Low-Temperature, Low-Pressure Metamorphism of Mn-rich Rocks in the Lienne Syncline, Venn

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 3 IttGES 767-783 1996

THOMAS THEYE1*, WERNER SCHREYER* AND ANDRE-MATHIEU FRANSOLET3

'INSTITUT FUR GEOWISSENSCHAFTEN, TECHNISCHE UNIVERSITAT, POSTFACH 3JJ9, D-JSOH BRAUNSCHWEIG. GERMANY'INSTITUT FOR MINERALOGIE, RUHR-UNIVERSITAT, D-44780 BOCHUM, GERMANY'LABORATOIRE DE MINERALOGIE, BATIMENT B18, UNIVERSITE DE LIEGE, ALLEE DU 6 AOUT, B-4000 5ART TILMAN (LIEGE), BELGIUM

Low-Temperature, Low-PressureMetamorphism of Mn-rich Rocks in theLienne Syncline, Venn-Stavelot Massif(Belgian Ardennes), and the Role ofCarpholite

Low-grade Mn-rich metamorphic rocks of the Lienne syncline(western part of the Venn—Stavelot Massif, Belgian Ardennes)have been re-examined to evaluate the petrological significanceof carpholite proper, Mn2+Al2[Si206J(0H)4. MetamorphicP-T'conditions of these rocks are estimated to be ~300°Q 1-2kbar, which is in accordance with the exclusive occurrence ofcarpholite in low-P rocks such as hydrothermal environmentselsewhere. Carpholite of the Lienne syncline exclusively occurs inquartz-rich segregations. Its composition is close to end^member.Thermodynamic calculations confirm that carpholite is a stable

phase at low-pressure—low-temperature conditions, in contrast toferro- and magnesiocarpholite, which are high-pressureminerals. No information is available on the high-P behaviourof carpholite. The occurrence of carpholite is partly closelyassociated with spessartine-bearing country rocks, or carpholiteis altered to assemblages with spessartine, sudoite, chlorite,muscovite and paragonite. Spessartine in these rocks containsminor amounts of hydrogarnet component {(H/4)/[Si+ (H/4)] = 0-03-0-06}. The presence of carpholite-spessartineassemblages in these low-P rocks is in contrast to high-pressuremetamorphic rocks from other areas, where parageneses such asfem/magnesiocarpholite—chloritoid or magnesiocarpholite-chlorite-kyanite occur. The appearance of carpholite—garnetassemblages in low-P Mn-rich rocks can be explained by con-trasting phase relations because of a high Mn—Mg partitioncoefficient between the minerals under consideration. In rhodo-chrosite-bearing veins in the Lienne syncline, nearly completereplacement of carpholite by spessartine and chlorite is due tothe continuous reaction carpholite + rhodochrosite + quartz =

spessartine + chlorite + H2O + CO 2, which defines a very lowXco, in the temperature range under consideration. It is sug-gested that spessartine (possibly containing some hydrogarnetcomponent), during prograde metamorphism at low pressure,becomes stable at a temperature of ~300°C

KEY WORDS: carpholiU; sptssartine; sudoilt; Vmt-Stanelot Massif;Ardemes

INTRODUCTIONThe most common members of the carpholite groupcan be represented by the general formula(Fc2+

>Mn2+,Mg) (Al,Fe3+)2[Si2O6] (OH,F)4. Mineralnames so far defined are for the end-members con-taining Mn, Fe and Mg as divalent cations, i.e. car-pholite, ferrocarpholite and magnesiocarpholite (e.g.De Roever, 1951; Goffe et al., 1973; Mottana &Schreyer, 1977; Viswanathan & Seidel, 1979).Fluorine occurs as a minor but significant constituentin natural carpholite group minerals. Ferric iron isgenerally very low. A non-stoichiometric type ofcarpholite containing K, Li and F was recentlydescribed by Ghose et al. (1989).

The members of the carpholite group occur inhydrothermal or low-grade metamorphic environ-ments rich in aluminium such as metapelites, meta-bauxites and related vein mineralizations. Recentfindings of magnesio- and ferrocarpholite attracted

•Comaponding author. Hue x49|f531/3918131.e-mail; [email protected] © Oxford Univenity Preo 1996

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JOURNAL OF PETROLOGY VOLUME 37 NUMBER 3 JUNE 1996

particular attention because they solely occur inhigh-pressure-low-temperature metamorphic rocksformed at pressure >7 kbar at 30O-450°C. Thesetwo minerals can now be regarded as indicators forsuch a type of metamorphism (Seidel, 1978; Goffe,1982; Chopin & Schreyer, 1983; Goffe & Chopin,1986; Schreyer, 1988; Theye et al., 1992; Vidal et al,1992). In contrast, the long known manganese end-member of the carpholite group is only known inlow-P-low-7" metamorphic and hydrothermalenvironments. Carpholite was first described byWerner in 1817 [cited by Dana (1914)] in hydro-thermal veins of a tin mine in Schlaggenwald, nowSlavkov in the Czech Republic, where it is associatedwith rhodochrosite and fluorite. Examples of occur-rences of carpholite in low-grade metamorphic Mn-rich pelitcs include, for example, the Wippra Zone ofthe Harz, Germany (Milgge, 1918) and the Venn-Stavclot Massif in the Ardennes, Belgium (DeKoninck, 1879; Fransolet, 1972). Mottana &Schreyer (1977) compiled the then known occur-rences of the carpholite group.

A recent re-investigation of low-grade meta-morphic rocks in the Wippra Zone of the Harz(Theye & Siedel, 1993) demonstrated that mineralparageneses involving carpholite are well suited for acharacterization of metamorphic P~T conditions. Inthis paper, we are presenting new data on car-pholite-bearing rocks from the Lienne syncline in thewestern part of the Venn-Stavelot Massif, withemphasis on their phase relationships, geothermo-barometry and petrological significance. These dataare compared with data on Mn-rich rocks from otherparts of the Venn—Stavelot Massif and from theWippra Zone. Particular attention is also paid to therelation between carpholite and spessartine, and tothe appearance of spessartine during progrademetamorphism.

GEOLOGICAL SETTING OFTHE VENN-STAVELOTMASSIFThe Venn—Stavelot Massif of the Belgian Ardennesrepresents a large block of Cambro-Ordovicianmetasediments belonging to the Rhenohercynian ofthe Variscan orogenic belt. From overlyingDevonian and Carboniferous rocks, the Venn—Sta-velot Massif is separated by an angular unconformity[for a recent outline of the regional geology, seeFielitz (1992)]. The age of major deformation andmetamorphism in the Venn—Stavelot Massif isproved to be Upper Carboniferous although alreadyin the Caledonian period deformation took place

(Schreyer, 1975; Kramm, 1982; Geukens, 1984;Kramm et al., 1985a,A).

Metamorphism of the Venn-Stavclot Massif is ofvery low to low-grade type in the sense of Winkler(1979). The highest-grade rocks are present in theSE portion of the massif (region of Recht-Ottre—Lierneux). Kramm (1982) and Kramm et al. (19854)estimated 360-420°C at 2 kbar, based on the pre-sence of andalusite, chloritoid and spessartine as wellas on white K-mica composition. A lower tem-perature seems to hold, however, for the Liennesyncline in the western part of the massif (Kramm,1982), to be discussed here.

