Journal of African Earth Sciences 37 (2003) 207–227

www.elsevier.com/locate/jafrearsci

A review of Archaean and Paleoproterozoic evolution of theIn Ouzzal granulitic terrane (Western Hoggar, Algeria)

Khadidja Ouzegane a, Jean-Robert Kienast b,*, Abderrahmane Bendaoud a,Amar Drareni a

a Laboratoire de G�eeodynamique, G�eeologie de l’Ing�eenieur et de Plan�eetologie, Facult�ee des Sciences de la Terre, de G�eeographie et de l’Am�eenagement duTerritoire, U.S.T.H.B., B.P. 32 El Alia, Dar el Beida, Algiers 16111, Algeria

b Laboratoire de G�eeosciences Marines, C.N.R.S.––IPGP, UMR 7097, Universit�ee Paris 7, 4 place Jussieu, Tour 26, 3�eeme �eetage,Paris Cedex 05 75252, France

Received 20 September 2002; accepted 30 May 2003

Abstract

The In Ouzzal terrane (Western Hoggar) is an example of Archaean crust remobilized during a very-high-temperature meta-

morphism related to the Paleoproterozoic orogeny (2 Ga). Pan-African events (�0.6 Ga) are localized and generally of low intensity.

The In Ouzzal terrane is composed of two Archaean units, a lower crustal unit made up essentially of enderbites and charnockites,

and a supracrustal unit of quartzites, banded iron formations, marbles, Al–Mg and Al–Fe granulites commonly associated with

mafic (metanorites and garnet pyroxenites) and ultramafic (pyroxenites, lherzolites and harzburgites) lenses. Cordierite-bearing

monzogranitic gneisses and anorthosites occur also in this unit. The continental crust represented by the granulitic unit of In Ouzzal

was formed during various orogenic reworking events spread between 3200 and 2000 Ma. The formation of a continental crust made

up of tonalites and trondhjemites took place between P3200 and 2700 Ma. Towards 2650 Ma, extension-related alkali-granites

were emplaced. The deposition of the metasedimentary protoliths between 2700 and 2650 Ma, was coeval with rifting. The me-

tasedimentary rocks such as quartzites and Al–Mg pelites anomalously rich in Cr and Ni, are interpreted as a mixture between an

immature component resulting from the erosion and hydrothermal alteration of mafic to ultramafic materials, and a granitic mature

component. The youngest Archaean igneous event at 2500 Ma includes calc-alkaline granites resulting from partial melting of a

predominantly tonalitic continental crust. These granites were subsequently converted into charnockitic orthogneisses. This indi-

cates crustal thickening or heating, and probably late Archaean high-grade metamorphism coeval with the development of domes

and basins. The Paleoproterozoic deformation consists essentially of a re-activation of the pre-existing Archaean structures. The

structural features observed at the base of the crust argue in favour of deformation under granulite-facies. These features are

compatible with homogeneous horizontal shortening of overall NW–SE trend that accentuated the vertical stretching and flattening

of old structures in the form of basins and domes. This shortening was accommodated by horizontal displacements along trans-

pressive shear corridors. Reactional textures and the development of parageneses during the Paleoproterozoic suggest a clockwise P–

T path characterized by prograde evolution at high pressures (800–1050 �C at 10–11 kbar), leading to the appearance of exceptional

parageneses with corundum–quartz, sapphirine–quartz and sapphirine–spinel–quartz. This was followed by an isothermal

decompression (9–5 kbar). Despite the high temperatures attained, the dehydrated continental crust did not undergo any significant

partial melting. The P–T path followed by the granulites is compatible with a continental collision, followed by delamination of the

lithosphere and uprise of the asthenosphere. During exhumation of this chain, the shear zones controlled the emplacement of

carbonatites associated with fenites.

� 2003 Elsevier B.V. All rights reserved.

Keywords: Archaean; Paleoproterozoic; Hoggar massif; Tuareg shield; In Ouzzal; Very-high-temperature granulites

* Corresponding author. Tel.: +33-1-44-87-51-90; fax: +33-1-44-27-

39-11.

E-mail address: jrk@ccr.jussieu.fr (J.-R. Kienast).

0899-5362/$ - see front matter � 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.jafrearsci.2003.05.002

1. Introduction

During the Archaean, various cycles of geodynamic

evolution took place in several cratons whereby an

episode of island-arc accretion was followed by the

formation of tonalites, trondjhemites and granodiorites

Fig. 1. (A) Schematic map of the Tuareg shield showing the setting of

the In Ouzzal terrane within the Trans-Saharan Pan-African belt

(Black et al., 1994). (B) Geological sketch-map of the In Ouzzal ter-

rane and adjacent areas (Caby, 1996). AH: Ahnet Palaeoprotero-

zoic quartzites; AM: Amesmessa gold deposit; BE: In Bezeg granulite

unit; IA: In Assaka non-metamorphic Paleoproterozoic quartz-

ites; IAL: In Alarene layered complex; IGU: Iforas Granulite Unit; IH:

208 K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227

(TTG). This led to a period of reworking of the crustaccompanied by dome-and-basin tectonics, the

emplacement of calc-alkaline granites and the develop-

ment of high-grade metamorphism followed by a long

period of quiescence lasting up to 500 million years

(Choukroune et al., 1997; Condie, 1997). One of the

aims of this review is to highlight the timing as well as

the mechanisms of creation, deformation and differen-

tiation of a segment of crust from the Archaean to thePaleoproterozoic. Despite a Paleoproterozoic tectono-

metamorphic event (2000 Ma), these granulites preserve

the general characteristics of the precursors to the Ar-

chaean granite/greenstone belt-like terrains. The topics

of geological research on this kind of terrain, are to

characterize the nature of the protoliths, the relation-

ships between the meta-igneous and metasedimentary

units, the emplacement age of the protoliths and that ofmetamorphic events. Finally, the structural and meta-

morphic characterization of granulites is required to

establish the pressure–temperature paths followed by

these rocks during their evolution.

The Hoggar is composed of well preserved and lar-

gely reworked Archean (3200–2500 Ma) and Paleopro-

terozoic terranes (2000 Ma) and juvenile Pan-African

terranes (750–550 Ma). The main tectonic feature of thisarea is the system of lithospheric north–south shear

zones with large movements related to terranes collage

and continental collision (Black et al., 1994; Fig. 1A).

The In Ouzzal terrane forms an elongated N–S trending

block, more than 400 km long and 80 km wide in the

north around the In Hihaou massif, thinning out until it

disappears in the south near the Malian border, where it

is then offset towards the Iforas granulitic unit (Fig. 1B).The main relief rising several hundreds of metres above

the In Ouzzal granulitic reg (¼ desert pediment with

small pebbles) is made up of ignimbrite massifs dated at

530 Ma (Picciotto et al., 1965), and the Pan-African

granites of In Hihaou and In Eher in the West, Nahalet

and Tihimatine in the centre and Ihouhaouene in the

East (Fig. 2). The boundaries of the In Ouzzal granulitic

terrane with the branches of the Pan-African belt arerepresented by vertical wrench faults. The East-In

Ouzzal mylonitic margin has been studied in detail in

the areas of Tirek (Attoum, 1983) and Amesmessa

(Djemai, 1996), where it consists of a major vertical fault

with a dextral strike-slip component. The West-Ouzza-

lian wrench fault, on the other hand, has a sinistral

displacement (Caby, 1970), the last movements along

the fault coincided with the emplacement of the TinZebane alkaline–peralkaline dyke swarm (�592 Ma;

bFig. 1 (continued)

Ihouhahouene alkali-granite; IZ: In Zize volcanic complex, TAS:

Tassendjanet; TIS: Tissangenine; TI: Tirek gold deposit; TZ: Tin

Zebbane dyke complex.

Fig. 2. Geological sketch-map (simplified from Caby, 1970) showing

the location of areas mentioned in the text (black square) and position

of cross-section (Fig. 3).

K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227 209

Hadj-Kaddour et al., 1998). Moussine-Pouchkine et al.

(1988) pointed out that the extreme northern part of In

Ouzzal is overthrust onto the volcanogenic series of

Adrar Ahnet. In fact, arc-type formations and glauco-

phane-bearing metamorphic rocks indicate the existenceof a Pan-African subduction zone in this area (Caby and

Moni�ee, 2003, this issue; Mokri, personal communica-

tion). The recognition of the tectonic contacts and the

metamorphic contrast between the In Ouzzal granulitic

unit and the Pan-African belt (amphibolite and greens-

chist facies, respectively), along with the Archaean age

of the granulite protoliths, suggest that In Ouzzal is an

exotic Pan-African terrane made of Archean and Pa-leoproterozoic rocks (Kienast and Ouzegane, 1987;

Black et al., 1994; Caby, 1996).

