Contrasting chemical characteristics of granite and syenite from Perhentian islands, Peninsular Malaysia

Abstract

The Perhentian intrusion is a reversely zoned complex exposed over several islands off the coast of Peninsular Malaysia. The intrusion is made up of Perhentian Kecil syenite rimmed by more evolved Perhentian granite. The former consists of a variety of igneous rocks ranging in composition from syenitic to monzonitic and even gabbroic rocks whereas syenogranite dominated the latter pluton. Field relationships of the rocks suggest that the Perhentian granite is younger than Perhentian Kecil syenite. Both plutons show different trends in the Q-A-P classification. The essential minerals in the Perhentian Kecil syenite are K-feldspar, plagioclase, hornblende, augite, quartz, biotite, sphene, epidote, apatite, zircon and magnetite whereas K-feldspar, plagioclase, quartz, 'biotite, hornblende, allanite, zircon, epidote and opaque phase make up the Perhentian granite. As in the field and petrographic characteristics, geochemistry of both plutons also show a different behaviour. The differences are: 1. On a TAS diagram, the Perhentian Kecil syenite trend evolved towards the nepheline normative whereas the Perhentian granite rocks seems to evolved towards the quartz normative. 2. Both pluton show a different ACNK trend with Si02, thus the ACNK trend of the syenitic rocks increase whereas those from the granitic rock decrease with increasing Si02• 3. Plots of CaO and (Na20 vs ~O) vs Si02 emphasise the alkali calcic character of the syenitic rocks with alkali-lime index of 54.5, as well as very different character, in alkali term, of the Perhentian granite pluton in which the CaO and (N~O + ~O) curves do not intersect. This is due to the lower CaO and higher (N~O + ~O) contents of the granitic rocks which are constant over the Si02 ranges (71-75%). 4. Rocks from Perhentian Kecil syenite have high Sr, Ba and SrlY ratio compared to the Perhentian granite. All the Perhentian granite rocks plot below the line BalSr = 1 and can be considered as low Ba-Sr granite. 5. The Perhentian granite has low total REE (106-382) compared to the Perhentian Kecil syenite (224- 450). The granite also has more restricted LaJLuNratios (0.96-58.8) compared to the syenitic rock which has wider LaJL~ratios (30.7-218.5). Field, petrology and geochemical studies of the Perhentian rocks indicate that both Perhentian granite and Perhentian Kecil syenite are made up of individual batches of melt.