Gedrite-garnet-sillimanite-bearing granulites from Amessmessa area, south In Ouzzal, Hoggar, Algeria

J. metamorphic Geol., 1996, 14, 739–753

Gedrite–garnet–sillimanite-bearing granulites from Amessmessa area,south In Ouzzal, Hoggar, AlgeriaK. OUZEGANE, 1 S . D JEMAI 1 AND M. GUIRAUD2

1 Institut des Sciences de la Terre, U.S.T .H.B., B.P. 9, Dar El-Beıda, Algiers, Algeria2 Laboratoire de Mineralogie, URA 736, Museum National d ’Histoire Naturelle, 61, rue Buffon, 75005 Paris, France

ABSTRACT Some granulites from the Amessmessa area (south In Ouzzal unit, Hoggar) contain the peak assemblagegedrite+garnet+sillimanite+quartz that was used to estimate the P–T conditions of metamorphism.The rocks developed symplectites and corona textures by the breakdown of the primary paragenesis toorthopyroxene, cordierite and spinel. The successive parageneses formed in separate microdomainsaccording to a clockwise P–T path. Geothermometry, geobarometry and phase diagram calculationsindicate that the textures formed by decompression and cooling from 7–9 kbar and 850–900 °C to3.5–4.5 kbar and 700–800 °C. This P–T evolution is consistent with low to medium aH2O, between 0.4and 0.7, and is similar to the metamorphic conditions deduced in Al–Mg granulites from the north ofIn Ouzzal.

Key words: gedrite; granulite; Hoggar, In Ouzzal; Precambrian; P–T path.

the second is composed of metasediments with marbles,INTRODUCTION aluminous granulites and several elongated lenses

of basic to ultrabasic composition. In contrastThe Hoggar–Iforas region is composed of N–S seg-ments with alternating weakly and strongly metamor- to the northern part of the In Ouzzal unit, the

granulites in the Amessmessa area are retrogressed tophosed rocks, separated by mega-shear zones whichcut across the region from north to south (Fig. 1). greenschist facies along the Pan-African ductile-shear

zones.These structures are Pan-African (900–600 Ma) andCaby et al. (1981) interpreted them as a complete The dominant structures observed in the

Amessmessa area are large-scale folds with NE–SW-Wilson cycle with ocean opening and closing betweenthe west African craton and the Hoggar domain. In trending axial planes. These folds affect the early

foliation (S1) and lineation (L1) in both charnockitesthe recently published geodynamic model of Blacket al. (1994), this region is presented as a collage and metasedimentary series which are strongly flat-

tened and elongated (Haddoum, 1992; Haddoumof displaced terranes with their own lithologicalsequences, metamorphic regimes, magmatic and tec- et al., 1994).

The gedrite–garnet–sillimanite-bearing granulites,tonic events. Three major structural domains separatedby mega-shear zones can be recognized: the Western, which are the focus of this paper, are the first

occurrence reported in this region. They occur withinCentral and Eastern Hoggar. The western Hoggar,which contains the In Ouzzal granulite facies shield, the metasedimentary series interlayered with narrow

bands of sapphirine–orthopyroxene–sillimanite–garnet-divides the Pan-African belt, which is composed ofvolcano-detrital material metamorphosed at amphibo- quartz-bearing Al–Mg granulites and lenses of ortho-

pyroxene–garnet-bearing metabasic rocks. Only a fewlite to greenschist facies conditions, into an easternbranch and a western branch. The polycyclic Central occurrences of the gedrite–sillimanite association have

been reported in the literature (Robinson & Jaffe,Hoggar is composed of Eburnean (2000 Ma) gneissesreactivated and intruded by abundant granitoids 1969; Spear, 1982; Schumacher & Robinson, 1987;

Goscombe, 1992; Spear, 1993). They all correspond toduring the Pan-African orogeny. The Eastern Hoggarwas stabilized at an early stage of the Pan-African moderately high-P conditions and the breakdown of

this assemblage to cordierite and garnet, as it isepisode about 725 Myr ago.The Amessmessa region is located in the south of the generally described, implies a P–T path characterized

by unloading.In Ouzzal shield (Western Hoggar) and is separatedfrom the Pan-African belt by large-scale vertical ductile- The aim of this paper is to describe the textures,

mineral chemistry and phase relations of the gedrite–shear zones (Fig. 1). This region is composed ofArchaean rocks that were affected by the Eburnean garnet–sillimanite-bearing granulites in the Amess-

messa area and to constrain the P–T path, so far(2000 Ma) orogeny. Two series of granulite facies rocksare recognized: the first contains charnockites, whereas unknown for this region, by the utilization of

739

740 K. O UZE GAN E E T AL .

plagioclase and biotite. A primary coarse-grained,centimetre-scale, paragenesis breaks down to symplec-tites and coronas of secondary parageneses. Theprimary paragenesis is inferred to be garnet+gedrite+ sillimanite+quartz+plagioclase+ rutile+ilmenite+biotite. The secondary paragenesis involvesorthopyroxene+cordierite±spinel.

All the primary minerals are deformed; the rock hasa cataclastic or mylonitic texture indicating high strain,marked by the alignment of gedrite, quartz, sillimanite,biotite and ilmenite along L1. Gedrite appears aselongated, lenticular and boudinaged grains thatcommonly show undulose extinction, mechanical twin-ning and subgrains. Quartz occurs as discontinuousribbons that form lenses with asymmetric tails thatresults from flattening with a rotational component.Garnet is elongated in the mylonitic zones of the rock.All these features are consistent with ductile defor-mation at high temperature. In the zones preservedfrom intense deformation, garnet is highly fracturedand poikilitic and contains numerous quartz inclusions.The fractures are irregular and without any preferentialorientation. Coronas and symplectites are generallyundeformed.

