, Geochemistry and Tectonic significant of Mantle Peridotite from Bir Umq Ophiolite Complex, ‎Arabian Shield, Saudi Arabia ‎,

Journal Article
Abuamarah‎, ‎2.‎ Bassam A. . 2019
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Publication Abstract: 

The Bir Umq ophiolite is one of the most important ophiolitic successions in the Arabian Shield, and represents an excellent case for the study of the tectonomagmatic evolution of the earliest
Precambrian events in the juvenile part of the Arabian-Nubian Shield (ANS). It is a dismembered ophiolite, which includes a serpentinized peridotite with small amounts of gabbro and mélange,
and is overlain by the Sumayir formation. The mantle section of the Bir Umq ophiolite has been
pervasively sheared and folded during its emplacement and is extensively serpentinized, carbonated
and silicified, resulting in the common development of magnesite and listwaenite along
the shear zones. Listwaenite occurs in the form of upstanding ridges due to its resistance to
erosion. Antigorite is the main serpentine mineral, which, however, has low amounts of lizardite
and chrysotile, indicating that the present serpentinites formed by prograde metamorphism. The
ophiolitic rocks of Bir Umq have undergone regional metamorphism up to the greenschist to
amphibolite facies. The presence of mesh and bastite textures indicates harzburgite and dunite
protoliths. The serpentinized peridotite preserves rare relicts of primary minerals such as olivine,
pyroxene and Cr-spinel. The serpentinized ultramafics of Bir Umq have high Mg# [molar Mg/(Mg
+Fe2+); 0.90–0.93), low CaO, and Al2O3 contents similar to that of the environment of the
suprasubduction zone. Additionally, they are characterized by the depletion of some compatible
trace elements (e.g., Nb, Sr, Ta, Zr, Hf and REE), but show a wide variation in the Rb and Ba.
Moreover, they are enriched in some elements that have affinities for Mg-rich minerals such as Ni,
Cr, V, and Co. Fresh relics of olivine have high Fo (av. 0.91) and NiO (av. 0.42) contents, similar to
those in the mantle olivine. The fresh Cr-spinel has high Cr# (0.68) and low TiO2 content (av. 0.11),
similar to those in modern fore-arc peridotites. The composition of both orth- and clinopyroxenes
confirms the fore-arc affinity of the studied ultramafics. The present study indicates that the
protoliths of the serpentinized ultramafics of Bir Umq have high partial melt degrees, which is
consistent with the characteristics of ultramafic rocks formed in a subarc environment (fore-arc)
within a suprasubduction zone system.