Sodic-calcic amphiboles found from dolerite in the Nedamo Terrane, Northeast Japan

Abstract

    Sodic-calcic amphiboles (winchite and richterite) occur in dolerite in the Nedamo Terrane, Early Carboniferous accretionary complex. Sodic-calcic amphiboles occur generally in high-P/T metamorphic terranes such as the Sambagawa Terrane, the Suo Terrane etc., whereas they also occur in manganese ore deposits and alkali igneous rocks. Sodic-calcic amphiboles of a high-P/T metamorphic type tend to show the feature of high-Al₂O₃ and low-TiO₂. The sodic-calcic amphiboles of the dolerite in the Nedamo Terrane are difficult to be recognized as the high-P/T metamorphic type because they do not show the feature of high-Al₂O₃ and low-TiO₂, that is they contain quite low glaucophane component. However, the sodic-calcic amphiboles could be formed as a metamorphic mineral in a subduction zone considering that some sodic-calcic amphiboles which show low glaucophane component occur certainly in high-P/T metamorphic rocks, bulk chemistry of the dolerite shows the feature of within-plate tholeiite, occurrence of the sodic-calcic amphiboles is different from that of sodic-calcic amphiboles in alkali igneous rock or contact aureole, and the dolerite does not undergo alteration or metasomatism.