The Marbat metamorphic core-complex (Southern Arabian Peninsula): Reassessment of the evolution of a Neoproterozoic island-arc from petrological, geochemical and U-Pb zircon data

Accepted Manuscript The Marbat metamorphic core-complex (Southern Arabian Peninsula): reassessment of the evolution of a Neoproterozoic island-arc from petrological, geochemical and U-Pb zircon data Pierre Barbey, Yoann Denèle, Jean-Louis Paquette, Julien Berger, Jérôme Ganne, Damien Roques PII: DOI: Reference:

S0301-9268(17)30416-3 https://doi.org/10.1016/j.precamres.2017.12.013 PRECAM 4958

To appear in:

Precambrian Research

Received Date: Revised Date: Accepted Date:

19 July 2017 28 November 2017 4 December 2017

Please cite this article as: P. Barbey, Y. Denèle, J-L. Paquette, J. Berger, J. Ganne, D. Roques, The Marbat metamorphic core-complex (Southern Arabian Peninsula): reassessment of the evolution of a Neoproterozoic islandarc from petrological, geochemical and U-Pb zircon data, Precambrian Research (2017), doi: https://doi.org/ 10.1016/j.precamres.2017.12.013

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The Marbat metamorphic core-complex (Southern Arabian Peninsula): reassessment of the evolution of a Neoproterozoic island-arc from petrological, geochemical and U-Pb zircon data Pierre Barbey1, Yoann Denèle2, Jean-Louis Paquette3, Julien Berger2, Jérôme Ganne2, Damien Roques2 1

CRPG, UMR 7358, CNRS, Université de Lorraine, BP 20, F-54501 Vandœuvre-lès-Nancy

Cedex, France 2

GET, Université de Toulouse-CNRS-IRD-OMP, 14 Avenue E. Belin, F-31400 Toulouse,

France 3

Laboratoire Magmas et Volcans, Campus Universitaire des Cézeaux, 6 Avenue Blaise

Pascal, TSA 60026 – CS 60026, F-63178 Aubière Cedex, France

Corresponding author: Pierre BARBEY E-mail: [email protected]

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Abstract The Marbat basement (Sultanate of Oman) belongs to the Neoproterozoic accretion domain of the Arabian-Nubian shield. We present new geochronological, petrological and geochemical data as an extension of our previous study (Denèle et al., 2017) re-interpreting this basement as a metamorphic core complex (MCC). We showed that this MCC consists of a metamorphic unit (Juffa complex) separated by an extensional detachment from a plutonic unit (Sadh complex and Tonalite plutons). Geochemical data show that the Juffa metasediments correspond to volcanogenic graywackes, suggesting deposition in front of a juvenile magmatic arc. New in situ U–Pb zircon data show that the protolith of the Juffa paragneisses is Tonian in age (960–830 Ma). The Juffa complex recrystallized under amphibolite facies conditions (950 MPa, 630°C) corresponding to thermal gradient of 17–18°C/km, i.e. close to that observed in fore-arc environment subjected to tectonic underplating. It was then retrogressed (< 400 MPa, < 400°C) during exhumation of the MCC. Phase assemblages of the Sadh complex and Tonalite plutons record magmatic temperatures estimated at 700–840°C and 610–840°C from hornblende and zircon-saturation thermometry, respectively; pressures are grossly estimated at ca. 250 MPa. These data and structural evidence led us to consider that the Sadh complex is a plutonic unit emplaced during exhumation of the MCC. Hence, we re-interpret the previously published zircon core U–Pb ages of the Banded gneisses (ca. 860–830 Ma) as inherited, and zircon rim ages (ca. 815 Ma) as the age of their intrusion. This suggests that the parent magma of all these granitoids could have interacted with volcanic material equivalent to the Juffa complex. Overall, the Marbat area developed during the Tonian and involved (i) deposition of volcanosediments (Juffa complex) in an island-arc environment (960–830 Ma); (ii) metamorphism under middle pressure conditions during tectonic underplating at ca. 820–815 Ma, followed by emplacement of plutonic rocks in the upper crust during initiation of the MCC at ca. 815–810 Ma; and (iii) late-tectonic intrusions associated with MCC amplification at ca. 800–790 Ma (Mahall intrusives and Tonalite plutons).

Keywords Neoproterozoic; Arabian-Nubian shield; Oman; Metamorphic core complex; Zircon U–Pb; Tectonic underplating

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Highlights - The Juffa complex is a Tonian (ca. 850 Ma) juvenile volcano-sedimentary series - The Sadh complex is a plutonic unit (ca. 815–800 Ma) mantling the Marbat core complex - The Marbat core complex is an exhumed fore-arc crust with felsic lower crust

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1. Introduction

The Arabian-Nubian shield (ANS) corresponds to the northern part of the East-AfricanAntarctic Orogen, a Neoproterozoic transcontinental orogen, which runs from Antarctica to Egypt (Stern, 1994; Meert, 2003; Jacobs and Thomas, 2010), and result from the diachronous collision between East and West Gondwana (e.g. Kusky et al., 2003). The ANS, which extends along the Red Sea and Gulf of Aden (Fig. 1), is a complex association of intraoceanic arcs accreted in the 880-630 Ma period, with magmatism extending until ca. 570 Ma (Kröner, 1985; Windley et al., 1996; Whitehouse et al., 1998; Stoeser and Frost, 2006; Stern et al., 2010). Farther east, the Neoproterozoic rocks of the ANS, covered by the Cambrian to Phanerozoic sedimentary rocks of the Arabian platform, are exposed in small windows at the Marbat region (Sultanate of Oman) and Socotra Island (Yemen), which were close prior to the Oligo-Miocene rifting (d’Acremont et al., 2006; Leroy et al., 2010). These two areas are interpreted as sites of extensive calc-alkaline magmatism during the Neoproterozoic, linked to the development of an island-arc (Marbat area; Platel et al., 1987b; Mercolli et al., 2006), or to an Andean-arc / back-arc context (Socotra Island; Denèle et al., 2012).

The basement exposure of the Marbat area is an interesting area to study the sequence of development of a juvenile crustal segment, from the deposition and burial of sediments to its exhumation; and to study the relationships between the metamorphic and intrusive units. A number of interesting results (Würsten, 1994; Briner, 1997; Mercolli et al., 2006; Rantakokko et al., 2014) show that crustal growth occurred over a period of ca. 100 Ma, from 850 to 750 Ma. It includes the deposition of a thick clastic unit in arc environment, deformed and recrystallized as paragneiss under high-grade conditions (Juffa complex); and emplacement of calc-alkaline, mafic to silicic igneous rocks, recrystallized as orthogneiss under amphibolite facies conditions (Sadh complex) and juxtaposed to the Juffa complex. More recently, Roques (2016) and Denèle et al. (2017) have re-interpreted the Marbat region as a metamorphic core complex (MCC) exhumed in relation with syn-accretion strike-slip tectonics. They show that the contact between the Juffa complex (lower plate) and the Sadh complex (upper plate) corresponds to an extensional detachment; and suggest that the orthogneisses of the Sadh complex do not correspond to metamorphic rocks but to syntectonic plutonic rocks emplaced during the exhumation of the MCC. Besides, the ages of the source and sedimentation of the Juffa paragneisses still remain poorly defined from Nd model ages (Mercolli et al., 2006).

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This has led us to revisit the whole evolution of the Marbat area and to propose a revised model.

In this paper, we combine new petrological, geochronological and geochemical data to previously published results, in order to reassess the Tonian evolution of the Marbat metamorphic core complex and to compare it with the basement of the Island of Socotra. We focus our study (i) on determining the nature and age of the metasediments, (ii) on comparing the evolution of the P–T conditions in the paragneiss and orthogneiss complexes, and (iii) on establishing the relationships between the formation of the metamorphic core complex and the calc-alkaline plutonism.

2. Geological setting

The crystalline basement of the Marbat area (Platel et al., 1987a, b, c, d; Mercolli et al., 2006) has been subdivided into four main units (Fig. 2) sealed by Cryogenian/Ediacaran clastic sediments of the Huqf Supergroup (Allen, 2007; and references therein): Juffa complex, Sadh complex, Tonalite plutons and Late intrusives. Their main petrological, geochemical and geochronological features are presented below from the works of Mercolli et al. (2006) and Rantakokko et al. (2014).

(1) The Juffa complex consists of biotite-muscovite paragneisses containing layers of amphibolites and lenses of ultramafic rocks. The amphibolites and ultramafic lenses, reaching locally 10 m in thickness and 200 m in length, are located at the top of the complex (Würsten, 1994). Incipient anatexis of paragneisses is outlined by the development of leucosome and melanosome layers, parallel to the foliation. This complex is interpreted by Mercolli et al. (2006) as a tectonic assembly of allochthonous and heterochronous units. The gneisses are considered to represent a clastic sedimentary sequence involving protoliths with a minimum age of 1.3 Ga and deposited after 1000 Ma.

(2) The Sadh complex is subdivided into Banded gneisses and Mahall intrusives. The Banded gneisses correspond to a heterogeneous unit of mafic to silicic, anatectic gneisses intruded by amphibolite dykes. The gneisses, which underwent polyphased deformation and recrystallization under amphibolite facies conditions, are considered to be mainly of magmatic origin. The Mahall intrusives consist of diorites to tonalites intruding in the Banded 5

gneisses, deformed and recrystallized into amphibolite and gneisses. The Banded gneisses and Mahall intrusives were deformed together during the main tectonic event. Neodymium model ages suggest a rather homogeneous source for the Banded gneisses (960 < TDM < 910 Ma) and Mahall intrusives (950 < TDM < 880 Ma) (Mercolli et al., 2006). Crystallization ages of the magmatic protoliths of the Banded gneisses are estimated at 831 ± 7 Ma and 835 ± 6 Ma (Rantakokko et al., 2014), whereas emplacement of the Mahall intrusives is dated at ca. 799 ± 5 Ma (Mercolli et al., 2006).

(3) The Fusht (hornblendite, diorite and tonalite) and Hadbin (tonalites and granodiorites) plutons mark the end of the large-scale ductile deformation within the basement and are considered to be emplaced at the end of the main deformation event. Zircon U–Pb dating (Mercolli et al., 2006) shows that these rocks emplaced at 783 ± 7 Ma (Fusht) and 786 ± 6 Ma (Hadbin). To this group should be added the Marbat granodiorite dated at ca. 795 Ma (Rantakokko et al., 2014).

