History of the West African Neoproterozoic Ocean: Key to the geotectonic history of circum-Atlantic...

Gondwana Research xxx (2014) xxx–xxx

GR-01370; No of Pages 14

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History of the West African Neoproterozoic Ocean: Key to the geotectonic history ofcircum-Atlantic Peri-Gondwana (Adrar Souttouf Massif, Moroccan Sahara)

Andreas Gärtner a,⁎, Michel Villeneuve b, Ulf Linnemann a, Axel Gerdes c,d, Nasrrddine Youbi e,f,Omar Guillou e, Eh-Cherki Rjimati g

a Senckenberg Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Sektion Geochronologie, GeoPlasma Lab, Königsbrücker Landstraße 159, 01109 Dresden, Germanyb CEREGE, Aix-Marseille Université, Centre Saint-Charles, case 67, 3 place Victor Hugo, 13331, Marseille, Francec Institut für Geowissenschaften, Mineralogie, Goethe Universität Frankfurt, Altenhoeferallee 1, 60438 Frankfurt, Germanyd Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland 7602, South Africae Department of Geology, Faculty of Sciences-Semlalia, Cadi Ayyad University, Prince Moulay Abdellah Boulevard, P.O. Box 2390, Marrakech, Moroccof Centro de Geologia da Universidade de Lisboa (CeGUL), Faculdade de Ciências (FCUL), Departamento de Geologia (GeoFCUL), Campo Grande C6, 1749-016 Lisboa, Portugalg Directorate of Mining Development, Ministry of Energy, Mines, Water and Environment, Rabat, Morocco

⁎ Corresponding author.E-mail addresses: [email protected] (

univ-provence.fr (M. Villeneuve), ulf.linnemann@[email protected] (A. Gerdes), [email protected] (O. Guillou), [email protected] (E.-C. Rjim

http://dx.doi.org/10.1016/j.gr.2014.11.0111342-937X/© 2014 Published by Elsevier B.V. on behalf o

Please cite this article as: Gärtner, A., et al., HPeri-Gondwana (Adrar Souttouf ..., Gondwan

a b s t r a c t
a r t i c l e i n f o
Article history:Received 25 August 2014Received in revised form 12 November 2014Accepted 21 November 2014Available online xxxx

Handling Editor: W.J. Xiao

Keywords:U-Th-Pb and Lu-Hf geochronologyZirconMauritanidesIsland arcWest African Neoproterozoic Ocean

Bordered by the Archaean basement of the Reguibat Shield to the E and S, the Adrar Souttouf Massif is located inthe southern regions of the Moroccan Sahara and represents the northern part of the Mauritanide belt. Thecentral areas of this massif comprise the Dayet Lawda and Sebkha Matallah units that are mainly composed ofmafic rocks. Ten samples taken from these rocks yielded 531 zircon grains that were analysed with respect totheir morphology, U-Th-Pb, and Lu-Hf isotope composition. Additionally, 155 apatite grains from six sampleswere also dated by the U-Th-Pb method. Mostly well rounded zircon grains commonly showed brightcathodoluminescent (CL) overgrowth domains and leaching zones, indicating metamorphic overprint aroundunaffected oscillatory zoned core areas. All samples but one yielded two significant zircon age populations:~605 Ma and ~634 Ma with some inherited grains in the range of ~650–740 Ma and very scarcely up to ~1190Ma. For six samples theHf isotope composition suggests amajor contribution ofNeoproterozoic juvenilemagmasfrom depleted mantle source. The zircon Hf data for the remaining samples points to a predominant recycling ofolder Archaean to Palaeoproterozoic crust. Based on the geochemical composition of the rocks, an island arcsetting is assumed at the periphery of theWest African Craton close to the Cryogenian-Ediacaran boundary. Sub-sequent metamorphism during accretion and partial obduction onto the basement rocks took place at about 605Ma. Aminor Variscan overprint could be demonstrated for only one sample. This case study exemplifies the greatpotential of the widely occurring metamorphosed mafic and ultramafic rocks along the western margin of theWest African Craton for palaeogeographic and geodynamic reconstructions including the peri-Gondwananterranes during the Late Neoproterozoic.

© 2014 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

1. Introduction

Several occurrences ofmafic and ultramafic rocks are reported alongthewesternmargin of Africa. Some of them are interpreted to representremnants of Neoproterozoic oceanic crust, but yet remain undated. Theintroduction of a West African Neoproterozoic Ocean by Villeneuveet al. (2010) led to the question how the numerous mafic rocks alongthe recent Atlantic margin of Africa could be correlated and how thisparticular ocean may have evolved. One of the largest areas of rocks

A. Gärtner), [email protected] (U. Linnemann),a (N. Youbi), guillou.omar@ati).

f International Association for Gondw

istory of the West African Nea Research (2014), http://dx

potentially related to such an oceanic or island arc setting is located inthe Adrar Souttouf Massif (Moroccan Sahara). Although there areseveral maps at large scales and much detail recently published by theGeological Survey of Morocco, wide areas remain unmapped. Thisapplies particularly for major parts of the units, which are mainlycomposed of mafic rocks related to oceanic crust.

The Adrar Souttouf and the Dhlou Massif are both forming theSouttouffide Belt, which is the northern part of the CarboniferousMauritanide Belt in the Moroccan Sahara (Dacheux, 1967; Villeneuveet al., 2006; Villeneuve, 2008; Michard et al., 2010). A complex andpolyphased geologic history including a thin skin model for nappe tec-tonics of the four main units of the Adrar Souttouf Massif has alreadybeen assumed by several authors (e.g. Sougy, 1962a; Bronner et al.,1983; Marchand et al., 1984; Villeneuve et al., 2006; Michard et al.,2010; Gärtner et al., 2013a). First attempts to reconstruct the complex

ana Research.

oproterozoic Ocean: Key to the geotectonic history of circum-Atlantic.doi.org/10.1016/j.gr.2014.11.011

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2 A. Gärtner et al. / Gondwana Research xxx (2014) xxx–xxx

evolution of this massif using several geochronological methods weredone by Villeneuve et al. (2006), Gärtner et al. (2013a), whileMontero et al. (2014) give valuable information about the adjoiningparts of the Reguibat Shield. The following study aims to a precise agedetermination of the intrusion as well as the metamorphic overprintof several magmatic rocks that are possibly Neoproterozoic in age andrelated to oceanic and island arc settings (Youbi et al., submitted forpublication). This paper presents new U-Pb ages of zircon and apatitecombined with zircon Hf isotope analyses and zircon morphology. Aregionally limited reconstruction of the West African NeoproterozoicOcean's evolution is proposed.

2. Geological setting

Situated at the western margin of the Archaean Reguibat Shield ofthe West African Craton, the Adrar Souttouf Massif represents thenorthern section of the Mauritanides (Sougy, 1962a). A subdivision ofthe investigated area into four main structural domains was introducedby Villeneuve et al. (2006, 2010). The geotectonic units are named fromwest to east as follows: Oued Togba, Sebkha Gezmayet, Dayet Lawda,and Sebkha Matallah. All of them show a NNE-SSW trending strikeand are thrusted top-SE onto the western Reguibat basement and / ora thin Ordovician to Devonian sedimentary succession termed as DhloatEnsour Group (Michard et al., 2010, and references therein). The thinskin model was introduced by Sougy (1962a) and supported by mostof the later authors. Polyphased metamorphic overprint of the entireregion in the course of Neoproterozoic and Variscan-Alleghanian orog-enies can be inferred from the dominant occurrence of rocks showingall stages of metamorphismwithin the Adrar Souttouf Massif. Geochro-nologic data corroborates the hypothesis of a complex polyphased geo-logic history of the entire area (Villeneuve et al., 2006; Gärtner et al.,2013a). Both of the western Oued Togba and Sebkha Gezmayet unitsare mainly composed of high-grade metamorphic rocks intruded bygranitoids and, locally, by mafic and ultra-mafic dykes (Villeneuveet al., 2006; Michard et al., 2008, 2010). Recent radiometric datafrom the Oued Togba and Sebkha Gezmayet units suggest theirperi-Gondwanan origin. The zircon age distribution patterns of thefirst mentioned unit is interpreted to show Avalonian affinities, whilethe latter one is highly likely linked to Meguma (Gärtner et al.,2013a). The Dayet Lawda and parts of the Sebkha Matallah units areinterpreted to contain rocks potentially derived from oceanic crust,forming an ophiolitic massif of approximately 220 km length andabout 45 km width within the Adrar Souttouf Massif (Gasquet et al.,2008; Rjimati and Zemmouri, 2011; Fig. 1).

