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Journal of African Earth Sciences 39 (2004) 187–192

Granulitic metamorphism in the Laouni terrane(Central Hoggar, Tuareg Shield, Algeria)

Abderrahmane Bendaoud a,*, Amel Derridj b, Khadidja Ouzegane a,Jean-Robert Kienast c

a Faculte des Sciences de la Terre, de Geographie et d�Amenagement du Territoire, U.S.T.H.B., B.P. 32, Dar el Beida, Algiers, Algeriab Faculte des Hydrocarbures et de la Chimie, Universite m�Hamed Bougara, 35000 Boumerdes, Algeria

c Laboratoire de Petrologie, CNRS UMR 7097 IPGP, Universite de Paris 7, Tour 26-O, 4 place Jussieu, 75252 Paris, France

Available online 30 September 2004

Abstract

In the Laouni terrane, which belongs to the polycyclic Central Hoggar domain, various areas contain outcrops of formations

showing granulite-facies parageneses. This high-temperature metamorphism was accompanied by migmatization and the emplace-

ment of two types of magmatic suite, one of continental affinity (garnet pyroxenites and granulites with orthoferrossilite–fayalite–

quartz), and the other of arc affinity (layered metanorites). Paragenetic, thermobarometric and fluid-inclusion studies of the

migmatitic metapelites and metabasites make it possible to reconstruct the P–T–aH2O path undergone by these formations. This

path is clockwise in the three studied areas, being characterized by a major decompression (Tamanrasset: 10.5kbar at 825 �C to

6kbar at 700 �C; Tidjenouine: 7.5kbar at 875 �C; to 3.5kbar at 700 �C; Tin Begane: 13.5kbar at 850 �C; to 5kbar at 720 �C), followedby amphibolitization that corresponds to a fall of temperature (from 700 to 600 �C) and an increase in water activity (from 0.2–0.4 to

almost 1).

The main observed features are in favour of petrogenesis and exhumation related to the Eburnean orogeny. However, the lacks

of good-quality dating work and a comparison with juvenile Pan-African formations having undergone high-pressure metamor-

phism, in some cases reaching the eclogite facies, do not rule out the possibility that high-temperature parageneses are locally

due to Pan-African events.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Laouni; Hoggar; LATEA; Granulite; Eburnean Pan-African

1. Introduction

The Central Hoggar is located at the core of the Tua-reg Shield (Fig. 1(A)), providing an example of a Pre-

cambrian domain that has undergone a polycyclic

history. Indeed, the available dating suggests that this

domain consists of Eburnean crust (Paleoprotero-

zoic � 2000Ma), including some Archaean zones, onto

0899-5362/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jafrearsci.2004.07.050

* Corresponding author. Tel: +213 21 24 76 47; fax: +213 21 24 76

47.

E-mail address: abderbendaoud@hotmail.com (A. Bendaoud).

which juvenile Pan-African (850–550Ma) material was

locally accreted (Liegeois et al., 2003; Peucat et al.,

2003). Thus, the set of formations in this area corre-sponds to a superposition of Pan-African nappes

reworking Archean–Eburnean material to a variable ex-

tent. The base of some of these nappes is picked out by

relatively well-preserved eclogitic layers. During the

Pan-African orogeny (850–550Ma), to which the region

owes its current structure, the Tuareg Shield was built

up from the amalgamation of different terranes and

was involved in two major collisions, one with the WestAfrican Craton and the other with the Saharan Craton

(Black et al., 1994).

Fig. 1. Geological maps of the studied areas, and their situation within the Hoggar: (A) Tuareg Shield (Black et al., 1994); (B) Tamanrasset area

(Ouzegane et al., 2001); (C) Tidjenouine area (Bertrand et al., 1986); (D) Tin Begane area (Derridj et al., 2003).

