The Calzadilla Ophiolite (SWIberia) 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 minimumthickness of c. 1000mand several ultramafic slices that appear imbricatedwith Edi- acaran 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 associatedwith fore-arc settings. U-Pb zir- con dating of the gabbroic rocks suggests that the igneous protoliths crystallised at c. 600Ma and were extracted directly fromthe mantle, according to the juvenile isotopic sources revealed by the Hf isotopic composition of the dated zircons. At c. 540Ma the U-Pb isotopic systemwas affected by a partial resetting eventwith moderate gen- eration of newzircon. According to these data, it is considered that the CalzadillaOphiolite 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 domainwas probably proximal to theWest Africa Craton. It is very like- ly that the ductile deformation, metamorphismand tectonic imbrication of the Calzadilla Ophiolite and the Serie Negra occurred at c. 540 Ma, during an increase in the subduction rate and a significant decrease of the subduc- tion 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.