Ar-40/Ar-39 dates of emplacement-related metamorphic rocks beneath the Samail ophiolite in Oman show that cooling to <525 degrees C occurred within similar to 1 million years of igneous crystallization of the ophiolite. This unexpectedly short time span and rapid cooling means that old, cold continental or oceanic lithosphere must have been adjacent to the ophiolite during spreading and then been thrust beneath the ophiolite almost immediately afterward.