Early Neoproterozoic rift-related magmatism in the Anti-Atlas margin of the West African craton,...

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ARTICLE IN PRESSG ModelRECAM-4114; No. of Pages 10

Precambrian Research xxx (2014) xxx–xxx

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Precambrian Research

jo ur nal homep ag e: www.elsev ier .com/ locate /precamres

arly Neoproterozoic rift-related magmatism in the Anti-Atlas marginf the West African craton, Morocco

. Javier Álvaroa,∗, André Poucletb, Hassan Ezzouhairi c, Abderrahmane Soulaimanid,l Hafid Bouougrid, Andrés Gil Imaze, Abdelilah Fekkakc

Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Spain1383 rue de la Source, 45160 Olivet, FranceDépartement de Géologie, Université Chouaib Doukkali, 24000 El Jadida, MoroccoDépartement de Géologie, Faculté des Sciences – Semlalia, Université Cadi-Ayyad, P.O. Box 2390, 40000 Marrakech, MoroccoDepartamento Ciencias de la Tierra, Universidad de Zaragoza, 50009 Zaragoza, Spain

r t i c l e i n f o

rticle history:eceived 1 October 2013eceived in revised form 2 October 2014ccepted 6 October 2014vailable online xxx

eywords:

a b s t r a c t

The Moroccan Anti-Atlas contains the relics of a Tonian(?)-early Cryogenian rift developed along thenorthern margin of the West African craton. A distinct tholeiitic volcanic suite of lava flows, sills anddikes is encased in the Jbel Lkest Group (Kerdous inlier), the Taghdout Group (Zenaga inlier), the U1–U4units of the Bleida-Tachdamt Group (Bouazzer Elgraara inlier), and the Lower Sedimentary Ensembleand Middle Volcanic Unit of the Tizi n’Taghatine Group (Siroua, Zenaga and Bouazzer Elgraara inliers).Two magmatic groups are distinguished that originated from a spinel–garnet transition zone ranging

ixed platformequencesholeiitic basaltodinia

from zero garnet content to moderately enriched residual garnet content in the source, and partialmelting degrees decreasing from 10% to 3%. Incompatible element patterns indicate contribution ofboth asthenospheric and lithospheric components suggesting an initial rifting geotectonic context,with lithosphere thinning and asthenosphere upwelling, similar to other complex systems of rifts that

the E
ithosphere thinningstenosphere upwelling
dissected Rodinia during

. Introduction

The West African craton (WAC) consists of two Archaean–alaeoproterozoic metamorphic and magmatic shields separatedy two intracratonic sedimentary basins. The WAC was constructeduring the Archaean and the ∼2 Ga Palaeoproterozoic Eburneanrogeny. After a long Mesoproterozoic interval of quiescence (ca..7–1.0 Ga), except emplacement of some mafic dikes (1.41 Ga;l Bahat et al., 2013; Söderlund et al., 2013), the margins of theAC were affected by several Early-Neoproterozoic rifting events

eflecting continental break-up. Finally, at the end of the Neopro-erozoic, the WAC was subjected to various stages of convergencen all of its margins, to the North along the Anti-Atlas, to the Eastlong the Trans-Saharan belt, to the South along the Rockelides and

Please cite this article in press as: Álvaro, J.J., et al., Early NeoproterozAfrican craton, Morocco. Precambrian Res. (2014), http://dx.doi.org/1

assarides, and to the West along the Mauritanides (Hefferan et al.,000; Ennih and Liégeois, 2008; Gasquet et al., 2008; Villeneuve,008).

∗ Corresponding author.E-mail addresses: [email protected] (J.J. Álvaro), [email protected]

A. Pouclet), ezzouhairi [email protected] (H. Ezzouhairi), [email protected]. Soulaimani), [email protected] (E.H. Bouougri), [email protected] (A.G. Imaz),[email protected] (A. Fekkak).

ttp://dx.doi.org/10.1016/j.precamres.2014.10.008301-9268/© 2014 Elsevier B.V. All rights reserved.

arly Neoproterozoic.© 2014 Elsevier B.V. All rights reserved.

One of these Early Neoproterozoic rifts, preserved in the Anti-Atlas, encompasses a ∼400 km long domain that forms a roughlyENE–WSW-trending belt of disconnected inliers. In the centralAnti-Atlas, remnants of the rift are exposed in the inliers of Ker-dous, Tagrara de Tata, Agadir Melloul, Zenaga, Siroua and BouazzerElgraara (Fig. 1), which contain evidence of its entire history, frombirth (ca. 900–700 Ma) to senescence and final Pan-African closureand suturing (ca. 660–605 Ma). Rifting on the Eburnean basementtook place by the development of fault-controlled troughs intowhich the volcanosedimentary succession of the Tizi n’TaghatineGroup (Bouougri and Saquaque, 2004) was deposited associatedwith hydrothermal activity (Leblanc and Billaud, 1978, 1990;Leblanc and Lancelot, 1980; Leblanc and Moussine-Pouchkine,1994; Fekkak et al., 2003; Thomas et al., 2004). The aim of thispaper is to characterize the nature and evolution of the Early Neo-proterozoic rift recorded in the Anti-Atlas margin of the WAC,based on (1) the stratigraphic relationships of the rift-related mixed(carbonate–siliciclastic) sediments and igneous rocks; and (2) thepetrological and geochemical features of mafic rocks.

oic rift-related magmatism in the Anti-Atlas margin of the West0.1016/j.precamres.2014.10.008

2. Geological setting and stratigraphy

The Anti-Atlas Mountains are structurally divided by theAnti-Atlas Major Fault (AAMF in Fig. 1) into two major

dx.doi.org/10.1016/j.precamres.2014.10.008


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ARTICLE IN PRESSG ModelPRECAM-4114; No. of Pages 10

2 J.J. Álvaro et al. / Precambrian Research xxx (2014) xxx–xxx

utcroM

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Fig. 1. Geological sketch of the Precambrian oodified from Gasquet et al. (2005).

ectonostratigraphic domains: (1) the western and central Anti-tlas, SW and along the AAMF; and (2) the eastern Anti-Atlas,ituated NE of the AAMF to the SAF (South Atlas Fault). Palaeo-roterozoic rocks of the western Anti-Atlas are not exposed inhe eastern Anti-Atlas and their absence has direct implicationsor the northern limit of the WAC (Ennih and Liégeois, 2001,008; Bouougri, 2003; Gasquet et al., 2005). Ophiolitic rocksound in the central Anti-Atlas along the AAMF mark the loca-ion of obducted Neoproterozoic oceanic lithosphere relics ontohe northern margin of the WAC, and broadly coincide with theorthern limit of exposed Palaeoproterozoic basement (Walsht al., 2012). Whereas the AAMF represents a significant Neopro-erozoic suture zone, the SAF played key roles in (1) Cambrianalaeogeographic reconstructions (marking the limit between thediacaran–Cambrian rifted platform and its basinal counterpart;estombes et al., 1985; Álvaro et al., 2008); and (2) Variscanodels (marking a sharp contact in the style of Variscan defor-ation; Soulaimani and Burkhard, 2008; Michard et al., 2010). Our

aper offers no arguments to the debate of the northern bound-ry of the WAC as the shallower marine sediments simply pointo the setting of the WAC coastline, in a rifting context, receivingediments from a southern source area (Bouougri and Saquaque,004).

