'I' type features in the so-called 'S' type Western Belt granites, Peninsular Malaysia

Azman A. Ghani (2000) 'I' type features in the so-called 'S' type Western Belt granites, Peninsular Malaysia. Bulletin of the Geological Society of Malaysia, 44. pp. 133-138. ISSN 2637-109X

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Abstract

The Western Belt granites of Peninsular Malaysia have been considered as constituting exclusively ‘S’ type granites. The ‘S’ type features in the granites are, (a) high initial ⁸⁷Sr/⁸⁶Sr isotope ratio, > 0.710, (b) low Na₂0 content, < 3.2% Na₂O in rocks with – 5% K₂O, (c) narrow range of felsic rock (Si0₂: 65.95 to 77.4%), (d) high K₂O/Na₂O ratio, 1.4-2.8 (S type: 0.9-3.2), (e) usually ilmenite-bearing and (f) contain pelitic or quartzose metasedimentary xenoliths. Despite the similarities above, the Western Belt granites also have some characteristics which are more characteristic of ‘I’ type than those of the ‘S’ type granites, namely (a) AI-rich minerals such as sillimanite and cordierite are absent except some andalusite, (b) occurrence of primary wedge sphene and pale green amphibole especially in the northern part of the batholith, (c) pinkish K-feldspar crystals (usually as phenocrysts) present, (d) occurrence of mafic, hornblende-bearing enclaves, (e) increasing ACNK value with Si0₂, (f) increasing peraluminosity towards the most differentiated rocks (‘S’ type granite: increasing peraluminosity towards the most mafic varieties) and (g) P₂0₅ vs Rb trend is similar to the ‘I’ type granite. The aspects of the Western Belt granites highlighted in this paper warrants more serious and in-depth study in order to understand the nature of the granite source rock(s) (cf. Chakraborty, 1994). They are clearly some differences between the Western Belt protolith compared to those of the southern Australian rocks.

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