Lelubre (1952) conducted the first petrological studyof In Ouzzal; he stressed the original nature of the

series exhibiting ‘‘In Ouzzal facies’’. He compared them

with the charnockites of India and proposed that

these granulites were much older than the terrains that

surround them. In the area around In Hihaou and

Tekhamalt, a granulitic complex was described briefly by

Giraud (1961) and Le Fur (1966). This complex is com-

posed of charnockites, nodular marbles, garnet–graphitegneisses, quartzites with magnetite and quartzites with

garnet–orthopyroxene–sillimanite (Giraud, 1961). Litho-

logically the In Ouzzal formations are very diversified

(Kienast et al., 1996) and are composed mainly of two

units: (i) a lower crustal unit made up essentially of

enderbitic orthogneisses, and (ii) a supracrustal unit

composed of quartzites, banded iron formations, mar-

bles, Al–Mg granulites and Al–Fe granulites commonlyassociated with meta-igneous material such as mafic

(metanorites and garnet pyroxenites) and ultramafic

(lherzolites, harzburgites and pyroxenites) lenses, as well

as anorthosites and cordierite-bearing monzogranitic

gneisses. Regarding the supracrustal unit, the most ori-

ginal feature is the abundance and the variety of Al–Mg

granulites. These rocks contain parageneses which indi-

cate a very-high-temperature metamorphism at about1000 �C for a pressure of 10–11 kbar. The marbles with

wollastonite–scapolite–calcite–quartz, anorthite–grossu-

larite (Benyahia, 1996; Boureghda, 2000; Ouzegane

et al., 2002) and the spinel–quartz quartzites with or

without corundum (Ait Djafer, 1996; Guiraud et al.,

1996; Badani-Djebloun, 1998) also confirm the extreme

temperature conditions of this metamorphism.

2. Principal lithological types

2.1. Enderbitic and charnockitic orthogneisses

The charnockitic and enderbitic unit is very exten-

sively exposed, covering most of the area between Oued

Tin Tchik Tchik and Tekhamalt (Fig. 2, Ouzegane,

1987; Ouzegane and Kienast, 1996) and form the major

part of the outcrops in the areas of Tirek and Ames-messa (Fig. 1B). These rocks are also found in the areas

of Roccan, Ihouhaouene and Tikechitine (Fig. 2) asso-

ciated with marbles and alumino-magnesian granulites.

The foliation trajectories delineate dome structures

occupied by the charnockites or enderbites, whereas the

metasedimentary formations occur in basins (Fig. 3;

Haddoum et al., 1994). Foliation is generally sub-verti-

cal and parallel to the lithological layering, reflecting theeffects of tectonic transposition. The stretching linea-

tions show highly variable but generally steep plunges

(70–90�). Based on the structural, geochemical and

geochronological (U/Pb on zircon) studies, three major

Fig. 3. Schematic NNW-SSE cross-section summarizing the style of deformation and structural relationships between Archean charnockitic gneisses

and supracrustal series of the In Ouzzal terrane.

210 K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227

groups of orthogneisses have been distinguished (Had-

doum et al., 1994; Peucat et al., 1996):

• the first group corresponds to orthogneisses of the

trondhjemite, tonalite and granodiorite type (TTG)

that are dated at between 3200 and 2700 Ma;• the second group corresponds to alkali-granitic or-

thogneisses dated at 2650 Ma;

• the third group corresponds to a calc-alkaline suite

made up of granodiorites and monzogranites em-

placed close to 2500 Ma.

The first group comprises folied enderbitic ortho-

gneisses exposed in the areas of Tin Tchik Tchik, Roc-can and Tickechitine. The structure of these rocks is

generally banded with a foliation defined by dark layers

containing pyroxenes. Orthogneisses of trondhjemitic to

tonalitic composition exhibit varied mineral associa-

tions. The trondhjemitic gneisses are composed of

hypersthene, quartz, oligoclase and accessory minerals

such as magnetite, ilmenite, apatite and zircon. In cer-

tain cases, perthites and biotite also occur aroundhypersthene. In addition to hypersthene, tonalitic or-

thogneisses are composed of clinopyroxene, plagioclase

(An35–40), perthite, quartz, rare biotite, magnetite,

ilmenite, apatite and zircon.

The second group includes charnockitic ortho-

gneisses generally forming small ranges that rise more

than 60 m above the reg, including Tekhamalt Tan

Affela and Tekhamalt Tan Ataram (Fig. 2). Theserocks occur also in the reg of Roccan and Oued

Ihouhaou�eene (Fig. 2). The distinctive feature of these

rocks is the presence of the association perthite-quartz-

ferro-augite, to which can be added oligoclase and

green hornblende. The accessory minerals are apatite,

zircon, magnetite, ilmenite, pyrochlore and chevkinite

(Acef et al., 2001).

The third group consists of calc-alkali orthogneisses

forming a granodioritic to monzogranitic suite intruded

into the preceding groups, but which are also associated

with the supracrustal formations. The mineralogical

composition of granodioritic orthogneisses includes

plagioclase (An30), quartz, perthites and hypersthene,with rare biotite and, sometimes, green hornblende

(Lassel, 1990). Monzogranitic orthogneisses contain

more biotite than the granodioritic orthogneisses, while

hornblende is not observed (Peucat et al., 1996).

Orthogneisses of the first group define a trondjhemitic

differentiation trend, with K2O/Na2O ratios <0.5 (Bar-

ker, 1979). The spiderdiagram plots normalized to the

primitive mantle show patterns for most of the elementsthat agree with Archaean TTG suites, e.g. negative

anomalies in Ta, Nb, P and Ti, but a positive anomaly in

Pb (Fig. 4). The low contents of Ta (<0.5 ppm), Nb (9–12

ppm), Y (<20 ppm) and Yb (<1.6 ppm) are comparable

with values from igneous rocks of present-day island

arcs. As with the Archaean granitoids, the In Ouzzal

gneisses with TTG composition show low contents of Yb

and a strong fractionation of the REE, suggesting thatthe source could have been garnet amphibolites. Their

contents of Y (7–18 ppm) and Sr/Y ratios (18–62) may be

explained by variable quantities of garnet in the residue

and varied pressure conditions of formation (Peucat

et al., 1996). The U–Pb zircon dating suggest at least two

emplacement episodes at 3.2 and 2.7 Ga and a probable

reworking of an older crust in the case of the TTG dated

at 2.7 Ga, with some of these rocks having TDM modelages of 3.3 Ga (Peucat et al., 1996). Most of these or-

thogneisses yield TDM model ages around 3.2 Ga, in good

agreement with the U–Pb zircon ages, thus supporting

the relative immobility of Sm and Nd, and all the rare

earth elements (REE). The calculated eNd at 3.2 Ga

ranges from +0.4 to +5.3, with a concentration of values

around +4 (Peucat et al., 1996) that corresponds to the

0.1

1

10

100

1000

Cs Rb Ba Th U K Ta Nb La Ce Pb Sr Nd P Hf Zr Sm Ti Tb Y Yb

TTG

CA2

CA1

Alk3

Sam

ple/

Prim

itive

Man

tle

Fig. 4. Spider diagram of Archaean granitic orthogneisses. Data after Peucat et al. (1996). Primitive mantle normalization values of McDonough

et al. (1992).

K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227 211

depleted mantle at 3.2 Ga. Conversely, the ISr values at

3.2 Ga are much too high since they vary from 0.7021 to

0.7065. This illustrates that these rocks have undergone a

Rb loss.

The second orthogneiss type shows alkali-type com-

positions ranging from granodiorite–monzogranite togranite (SiO2 ¼ 69–75 wt.%). With the exception of the

most felsic members, which display a slightly peralumi-

nous trend, the remaining samples are metaluminous.

They are rich in K2O (>6 wt.%), low in Na2O (<2.9

wt.%), and have very low XMg ratios (0.07–0.11). Their

composition suggests a rift-type magmatism (Peucat

et al., 1996). The enrichment in incompatible elements

such as Ta, Nb, Y, and especially Ba (>1000 ppm), Hfand Zr (500–600 ppm), as well as their high Ga/Al ratios

(Peucat et al., 1996) also shows an affinity with intra-

plate granites and particularly the type-A2 granites of

Eby (1992). Apart from the most mobile elements and

Hf and Zr, the trace element contents of the alkali-

orthogneisses (Fig. 4) are very close to those of the

Archaean ALK3-type granites of Sylvester (1994). For

example, the REE are enriched and show relativelyweak fractionation ((La/Yb)N ¼ 6–10), in particular the

HREE ((Gd/Yb)N <1.7). On the other hand, Hf and Zr

contents are typical of present-day alkali-granites. Sev-

eral authors (Anderson, 1983; Creaser et al., 1991)

attribute the origin of this type of granite to partial

melting of tonalites. However, the binary diagrams trace

elements rule out any genetic link with the sampled

TTGs. The Rb/Sr pair makes it possible to calculate anisochron at 2.61 ± 0.07 Ga with an initial ISr ratio of

0.709 ± 0.002 (MSWD¼ 10). This is consistent with the

result obtained by the U–Pb method on zircons, which

yields an age of 2.65 Ga (Peucat et al., 1996). The eNd

values corresponding to 2.65 Ga are low and relatively

homogeneous, being located around )6.5, whereas TDM

model ages vary between 3.7 and 3.3 Ga. Thus, the

isotopic compositions of Sr and Nd suggest the

reworking of an older source. However, these alkali

granites also exhibit low dO18 values (5.95–7.7&; Peucat

et al., 1996), suggesting a re-equilibration at high tem-

perature with their surrounding metasedimentary rocks,which have a low dO18. This implies that the alkali-

granites have a juvenile origin (or at least are derived by

remelting of a juvenile material, for example of met-

adiorites). For the type-A granites, the juvenile source

could correspond mainly to the lower part of the con-

tinental lithosphere enriched in HFSE and the upper

part of the asthenosphere, as suggested by Li�eegeois et al.

(1998). The isotopic compositions would thus resultfrom a previously enriched lithosphere or a crustal

contamination process (Li�eegeois et al., 1998; Bonin et al.,

1998).

