Petrogenesis and tectonic setting of late Precambrian ensimatic volcanic rocks, central eastern desert of Egypt

Abstract

Abstract Early stages in the geologic evolution of the central eastern desert of Egypt (CED) reflect an intense episode of ensimatic volcanic activity similar to modern magmatism of the ocean floors and island arcs. This paper reports results from studies of the petrology and petrogenesis, and interprets the significance of these Late Precambrian volcanic rocks. A three-fold stratigraphy is preserved in the basement of the CED. A basal section of oceanic crust includes ultramafics, gabbros and pillowed basalts. These older metavolcanics (OMV) are conformably succeeded by dominantly volcanogenic metasediments, which are in turn succeeded by a dominantly andesitic, calc-alkaline sequence of younger metavolcanics (YMV). The OMV and YMV are largely restricted to the CED in Egypt, but analogous terranes are found in northern Arabia. (40–400 ppm) and Ni (30–260 ppm). They are poor in K 2 O (0.05–0.92%), Rb (0.3–5.0 ppm) and Ba (11–89 ppm). On Ti-Zr-Cr-V-Ni-P discriminant diagrams, the OMV plot in the field of modern abyssal tholeiites. High K/Rb (450–1800) and light REE depletions support this inference, although K/Ba (25–45) is lower than modern mid-ocean ridge basalts (MORB). The sum of OMV geochemical characteristics requires that these magmas were derived by the fractional fusion of the mantle. It is suggested that the OMV were generated by 20–25% fractional melting of previously depleted mantle at depths of less than 60 km. Relatively little fractionation accompanied ascent to the surface, where the OMV were erupted in a primitive crustal environment, either a small oceanic rift or a back-arc basin. Metamorphism of the YMV resulted in little elemental redistribution. These andesites have sub-alkaline clinopyroxenes and major-element geochemical characteristics indistinguishable from modern calc-alkaline andesites. YMV andesites in the central and western CED have K/Rb = 400–600, K/Ba = 20–40 and are light REE-enriched and heavy REE depleted. High concentrations of Cr (50–150 ppm) and Ni (20–100 ppm) and low initial 87 Sr/ 86 Sr ratios (0.7028–0.7030) indicate that these magmas were generated by melting in the mantle. Modelling studies and consideration of experimental data indicate that these andesites were formed by 2–10% fractional fusion of hydrous, undepleted, garnet therzolite at depths of 65 km or more in the mantle. The data show that an intense episode of instability, convection, and widespread melting occurred in the mantle beneath Afro-Arabia at the end of the Precambrian.