MINERAL CHEMISTRY OF A PERALKALINE COMBEITE-LAMPROPHYLLITE NEPHELINITE FROM OLDOINYO-LENGAI, TANZANIA

A peralkaline nephelinite lava ([Na+K]/Al 2.15) from the active carbon atite volcano Oldoinyo Lengai, contains combeite, Ba lamprophyllite, a phase with affinities to delhayelite, CeSrNb perovskite, a CaNa phosp hate high in Sr, Ba and K, and peralkaline glass; in addition to Fe-ri ch nepheline, aegirine-rich clinopyroxene and FeK-rich sodalite. The h igh alkali concentrations relative to alumina in the bulk rock could n ot have been achieved by fractionational crystallisation of the known Al-rich phenocryst phases (nepheline and sodalite) and some other proc ess must be invoked.