K-rich Basalt in the Bukit Mersing area, Third Division, Sarawak

The basalts exposed near Bukit Mersing, Sarawak are early Eocene in age, and li e conformably over the highly folded , imbricated metasediments of th e Rajang Group. Initi ally thought to be ophiolite, it lies close to the "Tatau-Bukit Mersing Line" , which was thought to be a major thrust fault or terrane suture, containi ng obducted ocean floor material. The one analysis published in 1957 showed a K20 value of 2.82%, too high for ocean floor basalt produced at a spreadin g center. New analyses confirm that K contents in these rocks are hi gh. The K is hosted in K-feldspar rims around pl agiocla se phenocrysts. Based on the chemistry, and on field relation ships , the Tau Range basalt is not ophi o lit e, but is likely to be Oceanic Island basalt, developed over oceanic crust, caused by short-lived hot spot magmatism.