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Earth and Planetary Science Letters 393 (2014) 83–93

Contents lists available at ScienceDirect

Earth and Planetary Science Letters

www.elsevier.com/locate/epsl

Early Carboniferous (∼357 Ma) crust beneath northern Arabia:Tales from Tell Thannoun (southern Syria)

Robert J. Stern a,∗, Minghua Ren b, Kamal Ali c, Hans-Jürgen Förster d,Abdulrahman Al Safarjalani e, Sobhi Nasir f, Martin J. Whitehouse g,Matthew I. Leybourne h, Rolf L. Romer d

a Dept. of Geosciences, University of Texas at Dallas, Richardson, TX 75083-0688, USAb Dept. Geosciences, University of Nevada, Las Vegas, Las Vegas, NV 89154-4010, USAc Faculty of Earth Sciences, King Abdulaziz University, Jeddah, Saudi Arabiad GFZ German Research Centre for Geosciences, Potsdam, D-14473, Germanye Department of Environmental Sciences, Damascus University, Damascus, Syriaf Dept. Earth Sciences, Sultan Qabos University, Muscat, Omang Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, SE-104 05, Swedenh ALS Geochemistry, 2103 Dollarton Hwy, North Vancouver, BC, V7H 0A7, Canada

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 August 2013Received in revised form 12 January 2014Accepted 18 February 2014Available online 12 March 2014Editor: T.M. Harrison

Keywords:HercynianU–Pb zirconxenolithgeochemistrySr–Nd–Pb isotopesSyria

Continental crust beneath northern Arabia is deeply buried and poorly known. To advance our knowledgeof this crust, we studied 8 xenoliths brought to the surface by Neogene eruptions of Tell Thannoun,S. Syria. The xenolith suite consists of two peridotites, one pyroxenite, four mafic granulites, and onecharnockite. The four mafic granulites and charnockite are probably samples of the lower crust, andtwo mafic granulites gave 2-pyroxene equilibration temperatures of 780–800 ◦C, which we take to reflecttemperatures at the time of formation. Peridotite and pyroxenite gave significantly higher temperaturesof ∼900 ◦C, consistent with derivation from the underlying lithospheric mantle. Fe-rich peridotite yieldedT ∼ 800 ◦C, perhaps representing a cumulate layer in the crust. Three samples spanning the lithologicrange of the suite (pyroxenite, mafic granulite, and charnockite) yielded indistinguishable concordant U–Pb zircon ages of ∼357 Ma, interpreted to approximate when these magmas crystallized. These igneousrocks are mostly juvenile additions from the mantle, as indicated by low initial 87Sr/86Sr (0.70312 to0.70510) and strongly positive initial εNd(357 Ma) (+4 to +9.5). Nd model ages range from 0.55 to0.71 Ga. We were unable to unequivocally infer a tectonic setting where these melts formed: convergentmargin, rift, or hotspot. These xenoliths differ from those of Jordan and Saudi Arabia to the south in fourprincipal ways: 1) age, being least 200 Ma younger than the presumed Neoproterozoic (533–1000 Ma)crust beneath Jordan and Saudi Arabia; 2) the presence of charnockite; 3) abundance of Fe-rich mafic andultramafic lithologies; and 4) the presence of sapphirine. Our studies indicate that northern Arabian platelithosphere contains a significant proportion of juvenile Late Paleozoic crust, the extent of which remainsto be elucidated. This discovery helps explain fission track resetting documented for rocks from Israeland provides insights into the nature of Late Paleozoic (Hercynian) deformation that affected Arabia nearthe Persian Gulf.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

We are interested to better understand the origin and evolu-tion of the continental crust of the Arabian Plate as exemplary

* Corresponding author.E-mail addresses: [email protected] (R.J. Stern), [email protected]

(M. Ren), [email protected] (K. Ali), [email protected] (H.-J. Förster),[email protected] (A. Al Safarjalani), [email protected] (S. Nasir),[email protected] (M.J. Whitehouse), [email protected](M.I. Leybourne), [email protected] (R.L. Romer).

of how continental crust forms and evolves. Basement expo-sures in the Arabian Shield in the southern part of the Plateindicate that this crust mostly formed in Neoproterozoic time,900–540 Ma (Fig. 1A) (Brew et al., 2001; Stern and Johnson,2010). It is often assumed that all Arabian continental crust isof similar Neoproterozoic age but this assumption is untestedbecause northern Arabia is deeply buried beneath Phanerozoicsediments. Recent results suggest that the buried crust of N. Ara-bia has a more complicated history than heretofore imagined.

http://dx.doi.org/10.1016/j.epsl.2014.02.0430012-821X/© 2014 Elsevier B.V. All rights reserved.


84 R.J. Stern et al. / Earth and Planetary Science Letters 393 (2014) 83–93

Fig. 1. (A) Main outcrops of Cenozoic volcanic rocks in the northwestern part of the Arabian Plate (modified after Lustrino and Sharkov, 2006). Note the northern limitsof exposed basement of the Arabian Shield (light green). (B) Simplified geological sketch map of western Syria (Lustrino and Sharkov, 2006), showing location of crustalprofile C–C′ . DST = Dead Sea Transform. (C) Crustal profile, modified after gravity model of Brew et al. (2001), further constrained by receiver functions and independentdeterminations of depth-to-basement.

For example, ∼1.0 Ga crust has been documented at the north-ern limit of the Shield in Sinai (Fig. 1A) (Be’eri-Shlevin et al.,2012) and detrital zircons of this age are abundant in Cambro–Ordovician sandstones from the region (Meinhold et al., 2013),suggesting that Mesoproterozoic crust may be buried beneath thick

sediments of northern Arabia (Fig. 1). On the other hand, buriedcrust may include Phanerozoic material, consistent with the ob-servation that northern Arabia was affected by the Hercynianorogeny in Carboniferous time, possibly reflecting subduction ofthe Paleotethys and terrane accretion along the northern margin


R.J. Stern et al. / Earth and Planetary Science Letters 393 (2014) 83–93 85

of Gondwana (Husseini, 1992; Konert et al., 2001). Furthermore,fission-track ages of Precambrian zircons from Israel indicate thata major episode of uplift and cooling occurred in Late Devonian–Early Carboniferous time (328–373 Ma) (Konert et al., 2001).

