Eclogites at the north-western margin of the Bohemian Massif: A review
Okrusch, Martin; Matthes, Siegfried; Klemd, Reiner; O'brien, Patrick J.; Schmidt, Klaus
European Journal of Mineralogy Volume 3 Number 4 (1991), p. 707 - 730
published: Aug 27, 1991
ArtNo. ESP147050304001, Price: 29.00 €
Abstract Eclogitic relics testifying to one or more early events of high-pressure metamorphism are present in different tectonic units of the NE Bavarian crystalline basement forming the north-western margin of the Bohemian Massif. Eclogites and their amphibolized derivatives are most frequent in the Munchberg nappe complex where they form constituents of the uppermost crystalline nappe, the so-called Hangendserie. The eclogites and eclogite-amphibolites are conformably intercalated within metasediments. According to their major, trace and REE geochemistry, two types of eclogite can be distinguished. Dark eclogites exhibit a MORB-like composition, whereas light, often kyanite-bearing eclogites are presumably derived from a high-Al gabbroic protolith, in part plagioclase-rich cumulates. Judging from the stable assemblage kyanite + omphacite (with jadeite contents up to 50 mole-%), pressure exceeded about 20 kbar in a temperature range of 600-650 °C, as estimated by garnet-omphacite thermometry. Veins filled with high-pressure minerals like kyanite or omphacite are possibly the result of hydraulic fracturing under high water-vapour pressures. Radiometric dating points to a minimum age of 395 Ma for the final stage of the high-pressure event. Isothermal uplift led to the formation of various breakdown assemblages. A penetrative medium-pressure overprint, radiometrically dated at about 380 Ma, transformed the eclogites partly or totally into (garnet-)amphibolites. Mineral assemblages in the interlayered metasedimentary host rocks of the eclogites, including staurolite and kyanite, indicate pressures of 5-6 kbar and temperatures of 550-650 °C for this amphibolite-facies metamorphism. High-pressure relics in the metasediments are rarely present, but indicate a common tectono-metamorphic history with the intercalated eclogites. In the Erbendorf-Vohenstrauss zone, the only notable eclogitic relic is poorly exposed near Kaimling where it is associated with serpentinite and retrograded pyroxenite. This eclogite is widely overprinted, under medium-pressure conditions, by the penetrative amphibolite-facies metamorphism which affected the Erbendorf-Vohenstrauss zone 380 Ma ago. Among the various occurrences of eclogitic relics in the Moldanubicum sensu strictu, the retrograded eclogite body at Winklarn is the most prominent. It is associated with peridotites and garnet pyroxenites and intercalated within Moldanubian cordierite-sillimanite gneisses. According to the record in the metabasites, the P-T evolution probably started in the amphibolite facies and continued through the eclogite (710 °C, > 15 kbar), high-pressure granulite (800 °C, 12 kbar), medium-pressure granulite (700 °C, 7-9 kbar) and low-pressure amphibolite facies. This latest overprint, well documented in the adjacent cordierite-sillimanite gneisses, is radiometrically dated at about 330 Ma.