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Journal of African Earth Sciences 39 (2004) 285–293

Neoproterozoic granitoids associated with the Bou-Azzerophiolitic melange (Anti-Atlas, Morocco): evidenceof adakitic magmatism in an arc segment at the NW

edge of the West-African craton

E.H. Beraaouz a,*, M. Ikenne a, A. Mortaji a, A. Madi b, M. Lahmam c, D. Gasquet d

a Dept. de Geologic, Faculte des Sciences, BP. 8106 Agadir, Moroccob CTT. Bou-Azzer, groupe ONA, Morocco

c Dept, de Geologie, Faculte des Sciences Ben M’sik Casablanca, Moroccod Lab. EDYTEM, Univer. de Savoie, CISM, Campus Scientifique, F. 73376 Le Bourget du Lae Cedex, France

Available online 1 October 2004

Abstract

The Neoproterozoic intrusions of the Bou-Azzer El Graara inlier consist of metaluminous, medium to high-K, I-type granitoids.

Two groups of granitoids can be distinguished based on chemistry and isotopic signature: (1) the early (�670Ma) medium-K calc-

alkaline, pre-collisional diorites of Ousdrat, Bou-Azzer, Bou-Izbane, and Ait-Hmane, with less fractionated REE patterns

(2.6 < (La/Yb)N < 12.1); (2) the late (�615Ma) high-K calc-alkaline, post-collisional granodiorites of Bleida characterized by rela-

tively more fractionated REE patterns (8.9 < (La/Yb)N < 12.6).

These rocks show characteristics typical of arc magmatic rocks (depleted in Ti, Nb and Ta) and display, to various degrees, an

adakitic signature (Al2O3 > 15%, 3 < %Na2O < 6.4, Yb < 1.8ppm, Y < 20ppm and isotopic ratios of Sr and Nd similar to the

ophiolitic rocks). However their La/Yb and Sr/Y are relatively low in most of the samples.

The origin of these arc magmas is not completely understood. In this paper we argue that some of these rocks probably contain

components of adakitic melts. The early group was produced by partial melting of subducted oceanic crust followed by interaction

of the melt with the overlying mantle wedge, and the late group by dehydration melting of underplated basalts in the lower crust in

the garnet stability field.

� 2004 Published by Elsevier Ltd.

Keywords: Neoproterozoic; Granitoid; Calc-alkalic; Adakitic; Slab

1. Introduction

Arc magmas are generally considered to be derived

from mantle wedge hydrous melting induced by fluids

released from a subducting slab (e.g., Gill, 1981; Arcu-

lus, 1994; Prouteau et al., 2001). Understanding of the

petrogenesis of arc magmas has evolved from general-

0899-5362/$ - see front matter � 2004 Published by Elsevier Ltd.

doi:10.1016/j.jafrearsci.2004.07.040

* Corresponding author.

E-mail address: beraaouz@hotmail.com (E.H. Beraaouz).

ized conceptual models to quantitative interpretations.Regional studies reveal the nature of the tectonic envi-

ronment, mantle composition, crustal thickness, effects

of H2O, and subduction zone geometry. The slab melts,

in this particular type of arc magmatism, are generally in

association with unusual geodynamic or/and thermal

conditions: when a young and hot oceanic slab sub-

ducts, the geothermal gradient along the Benioff plane

is high. In these conditions the melting temperature isreached before the slab dehydration point. Melting of

such a hot subducted slab produces adakitic magma

286 E.H. Beraaouz et al. / Journal of African Earth Sciences 39 (2004) 285–293

(Defant and Drummond, 1990; Peacock et al., 1994;

Yogodzinsky et al., 1995; Maury et al., 1996; Martin,

1999; Prouteau et al., 1999, 2001).

Adakites and rocks with adakitic features have multi-

ple origins: (1) melting of a young and hot subducted

oceanic crust (Defant and Drummond, 1990), (2) water-undersaturated partial melting of underplated mafic

lower crust or previously subducted oceanic crust (Pea-

cock et al., 1994; Barnes et al., 1996), (3) shallow level

crust assimilation and fractional crystallization (AFC)

of basaltic magma (Castillo et al., 1999), (4) melting of

source with significant components of subducted sedi-

ments (Shimoda et al., 1998).

The aim of this study is (i) to describe the geochemi-cal characteristics of the Neoproterozoic granitoids of

the Bou-Azzer El Graara inlier (BEI), (ii) to present cur-

rent interpretations of geochemical data and (iii) to test

if these intrusions represent arc-related or collision-

related magmatism.