188 A. Bendaoud et al. / Journal of African Earth Sciences 39 (2004) 187–192

Several authors point out the remarkable structural

and compositional unity of the ortho- and para-derivedamphibolitic formations (some with granulite-facies rel-

ics) making up the Laouni, Azrou, N�Fad, Tefedest andEgere-Aleksod blocks that occupy the entire central-

western part of Central Hoggar (Fig. 1(A)). According

to Latouche et al. (2000) and Liegeois et al. (2003), these

four terranes made up an Archean–Eburnean micro-

continent known as LATEA. During the Pan-African

orogeny, it appears that LATEA formed a passive mar-gin that was involved in two collisions with arc-type

terranes: in the West, the Iskel terrane of Mesoprotero-

zoic age stabilized towards 840Ma (Caby, 2003) and, in

the East, the Serouanout terrane of unknown age, but

including a large part of apparently juvenile material

(Black et al., 1994). During the post-collisional phase,

this micro-continent was dismembered following hori-

zontal movements along major shear-zones (Liegeoiset al., 2003).

The present study concerns the metamorphic history

of three areas (Tamanrasset, Tidjenouine and Tin Beg-

ane) where granulitic formations are exposed. The rocks

show granulite- and amphibolite-facies parageneses.

These three areas all are located within the Laouni

terrane that constitutes the SW part of Central Hoggar

(Fig. 1A).

2. Tamanrasset

The area of Tamanrasset is made up of migmatitic

gneisses and metasediments with numerous intercala-

tions of metabasites (Fig. 1B). The various rock-types

are commonly retrogressed into the amphibolite facies,and even into the greenschist facies near the mylonitic

zones that form belts around this area. However, remark-

ably preserved lenses of metabasite containing garnet

(Grt)–clinopyroxene (Cpx)–orthopyroxene (Opx)– horn-

blende (Hbl)–plagioclase (Pl)–quartz (Qtz) (Fig. 2(A)) al-

low us to estimate P–T path evolution, by combining the

study of phase relations with mineral equilibria calcu-

lated by the Thermocalc program of Powell and Holland(1988), conventional and automatic geothermobarome-

try, and fluid inclusions (Ouzegane et al., 2001). A meta-

morphic peak is calculated at approximately 825 ± 25 �Cand 10 ± 1kbar (Fig. 3(A)). Then, these rocks underwent

a retrograde evolution leading to conditions of 700 ±

50 �C at 6.5 ± 1kbar, with H2O activity remaining

Fig. 2. Photomicrographs of representative metamorphic rocks from Tamanrasset, Tidjenouine and Tin Begane areas. (A) Breakdown of

garnet + clinopyroxene + quartz to orthopyroxene + plagioclase + hornblende and development of fine lamellae of amphibole along clinopyroxene

cleavage planes in garnet–pyroxenites of Tamanrasset. (B) Metapelite from Tamanrasset showing the sillimanite–biotite–plagioclase–quartz–K-

feldspar assemblage. (C) Metapelite from Tidjenouine showing the breakdown of garnet and quartz to orthopyroxene + cordierite. Spinel-cordierite

symplectites result from the reaction: garnet + sillimanite) spinel + cordierite. Note that in cracks of garnet it developed orthopyroxene-spinel-

cordierite symplectites. (D) Exsolutions of garnet with preferred orientations in aluminous orthopyroxene of sillimanite–free metapelite from

Tidjenouine. (E) Inclusion of kyanite armoured by garnet in garnet–sillimanite–biotite metapelite from Tin Begane. (F) Representative assemblages

of garnet pyroxenites from Tin Begane, showing orthopyroxene-plagioclase intergrowths and orthopyroxene corona around quartz, resulting from

the reaction: garnet + clinopyroxene + quartz ) orthopyroxene + plagioclase (G) Retrogressed garnet pyroxenite from Tamanrasset showing the

breakdown of orthophroxene + hornblende to cummingtonite + plagioclase + quartz.

