PETROGRAPHY, MINERALOGY AND GEOCHEMISTRY OF THE …1-2)/Art03.pdf · 2012. 9. 24. · Revista de la Sociedad Geológica ... geoquímica final es debida a la contribución de contaminación

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Revista de la Sociedad Geológica de España, 21(1-2), 2008

The northern domain of the Central AtlanticMagmatic Province (CAMP) is mainly characterized bylarge, NE trending, extrabasinal dykes that occur inboth the eastern and western sides of the Atlantic - theIberian Massif, the Anti Atlas Ranges and the coastalplain of the Canadian Maritime Provinces. Thesedykes, as part of the CAMP, are related to riftingprocesses that lead to the rupture of Pangaea and thesubsequent opening of the central Atlantic (May, 1971).In Southern Morocco, in the central part of the AntiAtlas, this northern domain of the CAMP is representedby a long, more than 200 km, dyke known as the GreatDyke of Foum-Zguid (Leblanc, 1973). Nevertheless, inthe western part of the Anti Atlas, another NE trendingdolerite dyke is labelled as «Hercynian magmatism» inregional maps, despite its general resemblances withthe CAMP’s dykes.

PETROGRAPHY, MINERALOGY AND GEOCHEMISTRY OF THEIGHREM DIABASE DYKE (ANTI-ATLAS, SOUTHERN MOROCCO)

A. Touil1, R. Vegas2, A. Hafid1, R. Palomino2, A. Rizki1, A. Palencia3 and V.C. Ruiz3

1 Département des Sciences de la Terre, Faculté des Sciences et Techniques Guéliz, BP 549, Marrakech, Morocco. [email protected]

2 Departamento de Geodinámica, Facultad de Ciencias Geológicas, Universidad Complutense, 28040 Madrid.3 Departamento de Física de la Tierra y el Cosmos, Facultad de Ciencias Físicas, Universidad Complutense, 28040 Madrid.

Abstract: The Ighrem dyke is one of the two great doleritic dykes of the Central Atlantic MagmaticProvince (CAMP) occurring in the Anti-Atlas Ranges (Southern Morocco). Its chemical compositioncorresponds to quartz-normative tholeiites showing, away from the dyke’s borders, an increase in theSi, Zr and Ce and a decrease in the Fe2O3

*, Ti O2 and Nb contents respectively. The bulk chemicalcompositions, together with those from pyroxenes, substantiate the character of continental tholeiitesfor the rocks forming the dyke. Trace element ratios Zr/Nb = 12 to 17 and (La/Ce)N ~ 1.29 are indicativeof an enriched mantle source akin to that of the E-MORB. Fractional crystallisation is the mainmechanism of differentiation for these rocks and their final geochemical signature is due to thecontribution of crustal contamination and secondary hydrothermal alterations.

Key words: Central Atlantic Magmatic Province, tholeiite, dolerite, Ighrem dyke, Southern Morocco.

Resumen: El dique de Ighrem puede considerarse como uno de los dos grandes diques doleríticos dela Provincia Magmática del Atlántico Central que afloran en la región correspondiente a las montañasdel Anti-Atlas (Sur de Marruecos). Su composición química concuerda con la de las toleitas concuarzo normativo y muestra, desde los bordes al centro, un aumento en el contenido de Si, Zr y Ce yuna disminución en el de Fe2O3

*, Ti O2 y Nb. Las composiciones globales de las rocas de este dique,junto con las de los piroxenos, confirman su carácter de toleitas continentales. Las ratios Zr/Nb = 12-17 y (La/Ce)N ~ 1.29 son indicativas de un manto enriquecido semejante al de E-MORB. Lacristalización fraccionada es el mecanismo principal para diferenciación de estas rocas, cuya signaturageoquímica final es debida a la contribución de contaminación cortical y a las alteraciones hidrotermalessecundarias

Palabras clave: Provincia Magmática del Atlántico Central, toleitas, doleritas, dique de Ighrem, Surde Marruecos.

