1 s2.0 S1342937X20301775 Main

Gondwana Research 86 (2020) 203–221
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Gondwana Research
journal homepage: www.elsevier.com/locate/gr
Petro-tectonic evolution of metamorphic sole of the Semail

ophiolite, UAE
Soujung Kim a,b, Yirang Jang c,⁎, Sanghoon Kwon a, Vinod O. Samuel a, Sung Won Kim c, Seung-Ik Park d,
M. Santosh e,f,g, Sotirios Kokkalas h
a
Department of Earth System Sciences, Yonsei University, Seoul 03722, Republic of Korea
b
Presently at School of Geography and Earth Sciences, McMaster University, Hamilton, ON L8S 4K1, Canada
c
Geology Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea
d
Department of Geology, Kyungpook National University, Daegu 41566, Republic of Korea
e
School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China
f
Department of Earth Sciences, University of Adelaide, Adelaide, SA, Australia
g
Yonsei Frontier Lab, Yonsei University, Seoul 03722, Republic of Korea
h
Department of Geology, University of Patras, 26500, Greece
a r t i c l e i n f o a b s t r a c t
Article history: The Semail ophiolite located in the eastern part of the Arabian platform preserves remnants of ocean plate stra-
Received 26 March 2020 tigraphy and related metamorphic sole. To understand the petro-tectonic evolution of a metamorphic sole during
Received in revised form 21 May 2020 subduction to obduction processes, here we investigate the garnet metagabbros from the metamorphic sole and
Accepted 21 May 2020
the tonalites which intruded the mantle section of the Khor Fakkan Block. We present results from petrology,
Available online 2 July 2020
geochemistry, zircon U-Pb, Hf and O isotope analyses and phase equilibria modeling. The garnet metagabbro
Handling Editor: A. Festa samples have E-MORB-type enriched-mantle compositions with zircon dates of ca. 89–96 Ma, and positive εHf
(t) values ranging from 5.6 to 10.0. The tonalite is peraluminous with those range of ca. 87–92 Ma, and a range
Keywords: of positive εHf(t) values of 5.1–10.0. The similarity in εHf values from both the garnet metagabbro and tonalite
Semail ophiolite samples suggests a strong relevance to their mantle source, indicating the role of subducted material during
Garnet metagabbro their formation. In contrast, the δ18O(zircon) values show distinctly different values of high δ18O(zircon) of
Tonalite ~13–16‰ for the tonalite and ~ 5–8‰ for the metagabbro samples, reflecting variations in the role of surface-
Mantle metasomatism derived source materials. The phase equilibria modeling of the garnet metagabbro shows high-pressure amphib-
Zircon Hf and O isotopes
olite facies metamorphism that preceded the peak granulite facies metamorphism, followed by lower pressure
hydration and decompression. This clockwise P-T path might reflect partial melting and differentiation of mantle
wedge section above subducted slab. Our results provide insights into the complex processes within a supra-
subduction zone, implying differences in degree of partial melting of the ocean plate stratigraphic sequences in-
cluding recycled oceanic slab and surface-derived marine sediments that were subsequently interacted with hy-
drothermally altered mantle at a mantle wedge during subduction to obduction processes that formed the Semail
ophiolite during the Upper Cretaceous.
© 2020 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
1. Introduction 2017; Li et al., 2018, 2019; Saccani et al., 2018; Jiang and Zhu, 2019;
Zheng et al., 2019). The Semail ophiolite, also known as the United
Subduction zones mark Earth's convergent margin plate boundary Arab Emirates (UAE)-Oman ophiolite complex, located in the eastern
where the oceanic lithosphere interacts with the upper mantle, generat- part of the Arabian platform is the largest and most well studied
ing diverse magma types (Bebout et al., 2017). The obducted oceanic ophiolite complex on the globe (e.g. Juteau et al., 1988; Boudier and
lithosphere along such margins provide insights into the exhumed do- Juteau, 2000; Python et al., 2008; Goodenough et al., 2010; Searle
mains of deeply subducted oceanic crust and upper mantle (e.g. Searle et al., 2015; Duretz et al., 2016; Soret et al., 2017). It preserves the rem-
and Malpas, 1980, 1982; Gnos, 1998; Searle and Cox, 2002; Thomas nants of typical ocean plate stratigraphy and the related metamorphic
and Ellison, 2014; Roberts et al., 2016; Luo et al., 2017; Saha et al., sole providing an excellent natural laboratory to investigate the petro-
tectonic evolution during subduction to obduction events in a conver-
gent plate margin (e.g., Searle and Cox, 2002; Rioux et al., 2013;
⁎ Corresponding author. Goodenough et al., 2014; Spencer et al., 2017; Joun et al., 2019). The
E-mail address: yirang@kigam.re.kr (Y. Jang). metamorphosed mafic rocks in this complex, especially garnet
https://doi.org/10.1016/j.gr.2020.05.013
1342-937X/© 2020 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
204 S. Kim et al. / Gondwana Research 86 (2020) 203–221
amphibolite in the metamorphic sole, represent crustal rocks that are 2. Geological setting
subducted to great depth and exhumed back to the surface during
obduction along the Arabian continental margin (Searle and Malpas, The Alpine-type Semail ophiolite is exposed over 500 km from the
1980, 1982; Gnos, 1998; Searle and Cox, 2002; Thomas and Ellison, northeast coast of Musandam Peninsula of Oman all the way to the
2014). These rocks might represent mid-ocean ridge basalt (MORB) of United Arab Emirates (UAE) (Fig. 1). It is the product of Late Cretaceous
the subducted oceanic slab that was scraped off and accreted to the obduction of the Neotethyan oceanic lithosphere onto Middle Permian
upper plate during initial stage of the supra-subduction zone (SSZ) pro- to Middle Cretaceous shelf carbonate sequences of the Arabian passive
cess (e.g. Boudier and Coleman, 1981; Boudier et al., 1988; Cowan et al., continental margin (Glennie et al., 1973; Searle et al., 1983, 1990;
2014; Joun et al., 2019). The protolith of the metamorphic rocks possibly Lippard et al., 1986; Nicolas, 1989; Searle and Cox, 1999, 2002). The
represents mafic magma that was generated either from the partial ophiolite belt preserves large slices of ocean plate stratigraphic section
melting of a subducted slab (e.g. Searle and Malpas, 1980, 1982; from mantle, lower crust, sheeted dikes, and pillow lavas of mid-ocean
Pearce et al., 1981; Searle and Cox, 1999, 2002; Warren et al., 2003, ridge basalt affinity, marking the Phase 1 magmatism, with intercalated
2005; Whattam and Stern, 2011) or from the partial melting of a hy- pelagic sediments lying on top of the fault-bounded metamorphic sole
drous mantle wedge (Spencer et al., 2017; Wang et al., 2017). (Searle and Malpas, 1980; Nicolas et al., 2000) (Fig. 2). In the Aswad
In this study, we investigate the petrologic, geochemical and zircon Block, the ultramafic rocks of the mantle section are intercalated with
U-Pb-Hf-O isotopic features of the garnet metagabbro preserved in the deformed gabbro (Figs. 2 and 3a). This earlier oceanic crust is intruded
metamorphic sole and the tonalite intruded mantle section of the by Phase 2 magmatic rocks that consist of high-level gabbro, dolerite,
Khor Fakkan Block of the Semail ophiolite. Our results suggest that basalt, pyroxenite, wehrlite and tonalite (Fig. 2) (Koepke et al., 2009;
these rocks were originated by mixing of partial melting of subducted Goodenough et al., 2010, 2014) with hydrous SSZ geochemical affinity
slab/supracrustal material, and their subsequent interaction with hy- (e.g., Godard et al., 2003; Warren et al., 2005; Goodenough et al.,
drothermally altered mantle at a mantle wedge. The results in turn 2010; Rioux et al., 2012, 2013; MacLeod et al., 2013; Haase et al.,
will provide important clues on the role of subducted slab and mantle 2015). In the Dadnah area of the Khor Fakkan Block, the newly found
wedge in generating the Semail ophiolite. tonalite intruded harzburgite of the mantle section (Figs. 2 and 3b).