(4) The Late intrusives comprise the Leger granite body (ca. 720 Ma), aplite–pegmatite dykes (ca. 750 Ma), and basaltic to rhyolitic dykes (Shaat dyke swarm) emplaced at ca.750–700 Ma (Worthing, 2005; Mercolli et al., 2006; Bowring et al., 2007; Roques, 2016).

Metamorphic P–T conditions for the Juffa group, initially estimated by Würsten (1994), were reassessed by Briner (1997) using the TWEEQU program (Berman, 1988, 1990, 1991). The results synthesized in Mercolli et al. (2006) show that the peak conditions in the paragneisses, estimated at 650°C – 650 MPa, are followed by retrogression at ca. 600°C down to 350 MPa. The Banded gneisses are regarded as polymetamorphic and recrystallized at higher temperature than the paragneisses of the Juffa complex; whereas the Mahall intrusives are considered as monophased and recrystallized under conditions similar to those of the Juffa complex. P–T conditions in the Banded gneisses were estimated from mineral assemblage in calc-silicate rocks using TWEEQU, and from plagioclase-amphibole thermometry and Al-inhornblende barometry on the tonalitic and granodioritic orthogneisses, using the calibrations of Holland and Blundy (1994) and Schmidt (1992), respectively. Estimates are 636°C – 460 MPa from the calc-silicate rocks and 710°C – 460 MPa from amphibole thermobarometry. A three-stage evolution is proposed for the Sadh complex (Briner, 1997; Mercolli et al., 2006), involving: (i) the migmatisation of the Banded gneisses under high P–T conditions; (ii) then, their juxtaposition to the Juffa complex during subduction, followed by the emplacement of 6

the Mahall intrusives; and finally (iii) deformation and partial melting of both the Juffa and Sadh complexes under low-P, high-T conditions. U–Pb dating on zircon fractions and Pb–Pb isochron age on garnet from Juffa paragneisses yielded dates of 811 ± 9 Ma and 809 ± 17 Ma, respectively, for the metamorphic peak (Mercolli et al., 2006), in agreement within uncertainties with the age range (820–815 Ma) given by Rantakokko et al. (2014).

Overall and according to Mercolli et al. (2006), the Neoproterozoic evolution of the Marbat basement include (i) the deposition of a clastic sedimentary series (Juffa complex) partly derived from the erosion of an old crystalline basement; (ii) intrusion and recrystallization of the magmatic protoliths of the Banded gneisses followed by their juxtaposition to the Juffa complex in relation with the development of an oceanic island arc; (iii) emplacement of the Mahall intrusives and their deformation under high-T amphibolite facies conditions related to subsequent arc collision; and (iv) intrusion of the Tonalite plutons.

Recent field and structural data have shown that the Neoproterozoic Marbat basement corresponds to a MCC exhumed in a releasing bend along a dextral strike-slip fault system (Roques, 2016; Denèle et al., 2017). According to these authors, the paragneisses (Juffa complex) correspond to the lower plate rocks of the MCC forming a gneiss dome structure, with gently dipping foliation planes and a NE-SW long axis. The orthogneisses and the granitoids of the Sadh complex and Tonalite group constitute the upper plate rocks of the MCC (Fig. 2a, b). Mineral lineations are associated with top-to-the-south shearing in the southern part of the gneiss dome, and top-to-the-north shearing in its northern part; the deformation is mainly coaxial along the dome axis. Microstructures in the paragneisses show homogeneous high to middle temperature (MT–HT) deformation, overprinted by dense lowgrade C’ shear bands close to their contact with the orthogneisses. This suggests syn-cooling strain localization along the contact, which is interpreted as a detachment. Deformation in the overlying orthogneissic and igneous envelope is strongly heterogeneous (encompassing magmatic to mylonitic microstructures), but structurally consistent with that of the paragneisses. HT to MT gneissic and mylonitic structures are widespread to the west and along the detachment (Banded gneisses), whereas in the coastal area foliation trajectories wrap around zones showing mainly magmatic microstructures (corresponding grossly to the Mahall intrusives). In the eastern termination of the dome, the detachment oriented NE-SW branches onto a large-scale N10° shear zone that cuts across the magmatic to submagmatic fabric of the Fusht pluton. The whole structural pattern shows a continuum of deformation 7

from high- to low-temperature conditions, related to exhumation of the gneiss dome. This exhumation is synchronous with emplacement of calc-alkaline magmas more intensely deformed close to the detachment (Banded gneisses) than farther south where the magmatic fabrics are preserved (Mahall intrusives), and farther east where the Fusht and Hadbin plutons were emplaced later.

3. LA–ICP–MS zircon U-Pb data on the Juffa complex

The Marbat basement has been the subject of geochronological studies, more particularly focused on the (meta)-plutonic rocks (e.g. Mercolli et al., 2006; Rantakokko et al., 2014). Here, we aim at estimating the age of the protolith of the sediments of the Juffa complex from in situ analysis of detrital zircon grains. Two paragneiss samples (MR-09 and MR-13) were selected for in situ zircon U–Pb dating by laser ablation–inductively coupled plasma-mass spectrometry (LA–ICP–MS; Laboratoire Magmas et Volcans, Clermont-Ferrand). The samples consist mainly of plagioclase, K-feldspar, quartz, muscovite and subordinate amounts of biotite, garnet, ilmenite and zircon.

Zircon separation was carried out at CRPG-CNRS (Nancy). Grains were concentrated from crushed rock samples by means of heavy liquids and then magnetic separation (Frantz isodynamic separator). Grains were handpicked, mounted in epoxy resin, grinded and polished

to

expose

their

core.

Morphologies

and

textures

were

imaged

with

cathodoluminescence (CL) imaging techniques, using a JEOL JSM-6510 scanning electron microscope (CRPG-CNRS, Nancy). The LA–ICP–MS method used for dating is reported in Paquette et al. (2014) (see details in Supplementary material, §. SM1). U, Th and Pb concentrations were calibrated relative to the certified GJ-1 zircon standard (Jackson et al., 2004). Concordia diagrams were generated using the Isoplot/Ex v.2.49 software package (Ludwig, 2001). The whole U–Pb data set is given in Supplementary material (Table SM2).

Most zircons occur as elongated or stubby grains, with size ranging from 100 to 250 µm. They are pale-yellow, translucent and generally inclusion free; they are mostly subhedral to euhedral, though some grains are rounded or broken. CL imaging of their internal texture (Fig. 3) shows that most grains consist of a large internal part, commonly surrounded by a thin bright rim. The internal part shows, in most cases, well-developed, concentric and facetted oscillatory zoning, while some grains show alternate CL dark and bright stripes (e.g. 8

Fig. 3, grain MR-13/111). The bright rim, when present, is thin, continuous or discontinuous, and generally homogeneous, even though some rims may show patchy textures (e.g. Fig. 3, grains MR-13/05, /72, /89). The rims may simply surround euhedral facetted grains (e.g. Fig. 3, grains MR-13/92, /93), or mantle corroded grains, truncating the internal oscillatory zoning (e.g. Fig. 3, grain MR-13/01, /19).

A set of 124 grains were analysed (107 grains for MR-13 and 17 for MR-09), with analyses made mostly in the centre of the grains to record possible inherited cores. Among these 124 analyses obtained for the cores, a set of 96 analyses (84 grains for MR-13 and 12 grains for MR-09) allow individual Concordia ages to be calculated (Ludwig, 1998). In order to minimize age uncertainty and to prevent any arbitrary filter of the discordancy limit on 206

Pb/238U apparent ages, we only selected analyses providing individual Concordia ages. All

data are shown in a Concordia plot (Fig. 4a) and reported in the Supplementary Table SM2. In sample MR-09, the 12 analyses yield Concordia ages ranging from 955 to 832 Ma, whereas in sample MR-13 the 84 analyses spread over a larger range of Concordia age (940–792 Ma). The data set presented in a frequency diagram of Concordia ages (Fig. 4b), forms a subGaussian distribution with an enlarged tip. About 94 % of the Concordia ages range from 910 to 810 Ma; the remaining data consist of a few slightly older concordant analyses (up to 955 Ma), and two analyses with younger ages. Concentrations in U are moderate, though lower in sample MR-13 (20–198 ppm) than in sample MR-09 (202–373 ppm), suggesting that there is probably no metamict zircon grains. Th/U ratios overlap in the two samples and range from 0.4 to 1.0, values lower than 0.1 only occurring in discordant zircon crystals. A weighted mean of the Concordia ages from the whole dataset, excluding the 7 extreme ages reported above, yields an age of 862 ± 5 Ma (n = 89, MSWD = 4.4). This large MSWD demonstrates that the zircon population can be considered as heterogeneous in age within individual uncertainties and that it rather consists in several discrete age groups. Considering the visible breaking points on the evolution curve of the Concordia ages reported in Fig. 4c, five successive age groups can be proposed at 820 ± 8 Ma (n = 9), 836 ± 6 Ma (n = 15), 849 ± 6 Ma (n = 12), 863 ± 5 Ma (n = 21) and 884 ± 4 Ma (n = 25), respectively.

We also tried to analyse rims in an attempt at estimating their significance and age. On 31 rims, we obtained 22 individual Concordia ages (18 for sample MR-13 and 4 for sample MR09) reported in a Concordia plot (Fig. 4d). They do not show clear differences in age with regard to cores, with individual Concordia ages ranging from 885 to 772 Ma. Nevertheless, 9

the 4 rim analyses with the younger Concordia ages (i.e. < 830 Ma) show Th/U ratios lower than 0.08. Many rims with very low Th/U ratios are the most discordant. The other 18 analyses with the oldest ages (885–831 Ma) mostly show the highest Th/U ratios. Interestingly, zircon grain #100 from sample MR-13 is characterized by a low Th/U metamorphic rim concordant at 817 ± 21 Ma and a high Th/U zoned magmatic core concordant at 834 ± 19 Ma. Overall, though most of the analysed rims are closely similar to cores and can be considered to be of primary magmatic origin, some rims show evidence of metamorphic recrystallization at ca. 815 Ma.

