granulite facies metamorphism and retrogressive evolution of the monts du lyonnais metabasites...

Lithos, 18 (1985) 97-113 97 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Granulite facies metamorphism and retrogressive evolution of the Monts du Lyonnais metabasites (Massif Central, France)

ERIC DUFOUR

Laboratoire de POtrologie, UA 726, DOpartement des Sciences de la Terre, UniversitO Claude Bernard - Lyon, 1, 27-43 Boulevard du 11 Novembre, 69622 Villeurbanne Cedex (France)

LITHOS

0 Dufour, E., 1985. Granulite facies metamorphism and retrogressive evolution of the Monts du kyon-

nais metabasites (Massif Central, France). Lithos, 18: 97-113.

Garnet-bearing pyriclasites have been found within the Monts du Lyonnais formation near St. Andr6- la-C6te, in the eastern part of the Massif Central. The rocks appear to have differentiated from a tholei- itic magma of continental affinity and they may be genetically related to quartzofeldspathic granulites considered to be the products of crustal melts.

By means of electron microprobe analyses, the consistent pressure and temperature estimates based essentially on pyroxene-garnet equilibria show that the peak of metamorphism was at about 860°C and 10 kbar. Furthermore this study tends to confirm the affiliation of Monts du Lyonnais granulites to the Early Variscan (granulite-eclogite facies) metamorphism which extends from Bohemia to Spain through central Europe. This event may be expected in the vicinity of an ancient suture zone where a continental collision followed a subduction.

(Accepted after revision January 21, 1985)

Introduction

In recent years petrographical research has shown that several terranes of the Massif Central preserve disequilibrium mineral parageneses which record a sequence of superimposed metamorphic events (Forestier and Lasnier, 1969; Marchand, 1974; Lasnier, 1977). The early stage o f such a metamorphic evolution is characterized by granulitic assemblages.

Occurrences of the granulite facies are now well known in several parts of the Massif Central: in Bourbonnais (Genti lhomme, 1975), in Cezallier (Mathonnat, 1983), in Haut-Allier (Marchand, 1974; Lasnier, 1977), in Limousin (Santallier, 1981), in Rouergue (Nicollet, 1978) and in the Monts du Lyonnais (Blanc, 1981; Dufour, 1982) (see Fig. 1).

This paper presents a detailed account of the mineralogy of the Monts du Lyonnais granulites

0024-4937/85/$03.30 © 1985 Elsevier Science Publishers B.V.

with the help of chemical analyses. The principal object is to estimate both the initial

physical conditions of metamorphism and the sub- sequent retrogressive evolution in order to elucidate, in relation with the tectonism, the history of the Monts du Lyonnais series.

Geological setting

In the eastern part of the Massif Central, the Monts du Lyonnais series consist of polymetamorphic granulite- and amphibolite-facies rocks of wide compositional ranges, The granulitic rocks were recognized for the first time in relics near Orlienas (Da- voine, 1975), then in an area over more than 15 km long to 3 km wide (Fig. 1).

The Monts du Lyonnais granulites mainly occur as layers of banded garnet-pyroxene granulites

98

/ (1) Monts du L y o n n a i s ; J

/ (2) Bourbonnais ; /

/ (3) Ceza l l ie r ; / (4) Haut Allier ; /

/ (5) Limousin ;

(6) Rouergue ;

du ~o~t s

l es

St-Andr~- la -C~te

Cozan

Fig. 1. Geographical setting of the Monts du Lyonnais (Massif Central, France).

Lyonn ais

gr ignals ff

Orlienas • • ' 3

•

S t_v i n t en t

S

1 t

5 Ikm

interstratified in garnet-quartz-bearing rocks (leucocratic orthogranulites). Major and trace element data (Table 1) of the metabasites tend to support a volcanic origin (within plate basalt; Dufour, 1982).

Associated metapelites display the assemblage mesoperthite-garnet-kyanite and/or sillimanite where cordierite and spinel develop during the retrogres-

sion. This granulite area is surrounded to the north by migmatitic biotite gneisses of granitic composition and to the south by gneisses metamorphosed under amphibolite-facies conditions, probably of sedimentary origin, which vary between quartzofeldspathic types and sillimanite- and biotite-rich types.

T A B L E 1

Whole-rock analyses of metabasites from Monts du L y o n n a i s

Sample BR1 CO2 CO3 O R 3 4 SA26 S A 2 7 SV96

SiO 2 (wt .%) 49 .05 43 .33 46 .66 43 .22

AI203 13.75 13.90 16.05 16.10 Fe203 1.13 4.51 1.18 2.62

FeO 13.33 11.54 10.10 12.10 MgO 4 .50 7.24 6 .17 5.30

CaO 9.09 11.09 11.46 9 .40

N a 2 0 2.93 1.19 2.67 3.35

K 2 0 0 .37 0.51 0 .62 0 .69

TiO 2 3 .97 3.92 2.00 3.56

P2Os 0.60 0 .49 0 .27 1.32 MnO 0.23 0 .25 0 .18 0 .26

H 2 0 + 0 .86 1.31 1.65 1.23

H 2 0 0 .14 0 .06 0.06 0.12

Total 99.95 99 .34 99 .07 99 .27

Ba ( p p m ) 127 116 98 538

Rb 13 14 19 12

Sr 347 95 263 644 Y 50 67 34 42

Zr 288 257 126 229 Nb 49 44 18 43

49 .42 52.10 46.91

13.10 13.85 15.00

1.86 1.19 1.55

13.64 9.39 10.10

3.87 6 .02 7.92

8.17 8.80 10.71

2.97 3.32 2.81

0.72 1.20 0 .78

3.29 2.01 1.99 0.85 0.21 0 .23

0 .22 0 .14 0 .18

1.18 1.55 1.18

0 .14 0.12 0.11

99 .43 99 .90 99 .47

307 211 142

21 36 45 334 170 154

64 28 32

197 153 149 63 21 26

Major element analysis by means of wet chemical and rare elements by X-ray fluo- r escence techniques. BR = Brignais; CO = C o z a n : O R = Orlienas; SA = Sa in t -AndrE-

ta-Cg~te; SV = Saint-Vincent.

99

Whole-rock Rb/Sr isochron data of garnet-leucocratic orthogranulites give a Cambro-Ordovician age of 497 -+ 8 Ma (Duthou et al., 1981) which expresses the emplacement of acid protoliths contemporary with the intrusions of mafic and ultramafic melts into a continental crust about 500 Ma ago (U-Pb on zircon; Gebauer and Grtinenfelder, 1982). In the southern part, some acidic gneisses of leucocratic composition represent intrusions into the volcanic sequence at the time of the granulite-facies metamorphism during the end of Ordovician (Dufour et al., 1983).

Petrography of the metabasites

The main constituents of the mafic granulites are plagioclase, pyroxenes, garnet and sometimes hornblende. Only garnet blasts up to 2 mm in diam- eter can be recognized macroscopically.

Under the microscope, the rocks are characterized by a granoblastic texture with some por- phyroblasts of garnet and hornblende. They are fine- to medium-grained with an average grain size between 0.1 and 1 ram. The mafic granulites of Monts du Lyonnais are classifed according to the diagram adapted from Mehnert (1972), as listed in Table 2. The following paragenesis: garnet + clino-

pyroxene + orthopyroxene 1 + plagioclase 1 + rutile -+ brown hornblende is assumed to have formed in equilibrium during the main stage of granulitic metamorphism. This paragenesis is considered as stable because contacts can be observed among all the mineral participating in the assemblage.

