Petrogenesis of Middle Triassic andesite in Sayaburi area, Laos: Constraints from whole-rock geochemistry, zircon U-Pb geochronology, and Sr-Nd isotopes

Abstract

Despite the presence of a large area of andesite in the Sayaburi Province of Laos, it has received very little attention. Based on a combination of detailed field investigations, geochronology and geochemical analysis, this study aims to explore the geochemical, Sr-Nd isotopic, and source rock characteristics, as well as the genesis and tectonic setting of the andesite in this region. In the Sayaburi Province, the andesite zircon U-Pb age is (241.2±1.2) Ma. The andesite rock is classified in the metaluminous-weak peraluminous calc-alkaline series. The light rare-earth elements (LREEs) are enriched and characterized by clear fractionation, whereas heavy rare-earth elements (HREEs) are relatively depleted and have no signs of fractionation. The average δEu is 0.96 with weak-or-no Eu anomalies. It is enriched in large ion lithophile elements such as Rb and K, while depleted in high field-strength elements such as Nb, Ta, P and Ti. For andesites in the Sayaburi Province, the (87Rb/86Sr)t value ranges in 0.702849–0.704687, the eNd(t) value is between 3.53 and 4.77, the tDM(t) value ranges in 633–835 Ma, and the tDM2(t) ranges in 625–724 Ma. The results based on the synthesis of petrology, geochemistry, and regional tectonic background studies show that 1) the andesitic magma source in the study area is an enriched mantle, which is modified by subduction zone fluids; 2) the geotectonic background environment of the andesite in Sayaburi area is the continental island arc environment and related to the tectonic evolution of Jinghong-Nan-Uttaradit back-arc basin, which reflects that the magmatic source is enriched with a mantle wedge component modified by a subduction zone fluid (or melt).