Petrogenesis and tectonic setting of late Precambrian ensimatic volcanic rocks, CED of Egypt. Precambrian Res

Department of Terrestrial Magnetism, 5241 Broad Branch Road N.W., Washington, DC 20015 U.S.A.
Precambrian Research (Impact Factor: 5.66). 12/1981; 16(3):195-230. DOI: 10.1016/0301-9268(81)90013-9


Early stages in the geologic evolution of the central eastern desert of Egypt (CED) reflect an intense episode of ensimatic volcanic activity similar to modern magmatism of the ocean floors and island arcs. This paper reports results from studies of the petrology and petrogenesis, and interprets the significance of these Late Precambrian volcanic rocks.A three-fold stratigraphy is preserved in the basement of the CED. A basal section of oceanic crust includes ultramafics, gabbros and pillowed basalts. These older metavolcanics (OMV) are conformably succeeded by dominantly volcanogenic metasediments, which are in turn succeeded by a dominantly andesitic, calc-alkaline sequence of younger metavolcanics (YMV). The OMV and YMV are largely restricted to the CED in Egypt, but analogous terranes are found in northern Arabia. (40–400 ppm) and Ni (30–260 ppm). They are poor in K2O (0.05–0.92%), Rb (0.3–5.0 ppm) and Ba (11–89 ppm). On Ti-Zr-Cr-V-Ni-P discriminant diagrams, the OMV plot in the field of modern abyssal tholeiites. High K/Rb (450–1800) and light REE depletions support this inference, although K/Ba (25–45) is lower than modern mid-ocean ridge basalts (MORB). The sum of OMV geochemical characteristics requires that these magmas were derived by the fractional fusion of the mantle. It is suggested that the OMV were generated by 20–25% fractional melting of previously depleted mantle at depths of less than 60 km. Relatively little fractionation accompanied ascent to the surface, where the OMV were erupted in a primitive crustal environment, either a small oceanic rift or a back-arc basin.Metamorphism of the YMV resulted in little elemental redistribution. These andesites have sub-alkaline clinopyroxenes and major-element geochemical characteristics indistinguishable from modern calc-alkaline andesites. YMV andesites in the central and western CED have K/Rb = 400–600, K/Ba = 20–40 and are light REE-enriched and heavy REE depleted. High concentrations of Cr (50–150 ppm) and Ni (20–100 ppm) and low initial 87Sr/86Sr ratios (0.7028–0.7030) indicate that these magmas were generated by melting in the mantle. Modelling studies and consideration of experimental data indicate that these andesites were formed by 2–10% fractional fusion of hydrous, undepleted, garnet therzolite at depths of 65 km or more in the mantle.The data show that an intense episode of instability, convection, and widespread melting occurred in the mantle beneath Afro-Arabia at the end of the Precambrian.

