The Umm Naggat metagabbro-diorite suite (UNGD) forms a part of the mafic intrusive province in the Arabian-Nubian Shield. The rocks range in composition from gabbro through diorite to quartz diorite. The suite forms a compositional continuum with a wide major element variation, particularly in terms of SiO 2 (49–61%), CaO (5–11 %) and MgO (3–10%) content. Geochemically, the suite has a tholeiitic/calc-alkaline affinity typical of subduction-related rocks. The compatible behaviour of Mg, Ca, Fe, Cr, Ni and Sc, together with the incompatibility of Na, K, Ba, Rb, Zr and Y, suggest that the evolution of the suite is largely controlled by crystal melt fractionation. In the Egyptian Shield, three chemically different groups of the orogenic-related mahc magmatism are discriminated: the I-type gabbro (group I), to which the UNGD complex belongs, is dominantly a calc-alkaline, arc-related suite characterized by a rather more fractionated trend with evolution towards more silica-rich compositions; the O-type gabbro (group II) includes the ophiolitic-related metagabbros with typical tholeiitic affinity and closely resembling the mid-ocean ridge basalts (MORB); the Y-type gabbro (group III) encompasses calc-alkaline/ tholeiitic fresh gabbros that have a more primitive and less fractionated character compared to group I. The UNGD suite is characterized by low abundances of HFS elements such as Nb, Ti, Zr and Y, which reflects a depleted (relative to MORB) mantle wedge overlying the subducted slab. The addition of a slab-derived fluid to such a depleted source is responsible for the relatively high abundances of LIL elements, particularly K, Rb, Ba, Th and Sr. Subsequently, the primary magma was differentiated largely by fractional crystallization of plagioclase, clinopyroxene, amphibole and magnetite to generate the more evolved rocks in the UNGD suite.