Abstract

Miocene 'limestones' around Tinderet, noted for their mammalian fossils, but recognized as in part carbonatitic, are now shown to be carbonatite tuffs and volcaniclastics. They contain ejectamenta of fenites, sövites and blocks and bombs of carbonatite lava which have quenched trachytoidal and flow-banded textures and consist largely (90%) of calcite, plus apatite and magnetite. The principal primary constituent is inferred to have been nyerereite (cf. the natrocarbonatite of Oldoinyo Lengai) now pseudomorphed by calcite, and accompanied by primary tabular calcite. Phase relations and other occurrences of such rocks are discussed. The tuffs comprise lithic and crystal lapilli, Pele's tear-drop and accretionary lapilli types.

The silicate content of the tuffs and bombs is remarkably low (c. 3%), and their eruption preceded major flows of nephelinitic agglomerates. We suggest this sequence followed simultaneous separation of immiscible silicate and natrocarbonatite magmas at depth, but the greater energy and mobility of the carbonatite enabled it to surface first. These pyroclastics (and one carbonatite plug) form an outpost of the Homa Bay petrographic province.

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