Peralkaline silicate lavas at Oldoinyo Lengai, Tanzania

Abstract A detailed study of Oldoinyo Lengai has led to the recognition of two major cone-building stages. An early, predominantly phonolitic stage, Lengai I, forms the southern cone. The recent nephelinitic Lengai II developed following a major sector collapse event over Lengai I. Petrography of Lengai II lavas show that nephelinite is combeite- and wollastonite-bearing. All Oldoinyo Lengai lavas are peralkaline and highly evolved in terms of low Mg#, Ni and Cr values. Within the unique Lengai II combeite–wollastonite–nephelinite (CWN) peralkalinity increases with time to extreme values (Na + K)/Al = 2.36. Mineralogical expression of peralkalinity is the presence of combeite and Na-rich clinopyroxene. In addition, exceptionally high Fe 2 O 3 (up to 10.28 wt.%) in nepheline is an indicator for alumina deficiency. Combeite also shows high Fe 3+ . Phonolite and CWN of Lengai I and Lengai II show similarly enriched LILE and LREE values and generally parallel patterns in PM normalized and REE plots.