Touil, A., Vegas, R., Hafid, A., Palomino, R., Rizki, A., Palencia, A. and V.C. Ruiz, V.C. (2008):Petrography, mineralogy and geochemistry of the Ighrem diabase dyke (Anti-Atlas, Southern Morocco).Revista de la Sociedad Geológica de España, 21 (1-2): 25-33.

This dyke, which constitutes the object of this study,is named here as Ighrem after the nearest eponym mainvillage in Southern Morocco. It runs parallel to theLate Jurassic Foum-Zguid great dyke, between AitAbdellah and Taliwine (Figs. 1 and 2), cross-cuttingboth the Precambrian basement exposures and thePalaeozoic cover flexural folds of the Variscanstructures.

So far, this dyke has not been the object of anymineralogical or geochemical study. Nonetheless,according to its geological setting, the Ighrem dyke canbe considered as equivalent of that of Foum Zguid,which is formed by doleritic rocks with tholeiiticaffinity (Bertrand, 1991; Aarab et al., 1994), and forwhich a whole-rock, K/Ar age of 187 ± 4 Ma (Hailwoodand Mitchell, 1971; Bertrand and Westphal, 1977) anda plagioclase, 40Ar/39Ar age of 196.9 ± 1.8 Ma (Sebai et

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A. Touil, R. Vegas, A. Hafid, R. Palomino, A. Rizki, A. Palencia and V.C. Ruiz

al., 1991) have been published. The aim of this paper isto present a mineralogical and geochemical (major,trace and REE elements data) study of the Ighrem dyke.A comparison with related Foum Zguid and Messejana-Plasencia dykes has been also outlined with the purposeof illustrate the main features of this magmatism inrelation with its regional context.

Petrography

The Ighrem dyke is subvertical and extends in aNE direction over almost 200 km, between AitAbdellah and Taliwine with constant thickness ofabout 30 m. It cross-cuts at high angle the different

formations of the western Anti-Atlas ? Precambrian,late Ediacaran (Adoudounian and Taliwinian) andlower Palaeozoic (Fig. 2). Unlike the exceptionallywell-exposed Foum-Zguid dyke, which forms acontinuous crest amid the surrounding Palaeozoicsedimentary rocks, the Ighrem dyke is often coveredby Quaternary deposits and appears segmented andhighly weathered, occupying the thalwegs orientedalong its trace. The best exposures are rare ands i tua ted to the nor thwest of the Ai t Abdal lahboutonnière, where the outcrops contain rather freshrocks.

The Ighrem dyke is composed essentially ofdoleritic rocks and exhibits, as most of the doleritedykes described in the literature (Azambre et al., 1987;Bertrand, 1991; Touil et al., 1999; Hafid et al., 2001), avariety in grain size and textures that can be related tomagma cooling rates, as well as to flow-differentiationeffects. There is no petrological variation along dedyke.

According its grain size, two different facies can bedistinguished:

- A coarse- to medium-grained (1 to 2 mm)main facies that shows a doleritic texture andcontains locally apparent pegmatitoid pocketswith a diameter of about 5 cm. In this facies, asubvertical magmatic layering parallels thedirection of the dyke. The layering is materia-lized by an alternation of dark- and light-co-loured bands which are relatively rich in pla-gioclase. This facies can evolve locally to alight-coloured, metre-sized facies with gab-broic textures (2-3 mm), occupying the innerpart of the dyke.

Figure 1.- Main tectonic units of Morocco and location of the Ighrem(I) and Foum-Zguid (FZ) dykes.

Figure 2.- Simplified geological sketch of the central Anti-Atlas showing the situation of the Ighrem and Foum Zguid dykes. 1: Precambrianformations; 2: Ediacarian (Adoudounian) 3: late Ediacarian (Taliwinian): 4: Cambrian; 5. Ordovician-Silurian; 6. Devonian; 7: post-Palaeozoiccover; 8: Mesozoic dolerite dykes.

