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.

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Figure 1. Simplified geological map of the Oman Mountains, after 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).

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Figure 2. Palinspastic reconstruction of the Semail ophiolite immediately prior to emplacement, according to the subduction-zone model presented here, after the models of Searle and Malpas (1980, 1982), Pearce et al. (1981), and Lippard et al. (1986). The lower mid-oceanic-ridge basalts (MORB) of the ophiolite complex (Geotimes unit) are overlain by the immature island-arc tholeiites (IAT) of the Lasail unit. It is possible that the boundary between these two volcanic units represents the initiation of subduction beneath the ophiolite. In this case earlier mantle flow lines would relate to the upwelling of residual mantle harzburgite beneath the magma chamber, and these fabrics were superimposed by subduction-obduction–related fabrics marking the early high-temperature shearing during the emplacement process.

Magma generated in the magma chamber at as much as 500 m of a basalt-andesite-dacite- alliferous sediments or umbers, within and on the base of the crust was fed upward via the rhyolite fractionation sequence, which is strongly top of the pillow lavas, overlain by pelagic sedi- sheeted dike complex to the extrusive pillow depleted relative to N-MORB and has been inter- ments, including radiolarian mudstones. lavas. At the top of the magma chamber dikes can preted as immature island-arc tholeiites, erupted be seen coalescing between gabbroic screens, at discrete volcanic centers above a subduction Age of the Semail Ophiolite and at higher structural levels dikes indicate zone (Pearce et al., 1981). Highly depleted 100% crustal extension. The dikes trend parallel clinopyroxene-phyric basalts are overlain by U-Pb zircon ages from plagiogranites in the to the ridge axis and are roughly arcuate along the about 100 m of mildly alkaline, transitional Semail ophiolite range from 97.3 to 93.5 ± 0.25 strike of the Oman Mountains with little segmen- within-plate basalts (Salahi unit) erupted off axis, Ma with a mean age of 94.8 ± 0.1 Ma (Tilton tation by crosscutting transform-fault structures probably along small fissures. et al., 1981). These ages are crystallization ages (MacLeod and Rothery, 1992). The upper part of Whereas the upper volcanic units are well ex- of the late-stage differentiates in the plutonic the Semail ophiolite consists of as much as 2 km posed along the northern Oman Mountains, de- sequence and are interpreted as recording the of pillow lavas which, in northern Oman, have tailed studies of the crustal sequence in the time of formation of the ophiolite. Radiolarian been subdivided into four major units (Pearce southeastern Oman Mountains show only the ages from pelagic sediments interbedded et al., 1981; Alabaster et al., 1982). The lower lower MORB basalts, and hence Coleman within the upper volcanic units are Cenoman- Geotimes unit comprises clinopyroxene and pla- (1981), Hopson et al. (1981), and Boudier et al. ian–early Turonian (Tippit et al., 1981) and cor- gioclase-phyric basalts that are slightly depleted (1985, 1988) interpreted the ophiolite as having relate closely with the igneous zircon ages. The relative to normal mid-oceanic-ridge basalt been generated at a mid-oceanic ridge. The high- Cenomanian marks the timing of final stable (N-MORB). The overlying Lasail unit comprises est levels of the ophiolite show Fe-Mn–rich met- carbonate sedimentation on the Arabian plat-

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Figure 3. The mid-oceanic-ridge model for the formation and emplacement of the Semail ophiolite, after Hopson et al. (1981), Pallister and Hopson (1981), Boudier et al. (1985), Nicolas et al. (1988), Boudier and Nicolas (1995), and Hacker (1991, 1994). Note that if the site of initiation of the Semail thrust was at the actual ridge, the footwall (protoamphibolites of the metamorphic sole) would have to be the same age and composition as the ophiolite volcanic rocks above.

form and along the shelf margin (Sumeini Sumeini Window, North Oman Mountains quence peridotites during initial oceanic detach- Group rocks) before the shelf margin collapsed ment along a thrust-fault zone that extended at to form the Aruma foreland basin, into which Some 80 m of fine- to medium-grained horn- least 12–14 km below the oceanic Moho (Fig. 2). the allochthonous thrust sheets were emplaced blende + plagioclase amphibolites are immedi- Protolith rocks of the Haybi complex, ocean- (Glennie et al., 1973, 1974). ately below the basal harzburgites of the ophiolite floor basalts and sedimentary rocks, were sub- sequence in the Sumeini window (Fig. 4). The up- ducted to this depth along a northeast-dipping AMPHIBOLITE-GREENSCHIST FACIES per 20–40 m also contain almandine garnet and subduction zone during or immediately after METAMORPHIC SOLE IN OMAN clinopyroxene. Thin bands of calc-silicate marbles formation of the Semail ophiolite during Ceno- with hornblende, grossularite garnet and diopside, manian-Turonian time and welded to the base of Detailed structural mapping along the base of and muscovite + piemontite ± diopside quartzites the ophiolite. During subsequent exhumation and the Semail ophiolite has distinguished a distinct are intercalated within the amphibolites. At the thrusting, shallower-level, less-metamorphosed thrust sheet termed the Haybi complex, separating highest structural levels within the metamorphic greenschist facies rocks were formed at lower the imbricated Hawasina sedimentary rocks be- sole partial melting produced small monzodioritic P-T conditions along the base of the amphibo- low from the Semail ophiolite peridotites above melt pods composed of K-feldspar and pargasitic lites. Later brittle thrusting transported the entire (Searle and Malpas, 1980, 1982). The rocks hornblende (Searle and Malpas, 1980, 1982). ophiolite sequence, plus its metamorphic sole within the Haybi complex include amphibolite Late-stage calcium metasomatism produced welded onto the base, over all underlying units. and greenschist facies metamorphic rocks of the rodingites composed of hydrogrossular, diopside, Immobile trace element geochemistry of the sole, relatively unmetamorphosed alkalic and micas, and calcite. The metamorphic fabric is par- subophiolite amphibolites shows that the protolith tholeiitic volcanic rocks of mainly Triassic age allel to the banding in the lowermost harzburgites was not similar to the Semail ophiolite volcanic (Haybi volcanic group; Searle et al., 1980), Late and lherzolites of the mantle sequence, indicating rocks nor to the highly alkalic Triassic alkalic Permian and Late Triassic Oman exotic limestone that the latter fabric within the banded ultramafic basalts and ankaramites of the Haybi volcanic blocks (Searle and Graham, 1982) and various unit is also related to emplacement. rocks (Searle et al., 1980; Searle and Malpas sedimentary and tectonic melanges. The first de- Structurally beneath the amphibolites and sep- 1982). The amphibolites have geochemical tracer tailed studies of the metamorphic sole rocks in arated by a strongly sheared contact are epidote patterns similar to MORB or slightly enriched Oman were carried out by Searle (1980) and amphibolites and a variety of greenschist facies MORB, as do some tholeiitic or transitional vol- Searle and Malpas (1980, 1982) in northern rocks. Muscovite-, piemontite-, and stilpnome- canic rocks within the underlying Haybi complex. Oman (Sumeini, Asjudi, Haybi, and Hawasina lane-bearing quartzites are the most common The banded marbles appear to be metamorphosed window areas) and by Ghent and Stout (1981), rocks, followed by banded marbles, rare pelites, exotic-type carbonates, and the piemontite-bearing Lanphere (1981), and Hacker and Mosenfelder and metavolcanic rocks. All the amphibolite and quartzites were probably manganiferous cherts de- (1996) in the Wadi Tayyin area, southeastern greenschist facies rocks have been thrust over un- posited in the distal part of the ocean basin. Tem- Oman Mountains. The metamorphic sole in the metamorphosed sedimentary rocks, Haybi vol- peratures of metamorphism reached 670–740 °C, United Arab Emirates was studied by Alleman canic rocks, and melanges along a brittle thrust although temperatures could have reached ~800 and Peters (1972), Searle (1980), Bucher (1991), fault. Metamorphic isograds have been strongly °C to produce the tonalitic partial melts within the Stoessel and Ziegler (1985), Gnos and Kurz condensed during emplacement-related shearing, upper amphibolites (Searle and Malpas, 1980). (1994), and Gnos and Nicolas (1996). In Oman and pressure-temperature (P-T) conditions show The dynamothermal metamorphism was synchro- the most complete and intact section through the that the metamorphic gradient is inverted. Heat nous with intense shearing during which shear metamorphic sole is in the Sumeini window. must have been derived from the mantle se- stresses of ~100 MPa were attained.

