Unraveling the emplacement history of a Portuguese post-tectonic Variscan pluton using magnetic fabrics and gravimetry

The Lamas de Olo pluton is a post-tectonic Variscan pluton located in northern Portugal. It is a composite pluton comprising three outcropping biotite granites, similar in mineralogical composition but with different textures and different biotite and magnetite contents: Lamas de Olo, Alto dos Cabeços, and Barragem. Two magnetic fabric techniques, anisotropy of magnetic susceptibility and anisotropy of anhysteretic remanent magnetization, were used to test the coaxiality of magnetite and biotite fabrics. Comparing both magnetic tensors, the fabric of magnetite and paramagnetic minerals was found to be coaxial. The magmas ascended passively through opening fractures and were controlled by the intersection of ENE-WSW, NNW-SSE and NNE-SSW fault systems. The gravimetric study shows that the Lamas de Olo is an asymmetric, wedge-shaped pluton with a thickness of ca. 2.5–3.5 km. The kinematic model points to the presence of a feeder zone following an ENE-WSW direction, with two roots for Lamas de Olo and Alto dos Cabeços granites. Regarding Barragem granite, several conduits were identified along an NNE-SSW fault system, subparallel to the Verín-Régua-Penacova fault. Comparing the shape of three post-tectonic biotite plutons with same age and located along the same regional structure, the Verín-Régua-Penacova fault, we observed that the geometry is mainly conditioned by the nature of the host rocks and characteristics of the crustal structure. Wedge-shaped plutons are intruded into granites, and flat-shaped plutons are hosted in metasediments.