Minerals belonging to the primary assemblage arenever observed in mutual contact and are alwaysseparated by either symplectite or corona structures.The shape and size of the primary minerals depend onthe extent of the breakdown reaction. The best exampleis garnet which has a variable size (0.2–5 mm) and aFig. 1. Location map and simplified geological features of thevery irregular shape and which can be completelyPharusian belt (Western Hoggar, Algeria).replaced by an orthopyroxene–cordierite symplectitemimicking its initial shape (Fig. 2a). Coronas andsymplectites are intimately associated in the breakdowngeothermobarometers and of the computer programreaction textures. For example, at the contact between (Powell & Holland, 1988).garnet and quartz, quartz is rimmed by a corona oforthopyroxene, whereas garnet is mantled by a

PETROGRAPHY cordierite+orthopyroxene symplectite (Fig. 2a,b). Thetype of corona and of symplectite varies according toThe mineral assemblages and the modal amounts ofthe local modal composition:the different phases are presented in Table 1. The rock1 cordierite+orthopyroxene symplectite rims garnetis mainly composed of orthopyroxene, cordierite andand gedrite at the contact with quartz (Fig. 2a–c);quartz, with minor amounts of gedrite, garnet and2 cordierite+orthopyroxene+spinel symplectite rimsilmenite and trace amounts of rutile, sillimanite, spinel,garnet and gedrite at their mutual contact in areasdevoid of quartz (Fig. 2d,e);

Table 1. Abbreviations and representative mineral assemblages 3 a corona of cordierite or even a matrix of granoblas-and modal composition (vol.%) of gedrite–garnet–sillimanite– tic cordierite separates garnet, sillimanite and quartzquartz-bearing granulites. in areas devoid of gedrite;Abbreviations 4 garnet is rimmed by cordierite and sillimanite byBiotite Bt Orthopyroxene Opx cordierite+spinel symplectite in areas devoid of bothCordierite Crd Plagioclase Pl gedrite and quartz (Fig. 2f ).Garnet Grt Quartz QzGedrite Ged Rutile Rut Ilmenite is always rimmed by orthopyroxene andIlmenite Ilm Sillimanite Sil can be involved in the cordierite+orthopyroxeneOrthoamphibole Oam Spinel Sp

symplectite and cordierite+orthopyroxene+spinelModal composition symplectite. Plagioclase appears to be part of theSample Ged Grt Opx Crd Qz Sil Sp Bt Pl Ilm Rut breakdown reaction of gedrite because inclusions foundTir67A 3 8 37 29 20 X X X X 2 X in symplectitic cordierite are richer in Na than matrixTir67A3 18 3 41 25 10 X X X X 2 X plagioclase. Rutile is associated with large crystals ofX: <1% primary ilmenite and biotite, and is only found in the

G ED RIT E– G AR NE T– SI LL IMAN IT E- BEARI NG GR AN UL I TE S 741

Fig. 2. (a) Photomicrograph of granulite from Amessmessa showing the breakdown of garnet (Gt)+quartz (Qz)+gedrite (Ged) toorthopyroxene (Opx)+cordierite (Cd). Primary garnet and gedrite pseudomorphs are rimmed by a symplectic intergrowth ofcordierite and orthopyroxene. These symplectites are in turn rimmed by coarser aggregate coronas of orthopyroxene in directcontact with primary quartz. (b) Photomicrograph showing the breakdown of garnet+quartz to orthopyroxene+cordierite. Quartzis rimmed by orthopyroxene and fine intergrowths of cordierite (white) and orthopyroxene occur along microfractures of garnet. (c)Photomicrograph that shows the breakdown of gedrite+quartz to orthopyroxene+cordierite. Note the large grain of gedritesurrounded by fingerprint-like intergrowth of cordierite–orthopyroxene and orthopyroxene corona mantling quartz. (d )Photomicrograph of orthopyroxene–cordierite–spinel (Sp) symplectite developed at the contact of large primary garnet and gedritegrains. Note sillimanite completely pseudomorphed by cordierite–spinel symplectite at the top left. (e) Close-up view of gedritereacting out to orthopyroxene+cordierite+spinel. Spinel (black) occurs as fine granules intergrown with orthopyroxene andcordierite that replaces gedrite porphyroblast. (f ) Photomicrograph showing the breakdown of garnet+sillimanite tocordierite+spinel, where spinel in fine droplets grows around sillimanite crystals away from garnet.


quartz-rich areas as primary elongated crystals or Garnet is almost exclusively an almandine–pyropesolid solution: almandine ranges from 45 to 75% andalong cracks in gedrite.pyrope from 22 to 50%, the sum of spessartine,andradite and uvarovite is always <1% and the

M INERAL CHEM ISTRY grossular content does not exceed 3% (Table 3).Garnets are zoned with the cores always richer in MgRepresentative analyses are listed in Tables 2–8. The

analyses have been performed with a CAMEBAX than the rims (Fig. 4). The largest core–rim difference(from Pyp48Alm52 to Pyp25Alm75 ) is observed in themicroprobe at the CAMPARIS centre (University of

Paris VI). The operating conditions were 15-kV garnet found at the contact with quartz (Fig. 4).Orthopyroxene varies with respect to the texturalaccelerating voltage and 10-nA sample current. Natural

silicates and synthetic oxides were used as standards type (Table 4, Fig. 5). In quartz-bearing textures,orthopyroxene is richer in Fe in garnet-bearingfor all elements, except for fluorine and zinc, which

were calibrated on fluorite and sphalerite, respectively. domains (XMg=47–51) than in gedrite-bearingdomains (XMg=62–68). For each type of texture,Gedrite has a formula based on 23 oxygens (Table 2)

with Fe3+ recalculated according to Spear (1980). As coronitic orthopyroxene has the same XMg as symplec-titic orthopyroxene, but has lower Al2O3. The Al2O3Robinson et al. (1971) have emphasized the importance

of Na in the A site of gedrite, the appropriate of orthopyroxene in corona and in symplectites,respectively, is 1.2–2.2 wt% and 2.9–5.3 wt% at therecalculation corresponds to all Na assigned to the

A-site. This method gives the lowest Fe3+ compatible contact with gedrite and 1.4–2.1 wt% and 2.1–2.9 wt%at the contact with garnet. There is no significantwith stoichiometry (Stout, 1972; Brady, 1974; Robinson

et al., 1982). The composition of orthoamphiboles is difference in orthopyroxene composition between theorthopyroxene+cordierite+spinel symplectite at thevery homogeneous; it is characterized by high Al2O3(15.7–17.6 wt%) and Na2O (1.9–2.21 wt%) and low garnet–gedrite contact and the orthopyroxene+cordierite symplectite at the quartz–gedrite contact.CaO (0.4–1.0 wt%). According to the IMA classifi-

cation (Leake, 1978), the orthoamphibole is a gedrite Two groups of XMg values in cordierite compositionsare recognized according to the type of texture(Fig. 3a), and on the (Na+K)A vs. AlIV diagram plots

very close to the ‘Ideal Gedrite’ point of Robinson (Table 5): the XMg ratio is the lowest at the contactbetween garnet and sillimanite (XMg=69–77) and theet al. (1971) (Fig. 3b). These gedrites are slightly

enriched in Fe at the contact with cordierite–orthopy- highest in gedrite-bearing textures (XMg = 81–82).Spinel is very poor in Cr2O3 (0.04–0.64 wt%) androxene symplectite: their XMg [100Mg/(Mg+Fe2+)]

varies from 70 to 64 from core to rim. ZnO (0.03–0.12 wt%) (Table 6). Spinel in symplectite

Table 2. Chemical compositions of gedrite.Analyses recalculated according to Spear(1993).

