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

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1 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 1 PAGES Petrology of High-Pressure Metapelites from the Adula Nappe (Central Alps, Switzerland) CHRISTIAN MEYRE 1, CHRISTIAN DE CAPITANI 1, THOMAS ZACK 2 AND MARTIN FREY 1 1 MINERALOGISCH-PETROGRAPHISCHES INSTITUT DER UNIVERSITÄT BASEL, BERNOULLISTRASSE 30, CH-4056 BASEL, SWITZERLAND 2 MINERALOGISCH-PETROGRAPHISCHES INSTITUT UNIVERSITÄT GÖTTINGEN, GOLDSCHMIDTSTRASSE 1, D GÖTTINGEN, 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 structurally Alps, 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-pressure high-pressure relics, even though most of the felsic lithologies were terranes in the world (e.g. Dabie-Shan, China; Caleretrogressed during a later amphibolite facies overprint. Calculations donides, Norway), only very few relics of high-pressure of equilibrium phase diagrams of whiteschist and sodic whiteschist metamorphism have been reported within felsic lithassemblages reveal conditions of P>20 kbar at T ~650 C. ologies (metapelitic rocks, orthogneisses) until now. This Common 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 eclogite and temperature. Calculations of the peak pressure conditions in the facies conditions or only part of it, that is, the eclogite investigated metapelite samples are in good agreement with analogue lenses, which were buried to great depths and later calculations of eclogite samples. These results combined with struc- emplaced in amphibolite facies country rocks. This latter tural 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 event under amphibolite facies conditions. In contrast, the assumption that the entire Adula nappe underwent eclogite facies conditions presumes KEY WORDS: Adula nappe; Central Alps; equilibrium phase diagram; a continuous retrograde evolution from high-pressure geothermobarometry; high-pressure metapelite conditions to amphibolite and upper greenschist grade within a single pressure temperature loop (Löw, 1987; Meyre & Puschnig, 1993; Partzsch et al., 1995b). Because the main foliation of the Adula nappe can be correlated INTRODUCTION with Oligocene structures in the allochthonous Mesozoic The regional high-pressure metamorphism under eclogite cover of the Penninic nappes ( Bünderschiefer metafacies conditions in the Adula nappe (Central Alps, Switzerland; sediments) and in the overlying nappes as well, this main Fig. 1) is well known mainly from investigations foliation must have developed in Late Eocene to Early Corresponding author. Telephone: Fax: meyre@ubaclu.unibas.ch Oxford University Press 1999

2 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 1 JANUARY 1999 Fig. 1. Simplified tectonic map of the Adula nappe Cima Lunga unit. The localities of the investigated samples are Zapporthorn (east of San Bernardino) and Trescolmen (east of Mesocco). REGIONAL GEOLOGY The Adula nappe belongs to the Penninic domain of the Swiss Central Alps and represents the palaeogeographically southernmost part of the European margin at a time before the closure of the Valais trough in the early Eocene (Schmid et al., 1990, 1996). In contrast to the Tambo nappe in the hanging wall and the Simano nappe in the footwall, the Adula nappe is characterized by a regional high-pressure metamorphism. The metamorphic conditions of this event gradually increase from north (blueschist facies conditions; C, kbar at Vals) to south (very high-p conditions; C, kbar at Alpe Arami; Heinrich, 1986). This highpressure metamorphism is followed by fast exhumation to amphibolite and greenschist facies conditions. Whereas eclogite assemblages are widespread in the core of metabasic lenses, high-pressure relics in felsic lithologies (metapelite and leucocratic gneisses) are concealed or even completely destroyed. Five deformation phases can be distinguished in the middle Adula nappe, four of them related to the exhumation process. A more detailed description of the structural evolution of the Adula nappe has been given by Löw (1987), Meyre & Puschnig (1993), Partzsch & Meyre (1995) and Partzsch (1998). The Sorreda deformation phase (D 1 ) is correlated with the imbrication of Mesozoic sediments ( Triassic quartzites and marbles) and the pre-mesozoic basement of the European margin. Löw (1987) suggested prograde conditions under elevated pressures for D 1. The Trescolmen phase (D 2 ) occurred under eclogite facies conditions and is represented by recrystallized omphacite and elongated garnet aggregates that form a distinct foliation in eclogite lenses. This deformation phase took place under early retrograde conditions. The Zapport deformation phase (D 3 ) is associated with the main foliation in the middle Adula nappe. Deformation started under high-pressure conditions and continued well into amphibolite facies con- ditions. This main foliation affected also the overlying Tambo and Suretta units and was therefore still active when the Penninic nappes were piled up. Related to the main foliation is an apparent stretching lineation as well as isoclinal folding. The fourth deformation phase ( Leis phase; D 4 ) appears as open folds in gneisses and crenulation in lithologies with a high amount of sheet silicate minerals. Metamorphic conditions are lower amphibolite to upper greenschist facies. The latest visible deformation phase ( Carassino phase) mainly affects the frontal part of the Adula nappe in the north