Al-compositional variation in ophiolitic chromitites from the south Eastern Desert of Egypt: Petrogenetic implications

Abstract

Podiform chromitite bodies of variable sizes are hosted in the serpentinized ophiolitic dunite-harzburgite of the Arabian Nubian Shield in Arais (AR) and Malo Grim (MG) areas in the southern Eastern Desert of Egypt. The composition of chromian spinel in most of the studied chromitite samples has not changed significantly from its magmatic composition. However, the fractured and disseminated-textured ores were slightly metamorphosed under the lower greenschist facies (AR) to the transitional greenschist–amphibolite facies (MG). Alteration is recorded in individual chromite grains in the form of optical and chemical zoning. Core to rim chemical trends are expressed by MgO- and Al2O3- decrease, mainly compensated by FeO and Cr2O3 increases. The unaltered chromian spinel in these bodies displays a large range of Al estimating Cr# (Cr/Cr + Al) atomic ratio from 0.57 to 0.89. In AR chromitite, the Al-richer chromite interstitially occurs within the Al-poorer one, whereas in MG chromitite the variation of Al is between massifs. The calculated compositions of melts in equilibrium with chromian spinel of Al-richer and Al-poorer chromites are island arc low-Ti tholeiites and boninites, respectively. The formation of high-Cr chromitites is interpreted as a result of the extensive reaction of harzburgite with migrating island arc tholeiite melts of boninitic affinity. Melt–rock reaction produced boninitic melts and porous dunitic channels in which the mixing/mingling of melts promoted crystallization of monomineralic high-Cr chromian spinel. The presence of Al-richer chromite interstitially in the Al-poorer chromite in the AR ore was most probably due to metasomatism by Al-rich melts, which may be related to the intrusion of the Al-rich websterite. In MG, on the other hand, the wide distribution of Al in chromitites in small district most probably reflects temporal and/or spatial variations in the types of melt (boninitic and tholeiitic) that were generated from, and emplaced in, subarc mantle domains in a supra-subduction zone environment. Key words: Al-compositional variation, podiform chromitite, mineral chemistry, alteration, tholeiitic and boninitic melts, supra-subduction zone, south Eastern Desert of Egypt.