Orthogneisses belonging to the third group have

compositions typical of calc-alkaline suites (Peucat et al.,

1996). They are divided into two units. The first unit

consists of granodioritic to monzogranitic gneisses of

very slightly peraluminous composition (A/CNK¼1.01–1.08) having XMg (0.22–0.36) typical of Archaean

calc-alkali granites as defined by Sylvester (1994). Ele-

mental abundances normalized to the primitive mantle

show patterns similar to those observed in orogenic

magmatic suites, with negative anomalies in Ba, Ta, Nb,

Sr, P and Ti (Fig. 4). However, just like the Archaean

TTGs, these rocks yield highly fractionated REE ((La/

Yb)N �50) with a concave-upward pattern for theHREE. In addition to the low contents of TiO2 and V,

this suggests a residue containing garnet (Sylvester,

1994). Except for the most mobile elements (Cs, U,

Rb and Cu), the trace element composition of these

ancient granites is comparable to the Archaean CA2-

type calc-alkaline granites of Sylvester (1994). This

212 K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227

author attributes their origin to the fusion of a tonaliticArchaean lower crust. Such an origin implies a thick-

ening of the crust or heat event and the prior existence of

TTG. The second unit shows a calc-alkaline trend and is

made up of orthogneisses with monzogranitic compo-

sition. Their major element composition is typical of

post-orogenic highly fractionated calc-alkaline granites

(Peucat et al., 1996). Their trace element contents re-

semble those of the Archaean calc-alkaline granites oftype CA1. According to Sylvester (1994), such granites

would be derived from the melting of tonalites inter-

stratified with metasedimentary rocks, at shallower

depths than for the CA2, without leaving garnet in the

residue. The U/Pb zircon data indicate an emplacement

age of 2.5 Ga for both types of calc-alkali-granites, with

a minimum protolith age of 2.94 Ga (Peucat et al.,

1996). The Sm/Nd isotopic data for the first unit, withCA2-type composition, provide model ages of around

3.1 Ga. The second unit has more scattered TDM, be-

tween 3.05 and 3.5 Ga. Peucat et al. (1996) attribute

these values to the presence of an older tonalitic pro-

tolith.

2.2. Supracrustal mafic and ultramafic meta-igneous

granulites

The mafic granulites (metanorites and garnet pyroxe-

nites) are mostly located around In Hihaou, but they

also occur in the areas of Tekhamalt, Ihouhaou�eene andAmesmessa (Figs. 1 and 2). They occur in boudinaged

lenses parallel to the foliation in Al–Mg granulites, ban-

ded iron formations, marbles, peridotites, enderbitic and

charnockitic orthogneisses. Two types of metamorphosed

mafic rocks––with or without garnet––are distinguished

in the field. The metanorites are composed of salitic clin-

opyroxene (Ca48Mg40Fe12), orthopyroxene (XMg ¼ 0:65),

plagioclase (An48 to An70), pargasite, magnetite andsometimes biotite. The garnet pyroxenites are composed

of two quite distinct parageneses. The primary assemblage

is characterized by the presence of garnet, clinopyroxene,

pargasite, plagioclase (labradorite), magnetite, rutile and

quartz. This assemblage is involved in reactions leading to

the development of orthopyroxene–plagioclase symplec-

tites following decompression (Kienast and Ouzegane,

1987; Djemai, 1996).The peridotites appear especially in lenticular masses

several hundreds of metres thick in the northern part of

In Ouzzal (In Hihaou and Tekhamalt), frequently

associated with pyroxenites, anorthosites and with me-

tasedimentary formations with which they are folded.

These banded rocks generally show a very advanced

degree of serpentinization. These peridotites can be

classified into two groups (Bendaoud et al., 2001): (1)lherzolites containing olivine (Fo88), diopside, orthopy-

roxene (XMg ¼ 0:87), spinel (24–42% Cr2O3) and (2)

harzburgites containing olivine, orthopyroxene and

magnetite. The pyroxenites are characterized in the fieldby a light green colour, and are composed essentially of

orthopyroxene (XMg ¼ 0:75–85) and pargasite (XMg ¼0:82–0:90) showing equilibrium textures with triple

points. The chromium-rich (6–33% weight oxide) brown

spinel forms interstitial fine crystals or inclusions in

pargasite or enstatite. Plagioclase (An80–99) is ubiquitous,

appearing in patches, whereas phlogopite (XMg ¼0:87–0:92) and diopsidic clinopyroxene (Ca49Mg45Fe6)are rarer.

All the mafic and ultramafic rocks clearly show a

mobility in certain elements such as Rb, U and Th, as

indicated by Rb/Sr (0.01–3.44), La/Th (<6) and Th/U

(<8) ratios that are abnormal for igneous rocks (Peucat

et al., 1996). The same probably applies to Na and K.

The samples with the highest MgO contents (>20%) are

cumulates, as shown by their contents of Ca, K, P and Tiand certain trace elements such as Sm and Yb that are

depleted relative to the primitive mantle. These rocks

display chondritic values for Al2O3/TiO2, CaO/Al2O3

and (Gd/Yb)N , with average ratios of 22.3, 0.85 and 1.05,

respectively (Bendaoud et al., 2002). The metanorites

have MgO between 7.5 and 13.5 wt.%, and are highly

magnesian tholeiites. Their REE contents and abun-

dance patterns are comparable with those of Archaeantholeiites (Fig. 5). Some metanorites, like those of Roc-

can and Tin Tchik Tchik, plot in the fields of calc-alka-

line rocks. Even if some of these rocks are clearly

depleted in the most mobile elements (Cs, Rb, Th and U),

they yield REE patterns and spiderdiagram similar to

those of present-day highly magnesian andesites (Fig. 5).

Not all of these rocks could be dated. They yield ISr ratios

that remain high even at 3.2 Ga (0.7046–0.7072) and TDM

model ages which vary from 3136 to 4193 Ma (Peucat et

al., 1996). These values indicate that the element pairs

Rb/Sr and Sm/Nd were strongly disturbed by meta-

morphic alteration and/or events subsequent to their

emplacement. This is often the case for the mafic and

ultramafic rocks from Archaean greenstone belts and

particularly those having undergone high-grade meta-

morphism (Gruau et al., 1992; Arndt, 1998).

2.3. Supracrustal metasedimentary formations

2.3.1. Al–Mg granulites

Alumino-magnesian granulites are exceptionally well

developed near Adrar In Hihaou and in the Tekhamalt

area, but also occur more rarely in the areas of

Ihouhaouene and Amesmessa (Figs. 1 and 2). These

granulites are generally interstratified with rocks of

sedimentary origin such as banded iron formations and

marbles. They are also in direct contact with meta-

igneous rocks such as enderbites, charnockites, metan-orites or peridotites. A first distinction can be made

between two types of alumino-magnesian granulite

displaying a clear contrast in the field (Kienast and

Fig. 5. Chondrite-normalized REE patterns for Archaean mafic and ultramafic granulites. Normalizing values are those of Boynton (1984). Data

after Peucat et al. (1996).

K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227 213

Ouzegane, 1987). One type of granulite is rich in quartz,

while the other is quartz-free. However, the majority of

these granulites are rich in quartz and form layers with a

brown patina of variable thickness (from one to several

metres). They are dense massive rocks with a sometimeswell-marked foliation. Garnet and orthopyroxene por-

phyroblasts show a ductile behaviour expressed by an

elongation of the grains materialising the sub-vertical

stretching lineations as well as the torsion of orthopy-

roxene cleavage planes. This provides evidence of high-

temperature deformation by diffusion-creep at the grain

boundaries. Within these quartzitic layers, occur quartz-

free lenses of varied sizes (a few cubic centimetres toseveral cubic metres), easily recognizable in the field

because of their blue colour due to the abundance of

sapphirine. All these rocks are remarkable in exhibiting

the association orthopyroxene–sillimanite along with an

extremely diverse mineralogy made up of cordierite,

biotite, spinel, corundum, plagioclase, potassic feldspar,

ilmenite and rutile. Like sapphirine, garnet can be an

abundant mineral in certain layers. Based on the pres-ence of certain index minerals and their silica content,

alumino-magnesian granulites can be classified into five

groups (Bernard-Griffiths et al., 1996; Ouzegane and

Kienast, 1996; Fig. 6): spinel-bearing Al–Mg granulites

(SiO2: 37–40%), corundum-bearing Al–Mg granulites

(SiO2: 40–45%), quartz-free Al–Mg granulites (SiO2: 43–

45%), quartz-bearing Al–Mg granulites (SiO2: 46–63%)

and quartzitic Al–Mg granulites (SiO2: 65–85%).

