Some Like It Cold: The “Conundrum of Samail” Revisited

摘要

Abstract Most tectonic models consider that the “Samail subduction zone” was the only subduction zone at the mid‐Cretaceous convergent Arabian margin. We report four new Rb‐Sr multimineral isochron ages from high‐pressure (HP) rocks and a major shear zone of the uppermost Ruwi‐Yiti Unit of the Saih Hatat window in the Oman Mountains of NE Arabia. These ages demand a reassessment of the intraoceanic suprasubduction‐zone evolution that formed the Samail Ophiolite and its metamorphic sole in the Samail subduction zone. Our new ages constrain waning HP metamorphism of the Ruwi subunit at ～99‐96?Ma and associated deformation in the Yenkit shear zone between ～104 and 93?Ma. Our ages for late stages of deformation and HP metamorphism (thermal gradients of ～8–10°C?km?1) overlap with published ages of ～105‐102?Ma for Samail‐subduction‐zone prograde‐to‐peak metamorphism (thermal gradients of ～20–25°C?km?1), subsequent decompressional partial melting of the metamorphic sole and suprasubduction‐zone crystallization of the Samail Ophiolite (thermal gradients of ～30°C?km?1) between ～100 and 93?Ma. Thermal considerations demand that two subduction zones existed at the mid‐Cretaceous Arabian margin. High‐pressure metamorphism of the Ruwi‐Yiti rocks occurred in a mature, thermally equilibrated “Ruwi subduction zone” that formed at ～110?Ma. Initiation of the infant, thermally unequilibrated and, thus, immature, outboard intraoceanic Samail subduction zone occurred at ～105?Ma. The Samail Ophiolite and its metamorphic sole were then thrust over the exhuming Ruwi‐Yiti HP rocks and onto the Arabian margin after ～92?Ma, while the bulk of the Saih Hatat HP rocks below the Ruwi‐Yiti Unit started to be underthrust in the Ruwi subduction zone.