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Journal of African Earth Sciences 48 (2007) 301–313

Emplacement of ellipsoid-shaped (diapiric?) granite: Structuraland gravimetric analysis of the Oulmes granite

(Variscan Meseta, Morocco)

Abdelfatah Tahiri a, J. Fernando Simancas b,*, Antonio Azor b, Jesus Galindo-Zaldıvar b,Francisco Gonzalez Lodeiro b, Hassan El Hadi c, David Martınez Poyatos b,

Ana Ruiz-Constan b

a Department of Geology, Institut Scientifique, BP 703 Rabat-Agdal, Moroccob Departamento de Geodinamica, Universidad de Granada, Campus de Fuentenueva s/n, 18071 Granada, Spain

c Department of Geology, Sciences Faculty, Ben Msik-Sidi Othmane, Casablanca, Morocco

Received 16 June 2006; received in revised form 17 January 2007; accepted 20 April 2007Available online 6 May 2007

Abstract

The Oulmes granite is a NE–SW elongated stock intruded in Ordovician metasedimentary rocks which crop out at the core of a regio-nal anticlinorium of the Moroccan Variscan Meseta. The stock, dated at around 300 Ma, is made up of peraluminous two-mica granite,with subordinate amounts of muscovite leucogranite. Mineral composition and textural features of both the granite and the thermalaureole enable us to constrain the P–T conditions during magma intrusion in �250–300 MPa and �600 �C at the contact betweenthe granite and its host rock. Emplacement of the Oulmes granite postdates the main regional structures and is coeval with the late stagesof Variscan compression. Prior to granite intrusion, the host rocks were affected by a main deformation phase responsible for the devel-opment of a penetrative slaty cleavage and regional-scale folds.

Granite emplacement gave way to a 2 km-thick strain aureole, which includes both the uppermost part of the stock and the surround-ing host rocks. The strain aureole is characterized by a gently-dipping planar-linear fabric with a generally weak N–S oriented stretchinglineation. In the granite, the fabric of the strain aureole consists of a solid-state, locally mylonitic, foliation. In the host rocks, a secondaryfoliation develops which crenulates the older regional foliation. Magmatic structures, mainly consisting of steeply dipping N–S orientedmagmatic foliation, are preserved in the central part of the pluton. The 3D shape of the granite has been modelled from gravity data and,together with the observed magmatic flow along subvertical planes and the strain recorded in the aureole, leads us to envisage a diapiric-dominated mechanism for the emplacement of the Oulmes granite. Drawing on the petrogenesis of peraluminous granites, the diapiricascent of the granite probably has not exceeded 10 km.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Granite emplacement; Diapirism; Gravimetry; Variscan orogen; Morocco

1. Introduction

Granites and genetically related anatectic gneiss domesare fundamental expressions of vertical flow in the crust(e.g. Whitney et al., 2004). A main goal of the studies con-

1464-343X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jafrearsci.2007.04.005

* Corresponding author. Tel.: +34 958243353; fax: +34 958248527.E-mail address: simancas@ugr.es (J.F. Simancas).

ducted on granite emplacement is to provide informationon the thermo-mechanical interaction between melts andtheir enveloping crust. Concerning granites, an importantissue is how far diapirism alone can move granitic meltsupwards in the crust, i.e. how far can pure Raileigh–Taylorinstability – assisted only by melt-induced weakness in thehost rocks – cause granitic magma ascent through the crust(e.g. Burov et al., 2003). Vis-a-vis with diapirism, granitic

302 A. Tahiri et al. / Journal of African Earth Sciences 48 (2007) 301–313

magma ascent related to weakness pathways, such as frac-ture conduits, must be considered (e.g. Petford et al., 1993;Clemens, 1998).

Field (geological and geophysical) studies of granitesand their thermal/strain aureoles can be helpful for testingtheoretical models. In this respect, the three-dimensional(3D) shape of granite bodies, as determined from gravimet-ric or seismic surveys, is a significant aspect of fieldresearch. Two end-member 3D shapes can be distinguishedas correlative with different ascent/emplacement mecha-nisms: on the one side, shapes like mushrooms or blobsmight be considered to be consistent with diapiric-domi-nated uprise, though in situ ballooning (Ramsay, 1989)must be considered too; on the other, laccolithic/lopolithicgranite bodies are more probably related to fracture-assisted ascent (e.g. McCaffrey and Petford, 1997).

Small-sized intrusions, such as stocks, usually featuringa single magma pulse, are good preliminary targets in orderto tackle the major controversial issue of batholith building(e.g. Glazner et al., 2004; Zak and Paterson, 2005). Despiterepresenting very different magma volumes accumulatedalong different time-spans, batholiths and stocks mustshare fundamental processes concerning ascent andemplacement of magma. In other words, stocks may beviewed as due to single intrusive events from which a com-plex batholith may have been composed by successiveamalgamation.

This work reports a study of the Oulmes granitic stock.Important constraints on its emplacement in the uppercrust can be derived from the structural analysis of thegranite and its host rock. Furthermore, a gravimetric sur-vey has been performed in order to gain knowledge ofthe geometry of the buried part of the granite. The combi-nation of a 3D geometry for the stock, attitude of the mag-matic foliation, deformation in and around the granitebody, as well as thermal/metamorphic data of the graniteand the host rocks, enables us to propose an emplacementmechanism dominated by diapirism. Our conclusion is atodds with the common view that most of the plutons inthe Moroccan Meseta are emplaced under the control ofstrike-slip structures (e.g. Lagarde et al., 1990).

