Tectonic setting, origin, and obduction of the Oman ophiolite

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 km(2), si milar to 550 km long, similar to 150 km wide); it was emplaced onto the Ara bian continental margin during Late Cretaceous time. The ophiolite originat ed 96-94 Ma at a spreading center above a northeast-dipping subduction zone associated with initiation of immature island-are tholeiitic lavas (Lasail are) at the highest levels of the ophiolite. Simultaneous underthrusting o f 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 similar to 850 degre es C, dated as 95-93 Ma. Subduction cannot have been initiated at a mid-oce anic 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, U nited Arab Emirates, similar to 870 m of granulite facies rocks (enstatite + spinel +/- diopside quartzites, garnet + diopside + wollastonite calc-sil icate marbles, clinopyroxene-bearing amphibolites) were formed at temperatu res similar to those of the garnet + diopside amphibolites of the Oman sole , 800-850 degrees C, but at slightly higher pressures, as much as 9 kbar, T hey are interpreted as deeper level metamorphosed continental margin sedime ntary rocks exhumed by out-of-sequence thrusting placing granulites over ma ntle sequence harzburgites during the later stages of obduction, Subduction of the Arabian continental crust beneath the obducting Semail ophiolite to similar to 78-90 km depth has been proven by thermobarometry of the As Sha h eclogites (to 20-23 kbar) in the eastern sector. In the United Arab Emira tes the subducted continental crust began to partially melt, producing unus ual biotite +/- muscovite +/- garnet +/- tour maline +/- cordierite +/- and alusite-bearing granites that intrude the uppermost mantle sequence harzbur gites and lowermost crustal sequence cumulate gabbros of the ophiolite, We suggest that the entire leading (northeast) edge of the Arabian plate was s ubducted 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-thi ckness ophiolite section, led to blueschist, amphibolite, and granulite fac ies conditions in the metamorphic sole, and crustal melting in the subophio lite basement produced leucocratic granites that intruded up as dikes throu gh the obducted ophiolite. A model for ophiolite obduction is presented, wh ich accounts for all the structural and metamorphic conditions reported fro m the Oman Mountains.