Sm-Nd isotope and trace element evidence for heterogeneous igneous protoliths of Variscan mafic blueschists in the East Krkonoše Complex (West Sudetes, NE Bohemian Massif, Czech Republic)

Abstract

Mafic blueschists of the East Krkonoše Complex (EKC, Krkonoše-Jizera Block, central West Sudetes) are glaucophane-bearing metabasites partly overprinted to greenschist facies assemblages. Major and trace element concentrations and Sm-Nd radiogenic isotopes were studied with the aim to evaluate the chemical affinities of their igneous protoliths of Early Ordovician age (485 ± 4 Ma, [1]), and draw inferences on their paleotectonic setting. The distribution of immobile incompatible trace elements patterns (e.g., Th/Nb from 0.07 to 0.1) and Nd isotope signatures (?Ndi = +5.9 to +7.7) of most mafic samples correspond to those of modern basalts of MORB—and OIB-types, respectively, free of significant interactions with materials from the continental crust. Basalts of this compositional diversity may be generated during the early stages of sea-floor spreading, but may also build sea-floor volcanoes (seamounts) situated close to mid-ocean ridges. One sample displays evidence for significant crustal contribution (Th/Nb = 0.4, ?Ndi = + 3.2), while another one, with deep Nb anomaly (Nb/Nb*= 0.33) combined with LREE depletion (147Sm/144Nd = 0.2018) and strongly radiogenic Nd isotope signature (?Ndi = + 7.4), is reminiscent of supra-subduction lavas formed in the absence of recycling of sediments of continental provenance. The igneous precursors of the EKC mafic blueschists were extracted from diverse mantle sources, that probably reflect a range of tectonic environments: incipient sea-floor spreading subsequent to lithospheric break-up heralded by bimodal volcanism [2], oceanic seamounts and, possibly, fragments of supra-subduction oceanic crust. The occurrence of such a heterogeneous igneous assemblage, combined with the presence of HP-LT metamorphic overprinting, suggests that all these mafic bodies were scrapped off subducted oceanic crust and its attached passive margin, and incorporated into an accretionary complex during the final closure of an oceanic domain, in Late Devonian - Early Carboniferous times.