4. Re-evaluation of the P–T conditions

In their metamorphic core complex model, Denèle et al. (2017) consider that the Sadh complex does not correspond to polyphased metamorphic rocks, but are syntectonic plutonic rocks. This interpretation implies that thermometric estimates have to be consistent with “magmatic temperatures”, whatever thermometer is used. We have, therefore, re-evaluated the P–T data in the Juffa complex, Sadh complex and Tonalite plutons, using recent thermodynamic database and thermobarometric calibrations.

Peak P–T conditions for the Juffa group have been determined from two paragneiss samples (MI-11 and MI-44) in the CaTiNKFMMnASH system using the version 6.6.8 (11-2013) of Perple_X (Connolly, 2009), for H2O content corresponding to the loss on ignition values obtained from whole-rock analyses. As shown by Denèle et al. (2017), the observed mineral assemblage (garnet, biotite, muscovite, plagioclase core, quartz and ilmenite) gives peak conditions at around 950 MPa and 630°C; retrograde conditions are estimated at < 400 MPa and 400°C using plagioclase rim, and 300 MPa and 350°C using the chlorite-phengite multiequilibrium approach (Ganne et al., 2012).

P–T conditions of magmatic crystallization for the orthogneisses and igneous rocks of the Sadh and Tonalite plutons were estimated using amphibole thermobarometry and zircon saturation thermometry. Amphibole composition shows a dependence on temperature and pressure, and this method has been widely used to estimate emplacement pressures and temperatures of calc-alkaline igneous rocks (e.g., Hammarstrom and Zen, 1986; Hollister et al., 1987; Johnson and Rutherford, 1989; Schmidt, 1992; Blundy and Holland, 1990; Blundy et al., 2006; Costa et al., 2013; Turner et al., 2013; Brenna et al., 2014; Leuthold et al., 2014). 10

In this study, we applied the thermobarometer RR2012 (Ridolfi and Renzulli, 2012), which is calibrated for calc-alkaline and alkaline igneous rocks over a pressure range of 130 to 2200 MPa. This calibration has been recently under scrutiny (e.g., Walker et al., 2013; Shane and Smith, 2013; Erdmann et al., 2014; Kiss et al., 2014). Putirka (2014) considers that it gives satisfactory results when compared to experimental results involving a broad range of starting compositions (basalt to rhyolite) and pressures (200-800 MPa). Nonetheless, Erdman et al. (2014) shows that if calculated temperatures are reliable estimates of the crystallization temperatures, calculated pressures do not reflect crystallization conditions but rather compositional variations. Therefore, temperature estimates given below can be considered with confidence (Ganne et al., 2016), while pressure estimates have to be regarded more cautiously (Margirier et al., 2016; Ganne et al., 2017). In-situ major element abundances were measured on thin section with an electron microprobe CAMECA SX-Five (15 kV, 10 nA) at Centre de Microcaractérisation Raimond Castaing (Toulouse). Chemical data collected on amphiboles (Denèle et al., 2017) were filtered on the basis of pressure-related “apparent percentage error” (APE) values calculated with the RR2012 thermobarometric method. We have also excluded amphiboles whose chemistry did not match the dedicated range of chemical composition and structural formulae.

The following temperature estimates were obtained from amphibole thermobarometry (Fig. 5a-d): 756–838°C (average: 802 ± 14°C) for the Banded gneisses, 776–838°C (average: 801 ± 11°C) for the Mahall intrusives, and 705-811°C (average: 753 ± 21°C) for the Fusht pluton. Pressure estimates are: 180–290 MPa (average: 230 ± 30 MPa) for the Banded gneisses, 170– 320 MPa (average: 230 ± 40 MPa) for the Mahall intrusives, and 90–200 MPa (average: 150 ± 30 MPa) for the Fusht pluton. These pressure estimates are in the same range as those obtained for the retrograde stage of the Juffa complex. Besides, we have attempted to estimate the temperatures of emplacement of the parent magmas of the Banded gneisses, Mahall intrusives and Tonalite plutons, using the zircon solubility model of Watson and Harrison (1983). We have selected whole-rock analyses with silica contents ranging from 56 to 76 wt.% (i.e. within the compositional range of experimental zircon-saturated melts) using our data set and that of Würsten (1994). The estimated temperatures fall in the following ranges: 613–838°C (average: 742 ± 54°C) for the Banded gneisses, 710-838°C (average: 778 ± 39°C) for the Mahall intrusives, 655–815°C (average: 752 ± 38°C) for the Fusht pluton, and 652– 797°C (average: 747 ± 37°C) for the Hadbin pluton. Although these temperatures should be considered as maximum estimates owing to the presence of an inherited component in the 11

zircons, it is interesting to note the overall concordance between the temperatures obtained from amphibole thermometry and those yielded by whole-rock Zr saturation thermometry. This data set shows that there is an important inversion in the peak temperature estimates between the Juffa complex (ca. 650°C) and the overlying Banded gneisses (ca. 800°C), a point already outlined by Briner (1997) that will be addressed farther in the Discussion section.

5. Geochemical data

Major and trace elements and Sr and Nd isotopic compositions were determined at the Service d'Analyse des Roches et Minéraux (SARM, CRPG-CNRS, Nancy). We selected 23 samples from the Marbat basement, and 2 paragneiss samples from the Sherubrub area (Socotra Island, Denèle et al., 2012). Analytical methods are reported in Supplementary material (§. SM3). The data are given in Tables 1 for major and trace elements, in Table 2 for rare earth elements (REE), and in Table 3 for isotopic compositions. Regarding major elements, our data are consistent with the large data base of Würsten (1994).

5.1. Juffa complex

Six samples were selected among the main rock types of the Juffa complex: two amphibolites, three intermediate biotite-muscovite gneisses, and one silicic biotite-muscovite gneiss. In a Fe2O3 + CaO + TiO2 – Al2O3 diagram (Fig. 6a), our data along with those of Würsten (1994) plot close to the mean composition of gabbros (amphibolites), within the field of graywackes (intermediate biotite-muscovite gneisses), and towards the most differentiated end of the magmatic trend (silicic biotite-muscovite gneiss).

Amphibolites show silica contents ranging from 45.5 to 53.5 wt.%, with Mg# values of 22 to 56 (Würsten, 1994). The two amphibolite samples we have analysed show a marked mafic composition (SiO2 around 46 wt.%, 6.2 < MgO < 10.3 %, 10.0 < CaO < 13.2 %), but with distinct Fe/Mg ratios. One sample is magnesian (Mg# = 55, MgO = 10.30 wt.%) and shows high concentrations in CaO, Cr and Ni (13.23 %, 1350 ppm and 348 ppm, respectively), but low concentrations in Nb, Ta, Zr, Hf, Y and REE. These characteristics are indicative of the cumulate nature of this amphibolite, probably involving clinopyroxene. The other sample is ferroan (Mg# = 29, MgO = 6.24 wt.%) and shows high contents in FeOtot, TiO2 and V (15.41 12

wt.%, 2.38 wt.% and 294 ppm, respectively), as well as in Zr and Y (122 and 50 ppm). This together with chondrite-normalized REE concentrations around 30 suggests that this sample could correspond to evolved (Fe-Ti-rich) mafic material, probably similar to a ferro-gabbro. In spite of distinct Ti and V contents, these two amphibolites show comparable Ti/V ratios (22/29), within the range of Würsten’s data (17 < Ti/V < 85; average 42); these ratios are comparable to those observed in mid-ocean-ridge basalts (MORB) (Shervais, 1982). The rare earth element patterns of these two samples (Fig. 7a) are depleted in light REE (0.5 < [La/Yb]N < 0.6), with flat heavy REE patterns ([Gd/Yb]N = 1.1) and almost no Eu anomaly (0.92 < Eu/Eu* < 1.06), resembling the pattern of average N-MORB. Multi-element plots (Fig. 7b) show strong positive anomalies in K, Rb, Ba (and Sr for one sample), and a negative anomaly in Nb and Ta. The Th/Yb (0.03/0.06), Nb/Yb (0.49/0.40) (Fig. 8a) and Ta/Yb (0.04/0.03) ratios are close to those of the depleted mantle, and the Zr/Nb ratios (47/39) are in the range of N-MORB (> 30 according to Wilson, 1989). However, the small negative anomalies observed for Nb and Ta, La/Nb ratios (1.76–1.81) higher than that of average NMORB (1.15 according to Gale et al., 2013), and the positive peaks in some large ion lithophile elements (K, Rb and Ba) in multi-elements plots are more comparable to tholeiitic fore-arc basalts such as those sampled in the Izu-Bonin-Marianna arc system (Reagan et al., 2010).

The major element composition of the Juffa gneisses plot mostly within the field of graywackes close to the magmatic trend (Fig. 6a), with silica contents ranging from 61 to 73 wt.% (Würsten, 1994). Among the samples we have analysed, three fall in this domain, with the following characteristics: (i) silica content close to that of andesites (61–64 wt.%), (ii) peraluminous composition, with A/CNK ratios (molar Al2O3 / [CaO + Na2O + K2O]) ranging from 1.4 to 1.6 (1.4 ≤ A/NK ≤ 2.4); (iii) high K2 O contents (0.85 < K2O/Na2O < 1.33); and (iv) ferroan chemistry (21 ≤ Mg# ≤ 26). The silicic gneiss sample is distinguishable only by higher SiO2 content (75.5 %). Zr/TiO2 (0.03–0.06) and Nb/Y (0.23–0.25) ratios corroborate the andesitic to rhyodacitic chemistry of these samples. The concentrations in Ba (400–720 ppm), Rb (56–98 ppm) and Sr (109–288 ppm) are variable, with Rb/Sr ratios ranging from 0.20 to 0.64. The REE patterns (Fig. 7a) of the intermediate gneiss samples are fractionated (4.7 < [La/Yb]N < 5.3), with flat heavy REE (0.9 < [Gd/Yb]N < 1.3), rather high chondritenormalized Yb contents (21.9 < YbN < 26.1) and negative Eu anomalies (0.65 < Eu/Eu* < 0.69). These features are similar to those of the North-American Shale Composite (Gromet et al., 1984) and of the quartz-intermediate to quartz-rich Phanerozoic graywackes (Taylor and 13

McLennan, 1985). The silicic gneiss sample is distinguished by its more negative Eu anomaly (Eu/Eu* = 0.49) and more concave-upward heavy REE. Multi-element plots (Fig. 7b) show fractionated patterns, with negative anomalies in Nb, Ta, P and Ti. The silicic gneiss is more fractionated, with the same but more marked negative anomalies, plus anomalies in Zr and Eu. These patterns are comparable to the average subducted sediments (Planck and Langmuir, 1998). Sr and Nd isotopic compositions were determined for one intermediate gneiss and the silicic gneiss. Initial

87

Sr/86Sr(820 Ma) ratios are low (0.7016 and 0.7026, respectively);

εNd(820 Ma) values (+4.1 and +4.6) and model ages (TDM = 1210 and 1124 Ma) are consistent with the data of Mercolli et al. (2006) (+1.5 ≤ εNd(820 Ma) ≤ +4.5; 1085 ≤ TDM ≤ 1419 Ma) recalculated at 820 Ma and with 143Nd/144Nd = 0.51315 for the depleted mantle (White, 1997).

