petrology of high-pressure metapelites from the adula nappe

JOURNAL OF PETROLOGY VOLUME 40 NUMBER 1 PAGES 199–213 1999

Petrology of High-Pressure Metapelitesfrom the Adula Nappe (Central Alps,Switzerland)

CHRISTIAN MEYRE1∗, CHRISTIAN DE CAPITANI1, THOMAS ZACK2

AND MARTIN FREY1

1MINERALOGISCH-PETROGRAPHISCHES INSTITUT DER UNIVERSITAT BASEL, BERNOULLISTRASSE 30, CH-4056 BASEL,

SWITZERLAND2MINERALOGISCH-PETROGRAPHISCHES INSTITUT UNIVERSITAT GOTTINGEN, GOLDSCHMIDTSTRASSE 1,

D-37077 GOTTINGEN, GERMANY

RECEIVED JANUARY 12, 1998; REVISED TYPESCRIPT ACCEPTED JUNE 15, 1998

High-pressure metamorphism in the Penninic Adula nappe (Central on eclogite lenses, which are abundant in the structurallyAlps, Switzerland) reached eclogite facies conditions. Besides abund- upper part of this nappe (Heinrich, 1986; Droop et al.,ant mafic eclogite lenses, very few metapelitic rocks also preserved 1990; Meyre et al., 1997). As in many other high-pressurehigh-pressure relics, even though most of the felsic lithologies were terranes in the world (e.g. Dabie-Shan, China; Cale-retrogressed during a later amphibolite facies overprint. Calculations donides, Norway), only very few relics of high-pressureof equilibrium phase diagrams of whiteschist and sodic whiteschist metamorphism have been reported within felsic lith-assemblages reveal conditions of P>20 kbar at T ~650°C. ologies (metapelitic rocks, orthogneisses) until now. ThisCommon metapelitic assemblages (garnet + phengite + kyanite led to discussions on whether the entire Adula nappe+ quartz ± paragonite) are stable over a wide range in pressure underwent a high-pressure metamorphism under eclogiteand temperature. Calculations of the peak pressure conditions in the facies conditions or only part of it, that is, the eclogiteinvestigated metapelite samples are in good agreement with analogue lenses, which were buried to great depths and latercalculations of eclogite samples. These results combined with struc- emplaced in amphibolite facies country rocks. This lattertural investigations support a single P–T loop for this area with assumption would presume two different pressure–a Tertiary high-pressure event (Late Eocene) that affected the entire temperature loops: first an Eo-Alpine or even a pre-Adula nappe. Alpine high-pressure event (e.g. Biino et al., 1997), later

overprinted by the Alpine ‘Lepontine’ metamorphic eventunder amphibolite facies conditions.

In contrast, the assumption that the entire Adulanappe underwent eclogite facies conditions presumes

KEY WORDS: Adula nappe; Central Alps; equilibrium phase diagram; a continuous retrograde evolution from high-pressuregeothermobarometry; high-pressure metapelite conditions to amphibolite and upper greenschist grade

within a single pressure–temperature loop (Low, 1987;Meyre & Puschnig, 1993; Partzsch et al., 1995b). Becausethe main foliation of the Adula nappe can be correlated

INTRODUCTION with Oligocene structures in the allochthonous Mesozoiccover of the Penninic nappes (‘Bunderschiefer’ meta-The regional high-pressure metamorphism under eclogitesediments) and in the overlying nappes as well, this mainfacies conditions in the Adula nappe (Central Alps, Swit-

zerland; Fig. 1) is well known mainly from investigations foliation must have developed in Late Eocene to Early

∗Corresponding author. Telephone: 41 61 267 36 28. Fax: 41 61 26728 81. e-mail: [email protected] Oxford University Press 1999

JOURNAL OF PETROLOGY VOLUME 40 NUMBER 1 JANUARY 1999

REGIONAL GEOLOGYThe Adula nappe belongs to the Penninic domain of theSwiss Central Alps and represents the palaeo-geographically southernmost part of the Europeanmargin at a time before the closure of the Valais troughin the early Eocene (Schmid et al., 1990, 1996). In contrastto the Tambo nappe in the hanging wall and the Simanonappe in the footwall, the Adula nappe is characterizedby a regional high-pressure metamorphism. The meta-morphic conditions of this event gradually increase fromnorth (blueschist facies conditions; 450–550°C, 10–13kbar at Vals) to south (very high-P conditions; 750–900°C,18–35 kbar at Alpe Arami; Heinrich, 1986). This high-pressure metamorphism is followed by fast exhumationto amphibolite and greenschist facies conditions. Whereaseclogite assemblages are widespread in the core of me-tabasic lenses, high-pressure relics in felsic lithologies(metapelite and leucocratic gneisses) are concealed oreven completely destroyed.

Five deformation phases can be distinguished in themiddle Adula nappe, four of them related to the ex-humation process. A more detailed description of thestructural evolution of the Adula nappe has been givenby Low (1987), Meyre & Puschnig (1993), Partzsch &Meyre (1995) and Partzsch (1998). The ‘Sorreda’ de-formation phase (D1) is correlated with the imbricationof Mesozoic sediments (Triassic quartzites and marbles)Fig. 1. Simplified tectonic map of the Adula nappe–Cima Lunga unit.

The localities of the investigated samples are Zapporthorn (east of San and the pre-Mesozoic basement of the European margin.Bernardino) and Trescolmen (east of Mesocco). Low (1987) suggested prograde conditions under elevated

pressures for D1. The ‘Trescolmen’ phase (D2) occurredunder eclogite facies conditions and is represented by

Oligocene time (Schmid et al., 1996). This implication as recrystallized omphacite and elongated garnet aggregateswell as geochronologic data from the Cima Lunga unit that form a distinct foliation in eclogite lenses. This(Becker, 1993; Gebauer, 1996) and the assumption of a deformation phase took place under early retrogradesingle continuous pressure–temperature path for the conditions. The ‘Zapport’ deformation phase (D3) is as-Adula nappe imply a Tertiary age for the high-pressure sociated with the main foliation in the middle Adulametamorphism. nappe. Deformation started under high-pressure con-

One way to support this hypothesis is to find high- ditions and continued well into amphibolite facies con-pressure relics in felsic lithologies and to demonstrate that ditions. This main foliation affected also the overlyingthey experienced the same metamorphic and structural Tambo and Suretta units and was therefore still activeevolution as the eclogite lenses. Heinrich (1982) described when the Penninic nappes were piled up. Related to thewhite mica with highly phengitic cores in metapelitic main foliation is an apparent stretching lineation as wellsamples and proposed a regional high-pressure meta- as isoclinal folding. The fourth deformation phase (‘Leis’morphism for the entire Adula nappe (Heinrich, 1986). phase; D4) appears as open folds in gneisses and cren-

