Decompression reactions and P–T conditions in high-pressure granulites from Casares-Los Reales units of the Betic-Rif belt (S Spain and N Morocco)

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Journal of African Earth Sciences 39 (2004) 375–383

Decompression reactions and P–T conditions in high-pressuregranulites from Casares-Los Reales units of the Betic-Rif belt

(S Spain and N Morocco)

Faouziya Haissen a,*, Antonio Garcia-Casco b,Rafael Torres-Roldan b, Abdelmouhsine Aghzer a

a Laboratoire de Geodynamique, Departement de Geologie, Faculte des Sciences, Universite Chouaıb-Doukkali, 24000 El Jadida, Moroccob Departamento de Mineralogia-Petrologia, Universidad de Granada, Fuentenueva s/n, 18002 Granada, Spain

Available online 22 September 2004

Abstract

High-pressure granulites are exposed in the Casares-Los Reales group (internal zones of Betic-Rif belt, S Spain–N Morocco) as

part of the crustal envelope of Beni Bousera-Ronda Peridotites. They are mostly metapelitic but include intercalations of mafic com-

position. The metamorphic history is marked by the preservation of early high-pressure assemblages together with secondary low-

pressure assemblages suggesting a state of textural and compositional disequilibrium. The P–T path constrained by geothermobar-

ometry and reaction textures from mafic and pelitic lithotypes passes from P800 �C/15kbar to 600 �C/5kbar, to indicating a strong

decompression related to cooling, followed by a near-isobaric cooling 430 �C and 4kbar. Such P–T evolution of granulites is thought

to reflect some sort of rapid tectonic collapse of crust previously thickened through collision.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Betic-Rifean granulites; Isothermal decompression; Disequilibrium textures; Extensional collapse

1. Introduction

Mineral reaction textures are a record of particular

P–T conditions that the rocks underwent. The low diffu-

sivity of some components in granulite-facies conditions

allows them to preserve textures and compositions of

both the early high-pressure mineral assemblages and

retrograde ones, as well as the reaction mechanisms.

Generalized reactions governing the development of

symplectites and coronas can be readily inferred basedon the textural features. If the P–T position of the reac-

tions of these textures are known, these latter can be

used to constrain the pressure–temperature path fol-

lowed by a particular rock during its metamorphic evo-

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

doi:10.1016/j.jafrearsci.2004.07.030

* Corresponding author. Fax: +00 212 22259652.

E-mail address: [email protected] (F. Haissen).

lution (e.g. Ellis et al., 1980; Droop and Bucher-

Nurminen, 1984; Sandiford et al., 1987). The P–T pathconstructed may be useful to extract some information

relative to the tectonic setting of the metamorphism

(e.g. Harley, 1989) and to the global tectonometamor-

phic evolution of a particular region.

The Casares-Los Reales units of the Betic-Rif belt

offer a good example for studying decompression reac-

tions associated with basic and pelitic high-pressure

granulites, which were previously studied by Kornpro-bst (1974) and Loomis (1977). In this work, we apply

the most recent results of experimentally determined

reactions in granulite-facies conditions and we used

modern thermobarometric calibrations. The P–T paths

contructed from reactional textures and thermobaro-

metric study reveal an isothermal decompression sug-

gesting a tectonic exhumation process. This latter

mailto:[email protected]

376 F. Haissen et al. / Journal of African Earth Sciences 39 (2004) 375–383

corresponds to an extensional tectonic setting which is

related to Late Oligocene–Early Miocene thinning of

the Alboran domain.

2. Geological setting, lithotypes, mineral assemblages andmineralogy

The studied samples come from two regions of the

western Mediterranean: Sierra Bermeja massif in the

Western Betic Zone and Beni Bousera massif (internal

zones of Betic-Rifean belt, S Spain and N Morocco

respectively, Fig. 1). Alpujarride rocks outcropping

there belong to the Casares-Los Reales group of Monieet al. (1994). The tectonostratigraphic section of the

