Pliocene Cinder Cone Chains Adjacent to the Main Ethiopian Rift: Precursors to Rift-based Quaternary Zones of Focused Magmatic Intrusion?

The Main Ethiopian rift (MER) is the premier example of rifting that is actively extending a continent. A key target in understanding the transition from continental rifting to oceanic spreading are the Quaternary linear magmatic-tectonic belts that extend discontinuously along the rift north of 6°N (Wonji Fault Belt and Silti-Debre Zeyit Fault Zone). However, uncertainties remain as to how these zones of focused magmatic intrusion initiate. Possible precursors to the rift floor Quaternary linear magmatic-tectonic belts are preserved in the central MER. Here, Pliocene linear magmatic chains adjacent to the rift on eastern and western plateaus share many of the same morphological features as the later rift-floor magmatic-tectonic belts, and allow for a temporal probe of mantle sources and magmatic plumbing systems of the evolving rift. The Pliocene Akaki belt is located along the western Ethiopian plateau margin adjacent to the rift-floor Quaternary Silti-Debre Zeyit Fault Zone. Magmas within the Akaki belt extend to some of the most primitive recorded in the rift (up to ~14% MgO), and follow an augite-dominated fractionation path that is consistent with crystal fractionation at deep crustal levels. This interpretation is supported by geophysical evidence of melt present in the lower crust in this region. The magmatic plumbing system of the Akaki magmas is inferred to be similar to that of the adjacent Silti-Debre Zeyit Fault Zone. Both magmatic belts are dominated by fractionation at deep levels within the crust, and have abundant megacrysts of augite, feldspar and olivine - evidence of magma stalling. By comparison, lavas from the Wonji Fault Belt - the regionally dominant magmatic belt located adjacent to the eastern rift margin - have fractionated at much shallower levels and have few megacrysts. These data suggest a dominantly spatial control of the magma plumbing systems in the rift, with little temporal changes along the western rift margin during the Pliocene- Quaternary magmatic episodes. The trace element characteristics of more primitive (>7% MgO) Akaki lavas are comparable to other MER magmas, but show slightly elevated values of Ba/Rb and weakly negative K anomalies in primitive-mantle normalized plots. We interpret these characteristics as evidence for small quantities of amphibole in the mantle source of the Akaki lavas. By contrast, minor phlogopite is present in the source of adjacent Quaternary Silti-Debre Zeyit Fault Zone lavas, implying a temporal shift in mantle source lithologies of magmas along the western rift margin. The basis of this temporal shift and the origin of these contrasting source lithologies remains uncertain. While the geochemical signature of amphibole in the source of a magma indicates some contribution from the lithospheric mantle, the origin of this amphibole is equivocal and could be equally derived from modern or ancient metasomatic events. Easily fusible phases such as amphibole and phlogopite may be important in melt generation, and future isotopic characterization of the Akaki lavas will help constrain unresolved questions surrounding the origin of these phases and their role in magmatism in the central MER.