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Figure 4. Photo of the northwest Su- meini window, northern Oman, showing the classic subophiolite metamorphic sole with garnet + clinopyroxene amphibolites underlying banded harzburgites, lherzolites, and dunites of the ophiolite along the Semail thrust.

Figure 5 shows a P-T grid with the approxi- melanges are probably distal equivalents of the The presence of hornblende and plagioclase in mate calculated P-T paths for all the metamor- Aruma group shales in the foreland basin (Glennie the amphibolites requires temperatures above phic zones in the northern Oman Mountains de- et al., 1973; Robertson, 1987) and the Ruwi 475 °C and the presence of clinopyroxene in the scribed herein. The subophiolite amphibolites melange in the Muscat area. upper levels indicates that temperatures could were dominantly basalts, which together with have reached 780 °C. Fe and Mg partitioning be- distal oceanic sedimentary rocks, were rapidly Wadi Tayyin, Southeast Oman Mountains tween garnet and clinopyroxene were used by subducted beneath a young, still-hot ophiolite. Searle and Malpas (1980), Ghent and Stout The heat source came from the hot peridotites The classic section through the metamorphic (1981), and Hacker and Mosenfelder (1996) to thrust above cold material, so the resulting P-T sole in southeastern Oman occurs in the Green obtain peak temperatures from the sole, but this loop was counterclockwise. Pool area of Wadi Tayyin (Ghent and Stout, also depends on pressure and the Ca content of 1981; Hacker and Mosenfelder, 1996), where garnet. Peak metamorphic temperatures at Wadi Haybi-Hawasina window about 250 m of amphibolite facies amphibolites Tayyin are thought to be 775–875 °C (Ghent and (Central Oman Mountains) and quartzites are beneath the peridotite. Both Stout, 1981; Hacker and Mosenfelder, 1996). upper and lower contacts are thrusts, and minor Hacker (1994) and Hacker et al. (1996) demon- In the central Oman Mountains, the base of the imbricate faults also occur within the sequence. strated from 40Ar/39Ar dating of hornblendes in Semail ophiolite also shows the metamorphic Here the uppermost few meters are also com- the metamorphic sole that metamorphism oc- sole amphibolites and greenschists along the Se- posed of garnet + clinopyroxene + hornblende curred within 2 m.y., during or immediately fol- mail thrust, but in many areas the original struc- gneiss with rare pods of partial melt composed of lowing ophiolite formation, which can be taken tural sequence has been broken up by strong plagioclase + quartz + apatite ± epidote ± horn- as 94.8 Ma, the average U-Pb age from zircons in shearing and imbrication associated with em- blende ± clinopyroxene. Structurally below are plagiogranite. The complete spread of 40Ar/39Ar placement. Melanges containing blocks of am- hornblende + plagioclase amphibolites and hornblende ages is 95.7–92.6 ± 0.6 Ma. The phibolites and greenschists encased within a ser- quartzites (metacherts) in a sequence similar to model whereby detachment was initiated at a pentinite matrix mark the base of the ophiolite. that at Sumeini. Ghent and Stout (1981) reported spreading center (Fig. 3), following the models of The serpentinized harzburgites and lherzolites greenschist facies assemblages below the amphi- Coleman (1981) and Boudier et al. (1985), was have acted as a ductile detachment horizon on bolites, but Hacker and Mosenfelder (1996) pre- derived from work in the southeastern Oman which the ophiolite has moved, breaking up the sented data that indicate that these rocks are Mountains, where the Semail arc-related lavas amphibolite sole beneath. Intact sequences do oc- lower amphibolite facies and that they have been are not exposed. Hacker (1991) and Hacker et al. cur notably along the northeast margin of the thrust over unmetamorphosed sedimentary rocks (1996) also supported the model of detachment Hawasina window (Searle and Cooper, 1986), without intermediate greenschists. Amphibolite initiating at the ridge, but also suggested that a where a section similar to that described from the facies mineral paragenesis have been overprinted transform-fault setting might be an appropriate Sumeini Window is present. The metamorphic by greenschist and prehnite-pumpellyite facies site of detachment. However, the narrow overlap sole or the serpentinite melange tectonically minerals with increasing chlorite content down- in time between ophiolite formation and amphi- overlies Haybi volcanic rocks with Triassic ex- section. bolite facies metamorphism during thrusting in otic reefal limestone blocks, and sedimentary The lack of pelitic assemblages in the meta- the sole surely suggests that subduction processes melanges of possible Late Cretaceous age. These morphic sole makes thermobarometry difficult. were operating at depth below the mantle se-

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Figure 5. Pressure-temperature (P-T) diagram showing the proposed P-T paths for all the major metamorphic units described from the Oman Mountains. The P-T data used for the As Sifah eclogites and Hulw blueschists are from Searle et al. (1994). Squares with parallelogram uncertainty boxes are from the garnet- clinopyroxene-phengite thermo- barometer. Reference curves are equilibria calculated with the data set of Holland and Powell (1990). P-T data for the carpholite zone and Ruwi melange are from Goffé et al. (1988) and El-Shazly (1994), for the Sumeini amphibolites, from Searle (1980) and Searle and Malpas (1980), for the Wadi Tayyin amphibolites from Hacker (1991), and for the Asimah and Bani Hamid granulites, from Gnos (1992, in press).

