Abstract
The Jurassic accretionary complex of the Ashio belt, Japan, consists mainly of sandstone and mudstone, although metabasalt, chert, and limestone are commonly observed in the Hayade-gawa area, Gosen region, Niigata prefecture. The metabasalt bodies, which are considered to occur as blocks in the accretionary complex, consist of massive lava, pillow lava, and hyaloclastite of generally low vesicularity (<5 vol.%), although highly vesicular rocks are locally observed. The metabasalts are classified into five petrographic types based on phenocryst assemblage: Pl, Pl+Ol, Pl+Ol+Cpx, Pl+Cpx, and aphyric. The whole-rock compositions of 10 metabasalt samples show an oceanic island basalt (OIB) affinity. Metabasalts from the southern part of the study area show higher Zr concentrations, higher Nb/Zr and La/Zr ratios, and lower TiO2/Zr and Y/Zr ratios than those in the northern part. Chondrite-normalized REE patterns show an enrichment in LREEs (similar to OIB) that is most pronounced in the Zr-rich metabasalts from the southern area. Clinopyroxene from 16 samples shows a wide range of Mg# (0.67-0.85), and clinopyroxene chemistry indicates that the magma had affinities with non-orogenic tholeiite and alkali basalt. Clinopyroxenes with low Mg# are divided into two groups: a high-Ti group from the southern area and low-Ti group from the northern area. In contrast, clinopyroxenes with high Mg# have similar Ti contents in both areas. Chromian spinels from 11 samples also have intraplate basalt signatures, similar to those of Hawaiian tholeiitic and alkali basalts. In summary, the whole-rock and mineral chemistry data indicate that the metabasalts in the Hayade-gawa area originated from oceanic island volcanism, with oceanic alkali basalt in the southern area and oceanic tholeiitic basalt in the northern area.
