Metamorphism and 40Ar/39Ar dating of the Halaban Ophiolite and associated units: evidence for two-stage orogenesis in the eastern Arabian Shield

The Arabian Shield has long been recognized as a region where plate-tectonic processes have been in action during most of the Late Proterozoic resulting in the amalgamation of the shield's five constituent terranes along four major suture zones. Studies thus far carried out on these collisional belts have concentrated on the origin and dating of intermediate to acidic igneous rocks, and very little emphasis has been placed on the understanding of metamorphic processes operative during plate convergence. The Al-Amar Suture separates the Afif microplate from the Ar Rayn Block in the easternmost section of the Shield. Among several ophiolite occurrences in this suture, the Halaban Belt is by far the largest and preserves within and around it a record of a number of thermal/structural events related to the opening and eventual closure of a supra-subduction zone back-are basin that existed in the period 695-680 Ma. The bimodal nature of 40Ar/39Ar dates from the Halaban Ophiolite and associated units is indicative of a two-stage orogenesis climaxing at 680 and 600 Ma, instead of the previously proposed model of a single orogenic episode between 670 and 630 Ma. The first stage (680 Ma) is believed to be related to basin closure and ophiolite emplacement as the Ar Rayn island are collided with the Afif microcontinent. The second episode (600 Ma) was the outcome of a major collisional event between the Arabian craton and a large continental mass east of the Ar Rayn Block.