J. GANNE 1 *, F. BUSSY 2 AND O. VIDAL 3

Transcription

1 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 PAGES 1281± Multi-stage Garnet in the Internal BriancË onnais Basement (Ambin Massif, Savoy): New Petrological Constraints on the Blueschist-facies Metamorphism in the Western Alps and Tectonic Implications J. GANNE 1 *, F. BUSSY 2 AND O. VIDAL 3 1 LABORATOIRE DE GEODYNAMIQUE DES CHAIÃ NES ALPINES, CNRS, UMR 5025, UNIVERSITE DE SAVOIE, DOMAINE UNIVERSITAIRE, F-73376, FRANCE 2 INSTITUTE OF MINERALOGY AND GEOCHEMISTRY, BFSH-2, UNIVERSITY OF LAUSANNE, LAUSANNE, CH-1015, SWITZERLAND 3 LABORATOIRE DE GEODYNAMIQUE DES CHAIÃ NES ALPINES, CNRS, UMR 5025, UNIVERSITE JOSEPH FOURIER, MAISON DES GEOSCIENCES, B.P. 43, GRENOBLE, FRANCE RECEIVED FEBRUARY 25, 2002; ACCEPTED FEBRUARY 26, 2003 Three types of garnet have been distinguished in pelitic schists from an epidote±blueschist-facies unit of the Ambin and South Vanoise BriancËonnais massifs on the basis of texture, chemical zoning and mineral inclusion characterization. Type-1 garnet cores with high Mn/Ca ratios are interpreted as pre-alpine relicts, whereas Type-1 garnet rims, Type-2 inclusion-rich porphyroblasts and smaller Type-3 garnets are Alpine. The latter are all characterized by low Mn/Ca ratios and a coexisting mineral assemblage of blue amphibole, high-si phengite, epidote and quartz. Prograde growth conditions during Alpine D 1 high-pressure (HP) metamorphism are recorded by a decrease in Mn and increase in Fe (Ca) in the Type-2 garnets, culminating in peak P±T conditions of 14±16 kbar and 500 C in the deepest parts of the Ambin dome. The multistage growth history of Type-1 garnets indicates a polymetamorphic history for the Ambin and South Vanoise massifs; unfortunately, no age constraints are available. The new metamorphic constraints on the Alpine event in the massifs define a metamorphic T `gap' between them and their surrounding cover (BriancËonnais and upper Schistes Lustres units), which experienced metamorphism only in the stability field of carpholite± lawsonite (T C). These data and supporting structural studies confirm that the Ambin and South Vanoise massifs are slices of `eclogitized' continental crust tectonically extruded within the Schistes Lustres units and BriancËonnais covers. The corresponding tectonic contacts with top-to-east movement are responsible for the juxtaposition of lower-grade metamorphic units on the Ambin and South Vanoise massifs. KEY WORDS: Alpine HP metamorphism; Ambin and South Vanoise BriancËonnais basements; metamorphic gaps; multistage garnets; Western Alps INTRODUCTION For a long time [Bocquet (Desmons), 1974a, 1974b; Borghi et al., 1999, and references therein] all garnets in micaschists of the so-called `polymetamorphic' basements of the Ambin and South Vanoise massifs (Fig. 1a), and more generally from the BriancË onnais domain, have been regarded as relics of a pre-alpine Barrovian metamorphism. Although a few workers suspected the existence of Alpine garnets (Ellenberger, 1958; Goffe, 1977; Caby, 1996), distinguishing pre- Alpine from Alpine garnets was an unresolved issue. *Corresponding author. Telephone: (33) Jerome.ganne@univ-savoie.fr Journal of Petrology 44(7) # Oxford University Press 2003; all rights reserved

2 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 JULY 2003 Fig

3 GANNE et al. HIGH-PRESSURE METAMORPHISM, WESTERN ALPS Fig. 1. (a) Geological map of the French±Italian Western Alps, mainly from the Annecy 1/ map sheet (Debelmas et al., 1989): the Penninic domain consists of reactivated basement, Palaeozoic and Mesozoic±Cenozoic sediments and Mesozoic oceanic remnants. SVA, South Vanoise; GSB, Gran Saint Bernardo; DM, Dora Maira; GP, Gran Paradiso; MR, Monte Rosa; SL, Sesia Zone; LA, Lanzo massif. The Ambin massif is outlined by the rectangle. (b) Simplified metamorphic map of the Western Penninic domain, mainly after Goffe & Chopin (1986), Pognante (1991) and Desmons et al. (1999a). Each shade of grey corresponds to the inferred high-pressure peak of metamorphism recorded by the Penninic units (mostly overprinted by subsequent low-pressure metamorphism. (1) Very HP eclogite facies (UHP), including kyanite eclogite and pyrope±coesite whiteschists; (2) high-t blueschist facies (i.e. eclogite facies), including widespread paragonite±zoisite eclogite, and characterized by the glaucophane±garnet assemblage; (3) medium-t blueschist facies characterized by the glaucophane±epidote assemblage; (4) low-t blueschist-facies characterized by the glaucophane±lawsonite carpholite assemblage; (5) widespread greenschist facies characterized by the chlorite±albite±pumpellyite lawsonite±carpholite assemblage. Metamorphic isograds (A, lawsonite ; B, carpholite ) were discussed by Goffe & Chopin (1986). The External domain is characterized by very low-grade metamorphic assemblages (laumonite, pyrophyllite±prehnite±pumpellyite). (c) Structural maps of the Ambin massif. This consists of three superimposed tectonic nappes: the deeper Clarea Nappe preserves traces of an early HP±LT deformation (D 1 ) linked to a north±south stress field. The middle and upper nappes (Ambin and Schistes Lustres Nappes, respectively), consist of oceanic, BriancË onnais covers and basement slices that are affected by a later deformation (D 2 ). D 2 is linked to a pervasive shearing event with top-to-the east movement direction. The geological cross-section A±B is shown in Fig. 16, with more explanation. The discovery of high-pressure mineral inclusions (generally regarded as being of Alpine age) in small garnets from the Ambin and South Vanoise massifs provided evidence for the existence of multi-stage garnet growth (Alpine and pre-alpine; Ganne, 1999). The occurrence of pre-alpine garnet is consistent with other assumed low-pressure±high-temperature (LP±HT) pre-alpine metamorphic relics in the Ambin massif, such as biotite, muscovite, hornblende or staurolite and sillimanite pseudomorphs [Gay, 1971; Bocquet (Desmons), 1974a, 1974b; Callegari et al., 1980; Desmons, 1992; Borghi et al., 1999; Desmons et al., 1999b]. The objective of this study is to characterize the various generations of garnet on the basis of their mineral inclusions, chemical composition and typology. In particular, systematic 2D X-ray element mapping (e.g. Matsumoto & Hirajima, 2000) has been undertaken and interpreted in the light of excellent models developed by Hollister (1966), Kretz (1973), 1283