The two paragneiss samples from the Sherubrub basement area in the Socotra Island are compositionally close to that of the Juffa metasediments (Fig. 6a), with intermediate, peraluminous and ferroan chemistry (SiO2 ca. 65 wt.%; 1.7 ≤ A/CNK ≤ 2.2; 18 ≤ Mg# ≤ 21). Their REE and multi-element patterns (Fig. 7a, b) are also close to those of the Juffa paragneisses, with negative anomalies in Nb, Ta, P and Ti; and with fractionated REE (5.6 < [La/Yb]N < 8.0), rather high chondrite-normalized Yb contents (19.1 < Yb N < 23.0) and negative Eu anomalies (0.63 < Eu/Eu* < 0.65).

5.2. Banded gneisses and Mahall intrusives (Sadh complex)

The Sadh complex consists of rocks ranging compositionally from gabbro to granite, generally strongly deformed such as the Banded gneisses (Fig. 9a, c), but still showing locally preserved igneous features (magmatic textures, dykes, etc.) especially in the Mahall intrusives. Eight samples were selected: four among the Banded gneisses (one amphibolite and three orthogneisses), and four among the Mahall intrusives (two gabbroic rocks and two tonalites). All samples show major element compositions consistent with the data of Würsten (1994), and plot along the magmatic trend in the Fe2O3 + CaO + TiO2 – Al2O3 diagram (Fig. 6b). There appears no significant difference between the Banded gneisses and Mahall intrusives in terms of major element composition: the sample suite is magnesian and metaluminous, and belongs to the medium-K calc-alkaline series (Fig. 8a-d), with silica contents ranging from 41.96 to 76.10 wt.% (Würsten, 1994).

14

Among the three more mafic rocks associated with the Banded gneisses and Mahall intrusives, two show low silica (46.08 and 46.3 wt.%) and high CaO (8.88 and 8.97 %), FeOtot (10.25 and 11.55 %) and TiO2 (1.68 and 1.81 %) contents, with Mg# values between 44 and 56. The third sample has higher silica (51.59 %), alumina (19.84 %) and Sr (939 ppm) contents indicative of a composition richer in plagioclase. Their concentration in V (155-275 ppm) is rather high with Ti/V ratios (35–47) typical of calc-alkaline lavas. The ratios Th/Yb (0.14–0.35), Nb/Yb (1.3–2.2) and Ta/Yb (0.09–0.15) are slightly outside the « asthenospheric mantle trend » (Fig. 8e); and the Zr/Nb ratios are lower (20 and 26 for the Mahall mafic rocks) or similar (42 for the Banded gneiss amphibolite) to that of N-MORB; nonetheless, these samples show negative anomalies in Nb and Ta. The intermediate to silicic samples from both the Banded gneisses and Mahall intrusives show silica contents ranging from 57.5 to 73.3 wt.%, saturation in alumina (0.9 < A/CNK < 1.0), a sodic chemistry (0.20 ≤ K2O/Na2O ≤ 0.98) and rather elevated Mg# values for felsic rocks (from 23 to 46). The Ti/V ratios of all these intermediate and silicic rocks (52–104) are similar to those of calc-alkaline suites with SiO2 > 58 % (Shervais, 1982). Rb contents are low (29 < Rb < 40 ppm), while contents in Sr (235–749 ppm) and especially Ba (647–2149 ppm) are high.

The REE patterns show two groups of samples (Fig. 7c). One group corresponding to the mafic rocks and the tonalites (SiO2 < 60 %), shows moderately fractionated patterns ([La/Yb]N = 2.65–5.26), with high heavy REE ([Yb]N > 15.5) and Y (25–42 ppm) contents, and weak Eu anomalies (0.93–1.13). The other group (SiO2 > 70 %) has more fractionated patterns ([La/Yb]N = 21.0–29.1), with significantly lower heavy REE ([Yb]N = 3.0–6.3) and Y (5–13 ppm) contents, and negative or positive Eu anomalies (Eu/Eu* = 0.67, 1.25, 1.89). Sr/Y ratios (14–58) are similar to those observed in arc formations (e.g. Moyen and Martin, 2012; Jagoutz et al., 2013). Multi-element diagrams (Fig. 7d) show common features for all these samples: enrichment in large ion lithophile elements and marked negative anomalies in Th, U, Nb and Ta. On the whole, the patterns closely resemble that of arc lavas and group 1 / group 2 plutonic rocks from the Kohistan batholith (Jagoutz et al., 2013). Sr and Nd isotopic compositions determined for two mafic rocks and three silicic rocks from the Banded gneisses and 143

Mahall

intrusives

show

homogeneous

87

Sr/86Sr(820 Ma)

Nd/144Nd(t820 Ma) (0.51184–0.51187) initial ratios. εNd (820

Ma)

(0.7024–0.7028)

and

values (+5.0 to +5.7) and

model ages (970 ≤ TDM ≤ 948 Ma) are grossly consistent with the data of Mercolli et al. (2006) recalculated (+6.0 ≤ εNd (820 Ma) ≤ +6.8; 871 ≤ TDM ≤ 809 Ma).

15

5.3. The tonalite plutons

Nine samples were selected, representing the Fusht (5), Hadbin (2) and Marbat (2) plutons. Major element compositions are consistent with the data of Würsten (1994), and plot along the magmatic trend in the Fe2O3 + CaO + TiO2 – Al2O3 diagram (Fig. 6b). The three plutons are magnesian, metaluminous to slightly peraluminous (0.64 < A/CNK < 1.12), and belongs to the medium-K calc-alkaline series (Fig. 8a-d). Silica contents range from 43.47 wt.% for the most mafic sample to 75.48 wt.% for the leucotonalitic ones (Würsten, 1994).

Whole-rock compositions of our samples range from diorite to tonalite, with silica contents varying from 51.4 % for the mafic rock to 72.6 % for the most silicic rocks. Samples are saturated in alumina (0.8 < A/CNK < 1.1) and sodic (0.18 ≤ K2O/Na2O ≤ 0.51), with high Mg# (40–51). Concentrations in Rb are low (13–37 ppm), whereas they are high for Sr (376– 989 ppm) and Ba (539–1235 ppm). V and Ti contents (114 and 207 ppm, 0.85 and 1.58 %) are high in the more mafic samples (SiO2 < 60 %), and low in the silicic rocks (11–61 ppm, 0.12–0.62 wt.%); Ti/V ratios ranging from 44 to 85. The mafic sample shows low Zr/Nb (21.3) and Sr/Y (14) ratios, but high Y content (40 ppm), and high Th/Yb (0.4), Nb/Yb (2.2) and Ta/Yb (0.15) ratios compared to the asthenospheric mantle.

The REE patterns (Fig. 7e) are fractionated ([La/Yb]N = 3.6–18.2) with concave-upward heavy REE. The mafic samples (SiO2 < 60%) show the less fractionated patterns ([La/Yb]N = 3.6 and 8.6) and high Yb and Y contents ([Yb]N = 24.4 and 7.9; Y = 17 and 40 ppm); there is no Eu anomaly (Eu/Eu* = 1.04). In the more silicic rocks (SiO2 > 63 %), the REE are more fractionated ([La/Yb]N = 9.7–18.2), with significantly lower Yb and Y contents ([Yb]N = 1.4– 4.8; Y = 2–9 ppm) leading to high Sr/Y (most samples > 60), and positive Eu anomalies (Eu/Eu* = 1.2–2.1), suggesting that garnet, hornblende and plagioclase played a significant role in their differentiation. Multi-element diagrams (Fig. 7f) show enrichment in large ion lithophile elements, negative anomalies in Th, U, Nb, Ta, P and Ti, and a positive anomaly in Sr. They are similar to group 2 granitoids of the Kohistan batholith (Mesozoic oceanic arc; Jagoutz et al., 2013; Fig. 10) albeit with lower Th and U concentrations. Sr and Nd isotopic compositions determined for one sample from the Fusht pluton give 143

87

Sr/86Sr(820

Ma)

and

144

Nd/ Nd(820 Ma) initial ratio of 0.7028 and 0.51185, respectively; εNd (820 Ma) values at +5.2

and Nd model age of 943 Ma. The data of Mercolli et al. (2006) for the Fusht and Hadbin

16

plutons (recalculated) give εNd(820 Ma) values of +5.8 and +5.9, and TDM of 878 and 866 Ma, respectively.

6. Discussion

Based on the new geochronological data obtained on the Juffa metasediments, on the reevaluation of the P–T conditions and on additional whole-rock compositional data, we revisit the evolution of the Neoproterozoic basement of the Marbat area. A short comparison will be made with data available for the Socotra Island that was close to Marbat prior to the opening of the Gulf of Aden.

6.1. P–T data and the nature of the Sadh complex

Our thermobarometric data for the peak (630°C – 950 MPa) and retrograde (350–400°C and 350–400 MPa) metamorphic conditions in the Juffa complex are comparable to the previous estimates of 650°C – 600 MPa for the peak, and 600°C – 350 MPa for the retrograde stage (Würsten, 1994; Briner, 1997; Mercolli et al., 2006), though they differ by higher pressures at the metamorphic peak and lower temperatures for the retrograde re-equilibration. The difference in pressure estimates does not seems to be due to sampling, our samples having been collected in the same area as those of the previous studies, but might be attributed to the use of different calibrations, or more probably to variable retrograde re-equilibration of phase assemblages in the samples studied by Würsten and Briner. In this respect, it is worth of note that one sample studied by Würsten gave a pressure estimate of 850 MPa. Our temperature estimate for the Banded gneisses (800°C ± 14°C) is significantly higher than the 710°C estimate of Briner (1997), therefore indicating that the temperature inversion between the Juffa and Sadh complexes is even greater than previously considered. Conversely, our pressure estimate (ca. 250 MPa), although it should be taken cautiously, is consistent with the pressure estimate for retrograde reequilibration (ca. 450 MPa) given by Mercolli et al. (2006).

