Pan-African Younger Granitoids of the Southern Eastern Desert, Egypt: Geology, Geochemical Constraints, and Mineralization

Abstract

Younger granitoids of the Gabal El Farayid—Gabal El Marafawy district in the southern Eastern Desert, Egypt are compositionally syenogranites. Based on petrological and geochemical studies, they are classified as two distinct types: the biotite-hornblende subsolvus El-Marafaway I-type syenogranite; and the biotite-bearing hypersolvus El-Farayid A-type syenogranite. The subsolvus El-Marafawy syenogranite is characterized by low Rb/Sr ratios and high field strength element (HFSE) concentrations (Nb, Ta, Y), fractionated LREEs with flat HREEs, high LREE/HREE ratios, negative Nb-Ta anomalies, and lack of negative Ba and Eu anomalies. The hypersolvus El-Farayid syenogranite (HFS) is characterized by high Rb/Sr ratios and HFSE concentrations, slightly fractionated (HFSI) to unfractionated (HFSII) LREE with a positive slope for the HREE, absence of negative Nb-Ta anomalies and significant negative Ba, Ce, Zr, and Eu anomalies. Based on field and geochemical data, the late-tectonic subsolvus El-Marafawy syenogranite may have been produced by dehydration partial melting of an amphibolitic source, followed by high-pressure fractionation. The post-tectonic hypersolvus El-Farayid syenogranite probably formed from a residual magma by crystallization of biotite, plagioclase, K-feldspar and accessory monazite, allanite, and titanite in a shallow-level magma chamber emplaced along strike-slip faults of a convergent margin. Generally, the syenogranites were generated during a period of rapid tectonic transition from crustal thickening during subduction (subsolvus El-Marafawy syenogranite) to crustal thinning during extension (hypersolvus El-Farayid syenogranite), which represents the final magmatic stage of an extensive arc system. U and Th contents in the above rock types are comparable, indicating that these elements are mainly contained in accessory minerals that controlled their distributions.