Mineralogically, manganese-rich metasediments ofthe Upper Salmian (middle Ordovician) are of par-ticular interest. In the region of Recht—Ottre—Lierneux, these rocks contain various assemblagesinvolving silicate minerals such as spessartine, anda-lusite-kanonaitc ('viridine'), sudoite, Fe—Mn-richmembers of the chloritoid group, and also raremineral species such as ardennite and davreuxite[Kramm, 1979; Theye & Fransolet, 1994; Pasero etal., 1994; for a compilation see Melon et al. (1976)and Fransolet et al. (1977, 1984)]. Carpholite onlyoccurs in the lower-grade rocks of the Lienne syn-cline.

In contrast, in the Mn-poor metasediments of theunderlying Lower Salmian (lower Ordovician), noP-T indicative mineral assemblages are known.

PETROGRAPHY OF THELIENNE SYNCLINE ROCKSThe petrographic description is based on —100 thinsections of rocks from the Lienne syncline. Theobserved mineral assemblages are compiled in Table1. Unless otherwise indicated, all descriptions referto rocks of the Upper Salmian in the Lienne syn-cline.

Metapelites and metapsammitesThe majority of rocks consist of green or red meta-pelites and metapsammites. Particularly in meta-psammites, detrital relics such as quartz grains andmuscovite flakes are often well preserved. Altern-ating layers distinguished by their grain size and bythe amount of haematite reflect sedimentary beddingthat is preserved on a millimetre to centimetre size.

A ubiquitous metamorphic foliation S\, which, inmost cases, appears parallel to sedimentary bedding,is defined by parallel alignment of phyllosilicates andelongated quartz grains. Occasionally, a crenulationcleavage S% oblique to S\ is developed. Quartz-richsegregations are parallel to or crosscut the foliation.

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THEYE ttd. CARPHOLITE IN Mt»-RICH ROCKS

Table 1: Mineral assemblages ofSalmian

(Ordovician) rocks in the Lienne syncline (western

part of the Venn—Stavelot Massif)

Upper Satnkn

Chlortioid tchistt

Qtz-M»-Chl-Cld

Qtz-Ms-Chl-CId-HaBm

Red schists

Qtz-Ms-Pg-Chl-Hiem

Qte-Ms-Pg-Chl-Sud-Haem

Qtz-M s-Pg-S ud-G rt-H I em

Spessartine-rich layers

Qtz-Ms-Pg-Cht-Sud-Grt

Carpholite- bearing segregations

Qtz-Cph-(Chl)-(SudMGrt)-(M«)-(Po)

Qtz-Cph-ftds

Manganese ore

Qtz-Ms-Rds-Kut-Haem

Qtz-ChWSrt-Rds-Kut

Qte-Cph-Rds-Chl-Grt-(Ms)-(Chl)-<Grt)

Lower Sahnhn

Qtz-Ms-Chl

Minerals replacing carpholite are shown in parentheses. Chi,chlorite; Cph, carpholite; Cld, chloritoid; Grt, gamet Haem,haematite; Kin, kaolinite; Kut, kutnahorite; Mgs, magnesite; Ms,muscovite; Pg, paragonite; Prl, pyrophytlite; Qtz, quartz; Rds,rhodochrosite; Sud, sudoite; Sps, spessartine.

A first type of metasediments is represented bygreen chloritoid schists that consist mineralogicallyof chloritoid (up to 0-1 mm in size), chlorite, mus-covite and quartz. They are intercalated with redschists containing fine-grained haematite in addition.The phyllosilicates arc oriented parallel to thefoliation, and chloritoid is grown syn- to post-tec-tonically with respect to the foliation Su but pre-£2.No microstructural indications of non-equilibriumbetween the minerals have been observed. Quartz-rich segregations present in this rock type containquartz, chlorite, haematite and small crystals ofrhodochrosite.

Besides chloritoid-bearing metapelites and meta-psammites, red to purple schists occur that aredevoid of chloritoid and rich in fine-grained hae-matite. Mesoscopically, these rocks are characterizedby smooth and shiny foliation planes owing to veryfine-grained, well-aligned sheet silicates. The mono-tonous mineral assemblage of this rock type isquartz, muscovite, paragonite, a mixed-layer

mineral muscovite-paragonite [see Frey (1969)],chlorite, haematite, and, in some cases, sudoite andspessartine (up to 0-03 mm in size).

Centimetre-sized white layers arc occasionallypresent as intercalations within the red schists devoidof chloritoid. They are rich in spessartine associatedwith some quartz, muscovite, paragonite, chloriteand sudoite. These spessartine-rich layers are similarto the so-called coticules that are mined as whet-stones in the Recht—Ottre-Lierneux region. Thepctrological significance of these rocks will be thesubject of a further paper.

In contrast to the manganese- and haematite-richschists of the Upper Salmian, metasediments of theLower Salmian generally do not contain specificmanganese minerals and arc poor in haematite.These rocks contain the monotonous mineral assem-blage quartz— muscovite-trioctahedral chlorite.

Carpholite-bearing segregationsCarpholite in the Lienne area has been found onlywithin quartz-rich segregations in the vicinity ofMeuville and Bicrlcux. The carpholite segregationsare, like the spessartine-rich layers, restricted to redor purple schists devoid of chloritoid. Minera-logically, the segregations consist of quartz, car-pholite (and its alteration products; see below) and,in one specimen, rhodochrosite. In some cases, car-pholite is in close contact with spessartine-rich layersof the country rocks without any microstructuralindications of non-equilibrium between theseminerals.

Needle-like crystals of carpholite (up to somemillimetres long) occur within quartz-rich segrega-tions which are, in most cases, developed parallel tothe foliation of the country rock. Often, the car-pholite needles are also orientated parallel tofoliation. After the main crystallization stage of car-pholite, many segregations arc affected by crack-sealdominated extension processes. Cracks are mainlysealed with quartz, occasionally accompanied byminor carpholite indicating that this extensionalready began in the stability field of carpholite.However, in some cases, partly related to thisextension, carpholite is replaced by various mixturesof chlorite, sudoite, paragonite and muscovite.Occasionally, small amounts of garnet are alsopresent in these alteration products (see Fig. 1). Allstages from completely fresh to completely pseudo-morphed carpholite have been observed. Fur-thermore, as products of superficial weathering,carpholite may be altered into black, X-ray amor-phous aggregates of Mn-oxides.

In one rock sample (22646), carpholite occurs

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(b)

-X

o

fo

w

Ofoo<

s

Fig. 1. Photomicrographs of carpholite-bearing segregations in Mn-ore. (a) Fibrous carpholite nearly completely replaced by garnet (dark, high relief) and chlorite. Thin fibres offresh carpholite are included in quartz (light matrix), (b) Pseudomorphs, partly with fibrous outlines, of chlorite (grey) and garnet (dark, high relief) after carpholite. Rhodochroute

showing corroded outlines is present in the upper left. The light matrix u quartz.

