METAMORPHISM AND AR-40 AR-39 DATING OF THE HALABAN OPHIOLITE AND ASSOCIATED UNITS - EVIDENCE FOR 2-STAGE OROGENESIS IN THE EASTERN ARABIAN SHIELD/

The Arabian Shield has long been recognized as a region where plate-te ctonic processes have been in action during most of the Late Proterozo ic resulting in the amalgamation of the shield's five constituent terr anes along four major suture zones. Studies thus far carried out on th ese collisional belts have concentrated on the origin and dating of in termediate 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 fr om the Ar Rayn Block in the easternmost section of the Shield. Among s everal ophiolite occurrences in this suture, the Halaban Belt is by fa r the largest and preserves within and around it a record of a number of thermal/structural events related to the opening and eventual closu re of a supra-subduction zone back-are basin that existed in the perio d 695-680 Ma. The bimodal nature of Ar-40/Ar-39 dates from the Halaban Ophiolite and associated units is indicative of a two-stage orogenesi s climaxing at 680 and 600 Ma, instead of the previously proposed mode l 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 emp lacement as the Ar Rayn island are collided with the Afif microcontine nt. 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.