U-Pb and Rb-Sr geochronology of the Western Ethiopian Shield; with Suppl. Data 90-18

Five U-Pb zircon and four Rb-Sr whole-rock isochron dates constrain the ages of plutonic rocks and their subsequent history within the Western Ethiopian Shield. The U-Pb data show plutonic activity between 830 and 540 Ma, similar to the time span established for the Pan-African rocks of the Southern Red Sea Hills of the Sudan and the Jeddah terrane of Saudi Arabia. U-Pb zircon ages of 828 +9/-2 and 814 + or - 2 Ma, obtained from pre-kinematic plutons of the Birbir Domain, date the time of their emplacement. One of these plutons (Goma) yielded a whole-rock Rb-Sr isochron date of 759 + or - 18 Ma. Granite sheets within the high-grade Baro Domain, believed to be anatectic, were dated by U-Pb at 783 +19/-14 Ma. The 780-760 Ma event is interpreted as a period of Pan-African regional metamorphism that correlates with the age of low-grade metamorphism of arc-related rocks elsewhere in northeast Africa. Other reset Rb-Sr whole-rock isochrons from the Western Ethiopian Shield that cluster around 635 Ma provide evidence of significant isotopic homogenization, probably associated with major transcurrent movement parallel to a plate margin. A U-Pb lower-intercept age of 582 +29/-33 Ma from Baro granite sheets denotes a second high-grade metamorphic event and correlates with the age of Mozambiquean amphibolite-facies metamorphism in northwest Kenya. Late- to post-kinematic plutons at 571 +11/-3 and 541 +10/-16 Ma place a minimum age limit on deformation and metamorphism.