In this study we concentrate on the petrologic evolution ulation in lithologies with a high amount of sheet silicateof high-pressure metapelitic rocks of the middle Adula minerals. Metamorphic conditions are lower amphibolitenappe. We apply the thermodynamic approach of Gibbs to upper greenschist facies. The latest visible deformationfree energy minimization to calculate stable assemblages phase (‘Carassino’ phase) mainly affects the frontal partand present some possible mineral equilibria for these of the Adula nappe in the north and fades out towardsrocks. From these petrologic calculations and from tex- the south. In the geographically central part of the nappe,tural observations we conclude that the metamorphic the ‘Carassino’ phase is related to a slight undulationevolution of the investigated metapelitic samples is in (occasional kinking) of gneisses.agreement with the pressure and temperature evolution Detailed descriptions of the regional geology of theof the eclogite rocks and with the structural evolution of Adula nappe have been given by Frischknecht (1923),

Heinrich (1983, 1986), Low (1987) and Partzsch (1998).the middle Adula nappe as well.

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MEYRE et al. HP METAPELITES FROM THE ADULA NAPPE

PETROGRAPHY AND MINERAL Table 1: Selected electron microprobe

analyses of garnet used to calculate theCHEMISTRYThe investigated samples of this study were collected at equilibrium phase diagrams; end-memberthe localities Zapporthorn (MF2643) and Trescolmen distribution after Deer et al. (1967)(CHM1, CHM39 and Z6-50-12) in the central part ofthe Adula nappe (see Fig. 1).

Sample: Z6-50-12 MF2643 CHM1 CHM39We analysed the minerals of samples CHM1, CHM39

Analysis no.: 11 23 41 94and MF2643 with a JEOL JXA-8600 electron micro-probe at the University of Basel (Switzerland). The micro- SiO2 38·18 39·74 37·79 39·61

probe is equipped with Voyager software by Noran TiO2 0·02 0·09 0·03 0·06

Instruments. The operating conditions were set at 15 kV Al2O3 21·52 22·32 20·40 22·04

and 10 nA, and the correction procedure was PROZA FeO 29·37 24·22 33·68 24·96(Bastin & Heijligers, 1990). Minerals of sample Z6-50-12 MnO 0·76 0·02 0·75 0·47were analysed with a Cameca SX51 electron microprobe MgO 5·65 8·43 6·57 8·11with conditions of 15 kV and 20 nA at the University of CaO 4·84 6·71 0·69 5·90Heidelberg (Germany). The correction procedure was a Na2O 0·02 0·03 0·05 0·06ZAF routine. For all analyses natural and synthetic K2O 0·02 0·02 0·02 0·01crystals were used as standards.

R 100·39 101·58 99·98 101·22Tables 1, 2 and 3 show microprobe analyses of garnet,Normalized to 24 oxygenswhite mica, talc, biotite, amphibole, and jadeitic pyr-Si 5·98 5·99 6·00 6·02oxene. These analyses are representative for the as-Ti 0·00 0·01 0·00 0·01semblages that were stable at conditions of the high-

pressure metamorphism or the amphibolite facies over- Al 3·97 3·97 3·82 3·95

print. Fe 3·85 3·05 4·47 3·17

The phengite analyses in Table 2 are projected to Mn 0·10 0·00 0·10 0·06the ternary system with the end-members muscovite, Mg 1·32 1·89 1·56 1·84aluminoceladonite and ferro-aluminoceladonite. A de- Ca 0·81 1·08 0·12 0·96composition into these three end-members cannot com- Na 0·01 0·01 0·02 0·02prehensively characterize the measured micas (e.g. the K 0·00 0·00 0·00 0·00paragonite component is completely ignored). This sim-

R 16·04 16·02 16·09 16·01plification is necessary to use the available solid solutionX(Grs+Adr) 0·13 0·18 0·02 0·16model for white mica [modified after Massonne &X(Prp) 0·22 0·31 0·25 0·30Szpurka (1997)] in our computations (see the sectionX(Alm) 0·63 0·51 0·72 0·53‘Equilibrium phase diagrams’).X(Sps) 0·02 0·00 0·01 0·01In the following, mineral abbreviations are after Kretz

(1983) and Bucher & Frey (1994), and Wm indicateswhite mica.

within the main foliation (Fig. 2). No feldspar was detectedin this sample.Garnet–white mica–kyanite schist from

Garnet is essentially unzoned. Slightly decreasing PrpTrescolmen (CHM1)values towards the rim are interpreted as diffusional re-Sample CHM1 comes from a lens of several metresequilibration (Fig. 3). Phengite is zoned with respect toof metapelite (garnet–white mica–kyanite schist) withinSi content, with values up to Si = 3·4 p.f.u. in the coreorthogneisses from the Trescolmen area (Swiss co-decreasing to Si = 3·1 in the rim. Phengitic white micaordinates: 733.910/139.460). Large garnet crystals, upgrown in shearbands (related to late stages of ‘Zapport’)to 2 cm in size, with inclusions of kyanite (several mil-systematically reveals low Si values (Si = 3·1 p.f.u.) andlimetres in size), white mica and quartz are abundant.is unzoned. Representative analyses of garnet, phengiteBesides quartz and kyanite further main components ofand paragonite of sample CHM1 are listed in Tables 1this rock are phengitic muscovite and paragonite definingand 2.the distinct main foliation [‘Zapport’ deformation phase;

We assume that the main foliation began to evolveafter Low (1987) and Partzsch et al. (1995a)]. Biotiteunder high-pressure conditions (core of phengitic whiteonly occurs as a retrograde product of muscovite. Small

anhedral staurolite grows in pressure shadows of garnet mica) and continued under amphibolite facies conditions

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Table 2: Representative electron microprobe analyses of phengite, paragonite, talc and biotite