2. The Oulmes granite and its thermal aureole

The geodynamic setting of the Variscan granitoids ofMorocco has been recently reviewed (Gasquet et al., 1996;El Hadi et al., 2006) and the existence of a regional patternof magmatism has emerged, which can be correlated withother regions of the Variscan orogen in Europe. As a dis-tinctive zone, the Central Palaeozoic Massif of Morocco ischaracterized by the presence of granitoids with peralumi-nous character and other features making them to resembleS-type granites (Mahmood and Bennani, 1984). One of themost representative granitic bodies in this zone is theOulmes granite, which constitutes a stock of 9 · 4 km,elongated in the NE–SW direction. The host rocks areOrdovician metasedimentary rocks (phyllites and meta-

sandstones), which crop out in the core of a regional antic-linorium located in the Central Meseta of the MoroccanVariscides (Fig. 1a). Many decimetre- to metre-scale veinsof quartz (±cassiterite/wolframite) cross-cut both the topof the granite and the host rock; henceforth, these veins willbe referred to as quartz veins. Compositionally, the Oulmesgranite is rather homogeneous and free of xenoliths. Petrog-raphycally, it is made up of two-mica granite (±cordierite),with subordinate amounts of muscovite leucogranite(±tourmaline, ±garnet) (Boutaleb, 1988). Geochemically,the granite is strongly peraluminous and highly potassic.In spiderdiagrams, it shows troughs at Ba, Sr and Ti, whichmay be due to some fractional crystallization of feldsparsand Fe–Ti ores (Boutaleb, 1988; Baudin et al., 2001). Inter-estingly, biotites are strongly aluminous (Boutaleb, 1988).An initial 87Sr/86Sr ratio of 0.7176 has been obtained byMrini et al. (1992); such ratio could have been affected bya late hydrothermal opening of the system, but the authorstake for granted an initial 87Sr/86Sr ratio over 0. 710. Thus,petrographic, geochemical and isotopic data all togetherpoint consistently to a crustal origin, from a peraluminoussource (most probably, metasediments), for the Oulmesgranite. The age of this granite is latest Carboniferous:298 ± 3 Ma (Rb/Sr whole-rock); 296 ± 3 Ma (SHRIMPU/Pb on zircon,); 308 ± 8 Ma (U/Pb on monazite) (Mriniet al., 1992; Baudin et al., 2001).

The stock is surrounded by an aureole of thermal meta-morphism with an approximate thickness of 1.3 km(Fig. 1). The mineral association in the internal zone ofthe thermal aureole is: quartz, muscovite, brown-red bio-tite, andalusite, ±cordierite, ±tourmaline (tourmaline isvery abundant locally, forming tourmalinites); sillimanite(fibrolite), staurolite and garnet have been reported as rareoccurrences (Baudin et al., 2001). Textural relationshipsindicate that porphyroblast growth in the hornfelses wassyn-kinematic with respect to a flat or gently-dipping cren-ulation foliation developed in the aureole as a consequenceof granite intrusion (see next section).

The depth of granite emplacement can be roughly esti-mated (Fig. 2) by taking into account the following data:(i) the stability of andalusite in the innermost zone of thecontact aureole (sillimanite is exceptional); (ii) the presenceof cordierite in the aureole (garnet is rare) and (iii) the for-mation of scarce staurolite. Fig. 2 shows several experimen-tally calibrated reactions that help constraining the P–Tconditions at the level of granite emplacement, even thoughthe stability fields of garnet, cordierite and staurolite (reac-tions 4 and 5 in Fig. 2), as well as those of andalusite andsillimanite (reactions 3 and 6 in Fig. 2) cannot be consid-ered to be neatly defined. However, a small area in theP–T space can be depicted in which the observed petrogra-phy reasonably agrees with experimentally calibratedpetrogenetic grids. These conditions are 250–300 MPa (8–10 km-deep) and �600 �C at the contact between the gran-ite and its host rocks (Fig. 2). The presence of igneousmuscovite is not a further constraint because it has beenreported at pressures lower than experimentally predicted,

Oulmès

Tarmilat

68

75 7277

8070

42

65

80 70

70

60

4047

5348

35

33

25

10 7

45

4544

30 20 550

6460

88

36

80

5535 32 30

15 30

35

8

4535

30

15

5

14

15

18

30

8

43 35

33

65

75

75

20 17

23

30

30

30

18

5

44

65Bou

Lahm

a

e

yel

Ri v r

73

Pliocene-Quaternary volcanics

Oulmès granite

Or 1

Or 1Or 2

Or 1

Or 2

Or 2Or 3

Or 3

S-D

Or 3S-D

Or 2Or 3

Or 1

Or 1

Lower OrdovicianMiddle OrdovicianUpper OrdovicianSilurian-Devonian

Boundary of the thermal aureoleBoundaries of the strain aureoleAxial trace of F1 folds

(S ) bedding0

Local vergence of F1

(S ) strain aureole foliation in the host rockA

(S ) strain aureole foliation in the graniteS

stretching lineations associated to Sidem with sense of top movement

A Sand Sa)b)

(S ) Magmatic foliationM

0 1 2 km

NPlunge of F1 folds

12

4

3

1 Regionaldeformation

2 Strain aureolein host rock

3 Strain aureole ingranite

4 Granite below thestrain aureole

b c

I

II3325´

0

6 5´0

R

i f

8º W

High Atlas

Anti- Atlas

RabatAtlantic Ocean Mediterranean Sea

MoroccanMeseta

200 km30º N

a

Oulmèsarea

Lithological contacts

Fig. 1. (a) Location of the Oulmes area in the Moroccan Variscan Meseta. Palaeozoic and Neoproterozoic outcrops have been depicted in grey. (b)Geological map of the Oulmes granite and its host rock. I–II marks the cross-section shown in Fig. 4. (c) Sketch of the four deformation zonesdistinguished in/around the Oulmes granite. See text for further explanations.

