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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 mountains 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, isotropic gabbros, late cross- intruded by hornblende pegmatites. The Bani Hamid granulites represent cutting wehrlites, sheeted dikes, and pillow lavas with interleaved 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

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

0 10 25°0′N km

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

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

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.

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.

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). The 206Pb/238U zircon ages from garnet amphibolites and tonalitic partial melt pods and veins from the Wadi Tayyin metamorphic sole locality in Oman range from 94.69 ± 0.12 to 94.48 ± 0.23 Ma (Warren et al., 2005; Rioux et al., 2013), suggesting that peak metamorphism slightly postdated ridge magmatism at this locality. Styles et al. (2006) reported “concordia ages” of 95.29 ± 0.21 Ma and 95.69 ± 0.25 Ma for felsic leucocratic melts from amphibolite sole exposures in the Masafi area (UAE). The timing of ridge-related magmatism has not been dated in the UAE, but is constrained to be older than 95.74 ± 0.32 to 96.40 ± 0.29 Ma (Styles et al., 2006; Goodenough et al., 2010), indicating that the sole metamorphism and partial melting were either synchronous with or slightly postdated ridge magmatism in the UAE. Unpublished 206Pb/238U zircon dates showing that leucocratic melting in the Sumeini sole locality in northern Oman was synchronous with ridge magmatism were reported by Rioux et al. (2014). The Bani Hamid thrust sheet in the northern Oman mountains is a 1.2-km- thick sheet of high-temperature granulites that have been thrust into the harzburgite-dunite section of the mantle sequence (Fig. 5). As far as we are aware, such thick granulites are not known from any other subophiolite metamorphic sole. The Semail ophiolite has been structurally repeated by out-of-sequence thrusting along the Bani Hamid thrust and the granulites have been exhumed from depths of ~20–25 km beneath the already obducted ophiolite. The Bani Hamid rocks include enstatite + diopside quartzites (± hornblende, cordierite, sapphirine), diopside + andradite garnet + wollastonite + scapolite calc-silicates, and amphibolites (hornblende + plagioclase ± clinopyroxene ± biotite) with localized partial melting, intruded by hornblende pegmatites (Gnos and Kurz, 1994; Gnos, 1998). Published U-Pb zircon dates from Bani Hamid rocks range from 93.2 to 91.8 Ma (Styles et al., 2006), younger than the ophiolite and the normal amphibolite sole rocks. The Bani Hamid granulites represent metamorphosed dolomitic cherts and calcareous turbidites associated with alkali volcanics, probably derived from the distal Haybi Complex, and are tight to isoclinally folded at all scales (Searle et al., 2014). In the UAE a series of leucocratic dikes ranging from andalusite-cordier ite-biotite monzogranites to garnet-tourmaline leucogranites intrude into the mantle sequence of the Khawr Fakkan block (Peters and Kamber, 1994; Cox et al., 1999). These crustal melt granites are thought to have been derived from partial melting of a pelitic metasedimentary source beneath the ophiolite (Searle and Cox, 2002; Searle et al., 2014). These dikes are very distinct Figure 5. Simplified metamorphic-tectonic stratigraphy of the Bani Hamid and Wadi Ham sequences. Ol—olivine; Opx— from the plagiogranites formed by partial melting of hornblende gabbros in orthopyroxene; Chr—chromium; Sp—spinel; Cpx—clinopyroxene. the roof of the magma chamber that are present along the length of the Semail

ophiolite (Rollinson, 2009). Styles et al. (2006) reported a single date of 91.38 ± tremely complex nonsymmetrical folds along the footwall of the Shis thrust 0.23 Ma for a mixed tonalite-microgabbro from the Dadna area. that truncates fold axes in the footwall rocks. The Bani Hamid thrust along In this paper we present results from new structural mapping of the Bani the structural base of the Bani Hamid granulite facies rocks continues into the Hamid granulites and new thermobarometry results and pseudosection harzburgite mantle sequence to the northeast as the granulite amphibolites of modeling of high-temperature metaalkali basalts and orthopyroxene-bearing Bani Hamid plunge beneath the exposure level. quartzites. Peak metamorphic conditions are supplemented by Zr-in-titanite thermometry from granulite facies quartzite, and new timing constraints are provided by high-precision U-Pb zircon dating of an amphibolite and small- PETROGRAPHY scale leucocratic dike and laser ablation U-Pb titanite dating of quartzite and marble samples. We use these data to constrain depths and timing of meta- The Bani Hamid metamorphic rocks are very distinct from the normal sub- morphism. We use our structural mapping constraints to restore the Bani ophiolite metamorphic sole. They are mainly composed of quartzites, calc-sili Hamid thrust sheet in relation to the known geometry of the Semail ophiolite cates, and marble with minor amphibolite and no pelites. They are uniformly thrust sheet and present a tectonic evolution based on our new data com- high-temperature assemblages of granulite or upper amphibolite facies (Alle- bined with U-Pb timing constraints (Styles et al., 2006; Rioux et al., 2012a, mann and Peters, 1972; Searle and Malpas, 1980; Gnos and Kurz, 1994; Gnos 2013). We speculate on the composition and structure of the lower crust as and Nicolas, 1996; Searle and Cox, 2002). imaged on recently acquired deep seismic profiles across the mountains in northern Oman and the UAE (Rouré et al., 2006; Tarapoanca et al., 2010; Naville et al., 2010). We integrate geological constraints from the northeast- Quartzites ern Oman mountains where high-pressure eclogites have been exhumed east of Muscat (As Sifah eclogites; Searle et al., 1994, 2004; Warren et al., Three different types of quartzite are found in the Bani Hamid and Wadi 2003; Agard et al., 2010), and from the northern mountains where the Bani Ham thrust slices. Clinopyroxene quartzites (Ca-Mg) are the most common Hamid granulites crop out, to formulate a model for ophiolite obduction and and occur as massive bands interbedded with marbles and calc-silicates. They emplacement. contain quartz + clinopyroxene + andradite garnet + hornblende + titanite ± enstatite and show a variety of deformation fabrics from coarsely annealed recrystallized grains to mylonites (Fig. 7). The second type is more aluminous STRUCTURAL SETTING OF THE BANI HAMID GRANULITES (Mg-Al) orthopyroxene quartzites containing quartz + aluminous enstatite + cordierite + spinel ± kyanite + biotite + chlorite. Enstatites may form aggre- Two major outcrops of granulite-amphibolite facies metamorphic rocks are gates enclosing magnetite, hematite, green spinel, phlogopite, plagioclase, present in the northern ophiolite (Khawr Fakkan block), the Wadi Ham thrust and sapphirine (Gnos and Kurz, 1994). Cordierites contain fibrous sillimanite slice, and the Bani Hamid thrust sheet. The Wadi Ham sheet is structurally and spinel inclusions. Occasionally both diopside and enstatite are found in beneath the Bani Hamid thrust sheet and is cut by the Wadi Ham fault (Fig. 2). the same quartzite layer. These assemblages are found mainly in the struc- The Bani Hamid thrust, which places the granulite-amphibolite facies rocks turally lower western part of the Bani Hamid thrust sheet immediately above over a lower ultramafic unit, continues to the southwest along the top of the the Bani Hamid thrust. Gnos and Kurz (1994) described sapphirine + corun- Wadi Ham granulite amphibolites. This fault is one of several late-stage out-of- dum assemblages confined to a single Al-rich layer in Wadi Madhah, along the sequence thrusts that tectonically repeats the entire ophiolite and underlying northwestern part of Bani Hamid. Sapphirine occurs together with spinel and metamorphic rocks. Another such thrust is the Masafi-Dibba thrust that places shows small blebs of magnetite or hematite. The third and least common type the entire Khawr Fakkan ophiolite slice in the eastern mountains over a struc- is Mn-rich cordierite quartzites that commonly contain quartz + pyroxmang- turally lower unit composed entirely of mantle peridotites. ite + cordierite + spessartine garnet + sillimanite + piemontite or other Mn-rich The internal structure of the Bani Hamid thrust sheet is dominated by silicates. Retrogressive reactions show cordierite breaking down to the green- large-scale west-verging recumbent fold nappes (Fig. 6). These large-scale schist facies association kyanite + chlorite (Figs. 8A, 8B) and the formation of folds affect the entire 1.2 km section, and restoration shows a minimum of biotite + quartz symplectites, typical of retrogressed granulites (Figs. 8C, 8D). 130 km shortening. Small-scale tight to isoclinal folds are present within more None of these compositions is typical of clastic sedimentary quartzites, competent layers, whereas the marble bands show completely incompetent and they are most likely derived from cherts. Gnos and Kurz (1994) presented flow folds thickening into fold hinges. A large-scale out-of-sequence thrust major and trace element analyses of a variety of Bani Hamid quartz-rich rocks, fault termed the Shis thrust has been mapped in the middle of the Bani Hamid and noted that the trace element patterns are atypical for quartzites and, par- thrust slice, placing recumbently folded quartzites with minor marble bands ticularly for the Ca-Mg-quartzites, show relatively high concentrations of ele over a dominantly marble unit (Fig. 6). Calc-silicates and marbles show ex- ments typical of basalts. Possible protoliths include hydrothermally altered

Figure 6. Photograph and annotated section of the Bani Hamid sequence along Wadi Shis, looking southwest. Cpx—clino pyroxene.

Figure 7. (A) Highly strained clinopyroxene quartzite interbanded with a marble horizon, Wadi Figure 8. Retrogressive reactions and textures from Bani Hamid. (A, B) Cordierite (Crd) break- Shis. (B) Thin section of a well-annealed diopside quartzite. (C) Thin section of the same lithol- ing down to kyanite (Ky) + chlorite (Chl) in cordierite quartzites. Qtz—quartz. (C, D) Biotite + ogy showing retrogressive deformation fabric with formation of subgrains and bulging recrys- quartz symplectites. Plag—plagioclase; Opx—orthopyroxene. (E) Wollastonite (Woll) showing a tallization. calcite + quartz rim. (F) Diopside with garnet rim.

silicified basalts, cherts with diagenetic dolomite, or cherts mixed with vol- Calcareous Rocks canoclastic material, in keeping with their association with amphibolites. The mineral assemblage and major element composition of enstatite-cordierite Typical calc-silicate and impure marble assemblages contain calcite and quartzites indicate that the added material is close to the composition of chlo- wollastonite with varying amounts of diopside, large brown andradite-grossu rite, another common product of the hydrothermal alteration of basic volcanic lar garnet, and minor amounts of quartz, plagioclase, and titanite (Fig. 9). The rocks (cf. Vallance, 1967). The Mn quartzites, however, represent more charac- marble bands show extreme deformation with extensive flow folding and teristic oceanic cherts. All three types of quartzite show wide variation in the small enclaves of isoclinally folded calc-silicates and quartzites completely proportion of quartz to silicate minerals, and include bands and segregations enclosed in marble (Figs. 10A, 10B). Similar mixed calc-silicate, marble, and dominated by the silicate assemblage. quartzite bands are found in the normal metamorphic sole, for example in

Figure 9. Calc-silicate lithologies from Bani Hamid. (A) Euhedral andradite garnets in a calc-silicate, Wadi Shis. (B) Thin section showing wollastonite in calc-silicate. (C) Clinopyroxene rich pod (green) within calc-silicate. (D) Mafic diopside-rich pod showing a fold closure enclosed within calc-silicate. (E) Thin section of calc-silicate showing brown andradite garnet, green diop side, and white scapolite. (F) Granoblastic marble showing wollastonite crystal bottom right.

the Sumeini window (Searle and Malpas, 1980, 1982; Searle and Ali, 2009), where they are also thought to represent metamorphosed Haybi Complex distal sedimentary rocks.

