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Petrology of the greenstones of the Lower Sorachi Group in the Sorachi–Yezo Belt, central Hokkaido, Japan, with special reference to discrimination between oceanic plateau basalts and mid‐oceanic ridge basalts
Author(s) -
Nagahashi Tohru,
Miyashita Sumio
Publication year - 2002
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1046/j.1440-1738.2002.00361.x
Subject(s) - basalt , geology , geochemistry , phenocryst , olivine , mantle (geology) , fractional crystallization (geology) , partial melting , plateau (mathematics) , oceanic crust , petrology , subduction , volcanic rock , paleontology , volcano , tectonics , mathematical analysis , mathematics
The Lower Sorachi Group of the Sorachi–Yezo Belt in central Hokkaido, Japan is a peculiar accretionary complex characterized by numerous occurrences of greenstones (metabasalts and diabases), which are mostly composed of aphyric basalts. Clinopyroxene‐rich phenocryst assemblage in phyric basalts is different from olivine–plagioclase assemblage in mid‐oceanic ridge basalts (MORB). The greenstones are geochemically uniform, and show a lower‐Ti trend than MORB in an FeO*/MgO‐TiO 2 diagram, mostly plotting on the island arc tholeiite (IAT) field in a TiO 2 −10MnO−10P 2 O 5 diagram. In a MORB‐normalized spider diagram, the greenstones show a flat pattern from P to Y, which are lower than those of normal mid‐oceanic ridge basalt (N‐MORB). These indicate that the greenstones were derived by a higher degree of partial melting from a depleted mantle similar to a N‐MORB source, and experienced olivine–clinopyroxene fractional crystallization. However, a positive spike of Nb in the spider diagram cannot be explained, and may be attributed to mantle heterogeneity. These characteristics are analogous to those of oceanic plateau basalts (OPB) such as in Ontong Java Plateau, Manihiki Plateau and Nauru Basin, suggesting that the greenstones in the Lower Sorachi Group are of oceanic plateau origin. The present study proposes new field divisions to distinguish OPB from MORB in the conventional FeO*/MgO–TiO 2 and TiO 2 −10MnO−10P 2 O 5 diagrams.