2.1. Dayet Lawda unit

The Dayet Lawda unit consists mainly of metamorphosedmafic to ul-tramafic rocks (gabbro, gabbro-diorite, olivine gabbro, anorthosite,serpentinite, etc.; Arribas, 1968). This unit still remains unmapped in larg-er scales than 1:1.000.000. Nevertheless, Villeneuve et al. (2006) intro-duced a subdivision into four sub-units for the central and northernparts, referred to as “formations” including several K-Arwhole-rock ages:

• Formation A is situated at the westernmargin of the Dayet Lawda unitand overthrusted by the rocks of the eastern Sebkha Gezmayet unit.This formation is mainly composed of amphibolitic gneisses, granitesand doleritic dykes. A K-Arwhole-rock age of 396± 9Mawas obtain-ed for one of the gneisses, which is inferred to have been affected by asubsequent metamorphic event. Therefore, the age is interpreted toreflect rejuvenation. One sample (D72)was taken from this formationfor isotopic analyses.

• Formation B forms the northernmost part of the Dayet Lawda unit andis bordered by thrusts against the Sebkha Gezmayet and SebkhaMatallah units. The formation is dominated by olivine gabbro, withone of them dated at 1061 ± 25 Ma (K-Ar). Latest field observations

Please cite this article as: Gärtner, A., et al., History of the West African NePeri-Gondwana (Adrar Souttouf ..., Gondwana Research (2014), http://dx

made by our working group in 2014 gave evidence for the occurrenceof pyroxenitic mantle as well as leucocratic granite xenoliths / dykeswithin the gabbro and a possible pop-up structure along the westernmargin of formation B. This structure includes an intercalation of am-phibolites and orthogneisses with locally occurring marble lenses.Penetrative deformation and steeply dipping foliation classify thepop-up structure as oneof themost important geotectonic boundariesbetween the different units of the Adrar Souttouf Massif. This newlydiscovered structure could be traced from the road Dakhla-Aoucertto the Sebkha Tidsit and is most probably evolved along the entirewestern margin of the Dayet Lawda unit.

• Formation C is located in the central regions of the Dayet Lawda unit.Its borders against formation D are not defined south of ca. N22°10′due to the poor outcrop situation and difficult access. Most frequentlyoccurring rocks are amphibolites, granites, gabbros, peridotites andbasalts. One basalt sample was dated at 733 ± 17 Ma (K-Ar), whichis inferred to represent the intrusion age (Villeneuve et al., 2006).A K-Ar whole-rock age of 274 ± 12 Ma and a K-Ar feldspar age of251 ± 6 Ma from a gabbro (Villeneuve et al., 2006) are bothinterpreted to depict the rejuvenation of a possibly Variscan protolith.Within Formation C there is a possibly basement-related granitic inli-er. Potentially slightly rejuvenated amphiboles of a cataclastic felsicrock sample yielded an age of 953 ± 23 Ma (Villeneuve et al., 2006).Rock specimens were sampled at two localities: D57 from FormationC and D52 from the granitic inlier.

• Formation D comprises the eastern regions of the Dayet Lawda unitand is built up from metamorphosed gabbros and gabbro-diorites.Because the rocks of this formation are thrusted over the SebkhaMatallah unit, they are supposed to show similarities to the rocks ofthe four ultramafic domains within the latter unit (see below). K-Arintrusion ages between ~484 and 514 ± 12 Ma are known fromthree localities within this formation. A quartzitic diorite analysedby Gärtner et al. (2013a) did not contain zircons for age determina-tion. Therefore, a set of five samples (D76, D178, D178a, D180,D180b) was taken from different localities of this formation.

2.2. Sebkha Matallah unit

This unit comprises several types of metamorphic rocks, such asphyllites, migmatites, orthogneisses and amphibolites, accompaniedby numerous granitoid intrusions. These rocks crop out widespread inmajor parts of the eastern Sebkha Matallah unit (Rjimati et al., 2002a,b,c,d,e, 2010, 2011; Rjimati and Zemmouri, 2011). Four domains ofmafic to ultramafic rocks are interpreted as klippen structures (Sougy,1962a; Bronner et al., 1983; Marchand et al., 1984) and are namedEntajjat, Mades, Tuisten and Agreb Labghar occur in the southwesternarea of this unit (Fig. 1, Agreb Labghar is outside the map). They aredominated by gabbro, diorite and anorthosite and are supposed to belinked to the thrusting of the Dayet Lawda unit over the SebkhaMatallah unit (Sougy, 1962a,b; Bronner et al., 1983; Marchand et al.,1984; Villeneuve et al., 2006; Michard et al., 2008). Further bodies ofsimilar composition but smaller dimensions are known. All of themafic and ultramafic rocks are grouped into the Agrour Lafras suite,which is expected to be Archaean to Palaeoproterozoic in age (Rjimatiand Zemmouri, 2011). There are only few radiometric ages publishedon minerals from the Sebkha Matallah unit: An age of 1110 ± 26 Maon amphiboles of a gabbro-diorite representsmost likely an event of re-juvenation, while the host rock displays a K-Ar age of 1761 ± 42 Ma(Villeneuve et al., 2006). More recent data on zircon are given byMontero et al. (2014), who describe a gneissose noritic gabbro anortho-site with an age at 2435 ± 9 Ma and a leucotonalite having twodiscordias with upper intercepts at 2936 ± 10 and 3112 ± 8 Ma. Atotal amount of ten samples was taken from the Sebkha Matallah unit(D62, D63, D158a, D162, D174, D175, D176a, D177, D181, D182c) for



Kalsilite-nepheline sye-nites of Aoucert (~2.46 Ga)

Meso-Cenozoic coastal basins: sediments

Oued Togba unit (Avalonia affinity): granitoids, quartzites,orthogneissesSebkha Gezmayet unit (Megu-ma affinity): granitoids, quartz-ites, orthogneissesFormation A: amphibolitic gneisses, granites, dolerites

Formation B: gabbros, pyroxe-nites, leucogranite dykes

Fm. C: amphibolites, granites, gabbros, peridotites, basalts

Formation D: metamorphosedgabbros, gabbro-diorites

deformed mafic and ultramafic rocks

schists, migmatites, mylonites, orthogneisses

mica schists

Neoproterozoic quartzites

Proterozoic mafic dykes

Archaean metasediments

Day

et L

awd

a u

nit

Seb

kha

Mat

alla

h u

.Dhloat Ensour unit: Ordovi-cian to Devonian sediments

Basement inlier: granitoids, diorites

Archaean mafic rocks

Archaean basement of the Reguibat Shield

faults / thrusts

wadis

main roads

sebkhas

Wes

t A

fric

an C

rato

n

EntajjatEntajjat

MadesMades

TuistenTuisten50 km50 km

23°N23°N

22°N22°N

15°W15°W16°W16°W

AoucertAoucert

D182cD182c

D181D181

D180, D180bD180, D180bD178D178, D178a, D178a

D177D177D176aD176a

D175D175D174D174

D162D162D158aD158a

D135D135D76D76

D72D72

D63D63D62D62D57D57

D52D52D49D49

15°W15°W16°W16°W

AoucertAoucert

23°N23°N

22°N22°N

Dayet Lawda unit Sebkha Matallah unitSebkha Gezmayet u.Pop-up

structure

Fig. 1. Location, geological setting and tectonic interpretation of themafic and ultramafic rocks of the Dayet Lawda and SebkhaMatallah units within the Adrar SouttoufMassif in southernMoroccan Sahara, NWAfrica. Samples that contained zircons are indicated in yellow. The compilation of themap and the ages is based on Bea et al. (2013), Gärtner et al. (2013a),Monteroet al. (2014), Rjimati et al. (2002a,b,c,d,e,f, 2010, 2011), Rjimati and Zemmouri (2011) and Villeneuve et al. (2006).

3A. Gärtner et al. / Gondwana Research xxx (2014) xxx–xxx

further geochronologic analyses. Sample D135was taken from a dyke inthe basement of the Reguibat Shield below Ordovician quartzites of theDhloat Ensour unit at the eastern margin of the Sebkha Matallah unit(Fig. 1).