The influence of retrograde metamorphism is common over the whole range of the Monts du Lyonnais series. The first stage of retrogression is the replacement of garnet by vermicular orthopyroxene 2 and plagioclase 2 coronas (Fig. 2A). These reaction coronas mark the transition between high- and intermediate-pressure granulite-facies metamorphism (De Waard, 1965). These granulite coronas correspond to relics which have escaped a tectonic rehomogenization. A later stage of retrogression leads to the growth of amphibolite-facies assemblages. The following microscopical changes can be observed. The reaction coronas show the total consumption of garnet (Fig. 2B) so that amphibolitization of granulites proceeds as described for mafic rocks by Sills (1983) and for acid gneisses by Beach (1974). Clinopyroxene, orthopyroxene 1 and sometimes ilmenite (Fig. 2C) are rimmed by green hornblende. The late stage of retrogression is the replacement of clinopyroxene by hornblende and hornblende- quartz intergrowths while garnet is surrounded by small laths of biotite.

T A B L E 2

M i n e r a l a s s e m b l a g e s o f s a m p l e s i n v e s t i g a t e d

S a m p l e P l a g i o p y r i - H o r n . p l a g i o p y r i g a r n i t e P y r i - P y r i b o l i t e A m p h i b o l i t e

g a r n i t e c l a s i t e

O R 4 S A 1 8 B R 1 S A 2 6 S A I O I S V 9 6 B R I O B R l l C O 3 C O 2 S A 2 7

A s s e m b l a g e

G a r n e t + * + + *

C l i n n p y r o x e n e + + + +

O r t h o p y r o x e n e 1 + + - -

O r t h o p y r o x e n e 2 + + +

B r o w n h o r n b l e n d e + *

G r e e n h o r n b l e n d e - •

Q u a r t z + + - -

P l a g i o c l a s e 1 * * + * +

P l a g i o c l a s e 2 + +

B i o t i t e 2 - + -

O p a q u e s + + + + +

A p a t i t e + + + + -

S p h e n e

Z i r c o n - - - + -

_ r

+ + +

- - + *

, ¢

+ ,

+ _ _ +

+ + * , *

+ - - _

+ + + + ,

- - + _ + - -

+

_ _ _ + - -

* = 2 5 v o l . % : + : 2 0 5 v o l . % ; - - = 5 v o l . % .

100

Fig. 2. Thin-section photographs of granulite metabasites from the Monts du Lyonnais. A. Replacement of garnet by vermicular orthopyroxene 2 and plagioclase 2 coronas. B. Orthopyroxene 2 and plagioclase 2 association after the total consumption of garnet. C. Ilmertite rimmed by green hornblende. D. Centripetal growth of coronas inside a garnet.

101

Mineral chemistry

Analytical procedures

Mineral in polished carbon-coated thin sections were analyzed for all major elements (plus Cr and Ni) with an automated Camebax electron microprobe at the University of Nancy and at the University of Clermont-Ferrand.

At the Universities of Clermont II and Nancy I operating conditions were 15 kV accelerating poten- tial and 10 nA sample current.

Garnet

Garnet is a common mineral at Monts du Lyon- nais and is present both in felsic and mafic granulites. In most cases the anhedral grains are devoid of inclusions although a few garnets (sample SA18) include quartz, clinopyroxene, plagioclase and ilmenite.

Compositions of garnet from 4 samples of mafic granulites are listed in Table 3. They are almandine garnets containing appreciable amounts of pyrope (14-30%) and grossular (16-26%), and minor

T A B L E 3

Electron microprobe analyses o f garnet

Sample O R 4 S A 1 8

(1) (16) (35) ( 1 1 2 ) ( 1 1 3 ) core rim rim core rim

BR10 B R l l

( 1 2 5 ) ( 1 2 6 ) (59) ( 2 4 0 ) ( 2 3 9 ) core rim core core rim

SiO 2 (wt .%) 3 6 . 7 4 3 8 . 5 5 3 7 . 8 3 3 9 . 0 6 3 8 . 0 7 AI203 2 1 . 7 3 2 1 . 0 5 2 1 . 1 0 2 2 . 1 6 2 1 . 8 0 F e O t 2 3 . 6 2 2 8 . 6 2 2 8 . 0 6 2 3 . 5 4 2 7 . 9 7 MgO 5 .76 4 . 1 0 3 .82 6 . 2 0 4 . 0 3 M n O 1 .08 1 .89 1 .94 0 . 4 4 1.21 C a O 9 .49 6 . 2 8 7 .56 9 . 1 8 6 .29 K 2 0 0 . 0 0 0 .00 0 .00 0 . 0 0 0 .01 N a 2 0 0 . 0 0 0 . 0 0 0 . 0 0 0 .05 0 . 0 0 TiO 2 0 .04 0 . 0 4 0 . 0 0 0 .05 0 .19 NiO 0 .12 0 . 1 0 0 .05 0 . 0 0 0 . 0 0 C r 2 0 3 0 . 0 0 0 . 0 0 0 .00 0 . 0 0 0 . 0 0

Total 1 0 0 . 5 8 1 0 0 . 6 3 1 0 0 , 3 6 1 0 0 . 6 8 9 9 . 5 7

Cat ion n u m b e r s o n the basis o f 2 4 o x y g e n s :

Si 5 . 9 8 0 6 . 0 5 4 5 . 9 7 7 5 . 9 8 8 6 . 0 0 6 AI 3 . 9 4 9 3 . 8 9 2 3 . 9 2 5 4 . 0 0 0 4 . 0 4 9 Fe 3 . 0 4 9 3 . 7 5 9 3 . 7 0 8 3 . 0 1 8 3 . 6 9 0 Mg 1 . 3 2 5 0 . 9 6 0 0 . 9 0 0 1 . 4 1 7 0 . 9 4 8 Mn 0 . 1 4 1 0 . 2 5 1 0 . 2 6 0 0 . 0 5 7 0 . 1 6 2 Ca 1 . 5 7 0 1 . 0 5 7 1 . 2 8 0 1 . 5 0 8 1 . 0 6 3 K 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 2 Na 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 1 5 0 . 0 0 0 Ti 0 . 0 0 5 0 . 0 0 5 0 . 0 0 0 0 . 0 0 6 0 . 0 2 3 Ni 0 . 0 3 0 0 . 0 2 5 0 . 0 1 3 0 . 0 0 0 0 . 0 0 0 Cr 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0

Total 1 6 . 0 4 9 1 6 . 0 0 2 16 .061 1 6 . 0 0 8 1 5 . 9 4 2

M o l e p e r c e n t e n d m e m b e r s :

Almandine 5 0 . 1 3 6 2 . 3 8 6 0 . 3 1 5 0 . 3 2 6 3 . 1 0 Pyrope 2 1 . 8 0 1 5 . 9 4 1 4 . 6 4 2 3 . 6 4 1 6 . 2 0 Spessartine 2 .33 4 . 1 8 4 . 2 3 0 .96 2 .75 Grossular 2 5 . 6 7 17 .43 2 0 . 8 2 2 5 . 0 9 1 7 . 5 9 Andradite 0 . 0 7 0 . 0 7 0 . 0 0 0 . 0 8 0 .35 Ouvarovite 0 . 0 0 0 . 0 0 0 .00 0 . 0 0 0 . 0 0

