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

Abstract

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‐TiO2 diagram, mostly plotting on the island arc tholeiite (IAT) field in a TiO2−10MnO−10P2O5 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–TiO2 and TiO2−10MnO−10P2O5 diagrams.

Oceanic plateau accretion inferred from Late Paleozoic greenstones in the Jurassic Tamba accretionary complex, southwest Japan

“…(1996, 2002); Sorachi, Japan, Kimura et al . (1994) and Nagahashi and Miyashita (2002); and Mikabu, Japan, Tatsumi et al . (1998).…”

“…In fact, accreted oceanic plateaus of Late Jurassic to Cretaceous formation ages have been reported from the accretionary complexes of Japan: the Sorachi‐Yezo Belt in central Hokkaido (Kimura et al . 1994; Nagahashi & Miyashita 2002) and the Mikabu Belt in southwest Japan (Ozawa et al . 1999).…”

Petrochemical evidence for off‐ridge magmatism in a back‐arc setting from the Yakuno ophiolite, Japan

“…The similar geochemical characteristics of MORB, back‐arc basin basalt (BABB) and oceanic plateau basalt (OPB) have prevented geochemical discrimination. Nagahashi and Miyashita (2002) suggested petrographical and geochemical differences between MORB and OPB: olivine and plagioclase dominates the phenocryst assemblage of MORB, but olivine and clinopyroxene prevails in that of OPB; and OPB has a lower Ti and P and a higher Nb content than N‐MORB. These characteristics, however, are not useful for the discrimination of MORB, BABB and OPB, due to significant overlap between BABB and OPB.…”

Stratigraphic and Petrological Insights into the Late Jurassic– Early Cretaceous Tectonic Framework of the Northwest Pacific Margin

“…The origin of these basaltic rocks is controversial, and several tectonic models have been proposed. Most of the models atributed the origin of these basalts to the mid-ocean ridge, back-arc basin, or oceanic plateau [4,9,16,19,28,31].…”

“…On the other hand, the oceanic plateau is also considered to be a strong candidate for the origin of the basaltic rocks of the Gokurakudaira Formation [4,9,16]. Large igneous province formation and interactions between depleted mantle and continental crust could also explain the coexistence of basalts of the Gokurakudaira Formation, as well as the andesitic volcaniclastic rocks of the Hachimoriyama Tuf Member and terrigenous sediments of the Shinpaizawa Sandstone Member (Figure 9A-1, A-2).…”