Eclogites

Abstract

Eclogites are rocks of gabbroic-basaltic composition and consist primarily of two minerals: a grass-green clinopyroxene called omphacite and a red or red-brown garnet. In spite of the basaltic composition, plagioclase does not occur in eclogites. Petrogenetically, this is very significant. Eskola attributed the origin of eclogites to very high pressures, substantiated by modern experimental work. Eskola also established an “eclogite facies.” However, eclogite zones cannot be mapped in metamorphic terrains. Eclogites are usually encountered as bands or lenses in metamorphic complexes of different grades: in the granolite zone, [almandine]-high and -medium-grade metamorphism, low grade, and [glaucophane-lawsonite]-very-low grade. Eclogites also occur as inclusions in kimberlites and basalts and as bands and layers in dunites and peridotites. In some localities, they appear to have been tectonically squeezed into an alien environment. Eclogites therefore occur in a wide range of environments, as has been pointed out, among others, by Smulikowski (1964).