3 8 . 6 5 3 7 . 8 6 3 9 . 3 8 3 8 . 0 3 3 7 . 9 8 2 2 . 4 2 2 2 . 1 8 2 1 . 8 6 2 2 . 7 1 2 2 . 0 8 2 3 . 7 4 2 7 . 2 8 2 3 . 9 3 2 4 . 6 5 2 5 . 2 0

5 .66 5 .88 8.01 7 .93 6 .69 0 .50 0 .57 1 .10 1 .14 1 .73 9 .33 6 . 7 8 6 .42 6 .45 6 . 0 4 0 .01 0 . 0 0 0 . 0 0 0 . 0 0 0 .04 0 .03 0 .00 0 .00 0 . 0 0 0 . 0 0 0 . 1 8 0 . 1 0 0 .00 0 .09 0 . 0 6 0 . 0 0 0 . 0 0 ~ 0 . 0 0 0 . 0 0 0 .00 0 . 0 0 0 .00 0 . 0 0 0 . 0 4 0 .04

1 0 0 . 5 2 1 0 0 . 6 5 1 0 0 . 7 0 1 0 1 . 0 4 9 9 . 8 6

5 . 9 4 8 5 . 8 8 5 6 . 0 1 4 5 . 8 2 5 5 . 9 1 4 4 . 0 6 3 4 . 0 5 9 3 . 9 3 0 4 . 0 9 5 4 . 0 4 8 3 . 0 5 5 3 . 5 4 6 3 . 0 5 6 3 . 1 5 7 3 . 2 8 1 1 . 2 9 8 1 . 3 6 2 1 .823 1 .810 1 .553 0 . 0 6 5 0 . 0 7 5 0 . 1 4 2 0 . 1 4 8 0 . 2 2 8 1 . 5 3 8 1 . 1 2 9 1 . 0 5 0 1 .059 1 . 0 0 8 0 . 0 0 2 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 8 0 . 0 0 9 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 2 1 0 . 0 1 2 0 . 0 0 0 0 . 0 1 0 0 . 0 0 7 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 5 0 . 0 0 5

1 6 . 0 0 0 1 6 . 0 6 8 1 6 . 0 1 6 1 6 . 1 0 9 16 .051

5 1 . 4 0 5 8 . 0 7 5 0 . 3 4 5 1 . 1 9 5 4 . 1 0 2 1 . 8 3 2 2 . 3 0 3 0 . 0 2 2 9 . 3 4 2 5 . 5 9

1 .10 1 .24 2 .34 2 .40 3 .76 2 5 . 3 5 18 .23 17 .30 16.81 16 .32

0 .32 0 .16 0 . 0 0 0 .16 0 .13 0 . 0 0 0 . 0 0 0 .00 0 . 1 0 0 . 1 0

A l m a + Spes

9 o

8 o

7 o

102

Gros 4- Andr Pyrope

Fig. 3. P lo t o f a n a l y s e d g a r n e t s f r o m M o n t s du L y o n n a i s

m e t a b a s i t e in t he t e r n a r y d i a g r a m : a l m a n d i n e + s p e s s a r t i n e -

p y r o p e - g r o s s u l a r + a n d r a d i t e . (o = O R 4 ; z~ = S A 1 8 ; * = B R 1 0

a n d B R l l ; a r r o w s i n d i c a t e e v o l u t i o n f r o m co re t o r im in

g a r n e t c o m p o s i t i o n . )

amounts of spessartine (1--4%). The low spessartine content is partly due to the deficiency of their host rocks in MnO.

Individual grains of garnet are zoned. Relative to the cores, the rims are generally depleted in Ca, enriched in Mn, and have a lower Mg/Fe ratio (Fig. 3; Table 3). Such a distribution of the Ca content across the garnet appears to be very common in the high-P facies rocks (Bryhni and Griffin, 1971; Kurat and Scharbert, 1972; Schmid and Wood, 1976; Yardley, 1977), while inverse Ca zoning is very scarce (Kornprobst, 1977). The observed zoning was interpreted as the result of a decrease in pressure and temperature during the growth of the garnet (Kurat and Scharbert, 1972; Cygan and La- saga, 1982) but it could also result from a re-equilibration at low P of a garnet already crystallized at higher P.

Clinopyroxene

Clinopyroxene is found in apparent textural equilibrium with orthopyroxene 1 and with garnet in several Monts du Lyonnais metabasites (in par- ticular OR4 and BR10).

Electron microprobe analyses of clinopyroxenes are listed in Table 4. The cation numbers are pre-

T A B L E 4

E l e c t r o n m i c r o p r o b e ana lyses o f c l i n o p y r o x e n e s

S a m p l e O R 4 S V 9 6 B R I O

(8) (13) (7) (41) (62)

SiO 2 4 9 . 1 2 5 0 . 1 0 5 0 . 4 3 4 8 . 8 9 4 9 . 9 0 AI203 3 . 1 4 2 .98 2 .07 5 .24 3 .97 F e 2 0 3 4 . 7 9 2 .86 0 .65 1 .38 3 .02 FeO 8 .60 10 .16 1 3 . 3 9 1 4 . 8 2 5 .23 MgO 11 .43 1 1 . 0 7 10 .16 9 .71 13 .43 M n O 0 . 2 4 0 . 2 7 0 . 3 9 0 .21 0 .33 C a O 2 1 . 5 6 2 2 . 2 1 2 1 . 7 2 1 7 . 3 7 2 2 . 1 9 K 2 0 0 . 1 5 0 . 0 0 0 . 0 0 0 . 3 4 0 .06 N a 2 0 0 .39 0 . 3 4 0 .19 0 . 7 4 0 . 4 7 TiO 2 0 .35 0 . 2 9 0 . 3 7 0 . 6 4 0 . 7 4 NiO 2 0 . 0 0 0 . 0 0 0 .01 0 . 0 0 0 .02 C r 2 0 3 0 . 0 0 0 . 0 0 0 .06 0 . 0 0 0 . 2 0

T o t a l 9 9 . 7 7 1 0 0 . 2 8 9 9 . 4 4 9 9 . 3 4 9 9 . 5 6

Cation n u m b e r s on the basis o f 6 o x y g e n s normal i zed to 4 metals:

Si 1 . 8 6 9 1 . 8 9 7 1 .939 1 . 8 7 9 1 .865 A1TM 0 . 1 3 2 0 . 1 0 3 0 . 0 6 1 0 . 1 2 2 0 . 1 3 5 A1VI 0 . 0 0 9 0 . 0 3 0 0 . 0 3 3 0 . 1 1 6 0 . 0 4 0 Fe 3+ 0 . 1 3 7 0 . 0 8 1 0 . 0 1 9 0 . 0 4 0 0 . 0 8 5 Fe 2+ 0 . 2 7 4 0 , 3 2 2 0 . 4 3 1 0 . 4 7 6 0 , 1 6 4 Mg 0 . 6 4 8 0 . 6 2 5 0 . 5 8 2 0 , 5 5 6 0 , 7 4 8 Mn 0 , 0 0 8 0 . 0 0 9 0 . 0 1 3 0 , 0 0 7 0 , 0 1 0 Ca 0 . 8 7 9 0 . 9 0 1 0 . 8 9 5 0 . 7 1 5 0 . 8 8 9 K 0 , 0 0 7 0 . 0 0 0 0 . 0 0 0 0 , 0 1 7 0 , 0 0 3 Na 0 . 0 2 9 0 , 0 2 5 0 . 0 1 4 0 . 0 5 5 0 , 0 3 4 Ti 0 , 0 1 0 0 . 0 0 8 0 . 0 1 1 0 . 0 1 8 0 . 0 2 1 Ni 0 . 0 0 0 0 . 0 0 0 0 . 0 0 1 0 . 0 0 0 0 . 0 0 1 Cr 0 . 0 0 0 0 . 0 0 0 0 . 0 0 2 0 , 0 0 0 0 , 0 0 6

