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

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.