2. Geological background

The Precambrian Anti-Atlas outcrops in several in-

liers within the Phanerozoic cover (Fig. 1a). It resulted

from two superposed orogens (Choubert, 1963; Charlot,

1976): an Eburnean orogen, Paleoproterozoic (Precam-

brian I in local nomenclature) in age, and a younger,

Neoproterozoic Pan-African (Precambrian II) orogen.

Fig. 1. (a) The Precambrian inliers of the Moroccan Anti-Atlas (from Ch

SAP = South Atlasic Fault and AAMF = Anti-Atlas Major Fault. (b) Simpl

intrusions (modified from Leblanc, 1975).

Geological data from the BEI (Leblanc, 1975, 1981;

Leblanc and Lancelot, 1980; Bodinier et al., 1984) show

that it is mainly made up of three units (Fig. 1b). The

lower unit (Precambrian II) comprises an ophiolitic

complex with platform sedimentary deposits (lime-

stones, quartzites and arenaceous lutites) dated by Rb/Sr method at 788 ± 9Ma (Clauer, 1976). This unit is in-

truded by diorite-granodiorite plutons. The intermediate

unit (Precambrian II–III) with detrital deposits known

as ‘‘the Tidilline series’’ covers the ophiolitic melange.

The upper unit (Precambrian III), formed of a thick vol-

canic and volcano-detritic series, covers unconformably

the preceding units. The Pan-African tectonic events

comprised two stages. The first one (B1), which affectsonly the lower unit, is associated with regional N90–

120�E schistosity and a coevolved of ductile sinistral east

west shear zones. The second stage (B2) affects both low-

er and intermediate units. It is characterized by east-west

upright folds with axial-plane schistosity.

A suture zone, marked by the Anti-Atlas Major

Fault, separates the rifted West African Craton to the

South from late Precambrian forearc outcrops to theNorth. The inlier�s central part includes ophiolites,

blue-schist melanges, synkinematic granitoids, and late-

tectonic transpressional basins (Saquaque et al., 1989a;

Hefferan et al., 1992, 2002). However, recently these

ophiolites have been interpreted as allochthonous Pan-

African ocean crustal slices, thrust onto theWest African

Craton (WAC) passive margin sequence (Ennih and

oubert, 1963) and localization of Bou-Azzer El Graara studied area.

ified geology of Bou-Azzer Elgraraa Inlier with localization of studied

E.H. Beraaouz et al. / Journal of African Earth Sciences 39 (2004) 285–293 287

Liegeois, 2001). These authors propose that the northern

limit of the WAC is located at the South Atlas Fault and

not at the Anti-Atlas Major Fault.

This paper investigates the Neoproterozoic granitoids

of Ait-Hmane (AH), Bou-Azzer-Bou Offroukh (BB),

Bou-Izbane (BI), Ousdrat (OS) and Bleida (BL) (Fig.1b). Descriptions of these synkinematic intrusions are

given in Saquaque et al. (1989b) and references therein.

They often display a N 125�E plano-linear fabric (linea-

tion and foliation) that is parallel to the regional schist-

osity, attributed to the major Pan-African tectonic phase

(B1 of Leblanc, 1975) dated at 685 ± 15Ma by Clauer

(1976). Previously published ages for these granitoids

are 615 ± 12Ma (U–Pb, zircons) for granodiorites fromBL (Ducrot, 1979); 614 ± 13Ma (whole rock Rb–Sr) for

diorites from BL (Mrini, 1993); 667 ± 11Ma (U–Pb, zir-

cons) (Mrini, 1993) and 653 ± 1.5Ma (U–Pb, zircons)

(Inglis et al., 2003) for quartz-diorites from BB; 678 ±

10Ma (U–Pb, zircons) (Ducrot, 1979) and 640 ± 1.5Ma

(U–Pb, zircons) (Inglis et al., 2003) for quartz-diorites

from OS.

These intrusions are organized in medium-sized mas-sifs and stocks that are either roughly circular in plan

(i.e. OS) or, more commonly, WNW-ESE elongated

(i.e. AH and BB), and intrude sedimentary and vol-

cano-sedimentary series and ophiolitic melange.

The intrusions are generally differentiated and com-

prise more intermediate than silicic rocks. They include

diorites, quartz-diorites and monzodiorites. Granodior-

ites are very important in BL but subordinate in theother intrusions. The contact between diorite and grano-

diorite is intrusive in BL, but in the other intrusions the

transition from mesocratic to leucocratic rocks is grad-

ual and marked by an increase of quartz at the expense

of amphibole.

