Granulites and related rocks in variscan median Europe: A dualistic interpretation

Abstract

On the basis of differences in structural, petrological and radiometric features, two main clear-cut groups can be distinguished for the granulites of the Variscan median Europe. Group I comprises lenses of eclogite-granulite scattered in the internal zones of the Hercynian belt, whereas Group II includes granulitic slices related to deep-seated Alpine lineaments and xenoliths scavenged by recent volcanoes. Group I granulites belong to early thrust nappes of the Variscan orogen; they display high-pressure assemblages and are commonly associated with eclogites and garnet-peridotites. From radiometric data, this HP granulitic-eclogitic metamorphism happened around 450-400 Ma throughout median Europe. A subduction context is suggested by the low T/ P gradient, the occurrence of eclogites and the contemporaneity with some blueschist-facies rocks. Group II granulites are characterized by massive occurrences, intermediate-pressure granulite-facies parageneses and the absence of eclogites. Granulitic paragneisses often display a pronounced depletion in granitophile elements and may appear as degranitized restites. Numerous basic-ultrabasic complexes occur and are interpreted as deep-seated, layered igneous intrusions emplaced during the granulitic metamorphism. Radiometric dating indicates a late Hercynian (ca. 300 Ma) age. The high T/ P gradient, the occurrence of synchronous basic intrusions and the post-collision context suggest a granulitic event due to a major thermal anomaly. The Variscan cycle is thus characterized by two granulite-forming episodes. “Older granulites” of the HP type result from an early compressive stage in a crustal subduction context and “younger granulites”, from a major thermal event which originated in the upper mantle and annealed the deepest parts of the Variscan belt. A broadly similar duality in the genesis of granulitic rocks may be anticipated in other collisional belts.