Sample 67A 67A 67A 67A 67A 67A 67A 67A 67A3 67A3 67A3Core Rim Core Rim Rim/crd

SiO2 43.45 42.17 42.12 41.97 42.53 42.20 42.17 42.09 43.96 43.14 43.35TiO2 0.59 0.65 1.01 0.58 0.53 0.53 0.60 0.77 0.64 0.49 0.68Al2O3 15.82 16.33 16.08 16.08 16.18 16.55 16.41 15.73 16.70 17.63 17.10Cr2O3 0.07 0.18 0.17 0.04 0.05 0.10 0.00 0.00 0.10 0.12 0.03MgO 17.90 17.61 16.90 16.69 18.20 18.10 17.64 17.33 18.88 18.61 18.89FeO 15.98 15.78 16.82 16.63 15.61 15.10 16.15 16.32 15.12 14.52 14.57MnO 0.17 0.20 0.14 0.11 0.07 0.11 0.14 0.18 0.08 0.17 0.07CaO 0.86 0.80 1.02 0.86 0.90 0.87 0.89 0.92 0.60 0.52 0.62Na2O 2.04 1.96 2.08 2.10 2.05 2.07 2.10 1.91 2.07 2.03 2.12K2O 0.00 0.01 0.00 0.01 0.03 0.00 0.00 0.00 0.00 0.05 0.01H2O 1.62 1.45 1.61 1.58 1.58 1.77 1.60 1.54 0.00 0.00 0.00F 0.96 1.25 0.92 0.94 1.01 0.58 0.94 1.04 0.77 0.59 0.60Cl — — — — — — — — 0.17 0.00 0.08Total 99.46 98.39 98.87 97.59 98.74 97.98 98.64 97.83 99.09 97.87 98.12Si 6.29 6.17 6.17 6.22 6.18 6.16 6.15 6.20 6.24 6.17 6.19Aliv 1.72 1.83 1.83 1.78 1.82 1.84 1.85 1.80 1.76 1.84 1.81Alvi 0.98 0.99 0.94 1.03 0.95 1.00 0.97 0.93 1.03 1.14 1.06Ti 0.06 0.07 0.11 0.07 0.06 0.06 0.07 0.09 0.07 0.05 0.07Fe3+ 0.03 0.12 0.06 0.01 0.17 0.13 0.15 0.15 0.01 0.01 0.01Cr 0.01 0.02 0.02 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.00Mg 3.86 3.84 3.69 3.69 3.94 3.94 3.84 3.81 3.99 3.96 4.02Fe2+ 1.91 1.81 2.00 2.05 1.73 1.72 1.82 1.86 1.78 1.72 1.73Mn 0.02 0.03 0.02 0.01 0.01 0.01 0.02 0.02 0.01 0.02 0.01Ca 0.13 0.13 0.16 0.14 0.14 0.14 0.14 0.15 0.09 0.08 0.09NaM4 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00NaA 0.57 0.56 0.59 0.60 0.58 0.59 0.59 0.55 0.57 0.56 0.59K 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00Total 15.57 15.56 15.59 15.61 15.58 15.59 15.59 15.55 15.57 15.57 15.59OH 1.56 1.42 1.57 1.56 1.54 1.73 1.57 1.52 1.61 1.74 1.71F 0.44 0.58 0.43 0.44 0.46 0.27 0.43 0.49 0.34 0.27 0.27Cl — — — — — — — — 0.04 0.00 0.02XMg 66.9 68.0 64.8 64.2 69.5 69.6 67.8 67.2 69.1 69.7 69.9


Fig. 3. (a) Compositions of orthoamphibolesafter Leake, (1978). (b) Plot of (Na+K)A vs.AlIV in gedrite.

Table 3. Chemical compositions of garnet. Analyses recalculated on 12 oxygens.

Sample 67A 67A 67A 67A 67A 67A 67A 67A 67A 67A 67A 67A 67A 67A 67A 67A 67A 67A 67A 67Act sym core core core cy sym ct Qz ct opx ct sym core ct sym

Texture Grt+Qz=Opx+Crd Grt+Qz=Opx+Crd Gt+Ged=Opx+Cd+Sp

SiO2 37.74 38.22 38.64 39.06 39.28 39.67 39.36 39.80 39.34 38.90 38.72 37.59 37.39 37.18 37.16 37.29 38.98 38.87 38.97 38.15TiO2 0.00 0.09 0.03 0.07 0.05 0.00 0.02 0.02 0.00 0.05 0.03 0.00 0.00 0.02 0.02 0.00 0.04 0.05 0.16 0.07Al2O3 21.35 21.68 22.02 22.07 22.12 21.70 22.06 22.10 22.15 22.12 21.94 21.27 21.07 21.28 21.30 21.21 22.43 22.51 22.40 21.56Cr2O3 0.03 0.11 0.02 0.12 0.08 0.17 0.00 0.02 0.00 0.03 0.03 0.02 0.02 0.03 0.00 0.05 0.13 0.17 0.05 0.15MgO 6.81 9.35 10.25 11.18 11.23 12.19 12.28 12.40 11.97 11.27 9.91 6.42 6.22 5.77 5.67 5.90 12.84 13.66 13.35 10.53FeO 31.51 28.23 26.33 25.32 25.14 23.98 23.99 23.67 24.56 25.62 27.93 31.77 32.48 32.52 32.64 32.99 24.72 22.61 23.31 27.27MnO 0.65 0.23 0.38 0.22 0.34 0.31 0.22 0.41 0.40 0.18 0.48 0.82 0.95 0.82 0.93 0.98 0.34 0.27 0.23 0.27ZnO 0.07 0.03 0.02 0.00 0.05 0.00 0.02 0.00 0.04 0.04 0.00 0.07 0.07 0.00 0.10 0.00 0.00 0.00 0.04 0.03CaO 0.79 0.97 1.09 0.97 0.91 1.05 1.00 0.97 0.89 0.97 1.00 0.98 0.94 0.85 1.02 0.88 0.79 0.80 0.81 0.95Total 98.95 98.91 98.78 99.01 99.20 99.07 98.95 99.39 99.35 99.18 100.04 98.94 99.14 98.47 98.84 99.30 100.27 98.94 99.32 98.98

