Retrograde evolution in the Central Vosges mountains (northeastern France): implications for the metamorphic history of high-grade rocks during the Variscan orogeny

387

Tectonophysics, 205 (1992) 387-407

Elsevier Science Publishers B.V., Amsterdam

Retrograde evolution in the Central Vosges mountains (northeastern France) : implications for the metamorphic history of high-grade rocks during the Variscan orogeny * Louis Latouche, Jacques Fabrk Laboratoire de Mimkalogie, U.R.A. C. N.R.S. 736, M&urn

and Michel Guiraud National d’Histoire Naturelle, Paris, France

(Received January 16, 1990; revised version accepted August 18, 1991)

ABSTRACT Latouche, L., Fabrics, J. and Guiraud, M., 1992. Retrograde evolution in the Central Vosges mountains (northeastern France): implications for the metamorphic history of high-grade rocks during the Variscan orogeny. Tectonophysics, 20.5: 387-407.

Metamorphism in the Vosges massif is characterized by rocks formed under a wide range of pressures and temperatures: high-pressure rocks (garnet peridotites and eclogites facies rocks) are found together with low-pressure rocks (garnetcordierite stability field). Hitherto, metamorphism has been interpreted within the framework of an identical P-T evolution for all rock types corresponding to either two separate geotectonic events or a unique P-T evolution from high-pressure to low-pressure conditions. We studied gneisses from the Vosges massif in order to constrain the P-T-t path they followed. P-T estimation indicates that gneisses equiiibrated between 5 and 7 kbar during peak metamorphism time. Rocks later experienced cooling from temperatures above 650°C down to 550°C. It is possible that the entire P-T evolution of the gneisses took place within the lower part of the amphibolite facies; which is inconsistent with the P-T evolution of the high-pressure rocks. The noteworthy absence of muscovite + staurolite assemblages is attributed to conditions of low H,O and low 0, activity. The consequences for the geodynamics in the Vosges massif are that at least two separate geotectonic events are required in order to explain the juxtaposition of high-pressure rocks with low-pressure amphibolite facies rocks.

* Correspondence

to: L. Latouche, Laboratoire

de Mineralogie, U.R.A.C.N.R.S. 736, Museum National d’Histoire Naturelle, 61 rue Buffon, 75005 Paris, France.

Introduction Granulite facies rocks were described for the first time in the Vosges massif (Fig. 1) by von Eller (19611. Later, Hameurt (1967) demonstrated that the basement compiex of the Central Vosges underwent polymetamorphism and was part of the Moldanubian Zone of the European Hercynian belt. Pin and VieIzeuf (1983) recognized two main groups of granulites in Variscan median Europe. “Type I” granulites are highpressure rocks associated with eclogites and garnet peridotites, and formed during the early stage of the Variscan orogeny (430-400 Ma). “Type II” HO-1951/92/$05.~

granulites display medium- to low-pressure paragenesis and are characteristic of the Late Variscan (about 300 Ma) crust formation. The Vosgesian granulites were ascribed to the Type I by Pin and Vielzeuf (1983) on account of their association with garnet peridotites and scarce eclogites and the occurrence of relict kyanite (Hameurt, 1967; Fluck, 1980). However, geothe~o-baromet~ undertaken on some aluminous granulites from the Sainte-Marie-aux-Mines area (Fig. 2), indicated a pressure between 8 and 6 kbar, significantly lower than the value of 13 kbar accepted elsewhere for Type I granulites (Pin and Vielzeuf, 1988). Therefore, it was suggested that the ~l~etamo~hism described by Hameurt (1967) could correspond to the overprinting of a high-pressure metamorphic stage by a low-pressure one (Pin and Vielzeuf, 1988). Further studies by Fabrics and Latouche (1988) and

0 1992 - Elsevier Science Publishers B.V. All rights reserved

I

Fig.

1. Location

I = high-grade

map

1

2

3 _I

of the Vosges

metamorphic

nappes;

mountains

and

of the Central

2 = Cambrian-Silurian

elastics;

Vosges

granulites

3 = Middle

(after

Wickert

Devonian-Lower

and

Eisbacher,

Carboniferous

1988).

sedimentary

rocks and volcanics.

Rey et al. (1989) confirmed the polymetamorphic nature of the Vosgesian metamorphism. They concluded that the structure of the Sainte-Marieaux-Mines area results from decompression from pressures above 9 kbar down to 5 kbar during syn-metamorphic thrusting of a granulite facies unit over a less metamorphic gneissic unit.

Fig. 2. Geological Group.

sketch map of the Central

WA4 = Western

Migmatitic

unit; UC = Urbeis-Combrimont

Vosges.

I = Varied

Our mineralogical and petrological data on rock samples selected from the polymetamorphic basement in the Central Vosges massif allow us to challenge this theory of P-T evolution. The results presented in this paper show that cooling rather than decompression is the process recorded in gneisses during this multi-stage evolution.

Group;

2 = Leptynitic

unit; SMM = Sainte-Marie-aux-Mines

unit; C = Combrimont. 22-3,

Samples 22-4,

in Combrimont

23-2,

23-3,24,

granulites;

3 = migmatites;

unit; LCM = La-Croix-aux-Mines 24-l.

are: 21-1,

21-l’,

21-2,

21-3,

4 = Monotonous

unit; TE = Trois-Epis 21-3’.

22, 22’, 22-2,

RETROGRADE

EVOLUTION

IN THE CENTRAL

VOSGES

389

MOUNTAINS

Geological setting The Central Vosges massif comprises medium to high grade polymetamorphic areas separated by various intrusive granitoids. From the west to the east (Fig. 2), five units of metamorphic rocks have been defined (von Eller et al., 1970): the Western Migmatitic (WM) unit, the La-Croixaux-Mines (LCM) unit, the Urbeis-Combrimont (UC) unit, the Sainte-Marie-aux-Mines (SMM) unit and the Trois-Epis migmatic (TE) unit. Besides the division in several tectonic units, the Vosgesian formations can be divided into four lithological groups (von Eller, 1961; Hameurt, 1967): (1) the “Monotonous” Group which mostly comprises sillimanite gneisses; (2) the “Varied” group composed, according to Fluck (19801, of kinzigitic garnet gneisses (the La Fonderie type formation, Fig. 31, coarse khondalite-kinzigite garnet gneisses with rare lenses of talc-silicate rocks, marbles and basic granulites (the “pearl gneiss” formation) and amphibolitized mafic granulites interlayered with garnet gneisses and lenses of retrogressed spine1 peridotites (the “amphibolites formation”); (3) the granulitic leptynites characterized by the occurrence of garnet peridotites; (4) migmatites, locally known as the migmatites of Gerbepal, Kaysersberg or Trois-Epis (von Eller, 1961; Hameurt, 1967) Gneisses from the Monotonous Group derived from a quartz-pelitic protolith; whereas the khondalitic-kinzigitic gneisses from the Varied Group would represent a sedimentary sequence mainly composed of graywackes, pelites and talc-alkaline volcanic rocks (Fluck, 1980; Miiller, 1989). The Monotonous Group is the single component of the UC unit; it also occurs in the eastern part of the SMM unit and possibly in the central part of the LCM unit (Hameurt, 1967). The Varied Group is present in the northern part of the SMM unit and in the LCM unit. The granulitic leptynites outcrop in the southwestern part of the SMM unit (Co1 des Bagenelles area) and in the upper part of the WM unit. The migmatites lie in the lower part of the WM unit and are the only component of the TE unit.