and fades out towards the south. In the geographically central part of the nappe, the Carassino phase is related to a slight undulation (occasional kinking) of gneisses. Oligocene time (Schmid et al., 1996). This implication as well as geochronologic data from the Cima Lunga unit (Becker, 1993; Gebauer, 1996) and the assumption of a single continuous pressure temperature path for the Adula nappe imply a Tertiary age for the high-pressure metamorphism. One way to support this hypothesis is to find highpressure relics in felsic lithologies and to demonstrate that they experienced the same metamorphic and structural evolution as the eclogite lenses. Heinrich (1982) described white mica with highly phengitic cores in metapelitic samples and proposed a regional high-pressure metamorphism for the entire Adula nappe (Heinrich, 1986). In this study we concentrate on the petrologic evolution of high-pressure metapelitic rocks of the middle Adula nappe. We apply the thermodynamic approach of Gibbs free energy minimization to calculate stable assemblages and present some possible mineral equilibria for these rocks. From these petrologic calculations and from textural observations we conclude that the metamorphic evolution of the investigated metapelitic samples is in agreement with the pressure and temperature evolution Detailed descriptions of the regional geology of the of the eclogite rocks and with the structural evolution of Adula nappe have been given by Frischknecht (1923), the middle Adula nappe as well. Heinrich (1983, 1986), Löw (1987) and Partzsch (1998). 200

3 MEYRE et al. HP METAPELITES FROM THE ADULA NAPPE PETROGRAPHY AND MINERAL Table 1: Selected electron microprobe CHEMISTRY analyses of garnet used to calculate the The investigated samples of this study were collected at equilibrium phase diagrams; end-member the localities Zapporthorn ( MF2643) and Trescolmen (CHM1, CHM39 and Z ) in the central part of distribution after Deer et al. (1967) the Adula nappe (see Fig. 1). Sample: Z MF2643 CHM1 CHM39 We analysed the minerals of samples CHM1, CHM39 Analysis no.: and MF2643 with a JEOL JXA-8600 electron microprobe at the University of Basel (Switzerland). The micro- SiO probe is equipped with Voyager software by Noran TiO Instruments. The operating conditions were set at 15 kv Al2 O and 10 na, and the correction procedure was PROZA FeO (Bastin & Heijligers, 1990). Minerals of sample Z MnO were analysed with a Cameca SX51 electron microprobe MgO with conditions of 15 kv and 20 na at the University of CaO Heidelberg (Germany). The correction procedure was a Na 2 O ZAF routine. For all analyses natural and synthetic K 2 O crystals were used as standards. Tables 1, 2 and 3 show microprobe analyses of garnet, Σ white mica, talc, biotite, amphibole, and jadeitic pyr- Normalized to 24 oxygens oxene. These analyses are representative for the as- Si semblages that were stable at conditions of the highpressure Ti metamorphism or the amphibolite facies over- Al print. Fe The phengite analyses in Table 2 are projected to Mn the ternary system with the end-members muscovite, Mg aluminoceladonite and ferro-aluminoceladonite. A de- Ca composition into these three end-members cannot com- Na prehensively characterize the measured micas (e.g. the K paragonite component is completely ignored). This sim- Σ plification is necessary to use the available solid solution model for white mica [modified after Massonne & X(Grs+Adr) Szpurka (1997)] in our computations (see the section X(Prp) Equilibrium phase diagrams ). X(Alm) In the following, mineral abbreviations are after Kretz (1983) and Bucher & Frey (1994), and Wm indicates white mica. X(Sps) Garnet white mica kyanite schist from Trescolmen (CHM1) Sample CHM1 comes from a lens of several metres of metapelite (garnet white mica kyanite schist) within orthogneisses from the Trescolmen area (Swiss co- ordinates: / ). Large garnet crystals, up to 2 cm in size, with inclusions of kyanite (several mil- limetres in size), white mica and quartz are abundant. Besides quartz and kyanite further main components of this rock are phengitic muscovite and paragonite defining the distinct main foliation [ Zapport deformation phase; after Löw (1987) and Partzsch et al. (1995a)]. Biotite only occurs as a retrograde product of muscovite. Small anhedral staurolite grows in pressure shadows of garnet within the main foliation (Fig. 2). No feldspar was detected in this sample. Garnet is essentially unzoned. Slightly decreasing Prp values towards the rim are interpreted as diffusional reequilibration (Fig. 3). Phengite is zoned with respect to Si content, with values up to Si = 3 4 p.f.u. in the core decreasing to Si = 3 1 in the rim. Phengitic white mica grown in shearbands (related to late stages of Zapport ) systematically reveals low Si values (Si = 3 1 p.f.u.) and is unzoned. Representative analyses of garnet, phengite and paragonite of sample CHM1 are listed in Tables 1 and 2. We assume that the main foliation began to evolve under high-pressure conditions (core of phengitic white mica) and continued under amphibolite facies conditions 201

