Hornblende gabbros of Wadi Az Zarib area,Central Eastern Desert,Egypt: opaque mineralogy,geochemistry, tectonic setting,and petrogenesis

A small intrusive fresh gabbroic mass intrudes the Neoproterozoic metasediments and Dokhan volcanics of Wadi Az Zarib area, Central Eastern Desert. It is composed of hornblende gabbros and leuco-hornblende gabbros. Their petrography, opaque mineralogy, and geochemistry are addressed to elucidate their tectonic setting and petrogenesis. They represent a subduction-related calc–alkaline magma that evolved in an island arc setting. In terms of maturity, the supposed arc represents an intermediate stage between continental arc and active continental margin. Thermobarometry and physical–chemical data of the parent magma as deduced from compositions of amphiboles, biotite, and plagioclase indicate crystallization temperatures of 931–825 °C at pressures of 6.16–4.01 kbar and H₂O_melt of 6.4–5.2 wt%. Data, as presented, argue in favor of fractional crystallization mechanism to be accounted to the present suite to interpret the observed variations. The evolution of the suite from hornblende gabbros to leuco-hornblende gabbros was accompanied by decreasing of MgO, CaO, Cr, and Ni with simultaneous increasing of Al₂O₃, TiO₂, Na₂O, K₂O, Ba, Rb, Sr, La, and Ce. Residuals calculated during mass balance fractional crystallization modeling suggest that brown and green hornblendes are the main fractionated phases which derived the melt composition towards the leuco-hornblende gabbros.