Tracking the transition from subduction-related to post-collisional magmatism in the north Arabian-Nubian Shield: A case study from the Homrit Waggat area of the Eastern Desert of Egypt

Field and geochemical observations of the granitoids of the Homrit Waggat area in the central Eastern Desert of Egypt reveal two magmatic phases. The early phase of weakly deformed subduction-related calc-alkaline rocks includes tonalite and granodiorite. We name the later phase the Homrit Waggat Pluton (HWP); it includes undeformed syenogranite, alkali feldspar granite, and minor albitized granite. The tonalite and granodiorite have distinct negative Nb-Ta anomalies and lower alkalis, REE, Nb, Zr, and Hf than the HWP. The early magmatic pulse is a subduction-related suite, likely generated by underplating of mantle-derived magmas that triggered partial melting of mafic lower crust; mixing of these melts led to intermediate magma that further fractionated to tonalite and granodiorite. The HWP granites of the late magmatic pulse are transitional from a subduction-related to an anorogenic within-plate environment, plausibly generated by post-collisional lithosphere delamination. Although the parent magma of the HWP was I-type, extensive fractional crystallization produced residual liquids with A(2)-type character. Albitized granites are found only along the outer margin of the HWP, and contacts with the alkali feldspar granite are gradational, suggesting fluid interactions at a late stage of crystallization. The original textures of the albitized granites are preserved, but their bulk composition was modified by the production of Na-rich minerals and the removal of K, REE, and some trace elements by fluids.