Revisiting gabbroic rocks in the Egyptian-Nubian Shield: new perspectives on the tectonic evolution and classification

This contribution reconceives the classification and tectonic evolution of gabbroic rocks in the Egyptian segment of the Arabian-Nubian Shield (ANS), where gabbroic intrusions of diverse ages and geodynamic settings offer an opportunity to give new insights into the tectonomagmatic evolution of the shield’s crust. Early classifications were mainly based on field criteria and mineral compositions. Four distinct gabbro types have been identified: ophiolitic metagabbros, arc-related metagabbros, collision-related gabbros, and anorogenic alkaline gabbros; each reflects a unique petrogenesis and tectonic setting. This new refined classification elucidates the petrogenesis of Egyptian gabbros and emphasizes their formation in peculiar tectonic processes, including mid-ocean ridge spreading, subduction, continental collision, post-collisional extension, and within continental plate up-arching and rifting. Ophiolitic and arc-related gabbros are affiliated with the early pre-collision stages, while collision-related gabbros are associated with the ANS’s collisional phases. The latter Phanerozoic within-plate extension facilitated the formation of anorogenic alkaline gabbros as the least evolved members in some alkaline ring complexes. This review provides a deeper overview and fills gaps in the interpretation of the complex magmatic and tectonic processes that influenced the Arabian-Nubian Shield from the Early Neoproterozoic to the Late Cretaceous period.