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THE IGHREM DIABASE DYKE, SOUTHERN MOROCCO

- A dark-coloured, fine-grained (< 0.5 mm)and centimetre-sized marginal facies that ex-hibits a microdoleritic, frequently porphyri-tic, texture.

The transition between these facies is continuousand progressive. On the contrary, no longitudinalvariation has been observed. Moreover, the twodifferent facies present the same mineralogicalassembly with a weak variation in the proportion ofthe constituent minerals. The primary minerals are ingenera l ra ther wel l p reserved and cons is t o fplagioclase and pyroxene, making up to 90% of therocks . The res t o f the assemblage inc ludestitanomagnetite, ilmenite, apatite, quartz, hornblendeand locally biotite. The secondary minerals comprisealbite, actinolite, chlorite and calcite, and someamount of ferro-hornblende as well.

The rocks show the effects of a heterogeneoushydrothermal transformation, which can eventuallycause the more or less complete obliteration of theprimary mineralogy, mainly at the dyke margins.

Mineralogy

Mineralogical analyses have been performed at theCentro de Microscopía Electrónica Luis Bru of theComplutense University, Madrid, using a JEOL JXA-8900 M electron microprobe. The operating conditionswere an accelerating voltage of 15 kV and ZAF matrixcorrection routine. Counting times for peak andbackgrounds were 20 and 10 s; a 20 mA beam and aspot size between 5 and 1ì were used. As analyticalstandards, those from Jarosevictch et al. (1980) wereemployed.

Primary minerals

PlagioclaseThis is the most abundant phase and its crystals are

arranged in idiomorphic to subidiomorphic laths,conferring an intergranular texture to the rock. Itappears occasionally in the form of zoned phenocrysts,mostly in the dyke margins, within the fine-grainedfacies. Its primary zoning is marked by the alterationminerals sericite and/or epidote. The relativelyunaltered crystals show generally homogeneous,labradorite-type (An62 – An48), compositions (Fig. 3),the most sodic of them corresponding to the mostexternal part of their rims.

ClinopyroxeneIt appears as idiomorphic to subidiomorphic

phenocrysts, as well as small intergranular crystals,both of augite (Wo28-47En21-47Fs16-40) composition(Fig . 4) . P igeoni te occas ional ly occurs e i thercrystallising at its rims or forming isolated smallcrystals.

Chemical variations between the rims and coresof the crystals are significant and characterized by a

progressive decrease in Ti, Al, Na and Mg, and by anincrease of Si, Fe and Ca toward the rims (Table I). Ad i ff e ren t i a t ion p rocess caused by f r ac t iona lcrystallization can explain these variations.

At dyke sca le , there a re no d i ffe rences inchemical composition between clinopyroxenes fromthe coarse-grained facies (gabbroic texture) andthose from the intermediate facies (doleritic texture).However, clinopyroxenes from the fine-grainedmarginal facies show a relatively high Al and Tiaverage content than those from both central andintermediate facies (Fig. 4).

In the Igh rem Dyke , t he aug i t e -p igeon i t eassociat ion is character ised by the absence of

Figure 3.- Projection of the Ighrem dyke feldspars on the Ab-Or-Andiagram. Open circles: coarse-grained plagioclase facies; black circlesfine-grained plagioclase facies; open squares: coarse-grained K-feldspar facies; black squares: K-feldspar associated to micropegmatite.

Table I.- Representative pyroxene analyses of the Ighrem Dyke.

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intermediate compositions between both minerals.This association has been also described in thePlasencia-Mesejana dyke (Andonaegui et al., 2005)and is symptomatic of the continental tholeiites, afeature confirmed by the low Al, Ti and Ca contentsin the augites (Leterrier et al., 1982; Bertrand, 1991).In the Ti vs. Ca and Ti vs. Ca + Na Ti-Ca diagram,which allows discrimination between orogenic andnon-o rogen ic basa l t s , t he ana lysed pyroxenecrystals, (and especially their cores), are clearlyplotted on the field corresponding to non-orogenicbasalts (Fig. 4) The points situated in the field oforogenic basalts correspond to the crystal rims whichare relatively enriched in Ca.

