Structure of the metamorphic sole to the Oman Ophiolite, Sumeini Window and Wadi Tayyin: implications for ophiolite obduction processes

Abstract

AbstractTwo localities where the inverted metamorphic sole of the Oman Ophiolite are best exposed, Sumeini Window and Wadi Tayyin, have been mapped and sampled in greater detail. In both areas an inverted pressure and temperature profile is exposed beneath the Semail Thrust, the base of the ophiolite, from garnet+clinopyroxene-bearing granulite to hornblende+plagioclase amphibolite down through epidote amphibolite and a variety of greenschist facies metasediments, dominantly cherts, marbles and quartzites. Thermobarometry on garnet- and clinopyroxene-bearing amphibolites immediately beneath the contact with mantle sequence harzburgites shows that the upper sole rocks formed at pressure–temperature (P–T) conditions of 770–900 °C and 11–13 kbar, equivalent to depths of 30–40 km in oceanic lithosphere. Heat for metamorphism can only have been derived from the overlying mantle sequence peridotites. Pressures are higher than can be accounted for by the thickness of the ophiolite (15–20 km). Timing of peak metamorphism was synchronous with the formation of the ophiolite gabbroic–trondhjemite crustal sequence and eruption of the pillow lavas (Cenomanian; 96–95 Ma). Metamorphic sole rocks have been structurally repeated by imbricate thrusting, casting doubt on previous estimates of thermal gradients. All the data support a subduction zone setting for metamorphism and a supra-subduction zone environment for ophiolite formation during the Cenomanian.