2.3. Further occurrences of mafic and ultramafic rocks at the westernmargin of the West African Craton

The mafic and ultramafic rocks of the Dayet Lawda and the SebkhaMatallah units are interpreted to represent a vestige of the WestAfrican Neoproterozoic Ocean (Gärtner et al., 2013a) proposed byVilleneuve et al. (2010). Although there is no simple connection be-tween the northern and the central Mauritanides (Le Goff et al., 2001;Caby and Kienast, 2009), potentially equivalent and / or correlating


Neoproterozoic units of oceanic origin occur at several localitiesalong the western margin of the West African Craton (Villeneuve andDallmeyer, 1987; Villeneuve et al., 2006; Villeneuve, 2008). One ofthem is represented by the amphibolites and eclogites of the TarfMagneïna unit, Mauritania, which is located in the southern extensionof the Adrar SouttoufMassif. Two eclogite samples of this region yieldedzircon ages at 593 ± 10 and 595 ± 7 Ma which are interpreted to timethe intrusion of the gabbroic protolith (Le Goff et al., 2001). The sameauthors argue that Sm-Nd-ages at 325 ± 43 and 333 ± 25 Ma point tometamorphic overprint during the Variscan-Alleghanian orogeny. Sim-ilar rocks are described by several authors for the central and southernparts of the Mauritanides (e.g. Dia, 1984; Le Page, 1983; Villeneuve,2005; Caby and Kienast, 2009 and references therein). Ages at between620 and 645Ma for two series of these rocks in southernMauritania are



all zircons, 90–110% conc., n=294/533

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

540

640

740

840

940

1040

1140

1240

1340

1440

1540

1640

1740

1840

1940

2040

2140

2240

2340

2440

2540

2640

2740

2840

2940

3040

3140

3240

3340

3440

Pro

babi

lity

0

5

10

15

20

25

30

35

40

45

50

55

60Frequency

all zircons between 540 and 750 Ma, 90–110% conc.n=273/533

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

540

640

740

Pro

babi

lity

051015202530354045505560

Frequency

slighthly discordant grains of sample D181

605

634

data-point error ellipses are 2σ

238UPb206

207Pb235U

D181, gabbroic diorite, n = 17/54, apatite

Intercepts at287 ± 160 & 4899 ± 76 [± 77] M a

MSWD = 0.57

4500

3500

2500

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 20 40 60 80 100 120


238UPb206

Intercepts at0 ± 0 and 605.9 ± 4.1 [± 6.1] M a

MSWD = 0.066

207Pb235U

D181, gabbroic diorite, n = 111/130, 90-110% conc.

630

610

590

570

550

530

0.084

0.088

0.092

0.096

0.100

0.104

0.66 0.70 0.74 0.780.82

0.86

0.90

720

680

640

600

560

520

0.08

0.09

0.10

0.11

0.12

0.7 0.8 0.9 1.0 1.1


238UPb206

C oncord ia A ge = 597.7 ± 3.7 M a (2σ, decay-const. e rrs . inc luded),

MSWD (of concordance) = 0.34, Probability (of concordance) = 0.56

207Pb235U

D177 metagabbro, n = 6/6, 90-110% conc.

0.094

0.095

0.096

0.097

0.098

0.099

0.75 0.77 0.790.81

0.83

580

590

600

610

640

620

600

580

0.092

0.094

0.096

0.098

0.100

0.102

0.104

0.106

0.78 0.82 0.86 0.90


238UPb206



207Pb235U

D162, gabbroic diorite, n = 38/82, 90-110% conc.

0.096

0.098

0.100

0.102

0.104

0.74 0.78 0.82

0.86

0.90

0.94

580

600

700

660

620

580

0.090

0.094

0.098

0.102

0.106

0.110

0.114

0.088

0.7 0.8 0.9 1.0 1.1 1.2


238UPb206



207Pb235U

D158a, amphibolite gneiss, n = 10/16, 90-110% conc., apatite

0.115

0.117

0.119

0.121

0.123

0.85 0.95 1.05

1.15

1.25

710

730

750

0.256

0.260

0.264

0.268

0.272

0.276

3.3 3.4 3.5 3.6

3.7

3.8

1500

1540

1580 C oncord ia A ge = 1533 ± 10 M a (2σ, decay-const. e rrs . inc luded),


1000

1400

1800

2200

2600

0.0

0.2

0.4

0.6

0 4 8 12 16

data-po in t e rro r e llipses are 2σ

238UPb206

cIntercepts at

0 ± 0 & 3101 ± 6 M a [±10] MSWD = 1.6

207Pb235U

D158a, amphibolite gneiss, n = 19/30, 90-110% conc.

bIntercepts at

0 ± 0 & 3018 ± 7 M a [±10] MSWD = 0.079

aIntercepts at

0 ± 0 & 2885 ± 5 M a [±8] MSWD = 0.45

a b c

3000

2600

2200

1800

1400

0.0

0.2

0.4

0.6

0 4 8 12 16 20 24


238UPb206

207Pb235U

D63, amphibolite, n = 24/35, apatite

bIntercepts at

0 ± 0 & 4479 ± 14 [±17] M aMSWD = 0.29

aIntercepts at

0 ± 0 & 3617 ± 21 [±22] M aMSWD = 0.056

a b

4500

3500

2500

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80


238UPb206



207Pb235U

D63, amphibolite, n = 37/87, 90-110% conc.

740

700

660

620

580

0.09

0.10

0.11

0.12

0.75 0.85 0.95 1.05 1.15

0.093

0.095

0.097

0.099

0.101

0.103

0.72 0.76 0.80

0.84

0.88

580

600

620

0.101

0.103

0.105

0.107

0.109

0.83 0.85 0.87 0.890.91

0.93

0.95

0.97

630

650

670C oncord ia A ge = 650.4 ± 4.3 M a

(2σ, decay-const. e rrs . inc luded), MSWD (of concordance) = 0.93,

Probability (of concordance) = 0.34


238UPb206



207Pb235U

D62, microgabbro, n = 17/50, 90-110% conc.

0.093

0.095

0.097

0.099

0.101

0.70 0.74 0.78 0.82

0.86

0.90

580

600

620

1200

1000

800

600

0.07

0.09

0.11

0.13

0.15

0.17

0.19

0.21

0.23

0.2 0.6 1.0 1.4 1.8 2.2


238UPb206



207Pb235U

D178, metadiorite, n = 37/100, 90-110% conc.

0.092

0.094

0.096

0.098

0.100

0.102

0.104

0.74 0.76 0.78 0.80 0.82

0.84

0.86

0.88

570

590

630

1100

1000

900

800

700

600

500

0.07

0.09

0.11

0.13

0.15

0.17

0.19

0.4 0.8 1.2 1.6 2.0


238UPb206



207Pb235U

D57, black volcanic tuff, n = 7/9, 90-110% conc.

0.095

0.097

0.099

0.101

0.78 0.80 0.82

0.84

590

600

610

620

620

610

600

590

580

0.093

0.095

0.097

0.099

0.101

0.0 0.4 0.8

1.2

1.6

apatite

650

640

630

620

610

600

590

0.095

0.097

0.099

0.101

0.103

0.105

0.107

0.79 0.81 0.83 0.85 0.87 0.89 0.91


238UPb206



207Pb235U

D52, diorite, n = 18/35, 90-110% conc.700

660

620

580

0.088

0.092

0.096

0.100

0.104

0.108

0.112

0.116

0.75 0.85 0.95 1.05

0.091

0.093

0.095

0.097

0.099

0.101

0.103

0.70 0.74 0.78 0.82

0.86

0.90

630

610

590

570


238UPb206



207Pb235U

D49, rhyolitic tuff, n = 4/4, 90-110% conc.

0.094

0.096

0.098

0.100

0.102

0.77 0.79 0.81 0.83 0.85

600

620

S e

b k

h a

M

a t

a l

l a

h

u

n i

tD

ayet

Law

da

u.

S. G

ezm

ayet

u.

610

620


Please cite this article as: Gärtner, A., et al., History of the West African Neoproterozoic Ocean: Key to the geotectonic history of circum-AtlanticPeri-Gondwana (Adrar Souttouf ..., Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.11.011



given by Lahondère et al. (2005), while occurrences of metagabbros inthe Bassaride Belt of central southern Senegal were dated at 621 ±8 Ma by Fullgraf et al. (2010). Further rocks related to Neoproterozoicoceanic crust in the Rokelides are described by Lytwyn et al. (2006)and Villeneuve (2008).

Another region comprising several ophiolitic structures is situated inthe Anti-Atlas. With ages between 762 +1/−2 and 697 ± 8 Ma, therocks of the Bou Azzer, Iriri, and Sirouer mafic terranes are significantlyolder than similar structures in the Mauritanides, Bassarides, andRokelides (Thomas et al., 2002; Samson et al., 2004; El Hadi et al.,2010; Liégeois et al., 2013; Blein et al., 2014). The obduction of the Iririophiolites is supposed to be represented by zircon overgrowth rimsdated at 663 ± 13 Ma, while protolith emplacement in an island arc isinterpreted to have taken place at 743 ± 14 Ma inferred from zirconcores (Thomas et al., 2002). Based on further radiometric data, thephase of obduction is assumed to have ended latest at ca. 630 Ma(Thomas et al., 2002; Inglis et al., 2004; El Hadi et al., 2010).

3. Methods

A total of 20 samples from the Sebkha Gezmayet, Dayet Lawda, andSebkha Matallah units were selected in order to determine zircon mor-phology, U-Th-Pb ages, and Lu-Hf isotope analyses. Prior to mounting,CL-imaging and analyses via Laser Ablation with Inductively CoupledPlasmaMass Spectrometry (LA-ICP-MS), themorphology of each zircongrain and its crystal faceswere studied using a scanning electronmicro-scope (SEM) to gain information about possible metamorphism andmelt characteristics during crystal growth (Pupin, 1980; method de-scribed in Gärtner et al., 2013b). U-Th-Pb isotope analyses have beenperformed using an Element 2 XR coupled to a New Wave FX 193 nmArF Ecximer laser system at the GeoPlasma Lab Dresden. Lu-Hf isotopeanalyses have been done using a Thermo-Finnigan NEPTUNE coupledto a RESOlution M50 193 nm ArF Excimer laser system at GoetheUniversity Frankfurt (Gerdes and Zeh, 2006, 2009).