In this study we address the age and nature of crust beneathsouthern Syria, using eight lower-crust and upper mantle xeno-liths brought to the surface by Neogene basaltic volcanism at TellThannoun in southwestern Syria. We report mineral compositions,whole-rock major and trace-element compositions, whole-rock Sr–Nd–Pb isotope signatures, and U–Pb zircon ages and use theseresults to show for the first time that northern Arabia containsa significant tract of Late Paleozoic juvenile crust. Discovery ofthis new crustal tract illuminates the nature of enigmatic Hercy-nian thermal effects and deformation that affected large portionsof northern Arabia.

2. Geological setting and previous work

Exposed rocks in Syria are entirely of Phanerozoic age (Fig. 1A).Crystalline basement is generally buried >6 km deep beneathPhanerozoic sediments and has not been penetrated by drilling(Brew et al., 2001) (Fig. 1C). Little is known about this crust beyondits geophysically-determined density and thickness and inferredsubdivision into mafic lower and felsic upper crust (Fig. 1C) (Brewet al., 2001). Western Syria – the region of interest – is composedof two stable platforms – the Aleppo Platform in the north and theRutbah Platform in the south – separated by the NE–SW trend-ing Palmyride fold-and-thrust belt (Fig. 1B). The Palmyrides arethought to mark an intracontinental rift which inverted in latestCretaceous to Late Eocene time (Salel and Séguret, 1994). Crustalstructure differs on either side of the Palmyrides, thicker (∼30 kmof crust plus ∼8 km sediments) beneath the Rutbah Uplift to thesouth than beneath the Aleppo Plateau to the north (∼25 km plus∼6 km sediments) (Brew et al., 2001).

There are several extensive Cenozoic basalt fields – known asharrats – on the Arabian Plate. Harrat Ash Shaam is by far thelargest of these, covering ∼40,000 km2 and extending from Israelon the west through SW Syria, eastern Jordan, and into northernSaudi Arabia (Fig. 1A). Similar to lavas of other Arabian-Plate har-rats, those of Harrat Ash Shaam are mostly alkali basalt, basanite,and tephrite generated by low (<10%) degrees of upper mantlemelting (Krienitz et al., 2007; Shaw et al., 2003). Harrat As Shaamvolcanism started about 24 Ma and continued to recent times,with a hiatus between 13 and 7 Ma (Krienitz et al., 2007). TellThannoun, the focus of this study, is located at 32◦53′ N, 36◦37′ Ein Harrat As Shaam in Syria and is part of an inlier of Pleistocenebasalt that is surrounded by Holocene lavas (Krienitz et al., 2007).Tell Thannoun is constructed on the Rutbah Uplift and the xeno-liths we studied sample this lithosphere (Fig. 1B).

There are several locations in western Arabian harrats whereeruptions of mafic lava brought xenoliths of deep crust and uppermantle material to the surface. A lot of work has been done onxenoliths from Saudi Arabia, Jordan, Israel, and Syria (Ghent et al.,1980; Coleman et al., 1983; Kuo and Essene, 1986; McGuire, 1988a,1988b; Stein and Katz, 1989; Nasir, 1992, 1995; Stein et al., 1993;Al-Mishwat and Nasir, 2004; Krienitz and Haase, 2011; Medarisand Syada, 1999, 1998; Nasir and Safarjalani, 2000; Safarjalani etal., 2009, 2004). Previous studies on lower crustal xenoliths of Ara-bia show that these form a part of a gabbroic intrusive complexthat underlies the Arabian Plate, and may represent mafic rootsof one or more arc complexes of Pan-African (Neoproterozoic) age(Nasir, 1995; Al-Mishwat and Nasir, 2004). Significant differenceshave been noted between lower crustal xenoliths of Syria andthose from elsewhere on the Arabian Plate. Al-Mishwat and Nasir(2004) note that xenoliths from Tell Thannoun are rich in amphi-bole (up to 30%) and contain kaersutite, phases that are rare from

xenolith suites from Arabia to the south. Some Syrian xenolithsare also enriched in High Field Strength Elements (HFSE; e.g. Ti,Nb), Large Ion Lithophile Elements (LILE; e.g. K, Rb) and Light RareEarth Elements (LREE) (Al-Mishwat and Nasir, 2004).

We studied a suite of Tell Thannoun xenoliths that was col-lected by AAS and HJF in August 2009 and earlier by AAS andhis students from a single flow unit in an area of several hun-dred square meters on the flank of Tell Thannoun. The flow in thisarea is crowded with xenoliths up to 30 cm long and occasionallybigger. Lower crust and mantle xenoliths occur together, and up-per crust xenoliths are also present. Most xenoliths are weathered.Sixteen of the freshest xenoliths were studied by HJF and eight ofthese are subjects of this study.

3. Analytical methods

This section outlines the analytical techniques that we used;further details are provided in Supplementary document 1. Min-eral chemistry was obtained by electron probe microanalysis at theUniversity of Texas at El Paso (UTEP) and University of Nevada,Las Vegas (UNLV). Mineral analyses are reported in Supplemen-tary documents 2A–2H. The two-pyroxene thermometer of Breyand Köhler (1990) was used where possible. The two-oxides ther-mometer formulated by Andersen and Lindsley (1988) was appliedfor suitable samples. Oxide compositions were projected using theprogram QUILF95 (Andersen et al., 1993).

