The late Ediacaran (580–590 Ma) onset of anorogenic alkaline magmatism in the Arabian–Nubian Shield: Katherina A-type rhyolites of Gabal Ma’ain, Sinai, Egypt

Abstract

The Katherina Volcanics of Gabal Ma’ain in the Sinai comprise an Ediacaran (580–590Ma) approximately 450m thick succession dominated by porphyritic rhyolite lava flows with subordinate related pyroclastics. These volcanics unconformably overlie the calc-alkaline Younger Granites (≥590Ma) and are intruded by alkaline granitoids (578±8Ma). The rhyolites have a potassic alkaline affinity and peraluminous to slightly metaluminous character. They exhibit many of the classic features of A-type magmas, including enrichment of incompatible elements, such as Zr, Nb, Y, Ga, Zn and Ce and total REE, as well as high FeO*/(FeO*+MgO) and 10,000*Ga/Al2O3 ratios. The A-type rhyolites have LREE-enriched patterns with pronounced negative Eu anomalies that are comparable with typical REE profiles for “hot-dry-reduced rhyolites”. Saturation thermometry has yielded zircon and apatite crystallization temperatures ranging between 913 and 925°C and 669 and 931°C, respectively. The investigated trace element patterns indicate that the Katherina A-type rhyolites were very likely to have evolved through simple fractional crystallization of a parental magma derived from an enriched (most probably asthenospheric) mantle source, supplemented by a crustal component inherited from pre-collision subduction events, or a ‘recycled component’ in the source. Katherina A-type rhyolites were likely erupted in a within-plate setting. The eruption of these rhyolites marks the onset of the anorogenic period during which the rigid massif (Arabian–Nubian Shield) was subjected to post-collisional tensional stresses and intra-plate rifting.