Continental back-arc basin origin of some ophiolites from the Eastern Desert of Egypt

Summary Geochemical and petrographical data of three ophiolitic pillow metavolcanic occurrences from the central Eastern Desert of Egypt are presented. The investigated rocks show a subalkaline, tholeiitic affinity. Chemical data indicate that the metavolcanics have transitional within-plate basalt to island-arc basalt features, which are characteristics of basalts formed in ensialic back-arc basins. The association of the investigated ophiolites with volcanoclastic metasedimentary rocks of marine to continental facies is a further confirmation of their ensialic evolution. This suggestion, along with the geochronologic, isotopic and crustal growth rate evidences, revives interest in models that involve contribution from a pre-Pan-African continental crust at least in the southern part of the Egyptian Shield. Mixing between a depleted mantle-derived magma and an enriched crustal melt, a process similar to AFC (assimilation and fractional crystallization), is suggested for the evolution of the investigated rocks. This study provides evidence for formation of some ophiolites in the Eastern Desert of Egypt in continental (ensialic) back arc basins.