Late Permian ultrapotassic rhyolites in SE Thailand: evidence for a Palaeotethyan continental rift basin

Abstract

The volcanic rocks in the Chanthaburi zone are rarely reported, but are important in investigating the tectonic evolution of the Palaeotethyan Ocean in SE Thailand. Four rhyolitic samples from Ko Chang Island yield zircon ages of 258-254 Ma, confirming the presence of Late Permian volcanic rocks in SE Thailand. These rocks consist of Group 1 rhyolites and Group 2 rhyolitic ignimbrites and have high K2O contents of 4.92-7.10 wt% and A/CNK values of 1.10-1.69. They are enriched in the light rare earth elements, Rb, Th, U, Zr and Y and show negative anomalies of Ba, Sr, Nb, Ta and Ti, with clear Eu anomalies. Their whole-rock epsilon(Nd) (t) values range from -1.7 to -3.1. Zircon in situ epsilon(Hf) (t) and delta O-18 values range from 0.0 to +5.6 and from 8.2 to 9.6 parts per thousand, respectively. They belong to the group of peraluminous, ultrapotassic A-type rhyolites and were derived from partial melting of a mixed source of Mesoproterozoic metasedimentary rocks with a component of juvenile mafic crust. These ultrapotassic rhyolites formed in a continental rift setting in response to the rollback of a subducted Palaeotethyan oceanic slab beneath the Indochina Block. Combining these results with previous geological observations, we propose that there were some sporadically distributed continental rift basins along the Eastern Palaeotethyan domain during the Permian.