Metabasic Rocks

The metabasic units consist of amphibolite and pyroxene granulite, generally with a medium-grained granoblastic texture (~2 mm) and a weak gneissic fabric. While some are simple hornblende + plagioclase ± clinopyroxene amphibolites, a significant proportion are biotite bearing with abundant opaque oxides. The dominant oxide phase is titanohematite. These features suggest that an important protolith is alkali basalt rather than a mid-oceanic ridge basalt (MORB)-like tholeiite. These alkali basalts have a geochemical signature (high bulk Ti) almost identical to that of the alkali basalts in the Haybi Complex (Searle et al., 1980; Searle, 1984). Metamorphosed alkali peridotites (jacupi- rangites, wehrlites) and gabbros were reported (Searle, 1984) from the Wadi Ham metamorphic sequence; this supports the inference that these rocks are metamorphosed equivalents of the Haybi Complex. Lenticular pods of green, almost pure diopside occur along the uppermost part of the Bani Hamid thrust sheet along Wadi Shis. These unusual rocks could be the result of high-temperature calcium-rich metasomatic fluids pumped along the bounding thrust immediately above.

THERMOBAROMETRY AND MICROSTRUCTURES

Determining precise metamorphic conditions for the Bani Hamid granulites has proved problematic because of the lack of rock types and assemblages amenable to conventional thermobarometry. Metapelites are lacking, and metabasic rocks do not contain garnet. Gnos and Kurz (1994) summarized Brey and Köhler (1990). This geothermometer is calibrated for peridotites, the metamorphic geology and described an occurrence of sapphirine coexist- rather than for metabasic and quartz-rich rocks. Nevertheless, the results ap- ing with quartz and aluminous spinel, potentially diagnostic for ultrahigh-tem- pear plausible in comparison with geothermometry on related rocks (Gnos perature metamorphism, in an oxidized enstatite-bearing Mg-Al quartzite. The and Kurz, 1994) and in the context of phase equilibrium constraints. Limits high Fe3+ content of the sapphirine and its intimate association with compos- to pressure were based on experimentally determined constraints: a mini- ite titanohematite–magnetite–spinel oxide grains gave rise to uncertainty over mum of 6.5 kbar was determined from the composition of spinel enclosed the phase relationships and equilibration temperature of the assemblage, and in cordierite, calibrated against the experiments of Seifert and Schumacher so Gnos and Kurz looked to independent methods of pressure-temperature (1986), and a maximum of 9 kbar was calculated from the upper stability limit

(P-T ) estimation; they cited temperatures of 830 ± 20 °C for quartzites and of hydrous Mg-cordierite adjusted for Fe (XMg = 0.9) and the likely volatile 835 ± 44 °C for amphibolites, using the two-pyroxene geothermometer of content of the cordierite.

garnets. Multiequilibrium results using THERMOCALC (Powell and Holland, 1988) in average P-T and average P mode gave 838 ± 286 °C, 14.7 ± 2.8 kbar for a metabasic granulite (OM135), 10.5 ± 1.1 kbar at assumed 850 °C for an Mg-Al quartzite (JC107), and 12.9 ± 2.0 kbar at 860 °C for a Ca-Mg quartzite. These diverse and imprecise pressure results are significantly higher than those of Gnos and Kurz (1994), and overlap those for the main ophiolite sole (Cowan et al., 2013). For this study, two samples were analyzed in order to apply modern equilibrium phase diagram techniques to the problem. The metabasic biotite two-pyroxene granulite OM135 investigated by Cox (2000) was chosen for reanalysis, as it shows the fullest assemblage and least retrogression of the available suite. The peak assemblage is inferred to be plagioclase + clinopyroxene + orthopyroxene + biotite + hematite. Green hornblende forms rims on pyroxene and is interpreted as a retrograde phase. Orthopyroxene shows partial breakdown to amphibole and a fine-grained aggregate with the composition of talc. The second sample is AC13–34, a quartz-rich cordierite-orthopyroxene A gneiss of the Mg-Al-quartzite lithology collected during field work in this study. It has a coarse-grained gneissic texture, and the high-grade minerals show relatively little alteration. Mineral analyses were performed on a JEOL JSM-840A scanning electron microscope (SEM) in the Department of Earth Sciences, University of Oxford, which is equipped with an Oxford Instruments Isis 300 energy-dispersive analytical system. The accelerating voltage was 20 kV, with a beam current of 6 nA, and a live counting time of 50 s. The system was calibrated with a range of natural and synthetic standards, and a ZAF correction procedure was used. The beam current was checked regularly and the count rate calibrated every 120 min using a cobalt metal standard. Representative mineral analyses are listed in Table 1. The rocks show high-variance assemblages and define few useful geothermometers and barometers. However, it is significant that neither the meta basite nor the Mg-Al gneiss contains garnet, in contrast to the main metamorphic sole, in which garnet-clinopyroxene amphibolite and relic garnet granulite enclaves are characteristic at the highest grade. In addition, hornblende-bearing metabasites have locally undergone partial melting. A more useful ap- proach, which can incorporate these constraints on P-T conditions, is to use B calculated equilibrium assemblage diagrams (pseudosections). Figure 10. Deformation textures in calc-silicates from Wadi Shis. (A) Bands of marble enclos- Bulk compositions for the two samples were calculated by combining ing detached pods of highly deformed refolded isoclinal folds in calc-silicate. The marble mean microprobe analyses of homogeneous major phases in proportion to band is bounded by more competent layers of clinopyroxene (Cpx) quartzite on both sides. their modal volume in the rock (cf. Carson et al., 1999). Volume proportions (B) High-temperature deformation fabrics in wollastonite-bearing marble, Wadi Shis. were determined from area measurement using optical microscopy, SEM back- scattered electron imaging, and image analysis of scanned thin sections using the software JMicrovision (Roduit, 2007). The pseudosections were calculated Conventional and multiequilibrium thermobarometry was used by Cox using the software THERMOCALC (Powell and Holland, 1988; Powell et al., 1998) (2000) on a variety of rock types, with scattered results, some of which were version 3.33, using the internally consistent thermodynamic data set of Hol- cited in Searle and Cox (2002). Garnet-clinopyroxene thermometers applied to land and Powell (1998) updated to version 5.5 (November 2003). Solid-solution

calc-silicate assemblages gave results that differ greatly between calibrations, phases were modeled in the system NCKFMASHTO (Na2O-CaO-K2O-FeO-MgO-

presumably because of the unsuitable chemical composition of the Ca-rich Al2O3-SiO2-H2O-TiO2-O) using activity formulations for amphibole from Diener

TABLE 1. REPRESENTATIVE MINERAL ANALYSES USED FOR GEOTHERMOBAROMETRY Sample AC13-34 AC13-34 AC13-34 AC13-34 AC13-34 AC13-34 OM-135 OM-135 OM-135 OM-135 OM-135 OM-135 Mineral Opx Opx Crd Pl Ilm HemCpx OpxPlHbl Bt Hem Analysis core rim mean representative mean of 4mean of 3meanmeanmeanmeanmeanmean

SiO2 48.53 52.69 49.40 51.10 0.14 0.15 51.8654.73 53.7248.20 37.630.07

TiO2 0.18 0.07 0.00 0.07 46.6219.53 0.32 0.08 0.02 0.67 2.70 18.06

Al2O3 7.79 3.06 34.06 32.31 0.26 0.34 3.20 1.87 29.568.8015.32 0.26

Cr2O3 0.11 0.04 0.05 0.07 0.30

Fe2O3 2.52 0.69 12.3561.57 3.23 65.63 FeO 18.59 18.54 3.55 0.00 37.3116.72 6.05 13.900.255.7811.04 15.53 MnO 1.40 1.59 0.27 0.00 3.910.670.290.630.010.210.150.36 MgO 21.29 24.04 11.30 0.05 0.23 0.14 14.2028.110.0516.51 17.450.12 CaO 0.19 0.06 0.01 14.33 0.04 0.03 23.570.5212.33 12.910.030.03

Na2O 0.00 0.00 0.09 3.27 0.060.010.080.004.200.670.10 0.00

K2O 0.00 0.00 0.00 0.09 0.020.000.030.010.080.569.84 0.02 Total 100.48 100.73 98.69 101.22 100.95 99.1699.71 99.89100.2897.61 94.28100.38 Oxygens 6618 833 66823 22 3 Si 1.791 1.923 4.963 2.295 0.003 0.0041.923 1.9602.422 6.8785.586 0.002 Ti 0.005 0.002 0.000 0.003 0.8780.385 0.0090.002 0.0010.072 0.3020.350 Al 0.339 0.132 4.033 1.7110.008 0.0100.140 0.0791.571 1.4812.681 0.008 Cr 0.0030.001 0.0020.007 0.0000.006 Fe3+ 0.070 0.019 0.233 1.2130.348 1.283 Fe2+ 0.574 0.566 0.298 0.000 0.7810.366 0.1880.416 0.0100.691 1.3730.337 Mn 0.044 0.049 0.023 0.000 0.083 0.0150.009 0.0190.000 0.0250.019 0.008 Mg 1.171 1.308 1.692 0.003 0.0090.005 0.7851.500 0.0033.5103.858 0.005 Ca 0.008 0.002 0.001 0.690 0.001 0.0010.937 0.0200.596 1.9750.005 0.001 Na 0.000 0.000 0.018 0.285 0.003 0.0010.006 0.0000.367 0.1860.029 0.000 K 0.000 0.000 0.000 0.005 0.0010.000 0.0020.000 0.0050.102 1.8640.001 Sum 4.000 4.000 11.029 4.992 2.000 2.0004.000 3.9984.977 15.275 15.718 2.000 XMg 0.671 0.698 0.850 0.0110.014 0.8070.783 0.2490.836 0.7380.014 Note: Opx—orthopyroxene; Crd—cordierite; Pl—plagioclase; Ilm—ilmenite; Hem—hematite; Cpx—clinopyroxene; Hbl—hornblende; Bt—biotite.

et al. (2007) and Diener and Powell (2012), garnet, biotite and silicate liquid from as Ca-Tschermak’s molecule, and the Na2O content is low. The plagioclase White et al. (2007), feldspars from Holland and Powell (2003), and Fe-Ti oxides composition is a labradorite with 58% ± 2% anorthite. Biotite flakes oriented in from White et al. (2000). The Green et al. (2007) activity model for clinopyrox- the gneissic foliation have a uniform composition for most elements, with as

ene was found to be unsuitable for high-T pyroxenes containing significant much as 3 wt% TiO2, but show a spread in XMg values from 0.67 to 0.79. The Ca-Tschermak and enstatite solid solution and low jadeite, and an alternative abundant oxide phase is a titanohematite that forms composite grains with

formulation in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2, extended from the some magnetite. The green amphibole, which forms rims on pyroxene grains,