Whole-rock geochemical analyses were performed at ALS Geo-chemistry, Vancouver, BC, Canada. Major elements were deter-mined by Inductively Coupled Plasma – Atomic Emission Spec-troscopy (ICP-AES) following lithium metaborate/lithium tetrabo-rate (LiBO2/Li2B4O7) fusion. Trace and rare earth elements weredetermined by Inductively Coupled Plasma – Mass Spectrometry(ICP-MS). Carbon and S contents were measured by heating in aLECO induction furnace. The base metals and elements commonlyassociated with sulfide phases (Ag, As, Cd, Co, Cu, Mo, Ni, Pb,Zn) were determined by Inductively Coupled Plasma – Mass Spec-troscopy (ICP-MS). Normative mineralogy was calculated assumingFe3+/Fe total = 0.1. Complete major and trace element data areprovided in Supplementary document 3.

U–Pb zircon ages were determined using a CAMECA IMS-1270secondary ion mass spectrometer (SIMS) at the Swedish Museumof Natural History, Stockholm, Sweden. A total of 64 spots from fivesamples (TT06, TT08, TT13, TT14, TT16) were analyzed (completedata in Supplementary document 4). Results were plotted as two-sigma error ellipses on concordia diagrams (Tera and Wasserburg,1972). Data were processed using SQUID (Ludwig, 2001a) and Iso-plot (Ludwig, 2001b) software. Weighted mean ages are given withtwo-sigma errors.

Radiogenic isotopic compositions (Sr, Nd, Pb) were determinedat Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany.Strontium and neodymium isotopic compositions were determinedon a Triton multi-collector mass-spectrometer using dynamicmulti-collection. 87Sr/86Sr for NBS 987 was 0.710249 ± 0.000005(2-sigma for 20 analyses). 143Nd/144Nd value of the La Jolla Ndstandard was 0.511855 ± 0.000006 (2-sigma for 14 analyses). An-alytical uncertainties of the individual measurements are reportedas 2-sigma. Nd-model ages (TDM) were calculated using the algo-rithm of Nelson and DePaolo (1984). Lead was determined on aFinnigan MAT262 multi-collector mass-spectrometer using staticmulti-collection. Instrumental fractionation was corrected with0.1%/a.m.u. as determined from repeated measurements of NBS981. Accuracy and precision of reported Pb-isotope ratios are betterthan 0.1% at the 2-sigma level.



4. Results

4.1. Petrology

Table 1 summarizes the principal petrologic features of theeight studied xenoliths. The xenolith suite is very diverse, includ-ing two peridotites (TT05 and 15), one pyroxenite (TT06), fourmafic granulites (TT01, 13, 14, and 16), and one charnockite (TT08).All samples contain both ortho- and clinopyroxene. Orthopyrox-enes in mafic granulites encompass a narrow compositional range,EN65-72 (Table 1). The samples – including the peridotites – areremarkably free of tectonic fabric, but charnockite TT08 showsstrong layering of silicate and oxide minerals (Fig. 2D). One peri-dotite could be fertile mantle (TT05) but the other (TT15) is un-usually Fe-rich (23.5% Fe2O3; Table 2), a composition not commonin mantle rocks. TT15 may be a cumulate, an interpretation that isconsistent with observed layering of dark and light color mineralsbands. Orthopyroxene in TT15 is ∼EN76 and compositionally sim-ilar to that in the pyroxenite (TT06) and peridotite (TT05). TT06sapphirine-bearing pyroxenite is the first reported occurrence ofsapphirine from the lower continental crust of Arabia (sapphirineis primarily a product of high grade metamorphism of rocks poorin silica and rich in magnesium and aluminum). Some of the maficgranulites show faint layering, suggesting a cumulate origin (TT01,TT13, TT14). Four mafic granulites contain plagioclase with a re-stricted compositional range, AN35-42. Ilmenite and fluorapatitewere identified in five samples (cf. Table 1). Four mafic granulitescontain amphibole. One mafic granulite (TT16) also contains garnetand biotite. TT08 is a charnockite (60.7% SiO2, 0.66% MgO, 5.35%K2O), with an unusually broad range of mineral compositions. Ithas very little modal quartz and is essentially a granulitic syenite,with ∼90% perthitic feldspar (plagioclase = AN15).

Because there are no suitable phases to use for calculating equi-libration pressures, we cannot directly constrain from what parts ofthe lithosphere these samples were derived. The four mafic gran-ulites and charnockite were most likely derived from the crust,and two of these yield equilibration T = 780–800 ◦C, which wetake to reflect lower crustal temperatures at the time of forma-tion. Peridotite TT05 and pyroxenite TT06 give significantly higherT ∼ 900 ◦C and these may be from the underlying lithosphericmantle. Fe-rich peridotite TT15 yields T ∼ 800 ◦C and may repre-sent a cumulate layer that formed near the base of the crust.

4.2. Geochemistry:

Analyzed samples are reasonably fresh, as demonstrated by Losson Ignition <1 wt.%. Still, some ilmenite is altered to hematite.SiO2 content varies widely, from 34.7 wt.% for Fe-rich peridotiteTT15 to 60.7 wt.% for charnockite TT08 (Fig. 3). Three major litho-logic groups can be distinguished from MgO contents: ultramafic(18–41 wt.% MgO); mafic (6.2–8.3 wt.% MgO), and felsic (<1 wt.%MgO; Fig. 3). Only Fe-peridotite TT15 contains normative corun-dum, attesting to the largely igneous nature of the suite. All sam-ples except the charnockite contain normative olivine and fivexenoliths contain minor normative nepheline (Supplementary doc-ument 4).

Ultramafic xenoliths: Mg# (= 100∗ atomic Mg/Mg + Fe) is∼90 for peridotite TT05 and pyroxenite TT06; these also contain∼2900 ppm Cr and 635–1950 ppm Ni, as might be expected formantle rocks. In contrast, Fe-rich peridotite TT15 has Mg# ∼ 69 inspite of containing almost 5300 ppm Cr and 1000 ppm Ni.

