A fore-arc setting of the Gerf ophiolite, Eastern Desert, Egypt: Evidence from mineral chemistry and geochemistry of ultramafites

Abdel‐Karim, Abdel‐Aal M.; Ali, Shehata; Helmy, Hassan M.; El-Shafei, Shymaa A.

doi:10.1016/j.lithos.2016.05.023

Cited by 66 publications

(68 citation statements)

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“…These authors inferred a back arc or an inter‐arc basin origin based on the chemical composition of the ophiolitic rocks. This interpretation conflicts with the observation that most boninites are now found in the fore‐arcs of the interoceanic arcs (Abdel‐Karim et al, , ; Dilek, Furnes, & Shallo, ; Farahat, ). The analysed primary chromites of the studied three areas lie in the supra‐subduction zone field (Figure c).…”

Section: Discussionmentioning

confidence: 76%

“…The ophiolitic serpentinites underwent progressive metamorphism (regional) causing alteration of Al‐rich chromite to Cr‐magnetite. This transferred the peridotites from the greenschist facies to the lower amphibolite facies at T = 500–550 °C (Abdel‐Karim, Ali, Helmy, & El‐Shafei, ; Hamdy & Lebda, ).…”

Section: Discussionmentioning

confidence: 99%

“…The ophiolitic serpentinites underwent progressive metamorphism (regional) causing alteration of Al-rich chromite to Cr-magnetite. This transferred the peridotites from the greenschist facies to the lower amphibolite facies at T = 500-550°C (Abdel-Karim, Ali, Helmy, & El-Shafei, 2016;Hamdy & Lebda, 2011). Rollinson (2005) argued that the compositional variation in ophiolitic chromites are related to the process of a melt-rock reaction rather than different tectonic environment in addition, the presence of water in the melt is thought to be necessary for the crystallization of ophiolitic chromite (Edwards, Pearce, & Freeman, 2000).…”

Section: Alteration and Metamorphismmentioning

confidence: 99%

“…Barnes & Roeder, 2001; Gonzálea-Jimènez, Kerestedjian, Proenza, & Gervilla, 2009;Proenza, Zaccarini, Escayola, et al, 2008;Zhou, Robinson, Malpas, & Zijin, 1996). Characteristic features of chromites hosted in the Pan-African ophiolites of Egypt are given by Ahmed, Arai, and Attia (2001), Ahmed (2007), Farahat (2008), Hamdy and Lebda (2011) and Abdel-Karim et al (2014, 2016.…”

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confidence: 99%

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Mineralogy and chemical aspects of some ophiolitic metaultramafics, central Eastern Desert, Egypt: Evidences from chromites, sulphides and gangues

Abdel‐Karim

El-Shafei

2017

Geological Journal

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Handling editor: P. HorváthChromites and sulphides are the most common ore minerals in the ophiolitic metasomatized ultramafics (~790 Ma) of Um Halham, Fawakhir and Barramiya, central Eastern Desert, Egypt.These ultramafics exhibit variable degrees of alteration and metasomatism and include massive serpentinites (serpentinized peridotite and serpentinite), tremolite-talc rocks, talc-carbonate rocks, listwaenite-like rocks and typical listwaenite.The alteration of chromite to Cr-magnetite was accompanied by the formation of chloritic aureoles due to the release of Al and Mg from chromite. Textural and compositional features of the chromites suggest a greenschist up to lower amphibolite facies metamorphism (at 500-600°C), which is facial with the host ultramafics. The chromites exhibit a boninitic affinity and a forearc suprasubduction setting of the present ophiolite assemblages. The variability of chromite chemistry indicates a melt-rock reaction, together with the water-bearing melt which is necessary for crystallizing chromite. Pentlandite, pyrrhotite, arsenopyrite, gersdorffite and pyrite are the most common sulphide minerals. Gold occurs either as an invisible phase associated with gersdorffite, arsenopyrite, and As-rich pyrite or as native nuggets in listwaenites of Barramiya area. Gangue metasomatized phases associated with these ore minerals are identified as sericite, carbonate and chlorite. The investigated sericite ranges in composition from mariposite (>0.5% Cr) to fuchsite (>1% Cr), which indicates typical listwaenites. Five varieties of carbonate minerals are identified; magnesite, breunnerite, dolomite, calcite and ankerite.The CO 2 -rich fluids are released during progressive decarbonation reactions in carbonate-bearing metasediments, whereas Ni and As-rich fluids are attributed to nearby granitic intrusions affected on serpentinites and leached Ni and precious metals like gold forming As-Ni-rich fluids. These metasomatic fluids are thought to reflect a range of melts derived from a compositionally evolving source during subduction initiation in a forearc environment.

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Section: Discussionmentioning

confidence: 76%

Section: Discussionmentioning

confidence: 99%

Section: Alteration and Metamorphismmentioning

confidence: 99%

mentioning

confidence: 99%

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Mineralogy and chemical aspects of some ophiolitic metaultramafics, central Eastern Desert, Egypt: Evidences from chromites, sulphides and gangues

Abdel‐Karim

El-Shafei

2017

Geological Journal

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“…Remnants of ultramafic rocks produced in mid-oceanic ridges (Khedr, Arai, Python, & Tamura, 2014) or subduction zones (Khedr & Arai, 2013;Yellappa et al, 2014;Abdel-Karim, Ali, Helmy, & El-Shafei, 2016) occur within collisional sutures and can be discriminated using geochemical proxies (Kamenetsky, Crawford, & Meffre, 2001;Maulana, Christy, & Ellis, 2015). The serpentinisation and magnetite mineralisation associated with ultramafic rocks are not only related to the tectonic setting but are also controlled by the geochemical conditions during serpentinisation process, such as the protolith composition, temperature, silicon activity, and oxygen fugacity (Frost, Evans, Swapp, Beard, & Mothersole, 2013;Katayama, Kurosaki, & Hirauchi, 2010;Khedr & Arai, 2018;Nozaka, Wintsch, & Meyer, 2017).…”

