METAMORPHISM AND DEFORMATION ALONG THE EMPLACEMENT THRUST OF THE SAMAIL OPHIOLITE, OMAN

The base of the Samail ophiolite is a metamorphic shear zone that prov ides key information about ophiolite emplacement. The classic exposure of this metamorphic sole at Wadi Tayin consists of a few meters of ga rnet-clinopyroxene mafic gneiss and 80 m of upper amphibolite facies m afic and quartzose tectonite, overlying 150 m of lower amphibolite fac ies mafic and sedimentary schist. The sole is bounded by faults but is internally coherent. Peak metamorphic temperatures decrease exponenti ally, from similar to 825 degrees C at the top of the sole to similar to 500 degrees C at its base, as documented by changes in grain size, hornblende composition, the disappearance of chlorite and epidote, gar net-clinopyroxene element partitioning, and garnet-hornblende partitio ning. The inverted peak temperature gradient decreases from > 4000 deg rees C/km in the upper 50 m to similar to 100 degrees C/km in the lowe r 150 m. Contrary to earlier reports,greenschist-facies tectonites are not present at Wadi Tayin, although the entire sole underwent greensc hist-facies retrogression, driven by the influx of fluids from the ove rlying peridotite or underlying carbonate. Calculations based on an am phibolite constitutive relation reveal that, although the section repr esents a lithologic continuum, the lower 200 m of the sole is a relati vely undeformed block whereas the uppermost 25 m represent material in itially separated by tens to a couple of hundreds of kilometers and no w integrated into a single column.