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Tectonic Setting, Origin, and Obduction of the Oman Ophiolite

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Tectonic Setting, Origin, and Obduction of the Oman Ophiolite Tectonic setting, origin, and obduction of the Oman ophiolite Mike Searle* Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, United Kingdom Jon Cox } ABSTRACT maline ± cordierite ± andalusite–bearing Muscat–As Sifah area in the southeastern moun- granites that intrude the uppermost mantle tains. Regional high pressure assemblages along The Semail ophiolite in the Oman Moun- sequence harzburgites and lowermost crustal the north side of the Saih Hatat dome include tains is the world’s largest and best preserved sequence cumulate gabbros of the ophiolite. carpholite-bearing metasedimentary rocks (Goffé thrust sheet of oceanic crust and upper mantle We suggest that the entire leading (northeast) et al., 1988), lawsonite-bearing metabasalts (>10 000 km2, ~550 km long, ~150 km wide); it edge of the Arabian plate was subducted be- (El-Shazly, 1994), glaucophane + garnet + chlori- was emplaced onto the Arabian continental neath the ophiolite during the final stages of toid blueschists, and eclogite facies metapelites margin during Late Cretaceous time. The ophi- obduction leading to eclogitization of the and metabasites (El-Shazly et al., 1990; Searle olite originated 96–94 Ma at a spreading center crustal rocks. Higher temperatures and pres- et al., 1994). We then discuss emplacement mod- above a northeast-dipping subduction zone as- sures in the United Arab Emirates sector, pos- els for the Oman ophiolite that satisfy the struc- sociated with initiation of immature island-arc sibly due to a thicker or double-thickness tural and metamorphic constraints of the ophiolite tholeiitic lavas (Lasail arc) at the highest levels ophiolite section, led to blueschist, amphibo- itself and all the metamorphic rocks beneath the of the ophiolite. Simultaneous underthrusting lite, and granulite facies conditions in the ophiolite. of Triassic (and Jurassic[?]) mid-oceanic-ridge metamorphic sole, and crustal melting in the basalt and alkalic volcanic rocks beneath >12 subophiolite basement produced leucocratic SEMAIL OPHIOLITE km of upper mantle depleted harzburgites pro- granites that intruded up as dikes through the duced garnet + clinopyroxene amphibolites obducted ophiolite. A model for ophiolite ob- The Semail ophiolite comprises a large intact formed at temperatures of ~850 °C, dated as duction is presented, which accounts for all thrust slice composed of ~8–12 km of upper 95–93 Ma. Subduction cannot have been initi- the structural and metamorphic conditions mantle peridotites and 4–7 km of oceanic crustal ated at a mid-oceanic ridge, otherwise the pro- reported from the Oman Mountains. rocks (Lippard et al., 1986; Nicolas, 1989). Fig- tolith of the amphibolites in the metamorphic ure 2 shows the Late Cretaceous ophiolite stratig- sole would be the same age and composition as INTRODUCTION raphy in the context of the model proposed in this the ophiolite volcanic rocks above. In the north- paper, that of the ophiolite originating as a supra- ern part of the Oman Mountains in the Bani Two fundamental questions concern ophiolite subduction zone piece of oceanic crust and upper Hamid area, United Arab Emirates, ~870 m of complexes world-wide. (1) In what tectonic setting mantle. It is possible that the earlier oceanic ridge granulite facies rocks (enstatite + spinel ± diop- were the ophiolites originally formed (e.g., mid- tectonic setting was superseded by a later sub- side quartzites, garnet + diopside + wollastonite oceanic ridge, transform fault, or a suprasubduc- duction-zone tectonic setting. Figure 3 shows the calc-silicate marbles, clinopyroxene-bearing tion zone marginal basin)? (2) How were these principal alternative model, whereby the Semail amphibolites) were formed at temperatures dense (3.0–3.3 g/cm3) oceanic mantle and crustal ophiolite was produced at a mid-oceanic ridge similar to those of the garnet + diopside amphi- rocks emplaced onto more buoyant, less-dense and obduction was initiated at the ridge itself. Al- bolites of the Oman sole, 800–850 °C, but at (2.7–2.8 g/cm3) continental crust? The Semail though the ophiolite was emplaced as one inter- slightly higher pressures, as much as 9 kbar. ophiolite complex of the Oman Mountains in east- nally consistent thrust sheet, it was subsequently They are interpreted as deeper level metamor- ern Arabia (Fig. 1) is not only probably the largest disrupted by various normal faults and out-of- phosed continental margin sedimentary rocks and best exposed such ophiolite anywhere, but it is sequence thrusts. Most of the mantle sequence is exhumed by out-of-sequence thrusting placing also the most extensively studied (e.g., Glennie composed of tectonized harzburgite, which rep- granulites over mantle sequence harzburgites et al., 1973, 1974; Coleman, 1981; Lippard et al., resents the residual mantle from partial melting during the later stages of obduction. 