The petrogenesis of late Precambrian felsic alkaline magmatism in south Sinai, Egypt

Abstract

Alkaline felsic magmatism represents the final phase of the late Precambrian acid magmatism in south Sinai. The studied felsic suites occur as dyke-like intrusions, exhibit features characteristic of A-type granites. There were two main cycles of magmatic activity separated by a period of magmatic quiescence, which led to a composition gap between them. The earlier cycle includes quartz syenite that represents the precursor of the A-type granite series in south Sinai, whereas the second cycle includes more felsic volcanic-subvolcanic intrusive complexes. Intrusion of the two cycles was controlled by the preexisting structure and individual igneous bodies were emplaced in an extensional tectonic regime during a phase of fracturing and uplift at the end of the Pan-African orogeny. The high variability in geochemical signature of A-type magmas in south Sinai suggests a variety of petrologic processes and reservoirs which reflects the participation of an intraplate mantle reservoir and evolved continental crust. Kenrichment in the rhyolite is a primary feature and attributed to the effect of volatiles which accumulated in the upper part of the magma chamber. In general, A-type magmas in south Sinai can be differentiated according to their magma source and tectonic setting into (1) non-primitive A-type magma (A_Np) and (2) primitive A-type magma (A_p). The non-primitive A-type magma is commonly known in Sinai as Iqna granite, which belongs to the highly fractionated, late- to post-orogenic, calc-alkaline 1- type granites. The primitive A-type magma shows within-plate geochemical characteristics and was emplaced in an anorogenic setting. It is distinguished further into: (i) a magma having characteristics indicative of OIB-mantle contribution (A₁)' and (ii) magma derived from crustal sources (A₂).