REGIONAL GEOLOGY

The Adula nappe belongs to the Penninic domain of the Swiss Central Alps and represents the palaeogeographically southernmost part of the Europeanmargin 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; 450-550°C, 10-13 kbar 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 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<sub>1</sub>) 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<sub>1</sub>. The `Trescolmen' phase (D₂) 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<sub>3</sub>) is associated with the main foliation in the middle Adula nappe. Deformation started under high-pressure conditions and continued well into amphibolite facies conditions. 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; D4) 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.

Detailed descriptions of the regional geology of the Adula nappe have been given by Frischknecht, (1923), Heinrich, (1983, , 1986), Löw, (1987) and Partzsch, (1998).

PETROGRAPHY AND MINERAL CHEMISTRY

The investigated samples of this study were collected at the localities Zapporthorn (MF2643) and Trescolmen (CHM1, CHM39 and Z6-50-12) in the central part of the Adula nappe (see Fig. 1).

We analysed the minerals of samples CHM1, CHM39 and MF2643 with a JEOL JXA-8600 electron microprobe at the University of Basel (Switzerland). The microprobe is equipped with Voyager software by Noran Instruments. The operating conditions were set at 15 kV and 10 nA, and the correction procedure was PROZA (Bastin & Heijligers, 1990). Minerals of sample Z6-50-12 were analysed with a Cameca SX51 electron microprobe with conditions of 15 kV and 20 nA at the University of Heidelberg (Germany). The correction procedure was a ZAF routine. For all analyses natural and synthetic crystals were used as standards.

Tables 1, 2 and 3 show microprobe analyses of garnet, white mica, talc, biotite, amphibole, and jadeitic pyroxene. These analyses are representative for the assemblages that were stable at conditions of the high-pressure metamorphism or the amphibolite facies overprint.

Table 1. Selected electron microprobe analyses of garnet used to calculate the equilibrium phase diagrams; end-member distribution after Deer et al. (1967)

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

Table 3. Selected electron microprobe analyses of amphibole and pyroxene

The phengite analyses in Table 2 are projected to the ternary system with the end-members muscovite, aluminoceladonite and ferro-aluminoceladonite. A decomposition into these three end-members cannot comprehensively characterize the measured micas (e.g. the paragonite component is completely ignored). This simplification is necessary to use the available solid solution model for white mica [modified after Massonne & Szpurka, (1997)] in our computations (see the section `Equilibrium phase diagrams').

In the following, mineral abbreviations are after Kretz, (1983) and Bucher & Frey, (1994), and Wm indicates white mica.

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 coordinates: 733.910/139.460). Large garnet crystals, up to 2 cm in size, with inclusions of kyanite (several millimetres 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.

Figure 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.

Garnet is essentially unzoned. Slightly decreasing Prp values towards the rim are interpreted as diffusional re-equilibration (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.

Figure 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.

We assume that the main foliation began to evolve under high-pressure conditions (core of phengitic white mica) and continued under amphibolite facies conditions (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: 733.600/139.600) 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 hasa 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).

Figure 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.

Garnet is essentially unzoned in the core (core composition: Alm₅₁Prp₃₃Grs₁₅Sps₁). A slight zonation towards the rim with respect to pyrope and grossular (Alm₅₀Prp₂₈Grs₂₁Sps₁) can be observed.

Symplectic pseudomorphs of albite and amphibole after omphacite occur interstitially between garnet and quartz. Tables 1 and 2 show microprobe analyses of garnet and phengite of sample CHM39.

Sodic whiteschist from Trescolmen (Z6-50-12)

At Trescolmen, rare metapelitic rocks with a high-pressure paragenesis occur as blocks below the Cima di Gagela (Swiss coordinates 733.650/139.250). The investigated sample (Z6-50-12) has three zones: a phengite-rich layer followed by a 2 cm wide quartz-rich layer grading into eclogite that consists mainly of garnet and clinopyroxene. Jadeitic clinopyroxene and Mg-glaucophane are present in the intermediate quartz-rich layer coexisting with quartz, garnet, kyanite, phengite, and rutile. The foliation in all three zones is parallel to the layering and defined by aligned phengite, amphibole and clinopyroxene, respectively.

A similar assemblage with jadeite and Mg-glaucophane was described by Kienast et al., (1991) from the Dora Maira Massif and called `sodic whiteschist', a term also used here.

In thin section, garnet, jadeite, Mg-glaucophane, kyanite and quartz can be defined as primary phases and represent the peak pressure assemblage. Zircon, allanite and rutile occur as accessory phases. Recrystallization still under high-pressure conditions (`Trescolmen' phase) led to the formation of paragonite and magnesite that grew around kyanite and glaucophane.

Retrogression under amphibolite conditions led to the formation of fine-grained symplectite consisting of plagioclase and amphibole around jadeite and glaucophane, with jadeite more strongly affected than glaucophane.

Garnet shows a distinct and continuous zoning (Fig. 5) without any break in the pattern except for the outermost rim. Fe and Mn decrease towards the rim whereas Mg increases, typical for growth zoning under prograde conditions. In contrast, Ca shows an almost flat pattern with only a slight but noticeable increase from core to rim. Only at the outermost rim (25 µm) does Ca content drop significantly from Grs₁₅ to Grs₈. This drop is probably due to the formation of amphibole and plagioclase (An_11-18) in symplectite around garnet during late retrogression.

