Petrogenesis of post-orogenic Pan-African rare-element granitic pegmatites in the western Arabian-Nubian Shield, Aswan area, southern Egypt

Abstract

• Aswan pegmatites evolved via fractional crystallization of the parental granitic magma . • The rare-element pegmatites were formed in a post-orogenic setting. • Post-magmatic hydrothermal fluids play an important role in their formation. Pegmatites with rare-element mineralization from the Aswan area in southern Egypt intruded the post-orogenic Pan-African granitic rocks and the associated gneisses. The granitic pegmatites show a regular pattern of concentric zoning and display a weak affinity to the Nb-Y-F (NYF) pegmatite family. These pegmatites consist mainly of K-feldspar, albite, quartz, and subordinate muscovite, accessory minerals include Nb-Ta oxide minerals, monazite-(Ce), xenotime-(Y), zircon, rutile, and magnetite. Field observations combined with the petrography, mineralogy and geochemistry of pegmatites and associated rocks in the Aswan area indicate that the pegmatites evolved via fractional crystallization of the parental granitic magma with an effective role for the post-magmatic alterations. The wide compositional variations of Nb-Ta minerals in term of their major and trace elements reflect the pegmatite formation stages. Primary magmatic generation of columbite-(Fe) occurs as homogeneous grains or showing oscillatory zoning with high Nb/Ta and Fe/Mn ratios. Late ixiolite shows patchy and irregular zoning with high Sc and W replacing the earlier regular texture, indicating metasomatic replacement. Electron microprobe U–Th–Pb dating of monazite from the studied pegmatites yielded post-orogenic Pan-African age of 578 ± 5 Ma, comparable with age of the fine-grained granite in this area, but it is younger than the previously estimated U–Pb zircon age of surrounding the Monumental granites and gneisses (∼600 to 620 Ma). The similarity in age and geochemical characteristics of the pegmatites and the fine-grained granite proved their genetic relationship.