The rocks show a granular fabric and vary in colour

from dark grey diorites and light grey tonalites to white

and pinkish granodiorites. In thin sections, these rocksdisplay granular textures, most often medium- to fine-

grained, and some have a tectonic fabric. Plagioclase

(An05-48) is the main phase and is commonly subhedral.

Other minerals in these rocks include amphibole (mostly

Mg-hornblende), biotite, magnetite, quartz and a little

K-feldspar. Accessory minerals are apatite, zircon and

titanite (allanite is rarely observed).

All these rocks exhibit petrographical evidence forhydrothermal alteration at different degrees and green-

schist facies metamorphism associated with the B1 defor-

mation. The secondary minerals are chlorite, epidote,

sericite, actinolite, leucoxene, calcite and Fe-oxides.

3. Geochemical characteristics

Geochemical analyses were carried out at the com-

mon laboratory of chemical analyses (CRPG-CNRS,

Nancy) by ICP-MS. Major and trace element data of

representative samples from studied rocks are given in

Table 1.

Comparison of relatively fresh and altered samples

indicates that hydrothermal alteration and/or metamor-

phism has changed the large-ion lithophile elements inthe rocks (LILE; Rb, Ba, K, Sr) through pervasive

mobilization of these soluble components. To avoid sec-

ondary effects, the focus of this investigation is concen-

trated on the less mobile rare earth elements (REE) and

high field strength elements (HFSE). This group of com-

ponents, when considered jointly with Nd and Sr isotope

data, provides an insight into the crustal and mantle

processes involved in petrogenesis.The silica contents vary from 50.5% to 73.9% with the

majority of analyses having intermediate contents. The

BI massif contains the more felsic rocks SiO2: (55.4–

73.99%), which are characterized by high Al2O3

(>15%) and Na2O (3.1–6.5%) contents.

The mesonormative quartz, K-feldspar and plagio-

clase ternary plot Fig. 2 (Le Maitre, 1989) shows that

BI consists of monzodiorite and granodiorite, BB of dior-ite with or without quartz and granodiorite, OS and AH

of Qz-diorite, tonalite and monzodiorite. Samples of AH

display an evolution toward monzogranites, whereas

those of the other massifs have a tendency of evolution

toward the field of the tonalites. In this diagram, I-type

and calc-alkaline affinity are indicated. The plot of grani-

toids with greater than 10% normative quartz, in ternary

Albite-Anorthite-Orthoclase normative compositions(not shown), shows that samples from BL plot in the gra-

nodiorite and tonalite fields, and samples from OS, BB

and BI plot within the trondhjemite–tonalite fields.

In a SiO2 versus K2O diagram Fig. 3, the rocks range

from medium-K to high-K domains. Most of the grano-

diorites from BL manifest high-K character. In the other

massifs the majority are medium-K.

Except for some samples from BL and BB, the A/CNK ratio (Zen, 1986) is lower than 1.1 and illustrate

a metaluminous to slightly peraluminous character.

The samples are characterized by low Y (2.1–21.4)

and Th (0.26–6.9) contents. The Sr values range from

143 to 1052 but the Sr/Y ratios are not high and vary

from 12 to 93.

Comparison of the REE patterns of representative

samples (Fig. 4) allows us to distinguish three groups:(i) granodiorite of BL with a relatively steep REE pat-

tern ((La/Yb)N of 6.8–12.6) and Eu anomalies (0.8–

1.1), and relatively fractional LREE ((La/Sm)N from

2.76 to 4.36); (ii) quartz-diorite from BB with a sigmoi-

dally shaped REE pattern ((La/Yb)N from 5.1 to 20.3)

and a discrete Eu anomaly (0.9–1.1), and more frac-

tional LREE ((La/Sm)N from 1.8 to 5.2) and (iii) rocks

from AH, OS and BI with less fractional patterns((La/Yb)N of 2.6–12.1) without a meaningful Eu

anomaly (0.8–1.3).

