Petrochemistry of the so-called Haneyama lavas

Abstract

So-called “Haneyama-lavas”, as named by T. Matsumoto, are widely distributed in central Kyushu, especially in Oita Prefecture. These lavas are mostly glassy and their texture shows various types, namely, spherulitic, fluidal and perlitic. Phenocrysts are plagioclase (An 27-45%) and sporadic mafic minerals. Their chemical compositions are shown in Table 1 and their normative compositions in Table 3. As compared with the average chemical compositions of Japanese volcanic rocks, the Haneyama lavas are rich in alkalies, and poor in CaO, FeO and MgO. Normative feldspars are rich in Or and Ab, and poor in An. Wo is always absent. Chemical compositions and norm of spherulitic rock and spherulite in Haneyama lavas are shown in Table 5. Compared with the host rock, the spherulite is rich in SiO2, FeO, CaO and especially alkalies and the composition lies near the eutectic line between quartz and alkali feldspar.