Petrogenesis of Bir Madi Gabbro-Diorite and Tonalite-GranodioriteIntrusions in Southeastern Desert, Egypt: Implications forTectono-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 Tonalite-Granodiorite Suite (TGrS). The GDC has a weak tonalitic to strong calc-alkaline character and ismade up of olivine gabbro, hornblende gabbro, diorite and monzodiorite. The olivine gabbro is characterized by abun-dance of augite and labradorite with pseudomorphic serpentine. The hornblende gabbro is mainly composed of horn-blende, labradorite, andesine and minor amounts of quartz with or without augite. The diorite consists essentially ofandesine, hornblende, biotite and quartz. The GDC is compositionally broad, with a wide range of SiO2 (46-57 %) andpronounced enrichment in the LILE (Ba and Sr) relative to the HFSE (Nb, Y and Zr). The GDC rocks exhibit petrologicaland geochemical characteristics of arc-related mafic magmas, derived possibly from partial melting of a mantle wedgeabove 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 amphibolite enclaves andfoliated gabbroic xenoliths. Based on the field evidence and geochemical data, the GDC and TGrS are not related to asingle magma type through fractional crystallization. The presence of microgranular amphibolite enclaves in the tonaliticrocks suggest against their generation by partial melting of a mantle-derived basaltic source. The tonalitic magmaoriginated from partial melting of an amphibolitic lower crust by anatexis process at a volcanic arc regime duringconstruction of the Arabian-Nubian Shield. Fractional crystallization of K-feldspar and biotite gave more developedgranodiorite variety from the tonalitic magma. The gabbroic xenoliths are similar in the chemical composition to theinvestigated metagabbros. They are incompletely digested segments from the adjacent metagabbro rocks incorporatedinto the granitic magma through an assimilation process.