Geochemistry and Petrogenesis of a Late Proterozoic Volcanic Sequence in the Magal Gebriel Area, South Eastern Desert, Egypt

Abstract

Metabasalts and meta-andesites together with small amounts of metadacite constitute the Magal Gebriel volanic suite. Restricted amounts of unmetamorphosed rhyolitic rocks are also present. The metavolcanics show a wide range of Mg numbers (0.38 - 0.71), and are enriched in LILE and LREE. The rhyolitic rocks are very rich in Si02 (72.0-76.6 wt.%), low in Ti02, Sc, V and high in LILE and LREE. The geochemical trends defined by the basaltic to dacitic volcanip rocks suggest fractional crystallization from a basaltic liquid by removal of pyroxene, plagioclase and opaque minerals. The Magal Gebriel metavolcanics (MGM) posses the geochemical characteristics of an island arc tectonic setting while those of the rhyolitic rocks posses that ofwithin-plate tectonic setting. The nearly flat HREE patterns of the studied metavolcanic rocks {(Gd/Yb)N= 1.2-1.5} suggest that they originated from a garnet-free peridotite source. The parent magma may represent low-degree (5 - 8%) partial melts of a depleted mantle. Fractional crystallization of the primitive magma yielded the different volcanic rocks of Magal Gebriel volcanic suit. The enrichment of the subarc mantle in the sub-duction-derived component in Magal Gebriel area was modelled. The calculated concentrations of some mobile elements are generally lower than those estimated for other island arcs in western Pacific and Mexico but with a similar order of enrichment: Ba > Rb > K > La > Sr > Ce. The rhyolitic rocks have REE patterns that are nearly parallel to those of the metavolcanic rocks suggesting similar parental magmas. However, the rhyolitic mother magma must have subjected to severe differentiation and extraction of large amount of plagioclase as indicated by the strong negative Eu anomalies and low Sr contents.