GARNET, TITANITE, AND MONAZITE PETROCHRONOLOGY OF WADI TAYIN, OMAN: IMPLICATIONS FOR OPHIOLITE OBDUCTION AND METAMORPHIC SOLE ASSEMBLY

Metamorphic soles – deformed crustal rocks welded to the base of ophiolites – are critical records of subduction initiation and ophiolite emplacement. High-grade assemblages from the metamorphic sole of the Semail Ophiolite yield peak metamorphic pressures of 11–13 kbar, yet the structural thickness of the ophiolite is only 15–20 km. This suggests that either i) the ophiolite was initially 40–50 km thick and was thinned after obduction, or ii) the ophiolite was initially <20 km thick and high-pressure sole rocks were exhumed from greater depth. To address these hypotheses, we dated garnet, monazite, and titanite from the Wadi Tayin metamorphic sole locality in Oman. A garnet-bearing metapelite from ~15 m below the basal ophiolite thrust records peak conditions of ~5 kbar and ~600°C, with no evidence of garnet resorption; a four-point garnet-WR regression through multiple garnet size fractions yields a Lu-Hf date of 95.8±0.5 Ma (MSWD = 0.3). Monazite from the same sample records Th-corrected LA-ICP-MS U-Pb dates ranging from ~97–92 Ma, with a weighted mean date of 94.0±0.8 Ma (MSWD = 1.8). Titanite from a nearby mafic amphibolite yielded an LA-ICP-MS U-Pb isochron date of 92.2±1.7 Ma (MSWD = 1.1), which is interpreted as cooling of high-grade rocks below ~650°C. These data show that the lower-grade sole metapelite was metamorphosed prior to or coeval with granulite-facies melting (94.82±0.035 Ma: Rioux et al., 2016) at a different point along the same subduction channel. Existing P-T data (Cowan et al., 2014) show that the highest-grade sole rocks remained hot during the initial stages of exhumation, yet the absence of a high-P overprint shows that the metapelite never re-equilibrated at depths >20 km. Our data therefore favor juxtaposition of amphibolite/granulite-facies slivers exhumed from depth with the lower-grade metapelite at 15–20 km depth by ~92 Ma, before the entire package was emplaced onto the Arabian continent.