Abdel-Fattah M. Abdel-Rahman

The Rb-Sr geochronological evolution of the Ras Gharib segment of the northern Nubian Shield

Rb-Sr whole-rock isochron ages have been determined for six igneous rock suites from the Ras Gharib. These represent all major crustal components of the northern Nubian Shield and are, from oldest to youngest: diorite-tonalite, extrusive rocks, granodiorite-adamellite and leucogranite, muscovite trondhjemite (orogenic Pan-African suites), dyke swarms and anorogenic peralkaline granites. The ages obtained are: 881 ± 58 Ma, 620 ± 16 Ma, 552 ± 7 Ma, 516 ± 7 Ma, 493 ± 7 Ma and 476 ± 2 Ma. The initial 87Sr/86Sr ratios of the Pan-African suites and the dykes are in the range 0.7039 to 0.7047. The initial ratio of the anorogenic peralkaline granites is 0.7110 ±0.0012, higher than any previously reported for igneous rocks from the Egyptian belt. The 493 ± 7 Ma age of the dyke swarms represents a younger limit for the Pan-African compression–accretion event and an upper (older) limit for an extensional tectonic regime. Linked with this is the anorogenic peralkaline granite magmatism (476 ±2 Ma), which is the last manifestation of igneous activity in this region. The low initial ratios of the Pan-African suites provide no support for the existence of an older (Archaen) sialic continental crust in the region. The anorogenic peralkaline granite with the high initial ratio was produced by partial melting of earlier Pan-African suites. The age data are used to construct a tentative correlation scheme for shield rocks of Egypt, Sudan and Saudi Arabia.

The Mount Gharib A-type granite, Nubian Shield: petrogenesis and role of metasomatism at the source

The Mount Gharib peralkaline A-type complex (476±2 Ma), located in the Nubian Shield of Egypt, contains sodic-calcic to sodic amphiboles, accessory astrophyllite, zircon, fluorite, apatite, allanite, aenigmatite, elpidite(?) and ilmenite. This “within plate” hypersolvus suite is enriched in large-ion lithophile (LIL) and high field-strength (HFS) elements, and characterized by a fractionated REE pattern (Ce/Yb=49) and a significant negative Eu anomaly. A fine-grained acicular-amphibole-bearing roof facies shows further enrichment in the LIL and HFS elements. The suite was emplaced in a Pan-African granodiorite-adamellite host, which it locally metasomatized. The affected rocks contain hydrothermal albite, end-member arfvedsonite, astrophyllite, and levels of the LIL and HFS elements intermediate between those in the peralkaline granite and the roof facies. Trace element and isotopic modeling of this A-type granite, with its high initial 87Sr/86Sr value (0.7110), documents an active role of the lithosphere in magma generation. Lithospheric extension, expressed by regional dyke-swarms, was caused by cooling, fracturing and relaxation of the thin, newly formed Pan-African crust. Localized partial melting took place in an open system, possibly as a result of an influx of alkali-rich fluid derived from a sublithospheric source. Metasomatic reactions similar to those observed in the metasomatized wallrocks are considered to have played an important role just prior to the onset of anatexis and generation of the A-type melt.

Pan-African volcanism: petrology and geochemistry of the Dokhan Volcanic Suite in the northern Nubian shield