CMAS (CaO-FeO-MgO-Al2O3-SiO2) model developed in Green et al. (2012), was is dominantly magnesiohornblende (6.5–7.1 Si per formula unit, p.f.u.) with a

supplied by T.J.B. Holland (2014, personal commun.). low XNa,M4 of 0.035 and a relatively small content of Na and K in the A site (up to ~0.3). Actinolite is a minority with ~7.6 Si p.f.u. These analyses are consistent with a retrograde origin for amphibole. Metabasite (OM135) Two-pyroxene geothermometers appropriate for metabasic rocks (e.g., Lindsley, 1983) indicate temperatures between 700 and 900 °C but are insuffi- Pyroxene compositions are relatively uniform and do not show obvious ciently precise to improve on the existing peak temperature estimates of be-

compositional zoning. Orthopyroxene has ~2 wt% Al2O3 and ~0.5 wt% CaO. tween 800 and 900 °C. Modeling in granulite facies rocks, where aqueous fluid

The clinopyroxene contains ~3.2 wt% Al2O3, which is accommodated mostly is not in excess, requires attention to the amount and behavior of the compo-

nent H2O. In OM135 the relevant constraints on successful modeling are that water content of the modal minerals and assuming that the cordierite con-

hornblende occurs only as a retrograde phase, and that partial melting has tained ~0.5 H2O per formula unit. The peak assemblage field in the P-T pseudo locally occurred in similar, hornblende-bearing metabasites. Therefore, condi- section (Fig. 11C) is constrained by presence of cordierite (with or without tions for the peak assemblage should be located in an amphibole-free field at biotite) and the absence of garnet or sillimanite or evidence for melting. The or near the solidus. inferred field is between ~750 °C and the calculated solidus at ~900 °C, and In OM135 the calculated assemblage proved to be sensitive to the bulk at pressures below the curve defining the appearance of garnet, which has a

H2O content. At 0.5 mol% H2O hornblende is a ubiquitous phase up to and shallow P-T slope and is at 7.5–8 kbar in the range 750–900 °C, and the limit for

beyond the solidus. Figure 11A shows an isobaric T-m(H2O) section, on which cordierite, which is at ~6.5 kbar at 800 °C. the field of hornblende-bearing assemblages terminates toward low water The key compositional variable is the Al content of orthopyroxene. This is contents, with a steep boundary in the temperature range of interest at ~0.5 a geothermometer in garnet-bearing assemblages, as investigated by Hensen

mol% H2O. On this basis, a water content of 0.4 mol% was chosen for locating and Harley (1990), Carrington and Harley (1995), and Harley (1998). In the nat- the granulite facies equilibration conditions in P-T space. Figure 11A also pre- ural pyroxene and in the solution model used in this study, this is quantified

dicts that at higher bulk-rock H2O contents, hornblende-bearing two-pyroxene as the mole fraction of Al in the M1 site (XAl,M1). The values of this parameter granulite, and clinopyroxene-amphibolite are stable both below and above the are a little lower than the tetrahedral Al content, and of the commonly used solidus between 800 and 900 °C. This is consistent with the observed range of parameter 0.5× (total Al per 6 oxygens) because of the presence of Fe3+ in peak assemblages in Bani Hamid metabasites and with the appearance of melt M1. The calculated isopleths in the garnet-orthopyroxene fields in Figure 11C segregations in amphibole-bearing varieties. It suggests that the variation in are a little shallower in slope than those plotted by Harley (1998; Fig. 4C), but mineral assemblage may be controlled as much by initial water content as by agree well in position in the pressure range 7–10 kbar. In the absence of garnet variation in other major components. and in coexistence with cordierite, the controlling equilibrium is likely to be

The calculated P-T field for the observed solid assemblage (Fig. 11B) occurs 2MgAl2SiO6 (in orthopyroxene) + 3SiO2 = Mg2Al4Si5O18 (cordierite), for which over a broad area bounded at high pressure by the incoming of garnet and at the end-member equilibrium has a large positive volume change, suggesting

high temperature by the solidus. The position of the solidus at a given H2O that the isopleths should be sensitive to pressure. As expected, the isopleths of

content should be of reasonable reliability, because the melt composition is XAl,M1 have a shallow slope in the relevant fields in Figure 11C.

trondhjemitic and well approximated by the melt model of White et al. (2007). In the natural pyroxene, the value of XAl,M1 depends on the accuracy and However, relationships at higher temperature carry uncertainty, because the precision of the entire analysis, and particularly on Si and on the effect of re- activity model for silicate liquid is optimized for granitic compositions and calculation for Fe3+. The total Al content of the core composition group varies 3+ the melting of pelites, rather than for metabasites. from 0.28 to 0.34 Al p.f.u. With allowance for Fe in M1 the mean value of XAl,M1 is 0.135 ± 0.01. This isopleth is in the upper part of the assemblage field just above 6 kbar, in the temperature range 780–910 °C. Mg-Al Quartzite (AC13–34)

This rock is dominated by porphyroblasts of pinkish-brown aluminous en- Peak P-T Conditions

statite (>35 vol%) with uniform XMg = 0.67 ± 0.02 but with varying Al2O3 content.

The maximum Al2O3 of 7.72 wt% was recorded in the core of a 3 mm grain. The P-T estimates for these rocks, and particularly the determination of Rim compositions ranged down to 3.92 wt%, and rim composition profiles over maximum pressure, depend on the calculated position of field boundaries length scales of 20–150 µm consistent with diffusional closure on cooling were in high-variance assemblages. The data set uncertainties as calculated by recorded using qualitative line scans. Cordierite (6–8 vol%) forms small grano- THERMOCALC and based on the least-squares fit to end-member enthalpies

blasts of uniform composition with XMg = 0.85. A small amount of biotite occurs are small, typically ±2–10 °C, or 0.2–0.4 kbar, both on assemblage bound mostly as ragged flakes associated with orthopyroxene and much or most is aries and on composition isopleths. The position of low-variance assemblage likely to be of retrograde origin. Plagioclase is partly sericitized; fresh gains are boundaries is controlled largely by end-member properties and is insensitive calcic (~70% anorthite). The Fe-Ti oxides form complex composite crystals com- to bulk composition. However, for the boundaries of high-variance assem- monly made up of titanohematite showing lamellar exsolution of ilmenite, sur- blages, such as the incoming of garnet in the metabasite OM135, uncertainty rounded by associated magnetite and matrix phases with a moat of Mn-bearing in the bulk composition will dominate. Analytical uncertainty on microprobe ilmenite (see Table 1 for representative analyses). The peak oxide phase is likely analyses adds a small contribution to the bulk composition calculation. In addi to have been a titanohematite with significantly higher Ti content. tion, a certain amount of bias in selecting, grouping, and averaging analyses The modeled bulk composition used an estimated water content appro- is almost unavoidable, as is the extent to which analyses are projected into priate for the near-anhydrous high-grade assemblage, based on the nominal the system that can be modeled, according to the concept of an ideal analysis

1100 12 0 A C 2 OM135 AC13-34 0.08 0.09 0.1 0.11 Cpx Liq 0.1

Grt 11 0.13 Opx Liq Grt Opx Grt Opx 1000 Cpx Bt Cpx Bt Liq Kfs Liq Bt Kfs 0.14 10

0.15 Cpx Opx Bt Liq

9 0.16 ) 900 Hbl Cpx Opx Bt Liq Hbl Cpx Bt Liq Cpx Opx Bt bar)

8 Grt Opx Sil Opx Sil Hbl Cpx Bt Bt Kfs Kfs Liq Opx Liq essure (k emperature (°C Pr T 800 Cpx Opx Bt Kfs 0.16 Hbl Cpx Opx Bt 7 Opx Sil Bt Kfs 0.15 Opx Crd Kfs Liq 6 0.14 Opx Crd 700 0.13 Opx Crd Liq Bt Kfs Opx Crd Kfs 0.12 5 All assemblages 0.11 All assemblages contain Pl, Hem, Mag contain Qz, Pl, Ilm 0.10 600 4 00.1 0.2 0.30.4 0.5 0.6 0.7 0.8 0.9 1 700750 800850 900 950 1000 1050 1100 Mole % H2O Temperature (°C)

14 B OM135 Cpx Grt Cpx Grt 13 Bt Qz Bt Liq Cpx Grt t Bt Mag Liq Bt Kfs Qz x Gr 12 Cp t Bt x Gr Cp Figure 11. Calculated phase relations for Bani Hamid samples, using bulk compositions (in mol% g 11 oxides). OM135: H O 0.40, SiO 44.955, Al O 10.010, CaO 13.676, MgO 8.774, FeO 13.481, K O t Bt Ma 2 2 2 3 2 x Gr t Cp 0.365, Na2O 2.174, TiO2 1.916, O 4.248. AC13–34: H2O 0.196, SiO2 64.390, Al2O3 3.994, CaO 0.548, x Gr MgO 16.720, FeO 10.609, K O 0.192, Na O 0.307, TiO 1.796, O 1.249. (A) T–m(H O) section for 10 Cp s 2 2 2 2 Bt Kf OM135 metabasite. Solidus emphasized with solid purple curve. Peak assemblage is in field

bar) labeled Cpx Opx Bt. Abbreviations: Cpx—clinopyroxene, Opx—orthopyroxene, Bt—biotite, Liq— liquid, Pl—plagioclase, Kfs—K-feldspar, Hem—hematite, Mag—magnetite, Hbl—hornblende. 9 Cpx Bt Vertical dotted line indicates the water content chosen for the pressure-temperature (P-T ) sec- Mag Liq ssure (k tion. (B) P-T section for OM135 metabasite, with solidus emphasized in solid purple and low-P

Pre 8 limit of garnet (Grt) stability marked by dashed red curve. Observed peak assemblage occupies Cpx the large field labeled Cpx Opx Bt Mag. Qz—quartz. (C) P-T section for orthopyroxene-quartzite Cpx Opx Bt Mag Opx AC13-34. Solidus emphasized in purple, low-P limit of garnet stability in red, high-P limit of Bt Kfs cordierite (Crd) stability in blue. Contours of X in orthopyroxene are shown for subsolidus 7 Mag Al,M1 Cpx Mag fields. The area consistent with the observed peak assemblage and Al content of Opx is shaded Liq in gray. Ilm—ilmenite; Sil—sillimanite. 6 Cpx Opx Bt Mag 5 Liq All assemblages contain Pl, Hem 4 700750 800850 900 950 1000 1050 1100 Temperature (°C)

discussed by Powell and Holland (2008). In practice, however, the largest un- observed Th/U of modern mid-oceanic ridge basalts, as discussed in Rioux certainty is probably the quantification of modal proportions of minerals for et al. (2015). All 206Pb/238U dates discussed here for these samples are Th the bulk composition. A test in which the volume proportions of all phases in corrected. OM135 were randomly varied with a normally distributed relative uncertainty Isotope dilution–thermal ionization mass spectrometry (ID-TIMS) U-Pb data of 10% (2s) propagated to an uncertainty of ~±30 °C (2s) in the position of the are reported in Supplemental Table 11 and plotted in Figure 12. Zircons from orthopyroxene-out curve, ±20 °C for the solidus curve over the pressures of 131205M04 had low U concentrations and yielded 0.3–2.1 pg of radiogenic Pb interest, and ±0.9 kbar for the incoming of garnet at 850 °C. In contrast, the (Pb*), resulting in relatively large uncertainties for 4 of the 6 dated fractions. position of the garnet-in boundary in the orthopyroxene-quartzite AC13–34 is The data define a single concordant population (mean square of weighted de-

