ECLOGITES WITH A SHORT-LIVED GRANULITE-FACIES OVERPRINT IN THE MOLDANUBIAN ZONE, CZECH-REPUBLIC - PETROLOGY, GEOCHEMISTRY AND DIFFUSION MODELING OF GARNET ZONING

The petrology and geochemistry of a newly discovered suite of high-pre ssure garnet+clinopyroxene-bearing rocks from the Monotonous Series of the Moldanubian Zone of the Bohemian Massif, southwest Czech Republic have been investigated. Three types [common eclogites, quartz +/- kya nite +/- (clino)zoisite eclogites and garnet-hornblende-clinopyroxenit es] are distinguished by petrography and geochemistry. All underwent a significant degree of partial breakdown under granulite and amphiboli te facies conditions during exhumation. Important features include the growth of orthopyroxene in breakdown domains after garnet and omphaci te and anorthite + spinel +/- corundum +/- exceedingly peraluminous sa pphirine replacing kyanite. Garnet zoning and inclusion patterns suppo rt a prograde evolution from low pressures for at least some of the sa mples. The post-eclogite stage granulite facies overprint indicates th at high temperatures prevailed during exhumation, but preservation of zoning in some garnets and the results of diffusion modelling suggest that this overprint took place over a very short timescale. The geoche mical and petrological results allow characteristic differences to be recognized between these eclogites and metabasites found in other tect onic units of South Bohemia and consequently the assigning of all high -pressure rocks to a single, now disrupted, tectonic unit is a gross s implification that seriously misrepresents the tectono-metamorphic his tory of the region.