Casares-Los Reales group is typically composed of

metapelitic granulites (directly overlying the ultramafic

rocks) and gneisses that grade upwards to lower-grade

metapelites and marbles (Loomis, 1972a; Kornprobst,

1974; Torres-Roldan, 1981). In the studied regions, ma-

fic granulites are hosted by metapelitic associations

(kyanite + quartz + plagioclase + sillimanite + gar-net + biotite + K-feldspar) and concentrated mainly

near the contact with ultramafic rocks (Bouybaouene

Fig. 1. (a) Sketch map showing the location of the Alboran domain, with t

bermeja massif (the western segment of the Betic Zone) (b) and the Beni-Bou

dorsale, (3) Nieves units, (4) Casares-Los Reales Units, (5) Federico units, (6)

miocene formations.

et al., 1998). These mafic intercalations outcrop as

metric strat (2–5m) or sporadically as centimetric lenses

often elongated in concordance with the main foliation

of the country rocks. Samples selected for analysis were

those, which have best preserved primary fabrics and

mineral assemblages. However, all samples display atleast some textural disequilibrium features indicating

partial replacement of the early, high P–T metamorphic

assemblages by retrograde mineral phases. The mineral

assemblages of the varied lithotypes are summarized in

Table 1. The mineral abbreviations used are after Kretz

(1983).

2.1. Mafic granulites

The studied mafic granulites of the Casares-Los Rea-

les group contain the equilibrium assemblage gar-

net + plagioclase + clinopyroxene ± quartz + rutile with

different retrograde minerals such as orthopyroxene,

pargasitic hornblende, ilmenite, calcic plagioclase and

biotite. The main prevalent texture is granoblastic with

some porphyroblasts of garnet and clinopyroxene. Gar-net occurs as large lenticular matrix grains or lobate

and resorbed porphyroblasts, up to 5mm in diameter,

he ultrabasic masifs shown in black. Geological sketch map of Sierra

sera massif (internal Rif, N Morocco) (c). (1) External Zones, (2) Betic

Blanca Units, (7) Malaguides-Ghomarides, (8) Flyschs, (9) Syntectonic

Table 1

Lithotype varieties of the Casares-Los Reales units with the corresponding mineral assemblages

Lithotype Mineral assemblage

Pelitic granulites Grt–Pl–Kfs–Ky–Bt–Qtz–Rt–Gr–Ap–Zr (Sill + Bt,Spl + Crd,Ilm)

Qtz-absent mafic granulite Cpx–Grt–Pl–Rt ± Zr ± Ap ± Sph-(Pl,Ilm,Hbl)

«Fe-rich» mafic granulites Cpx–Grt–Pl–Qtz–Rt ± Spl ± Spl-(Hbl,Pl,Ilm)

«Mg-rich» mafic granulites Cpx–Grt–Pl–Qtz–Rt ± Ap ± Gr ± Zr (Bt,Ilm,Hbl,Pl,Opx)

The retrograde phases are shown in parentheses. Mineral abbreviations are after Kretz (1983).

F. Haissen et al. / Journal of African Earth Sciences 39 (2004) 375–383 377

containing inclusions of clinopyroxene, plagioclase and

quartz. They are always mantled by well-developed

coronas and/or symplectites. In the Fe-rich lithotypes,

a symplectitic texture composed by medium-grained

intergrowth of hornblende–plagioclase occurs around

almandine-rich garnet (alm57prp8–32grs15–37sps5). In the

Mg-rich lithotypes, pyrope-rich garnet (prp37–62alm27–48

grs13–25sps0–1.9) is mantled by an inner fine-grained sym-plectite composed by pseudomorphs of plagioclase–

orthopyroxene–spinel and an outer coarser-grained

orthopyroxene–plagioclase corona in contact with

quartz. In this symplectitic texture, the presence of spi-

nel was not mentioned in studies of Loomis (1977) and

Kornprobst (1974) on the same rock types. Generally,

garnet shows a narrow, irregular compositional zoning

which results from retrograde local exchange with adja-cent phases (invariably an increase in almandine content

within fine grain rims, Table 2). Clinopyroxene is abun-

dant in those rocks as granoblastic or porphyroblastic

grains always rimmed by hornblende, or as inclusions

in garnet. The porphyroblasts are generally substituted

by plagioclase–hornblende patches and the matrix

grains show very fine plagioclase exsolutions. A jadeitic

composition (Na- and Al-rich) is preserved in the coreswhilst rims are diopsidic (Mg-, Ca-, Si- and sometimes

