Vestiges of late Cambrian arc-related magmatism within the Variscan belt of Europe: Zircon geochronological and whole-rock geochemical data from the Leszczyniec Metaigneous Complex, Sudetes, Poland

Abstract

We present major oxides, trace elements, Sr-Nd-Hf isotope and zircon age data for rock samples from the Leszczyniec Metaigneous Complex (LMC) in the Polish Sudetes. The LMC comprises low/medium-grade metamorphic mafic rocks of basaltic composition that are intimately associated with felsic and amphibole-bearing gneisses. Magmatic precursors to all varieties of gneisses were emplaced between c. 515 Ma and 495 Ma. The age of the metabasite remains unconstrained, but the calculated two-stage Nd model age (0.54 Ga) of the sample with highest radiogenic Nd value may in this case represent a maximum age. This is consistent with the oldest single zircon ages produced for the gneisses that may represent inheritance. The most prominent geochemical feature of the studied samples is a depletion in the HFSE, most notably Nb and Ta, which can be observed in all studied lithologies. The high abundance of basic rocks with troughs at Nb in primitive mantle-normalized multi-elemental diagrams, combined with a limited degree of REE fractionation and highly radiogenic Nd and Hf isotopic compositions favour an intraoceanic volcanic arc tectonic setting for emplacement of the LMC protoliths. The derivation of compositionally diverse magmatic precursors to gneisses can be explained by the combination of slab sediment melts at relatively low-pressure/temperature (high Nb/Y felsic gneiss), heterogeneous mixing of those melts and slab derived fluids with a highly depleted mantle (amphibole-bearing gneiss/metagabbro), and low-degree partial melting of metabasite of a former back-arc domain (low Nb/Y felsic gneiss). This study demonstrates that late Cambrian magmatism, widespread across terranes included in the Variscan belt, cannot be unequivocally attributed to continental rifting. Metaigneous suites belonging to the Variscan Middle and Upper Allochthon, such as the LMC, were presumably derived from marginal basins developed within a convergent tectonic setting outboard of Gondwana or Baltica, which may have remained active until the end of the Cambrian.