Petrography and geochemistry of the volcanic rocks in Changwat Uthai Thani and Nakhon Sawan, Central Thailand

Abstract

Volcanic and plutonic associated rocks are widespread throughout the western part of Changwat Uthai Thani and Changwat Nakhon Sawan. They occur as stocks, dykes, lava flows, and explosive ejectas. Field as well as petrography and geochemistry evidences suggest that the plutonic rocks in the area are shallow emplaced and are genetically related to the capped volcanic rocks. In addition all magmatic activities are deduced to have taken place during the period of post-Permian and Middle Triassic ages. The compositions of these two different modes of origin of rocks are ranging from intermediate to silicic. Diorite and andesite are the intermediate rocks and granite, rhyolite, and rhyolitic tuff are the silicic ones. Geochemical data and their variation diagrams indicate that these plutonic and volcanic rocks belong to the same differentiation trend of calc-alkaline series. Diorite and/or andesite exposed in the study area though are apparently too small in volume when compared with the silicic rocks, they are believed to be the parental magma which subsequently other intermediate and silicic rocks, either plutonic or volcanic, were derived by process of magmatic differentiation. Bunopas (1981) has suggested that along the western edge of Khorat plateau (eastern belt) and central Chao Phraya plain (western belt) which plutonic and volcanic rocks of the present study belong to, were actually former volcanic arcs with subducted plates underneath. Therefore, the primary intermediate magmas in the area concerned are having higher tendency to be derived from partial melting of hydrous mantle material (Mysen and Boettcher, 1975 a,b) or subducted oceanic floor (Green and Ringwood, 1968) rather than from differentiation of basaltic magma (Brown, 1928; Osborne, 1959, 1969; Eggler, 1972; Holloway and Bernham, 1972; Nicholls and Ringwood, 1973; Allen et a., 1975; Cowthorn and O’Hara,1976). Abundance of intermediate and silicic rocks of calc-alkaline series of Permian-Triassic ages also suggests that igneous activities of both the eastern and western belts develop in the regions of active continental margins where the crust is more than 30 km. thick (Miyashiro, 1975 : Green, 1982).