L7-A-2,

7-A-2; 7-A-3

Fig. 3. Geological

sketch

‘9,

9;

12-8,

13

map of the Sainte-Marie-aux-Mines

area.

I = La Fonderie

Formation;

2 = Pearl

tion;

3 = Amphibolite

Formation;

4 = Monotonous

5 = spine1

peridotites;

granulites;

F = La Fonderie;

I’Hfte;

CH = Le

Bagenelles. samples

Samples

6 = garnet

Chaufour;

peridotites;

E = Echety;

Rauenthal

FormaGroup;

7 = leptynitic

SP = St Pierre-sur-

R = Rauenthal;

in La Fonderie:

St-Pierre-sur-PHPte,

Gneiss

5-2, and

7-A-2, 7-A-2’, 7-A-3, 9, 9’, 12-8,

B = Co1 des 5-4,

5-7,

5-10;

Le Chauffour: 13.

Fluck (1971) acknowledged the geochemical difference and the tectonic boundary between the Varied Group gneisses and the granulitic leptynites but, later, Bonhomme and Fluck (1974) and Fluck (1980) considered that this boundary between the gneisses and the leptynites corresponded to a metamorphic isograd.

I

According to Hameurt (1967) and Fluck (1980) three tectonometamorphic events can be distinguished within the polymetamorphic granulites of the Varied and Monotonous Groups: the first event (Gl) produced kyanite, which is elongated within a locally preserved early foliation in some places. A widespread event (G2) re-oriented the previous structures into tight isoclinal folds and large prismatic sillimanite crystals developed under granulite facies conditions. This event corresponds to thrusting of the Varied Group onto the Monotonous Group. The age of this event has been estimated at 386 of-15 Ma by whole-rock Rb-Sr isotope data (Bonhomme and Fluck, 1981). It is in good agreement with the whole-rock RbSr age determined for the emplacement of the “granite fondamental” during the G2 event at 381 + 18 Ma (Hameurt and Vidal. 1973). The third stage (G3) corresponds to retrogression to the lower amphibolite facies. According to Blumenfeld and Bouchez (1988), deformation related to this G3 stage is mainly observed in granites emplaced concurrently to sub-horizontal shearing and thrusting to the southwest of the WM and TE units onto the SMM and the LCM units. This event has been dated at 340 i: 30 Ma (Rb-Sr whole-rock isotope data; Bonhomme and Fluck, 1981). La

Fonderie

L 0

’ 1

Pearl

C’ltt

I.‘1 .\I

The age of the first metamorphic episode, Gl, is not known but is estimated at 420-430 Ma by comparison with isotopic data on high-pressurehigh-temperature paragenesis from neighbouring parts of the Moldanubian Zone, such as Schwarzwald (Schleicher and Kramm, 1986). Petrography

Of the 29 samples collected, 17 were studied with a microprobe. We sampled garnet gneisses in three areas along a S-N trend across the central metamorphic unit in order to compare assemblages in the Varied and Monotonous Groups (Fig. 2, Fig. 3b). These three areas are found within the SMM and UC units: outcrops of Varied Group rocks are located in the SMM unit (garnet gneisses group only), near La Fonderie (between Echery and Sainte Marie-aux-Mines), and in the triangle between Saint Pierre-surI’Hbte, Rauenthal and Chauffour (Fig. 3b), where the “Pearl Gneiss formation” predominates. Outcrops of Monotonous Group rocks are located in the UC unit near Combrimont (Fig. 2), where gneisses surround a small body of eclogite (Hameurt, 1967). In hand specimen, all the gneisses are, on the whole. fine-grained rocks with a pervasive foliagneiss

Combrimont

I 2mm

Fig. 4. Summary

l.AI‘Ot

of the textures

found in the gneisses

RETROGRADE

EVOLUTION

IN THE CENTRAL

VOSGES

MOUNTAINS

tion. In detail they comprise two types of layers: fine grained layers where crystals up to 5 mm are embedded within a matrix l-10 mm in size, which alternates with very fine-grained layers displaying a grain size reduction in the ratio 1:lO and with an equigranular texture. The mixing in different proportions of these two kinds of layers leads to two types of gneiss: augen type gneiss (hence their name of “pearl gneiss”), where the first type of layer predominates, and mylonite, as found at La Fonderie and in the UC unit. In thin section (Fig. 41, the fine-grained layers are made of porphyroclasts of garnet, plagioclase, sillimanite, biotite or cordierite, surrounded by the foliation, which is made of millimetre-sized crystals of quartz, feldspar and biotite. In mylonitic layers small, stretched, more-or-less poikiloblastic, crystals (0.1-0.2 mm) of garnet and plagioclase are surrounded by flexuous quartzofeldspathic ribbons. In garnet gneisses from La Fonderie (samples 5-7, Fig. 5a), garnet is transformed into strongly flattened polycrystalline aggregates made of polygonal, relatively evengrained, arrangements of subgrains. The same relationships may be observed in sillimanite-rich layers: even if low-energy forms of sillimanite (e.g.(llO}) are very stable, the equivalent of polygonal grain-size reduction exists around large and flexuous sillimanite porphyroclasts. These textures, containing coexisting syn- and postkinematic minerals, are typical of strong uniaxial deformation. Table 1 displays the mineralogy of the samples studied. Typical porphyroclastic garnets display a clear textural zoning: a core rich in minute inclusions, mostly rutile, parallel to {llO}, is surrounded by an inclusion-free rim. Other large crystals display a different textural zoning: a thick median zone, rich in inclusions of quartz, plagioclase, biotite, graphite, ilmenite and globular rutile, separates the core from an outer zone, both of which are free of mineral inclusions. These garnets are generally ante- to syn-kinematic crystals enclosed in the foliation (Fig. 5b). Garnet porphyroblasts are devoid of inclusions and display syn- to post-kinematic features (Fig. 5~). Cordierite shows up in thin section, either as fractured and often pinitized porphyroclasts or as

