Notes on petrography and rock-forming mineralogy (17)

Abstract

The chemical variation of pumpellyite on a thin section scale has been determined in two prehnite-pumpellyite facies metabasites. These rock specimens are characterized by the presence of abundant hornblende relics and the high distribution-density of veinlets. Pumpellyites occurring in and around prehnite-rich veinlets have been examined for one specimen and those in the same mineral assemblage replacing original plagioclase crystals for the other specimen. Microprobe analyses give evidence of wide compositional variations (ΣFe2O3=3.4-17.9 wt. % in the former and 6.5-12.6 wt. % in the latter). Pumpellyite crystals even in the same pseudomorph after plagioclase have fairly varying compositions (ΣFe2O3=9.7-12.6 wt. %). The chemically sympathetic variations of coexisting pumellyite, epidote and prehnite can be explained in terms of the presence of several local domains with different effective bulk chemical compositions which were essentially controlled by the degree of the interaction between preexisting phases and circulating veinlet-forming fluids during metamorphism. These individual domains appear to be roughly in chemical equilibrium because of the consistent Al-Fe distribution between coexisting pumpellyite, epidote and prehnite.