Showing papers by "William K. Hart published in 1989"

Basaltic volcanism in Ethiopia: Constraints on continental rifting and mantle interactions

Abstract: Middle to late Cenozoic mafic lavas from the Ethiopian volcanic province exhibit considerable chemical and isotopic diversity that is linked to eruption age and eruption location. These variations provide a geochemical framework in which continental rifting can be examined. Trace element and Sr, Nd, and Pb isotopic data are interpreted to indicate involvement of up to two depleted and two enriched mantle reservoirs throughout Cenozoic rift development in Ethiopia. Superimposed on the characteristics imparted by varying degrees of melting of these distinct reservoirs are the effects of crystal fractionation and, in some instances, crustal contamination. Initial stages of Oligocene rifting and volcanism, as manifested by the rift-bounding plateau flood basalts, are attributed to asthenospheric upwelling and melting of a heterogeneous, enriched subcontinental lithospheric mantle. Mildly alkaline lavas were produced from an enriched source with characteristics similar to those of the inferred source of other mantle-derived lavas and xenoliths from east Africa (LoNd array, EMI to HIMU). Contemporaneous tholeiitic lavas were derived from a source similar to that producing oceanic basalts from Samoa and the Society Islands (EMII). As lithospheric thinning and rifting continued into the Miocene, upwelling depleted asthenosphere (depleted OIB reservoir, PREMA) interacted with the lithospheric sources producing lavas with hybrid elemental and isotopic characteristics (11–6 Ma plateau and rift margin basalts). Crustal contamination is most evident in the Oligocene to Miocene plateau basalts and is suggested to have taken place primarily at middle to lower crustal levels during initial stages of continental rifting. By 4–5 Ma b.p. continental breakup had begun in Afar, with basalts during this period being derived almost entirely from a depleted PREMA-type reservoir. In the Main Ethiopian Rift, where continental breakup is less advanced, young rift basalts retain a geochemical signature consistent with enriched (LoNd)-depleted (PREMA) mantle hybridization. During the Holocene, proto-oceanic crust and oceanic crust characterize the Afar and Red Sea/Gulf of Aden, respectively, and input from a depleted MORB source first becomes apparent. Chronologic and tectonic control on mantle melting, mantle reservoir interactions, and crust-mantle interactions is a theme common to many extensional regions. Another common feature is the apparent role of a depleted PREMA-type reservoir in these regions, supporting the idea that this reservoir is located at depth within the convecting asthenosphere. Involvement of enriched mantle during continental extension-related magmatism is also prevalent, but the geochemical signature of this component varies from region to region, suggesting a strong link to local crust formation history and local enrichment events such as subduction-driven lithospheric recycling.