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

Resumen

Pyroxene (diopside or augite) is present in al/ rocks of the Miocene Volcanic Complexes on Fuerteventura. Pyroxene phenocrysts are specially abundant in basa/ts, in which they represent a third of al/ phenocrysts. The En/(En + Fs) ratio decreases from core to rim in the scarce zoned phenocrysts and is a/ways Jower in the sma/1 crystals forming the groundmass. The En-content is a/so e/ose/y re/ated to the total-rock weight percent MgO. The variation in a rock sample is, sometimes, as large as the one observed as a whole. The total range is En85_69 in basalts, En81_63 in trachybasa/ts, En77_64 in basa/tic trachyandesites, En74_68 in trachyandesites and En70_62 in trachytes. The maximum En-content of basalts remains constant regard/ess the rock MgO concentration. This is due to crystallisation of pyroxene after an important amount of o/ivine, when the residual liquid is strongly Mg-depleted.The Ca+Na - Ti concentrations are those typica/ of a/kaline rocks, showing an evident increase in Wo-component With increasing rock alkalinity. The Wo-component decreases from phenocrysts to groundmass in mildly a/kaline rocks, remains constant in middle a/kaline rocks and increases in strongly alkaline and ultra-alka/ine rocks.