2.3.2. Al–Fe granulites and quartzites

Al–Fe granulites containing garnet and sillimanite are

well represented in the area of Ihouhaouene (Fig. 2) near

the carbonatite occurrences (Ait Djafer and Ouzegane,

1998). They crop out in an extremely variable way in

stratified massive bands that form hills rising 10 m

above the reg between carbonatite centres 1 and 2

(Ouzegane, 1987), sometimes in discontinuous disruptedbands forming lenses from 10 to 20 cm in size. Quartzo-

feldspathic veinlets are distributed in the fold hinges,

giving these rocks a migmatitic appearance (Ait Djafer,

1996; Ait Djafer and Ouzegane, 1998). In thin section,

these veinlets are composed of quartz in plates with

sutured outlines associated with plagioclase (An33) and

potassic feldspar. Primary garnet (XMg ¼ 0:2–0:4) is

systematically surrounded by cordierite, whereas thespinel occurs as vermiculated grains (XMg ¼ 0:15–0:39),

that grow out from sillimanite to form symplectites with

cordierite (XMg ¼ 0:64 to 0.87). The synchronous

development of cordierite and spinel is explained by the

breakdown of garnet in contact with sillimanite during

decompression. Some large primary spinel crystals are

isolated from quartz by a corona of cordierite (Ait

Djafer and Ouzegane, 1998).Gedrite–garnet–sillimanite granulites appear in cm-

scale lenses associated with Al–Mg granulites containing

sapphirine–orthopyroxene–sillimanite, and with garnet

pyroxenites in the area of Amesmessa (Djemai, 1996;

Ouzegane et al., 1996). The gedrite–sillimanite associa-

tion is rare, existing only in few documented localities

worldwide (Robinson and Jaffe, 1969; Spear, 1982;

Goscombe, 1992; Spear, 1993). The primary paragenesisis made up of garnet (pyrope: 22–50%, almandine: 45–

75%), gedrite (XMg ¼ 0:64–0:70), sillimanite, quartz,

plagioclase (An25–42), rutile, ilmenite and biotite

(XMg ¼ 77–78). The minerals belonging to the primary

Fig. 6. Classification of Al–Mg and Al–Fe granulites according to The Mg/Mg+Fe (in mole proportions) versus SiO2 (wt.%) plot showing fields for

quartz-free granulites (SiO2 < 45%) and quartz-bearing granulites (SiO2 > 45%). This diagram also distinguishes garnet-free Al–Mg granulite (Mg/

Mg+Fe >0.80) from garnet-bearing Al–Mg and Al–Fe granulites (Mg/Mg+Fe> 0.80); orthopyroxene-free granulites (XMg < 0:50Þ are also shown

as a separate field from orthopyroxene-bearing granulites. Data reported in Bernard-Griffiths et al. (1996), Badani-Djebloun (1998) and Ouzegane

et al. (2003).

214 K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227

paragenesis are not observed in direct contact with each

other. Instead, they are always separated by coronas or

symplectites related to the secondary paragenesis made

up of cordierite-orthopyroxene (with or without spinel)

formed following decompression. The newly formed

orthopyroxene displays an enstatite composition near

47–51 mol% when it develops from the reaction: garnet +quartz) orthopyroxene+ cordierite, but is more mag-

nesian (enstatite 62–68%) when gedrite participates in

the reaction in the presence of quartz. The cordierite–

orthopyroxene–spinel symplectites develop at the con-

tacts between gedrite and garnet in the microdomains

without quartz. Garnet is surrounded by a corona of

cordierite and sillimanite is surrounded by cordierite-

spinel symplectites in the microdomains lacking quartzand gedrite, suggesting a decompression reaction: gar-

net + sillimanite) cordierite + spinel. Cordierite alone,

forming a corona, separates garnet from sillimanite and

quartz in the absence of gedrite (Ouzegane et al., 1996).

Al–Fe quartzites containing hercynite–quartz are less

rare than the preceding rock-types; they form discontin-

uous layers generally related to the shear zones at Alouki,

Ihouhaouene and Tin Tchik Tchik, where they are asso-ciated with rocks of sedimentary origin (Fig. 2). The

development of garnet and sillimanite in coronas or

symplectites is explained by a reaction of the type:

hercynite+magnetite+ quartz) almandine+ sillimanite

or hercynite+ quartz) almandine+ sillimanite (Badani-

Djebloun et al., 2000). We also sometimes observe

thin garnet coronas around the sillimanite and magne-

tite. The following reaction accounts for these tex-tures: quartz+ sillimanite +magnetite) almandine+O2

(Helal, 1987).

Al–Mg and Al–Fe granulites are composed essen-

tially of the components SiO2–Al2O3–FeO–MgO,

accounting for a sum higher than 90% of the total oxi-

des. Fig. 6 shows a plot of Mg/Mg + Fe mole ratio

versus SiO2 for a set of whole-rock samples. This dia-

gram allows to distinguish granulites containing primary

garnet (XMg ¼ 0:30–0:80) from those which do notcontain any (XMg ¼ 0:82–0:96), as well as granulites with

quartz (SiO2 > 45%) from quartz-free varieties (SiO2 <

45%). Al–Fe granulites exhibit XMg ratios ranging be-

tween 0.3 and 0.48, and this chemical characteris-

tic prevents the development of the orthopyroxene +

sillimanite or sapphirine-bearing association. This is in

contrast with Al–Mg granulites (XMg > 0.50), which are

characterized by the omnipresence of these associations.The major and trace geochemistry of Al–Mg granulites

from In Ouzzal, especially the very high contents of Cr

(400–2500 ppm), Ni (100–1100 ppm), Co (50–80 ppm),

cannot be explained by a process of partial melting. The

preservation of their oxygen isotope composition (Ber-

nard-Griffiths et al., 1996) suggests rather that their

particular refractory chemistry is of pre-metamorphic

origin. Thus, the lowest dO18 values––varying from +5to +6&––are recorded in Al–Mg granulites from the

area of Tekhamalt. Although these values do not cor-

respond to the dO18 of ordinary pelites, they are com-

parable with those of chlorites, a mineral produced by

the hydrothermal alteration of mafic to ultramafic

materials in the marine environment (Rahmani, 1992;

Bernard-Griffiths et al., 1996). These metasedimentary

rocks have dO18 values between 5 and 9.5&, corre-sponding to a mixture, in variable proportions, between

a mature detrital component (composed essentially of

K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227 215

quartz) derived from the erosion of a granitic protolithand an extremely immature component, resulting from

hydrothermal alteration of mafic to ultramafic proto-

liths. This latter component is of chloritic type, rich in

Al, Mg, Cr, Co and Ni, with possible additions of

chromium-rich spinel and small proportions of illite

and/or muscovite to account for the K contents of these

granulites (0.3–2%, Bernard-Griffiths et al., 1996). The

very high contents of chromium (3960 ppm) and nickel(up to 1100 ppm) in the Al–Mg granulites of In Ouzzal

is a typical feature of early Archaean immature sedi-

ments (Bernard-Griffiths et al., 1996). Al–Fe granulites

also exhibit Cr, Ni and Co contents that are comparable

with Archaean and end-Archaean shales, since they

result––like the Al–Mg granulites––from the mixing of

chloritic and detrital quartz components (Badani-Djeb-

loun, 1998). The Al–Mg quartzites and Al–Fe quartzitesshow SiO2 contents varying from 62 to 85 wt.% and

chromium contents of up to 100–200 ppm, and are

characterized by the presence of detrital chromite in the

sedimentary protoliths. These rocks resemble the fuch-

site-bearing quartzites of the Archean cratons (Eriksson

et al., 1997).

Detrital zircons from the Al–Fe quartzites of Ihou-

haou�eene show growth structures with ancient cores andcoronas formed during the granulite-facies event at 2

Ga. Inherited zircon cores indicate ages that are scat-

tered between 3100 and 2700 My (Lancelot et al., 1976;

Helal, 1987; Peucat et al., 1996). These geochronological

data indicate that the deposition of the supracrustal

series probably took place after 2 700 Ma (Peucat et al.,

1996). Inherited zircons present in the Al–Mg granulites

show a spectrum of ages ranging between 2700 and 3200Ma, which corresponds to the emplacement of the var-

ious generations of TTG (Peucat et al., 1996). This im-

plies that part of silica present in these rocks had a

detrital origin, possibly derived from the erosion of pre-

existing Archaean granites at In Ouzzal.

2.3.3. Magnetite-bearing quartzites (banded iron forma-

tion)

The main outcrops of banded iron formation occur in

the Alouki massif, where they are involved in map-scale

folds that can be followed over more than 20 km. These

rocks occur also in the areas of Ihouhaouene, Tin TchikTchik, Tekhamalt and Amesmessa in more or less con-

tinuous beds from 1 to 50 m thick repeated by isoclinal

folds (Fig. 2). While they display a practically constant

association with marbles and metanorites, they are also

encountered in the area of Ihouhaouene in direct con-

tact with fenites (carbonatite centres 1 and 2; Ouzegane,

1987; Guiraud et al., 1996). Throughout these areas, the

iron-bearing formations and their associated rocks makeup supracustal sequences forming basins (Fig. 3) that

are separated from domes of enderbites. The mineralogy

of the banded iron formation is generally simple, being

composed of quartz and magnetite. Magnetite is eitherdisseminated in the rock, or clustered together in beds

varying in thickness from 1 cm to 1 m. These rocks are

sometimes highly deformed with thin levels of pseu-

dotachylite. There are also some banded iron formations

with more varied mineralogy, notably containing asso-

ciations with corundum–quartz (Ihouhaouene; Guiraud

et al., 1996) and garnet–orthoferrosilite–almandine–

spinel (Tekhamalt; Ouzegane and Kienast, 1996;Badani-Djebloun, 1998).

2.3.4. Marbles

The marble layers are generally limited to isolated

lenticular bands (a few metres to 100 m thick), but some

very extensive outcrops are observed in the areas of Illa-saka, Ihouhaouene, Roccan, Tekhamalt (Fig. 2), Tirek

and Amesmessa (Fig. 1A). They generally form folded

layers with vertical axes, like the associated banded iron

formations. The In Ouzzal marbles exhibit a wide miner-

alogical diversity. Two main groups can be distinguished,

dolomitic marbles characterized by a paragenesis with

dolomite–olivine–spinel–phlogopite–amphibole–clinopy-

roxene, and dolomite-free marbles with calcite–quartz–grossularite–andradite–wollastonite (Benyahia, 1996;

Boureghda, 2000). Scapolite-bearing marbles are associ-

ated with these two categories, but are found in lesser

proportions.