The traditional lithostratigraphic subdivision of the Precam-rian of the Atlas Mountains into three broad “lithosomes” (e.g.,houbert, 1963), the so-called P-I (Archaean or Palaeoproterozoic),-II (Lower and Middle Neoproterozoic) and P-III (Upper Neopro-erozoic), was widely followed until ∼2000 (Thomas et al., 2002).ecent mapping and the increase of available radiometric ages haveenerated a proliferation of lithostratigraphic nomenclature due tohe abundance of Precambrian inliers, each of which has commonlyeceived its own terminology. The P-II lithosome analyzed in thisaper is bounded by two major unconformities, a lower angulariscordance covering Palaeoproterozoic rocks and the Pan-Africanngular discordance. The lower P-II or Anti-Atlas SupergroupThomas et al., 2004) comprises three discrete sequences relatedo (1) rifting and break-up of the northern margin of the WAC


on which this paper is focused), (2) production of oceanic crustpreserved as metamorphic ophiolitic remnants), and (3) island-arcormation (thrust sheets of calk-alkaline metavolcanic and plutonicocks).

ps of the High Atlas and Anti-Atlas, Morocco.

The lower part of the Anti-Atlas Supergroup shows a progres-sive change of sedimentary facies from SW to NE representinga broad proximal–distal trend in a rifting context: conglomer-ates and sandstones dominate in the Ifni and Kerdous inliers;and quartzites, carbonates and shales (Choubert’s “Série des Cal-caires et Quartzites”) in the Igherm, Siroua, Zenaga and BouazzerElgraara inliers (Bouougri and Saquaque, 2004; Thomas et al., 2004)(Fig. 1). This lithological arrangement has led to a proliferationof lithostratigraphic terminology: e.g., the Jbel Lkest Group in theKerdous inlier (Hassenforder, 1987), the Bleida-Tachdamt Groupin the Bouazzer Elgraara inlier (Leblanc and Moussine-Pouchkine,1994) and the Taghdout Group in the northern edge of the ZenagaComplex and in highly deformed and metamorphosed tectonicslices of the AAMF (Thomas et al., 2004). Correlation between thesegroups (recently included in the Lkest-Taghdout Group; Gasquetet al., 2005, 2008; Álvaro et al., 2014) is both lithostratigraphic andstructural. The Jbel Lkest, Bleida-Tachdamt and Taghdout groupsare post-Palaeoproterozoic volcanosedimentary packages predat-ing the main Pan-African tectonic event. The only lithostratigraphiccorrelation linking the Siroua-Zenaga and Bouazzer Elgraara inlierswas proposed by Bouougri and Saquaque (2004), which is in needof revision (Fig. 2).

The oldest sampled sills are embedded in the mixed (carbonate–siliciclastic) Jbel Lkest Group (Hassenforder, 1987) of the Kerdousinlier, and in the Agouny Formation (Taghdout Group; Thomas et al.,2004) or Taghdout Formation (Tizi n’Taghatine Group; Bouougriand Saquaque, 2004) of the Zenaga inlier. Their metamorphicand structural patterns allow differentiation from a swarm ofdoleritic dikes, named Ifzwane Suite by Thomas et al. (2004), whichinclude dolerites ranging in age from Palaeoproterozoic to Neo-proterozoic (Walsh et al., 2002; Kouyaté et al., 2013). The latteris also recognized in the U1 unit of the Bleida-Tachdamt Group(Leblanc and Moussine-Pouchkine, 1994) or Ifarkhs-n-Tirsal For-mation (Bouougri and Saquaque, 2004) at Tachdamt (BouazzerElgraara inlier). Younger sampled lava flows are interbedded inthe U2–U4 units of the Bleida-Tachdamt Group (Leblanc andMoussine-Pouchkine, 1994) and in the Middle Volcanic Unit and


Upper Sedimentary Ensemble (Bouougri and Saquaque, 2004) ofthe Siroua and Bouazzer Elgraara inliers.

Mafic sills and doleritic dikes are also known in the Igherminlier (Hafid et al., 1998; El Aouli et al., 2001). Doleritic dike



J.J. Álvaro et al. / Precambrian Research xxx (2014) xxx–xxx 3

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ig. 2. Stratigraphic chart of the Jbel Lkest Group in the Kerdous inlier (HassenfordThomas et al., 2004) and Tizi n’Taghatine (Bouougri and Saquaque, 2004) groupsormation; DS1–DS5 represent Bouougri and Saquaque’s (2004) depositional seque

warms crosscut the Palaeoproterozoic basement of the Zenaga andguerda-Taïfast inliers (El Janati et al., 2013). The youngest dikes areated ca. 885 Ma (Kouyaté et al., 2013). Many other mafic dikes areecorded in the Siroua, Tagragra d’Akka, Ifni, and Bas Drâa inliersIkenne et al., 1997; Touil et al., 1999; El Aouli et al., 2010). Some ofhese dikes can be correlated with the Cryogenian mafic intrusionsf the Zenaga and Kerdous inliers. Others are either older and linkedo the Palaeoproterozoic story, as in the Tagragra de Tata, Zenagand Iguerda-Taïfast inliers (Walsh et al., 2002; Kouyaté et al., 2013),r younger and related to the Ediacaran Atlasic Volcanic ChainPouclet et al., 2007; El Aouli et al., 2010). For instance, the maficikes of the Bas Drâa inlier crosscut the granite of Taourgha dated at75 ± 4 Ma (Aït Malek et al., 1998). The calc-alkaline mafic dikes ofhe Igherm, Kerdous, and Ifni inliers postdate the Tizi n’Taghatinend Jbel Lkest groups and their tholeiitic mafic rocks (El Aouli et al.,001).

