Petrogenesis of Permian–Triassic felsic igneous rocks along the Truong Son zone in northern Laos and their Paleotethyan assembly

Abstract

Felsic igneous rocks in northern Laos are key elements of the Truong Son and Song Ma tectonic zones and are important for constraining the Paleotethyan evolution in southeastern Asia. This paper presents new geochronological, geochemical and Sr–Nd–Hf–O isotopic data from granitoids and rhyolites in northern Laos. Geochronological data show that these samples have zircon U-Pb ages ranging from ca. 281 Ma to 221 Ma. These samples can be subdivided into four groups based on their lithology and geochemical composition. Group 1 samples are granodiorite and granite with I-type geochemical characteristics. They have lower 87Sr/86Sr (i) ratios (0.7036–0.7059) and higher εNd (t) values (+0.7 − +0.8) than Group 2 biotite granite samples (87Sr/86Sr (i) = 0.7087–0.7129 and εNd (t) = −5.3 to −9.2) and Group 3 rhyolite samples (87Sr/86Sr (i) = 0.7092–0.7103 and εNd (t) = −5.9 to −8.9). Zircon grains from Group 1 samples yield positive εHf (t) values of +5.1 − +13.8, TDM ages of 0.37–0.71 Ga and δ18O values of 5.6–6.3‰. In contrast, zircons from Group 2 and Group 3 samples have mainly negative εHf (t) values (−9.6 to +1.6 and −4.7 to −14.7), older TDM ages (1.08–1.70 Ga and 1.42–1.97 Ga), and higher δ18O values (5.7–10.3‰ and 6.7–8.0‰), respectively. Our geochemical data suggest that the Group 1 granite samples are the products of the partial melting of newly underplated mafic rocks, whereas the Group 2 and 3 samples originated from a mixed source of “ancient” metaigneous and metagreywacke components. Group 4 samples include Late Triassic I-type granodiorite and granite and have εNd (t) = −1.9 to −3.9, 87Sr/86Sr (i) = 0.7079–0.7084, zircon εHf (t) = 0.0 − +7.5, and δ18O = 5.1–6.9‰, indicating a derivation from a metagreywacke source with a component of juvenile mafic crust. Our data suggest that the Permian felsic igneous rocks in northern Laos formed in an arc setting related to the Song Ma Paleotethyan branch/BAB subduction, whereas the Late Triassic I-type granitoids formed in a post-collisional setting in response to the gravitational collapse of thickened crust during the collision of the South China and Indochina blocks. The temporal-spatial pattern of igneous rocks along the Truong Son zone is related to the Paleotethyan evolution and resembles that along the Ailaoshan-Song Ma zone and its eastward extension in northern Hainan.