To ta l 4 . 0 0 0 4 , 0 0 0 4 , 0 0 0 4 . 0 0 0 4 . 0 0 0

Mole p e r c e n t end m e m b e r s :

En 3 5 . 8 2 3 3 . 6 6 3 0 , 0 2 3 1 . 7 0 4 1 . 3 3 Fs 1 5 . 5 9 1 7 . 8 2 2 3 . 8 4 2 7 . 5 4 9 .56 Wo 4 8 . 5 9 4 8 . 5 2 4 6 . 1 4 4 0 . 7 6 4 9 . 1 1

sented in a recalculated way (O = 6, M = 4), showing maximum Fe 3÷ values consistent with the assump- tion of stoichiometry (Vieten and Hamm, !978). These clinopyroxenes can be termed salites (Fig. 4), except for a few Ca-poor ones which plot in the field of augites (Deer et al., 1978). An increase in the Fe/Mg ratio goes along with a slight decrease in Ca, as shown in Fig. 4. The jadeite and Tschermak's molecule components are very low in comparison with other typical clinopyroxenes of the granulite facies.

103

Ca

/ /

MI 1 0 3 0 5 0 F e + M n

Fig. 4. Plot o f analysed orthopyroxenes and cl inopyroxenes in the pyroxene quadrilateral (z~ = OR4; o = SA18; o = SV96; * = BR10 and B R l l ) .

Orthopyroxene

Orthopyroxene is the main ferromagnesian phase in most of the metabasites investigated. This ferromagnesian mineral occurs in two types of genera- tions: primary phase (Opx 1) is composed of large crystals with minor inclusions of quartz and opaque minerals while secondary (Opx 2) occurs in vermicular granules essentially found around or instead of garnets.

Electron microprobe analyses are listed in Table 5. As for clinopyroxene, the cation numbers are presented in a recalculated way (O = 6, M = 4). All these pyroxenes are poor in Al, so that this method produces a deficiency in A1vI in the analysed

T A B L E 5

E l e c t r o n m i c r o p r o b e ana lyses o f orthopyroxenes

S a m p l e O R 4 S A 1 8 S V 9 6 BR11

(23) (38 ) ( 1 3 1 ) ( 1 2 9 ) (14) (49) ( 2 4 4 ) ( 2 5 6 ) O p x 1 O p x 2 O p x 1 O p x 2 O p x 1 O p x 2 O p x 1 O p x 2

SiO 2 (wt .%) 5 0 . 1 2 5 0 . 4 4 5 0 . 0 7 5 0 . 9 8 4 9 . 9 1 5 0 . 3 6 5 1 . 3 9 50 .41 AI203 1 .24 0 . 8 4 0 .91 1.02 0 .64 0 .87 1 .20 1.52 F % O 3 0 . 6 7 1.81 2 .38 0 . 0 0 0 .43 0 . 0 0 2 .47 4 . 4 3 FeO 3 0 . 6 3 2 8 . 8 7 2 8 . 3 8 2 9 . 8 9 3 3 . 5 6 3 3 . 3 8 2 1 . 6 8 2 0 . 1 8 MgO 1 5 . 7 3 16 .65 1 7 . 1 0 1 6 . 7 4 1 3 . 9 9 14 .09 2 1 . 7 0 2 1 . 7 8 MnO 0 .55 0 . 6 6 0 . 4 7 0 . 3 8 0 .71 0 .55 0 .46 0 .66 C a O 0 . 5 6 0 .62 0 .31 0 . 4 0 0 .25 0 .78 0 . 4 4 0 .56 K 2 0 0 .01 0 . 0 3 0 . 0 5 0 .02 0 . 0 0 0 . 0 4 0 .06 0 . 0 0 N a 2 0 0 . 0 0 0 .05 0 . 0 0 0 . 0 0 0 . 0 3 0 .02 0 . 0 0 0 . 0 0 TiO 2 0 . 0 4 0 .01 0 . 0 0 0 .15 0 . 0 0 0 . 1 0 0 .11 0 . 0 8 C r 2 0 3 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 .01 0 . 0 0 0 .02 0 . 1 8

T o t a l 9 9 . 5 5 9 9 . 9 8 9 9 . 6 7 9 9 . 5 8 9 9 . 5 3 1 0 0 . 1 9 9 9 . 5 3 9 9 . 8 0

Cation n u m b e r s on the basis o f 6 o x y g e n s normal i z ed to 4 metals."

Si 1 .961 1 . 9 5 7 1 . 9 4 6 1 . 9 7 7 1 . 9 8 0 1 . 9 7 9 1 .936 1 . 8 9 8 Al TM 0 . 0 3 9 0 . 0 4 3 0 . 0 5 5 0 . 0 2 3 0 . 0 2 0 0 . 0 2 1 0 . 0 6 4 0 . 1 0 2 A1VI 0 . 0 1 8 0 . 0 0 0 0 . 0 0 0 0 . 0 2 4 0 . 0 1 0 0 . 0 1 9 0 . 0 0 0 0 . 0 0 0 Fe s+ 0 . 0 2 0 0 . 0 5 3 0 . 0 6 9 0 . 0 0 0 0 . 0 1 3 0 . 0 0 0 0 . 0 7 0 0 . 1 2 5 Fe E+ 1 . 0 0 2 0 . 9 3 7 0 . 9 2 2 0 . 9 7 0 1 .113 1 . 0 9 7 0 . 6 8 3 0 . 6 3 6 Mg 0 . 9 1 7 0 . 9 6 3 0 . 9 9 0 0 . 9 6 8 0 . 8 2 7 0 . 8 2 5 1 . 2 1 9 1 .222 Mn 0 . 0 1 8 0 . 0 2 2 0 . 0 1 5 0 . 0 1 2 0 . 0 2 4 0 . 0 1 8 0 . 0 1 5 0 . 0 2 1 Ca 0 . 0 2 3 0 . 0 2 6 0 . 0 1 3 0 . 0 1 7 0 . 0 1 1 0 . 0 3 3 0 . 0 1 8 0 . 0 2 3 K 0 . 0 0 0 0 . 0 0 1 0 . 0 0 2 0 . 0 0 1 0 . 0 0 0 0 . 0 0 2 0 . 0 0 3 0 . 0 0 0 Na 0 . 0 0 0 0.0_04 0 . 0 0 0 0 . 0 0 0 0 . 0 0 2 0 . 0 0 2 0 . 0 0 0 0 . 0 0 0 Ti 0 . 0 0 1 0 . 0 0 0 0 . 0 0 0 0 . 0 0 4 0 . 0 0 0 0 . 0 0 3 0 . 0 0 3 0 . 0 0 2 Cr 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 1 0 . 0 0 5

To ta l 4 . 0 0 0 4 . 0 0 0 4 . 0 0 0 4 . 0 0 0 4 . 0 0 0 4 . 0 0 0 4 . 0 0 0 4 . 0 0 0

M o l e per c e n t e n d m e m b e r s :

En 4 6 . 7 9 4 9 . 4 4 5 1 . 0 3 4 9 . 2 1 4 1 . 8 7 4 1 . 8 2 6 3 . 0 0 6 4 . 2 5 Fs 5 2 . 0 4 4 9 . 2 3 4 8 . 3 0 4 9 . 9 3 5 7 . 5 7 '56.51 3 6 . 0 7 3 4 . 5 4 Wo 1 .17 1 .33 0 . 6 7 0 . 8 6 0 . 5 6 1 .67 0 .93 1.21

104

grains. The FS (ferrosilite) amounts range from 38% to 58%, but such a variation appears to be related to chemical changes in the host rocks. Most of the orthopyroxenes can be termed ferrohypersthenes (Fig. 4) whilst those of BRl l are hypersthenes s.s. (Deer et al., 1978), reflecting the higher molar ratio MgO/(MgO + FeO t + MnO) of the host rock.

