Hornblende gabbros of Wadi Az Zarib area, Central Eastern Desert, Egypt: opaque mineralogy, geochemistry, tectonic setti
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ORIGINAL PAPER
Hornblende gabbros of Wadi Az Zarib area, Central Eastern Desert, Egypt: opaque mineralogy, geochemistry, tectonic setting, and petrogenesis Mohamed W. Ali-Bik & Said H. Abd El Rahim & Wael Abdel Wahab
Received: 2 April 2013 / Accepted: 24 June 2013 # Saudi Society for Geosciences 2013
Abstract 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 H2Omelt 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. Keywords Hornblende gabbros . Subduction related . Calc–alkaline magma . Petrogenesis
Introduction The Egyptian gabbroic rocks are widely distributed in all tectonic environments from ophiolitic to continental settings (e.g., Takla et al. 1981; El Gaby et al. 1990; Hassan and Hashad 1990; Ali Bik 1999a). Based on chronologic data, field relations, and tectonicsettings,fourgroupsofgabbroicrocksaredistinguished (e.g., Hassan and Hashad 1990): (a) ophiolitic metagabbro, (b) island arc intrusive metagabbro–diorite complex, (c) intrusive fresh gabbros±ultramafic associations, and (d) gabbroic rocks associated with Phanerozoic ring complexes. The first three groups are of Neoproterozoic age and related to Pan-African basement rocks. The studied Az Zarib gabbros pertain to the third group of gabbroic rocks which are widely distributed as small intrusive magmatic masses in the Egyptian basement belt. These small basic rocks are mantle derivates and pertain to the tectogenetic association in sense of El Gaby et al. (1990), which substantiates the evolution of the crust towards the continental margin Cordilleran stage. In the Egyptian basement sequence, these gabbros post-date the calc–alkaline older granitoids and pre-date the calc–alkaline younger granitoids (e.g., Akaad and Noweir 1969; El Ramly 1972). The present work is concerned with mineralogy, g
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