Sample: CHM39 CHM39 CHM1 CHM1 Z6-50-12 CHM1 Z6-50-12 MF2643 MF2643 MF2643

Analysis no.: 45 51 5109 22-47 39phe 5134 57pg 38pg 9tc 13bt

Phe (rim) Phe (core) Phe (rim) Phe (core) Phe Pg Pg Pg Tc Bt

SiO2 50·86 52·12 47·73 49·35 51·75 46·49 46·11 46·95 61·01 39·72

TiO2 0·44 0·33 0·47 0·59 0·34 0·20 0·10 0·36 0·03 0·54

Al2O3 30·25 28·90 32·95 26·77 26·44 38·69 39·64 38·19 1·19 17·25

FeO 1·29 1·28 1·91 2·64 1·13 0·65 1·07 0·59 2·97 7·42

MnO 0·00 0·00 0·00 0·00 0·03 0·00 0·00 0·00 0·04 0·00

MgO 3·78 4·05 1·54 2·65 4·27 0·13 0·32 0·71 28·78 20·90

CaO 0·00 0·08 0·00 0·01 0·02 0·22 0·24 0·29 0·02 0·06

Na2O 0·81 0·54 1·31 0·64 0·73 6·04 7·44 7·03 0·10 1·06

K2O 10·20 10·01 9·58 10·58 9·78 2·15 0·37 1·21 0·00 7·81

R 97·63 97·31 95·49 93·23 94·47 94·57 95·29 95·33 94·14 94·76

Normalized to 22 oxygen

Si 6·59 6·75 6·34 6·77 6·90 6·02 5·91 6·03 7·91 5·67

Ti 0·04 0·03 0·05 0·06 0·03 0·02 0·01 0·03 0·00 0·06

Al 4·62 4·41 5·16 4·33 4·15 5·90 5·99 5·78 0·18 2·90

Fe 0·14 0·14 0·21 0·30 0·13 0·07 0·12 0·06 0·32 0·89

Mn 0·00 0·00 0·00 0·00 0·00 0·00 0·00 0·00 0·00 0·00

Mg 0·73 0·78 0·30 0·54 0·85 0·03 0·06 0·14 5·56 4·44

Ca 0·00 0·01 0·00 0·00 0·00 0·03 0·03 0·04 0·00 0·01

Na 0·20 0·14 0·34 0·17 0·19 1·52 1·85 1·75 0·03 0·29

K 1·69 1·65 1·62 1·85 1·66 0·36 0·06 0·20 0·00 1·42

R 14·01 13·91 14·02 14·02 13·92 13·94 14·04 14·02 14·01 15·68

Si p.f.u. 3·29 3·37 3·17 3·38 3·45

Fe/(Fe+Mg) 0·161 0·151 0·410 0·359 0·129 0·737 0·655 0·318 0·055 0·166

K/(Na+K) 0·89 0·92 0·83 0·92 0·90 0·19 0·03 0·10 0·83

X(Ms)∗ 0·71 0·63 0·83 0·62 0·55

X(Cel)∗ 0·25 0·32 0·10 0·25 0·39

X(Fe-Cel)∗ 0·05 0·06 0·07 0·13 0·06

∗X(Ms)=4−Si p.f.u.; X(Cel)=[1−X(Ms)]×Mg/(Fe+Mg); X(Fe-Cel)=[1−X(Ms)]×Fe/(Fe+Mg).

a distinct foliation defined by white mica flakes (phengite).(growth of staurolite, phengite in shearbands). This struc-Garnet and kyanite are abundant besides the majortural and metamorphic evolution can be observed withincomponent quartz. Minor components are apatite, rutilemetapelites of the entire Adula nappe (Heinrich, 1982;and ilmenite. Rims of phengitic white mica are epi-Low, 1987; Meyre & Puschnig, 1993; Partzsch, 1998).tactically overgrown by biotite (see Heinrich, 1982). Phen-gite is zoned in silica content from core (Si ~3·4 p.f.u.)to rim (Si ~3·3 p.f.u.) as a result of diffusion (see below).

Garnet–white mica–kyanite quartzite from This zonation can be systematically observed in me-Trescolmen (CHM39) tapelites of the upper Adula nappe and seems to be

related to equilibration during decompression (Heinrich,For the Trescolmen locality, Heinrich (1982), Meyre &1982; Partzsch, 1998).Puschnig (1993) and Partzsch (1998) have described a

Garnet is essentially unzoned in the core (core com-crucial outcrop (Swiss coordinates: 733.600/139.600) thatposition: Alm51Prp33Grs15Sps1). A slight zonation towardsshows a mafic lens folded under eclogite facies conditions.the rim with respect to pyrope and grossularIn the fold hinge, high-pressure relics are preserved in a

metasedimentary quartzitic rock (Fig. 4). This rock has (Alm50Prp28Grs21Sps1) can be observed.

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Gagela (Swiss coordinates 733.650/139.250). The in-Table 3: Selected electron microprobevestigated sample (Z6-50-12) has three zones: a phengite-

analyses of amphibole and pyroxene rich layer followed by a 2 cm wide quartz-rich layergrading into eclogite that consists mainly of garnet and

Sample: MF2643 MF2643 Z6-50-12 Z6-50-12 clinopyroxene. Jadeitic clinopyroxene and Mg-glau-Analysis no.: Am1/5 Am1/6 50-12-23a 50-12-17c cophane are present in the intermediate quartz-rich layer

Bar Bar Gln Jd coexisting with quartz, garnet, kyanite, phengite, andrutile. The foliation in all three zones is parallel to theSiO2 56·62 55·60 58·17 58·09layering and defined by aligned phengite, amphibole andTiO2 0·17 0·19 0·07 0·00clinopyroxene, respectively.Al2O3 11·73 11·63 12·45 20·84

A similar assemblage with jadeite and Mg-glaucophaneFeO 4·82 5·89 4·20 3·04was described by Kienast et al. (1991) from the Dora

MnO 0·00 0·00 0·05 0·04Maira Massif and called ‘sodic whiteschist’, a term also

MgO 16·71 16·14 13·50 1·78used here.

CaO 3·69 4·18 0·70 2·48In thin section, garnet, jadeite, Mg-glaucophane, ky-

Na2O 5·10 4·90 7·46 13·29 anite and quartz can be defined as primary phases andK2O 0·16 0·16 0·08 0·00 represent the peak pressure assemblage. Zircon, allaniteH2O∗ 2·27 2·25 2·23 — and rutile occur as accessory phases. RecrystallizationR 101·27 100·94 98·91 99·56 still under high-pressure conditions (‘Trescolmen’ phase)

led to the formation of paragonite and magnesite thatNormalized to: 23 oxy 23 oxy 23 oxy 4 catgrew around kyanite and glaucophane.Si 7·48 7·40 7·83 2·00

Retrogression under amphibolite conditions led to theAl(iv) 0·52 0·60 0·17 0·00formation of fine-grained symplectite consisting of plagio-Al(vi) 1·30 1·22 1·81 0·84clase and amphibole around jadeite and glaucophane,Ti 0·02 0·02 0·01 0·00with jadeite more strongly affected than glaucophane.Fe3+ 0·53 0·66 0·19 0·05

