Subduction- and non-subduction-related igneous rocks in the Central European Variscides: geochemical and Nd isotope evidence from the Kłodzko Metamorphic Complex, Polish Sudetes

Abstract

The Kłodzko Metamorphic Complex (KMC) in the Central Sudetes is a composite outcrop of pre-Upper Devonian metasedimentary and metaigneous rocks, formed of several thrust units. The metaigneous rocks are geochemically diversified, and were interpreted to reflect a complex geodynamic setting of emplacement. The association of large amounts of felsic and mafic rocks is reminiscent of the model of Cambro-Ordovician bimodal, rift-related suites developed along the northern periphery of Gondwana. However, the felsic rocks are potash-poor, calc-alkaline in character, while the associated mafic rocks are, in part, metagabbros and cumulates resembling N-MORB, which is consistent with neither typical ensialic rift nor evolved MOR tectonic environments. Combined with published data, our new geochemical and Nd isotope results show that the metabasic rocks of the northeastern part of the KMC, not associated with felsic volcanics, are of within-plate type, with an ϵNd 400 (assuming approximate youngest possible Silurian/Devonian age) of +6.8, typical of magmas derived from time-integrated depleted mantle sources. The metagabbros of the southwestern part of the KMC (associated with felsic rocks) range from slightly enriched to depleted rocks, and their ϵNd 560 (assuming a Neoproterozoic age, K. Turniak, personal communication) scatters from +2.2 to +8.6, suggesting that hybrid sources and/or variable degrees of crustal contamination of a strongly depleted mantle source were involved. The intermediate and acidic rocks are peraluminous to metaluminous rhyolites, rhyodatites/dacites, and andesites (and volcaniclastics), with Na 2O > K 2O and large negative anomalies of Nb, Sr, and Ti. Their highly variable, but distinctly positive, ϵNd 560 values (from +2.9 to +8.6, mostly clustered around +5.5) overlap those measured in the associated metagabbros, thereby substantiating close genetic relationships. Metarhyolites produced by crustal melting are conspicuously missing. A subduction-related environment is suggested for this peculiar mafic–felsic association of probable Neoproterozoic age. The overall geochemical variations, combined with the new age data, indicate that the KMC is a composite domain that displays fragments of a Pan-African active margin, telescoped during Variscan nappe tectonics, with rift-related volcanics and passive margin sequences probably related to the Palaeozoic break-up of Gondwana.