High-pressure-low-temperature metamorphism and polyphase alpine deformation at Sant'Andrea di cotone (eastern corsica, france)

Tectonophysics, 78 (1981) 419-451 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

HIGH-PRESSURE-LOW-TEMPERATURE POLYPHASE ALPINE DEFORMATION (EASTERN CORSICA, FRANCE)

J.M. CARON I, J.R. KIENAST

419

METAMORPHISM AND AT SANT’ANDREA DI COTONE

2 and C. TRIBOULET

2

1 Institut de Ge’ologie et E.R.A. no. 07 0887 - 1, rue Blessig, 67084 Strasbourg, Cedex (France) 2 Dkpartement de Pe’trologie et L.A. au CNRS - 4, Place Jussieu, 75230 Paris, Cedex 05 (France) (Received January 26,198l)

ABSTRACT Caron, J.M., Kienast, J.R. and Triboulet, C., 1981. High-pressure-low-temperature metamorphism and polyphase Alpine deformation at Sant’Andrea di Cotone (Eastern Corsica, France). In: G.S. Lister, H.-J. Behr, K. Weber and H.J. Zwart (Editors), The Effect of Deformation on Rocks. Tectonophysics, 78: 419-451. A quarry, located in the “Schistes lustres” of eastern Corsica, has been studied in detail for both structural geology and metamorphic petrology. The superimposed structures belong to three successive generations. For each generation, folds, foliations, lineations and behaviour of metamorphic minerals are briefly described. Attention is devoted to the connection between deformation-recrystallization processes of minerals in thin section and macroscopic contrasts in ductility. Typical HP-LT metamorphic minerals were developed during the two first generations of structures and were little transformed during the third. Clinopyroxenes, garnets, blue amphiboles, phengites, chlorites, stilpnomelane, piemontite, lawsonite, pumpellyite and deerite occur in different lithologies. P-T conditions of metamorphism are estimated to 8 kbar and 3OO’C. fo, and Xcoz in metamorphic fluids are specified. Furthermore, evolution and behaviour of metamorphic fluids are described in this example of HP-LT metamorphism, with emphasis on interactions between these fluids, deformation processes and mineral assemblages.

INTRODUCTION

Eastern Corsica is mainly made up of “Schistes lust&“, which have undergone an Alpine high-pressure-low-temperature (HP-LT) metamorphism of glaucophane-lawsonite type (Lacroix, 1893-1910, 1897; Nentien, 1897; Orcel, 1924; Brouwer and Egeler, 1952; Peterlongo, 1968; Amaudric du Chaffaut et al., 1976) and polyphase Alpine deformation (Caron, 1977, Sauvage-Rosenberg, 1977; Caron et al., 1979). In this kind of metamorphism, occurrence of jadeite (Autran, 1964) and other minerals (Autran, in 0040-1951/81/0000-0000/$02.50

@ 1981 Elsevier Scientific

Publishing Company

420

0

_

20 km _~

Fig. 1. Location of the Sant’Andrea di Cotone quarry on a structural sketch map of Corsica (after Caron and Bonin, 1980). 1 = metamorphic pregranite series and Palaeozoic rocks; 2 = gabbros-diorites; 3 = granodiorites and porphyritic monzogranites (zone A); 4 = granodiorites and tonalites (zone B); 5 = granodiorites and monzogranites (zone C); 6 = leucocratic granites; 7 = talc-alkaline vulcanism; 8 = subalkaline potassic series; 9 = anorogenic alkaline complexes; 10 = granites of Eastern Corsica; 11 = autochtonous series; 12 = San Angelo unit; 13 = Corte unit; 14 = Santa Lucia unit; 15 = BagliaconeRiventosa series; 16 = Castagniccia series; I7 = ophiolites; 18 = Santo Pietro di Tenda series; 19 = Inzecca series; 20 = superficial allochtonous units; 21 = Neogene and Quaternary deposits.

421

-1

. Cd&T, ~:J;,+;;

2

*s**’

3

;o”eo:

4

\

tk

Fig. 2. Front-view of the main quarry. 1 = metabasites; quartzites and marbles; 4 = jadeite-gneisses; 5 = marbles.

5

2 = quartzites;

3 = piemontite-

Durand-Delga et al., 1978) was specially mentioned many years ago in the Sant’Andrea di Cotone quarry. A detailed structural and petrographic study of this quarry is presented here. The quarry lies near the eastern margin of Corsican “Schistes lustres” (Fig. 1) about 2 km W of Sant’ Andrea di Cotone. The investigations made in the main quarry have been completed by observations on an adjacent smaller quarry. In both quarries, 5 to 15 m wide tectonic lenses are bounded by shear zones (Fig. 2). Within each lens, several generations of superimposed folds and foliations can be clearly observed. The deformed rocks - metabasites, quartzites (metacherts), gneisses and marbles - all belong to the “Santo-Pietro-di-Tenda” series. This lithostratigraphic series (Fig. 3) (Caron and Delcey, 1979) corresponds to the top of an ophiolitic sequence and to the base of its sedimentary cover. Compared with other postophiolitic sedimentary series from Corsica, the Alps and the northern Appennines, this Santo-Pietro-di-Tenda series is probably Upper Jurassic. Nevertheless, it

marbles

gneisses

and

quartzites metabasites

Fig. 3. Schematic Caron and Delcey,

lithostratigraphic 1979).

columm

of the Santo

Pietro

di Tenda

series (after

422

differs from them by an usual abundance of dolomitic and rhyolitic pebbles within quartzites and marbles; gneisses also represent former rhyolitic arkoses intercalated in metaradiolarites. The diversified primitive lithology is reflected by various metamorphic parageneses on a very small outcrop area. Moreover, the different beds delineate very accurately the successive structures at outcrop and sample scales. These rocks are, therefore, well-suited to study the relations between metamorphic crystallizations and deformations. An extensive description of this quarry will be given elsewhere (Caron and Bonin, 1980). STRUCTURES

Superimposed

strut tures

At different scales (outcrop, sample, thin section) many pictures allow the distinction of three generations of penetrative structures (Fig. 4). (1) Isoclinal folds (F,) with acute hinges are associated with a metamorphic schistosity S,. (2) The most obvious folds (F,) are flattened, with rounded hinges, and they deform the former structures. (3) An irregularly distributed crenulation cleavage (S,) cuts off both limbs of F2 folds and draws an axial surface of undulations (F3). Major structures of the same generations account also for the map distribution of the major units. The features of minor structures and the behaviour of metamorphic minerals are described below. First generation Features structures.

of first

synmetamorphic

structures

are often

blurred

by later

F, folds. First phase folds, F,, are recognized where contrasting lithology occurs (e.g. interbedded quartzites and marbles; Fig. 5). They occur as acute ravelled strips, being less acute in metabasites. The interference patterns between F, and F2 folds indicate that both generations are approximately coaxial. S, axial-plane foliation. A well-developed metamorphic schistosity is outlined by parallelism between primary bedding and array of most metamorphic minerals (amphiboles, lawsonites, micas) (Fig. 6). This schistosity is disturbed around competent dolomitic and rhyolitic pebbles. L, lineation. Some mineral lineations been parallel to F, folds axes.