391

symplectites, together with green spine1 in contact with garnet and sillimanite. Primary rutile is included within garnet, whereas secondary rutile is present as sagenite in biotite altered to chlorite. Chlorite and muscovite and fibrolitic sillimanite are always secondary phases, whereas biotite displays only primary features, either included within minerals or within the foliation. Biotite is frequently altered into chlorite + muscovite. Three rock types have been distinguished on the basis of the primary mineral assemblage: (1) Garnet-biotite-bearing gneisses (samples 5-2, 5-4, 5-7, 5-10, 7-A-2, 7-A-2’, 7-A-3, 9, 9’, 12-8, 13, 21-3, 21-3’, 22-1, 22-2, 22-3, 22-4, 24, 24-l). This rock type represents 90% of outcrops and 75% of the samples studied. They are composed of (in decreasing abundance): quartz, plagioclase, biotite and garnet. Accessory minerals are sillimanite, cordierite, ilmenite, rutile and graphite. An important feature of these rocks is that K-feldspar is present only in a few samples and in very minor amounts. Some cordierite grows within symplectite at the contact with garnet (Fig. 5d). Porphyroclastic cordierite is pinitized, plagioclase is sericitized and biotite is altered to chlorite and sagenite. Secondary fibrolitic sillimanite has been found in sample 21-3’ (Fig. 5e). Chlorite and muscovite develop around some garnets or after biotite or along surfaces discordant to the main foliation. (2) Sillimanite-biotite-bearing gneisses (samples 21-1, 21-l’, 21-2): these are porphyroclastic rocks with a pervasive foliation. They are composed of (in decreasing abundance): quartz, plagioclase, biotite and sillimanite. Accessory minerals are muscovite, ilmenite, chlorite, spine1 and graphite. Coarse prismatic, apparently boudinaged, sillimanite is separated from biotite by muscovite. In addition, in some places green spine1 grows along with ilmenite and rutile at the apex of sillimanite (Fig. 5f). Chlorite can occur as an alteration product of biotite. (3) Cordierite-biotite-bearing gneisses (samples 22, 22’, 23-2, 23-3): these are porphyroclastic rocks with a pervasive foliation. They are composed of (in decreasing abundance): quartz, plagioclase, cordierite, biotite and sillimanite. Ac-

RETROGRADE

EVOLUTION

IN THE

CENTRAL

VOSGES

393

MOUNTAINS

TABLE 1 Mineral assemblages in the samples studied Mineral

Sample 5-2

Garnet Biotite Sillimanite Cordierite Quartz Plagioclase K-feldspar Ilmenite Rutile Spine1 Muscovite Chlorite Graphite

garnet biotite sillimanite cordierite quartz plagioclase k-feldspar ilmenite Wile spine1 muscovite chlorite graphite

5-4

++ 11

++ + + < 1 + i

5-7

i-l-i11 1 1s + +

2 2 +

+ i S <2 <2 +

21-2

21-3

11 1 +++ + + +
<+ 1

+++ -I-++ +

22 2 +

++ 1

5-10

7-A-2

l-A-2’

l-A-3

+++ 11 11 IS + +

+ 11

++ ii

I

9

9’

12-8

t-f

+++

++


1



111

13 + 1

+++ ++

++ ++

<+ <+

+

+ i

+ i

<+ +++
2 2 +

<2 2 -t

<2 2 +

2 +

<2 2 +

22-1

22-2

22-3

22-4

23-2

23-3

<+ <+ <+ <+ 1 11 11 1 < 2(f) < 1 2(f) < 1 2(f) 11 11 + +++ +++ +++ +++ +++ +++ +++

<+ 1

+ 1

<+ 1

1

+ +++

<+ +++

+ +++

<+

+

++ + +
21-3’

+

22 2 +

++ +++

+++ +++

+

+ i

+

i 2 22 +

2 2 +

i

i 2 2 +

++ +++

S 2 + 22

22’

f

+

2(sd

2(sg)



<+

ii

+

1 +++ f
1 < 12(f) 1 +++ + +

2&S) <2 <2 +

<2 2 +

<2 2 +

2 2 +

21-1

21-1’

11 11

II 11

++ ++

++ ++

+

+



24

24-l

f 11

i”++ ++ 1 +

<+ 1
2(sg) 2 +

2 2 +

2 2 +

2 +

1 = primary phase: 2 = secondary phase; + = phase stable with both primary and secondary phases: < , + , + +, + + + = abundance from rare (<) to more than 50% (+ + + 1; legend is also valid for symbols 1 and 2. Special features: s = mineral occurring in symplectite; i = inclusion; f = fibrolite; sg = sagenite.

cessory minerals are ilmenite, muscovite and graphite. Thin sections are characterized by secondary fibrolitic sillimanite embayed within muscovite, biotite and cordierite (Fig. 5g). Rare garnet po~hyroblasts (in samples 22 and 22’) and one prismatic sillimanite crystal within plagioclase (in sample 23-3) have been found.

Mineral chemistry Minerals have been analyzed using a Camebax electron microprobe. Operating conditions were 15 kV and 10 nA, with a counting time of 20 s, except for volatile elements in biotite and plagioclase. Table 2 displays some selected analyses.

Fig. 5. Microphotographs of the textural relationships in the rocks studied. (a) Folyc~stalline aggregates of garnet (sample 5-7). fb) Ante- to syn-kinematic garnets (sample 7-A-2’). (cl Porhyroblastic garnet coexisting with chlorite (sample 24). (d) Cordierite-spine1 symplectite between garnet and sillimanite (sample 5-10). (e) Secondary fibrolitic sillimanite in muscovite (sample 21-3’). (f) Spine1 with ilmenite and rutile at the apex of sillimanite (sample 21-l’). (g) Secondary fibrolitic sillimanite within muscovite, biotite and cordierite (sample 23-3). (h) Overgrowth of garnet at the contact with chlorite (sample 9’). 7Z = transition zone; 0 = overgrowth.

g

269

254

25.45

25.71

28.40

5.989 0.000 4.077 0.001 3.225 0.000 0.000 0.077 2.312 0.284 0.015 0.000 15.981

5.972

0.013

4.035

0.007

3.266

O.ooO

0.000

0.080

2.368

0.255

0.000

0.000

15.995

5.987

0.000

3.946

0.000

3.656

0.000

0.000

0.119

1.888

0.348

0.015

0.000

16.018

Si

Ti

Al

Cr

Fe

Zn

NiO

Mn

Mg Ca

Na

K

Sum

99.67

1.010 0.781 0.021

0.188 0.006

0.295

15.968

15.990

Abbreviations as indicated in the text. No. = the analysis reference.