Table 1

Major (wt.%) and trace element (ppm) abundances in representative samples from studied granitoid plutons of Bou-Azzer El Graara inlier

N� Ait-Hmane Bou-Izbane Bou-Azzer Ousdrat Bleida

BA9 BA10 AH1 AH2 AH3 BZ6 BZ8 BI04 BI7 BI8 BB3 BB1 BB2 BO17 BO18 OS4 OS8 BL12 BL13 BL15 BL6 BL7 BL8

SiO2 50.15 52.20 56.80 57.40 56.73 73.99 61.57 64.10 55.56 64.08 56.77 63.32 61.78 54.06 52.29 52.25 63.92 62.29 63.63 63.36 59.10 58.64 64.05

Al2O3 18.82 19.76 17.67 17.85 17.18 13.24 16.00 15.91 19.27 15.92 17.41 16.63 17.91 16.79 16.19 18.13 14.46 16.02 15.85 16.11 14.36 14.04 16.27

Fe2O3 8.92 7.08 6.94 6.74 7.40 1.70 4.93 4.56 5.78 3.89 5.64 5.00 4.41 7.08 8.65 9.28 6.01 5.67 5.37 4.79 7.63 8.03 5.13

MnO 0.15 0.14 0.13 0.11 0.14 0.04 0.08 0.06 0.14 0.05 0.10 0.11 0.07 0.12 0.12 0.12 0.08 0.12 0.13 0.11 0.15 0.15 0.10

MgO 4.12 3.36 2.63 2.54 3.53 0.60 2.89 2.24 4.26 2.36 0.16 2.17 4.16 3.71 2.96 4.45 2.88 1.99 1.76 1.87 5.49 6.01 1.76

CaO 7.15 7.79 5.84 5.65 6.29 1.22 2.55 4.00 5.26 2.64 8.65 4.82 0.64 5.00 5.97 4.39 3.18 3.11 2.53 3.50 6.27 5.55 4.18

Na2O 4.50 3.61 4.98 3.61 3.38 6.45 5.87 4.15 4.26 5.11 6.44 3.59 4.79 4.67 4.46 4.13 4.09 3.23 3.31 3.37 2.08 2.13 3.27

K2O 0.30 1.24 1.18 2.20 1.60 0.43 0.55 1.17 1.94 1.38 0.27 1.31 2.11 0.74 0.75 1.17 1.02 2.77 3.13 2.72 1.67 1.77 2.28

TiO2 0.85 0.58 0.67 0.58 0.74 0.13 0.65 0.53 0.41 0.55 0.57 0.51 0.57 0.78 0.88 1.67 1.16 0.55 0.54 0.50 0.71 0.71 0.55

P2O5 0.21 0.27 0.17 0.18 0.19 0.08 0.14 0.12 0.12 0.14 0.18 0.19 0.17 0.20 0.21 0.17 0.27 0.21 0.23 0.23 0.16 0.17 0.21

LOI 4.68 3.85 2.83 3.02 2.67 1.93 4.57 2.99 2.90 3.67 3.68 2.20 3.20 6.68 7.34 4.08 3.28 3.94 3.41 3.32 2.30 2.73 2.09

Total 99.85 99.88 99.84 99.88 99.85 99.81 99.80 99.83 99.90 99.79 99.87 99.85 99.81 99.83 99.82 99.84 100.35 99.90 99.89 99.88 99.92 99.93 99.89

As 2.5 3.1 9.1 8.4 6.1 5.6 7.9 2.9 5.1 1.7 24.6 5.5 17.5 9.7 6.2 8.3 5.8 1.8 2.4 1.6 4.2 3.9 3.2

Ba 285 385 323 526 372 203 150 198 477 153 81 249 260 134 148 240 177 817 848 634 597 629 637

Ce 20.6 25.7 26.2 32.5 29.8 32.5 12.6 18.3 12.1 26.5 16.8 62.7 13.9 16.5 11.7 20.6 28.3 50.0 39.7 55.0 42.7 47.7 44.3

Co 87.5 91.3 36.7 43.3 43.7 151.0 70.7 53.0 72.9 67.0 66.7 73.5 46.5 74.0 64.2 35.8 87.7 128.0 150.0 138.0 89.2 66.8 56.1

Cr 7.7 25.4 19.7 17.9 60.8 9.9 32.5 32.4 84.6 30.8 17.2 22.0 35.1 77.6 64.8 23.9 40.6 12.7 12.4 13.8 457.4 391.2 27.1

Cs 0.3 0.9 0.6 1.0 1.2 0.2 0.8 2.8 1.5 1.9 <L.D. 0.7 2.2 1.4 1.6 2.3 1.4 1.4 1.3 1.7 1.5 1.1 1.2

Dy 3.1 2.6 3.4 3.5 3.8 1.6 2.0 2.4 1.8 1.8 2.0 2.0 0.6 2.5 1.9 3.7 3.1 3.1 2.9 2.6 3.3 3.3 2.7

