Geochemical characteristics of amphibolites in the Oki metamorphic rocks, Oki-Dogo Island, southwestern Japan: mixed occurrence of amphibolites with different geochemical affinity

Abstract

Geochemical analyses were performed on amphibolites in the Oki metamorphic rocks in Oki-Dogo Island, southwestern Japan. The amphibolites occur as layers or boulders in the pelitic and psammitic gneisses. Major and trace element characteristics suggest tholeiitic or alkaline chemical affinities and are divided into two groups (Group I and II). Group I amphibolites occurring as layers are characterized by low SiO₂ (43.5-46.6%) and high TiO₂ (>2.6%) and P₂O₅ (>0.5%), whereas Group II amphibolites by high SiO₂ (46.5-47.6%) and low TiO₂ (<1.6%) and P₂O₅ (<0.2%). Relative enrichment of incompatible elements and their high ratios (such as Zr/Y, Ti/ Y, Ta/Yb), and light rare-earth element (LREE) enriched patterns for Group I samples signify that the protoliths of Group I amphibolites correspond to within-plate basalt. In contrast, Group II amphibolites, marked by low contents of incompatible elements and nearly flat REE patterns, are further classified into three sub-groups (IIa, IIb and IIc). Group IIa and IIb amphibolites occur as boulders and the IIc as a layer in the psammitic gneissses. Group IIa and IIc amphibolites resemble tholeiitic basalt in oceanic island-arc tectonic setting, whilst the IIb amphibolite boulder represents chemical similarity to the E-type MORB. With exception of boulder-like amphibolites, the predominance of the Group I amphibolites suggests that the protolith formation of amphibolites (and clastic sedimentation) occurred in an intracontinental tectonic setting.