The temperature inversion has been previously interpreted (Briner, 1997; Mercolli, 2006) in terms of a metamorphic evolution involving first the recrystallization of the Banded gneisses at high temperatures and then their juxtaposition to the Juffa complex along a retrograde path, followed by the intrusion of the Mahall intrusives and Tonalite plutons. Based on three main arguments, we consider that the Sadh complex do not correspond to a metamorphic unit, but 17

more simply to a synkinematic plutonic unit emplaced during the formation of the metamorphic core complex.

(1) As demonstrated by Denèle et al. (2017), the structural patterns of the Marbat area (foliation and lineation trajectories, kinematic indicators) show that the deformation of the Juffa and Sadh complexes results from the same major tectonic event corresponding to the exhumation of the metamorphic core complex. As a consequence, the temperature inversion cannot be attributed to distinct tectono-metamorphic events, therefore questioning the metamorphic nature of the Sadh complex. Field and compositional data on the Sadh complex are typically those of plutonic rocks encompassing a wide range of rock types (diorite, tonalite, granodiorite, granite) as previously outlined (Mercolli et al., 2006; Rantakokko et al., 2014). As shown from structural data, the Banded gneisses and Mahall intrusives differ only by the intensity of their deformation, more pronounced in the Banded gneisses that are close to the detachment, than in the Mahall intrusives that occur farther south.

(2) There is a good concordance between temperatures determined from whole-rock Zr content (zircon saturation thermometry) considered to give an estimate of temperatures prevailing during magma emplacement and crystallization, and those determined from amphibole thermometry. Moreover, it is worth of note that the temperatures recorded by the Banded gneisses (Hbl: 802 ± 14°C, Zrn: 742 ± 54°C) are similar to those of the other units clearly identified as intrusive (Hbl: 801 ± 11°C, Zrn: 778 ± 39°C for the Mahall intrusives; Hbl: 753 ± 21°C, Zrn: 752 ± 38°C for the Fusht pluton). All these temperature estimates are above solidus, therefore suggesting that hornblende equilibrated under magmatic conditions. Another interesting point is that the pressures corresponding to these magmatic conditions not only are largely lower than those of the metamorphic peak but also correspond to the retrograde conditions determined in the Juffa paragneisses. This shows that the thermal peak is not synchronous in both units (the Juffa complex and Banded gneisses), even though their deformation results from the same major tectonic event; and further suggests that the emplacement of the calc-alkaline magmas at the origin of the Sadh complex could have started after the metamorphic peak, during exhumation of the MCC.

(3) Structural data show that the intensity of the orthogneissification depends on the position of the intrusions with respect to the detachment, and on their timing with respect to the main deformation event. In this regard, the “migmatitic” appearance of the Banded gneisses could 18

not be the result of partial melting related to a metamorphic event, but more likely could be due to deformation of magmas close to their solidus (“submagmatic” deformation), as far as the melt topology at melt fractions < 0.4 is independent from the melting or crystallizing state of the rocks, but is determined by differential stress and finite strain (see discussion in Rosenberg and Handy, 2005).

On the whole, we consider that the Sadh complex is a plutonic unit mantling the Marbat gneiss dome, a configuration not uncommon in metamorphic core complexes worldwide, well-illustrated for instance in the Aegean Sea (see Denèle et al., 2011; Rabillard et al., 2015). Besides, we note that the metamorphic gradient recorded by the Juffa paragneisses (ca. 17– 18°C/km) is close to that observed in sedimentary rocks that underwent tectonic underplating beneath fore-arc crust (Ducea et al., 2009).

6.2. Interpretation of the zircon U–Pb data

Zircon core U–Pb data from two samples of the Juffa complex allow the origin and age of the paragneiss protolith to be discussed. The mostly euhedral morphology of the zircon grains suggests weak erosion and, therefore, limited transport of the detrital fraction. The Th/U ratios (0.4–1.0) of most cores suggest a magmatic origin without significant metamorphic dissolution and recrystallization. The 96 grains (on 124 grains) yielding a Concordia age provide Neoproterozoic ages, the oldest ones not exceeding 955 Ma. Our data are also consistent with the estimated 820–810 Ma age range for the metamorphism (Mercolli et al., 2006; Rantakokko et al., 2014). Indeed, it is reasonable to consider that the younger cores belonging to the 820 ± 8 Ma age group result from partial metamorphic resetting. Additionally, a single grain shows a metamorphic rim dated at 817 ± 21 Ma.

The zircon rims yield U-Pb ages generally not significantly different from those of the cores. The rims characterized by the oldest ages (885–831 Ma) and the highest Th/U ratios (Th/U > 0.1) are probably of primary magmatic origin without significant changes. Conversely, the rims with the younger ages and the lowest Th/U ratios (< 0.1) were probably partially reset during the metamorphic event. All these data suggest that the detrital fraction of the Juffa metasediments was dominantly Neoproterozoic, probably mostly around 850 Ma; and that the effects of the metamorphism remained very limited, without systematic and significant

19

dissolution and recrystallization, in agreement with peak temperatures (630°C) that remained close to the water-saturated granite solidus.

The ca. 850 Ma ages for the source of the Juffa metasediments, the plutonic nature of the Banded gneisses and their emplacement contemporaneous with the formation of the metamorphic core complex, are not easily reconcilable with a chronological evolution considering that magmas emplaced in the 850–830 Ma interval and were deformed and metamorphosed at ca. 820–815 Ma, as suggested by Mercolli et al. (2006). This apparent inconsistency can be ruled out by examining the zircon internal textures of the samples from the Banded gneisses analysed by Rantakokko et al. (2014; their Table 2 and Fig. 3). One of their sample (felsic vein OM05-38) is particularly illustrative in this respect. The zircon grains consist of zoned or homogeneous cores with a concordant age of 841 ± 6 Ma, surrounded and often truncated by large zoned rims with a concordant age of 815 ± 4 Ma (Rantakokko et al., 2014; e.g., their Fig. 5e, f). Though more complex and less clear, their other samples show similar features, with older cores surrounded by younger rims, as for example grain 14 of sample OM05-39, with 206Pb/238U ages of 837 ± 8 and 815 ± 8 Ma, respectively (Rantakokko et al., 2014; their Fig. 3b). Lastly, it is also interesting to consider the zircon grains from the Marbat granodiorite emplaced at 791 ± 10 Ma, showing inherited cores with a Concordia age of 853 ± 10 Ma (Rantakokko et al., 2014). In our samples, the zircon group at 820 ± 8 Ma may be easily interpreted in terms of metamorphism. At the opposite, the other zircon groups at 836 ± 6 Ma, 849 ± 6 Ma, 863 ± 5 Ma and 884 ± 5 Ma are less easy to interpret. Nevertheless, the lack of evidence of well-defined age peaks rather favour regional-scale inherited zircons than a continuous and long magmatic activity, intrusions commonly resulting from episodic short time pulses. Moreover, structural and petrological data show that the Sadh complex was emplaced during exhumation of the dome, and even probably after the metamorphic peak; therefore, the intrusion age can hardly be older than 820 Ma. On the whole, we suggest to consider the zircon cores with ages older than 820 Ma and extending up to 860 Ma, as an inherited component from the source of the magmas that gave rise to the calc-alkaline intrusions (Sadh complex, Tonalite plutons). The overlap of these inherited ages with the ages of the metasedimentary rocks of the Juffa complex suggests that this complex (or comparable formations) could have potentially contributed to the plutonism. The zircon rims with ages close to 815 Ma are likely to represent the age of emplacement and synkinematic crystallization of the magmas during the formation of the MCC, which would be in good agreement, within uncertainties, with the date of the metamorphic event in the 20

Juffa complex, estimated by Mercolli et al. (2006) at 811 ± 9 and 812 ± 10 Ma (zircon IDTIMS data, upper intercepts), and 809 ± 17 Ma (garnet Pb-Pb isochron age). The Mahall intrusives (799 ± 5 Ma), hornblende gabbro-diorite in the Banded gneisses (796 ± 14 Ma) and the Tonalite plutons (791 ± 10 Ma for the Marbat granodiorite, and 783 ± 7 and 786 ± 6 Ma for the Fusht and Hadbin plutons), emplaced later, are less and less affected by the deformation.

Although a comparable lithological succession seems to emerge for the Socotra Island, the age range for plutonic rocks appears to be wider, i.e. 860–800 Ma, with the late granite bodies and rhyolitic extrusive rocks being dated at around 720 Ma (Hadibo granite) (Denèle et al., 2012; Pease et al., 2012, 2014; Whitehouse et al., 2012). However, worth of note are the older concordant ages (858 ± 11 and 856 ± 12 Ma) obtained from inherited zircon cores of the Mt Haggier pink granite and gabbro enclave, suggesting that the source of the granites could be of the same age as the Juffa metasedimentary rocks (Denèle et al., 2012); and the 1060–800 Ma age range for the zircons of Sherubrub paragneisses with a discordant grain older than 2350 Ma (Whitehouse et al., 2012). These data seem to indicate that the Socotra metasediments could be of the same age as the Juffa ones, and that an old crustal component could have possibly contributed to the sedimentary budget. Nonetheless, the highly positive εNd(820 Ma) values, the low Sr initial ratios and the absence of old inherited cores in zircons favours to the first order a juvenile source for the metasediments.

6.3. Geotectonic context

Geochemical, geochronological and P-T data show that the Neoproterozoic Marbat gneiss dome consists of two main units: a Tonian volcano-sedimentary unit corresponding to the Juffa complex; and a plutonic unit encompassing the Banded gneisses, Mahall intrusives and Tonalite plutons, corresponding to magmatic activity extending from ca. 815 to 790 Ma, and accompanying the exhumation of the Marbat metamorphic core complex, as discussed below.