The Late Proterozoic Dokhan volcanic suite (620 Ma) of the northern Nubian shield is the product of Late Pan-African volcanism. The suite covers the entire spectrum from basalt to high-silica rhyolite and occurs as two units: a dark-coloured unit containing basalt-andesite-dacite, and a light-coloured unit encompassing dacite-rhyodacite-rhyolite. The latter unit is made up largely of ash flow tuffs and ign-imbrites that are locally interstratified with basalt and andesite lava flows. The suite forms a continuum in composition with a wide range of Si0 2 (48–77 wt%), CaO (0.1–8.9 wt%), Sr (81–906 ppm), Zr (85–340 ppm) as well as most other elements, and is moderately enriched in incompatible elements, including rare earth elements (REE). The suite exhibits fractionated, subparallel REE patterns that are similar overall to Andean andesites and ignimbrites. Well-defined major and trace element trends and fractionated REE profiles are consistent with a fractionated basalt to rhyolite calc-alkaline magma series. It is a typical calc-alka-line orogenic complex and exhibits mineralogical-geochemical traits of arc-related volcanism. The suite neither resembles products of extensional nor transitional tectonic regimes as previously thought, but was produced in a subduction-related tectonic environment. The mafic nature of the least-evolved rocks of the suite, along with its relatively low initial 87 Sr/ 86 Sr ratio (0.7039) are considered to indicate a mantle source. A mantle-derived basaltic magma fractionated, with amphibole and plagioclase dominating the fractionating assemblage, to produce the more felsic varieties, as suggested by major and trace element fractionation modelling.

Tectonic-magmatic stages of shield evolution: the Pan-African belt in northeastern Egypt

Abstract The Arabian-Nubian shield illustrates an example of plate tectonics during the Pan-African orogenic event (ca. 950-550 Ma); it is considered one of the most remarkable Proterozoic shield areas yet known. Although there is now a general agreement that the shield has evolved by a series of magmatic arcs and terrain accretion, the evolution of its northernmost part exposed in northeastern Egypt remains controversial. Two contrasting tectonic models have been proposed for the evolution of this region: regional crustal extension and magmatic-arc regimes. There is no convincing evidence of older (Archean) sialic materials within the region, although the region is flanked by Archean crust. The Neoproterozoic crustal components exposed in northeastern Egypt preserve a record for its evolution. A large gabbro-diorite-tonalite complex (GDT, 881 Ma) emplaced during the early stage of the Pan-African orogeny is geochemically primitive, exhibits a low initial 87 Sr 86 Sr ratio (0.704) and shows trace-element characteristics of oceanic-arc-related lavas. The GDT complex was produced (by fractionation) from a mantle-derived tholeiitic magma formed within an island-arc tectonic environment. A synorogenic stage characterized by extensive volcanism that produced the Dokhan volcanic suite (DVS, 620 Ma) forms a continuous basalt to rhyolitic ignimbrite calc-alkaline magma series, exhibiting features of arc-related volcanism, with REE profiles similar overall to Andean andesites and ignimbrites. The Dokhan suite was produced in a continental-arc setting. A granodiorite-adamellite-leucogranite composite batholith (552 Ma) emplaced during the late stage of the Pan-African orogeny exhibits typical features of I-type complexes, trace element traits of volcanic-arc granites, and is also interpreted to have been formed in an Andean-type setting. A trondhjemite pluton was also formed at this late orogenic stage, by partial melting of GDT host rocks at depth. Cooling and relaxation of the newly formed Pan-African crust caused extensive fracturing, which was followed by intrusion of NE-SW- and NNW-SSE-trending dyke swarms at 493 Ma. This episode marks a fundamental inversion from orogenic compressional to anorogenic extensional processes around the Neoproterozoic-Paleozoic boundary. Anorogenic magmatism was locally associated with such extension-induced structures. The Mount Gharib peralkaline granite (476 Ma), for example, exhibits trace element traits of A-type, within-plate granites, and a high initial 87 Sr 86 Sr ratio (0.711), and was formed in a rift environment. Thus, the region is characterized by lengthy episodes of discontinuous subduction (∼ 880-550 Ma), during which it evolved to form a primitive crust by early Pan-African island arcs and late Pan-African continental arcs. This was followed by an even longer period of crustal extension (∼ 550-90 Ma) that produced localized anorogenic magmatism.

