Olivine‐spinifex basalt from the Tamba Belt, southwest Japan: Evidence for Fe‐ and high field strength element‐rich ultramafic volcanism in Permian Ocean

• Published in: Island Arc
• Main Authors: Ichiyama, Yuji, Ishiwatari, Akira, Koizumi, Kazuto, Ishida, Yoshito, Machi, Sumiaki
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2007
• Subjects: Kola Peninsula; Pechenga; kola peninsula
• Online Access: http://dx.doi.org/10.1111/j.1440-1738.2007.00590.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1440-1738.2007.00590.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1440-1738.2007.00590.x

Abstract Permian basalt showing typical spinifex texture with >10 cm‐long olivine pseudomorphs was discovered from the Jurassic Tamba accretionary complex in southwest Japan. The spinifex basalt occurs as a river boulder accompanied by many ferropicritic boulders in a Permian chert‐greenstone unit. Groundmass of this rock is holocrystalline, suggesting a thick lava or sill for its provenance. Minor kaersutite in the groundmass indicates a hydrous magma. The spinifex basalt, in common with the associated ferropicritic rocks, is characterized by high high field strength element (HFSE) contents (e.g. Nb = 62 ppm and Zr = 254 ppm) and high‐HFSE ratios (Al 2 O 3 /TiO 2 = 3.9, Nb/Zr = 0.24 and Zr/Y = 6.4) unlike typical komatiites. The spinifex basalt and ferropicrite might represent the upper fractionated melt and the lower olivine‐rich cumulate, respectively, of a single ultramafic sill (or lava) as reported from the early Proterozoic Pechenga Series in Kola Peninsula. Their parental magma might have been produced by hydrous melting of a mantle plume that was dosed with Fe‐ and HFSE‐rich garnet pyroxenite. The spinifex basalt is an evidence for the Pechenga‐type ferropicritic volcanism taken place in a Permian oceanic plateau, which accreted to the Asian continental margin as greenstone slices in Jurassic time.