Supplemental Table 1. ID-TIMS U-Pb zircon data

Composition Isotopic Ratios Dates

Uncorrected Th-corrected (Th/Umelt = 2.63 ± 0.90) Uncorrected Th-corrected (Th/Umelt = 2.63 ± 0.90) Pb*/ b Pb*c Pbcc Th/d206 Pb/e208 Pb/f206 Pb*/f 2 207 Pb*/f 2 207 Pb*/f 2 corr.g206 Pb*/f 2 207 Pb*/f 2 corr.g 206 Pb/h 2 207 Pb/h 2 207 Pb/h 2 206 Pb/h 2 207 Pb/h 2 not sensitive to bulk composition variation. This is largely due to the low Ca viates [MSWD] of the weighted mean = 0.54), and the 2 most precise analyses Pbc (pg) (pg) U 204 Pb 206 Pb 238 U(% err) 235 U(% err) 206 Pb* (% err) coef. 238 U(% err) 206 Pb*(% err) coef. 238 U(abs) 235 U(abs) 206 Pb (abs) 238 U(abs) 206 Pb (abs)% disc.i

131205M04, amphibolite z1 11.1 2.05 0.18 0.470 690.4 0.14987 0.0147615 0.151 0.09862 1.69 0.04848 1.63 0.426 0.01477540.151 0.04843 1.63 0.425 94.463 0.141 95.501.54121.5 38.5 94.552 0.141 119.238.522.2 z2 6.8 1.34 0.20 0.503 425.7 0.16046 0.0147392 0.225 0.09586 2.68 0.04719 2.62 0.324 0.01475290.225 0.04715 2.62 0.323 94.322 0.211 92.952.38 57.9 62.4 94.409 0.211 55.7 62.4 -63.0 206 238 z3 3.2 0.67 0.21 0.505 211.9 0.16115 0.0147505 0.478 0.09572 6.36 0.04708 6.20 0.364 0.01476410.478 0.04704 6.20 0.363 94.393 0.448 92.825.64 52.4 148.094.4800.448 50.2 148.1-80.0 content of the rock, leading to the control of garnet and cordierite stability yielded Pb/ U dates of 94.55 ± 0.14 Ma and 94.41 ± 0.21 Ma. We interpret z4 1.5 0.30 0.21 0.427 107.4 0.13631 0.0148046 0.945 0.09886 11.85 0.04845 11.59 0.305 0.01481880.943 0.04841 11.590.305 94.737 0.888 95.7310.82 120.5273.2 94.827 0.888 118.2273.3 21.4 z5 2.0 0.53 0.27 0.434 139.3 0.13835 0.0147095 0.808 0.10068 8.07 0.04966 7.90 0.258 0.01472370.806 0.04961 7.90 0.258 94.133 0.755 97.407.49 178.2184.2 94.223 0.754 175.9184.3 47.2 z6 2.2 0.86 0.40 0.307 155.3 0.09787 0.0147299 0.527 0.09934 6.91 0.04894 6.80 0.253 0.01474490.526 0.04889 6.80 0.252 94.263 0.494 96.176.34 143.7159.5 94.358 0.493 141.3159.6 34.4

131205M05, plagioclase+amphibole dike iz1 18.6 26.38 1.42 0.398 1164.5 0.12699 0.0144746 0.087 0.09572 0.84 0.04798 0.82 0.276 0.01448900.087 0.04793 0.82 0.274 92.641 0.080 92.810.7597.319.592.7320.080 94.9 19.5 4.8 iz2 86.1 34.99 0.41 0.277 5515.7 0.08834 0.0144489 0.060 0.09571 0.22 0.04806 0.19 0.538 0.01446410.060 0.04801 0.19 0.536 92.477 0.055 92.810.20101.3 4.692.5740.056 98.8 4.68.7 by well-constrained equilibria in the subsystem FMAS(H) [FeO-MgO-Al2O3- these dates to reflect the timing of metamorphic zircon growth. Zircons from iz3 89.1 33.15 0.37 0.362 5578.6 0.11536 0.0144838 0.072 0.09575 0.23 0.04797 0.20 0.531 0.01449840.072 0.04792 0.20 0.529 92.699 0.066 92.850.2096.74.7 92.792 0.066 94.3 4.74.2 z1 69.3 35.01 0.51 0.431 4258.0 0.13760 0.0144743 0.100 0.09580 0.28 0.04802 0.25 0.451 0.01448850.100 0.04797 0.25 0.450 92.639 0.092 92.890.2599.35.9 92.729 0.092 97.0 5.96.7 z2 94.8 26.02 0.27 0.496 5723.0 0.15825 0.0145032 0.064 0.09593 0.25 0.04800 0.23 0.444 0.01451700.064 0.04795 0.23 0.440 92.822 0.059 93.020.2398.05.5 92.910 0.059 95.7 5.55.3 z3 44.3 8.07 0.18 0.604 2605.7 0.19271 0.0144962 0.074 0.09591 0.48 0.04801 0.45 0.393 0.01450930.074 0.04797 0.45 0.390 92.778 0.068 93.000.43 98.6 10.8 92.861 0.069 96.5 10.8 5.9 z4 96.9 21.44 0.22 0.689 5564.0 0.21983 0.0144994 0.064 0.09577 0.25 0.04793 0.23 0.501 0.01451190.065 0.04788 0.23 0.494 92.798 0.059 92.860.2394.65.4 92.878 0.060 92.5 5.41.9 z5 83.2 23.35 0.28 0.465 5064.5 0.14849 0.0144953 0.056 0.09581 0.24 0.04796 0.22 0.449 0.01450920.057 0.04792 0.22 0.445 92.772 0.052 92.900.2296.35.3 92.860 0.052 94.0 5.33.7 SiO2 (H2O)]. 131205M05 yielded precise results that define a spread of data along concor- z6 15.5 13.17 0.85 0.390 976.1 0.12439 0.0144836 0.200 0.09538 1.19 0.04778 1.15 0.290 0.01449800.200 0.04774 1.15 0.290 92.697 0.184 92.501.05 87.5 27.2 92.789 0.184 85.2 27.2 -5.9

a All digestions carried out in Parr acid digestion vessels held at 210ºC for 48–72 hours. 206 238 b Ratio of radiogenic to common Pb. c Total radiogenic Pb, common Pb and U (picograms). On this basis, the best estimate of peak equilibration for AC13–34 is 850 ± dia, with Pb/ U dates of 92.906 ± 0.059 to 92.572 ± 0.055 Ma. To image the d Th/U ratio calculated from 208 Pb/206 Pb and the 206 Pb/ 238U date of the sample. e Fractionation and spike corrected isotopic ratios, reduced using EARTHTIME ET535 tracer calibration v.3. f Fractionation, spike, and blank corrected radiogenic isotope ratios. Laboratory blanks were corrected using 206Pb/ 204Pb = 18.416 ± 0.698, 207Pb/ 204Pb = 15.358 ± 0.452, 208 Pb/ 204Pb = 37.461 ± 1.470. g Correlation coefficient of radiogenic 207Pb* /235U and 206 Pb*/238 U. h Dates (Ma) calculated using 238U/235 U = 137.818 (Hiess et al., 2012), and decay constants of 238U = 1.5513 x 10-10 yr-1 and 235 U = 9.8485 x 10-10 yr-1 (Jaffey et al., 1971). 60 °C and 6.3 ± 0.5 kbar. This is consistent with the assemblage field for the meta structure of the 131205M05 zircons, we mounted 14 grains and imaged them i % discordance = 100 - (100 * ( 206 Pb/238 U date) / (207Pb/ 206 Pb date)) basite OM135, and may be taken as the best estimate of peak equilibration by cathodoluminescence (CL). The imaged grains had relatively simple zoning 1Supplemental Table 1. Isotope dilution–thermal ion- for the Bani Hamid granulites. It plots at the low end of the pressure range es- (Supplemental Fig. 12), with no clear evidence for the presence of older cores ization mass spectrometry (ID-TIMS) U-Pb zircon data. Please visit http://dx .doi .org /10 .1130/GES01199 .S1 or timated by Gnos and Kurz (1994). In contrast, the variable and higher pressure or younger rims. After imaging, we plucked three of the imaged zircons out of the full-text article on www.gsapubs.org to view Sup- estimates of Cox (2000) were based on imprecise multiequilibrium thermo the mount for dating (iz1–iz3). Given the absence of clear cores or rims in the plemental Table 1. barometry that was not checked for consistency with the observed mineral imaged zircons, we interpret the spread in the U-Pb dates from this sample to 3 iz1iz2 iz3 assemblages. Assuming a relatively dense overburden of 2900 kg/m that reflect assimilation of slightly older whole zircons from adjacent amphibolites includes mantle material, this pressure is equivalent to peak burial at 22.5 ± and melt pods. Following this interpretation, the youngest zircon date provides 2 km. Neglecting the constraint of the Al content of orthopyroxene, the upper the best estimate of the crystallization age of the dike (92.572 ± 0.055 Ma). limit of the observed assemblage field and the incoming of garnet are at ~7.5 Styles et al. (2006) dated four samples from the Bani Hamid thrust sheet iz4iz5 iz6 kbar, placing a maximum depth of burial for the Bani Hamid unit at ~27 km. by ID-TIMS U-Pb zircon geochronology. Reported dates are “concordia dates” calculated in Isoplot (Ludwig, 2003). The dates are not Th corrected, and cor- recting the ages would likely increase the reported dates by 80–100 k.y. A date U-Pb GEOCHRONOLOGY of 92.43 ± 0.15 Ma was obtained from a felsic melt pod in an amphibolite, while iz7iz8 iz9 an impure marble gave a zircon date of 92.72 ± 0.39 Ma. Also reported were U-Pb Zircon Geochronology by Isotope Dilution– dates for a granite intruding calc-silicate at 93.22 ± 0.29 Ma and a mafic peg- Thermal Ionization Mass Spectrometry matite sill intruding quartzite at 91.84 ± 0.18 Ma. The dates from the felsic melt

iz10 iz11 iz12 pod and impure marble are within uncertainty of our new dates from the small In order to date the timing of metamorphism in the Bani Hamid thrust sheet, discordant vein, suggesting peak metamorphism and melting ca. 92.6 Ma. we carried out isotope dilution–thermal ionization mass spectrometry U-Pb zircon dating on an amphibolite (131205M04; Universal Transverse Mercator, iz13 iz14 UTM, coordinates 0423233 2795438) and a crosscutting dike (131205M05; UTM U-Pb Titanite Geochronology, Titanite Trace Element Analysis, and 0423284 2795477). The sampled dike is ~5 cm wide, consists of plagioclase + Zr-in-Titanite Thermometry by Laser-Ablation Split-Stream Inductively amphibole, and crosscuts the amphibolite foliation; the dike composition is Coupled Plasma Mass Spectrometry similar to other nearby leucocratic pods and segregations concordant with the Figure DR1. Cathodoluminescence (CL) images of zircons from sample 131205M05. Zircons iz1–iz3 were plucked out of the mounts and dated after imaging (Figure 12, Table DR1). CL images amphibolite foliation, and all are probably related to localized melting in this All titanites in this study were analyzed in thin section by laser-ablation were taken on a JEOL Superprobe JXA-733 at the Massachusetts Institute of Technology. area. Zircon dissolution and U-Pb analyses were carried out in the radiogenic split-stream inductively coupled plasma mass spectrometry at the Univer- 2Supplemental Figure 1. Cathodoluminescence (CL) isotope laboratory at the Massachusetts Institute of Technology (Cambridge, sity of California, Santa Barbara; U-Pb and trace element data were collected images of zircons from sample 131205M05. Zircons Massachusetts). Single zircons and grain fragments were dissolved following simultaneously on the same spot analyses. U-Pb data were reduced using BLR iz1–iz3 were plucked out of the mounts and dated after imaging (Fig. X; Supplemental Table 1). CL images the chemical abrasion method (Mattinson, 2005), modified for single grain (Aleinikoff et al., 2007) as a primary titanite standard and at least two secondary were taken on a JEOL Superprobe JXA-733 at the analyses, as described in Rioux et al. (2012b, supplemental material). The standards were included in each run as further monitors of accuracy. Trace ele Massachusetts Institute of Technology. Please visit 206Pb/238U dates were corrected for initial exclusion of 230Th during zircon crys- ments were reduced with 43Ca as an internal standard, assuming 19.25 wt% http://dx .doi .org /10 .1130/GES01199 .S2 or the full- text article on www.gsapubs.org to view Supplemen- tallization using the Th/U of the analyzed zircon, calculated from the mea- total Ca in titanite. We used NIST SRM 610 glass (Pearce et al., 1997; Rocholl tal Figure 1. sured 206Pb/238U date and 208Pb/206Pb, and a melt Th/U = 2.63 ± 0.90, based on et al., 1997) as a primary standard for all trace elements in JC98 and for rare