Fe-rich) in both porphyroblasts and inclusions (Table

2). Some specific compositional features are observed

such as the high amount of Ti (up to 1.5wt%) in

quartz-free granulites and in Fe-rich quartz-bearing

ones. The Mg-rich granulites show a high content of

Mg (0.68–0.85 atoms p.f.u.) and low content of Fe

(0.07–0.13 atoms p.f.u.). As for the Fe-rich granulites,they are Ca- and Si-rich (0.91–0.98 and 1.89–1.98 atoms

p.f.u.) and Na- and Al-poor (0.015–0.055 and 0.042–

0.147 atoms p.f.u.). Some compositional variations

reflect retrograde exchange reactions that operated be-

tween clinopyroxene and adjacent phases. Secondary

orthopyroxene implicated in reaction textures is En76,

Al-poor (XAl = 0.006–0.065 atoms p.f.u.). Matrix gra-

noblastic plagioclase has an andesitic composition(Ab40–75) (Table 2) with the highest content of albite pre-

served in cores whilst rims are enriched in the anorthite

component. Plagioclase grains closed in garnet and

clinopyroxene are slightly richer in Ca, but show the

same compositional rim-core variation as matrix grains.

Plagioclase intergrown with hornblende is An96, ob-

served in symplectitic texture in Fe-rich lithoypes,

whereas it is more sodic (An80) when intergrown with

orthopyroxene-spinel in symplectitic texture of Mg-rich

samples and, is An60 when intergrown with orthopyrox-

ene as a coronitic reaction sequence in the garnet–quartz

contact zone. It is thus apparent that retrograde plagio-

clase is appreciably more calcic than the early stable

high P–T plagioclase. Such gain in anorthite content is

clearly related to a loss of grossular component fromthe coexisting garnet. Variable amounts of pargastic

hornblende occur as retrograded rims or patches in

clinopyroxene or implicated in kelyphitic textures (Table

2). Compositions vary locally depending on both whole-

rock chemistry and reactant phase compositions. Retro-

grade phlogopite is present in Mg-rich quartz-bearing

granulites replacing garnet or associated with horn-

blende, showing the highest Ti content when adjacentto ilmenite or rutile and the highest Mg/(Mg + Fe) con-

tent in contact with garnet (Table 2). Such variations re-

flect different locations of growth of this phase.

It is worth noting that, beside this dominant type of

mafic granulite, kyanite-bearing granulites locally occur

in the Beni Bousera Massif envelope (Bouybaouene

et al., 1998). These peculiar high-pressure granulites

contain sapphirine in their disequilibrium symplectitictextures.

2.2. Pelitic granulites

Pelitic granulites of the Casares-Los Reales units are

medium-grained garnet–biotite–plagioclase–K-feldspar–

kyanite–quartz rocks with some retrograde phases such

as sillimanite, biotite, ilmenite and, in the ost retro-graded samples, spinel and cordierite (Table 1). The

prevalent fabric is porphyroclastic with elongate mineral

grains showing some preferred orientation and recrystal-

lized ribbon-textured quartz. Primary minerals of these

rocks are essentially homogeneous. Garnet is abundant

as corroded porphyroblasts showing a sieve appearance

with inclusion of biotite, kyanite, rutile, plagioclase and

quartz. The composition is alm47–65prp21–30grs5–30sps0–

1.8 with the highest almandine content presented by

grain rims (Table 3). The shape and smooth curvature

of this Fe-enrichment profile is consistent with diffu-

sion-controlled re-equilibration of an originally homo-

geneous garnet grain. The majority of the sillimanite,

occurring as fibrolite mats, in these granulites shows evi-

dence of having replaced kyanite. A phlogopitic biotite

Table 2

Representative chemical analyses of mineral phases of basic granulites. C: core, R: rim

Sample no. FH-18 FH-17 FH-17 FH-17 FH-20A FH-17

Hbl Bt Pl Grt(R) Grt(C) Ilm Cpx(R) Cpx(C)