A. Bendaoud et al. / Journal of African Earth Sciences 39 (2004) 187–192 189

around 0.2 (Ouzegane et al., 2001). This is illustrated

(Fig. 2(A)) by the destabilization of the primary assem-

blage containing Grt (Alm57–Py17–Grs25–Sps3;XFe = 0.53)–Cpx (XFe = 0.46)–Opx1 (XFe = 0.57)–Pl1(An50)–Hbl1 (pargasite)–Qtz into a secondary assem-

blage with Opx2 (XFe = 0.61)–Pl2 (An80)–Hbl2 (magne-

sio-hornblende). The conditions of pressure and

temperature for the secondary assemblages are compara-

ble with those obtained in the surrounding migmatitic

metapelites containing a garnet–biotite (Bt)–sillimanite

(Sill)–quartz–plagioclase–K-feldspar (Kfs) assemblage(Fig. 2B).

Amphibolitization in the metabasites leads to the

appearance of a third generation of parageneses (Fig.

2(G)) characterized by the presence of cummingtonite(Cum), which reflects an increase in H2O activity and

a fall of temperature.

3. Tidjenouine

The granulitic formations of Tidjenouine are predom-

inately composed of migmatitic gneisses (Fig. 1(C)) thatare much better preserved than in the Tamanrasset area.

Fig. 3. P–T paths evolution of Tamanrasset granulites—with inclusion isochors (solid straight line, V in cm3/mol) inferred from Th histograms

(grey = retrogressed garnet and white = metapelite). V = molar volume, d = CO2 density in g/cm3 (A); Tidjenouine (B) and Tin Begane (C). The P–T

conditions have been computed essentially with average P–T Thermocalc method (Powell and Holland, 1988).

190 A. Bendaoud et al. / Journal of African Earth Sciences 39 (2004) 187–192

Tidjenouine is the only area of the Laouni terrane where

the granulite-facies metamorphism has been dated, yield-

ing an age of 2069 ± 49 Ma (U–Pb on zircon, (Bertrand

et al., 1986), recalculated following Ludwig, 1999 by Lie-

geois et al., 2003). Two suites of metabasites have been

described (Bendaoud, 1999; Ouzegane et al., 2000; Bend-

aoud et al., 2003). The continental suite (garnet pyroxe-

nites, amphibolites and granulites with fayalite) reflectsintra-plate magmatism, whereas the other suite has an

arc-type affinity (layered metanorites). The migmatitic

metapelites show a great paragenetic diversity in relation

to the variations of the XMg and XAl ratios in these rocks.

Thus, various parageneses have been distinguished based

on the presence or absence of a certain number of miner-

als such as orthopyroxene, gedrite, sillimanite, corun-

dum and quartz. The metapelites show a progradeevolution, during which these formations started to melt.

The peak of the metamorphism was reached at

875 ± 50 �C and 7.5 ± 1kbar (Fig. 3(B)). This was fol-

lowed by decompression and a fall in temperature that

brought the rocks to conditions of 700 �C and 3–4kbar.

The earliest reactions led to the appearance of garnet fol-

lowing the destabilization of biotite or gedrite, in relation

to reactions of the type: Bt + Sill + Qtz ± Pl ) Grt +

Melt + Kfs ± Crd and Ged + Sill + Qtz ) Grt + Crd +

Melt. The peak is represented by the presence of alumi-

nous orthopyroxene which later exsolved garnet in the

metapelites without sillimanite. In the magnesian metap-

elites with sillimanite, on the other hand, the highest

pressures are indicated by the composition of garnets

that reach an XMg value of 51%. During decompression

from 7 to 4kbar, cordierite appeared in coronas aroundgarnet or biotite, as well as in symplectites with spinel

(Spl) and/or orthopyroxene, according to reactions such

as Grt + Sill ) Crd + Spl; Bi + Sill ) Crd + Spl + Kfs

(melt or vapour); Grt + Qtz ) Opx + Crd (Fig. 2(C))

and Grt ) Spl + Crd + Opx + P1.