4 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 JULY 2003 Tracy et al. (1976), Yardley (1977), Tracy (1982), 1994), Ghent (1986) and Spear (1993) on zoning in garnet and its significance in terms of metamorphic evolution. New thermobarometric data on these Alpine garnets provide improved constraints on the HP Alpine metamorphic conditions recorded within the Ambin and South Vanoise massifs, and allow a reconsideration of these massifs within the `metamorphic belt' of the Western Alps (Fig. 1b; Goffe & Chopin, 1986; Pognante, 1991; Agard et al., 2001). This study also raises the question of the possible existence of `eclogitized' continental crust (garnet- or jadeite-bearing rocks) in a more external position than the Piemontais (Spalla et al., 1996; Schwartz et al., 2000) with implications for the exhumation mechanisms of HP rocks in the Alps. GEOLOGICAL SETTING OF THE AMBIN AND SOUTH BRIANCË ONNAIS BASEMENTS The main tectonic domains of the Western Alps are represented in Fig. 1a. They are, from west to east, the Dauphinois or Helvetic external domain (European origin), the middle or intermediate Penninic domain (Valaisan, BriancË onnais and SubbriancË onnais) and the internal Penninic domain, the last being constituted by the so-called Liguro-Piemontais units (oceanic suture of the Schistes Lustres) and Internal Crystalline Massifs (basement units of European and Austroalpine origin). The main characteristics of these domains have been summarized in review papers such as those by Stampfli & Marchant (1995), Escher et al. (1997) and Debelmas et al. (1998). Generally speaking, deformation becomes increasingly ductile (Debelmas & Lemoine, 1970) and metamorphism increasingly high (e.g. Desmons et al., 1999a) from the external to the internal part of the Alpine chain. The latter culminated in the so-called Lepontine thermal dome in the central Alps (Steck & Hunziker, 1994; Todd & Engi, 1997). Of particular interest is the record of a high- to ultra-high-pressure±low-temperature (HP±LT) metamorphic event in the internal domains and in the intermediate domains [see Duch^ene et al. (1997) for review] to a lesser extent (Fig. 1b); it is absent from the outer part of the belt. Since Bearth's (1952) pioneering work, field and petrological studies on mafic (e.g. Kienast et al., 1991; Lardeaux & Spalla, 1991; Pognante, 1991; Scambelluri et al., 1991) and pelitic rocks (Goffe & Chopin, 1986; Pognante, 1991; Agard et al., 2001, among many others) have led to considerable progress in the characterization and distribution of Alpine metamorphic assemblages in the Western Penninic Alps. Five main facies can be distinguished in the intermediate and internal units: (1) an ultra-hp (UHP) eclogite facies, including kyanite eclogite and pyrope±coesite whiteschists; (2) a high-t blueschist facies, including widespread paragonite±zoisite eclogite, and characterized by a glaucophane±garnet assemblage; (3) a medium-t blueschist facies characterized by a glaucophane±epidote assemblage; (4) a low-t blueschist facies characterized by a glaucophane±lawsonite carpholite assemblage; (5) a widespread greenschist facies characterized by a chlorite±albite±pumpellyite lawsonite±carpholite assemblage. This so-called `Penninic metamorphic belt' (Goffe & Chopin, 1986; Pognante, 1991) has been classically interpreted as resulting from an eastward-dipping subduction zone (Dal Piaz et al., 1972). The metamorphic zonation of the Penninic domain, however, is not always strictly adhered to on closer inspection. The geometry and setting of the presentday boundaries between the metamorphic units result essentially from the post-collisional tectonic evolution of the Alpine belt. The Ambin and South Vanoise basement massifs belong to the BriancË onnais Zone, which is interpreted by most workers as palaeogeographically issued from the European passive margin (Lemoine & de Graciansky, 1988) or as an allochthonous terrane (Stampfli & Marchant, 1995; Bertrand et al., 1996). They form dome-shaped basement windows (Fig. 1b and c) cropping out beneath allochthonous metamorphic envelopes of various origins (BriancË onnais Mesozoic units, ocean-derived Liguria±Piemont zone units). The origin and pre-alpine tectonometamorphic evolution of these basement units are still poorly known. To simplify nomenclature and description, we will distinguish three main lithotectonic groups within them, which we will call `nappes'. The latter are separated by major tectonic discontinuities, which are thought to have a stratigraphic significance (Michel, 1957; Gay, 1971; Ganne et al., 2003). These three nappes are, from bottom to top (Fig. 1c): (1) the Clarea Nappe, consisting of pre- Permian rocks; (2) the Ambin Nappe, consisting of slices of pre-permian basement, Permo-Triassic and Triassic to Eocene metasediments; (3) the `Schistes Lustres' Nappe, consisting of Jurassic to Cretaceous allochthonous oceanic metasediments from Liguria± Piemont. This lithostratigraphy has been established on the basis of published work, especially from the Lanslebourg±Mont d'ambin (Fudral et al., 1994) and Modane (Debelmas et al., 1989) 1/ map sheets. All the garnet-bearing micaschists described in this study were collected from the Clarea Nappe. The latter consists of banded micaschists, fine-grained amphibolites associated with glaucophanites and prasinites 1284

