Phase relations of pumpellyite-actinolite facies metabasites in the Sanbagawa metamorphic belt in central Shikoku, Japan

Abstract

Sanbagawa metabasites metamorphosed at conditions near the upper limit of the pumpellytic-actinolite facies were examined in terms of phase equilibria in the five component system Al2O3Fe2O3CaOMgOFeO. The Fe3+ content of epidote measured as XEpFe (=Fe/(Fe + Al) of epidote) in the assemblage epidote-chlorite-actinolite-pumpellyite decreases gradually towards the higher-grade, pumpellyite-free areas. The progressive change in XFeEp can be detected within one metabasite bed 200 meters thick near the upper limit of the pumpellyite-actinolite facies. The MgFe2+ substitution, as expressed by variation of Fe/(Fe + Mg) in chlorite (0.40-0.55) has little effect on the Fe3+ + Al) ratios of epidote and pumpellyite in the above-mentioned assemblage. The lowet XFeEp in the pumpellite-bearing assemblage is 0.15 and hence the upper limit of the pumpellyite-actinolite facies is defined by the appearance of an epidote-chlorite-actinolite assemblage with XEpFc = C.15. © 1982.