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 |
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| Authors: | B Wolde |
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| Institution: | 1. Department of Geology, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia
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| 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. |
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| Keywords: | Main Ethiopian rift Transitional basalt magmatism Lithospheric extension Plateau magmatism Thermal anomaly |
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