Evolution of rhyolitic magma from Ata caldera: progressive melting model for chemical variation

In order to understand a magmatic evolution of caldera forming rhyolitic eruption, essential fragments from Ata caldera, which is one of the four gigantic Quaternary caldera in Kyushu Japan, have been analyzed on bulk rock and glass chemistry. Rhyolitic magmas formed Ata caldera have two different rock types: pyroxene dacite and hornblende rhyolite. Phenocrystic minerals of hornblende rhyolite consists horblende, plagioclase, and quartz. On the other hand, pyroxene dacite is plagioclase as a main phenocrystic mineral, accompanied by small amounts of fine grain pyroxenes. Concentration of LIL elements, such as K2O, Ba, Na2O, Rb, in both types are the same level, but concentration of HFS elements, such as Nb, Y, Zr and Zn, decrease with increasing SiO2.Geochemical contrast between them are also obvious in the diagram of REE abundance pattern. REE patterns of hornblende rhyolites have lower than basaltic rocks and are depleted in MREE. REE patterns of pyroxene dacite are equal or even lower than that of basalt and higher than hornblende dacite. The REE pasterns imply that hornblende, which has high distribution coefficients in MREE should be residual mineral for hornblende dacite and is not liquidus facies in pyroxene dacites. Rhyolitic magmas are produced by the dehydration melting reaction which can be drawn as following equation: Pl(1) + Bi + Qz + K-feld + Hbl to melt(1) + Pl(2) + Qz + Hbl + Opx. In this reaction, accessory minerals may not be involved due to their small solubility such low temperature. When the source region become hotter, then the reaction advance further. Above equation will change to following one: Pl(2) + Qz + Hbl +- Opx +- Mt to melt(2) + Pl(3) + Cpx + Opx + Mt Incompatible elements may increase in the melt due to breaking down of hornblende and dissolving accessory minerals in more higher condition, both minerals have high distribution coefficients for HFSE. Geochemical characteristics of essential fragments from Ata caldera arranged in the volcano stratigraphic sequences can be explained reasonably by dehydration melting process in the source region than fractionation and contamination in the magma chamber.