The Lindsly’s (1983) geothermometer, applied tothe pigeonite-augite phase equilibria, for a pressureof 5 Kbar, gave temperatures comprised between1000 and 1100°C. These temperatures are rathercomparable to those obtained in other Mesozoicdolerite dykes (Azambre et al. , 1987; Bertrand,1991; Demant and Morata, 1996).Late crystallisation minerals

AmphiboleThis phase is very frequently present on the

medium- and coarse-grained facies containingmicropegmatites. As in several dolerites described inthe literature (Azambre et al., 1987 and Morata et al.,

1997 and Hafid et al., 2001), the amphibole of theIghrem dyke is a green-brownish amphibole occurringas small idiomorphic to subidiomorphic crystalsclosely related to micropegmatites. The morphologyand chemical composition of this amphibole point outto i t s la te crys ta l l i sa t ion , together wi th f lu idenrichment for the residual liquid. All of the analysedamphiboles are calcic, after the Leake et al. (1997)classification, and characterised by high contents ofalkaline elements (0.51<(Na + K)A< 0.76 a./f.u.) andtitanium (0.15<Ti<0.21 a./f.u.). They correspond toferro-edenite, to edenitic ferro-hornblende and even toedenite (Fig. 5). These relatively high Ti contents inamphibole can be explained by an enrichment of thiselement in the residual liquid, a characteristic oftholeiitic magmas.

Crystal l isa t ion temperatures and pressures ,estimated with the Otten’s (1984), Ti-in-amphibolegeothermometer (T(ºC) =1240*Ti (a./f.u.) + 545), andthe Schmidt’s (1992), Al-geobarometer (P(kbar) = -3.01 +4.76* Altotal (a./f.u.)), range between 809 to 760º C and2.5 to 3 kbar respectively (Table II).

BiotiteIt is scarce and occurs mainly in the coarse central

facies forming small, strongly pleochroic, red-brownish lamellae, always occupying an intersertalposition in association with quartz and hornblende.

Figure 4.- A) projection ofthe pyroxene compositionson the Wollastonite-E n s t a t i t e - F e r r o s i l i t ediagram (Morimoto et al.,1988); B) compositions ofthe pyroxene on the Ti vs.Ca and Ti vs . Ca+Nadiagrams (Leterrier et al.,1982). Black circles:coarse-grained facies;open circles: fine-grainedfacies.


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Systematic chloritisation of the biotite is shown by adecrease of K2O content (7.74< K2O<9.98%). Theamounts of Fe and Ti are high (1.8<TiO2<4.06% and0.63<XFe<0.77%). The high amount of titaniumindicates a rather high temperature of crystallisation.

Quartz, K-feldspar and micropegmatiteBoth phases are ubiquitous, either in isolated

crystals or in association forming micropegmatites, thelatter being profuse in the relatively differentiatedcoarse-grained facies. The individual K-feldsparcrystals display sodic orthoclase (Ab32-51Or42-65)composi t ions, whereas those associated to themicropegmatites are orthoclase (Fig. 3).

Accessory mineralsThe most abundant accessory minerals are

t i tanomagnet i te and i lmeni te , both formingsubidiomorphic to alotriomorphic crystals that fill upthe inter-granular spaces. Apatite is rather common andoccurs as thin needles closely related to the final stagesof crysta l l isa t ion, par t icular ly to the adjacentmicropegmatite and quartz.

Secondary minerals

The secondary minerals that characterize theconditions of the green-schist’s facies, correspond to

chlorite, epidote, albite, calcite and leucoxene. Tothese minerals, amphibole can be sometimes added, itsdistribution and abundance varying from one sampleto another. This amphibole e i ther appears ininterstitial idiomorphic crystals or forms relativelycontinuous aureoles at the rims of the pyroxene. Itcorresponds to ferro-hornblende and actinolitic ferro-hornblende, characterized by relatively low contentsin alkaline elements (0.22<(Na+K)A<0.5 a./f.u.) andtitanium (0.08<Ti<0.18 a./f.u) (Fig. 5).