quence at the same time the crustal sequence was lonitic quartzites and quartz-feldspar-cordierite by the presence of amphibole and two pyroxenes. being formed. We propose that the subduction gneisses, gray to blue-green, which often weather However, modal variations are large, perhaps re- model of Figure 2 can be applied to all segments to a rusty red-orange. Granulite facies is indicated flecting protolith heterogeneity, and subsequent of the Oman ophiolite. by the presence of two pyroxene-amphibole as- alteration to epidote amphibolite facies assem- semblages. These quartzose lithologies are hetero- blages is common. Enstatite and a more common GRANULITE-AMPHIBOLITE FACIES geneous, both laterally and vertically, with variable colorless to pale green diopsidic clinopyroxene METAMORPHIC ROCKS, degrees of schistosity and grain size; composi- are patchily developed in the amphibolites. There UNITED ARAB EMIRATES tional layering is manifested by thin monominer- is a distinct variation in mineral chemistry and alic spinel and pyroxene layers. The quartzose modal proportions between the core and marginal Bani Hamid Granulites lithologies are most common close to the thrust amphibolite units. The former have limited modal surfaces that bound the Bani Hamid area, and there clinopyroxene, andesine, and hastingitic amphi- A structurally and petrographically distinct are systematic variations in grain size, degree of bole; the latter have more abundant clinopyrox- metamorphic sole is exposed in the Bani Hamid metamorphic segregation, and strength of the my- ene, labradoritic plagioclase, and pargasitic am- area of the United Arab Emirates around the vil- lonitic fabric of the quartzites, depending on dis- phibole. Apatite, opaque ore minerals, and lages of Shis and Hajar in the east and Bulaydah tance from the bounding thrusts. In addition, fluid multigenerational sphene are common accessory in the southwest (Fig. 6). Here the metamorphic metasomatism, presumably derived from the adja- phases. Throughout the Bani Hamid area, stro- rocks have been thrust over a lower harzburgite cent mantle sequence harzburgites, has altered the matic (foliation parallel) leucocratic partial melt unit of the ophiolite along an out-of-sequence primary metamorphic mineralogy, and the mar- pods and lenses to 20 cm are found in the amphi- thrust. The metamorphic rocks are much thicker ginal quartzites contain uralitic amphiboles, epi- bolites; a typical tonalitic assemblage includes than elsewhere in Oman and are surrounded by dote group minerals, and phyllosilicates. The coarse-grained andesine, quartz, hornblende, and mantle sequence harzburgites, but the entire sec- quartzites in the core of the thrust sheet tend to be biotite. The presence of such evidence of melting tion has been tectonically repeated (Searle, 1980; intercalated with amphibolites and calc-silicates. can provide an independent figure for the peak Gnos, 1992). The metamorphic grade is also They have a less well-defined fabric, weaker com- metamorphic temperature obtained. Typical am- higher than elsewhere in Oman with about 900 m positional layering, and more modal feldspar and phibolite assemblages include diopsidic clinopy- of upper amphibolite and granulite facies rocks. cordierite, the latter with inclusions of matted roxene + enstatite + pargasitic hornblende + Lower grade rocks are absent. Previous work in fibrolitic sillimanite. Also, distinctive pinkish labradorite + magnetite + sphene, diopsidic the area (Stossel and Zeigler, 1985; Bucher and piemontite quartzites are more common, and this clinopyroxene + hastingitic hornblende + ande- Kurz, 1991; Gnos, 1992; Gnos and Kurz, 1994) Mn-rich lithology has been taken as evidence for a sine + apatite + magnetite, and hornblende + an- has concentrated on the petrography and mineral marine source for the protoliths. desine + epidote + biotite + spinel. chemistry of the quartzo-feldspathic gneisses that The amphibolite lithologies are dark brown to Perhaps the most spectacular lithologies found bound the metamorphic thrust sheet. black, medium-grained, well-foliated rocks with a in the Bani Hamid area, both in terms of petrology The Bani Hamid rocks are dominated by my- typical granulite-grade assemblage characterized and field relations, are the calc-silicate and marble

Geological Society of America Bulletin, January 1999 109

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units. In the western part of the area, around Bu- high-pressure metamorphic rocks were recog- range of country rocks intruded encompasses tec- laydah, the calc-silicates have limited outcrops nized in the Dibba zone by the presence of glauco- tonized and serpentinized harzburgites, dunites, and tend to occur as pods, lenses, or thin continu- phane and crossite-bearing metabasalts although dikes of plagioclase wehrlite, and cumulate gab- ous layers to 5 m thick intercalated within the the garnet + diopside amphibolites appear to be bros. The dikes show a wide range in composi- more abundant amphibolites and quartzites. How- very similar to the sole rocks elsewhere in Oman tion from quartz diorite–tonalite–granodiorite to ever in the eastern part of the Bani Hamid area, (Searle, 1980; Searle and Malpas, 1980, 1982). biotite granite, andalusite + cordierite + biotite around Wadi Shis, calc-silicates are the dominant Pressure estimates from the amphibolites in granite, and garnet + tourmaline leucogranite. lithology and they exhibit spectacular ptygmatic Sumeini and Wadi Tayyin were imprecise because Some, notably those with muscovite, biotite, folding on all scales, as well as euhedral porphy- of the very low jadeite content and the lack of tourmaline, garnet, andalusite, and cordierite roblasts of garnet and pyroxene to 50 cm. Unlike quartz in the assemblage (Searle and Malpas, (Fig. 8), undoubtedly have a continental crustal the quartzites and amphibolites, which generally 1980; Ghent and Stout, 1981). The recent discov- source. Others, notably the more trondhjemitic have a foliation defined by mineral-preferred ori- ery of kyanite within garnet amphibolites in the compositions (Fig. 9), could be related to an entations, the calc-silicate units have ptygmatic Asimah area, the United Arab Emirates, has en- ophiolitic source, although it is difficult to ex- folds defined by compositional and modal varia- abled more precise P-T conditions to be es- plain their structural position intruding the upper tions. Microstructurally they tend to be granoblas- tablished (Gnos, in press). Two pyroxene and mantle and lower crust. Most trondhjemites or tic, especially the carbonate-rich marbles. Typical garnet-clinopyroxene barometry indicates peak plagiogranites occur within the upper part of the marble assemblages include calcite + diopside + temperatures of 800 ± 50 °C for garnet amphibo- isotropic gabbros at midcrustal levels and are K-feldspar + spinel, calcite + wollasonite + scapo- lites at Asimah with pressure ≥11 ± 2 kbar (Hacker rarely, if ever, within the mantle (Fig. 2). Many of lite + garnet + K-feldspar, and calcite + chon- and Gnos, 1997; Gnos, in press). Thus it appears these dikes trend approximately east-west or drodite + quartz + K-feldspar + diopside. Typical that pressures are slightly higher in the Asimah east-northeast–west-southwest and cut fabrics in calc-silicate assemblages include grossularite metamorphic sole rocks than in the Bani Hamid the harzburgites and websterites and banding in garnet + wollastonite + diopside + scapolite granulites (Fig. 5). All of these subophiolite meta- the cumulate gabbros. They are clearly intrusive, + K-feldspar, diopside + piemontite + rhodo- morphic rocks have counterclockwise P-T paths and several are zoned with pegmatitic margins. chrosite, diopside + grossularite + wollastonite compatible with rapid burial of cold material be- The intrusive sequence is unusual with xenoliths + spinel, merwinite + clinozoisite + spinel neath a hot peridotite heat source. Rapid heating of felsic material enclosed in more mafic, but + K-feldspar + calcite, and grossularite + epidote during burial beneath the harzburgite mantle se- clearly genetically related, material. Many dikes + diopside + idocrase + K-feldspar + spinel. Rare quence that was at temperatures of 1100–1200 °C also contain xenoliths and schlieren of harzbur- minerals encountered included merwinite, beryl, was followed by decompression and cooling dur- gite and gabbroic country rocks. rhodochrosite, and johannsenite. ing exhumation related to uplift along the Semail The rocks are typically medium- to coarse- The abundance of quartzite and marble litholo- thrust during emplacement. The approximate po- grained, holocrystalline, leucocratic, equigranular, gies within the Bani Hamid granulites and the gen- sitions and depths of the high pressure units in the hypabyssal igneous rocks, and their composition eral lack of pelitic rocks suggest protoliths of con- United Arab Emirates is shown on the reconstruc- varies from intermediate to acid. From the quartz tinental-margin–type sedimentary rocks, rather tion sections in Figure 7. Whereas the protoliths of diorites to the tonalites to the leucogranites, horn- than the distal cherts and oceanic basalts (Haybi the Asimah rocks appear to be distal Haybi com- blende is superseded by biotite and the proportion complex) that form the protolith of the subophio- plex basalts and sedimentary rocks and can thus be of mafic minerals decreases from ~20% to <5%. lite amphibolites. However, the Bani Hamid rocks assigned to the metamorphic sole sensu stricto, the This is accompanied by an increase in modal mi- resemble lithologies in the metamorphic sole else- Bani Hamid granulites may be a slice of continen- croperthitic microcline from zero to ~50%, and a where in the Oman Mountains, except in the rela- tal subophiolite “basement” plucked from beneath decline in the modal proportion and Ca content of

