Structure of the NE Oman Mountains: a review of robust geochronological constraints for tectonic models of ophiolite obduction and continental subduction

The Late Cretaceous Semail ophiolite, Oman was emplaced onto the previously passive continental margin of Arabia. Recent geochronology from the ophiolite, its metamorphic sole, and associated high-pressure (HP) metamorphic rocks have resulted in a proliferation of new tectonic models explaining the sequence of events associated with ophiolite obduction and later continental subduction. These models have argued for (a) subduction initiation ∼8 Myr prior to Semail ophiolite formation, (b) two distinct HP metamorphic events, including HP subduction metamorphism before and after ophiolite emplacement, and (c) the presence of two separate, synchronous subduction zones operating during ophiolite formation and emplacement. We review published data and demonstrate that these new models are unsupported by robust geochronological and geological constraints; rather, the existing data are best explained by the initiation of a single NE-dipping subduction zone by ∼96.7 Ma, over which the ophiolite crystallized and along which the continental margin eventually subducted beneath it. The overlap between U-Pb zircon age dates on the Semail ophiolite crustal sequence (96.1–95.2 Ma), and U-Pb zircon (96.7–95.2 Ma) and internally consistent Lu-Hf garnet (97.9–93.0 Ma) dates from the metamorphic sole indicate synchronous Cenomanian ophiolite crystallization and sole metamorphism, consistent with ophiolite formation in a supra-subduction zone environment. By ∼79 Ma, the Oman continental margin reached the subduction zone, and Early Permian shelf carbonates and basement slices were dragged down to depths ∼90–100 km and metamorphosed to eclogite facies (≥20 kbar), which is recorded by Sm-Nd, U-Pb, and Rb-Sr ages from multiple phases (garnet, zircon, rutile, phengite).