Abstract

Gneisses of the Góry Sowie Block, of probable Late Precambrian–Early Cambrian age, contain numerous small metabasite bodies which may have originated as dykes or sills. Many metabasites have phase assemblages and textures which show that they suVered early granulite-facies and later amphibolite- facies metamorphism, although some exhibit no evidence for the earlier event. Trace element geochemistry enabled four metabasite groups to be distinguished: (a) a dominant meta-tholeiite group (Sowie group) probably incorporating more than one set of intrusive rocks; (b) meta-tholeiites characterized by higher Nb/Y (high Nb/Y group); (c) Ti-poor metabasites (Myslêcin group) that exhibit strongly depleted HFSE; (d) alkali metabasalts (Wlóki group). Sowie group meta-tholeiites were also divided according to their dominant host lithology: granitoid gneiss, migmatitic paragneiss or gneiss containing relict granulite-facies rocks. Those in granitoid gneiss are compositionally restricted (although they fall within the same compositional range as other Sowie meta-tholeiites), and may comprise both early metabasites and a later intrusive group, postdating granite emplacement. Both the latter metabasites and the granitoid gneiss lack phase assemblages indicative of granulite facies metamorphism: this suggests that both were emplaced after the granulite-facies metamorphic event, but before or during amphibolite-facies metamorphism. An Early Ordovician emplacement age for syn-metamorphic granitoid gneiss shows that the Góry Sowie Block underwent an Ordovician (early Caledonian) metamorphic event, following metabasite emplacement, or was at this grade during the Ordovician. Nonetheless, the chemistry of all the metabasite groups and their geological setting is consistent with emplacement either through continental crust undergoing extension at a passive continental margin, or during the formation of an intracratonic basin before subsequent metamorphism recorded a local compressional event.

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