GeolCarp_Vol50_No4_283

GEOLOGICA CARPATHICA, 50, 4, BRATISLAVA, AUGUST 1999

283293

PHASE RELATIONS AND P-T PATH OF CORDIERITE-BEARING

MIGMATITES, WESTERN TATRA MOUNTAINS,

WESTERN CARPATHIANS

LÍVIA LUDHOVÁ

and MARIAN JANÁK

Department of Mineralogy and Petrology, Faculty of Science, Comenius University, Mlynská dolina, 842 15 Bratislava,

Slovak Republic; ludhova@fns.uniba.sk

Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 842 26 Bratislava, Slovak Republic; geolmjan@savba.sk

(Manuscript received December 3, 1998; accepted in revised form March 17, 1999)

Abstract: Cordierite, as a product of Variscan regional metamorphism and exhumation, has been recognized in the

migmatitic metapelites of the Western Tatra Mountains, Western Carpathians. The following cordierite-producing reac-

tions have been deduced from reaction textures and phase equilibria in the system KFMASH: a) garnet decomposition

according to the reaction garnet + sillimanite + quartz + fluid = cordierite during decompression at a temperature of

~700 ºC down to a pressure of ~5 kbar; b) dehydration-melting of biotite by the reaction biotite + sillimanite + quartz =

cordierite + K-feldspar + melt due to further pressure drop to ~4 kbar at a constant, or increasing temperature to ~750 ºC;

c) melt crystallization in the leucosomes according to the reaction melt = cordierite + quartz + K-feldspar + biotite +

fluid, during cooling. Cordierite pinitization and its replacement by fine-grained phengitic white mica and pale green

Mg-rich biotite according to the reactions cordierite + sillimanite + K-feldspar + fluid = phengitic muscovite + quartz

and cordierite + K-feldspar + fluid = biotite + sillimanite + quartz took place during subsolius retrogression at a tempera-

ture of ~ 600 ºC and pressure of ~3 kbar. The origin of cordierite was also controlled by the local, more magnesian bulk

composition of cordierite-bearing metapelites. The presence of cordierite is indicative of a high-temperature/low-pres-

sure stage during the Variscan orogenic collapse in the pre-Alpine basement of the Western Carpathians.

Key words: Variscan orogen, Western Tatra Mountains, dehydration-melting, migmatites, cordierite.

Introduction

Cordierite is a typical phase of the contact or regional low-

pressure metamorphic assemblages (Spear 1993; Bucher &

Frey 1994). In the Western Carpathians crystalline complexes,

cordierite was for a long time known only as a product of low-

pressure contact metamorphism, in assemblages with an-

dalusite. It was described in the contact zones of the Modra

granodiorite in the Malé Karpaty Mountains (Korikovsky et al.

1985) and Rochovce Granite in the Veporic Superunit (Vrána

1964; Korikovsky et al. 1986; Vozárová 1990). In recent years,

the presence of cordierite in gneisses and migmatites was re-

ported from the Ve¾ká Fatra Mountains (Janák & Kohút 1996)

and the Western Tatra Mountains (Ludhová 1996; Ludhová &

Janák 1996; Janák et al. 1999a,b). Here, cordierite occurs in

metapelitic migmatites, in assemblages with sillimanite and

garnet, similar to the rocks in the Ve¾ká Fatra Mountains. The

purpose of this study is to present details on the cordierite-

forming reactions and phase equilibria in the metapelitic

FMASH system, and to discuss a possible metamorphic P-T

path followed by cordierite-bearing rocks with respect to the

tectonometamorphic evolution of the Tatra Mountains (Janák

1994; Janák et al. 1996, 1999a,b).

Geological setting

The Tatra Mountains represent a typical core complex lo-

cated in the northernmost sector of the Tatric Superunit in the

Western Carpathians. They represent a key area for the study

of the eastern and south-eastern continuation of the Variscan

basement within the Alpine-Carpathian orogenic belt in Cen-

tral Europe (Krist et al. 1992).