The identification of the protoliths of the dolomitic

marbles was established by Fourcade et al. (1996), based

on the chemical composition and isotopic characteristics

of these marbles. The high Al and Mg contents of thedolomitic marbles could be due to the addition of a

chloritic component derived from the hydrothermal

alteration of a mafic/ultramafic source during carbonate

sedimentation. The carbonate sediment protolith would

be composed of varied proportions of calcite, dolomite

and chlorite. During granulite-facies metamorphism,

this impure dolomite produced forsterite–spinel marbles

with or without diopside. The isotope signatures ofoxygen (dO18 ¼ 7.9–18.9&) and carbon (dC13 ¼)0.8&

to )4.2&) are very heterogeneous, indicating that the

marbles of In Ouzzal have preserved their pre-meta-

morphic characteristics. The preservation of such an

isotopic heterogeneity implies the absence of homoge-

nizing fluids or percolation of CO2 of mantle origin

during the granulite-facies event (Fourcade et al., 1996).

2.3.5. Al–Mg–Ca granulites (paleoskarns)

Al–Mg–Ca granulites crop out especially in the area

of Tekhamalt (Fig. 2), in direct contact with charnoc-

kitic orthogneisses of alkali-granitic type, and are sys-

tematically associated with the olivine–spinel marbles as

in the areas of Alouki, In Hihaou, Tirek and Tekhamalt.They are banded rocks remarkable by the presence of

the association spinel–pargasite–anorthite and fassaite-

type clinopyroxene, with occasional pyrochlore and

216 K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227

relics of calcite. In the area of Alouki, there are rareassociations containing musgravite–sapphirine–spinel–

phlogopite–corundum–pyrochlore–scheelite–monazite

intercalated with layers of pargasite–clinopyroxene–

musgravite–spinel–anorthite–pyrochlore and calcite (Be-

nyahia, 1996). Musgravite, a beryllium-rich mineral

(5–6% Be; Boumaza-Benyahia et al., 2001), is very rare,

since there are only five known occurrences worldwide.

In the area of Tekhamalt, the dolomitic marbles incontact with charnockites (alkali-granitic orthogneisses)

develop Al–Mg–Ca granulites with diopside–anorthite–

spinel. Fourcade et al. (1996) interpret these rocks as

resulting from decarbonatization, i.e. paleoskarns,

based on their high W and Ag contents (300 ppm and

325 ppb, respectively). The process of decarbonatization

took place before the Paleoproterozoic granulite-facies

metamorphism, and is probably contemporary with theemplacement of alkali-granitic orthogneisses at around

2.65 Ga (Fourcade et al., 1996). In the area of Alouki,

Al–Mg–Ca granulites containing musgravite also result

from a metasomatic process related to these charnock-

itic granites (rich in Nb, Ta and Be) in contact with

dolomitic limestones (Benyahia-Boumaza et al., in

preparation).

2.4. Paleoproterozoic magmatic activity

The magmatic events that accompanied the Paleo-proterozoic granulite-facies metamorphism have re-

sulted in the emplacement of anorthosites, small masses

of cordierite-bearing monzogranitic gneisses (Peucat

et al., 1996) and carbonatites. In the area of Tirek,

syenites intruded along the shear zones over a distance

of more than 50 km (Haddoum, 1992) yield U/Pb zircon

dates of 1 991 ± 20 Ma (Semiani, 1995).

2.4.1. Anorthosites

Anorthosites crop out either in the form of metre-

scale disrupted lenses associated with the supracrustal

series or in more massive layers attaining 1 km thick.The massive anorthosites exposed in the area of Tike-

chitine show stratified structures evocative of cumulate

textures. Pyroxenes are variably distributed, sometimes

concentrated in more or less well defined beds of a few

decimetres to a few centimetres thick. In outcrop, we can

observe a sharp or gradual transition from anorthosites

to leuconorites, with field relations similar to those seen

in layered complexes (Ashwal, 1993). Pyroxene alsooccurs in metre-scale veins in the metanorites and garnet

pyroxenites, where these veins are folded along with the

country rocks. Plagioclase (labradorite–bytownite)

forms the essential mineral of the rock, and coexists with

orthopyroxene or clinopyroxene, accessory minerals

such as titanite, apatite, spinel and ilmenite and, less

constant, quartz, antiperthite and hornblende. Garnet

can be present surrounded by reaction rims with horn-blende-plagioclase. Very rare zircon is also observed.

The only dated anorthosite was sampled in the Alo-

uki area, where this rock-type occurs as veins (50 cm)

associated with metanorites. This rock shows a com-

position similar to the plagioclase-amphibole cumulates

observed in certain layered mafic intrusions, yielding an

age of 2002 ± 7 Ma (U/Pb on zircon; Peucat et al., 1996).

However, Sr–Nd isotopic analyses on ten samples froma more massive anorthositic unit in the Tikechitine area

yield older dates (Archaean, with an age around 3 Ga,

A€ııt-Djafer et al., in preparation).

2.4.2. Cordierite-bearing monzogranitic gneisses

Cordierite-bearing monzogranitic gneisses occurthroughout the In Ouzzal terrane, from North to South.

They form massive leucocratic layers, sometimes

exceeding 100 m in thickness and showing the same

deformation characteristics as the supracrustal series.

These rocks occur also in cm-thick veinlets associated

with Al–Mg granulites or cutting the metanorites. From

the chemical point of view, they represent metamor-

phosed S-type granites (Peucat et al., 1996). Quartz oc-curs in equant patches or polycrystalline plates.

Plagioclase is oligoclase; antiperthite coexists with fine

perthites that are sometimes transformed into micro-

cline. The rocks contain also biotite, garnet, cordierite,

ilmenite and zircon. These rocks show a peraluminous

composition (A/CNK¼ 1.05–1.08) and are very rich in

SiO2 (�78 wt.%), K, Ba and Rb, and poor in Ti, Al, Fe,

Mn, Mg, P, Ta, Nb, Ga, Y, Zn and REE. SHRIMP U–Pb dating of overgrowths and euhedral clear zircon

yielded 207Pb/206Pb age of 1983 ± 15 Ma (Peucat et al.,

1996).

2.4.3. Carbonatites and fenites

In the area of Ihouhaouene, numerous carbonatitemassifs (Fig. 2) occur that are systematically associated

with fenites at their contact with the granulites (Ou-

zegane, 1987; Ouzegane et al., 1988; Bernard-Griffiths

et al., 1988; Carpena et al., 1988; Fourcade et al., 1996).

These rocks display some very particular features, dif-

fering from usual carbonatites by the lack of felds-

pathoids and the abundance of wollastonite which

coexists with calcite and quartz. These rocks show anextreme mineralogical diversity, containing carbonates

(calcite, bastnaesite), phosphates (apatite, monazite and

britholite) and silicates (clinopyroxene, amphibole, bio-

tite, and potassic feldspar). Accessory minerals include

titanite, allanite, fluorite, magnetite and grossularite–

andradite garnet. In the field, a relative chronology

comprising several stages can be established for the

emplacement of the carbonatite complex (Ouzeganeet al., 1988; Fourcade et al., 1996). Two carbonatite

generations are distinguished. The fenites are cut by

sovite veins with mylonitized margins. These carbonatite

K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227 217

veins (1–500 m thick and up to 2 km long) can beanastomosed, forming a true network. The fragments in

the carbonatite breccias are made up of fenites rich in

clinopyroxene clusters surrounded by grossularite–

andradite, and potassic feldspar xenoliths. In the matrix

cementing the fragments, calcite predominate (50–70%).

These rocks are characterized by pink apatites (0.5–1.5%

REE, Ouzegane et al., 1988). The second generation of

intrusions is represented by pegmatitic carbonatite veinsof restricted size (maximum 5 m thick) cuts the brecci-

ated carbonatites. Very large crystals of apatite (up to 1

m; Fourcade et al., 1996) make up to 50% of the modal

composition. They contain many microinclusions such

as monazite, quartz, bastanaesite and britholite. The

britholite in exsolution in apatite can occupy more than

40% of the volume of the apatite. Substitutions of the

type P + Ca)REE +Si can account for the exception-ally high rare earth contents of these apatites (1–7%

REE) and of the britholites (30–62% REE, Ouzegane

et al., 1988). The brecciated carbonatites systematically

cut across two types of fenite––red or white––located at

the margins of the complexes in direct contact with the

granulites, from which they are derived by metasomatic

transformation (Fourcade et al., 1996). The wollas-

tonite-bearing white fenites show a clear foliation con-cordant with the foliation of the granulites, with vertical

to subvertical dips. The red fenites are rocks with

banding formed of dark clinopyroxene and pale-red

layers rich in potassic feldspar. Garnet, titanite, calcite,

apatite and quartz are in small amounts. Wollastonite

can be well developed in the white fenites, associated

with potassic feldspar, garnet and apatites rich in

britholite (Fourcade et al., 1996).The compositions of carbonatites and fenites from

the Ihouhaouene massifs are characterized by high

contents in light REE (LREE), Th, U, F, Ba and Sr, but

are relatively depleted in Nb, Ta, Hf, Zr and Ti (Ou-

zegane et al., 1988; Bernard-Griffiths et al., 1988;

Fourcade et al., 1996).The emplacement of carbonatites

in the Ihouhaouene area has been dated at 1 994 ± 15

Ma by U/Pb zircon (Bernard-Griffiths et al., 1988),which confirms the earlier result of All�eegre and Caby

(1972) who obtained an age of 2 090 ± 20 Ma on a flu-

oro-apatite rich in thorium. Carbonatite emplacement is

coeval with the functioning of ductile shear zones under

granulite-facies conditions (Ouzegane et al., 1988;

Fourcade et al., 1996). The isotopic signatures in the

fenites and the carbonatites represents one of the most

particular features of the Ihouhaouene massifs. In termsof eNd (¼)6.3 to +0.9) and ISr (0.7093–0.7104) at 1994

Ma, the radiogenic isotope composition of the carbon-

atites and d18O values of metasomatic clinopyroxenes

(¼ 6.9–8&), indicate that the fluids from which they

derive underwent an intense interaction with the crust

during their ascent. The isotopic composition of carbon

(d13C¼)3.5& to +9.7&) suggests that these fluids

probably originate from the mantle by degassing ofmantle-derived magmas at great depth (Fourcade et al.,

1996). The migration of fluids through the crust via

major shear zones could have redistributed the elements

dissolved in these fluids (starting from orthogneisses

with TTG composition), particularly the LREE, K, Sr

and Th (Fourcade et al., 1996).