Fig. 1. Simplified geologic map of the UAE part of the Semail ophiolite including the Khor Fakkan and Aswad blocks (modified after Thomas and Ellison, 2014; Joun et al., 2019). The large-
scale inset map presents the distribution of the Semail ophiolite in the Arabian Peninsula (modified after Lippard et al., 1986). The box within the inset map indicates the location of the
study area.
S. Kim et al. / Gondwana Research 86 (2020) 203–221 205
Fig. 2. Geologic map of the northern Semail ophiolite, Fujairah, UAE (modified after Thomas and Ellison, 2014; Roberts et al., 2016). Red circles indicate the sample locations.
Isotopic ages suggest that the Phase 1 and 2 magmatic activities overlap but is not clearly defined due to discontinuous exposures in the field.
in a short range of ca. 98 to 93 Ma (Joun et al., 2019 and references The origin of the latter meta-igneous rocks has been interpreted as
therein). Apart from the magmatic history composed of the ophiolite deep (N30 km) transfer of highly metamorphosed rocks that were
crust, obduction history of the Semail ophiolite has been studied by var- scraped off either from the subducting slab of mid-ocean ridge origin
ious workers based on the nature of high-pressure metamorphism be- or from the overriding plate during the slightly younger magmatism
neath the ophiolite complex (Gnos and Kurz, 1994; Gnos, 1998; Cox, (Goodenough et al., 2014; Searle et al., 2015). The process occurred ei-
2000; Searle and Cox, 2002; Styles et al., 2006; Cowan et al., 2014; ther through typical arc-type subduction zone magmatism or by mixing
Thomas and Phillips, 2014). Among the exposures of these metamor- of hydrous mafic melt derived from partial melting of the mantle wedge
phic rocks, the Asimah-Masafi area is the largest one in the Semail with melts originated from the partial melting of pelagic sediments
ophiolite complex (Fig. 2). The metamorphic sole of the Semail ophiolite under SSZ environment (e.g. Searle and Malpas, 1980, 1982; Boudier
in general consists of a highly condensed sequence of the high- and Coleman, 1981; Pearce et al., 1981; Boudier et al., 1988; Searle
temperature sole that structurally overlies the low-temperature sole and Cox, 1999, 2002; Warren et al., 2003, 2005; Whattam and Stern,
along a highly deformed ductile shear zone (Gnos, 1992; Searle et al., 2011; Cowan et al., 2014; Spencer et al., 2017; Joun et al., 2019). Previ-
2015). On the contrary, the metamorphic sole preserved at Masafi ous geochronological studies indicate that the formation of the oceanic
area of this study represents a mélange-type complex, consisting of am- crust constituting the ophiolite, as well as related magmatism and
phibolite to granulite facies metasedimentary (e.g. marble and quartz- metamorphism through the intra-ocean subduction and ophiolite
ite) and mafic meta-igneous (e.g. amphibolite and metagabbro) rocks obduction occurred within a short period of ca. 2–3 Ma (Hacker et al.,
(Fig. 3c, d). Their contact relationship with the surrounding mantle sec- 1996; Styles et al., 2006; Goodenough et al., 2010; Rioux et al., 2012,
tion has been reported as a major thrust boundary (Searle et al., 2015), 2013, 2016; Jacobs et al., 2015; Roberts et al., 2016). The metamorphism
Fig. 3. Outcrop photographs showing (a) ultramafic rocks of the mantle section in the Aswad Block (25°08′14.82″N, 56°13′56.01″E), (b) tonalite intruding harzburgite of the mantle section
in the Dadnah area of the Khor Fakkan Block (25°30′41.22″N, 56°18′46.37″E), and (c)–(d) garnet metagabbros from the mélange complex in the Masafi area of the Khor Fakkan Block
(25°19′29.33″N, 56°09′27.11″E; 25°19′29.33″N, 56°09′25.75″E).
is characterized by either anticlockwise P-T path (Searle and Malpas, (ICP-AES) (ENVIRO II; Thermo Jarrel-Ash) and ICP mass spectrometry
1980; Gnos and Nicolas, 1996; Hacker et al., 1996; Hacker and Gnos, (ICP-MS) (Optima 3000; Perkin-Elmer) at Activation Laboratories, Ltd.
1997; Gnos, 1998; Searle and Cox, 2002) with the inverse metamorphic Canada. Analytical uncertainties ranged from 1% to 3%.
gradient (Boudier et al., 1988; Hacker et al., 1996; Cowan et al., 2014;
Searle et al., 2015) or clockwise P-T path with similar peak P-T condi- 3.2. LA-MC-ICPMS zircon U-Th-Pb and Hf isotope analyses
tions due to mechanical coupling of the subducting plate rocks into
the overriding plate (Soret et al., 2017). The role of hydrous Phase 2 in- Zircon grains in the garnet metagabbros, tonalites and olivine
trusions in the overriding plate (Haase et al., 2015; Spencer et al., 2017) websterite were separated using conventional sieving and water pan-
during their accretion-exhumation processes has also been emphasized. ning techniques. After eliminating the magnetic fraction from the con-
centrates with a neodymium magnet, zircon grains were hand-picked
3. Sampling and analytical methods under a binocular microscope. The separated zircons were mounted
with FC1 (1099 Ma; Paces and Miller, 1993) and SL13 (U: 238 ppm;
We present U-Pb, Hf and O isotope data for zircon grains, and major, Claoue-Long et al., 1995) reference zircons and polished to obtain
trace and rare earth elements (REE) for whole-rock samples of the rep- their internal textures. Zircon grains in mount were photographed
resentative garnet metagabbro (sample # UAE57, UAE62) that were tra- under an optical microscope, and their internal zoning was imaged by
ditionally called as garnet amphibolite within the Semail ophiolite cathodoluminescence (CL) using a JEOL 6610LV scanning electron mi-
metamorphic sole in Masafi area of the Khor Fakkan Block. These croscope. The zircon U-Th-Pb isotopic analyses were carried out using
rocks are mainly composed of garnet, clinopyroxene, and amphibole a Nu Plasma II multi-collector inductively coupled plasma mass spec-
with minor plagioclase, quartz, zoisite, rutile and titanite (Fig. 4a–d). trometer equipped with a New Wave Research 193-nm ArF excimer
We also carried out the same analyses for the Phase 2 tonalite (sample laser ablation system (LA-MC-ICPMS) at the Korea Basic Science Insti-
# UAE55), commonly called plagiogranite which intruded into the man- tute (KBSI), Korea. Analyses were performed with a spot diameter of
tle section, in the Dadnah area of the Khor Fakkan Block. This rock is 10–15 μm for tonalites and 20–25 μm for garnet metagabbros/olivine
composed of biotite, quartz, plagioclase, and K-feldspar (Fig. 4e). The websterite, a repetition rate of 5 Hz, and an energy density of 2–5 J/
Phase 1 olivine websterite (sample # UAE68) from the Aswad Block is cm2. Helium (970 mL/min) was used as a carrier gas. The background
composed mainly of olivine, clinopyroxene, and orthopyroxene, with intensities, dwell time, and wash out time were 30 s, 30 s, and 20 s, re-
euhedral grains of plagioclase, serpentinite, and mica (Fig. 4f). spectively. Signal intensities were collected from each collector every
0.2 s (integration time). Raw data were corrected for background,
3.1. Whole-rock geochemical analysis laser-induced elemental fractionation, mass discrimination, and drift
in ion counter gains. U-Pb isotope ratios were calibrated by concordant
All samples were analyzed for major, trace, and rare earth element reference zircon 91500 (1065 Ma; Wiedenbeck et al., 1995), which was
(REE) abundances. The whole-rock samples were crushed to 90% pass- used at the beginning and end of each analytical session and regular in-
ing through a 10-mesh sieve and were spilt to yield approximately tervals during each session, according to protocols adapted from
250 g. Then the spilt samples were pulverized to 95% passing through Andersen (2002). A correlation of signal versus time was also assumed
a 200-mesh sieve to provide homogeneous and representative sample. for the reference zircons. All ages were calculated with 2σ error, and
The concentrations of the major, trace and REEs in each sample were an- data reduction was conducted using Iolite 2.5 (Paton et al., 2011) and
alyzed using inductively coupled plasma atomic emission spectrometry Isoplot 3.71 (Ludwig, 2008) software.