Si 2.99 2.98 2.99 2.98 2.99 3.00 3.00 3.01 2.99 2.98 2.98 2.99 2.98 2.99 2.98 2.98 2.95 2.94 2.95 2.96Ti 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00Al 2.00 1.99 2.01 1.99 1.99 1.94 1.98 1.97 1.98 2.00 1.99 2.00 1.98 2.01 2.01 2.00 2.00 2.01 2.00 1.97Cr3+ 0.00 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.01Mg 0.81 1.09 1.18 1.27 1.27 1.38 1.40 1.40 1.35 1.29 1.14 0.76 0.74 0.69 0.68 0.70 1.45 1.54 1.51 1.22Fe2+ 2.09 1.84 1.70 1.62 1.60 1.52 1.53 1.50 1.56 1.64 1.80 2.12 2.16 2.18 2.19 2.20 1.56 1.43 1.48 1.77Mn 0.04 0.02 0.03 0.01 0.02 0.02 0.01 0.03 0.03 0.01 0.03 0.06 0.06 0.06 0.06 0.07 0.02 0.02 0.02 0.02Zn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00Ca 0.07 0.08 0.09 0.08 0.07 0.09 0.08 0.08 0.07 0.08 0.08 0.08 0.08 0.07 0.09 0.08 0.06 0.06 0.07 0.08Total 8.00 8.00 8.00 7.96 7.96 7.95 8.01 7.99 7.98 8.01 8.02 8.01 8.01 8.01 8.01 8.02 8.05 8.01 8.03 8.04

XMg 27.8 37.1 41.0 44.0 44.3 47.5 47.7 48.3 46.5 43.9 38.7 26.5 25.4 24.0 23.6 24.2 48.1 51.8 50.5 40.8Pyr 0.27 0.36 0.39 0.43 0.43 0.46 0.46 0.47 0.45 0.43 0.37 0.25 0.24 0.23 0.22 0.23 0.47 0.50 0.49 0.40Alm 0.70 0.61 0.57 0.54 0.54 0.51 0.51 0.50 0.52 0.54 0.59 0.70 0.71 0.73 0.73 0.72 0.51 0.47 0.48 0.57Spe 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.02 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01Gro 0.02 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0.03 0.03 0.03 0.02 0.03 0.03 0.02 0.02 0.02 0.03


garnet+sillimanite+quartz=cordierite , (3 )garnet+sillimanite=cordierite+spinel , (4 )

gedrite+quartz=orthopyroxene+cordierite , (5 )gedrite=orthopyroxene+cordierite+spinel . (6 )

Plagioclase and ilmenite are involved in the reactiontextures. However, the phase relationships can beapproximated by the FeO–MgO–Al2O3–SiO2–H2O(FMASH) system, provided that the presence of Nain gedrite is taken into account by projecting composi-tions from albite. Because of the reaction texture andthe zoning profiles in minerals, especially garnet, it isnot possible to determine the equilibrium compositions.To understand the evolution of the mineral composi-tion with change in mineralogy, the parageneses of thequartz-bearing domain were plotted on an Al–Fe–Mgdiagram assuming quartz and water in excess (Fig. 6a).This diagram shows that the increase in Fe in garnetcan be correlated with the divariant reaction (3)garnet+sillimanite=cordierite. This divariant reactionoccurs for a P–T domain in which gedrite+sillimaniteis not stable. This diagram shows also that thebreakdown of garnet+gedrite to orthopyroxene+cordierite is almost a degenerate univariant reactionwhich implies that gedrite+sillimanite is not stable inthe presence of quartz. In order to compare quartz-and spinel-bearing assemblages and to study the effectof bulk rock composition, a projection from garnet ispresented in Fig. 6(b) as this mineral is involved infour of the recognized equilibria. The mineral composi-tions have been projected from water and garnet ontothe Al–Fe0.6Mg0.4–Si (A–FM–S) plane (Fig. 6b). InFig. 4. Zoning profile in garnet surrounded by orthopyroxene–

cordierite symplectite. this projection gedrite plots within the tie-triangleorthopyroxene–cordierite–spinel, as is observed in therocks. The compatibility diagram is consistent with

with cordierite and orthopyroxene is more magnesian the reactions deduced from Fig. 6(a): equilibria (3) and(XMg=37) than that formed at the garnet–sillimanite (4) correspond to divariant reactions which can occurcontact (XMg=25–35). only if the gedrite–sillimanite tie-line is not stable. The

Plagioclase at the margin of symplectites has higher diagram also shows that the breakdown of gedrite toanorthite content (An36–42 ) than plagioclase included spinel+cordierite+orthopyroxene should occur afterin cordierite (An25–29) (Table 7). the breakdown of the gedrite+quartz tie-line. The

Biotite is close to the phlogopite end-member nature of equilibrium depends on the bulk composition(Table 8) characterized by XMg in the range 77–78, of micro domains: sillimanite-bearing equilibria involv-Al2O3 in the range 14.8–16.1 wt% and TiO2 in the ing Fe-cordierite and Fe-spinel occur for aluminousrange 1.8–4.1 wt%. Fluorine does not exceed 1.2 wt%. bulk compositions, whereas gedrite-bearing equilib-

ria involving Mg-cordierite and Mg-spinel occurfor Fe–Mg bulk compositions, and spinel-bearing

PARAGENETIC EVOLUTION AND ESTIM ATION assemblages only form in quartz-absent domains.OF P–T CONDITIONS The physical conditions can be evaluated throughMinerals of the primary paragenesis are inferred to be conventional geothermobarometry although applyinggarnet, gedrite, sillimanite, quartz, plagioclase, rutile, these techniques to these rocks is made difficult by theilmenite and biotite. The partial breakdown reactions presence of strongly zoned minerals, in particularwhich can be recognized are garnet, and local compositional variations. Equilibria

used for estimating pressure in the rocks discussed ingarnet+gedrite+quartz=orthopyroxene+cordierite , this paper are: garnet, plagioclase, sillimanite and(1) quartz (GASP: Newton & Haselton, 1981; Ganguly &garnet+gedrite=orthopyroxene+cordierite+spinel , Saxena, 1984; Hodges & Crowley, 1985; Koziol, 1989),

garnet–orthopyroxene–plagioclase–quartz (Newton &(2)


Table 4. Chemical compositions of orthopyroxene. Analyses recalculated on 6 oxygens.

