Mohamed A. Obeid

Serpentinized Peridotites at the North Part of the Wadi Allaqi District (Egypt): Implications for the Tectono‐Magmatic Evolution of Fore‐arc Crust

The Neoproterozoic Allaqi-Heiani suture (800–700 Ma) in the south Eastern Desert of Egypt is the northernmost linear ophiolitic belt that defines an arc-arc suture in the Arabian-Nubian shield (ANS). The Neoproterozoic serpentinized peridotites represent a distinct lithology of dismembered ophiolites along the Allaqi-Heiani suture zone. The alteration of peridotites varies, some contain relicts of primary minerals (Cr-spinel and olivine) and others are extremely altered, especially along thrusts and shear zones, with development of talc, talc-carbonate and quartz-carbonate. The fresh cores of the chromian spinels are rimmed by ferritchromite and Cr-magnetite. The fresh chromian spinels have high Cr# (0.62 to 0.79), while Mg# shows wider variation (0.35–0.59). High Cr# in the relict chromian spinels and Fo content in the primary olivines indicate that they are residual peridotites after extensive partial melting. The studied ophiolitic upper mantle peridotites are highly depleted and most probably underwent high degrees of partial melting at a supra-subduction zone setting. They can be produced by up to ∼20%–22% dynamic melting of a primitive mantle source. The mineralogical and geochemical features of the studied rocks reflect that the mantle peridotites of the north part of the Wadi Allaqi district are similar to the fore-arc peridotites of a supra-subduction zone.

Mineralogy, geochemistry, and geotectonic significance of the Neoproterozoic ophiolite of Wadi Arais area, south Eastern Desert, Egypt

ABSTRACT The dismembered ophiolites in Wadi Arais area of the south Eastern Desert of Egypt are one of a series of Neoproterozoic ophiolites found within the Arabian–Nubian Shield (ANS). We present new major, trace, and rare earth element analyses and mineral composition data from samples of the Wadi Arais ophiolitic rocks with the goal of constraining their geotectonic setting. The suite includes serpentinized ultramafics (mantle section) and greenschist facies metagabbros (crustal section). The major and trace element characteristics of the metagabbro unit show a tholeiitic to calc-alkaline affinity. The serpentinized ultramafics display a bastite, or less commonly mesh, texture of serpentine minerals reflecting harzburgite and dunite protoliths, and unaltered relics of olivine, orthopyroxene, clinopyroxene, and chrome spinel can be found. Bulk-rock chemistry confirms harzburgite as the main protolith. The high Mg# (91.93–93.15) and low Al2O3/SiO2 ratios (0.01–0.02) of the serpentinized peridotite, together with the high Cr# (>0.6) of their Cr-spinels and the high NiO contents (0.39–0.49 wt.%) of their olivines, are consistent with residual mantle rocks that experienced high degrees of partial melt extraction. The high Cr# and low TiO2 contents (0.02–0.34 wt.%) of the Cr-spinels are most consistent with modern highly refractory fore-arc peridtotites and suggest that these rocks probably developed in a supra-subduction zone environment.

Shear zone-hosted gold mineralization in the Late Proterozoic rocks of El Beida area, south Eastern Desert, Egypt

Abstract The El Beida area of the Eastern Desert, Egypt, comprises a wide variety of Late Proterozoic rocks. It is occupied by ophiolitic serpentinites, metagabbros and pillow metabasalts, an island-arc assemblage of metasediments and metavolcanics, and syntectonic granitoids. The serpentinites occur as a small, NNW stretched slice obducted over the metavolcanics along a carbonatized shear zone. The obducted rocks are characterized by development of talc–carbonates, magnesite pockets and quartz–carbonate veinlets. The El Beida area is dissected by another northwest thrust fault that runs through the pillow metabasalts and metavolcanics. Quartz veins and hypogene alteration processes (i.e. silicification and chloritization) are well developed within the thrusting plane, forming a diverse shear zone. The El Beida gold deposit is localized in two shear zones (northwest–southeast) confined to the serpentinite and pillow metabasalts. Gold mineralization is associated mainly with the early (D 1) deformation event in the area. The shear zones were affected by two stages of influx of hydrothermal fluids during plate destruction of the Late Proterozoic rocks. Mineralogically, the shear zones contain chromite, magnetite, hematite, ilmenite, pyrite, chalcopyrite, goethite, limonite, covellite, bornite and gold. The carbonatized shear zone (SiO2<50%) within the obducted area of the serpentinite rocks has a very low Au content (0.24–0.45 ppm). The silicified shear zone of the pillow metabasalt rocks, however, is richer in SiO2(>80%) and contains up to 45 ppm Au. Gold is observed as fine grains (ca 0.3 mm) and minute blebs (20-0.5 µm) intergrown with quartz or as fine, irregular specks associated with hematite and malachite. Gold is also found as minute, irregular specks (ca 10 µm) remobilized from the altered hematite. Analyses of gold specks in the quartz veins and hematite of the silicified shear zone gave compositions of 92 and 94% Au, respectively.

