The Precambrian rocks of Ethiopia and Somalia form the link between two important orogenic belts of Africa: the Upper Proterozoic, low-grade, juvenile arc complexes of the Arabian-Nubian Shield to the north in northeast Africa and Saudi Arabia and the predominantly medium- to high-grade, partly reworked gneissic terrain of the Mozambique Belt to the south in east Africa. Granitoids from southern Ethiopia are dominantly I-types having higher CaO and Na2O/K2O and lower Zr and Rb/Sr than granitoids from eastern Ethiopia and Somalia of comparable SiO2 values. In southern Ethiopia three periods of magmatism are identified on the basis of single zircon 207Pb/206Pb evaporation ages at ~850, ~750-700 and ~650-550 Ma, with a mean age of ~700 Ma, and these correlate well with events documented from other parts of Ethiopia and the Arabian-Nubian Shield. The eNd(700 Ma) and eSr(700 Ma) values range from -1.2 to +3.2 and -13.4 to +3.7, respectively. These isotope ratios preclude any significant contribution from much older continental crust in the generation of these rocks. Nd mean crustal residence ages, based on depleted mantle model, range from 1.11 to 1.54 Ga. These data support the model that southern Ethiopia constitutes part of the Arabian-Nubian Shield.
In contrast, granitoids from eastern Ethiopia and Somalia are dominantly to exclusively S-types. In eastern Ethiopia early to middle Proterozoic zircon ages (781-2489 Ma) are found. The eNd(700 Ma) and eSr(700 Ma) values range from -4.3 to -14.8 and +33.3 to +94.4, respectively. Nd mean crustal residence ages range from 1.77 to 2.21 Ma. These data are in contrast with the data from the western and southern parts of Ethiopia and indicate reworking of pre-Pan-African crust in eastern Ethiopia.
Variations in granitoid chemistry on a regional scale suggest the existence of two separate basement terrains between southern and eastern Ethiopia, which may be separated by a major tectonic line now concealed by Phanerozoic rocks. This tectonic line may represent a major tectonic boundary between the juvenile Arabian-Nubian Shield to the west and a pre-Pan-African gneissic terrain to the east.