The crystalline basement of the Tatra Mountains is com-

posed of pre-Mesozoic metamorphic rocks and granitoids,

overlain by Mesozoic and Cenozoic sedimentary cover se-

quences and nappes. Metamorphic rocks are more abundant in

the western partthe Western Tatra Mountains (Fig. 1),

whereas granites are more abundant in the eastern partthe

High Tatra Mountains. Within the basement, two superim-

posed tectonic unitslower and upper, differing in lithology

and metamorphic grade, have been distinguished (Kahan

1969; Janák 1994). The lower tectonic unit is composed of

staurolite, kyanite and fibrolite sillimanite-bearing mica-

schists. Two metamorphic zones (Fig. 1), staurolite-kyanite

and kyanite-sillimanite were distinguished (Janák et al. 1988).

The upper tectonic unit is lithologically variable. Near the

base of the unit, orthogneisses and banded amphibolites with

eclogite relics prevail, belonging to the kyanite metamorphic

zone (Janák 1994; Janák et al. 1996; 1999a,b). Higher levels are

composed of migmatites and gneisses, intruded by a sheet-like

granitoid pluton. They belong to the sillimanite metamorphic

zone with no high-pressure relics (Janák et al. 1988; Janák

1994; Janák et al. 1999a,b). Cordierite-bearing migmatites occur

only rarely. They have been found near Jeová (Fig. 1), in mig-

matites of diatexite type (Ludhová 1996; Ludhová & Janák

1996; Janák et al. 1999a,b), which may be regarded as roof pen-

dants of a granite pluton (e.g. Gorek 1956; Kahan 1969).

284 LUDHOVÁ

and JANÁK

A polyphase, Variscan and Alpine deformation (Fig. 1) un-

der distinct P-T conditions was recognized in the Western

Tatra Mountains (Kahan 1969; Fritz et al. 1992; Janák 1994).

The Variscan D

deformation is defined by a mineral linea-

tion in metamorphites and is related to the top-to-the-south-

east thrusting of the upper unit onto the lower one. Subse-

quent D

deformation due to dextral or top-to-the-east shear

has been recognized in the migmatites of the sillimanite zone

as well as in the marginal zones of the granitoid pluton. The

deformation is attributed to the Variscan orogen-parallel

extension. Alpine D

deformation in brittle conditions is

manifested by top-to-the-north-west shear. The last major D

deformation is related to updoming and normal faulting in

north-south to north-west-south-east direction during Tertia-

ry extension.

The oldest tectonometamorphic events in the Tatra Moun-

tains seem to be Early Paleozoic, between 380 and 420 Ma

(Rb-Sr whole rock isochron of Burchart (1968); zircon single

grain data from orthogneisses of Poller et al. (1997)). Ac-

cording to Rb-Sr isochrons, the granitoid magmatism oc-

curred at 290310 Ma (Burchart 1968) or 340350 Ma

(Gawêda 1995). New zircon single-grain data from the West-

ern Tatra granites (Todt et al. 1998) yield ages of 340360

Ma. The cooling ages of micas from the granites and migma-

tites range from 330 to 300 Ma (

Ar/

Ar method, Maluski

et al. 1993; Janák 1994), reflecting the late-Variscan exhu-

mation and the absence of a higher-temperature Alpine reju-

venation.

Petrography, reaction textures and mineral chemistry

At the contact of granitoid intrusion with metamorphic

rocks, a continuous change from inhomogeneous stromatitic

and diatexitic migmatites to massive granite can be observed.

The stromatitic migmatite comprises leucosomes of quartz-

plagioclase-K-feldspar, melanosomes of biotite and/or meso-

some. The mesosome mineral assemblage consists of garnet,

K-feldspar, sillimanite, biotite, cordierite, plagioclase, quartz

and muscovite. As minor minerals, zircon, monazite, xeno-

time, ilmenite, sphene, corundum and allanite were detected.

The chemical compositions of minerals were determined

using electrone microprobe JEOL 733 at Geological Survey

of the Slovak Republic in Bratislava. A point beam with op-

erating conditions of 10 nA and 15 kV was used. The data

were reduced by the ZAF method. Mineral abbreviations in

this paper are according to Kretz (1983).