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THEYE etaL CARPHOLTTE IN Mn-RICH ROCKS

associated with rhodochrosite. These two mineralsseem to have been formed more or less con-temporaneously. Minor amounts of muscovite arcpresent, in this sample, along late shear planes.

Manganese oreA further rock type intercalated in red schists con-sists of layers of manganese ore which have beenmined until 1934 (Fourmarier & Calembert, 1942;Berger, 1965). They are conformable sedimentaryhorizons rich in manganese carbonate mineralsaccompanied by quartz, white mica and haematite.However, intensive veining which includesdeposition of quartz, haematite, rhodochrosite (up tosome millimetres in size), kutnahoritc, spessartincand Mn-rich chlorite also indicates significantmobilization of manganese during metamorphism.Rhodochrosite and kutnahoritc arc partly alteredinto Mn-oxidcs.

The vein mineralogy is of particular interest. Themicrostructure indicates simultaneous growth ofquartz and carbonate minerals. Chlorite andspessartine, however, partly seem to replace theMn-carbonates (Fig. lb). Besides coarse chlorite-spessartine aggregates, these two minerals (partlyaccompanied by small amounts of muscovite) alsooccur in the form of columnar or fibrous pseudo-morphs with rhombic cross-sections that are bestinterpreted as pseudomorphs after carpholite (Fig.la, b). This interpretation is corroborated by thepresence of tiny fibres of carpholite which are pre-served as inclusions in quartz.

MINERAL CHEMISTRYMethodsMineral analyses were obtained using a wavelength-dispersive Gameca electron microprobe. Measure-ment conditions were 15 kV, 10-15 nA, 10-20 scounting time. Standards were albite (Na), ortho-clase (K), olivine (Si, Mg), anorthite (Ga, Al),Fe2O3 (Fe), MnTiO3 (Mn.Ti), BaSO4 (Ba), ZnS(Zn), fluorite (F) and vanadinite (V), except forspecimens 89/40a, 89/30x, 89/40z and 22646. Forthese specimens, we used andraditc glass (Ca, Fe),pyropc (Si, Al, Mg), jadeite (Na), topaz (F), spes-sartine (Mn), K-silicate glass (K), Ba-glass (Ba),zincite (Zn), V-metal (V), and rutile (Ti). Someanalyses show low oxide totals, which may beattributed to the small grain size of many of theanalysed minerals.

CarpholiUCarpholite present in quartz-rich segregations of

haematite-rich metapelites forms yellow crystals withneedle-like habit (up to some millimetres long). Thechemical composition of carpholite (Table 2a) israther constant in terms of the divalent cations,which are mainly manganese and magnesium (Fig.2a). ATFe* = Fe2+/(Fe2° + Mn + Mg) is generally< 0 1 , A'M,, varies between 0-9 and 0-8, and XMg

varies between 0-1 and 0-2. Only one specimen (89/2c) tends to more Mg- and Fe-rich compositions.Fluorine varies between 0 and 0-5 wt%. Potassiumdocs not occur in detectable amounts. The presenceof significant vanadium in carpholite that coexistswith V-rich ardennitc is noteworthy [an analysis ofardennitc coexisting with carpholite of specimen 89/95 has been given by Pascro et al. (1994)]. In spe-cimen 89/85, carpholite contains up to 3-5 wt %V2O3 corresponding to 8 mol % of the trivalentcations. Carpholite not coexisting with ardennitcshows a low vanadium content (e.g. 89/81: 0-3 wt%V2O3 corresponding to X\ — 0-007). Fransolet(1972) analysed a V2O3 content of 0-06 wt % incarpholite from Meuville. The low trivalent totals ofsome carpholite analyses in Table 2a, for whichvanadium was not determined, may be due to a sig-nificant content of this clement.

Chloritoid

The mineral analyses show that chloritoid in meta-pelites and metapsammites is poor in magnesiumend-member (<8 mol %; Table 2b; Fig. 2b; andTheye & Fransolet, 1994). Ferrous iron and man-ganese are present in nearly equal amounts.

ChloriteIn contrast to chloritoid, coexisting chlorite is rela-tively poor in manganese. This chlorite containsferrous iron and magnesium as major divalentcations (Table 2b). The divalent cations are regu-larly partitioned between chlorite and coexistingchloritoid (Fig. 2b). Chlorite replacing carpholite inquartz-rich segregations is poorer in ferrous iron andricher in manganese (Table 2a; Fig. 2a). As alreadydescribed by Schreycr et al. (1986), two distinctpopulations of chlorite arc present in the manganeseore (Table 2c; Fig. 2c). A relatively Mn-poor, Mg-rich chlorite occurs replacing carpholite, whereasMn-rich chlorite is analysed in coarse chlorite-spessartine aggregates (Fig. 2c). Based on micro-structural features of coexisting chlorites with dif-ferent compositions, Schreycr et al. (1986) suggesteda miscibility gap in Mn—Mg-Fc chlorites and, fur-thermore, a chemical evolution of vein minerals fromMn rich to Mn poorer and Mg rich.

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JOURNAL OF PETROLOGY VOLUME 37 NUMBER S JUNE 1996

Table 2: Representative mineral analysesfromfour different rock types in the Lienne area; average ofn

analyses (n,a\, not determined; — , <0-05wt%)

wt%:

No.:

IT.

SK)2

T102

AljO,

VjO,

FeO

MnO

MgO

ZnO

CaO

BaO

NajO

K2O

F

E

Si

Ti

Al

V

Fe*"

Fe2*

Mn

M S

Zn

£

F

(a) CjrphoTrte-bearing tagragatians (mineral* replacing carphoGta in parentheses)

Cph

89/2b

6

36-98

0-22

27-20

2-84

0-91

19-13

1-27

0-10

—

—

—

—

0-10

87-76

8 (0 . OH)

2016

0009

1-797

0-128

0000

0043

0-908

0-106

0004

2-995

0-012

Cph

89/2c

6

38-08

0-23

30-76

n j .

2-65

14-00

3-14

0-08

—

—

—

—

0-30

89-14

2034

0-009

1-936

0-000

0018

0096

0-633

0-260

0003

2-944

0051

Cph

89/3

6

36-62

0-28

28-94

n.d

1-02

17-54

1-68

—

—

—

—

—

0-20

86-28

2044

0-012

1-904

0-000

0032

0015

0-829

0-140

0-000

2-932

0032

Cph

89/40a

8

36-82

0-15

28-98

nA.