Sample Tir67A Tir67A Tir67A Tir67A Tir67A Tir67A3 Tir67A3 Tir67A3 Tir67A3 Tir67A3 Tir67A3Texture in Ged symp symp symp ct.Ilm ct.Ilm corona corona corona corona corona

Ged+Qz=Opx+Crd

SiO2 50.21 50.03 49.89 50.81 50.22 52.10 52.48 53.27 53.60 52.19 52.86TiO2 0.21 0.14 0.21 0.18 0.14 0.32 0.18 0.07 0.04 0.46 0.14Al2O3 4.37 4.32 4.41 4.51 3.78 2.19 2.15 1.80 1.17 1.60 2.11Cr2O3 0.17 0.07 0.10 0.14 0.23 0.03 0.03 0.03 0.03 0.00 0.03MgO 21.22 20.88 20.92 21.91 21.06 22.84 23.10 23.49 23.06 21.90 22.49FeO 22.37 22.45 22.47 21.01 22.05 21.57 21.36 21.09 21.00 22.26 21.84MnO 0.22 0.16 0.25 0.25 0.22 0.19 0.29 0.19 0.16 0.06 0.22ZnO 0.00 0.11 0.00 0.00 0.07 0.00 0.11 0.07 0.00 0.11 0.00CaO 0.20 0.10 0.12 0.10 0.17 0.10 0.08 0.10 0.15 0.20 0.15Na2O 0.03 0.03 0.05 0.01 0.03 0.00 0.03 0.00 0.00 0.01 0.01K2O 0.00 0.02 0.00 0.00 0.00 0.04 0.02 0.02 0.00 0.00 0.00F 0.00 0.00 0.00 0.12 0.07 0.03 0.09 0.00 0.00 0.02 0.00Total 99.00 98.30 98.43 99.04 98.03 99.42 99.94 100.14 99.20 98.83 99.89

Si 1.89 1.90 1.89 1.89 1.91 1.94 1.94 1.96 1.99 1.96 1.96Ti 0.01 0.00 0.01 0.01 0.00 0.01 0.01 0.00 0.00 0.01 0.00Al 0.19 0.19 0.20 0.20 0.17 0.10 0.09 0.08 0.05 0.07 0.09Cr3+ 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00Mg 1.19 1.18 1.18 1.22 1.19 1.27 1.27 1.29 1.28 1.23 1.24Fe2+ 0.70 0.71 0.71 0.66 0.70 0.67 0.66 0.65 0.65 0.70 0.68Mn 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01Zn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ca 0.01 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.01 0.01 0.01Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00F 0.00 0.00 0.00 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.00XMg 62.8 62.4 62.4 65.0 63.0 69.1 65.8 66.5 66.2 63.7 64.7

Fig. 5. Compositions of orthopyroxenesplotted in Al2O3–FeO–MgO (mol%)diagram.

Perkins, 1982; Bohlen et al., 1983; Moecher et al., concur within their quoted errors: the pressure rangesare, respectively, 8.4–9.3 kbar (Koziol ), 7.3–8.2 kbar1988) and garnet–rutile–ilmenite–plagioclase–quartz

(GRIPS: Bohlen & Liotta, 1986). Temperature was (Newton & Haselton), 7.8–8.5 kbar (Ganguly &Saxena) and 7.0–7.7 kbar (Hodges & Crowley).estimated using the Fe–Mg exchange between garnet

and orthopyroxene (Harley, 1984; Sen & Bhattacharya, Pressure estimates based on the GRIPS barometer(7.6–8.2 kbar) are consistent with pressures using the1984; Lee & Ganguly, 1988) and between garnet and

cordierite (Holdaway & Lee, 1977; Perchuk & GASP calibration. For the secondary assemblages,estimated pressures using garnet-rim–orthopyroxene–Lavrent’eva, 1981). Maximum pressure calculated

using the GASP calibration is in the range 7.0–9.3 plagioclase–quartz fall in the range 4.4–7.2 kbar at900 °C which indicates that the symplectites formedkbar at 900 °C for the minerals of the primary

paragenesis and core garnet compositions. The com- by decompression. The temperature ranges obtainedfor garnet rims are 824–913 °C using the Sen &parison of the four calibrations indicates that they


Table 5. Chemical compositions of cordierite. Analyses recalculated on 18 oxygens.

Sample 67A 67A 67A 67A 67A 67A 67A 67A 67A 67A 67A 67ARemarks symplectite/Sp symplectite/OpxTexture Ged+Qz=Opx+Crd Sil+Grt=Crd+Sp Grt+Qz=Opx+Crd Ged+Grt=Opx+Crd+Sp

SiO2 49.16 49.58 48.78 48.84 48.83 48.80 49.03 49.05 48.59 48.64 48.48 48.19TiO2 0.00 0.00 0.10 0.00 0.00 0.03 0.07 0.00 0.08 0.04 0.03 0.05Al2O3 33.88 33.50 33.40 33.64 33.44 33.03 33.34 33.31 33.26 33.39 33.00 33.86Cr2O3 0.04 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.02 0.00 0.00MgO 11.02 10.85 10.59 10.31 9.76 9.05 9.94 9.63 9.06 9.09 10.91 10.78FeO 4.74 4.31 4.07 5.64 6.57 6.78 6.42 6.39 6.85 7.15 4.37 4.26MnO 0.00 0.01 0.01 0.03 0.03 0.10 0.13 0.10 0.15 0.08 0.00 0.00ZnO 0.00 0.00 0.03 0.00 0.04 0.05 0.01 0.00 0.04 0.00 0.00 0.00CaO 0.05 0.05 0.06 0.01 0.02 0.10 0.05 0.00 0.07 0.08 0.12 0.06Na2O 0.16 0.13 0.28 0.09 0.10 0.11 0.13 0.12 0.13 0.11 0.15 0.09K2O 0.05 0.01 0.47 0.00 0.00 0.00 0.02 0.00 0.00 0.07 0.01 0.01F 0.14 0.00 0.03 0.00 0.00 0.00 0.00 0.19 0.00 0.00 0.00 0.08Total 99.23 98.44 97.83 98.57 98.80 98.08 99.14 98.80 98.22 98.68 97.06 97.38

Si 4.93 5.00 4.97 4.96 4.97 5.00 4.97 4.97 4.98 4.967 4.97 4.92Ti 0.00 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00Al 4.01 3.99 4.01 4.03 4.01 3.99 3.98 3.98 4.02 4.021 3.99 4.07Cr3+ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Mg 1.65 1.63 1.61 1.56 1.48 1.38 1.50 1.46 1.38 1.38 1.67 1.64Fe2+ 0.40 0.36 0.35 0.48 0.56 0.58 0.54 0.54 0.59 0.61 0.38 0.36Mn 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.00 0.00Zn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ca 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.00 0.01 0.01 0.01 0.08Na 0.03 0.03 0.06 0.02 0.02 0.02 0.03 0.02 0.03 0.02 0.03 0.02K 0.01 0.00 0.06 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00F 0.04 0.00 0.01 0.00 0.00 0.00 0.00 0.06 0.00 0.00 0.00 0.03XMg 80.5 81.8 82.2 76.5 72.6 70.4 73.4 72.9 70.2 69.4 81.6 81.8

Table 6. Chemical compositions of spinel. Analyses recalculated on 32 oxygens and 24 cations.

