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Journal of the Geological Society

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The Jabel Sayid complex, Arabian Shield: geochemical constraints on the origin of peralkaline and related granites

¹ Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK76AA
² Faculty of Earth Sciences, King Abuldaziz University, Jeddah, Saudi Arabia

The Jabel Sayid Complex comprises two principal rock types which require distinct source regions. An earlier intrusion (572 ± 24 Ma) of biotite-hornblende monzogranite displays selective enrichment of LIL elements and depletion of HREE characterizing a calc-alkaline magma derived from a garnet-bearing source within a hydrated mantle wedge. The younger intrusion (534 ± 40 Ma) is an aegirine-arfvedsonite peralkaline granite which can be derived neither by fractionation of a calc-alkaline magma, nor by closed-system anatexis of a crustal source. This granite is enriched in HFS elements and strongly depleted in Sr, Ba and Eu, and requires the contribution of a fluorine-bearing fluid phase. This volatile phase has resulted in a range of metasomatic rock types including an aplitic rim which carries extreme levels of HFS elements and is a potential economic source of U, and a red granite characterized by widely ranging trace element abundances and strong Eu depletion. The peralkaline granite is typical of siliceous melts derived from an intra-plate tectonic environment, with trace elements strongly controlled by volatiles which are probably mantle-derived. The role of crustal contamination on such granites can neither be proved nor excluded.