Journal of African Earth Sciences 58 (2010) 81–88

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Journal of African Earth Sciences

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Lower Cryogenian calc-alkaline mafic rocks of the Western Anti-Atlas (Morocco):An example of orogenic-like magmatism in an extensional setting

El Hassan El Aouli a, Dominique Gasquet b,*, Alain Cheilletz c

a LAGAGE, Département de Géologie, Faculté des Sciences, Université Ibn Zohr, B.P. 8106, Agadir, Moroccob EDYTEM, Université de Savoie, CNRS, Campus Scientifique, F 73376 Le Bourget du Lac Cedex, Francec CRPG and ENSG, Nancy Université, BP 20, 54501 Vandoeuvre lès Nancy, France

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

Article history:Received 23 February 2009Received in revised form 29 December 2009Accepted 29 January 2010Available online 8 February 2010

Keywords:Calc-alkaline mafic rocksLower CryogenianPalaeoproterozoic subductionIgherm–Kerdous–IfniWestern Anti-AtlasMorocco

1464-343X/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.jafrearsci.2010.01.010

* Corresponding author. Address: Université de SaUMR 5204, Campus Scientifique, 73376 Le Bourget du03 79 86 45; fax: +33 03 79 75 87 77.

E-mail addresses: hassan.elaouli@laposte.net (E.H. Euniv-savoie.fr (D. Gasquet), cheille@crpg.cnrs-nancy.f

The lower Cryogenian mafic magmatism from the Igherm, Ifni and Kerdous inliers (Moroccan WesternAnti-Atlas) have calc-alkaline, tholeiitic and alkaline affinities. The calc-alkaline dolerite dykes and gab-bros bodies emplaced before the conglomeratic formations of the Upper Cryogenian and after the tholei-itic mafic rocks that characterize the pre-Pan-African rifting. They are similar to rocks from orogenicsetting and characterized by high LILE, Th, Ce, P, Sm contents and La/Nb ratio and a low HFSE content withnegative anomalies in Nb, Zr and Ti. The geodynamic environment of the sedimentary country rocks cor-responds to that of a passive margin in a distensive tectonic context. The calc-alkaline affinity of thesemagmas can be attributed to the influence of a Palaeoproterozoic subduction zone that contributed tothe enrichment of the sub-continental mantle. During the extensional event of the Pan-African orogene-sis, the mantle would have produced tholeiitic, alkaline and/or transitional magmas before melting(caused by adiabatic decompression) reached the enriched sub-continental mantle. This mantle was pre-viously enriched during the Eburnean subduction, where it would have generated calc-alkaline magmas.

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1. Introduction

The Moroccan Anti-Atlas range is a particularly rich area for thestudy of mafic magmatism, as numerous mafic dyke swarms,sometimes associated with intrusive massifs, were emplaced overa period of time extending from the Palaeoproterozoic (Choubert,1963; Gasquet et al., 2004) to the Jurassic (Bertrand and Prioton,1975). The Cryogenian mafic rocks analyzed during the presentstudy are of particular interest because they formed on the north-ern edge of the West-African Craton (Pouclet et al., 2007) at thesame that a back-arc basin was opening further east in the CentralAnti-Atlas, as indicated by the presence of ophiolites at Bou Azzer(Leblanc, 1975) and Siroua (El Boukhari, 1991; Chabane, 1991).

Hafid et al. (1998) and Hafid (1999) reported that the Cryoge-nian mafic rocks of the Igherm, Adrar Izazen, Iguerda-Taïfast, Aga-dir Melloul, Zenaga and Kerdous inliers (Central and Western Anti-Atlas) are relatively homogenous and have a tholeiitic affinity.Such homogenous compositions have also been noticed in thenortheastern Anti-Atlas (Saghro) for the Kelaat Mgouna and Bou-malne Cryogenian basalts (Fekkak et al., 1999, 2001) and the

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l Aouli), dominique.gasquet@r (A. Cheilletz).

Tagmout gabbros (Mokhtari et al., 1995). All these mafic rocksseem to be related to the Neo-Proterozoic extension episode ofthe northern part of the West-African Craton.

However, subsequent work in the Southwestern Anti-Atlas hasshown that these rocks are not as homogenous as first thought, as,in addition to the mafic dykes with tholeiitic affinities, there areother dykes with transitional (Iguerda-Taïfast) or alkaline (Igherm)affinities. In addition, the latest studies have revealed the presenceof other mafic bodies and dykes with calc-alkaline affinities in theIgherm inlier (El Aouli, 2004). This association of tholeiitic, alkalineand/or transitional magmas with calc-alkaline magmas has beenalso recently found in the Northeastern Anti-Atlas (Lower Cryoge-nian basins of Sidi Falah, Kelaat Mgouna and Boumalne-Imiter) in avolcanic arc or active margin sedimentary environment (Benziane,2007).

Coeval tholeiitic, alkaline and/or transitional mafic magmatismsin extensional settings have been described by several authors(e.g.: Kampunzu et al., 1984; Bertrand, 1991; Storey et al., 1992;Seymour and Kumarapeli, 1995; Kampunzu et al., 2000). However,the situation found in the Igherm inlier where calc-alkaline maficrocks coexist with tholeiitic and alkaline rocks in a geotectonic set-ting that does not seem to have evolved quickly, is much rarer.