Anorogenic magmatism: chemical evolution of the Mount El-Sibai A-type complex (Egypt), and implications for the origin of within-plate felsic magmas

The Mount El-Sibai alkaline granitic complex (eastern Egypt) forms an elongate body, which was emplaced at the extension of a NW-trending shear zone, within voluminous calc-alkaline Pan-African host rocks. The complex is hypersolvus in nature and is composed of perthite, quartz, alkali amphibole, Fe-rich biotite, and accessory zircon, apatite, fluorite, aenigmatite and ilmenite. Data on mineral chemistry show that the amphibole ranges in composition from hastingsite to pure end-member arfvedsonite, and the biotite is largely titaniferous annite. Geochemically, the complex is highly evolved in composition (with 72–78 wt% SiO 2 , and DI values of 85–98), is enriched in Rb (48–291 ppm), Nb (28–237 ppm), Y (47–269 ppm), Zr (58–618 ppm), Ga (17–41 ppm) and the REE (176–437 ppm), and depleted in Al, Mg, Ca, Sr and Eu. The complex exhibits a wide trace-element compositional range. The REE patterns are uniform, parallel to sub-parallel, fractionated ((La/Yb) n = 4.7), LREE enriched over HREE, and show prominent negative Eu-anomalies. The albitized facies of this complex shows the highest concentrations of large ion lithophile (LIL) and high field strength (HFS) elements. The complex exhibits mineralogical and chemical traits typical of within-plate A-type granites. Mount El-Sibai is interpreted to have been developed during a phase of cooling, relaxation, crustal attenuation, and fracturing of the newly formed Pan-African crust. Results of geochemical modelling indicate that the magma may have formed by a large degree of batch partial melting (F=0.65) of Pan-African calc-alkaline rocks, which had been metasomatized. Metasomatism of source rocks may have been caused by a Na–F-rich fluid phase compositionally similar to that which produced the albitized facies. The volatile flux may have caused fenitization-type reactions along fissures and re-activated Pan-African fractures prior to anatexis, and is considered to have played a role as an important agent of heat transfer. Temperature necessary for crustal anatexis is likely to have been produced as a result of shear heating, caused by a rapid change in the direction of plate motions beneath eastern Egypt during Early Palaeozoic times. The confining pressure must have been released by fissuring of the crust. Magma ascent may have been facilitated by reactivation of pre-existing faults and shear zones. This model may have wider implications for the generation of within-plate felsic magmas in other regions.

Cenozoic volcanism in the Middle East: petrogenesis of alkali basalts from northern Lebanon

Large discontinuous exposures of basaltic lava flows, ranging in age mostly from Miocene to Recent, are present in several localities extending from Sinai, Jordan, Palestine, Israel, to Lebanon and Syria (Dubertret, 1955; Baldridge et al . 1991; Mouty et al . 1992; Heimann et al . 1996; Shaw et al . 2003). These form the Cenozoic volcanic province of the Middle East, occurring mostly along or near the transform faulted boundary (the Dead Sea–Ghab transform fault system) between the Arabian and African plates and the Levantine subplate (Fig. 1⇓). Further to the south, several other extensive Cenozoic volcanic provinces occur in Arabia and east Africa (mostly Ethiopia). Figure 1. Regional geological map showing the distribution of the various Cenozoic volcanic provinces in east Africa, Arabia and the Middle East. The Cenozoic continental flood basalts in Ethiopia, Yemen, western Arabia and Jordan have been extensively studied (Camp & Roobol, 1989, 1992; Altherr, Henjes-Kunst & Baumann, 1990; Stein & Hofmann, 1992; Baker, Thirlwall & Menzies, 1996; Baker et al . 1997; Shaw et al . 2003). The Cenozoic volcanic province of Ethiopia is clearly related to the East-African rift system (Barberi et al . 1975; Mohr, 1983). The Quaternary intraplate basaltic volcanic field in Yemen appears to be the result of melting shallow mantle, perhaps in response to small amounts of lithospheric extension that were metasomatized and hydrated by the Afar plume during, or shortly after, Oligocene flood volcanism (Baker et al . 1997). The lavas of this basaltic field of western Yemen were subjected to variable degrees of contamination (0–20 %) of an Early Proterozoic to Late Archaean silicic lower crustal component (Baker, Thirlwall & Menzies, 1996; Baker et al . 1997). The Cenozoic continental flood basalt provinces of …