3. Dome and basins structure reworked by Paleoprotero-

zoic tectonics

Geologically, the In Ouzzal terrane is characterizedby NE–SW to ENE–WSW closed structures trending

that correspond to domes of enderbitic orthogneiss (Fig.

2). The supracrustal formations are made up of meta-

morphosed sedimentary rocks and mafic/ultramafic

igneous rocks that occupy the basins (Fig. 3) between

these domes (Haddoum, 1992; Haddoum et al., 1994).

In certain parts of In Ouzzal, the supracustal synforms

and orthogneiss domes exhibit linear corridors neartheir contacts corresponding to shear zones. The shear

zones are preferentially formed in quartzites and mar-

bles that are often ultramylonites (Alouki, Ihouhaoune

and Tirek). In these shear zones, the deformation related

to vertical kinematics interferes with ductile strike-slip

deformation, in which the horizontal stretching direc-

tions are superimposed on NE–SW sinistral shearing in

Alouki and ENE–WSW dextral shearing in Tin TchikTchik (Fettous, 2001). Locally, these shear zones host

granulitic pseudotachylites with orthopyroxene recrys-

tallisation occupying tiny pull-apart structures and cm-

scale gashes indicating a NE–SW stretching during

NW–SE shortening (Fettous, 2001). The formation of

these very-high-temperature pseudotachylites is proba-

bly due to the rheology of these completely dehydrated

rocks (Clarke and Norman, 1993). In the linear corri-dors, the folds are well expressed and have a sheath

geometry consistent with ductile deformation during the

transcurrent shearing (Haddoum, 1992). The folding

affects all lithologies. In the Ihouhaouene area, the fe-

nites have planar structures that are concordant with the

foliation of the surrounding granulites with horizontal

mineral lineations. The strike of the foliations is oblique

to the shear zones, thus making it possible to recognizeconjugate movements that are sinistral (NE–SW) as well

as dextral (ENE–WSW). These observations are com-

patible with the NW–SE shortening recorded in the

surrounding granulites. The emplacement of the first

generation of carbonatites occurred during this trans-

current shearing. The foliation of the fenites in the most

highly deformed domains is involved in sheath folds that

accommodate the horizontal displacements. These foldsare well studied in the mylonitic zones, being attributed

to intense deformation with a strong shearing compo-

nent. Various studies on the structures forming these

218 K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227

domes and basins (Haddoum, 1992; Haddoum et al.,1994; Fettous, 2001) suggest a strong resemblance with

structures documented in Archaean terrains such as the

Dharwar Craton in the south of India. In this latter

area, they are interpreted as being related to deforma-

tion due relative vertical displacements between gneissic

diapirs and greenstone belts rocks that are said to be

‘‘sagducted’’ (Bouhallier et al., 1995; Choukroune et al.,

1997). In the In Ouzzal terrane, the history of Paleo-proterozoic deformation was firstly interpreted in terms

of two successive stages (Haddoum et al., 1994), the first

corresponding to nappe tectonics and a second stage

characterized by faulting along transcurrent shear zones.

More recent studies (Fettous, 2001; Mahdjoub, personal

communication) refutes this and suggests that Paleo-

proterozoic tectonics consisted only of a re-activation of

earlier deformation. The structural features observed atthis level––corresponding to the base of the crust––

suggest that deformation took place entirely under

granulite-facies conditions. All the structures are com-

patible with a homogeneous NW–SE shortening (Fet-

tous, 2001). This horizontal shortening accentuates the

vertical stretching and flattening of old structures in the

form of basins and domes, which explains their elliptical

shape and their NE–SW and ENE–WSW elongation(Fig. 3). Both in the enderbitic orthogneisses and in the

metasedimentary formations, this shortening is ex-

pressed by the presence of a foliation and generally

vertical mineral stretching lineations. According to

Fettous (2001), the homogeneous shortening of the do-

mes and basins, the offset of transcurrent shear zones

and the formation of pseudotachylites would corre-

spond to three successive stages of deformation, allbeing Paleoproterozoic. However, it appears more likely

that these stages are contemporaneous. The vertical

stretching of structures is accommodated by horizontal

displacements along corridors of transpressive shearing,

as suggested by models for the formation of tectonic

domes and basins in the Dharwar Craton (Bouhallier

et al., 1993; Bouhallier et al., 1995).

4. Paleoproterozoic paragenetic evolution and metamor-

phic conditions

The ages of 2100–1950 Ma on minerals by Rb/Sr

method (All�eegre and Caby, 1972), U/Th/Pb on zircon

(Lancelot et al., 1976; Peucat et al., 1996) and Sm/Nd on

garnet (Ben Othman et al., 1984; Peucat et al., 1996)

imply that the In Ouzzal terrane acquired its metamor-

phic imprint during the Eburnean orogeny.

The granulites of In Ouzzal are particularly well

suited for the reconstruction of the metamorphic historybased on the chemical compositions of minerals and

rocks. This approach makes it possible to determine the

variations of pressure, temperature and XH2O. The esti-

mation of the conditions of pressure and temperaturehas been carried out using Al–Mg granulites (Ouzegane,

1987; Kienast and Ouzegane, 1987; Bertrand et al., 1992;

Ait Djafer, 1996; Boumaza, 1996; Mouri et al., 1996;

Ouzegane and Boumaza, 1996; Adjerid, 2002), Al–Fe

granulites (Ait Djafer, 1996; Djemai, 1996; Ouzegane

et al., 1996), corundum–magnetite quartzites (Guiraud

et al., 1996), calc-silicate granulites (Benyahia, 1996)

and garnet pyroxenites (Ouzegane, 1987; Kienast andOuzegane, 1987; Djemai, 1996). The most complete se-

quence of reactions could be identified in the alumino-

magnesian granulites, where a prograde metamorphism

is recorded under granulite-facies conditions at extreme

temperatures (Ouzegane, 1987; Kienast and Ouzegane,

1987; Bertrand et al., 1992; Ouzegane and Boumaza,

1996; Ouzegane et al., 2003). Although there is a con-

tinuum in the succession of parageneses, the metamor-phic history can be subdivided into two major stages

(Fig. 7): a prograde stage taking place at a pressure of

10–11 kbar characterized by a rise in temperature from

800 to 1050 �C, followed by a decompression from 9 to 5

kbar at temperatures close to 900–1000 �C (Ouzegane,

1987; Bertrand et al., 1992; Ouzegane and Boumaza,

1996). The prograde stage led to the development of

very-high-temperature metamorphic conditions. It ischaracterized by the significant growth of a number of

minerals and the formation of characteristic mineral

associations such as sapphirine–quartz and sapphirine–

spinel–quartz. The metamorphism during decompres-

sion is characterized by the formation of increasingly

fine-grained symplectites with cordierite, sapphirine,

orthopyroxene and spinel.

4.1. Prograde stage

This early stage, illustrated particularly in the system

FeO–MgO–Al2O3–SiO2 (FMAS), is characterized by theequilibrium of primary parageneses such as orthopy-

roxene–corundum–garnet, orthopyroxene–corundum,

garnet–spinel, garnet–spinel–corundum, garnet–quartz–

sillimanite–orthopyroxene, orthopyroxene–sillimanite

and orthopyroxene–sillimanite–quartz. In the more

complex chemical systems, occur variable amounts of

biotite, potassic feldspar, plagioclase and rutile. The

evolution of parageneses in the quartz-bearing Al–Mggranulites of In Ouzzal can be represented in a pressure–

temperature grid adapted to the FMAS system (Fig.

7). This diagram is established using the Thermocalc

software, which includes data on sapphirine (V 2.75

Holland and Powell, 1998). Ouzegane et al. (2003)

modified the sapphirine enthalpy calculated by Holland

and Powell (1998) in order to fit the curves calculated by

Thermocalc to the slopes of experimental curves deter-mined for reactions such as pyrope + corundum + spi-

nel) sapphirine (Ackermand et al., 1975). Fig. 7 shows

the position of the invariant points [Sp] and [Opx] cal-

Fig. 7. Petrogenetic grid calculated in the FMAS system using Thermocalc. The P–T paths are derived from the study of quartz-bearing rocks. See

the text for discussion.