The Igherm inlier (Western Anti-Atlas), which is less erodedthan its neighbors, is a key inlier for the study of mafic rocks be-cause of (i) the abundance of its dykes and intrusive bodies, (ii)its pivotal geodynamic position between the Western and Central

82 E.H. El Aouli et al. / Journal of African Earth Sciences 58 (2010) 81–88

Anti-Atlas, and (iii) the diverse range of the mafic rocks it contains(El Aouli et al., 2001a,b).

The main objective of the present study is to propose a geody-namic setting for the emplacement of these Neo-proterozoic maficrocks within the more general framework of the Anti-Atlas, and toinvestigate the relations between this magmatism and the pre-existing Pan-African and Eburnean subductions. In order to achievethis, we first characterized the field relations, petrography and geo-chemistry of the Cryogenian calc-alkaline mafic dykes and bodiesof the Igherm inlier by comparing them to the Ifni and Kerdousinliers.

2. Geological setting of the Anti-Atlas

The Proterozoic basement of the Anti-Atlas is divided into twodomains at each side of the WNW–ESE oriented Central Anti-AtlasFault Zone (CAAFZ, Fig. 1). The northeastern domain has beeninterpreted as a multi-deformed Neo-proterozoic Pan-African oro-genic segment that encompasses the Siroua, Saghro and Ougnatmassifs (Choubert, 1952; Leblanc, 1972, 1975; Leblanc and Lance-lot, 1980). However, the Neo-proterozoic magmatism of this do-main displays a Palaeoproterozoic source suggesting anunderlying Eburnean basement (Gasquet et al., 2005, Ennih andLiégeois, 2001). The southwestern domain consists of a Palaeopro-terozoic and Neo-proterozoic basement that was deformed by theEburnean and Pan-African orogenies (Charlot, 1978; Mrini, 1993;Lama et al., 1993), and covered by Ediacaran to Paleozoic forma-tions. The Bou Azzer inlier lies between these two domains, alongthe CAAFZ. This inlier consists mostly of a Pan-African ophioliticcomplex (Leblanc, 1975) that has recently been dated at the neigh-boring Siroua massif at 761 ± 2 Ma (U/Pb on zircons from plagiog-ranites; Samson et al., 2004). The Igherm, Ifni and Kerdous inliers –the focus of the present study – belong to the southwestern do-main on the northern margin of the West-African Craton (Mous-sine-Pouchkine et al., 1988). No Mesoproterozoic ages have everbeen found in the whole Anti-Atlas.

2.1. The Neo-proterozoic

The Neo-proterozoic of the Anti-Atlas has been divided into twosupergroups: the Cryogenian Anti-Atlas Supergroup (AASG) andthe Ediacaran Ouarzazate Supergroup (OSG) (Thomas et al.,

100

Agadir

EW

N

S

Kerdous

Akka

Igherm

Anti

Sidi

Atlanti

c oce

an

So

(b)

Siroua

500 km

(a)

?

?

15°N

30°N 15°W 0°

Abidjan

Agadir

Anti-Atlas

West AfricanCraton

Ouarzaz

Ifni

Bas Drâa

Zenaga

SAF

High Atla

south western domain

Iguerda-TaïfasAdrar Izazen

Bou

Fig. 1. Geological sketch map of the Anti-Atlas belt in southern Morocco a

2004). The Lower Cryogenian formations (Jbel Lkest Group, AASGpro parte) lie unconformably on the Palaeoproterozoic basement(Hassenforder, 1987; Gasquet et al., 2004). The bottom part ofthe AASG (limestones and quartzites) consists of epicontinentalplatform sediments, whereas the upper members (schists andstratified quartzites) are characteristic of an ocean basin environ-ment (Choubert and Faure-Mauret, 1973, 1976). Mafic rocks havebeen described within all the Cryogenian sedimentary sequencesof the AASG (Choubert and Faure-Mauret, 1973; Hafid et al.,1998; El Aouli et al., 2001a; El Aouli, 2004; Gasquet et al., 2004).The Upper Cryogenian, which records the late Pan-African event(Jeannette, 1981), is composed mostly of molassic conglomerates,with rhyolites, ignimbrites and acid tufs at the base (Choubertand Faure-Mauret, 1973). The Ediacaran OSG formations consistof conglomerates overlain by purple-blue pelites, preceded byandesitic flows in the northeastern part of the Igherm inlier (Chou-bert and Faure-Mauret, 1973) and, more generally across the Anti-Atlas, by extremely thick (up to 2000 m in the Ouarzazate area)piles of felsic volcanic rocks (notably rhyolites).