5 GANNE et al. HIGH-PRESSURE METAMORPHISM, WESTERN ALPS fabrics and linked to the HP metamorphic peak (M 1 ; this study). The D 1 event is well preserved in the Clarea Nappe, i.e. in the deeper part of the Ambin and South Vanoise massifs. Fig. 2. Mineralogy±deformation relationships in the Ambin massif (Alpine minerals). Grt, garnet; Jd, jadeite; Ep (Epi), epidote; Phe, phengite; Pg, paragonite; Cld, chloritoid; Gln, glaucophane; Czo, clinozoisite; Chl, chlorite; Act, actinolite; Bio, biotite; Ab, albite. (glaucophane chlorite) and rare marbles. These lithologies may represent a dominantly pelitic, flyschtype sequence with occasional mafic horizons (Gay, 1971; Pognante et al., 1984; Polino et al., 1999). Depending on the dominant mineral in the rocks we refer to them as glaucophane-bearing (GBM), albitebearing micaschists (ABM), or epidote-bearing micaschists (EBM). From a structural point of view, finite strain analysis reveals the existence of three, more or less diachronous, ductile to brittle±ductile deformation phases, characterized by specific types and/or vergence of structures. Structural and metamorphic data are presented in terms of D 1, D 2, D 3 (Fig. 2) and brittle events for the purpose of comparison with earlier descriptions in adjacent areas. A critical appraisal of this classification is beyond the scope of this study and is deferred to another paper (Ganne et al., 2003). The most obvious structures recognizable in the Ambin and Schistes Lustres Nappes are those related to the D 2 ( D 3 ) retromorphic deformations. These ductile to brittle± ductile shear events overprint pre-existing fabrics such as S 1 (D 1 event), which is the earliest Alpine schistosity clearly distinguishable from pre-alpine MINERAL ASSEMBLAGES AND MICROSTRUCTURAL RELATIONSHIPS More than 200 samples of garnet-bearing micaschists have been collected from the Ambin and South Vanoise massifs. Sixty-one thin sections were studied, 19 of which were then used for microprobe analysis. The typical mineralogy of the micaschists comprises garnet, white mica, quartz, albite, blue amphibole, chlorite, chloritoid, biotite, jadeite, epidote, calcite, accessory minerals (Gay, 1971). The habits of the garnets and the nature of their mineral inclusions provide important criteria allowing the establishment of a three-fold classification. Type-1 garnets are large (02±1 cm, Fig. 3) and display a contrasting rim outlined at its inner contact by a variety of inclusions (ˆ inclusion-rich rim), such as quartz, blue amphibole, epidote, white mica (colourless muscovite and greenish phengite), chlorite and/or biotite. Blue amphibole and phengite exhibit very sharp grain boundaries and may have formed in equilibrium with the garnet rim. Other mineral inclusions, such as biotite or muscovite, occur as clasts. Their irregular outline is often destabilized as shown by the appearance of chlorite, suggesting that these minerals are not in equilibrium with the garnet rim. Quartz crystals, generally fragmented in small clasts, show undulatory extinction. Garnet cores are poor in inclusions, but they may contain large biotite flakes. Such grains often lie at a high angle to the main Alpine schistosity (S 1 ) of the rock and may be truncated by the inclusion-rich rim. These Type-1 garnets are systematically wrapped by the main fabric of the micaschists (Fig. 3). Relationships between inclusion-rich rims, core and inclusions in the core suggest that the crystallization of the rim occurred under HP conditions in a simple shear deformation regime. During that event, the garnet cores appear to have rolled in the matrix of the rock, become blunted along their edges and sometimes fragmented. Thus the inclusionrich rim outlines a major tectonic event, which clearly separates two generations of garnet. Large Type-2 garnets (015±07 cm, Fig. 4) and small Type-3 garnets (d 5 05 mm, Fig. 5) never display inclusion-rich rims; on the contrary, white mica, blue amphibole and chlorite inclusions are dispersed throughout the whole crystal. Small Type-3 garnets occur within the S 1 fabric as syn-kinematic crystals 1285

6 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 JULY 2003 Fig. 3. (a) Large Type-1 garnet in a biotite-bearing micaschist (Ga-40b). Ab, albite; Gln, glaucophane; Jd, jadeite; Mus, muscovite; Bio1, biotite of first generation. (b) Sketch of Type-1 garnet from (a), showing the distribution of inclusions: glaucophane and phengite are found only in the rim. Chl, chlorite; Qtz, quartz; S 1, main Alpine schistosity; S 1, pre-alpine schistosity. A±B, trace of zoning profile shown in Fig. 3f. (c, d) X-ray maps of the same garnet: relative content of Ca and Fe varying from low (dark) to high (white). The straight contour lines of Fe suggest the original growth surface of euhedral crystals associated with biotite1; the Ca-rich rim cuts them. (e, f ) Compositional variations of garnets along the profile A±B in the Fe Mg±Ca±Mn diagram; black arrow indicates bell-shaped zoning of Mn suggesting that the garnet growth occurred during a gradual increase of P±T conditions. (f ) Mole proportion of Fe, Ca, Mg, Mn along the profile A±B. (Fig. 6a and b; GA-53); they preserve relicts of the S 1 fabric marked by trails of opaque minerals (rutile and titanite). Type-3 garnets occur sometimes, without particular orientation, in mineral aggregates (chloritoid, blue amphibole, epidote, phengite, garnet) wrapped by the S 1 schistosity (Fig. 6a and b). Such aggregates could represent pseudomorphs of pre- Alpine staurolite (Borghi et al., 1999), destabilized according to the reaction St Ky Grt $ Cld Qtz (Spear & Cheney, 1989; Mahar et al., 1997). Some Type-2 garnets appear to have resulted from the coalescence of several smaller Type-3 garnet grains (Fig. 7). Blue amphibole is abundant in some micaschists, associated with jadeitic pyroxene (glaucophanebearing micaschists), and scarce in others (albitebearing micaschists). It occurs commonly within the S 1 fabric as an elongated, syn-kinematic mineral, frequently boudinaged with the development of chlorite and actinolite in the fracture between the rods. Blue amphibole sometimes occurs associated with white 1286

