Petrochronology of Wadi Tayin Metamorphic Sole Metasediment, With Implications for the Thermal and Tectonic Evolution of the Samail Ophiolite (Oman/UAE)

摘要

Ophiolite metamorphic soles preserve important records of ophiolite emplacement, but there have been few detailed investigations into their non-mafic portions. We present new thermobarometric and petrochronologic data from a metasediment and mafic restite in the upper Wadi Tayin sole exposure in the Samail (Oman-UAE) Ophiolite. Thermodynamic modeling suggests metasedimentary garnet nucleation at similar to 4 kb, similar to 550 degrees C and final growth at 7.5 +/- 1.2 kbar, 665 +/- 32 degrees C, occurring by 93.0 +/- 0.5 Ma (Lu-Hf isochron). Zircon U-Pb dates of 106.9 +/- 2.3 (detrital) and 98.7 +/- 1.7 to 94.1 +/- 1.6 Ma (metamorphic) bracket the initiation of metamorphism, and monazite U-Pb dates from similar to 97-89 Ma suggest a lengthy period of growth or recrystallization. A mafic titanite U-Pb age of 92.2 +/- 1.8 Ma records the earliest possible juxtaposition of high- and lower-grade sole rocks. These and other data suggest that (i) the Wadi Tayin sole preserves an inverted metamorphic, metasomatic, and age gradient,(ii) metasediment metamorphism occurred during, or soon after, crystallization of the overlying ophiolite (= 96.5 Ma); and (iii) sole metasediments define a thermal gradient continuous with hotter, higher-P amphibolites. Some of these data conflict with existing models for sole formation, and we propose several hypotheses to explain them. Cooling of the sole below Ar closure by similar to 92 Ma suggests that strain rapidly partitioned away from the sole, leading to large-scale, thin-skinned thrust emplacement of the ophiolite 100 km across the continental margin and the late, cool underthrusting of the continental margin.