Mafic xenolits: There are two subgroups of mafic granulitesthat are mineralogically similar but have different chemical com-positions. The more primitive group (TT01 and TT13) is markedby AN40-42, EN69-72, moderate Mg# ∼ 62 to 69, and low Fe,Ti and P contents (7.6–10 wt.% Fe2OT

3, 0.48–1.19 wt.% TiO2 and Tabl

e1

Petr

olog

icsu

mm

ary,

Tell

Than

noun

Xen

olit

hs.

Min

eral

TT01

TT05

TT06

TT08

TT13

TT14

TT15

TT16

Mafi

cG

ranu

lite

Peri

doti

tePy

roxe

nite

Char

nock

ite

Mafi

cG

ranu

lite

Mafi

cG

ranu

lite

Peri

doti

teM

afic

Gra

nulit

e

Oliv

ine

Fo90

Fo66

–77

Spin

elCr

#0

0.12

0.03

14

Clin

opyr

oxen

eW

o43E

n35F

s15A

c7W

o45E

n46F

s4A

c4W

o48E

n44F

s4A

c3W

o39E

n33F

s22A

c4W

o42W

n37F

s13A

c7W

o41E

n37F

s16A

c6W

o45E

n40F

s11A

c4W

o43E

n36F

s15A

c6O

rtho

pyro

xene

Wo1

En69

Fs30

Wo1

En90

Fs9

Wo1

En88

Fs11

Wo1

En37

-60F

s39-

62W

o1En

72Fs

27W

o1En

67Fs

30W

o1En

76Fs

23W

o1En

65Fs

34Pl

agio

clas

eA

n42

An1

5A

n40

An3

8A

n35

Am

phib

ole

Kae

rsut

ite

Kat

opho

rite

Parg

asit

ePa

rgas

ite

Gar

net

Py34

Al4

7Gr1

5Ilm

enit

ex

xx

xx

Hem

atit

ex

xA

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tex

xx

xx

Oth

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iner

als

Sapp

hiri

neZi

rcon

,K-f

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ldsp

ar,b

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teT

◦ CBK

N78

090

089

682

580

0T

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UIL

F95

810

fO2

−12.

6D

iff

FMQ

1.65



Fig. 2. A) Landscape around Tell Thannoun volcano (background), Abdulrahman Safarjalani and Andrea Förster in foreground, B) Gabbroic xenolith in mafic lava; C) Planelight thin-section scan of TT06 sapphirine peridotite (∼3 cm wide), Ol (Fo90) + Opx (En89) + Diopside + Sp (Cr# = 3) T (BKN) = 900–930 ◦C; D) Plane light thin sectionscan of TT-08 charnockite (∼3 cm wide); E) Backscattered electron image of TT06 sapphirine pyroxenite, showing rim of sapphirine (sap) surrounding aluminous spineland relationships with orthopyroxene (opx) and clinopyroxene (cpx); F) Backscattered electron image of TT-13 layered mafic granulite: En71–72; Cpx; An38–59; Pargasiticamphibole; Sp Cr# ∼0.

Table 2Whole-rock Sr, Nd, and Pb isotope composition of xenoliths from Tell Thannoun, Syria.

Sample(lithology∗)

87Sr 87Sr(T)a 143Nd εNd(T)

a TDM

(Ga)b

206Pb 207Pb 204Pb 206Pbc 207Pbc 208Pbc

86Sr 86Sr 144Nd 204Pb 204Pb 204Pb 204Pb 204Pb 204Pb

TT 01 (MG) 0.704558 ± 3 0.70455 0.512762 ± 4 4.5 0.71 18.628 15.634 38.325 18.57 15.63 38.27TT 05 (Pd) 0.705139 ± 6 0.70510 0.513060 ± 7 6.2 – 18.242 15.658 38.192 18.15 15.65 38.16TT 06 (Px) 0.704670 ± 8 0.70421 0.513088 ± 4 9.5 – 18.118 15.618 37.965 17.99 15.61 37.94TT 08 (Ch) 0.704104 ± 5 0.70312 0.512640 ± 3 4.0 0.60 18.424 15.621 38.251 18.14 15.61 38.11TT 13 (MG) 0.703541 ± 9 0.70352 0.512704 ± 3 4.9 0.55 18.456 15.653 38.558 18.33 15.65 38.41TT 14 (MG) 0.703292 ± 8 0.70316 0.512750 ± 2 4.9 0.60 18.703 15.594 38.538 18.39 15.58 38.13TT 15 (Pd) 0.703641 ± 8 0.70340 0.512876 ± 2 4.9 – 18.299 15.644 38.259 18.17 15.64 38.21TT 16 (MG) 0.703324 ± 8 0.70327 0.512693 ± 2 4.3 0.63 18.607 15.612 38.399 18.50 15.61 38.32

∗ MG = mafic granulite; Pd = peridotite; Px = pyroxenite; Ch = charnockite.a 87Sr/86Sr(T) and εNd(T) were calculated for 357 Ma using λ87Rb = 1.42 ∗ 10−11 y−1 and λ147Sm = 6.54 ∗ 10−12 y−1, (147Sm/144Nd)0

CHUR = 0.1967, and(143Nd/144Nd)0

CHUR = 0.512638, respectively, and the concentration data.b Nd model ages according to Nelson and DePaolo (1984).c Lead isotope data recalculated to 357 Ma, using the contents of Pb, Th, and U and λ238U = 1.55125∗10−10 y−1; λ235U = 9.848∗10−10 y−1; λ232Th = 4.9475∗10−11 y−1.