An example of post-collisional appinitic magmatism with an arc-like signature: the Wadi Nasb mafic intrusion, north Arabian–Nubian Shield, south Sinai, Egypt

ABSTRACT We present new data for the Neoproterozoic mafic intrusion exposed in Wadi Nasb, south Sinai, Egypt (northernmost Arabian–Nubian Shield; ANS). The Nasb mafic intrusion (NMI) intrudes metasediments, Rutig volcanics, and diorite/granodiorite, and is intruded in turn by younger monzogranite and quartz-monzonite. Available geochronological data for the country rocks of the NMI provide a tight constraint on its age, between 619 and 610 Ma, during the hiatus between the lower and upper Rutig volcanics. The NMI is neither deformed nor metamorphosed, indicating post-collisional emplacement, and uralitization by late-magmatic and sub-solidus alteration is restricted to the margins of the intrusion. A quantitative fractionation model indicates a fractionating assemblage of 61% primary amphibole, 10% clinopyroxene, 28% plagioclase, 1% biotite, 0.4% apatite, and 0.15% Fe-Ti oxide. Contrary to the recent studies, we find that the nearby diorite of Gebel Sheikh El-Arab is not co-genetic with the appinitic gabbro of the NMI. Although there are volcanic xenoliths in the NMI, we find no chemical evidence requiring contamination by continental crust. A subduction-related signature in a post-orogenic intrusion requires the inheritance of geochemical tendencies from a previous subduction phase. Given that the fine-grained gabbro of the NMI is consistent with a near-primary mantle melt, we attribute this inheritance to persistence and later melting of the slab-modified mantle domains, as opposed to partial melting and assimilation of the juvenile continental crust. The fine-grained gabbro composition indicates derivation at temperature and pressure conditions similar to the sources of mid-ocean ridge basalts: mantle potential temperature near 1350°C and extent of melting about 7%. Such temperatures, neither so high as to require a plume nor so low as to be consistent with small degrees of melting of a volatile-rich source, are most consistent with a lithospheric delamination scenario, allowing the upwelling of fertile, subduction-modified asthenosphere to depths ≤50 km.

Geochemistry and petrogenesis of post-collisional alkaline and peralkaline granites of the Arabian-Nubian Shield: a case study from the southern tip of Sinai Peninsula, Egypt

ABSTRACT The southern Sinai Peninsula, underlain by the northernmost extension of the Arabian-Nubian Shield, exposes post-collisional calc-alkaline and alkaline granites that represent the youngest phase of late Neoproterozoic igneous activity. We report a petrographic, mineralogical and geochemical investigation of post-collisional plutons of alkaline and, in some cases, peralkaline granite. These granites intrude metamorphosed country rocks as well as syn- and post-collisional calc-alkaline granitoids. The alkaline and peralkaline granites of the southern tip of Sinai divide into three subgroups: syenogranite, alkali feldspar granite and riebeckite granite. The rocks of these subgroups essentially consist of alkali feldspar and quartz with variable amounts of plagioclase and mafic minerals. The syenogranite and alkali feldspar granite contain small amounts of calcic amphibole and biotite, often less than 3%, while the riebeckite granite is distinguished by sodic amphibole (5–10%). These plutons have geochemical signatures typical of post-collisional A-type granites and were most likely emplaced during a transition between orogenic and anorogenic settings. The parental mafic magma may be linked to lithospheric delamination and upwelling of asthenospheric mantle material. Differentiation of the underplated basaltic magma with contributions from the juvenile crust eventually yielded the post-collisional alkaline granites. Petrogenetic modelling of the studied granitic suite shows that pure fractional crystallization cannot quantitatively explain chemical variations with the observed suite, with both major oxides and several trace elements displaying trends opposite to those required by the equilibrium phase assemblage. Instead, we show that compositional variation from syenogranite through alkali feldspar granite to riebeckite granite is dominated by mixing between a low-SiO2 liquid as primitive or more primitive than the lowest-SiO2 syenogranite and an evolved, high-SiO2 liquid that might be a high-degree partial melt of lower crust.