2.2. The Palaeoproterozoic

The Palaeoproterozoic of the Anti-Atlas consists of micaschistsand calc-alkaline granites deformed during the Eburnean (Chou-bert, 1963; Choubert and Faure-Mauret, 1976; Oudra, 1988; Gas-quet et al., 2005). Syn-orogenic granitoids developed after2050 Ma, associated with calc-alkaline and peraluminous magmasderived from a mafic source (with a small Archaean crustal compo-nent) in a plate-convergence back-arc setting (Gasquet et al., 2001,2004, 2005, 2008; Mortaji et al., 2000; Mortaji, 2007). According toBenziane (2007), the Palaeoproterozoic in the Anti-Atlas is charac-terized by two distinguishable magmatic events both related to asubduction setting: the first at 2110–2080 Ma (trondhjemitic mag-matism) and the second at 2050–2030 Ma (calc-alkalinemagmatism).

2.3. Mafic magmatism

Mafic magmatism in the Anti-Atlas is represented by at leastfour generations of dykes: (i) Tholeiitic dykes of Palaeoproterozoicage. Dykes in the Tagragra de Tata have been dated at 2040 ± 6 Mausing the SHRIMP U/Pb method on zircons (Walsh et al., 2002;

km

Ordovician

Ediacaran

Late Ediacaran to Cambrian

Cryogenian and Palæoproterozoic

Ougnat

Imiter

- Atlas

Flah

Tagmout

Tamlalt

Anti-Atlas Fault Zone

Atlas

uth

Central

ateSaghro

Fault

s

north-eastern domain

t

Azzer Undifferentiated post-Ordovician cover

Kelaat MgounaBoumalne

nd location of the three studied inliers (bold). SAF: South Atlas Fault.

E.H. El Aouli et al. / Journal of African Earth Sciences 58 (2010) 81–88 83

Benziane, 2007), and a microgranite from the Tafeltast inlier,which is cartographically and structurally associated to the maficdykes in this inlier and to those in the Tagragra d’Akka inlier, hasbeen dated at 1760 ± 3 Ma (Gasquet et al., 2001 and Gasquetet al., 2004). (ii) Cryogenian dykes with tholeiitic and alkaline affin-ities, coeval with the opening of back-arc oceanic basin in the Cen-tral Anti-Atlas (Clauer et al., 1982).

U/Pb ages of 761 ± 2 Ma have been obtained using zircons fromplagiogranites associated with the Siroua ophiolites (Samson et al.,2004). (iii) Dacite dykes in the Tafeltast and Tagragra d’Akka in-liers, dated at 600 ± 5 Ma using the SHRIMP U/Pb method (Gasquetet al., 2004). (iv) Late Pan-African mafic dykes, including those thatcrosscut the Taourgha granite, dated at 575 ± 4 Ma (U/Pb ages, AïtMalek et al., 1998) using samples from the Bas Drâa inlier (Ikenneet al., 1997).

3. Geological setting of the calc-alkaline mafic dykes of theIgherm, Kerdous and Ifni inliers

The calc-alkaline mafic rocks of the Igherm and Kerdous inliersoutcrop in the form of numerous dolerite dykes that trend NW–SEto N–S and that range from several meters to several tens of metersin length and from 0.5 to 1.5 m in width, together with hundred-meter scale mafic bodies that do not show any magmatic fabricat outcrop. They intrude the schist-greywackes and granites ofthe Palaeoproterozoic basement, and into Lower Cryogenian lime-stones and quartzites. They precede the Upper Cryogenian con-glomeratic formations. The limestones in direct contact with themafic bodies have been metamorphosed to greenschist facies(Choubert and Faure-Mauret, 1973), whereas the quartzites havecentimeter-scale borders with a horny aspect and a very fine grain.The Palaeoproterozoic and Cryogenian formations and the maficrocks are affected by the regional NE–SW trending Pan-Africanschistosity. Relative chronology shows that the calc-alkaline dykesintrude both the calc-alkaline mafic bodies (present study) and thedykes with tholeiitic affinities (El Aouli et al., 2001a; El Aouli,2004). On the other hand, no relationship has been observed atoutcrop between the mafic dykes, mafic bodies and alkaline dykes.The latter, like the calc-alkaline dykes, intrude the Palaeoprotero-zoic basement and the tholeiitic mafic bodies, and have not beenobserved in Lower Cryogenian quartzites (El Aouli et al., 2001a;El Aouli, 2004; El Aouli et al. 2004).

The calc-alkaline mafic dykes of the Ifni inlier cut across boththe Palaeoproterozoic (Alouzad) and the Neo-proterozoic (Mesti)granites.

4. Petrography and geochemistry of the calc-alkaline maficrocks

The calc-alkaline mafic dykes are recognizable on the field bytheir grey color and their fine-grained texture and mm-sizedphenocrysts with respect of the tholeiitic and alkaline dykes whichare green or red colored and show a medium or coarse grained tex-ture. However, no distinction is possible on the field between thedifferent mafic bodies.

