The metamorphic sole along the base of the Semail ophiolite in Oman records the earliest
thrust slice subducted and accreted to the base of the ophiolite mantle sequence. In the Bani Hamid
area (United Arab Emirates) a c. 870 m thick thrust slice of granulite facies rocks includes garnet+
diopside amphibolites, enstatite+cordierite+sillimanite+spinel±sapphirine quartzites, alkaline
mafic granulites (meta-jacupirangites) quartzo-feldspathic gneisses and calc-silicates. The latter contain
garnet+diopside+scapolite+plagioclase±wollastonite. P–T conditions of granulite facies
metamorphism are in the range 800–860°C and 10.5±1.1 kbar to 14.7±2.8 kbar. Garnet+clinopyroxene+hornblende+plagioclase amphibolites from the metamorphic sole record peak P–T conditions
of 840±70°C and 11.6±1.6 kbar (THERMOCALC average P–T mode) and 840–870°C and
13.9–11.8 kbar (conventional thermobarometry) with low degrees of partial melting producing very
small melt segregations of tonalitic material. Pressure estimates are equivalent to depths of 57–46 km
beneath oceanic crust, much deeper than can be accounted for by the thickness of the ophiolite.
40Ar39Ar hornblende ages from the amphibolites range from 95–93 Ma, synchronous with formation
of the plagiogranites in the ophiolite crustal sequence (95 Ma), eruption of the Lasail (V2) volcanic
sequence and deposition of Cenomanian–Turonian radiolaria in metalliferous sediments between the
Geotimes (V1) and Lasail (V2) lavas. Protoliths of the metamorphic sole were Triassic–Jurassic and
early Cretaceous Haybi volcanic rocks, Exotic limestones and quartzites and were clearly not equivalent
to the Semail ophiolite rocks, showing that initiation of subduction could not have occurred at the
ridge axis. Heat for metamorphism was derived from the mantle sequence harzburgites and dunites
which were at or around 1100–1500°C. All data from the sub-ophiolite metamorphic sole in Oman
and the United Arab Emirates indicate that the ophiolite was formed in a Supra-Subduction zone setting
and that obduction occurred along a NE-dipping high-temperature subduction zone during Late
Cretaceous times.
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