Fig. 4. Representative photomicrographs showing mineral assemblages of (a)–(d) garnet metagabbros from the Masafi area and (e) tonalite from the Dadnah area in the Khor Fakkan
Block, and (f) olivine websterite in the Aswad Block in the UAE part of the Semail ophiolite. Abbreviation: Amph, amphibole; Bt, biotite; Grt, garnet; Kfs, K-feldspar; Ol, olivine; Pl,
plagioclase; Px, pyroxene; Qtz, quartz; Ttn, titanite.
Hf isotopic compositions of the same zircons for LA-MC-ICPMS U-Pb 3.3. SHRIMP zircon U-Th-Pb and Oxygen isotope analyses
age determinations were measured using LA-MC-ICPMS at the KBSI.
Among the Nu Plasma II mass spectrometer collectors, 10 Faraday col- Zircon U-Th-Pb isotope analyses from the garnet metagabbro and
lectors were set to simultaneously detect Yb-Lu-Hf isotopes in the fol- tonalite samples were also performed using a SHRIMP-IIe/MC ion mi-
lowing array: 172Yb (low 4), 173Yb (low 3), 174(Yb + Hf) (low 2), 175Lu croprobe at the IBERSIMS laboratory of the CIC-University of Granada,
(low 1), 176(Yb + Lu + Hf) (axial), 177Hf (high 1), 178Hf (high 2), Spain. The zircon preparation procedures are the same as described in
179
Hf (high 3), 180Hf (high 4), and 182W (high 6). Instrument parameters 3.2. Separated zircon grains from samples were mounted on a 3.5 cm di-
include a spot size of 50 μm, a 10 Hz repetition rate, and an energy den- ameter epoxy SHRIMP megamount with reference zircons (Ickert et al.,
sity of 6–8 J/cm2. He (650 mL/min) and N2 (2 mL/min) were used as car- 2008). Once polished, zircon grains were studied by optical (reflected
rier gases for higher Hf isotope intensities (Iizuka and Hirata, 2005). The and transmitted light) and scanning electron microscopy (secondary
background intensities, dwell time, and wash out time were 30 s, 60 s, electrons and cathodoluminescence), coated with a 12 nm thick gold
and 20 s, respectively. Signal intensities for each detector were collected layer. The SHRIMP U-Th-Pb analytical method roughly followed that de-
every 0.2 s (integration time). 176Lu and 176Yb on the 176Hf signal inter- scribed by Williams and Claesson (1987) and is described in detail at
ference were corrected using the methods of Chu et al. (2002) and www.ugr.es/~ibersims/ibersims. All isotopes were acquired using oxy-
Vervoort et al. (2004), respectively. The mass bias of measured Hf isoto- gen ion beams of diameter about 20 μm. Each selected spot is rastered
pic ratios was corrected to 179Hf/177Hf = 0.7325, using an exponential with the primary beam for 120 s before the analysis and then analyzed
correction law (Russel et al., 1978; Patchett et al., 1981). To evaluate by six scans, following the isotope peak sequence 196Zr2O, 204Pb,
the precision and accuracy of 176Hf/177Hf ratios, 91500 (0.282297; 204.1
background, 206Pb, 207Pb, 208Pb, 238U, 248ThO, 254UO. Every peak of
Griffin et al., 2000) and Plešovice (0.282482; Sláma et al., 2008) refer- every scan is measured sequentially 10 times with the following total
ence zircons were repeatedly analyzed at the beginning and end of counting times of 20 s for every Pb mass per scan. All calibration proce-
each analytical session, and regular intervals during each session. All ra- dures were performed on the standards included on the same mount.
tios were calculated with 2σ error, and data reduction was conducted Uranium concentration and U/Pb ratios were calibrated using the SL13
using the Iolite 2.5 software (Paton et al., 2011). (U: 238 ppm; Claoue-Long et al., 1995) and TEMORA (417 Ma; Black
Fig. 5. Geochemical classifications for (a) the Masafi garnet metagabbro in the total alkali versus SiO2 (TAS) diagram (after Cox et al., 1979), (b) the Aswad olivine websterite in the Ol-Opx-
Cpx ternary diagram (after Streckeisen, 1976) and (c) the Dadnah tonalite in the normative An-Ab-Or ternary diagram (after O'Connor, 1965).
et al., 2004) reference zircons, respectively. Mass calibration was done 0.3‰; Black et al., 2004) was measured in every three unknowns for
on the REG zircon (ca. 2.5 Ga, very high U, Th, and common lead con- calibration.
tent). Data reduction was done with the SHRIMPTOOLS software (avail-
able from www.ugr.es/˜fbea) using the STATA™ programming
language, which is a new implementation of the PRAWN software orig-
inally developed for the SHRIMP. The errors of spot analyses of the U-Pb
isotopes were reported with 1σ-level. 3.4. Electron probe microanalysis and phase equilibria modeling
Oxygen isotopes of the zircon grains examined for SHRIMP U-Pb age
determinations were analyzed following the method described by Petrographic studies are also carried out for phase equilibria model-
Ickert et al. (2008). The primary beam consisted of a 15 kV and 3 to ing on polished thin sections of the garnet metagabbro samples, using a
3.5 nA Cs ion beam focused to produce a 17 × 20 μm elliptical spot on JEOL JXA-8100 Superprobe, Electron Probe Micro Analyzer (EPMA),
the sample. The electrical charge of the non-conductive zircons caused housed at the Department of Earth System Sciences, Yonsei University,
by the primary beam was neutralized with an electron beam that im- Korea. Based on that, the P-T isochemical phase diagram sections were
pacted the sample in a ~200 μm diameter spot concentric with the Cs modeled using effective bulk composition on the chemical system
beam. The source slit was fixed at 150 μm and the secondary 18O and Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2. The whole rock analysis
16
O were measured simultaneously in static mode on two Faraday of mélange garnet metagabbro samples obtained from XRF showed a
cups with entry slits of 300 μm. The sample was pre-sputtered for maximum of 2–3% difference for two samples. Judging by the difference
180 s before analysis. During the last 90 s of the pre-sputtering time, does not create a significant change in the pseudosection, the result
the secondary beam and the electron beam were focused to obtain the from the two samples was averaged (see column F to I in Supplemen-
maximum signal on the Faraday cups. The measurement consisted of tary Table 1). Further, elements weighting b1% were not considered.
two sets of six scans, each scan lasted 10 s. The mass fractionation Therefore, the bulk chemistry of the sample considered in the
caused by the Earth's magnetic field was compensated with Hemholtz pseudosection are SiO2 = 45.96, TiO2 = 1.61, Al2O3 = 16.47, FeO =
coils operated at −309 mA. This compensation also eliminated the 12.15, MgO = 6.75, CaO = 11.71, Na2O = 2.53, H2O = 1.71 (wt%).
mass fractionation related to the horizontal steering of the secondary The phase diagram was calculated using the PERPLE_X 6.7.9 version
beam. Electron-induced secondary-ion emission (EISI) was measured (Connolly, 1990, 2005) based on the thermodynamic data set of
for 10 s both at the beginning of each scan and at the end of the mea- Holland and Powell (1998) and solid solution model of garnet
surement, it was then and subtracted accordingly. Data reduction was (Holland and Powell, 1998), clinopyroxene (Holland and Powell,
performed using the POXI software developed by Peter Lanc and Peter 1996), plagioclase (Newton et al., 1980), amphibole (Dale et al., 2000)
Holden at the ANU. The zircon standard TEMORA-2 (δ18O = 8.2 ± and melt (White et al., 2001).