___-__ _____.

0.000

0.000

0.009

J6.029

0.000

0.299

1.362

0.066

0.188

0.000

0.000

0.000

2.284

0.000

0.000

3.896

4.112

3.275 0.000

4.073 0.005

4.144

4.095 0.004

0.006

0.010

0.000 0.000

5.937

5.952

5.967

0.03 100.05

99.98

99.79

100.46

100.39

0.00

100.86

0.24

0.07

0.02

0.00

0.00

0.00

0.00

0.00

0.05

0.00

0.05

K,O Sum

Na,O

4.60

1.12

1.80

1.75

1.57

2.11

2.23 4.28

1.42 5.82

0.51

8.23

MgO CaO

10.03

0.60 10.24

0.62

10.46

0.91

MnO

0.00

0.00

0.00

0.00

0.00

0.00

NiO

0.00

0.00

5.012

0.014

0.532

0.464

0.003

0.000

0.000

0.000

0.000

0.000

1.478

0.000

2.522

6.08

9.61

0.04

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

5.016

0.008

0.499

0.513

0.003

0.002

0.000

0.000

0.006

0.000

1.494

0.000

2.491

99.66

0.13

5.69

10.58

0.05

0.04

0.00

0.00

0.00

0.16

0.01

0.00

29.41

0.00

27.99

0.00

27.79

0.00

0.00

0.00

0.00

0.51

Fe@, zno

Cr*Os Fe0 31.29

0.04

0.03

0.00

0.01

0.06

0.00 25.63

21.82

22.38

22.74

22.83

22.54

21.75

AW,

0.00

55.02

55.90

0.05

0.08

0.00

0.00

0.11

37.48

37.89

39.05

39.53

39.32 0.00

PI 3

PI 2

440

g

0.00

436

g

g

38.90

1

9

9

22-1

SiO,

g

249

g

Mineral

NO.

22

9’

9’

TiO,

9’

Sample

9

component

Mineral/

Selected analyses

TABLE 2

74

4.99 1

0.015

0.544

0.422

0.002

0.002

o.oOU

0.000

0.000

0.000

1.433

0.001

2.571

99.13

0.26

6.21

8.71

0.03

0.06

0.00

0.00

0.00

0.00

15.385

1.758

0.056

0.000

3.090

0.011

0.000

0.000

1.467

0.020

2.941

0.647

5.394

96.26

9.46

0.20

0.00

14.23

0.09

0.00

0.00

0.00

12.04

0.17

17.13

26.92 0.00

5.91

15.549

1.861

0.108

0.000

3.351

0.010

0.000

0.000

1.364

0.024

2.817

0.655

5.360

96.17

9.98

0.38

0.00

15.38

0.08

0.00

0.00

0.00

11.16

0.21

16.35

5.96

36.67

bi 8

bi

37.03

9

5-4

0.03

56.91

PI 60

9’

15.691

1.779

0.112

0.000

2.767

0.000

0.000

0.000

2.324

0.000

2.911

0.323

5.475

95.20

9.18

0.38

0.00

12.22

0.00

0.00

0.00

0.00

18.29

0.00

16.26

2.83

36.04

98

bi

22-l

I 1.057

0.000

0.020

0.003

I .624

0.011

0.000

0.000

0.426

0.008

4.033

0.001

4.93 1

98.96

0.00

0.10

0.03

10.79

0.13

0.00

0.00

0.00

5.05

0.10

33.90

0.01

48.85

2

cd

5-10

11.028

0.000

0.016

0.000

1.298

0.012

0.000

0.000

0.680

0.002

4.080

0.002

4.937

97.30

0.00

0.08

0.00

8.38

0.14

0.00

0.00

0.00

7.83

0.03

33.31

0.02

47.51

12

cd

22

24.022

0.006

0.124

0.005

1.263

0.041

0.000

1.351

5.182

0.206

15.809

0.005

0.030

99.13

0.02

0.28

0.02

3.70

0.21

0.00

7.99

0.0

27.05

1.14

58.56

0.03

0.13

SP

21 -1‘

: -: >

2 ; ; I

6

_

RETROGRADE

EVOLUTION

IN THE

CENTRAL

VOSGES

395

MOUNTAINS

TABLE 3 Summary of P-T estimations for peak conditions Garnet-biotite (Hodges and Spear, 1982) 73 pairs Regression line at the origin gives Ln K, = 1.5192 and T = 660°C at 6 kbar Garnet-cordierite, T"Ccalculated at P = 5 kbar Sample qtr. pairs Perchuk and Lavrent’eva Holdaway and Lee Garnet-plagioclase-sillimanite-quartz Samples qu. pairs P (kbar) Garnet-rutile-ilmenite-sillimanite Samples qu. pairs P (kbar)

22 6 670-720 720-780

(Newton and Haselton, 19801, P kbar calculated at T = 700°C 9’ 9 9 11 4.9-5.5 4.7-6.2 (Bohlen et al., 1983), P kbar calculated at T = 700°C 5-4 2 7

Garnet: although garnets are all of almandine type, they display large variations between sampies (Fig. 6): values of X,, = Mg/Mg + Fe + Ca

9’ 5 6

+ Mn are in the range 0.09-0.24 for the UC unit and in the range 0.22-0.42 for the SMM unit. Two groups of garnet can be distinguished on the

0.2

X PJ

0.1

,\

1

0.3 Fig. 6. Garnet composition =Xp,

XPY

0.4

versus Xo, diagram for samples from SMM and UC units. Numbers refer to samples (571 analyses).

0.10. Mn is negligible

in all the analyzed

samples

from Kpe = Mn/Mg + Fe + Ca + Mn ranges 0.0 1 to 0.11). A common trend is found in compositions \-.

I be

CaO

4 -l-Vi2 --^

of

within

- 0.20

porphyroblasts

each sample;

to the contact

--F--U__-_

whereas

MnO I

2oop q

0

m

and

variation

to plagioclase

the spread

ticular

zoning.

spends

to a plateau

in X,,

porphyroclasts

in Xc.;, is only due inclusions

The central

X,

in X,,

by 30%. Garnets

rimmed (10-20

by a thin

the sharp

values.

and discontinuous

grn) of secondary decrease

garnet

towards

corrc-

(Fig. 7a) with

edge,

decreases

to a par-

part of garnets

of constant

only a 5% variation X,,

(Fig. 6)

corresponds

Towards

the

arc often overgrowth

and, in this case,

the edge constitutes

the transition zone between the central part and the overgrowth (Fig. 7b). It is worth noting that this overgrowth Fig. 7. Garnet

zoning

profiles

in samples

9 and

chlorite

13 (SMM

however,

5

ranging

with

(Fig. 5h).

two groups

of biotite

(Fig. Xa), Ti correlation;

do appear:

one

characterized by low Al”’ CXZ’ = 0.03-0.09) and one characterized by high Al”’ CXZ’ = 0.10-0.20). Both the high Al”’ and low Al”’ group biotites

basis of X,,, = Ca/Mg + Fe + Ca + Mn: high Ca garnets have a XgrO ranging from 0.10 to 0.20 and have an X,,,

only at the contact

Biotite: In an X.,.i/X,4,~~ diagram and Al”’ do not show any significant

unit).

low Ca garnets

is found

from 0.02 to

015-

a 3 t= c=& + biL” 0.10

-

0.05

-

AI”’ Fe+Mg+Ti+Ai”’

T1 Mg+Fe

Fig. 8. Biotite

low

Al

I

I

0.05

0 10

I (I 15

T1 Mg+Fe

I

020

high Al

composition Numbers refer to samples (135 analyses). (a) Ti/(Ti + Mg + Fe + AI”‘) versus AI”‘/(Ti AI”‘) diagram. (b) Ti/Mg + Fe versus Mg/Mg + Fe diagram for high Al and low Al biotites.

