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

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ID: 308289
2021
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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.
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Authors Ayman E. Maurice, Mokhles K. Azer, Paul D. Asimow, Fawzy F. Basta, Hassan M. Helmy, Tomoyuki Shibata
Journal american journal of science
Year 2021
DOI
10.2475/10.2021.02
URL
https://doi.org/10.2475/10.2021.02
Keywords Keywords not found

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