El Olivino de las rocas volcánicas miocenas de la isla de Fuerteventura (Islas Canarias)

Abstract

Olivine phenocrysts are always present in the Miocene basalts of Fuerteventura, whilst they are scarce in the trachybasaltic types. Notwithstanding, olivine is common as part of the groundmass in these trachybasalts. This mineral is completely absent in the trachyandesites. When every single rock is considered separately, the Fo-content of olivines is always higher in phenocrysts than in small crystals fo rm in g the groundmass. So, the o liv in e phenocrysts in basalts are Follh7l and Fo7, u those fou nd in the groundmass. In trachybasalts these values are respectively lower, Fo7,l 74 the scarce phenocrysts and Fom the more abundant crystals contained in the fine grained matrix.No difference related to rock alkalinity is appreciated. Differences in the olivine behaviour among all three the Volcanic Complexes that constitute the Old Basaltic Series are not either observed.The Mg-richer phenocrysts in the less differentiated basaltic types show compositional features which evidence equilibrium with the bearing rock. On the contrary, those in more differentiated basalts should be interpreted as inherited from Mg-richer magmas