Kokipie (Sagat)

Kokipie Hill lying on the border with Kenya is the only significant remnant of a volcanic cone that was fed from a heart-shaped vent lying to the northwest. The hill is built of agglomerates overlying about 30 m of horizontally bedded grits, marls and conglomerates consisting almost exclusively of basement fragments. The agglomerates contain blocks up to 5 m in diameter of nephelinite but these are strongly altered with abundant patches of natrolite. Two smaller hills are the only remnants of the cone. The vent is about 1000x600 m and most of the margin is marked by a zone of explosion breccia of fragmented basement rocks. Within the vent are agglomerates, banded tuffs and a range of intrusive rocks including ijolite, melteigite and nepheline syenite. The ijolites and melteigites consist predominantly of pyroxene, varying from titanaugite to aegirine, and nepheline with magnetite and accessory titanite, apatite and biotite. Occasional melilite ijolites are present and olivine has been identified in a melteigite, within which amphibole also occurs. One group of the ijolites contains orthoclase and these rocks pass into nepheline syenites with a maximum feldspar content of 29% (Trendall, 1962). The vent agglomerates contain fragments of all the principal igneous rock types together with some derived from the basement as well as finely banded tuffs. Cutting the vent and extending for several kilometres to the south, and into Kenya, are tinguaite dykes which vary between 2 and 6 m in width, but south of Kokipie Hill also develop as thicker bodies. The tinguaites comprise phenocrysts of nepheline and occasional sanidine and aegirine in a trachytic groundmass of aegirine needles, feldspar and nepheline and sparse sodic amphibole; zeolitization is sometimes extensive and concentrated as spherulitic structures (Searle, 1952). An intrusion of ijolite occurs on the international border southwest of Kokipie and a smaller one to the northeast. Detailed petrography will be found in Searle (1952) and Trendall (1962).