The mafic–ultramafic rock association of Loderio–Biasca (lower Pennine nappes, Ticino, Switzerland): Cambrian oceanic magmatism and its bearing on early Paleozoic paleogeography

Abstract

Dismembered relics of mafic and ultramafic rock in high-grade basement rocks often record pre-collisional stages of deep lithospheric and/or oceanic mafic magmatism in an orogen. A lens of amphibolites and serpentinites, intercalated between the crystalline Simano and Leventina nappes (lower Penninic nappes, Central Alps, Switzerland) was investigated for the protolith age and chemical and isotopic composition. Despite the polyphase high-grade metamorphic overprinting, primary isotopic and chemical relationships are still preserved and are indicative of an origin in an active margin situation. Trace element geochemistry and Nd isotopes of amphibolites argue for the fractionated gabbros as protoliths, which were formed by the partial melting of the upper mantle. The rocks show a variation of ε Nd values from +7.3 to +4.2 and Nd model ages up to 2 Ga, taken as evidence for the contamination by a geochemically enriched, old lithospheric source. Sr isotopes are heavily disturbed through the percolating crustal fluids after the emplacement into the continental crust. U–Pb age determination of zircon from two amphibolites using both in situ (SHRIMP) and conventional methods converge at an age of 518±11 Ma for the crystallization of the protoliths. The rock association of Loderio–Biasca is a further example from the Alpine basement recording an oceanic domain between Gondwana and the different Gondwana-derived microcontinents in the Cambrian–Ordovician times. Remnants of this ocean were incorporated into the accretionary wedges that formed during the subduction/collision events from Ordovician to Carboniferous, commonly summarized as the Caledonian and Variscan orogeny, and are today spread over the so-called Saxothurigian and Moldanubian domain of the Variscan orogen. These domains consist, thus to large extents, of heterochronous accretionary wedge sequences.