For a detailed description of sample preparation, morphologicalstudies, U-Th-Pb dating and Lu-Hf isotope analyses, please refer to par-agraphs 3.1 to 3.3 in the electronic supplement to this paper. Instrumentsetup details are provided in Supplementary Table 1. All supplementarydata is available from the journal website.

4. Results

Of the 20 samples, 10 did not contain any zircon or apatite grains(D72, D76, D135, D174, D175, D176a, D178a, D180, D180b, D182c),whereas 531 zircons and 155 apatites could be extracted from theremaining 10 samples. All of the zircons were studied with respect totheir morphology. Classification of morphological features is done asdescribed in Gärtner et al. (2013b), while interpretation of internaltextures is in dependence on Corfu et al. (2003). Any characterisationof zircon morphotypes follows Pupin (1980). Isotopic measurementsfor U-Th-Pb were done at 533 spots on 493 zircon grains of which 294yielded ageswith a concordance between 90 and 102% (SupplementaryTable 2). The 158 spots on 155 apatites gave 11 ages within the men-tioned interval. Only grains with a concordance between 90 and 110%were utilised for interpretative purposes. The minimum confidencelevel for the calculation of concordia ages is set at 95%. All morphologicas well as the isotopic data obtained via LA-ICP-MS techniques are pro-vided in Supplementary Tables 2 to 4 (available from the journalwebsite). All concordant and further relevant grains of each sampleare plotted in concordia diagrams (Fig. 2). An additional binnedfrequency and probability density plot (Fig. 2) is given to facilitate asummary visualisation of primary magmatic and metamorphic events

Fig. 2. Concordia plots of U-Pb zircon and apatite data for all samples of this study coprobability density plot in the upper right figure shows the main magmatic and metamthe text.


in the working area. The Th-U elemental ratio, which indicates whethera zircon preferentially lost Th with respect to U during metamorphicrecrystallisation, and if it was formed in a felsic or a mafic meltis given for each analysed grain, but only interpreted in case of concor-dance.Measurements for Lu-Hf isotopicswere carried out on all concor-dant zircons of each sample. Finally, 252 grains gave useful Lu-Hf data(Fig. 3). A detailed description of the geochemical and further petrologicfeatures for the analysed samples is given by (Youbi et al., submitted forpublication; Supplementary Table 5; Fig. 4).

4.1. D49 rhyolitic tuff, Sebkha Gezmayet unit, close to Formation A of theDayet Lawda unit, N22°22′12.00″, W15°25′26.64″

Only two zircon grains could be extracted from this nearly unde-formed, fine grained and parallel bedded rhyolitic tuff, having lengthsof 96 and 160 μm and widths of 62 and 85 μm. The resulting meanelongation is 0.59 ± 0.08, while the roundness is 3 and 4, respectively(average is 3.50). One of the grains shows the S19 morphotype. Bothof them show very dark central regions with low contrast and hardlyvisible oscillatory zoning, which are encircled by thin, bright over-growth rims. Two spots were set on the central part of each zircon. Allof them yield concordant ages between 604 ± 12 and 608 ± 13 Maresulting in a common concordia age at 606 ± 6 Ma (Fig. 2). Withrespect to theweak deformation, the poorly rounded grains and the un-disturbed character of the analysed areas, this age is interpreted to datethe time of intrusion. Th-U isotopic ratios vary between 0.90–0.98and 0.38–0.41. Both grains were analysed for their Lu-Hf isotopics.They yield εHf(t) ratios of 5.9 and 6.3, corresponding to TDM ages of0.90–0.92 Ga (Fig. 3).

4.2. D52, diorite, granitic inlier within the Dayet Lawda unit, Formation C,N22°21′06.60″, W15°20′26.94″

This slightly deformed, medium-grained diorite sample shows ran-domly scattered melanocratic domains enriched in mafic minerals andwas taken at the western margin of the granitic inlier within FormationC of the Dayet Lawda unit. A total amount of 35 zircons ranges between50 and 201 μm in length and 30 to 79 μm in width, with a resultingmean elongation of 0.54 ± 0.13. The roundness of the grains is 4 to 8with an average value of 6.49. Due to this, themorphotypes of 13 grainscould be defined: S22 (2), S23 (4), S24 (7). Internal textures are domi-nated by oscillatory zoning, with the exception of one. However, 60%of the crystals have very thin, bright overgrowth rims. Each zircon wasanalysed with one spot. 18 of these measurements yield concordantages in a range from 592 ± 16 to 663 ± 21 Ma. A concordia age at603± 5Ma could be calculated from the youngest population compris-ing 12 grains up to 608±16Ma (Fig. 2). This agemost likely reflects thetime of intrusion, which is corroborated by the absence of metamorphiccharacteristics of the zircon grains and the veryweak deformation of therock. The Th-U values are in a range from 0.24 to 1.07 for all, and 0.28 to0.92 for concordant grains. Measurements of Lu-Hf isotopics of 13concordant zircons resulted in εHf(t) ratios in a range of −27.9 to 8.1and TDM ages from 1.82 to 3.67 Ga (Fig. 3).

4.3. D57 black volcanic tuff, Dayet Lawda unit, Formation C, N22°20′30.72″,W15°18′51.00″

The fine grained, deformed black volcanic tuff is of basaltic-andesiticto andesitic composition and shows foliation due to alignment ofmineral grains. Sample D57 contained nine zircon grains. Theyshow lengths in a range from 78 to 164 μm and widths between 43and 127 μm. The mean elongation is 0.63 ± 0.11. Roundness is within

llected from the Sebkha Gezmayet, Dayet Lawda, and Sebkha Matallah units. Theorphic events in this unit based on U-Pb zircon ages. Further details are given in



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Fig. 3. Diagram of εHf(t) ratios versus concordant U-Pb zircon age of all samples. The small inset figure shows the data for all zircon grains younger than 750 Ma. Note the wide spread ofvalues for samplesD52, D177, andD178 indicating potential crustalmixing. Datawere calculatedusing the decay constant of 1.867 × 10-11 (Scherer et al., 2001), and the CHURparametersof Bouvier et al. (2008). Further details and references of depleted mantle evolution are given in Gerdes and Zeh (2006, 2009) and Dhuime et al. (2011).


the classes 7 to 9 (average value is 8.00). Because of these highlyrounded grains, a definition ofmorphotypeswas impossible. CL imagingrevealed bright overgrowth rims enclosing either magmatic (6) ormetamorphic (3) core domains. One spot was set on each grain to ana-lyse U-Th-Pb isotopics. Seven of them yield concordant ages between607±11 and 634± 12Ma. A concordia age calculated of the five youn-gest grains with ages up to 613 ± 11 Ma is at 610 ± 5 Ma (Fig. 2). Thisage is interpreted to date the metamorphic overprint of the rock. Th-Uvalues are between 0.05 and 0.53 for all zircons aswell as for concordantones, while eight of the nine grains have values above 0.38. TheεHf(t) ratios of the concordant grains are between 6.2 and 10.2 withTDM ages at 0.70 and 0.90 Ga (Fig. 3).

Five apatite grains of this sample were analysed for their U-Th-Pbisotopic composition. Three of them contain too little U to give mean-ingful results. One of the remaining two grains yielded a concordantage of 599 ± 16 Ma, which is in accordance to the obtained zirconages. The other apatite is highly discordant, but shows a similar206Pb-238U-age at ca. 593 Ma.

4.4. D178 metadiorite, Dayet Lawda unit, Formation D, N22°06′05.52″,W15°20′37.14″

The metadiorite D178 is medium-grained, deformed and locallyshows stretched leucocratic domains. One hundred and eighteenzircons were extracted from this sample. They have lengths of 42 to229 μm, widths of 39 to 142 μm, and a resulting mean elongation of0.70 ± 0.14. The roundness of the grains comprises classes 5 to 10with an average value of 7.79. 21 grains are definable with respect totheir morphotypes: S3 (4), S18 (2), S19 (1), S20 (1), S22 (2), S23 (1),S24 (6), S25 (4). Bright overgrowth rims are surrounding very darkcores with low or no contrast. Nevertheless, at least ten of them showoscillatory zoning indicating a magmatic origin. Analyses of U-Th-Pbisotopic composition were done setting 100 spots on 90 grains. Thirtyseven spots on 36 grains yield concordant ages between 596 ± 10 and


1029 ± 168 Ma (the 206Pb/238U age is 1015 ± 18 Ma). Of them, 23grains have ages between 596 and 613 Ma resulting in a concordia ageat 606± 2Ma (Fig. 2), which is thought to time themetamorphic over-print of the rock. Older grains occur at 620 to 648 Ma, 682 and 683 Ma,739 Ma and 1029 Ma. Obtained Th-U values are between 0.02 and 2.31for all grains, and vary from 0.02 to 1.44 for concordant ones. Lu-Hfanalyses were executed for 26 concordant zircons. Their εHf(t) ratiosrange from−7.6 to 5.9 with TDM ages between 0.92 and 1.69 Ga (Fig. 3).