4 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 1 JANUARY 1999 Table 2: Representative electron microprobe analyses of phengite, paragonite, talc and biotite Sample: CHM39 CHM39 CHM1 CHM1 Z CHM1 Z MF2643 MF2643 MF2643 Analysis no.: phe pg 38pg 9tc 13bt Phe (rim) Phe (core) Phe (rim) Phe (core) Phe Pg Pg Pg Tc Bt SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Σ Normalized to 22 oxygen Si Ti Al Fe Mn Mg Ca Na K Σ Si p.f.u Fe/(Fe+Mg) K/(Na+K) X(Ms) X(Cel) X(Fe-Cel) X(Ms)=4 Si p.f.u.; X(Cel)=[1 X(Ms)] Mg/(Fe+Mg); X(Fe-Cel)=[1 X(Ms)] Fe/(Fe+Mg). (growth of staurolite, phengite in shearbands). This structural and metamorphic evolution can be observed within metapelites of the entire Adula nappe (Heinrich, 1982; Löw, 1987; Meyre & Puschnig, 1993; Partzsch, 1998). Garnet white mica kyanite quartzite from Trescolmen (CHM39) For the Trescolmen locality, Heinrich (1982), Meyre & Puschnig (1993) and Partzsch (1998) have described a crucial outcrop (Swiss coordinates: / ) that shows a mafic lens folded under eclogite facies conditions. In the fold hinge, high-pressure relics are preserved in a metasedimentary quartzitic rock (Fig. 4). This rock has a distinct foliation defined by white mica flakes (phengite). Garnet and kyanite are abundant besides the major component quartz. Minor components are apatite, rutile and ilmenite. Rims of phengitic white mica are epitactically overgrown by biotite (see Heinrich, 1982). Phengite 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). This zonation can be systematically observed in metapelites of the upper Adula nappe and seems to be related to equilibration during decompression (Heinrich, 1982; Partzsch, 1998). Garnet is essentially unzoned in the core (core com- position: Alm 51 Prp 33 Grs 15 Sps 1 ). A slight zonation towards the rim with respect to pyrope and grossular (Alm 50 Prp 28 Grs 21 Sps 1 ) can be observed. 202

5 MEYRE et al. HP METAPELITES FROM THE ADULA NAPPE Table 3: Selected electron microprobe Gagela (Swiss coordinates / ). The investigated sample (Z ) has three zones: a phengiteanalyses of amphibole and pyroxene rich layer followed by a 2 cm wide quartz-rich layer grading into eclogite that consists mainly of garnet and Sample: MF2643 MF2643 Z Z clinopyroxene. Jadeitic clinopyroxene and Mg-glau- Analysis no.: Am1/5 Am1/ a c cophane are present in the intermediate quartz-rich layer Bar Bar Gln Jd coexisting with quartz, garnet, kyanite, phengite, and SiO rutile. The foliation in all three zones is parallel to the TiO layering and defined by aligned phengite, amphibole and clinopyroxene, respectively. Al 2 O A similar assemblage with jadeite and Mg-glaucophane FeO was described by Kienast et al. (1991) from the Dora MnO Maira Massif and called sodic whiteschist, a term also MgO used here. CaO In thin section, garnet, jadeite, Mg-glaucophane, ky- Na 2 O anite and quartz can be defined as primary phases and K 2 O represent the peak pressure assemblage. Zircon, allanite H 2 O and rutile occur as accessory phases. Recrystallization Σ still under high-pressure conditions ( Trescolmen phase) Normalized to: 23 oxy 23 oxy 23 oxy 4 cat led to the formation of paragonite and magnesite that Si grew around kyanite and glaucophane. Al(iv) Retrogression under amphibolite conditions led to the Al(vi) formation of fine-grained symplectite consisting of plagioclase and amphibole around jadeite and glaucophane, Ti with jadeite more strongly affected than glaucophane. Fe Garnet shows a distinct and continuous zoning (Fig. 5) Fe without any break in the pattern except for the outermost Mn rim. Fe and Mn decrease towards the rim whereas Mg Mg increases, typical for growth zoning under prograde Ca conditions. In contrast, Ca shows an almost flat pattern Na with only a slight but noticeable increase from core to K rim. Only at the outermost rim (25 μm) does Ca content OH drop significantly from Grs 15 to Grs 8. This drop is prob- Σ ably due to the formation of amphibole and plagioclase X(Jd) 0 84 (An ) in symplectite around garnet during late retro- X(Di) 0 08 gression. X(Hd+Ae) 0 08 Clinopyroxene is close to jadeite end-member composition [X Jd = ; classification after Morimoto Calculated H 2 O. (1988)], whereas primary amphibole is close to pure Mineral analyses of sample Z were used to calculate Mg-glaucophane, with X(Na,M4) up to 0 95, Al(vi)/ equilibrium phase diagrams. Amphibole normalized to 23 [Fe 3+ + Al(vi)] = 0 94 and Mg/(Mg + Fe) = oxygens (Tindle & Webb, 1994; Leake et al., 1997). Pyroxene Mineral analyses of sample Z are listed in analysis normalized to four cations (after Morimoto, 1988). Tables 1, 2 and 3. Symplectic pseudomorphs of albite and amphibole after 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 Santini garnet and phengite of sample CHM39. [1991, see also Frey et al. (1992); Swiss coordinates: / ]. A large eclogite lens of 60 m length lies within leucocratic gneisses. Metapelite occurs at the Sodic whiteschist from Trescolmen (Z6-50- rim of the lens and as metasomatized veins of several 12) centimetres thickness within the boudin. The mineralogy At Trescolmen, rare metapelitic rocks with a high-pressure paragenesis occur as blocks below the Cima di typical whiteschist association of talc + kyanite + of the associated metapelite and the veins represents a white 203

