Late Ediacaran crustal thickening in Egypt: Geochemical and isotopic constraints from post-collisional volcanism in southern Sinai, Egypt

A number of Late Ediacaran post-collisional volcanic sequences are exposed in southern Sinai, which represents the extreme northern tip of the Arabian-Nubian Shield (ANS). To clarify the age and geochemical characteristics of such volcanism, two localities were selected for the present study: the Meknas and Iqna Shar’a volcanics. These undeformed and unmetamorphosed sequences include intermediate to felsic subaerial lava flows, tuffs and ignimbrites accompanied by deposition of immature clastic sediments. New SIMS U-Pb dating of zircons from two samples of the Meknas lava flows yielded ages of 593 ± 12 and 616 ± 1 Ma, while zircons from three samples of the Iqna Shar’a volcanics yielded ages of 600 ± 6, 616 ± 4, and 617 ± 6 Ma. Combined with field evidence, the zircon ages enable us to recognize two phases of post-collisional volcanic activity in southern Sinai, at 592-600 Ma and 616-617 Ma. Geochemically, the volcanic rocks of the two successions display large silica variations and are mostly medium- to high-K calc-alkaline rocks. The lower units of the earlier phase consist of andesite and dacite, whereas the upper units of the later phase are more evolved, rhyodacite to rhyolite. The evolved rhyolites of the second phase have characteristics that are transitional to alkaline A-type magmas, but this is attributed to extensive fractionation and does not require a change in the tectonic regime. Geochemically, the Meknas and Iqna Shar’a volcanics are enriched in most LILE and depleted in most HFSE. Moreover, they are generally enriched in LREE relative to HREE and characterized by moderate degrees of REE fractionation [(La/Yb)_N = 7.0-12.8)]. They evolved from high-K calc-alkaline magmas that were generated in a post-collisional regime. Despite the temporal gap, it appears that all lavas in each locality are cogenetic and formed via fractional crystallization from a common parental melt. Although they erupted in a post-collisional setting, both volcanic suites display geochemical fingerprints of subduction influence, interpreted to reflect remelting of previously formed arc material ca. 750-650 Ma in age. These magmas were derived from the mafic lower crust, which likely melted due to lithospheric delamination. This is consistent with the Hf isotope ratios of their zircons, which consistently yield positive _Hf(t) values (+3.2 ±1.5 and +4.3 ± 1.7, from Iqna Shar’a; +2.6 ± 2.3 and +5.3 ± 1.7 from Meknas). The 50-150 Ma time span between emplacement of this lower crust and its remelting was insufficient for its Hf isotope ratio to evolve to negative values considered representative of an ancient crustal source. Contamination by upper continental crust and fractional crystallization were responsible for the variation observed within the studied volcanic suites.