Bulletin of the Geological Society of Malaysia, Volume 12, Aug. 1980, pp. 79 – 86
Department of Geology, University of Malaya, Kuala Lumpur, Malaysia
Abstract: The alkali basaltic rocks of Kuantan range from alkali olivine basalt to hawaiite. They are nepheline to hypersthene normative and straddle the critical plane of silica undersaturation of the basalt tetrahedron. The nepheline normative rocks have lower FeO*/(FeO* + MgO) and higher normative An/(An + Ab). With increasing FeO*/(FeO* + MgO), the normative nepheline decreases in amount and then gives way to hypersthene. Apparently, the nepheline normative alkali basalt magma has fractionated towards the hypersthene normative compositions which implies that the Kuantan alkali basalts have evolved under conditions where the 01-Di-PI plane is not a thermal barrier. This is an uncommon type of differentiation and can be classified, following Miyashiro (1978), as straddle-B type. This evolutionary trend conforms well with the recent experimental results of Presnalt et. al. (1978) that suggest that alkali basalt magmas can yield tholeiitic residual liquids by fractional crystallization under 4 to 12 kb pressure.