Fig. 6. Plots of major element versus SiO2 for the Masafi garnet metagabbros, together with that from the garnet amphibolites of the Semail ophiolite (Searle and Malpas, 1982; El Tokhi
et al., 2018).
4. Whole-rock geochemical characteristics element variation of the Masafi garnet metagabbro samples from this
study is presented in Fig. 6. The results of previous studies from a garnet
Major, trace, and REE compositions of representative samples of the amphibolite of the Semail ophiolite (Searle and Malpas, 1982; El Tokhi
garnet metagabbros (UAE57 and UAE62) from Masafi area (Masafi gar- et al., 2018) are included for comparison. The data show relatively
net metagabbro hereafter) and the Phase 2 tonalite (UAE55) in Dadnah high Al2O3, Fe2O3, Na2O and K2O in the SiO2 content ranging from 45
area (Dadnah tonalite hereafter) of the Khor Fakkan Block, and Phase 1 to 50% (Fig. 6). On the major element variation diagrams, together
olivine websterite (UAE68) from the Aswad Block (Aswad olivine with felsic granitoids from Rollinson (2015) and Joun et al. (2019), the
websterite hereafter) of the Semail ophiolite are listed in Supplemen- felsic granitoids including the Dadnah tonalite from this study show
tary Table 1. In the total alkali versus SiO2 diagram, the Masafi garnet negative correlations between SiO2 and Al2O3, Fe2O3 and MgO (Fig. 7).
metagabbro plots in the field of the gabbro (Fig. 5a). On the Ol-Opx- The Dadnah tonalite has high SiO2 (71–76%) content with low total al-
Cpx ternary diagram, normative mineral contents of the ultramafic sam- kalis, and plot on the peraluminous field (Fig. 7), typical of S-type gran-
ples are plotted within olivine websterite and lherzolite fields, but the ite. The chondrite-normalized REE patterns (Sun and McDonough,
dominant pyroxene in the sample and thin-section indicates it as an ol- 1989) of the Masafi garnet metagabbro samples from this study are
ivine websterite (Fig. 5b). The Dadnah tonalite plots in the tonalite field shown in Fig. 8a. They show LREE-enriched patterns including La, Ce,
on the An-Ab-Or CIPW-normative ternary diagram (Fig. 5c). The major Pr, Nd and Sm, implying the influx of incompatible elements during
Fig. 7. Plots of major element versus SiO2 for the Dadnah tonalites. Previous studies (Rollinson, 2015; Joun et al., 2019) from the felsic granitoids of the Semail ophiolite are plotted for
comparison. Note that the Dadnah tonalite is plotted in the peralumious field on the A/NK versus A/CNK diagram (after Maniar and Piccoli, 1989).
their formation. The spider diagram of trace elements normalized to the the sample UAE62A show angular to rounded and multi-faceted shapes
primitive mantle (Sun and McDonough, 1989) for the garnet with a complete lack of zoning in some cases. The oscillatory zoning is
metagabbro samples shows LILE anomaly including Cs, Rb, and K rarely observed, with thin overgrowth rims that could not be analyzed
(Fig. 8b), indicating the effects of alteration. The positive Eu anomaly (Fig. 9a). The zircon sizes are ranging from 50 to 180 μm in diameter.
with negative Gd and Y anomalies in the graph also provides clues of These zircons have low U and Th contents ranging from 2 to 106 ppm
mixing (Fig. 8b) (Pilet et al., 2011). and 0.5 to 154 ppm, respectively (Supplementary Table 2). The concor-
dant zircon data show 206Pb/238U dates ranging from ca. 88.8 to 95.7 Ma
5. LA-MC-ICPMS zircon U-Pb ages and Hf isotope compositions (n = 16 of 28) (Fig. 9b), whereas the zircon ages of ca. 94.5 and 94.9 Ma
are inferred from the oscillatory zoned zircons from different areas of
We analyzed the zircon grains from the Masafi garnet metagabbro the metamorphic sole (Rioux et al., 2013). The spread in zircon dates
(UAE62A; identical sample with UAE62 in the whole-rock geochemis- may reflect the effect of later geological event (Fig. 9b). These zircon
try), the Aswad olivine websterite (UAE68) and the Dadnah tonalite dates are overlapped with previously reported medium to high-grade
(UAE55A-1, UAE55A-2; identical samples with UAE55 in the whole- metamorphism of the Semail ophiolite (Styles et al., 2006; Searle
rock geochemistry) samples. The U-Th-Pb results are listed in Supple- et al., 2015; Roberts et al., 2016). The zircon grains from the sample
mentary Table 2, and are illustrated on concordia diagrams in Fig. 9 UAE68 show angular to rounded shapes (80–200 μm) with sector or
with representative CL images of zircon grains. The zircon grains from faint zoning (Fig. 9c). They have low U and Th contents ranging from
shows εHf(t) values plotted against U-Pb ages for the zircon grains an-
alyzed from Masafi garnet metagabbro, the Aswad olivine websterite,
and the tonalite samples. The εHf(t) values are all positive, ranging
from 5.6 to 10.0 for the zircon dates 89–96 Ma (UAE62A), 14.5–22.6
for the dates of 91–96 Ma (UAE68), and 5.1 to 10.0 for the dates of
87–92 Ma (UAE55A-1, UAE55A-2) (Supplementary Table 3; Fig. 10).
6. SHRIMP zircon U-Pb ages and Oxygen isotope compositions
Zircon grains were also analyzed using a SHRIMP-IIe/MC for the

Masafi metagabbro (UAE62B; identical sample with UAE62 in the
whole-rock geochemistry) and the Dadnah tonalite (UAE55B; identical
sample with UAE55 in the whole-rock geochemistry). The U-Th-Pb re-
sults are listed in Supplementary Table 4, and are illustrated on
concordia diagrams in Fig. 11 with representative CL images of zircons.
The sample UAE62B has a euhedral to subhedral and predominantly
transparent oval or short prismatic shape zircon population, which
have a length of 70–200 μm and are dominated by oscillatory zoning
(Fig. 11a). These zircons have extremely low U and Th contents ranging
from 5 to 48 ppm and 0.8–12.5 ppm, respectively, and low Th/U ratio
ranging from 0.064 to 0.455 (Supplementary Table 4). The results
show the 206Pb/238U apparent dates ranging from ca. 87.8 to 124.1 Ma,
probably reflecting the metamorphic effect (Fig. 11b). They yield a
weighted mean age of 98.0 ± 1.5 Ma with high MSWD of 9.4 (n =
42) excepting for four dates among the analyzed points. The sample
UAE55B contains abundant euhedral- and long prismatic-shaped zircon
population, which are translucent light-brown to colorless. In the CL im-
ages, the euhedral- to prismatic-shaped grains show a length of
50–200 μm and a length to width ratio of 1:1 to 5:1 showing obvious os-
cillatory and sector zoning in the CL images (Fig. 11c). They show a
range of U contents (148–545 ppm) and moderate Th/U ratios
Fig. 8. Plots of (a) chondrite-normalized rare earth element (REE) and (b) primitive (0.598–0.719) (Supplementary Table 4). Thirty spots were analyzed
mantle-normalized trace element distribution diagrams for the Masafi garnet on the representative zircon grains with mostly concordant zircon
metagabbros (after Sun and McDonough, 1989). dates ranging from ca. 87.2 to 93.1 Ma, yielding a weighted mean
206
Pb/238U age of 89.98 ± 0.62 Ma (MSWD = 10.1; n = 30) (Fig. 11d).