Cordierite is often difficult to recognize, both in macro- and

microscale, because of its intensive pinitization. Only islands

of pure cordierite are preserved in the cores of the pinitized

grains (Fig. 2A,B); this fresh cordierite has a nearly constant

composition of Mg/Mg+Fe ~0.60 (Table 1). Three genetically

different types of cordierite have been distinguished:

1) in direct contact with garnet relics, without sillimanite

and/or biotite between them (Fig. 2C,D);

2) in direct contact with sillimanite and/or biotite. In this

case, i) sillimanite and/or biotite have resorbed garnet and

created the rims around large garnet grains (Fig. 2E); pin-

Pre-Alpine basement

Lower unit

mica schist

Upper unit

migmatite

orthogneiss

amphibolite

granite

Variscan thrust

metamorphic isograd

Variscan

D1 deformation

Variscan

D2 deformation

Alpine

D3 deformation

Kyanite zone

2 km

Baranec

Jeová

Kyanite-Fibrolite zone

Sillimanite zone

lit

ZT 175

Sil

Western Tatra

ZT 34/95

ZT 16/97

19.75°E

49.25°N

Sil

Fig. 1. Simplified geological map of the Western Tatra Mountains with the location of cordierite-bearing samples.

PHASE RELATIONS AND P-T PATH OF CORDIERITE-BEARING MIGMATITES 285

itized cordierite is not in direct contact with garnet, or ii) sil-

limanite and/or biotite are dominant phases in the flakes of

pinitized cordierite and garnet is preserved only as small rel-

ics (Fig. 2F), or iii) sillimanite and/or biotite are the only

phases (garnet is missing) in contact with pinitized cordierite

(Fig. 2G,H);

3) in leucocratic quartz + plagioclase + K-feldspar bearing

domains (Fig. 2I). The worm-like inclusions of undeformed

quartz are characteristic for this type of cordierite (Fig. 2I).

Garnet grains are of different sizes and they are strongly re-

sorbed by sillimanite and/or biotite (Fig. 2E,J), cordierite (Fig.

2C,D) or all of them. Most of the garnet is inclusion free, in-

clusions of quartz, sillimanite and biotite (fresh or chlorotized)

were only recognized in some garnet grains. The core compo-

sition values in all samples (Table 2) are 71.875.4 % almand-

ine, 5.314.3 % spessartine, 11.116.3 % pyrope and 1.92.7 %

grossular. Rim compositions (Table 2) are 70.974.3 % al-

mandine, 9.516.2 % spessartine, 10.113.9 % pyrope and

1.92.7 % grossular. The differences between core and rim

compositions are minimal. The highest core-rim composition-

al difference was observed in the spessartine end member,

which is ~3.7 % higher in the rim than in the core of the indi-

vidual garnet grain (Table 2, Grt2). Consequently, composi-

tional differences reflected by spessartine and almandine in-

crease, and pyrope as well as Fe/Fe+Mg ratio decrease from

the core to the rim, indicate a retrogression of the garnet.

Biotite is present as two distinct types. Dark brown biotite

together with sillimanite defines the metamorphic foliation

(Fig. 2K). The biotite Fe/Fe+Mg ratio is in the range 0.48

0.61 (Table 3). No compositional differences between biotite

in the matrix and biotite inclusions in garnet were observed.

Consequently, we consider biotite inclusions in garnet as

pseudoinclusions resorbing garnet interior. It is nearly im-

possible to recognize potentially true inclusions trapped

during garnet growth. Only minor biotite chloritization has

been observed.

A different type of biotite is pale green biotite, intergrown

with muscovite and forming randomly oriented porphyroblasts

around pinitized cordierite (Fig. 2A,B). This type represents a

product of cordierite resorbtion (pinitization). As a conse-

quence, it is TiO

depleted, which causes its green colour and

more Mg-rich, with Fe/Fe+Mg ratio ~0.45 (Table 3).

White micas are present in several genetically different

types (Table 4).

Randomly oriented lath-shaped porphyroblasts corre-

sponding to muscovite (Ms1) with a moderate phengite com-

Table l: Chemical composition of cordierite.

