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Research ArticleArticle

The kabr El-Bonaya peridotites, Southeastern Sinai, Egypt: petrology, geochemistry, and metamorphism of Neoproterozoic arc ultramafic cumulates

Ayman E. Maurice, Mokhles K. Azer, Paul D. Asimow, Fawzy F. Basta, Hassan M. Helmy and Tomoyuki Shibata
American Journal of Science December 2021, 321 (10) 1445-1496; DOI: https://doi.org/10.2475/10.2021.02
Ayman E. Maurice
*Geology Department, Faculty of Science, Helwan University, 11790 Cairo, Egypt
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  • For correspondence: Ayman.Maurice@yahoo.com Ayman.Maurice@science.helwan.edu.eg
Mokhles K. Azer
**Geological Sciences Department, National Research Centre, 12622-Dokki, Cairo, Egypt
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Paul D. Asimow
***Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, 91125, USA
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Fawzy F. Basta
§Geology Department, Faculty of Science, Cairo University, 12613 Giza, Egypt
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Hassan M. Helmy
§§Geology Department, Faculty of Science, Minia University, 61519 Minia, Egypt
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Tomoyuki Shibata
§§§Graduate School of Advanced Science and Engineering, Hiroshima University, Hiroshima, Japan
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Abstract

Two small, isolated ultramafic masses in the northeastern part of the Wadi Kid area, southeast Sinai, are composed of variably serpentinized harzburgite and lherzolite with minor talc-anthophyllite rock. The primary phases are dominantly olivine, orthopyroxene and Cr-spinel; clinopyroxene, amphibole, and phlogopite are also found in lherzolite samples. The whole-rock Mg# of harzburgite samples (89–91) is higher than that of lherzolite (average 82). The harzburgite samples contain olivine with higher Mg and Ni contents, orthopyroxene with higher Mg#, and Cr-spinel with higher Cr content than do the lherzolite samples. The REE patterns of clinopyroxene and amphibole in lherzolite are most consistent with a cumulate origin. Although several compositional characteristics of the harzburgites resemble those of residual mantle, in detail the Cr2O3 and Al2O3 contents of fresh Cr-spinel in harzburgite are different from those found in mantle samples or in any of the Neoproterozoic ophiolitic peridotites throughout the Arabian-Nubian Shield. Thus, all the ultramafic rocks at Kabr El-Bonaya are best explained as ultramafic cumulates, with harzburgite consisting of early-formed cumulate phases and lherzolite containing later-formed cumulate phases with higher REE abundances, primary hydrous minerals, evolved primary silicates, and high TiO2 (0.77 wt.%) and Al2O3 (18 wt.%) contents in Cr-spinel. The trace-element characteristics of the rocks indicate a subduction-related parental magma: whole-rock chondrite-normalized REE patterns are LREE-enriched; calculated fO2 values are elevated (+2.47 to +3.39 log units above the fayalite-magnetite-quartz buffer); and computed N-MORB-normalized trace element patterns for melts in equilibrium with clinopyroxene and amphibole have negative Nb-Ta anomalies and enrichment in large-ion lithophile elements. The low Al2O3/SiO2 ratios (0.007–0.040) of harzburgite samples and the low TiO2 contents and high Cr# of their Cr-spinel indicate derivation from a mantle source that experienced high-degree partial melting. From these characteristics, we infer a boninitic parental melt for the harzburgite. We offer an illustrative quantitative fractionation model that can explain the successive derivation of harzburgite and lherzolite cumulates along a single equilibrium, polybaric cooling path. We conclude that the Kabr El-Bonaya ultramafic cumulates represent the exposed roots of a Neoproterozoic island arc that was caught in the collision between East and West Gondwana.

  • Neoproterozoic
  • Island Arcs
  • Ultramafic cumulates
  • Arabian-Nubian Shield
  • Sinai
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American Journal of Science: 321 (10)
American Journal of Science
Vol. 321, Issue 10
1 Dec 2021
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The kabr El-Bonaya peridotites, Southeastern Sinai, Egypt: petrology, geochemistry, and metamorphism of Neoproterozoic arc ultramafic cumulates
Ayman E. Maurice, Mokhles K. Azer, Paul D. Asimow, Fawzy F. Basta, Hassan M. Helmy, Tomoyuki Shibata
American Journal of Science Dec 2021, 321 (10) 1445-1496; DOI: 10.2475/10.2021.02

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The kabr El-Bonaya peridotites, Southeastern Sinai, Egypt: petrology, geochemistry, and metamorphism of Neoproterozoic arc ultramafic cumulates
Ayman E. Maurice, Mokhles K. Azer, Paul D. Asimow, Fawzy F. Basta, Hassan M. Helmy, Tomoyuki Shibata
American Journal of Science Dec 2021, 321 (10) 1445-1496; DOI: 10.2475/10.2021.02
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    • INTRODUCTION
    • GEOLOGIC SETTING
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    • ANALYTICAL METHODS
    • SERPENTINE PHASES
    • MINERAL CHEMISTRY
    • WHOLE-ROCK GEOCHEMISTRY
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Keywords

  • Neoproterozoic
  • Island Arcs
  • Ultramafic cumulates
  • Arabian-Nubian Shield
  • Sinai

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