The albite, chlorite, actinote, epidote, leucoxene and quartzparagenesis shows that the calc-alkaline mafic rocks have under-gone secondary metamorphism in the ‘‘greenschist” facies. The flu-idal and porphyritic microlitic textures found along the edges ofthe dykes or bodies gives way to a fine intersertal texture in theircenters. The groundmass is cryptocrystalline and rich in calcite,chlorite and quartz. These rocks contain particularly large quanti-ties of plagioclase, which can account for as much as 45–50% ofthe total volume of the rock. Chlorite and iron oxide pseudomorphshave the characteristic shapes of amphibole and olivine. Micro-

phenocrysts of plagioclase have been transformed into calciteand albite (An05). Subeuhedral oxide and sulphide minerals arefound in both the plagioclase and the chlorite pseudomorphs offerro-magnesian minerals. The primary mineral paragenesis ofthe gabbro bodies consists of plagioclase, uralitized pyroxene, opa-que minerals and rare quartz-albite micropegmatite. The texture isgenerally intersertal along the borders of the mafic bodies, becom-ing ophitic to sub-ophitic or phaneritic in the centers.

Thirty samples of the least altered rocks were analyzed by theRock and Mineral Analysis Department at CRPG-CNRS (Nancy)using ICP-AES and ICP-MS (Table 1), and following the proceduresdescribed by Carignan et al. (2001). Samples with high contents ofsecondary magnetite, iron oxides, quartz and calcite were ex-cluded from the analysis. Analyses of major, incompatible traceelements and rare earth elements, which are considered to showvery little mobility during alteration processes (Pearce and Cann,1973; Floyd and Winchester, 1975; Tarney et al., 1979), revealedhigh Al2O3 contents (14.10–20.64%) and low concentrations of(Fe2O3)t (7.00–12.94%), TiO2 (0.68–1.60%), Zr (66 to 257 ppm),Nb (4.9 to 9.57 ppm), Y (9.7–30.4 ppm) and V (105 to 242 ppm)compared to the high La/Nb ratios (2.09 to 5.55). The low V con-tents, and their progressive decrease with respect to FeO�/MgO,together with the absence of iron enrichment (Miyashiro, 1974;Miyashiro and Shido, 1975), indicate a calc-alkaline character(Fig. 2). The very low variations in TiO2 (0.68–1.60%) with respectto FeO�/MgO (1.07–2.13) reflect an isotitanium character that istypical of orogenic zones (Bébien, 1980) (Fig. 2) as also indicatedin Cabanis and Lécolle’s (1989) Y/15-La/10-Nb/8 diagram (Fig. 3).The Ti/V ratios (17.58–61.94) of these dolerites and gabbros be-long within the field of mid-ocean ridge or back-arc basalts inShervais’s (1982) Ti/1000-V diagram. However, they demonstratea clear calc-alkaline affinity (Fig. 4). When normalized to thechondrite of Evensen et al. (1978), their REE patterns show (La/Sm)N, (La/Yb)N and (Gd/Yb)N ratios of between 1.19 and 3.42,2.79 and 20.52, and 0.96 and 4.20, respectively, which indicatehigh enrichment in LREEs and a slight fractionation of the HREEs(Fig. 5). The weak positive Eu anomaly can be related to the rela-tive richness of these rocks in plagioclase due to the original mag-ma composition.

5. Discussion

The presence in the WAA Cryogenian sedimentary formations ofa calc-alkaline mafic magmatism nearly coeval with tholeiitic andalkaline magmas poses the problem of the source of the magmasand the geodynamic setting of their emplacement. This problemwas addressed by looking at the nature of the sedimentary rocksand by investigating the geochemistry of the magmatic rocks.The Pan-African geodynamic setting of the Anti-Atlas was thenidentified by comparing it with geodynamic settings described inother parts of the world.

5.1. Sedimentary environment of the surrounding Cryogenian series ofthe WAA

The Lower Cryogenian sedimentary environment in which theWAA calc-alkaline magmatism was emplaced evolved from an epi-continental platform (limestones and quartzites) to an ocean basin(schists and stratified quartzites) (Choubert and Faure-Mauret,1973, 1976). This environment may correspond to pre-Pan-Africanrifting at the same time that a Pan-African back-arc oceanic basinwas opening in the Central Anti-Atlas (200 km east of Igherm), asindicated by the Bou Azzer and Siroua ophiolites (Leblanc, 1975;El Boukhari, 1991).

Table 1Representative major (wt.%) and trace (ppm) element analyses of the calc-alkaline mafic rocks from Igherm, Ifni and Kerdous inliers. Analytical methods: major element concentrations were determined by ICP-AES, and trace elements byICP-MS at CRPG-CNRS (Nancy, France). Analytical uncertainties are estimated at 2% for major elements, and at 5% or 10% for trace-element concentrations (except REE) higher or lower than 20 ppm, respectively. Precision for REE isestimated at 5% when chondrite-normalized concentrations are >10 and at 10% when they are lower. LOI: loss on ignition, nd: not determined, <DL: below detection limit.

