Gabbroic rocks from the southern Malay Peninsula and their relation to similar rocks of other orogenic zones

Abstract

The Linden and Gombak gabbroic stocks outcrop as two discrete bodies in the southern tip of the Malay Peninsula. They may be referred to as batholithic gabbros. which occur as relatively minor components within orogenic belts in association with large. intermediate to acidic batholiths. The Linden stock is mainly of cumulate-textured olivine eucrite and allivalite. They are characterised by an entirely unzoned anorthite (An 92). The Gombak stock is predominantly of medium grained hypidiomorphic granular norite. which is invariably olivine-free and contains both augite and Ca-poor pyroxene (either primary orthopyroxene or inverted pigeonite). This is only the second reported occurrence of inverted pigeonite in batholithic gabbros. Systematic mineralogical. textural and petrochemical variations demonstrate that the gabbroic rocks of the Linden and Gombak stocks constitute a differentiated series derived from a single parental magma. Allivalite and olivine eucrite of the Linden stock represent crystal cumulates resulting from high level fractionation of a high-alumina. low-potash. basalt parental magma which subsequently crystallised apparently non-cumulate Gombak norite. Ca-poor pyroxene of the Linden-Gombak suite demonstrates a change in texture from ophitic in olivine-bearing Linden cumulates to granular in olivine-free Gombak norite. A similar textural change involving Ca-rich or Ca-poor pyroxene. on the cessation of olivine crystallisation. is apparently common amongst gabbroic associations and more important than previously considered. The Linden-Gombak gabbroic association and similar batholithic gabbro suites of other orogenic belts are interpreted as unroofed magma chambers representing the root zones of palaeo calc-alkaline volcanoes by virtue of the similarity between these gabbroic rocks and the cognate plutonic xenoliths of calc-alkaline volcanics.