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Electron probe microanalyses of biotite and hornblende are presented for a variety of granitoids from Peninsular Malaysia and Sumatra. Most biotites from the Main Range have Fe/(Fe+Mn+Mg) mole ratios greater. than 0.62, with exceptions at Sg. Sok and the Cameron Highlands, which have !-type characteristics. The Eastern Belt granitoids commonly contain hornblende and biotite. The biotite shows a wide range ofFe/(Fe+Mn+Mg), with higher values from the known tin mineral­ ized areas of Jemaluang and Kuantan. Hornblendes and co-existing biotites generally exhibit equilibrium partitioning of Fe/Mg proving a magmatic origin for both. The total aluminium in the amphiboles prove an epizonal environment of emplacement for the Eastern Belt granitoids and an even higher sub-volcanic environment for theSE province. The known occurrences of amphibole in the Main Range are few (Sg. Sok in Kedah and Bk. Berapit in Perak). They are of actinolitic hornblende and exhibit a non­ equilibrium partitioning of Fe/Mg with the biotite. They are probably of hydrothermal origin. An unusually green coexisting biotite and hornblende in the Gunung Benom area of the Central Belt appears to represent a deeper level of emplacement than the Main Range or Eastern Belt. The biotite-hornblende assemblage is not in equilibrium and the biotite is unusually enriched in magnesium. A reconnaissance overview of the chemical variation of biotites and hom­ blendes of Malaysian and Sumatran granitoids is presented. The samples analysed in the present study are listed in Table 1. The majority of the samples are from Peninsular Malaysia, with three granitoids from Sumatra and one trondhjemite from the gabbro layer of the Sabah ophiolite included for compari­ son. Brief reference is also made to feldspar, chlorite and muscovite analysis made during the course of the study. All analyses were carried out on carbon­ coated polished thin sections in an electron probe microanalyser using a computer-controlled energy dispersive spectrometer for all elements except fluorine, for which a wavelength spectrometer was us.ed. The preliminary results of this study were presented at the GEOSEA IV Conference in Manila in November 1981, but were not compiled for publication in the proceedings. Liew (1983) carried out a considerable number of microprobe analyses of such minerals as feldspar, biotite, muscovite, tourmaline, rare garnet, hom-

Chemical variation of biotite and hornblende in some Malaysian and Sumatran granitoids