Abstract

Andesite in Sa Thay area is gray to dark gray, displays a flow or block structure and porphyric texture with main phenocryst of plagioclase (andezine) accounting for about 15-40%, less amphibole and pyroxene. The matrix of rock has an andesite texture, interlaminar or microcrystalline structure, consisting of microcrystals of plagioclase (andesine), biotite, hornblende, pyroxene, and volcanic glass. Colored minerals are often chloritized and epidotized. Zircons separated from an andesite sample collected in Sa Thay area, Kon Tum province were analysed for their U-Pb isotope composition by LA-ICP-MS method. The analytical results of U-Pb isotope composition of zircon grains show that the crystallization age of andesite is 265.6±6.9 Ma. In addition, there are older age values concentrated in the range of 443-565 Ma, indicating contaminaton with materials of country rocks during the andesitic magma moved to higher level in the crust. This crustal material has an early Paleozoic age, this period is quite close to the magmatic activities of the Ordovician - Silurian period in the Kon Tum massif.