(e.g. amphibole

needles)

may have

423

2m

Fig. 4. Superimposed structures of successive generations, at different scales. a. Left side of the main quarry (1 = metabasites; 2 = gneisses; 3 = marbles). b. Alternating quartzites (dots) and marbles; small Eastern quarry. c. Metaradiolarian schists, right side of the main quarry.

424

I

10 cm

Fig. 5. Profiles

of some minor F,-folds.

Behauiour

of minerals. The appearance of minerals such as glaucophane, lawsonite, and white micas, occurred probably during or at the end of the development of S, schistosity. Some of these minerals, and all jadeites and garnets appeared prior to Sz cleavage.

Fig. 6. Schistosity S1 in metabasites. Plane-polarized light.

Gl = glaucophane,

’ La = lawsonite;

Ph = phengite.

425

Second generation Fz folds. They are tight (interlimb angle: 20-50”), with rounded hinges (Fig. 7). On cross section, they have convergent isogons (class lC, Ramsay, 1967). The shape of profile sections is slightly variable, depending on the lithology in which they occur. An attempt to estimate these differences was made by measuring the variations of orthogonal thickness of folded beds on cross sections. The different curves (Fig. 8) show a flattening amount increasing from metabasites to quartzites and gneisses, and to marbles. This is interpreted as the result of a contrast in ductility between adjacent lithologies: during the flattening of Fz folds, metabasites were more competent than quartzites and gneisses, which in turn were more competent than marbles. The ductility contrast emphasized above results also in different orientations of F2 fold axes (Fig. 9). During the evolution of F2 folds, for instance, a disharmony appears between the outer arc of folds in metabasites and the inner arc of enveloping folds in marbles. In this way, the axes of Fz folds in marbles are strongly reoriented towards mineral lineation (stretching lineation), while in metabasites they remain oblique to this lineation. Sz axial-plane foliation. There is commonly no cleavage S2 in metabasites; a thin crenulation cleavage S2 occurs in micaceous quartzites and in schists; the first schistosity is more completely transposed in a new S2 schistosity in micaceous gneisses, and particularly in marbles where it is generally the only foliation.

Fig. 7. Profiles of some minor Fz-folds (2-10 c. Marbles.

cm wide). a. Metabasites. b. Gneisses.

426

” marbles

\

65%

0 !-_____~_____

_~~_

Fig. 8. Mean curves t& (after in cross-sections of F, folds.

J Ramsay,

1967)

related

to orthogonal

thickness

variations

L2 line&ion. Besides intersection and microfold lineations, a mineral lineation is outlined by a preferred orientation of glaucophane needles and by the great axes of elongate quartz or calcite crystals or aggregates. It corresponds

metabasites

W

i’

,

/

I-

gxx3

I

I

E

*7

I

*a l* “2

ii

dN 160,25S a

b

Fig. 9. Disharmony between metabasites and marbles in F, folds, resulting in the reorientation of Fz axes in marbles. a. Measurements made on the area represented on Fig. 4a (equal-angle projection, upper hemisphere). b. Sketch-representation of this reorientation.

421

a

b

C

Fig. 10. Reorientation and recrystallization of glaucophanes in quartzites (drawing from photographs). a. mm-wide glaucophanes are randomly oriented in the St schistosity. b. Glaucophanes, mechanically rotated, tend to be reoriented toward the stretching lineation 152. c. Former large relict glaucophanes are surrounded by thin newly formed glaucophane needles outlining the Lz lineation.

to a stretching lineation, the relation pointed out above (Fig. 9). Behauiour

of which

with

fold axes has been

of minerals. Former metamorphic minerals are strained by penetrative deformations of the second generation. As a function of the lithology, some minerals become unstable and are replaced; others are strained, but not obviously transformed; most recrystallize more or less completely. Garnets in metabasites (which may be replaced by chlorite), jadeites (the rims of which are transformed into phengites) and lawsonites in marbles (pseudomorphosed by calcite and phengite) belong to the first category. However, most minerals from metabasites, such as glaucophanes and lawsonites, are strained but still remain unaltered during this deformation. Recrystallization can already be macroscopically recognized for glaucophanes in quartzites (Fig. 10): former large glaucophane crystals (1 mm wide) have been randomly oriented in S, schistosity; during DZ deformation, they were mechanically rotated toward the stretching lineation and, moreover, new thin glaucophane needles were growing up along this new lineation. In thin section, former large glaucophane crystals show an irregular extinction, while new needles have no internal deformation (Fig. 11). At least some of these new needles seem to have their origin in chips of former ones, by recrystallization processes. Elsewhere, glaucophane or white micas previously developed in S, schistosity are recrystallized around Fz microfolds and form polygonal arcs (Spry, 1974). In carbonate-poor rocks, lawsonite too may recrystallize and armour Fz crenulations (Caron, 1974). Some differences in behaviour between quartz and calcite have to be pointed out. Quartz grains show evidence of plastic intracrystalline deformation (undulose extinction, deformation bands, sub-basal deformation lamellae; Hobbs et al., 1976), of recovery (subgrains) and recrystallization (serrated grain boundaries, less deformed new grains) (Fig. 12). In this way, they become elongated parallel to SZ foliation. On the contrary, in carbonate rocks, quartz grains seem relatively undeformed and have a near spherical shape in a calcitic matrix. In such rocks, calcite has undergone strong intracrystalline deformation and recrystallization, which leads to a grain size reduction. Moreover, calcite has been deformed by dissolution-crystalliza-

428

Fig. 11. Different generations of glaucophanes in metaradiolarian schists. Large older deformed glaucophanes (b), with core rich in quartz droplets (a), and smaller newformed or recrystallized needles (c). Plane-polarized light.

429

tion processes, as emphasized by boudinaged amphiboles and quartz sealed with new calcite (Fig. 13). These differences in behaviour between minerals from different lithologies account for the macroscopic ductility contrasts between metabasites, quart-

Fig. 12. For caption

see p. 430.

