PETROLOGY AND GEOCHEMISTRY OF CENOZOIC BASALTS FROM AYRA-LALO KILE AREA, WESTERN ETHIOPIA: IMPLICATION FOR PLUME-LITHOSPHERE INTERACTION AND EVOLUTION OF MANTLE SOURCES
Creators: name: DEMISE TADESSA BONJE
Publisher: name: National Academic Digital Repository of Ethiopia
Year: 2019
Subjects: lkaline basalts, Crustal contamination, Lower basalt, Mantle source, Plume, Tholeiite basalts, Upper basalt

description: Petrographic, major and trace-element results are presented for basalts (MgO ranges: 3.79-11.15 wt. %) from the Ayra-Lalo Kile area, western Ethiopian Plateau. Two magmatic activities are recognized: 1) lower basalt (alkali basalt) and upper basalt (transitional tholeiitic basalts). Petrographic study demonstrated that the lower basalt is characterized by both aphyric and phyric texture (phyric basalts contain 15-30% phenocrysts of dominantly olivine and pyroxene minerals). The upper basalt is phyric in nature with dominant plagioclase and subordinate pyroxene (about 20-35% phenocrysts). Geochemical characteristics of both basaltic suites display remarkable peaks at Nb, Ta and negative troughs at K, Pb. These geochemical signatures indicate crustal contamination has no great role in the genesis of studied basalts; hence reflect the geochemical characteristics of the mantle sources. Even though, the effect of the crustal contamination was not much more evident in both upper and lower basalts; upper basalt relatively exhibit minor sign of crustal contamination of basaltic magmas involving crustal input. The positive correlations of Ni and V with MgO content may indicate that olivine and clinopyroxene fractionation respectively; which was more dominantly revealed in lower basalt than in upper basalt. The variations in the contents of major and incompatible trace elements ratios of upper and lower basalts are reliable to provide evidence for the involvement of distinct mantle source components in the petrogenesis of these basalts. The lower basalt has relatively high TiO2, P2O5, Nb/La, and strongly enriched with LREE and depleted with HREE; whereas upper basalt has low TiO2, P2O5, Nb/La, and flat HREE. These distinct characteristics of rock type may indicate source heterogeneity between these two groups; since fractional crystallization and partial melting of the same source cannot result such deviation. Upper basalt has higher (La/Nb, Zr/Nb, Ba/Nb) ratios; whereas lower basalt has low La/Nb, Zr/Nb, Ba/Nb) ratios which bear a close similarity to sources of ocean island basalts (OIB). These geochemical signatures in the lower basalt may imply enriched or mantle plume component; whereas the geochemical characteristics of the upper basalt indicate a depleted mantle component probably contributions from depleted asthesphere interacted with lithospheric mantle. Thus, lower basalt was derived from an OIB-like mantle component that corresponds to the initial material of the Afar mantle plume. From plots of (Tb/Yb)N versus (La/Sm)N, shows that the upper basalt may be produced from a shallow level ii partial melting of spinel containing mantle source, which was accompanied by minor sign of crustal material; whereas, lower basalt may characterized by partial melting of garnet containing mantle source at the deeper-level. In regional correlation, the characteristic of upper basalt is relatively similar to the low-titanium (LT) and high-titanium 1(HT1)-type basalts from northwestern Ethiopian plateau basalts; while lower basalt resembles high-titanium (HT2)-type basalts from northwestern Ethiopian plateau flood basalts.<br>