App l i ca t ion o f t he Ot t en ’s (1984)geo the rmomete r and the Schmid t ’s (1992)geoba romete r t o t hese amph ibo le s y i e ldedcrystallisation temperatures between 600 and 700º Cand pressures lower than 2 kbar (Table II). Thisimplies that the ouralitisation of the clinopyroxene isprior to the hydrothermal late transformations,taking place during a late-magmatic stage (600-700º

Figure 5.- Position of the amphiboles on the Leake’s et al. (1982)diagrams: A) primary amphiboles; B) secondary amphiboles.

Table II.- Representative amphibole analyses of the Ighrem Dyke.


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Figure 6.- Comparison of variations in some major and trace elements in function of SiO2 between the Ighrem dyke (black circles: fine-grained marginal facies , open circles: coarse-grained facies, this study) and the Foum-Zguid dyke (open squares, after Bertrand et al., 1982 andBertrand, 1991).


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C) related to the increase of P 2H O at the end of therock’s crystallisation.

Whole rock geochemistry

For whole rock geochemistry, the collected sampleswere analysed at Activation Laboratories, Ontario,Canada. Sample preparat ion and measure ofconcentration was performed by lithium metaborate/tetraborate fusion-ICP for major elements and by ICP/MS for minor elements. All the elements were analysedunder the control of certified international standardsand the reproducibility is found to be better than 2%.Further details can be found at http://actlabs.com

All the analysed rocks present a ra therhomogeneous andesite basalt-type composition (TableIII): 50.93<SiO2<53.35%; 12.08<Al2O3<12.83%;1 3 . 7 2 < F e 2 O * 3 < 1 6 . 6 5 % ; 3 . 3 5 < M g O < 3 . 9 1 % ;1.51<TiO2<1.64%; 4.36<Na2O + K2O<5.77.

The contents of MgO (3.35<MgO<3.91%) and Niand Cr (generally lower than 20 ppm) suggest theprovenance from a fractionated liquid for the Ighremrocks, while the low MgO content indicates the evolvedcharacter of the Ighrem dyke. The most basic termsoccur in the border facies and the increase in SiO2content toward the central part of the dyke isaccompanied by a decrease in Fe2O*3, TiO2, and Nb, aswell as by an increase in Zr, Y and Ce (Fig. 6). Thevariations in CaO, Na2O, K2O, Rb, Sr and Ba contentsshow a dispersion that cannot by explained by a simplecrystallisation process; instead, they point out to acertain effect of secondary alterations which is relatedwith the higher L.O.I. content of the most alteredsamples. So, to characterize chemically these rocks, weavoided the use of these elements. On the other hand,the observed parallelism among the rare-earths spectra(Fig 7A), in particular between the least differentiated

border and the inner part of the dyke (Fig. 7B),indicates that the main differentiation process for theserocks is the fractional crystallisation.

The available analyses for the analogous Foum-Zguid (Bertrand et al., 1982; Bertrand, 1991) andMessejana-Plasencia (Cebriá et al., 2003) dykes differfrom those for the Ighrem dyke here presented. TheIghrem analyses show a contrasted evolution of thecompositions that is marked by the increase in Fe2O*3,TiO2, P2O5, K2O, Ba, Nb and Ce contents and thedecrease in Al2O3, MgO and CaO during crystallisation.On the contrary, taking into account its constant andlow MgO content , the Ighrem rocks are thuscomparatively richer in Fe2O*3, TiO2, P2O5, and K2Oand trace incompatible elements, and poorer in Al2O3,CaO (Figs. 6 and 7). This means that all three dykesmust belong to same province, although the Ighremdyke represents the most evolved stage.