The 20 apatite grains found in this sample were analysed with 23spots for their U-Th-Pb isotopics. But only one grain contained enoughU (7 ppm) to generate an age, which is highly discordant. Thus, theapatites of this sample can not be used for further interpretativepurposes.

4.5. D62, microgabbro, Sebkha Matallah unit, Agrour Lafras suite, N22°18′15.60″, W15°05′34.14″

This medium-grained microgabbro is characterised by layers ofaligned minerals suggesting metamorphic overprint. A total amount of43 zircon grains were extracted from sample D62. The length of thecrystals is between 63 and 180 μm and the width ranges from 41 to135 μm, resulting in a mean elongation of 0.77 ± 0.19. The grains arepoorly, to completely, rounded (classes 4 to 10; average value is 7.42).Seven of them could be defined with respect to their morphotypes:S19 (1), S20 (3), S24 (2) and S25 (1). All grains have significant signsof metamorphic overprint that reach from bright very thin overgrowthrims (23%) and broad rims (23%) to totally homogenised crystals (54%).The inner parts of the non-homogenised zircons show oscillatorygrowth patterns, which are typical for magmatic grains. Each of the 43zircons were analysed for U-Th-Pb isotopics with a total of 50 spots.Concordant ages could be obtained from 17 spots at 16 grains andrange from 600 ± 12 to 1190 ± 32Ma. The concordia age of the youn-gest populationwas calculated at 602± 2Ma (Fig. 2) and is interpretedto reflect the age of metamorphism. Further zircon grains are at



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Fig. 4. Plots of geochemical data for all samples from themafic rocks of the Adrar Souttouf Massif except D52. Upper left: Chemical classification of the sampled volcanic, subvolcanic andplutonic rocks of this study based on SiO2 versusNa2O+K2O ratios (Lebas et al., 1986); Upper right: Spider diagram of the REE composition of all studied samples, except D52, normalisedto primitvemantle (Sun andMcDonough, 1989); Lower left: Ti-V discrimination diagram of Shervais (1982), indicating the different tectonic settings during the formation of all basaltic,doleritic and gabbroic samples of this study; Lower right: P2O5-MnO-TiO2 discriminant for ocean island, MORB, arc basalts, and boninites of Mullen (1983), including all basaltic, doleritic,gabbroic and basaltic andesites of this study. All data used for the compilation of this figure is given in Supplementary Table 5.


615–620 Ma, around 630 Ma, 671 Ma, 732–744 Ma, and 1190 Ma. Anoteworthy population of four zircons yields a concordia age at648 ± 7 Ma. Grain D62-040 is the only one which has concordantages for the core (732± 16 Ma) and the rim (646 ± 13Ma). MeasuredTh-U values are in a range from 0.18 to 1.39 for all grains and from 0.32to 1.39 for concordant zircons, respectively. Analyses of Lu-Hf isotopicsof 17 concordant zircons gave εHf(t) ratios between 4.6 and 11.3 withTDM ages from 0.65 to 1.44 Ga (Fig. 3).

None of the 25 apatite grains extracted from this sample gave aconcordant U-Pb age and only two grains show any values at all. Bothof them are highly discordant, have U contents between 5 and 3 ppm,while the remaining grains are assumed to have even less, and thusare below the detection limit for the applied techniques. Therefore, noapatite ages are available for this sample.

4.6. D63, amphibolite, Sebkha Matallah unit, Agrour Lafras suite, N22°17′34.44″, W15°03′15.48″

The medium- to coarse-grained amphibolite sample D63 is of gab-broic composition. Most of its crystals are aligned, which is supposedto reflect metamorphic overprint. The sample contained 70 zirconsthat are between 57 and 270 μm in length and show widths of 38 to141 μm. The resulting mean elongation is 0.69 ± 0.15. Except onegrain having a roundness of 4, the other zircons are determined to clas-ses 6 to 10 with an average value of 8.24. Seven zircons were definedwith respect to their morphotypes, which are S8, S13, S20, S23, S24(2), and J3. All grains show indications of metamorphic overprints.


About 29% having bright overgrowth rims enclosing magmatic coredomains, while 49% exhibiting overgrowth around metamorphiccores. The remaining zircons are largely homogenised. Eighty sevenspots were set on 67 grains to gain U-Th-Pb isotopic data, with 37spots on 37 grains having concordant ages in a range from 601 ± 14to 709 ± 14 Ma. The youngest population comprises eight zircons upto 610 ± 18 Ma with a resulting concordia age at 606 ± 5 Ma (Fig. 2).This age is assumed to represent the metamorphic event that affectedinternal textures of the zircon grains. Further age groups occur succes-sively until 654 Ma and from 687 to 709 Ma. A Concordia age at650 ± 4 Ma could be calculated for the largest cluster of ten grains be-tween 646 ± 15 Ma and 654 ± 14 Ma. Additionally, the bright over-growth rims of 20 grains were analysed. None of them yielded aconcordant age, but all of them showed significantly low U values rang-ing between 1 and 19 ppm (arithmetic mean = 4.75 ppm). ObtainedTh-U values are between 0.09 and 1.06 for all zircons, which is thesame range for the concordant ones. The εHf(t) ratios of 32 concordantzircons are between 6.4 and 12.5,while TDM ages are in a range of 0.60 to0.90 Ga (Fig. 3).

A total amount of 35 apatite grainsweremeasured for their U-Th-Pbisotopic composition. Ten of them contained not enough U for any dataacquisition. Except one grain, the remaining apatites were split into twogroups, with each of them forming a discordia characterised by a lowerintercept forced through zero. The first discordia comprises 18 grainsand has an upper intercept at 4479 ± 14 Ma, while the second ones isat 3617 ± 21 Ma. Due to these values, all apatites are supposed tocontain to much common lead for further interpretative purposes.




4.7. D158a, amphibolite gneiss, easternmost Sebkha Matallah unit,Aghaylas suite, N22°12′25.80″; W14°51′07.44″

Medium-grained amphibolite gneiss D158a shows foliation and is oftrachyandesitic composition. The 29 zircon grains that were found arebetween 83 and 354 μm in length, 52 to 231 μm in width and have amean elongation of 0.67 ± 0.13. All of them are well rounded tocompletely rounded (classes 7 to 10; average value is 8.17) and thus,do not show any complete crystal faces. Every zircon shows internaltextures indicative of having experienced metamorphism, although ofdifferent stages. Thirty spots were set on the 29 zircon grains, while19 of them yield concordant ages between 1884 ± 10 Ma and3216 ± 15 Ma. Single zircon grains are at 1884 ± 10 Ma, 2624 ± 14Ma, and 3216 ± 15 Ma. Twenty one of the 26 remaining grains definethree discordia lines, whose lower intercepts are forced through zero,whereas their upper intercepts are at 2885 ± 5 Ma, 3018 ± 7 Ma, and3101 ± 6 Ma (Fig. 2). The latter three ages are interpreted to representmajor magmatic events, while the maximum age of deformation isdefined by the youngest concordant zircon (1884 ± 10 Ma). Th-Uvalues range from 0.06 to 1.50 for all and from 0.10 to 1.04 forconcordant grains. Lu-Hf isotopics of 18 concordant zircons showεHf(t) ratios from −9.9 to 11.6, whereas TDM ages range from 1.99 to3.44 Ga (Fig. 3).

Of the 16 apatites analysed for U-Th-Pb isotopics, 10 have concor-dant ages between 726 ± 12 Ma and 2976 ± 12 Ma. Two populationswith each comprising 4 grains allowed the calculation of Concordiaages at 731 ± 6 Ma and 1533 ± 10 Ma. The younger one can beinterpreted as the last metamorphic event with temperatures allowinga reopening of the apatite lattice.

