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Research Paper GEOSPHERE Structure and metamorphism beneath the obducting Oman ophiolite: Evidence from the Bani Hamid granulites, northern GEOSPHERE; v. 11, no. 6 Oman mountains doi:10.1130/GES01199.1 M.P. Searle1, D.J. Waters1, J.M. Garber2,3, M. Rioux3, A.G. Cherry1,*, and T.K. Ambrose1 18 figures; 1 table; 5 supplemental files 1Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK 2Department of Earth Sciences, 1006 Webb Hall, University of California, Santa Barbara, California 93106, USA 3Earth Research Institute, 6832 Ellison Hall, University of California, Santa Barbara, California 93106, USA CORRESPONDENCE: [email protected] CITATION: Searle, M.P., Waters, D.J., Garber, J.M., ABSTRACT zircon and titanite U-Pb dates span ca. 94.5–89.8 Ma. Peraluminous granitic Rioux, M., Cherry, A.G., and Ambrose, T.K., 2015, Structure and metamorphism beneath the obducting dikes intruding the mantle sequence peridotites are as young as 91.4 Ma and Oman ophiolite: Evidence from the Bani Hamid gran- The Cretaceous Semail ophiolite (northern Oman and the United Arab likely reflect localized partial melting of crustal material during the late stage ulites, northern Oman mountains: Geosphere, v. 11, Emirates) includes an intact thrust slice of Tethyan oceanic crust and upper of the obduction process. A minimum of 130 km shortening is recorded by res- no. 6, p. 1812–1836, doi:10.1130/GES01199.1. mantle formed above a northeast-dipping subduction zone that was the site toration of the major folds within the Bani Hamid thrust sheet, and more than of initiation of obduction. The normal metamorphic sole of the Semail ophio- 30 km offset has occurred along the west-directed breaching out-of-sequence Received 8 May 2015 Revision received 20 July 2015 lite comprises a highly condensed sequence of hornblende + plagioclase ± Bani Hamid thrust. These rocks may be representative of deep-level duplexes Accepted 23 September 2015 garnet amphibolites with small enclaves of garnet + clinopyroxene granulites imaged on recent seismic sections across the mountains of northern Oman– Published online 2 October 2015 immediately beneath the mantle sequence peridotites, tectonically underlain United Arab Emirates. by a series of epidote amphibolite and greenschist facies lithologies in a highly deformed ductile shear zone. Peak metamorphic conditions of 770–900 °C and 11–15 kbar indicate metamorphism at depths far greater than can be ac- INTRODUCTION counted for by the preserved thickness of the ophiolite (~15 km). In the moun- tains of northern Oman, the 1.2-km-thick Bani Hamid thrust sheet is com- The Semail ophiolite of northern Oman and the United Arab Emirates (UAE) posed of intensely folded granulite and amphibolite facies rocks within mantle (Figs. 1 and 2) is thought to represent a slice of oceanic crust and upper mantle sequence peridotites, exhumed by late-stage out-of-sequence thrusting along formed at a fast-spreading ridge (Glennie et al., 1973; Lippard et al., 1986; Nico- the Bani Hamid thrust. The Bani Hamid thrust slice includes two-pyroxene las et al., 1989; Searle and Cox, 1999; Goodenough et al., 2010) during the Ceno- quartzites (± hornblende, cordierite, sapphirine), diopside + andradite garnet + manian (Tilton et al., 1981; Warren et al., 2005; Rioux et al., 2012a, 2013). The wollastonite + scapolite marbles and calc-silicates and amphibolites (horn- ophiolite preserves a complete oceanic crustal sequence as much as 7–8 km blende + plagioclase ± clinopyroxene ± biotite) with localized partial melting, thick, including layered peridotites and gabbros, iso tropic gabbros, late cross- intruded by hornblende pegmatites. The Bani Hamid granulites represent cutting wehrlites, sheeted dikes, and pillow lavas with inter leaved radiolarian metamorphosed cherts and calcareous turbidites probably derived from the cherts. The crustal section is underlain by an ~12–15 km thickness of upper distal Haybi Complex and Oman Exotic limestones, which have an alkali ba- mantle harzburgites and dunites (Fig. 3). New high-precision 206Pb/238U zircon saltic substrate. Metamorphic modeling using the program THERMOCALC in dates suggest that the early magmatism associated with the lower Geotimes the system NCKFMASHTO (Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O) or V1 volcanism in the Oman part of the ophiolite occurred during the narrow gives peak pressure-temperature conditions of 850 ± 60 °C and 6.3 ± 0.5 kbar, time span of ca. 96.1–95.5 Ma, thus dating formation of the ophiolite crustal a pressure that is much lower than that of the metamorphic sole, suggesting sequence (Rioux et al., 2012a, 2013). Late gabbros, trondhjemites, and tonalites a different origin. The 206Pb/238U zircon dates indicate that the gabbroic crust have U-Pb zircon dates of 95.3–95.1 Ma (Rioux et al., 2013), likely related to of the ophiolite formed by ridge magmatism from before 96.1 to 95.5 Ma. The subduction below the ophiolite. 