GEOCHEMICAL EVIDENCE FOR PROGRESSIVE, RIFT-RELATED EARLY PALEOZOIC VOLCANISM IN THE WESTERN SUDETES

The volcanigenic rocks of the Kaczawa Mts (western Sudetes, Poland), i n the eastern Variscides, show changing geological and geochemical evo lution during early Palaeozoic time. The lower part of the succession (Cambrian (?)-Ordovician) has three components. 1. Shallow marine to s ubaerial metabasalts, associated with limestones and volcaniclastics. The lavas are dominantly of a transitional tholeiitic-alkaline type an d their trace element patterns typically represent a rift-related envi ronment. They pass laterally (and upwards ?) into more depleted basalt s resembling enriched MORB, with Nd-isotopic characteristics indicatin g contamination by continental crust. 2. Interlayered rhyodacitic lava s and volcaniclastics which show negative epsilon(Nd) values, suggesti ng formation of the original magma by crustal melting. 3. An overlying alkaline bimodal suite of lavas and volcaniclastic rocks, as well as alkaline metabasites of shallow-intrusive character. The geochemistry of the latter resembles oceanic island volcanics, but they may well ha ve been emplaced in the same evolving rift environment. The upper part of the Kaczawa succession, Ordovician-Silurian (?) in age, comprises a thick and monotonous sequence of deep-marine pillowed and massive me tabasalts, associated with black shales and cherts. These lavas exhibi t MORB trace element characteristics, with minor evidence of crustal c ontamination. During this stage of rifting, true oceanic crust probabl y formed. It is thus suggested that the studied part of the Kaczawa su ccession developed in a progressively evolving rift, initially within an ensialic environment, and finally reaching the stage of a basin und erlain by oceanic-type crust. Together with similar Cambrian-Ordovicia n volcanic-sedimentary associations, widely distributed in western Eur ope, from Portugal, through France and Germany, they represent a recor d of the Early Palaeozoic rifting in the northern periphery of Gondwan a.