Garnet shows a distinct and continuous zoning (Fig. 5)Fe2+ 0·00 0·00 0·29 0·04

without any break in the pattern except for the outermostMn 0·00 0·00 0·01 0·00

rim. Fe and Mn decrease towards the rim whereasMg 3·29 3·20 2·71 0·09

Mg increases, typical for growth zoning under progradeCa 0·52 0·60 0·10 0·09

conditions. In contrast, Ca shows an almost flat patternNa 1·31 1·26 1·95 0·89 with only a slight but noticeable increase from core toK 0·03 0·03 0·01 0·00 rim. Only at the outermost rim (25 lm) does Ca contentOH∗ 2·00 2·00 2·00 — drop significantly from Grs15 to Grs8. This drop is prob-R 16·99 16·99 17·06 4·00 ably due to the formation of amphibole and plagioclase

(An11–18) in symplectite around garnet during late retro-X(Jd) 0·84gression.X(Di) 0·08

Clinopyroxene is close to jadeite end-member com-X(Hd+Ae) 0·08position [XJd = 0·80–0·91; classification after Morimoto(1988)], whereas primary amphibole is close to pure∗Calculated H2O.Mg-glaucophane, with X(Na,M4) up to 0·95, Al(vi)/Mineral analyses of sample Z6-50-12 were used to calculate

equilibrium phase diagrams. Amphibole normalized to 23 [Fe3++Al(vi)] = 0·94 and Mg/(Mg+ Fe) = 0·89.oxygens (Tindle & Webb, 1994; Leake et al., 1997). Pyroxene Mineral analyses of sample Z6-50-12 are listed inanalysis normalized to four cations (after Morimoto, 1988).

Tables 1, 2 and 3.

Symplectic pseudomorphs of albite and amphiboleafter omphacite occur interstitially between garnet and Whiteschist from Zapporthorn (MF2643)quartz. Tables 1 and 2 show microprobe analyses of The outcrop of this sample was first described by Santinigarnet and phengite of sample CHM39. [1991, see also Frey et al. (1992); Swiss coordinates:

729.100/148.350]. A large eclogite lens of 60 m lengthlies within leucocratic gneisses. Metapelite occurs at the

Sodic whiteschist from Trescolmen (Z6-50- rim of the lens and as metasomatized veins of several12) centimetres thickness within the boudin. The mineralogy

of the associated metapelite and the veins represents aAt Trescolmen, rare metapelitic rocks with a high-pres-sure paragenesis occur as blocks below the Cima di typical whiteschist association of talc+ kyanite+white

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Fig. 2. Schematic micrograph of garnet clast within a matrix consisting of phengitic muscovite, paragonite and quartz (sample CHM1). Anhedralstaurolite and biotite are growing in pressure shadows of garnet within the main foliation, which belongs to late stages of the ‘Zapport’deformation phases [see text, and Low (1987) and Partzsch (1998)]. Additional phases are kyanite and rutile. No feldspar could be observed.

Fig. 4. Schematic view of outcrop from Trescolmen area (see Meyre& Puschnig, 1993). The metabasic lens represents a fold with an eclogitecore. The rim is overprinted under amphibolite facies conditions.Fig. 3. Profile of single garnet clast in sample CHM1. The core isQuartz-rich metapelite (assemblage Qtz+Grt+Ky+ Phe+Rt) isunzoned whereas a weak zonation caused by diffusion is restricted toenclosed in the hinge of the fold. S2 refers to the ‘Trescolmen’the outermost rim. A semi-inclusion of white mica (at ~0·2 mm fromdeformation phase, S3 to the ‘Zapport’ deformation phase.rim) is connected to the matrix. The zonation around white mica

therefore reflects rim composition.

grain boundaries overgrow the peak pressure assemblage.mica+ garnet (Schreyer, 1973, 1977). Major com- No reaction rim or symplectite occurs around barroisiticponents are garnet, talc, paragonite, and kyanite (see amphibole. White mica always has paragonitic com-Tables 1 and 2 for chemical analyses). Minor components position with low potassium content.include barroisitic amphibole, phlogopite, quartz, rutile,and staurolite (as small inclusion in garnet). Two differentmetamorphic stages can be observed: the peak pressure

EQUILIBRIUM PHASE DIAGRAMSassemblage (garnet+ talc+ paragonite+ kyanite±quartz) is affected by retrograde phlogopite that mainly The equilibrium phase diagrams (Figs 6–9) for metapelitic

samples from the middle Adula nappe were calculatedreplaces talc [re-equilibration event; see Meyre et al.(1997)]. Barroisitic amphibole (representative analyses in with the computer code THERIAK-DOMINO (de Cap-

itani & Brown, 1987; de Capitani, 1994). This programTable 3) appears to be stable under conditions of the re-equilibration event because euhedral grains with straight performs a Gibbs free energy minimization for a set of

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Fig. 5. Profile (rim to rim) of single garnet clast in sample Z6-50-12.From the zonation (decreasing Sps content and increasing Prp contentfrom core to rim) a prograde growth is suggested.

minerals (internally consistent database) in a given P–TFig. 6. Equilibrium phase diagram of sample CHM1 from Trescolmenspace. For a detailed description of DOMINO and itscalculated with the computer program DOMINO of de Capitaniabilities, the reader is referred to Biino & de Capitani (1994). Equilibrium assemblages are computed by Gibbs free energy

(1995) and to the homepage of the THERIAK-DOM- minimization on the basis of a specific bulk composition (see Table 5).The peak pressure assemblage (Grt+ Phe+ Pg+Ky+Qtz+H2O;INO software package (http://therion.minpet.unibas.ch/striped area) is overprinted by biotite and staurolite (dark grey andminpet/groups/theruser.html). An application of this ap- light grey area). The bold line limits the feldspar stability field to lower

proach to mafic eclogites of the middle Adula nappe has temperatures.been previously presented by Meyre et al. (1997).

The internally consistent mineral database ( JUN92)used for the calculations is an updated version of thatof Berman (1988). A table of the updated and newthermodynamic data that were used in this study andwere not published by Berman (1988) are listed in Table 4.In our calculations, solid solution models were consideredfor the following minerals: phengite, garnet, omphacite,feldspar, paragonite, and biotite (Table 5).

