map for the whole of the Indonesian Archipelago which includes Sumatra (Clarke 1990). Subsequently BGS undertook a similar but smaller project in southern Sumatra in order to upgrade geological mapping and mineral exploration programmes which were being conducted by the Indonesian Geological Research and Development Centre (GRDC) and DMR. As part of this programme a specific effort was made to investigate the granites of this region. A combined granite workshop/regional mapping programme resulted in the identification of many granite units within batholiths such as Lassi, Bungo and Garba, as well as numerous isolated plutons. Full geochemical and isotopic analyses were provided for these granites (McCourt & Cobbing 1993; McCourt et al . 1996). Gasparon & Varne (1995) have provided further