SiO2 39.91 39.66 SiO2 39.69 39.77 SiO2 58.12 SiO2 40.68 40.69 FeO 43.29 SiO2 (%) 51.93 51.04

TiO2 0.55 0.60 TiO2 3.02 3.13 Al2O3 26.71 TiO2 0.06 0.06 TiO2 53.31 TiO2 0.55 0.68

Al2O3 17.64 18.84 Al2O3 14.85 14.77 CaO 8.50 Al2O3 22.93 25.89 Cr2O3 0.00 Al2O3 5.90 8.69

Cr2O3 0.11 0.13 FeO 7.50 7.12 Na2O 6.28 Cr2O3 0.05 0.02 Al2O3 0.03 Cr2O3 0.18 0.13

FeO 12.05 12.03 MgO 20.01 20.43 K2O 0.55 FeO 15.39 13.34 MnO 0.59 FeO 3.83 3.77

MnO 0.10 0.08 CaO 0.02 0.02 Total 100.15 MnO 0.28 0.21 MgO 1.41 MnO 0.04 0.05

MgO 11.71 11.45 Na2O 0.31 0.29 MgO 12.19 13.04 ZnO 0.03 MgO 14.51 13.34

CaO 11.19 11.38 K2O 9.36 9.31 CaO 8.84 7.16 V2O5 0.32 CaO 22.48 21.25

Na2O 3.05 3.02 F 0.67 0.60 Total 100.42 100.40 Total 99.01 Na2O 0.80 1.17

K2O 0.05 0.07 Total 96.25 96.18 Total 100.23 100.10

Total 96.35 97.25

O = 23 (13-CNK) O = 20 O = 8 O = 12 O = 6 O = 6

Si 5.841 5.753 Si 5.723 5.718 Si 2.597 Si 2.998 2.947 Ti 2.047 Si 1.889 1.856

[iv]Al 2.159 2.247 [iv]Al 2.277 2.282 Al 1.407 Ti 0.003 0.003 Al 0.002 [iv]Al 0.111 0.144

[vi]Al 0.886 0.975 Ca 0.407 Al 1.992 2.209 Cr 0.000 [vi]Al 0.142 0.228

Cr 0.012 0.015 [vi]Al 0.247 0.222 Na 0.547 Cr 0.003 0.001 Fe 1.849 Cr 0.005 0.004

Fe3+ 0.753 0.726 Ti 0.327 0.338 K 0.031 Fe 0.949 0.808 Mn 0.025 Ti 0.015 0.018

Ti 0.060 0.066 Fe 0.905 0.856 Total 4.989 Mn 0.018 0.013 Mn 0.025 Ti 0.015 0.018

Mg 2.555 2.475 Mg 4.301 4.378 Mg 1.412 1.743 V 0.011 Mg 0.787 0.723

Fe2+ 0.721 0.734 Total [VI] 5.780 5.794 ab 0.555 Ca 0.698 0.555 Total 3.935 Ca 0.876 0.828

Mn 0.012 0.009 an 0.413 Total 8.073 8.280 Na 0.057 0.083

[M4]Ca 1.755 1.769 Ca 0.002 0.003 or 0.032 alm Fe3+ 0.000 0.000

[M4]Na 0.245 0.231 Na 0.088 0.081 alm 0.316 0.290 Fe2+ 0.117 0.115

[A]Na 0.622 0.618 K 1.722 1.708 Sps 0.006 0.005 Total 4 4.001

[A]K 0.009 0.012 Total [XI] 1.812 1.792 Grs 0.232 0.200

Total 15.63 15.63 prp 0.446 0.506 XMg 0.871 0.863

Mg/Fe 1.412 1.743

378

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Table 3

Representative chemical analyses of mineral phases of pelitic granulites

Sample no. FH-3B

Bt Kfd(Mtx) Pl(Mtx) Grt(C) Grt(R)

