The volcanigenic rocks of the Kaczawa Mts (western Sudetes, Poland), in the eastern Variscides, show changing geological and geochemical evolution during early Palaeozoic time. The lower part of the succession (Cambrian (?)–Ordovician) has three components. 1. Shallow marine to subaerial metabasalts, associated with limestones and volcaniclastics. The lavas are dominantly of a transitional tholeiitic–alkaline type and their trace element patterns typically represent a rift-related environment. They pass laterally (and upwards ?) into more depleted basalts resembling enriched MORB, with Nd-isotopic characteristics indicating contamination by continental crust. 2. Interlayered rhyodacitic lavas and volcaniclastics which show negative ɛ Nd values, suggesting 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 have 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 metabasalts, associated with black shales and cherts. These lavas exhibit MORB trace element characteristics, with minor evidence of crustal contamination. During this stage of rifting, true oceanic crust probably formed. It is thus suggested that the studied part of the Kaczawa succession developed in a progressively evolving rift, initially within an ensialic environment, and finally reaching the stage of a basin underlain by oceanic-type crust. Together with similar Cambrian-Ordovician volcanic-sedimentary associations, widely distributed in western Europe, from Portugal, through France and Germany, they represent a record of the Early Palaeozoic rifting in the northern periphery of Gondwana.

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An Early Palaeozoic (Ordovician ?) metamudstone sequencc near Wojcieszow, Kaczawa Mts, Western Sudetes, Poland, contains numerous metabasite sills, up to 50m thick. These subvolcanic rocks are of within-plate alkali basalt type. Primary igneous phases in the metabasites, clinopyroxenc (salite) and kaersutite, are veined and partly replaced by complex metamorphic mineral assemblages. Particularly, the kaersutite is corroded and rimmed by zoned sodic, sodic-calcic and calcic amphiboles. The matrix is composed of adnolite, pycnochlorite, albite (An 5 0.5%). epidote (Ps 27-33), titanite, calcite, opaques and, occasionally, biotite, phengite and stilpnomelane.The sadic amphiboles are glaucophane to crossite in composition with NaB from 1.9 to 1.6. They are rimmed successively by sodic-calcic and calcic amphiboles with compositions ranging from magnesiofem-winchite to actinolite. No compositions between NaB = 0.92 and NaB = 1.56 have been ascertained.The textures may be interpreted as representing a greenschist faaes overprint on an earlier blueschist (or blueschist-greenschist transitional) assemblage. The presence of glaucophane and no traces of a jadeitic pyroxene + quartz association indicate pressures between 6 and 12 kbar during the high-pressure episode. Temperature is difficult to assess in this metamorphic event. The replacement of glaucophane by actinolite + chlorite + albite, with associated epidote, allows rastriction of the upper pressure limit of the greenschist recrystallization to <8 kbar, between 350 and 450" C. The mineral assemblage representing the greenschist episode suggests the P-T conditions of the high-pressure part of the chlorite or lower biotite zone. The latest metamorphic recrystallization, under the greenschist facies, may have taken place in the Vistan.