Ech. Boutonnière Igherm Ifni Kerdous

ITH 3b ITH 3c TAS 1b TAS 3 TAS 4 TAS 5 TAS 7b TAS 8 ISS 2c TAS 16 ISS2 TAL 2 TNW 2 TAS 03 TAS 08 TAS 16b MES1 MES2 CAR2 OSI1 KER 4 KER 7

SiO2 52.62 52.19 50.28 47.20 47.57 50.12 51.71 48.00 51.37 46.40 52.40 52.33 49.10 47.79 47.92 46.84 52.53 51.18 50.42 49.86 54.38 54.45Al2O3 14.85 14.87 15.36 20.64 19.03 16.10 15.51 17.10 16.23 17.10 14.61 15.29 16.39 20.86 17.21 17.47 16.91 16.47 15.60 16.88 18.46 16.60TiO2 1.01 1.02 0.76 1.12 1.41 0.86 0.91 1.38 0.88 1.12 0.68 1.16 0.90 1.14 1.40 1.20 1.30 1.09 1.60 1.43 0.51 1.41Fe2O3t 11.25 11.15 9.80 9.91 9.07 10.96 9.35 12.81 9.16 12.91 10.16 12.20 9.95 10.01 12.94 13.14 7.65 8.87 9.79 10.18 7.35 8.01MnO 0.17 0.17 0.15 0.17 0.15 0.17 0.16 0.22 0.16 0.26 0.17 0.16 0.19 0.18 0.21 0.25 0.21 0.17 0.15 0.31 0.09 0.09MgO 6.05 6.05 5.87 4.72 4.16 5.05 5.41 6.05 6.19 6.83 6.75 5.16 5.58 4.76 6.04 6.85 4.96 7.21 6.57 7.46 5.01 4.67CaO 7.16 7.41 9.41 7.33 9.46 9.28 7.37 7.20 7.59 7.63 8.66 6.77 7.98 7.44 7.35 7.91 4.78 7.71 8.17 5.87 8.56 6.77Na2O 3.4 3.27 4.48 3.72 3.06 2.95 5.25 2.95 3.06 2.91 2.61 2.31 3.21 3.60 2.85 2.87 4.36 2.56 2.23 2.44 2.91 3.94K2O 0.60 0.62 0.34 1.20 1.56 0.30 0.52 0.83 1.88 0.56 0.91 0.96 2.93 1.21 0.83 0.60 3.44 1.58 1.65 2.90 1.07 2.26P2O5 0.14 0.15 0.20 0.30 0.39 0.22 0.34 0.34 0.28 0.29 0.20 0.16 0.26 0.24 0.28 0.21 0.40 0.33 0.68 0.48 0.12 0.30P.F 2.55 2.98 3.16 2.67 2.95 3.81 2.62 2.76 2.75 2.61 2.66 3.34 3.46 2.70 2.89 2.60 3.18 2.90 2.95 2.08 1.34 1.12Total 99.80 99.88 99.81 98.98 98.81 99.82 99.15 99.64 99.55 98.62 99.81 99.84 99.95 99.93 99.92 99.94 99.72 100.1 99.80 99.90 99.80 99.62

ppmBa 227 228 120 519 497 135 180 457 1400 323 200 245 380 496 418 345 531 586 657 751 214 974Be 1.20 1.07 1.00 0.80 1.00 1.29 1.00 1.10 1.00 1.20 1.60 0.81 0.83 1.08 0.52 1.03 1.88 <L.D. <L.D. 1.99 0.55 0.83Co 30 69 36 42 38 44 47 46 28 66 54 48 35 46 53 63 34 39 36 35 24 24Cr 66 62 105 27 31 62 59 73 144 120 98 7 172 23 62 114 85 445 203 156 145 100Ga 16 19 5 9 5 5 20 5 26 18 23 nd 19 23 20 23 20 21 20 22 nd ndCu 51 44 8 6 9 8 7 11 31 8 7 3 2 3 5 5 6 66 45 <L.D. 11 7Ni 56 57 27 50 52 19 26 53 64 68 30 18 64 52 48 60 38 124 101 90 18 39Nb 7.0 6.4 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 6.8 4.5 5.4 5.4 4.9 7.0 6.9 12.8 9.6 2.9 9.6Rb 20 20 13 42 57 12 15 26 74 19 19 20 126 44 26 17 175 46 47 196 36 70Sc 33 33 39 17 21 38 26 23 22 25 41 23 nd 14 22 24 nd nd nd nd 19 12Sr 253 257 232 595 508 338 227 482 168 595 248 220 103 576 441 613 540 519 647 136 339 651V 236 242 181 144 163 225 184 171 167 179 232 286 157 141 170 184 134 155 162 139 105 148Zn 112 127 83 84 80 95 110 66 140 121 130 101 155 95 122 151 203 161 116 624 76 87Zr 96 101 75 82 89 85 123 90 106 71 66 114 117 88 87 79 161 123 257 182 75 153Th 1.80 2.95 5.00 5.00 5.00 5.00 9.00 7.00 7.00 5.00 6.00 nd 2.62 1.27 1.13 1.09 3.86 1.17 2.70 2.50 nd ndTa nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 0.75 0.56 0.82 0.62 nd ndY 20.99 20.42 16.62 16.41 20.26 19.90 18.63 20.75 35.34 29.99 30.40 24.20 21.30 16.24 19.40 16.59 24.15 14.67 27.52 12.71 12.60 9.67U 1.00 0.55 nd nd nd nd nd nd nd nd nd nd 1.88 0.17 0.14 0.16 2.12 0.42 0.90 2.01 nd nd

