The Calzadilla Ophiolite (SW Iberia) and the Ediacaran fore-arc evolution of the African margin of Gondwana

Abstract

In the Ossa Morena Domain of SW Iberia, the Calzadilla Ophiolite is formed by an ensemble of ultramafic and mafic rocks that seem to represent a fragment of an oceanic Moho transition zone. The ophiolite consists of a main sheet with a minimum thickness of c. 1000m and several ultramafic slices that appear imbricated with Ediacaran siliciclastic rocks of the so called Serie Negra. The mafic rocks of the Calzadilla Ophiolite show an extreme depletion in some HFSE such as Nb, Zr, Th, Hf and Ta, low TiO2 contents and high MgO, which allow identifying them as magmatic types of boninitic affinity, such as those frequently associated with fore-arc settings. U-Pb zircon dating of the gabbroic rocks suggests that the igneous protoliths crystallised at c. 600Ma and were extracted directly from the mantle, according to the juvenile isotopic sources revealed by the Hf isotopic composition of the dated zircons. At c. 540Ma the U-Pb isotopic system was affected by a partial resetting event with moderate generation of new zircon. According to these data, it is considered that the Calzadilla Ophiolite was formed in a supra-subduction zone setting, very likely in a fore-arc domain, during a roll-back episode affecting the peri-Gondwanan subductive slab. The fore-arc domain was probably proximal to the West Africa Craton. It is very likely that the ductile deformation, metamorphism and tectonic imbrication of the Calzadilla Ophiolite and the Serie Negra occurred at c. 540Ma, during an increase in the subduction rate and a significant decrease of the subduction angle of the oceanic slab. The information provided by the Calzadilla Ophiolite allows to better constrain the geodynamic evolution of the African margin of Gondwana.