Synchronous formation of the metamorphic sole and igneous crust of the Semail ophiolite: New constraints on the tectonic evolution during ophiolite formation from high-precision U–Pb zircon geochronology

Abstract

The Semail (Oman-United Arab Emirates) and other Tethyan-type ophiolites are underlain by a sole consisting of greenschist-to granulite-facies metamorphic rocks. As preserved remnants of the underthrust plate, sole exposures can be used to better understand the formation and obduction of ophiolites. Early models envisioned that the metamorphic sole of the Semail ophiolite formed as a result of thrusting of the hot ophiolite lithosphere over adjacent oceanic crust during initial emplacement; however, calculated pressures from granulite-facies mineral assemblages in the sole suggest the metamorphic rocks formed at >35 km depth, and are too high to be explained by the currently preserved thickness of ophiolite crust and mantle (up to 15-20 km). We have used high-precision U-Pb zircon dating to study the formation and evolution of the metamorphic sole at two well-studied localities. Our previous research and new results show that the ophiolite crust formed from 96.12-95.50 Ma. Our new dates from the Sumeini and Wadi Tayin sole localities indicate peak metamorphism at 96.16 and 94.82 Ma (± 0.022 to 0.035 Ma), respectively. The dates from the Sumeini sole locality show for the first time that the metamorphic rocks formed either prior to or during formation of the ophiolite crust, and were later juxtaposed with the base of the ophiolite. These data, combined with existing geochemical constraints, are best explained by formation of the ophiolite in a supra-subduction zone setting, with metamorphism of the sole rocks occurring in a subducted slab. The 1.3 Ma difference between the Wadi Tayin and Sumeini dates indicate that, in contrast to current models, the highest-grade rocks at different sole localities underwent metamorphism, and may have returned up the subduction channel, at different times.

Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite, Oman and UAE)

“…Partial melting would have occurred during the formations of the metamorphic sole ( c . 95–94 Ma; Rioux et al., , ; Searle, Cherry, Ali, & Cooper, ) and at similar P–T conditions (~1 GPa; Moyen & Stevens, ; Rollinson, ), with melt fractions of ~20 vol.% estimated by Rollinson () to generate the granitoid intrusions.…”

“…Evidence of partial melting, however, is scarcely observed in the Oman/UAE metamorphic soles and restricted to rare tonalitic to trondhjemitic pods interpreted as evidencing melting of the sole at peak metamorphic conditions (Cowan et al., ; Gnos, ; Hacker & Mosenfelder, ; Searle & Malpas, ), as supported by U–Pb ages ( c . 95–94 Ma; Rioux et al., ; Searle et al., ) and P–T conditions (770–900°C and 1.1–1.4 GPa; Cowan et al., ) similar to that of the HT sole. Accordingly, thermodynamic modelling of the mafic HT sole shows that a melt fraction <6 vol.% is expected at these temperature–depth conditions; Figure ).…”

“…Uranium‐lead zircon and rutile dating of mafic and felsic samples of the ophiolite crust display a cluster of ages at c . 96–95 Ma (Figure c; Goodenough et al., ; Rioux et al., , , ; Roberts, Thomas, & Jacobs, ; Tilton et al., ) indicating that most of the ophiolite was formed within c . 2 Ma.…”

“…Despite being the most‐studied ophiolite, its exact petrological nature and origin remain debated (see Rioux et al., , ; for extended discussion). While the structure of the ophiolite, mostly composed of variously tectonized harzburgite with a typical near‐axis mantle flow fabric, advocates for formation beneath an ocean ridge (Ceuleneer, Nicolas, & Boudier, ; Nicolas et al., ), the geochemistry of the crust displays two distinct patterns highlighting a more complex geodynamic environment of formation.…”

The Tell-Rif orogenic system (Morocco, Algeria, Tunisia) and the structural heritage of the southern Tethys margin

“…Such a configuration is unusual because, in general, the subducted domain is the oceanic one. However, a comparison comes to mind with the Oman belt where tectonic windows exhibiting HP-LT metamorphism affecting continental crust support the non-metamorphic Semail ophiolite (see Duretz et al, 2016;Goffé et al, 1988;Rioux et al, 2016).…”