Integration of Whole-Rock Geochemistry and Mineral Chemistry Data for the Petrogenesis of A-Type Ring Complex from Gebel El Bakriyah Area, Egypt

Abstract

El Bakriyah Ring Complex (BRC) is a prominent Neoproterozoic post-collisional granite suite in the southern part of the Central Eastern Desert of Egypt. The BRC bears critical materials (F, B, Nb, and Ta) in appreciable amounts either in the form of rare-metals dissemination or in the form of fluorite and barite vein mineralization. The complex consists of inner syenogranite and outer alkali feldspar granite that have been emplaced in a Pan-African assemblage made up of granitic country rocks (granodiorite and monzogranite), in addition to post-collisional fresh gabbro as a part of the Arabian-Nubian Shield (ANS) in northeast Africa. Granites of the BRC are characterized by enrichment in silica, alkalis, Rb, Y, Ga, Nb, Ta, Th, and U and depletion in Sr, Ba, and Ti. Geochemical characterization of the BRC indicates that the magma is a crustal melt, which originated from the partial melting of metasedimentary sources. Concentrations of rare-earth elements (REEs) differ in magnitude from the ring complex and its granitic country rocks but they have similar patterns, which are sub-parallel and show LREEs enrichment compared to HREEs. The presence of a negative Eu anomaly in these rocks is related to plagioclase fractionation. The abundance of fluorine (F) in the different granite varieties plays an important role in the existence of a tetrad influence on the behavior of REEs (TE1, 3 = up to 1.15). Geochemical parameters suggest the crystallization of the BRC granite varieties by fractional crystallization and limited assimilation. Mn-columbite and Mn-tantalite are the most abundant rare-metals dissemination in the BRC granite varieties. We present combined field, mineralogical and geochemical data that are in favor of magma originating from a metasedimentary source for the BRC with typical characteristics of A-type granites. Our geodynamic model suggests that the Gebel El Bakriyah area witnessed the Neoproterozoic post-collisional stage of the ANS during its late phase of formation. This stage was characterized by the emplacement of fresh gabbros followed by the syenogranite and alkali-feldspar granite of the BRC into an arc-related assemblage (granodiorite and monzogranite). It is believed that the mantle-derived magma was interplated and then moved upward in the extensional environment to a shallower level in the crust owing to events of lithospheric delamination. This presumably accelerated the processes of partial melting and differentiation of the metasedimentary dominated source (Tonian-Cryogenian) to produce the A-type granites building up the BRC (Ediacaran).