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

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 Na2O and Na2O/K2O and lower Cr and Ni than granitoids with comparable SiO2 values from eastern Ethiopia. In southern Ethiopia three periods of magmatism are identified on the basis on single zircon Pb-207/Pb-206 evaporation ages, namely at similar to 850, similar to 750-700 and similar to 650-550 Ma, and these correlate well with events documented from other parts of Ethiopia and the Arabian-Nubian Shield. The initial epsilon(Nd(700 Ma)) and epsilon(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 Palaeo-Neoproterozoic zircon ages (781-2489 Ma) are found. Initial epsilon(Nd(700 Ma)) and epsilon(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. (C) 1998 Elsevier Science Limited.