Fig. 3. Harker variation diagrams for four oxides vs. SiO2: A) FeO∗/MgO showing tholeiitic – calc-alkaline boundary of Miyashiro (1974); B) MgO; C) Al2O3; D) K2O. Notethe exceptionally large range in compositions, from normal peridotite to Fe-rich peridotite to pyroxenite to mafic granulite to felsic rock (charnockite). Syrian xenoliths arecompared to xenoliths from other harrats to the south in Arabia (data sources: USGS, 1984; Al-Mishwat and Nasir, 2004; Ghent et al., 1980; McGuire and Stern, 1993;Nasir, 1995; Nasir and Safarjalani, 2000).

0.05–0.19 wt.% P2O5). The more fractionated group of mafic gran-ulites (TT14 and TT16) is characterized by AN35–38, EN65–68, lowMg# ∼ 42 to 67, and high Fe, Ti and P contents (16.5–17.2 wt.%Fe2OT

3, 3.34–5.19 wt.% TiO2 and 0.75–1.68 wt.% P2O5, respectively).Fractionated mafic granulites plot in the tholeiitic field whereasmore primitive granulites plot in the calc-alkaline field (Fig. 3A).Mafic xenoliths are aluminous, with 16.45 to 20.8 wt.% Al2O3.

Felsic xenolith: Only the charnockite (TT08) contains a smallamount (0.3 wt.%) of normative quartz. In spite of these alka-line tendencies, this is a low- to medium-K suite (Fig. 3D), withthe exception of the high-K charnockite TT08 containing 5.35 wt.%K2O, which plots in the tholeiitic field and is classified as ferroancharnockite (Frost and Frost, 2008).

Trace element abundances for the suite are also broadly con-sistent with our inference that these xenoliths – possibly exceptof peridotite TT05 and pyroxenite TT06 – formed from enrichedmagma. Trace element abundances further distinguish the maficgranulites. For example, the more primitive group has higher Srconcentrations but lower total rare earth element (REE) contentscompared to more fractionated mafic granulites (1205–2040 ppmSr and ∼43 ppm REE vs. 682–943 ppm Sr and 157–359 ppmREE) (Fig. 4). Charnockite TT08 has moderate Sr and REE con-tents (336 and 154 ppm, respectively), but remarkably high con-tents of Ba and Zr (6100 and 1370 ppm, respectively). All sam-ples are moderately to strongly LREE-enriched, as demonstratedby chondrite-normalized La/Yb ratio [(La/Yb)n]. Ultramafic rocksshow (La/Yb)n = 1.6 to 3.3, further indicating that these are notsamples of depleted mantle. Peridotite TT05 and pyroxenite TT06are distinguished by deep negative Ce-anomalies, a feature not

shown by the other samples; Ce anomalies in peridotites wereshown by Neal and Taylor (1989) to indicate subduction influence.Both groups of mafic granulites and charnockite are LREE-enriched,with (La/Yb)n = 4.5 to 17 (Fig. 4). Mafic granulites have mod-est to strong positive Eu-anomalies (Eu/Eu∗ = 1.06 to 1.78) andcharnockite TT08 has a remarkably high positive Eu-anomaly, withEu/Eu∗ = 6.3.

These results reveal a broadly consanguinous suite that is char-acterized by strong Fe- and Ti enrichment (Fig. 3). This furthersuggests that a broadly tholeiitic suite crystallized deep in thecrust and lithospheric mantle under low fO2 conditions.

4.3. Geochronology

All eight samples hosted zircon grains in quiet variableamounts. We could separate a few grains from the peridotites:TT05 (5 grains) and TT15 (one grain). Sapphirine pyroxenite TT06contained a dozen grains. All four of the mafic granulites yieldedabundant zircon. Charnockite TT08 yielded thousands of grains.On the basis of yield, five samples were selected for U–Pb zircondating (TT06, TT08, TT13, TT14, and TT16; analytical results areprovided in Supplementary document 1). Zircons were first imagedusing cathodeluminescence (CL) for xenocrystic cores which werenot identified; examples of the CL images are provided in Supple-mentary document 6. Three samples (pyroxenite TT06, charnockiteTT08, and primitive mafic granulite TT13) gave robust, concor-dant, and analytically indistinguishable ages. Pyroxenite TT06 gavea robust 206Pb/238U age for 9 grains of 358 ± 6 Ma (Fig. 5A).Charnockite TT08 gave a 206Pb/238U age of 355±6 Ma for 11 grains



Fig. 4. Chondrite-normalized REE plot. Open circle for TT05 indicates Ce < 0.5 ppm, Syrian xenoliths are compared to xenoliths from other harrats to the south in Arabia(data sources: USGS, 1984; Al-Mishwat and Nasir, 2004; Ghent et al., 1980; McGuire and Stern, 1993; Nasir, 1995; Nasir and Safarjalani, 2000).

(Fig. 5B). Mafic granulite (TT13) gave a 206Pb/238U age for ninegrains of 359 ± 7 Ma (Fig. 5C). Th/U for zircons from these threesamples averaged 0.75 indicating that these formed by igneousprocesses (Hoskin and Schaltegger, 2003). We interpret these threeages to approximate when the magmas forming these rocks solid-ified in the lower crust, at ∼357 Ma.

The two fractionated mafic granulites yielded plenty of zir-cons but scattered ages (Supplementary document 5). TT14 gave206Pb/238U ages that scatter from 38 to 222 Ma.

TT16 yielded 206Pb/238U ages that range from 41 to 193 Ma. Weinterpret the results to reflect loss of Pb from the zircons. Thesetwo samples have zircons characterized by lower mean Th/U, aver-aging 0.46 for TT14 and 0.62 for TT16. On the basis of these results,it cannot be unambiguously established whether the magmas thatformed these rocks originated at the same time as TT06, TT08, andTT13 or not.

