Geochemistry and mineral chemistry of granitic rocks from west Wadi El Gemal area, southern Eastern Desert of Egypt: Indicators for highly fractionated syn- to post-collisional Neoproterozoic felsic magmatism

Abstract

Leucogranite, pegmatite, and aplite from selected areas in the Wadi El Gemal area in the southern Eastern Desert of Egypt were investigated geochemically for their petrogenesis. These rocks represent a significant episode of felsic magmatism during the late stage of the Pan-African orogeny in the evolution of the Arabian–Nubian Shield (ANS) during the Late Neoproterozoic. On a petrographic basis, the leucogranite is sometimes garnetiferous and can be distinguished into monzogranite, syenogranite, and alkali feldspar granite. The analyses of muscovite, biotite, garnet, and apatite reveal the magmatic nature of the studied leucogranite. The investigated leucogranite, pegmatite, and aplite are alkali-calcic, calc-alkaline, and peraluminous. The peraluminous nature of these rocks is evidenced by using the chemical analyses of biotite. These studied rocks show a slight enrichment in light rare-earth elements (LREEs) and large-ion lithophile elements (LILE, especially Rb and Th), with an insignificant depletion of heavy rare-earth elements (HREEs). On a geochemical basis, the leucogranite, pegmatite, and aplite in the study area crystallized from multiple-sourced melts that include mafic, metagraywake, and pelitic. They were derived from melts generated at crystallization temperatures around 568–900 °C for leucogranite, 553–781 °C for pegmatite, and 639–779 °C for aplite based on the Zr saturation geothermometers, and at a pressure around 0.39–0.48 GPa, i.e. shallow depth intrusions. The studied felsic rocks have strong negative Eu anomalies, which are very consistent with an upper crust composition, indicating fractionation of feldspar cumulates. Also, they show a moderate La/Sm ratio indicating combined magmatic processes represented by partial melting and fractional crystallization. Integration of whole-rock chemical composition and mineral microanalysis suggests that felsic magmatism in the west Wadi El Gemal area produced voluminous masses of syn- to post-collisional granite, pegmatite, and aplite. An evolutionary three-stage model is presented to understand late magmatism in the ANS in terms of a geodynamic model. Such a model discusses the propagation of felsic magmatism in the ANS during syn-collisional to post-collisional stages.