Fig. 12. Deformation and recrystallization of quartz during the second generation of structures. (a) Crossed polar-s: strongly deformed quartz grain, with sub-basal deformation lamellae and subprismatic deformation bands (approximately parallel to Sz cleavage); nucleation of new small grains occurs along deformation lamellae. (b) Crossed polars: slightly deformed quartz grains have undergone recrystallization, as evidenced in planepolarized light(c) by the former microfolding of an enclosed chlorite.

zites and gneisses, and marbles. The more the rock contains recrystallized minerals, the more the rock is strained.

deformed

and

Third generation

Structures

of third generation

are irregularly

distributed.

F3 folds.

They deform, among other structures, F2 folds and L2 lineations. Interference patterns of both generations of folds, F3 profile sections, and patterns of L2 lineations deformed around F3 folds, point out that buckling acted as the dominant deformation process. S3 axial-plane

foliation. A spaced crenulation cleavage S3 is restricted almost only to quartzites. In metabasites, microfractures and microveins may be related to this generation. L3 lineations.

axes.

In quartzites,

an intersection

lineation

is parallel

to F3 fold

Fig.

13.

Solution

transfer

(Ca) sealing microboudinaged

of calcite crossite

the second generation of structures. Calcite (Cr) in (a) and quartz (Q) in (b). Crossed polars.

during

Behaviour of minerals. Most metamorphic minerals recrystallize. They are, however, not clearly altered.

The obvious

feature

is an inverted

behaviour

are deformed

of quartz

and do not

and calcite,

com-

432

pared to the second generation of the structure. Quartz is now more easily deformed than calcite and migrates in solution: besides plastic deformation, it seals microveins and microboudinaged minerals during this third generation of structures.

Recapitulation Features of the three generations of structures are summarized in Table I. From this table, it appears that structures in the metabasites are mainly con-

TABLE

I

Summarized features of the successive generations of structures Metabasites

Quartzites-gneisses

Marbles

isoclinal; acute or rounded hinge

isoclinal; acute hinge

isoclinal; acute hinge

axial-plane foliation Si

schistosity

schistosity

schistosity

lineation L 1

mineral lineation?

?

?

newformed or recrystallized minerals

glaucophane, lawsonite, phengite, garnet

quartz, jadeite, phengite, lawsonite, glaucophane

calcite, quartz, phengite, lawsonite, glaucophane

tight; flattened or chevrons

subisoclinal; flattened

First generation: folds F1

Second generation: tight, flattened folds F2

axial-plane foliation Sz

none

thin crenulation cleavage or transposed schistosity

transposed schistosity

lineation L,

microfolds; rare mineral lineation

microfolds and mineral lineation

rare mineral lineation

newformed or recrystallized minerals

phengite (lawsonite)

quartz, phengite, calcite, lawsonite, glaucophane

calcite, quartz, phengite, (glaucophane)

undulations

open folds or chevrons

open folds or chevrons

axial-plane foliation Ss

none (fractures)

spaced crenulation cleavage

rare crenulation cleavage

lineation LJ

none

intersection microfolds

rare intersection

newformed or recrystallized minerals

(albite, pumpellyite, epidote)

quartz, (phengite, chlorite)

Third generation: folds F3

and

(calcite, quartz)

433

netted to the first generation, those third, those of marbles to the second. METAMORPHIC

of quartzites

to the first, second

and

PETROLOGY

The structures briefly described above were formed during a metamorphism, the physico-chemical conditions of which, in relation with mineralogical parageneses, will be studied in the following section. TABLE

II

List of the different hysterogen

omphacite garnet

Fe

garnet

Ca-Mn

blue

observed

parageneses

(A: relict mineral;

B: typomorphic

amphibole

lawsonite

A A B

B

B

B

B

B

B

B

B

B

epidote

C

pumpellyite

C

rutile B

chlorite

BB

phengite

BBBBBBBBBB

B

B

B

B

B

B

B

B

B

albite

c B

B

B

jadeite

quartz

B

A

sphene

B

B

B

B

c

c

B

B

B

B

B

B

aegyrine

B

riebeckite

B

magnetite

B

BB

B

B

B

B B

B

B

hematite

B

B

braunite

B

piemontite

B

stilpnomelane

dolomite

B

BBBBBBBBBBBBBB

B

deerite

calcite

mineral;

mineral).

B B

B

BBBBBBB B

B

C:

434

Mineral associations Metamorphic parageneses observed in the Sant’Andrea di Cotone quarry belong to the blueschist facies. They are summarized in Table II for the different lithologies. Some features need to be pointed out. Metabasites. Metabasites are either massive or banded. In some cases, former tuffs (with relict magmatic pyroxenes) can be recognized. In other cases, the metabasites were probably pillow lavas and/or pillow breccias. The typomorphic association, with glaucophane, lawsonite and phengite, is labelled “B”. Besides this paragenesis older metamorphic minerals, (labelled “A”) have been observed as relicts: omphacites and l-cm wide garnets, which are secondarily transformed into chlorite and small discrete garnet typomorphic assemblages. The existence of an early eclogitic paragenesis is thus evidenced. Finally, in rare thin sections, late hysterogen pumpellyite (“C”) occurs within lawsonite pseudomorphs, or in veins. Quartzites. Metabasites are often in contact with various siliceous beds: metacherts, metashales and iron-rich quartzites. Quartzites are often hematite-rich, but at the contact between hematite-quartzites and metabasites occurs a metasomatic rock containing quartz + deerite + aegyrine + riebeckite + hematite + magnetite. Gneisses. Jadeite gneisses represent former rhyolite pebbles (in quartzites and marbles) and former rhyolitic arkoses interbedded with quartzites. Sometimes, they contain K-feldspars in which jadeite occurs, replacing former perthitic albite (A. Autran, pers. commun., 1979). Such an occurrence has been described in the Western Alps (Lefevre and Michard, 1965; Compagnoni and Maffeo, 1973; Saliot, 1978). Marbles. Marbles are made of alternating beds more or less quartz-rich. Their mineral paragenesis (e.g. presence of stilpnomelane or piemontite-braunite) depends on variations in bulk rock chemistry. Metamorphic

minerals

Most of the metamorphic minerals from the studied quarry have been analyzed by microprobe: automatic analysis “TRACOR” or spectrometry, as described in Triboulet (1978). Some representative analyses are given in Table III, which synthesizes results of 235 punctual analyses. Clinopyroxenes. Three types of metamorphic clinopyroxenes have been found (Fig. 14): relict omphacites (“A”) from a warmer phase, and jadeiteaegyrine, typomorphic of glaucophane-lawsonite facies, from two different sequences.