The age of the ocean opening that renewed sedimentationn the northern WAC margin after the Eburnean orogeny andhe relative Mesoproterozoic gap is poorly constrained. In theouazzer Elgraara inlier, the thermal metamorphism linked to themplacement of gabbroic dikes, which is assumed to belong to theift-related magmatic event, has been dated at 788 ± 10 Ma (Clauert al., 1982). In the Siroua and Bleida inliers, Thomas et al. (2002,004), Samson et al. (2004), D’Lemos et al. (2006) and El Hadi et al.2010) obtained a bracket age of 762–700 Ma for a post-rift episodef island-arc growth and subsequent deformation. Detrital 880 Mald zircons from the Saghro Group (Liégeois et al., 2006) may sug-est a still older minimum age if these zircons are attributed to theift-related magmatic event, a suggestion that fits recent dates byouyaté et al. (2013) obtained from doleritic dikes of Zenaga and

guerda n’Taifast inliers. The studied rift-related lava flows, dikesnd sills of dolerites and gabbros are not directly dated but, based onhe above-reported geochronological constraints, a Tonian(?)-earlyryogenian age fits well with the rifting episode analyzed below.


. Volcanic products

Study of the Early Neoproterozoic rift-related magmatic eventn the Anti-Atlas is based on detailed examination of exposures

87) and the Bleida-Tachdamt (Leblanc and Moussine-Pouchkine, 1994), Taghdoute Siroua, Zenaga, and Bouazzer Elgraara inliers of the Anti-Atlas; Mim: Mimount

in the Kerdous, Siroua, Zenaga and Bouazzer Elgraara inliers. Geo-chemical data from mafic rocks are based on 30 chemical analysesof representative samples from the Jbel Lkest Group of the Ker-dous (carbonate-free) and the Lower Sedimentary Ensemble andMiddle Volcanic Unit of the Siroua, Zenaga and Bouazzer Elgraara(carbonate-bearing) inliers. Major, trace, and rare-earth elementswere determined by inductively coupled plasma mass spectrom-etry (ICP-MS) at ACTLABS in Canada and CRPG in Nancy (France).Precision for major and trace elements is usually better than 2% and5–10%, respectively.

3.1. Petrography

The reported lava flows, sills and dikes consist of microgabbros,dolerites, microdolerites and basalts displaying intersertal or inter-granular to microlitic textures more or less porphyritic. Magmaticminerals have been severely transformed to secondary mineralsowing to low-grade metamorphic process. Feldspars are changedto mixed albite and epidote. Pyroxenes are replaced by actinoliteand chlorite, and olivine by magnesian chlorite or smectite. Theyare only preserved as small relics in the core of large crystals. Theformer hyalo-microlitic groundmass is changed to a fine mixture ofactinolite, epidote, albite, sericite, iron oxides, calcite, and quartz.

The mineral composition is dominated by feldspar that crystal-lized before or simultaneously with clinopyroxene. Orthopyroxeneis common, as well as olivine though this mineral is partly cor-roded and poorly preserved. Oxides, mainly titanomagnetite, arelate idiomorphic crystals that can be abundant in evolved rocks. Allthese petrographical features are symptomatic of olivine tholeiitecompositions. OPX-rich cumulate and olivine-rich cumulates arerecorded at Tachdamt, and feldspar-rich cumulates at Kerdous.

3.2. Geochemistry

We selected thirty rocks for chemical analyses. Six of them were


discarded because of their high loss on ignition and leaching ofalkalis during post-magmatic processes. The remaining analyses(24) are given in Table 1. The rocks are mafic, moderately alkalineand sodic (44.0 < SiO2 wt% < 53.8; 1.5 < Na2O + K2O wt% < 5.9). Their


Please cite this article in press as: Álvaro, J.J., et al., Early Neoproterozoic rift-related magmatism in the Anti-Atlas margin of the WestAfrican craton, Morocco. Precambrian Res. (2014), http://dx.doi.org/10.1016/j.precamres.2014.10.008


4 J.J. Álvaro et al. / Precambrian Research xxx (2014) xxx–xxx

Table 1Selected chemical analyses of doleritic and basaltic rocks from the Lower Neoproterozoic Tizi n’Taghatine and Jbel Lkest groups in the Taghdout, Tizi n’Taghatine, N’Kob,Tachdamt, Bleïda, and Kerdous inliers; LOI, loss on ignition. Analyses have been performed at Actlabs (Canada) and CRPG-CNRS (Nancy, France). Major element concentrationswere determined by ICP-AES and trace elements by ICP-MS. Analytical uncertainties are estimated at 2% for major elements and at 5% to 10% for trace elements dependingon their concentrations higher or lower than 10 chondrite-normalized contents. Mg# = 100Mg/(Mg = Fe2+); Fe2+ is calculated using a ration value of 0.15 for Fe2O3/FeO wt%.La/YbPM, ratio of normalized values to the Primitive Mantle of Sun and McDounough (1989).

Site Taghdout Tizi n’Taghatine N’Kob Tachdamt

# Tag 3 Tag 1 Tag 2 Htz 3 Nk 1 Nk38 Nk749 Tach 8 Tach 3 Tach 4 TCHF2 TCHF5Group 1 1 1 1 1 2 2 1 1 1 1 1

SiO2 46.01 47.71 47.96 48.04 49.44 49.78 50.91 53.84 47.31 47.30 47.82 46.33TiO2 1.08 1.75 1.32 1.32 2.20 3.20 3.04 0.99 2.26 1.80 1.73 1.69Al2O3 14.71 14.27 15.17 13.73 13.22 11.87 13.07 13.98 13.76 14.54 13.95 14.36Fe2O3 12.49 13.74 13.18 12.85 14.15 13.24 12.68 8.11 14.61 13.11 13.63 14.60MnO 0.16 0.19 0.17 0.20 0.54 0.21 0.25 0.15 0.33 0.24 0.23 0.21MgO 8.94 6.73 6.96 6.95 6.75 5.43 6.04 10.11 6.97 6.86 7.58 6.94CaO 9.75 10.36 9.64 11.45 6.24 6.76 7.57 6.21 9.60 8.23 8.38 11.33Na2O 1.17 2.48 1.90 2.18 4.44 5.63 3.24 2.67 2.61 3.31 3.38 1.88K2O 0.61 0.83 0.91 0.10 0.30 0.23 0.65 0.50 0.35 0.25 0.05 0.07P2O5 0.09 0.12 0.12 0.09 0.17 0.31 0.41 0.14 0.21 0.13 0.21 0.20LOI 3.70 2.48 3.61 3.29 2.07 2.34 1.51 3.95 2.91 2.87 2.89 2.34Total wt.% 98.71 100.66 100.94 100.20 99.52 99.00 99.37 100.65 100.92 98.64 99.85 99.95