Nature and geodynamic setting

The early crystal l isat ion of plagioclase andpyroxene, with respect to Ti-Fe oxides and quartz, isconsidered as typical of tholeiitic series. A tholeiiticcharacter is also well documented by the pyroxenecomposition and confirmed by the whole-rock chemicalcomposition. Accordingly, in the FeO*/MgO ratio asfunction of SiO2 diagram (Fig. 8), the studied rocks fallwithin the field defined by Miyashiro (1974) ascorresponding to tholeiitic basalts, while those of FoumZguid and Messejana-Plasencia are astride both fieldsof the calco-alkaline and tholeiitic basalts. This affinityis also confirmed by their position in the Winchesterand Floyd’s (1977) Nb/Y vs. Zr/P2O5 diagram (Fig. 8).The Ighrem rocks present Nb/Y = 0.19 to 0.27, Zr/P2O5= 0.051 to 0.078 and Ti/V = 44 to 49 ratios, similar tothose of Mesozoic continental tholeiites (Dupuy and

Figure 7.- A) Chondrite normalized REE elements (Sun et McDonough, 1989) for the Ighrem rocks. Comparison with the Messejana-Plasencia(MP) and Foum Zguid (FZ) dykes (field between dotted lines); B) Variations of REE contents between the border (sample TZ 14A) and the centre(sample TZ 14G) of the Ighrem dyke. Both samples are distant 12 m.


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Dostal, 1984; Alibert, 1985; Bertrand, 1991; Caroff etal., 1995; Cebriá et al., 2003).

The REE spectra (Fig. 7) indicate that the Ighremrocks are relatively richer in REE than their equivalentones from Foum Zguid (Bertrand, 1991) and Messejana-Plasencia (Cebriá et al., 2003) dykes. This is in part dueto the more evolved character of the Ighrem rocks withrespect to those of the two other dykes. The LREE ratiosare particularly higher in the Ighrem Dyke ((La/Sm)N =2.78-2.98) than in the dykes of Foum Zguid ((La/Sm)N =1.61-2.14) and Messejana-Plasencia ((La/Sm)N = 1.61-

1.88). However, the HREE ratios are comparable ((Tb/Lu)N = 1.06-1.18 in Ighrem and Foum Zguid and 1.23-1.36 in Messejana-Plasencia).

The affinity of the Ighrem rocks with continentaltholeiites is very evident in the MORB normalizedmulti-elements diagram (Fig. 9). The studied rocks arecharacterized, as is common in other continentaltholeiites, by enrichment in highly incompatibleelements (Rb, Ba, Th, K, La, Ce and Zr) and a generallynegative slope of the spectre. In particular, the Ighremtholei i tes show an important enr ichment inincompatible elements and a pronounced negativeanomaly in Nb with respect to the continental tholeiitesof the Foum-Zguid and Messejana-Plasencia dykes.

The strong negative anomaly in the Nb content ofthe Ighrem rocks is rather dependent on the relativelyhigh contents of K2O in these rocks since the contentsin Nb are comparable to those of the Messejana-Plasencia and Foum-Zguid rocks. These high contentsin K2O seem result from the effect of secondaryalterations, as suggested by the strong dispersal ofpoints on the diagram of the figure 6. The relative andweak negative anomaly of the Ti content in the Ighremtholeiites can be explained by the crystallization offero-titaneous oxides during the continuous growing ofthe other incompatible elements in the residual liquids,as shown by the evolution of these elements on thediagrams of figure 6.

Nature of the source and discussion

The rocks of the Ighrem dyke are characterizedby small contents in Cr and Ni (<20 ppm) which

Table III.- Chemical analyses of the Ighrem Dyke. FG: marginal fine-grained facies; CG: central coarse-grained facies.

Figure 8.- FeO*/ MgO vs. MgO (Miyasshiro, 1974); Nb/Y vs. Zr/P2O5 (Winchester and Floyd, 1977); V vs. Ti/1000 (Shervais, 1982)and TiO2 vs. Y/Nb (used by De Kock et al., 2000) diagrams.Comparisons with Messejana-Plasencia (MP) and F o u mZguid (FZ).


