POSSIBLE ORIGIN OF THE OPHIOLITES OF EASTERN DESERT, EGYPT, FROM GEOCHEMICAL PERSPECTIVES

Recent recognition of a number of ophiolites from the Precambrian basement of the central and southern subregions of the Eastern Desert region of Egypt is documented along with their field and geochemical perspectives. A synthesis is presented based on previously available chemical data on major and trace elements of 475 carefully filtered ophiolitic rock samples and their mineral constituents. The data has been partitioned into that for five units of rock-types spanning 93 samples of ultramafics (UM), 128 of metagabbros (MG), 20 of sheeted dykes (SHD), 206 of metavolcanics (MV), and 28 of metasediments (MS). Similarities in the values for lavas and sheeted dykes appear to be a manifestation of their cogenetic origin. Amongst the Eastern Desert ophiolites (EDO), those belonging to the harzburgite subtype are prevalent, although a few lherzolite subtype ophiolites occur in the Central Eastern Desert (CED) subregion. Differentiation of the parental magmas led to the formation of volcanics at their latest stages, and resulted in the igneous rock series of subalkaline, mostly tholeiitic character. Distinctions between the ophiolites from CED and Southern Eastern Desert (SED) subregions are emphasized. Whole-rock geochemistry inclusive of REE variations indicate complex tectonic settings. Gerf ophiolite of SED revealed affinities to normal MORB type genesis. Protoliths of greywackes around ophiolites indicate the existence of both oceanic and continental island arcs. Mineral chemistry of clinopyroxenes, amphiboles, plagioclase feldspars, chromites, serpentines, and iron oxides are also used in comparisons with other known ophiolites of different ages. In many aspects, such as pyroxene chemistry, the implied geotectonic settings of the Eastern Desert ophiolites belong to a wider span of settings than the previously expressed back-arc basinal origin of all these ophiolites.