Constraints of Late Triassic mafic-felsic volcanic rocks in northwestern Laos on the Eastern Paleotethyan post-collisional setting

Abstract

Early Mesozoic volcanic rocks in northwestern Laos are rarely reported and important for investigating the Paleotethyan tectonic evolution and regional correlation in SE Asia. In this study, New zircon ages and whole-rock geochemical data are presented for the newly identified volcanic rocks in northwestern Laos. Twelve volcanic rocks yield zircon U–Pb ages of 228–234 Ma. The volcanic rocks in northwestern Laos contain basalt, basaltic andesite, andesite, and rhyolite and can be geochemically subdivided into three groups. Both Group 1 and 2 samples belong to the calc-alkaline mafic-intermediate volcanic rocks. Group 1 has εNd (t) values of −0.52 to +1.20, εHf (t) values of +0.9 to +3.6 and δ18O values of 6.3–7.0‰. In contrast, Group 2 has lower εNd (t) values of −2.82 to −5.89 and εHf (t) values of −4.0 to +1.4 and higher δ18O values of 6.6–8.1‰ than Group 1. Group 3 samples are high-K calc-alkaline rhyolites with high εNd (t) (−0.08 to +0.74) and positive εHf (t) (+4.6 to +13.2) values. Geochemical characteristics suggest that the Group 1 and 2 samples were derived from a metasomatized lithospheric mantle modified by sediment-derived components, and Group 3 rhyolites were derived from partial melting of juvenile mafic rocks with a crustal component. Our results, along with the geological observations and reported ages suggest that the Late Triassic volcanic rocks in northwestern Laos were formed in a post-collisional setting, and provide the key studies for the connection between the Lancangjiang and Chiang Khong-Lampang-Tak zones.