1-66

17-93

1-64

—

—

—

—

—

0-20

86-38

2-007

0008

1-914

0-000

0-070

0-008

0851

0-137

0000

2-986

0-033

Cph

89/40x

3

37-90

0-17

30-60

n.d

0-97

17-39

1-94

0-04

—

—

—

—

0-60

89-51

2-038

0-007

1-940

0-000

0-016

0-027

0-792

0-166

0-002

2-940

0-083

Cph

89/402

2

36-25

0-18

29-40

0-24

1-21

18-30

1-49

—

—

—

—

—

0-30

87-37

2011

0O08

1-922

0-011

0-046

0-011

0-861

0-123

0-000

2-982

0-044

Cph

89/72c

3

37-63

0-41

29-10

n.d

2-89

16-50

1-72

0-10

—

—

—

—

0-30

88-66

2049

0-017

1-887

0-000

04)63

0-089

0-761

0-139

0-004

2-920

0-046

Cph

89/81

2

36-45

0-24

29-70

0-32

0-79

17-46

2-09

—

—

—

—

0-20

87-24

2-011

0010

1-931

0-014

0-031

0-008

0-816

0-172

0000

2-980

0-035

Cph

89/96

6

37-12

0-17

27-94

3-49

1-19

18-62

1-14

n.d

—

n.d

n.d

n.d

n.d

89-67

2-023

0007

1-795

0-163

0026

0028

0-860

0093

0000

2-962

0-000

Cph

22646

6

35-63

0-21

28-62

0-05

2-98

17-79

1-24

n.d.

—

—

—

—

86-53

2001

0-009

1-894

0002

0092

0048

0-846

0-104

0-000

2-996

0-000

Sudoite

The di/trioctahedral chlorite mineral sudoite hasbeen detected using the intensity ratio of its basal X-ray peaks (Fransolet & Schreyer, 1984):

R ~ -^(003)/1/(002) + (004)] •

The R value of 'normal' trioctahedral chlorite is inthe range 0-1-0-3, whereas R is 0-7-0-8 for sudoite(Fransolet & Schreycr, 1984; Theye et al., 1992).Intermediate R values can be reasonably related to

mixtures of sudoite and chlorite. Most of the meta-pelites and metapsammites in the Lienne synclinearea show R values of 0-12—0-27. However, inter-mediate R values of 0-35-0-64 indicating the pre-sence of sudoite in addition to trioctahedral chloritehave also been measured in some red schists of thisregion. Microprobe analyses of sudoite occurring asrock-forming mineral in red schists as well as ofsudoite replacing carpholite in quartz-rich segrega-tions show that this mineral is always close to themagnesium end-member (Tables 2a, 2d; Fig. 2a).

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THEYE ttd. CARPHOLITE IN Mn-RICH ROCKS

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s

i

i

i

0-10

o

o

94-7

793

-26

93-4

6

CO

3

5

o mi 8 8 9 8 8 3 8« n r « 6 < : - 6 6 6 6

S ^ O ^ r c o o o c o r ^ e o Q U

6 r 6 6 j i 6 6 n

T 9 n-CO O IDCM r- CM

en o CD

CM O ID

C M O K ) 6 6 6 6 6

0 0 0 0 0

8 8 8 9 90 0 0 0 0

r 8 V Q o a S g

c o 6 i 6 A 6 6 & & 6 6 N 6 6 6 6 6

c b 6 •» o ^ - o 6 o 6 c b o c w 6 6 6 6 6

c o o « O ^ O O O O O O C M 0 0 0 6 6

C O O * O i ^ O O O O

CM O O O O O

A 6 6

C M C 0 0 9 c n c M C D O C Mg | ^ flD ^3 ^J *|j* r[|l g ^ ^ j ^ j

6 « : - 6 6 6 6 6 6 M

o - r- 6

t? 8 8 3 2 3 S 2 ? 9• - 6 6 1 6 6 A CD o

co 6 ^

cb 6 ^

op ^^ C p C p CQ ^ J (^P Q p ^^C# O) C^J C 7 0 0 c^^ C ) t ^ j C^)

c o o - O i ^ - o o o o

§O P » N O J

w * *~ ^ Q ffiO M O Q O O

6 M

8 8 2 P co6 6 6 6 6

S g CM en

SCO

I;-16-1 I

> £ 5 S i5

I S T- c o o •<* o « - b o o o b o M

O O M O O O O O

CD IO Q M in»~ O cn r* 009 9 <- b 30 0 6 6 6

r fi US IOo o to r*

6 6 6 6 6

W 5 I WA A

5o

6

6

S9o

9o o

S co

i E

773

Dow




Table 2: continued

Wt9fc

No.:

rr.

SiO2

TI0 2

AI2O3

FoO

MnO

MgO

ZnO

CaO

BaO

Na2O

K20

Si

Al

I

Tl

Al

Fe1*

Fo2+

Mn

Mg

Zn

X

Ca

Ba

Na

K

L

(b) Chloritoid-bearing mstapelites and metapsammites [compositlom of chloritoid taken from Theye & Fransolet (1994)]

Cld

89/60

5

23-98

—

38-98

12-80

13-88

108

—

—

—

—

—

90-72

12(0, OH)

2035

0000

3-899

0060

0-848

0-997

0-139

0000

5-943

0000

0000

0000

0000

0000

Cld

89/63a

3

23-93

—

39-33

12-63

13-13

1-17

—

—

—

—

—

90-19

2-037

0000

3-946

0028

0-873

0-947

0-148

0-000

6-940

0-000

0000

0-000

0-000

0-000

Cld

89/70a

3

2403

—

39-33

1200

13-93

1-30

0 0 6

—

—

—

—

90-65

2038

0-000

3-928

0-036

0-816

1000

0-164

0-O04

5-947

0-000

0-000

0-000

0-000

0-000

Chi

89/60

4

24-29

0-05

24-65

1805

5-82

12-98

—

—

—

—

0-O8

85-92

14(0. OH)

2-582

1-418

4-000

0-OO4

1-670

1-605

0-525

2-067

0-000

5-861

0-000

O-OOO

0-000

0-010

0-010

Chi

89/63a

3

24-82

—

25-29

18-85

5-51

12-76

—

—

n.d.

—

—

87-23

2-697

1-403

4000

0000

1-714

1-650

0-488

1-991

0000

6-843

0000

0000

0000

0000

0000

Chi

89/70a

3

24-88

—

25-64

17-34

4-71

12-94

0-04

—

—

—

0-30

85-86

2-627

1-383

4000

0-000

1-796

1-628

0-419

2-029

0-003

5-773

0-000

0-000

0-000

0-040

0-040

Us

89/63a

1

4605

0-11

34-41

2-67

007

0-62

—

—

n.d.

0-94

9-20

9407

11 (0, OH)

3093

0-907

4000

0006

1-817

0-150

0004

0-062

0000

2039

0000

0-000

0-122

0-788

0-910

Ms

89/70a

1

45-60

—

30-83

4-40

0-63

1-70

—

—

—

0-39

1001

93-56

3-141

0-859

4-000

0-000

1-644

0-263

0-037

0-175

0-000

2-109

0-000

O-OOO

0-052

0-880

0-932

Rds

89/60

1

—

—

3-21

5002

1-90

—

1-45

006

—

—

66-64

1 cation

0054

0-858

0057

0000

0031

0000

1000

Garnet

Garnet with composition close to spessartine hasbeen observed as rock-forming mineral in metapel-ites and replacing carpholite in quartz-rich segrega-tions. For the divalent cations, only minor amountsof Ca (CaO: 1-5-4-6 wt%) and Fe (FeO: 01-11wt%) are present besides Mn. The MgO content isalways below 01 wt% (Tables 2a, 2c, 2d). Normal-izing the garnet analyses to 12 (O), the content of Si

per formula unit (p.f.u.) is significantly below 3-0and the ratio of M2 /M3+ deviates from 3/2.Moreover, the oxide totals are too low. This suggeststhat some hydrogarnet component is present ingarnet from the Lienne syncline. Recalculating thegarnet structural formulae to make Si + (H/4) = 3results in 009-0-17 (H/4) p.f.u. (corresponding to0-7-1-2 wt% H2O). After this procedure, the ratioof M"I+/M3 is satisfyingly close to 3/2. Furtherevidence for the presence of a hydrogarnet com-

774

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THFYEelaL CARPHOLTTE IN Mn-RICH ROCKS

Table 2: continued Table 2: continued

w t *

No.:

rr.

