Abstract

The Semail ophiolite in the Oman Mountains is the world's largest and best preserved thrust sheet of oceanic crust and upper mantle (>10 000 km2, ∼550 km long, ∼150 km wide); it was emplaced onto the Arabian continental margin during Late Cretaceous time. The ophiolite originated 96–94 Ma at a spreading center above a northeast-dipping subduction zone associated with initiation of immature island-arc tholeiitic lavas (Lasail arc) at the highest levels of the ophiolite. Simultaneous underthrusting of Triassic (and Jurassic[?]) mid-oceanic-ridge basalt and alkalic volcanic rocks beneath >12 km of upper mantle depleted harzburgites produced garnet + clinopyroxene amphibolites formed at temperatures of ∼850 °C, dated as 95–93 Ma. Subduction cannot have been initiated at a mid-oceanic ridge, otherwise the protolith of the amphibolites in the metamorphic sole would be the same age and composition as the ophiolite volcanic rocks above. In the northern part of the Oman Mountains in the Bani Hamid area, United Arab Emirates, ∼870 m of granulite facies rocks (enstatite + spinel ± diopside quartzites, garnet + diopside + wollastonite calc-silicate marbles, clinopyroxene-bearing amphibolites) were formed at temperatures similar to those of the garnet + diopside amphibolites of the Oman sole, 800–850 °C, but at slightly higher pressures, as much as 9 kbar. They are interpreted as deeper level metamorphosed continental margin sedimentary rocks exhumed by out-of-sequence thrusting placing granulites over mantle sequence harzburgites during the later stages of obduction.

Subduction of the Arabian continental crust beneath the obducting Semail ophiolite to ∼78–90 km depth has been proven by thermobarometry of the As Sifah eclogites (to 20–23 kbar) in the eastern sector. In the United Arab Emirates the subducted continental crust began to partially melt, producing unusual biotite ± muscovite ± garnet ± tourmaline ± cordierite ± andalusite–bearing granites that intrude the uppermost mantle sequence harzburgites and lowermost crustal sequence cumulate gabbros of the ophiolite. We suggest that the entire leading (northeast) edge of the Arabian plate was subducted beneath the ophiolite during the final stages of obduction leading to eclogitization of the crustal rocks. Higher temperatures and pressures in the United Arab Emirates sector, possibly due to a thicker or double-thickness ophiolite section, led to blueschist, amphibolite, and granulite facies conditions in the metamorphic sole, and crustal melting in the subophiolite basement produced leucocratic granites that intruded up as dikes through the obducted ophiolite. A model for ophiolite obduction is presented, which accounts for all the structural and metamorphic conditions reported from the Oman Mountains.

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