A few grains are zoned: relative to cores, the rims are generally depleted in Mg and enriched in Fe and Mn, which is similar to the garnet zoning pro- file. No systematic variations were observed between orthopyroxene 1 of the matrix and orthopyroxene 2 from the coronas around garnets (Table 5). Also the observed absence of significant composition differences between orthopyroxene 1 and ortho-

pyroxene 2 grains leads one to suspect that all the pyroxenes may have re-equilibrated during the retrograde history. All these orthopyroxenes have a metamorphic origin according to Rietmeijer's (1983) diagram.

Amphibole

Amphiboles of green, brownish-green and brown colour are primary constituents in the granulite- facies assemblage of the studied metabasites. In retrogressed rocks, they are accompanied by a second-generation amphibole, generally of bluish-green colour, which formed in the coronas at the expense of pyroxene and occasionally ilmenite (Fig. 2C).

T A B L E 6

Electron microprobe analyses of amphiboles

Sample OR4 SV96

(15) (30) (4) (6) Amp 2 Amp 2 Amp 2 Amp 2

(rim llm) (rim Cpx)

BRIO

(13) (63) (65) Amp 2 Amp 1 Amp 1

SiO2 (wt .%) 43 .56 43 .07 A1203 10.23 10.47 Fe203 4.61 3.29 FeO 13.65 14.59 MgO 10.27 9.70 MnO 0.22 0 .06 CaO 11.52 11.51 K 2 0 1.31 1.42 N a 2 0 1.05 0.95 TiO 2 1.58 1.63 NiO 0 .00 0 .00 Cr203 0 .00 0 .03

Total 98.00 96.72

Cation numb ers on the

Si 6 .510 AI TM 1.490

AI V! 0 .311

Cr 0 .000 Ni 0 .000 Ti 0 .178 Fe 3+ 0 .518 Mg 2 .288 Fe 2+ 1.705

Fe 2+ 0.001 Mn 0 .028 Ca 1.845 Na 0 .126

Na 0 .178 K 0 .250

]~A 0 .428

43 .74 46 .73 43 .84 42 .17 43 .59 11.26 8.57 11.33 13.22 11.55

2.55 5.94 3.05 3.47 3.28 16.92 13.73 16.40 9 .77 9 .28

8.89 10.32 8.67 12.07 12.93 0.25 0 .14 0.27 0.16 0.12

11.91 11.43 11.44 11.28 11.37 1.01 0.51 0.82 1.10 0.86 1.17 0 .84 1.14 1.91 1.78 1.38 1.08 1.64 2.26 1.75 0 .07 0.02 0 .10 0 .00 0 .07 0 .00 0.06 0.07 0.07 0 .10

99 .15 99 .37 98.75 97.49 96.68

basis of 23 oxygens:

6 .529 6 .509 6 .828 6.525 6 .215 6 .439 1.471 1.491 1.172 1.475 1.784 1.561

0 .398 0 .493 0 .303 0.511 0 .509 0 .448 0 .004 0.O00 0 .007 0 .008 0 .008 0 .012 0 .000 0 . 017 0 .005 0 .024 0 .000 0 .0 1 7 0 .186 0 .154 0 .119 0 .184 0 .2 5 0 0 .194 0 .374 0 .285 0 .653 0.341 0.385 0 .364 2 .192 1.972 2 .247 1.923 2.651 2 .846 1.846 2 .089 1.666 2 .009 1.197 1.119

0 .004 0 .017 0 .012 0 .033 0 .008 0 .028 0 .008 0 .032 0 .017 0 .034 0 .020 0 .015 1.870 1.899 1.790 1.824 1.783 1.800 0 .118 0 .052 0.181 0 .109 0 .189 0 .157

0.161 0 .286 0 .057 0 .220 0 .357 0 .353 0 .275 0 .192 0 .095 0 .156 0 .207 0 .162

0 .436 0 .478 0 .152 0 .376 0 .564 0 .515

105

Secondary amphiboles have never been found to mantle green-brown primary amphiboles.

Electron microprobe analyses of both types are listed in Table 6. The cation numbers are presented in a recalculated way (O = 23, M = 13) in order to estimate the Fe a÷ amount, in following the recal- culation method of Holland and Richardson (1979). According to the terminology of Leake (1978) the primary amphiboles are ferroan-pargasitic hornblendes or ferroan pargasites while the second generation amphiboles are tschermakitic- or magnesio- or even ferro-hornblendes.

Furthermore the analyses show that the ratio Mg/(Fe 2÷ + Mg) = 0.57 is similar in clinopyroxene (anal. No. 7) and its surrounding hornblende (anal. No. 8). Furthermore the lower amount of Ti of this secondary hornblende can be considered (Sen and Oliver, 1981) as a simple consequence of the low Ti content of the reactants (clinopyroxene here, and orthopyroxene 1 or plagioclase 1) that would produce this hornblende.

In spite of the low AIVI contents (Raase, 1974) the high, though variable, Ti or K contents allow us to compare these hornblendes with other granulite- facies amphiboles (Binns, 1965; Coolen, 1980; Sills, 1983).

Plagioclase

In all the investigated granulite-metabasites, plagioclase 1 is a major constituent, sometimes ex- ceeding 25% in volume (Table 2). Its An-Ab-Or molar ratios, calculated from electron microprobe analyses, are listed in Table 7. They are calcic plagioclases ranging from An42 in a quartz-bearing plagio- pyrigarnite (SA18) to An84 in a pyribolite (BRI1). Zoning of plagioclase 1 is not common but in some samples the grains are weakly but normally zoned although the An contents do not vary more than 2 mole%.

The fine-grained secondary plagioclase 2 of the coronas, inside (Fig. 2D), around (Fig. 2A), or in association with Opx 2 instead (Fig. 2C), of garnets have a composition over 85 mole% An. This high An content is in agreement with a generalized de- stabilization of the garnet-clinopyroxene pair according to the reactions of Manna and Sen (1974) where An-plag. 2 > An-plag. 1 (see Section "Mineral reactions related to the retrogressive evolution").