tive proportions of amphibolites, quartzites, and by a later out-of-sequence thrust and emplaced plagioclase from ~50% to ~10% and An40 to An20, marbles. Estimates of the peak temperature and into the midst of the obducted mantle sequence ul- respectively. Minerals characteristic of aluminous pressure of the Bani Hamid granulites (850 °C, tramafic rocks of the ophiolite (Fig. 7). melts include a pink-gray Fe-rich garnet, lepido- 6.5–9 kbar; Gnos, 1992) are similar to the highest lite, topaz, and tourmaline. The dikes sampled and grade of garnet-clinopyroxene amphibolites from ANATECTIC GRANITES INTRUDING described also have a metamorphic assemblage in- Sumeini (Searle, 1980), Wadi Tayyin (Ghent and THE SEMAIL OPHIOLITE cluding epidote, clinozoisite, pumpellyite, chlorite Stout, 1981; Hacker and Mosenfelder, 1996), and (often after primary igneous biotite), and prehnite, Asimah (Gnos, 1997, personal commun.). The In the northern Oman Mountains there is evi- which is generally restricted to veins, although it great thickness of the Bani Hamid sole rocks can dence that the subducting Arabian continental may be interleaved with chlorite as pseudomorphs be explained by widespread isoclinal folding and crust partially melted to produce peraluminous after biotite. postmetamorphic thrusting. The thrusting history potassic granites (Fig. 7). These granites intruded Such metamorphic assemblages are character- of the high-pressure rocks and obduction model up through the obducting ophiolite during the istic of the prehnite-pumpellylite facies, associ- for the northern Oman Mountains is discussed at later stages of emplacement. In the United Arab ated with hydrothermal alteration and P-T condi- the end of this paper. Emirates the dikes are restricted to the eastern tions of ~300 °C/3 kbar. The country rocks also side of the mantle sequence outcrop, at least from appear to have undergone a similar degree of Asimah-Dibba Zone, United Arab Emirates Bulaydah village northeast to Ras Dibba (Fig. 6). metamorphism, with extensive serpentinization These intrusions, in the form of irregular, often of the ultramafic rocks and uralitization of gab- The metamorphic sole beneath the Semail ophi- anastomosing dikes and lenses to 100 m in length broic pyroxenes. This conclusion is consistent olite is thicker and more widespread in the Dibba and 5 m in width, occur consistently in a similar with earlier work on Pb isotopes of similar rocks zone in the far north of the ophiolite outcrop than structural position in the upper part of the mantle nearby (Briqueu et al., 1991), which showed that anywhere else in the Oman Mountains (Alleman sequence and in the lower crustal sequence cu- high 207Pb/204Pb and 208Pb/206Pb ratios were due and Peters, 1972; Searle, 1980). Intermediate mulate gabbros (Peters and Kamber, 1994). The to crustal contamination, possibly from hy-

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Figure 7. Ophiolite obduction model for the northern part of the Oman ophiolite in the United Arab Emirates showing the preemplacement structural position of the Musandam shelf, slope (Sumeini complex), basin (Hawasina complex), and trench (Haybi complex), as well as the formation of the subophiolite amphibolite and greenschist facies rocks and the Bani Hamid granulites. Subduction of the leading edge of the Arabian continental crust led to crustal melting beneath the Semail ophiolite, and formation of leucogranites that were intruded up through the mantle sequence as dikes.

drothermal fluids. Any model of the origin and mation includes quartz subgrains, sutured grain Age and Origin of Granite Dikes age of the leucogranitic dikes must also account boundaries, and undulose grain extinction in less for this metamorphism. deformed examples. Gneissic fabrics are defined The precise age of these granitic dikes is un- In addition the majority of the dikes exhibit a by a grain-shape–preferred orientation of quartz, clear and is currently being determined from sometimes considerable degree of deformation, laths of mica, and trains of feldspar augen. The U-Pb zircon and monazite dating; published K-Ar with the development of augen gneisses and orientation of these fabrics appears to be random. cooling ages are variable. A biotite aplite cutting quartzose mylonites. The most heavily deformed However, biotite-defined gneissic fabrics within the harzburgite near Sharm in the United Arab rocks are the tonalitic and leucogranitic dike the biotite-rich pegmatitic margins of several Emirates gave a K-Ar biotite age of 85 ± 3 Ma components. Microstructural evidence of defor- dikes are subparallel to the dike walls. (Searle, 1980). White micas from garnet-bearing

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Figure 8. Typical leucogranitic dike intruding mantle sequence harzburgites at Wadi Zikt, west of Dadnah, the United Arab Emirates. This dike contains a pegmatitic tourmaline 14 cm long, andalusite, cordierite, biotite, and muscovite.

Figure 9. Biotite granite dikes and veins intruding harzburgites, wehrlites, and cumulate gabbros of the uppermost mantle and lower crustal sequence of the ophiolite, Ras Dadnah cove, United Arab Emirates.

granites in the United Arab Emirates have yielded by three different processes: (1) as end products having an ophiolitic origin. Peters and Kamber 40Ar/39Ar ages of 89.1 and 89.9 ± 0.4 Ma (Hacker of extreme fractionation of the ophiolite late in- (1994) suggested that fluids released from the de- et al., 1996). Other hornblende-bearing granites trusive series, (2) as partial melting of the meta- hydration of the metamorphic sole could infil- have 40Ar/39Ar ages of 93.8 ± 0.2 Ma (Hacker morphic sole amphibolites, and (3) as continental trate upward into the still-hot ophiolite, triggering et al., 1996), indistinguishable from hornblende crustal melts originating from the subducted Ara- partial melting. However, the volume of partial ages from the metamorphic sole, but these gran- bian crust beneath the Semail ophiolite. Although melting within the amphibolitic sole is extremely ites are petrologically distinct from the garnet- the more trondhjemitic compositions could be small (Searle, 1980) and the amphibolites and mica leucogranites at Ras Dadnah and we think explained by extreme fractionation of the quartz quartzites would probably not contain sufficient that no correlation can safely be made between diorite–tonalite–granodiorite late intrusive series, volatiles to induce melting such a distance upsec- the two types. it is almost impossible to explain the garnet + tion. The tonalitic composition of the partial The origin of the granites could be explained tourmaline + cordierite + two-mica granites as melts (Searle and Malpas, 1982) is also not simi-

