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Volcanic composition in relation to tectonics in the ethiopian rift system: A preliminary investigation

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Conclusions

A study of the regional chemistry of the Ethiopian silicic volcanics confirms previous suspicions that the rift and plateaux form distinct magmatic sub-provinces (Molly, 1934;Merla, 1963). The rift silicics are typically pantelleritic, being strongly enriched in Na, and also possibly Fe and Ca relative to the comenditic silicics typical of the plateaux. The distinction is most marked in central Ethiopia, where the regional Bouguer gravity anomalies show their lowest values at the centre of the Ethiopian swell (Baker andMohr, 1970).

In attempting to explain the geochemical differences between the rift and plateau silicics in Ethiopia, the following alternative hypotheses have been briefly examined but none appears adequate: 1. differing degree of fractionation; 2. assimilation and reaction with thick continental crust under the plateau; 3. differing depths of melting. Whilst the writer tends to agree withNoble (1968) in his conclusion that there is close consanguinity between comendites and pantellerites, the absolute enrichment of sodium (and iron) without concomittant depletion of potassium seems to indicate another phenomenon active under the rift zone. Thus the most extreme alkalienrichment in the Ethiopian province occurs in the peralkaline trachytes (solvsbergites and tinguaites) where total alkalis may reach as high as 16%, yet without the dominance of sodium over potassium which is found in the pantellerites, and with low Fe/Al.

It is therefore tentatively suggested, from the preliminary data available, that the genesis of the rift pantellerites requires a source of soda and iron additional to that available in fractionation or partial melting processes from a trachyte parent. This source may be similar to that supplying the ferride-rich hot brines of the Red Sea and the Afar region of the Ethiopian rift (Holwerda andHutchinson, 1968).

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Mohr, P.A. Volcanic composition in relation to tectonics in the ethiopian rift system: A preliminary investigation. Bull Volcanol 34, 141–157 (1970). https://doi.org/10.1007/BF02597782

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