THE GABAL-GERF COMPLEX - A PRECAMBRIAN N-MORB OPHIOLITE IN THE NUBIAN SHIELD, NE AFRICA

We report geochemical and isotopic data for tectonically dismembered units of the Cabal Gerf mafic-ultramafic complex, the largest Neoproterozoic (Pan-African) ophiolite in the Arabian-Nubian Shield and located near the Red Sea in the border region between Egypt and the Sudan. The complex consists of basaltic pillow lavas, sheeted dykes, isotropic and layered gabbros and an ultramafic melange, all in tectonic contact along thrust sheets. Major- and trace-element data, including REE, for the pillow lavas and sheeted dykes are indistinguishable from modern high-TiN-MORB. Chemical variations in the various rock types can be ascribed to fractionation and accumulation involving olivine, clinopyroxene and plagioclase. A comparison with chemical data from ophiolites of the Arabian-Nubian Shield and elsewhere in the world shows the Cabal Gerf complex to be the only Precambrian ophiolite with N-MORB chemistry, and we suggest that its basalts and sheeted dykes originally formed in a major ocean basin. Sm and Nd isotope analyses combined with published zircon data suggest an age of similar to 750 Ma for the time of igneous crystallization of the Cabal Gerf complex. epsilon(Nd) initial values vary between + 6.5 and + 8.8, some of the highest yet reported for Neoproterozoic mantle-derived rocks. Pb isotopic data for the basalts and sheeted dykes are similar to modem N-MORB, while the gabbros are more akin to island are and back-are basin rocks. We ascribe their elevated Pb-207/Pb-204 ratios to mixing of a small amount of pelagic sediment with the magma source of the gabbros during subduction and subsequent melt generation above a subduction zone. The pillow basalts, sheeted dykes and gabbros were brought together by tectonic stacking during the obduction process when collision of island are complexes with the active margin of the African continent occurred during an accretion event similar to 600-700 Ma ago.