Mg Ca Na K Ti

A11v A1v1 Fe3+ Fez+ MIl

Si

TiO,

K2O

M@ CaO Na20

Fe2 03 MnO

III

0.001

0.212 0.294 0.085 0.002 0.407 0.454 0.506

2.019

101.01

0.03

2.80 10.79 0.07 7.55 11.72 7.22

55.85 4.98

C77222a omph

representative

tot. Fe0 Fe0

A1203

SiOz

Some

TABLE

0.021 0.027 0.908

0.055

1.958 0.042 0.976

100.26

0.41 0.74 13.83

57.82 25.51 1.95

1.990 0.010 0.238 0.567 0.193 0.006 0.078 0.114 0.814

101.14

6.19 20.19 0.18 1.40 2.86 11.27

53.41 5.64

ccl78

aeg

cc16

of metamorphic

jd

analyses

1.984 0.016 1.577 0.116 0.687 0.604

3.012

101.83

1.79 6.02 7.36

39.82 21.97 24.87

gtl

ccl73

minerals

2.959 0.041 1.966 0.034 0.868 1.087 0.034 1.050

100.88

16.14 0.29 12.31

37.19 21.40 13.55

gt2

ccl73

and related

1.762 0.123 1.969 0.014 1.133 0.064 1.938 0.005 0.003

8.008

97.38

16.54 1.14 0.12 5.34 0.42 7.00 0.03 0.03

56.26 10.50

gl

0.952 0.688 2.647 0.039 0.702 0.038 1.970

8.042

97.10

0.19

21.15 6.11 0.31 3.15 0.24 6.79

53.76 5.40

cc178 rieb

formulae.

c7833b

structural

0.449 0.004 0.608 0.031 0.003 1.878 0.021

7.737 0.263 3.250

95.92

0.02 3.02 0.21 0.01 10.93 0.20

55.42 22.13 3.98

phg

cc16

0.341

0.033 0.040

11.627 0.373 0.888 4.520 11.628 0.711

98.71

1.32

0.09 0.06

40.42 17.46 2.44

33.81 3.11

cc178 deer

226Bb

0.312 0.059 0.846 3.067 0.296

5.288

6.068

94.53

0.45 3.68 18.56 0.99

39.35 29.08 2.42

pump

436

Others

/L-JG~~fi Aegyrine

Jadeite

Fig. 14. Composition of clinopyroxenes in Essene and Fyfe’s (1967) diagram. A : relictual ciinopyroxenes from metabasaites; B = typomorphic jadeites from gneisses (arrow from core to rim); B’ = pyroxenes from a deerite-bearing rock (arrow from core to rim).

Omphacitic pyroxenes are relictual, preserved inside large glaucophane plus lawsonite aggregates. In Essene and Fyfe’s (1967) diagram (Fig. 14), they plot in aegyrine-augite, chloromelanite and omphacite fields (they are Na-poor and Fe3+ -rich). These pyroxenes are rather typical of Fe-rich metabasites crystallized in eclogite facies (Lombard0 et al., 1978). Jadeites are typomorphic in gneiss (meta-arkoses and metarhyolites). They are very pure. Sometimes, a zoning from aegyrine 25% (core) to pure jadeite Almandine

Grossular Andradite

+

+ Pyrope

Spessartine

Fig. 15. Composition of garnets. A = relictual garnets from metabasites; B = typomorphic garnets from metabasites and marbles.

431

(rim) can be seen. These compositions are similar to pyroxene analyses in gneisses from eastern Corsica (Essene, 1969): Jd 94-Aeg 2-Oth 4. Zoned aegyrine occurs together with riebeckite in a deerite-bearing rock (Table III). Unlike the jadeite observations, the ferric iron content increases from core (chloromelanite) to rim (aegyrine). Such pyroxene compositions have been observed in Fe-rich oxidized rocks from Syros, Greece (Bonneau et al., 1980), with riebeckite and derrite also.

Garnets. Garnets occur both in metabasites

and marbles. Garnets from metabasites belong to two successive generations, A and B; only typomorphic garnet (B) is present in marbles. Former garnets occur either as 1 cm-wide phenoblasts, or as armoured relicts within typomorphic glaucophanes. These garnets, almandine-rich and pyrope-poor (Fig. 15) are characteristic of eclogitic facies (see analyses from Mt. Viso area, Lombard0 et al., 1978). Small typomorphic garnets are costable with chlorite, and occur as reactional rims around eclogitic almandine, or they are scattered as discrete crystals in metabasites and marbles. In both types of rocks, they are spessartine-rich (Fig. 15). The occurrence of such Mn-garnets in glaucophane-lawsonite facies has rarely been mentioned (De Roever, 1950; Brouwer and Egeler, 1952). They can be compared with garnets from greenschist facies metabasites in which the Mn content increases the garnet stability towards low temperatures (Sturt, 1962; Atherton, 1965). At higher pressures but still low temperatures, in the glaucophane-lawsonite facies, garnet is Mn-rich also; richer than in zoi’site-glaucophane schists and eclogites.

Fe_Glaucophane

Riebeckite

-r--r--l

Glaucophane

Mg-Riebeckite Fe3’ Fe3++ A

I”’

Fig. 16. Composition of blue amphiboles, in Miyashiro’s (1957) basites and marbles; open circles = deerite-bearing rock.

diagram. Dots = meta-

438

Blue amphiboles. Blue amphiboles are abundant in all types of rocks, except gneisses which are not rich enough in iron. The compositions have been plotted in Miyashiro’s diagram (1957) (Fig. 16). The Fe3+ content of the amphiboles has been calculated with Papike’s (1974) method. As in amphiboles from Calabria (Hoffman, 1972) and Milos, Greece (Kornprobst et al., 1979), Fe-content decreases from core to rim in zoned crystals. No significant differences have been observed here between blue amphiboles from different lithologies, or from different tectonic generations. At Sant’Andrea di Cotone, the rather high Fe*+/(Fe’+ + Mg) ratio in blue amphiboles is characteristic of low-temperature metamorphism. Similarly, in the Western Alps, ferroglaucophane occurs in low-temperature metabasites from Queyras (Mevel, 1975; Triboulet, 1977, 1978) while glaucophane S.S. is present in the higher-temperature Sezia zone (Liebeaux, 1975), like in Syros and Sifnos islands (Aegean Sea) with respect to Milos. This decrease of Fe”/( Fe2’ + Mg) ratio with increasing temperature seems to be related to the development of almandine garnet; at low temperature, in a lawsonite stability field, iron enters blue amphibole which is thus a ferroglaucophane; at higher temperature, in glaucophane-zo’isite s.1. stability field, a Mg-rich blue amphibole (glaucophane s.s.) occurs together with a Fe*+-rich garnet (almandine). This distribution may be expressed by a continuous reaction: Fe-glaucophane with lawsonite gives Mg-glaucophane with almandine. A similar phenomenon has been described in New Caledonia (Black, 1977) where an isograd corresponds to the reaction: lawsonite

+ Fe-chlorite

+ crossite + epidote

Some zoned blue amphiboles

+ almandine

show rather important

+ Mgchlorite + glaucophane

edenitic

substitutions.