+ Mg + Fe +

391

RETROGRADEEVOLUTION1NTHECENTRALVOSGESMOUNTAlNS a Plagioclases m

24-1

-

-

24 -

22-4 m

22 -

-22-I

13 9-

1 UC

22-3

22-1

Y7-A-2’ -

5-7 5-4 m

XAn%

Fig. 9. Plagioclase and cordierite compositions in SMM and UC units.

occur in the UC unit gneisses, whereas only low Al”’ group biotites are found in the SMM unit gneisses. The low Al”’ group displays variations in X,, values (X,, = Mg/Fe + Mg) from 0.42 to 0.75 and in XTi values (XTi = Ti/Fe + Mg) from 0.04 to 0.16, whereas the high Al”’ group displays variations in X,, values from 0.40 to 0.55 and in XTi values from 0.04 to 0.13 (Fig. 8b). There is a rough positive correlation between X Mg and Xri in the SMM biotites (Fig. 8b). Biotite crystals, whatever their grain size, are

mostly unzoned. In each single sample, biotites within the fine grained matrix constantly have lower XM, values than those enclosed within garnet. The variation within each type-matrix or inclusion-never exceeds lo%, whereas the variation between the two types can reach 20%. Plagioclase: compositions range, on average, from oligoclase to andesine; one inclusion of pure albite in sample 24-l and one of labradorite An 60_63in sample 22-l have been found in garnet. A general trend is that high X, plagioclase is found together with high Xc, garnets (Fig. 9a). The range of compositions are small (Fig. 9a) except for samples 9 and 7-A-2’, in which inclusions of various composition in garnet occur. Whatever site they occur in, either as isolated coarse crystals (2-3 mm) or as small grains (O.l0.2 mm) within a blastomylonitic matrix, plagioclases display no significant optical zoning. K-feldspar: this is Ors5_s6 in samples 5-4 and 5-10 @MM unit gneisses) and Or,, in sample 24-l (UC unit gneisses). Sillimanite: this is very poor in Fe3+in all the samples analyzed (Fe3+ varies from 0 to 0.04 per formula unit). Spinef: all the spinels belong to the hercynitespinel-gahnite solid solution. Spine1 (associated with ilmenite and rutile enclosed in muscovite)

‘.* T Al”‘-1 0.7--

0.6--

0.5--

0.4--

~ZZ,',~,~,Z~,\Zr\Z,',',~,',~,',~ ',',',',',~,~,~,~,~,~,~,~,~,~,~,~,~,~ ,,,,:,:,:,:,:,:,:,:,:,:,:,:,:,:,:,:,:,:,: .'.'r'~'.'.'.'~'.'.'r'~'~'.',',','~'.'.'~ ,,,,,,,,,,,,,,,,I,,, :,:+:,;,:*:z:,:,: ai :;,:,;,:,:+: .'.'.'.'.'.'.'.'.'. C.'.'.'.'.'. .'.'.'~'.'\'.'cir'i.'~.'.'.','.'.'.'. .'.'r'r'~'.'.'~'.'.'.'.'.'~'.'.'.'~'.'.'.', ,,,,,\,.,~,_,<_,_.,.,~,/ ,I,,,

0.3--

0.2--

0.1 --

0.;

0.5

0.6

0.7

0.8

Fig. 10. Chlorite compositions = Al”’ - 1 versus XM,, diagram (76 analyses). Classification after Bailey (1980).

39x

I_ I ,\-IOI

which surrounds sillimanite porphyroclasts in the biotite-sillimanite gneisses, has a higher Zn content and a lower Mg content (X,, = Zn/Zn + Fe + Mg in the range 0.11-0.17 and Mg/Fe + Mg in the range O.ZO-0.241, than spine1 associated with cordierite, garnet and silIimanite in garnetrich aggregates in the garnet-biotite bearing gneisses (X,, in the range 0.02-0.03, Mg/Fe + Mg in the range 0.35-0.371. Plotting all the compositions onto the ternary diagram Zn-Mg-Fe, which relates the spine1 composition to the most common geological environments (fig. 12 of Spry and Scott, 19861, clearly indicates an aluminous metasediment protolith for the spinel-hearing rocks in the Varied Group and the Monotonous Group. Cordierite: the range of compositions are small within each sample (Fig. 9bl; with cordierites in the UC unit being more magnesian than those in the SMM unit. Large poikilitic crystals and crystals in symplectites in samples 5-4 and S-10 do not have significantly different compositions. Zlmenite: this is almost pure ilmenite in all the samples analyzed. The Xilm ranges from 0.94 to 0.97 with very little Fe’“, Mg or Mn.

< Ill.

i-1 Al

Chlorites: chlorites belong to the solid solution clinochlore-chamosite (classification of Bailey, 1980). There is a large variation in the X Mg values between samples, mirroring the variation observed in garnets and biotites (Fig. Ill). Two types of chlorites can be clearly distinguished within each sample: (1) chlorites formed by the direct alteration of biotites, which have the same X ML!ratio as the biotites: and (2) neobiasts of chlorite. which have a higher X,, value than the coexisting biotite. That the chlorite formed after biotite has the same XM, value as the biotite suggests that equilibrium was not reached in this case. Conversely, neoblasts of chlorite can be considered as equilibrium chlorites. Influence of rock chemistry According to Hameurt (19671, bulk rock compositions of the Monotonous Group and the Varied Group gneisses are not significantly different. However, some differences in mineral chemistry can be interpreted as bulk rock composition differences. Biotite compositions are divided in two separate groups on the basis of the AI”’ content, kf

-4 +ilm +Pl SW

+bi

,23-2

d

(224

1

Fig. 11. Projection of peak conditions parageneses. Phases in the system CKFMASHT have been projected from excess phases q, ilm, Pl, I3,O and bi onto the plane kf-g-cd. Theoretical phases as close to measured ones as possible have been used. Circles indicate where the rocks plot using the respective modal proportions of the projected minerals.

