The Ediacaran Ferani and Rutig volcano-sedimentary successions of the northernmost Arabian-Nubian Shield (ANS): New insights from zircon U–Pb geochronology, geochemistry and O–Nd isotope ratios

Abstract

Late Neoproterozoic (Ediacaran) evolution of the northernmost segment of the Arabian-Nubian Shield (ANS) included vast intrusion of granitoids and development of volcano-sedimentary basins. Here we describe two of such volcano-sedimentary successions (Rutig and Ferani) exposed in southern Sinai, Egypt. The successions are dominated by intermediate to silicic volcanics of mostly high-K calc-alkaline affinity, interbedded with immature sediments. Minor andesites form the basal parts of both successions, whereas the dominant volcanics are dacites to rhyolites. The latter are highly fractionated and include A-type granite affinities. In the Rutig succession, the basal andesites have characteristics of low-silica adakitic (LSA) rocks. New zircon U–Pb dating by SIMS of dacites and rhyolites from both successions yielded ages between ca. 627 and 593 Ma. Combined with field evidence and previous age data from interbedded conglomerates, the Rutig succession is divided into a lower and an upper part recording distinct volcano-sedimentary cycles. The lower Rutig volcanic rocks of ca. 630–615 Ma and the associated sediments were folded and then unconformably overlain by the upper Rutig succession (615–590 Ma). Dacites and rhyolites from the lower and upper parts of the Ferani succession record ages of 607–593 Ma overlapping with the end of volcanic activity in the upper Rutig. SIMS δ 18O of dated zircons from dacites in the Rutig and Ferani are all within the mantle range for zircon, and show no consistent trends associated to cores and rim domains. Coupled with minor inheritance in the Rutig and Ferani volcanics this indicates little crustal contamination during evolution of the magmatic suites. Whole-rock ɛNd( t) for the volcanics of the Rutig range between +2.7 and +3.2, whereas slightly but significantly higher values of +3.5 to +4.7 are recorded in the Ferani volcanics. The values are consistent with the juvenile character of the ANS magmas, and also further corroborate models of geographical isotope provinciality in this segment of the shield. Detailed comparison of geochemical patterns and timing of volcanic activity confirms that Rutig and Ferani, as well as several other volcano-sedimentary successions of Sinai correlate with the Dokhan Volcanics of the Eastern Desert of Egypt. The Dokhan Volcanics including those exposed in the Rutig and Ferani successions compare well in terms of age and geochemical patterns with the local 635–590 Ma calc-alkaline plutonic suite, but some volcanic rocks display geochemical similarities with a younger (608–580 Ma) alkaline plutonic suite. This variance may in part reflect a change in the tectonic regime at ca. 610–600 Ma with a change to more alkaline compositions, as reflected in the plutonic suites. However, the occurrence of early (ca. 620 Ma) volcanic rocks of highly fractionated A-type granite affinity in the Rutig succession has not been documented in the calc-alkaline plutonic suite and requires a different explanation. We interpret the Ediacaran magmatic activity in the northernmost ANS to have occurred in a post-collisional regime and note for the occurrence of ca. 635–630 Ma adakitic rocks, including the andesites of the lower Rutig. These may record early manifestations of lithospheric delamination involving parts of mafic lower crust that melted in the process to form adakitic magmas. However, major asthenospheric upwelling, which heated the lithospheric mantle and the crust and triggered vast plutonic and volcanic activity, peaked much later, between 610 and 590 Ma. This later event was also associated with the transition between high-K calc-alkaline and alkaline magmatic activity in this region.