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lar to the composition of the biotite granites or (El-Shazly, 1994). Metabasalts containing the as- ing from this northern margin of the Saih Hatat the two-mica, garnet-bearing granites. semblage chlorite + lawsonite + quartz + epidote window, including the Mesozoic shelf carbonates, We suggest that the most likely source for ± pumpellyite ± albite occur within a matrix con- the Sumeini slope units, and the allochthonous these granitic dikes is from partial melting of taining carpholite. Beneath this unit carpholite- Hawasina and Haybi thrust sheets. Between subducted continental crustal basement beneath bearing mudstones and quartz-mica schists con- Qantab and Yiti, and around Bandar Khuyran, the Semail ophiolite during the late stages of obtain blocks of metacarbonate, chert, and some Tertiary neoautochthonous limestones rest di- duction (Fig. 7). This is supported by a prelimi- metabasalts. The Ruwi melange was mapped as rectly upon the shelf units (Fig. 10). ε nary Nd value of –16.5 from a garnet-mus- part of the Late Cretaceous Muti Formation The stratigraphy of these shelf units and their covite-tourmaline leucogranitic dike from Ras (Aruma Group) by Glennie et al. (1974) and precise ages are poorly constrained, although the Dadnah (R. Pederson, 1997, personal commun.), Robertson (1987), but its rock types, structural original work of Glennie et al. (1974) ascribed which suggests that the material was derived position, and metamorphism all suggest that it the Mahil Formation to the Triassic and the un- from the melting of continental rock. K-Ar cool- more likely forms part of the Haybi complex, the derlying Saiq Formation to the Permian on the ing ages (95–90 Ma, Gnos and Peters, 1993; or thrust slice immediately underlying the Semail basis of fossil assemblages. Subsequent work, ca. 85 Ma, Searle, 1980) are similar to ages of peridotites (Searle and Malpas, 1980). A late- particularly by Goffé et al. (1988) and El-Shazly high-pressure eclogite facies metamorphism in stage out-of-sequence thrust carrying a slice of et al. (1990), has defined limited and variable the southeast Oman Mountains (Searle et al., Triassic Mahil Formation shelf carbonate show- metamorphic zones, on the basis of metapelitic 1994). A similar continental crustal subduction ing a spectacular east-verging recumbent fold and metabasic assemblages respectively. Searle model used by Searle et al. (1994) to explain the cuts across the top of the Ruwi melange. These et al. (1994) divided the sequence into a number structures and P-T conditions of the eclogites in rocks, together with the Muttrah peridotite, are of structural-metamorphic units, each clearly southeast Oman could be applied to the granulite unconformably covered by Paleocene-Eocene structurally divided (Fig. 10). Since the high- facies metamorphic sole rocks at Bani Hamid unmetamorphosed limestones. pressure metamorphism predated the final re- and the granitic dikes at Ras Dadnah in the Thermobarometry indicates that P-T conditions gional deformation, the major structural features northern part of the ophiolite belt (Fig. 7). ranged from 3 to 6 kbar and 250 to 300 °C for the now delineate the metamorphic zones. Ruwi melange and >6 to 8 kbar and >310 °C for The Yiti unit is the northernmost of the HIGH-PRESSURE the basement units (El-Shazly, 1994). Metamor- “nappes” and consists largely of Mahil Forma- BLUESCHIST-ECLOGITE phic grade increases down structural section from tion calc schists; intercalated carpholite-bear- FACIES ROCKS IN NORTHEAST OMAN lawsonite-albite facies in Ruwi to blueschist facies ing metapelites and crossite-epidote–bearing in underlying units (El-Shazly, 1994; Searle et al., metabasites are characteristic of a low-grade Whereas the metamorphic sole throughout 1994). Both the structure and metamorphic condi- high-pressure regime (Fig. 5). The unit is Oman–United Arab Emirates represents a high- tions of rocks in the Ruwi to Quryat region of bounded to the south by the Yenkit shear zone temperature amphibolite-granulite facies event northeast Oman are very different from anywhere (Searle et al., 1994), which trends ~075° and during subduction of oceanic basalts and sedi- else beneath the Semail ophiolite (Fig. 5). In this dips north. The shear-zone geometry, with the mentary rocks beneath the ophiolite during dis- region, along the leading edge of the Arabian con- Triassic Mahil Formation overlying Permian placement, in the southeastern part of the Oman tinental margin, the shelf carbonates were in- Saiq Formation units, suggests an extensional Mountains from the Muscat area to the Quryat tensely folded and thrusted during the obduction regime, and the rotation of fold axial planes, area (Fig. 10), a regional high-pressure belt con- event. The basal Tertiary unconformity rests on all with the presence of minor imbricate duplexes, tains carpholite-bearing metasedimentary rocks units from pre-Permian basement to peridotites of suggests a component of dextral motion in ad- and blueschist and eclogite facies metabasalts and the Semail ophiolite, showing that all deformation, dition to the dip-slip motion. metapelites (Goffé et al., 1988; El-Shazly et al., except relatively minor folding, and metamor- South of the Yenkit shear zone, which crops 1990; El-Shazly and Liou, 1991; Searle et al., phism was in Late Cretaceous time. Large struc- out in Wadis Mayh and Hulw, a thick sequence of 1994). This metamorphism affects the allochtho- tural sections of the allochthonous Hawasina and dolomitic limestones, with subordinate metaba- nous thrust sheets beneath the ophiolite, the entire Haybi complex are missing between the shelf car- sic units, extends over a considerable area. These shelf carbonate sequence, as well as the pre-Per- bonates and the ophiolite and the entire 4–5 km rocks have been structurally repeated in a number mian basement along the north margin of the Saih crustal section of the ophiolite has been removed, of shear zones that can be traced laterally be- Hatat dome. A more detailed map of the eclogite either by normal faulting or by erosion, prior to tween their clear expressions in the wadis. The and blueschist facies area from As Sifah to Wadi deposition of the upper Maastrichtian and lower consistent north-dipping nature of these shear Hulw is shown in Figure 11 with a cross-section Tertiary limestones. Normal subophiolite amphi- zones, with south-facing isoclinal folds, is con- in Figure 12. The major structural and metamor- bolite facies rocks are missing in this region where sistent with the regional thrusting direction in phic units were described by Goffé et al. (1988) blueschist and carpholite-grade rocks occur be- southeast Oman during ophiolite obduction and Searle et al. (1994), who also presented a neath the Tertiary limestone cover. (north-northeast to south-southwest). The south- model showing the subduction of the leading edge ernmost of these shear zones, in Wadi Hulw, sep- of the Arabian continental crust northeastward be- Saih Hatat Blueschist-Carpholite Zone arates the south-facing structures described here neath the ophiolite during the latest stages of ob- from large-scale recumbent north-northeast– duction (Fig. 13) that explains the metamorphism Immediately beneath the Ruwi melange, sepa- facing sheath folds to the south. Goffé et al. and P-T conditions of each thrust sheet and unit. rated by an arcuate, east-west–striking and north- (1988) described metapelites from this unit with dipping fault, lies a thick sequence of Permian- characteristic high-pressure mineral assemblages Ruwi Melange Triassic, folded and thrusted metamorphosed such as carpholite + kaolinite + pyrophillite; the shelf carbonates, termed the “Muscat nappes” by coexistence of the former two minerals suggest Beneath the Muscat-Muttrah peridotite a Michard et al. (1984). A great thickness of the P-T conditions of <280 °C and >7–8 kbar unique melange crops out around the Ruwi area normal subophiolite sequence is structurally miss- (Chopin and Schreyer, 1983). Furthermore, the