SiO2 38.00 37.27 SiO2 65.01 57.31 SiO2 38.77 38.20

TiO2 3.58 3.29 Al2O3 18.93 26.63 TiO2 0.36 0.15

Al2O3 17.18 16.64 CaO 0.14 8.82 Al2O3 21.93 21.70

FeO 13.59 13.75 Na2O 1.46 6.50 Cr2O3 0.07 0.06

MgO 13.48 14.17 K2O 14.07 0.44 FeO 26.38 28.92

CaO 0.05 0.02 Total 99.60 99.71 MnO 0.51 0.80

Na2O 0.06 0.08 MgO 7.06 5.72

K2O 8.98 9.18 CaO 5.95 5.14

F 0.25 0.31 Total 101.03 100.67

Total 95.47 95.09

O = 20 O = B O = 12

Si 5.609 5.559 Si 2.989 2.579 Si 2.976 2.978

[iv]Al 2.391 2.441 Al 1.025 1.413 Ti 0.021 0.009

Ca 0.007 0.425 Al 1.984 1.993

[vi]Al 0.597 0.485 Na 0.131 0.570 Cr 0.004 0.004

Ti 0.397 0.369 K 0.825 0.025 Fe 1.693 1.885

Fe 1.678 1.716 Total 4.976 5.012 Mn 0.033 0.053

Mg 2.965 3.151 Mg 0.477 0.353

Total [VI] 5.638 5.721 Ca 0.489 0.429

Total 7.678 7.703

Ca 0.008 0.002

Na 0.016 0.024 ab 0.136 0.559 alm 0.560 0.622

K 1.692 1.747 an 0.007 0.417 sps 0.011 0.017

Total [XII] 1.716 1.774 or 0.857 0.025 grs 0.162 0.142

prp 0.267 0.219

Mg/Fe 0.477 0.353

C: core, R: rim, Mtx: matrix.


is present as primary grains dispersed in matrix (Table 3)

or as a secondary phase replacing garnet. Plagioclase is

present as antiperthitic porphyroblasts (An37–44Ab54–60

Or0–0.2), neoblastic grains (An37–48Ab48–60Or0–10) or as

inclusions with more calcic compositions (An43–51

Ab45–54(Or0–0.3), in garnet. K-feldspar matrix grain com-

positions are An0.7–10Ab13–19Or70–86 antiperthitic com-

position is An0.5–1Ab11–15Or83–87.

3. Reactional history


Mafic granulites exhibit several textures such as sym-

plectites and/or coronas around relic garnet, and rims ormantles of hornblende on clinopyroxene. These textures

are more developed in quartz-bearing assemblages and

are composed by secondary minerals (plagioclase,

orthopyroxene, hornblende, spinel) with compositions

totally different from that of primary minerals (plagio-

clase, garnet, clinopyroxene).

3.1.1. Mg-rich quartz-bearing granulites

An important microtextural feature of such rocks is

that their pyrope-rich garnet exhibits an outer coronitic

reaction boundary and an inner symplectitic rim. The

inner fine-grained symplectite is composed by vermicu-

lar or graphic intergrowths of plagioclase, orthopyroxene

and spinel, whereas the outer coarse-grained corona

comprise plagioclase and orthopyroxene. This outer

coronas is observed only in zones where garnet is in con-

tact with quartz. Textural relationships observed in

these rocks suggest that the formation of the inner sym-

plectite around the garnet is the latest event recorded intheir retrograde evolution and is then posterior to the

formation of the outer corona.

The formation of the outer Pl + Opx corona is related

to the following reaction:

Grt þ Cpx þ PlMatrix þ Qtz ¼ Pl þ Opx ð1ÞSimilar corona textures have been reported from the

Betic-Rif granulites (Kornprobst, 1974) as well as other

granuliteterranes (e.g. Dufour, 1985; Schenk, 1984;

Boullier and Barbey, 1988; Sandiford et al., 1988;

Harley, 1989; Messiga and Bettini, 1990; Thost et al.,

1991; Ouzegane et al., 2001).

The Opx + Pl + Spl assemblage localized at the inner

symplectite is attributed to the reaction:

Grt þ Ab ¼ Opx þ Spl þ Pl ð2ÞThe presence of Spl in these textures was not men-

tioned in Loomis (1977) work about the same granulite

terrane. In other granulitic rocks elsewhere, coexisting


coronas and symplectic textures around garnet spinel

are only observed in symplectic textures from quartz-

free conditions (e.g. Messiga and Bettini, 1990; Harley,

1989; Thost et al., 1991) as indicated by Harley (1989).

