ON THE CLASSIFICATION AND ORIGIN OF THE NEOPROTEROZOIC OPHIOLITIC MELANGE OF THE CENTRAL EASTERN DESERT, EGYPT: GEOLOGICAL AND GEOCHEMICAL STUDIES

The Eastern Desert of Egypt is a part of the Neoproterozoic Arabian–Nubian Shield (ANS). Neoproterozoic ophiolites and ophioitic mélange are common in the Arabian–Nubian Shield (ANS). Field geology, petrography, whole rock chemistry and mineral chemistry have been used to investigate the petrology and origin of the ophiolitic rocks and their significance in the classification and origin of the ophiolitic mélange of the central Eastern Desert of Egypt.

Different occurrences of ophiolitic melange in the Central eastern Desert of Egypt have subjected to investigation. The melange is composed of exotic and native blocks and fragments of variable sizes and types set in a fine sheared volcaniclastic matrix. The predominant exotic blocks in the mélange are mainly ophiolitic and include serpentinites and metamorphosed ultramafics, metagabbros, massive and pillowed metabasalts and pelagic sediments. These ophiolites commonly occur as dismembered blocks and fragments, together with native fragments of metavolcanics and their metapyroclastic counterparts. Ophiolitic blocks and fragments of the ophiolitic melange occur as mountain-size sheets and blocks, tectonic slices, blocks-in-matrix, and fragments up to boulder size. Based on the mode of occurrences of these exotic ophiolitic components, the ophiolitic mélange of the Central Eastern Desert is classified and mapped into tectonic mélange, olistostrome and olistostromal mélange.

The chemical analyses of primary chrome spinels in metamorphosed ultramafic rocks have been used for petrogenetic and petrotectonic indications. Compositional variation of the investigated chrome spinel clearly indicates three distinct groups based on their cr#: group I has low Cr# (0.3-0.42), group II has moderate Cr# (0.45-0.68), whereas group III has high Cr# (0.78-0.88). Two types of ultramafic rocks in the ophiolitic mélange have been recognized, which display mid-ocean ridge (MOR) and suprasubduction zone (SSZ) tectonic setting. The MOR-type rocks include lherzolite and Cpx-bearing harzburgite with low-Cr# (<0.6), whereas the SSZ-type rocks include the Cpx-free harzburgite and dunite with high-Cr#. Ophiolitic metabasalts mostly define a subalkaline suite characterized by low K and moderate to high Ti contents, that has tholeiitic affinities.

The metamorphosed ultramafic represent fragments of oceanic lithosphere that formed and/or modified in an arc/back-arc environment and are comparable to those of ophiolitic harzburgite and Alpine ophiolites. The ophiolitic metabasalts within the mélange show similarity to MORB and marginal basin tectonic setting. The whole rock associations of the ophiolitic melange have been subjected to different events of deformation and metamorphism. Ophiolitic components have incorporated into the mélange through sedimentary and/or tectonic processes. Back-arc or inter-arc basin is favored to be the possible site for the formation of the 'ophiolitic mélange'.