K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227 219

culated with a H2O values of 0.3 and 0.5. In this case,

the calculated spinel invariant point (9–10.5 kbar, 1050

�C) is compatible with the experimental data of Hensen

and Green (1973), Bertrand et al. (1991) and Harley and

Motoyoshi (2000). These authors give the following

conditions for the invariant point: 10.1 ± 1 kbar at 1020

�C, 9.5 ± 1 kbar at 1030 �C and 10.5 ± 1 kbar at 1050 �C,

respectively. In quartz-bearing Al–Mg granulites, theearliest reaction is garnet + quartz) orthopyroxene +

sillimanite (Fig. 7). The conditions of crystallization

during this early stage are estimated at around 9.5–

11.5 kbar and 1000 �C, using the calibration of the

garnet–hypersthene–sillimanite–quartz equilibrium as a

geobarometer (Bertrand et al., 1992); this result is in

agreement with the T–X diagram calculated by ‘‘Ther-

mocalc’’ at a pressure of 10.5 kbar (Fig. 8A). At 10 kbarand low fO2 conditions, the metamorphic peak tem-

perature is expressed by the association sapphirine–

quartz. Bertrand et al. (1991) show that this association

is stable above 1050 �C. This paragenesis can be formed

due to the crossing of the univariant reaction orthopy-

roxene + sillimanite) garnet + sapphirine + quartz (Fig.

8A). Orthopyroxene–quartz–sapphirine symplectites

around garnet have only been observed in the quartz-ites of Tekhamalt, along with evidence for the divari-

antreaction orthopyroxene + sillimanite) sapphirine +

quartz (Rahmani, 1992; Bertrand et al., 1992; Adjerid

et al., 2002; Adjerid, 2002). These symplectites appear to

develop in chemically distinct microdomains, since

reaction products involving garnet are richer in iron

than those lacking garnet (Fig. 8A). On the P–T grid

(Fig. 7), associations with sapphirine +quartz appear to

the right of the [Sp] invariant point, thus reflecting the

highest temperatures attained in the granulites.During the sequence of early prograde reactions,

garnet becomes enriched in iron (XMg decreasing from

0.64 to 0.50) and the Al2O3 content of the orthopyrox-

ene increases. The Al2O3 content of sapphirine decreases

in relation to a substitution of the type MgSi@AlAl

(Ouzegane, 1987; Bertrand et al., 1992; Ouzegane et al.,

2003). These observations, combined with thermody-

namic calculations and the topological constraints,clearly demonstrate that the high-pressure stage (10–11

kbar) is accompanied by a significant increase in tem-

perature (from 800 to 1050 �C and Fig. 7) under isobaric

conditions (Bertrand et al., 1992; Ouzegane and

Boumaza, 1996). During formation of the sapphi-

rine + quartz association, the temperature is extreme as

shown by the low value of Kd between hypersthene and

garnet (Kd: 2.06–1.99), the high Al2O3 content of theorthopyroxene (which reaches 11.8%) and the high py-

rope content in the garnet (maximum at 50–55 mol%).

Fig. 8. T–X pseudosections calculated in the FMASH system. Prograde paths for (A) quartz-bearing Al–Mg granulites and (B) corundum-bearing

Al–Mg granulites according to their XMg bulk composition. See text for discussion.

220 K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227

Thermometry based on the Fe–Mg exchange equilib-

rium and the solubility of alumina between garnet and

orthopyroxene yields temperatures close to 970 ± 70 �Cat 10 ± 1.5 kbar (Bertrand et al., 1992). This is com-

patible with the association sapphirine–quartz and the

grids calculated by Thermocalc. In addition, the Al–Mg

granulites of Ihouhaouene show reactions involving

phlogopite at very-high-temperature (900–1000 �C)

without partial melting in a liquid-absent KFMASH

system (Mouri et al., 1993; Mouri, 1995). One of the

hypotheses proposed by Mouri et al. (1996) to explainthe stability of the phlogopite + quartz assemblage at

very high temperature––without melting––is the pres-

ence of fluorine, which stabilizes biotite under extreme

granulite-facies conditions as shown by the experimental

work of Carrington and Harley (1995). Indeed, biotites

in the Al–Mg granulites of In Ouzzal have high fluorine

contents attaining 2–4%.In Al–Mg granulites with corundum or spinel, the

earliest reactions lead to the development of sapphirine

in large crystals (1–10 cm). The corundum is separated

from the orthopyroxene by a double corona of sapphi-

rine armouring the corundum and sillimanite, thus

suggesting the reaction orthopyroxene + corundum)sapphirine + sillimanite (Ouzegane, 1987; Bertrand et al.,

1992; Ouzegane et al., 2003). At 9 kbar, the associationhypersthene–corundum–sapphirine–sillimanite provides

temperatures of 795 �C for magnesian assemblages

and 850 �C for assemblages richer in iron (Bertrand

et al., 1992), which seems to indicate an increase in

K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227 221

temperature of 55 �C under isobaric conditions. Allthe prograde reactions are characterized by equilib-

rium mineral growth and textures with triple points.

A number of very rare prograde reactions (Ouzegane,

1987; Bertrand et al., 1992; Ouzegane et al., 2003) can

proceed until the complete disappearance of corundum

or spinel. These include the reactions: orthopyroxene +

corundum) garnet + sapphirine + sillimanite (Fig. 8B),

garnet + corundum + spinel) sapphirine and garnet +spinel)sapphirine+orthopyroxene. The prograde stage

can also be recognized in the scapolite marbles of

Ihouhaouene: in the system CaO- Al2O3–SiO2-Vapour

(CASV), the growth of wollastonite takes place at the

expense of calcite + quartz via the reaction cal-

cite + quartz)wollastonite + CO2. This reaction occurs

in response to an increase in temperature from 800 up

to 1000 �C at a pressure of around 10–11 kbar (Be-nyahia, 1996; Ouzegane et al., 2002). In the banded iron

formation of Ihouhaouene, the most extreme conditions

of pressure and temperature are represented by the

association corundum–quartz, with equilibrium condi-

tions estimated at 12 kbar and 1100 �C (Guiraud et al.,

1996).

4.2. Decompression stage

All primary and intermediate associations developed

during the prograde stage are destabilized during a

metamorphic evolution characterized by a significantdrop in pressure down to 5 kbar (Ouzegane, 1987;

Bertrand et al., 1992; Mouri et al., 1996; Ouzegane and

Boumaza, 1996; Adjerid, 2002; Ouzegane et al., 2003). A

P–X diagram was constructed in the FMAS system

using the program Thermocalc (Holland and Powell,

1998). This diagram proves particularly useful in

describing the evolution of the various assemblages as a

function of XMg in the rocks during a lowering of pres-sure. We fixed an aH2O value of 0.3 for the calculation of

this diagram (Fig. 9), thus taking account of the low

water activities encountered in granulite-facies rocks.

Fig. 9 shows the successive parageneses found in Al–Mg

granulites without garnet (XMg of the rock >0.80),

granulites with garnet (XMg ranging between 0.50 and

0.80) and Al–Fe granulites (XMg: 0.30–0.40). In the most

magnesian Al–Mg granulites with quartz (XMg >0.80,Fig. 9), the orthopyroxene, sillimanite and quartz are

systematically isolated by a corona of cordierite, sug-

gesting the reaction orthopyroxene+ sillimanite +

quartz) cordierite. This reaction occurs at a pressure

around 9 kbar at a temperature of 950 �C. This is in

agreement with the work of Annersten and Seifert

(1981), who show that magnesian cordierite can be

stable up to 11 kbar at 1000 �C. However, withincreasing iron in the system, this mineral can be sta-

ble up to 9± 0.5 kbar. The reaction orthopyroxene +

sillimanite) sapphirine + cordierite, which occurs in

Al–Mg granulites with corundum (XMg of the rock canreach 0.96; Ouzegane et al., 2003) as well as in the

quartz-free microdomains of Al–Mg granulites, indi-

cates pressures similar to the preceding reaction. In Al–

Mg granulites richer in iron (XMg close to 0.80), the

earliest divariant parageneses are represented by garnet–

sapphirine–sillimanite–quartz (Fig. 9). During the fall in

pressure, reactions involving garnet, such as the divariant

reaction garnet + sillimanite ) sapphirine + cordierite,take place before the crystallization of symplectites with

cordierite-spinel (Fig. 9). In the corundum-bearing Al–

Mg granulites and the Al–Mg granulites with or without

quartz, one of the latest reactions is orthopyroxene +

sapphirine) cordierite + spinel, which occurs at around

5–6 kbar (Fig. 9). In these Al–Mg granulites, various

chemical microdomains appear reflecting the different

XMg in minerals. Thus, between 7 and 8 kbar, the reac-tion garnet + sillimanite) cordierite + sapphirine is re-

lated to microdomains richer in iron than those involved

in the reaction garnet) orthopyroxene + cordierite +

sapphirine. Al–Fe granulite textures indicate that the

cordierite coronas and cordierite-spinel symplectites

were formed by decompression via the following reac-

tions: garnet + sillimanite + quartz) cordierite, garnet +

sillimanite) cordierite + spinel, garnet + sillimanite)cordierite + spinel + quartz. The latest-stage reaction

occurs following the growth of cordierite at the expense

of spinel–quartz, in relation to a fall in temperature to

750 �C at 5–6 kbar (Kienast and Ouzegane, 1987). The

marbles of In Ouzzal allow us to constrain the role of

fluids such as CO2 during decompression. In the scapo-

lite marbles, the decompression from 10 to 5 kbar is

marked by the appearance of anorthite and calcitefollowing the breakdown of meionite. The stability of

parageneses with calcite–quartz–grossularite–meionite–

anorthite provides evidence for XCO2 lower than 0.5

(Boureghda, 2000; Ouzegane et al., 2002). In the olivine-

spinel marbles, the reaction: 2 forsterite + 4 calcite +

2CO2 ) diopside + 3 dolomite takes place at XCO2 be-

tween 0.7 and 0.9 for temperatures close to 800–900 �C(Boureghda, 2000; Ouzegane et al., 2002). It appearsclearly that the buffering of XCO2 is internal, in relation

to the onset of various reactions at different stages in the

history of these rocks. Thus, CO2 is consumed during

the prograde stage in the scapolite marbles by the reac-

tion grossularite +CO2 ) 3 wollastonite + 2 calcite +

meionite. On the other hand, CO2 is released during the

late stages by the reaction meionite + 5 wollastonite) 3

grossularite + 2 quartz + CO2, at the same time as garnetcoronas are developed in association with quartz (Be-

nyahia, 1996; Ouzegane et al., 2002). As in the Archaean

marbles of Antarctica (Fitzsimons and Harley, 1994),

there is no exchange of CO2 between the surrounding

charnockitic rocks and the marbles, the latter behaving

like a closed system. This is in agreement with the very

heterogeneous isotopic signatures of these marbles,

P (

kbar

)

Fig. 9. P–X pseudosection calculated in the FMASH system for quartz-bearing, quartz-free Al–Mg granulites and Al–Fe granulites. The decom-

pression paths are drawn according to XMg bulk-rock composition. See the text for discussion.

