Convergent metamorphism of eclogite and garnet-bearing ultramafic rocks at Lien, West Norway

Abstract

Eclogite and garnet peridotite and pyroxenite are important features in the supracrustal sequence of the Basal Gneiss region in West Norway. Three modes of origin have been proposed for these rock types: (1) both eclogite and garnet-bearing ultramafic rocks equilibrated in the upper mantle and subsequently were tectonically emplaced in their present positions^1–3, (2) the eclogites were produced by in situ metamorphism of a crustal sequence, but the ultramafic rocks originated in the upper mantle and were tectonically emplaced^4–10, and (3) the ultramafic rocks were derived from the mantle but were emplaced before the eclogite facies metamorphic event and have shared a common metamorphic history, at least in part, with the crustal eclogites¹¹. I report here the application of recently developed geothermometers/barometers to garnet-bearing assemblages in the Lien peridotite at Almklovdalen and to adjacent eclogites in surrounding supracrustal gneisses which has documented a retrograde metamorphic trend from 820 °C, 28.1 kbar to 645 °C, 17.6 kbar in the ultramafic rocks and a prograde trend in the eclogites which converges with that of the ultramafic rocks. The Lien peridotite seems to have been derived from the upper mantle in eclogite facies conditions and emplaced into unusually thick continental crust during an eclogite facies metamorphic event.