3.1.2. Fe-rich quartz-bearing mafic granulites

Whereas a coronitic structure is always present as an

external rim on symplectite in Mg-rich lithologies, the

Fe-rich ones only display a symplectitic microstructure

composed by small finger-like crystals of plagio-

clase + hornblende, both growing perpendicular to the

garnet boundaries. This hydrous symplectite becomes

coarser-grained moving away from the garnet bounda-

ries and develops small granoblastic aggregates of horn-blende and plagioclase. Garnets are sometimes

completely replaced by this symplectite. The reaction

proposed for this case is:

Grt þ Cpx þ PlMatrix þ H2O ¼ Hbl þ Pl ð3Þwhere the unique possible source of H2O is represented

by the enclosing metapelites.

3.2. Pelitie granulites

The following reaction is deduced for pelitic rocks:

Grt þ Kfs þ Rt þ H2O þ Pl1

¼ Bt þ Pl2 þ Als þ Ilm þ Qtz ð4Þ

with Pl1 the anthiperthitic plagioclase and Pl2 the neob-

lastic plagioclase. Reaction (4) can be considered as the

multicomponent equivalent in the KFMASH system of

the reaction:

Grt þ Kfs þ H2O ¼ Bt þ Sill þ Qtz ð5ÞSuch garnet decomposition is considered as being

characteristic of granulitic terranes that evolved through

a near-isothermic decompressional P–T path (Harley,

1989; Carswell and O�Brien, 1993).

4. Thermobarometry


Calculations of P–T conditions are difficult to per-

form on such texturally complex rocks and in fact, these

rocks have relatively few phases that can be inferred to

have once been in equilibrium. However, the granoblas-

tic texture is well preserved in most mafic assemblages

and the presence of a chemical zoning in garnet, clinopy-

roxene and plagioclase of the principal assemblage whencombined with reaction textures (corona and symplec-

tites) yield at least two sets of thermobarometric infor-

mation. Peak P–T conditions are calculated from core

compositions of early phases Grt–Cpx–Pl–Qtz and

inclusions; the retrograde part of the P–T path can be

constrained from the re-equilibrated rim compositions

of primary phases and the late reaction texture phase

compositions. The most suitable mineralogy to deter-

mine the pressure of formation of this granoblastic tex-

ture is found in the quartz-bearing samples; in quartz-

free ones, only temperatures can be estimated. Thequartz-bearing samples permit us to use many thermom-

eter (Cpx–Grt, Opx–Grt, Bt–Grt) and barometer formu-

lations involving garnet, clinopyroxene, orthopyroxene,

plagioclase, quartz, and biotite (GADS, GAPES). The

Fe2+–Mg2+ partitioning between garnet and clinopyrox-

ene provides a good means of determining the equilibra-

tion temperature of clinopyroxene-bearing granulite.

The GADS barometry (Newton and Perkins, 1982)and garnet–clinopyroxene thermometry (Ellis and

Green, 1979; Krogh, 1988; Pattison and Newton,

1989) were applied; however only the results of the cal-

ibration of Ellis and Green (1979) were used since those

of Pattison and Newton (1989) can not be used in some

samples as their garnet XMg is higher than 0.125–0.6

atoms p.f.u., i.e. outside of the range for which the cal-

ibration was proposed. In quartz-free granulites, thesame calibrations based on the thermometer Grt–Cpx

were used.

The core compositions of early phases Grt–Cpx–Pl–

Qtz and their inclusions, yielded peak temperature con-

ditions dispersed between 800 and 1000 �C at pressures

of 14–16kbar. The retrograde post-peak P–T conditions

constrained from the re-equilibrated rim compositions

of primary phases and the late reaction texture phasecompositions, are dispersed between 700 and 900 �C at

9–13kbar.

4.2. Pelitic granulites

For these garnet–kyanite assemblages, the GASP

barometer is applicable because the compelling minera-

logical evidence, described in previous sections, showsthat the loss of grossular component from garnet and

gain of anorthite component in plagioclase can be

attributed to the multivariant continuous reaction

equivalent of GASP operating during the retrograde his-

tory of these granulites. Since its initial calibration and

application by Newton and Haselgon (1981), this

barometer has been refined on the basis of thermody-

namic data (Powell and Holland, 1988) and further di-rect experimentation (Koziol and Newton, 1988, 1989).