1986; Nicolas et al., 1988; Robertson et al., 1990). of fertile primary spinel lherzolite at pressures in Subduction of the Arabian continental Here we review the geology of the Oman ophiolite excess of 20 kbar. Early high-temperature or- crust beneath the obducting Semail ophiolite and in particular the classic high-temperature am- thopyroxene fabrics in the harzburgites probably to ~78–90 km depth has been proven by ther- phibolite and greenschist facies metamorphic sole represent mantle flow patterns that appear to mobarometry of the As Sifah eclogites (to along the base of the ophiolite. We then discuss the show the presence of mantle diapirs below the 20–23 kbar) in the eastern sector. In the structural evolution of two high-pressure regions ridge (Ceuleneer et al., 1988; Nicolas et al., United Arab Emirates the subducted conti- in the Oman Mountains. We first describe some 1988). Mantle diapirs represent the feeders for nental crust began to partially melt, produc- unique subophiolite high-temperature, medium- to the ridge segment and magma chambers are pre- ing unusual biotite ± muscovite ± garnet ± tour- high-pressure granulite facies rocks in the Bani sumably centered above the diapirs (Boudier and Hamid thrust sheet in the United Arab Emirates. Nicolas, 1995). High-temperature lineations *E-mail: [email protected]. The second high-pressure region is in the were formed by plastic flow of upwelling mantle GSA Bulletin; January 1999; v. 111; no. 1; p. 104–122; 13 figures. 104 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/1/104/3382981/i0016-7606-111-1-104.pdf by guest on 01 October 2021 OMAN OPHIOLITE Figure 1. Simplified geological map of the Oman Mountains, af- ter Glennie et al. (1974) and Open University Oman ophiolite Map Sheets 1–4. that diverges away from the ridge axis below the late sequence overlain by isotropic gabbros, dio- crustal sequence suggests open-system fraction- Moho. These earlier mantle fabrics have been rites, and trondhjemites (high level intrusives). ation of a continually replenished magma cham- overprinted along the base by a strongly banded The cumulate sequence layered peridotites and ber (Browning and Smewing, 1981). Numerous harzburgite-lherzolite-dunite unit (banded ultra- gabbros represent the floor of the magma cham- magma chamber models have been proposed mafic unit; Searle and Malpas, 1980, 1982) ber and are composed of a variety of rocks con- based on field mapping of the cumulate se- showing emplacement-related fabrics. Dunite taining the primary minerals olivine, orthopyrox- quence and high level intrusives (e.g., Pallister pods and lenses with small chromite bodies occur ene, chrome spinel, clinopyroxene, hornblende, and Hopson, 1981; Browning and Smewing, toward the upper levels of the mantle sequence. and plagioclase. Rhythmically layered dunites 1981; Nicolas et al., 1988; Boudier and Nicolas, The petrological Moho is the base of the crustal (olivine + chrome spinel), wehrlites (olivine + 1995, Nicolas, 1989). The Oman ophiolite is sequence and is marked by the appearance of pla- clinopyroxene + chrome spinel), olivine gabbros generally thought to represent a fast-spreading gioclase. Melts generated in the upper mantle (olivine + clinopyroxene + plagioclase cumu- ridge with an open, continually replenished were formed in linear magma chambers in which lates), and two-pyroxene gabbros or gabbronorite magma chamber. The along-strike continuity of convecting magma was being fed in batches from (olivine + clinopyroxene + orthopyroxene + pla- the cumulate sequence peridotites and gabbros below and generated melts feeding upward gioclase) show gravity-settling cumulate tex- suggests a long, linear magma chamber. Trond- through the sheeted dikes above. tures. Late intrusive wehrlites have discordant hjemites or plagiogranites, composed of quartz Tholeiitic picritic melts generated the crustal contacts and intrude higher levels of the crustal and plagioclase with minor ferromagnesian min- sequence which comprises as much as 4 km of sequence (Juteau et al., 1988). The cyclic nature erals, represent the final end product of magmatic layered mafic and ultramafic rocks of the cumu- of the cumulate layered series at the base of the differentiation (Pedersen and Malpas, 1984). Geological Society of America Bulletin, January 1999 105 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/1/104/3382981/i0016-7606-111-1-104.pdf by guest on 01 October 2021 SEARLE AND COX Figure 2. Palinspastic reconstruction of the Semail ophiolite immediately prior to emplacement, according to the subduction-zone model pre-
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