Sc 31 43 37 44 40 34.2 30.7 24 42 42V 240 381 289 347 383 367 329 160 407 396 336 339Cr 170 120 210 140 120 161 171 590 180 210 277 267Co 52 46 48 47 45 25 35 37 46 45 54 56Ni 170 90 110 90 90 98 108 270 100 110 106 122Cu 1780 40 320 160 90 59 94 <10 100 150 100 117Zn 120 120 180 120 450 546 134 70 170 130 132 126Ga 16 19 20 18 18 15 21 19 20 23Ge 1.9 2.2 2.5 2.3 2 1.6 2.1 1.9 1.6 1.8Rb 18 25 31 2 6 14 28 19 7 6 1 2Sr 266 317 282 187 105 156 346 183 230 210 222 346Y 15.5 23.7 22.6 23.7 27.6 32 35 16.6 28.2 25 26.9 25.9Zr 57 88 85 83 116 202 260 96 121 97 106 104Nb 5.3 6.1 9.8 6 7.5 23 25 6.4 7.8 5.3 6.1 5.9Cs 0.6 1 1.1 1 0.2 0.9 0.5 1 0.3 <L.D.Ba 864 267 372 57 122 31 298 237 160 207 58 45Hf 1.6 2.5 2.3 2.1 3.2 2.5 3 2.7 2.8 2.8Ta 0.3 0.39 0.42 0.34 0.44 0.39 0.44 0.31 0.49 0.46Th 0.72 0.6 2.7 1.03 0.8 2.57 0.98 0.78 0.62 0.63U 0.18 0.17 0.35 0.28 0.21 1.02 0.21 0.17 0.18 0.19

La 6.90 6.70 20.60 8.00 8.30 20.50 26.60 12.30 10.20 7.80 7.10 6.70Ce 17.60 18.40 46.50 20.10 22.70 53.90 62.10 27.80 26.40 21.30 19.10 18.20Pr 2.24 2.55 5.34 2.55 3.17 3.23 3.48 2.87 2.71 2.77Nd 10.50 13.00 22.00 12.00 15.50 30.50 39.20 13.60 16.80 14.10 14.60 13.10Sm 2.74 3.63 4.57 3.26 4.51 7.69 8.54 3.21 4.60 3.97 4.29 3.90Eu 0.96 1.36 1.47 1.14 1.48 2.43 2.41 0.97 1.50 1.46 1.39 1.38Gd 2.77 4.09 4.23 3.59 4.76 7.12 7.50 3.05 4.83 4.23 4.67 4.48Tb 0.48 0.70 0.70 0.64 0.83 0.50 0.87 0.77 0.73 0.70Dy 2.84 4.22 4.03 3.81 5.00 6.26 6.56 2.88 4.91 4.55 4.48 4.53Ho 0.57 0.85 0.80 0.78 0.98 0.58 0.98 0.91 0.99 0.94Er 1.64 2.37 2.26 2.28 2.72 2.84 2.90 1.66 2.68 2.50 2.42 2.52Tm 0.24 0.35 0.32 0.35 0.40 0.25 0.38 0.36 0.39 0.36Yb 1.55 2.20 2.08 2.23 2.61 2.35 2.59 1.57 2.49 2.26 2.50 2.26Lu 0.24 0.35 0.32 0.35 0.39 0.42 0.44 0.24 0.37 0.37 0.35 0.35

Mg# 61.68 52.41 54.28 54.87 51.75 47.97 51.71 73.70 51.75 54.05 55.56 51.66La/YbPM 3.19 2.19 7.11 2.57 2.28 6.26 7.37 5.62 2.94 2.48 2.04 2.13

Site Bleïda Kerdous

# TCHF6 TCHF8 TCHF10 Tch 37 Tach 7 Tach 6 Tch 36 Tach 2 Bld 5 Bld 3b KerF5 KerF6Group 1 1 1 1 1 evolved 1 evolved 2 2 1 2 1 1

SiO2 45.08 46.80 46.62 47.77 49.06 49.57 52.79 47.65 45.97 49.36 43.98 51.82TiO2 1.77 1.63 1.79 1.46 4.10 3.49 1.74 3.91 1.79 3.39 0.90 0.90Al2O3 14.75 14.20 14.05 14.56 13.18 12.52 15.41 13.92 14.64 13.46 16.47 13.50Fe2O3 14.72 14.53 15.19 10.99 16.41 16.53 7.26 12.85 13.12 14.94 13.60 10.77MnO 0.23 0.24 0.27 0.22 0.18 0.21 0.13 0.13 0.22 0.16 0.18 0.17MgO 7.74 7.69 7.45 10.44 4.46 3.76 6.15 4.93 5.87 5.32 10.31 8.44CaO 9.94 8.20 6.87 5.84 4.13 4.82 5.20 7.19 10.51 5.97 7.85 9.27Na2O 2.16 2.69 3.82 3.00 4.12 4.03 4.39 3.45 1.48 4.14 2.21 2.73K2O 0.08 0.11 0.11 1.22 0.04 0.02 1.27 2.18 0.03 0.06 0.20 0.41P2O5 0.20 0.24 0.20 0.19 0.58 0.55 0.37 0.42 0.13 0.38 0.17 0.15LOI 3.19 3.42 3.40 4.03 4.32 3.42 5.57 3.28 4.63 3.80 3.97 1.94Total ppm 99.86 99.75 99.77 99.72 100.58 98.92 100.28 99.91 98.39 100.98 99.84 100.10

Sc 30 42 40 13 30 41 30V 367 336 340 184 370 278 135 362 341 431 58 214Cr 278 233 236 700 <20 <20 180 120 210 100 252 284Co 59 57 54 44 38 33 31 31 44 37 54 34Ni 117 113 109 150 60 50 150 90 100 90 50 45Cu 50 43 71 10 20 10 <10 10 10 10 9 18Zn 137 144 148 190 140 150 110 110 140 110 149 122Ga 23 20 20 17 22 23 19 20 25 22 17 14Ge 1.8 1.5 1.4 1.5 3.7 3.3 1.8 2.6 2.9 1.8 1.2 1.5Rb 2 2 3 39 1 1 28 33 1 1 4 19Sr 350 166 242 196 198 99 353 184 153 144 347 210Y 27.5 23.6 27.1 20.3 58.8 64.7 16.3 32.7 26.1 31.1 12 15.8Zr 109 108 109 131 277 311 197 240 99 208 55 69Nb 5.9 5 6.5 16.5 23.5 25.2 27.8 25.6 7.2 23.2 5 4.7Cs 0.21 <L.D. 0.2 1.1 0.3 0.2 0.6 0.4 0.2 0.2 0.2 0.5Ba 35 35 56 523 108 92 563 485 36 92 102 170Hf 2.8 2.7 2.9 3.2 7 7.8 4.7 5.9 2.8 5.2 1.5 2.2Ta 0.41 0.39 0.53 1.02 1.51 1.57 1.65 1.71 0.43 1.38 0.37 0.4Th 0.6 0.54 0.73 2.58 5.15 5.61 5.5 3.54 1.53 4.39 0.39 1.83U 0.17 0.14 0.21 0.77 1.38 1.51 1.55 0.79 0.21 0.87 0.1 0.43




