CONTRAINTS ON PAN AFRICAN METAMORPHISM OF THE WESTERN ETHIOPIAN SHIELD

The Precambrian rocks of Western Ethiopia comprise a juvenile metavolcano-sedimentary Birbir domain, similar to rocks of the Arabian-Nubian Shield (ANS), bounded by higher-grade, gneissic rocks (Baro and Geba domains) presumed to be a northern continuation of the Mozambique Belt (MB). The area was affected by the Pan African (~900-550 Ma) tectonothermal event, resulting in the development of N-S trending structures, polyphase intrusion and metamorphism. Porphyroblast-matrix relationships suggest that the peak of metamorphism occurred at an early stage relative to the development of Pan African structures. Thermobarometric calculations on Geba and Baro gneisses yield mid- to upper-amphibolite facies conditions for this event (T=650-700 ^oC, P=7-10 kbar). The abundant metabasite parageneses within the Birbir domain generally indicate greenschist to lower-amphibolite facies conditions. Calculations on a low-variance garnet-chloritoid-staurolite-schist from a metamorphosed clastic shelf sequence within the Birbir domain gives results of 560-580 ^oC at ~7-10 kbar. Such high pressures have not previously been recorded in the ANS of Western Ethiopia and, when combined with lithological considerations, may imply the existence of a discrete lithostructural unit. The Birbir domain appears to be distinct not only in protolith types but also in metamorphic grade from the gneissic domains. The contacts of the Birbir with the Geba and Baro have been identified as tectonic and the margins of the Birbir established to be a major transcurrent shear belt. The combined evidence implies that the domains are discrete and allochthonous. Juxtaposition was probably the result of oblique closure following rifting. The Pan African metamorphic peak was evidently attained prior to, or during, the earliest stages of closure implying an overall anti-clockwise P-T-t evolution. The age of formation of the gneisses and their protoliths and their assumed link with the MB has yet to be established. [FWF P13643-GEO]