Neoproterozoic Ophiolites in the Arabian Shield: Field Relations and Structure, Kingdom of Saudi Arabia

SGS-OF-2003-1 Peter R. Johnson, Fayek H. Kattan, and Ahmed M. Al-Saleh
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ABSTRACT
Ophiolites make up a small but tectonically important part of the Arabian shield.  Where most complete, they consist of serpentinized peridotite, gabbro, dike complex, basalt, and pelagic rocks.  However, because of folding and shearing, the majority of the ophiolites lack one or more of these diagnostic lithologies.  Nonetheless, the incomplete assemblages are identified as ophiolites because they minimally include peridotite and gabbro, in many cases are associated with basalt, and in all cases show evidence of emplacement by thrusting and shearing rather than intrusion.  The ophiolites range in age from ~870 Ma to ~695 Ma, documenting a 200 million year period of oceanic magmatism in the Arabian shield, and are caught up in ~780 Ma- ~680 Ma suture zones that reflect a 100-million year period of terrane convergence.  All the ophiolites are strongly deformed, metamorphosed, and altered by silicification and carbonatization.  Low-grade greenschist facies metamorphism predominates, but in places the rocks reach amphibolite grades.  Alteration resulted in the development of listwaenite, particularly in shear zones, and locally the only evidence that mafic-ultramafic rocks underlie a given area is the presence of upstanding ridges of listwaenite that are resistant to erosion.  S/C fabrics are widespread and indicate that the ophiolites were affected by both strike-slip and vertical displacements.  Variations in senses of shear observed along and across the strike evidence considerable strain partitioning during deformation.  However, prevailing senses of shear can be discerned for several of the ophiolites that, in conjunction with other structural observations, indicate the main shear trajectories of the shear zones containing the ophiolites.  Jabal Ess, Jabal Tharwah, and Bi’r Umq ophiolites were emplaced during periods of dextral transpression on the Yanbu and Bi’r Umq sutures, respectively.  The Bi’r Tuluhah ophiolite was emplaced during sinistral transpression of the Hulayfah-Ad Dafinah-Ruwah suture, and the Halaban ophiolite was emplaced during west-directed convergence on the Halaban suture.

 

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Johnson, P.R., Kattan, F.H., and Al-Saleh, A.M., 2003, Neoproterozoic ophiolites in the Arabian shield: field relations and structure, Kingdom of Saudi Arabia: Saudi Geological Survey Open-File Report SGS-OF-2003-1, 29 p., 16 figs., 1 table.