Abstract:
Khao Tham volcanics is a part of Lam Narai volcanic field, located 7 kilometers north of Amphoe Srabot, Changwat Lopburi. This project is aimed to study petrography, geochemistry and mineral chemistry for classification and correlation of volcanic rocks in the study area. Based on petrographic characteristics, Khao Tham volcanic rocks can be classified into 6 types, i.e., rhyolites, vitrophyric rhyolites, rhyolitic tuff, andesites, perlitic pitchstones and devitrified perlites. Rhyolites consist of microcrystalline feldspar and tridymite with vitrophyric groundmass. Smaller amounts of microcrystalline feldspar and tridymite are found in vitrophyric rhyolites. Andesites consist mostly of microcrystalline feldspar and biotite. Rhyolitic tuff is composed mainly of vitrophyric ash groundmass. Perlitic pitchstones show perlitic cracks and vitrophyric texture. Spherulitic texture is found in devitrified perlites. Feldspar found in these rocks are classified based on mineral chemistry as sanidine. Geochemically, Khao Tham volcanic rocks contain similar major and minor oxides ranging within ranges of 68-78 wt% SiO2, 13-15 wt% Al2O3, 3-5 wt% K2O, 0.1-5 wt% Na2O. However, the lesser amounts of Fe2O3 and TiO2 of fine-grained rhyolites indicate that this rock type provides good quality feldspar appropriate for industry. In addition, mineral chemical analyses also indicate that Khao Tham volcanic rocks contain mainly alkali feldspar, particular Sanidine.