Mercolli and co-workers (Würsten, 1994; Briner, 1997; Mercolli et al., 2006), have shown that the Juffa complex is a metasedimentary series including amphibolites and ultramafic rocks; the ultramafic rocks (not considered in this study) are interpreted as harzburgite slices tectonically introduced into the Juffa complex during subduction, whereas the amphibolites are considered as tholeiitic basalts chemically close to fore-arc basalts, an interpretation 21

supported by our trace and REE data set (Fig. 7b). The volcanogenic nature of the Juffa metasedimentary rocks is suggested by their composition close to andesite and rhyolite with positive εNd(820 Ma) values (+1.5 to +4.5), and the presence of a zircon population with euhedral morphologies and Neoproterozoic ages (< 960 Ma). Nevertheless, their saturation in alumina, the local presence of sillimanite reported by Mercolli et al. (2006), their REE patterns close to average fine-grained sedimentary rocks, suggests sedimentary reworking as previously proposed (Würsten, 1994; Briner, 1997). These characteristics lead us to consider that the protolith of the Juffa gneisses corresponds dominantly to volcanogenic graywackes predominantly of juvenile origin, such as accretionary wedge sediments alimented by volcanogenic material derived from the arc crust. However, their Nd isotopic composition with older model ages (1000 – 1450 Ma) could indicate the presence of an older recycled crustal component for which two origins can be envisaged: either a clastic sedimentary contribution derived from the erosion of a continental crust, which is for instance supported by the zircon data of Whitehouse et al. (2012) on the Socotra metasediments; or more likely volcano-sediments issued from magmas formed in a mantle modified by a subduction component, which seems to be supported by the absence of zircon cores older than 956 Ma in the Juffa metasediments. The two whole-rock analyses of the Sherubrub paragneisses (Socotra) show composition and zircon U–Pb ages close to those of the Juffa paragneisses suggesting that the Marbat and Socotra metasediments could correspond to contemporaneous metasedimentary series consisting mainly of volcanogenic graywackes emplaced during the Tonian, in an island-arc context.

The plutonic unit consists of variably deformed igneous rocks (Banded gneisses, Mahall intrusives and Tonalite plutons) accompanying the exhumation of the Marbat core complex. These rocks range in composition from gabbro to granite, share common chemical characteristics (e.g. calc-alkaline signature, positive εNd(820 Ma) values), but are distinguishable by their amount of deformation and their age of emplacement. The oldest and the more deformed rocks correspond to the Banded gneisses emplaced at ca. 815 Ma, synchronously with the initiation of exhumation of the MCC; the Mahall intrusives, emplaced at ca. 800 Ma, still show significant amount of deformation; and the Tonalite plutons, the less deformed and emplaced at ca. 790 Ma, mark the end of the exhumation process. All the rocks show the characteristic negative anomalies in Nb, Ta, P and Ti, typical of arc magmatism. The homogeneity of the εNd(820 Ma) values (+5.0 to +5.9) and Nd model ages (970 < TDM < 866

22

Ma), along with the presence in zircon grains of inherited cores with concordant ages in the range 860–840 Ma, show that the parent magmas have interacted with a homogeneous Tonian source, probably equivalent to the Juffa and Sherubrub volcanogenic graywackes. The REE concentrations in the intermediate and silicic plutonic rocks show that they arose from two types of magma. One is characterized by high chondrite-normalized Yb contents (Yb N > 20) and is found only in the Banded gneisses and Mahall intrusives. The other, corresponding to the most siliceous samples and occurring in all rock groups but systematically in the late tectonic Tonalite plutons, shows low Yb contents (1.7 < YbN < 7.6) and concave-upward middle implying the presence of garnet and amphibole in the residual mineral phase assemblage. The magmatic precursor of the Banded gneisses, Mahall intrusives and Tonalite plutons are comparable in composition to the plutonic rocks of the Kohistan batholith forming the upper to middle crust of a mature Mesozoic oceanic arc (type 1 and type 2 plutonic rocks in Jagoutz et al., 2013; Fig. 10). Specifically, samples displaying low HREE contents (La/YbN > 10), high Sr/Y ratios and high Mg# (~40) are typically formed in thick and mature arcs where the crust can reach more than 30 km depth. Felsic magmas with these peculiar geochemical fingerprints are formed either by high pressure (> 0.8 GPa) garnet and hornblende fractionation from primitive hydrous arc basalts (Jagoutz et al., 2013), or incongruent garnet-present remelting of lower crustal hydrous basic rocks (Garrido et al., 2006).

The plutonic rocks of the Socotra Island, contemporaneous with the plutonism of the Marbat area (granites and gabbros from Mt Haggier and Sherubrub area; Denèle et al., 2012) show compositional features comparable to those of the Marbat plutonic rocks, such as negative anomalies in Nb, Ta, Sr, P, Ti, and a dominantly metaluminous chemistry. However, they are distinguishable by high heavy REE contents (9.3 < YbN < 113.7), and by a trace element signature transitional with within-plate granites (Fig. 8f), suggesting a more evolved character and generation at lower pressures than the Marbat plutonic rocks.

6.4. The Marbat island-arc: a scenario

A scenario based on two lithospheric-scale cross-sections at ca. 820–815 Ma and for the period 815–790 Ma (Fig. 11) is proposed in an attempt to account for the presence of volcanogenic metasedimentary rocks (Juffa complex) at 35 km depth before the emplacement of voluminous, mafic to felsic, 815–790 Ma plutons in the upper crust. 23

The oldest event recorded in the Marbat basement corresponds to deposition of the protolith of the volcano-sedimentary rocks of the Juffa complex that took place probably mostly around 850 Ma. These sediments were deposited in a basin located close to a juvenile island-arc system. At around 820–815 Ma, formation of a fore-arc crust during the initiation of an intraoceanic subduction (e.g. Whattam and Stern, 2011) led to emplacement of basalts or gabbros, the protoliths of the amphibolite lenses. An accretionary wedge was developed in front of this fore-arc domain, in association with tectonic underplating of sediments under the fore-arc crust (e.g. Von Huene and Scholl, 1991; Gutscher et al., 1998). Underplating occurred at ca. 820–815 Ma, i.e. the age of the metamorphic peak in the paragneisses of the Juffa complex recording burial of sedimentary rocks to about 35 km depth, along a gradient of 17–18°C, typical for sediment underplating (Ducea et al., 2009).

The second stage of our scenario corresponds to the growth of a mature island-arc on the old fore-arc domain. This configuration implies slab roll-back and steepening to explain the migration of the arc front towards the trench (Fig. 11a, b). We have demonstrated that the Marbat magmatic arc is dominated by strike-slip tectonics during this event (Denèle et al., 2017), implying strain partitioning in the upper plate of the subduction zone submitted to slab roll-back. Extension could be restricted to the back-arc region while the magmatic arc and the trench recorded strike-slip tectonics and shortening, in a whole oblique convergence geodynamic system. Relationships between shortening and extension in the upper-plate is not uncommon in active subduction zone submitted to slab roll-back, such as the Aegean domain above the Hellenic subduction (e.g. Jolivet et al., 2013). We consider that at this time (i.e. 815–790 Ma) voluminous magmatism has strongly reworked the old fore-arc crust. However, the formation of a releasing bend in the strike-slip system have locally preserved the old configuration of the fore-arc crust underplated by the Juffa metasedimentary rocks (Fig. 11b). The formation and evolution of the releasing bend in the lower crust (i.e. MCC) did not allow calc-alkaline magma transfer as attested by the lack of calc-alkaline granitic bodies in the paragneiss domain in contrast to the voluminous magmatic activity in the upper-crust. Melt has been transferred efficiently from deep crust levels to shallow plutons above the main detachment in strike-slip shear zone, whereas the formation of the releasing bend with subhorizontal fabrics in the lower crust shut off melt transfer. Preservation of middle temperature – middle pressure metamorphic assemblages in the Marbat area suggests that the formation of the releasing bend and the exhumation of the Juffa complex began during the 24

initiation of the mature arc, precluding heating of the paragneisses at the base of the arc. Formation of the MCC is associated with a decoupling at the base of the felsic crust (see Denèle et al., 2017; and references therein). As mature arcs are classically associated with mafic underplating at the base of the crust (e.g. Calvert et al., 2013), we consider that the decoupling at the base of the MCC could have occurred above a hypothetical mafic root of the arc (Fig. 11c), considering that the contact between metasedimentary and mafic rocks must correspond to a major rheological contrast.

In this scenario, the heterogeneous nature of the Juffa complex, characterized by an association of sedimentary rocks (paragneisses) and magmatic (amphibolites) and mantle (ultramafic) boudins, could be accounted in two different ways. This association could be linked to the extensive event that controlled the MCC exhumation. Extension could lead to boudinage of the competent crustal layer corresponding to the mantle and magmatic parts of the fore-arc crust, sandwiched between two incompetent layers, i.e. the fore-arc basin intruded by granitoids and the underplating sediments (Fig. 11). An alternative view is to consider that this association is partly inherited from the compressive event that led to underplating of the sediments, with involvement of the fore-arc crust and mantle in the imbrication zone. The succession of events involving tectonic underplating of sediments in an intra-oceanic subduction zone corresponds to a process of relamination of the arc crust (Hacker et al., 2011). However, our model for the Marbat area do not involve diapiric ascent but tectonic process. Relamination of sediments by diapiric ascent from the slab interface (i.e. Hacker et al., 2011) seems to imply significant heating of the sediment diapir, while the P–T path observed in the Marbat paragneisses do not imply temperature > 650°C.

7. Conclusion

New geochronological, geochemical and P–T data, combined with structural data show that the Marbat basement corresponds to a metamorphic core complex consisting of two main units:

(1) A Tonian metasedimentary unit (Juffa complex) mainly comprised of volcanogenic graywackes emplaced over the 960–830 Ma period and associated with fore-arc type basalts. Geochemical data confirm the dominantly juvenile nature of this unit and the contribution of an older recycled component, either as a detrital fraction arising from the erosion of an older 25

continental crust, or through the parent magmas of the volcanogenic graywackes recycling a crustal component, or both. As previously shown, this unit recrystallized under mediumtemperature amphibolite facies conditions at ca. 820–815 Ma, and was then exhumed as a MCC in a strike-slip context.

