The West Sudetes (NE margin of the Bohemian Massif) consist of a complex mosaic of several tectonometamorphic units juxtaposed during the Variscan orogeny. The polyphase Variscan tectonothermal development of the West Sudetes was determined by 4~ ages of single grains and mineral concentrates. Late Famennian (359 Ma) mica ages from the high-grade G6ry Sowie Block suggest continuous uplift after a Late Devonian high temperature-low pressure (HT-LP) event contemporaneous with the end of subduction-related high pressure-low temperature (HP-LT) metamorphism in the East Krkono~e Complex.Mid-Late Devonian high pressure events in the Krkonoge-Jizera Terrane and Orlica-Snieznik Dome are followed by coeval high temperature events between 345 and 335 Ma (Vis6an). The latter are interpreted as consequence of uplift, and decompression during overthrusting of both complexes on their forelands. Subsequent small-to large-scale shear movements dated at around 325-320 Ma (early Namurian) affected the Orlica-Snieznik Dome, Krkono~e-Jizera Terrane. including the Intra-Sudetic Fault. and also the eastern Lusatian Granitoid Complex. They were accompanied by contemporaneous emplacement of the Krkonoge-Jizera pluton. The upper limit of the tectonometamorphic and magmatic activity is dated at 314-312 Ma (Namurian/Westphalian boundary). The final juxtaposition of the diversified tectonometamorphic units, which constitute the West Sudetes, took place in early Namurian times. Downloaded from T h e K r k o n o g e -J i z e r a T e r r a n e

Cambrian and Ordovician-Middle Devonian sequences of two successive Early Palaeozoic basins of the Barrandian unconformably overlie Cadomian basement in the Bohemian Massif NW interior (Teplµ-Barrandian unit) which is the easternmost peri-Gondwanan remnant within the Variscides. Correlation of stratigraphy and geochemistry of the Early Palaeozoic siliciclastic rocks elucidated sediment provenances. Sandstones of the Middle Cambrian Příbram-Jince Basin were derived from a Cadomian Neoproterozoic island arc. The source area of the Ordovician shallow-marine siliciclastics of the successor Prague Basin is a dissected Cadomian orogen. Late Cambrian acid volcanics of the Barrandian and Cambrian (meta)granitoids emplaced in the W part of the Teplµ-Barrandian Cadomian basement are also discernible in these sediments. Old sedimentary component increased during the Ordovician. Early Llandovery siliciclastic rocks show characteristics of an abruptly weakened supply of terrigenous material and an elevated proportion of synsedimentary basic volcanics as a result of Silurian transgression. Emsian siliciclastics (intercalated in the Late Silurian to Early Devonian limestone suite) presumably comprise an addition of coeval basic/ultrabasic volcaniclastics. Middle Devonian flysch-like siliciclastics indicate reappearance of Cadomian source near the Barrandian during early Variscan convergences of Armorican microplates that preceeded accretion of the Teplµ-Barrandian unit within the Bohemian Massif terrane mosaic.

The Early Palaeozoic East Krkonos Ïe Complex (EKC) situated in the central West Sudetes, NE Bohemian Massif, is a volcanosedimentary suite containing abundant ma®c 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 be related to a rising mantle diapir associated with intracontinental rifting. At the early stage, limited melting of an upwelling asthenosphere produced alkali basalts and enriched tholeiites which compositionally resemble oceanic island basalts. A later stage of rifting with larger degrees of melting at shallower depths generated tholeiitic basalts with E-MORB to N-MORB characteristics.The values of ( 87 Sr/ 86 Sr) i 0.706 and E Nd500 À5 AE 1 of the porphyroids (metarhyolites) as well as the lack of rocks with intermediate compositions 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 with abundant terrigenous metasediments suggest that the felsic±ma®c volcanic suite was generated during intracontinental rifting. This process, widespread in Western and Central Europe during the Early Palaeozoic, is evidence of large-scale fragmentation of the northern margin of the Gondwana supercontinent.