The results of the SHRIMP oxygen isotope analyses of the same zir-
10 to 100 ppm and 3 to 48 ppm, respectively, the Th/U ratio ranging con grains are presented in Supplementary Table 5, and are illustrated
from 0.223 to 0.518 (Supplementary Table 2). These zircons show in Fig. 12. The zircon grains from the Masafi garnet metagabbro
206
Pb/238U apparent dates ranging from ca. 90.8 to 96.1 Ma (n = 23), ex- (UAE62B) have δ18O(zircon) values ranging from 4.96‰ to 8.33‰
cluding two spots that are statistical outliers based on the t-test (mean 6.05 ± 0.16‰, n = 17), having 18O/16O ratios of
(Fig. 9d). On the contrary, zircon grains from tonalite samples 0.002015–0.002022. On the other hand, zircon grains from the Dadnah
(UAE55A-1, UAE55A-2) from the Dadnah area show well defined oscil- tonalite (UAE55B) have much higher δ18O(zircon) values ranging from
latory/sector zoning. Most zircon grains have the euhedral- to 12.80‰ to 15.53‰, and a weighted mean δ18O(zircon) of 14.32 ±
prismatic-shapes showing a length of 100–300 μm, and their aspect ra- 0.25‰ (n = 22) with 18O/16O ratios of 0.002032–0.002036.
tios are vary ranging from 1:1 to 6:1 (Fig. 9e, g). They show a range of
188–673 ppm U contents and moderate Th/U ratios (0.197–0.891) 7. Metamorphic P-T conditions of garnet metagabbros
(Supplementary Table 2). The zircon data from each tonalite sample
shows two age groups with slight differences. The results from the sam- The Masafi garnet metagabbro within the mantle section of the Khor
ple UAE55A-1 show zircon dates ranging from ca. 87.0 to 88.3 Ma (n = Fakkan Block is composed of garnet, clinopyroxene, amphibole with
6) and from ca. 89.8 to 91.7 Ma (n = 19), while the older group yield minor plagioclase, zoisite, ilmenite, titanite, zircon, rutile and magnetite
weighted mean 206Pb/238U age of 90.6 ± 0.16 Ma (MSWD = 1.5; n = (Fig. 13). Zoisite and rutile grains are present as inclusions within the
19) (Fig. 9f). The younger and older groups of zircons from the sample garnet and clinopyroxene (Fig. 13a). Ilmenite grains occur inside
UAE55A-2 show zircon dates range from ca. 87.7 to 89.6 Ma (n = 11) titanite, which is surrounded by amphibole (Fig. 13b, c). Several in
and from ca. 90.8 to 91.9 Ma (n = 14), respectively (Fig. 9h). The calcu- situ partial melt pockets/pods enriched in plagioclase and K-feldspar
lated weighted mean ages are 89.25 ± 0.17 Ma (MSWD = 0.64, n = 10 are noted inside garnet and amphibole rich areas (Fig. 13b–f). Pyrox-
of 11) and 91.24 ± 0.19 Ma (MSWD = 0.33, n = 14), respectively. enes grow either at the rim of garnet or are located inside the amphibole
The same zircon grains from the Masafi garnet metagabbro regime (Fig. 13a, c, e). Plagioclase is scattered in diverse forms, and
(UAE62A) and the Aswad olivine websterite (UAE68) have 176Lu/177Hf those at the rim of amphiboles are anhedral, and inside garnet and am-
ratios of 0.000033–0.000926 and 0.000943–0.046419, respectively, phibole is mostly subhedral. Grains with size b 10 μm occur inside the
and present-day 176Hf/177Hf ratios of 0.282870–0.283000 and melt pockets/pods (Fig. 13a, b).
0.283129–0.283360, respectively (Supplementary Table 3). The same Metamorphic P-T condition for the Masafi garnet metagabbro sam-
zircon grains from the tonalite samples UAE55A-1 and UAE55A-2 have ples based on mineral chemical composition is shown on a
176
Lu/177Hf ratios of 0.001616–0.004620 with present-day 176Hf/177Hf pseudosection (Figs. 14 and 15; Supplementary Table 6). The XMg
ratios of 0.282889–0.283002, and 176Lu/177Hf ratios of (MgO/MgO + FeO) of garnet in the garnet metagabbro is 0.16–0.23,
0.002462–0.004485 with present-day 176Hf/177Hf ratios of and the chemical composition indicates dominantly almandine
0.282865–0.282984, respectively (Supplementary Table 3). Fig. 10 (Fig. 14a). However, the Fe content decreases from core to rim. The
Fig. 9. Scanning electron microscope cathodoluminescence (CL) images of sectioned zircon grains and concordia plots of LA-MC-ICPMS U-Pb zircon age dating from the garnet metagabbro
(UAE62A) (a) and (b), olivine websterite (UAE68) (c) and (d), tonalites (UAE55A-1, UAE55A-2) (e)–(h). Age and Hf analysis spots are represented by solid circles with black numbers and
dashed circles with blue numbers, respectively.
XMg (MgO/MgO + FeO) of the pyroxene from the garnet metagabbro is recognized based on composition (Leake et al., 1997): hornblende of
in the range of 0.55–0.58, and pyroxene inclusions in garnet show calcic group and actinolite (Fig. 14d).
XMg = 0.67–0.69, which indicates dominant diopside content It is noticed that prograde stage assemblages like amphibole, plagio-
(Fig. 14b). The plagioclase grains grown at the rim of amphibole show clase, zoisite and rutile present in peak garnet and clinopyroxene
andesine to labradorite composition, indicating an intermediate mem- (Fig. 13). This suggests that the reactions were not complete and
ber of the plagioclase series in the range of XAn = 0.47–0.52 (Fig. 14c). minor quartz should be present in the prograde and peak stages. The
Other plagioclase grains in matrix and inclusions within garnet are an- formation of retrograde stage sphene and amphibole supports the reac-
orthite within the range of XAn = ~1.0. Two types of amphiboles are tions that were driven in the presence of quartz. The silica
Fig. 10. Plots of εHf(t) values versus 206Pb/238U age for zircons from the olivine websterite Fig. 12. Plots of δ18O values versus 206Pb/238U age for zircons from the garnet metagabbro
(UAE68), garnet metagabbro (UAE62A) and tonalites (UAE55A-1, UAE55A-2). (UAE62B) and the tonalite (UAE55B). The δ18O values of the mantle zircons are from
Valley et al., 2005.
undersaturated stage or phases are not observed both in the thin-

sections and the pseudosection. The pseudosection calculated using and 1.35 GPa by comparing the pseudosection isopleths with the ana-
the bulk chemistry of samples shows that quartz was stable at the pro- lyzed mineral composition, equivalent to a crustal depth of
grade, peak and retrograde stages under silica saturated system. This ~25–30 km. Following the identification of the peak metamorphic
suggests that a minor amount of quartz was present in these rocks dur- stage, the prograde and retrograde evolution is identified by correlating
ing entire processes, which is identified in the matrix and melt pockets assemblages in the phase diagram with that observed in the mineral
along with K-feldspar and plagioclase (Figs. 4 and 13). textures (Figs. 13, 14, 15). An initial prograde condition was identified
Garnet with maximum XMg = 0.23 and clinopyroxene with maxi- based on the stability of zoisite and rutile in the phase diagram
mum XMg = 0.69 indicate a peak P-T condition of around 700–750 °C (Fig. 15). These assemblages are present as inclusions in garnet. The
Fig. 11. Scanning electron microscope cathodoluminescence (CL) images of sectioned zircon grains and concordia plots of SHRIMP U-Pb zircon age dating from the garnet metagabbro
(UAE62B) (a) and (b), and the tonalite (UAE55B) (c) and (d). Age and Oxygen analysis spots are represented by solid circles with black numbers and dashed circles with pink
numbers, respectively.
Fig. 13. Representative back-scattered electron (BSE) images showing mineral assemblages of garnet metagabbros from the Masafi area in the Khor Fakkan Block of the Semail ophiolite.