19 Reads
  • Source
    • "The basement complex in the Central Eastern Desert displays strong ensimatic affinities and consists mainly of an island arc complex which is overthrusted by dismembered ophiolitic sequence and intruded by syntectonic grey granitoides (Stern 1981; Ries et al., 1983; Sturchio et al., 1983; El Ramly et al., 1984). These rock units are overlain by local occurrences of molasse-type Hammamat sediments, Dokhan volcanics and are intruded by post-orogenic younger granites (Grothaus et al., 1979; Greenberg 1981; Ries et al., 1983). "
    [Show abstract] [Hide abstract]
    ABSTRACT: The present study embodies the results of field work and laboratory investigations and attempts to present the geology, petrography, geochemistry and petrogenesis of the Ediacaran Dokhan Volcanics at Wadi Zareib, Central Eastern Desert, Egypt. The investigated area is located along the Red Sea Coast and most common rock types among the Dokhan Volcanics are rhyolites and trachyandesites are commonly interbedded with lava flows successions. Microphenocrysts of quartz, alkali feldspars and plagioclase set in a fine-grained groundmass of microcrystalline to felsitic aggregates of quartz and plagioclase together with chlorite, epidote, sericite and hematite. Major element oxides and trace element data suggest that the trachyandesites are genetically related through crystal fractionation. The trace elements Y, Pb, Cu, Zr and Ga show negative anomalies (enriched) relative to Sr and Ba element. Wadi Zareib Dokhan Volcanics were originated from typical calc-alkaline reflecting the magmatic differentiation of metaluminous to peraluminous magma type. The geochemical trends of major oxides and trace elements of the studied volcanic rock varieties may suggest their co-magmatic nature. The Wadi Zareib Dokhan Volcanics display geochemical characteristics of both orogenic arc-type and anorogenic within-plate environments, suggesting eruption in a transitional “post-collisional tectonic setting. These rocks were derived from a single magma and suffered subsequence fractional crystallization.
  • Source
    • "In (a) and (b) the fields of ophiolitic metavolcanics (older metavolcanics, OMV), island arc metavolcanics (younger metavolcanics, YMV) and Dokhan Volcanics (DV) are from Ali K.A. et al. (2009), Abdel Rahman (1996), Basta et al. (1980), Eliwa et al. (2006), Moghazi (2003), Stern (1981), and Stern and Gottfried (1986). plagioclase, K-feldspar and biotite) and primary volcanic textures are still preserved. "
    [Show abstract] [Hide abstract]
    ABSTRACT: The Kid Group is one of the few exposures of Neoproterozoic metavolcano-sedimentary rocks in the basement of southern Sinai in the northernmost Arabian–Nubian Shield. It is divided into the mostly metamorphosed volcaniclastic Melhaq and siliciclastic Um Zariq formations in the north and the mostly volcanic Heib and Tarr formations in the south. The Heib, Tarr, and Melhaq formations reflect an intense episode of igneous activity and immature clastic deposition associated with core-complex formation during Ediacaran time, but Um Zariq metasediments are relicts of an older (Cryogenian) sedimentary sequence. The latter yielded detrital zircons with concordant ages as young as 647 ± 12 Ma, which may indicate that the protolith of Um Zariq schist was deposited after ~ 647 Ma but 19 concordant zircons gave a 206Pb/238U weighted mean age of 813 ± 6 Ma, which may represent the maximum depositional age of this unit. In contrast, a cluster of 11 concordant detrital zircons from the Melhaq Formation yield a weighted mean 206Pb/238U age of 615 ± 6 Ma. Zircons from Heib Formation rhyolite clast define a 206Pb/238U weighted mean age of 609 ± 5 Ma, which is taken to approximate the age of Heib and Tarr formation volcanism. Intrusive syenogranite sample from Wadi Kid yields a 206Pb/238U weighted mean age of 604 ± 5 Ma. These constraints indicate that shallow-dipping mylonites formed between 615 ± 6 Ma and 604 ± 5 Ma. Geochemical data for volcanic samples from the Melhaq and Heib formations and the granites show continuous major and trace element variations corresponding to those expected from fractional crystallization. The rocks are enriched in large ion lithophile and light rare earth elements, with negative Nb anomalies. These reflect magmas generated by melting of subduction-modified lithospheric mantle, an inference that is further supported by εNd(t) = + 2.1 to + 5.5. This mantle source obtained its trace element characteristics by interaction with fluids and melts from subducting oceanic crust during the Late Cryogenian time, prior to terminal collision between fragments of East and West Gondwana at ~ 630 Ma. Positive εNd(t) values and the absence of pre-Ediacaran zircons in all but Um Zariq metasediments indicate minor interaction with Cryogenian and older crust. A model of extensional collapse following continental collision, controlled mainly by lithospheric delamination and slab break-off is suggested for the origin of the post-collision volcanics and granites at Wadi Kid. No evidence of pre-Neoproterozoic sources was found. Kid Group Ediacaran volcanic rocks are compositionally and chronologically similar to the Dokhan Volcanics of NE Egypt, which may be stratigraphic equivalents.
    Lithos 11/2012; 154:147–165. DOI:10.1016/j.lithos.2012.07.003 · 4.48 Impact Factor
  • Source
    • "Im kristallinen Grundgebirge treten drei Gruppen vulkanischer Gesteine auf, die basische bis intermediäre, seltener saure (also SiO 2 -reiche) Zusammensetzungen aufweisen. Die beiden älteren Serien, die im Zentral-und Südteil der ägyptischen Ostwüste weit verbreitet sind, wurden metamorph überprägt; sie werden daher als Ältere und Jüngere Meta-Vulkanite bezeichnet (Stern 1981). Aufgrund geochemischer Kriterien lässt sich vermuten, dass die vulkanischen Gesteine der beiden älteren Gruppen ursprünglich im Bereich über einer Subduktionszone gefördert wurden, in dem Kompressions- Tektonik vorherrschte. "

Show more