The activity model of Berman (1990) was adopted for

garnet and those for plagioclase were computed after

Fuhrman and Lindsley (1988) and Elkins and Grove

(1990). Cores of garnet and plagioclase are used to cal-

culate pressure of early high-grade assemblage and rim

compositions in immediate contact with one another

for retrograde conditions. For estimating temperatureof those assemblages, no thermometer is better than

the GARB thermometer. Several calibrations are now


available for this thermometer from which we used those

of Thompson (1976), Ferry and Spear (1978), Hodges

and Spear (1982), Ganguly and Saxena (1984), Indares

and Martignole (1985) and Berman (1990). Accordingly,

the P–T conditions obtained are scattered in temperature

(600–780 �C) for peak assemblage at 12–15kbar, and areat 400–530 �C for the retrograde assemblage at 2–5kbar.

This mismatch in temperature is due to the fact that the

differences in temperatures calculated using each ther-

mometer calibration may exceed 100 �C, which is not

the case for the calibrations of the barometer, that yield

near-identical pressures ±0.5–1kbar. The presence of

antiperthitic plagioclase porphyroblasts allows us to ap-

ply the two-feldspar thermometry using the calibrationof Elkins and Grove (1990) and that of Fuhrman and

Lindsley (1988). Temperatures deduced lie in the interval

500–600 �C, the P–T conditions obtained from the cali-

bration of Elkins and Grove (1990) are 580 �C and

±4kbar, whilst the pressures deduced from the formula-

tion of Fuhrman and Lindsley (1988) are widespread be-

tween 1 and 20kbar. The high scattering of pressures in

this case indicates that the model of Elkins and Grove(1990) describe better the behaviour of the solid solution

of feldspar than that of Fuhrman and Lindsley (1988).

Fig. 2. P–T grid combining thermobarometric results of pelitic and

mafic granulites. GARB thermometer and GASP barometer are used

to deduce P–T conditions for cores and rims of principal phases, solvus

thermetry of feldspaths is used. Abbreviations: E and G = Elkins and

Grove (1990), F and L = Fuhrman and Lindsley (1988). Reaction

Bt + Als + Qtz = Grt + Crd + Kfs + H2O is calculated from the data

base of Holland and Powell (1990). Error ranges calculated are: 3%

relative for atomic proportions of elements in mineral (Grt, Cpx, Opx

y Pl), ±50�C for GASP barometer, ±1kbar for GARB thermometer

and ±0.5kbar for pressure of GASP reaction between pure phases,

0.01% relative for molar volume of pure phases of GASP reaction and

20% relative for Margules parameters (WH, WS, WV) of Berman

(1990) model for garnet and Elkins and Grove (1990) for plagioclase,

no error was considered for thermodynamic properties for GARB

reaction, nor for enthalpy and entropy of GASP reaction. Note that

are represented on the graph only one P–T peak condition (650�C/

12.5kbar) and one for retrograde conditions (430�C/4kbar) from

GARB thermometer of Berman (1990) and GASP barometer.

5. P–T path, discussion and conclusions

The mafic granulites of the Casares-Los Reales units

are an additional example of Alpujarride rocks deeplymarked by a textural and compositional disequilibrium

state. Such disequilibrium is traduced by the coexistence

of incompatible phases (presence of spinel in

Opx + Pl + Spl kelyphites of quartz-bearing granulites),

by incomplete reactions (as testified by the multiple reac-

tion textures observed) and/or by the compositional het-

erogeneity. The presence of spinel in OpxK + PlK + Spl

kelyphites (in zones of contact of garnet with quartz)next to OpxC + PlC corona indicates the role of effective

barrier of this corona texture against the diffusion of

chemical elements between primary phases (matrix plag-

ioclase, clinopyroxene and garnet) and the secondary

phases (corona orthopyroxene and corona plagioclase).