Phengites. White mica is widespread in all rocks. The phengitic substitution (excess of SI“+ in the tetrahedral sites, with respect to pure muscovite endmember) varies from Si 3.4 to Si 3.7, with a maximum of about 3.6 (Fig. 17). According to Velde (1967), this high celadonite content substitution should be typical of high pressure and/or low temperature. This phengitic substitution is for instance higher than in zoi’site-almandine blueschists from the inner Alps, Syros and Sifnos islands, or south Brittany (ile de Groix; Triboulet, 1974), with Si 3.2-Si 3.4. Chlorites. They show a negative correlation between total Fe and Si on Foster’s (1962) diagram (Fig. 18). The most Fe-rich chlorites are costable with Mn-garnet, around a former eclogitic garnet. Piemontite. Mn-epidote occurs in quartzitic marbles, together with braunite. It is Mn-richer and more oxidized than Alpine piemontites associated with braunite, which are more oxidized than piemontites from spessartine-bearing rocks (Dal Piaz et al., 1979).

439

of

Number analyses t

I

I

3.7

3.6

3.5

substitution.

Phengitic

Fig, 17. Histogram of phengitic substitution.

Deerite. It is an unusual Fe-rich silicate which occurs in some blueschists (Agrell et al., 1965; Agrell and Gay, 1970; Bocquet and Forette, 1973; Langer et al., 1977; Schliestedt, 1978). As in California, it is here Mn-rich and occurs under hematite + magnetite buffer, with aegyrine and riebeckite. Stilpnomelune. few oxidized basites.

Ferrous brown stilpnomelane has been observed only in a garnet(spessartine)-bearing marbles, in contact with meta-

Pumpellyite. Pumpellyite is rare in the Sant’Andrea di Cotone quarry. It replaces typomorphic lawsonite or it fills up late veins (retromorphose and/or P-fluid variations). This lightly coloured pumpellyite is Al-rich and it belongs to blueschists-pumpellyite field (Coombs et al., 1975).

FTg 0.8

0.6

1

Thuringite

Chamosite

Ripidolite

Brunsvigite

Diabant

ite

0

0.4

0

1

.

0.2

I Sheridanite

Cl i nochlore

. Penninite l

Si 6 Fig. 18. Composition of chlorites in Foster’s (1962) diagram. Open circles = chlorites in equilibrium with garnet and’lawsonite in metabasites.

.

440

Conditions

of metamorphism

Polyphase metamorphism. As already pointed out, three successive phases of metamorphism can be recognized in metabasites : (A) Relictual phase characterized by omphacite and Fe-Mg-rich garnet (almandine). (B) Typomorphic parageneses, where glaucophane, lawsonite, chlorite, phengite and occasionally Mn-rich garnet (spessartine) are associated. (C) Late hysterogen minerals such as pumpellyite, chlorite, albite, epidote or calcite. Let us now specify some reactions which lead from one metamorphic phase to the following one. From A to B, the most obvious transformation in thin section is the destabilization of older garnet and of omphacite. For garnet, the inclusive transformation can be expressed by the reaction: 10 garnet A(MnO. ,Fe,.,Ca,.,) + 2.5 chlorite

+ 20 Hz0 = 1 garnet B(MnlCalFel

(Als. 1Feg.7Si5.2) + 2.5 lawsonite

For omphacite destabilization, different reactions on the composition of the starting mineral : 5 omphacite

1

+ 4 quartz can be involved, depending

+ 5 garnet A + 14 Hz0 * 2 ferroglaucophane

+ 2 lawsonite

+ 1 chlorite

for jadeite-rich

omphacite;

3.3 omphacite

+ 2 garnet A + 3.33 Hz0 * 1.33 ferroglaucophane

+ 1 lawsonite

+ 1 garnet B

for less jadeite-rich 8.66 omphacite + 8 lawsonite

omphacite;

+ 11 garnet A + 31.33 Hz0 + 1.33 ferroglaucophane + 3 chlorite

+ 2.68 quartz

for jadeite-poor omphacite. The transition from A (former eclogites) to B (later blueschists) corresponds thus to an hydration of metabasites. From B to C, the development of pumpellyite at the expense of lawsonite can be accounted for by the following reaction (Guitard and Saliot, 1970): 23 lawsonite

+ 17 calcite + 2 chlorite

+ 10 pumpellyite

+ 8 quartz

+ 17 CO* + 19 Hz0

The pumpellyite formed by this reaction is Al-rich, as in the performed analyses. It must be pointed out that, although these transformations are clear in metabasites, polyphase metamorphic is not so obvious in the other lithologies.

441

P-T conditions. An estimation of P-T conditions of typomorphic parageneses can be attempted in two ways: using reactions available in the literature (especially for metabasite associations), and using distribution coefficients between associated minerals. First, glaucophane-lawsonite association must occur (Fig. 19a) within the stability field of lawsonite versus zoi’site (Newton and Kennedy, 1963) and versus laumonite (Thompson, 1970; Liou, 1971; Nitsch, 1972), within the stability field of glaucophane (Maresch, 1977) at a pressure high enough for a jadeite-rich pyroxene to be stable (Essene et al., 1967; Newton and Smith, 1967; Newton and Kennedy, 1968). Then, temperature is estimated by means of Bickle and Powell’s (1977) thermometer, based upon iron partitioning between dolomite and Mg calcite. Care must be taken however, because of the thermodynamic approximations in the method itself, and because of the low analytic precision at such low Fe- and Mgcontents. Nevertheless, this leads to a temperature of about 300-350°C (Fig. 19b). Further, phengite-garnet equilibrium, experimentally tested by Krogh and Raheim (1978). can be used with caution because of the Fe oxidation state in phengites and the Mn effect in garnets. The intersection of this phengitegarnet curve (3685 + 77.1 P)/(Log KD + 3.52) = T with KD = [(Fe/Mg),t]/ [(Fe/Mg),,] and calcitedolomite curve gives an estimation of the P-T con-

15-

10-

Lw _---__-I Laum

5I

l

300

200

400

T(OC)

b

Fig. 19. P-T conditions of metamorphism for the studied rocks. a. Stability fields of jadeite + quartz (Jd + Q) (Newton and Kennedy, 1968), jadeite 82% (Jd 82) (Newton and Smith, 1967), lawsonite (Lw) (Nitsch, 1972), laumontite (Laum) (Nitsch, 1972; Liou, 1971) and glaucophane (GI) (Maresch, 1977). b. Estimated P-T conditions from analytical data; (I): KD phengite--garnet in metabasites = 58.49, after Krogh and Raheim (1978); (2) from Fe in dolomite and Mgcalcite, after Bickle and Powell (1977) (Fe in dolomite = 0.053, Fe in calcite = 0.001, Mg in calcite = 0.020).

