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

摘要

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/Al_2O_3 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.