Composition, age, and origin of the ~620 Ma Humr Akarim and Humrat Mukbid A-type granites: No evidence for pre-Neoproterozoic basement in the Eastern Desert, Egypt

Abstract

The Humr Akarim and Humrat Mukbid plutons, in the central Eastern Desert of Egypt, are late Neoproterozoic post-collisional alkaline A-type granites. Humr Akarim and Humrat Mukbid plutonic rocks consist of subsolvus alkali granites and a subordinate roof facies of albite granite, which hosts greisen and Sn-Mo-mineralized quartz veins; textural and field evidence strongly suggest the presence of late magmatic F-rich fluids. The granites are Si-alkali rich, Mg-Ca-Ti poor with high Rb/Sr (20-123), and low K/Rb (27-65). They are enriched in high field strength elements (e. g., Nb, Ta, Zr, Y, U, Th) and heavy rare earth elements (La n/Yb n = 0. 27-0. 95) and exhibit significant tetrad effects in REE patterns. These geochemical attributes indicate that granite trace element distribution was controlled by crystal fractionation as well as interaction with fluorine-rich magmatic fluids. U-Pb SHRIMP zircon dating indicates an age of ~630-620 Ma but with abundant evidence that zircons were affected by late corrosive fluids (e. g., discordance, high common Pb). εNd at 620 Ma ranges from +3. 4 to +6. 8 (mean = +5. 0) for Humr Akarim granitic rocks and from +4. 8 to +7. 5 (mean = +5. 8) for Humrat Mukbid granitic rocks. Some slightly older zircons (~740 Ma, 703 Ma) may have been inherited from older granites in the region. Our U-Pb zircon data and Nd isotope results indicate a juvenile magma source of Neoproterozoic age like that responsible for forming most other ANS crust and refute previous conclusions that pre-Neoproterozoic continental crust was involved in the generation of the studied granites. © 2012 Springer-Verlag.