Table 1 (Continued )

Site Bleïda Kerdous

# TCHF6 TCHF8 TCHF10 Tch 37 Tach 7 Tach 6 Tch 36 Tach 2 Bld 5 Bld 3b KerF5 KerF6Group 1 1 1 1 1 evolved 1 evolved 2 2 1 2 1 1

La 8.00 5.90 7.50 16.10 33.00 33.80 34.30 30.80 12.80 35.20 6.40 10.30Ce 19.50 15.70 19.40 35.70 78.40 79.80 68.10 71.00 32.10 84.20 14.10 21.80Pr 2.83 2.33 2.80 4.23 9.70 10.10 7.17 8.83 3.92 10.10 2.07 3.00Nd 14.00 11.80 14.40 17.80 43.10 45.00 27.80 39.70 17.90 42.70 9.30 12.90Sm 4.11 3.57 3.92 3.95 10.80 11.30 5.78 9.93 4.46 8.80 2.20 3.10Eu 1.49 1.23 1.43 1.23 3.26 3.18 1.78 3.21 1.54 2.71 1.12 0.88Gd 4.81 4.10 4.39 3.74 10.40 11.20 4.54 8.76 4.70 7.28 2.22 3.05Tb 0.76 0.65 0.70 0.63 1.81 1.88 0.64 1.27 0.82 1.11 0.35 0.50Dy 4.72 4.15 4.40 3.62 10.50 10.90 3.25 6.53 4.76 5.81 2.14 2.88Ho 0.95 0.87 0.92 0.71 2.04 2.14 0.56 1.15 0.96 1.07 0.43 0.58Er 2.49 2.27 2.53 1.94 5.64 6.04 1.50 2.87 2.57 2.88 1.20 1.59Tm 0.37 0.34 0.37 0.29 0.82 0.88 0.20 0.39 0.38 0.39 0.18 0.23

5.60.8

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FmtT1e

Yb 2.29 2.44 2.59 1.80 5.16

Lu 0.33 0.36 0.36 0.29 0.83

Mg# 54.18 54.34 52.44 68.11 37.93

La/YbPM 2.51 1.74 2.08 6.42 4.59

g number (100Mg/Mg + Fe2+) ranges from 74 to 68 in cumula-ive rocks, 66 to 44 in moderately evolved basaltic rocks, and to4 in the most evolved rocks that are enriched in Fe and Ti. Inhe CIPW normative composition Qtz-Ab + Or-Hy-Ol-Ne combinedernary diagram (Fig. 3), the rocks are saturated olivine-tholeiites tolivine-basalts. However, the large spread of the normative compo-itions is due to significant variations of the alkali contents modifiedy post-magmatic processes. This is exemplified by two tholeiiticnd alkali-basaltic rocks from N’Kob (red squares) that show veryifferent normative compositions but similar petrographical andrace element compositions. It is assumed that most of the rocksre saturated basalts intermediate between olivine-tholeiite andlivine-basalt. Because of the leaching of alkalis, it is not possibleo use nomenclature based on the alkali content.

The trace element compositions are consistent with moderatelylkaline and evolved basaltic rocks. The more magnesian basaltsre fairly enriched in Cr and Ni. The Primitive Mantle normalizedncompatible element diagram displays a moderate enrichment inhe more incompatible elements, with the exception of the mobilelements Rb and Ba betraying erratic contents due to metamorphicnd alteration effects (Fig. 4A). Post-magmatic mobility of Th and Us suspected in some samples. Two magmatic groups (Groups 1 and) can be discriminated on the basis of their trace element abun-ances and the more incompatible element enrichment. Group 1omprises most of the mafic rocks from all the sites, with rarearth element (REE) contents ranging from 43 to 115 ppm and a


a/YbPM ratio of 1.7–5.6. It includes a few evolved basalts having areater concentration of REE for the same La/YbPM ratio. Group 2 isimited to some basalts of N’Kob, Tachdamt, and Bleïda. It is richern REE and more fractionated with a La/YbPM ratio of 6.3–19.7. Both

ig. 3. Normative composition Qtz-Ab + Or-Hy-Ol-Ne combined ternary diagram ofafic rocks of the Tizi n’Taghatine and Jbel Lkest groups. T, tholeiite; Ol-T, olivine-

holeiite; Ol-B, olivine-basalt; Alk-B, under-saturated alkaline basalt; Bs, basanites.he limit between Ol-T and Ol-B corresponds to Hy normative content c. 15%. Group

and Group 2 rocks after the trace element analyses. Tachdamt 2 refers to twovolved rocks of ferro-tholeiite composition.

2 1.25 2.34 2.48 2.38 1.20 1.518 0.19 0.36 0.39 0.37 0.19 0.23

4 65.57 46.31 50.15 44.46 63.02 63.792 19.69 9.45 3.70 10.61 3.83 4.89

groups are characterized by weak Nb and Ta negative anomalies,(Nb/La)PM and (Ta/La)PM averaging 0.72 and 0.84, and 0.83 and0.80 in the Group 1 and Group 2, respectively. Weak positive Sr-anomalies (Sr/Sr* = 1.6–1.7) are observed in some rocks of Taghdoutand Tachdamt. The highest Sr-anomaly value, combined with Eupositive anomaly (Sr/Sr* = 2.3; Eu/Eu* = 1.6), is observed in a doleritefrom Kerdous, in agreement with its plagioclase-cumulative tex-ture. Negative Sr-anomalies are common (Sr/Sr* = 0.7–0.2) and canbe due either to plagioclase fractionation, namely in the evolvedrocks of Tachdamt, or, more probably to secondary processes ofalteration and weathering as Sr is a mobile element. No signifi-cant Ti-anomalies are observed, except limited positive anomalies(Ti/Ti* = 1.1) in two rocks of N’Kob and Tachdamt, and negativeanomalies (Ti/Ti* = 0.8–0.7) in some rocks of Taghdout, Tachdamt,and Kerdous suggesting fractionation of a Ti-bearing mineral phase.