7 GANNE et al. HIGH-PRESSURE METAMORPHISM, WESTERN ALPS Fig. 4. Large Type-2 garnets in a biotite-bearing micaschist (Ga-70). (a) Ga-70-2: part of this large garnet is replaced by chlorite (around the crystal). (b) Sketch of Type-2 garnet from (a), showing the relationship between garnet and the S 1 fabric: high-si substituted phengite is found along the external edge. Phe, phengite; Mus, muscovite; Qtz, quartz; S 1, main Alpine schistosity; S 1, pre-alpine schistosity. A±B, trace of zoning profile shown in (d). (c) Compositional variations of garnets along the profile in Fe Mg±Ca±Mn diagram. (e) Compositional variations of the garnet Ga-70/P1 along the profile C±D (f) in Fe Mg±Ca±Mn diagram. Black arrow indicates half bell-shaped zoning of Mn suggesting that these garnets grew during a gradual increase of P±T conditions and probably during a rotational deformation. The dispersion along the Ca(Fe,Mg) 1 vector (white arrow) with depletion of Fe towards the edge of crystal is discussed in the text. (f) X-ray map of Ga-70/ P1 garnet: relative content of Mn varying from low (dark) to high (white). mica as inclusions in all garnet types (Fig. 8). In the BriancË onnais domain, this HP±LT mineral is classically regarded as being diagnostic of Alpine metamorphism. Epidote, pale green in plane-polarized light, is abundant in the epidote-bearing micaschists. It grows within the S 1 fabric as an elongated syn-kinematic mineral, associated with titanite, blue amphibole and phengite. 1287

8 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 JULY 2003 Fig. 5. (a) Small Type-3 garnet (BSE image) in a glaucophane-bearing micaschist (Ga-53). (b) Sketch of Type-3 garnet from (a), showing the distribution of inclusions: glaucophane and phengite are disseminated everywhere through garnet. Grt, garnet; Cld, chloritoid; Phe, phengite; Chl, chlorite; S 1, main Alpine schistosity. A±B, trace of zoning profile shown in (e). (c) X-ray map of this garnet: relative ratio of Mn varying from low (dark) to high (white); metamorphic significance of the thin-rim at the extreme edge of the garnet and along the contact with phengite inclusion is discussed in the text. (d) Compositional variations of garnets along A±B profile in Fe Mg±Ca±Mn diagram. White arrow indicates FeCa 1, CaFe 1 exchanges in garnets (see text for explanation). S 1 may be folded within the internal Type-2 garnet (Figs 9 and 10b) during the Alpine D 1 shearing event. Chloritoid occurs in garnet±glaucophane±epidote± phengite aggregates, which possibly represent staurolite pseudomorphs (Fig. 6a and b). Sometimes it grows within the S 1 fabric (Fig. 6a and b) as an elongated syn-kinematic mineral, associated with white mica. Chloritoid is never observed as inclusions in garnet, unlike in the Alpine garnets from the Dora Maira massif (Matsumoto & Hirajima, 2000). White mica generally appears as pale green, fine plates, elongated in the S 1 fabric, as well as very fine fringes around pre-alpine muscovite (Fig. 6a and b). It is also present as inclusions, either dispersed with blue amphibole in Type-2 (Fig. 4) and Type-3 garnets or concentrated with blue amphibole along the rim of large Type-1 garnets (Fig. 3). Two generations of biotite occur sporadically in the micaschists. The first one is considered to be pre-alpine in age (Monie, 1990); it occurs as large plates (up to 3±5 mm), sometimes twisted and kinked in strongly deformed rocks, or sheared as fine plates commonly reoriented in the S 1 Alpine fabric (Figs 3b and 7a). Biotite of a second generation crystallizes around the rim of Type-1 garnets, as well as forming elongated syn-kinematic crystals parallel to the S 1 fabric (Figs 8a and 11c). It also crystallizes in the axial plane of Alpine folds, truncating the large pre-alpine biotites. More rarely, it crystallizes around blue amphibole. We associate this second generation of biotite with an Alpine metamorphic stage. MINERAL CHEMISTRY Analytical methods Microprobe analyses were carried out at the University of Lausanne with a CAMEBAX SX50. Counting times were 15±30 s per element on peak and 5±30 s on background depending on concentration. The accelerating 1288

