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

Abstract

The Neoproterozoic rocks of Bir Madi area, south eastern desert, comprises of Metagabbro-Diorite Complex  (GCD) and Tonalite-Granodiorite Suite (TGrS). The GDC has a weak tonalitic to strong calc-alkaline character and is made up of olivine gabbro, hornblende  gabbro, dionite 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. TheGDC is compositionally broad.,with a wide range of SiO₂, (46-57 %) and pronounced enichment 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 calcalkaline affinity and show the geochemical signatures of I-type granitoids. The TGrS contains 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 prcsence of microgranular amphibolite enclavcs in the tonalitic rocks suggest against their generation by partial melting of a mantle-derived basaltic source. The (tonalitic magma originated from partial melting of an amphibolitic lower crust by anatexis process at a volcanic arc regime during construction of 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.