Sample 67A 67A 67A 67A 67A 67A 67A 67A 67A4 67A4 67A4symplectite (Crd) symplectite Opx−Crd symplectite (Crd)

Texture Grt+Sil=Crd+Sp Ged+Grt=Opx+Crd+Sp Grt+Sil=Crd+Sp

SiO2 1.95 0.52 0.04 0.05 0.02 0.02 0.24 0.21 0.03 0.04 0.01TiO2 0.09 0.12 0.05 0.14 0.04 0.10 0.04 0.09 0.02 0.04 0.11Al2O3 58.48 59.63 59.40 59.27 58.55 59.20 59.07 60.80 63.28 61.86 62.50Cr2O3 0.35 0.35 0.04 0.28 0.64 0.30 0.30 0.37 0.36 0.60 0.60MgO 6.07 6.34 6.16 6.27 5.92 6.03 6.19 9.13 8.59 8.25 8.51FeO 32.42 31.92 32.33 33.16 32.99 32.64 32.07 27.71 28.03 28.25 28.68MnO 0.05 0.19 0.06 0.15 0.16 0.09 0.10 0.05 0.04 0.11 0.07ZnO 0.00 0.05 0.12 0.00 0.03 0.00 0.03 0.11 0.06 0.00 0.06CaO 0.00 0.04 0.02 0.00 0.00 0.00 0.01 0.01 0.00 0.01 0.01Na2O 0.00 0.00 0.02 0.01 0.00 0.00 0.00 0.01 0.01 0.01 0.01K2O 0.02 0.00 0.00 0.01 0.02 0.01 0.00 0.00 0.00 0.01 0.00F 0.16 0.00 0.10 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.01Total 99.59 99.16 98.34 99.34 98.37 98.39 98.05 98.49 100.42 99.20 100.57

Si 0.43 0.12 0.01 0.01 0.01 0.01 0.05 0.05 0.01 0.01 0.00Ti 0.02 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.00 0.01 0.02Al 15.28 15.68 15.73 15.60 15.60 15.73 15.72 15.75 16.05 15.95 15.90Cr 0.06 0.06 0.01 0.05 0.11 0.05 0.05 0.06 0.06 0.10 0.10Fe3+ 0.00 0.00 0.31 0.29 0.27 0.18 0.11 0.06 0.00 0.00 0.00Mg 2.01 2.11 2.06 2.09 2.00 2.03 2.08 2.99 2.76 2.69 2.74Fe2+ 6.01 5.96 5.76 5.91 5.97 5.97 5.95 5.03 5.04 5.17 5.18Mn 0.01 0.04 0.01 0.03 0.03 0.02 0.02 0.01 0.01 0.02 0.01Zn 0.00 0.01 0.02 0.00 0.01 0.00 0.01 0.02 0.01 0.00 0.01Ca 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00Na 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00K 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00F 0.11 0.00 0.07 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01Total 23.94 23.99 24.00 24.00 24.00 24.00 24.00 24.00 23.94 23.97 23.98XMg 25.0 26.1 26.4 26.1 25.1 25.3 25.9 37.3 35.3 34.2 34.6

Bhattacharya (1984) calibration, 741–806 °C after exchange equilibria are diffusion controlled, thetemperatures calculated may be considered as closureHarley (1984) and 776–858 °C using Lee & Ganguly

(1988). The garnet–cordierite thermometer using the temperatures. The wide range of temperatures recordedin the rocks could reflect continuous Fe–Mg re-Holdaway & Lee (1977) and Perchuk & Lavrent’eva

(1981) calibrations yields lower temperature, 751– equilibration during cooling but it can be assumedthat the peak temperature reached at least 850 °C.755 °C and 683–687 °C, respectively. Since Fe–Mg


Table 7. Chemical compositions of plagioclase. Analysesrecalculated on 8 oxygens.

Sample 67A 67A 67A 67A 67A4 67A4corona Crd−Sp

Texture Grt+Sil=Crd+Sp margin corona

SiO2 57.48 57.80 57.44 60.77 61.02 61.24TiO2 0.03 0.00 0.00 0.06 0.03 0.03Al2O3 26.25 26.17 26.47 24.24 25.01 24.51Cr2O3 0.00 0.00 0.06 0.00 0.00 0.03MgO 0.01 0.01 0.01 0.00 0.00 0.02FeO 0.21 0.13 0.59 0.19 0.03 0.05MnO 0.05 0.00 0.00 0.03 0.00 0.00ZnO 0.03 0.06 0.03 0.06 0.00 0.00CaO 8.24 7.98 8.77 6.01 5.78 5.32Na2O 6.55 6.65 6.64 7.86 8.45 8.58K2O 0.17 0.23 0.05 0.33 0.21 0.21F 0.00 0.00 0.00 0.00 0.00 0.03Total 99.03 99.03 100.05 99.55 100.54 100.03

Si 2.60 2.61 2.58 2.72 2.70 2.72Ti 0.00 0.00 0.00 0.00 0.00 0.00Al 1.40 1.39 1.40 1.28 1.30 1.28Cr3+ 0.00 0.00 0.00 0.00 0.00 0.00Mg 0.00 0.00 0.00 0.00 0.00 0.00Fe2+ 0.01 0.01 0.02 0.01 0.00 0.00Mn 0.00 0.00 0.00 0.00 0.00 0.00Zn 0.00 0.00 0.00 0.00 0.00 0.00Ca 0.40 0.39 0.42 0.29 0.27 0.25Na 0.57 0.58 0.58 0.68 0.73 0.74K 0.01 0.01 0.00 0.02 0.01 0.01F 0.00 0.00 0.00 0.00 0.00 0.00An 0.41 0.39 0.42 0.29 0.27 0.25Ab 0.58 0.59 0.58 0.69 0.72 0.74Or 0.01 0.01 0.00 0.02 0.01 0.01

Table 8. Chemical compositions of biotite. Analysesrecalculated on 22 oxygens.

