Mineral assemblages change of the lawsonite-blueschist with an increase of temperature and pressure in the Hakoishi sub-unit of the Kurosegawa belt in Yatsushiro area

Abstract

We investigated the mineral assemblage changes within the lawsonite-blueschists in the Hakoishi sub-unit of the Kurosegawa belt in Yatsushiro area. Following mineral assemblages are observed in studied area from east to west; pumpellyite (Pmp) + Na-amphibole (Na-Amp), lawsonite (Lws) + Pmp + Na-Amp, Lws + Na-Amp + Na-pyroxene. Newly constructed petrogenetic grid in NCMASH system shows such changes are derived from pressure/temperature increase. Compositional changes of amphiboles and pyroxenes, in addition to above-mentioned assemblage changes, suggest that studied lawsonite-blueschist was a part of the oceanic crust subducted at the depth of 18-25km.