Petrogenesis of Bir Madi Gabbro-Diorite and Tonalite-Granodiorite Intrusions in Southeastern Desert, Egypt: Implications for Tectono-Magmatic Processes at the Neoproterozoic Shield

Abstract

The Neoproterozoic rocks of the Bir Madi area, south eastern desert, comprise a Metagabbro-Diorite Complex
(GDC) and a Tonalitc-Granodiorite Suite (TGrS), The GDC has a weak tonalitic to strong calc-alkaline character and is
made up of olivine gabbro, hornblende gabbro, diorite and monzodiorite. The olivine gabbro is characterized by abundance of augite and labradorite with psuedomorphic  serpentine. The hornblende gabbro is mainly composed of hornblende, labradorite, andesine and minor amounts of quartz with or without augite. The diorite consists essentially of andesine, hornblende, biotite and quartz. The GDC is compositionally broad.,with a wide range of SiO₂  (46-57 %) and pronounced enrichment in the LILE (Ba and Sr) relative to the HFSE (Nb,Y and Z). The GDC rocks exhibit petrological and geochemical characteristics of arc-related mafic magmas, derived possibly from partial melting of a mantle wedge above an early Pan-African subduction zone of the Neoproterozoic Shield. The tonalite and granodiorite have a calc alkaline affinity and show the geochemical signatures of I-type granitoids. The TGrS contains an amphibolite enclaves and foliated gabbroic xenoliths. Based on the field evidence and geochemical data, the GDC and TGrS are not related to a
single magma type through fractional crystallization. The precsence of microgranular amphibolite enclaves in the tonalitic
rocks suggest against their generation by partial melting of a mantle-derived basaltic source. The tonalitic magma originated partial melting of an amphibolite lower crust by anatexix process at the volcanic arc regime during  constnuction of the Arabian-Nubian Shield. Fractional crystalization of K-feldspar and biotite gave more developed granodiorite variety from the tonalitic magma. The gabbroic xenoliths are similar in the chemical composition to the investigated metagabbros. They are incompletely digested segments from the adjacent metagabbro rocks incorporated into the granitic magma through an assimilation process.