East African Rift System (EARS) Plume Structure: Insights from Quaternary Mafic Lavas of Turkana, Kenya

Quaternary mafic lavas from Lake Turkana (northern Kenya) provideinformation on processes operating beneath the East AfricanRift in an area of anomalous lithospheric and crustal thinning.Inferred depths of melting beneath Turkana (15–20 km)are shallower than those recorded elsewhere along the rift,consistent with the anomalously thin crustal section. The maficlavas have elevated incompatible trace element contents whencompared with mid-ocean ridge basalts, requiring an enrichmentevent in the source region. Basalts with low Sr isotopic ratios(     

Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-{micro} Volcanism in Africa

The East African Rift System is important to understanding plume-initiatedrifting as manifest in the geochemistry of mafic lavas eruptedalong the rift throughout its evolution. We present new datafrom high-MgO Tertiary lavas from Turkana, northern Kenya, toinvestigate regional melt source components, to identify thedepths and degrees of melting, and to characterize spatiallyand temporally the chemical structure of the underlying mantle.The Turkana area is a region of high lithospheric extensionthat sits between two topographic uplifts thought to be surfaceexpressions of one or more upwelling mantle plumes. Thinningof local crust is believed to be accompanied by widespread removalof the mantle lithosphere, causing the asthenosphere to be inclose contact with the overlying crust. New geochemical dataon basanites, picrites and basalts (MgO >7 wt %) tightlyconstrain the primary melt source regions of Tertiary volcanism.Initial isotopic signatures (¹⁴³Nd/¹⁴⁴Nd = 0·51267–0·51283,⁸⁷Sr/⁸⁶Sr = 0·7031–0·7036) and trace elementabundances (Ce/Pb 30, La/Nb = 0·6–0·8 andBa/Nb = 3–10) in these lavas are consistent with derivationfrom sub-lithospheric sources. Basalts and picrites eruptedbetween 23 and 20 Ma have Sr–Nd–Pb–He isotopiccharacteristics indicative of high-µ influence, recordhigh depths and degrees of partial melting, and are associatedwith rift propagation to the north and south. Accordingly, theselavas sample a source region that is geochemically distinctfrom that reflected both in Oligocene Ethiopian flood basaltsand in the modern Afar region. The geochemical data supportnumerical and theoretical models as well as tomographic resultsproviding for a complex thermal structure in the mantle beneathEast Africa and are interpreted to reflect isotopically distinctplume heads beneath Tanzania and Afar that are derived fromthe chemically heterogeneous South African superplume. KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar     

Petrogenesis of Evolved Basalts and Rhyolites at Austurhorn, Southeastern Iceland: the Role of Fractional Crystallization

The Austurhorn intrusive complex in southeastern Iceland representsthe evolved hypabyssal remains of an eroded Tertiary (6–7Ma) central volcano. The complex consists of a layered gabbrointrusion, a composite granophyric stock, and abundant maficand felsic dikes. Mineralogical and geochemical trends amongcontemporaneous, compositionally diverse liquids from the complexprovide insight into the genesis of evolved basalts and rhyolitesin Iceland that are difficult to infer from studies of onlylavas. Mafic and felsic samples have comparable ranges in incompatibletrace element ratios (Ba/La and P/Ce) and Sr- and Pb-isotopes(O'Nions and Pankhurst, 1973; B. Hanan, pers. comm., 1988),suggesting derivation from a common parental composition. Majorand trace element variations throughout the Austurhorn suiteare consistent with fractional crystallization of the observedphenocrysts. Quartz-normative basalts were derived from parentalbasalt containing 7.8 wt.% MgO by extensive low-pressure crystallizationof olivine, augite, plagioclase, magnetite, and ilmenite. Thefractionating assemblage is consistent with the observed mineralogyof associated gabbro. Moreover, the cumulus mineralogy of thegabbro provides evidence for fractionation processes in a compositionalinterval not represented by dikes and sills (i.e., 54–63wt.% SiO₂).Diversity among the mafic dikes reflects severaladditional factors: (1) crystallization under conditions ofvariable oxygen fugacity; (2) separate mantle melting eventsthat produce different Ce/Yb values; (3) contamination of somemafic dikes at depth, presumably by interaction with felsicmagmas. Major and trace element trends among most felsic samples canbe modeled by fractionation of the observed mineral phases:plagioclase, K-feldspar, clinopyroxene, ilmenite, apatite, allanite,and zircon. Although crustal melting has been proposed for generatingIcelandic rhyolites, most Austurhorn felsic samples are unlikeliquids derived by melting of hydrated basalts. In particular,apatite and zircon have controlled the abundances of Zr, Hf,and the REE in the felsic rocks, but they are unlikely to beresidual phases during partial melting of basalt. One felsicdike, interpreted as a melt of an evolved source, shows petrographicevidence of in situ anatexis and also has anomalous trace elementabundances and unusually high ₂₀₆Pb/₂₀₄Pb.