Abbreviation: Amph, amphibole; Cpx, clinopyroxene; Grt, garnet; Ilm, ilmenite; Pl, plagioclase; Px, pyroxene; Rt, rutile; Ttn, titanite; Zo, zoisite.
presence of zoisite and rutile inclusions in garnet (Fig. 13) indicates pro- observed in the thin-sections should represent intense melting and
grade high-pressure conditions during the initial metamorphic stage melt loss during the peak metamorphism. Finally, the retrograde stage
(Smith, 1988; Carswell, 1990). During this stage, the P-T path in the is identified based on the stability of titanite and ilmenite. These min-
phase diagram evolves from low- to high-pressure transition along erals are present on the grain boundaries of garnet, amphibole and py-
the thermal gradient typically observed in subduction settings (Liu roxene (Fig. 13). Titanite, which grows along with ilmenite (Fig. 13), is
et al., 1996), where the protolith of metagabbro crystallized considered as a mineral that can be stabilized at lower temperatures
garnet along with zoisite and rutile. Following this, the P-T path takes and pressures during later hydration and exhumation (Liou et al.,
a sharp turn towards the region of the high thermal gradient. At this 1998). The results above suggest a clockwise path for the Masafi garnet
stage, the rock underwent partial melting (appearance of melt phase) metagabbro during its metamorphic evolution (Fig. 15).
and clinopyroxene starts appearing in the phase diagram. The calcula-
tion shows that ~10 vol% melt could be produced at the peak conditions 8. Discussion
(Supplementary Fig. 1). The abundant melt pockets are observed in the
thin-sections (Fig. 13). Quantitatively, a hundred of such melt pockets 8.1. Petrogenesis of the garnet metagabbro and tonalite
having an average size of ~0.0016 cm2 are identified in a thin-section
scale (3.5 × 2.5 cm2). It gives an approximate 2% melt pocket in the The metamorphic sole rocks of this study from the Semail ophiolite
thin-section. This suggests that sufficient amounts of melt were lost represent mélanges consisting of both sedimentary (marble and quartz-
from the rock during metamorphism. About 5–8 vol% melt is considered ite) and igneous (garnet metagabbro) protoliths (Fig. 3) occurred along
as the conditions for melt loss (Diener and Fagereng, 2014) and such major thrust boundaries, and subjected to amphibolite to granulite fa-
systems could retain a 1–2% melt (Sawyer, 2001). Melt pockets cies metamorphism. In the Semail ophiolite, the high-grade
Fig. 14. Mineral composition of (a) garnet, (b) clinopyroxene, (c) plagioclase and (d) amphibole from Electron probe micro-analysis (EPMA).
metamorphic sole rocks, commonly called garnet amphibolite, have the Masafi garnet metagabbros are plotted in the tholeiitic to transi-
been interpreted by either a mid-ocean ridge basalt (MORB) source tional basalt fields of E-MORB setting, indicating their evolution from
from partial melting of the subducted oceanic slab that melted during enriched mantle source via within plate enrichment (Fig. 16a). In addi-
the initial stages of the supra-subduction zone (e.g. Boudier and tion, the Hf/3-Th-Ta plot (Wood, 1980) of the garnet metagabbros also
Coleman, 1981; Boudier et al., 1988; Cowan et al., 2014) or a mafic shows their E-MORB geochemical characteristics (Fig. 16b). These are
magma with enriched mantle composition that was generated on top contrary to the N-MORB characteristics with tholeiite trend from
of a subducted slab (e.g. Searle and Malpas, 1980, 1982; Pearce et al., Phase 1 volcanic/magmatic units of the Semail ophiolite (Pearce et al.,
1981; Searle and Cox, 1999, 2002; Warren et al., 2003, 2005; 1981; Alabaster et al., 1982). The E-MORB characteristics generally indi-
Whattam and Stern, 2011). cate the influence of an enriched mantle reservoir, implying that chem-
The first line of evidence for the source magma comes from the geo- ical components released by partial melting of the subducted oceanic
chemical characteristics of the Masafi garnet metagabbro preserved in slab might have played an important role for the genesis of the garnet
the metamorphic sole. The enrichment of LREE indicates the influx of metagabbro preserved in the metamorphic sole of the Semail ophiolite
fluid enriched in incompatible elements during the formation of these in the Masafi area (Pearce and Peate, 1995; Hawkesworth et al., 1997).
rocks (Fig. 8a). Depletion of the HSFE in the primitive mantle- The second evidence is from the zircon Hf and oxygen isotope com-
normalized trace element patterns indicates the presence/crystalliza- positions from the same garnet metagabbro samples. Zircon is a robust
tion of garnet during the formation of gabbroic partial melt (Fig. 8b). mineral which is resistant to Hf mobility and contamination (e.g. Iizuka
Positive Eu anomaly indicates that plagioclase crystals are not settled et al., 2017 and references therein). For geochemical comparisons, Hf
in the residue, but mostly present in the gabbroic melt (Fig. 8b). The analysis has been carried out for the Phase 1 Aswad olivine websterite
large negative Y anomaly might be attributed to the settling of garnet representing a mantle section, and both Hf and oxygen isotope analyses
and pyroxene (Fig. 8b). In the Ta/Yb vs. Th/Yb diagram (Pearce, 1983), for the Phase 2 Dadnah tonalite samples, commonly called plagiogranite
ophiolite in UAE and Oman. They suggested that these granitoids having
extremely high δ18O(zircon) values indicate that pelitic/siliceous sedi-
ments were incorporated during the partial melting, and were intruded
into the mantle section of the ophiolite during subduction stage at ca.
99.8 Ma before obduction. On the other hand, Joun et al. (2019) re-
ported a possible source for the Phase 1 low δ18O(H2O) tonalite (ca.
98.6–94.9 Ma) in Dadnah area of the Semail ophiolite, suggesting that
it most likely formed from a complex process involving partial melting
of the subducted recycled oceanic slab having MOR-type composition
with minor contamination of oceanic sediments that were subsequently
interacted with hydrothermally altered sub-arc mantle during the ini-
tial stage of the supra-subduction. The Phase 2 peraluminous S-type
tonalite from this study in the Dadnah area of the Semail ophiolite sec-
tion has more similar characteristics with Phase 1 tonalite from Spencer
et al. (2017) in terms of its geochemical composition and oxygen iso-
tope characteristics, regardless of their age difference (Fig. 12). It is no-
table that both the Phase 1 and Phase 2 peraluminous granitoids from
the Semail ophiolite have high δ18O(zircon) values (14–28‰ and
13–15‰), requiring mixing of melted subducted marine sediments
with mafic magma that was generated by the hydrous partial melting
of a mantle wedge metasomatized by fluids released from the
subducted slab (Spencer et al., 2017; Wang et al., 2017). The
metagabbro samples have more evolved compositions (δ18O(zircon) =
5–8‰) from the mantle values (δ18O(zircon) = 5.3 ± 0.3‰; Valley
et al., 1998, 2005) (Fig. 12). These differences might reflect the differ-
Fig. 15. Phase equilibria diagram showing a clockwise path for the garnet metagabbros ences in the amount of subduction-related chemical components for
from the Masafi area in the Khor Fakkan Block of the Semail ophiolite, UAE. their petrogenesis (Valley et al., 2005). The presences of amphibole
and anorthite without inclusion as well as feldspar melts inside the gar-
that intruded into the mantle section of the Khor Fakkan Block. In this net support the hydrous nature of the mantle wedge during protolith
study, the zircons from the garnet metagabbro yielded positive εHf formation of the garnet metagabbro (e.g. Beard, 1986;
(t) values of 5.6 to 10.0 for corresponding zircon dates range from 89 Panjasawatwong et al., 1995; Sisson et al., 1996; Takagi et al., 2005)
to 96 Ma, and 5.1 to 10.0 for the dates range from 87 to 92 Ma for the (Fig. 13).
