Petrology of a Neoproterozoic Alaskan-type complex from the Eastern Desert of Egypt: Implications for mantle heterogeneity

Khedr, Mohamed Zaki; Arai, Shoji

doi:10.1016/j.lithos.2016.07.016

Cited by 51 publications

(37 citation statements)

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“…In this case, the CaO content is related to the presence of clinopyroxene, which has been verified by petrography and EPMA analyses. In addition, the Fo content of olivine is lower than 85 in our rocks, similar to that of Alaskan‐type intrusions (Khedr & Arai, ). On the other hand, the Mg# of clinopyroxene shows marked variation (60–87), which is different from the Oman abyssal peridotites (90–96; Khedr et al, ) and the Happo‐One forearc peridotites (96–98; Khedr, Arai, Tamura, & Morishita, ).…”

Section: Discussionsupporting

confidence: 82%

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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Section: Discussionsupporting

confidence: 82%

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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“…Alaskan-type intrusions in the Eastern Desert typically have Cr# ranges between 0.31 and 0.90 and Fe 3+ # between 75 and 55. The spinels are characteristically Al–Mg poor, similar to those seen in Yubdo (Helmy & Mogessie, 2001; Helmy & El Mahallawi, 2003; Farahat & Helmy, 2006; El-Rahman et al 2012; Khedr & Arai, 2016).…”

Section: Resultssupporting

confidence: 55%

“…The Neoproterozoic ophiolites and associated ultramafic–mafic intrusions in the Eastern Desert of Egypt have been suggested to be 890–690 Ma (Stern et al 2004; Azer & Stern, 2007; Abdel-Karim et al 2016). The northern Arabian–Nubian Shield is made up of both ophiolitic mafic–ultramafic rocks (Stern et al 2004; Ahmed, 2013; Khedr & Arai, 2016) and Alaskan-type ultramafic–mafic rocks (Helmy & Mogessie, 2001; Helmy & El Mahallawi, 2003; Farahat & Helmy, 2006; El-Rahman et al 2012; Khedr & Arai, 2016).…”

Section: Resultsmentioning

confidence: 99%

“…It has been suggested that they represent Alaskan-type, concentrically zoned intrusions, which were emplaced into an extensional arc or back-arc environment (Mogessie, Belete & Hoinkes, 2000; Braathen et al 2001; Grenne et al 2003). These small elliptical bodies are common in the northern parts of the Arabian–Nubian Shield in the Eastern Desert of Egypt: Gabbro Akarem (Helmy & El Mahallawi, 2003; El-Rahman et al 2012), Genina Gharbia (Helmy et al 2014), Abu Hamamid (Helmy et al 2015) and Dahanib (Khedr & Arai, 2016).…”

Section: Geological Setting/backgroundmentioning

confidence: 99%

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The origin of the ultramafic rocks of the Tulu Dimtu Belt, western Ethiopia – do they represent remnants of the Mozambique Ocean?

et al. 2017

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The East African Orogen contains a series of high-strain zones that formed as Gondwana amalgamated. The Tulu Dimtu shear belt is one of these N–S structures within the Barka–Tulu Dimtu zone in western Ethiopia, and contains ultramafic bodies of equivocal origin. Identifying the petrogenetic origin of these enigmatic rocks provides evidence for the geodynamic significance of these shear zones. Owing to their altered state, these ultramafic rocks’ well-preserved chrome spinels provide the only reliable evidence for their source and tectonic affiliation. Chrome spinels have high Cr2O3 (30.04–68.76 wt %), while recalculated Fe2O3 (< 2 %) and TiO2 (0.01–0.51 %) values are low. The Cr# (molar Cr3+/Cr3+ + Al2+) and Mg# (Mg2+/Mg2+ + Fe2+) have averages of 0.88 and 0.22, respectively. Based on olivine–spinel equilibria, the calculated fO2 values (FMQ +3.03) for the dunites reveal a highly oxidized environment. This spinel chemistry (high Cr# > 0.6 and low Ti) supports a supra-subduction origin, with an oxidized mantle source more refractory than depleted MORB mantle (DMM). These spinel compositions indicate that some ultramafic bodies in western Ethiopia, including those from Daleti, Tulu and Dimtu, are serpentinized peridotites emplaced as obducted ophiolite complexes. By contrast, the ultramafic rocks from the Yubdo locality have a different spinel chemistry, with strong affiliation with igneous spinels formed in Alaskan-style mafic intrusions. These collective results suggest that regardless of their origin as supra-subduction ophiolites or as Alaskan-type intrusions, these spinels were formed on a convergent-subduction margin.