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

Clinopyroxene is close to jadeite end-member composition [X_Jd = 0·80-0·91; classification after Morimoto, (1988)], whereas primary amphibole is close to pure Mg-glaucophane, with X(Na,M4) up to 0·95, Al(vi)/[Fe³⁺ + Al(vi)] = 0·94 and Mg/(Mg + Fe) = 0·89. Mineral analyses of sample Z6-50-12 are listed in Tables 1, 2 and 3.

EQUILIBRIUM PHASE DIAGRAMS

The equilibrium phase diagrams (Figs 6- 9) for metapelitic samples from the middle Adula nappe were calculated with the computer code THERIAK-DOMINO (de Capitani & Brown, 1987; de Capitani, 1994). This program performs a Gibbs free energy minimization for a set of minerals (internally consistent database) in a given P-T space. For a detailed description of DOMINO and its abilities, the reader is referred to Biino & de Capitani, (1995) and to the homepage of the THERIAK-DOMINO software package (http://therion.minpet.unibas.ch/minpet/groups/theruser.html). An application of this approach to mafic eclogites of the middle Adula nappe has been previously presented by Meyre et al., (1997).

Figure 6. Equilibrium phase diagram of sample CHM1 from Trescolmen calculated with the computer program DOMINO of de Capitani, (1994). Equilibrium assemblages are computed by Gibbs free energy minimization on the basis of a specific bulk composition (see Table 5). The peak pressure assemblage (Grt + Phe + Pg + Ky + Qtz + H₂O; striped area) is overprinted by biotite and staurolite (dark grey and light grey area). The bold line limits the feldspar stability field to lower temperatures.

Figure 7. Equilibrium phase diagram for sample CHM39 from a folded eclogite lens at Trescolmen (see Fig. 4). The peak pressure assemblage Grt + Phe + Ky + Qtz + H₂O (striped area) is overprinted by biotite (grey area). Below 500°C the diagram is unreliable (dashed lines).

Figure 8. Equilibrium phase diagram for a sodic whiteschist from Trescolmen (sample Z6-50-12). The pressure and temperature conditions of the re-equilibration event (Gln + Grt + Pg + Qtz + H₂O; 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 + H₂O is marked by the striped field.

Figure 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: re-equilibration event).

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).

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

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

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 consistent with the garnet solution model of Berman, (1990) and the feldspar solution model of Fuhrman & 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 solution (mixing on site) between phlogopite and annite.

Bulk compositions of the investigated samples were derived from representative single analyses of the minerals 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 H₂O 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 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 high-pressure 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.

Table 6. Bulk composition of investigated samples used for DOMINO calculations

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 + H₂O (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) = 0·19-0·22.

The computation of sample CHM39 was performed in the sodium-free CKFMASH system, because sodium-bearing 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').

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 + H₂O) observed in sample CHM39 is stable over a large P-T interval (Fig. 7). The upper pressure limit is constrained by the quartz-coesite transition, 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 consideration 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 + H₂O 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 Z6-50-12 from Trescolmen (Fig. 8) is characterized by the stability field of the peak pressure assemblage Gln + Omp + Grt + Ky + Qtz + H₂O 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 + H₂O, which was reached in the re-equilibration 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 Z6-50-12, 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 + H₂O. With this diagram, we reach the actual limits of thermodynamic 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 Z6-50-12 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 (Ca₃Al₂Si₃O₁₂), almandine (Fe₃Al₂Si₃O₁₂) and pyrope (Mg₃Al₂Si₃O₁₂) in garnet and aluminoceladonite (KMgAl[Si₄O₁₀](OH)₂) in phengite (Fig. 10a and b).

Figure 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.

The composition of garnet within the large stability field 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 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 = 0·10) 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).

DISCUSSION

From the above results, we can confine the pressure and temperature conditions of two distinct events (`Trescolmen' 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.

Figure 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 Z6-50-12, sodic whiteschist). The `Trescolmen' phase (re-equilibration event) is constrained by the overlap of field (2) with the stability field of amphibole-bearing 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).

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 + H₂O of the sodic whiteschist sample Z6-50-12 seems to be best suited to define the high-pressure climax (~25 kbar at ~600-700°C). The observed peak pressure assemblages 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 produce an unzoned garnet, which is not affected during part of the retrograde path because garnet remains stable with (almost) the same chemical composition.

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 (Z6-50-12) mineralogy as well as `normal' metapelitic assemblages (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). Furthermore, 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 + H₂O equilibrated at conditions of 14 kbar and 650°C (sample CHM39) and 10-11 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.

ACKNOWLEDGEMENTS

We thank T. Thoenen for performing electron microprobe (EMP) measurements on sample MF2643. Fruitful collaboration with T. Nagel, J. H. Partzsch and E. Zinngrebe is gratefully acknowledged. We are thankful for the constructive and helpful reviews by T. Holland, G. Markl and O. Vidal. Motivating late-night debates of C.M. with C. Manning, A. Matthews and J. Amato during the 5th International Eclogite Conference, Ascona, are appreciated. We thank S. Th. Schmidt and H.-P. Meyer for help at the EMPs at the Universities of Basel and Heidelberg, respectively. This study is part of the Ph.D. thesis of C.M., which was financially supported by Swiss National Science Foundation Grants 20-39130.93 and 20-45270.95.

REFERENCES