9 GANNE et al. HIGH-PRESSURE METAMORPHISM, WESTERN ALPS Fig. 6. Structural and mineralogical evidence to argue that D 1 (i.e. the first Alpine deformation recognized in the basement) and M 1 (i.e. the peak of Alpine metamorphism recorded by typical HP mineral assemblages) are synchronous. (a, b) Small Type-3 garnets (diameter 505 mm) occur commonly in chloritoid±glaucophane±phengite aggregates possibly representing staurolite pseudomorphs (Ga-53). (b) is a sketch of the photomicrograph (a). These garnets contain numerous inclusions of high-si substituted phengites and glaucophane. Sometimes they occur within the S 1 fabric as syn-kinematic minerals (b): they preserve in their core relicts of the S 1 fabric marked by trails of opaque minerals. This S 1 relict may be folded within the internal garnet and suggests a D 1 non-coaxial deformation. 1, garnet (Grt); 2, chloritoid; 3, glaucophane; 4, mixture of Alpine HP minerals in possible pseudomorphs of staurolite; 5, inherited muscovite underlining a pre-alpine schistosity (S 1 ); 6, high-si substituted phengite underlining the main Alpine schistosity (S 1 ). voltage was 15 kv for a beam current of 10±20 na, depending on the analysed species. Natural silicates were used as standards. Thirty-seven zoning profiles and 26 X-ray maps (2D) of elements (Ca, Fe, Mg and Mn) were obtained for Ambin and South Vanoise garnets. Compositional data for garnet, biotite, glaucophane, jadeite, phengite, chloritoid and clinozoisite are reported in Tables 1±5. The complete dataset may be downloaded from the Journal of Petrology website at Zoning patterns of garnet Large garnets Type-1 core garnets (Fig. 3) are a solid solution of almandine (X Alm ˆ 058±070), grossular (X Grs ˆ 008±015), spessartine (X Sps ˆ 009±030, exceptionally 060) and pyrope (X Prp ˆ 004±010). These garnets also display a strong growth zoning characterized by FeMn 1 (FeCa 1 ) exchange. X Sps is always higher than X Grs. Close to the inclusion-rich rim, X Sps, X Grs and X Prp contents converge toward a mean value of 10%, whereas X Alm is maximum at 70%. Type-1 rims show (Figs 3 and 12) an abrupt increase in Ca at the expense of Mn Fe (X Grs ˆ 20±25%). Zoning is shown between only Mn and Ca. Type-2 garnets (Fig. 4) are a solid solution of almandine (X Alm ˆ 075±052), grossular (X Grs ˆ 019±037), spessartine (X Sps ˆ 00±016) and pyrope (X Prp ˆ 001± 008). These garnets display strong asymmetric zoning (Fig. 9b): FeMn 1 exchange (see also CaMn 1, Fig. 8g) with depletion of Mn towards the outer part of the crystal. The Ca content is rather constant inside the crystal; a slight decrease or increase near the edge of the crystal may result from an Fe(Mn,Ca) 1 or CaMn 1 exchange (Fig. 4d). In contrast to the Type-1 garnet, X Sps is always lower than X Grs. Small garnets Type-3 small garnets (Fig. 5) are a solid solution of almandine (X Alm ˆ 064±084), grossular (X Grs ˆ 018±030), spessartine (X Sps ˆ 0014±014) and pyrope 1289

10 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 JULY 2003 Fig. 7. (a) Fe Mg±Ca±Mn diagram showing the range of chemical zoning of large Type-2 garnets in a glaucophane-bearing micaschist (Ga-72). (b) X-ray map: concentration in Mn varying from low (dark) to high (white): this garnet results from a coalescing of several small Mn-rich garnets; A±B, trace of zoning profile shown in (c). (c) Graphic showing, along the A±B profile, the variations of Fe, Mg, Ca and Mn in garnet and Si 4 in phengites. Black arrow in (a) and (c) indicates bell-shaped zoning of Mn suggesting that garnet grows during a gradual increase of P±T conditions (inclusions of phengites gradually substituted, from core to rim, with Si 4 ). It should be noted that low-si substituted phengite inclusions are located in the high-mn garnet composition. (X Prp ˆ 001±008). Crystals display a very faint zoning: Mg(Fe,Ca) 1 exchange with Fe or Ca depletion at the edge of the crystal. The chemical compositions measured at the periphery of Type-2 garnets and in the rim of Type-1 garnets (Ca-rich) are similar to those of many of the small Type-3 garnets (Fig. 5d; Table 1). Depletion of Fe, Mg and Ca at the extreme edge of these small garnets (Fig. 5e), correlated with an increase in their Mn content, is interpreted as resulting from a late diffusion process during the retrograde P±T path (D 2 mineral assemblages). A similar type of zoning can be observed along the contact between inclusions and garnet (Fig. 5c) or at the extreme edge of Type-2 (Fig. 4f ) and Type-1 garnets. Sodic phases Blue amphibole (Table 2) occurs as inclusions in garnets. According to Leake's (1978) nomenclature, it is Fe-glaucophane (1) in small Type-3 garnets, (2) in the 1290

11 GANNE et al. HIGH-PRESSURE METAMORPHISM, WESTERN ALPS Fig. 8. Bimodal distribution of large Type-1 and Type-2 garnets in the same thin section (Ga-40b: biotite-bearing micaschist). (a) Photomicrograph and (b) sketch showing the relationship between garnet and the main Alpine schistosity (S 1 ); Ab, albite; Gln, glaucophane; Jd, jadeite; Mus, muscovite; Bio1, biotite of first generation; Bio2, biotite of second generation; Qtz, quartz; S 1, main Alpine schistosity; S 1, pre-alpine schistosity. X-ray maps of these garnets: relative content of Ca (c, d) and Fe (f ) varying from low (dark) to high (white). The Carich rim marks the boundary between Alpine (rim) and pre-alpine garnet (core, [1]). (e) Sketch of the Type-2 garnet occurring in the lower part of (d): the distribution of blue amphibole inclusions should be noted. Fe-Rieb, ferro-riebeckite; Fe-Gln, ferro-glaucophane. A±B, trace of zoning profile shown in (g). (f) X-ray map of this garnet (g) showing correlation between chemical zoning and the distribution of blue amphibole inclusions inside the garnet: the highest-pressure amphiboles are located in the external Mn-poor edge. 1291

12 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 JULY 2003 Fig. 9. Chemical zoning of large Type-2 garnets in an epidote-bearing micaschist (Ga-104). (a, b) X-ray maps: concentration in Mn and Ca varying from low (dark) to high (white). A±B, trace of zoning profile shown in (d) and in the Fe Mg±Ca±Mn diagram (c). Black arrow in (c) indicates bell-helicitic zoning of Mn suggesting that garnet growth took place during a rotational deformation with a gradual increase of P±T conditions (transition of epidote to clinozoisite composition from core to rim). It should be noted that along the profile (dotted lines) X Alm decreases and X Grs increases around the epidote inclusions (Fig. 10a): the higher the amount of ferric iron in epidote (see table in Fig. 10a), the higher are the X Alm and X Grs peaks (d). external edge of large Type-2 garnets and (3) in the rim of large Type-1 garnets, and Fe-riebeckite in the internal part of large Type-2 garnets. In the rock matrix, Fe-glaucophane can display a late chemical zoning with depletion of Na at the edges of the crystal. This chemical zoning does not affect the Feglaucophane inclusions in garnets. In the rocks strongly affected by the retrograde metamorphism, blue amphiboles recrystallized to a stable association of chlorite±actinolite±slightly substituted phengite. It should be noted in Table 3 that glaucophane inclusions in garnet have a lower Mg content (18±568 mol %) and a higher Mn content (012±023 mol %) compared with matrix glaucophane. Na-clinopyroxenes occur as many small-grain assemblages associated with albitic plagioclase in the ABM-matrix (X Jd 088) or with the blue amphibole in the GBM (X Jd 055). Jadeite has never been observed as inclusions in garnet. Epidote There is a significant range of composition from epidote inclusions in garnet to clinozoisite in the matrix. Clinozoisite has an Fe 3 /(Al Fe 3 ) ratio ranging from 025 to 027 (Table 2). 1292

