Geochemistry, PbPb single zircon ages and NdSr isotope composition of Precambrian rocks from southern and eastern Ethiopia: implications for crustal evolution in East Africa

Abstract

Geochemical and isotope data for granitoid rocks from southern and eastern Ethiopia delineate the presumed margin of the Pan-African juvenile terrain of the Arabian-Nubian Shield against an older crustal segment of unknown origin extending from eastern Ethiopia to northern Somalia. Granitoids from southern Ethiopia have higher Na 2O and Na 20 K 2 O and lower Cr and Ni than granitoids with comparable SiO 2 values from eastern Ethiopia. In southern Ethiopia three periods of magmatism are identified on the basis on single zircon 207 Pb 206 Pb evaporation ages, namely at ∼850, ∼750-700 and ∼650-550 Ma, and these correlate well with events documented from other parts of Ethiopia and the Arabian-Nubian Shield. The initial ϵ Nd(700 Ma) and ϵ sr(700 Ma) values range from −1.2 to +3.2 and from −13.4 to + 3.7, respectively, which precludes any significant contribution from much older continental crust in the generation of these rocks. Neodymium mean crustal residence ages, based on a depleted mantle model, range from 0.96 to 1.26 Ga. These data support the interpretation that southern Ethiopia constitutes part of the Arabian-Nubian Shield. In contrast, granitoids from eastern Ethiopia show geochemical features of S-type granites. In eastern Ethiopia Pal aeo-Neoproterozoic zircon ages (781–2489 Ma) are found. Initial ϵ Nd(700 Ma) and ϵ sr(700 Ma) values range from−4.3 to −18.3 and + 33.3 to + 99.8, respectively. Neodymium mean crustal residence ages range from 1.62 to 2.88 Ga. These data, in comparison to the western and southern parts of Ethiopia, are indicative of considerable reworking of pre-Pan-African crust. Variations in age, SrNd isotope ratios and chemistry of the granitoids on a regional scale also suggest the existence of two separate basement terrains between southern and eastern Ethiopia, which may be separated by a tectonic line now concealed by Phanerozoic rocks. This tectonic line may represent a major tectonic boundary between the juvenile Arabian-Nubian Shield in the west and a pre-Pan-African gneissic terrain to the east, thus delineating the eastern margin of the Arabian-Nubian Shield.