Cadomian magmatism and metamorphism at the Ossa Morena/Central Iberian zone boundary, Iberian Massif, Central Portugal: Geochemistry and P–T constraints of the Sardoal Complex

Abstract

A well preserved Cadomian basement is exposed in the Iberian Massif, Central Portugal, at the Ossa Morena/Central Iberian zone boundary, which allows the determination of reliable geochemical data. A sequence of Cadomian and Variscan magmatic and tectonometamorphic events has been already described for this area and are documented in other areas of the Avalonian–Cadomian orogen. However, the geochemical information concerning the Cadomian basement for this area is still limited. We present whole rock geochemical and oxygen isotopic information to characterize the igneous protoliths of the Sardoal Complex, located within the Tomar–Badajoz–Córdoba Shear Zone, and identify their tectonic setting. We use detailed petrography, mineral chemistry and P–T data to characterize the final Cadomian tectonometamorphic event. The Sardoal Complex contains orthogneiss and amphibolite units. The protoliths of the orthogneiss are calc-alkaline magmas of acid composition and peraluminous character that were generated in an active continental margin in three different stages (ca. 692Ma, ca. 569Ma and ca. 548Ma). The most significant processes in their petrogenesis are the partial melting of old metasedimentary and meta-igneous crust at different crustal levels and the crystal fractionation of plagioclase, alkali feldspars, apatite, zircon and Fe-Ti oxides. The protoliths of the amphibolite, older than ca. 540Ma, are tholeiitic and calc-alkaline magmas of basic composition that display N-, T- and E-MORB affinities. They were generated in an active continental margin. Crustal contamination and fractional crystallization of hornblende and diopside were involved in their petrogenesis. However, the fractional crystallization was not significant. The magmatic activity recorded in the Sardoal Complex indicates the existence of a long-lived continental arc (ca. 692–540Ma) with coeval felsic and mafic magmatism. The final stage of the Cadomian metamorphism is usually represented in other areas of the Cadomian basement as a LP–HT metamorphic event. However, the P–T data obtained by thermodynamic modelling indicates medium pressure/high temperature conditions at ca. 540Ma. These data suggest that the Sardoal Complex represents a deeper level of the exhumed Cadomian basement where the final stage of the Cadomian metamorphism was recorded.