In samples CHM1 and CHM39, phengite was com-puted with a modified solid solution model of Massonne& Szpurka (1997). In addition to the published model byMassonne & Szpurka (1997) we included an extrapolationinto the ternary system according to Kohler (1960;Table 5). Kohler’s equation avoids a ternary parameterand is solely based on a geometrical extrapolation. Fora discussion of Kohler’s expansion, the reader is referredto the studies by Choi (1988) and Kirschen & De Capitani(1998). The solid solution model of Berman (1990) wasused for garnet. The other solution models do not in-corporate Mn end-members. The omphacite solid so-lution model by Meyre et al. (1997) was fitted to be Fig. 7. Equilibrium phase diagram for sample CHM39 from a foldedconsistent with the garnet solution model of Berman eclogite lens at Trescolmen (see Fig. 4). The peak pressure assemblage

Grt+ Phe+Ky+Qtz+H2O (striped area) is overprinted by biotite(1990) and the feldspar solution model of Fuhrman &(grey area). Below 500°C the diagram is unreliable (dashed lines).Lindsley (1988). Paragonite in sample MF2643 and Z6-

50-12 was modelled according to Chatterjee & Froese(1975). In this study, biotite is defined as binary and ideal Bulk compositions of the investigated samples were

derived from representative single analyses of the mineralssolution (mixing on site) between phlogopite and annite.

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for a whole-rock sample. That is, with DOMINO wecompute a local equilibrium of an observed assemblagerestricted to a small closed system.

The use of a ‘bulk composition’ that represents high-pressure conditions allows us to estimate these conditionsand to predict possible equilibria during the retrogradeevolution. This is only possible within a restricted pres-sure–temperature range, because all equilibrium phasediagrams are calculated with a constant bulk chemistry.Therefore, processes that affect the bulk (e.g. inflow oroutflux of fluids, minerals not in equilibrium, etc.) cannotbe entirely modelled. Simply using a whole-rock analysisby X-ray fluorescence would lead to uninterpretableresults, because parts of the sample that are not involvedin the equilibrium (e.g. cores of zoned minerals) wouldstrongly influence the calculated assemblages.

The bulk compositions used for calculations are listedin Table 6.

Calculation of stable assemblagesFig. 8. Equilibrium phase diagram for a sodic whiteschist from Tresco-

The equilibrium phase diagram (in the CNKFMASHlmen (sample Z6-50-12). The pressure and temperature conditions ofthe re-equilibration event (Gln+Grt+ Pg+Qtz+H2O; grey area) system) for sample CHM1 (Fig. 6) displays a large stabilityare in agreement with calculations of eclogite rocks from the same field for the assemblage Grt+ Phe+ Pg+ Ky+ Qtzlocality (Meyre et al., 1997). The peak pressure assemblage + H2O (striped field in Fig. 6). This assemblage isGln+Omp+Grt+Ky+Qtz+H2O is marked by the striped field.

believed to represent peak metamorphic conditions ofthe sample. The upper pressure limit (at ~25 kbar) forthis bulk composition is given by the stability of coesiteas well as omphacite at the expense of paragonite. Thebold line in Fig. 6 is the low-temperature limit of thefeldspar stability field. Towards lower pressures and tem-peratures, staurolite and biotite appear. Below 13 kbar,biotite is stable in addition to the peak pressure as-semblage (dark grey field). Staurolite appears below ~9kbar at 600°C (light grey area in Fig. 6).

This evolution under amphibolite facies conditionsis in good accordance with the observation of newlycrystallized, anhedral staurolite in pressure shadows ofgarnet and the epitactic overgrowth of white mica bybiotite. Staurolite growth seems to be the latest stage ofmetamorphism recorded in this area. However, one hasto be aware that only the ferrous end-member of staurolitewas considered and mixing properties towards the mag-nesium end-member were not included. This results inFig. 9. Qualitative phase diagram for whiteschist sample MF2643

(Zapporthorn locality). Talc and paragonite are overprinted by biotite small stability fields for staurolite-bearing assemblagesin the presence of garnet (striped area: peak pressure; grey area: re- compared with natural observations. The measured Mgequilibration event). content in staurolite from sample CHM1 is Mg/

(Mg+ Fe) = 0·19–0·22.The computation of sample CHM39 was performedthat are believed to be in equilibrium during eclogite

facies conditions combined with their modal proportions. in the sodium-free CKFMASH system, because sodium-bearing phases occurring in CHM39 are only within aThe modal proportions of the relevant phases are es-

timated by eye. The pure phases quartz and H2O are fine-grained symplectite consisting of Ab-rich plagioclaseand sodic–calcic amphibole. This symplectite is obviouslyadded in excess to ensure their presence in all com-

putations. The result is a simplified ‘bulk composition’, not in equilibrium with the rest of the sample (see thesection ‘Petrography and mineral chemistry’).valid only for a part of a thin section but not representative

206

ME

YR

Eet

al.H

PM

ET

APE

LIT

ES

FRO

MT

HE

AD

UL

AN

APPE

Table 4: Thermodynamic properties of end-members that are updated or were not published by Berman (1988)

Mineral Formula Comments∗ Ho(J) So(J/K) Vo(J/bar)

k0 k1 k2 k3 k6

v1 v2 v3 v4

Annite KFe3[AlSi3O10](OH)2 (1) −5142000·00 421·01001 15·483

727·20800 −4775·040 −13831900 2119060000 0·00000

3·44473262 0·00000000 −0·16969784 0·00000000

Al-Celadonite KAlMg[Si4O10](OH)2 (2) −5832415·00 288·5270 13·870

645·91500 −4129·540 −13864470 1978171000 0·00000

3·35270000 0·00000000 −0·17169000 0·00042950

Fe–Al-Celadonite KAlFe[Si4O10](OH)2 (2) −5492287·00 303·1480 13·962

664·75500 −4553·110 −12459890 1871056000 0·00000

3·30000000 0·00000000 −0·17000000 0·00040000

Glaucophane Na2Mg3Al2[Si8O22](OH)2 (3) −11960500·00 535·0000 26·050

1717·50000 −19272·000 7050000 0·00000000 −0·12107

2·20000000 0·00088000 −0·11600000 0·00029000

Hedenbergite CaFeSi2O6 (4) −2837144·22 174·3314 6·795

296·70260 −1239·150 −7416518 893501213 0·00000

2·37026500 0·00050310 −0·07900000 0·00003923

Staurolite Fe4Al18Si7·5O44(OH)4 (4) −23765364·00 1005·3270 44·676

2577·67407 −16265·359 −61185888 8692047872 0·00000

1·79363460 0·00090396 −0·08000000 0·00000000

∗Comments: (1) Properties derived by Berman (1990), modified by McMullin et al. (1991). (2) Properties derived by Massonne & Szpurka (1997). (3) Propertiesderived by Evans (1990). (4) Provisional properties of database JUN92.RGB [update of Berman (1988)].Cp=k0+k1T−0·5+k2T−2+k3T−3+k4/T+k5T+k6T2; V(P,T)/V(1,298)=1+v1(T−298)+v2(T−298)2+v3(P−1)+v4(P−1)2. Units are in J, K and bar, v1, v2, v3 terms need tobe divided by 105, v4 by 108. k4 and k5 are zero for all end-members.

