Spatial and temporal variations in the compositions of Upper Miocene to Recent basic lavas in the northern main Ethiopian rift: Implications for the causes of Cenozoic magmatism in Ethiopia

Abstract

The rate of lithospheric extension has previously been suggested as the most important factor governing the compositions of magmas generated in the Cenozoic Ethiopian volcanic province (CEVP). However, the distribution and chemistry of volcanic rocks extending from the western plateau margin at Addis Ababa to the rift floor in Nazret, northern sector of the main Ethiopian rift (MER), suggest that transitional magmatism in the region may have been triggered by an increase in the amount of lithospheric extension in the Early Pliocene. The rocks occur across an area of variable crustal thickness and show a general age progression from Upper Miocene (^9 Ma) to Recent toward the rift. Alkalic basalts are extensive in the western part of the rift and along its margin but are found only locally within the rift, whereas transitional basalts are found within the rift only. Both types of basalts appear to have been derived from a common mantle source. In contrast, alkalic and transitional basalts on the Ethiopian plateau are mutually exclusive in terms of their spatial distribution, but exhibit a compositional contiguity which suggests that transitional magmas on the plateau formed at the expense of alkalic magmas, i.e. by equilibration of alkalic magmas at relatively shallow depth. The alkalic basalts bear clear record of a decrease in the degree of partial melting with time, suggesting that magmatism on the plateau was possibly triggered by a transient thermal anomaly.

Kinematics of the Danakil microplate

“…Stratoid lavas in central Afar [44] may be a manifestation at V4 Ma of the onset of this spreading, also roughly synchronous with the onset of sea£oor spreading in the southern Red Sea. If sea£oor spreading indeed initiated in this period, the heating and consequent weakening of the lithosphere in Afar and, possibly, nascent ridgepush forces may have promoted self-sustaining accretion, which may explain why the presentday instantaneous pole is applicable to past movements in Afar.…”

Compositional Differences between Felsic Volcanic rocks from the Margin and Center of the Northern Main Ethiopian Rift

“…Rock samples with SiO 2 < 53 wt.% are defined as basic, whereas samples with SiO 2 > 53 wt.% are classed as felsic. In common with many bimodal suites in continental rifts and oceanic islands, the basic rocks at the rift margin are generally alkaline or transitional, whereas rift center equivalents are typically transitional (Wolde, 1996) (Fig. 2).…”

“…Reevaluation of seismic refraction data for the region by Makris and Ginzburg (1987) revealed thinning of the crust from 44 km thick at Addis Ababa to 30-26 km in the center of the MER to the east. This led Wolde (1996) to regard the volcanic sequences at Debre Zeit and to its east as rift center, and Miocene to Pliocene volcanic rocks in the Addis Ababa region, west of Debre Zeit as rift margin.…”

“…The Ethiopian volcanic province is dominated by up to 300,000 km 3 of generally fissure-fed MidTertiary basic (SiO 2 < 53 wt.%) volcanic rocks, and minor associated felsic (SiO 2 > 53 wt.%) products. However, the proportion of felsic products in the Ethiopian rift valley itself is high, reaching about 90% of the total volume (Mohr, 1992 (Morton et al, 1979;WoldeGabreil et al, 1990;Wolde, 1996;Stewart and Rogers, 1996;Chernet et al, 1998;Pik et al, 1998;Ayalew et al, 2002). Recent geochemical and isotopic studies have focused on the Oligocene-Miocene to Quaternary basic-felsic volcanism that accompanied the formation of the MER.…”

“…1). (Wolde, 1996) and the Ethiopian Plateau (EP, Stewart and Rogers, 1996). b) Volcanic and tectonic structures of the MER.…”