tonalite samples (Fig. 10; Supplementary Table 3). The Th/U ratios of In summary, geochemical evidence in conjunction with zircon Hf
the analyzed dates indicate possible metamorphic and magmatic ori- and oxygen isotope data from the garnet metagabbro in the metamor-
gins for the garnet metagabbro and tonalite samples, respectively. phic sole and tonalite that intruded the mantle section of the Semail
Those from the olivine websterite yielded positive εHf(t) values of ophiolite indicate that positive εHf(t) and high δ18O(zircon) values
14.5 to 22.6 for the date range of 91–96 Ma (Fig. 11; Supplementary might be the mixing product of partial melting of subducted slab and
Table 3). Hf is an incompatible element that remains predominantly in supracrustal material on top of it with hydrothermally altered mantle
the depleted mantle during progressive partial melting (e.g. Iizuka wedge. The similarity and difference in geochemical characteristics re-
et al., 2017 and references therein). The previous data of positive εNd flect changes in rate and style of recycled supracrustal material-
values for protolith of the metamorphic sole rocks from the Semail hydrous mantle interaction during subduction to obduction processes.
ophiolite were interpreted as the product of a mixture of mafic
magma with a sedimentary component (Haase et al., 2015; Rioux 8.2. Petro-tectonic evolution of the metamorphic sole of the Semail
et al., 2016). On the contrary, trondhjemite dike in mantle section and ophiolite, UAE
the tonalite pod from the metamorphic sole show negative εNd values
(Rioux et al., 2013), indicating partial melting of subducted oceanic The metamorphic sole preserved in the Masafi area from this study
slab and/or sediment for their petrogenesis (Patchett et al., 2004; occurs within a tectonic zone bounded by major thrusts at the base of
Faure and Mensing, 2005; Bouvier et al., 2008), having positive εHf the Semail ophiolite, where slivers originated from ocean plate strati-
values that are comparative to the result of this study. Both garnet graphic sequence metamorphosed up to the granulite facies including
metagabbro and tonalite samples from this study have identical εHf marble, quartzite and garnet metagabbro are preserved within the mé-
values (Fig. 10), suggesting a strong relevance to their mantle source lange. The normal metamorphic sole with narrow (~50–150 m) section
(Patchett et al., 2004; Faure and Mensing, 2005; Bouvier et al., 2008). of amphibolite and greenschist facies units has been interpreted to be
The results of vertical to high-angle slope on εHf(t) vs. age diagram in- formed by heat transfer from the incipient mantle wedge towards the
dicate, for both the Masafi garnet metagabbro and the Dadnah tonalite top of the subducting plate with inverse metamorphic gradient, waning
samples, either mixing of crustal material and depleted upper mantle away from the overriding obducted slab as an extreme form of thermal
or rapid evolution of nascent crust (Fig. 10). metamorphism during the first million years of intra-oceanic subduc-
Oxygen isotope (δ18O) provides a powerful tool to detect the contri- tion (Searle and Malpas, 1980; Boudier et al., 1988; Hacker et al.,
bution of crustal material to mantle-derived melt thorough transport of 1996; Hacker and Gnos, 1997; Gnos, 1998; Searle and Cox, 1999,
sediments to the mantle via subduction (e.g. Spencer et al., 2017). In 2002; Wakabayashi and Dilek, 2003; Cowan et al., 2014; Searle et al.,
contrast to the identical positive εHf values (Fig. 10), both the Masafi 2015; Soret et al., 2017). This type of early stage subduction process
garnet metagabbro and the Dadnah tonalite samples from this study was suggested as an obduction-related metamorphism at 700–900 °C
yield distinctly different values of high δ18O(zircon) of ~13–16‰ for and 0.4–1.3 GPa, mostly based on garnet–clinopyroxene thermo-
the tonalite and ~5–8‰ for the metagabbro samples (Fig. 12). Recently, barometry (Ghent and Stout, 1981; Searle and Malpas, 1982; Gnos
Spencer et al. (2017) reported unusually high δ18O(zircon) values (14 to and Kurz, 1994; Hacker et al., 1996; Hacker and Gnos, 1997; Gnos,
28‰; values relative to Vienna standard mean ocean water [VSMOW]) 1998; Searle and Cox, 2002; Cowan et al., 2014) during ca. 95 to
from Phase 1 peraluminous, granitoid samples from the Semail 92 Ma (Styles et al., 2006; Roberts et al., 2016). Previous P-T estimates
based on multi-equilibrium thermo-barometry reported similar esti-

mates of ~770 °C and ~1.1–1.4 GPa for amphibolites from Sumeini and
Wadi Tayin area of the Semail ophiolite (Searle and Cox, 2002; Cowan
et al., 2014). Recently, Soret et al. (2017) suggest subdivision of meta-
morphic sole, from top to bottom, representing P-T conditions of
850 °C and 1 GPa, 725 °C and 0.8 GPa, and 530 °C and 0.5 GPa based
on thermodynamic modeling. The phase equilibria modeling supported
by the mineral inclusions, such as zoisite and rutile in garnet, indicates
high-pressure metamorphism preceding the peak metamorphism
(Skjerlie and Patiño Douce, 2002). The phase equilibria modeling of
the Masafi garnet metagabbro from this study shows similar high-
pressure amphibolite facies metamorphism of ~1.2 GPa and ~660 °C
that preceded the peak granulite facies metamorphism of ~1.35 GPa
and ~770 °C, followed by a lower pressure hydration and decompres-
sion stages (Fig. 15).
In the previous studies, garnet amphibolite from the normal
ophiolite section of this area has been interpreted to have formed dur-
ing intra-oceanic subduction initiation at depths of ~40 km when the
subducted oceanic crust plate is heated by the overlying mantle, scraped
off and accreted to the upper plate, indicating their N-MORB type
subducted slab origin with normal mantle δ18O values of b6‰ via either
counter-clockwise or clockwise P-T paths at ca. 90–98 Ma (Searle and
Cox, 1999, 2002; Cowan et al., 2014; Searle et al., 2015; Soret et al.,
2017; Guilmette et al., 2018). However, this is contrary to the E-
MORB-type sources with positive εHf values and more evolved δ18O
compositions from 5 to 8‰ from the garnet metagabbro from this
study (Figs. 10, 12), that might be originated by complex processes
within a supra-subduction zone through a clockwise P-T path during
similar ages (Figs. 9, 11, 15).
Based on our petrologic, geochemical and zircon U-Pb-Hf-O isotopic
features together with previously published data from the study area,
we make an attempt to propose a petro-tectonic model with summary
chart for the ages of the Semail ophiolite, UAE (Fig. 17). As described
above, high-pressure amphibolite metamorphism preceded the peak
granulite metamorphism, and is followed by decompression and melt-
ing (Fig. 15). This is overlapped with the felsic intrusion ages reported
from the UAE part of the Semail ophiolite (Joun et al., 2019; this
study). Since the ages of the metamorphic sole, the ophiolite formation
and the felsic magmatism closely overlapped (Fig. 17b), a mechanism
including partial melting of the subducted ocean plate stratigraphic sec-
Fig. 16. Plots of (a) Ta/Yb vs. Th/Yb (Pearce, 1983) and (b) Hf–Th–Ta discrimination
tion with synchronous formation/exhumation of the metamorphic sole diagrams (Wood, 1980) for the garnet metagabbros from the Masafi area in the Khor
is required at an incipient supra-subduction zone (Boudier and Fakkan Block. Vectors for Ta/Yb vs. Th/Yb diagrams show the influence of subduction
Coleman, 1981; Boudier et al., 1988; Cowan et al., 2014; Goodenough component, within-plate component, crustal contamination, non-subduction basalts
et al., 2014; Haase et al., 2015). The chemical components and fluids re- trend and fractional crystallization. Abbreviation: N-MORB, normal type mid-ocean
ridge basalts; E-MORB, enriched type mid-ocean ridge basalts; OIB, ocean island basalt;
leased by partial melting from the subducted older and denser oceanic WPB, within plate basalts; CAB, calc-alkali basalt; IAT, island arc tholeiite; WPA, within
crust (older than Cenomanian in age N ca. 94 Ma) and surface-derived plate alkaline-basalt; WPT; within plate tholeiite. The symbols are the same as in Fig. 8.