Er 1.7 1.4 1.8 1.9 2.2 0.8 1.2 1.3 0.9 1.0 1.0 1.0 0.2 1.2 0.9 1.7 1.5 1.5 1.4 1.5 1.8 1.8 1.4

Eu 1.1 1.3 1.0 1.1 1.3 0.6 0.6 0.7 0.8 0.6 0.8 1.1 0.3 1.0 0.6 1.4 1.0 1.3 1.3 1.3 1.2 1.1 1.3

Ga 20.5 21.3 19.3 18.8 17.5 11.7 14.1 16.7 18.9 14.7 22.7 17.4 21.5 17.2 19.2 20.7 16.3 19.7 20.1 19.8 17.2 18.2 18.5

Gd 3.3 3.0 3.6 3.8 4.3 0.9 2.1 2.4 1.9 2.0 2.5 2.6 0.7 3.0 2.1 4.0 3.7 3.8 3.6 3.5 3.6 4.0 3.0

Ge 1.3 1.3 1.4 1.4 1.3 0.9 0.7 1.1 1.2 0.7 2.7 1.2 0.9 1.0 1.1 1.6 1.0 1.2 1.3 1.4 1.4 1.4 1.3

Hf 2.4 1.9 3.0 3.0 4.0 2.5 2.8 3.3 2.5 3.9 2.8 2.6 2.8 1.8 1.7 2.5 8.7 4.0 4.1 3.8 3.6 4.1 3.8

Ho 0.6 0.5 0.7 0.7 0.8 0.3 0.4 0.4 0.4 0.4 0.4 0.3 0.1 0.5 0.4 0.6 0.6 0.5 0.5 0.5 0.7 0.7 0.5

La 8.8 11.5 11.3 14.4 12.8 16.4 4.9 7.8 5.4 11.8 7.2 31.1 6.1 6.7 4.8 9.5 12.3 25.6 20.0 28.9 20.5 22.7 22.4

Lu 0.3 0.2 0.3 0.4 0.4 0.2 0.2 0.2 0.1 0.2 0.1 0.2 0.0 0.2 0.1 0.2 0.2 0.3 0.3 0.2 0.3 0.3 0.2

Nb 3.2 2.5 4.4 4.7 5.0 4.2 2.4 2.7 1.2 3.0 4.2 5.0 4.4 2.5 2.8 3.0 5.0 10.2 11.8 9.7 6.3 6.5 9.3

Nd 12.1 13.4 14.4 18.5 18.5 12.6 7.6 10.3 8.0 13.3 11.0 23.8 5.7 11.5 7.3 14.2 17.3 22.3 18.9 23.6 22.0 22.7 20.3

Pr 2.8 3.2 3.5 4.4 4.2 3.6 1.8 2.5 1.8 3.6 2.4 7.0 1.7 2.4 1.6 3.0 3.9 5.6 4.8 6.2 5.5 5.8 5.2

Rb 5.6 33.9 24.9 44.1 38.4 7.4 14.0 27.2 50.1 38.3 6.4 25.2 67.8 23.5 23.6 32.5 28.0 79.8 90.4 70.2 52.1 56.6 61.7

Sm 3.1 3.0 3.6 4.2 4.1 2.3 1.9 2.6 1.8 2.6 2.5 3.7 1.0 2.9 1.7 3.9 3.9 4.3 3.8 4.2 4.7 4.6 4.0

Sr 305 493 374 303 327 143 161 214 492 108 1052 373 149 391 246 297 197 433 449 393 >70 298 498

Ta 1.2 1.3 0.7 0.9 0.9 2.7 0.9 1.1 0.9 1.3 1.3 1.7 1.0 0.9 0.6 0.5 1.6 2.8 3.3 2.8 1.2 1.2 1.9

Tb 0.5 0.4 0.5 0.5 0.6 0.3 0.3 0.4 0.3 0.4 0.3 0.4 0.1 0.4 0.3 0.6 0.5 0.5 0.5 0.5 0.6 0.6 0.5

Th 1.0 2.8 2.0 2.0 2.5 3.5 0.9 1.5 0.9 2.4 0.6 4.8 3.7 0.6 0.8 0.8 0.7 5.9 5.7 6.9 4.0 4.3 5.2

Tm 0.3 0.2 0.3 0.3 0.4 0.1 0.2 0.2 0.1 0.2 0.2 0.1 0.0 0.2 0.1 0.2 0.2 0.2 0.2 0.2 0.3 0.3 0.2