In the metapelites without sillimanite and with pri-

mary orthopyroxene, the exsolution of garnet by an alu-

minous primary orthopyroxene (Al2O3 > 6.5wt%)indicates a stage of falling temperature at relatively high

pressures (Fig. 2(D)). The final stage (600 �C at 3–4kbar)

corresponds to a fall of temperature that, for example, is

reflected in gedrite-bearing granulites by parageneses

with anthophyllite.

The same P–T path is exhibited by granulites with

fayalite–quartz–ferrosilite (Bendaoud et al., 2003). The

physical conditions of peak assemblage, garnet–clinopy-

A. Bendaoud et al. / Journal of African Earth Sciences 39 (2004) 187–192 191

roxene–amphibole–plagioclase–quartz, correspond to

pressures of 7.1 ± 1kbar at temperatures of 880 ± 60 �Cand a H2O of 0.2. During decompression, the clinopy-

roxene in contact with garnet and quartz initially broke

down into orthopyroxene + plagioclase. The P–T condi-

tions computed for this paragenesis are around 750 �Cand 5kbar. Then the orthopyroxene reacted with garnet

to produce fayalite + plagioclase symplectites. The latest

stage corresponds to the orthopyroxene–fayalite–quartz–

plagioclase assemblage reflecting low pressures �3–

4kbar at temperature of 700�C.

4. Tin Begane

The area of Tin Begane (Fig. 1(D)), located at 110km

to the South of Tamanrasset, shows formations equiva-

lent to those of the preceding areas. However, it is char-

acterized by the persistence of kyanite (Ky) relics in the

metapelites and the presence of eclogitic layers in the

southern part of the area. The metasedimentary succes-

sion is made up of olivine-spinel marbles intercalatedwith garnet–sillimanite–biotite metapelites containing

kyanite relics (Fig. 2(E)) armoured by garnet. In this

area, there are also found iron-bearing quartzites with

hercynite–almandite–faylite and lenses of metabasite

both with and without garnet. Mylonitic contacts sepa-

rate the metasedimentary succession from migmatitic

orthogneisses. The metabasites of the northern part of

Tin Begane show geochemical compositions essentiallycomparable with continental tholeiites (Derridj, 2000).

They differ from the metabasites of the preceding areas

by the presence of three successive parageneses, the lat-

est contains spinel. The primary paragenesis is Grt–Cpx

(A12O3 = 7.15wt%; Na2O = 1.15wt%; XMg = 0.80)–Pl

(An47)–Hbl–Qtz–Rutile. This paragenesis is remarkable

for the composition of the garnet, with contents of py-

rope and grossularite reaching 38% and 30%, respec-tively (Alm32 Py38 Grs30), values that are characteristic

of very-high pressure. Thermobarometric studies of this

paragenesis indicate conditions of 13.5 ± 1.5kbar and

860 ± 60 �C (Fig. 3(C)). These conditions are located at

the boundary between the granulite and the eclogite

facies. The secondary paragenesis, with Opx–Pl (An60) ±

Grt2 (Alm47 Py31 Grs20 Sps2) ± Hbl2, results primarily

from the reactions: Grt + Cpx + Qtz ) Opx + Pl (Fig.2(F)) and Grt + Hbl1 + Qtz ) Opx + Pl + Hbl2 (symp-

lectitic stage 1). The latest paragenesis is formed at the

expense of garnet in cracks, owing to the reaction:

Grt ) Opx + Spl + Pl (An95) (symplectitic stage 2).

The corresponding conditions for these two parageneses

are 10.7 ± 1.3kbar at 790 ± 60 �C and 4.8 ± 1.3kbar at

750 �C, respectively (Derridj et al., 2003). The metapelites

of the country rocks and the olivine-clinopyroxene mar-bles yield conditions equivalent to those recorded by the

two later parageneses of the metabasites.

