Whole-rock geochemistry of metamorphosed mafic rocks from Mt. Sangun area, Fukuoka, Kyushu

Abstract

Geochemical data of metamorphosed mafic rocks can be a powerful tool for inferring the tectonic setting of their protoliths using discrimination diagrams. The metamorphic rocks from the Mt Sangun area consist mainly of mafic and pelitic rocks. In this study, we performed geochemical analyses of the metamorphosed mafic rocks from the Mt. Sangun area and plotted the data on discrimination diagrams. The rocks can be divided into two types: fine- and coarse-grained amphibolite. Most of the fine-grained amphibolites plot on the differentiation trend of basaltic magma; however, some of the coarse-grained amphibolites record accumulation according to the SiO₂/Al₂O₃ - Mg# (100×MgO/(MgO+FeO*)) diagram. The fine-grained amphibolites are geochemically similar to mid-ocean ridge basalt (MORB) or back-arc basin basalt (BABB). In contrast, the coarse-grained amphibolites, excluding samples with cumulate characteristics, resemble volcanic arc basalts (VAB).