6 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 1 JANUARY 1999 Fig. 2. Schematic micrograph of garnet clast within a matrix consisting of phengitic muscovite, paragonite and quartz (sample CHM1). Anhedral staurolite 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 Löw (1987) and Partzsch (1998)]. Additional phases are kyanite and rutile. No feldspar could be observed. Fig. 3. Profile of single garnet clast in sample CHM1. The core is unzoned whereas a weak zonation caused by diffusion is restricted to the outermost rim. A semi-inclusion of white mica (at ~0 2 mm from rim) is connected to the matrix. The zonation around white mica therefore reflects rim composition. Fig. 4. Schematic view of outcrop from Trescolmen area (see Meyre & Puschnig, 1993). The metabasic lens represents a fold with an eclogite core. The rim is overprinted under amphibolite facies conditions. Quartz-rich metapelite (assemblage Qtz + Grt + Ky + Phe + Rt) is enclosed in the hinge of the fold. S2 refers to the Trescolmen deformation phase, S3 to the Zapport deformation phase. grain boundaries overgrow the peak pressure assemblage. mica + garnet (Schreyer, 1973, 1977). Major com- No reaction rim or symplectite occurs around barroisitic ponents 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 different metamorphic stages can be observed: the peak pressure assemblage (garnet + talc + paragonite + kyanite ± EQUILIBRIUM PHASE DIAGRAMS quartz) is affected by retrograde phlogopite that mainly The equilibrium phase diagrams (Figs 6 9) for metapelitic replaces talc [re-equilibration event; see Meyre et al. samples from the middle Adula nappe were calculated (1997)]. Barroisitic amphibole (representative analyses in with the computer code THERIAK-DOMINO (de Capitani Table 3) appears to be stable under conditions of the reequilibration & Brown, 1987; de Capitani, 1994). This program event because euhedral grains with straight performs a Gibbs free energy minimization for a set of 204

7 MEYRE et al. HP METAPELITES FROM THE ADULA NAPPE Fig. 5. Profile (rim to rim) of single garnet clast in sample Z From the zonation (decreasing Sps content and increasing Prp content from core to rim) a prograde growth is suggested. minerals (internally consistent database) in a given P T space. For a detailed description of DOMINO and its Fig. 6. Equilibrium phase diagram of sample CHM1 from Trescolmen calculated with the computer program DOMINO of de Capitani abilities, 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). INO software package ( The peak pressure assemblage (Grt + Phe + Pg + Ky + Qtz + H 2 O; striped area) is overprinted by biotite and staurolite (dark grey and minpet/groups/theruser.html). An application of this aplight 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 that of Berman (1988). A table of the updated and new thermodynamic data that were used in this study and were not published by Berman (1988) are listed in Table 4. In our calculations, solid solution models were considered for the following minerals: phengite, garnet, omphacite, feldspar, paragonite, and biotite (Table 5). In samples CHM1 and CHM39, phengite was computed with a modified solid solution model of Massonne & Szpurka (1997). In addition to the published model by Massonne & Szpurka (1997) we included an extrapolation into the ternary system according to Kohler (1960; Table 5). Kohler s equation avoids a ternary parameter and is solely based on a geometrical extrapolation. For a discussion of Kohler s expansion, the reader is referred to the studies by Choi (1988) and Kirschen & De Capitani (1998). The solid solution model of Berman (1990) was used for garnet. The other solution models do not incorporate Mn end-members. The omphacite solid solution model by Meyre et al. (1997) was fitted to be Fig. 7. Equilibrium phase diagram for sample CHM39 from a folded consistent with the garnet solution model of Berman eclogite lens at Trescolmen (see Fig. 4). The peak pressure assemblage (1990) and the feldspar solution model of Fuhrman & Grt + Phe + Ky + Qtz + H 2 O (striped area) is overprinted by biotite Lindsley (1988). Paragonite in sample MF2643 and Z6- (grey area). Below 500 C the diagram is unreliable (dashed lines) was modelled according to Chatterjee & Froese (1975). In this study, biotite is defined as binary and ideal Bulk compositions of the investigated samples were solution (mixing on site) between phlogopite and annite. derived from representative single analyses of the minerals 205

8 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 1 JANUARY 1999 for a whole-rock sample. That is, with DOMINO we compute a local equilibrium of an observed assemblage restricted to a small closed system. The use of a bulk composition that represents highpressure conditions allows us to estimate these conditions and to predict possible equilibria during the retrograde evolution. This is only possible within a restricted pressure temperature range, because all equilibrium phase diagrams are calculated with a constant bulk chemistry. Therefore, processes that affect the bulk (e.g. inflow or outflux of fluids, minerals not in equilibrium, etc.) cannot be entirely modelled. Simply using a whole-rock analysis by X-ray fluorescence would lead to uninterpretable results, because parts of the sample that are not involved in the equilibrium (e.g. cores of zoned minerals) would strongly influence the calculated assemblages. The bulk compositions used for calculations are listed in Table 6. Fig. 8. Equilibrium phase diagram for a sodic whiteschist from Trescolmen (sample Z ). The pressure and temperature conditions of the re-equilibration event (Gln + Grt + Pg + Qtz + H 2 O; grey area) are in agreement with calculations of eclogite rocks from the same locality (Meyre et al., 1997). The peak pressure assemblage Gln + Omp + Grt + Ky + Qtz + H 2 O is marked by the striped field. Fig. 9. Qualitative phase diagram for whiteschist sample MF2643 (Zapporthorn locality). Talc and paragonite are overprinted by