RETROGRADE

EVOLUTION

IN THE CENTRAL

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399

MOUNTAINS

which is related to rock chemistry (Indares and Martignole, 1985a). In addition, garnet, biotite and chlorite compositions vary with respect to X Mg from one sample to an other. Finally, the difference in the bulk rock composition can be seen in Fig. 11: all the SMM unit gneisses are located close to garnet (only sample 12-8 contains a significant amount of sillimanite) whereas the UC unit gneisses are more widespread, with samples located close to the three apices: garnet, sillimanite and cordierite. Therefore, although the differences in bulk rock compositions of gneisses are small, they are large enough to alter the mineralogy and mineral chemistry significantly.

mary assemblages deduced from observation of thin sections are parageneses which are stable under the same P-T conditions. Towards the low-pressure side, the stability of these parageneses is limited to pressures above 4 kbar by the stability of biotite + sillimanite in KFMASH (Grant, 1985): biotite + sillimanite + quartz = garnet + cordierite + K-feldspar + water (1) Towards the high-temperature side, the paragenesis stability is restricted to temperatures below 850°C by the KMASH reaction (Grant, 1986): quartz + biotite = K-feldspar + orthopyroxene

Paragenetic

analysis

Discarding muscovite and chlorite as secondary phases, most primary assemblages can be represented by the system NCKFMASHT (Na,OCaO-K,O-FeO-MgO-Al,O,-SiO,-H,OTiO,). Phases in excess are quartz, ilmenite, plagioclase and biotite. Moreover, water is considered as being in excess. As Na is only present in plagioclase, we consider that Na and Ca are not independent components and, therefore, we projected the assemblages onto the plane K-feldspar-garnet-cordierite from the excess phases (Fig. 11). This projection shows that all the pri-

+ liquid

(2)

Towards the low-temperature side, the paragenesis stability is confined by the occurrence of sillimanite + K-feldspar to temperatures above 700°C. Some reactions can be deduced from the observation of thin sections. One kind of reaction is the formation of spine1 symplectites devoid of quartz (Fig. 5d and 5f). Figure 12 displays the compatibility diagrams corresponding to spine1 forming between garnet and sillimanite (Fig. 5d); Fig. 12a shows that spine1 + cordierite forms as garnet becomes enriched in Fe and that the con-

KFMASHTZ Zn

g

“FeMgAl”

(4 Fig. 12. Qualitative compatibility diagrams for the phase relationships in cd + sp symplectites. Mineral compositions of samples analyzed have been used. Dot indicates where the global composition should project. (a) Using KFMASHT. (b) Using KFMASWZ.

KFMASHTZ KFMASHT +bi +ru +ilm

+mu

Zn

+bi +ru +ilm 4-w

SP

4-W

(4

b)

Fig. 13. Qualitative compatibility diagrams for the phase relationships in mu + sp symplectites. Mineral compositions of samples analyzed have been used. Dot indicates where the global composition should project. (a) Using KFMASIIT. fb) Using W~IIASHT~.

tinuous reaction garnet + sillimanite = cordierite + spine1 proceeds within the stability field of biotite + siIlimanite but outside the stability field of spine1 + quartz in the FMAS system. However, Zn must also be taken into account as this com-

ponent is not negligible in spinel; quartz is theoretically stable with Zn-rich spinel, but only with a spine1 whose composition is richer in Zn than the only one in equilibrium with garnet found in these in rocks (Fig. 12b). Figure 13 displays the

Fig. 14. Successive phase relationships in the gneisses. (a)-(f) Phases in the system CKFMASTH projected from q, pl, ilm, bi and H,Q onto the plane kf-g-chl. (g) Phases in the system CKFMASTH projected from q, pl, ilm, mu and Hz0 onto the AFM plane (after Thompson, 1957).

RETROGRADE

EVOLUTlON

IN THE

CENTRAL

VOSGES

compatibility diagrams corresponding to spine1 forming at the apex of sillimanite (Fig. 5f); in this case spine1 forms as the sillimanite-muscovite tie-line moves towards lower values of Mg/Mg + Al, which corresponds to biotite becoming more Fe-rich in the projection adopted for Fig. 13a. As in the previous reaction, quartz is theoretically stable only with Zn-spine1 (Fig. 13b). Other retrograde reactions lead to the formation of muscovite and/or chlorite. Equilibrium between muscovite, chlorite and garnet, which was observed in thin section, implies a multi-stage retrogression from primary paragenesis. One such possible evolution is presented in Fig. 14: Fig. 14a represents the primary paragenesis (as in Fig. 111. The rare retrogression assemblage involving cordierite, chlorite and K-feldspar (Fig. 14b) is found in sample 23-2. Formation of fibrolite could occur by reaction of some Fig. 14c assemblages to Fig. 14d assemblages (sample 21-3’) and reaction of some Fig. 14d assemblages to Fig. 14e assemblages (samples 22 and 22’). Figure 14f displays the most retrogressed assemblages found in the samples studied. It is worth noting that garnet is produced during the late stage of retrogression (Figs. 14e,f). Moreover, in the divariant

Fig. 15. Petrogenetic

grid in the

KFMASH

401

MOUNTAINS

system projected

field muscovite + garnet + biotite + chlorite (Fig 14g), alteration of biotite into chlorite occurs as chlorite, biotite and garnet become enriched in Fe and retrogression leads to the disappearance of biotite, for bulk compositions where the Al/Fe + Mg ratio is located between garnet and chlorite in Fig. 14g. These features are consistent with the presence of an Fe-rich overgrowth around garnets in contact with chlorite devoid of biotite. This overgrowth is similar to those described by Rumble and Finnerty (1974) for Mn. In our case, primary garnet is firstly consumed during retrogression and forms only at the latest stage; hence the sharp difference in composition observed between the overgrowth and the central part of garnets. Thus, the evolution of paragenesis is consistent with a continuous retrogression from the sillimanite + K-feldspar stability field to the garnet + chlorite + muscovite + biotite stability field. This retrograde P-T path implies, however, that staurolite is metastable with respect to the current petrogenetic grids (Harte and Hudson, 1979; Holland and Powell, 1990). This inconsistency disappears if the influence of the fugacity of oxygen is considered. Holland and Powell ex-

from bi, q and H,O

(1990). bi and q are in excess but have been left in italics to show the relevance Arrows

and shaded

areas indicate

modified

for very low fo,

of the reactions

the P-T-t

path.

from Holland

with respect

and Powell

to phase relationships.

402

plained the discrepancy between their calculated grid and the Harte and Hudson’s grid based on natural assemblages by considering the effect of f;,_. According to Holland and Powell, lowering j’o, tends to decrease the stability field of chloritoid + biotite and staurolite + biotite, so that metastable reactions in Hudson and Harte’s grid become stable. Following the same logic, at fo, conditions lower than those of the Holland and Powell’s grid, the stability field of staurolite would disappear at low to medium pressures, giving way to the stability of the reactions cd + bi = mu + chl + sill and cd + kf + bi = mu + chl deduced from the paragenetic analysis. The widespread occurrence of graphite supports the assumption that very low fbz conditions existed during metamorphism and, therefore, a petrogenetic grid valid for very low fo, conditions was drawn from Holland and Pow& grid in order to interpret the succession of paragenesis observed in the gneisses. The petrogenetic grid shown in Fig. 15 was obtained by extending the staurolite out reactions in the Holland and Powell’s grid. The two reactions g + mu + chl = cd + bi and g + cd + mu = bi + sill intersect at the stable invariant point (st,ctd,kf) located at about 600°C and 5 kbar; this creates a nehvork of stable reactions. The reaction mu + chl + bi = cd + kf is located between the reactions mu = sill + kf and mu + sill + chl = cd + bi according to Schreinemaker’s rules. This grid shows that, as biotite + sillimanite is stable in most thin sections, retrogression from the peak temperature recorded in the rocks occurred entirely above the invariant point (st,ctd.kf), making the retrograde P-T path more or less isobaric. The high temperature part of the grid deduced from Harris (1981) and Hensen and Harley (1990) also constrains the P-T path: the FAS reaction g + sill = cd + sp is the continuous reaction responsible for the first type of symplectite (Figs. 5d, 12). The flat slope of this reaction indicates that symplectites formed by isothermal decompression. Concerning the P-T stability field of these symplectites, Monte1 et al. (1986) studied the effect of adding Zn to the FAS system on the reactions g + sill + q = cd and sp + q = cd. Although the calibration of the reaction g + sill = cd + sp is not available, results of Monte1 and