4.4. Radiogenic isotopes

Isotopic compositions of Sr, Nd, and Pb and initial ratios cal-culated at 357 Ma are listed in Table 2. Initial isotopic ratiosare shown in Fig. 6. Several observations are noteworthy: 1) Ini-tial 87Sr/86Sr ratios are low but variable (0.70312 to 0.70510),with the highest value belonging to peridotite TT05. 2) InitialNd-isotopic compositions are radiogenic, with εNd(357 Ma) = +4to +9.5. Most samples have a restricted range of εNd(357 Ma) =+4.0 to +4.9, only peridotite TT05 (εNd(357 Ma) = +9.5) and py-roxenite TT06 (εNd(357 Ma) = +6.2) are outside this range. Fivesamples have 147Sm/144Nd low enough (<0.165; Stern, 2002) togive meaningful Nd-model ages. Using the depleted mantle modelof Nelson and DePaolo (1984), these range from 0.55 to 0.71 Ga(Table 2). 3) Initial Pb-isotopic compositions vary significantly:206Pb/204Pb = 17.99 to 18.57, with the ultramafic rocks showingthe lowest 206Pb/204Pb and the mafic granulites having the high-est 206Pb/204Pb (Fig. 6B, C). The suite shows modestly elevated208Pb/204Pb relative to the Northern Hemisphere Reference Line(NHRL), similar to Indian Ocean MORB (Fig. 6B). 207Pb/204Pb is alsoelevated relative to NHRL (Fig. 6C), similar to what is observed formodern subduction-related igneous rocks. Charnockite TT08 has

mantle-like initial 87Sr/86Sr (0.70312), strongly positive εNd(357 Ma)(+4.0), and initial Pb-isotopic compositions that are in the mid-dle of the range for the 8 samples; all of which argue against asignificant input of older crust into this melt.

5. Discussion

The studied eight Tell Thannoun xenoliths sample the deep con-tinental crust and upper mantle beneath southern Syria. Geochem-ical, geochronological, and isotopic data suggest that this could bea broadly comagmatic suite, with the possible exception of lherzo-lite sample TT05. These xenoliths differ from better studied lowercrustal xenoliths from Jordan and Saudi Arabia in four principalways: 1) significantly younger age; 2) the presence of charnock-ite; 3) abundance of Fe-rich mafic and ultramafic lithologies; and4) the presence of sapphirine. There are not yet any publishedU–Pb zircon ages for xenolith samples from Jordan and Saudi Ara-bia lower crust but the association of especially the Saudi Arabianxenoliths with vast expanses of Neoproterozoic juvenile crust sug-gests formation in Neoproterozoic time (Stern and Johnson, 2010).This is at least 200 Ma older than the Mississippian (Early Car-boniferous) age of Tell Thannoun xenoliths. We conclude that thethree samples yielding concordant ∼357 Ma ages reflect an impor-tant crust-forming event and that the younger, discordant zirconages for two samples of more fractionated mafic granulite reflectincomplete resetting of the zircon U–Pb system, perhaps due toheating associated with Cenozoic volcanism.

In this section we explore the significance of our results forunderstanding the geologic evolution of northern Arabia, includ-ing considering other evidence for Late Paleozoic igneous activity(5.1); likely tectonic setting where these magmas formed (5.2); re-lationship of this crustal tract to those of similar age elsewhere inEurasia (5.3); and what our results for Tell Thannoun xenoliths re-veals about the nature of the crust beneath northern Arabia (5.4).

5.1. Other evidence for late paleozoic igneous activity in N. Arabia

Discovery of evidence for an Early Carboniferous igneousepisode beneath northern Arabia is consistent with several other



Fig. 5. Tera–Wasserburg concordia diagrams for a) TT06 sapphirine pyroxenite,b) TT08 charnockite, and c) TT-13 mafic granulite. Error ellipses are shown as twosigma error; age shown also includes 2 sigma error.

observations. First, zircon fission-track ages of 17 Precambrian sed-imentary and igneous rocks from deep boreholes and outcrops insouthern Israel and Sinai fall within the range 328–373 Ma (LateDevonian–Early Carboniferous) (Kohn et al., 1992). These resultswere interpreted to indicate total annealing of zircon fission tracksbut only partial resetting of coexisting titanite, constraining tem-peratures in this region (to the SW of Tell Thannoun) to about 225

Fig. 6. Isotopic compositions of Tell Thannoun xenoliths, corrected for 357 Ma ofradiogenic growth. A) Sr–Nd. Location depleted MORB mantle (DMM), prevalentmantle (PREMA), high-mu (HIMU) and enriched mantles 1 and 2 (EM1, EM2) andfields for MORB and Ocean islands basalt are from White (2010). B) 208Pb/204Pbvs. 206Pb/204Pb and C) 207Pb/204Pb vs. 206Pb/204Pb. Fields for DMM, EMI, EM2,HIMU, Indian MORB, and the Northern Hemisphere Reference Line (NHRL) are fromGarrido et al. (2007).

± 50 ◦C, followed by rapid uplift and cooling. Kohn et al. (1992)concluded that the Late Devonian–Early Carboniferous event wasconfined to a relatively narrow belt extending from the Gulf ofSuez area through NE Syria and SE Turkey, but this conclusion mayneed to be modified in light of the evidence for a major magmaticepisode suggested from Tell Thannoun xenoliths.

Another line of evidence indicating that Late Paleozoic igneousactivity was important for forming northern Arabian crust is pro-vided by hydrocarbon drilling results by Saudi ARAMCO in north-ern Saudi Arabia. According to unpublished reports (Afifi, 2013),mafic sills and dikes were penetrated and cored in several wellsin the Nafud basin, particularly the vicinity of the Cretaceous WadiSirhan graben, a few hundred km SSW of Tel Thannoun. Unpub-lished petrographic studies reveal two mafic suites: diabase and



alkaline lamprophyres. These were dated by K–Ar whole-rock tech-niques and the most reliable ages are 360 ± 18 Ma for the diabasesills (i.e., Late Devonian–Early Carboniferous), and around 80 Ma(Late Cretaceous) for the lamprophyres. The mafic sills intrude thePaleozoic section, particularly Silurian shales. Paleozoic volcanicrocks were not recognized and may have been removed by up-lift and erosion. The Paleozoic sills are about the same age as thesuite we have studied and are probably related.

