丹波帯の緑色岩  [in Japanese] Greenstones in the Tamba belt  [in Japanese]

The allochthonous greenstone blocks (altered basalts) in the Tamba belt, Southwest Japan are classified into two rock types, alkaline basalt and tholeiite from relict elinopyroxene and bulk rock chemistries. Each rock type occurs in different stratigraphic unit. Sm-Nd studies on the greenstones give three different ages. The initial ratios of these isotopes are also different for the rocks with different ages. Southern tholeiite and alkaline basalt give ε_<Nd> values of +3.0〜+5.1 whereas northern rocks show more depleted nature at ε_<Nd>=+7.6〜+8.2. Isotope as well as trace element geochemistries suggest that the original tectonic settings. are diverse. The inferred ages and tectonic natures are 335 Ma, oceanic island (ε_<Nd>=+3.1〜+5.1) for the southern tholeiite, 280 Ma (?), oceanic island (ε_<Nd>=+3.0〜+4.1) for the southern alkaline basalt and 300 Ma, more depleted oceanic island (ε_<Nd>=+7.6〜+8.2) for the northern tholeiite and alkaline basalt, respectively.