Composite-laccolith emplacement of the post-tectonic Vila Pouca de Aguiar granite pluton (northern Portugal): a combined AMS and gravity study

The Vila Pouca de Aguiar granite pluton, emplaced during the latest event o f the Variscan orogeny of northern Portugal, is here subjected to a detaile d study that combines magnetic fabric measurements and gravity modelling of its shape at depth. This laccolith, less than 1 km in thickness over appro ximate to 60% of its outcrop area, appears to be fed from its northern area , through narrow conduits, up to 5 km deep, belonging to a set of Y-shaped valleys that almost perfectly correspond to the local Regua-Verin fault-sys tem identified in the geological maps. A normal petrographical zonation, al ready identified geologically, appears to be rather progressive, although a gradient in magnetic suceptibility magnitude in-between the two main magma types is evidenced. It is suggested that the first to be emplaced and the least evolved granite type (Vila Pouca de Aguiar Granite) upwelled from the local, NE-trending fault-zone, acting as a dyke, and formed a thin sill wh ere NE-directed magma flow was dominant, at least close to the floor. The m ore evolved granite type (Pedras Salgadas Granite), located just above the main feeder zone, and deeply rooted at the intersection beween underlying f aults, is at the centre of a remarkably regular concentric distribution of the foliation trajectories. They may reflect the late doming of the laccoli th's northern part, coeval with a slight E-W extension of the inflating mag ma reservoir, as marked by the E-W-trending lineations. Along with ubiquito us magmatic to near-magmatic microstructures and particularly low anisotrop y magnitudes, such patterns can be entirely explained by magma movement wit hin its inflating reservoir. This composite laccolith, during emplacement o f which no interference with the regional strain pattern can be recorded, i s therefore considered as typical of post-tectonic emplacement.