Abstract
87/86Sr and 143/144Nd for late-Pliocene to Quaternary volcanic rock in Aso area in SW Japan, were determined to reveal temporal changes of the magma characteristics and a mechanism of magmatic system. Isotopic ratios of volcanic products at each period distributed in Sr-Nd isotopic diagram are as follows. 1) early pre-caldera period: separately distributed in two areas (component A: around (0.7040, 0.51285), component B: around (0.7044, 0.51270), 2) late pre-caldera period: distributed between component A and B along mantle array, 3) intercaldera period: distributed within the area of caldera forming period (excepting for ratios of Neko-dake), 4) post-caldera period: distributed between component A and B like that of late pre-caldera period. Source materials of volcanic products of each period are inferred from these results. Two components (A and B) of volcanic rocks in early pre-caldera period are related to source materials of volcanic products both in late pre-caldera and post-caldera period , or crustal material made this variation in these periods as contaminating to component A. Geochemical characteristics of post-caldera volcanic products are not same as that of inter-caldera and caldera period, and this implicates that magma process of post-caldera was changed.
