Volcanic stratigraphy and compositional trend of the pre-caldera lavas from Aso volcano, Japan: Contrasting volcanism between pre-caldera and caldera-forming stages

Aso caldera, located in central Kyushu island, Japan, was formed by four large-scale pyroclastic eruptions, i.e. Aso-1 (270 Ka) to Aso-4 (90 Ka). Pre-caldera volcanism (800-400 Ka) are effusive, however, its detailed stratigraphy has not been studied yet. Landslides caused by 2016 Kumamoto Earthquake, and recovering constructions revealed many lava outcrops. We established the stratigraphy of the pre-caldera lavas west of caldera rim, and examined compositional change of magmas before and after the caldera formation. We found at least eleven lava flow units with maximum thickness of 15 m. The lavas are often interbedded with collapse sediments and tuff breccia, the latter of which is sometimes very thick. The whole sequence of lavas are overlain by welded tuff of caldera-forming eruptions. The oldest rocks (800-600 Ka) are found near the ridge, where the largest landslide and collapse of Aso bridge occurred, indicating the existence of an eroded old volcanic edifice.

Pre-caldera lavas of the studied area are almost all andesites (SiO₂ =52.6 to 61.0 wt.%) with three distinct petrographic rock types: (A) olivine basaltic andesite, (B) hornblende andesite, and (C) two pyroxene trachy-andesite. The oldest rocks of the area (eroded volcanic edifice) are type (A). The early lava flow units are type (B), followed by a series of units mostly type (A). Type (C) rock is found among the last products closest to the caldera-forming eruptions. Disequilibrium mineral assemblage of olivine and hornblende is sometimes observed in both type (A) and (B) lavas. Pre-caldera lavas show a narrow compositional range and form a general trend of rapid increase of SiO₂ against Fe/Mg (calc-alkaline character). In contrast, Aso-1 and Aso-2 magma (pyroxene andesite to pyroxene dacite) show a general trend of rapid increase of Fe/Mg against SiO₂ (tholeiitic character; Hunter, 1998). Comparison of magma composition between pre-caldera and caldera-forming stages indicates contrasting evolutional processes, resulting from different magma supply systems.