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Figure 12. Geological section from Wadi Hulw to the As Sifah eclogites; see Figure 11 for location.

association of carpholite + pyrophillite + quartz As Sifah Eclogites ported and analyzed by Searle et al. (1994), who gives P-T conditions of ~320 °C and >7 kbar, ac- produced P-T estimates (23 kbar, 540 °C) suggest- cording to the petrogenetic grid of Vidal et al. The deepest structural unit in the Saih Hatat ing a burial depth previously unsuspected and (1992). In addition, locally inverted metamorphic window exhibits eclogite-grade metamorphism, matched only by coesite-bearing eclogites from gradients were found, which they ascribed to suggesting burial depths of ~78–90 km (Searle ultra high-pressure localities such as the Alps, the postmetamorphic thrusting. Microstructural et al., 1994), and is centered around the coastal Dabie Shan, and the Norwegian Caledonides analysis of metabasites by Searle et al. (1994) village of As Sifah in the east of the window (Coleman and Wang, 1995). More recent calcula- suggested a clockwise P-T evolution from a blue (Figs. 10 and 11). Field mapping has confirmed tions using the phengite-garnet geobarometer on amphibole–bearing to a greenschist facies the existence of a shallow, west-dipping detach- the As Sifah eclogites give pressure estimates of assemblage. ment fault that separates this unit from the over- about 20 kbar. These pressures require depths >78 The Hulw unit crops out south of these units lying low-grade dolomitic carbonates of the km using average densities of oceanic crust for the and to the south and west of the underlying high Hulw and Mayh units of the Permian Saiq eclogite facies metamorphism and require conti- grade As Sifah unit, from which it is separated by Formation (Fig. 12). Existing P-T work (Searle nental subduction for their burial and rapid ex- a major, arcuate, west-dipping extensional de- et al., 1994; Goffé et al., 1988) demonstrates that humation (Fig. 13). tachment (Figs. 11 and 12). The unit is domi- this is a major extensional feature, juxtaposing Structurally the As Sifah unit consists of a nated by large-scale nappe structures and sheath lithologies that were vertically separated by as number of high amplitude, short wavelength east- folds in dolomitic limestones, white quartzose much as 50 km under peak conditions. Less than northeast–northeast–verging asymmetric isoclinal schists, and extensive massive and schistose 10 km separates the units on the ground today. folds, and a later generation of east-west–trending metabasic sills and flows (El-Shazly et al., 1990; The As Sifah unit consists of a number of isoclinal folds, which flatten to the east to produce Searle et al., 1994); the latter are characterized by lithologies dominated by calcite + phengite + a relatively flat-lying domal structure centered on transitional blueschist-greenschist facies assem- quartz schists, with subordinate massive and schis- the village (Fig. 11). The dominant regional folia- blages differing only from those found in the tose metabasic units, quartz + phengite + garnet tion is defined by high-pressure phases (om- overlying “nappes” by their greater degree of ret- schists and thin layers and boudins of metabasic phacite, phengite), and is folded into D2 asym- rogression. The southern limit of this unit is a eclogite, characterized by the assemblage sodic metric folds that verge northeast, and were north-verging backthrust, where Paleozoic, low- clinopyroxene + garnet + phengite + glaucophane formed after the growth of garnet, but before all grade, pre-Permian Amdeh quartzites overlie the + rutile + quartz. All the units are affected by vari- the high-pressure mineral growth was complete. rocks of the Hulw unit, and in general the meta- able degrees of greenschist facies overprinting. Al- Later D3 extensional crenulation fabrics were de- morphic grade to the south of this boundary is in- though first reported by Le Metour et al. (1986, veloped throughout the unit, and were contempo- ferred to decline rapidly (Lovelock et al., 1981; 1990), the most extensive outcrops of massive raneous with continuing greenschist facies retro- Goffé et al., 1988). eclogite, along the coast north of As Sifah, were re- gression. Overall, P-T and petrographic evidence

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suggests a clockwise path for the unit (Fig. 5). Ex- oceanic ridge and obduction was initiated at the been initiated at the site of the ridge, which must humation of the eclogites occurred in two stages, ridge, as shown in Figure 3, then the footwall of have been to the northeast of the site of initiation first from eclogite to greenschist facies condi- the subduction zone (metamorphic sole and of the Semail thrust (Searle and Malpas, 1980). tions, and then a final exhumation along the foot- Haybi complex) should be the same age as the The only remaining setting wherein older, wall of extensional detachments (Searle et al., ophiolite itself. This is clearly not the case. The colder material could be juxtaposed against 1994). We now discuss emplacement models for Haybi complex is composed dominantly of older young, hot ophiolite would be a transform-fault the Semail ophiolite that must satisfy all the struc- ocean floor material such as Triassic alkalic setting. However, this transform fault would have tural and metamorphic criteria outlined here. basalts, ankaramites (with titan-augite and kaer- to have been oriented parallel to the continental sutite phenocrysts), and nephelinites, as well as margin in order to have formed the site of initia- TECTONIC SETTING OF tholeiitic volcanic rocks (Searle et al., 1980), Late tion of the Semail thrust. There is no structural THE OMAN OPHIOLITE Permian and Late Triassic Oman exotic limestone evidence for this in Oman, and there is little evi- blocks (Searle and Graham, 1982), Cretaceous dence for transform faulting at right angles to the Mid-Oceanic Ridge or Suprasubduction Zone melanges, and the metamorphic sole. Geochem- ridge either (MacLeod and Rothery, 1992). We Origin for the Semail Ophiolite? istry of the amphibolites in the metamorphic sole therefore reject both the mid-oceanic ridge set- shows that the amphibolites were not the same as ting and the transform fault setting for the tec- Two major tectonic settings have been pro- either the Semail ophiolite volcanic rocks or the tonic origin of the Semail ophiolite, which must posed for the origin of the Semail ophiolite: (1) a Haybi alkalic basalts. Immobile trace element have formed in a marginal basin above a north- typical mid-oceanic ridge (Fig. 3; Coleman, 1981; geochemistry clearly shows that the amphibolites east-dipping subduction zone, dipping away Hopson et al., 1981; Boudier et al., 1985, 1988; are closest to the Haybi tholeiitic volcanic rocks from the continental margin, as shown in Figure Boudier and Nicolas, 1995; Hacker, 1991; Hacker of Triassic or Jurassic age (Searle, 1980; Searle 2. Obduction must have initiated along the Se- et al., 1996), or (2) a suprasubduction-zone setting and Malpas, 1982). Thus the volcanic rocks being mail thrust at a trench where old, cold Triassic (Fig. 2; Searle and Malpas, 1980; Pearce et al., subducted along the footwall of the Semail thrust and Jurassic ocean-floor rocks were being sub- 1981; Alabaster et al., 1982; Lippard et al., 1986). were considerably older and colder than the ducted northeastward beneath the Semail ophio- The evidence hinges on the interpretation of the Cenomanian volcanic rocks erupted in the ophiolite during or immediately after formation of the volcanic sequence geochemistry, the structural lite. Obduction therefore cannot have been initi- Semail crustal sequence. restoration of the thrust sheets, and the timing of ated at the mid-oceanic ridge that formed the ophiolite formation relative to the timing of meta- ophiolite. Subduction of the Continental Crust morphism in the amphibolite facies sole. The vol- The coincidence in timing between the U-Pb canic units at the upper levels of the Semail ophi- zircon ages in the plagiogranites and the meta- It is mechanically extremely difficult for more olite are most complete in northern Oman, where morphic 40Ar/39Ar hornblende ages in the meta- buoyant continental crust with densities of about Pearce et al. (1981) and Alabaster et al. (1982) de- morphic sole (Hacker, 1994) also point to sub- 2.8 g/cm3 to be subducted beneath an ophiolitic termined the volcanic stratigraphy and, using duction of Haybi basalts beneath the ophiolite or oceanic crust with densities of about 3.0– trace element geochemistry, proposed an imma- mantle sequence to depths at least 12–14 km be- 3.3 g/cm3. Many obducted Tethyan ophiolites are ture island-arc setting for the Lasail unit volcanic low the Moho at the same time as the ophiolite relatively thin-skinned thrust sheets that have been rocks. Only the lower MORB-type volcanic rocks crustal sequence was forming and the same time thrust onto attenuated passive margin sequences. are present in south Oman (Bailey, 1981; Hopson as Cenomanian-Turonian radiolaria were form- In Oman the stratigraphic thickness of the Semail et al., 1981). ing the radiolarian oozes above the Semail vol- ophiolite is 14–20 km, although the structural If the Semail ophiolite originated at a mid- canic sequence. Clearly obduction cannot have thickness may not exceed 8–10 km. The leading

