International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

Constraints on the origin of gabbroic rocks from the Moldanubian-Moravian units boundary (Bohemian Massif, Czech Republic and Austria)

Pages: 175 - 191

Authors: JAROMIR ULRYCH, LUKAS ACKERMAN, VACLAV KACHLIK, ERNST HEGNER, KADOSA BALOGH, ANNA LANGROVA, JAN LUNA, FERRY FEDIUK, MILOS LANG, JIRI FILIP

Abstract: Gabbroic bodies from the Moldanubian Monotonous Group (Mariz) and the Moravian Vratenin Unit (other sites), often showing retrogressive recrystallization at their margins in the amphibolite-facies grade, have norite, gabbronorite, gabbro and hornblendite compositions. Gabbros with preserved coronitic textures are limited to the Vratenin Unit. The estimated equilibration temperatures derived from plagioclase–amphibole pairs and orthopyroxene Ca contents calculated for pressures 5–10 kbar overlap for coronitic (700–840 °C) and non-coronitic gabbroic rocks (680–850 °C). Although the Moldanubian (Mariz) gabbroic rocks are more Mg-rich compared to the Moravian gabbroids, they show crust-like La/Nb ratios of 2.1–6.6 characteristic of subduction-related magmatic rocks coupled with uniform low eNd values of +0.6 to +0.7. Apparent subduction-related features are probably caused by contamination by juvenile crust and/or by metamorphic fluid rich in incompatible elements during the Variscan metamorphism. Samples from Korolupy–Nonndorf and Mesovice have La/Nb ratios <1.7 and show negative correlations between La/Nb and eNd. Such decoupling between La/Nb and eNd could be attributed to contamination of the subduction-related parent magma by crustal material with higher La/Nb and lower eNd values. Samples from Uhercice show ambiguous geochemical patterns inherited from contamination by very old recycled material. Gabbroic rocks from Mariz should represent an underplated, partly layered cumulate body of continental tholeiite composition, strongly influenced by crustal contamination. In contrast, gabbroic bodies from the Vratenin Unit, having a close spatial relationship to the surrounding garnet amphibolites, were emplaced into a lithologically variable passive margin sequence probably during the Cadomian extension.

Keywords: Bohemian Massif, Moldanubian Unit, Moravian Unit, Sr-Nd isotopes, K-Ar ages, geochemistry, gabbroic rocks

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