442

---r-----i---------)

200

300

400

500

TPC)

Fig. 20. Oxygen fugacity in rocks from the quarry, at 8 kb and about 300°C, et al. (1978). 1, 2, 3, see text. Br = braunite; Gr = garnet; Hem = hematite; netite; Pm = piemontite; Rh = rhodonite; V = vapour.

after Brown Meg = mag-

ditions (Fig. 19b) : from 350°C - 13 kbar to 300°C - 8 kbar. This last value seems more reasonable, although such a pressure is difficult to explain by loading only, on the basis of geological data. Composition of metamorphic fluids. Oxygen fugacity is evaluated with help of Brown et al.% (1978) diagram, at 8 kbar. Considering a temperature about 300-350°C (Fig. 20) piemontite-braunite marbles (3) and hematite quart-

Fig. 21. CO2 content after Nitsch (1972). quartz; 20 = zoi’site.

of fluids in equilibrium with calcite-lawsonitequartz-bearing rocks, Cc = calcite; K = kaolinite; Lw = lawsonite; Py = pyrophyllite; Q =

443

zites (2) are the most oxidized; stilpnomelane-magnetite marbles (1) are the least oxidized. Such Fo2 variations in adjacent rocks seem not only to be correlated with the nature of premetamorphic rocks, as evidenced elsewhere (Chinner, 1960; Mueller, 1961; Chopin, 1979). In metasedimentary rocks, Fo2 appears indeed to be determined by the distance to the metabasites boundary, which acts as a reduction front. This distance depends not only on the sedimentation, but of course on the tectonic history also. Lawsonite-calcite-quartz assemblage is critical for the CO* content in the fluid phase (Fig, 21). The molar fraction of CO2 cannot exceed 2 or 3% for the involved temperatures (Nitsch, 1972). The same result is obtained from calcite + quartz + sphene assemblage stability (Ernst, 1972), which is observed at Sant’Andrea di Cotone too. However, Xcol must have been higher in marbles at the end of structural phase 2, since lawsonite does not recrystallize as in carbonate-poor rocks, but is replaced by calcite + phengite. RELATIONS

BETWEEN

DEFORMATION

AND METAMORPHISM

Relations between minerals and structures It has been pointed out above that structures of the successive generations 1, 2 and 3 are unequally developed in the different lithologies. Moreover, an evolution of metamorphic conditions is recorded by minerals (A, B and C). It is striking that different generations of structures have been formed mainly during B-type metamorphic conditions, as attested by recrystallization evidence. Recrystallizations are still varying in the different lithologies. A qualitative estimation of these variations has been attempted (Fig. 22). The continuity of metamorphic conditions is indicated by the lack of transitory alteration between crystallization and recrystallization of glaucophane, for instance. Differences in the recrystallization amounts of the main rock-forming minerals seem to answer for the ductility contrast between adjacent lithologies. This is particularly clear for the evolution of the second generation structures (flattening following buckling). Ductile deformation looks so much easier than recrystallization and occurs widely, probably for the following reasons : (a) New undeformed grains can store more energy in the form of lattice defects than older strained crystals can (Hobbs et al., 1976; White, 1977). (b) New grains are in general smaller than former ones, especially in marbles, and this grain-size reduction enhances the deformation of the aggregate by boundary-sliding processes (Schmid et al., 1977). (c) Deposition from the pore solution, connected with solution along surfaces under compressive stress, is an important deformation process of the bulk rock (Durney, 1972a, b).

Meta

basites

Cuartzites

.Gneisses

Marbles

Fig. 22. Qualitative estimation of relations between deformation and crystallizationrecrystallization as a function of time. A = amphibole; C = calcite; L = lawsonite; M = mica; Q = quartz. I, II, III: successive generations of structures. A, B, C: successive phases of metamorphism.

The deformability of the polymineralic aggregate most deformable mineral species in the rock. Behaviour

of metamorphic

is related

to that of the

fluids

This last fact emphasizes the mechanical role played by interstitial fluids. Microstructural and petrologic& considerations enable the evolution of these fluids to be outlined. The core of minerals like glaucophane, jadeite and lawsonite, may be very rich in quartz droplets (Fig. 11 and 23). This may correspond to a symplectic growth of the earliest part of these minerals and quartz, which would have been formed simultaneously. But, more probably, the metamorphic minerals grew between (or from the border of) previous quartz grains. Indeed, on the one hand, different quartz droplets enclosed in a same wider mineral do not have extinction for the same orientation of the section. On the other hand, lawsonite, which overprints a former tectonic layering, spreads out extensively at the expense of phengite-rich microlayers rather than of quartz-rich ones. This figure seems to be significant of very short-range migration of elements, and especially of Al (Carmichael, 1969). Such a weak migration is accounted for by diffusion processes (Fisher, 1973) rather than by percolation within a moving fluid. Thus, the system must at first have been closed with respect to fluid migration. Nevertheless, a high pressure of water-rich fluids must have occurred in some parts of these rocks at least, since lawsonite was growing during early stages of metamorphism. Then, probably as structures of the first generation proceeded, permeability and element migration increased, which account for the rim of glaucophane and

Fig. 23. Jadeite from gneisses, with quartz-rich core and pure jadeite rim. The outer margin is secondarily replaced by albite. Plane-polarized light.