Fig. 2. Estimated P–T conditions (grey ellipse) for the hornfelses of theinnermost thermal aureole. Experimentally calibrated mineral reactionsafter: (1) Willye (1977); (2) Chatterjee and Johannes (1974); (3) Holdaway(1971); (4) Holdaway and Lee (1977); (5) Richardson (1968). Theandalusite/sillimanite equilibrium (reaction 6) is after Pattison (1992).Ruled areas represent andalusite/sillimanite and cordierite/garnet transi-tions. Downward pointing arrow indicates an approximate P–T path forthe Oulmes granite, from the source region to the level of emplacement.See text for further explanations. Mineral abbreviations: ALS: aluminiumsilicate; AND: andalusite; CD: cordierite; GR: garnet; KF: k-feldspar;MS: muscovite; QZ: quartz; SIL: sillimanite; ST: staurolite.

A. Tahiri et al. / Journal of African Earth Sciences 48 (2007) 301–313 303

either because muscovite may be a relict phase or becauseminor components can slightly lower the magma solidusor change the muscovite stability field (Zen, 1988).

3. The structures in the Oulmes granite and its host rock

The outcrop of the Oulmes granite has a NE–SW elon-gated dome shape, with the boundary contacts of the gran-ite systematically dipping 20–45� towards the host rocks(Figs. 1b and 3a). A strain aureole can be observed whichaffects both the top of the granite itself and the host rocksurrounding the granite; the aureole has the same domalshape than the granite (Fig. 1b). The main structure inthe strain aureole is a solid-state foliation in the graniteand a crenulation foliation in the host rocks (Fig. 3b–e);these two foliations parallel each other and the boundarycontact of the granite (Fig. 2). Interesting observationson the deformation around the Oulmes granite were madeby Pique (1976), while Diot et al. (1987) provided the mostcomplete set of data on structures in the granite, as well asa model of emplacement that will be discussed later.

The strain aureole in/around the granite obliterates pre-vious structures both in the host rocks (mainly a regional

Fig. 3. (a) Outcrop view of the granite/host rock contact. (b) Foliation SA in the strain aureole, showing refraction at the contacts between a quartz veinand the metapelitic host rock. (c) Microscope photography showing the main regional foliation (S1) inside microlithons bounded by the strain aureolefoliation (SA). The growth of andalusite (And) is syn-kinematic with respect to SA. (d) Elongated andalusite crystals on the SA foliation. Note N–Spreferred orientation. (e) Solid-state foliation SS, affecting the top of the granite. (f) Steeply-dipping magmatic foliation SM, defined mainly by K-feldsparphenocrystals.

304 A. Tahiri et al. / Journal of African Earth Sciences 48 (2007) 301–313

foliation) and the granite (a magmatic foliation). Accord-ingly, four different zones can be distinguished as follows(Fig. 1b and c):

(1) The Palaeozoic metasediments cropping out far fromthe granite are characterized by bedding (S0) affectedby a slaty cleavage (S1), which is the axial-plane foli-ation of minor- and regional-scale F1 folds; F1 and S1

represent the main regional deformation. Locally, adisjunctive cleavage appears associated with late openfolds.

(2) The sudden, though progressive, appearance of acrenulation foliation (SA) in the host rock character-izes the strain aureole around the granite. SA is a foli-ation with low/moderate dip that crenulates ortransposes S0 and S1 (Fig. 4a); as a result, S0 and S1

eventually become microscopic relics at the interiorof thin microlithons (Fig. 3c). In previous studies(Pique, 1976; Aıtomar, 1985; Diot et al., 1987) SA

has been misinterpreted as S1 passively rotatedaround the granite, instead of as a new foliationrelated to granite emplacement. On the SA surfaces,a mineral lineation (LA) is sometimes defined by pre-ferred orientation of syn-kinematic andalusite prisms(Fig. 3d). Near the granite contact, abundant quartzveins are observed with steeply dipping attitude; theveins are affected by SA (Fig. 3b). At some places, lateopen folds and a disjunctive cleavage overprint the SA

crenulation cleavage.(3) The topmost (external) part of the granite is also

affected by a solid-state foliation (SS) (Figs. 1b and3e). Feldspars, micas and quartz show evidence of

Fig. 4. (a) Cross-section of the Oulmes granite and its host-rock (see location in Fig. 1b), including the observed relationships between bedding (S0), mainregional foliation (S1) and strain aureole foliations (SA in the host rock, SS in the granite top). Lower hemisphere equal-area projection of: (b) beddingpoles, (c) intersection lineations and microfold axes and (d) poles to the SA aureole foliation.

A. Tahiri et al. / Journal of African Earth Sciences 48 (2007) 301–313 305

plastic deformation (mantled porphyroclasts, micafishes, elongated quartz grains), and the granite takesthe appearance of an orthogneiss. Usually, there is astretching lineation (LS) trending N–S, mainlymarked by quartz elongation. The foliation SS inthe granite carapace shows low-moderate dips, beingeverywhere parallel to the foliation SA in the hornfel-ses (Figs. 1b and 4a), and the lineation LS has thesame N–S trend than the lineation LA in the hostrock. This parallelism indicates that these structuresare contemporary and define the strain aureole asso-ciated with the granite emplacement. Quartz veins,together with aplite dikes, are also found as in theadjacent hornfelses.

(4) The central granite outcrop is below the strain aure-ole, showing a substantially different fabric fromthe one in the topmost part. It can be recognized herea subtle foliation (SM) defined by the preferred ori-entation of unstrained crystals of feldspar andbiotite, surrounded by a matrix of unstrained quartz(Fig. 3f).