The nature, age and petrogenesis of the Cartier Batholith, northern flank of the Sudbury Structure, Ontario, Canada

Abstract The Cartier Batholith, exposed about 50 km northwest of Sudbury, Ontario, Canada, is a late Archean K-rich granitic batholith. Along with the Levack Gneiss Complex, it forms the footwall of the North Range of the Sudbury Igneous Complex. The Cartier Batholith, dominantly monzogranitic to granodioritic, is relatively homogeneous, unfoliated, and contains K-feldspar megacrysts. It shows several distinctive chemical features, including high concentrations of Zr, Th, U and LREE, and low concentrations of Nb, Ti, and HREE. Several discriminants suggest a post-orogenic tectonic environment for the batholith. This inference is consistent with a UPb age of 2642 ± 1 Ma obtained on zircon, only slightly younger than the inferred timing of high-grade metamorphism of the Levack Gneiss Complex. The Cartier Batholith could have formed as a result of approximately 33% partial melting of the Levack Gneiss Complex.

Mesozoic volcanism in the Middle East: geochemical, isotopic and petrogenetic evolution of extension-related alkali basalts from central Lebanon

Mesozoic picritic and alkali basalts from central Lebanon represent a significant part of an extension-related Upper Jurassic to Upper Cretaceous discontinuous volcanic belt which occurs throughout the Middle East. Volcanism was associated with an episode of intraplate extension that followed a period of continental break-up, where Mesozoic micro-continental blocks separated from Gondwana as the Neotethys ocean opened in Jurassic times. This volcanic episode produced mafic lava flows ranging in thickness from 5 to 20 m, along with some minor pyroclastic flows. These flows are stratigraphically intercalated with thick carbonate platform deposits. The basalts are made up of about 15–20% olivine (Fo 78–91 ), 30–35% clinopyroxene (salite), 40–50% plagioclase (An 56–71 ) and opaque Fe–Ti oxides (∼5%). Geochemically, the rocks exhibit a relatively wide range of SiO 2 (40.4 to 50.5 wt%) and MgO (5.1 to 15.5 wt%) contents, are relatively enriched in TiO 2 (1.7 to 3.7 wt%) and vary in composition from alkali picrite and basanite to alkali basalt. The Mg numbers range from 0.56 to 0.70, with an average of 0.63. The rocks are enriched in incompatible trace elements such as Zr (86–247 ppm), Nb (16–66 ppm) and Y (19–30 ppm). Such compositions are typical of those of HIMU-OIB and plume-related magmas. The REE patterns are fractionated ((La/Yb)N = 11), LREE enriched, and are generally parallel to subparallel. Elemental ratios such as K/P (1.1–4.7), La/Ta (11–13), La/Nb (0.57–0.70), Nb/Y (0.68–1.55) and Th/Nb (0.20–0.36) suggest that crustal contamination was minor or absent. This may be related to a rapid ascent of the magma, in agreement with the nature (mafic, oceanic-like) and the small thickness (about 12 km) of the Mesozoic crust of the Eastern Mediterranean region. The 143 Nd/ 144 Nd isotopic compositions of the lavas range from 0.512826 to 0.512886, and 87 Sr/ 86 Sr from 0.702971 to 0.703669, suggesting a HIMU-like mantle source. Trace element compositions indicate a melt segregation depth of 100–110 km, well within the garnet lherzolite stability field. The geochemical characteristics of the rocks are typical of within-plate alkali basalts, and suggest that the magmas were derived from a fertile, possibly plume-related, enriched mantle source. Petrogenetic modelling indicates that the magmas were produced by very small degrees of batch partial melting (F = 1.5%) of a primitive garnet-bearing mantle source (garnet lherzolite).