Figure 13. Tectonic model for the subduction-obduction evolution of the Oman ophiolite in the southeastern Oman Mountains, after Searle et al. (1994). (A) The palinspastically restored cross section across the Arabian continental margin with the positions of the major thrusts. Note that the shelf, slope (Sumeini), and basin (Hawasina) sedimentary rocks are laterally time-equivalent units. The continental crust thins toward the margin in the northeast. The heavy box shows the restored stratigraphic position of the protolith of the high-pressure As Sifah unit rocks, namely the Permian Saiq Formation along the continental margin. The crust beneath the distal Hawasina and Haybi units is dominantly alkalic basaltic to transitional tholeiitic basaltic rocks of latest Permian–Triassic and Jurassic age. The subophiolite amphibolites were formed at pressures of about 5–7 kbar (17–18 km depth) by accretion of Haybi-type basaltic crust to the base of the mantle sequence at least 12 km structurally beneath the Moho (M). Greenschists were subsequently accreted by underplating of Haybi complex sedimentary rocks beneath the amphibolites and metamorphosed at pressures between 2 and 4 kbar (7–14 km depth). The Semail ophiolite plus metamorphic sole was then thrust onto the Haybi complex and thrusting propagated southwestward with time. (B) During the subduction-obduction process, the continental margin rocks were subducted to depths of around 80–90 km where peak metamorphic pressures of 23 kbar are recorded by thermobarometry in the As Sifah unit eclogites. The subophiolite thrust sheets (Haybi and Hawasina complexes) were thrust onto the foreland and are mainly missing in the internal (northeast) parts of the Oman Mountains, where the ophiolite lies almost directly on the shelf carbonates. The subduction of continental margin material down to 80–90 km must have occurred during a phase of rapid plate motion during ophiolite obduction. (C) Exhumation of the high-pressure As Sifah unit rocks must also have occurred rapidly and soon after peak metamorphism, by a combination of buoyancy forces of the lighter continental material beneath a denser ophiolite, hydrodynamically driven return flow immediately back up the same subduction zone, and extensional normal faulting during uplift of the rocks along the footwall. The largest extensional fault is the contact between the Hulw and As Sifah unit that shows around 16 kbar of pressure dif- ference across it. The late stage culmination of the Saih Hatat window was accompanied by normal faulting around the margins. The Semail thrust beneath the ophiolite was folded around the shelf carbonate culmination and reactivated as a normal fault.

