Pegmatitoides in the horizontal basalts (series I) of lanzarote and fuerteventura Islands

Coarse-grained rocks found as veinlets, veins and dykes in the horizontal basaltic formation (Series I) of the Islands of Lanzarote and Fuerteventura (Canarian Archipelago) have been studied petrographycally and chemically. The term « pegmatitoides » as given byLacroix (1928, 1929) has been adopted for these coarse-grained rocks. Petrographycally, they are alkali gabbros, ranging from theralites to essexites and even to nepheline monzonitic types. The degrce of crystallization and the textural relationships vary within wide limits. Basalts that contain pegmatitoides are olivine basalts or even more basic types, such as oceanites. Chemical study of pegmatitoides with relation to their host basalts shows that pegmatitoides are richer in Si₂O, Al₂O₃, Na₂O, K₂O and poorer in CaO and MgO than their hosts-basalts. Hypotheses are presented in this study as to the genesis of these pegmatitoides.