Isotope geochronology in the Indonesian Tin Belt

Abstract

Granitic rocks on Belitung (Billiton), Bangka and the Tuju islands (Pulau Tudjuh) have a Rb-Sr isochron age of 217 ± 5 Ma with initial 87/Sr86Sr = 0.7152 ± 0.0029 (15 whole-rocks and 4 biotites). K-Ar ages of four hornblendes and two biotites average 214 ± 4 Ma. The granitic complex on Karimata has a Rb-Sr isochron age of 74 ± 2 Ma with initial 87Sr/86Sr = 0.7101 ± 0.0025 (8 whole-rocks), while an associated amphibolite has a K-Ar age of 78 ± 5 Ma. (Rb-Sr ages based upon d = 1.39 x 10-11/a; errors at 95% confidence level.) The granites have intruded into flysch-type sediments containing Norian fossils, while the regional stratigraphic and tectonic relationships strongly suggest that they are overlain by fossiliferous sediments of probably Rhaetian age. The age of 217 ± 5 Ma can thus be taken as the minimum age of the Norian, probably as representing the Norian/Rhaetian boundary. Cassiterite mineralisation is associated with both Upper Cretaceous and Upper Triassic granitic masses, but major tin deposits are related only to Upper Triassic plutons.