THE GRANULITE ECLOGITE FACIES TRANSITION - A COMPARISON OF EXPERIMENTAL WORK AND A NATURAL OCCURRENCE IN THE BERGEN ARCS, WESTERN NORWAY

A unique segment of the deeper parts of the Caledonian continent-continent collision zone is exposed in one of the thrust sheets in the Bergen Arcs of western Norway. It demonstrates that Precambrian granulite-facies rocks underwent fluid-controlled eclogitization on a regional scale. The transition from granulite to eclogite facies is sharp. It occurs over distances of centimetres to metres and follows shear zones and infiltration fronts. Major anastomosing shear zones, typically 50 to 100 m thick, with well-banded eclogite-facies rocks crosscut the lithological boundaries. They can be followed continuously for several kilometres. Breccias, where metre-size blocks of granulite float in an eclogite matrix, outcrop over areas of 0.5 km2. Areas comprising more than 80% eclogite occupy several square kilometres. This mixture of metamorphic facies indicates the metastable occurrence of granulite-facies rocks in the lower crust of modern and ancient orogenic belts. In contrast to earlier experimental work, such field relations imply that the granulite-eclogite facies boundary in nature can be sharp and independent of lithological boundaries. A partly eclogitized lower crust, of the kind exposed in the Bergen arcs can explain the 7.2-7.8 km/s Vp layer present in many crust-mantle transition zones. Anastomosing shear zones and areas of almost complete eclogitization represent potential lower crustal reflectors. © 1990.