U/Pb and Pb/Pb zircon ages from granitoid rocks of Wallagga area: constraints on magmatic and tectonic evolution of Precambrian rocks of western Ethiopia

The Precambrian rocks in western Ethiopia consist of high- and low-grade te rranes intruded by granitoids with a wide compositional spectrum. The forma tion ages of these granitoid rocks are, so far, poorly understood. Single-g rain zircon Pb/Pb evaporation and conventional U/Pb dating conducted on fou r granitoids places time constraints on their emplacement and tectonotherma l events. Three granitoid magmatic events were identified at 815 Ma, 700-73 0 Ma, and 620-625 Ma, which were marked by emplacement of the calc-alkaline Ujjukka granite and granodiorite, the anatectie Suqii-Wagga two-mica grani te and the Guttin K-feldspar megacrystic granite, and the anorogenic Ganjii monzogranite, respectively. We interpret the 815 Ma age to mark a major ma gmatic episode in this part of Africa. A tectonothermal event at similar to 630 Ma preceded the emplacement of the within-plate granitoids at 620-625 Ma. The decrease of ages from the calc-alkaline to anorogenic granitoids su ggests a shift of magmatic styles and tectonic setting of the granitoids ov er a period of 200 million years. The Suqii-Wagga and Guttin granites, repr esenting the granitoid population in the migmatitic terrane, formed as part of the successive evolution of the granitoid magmatism in the region. The presence of xenocrystic zircons of Mesoproterozoic ages in both granitoid p opulations emplaced into the low-grade volcanosedimentary sequence and the high-grade, often migmatitic, gneisses suggest contribution of pre-Pan-Afri can crust to the origin and evolution of the granitoids. Conventional U/Pb studies of zircons from the Guttin K-feldspar megacrystic granite and the G anjii monzogranite yielded upper intercept ages of similar to 3 Ga and simi lar to 2 Ga, respectively, possibly indicating the presence of reworked Arc hean-Proterozoic crustal material.