Sample 67A 67A 67Arim core rim

Fig. 6. (a) Projection from quartz and H2O onto theTexture in quartzAl–Fe–Mg diagram. Inferred primary relationships are shownby dashed lines. (b) Projection from garnet and H2O onto theSiO2 38.26 37.81 36.58plane Al–Fe0.6Mg0.4–Si inferred for the studied rocks.TiO2 4.13 3.34 3.13

Al2O3 14.87 15.02 14.83Cr2O3 0.02 0.00 0.07 (1988) and the internally consistent thermodynamicMgO 18.04 18.78 18.82FeO 8.94 9.50 10.19 data-set of Holland & Powell (1990). Phase equilibriaMnO 0.02 0.13 0.00 were calculated in the FMASH system with the activityZnO 0.09 0.00 0.04

models given in the appendix. An FMASH petrogeneticCaO 0.03 0.10 0.08Na2O 0.16 0.16 0.06 grid involving garnet, orthopyroxene, orthoamphibole,K2O 8.86 8.16 7.52 sillimanite, cordierite, spinel, quartz and water forH2O 4.00 4.02 3.68F 0.16 0.08 0.62 aH2O

=1 is presented in Fig. 7 and two pseudosectionsTotal 97.58 97.09 95.63 corresponding, respectively, to an Si–Al bulk composi-Si 5.62 5.58 5.45 tion and an Fe–Mg bulk composition are presented inAl 2.57 2.62 2.61 Fig. 8. For the sake of clarity, only XgarnetFe isoplethsTi 0.46 0.37 0.35Cr3+ 0.00 0.00 0.01 are shown in the pseudosections as this parameterMg 3.95 4.13 4.18 displays the largest range in variation. Both pseudosec-Fe2+ 1.10 1.17 1.27 tions account qualitatively for the paragenetic evolutionMn 0.00 0.02 0.00Zn 0.01 0.00 0.00 and for the changes in mineral chemistry. The increaseCa 0.01 0.02 0.01 in XgarnetFe corresponds to decompression; this result isNa 0.05 0.05 0.02

confirmed by the XgarnetFe isopleths in spinel-bearingK 1.66 1.54 1.43Total 15.42 15.49 15.32 assemblages which have also a flat slope and areOH 3.92 3.96 3.66 located at lower pressure. Moreover, the pseudosec-F 0.08 0.04 0.34XMg 78.2 77.9 76.7 tions show that cordierite+orthopyroxene form in

Fe–Mg bulk compositions whereas only cordieriteforms after garnet+sillimanite in Si–Al bulk composi-The stability conditions of the mineral assemblages

described in the previous section can also be estimated tions followed by the appearance of cordierite+orthopyroxene in garnet-bearing areas. The pseudo-using phase diagrams calculated with the computer

program version 2.3 of Powell & Holland sections also show that the transition orthoamphi-


Fig. 7. Petrogenetic grid calculated in theFMASH system at aH2O = 1 and involvinggarnet, orthoamphibole, orthopyroxene,sillimanite, cordierite, spinel and quartz.

bole=orthopyroxene is discontinuous and thereforeDISCUSSIONthat the crystallization of orthopyroxene implies an

increase in temperature. However, in the rock system According to the petrogenetic grid and the pseudo-sections, the pressures estimated by geobarometrywhich involves Na and Ca, the decompression reaction

could be orthoamphibole+garnet=orthopyroxene+ for primary and secondary parageneses are consis-tent with conditions below the stability of thecordierite+plagioclase+H2O which does not neces-

sarily occur with an increase in temperature. The orthoamphibole+sillimanite assemblage. In contrast,the maximum temperature estimated by geothermobar-pseudosections are also consistent with the fact that

the breakdown of orthoamphibole does not produce ometry corresponds to the stability of orthoamphiboleinstead of orthopyroxene, yet orthopyroxene is stablea rim of orthopyroxene around orthoamphibole,

but a corona of cordierite+orthopyroxene symplectite. in the mineral assemblages. Moreover, in the naturalsystem, orthoamphibole would be stabilized by Na, inThey also show that garnet+sillimanite+

orthoamphibole is stable at a lower temperature the orthopyroxene stability field, and therefore oneshould assume that the peak temperature is higherthan garnet+sillimanite and than garnet+orthoamphi-

bole or orthopyroxene. Therefore, as the textures than 1000 °C according to the grid (Fig. 7). However,such a high temperature is probably inconsistent withhave recorded decompression from these two latter

stability fields, the occurrence of garnet+ the orthoamphibole–orthopyroxene equilibrium innatural assemblages.sillimanite+orthoamphibole at the early stage of

evolution should imply an increase in temperature The calculated grid is in agreement with thepetrogenetic grid presented by Spear (1993) whichprior to decompression. However, although the pri-

mary phase relationships have been lost, sillimanite implies that the orthoamphibole+sillimanite associ-ation breaks down in the absence of staurolite betweenand orthoamphibole are found in separate chemical

microdomains and therefore there is no evidence that 700 °C–5 kbar and 800 °C–10 kbar. It is also inagreement with the grid proposed by Xu et al. (1994)this increase in temperature has really been recorded.

Thus, although the rocks must have experienced which shows that the univariant reaction ortho-amphibole+sillimanite+quartz=cordierite+garnet isheating at some stage in order to reach peak conditions,

it is not possible to estimate at which pressure this located at a pressure around 8 kbar at 800 °C. There-fore, the pressure estimate seems realistic but tem-heating took place with the present data. We conclude

that the rocks have only recorded decompression from perature appears to be overestimated, although thepartitioning of Al into orthoamphibole instead ofequilibrium pressure of the primary assemblages

sillimanite+garnet+quartz and orthoamphibole+ orthopyroxene should shift the orthoamphibole–ortho-pyroxene equilibrium towards higher temperature. Atgarnet+quartz, to pressure corresponding to second-

ary, cordierite-bearing assemblages. least two hypotheses can be proposed to explain the


Fig. 8. Pseudosections calculated in theFeO–MgO–Al2O3–SiO2 system. All theassemblages contain quartz. (a) Si–Al-richbulk composition; (b) Fe–Mg-rich bulkcomposition.

