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

Abstract

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 in situ 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 (urn:x-wiley:02634929:media:jmg12314:jmg12314-math-0001 << Ptotal). Thus, eclogites had to form within the anhydrous mantle rather than the host hydrous crust. Finally, there was doubt as to whether the necessary P–T conditions could be generated in continental crust, even when tectonically thickened. The arguments for an in situ origin were based largely on external eclogites. Thin sections showed garnet cores with amphibolite facies inclusions and rims with eclogite facies minerals suggesting prograde metamorphism. Similarly, core to rim changes in mineral chemical composition were consistent with increasing P–T. Coesite and microdiamond were found in both eclogites and host gneisses. Finally, thermobarometry showed burial depths increased from SE to NW across the WGR. Breakthroughs occurred when old assumptions were discarded. Eclogite recrystallization actually can occur in the presence of water. Eclogites and garnet peridotite and pyroxenites had completely different histories. They give different ages, formed under different P–T conditions, and have different geochemical fingerprints. The debate was finally resolved when it became generally accepted that continental crust could subduct into the mantle. Thus, it could subduct to eclogite facies depths where, simultaneously, peridotites could be inserted from the overlying mantle wedge. Both sides of the debate were correct! However, eclogites recrystallized “in situ” only because the enclosing crust was deep in the mantle and garnet peridotites did invade continental crust as solids, but only because the crust was below a mantle wedge. The “Great Debate” was fierce at times, but it led to the modern understanding that continental subduction is a vital part of mountain building