Prograde eclogite in the Gfohl Nappe, Czech Republic: new evidence on Variscan high-pressure metamorphism

A layer of relict, high-temperature, prograde eclogite has been discovered within felsic granulite of the Gfohl Nappe, which is the uppermost tectonic unit in the Moldanubian Zone of the Bohemian Massif; the easternmost of the European Variscan massifs. Pressure-temperature conditions for eclogite (greater than or equal to 890 degrees C, 18.0 kbar) and felsic granulite (c. 1000 degrees C, 16 kbar) place early metamorphism of the polymetamorphic Gfohl crustal rocks within the eclogite facies, and preservation of prograde compositional zoning in small garnet grains in high-temperature eclogite requires very rapid heating, as well as cooling. Mantle-derived garnet and spinel-garnet peridotites are associated with the high temperature-high pressure crustal rocks in the Gfohl Nappe, and this distinctive lithological suite appears to be unique among European Phanerozoic orogenic belts, implying that tectonic processes during the late stages in evolution of the Variscan belt were different from those in the Caledonian and Alpine belts. The unusually high temperatures and pressures in Gfohl crustal rocks, mineralogical evidence for rapid heating and cooling, juxtaposition of lithospheric and asthenospheric mantle with crustal rocks, and widespread production of late-stage granites indicate that culmination of the Variscan Orogeny may have been driven by lithospheric delamination and asthenospheric upwelling.