From back-arc to rifted margin: Geochemical and Nd isotopic records in Neoproterozoic?-Cambrian metabasites of the Bystrzyckie and Orlickie Mountains (Sudetes, SW Poland)

Abstract

Metamorphosed during the Variscan orogeny, sediments of the ca. 560Ma Młynowiec Formation and ca. 530Ma Stronie Formation in the Bystrzyckie and Orlickie Mountains (Central Sudetes, Poland) contain metabasites with a range of basaltic compositions. Immobile trace element and Nd isotope features allow distinction of dominant, either E-MORB-like (Group 1: Zr/Nb 9–20, εNd530 +2.6 to +6.7) or mildly enriched N-MORB-like tholeiites (Group 2: Zr/Nb 21–27, εNd530 +0.2 to +6.7), and scarce but genetically important OIB-like alkaline (Group 3: Zr/Nb 5, εNd530 +2.2) or depleted tholeiitic rocks (Group 4: Zr/Nb 67, εNd530 +7.9). Neither the radiogenic age nor age relationships between these four groups are known. However, field evidence suggests that the metabasites are younger than the Młynowiec Formation and that their emplacement must have been coeval with the accumulation of the Stronie Formation sediments. The OIB affinity of Group 3 is interpreted to reflect an enriched mantle (EM)-type asthenopheric source whilst the groups of tholeiitic rocks indicate involvement of depleted (locally slightly residual) MORB-type mantle (DMM). Several geochemical signatures, the decoupling between Nd isotope and trace element characteristics, and melting models indicate variable enrichment of the DMM-like source, here ascribed to asthenosphere-derived OIB-like melts (Group 1 and 2) and a contribution from a supra-subduction zone (Group 2 and 4). Based on contrasting back-arc basin (BAB)- and within-plate-like affinities of the metabasites, and on petrogenetic constraints from the spatially related infill of the Stronie Formation rift basin, the studied magmatic episode is suggested be related to cessation of the supra-subduction zone activity, presumably induced by ridge-trench collision. This event might have led to slab break-off, the development of a transform plate boundary, opening of a slab window and upward migration of sub-slab enriched asthenosphere. Decompression melting of the upwelling asthenosphere could then have produced OIB-like melts which segregated and infiltrated into the mantle of the former subduction zone, with randomly distributed slab-derived components. In an extensional regime, magmas generated at shallow levels from heterogeneous mantle regions were emplaced within sedimentary rocks of the overlying rift basin. The vestiges of subduction-related processes and within-plate style of mantle enrichment suggest that the metabasites could be related to final stages of the Cadomian orogeny and incipient Early Palaeozoic rifting of Gondwana that heralded the opening of the Rheic Ocean.