U 0.3 0.8 0.5 0.6 0.6 0.6 0.3 0.4 0.3 0.6 0.4 0.7 0.5 0.2 0.3 0.4 0.3 3.1 5.1 3.8 1.3 1.2 2.6

V 217 134 130 116 142 11.0 85.0 74.3 63.9 82.8 130.0 70.2 115.1 150.0 171.0 225.2 136.2 61.4 51.7 52.5 134.9 144.1 54.7

Y 16.9 14.7 18.4 19.0 21.4 9.3 11.1 12.8 10.0 9.1 11.3 9.2 2.1 14.5 10.7 17.2 15.2 16.7 16.2 15.2 17.8 18.2 13.3

Yb 1.7 1.5 2.2 2.2 2.6 0.9 1.3 1.5 0.9 1.1 1.0 1.0 0.2 1.3 1.0 1.4 1.5 1.6 1.5 1.6 2.0 2.0 1.5

Zn 109 88 75.7 73.5 88.3 40.8 103 56.0 114.6 45.5 14.5 84.8 73.6 94.0 92.5 92.6 66.0 119.0 101.0 76.2 182.1 182.5 84.2

Zr 101 71.4 115 110 150 76.8 129 110.1 99.6 140.7 116 100.2 102.4 71.9 67.2 88.8 358.2 156.0 150.0 155.0 140.3 160.8 140.3

288

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aouzet

al./JournalofAfrica

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Scien

ces39(2004)285–293

Fig. 2. Quartz-Orthoclase-Plagioclase ternary plot based on CIPW normative values (Lameyre, 1987; Le Maitre, 1989) for studied granitoids. Fields

for I-, S- and A-type granites are from Bowden et al. (1984).

Fig. 3. Plot of studied rocks within the K2O versus SiO2 classification

diagram of Peccerillo and Taylor (1976).

Fig. 4. Chondrite-normalized variation diagrams of rare earth element

abundance (Evensen et al., 1978) for main rocks of the different studied

massifs. (Legend: BL = Bleida, AH = Ait-Hmane, BI = Bou-Izbane,

BB = Bou-Azzer and OS = Ousdrat.)

E.H. Beraaouz et al. / Journal of African Earth Sciences 39 (2004) 285–293 289

4. Tectonic setting and genesis

A primitive-mantle-normalized (Sun and

McDonough, 1989) multi-element diagram (Fig. 5)

shows typical magmatic arc signatures for almost all

the samples. HFSE (Nb, Ta, Hf, Zr, Ti) depletion is a

characteristic of this tectonic environment and is most

conspicuously marked by Ta and Nb trough (Wilson,

1989). Depletion of the entire suite of HFSE is seen

throughout the rock groups with the exception of Zr,which is enriched in samples from OS and BB. Mobile

Fig. 5. Mantle-normalized trace element abundance patterns (Sun and

McDonough, 1989) of representative studied granitoids, compared

with experimental hybridized slab melt at 3.5GPa (Rapp et al., 1999).

Ti anomaly with horizontal bar separate the range of Ti in rutile-

saturated slab melts at 3–4GPa and 1075–1100�C (downward pointing

arrow above the horizontal bar at 3 times the primitive mantle

abundance of Ti) and in slab melts coexisting with amphibole-bearing

eclogitic residue at 1.5–3.2GPa and 800–1000 �C (downward pointing

arrow below horizontal bar) is after Rapp et al., 1999. (Legend:

BL = Bleida, AH = Ait-Hmane, BI = Bou-Izbane, BB = Bou-Azzer

and OS = Ousdrat.)

Fig. 7. (87Sr/86Sr) versus eNd plot for Bou-Azzer El Graara granitoids

and associated rocks. Data are from Mrini (1993).

290 E.H. Beraaouz et al. / Journal of African Earth Sciences 39 (2004) 285–293

elements like the LILEs (Ba, K, Sr) are relatively en-

riched in all samples. Enrichment of these elements is as-

cribed to fluids from subducted sediments or subducted

basalts and hence is a further indication of subduction

related magmatic rocks.

In the diagram (Fig. 6) of Liegeois et al. (1998),opposing the mean of the NYTS value of the Yenchi-

chi–Talabit series, the values of the SNX and SNY

parameters lie close to the origin for the samples of

BI, OS, AH (except for one sample) and BB, and vary,

respectively, from 0.28 to 0.7 and from 0.1 to 0.7, com-

parable to those of the pre-collisional TTG and syn-

thrust HKCA. The values of SNX (0.82–1.5) and SNY

of about 0.5 place the samples of BL in the domain ofthe post-collisional HKCA and Yenchichi–Talabit

series.