biotite in the presence of garnet (striped area: peak pressure; grey area: reequilibration event). that are believed to be in equilibrium during eclogite facies conditions combined with their modal proportions. The modal proportions of the relevant phases are estimated by eye. The pure phases quartz and H 2 O are added in excess to ensure their presence in all computations. The result is a simplified bulk composition, valid only for a part of a thin section but not representative Calculation of stable assemblages The equilibrium phase diagram (in the CNKFMASH system) for sample CHM1 (Fig. 6) displays a large stability field for the assemblage Grt + Phe + Pg + Ky + Qtz + H 2 O (striped field in Fig. 6). This assemblage is believed to represent peak metamorphic conditions of the sample. The upper pressure limit (at ~25 kbar) for this bulk composition is given by the stability of coesite as well as omphacite at the expense of paragonite. The bold line in Fig. 6 is the low-temperature limit of the feldspar stability field. Towards lower pressures and temperatures, staurolite and biotite appear. Below 13 kbar, biotite is stable in addition to the peak pressure assemblage (dark grey field). Staurolite appears below ~9 kbar at 600 C (light grey area in Fig. 6). This evolution under amphibolite facies conditions is in good accordance with the observation of newly crystallized, anhedral staurolite in pressure shadows of garnet and the epitactic overgrowth of white mica by biotite. Staurolite growth seems to be the latest stage of metamorphism recorded in this area. However, one has to be aware that only the ferrous end-member of staurolite was considered and mixing properties towards the magnesium end-member were not included. This results in small stability fields for staurolite-bearing assemblages compared with natural observations. The measured Mg content in staurolite from sample CHM1 is Mg/ (Mg+ Fe) = The computation of sample CHM39 was performed in the sodium-free CKFMASH system, because sodiumbearing phases occurring in CHM39 are only within a fine-grained symplectite consisting of Ab-rich plagioclase and sodic calcic amphibole. This symplectite is obviously not in equilibrium with the rest of the sample (see the section Petrography and mineral chemistry ). 206

9 Table 4: Thermodynamic properties of end-members that are updated or were not published by Berman (1988) Mineral Formula Comments H o (J) S o (J/K) V o (J/bar) 207 k 0 k 1 k 2 k 3 k 6 v 1 v 2 v 3 v 4 Annite KFe 3 [AlSi 3 O 10 ](OH) 2 (1) Al-Celadonite KAlMg[Si 4 O 10 ](OH) 2 (2) Fe Al-Celadonite KAlFe[Si 4 O 10 ](OH) 2 (2) Glaucophane Na 2 Mg 3 Al 2 [Si 8 O 22 ](OH) 2 (3) Hedenbergite CaFeSi 2 O 6 (4) Staurolite Fe 4 Al 18 Si 7 5 O 44 (OH) 4 (4) MEYRE et al. HP METAPELITES FROM THE ADULA NAPPE Comments: (1) Properties derived by Berman (1990), modified by McMullin et al. (1991). (2) Properties derived by Massonne & Szpurka (1997). (3) Properties derived by Evans (1990). (4) Provisional properties of database JUN92.RGB [update of Berman (1988)]. Cp=k 0 +k 1 T 0 5 +k 2 T 2 +k 3 T 3 +k 4 /T+k 5 T+k 6 T 2 ; V(P,T)/V(1,298)=1+v 1 (T 298)+v 2 (T 298) 2 +v 3 (P 1)+v 4 (P 1) 2. Units are in J, K and bar, v 1,v 2,v 3 terms need to be divided by 10 5, v 4 by k 4 and k 5 are zero for all end-members.

10 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 1 JANUARY 1999 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 a Ann =(X M Fe) 3 ; a Phl =(X M Mg) 3 Comments: (1) Margules parameter of binary joins from Massonne & Szpurka (1997) and ternary expansion according to Kohler (1960): ΔG excess = (W iijx i2 X j ). Phengite solid solution was considered for the calculation of sample CHM1 and CHM39. X i +X j (2) In this study the quaternary end-member Sps, calibrated by Berman (1990) was not considered. (3) The structural transition of albite was included in the Landau formulation of Salje et al. (1985). (4) Paragonite solid solution was considered for the calculation of samples MF2643 and Z Sample Table 6: Bulk composition of investigated samples used for DOMINO calculations Bulk composition (modal proportion, mineral composition) CHM1 40 Phe(Ms 0 67 Cel 0 33 )+10 Grt(Alm 0 73 Prp 0 25 Grs 0 02 ) +10 Pg+20 Ky+100 Qtz+100 H 2 O CHM39 10 Phe(Ms 0 70 Cel 0 25 FeCel 0 05 )+20 Z MF2643 Grt(Alm 0 51 Prp 0 33 Grs 0 16 )+20 Ky+50 Qtz+100 H 2 O 20 Grt(Alm 0 65 Prp 0 22 Grs 0 13 )+10 Omp (Jd 0 87 Di 0 07 Hd 0 06 )+20 Ky+10 Gln+30 Qtz+100 H 2 O 20 Grt(Alm 0 54 Prp 0 29 Grs 0 17 )+40 Tlc +20 Wm(Pg 0 88 Ms 0 12 )+100 Ky+100 Qtz+100 H 2 O Because we are not able to reconstruct the composition of the former omphacite from the observed symplectite, we excluded the symplectitic phases from consideration. Therefore the computation of sample CHM39 represents only a part of the high-pressure assemblage (without omphacite). The part of the high-pressure assemblage (Grt + Phe + Ky + Qtz + H 2 O) observed in sample CHM39 is stable over a large P T interval (Fig. 7). The upper pressure limit is constrained by the quartz coesite trans- ition, whereas to lower temperatures, the absence of chlorite is a limiting, but weak constraint for this assemblage. Because chlorite was only considered as Mg end-member clinochlore, the equilibrium phase diagram remains ambiguous below 500 C. However, the con- sideration of a chlorite solution model (e.g. an Fe Mg chlorite) would result in increased stability fields of the chlorite-bearing assemblages. Therefore we conclude that the low-temperature limit