Mineralogical and Geochemical Constraints on the Postcollisional Mafic Magmatism in the Arabian-Nubian Shield: An Example from the El-Bakriya Area, Central Eastern Desert, Egypt

Abstract

In this work, we present new field observations, geochemical data, mineral chemistry, and interpretations from the late Neoproterozoic El-Bakriya mafic intrusion in the central Eastern Desert of Egypt to enhance our knowledge and understanding of the formation of the juvenile continental crust in the Arabian-Nubian Shield (ANS). The field relationships indicate that the gabbroic intrusion is younger than the syntectonic granodiorite but older than the postcollisional El-Bakriya granites. The El-Bakriya mafic intrusion is neither deformed nor metamorphosed and preserves typical primary mineralogical features and texture. The intrusion is composed of three main varieties of gabbro with prominent gradational contacts, namely, troctolite, olivine gabbro, and hornblende gabbro. Both megascopic and imperceptible layering is recognized in the intrusion. El-Bakriya gabbros are characterized by an enrichment in large-ion lithophile elements relative to high field strength elements and a noticeable alkaline affinity owing to fractional crystallization. Different varieties of gabbro show continuous linear composition trends in all major- and trace-element variation diagrams, indicating a cogenetic origin source. The initial magma was an alkaline mafic melt that derived from the mantle with minor crustal assimilation before its emplacement in the upper crust in a stable postcollisional cratonic setting. Mineralogical and geochemical data are combined evidence of an asthenosphere upwelling and lithosphere delamination scenario for the evolution of the El-Bakriya mafic intrusion. On the basis of geothermobarometric calculations, the gabbroic rocks crystallized at a pressure of 5–6.8 kbar and a temperature of 750°C–900°C. The petrological, geochemical, and mineralogical characteristics of the El-Bakriya mafic intrusion are akin to those of the Egyptian postcollisional younger gabbros. These gabbros are totally different from the metamorphosed oceanic and arc-related gabbros, which constitute the earlier juvenile crustal components that formed during the development of the ANS in the late Neoproterozoic.