Middle/Late Cambrian intracontinental rifting in the central West Sudetes,NE Bohemian Massif (Czech Republic): geochemistry and petrogenesis of the bimodal metavolcanic rocks

The Early Palacozuic East Krkonose Complex (EKC) situated in the central We st Sudetes, NE Bohemian Massif, is a volcano sedimentary suite containing a bundant mafic and felsic volcanics metamorphosed to greenschist facies. The trace element distribution patterns and Nd isotope signatures (E-Nd500 = + 3 1 to + 6.6) of the metabasites (metabasalts) indicate that they may b e related to a rising mantle diapir associated with intracontinental riftin g. At the early stage, limited melting of an upwelling asthenosphere produc ed alkali basalts and enriched tholeiites which compositionally resemble oc eanic island basalts. A later stage of rifting with larger degrees of melti ng at shallower depths generated tholeiitic basalts with E-MORB to N-MORB c haracteristics. The values of ((87)Sd/Sr-86), = 0.706 and E-Nd500 = -5 +/- 1 Of the porphyr oids (metarhyolites) as well as the lack of rocks with intermediate composi tions suggest that the felsic rocks were formed by a partial melting event of continental crust triggered by mantle melts. The geochemistry of the EKC bimodal metavolcanics and their. association wi th abundant terrigenous metasediments suggest that the felsic-mafic volcani c suite was generated during intracontinental rifting. This process, widesp read in Western and Central Europe during the Early Palaeozoic, is evidence of large-scale fragmentation of the northern margin of the Gondwana superc ontinent. Copyright (C) 2001 John Wiley & Sons.