Fe-Mg partition and miscibility gap between coexisting calcic amphiboles from the southern Abukuma Plateau, Japan

Abstract

Lamellar intergrowths of actinolite and hornblende or aluminous actinolite occur in metamorphosed igneous rocks in the Hitachi metamorphic district, southern Abukuma Plateau. Electron microprobe analyses of five pairs are presented. The Fe-Mg partitioning and the miscibility gap are discussed in terms of an Mg-Fe-AlIV diagram. The Fe-Mg partition coefficients depend on the AlIV contents in hornblendes in a manner indicating that the pairs are close to equilibrium. Calcic amphibole pairs of high Fe/Mg ratio are richer in AlIV than those with low ratio. The AlIV content of the coexisting actinolite first increases with rising temperature, but then decreases as the temperature increases further. On the other hand, the AlIV content of coexisting hornblende-aluminous actinolite successively decreases with rising temperature.