Late Neoproterozoic strongly peraluminous leucogranites, South Eastern Desert, Egypt--petrogenesis and geodynamic significance.

Late Neoproterozoic garnet-bearing leucogranites are developed locally along thrust faults in the South Eastern Desert, Egypt. This work presents field observations, whole rock major and trace element abundances, Rb--Sr isotope data and mineral chemistry for three occurrences in the Sikait-Nugrus area. Field observations show that the leucogranites cut the faults and their contact with the country rocks is sharp with no indication of contact metamorphism. They were intruded into a low-grade metamor-phosed ophiolitic melange and a high-grade metamorphosed metasedimentary succession of biotite schist composition. Numerous biotite schist enclaves, having irregular and diffuse contacts, are recorded within the leucogranites. Whole rock Rb--Sr ages of the leucogranites from two different localities are 610 ± 20 and 594 ± 12 Ma respectively; they are interpreted as emplacement ages. The leucogranites contain more than 70% SiO₂, and they are strongly peraluminous (A CNK >1.1) with low TiO₂,Fe₂O₃ *, MgO, CaO, Ba, Sr, LREE, Eu/Eu* and Sr/Ba and high Rb, Rb/Zr, Rb/Sr and Rb/Ba. These geochemical parameters and the low initial ⁸⁷Sr/⁸⁶Sr ratios (∼ 0.703) indicate crustal derivation by dehydration partial melting from a juvenile protolith similar to the exposed biotite-rich metasediments. Models for the tectonic setting of these leucogranites suggest their emplacement during an extensional tectonic stage that follows continental collision. It is proposed that crustal heating, caused by decompression along shear zones, is responsible for the production of these granitic melts. The results support previous hypotheses and further document a regional late Neoproterozoic extensional tectonic event, which is probably related to the initial break-up of Gondwana. [ABSTRACT FROM AUTHOR]