Following a petrological and geochronological study

(Sm–Nd on garnet) in the south of this same area, Lie-

geois et al. (2003) determined a metamorphic evolution

path (Fig. 3(C)) at the base of the Pan-African nappes

that passes successively through the following condi-

tions: 17kbar at 790 �C (eclogite �690Ma), 12kbar at830 �C (garnet amphibolite �686Ma), 8kbar at 700 �C(kyanite–garnet gneisses) and 4 kbar at 500 �C (green-

schist �530My).

5. Discussion and conclusion

In several areas of the Laouni terrane, observed gran-ulitic formations are commonly associated with an

important migmatitic event. Petrological, thermobaro-

metric and fluid-inclusion studies allow us to determine

a clockwise path with a decompression stage generating

spectacular coronitic and symplectitic textures in both

the para- and ortho-derived metamorphic units. The suc-

cession of parageneses during this decompression is a

function of various chemical compositions. The metap-elites or microdomains rich in silica and magnesium

are characterized by the appearance of an orthopyrox-

ene–cordierite association at the expense of garnet,

quartz and biotite in the absence of sillimanite. The

metapelites and microdomains rich in aluminium and

iron make up assemblages with spinel-cordierite, but

without orthopyroxene, following the destabilization

of garnet, sillimanite and biotite. This explains theabsence of orthopyroxene in Tin Begane and Tamanras-

set, where the metapelitic formations are highly iron-

enriched and aluminous.

During the prograde stage, the formation of inclu-

sions in the core of primary garnet indicates that the

kyanite field was traversed in the Tin Begane area, while

only sillimanite is present in Tamanrasset and Tidjenou-

ine. In the three studied areas, the peak of temperature issituated at around 850 ± 25 �C (Fig. 3), whereas the

pressure peak is very variable since it ranges from

7.5kbar (Tidjenouine) through 10.5kbar (Tamanrasset)

to 13.5kbar (Tin Begane). This implies the possibility

that different structural levels can be observed in the

Laouni terrane. Two types of metabasites are also dis-

tinguished, one with tholeiitic intra-plate affinity and

the other showing arc affinity.Although Bertrand and Jardim de Sa (1990) and

Ouzegane et al. (2001) propose that the observations

are more likely compatible with an Eburnean orogeny,

one of the fundamental problems in the interpretation

of this metamorphism and the nature of the metabasites

involved is the lack of precise dating. Several geologists

prefer to attribute a Pan-African age to the granulites,

integrating the petrogensis and exhumation of theserocks into the pan-African orogeny of Central Hoggar

(e.g. Caby, 2003; Barbey et al., 1989). However, the

192 A. Bendaoud et al. / Journal of African Earth Sciences 39 (2004) 187–192

age of �2000Ma obtained on the Tidjenouine forma-

tions indeed seems to refer to the age of the granulitic

protolith. In a critical review of the published U–Pb ages

from Hoggar, Bertrand (1998, unpublished—written

communication) indicates that the zircons dated at

2069 ± 49Ma in Tidjenouine granulites (Bertrandet al., 1986) are clearly metamorphic—with rounded

spheroidal shapes—and that the lower intercept ob-

tained here (at around 530Ma) is an analytical artefact.

In the Tin Begane area, the isotopic data (Rb–Sr and

Sm–Nd on whole rocks) of various rock types with gran-

ulitic relics would appear to indicate Pan-African ages

or at least an important Pan-African reworking (Lie-

geois, written communication). In the same way, thepressure–temperature path determined on these granu-

lites does not appear to contradict, within the limits of

uncertainty, the path given for the metabasites contain-

ing eclogitic relics (Fig. 3(C)). Dating work in progress

on garnets (Sm–Nd) from the granulitic parageneses

should soon make it possible to resolve these uncertain-

ties and provide a better understanding of the geody-

namic evolution of these areas.

Acknowledgments

We thank P. Holtta and L. Solari, reviewers, for their

constructive and very helpful remarks that have signifi-

cantly improved the quality of this work. M.S.N. Car-

penter is thanked for help with the English. We alsothank Abdelkader Regagda for the field facilities. This

work is a contribution to the projects PNR AU 19943

and to NATO EST/CLE 979766.

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