EVOLUTION OF NATROCARBONATITE FROM A WOLLASTONITE NEPHELINITE PARENT - EVIDENCE FROM THE JUNE, 1993 ERUPTION OF OLDOINYO-LENGAI, TANZANIA

Lapilli and unusually thick, high viscosity lavas extruded from Oldoin yo Lengai in Tune, 1993 are composed of crystal-rich natrocarbonatite containing a small proportion of porphyritic silicate spheroids. The s ilicate spheroids are of wollastonite nephelinite mineralogy, but they also contain natrocarbonatite mineral grains and aggregates (a) in gl ass inclusions within nepheline and pyroxene phenocrysts, and (b) with in the fine-grained silicate groundmass of the spheroids. The chemical relationships of the natrocarbonatite component in the spheroids to t he dominant silicate fraction are consistent with liquid immiscibility , as are the multiple stages of unmixing. These direct observations co nfirm the intimate relationship between natrocarbonatite and wollaston ite nephelinite inferred both from the field relationships at Oldoinyo Lengai, and from low-PT experimentation. The plutonic equivalent (wol lastonite ijolite) of the magma type found to be parental to natrocarb onatite at Oldoinyo Lengai does occur in other carbonatite complexes o f differing ages and wide geographical distribution; hence it is possi ble that the generation of natrocarbonatite is not unique to Oldoinyo Lengai. Certain features in the matrix of the carbonatite flows (coars er grain size, breakdown of solid-solutions observed in earlier, rapid ly chilled lavas) are attributed to the relatively slow cooling of the se atypically thick flows.