A third line of evidence comes from evidence for Late Paleo-zoic ‘Hercynian’ (∼380–280 Ma) tectonic movements in northernArabia, which were particularly important for controlling majorN–S structures in Saudi Arabia oilfields (Faqira et al., 2009). Her-cynian instability in northern Arabia is reflected in structures likethe north-trending Al-Batin Arch (Faqira et al., 2009). An extensiveperiod of erosion due to Early Carboniferous uplift is preserved aswhat is known as the Hercynian Unconformity (Faqira et al., 2009).This unconformity is well known from the Greater Ghawar UpliftProvince in northern Arabia, where about 1100 m of section waseroded away (Wender et al., 1998). The cause of this deformationhas not been satisfactorily explained. Heretofore no relationshipbetween crustal movements and igneous activity has been iden-tified in northern Arabia, but our results demonstrate that EarlyCarboniferous geologic evolution involved igneous activity in theregion.

5.2. Tectonic setting

We investigated whether or not tectonic setting could be in-ferred from chemical and isotopic compositions. Major elementdata indicate that xenoliths representing crustal rocks tend to-ward silica-undersaturation. Crustal rocks and Fe-rich peridotitedemonstrate strong Fe- and Ti enrichment indicating that thisis a strongly tholeiitic suite that evolved under low fO2 condi-tions, which is consistent with abundant ilmenite and stronglypositive Eu anomalies. The high Zr contents of charnockite TT08(1370 ppm) require high T and dry conditions, although such highconcentrations may also partly reflect zircon accumulation. Usingprocedures outlined in Hanchar and Watson (2003), we calculatezircon saturation T ∼ 950 ◦C. Such high T imply dry conditions,more consistent with a rift than an arc setting.

Another petrotectonic insight is provided by the sapphirine py-roxenite sample (TT06). Experimental and natural rock data showthat assemblages of clinopyroxene + spinel + sapphirine like thatin TT06 coexist at 8–15 kbar and 800–900 ◦C (Su et al., 1997). Suet al. (1997) argued that such conditions are best satisfied in atectonic setting where basaltic magma is underplated and there issignificant interaction between infiltrating basaltic melts and man-tle peridotite.

Trace-element and isotope data for Tell Thannoun xenoliths donot show a consistent picture, possibly indicating that the sourcehad been affected by several different processes. For instance, peri-dotite TT05 and pyroxenite TT06 have Nd isotopic compositionsindicative for a strongly depleted source, whereas their Sr–Ndisotopic decoupling and their deep negative Ce-anomalies mightbe expected in rocks that were affected by subduction-relatedfluids. In contrast, the mafic xenoliths are derived from an en-riched magma and have N-MORB normalized trace element pat-terns that do not show negative Nb–Ta anomalies characteristic ofsubduction-related igneous rocks.

5.3. How does Late Paleozoic crust of N. Arabia relate to other crustaltracts of similar age in the region?

Throughout most of its history, Arabia was an integral part ofthe supercontinent Gondwanaland, and northern Arabia was situ-ated near its margin. The Late Paleozoic to Early Mesozoic evolu-

tion of this part of Gondwanaland was dominated by Late Devo-nian opening and Early Triassic closing of the Paleo-Tethyan Ocean(Stampfli and Borel, 2002). Such a setting also agrees with whatis known about the Mishto Magmatic Complex (MMC) in northernIran, which consists of a gabbro and sheeted dike complex thathad intruded into Precambrian continental basement (Saccani etal., 2013). MMC gabbros yield a U–Pb zircon age of 356.7±3.4 Ma,indistinguishable from Tell Thannoun xenolith ages. MMC igneousrocks include N-MORB and P-MORB varieties, interpreted as pro-duced by interaction between depleted MORB-type asthenosphereand enriched mantle. Saccani et al. (2013) inferred that the MMCrepresents an Early Carboniferous magmatic event developed dur-ing rifting to open Paleo-Tethys.

The pattern of overall convergence between Gondwana andLaurussia changed in the Late Devonian to Early Carboniferoustime because of the collision of these two continents to form muchof the crust of modern Europe. Continued subduction betweenGondwana and Laurussia occurred thereafter farther to the west,resulting both in the closure of the remaining Rheic ocean and theopening of the Paleo-Tethys (Kroner and Romer, 2013). Thus, 420to 320 Ma old igneous rocks in the central and western part of theplate boundary zone between Gondwana and Laurussia are relatedto subduction and collision (as summarized in Kroner and Romer,2013), whereas igneous rocks farther to the east show geochem-ical fingerprints indicating rifting and formation of oceanic crust.The c. 357 Ma old mantle-derived rocks of SW Syria and Iran mayreflect this west-to-east transition from convergence to extension.

5.4. What do Tell Thannoun xenoliths reveal about crust beneathnorthern Arabia?

Tell Thannoun xenoliths indicate that much of the crust be-neath SW Syria formed in Early Carboniferous time, ∼357 Ma.A schematic representation of what these xenoliths reveal aboutthe crust and upper mantle is shown in Fig. 7, superimposed onthe crustal profile of Brew et al. (2001). Note that the geophysicalprofile that this is based on lies ∼80 km east of Tell Thannoun(Fig. 1C), so the relative position of the crust sampled by TellThannoun, especially relative to the inferred intrusion in Fig. 7,is poorly constrained. Nevertheless, the interpreted profile showsthe possibility that the more fractionated group of mafic granulitessample inferred intrusions in the Rutbah block (Brew et al., 2001).