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Table 5: Solution models used for the calculation of equilibrium phase diagrams

Phase Solution model Comments∗ References

Phe ternary (Ms, Cel, FeCel), non-ideal (1) Massonne & Szpurka (1997)

Grt ternary (Grs, Prp, Alm), mixing on site, non-ideal (2) Berman (1990)

Omp ternary (Di, Jd, Hd), non-ideal Meyre et al. (1997)

Fsp ternary (Ab, K-fsp, An), Landau transition, non-ideal (3) Fuhrman & Lindsley (1998)

Pg binary (Ms, Pg), non-ideal (4) Chatterjee & Froese (1975)

Bt binary (Phl, Ann), mixing on site, ideal

aAnn=(XMFe)3; aPhl=(XM

Mg)3

∗Comments: (1) Margules parameter of binary joins from Massonne & Szpurka (1997) and ternary expansion according to

Kohler (1960): DGexcess=(WiijXi

2Xj)Xi+Xj

. Phengite solid solution was considered for the calculation of sample CHM1 and CHM39.

(2) In this study the quaternary end-member Sps, calibrated by Berman (1990) was not considered. (3) The structuraltransition of albite was included in the Landau formulation of Salje et al. (1985). (4) Paragonite solid solution was consideredfor the calculation of samples MF2643 and Z6-50-12.

the low-temperature limit of the assemblage Grt + PheTable 6: Bulk composition of investigated+ Ky + Qtz + H2O is a minimum.

samples used for DOMINO calculations The sample re-equilibrated in the biotite stability fieldat ~14 kbar and ~650°C during the overgrowth of white

Sample Bulk composition (modal proportion, mineral mica by biotite (see the section ‘Calculation of isopleths’).composition) The equilibrium phase diagram in the CNKFMASH

system for the sodic whiteschist Z6-50-12 from Trescol-CHM1 40 Phe(Ms0·67Cel0·33)+10 Grt(Alm0·73Prp0·25Grs0·02) men (Fig. 8) is characterized by the stability field of the

+10 Pg+20 Ky+100 Qtz+100 H2O peak pressure assemblage Gln + Omp + Grt + KyCHM39 10 Phe(Ms0·70Cel0·25FeCel0·05)+20 + Qtz + H2O at ~25 kbar and ~650°C (striped

Grt(Alm0·51Prp0·33Grs0·16)+20 Ky+50 Qtz+100 H2O field in Fig. 8). This assemblage is constrained by theZ6-50-12 20 Grt(Alm0·65Prp0·22Grs0·13)+10 Omp quartz–coesite transition towards higher pressures and

(Jd0·87Di0·07Hd0·06)+20 Ky+10 Gln+30 Qtz+100 H2O by the absence of glaucophane towards higher tem-MF2643 20 Grt(Alm0·54Prp0·29Grs0·17)+40 Tlc peratures. At lower pressures, paragonite replaces om-

+20 Wm(Pg0·88Ms0·12)+100 Ky+100 Qtz+100 H2O phacite and kyanite, resulting in the assemblage Gln +Grt+ Pg+ Qtz+ H2O, which was reached in the re-equilibration event during the ‘Trescolmen’ deformationphase (grey field in Fig. 8). Below ~19 kbar, the feldsparBecause we are not able to reconstruct the compositionstability field restricts the re-equilibration assemblage toof the former omphacite from the observed symplectite,temperatures below 700°C. For sample Z6-50-12, wewe excluded the symplectitic phases from consideration.have chosen a phengite-free assemblage for calculationTherefore the computation of sample CHM39 representsbecause of inconsistencies between the used solid solutiononly a part of the high-pressure assemblage (withoutmodels. For unknown reasons, the solid solution modelomphacite).for phengite (Massonne & Szpurka, 1997) does not agreeThe part of the high-pressure assemblage (Grt + Phewell with the muscovite–paragonite model of Chatterjee+ Ky + Qtz + H2O) observed in sample CHM39 is& Froese (1975). This results in a large number of smallstable over a large P–T interval (Fig. 7). The upperstability fields of unrealistic assemblages. Glaucophanepressure limit is constrained by the quartz–coesite trans-was taken into account as Mg end-member.ition, whereas to lower temperatures, the absence of

The qualitative equilibrium phase diagram for samplechlorite is a limiting, but weak constraint for this as-MF2643 (Fig. 9) is restricted to only a part of the observedsemblage. Because chlorite was only considered as Mgassemblages (peak pressure and re-equilibration event).end-member clinochlore, the equilibrium phase diagramThe main feature that can be deduced from this diagramremains ambiguous below 500°C. However, the con-is the biotite overprint of the peak pressure assemblagesideration of a chlorite solution model (e.g. an Fe–MgGrt + Pg + Tlc + Ky + Qtz + H2O. With thischlorite) would result in increased stability fields of the

chlorite-bearing assemblages. Therefore we conclude that diagram, we reach the actual limits of thermodynamic

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modelling. The crucial input parameters of the cal-culations are—besides the bulk composition of the equi-librium phases—the solid solution models for the mixingphases, which are often not yet calibrated. The mainproblem with the investigated sample is the thermo-dynamic behaviour of amphibole with barroisitic to glau-cophane composition. Whereas in sample Z6-50-12amphibole is glaucophane of almost pure end-membercomposition, amphibole in sample MF2643 is of bar-roisitic composition (Table 3). This barroisite could not bethermodynamically modelled because of the still unknownthermodynamic mixing behaviour between sodic, sodic–calcic and calcic amphiboles. Because of the absence ofa reliable solution model for sodic–calcic amphiboles wewere unable to model the entire observed assemblage ofMF2643.

Calculation of isoplethsThe results of isopleth calculations strongly depend onbulk composition and the variance of the assemblage.They are valid only for the specific sample they arecalculated for. For sample CHM1 and CHM39, wecomputed isopleths for the end-members grossular(Ca3Al2Si3O12), almandine (Fe3Al2Si3O12) and pyrope(Mg3Al2Si3O12) in garnet and aluminoceladonite (KMgAl-[Si4O10](OH)2) in phengite (Fig. 10a and b).