TABLE 7

Compositions of plagioclases calculated from electron mic.o- probe analyses

(a) Primary plagioclase 1

Sample OR4 SAI8 SV96 BR10 BRI1 (36) (132) (9) (67) (257)

An (mole%) 82.5 42.7 78.7 73.8 82.1 Ab 17.4 55.9 21.1 26.1 17.4 Or 0.1 1.4 0.2 0.1 0.5

(b) Secondary plagioclase 2

OR4 SA18 SV96 BR10 BRl l (6) (114) (43) (63) (251)

An(mole%) 86.7 86.7 86.0 93.6 92.2 Ab 13.3 12.9 13.4 6.4 7.5 Or 0.0 0.4 0.6 0.0 0.5

Fe-Ti oxides

The predominant primary oxide phase in the granulite-metabasites of Monts du Lyonnais is ilmenite accompanied by minor amounts of rutile and more rarely magnetite. The distribution of Fe-Ti oxides seems to show a consistent trend. Indeed the rutile-bearing assemblages appear to be the most granulitic while the magnetite-bearing ones occur during the retrogression. Sometimes ilmenite destabilized becomes surrounded by green hornblende (Fig. 2C). Representative microprobe analyses of optically pure phase are listed in Table 8.

Except for a low decrease of Mg ratio, ilmenite SV96 does not suggest significant zoning, particularly when considering the high sum of core.

According to Carmichael's (1967) and Rumble's (1973) methods, the FeO/Fe203 ratio was calculated by combining equivalent amounts of Mg, Mn, and Fe with Ti to form the geikielite, pyrophanite and ilmenite molecules (Table 8). After this calculation the excess in Fe was used to form the hematite molecule. The calculated Fe2Oa content ranges from 0.71 to 4.95 wt.%. Furthermore, these ilmenites have low MnO content, comparable to ilmenites described in granulite-facies rocks (Grew, 1980).

106

T A B L E 8

E l e c t r o n m i c r o p r o b e ana lyses o f Fe-Ti o x i d e s

I lmen i t e Magnetite

S a m p l e S A 1 8 S V 9 6 S A 1 8 ( 1 1 9 ) (1) (2) (3)

co re r im

SiO 2 (wt .%) 0 . 1 6 0 . 0 0 0 .05 0 . 0 0 AI203 0 . 0 0 0 . 0 0 0 . 0 8 0 .06 F e 2 0 3 0 .71 2 .53 2 .88 6 8 . 6 2 FeO 4 4 . 6 0 4 5 . 6 5 4 4 . 5 9 3 0 . 6 3 MgO 0 .95 0 . 6 7 0 . 5 4 0 .05 MnO 0 . 3 7 0 .41 0 . 4 4 0 . 1 0 C a O 0 . 0 5 0 . 0 8 0 .20 0 . 1 0 N a 2 0 0 . 0 3 0 .01 0 . 0 8 0 . 0 3 TiO 2 5 1 . 9 0 5 2 . 5 3 5 1 . 0 9 0 .05 NiO 0 . 0 0 0 . 0 0 0 .02 0 . 0 0 C r 2 0 3 0 . 0 0 0 .02 0 .01 0 . 0 0

T o t a l 9 8 . 7 7 1 0 1 . 8 8 9 9 . 9 6 9 9 . 6 4

Cation numbers on the basis o f 6 oxygens for ilmenite and 32 oxygens for magnetite:

Si 0 . 0 0 8 0 . 0 0 0 0 . 0 0 3 0 . 0 0 0 A1 0 . 0 0 0 0 . 0 0 0 0 . 0 0 5 0 . 0 2 2 Fe 3+ 0 . 0 2 7 0 . 0 9 4 0 . 1 0 9 15 .941 Fe 2+ 1 .892 1 . 8 8 5 1 . 8 7 8 7 . 9 1 6 Mg 0 . 0 7 2 0 . 0 4 9 0 . 0 4 0 0 . 0 2 3 Mn 0 . 0 1 6 0 . 0 1 7 0 . 0 1 8 0 . 0 2 6 Ca 0 . 0 0 3 0 . 0 0 4 0 . 0 1 0 0 . 0 3 3 Na 0 . 0 0 3 0 . 0 0 1 0 . 0 0 6 0 . 0 1 8 Ti 1 . 9 7 9 1 .951 1 .935 0 . 0 1 2 Ni 0 . 0 0 0 0 . 0 0 0 0 . 0 0 1 0 . 0 0 0 Cr 0 . 0 0 0 0 . 0 0 1 0 . 0 0 0 0 . 0 0 0

T o t a l 4 . 0 0 0 4 . 0 0 2 4 . 0 0 5 2 3 . 9 9 1

End members (neglecffng Si02, Al203 and CaO):

Ilmenite s.s. 94 .3 92 .2 9 1 . 8 P y r o p b a n i t e 0 .8 0 .8 0 .9 Geik ie l i te 3.6 2 .4 2 .0 H e m a t i t e 1.3 4 .6 5.3

Metamorphic evolution of the Monts du Lyonnais metabasites

Phase relations during the "early" phase of metamorphism

As silica is present in excess (quartz is a constituent of all granulite-metabasites of Monts du Lyonnais; Table 2), the ACF diagram can be used (Winkler, 1979). Garnet, clinopyroxene, orthopyroxene t and plagioclase 1 (and even sometimes brown or green hornblende 1) are in mutual contact with each other and are considered in equilibrium.

The variations of mineral compositions expressed by the ACF projection are rather small. The phase assemblage is typical of the granulite facies (Winkler, 1979).

Mineral reactions related to the retrogressive evolution

The replacement of garnet by vermicular orthopyroxene 2-plagioclase 2 is common in the Monts du Lyonnais granulite-metabasites (Fig. 2A) and results from the breakdown of the garnet-clinopyroxene association. According to Manna and Sen (1974) and McLelland and Whitney (1977), this pair reacts to form An-rich plagioclase 2, orthopyroxene 2 and Fe-Ti oxides. Furthermore, as plagioclase 2 is poor in albite and as clinopyroxene is poor in jadeite, plagioclase 1 is obviously also involved in the reaction. The following equation (1), which is balanced for all elements with a negligible 0.1 cation difference, can be written:

6Fe 1.7oCa0.86Mgo.44Si3A12012 +

(garnet)

Feo.4sMg0.64Ca0.aaNa0.oaSi206 +

(clinopyroxene)

2Ca0.48Nao.s2Si2.6All.408 + 4TIO2

(plagio 1 - andesine) (rutile)

4Fel.9aMgo.o7TiO3 + 3Fe0.98 Mgo.gaCa0 .o4Si206

( i lm en i t e ) (orthopyroxene 2)

+ 8Cao.86Nao.a4A11.9Si2.108 + 2SIO2 (1)

(plagio 2 - bytownite) (quartz)

This reaction resulting from increasing temperatures and/or decreasing pressures was frequently used to distinguish the high-pressure field from the low-pressure field of the granulite facies (De Waard, 1965). Similar reaction coronas between garnet and clinopyroxene have been described in other granulite-facies terranes (Kornprobst, 1971, 1977; Lasnier, 1977; Loomis, 1977; Coolen, 1980; An- drieux, 1982, etc.).

