Miocene Volcanic Rocks Over the Area of Chenonja-bong and Siru-bong, Jinhae (1): Petrography and Petrochemical Characteristics

The Miocene andesite and basalt intruded into and/or extruded on the Cretaceous volcanic and granitic rocks over the area of Chenjabong and Sirubong in the vicinity of Jinhae, southern part of Kyongsang basin. The K-Ar ages of the younger volcanic rocks are from 16 Ma (Sirubong andesite) to 10 Ma (Cheonjabong basalt), which indicate the Miocene volcanism in the outer part of the Tertiary basin in the Korean peninsula. The volcanics are divided into Chenjabong andesite, Cheonjabong basaltic andesite, Sirubong andesite and Cheonjabong basalt. The Cheonjabong andesite is composed of phenocrysts of clinopyroxene and plagioclase () and groundmass with lath-like plagioclase () and glass. The Cheonjabong basaltic andesite is composed of plagioclase phenocryst () with plagioclase lath () and glass in groundmass. The Sirubong andesite is only consisted of plagiocalse lath () and glass with absence of phonocryst. The Cheonjabong basalt shows typical porphyritic texture with phenocrysts of olivine () and clinopyroxene. The groundmass of the Cheonjabong basalt is composed of microphenocrysts of olivine, clinopyroxene and plagioclase () and plagioclase laths () showing pillotaxitic and intergranular texture. The Cheonjabong andesite, Cheonjabong basaltic andesite, Sirubong andesite are belong to calc-alkialine but the Cheonjabong basalt is alkaline basalt. By tectonic discrimination diagrams the parental magmas of the volcanic rocks have occurred boundary.