Garnet zoning and reaction textures in overprinted eclogites, Bohemian Massif, European variscides: a record of their thermal history during exhumation

Abstract

Eclogites from different parts of the Variscan basement of the Bohemian Massif show a variation in temperatures recorded at peak pressures as well as during subsequent exhumation. These differences are reflected in the extent of modification of pre-, syn- and post-eclogite facies garnet compositions and by the types of reactions taking place at garnet rims. Münchberg Massif eclogites formed at conditions of 600–650°C, > 20 kbar, were mainly overprinted in the amphibolite facies field, and contain garnets with an old homogeneous core and often well preserved idiomorphic growth zoning to Mg-rich rims. In the Mariánské Lázně Complex the eclogite facies stage (640–715°C, < 19.5 kbar) was followed by a weak granulite facies stage, with the formation of orthopyroxene and sapphirine, and a more pervasive amphibolite facies overprint. Despite the slightly higher temperatures, strong compositional profiles in garnet have not been smoothed or homogenised. In the Moldanubian Zone (eclogite stage 700–800°C, > 15 kbar), many eclogites show well developed granulite facies overprints with widespread orthopyroxene growth in garnet and omphacite domains, spinel and/or sapphirine after kyanite, and in some cases even a low-pressure high-temperature overprint that produced Fe-rich olivine. The latter rocks clearly endured higher temperatures during breakdown but, remarkably, strong growth zoning and sharp overgrowth features are preserved in some cases. Diffusion modeling shows that the preservation of zoning in garnet of rocks that have experienced granulite facies temperatures is only possible if the time spent at these conditions is short ( < 1 Ma). This time scale is inconsistent with a smooth P-T-t path as expected from thermal modeling for erosion- or extension-driven exhumation and implies that these Variscan rocks experienced short-lived thermal events during their exhumation history.