Abstract
Volcanic activity in the Aso area of central Kyushu, SW Japan, is divided into three stages based on caldera formation: a pre-caldera stage, a caldera-forming stage, and a post-caldera stage. We established the stratigraphy, and analyzed the whole-rock Sr and Nd isotopic compositions of volcanic products distributed in the NW wall of Aso Caldera to investigate the eruption style and magma-plumbing system of pre-caldera volcanism. Multiple andesitic lavas with minor dacitic lava make up a large part of the caldera wall. This eruption style of pre-caldera volcanism is difference from that of the caldera-forming stage, which is characterized by gigantic pyroclastic eruptions. The cause of this transition in eruption style from the pre-caldera to caldera-forming stages is probably a decrease in the rate of extensional crustal strain in the area. The whole-rock isotopic compositions of the pre-caldera volcanic products are largely similar to those of the caldera-forming volcanic products, indicating that the magmas of these volcanic products had a common origin. Previous petrological studies have suggested that the silicic magma of the caldera-forming stage originated from crustal melting; consequently, it is also considered that the magma of the Pre-caldera stage formed via this process. The pre-caldera volcanic products show a wide range of 87Sr/86Sr values (0.7042-0.7045), whereas the products of the caldera-forming stage show a lower and narrower range of values (0.7040-0.7042). This observation indicates isotopic heterogeneity of the crust and differences in the composition of the crust that melted during the pre-caldera and caldera-forming stages.