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indicate that they derivate from a differentiatedliquid. Chemical variations within these rocks can beexplained by means of a fractional crystallisationprocess. Other factors, difficult to evaluate, such ascrustal contamination and late secondary alterations,could contribute to the final signature of the rocks.

The Ighrem tholeiites are characterized by Zr/Nb =12 to 17 and (La/Ce)N ~ 1.29 ratios, supporting aderivation from an enriched-type E-MORB or OIBsource (Fodor and Vetter, 1984; Sun and McDonough,1989). The important enrichment in LILE and lightrare-earths elements, together with the negative Nbanomaly, poses a difference between the Ighrem rocksand the E-MORB that can be interpreted as a result ofcrustal contamination during the ascent of the magmathrough the continental crust (Dostal et al., 1983;Dupuy and Dostal, 1984; Cox and Hawkesworth, 1985;Wilson, 1989). This is also supported by the high Ba/Zr= 2.1 to 2.12, Th/Nb = 0.51 to 0.75 and Th/La = 0.25 to0.29 ratios (Fitton et al., 1995; Fodor and Vetter, 1984).In the Th/Yb vs. Ta/Yb diagram – used by Pearce(1984) for the characterisation of contamination types– it is possible to consider the Ighrem tholeiiticcompositions as derived from an E-MORB-type magmacontaminated by a schistose Archaean crust (Fig. 10).

Conclusion

A new large, dolerite dyke must be considered as partof the continental tholeiites of the CAMP in the AntiAtlas of Southern Morocco. This dyke has been studiedfor the first time from a petrological point of view and

shows a clear correspondence with the other two largedykes described in the eastern side of the Atlantic –Foum Zguid in the Anti Atlas and Messejana-Plasenciain the Iberian Peninsula.

The bulk chemical compositions of the Ighrem rocks,together with those of their pyroxenes, reassert thecharacter of continental tholeiites for these rocks. Multiplefactors such as differentiation by fractional crystallisation,crustal contamination and secondary alteration shouldcontribute to the final geochemical signature of theIghrem rocks. Its original magma should derivate from anenriched mantle source, similar to that of the E-MORB

Unlike the Foum-Zguid and Plasencia-Messejanadykes, the Ighrem Dyke does not contain olivine in itsmineral assemblage and shows rather homogenouscompositions characterized by a relatively high contentin FeO*3, TiO2, P2O5 and K2O and by a low content inAl2O3, MgO, CaO and incompatible elements. Thissuggests that the Ighrem Dyke represents a more evolvedstage of differentiation than the other two dykes. Inaddition, this may probably indicate a major degree ofcrustal contamination and hydrothermal alteration.

Acknowledgements

This study has been funded by the AECI (Spanish Agencyfor International Cooperation) projects 195/03 and 195/04. Weare also grateful to the Centro de Microscopía Electrónica,Complutense University, Madrid, especially to A. Fernández-Larios.

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Figure 10.- Ighrem dyke plots in the Th/ Yb vs. Ta/Yb (Pearce, 1984)diagram. C: crustal contamination; W: mantle enrichment.Values for E-MORB, N-MORB, OIB and PM after Sun andMcDonough (1989); values for the upper continental crust (UC) andthe schistose Archaean crust (AC) after Taylor and McLeanan (1985).

Figure 9.- MORB normalized diagram showing the position of deIghrem rocks with respect to some geodynamic settings and to therelated Foum Zguid and Messejana Plasencia dykes. Values forprimitive mantle (PM) and oceanic-island basalts (OIB), after Sun andMcDonough (1989); for Foum Zguid after Bertrand (1991) and forMessejana Plasencia after Cebriá et al., (2003).


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Manuscrito recibido el 15 de mayo de 2006Aceptado el manuscrito revisado el 21 de diciembre de 2006


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