3.1. The structure over the Oulmes granite

The geological cross-section of the Oulmes area shownin Fig. 4a is based on our structural observations, coupledwith supporting evidence from geological mapping (Baudinet al., 2001). The macrostructure over the Oulmes granite

consists of an asymmetric, moderately inclined, south-east-vergent anticline (F1 fold; Fig. 4a–c) with an associ-ated axial-plane slaty cleavage (S1). The steep limb of theanticline has been intensely deformed on top of the granite.The F1 anticline has been folded as a whole by an opendome cored on the granite (Fig. 4a and d). This open foldrepresents a second generation of regional structuresrelated to further (but moderate) subhorizontal shortening,as attested by the local development of minor open foldsand spaced foliation compatible with the dome structure.

The Oulmes area is bounded by two faults located to theSE and NW, partially shown in Fig. 1 (Baudin et al., 2001).The NW fault (Rhilat fault) has a NE–SW strike anddownthrows the NW block (Fig. 4a). The SE fault (Oulmesfault) strikes NNE–SSW and downthrows the SE block.Thus, the Rhilat and Oulmes faults make the Oulmes areaappear as a horst (Fig. 4a). Nevertheless, the full kinemat-ics of these faults seems to include dextral displacementtoo (Baudin et al., 2001). Actually, the Oulmes fault isconsidered to be a branch of the kinematically complexSmaala–Oulmes fault system (Tahiri, 1994; Hoepffneret al., 2006).

The F1 southeast-vergent anticline is a major structureof the main phase of Variscan deformation in the region.Its age is constrained between the Westphalian A, whichis folded, and the Westphalian D, which lays unconform-able over the folded rocks (Tahiri, 1994). Considering thatthe most probable age for the Oulmes granite is 300 Ma

306 A. Tahiri et al. / Journal of African Earth Sciences 48 (2007) 301–313

(see radiometric ages above), the stratigraphic dating ofthe F1 deformation at around 310 Ma is consistent withthe geometric and microstructural evidence, which showsthat the granite emplacement deforms the F1 anticline(Fig. 4a). This interpretation is at variance with previousones considering (i) the emplacement of the granite as syn-chronous with respect to F1 folding and, therefore, (ii) themain foliation in the strain aureole (SA) as equivalent to S1

(Aıtomar, 1985; Diot et al., 1987).

3.2. The strain aureole

The thickness of the strain aureole is about 2 km, with1.3 km developed in the host rock and 0.7 km in the granite(Figs. 1b and 4a). The upper/external boundary of thestrain aureole coincides with the external boundary of thethermal aureole, as defined by the appearance of slates withbiotite spots. This coincidence points to strain softeningdue to temperature increase in the host rocks. The aureolefoliation SA in the host rock moulds around the top of thegranite (Figs. 1b and 4d).

The structural elements in the strain aureole are sketchedin Fig. 5a. Beds have been deformed by strain and rotation(see also Fig. 4a). The N–S stretching lineation (LS), which

Fig. 5. (a) Sketch of the structural features of the host rock and the upper partlineation in the granite carapace. In the lower diagram, from Diot et al. (1987showing the strike of quartz veins (80 data). Lower part: poles to quartz veins;hemisphere projection of magmatic foliation (SM) planes. In the lower diagram6.4%.

is observed on SS planes of the granite, is parallel to the lin-eation (LA) defined by andalusite crystals in the host rock,and perpendicular to boudin axes (Fig. 5a and b). Neverthe-less, LS is poor or not developed at some areas, in agreementwith the rare occurrence of chocolate tablet boudinage in thehost rock. The preferred orientation of andalusite crystals inthe hornfelses can be used to calculate the local strain ellipseon the SA planes (Ramsay and Huber, 1983). Unfortunately,suitable outcrops are scarce and the computed X/Y ratio of2.2 in one outcrop might not be representative of the strain inthe aureole. The majority of veins and dikes are steeply dip-ping and E–W striking, but there is some dispersion (Fig. 5c).We interpret the dispersion of veins as indicative of: (i) finitepositive extension in a range of orientations, though withmaximum extension along the N–S direction; (ii) line rota-tion under strain (see next paragraph). As shown in Fig. 5band c, our structural data are basically in agreement with cor-responding sets of data from Diot et al. (1987). The onlynoticeable discrepancy is in the orientation of dikes, withmaximum E–W frequency from our data against maximumESE–WNW frequency from Diot et al. (1987).

The coincidence between the upper/external boundariesof the strain aureole (appearance of SA) and the thermalaureole (biotite-in isograd) (Fig. 1b), together with experi-

of the granite. (b) Lower hemisphere equal-area projection of the stretching), there are 32 data, with contours at 3–6–9–12%. (c) Upper part: diagram176 data and contours at 0.5–1–2–3–5% (from Diot et al., 1987). (d) Lower, from Diot et al. (1987), there are 62 data, with contours at 1.6–3.2–4.8–

Fig. 6. Lower hemisphere stereoplot of minor fault planes in the Oulmesgranite. These faults are thought to be active at the end of graniteemplacement or shortly after. See text for further explanations.