0.0151 0.01456 AB96.5 131205M04 131205M05 93.1 amphibolite 96.0 plag-amph dike 93.0 995.5.5 0.0149 0.01452 92.99292.9 U 95.0 U 238 238 92992.82.82 8 94.5 Pb/ Pb / 92.7.7 206 206 0.0147 94.0 0.01448 3Supplemental Text File. Titanite analytical methods. Please visit http://dx .doi .org /10 .1130/GES01199 .S3 92.6 93.5 or the full-text article on www.gsapubs.org to view the Supplemental Text File. 92.5 93.0 Primary TE RM: Primary TE RM: Primary TE RM: 0.01444 NIST SRM 610 TNT150 TNT1500 0.0145 92.4 10000 10000 10000

1000 1000 1000 JC106 100 100 100 10 10 10 0.085 0.095 0.105 0.115 0.094 0.095 0.096 0.097

(titanite unknown) 1 1 1

0.1 0.1 0.1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho YErTmYbLu La Ce Pr Nd Sm Eu Gd Tb Dy Ho YErTmYbLu La Ce Pr Nd Sm Eu Gd Tb Dy Ho YErTmYbLu 207Pb/235U 207Pb/235U

10000 10000 10000

1000 1000 1000 BLR 100 100 100 Figure 12. U-Pb concordia diagrams of single-grain and grain fragment zircon isotope dilution–thermal ionization mass spectrometry dates (ages on concordia are in Ma). (titanite RM) 10 10 10 1 1 1 238 235 La Ce Pr Nd Sm Eu Gd Tb Dy Ho YErTmYbLu La Ce Pr Nd Sm Eu Gd Tb Dy Ho YErTmYbLu La Ce Pr Nd Sm Eu Gd Tb Dy Ho YErTmYbLu All data are corrected for initial Th exclusion. Gray bands represent 2σ uncertainties on concordia based on decay constant uncertainties of 0.107% ( U) and 0.136% ( U) 10000 10000 10000 (Jaffey et al., 1971). Data reduction and plotting were done using the Tripoli and U-Pb_redux software packages (Bowring et al., 2011; McLean et al., 2011). Abbreviations: 1000 1000 1000 C5-I3B 100 100 100 plag—plagioclase; amph—amphibole. (titanite RM) 10 10 10

1 1 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho YErTmYbLu La Ce Pr Nd Sm Eu Gd Tb Dy Ho YErTmYbLu La Ce Pr Nd Sm Eu Gd Tb Dy Ho YErTmYbLu

10000 10000 10000

1000 1000 1000 Y1710C5 100 100 100

(titanite RM) 10 10 10

1 1 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho YErTmYbLu La Ce Pr Nd Sm Eu Gd Tb Dy Ho YErTmYbLu La Ce Pr Nd Sm Eu Gd Tb Dy Ho YErTmYbLu earth elements (REE) in JC106; TNT150 titanite glass (Klemme et al., 2008) was quartz, and therefore aSiO2 is uncertain, we did not attempt Zr-in-titanite ther- Figure DR2: Chondrite-normalized REE patterns for four titanite samples. Note the similar pattern regardless of trace element (TE) primary reference material (RM) used. used as a primary standard for Zr in JC106. Al, Si, Fe, Ti, Zr, and other elements mometry for this sample. were included in the trace element routine for both samples to monitor for Sample JC106 is a diopside quartzite, with hornblende and epidote over- 4Supplemental Figure 2. Chondrite normalized rare contamination by other phases. All chondrite-normalized REE patterns were growths on diopside interpreted as retrograde phases (Fig. 13B). Titanite occurs earth element patterns for four titanite samples. Note the similar pattern regardless of trace element (TE) calculated using chondritic values from McDonough and Sun (1995). Further in two textural settings: smaller, 200–400-µm-diameter subhedral grains are primary reference material (RM) used. Please visit details of the method are contained in the Supplemental Text File3 and Supple- present in the quartz-dominated matrix, whereas larger subhedral to euhedral http://dx .doi .org /10 .1130/GES01199 .S4 or the full- mental Figure 24, and additional information is available in Spencer et al. (2013) grains (to 1200 µm diameter) are associated with diopside-dominated ag- text article on www.gsapubs.org to view Supplemen- tal Figure 2. and Kylander-Clark et al. (2013). Complete U-Pb and trace element data are gregates. Grains from both textural settings were analyzed, and define three available in Supplemental Table 25. Stated 2s date uncertainties in this study distinct trace element populations: (1) a light (L)REE-enriched, high-Zr pop- Supplemental Table 2. Complete titanite U-Pb and trace element LASS data

Sample Composition Isotopic Ratiosa Trace Elementsa calculated with NIST SRM 610 as primary RM Pb U 238U/ 207Pb/AlSiP Ti VCrFe include in-run errors and decay constant errors only; titanite dates determined ulation defined by spots within the core of a single matrix grain, (2) a less ppm ppm 206Pb ± 2 SE 206Pb ± 2 SE ppm ± 2 SE ppm ± 2 SE ppm± 2 SE ppm± 2 SE ppm± 2 SE ppm± 2 SE ppm JC98, marble 10.4917.26 43.84 1.56 0.3634 0.0121 1.32E+04 1.60E+03 1.30E+05 2.00E+04 150262.48E+053.30E+04347 30 30.1 6.610040 20.4418.42 49.33 1.59 0.3080 0.0117 1.32E+04 1.30E+03 1.56E+05 1.90E+04 192442.86E+052.70E+04423 42 26.2 4.21.05E+04 30.5213.68 40.31 1.32 0.4000 0.0144 #1.62E+04 2.50E+03 1.42E+05 1.10E+04 160212.89E+052.60E+04389 23 32.6 3.79280 by this method have an external uncertainty of 2% (explained in Supplemental LREE-enriched, lower Zr population defined by matrix rims and some whole 40.6610.36 28.41 1.34 0.5300 0.0184 1.59E+04 2.50E+03 1.42E+05 1.90E+04 186282.74E+058.90E+03383 45 33.7 4.39.80E+03 50.3419.58 55.01 1.90 0.2412 0.0101 1.40E+04 1.90E+03 1.49E+05 8.70E+03 158172.63E+053.70E+04395 39 31.3 7.410220 60.3310.32 45.58 2.02 0.3200 0.0182 1.50E+04 1.60E+03 1.49E+05 1.20E+04 175353.08E+052.40E+04401 50 32.2 6.21.02E+04 70.2611.84 58.21 3.01 0.2050 0.0156 1.45E+04 1.40E+03 1.41E+05 1.70E+04 149412.84E+051.30E+04417 34 25.2 3.69400 80.5314.09 38.51 1.66 0.4240 0.0164 1.47E+04 1.20E+03 1.46E+05 8.50E+03 165203.11E+052.40E+04426 35 28.6 6.59740 Text File). Zr-in-titanite temperatures were calculated using the Hayden et al. matrix grains, and (3) an LREE-depleted, low-Zr population from large grains in 90.2421.51 64.94 2.23 0.1352 0.0093 1.60E+04 1.60E+03 1.49E+05 1.50E+04 182332.75E+052.60E+04368 28 34.9 4.911280 10 0.61 14.91 36.95 1.48 0.4480 0.0158 14840 5801.44E+05 1.70E+04 203382.86E+053.60E+04409 42 35.2 4.81.01E+04 11 0.48 29.04 55.25 1.54 0.2180 0.0082 1.34E+04 1.20E+03 1.52E+05 2.30E+04 194532.94E+054.80E+04454 67 34.7 3.31.14E+04 12 0.29 15.52 59.74 2.15 0.1760 0.0115 1.54E+04 1.80E+03 1.65E+05 1.60E+04 174342.71E+051.60E+04438 57 31.3 5.810490 13 0.50 13.03 39.23 1.58 0.3840 0.0151 1.54E+04 1.70E+03 1.58E+05 1.80E+04 158393.00E+052.80E+04420 34 30.7 5.51.09E+04 14 0.35 13.59 50.13 2.11 0.2710 0.0123 1.43E+04 1.10E+03 1.44E+05 1.80E+04 185312.85E+053.20E+04414 38 27.5 3.51.03E+04 (2008) calibration; we report propagated external 2s uncertainties on the tem- contact with diopside (Fig. 13B). The first two populations yield appreciable U 15 0.23 12.89 58.89 2.51 0.2000 0.0126 13330 8601.48E+05 7.10E+03 190262.65E+052.30E+04383 30 35.5 5.89.50E+03 16 0.45 21.38 49.85 1.65 0.2802 0.0099 1.43E+04 1.30E+03 1.41E+05 2.10E+04 169222.82E+052.90E+04390 35 31.6 5.911160 17 0.24 17.54 59.17 2.17 0.1378 0.0103 1.39E+04 2.10E+03 1.45E+05 7.60E+03 172302.63E+052.50E+04360 30 34.9 6.11.09E+04 18 0.27 14.34 56.15 2.31 0.1710 0.0125 1.54E+04 1.40E+03 1.54E+05 1.80E+04 197252.96E+052.50E+04449 53 34.9 4.510770 19 0.51 19.33 45.21 1.62 0.3180 0.0145 1.38E+04 1.10E+03 1.59E+05 1.80E+04 209352.79E+051.70E+04435 29 40.4 4.91.19E+04 20 0.40 20.45 52.47 1.56 0.2710 0.0123 1.61E+04 2.20E+03 1.50E+05 1.20E+04 182412.93E+053.00E+04404 43 34.5 6.71.17E+04 perature results, which include errors in Zr abundance, pressure, aTiO and the for dating, whereas the third population had low U concentrations and the Pb 21 0.53 22.26 60.75 1.86 0.1751 0.0095 1.50E+04 1.20E+03 1.51E+05 9.70E+03 180192.78E+052.70E+04385 41 34.9 6.511940 2 22 0.25 22.4 61.69 1.99 0.1582 0.0082 1.51E+04 1.20E+03 1.45E+05 1.50E+04 177202.93E+051.30E+04410 39 36.8 411170 23 0.26 20.03 61.27 2.09 0.1504 0.0099 1.41E+04 1.50E+03 1.31E+05 1.50E+04 175212.72E+052.50E+04372 34 29.4 51.08E+04 24 0.35 18.58 54.35 2.03 0.2519 0.0108 1.55E+04 2.60E+03 1.49E+05 1.40E+04 193262.79E+054.30E+04398 50 29.8 3.913010 25 0.61 16.6 37.20 1.30 0.4412 0.0126 1.50E+04 1.10E+03 1.42E+05 5.60E+03 176342.80E+051.30E+04408 29 28.5 51.22E+04 thermodynamic calibration of the thermometer. analyses were dominated by common Pb; only those spots from the third pop- 26 0.29 21.04 58.28 1.82 0.1937 0.0079 1.66E+04 1.60E+03 1.46E+05 1.80E+04 181312.94E+054.20E+04375 28 36.1 3.11.19E+04 27 0.30 20.25 59.95 2.07 0.1769 0.0079 1.59E+04 3.10E+03 1.31E+05 1.40E+04 200342.72E+053.80E+04372 41 35.8 2.91.14E+04 28 0.53 22.42 62.07 2.29 0.1392 0.0086 13700 9901.45E+05 9.60E+03 174232.79E+051.20E+04359 25 31.4 2.812000 29 0.13 16.61 59.74 2.24 0.1730 0.0093 1.40E+04 2.10E+03 1.52E+05 1.70E+04 225292.68E+052.80E+04374 36 31 6.512290 30 0.22 19.82 59.74 1.86 0.1655 0.0084 1.48E+04 1.80E+03 1.52E+05 1.70E+04 153242.89E+053.10E+04363 60 33.2 1.81.27E+04 31 0.18 19.72 61.69 1.79 0.1474 0.0097 1.44E+04 1.90E+03 1.35E+05 1.70E+04 100162.56E+052.50E+04374 28 31.9 6.11.17E+04 Sample JC98 is a calcite + diopside + wollastonite + plagioclase + titanite + ulation with U above detection limits (and therefore sufficient precision in the 32 0.27 17.24 55.16 1.83 0.2150 0.0109 14760 9401.43E+05 1.30E+04 159242.84E+051.40E+04375 14 33.2 5.412740 33 0.30 17.01 54.70 1.67 0.2420 0.0120 1.43E+04 1.00E+03 1.41E+05 1.10E+04 160182.53E+053.00E+04364 34 25.5 5.21.19E+04 34 0.22 19.06 63.41 2.31 0.1459 0.0093 1.57E+04 1.60E+03 1.48E+05 1.90E+04 181362.79E+054.00E+04379 51 29.4 3.21.22E+04 35 0.28 17.1157.11 2.04 0.2022 0.0100 1.43E+04 1.90E+03 1.51E+05 1.80E+04 158272.64E+054.40E+04386 47 28.3 3.811390 36 0.19 15.96 58.21 2.14 0.2016 0.0107 1.44E+04 1.20E+03 1.46E+05 1.90E+04 195202.48E+052.30E+04340 42 25.1 6.511480 238 206 37 0.16 16.68 57.74 2.06 0.1741 0.0089 1.35E+04 1.20E+03 1.32E+05 1.10E+04 177212.63E+051.70E+04354 36 30.6 3.91.16E+04 garnet marble (Fig. 13A). Minor alteration of diopside, wollastonite, and U/ Pb ratio) were included in the isochron calculation. All three populations 38 0.18 17.7 59.35 2.12 0.1578 0.0095 1.43E+04 1.10E+03 1.46E+05 1.00E+04 143272.95E+051.30E+04369 35 38.9 3.61.22E+04 39 0.20 17.94 59.63 1.80 0.1770 0.0116 1.36E+04 1.40E+03 1.38E+05 9.10E+03 120162.60E+052.20E+04343 39 37.4 1.711600 40 0.21 18.47 63.05 2.03 0.1560 0.0091 1.48E+04 1.20E+03 1.44E+05 1.60E+04 144272.75E+052.60E+04348 12 28.2 4.312280 41 0.29 17.51 54.76 2.08 0.2120 0.0118 1.55E+04 1.80E+03 1.44E+05 8.90E+03 188352.79E+051.90E+04358 23 40.8 6.91.22E+04 42 0.82 21.09 36.75 1.17 0.4456 0.0127 1.41E+04 1.10E+03 1.33E+05 1.80E+04 190212.64E+051.80E+04350 37 30.4 21.12E+04 43 0.23 17.47 57.44 1.88 0.2060 0.0117 1.56E+04 1.60E+03 1.49E+05 1.20E+04 152202.86E+051.90E+04370 29 33.7 5.812260 plagioclase are observed in thin section; calcite, titanite, and garnet are pris- plot on a single, well-defined isochron. The regressed lower intercept date for 44 0.20 18.01 60.98 2.13 0.1563 0.0091 15520 9001.45E+05 7.80E+03 119242.74E+052.70E+04388 22 33.2 4.212520 45 0.17 16.86 59.74 2.06 0.1590 0.0105 1.48E+04 1.20E+03 1.43E+05 1.80E+04 146192.86E+052.20E+04396 28 32.8 4.21.29E+04 tine. Subhedral 100–300-µm-diameter titanites generally occur in association this sample is 89.8 ± 1.5 Ma (MSWD = 0.79). 5Supplemental Table 2. Complete titanite U-Pb and with diopside and plagioclase, and more rarely as isolated grains surrounded Zr-in-titanite temperatures were calculated for JC106 at the peak pressure trace element laser-ablation split-stream (LASS) data. by calcite. Two ~200-µm-diameter grains in contact with diopside define a reported earlier in this study (6.3 ± 0.5 kbar), and values of a = 1.0 and a = Please visit http://dx .doi .org /10 .1130/GES01199 .S5 SiO2 TiO2 or the full-text article on www.gsapubs.org to view single age and trace element population (Fig. 13A). The regressed lower inter 0.75 ± 0.25 were adopted to account for the presence of quartz and titanite Supplemental Table 2. cept date for this sample is 91.0 ± 1.1 Ma (MSWD = 1.4). Because JC98 lacks without rutile. Weighted mean averages of these results yield a temperature

