Evidence for Late Cambrian Arc-Related Magmatism along the Northern Margin of Gondwana: Zircon Geochronological and Geochemical Data from the Leszczyniec Metaigneous Complex, Sudetes, Poland

We present combined whole-rock geochemical and isotope, and zircon age data from the Leszczyniec Metaigneous Complex (LMC) in the Polish Sudetes. The LMC comprises relatively low-grade metamorphic mafic rocks of basaltic composition that are intimately associated with plagiogranite and tonalite gneisses. The ages obtained from six samples are interpreted as time of zircon crystallization associated with the magma emplacement. Both felsic and mafic magmas were generated from c. 511 Ma until 492 Ma. The most prominent geochemical feature of the studied samples, observed in all lithologies and chemical groups, is a depletion in the HFSE, most notably Nb and Ta combined with an enrichment in LILE. The high abundance of basic rocks with negative Nb anomaly, the limited degree of REE fractionation, and the highly radiogenic Nd and Hf isotopic compositions favour a volcanic arc tectonic setting for emplacement of the LMC protoliths. Only felsic gneisses have high La N /Yb N ratios and non-radiogenic Nd and Hf isotopic compositions, compatible with significant melting of crustal component, but zircon U-Pb geochronology records no older inherited component, precluding substantial assimilation of a typical crustal source. Nonetheless, significant geochemical and isotopic variations are hard to reconcile with origin from a singular parental magma, instead they are explained by the variable input of different slab-derived components and fractional crystallisation processes. Precursors to some chemical groups of metabasites originated from melting of the depleted mantle while other metabasites together with hornblende-bearing gneisses represent a boninite-type volcanic series. The latter are inferred to have been emplaced in a late-stage supra-subduction setting associated with upwelling asthenospheric mantle and high heat-flow regime following slab rollback. The characteristics of igneous precursors to the LMC are comparable to those of the Variscan Allochthon and are consistent with their origin within a convergent tectonic setting outboard of the Gondwana margin.