4.8. D162 gabbroic diorite, Sebkha Matallah unit, Agrour Lafras suite,N22°15′05.04″, W14°59′04.98″

Sample D162 is a medium- to coarse-grained gabbroic diorite.Aligned crystals indicate metamorphic overprint which contained 88zircons. They have lengths between 58 and 210 μm and widths of 37to 132 μm resulting in a mean elongation of 0.73 ± 0.13. Except onegrainwhich is poorly rounded (class 4), the remaining ones are roundedto completely rounded (classes 6 to 10) with an average value of 7.97.Despite the high roundness, 17 grains have definable morphotypeswhich are: S13 (1), S17 (2), S18 (1), S19 (2), S22 (1), S24 (4), S25 (4),J5 (1), and D (1). All of the grains show thin, bright overgrowth rimsaround slightly metamorphosed core domains, with some of themexhibiting sporadically occurring remnants of wide oscillatory zoning.Eighty two spots were set on 82 zirconswith 38 of them having concor-dant ages. The youngest grain has an age of 603 ± 11 Ma, the oldestis 699 ± 11 Ma, whereas the youngest population comprises six grainsup to 612 ± 12Ma. Thus, the calculated concordia age is at 608± 5Ma(Fig. 2) and is assumed to time themetamorphic event that affected thedolerite. Obtained Th-U values for all grains are between 0.19 and 0.70,while concordant zircons show values of 0.19 to 0.54. Furthermore, ob-tained εHf(t) ratios range from 7.6 to 18.0, while TDM ages are between0.27 and 0.85 Ga (Fig. 3).

4.9. D177 metagabbro, Sebkha Matallah unit, base of the Entajjat domain,Agrour Lafras suite, N21°59′10.14″, W15°12′54.42″

The coarse-grained metagabbro sample D177 shows alternatinglayers of its different mineral phases that are typical for metamorphicoverprint. Six zircons were extracted from. this rock. Their lengthsrange from 77 to 174 μm, whereas widths are between 65 and 128μm.The mean elongation is 0.81 ± 0.17. Obtained classes of roundness are7 to 9, resulting in an average value of 7.83. Nevertheless, one grainwas definable with respect to its morphotype (S19). All zircons showcharacteristics of a metamorphic overprint accompanied by brightovergrowth rims. Furthermore, two grains show remnants of wide


oscillatory zoning, indicating magmatic origin. Each zircon grain wasanalysed with one spot for U-Th-Pb isotopic composition. Remarkably,all six measurements yield concordant ages between 597 ± 7 and632 ± 8 Ma, with five of them clustering around 597 and 598 Ma, re-spectively. Thus, the calculated concordia age that is thought to indicatethemetamorphic overprint is at 598± 4Ma (Fig. 2). The obtained Th-Uvalues are in narrow confines with a minimum of 0.19 and a maximumof 0.25. Additional measurements of Lu-Hf isotopics at five concordantzircons gave εHf(t) ratios in a range of −0.6 to 4.7 with resulting TDMages between 0.98 and 1.27 Ga (Fig. 3).

4.10. D181 gabbroic diorite, Sebkha Matallah unit, NW margin of theEntajjat domain, N22°02′58.50″, W15°09′40.08″

The medium-grained gabbroic diorite D181 exposes signs of weakmetamorphic overprint like preferred orientation of minerals. Thisrock contained 131 zircons having lengths from 44 to 249 μm andwidths in between 38 and 119 μm. The resulting mean elongation is0.67 ± 0.13. The zircons are rounded within classes 5 to 10 having anaverage value of 7.98. Despite this, 24 grains are definable with respectto their morphotype: S8 (2), S18 (4), S19 (3), S23 (3), S24 (9), S25 (2),P3 (1). All grains show bright overgrowth rims, whereas darkmagmaticcore domains are present in 27% of all zircons. Another 66% of the grainshave dark centres of low contrast with indications of metamorphicoverprint. The remaining zircons are completely homogenised andshow bright CL-signals. A total amount of 130 spots was set on 130grains yielding 111 concordant ages. They are between 547 ± 10 and697±14Ma, but all grains younger than ~590 are assumed to be slight-ly discordant. Therefore, a discordia was calculated for all 74 grainshaving ages from 547 to 611 Ma. The lower intercept is forced through0, while the upper intercept is at 606 ± 4 Ma (Fig. 2), and thus isinterpreted to define the age of metamorphism. Older ages occur at617 to 669 Ma and at 681 to 697 Ma. Th-U values are in a range from0.16 to 0.98 for all, and between 0.26 and 0.98 for concordant zircons.104 concordant zircons were analysed with respect to their Lu-Hfisotopics. Obtained εHf(t) ratios are between 5.6 and 18.2, while TDMages range from 0.31 to 0.91 Ga (Fig. 3).

U-Th-Pb measurements were done on 54 apatite grains. Most ofthem contained too few U for any data. However, a group of 17 grainsforms a discordia with a lower intercept at 287 ± 160 Ma, while theupper intercept is at 4899 ± 7 Ma. The unreliable high age of theupper intercept is most likely caused by incorporation of commonlead. Nevertheless, the lower intercept can be used as a hint to a furthermetamorphic overprint during the Variscan-Alleghanian orogeny.

5. Discussion

Geochemical data determined on 18 mafic to ultramafic rocksamples in the Dayet Lawda and Sebkha Matallah units indicate thepresence of oceanic crust and island arc fragments (Youbi et al.,submitted for publication; Supplementary Table 5; Fig. 4). New U-Pbages and Lu-Hf isotope determinations obtained from zircon and apatiteof ten samples indicate the presence of juvenile crustal material in theEdiacaran. The ages occur in two peaks at ~605Ma and ~634Ma, exceptsample D158a, which is significantly older, and hence, will be discussedseparately. This age spectrum is not consistent with previouslypublished K-Ar data from the Sebkha Gezmayet and Dayet Lawdaunits of the Adrar Souttouf Massif (Villeneuve et al., 2006), but similarto the zircon ages obtained by Le Goff et al. (2001) in the Tarf Magneïnaunit in its southern extension. At least two major metamorphic eventsare assumed to have affected this region since Neoproterozoic times inthe course of the Ediacaran and Variscan-Alleghanian orogenies(Le Goff et al., 2001; Villeneuve et al., 2006; Murphy and Nance, 2008;Michard et al., 2010; Villeneuve et al., 2010). Contrary to hypothesesthat assume oceanic crust or locally occurring intrusions of Palaeozoicage (Villeneuve et al., 2006) similar to the Mauritanide Belt (Fullgraf




et al., 2010), the obtained zircon age spectrum suggests that neither theDayet Lawda, nor the SebkhaMatallah unit show evidence of Palaeozoic(Rheic Ocean) oceanic crust or other magmatic activities.

5.1. Origin of the zircons

For the interpretation of the isotopic data it is necessary to estimatewhether the zircons originate from mafic and ultramafic melts or arerecycled from pre-existing rocks intruded by the magma. Some com-mon attributes of zircons derived from mafic to ultramafic melts are:Th-U elemental ratios above 1, Ti-in-zircon thermometrical results sug-gesting high temperatures, low Ti and REE concentrations, as well as in-dicative crystal faces (Pupin, 1980; Belousova et al., 2002; Fu et al., 2008;Wang et al., 2011; Fig. 5). Although a high percentage of all crystalswerevery rounded, which seems to be common for somemafic melts (Hollisand Sutherland, 1985; Tietz, 2003), a number of 89 zircons was defin-able with respect to their morphotypes. Fig. 6 visualises that most ofall zircons have morphotypes, which are linked to high temperaturesand show typical distribution patterns for mafic rocks (e.g. Pupin,1980; Räsänen and Huhma, 2001; Tietz, 2003; Yu et al., 2010). Th-Uvalues of the concordant zircons are predominatly b 1, which is mostlikely caused by recurrent metamorphic overprint with accompanyingTh loss (Geisler et al., 2001, and references therein; Fig. 5). There arelow abundances of inherited zircons older than 700 Ma (Fig. 2), which

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Th/U

Fig. 5. Th-U elemental ratios of 1921 90–110% concordant zircon grains from mafic, intermsupplement) in comparison to all concordant zircon grains of this study; note varying scales (R


leads to the assumption of juvenile rockswithout crustal contamination.This is also corroborated by εHf(t) ratios and TDM ages obtained fromLu-Hf measurements that indicate juvenile crust as source for the zir-cons formost of the samples (see below, Fig. 3). Nevertheless, numerouszircons do not show the typical textures related to mafic and ultramaficorigin, as the vast majority of the investigated grains have very roundedbut smooth surfaces. Most of them also show bright overgrowth rims ofdifferent width or even homogenisation. Both characteristics areassumed to indicate at least two events affecting the originally euhedralzircons that are documented by few grains with relicts of former crystalfaces. A chronology of the events can be deduced from SEM photo-graphs and CL images. In a probable first step, the zircons wererounded, which was possibly caused by changing melt compositionscharacterised by decreasing Zr abundance and resulting disequilibriumof zircons with the melt. Thus, dissolution of zircon crystals (meltingcorrosion) would have lasted until saturation in the melt (Harrisonand Watson, 1983; Tomaschek et al., 2003; Schwartz et al., 2010) orfinal rock formation in cause of subsequent cooling had been achieved.Later on, a hypothesisedmajormetamorphic event caused the overprintaccompanied by U loss of many grains (Geisler et al., 2001), which canalso be inferred for the remarkably number of apatite grains totally de-pleted in U. Both features are represented by bright overgrowth rimsthat are depleted in U by 90% and more compared to the core domains(Fig. 7).

