El evento de fusión parcial en el dominio continental de la banda metamórfica de Aracena (Macizo Ibérico meridional): condiciones estructurales, geoquímicos e isotópicos

Abstract

The high temperature/low pressure metamorphic event that affected the high grade zone of the continental domain of the Aracena metamorphic belt gave place to numerous migmatites. In this work, aluminous migmatites from the continental domain have been studied. Three leucosomes from one of these migmatites have been analysed. Spidergram patterns show moderate enrichment in REE, low REE fractionation and positive Eu anomalies, which are interpreted as the effect of disequilibrium partial melting. The Rb-Sr isotope study supports this interpretation. The disequilibrium partial melting process is usually related to moderate melt proportion generated at high temperature and/or melting associated with a non-coaxial deformation path. Moreover, the presence of diffusive leucosome limits indicates that after partial melting, segregation took place by percolation. Trace elements diagrams point to a convergent plate tectonic setting for the onset of migmatization. It is proposed that during partial melting, isotopic equilibrium was attained at the centimetric scale. Rb-Sr isochrons have been calculated from samples of different migmatites, and have yielded late variscan ages (313 ± 10 – 326 ± 8 Ma) for the partial melting event that took place in the continental domain. Structures observed in aluminous migmatites from the continental domain such as viscous folds, agmatic structures or axial-planar leucosomes indicate that the partial melting event was approximately syn-tectonic with respect to the mian deformational phase registered in the continental domain, which was responsible for the development of the main migmatitic foliation. Nevertheless, residual quartz-rich melts crystallised under static kinematic conditions, once the main deformation event had stopped.