The composition of garnet within the large stabilityfield of the observed assemblage Grt + Phe + Ky +Qtz + H2O is almost constant for sample CHM39(Fig. 10b; compare Fig. 7). For sample CHM1 (Fig. 10a),the garnet composition is more variable but still muchmore uniform than that of phengite. The wide spacingof isopleths is a possible reason for the absence of azonation in garnet: if garnet grows within a small P–Twindow at peak pressure conditions, it attains a com-position that is stable during almost the entire isothermaldecompression (from 22 kbar to 10 kbar at 650°C).

During the retrograde path, phengite adapts its com-position continuously to the changing equilibrium con-ditions. This results in a strong zonation in silica content(celadonite component) from core to rim. The pressure(and temperature) dependence of phengite compositionfor these bulk chemistries can therefore be used for

Fig. 10. Calculated isopleths for aluminoceladonite component inthermobarometric estimations. In Fig. 10a and b isoplethsphengite and of pyrope, grossular and almandine in garnet. Isopleths

of aluminoceladonite content in phengite are plotted, of aluminoceladonite are based on a modified solid solution model forphengite by Massonne & Szpurka (1997). Garnet isopleths are basedbased on a modified solid solution model for phengite ofon the solution model by Berman (1990). Bold lines refer to measuredMassonne & Szpurka (1997). These isopleths are validrim compositions of phengite and garnet; dashed lines refer to coreonly for the bulk chemistries used to calculate the equi- compositions of zoned phengite. The circles represent the conditions

librium phase diagrams of CHM1 and CHM39 in com- of re-equilibration, that is, the region of crosscutting of the isopleths.(a) Isopleths, valid for the bulk composition of sample CHM1 (seebination with the considered solid solution models (FigsFig. 6 and Table 5). Sample CHM1 re-equilibrated at ~10–11 kbar6 and 7). For the bulk composition of sample CHM1 and ~650°C. (b) Isopleths, valid for the bulk composition of sample

the zonation of the silica content in phengite from Si CHM39 (see Fig. 7 and Table 5). Sample CHM39 re-equilibrated at~14 kbar and ~650°C.~3·38 p.f.u. (XCel= 0·25) in the core to Si ~3·17 (XCel=

209


0·10) in the rim reflects a pressure decrease from ~21kbar (core) to ~11 kbar (rim) at 650°C. The zonation ofthe silica content in phengite for sample CHM39 rangesfrom Si ~3·4 p.f.u. (XCel = 0·32) in the core to Si ~3·3p.f.u. (XCel = 0·25) in the rim. This reflects a pressuredecrease from ~19 kbar (core) to ~14 kbar (rim) at650°C.

To estimate the relevance of the pressure and tem-perature determination, it is important to evaluate theuncertainties of the calculations. The position of theboundaries of the different stability fields mainly dependson the calibration of the defined solid solution modelsand the uncertainty of the thermodynamic end-memberdata. This results in a moderate relative error betweenthe different stability fields, but the absolute position inpressure and temperature may vary within ~±1 kbarand ~±50°C.

Expected uncertainties of mineral compositions withinthe stability fields for the simplified systems are essentiallygiven by the uncertainty of the experimental data. Adirect comparison with natural systems may include evenlarger uncertainties, because no activity corrections forminor elements (e.g. Cr, Mn, Zr, etc.) are considered inthe solid solution models. As a rule of thumb, we assumean uncertainty of±2 mol %, which roughly correspondsto the error of electron microprobe measurements. Thiserror is also reflected in the isopleth computations(isopleths are displayed in steps of 2 mol % in Fig. 10aand b).

Fig. 11. Proposed P–T path for investigated metapelitic rocks of themiddle Adula nappe. This path is in accordance with the evolution ofeclogites from the same area (Meyre et al., 1997). The striped field (1)represents peak pressure conditions; in light grey (2) the stability fieldDISCUSSIONof paragonite overprint is marked (both for sample Z6-50-12, sodic

From the above results, we can confine the pressure and whiteschist). The ‘Trescolmen’ phase (re-equilibration event) is con-strained by the overlap of field (2) with the stability field of amphibole-temperature conditions of two distinct events (‘Tresco-bearing eclogite (3) from the same area (Meyre et al., 1997). Thelmen’ and ‘Zapport’ phases) during the tectonic andisopleths for celadonite content in phengite from sample CHM1 are

metamorphic evolution of the middle Adula nappe helpful to model the retrograde path. Sample CHM1 re-equilibrated(Fig. 11). These conditions coincide very well with the at ~9–10 kbar and ~650°C (‘Zapport’ phase). The wet solidus of

granite is taken from Huang & Wyllie (1974).published P–T path of Meyre et al. (1997), based ongeothermobarometry of mafic eclogites, as well as with theproposed P–T path of Partzsch (1998) for the structurallyupper part of the Adula nappe. Meyre et al. (1997) mainly kbar at ~600–700°C). The observed peak pressure as-focused on the ‘Trescolmen’ phase, where amphibole semblages of the other investigated samples are stableoverprints the peak pressure assemblage Grt+ Omp+ over a large area in P–T space. This is particularly trueKy + Qtz in mafic eclogites. Partzsch (1998) dealt with for samples CHM1 and CHM39. The high variance ofthe geodynamic evolution and the regional context of the system is due to the fact that only few phases arethe high-pressure metamorphism of the Adula nappe. involved in the equilibrium (garnet is the only phase

containing calcium, paragonite component in white micafixes sodium, etc.). The absence of zonation in garnets

Peak pressure conditions of samples CHM1 and CHM39 can be explained by thishigh variance during the retrograde path. Growth withinIn this study [as well as in that of Meyre et al. (1997)]a small P–T field under prograde conditions might pro-peak pressure conditions cannot be very well constrained.duce an unzoned garnet, which is not affected duringThe assemblage Omp + Grt + Gln + Ky + Qtz +part of the retrograde path because garnet remains stableH2O of the sodic whiteschist sample Z6-50-12 seems to

be best suited to define the high-pressure climax (~25 with (almost) the same chemical composition.