A C

Figure 13. Titanite U-Pb and trace element data from Bani Hamid. (A) Tera-Wasser burg concordia diagram and chondrite-normalized rare earth element (REE) plot for marble sample JC98. The data define a single age and trace element population. CI—carbonaceous chondrite; MSWD—mean square of weighted deviates. (B) Tera-Wasserburg concordia diagram and chondrite-normalized REE plot for quartzite sample JC106. The data yield three distinct trace element populations, identified by different colors and discussed in the text. One of the populations (in green) contained only a subset of spot analyses with sufficient U for isotopic analysis. The three trace element populations plot on a single isochron. (C) Zr-in-titanite temperatures and propagated external 2σ uncertainties for the three trace element populations in JC106; colors match those in B. The legend for all symbols is shown in the inset box and is discussed in the Supplemental Text File.

of 792.7 ± 28.8 °C for the first trace element population (matrix cores), 743.3 ± Pb closure temperatures for the analyzed titanite grains, for cooling rates of 26.2 °C for the second population (matrix rims and some whole grains), and 10–100 °C/m.y., are <700 °C for both JC98 and JC106 (Dodson, 1973; Cherniak, 732.0 ± 25.7 °C for the third population (grains in diopside aggregates) (Fig. 1993). These relationships indicate that the reported titanite dates are cooling 13C). Along with textural constraints, the calculated Zr-in-titanite tempera- ages from high temperature. The U-Pb zircon and titanite dates suggest a pro- tures suggest that titanite neo- or recrystallization occurred at or near peak tracted metamorphic history over 4.8 ± 1.5 m.y. from 94.55 ± 0.14 Ma to 89.8 ± metamorphic conditions and continued during cooling. However, calculated 1.5 Ma for the Bani Hamid granulites.

METAMORPHIC SUMMARY AND PROTOLITHS

The tectonic stratigraphy of the Bani Hamid granulites appears to be a sequence with metaalkali basalts along the base overlain by carbonates, calc-silicates, and quartzites at higher levels. Almost no clastic pelitic rocks are exposed. There are several possible unmetamorphosed protolith sources, notably (1) the Sumeini Group shelf-slope carbonates, (2) proximal Hamrat Duru Group turbidites, in which there are stratigraphic horizons of carbonates (e.g., Nayid Formation), sandstones (Guwayza Formation), and cherts (e.g., Sidr Formation), (3) distal Hawasina thrust sheet composed mainly of abyssal radiolarian cherts, and (4) Haybi Complex thrust sheet immediately beneath the Semail ophiolite, composed mainly of Late Triassic Oman Exotic reefal limestones above an alkali basaltic substrate, cherts, and mélanges. The Sumeini Group comprises more than 700 m stratigraphic thickness of well-bedded limestone conglomerates and calcarenites of platform margin and slope facies origin (Fig. 4) with almost no chert or volcanic components (Watts, 1987, 1990; Searle and Cooper, 1986; Cooper et al., 2014). The stratigraphy of the Sumeini Group directly correlates to the time-equivalent shelf carbonates on the Arabian autochthon (Glennie et al., 1973, 1974; Robertson and Searle, 1990; Searle et al., 1983, 1990). Although the Bani Hamid calc-silicates could be derived from Sumeini-like lithologies, the lack of siliceous cherts or volcanics makes this unlikely to be the protolith of the Bani Hamid granulites. The shelf slope-proximal basin facies Hamrat Duru Group shows a range of lithologies including limestone and sandstone turbidites, two horizons of cherts (Late Triassic and Late Jurassic), and some shaley horizons, with rare basaltic sills (Cooper, 1988, 1990; Bernoulli and Weissart, 1987; Béchennec et al., 1990; Blechschmidt et al., 2004). These rocks could be suitable protoliths for the Bani Hamid granulites except for the lack of major volcanic horizons and the fact that, whereas the quartzite and carbonate units in Bani Hamid are relatively thick, the Hamrat Duru units are thin and interbedded. The distal Hawasina Complex rocks are unlikely protoliths of the Bani Hamid granulites, because although the abundant cherts would be suitable source rocks for the two-pyroxene quartzites, there is almost no carbonate material and only a very few minor basaltic intrusives in the Halfa, Hulw, and Shamal Formations (Glennie et al., 1974). The most suitable protolith sources for the Bani Hamid granulites are Figure 14. Composite tectonic-stratigraphic section through the distal Hawasina Complex and the the rocks seen in the most distal allochthonous unit beneath the ophio- Haybi thrust sheet in the central Oman mountains showing potential protoliths of the Bani Hamid granulites. Abbreviations: L.—lower; U.—upper; Fm—formation; Trias—Triassic; Jur—Jurassic. lite, the Haybi Complex thrust sheet (Fig. 14). These rocks are immediately beneath the mantle sequence harzburgites and its metamorphic sole where attached, and include Oman Exotic reef limestones as much as 800 m thick, ces, occasionally with blocks of subophiolite metamorphic sole (Searle and which usually have an alkali basaltic basement (Searle et al., 1980; Searle and Malpas, 1980, 1982; Searle and Cox, 1999; Cowan et al., 2013). We propose that Graham, 1982; Pillevuit et al., 1997). Exotic limestones such as Jebel Misht in these rocks, underthrust to middle or lower crust levels (~22–27 km depth) the central Oman mountains (Fig. 15) are Late Triassic oceanic seamounts form- during the later stages of obduction, were metamorphosed under high-T gran- ing carbonate guyots above an alkali basaltic substrate immediately beneath ulite facies conditions, and were subjected to tight isoclinal folding and intense the Semail ophiolite thrust sheet. Other exotics (e.g., Jebel Kawr) are overlain ductile strain. Due to density and buoyancy, these siliceous and carbonate-rich by a cap of latest Triassic–Jurassic radiolarian cherts reflecting a drowning of rocks were unsubductable and therefore must have choked the trench or sub- the oceanic guyot at the end of the Triassic (Fig. 14). The Haybi Complex also duction zone before being exhumed by out-of-sequence thrusting along the includes trench-type mélanges with both serpentinite and sedimentary matri- Bani Hamid thrust.

