Eclogite facies metamorphism in the lower continental crust

Abstract

Summary Occurrences of medium-temperature eclogite-facies assemblages in the Norwegian Caledonides, Western Alps and Central European Variscides are reviewed with reference to a generalized tectono-thermal model for their stabilization in continental plate collision zones involving A-type subduction. It is demonstrated that the respective pressure-temperature-time paths for lower continental crust and uppermost mantle during such orogenesis contrast with that for upper crust, and that mineralogical features, such as chemical zoning in garnets, may monitor the tectonic location and thermal evolution of different crustal levels in the resultant thrust-nappe stack. Phanerozoic orogenic belts of this type typically show evidence of the generation of early eclogite-facies assemblages subjected to partial greenschist- or amphibolite-facies overprint some 40–60 Ma later. The survival and ultimate surface exposure of the high-pressure assemblages requires that their exhumation is rapid (mean uplift rate around 1 mm/year) relative to the rate of thermal relaxation in the tectonically thickened continental crust.