The crust beneath northern Arabia (including Syria) is generallyconsidered to be a buried extension of Neoproterozoic crust likethat exposed in the Arabian–Nubian Shield to the south. The onlyevidence we found in favor of this interpretation are the ∼0.55to 0.71 Ga Nd-model ages reported for five of the Tell Thannounxenoliths (Table 2). Nd-model ages are often a few hundred mil-lion years older than crystallization ages, especially if older crustalmaterial is recycled (Arndt and Goldstein, 1987). Therefore there isno compelling argument in support of interpreting Syrian crust ashaving formed in Neoproterozoic time, particularly because we didnot find a single zircon of Neoproterozoic age in the five samplesand 64 grains we dated. We prefer to interpret our results as in-dicating that much of the crust beneath SW Syria formed due tothe significant addition of magmas at ∼357 Ma. This is the firstdirect evidence for the age of the crust in this region, with ma-jor implications for our understanding of the geologic evolutionof the northern Arabian Plate. If this crust is Early Carboniferousin age, what is the location and nature of the boundary betweenthis crust and undoubted Neoproterozoic crust to the south? Be-cause Tell Thannoun lies ∼33◦ N, close to the Jordan border andNeoproterozoic rocks outcrop in Jordan as far north as 31◦ N, thetransition between ANS Neoproterozoic crust and Paleozoic crustprobably lies beneath northern Jordan if the crustal boundary hasa Tethyan E–W trend.



Fig. 7. Cross section of Brew et al. (2001) showing where xenoliths studied here might have originated. Source of Tell Thannoun magma must reflect low degrees of meltingnear the base of the lithosphere, perhaps deeper than shown here. Lithospheric mantle samples include peridotite (Pd) sample TT05 and sapphirine pyroxenite (SaPx) sampleTT06. Fe-rich peridotite (FePd) sample TT15 may have formed as a cumulate in the lower crust. More primitive group mafic granulite (PrMG) samples TT11 and TT13 maybe representative of mafic lower crust materials. More fractionated group mafic granulites (FrMG) samples TT14 and TT16 may also be derived from the mafic lower crust orcould come from a younger (Cretaceous?) intrusion. Charnockite sample TT08 probably originates in the lower crust. Significance of Palmyride orogen is not clear; if it marksa suture then age of crust beneath the Aleppo plateau is unconstrained.

Whatever the location and nature of its boundary with Neopro-terozoic crust to the south, Early Carboniferous crust beneath SWSyria may only extend northwards as far as the NE–SW-trendingPalmyride orogen. The Palmyride orogen has been reactivated sev-eral times, evolving as a rift basin in Late Paleozoic through Creta-ceous time before Cenozoic compression inverted it into a fold-and-thrust belt (Brew et al., 2003). Geophysical studies indicatethat the Palmyride orogen marks an important suture zone butour results compel us to challenge the inference that it origi-nated in Neoproterozoic time (Seber et al., 1993). Collision to formthe original Palmyride orogen must be younger than the flankingcrustal tracts that it sutures, which must be early Carboniferous oryounger. Given the evidence for Hercynian deformation in north-ern Arabia discussed in 5.1, we conclude that the Palmyride suturefirst formed as result of early Carboniferous collision. This inter-pretation is further supported by the intensification of Hercynianstructures from SE to NW across northern Arabia as the Palmyrideorogen is approached, especially the presence of the NE–SW trend-ing Nafud basin, which contains by far the thickest accumulationof Permo-Carboniferous sediments of the Arabian Peninsula (seeFig. 17 of Faqira et al., 2009). We further note the different base-ment depth, crustal thickness, and density on either side of thePalmyrides (Fig. 7) and take these to represent distinct crustalblocks that were sutured along the Palmyride trend. In this case,the documentation of ∼357 Ma crust beneath SW Syria and per-haps parts of N. Jordan requires a major reconsideration of thegeologic evolution of N. Arabia.

6. Conclusions

We have shown, through petrological, geochemical and isotopicanalyses and U–Pb zircon age dating of mantle and lower crustalxenoliths from Tell Thannoun, Syria, that the continental crust be-neath northern Arabia is fundamentally different from Neoprotero-zoic crust exposed in the Arabian–Nubian Shield to the south. The

eight xenoliths that we studied span a large compositional andmineralogical range and include primitive and Fe-rich peridotites,a sapphirine-bearing pyroxenite, mafic granulites, and a charnock-ite. Geochemical and isotopic data suggest that these diverse xeno-liths are broadly co-magmatic. Sr and Nd isotopic compositionsindicate that all formed from depleted mantle, although major andtrace element signatures are ambiguous as to a distinctive tectonicsetting. U–Pb zircon ages for three of the xenoliths are concordantwith an age of ∼357 Ma, the age of crystallization, at least 200 Mayounger than the Neoproterozoic crust (533–1000 Ma) underlyingother parts of the Arabia Plate. These new data reveal a previouslyunknown crustal tract that formed via Carboniferous magmatism.Furthermore, the age and composition of the Tell Thannoun xeno-liths and other evidence for Carboniferous magmatism sheds newlight on some otherwise puzzling observations in northern Ara-bia, such as evidence for Late Paleozoic heating and deformation.These results and considerations indicates that we have yet muchto learn about the geological evolution of northern Arabia.

Acknowledgements

Thanks to Jonathon Stebbins, who figured out that the mysterymineral in TT06 was sapphirine, to Jonathon Price, who helpedwith calculation of zircon saturation temperature for TT08, and toAbdulKader M. Afifi (Saudi ARAMCO) for comments about buriedPaleozoic igneous rocks of northern Saudi Arabia. Thanks also totwo anonymous referees for constructive comments and criticism.This is UTD Geosciences Contribution #12XX.

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2014.02.043.



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