The Upper Proterozoic ophiolite mélange zones of the easternmost Arabian shield

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Abstract
The Arabian-Nubian shield was produced by the episodic cratonization and accretion of intra-oceanic arc systems over the period 1100–500 Ma BP. The easternmost part of the Arabian shield is dominated by the N-S Al Amar fault zone, long believed to represent an easterly-inclined Upper Proterozoic suture between either two island arc systems or oceanic and continental lithosphere. In the area studied there are two N–S ophiolitic mélange zones which separate a central belt of metamorphosed trench sediments (Abt Schists) from flanking granitoid zones that represent the plutonic cores of arc systems. The mélange blocks are of layered gabbro, chromitite, homogeneous gabbro with dykes, plagiogranite, sheeted dykes and pillow lavas set in a sheared serpentinite matrix. The blocks are large ( c. 1 km) and gabbroic in the western zone and smaller (1–100 m) and of fine-grained basic rocks in the eastern zone; all are compositionally similar and petrographically typical of rock types found in ophiolite complexes elsewhere. However, the Zr, Ti and Nb content of all these rocks is too low even for the back-arc, marginal basin setting commonly invoked for ophiolite genesis. To explain the unusually low high field strength element abundances, it is suggested that either these rocks were generated in or near a transform fault zone and then deposited in an oceanic trench when they began to be subducted, or that their boninitic character identifies them as the products of fore-arc igneous activity and, by analogy with the tectono-magmatic evolution of the W. Phillipine-Mariana region in the W Pacific, they mark the site of arc separation prior to back-arc sea-floor spreading; the latter explanation is preferred.