Eclogite-Bearing Amphibolites from the Appalachian Mobile Belt, Northwest Newfoundland: Dry versus Wet Metamorphism

Abstract

Eclogite-bearing amphibolites from the western marginal metamorphic zone of the Appalachian mobile belt (the orthotectonic zone), northwest Newfoundland, were derived from basaltic dikes metamorphosed in situ under conditions of the upper greenschist to lower amphibolite facies. Where they occur in "dry" basement, the basic dikes have been recrystallized to eclogite, and in the "wet" sedimentary cover to a more typical amphibolite assemblage, thus, reflecting only differing under similar pressure and temperature conditions. The eclogites do not, therefore, represent the eclogite facies of metamorphism sensu stricto. This is compatible with the (disputed) extrapolation of experimentally determined phase boundaries which suggests that eclogite mineralogy may be stable in dry basaltic rocks along normal geothermal gradients in continental crust. It is further suggested that kelyphitic rims around the garnets are primary progressive metamorphic textures, explicable perhaps in terms of pre-eutectoidal types of reaction and gradients. Also, atoll garnets manifest primary prograde growth rather than selective (retrograde) replacement of the cores of preexisting crystals.