Anorogenic magmatism: chemical evolution of the Mount El-Sibai A-type complex (Egypt), and implications for the origin of within-plate felsic magmas

The Mount El-Sibai alkaline granitic complex (eastern Egypt) forms an elong ate body, which was emplaced at the extension of a NW-trending shear zone, within voluminous calc-alkaline Pan-African host rocks. The complex is hype rsolvus in nature and is composed of perthite, quartz, alkali amphibole, Fe -rich biotite, and accessory zircon, apatite, fluorite, aenigmatite and ilm enite. Data on mineral chemistry show that the amphibole ranges in composit ion from hastingsite to pure end-member arfvedsonite, and the biotite is la rgely titaniferous annite. Geochemically, the complex is highly evolved in composition (with 72-78 wt% SiO2, and DI values of 85-98), is enriched in R b (48-291 ppm), Nb (28-237 ppm), Y (47-269 ppm), Zr (58-618 ppm), Ga (17-41 ppm) and the REE (176-437 ppm), and depleted in Al, Mg, Ca, Sr and Eu. The complex exhibits a wide trace-element compositional range. The REE pattern s are uniform, parallel to sub-parallel, fractionated ((LaNb)(n) = 4.7), LR EE enriched over HREE, and show prominent negative Eu-anomalies. The albiti zed facies of this complex shows the highest concentrations of large ion li thophile (LIL) and high field strength (HFS) elements. The complex exhibits mineralogical and chemical traits typical of within-plate A-type granites. Mount El-Sibai is interpreted to have been developed during a phase of coo ling, relaxation, crustal attenuation, and fracturing of the newly formed P an-African crust. Results of geochemical modelling indicate that the magma may have formed by a large degree of batch partial melting (F=0.65) of Pan- African calc-alkaline rocks, which had been metasomatized. Metasomatism of source rocks may have been caused by a Na-F-rich fluid phase compositionall y similar to that which produced the albitized facies. The volatile flux ma y have caused fenitization-type reactions along fissures and re-activated P an-African fractures prior to anatexis, and is considered to have played a role as an important agent of heat transfer. Temperature necessary for crus tal anatexis is likely to have been produced as a result of shear heating, caused by a rapid change in the direction of plate motions beneath eastern Egypt during Early Palaeozoic times. The confining pressure must have been released by fissuring of the crust. Magma ascent may have been facilitated by reactivation of pre-existing faults and shear zones. This model may have wider implications for the generation of within-plate felsic magmas in oth er regions.