Petrology and geochemistry of granulite clasts in the Visean Luleč conglomerate, Kulm in central Moravia, Czech Republic


Authors: Vrána S, Novák M

Published in: Bulletin of Geosciences, volume 75, issue 4; pages: 405 - 412;

Keywords: Granulite clasts, Luleč conglomerate (Kulm, Upper Visean), Chemical composition of rocks and minerals, Granulite typology, Central Moravia,

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Ten samples of granulite clasts from the Upper Visean conglomerate in the Luleč quarry, 22 km NE of Brno, central Moravia, represent four different types of granulite. Felsic garnet granulites (± biotite) of calc-alkali leucogranite composition, carrying garnet Alm70Prp27-29Grs01-03, is the most common type. One sample of this group contains sillimanite ± hercynite pseudomorphs after kyanite mantled by plagioclase. One sample of unfoliated felsic garnet granulite, nearly medium-grained, features a steep REE pattern indicating monazite fractionation and prominent retrogressive zoning in garnet ranging from Prp30 to Prp13, associated with extensive biotitization. A single sample of high-pressure biotite-garnet granulite of paragneiss-like composition contains garnet Alm48Prp32Grs25-20 with a decompression zoning to Alm56Prp32Grs12. Highly peraluminous cordieritegarnet-sillimanite granulite contains garnet Alm70Prp27Grs03, mesoperthite, antiperthite and coarse prismatic sillimanite. This rock type shows a flat REE distribution pattern and could represent restite after extraction of partial melts of felsic garnet granulites. Granulites in the conglomerate represent mainly low- to medium-pressure types featuring a strong decompression annealing or crystallization. The felsic granulites are compositionally comparable to some low- medium-P types of granulites exposed in the present erosional section of the Moldanubian Zone and dated mainly at 339.8 ± 2.6 Ma. The cordierite-garnet-sillimanite granulites were apparently exposed over significant areas during Upper Visean and their association with felsic garnet granulites provides situation no longer observed in the present erosional section. Felsic granulites with preserved kyanite, significantly represented in the present erosional section of the Moldanubian Zone, are absent in the sample set studied. This may suggest their relative rarity in the Visean source region. The presence of the described types of granulites in the Upper Visean conglomerate indicates high rates of erosion on the order of several mm per year. Heterogeneity in granulite typology and composition in the source areas of granulite clasts is indicated. Study of a larger sample set will probably result in identification of additional types of granulites.