CONSTRAINING THE TIMING OF PROGRADE METAMORPHISM IN THE SOLE OF THE SEMAIL OPHIOLITE: COUPLED ZIRCON TIMS U-PB DATING AND TRACE ELEMENT ANALYSES FROM THE MASAFI SOLE EXPOSURE, UNITED ARAB EMIRATES (U.A.E.)

The Semail (Oman-U.A.E.) ophiolite is the largest and most extensively studied ophiolite in the world. Several lines of evidence suggest the ophiolite formed in a supra-subduction zone setting, potentially during subduction initiation. The ophiolite is underlain by an inverted metamorphic sole, consisting of greenschist- to granulite-facies metabasalts and metasediments. Geochronologic studies of these rocks can determine the timing of initial lower-plate burial relative to upper-plate extension, constraining the tectonic development of the ophiolite. Previous high-precision ID-TIMS zircon U-Pb dating of upper-level gabbros indicates rapid formation of the ophiolite crust from 96.12–95.50 Ma. In comparison, ID-TIMS zircons dates from leucocratic pods from the Sumeini and Wadi Tayin sole localities yield dates of 96.16 ± 0.022 Ma and 94.82 ± 0.035 Ma, respectively. Here we present new data from the Asimah-Masafi sole exposure at the northern end of the ophiolite in the U.A.E. To directly date peak metamorphism, we targeted garnet-clinopyroxene bearing amphibolites and used combined zircon ID-TIMS U-Pb dating and LA-ICP-MS trace-element analyses to link zircon dates to metamorphic processes. Zircons from five samples show a range from steep (Yb/Dy > 5) to shallow HREE (Yb/Dy < 5) slopes. We attribute spots with steep HREE slopes to pre-garnet zircon growth and spots with shallow HREE slopes to syn- or post-garnet zircon growth. In preliminary ID-TIMS data from two samples, HREE-enriched zircon have older TIMS U-Pb dates compared to HREE-depleted zircon from the same sample, which we interpret to bracket the timing of prograde garnet growth to ~96.6–96.2 Ma and ~95.6–95.3 Ma at the Khubakhib and Asimah localities, respectively. Our dates are similar to the previous U-Pb zircon dates from the sole, but significantly younger than recently reported Lu-Hf garnet dates (~104 Ma; Guilmette et al., 2018).