of the assemblage Grt + Phe + Ky + Qtz + H 2 O is a minimum. The sample re-equilibrated in the biotite stability field at ~14 kbar and ~650 C during the overgrowth of white mica by biotite (see the section Calculation of isopleths ). The equilibrium phase diagram in the CNKFMASH system for the sodic whiteschist Z from Trescolmen (Fig. 8) is characterized by the stability field of the peak pressure assemblage Gln + Omp + Grt + Ky + Qtz + H 2 O at ~25 kbar and ~650 C (striped field in Fig. 8). This assemblage is constrained by the quartz coesite transition towards higher pressures and by the absence of glaucophane towards higher temperatures. At lower pressures, paragonite replaces omphacite and kyanite, resulting in the assemblage Gln + Grt + Pg + Qtz + H 2 O, which was reached in the reequilibration event during the Trescolmen deformation phase (grey field in Fig. 8). Below ~19 kbar, the feldspar stability field restricts the re-equilibration assemblage to temperatures below 700 C. For sample Z , we have chosen a phengite-free assemblage for calculation because of inconsistencies between the used solid solution models. For unknown reasons, the solid solution model for phengite (Massonne & Szpurka, 1997) does not agree well with the muscovite paragonite model of Chatterjee & Froese (1975). This results in a large number of small stability fields of unrealistic assemblages. Glaucophane was taken into account as Mg end-member. The qualitative equilibrium phase diagram for sample MF2643 (Fig. 9) is restricted to only a part of the observed assemblages (peak pressure and re-equilibration event). The main feature that can be deduced from this diagram is the biotite overprint of the peak pressure assemblage Grt + Pg + Tlc + Ky + Qtz + H 2 O. With this diagram, we reach the actual limits of thermodynamic 208

11 MEYRE et al. HP METAPELITES FROM THE ADULA NAPPE modelling. The crucial input parameters of the calculations are besides the bulk composition of the equilibrium phases the solid solution models for the mixing phases, which are often not yet calibrated. The main problem with the investigated sample is the thermodynamic behaviour of amphibole with barroisitic to glaucophane composition. Whereas in sample Z amphibole is glaucophane of almost pure end-member composition, amphibole in sample MF2643 is of barroisitic composition (Table 3). This barroisite could not be thermodynamically modelled because of the still unknown thermodynamic mixing behaviour between sodic, sodic calcic and calcic amphiboles. Because of the absence of a reliable solution model for sodic calcic amphiboles we were unable to model the entire observed assemblage of MF2643. Calculation of isopleths The results of isopleth calculations strongly depend on bulk composition and the variance of the assemblage. They are valid only for the specific sample they are calculated for. For sample CHM1 and CHM39, we computed isopleths for the end-members grossular (Ca 3 Al 2 Si 3 O 12 ), almandine (Fe 3 Al 2 Si 3 O 12 ) and pyrope (Mg 3 Al 2 Si 3 O 12 ) in garnet and aluminoceladonite (KMgAl- [Si 4 O 10 ](OH) 2 ) in phengite (Fig. 10a and b). The composition of garnet within the large stability field of the observed assemblage Grt + Phe + Ky + Qtz + H 2 O is almost constant for sample CHM39 (Fig. 10b; compare Fig. 7). For sample CHM1 (Fig. 10a), the garnet composition is more variable but still much more uniform than that of phengite. The wide spacing of isopleths is a possible reason for the absence of a zonation in garnet: if garnet grows within a small P T window at peak pressure conditions, it attains a composition that is stable during almost the entire isothermal decompression (from 22 kbar to 10 kbar at 650 C). During the retrograde path, phengite adapts its composition continuously to the changing equilibrium conditions. This results in a strong zonation in silica content (celadonite component) from core to rim. The pressure (and temperature) dependence of phengite composition for these bulk chemistries can therefore be used for thermobarometric estimations. In Fig. 10a and b isopleths of aluminoceladonite content in phengite are plotted, based on a modified solid solution model for phengite of Massonne & Szpurka (1997). These isopleths are valid only for the bulk chemistries used to calculate the equilibrium phase diagrams of CHM1 and CHM39 in combination with the considered solid solution models (Figs 6 and 7). For the bulk composition of sample CHM1 the zonation of the silica content in phengite from Si ~3 38 p.f.u. (X Cel = 0 25) in the core to Si ~3 17 (X Cel = Fig. 10. Calculated isopleths for aluminoceladonite component in phengite and of pyrope, grossular and almandine in garnet. Isopleths of aluminoceladonite are based on a modified solid solution model for phengite by Massonne & Szpurka (1997). Garnet isopleths are based on the solution model by Berman (1990). Bold lines refer to measured rim compositions of phengite and garnet; dashed lines refer to core compositions of zoned phengite. The circles represent the conditions of re-equilibration, that is, the region of crosscutting of the isopleths. (a) Isopleths, valid for the bulk composition of sample CHM1 (see Fig. 6 and Table 5). Sample CHM1 re-equilibrated at ~10 11 kbar and ~650 C. (b) Isopleths, valid for the bulk composition of sample CHM39 (see Fig. 7 and Table 5). Sample CHM39 re-equilibrated at ~14 kbar and ~650 C. 209