However, some diffusional processes have operated be-

tween corona plagioclase and kelyphitic plagioclase to

modify the composition of this latest phase from the ini-tial composition (100% anorthite resulting from direct

breakdown of grossular component of garnet) towards

composition of 60% anorthite. Such processes have

operated in restricted domains and only when corona

and kelyphite textures coexist (Mg-rich quartz-bearing

granulites). In the case of Fe-rich granulites, kelyphites

are composed of plagioclase, enriched in anorthite

(An � 100%), and hornblende. Modelling of reactionsyielding to those textures indicate that the kelyphites re-

sult from direct breakdown of garnet with implication of

some primary phases as clinopyroxene, and that corona

textures grew in incompatibility zones of the association

garnet–quartz. These reactions were reported in litera-

ture as indicating an important drop of pressure during

the retrograde path. Such textural indices have been of

great use when evaluating the P–T path of thosegranulites.

In spite of the fact that the application of reversible

process thermodynamic techniques for the deduction

of the P–T conditions of these rocks include a strong

uncertainty, together with the presence of disequilibrium

in their associations, a P–T path which take is into con-

sideration all their textural and compositional character-

istics is proposed (Fig. 2). Thermobarometrycalculations indicate that there is a remarkable agree-

ment between the estimates in both pelitic and mafic

granulites, suggesting that these rocks experienced the

same thermotectonic events. Integrating these thermo-

barometric data, reaction textures and mineral composi-

tions, the P–T evolution suggests that these peak

granulitic assemblages were formed at T P 800 �C and

pressure of 15kbar, and the retrograde assemblagesexperienced near-isothermal decompression to <500 �Cand 4–5kbar. Despite the fact that these high values

for the peak assemblages are surprising, we believe them


to be meaningful in view of the remarkable consistency

of the calculated results.

Considering only mafic lithotypes, the deduced path

is limited to a near-isothermal decompressional stage

(DP � 10kbar, Fig. 2), granulites developed their dis-

equilibrium textures during this decompressional stage.The principal foliation S2–3 of the pelitic granulites is

attributed as well to this stage of evolution. The end

of this decompressional stage is dated at 22 Ma

(e.g. Monie et al., 1994; Sosson et al., 1998; Sanchez-

Rodriguez and Gebauer, 2000). Several high exhuma-

tion rates were proposed according to radiometric data

(e.g. Monie et al., 1994; Tubia et al., 1997). According

to data obtained from the pelitic granulites, the decom-pression is followed by a near-isobaric cooling (Fig. 2).

This final stage of P–T path is equally, reported also

in other Alpujarride units, and is dated to Late Oligo-

cene–Early Miocene (Zeck et al., 1989, 1992; Monie

et al., 1994). Radiometric data currently available for

the western Alpujarrides indicate that the final stage of

cooling of the P–T path occurred with a high cooling

rate of 250–450 �C/Ma in the range 19–22 Ma. Such ra-pid cooling suggests an exhumation rate that can reach

3km/Ma at this metamorphic stage in the Casares-Los

Reales units. This very fast P–T evolution of Casares-

Los Reales units allowed the preservation of all these

disequilibrium characteristics in these granulites, as well

as in others Alpujarride rock types (for example Torrox

gneisses: Garcia-Casco, 1993; Garcia-Casco et al., 1993;

Garcia-Casco and Torres-Roldan, 1996).The decompressional P–T path observed in western

Alpujarrides is consistent with the models proposed

for granulites in a thickened crust undegoing extensional

collapse (e.g. Albarede, 1976; England, 1987; Thompson

and Ridley, 1987; Ellis, 1987; Harley, 1989; Sandiford,

1989). The extensional collapse associated with the

opening of the Alboran Sea was related to the convec-

tive removal (Platt and Vissers, 1989; Vissers et al.,1995) or delamination (Channell and Mareschal, 1989;

Docherty and Banda, 1995) of the lithospheric root be-

neath the previously thickened crust. The thickening

event is related to an Hercynian (Michard et al., 1991;

Bouybaouene et al., 1998; Chalouan et al., 2001) or

Alpine orogeny (Montel et al., 2000).

Acknowledgments

The authors are grateful to Michard and Ouzegane

for their critical comments.

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Documents

Decompression reactions and P–T conditions in high-pressure granulites from Casares-Los Reales units of the Betic-Rif belt (S Spain and N Morocco)