Published:
Central Library of the Slovak Academy of Sciences, 2010
Published in:
Geologica Carpathica, 61 (2010) 3, Seite 175-191
Language:
Not determined
DOI:
10.2478/v10096-010-0009-0
ISSN:
1336-8052;
1335-0552
Origination:
Footnote:
Description:
Constraints on the origin of gabbroic rocks from the Moldanubian-Moravian units boundary (Bohemian Massif, Czech Republic and Austria)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 εNdvalues 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 εNd. Such decoupling between La/Nb and εNdcould be attributed to contamination of the subduction-related parent magma by crustal material with higher La/Nb and lower εNdvalues. 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.