222 K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227

which indicate they have preserved their pre-metamor-

phic features (Fourcade et al., 1996).

5. Interpretation and conclusion

The In Ouzzal terrane represents an Archaean block

reactivated in the Paleoproterozoic, which enables us

to investigate magmatic, metamorphic and structuralfeatures specific to the Archaean period and propose

a model for the formation of very-high-temperature

Paleoproterozoic granulites.

5.1. Archaean evolution (3.3–2.5 Ga)

Archaean geodynamic evolution can be approached

primarily owing to geochemical and geochronological

data obtained on the granitic rocks. The sialic material

of the In Ouzzal terrane was mostly formed during the

early Archaean, as shown by the TDM model ages which

are older than 3 Ga for all the felsic rocks of igneous

origin (All�eegre and Caby, 1972; Lancelot et al., 1976;

Ben Othman et al., 1984; Peucat et al., 1996). The oldestrocks dated in the In Ouzzal terrane are enderbitic or-

thogneisses of tonalitic and trondhjemitic composition

sampled from the areas of Tin Tchik Tchik and Roccan,

which yield ages around 3.3–3.2 Ga (U/Pb zircon, Peu-

cat et al., 1996). The isotopic data indicate that the

igneous suites correspond to juvenile material. Such a

sialic crust, made up of TTG and rocks of andesitic

affinity, was probably formed by accretion of island arcs

in a scenario resembling that of present-day plate tec-

tonics (Choukroune et al., 1997; De Wit, 1998). A new

suite of TTG was emplaced at 2.7 Ga. The isotopic dataon these TTG suggest the reworking––at least partial––

of an older crust (Nd model ages¼ 3.3 Ga) that could

have been formed during an earlier stage. Towards 2.65

Ga, A-type granites were emplaced with the chemical

signatures of rift-related granites. The mafic and ultra-

mafic granulites of In Hihaou, could also be of this age.

Detrital zircons yielding ages of 2.7 Ga in the metase-

dimentary rocks and the formation of skarns at thecontact between the marbles and the A-type granites

(2.65 Ga, U/Pb zircon) make it possible to date the

deposition of the metasedimentary formations at about

this period.

The presence of calc-alkaline granites of type CA1

and CA2, dated at 2.5 Ga (U/Pb zircon) reflects the

melting of a lower and middle crust showing a tona-

litic affinity acquired during the preceding stages andtransformed into granulites (Peucat et al., 1996). The

development of domes and basins structures during

K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227 223

homogeneous shortening at this stage fits with the modelof deformation of the Archaean crust proposed by

Bouhallier et al. (1995) and Choukroune et al. (1997) for

the Dharwar Craton in India.

5.2. Extreme metamorphic conditions during the Paleo-

proterozoic (2 Ga)

Because of the particular features displayed by the

IOGU, we cannot apply the simple model of a meta-

morphic belt formed by continental collision as in

modern mountain belts. The In Ouzzal terrane presentsa number of particularities, including: very-high-tem-

perature metamorphism up to 1100 �C (Kienast et al.,

1996; Guiraud et al., 1996; Ouzegane and Boumaza,

Fig. 10. Cartoons showing preferred models for the tectono-metamorphic evo

lithospheric thermal barrier and uprise of asthenosphere during an Eburnean

characteristic parageneses are also shown.

1996; Harley, 1998), limited partial melting despite thehigh temperatures involved, a style of deformation

comparable with that observed in Dharwar type (India)

Archaean cratons, the emplacement of syn-granulite-

facies carbonatites with a very peculiar geochemistry,

and the presence of anorthosites. The major question

posed by the development of the In Ouzzal granulitic

terrane during the Paleoproterozoic, with its extreme

metamorphic conditions, concerns the heat source forthis very-high-temperature event. Some other questions

are linked, such as the relationships that might exist

between anorthosites, carbonatites and the very-high-

temperature metamorphism, as well as the regional

tectonic style associated with the metamorphism. The

syn-granulite-facies deformation of In Ouzzal provides

lution and emplacement of carbonatites subsequent to delamination of

event in the In Ouzzal granulitic terrane. Evolution of P–T paths and

224 K. Ouzegane et al. / Journal of African Earth Sciences 37 (2003) 207–227

evidence for a homogeneous NW–SE shortening. Thestructures indicate a re-activation of Archaean vertical

stretching of the domes and basins, marked by the

vertical foliation observed in lithologies making up

these structures. The accentuation of this stretching is

compatible with a homogeneous shortening, trending

NW–SE on the regional scale, and accommodated by

transpressive shear zone corridors. We suggest that an

orogenic episode occurred at �2.1–2.0 Ga, during whichthe lower crust underwent homogeneous shortening. At

the same time, there was an extreme heating event re-

lated to an upwelling of the asthenosphere (Fig. 10). The

study of the Paleoproterozoic granulite-facies meta-

morphism of In Ouzzal indicates a clockwise P–T path

with an increase of temperature at constant pressure

(800–1050 �C, 10–11 kbar), followed by an isothermal

evolution with a significant fall in pressure (9–5 kbar).Based on these metamorphic data, we propose an evo-

lution for the In Ouzzal terrane in three principal epi-

sodes or stages (Fig. 10).

• The first stage corresponds to a continuation of Ar-

chaean domes and basins tectonics, which became

accentuated with time. During this stage, the block

must undergo both shortening and thickening (Fig.10, stage 1). The metamorphic evolution results in

an increase of pressure (up to 10 kbar) for classical

temperatures in the granulite-facies at the base of

the crust (700–800 �C).

• From the metamorphic point of view, the following

stage (Fig. 10, stage 2) is of fundamental importance,

since during this episode the In Ouzzal lower crust

underwent a strong reheating leading to extreme tem-peratures (up to 1100 �C) at constant pressure or in

slight decompression. Such temperatures, even at

the base of the crust, imply a considerable heat con-

tribution from depth, most probably linked to an

upwelling of the asthenosphere contemporary with

an important delamination of the lithospheric man-

tle. To bring the lower crust at T > 1000 �C, this

could be a result of frontal hypercollision that couldremove completely the lithospheric mantle bringing

the asthenosphere (1300 �C) close to the Moho (Black

and Li�eegeois, 1993). However, the crust became dehy-

drated during several periods of Archaean melting.

As a result, the crust is no longer fertile and cannot

undergo significant partial melting. Since heat is no

longer taken up by the formation of granites, the tem-

perature is not buffered to a maximum value by melt-ing processes around 700 �C or 800 �C. Therefore,

heat will be able to diffuse without any hindrance

and lead to extreme conditions of metamorphism

(this mechanism was already suggested by Vielzeuf

et al., 1990).

• As illustrated on Fig. 10, stage 3 represents the set of

phenomena preceding the exhumation of the oro-

genic belt. Indeed, such an important upwelling ofthe asthenosphere associated with greatly enhanced

heat flow is generally accompanied by extensional

tectonics. Although the importance of this extension

is difficult to assess in the In Ouzzal terrane, it may

be expressed by the emplacement of anorthosites

and carbonatites related to shear zones channelling

the ascent of CO2-rich fluids characteristic of the

mantle. Thus, the extreme metamorphic temperaturesas well as the presence of anorthosites and carbona-

tites would represent the effects of a phenomenon

whose timing remains to be defined. However, a ther-

mal anomaly of this magnitude is evidently unstable,

which necessarily implies a return to a more normal

regime characterized by a fall in temperature coupled

with decompression. This stage of decompression,

recorded in all the In Ouzzal formations, could corre-spond to the stress relaxation and isostatic re-equili-

bration that accompanies the recovery of the

lithospheric mantle during the post-collisional post-

orogenic quieter conditions, leading back to the re-

establishement of a thick cratonic lithosphere and

thus to the thermal equilibrium state (Black and

Li�eegeois, 1993), that will protect the In Ouzzal ter-

rane during the Pan-African orogeny.

Acknowledgements

We are indebted to J.P. Li�eegeois and M. De Wit for

detailed and constructive reviews. M.S.N. Carpenter

and J. Boissonnas are thanked for improving the Eng-

lish. This work was financed by cooperative programmebetween the Universities of Algiers (USTHB), and Paris

7 (00 MDU 476 ‘‘H�eeritage �eeburn�eeen et structuration

pan-africaine du Hoggar: �eetude g�eeologique et g�eeophy-

sique), and by ORGM in Algeria for logistic support

during field work’’. We are also extremely grateful for

the support of ICESA project.

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