TEMPORAL EVOLUTION OF THE SAMAIL (OMAN-UAE) OPHIOLITE

High-precision isotope dilution-thermal ionization mass spectrometry U-Pb zircon dating from the Samail (Oman-UAE) ophiolite provides a detailed timeline for the formation of the ophiolite and metamorphic sole. Geochemical, geochronological, and thermobarometric data suggest that the Samail ophiolite formed in a subduction zone setting, likely during subduction initiation. As a result, the U-Pb dates from the ophiolite can potentially provide new insight into both ophiolite formation and subduction initiation processes.

U-Pb dates from intrusive rocks in the crust and mantle can be divided into three phases: (1) Formation of the crust through primarily decompression-related V1 magmatism from 96.1–95.6 Ma; (2) V2 magmatism related to H₂O-fluxed mantle melting from 95.6–95.2 Ma; and (3) Intrusion of slab-derived felsic dikes from 95.2–95.0 Ma. Taken together, the dates suggest rapid ophiolite formation, consistent with current estimates for the timescales of subduction initiation based on geodynamic models and observations from the Izu-Bonin-Mariana forearc.

The relative timing of peak metamorphism in the sole of the ophiolite can provide insight into the mechanism of subduction initiation (spontaneous versus induced; Guilmette et al., 2018); however, the timing of sole metamorphism in the Samail ophiolite has been controversial. Combined zircon U-Pb dates, HREE slope, and Ti-in-zircon temperatures record prograde-to-peak zircon and garnet crystallization between 96.7–95.2 Ma, immediately predating and overlapping the crystallization of the overlying ophiolite crust. These dates are consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. A gradient in the initiation of high-grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate along the length of the ophiolite.