marine sediments overridden by young and hot asthenospheric mantle
lowers the temperatures that are required for the partial melting in the
mantle wedge (Murphy, 2006; Joun et al., 2019). Our phase equilibria
modeling suggests metasomatized high-pressure amphibolite facies mélange garnet metagabbro samples from this study (Figs. 15, 17b).
conditions before partial melting and the peak granulite facies meta- Felsic intrusions with similar ages from various areas of the Semail
morphism. This indicates that the gabbroic protolith was at a depth of ophiolite (e.g. Joun et al., 2019 and references therein) show various
~25–30 km, and was metasomatized by fluids released from the magmatic processes with diverse source origin, having both
subducted slab (Spencer et al., 2017; Wang et al., 2017), and is followed peraluminous E-MORB compositions with high δ18O values and
by decompression and partial melting that are coincided with the intru- metaluminous N-MORB type sources with mantle δ18O values regard-
sion of the subducted sediment-originated felsic magma into the mantle less of its ages from ca. 89 to 100 Ma (Rollinson, 2009, 2015; Spencer
wedge (Spencer et al., 2017; this study) (Fig. 17b). This partial melting et al., 2017; Joun et al., 2019). The discontinuous sedimentary cover
phase produced the garnet-pyroxene peak assemblage at the higher along and across the slab interface can be responsible for these various
temperature (Fig. 15). The intense hydration during the final stage at felsic intrusions with wide age ranges observed in the Semail ophiolite
amphibolite facies conditions could have formed amphibole enriched via transport of melted ocean plate stratigraphic section to the hydrous
assemblage along with titanite and ilmenite (Fig. 13). Arc magmatism mantle wedge during subduction to obduction processes.
or forearc extension represented by felsic magmatism (e.g.
plagiogranite and tonalite) in subduction setting (Murphy, 2006; 9. Conclusion
Wang et al., 2017; Joun et al., 2019), which predates obduction, might
generate this condition, and finally obduction and exhumation at ca. Based on the petrologic, geochemical and zircon U-Pb, Hf and O iso-
70 to 93 Ma reduced both temperature and pressure conditions of the topic features of the garnet metagabbro preserved in the metamorphic
Fig. 17. Petro-tectonic model for the formation of the metamorphic sole and the felsic magmatisms (a) and the summary chart for the geochronologic data (b) of the Semail ophiolite, UAE.
Data from (1) McCulloch et al. (1981), Hacker and Gnos (1997), Perrin et al. (2000), Weiler (2000), Warren et al. (2005); (2) Gnos and Peters (1993), Hacker and Gnos (1997), Warren
et al. (2005), Styles et al. (2006), Cowan et al. (2014), Searle et al. (2015), Rioux et al. (2016), Soret et al. (2017), Guilmette et al. (2018), this study; (3) Tilton et al. (1981), Hacker et al.
(1996), Cox et al. (1999), Warren et al. (2005), Spencer et al. (2017), Joun et al. (2019) and references therein, this study; (4) Hacker and Gnos (1997), Jacobs et al. (2015), Rollinson
(2015), Roberts et al. (2016), Joun et al. (2019) and references therein.
sole and the tonalite intruded the mantle section of Khor Fakkan Block CRediT authorship contribution statement
of the Semail ophiolite presented in this study, we suggest the new in-
sights into their petro-tectonic evolution during subduction to Soujung Kim: Investigation, Data curation, Validation, Writing -
obduction processes: original draft, Writing - review & editing, Visualization. Yirang Jang: In-
vestigation, Data curation, Validation, Writing - original draft, Writing -
1. The positive εHf(t) and high δ18O(zircon) values from the garnet review & editing, Visualization. Sanghoon Kwon: Conceptualization,
metagabbro in the metamorphic sole and the tonalite that intruded Writing - original draft, Supervision, Funding acquisition. Vinod O.
the mantle section of the Semail ophiolite might reflect partial melt- Samuel: Data curation, Validation. Sung Won Kim: Data curation,
ing of the subducted slab/supracrustal material, and their subse- Funding acquisition. Seung-Ik Park: Data curation, Validation. M.
quent interaction with hydrothermally altered mantle at a mantle Santosh: Conceptualization, Methodology. Sotirios Kokkalas: Concep-
wedge, reflecting changes in rate and style of their interaction during tualization, Investigation.
subduction to obduction processes.
2. The phase equilibria modeling of the garnet metagabbro shows high-
Declaration of competing interest
pressure amphibolite facies metamorphism that preceded the peak
granulite facies metamorphism (around 700–750 °C and 1.35 GPa),
The authors declare that they have no known competing financial
followed by lower pressure hydration and decompression showing
interests or personal relationships that could have appeared to influ-
clockwise P-T path, which might reflect partial melting and differen-
ence the work reported in this paper.
tiation at a mantle wedge above the subducted slab.
3. The heterogeneous geochemistry from the metamorphic sole rocks
Acknowledgments
and the felsic intrusions from this study together with those from
previous researches might be the result of the degree of partial melt-
This research was supported by the Korea Institute of Geoscience
ing of the ocean plate stratigraphic sequences including recycled oce-
and Mineral Resources (KIGAM) research project (2015-11-1637; De-
anic slab and surface-derived marine sediments into a magma that
velopment of IOR/EOR technologies and field verification for carbonate
interacted with the hydrous mantle wedge during subduction to
reservoir in UAE), funded by the Ministry of Science and ICT (Informa-
obduction processes that formed the Semail ophiolite.
tion, Communication and Technology), Korea to S. Kwon. SK also appre-
Supplementary data to this article can be found online at https://doi. ciates the partial support by the NRF-2017R1A6A1A07015374 (Multi-
org/10.1016/j.gr.2020.05.013. disciplinary study for assessment of large earthquake potentials in the
Korean Peninsula) and NRF-2019R1A2C1002211 through the National Gnos, E., 1992. The Metamorphic Rocks Associated With the Semail Ophiolite (Sultanate
of Oman and United Arab Emirates). University of Berne, Switzerland (Ph.D. thesis).
Research Foundation of Korea funded by the Ministry of Science and Gnos, E., 1998. Peak metamorphic conditions of garnet amphibolites beneath the Semail
ICT. Y. Jang appreciate the support by a Basic Research Project ophiolite: implications for an inverted pressure gradient. Int. Geol. Rev. 40, 281–304.
(GP2020-003; Geological survey in the Korean Peninsula and publica- Gnos, E., Kurz, D., 1994. Sapphirine-quartz and sapphirine-corundum assemblages in
metamorphic rocks associated with the Semail ophiolite (United Arab Emirates).
tion of the geological maps) of the KIGAM, funded by the Ministry of Sci-
Contrib. Mineral. Petrol. 113, 325–332.
ence and ICT. This work represents part of S. Kim's M.S. Research Gnos, E., Nicolas, A., 1996. Structural evolution of the northern end of the Oman Ophiolite
performed at Yonsei University. S. Kim, Y. Jang and S. Kwon equally con- and enclosed granulites. Tectonophysics 254, 111–137.
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Gondwana Research 86 (2020) 203–221

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/gr

Petro-tectonic evolution of metamorphic sole of the Semail

6. SHRIMP zircon U-Pb ages and Oxygen isotope compositions

Zircon grains were also analyzed using a SHRIMP-IIe/MC for the

undersaturated stage or phases are not observed both in the thin-

based on multi-equilibrium thermo-barometry reported similar esti-

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