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. 40 Ar/ 39 Ar 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.

Forward Kinematic Modelling of a Regional Transect in the Northern Emirates Using Geological and Apatite Fission Track Age Constraints on Paleo-Burial History

“…Ar ages between~88 and 96 Ma of micas of the metamorphic sole (Miller et al 1999;Searle and Cox 2002).…”

New constrains on the thickness of the Semail ophiolite in the Northern Emirates

“…Numerous~95-Ma-old plagiogranites plug the Semail Ophiolite (Searle and Cox 2002) and were recently mapped by BGS within the Emirati part of the Semail Ophiolite, both near the current erosional top of this unit near Fujjairah, and near the sole thrust of this unit, in the core of the Masafi nappes anticline Styles et al 2006;Goodenough et al 2010). Zircons and other heavy minerals extracted and separated for the purpose of radiometric dating of these plutons, were also helpful for controlling the timing of tectonic uplift and unroofing of the ophiolite.…”

Greenschist-facies sub-ophiolitic metamorphic rocks of Andaman Islands, Burma–Java subduction complex

“…Thin sheets of metamorphic rocks associated with ophiolites have been reported from many locations worldwide (e.g., Coleman, 1981;Woodcock and Roberston, 1977;Hacker and Gnos, 1997;Searle and Cox, 2002;Celik, 2007). The origin of such ophiolite related metamorphic rocks from the Tethyan belt has been reviewed by Woodcock and Roberston (1977).…”

“…The origin of such ophiolite related metamorphic rocks from the Tethyan belt has been reviewed by Woodcock and Roberston (1977). A number of good works have been carried out from well exposed thick metamorphic sole of the Semail ophiolite Malpas, 1980, 1982;Bucher, 1991;Hacker and Mosenfelder, 1996;Hacker and Gnos, 1997;Gnos, 1998;Searle and Cox, 2002), continuous and undisrupted section of the Newfoundland ophiolite (Malpas et al, 1973;Malpas, 1979) and ophiolites of Turkey (e.g., Vergili and Parlak, 2005;Parlak et al, 2006Parlak et al, , 2007Celik, 2007). The metamorphic sole of the Semail ophiolite displays an inverted sequence from amphibolite to greenschist-facies metamorphism.…”

“…The metamorphic sole of the Semail ophiolite displays an inverted sequence from amphibolite to greenschist-facies metamorphism. For such continuous sections, the thermal and dynamic modeling are feasible but are limited for the disrupted occurrence of the metamorphic rocks of Andaman (e.g., Jamieson, 1979;Malpas, 1979;Searle and Cox, 2002). It is widely considered that metamorphism in sub-ophiolitic sole, develops at the contact of colder subduction crust with a hot hanging wall mantle (future ophiolite) and the metamorphic rocks are attached to the hanging wall (e.g., Hacker and Gnos, 1997).…”