Table 2: Chemical composition of garnet.

sample

ZT 34/95 ZT 34/95 ZT 34/95 ZT 34/95 ZT 175 ZT 175 ZT 16/97 ZT 16/97

Grt1

Grt2

Grt3

Grt4

core

rim

core

rim

core

rim

core

rim

SiO

36.93

37.46

36.94

36.51

38.28

37.39

36.66

36.52

21.48

21.44

20.90

21.32

20.82

21.13

20.85

20.81

TiO

0.00

MgO

2.93

2.48

3.40

2.81

4.13

3.76

3.83

3.51

FeO+

31.43

31.44

31.96

30.95

34.04

34.19

33.85

33.32

MnO

6.19

7.02

4.93

6.45

2.51

2.38

3.46

4.22

CaO

0.65

0.66

0.93

0.92

0.93

0.91

0.77

Total

99.67 100.60

99.06

98.96 100.72

99.73

99.43

99.15

Recalculated on the basis of 12 oxygens

2.984

3.006

2.999

2.976

3.039

3.005

2.973

2.975

2.046

2.028

2.000

2.049

1.949

2.000

1.994

1.998

0.000

0.353

0.297

0.411

0.341

0.489

0.450

0.463

0.426

2.124

2.110

2.170

2.110

2.260

2.298

2.296

2.270

0.424

0.477

0.339

0.445

0.169

0.162

0.238

0.291

0.056

0.057

0.081

0.080

0.079

0.078

0.067

Fe/Fe+Mg 0.857

0.877

0.841

0.861

0.822

0.836

0.832

0.842

P rp

0.119

0.101

0.137

0.115

0.163

0.151

0.139

Alm

0.718

0.717

0.723

0.709

0.754

0.769

0.749

0.743

Sps0.143

0.162

0.113

0.150

0.056

0.054

0.078

0.095

Grs0.019

0.019

0.027

0.026

0.022

FeO+ = total Fe as FeO

sample

ZT 34/95 ZT 34/95 ZT 34/95 ZT 175 ZT 175

SiO

47.44

50.01

47.05

48.58

48.92

32.78

33.34

32.83

31.93

32.26

TiO

0.00

MgO

6.95

6.54

7.04

7.20

7.16

FeO+

8.62

6.53

8.65

8.85

8.70

MnO

0.33

0.23

0.21

0.23

0.22

CaO

0.00

0.72

0.94

0.30

0.51

0.63

0.00

0.03

0.00

Total

96.84

97.62

96.08

97.30

97.88

Recalculated on the basis of 18 oxygens

4.982

5.135

4.971

5.071

4.059

4.035

4.090

3.929

3.943

0.000

1.008

1.000

1.109

1.120

1.106

2 +

0.757

0.561

0.764

0.773

0.754

0.029

0.020

0.019

0.020

0.019

0.000

0.147

0.187

0.061

0.103

0.127

0.000

0.004

0.000

Fe/Fe+Mg

0.410

0.359

0.408

0.405

FeO+ = total Fe as FeO

Table 3: Chemical composition of biotite.

