[Abstract] In the Marão mountain occur hercynian mafic veins cutting the Schistose Formation of the Landeilian age. The geochemistry of these rocks present high concentrations on Ti ,P, Zr, Nb, Ba, and REE (Ti O2= 1.23-1,39 %, P2O5= 1.32-1,60 %, Zr = 565-641 ppm, Nb= 33-46 ppm, Ba = 2531-3219 ppm, ΣREE = 295-430 ppm) and indicate significant REE fractionation (CeN/ Y bN= 10.46) similar to that reported for alkaline mafic rocks. These characteristics and their patterns in THOMPSON (1984) diagram are typical of alkaline rocks. The dikes intrusion occurred in a late stage of the tectonic evolution of the Centro-Iberian autochthon prior to the granitic magmatic phase. Their intrusion was facilitated, either by the reorientation of the stress field due to the emplacement of the NW Iberian nappe complex or local extensions common in transpressive regimes. This geodynamical context could produce the generation of alkaline magmatism due to partial melting of the upper mantle, induced by adiabatical decompression in the major crustal NW-SE shear zones