206Pb/238U zircon dates from the metamorphic sole range from 95.7 to 94.5 Ma, The Oman ophiolite forms the structurally highest of a series of thrust and suggest that metamorphism and melting was either synchronous with or sheets placing progressively more distal Tethyan units (Haybi Complex) over slightly postdated ridge magmatism. The Bani Hamid granulites are younger; more proximal thrust slices (Hawasina Complex and Sumeini Group). The en- For permission to copy, contact Copyright tire allochthon was emplaced into a Late Cretaceous flexural foreland basin Permissions, GSA, or [email protected]. *Present address: Statoil, Sandsliveien 30, Bergen, Norway (Aruma Group) that is above the middle Permian to middle Cretaceous shelf © 2015 Geological Society of America GEOSPHERE | Volume 11 | Number 6 Searle et al. | Granulite sole Oman ophiolite Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/6/1812/4333736/1812.pdf 1812 by guest on 30 September 2021 Research Paper 56°0′N 56°30′N A B t Oman Thrust Dibba Faul Dibba Location of Fig A Hagab t Thrus ( Semail O m Muscat Jebel Agah a n - Dadna UA E M ou ntai 25°30′N ns 0 100 Jebel Qamar km Aruma group sheeted dikes later gabbro ( massive gabbro Thrust ( Khor Fakkan e layered gabbro Figure 1. Geological map of the Oman ( Manama ophiolit mountains showing location of the Bani Semail Moho transition zone Hamid area. UAE—United Arab Emirates. Masafi dunite ( harzburgite 25°15′N Masafi metamorphic sole Wadi Ham Faul Bani Hamid granulites/amphibolites Dibba volcanics t Haybi complex Hawasina complex Fujairah City Dibba Zone Sumeni group Musandam shelf carbonates N 0 10 25°0′N km GEOSPHERE | Volume 11 | Number 6 Searle et al. | Granulite sole Oman ophiolite Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/6/1812/4333736/1812.pdf 1813 by guest on 30 September 2021 Research Paper MUSANDAM Dibba Jebel Fig.11 Yibir Dibba faultD2 Rul Dibba D4 E Ras Dadnah ZON D4 J. Agah Hagab thrust DIBBA Badiyah J. Qamar t us hr l T ai Masafi D1 m e S Dhaid Khawr Bani Fakkan Wa Hamid di Ham faul D1 t D2 J. Faiyah Fujairah GULF OF OMAN Khawr Kasba Rawdha ST Hatta faul t S e m a il thrust Jebel Wadi Shinas S u Sumeini HyT Rayy m e in i Sumeini t h r Window u s t Jebel Ghawil Figure 3. Generalized section through the ~20-km-thick Semail ophiolite thrust sheet tectonic Figure 2. Landsat photo of the mountains of northern Oman–United Arab Emirates showing key structural stratigraphy, showing the general structures and lithology of the oceanic crust and upper features and location of the Bani Hamid thrust sheet. Also shown are lines of the D1 and D4 seismic pro- mantle rocks. files. J.—Jebel; ST—Semail thrust; Hyb—Haybi thrust. GEOSPHERE | Volume 11 | Number 6 Searle et al. | Granulite sole Oman ophiolite Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/6/1812/4333736/1812.pdf 1814 by guest on 30 September 2021 Research Paper carbonate sequence (Fig. 4). Clues to how the ophiolite was obducted onto the garnet + clinopyroxene granulite, epidote amphibolite, and a variety of former passive continental margin of Arabia are mainly in the metamorphic greenschist facies metasedimentary rocks including piemontite (Mn epidote) sole of the ophiolite as well as an enigmatic and unique thrust slice, the Bani quartzites, marbles, and rare metavolcanics. The sole records an inverted and Hamid thrust sheet, composed of granulite facies partly retrogressed to am- highly condensed P-T gradient and shows intense mylonitic fabrics (Searle phibolite facies assemblages in the mountains of northern Oman–UAE. and Malpas, 1980, 1982; Ghent and Stout, 1981; Hacker and Mosenfelder, The metamorphic sole to the ophiolite consists of a narrow thrust slice 1996; Gnos, 1998; Searle and Cox, 2002; Searle and Ali, 2009). Peak P-T condi- of hornblende + plagioclase ± garnet amphibolites with small enclaves of tions in sole localities in Oman are 770–900 °C and 11–13 kbar (Cowan et al., Figure 4. Restoration of the thrust sheets in the mountains of northern Oman showing a palinspastic reconstruction of the continental shelf, slope, and Tethyan basin to the Semail ophiolite prior to obduction (after Searle et al., 2014). Fm.—formation; U.—upper; Mid.—middle. GEOSPHERE | Volume 11 | Number 6 Searle et al. | Granulite sole Oman ophiolite Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/6/1812/4333736/1812.pdf 1815 by guest on 30 September 2021 Research Paper 2013), equivalent to depths of 30–40 km in oceanic lithosphere. The implied depths of metamorphism are higher than can be explained by the preserved thickness of oceanic lithosphere in the ophiolite. Restoration of thrust sheets beneath the Semail ophiolite complex in the northern Oman mountains show at least 400 km of shortening in the proximal (Sumeini Complex), slope and basin (Hawasina Complex), and distal trench (Haybi Complex) sedimentary rocks structurally beneath the ophiolite (Cooper, 1988; Searle and Cox, 1999; Searle, 2007).

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