2000 2500 3000 Age in Ma

Th/U mantle = 3.70 (Rogers & Adams, 1969)

Th/U mean of mafic and intermediaterocks = 0.93 (Wang et al., 2011)

) zircons from this study and from literature

D49, D52, D57, D62, D63, D162, D177, D178, D181D158a

mafic rocks, n = 824intermediate rocks, n = 339metamorphosed mafic rocks, n = 558

this study, n = 294 literature, n = 1921

40 660 680 700 720 740 Ma

D49D52D57D62D63D162D177D178D181

ediate and metamorphosed mafic rocks compiled from the literature (see electronicogers and Adams, 1969).



All definable grains of this study, n = 89

Percentage

0< 2,5< 5,0< 7,5< 10,0< 12,5< 15,0> 15,0

~500°C

~900°C

S17 no visible faces

Classification of zirconmorphotypes (Pupin 1980)

B AB1 AB2 AB3 AB4 AB5 A C

H L1 L2 L3 L4 L5 G1-3 I

Q1 S1 S2 S3 S4 S5 P1 R1

Q2 S6 S7 S8 S9 S10 P2 R2

Q3 S11 S12 S13 S14 S15 P3 R3

Q4 S16 S17 S18 S19 S20 P4 R4

Q5 S21 S22 S23 S24 S25 P5 R5

E J1 J2 J3 J4 J5 D F

tem

per

atu

re

Morphotypes forming themajor clusters in this study

S23 S24 S25

Fig. 6. Relative abundance of zircon morphotypes according to Pupin (1980), compiled from all 89 definable grains of this study. Photographs on the lower left show a zircon grain withdefinable crystal faces and a very rounded grain that cannot be defined (scale is 20 μm). The sketches in the lower right visualise the most abundant morphotypes of this study.


5.2. An island arc in the periphery of the West African Craton at theCryogenian-Ediacaran boundary?

A passive margin with sediment accumulation is supposed for thisregion of the West African Craton during the Mid-Neoproterozoic(Bouougri and Saquaque, 2004). These sediments form the easternpart of the Sebkha Matallah unit, were most likely deposited in a shal-low marine environment and get coarser from W to E (Rjimati et al.,2002f; Villeneuve et al., 2006; Michard et al., 2010; Fig. 1). Geochemicalanalyses of the westward occurring mafic and ultramafic rocks suggestan oceanic and island arc setting during formation (Youbi et al.,submitted for publication; Fig. 4). Despite four zircon grains yieldingages at 1029Ma (D178), 732, 744, and1190Ma (D62), there is no inher-itance older than ~700 Ma, while youngest zircons are ~590 Ma. Giventhe age spectrum of all samples, except D158a, two significant clusterscan be distinguished at 605.1 ± 1.2 and 634.2 ± 1.2 Ma when usingthe 95% confidence level for calculation (Fig. 2). Both of the agesare interpreted to represent a major magmatic or tectonic event,

0

10

20

30

40

50

60

0 50 100 150 200 250

U (ppm) core

U (p

pm) r

im

depletion = 95%depletion = 90%

depletion = 75%dep

letio

n = 5

0%

U concetration (ppm) of cores and corresponding rims

Fig. 7. Relative degree of U-depletion in per cent between bright rims and dark core zonesof selected zircon grains. Photographs in the upper right part of the figure show typicalgrains showing this trend of depletion (scalebar = 100 μm).


respectively. Therefore, the 634 Ma age is interpreted to reflect themain peak of island arc formation due to oceanward subduction(Villeneuve et al., 2010; Murphy et al., 2013). This age is in accordanceto other coeval subduction-related magmatic events along the westernmargin of the West African Craton (Dallmeyer and Villeneuve, 1987;Villeneuve and Dallmeyer, 1987; Dallmeyer and Lécorché, 1990;Hefferan et al., 2000; Doblas et al., 2002; Condie, 2003; Lahondèreet al., 2005; Gasquet et al., 2008; Caby and Kienast, 2009; El Hadiet al., 2010; Fig. 8). Most of them are likely linked to the initialsubduction of the West African Neoproterozoic Ocean (WANO) sensuVilleneuve et al. (2010) and Villeneuve and Marcaillou (2013). The zir-cons of samples D49, D57, D62, D63, D162, and D181 show εHf(t) ratiosin a relatively narrow range from 5.6 to 18.2, indicatingmajor contribu-tion of juvenile magmas directly derived from mantle sources. A num-ber of values are above the depleted mantle array, but within thelimits supposed for MORB (Fig. 3). Obtained εHf(t) ratios of samplesD52, D177, and D178 are significantly lower, ranging between −27.9and 8.1, and hence are interpreted to reflect crustal mixing of Archaeanto Proterozoic precursors with juvenile Neoproterozoic components(e.g. Gerdes and Zeh, 2006; Abati et al., 2012; Hofmann et al., 2013).Comparable detrital U-Pb zircon ages and Hf values have been reportedby Abati et al. (2010, 2012) and are also thought to be linked to a poten-tial island arc, although located further north in the Anti-Atlas Belt.Notwithstanding the published work about the oceanic remnantswithin the Neoproterozoic belts the western margin of the WestAfrican Craton, there is a lack of reliable geochronologic data for coevaloccurring mafic and ultramafic rocks in this region, which currentlyhampers a more precise reconstruction of the overall evolution of thisparticular ocean.

5.3. Widespread metamorphic overprint during Ediacaran orogeny atca. 605 Ma

The large amount of ages reflecting an event at ~605 Ma (Fig. 2) ismost likely linked to the accretion of the precursing terranes of the re-cent Oued Togba and Sebkha Gezmayet units to theWest African Cratonin the course of terminal subduction of theWest African NeoproterozoicOcean (Villeneuve et al., 2010; Gärtner et al., 2013a; Fig. 8). This eventseems to have affected all of the investigated rocks except D49 and



650-630 Ma

635

W A N O

610-570 Ma

605 met.605589577

607

I A P E T U S

520-470 Ma

511506

I A P E T U S

R H E I C

420-400 Ma

416407

R H E I C

ca. 370 Ma

369

R H E I C

320-270 Ma

312270

Fig. 8. Hypothesised tectonic model of the possible geodynamic evolution of the AdrarSouttouf Massif: 650–630Ma: Initial opening of the Iapetus Ocean and beginning subduc-tion at thewesternmargin of theWest African Craton lead to the formation of an island arcin the during the ongoing closure of theWest African Neoproterozoic Ocean. 610–570Ma:Ongoing spreading of the Iapetus and final closure of the West African NeoproterozoicOcean reslts in terrane (proto-Oued Togba and proto-Sebkha Gezmayet units) and islandarc (Dayet Lawda unit) accretion, including partly obduction of the latter one and theSebkha Matallah unit onto the West African Craton basement. 520–470 Ma: Initial open-ing of the Rheic Ocean leads to rifting of Avalonian-Carolinian and possibly Megumanterranes, while the Iapetus Ocean is closing. 420–400 Ma: Final accretion of the terranesto the eastern margin of Laurussia. ca. 370 Ma: Closure of the Rheic Ocean. 320–270 Ma:Collision of Laurussia and Gondwana results in the formation of the Variscan-AlleghanianOrogen and leads to recurrentmetamorphic overprint accompanied by repeated thrustingof the Oued Togba, Sebkha Gezmayet, Dayet Lawda, and Sebkha Matallah units onto thebasement of the West African Craton.


D52, as the rocks themselves and nearly all zircons show typical charac-teristics of metamorphic overprint, like bright overgrowth rims deplet-ed in U and relatively low Th-U ratios (Figs. 5, 7). Gasquet et al. (2005)report a minor metamorphic event in the adjacent Anti-Atlas regionat ca. 605Ma,which is potentially a result of the hypothesised accretion.Ongoing melting of oceanic crust remnants beneath the Sebkha


Gezmayet and Oued Togba units potentially led to basaltic-andesitic(D57) and rhyolitic (D49) volcanism at the western regions of theDayet Lawda unit at 606 ± 6 and 607 ± 11 Ma, respectively. Ages at589, 603 and 611 Ma obtained from deformed granites to graniticgneisses (samples D13, D68, D102 in Gärtner et al., 2013a) situated atthe eastern margin of the Sebkha Gezmayet unit correspond well toboth of thementioned volcanic events (Fig. 8). The recent geologic situ-ation is interpreted as four thin nappes thrusted onto the basement ofthe West African Craton (Sougy, 1962a; Villeneuve et al., 2006;Michard et al., 2008, 2010). As a drifting Oued Togba and SebkhaGezmayet units towards Laurentia is assumed during the opening ofthe Rheic Ocean (Gärtner et al., 2013a), the initial obduction of theDayet Lawda unit onto the Sebkha Matallah unit is supposed to havetaken place during the Ediacaran orogeny at about 610 to 570 Ma(Fig. 8). Therefore, the ages at 593 and 595 Ma for an eclogite in thesouthern extension of the Adrar Souttouf Massif reported by Le Goffet al. (2001) may not reflect the age of the protolith, but of the firstmetamorphic overprint and, thus, would be in line to the data ofthis publication. With respect to the slightly younger ages indicatingmetamorphism towards S, the final closure of the West AfricanNeoproterozoic Ocean in the area of the Adrar Souttouf Massiv and itssouthern extension potentially proceeded diachronously from N to S.This is also consistent to the younger ages given by Villeneuve et al.(2010). Accordingly, the Adrar Souttouf Massif is the first area, inwhich remnants of the proposed oceanic tract between Avalonia(Oued Togba unit) and Meguma (Sebkha Gezmayet unit) on the oneside and the West African Craton (Murphy et al., 2013) on the othercould be identified.

