Bimodal zircon ages from Natash volcanics (southeast Egypt) and the link between eruption mechanisms and Late Cretaceous tectonics

Abstract

Trachyte plugs from the Natash volcanic field have been precisely dated using in situ U-Pb geochronology on zircon grains at similar to 93Ma. Available zircon ages thus suggest that the overall volcanic history at Natash was probably shorter than previously estimated based on Rb-Sr and K-Ar ages, which are more sensitive to element mobility. Basaltic flows and trachyte plugs were produced by a largely synchronous, Upper Cretaceous volcanic activity that occurred in the Natash area at the onset of the extensional fracturing that preceded and accompanied the doming of the Afro-Arabian Shield. Extraction and eruption of volumetrically dominant basalts from deeper lithospheric levels were ruled by major NW-SE strike-slip faults, whereas both major and subordinate fault systems activated by successive tectonic pulses favoured the ascent of more evolved melts (i.e. trachytes and rhyolites) from shallow-level magma chambers. The study revealed also the presence of zircon xenocrysts in some trachyte plugs associated or not to magmatic zircon. These xenocrysts have pre-Cambrian ages (similar to 681Ma) matching those of zircons from basement rocks in the area (similar to 700Ma) and were most probably incorporated during the emplacement of trachyte plugs at shallow depths. The strictly bimodal ages recorded by zircons suggest a very large time gap (>500Ma) between the end of the Pan-African Orogeny and the onset of new tectono-magmatic activity in the Natash area.

Hidden but Ubiquitous: The Pre-Rift Continental Mantle in the Red Sea Region

“…These variations are in agreement with the two-stages process of magmatic metasomatism from hydrous-melts, followed by dehydration at the base of the lithosphere as proposed by Stein et al (1997). Alternatively, Abu El-Rus et al (2019) proposed that the ancient refertilization was driven by carbonatite melts, in part following suggestions of Weiss et al (2016) for the origin of the oceanic HIMU. They pointed out that Hf-Nd-Sr-Pb isotopes and traceelement patterns support a genetic relationship between HIMU lavas and metasomatized carbonated peridotite sources in the subcontinental lithospheric mantle (SCLM) (Tilton and Bell, 1994;Bizimis et al, 2003).…”

“…During the metasomatic event, the interaction of a depleted mantle with a deep carbonatite melt can produce fertile lithologies having high U-Th/Pb, low Sm-Nd, Lu-Hf and Rb-Sr ratios, developing an HIMU character over time. The very low Al 2 O 3 contents (Figure 5C), the high Nb/Zr-Hf ratios, and the depletions in alkali (Rb, Cs, K) compared to other highly incompatible elements (Ba, Th, U, La) of the Marthoum pipes are consistent with this scenario (see Abu El-Rus et al, 2019).…”