Zabargad (St. John’s) Island: an uplifted fragment of sub-Red Sea lithosphere

Abstract

Zabargad is a small island located in the Red Sea about 50 km W of its axis, between 23º and 24ºN. The island is not volcanic but probably represents an uplifted block of upper mantle and crustal rocks, and yields information on the nature of the underlying lithosphere. Peridotites are the main rock type, consisting of exceptionally fresh spinel lherzolites, which equilibrated last at a depth >30 km in the mantle. Other ultramafic facies, some representing rocks which mixed with a basaltic melt fraction, and some which underwent metasomatic exchange at depth, are also present. The peridotite bodies are in tectonic contact with a metamorphic formation consisting of gneissic and amphibolitic rocks similar to those outcropping in the Pan African metamorphic basement of eastern Egypt, Peridotites and metamorphic rocks are both crossed by several generations of basaltic dykes. The sedimentary Zabargad Formation, of probable Cretaceous or Palaeocene age, consists of alternations of silicified limestones, sandstones, black shales and phosphoritic lenses, and is overlain by evaporites, probably of Miocene age, and by reef limestone units, one probably of early Pleistocene age, the other probably deposited during the last interglacial. Various formations are affected by localized metasomatic mineralizations (recrystallized large, gem-quality olivine, cancrinite, scapolite. garnierite). The uplift of upper mantle ultramafic bodies with fragments of crust, which created the island, might have occurred in connection with the early stages of development of the rift which preceded the formation of the Red Sea. An alternative explanation, that the Zabargad peridotites are remnants of a Pan-African Palaeozoic ophiolitic complex, is possible but less likely.

Early magmatism in the greater Red Sea rift: timing and significance

“…The geology of Zabargad is much more complex than that of the Brothers, with exposures of high-grade gneiss, unserpentinized peridotite, Mesozoic and Cenozoic sedimentary strata, Pleistocene coral terraces, and mafic dikes and sills (El Shazly and Saleeb Roufaiel 1977;Bonatti et al 1983). Bonatti et al (1983) first suggested that the Zabargad peridotites represented an important component of Red Sea rifting and proposed that they were emplaced at a depth of ϳ30 km within the proto-Red Sea rift mantle. Subsequent incipient fracture zone tectonics brought them to their position near sea level.…”

Northern Red Sea: Nucleation of an oceanic spreading center within a continental rift

“…Diabase dikes and sills have been described from Zabargad [e.g., El Shazly et al , 1974; Bonatti et al , 1983; Nicolas et al , 1987; Bosworth et al , 1996] and the Brothers Islands [ Shukri , 1944; Taviani et al , 1984]. Both locations are uplifted slivers of crustal and mantle rocks, with prerift sedimentary rocks present on Zabargad [e.g., Bonatti et al , 1983; Bosworth et al , 1996]. 40 Ar/ 39 Ar [ Villa , 1990] and U‐Pb [ Oberli et al , 1987] dating suggests Miocene ages of 18.5–20 Ma for the dikes on Zabargad.…”

Architecture of rifted continental margins and break-up evolution: insights from the South Atlantic, North Atlantic and Red Sea–Gulf of Aden conjugate margins

“…The only indication of exhumed mantle rocks in the central Red Sea has been found on the small Zabargad island, which is formed by relatively fresh peridotites associated with basement rocks and small patches of Neogene sediments (Bonatti et al 1986). According to some authors, this occurrence might be related to compressional transform faults and uplift of oceanic blocks (Bonatti et al 1981;Bonatti et al 1983). …”