Complex ages of five colour and morphology populations of zircon from the Jabal Radwa alkali granite, Kingdom of Saudi Arabia

Abstract The Jabal Radwa pluton (referred to here as the Radwa alkali granite), northwestern Arabian Shield, Kingdom of Saudi Arabia, is a near-circular exposure of peralkaline granite. Previous TIMS U–Pb age determinations of multigrain zircon fractions suggested an age of ∼480 Ma, at least 100 m.y. younger than the youngest known rocks of the Arabian Shield. A single sample of the Radwa alkali granite contains dark brown, green, blue, tan and yellow colour populations of zircon, plus yellow titanite. The ages of dark brown zircon were unobtainable by SHRIMP due to large matrix effects caused by high concentrations of REE and other trace elements. Green and blue zircon grains were determined to be ∼518 Ma. Tan zircon forms prismatic crystals that resemble those typical of igneous zircon. Three analytical investigations for tan zircon resulted in a SHRIMP age of 501.3 ± 3.9 and LA-ICP-MS ages of 506.7 ± 7.8 and 501.2 ± 5.4 Ma, confirming the uncharacteristically young age of this granite. Yellow zircon yielded a LA-ICP-MS age of 493 ± 13 Ma. Dark brown, green and blue zircon are considered to be xenocrysts, whereas tan and yellow zircon are igneous in origin. REE distribution patterns define two groups of zircon. Group 1 is composed of tan, yellow and dark brown zircon that have HREE-enriched patterns typical of igneous zircon. Group 2 is composed of green and blue zircon that have essentially horizontal REE patterns typical of hydrothermal zircon. Because essentially all Proterozoic magmatism in the Arabian Shield ended by ∼570 Ma, magmatism recorded by the Radwa alkali granite might represent igneous activity associated with the Cadomian orogeny and southward subduction beneath the northern edge of Gondwana.