Origin of anorthite and olivine megacrysts in island-arc tholeiites: petrological study of 1940 and 1962 ejecta from Miyake-jima volcano, Izu-Mariana arc

Although aphyric tholeiites were discharged from nearly the same fissures during 1940 and 1962 eruptions of Miyake-jima volcano, some of the 1940 rocks are characterized by the presence of anorthite (to 3 cm) and olivine (to 4 mm) megacrysts. We focus on the assemblage and composition of crystal-clots to discuss magmatic processes, because minerals in the same type of clots must have crystallized at the same time. Three types of clots are identified, megacryst (M), basaltic (B; 1190–1175°C) and andesitic (A; 1150–1080°C). The M-type crystal-clots are characterized by anorthite (An87–96) and olivine (Fo78–86) megacrysts. Major mafic minerals of the B-type and A-type crystal-clots are olivine-clinopyroxene without magnetite, and pyroxenes with magnetite, respectively. The 1940 megacryst-bearing rocks contain all the three types of clots, whereas 1940 megacryst-free rocks contain only A-type and B-type. However, megacrysts and M-type clots show petrographic features such as kink-bands of olivine, spherical olivines in anorthite, and wide homogeneous cores of anorthite, suggesting that these minerals may not be comagmatic phenocrysts but xenocrysts. Chemical compositions and crystal size distribution plots of these crystals are identical to those of plutonic xenoliths erupted from one of the 1940 fissure. Thus, we concluded that megacrysts and M-type clots are xenocrysts derived from the same sources as plutonic xenoliths. We consider that the magma plumbing system of the 1940 eruption consists of a deeper basaltic magma storage system and a shallower andesitic one. The plutonic body might be located under the northeastern part of the volcano between two magma storage systems. The basaltic magma captured plutonic xenoliths, and then mixed with the andesitic magma during the 1940 eruption. Whether erupted rocks contain xenocrysts or not must depend on the spatial relationship between ascending basaltic dykes and location of the deep-seated plutonic body. On the other hand, the 1962 ejecta contain only A-type crystal-clots, indicating that only shallower andesitic magma was discharged. Although anorthite megacrysts are often recognized in island-arc tholeiites, calcic plagioclase could crystallize not from nearly dry tholeiites but from H2O-saturated conditions. This could be achieved by closed system differentiation and/or addition of H2O from the crust, such as plutonic conditions.