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

Azer M.

Acta Geologica Polonica

2006

Vol. 56, no. 4

463-484

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 enolved 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 (ANP) and (2) primitive A-type magma (AP). 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 I-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 (A[1]), and (ii) magma derived from crustal sources (A[2]).