Cold plutonism in the Arabian–Nubian Shield: evidence from the Abu Diab garnet-bearing leucogranite, central Eastern Desert, Egypt

Abstract

The Neoproterozoic Abu Diab garnet-bearing leucogranite (SiO 2 73.7 – 76.8 wt%; total alkalis 8.1 – 9.3 wt%) was emplaced during the late to post Pan-African Orogeny. The pluton is almost homogeneous in composition, weakly peraluminous (molecular Al 2 O 3 /(CaO + Na 2 O + K 2 O) = 1.04 – 1.12) and approaches the minimum melt composition in the water-saturated Qz–Ab–Or system. The chondrite-normalized patterns are almost unfractionated, with weak tetrad effects and prominent negative Eu anomalies. Ba, Sr, P, Eu and Ti show strong negative anomalies in primitive-mantle normalized multi-element diagrams. Magmatic garnet is dominantly a spessartine (66 – 67 mol %)–almandine (27 – 29 mol %) solid solution. Our results indicate that the Abu Diab granite magma formed from low-degree eutectic melting of a felsic pelite source in the lower to middle continental crust. Zirconium saturation thermometry indicates magma generation temperatures of 732 ± 28°C (2σ), placing the Abu Diab leucogranite in the ‘cold granite’ group. This indicates that muscovite breakdown is the dominant reaction in the melt source. The high normative Or, the increasing Rb/Sr concomitant with decreasing Sr and Ba, and the sharp negative Eu anomalies are consistent with fluid-absent melting. Field relations and petrographic observations indicate the emplacement of Abu Diab granites after the closure of the Mozambique Ocean and collision of East and West Gondwana, most probably during fast adiabatic uplift and decompression of the thickened crust. Supplementary material: The locations of the peraluminous leucocratic granites in the Arabian-Nubian and other supplementary data for this paper are available at https://doi.org/10.6084/m9.figshare.c.3815293.