Subduction zone metamorphism during formation and emplacement of the Semail ophiolite in the Oman Mountains

Abstract

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.