Petrogenesis of the post-collisional rare-metal-bearing Ad-Dayheen granite intrusion, Central Arabian Shield

Abstract

At Hadhb't Ad-Dayheen, in the central Arabian Shield, a post-collisional igneous complex called the Ad-Dayheen intrusion is exposed. It was emplaced in the Early Ediacaran (613–625 Ma), during the final tectono-magmatic stage of Arabian Shield development. Despite limited and discontinuous ring-shaped outcrops due to alluvial cover and later faulting, three pulses of intrusion can be recognized in the field: an early pulse of monzogranite; a second pulse of syenogranite and alkali feldspar granite; and a final pulse of alkaline and peralkaline granite, mineralized microgranite, and pegmatite. Samples show distinctively low contents of CaO, MgO, and Sr in contrast to elevated concentrations of alkalis, Rb, Nb, Y, Ta, Hf, Ga, Zr and rare-earth elements (REE); these are common characteristics of post-collisional rare-metal-bearing A-type granites. The suite displays positive Nb–Ta anomalies and pronounced negative Eu anomalies (Eu/Eu* = 0.11–0.35). The alkaline/peralkaline granites and microgranite of the Ad-Dayheen intrusion feature disseminated mineralization, whereas mineralization is localized in the pegmatite. The primary magma feeding the Ad-Dayheen intrusion was mostly generated by partial melting of the juvenile crust of the Arabian Shield, with a minor mantle contribution. We argue that an episode of lithospheric delamination led to crustal uplift, erosional decompression, and generation of mantle melts that supplied heat to drive crustal melting. The anatectic deep crustal melts assimilated a F-bearing component that also added rare metals to the magma. Each pulse can be described by a fractional crystallization model, but the parental liquid of each subsequent pulse was first modified by further addition of fluorine and rare metals and loss of CaO, Sr, Ba, and Eu due to fluorite fractionation. Texture and morphology of the ore minerals indicate that mineralization (U, Th, Zr, Nb, Ta, Y, Hf and REE) took place in two stages: a magmatic stage coinciding with emplacement of the intrusion, followed by a hydrothermal stage. The magmatic process enriched the residual melt in high field strength elements (HFSE) and REE. The later hydrothermal stage further localized these elements and increased their concentrations to economic grades. The pegmatite is highly mineralized and contains high concentrations of U (81–179 μg/g), Th (244–600 μg/g), Zr (2397–14,927 μg/g), Nb (1352–2047 μg/g), Ta (96–156 μg/g), Y (828–2238 μg/g), Hf (131–377 μg/g) and ∑REE (1969–4761 μg/g).

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