jadeite being devoid of quartz inclusions. Such behaviour is also compatible with transient eclogite assemblages, which are then replaced by more hydrated parageneses. The distribution of these eclogite assemblages was probably very irregular at the scale of the outcrop, and this is another argument for irregular content (e.g. higher in tuffites than in pillow lavas) and mobility of water during the first stages of the metamorphic evolution. During the second generation of structures, element migration (and probably fluid migration) was more important than previously, as evidenced by sealing of microboudinaged minerals with calcite (Fig. 13). The redox system in the fluids was mainly buffered by bulk rock mineralogic composition (see above). The activity of CO* was tending to increase in fluids, especially in marbles, but also in quartzites. Calcium migrated over relatively large ranges (Ca-silicate lawsonite grew up along cleavage surfaces in gneisses), and this transport may have been achieved by fluid motion. Evidences of fluid motion are better preserved on structures of the third generation. For instance (Fig. 24) a trail of fluid inclusions outlines the S3 crenulation zone. Although the S3 surface neither stands out nor is continuous, it is obvious that metamorphic fluids have been introduced along this zone of weakness. This injection of fluids enhances plastic intracrystalline deformation of quartz (undulose extinction limited to inclusion rich grains;

446

447

Fig. 24. Fluids in a 5’s crenulation zone. a. The S’s crenulation zone is marked by a trail of inclusions (plane-polarized light). b. Detail of these fluid inclusions (plane-polarized light). c. The same as a, with crossed polars, showing undulose extinction in inclusionrich quartz grains. d. Detail of a broken unaltered lawsonite, bented along the crenulation zone: the microveins are filled up by quartz growing from the surrounding grains.

Fig. 24a, c). Moreover, despite the lack of bubbles study, these fluids must have a very high water content: along the S3 zone, lawsonite is broken up, but not transformed at all (see Fig. 21). This rehydration of the rocks enables migration of silica rather than migration of calcium: the third generation microveins are sealed by quartz (Fig. 24d) rather than by calcite. Thus, the microstructural development and the metamorphic history go on together, by interaction with metamorphic fluids (Fyfe et al., 1978), the behaviour of which is related to both the mechanical and structural evolution of the studied rocks. CONCLUSIONS

(1) The metamorphic rocks from the Sant’Andrea di Cotone quarry are typical of high pressure--very Eow temperature conditions (about 8 kbar, 300°C). These mineral assemblages are well preserved, because after developing during a first generation of structures, they recrystallize extensively during a second and they are not very transformed during the third generation of structures. (2) The persistence by recrystallization of such HP-LT metamorphic minerals is quite remarkable. On the strength of available isotopic data (40Ar39Ar measurements on glaucophanes and phengites; Maluski, 1977), such minerals have survived and recrystallized from about 90 Ma to about 35-40 Ma. It seems to us that these recrystallizations are related to the progressive hydration of metamorphic rocks during the slow rise of the deep part of an Alpine orogen.

448

(3) During this long-lived evolution, the microscopic behauiour of different minerals accounts for differences in deformability on a macroscopic scale. For instance, during the second generation of structures, scarce recrystallization corresponds to relatively little deformation in the metabasites. At the same time, the deformability of quartzites is higher, due to the behaviour of quartz. The solution-deposition processes, prominent for calcite, account for the high ductility of marbles. (4) These features are correlated with the metamorphic fluids’ behaviour, as approached by both microstructural and petrological observations: the deformation of rocks, in this metamorphic facies, tends to produce zones or surfaces of weakness along which fluids can migrate. In turn, this fluid motion tends to enhance deformation by both mechanical and chemical controls. Thus, one can see the history of such rocks as a succession of alternating stages. Some are heterogeneous, with weakness surfaces, the others are more homogeneous, with more penetrative structures. As the evolution of these rocks goes on, the homogeneous stages of deformation are less and less important, and fluid motion occurs rather along well-defined zones than through the whole mass of the rock. ACKNOWLEDGEMENTS

We gratefully acknowledge Albert Autran, Jean-Philippe Eissen, Paul De Fraipont, Bernard Mahwin and Catherine Viaux for allowing us to make use of the samples they have been collecting in the Sant’Andrea di Cotone quarry. We are indebted to Genevieve Pagand for typing the manuscript, and to Michell Ernewein and Joseph Gruner for helping in photographic work. Norbert Clauer, Jose Honorez, Nicole Liewig, Catherine Mevel and Hubert Whitechurch have kindly discussed our interpretations, and have reviewed parts of our manuscript. The Bureau de Recherches Geologiques et Minieres is thanked for its partial financial support, in the prospect of the 26th Geological International Congress. REFERENCES Agrell, S.O. and Gay, M., 1970. De la deerite dans les Alpes France-Italiennes. Bull. Sot. Fr. Mineral Cristallogr., 93 : 263-264. Agrell, S.O., Bown, M.G. and MacKie, D., 1965. Deerite, howieite and zussmanite, three new minerals from the Franciscan of the Laytonville district, Mendacino Co., California. Am. Mineral., 50: 278. Amaudric du Chaffaut, S., Kienast, J.R. and Saliot, P., 1976. Repartition de quelques mineraux du metamorphisme alpin en Corse. Bull. Sot. Geol. Fr., 18: 1179-1182. Atherton, M.P., 1965. The composition of garnet in regionally metamorphosed rocks. In: W.S. Pitcher and G.W. Flinn (Editors), Controls of Metamorphism. Oliver and Boyd, Edinburgh, pp. 281-290.