pull mechanism must have been responsible for subduction of basaltic crust to ~40 km, and slab break-off may have been responsible for the exhumation of the sole rocks. A density and buoyancy contrast between amphibolite-granulite sole rocks, containing 40%–50% plagioclase, compared to the ultramafic mantle rocks (harzburgites, dunites) was a major factor in the buoyancy-driven return flow of the sole rocks. The old (Triassic?) oceanic crust that was attached to the subducting plate was metamorphosed to eclogite, which has a density similar to the surround- ing mantle rocks and could have therefore broken off from the more buoy- ant amphibolite sole and continued to sink into the mantle. The exhumation of the amphibolite-granulite sole following slab break-off resulted in intense isoclinal folding and mylonitization during its return back up the same subduction channel. The sole amphibolite-granulites were exhumed from 25 to 40 km depth up to higher levels, and successive thrust slices of epidote amphibolites and greenschists were then accreted to their base, forming an inverted metamorphic field gradient (Fig. 17B). The exhumation of the sole rocks was extremely rapid, as deduced from almost synchronous 40Ar/39Ar cooling ages of hornblende at 93.5 Ma and muscovite and biotite ages of 92.4–89.2 Ma Figure 15. Aerial photograph of the Jebel Misht exotic, a 1000-km-thick Late Triassic carbon- (Hacker, 1994; Hacker et al., 1996). The inverted metamorphic gradient in the ate seamount above alkali basalt substrate and surrounded by deep-water cherts of the distal Hawasina Complex. The Semail thrust carrying the entire ophiolite above is along the eastern metamorphic sole clearly shows that the heat source for metamorphism was margin of the Jebel Misht exotic (right). in the overlying mantle, but a deeper mantle than that preserved in the ophio lite. As the rocks in the subduction channel exhumed, ductile shearing gave way to brittle thrusting and high-level imbrication, as seen along the Masafi TECTONIC MODEL FOR OPHIOLITE OBDUCTION corridor in UAE and various tectonic windows along the eastern margin of the AND EMPLACEMENT Oman mountains, for example, the Sumeini, Asjudi, and Hawasina windows and along Wadi Tayyin in the eastern mountains (Searle, 1985, 1988a, 2007; We propose a tectonic model for the evolution of the Bani Hamid granulites Cowan et al., 2013). based on our new mapping, structural, thermobarometric, and U-Pb age con- As the ophiolite obduction process continued, the Haybi Complex thrust straints (Fig. 16). The normal metamorphic sole comprising the amphibolites sheet was progressively underthrust beneath the ophiolite together with its with granulite enclaves (Masafi, UAE; Sumeini window and Wadi Tayyin in accreted metamorphic sole (Fig. 17C). These predominately carbonate and Oman), formed during subduction initiation at depths of ~40 km (Fig. 17A). quartzite rocks were unable to subduct to more than middle or lower crustal The amphibolites with granulite enclaves were exhumed from ~40 km depth to depths due to low density and buoyancy and were therefore jammed against ~20 km and accreted onto the base of the ophiolite during the initial obduction the subduction zone at depths of ~22–27 km. Continued compression led to event (Searle and Malpas, 1980, 1982; Gnos, 1998; Cowan et al., 2013). U-Pb tight isoclinal folding at all scales in the Bani Hamid granulites. Simple resto- zircon dating shows that the ophiolite crustal sequence gabbros and trond ration of the large-scale folds shows a minimum of 130 km internal shortening, hjemites crystallized at 96.1–95.5 Ma (Tilton et al., 1981; Warren et al., 2005; and the west-directed breaching out-of-sequence Bani Hamid thrust shows a Styles et al., 2006; Rioux et al., 2012a, 2013). Current U-Pb dates from the meta minimum of 30 km offset. At higher structural levels, the metamorphic sole, morphic sole suggest that metamorphism and melting in the sole may have Haybi Complex, and Hawasina Complex thrust sheets were imbricated by been either synchronous with or rapidly followed ridge magmatism (Warren thin-skinned thrust processes and subsequently folded across the Dibba zone et al., 2005; Styles et al., 2006; Rioux et al., 2013). We take the U-Pb dates, along (Searle, 1988a). with observed geochemical differences between the V1 volcanic rocks and The final stage in the evolution of the Bani Hamid granulites involved late- normal MORB (MacLeod et al., 2013) to support a suprasubduction zone tec- stage west-vergent out-of-sequence thrusting along the Bani Hamid thrust tonic setting of the ophiolite (Searle and Malpas, 1980, 1982; Pearce et al., 1981; (Fig. 17D). This late thrust places a more outboard complete ophiolite unit Lippard et al., 1986; Rioux et al., 2013; Searle et al., 2003, 2004, 2014), where (Khawr Fakkan block) along the hanging wall onto a more inboard mantle se- amphibolite-granulite metamorphism at depths of ~40 km was occurring in quence unit to the west. Another late-stage breakback thrust, the Masafi-Dibba the mantle beneath the ophiolite at the same time that ridge-related gabbros thrust, cuts through the ophiolite, truncating the lower mantle sequence and and trondhjemites in the ophiolite crust were crystallizing. Some sort of slab attached amphibolite-greenschist sole along the Masafi corridor in its footwall.

Ma SEMAIL METAMORPHIC GRANITE OPHIOLITE SOLE BANI HAMID DYKES GRANULITE OMAN UAE OMAN UAE OMAN UAE 87 N Out-of-sequence 88 Bani Hamid thrust

CONIACIA Fold-thrust thrust

Quartzite 90 Figure 16. Late Cretaceous time chart showing all published U-Pb age data from the Semail ophiolite, the normal metamorphic sole, the Bani Hamid thrust 91 Marble sheet (amph—amphibolite), and the peraluminous granite dikes in Oman and the United Arab Emirates (UAE) (Warren et al., 2005; Styles et al., 2006; Rioux et al., Pegmatite 2012a, 2013). Each data point represents 92 TURONIAN Crustal melting a single dated sample. Dates from Rioux Felsic pod below ophiolite et al. (2012a, 2013) are the youngest pre- Felsic pod cise single grain date from each sample.

ACEOUS Marble Bani Hamid The Styles et al. (2006) and Warren et al. 93 Granulite (2005) dates are not corrected for initial Granite metamorphism exclusion of 230Th during zircon crystallization, which would increase the dates by ~80–100 k.y. Age uncertainties are 2σ. TE CRET 94 Two dates from Styles et al. (2006) are LA plotted under both the Bani Hamid and granite dikes columns. We exclude four Amph. dates from Warren et al. (2005) from ridge- Peak Metamorphism related (V1) tonalites and trondhjemites 95 Sole because the analyses were done before

yyi n the widespread adoption of the chemical Subduction initiation Ta abrasion method and subsequent analy-

di ses of one of the samples by Rioux et al. Ophiolite formation

96 Wa (2014) suggested that the reported dates may be affected by Pb loss. Masafi

97 V2 plutonism CENOMANIAN Axial V1 plutonism 98 V2 plutonism Styles et al. 2006 Rioux et al. 2012, 2013 Warren et al. 2005 99 This Paper zircon This Paper titanite

100

km km WNW ESE 20 20 WNW ESE Sumeini Hawasina Haybi complex complex complex 10 Semail Ophiolite 10 Haybi Oman Semail Amphibolite Thrust Exotics Thrust metamorphic sole Hamrat Duru Distal Exotic >100 km? Group units limestones 0 Crustal 0 SEMAIL OPHIOLITE sequence Arum Muti Gabbro a Crustal Haw HAYBI S Musandam Moho asina em sequence Thr ail shelf carbonates ust Ha H T Oceani Pill wasi ay hr ow la Mantle SH bi us D –10 –10 EL na Th t y vas F A ru c Moho Ga ke Greenschist sole us st b s sequence aq Mantle bros BANI sequence futur HAMID e Semail thrust Sumeini Gp –20 Arabian Proterozoic –20 Shelf carbonates + basement basement duplexes . Dadnah Bt-granite Amphibol Arabia n ba me dikes Continental Moho sem l –30 –30 ent t sourc e ite sole

–40 A ~ 96.0 Ma –40 C ~ 94 – 93 Ma Eclogite Slab Break off km 20 km ESE WNW ESE WNW 20 Masafi Bani Hamid thrust thrust Musandam Dibba Masafi Semail Ophiolite 10 Shelf zone Gulf of Oman 10 SEMAIL OPHIOLITE Semail Thrust 0 ARUMA Crus Exotic YBI sequen Masafi MUSANDAM H HA tal K AW 0 Haybi Metamorphic SHELF ASIN A Mantle Moh ce Jr shelf Sumeini Hamrat Melang sole CARBONATES Tr o carbonates Duru e BA sequence Pm Mantle –10 G NI HAM sequenc Crusta RA NU sequence ID LIT E –10 Moho l Arabian e Arabian basement –20 SU basement MEINI Bani Hami –20 protolith d Continental Moho Moho –30 AMPHIBOLITE ? –30 GRANULITE –40 D ~ 92.91 Ma ?ECLOGITE –40 B ~ 95 Ma

Figure 17. Model for the formation and emplacement of the Semail ophiolite in the northern Oman mountains showing (A) the relative positions of the ophiolite and the “normal” metamorphic sole during the Cenomanian, (B) the exhumation of the sole amphibolites and transition from subduction zone deformation to thin-skinned fold-thrust belt of the Haybi and Hawasina Complexes. (C) Choking of the subduction zone by the arrival of unsubductable Bani Hamid high-temperature quartzites, marbles and calc-silicates (Bt—biotite). Peak metamorphic ages suggest the mantle wedge above is still very hot at this time. (D) Exhumation of the Bani Hamid thrust sheet by out-of-sequence thrusting along the Bani Hamid thrust and insertion into the mantle sequence.

