Metamorphism and deformation along the emplacement thrust of the Samail ophiolite, Oman

The base of the Samail ophiolite is a metamorphic shear zone that provides key information about ophiolite emplacement. The classic exposure of this metamorphic sole at Wadi Tayin consists of a few meters of garnet-clinopyroxene mafic gneiss and 80 m of upper amphibolite facies mafic and quartzose tectonite, overlying 150 m of lower amphibolite facies mafic and sedimentary schist. The sole is bounded by faults but is internally coherent. Peak metamorphic temperatures decrease exponentially, from ∼ 825°C at the top of the sole to ∼ 500°C at its base, as documented by changes in grain size, hornblende composition, the disappearance of chlorite and epidote, garnet-clinopyroxene element partitioning, and garnet-hornblende partitioning. The inverted peak temperature gradient decreases from > 4000°C/km in the upper 50 m to ∼ 100°C/km in the lower 150 m. Contrary to earlier reports, greenschist-facies tectonites are not present at Wadi Tayin, although the entire sole underwent greenschist-facies retrogression, driven by the influx of fluids from the overlying peridotite or underlying carbonate. Calculations based on an amphibolite constitutive relation reveal that, although the section represents a lithologic continuum, the lower 200 m of the sole is a relatively undeformed block whereas the uppermost 25 m represent material initially separated by tens to a couple of hundreds of kilometers and now integrated into a single column.