Lower Paleozoic rifting event in Central Iberian Zone (central-north Portugal): Evidence from elemental and isotopic geochemistry of metabasic rocks

Abstract

The occurrence of Lower Paleozoic mafic magmatic rocks in the Central Iberian Zone (CIZ) of the Variscan Orogen is rare. Amphibolites and metagabbros embedded in the metasediments of the Douro-Beiras Supergroup outcrop at Farminhão, Viseu (central-north Portugal). The protoliths of these two rock types are tholeiites presenting different isotopic signatures (εNd480 = +4.63 to +4.93 and +5.74 to +7.67) and incompatible element ratios (normalized La/Lu up to 4.5 and down to 0.7), respectively which suggests they are not cogenetic. The closely related meta-ultramafic rocks are considered as cumulates generated from the magmas that originated the metagabbros. The elemental and isotopic features of the metagabbros are similar to those reported for amphibolitic occurrences in Tenzuela (near Segovia, Spain) which allows the proposition of a similar age for the mafic rocks of Farminhão. Despite the depleted characteristics of the studied rocks, they are interpreted as having been formed during a continental rifting process characterized by variable degrees of stretching, some 100 Ma after the end of the deposition of the Douro-Beiras Supergroup. This rifting event marks the onset of the Variscan Cycle (s.l.) in the Central Iberian Zone. The occurrence of these metabasic rocks near the confluence between the Porto-Viseu Metamorphic Belt and the Juzbado-Penalva do Castelo Shear Zone suggests that these first-order structures may have worked as weakness zones constraining the ascent of magmas during the Ordovician. The Lower Ordovician metabasic rocks here studied are chemically similar to the abundant lower to medium Cambrian magmatic rocks of the Ossa Morena Zone, also in Iberia, further reinforcing the diachronous character of the opening of the Rheic Ocean that later propagated to the eastern sectors of the European Variscan Belt.