SiO2

T102

AljO,

v 2 o,FeO

MnO

MgO

ZnO

CaO

BaO

Na2O

K20

F

Z

(c) Segregation In manganese ore; carpholite replaced by

garnet and the Mn-poorer chlorite of the two is present only

as relics

Cph

93/7

2

35-73

0-O5

29-53

006

0-63

19-29

1-36

004

—

0-10

86-79

Grt

93/7

3

34-60

007

19-74

0-08

0-06

3906

000

0-12

3-33

0-03

—

—

97-08

Ms

93/7

2

47-29

0-01

32-90

0-06

2-44

0-47

100

0-07

0-09

0-29

0-28

9-66

0-10

94-66

Chi

93/7

1

26-27

—

20-98

—

6-41

21-90

12-80

0-42

0-23

—

87-01

Chi

93/7

1

27-73

002

21-48

—

9-46

7-65

19-92

0-09

0-14

—

86-49

Rds

93/7

1

—

—

—

—

—

55-53

0-35

309

—

—

58-97

Rds

93/7

1

—

—

—

—

1-93

48-96

0-92

6-04

0-03

—

—

57-87

wt%;

No.:IT.

SiO2

TiOj

v2o,FeO

MnO

MgO

ZnO

CaO

BaO

Na20

K20cr

(d) Spessartine-bearing red schist

Ms

89/2b3

45-73

0-13

34-67

1-52

0O4

0-53

0-02

0-18

1-57

8-34

92-73

Grt

89/2b6

34-26

0-18

19-98

0-14

38-63

0-06

2-80

—

—

—

9605

Sud

89/2b1

32-84

—

33-71

205

1-36

13-16

0-10

_

0-13

83-35

8 12 11 14 (O,OH) 1 cation

Si 1-999 2-888 3-162 2-715 2-818

Al 0-839 1-285 1-182

H/4' 0-112

I 3000 4-000 4000 4000

Ti 0002 0-004 0-001 0000 0-002

Al 1-947 1-942 1-754 1-371 1-390

V 0003 0-005 0003 0-000 0000

Fe*- 0-029 0-000 0000 0-000 0-000

Fe2 + 0000 0-004 0-137 0-486 0-804 0000 0032

Mn 0-914 2-760 0-027 1-993 0-658 0-925 0-814

Mg 0-114 0000 0-100 2-050 3-017 0010 0-027

Zn 0-002 0007 0-003 0-033 0-000 0000 0-000

Z 3011 2-025 6-933 6-871

Ca OOOO 0-298 0-008 0-000 0-010 0-065 0-127

Ba 0-000 0-001 0-008 0-000 0-000 0-000 0-000

Na 0-000 0-000 0-037 0-048 0-028

K 0-000 0000 0-825 0-000 0000

Z 0-000 5021 0-876 0-048 0-038 1-000 1000

Si

Al

H/4-

Z

Ti

Al

V

Mn

Mg

Zn

Z

Ca

Ba

Na

K

Z

11

0-007

12 14

3-108

0-894

4-000

0-OO6

1-880

0-000

0-000

0-086

0-002

0054

0-000

2-028

0-O02

0-OO5

0-206

0-722

0-935

2-876

0-124

3-000

0-011

1-977

0-000

0-000

0-010

2-747

0-000

0-004

0-262

0-000

0-000

0-000

5-001

3-108

0-892

4000

0000

2-868

0-000

0-000

0-162

0-109

1-857

0-007

6-003

0000

0000

0-000

0-016

0-016

0-0000-018 0-021 0-000 0-000

•Calculated to make Si+(H/4) = 3000.

775

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JOURNAL OF PETROLOGY VOLUME S7 NUMBER 3 JUNE 1996

(a) (b)

Fe2+ Mg2+

(c)

Chi

Fe'2+ Mg2+

Fig. 2. Distribution of divalent cations Fe2+, Mn + and Mg + in minerals from the Lienne lyndinc. (a) Carpholite-bearing segregationswith carpholite (circles) partly replaced by garnet (diamond), chlorite (triangles) and sudoite (squares). The black dot represents thecarpholite from Meuville described by Fransolet (1972). Dashed line encompasses the range of carpholite composition in rodu of theWippra Zone, Han (Theye & Siedel, 1993). (b) Chloritoid-chlorite-bearing metasediments. Coexisting minerals are connected bytie-lines, (c) Segregation in manganese ore (sample 93/7). Carpholite (open circles) u partly replaced by garnet (open diamond) andrelatively Mn-poor chlorite (open triangles). Mn-rich chlorite (filled trianglej) occurs as coarse aggregate microstxucturally unrelated to

carpholite.

ponent of the analysed garnets is indicated by theirlattice constants. Four determined values for garnetsfrom the Lienne syncline (93/7: 11-657 A; 89/81:11-569 A; 89/2b: 11-648 A; 89/3: 11-648 A) are sig-nificantly higher than those of OH-free Mn—Cagarnet (Ito & Frondel, 1968; Fig. 3), which is inaccordance with the behaviour of the grossular-

hydrogrossular series [Shoji, 1974; in Meagher(1982)]. Hsu (1968) observed that the lattice con-stants of spessartine synthesized below 500°C aresignificantly higher than those of spessartine pro-duced at higher temperature. This is also attributedto the presence of a hydrospessartine component inthe low-temperature products (Hsu, 1968).

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THEYEttaL CARPHOLTTE IN Mn-RICH ROCKS

11.87 T

11.66 -

11.63 -

*•=• 1134(0

1132 -

11310.00 0.03

SpsO10 0.15

X(Grs)0.20 ois

Grs-»>Fig. 3. Lattice constant! of four spessartine sample* from theIicnne lyndine plotted against XOr, of garnet. Compared withthe lattice comtanti of OH-free ipessartine-grossular garnet (Ito& Fronde], 1968), the determined values are significantly higher.

White micaWhite mica minerals comprise muscovite, paragoniteand a mixed-layer mineral muscovite—paragonite(see Frey, 1969; Kramm, 1980). These mineralsoccur either rock forming in metapelites or replacingcarpholite in quartz-rich segregations. Analyses ofmuscovite show a low Si content of 3-0—3-l Si p.f.u.(Tables 2a-2d).

RhodochrositeThis mineral is present in the manganese ore (seebelow) and in segregations. It contains variableamounts of Ca, Fc and Mg (Tables 2a—2c).

