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

Abstract

Gabbroic bodies from the Moldanubian Monotonous Group (Maříž) and the Moravian Vratěnín 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 Vratěnín 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 (Maříž) 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 ε Nd 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 Mešovice have La/Nb ratios <1.7 and show negative correlations between La/Nb and ε Nd . Such decoupling between La/Nb and ε Nd could be attributed to contamination of the subduction-related parent magma by crustal material with higher La/Nb and lower ε Nd values. Samples from Uherčice show ambiguous geochemical patterns inherited from contamination by very old recycled material. Gabbroic rocks from Maříž should represent an underplated, partly layered cumulate body of continental tholeiite composition, strongly influenced by crustal contamination. In contrast, gabbroic bodies from the Vratěnín 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.

Implication of corona formation in a metatroctolite to the granulite facies overprint of HP–UHP rocks in the Moldanubian Zone (Bohemian Massif)

“…Some of these rocks still preserve original olivine and coronitic textures formed by orthopyroxene, clinopyroxene, amphibole and spinel. The K–Ar dating of amphibole and biotite (Ulrych et al ., ) provided Variscan ages of 340–300 Ma interpreted to record timing of metamorphism; however, in two localities, the amphibole yielded Pre‐Variscan ages of 570–550 Ma believed to represent protolith ages.…”

“…This area is located at the contact with the Brunovistulian Block, where different geological units are superimposed and strongly tectonically imbricated (in the so‐called Drosendrof stack or window) with uncertain ancestry to either the Moldanubian Zone or the Brunovistulian Block (e.g. Dudek & Jenček, ; Högelsberger, ; Houzar & Novák, ; Houzar, ; Racek et al ., ; Ulrych et al ., ). The rock pile sequence with a high proportion of mafic members of ophiolitic affinity and c .…”

“…Ulrych et al . () summarized the petrographical and geochemical characteristics of several partly metamorphosed bodies of gabbro and norite in the eastern part of the Moldanubian Zone. They form ten to several hundred metre bodies that occur within migmatite of the Monotonous Unit and sillimanite gneiss of the Varied Units (Fig.…”

“…Different varieties of gabbro‐noritic rocks from the eastern part of the Moldanubian Zone were recently investigated by Ulrych et al . (). These include coronitic gabbros, some of which still preserve olivine.…”

“…These include coronitic gabbros, some of which still preserve olivine. K–Ar mineral dating on such rocks provided two contrasting ages, the pre‐Variscan dates that were interpreted as probable crystallization ages of the protolith and Variscan ages reflecting metamorphic overprint (Ulrych et al ., ).…”