sample

ZT 34/95 ZT 34/95 ZT 34/95 ZT 175 ZT 175 ZT 175 ZT 34/95 ZT 34/95

Bt1 Bt1 rim/

Bt2mx Bt3 ingrt

Bt4mx

Bt4mx ne ar Crd ne ar Crd

core

Grt1

core

rim

da rk

pa le

brow n

gree n

SiO

35.08

34.96

35.35

35.09

34.59

35.11

36.19

36.37

A l

19.35

19.73

19.22

19.78

18.79

19.45

23.09

21.63

TiO

2.65

2.23

2.49

2.13

3.10

2.72

0.00

MgO

7.26

7.81

7.92 10.66

9.14

8.76 11.06 11.36

FeO +

20.66

19.16

20.85

17.59

19.82

20.00

16.05

16.59

MnO

0.26

0.19

0.02

0.11

0.08

0.23

0.30

CaO

0.01

0.00

0.02

0.00

N a

0.14

0.23

0.15

0.10

0.17

0.08

0.23

0.24

9.51

9.44

9.94

8.98

8.64

8.92 10.36

9.92

Total

94.92

93.76

96.11

98.37

98.33

95.14

97.21

96.42

Recalculated on the basis of 22 oxygens

5.401

5.407

5.389

5.331

5.320

5.350

5.302

5.384

A lIV

2.599

2.593

2.611

2.669

2.680

2.650

2.698

2.616

A lVI

0.913

1.005

0.843

0.874

0.727

0.844

1.291

1.158

0.307

0.259

0.286

0.243

0.359

0.312

0.000

1.666

1.800

1.799

2.414

2.095

1.989

2.416

2.507

2.660

2.478

2.659

2.235

2.549

2.553

1.967

2.054

0.034

0.025

0.003

0.014

0.010

0.028

0.037

0.002

0.000

0.003

0.000

N a

0.042

0.069

0.043

0.029

0.051

0.024

0.066

0.070

1.868

1.863

1.976

1.741

1.695

1.734

1.936

1.874

Fe/Fe+Mg 0.615

0.579

0.596

0.481

0.549

0.562

0.449

0.450

Sign "/" means touching of tw o phases, mx = matrix, ingrt = inclusion in garnet
FeO + = total Fe as FeO

PHASE RELATIONS AND P-T PATH OF CORDIERITE-BEARING MIGMATITES 287

Fig. 2. Photomicrographs of: A pinitized cordierite with islands of pure cordierite; marked section is shown in detail at B cordi-

erite and typical porphyroblasts of intergrowing muscovite + pale green biotite; C , D garnet relics within the pinitized cordierite;

E pinitized cordierite touching fibrolitic sillimanite resorbing garnet; F sillimanite + biotite surrounded by cordierite with pres-

ence of garnet relics; G , H sillimanite + worm-like quartz (H) within the pinitized cordierite without garnet presence; I pin-

itized cordierite with worm-like quartz within the leucosome domains; J sillimanite and biotite resorbing garnet; K prismatic sil-

limanite + dark brown biotite defining metamorphic foliation; L myrmekite.

ponent are distributed in the matrix. They are considered as a

product of K-feldspar destabilization during retrogression.

Muscovite (Ms2) intergrown with pale green biotite (Fig.

2A,B) is chemically identical with Ms1.

Fine-grained white mica (Ms3) as a product of cordierite

pinitization (Fig. 2A,B) has a phengite composition, with

+Mg

up to 1.51.

Sillimanite is present both in the form of prismatic silli-

manite (Fig. 2J,K) and fibrolite (Fig. 2E). It is closely associ-

ated with dark brown biotite.

Plagioclase composition ranges between An

and An

and

corresponds to oligoclase-andesine composition (Table 5). In in-

dividual

plagioclase grains, slight anorthite enrichment (

∆

An =

12 %) toward the rim has been detected, especially near the

contact with garnet. Plagioclase is only locally sericitized.

K-feldspar (Table 5), together with plagioclase and quartz,

is present in leucocratic domains. Locally, myrmekite has

been developed (Fig. 2L).

Quartz in the matrix is often recrystallized and segregated

into bands.

The composition of garnet, cordierite, biotite and white

micas is shown on the AFM projection in Fig. 3.

Table 4: Chemical composition of white micas.

sample

ZT34/95 ZT34/95 ZT34/95

ZT175

ZT175 ZT16/97a ZT16/97b

Ms1

Ms2

Ms3

SiO

46.10

46.64

46.49

47.32

47.14

48.83

47.31

35.09

35.52

30.09

31.77

31.52

31.51

31.65

TiO

0.00

0.01

0.00

MgO

0.70

0.72

4.82

2.50

2.61

1.77

2.11

FeO+

1.42

1.53

4.87

3.38

3.28

2.08

2.88

MnO

0.00

0.01

0.03

0.04

CaO

0.00

0.04

0.01

0.00

0.63

0.67

0.20

0.17

0.25

0.15

0.22

10.95

10.86

9.91

9.72

9.91

9.94

10.26

Total

94.89

95.95

96.38

94.92

94.73

94.30

94.47

Recalculated on the basis of 22 oxygens

6.182

6.180

6.238

6.355

6.353

6.540

6.388

AlIV

1.818

1.820

1.762

1.645

1.647

1.460

1.612

AlVI

3.729

2.997

3.385

3.361

3.515

3.426

0.000

0.001

0.000

3 +

0.000

0.139

0.142

0.964

0.500

0.524

0.354

0.425

2 +

0.159

0.169

0.547

0.380

0.370

0.233

0.325

0.000

0.001

0.003

0.005

0.000

0.006

0.001

0.000

0.165

0.173

0.052

0.044

0.065

0.038

0.057

1.874

1.836

1.696

1.665

1.704

1.698

1.768

Fe/Fe+Mg

0.534

0.543

0.362

0.432

0.414

0.397

0.433

FeO+ = total Fe as FeO

▲

288 LUDHOVÁ

and JANÁK

Table 5: Chemical composition of feldspars.

