A PAN-AFRICAN ORIGIN AND UPLIFT FOR THE GNEISSES AND PERIDOTITES OF ZABARGAD-ISLAND, RED-SEA - A ND, SR, PB, AND OS ISOTOPE STUDY

A Sr, Nd, Pb, and Os isotopic study of peridotites and granulite-facie s gneisses from Zabargad Island in the Red Sea suggests that the tecto nothermal, petrogenetic, and geochemical evolution of these rocks occu rred largely during the Pan African Orogeny rather than the recent ope ning of the Red Sea. Sm-Nd model ages and whole rock errorchrons indic ate that spinel lherzolites and gneisses differentiated from a common depleted mantle source about 700 Ma. The lherzolites were mylonitized, metasomatized, and amphibolitized during a structural event that juxt aposed the peridotites with the gneiss complex and uplifted the gneiss /peridotite complex to relatively shallow crustal levels. Most radiome tric dating schemes suggest a Pan African age for this event. The gnei sses generally have lower Nd-143/Nd-144, Sr-87/Sr-86, Pb-208/Pb-204, P -207/Pb-204 and Pb-206/Pb-204 ratios than the peridotites. They extend linear trends defined by the spinel and amphibole peridotites on Sr-N d, Sm-Nd, and Pb-Pb diagrams, suggesting the gneisses were either the source or buffering medium for the Pan African metasomatism. Only one post-Pan African event had a significant effect on the geochemistry of the gneiss/peridotite complex: shallow level metasomatism by ultrahot (750-900 degrees C) hypersaline solutions with high Sr-87/Sr-86 (appr oximate to 0.710) ratio led to the development of gem-quality olivine crystals as well as low-pressure mineral assemblages in the peridotite s, gneisses and younger rocks. Plagioclase-rich assemblages with appar ent igneous textures (''troctolites'') that are most common in the sou thern peridotite body may have formed by interaction of these fluids w ith peridotite (i.e., are ''pseudo-troctolites''). Metasomatism change d the Sr-87/Sr-86, Sm/Nd, and Re/Os ratios of the plagioclase peridoti tes making them unsuitable representatives of the Pan African mantle.