Petrological study on Tertiary volcanic rocks from northeastern Shikoku

Abstract

Calc-alkaline basalt-rhyolite suite and high-alumina basalt were erupted in northeastern Shikoku Island, which consists of a part of ‘Setouchi petrographic province’ during Miocene to Pliocene in age. The volcanic activity of this area can be divided into the following four stages: 1) rhyolitic to biotite andesitic pyroclastic deposites characterized by the presence of garnets, 2) hornblende bearing andesitic lavas and their pyroclastics, 3) calc-alkaline basalts and andesites forming small domes or buttes, and 4) rhyolite to biotite dacite, andesite and high-alumina basalt flows and their pyroclastics. The first three erupted in upper Miocene and the last erupted in Pliocene time. Some porphyritic volcanics are characterized by high MgO and low SiO2 and Al2O3 content. However, calculated groundmass compositions as well as aphyric andesites reveal the nature of the normal calc-alkaline series of Japan. There are differences of mafic mineral assemblage between calc-alkaline basalt and high-alumina basalt, but no distinct chemical difference. It may be suggested that both basalts were derived from the same kind of magma, and that the mineralogical differences were related to magmatic temperature.