Although in most observations this first reaction is centrifugal (coronas around garnet; Fig. 2A), it is also not unusual to see a centripetal growth (coronas inside garnet; Fig. 2D). When destabiliza- tion of garnet occurs at the core rather than in sur-

107

rounding coronas, the intervention of clinopyroxene cannot be argued, and the reaction (2) is balanced as follows:

3Fel.3oCal.o2Mg0.68SiaA12 O12 +

(garnet)

Cao.4sNao.s2Si2.6All.4Oa + TiO2 + SiO2

(andesine) (rutile) (quartz)

2Feo.9aMgo.98Cao.04Si206

(orthopyroxene 2)

+ 4Cao.87Nao.13Si2.1A11.908 +

(bytownite)

Fe 1.93Mgo.oTTiO3 (2)

(ilmenite)

In the last stages, retrogression is marked by hydratation of the metabasite-granulites to produce amphibole and sometimes biotite. According to Sills (1983) orthopyroxene is at first replaced by a pale fibrous cummingtonite oriented parallel to the orthopyroxene cleavage. As retrogression con- tinues, clinopyroxene is rimmed by a green hornblende and sometimes by hornblende-quartz intergrowths, forming corona textures. These coronas are always concentrated at the core and absent from the rim of replaced pyroxene and increase in size towards these cores, similar to the descrip- tions of Beach (1974). Elsewhere it is not possible

to distinguish hornblende replacing orthopyroxene from that replacing clinopyroxene.

Many hydratation reactions producing amphibole were written especially by Himmelberg and Phinney (1967), Kerr (1979), Okrusch et al. (1979), Beach (1980), Corbett and Phillips (1981), Vielzeuf (1982) or Sills (1983), but these reactions are not chemical- ly balanced.

Evolution of metamorphic conditions and geological consequences

Mineral geo thermometry

Different types of geothermometers are avail- able to estimate the temperatures of metamorphism, based on the composition of coexisting minerals. In consideration of the critical study of Bohlen and Essene (1979), the reviews of Ferry (1982) and following Rollinson (1981) only the usual thermometers in granulites have been used here. Yet there are some variations in temperature both between different thermometers applied to the same mineral pair and on the same thermometer between different mineral pairs. These variations are a complex function of differences in calibration and thermodynamic formulation, of the speed at which different cations move between a mineral pair (Loomis, 1977; Nishiyama, 1983) and of real temperature differences preserved by incomplete re-equilibra-

TABLE 9

Temperature estimates (°C) for the metabasite granulites

Sample (1) (2) (3) (4) (S) (6) (7) (8) (9) (10) (11) (12)

OR 4 (rim) 759 806 844 784 811 809 715 675 725

SA18 (Opx 1) 815 735 (Opx 2) 722

SV96 (Opx 1) 772 806 633 641 640

BR10 (Core) 803 807 860 (rim) 801 804 825 790 780

BRI 1 (core) 867 (rim) 720 703 (Opx 2) 713

(1) T (°C) Gar-Cpx (Ellis and Green, 1979); (2) T(°C) Gar-Cpx (Saxena, 1979); (3) T(°C) Gar-Cpx (Dahl, 1980); (4) T (°C) Cpx-Opx (Wood and Banno, 1973); (5) T (°C) Cpx-Opx (Wells, 1977); (6) T (o C) Gar-Opx (Dahl, 1980); (7) T (°C) llm-Cpx (Bishop, 1980); (8) T (o C) llm-Opx (Bishop, 1980); (9) T (°C) Hor-Pla (Spear, 1981); (10) T (°C) Gar-Bio (Thompson, 1976); (11) T (°C) Gar-Bio (Holdaway and Lee, 1977); (12) T (o C) Gar-Bio (Ferry and Spear, 1978).

108

tion during cooling (Loomis, 1979). Another possible cause of the observed temperature variation is the inadequate estimation of Fe 3÷ (Okrusch et al., 1979). So, according to Griffin et al. (1979), cal- culations might be biased to higher K d values (lower temperatures) owing to a possible overestimate of Fe 3÷ in the clinopyroxenes. In consideration of these problems each result cannot be used as a quan- titative thermometer but only as a "link in the chain" to be assessed with the values obtained from other thermometers.

Table 9 lists the highest estimate temperatures. They allow us to propose temperatures of 860 - 870°C for the thermal peak of metamorphism, on the base of garnet-pyroxene equilibria (grain core analyses). However, these temperatures only rely on the calibration method of Dahl (1980) which seems to give always high results (Table 9) in comparison with other calibrations. The average temperatures based on garnet-clinopyroxene-orthopyroxene 1 equilibria (grain rim analyses) range from 785 ° to 815°C.

Temperatures of 710-720°C seem to mark the growth of vermicular orthopyroxene 2-plagioclase 2, whereas the appearance of hornblende 2, ilmenite and biotite during the later period of retrogression indicates temperatures between 640 ° and 735°C.

Mineral geobarometry

Recently, geobarometers based on the assemblages garnet-orthopyroxene (clinopyroxene)-plagioclase- quartz (Wells, 1979; Newton and Perkins, 1982) and clinopyroxene-plagioclase-quartz (Ellis, 1980) have been calibrated starting from experimental measurements. These barometers are based on large volume change reactions and are therefore well suited for pressure measurement. Furthermore they appear more sensitive than geobarometers based on only one mineral, such as the solubility of A1 in orthopyroxene (Wood, 1974) or the jadeite content of clinopyroxene (Currie and Curtis, 1976). Indeed, in the first case pressure estimates are very sensitive to the assumed distribution of A1 between the tetrahedral and octahedral (M1) sites in orthopyroxene. If A1 is assumed to be distributed between the sites such that AIlV + Si = 2, then the anaount of AI VI is considerably less, or even null, than that of AIIV (Table 5). So the application of the Wood's (1974) barometer seems doubtful even

if Wood tried to circumvent this problem by divid- ing A1 equally between the M1 and tetrahedral sites but this appears only an artifice. In fact, Rollinson (1981) has pointed out that a variation in T of 100°C can imply a pressure variation up to 4 kbar.

Similarly, the results of Coolen (1980) suggest that the application of the jadeite content of clinopyroxene in granulite rocks as geobarometer is rather limited because of the low jadeite content of these clinopyroxenes (Table 4).

From this short discussion it follows that only the garnet-clinopyroxene or orthopyroxene-quattz- plagioclase and clinopyroxene-plagioclase geobarometers based on the anorthite breakdown reaction are believed useful to be used here. Pressures estimated for the metabasite granulites of Monts du Lyonnais are listed in Table 10.

The clinopyroxene barometer indicates pressures of 8.0 and 9.8 kbar for the primary assemblage while the use o f the orthopyroxene barometer for the same assemblage produces a range from 7.8 to 9.4 kbar for the peak of metamorphism. The pressures estimated from rims equilibria fall between 7.2 and 8.0 kbar whereas the corona assemblages developed at pressures between 5.2 and 6.4 kbar. These estima- tions are consistent with the results from other areas (Wagner and Crawford, 1975; Stormer and Whitney, 1977; Behr, 1978; Coolen, 1980; Viel-

TABLE 10

Pressure estimates (in kilobars) for the metabasite granulites

Sample Geother- (1) (2) (3) (4) mometer (°c)

OR4 800 9.8 9.8 (rim) 800 7.2 8.0 (Opx 2) 800 5.8 5.5

SA18 (Opx 1) 800 9.4 8.8 (Opx2) 700 5.2 5.2

SV96 (core) 800 9.0 (rim) 800 6.3

BRIO 800 6.9 9.9

BR11 (core) 850 8.3 7.8 (rim) 700 7.8 7.0 (Opx 2) 700 6.2 6.4

(1) = P (kbar) Gar-Cpx-Pla-Qtz (Newton and Perkins, 1982); (2) = P (kbar) Gar-Opx-Pla-Qtz (Newton and Perkins, 1982); (3) = P (kbar) Gar-Opx-Pla-Qtz (Wells, 1979); (4) = P (kbar) Cpx-Pla-Qtz (Ellis, 1980).