13 GANNE et al. HIGH-PRESSURE METAMORPHISM, WESTERN ALPS Fig. 10. Structural and mineralogical evidence to argue that Alpine Type-2 garnet growth took place during a rotational deformation with a gradual increase of P±T conditions (Ga-104; see Fig. 9). Epidote and titanite grow within the S 1 fabric as elongated syn-kinematic minerals, associated with glaucophane and phengite. The S 1 fabric, progressively refolded during the shearing, is `quenched' within the internal Type-2 garnet (b). From core to rim, epidote inclusions change toward clinozoisite composition (note also that phengite in the matrix is more substituted than phengite inclusions). 1293

14 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 JULY 2003 Fig. 11. Textural and chemical relations between pre-alpine cores [1] and Alpine fabric. (a, b) X-ray maps of a Type-1 garnet in a biotitebearing micaschist: concentration in Fe and Ca varying from low (dark) to high (white). (c) Sketch of Type-1 garnet from (a) and (b): the inclusion-poor garnet core is fragmented in two parts during a simple shear deformation regime and wrapped by the main Alpine fabric (S 1 ). Inclusion-rich rims of Alpine garnets, high-si substituted phengites and Alpine biotite (Biotite2; see text for detail) grow at the expense of the pre-alpine clasts forming a strain fringe. Spot analyses have been performed to check the Alpine (rims, [2]) and pre-alpine (cores, [1]) composition of Type-1 garnet. Pre-Alpine muscovites are destabilized to phengite. Chloritoid Chloritoid has an X Mg ratio that varies between 008 and 014 with no significant variations from core to rim (Table 2). White mica White mica is phengite, paragonite or muscovite. Phengite inclusions in garnets have low Na contents and Si 4 values between 335 and 355 (Table 2). 1294

15 GANNE et al. HIGH-PRESSURE METAMORPHISM, WESTERN ALPS Table 1: Representative chemical compositions and structural formulae of garnets of Type-1, -2 and -3 Sample: Ga-40b Ga-40b Ga-40b Ga-70 Ga-70 Ga-53 Analysis: 1/33 1/16 1/1 2/37 2/2 37 Mineral: Grt Grt Grt Grt Grt Grt Rock: ABM ABM ABM ABM ABM GBM Type: Type-1 Type-1 Type-1 Type-2 Type-2 Type-3 Position: Core Rim 1 Rim 2 Rim Core Core SiO TiO Al 2 O FeO MnO MgO CaO Total Oxygens Si Ti Al Fe Mn Mg Ca Total X Mg % Alm % Sps % Prp % Grs GBM, glaucophane-bearing micaschists; ABM, albite-bearing micaschists; Rim 1, pre-alpine composition; Rim 2, Alpine composition. Syn-kinematic phengite, elongated within the two Alpine schistosities (S 1, S 2 ) is sometimes interlayered with paragonite: Si 4 values are between 310 and 360. In contrast to Alpine phengites, colourless muscovites linked to the pre-alpine events have very low Si contents (Si ) and high (Al total Na)/Si 4 ratios ( 15). both for the large pre-alpine biotite (Bio1; Monie, 1990) and for the small Alpine biotite (Bio2; this study). However, the microprobe analyses do not allow the distinction between oxychlorite, chloritized biotite and intergrown stilpnomelane and chlorite (Table 5). Rutile is nearly pure; ilmenite and titanite have homogeneous compositions. Other minerals Plagioclase is close to end-member albite in composition, with a maximum anorthite content of 001 mol %. Biotite (Table 4) and stilpnomelane are potentially present as retrograde phases. No significant chemical variations have been observed within a given crystal of biotite. The Mg/(Mg Fe) ratio (040±047) and Al VI content (between 048 and 065 atom p.f.u.) are similar DISCUSSION The occurrence of multi-stage garnet has been described in other HP units of the Western Alps (Desmons & Ghent, 1977; Borghi et al., 1985, 1994; Desmons, 1992; Sandrone & Borghi, 1992) and more particularly in the basement of the Dora Maira massif (Matsumoto & Hirajima, 2000). With the notable exception of a few studies (Ellenberger, 1958; Goffe, 1295

16 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 JULY 2003 Table 2: Composition of different types of biotite: Alpine (Bio2) and pre-alpine (Bio1) Sample: Ga-23 Ga-71 Ga-23 Ga-71 Ga-23 Ga-71 Ga-23 Ga-71 Analysis: Mineral: Gln Gln Jd Jd Phe Phe Cld Czo Rock: ABM GBM ABM GBM ABM GBM ABM GBM Position: mt incl mt mt incl incl mt mt SiO TiO Al 2 O Cr 2 O Fe 2 O FeO MnO MgO CaO Na 2 O K 2 O Total Oxygens Si Ti Al Cr Fe Fe Mn Mg Ca Na K Total Mn/Fe X Mg X Jd incl, inclusion; mt, matrix; GBM, glaucophane-bearing micaschists; ABM, albite-bearing micaschists. 1977; Caby, 1996), the multi-stage character of garnet has been systematically linked to pre-alpine metamorphic events in the BriancË onnais basement (Gay, 1971; Detraz & Loubat, 1984; Baudin, 1987; Debelmas et al., 1998; Borghi et al., 1999). On the basis of chemical composition and inclusion assemblages, we distinguish two generations of garnet in micaschists from a particular tectonic unitðthe Clarea NappeÐoccurring in the deeper part of the Ambin and South Vanoise basements. Two generations of garnets Glaucophane is a diagnostic mineral for Alpine HP metamorphism in the Ambin and South Vanoise BriancË onnais basements. Previous studies concluded that the pre-alpine metamorphism in the internal part of the Western Alps was mainly of low- to medium-pressure type (Desmons et al., 1999b). Therefore, we can use glaucophane as a indicator of Alpine-stage metamorphism. In the Clarea micaschists, large Type-2 garnets display chemical zoning with depletion of Mn and increase of Fe (Ca) toward the external edge of the crystal: the asymmetric bell-shaped zoning pattern defined by Mn suggests that garnet growth took place during a gradual increase of P±T conditions (Spear, 1993) and maintained surface equilibrium (Hollister, 1966; Kretz, 1973). Indeed, we obtain an excellent correlation (1) between the phengite distribution in 1296