210


The continuous retrogression of metapelitic rocks from‘Trescolmen’ phaseeclogite facies conditions to amphibolite facies conditionsThe assemblage Gln + Grt + Pg + Qtz + H2O(‘Lepontine’ metamorphism) that is presented in this(sample Z6-50-12), which represents the re-equilibrationstudy supports the geodynamic model of Schmid et al.event during the retrograde path (replacement of om-(1996) for the Tertiary orogeny in the Central Alps. Inphacite and kyanite by paragonite), is stable over a largethis model, the Adula nappe represents the southern tiparea in pressure and temperature. However, comparedof Europe in Early Paleocene time. During convergence,with the evolution of the mafic eclogite sample CHM30and finally, collision of the upper crust of the Apulianof Meyre et al. (1997) from the same locality, there is anmargin with the European margin (closure of the Valaisoverlapping field only at ~21 kbar and 650°C. In theocean during the Early to Late Eocene), the Adula nappeeclogite sample the re-equilibration event can be cor-was subducted to great depths. Forced extrusion parallelrelated with the ‘Trescolmen’ deformation phase (Meyreto the subduction shear zone seems to be the most likely& Puschnig, 1993; Meyre et al., 1997; Partzsch, 1998).mechanism for differential exhumation of the AdulaThis correlation is also reasonable for the sodic whiteschistnappe in respect to both higher and lower tectonicsample.units, which did not suffer eclogite facies metamorphismCalculations of isopleths of the solution phases (samples(Schmid et al., 1996).CHM1 and CHM39) support the estimated conditions

for the ‘Trescolmen’ phase and reveal additional in-formation on the pressure and temperature evolution(Fig. 11). The isopleth for a silica content of Si = 3·38p.f.u. implies a pressure of ~21 kbar at 650°C forthe assemblage Grt+ Phe+ Pg+Ky+Qtz+H2O in CONCLUSIONSsample CHM1. This is consistent with the assumed (1) In the Adula nappe high-pressure relics are notpressure conditions for eclogite samples of this area [re- restricted to the well-known eclogite lenses, but are alsoequilibration event of Meyre et al. (1997); see Heinrich found in metapelitic lithologies. The investigated samples(1986)]. However, pressure estimations for sample display whiteschist (MF2643) to sodic whiteschist (Z6-CHM39 indicate significantly lower pressures (~18–19 50-12) mineralogy as well as ‘normal’ metapelitic as-kbar at 650°C; dashed line in Fig. 10b), which are slightly semblages (Grt + Ky + Phe + Qtz ± Bt ± St)lower than the expected conditions of the re-equilibration (CHM1, CHM39).event and the ‘Trescolmen’ deformation phase (~21 kbar; (2) The calculation of equilibrium phase diagrams withMeyre et al., 1997). The low celadonite content of phengite the computer code DOMINO is well suited to modelcore in sample CHM39 may be explained by diffusional the evolution of these rocks at medium temperatures.homogenization of the silica distribution in phengite. However, peak metamorphic conditions cannot be con-

strained well because of the high variance of the observedassemblages as well as their large stability fields.

(3) The petrological data for the metapelitic samples‘Zapport’ phase presented in this study are in good agreement with

geothermobarometric studies on eclogite lenses of theThe investigated samples of this study reveal highersame area (Heinrich, 1986; Meyre et al., 1997). Fur-pressures for the ‘Zapport’ phase than the work ofthermore, metapelitic rocks of the Adula nappe displayPartzsch (1998) (6·5–8 kbar and 640–700°C): samplethe entire structural and petrological retrograde evolutionCHM1 equilibrated at ~11 kbar and 650°C, sampleof this Alpine unit.CHM39 at 14 kbar and 650°C, both in the stability field

(4) Pelitic rocks with the stable assemblage Grt+ Pheof biotite (filled circles in Fig. 10a and b). The absence± Pg + Bt + Ky + Qtz + H2O equilibrated atof feldspar in sample CHM1 is an important constraintconditions of 14 kbar and 650°C (sample CHM39) andtowards higher temperatures, especially for the ‘Zapport’10–11 kbar and 650°C (sample CHM1), and reflectphase (see Fig. 6). This limit is in accordance with theconditions in the late stages of the main deformationindependent observation that no melting of gneisses andevent (‘Zapport’ deformation phase).metapelitic rocks has been observed in this field area

(5) The presence of high-pressure relics in metapelitic(melting curve in Fig. 11).lithologies of the middle Adula nappe implies that notThe constant temperature during decompression isonly part of it (i.e. mafic eclogite lenses) but the entireprobably due to the influence of the ‘Lepontine’ meta-Adula nappe was subducted to great depths. This fact,morphism, which mainly heated up the southern part ofcombined with structural observations, makes a Tertiarythe Central Alps under amphibolite facies conditionsage for the high-pressure metamorphism of the Adula(Wenk, 1956; Todd & Engi, 1997) but also interfered

with the retrograde evolution of the middle Adula nappe. nappe most probable.

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Frey, M., Hunziker, J. C., Schmid, S. M., Thoenen, T. & Trommsdorff,ACKNOWLEDGEMENTSV. (1992). Bericht uber die Exkursion der Schweizerischen Min-

We thank T. Thoenen for performing electron micro- eralogischen und Petrographischen Gesellschaft zum Thema ‘Hoch-druck-Metamorphose in der Adula-Decke’ (29. September bis 5.probe (EMP) measurements on sample MF2643. FruitfulOktober 1991). Schweizerische Mineralogische und Petrographische Mit-collaboration with T. Nagel, J. H. Partzsch and E. Zinn-teilungen 72(2), 271–279.grebe is gratefully acknowledged. We are thankful for

Frischknecht, G. (1923). Geologie der ostlichen Adula. Beitrage zurthe constructive and helpful reviews by T. Holland,Geologischen Karte der Schweiz, N.F. 51, 65–93.

G. Markl and O. Vidal. Motivating late-night debates of Fuhrman, M. L. & Lindsley, D. H. (1988). Ternary feldspar modelingC.M. with C. Manning, A. Matthews and J. Amato during and thermometry. American Mineralogist 73, 201–215.

Gebauer, D. (1996). A P–T–t path for an (ultra?-) high-pressure ultra-the 5th International Eclogite Conference, Ascona, aremafic/mafic rock-association and its felsic country-rocks based onappreciated. We thank S. Th. Schmidt and H.-P. MeyerSHRIMP-dating of magmatic and metamorphic zircon domains.for help at the EMPs at the Universities of Basel andExample: Alpe Arami (Central Swiss Alps). In: Basu, A. & Hart, S.Heidelberg, respectively. This study is part of the Ph.D. (eds) Earth Processes: Reading the Isotopic Code. Geophysical Monograph,

thesis of C.M., which was financially supported by Swiss American Geophysical Union 95, 307–329.National Science Foundation Grants 20-39130.93 and Heinrich, C. A. (1982). Kyanite-eclogite to amphibolite facies evolution

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petrology of high-pressure metapelites from the adula nappe

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