La 25.65 13.34 12.96 14.65 16.38 13.64 13.06 15.49 17.27 27.77 23.04 13.70 18.55 14.77 14.22 12.81 30.16 17.91 43.07 24.27 13.72 20.96Ce 54.15 29.31 27.30 30.55 36.72 29.08 37.45 34.36 42.66 61.42 48.94 29.71 38.51 31.21 31.55 28.84 69.76 39.82 93.71 39.64 27.11 41.85Nd 32.30 15.57 13.86 17.17 21.26 14.69 28.71 19.76 27.97 34.57 24.25 15.84 21.33 17.15 17.99 16.42 39.73 22.28 48.43 26.24 12.79 21.59Sm 7.67 3.55 3.01 3.60 4.66 3.25 6.90 4.41 6.33 7.49 5.25 3.85 4.38 3.58 3.92 3.36 8.01 4.52 9.02 5.17 2.52 4.67Eu 2.32 1.18 0.91 1.51 1.69 1.03 1.39 1.63 1.96 3.16 1.82 1.13 1.00 1.49 1.43 1.31 2.41 1.63 2.60 1.12 0.98 1.77Gd 7.04 3.46 2.66 2.99 3.65 2.75 4.53 3.77 6.27 6.97 5.09 3.68 3.92 3.28 3.94 3.21 6.20 3.69 6.97 3.71 2.13 3.58Dy 5.62 3.42 2.85 2.95 3.64 3.16 3.55 3.55 6.99 6.22 5.74 3.96 3.46 2.80 2.98 2.68 4.65 2.87 5.38 2.55 2.08 2.21Er 2.84 1.97 1.71 1.58 1.86 1.99 1.94 1.96 4.02 3.44 3.72 2.22 2.07 1.47 1.63 1.55 2.32 1.42 2.66 1.26 1.20 0.85Yb 2.82 1.98 1.85 1.49 1.83 2.26 2.20 1.85 4.18 3.28 4.27 2.30 2.12 1.53 1.82 1.58 2.15 1.30 2.41 1.32 1.29 0.69Lu 0.50 0.31 0.28 0.23 0.28 0.35 0.33 0.29 0.66 0.50 0.68 0.36 0.33 0.24 0.28 0.26 0.33 0.21 0.36 0.21 0.22 0.10P

REE 140.9 74.1 67.4 76.7 92.0 72.2 100.1 87.1 118.3 154.8 122.8 76.8 95.7 77.5 79.8 72.0 165.7 95.6 214.6 105.5 64.0 98.3

(La/Yb)N 6.15 4.56 4.73 6.64 6.05 4.08 4.01 5.66 2.79 5.72 3.65 4.02 5.91 6.53 5.29 5.47 9.46 9.32 12.10 12.47 7.19 20.52(La/Sm)N 2.10 2.36 2.71 2.56 2.21 2.64 1.19 2.21 1.72 2.33 2.76 2.24 2.66 2.59 2.28 2.40 2.37 2.49 3.00 2.95 3.42 2.82(Gd/Yb)N 2.02 1.42 1.16 1.62 1.61 0.98 1.67 1.65 1.21 1.72 0.96 1.29 1.50 1.74 1.76 1.64 2.33 2.30 2.35 2.28 1.34 4.20La/Nb 3.66 2.10 2.59 2.93 3.28 2.73 2.61 3.10 3.45 5.55 4.61 2.03 4.11 2.72 2.64 2.62 4.31 2.60 3.36 2.53 4.81 2.19Ti/V 25.68 25.28 25.19 46.67 51.90 22.93 29.67 48.42 31.62 37.54 17.59 24.34 34.39 48.41 49.51 39.20 57.90 42.12 59.26 61.95 29.14 57.16Y/Nb 3.00 3.21 3.32 3.28 4.05 3.98 3.73 4.15 7.07 6.00 6.08 3.58 4.72 2.99 3.60 3.39 3.45 2.13 2.15 1.33 4.42 1.01

84E.H

.ElA

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African

EarthSciences

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0

0.5

1

1.5

2

0 1 2 3 4

FeO*/MgO

TiO

2(pp

m)

IghermIfniKerdous

A.T

I.T

CATH

Fig. 2. TiO2 vs. FeO�/MgO from Miyashiro and Shido (1975) of the calc-alkalinemafic rocks from Igherm, Ifni and Kerdous inliers. TH (tholeitic), CA (calc-alkaline).The boundaries between the AT (anisotitanous) and IT (isotitanous) domains areredrawn from Bébien (1980).

Fig. 3. Y/15-La/10-Nb/8 diagram (Cabanis and Lécolle, 1989) of the calc-alkalinemafic rocks from Igherm, Ifni and Kerdous inliers. (1) orogenic domain, (2) late topost orogenic intra-continental domain, (3) non-orogenic domain.

0

100

200

300

400

500

600

0 5 10 15 20

Ti/1000

V (

ppm

)

IghermIfniKerdous

Alkaline

MORB

Arc

Ti/V=20

Ti/V=50

Ti/V=10

Fig. 4. V vs. Ti/1000 diagram (Shervais, 1982) of the calc-alkaline mafic rocks fromIgherm, Ifni and Kerdous inliers.

