NEOPROTEROZOIC ZIRCONIUM-DEPLETED BONINITE AND THOLEIITIC SERIES ROCKS FROM ADOLA, SOUTHERN ETHIOPIA

In Adola, southern Ethiopia, mafic and ultramafic igneous rocks occur in narrow, 4-10 km wide, north-south-trending belts bounded by high-gr ade gneisses and migmatites. The mafic/ultramafic rocks are complexly deformed and metamorphosed in greenschist to lower amphibolite facies and are thought to be tectonically dismembered parts of an ophiolite c omplex. Preliminary geochemical and geochronological data highlight th at the high-grade rocks in southern Ethiopia and northern Kenya includ e a significant portion of juvenile rocks that were accreted at the sa me time as ophiolitic rocks at 885-765 Ma. This is also the time of wi despread oceanic magmatism and closure in the Arabian-Nubian Shield to the north. The Adola mafic rocks were previously described as island are tholeiites and mid-ocean ridge basalts (MORE), New chemical analys es on the Megado belt rocks reveal the presence of boninites and relat ed dacites interspersed with tholeiitic rocks. The Adola boninites are similar to the Cambrian boninites in western Tasmania in having relat ively low Zr/Sm (less than or equal to 32). Boninites with similarly l ow ratios have not been reported from elsewhere. The Adola tholeiites have high Ti/Zr (150-300). Mixing between tholeiite and boninite magma s may have resulted in elevated Ti/Zr (80-126) in some Adola boninites . Otherwise, Ti/Zr in the latter is low (20-40). Low Ti/Zr is characte ristic of Tertiary boninites in the west Pacific. The fact that both T i/Eu and Zr/Sm increase from the Adola and Tasmania type to the Tertia ry boninites at constant Ti/Zr suggests that Ti might be an element th at is also metasomatically added to the source of boninites and raises doubts about the role of amphibole in boninite petrogenesis.