3.3. Magmatic sources

The incompatible element profiles are characteristic of mantle-derived magmas. Rock analyses plot in the mantle array in theNb/Yb vs Th/Yb diagram (Fig. 5A) except a few rocks slightlyenriched in Th. Group 1 rocks resulted from higher degree of partialmelting than Group 2, or from a less enriched source. Consid-eration of the possible magmatic sources of the various basalticlavas is mainly based on the REE patterns. The Yb vs La/Yb dia-gram (Fig. 5B) indicates a decrease in the partial melting of theparent rock material and/or increasing enrichment of the sourcefrom Group 1 to Group 2. In addition, all the rocks are more orless evolved by progressive fractionation as shown by PrimitiveMantle normalized patterns (Fig. 4A). A garnet-bearing source issuspected if (Gd/Yb)N > 2 or (Tb/Yb)N > 1.8 (see Rooney, 2010). Inthe (La/Sm)C vs (Gd/Yb)C diagram (Fig. 5C), Group 1 originatedfrom a spinel-bearing source, and Group 2 from a garnet-bearingone. These relationships place constraints of the melting depth,the spinel/garnet limit being located between 80 and 90 km. Bothgroups display variations of degrees of melting or of the sourceenrichment. In the La/Sm vs Sm/Yb diagram (Fig. 5D), Groups 1and 2 are distinctly distributed. Their La/Sm values indicate that anenriched source is not realistic for parental magmas. Melt curvesof batch melting that may be convenient for these ratios suggestan undepleted to midly depleted theoretical source. Group 1 maficrocks may have resulted from ca. 10% of partial melting of spinellherzolite with no garnet. Group 2 rocks resulted from 5 to 3% ofpartial melting of spinel–garnet lherzolite. This result places themelting depth close to the spinel–garnet transition zone.

La/Ta and La/Nb ratios are generally used to discriminate litho-


spheric to asthenospheric mantle sources, higher values beingdevoted to the lithosphere with La/Ta > 22 and La/Nb > 1.8 (Coishand Sinton, 1992). The La/Ta ratio ranges from 14 to 49, averaging22 in Group 1, and from 18 to 25, averaging 21 in Group 2. The La/Nb


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atio ranges from 1.0 to 2.2, averaging 1.2 in Group 1, and from 0.9o 1.5, averaging 1.2 in Group 2. These values are indicative of ansthenospheric contribution and to a mixed asthenospheric andithospheric source.

.4. Geotectonic relationships

In a first step, the incompatible element profiles are used topproach the geotectonic signature of the basaltic rocks (Fig. 4B).he more mobile elements (Rb and Ba) are discarded in thisiscussion because of their erratic values. We can exclude aMORB-like source on the base of the heavy rare earth elementepletion and the more incompatible element enrichment of thewo groups. Initial rift tholeiite (IRT) and EMORB profiles fit wellith the Group 1 pattern. IRTs are generated during the thinning


f a lithospheric plate leading to a rift formation and resulted from contribution of both asthenospheric and lithospheric sources.ontinental tholeiite (CT) profile may be also convenient in spitef the Nb-, Ta-, and Ti-negative anomalies because many CTs

ig. 4. Primitive Mantle normalized incompatible element diagram. Normalization valueamples. Group 1 is moderately fractionated. Two evolved rocks are more enriched with pf the two groups. Average composition of initial rift tholeiites (IRT) after Holm (1985)

-MORBs, and oceanic island basalts (OIB) after Sun and McDounough (1989).

PRESSsearch xxx (2014) xxx–xxx

show weak Nb- and Ta-anomalies and no Ti-anomaly. All the CTsoriginate from lithospheric sources.

Group 2 rocks may have melted from the same (IRT, CT) sourceat greater depth with lower degree of partial melting. An OIB-likesource is not supported by the Nb and Ta behaviour of the Group 2rocks.

The IRT and CT signatures are supported in the La–Y–Nb andTi–Nb–Th discriminating diagrams (Fig. 6), while the EMORB signa-ture is clearly outside the La–Y–Nb field of our dataset (Holm, 1985;Cabanis and Lecolle, 1989). In the first diagram, the rocks plot in theintra-continental domain located between the orogenic (left) andnon-orogenic (right) domains. In the second diagram, the IRT andCT analytical data are used to distinguish two domains devoted tothe plate margin (IRT) and the within-plate (CT) geotectonic con-texts. The plotted rocks overlap the limit between the two domains


(excluding the Th-enriched contaminated samples). They displaythe same features than the lower Cambrian olivine basalts of thewestern High Atlas that are related to an ocean opening above acontinental thinned lithosphere (Pouclet et al., 2008).

s of Sun and McDounough (1989). Same symbols as for Fig. 3. (A) Profiles of all thearallel profiles. Group 2 is more fractionated and enriched. (B) Compositional rangeand Pouclet et al. (1995), continental tholeiites (CT) after Holm (1985), N-MORBs,




Fig. 5. Nb/Yb vs Th/Yb, Yb vs La/Yb, and La/Sm vs Sm/Yb diagrams to determine the possible mantle sources and the partial melting degrees of sources. Same symbols as forFig. 6A. In the Nb/Yb vs Th/Yb diagram, most of the mafic rocks plot in the mantle array, except a few ones that are enriched in Th and may have recorded crustal contamination.Group 1 rocks resulted from higher degree of partial melting than Group 2 ones, or from less enriched source. (B) Similarly, the Yb vs La/Yb diagram indicates a decrease orthe partial melting for the parental melts and/or increasing enrichment of the source from Group 1 to Group 2. In addition, all the rocks are more or less evolved by fractionalfractionation. (C) The (La/Sm)C vs (Gd/Yb)C diagram points to a spinel-bearing source for Group 1 and to a garnet-bearing one for Group 2, and then to a decreasing depthof melting. Normalization values of the chondrite C1 after Sun and McDounough (1989). (D) In the La/Sm vs Sm/Yb diagram, an enriched source is not realistic for parentalmagmas. Liquids compositions are calculated for batch melting of a fairly depleted mantle source. Melt curves are drawn for spinel–lherzolite, garnet–lherzolite, and a 50:50m olivinC ition c1 withl and M

tTtgCrc(

4

pamamwgr2

ixture of spinel– and garnet–lherzolite. Modal compositions of spinel–lherzolite (PX 10%, garnet 10%) are after Kinzler (1997) and Walter (1998). Mineral/melt part

mafic rocks may have resulted from ca. 10% of partial melting of spinel lherzoliteherzolite. Primitive Mantle, N-MORB, E-MORB, and OIB compositions are from Sun

Based on geochemical data, some doleritic dikes and sillshat intrude the neighbouring inliers of Siroua, Igherm, andagragra d’Akka can be compared with the mafic rocks ofhe Bleida-Tachdamt, Taghdout, Tizi n’Taghatine and Jbel Lkestroups, excluding the pre-Cryogenian dikes and the upperryogenian–Ediacaran dikes. They display the same compositionalange of Group 1 and Group 2, from tholeiitic to mildly alkalineompositions, thanks to more or less complete chemical analysesIkenne et al., 1997; El Aouli et al., 2001).