1

10

100

1000

La Ce Nd Sm Eu Gd Dy Er Yb Lu

Roc

k/C

hond

rite

IghermIfniKerdous

Fig. 5. Chondrite-normalized REE patterns (Evensen et al., 1978) of the calc-alkaline mafic rocks from Igherm, Ifni and Kerdous inliers.

E.H. El Aouli et al. / Journal of African Earth Sciences 58 (2010) 81–88 85

5.2. Geochemical characteristics of the calc-alkaline mafic rocks andthe sources of the magmas

The Cryogenian calc-alkaline mafic rocks of the Igherm, Kerdousand Ifni inliers show enrichment in elements with large-ion litho-phile elements (Sr, K, Rb, Ba, Th), depletion of elements with highfield strength elements and rare earth elements, and selectiveenrichment in Th, Ce, P and Sm, which is characteristic of calc-alka-line orogenic basalts (Fig. 6). More or less pronounced negativeanomalies for Nb, Zr, and Ti, and La/Nb ratios of more than 1.6(2.09 to 5.55) are classically considered to indicate ocean subduc-tion (Pearce, 1982) rather than a crustal influence (Thompson et al.,1984). These geochemical characteristics correspond to magmasthat are emplaced above or near a subduction zone, or in continen-tal domains during the first stages of rift or back-arc basin opening(Pearce, 1982).

In these zones, calc-alkaline orogenic rocks are generated bydehydratation of the subducted plate (Gill, 1981; Pearce, 1983; De-fant and Drummond, 1990), which leads to enrichment in the mostsoluble elements (silicon and incompatible LILE elements) in theoverlying mantle wedge. Consequently, this wedge may besubjected to a higher rate of partial fusion (Gallagher and Hawkes-worth, 1992) and thus generate mafic magmas of the calc-alkaline

Fig. 6. MORB-normalized diagram (Pearce, 1982) of the calc-alkaline mafic rocksfrom Igherm, Ifni and Kerdous inliers compared to calc-alkaline and transitionalbasalts from the volcanic arcs of Grenada (Antilles), Bogoslov (Aleutian Islands) andPenguin (Antarctic), (Pearce, 1982, 1983).

86 E.H. El Aouli et al. / Journal of African Earth Sciences 58 (2010) 81–88

association (Pearce, 1983). In these hydrous conditions, the decou-pling mechanism for incompatible HFSE (depleted) and LILE (en-riched) elements may be accentuated by the presence of minorphases, such as ilmenite, titanite, rutile and zircon, which becomestable in the residue (Kay, 1978; Yogodzinski et al., 1994; Saunderset al., 1980; Thompson et al., 1984).

5.3. Comparison with other examples of calc-alkaline magmatism

The calc-alkaline rocks of the Igherm, Kerdous and Ifni inliersshow similar Nb and Ti anomalies to the basalts in the Karoo Prov-ince of South Africa and the Ferrar Province of Australia (Fig. 7);these anomalies have been attributed to contamination of themantle source by fluids from subducted crust (Hergt et al., 1991).The WAA calc-alkaline rocks are also comparable (Fig. 6) withthe calc-alkaline basalts of the transitional volcanic arcs of theWest Indies, the Aleutians and Antarctica (Pearce, 1982), and theyshow similarities with the Eocene and Miocene basalts of thenorthwest USA and southern Canada, where a close association be-tween the calc-alkaline magmatism and extension has been dem-onstrated (Noblet, 1981; Hooper et al., 1995; Morris et al., 2000)(Fig. 8). In this latter setting, the sub-continental mantle, enrichedby prior subduction episodes, may provide a source for the calc-

Fig. 7. MORB-normalized diagram (Pearce, 1982) of the calc-alkaline mafic rocksfrom Igherm, Ifni and Kerdous inliers compared to low-Ti continental flow basalts(low-Ti-CFB) from Australia after Hergt et al. (1991).

Fig. 8. MORB-normalized spiderdiagrams of the calc-alkaline mafic rocks fromIgherm, Ifni and Kerdous inliers (Pearce, 1982) compared to the mafic calc-alkalinemagmatism from the Colville Igneous Complex (NE Washington State, USA) afterMorris et al. (2000).

alkaline magmas without active subduction (Ewart et al., 1992;Hooper et al., 1995).

In the case of the Colville Igneous Complex (NE WashingtonState, USA), the calc-alkaline magmatic rocks show an affinity withthe geochemical signatures of orogenic magmas from subductionzones (Morris et al., 2000), such as the calc-alkaline rocks of theWAA (Fig. 8). Morris et al. conclude that these calc-alkaline rocksresult from a regional extension that caused decompression melt-ing of the continental crust and intrusion of mantle materials. Inthis case, the calc-alkaline geochemical signature of the magmaswould be completely inherited from earlier Proterozoic subduc-tion(s). However, if geochemical characteristics point to the com-position of the source of the granitoids and not to theirgeotectonic setting, conditions of partial melting have to be metfor the considered source to generate magmas (Liégeois et al.,1998) and this depends from the geotectonic setting!