The Bani Hamid thrust is, therefore, a later structure cutting through, or breach- maline, andalusite, muscovite, and cordierite (Peters and Kamber, 1994; Cox ing, the overlying Semail ophiolite, and the upper thrust is also a breaching et al., 1999; Searle and Cox, 2002). These minimum melt peraluminous granites thrust cutting across the earlier Semail thrust as shown in Figure 17D. require a muscovite- or biotite-rich pelitic source. This presents a problem, be- At some stage between peak granulite metamorphism (Fig. 17C) and out-of- cause none of the Bani Hamid granulite lithologies are suitable source rocks. sequence thrust culmination (Fig. 17D), localized crustal melting resulted in pro- The normal metamorphic sole also contains lithologies (metabasaltic amphib- duction of small-scale tonalitic to leucogranitic melts ca. 91.4 Ma. These small olites, metacherts, metacarbonates in greenschists) that are unsuitable proto- granitic dikes commonly contain biotite and variable amounts of garnet, tour- lith source rocks for these crustal melts. Some uncommon shaley assemblages

are occasionally present in the mélanges in the Haybi Complex immediately 7. Current U-Pb zircon age data suggest that amphibolite facies metamor- beneath the ophiolite (Searle and Malpas, 1980, 1982). We propose that some phism in the sole may be synchronous with or immediately postdate forma- minor pelitic component of the Haybi Complex could be buried beneath the tion of the ophiolite crust by ridge magmatism (Wadi Tayyin, 94.7–94.5 Ma; eastern UAE part of the mountains, beneath the region where the granitic dikes Masafi, 95.7–95.3 Ma; Warren et al., 2005; Styles et al., 2006; Rioux et al., 2013). now occur (Khawr Fakkan block, Ras Dadnah area). More work is required to In contrast, U-Pb zircon ages on Bani Hamid rocks are younger, ranging from determine the precise sources of these enigmatic leucogranitic dikes. 94.5 to 91.8 Ma (Styles et al., 2006; Rioux et al., 2014; this study). Extremely high temperatures in the overlying mantle peridotites must have occurred for ~4 m.y. from 95.7 to ca. 92 Ma to explain the high-temperature granulites in DISCUSSION AND CONCLUSIONS both the normal metamorphic sole rocks (Cowan et al., 2013) and the Bani Hamid thrust sheet (this study). The U-Pb titanite ages from Bani Hamid (91.0 ± Comparisons of Bani Hamid with the Normal Metamorphic Sole 1.1 Ma and 89.8 ± 15 Ma, considering external uncertainties) reported here are interpreted as high-temperature cooling ages, and are significantly younger The Bani Hamid thrust sheet in northern Oman (Madhah enclave) and than both the ophiolite and the normal metamorphic sole. the UAE (Fujairah) differs from the normal metamorphic sole in several key respects. 1. Whereas the normal metamorphic sole rocks show a narrow (~50–150 m) Deep Seismic Constraints from the UAE section of amphibolites, with garnet + clinopyroxene granulite facies enclaves in the upper part, epidote amphibolites, and greenschist facies rocks, the Bani Four deep seismic profiles were shot by WesternGeco (2005) on behalf of Hamid thrust sheet shows a 1.2 km structural thickness of granulite facies mar- the Ministry of Energy of the UAE and preliminary results were presented by bles and diopside-enstatite-cordierite quartzites with minor amphibolites. Batty et al. (2004), Tarapoanca et al. (2010), and Naville et al. (2010). In addition, 2. The metamorphic sole rocks show a pronounced inverted metamorphic a new aeromagnetic survey in the UAE was carried out by Fugro (Batty et al., gradient beneath the Semail thrust along the base of the ophiolite; however, 2004; Styles et al., 2006). The northern seismic line (D4) crosses the Dibba the Bani Hamid rocks are all at granulite or uppermost amphibolite facies. zone and northernmost part of the ophiolite to Ras Dadnah and the southern 3. The sole rocks are found in the same structural position along the entire line (D1) crosses the main Khawr Fakkan ophiolite block and Bani Hamid thrust Oman mountains; however, the Bani Hamid granulites are only found in one slice just south of Masafi (Fig. 18). The two other lines are strike parallel, one location in the north. It is not clear whether these high-temperature granu- (D3) along the axis of the foreland inland from Ras al-Khaimah–Dubai, the other lites are also present at depth in Oman, but remain buried and unexposed, or (D2) along the middle of the ophiolite from the Dibba zone to south of Masafi. whether they are actually restricted to the northern mountains. However, in the These deep seismic lines were depth migrated and combined with industry UAE, in addition to the Bani Hamid metamorphism, the late leucocratic dikes lines, well data, and outcrop studies. Imaging structures beneath the ophiolite that intrude the mantle section provide evidence for a prolonged high-tem- slab is extremely problematic, but enhanced refraction and reflection stacks perature event (ca. 91.4 Ma; Styles et al., 2006). Similar young dikes have not were used to interpret possible deep structures beneath the mountains. The been identified in the Oman portion of the ophiolite (Rioux et al., 2012a, 2013), north-south D2 profile through the Masafi window shows a thick sedimentary suggesting a distinct tectonic history. pile below the ophiolite, but it was not possible to distinguish the shelf carbon- 4. Whereas the metamorphic sole rocks are always found along the base ate units from the Sumeini and Hawasina thrust sheets (Naville et al., 2010). of the ophiolite mantle sequence, the Bani Hamid granulite-amphibolite thrust Tarapoanca et al. (2010) showed thrust stacks cutting through the Permian– sheet is surrounded by mantle sequence peridotite, and has been thrust up by Cretaceous shelf carbonate on a trend south of the Musandam culmination. the out-of-sequence Bani Hamid thrust that breaks back in the sequence. Combined with exposed geology in the UAE and Oman parts of Musandam, 5. Although both the metamorphic sole rocks and the Bani Hamid rocks these thrusts are known to affect the pre-Permian basement, Permian–Meso are thought to be metamorphosed equivalents of the Haybi Complex (basal- zoic shelf carbonates, the Late Cretaceous foreland basin, and the Paleogene tic oceanic rocks, Mn-rich cherts, distal carbonates), the proportions of meta sedimentary cover. Thrust tip lines are truncated by the middle Miocene un- quartzite and marble in Bani Hamid is far greater. conformity in a seismic section west of Musandam (Ricateau and Riche, 1980; 6. Although the P-T conditions of the amphibolite-granulite sole rocks (770– Searle et al., 1983, 2014; Dunne et al., 1990). Other thrusts imaged on the seis- 900 °C; 11–15 kbar) imply a much deeper and more distal location in a subduc- mic sections cut through the subophiolite sheets and the shelf carbonates, tion zone setting beneath the ophiolite, the P-T conditions of the Bani Hamid but it is uncertain if these are late-stage ophiolite emplacement–related Late rocks (850 ± 60 °C; 6.3 ± 0.5 kbar) show a much shallower level of formation. Cretaceous thrusts or mid-Cenozoic (Musandam type) thrusts associated with The high temperatures from both the sole and the Bani Hamid rocks suggest the initial continental collision. The east-dipping normal fault bounding the that heat must have been provided from the hanging-wall mantle peridotites. southeast margin of Musandam and the northwest margin of the Dibba zone

Figure 18. Two seismic profiles across the northern Oman–United Arab Emirates (UAE) mountains showing interpreted structures (from Rouré et al., 2006; Tarapoanca et al., 2010; Naville et al., 2010). Abbreviations: Sgrp—supergroup; Cret., K.—Cretaceous; Pg—Paleogene; L.—lower; U.—upper. Reproduced with permission from Arabian Journal of Geosciences (Springer, Heidelburg).

(Searle, 1988a, 1988b; Searle et al., 2014) is shown clearly in the interpretative Carrington, D.P., and Harley, S., 1995, Partial melting and phase-relations in high-grade sections of Tarapoanca et al. (2010; Fig. 4). metapelites—An experimental petrogenetic grid in the KFMASH system: Contributions to Mineralogy and Petrology, v. 120, p. 270–291, doi:10 .1007/BF00306508. The thickness of the ophiolite is constrained as zero at Masafi to ~5 km Carson, C.J., Powell, R., and Clarke, G.L., 1999, Calculated mineral equilibria for eclogites in

depth beneath Dadnah on the east coast. Presumably the eastern margin of CaO-Na2O-FeO-MgO-Al2O3-SiO2-H2O: Application to the Pouébo terrane, Pam Peninsula, the Semail ophiolite is an east-dipping normal fault offshore Dadnah–Khor New Caledonia: Journal of Metamorphic Geology, v. 17, p. 9–24, doi:10.1046 /j.1525-1314 .1999.00177.x. Fakkan, in order to accommodate the thick Cenozoic sediments in the Gulf of Cherniak, D.J., 1993, Lead diffusion in titanite and preliminary results on the effects of radia- Oman offshore. We suggest that the stacked up thrust sheets imaged beneath tion damage on Pb transport: Chemical Geology, v. 110, p. 177–194, doi:10 .1016 /0009 -2541 the ophiolite on the D4 and D1 seismic lines could be equivalent units to the (93)90253-F. Cooper, D.J.W., 1988, Structure and sequence of thrusting in deep-water sediments during Bani Hamid metamorphic rocks or inboard equivalents of stacked up Sumeini ophiolite emplacement in the south-central Oman Mountains: Journal of Structural Geol- and Hawasina shelf margin thrust sheets. Only more detailed seismic imaging ogy, v. 10, p. 473–485, doi:10.1016/0191-8141(88)90035-1. will be able to constrain this deep subophiolite structure in more detail. Cooper, D.J.W., 1990, Sedimentary evolution and palaeogeographical reconstruction of the Mesozoic continental rise in Oman: Evidence from the Hamrat Duru Group, in Robert- son, A.F.H., et al., eds., The geology and tectonics of the Oman region: Geological Society, London, Special Publication 49, p. 161–187, doi:10.1144/GSL.SP.1992.049.01.11. ACKNOWLEDGMENTS Cooper, D.J.W., Ali, M.Y., and Searle, M.P., 2014, Structure of the northern Oman Mountains from the Semail Ophiolite to the foreland basin, in Rollinson, H., et al., eds., Tectonic evolution of This work was funded mainly by grants from the Ministry of Energy of the United Arab Emirates the Oman Mountains: Geological Society, London, Special Publication 392, p. 129–153, doi: and the Petroleum Institute, Abu Dhabi to M.Y. Ali, A.B. Watts, and Searle. Work by Rioux and 10.1144/SP392.7. Garber was supported by National Science Foundation (NSF) grant EAR-1250522. U-Pb zircon Cowan, R.J., Searle, M.P., and Waters, D.J., 2013, Structure of the metamorphic sole to the Oman dating was carried out by Rioux in the Massachusetts Institute of Technology (MIT; Cambridge, Ophiolite, Sumeini Window and Wadi Tayyin: Implications for ophiolite obduction pro- Massachusetts) radiogenic isotope laboratory (an NSF-supported facility). We thank Sam Bow- cesses, in Rollinson, H., et al., eds., Tectonic evolution of the Oman Mountains: Geological ring for access to the MIT laboratory, and for useful discussions about the U-Pb data. We are Society, London, Special Publication 392, p. 155–175, doi:10.1144 /SP392 .8. grateful to Mohammed Ali for logistics and discussions; Brad Hacker, Jon Cox, and Edwin Gnos Cox, J.S., 2000, Subduction-obduction related petrogenetic and metamorphic evolution of the for discussions; and A. Kylander-Clark for assistance with laser-ablation split-stream inductively Semail Ophiolite sole in Oman and the United Arab Emirates [Ph.D. thesis]: Oxford, UK, coupled plasma mass spectrometry analyses. University of Oxford, 169 p. Cox, J.S., Searle, M.P., and Pedersen, R.-B., 1999, The petrogenesis of leucogranitic dykes intruding the northern Semail Ophiolite, United Arab Emirates: Field relations, geochemistry and Sr/Nd isotope systematics: Contributions to Mineralogy and Petrology, v. 137, p. 267–287, REFERENCES CITED doi:10.1007/s004100050550. Agard, P., Searle, M.P., Alsop, G.I., and Dubacq, B., 2010, Crustal stacking and expulsion tectonics Diener, J.F.A., and Powell, R., 2012, Revised activity-composition models for clinopyroxene and during continental subduction: P-T-deformation constraints from Oman: Tectonics, v. 29, amphibole: Journal of Metamorphic Geology, v. 30, p. 131–142, doi:10 .1111 /j.1525-1314 .2011 TC5018, doi:10.1029/2010TC002669. .00959.x. Aleinikoff, J.N., Wintsch, R.P., Tollo, R.P., Unruh, D.M., Fanning, C.M., and Schmitz, M.D., 2007, Diener, J.F.A., Powell, R., White, R.W., and Holland, T.J.B., 2007, A new thermodynamic model for

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