The Motaghairat layered mafic-ultramafic intrusion in the Southern Eastern Desert (SED) of Egypt is relatively metamorphosed with no evidence for tectonic contacts with country rocks. This study deals with the geology, petrography, and mineral chemistry of the Motaghairat intrusion, as one of the best examples in the SED, to shed light on its emplacement thermodynamic conditions (P, T, ƒO2) and post-magmatic processes, including metasomatic history and microscale element transferring. The ultramafic rocks comprise plagioclase-bearing peridotite (dunite, lherzolite) and wehrlite with subordinate clinopyroxenite, while the mafic ones include gabbroic rocks with subordinate hornblendite and anorthosite, showing cumulate textures. The analyzed olivine grains are compositionally homogenous with narrow Fo content ranging from 85 in dunite to 81 in olivine gabbro, where their NiO content is generally low (< 0.3 wt%). Clinopyroxene is mainly diopside with subordinate augite (Wo30.76–49.21En39.53–56.81Fs4.52–19.60) and its Mg# decreases systematically from 0.92 in ultramafic rocks to 0.69 in mafic rocks. Orthopyroxene is enstatite in composition (Wo0.19 – 11.47En68.64–84.11Fs8.26–30.1) with a high Mg# (0.91) of ultramafic rocks relative to the low Mg# (0.7) of mafic ones. The mineral chemistry and back-scattered chemical mapping of coarse spinel grains reveal a marked heterogeneity with reversed chemical zoning represented by Cr-Fe-rich cores and Mg-Al-rich rims, forming Cr-spinel grains surrounded by a thin rim of Al-spinel. The occurrence of homogeneous Al-spinel (pale green) and Cr-spinel (dark brownish color) with rims of Al-spinel in ultramafic rocks suggests the formation of the former due to the interaction between spinel and melts, but the latter was formed because of a micro-scale element (Al, Mg, Cr, Fe) transfer between Cr-spinel and Al-rich silicate phases at the sub-solidus state. Some Cr-spinel grains show exsolved texture of Ti-rich ferritchromite due to subsolidus equilibration under slow cooling rates for the spinel-type solid solution. The Motaghairat mafic-ultramafic rocks fractionally crystallized from hydrous tholeiitic basaltic magma (parent melts). They are considered as a late-orogenic layered intrusion generated during the intra-arc rifting in the supra-subduction zone of the Arabian Nubian Shield. These rocks were generated in a slightly oxidizing (ΔNNO +0.03) to moderately oxidizing (ΔNNO +1.7) environment under low pressure (∼ 4.47 ± 0.33 kbar) and shallow depth (∼16.5 ± 1 km) conditions. Their estimated crystallization temperature shows wide ranges from ∼ 850 to 1200°C (∼1000 °C on average) because of variable fractional crystallization degrees that may, in turn, result in different magmatic compositions during cooling. The occurrence of fresh dunite, wehrlite, and clinopyroxenite dykes/veins in serpentinized peridotite, along with the mineral chemistry and textures of peridotites, are considered evidence of melt impregnation and melt–rock interaction during post-magmatic processes.

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