109

zeuf, 1980; Grew, 1981; etc.). Fortunately, these barometers have a very small temperature dependence. The appearance of hydrated minerals during retrogression into the amphibolite facies took place in the pressure range of 5.2-5.5 kbar at 700°C, in agreement with the P-T conditions estimated from the associated metapelites (Dufour, 1982).

Estimated P-T trajectory

The most prominent feature revealed by the mineral geothermometry and geobarometry in the Monts du Lyonnais is a trend of decreasing both temperature and pressure (Fig. 5). Similar patterns from granulite-metabasites have been con-

11

1 0

kbar

/ f /

/

/ / / "

/ / /

.... # ' J

/ /

/ /

3

2 i

i

J , R I \

\\ \

R \ \

I I 6 0 0 7 0 0 B O O g O 0

Fig. 5. P-T conditions estimated by thermodynamic data in Monts du Lyonnais metabasites. A - core mineral, "peak of metamorphism"; B - rim mineral compositions; C - corona formation; D - apparition of hydrated minerals (Arrows - retrograde evolution in time). (1) - lower limit for high-P granulite facies (Green and Ringwood, 1967); (2) - lower limit for intermediate-P granulite facies (Green and Ringwood, 1967); (3) - Cpx + Gar + Qtz --+ Opx 2 +

Pla 2 with XFe = 0.51 (Green and Ringwood, 1972); (R) - aluminosilicate (Richardson et al., 1969).

T °C P

structed for other areas in the Massif Central especially by Lasnier (1977) in Haut Allier and Santallier (1981) in Limousin.

In comparison with the model of "post-colli- sional" (subducted or thickened complexes) metamorphism (Albarede, 1976; Couturie and Korn- probst, 1977; England and Richardson, 1977; Ri- chardson and England, 1979; Oxburgh and England, 1980), there is no evidence for any temperature increase during the decompressional path. Similar post high P paths have been evidenced in various geological environments (Kornprobst et al., 1979; Vielzeuf, 1980; Krogh, 1982).

Such retromorphic evolutions may reflect a com- petition between thermal and mechanical exhumation (uplift and/or tectonic transport) before any thermal re-equilibration (thermal relaxation) (see also discussions in Thompson, 1981).

Modellised geological sequence for Monts du Lyon- nais metabasites deduced from P, T,t path

The Monts du Lyonnais region consists of a volcano-sedimentary series with intrusions of granite magmas, which has been affected by a high-grade metamorphic event. The following schematic sequence of geological development can be proposed from the study of metabasites.

The chemistry of these metabasites (Dufour, 1982) tends to support a volcanic origin with deposi- tion in a continental crust at 500 Ma (Gebauer and Griinenfelder, 1982). This period is also marked by a concomitant bimodal magmatism, with a peak at 497 Ma (orthogranulites of St. Andre-h-COte; Duthou et al., 1981) followed by some alkaline magmatic intrusions at 455 Ma (metaleucogranite of Cozan). This acid-basic igneous association may be considered a typical feature of tensional tectonic regimes. All these rocks were later affected by the high-grade metamorphism. During this event the mineral associations were completely re-equilibrated

- all primary igneous compositions have disap- peared - so that the cores of minerals could represent "metamorphic peak" compositions while rims evi- dently formed during the retrogression (Fig. 5). Thus the initial conditions of granulite-facies metamorphism in the Monts du Lyonnais can be estimated at 800-870°C and 7.8-9.9 kbar with a thermal peak around 860-870°C. This was followed by retrogression (Fig. 5 - main path) with both decreas-

110

ing temperatures and pressures. The conditions of 785-815°C and 7.2-8.0 kbar suggested by the data for mineral rims reflect a chemical re-adjustment of these minerals to the new physical conditions of metamorphism (intermediate-pressure granulite) im- posed by a retrogressive event concomitant to the regional uplift. This event continued with the development of vermicular coronas at 710-725°C and 7.2-8.0 kbar while complete retrogression begins under the physical conditions of 640-785°C and 5.2-5.5 kbar.

The age of the main phase of granulite facies metamorphism in the European basement still re- mains doubtful. Four possibilities have been suggested:

(1) upper Ordovician - 450 Ma - as in Saxony (J~iger and Watznauer, 1969), in Austria (Arnold and Scharbert, 1973) and in Lyonnais (Dufour, 1982);

(2) Silurian-Ordovician boundary - 432 Ma - as in Haut Allier (Ducrot et al., 1983);

(3) lower Silurian - 415 M a - as in Marvejols (Pin and Lancelot, 1982);

(4) and, even Carboniferous - 320 Ma - as in Limousin (Gebauer et al., 1981).

This last young age seems to be inconsistent with the Monts du Lyonnais evidence because late granitic intrusions without metamorphism intruded the granulite series at 332 Ma (Gay et al., 1981), during tectonic movements which postdate the main phase of metamorphism.

In the Monts du Lyonnais, as in Haut Allier, the granulite rocks seem to be already the consequence of an early event characterized by eclogitic assemblages. This high-P event may be regarded as a record of subduction (Kornprobst et al., 1980) which probably happened in the Monts du Lyonnais at the end of Cambrian (Gebauer and Grtinenfelder, 1982). In the upper Ordovician (Behr et al., 1980) the post eclogitic uplift would mark the development of the granulite-facies metamorphism. This model seems consistent first with the later metamorphic evolution described by Krogh (1982) in west Norway, second with the views of Pin and Vielzeuf (1983) concerning the Hercynian high-P granulites - their group I granulites - of which the metabasites of Monts du Lyonnais form a part.

Conclusion

In the Monts du Lyonnais, the conditions of pressure and temperature prevailing during the formation: (a) of the garnet, clinopyroxene, orthopyroxene 1 and plagioclase 1, assemblages; (b) of the orthopyroxene 2-plagioclase 2 coronas; and (c) of the hydrated assemblages correspond to the uplift accompanied by retrogressive metamorphism.

Pressure and temperature estimates based essentially on the pyroxene-garnet equilibria show that the peak of metamorphism occurred at 860°C and 10 kbar. This implies a crustal thickness of about 30 km. These results are in agreement with P-T estimates for other areas in the Variscan belt and with the interpretation of Burg et al. (1985).

Furthermore, this study tends to confirm the affiliation of the Monts du Lyonnais region to the diagonal axis which extends from the Moldanubian zone to Spain through central Europe, where granulites, eclogites and garnet peridotites are found. For many authors this axis may be regarded to indicate the vicinity of an ancient suture zone where a continental collision took place after a subduction.

Acknowledgements

The author would like to express his sincere thanks to W.L. Griffin and to E.R. Neumann for their critical review of the manuscript. He also appreciates J.M. Caron, M. Chenevoy, J.M. Lardeaux, B. Lasnier and D. Vielzeuf for their encouragements, numerous helpful discussions and constructive criticism on earlier versions of the text. He is in- debted to C. Uberty for typing the manuscript.

Financial assistance support for this research was provided by the E.R.A. 805 (CNRS) in the University of Lyon I.

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granulite facies metamorphism and retrogressive evolution of the monts du lyonnais metabasites...

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