房総半島三浦層群千畑層中の玄武岩礫
環伊豆地塊蛇紋岩帯の構成要素
1994 年 89 巻 3 号 p. 101-114
詳細
1994 年 89 巻 3 号 p. 101-114
Large amounts of basalt gravels are associated with serpentinite gravels in the Senhata Formation of the Miura Group (uppermost Miocene) in the Boso Peninsula, central Japan. The basaltic rocks possibly constitute a presumed mafic-ultramafic complex (“Fudoiwa serpentinite mass”) in the Tokyo Bay area, which belongs to the Circum-Izu Massif Serpentine Belt. They are slightly altered and have phenocrysts of olivine±clinopyroxene±plagioclase; they are sometimes picritic. Chromian spinel is common both as euhedral inclusions in olivine and as microphenocrysts, and clinopyroxene is often titanaugite. Bulk rock chemistry indicates that intraplate alkali basalts are dominant in the Senhata basalt gravels. Relic chromian spinel chemistry also suggests that the alkali basalts and the picritic basalts are of intraplate magma origin. The close association of intraplate basalts with the Fudoiwa serpentinites may be compatible with the presence of Ti-rich wehrlitic cumulates and Ti-rich metasomatites in the Fudoiwa mass reported by Arai et al. (1990). In the Circum-Izu Massif Serpentine Belt, alkali basalts tend to be associated with small-scale serpentinite masses, and tholeiitic ones, with large-scale masses.