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edge of the obducted ophiolite now lies 120 km stable hanging wall. This mechanism explains northeast-dipping subduction zone at 97–94 Ma. southwest of the known northeast limit of Arabian the large jumps in pressures going down struc- Subduction of Triassic (and Jurassic[?]) tholeiitic continental crust beneath the Batinah coast. The tural section across northern Saih Hatat. basalts along the subduction zone to depths of at underlying Hawasina and Haybi oceanic thrust least 12–14 km below the Moho and their meta- sheets, which palinspastically were situated be- Melting of the Continental Crust morphism to upper amphibolite facies occurred tween the continental margin and the ophiolite, re- during the time span 95–93 Ma, concomitantly store to a minimum of ~200 km or more. The ab- In the northern part of the Semail ophiolite belt with, or immediately after, crystallization of the solute minimum emplacement distance of the in the United Arab Emirates, the temperature at the ophiolite crustal sequence. These amphibolites Semail ophiolite was therefore ~320 km and more base of the ophiolite during obduction was proba- were welded onto the base of the mantle se- realistically 400 km. The major question now is bly slightly greater than that in the central and quence peridotites. As thrusting and exhumation how much of the continental slope and basin southeastern Oman Mountains. Hence, outcrops progressed, greenschist facies metamorphism at (Hawasina complex) was underlain by continental of the metamorphic sole are greater in extent and higher structural levels was imposed on Haybi crust, and what was its fate during the emplace- hotter, producing granulite facies assemblages. If a volcanic rocks, manganiferous cherts, exotic ment process? Searle (1984) described highly al- crustal subduction model similar to that of south- limestones, and shales, which were welded onto kalic ultramafic (wehrlites and jacupirangites) and east Oman (Searle et al., 1994; Fig. 13) is applied the base of the amphibolites. The ophiolite plus alkalic gabbro sills intruding the Haybi complex to the northern part, but the temperatures were its metamorphic sole was then emplaced a dis- volcanic sequence as well as some of the lower greater, it is quite possible that the continental crust tance of at least 350–400 km southwestward onto Hawasina thrust sheets. These alkalic basalts and could heat enough to partially melt (Fig. 7). Ana- the Arabian platform. Thrusts in general propa- sills span the Triassic, Jurassic, and Early Creta- texis would lead to production of leucogranitic gated down structural section with time, in se- ceous time and it seems likely from their structural magma, which would then separate from its quence toward the continental margin foreland, position that they represent long-lasting alkalic source and intrude upward through the obducted although some out-of-sequence thrusting and magmatism along the continent-ocean crust tran- ophiolite. The biotite granite dikes at Ras Dadnah normal faulting has been demonstrated (Searle, sition beneath the Oman Mesozoic passive mar- and elsewhere in the United Arab Emirates would 1985, 1988a). gin. The preemplacement northeastward limit of then be the highest level dikes intruding through In the southeastern Oman Mountains ther- continental crust therefore must have been some- the wedge of mantle sequence peridotite. Late- mobarometry of regional high-pressure meta- where within the sub-Hawasina basement between stage, out-of-sequence thrusting has repeated the morphic rocks has allowed estimation of paleo- the margin and the distal Hawasina thrust sheet. harzburgite-wehrlite mantle sequence plus meta- depths of metamorphism (Goffé et al., 1988; The As Sifah eclogites testify to the subduc- morphic sole package in the Bani Hamid area. The Searle et al., 1994; El-Shazly, 1994). The entire tion of continental crust below the Semail ophio- normal metamorphic sole comprising amphibo- continental margin in this sector was subducted lite to depths of about 78–90 km (Searle et al., lites and greenschists is exposed in the Asimah- to deeper levels than rocks in the central Oman 1994). The eclogite protoliths are basaltic flows Masafi corridor beneath the ophiolite and has Mountains, where no high-pressure metamor- and sills within Permian or older shelf facies car- been structurally imbricated within the Dibba phic rocks crop out at the surface. The deepest bonates that must also have been subducted to zone (Searle, 1980, 1988a; Gnos, 1992). We spec- metamorphism occurs in the As Sifah eclogites, these depths. The uplift or exhumation of these ulate that the thicker and structurally repeated where garnet + clinopyroxene + glaucophane + high-pressure rocks must have been related to ophiolite section, higher P-T conditions, and pres- phengite–bearing metabasalts and garnet + buoyancy and hydrodynamic reverse flow along ence of both granulite facies rocks and anatectic chloritoid–bearing metapelites were subducted the subduction zone as soon as subduction ceased granites in the United Arab Emirates sector, com- to depths of about 78–90 km. These rocks oc- (e.g., Platt, 1987). The thinned leading edge of pared to Oman to the southeast, may be due to the cur within quartzofeldspathic mica schists and Arabian continental crust was therefore pulled proximity of the continent-continent collision calc-schists that were part of either the Permian down the subduction zone until subduction ended zone of the Zagros region in Iran across the Saiq Formation shelf carbonate sequence or the and then rapidly exhumed along the same sub- nearby Musandam peninsula and Straits of Hor- pre-Permian basement. Textural and metamor- duction zone by buoyancy-driven return flow as- muz syntaxis (Searle, 1988b). phic evidence shows that the As Sifah eclogites sisted by active normal faulting between the evolved along a clockwise P-T path culminat- rapidly exhuming footwall and the relatively sta- EMPLACEMENT MODEL FOR ing at 20–23 ± 2.5 kbar and 540 ± 75 °C (Searle tic, cold, ophiolitic hanging wall (Fig. 13). Ero- THE OMAN OPHIOLITE et al., 1994). This is the first definitive example sion was not the primary factor leading to ex- of demonstrable continental crust having been humation although some extreme deformation of We have presented here (Fig. 7 for the northern subducted to such depths beneath an ophiolitic the shelf carbonate and pre-Permian basement sector; Fig. 13 for the southeast Oman Moun- hanging wall. Exhumation of the eclogites oc- sequence must have occurred along the northern tains) a new emplacement model for the obduc- curred in two stages, first by rapid emplace- margin of Saih Hatat. In addition, ~4–5 km of tion of the Oman ophiolite that satisfies all the ment against intermediate units of garnet + erosion of the obducting ophiolite sequence oc- structural, metamorphic, thermobarometric, crossite ± chloritoid blueschists at 20–25 km curred in the Muscat area, where crustal levels magmatic, and geochronological data discussed depth, and second by exhumation of the entire are not exposed and the sub-Tertiary unconform- herein. Palinspastic reconstruction of the entire high-pressure zone along extensional shear ity lies above harzburgites of the mantle se- Oman Mountains allochthon suggests that the zones (Searle et al., 1994). quence. Normal faulting is important in the ex- Semail ophiolite formed the most distal unit sep- In the northern Oman Mountains intermediate humation history of the high-pressure rocks of arated from the continental margin by the medium-pressure rocks have been reported in the northeast Oman, but this does not imply whole Sumeini Group slope facies carbonates, the form of crossite- and glaucophane-bearing meta- crustal extension and certainly not whole litho- Hawasina ocean-basin sediments, and the Haybi basalts in the Dibba zone (Searle, 1980), and sphere extension. Rather, the extension is rela- complex trench rocks. The Semail ophiolite was high-pressure kyanite-bearing assemblages were tive, between a rapidly exhuming footwall and a formed at a spreading center above a shallow reported by Gnos (in press) from garnet amphibo-

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lites in the Asimah area, the United Arab Emi- terial beneath the young, hot ophiolite occurred in rates would be 26 mm/yr. These would be mini- rates. Most of the subophiolite metamorphic a subduction setting. There is no evidence of a mum plate motion rates, because it is more likely rocks in the United Arab Emirates are similar to paleo-transform fault juxtaposing Haybi complex that thrust motion was differentially partitioned the amphibolite and greenschist facies sole else- rocks against the ophiolite at the site of obduction. between an early deep-seated subduction phase where in the Oman Mountains with the possible 5. Subduction of the leading edge of the Ara- and a later, more rapid, shallower thrust emplace- exception of the Bani Hamid area. Here, granulite bian continental crust to depths of about 78– ment stage. facies assemblages indicate P-T conditions of 90 km occurred beneath the Semail ophiolite metamorphism of 6–9 kbar and 850 °C (Gnos, during obduction, forming metabasaltic (sodic ACKNOWLEDGMENTS 1992). Protoliths for these granulites were clinopyroxene + garnet + glaucophane + phen- quartzites and carbonates from the continental gite) and metapelitic (garnet + chloritoid) eclog- This work was funded by National Envi- margin lithologies, in contrast to the distal oceanic ites (Fig. 13). These rocks formed along a ronment Research Council (NERC) grant sedimentary rocks and basalts that formed the clockwise P-T path, which culminated at 20–23 ± GT5/96/13/E to Searle and a NERC Ph.D. protoliths for the metamorphic sole sensu stricto. 2.5 kbar and 540 ± 75 °C. The eclogites occur grant to Cox. We are extremely grateful to within calc-schists and quartzofeldspathic schists, Hilal bin Mohammed Al-Azri for logistical CONCLUSIONS which must also have been to these depths. Ex- help in Oman and to David Willis and Judy humation occurred by buoyancy forces and hy- Willis in Muscat. We also thank Dave Waters 1. The Semail ophiolite formed during the drodynamically driven return flow as a result of and Tjerk Peters for discussions and Brad time span 97.3–93.5 ± 0.25 Ma (Cenomanian) the cessation of subduction. Hacker, Ed Ghent, and Ed Beutner for detailed from U-Pb ages of plagiogranites in the crustal 6. Regional high-pressure metamorphism and constructive reviews. sequence and radiolarian ages in oozes de- across the Saih Hatat region in the southeast Oman posited within the pillow-lava sequence. 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