A Precambrian trondhjemite boulder in Palaeozoic mudstones of NW Malaya

Abstract

A boulder of plutonic igneous rock, petrologically perhaps best termed a trondhjemite, which occurs as an isolated megaclast in slumped mudstones of the Singa Formation (mainly Carboniferous) at Pulau Tepor off NW Malaya, has been dated by the potassium–argon method. Two feldspar fractions, thought to represent alkali feldspar and plagioclase (though both are highly sodic) yield ages of 1029±15 Ma and 575±10 Ma, respectively, while a whole-rock sample yields an age of 264±4 Ma. The highest age is taken to give a minimum age for the rock, and the lower ages to indicate differential argon loss. This is the first radiometrically proved Precambrian rock from the Malay Peninsula, and tends to reinforce earlier inferences of a Precambrian continent as a source area for Palaeozoic sediments in the region.