Biotite and sillimanite from the prograde stage of meta-

morphic evolution have not been recognized. Their possible

relics would be modified by fast diffusion at high tempera-

tures (Spear 1991).

Garnet resorbtion by biotite and sillimanite indicates the re-

versal of the melting reaction (1). The grossular component of

garnet was simultaneously consumed according to the reac-

tion:

sillimanite + quartz + garnet = plagioclase (2)

which is documented by the development of plagioclase

around the garnet and enrichment of plagioclase by anorthite

close to the plagioclasegarnet contact.

Although cordierite has not been observed in sharp contact

with garnet, pinitized cordierite around garnet relics (cordier-

ite type 1, Fig. 2C,D) suggests, that cordierite grew directly

from garnet by the reaction:

garnet + sillimanite + quartz + fluid = cordierite (3)

On the other hand, cordierite, which is in direct contact

with sillimanite and/or biotite (Fig. 2EH), most probably

grew due to the reaction:

biotite + sillimanite + quartz + plagioclase = cordierite +

K-feldspar + melt

(4)

In some cases, it is evident that biotite and sillimanite

which are the reactants of reaction (4), are the products of

garnet resorbtion by reversal of reaction (1). This corresponds

to the microtextural observations as described above and it is

sample ZT34/95 ZT34/95 ZT34/95 ZT16/97a ZT175 ZT175 ZT34/95 ZT34/95

Plg1

Plg2

Plg3

Plg4

KfsKfs

core

rim

leuc

core

rim

SiO

62.92

64.31

63.70

63.18

64.04

63.11

63.24

64.10

23.07

23.24

23.60

23.80

22.71

24.28

19.00

18.93

MgO

0.00

FeO+

0.00

MnO

0.00

CaO

4.42

4.41

4.74

5.42

4.55

5.02

0.02

9.25

9.34

8.06

8.17

8.50

8.65

1.06

0.67

0.28

0.11

0.36

0.21

0.30

0.22

14.97

15.45

Total

99.94

101.40

100.46

100.79 100.11 101.28

98.29

99.17

Recalculated on the basis of 8 oxygens

2.788

2.803

2.796

2.772

2.822

2.758

2.964

2.978

1.205

1.194

1.221

1.231

1.180

1.251

1.050

1.037

0.000

2 +

0.000

0.210

0.206

0.223

0.255

0.215

0.235

0.001

0.795

0.789

0.686

0.695

0.726

0.733

0.096

0.060

0.016

0.006

0.020

0.012

0.017

0.012

0.895

0.916

0.206

0.240

0.265

0.224

0.240

0.001

0.779

0.788

0.738

0.722

0.758

0.748

0.097

0.061

0.016

0.006

0.022

0.012

0.018

0.012

0.902

0.938

leuc = in leucosome
FeO+ = total Fe as FeO

Fig. 3. AFM projection showing the composition of cordierite,

garnet, dark brown Fe-rich biotite Bt1, pale green Mg-rich biotite

Bt2, muscovite Ms1 and Ms2 and fine-grained phengite Ms3 from

cordierite bearing metapelites.

Metamorphic reactions

The observed microtextures indicate a sequence of meta-

morphic reactions. These are plotted in qualitative petroge-

netic grid in the KFMASH system with intermediate bulk Fe/

Fe+Mg composition (Fig. 4), according to Vielzeuf & Hollo-

way (1988). Suggested sequence of reactions and the P-T

path is demonstrated by arrows crossing equilibrium lines.