The initial isotopic composition of the rocks provides

information about the (mantle-) sources of the mag-

matic rocks as well as about processes by which their

chemical and isotopic compositions were modified.

The overall isotopic variability of the studied rocks of

BEI (Mrini, 1993) is relatively restricted, with valuesranging between 87Sr/86Sr: 0.705632–0.713087 for BL,

0.704688–0.716433 for OS and 0.704431–0.70811 for

BB; and 143Nd/144Nd: 0.512338–0.512689 for BL,

0.512804–0.513025 for OS and 0.512534–0.512694 for

BI.

The eNdi (Mrini, 1993) for these granitoids are posi-

tive and variable (eNd�670 Ma +4 to +1 for rocks from

BL and eNd�610 Ma = +8 to +5 for rocks from otherintrusions like those of ophiolitic rocks (eNd�T = +7.5

to +3).

The isotopic data display a sub-horizontal trend

across the mantle array in eNd versus 87Sr/86Sr diagram

Fig. 7, varying virtually only in Sr isotopic composition

Fig. 6. Plot of studied granitoids in SNX-SNY (sliding normalization

X and Y) diagram opposing the mean of the NYTS values (normal-

ization to the Yenchichi–Telabit series) characteristics for the alkaline-

peralkaline series and for the high-K calc-alkaline-shoshonitic series

(Liegeois et al., 1998).

and showing many features typical of adakites and high-

Mg diorite (sanukitoid) suites from the late Archaean

formations.

Some samples again bear many similarities to ada-

kites worldwide, as well as some of the ‘‘adakite’’ melts

produced in melting and peridotite assimilation experi-ments (Rapp and Watson, 1995; Rapp et al., 1999;

and mostly unpublished data communicated by Rapp).

The BEI samples are plotted in the La/Yb versus Yb

and Sr/Y versus Y diagrams (Figs. 8 and 9) and com-

pared to the experimental data from TTG or adakitic

melts from Rapp and Watson (1995) and Rapp et al.

(1999), produced by partial melting of four different ba-

salt compositions over a range of P–T conditions, inwhich the residual crystalline assemblage consisted of

eclogite (garnet + clinopyroxene + sphene/rutile) with

minor amounts of amphibole in some of the lower pres-

sure (<2.2GPa) experiments. In the La/Yb versus Yb

diagram (Fig. 8), the BEI samples plot in the lower

Fig. 8. La/Yb versus Yb for studied granitoids compared to experi-

mental adakite liquids at pressure range of 1–4GPa (Rapp and

Watson, 1995; Rapp et al., 1999).

Fig. 9. Sr/Y versus Y diagram for studied granitoids (limits after

Drummond and Defant, 1993).

E.H. Beraaouz et al. / Journal of African Earth Sciences 39 (2004) 285–293 291

range of the experimental field, but the clear signature of

residual garnet, a strong relative depletion in Yb, is

obvious. All samples have Yb < 2ppm, and the rela-

tively low La/Yb ratios can be attributed to low La con-centrations. However, in terms of Sr/Y versus Y (Fig. 9)

the situation is somewhat different, where study rocks

fall into the lower part of the field for experimental ada-

kites, and have low Sr/Y like high Mg-andesites from

the western Aleutians (Yogodzinsky et al., 1995) and

adakites from Mindanao in the Philippines (Sajona

et al., 1994). Again, the low Sr contents could be attrib-

uted to Sr loss during alteration and/or metamorphism,or indicate that the partial melting was relatively shal-

low and plagioclase could have remained stable in the

source region (Martin and Moyen, 2002). Another pos-

sible explanation is that fractionation of plagioclase and

amphibole in the crust has affected the distribution of Sr

and Y. Amphibole has a lower partition coefficient for Y

than garnet, so amphibole fractionation would tend to

drive liquids to higher Y concentrations, and plagioclasefractionation would tend to diminish Sr concentrations

in the melt.