Petrogenesis of anorogenic peralkaline granitic complexes from eastern Egypt

Abstract The Pan-African orogenic shield rocks of eastern Egypt were intruded by several anorogenic within- plate granitic complexes, including Mounts Abu-Kharif and El-Dob. These two massifs were emplaced at the intersection of a fault system and a shear zone. The two massifs are made up of hypersolvus peralkaline granites, consisting essentially of perthitic alkali feldspar (55-65 vol.%), quartz (30-35%), and alkali amphibole (ferrorichterite to arfvedsonite; 5–12%), with accessory zircon, apatite and ilmenite. The rocks are evolved in composition, are relatively enriched in Nb (53-75 ppm), Y (34-72 ppm), Zr (421-693 ppm), Ga (26-29 ppm), and the REE (294-562 ppm), and depleted in Al, Mg, Ca, Sr, Ba and Eu. The REE patterns are sub-parallel, LREE-enriched over HREE, and show prominent negative Eu anomalies. The rocks exhibit mineralogical and chemical traits typical of within-plate A-type granites. Rb-Sr radiometric age dating produced a Cambrian age of 522±21 Ma, and an initial 87Sr/86Sr ratio of 0.7080±0.0042. Thus, the investigated peralkaline granitic rocks were emplaced following the termination of the Pan-African orogeny. The rocks are interpreted to have formed in an extensional tectonic environment during a phase of cooling, relaxation, crustal attenuation, and fracturing of the newly-formed shield. Results of geochemical modelling indicate that the magma may have formed by a large degree of batch partial melting (F = 0.57) of Pan-African calc-alkaline shield rocks, which had been metasomatized possibly by a Na-rich fluid. The volatile flux may have caused fenitization-type reactions along fissures and re-activated Pan-African fractures prior to anatexis, and is considered to have played a role as an important agent of heat transfer. Shear heating, caused possibly by a rapid change in the direction of plate motions beneath eastern Egypt during the Early Palaeozoic, is likely to have produced temperatures necessary for crustal anatexis. The confining pressure must have been released by fissuring of the crust. Magma ascent may have been facilitated by reactivation of pre-existing Pan-African fractures.

The Euphrates volcanic field, northeastern Syria: petrogenesis of Cenozoic basanites and alkali basalts

The Plio-Quaternary Euphrates volcanic field of NE Syria includes large discontinuous exposures of basanitic and basaltic lava flows (1200 km2 in area). It represents the northern segment of the Cenozoic volcanic province of the Middle East and is located near the Bitlis collision suture. The rocks consist of olivine (15–20 %), clinopyroxene (30–35 %), plagioclase (45–55 %) and opaque phases. Chemically, the rocks are largely ultrabasic (SiO2 38.2–45.5 wt %, MgO 8.7–13.0 wt % and average Mg number of 0.65). They are enriched in incompatible trace elements such as Zr (133–276 ppm), Nb (25–71 ppm) and Y (17–28 ppm). The REE patterns are strongly fractionated ((La/Yb)N = 19.6), indicative of a garnet-bearing source. The 143Nd/144Nd isotopic compositions range from 0.512868 to 0.512940 (eNd = 4.5 to 5.9), and 87Sr/86Sr from 0.70309 to 0.70352. These chemical and isotopic compositions reflect strong affinities to OIB. Elemental ratios such as K/P (3.4), La/Ta (13) and La/Nb (0.77), and the low SiO2 values, suggest that the Euphrates magma was subjected to minimal crustal contamination. Petrogenetic modelling has been carried out using a variety of mantle source materials, different degrees of partial melting (0.1 to 10 %), and a number of scenarios including metasomatized sources. Modelling suggests that the magma could have been produced as a result of a small degree of partial melting of either (1) a garnet-bearing depleted source enriched with a small addition of metasomatizing fluids, or (2) a garnet-bearing fertile source. The overall chemical and petrological characteristics are more consistent with the generation of the Euphrates magma by a small degree of partial melting (F = 1 %) of a primitive, garnet-lherzolite mantle source, possibly containing a minor spinel component. The Neogene collision of the Arabian plate with Eurasia along the Bitlis suture resulted in reactivation (beneath the Euphrates basin) of deep-seated fractures, along which lavas may have penetrated the crust.