. Palaeogeographic implications

The palaeogeographic implications of the described riftingulses are dramatically controlled by Pan-African deformationnd subsequent opening of grabens and infill by post-Pan-Africanolasses. The main lithostratigraphic link between the Siroua

nd Zenaga inliers is the lateral continuity of the Oumoula For-ation (Bouougri and Saquaque, 2004) or Mimount Formation,


hich is structurally affected by the onset of the AAMF andeographically forms a subvertical, prominent E–W mountainange rising up to 400 m above the Zenaga plateau (Thomas et al.,004). The Oumoula (or Mimount) Formation is unconformably

e 53%, OPX 27%, CPX 17%, spinel 3%) and of garnet–lherzolite (olivine 60%, OPX 20%,oefficients for basaltic liquids are after the compilation of Rollinson (1993). Group

no garnet. Group 2 rocks resulted from 5 to 3% of partial melting of spinel–garnetcDounough (1989).

overlying the Palaeoproterozoic basement in the Siroua inlier andunconformably overlain by a half-graben infill of the OuarzazateSupergroup in the Zenaga inlier (Bouougri and Saquaque, 2004;Thomas et al., 2004). In addition, the base of the Bleida-TachdamtGroup in the Bouazzer Elgraara inlier is a tectonic contact onallochthonous units emplaced during the onset of Pan-Africanthrusts (Leblanc and Moussine-Pouchkine, 1994) (Fig. 2). Sedimen-tary and sequence-stratigraphic patterns of the Tonian(?)-earlyCryogenian volcanosedimentary complex (Bouougri et al., 1994;Leblanc and Moussine-Pouchkine, 1994; Bouougri and Saquaque,2000, 2004; Thomas et al., 2004; Álvaro et al., 2014; Clausen et al.,2014; Hefferan et al., 2014) can be related to the volcanic episodesdescribed above.

In the Zenaga inlier, the initiation of a stepwise rift opening ischaracterized by the unconformable deposition of conglomeratesand coarse-grained alluvial and fluvial facies on a Palaeoproterozoicbasement. At Taghdout, the base of the Tassarda Formation consistsof a monogenetic conglomerate and pebbly sandstone composed


of quartz, chert and composite clasts derived from the underlyingPalaeoproterozoic basement, dominated by migmatites. In con-trast, the basal conglomerates in Siroua are a polymictic facies as aresult of the heterogeneity of its basement (Thomas et al., 2004).


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Fig. 6. La–Y–Nb and Ti–Nb–Th discriminating diagrams after Cabanis and Lecolle(1989) and Holm (1985). Same symbols as for Fig. 5. Average compositions of IRTaSA

(sEaasda

iciwddie

oimFptsas

fter Holm (1985) and Pouclet et al. (1995), of CT after Holm (1985), and of OIB afterun and McDounough (1989). Area of lower Cambrian basalts from the western Hightlas after Pouclet et al. (2008).

The conglomerates pass upward into a depositional sequenceDS2 in Fig. 2) dominated by peritidal deposits, which progres-ively (so diachronously) onlapped the basement of the Bouazzerlgraara area. DS3 represents a new extensional pulse in the rift,s a result of which creation of accommodation space in the Sirouarea led to the onset of fluvio-deltaic deposits. The last depositionalequences (DS4 and DS5) represent progradation trends of fluvio-eltaic deposits recognized in the Siroua and Bouazzer Elgraarareas.

In addition to the broad SW–NE proximal-distal trend exhib-ted by the Tonian(?)-Cryogenian volcanosedimentary complex,orrelation of the Lower Sedimentary Ensemble sandstone bodiesndicates thinning of the fluvio-deltaic complexes to the SE together

ith slight thickening of the shaly interbeds and co-occurrence ofistinct carbonate interbeds. Facies associations and palaeocurrentata suggest that the platform strandline was oriented NW–SE with

ts continental source situated to the (present-day) south (Bouougrit al., 1994).

Syn-rift sediments developed successive progradational trendsf coarse-grained fluvio-deltaic systems with their setting stronglynfluenced by rift fault geometries (e.g., the Mimount/Oumoula For-

ation is absent in the Bouazzer Elgraara inlier and the Tanarouchtormation is only found in the Siroua inlier). Carbonates markhases of microbial productivity and high rates of chemical produc-


ion during transgression and highstand whereas the highest clasticupply rates occur during falling and low sea-level stands (Bouougrind Saquaque, 2004; Clausen et al., 2014). The clastic depositionalequences, underlain by erosive surfaces, represent alluvial and


fluvio-deltaic conglomerates and pebbly sandstones interpreted tocorrespond to relative lowstand periods when fault-related upliftin a footwall setting favoured erosion and siliciclastic supply. Sub-sequent transgression from regional subsidence resulted in marineflooding followed by carbonate productivity during transgressionand highstand. The record of at least three major depositionalsequences (DS2 to DS4) marked by lower coarse-grained allu-vial and/or fluvio-deltaic deposits and upper transgressive mixed(carbonate–siliciclastic) sedimentation marks the episodic dissec-tion of the rift into separate fault blocks contemporaneous withtilting. Fault-controlled subsidence and generation of structurallyinduced depocentres were accompanied by marine transgression,whereas progradation of fluvio-deltaic environments caused thedemise of carbonate production. Alternating episodes of riftingpulses and intervening tectonic quiescence are represented inthe depositional sequences, in which their basal parts representintervals of renewed palaeorelief sealing and not simply eustaticfluctuations in a passive margin. Syn-rift sequences character-ized by abrupt changes in thickness and facies associations alongnormal faults are not easily distinguishable due to possible faultreactivation during the subsequent Pan-African Orogeny and theobvious discontinuity among inliers, hampering the application ofrifting stratigraphic nomenclature (Bosence, 1998; Martins-Netoand Catuneanu, 2010).