12 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 1 JANUARY ) in the rim reflects a pressure decrease from ~21 kbar (core) to ~11 kbar (rim) at 650 C. The zonation of the silica content in phengite for sample CHM39 ranges from Si ~3 4 p.f.u. (X Cel = 0 32) in the core to Si ~3 3 p.f.u. (X Cel = 0 25) in the rim. This reflects a pressure decrease from ~19 kbar (core) to ~14 kbar (rim) at 650 C. To estimate the relevance of the pressure and temperature determination, it is important to evaluate the uncertainties of the calculations. The position of the boundaries of the different stability fields mainly depends on the calibration of the defined solid solution models and the uncertainty of the thermodynamic end-member data. This results in a moderate relative error between the different stability fields, but the absolute position in pressure and temperature may vary within ~±1 kbar and ~±50 C. Expected uncertainties of mineral compositions within the stability fields for the simplified systems are essentially given by the uncertainty of the experimental data. A direct comparison with natural systems may include even larger uncertainties, because no activity corrections for minor elements (e.g. Cr, Mn, Zr, etc.) are considered in the solid solution models. As a rule of thumb, we assume an uncertainty of ±2 mol %, which roughly corresponds to the error of electron microprobe measurements. This error is also reflected in the isopleth computations (isopleths are displayed in steps of 2 mol % in Fig. 10a and b). Fig. 11. Proposed P T path for investigated metapelitic rocks of the middle Adula nappe. This path is in accordance with the evolution of eclogites from the same area (Meyre et al., 1997). The striped field (1) represents peak pressure conditions; in light grey (2) the stability field of paragonite overprint is marked (both for sample Z , sodic whiteschist). The Trescolmen phase (re-equilibration event) is constrained by the overlap of field (2) with the stability field of amphibolebearing eclogite (3) from the same area (Meyre et al., 1997). The isopleths for celadonite content in phengite from sample CHM1 are helpful to model the retrograde path. Sample CHM1 re-equilibrated at ~9 10 kbar and ~650 C ( Zapport phase). The wet solidus of granite is taken from Huang & Wyllie (1974). DISCUSSION From the above results, we can confine the pressure and temperature conditions of two distinct events ( Tresco- lmen and Zapport phases) during the tectonic and metamorphic evolution of the middle Adula nappe (Fig. 11). These conditions coincide very well with the published P T path of Meyre et al. (1997), based on geothermobarometry of mafic eclogites, as well as with the proposed P T path of Partzsch (1998) for the structurally upper part of the Adula nappe. Meyre et al. (1997) mainly focused on the Trescolmen phase, where amphibole overprints the peak pressure assemblage Grt + Omp + Ky + Qtz in mafic eclogites. Partzsch (1998) dealt with the geodynamic evolution and the regional context of the high-pressure metamorphism of the Adula nappe. Peak pressure conditions In this study [as well as in that of Meyre et al. (1997)] peak pressure conditions cannot be very well constrained. The assemblage Omp + Grt + Gln + Ky + Qtz + H 2 O of the sodic whiteschist sample Z seems to be best suited to define the high-pressure climax (~25 kbar at ~ C). The observed peak pressure as- semblages of the other investigated samples are stable over a large area in P T space. This is particularly true for samples CHM1 and CHM39. The high variance of the system is due to the fact that only few phases are involved in the equilibrium (garnet is the only phase containing calcium, paragonite component in white mica fixes sodium, etc.). The absence of zonation in garnets of samples CHM1 and CHM39 can be explained by this high variance during the retrograde path. Growth within a small P T field under prograde conditions might pro- duce an unzoned garnet, which is not affected during part of the retrograde path because garnet remains stable with (almost) the same chemical composition. 210

13 MEYRE et al. HP METAPELITES FROM THE ADULA NAPPE Trescolmen phase The assemblage Gln + Grt + Pg + Qtz + H 2 O (sample Z ), which represents the re-equilibration event during the retrograde path (replacement of omphacite and kyanite by paragonite), is stable over a large area in pressure and temperature. However, compared with the evolution of the mafic eclogite sample CHM30 of Meyre et al. (1997) from the same locality, there is an overlapping field only at ~21 kbar and 650 C. In the eclogite sample the re-equilibration event can be cor- related with the Trescolmen deformation phase (Meyre & Puschnig, 1993; Meyre et al., 1997; Partzsch, 1998). This correlation is also reasonable for the sodic whiteschist sample. Calculations of isopleths of the solution phases (samples CHM1 and CHM39) support the estimated conditions for the Trescolmen phase and reveal additional information on the pressure and temperature evolution (Fig. 11). The isopleth for a silica content of Si = 3 38 p.f.u. implies a pressure of ~21 kbar at 650 C for the assemblage Grt + Phe + Pg + Ky + Qtz + H 2 Oin sample CHM1. This is consistent with the assumed pressure conditions for eclogite samples of this area [reequilibration event of Meyre et al. (1997); see Heinrich (1986)]. However, pressure estimations for sample CHM39 indicate significantly lower pressures (~18 19 kbar at 650 C; dashed line in Fig. 10b), which are slightly lower than the expected