Exotic orbicular grains (up to 0,5m in longitudinal direction) have been found at the Playa de Vallehermoso, in the miocene Basal Complex, which is the oldest geological unit of La Gomera (Canary Islands). These grains are present in rock fragments that are erosion residuals of nearby outcropping volcanic dykes. The petrological and mineralogical descriptions of the varying rocks of the Basal Complex and the adjacent units have been used to design a geological map, which is necessary to understand the geological context of these orbicular grains The orbicular grains are nodules of dunitic xenoliths, which are embedded in a fine-grained matrix of kaersutite, with minor amounts of chlorite, zoisite, sphen, diopside, calcite, magnetite and ilmenite. The contact between these zones is formed by narrow reaction rims, which have been generated by modal metasomatic reactions (olivine + kaersutite  tremolite + chlorite + titanite + magnetite). The complete mineralogy of the reaction rim is: tremolite, chlorite, sphen, magnetite, pyrite, chalkopyrite as well as relics of not completely reacted kaersutite. The relatively Fe rich and coarse grained olivines of the dunites are characterised by a Fo# (Mg/(Fe+Mg)) of 82 %, Ca contents of 500-5700 µg/gram, NiO of 0,13-0,28 mass% and MnO of 0,15-0,35 mass%. Furthermore, the olivines are marked by an enrichment in incompatible (LREE, Th, U, Ta, Nb) and mobile (K, Rb, Ba) elements due to metasomatic processes, compared to chondrite, PM and MORB contents. Visible evidences of the metasomatism are magnetite and carbonatic precipitates in fluid inclucions within the olivines. Based on this data, a formation of the dunites as restites through a melting process of mantle peridotite can be excluded. Instead the genesis of the originally dunite is determined by olivine accumulation in a fractionating OIB melt. Naturally decorated dislocations, occurring in dense networks within the olivines, act as documents of a strong plastic deformation affecting the olivines before they were carried to the surface. Most of the dislocations have been identified as mixtures of edge and screw dislocations. The idiomorphic spinel phases, which are included in the olivines are mainly chromium-rich ferro- magnetites. Particularities are spinel phases revealing a “donut texture”. These are specific spinels of circular shape which contain inclusions of intergrowth of pargasite and glas phases. Calcite veins, which cross most of the plutonic rocks, are hosting a large number of unregularly and regularly shaped fluid inclusions with an aqueous fluid. The average salinity of the fluids is 0,66 mass% NaCl, the molar volume is app. 21 cm3/mol and the density is 0,87 g/cm3. The fluid´s minimum homogenisation temperature has been determined as 210 °C. The veins formed due to hydrothermal alteration cells of infiltrating sea water and degasing CO2 in the basic rocks. The veins can be interpreted as the result of metasomatism, which liberated Ca-ions that combined with CO2 from a magmatic source to carbonic veins.