DATING THE SUBDUCTION OF THE ARABIAN CONTINENTAL MARGIN BENEATH THE SEMAIL OPHIOLITE, OMAN

Eclogites exposed along the NE margin of the Arabian continental plate beneath the Oman Semail ophiolite demonstrate that subduction of thinned continental crust to maximum pressures of 20 kbar, ca. 80km depth has occurred. Current debate centres on whether this high-pressure metamorphism occurred prior to, during, or following ophiolite obduction, which is known to have started ca. 94 My ago. We have obtained six concordant U-Pb zircon ages, using conventional TIMS methods, from two garnet-clinopyroxene-crossite-phengite eclogites from As Sifah, NE Oman, precisely constraining the age of high pressure metamorphism at 78.9 ± 0.2 Ma. Zircon occurs predominantly within, and along grain boundaries of, high pressure minerals (clinopyroxene and garnet) and contains inclusions of other high pressure mineral phases such as rutile. Zircon growth therefore occurred during or prior to eclogite facies metamorphism. This age demonstrates that the high pressure metamorphism of the As Sifah eclogites did not occur prior to ophiolite formation at 93-97 Ma, nor prior to ophiolite obduction at 93-94 Ma, but rather during continental subduction 15 My later. This new, precise, age of eclogite facies metamorphism demonstrates, perhaps uniquely, that thinned continental crust must have been subducted and exhumed beneath a hanging wall entirely composed of oceanic crust and upper mantle.