Petrological and geochemical characteristics of the Neoproterozoic magmatism at the Zabara area, Central Eastern Desert, Egypt: a window into the evolution of the Nubian Shield of Egypt

摘要

The Zabara area in the central Eastern Desert of Egypt is built up of the Shadli Group, comprising older ophiolitic rocks (metagabbro and metabasalt associated with metasediments) and younger metavolcanics. The studied metasediments (me-tagreywackes and metamudstones) have REE patterns characterized by slightly fractionated LREE, flat HREE and absence of an Eu anomalies. They have geochemical features similar to those of oceanic island arc metasediments. The ophiolitic metagabbro and metabasalt have similar major and trace element contents and show a tholeiitic affinity. They are characterized by very low levels of K_2O, Nb, Zr and Rb and flat REE patterns. Low Mg-numbers, Cr and Ni contents indicate a remarkably non primitive magma source. REE modelling reveals that the ophiolitic metagabbro and metabasalt could be derived from 25 percent and 10 percent non-modal batch melting, respectively, of spinel lherzolite source followed by fractional crystallization of olivine, clinopyroxene and plagioclase. The metavolcanics cover the entire spectrum from medium-K meta-andesite to high-K metadacite and metarhyolite with calc-alkaline affinity. They are apparently not primitive melts, they are characterized by low Cr, and Ni contents with low and variable Mg-numbers. The metavolcanics have slightly fractionated LREE and unfractionated, flat HREE patterns. The coherent major element variations and similar incompatible element ratios and REE patterns suggest a comagmatic origin. The REE geochemical modelling reveals that the metavolcanics represent a cogenetic fractionated sequence. The tholeiitic metagabbro, metabasalt and calc-alkaline metavolcanics have geochemical characteristics of both MORB and island arc tectonic environment. Rocks that are transitional between those from MORB and island arc environments are common in back-arc basins. Thus, I suggest that the rocks were erupted in a back-arc basin rather than an island arc setting to which they were generally assigned.