(2) A syn- to late-tectonic plutonic unit consisting of several magmatic intrusions emplaced all along the exhumation of the metamorphic core complex, but showing three magmatic peaks: highly deformed rocks emplaced at ca. 815 Ma (Banded gneisses), less deformed intrusions at ca. 800 Ma (Mahall intrusives), and late-tectonic intrusions at ca. 795–790 Ma (Tonalite plutons). The Banded gneisses and Mahall intrusives, grouped into the Sadh complex, do not correspond to a metamorphic unit as previously considered, but more simply to a plutonic unit mantling the MCC. Zircon U–Pb data showing the presence of inherited cores with concordant ages at ca. 860 Ma, and the REE patterns suggest that the parent magmas of this plutonic unit originate from partial melting of a source consisting in part of material equivalent in age to the Juffa complex.

The overall juvenile chemical signature of these two units corroborate the previous interpretations considering the Marbat area as a zone of accretion, which might have corresponded to fore-arc geodynamic context.

Acknowledgements This work was supported by CNRS-INSU SYSTER research grants. The manuscript benefited greatly from the constructive reviews of J. P. Liégeois and an anonymous reviewer. Thanks to E. Davy (CRPG-CNRS) for zircon separation and S. Al Kathiri for fruitful discussions.

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Figure captions Fig. 1. Geological sketch map of the Arabian-Nubian Shield (modified after Johnson et al., 2011; Fritz et al., 2013).

Fig. 2. The Marbat basement: (a) Geological map of the Marbat area showing the distribution of the main rock units and the foliation trajectories (after Mercolli et al., 2006; Roques, 2016; Denèle et al., 2017; and authors field work); (b) Cross-section (A-A’) across the north-eastern part of the Marbat metamorphic core complex.

Fig. 3. Cathodoluminescence images of zircon grains from the Juffa paragneiss. Analytical spots indicated by white dotted circles, with corresponding

206

Pb/238U ages and degree of

concordance.

Fig. 4. Zircon U-Pb plots for the Juffa group paragneiss: (a) Concordia diagram for all zircon core data; (b) distribution plot of Concordia ages for core data; (c) individual Concordia ages and error bar for the 96 concordant core data; (d) Concordia diagram for zircon rim concordant data.

Fig. 5. Results of amphibole thermobarometry on orthogneisses from the Sadh complex (Banded gneisses and Mahall intrusives), and on tonalites from the Fusht pluton, using the method of Ridolfi and Renzulli (2012) and the data set of Denèle et al. (2017). See text for further explanation.

Fig. 6. Fe2O3 + CaO + TiO2 vs. Al2O3 plot (La Roche, 1965) for (a) the paragneisses of the Juffa complex and Socotra Island (Sherubrub), and (b) the rocks from the Sadh complex and

34

Tonalite complexes. The data set of Würsten (1994) is shown for comparison. Dark solid line = igneous trend (ga = gabbro, di = diorite, gd = granodiorite, gr = granite).

Fig. 7. Rare earth element patterns and multi-element plots for the rocks of the Juffa complex and Socotra Island (a and b), Sadh complex (c and d), and Fusht, Hadbin and Marbat plutons (e and f). REE are normalised to chondrite according to Evensen et al. (1978); multi-element plots are normalized to the Primitive mantle of Sun and Macdonough (1989). NASC (North American Shale Composite) from Gromet et al. (1984); subducting sediments from Planck and Langmuir (1998); arc lavas (Vanuatu) from Peate et al. (1997); N-MORB from Sun and Macdonough (1989); field of quartz-intermediate and quartz-rich Phanerozoic graywackes from Australia and New Zealand from Taylor and McLennan (1985); field of Mariana forearc basalts from Reagan et al. (2010); and fields of Kohistan batholith type 1 and 2 plutonic rocks from Jagoutz et al. (2013).

Fig. 8. (a) to (d) Classification diagrams applied to rocks of the Marbat metamorphic core complex and Socotra Island: Alkali-lime index (a) and Fe-number (b) vs. silica (Frost et al., 2001); (c) molar Al2O3 / (Na2O + K2O) vs. Al2O3 / (CaO + Na2O + K2O) (Shand, 1943); (d) K2O vs. SiO2 diagram (Peccerillo and Taylor, 1976); (e) Th/Yb vs. Nb/Yb plot (Pearce, 1996); and (f) Rb vs. Y+Nb (Pearce et al., 1984).

Fig. 9. Field photographs of Banded gneisses of the southern termination of the dome (a-c), and of the Fusht (d-e) and Hadbin (f) plutons. (a) Magmatic layering transposed along the gneissic foliation in the Banded gneisses; dark, intermediate and light levels correspond respectively to diorite, granodiorite and leucogranite; note segregation of the lighter melts along shear planes. (b) Orthogneissified granodiorite (augengneiss) associated to amphibolite (dark) and aplite-pegmatite (light grey) dykes, transposed by the foliation. (c) Granodiorite (light grey) and diorite enclaves (grey on the right) cut by mafic dykes (dark) and transposed along the gneissic foliation in the Banded gneisses. (d) Magmatic layering in the southern part of the Fusht pluton; most of the outcrop consists of granodiorite (moderately dark) and diorite (dark) layers devoid of solid-state deformation; leucocratic veins occur at the contact between granodiorite and diorite. (e) Magma mingling in the eastern border of the Fusht pluton showing fragmented diorite layers in granodiorite; enclaves show incipient solid-state deformation and are associated with leucogranite suggesting partial remelting of the granitic

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material. (f) Magma mingling between gabbro and tonalite; mafic enclaves show incipient solid-state deformation.

Fig. 10. K2O vs. SiO2 and Sr/Y vs. Mg# for the Sadh complex and Tonalite plutons, compared to plutonic rocks of the Mesozoic Kohistan batholith (Jagoutz et al., 2013).

Fig. 11. Lithospheric-scale model for the Marbat island-arc. (a) Sediment underplating (ca. 820–815 Ma). Inset: zooming on the fore-arc structure. (b) Construction of the Marbat mature island-arc from 815 to 790 Ma. The layered mature arc is represented to highlight the presence of an old fore-arc crust with underplated sediments. The structure of this arc for the period 815–790 Ma was more complex, with magmatic reworking of the old fore-arc crust. This architecture with magmatic crust above and beneath fore-arc crust seems to be preserved only in a local releasing bend that corresponds to the Marbat area. Strain partitioning in the upper plate of the subduction zone for the period 815–790 Ma is shown by the red arrows. See text for additional explanations. (c) Three-dimensional crustal-scale structures of the Marbat metamorphic core complex at ca. 790 Ma (redrawn from Denèle et al., 2017).

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Highlights - The Juffa complex is a Tonian (ca. 850 Ma) juvenile volcano-sedimentary series - The Sadh complex is a plutonic unit (ca. 820–815 Ma) mantling the Marbat core complex - The Marbat core complex is an exhumed fore-arc crust with felsic lower crust

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Table 1. Major (wt.%) and trace (ppm) element compositions of selected samples from the Juffa, Sadh and Tonalite complexes. Juffa complex

Banded gneisses

Mahall intrusives

Sample n° Rock type

MI-26 Amph

MI-10 Amph

MR-6 Gn

MI-11 Gn

MI-25 Gn

MR-9 Gn

MR-31 MR-32 Amph Ton. Ogn

MI-28 Bt Ogn

MI-1 Bt Ogn

MR-24 Dio

MI-32 Gab

MR-23 Ton

MI-33 Ton

SiO2 Al2O3 FeOtot MnO MgO CaO Na2O K2O TiO 2 P2O5 LOI Total

45.51 12.70 15.41 0.27 6.24 9.99 1.97 0.75 2.38 0.21 3.72 99.31

46.54 14.58 8.33 0.14 10.30 13.23 1.52 0.41 0.71 0.06 3.25 99.17

60.94 17.49 6.69 0.14 2.41 1.83 2.84 2.41 0.96 0.17 3.70 99.65

61.51 18.63 5.21 0.12 1.77 1.78 3.70 3.68 0.76 0.13 2.17 99.50

63.52 16.37 4.97 0.08 1.30 2.24 2.17 2.88 0.79 0.14 5.34 99.84

75.46 13.33 1.38 0.07 0.47 0.68 3.69 3.21 0.22 0.05 0.99 99.54

46.32 16.35 10.25 0.19 7.40 8.97 3.23 1.90 1.81 0.40 2.45 99.38

57.47 18.05 6.29 0.14 2.42 5.53 5.84 1.18 1.08 0.44 1.38 99.89

70.49 15.24 2.57 0.05 0.63 2.33 4.96 2.47 0.34 0.10 0.76 99.97

71.83 14.37 2.07 0.04 0.36 1.13 4.45 4.34 0.31 0.06 0.68 99.66

46.08 18.38 11.55 0.21 5.24 8.88 3.57 1.45 1.68 0.41 1.99 99.57

51.59 19.84 6.77 0.16 4.28 7.98 4.78 1.62 0.89 0.31 2.86 101.13

60.33 16.97 5.91 0.18 2.04 5.02 5.03 1.31 1.13 0.43 1.15 99.57

73.32 14.70 1.73 0.04 0.58 1.98 5.22 2.20 0.32 0.08 0.81 100.99

99 53.8 222 20.5 2.0 3.5 2.6 88 10.5 53.0 242 0.2 0.2 0.1

38 53.0 1350 15.0 1.6 1.0 0.7 348 6.7 33.9 263 0.1 0.1 0.1

720 16.9 61 25.2 1.7 7.6 9.3 32 56.4 20.1 288 0.7 6.7 1.3

715 10.7 37 24.3 1.7 7.9 9.2 18 97.8 15.3 261 0.7 10.1 3.1

409 13.3 52 22.6 1.5 6.0 8.6 25 69.7 15.4 109 0.7 6.7 1.6

494 2.2 5 15.8 1.2 4.4 7.0 6 71.5 4.4 111 0.7 13.1 3.0

407 43.9 164 20.3 1.4 3.6 3.7 79 30.9 29.9 433 0.3 0.4 0.2

647 11.9 7 23.4 1.2 8.6 7.5 23 21.2 16.1 749 0.5 2.1 0.4

1527 3.7 13 18.9 1.1 4.7 3.7 6 28.8 362 0.2 0.9 0.2

2149 1.6 5 20.2 1.3 7.7 4.2 40.2 235 0.2 9.2 0.7

489 35.5 10 23.0 1.4 2.6 4.1 16 28.3 30.8 713 0.3 0.4 0.3

386 23.4 46 24.9 1.7 3.0 5.8 19 40.3 21.0 939 0.4 0.9 1.0

658 7.5 21.4 1.3 4.8 6.2 24 20.2 21.8 585 0.4 0.4 0.4

698 3.2 10 18.4 1.2 5.4 3.1 6 27.8 282 0.2 1.8 0.5

Ba Co Cr Ga Ge Hf Nb Ni Rb Sc Sr Ta Th U

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V Y Zr

494 49.7 122

195 16.9 27

109 38.6 286

68 39.9 285

85 34.7 219

13 27.6 131

229 30.7 155

124 33.4 350

22 8.6 182

18 13.3 282

275 25.9 104

155 25.2 115

77 42.2 225

22 4.9 214

Table 1. (Continued) Fusht pluton Sample n° Rock type SiO2 Al2O3 FeOtot MnO MgO CaO Na2O K2O TiO2 P2O5 LOI Total Ba Co Cr Ga Ge Hf Nb Ni Rb Sc Sr Ta Th U

