Geological setting of the Meatiq metamorphic core complex in the Eastern Desert of Egypt based on amphibolite geochemistry

Abstract

Large parts of the Central Eastern Desert of Egypt consist of Neoproterozoic nappes of ophiolite and island-arc volcanic rocks. Beneath these nappes metamorphic basement domes are exposed in tectonic windows such as the Meatiq basement. The mineral and whole rock chemistry of amphibolites included in the teconically lowermost granitoid gneiss, within the metasedimentary cover of the gneiss and within shear zones bordering the Meatiq basement dome, are investigated to determine their geodynamic setting. The amphibolites consist mainly of plagioclase, hornblende, quartz and, locally, clinopyroxene. Themetamorphic mineral assemblage is retrograded from upper amphibolite-facies peak metamorphic conditions to lower amphibolite-facies. The Niggli c-mg plot identifies them as orthoamphibolites, and whole rock chemistry indicates a basalt to basaltic andesite composition for most of the amphibolites. Immobile trace elements such as Zr, Y and Ti suggest that one group of amphibolites is derived from within-plate basalts and another belongs to the MORB tectonic setting. This is consistent with the result of the AFM plot, which indicates a tholeiitic composition for all the amphibolites. REE patterns confirm the trace element results, with a typical LREE depletion for the N-type MORB amphibolites. The first group of amphibolites is enriched in LREE and displays a concave pattern. The amphibolite inclusions in the Um Ba'anib gneiss record different geologic settings - from the formation of oceanic crust to within-plate basaltic magmatism - prior to the intrusion of the Um Ba'anib granitoid.