The presence of K-feldspar and garnet in the leucocratic

domains suggests that the early melt producing reaction was

dehydration melting of biotite reaction:

biotite + sillimanite + plagioclase + quartz = garnet + K-feldspar

+ melt

(1)

Fig. 4. A part of the qualitative petrogenetic grid in the KFMASH

system for metapelites of intermediate bulk composition (Vielzeuf

& Holloway 1988) with metamorphic reactions including cordier-

ite. Arrows represent suggested sequence of metamorphic reac-

tions and P-T path.

290 LUDHOVÁ

and JANÁK

Table 6: Summary of results from P-T calculations by the TWQ2 method.

composition, therefore they fall within the garnet + silliman-

ite + biotite triangle and not within the cordierite + silliman-

ite + biotite one (Fig. 6b) during the whole progress of the re-

versal of reaction (1).

Thermobarometric calculations

Pressure and temperature conditions were calculated by

the TWEEQU method (Berman 1991) with thermodynamic

data of Berman (1988, updated in March 1997). Non-ideal

activity models of garnet, biotite, cordierite (Berman & Ara-

novich 1996) and plagioclase (Fuhrman & Lindsley 1988)

were employed in the calculations. The results of P-T calcu-

lations are listed in Table 6 and the reconstructed P-T path is

shown in Fig. 8.

Large portions of leucosome together with garnet and K-

feldspar within indicate that dehydration melting curve of bi-

otite was crossed during the prograde stage of metamorphic

evolution. Widespread garnet resorbtion by biotite and silli-

manite indicates that this reaction also proceeded in the re-

verse sense. Because of the fast diffusion at high tempera-

ture, garnet and biotite compositions have been adjusted to

the actual pressure and temperature. Consequently, it is not

possible to reconstruct the real peak temperatures by garnet-

biotite exchange thermometry (Spear 1991). Therefore, the

garnet-biotite geothermometer and garnet-plagioclase-

quartz-sillimanite (GASP) geobarometer can yield only the

post-peak P-T conditions. In the calculations, we employed

the core compositions of garnet together with those of biotite

and plagioclase in the matrix. We assume that these might be

the most similar to the compositions during the peak condi-

tions, being least affected by retrogression. Such composi-

tions yield a temperature of 702+45 ºC and pressure of

5.3+1 kbar (Table 6).

The GASP reaction sillimanite + quartz + garnet = plagio-

clase used as the geobarometer is a net transfer reaction,

while the garnet-biotite geothermometer is the Fe-Mg ex-

change reaction. Moreover, diffusion rates in plagioclase are

very slow compared to diffusion rates in biotite. That means

that the closure temperature of GASP net-transfer reaction is

higher than that of the exchange garnet-biotite reaction (Flo-

rence & Spear 1995). Consequently, at the temperature of

700 ºC, the pressure was probably lower than the calculated

pressure of ~5 kbar (Fig. 8).

The calculated temperature of 700 ºC is ~50 ºC lower than

the temperature of the biotite dehydration melting reaction

according to Le Breton & Thompson (1988). This is consis-

tent with our assumption that only post-peak temperatures

can be reconstructed by thermobarometry. It is inferred that

true metamorphic peak conditions reached a temperature of

more than 750 ºC and a pressure of more than 6 kbar.

The position of equilibrium curve for the reaction (3) gar-

net + sillimanite + quartz + fluid = cordierite in the P-T space

was calculated using thermodynamic data of Berman (1988,

updated in March 1997). Continuous reaction and equilibri-

um curves for both Fe and Mg end-members are shown in

Fig. 8. Because of the flat slope of this reaction, decompres-

sion to less than 5 kbar is inferred to produce the first cordi-

erite generation.

Further cordierite formation together with a new portion of

melt is suggested by the reaction (4), i.e. biotite + sillimanite

+ quartz + plagioclase = cordierite + K-feldspar + melt. The

possible directions followed by the rock in crossing it (Figs.

4, 8), especially potential heating through heat advection

from the intrusion, are discussed below.