A. Tahiri et al. / Journal of African Earth Sciences 48 (2007) 301–313 307

mental and theoretical considerations on plastic strength ofquartz-rich rocks (e.g. Carter and Tsenn, 1987; Ranalli,1995), suggest that 400 �C may have been a threshold tem-perature for massive ductile flow in the aureole. Modelingof the thermal evolution around a magmatic intrusionshould take into account the rate of magma supply (e.g.Yoshinobu et al., 1998) but simpler models considering sin-gle magmatic intrusions can be applied to small plutons(e.g. Delaney, 1987). Here, we assume that: (i) the Oulmesgranite represents a single magma batch, (ii) the Oulmesgranite is 7 km wide (see Section 5), (iii) the initial temper-ature of the granite was 700 �C, (iv) the regional temper-ature of the country rock was around 300 �C and (v) heattransfer was made by conduction. With these assumptions,thermal models suggest that temperatures above 400 �Ccould be sustained for around a million years in most ofthe thermal aureole around the Oulmes granite, with thepossible exception of the most external parts, where criticalmaximum temperatures of 350–400 �C would still be main-tained for hundreds of years. Thus, in the host rock, sus-tained temperatures above those critical for plasticstrength would promote deformation by ductile flow atlow differential stress, which could be an efficient way ofcreating room for magma emplacement, as discussed later.

3.3. Magmatic flow

The central part of the Oulmes granite is not affected bythe solid-state foliation SA (Fig. 1b). However, the fabric inthe central part of the granite is not isotropic, thus showinga preferred shape orientation of unbroken and unstrainedcrystals of feldspar and biotite (Fig. 3f), in a matrix of ran-domly oriented quartz grains. Therefore, a planar mag-matic fabric (SM), related to magmatic flow, can bedefined. The attitude of SM is very different from the oneof the tectonic foliations SS (in the granite) or SA (in thehost rock) (see Fig. 1b and compare Figs. 4 and 5d): mostof the SM planes dip steeply and concentrate near the N–Sstrike. There are, however, some outcrops with intermedi-ate features, where the magmatic foliation is slightly foldedby a solid-state deformation (‘‘deformation a l’etat

intermediaire’’; Diot et al., 1987). We have no reliable dataabout the magmatic lineation (direction of flow) carried bythe SM planes, but Diot et al. (1987) argue that it was dom-inantly subhorizontal. We interpret this pattern of mag-matic structures as evidencing subvertical planar N–Smagmatic flow, with the direction of flow probably rotatingfrom vertical in the buried granite root, to subhorizontalnear the roof of the granite.

4. Regional stress during the emplacement of the oulmes

granite

Regional folds and thrusts of Late Carboniferous age inthe Central Meseta of Morocco strike NNE–SSW, suggest-ing that the dominant regional stress at that time was char-acterized by a WNW–ESE oriented major compressive

stress (r1), with vertical minor compressive stress (r3). Asdiscussed in Section 3.1 and shown in Fig. 4, the Oulmesgranite intruded some time after the development ofthese major structures. Thus, a question rises relative tothe regional stress by the time of granite emplacement.Some information is provided by the following pieces ofevidence:

(i) According to the most frequent trend of quartz veins(Fig. 5c), the dominant trend of r3 would have beenapproximately N–S.

(ii) The NE–SW oriented open dome over the granite is astructure indicating some horizontal WNW–ESEshortening, since it develops minor open folds(Fig. 4a). The precise timing of this mild deformationis uncertain, but its strong spatial association with thegranite suggests that it occurred near to or during thegranite emplacement.

(iii) Inside the granite, faulting on the walls of somequartz veins indicates normal and reverse displace-ments (Fig. 6). A palaeostress analysis with the SearchGrid method (Galindo-Zaldıvar and Gonzalez-Lode-iro, 1988) indicates that the dominant normal-slipfault set (Fig. 6) is compatible with NNE–SSW exten-sion (r3 25�! N200�E) and a palaeostress ellipsoid(r2–r3)/(r1–r3) = 0.98; therefore, the values of thevertical and the NW–SE stress axes were similar. Onthe other hand, the reverse slip component of a fewfaults (Fig. 6), which are not compatible with theabove palaeostress ellipsoid, points to a compressivestage with NW–SE oriented subhorizontal r1 axis.This compressive stage could be the one giving wayto the open dome over the granite.

As a whole, the above data suggest that granite emplace-ment took place under mild regional WNW–ESE compres-sive stress, after the development of the main regionaldeformation. Minor normal faults inside the granite andthe major normal faults bounding the Oulmes area(Fig. 4a) seem to be subsequent structures, indicating the

308 A. Tahiri et al. / Journal of African Earth Sciences 48 (2007) 301–313

complete decline of regional tectonic compression. Thus,the granite emplacement was definitely late-tectonic.

5. Gravimetric data and 3D model of the oulmes granite

The analysis of the negative Bouguer anomaly of granitebodies is a powerful tool to investigate their 3D shape (e.g.Vigneresse, 1990). In the region around the Oulmes granite,the regional Bouguer gravity values (Fig. 7a) progressivelydecrease southwards from values near 0 mGal on thethinned continental crust of the Saıss basin down to values

Measurement stations Profi

Profile A

Profile B

-24

-20-28

-32

-28

-36

-40

-40-44

-44

3708

3706

3704

3702

3700

3698

766 768 770 772

0

0

0

-5

TIDDAS

-25

-50 Bouguer anomaly( mGal)

OULMES

6.5ºW 6ºW

33ºN

33.5ºN

Tarmilat

Profi

leC

Tarmilat

KHENIFRA

a

b

Fig. 7. (a) Bouguer anomaly map of a wide area around the Oulmes granite, sh1981). (b) Bouguer gravity anomaly map of the Oulmes granite with locatiopositioned.

lower than �125 mGal on the High Atlas thickened conti-nental crust, as shown in the 1:500 000 and 1:200 000 Bou-guer anomaly maps of Morocco (Van den Bosch, 1981).The Oulmes granite is marked by a negative residual anom-aly of �25 to �30 mGal superimposed on that regionalanomaly (Fig. 7a). This local anomaly was roughly mod-elled by Van den Bosch (1981) as a half-spherical granitebody reaching 11 km-depth.

