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Ultrahigh-pressure grossular-rich garnetite from the Moldanubian Zone, Czech Republic

Stanislav VRÁNA and Jií FRDA

Czech Geological Survey, Klárov 3, 118 21 Praha 1, Czech Republic; e-mail: vrana@cgu.cz

A grossular-rich garnetite composed of - 80 vol. % garnet (Grs₆₃Alm₂₅Prp₇And₄) and 15 vol. % hercynite+anorthite symplectite after kyanite occurs as a boudin in migmatitic gneiss of the Gföhl unit in the easternmost part of the Moldanubian Zone, next to its overthrust on the Cadomian foreland in Moravia, Czech Republic. Garnet cores contain 0.10 to 0.18 wt. % Na₂O and up to 0.5 wt. % TiO₂. Both rutile and ilmenite are present as early inclusions in garnet. The UHP assemblage underwent partial decompression breakdown under granulite/amphibolite facies conditions, including the reaction garnet + kyanite hercynite + anorthite (An₉₉) and localised reaction of garnet to anorthite+aluminous augite+ferropargasite+titanite veinlets. A Ba-Al-Fe silicate loosely related to bafertisite is a minor phase tied to anorthite aggregates. The widespread preservation of Na-rich garnet indicates that Na diffusion was controlled by the limited extent of decompression reactions. The elevated sodium content in garnet, comparable to those of diamond-bearing UHP crustal rocks and diamondiferous eclogites, indicates imprint of sub-lithospheric high pressures, P = 4 GPa. The garnetite can be interpreted as a calc-silicate rock subducted to mantle depths and subsequently obducted with mantle peridotite and pyroxenites. This scenario is supported by published isotopic evidence on a Palaeozoic crustal component in some mantle-derived Variscan garnet pyroxenites and eclogites in the Gföhl unit. Very close similarity of garnetite composition to epidote (except Fe oxidation state and H₂O content) indicates that a nearly monomineralic epidosite is the most probable parent rock.

Key-words: ultrahigh-pressure metamorphism, Moldanubian Zone, grossular-rich garnetite, Na in garnet, decompression reactions, Ba-Al-Fe silicate.

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