I

IAI‘OI

(‘Ill

IFI

,\I

co-workers can still be used: the value of 4 kbar, estimated using the Zn value measured in spine1 in symplectites in both reactions, calibrated by Monte1 et al. (1986) constitutes a minimum estimation of pressure. This is because the spine1 theoretically in equilibrium with quartz should have a higher Zn content than the measured value (Fig. 12). This means that conditions of decompression are consistent with the peak conditions of metamorphism within the stability field of the paragenesis: garnet + biotite + sillimanite + cordierite + quartz. Interpretation of the second type of symplectite is not possible given the absence of experimental data on the system of interest; it can only be said that this type of symplectite is consistent with the muscovite + quartz stability field. Estimation

of P-T conditions

Metamorphic temperatures for the pelitic gneisses have been determined using geothermometers based on Fe-Mg exchange between garnet and biotite, and between garnet and cordierite. The paragenetic analysis showed that the zonation in garnet was related to the retrograde history of the gneisses. Therefore, when dealing with geothermometry and geobarometry, it is necessary to be cautious about the information it is possible to collect. Geothermometry and geobarometry have been applied to areas devoid of chlorite and muscovite in order to estimate the conditions of peak metamorphism and to areas where equilibrium compositions of garnet, muscovite and chlorite could be found in order to estimate the conditions of retrogression. Bulk rock chemistry is not negligible and has been taken into account. Peak of metamorphism

Because in granulite facies rocks important differences in K~p’“et-hiotitewithin a thin section (Bohlen and Essene, 1980) lead to a wide range of temperatures, Indares and Martignole (1985b) have assumed that garnet cores and matrix biotites isolated from garnet are generally not affected by late Fe-Mg exchange, and therefore

RETROGRADE

EVOLUTION

IN THE CENTRAL

VOSGES

403

MOUNTAINS

Pressure was estimated from the equilibria between garnet-plagioclase-AI-silicate-quartz (Newton and Haselton, 1981) and garnet-rutileAl-silicate-ilmenite-quartz (Bohlen et al., 1983). Activities for garnet and plagioclase were calculated using the method of Newton and Haselton (1981). The garnet-plagioclase geobarometer yields pressures of about 5-6 kbar at 700°C for samples from both the SMM and the UC units. Three samples (5-4, 5-10 and 9’) contain assemblages suitable for the application of the GRAIL geobarometer (Bohlen et al., 1983): rutile, ilmenite and sillimanite occur as independent grains within garnet. Rutile and sillimanite deviate only slightly from pure-component composition, so that their activities may be taken as 0.99. the activity of ilmenite was calculated on the basis of ideal mixing on-site. The pressure was estimated at around 6-7 kbar at 700°C; which is consistent with the garnet-plagioclase geobarometry.

are the most suitable for calculating peak temperatures. However, this method is flawed because of the possible non-equilibrium between garnet and the matrix biotite in dehydrated rocks such as granulites. Therefore, we preferred to apply the geothermometer of Ferry and Spear (1978), modified by Hodges and Spear (1982) in order to account for the activity of grossular component in garnet, to contacts between biotite and garnet central part. Nevertheless, Indares and Martignole correctly pointed out that temperatures calculated for granulite facies rocks needed to be corrected for the effect of Ti in biotite. The Ln K, values corrected for the effect of grossular content in garnet are correlated with XTi content in biotites (Fig. 16). The data can be interpreted with a single trend. Therefore, it is not inconsistent for all the gneisses (from both SMM and UC units) to have been formed under the same conditions. The value calculated from the intersection of the regression line with the Y axis (Ln K,) corresponds to the value of T calculated with a hypothetical Ti-free biotite in equilibrium with garnet. Given the uncertainties on the geothermometer equation and the regression line, this temperature is estimated at 660°C + 50°C at 6 kbar. With similar compositional restrictions as those for biotite-garnet thermometry, the cordierite-garnet geothermometers of Holdaway and Lee (1977) and of Perchuck and Lavrent’eva (1983) give values in the range 65o”C-700°C.

Retrogression

The mu + bi + pl + g + q geobarometer (Ghent and Stout, 1981) would have been a good method for estimating pressure during retrogression. However, due to the fact that equilibrium between chlorite and biotite in our samples is very uncertain, this geobarometer is unreliable. The garnet-chlorite geothermometer (Gram-

1.6

+

r

0

5-4

a

7-A-2'

09 n

9

l

22-l

0 .

22-3

0.61

224

1

0.04

I

I

0.06

I

0.08

I

0.10

0.12

‘Ti Fig. 16. Diagram

showing

result of linear least-squares

the linear

relationship

fitting. Intersection

between

Ln K,

corrected

for X,,

in garnet and XTi in biotite. The line is the to the origin leads to T = 660°C using the Hodges and Spear thermometer (1978).

bling, 1990) has been applied to contacts garnet overgrowths and “equilibrium” However,

given the scarcity

value

T = 610°C calculated

of

mometer

is only a rough

less, this estimation genetic

between chlorite.

of such contacts,

the

by this geother-

estimation.

is consistent

Neverthe-

with the petro-

grid in Fig. 15.

tent with low Xb,,o values than with high X,, (i values, there is no statistical difference between the runs. current

with

drated

rocks;

Retrogression studied

generally

contained

underwent

although

all the

retrogression,

they

well-preserved

primary

para-

genesis. As all the retrogression reactions require H,O as a reactant, this suggests that H,O was exhausted prior to the other reactants. Primary paragenesis is consistent with excess water (Figs. 11, 14). However, the question of whether pure Hz0 fluid was physically present in rocks at the time of the peak conditions of metamorphism needs to be addressed. That different parageneses, corresponding to different P-T conditions, coexisted within a single section supports the assumption of water exhaustion. The presence of symplectites supports the idea that very little fluid was present at the time of the peak conditions of metamorphism. As H,O is an important kinetic factor (it promotes diffusion), water deficiency could be responsible for the partial equilibrium between phases such as biotite and chlorite, and for the formation of symplectites. In addition, lowering the fH,o decreases the staurolite stability field in KFMASH (Laird, lY88), which has the same effect, with respect to phase relationships, as lowering f;,, (as considered in the petrogenetic grid in Fig. 15). This assumption is in agreement with Lamb and Valley (1985), who considered that many of the granulite facies rocks with low oxygen fugacities that they studied were also metamorphosed under vapour-free conditions. Runs of calculating P at various values of mix) were also underT and -GzO (H,O-CO, taken with THERMOC‘ALC (Powell and Holland, lY88) in order to estimate the value of atlzo consistent with the primary paragenesis. HOWever, although the results at T = 700°C give P values in the range 5-7 kbar and are more consis-

hydration

water

being

in the

as cooling

of previously exhausted

condehy-

during

the

path

The inferred

is not pervasive:

samples

the

of paragenesis

interpreted

process.