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Serpentinisation and magnetite formation in the Angwu ultramafic rocks from the central Bangong–Nujiang suture zone, Tibetan Plateau

Liu

Tao

et al. 2019

Geological Journal

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Serpentinised ultramafic rocks are widely distributed in the Bangong–Nujiang suture zone (BNSZ) and show close relationship with the regional mineralisation. Here, we investigate the serpentinised ultramafic rocks from Angwu in the central segment of the BNSZ, where the primary minerals include olivine, clinopyroxene, chromite, lizardite, magnetite, and chlorite. Based on a comprehensive analysis of the results from petrography, back‐scattered electron images (BSE), and energy dispersive spectrographs (EDS), as well as electron probe microanalysis (EPMA), the alteration process of the ultramafic rocks, including serpentinisation, can be divided into three stages: 1, formation of relatively Fe‐rich serpentine (Mg#, 0.82), with no precipitation of magnetite; 2, formation of relatively Mg‐rich serpentine (Mg#, 0.93), with precipitation of magnetite in serpentine veins; and 3, alteration of serpentine into chlorite. We computed the reactions associated with alteration process for the above stages. In addition, thermodynamic modelling in multicomponent mineral equilibrium phase diagrams indicate that during the serpentinisation process, the desilication of pyroxene in caused the enhancement of SiO2 activity and restricted the transformation from olivine to magnetite. The reaction temperature for the serpentinisation of these rocks was relatively high, and within this temperature interval (100–300°C), lizardite was stable, and precipitation of magnetite was not favoured. We also evaluate the factors influencing magnetite precipitation during serpentinisation of ultramafic rocks including the composition of primitive rocks and the activity of SiO2 and temperature. Our study provides insights on the mechanism of magnetite mineralisation associated with the serpentinisation of ultramafic rocks.

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New constraints on the tectono‐magmatic evolution of the Tidding‐Mayodia ophiolites, eastern Himalaya, northeast India

et al. 2021

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The Tidding‐Mayodia ophiolites (TMO) exposed along the Lohit and Dibang river valleys in eastern Himalaya that have been considered as the extension of the Indus‐Tsangpo Suture Zone ophiolites are revisited to review their petrogenetic‐tectonic origin. The ophiolites consist of depleted harzburgite and dunite with lesser amounts of mafic rocks (gabbro intrusives, mafic dykes) and carbonates. The serpentinized peridotites consist of antigorite, lizardite, olivine, Cr‐spinel, and bastite with minor sulfide minerals. From SEM‐EDS studies, sulfide minerals were observed to be associated mainly with magnetites. The main sulfide mineral is pentlandite with minor millerite that exists as inclusions inside the pentlandite grains. Elemental mapping of these sulfides shows that they are mainly Ni‐(Co‐)‐bearing sulfides. The olivines are highly forsteritic (Fo = 95–96) while the Cr‐spinels show distinct Cr‐magnetite rims with a chromite core (Cr# = ~93). The serpentinized peridotites have whole‐rock compositions of SiO2 <47 wt% and high MgO (>36.37 wt%) and low Al2O3 (<1.21 wt%), CaO (<0.82 wt%), indicating the depleted nature of the parent rocks. Highly fractionated LREEs as compared to HREEs [(La/Yb)N = 2.62–13.22], and REE and Cr spinel chemistry modelling suggests that the studied peridotites have formed from ~22% partial melting of a depleted spinel lherzolite source which later underwent interactions with a high‐temperature silicate melt that caused enrichment in LREE and Cr of spinels. The parental melt compositions of Cr spinel yield their formation during arc tectonism (Al2O3melt = 6.28–7.65 wt%, FeO/MgO = 1.00–1.33). Furthermore, Mn and Zn concentrations in spinels, the occurrence of Cr magnetite rim in Cr‐spinels, presence of secondary olivine with higher Fo (~98), and occurrence of low‐temperature re‐equilibrated sulfide minerals, indicate that the rocks were subject to low‐temperature metamorphism. Based on this evidence, combined with data from previous studies, a tectonic model has been proposed for the genesis of the studied ophiolites. This model shows that the ophiolites have formed from the entrapment of depleted N‐MORB mantle in the mantle wedge of an intra‐oceanic subduction zone. During the nascent forearc regime, this mantle wedge underwent interactions with high‐temperature melts, which caused changes in their chemistry. Moreover, the rocks underwent interactions with low‐temperature fluids in the mature forearc, which caused the formation of sulfides and metamorphozed these rocks.

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A fore-arc setting of the Gerf ophiolite, Eastern Desert, Egypt: Evidence from mineral chemistry and geochemistry of ultramafites

Cited by 66 publications

References 72 publications

Mineralogy and chemical aspects of some ophiolitic metaultramafics, central Eastern Desert, Egypt: Evidences from chromites, sulphides and gangues

Mineralogy and chemical aspects of some ophiolitic metaultramafics, central Eastern Desert, Egypt: Evidences from chromites, sulphides and gangues

Serpentinisation and magnetite formation in the Angwu ultramafic rocks from the central Bangong–Nujiang suture zone, Tibetan Plateau

New constraints on the tectono‐magmatic evolution of the Tidding‐Mayodia ophiolites, eastern Himalaya, northeast India

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