Chromian sodic pyroxene, phengite and allanite from the Sanbagawa blueschists in the eastern Kii Peninsula, central Japan

Abstract

Chromian aegirine – aegirine-augite, phengite and allanite occur in blueschists, which are interbedded with serpentinite conglomerates, in the Ise area of the Sanbagawa belt, central Japan. Aggregates of chromian phengite±sodic pyroxene are scattered in the blueschists, and often include detrital chromian spinel. Sodic pyroxene in the aggregate shows solid solution between aegirine and kosmochlor components with a maximum 12.8 wt.% Cr₂O₃ (kosmochlor component = 38 mol%). Sodic pyroxene also occurs as individual crystal in the matrix, and has lower Cr₂O₃ (up to 0.7 wt.%). Phengite has a maximum 17.8 wt.% Cr₂O₃ showing a substitution of Cr for Al in octahedral sites. Allanite occurs in an aggregate of chromian phengite, sodic pyroxene and detrital chromian spinel, and contains up to 7.5 wt.% Cr₂O₃. Chromium in sodic pyroxene, phengite and allanite is derived from detrital chromian spinel. Distinct difference in Cr₂O₃ content between sodic pyroxenes in the aggregate and matrix suggests limited diffusion of Cr during the recrystallization.