Geochemistry and mineralogy of the Jebel Aja Igneous Intrusion and the associated exotic pegmatites, Arabian Shield, Saudi Arabia

Abstract

We describe the Jebel Aja igneous intrusion (JAII), which sits at the farthest northeastern edge of the exposed Arabian Shield in Saudi Arabia. The JAII consists of late Neoproterozoic post-collisional A-type granites, and was emplaced during tectonic extension. It is a composite pluton consisting of a granitic core (monzogranite, syenogranite and alkali feldspar granite) surrounded by a later phase of alkaline and peralkaline granites. Pegmatites occur as pockets and dykes at the contact between the alkaline/peralkaline granites and syenogranite. The pegmatites have sharp contacts with their host granites and show variable colors (principally pink and white), grain sizes, textures, and mineral compositions. We report here that the white pegmatites host pockets with a unique mineralogy that includes the first observation of pyroxmangite, thortveitite and thalénite-(Y) in the whole Arabian Shield as well as a novel (Sc, Y, vacancy)-rich pyroxenoid and a vacancy-rich bustamite with excess octahedral Si. The pyroxmangite-dominated lithology also features accessory quartz, albite, spessartine, pyrochroite, muscovite, and fluorite. The presence of pyroxmangite rather than rhodonite, together with vacancy-rich and Si-excess pyroxenes, suggests an elevated pressure of emplacement. The granites of the JAII are highly evolved and have undergone extreme fractional crystallization of feldspars, leading to increases in the concentration of volatile and incompatible elements in the residual melt, which resulted eventually in injection of coeval pegmatites into the mostly solidified host granite. The syenogranite and alkaline/peralkaline granite of the JAII contain sodic amphibole and have geochemical characteristics common among within-plate rare-metal bearing A-type rocks. They have positive Nb-Ta anomalies that increase from syenogranite to alkaline/peralkaline granite. The pink pegmatite is highly mineralized and contains high concentrations of Nb (1540–1769 μg/g), Ta (103–136 μg/g), Y (1116–1616 μg/g), Zr (6362–9707 μg/g), Hf (215–264 μg/g), Th (278–384 μg/g), U (110–147 μg/g), and ∑REE (2334–3251 μg/g).