17 GANNE et al. HIGH-PRESSURE METAMORPHISM, WESTERN ALPS Table 3: Representative garnet (Grt), phengite (Phe), chloritoid (Cld), glaucophane (Gln) and clinozoisite (Czo) microprobe analyses and structural formulae used for THERMOCALC calculations Sample: Ga-40b Ga-53 Ga-22 Ga-22 Analysis: Mineral: Bio1 Bio2 Bio1 Bio2 Rock: ABM GBM ABM ABM Position: incl mt incl mt SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Total Oxygens Si Ti Al Fe t Mn Mg Ca Na K Total X Fe incl, inclusion; mt, matrix; GBM, glaucophane-bearing micaschists; ABM, albite-bearing micaschists. the garnets and their Tschermakitic substitution, the most substituted phengites being located at the edge of the crystal; (2) between the blue amphibole distribution in the garnets and their Al IV content, the Fe-glaucophane occurring at the edge of the crystal (Fig. 8g); (3) between the epidote distribution in the garnets and their Fe content, the Fe-clinozoisite occurring at the edge of the crystal and in the matrix. Glaucophane, clinozoisite and phengite inclusions may be attributed to the HP prograde stage of the Alpine metamorphism. Conversely, the chemical and textural discontinuity observed in large Type-1 garnets suggests the existence Table 4: Chemical compositions and mineral formulae of Type-2 garnets and glaucophane inclusions in sample Ga-22 Sample: Ga-22 Ga-22 Ga-22 Ga-22 Analysis: Mineral: Grt Grt Gln Gln Type: Type-2 Type-2 Position: Core Rim Core Rim SiO TiO Al 2 O Cr 2 O Fe 2 O FeO MnO MgO CaO Na 2 O K 2 O Total Oxygens Si Ti Al Cr Fe Fe Mn Mg Ca Na ÐÐ ÐÐ K ÐÐ ÐÐ Total % Alm % Sps % Prp % Grs Mn/Fe Grt, garnet; Gln, glaucophane. of at least two growth stages. The distribution of glaucophane inclusions indicates that only the rim of these garnets was developed during an Alpine HP metamorphic stage. As for small Type-3 garnets, the chemical composition of this rim displays a very faint growth zoning (FeMn 1 or CaMn 1 exchange): such a composition is similar to that measured at the edge of Type-2 garnet (Fig. 8g). Thus, we can postulate, 1297

18 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 JULY 2003 Table 5: Mineral parageneses and compositional ranges of the studied metapelites used for THERMOCALC calculations Albite-bearing micaschists (ABM) Glaucophane-bearing micaschists (GBM) Sample: Ga-22 Ga-23 Ga-40b Ga-55 Ga-156 Ga-51 Ga-53b Ga-53 Ga-70 Ga-71 Primary D 1 assemblage Phe (Si 4 ) 3.37± ± ± ± ± ± ± ± ± ±3.53 Grt % Alm 62.5± ± ±71 65±70 61±72 68±72 69± ± ± ±73 % Grs 7.9± ± ±22 18± ±23 18±23 19± ± ± ±27 % Sps 5.1± ± ± ±4.5 1± ± ± ±11.4 0±17 1±15.1 % Prp 6.8± ± ±8.8 5± ± ± ± ± ± ±9.5 Cld X Mg 0.07± ± ± ± ± ± ± ±0.08 Cpx X Mg 0.10± ± ± ± ± ± ± ±0.16 Gln X Mg 0.38± ± ± ± ± ± ± ± ± ±0.38 Rt **i **i ***m,i **m,i *m **m ***i ***m,i **m,i **m Ep m m ***m m Pg ***m,i **m,i ***m,i ***m,i ***m,i **m,i ***m,i *m,i ***m,i **m,i Opaque **ilm(m,i) **ilm(m,i) ***ilm(m,i) **ilm(m,i) *ilm(m,i) **ilm(m,i) ***ilm(m,i) **ilm(m,i) **ilm(m,i) **ilm(m,i) Secondary D 2 assemblage Phe (Si 4 ) 3.15± ± ± ± ± ± ± ± ± ±3.25 Ab ***m **m ***m ***m *m *m *m *m Pg **m *m ***m ***m *m *m ***m *m ***m **m Ep ***m **m **m **m Chl X Mg 0.38± ± ± ± ± ± ± Bt (Chl) ***m **m ***m *m *m **m **m Opaque **(ilm) **(ilm) ***(ilm) **(ilm) *(ilm) **(ilm) ***(ilm) **(ilm) **(ilm) **(ilm) *Rare. **Common. ***Frequent. i, inclusion within Grt; m, matrix mineral. (Si 4 ), Si content of phengite (on the basis of 22 oxygens). X Mg ˆ Mg/(Fe Mg); Bt (Chl), chloritized biotite; ilm, ilmenite. Mineral abbreviations are from Kretz (1983). according to the nature of inclusions (glaucophane, high-si substituted phengite) that small Type-3 garnets and the rim of large Type-1 garnets were linked to the Alpine metamorphism and document, in most cases, the last growth stages of an HP±LT prograde metamorphic event (Fig. 13). Unfortunately, we did not find any evidence to constrain the timing of growth of large, inclusion-poor garnet cores (Type-1). They could be either of pre- Alpine or of Alpine age. If Alpine, these garnet cores would have grown under low-pressure conditions of a very early metamorphic stage; partial resorption followed by a second growth stage under peak conditions (D 1 ) would then be expected. This interpretation is unlikely, because the asymmetrically zoned Type-2 garnets hosting glaucophane inclusions occur in the same thin sections as the large Type-1 garnets (Fig. 8). Therefore, we favour the idea of a pre-alpine growth for the core of large Type-1 garnets, as suggested by the inclusion of large biotite and muscovite [Bocquet (Desmons), 1974a, 1974b; Monie, 1990; Borghi et al., 1999). Mineral chemistry of the two generations of garnet On the basis of the distribution of Alpine inclusions in garnets, it is now possible to define compositional fields for Alpine and pre-alpine garnets (based on 2360 analyses). Ca and Mn are the best discriminating elements, whereas Fe and Mg are not and have been grouped in the ternary diagram of Fig. 14. Alpine garnets The main characteristic of Alpine garnets is their high Ca content, (20±37% mol wt), which is always higher than their Mn content. On an Fe,Mg±Ca±Mn diagram (Fig. 14), the dispersion of garnet along the 1298

