Magnetic fabric and 3D geometry of the Hombreiro-Sta. Eulaliapluton: Implications for the Variscan structures of eastern Galicia, NW Spain

Abstract

In this paper the internal structure and 3D geometry of the Hombreiro-Sta. Eulalia granitic pluton (Hercynian belt, NWSpain) are described. Characterization of the internal structure was performed from surface data, based on microstructural analysis and anisotropy of magnetic susceptibility studies. The geometrical features of the pluton morphology at depth were derived from gravity data. The pluton is made up of paramagnetic granites and leucogranites (K values ranging between 1.1 and 13.3 × 10−5 SI), with well developed magnetic fabrics (anisotropy degree P′ ≥ 1.05 for 78% of the sampling localities). The results obtained enable us to depict a flat-lying magnetic structure with associated N120°E-trending magnetic lineations. The gravity survey corroborates the subhorizontal laminar geometry of the pluton, which has an estimated thickness of 2 km and an almost planar floor. The 3D morphology is elongate at depth parallel to the linear internal fabric. This would imply that magma emplacement took place along this preferential direction. Whereas the deepest zones of the pluton trend WNW-ESE (elongate parallel to regional σ 1) it is almost rootless. We present data showing that the Hombreiro-Sta. Eulalia pluton emplacement was closely related (spatially and chronologically), along with other granitoid plutons from the Lugo Dome, with two major structures of eastern Galicia: the basal shear zone of the Mondoñedo nappe and the extensional Vivero fault.