5.4. Coexistence with the tholeiitic and alkaline rocks

The calc-alkaline rocks of the Igherm, Kerdous and Ifni inliersare emplaced in the same geological setting (intruded the LowerCryogenian series as bodies or dykes) and thus appears as nearlycoeval with the tholeiitic and alkaline mafic rocks.

Associations of alkaline and calc-alkaline rocks have frequentlybeen described in the Basin and Range Province of North America,where they are considered to mark extension phases (Fitton et al.,1988; Kempton et al., 1991; Bradshaw et al., 1993; Hooper et al.,1995; Hawkesworth et al., 1995). Evolution from calc-alkaline toalkaline magmatism in an extensional setting is linked to a changein the source of the magma, from a lithospheric source (calc-alka-line magmas) to an asthenospheric source (alkaline magmas), dur-ing the asthenosphere rises (Hooper et al., 1995).

There are two possible explanations for the coexistence be-tween calc-alkaline and tholeiitic-alkaline rocks in the WAA. Thefirst possibility is emplacement in a distal position during a transi-tion period between the oceanization phase and the subductionphase (El Aouli et al., 2001a; El Aouli, 2004), as described for thePan-African orogeny in the Central Anti-Atlas (Leblanc, 1975; ElBoukhari, 1991). In this case, the chemical compositions of themagmatisms would be influenced by the oceanization (tholeiitic,alkaline and/or transitional) and by the subduction (calc-alkaline).The second possibility is decompression melting of lithosphericsub-continental mantle enriched by a probable Palaeoproterozoicsubduction in the WAA (Gasquet et al., 2001, 2004 and Gasquetet al., 2005; Benziane, 2007). The only epsilon Nd data from litera-ture (�11 < eNd < �7; Gasquet et al., 2005) for the Neo-proterozoicHigh-K calc-alkaline magmatism seem to be in accordance with amantle metasomatized by a Palaeoproterozoic subduction.

5.5. Geodynamic setting in the Anti-Atlas

On a regional scale, calc-alkaline mafic rocks are poorly docu-mented in the AA excepted in the WAA where some dolerite dykesthat intrude the neighboring Bas Drâa and Tagragra d’Akka inliershave similar geochemical characteristics (Ikenne et al., 1995) andin the Siroua Massif of the Central Anti-Atlas where dolerite dykespartly derive from a crustal component related to an old subduc-tion (El Boukhari, 1991; Chabane, 1991; Touil et al., 1999). How-ever, this orogenic-like type mafic magmatism is distributedirregularly both across the Western Anti-Atlas (Bas Drâa, Igherm,Kerdous, Ifni and Tagragra d’Akka) and across each inlier, and thereis no obvious argument indicating an association with a subduc-tion zone. This suggests that the magma probably formed in a tec-tonic, magmatic and sedimentary setting undergoing extensionand by the decompression melting of a lithospheric sub-continen-tal mantle enriched by an earlier subduction (Palaeoproterozoic).

E.H. El Aouli et al. / Journal of African Earth Sciences 58 (2010) 81–88 87

Such a Palaeoproterozoic subduction is also suggested by the asso-ciation of calc-alkaline and peraluminous magmas found in theWAA (Gasquet et al., 2004 and Gasquet et al., 2005; Benziane,2007; Mortaji, 2007). The tholeiitic, transitional and alkaline rockswould then be the expression of the earliest magmatic activity ofthis Cryogenian extension, which took place before the meltingreached the lower layers of the lithospheric enriched mantle.

6. Conclusion

Our petrographical and geochemical study reveals the presenceof Cryogenian calc-alkaline mafic rocks in the Igherm, Kerdous andIfni inliers and for most of them an isotitanium character. Theserocks were emplaced before the conglomeratic formations of theUpper Cryogenian and after the tholeiitic mafic rocks that charac-terize the pre-Pan-African rifting.

Although mafic rocks with tholeiitic or alkaline affinities andwith anorogenic and intra-plate characteristics can be interpretedas the expression of an extensional setting marking the pre-Pan-African rifting in these inliers, the occurrence of calc-alkaline mag-mas in an extensional regime is more problematical. We attributethe calc-alkaline character of the magmas to the influence of a Pal-aeoproterozoic subduction zone that contributed to the enrich-ment of the sub-continental mantle. During the extensionalperiod of the Pan-African orogenesis, the mantle would have pro-duced tholeiitic, alkaline and/or transitional magmas before melt-ing (caused by adiabatic decompression) reached the enriched sub-continental mantle, where it would have generated calc-alkalinemagmas.

Acknowledgments

This study was supported by a scientific french-moroccan coop-eration grant awarded by the CNRS-CNRST (#SHS05/07) to EDY-TEM, UMR-CNRS 5204 (University of Savoie, Chambéry) andLAGAGE (University Ibn Zohr, Agadir). We would like to thank M.Ikenne and A. Mortaji for fruitful discussions. We also thank B. Bo-nin and J.P. Liégeois for their constructive review. The comments ofP. Henderson led to numerous improvements in the quality and theclarity of the English text.

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