In order to further constrain both the shape and thedepth of the granite, we have performed a more detailedgravity survey in this area. Gravity measurements were

les Granite boundary

mGal

-16

-20

-24

-28

-32

-32

-36

-36 -40

-44

-48

774 776 778

20 40 km

N

N

-75

-75

0

-100

5.5ºW 5ºW

Oulmes

AZROU

IFRANE

owing the NW–SE gradient of the regional anomaly (after Van den Bosch,n of the measurement points. Profiles A, B and C in Fig. 8 have been

A. Tahiri et al. / Journal of African Earth Sciences 48 (2007) 301–313 309

done following an irregular grid defined by the main roadscovering the granite outcrop and surrounding areas, exceptfor the southwestern border where a deep and abrupt valleyhampers to obtain accurate gravity data (Fig. 7b). Mea-surement points were positioned by GPS and a barometricaltimeter with precision of 0.5 m. Measurements were donein cycles of less than 3 h in order to allow linear correctionof gravimeter and altimeter drifts. Gravity values wereobtained with a Master Worden gravimeter, with tempera-ture compensation and precision of 0.01 mGal. The altim-etry at the base station has been obtained from thetopographic map, the real altimetry of each station beingfinally obtained after drift correction of barometric altime-try data. We have calibrated our relative gravity data withthe B90 measurement station of the Gravimetric Map ofMorocco (Rabat sheet), which is located in the eastern bor-der of the granite. Thus, we have obtained absolute gravitydata for each measurement point, which, in turn, were usedto calculate the Bouguer anomaly. A standard density forcrustal rocks of 2.67 g/cm3 was applied for Bouguer correc-tion. Moreover, a topographic correction has been per-

0

-2

-4

-6

Residual Anomaly Observed

Residual Anomaly Calculated

-6-8

-2

-22

-18

-10-1

Profile A

km0 2 4 6 8 10 12 14

NW SE

2.60 g/cm3

2.73 g/cm3

Granite

Metapelites

Fig. 8. Residual gravity anomaly models for three profiles (A, B and C) of the O2 1/2 D with a half-strike length of 5 km. See text for further explanation.

formed in order to appropriately reflect the effects of thedeep valley in the southwestern part of the granite, as wellas the smoother physiography in other parts of this area.This correction has been calculated for a radius of 22 kmof topographic influence, on the basis of a Digital Eleva-tion Model with approximately 90 m of pixel resolution,recently released by the NASA. The irregular grid of grav-ity measurements was interpolated using the krigingmethod in order to draw the Bouguer anomaly map.

The area investigated shows a NE–SW elongated nega-tive Bouguer anomaly, with minimum values of �45 mGalon the granite outcrop (Fig. 7b). The southwestern bound-ary of the anomaly cannot be established in detail since thesteep topography makes it impossible to obtain accurategravity data. These new gravity data provide details onthe Bouguer anomaly map, but the residual anomaly needsto be determined by taking into account the regional Bou-guer gravity data (Fig. 7a). According to the regional datagiven by the 1:500 000 and 1:200 000 gravity maps of Mor-occo (Van den Bosch, 1981) the residual anomaly in theOulmes granite results to be of about �25 mGal. This

0

-2

-4

-6

Dep

th

km

Res

idua

lAno

mal

y(m

Gal

)D

epth

km

6

4

Res

idua

lAno

mal

y(m

Gal

)

Profile B

-24

-22

-18

-20

-12-14-16

km0 2 4 6 8

w E

0 2 4 6 8km

0

-2

-4

-6

Dep

th

k

mR

esid

ualA

nom

aly

(mG

al)

-24-22

-18-20

-12-14-16

Profile C-10-8NESW

ulmes granite (see location in Fig. 7). Profiles A and B are 2D. Profile C is

310 A. Tahiri et al. / Journal of African Earth Sciences 48 (2007) 301–313

residual negative anomaly, characterized by a smooth var-iability of values, indicates that the granite is deeply rootedand NE–SW elongated, parallel to the regional structuraltrend.

No direct density data of the granite and the host rocksare available. However, the leucocratic composition of thegranite points to a low value among the range of granitedensities. Thus, a minimum density of 2.60 g/cm3 may bereasonably considered for the granite and a maximum den-sity of 2.73 g/cm3 for the metapelitic host rocks. Gravitymodelling considering lower density contrasts would yieldbodies with a bigger size than our model.

Gravity modelling was performed with the GRAVMAGsoftware (Pedley et al., 1993). The 2D residual gravitymodel of two transects orthogonal both to the elongatedOulmes granite outcrop and the gravity anomaly (Fig. 7,profiles A and B), indicates approximately circular cross-sections, reaching down to 7–8 km-depth. The deepest floorcould be a narrow flat surface, as suggested by the shape ofminima in the anomaly profiles (Fig. 8, profiles A and B).Additionally, we have modelled the geometry of the NEend of the granite body along a profile parallel to theNE–SW elongation of the granite outcrop and the gravityanomaly (Fig. 7, profile C). This geometry has been estab-lished through a 2 1/2D residual gravity model with 5 kmof mean half-strike, constrained by the two orthogonalprofiles A and B. This third section (Fig. 8, profile C) sug-gests that the granite extends with shallow dip for 2 kmbeyond its outcropping contact with the host rocks. Con-sidering the gravity profiles A, B, C and the outcroppingsurface of the granite, a chunky ellipsoidal shape isdeduced for the Oulmes granite pluton (Fig. 9b). Modelswith thinner shapes would not fit the observed anomalyprofiles.

Fig. 9. (a) The Oulmes granitic body positioned at its crustal level ofemplacement. Main flow lines in host rock and granite during emplace-ment are suggested; this flow might solve the room problem. (b) Sketch ofthe complete 3D shape of the granite, as defined by the gravity-modelledsections and the outcrop section.