temperature discrepancy between calculated and close to gedrite: typical values of XorthoamphiboleAlM2are 0.05

instead of 0.35. Fe–Mg ordering in orthopyroxene andobserved assemblages.1 The thermodynamic data for gedrite are poorly in orthoamphibole is not taken into account in the

activity model, but this simplification is not likely toconstrained by a single set of experiments (version 2.3 documentation). Propagating the uncer- explain the discrepancy between observed and calcu-

lated values.tainties leads to P–T estimates of 1020±84 °C and10.8±1.0 kbar and 967±43 °C and 9.6±0.4 kbar for 2 The reactions at the invariant points are dehydration

reactions and therefore lowering aH2Oshould lower thethe spinel-absent and the quartz-absent invariant

points, respectively. An increase of 80 kJ in the enthalpy temperature of the invariant points. According toNewton (1995), the presence of CO2-rich fluids duringof gedrite, representing <1% error, gives much lower

stability conditions at 788±66 °C and 9.7±1.2 kbar granulite facies metamorphism can explain that theorthoamphibole–orthopyroxene equilibrium buffersand at 728±12 °C and 8.5±0.4 kbar for the spinel-

absent and the quartz-absent invariant point, respect- aH2Oto a value as low as 0.4 in natural assemblages.

In order to check this hypothesis, the conditions ofively. However, in that case, the composition oforthoamphibole at the invariant points is close to the orthopyroxene-bearing equilibria were calculated for

aH2O=0.4 and 0.7. Results on the observed equilibriaanthophyllite–ferroanthophyllite solid solution and not


orthoamphibole+garnet+quartz+orthopyroxene+ The peak conditions are estimated at 7–9 kbar and850–900 °C and the formation of the spinel-bearingcordierite+H2O and orthoamphibole+garnet+

spinel+orthopyroxene+cordierite+H2O are pre- textures is estimated at 3.5–4.5 kbar and 700–800 °C.Calculated XorthoamphiboleAlM2

values are smaller than thesented in Fig. 9. There is a good agreement betweenthe analysed and calculated compositions except for analysed values, regardless of the method used to

calculate XAl from the structural formula, whereas theXorthoamphiboleAlM2, discussed below. For each aH2O, the

spinel-bearing assemblage forms at lower pressure and calculated and measured values of XorthoamphiboleAlM2are

the same. This discrepancy can have at least twolower temperature than the quartz-bearing assemblage.The mineral compositions in the assemblages explanations: orthoamphibole composition is not

strictly in the FMASH system, as Na, and to a lessergarnet+sillimanite+cordierite+spinel+H2O andgarnet+sillimanite+cordierite+quartz+H2O are extent Ca and Ti, is present in negligible amounts,

which may explain the high XAl values. Assuming thatconsistent with these conditions (Fig. 10). Thus,assuming a low aH2O

value, phase diagrams calculated the thermodynamic data for gedrite are accuratelyestimated, an alternative explanation, not necessarilywith the existing data set are consistent with indepen-

dent geothermometry and geobarometry estimates. inconsistent with the former, is that Al diffusion is

Fig. 9. Compositions of the mineralscalculated with respect to pressure andtemperature at different aH2O values andcompared with analysed compositions. (a)Paragenesis: Crd+Oam+Opx+Grt+Qz;(b) paragenesis: Crd+Oam+Opx+Grt+Sp. Thick lines indicate the range of theanalysed values and the correspondingpressure and temperature.


Fig. 10. P–T conditions calculated atdifferent aH2O for the analysed compositionsin parageneses (a) Qz+Crd+Sil+Grt and(b) Sp+Crd+Sil+Grt. Filled circlescorrespond to the conditions estimated forthe formation of Opx+Crd symplectiteswith Qz or Sp from Fig. 9. The grey linescorrespond to the P–T locations of thedifferent invariant equilibria obtained byfixing the mineral compositions to theanalysed values.

slower than Fe–Mg equilibration and therefore that also H. Haddoum for the numerous suggestions duringa field trip in Amessmessa. Comments on the manu-minerals have not equilibrated with respect to Al–Si

(e.g. Pattison & Begin, 1994). script by O. Mokhtari were very helpful. The paperalso benefited from the thorough reviews of F. Spearand J. C. Schumacher. This work is a contribution to

CONCLUSIONS the projects ‘Cartographie et Synthese Geologique duHoggar’ and AP 89 MES 116 supported by thePressure and temperature at the peak of metamorphism

in the gedrite–garnet–sillimanite-bearing granulites ORGM (Office National de la Recherche Geologiqueet Miniere), the University of Algiers (U.S.T.H.B.) inare estimated at 7–9 kbar and 850–900 °C. These

conditions are consistent with the presence of orthopy- Algeria, and URA CNRS 736, France.roxene–sillimanite–quartz–garnet–sapphirine in theAl–Mg granulites and of garnet–clinopyroxene–quartzin the metabasic rocks found in direct contact with

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cordierite acd=(1–x)2 (1–h)2Pattison, D. R. M. & Begin, N. J., 1994. Zoning patterns inorthopyroxene and garnet in granulites: implications for Fe-cordierite afcd=x2 (1–h)2geothermometry. Journal of Metamorphic Geology, 12, hydrous cordierite ahcd=(1–x)2 h2387–410.

Perchuk, L. L., Aranovich, L. Ya., Podlesskii, K. K., Lavrent’eva, garnet: variable is x (gt)=Fe2+/(Fe2++Mg)I. V., Gerasimov V. Yu., Fed–Kin, V. V., Kitsul, V. I., End-members mole fractions expressions are:Karsakov, L. P. & Berdnikov, N. V., 1985. Precambrian pyrope apy=(1–x)3granulites of the Aldan Shield eastern Siberia, U.S.S.R. Journalof Metamorphic Geology, 3, 265–310. almandine aalm=x3


orthoamphibole: variables are x(oa)=Fe2+/ End-members mole fractions expressions are:enstatite aen=(1–x) (1–y)(Fe2++Mg) and y (oa)=xAlM2End-members mole fractions expressions are: ferrosilite afs=x (1–y)Mg-Tschermak’s amgts =(1–x)yanthophyllite aanth=0.0625(1–x)7 (1–y)2 (2–y)4

Fe-anthophyllite afath=0.0625x7 (1–y)2 (2–y)4 spinel: variable is x (sp)=Fe2+/(Fe2++Mg)gedrite aged=(1–x)5 y4 (2–y)2 End-members mole fractions expressions are:spinel asp=(1–x)orthopyroxene: variables are x(opx)=Fe2+/

(Fe2++Mg) and y(opx)=xAlM1 hercynite aher=x

Gedrite-garnet-sillimanite-bearing granulites from Amessmessa area, south In Ouzzal, Hoggar, Algeria

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