The onset of the Middle Volcanic Unit or the U2–U3 units ofthe Bleida-Tachdamt Group represents a generalized episode ofvolcanic activity yielding a volcanosedimentary blanket lackingcarbonate production. Its end is marked by the final collapse ofrift branches: shale finally accumulated throughout the entire riftwhen the platform collapsed to form a trench slope, which was sub-sequently buried by turbidites (U4–U6 units or Upper SedimentaryEnsemble; Leblanc and Moussine-Pouchkine, 1994; Bouougri andSaquaque, 2004). The change from a sequence of terrestrial/marinealternations (Lower Sedimentary Ensemble) that show rapid lateralvariability of thickness and facies to an overlying, essentially sheet-like, mainly volcanic unit (Middle Volcanic Unit) can be interpretedto broadly represent a syn-rift/post-rift transition.

5. Onset of the Anti-Atlas rift in the Rodinia break-upcontext

Due to the lack of confident Early-Neoproterozoic palaeomag-netic data and the record of the Grenwillian orogen in the WAC,there are few constraints on the position of the latter in Rodinia.Trompette (1994, 1997) proposed the existence of a single WestAfrica-Amazonia-Río de la Plata megacraton in the Meso- and Neo-proterozoic, although their relative position in Rodinia is still amatter of debate.

The break-up of the supercontinent Rodinia was a prolongedepisode (ca. 860–570 Ma; Li et al., 2008 and references therein),which began with some isolated intrusions from South Chinaand Africa (870–850 Ma; Li et al., 2003a,b; Johnson et al., 2005),the Scandinavian Caledonides (ca. 870–845 Ma; Paulsson andAndreasson, 2002) and the Scottish promontory of Laurentia(Dalziel and Soper, 2001). Although these were interpreted as thefirst sign of a Rodinia superplume (Li et al., 2003a,b), the lat-ter displayed a widespread activity at ca. 825 Ma, as shown bymafic dike swarms, intra-continental mafic-ultramafic intrusions,and felsic intrusions commonly found in Australia (Wingate et al.,1998), South China (Li et al., 2003a,b), Tarim (Zhang et al., 2006),India (Radhakrishna and Mathew, 1996), Kalahari (Frimmel et al.,


2001), and the Arabian–Nubian terranes (Teklay et al., 2002). Inplaces, these intrusive rocks are unconformably overlain by similar-aged rift-related volcaniclastic successions, suggesting widespreadcontinental rifting. Rifting in South China shows remarkable


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imilarities to that of eastern Australia, featuring several episodesf magmatism and rifting in the ca. 830–700 Ma interval (Harlant al., 2003; Ling et al., 2003). Finally, magmatism at ca. 755–750 Maas the last major event crossing or breaking-apart Rodinia, as

ecognized in western Kalahari, South China, and western andoutheastern margins of Laurentia; Li et al., 2003a; McClellan andazel, 2014).

Due to the lack of a proper age control in the Lkest-Taghdoutroup, a direct correlation with any of the aforementioned volcanic

ntervals is not yet available. In any case, rifting in the northernargin of WAC necessarily precluded the onset of the metamorphic

riri island arc (Iriri Group; Álvaro et al., 2014; Hefferan et al., 2014),reated by subduction of oceanic crust (Thomas et al., 2002, 2004),here the Tasriwine ophiolite leucogranite of the Siroua inlier has

ielded an U–Pb age of 762 + 1/−2 Ma (Samson et al., 2004).During the break-up of Rodinia, a complex system of rifts diss-

cted the supercontinent. This was particularly the case in thextensional Pan-African realm between the West African and Balticratons and between the Congo and São Francisco cratons wherehe Pan-African oceans opened (e.g. configuration of Rodinia at00 Ma after Pisarevsky et al., 2003; Li et al., 2008; Kouyatét al., 2013). Numerous doleritic dikes intruded the sedimen-ary sequences (progradational fluvio-deltaic lowstands and mixederitidal to microbial reefal systems during transgression and high-tands) in the initial rift system of the west central-African margin,hich can be compared with the mafic rocks of the north-westernfrica margin. Their petrographical and chemical compositions areimilar to those of the contemporaneous Moroccan mafic rocksnd share their IRT and CT trace element patterns (Vicat andouclet, 1995; Vicat et al., 1996, 1997). Therefore, it is concludedhat the Tonian(?)-early Cryogenian magmatic activity recordedn the Anti-Atlas is related to initiation of a rift with lithospherichinning and asthenospheric upwelling. In such a context, the

agma genesis may have resulted from adiabatic decompression ofhe mantle.

. Conclusions

After a long Mesoproterozoic interval of relative quiescence (ca..7–1.0 Ga), the northern margin of the WAC recorded an Earlyeoproterozoic intracontinental rift, partly preserved in the Ker-ous, Siroua, Zenaga and Bouazzer Elgraara inliers of the Anti-Atlas.he rift-related rocks occur embedded within and crosscuttinghe Jbel Lkest, Bleida-Tachdamt, Taghdout and Tizi n’Taghatineroups, where two magmatic groups can be discriminated: Group

includes a few evolved basalts having greater concentrations ofEE for the same La/YbPM ratio, whereas Group 2 is limited toasalts richer in REE and more fractionated. Initial rift tholeiiteIRT) profile fits well with the Group 1 pattern, although conti-ental tholeiite (CT) may be also be considered, originating from

ithospheric sources. Group 2 rocks may have melted from theame source at greater depth with lower degree of partial melt-ng. Some doleritic dikes and sills that intrude the neighbouringnliers of Siroua, Igherm, and Tagragra d’Akka can be correlated

ith the mafic rocks of the Tizi n’Taghatine and Jbel Lkest groups.his magmatic activity is related to initiation of a rift with litho-pheric thinning and asthenospheric upwelling. In such a context,he magma genesis may have resulted from adiabatic decompres-ion of the mantle. During the break-up of Rodinia, a complexystem of rifts dissected the supercontinent. This was particularlyhe case in the extensional Pan-African realm between the Westfrican and Baltic cratons and between the Congo and São Francisco


ratons where the Pan-African oceans opened. Their petrographicalnd chemical compositions are similar to those of the contempo-aneous Moroccan mafic rocks and share IRT and CT trace elementatterns.

PRESSsearch xxx (2014) xxx–xxx 9

Acknowledgements

The staff of the Bleïda and Bou-Azzer mines is warmly thankedfor providing fieldtrip facilities and technical assistance. N. AitAyad and A. Charif (El Jadida) and S. Clausen (Lille) kindly helpedwith field sampling. Financial support for this work was providedby projects CGL2010-19491 and CGL2013-48877 from SpanishMINECO and EU-FEDER. We thank K. Hefferan and P. Mueller fortheir constructive manuscript reviews.

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Early Neoproterozoic rift-related magmatism in the Anti-Atlas margin of the West African craton,...

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