Reaction (6) cordierite + K-feldspar + fluid = biotite + sil-

limanite + quartz is, like reaction (4), a continuous reaction

and thus equilibrium curves for Fe and Mg end-members are

shown in Fig. 8. We assume that intensive pinitization of

cordierite occurred during cooling at pressures ~3 kbar. The

calculated temperature during retrogression, obtained from

garnet and cordierite rim compositions employed in garnet-

cordierite Fe-Mg exchange geothermometer, was 612+20 ºC

at an assumed pressure of 3 kbar (Table 6).

Effect of granite intrusion

A sheet-like granite intrusion, in contact with cordierite

bearing migmatites, could have been an extra heat source, in-

creasing the temperature during decompression and leading

P-T values calculated as intersections of G rt-B t geothermomether and G ASP geobaromether

Sample

Grt coresBt cores

Pl cores

P (bars

)

T (degC)

X Alm

X SpsX Prp

X GrsFe/Fe+Mg

X An

ZT 34/95

0.7148

0.1311

0.1284

0.1311

0.5063

0.2092

4316

622

ZT 34/95

0.7183

0.1433

0.1194

0.0190

0.6149

0.2023

4237

708

ZT 34/95

0.7230

0.1130

0.1371

0.0269

0.5963

0.2056

6697

749

ZT 175

0.7542

0.0563

0.1631

0.0264

0.5489

0.2242

6279

740

ZT 175

0.7561

0.0540

0.1635

0.0264

0.4807

0.2397

4860

665

ZT 175

0.7689

0.0542

0.1507

0.0262

0.5619

0.2397

5583

727

mean

5329

702

938

T values calculated with G rt-Crd geothermomether (pressure 3000bar)

Sample

Grt rims

Crd rims

P (bars)

T (degC)

X Alm

X SpsX Prp

X GrsX FeCrd

ZT 34/95

0.7175

0.1623

0.1009

0.0193

0.4039

3000

612

ZT 34/95

0.7097

0.1569

0.1143

0.0192

0.3545

3000

592

ZT 34/95

0.7087

0.1496

0.1147

0.0270

0.4038

3000

643

ZT 34/95

0.7161

0.1607

0.0974

0.0257

0.4012

3000

604

mean

3000

612

292 LUDHOVÁ

and JANÁK

We propose that cordierite has originated by several reac-

tions. First cordierite could have originated by the reaction

garnet + sillimanite + quartz + fluid = cordierite, during a

post-peak decompression to a pressure lower than 5 kbar.

Further cordierite-producing reaction biotite + sillimanite +

quartz + plagioclase = cordierite + K-feldspar + melt, could

take place during continuous decompression, possibly ac-

companied by limited (~50

ºC) increase in temperature due

to the heat advection from synkinematic granodiorite-tonalite

intrusion. Finally, cordierite could have been produced dur-

ing the cooling and crystallization of melt in the migmatite

leucosome. Subsolidus retrogression led to widespread

cordierite pinitization. The origin of cordierite was also con-

trolled by the more magnesian bulk composition of cordier-

ite-bearing migmatites.

We suggest that the origin of cordierite in the Western

Tatra Mountains is an important indicator of a high-tempera-

ture/low-pressure metamorphic stage, related to orogenic

collapse of Variscan orogen in the Western Carpathians.

There are close similarities to the Ve¾ká Fatra Mountains

(Janák & Kohút 1996) and well documented parts of the

Variscan orogen in Europe, for example Brittany (Jones &

Brown 1990; Brown & Dallmayer 1996) or the Iberian Mas-

sif (Escuder Viruete et al. 1997).

Acknowledgements: This paper is a part of Ph.D. study of

L.L. and was financially supported by the Grant of the

Comenius University UK/3865/98 Variscan exhumation of

high-grade metamorphic rocks in the crystalline complex of

the Tatra Mountains. We are grateful to Pavol Siman (Geo-

logical Survey, Bratislava) for help with the microprobe

analyses, Milan Kohút (Geological Survey, Bratislava), Pa-

vel Pitoòák (Geol. Inst. Academy of Science, Banská Bystri-

ca), Branislav Lupták (Comenius University, Bratislava) and

Jana Kotková (Geol. Inst. Academy of Science, Praha) for

their help during the field work. We thank Michael Brown,

Ján Spiiak and Vladimír Bezák for their helpful and con-

structive review of this paper.

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