Lithostratigraphy and geochemistry of Aojiki volcano and Sumiyoshiike and Yonemaru maars, Kamo Volcanic Field (Southern Kyushu), Japan

Abstract

The Kamo Volcanic Field (KVF), a partly preserved Quaternary volcanic sequence of three small-volume basaltic volcanoes located about 10 km northwest of the Aira caldera in the Kagoshima graben in southwestern Kyushu, Japan. It forms part of the volcanic arc of Japan and consists of the Late Pleistocene Aojiki volcano and two Holocene maars: Sumiyoshiike and Yonemaru. Here, we report the first results of a combined lithostratigraphic and petrogenetic study of these three volcanoes. Eruptive products of Aojiki volcano comprise an initial scoriaceous tephra, a lava flow and scoria fall from the cone. Sumiyoshiike and Yonemaru maar products are mainly scoria fall and pyroclastic surge deposits, which are closely related in time and were principally emplaced by phreatomagmatic explosive eruptions. Major and trace element data of the eruptive products suggest four groups of magmas. Aojiki scoria (G1: SiO2= 50.3-51.4) are the most evolved, followed by Aojiki tephra/lava (G2: SiO2 = 48.1-50.7), while Sumiyoshiike (G3: SiO2 = 47.0-48.5) and Yonemaru (G4: SiO2= 46.3-48.5) are relatively less evolved. These magmas are derived from the same spinel bearing (lithospheric) mantle source previously metasomatized by subducting fluids and might have evolved in independent magma chambers mainly by fractional crystallization and crustal contamination except for G1 magma which may have suffered some associated mixing.