Petrogenesis of anorogenic peralkaline granitic complexes from eastern Egypt

摘要

The Pan-African orogenic shield rocks of eastern Egypt were intruded by several anorogenic within-plate granitic complexes, including Mounts Abu-Kharif and El-Dob. These two massifs were emplaced at the intersection of a fault system and a shear zone. The two massifs are made up of hypersolvus peralkaline granites, consisting essentially of perthitic alkali feldspar (55-65 vol. percent), quartz (30-35 percent), and alkali amphibole (ferrorichterite to arfvedsonite; 5 - 12 percent), with accessory zircon, apatite and ilmenite. The rocks are evolved in composition, are relatively enriched in Nb (53-75 ppm), Y (34-72 ppm), Zr (421-693 ppm), Ga (26-29 ppm), and the REE (294-562 ppm), and depleted in Al, Mg, Ca, Sr, Ba and Eu. The REE patterns are sub-parallel, LREE-enriched over HREE, and show prominent negative Eu anomalies. The rocks exhibit mineralogical and chemical traits typical of within-plate A-type granites. Rb-Sr radiometric age dating produced a Cambrian age of 522+ -21 Ma, and an initial~(87)Sr/~(86)Sr ratio of 0.7080+ -0.0042. Thus, the investigated peralkaline granitic rocks were emplaced following the termination of the Pan-African orogeny. The rocks are interpreted to have formed in an extensional tectonic environment during a phase of cooling, relaxation, crustal attenuation, and fracturing of the newly-formed shield. Results of geochemical modelling indicate that the magma may have formed by a large degree of batch partial melting (F = 0.57) of Pan-African calc-alkaline shield rocks, which had been metasomatized possibly by a Na-rich fluid. The volatile flux may have caused fenitization-type reactions along fissures and re-activated Pan-African fractures prior to anatexis, and is considered to have played a role as an important agent of heat transfer. Shear heating, caused possibly by a rapid change in the direction of plate motions beneath eastern Egypt during the Early Palaeozoic, is likely to have produced temperatures necessary for crustal anatexis. The confining pressure must have been released by fissuring of the crust. Magma ascent may have been facilitated by reactivation of pre-existing Pan-African fractures.