5.4. Evidence for Variscan metamorphism and further evolution

The hypothesised drifting of the Oued Togba and Sebkha Gezmayetunits as a consequence of the opening of the Rheic Ocean (Gärtneret al., 2013a) limits the time of overthrusting of both onto the DhloatEnsour unit and the basement of the West African Craton to post-Ediacaran times. In accordance to the suggested Ediacaran obductionof the eastern parts of the Dayet Lawda and Sebkha Matallah units(Fig. 8), it has to be assumed, that the current existence of four thinskinned geotectonic units was achieved in several steps. This can alsobe deduced fromdecreasing degree of Variscanmetamorphism towardsE of the Adrar Souttouf Massif, which affected parts the zircon popula-tion of the Oued Togba and Sebkha Gezmayet units (Gärtner et al.,2013a). In one dolerite sample (D181, Fig. 2) of the Sebkha Matallahunit apatite grains underwent episodic Pb-loss, resulting in a discordiawith a lower intercept at 287±160Ma. Although the error is extremelylarge, that lower intercept age underlines the decreasing influence ofVariscan metamorphism towards the E. Accordingly, the Dayet Lawdaand Sebkha Matallah units were most likely shifted towards E a secondtime during the collision of Laurussia with the West African Craton.

Until this time, our model (Fig. 6) is in line with the establishedhypotheses concerning the movements of Meguma and the Avaloniaterrane assemblage during the Iapetus and Rheic Ocean evolution(e.g. Nance et al., 2010; Murphy et al., 2011, 2013). The opening of theAtlantic Ocean separated the North American terranes from the OuedTogba and Sebkha Gezmayet units that stuck at the western margin ofthe West African Craton.

5.5. Indication ofMesoproterozoic tectonic activity at thewesternmargin ofthe West African Craton

Sample D158a, an amphibolite gneiss of trachyandesitic composi-tion, was collected at the eastern margin of the Sebkha Matallah unit.All zircons of this sample are significantly older than every other grainof this study. A further difference is givenby thewide spreadof obtainedages. The main population clusters between ca. 2885 and 3100 Maindicate inheritance of zircons possibly derived from the basement of




the West African Craton (Chardon, 1997; Key et al., 2008; Schofieldet al., 2012; Montero et al., 2014), while the oldest zircon age at3216 ± 15 Ma reflects a magmatic event which is not yet known fromthe Reguibat Shield. A single zircon at 2624 ± 14 Ma is in accordanceto granitic magmatism in the adjacent Tiris Complex (Schofield et al.,2012). Obtained εHf(t) values for the zircons older than 2.5 Ga clusterbetween 11.6 and −9.9 and result in TDM ages ranging from 2.64 to3.44 Ga. More than 90% of all grains showing TDM ages older than2.99 Ga (Fig. 3). These values are very similar to the Hf data publishedfor the Reguibat basement by Key et al. (2008). The youngest zirconwas dated at 1884 ± 10 Ma and shows a TDM age of 1.99 Ga. This is inaccordance to the Eburnian-Birimian cycle of the Reguibat Shield(Schofield et al., 2006). Thus, a protolith age of approximately1884 Ma is assumed for the gneiss. Additional occurring apatites aresignificantly younger, with two age populations at 731 ± 6 Ma and1533 ± 10 Ma. At least the latter age has to be regarded as exotic forthe West African Craton and challenges the post-1700 Ma stability ofthe West African Craton postulated by Schofield et al. (2006, 2012).This is in agreement with recently published age determinations onseveral dykes of basement inliers between 1380 and 1650 Ma in theAnti-Atlas (El Bahat et al., 2013; Kouyaté et al., 2013; Söderlund et al.,2013) and older ages on biotite from the Amsaga area at 1494to 1575 Ma reported by Barrère (1967). Because there were no zirconsof these ages found in sample D158a, both of the apatite ages areinterpreted to be linked to a slight metamorphic overprint that affectedonly the apatite but not the zircon lattice due to higher closing temper-ature (e.g. Carrapa, 2010). Kouyaté et al. (2013) associate the 1650 Maage for the emplacement of mafic dykes to tectonic events during theassembly of Columbia (Meert, 2012; Nance et al., 2014; or Nuna sensuEvans and Mitchell, 2011). Following their hypothesis, the ~1530 Maapatite age is supposed to represent metamorphism due to the initialbreakup of Nuna. A second stage of metamorphic overprint is likely tobe recorded by the apatites at approximately 730 Ma. Similar ages areknown from the eastern margin of the West African Craton (Ennihand Liégeois, 2001; Caby et al., 2008), as well as from the Anti-Atlas(Thomas et al., 2002) and are potentially linked to the final breakup ofRodinia or the initial assembly of Gondwana (Evans, 2009; Bradley,2011; Youbi et al., 2013). The long range effects of these events presum-ably also affected the region which is now the eastern Sebkha Matallahunit, and thus, caused partial apatite rejuvenation.

6. Conclusions

A total amount of 20 samples was taken mainly from mafic and ul-tramafic rocks, indicating an island arc setting and intercalation withMORB (Youbi et al., submitted for publication; Supplementary Table 5;Fig. 4). Radiometric data of nine of the ten samples presented hereinare interpreted to reflect three major events. Two distinctive peaks inthe occurrence of zircon age populations at 634.2 ± 1.2 and 605.1 ±1.2 Ma are interpreted to time the main phase of island-arc relatedmagmatism andpeakmetamorphismdue to final accretion of the exoticOued Togba and Sebkha Gezmayet units to theWest African Craton, re-spectively. A third event is represented by mainly juvenile crust forma-tion during the Neoproterozoic for most of the samples as suggested bythe Hf analyses. In combination with morphological measurements, theHf data also emphasises the origin of the zircons from mantle-derived,mafic melts. Th-U ratios are consistently low, which is in line with theinterpretation of significant metamorphic overprint of the investigatedrocks during the Ediacaran (Gondwana-forming) and Palaeozoic(Variscan-Alleghanian) orogenies. A hint of Variscan metamorphicoverprint is given by the lower intercept age of one apatite sample at287 ± 160 Ma. The reinterpretation of published zircon data fromeclogites in northern Mauritania led to the hypothesis of a potentiallydiachronous closure of the West African Neoproterozoic Ocean. Thus,starting of this process at ca. 634 Ma in the area of the Adrar SouttoufMassif and continuing to the S are assumed.


One of the ten samples showed significantly older zircon and apatiteages than all the others. Located at the eastern margin of the AdrarSouttouf Massif, close to the outcropping basement of the WestAfrican Craton, this sample is interpreted to have been formed duringthe Eburnian-Birimian cycle of the Reguibat Shield. Several apatitesfound in this sample were dated at 731 ± 6 Ma and 1533 ± 10 Ma.With respect to recently published data, the latter age challenges theconventional viewpoint of absent Mesoproterozoic magmatic eventsand may result from metamorphism during the initial breakup ofColumbia (Meert, 2012; Nance et al., 2014). The 731 Ma apatite agecould possibly be caused by long range effects in the course of Rodinabreakup or Gondwana assembly, respectively.

Finally, this case study exemplarily shows the great potential of thewidely occurring metamorphosed mafic and ultramafic rocks alongthe western margin of the West African Craton for palaeogeographicand geodynamic reconstructions of this region during the LateNeoproterozoic.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.gr.2014.11.011.

Acknowledgements

We thank the civil and military authorities in Dakhla, Aoucertand Tichla (Southern Morocco), which facilitated our stay and ourmovement in these difficult areas. Thanks also to A. El Archi (ChouaïbDoukkali University) and A. Zemmouri (Geological Survey ofMorocco) for advices and guidance in the field. Furthermore we aregrateful for the fruitful comments of the reviewers that helped toimprove the manuscript and for the editing work of W. Xiao.

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History of the West African Neoproterozoic Ocean: Key to the geotectonic history of circum-Atlantic...

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