Geochemical and isotopic (Sr, Nd and Pb) characteristics of volcanic rocks from southwestern Ethiopia

Major, trace and isotope data are presented for volcanic rocks from southwestern Ethiopia. The studied samples consist of mafic and felsic rocks, with the mafic lavas having generally transitional chemical characteristics between alkaline and tholeiitic, a feature that is typical of the Ethiopian province. The mafic samples have low Mg numbers (Mg#<55, excluding sample E145) and low compatible element contents, such as Ni and Cr suggesting that these lavas have undergone fractionation en route to the surface. All the samples are strongly enriched in LREE and in other incompatible elements as well as in HFSE, such as in Nb and Ta. Sr-87/Sr-86 ratios show a relatively small range (0.7031-0.7039). Nd-143/Nd-144 ratios show limited variation (0.5127-0.5129). All analysed lavas are notable for their radiogenic and wide variation in Pb isotope ratios; Pb-206/Pb-204 from 18.44 to 19.94, Pb-207/Pb-204 from 15.57 to 15.72 and Pb-208/Pb-204 from 38.37 to 39.71. The slightly radiogenic Pb-207/Pb-204 and Pb-208/Pb-204 ratios can not be attributed to crustal contamination, because Sr-87/Sr-86 and Nd-143/Nd-144 ratios do not indicate evidence of contamination by crustal materials. Moreover, Sr and Nd isotopic ratios show essentially no correlation with major and trace element compositions. The slightly elevated Pb isotopic compositions in the southwestern Ethiopian mafic lavas are consistent with their derivation from a time-integrated U and Th enriched mantle source, which may be attributed to a hot mantle plume. Nonetheless, a subcontinental lithospheric mantle source is also involved for those mafic lavas with low Nd-143/Nd-144 and Pb-206/Pb-204 values. The felsic samples show substantial similarity in isotopic ratios with the mafic rocks, implying that they are petrogenetically related to the associated mafic lavas. (C) 1999 Elsevier Science Limited. All rights reserved.