Dating Continental Subduction Beneath the Samail Ophiolite: Garnet, Zircon, and Rutile Petrochronology of the As Sifah Eclogites, NE Oman

Studies of the high-pressure (HP) As Sifah eclogites in the NE Saih Hatat Window, Oman, have used different combinations of radiometric dating results (Ar/Ar, Sm-Nd vs. U-Pb, Rb-Sr) to interpret disparate tectonic models for the timing, geometry, and cause of continental subduction-including its relationship to the Samail Ophiolite. To determine the timing of continental subduction, we coupled petrochronological analyses of major (garnet) and accessory phases (zircon and rutile) from the highest grade metamorphic rocks (As Sifah eclogites) in the Saih Hatat. Early Permian (283.8 +/- 0.7 Ma) tuffaceous zircon cores are consistent with earlier interpretations that the As Sifah rocks were sourced from a distal portion of the Arabian continental margin. Data from a range of bulk compositions and metamorphic assemblages consistently suggest a single metamorphic event with garnet growth from similar to 81 to 77 Ma-though with slight, consistent offsets in the timing of metamorphic (re)crystallization between different lithologies. These new data confirm previous interpretations for continental HP metamorphism in a single NE-dipping subduction zone beneath the obducted Samail Ophiolite; there is no robust evidence for an similar to 110 Ma event or for SW-dipping subduction beneath the Arabian plate. Combined with other constraints, our data suggest that the As Sifah unit was subducted at rates similar to other small continental HP bodies, followed by two stages of cooling and exhumation-likely associated with the dragging to mantle depths by a mafic root, an initial phase of rapid exhumation from mantle depths, and a lengthy residence (<= 40 My) in the lower to middle crust. Plain Language Summary The subduction of continental material to mantle depths is of interest because (a) it is associated with the recycling of surface material to the deep mantle and (b) these types of rocks have been exhumed to the Earth's surface almost exclusively since ca. 1.0 Ga. The Saih Hatat window, Oman, is a unique expression of this process where buoyant continental rocks were apparently subducted and exhumed beneath a denser ophiolite (slice of oceanic crust and mantle). However, the timing and geometry of this process has been controversial. We measured isotopic dates and trace elements from several different minerals in a suite of representative samples from the the highest-grade, eclogite-facies metamorphic rocks in the Saih Hatat, and found that each gave the same range of dates (similar to 81-77 Ma). There are small variations due to changes in bulk composition, but the data firmly suggest a single subduction metamorphism episode. These results require the subduction of the Arabian continental margin after the obduction of the Samail Ophiolite, and suggest that the most deeply subducted, most outboard continental margin rocks were anchored to a dense mafic root before detaching, exhuming (first quickly, then more slowly), and eventually becoming exposed in tectonic windows through the ophiolite.