6. Discussion

In order to present an emplacement model for theOulmes granite, the most relevant points to be consideredare the following:

(i) The Oulmes granite deforms the main regional folds(Fig. 4), its intrusion having taken place in a moder-ately compressive context, under declining regionalVariscan compression.

(ii) Gravimetric modelling suggests a 3D geometry forthe Oulmes granite consisting in a NE–SW orientedellipsoid or cylinder, reaching down to 7–8 km-depth(Fig. 8). In our model, a reasonably low value ofgranite density has been chosen, mainly on thegrounds of petrographic composition, which meansa rather high density contrast with the host rocks.Accordingly, the inferred granite volume and depthcan be considered as minimum values.

(iii) The magmatic foliation in the lower part of the gran-ite indicates magmatic flow on N–S oriented subver-tical planes (Figs. 1b and 5d). This flow pattern canbe interpreted as magma ascent interacting withmoderate regional tectonic compression: the domi-nant regional stress state would have been: r1

WNW–ESE, r3 NNE–SSW and r2 vertical, in goodagreement with the N–S strike of magmatic foliationand the N–S trend of the magmatic flow direction.

(iv) A 2 km-thick strain and thermal aureole on top ofthe granite resulted from the interaction between thegranite and the host rock during the arrest of theintrusion. The strain aureole consists of a shell ofsolid-state ductile deformation in the granite and asurrounding carapace of highly strained host rock. Itshows moderate stretching in the N–S direction, whichwe interpret once again as resulting from the interac-tion between magma emplacement and mild regionalcompression. Unroofing by stoping was virtuallyzero, as attested by the total absence of xenoliths.

(v) Drawing on the strain and thermal evidences on theroof of the granite, we assume that an intense ther-mal aureole and coupled deformation would developat the walls of the intrusion.

Diot et al. (1987) emphasized the N–S magmatic flowand the N–S solid-state stretching, believing these featuresto be inconsistent with the radial pattern expected in theupper part of a diapir. This is why Diot et al. (1987)rejected diapirism and favoured emplacement controlledby regional strike-slip faulting. Let us discuss this interpre-tation in order to sustain our alternative.

First, we stress that the modelled 3D shape of the gran-ite (Figs. 8 and 9) does not fit easily with the theoretical andexperimental shapes of bodies controlled by strike-slipshearing (Roman-Berdiel et al., 1997). Second, we remarkthat N–S stretching is only moderate, with stretching line-ation varying from strong to faint and boudinage giving

A. Tahiri et al. / Journal of African Earth Sciences 48 (2007) 301–313 311

usually elongated boudins but sometimes chocolate tabletstructure. Indeed, we find that both the 3D shape of thegranite and its magmatic and solid-state flow pattern areconsistent with a diapir in a context of mild compressiveregional stress. Third, we have not found adequate sup-porting field evidence for a regional strike-slip shear zonewrapping up the granite; (i) gently plunging stretchinglineation has been observed only on top of the granite,but it is at the walls where such structure could be takenas indicative of strike-slip kinematics, (ii) increasing strainis restricted to and moulds around the pluton, withoutlinkage of this strain aureole with a regional shear zone;(iii) Recent mapping (Baudin et al., 2001) and our ownobservations do not confirm the curvature suggested byDiot et al. (1987) for the fold axial traces around the gran-ite, as if they entered a shear zone. To summarize, we con-tend that neither the 3D shape of the granite fits well withstrike-slip control nor compelling structural evidence existsat the outcrop for such control. Therefore, we prefer tointerpret the Oulmes stock as a diapiric-like body con-strained by mild regional compression, with shortening inthe direction WNW–ESE and stretching along the NNE–SSW direction. Diapiric emplacement carries with it somedegree of inflation at the emplacement level, which makesdiapirism and ballooning-type inflation two related mecha-nisms hard to be differentiated (Weinberg and Podladchi-kov, 1995). Nevertheless, in the Oulmes granite we findthat: (i) the magmatic foliation and the solid-state foliationare not parallel, but form high angle (Fig. 5a); (ii) the shellof solid-state deformation in the granite is too thin (lessthan 1 km) to account for the radial strain implicit in thetype of inflation envisaged by Ramsay (1989). Thus, we dis-card in situ ballooning-type inflation, in the strict sensedefined by Ramsay (1989). All the above arguments referto the way by which the magmatic body was emplaced inthe upper crust, and do not preclude any process relatedto the generation of magma at deeper structural levels.

Let us enlarge now the discussion towards the morespeculative issue of granite ascent. Experimental and theo-retical evaluation of the strength on metasedimentary rocksat temperatures around 600 �C and plausible geologicalstrain rates indicate that very low differential stresses areneeded to deform the host rock (Carter and Tsenn,1987). Thus, the differential stresses required might belower than the buoyant pressure of the granite (around25–30 MPa for a magma body as thick as the Oulmes gran-ite). The present-day outcrop shows the crustal level atwhich the granite could not penetrate further upwardbecause its thermal input onto the host rock was insuf-ficient to soften it. Nevertheless, at deeper levels, both thehost rock and the granite would have been hotter, themagma thus uprising by deforming and moving aside/down the host rock. Thus, in the middle crust, diapiricascent of a single magma batch as the one represented bythe Oulmes stock seems mechanically viable, i.e. the roomfor the rising granite may be provided by equivalent down-wards ductile flow of host rock (Fig. 9a). Obviously, at

higher/cooler levels this ascent mechanism would requirea greater volume of magma (Weinberg, 1996; Burovet al., 2003) or it is replaced by a combination of othermechanisms such as diking (Petford et al., 1993) emplace-ment along shear zones (Roman-Berdiel et al., 1997) orstoping (Paterson and Fowler, 1993).

