Petrological study on the basaltic rocks from Nishisonogi peninsula, Nagasaki prefecture, Japan.

Abstract

Petrographical and chemical studies have been made on basaltic rocks of the northern part of the Nishisonogi peninsula, Nagasaki prefecture. In these basaltic rocks following three groups are distinguished that is (1) Tholeiitic basalts, (2) Calc-akali basalts or andesites and (3) Alkali basalts in ascending order. Most of their chemical compositions vary in SiO2 content from 47 to 53 weight percent. These basaltic rocks are slightly higher in FeO+Fe2O3, and slightly lower in TiO2 and P2O5 contents than basaltic rocks of the adjacent areas. High-alumina basalts and high-alkali tholeiitic basalts of (1) are present in the same geological horizon and are concidered to be correlative with each other. It is considered that calc-alkali basalts in lower volcanic rocks are derived from tholeiitic basalt magma, however calc-alkali basalts in upper volcanic rocks are derived from alkali basalt magma. The former is lower in K2O and in Na2O contents than the latter. Those two groups above mentioned are well distinguished in SiO2-K2O diagram.