Pyroxenes in the miocene volcanic rocks of Fuerteventura (Canary islands)

Pyroxene (dio psid e o r augite) is p re se n t in a ll rocks o f th e M io c e n e V o lc a n ic C om plexes on Fuerteventura. Pyroxene phenocrysts are specially abundant in basalts, in which they represent a third of all phenocrysts.The En/(En+Fs) ratio decreases from core to rim in the scarce zoned phenocrysts and is always lower in the small crystals forming the groundmass. The En-content is also closely related 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 EnB569 in basalts, EnB163 in trachybasalts, En77M in basaltic trachyandesites, En74-6B in trachyandesites and En70 62 in trachytes.The m axim um En-content o f basalts remains constant regardless the rock MgO concentration. This is due to crystallisation of pyroxene after an important amount of olivine, when the residual liquid is strongly Mg-depleted.The Ca+Na - Ti concentrations are those typical of alkaline rocks, showing an evident increase in Wo-component with increasing rock alkalinity. The Wo-component decreases from phenocrysts to groundmass in mildly alkaline rocks, remains constant in middle alkaline rocks and increases in strongly alkaline and ultra-alkaline rock