Tectonic setting of Cadomian low-pressure metamorphism in the central Ossa-Morena zone (Iberian massif, SW spain)

Abstract

Abstract The upper Proterozoic Serie Negra (a monotonous, pre- and syntectonic, slate-greywacke series including volcanic episodes) crops out widely in the central area of the Ossa Morena Zone (SW Iberian Massif, Iberian Peninsula). This unit was affected by complex late Precambrian tectonothermal events (the Cadomian Orogeny) during which two phases of deformation (D1 and D2) are recognized in the 610-585 Ma time span. The second deformational episode (D2) was the most prominent and was related to progressive, low pressure/high temperature metamorphism (LP-HT) reaching the highest grade in the lower structural levels, where thermal evolution led to extensive migmatization and gave rise to the generation of different kinds of anatectic granitoids. In this work we present field, petrographic and analytical data dealing with the metamorphic evolution of a Cadomian LP-HT terrane from the central Ossa-Morena Zone (Iberian Massif, SW Spain), whose tectonic setting is unclear so far. The tectonic setting proposed herein for Cadomian D2 LP-HT events is discussed in terms of its relationship to plate convergence or to extensional tectonic regimes. Modelling of the P - T - t trajectories undergone by a crust moderately overthickened by the thrusting of thinned, sialic crust slices, reproduces well the observed P-T-t [aths if moderate or low uplift rate and high geothermal gradients are assumed for the uplift history. Numerical simulations combined with structural and metamorphic evidence support a compressional tectonic setting as responsible for the observed facies series distribution and deformation /metamorphism relationships. Furthermore, the regional geodynamic context (coeval high pressure metamorphism in neighboring areas of the Ossa-Morena Zone and extensive calc-alkaline magmatism), is consistent with such a tectonic setting. A correlation is tentatively proposed here between compressional Cadomian deformations in the Ossa-Morena Zone, Upper Proterozoic docking of Avalonian terranes in NE America and Cadomian orogenesis in the northern Armorican Massif.