conditions of the re-equilibration event and the Trescolmen deformation phase (~21 kbar; Meyre et al., 1997). The low celadonite content of phengite core in sample CHM39 may be explained by diffusional homogenization of the silica distribution in phengite. Zapport phase The investigated samples of this study reveal higher pressures for the Zapport phase than the work of Partzsch (1998) (6 5 8 kbar and C): sample CHM1 equilibrated at ~11 kbar and 650 C, sample CHM39 at 14 kbar and 650 C, both in the stability field of biotite (filled circles in Fig. 10a and b). The absence of feldspar in sample CHM1 is an important constraint towards higher temperatures, especially for the Zapport phase (see Fig. 6). This limit is in accordance with the independent observation that no melting of gneisses and metapelitic rocks has been observed in this field area (melting curve in Fig. 11). The constant temperature during decompression is probably due to the influence of the Lepontine metamorphism, which mainly heated up the southern part of the Central Alps under amphibolite facies conditions (Wenk, 1956; Todd & Engi, 1997) but also interfered with the retrograde evolution of the middle Adula nappe. The continuous retrogression of metapelitic rocks from eclogite facies conditions to amphibolite facies conditions ( Lepontine metamorphism) that is presented in this study supports the geodynamic model of Schmid et al. (1996) for the Tertiary orogeny in the Central Alps. In this model, the Adula nappe represents the southern tip of Europe in Early Paleocene time. During convergence, and finally, collision of the upper crust of the Apulian margin with the European margin (closure of the Valais ocean during the Early to Late Eocene), the Adula nappe was subducted to great depths. Forced extrusion parallel to the subduction shear zone seems to be the most likely mechanism for differential exhumation of the Adula nappe in respect to both higher and lower tectonic units, which did not suffer eclogite facies metamorphism (Schmid et al., 1996). CONCLUSIONS (1) In the Adula nappe high-pressure relics are not restricted to the well-known eclogite lenses, but are also found in metapelitic lithologies. The investigated samples display whiteschist ( MF2643) to sodic whiteschist (Z ) mineralogy as well as normal metapelitic as- semblages (Grt + Ky + Phe + Qtz ± Bt ± St) (CHM1, CHM39). (2) The calculation of equilibrium phase diagrams with the computer code DOMINO is well suited to model the evolution of these rocks at medium temperatures. However, peak metamorphic conditions cannot be constrained well because of the high variance of the observed assemblages as well as their large stability fields. (3) The petrological data for the metapelitic samples presented in this study are in good agreement with geothermobarometric studies on eclogite lenses of the same area (Heinrich, 1986; Meyre et al., 1997). Fur- thermore, metapelitic rocks of the Adula nappe display the entire structural and petrological retrograde evolution of this Alpine unit. (4) Pelitic rocks with the stable assemblage Grt + Phe ± Pg + Bt + Ky + Qtz + H 2 O equilibrated at conditions of 14 kbar and 650 C (sample CHM39) and kbar and 650 C (sample CHM1), and reflect conditions in the late stages of the main deformation event ( Zapport deformation phase). (5) The presence of high-pressure relics in metapelitic lithologies of the middle Adula nappe implies that not only part of it (i.e. mafic eclogite lenses) but the entire Adula nappe was subducted to great depths. This fact, combined with structural observations, makes a Tertiary age for the high-pressure metamorphism of the Adula nappe most probable. 211

14 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 1 JANUARY 1999 Frey, M., Hunziker, J. C., Schmid, S. M., Thoenen, T. & Trommsdorff, ACKNOWLEDGEMENTS V. (1992). Bericht über die Exkursion der Schweizerischen Min- We thank T. Thoenen for performing electron micro- eralogischen und Petrographischen Gesellschaft zum Thema Hochdruck-Metamorphose probe (EMP) measurements on sample MF2643. Fruitful in der Adula-Decke (29. September bis 5. collaboration with T. Nagel, J. H. Partzsch and E. Zinnteilungen Oktober 1991). Schweizerische Mineralogische und Petrographische Mit- 72(2), grebe is gratefully acknowledged. We are thankful for Frischknecht, G. (1923). Geologie der östlichen Adula. Beiträge zur the constructive and helpful reviews by T. Holland, Geologischen Karte der Schweiz, N.F. 51, G. Markl and O. Vidal. Motivating late-night debates of Fuhrman, M. L. & Lindsley, D. H. (1988). Ternary feldspar modeling C.M. with C. Manning, A. Matthews and J. Amato during and thermometry. American Mineralogist 73, the 5th International Eclogite Conference, Ascona, are Gebauer, D. (1996). A P T t path for an (ultra?-) high-pressure ultra- appreciated. We thank S. Th. Schmidt and H.-P. Meyer mafic/mafic rock-association and its felsic country-rocks based on for help at the EMPs at the Universities of Basel and SHRIMP-dating of magmatic and metamorphic zircon domains. Example: 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, National Science Foundation Grants and Heinrich, C. A. (1982). Kyanite-eclogite to amphibolite facies evolution of hydrous mafic and pelitic rocks, Adula Nappe, Central Alps. Contributions to Mineralogy and Petrology 81, Heinrich, C. A. (1983). Die regionale Hochdruckmetamorphose der Aduladecke, Zentralalpen, Schweiz. 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