2013

On the late Ediacaran initiation of the anorogenic alkaline magmatism in the Arabian-Nubian Shield: The A-type rhyolites of Gabal Ma'ain, Sinai, Egypt

Mohammed Z. El-Bialy ^a,* , Imbarak S. Hassen ^b

^a Geology Department, Faculty of Science, Port Said University, Port Said 42521, Egypt

^b Geology Department, Faculty of Science, Suez Canal University, Ismailia 41522, Egypt

* Corresponding author; Mohammed Z. El-Bialy (mzbialy@yahoo.com)

Abstract

The Katherina Volcanics of Gabal Ma'ain in the Sinai comprises an Ediacaran (580-590 Ma) approximately 450 m thick succession dominated by porphyritic rhyolite lava flows with subordinate related pyroclastics. These volcanics unconformably overlie the calc-alkaline Younger Granites (≥590 Ma) and are intruded by alkaline granitoids (578±8 Ma). 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 10000*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- 925 °C and 669-931 °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 undoubtedly 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.