Zoned epidote nodules from sedimentary rocks of the Eastport Formation, southwest New Brunswick

Abstract

Zoned epidote-rich nodules occur in shallow-water sedimentary rocks of the Eastport Formation in southwestern New Brunswick. The host rocks of the nodules are sandstone and shale displaying well preserved clastic texture and minimal metamorphic effects, whereas the recrystallized nodule cores consist of epidote and quartz, with minor proportions of prehnite and actinolite. Whole rock analyses show depletion of sodium, potassium, and magnesium in the nodule relative to the host as well as relative enrichment in calcium and iron. Chemical, mineralogical, and textural similarities to calcareous concretions lead to the conclusion that the nodules represent metamorphosed equivalents of ironstone concretions. The St. George pluton is thought to have been the source of heat for metamorphism. Differences in degree of recrystallization between the nodules and their host rocks are attributed to differences in original bulk composition. Although the primary textures in the pelitic and quartzofeldspathic host rocks remained essentially unchanged during very low grade metamorphism, the nodule cores, richer in calcium and iron, recrystallized into metamorphic assemblages.