COMPARISON WITH Mn-RICHROCKS OF THE WIPPRAZONE, HARZMetasediments of similar sedimentation age, com-positions and metamorphic assemblages to those ofthe Lienne syncline occur in the Wippra Zone (WZ)of the Harz, Germany (Siedel & Theye, 1993; Theye& Siedel, 1993). Temperature and pressure condi-tions were estimated by Theye & Siedel (1993) at~320°C, 3 kbar. Chloritoid compositions in the WZare richer in iron. They show, however, a similarMn/Mg to that in chloritoid schists of the Liennesyncline. In contrast to the Lienne syncline, the WZcarpholite also occurring within quartz-rich segre-gations shows a much more extensive compositionalspread, ranging from almost pure carpholite to

magnesiocarpholite with XMg up to 052 (Fig. 2a).The most Mg-rich carpholite coexists with sudoiteand chlorite without any microstructural sign ofnon-equilibrium. In contrast, sudoite in the Liennerocks only occurs as breakdown product of car-pholite, indicating that, in this case, the paragenesiscarpholite + sudoite probably does not representchemical equilibrium. These observations suggestthat, during vein formation in the WZ, Mn/Mg ofthe vein-forming fluid has significantly decreased.Mn-rich carpholite formed at the beginning was fol-lowed by magnesiocarpholite + sudoite. In theLienne area, the chemical evolution of carpholite-bearing veins is restricted to an Mn-rich episode.Rhodochrosite-bearing segregations occur both inthe Lienne area and in the WZ.

COMPARISON WITH HIGHER-GRADE ROCKS OF THEVENN-STAVELOT MASSIF INTHE RECHT-OTTRE-LIERNEUX AREAP-T conditions of age-equivalent metasediments inthe Recht-Ottre-Lierneux area are estimated to360-420°C, 2 kbar (Kramm, 1982; Kramm et al.,19854). A major difference from those of the Liennesyncline is the grain size of metamorphic minerals.In particular, chloritoid and garnet occur as milli-metre-sized blasts in the Recht-Ottre—Lierneuxarea, whereas these minerals are generally muchmore fine grained in the Lienne syncline. Differencesof the chemical compositions of minerals are alsostriking. Garnet in the Lienne syncline is virtuallypure spessartine containing some hydrogarnet com-ponent. In the Recht-Ottre—Lierneux area,however, garnet may contain significant amounts offerrous iron (up to 14 wt%). Moreover, the stoi-chiometry of the published analyses does not indicatethe presence of a hydrogarnet component (Kramm,1982; Bossiroy, 1984; Schreyer et al., 1992). Chlor-itoid in the Recht-Ottre—Lierneux area generallyshows a higher Mg/Mn ratio (at widely varying Xyc)than in the Lienne syncline (Theye & Fransolet,1994). A further mineralogical difference is the pre-sence of andalusite in the Recht-Ottre—Lierneuxarea. This mineral is not known from the Liennesyncline. However, hydrous aluminosilicates such askaolinite and pyrophyllite are also not observed inthe Lienne syncline, indicating that these rocks arepoorer in Al. Finally, carpholite has exclusively beendescribed in the Lienne syncline but not in theRecht—Ottre-Lierneux area.

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P-T CONDITIONS OFMETAMORPHISM IN THELIENNE SYNCLINEThe mineralogical and microstructural differencesclearly point to a lower temperature of meta-morphism in the Lienne syncline compared with theRecht-Ottre-Lierneux area of the Ardennes. Aminimum temperature of metamorphism for theLienne syncline can be calculated from the equi-librium chlorite + kaolinite/pyrophyllite = chloritoid+ quartz + H^O, using thermodynamic data pre-sented below and mineral compositions of chloritoidschists in Table 2b. Four rock specimens yieldminimum temperatures between 280 and 320°C(nearly independent of pressure). We estimate thattemperature of metamorphism in the Lienne synclineis ~300°C, which is considerably lower than in theRecht—Ottre—Lierneux area. No indications for themctamorphic pressure are present in the Liennesyncline. We assume that it is in the same range as inthe Recht-Ottre—Lierncux area, or slightly lower (1—2 kbar). These values are slightly lower than thoseestimated by Kramm (1982) for this region, butsimilar to those of the Wippra Zone.

PHASE RELATIONSHIPS ANDSTABILITY OF CARPHOLITEBASED ON CALCULATEDMINERAL EQUILIBRIAThe upper thermal breakdown of the high-pressuremetamorphic minerals ferrocarpholite and magne-siocarpholite occurs according to the reactions car-pholite sensu lato (s.l.) = chloritoid + quartz + HjO forFe-rich compositions and carpholite s.l. = chlorite +kyanite + quartz + H2O for Mg-rich compositions(Vidal et al., 1992). These reactions, which are con-tinuous with respect to Fe or Mg, lead to para-geneses involving carpholite s.l. + chloritoid orcarpholite s.l. + chlorite + kyanite. Assemblagesinvolving both carpholite s.l. and garnet, however,have never been observed in high-pressure meta-morphic (Mn-poor) rocks. In contrast, in Mn-richrocks of the Lienne syncline, we never observed car-pholite + chloritoid or carpholitc + chlorite + kyanite/andalusite (or hydrous Al-silicate) assemblages.Instead, we found the coexistence of carpholite (insegregations) with garnet (in adjacent layers) as wellas the replacement of carpholite by spessartine andchlorite.

To semi-quantitatively evaluate the fundamentaldifferences between phase relations of (1) Mn-poorhigh-pressure and (2) Mn-rich low-pressure car-

pholite-bearing assemblages, we calculated equi-librium curves using thermodynamic data. Theseequilibrium curves arc essentially isopleths alongwhich minerals with compositions as calculated oranalysed may coexist under changing P—T condi-tions. In a first approximation, phase relationships ofthe Mn-rich rocks of the Lienne syncline in theArdennes can be represented in terms of thechemical system MnO-MgO-Al2O3—SiC^-HjO(MnMASH), involving the phases garnet, car-pholite, chloritoid, chlorite, sudoitc, pyrophyllite,kaolinite, quartz and H2O. Because a complete set ofthermodynamic data is only available in the che-mical system MASH, but not for Mn minerals, weconfined our calculations to MASH equilibria,applying ideal mixing models to consider cationsother than Mg in the divalent cation sites. Thehydrogarnet component in garnet is not consideredin the calculations. In the case of chlorite, idealclinochlore, Mg5Al(AlSi30io)(OH)8, is used tobalance the equilibria. To account for theTschermak substitution in natural chlorite, a con-stant activity correction factor of 0-7 calculated withthe activity model of Vidal et al. (1992) from ana-lyses in Table 2 is applied [see Berman (1988)].

We used the thermodynamic data set of Berman(1988) supplemented by data for magnesiochloritoid(B. E. Patrick & R. G. Berman, unpublished, 1990),sudoite and magnesiocarpholite (Vidal et al., 1992).The calculations were performed with the PTAXand TWQ, software (Brown et al., 1988; Berman,1991).

