GEOCHEMISTRY AND AR-40-AR-39 GEOCHRONOLOGY OF THE MAFIC METAVOLCANIC ROCKS FROM THE RYCHORY MOUNTAINS COMPLEX (WEST SUDETES, BOHEMIAN MASSIF) - PALEOTECTONIC SIGNIFICANCE

The mafic metavolcanic rocks (blueschists and greenschists) of the Ryc hory Mountains crystalline complex (West Sudetes) experienced sea-floo r hydrothermal alteration (spilitization?) prior to regional metamorph ism. The metabasite geochemistry (namely trace element and REE abundan ces) indicates that the protolith was comparable in composition with ( 1) tholeiitic to transitional ocean-floor basalts and (2) transitional and alkaline intra-oceanic island basalts. Two main metamorphic event s affected the Rychory Mountains metabasites. In an earlier high press ure-low temperature metamorphic event, the rocks experienced blueschis t facies metamorphism. The results of Ar-40-Ar-39 geochronology studie s on phengites from the mafic blueschists date the end of the earlier metamorphism to 360 Ma. The greenschist metamorphic overprint followed around 340 Ma. The elongated bodies of mafic metavolcanic rocks are s ituated within the prominent NNE-SSW Leszczyniec shear zone following the trend of the Rychory Mountains and the Rudawy Janowickie Mountains . Both the geochemical affinities and the blueschist facies metamorphi sm of the metabasites suggest that this shear zone evolved from the Va riscan suture dividing western and central terranes of the West Sudete s. According to the radiometric age for the end of the high pressure-l ow temperature metamorphism, the terranes accreted during the Famennia n. A considerable time-span between the formation of the metabasite pr otolith and the blueschist metamorphism may indicate long-lasting subd uction of a large oceanic plate between Gondwana and Laurussia, possib ly accompanied by terrane accretion, prior to the Variscan orogeny.