12th GEOSEA 2012, Bangkok, Thailand
Bimodal Cenozoic Volcanism in Central Sarawak: Hot Spots or Extension?
Nur Iskandar Taib
University of Malaya, Kuala Lumpur
ABSTRACT
Several prominent Cenozoic volcanic edifices are found in the Upper Rajang Valley, in central Sarawak, Borneo. The late Eocene Bukit Mersing volcanics are potassic basalts, erupted during the deposition of the Bawang Member of the Rajang Group. Later tectonism has caused folding of the Rajang, and the volcanic layers now lie on their side, forming a prominent ridge. It has not been successfully dated using radiometric methods as yet. REE are highly enriched (La is enriched 400-500% over EMORB), with the steep chondrite-normalized patterns associated with OIBs and rift basalts. There is no Eu anomaly, and normalized incompatible element plots reveal a slight depletion in HFSE (i.e. Ta, Nb, Ti and Y). Pliocene to Recent volcanism is present in several locations – Usun Apau, the Hose and Niewenhuis Mountains, Linau-Balui, and several others across the Malaysian-Indonesian border. These volcanics form large plateaus, and are bimodal, early dacite being followed by a later balsaltic phase. The Linau-Balui basalts are thought to be very recent, since they were erupted on existing river terraces. Preliminary 40Ar/39Ar geochronology work reveals far older ages than suspected – 4 m.a. for Usun Apau dacites (Cullen, personal communication), and 2-2.5 m.a. for Usun Apau and Linau-Balui basalts. REE profiles for both Usun Apau and Linau-Balui basalts again show steep, enriched patterns, but the enrichment is less than that of the Bukit Mersing basalts (La enriched 50-400% over EMORB), and no HFSE depletion is seen. Bimodal volcanism is often associated with rifting or extension, however it should also be possible wherever a stratified magma chamber has developed. In the case of Borneo, there is no evidence of extension or rifting at the present time, nor are there reports of signs of imminent volcanism, such as hot springs or microseismicity. However, the region is heavily forested and access is difficult, so such signs may have gone unnoticed. There may be a causal link between late Cenozoic volcanism in Borneo and that found along the eastern edge of Sundaland – in Thailand, Vietnam, Hainan and as far north as Korea. 60 BOOK OF ABSTRACTS