Constant Mn—Mg partition coefficients derivedfrom references given in parentheses below wereassumed:

/3?"/Mg(Cph/Sud) = 35 (Harz; Theye & Siedel,1993)

A^n/Mg(Cld/Sud) = 161 (Ottre; T. Theye,unpublished analyses, 1994)

Jt^n/Mg(Cld/Grt) = 35 (Venn-StavelotMassif; Theye & Fransolet, 1994)

JC^n/Mg(Cld/Chl) = 26 (Venn-StavelotMassif; Kramm, 1982; Bossiroy, 1984).

In the MASH chemical system, the lower-pressurestability of magnesiocarpholite is defined at ~ 7 kbar(Fig. 4) by the reactions sudoite + quartz = car-pholite, chlorite + kaolinite = carpholite + quartz +H2O, and chlorite + pyrophyllite + H2O = carpholite+ quartz (Theye et al., 1992; Vidal et al., 1992).Because the Fe-Mg partition coefficients Cph/Chland Cph/Sud are not very far from one, the lower-pressure limits of the ferro- and of the magnesio-carpholite stability fields are similar.

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THEYE ctaL CARPHOLTTE IN Mn-RICH ROCKS

P (kbar)

8

Cph + Chi + Qtz= Grt + H2O —

Cph + Qtz =Grt + Kin + H p

-200 -300 -400 T(°C)

Fig. 4. Tentative P-T diagram with calculated MnMASH equi-libria. Thin lines are isopleths for J MnfCph) = 0'8 for the para-geneses Cph + Sud, Cph + Chi + Grt and Cph + Kin + Grt (H2Oand quartz in excess); labels ihow the corresponding end-memberequilibria. Daihed lines represent the lower-pressure stability ofmagnesiocarpholite. Thick line is ASH reaction as indicated. (For

mineral abbreviations, see Table 1.)

In contrast, in the MnMASH system, these equi-libria shift to lower pressure with increasing Mn/Mgof minerals owing to the high Mn—Mg partitioncoefficient between carpholite and sudoite orchlorite. In the limiting MnASH system, the stabilityfields of sudoite and of Mn-chlorite + kaolinite/pyro-phyllite have completely disappeared, that is, car-pholite is a stable mineral at very low pressureconditions (see Fig. 4). This result explains the pre-sence of carpholite in the Liennc syncline as well asin natural low-pressure environments such as inhydrothermal formations.

The isopleth for the carpholite—sudoite equi-librium calculated from the mineral compositions asmeasured in the Lienne syncline [-XMn(Cph) =0 -8 inFig. 4] is not meaningful, yielding negative pressuresat T> 200°C. Carpholite and sudoite of these rocks,consequently, probably do not represent chemicalequilibrium as already inferred from microstructuralfeatures.

Another interesting result is that, in theMnMASH system, already at relatively low Mn/(Mn + Mg), equilibria involving chloritoid becomemetastable with respect to parageneses with bothcarpholite and garnet. At high Mn/(Mn + Mg),these parageneses are carpholite + chlorite + garnetand carpholite + garnet + kaolinitc/pyrophyllite. Thecorresponding end-member equilibria are

carpholite + Mn-chlorite + quartz =spessartine + H2O (la)

carpholite + quartz =spessartine + kaolinite/pyrophyllite + H2O (2a)

[the supplement (a) refers to equilibria of the end-member system MnASH (-CO2), whereas equi-librium numbers without supplement refer to theenlarged system MnMASH (-CO2) as discussedbelow]. The calculations indicate that pure car-pholite would react, nearly independently ofpressure, at ~240°C according to equilibrium (la),and at 280°C according to (2a). Concerning (la), itshould be noted that the composition of Mn-chloriteas analysed in the Lienne rocks is not sufficientlyaluminous (see Fig. 6, below) to allow the simplerreaction Mn-chlorite + quartz to form spessar-tine + H2O, which was experimentally studied byHsu (1968). To balance this simpler reaction, achlorite with 2*33 Si p.f.u. is necessary. This value isfar from chlorite compositions in the Lienne syncline(Table 2). Moreover, the calculated low-tem-perature limit of the spessartine stability field is con-siderably lower than according to the experimentaldata of Hsu (1968). [For a discussion of the work ofHsu (1968), see below.]

In Fig. 4, the calculated isopleths A"Mn(Cph) =0-8as observed in the Lienne rocks are presented forequilibria (1) and (2). They indicate higher tem-peratures compared with the Mn end-membersystem: ~270°C (paragenesis with Cph + Chi + Grt)and 300°C (Cph + Grt + Kln/Prl), respectively.

The calculated temperatures for ^Y'v1n(CIph) =0-8are not very far from the estimated metamorphictemperature range derived for the Lienne syncline.However, as described above, kaolinite or pyro-phyllite have not been observed as decompositionproducts of carpholite, but muscovite and paragonitehave been observed, in addition to chlorite andgarnet. The formation of muscovite, paragonite andchlorite as alteration products of magnesiocarpholitewas also described by Goffe & Vidal (1992) in high-pressure metamorphic pelites. These workers inter-preted the appearance of white mica as due to adiminution of Na and K in the fluid phase ifpressure drops or temperature increases. These pro-cesses lead to the formation of chlorite and of whitemica (instead of pyrophyllite or kaolinite) at theexpense of carpholite. The same reasoning can beapplied to the rocks under consideration here.

Another environment that has to be discussed withrespect to mineral reactions involving carpholite isrhodochrosite-bearing segregations in the manganeseore. In a first approximation, phase relationships of

779

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these rocks can be represented in terms of thechemical system MnO-Al2O3-SiO2-H2O-CO2(MnASHC), involving the phases spessartine, car-pholite, Mn-chlorite, rhodochrosite and quartz, aswell as H2O and CO2 in the fluid phase. A Schrei-nemakers analysis with the variables Xco, and T(Fig. 5) shows that the upper thermal stability ofcarpholite in the presence of Mn-chlorite or rhodo-chrosite with quartz is defined by the reactions

carpholite + Mn-chlorite + quartz =2 spessartine + 6 H2O (1 a)

carpholite + rhodochrosite + quartz =spessartine + 2 H2O + 2 CO2. (3a)

Assuming the presence of all phases under con-sideration and buffering of the fluid phase by themineral assemblage carpholite + chlorite + rhodo-chrosite + spessartine, the overall reaction corre-sponding to the MnASHC invariant point in Fig. 5can be written

6 rhodochrosite + 2 carpholite + 2 quartz =Mn-chlorite + spessartine + 3 CO2. (6a)

(4a)

This reaction corresponds to the observation that,in segregations of the Mn ore, pseudomorphs aftercarpholite consist of chlorite and garnet and thatrhodochrosite shows corroded outlines (Fig. lb).

The natural minerals of the Lienne syncline,however, cannot be represented within the simplechemical system MnASHC, and at least magnesiumhas to be considered in addition. In Fig. 6, phasecompositions are represented in an A—Mg—(Mn + Fe)triangle (iron only plays a subordinate role). Themineral reaction which can be derived from thecrossing tie-lines shown is similar to reaction (6a):

Rds + Cph = Chi + Grt(with H2O, CO2, quartz in excess) (6)

but now in the enlarged chemical systemMnMASHC.

