The First Geological Investigation of Mekong River Rapids, Loei Province: Implications for a New Geologic Map and Tectonics in the Thai-Laos Boundary

The Mekong River Rapids outcrop in Loei Province appeared in the middle part of the Mekong River, a site of national geoheritage at the boundary between Thailand and Laos PDR. These rapids are not systematically classified on the geological map, and no studies have researched the rocks around the rapids in detail. This work aims to study petrography and the rocks’ chemical composition to explain the origin and to specify the names by field observation, petrography, and geochemistry. According to petrographical analysis, the studied rocks are classified into four groups: (1) a microcrystalline volcanic rock (alkaline rhyolite and rhyolite), (2) a garnet-pyroxene skarn, (3) a biotite-hornblende monzogranite, and (4) a dolomitic limestone. Major elements classify the studied rocks as rhyolite and dacite, while trace elements group them into rhyodacite and andesite fields. Geochemical data indicate that volcanic rocks generated from calc-alkaline magma erupted at the volcanic arcs related to subduction activities. Therefore, this work can hypothesize the geological model as granitic rocks intruded into carbonate hosts and metamorphosed into the skarn zone due to contact metamorphism. Meanwhile, the chill margin of the granite pluton rapidly formed as fine-grained igneous rocks with a volcanic eruption, which appeared at the Mekong Rapids.