Tholeiite from the Simien alkali basalt centre, Ethiopia

Abstract

SummaryThe Ethiopian Cainozoic volcanics associated with the African rift system comprise one of the world's most voluminous alkaline igneous provinces. The Simien Mountains are the remnants of a Miocene alkali olivine-basalt volcanic centre in the north-western part of this province. The end-phase activity at Simien featured intrusion of dyke-swarms of two trends, one parallel to the rift system, the other almost perpendicular to it. Dykes of the rift trend are typically alkaline, but a dyke sampled from the other trend proves to be an olivine-tholeiite. Its presence is interpreted, along with similar rocks from the Harar region in eastern Ethiopia, in terms of upper mantle rifting extending from the Gulf of Aden and Red Sea under the continental blocks of the Ethiopian swell.

Tholeiite from the Cenozoic alkaline volcanic province of Nigeria

“…This is the only lava having even remotely tholeiitic affinities to be recorded from the Nigerian alkaline volcanic province. The presence of tholeiitic lavas near or in alkaline provinces is not particularly rare (Benson, 1944;Le Bas & Mohr, 1970;Wimmenauer, 1970) and they can be abundant (Vincent, 1970) but they are normally fine grained basaltic rocks in which the orthopyroxene is characteristically a normative rather than a modal mineral, and olivine is present in at least small amounts. This rock is therefore unusual on three counts: its coarse texture, the absence of olivine, and the clumping of pyroxene.…”

“…The reasons why the thermal gradient beneath an overwhelmingly alkaline province should steepen sufficiently to locally generate tholeiitic magmas remain speculative (Le Bas & Mohr, 1970). There is no point in adding to the debate at present, except to point out that this alkaline province is almost totally devoid of contemporaneous faulting, unlike, for example, the Ethiopian and Rhinegraben provinces.…”

Estimating Groundwater Flow Velocity in Shallow Volcanic Aquifers of the Ethiopian Highlands Using a Geospatial Technique

“…This site is characterized by tilted and horizontally stratified hexagonal columnar jointing and the thick weathering features of Oligocene basaltic formations. In contrast, the welded pyroclastic flows are characterized by densely welded and lithic fragments associated with rhyolitic lava flows intercalated with ash and unwelded tuffs [37,45,[48][49][50][51][52]. The flood basaltic formation and welded to partially welded pyroclastic flows show variable transmissivity and yield of the aquifers where the weathered and fractured units are characterized by good water-bearing formation.…”