Bulletin of the Geological Society of Malaysia, Volume 16, Dec. 1983, pp. 71 – 85
1Department of Geology, University of Malaya, Kuala Lumpur, Malaysia
2lnstitute of Geological Sciences, Exhibition Road, London SW7 2DE, U.K.
3Fakulti Sains, Jabatan Alam Sekitar, Universiti Pertanian Malaysia, Serdang, Malaysia.
4Geological Research and Development Centre, Jalan Diponegoro 57, Bandung, Indonesia.
*Present address: c/o Robertson Research International, Llanrhos, Gwynedd LL30 ISA, U.K.
Abstract: Dolerite dykes and basaltic lavas, which are closely associated spatially in the Kuantan area of west Malaysia, differ in petrology, age, and in palaeomagnetic direction and are thus not genetically related.
The dykes intrude granite of Late Permian to Early Triassic age. Compositionally they range from olivine tholeiite to quartz tholeiite and have evolved from an olivine tholeiite magma through low pressure fractionation. The K-Ar ages for the dykes range from 97 ±2 Ma to 129 ±2 Ma, with a mean age of 104± 10 Ma (Cretaceous: base Aptian to Middle Cenomanian) favoured as most likely.
25 of the 27 dykes sampled have palaeomagnetic vector directions in the same population, with a mean of D = 333, I = 40. This corresponds to a palaeomagnetic pole at 58°N, 52°E, and α95=5.6°. One of the dykes has reversed magnetic polarity.
The lavas, which have been extruded over the granite, cover about 125 km2, with a total thickness of 20-25 m. The lavas are compositionally different from the dykes and include alkali olivine basalts, basanite, and olivine nephelinite. Basanites and alkali olivine basalts are probably genetically related while olivine nephelinite seems to have an independent origin. K-Ar ages for the lavas range from 2.5 ±0.1 Ma to 1.2 ±0.1 Ma, with average age of 1.7 ±0.2 Ma. The three lava types cannot be separated in terms of age. Four sites in the basalt sampled for palaeomagnetic studies, gave a mean palaeomagnetic direction of D = 174, I = 10, indicating that the basalts were extruded during the Matuyama reversed magnetic epoch.