Mafic rocks in a deep-crustal segment of the Variscides (the Góry Sowie, SW Poland): evidence for crustal contamination in an extensional setting

Abstract

Coronitic metagabbronorites (so-called hyperites) and metabasites occur within gneisses, migmatites and minor granulites in the high-grade metamorphic Gory Sowie Block in the Sudetes (SW Poland). Incompatible trace-element and eNd data, combined with field and petrographic evidence, suggest that three groups may be distinguished: (1) "enriched" amphibolites (Zr/Nb ca. 8, eNd500 +1 – +2.5), (2) "depleted" amphibolites (Zr/Nb >30, eNd500 +5 – +6), and (3) metagabbronorites (Zr/Nb 13–19, eNd500 scattered between +5.3 and –1). These characteristics favour an extensional within-plate setting, consistent with their field occurrence as small bodies scattered within metasedimentary and felsic metaigneous gneisses. In that setting, enriched mantle sources were partially melted to produce a range of basic magmas, showing affinities with continental tholeiites (within plate-type basalts). Presumably, in a more advanced stage of rifting, more depleted varieties, transitional to N-MORB, were emplaced. These mafic magmas were variably contaminated during their ascent in the crust. The metagabbronorites, with their well-preserved igneous textures, were previously interpreted as having been intruded after the main deformation and after the peak of amphibolite facies metamorphism (some authors suggesting a relation to the "circum Gory Sowie ophiolites"). However, the geochemical characteristics and P–T estimates suggest that the metagabbronorites are unrelated to the ophiolitic gabbros found in the neighbourhood of the Gory Sowie Block and, in contrast to them, have experienced a complex, polybaric P–T path.