Hadbin pluton

MR27 Gd

MR28 MMe

MI34 Ton

MI35 Ton

MI36 Ton

58.05 19.11 5.59 0.11 2.70 5.73 5.15 1.79 0.85 0.35 1.35 100.84

51.39 17.68 9.99 0.20 3.89 6.57 4.46 2.07 1.58 0.37 2.13 100.43

63.96 18.42 3.59 0.08 1.59 3.95 5.64 1.73 0.62 0.21 1.11 100.93

63.99 18.00 3.60 0.06 1.46 4.01 5.46 1.51 0.56 0.24 1.30 100.24

765 15.4 18 24.5 1.2 4.5 4.6 36 32.9 10.9 1072 0.3 1.2 0.6

825 24.6 25.2 1.7 4.5 8.8 10 37.4 29.2 551 0.6 1.5 1.1

687 8.8 15 23.4 1.0 4.6 4.1 7 29.4 830 0.2 1.5 0.3

684 8.2 12 22.5 0.9 4.4 3.8 7 23.9 989 0.2 0.9 0.4

Marbat pluton

MI38a Gd-light

MI38b Gd-dark

MI14 Gd

MI15 Gr

69.76 15.91 2.65 0.04 1.13 4.33 4.78 0.86 0.35 0.16 1.40 101.40

67.54 17.17 2.56 0.06 0.93 2.93 5.78 1.58 0.34 0.16 1.39 100.45

69.91 16.59 2.31 0.06 0.86 2.86 5.58 1.54 0.32 0.13 1.30 101.49

70.86 15.93 1.15 0.03 0.67 1.96 5.59 1.89 0.16 0.05 1.68 99.98

72.56 15.03 1.01 0.04 0.53 1.50 5.08 2.57 0.12 0.05 1.35 99.86

539 7.2 7 19.0 0.9 3.0 1.6 6 13.3 924 0.2 1.3 0.3

687 5.8 7 21.0 1.0 4.2 3.5 29.9 801 0.3 1.8 0.3

657 4.6 8 19.3 0.9 3.8 3. 4 27.4 755 0.3 1.8 0.3

959 1.8 9 16.3 0.9 3.2 2.4 5 27.2 484 0.2 1.0 0.2

1235 1.5 8 16.6 0.9 1.3 3.0 5 33.6 376 0.2 0.4 0.1

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V Y Zr

114 16.8 213

206 39.9 187

50 8.6 191

61 7.4 181

48 6.4 116

33 8.8 175

29 7.6 164

11 3.2 118

11 2.3 47

LOI = loss on ignition, - = below detection limit, FeOtot = total iron as FeO. Amph = amphibolite, Gn = gneiss, Bt Ogn = biotite orthogneiss, Dio = diorite, Gab = gabbro, Ton = tonalite, MMe = mafic microgranular enclave, Gr = granite

Table 2. Rare-earth element compositions of selected samples from the Juffa, Sadh and Tonalite complexes. Juffa complex Sample n° Rock type

MI-26 Amph

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

4.59 14.30 2.45 14.10 5.07 1.80 6.97 1.22 8.13 1.74 5.15 0.78 5.26 0.83

Banded gneisses

Mahall intrusives

MI-10 Amph

MR-6 Gn

MI-11 Gn

MI-25 Gn

MR-9 Gn

MR-31 MR-32 Amph Ton. Ogn

MI-28 Bt Ogn

MI-1 Bt Ogn

MR-24 Dio

MI-32 Gab

MR-23 Ton

MI-33 Ton

1.27 3.70 0.61 3.81 1.60 0.68 2.40 0.43 2.79 0.60 1.77 0.26 1.77 0.27

30.67 68.77 8.97 36.37 7.96 1.57 6.92 1.08 6.78 1.45 4.05 0.60 4.23 0.65

33.69 75.86 8.94 37.94 8.06 1.65 7.13 1.12 6.80 1.37 4.00 0.61 4.32 0.68

26.50 58.79 6.98 29.29 6.27 1.36 5.87 0.94 5.90 1.18 3.51 0.54 3.77 0.59

27.66 69.87 6.91 25.18 5.07 0.74 4.19 0.69 4.38 0.98 2.89 0.48 3.61 0.58

11.42 34.49 4.40 21.28 5.74 2.07 5.71 0.90 5.68 1.21 3.13 0.44 2.91 0.44

18.79 31.14 3.38 13.55 2.62 1.01 2.31 0.33 1.80 0.33 0.80 0.09 0.60 0.10

45.04 83.84 9.38 37.46 6.39 1.19 4.67 0.59 2.86 0.47 1.19 0.16 1.05 0.18

12.98 38.80 4.84 22.24 5.37 1.91 4.96 0.76 4.74 1.01 2.67 0.38 2.55 0.39

14.39 35.67 5.02 23.11 5.23 1.48 4.64 0.72 4.22 0.85 2.46 0.38 2.66 0.41

20.53 63.15 7.74 34.81 8.16 2.32 7.42 1.17 7.53 1.63 4.31 0.60 3.86 0.55

20.73 30.07 2.66 8.59 1.16 0.64 0.91 0.13 0.73 0.15 0.47 0.08 0.62 0.12

26.66 68.06 7.23 30.56 6.99 1.89 6.15 0.94 5.83 1.26 3.40 0.50 3.42 0.53

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Table 2. (Continued) Fusht pluton Sample n° Rock type

MR27 Gd

MR28 MMe

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

16.56 46.98 5.62 25.44 5.56 1.70 4.43 0.62 3.47 0.65 1.58 0.21 1.31 0.19

21.57 72.80 9.15 40.52 8.90 2.78 7.57 1.18 7.26 1.53 4.07 0.59 4.03 0.61

MI34 Ton 18.10 37.35 4.42 17.99 3.19 1.20 2.46 0.34 1.75 0.31 0.81 0.11 0.67 0.10

Hadbin pluton

Marbat pluton

MI35 Ton

MI36 Ton

MI38a Gd-light

MI38b Gd-dark

MI14 Gd

MI15 Gr

13.86 29.31 3.60 15.38 2.89 1.15 2.17 0.28 1.46 0.26 0.67 0.09 0.56 0.09

15.96 31.08 3.62 14.33 2.33 0.79 1.69 0.22 1.11 0.20 0.59 0.09 0.72 0.12

16.73 32.01 3.68 14.82 2.59 0.93 1.94 0.28 1.56 0.30 0.82 0.12 0.80 0.14

16.29 31.26 3.56 14.10 2.41 0.85 1.77 0.24 1.33 0.25 0.70 0.10 0.70 0.12

10.80 21.25 2.28 8.52 1.32 0.44 0.88 0.11 0.57 0.10 0.31 0.05 0.40 0.07

3.36 6.44 0.73 2.89 0.53 0.35 0.48 0.07 0.41 0.08 0.21 0.03 0.23 0.04

Amph = amphibolite, Gn = gneiss, Ogn = orthogneiss, Gab = gabbro, Dio = diorite, MMe = mafic micogranular enclave, Gd = granodiorite, Ton = tonalite, Gr = granite

Table 3. Sr and Nd isotopic data for whole rocks from the Marbat area. Juffa complex Sample n° Rb (ppm) Sr (ppm)

MR6 56.39 288.3

Banded gneisses MR9 71.46 111.3

MR28 37.44 550.8

Mahall intrusives MR31 30.86 433.4

MR32 21.21 748.8

MR24 28.34 713.5

MR23 20.22 585.3

Fusht pluton MR27 32.85 1072

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87

Rb (µmol) Sr (µmol) 87 Rb/86Sr 87 Sr/86Sr 2σ (87Sr/86Sr)t 86

0.1836 0.3229 0.5687 0.709239 0.000008 0.7026

0.2327 0.1247 1.8667 0.723497 0.000011 0.7016

0.1219 0.6170 0.1976 0.705084 0.000015 0.7028

0.1005 0.4855 0.2070 0.704835 0.000014 0.7024

0.0691 0.8387 0.0824 0.703775 0.000012 0.7028

0.0923 0.7992 0.1155 0.704165 0.000020 0.7028

0.0659 0.6556 0.1004 0.703969 0.000010 0.7028

0.1070 1.2008 0.0891 0.703878 0.000012 0.7028

Sm (ppm) 7.963 5.066 8.898 5.738 6.99 5.366 8.159 5.564 Nd (ppm) 36.37 25.18 40.52 21.28 30.56 22.24 34.81 25.44 147 Sm/144Nd 0.13236 0.12163 0.13276 0.16302 0.13828 0.14587 0.14170 0.13222 143 Nd/144Nd 0.512504 0.512471 0.512555 0.512747 0.512581 0.512630 0.512606 0.512558 2σ 0.000017 0.000006 0.000006 0.000004 0.000005 0.000004 0.000005 0.000006 (143Nd/144Nd)t 0.51179 0.51182 0.51184 0.51187 0.51184 0.51185 0.51184 0.51185 εNd(t) +4.1 +4.6 +5.1 +5.7 +5.0 +5.2 +5.2 +5.2 TDM (Ma) 1210 1124 1119 1211 1149 1167 1151 1106 CHUR composition used to calculate εNd values is 143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.1967; the decay constant is 6.54x1012 ; Nd model ages are calculated using 143Nd/144Nd = 0.511580 for the depleted mantle; (87Sr/86Sr)t, (143Nd/144Nd)t and εNd(t) calculated at 820 Ma.

42