Early Paleozoic trondhjemite, gneissic granite and alkali granite in southern Yunnan preserve important records for the Prototethyan tectonic evolution and regional correlations. Zircon ages suggest that these granitoids were emplaced during 476–436 Ma. The trondhjemites are characterized by high Na2O and low K2O contents, with εNd (t) values of −1.9 to −3.5 and εHf (t) values of −2.8 to +3.9. The trondhjemites were derived from an amphibolite source with a juvenile mafic component. The gneissic granites belong to the metaluminous low-K calc-alkaline series with εNd (t) value of −6.2 and εHf (t) values of −5.0 to −0.4. The alkali granites belong to the high-K calc-alkaline series and yield εNd (t) values of −10.1 to −10.7 and εHf (t) values of −7.9 to −2.3. The gneissic granites were derived from an “ancient” lower mafic crust, whereas the alkali granites were derived from a meta-sedimentary source. These granitoids were formed during the subduction of Prototethyan Ocean beneath the Simao Block, and can compare with those from the Truong Son and Tam Ky-Phuoc Son zones in southern Laos. Our study, along with these Early Paleozoic igneous from southern Laos, central Vietnam and Malay Peninsula suggests an arc-back-arc system along the northern margin of Gondwana.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5322386

Volcanic rocks in the Chanthaburi zone are rarely reported and important for investigating the tectonic evolution of Paleotethyan Ocean in SE Thailand. Four rhyolitic samples from the Ko Chang Island yield zircon ages of 254–258 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 LREEs, Rb, Th, U, Zr and Y, and show negative anomalies of Ba, Sr, Nb, Ta and Ti with obvious Eu anomalies. Their whole-rock εNd (t) values range from −1.7 to −3.1. Zircon in-situ εHf (t) and δ18O values range from 0.0 to +5.6 and 8.2‰ to 9.6‰, respectively. They belong to 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 subducted Paleotethyan oceanic slab beneath the Indochina Block. Combining previous geological observations, we propose that there are some sporadically distributed continental rift basins along the Eastern Paleotethyan domain during the Permian.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5635390