We further suggest that a relatively short vertical ascentfrom the source might take place for the Oulmes granite.This suggestion is based on the compositional characteris-tics of this granite, which include: presence of magmaticbiotite, muscovite and cordierite, high aluminous biotites,strong peraluminous and potassic character, and high ini-tial 87Sr/86Sr ratio (Boutaleb, 1988; Mrini et al., 1992; Bau-din et al., 2001). According to their chemical and isotopicfeatures, strongly peraluminous granites as the Oulmesgranite are thought to be derived from a peraluminoussource, either metapelites or gneisses of adequate composi-tion (e.g. Zen, 1988). Alkali loss to the surroundings orfractional differentiation could also originate peraluminos-ity, but these alternative origins can hardly be applied tostrongly peraluminous granites. Alkali loss to the sur-roundings must be a late-magmatic process, but the com-positional homogeneity and the presence of peraluminousearly magmatic minerals (aluminous biotite, cordierite) inthe Oulmes granite preclude this origin. On the other hand,in order to give strongly peraluminous granites by frac-tional differentiation large volumes of parental magmawould be needed (Zen, 1988), for which no evidence existsat Oulmes. Thus, the source of the Oulmes granite may wellhave been the anatexis of fertile middle crustal rocks atrelatively low temperatures, in accordance with the experi-mental work of Patino Douce and Harris (1998). On thesegrounds, the curves of granite solidus and muscovitedecomposition loosely constrain the relationships betweendepth of the source and emplacement level, under theassumptions that muscovite provided most of the waterand that ascent of granitic magma follows, at least in part,an adiabatic gradient (Hyndman, 1981a,b). As tentativelysuggested by the arrows in Fig. 2, a granite source not dee-per than 20 km is a reasonable proposition for the Oulmesgranite, the exact figure depending on the loss of heat dur-ing the ascent (near the source, magma ascent is expectedto be adiabatic). Thus, we suggest a relatively short diapiricjourney for the Oulmes granite.

Whether the diapiric emplacement proposed here for theOulmes granite can be applied to other granites in theregion or the more extended view of emplacement alongfractures created by strike-slip tectonics prevails (e.g.Lagarde et al., 1990; Hoepffner et al., 2005) is a questionthat needs additional studies, mainly gravimetric, con-ducted on those granites. A recent research on the magneticfabric of the Oulad Ouaslam granite (Boummane andOlivier, 2007), in the Jebilets Massif of the MoroccanMeseta, has concluded also against the dominant view oftranscurrent emplacement for the Moroccan granites,which suggests more variety of granite emplacement condi-tions than previously envisaged in the region.

312 A. Tahiri et al. / Journal of African Earth Sciences 48 (2007) 301–313

7. Conclusions

(i) A diapiric-dominated mechanism is suggested for theemplacement of the Oulmes granite, which is mainlysupported by the modelled 3D ellipsoidal geometry,the magmatic flow observed along subvertical planesand the strain/thermal aureole.

(ii) Simple considerations on plastic strength of the hostrock at the estimated conditions of temperature dur-ing magma emplacement suggest that the diapiricascent of the Oulmes granite would have been feasiblefrom a rheological point of view. At the emplacementlevel, subhorizontal flattening combined with N–Smagmatic and solid-state stretching affected the mag-matic diapir, as a response to coupled E–W and ver-tical compression.

(iii) The diapiric ascent of the Oulmes granite must havetaken place in a late-tectonic scenario, during thevanishing stages of the Variscan orogeny in theMoroccan Meseta. This type of emplacement defiesthe common belief of strike-slip tectonic control onthe emplacement of most Variscan plutons of theMoroccan Meseta.

(iv) Theoretical research on magmatic diapirism (Wein-berg and Podladchikov, 1995; Weinberg, 1996;Burov et al., 2003 and references therein) is funda-mental to improve our knowledge on vertical massflow in the crust. However, field work is necessaryto test the predictions made by the theoreticalmodels. In this respect, our study of the Oulmesgranite suggests that a granitic magma batch of4 km radius is likely to rise diapirically in the mid-dle crust. These results, which are comparable toothers for single stocks of similar volume (e.g.Galadı-Enrıquez et al., 2003), are in good agree-ment with the theoretical predictions made byBurov et al. (2003).

(v) Contrasted viscosities between basaltic and graniticmagmas point to different behaviours at the source.Thus, basaltic diapirs would never form becausebasaltic magma would be always drained by dikes(Weinberg, 1996), a prediction supported by geo-logical evidence. By contrast, the most reasonablefeeding mechanism for lower- to mid-crustal gra-nitic batholiths seems diapirism. By means of sucha mechanism, great volumes of granitic magmacould theoretically penetrate even to upper crustallevels (Burov et al., 2003), though the probabilityfor such a great volume of granitic magma to accu-mulate as single magma batches might be geologi-cally unlikely (Glazner et al., 2004). Thus, incontrast to diapiric-dominated granitic flow in thelower and middle crust, brittle failure channelizesmost of the granitic magma movement in the upper10–15 km of the crust, where mechanisms exploitingfractures can help to create space for the collectingmagma.

Acknowledgements

The research reported in this paper was made possiblethrough financial support by the Agencia Espanola deCooperacion Internacional (AECI), the Inter-AcademicMoroccan-Spanish Program projects 65/04/RE and 17/P/03 and the Spanish Ministry of Education and Sci-ence project TOPO-IBERIA CONSOLIDER-INGENIOCSD2006-00041. We thank Francisco Gonzalvez Garcıafor improving our English text. R. Weinberg, J.L. Bouchezand A. Fowler are acknowledged for their constructivecriticism.

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