The great eclogite debate of the Western Gneiss Region, Norwegian Caledonides: The in situ crustal v. exotic mantle origin controversy

An entertaining debate arose in the latter half of the 20th century among scientists working on the spectacular eclogite facies rocks that occur within metamorphic rocks of the Western Gneiss Region (WGR) of the Norwegian Caledonides. It resulted in part from Eskola's influential publication On the Eclogites of Norway who concluded, incorrectly, that mafic eclogites within gneisses (external eclogites) and garnetiferous ultramafic rocks within peridotite lenses had a common origin. The debate featured two end-member positions. One was that all these garnet-bearing assemblages, regardless of association, had an exotic origin, where they recrystallized at extremely high pressures and temperatures (P-T) in the mantle and then were tectonically inserted upward into the crust. The other was the insitu origin where this recrystallization occurred within the enclosing gneisses during regional metamorphism. Garnet peridotites and pyroxenites have compositions identical to ultramafic xenoliths in kimberlites and define P-T conditions that are appropriate to the upper mantle. Therefore, peridotite lenses were generally (and correctly) interpreted to be mantle fragments. However, some extended this exotic origin to external eclogites, particularly coarse-grained orthopyroxene- (and coesite-) bearing eclogites, which also formed at extremely high P-T. They noted an apparent pressure and temperature disequilibrium between anhydrous eclogites and the surrounding amphibolite facies gneisses. It was generally accepted that eclogites could form only in dry environments (PH2O<更多