Comparison of mantle normalized trace elements to

primitive-mantle (Fig. 5) of BEI samples, with several

experimental slab melts and their mantle-hybridized

equivalent (Rapp et al., 1999) shows that, the overall pat-

tern of BEI rocks correlates quite well with the pattern

for the experimental melt hybridized by the depleted per-idotite at 3.8GPa (note specifically the Th/U ratio, nega-

tive anomalies in Nb and Ti, overall fractionation of

LREE and LILE from HREE and Y). The Ti anomalies

(Rapp et al., 1999) are at levels slightly more than three

times the primitive mantle value for BL samples, indicat-

ing 3–4GPa and 1000–1100 �C P–T conditions for their

melting genesis; however they are less than three times

for the other massifs, suggesting lower P–T conditions

(1.5–3.2GPa and 800–1000 �C).

5. Discussion and conclusion

The BEI granitoids show typical characteristics of arc

magmatic rocks such as depletion in high field-strength

elements (HFSE) and enrichment in large-ion-lithophile

elements (LILE) relative to normal mid-ocean ridge

basalts.

On the basis of published ages, geochemical and

isotopic signatures we distinguished two groups of

granitoids in the BEI: the early (�670Ma) medium-Kcalc-alkaline, pre-collisional quartz diorite of OS, BB,

BI, and AH and the later (�615Ma) high-K calc-alka-

line, post-collisional granodiorites of BL.

Adakite and adakitic rocks possess distinctive geo-

chemical and isotopic features as outlined by various

authors (Defant and Drummond, 1990; Drummond

and Defant, 1993; Drummond et al., 1996; Peacock

et al., 1994; Maury et al., 1996).The other rocks produced in hot subduction zones

are high-magnesian andesite (HMA) (Yogodzinsky et

al., 1995; Stern and Kilian, 1996) with element signa-

tures noted above for the adakites, high Mg#, high Cr

and Ni abundances and LILE enrichment. These are

interpreted to be primary magmas generated by partial

melting of hydrous, LILE-enriched metasomatized man-

tle peridotite (Prouteau et al., 2001).Some of the studied granitoids have adakitic charac-

ters with low Th, Y and Yb, high Al2O3 and Na2O.

However, only some samples have high La/Yb and Sr/

Y values, which is similar to those of adakitic rocks.

Their Sr–Nd isotopic characteristics are similar to those

of the associated ophiolitic rocks (Fig. 7). The MORB-

like isotopic values of these rocks suggest derivation

from partial melting of oceanic basalts.Adakite and adakitic rocks are found in intraoceanic

island arc settings and in continental arcs.

Since the initial suggestion by Defant and Drum-

mond (1990) of the formation of adakites as slab melts

produced by the partial melting of young oceanic crust,

new information has been generated. The processes

that originate adakitic magmas are: (i) melting of a

young and hot subducted slab in a flat-fast and obliquesubduction; (ii) melting of underplated basalts in the

lower crust by hot spot; (iii) hybridization to modal

and/or cryptic metasomatism of the mantle by silica-rich

partial melt of the subducted slab (Maury et al., 1996;

Rapp et al., 1999); (iv) fluid-present melting, aside from

dehydration melting of oceanic crust (Prouteau et al.,

1999). All of these processes occur singly or in

combination.The differences that are evident between the studied

granitoids and the adakites are attributable to a number

292 E.H. Beraaouz et al. / Journal of African Earth Sciences 39 (2004) 285–293

of factors (alteration and/or metamorphism, intracrustal

fractionation and assimilation effects, etc.).

Those arc magmas with adakitic compositional fea-

tures probably have multiple origins. The pre-collisional

granitoids from BB, OS, BI and AH were probably ini-

tiated from adakitic magmatism, produced by partialmelting of an eclogitic oceanic crust and followed by

interaction of the melt with the overlying mantle wedge.

The most likely origin of post-collisional granodiorites

from BL is derivation through partial melting of metaig-

neous sources in the lower crust.

The occurrence of a possible relationship between

adakitic magmatism and precious and base metal miner-

alisation has already been recognized (Thieblemontet al., 1997 and references therein). With the knowledge

that an adakite belt is present in the BEI, it can be sur-

mised that a possible gold–copper mineralised belt may

also be present. This makes other Precambrian inliers in

the Anti-Atlas good exploration targets for gold–copper

mineralisation.

Acknowledgments

We are grateful to R.P. Rapp for kindly sending

experimental data and for his comments concerning

comparison between our samples and experimental

melts. Special thanks for Azizi Samir, director of Bou-

Azzer district mine for his help during the field work.

We sincerely acknowledge Dr. K. Goodenough and ananonymous referee for their constructive review to sig-

nificantly improve the paper. Special thanks are due to

Dr. Heather Moore for her helpful improvement of

the English of the manuscript.

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