Origin of felsic magmas in a large-caldera-related stratovolcano in the central part of NE Japan — Petrogenesis of the Takamatsu volcano

Abstract

Quaternary stratovolcanoes in NE Japan are sometimes underlain by large-caldera volcanoes. Rocks of the stratovolcanoes range from basalt (< 10%), andesite (> 80%) to high-silica dacite (< 10%), in contrast to those of the large-caldera volcanoes which are dominated by high-silica dacite. Thus the petrologic study of post-caldera volcanoes provides useful information on the genesis of felsic magmas in large-caldera volcanoes. The Takamatsu volcano, which is composed of calc-alkaline andesite to dacite, is one of the post large-caldera volcanoes ca. 40 km westward from the volcanic front in the central part of NE Japan. The Takamatsudake lava, composed of olv(±)–qtz(±)–opx–cpx–andesite to dacite (61.6–66.5 wt.% SiO 2), is covered by the Yamabushidake dacite composed of hbl(±)–qtz–opx–cpx–dacite (68.4–70.2%). Mafic inclusions (50.9–55.6%), which have a quench texture, having olv, opx(±), cpx(±) and plg phenocrysts, are found in these lavas. The intermediate rocks (Takamatsudake lavas and silica-rich mafic inclusions) possess features suggesting magma mixing, while such features are generally lacking in silica-poor mafic inclusions and the Yamabushidake dacite. All rocks plot on the same linear trends in co-variant diagrams, thus the intermediate rocks are formed by mixing between mafic and felsic end-member magmas. These are similar to silica-poor mafic inclusions (olv basalt, 1070–1080 °C) and the Yamabushidake dacite (hbl dacite, 820–850 °C). In the Aoso and Gassan volcanoes, examples of post large-caldera volcanoes from the front and rear arc of the central part of NE Japan, most of the rocks are also formed by mixing between mafic and felsic end-member magmas, although, in the case of the Aoso volcano, composition of the felsic end-member changed temporally. The following data on these post-large-caldera volcanoes suggest the mafic and felsic end-member magmas are cogenetic. (1) Increasing potassium content in mafic end-members of the three volcanoes from front to rear arc is a remarkable feature, but this feature is observed also in felsic end-members. (2) Sr isotopic data for the Takamatsu and Aoso volcanoes are rather constant in basalt to dacite. Fractional crystallization of phenocrystic minerals from the mafic end-member cannot explain the whole rock compositions of the felsic end-member. An alternative process to produce the felsic end-members is partial melting of solidified mafic end-member magmas and trace element compositions are consistent with this if hornblende gabbroic or amphibolitic residues are produced. This result, coupled with the compiled Sr isotopic data, show that the same process would take place in the generation of felsic magmas in the large-caldera volcanoes.