Inferred P-T-t

Water activity conditions

succession

is, therefore,

hydration

Discussion

The

gneisses

15. the formation plectite

indicates

P-T-t

path is presented

of cordierite that

in Fig.

+ sillimanite

decompression

sym-

occurred

during the period of peak conditions. This limited evolution is completely different from the cooling, essentially isobaric, P-T-t path corresponding to retrogression. The stability of sillimanite -t K-feldspar gives the peak metamorphism conditions at above 700°C. However, this limit is valid only for ati,o = 1 and, as discussed above, it is likely that water deficiency conditions prevailed during peak and early retrogression. Therefore, as the stability of sillimanite + K-feldspar is shifted towards lower temperatures, it is possible that peak metamorphism conditions occur between 650 and 7WC, as indicated by the geothermometric results. Since the most retrogressed assemblages were probably in equilibrium with HzO, an estimation of S50-600°C for the last conditions recorded by rocks is still valid. In this case cooling, as presented in Fig. IS, would be overestimated. Another consequence is that the peak metamorphism could have occurred within the stability field of staurolite + muscovitc iquartz + H,O in KFMASII, given the effect of H,O and O2 on staurolite-bearing assemblages. Also, the growth of spine1 + muscovite symplectites could be either related to the retrogression or interpreted higher phism.

as evidence al12,, domains

for occurrence during peak

in rocks 01 metamor-

Geological implications Due to the fact that very little water-rich fluid was present when primary paragenesis formed, the hydration process requires an external source of water to proceed. Mechanisms providing the necessary water are to be found in the late geo-

MOUNTAINS

405

logical events which happened in this part of the Hercynian belt. The retrogression occurred at medium to low pressure, in the vicinity of the andalusite-kyanite-sillimanite triple point, and it is tempting to relate retrogression of gneisses to the emplacement of andalusite-bearing granites below the UC and SMM units in the tectonic pile: release of fluids when the granite crystallized could have been a source of water. Moreover, recent tectonic studies (Rey et al., 1991) show that the latest movements along the shear fault planes corresponded to normal faulting. Therefore, granite formation with normal faulting suggests that some lithospheric thinning existed; if thinning is accompanied by high dT/dP gradients, P-T conditions for more dehydration and melting, followed by crystallization, would have occurred higher up the tectonic pile, hence the hydration observed in the gneisses. The P-T evolution at peak conditions of equilibration in gneisses more or less corresponds to isothermal decompression. How these conditions are related to the retrogression; that is, if is there a time gap between peak and retrogression or if the evolution from decompression to cooling during retrogression is continuous, is difficult to assess. However, the metamorphic paragenesis corresponding to events G2 and G3 as defined by Hameurt (1967) were dated by Bonhomme and Fluck (1981): metamorphic limestones and paragneisses devoid of retrogression were dated at 386 + 15 Ma, whereas retrogressed paragneisses and amphibolitized eclogites were dated at 340 + 30 Ma. As the peak conditions of metamorphism in our rocks are attributed to the G2 event and hydration to the G3 event, there would be a time gap between these events. This assumption is supported by the granitization occurring between 355 and 340 Ma (Hameurt and Vidal, 1973; Montigny et al., 1983). Another point of contention is the presence of remnants of high pressure rocks in the Vosges massif. We were not able to find any high pressure paragenesis in our samples, but they have been described elsewhere (kyanite in UC unit gneisses (Hameurt, 1967) and in SMM unit metapelites (Rey et al., 1989)). Moreover, garnet peridotites, eclogites and high-pressure granulites

imply pressures above 15 kbar for the metamorphic conditions. Therefore, incorporation of these high pressure rocks needs to be accounted for in any geodynamic model. Two ways have been proposed: one geotectonic event with a particular P-T-t path involving strong isothermal decompression (Fabrics and Latouche, 1988; Rey et al., 1989) or at least two separate geotectonic events (Hameurt, 1967; Fluck, 1980; Eisbacher et al., 1989). The unique geotectonic event model was based on the assumption that all the elements of the series (Monotonous and Varied groups, peridotites and eclogites) jointly followed the same P-T-t path. However, this assumption needs to be revised given our new data and the recent works on the Bohemian massif (Medaris et al., 1990; Carswell, 1991). In this area, garnet peridotites and spine1 peridotites originating from two different parts of the upper mantle occur together within gneisses (Medaris et al., 1990). Moreover, peridotites are allochthonous with respect to granulitic gneisses. In addition, Carswell (1991) considered that the juxtaposition of peridotites with granulites was not of primary origin with respect to the Variscan orogeny. Therefore, the high pressure rocks and the surrounding gneisses have probably not followed the same P-T-t path. Similarities are found in the Vosges massif: garnet peridotites are found only associated to granulitic leptynites (Van Eller, 1961; Hameurt, 1967). Also, as in the Bohemian massif, the high pressure granulitic leptynites represent the highest thrust sheet within the tectonic pile. Therefore, evidence of undoubtedly high pressures is restricted to the granulitic leptynites. If the other high pressure parageneses are taken into consideration, the occurrence of kyanite can been seen as confirmation of these high pressures. In fact, only the equilibrium between kyanite, K-feldspar and H,O is consistent with pressures above 12 kbar. Moreover, the petrological studies in the Vosges have, so far, failed to address the water deficiency question. It is also worth noting that our pressure estimation is not inconsistent with the transition between kyanite and sillimanite located within the aluminosilicate + K-feldspar field, between 6 and 7 kbar for the range 650-700°C. Therefore, we consider that the

RETROGRADE

EVOLUTION

IN THE

CENTRAL

VOSGES

gneisses from the Varied Group and the Monotonous Group never underwent high pressure metamorphism. The various formations comprising the Vosges series would have undergone different P-7‘ histories before their juxtaposition prior to or during the G2 event (386 + 15 Ma). However, an amphibolit~zed eclogite does outcrop at Combrimont, within the Monotonous group area (Hameurt, 1967). Although the field relationships with the gneisses arc not well defined, this eclogite could be caught up within the gneisses. Thus, a process similar to tectonic melange before the G2 event can be invoked in order create the lithological structure observed in the Vosges.

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