449

Autran, M.A., 1964. Description de l’association jadeite + quartz et des parageneses minerales associees dans les Schistes lustres de Sant’Andrea di Cotone (Corse). Bull. Sot. Fr. Mineral. Cristallogr., 87: XLIII-XLIV. Bickle, M.S. and Powell, R., 1977. Calcite-dolomite geothermometry for iron bearing carbonates. Contrib. Mineral. Petrol., 59 : 281-292. Black, P.M., 1977. Regional high-pressure metamorphism in New Caledonia: phase equilibria in the Ouegoa district. Tectonophysics, 43: 89-107. Bocquet, J. and Forette, M.C., 1973. Sur une deerite de l’ensemble des calcschistes piemontais, i Troncea (Italie). Bull, Sot. Fr. Mineral. Cristallogr., 96: 314-316. Bonneau, M., Geyssant, J., Kienart, J.R., Lepurier, C. and Maluski, H., 1980. Tectonique et metamorphisme Haute Pression d’lge Eocene dans les Hell&ides: exemple de l’ile de Syros (Cyclades, G&e). C.R. Acad. Sci., Paris, 291: 171-174. Brouwer, H.A. and Egeler, C.G., 1952. The glaucophane facies metamorphism in the Schistes lust& nappe of Corsica. K. Ned. Akad. Wet., Versl. Gewone Vergad. Afd. Natuurkd., Dl, 48: l-71. Brown, P., Essene, E.J. and Peacor, D.R., 1978. The mineralogy and petrology of manganese-rich rocks from St Marcel, Piemont, Italy. Contrib. Mineral. Petrol., 67: 227232. Carmichael, D.M., 1969. On the mechanism of prograde metamorphic reactions in quartzbearing pelitic rocks. Contrib. Mineral. Petrol., 20: 244-267. Caron, J.M., 1974. Rapports entre diverses “generations” de lawsonite et les deformations dans les Schistes lustres des Alpes cottiennes septentrionales (France et Italie). Bull. Sot. Geol. Fr., 16 : 225-263. Caron, J.M., 1977. Lithostratigraphie et tectonique des Schistes lust& dans les Alpes cottiennes septentrionales et en Corse orientale. Sci. Geol. Mbm., 48, 326 pp. Caron, J.M. and Bonin, B., 1980. GPologie de la Corse. 26e Congres Geol. Internat., Paris, G18-4: 80-90. Caron, J.M. and Delcey, R., 1979. Lithostratigraphie des Schistes lustres corses: diversite des series post-ophiolitiques, C.R. Acad. Sci., Paris, 288: 1525-1527. Caron, J.M., Delcey, R., Scius, H., Eissen, J.P., De Fraipont, P., Mahwin, B. and Reuber, I., 1979. Repartition cartographique des principaux types de series dans les Schistes lustres de Corse. C.R. Acad. Sci., Paris, 288: 1363-1366. Chinner, G.A., 1960. Pelitic gneisses with varying ferrous-ferric ratios from Glen-Glova, Angus, Scotland. J. Petrol., 1: 178-217. Chopin, C., 1979. Les parageneses ou oxyddes de concentrations manganesiferes des “Schistes lust&” de Haute Maurienne. Bull. Sot. Fr. Mineral. Cristallogr., 101: 514531. Compagnoni, R. and Maffeo, B., 1973. Jadeite-bearing metagranites 1.~. and related rocks in the Mount Mucrone Area (Sesia-Lanzo zone, Western Italian Alps). Schweiz. Mineral. Petrogr. Mitt., 53 : 355-378. Coombs, M.S., Nakamura, Y. and Vuagnat, M., 1975. Pumpellyiteactinolite facies schists of the Taveyanne Formation near La&che, Valais, Switzerland. J. Petrol., 17: 440-470. Dal Piaz, G.V., Di Battistini, G., Kienast, J.R. and Venturelli, G., 1979. Manganiferous quartzitic schists of the Piemonte Ophiolite Nappe. Mem. 1st. Geol. Mineral. Univ. Padova, 3 2. Durand-Delga, M., et al. 1978. Corse. Guides geologiques regionaux. Masson, Paris, 208 PP. Durney, D.W., 1972a. Deformation History of the Western unl.~~+;~ I’-,...-l’-la:m Switzerland. Unpubl. Ph.D. Thesis, London, 327 pp. Durney, D.W., 1972b. Solution transfer, an important geological deformation mechanism. Nature, 235: 315-317. Ernst, W.G., 1972. COz-poor composition of the fluid attending Franciscan and Sanbagawa low-grade metamorphism. Geochim. Cosmochim. Acta, 36: 497-504.

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451 reactions in the joint Newton, R.C. and Kennedy, G.C., 1963. Some equilibrium CaA12Si20s-HZO. J. Geophys. Res., 68: 2967-2984. Newton, MS. and Kennedy, G.C., 1968. Jadeite, analcite, nepheline and albite at high temperatures and pressures. Am. J. Sci., 266: 728-735. Newton, R.C. and Smith, J.V., 1967. Investigations concerning the breakdown of albite at depth in the earth. J. Geol., 75: 268-286. Nitsch, K.H., 1972. Das P-T-CO2 Stabilitatsfeld von Lawsonit. Contrib. Mineral. Petrol., 34: 116-134. Orcel, J., 1924. Notes mineralogiques et petrographiques sur la Corse. Bull. SOC. Hist. Nat. Corse, 461-464: 65-127. Papike, J.J., 1974. On the chemistry of clinoamphiboles. EOS Trans. Am. Geophys. Union, 55, (4): 469. Peterlongo, J.M., 1968. Les ophiolites et le metamorphisme a glaucophane dans le massif de 1’Inzecca et la region de Vezzani (Corse). Bull. Bur. Rech. Geol. Minieres (Fr.) Sect. 4: 17-79. Ramsay, J.G., 1967. Folding and Fracturing of Rocks. McGraw-Hill, New York, N.Y., 568 pp. Roever, W.P. De, 1950. Preliminary notes on glaucophane-bearing and other crystalline schists from South East Celebes and on the origin of glaucophane-bearing rocks. Proc. K. Ned. Akad. Wet., 53: 1455-1465. Saliot, P., 1978. Le metamorphisme dans les Alpes francaises. These inedite, Paris-Orsay, 192 pp. et microtectonique des Schistes lust&s et Sauvage-Rosenberg, M., 1977. Tectonique ophiolites de la vallee du Go10 (Corse alpine). These 3e cycle inedite, Montpellier, 83 PP. Schliestedt, M., 1978. Preliminary note on deerite from high-pressure metamorphic rocks of Sifnos, Greece. Contrib. Mineral, Petrol., 66: 105-107. Schmid, S.M., Boland, J.M. and Paterson, M.S., 1977. Superplastic flow in fine-grained limestone. Tectonophysics, 43: 257-291. Spry, A., 1974. Metamorphic Textures. Pergamon, Oxford, 350 pp. Sturt, B.A., 1962. The composition of garnets from pelitic schists in relation to the grade of regional metamorphism. J. Petrol,, 3: 181-191. Thompson, A.B., 1970. Laumonite equilibria and the zeolite facies. Am. J. Sci., 269: 267-275. Triboulet, C., 1974. Les glaucophanites et roches associees de l’ile de Groix (Morbihan, France): etude mineralogique et petrogenetique. Contrib. Mineral. Petrol., 45: 65-90. Triboulet, C., 1977. Stabilitd et relations de phases dans le systeme experimental Naz0-Alz0s-Mg0-Si0~-H~O. Applications 1 la petrologic des glaucophanites et des roches qui leur sont associbes. Thesis, Paris, 161 pp. (unpublished). Triboulet, C., 1978. Co-existing blue (sodic) and blue-green (calcosodic) amphiboles from fle de Groix, Morbihan, France. J. Petrol., 19: 653-668. Velde, B., 1967. Si4+ content of natural phengites. Contrib. Mineral. Petrol., 14: 250258. White, S., 1977. Geological significance of recovery and recrystallization processes in quartz. Tectonophysics, 39: 143-170.