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“…In support, we note that: (a) Ca‐amphibole is abundant in all rocks of the pluton (apart from the nodules); and (b) relict poikilitic textures occur locally, in which large amphiboles enclose either “olivine” (now pseudomorphed by talc + serpentine) or clinopyroxene. We also suggest that many of the amphiboles in these rocks may, at least in part, preserve their primary, igneous compositions, and such pargasitic compositions are common in mafic to ultramafic plutonic complexes elsewhere (e.g., Chen, Suzuki, Tian, Jahn, & Ireland, ; Daczko, Emami, Alliborne, & Turnbull, ; Khedr & Arai, ; Pettigrew & Hattori, ). Alternatively, if the pargasitic compositions in the matrix and chlorite‐amphibole rocks are due to metamorphism, then these rocks would not be isofacial with other rock types in the immediate field area, which are demonstrably in the greenschist facies.…”

Section: Discussionmentioning

confidence: 79%

Enigmatic nodule‐bearing rocks in the mafic to ultramafic Diana Mills pluton, Piedmont Province, Virginia

2018

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Handling Editor: M. SantoshThe Silurian Diana Mills pluton is a metamorphosed mafic-ultramafic body in the Piedmont Province of Virginia. Major rock types in the pluton include metadiorite and chlorite-amphibole rock, with lesser amounts of hornblendite. However, the most visually striking rocks, found at 4 locations, consist of tan-weathering nodules in a dark green matrix. The purpose of this study was to investigate the origin of these nodular rocks. All rocks show evidence of greenschistfacies metamorphism, but relict igneous textures are preserved locally. Apart from the nodules, Ca-amphibole is ubiquitous in all rock types. Nodules are dominated by serpentine or talc (+magnetite ± chromite), and many show a thin (≤5 mm) radially oriented shell of serpentine against adjacent matrix. The nodules are ultramafic (Mg#~80-90, up to7,563 ppm Cr, up to 2,038 ppm Ni), and their normative mineralogy is dominated by olivine and orthopyroxene. Thus, they represent metamorphosed harzburgites and pyroxenites. Matrix minerals are dominantly amphibole + chlorite, along with variable amounts of talc + magnetite. Some matrix samples are chlorite-rich, probably reflecting metasomatic reaction with nodules (i.e., they are small-scale "blackwalls").Matrix samples are also ultramafic (high Mg#, Cr, and Ni). The matrix of the nodule-bearing outcrops is essentially the same as other chlorite-amphibole rocks elsewhere in the pluton. We consider these rocks to represent emplacement of an original hornblende peridotite crystal mush (a mixture of crystals and hydrous melt), which locally carried harzburgitic nodules. The nodules most likely represent the earliest-formed cumulates from the Diana Mills parent magma.

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Petrology of a Neoproterozoic Alaskan-type complex from the Eastern Desert of Egypt: Implications for mantle heterogeneity

Cited by 51 publications

References 64 publications

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

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

The origin of the ultramafic rocks of the Tulu Dimtu Belt, western Ethiopia – do they represent remnants of the Mozambique Ocean?

Enigmatic nodule‐bearing rocks in the mafic to ultramafic Diana Mills pluton, Piedmont Province, Virginia

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