Also for this case, we calculated MnMASHCequilibria with the thermodynamic data mentionedabove. The equations of state used for the fluid phaseare from Haar et al. (1984) for H2O, Mader &Berman (1991) for CO2, and Kerrick & Jacobs(1981) for H2O-CO2 mixing. Because of therestricted validity of the Kerrick & Jacobs equations,equilibrium temperatures <350°C have been esti-mated using only reaction (1).

The result of the calculation performed for apressure of 2 kbar is shown in Fig. 7. The univariantequilibrium (6) (thick line in Fig. 7) terminates ininvariant points of the end-member systems

+ qtz,

Rds

Fig. 5. T-Xoo, diagram ihowing a Schreinemakera analyiii ofequilibria in the chemical system MnASHC, involving the phaieispessartine, Mn-chlorite, carpholite and rhodochrosite. H2O, CO2

and quartz in excess. (For mineral abbreviations, >ee Table 1.)

Mn+FeFig. 6. A-Mg-(Mn + Fe) ternary plot presenting mineralcompositions of coeziiting garnet, carpholite, Mn-poor chloriteand rhodochroiite at measured in specimen 93/7 (Table 2c).Ferrous iron ii present in significant amount* only in chlorite.The reaction Cph + Rdi = Grt + Chi can be derived from the cross-ing tie-lines indicated. H2O, COj and quartz in excess. (For

mineral abbreviations, see Table 1.)

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THEYE ctaL CARPHOLITE IN Mn-RICH ROCKS

t T-500

-400

-300

-200

Fig. 7. Calculated tentative T—XQQ, section with phase equilibriain the chemical system MnMASHC at 2 kbar with HjO, COj andquartz in excess. Thick line: univariant MnMASHC equilibrium(6). The values given at the black dots along this curve indicatethe changing X^ of carpholite. Thin line (3): iioplcth• Mn(Cph) = 0-8 for the paragenesis Cph + Rds + Grt. Dashedlines emanating from invariant point IP1 are univariant equilibriafor the end-member system MnASHG (la) Cph + Chl + Qtx =Grt + H,O, (3a) Cph + Rds/Mgs + Qtz = Grt + HjO + COj, (4a)Cph + Rds/Mgs + Qp! + H,O = Chl + COI, (5a) Chl + CO2 = Rds/Mgs + Grt + HjO. IP2: invariant point for the MASHC system.Rds/Mgs: rhodochrosite-magnesite in solid solution (for other

mineral abbreviations, see Table 1).

MnASHC (IP1) and MASHC (IP2), respectively.At each point of the univariant curve (6), the Mg/Mn of the minerals is fixed. The Mg-rich branch ofthe univariant curve is metastable with respect tominerals such as sudoite and pyrophyllite which arenot considered in the calculations (Mg-rich car-pholite is not stable at 2 kbar).

The calculated temperature for the invariant pointinvolving the isopleths for ZMn(Cph) = 0-8 is also at~250-300°C just as for the CO2-free system. Animportant new result is that, for the mineral com-positions under consideration, reaction (6), in theLienne syncline, may take place in the presence of avery CCvpoor fluid phase. The stable coexistence ofcarpholite and rhodochrosite as observed in one spe-cimen (22646) can be attributed to a more COj-richfluid phase. Breakdown of carpholite can be due toeither increasing temperature or decreasing A'co,-

The stoichiometric coefficients of equilibrium (6)considering the composition of carpholite in theLienne area [Mn/(Mg + Mn) =08; other mineralsaccording to element partition coefficients] are

2181 Rds + Cph + Qtz =0-929 Grt +0071 Chl+1-716 H2O + 2181 CO2.

Thus, only small amounts of chlorite will be pro-duced by the decomposition of Mn-rich carpholiteand rhodochrosite. Because significant quantities ofchlorite are present in the segregations of the Mnore, this mineral is believed to have already beenpresent associated with rhodochrosite and carpholiteduring the time of vein formation. A second gen-eration of (Mn-poorer) chlorite was then formedthrough the decomposition of carpholite (see Fig.2c). Possibly, this two-stage mineralogical evolutionof the segregation can account for the presence oftwo distinct chlorite compositions as described bySchreyer et al. (1986).

In nature, carpholite exclusively occurs in low-/"environments. No natural examples and no infor-mation are available on the high-P behaviour of thismineral, and calculations remain inconclusive aswell.

The calculations performed here explain in aqualitative manner many natural observations in theLienne syncline rocks such as the appearance of car-pholite at low pressure and temperature and thereaction relations between carpholite and garnet.The quantitative values calculated above can onlybe considered as an approximation. Besides un-certainties in thermodynamic data, the assumptionof P-T independent element partition coefficientsand of ideal mixing models, especially at low Mg/(Mg + Mn), introduces significant uncertainties. Thetemperature uncertainty of the calculations is esti-mated to be at least 30°C.

APPEARANCE OFSPESSARTINE DURINGPROGRADE METAMORPHISMExperimental investigations on the lower thermalstability of spessartine were carried out by Hsu(1968). Hsu observed growth of spessartine at theexpense of poorly defined 7 A Mn-chlorite andquartz at 488°C and 3 kbar, 415°C and 2 kbar, and370°C and 0-5 kbar under unbuffered oxygenfugacity conditions. No experiments, however, werereported showing the growth of Mn-chlorite andquartz at the expense of spessartine, but only thecontinued (metastable?) persistence of Mn-chlor-ite + quartz at low temperature. Moreover, theamount of the hydrospessartine component in garnetand of the Tschermak substitution in chlorite werenot determined in the run products of the equi-librium experiments. Therefore, the so-calledequilibrium curve Mn-chlorite + quartz = spessartine+ fluid drawn by Hsu (1968) was not really reversedand represents only the maximum lower thermal

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stability of spessartine. The first appearance of spes-sartine (possibly containing some hydrogarnet com-ponent) during prograde metamorphism, possiblyoccurs as low as ~300°C (at 1-2 kbar) as deducedfrom the reasoning presented here.

CONCLUSIONSThe phase-petrological analysis of very low gradeMn-rich metasediments of the Ardennes shows thatthese rock types have the potential to sensitivelycharacterize variables of metamorphism. At present,a full quantitative evaluation is hampered by theuncertainty or absence of thermodynamic data forMn minerals. To improve this situation, it would bedesirable to obtain more experimental data on Mnminerals, such as the upper thermal stabilities ofottrelite, Mn-chlorite and carpholite, as well asfurther data on the lower thermal stability of spes-sartine.

ACKNOWLEDGEMENTSThanks are due to Thomas Reinecke for microprobeanalyses on specimen 89/95 and for pointing out thepresence of vanadium in this specimen. Heinz-JurgenBernhard and Andreas Dressier assisted in themicroprobe work. Constructive reviews of M. Frey(Basel) and B. Goffe (Paris) are gratefully acknow-ledged.

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Low-Temperature, Low-Pressure Metamorphism of Mn-rich Rocks in the Lienne Syncline, Venn