19 GANNE et al. HIGH-PRESSURE METAMORPHISM, WESTERN ALPS Fig. 12. Chemical zoning in the rim of large Type-1 garnet. (a, c) X-ray maps of a large Type-1 garnet in a biotite-bearing micaschist (Ga- 22): concentration in Ca varying from low (dark) to high (white). The Ca-rich rim marks the boundary between Alpine (rim) and pre-alpine garnet (core). A±B and C±D are traces of zoning profiles shown in (b) and (d), respectively. Compositional variations of cores (e) and rims (f ) for large Type-1 garnets in the Fe Mg±Ca±Mn diagram. Black arrow in (f ) indicates zoning of Mn (d) suggesting that the Ca-rich rim has grown during a gradual increase of low P±T conditions. FeMn 1 vector characterizes a growth zoning during gradual increase of P±T conditions (increase of Fe and Ca toward the edge of crystals)ðthis is the dominant exchange. Conversely, the dispersion along the Fe,Mg $ Ca axis with alternating Fe (Fig. 5d) or Ca depletion towards the edge of crystal seems not very significant; it probably marks the last growth stage of Alpine garnet with respect to the prograde and/or retrograde P±T path. We observe the same dispersion among small garnets of a given rock (Fig. 5d; i.e. X Alm 064±084, X Prp 001±008, X Grs 018±030, X Sps 00±016). Pre-Alpine garnets Pre-Alpine garnets are systematically higher in Mn than in Ca. During their growth, under prograde 1299

20 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 JULY 2003 Fig. 13. Schematic P±T relationships between chemical zoning patterns for Alpine and pre-alpine garnets. (A) pre-alpine garnets (cores of large Type-1 garnets), original shape of crystal (euhedral) has sometimes been preserved. (B) Mn-rich Alpine Type-2 garnets (and more rarely of a few small Type-3 garnets): first steps of growth, inclusions of Fe-riebeckite and low-si substituted phengite. (C) Mn-poor composition for the large Type-2 garnets (external edge), the rim of large Type-1 garnet, and the most part of small Type-3 garnet: last steps of growth during the HP metamorphic peak, Fe-glaucophane and high-si substituted phengites. S 1, pre-alpine schistosity; S 1, main Alpine schistosity marked in some cases by oxides; Gt, garnet; St, staurolite pseudomorphs; Bio1, pre-alpine biotite; Bio2, assumed-alpine biotite; Type-1, -2 and -3 garnets are described in the text. White arrow indicates the FeCa 1, CaFe 1 exchanges with alternating Fe or Ca depletion towards the edge of crystal, which probably marks the last growth stage of Alpine garnet with respect to the prograde and/or retrograde P±T path (see text for details). P±T conditions (Spear, 1993), Type-1 garnets develop a concentric zoning in which the FeMn 1 exchange dominates (increase of Fe toward the external part of crystals). At the (assumed) end of their growth, i.e. before formation of the inclusion-rich rim (with quartz, glaucophane and phengite), these garnets had an X Alm content of 70% for X Prp, X Sps and X Grs contents around 10% (X Alm 058±070, X Prp 004±010, X Grs 008±015, X Sps 009±06). The CaMn 1 exchange is not very significant, with X Sps and X Grs contents oscillating around the mean value of 10% (Fig. 12b). This oscillatory zoning is probably linked to a rehomogenization phenomenon, which occurred at the beginning of the HP Alpine metamorphic stage. This average composition is close to that of pre-alpine garnets from the Dora Maira massif (Fig. 1a; Matsumoto & Hirajima, 2000, and reference therein), for similar mineral assemblages. Peak of Alpine metamorphism recorded in the basement This new garnet dataset provides a better constraint for estimating Alpine metamorphic P±T conditions. Calibrations of continuous and discontinuous reactions in the KMASH and NFMASH systems provide the opportunity of evaluating the P±T conditions in the metapelitic rocks, and more particularly the HP peak of metamorphism (M 1 ). To assess the validity of our estimates, we have combined the P±T evaluations obtained with the NFMASH (Na 2 O±FeO±MgO± Al 2 O 3 ±SiO 2 ±H 2 O) petrogenetic grid of Bosse et al. 1300

21 GANNE et al. HIGH-PRESSURE METAMORPHISM, WESTERN ALPS Fig. 14. Compositional variations of Alpine (A) and pre-alpine (B) garnets from Ambin and South Vanoise metapelites: 2360 analyses have been plotted in an Fe Mg±Ca±Mn diagram. Black arrow indicates bell-shaped zoning of Mn in Alpine and pre-alpine garnets suggesting a gradual increase of P±T conditions (Spear, 1993); white arrow indicates CaFe 1 and FeCa 1 exchanges in Alpine garnets (see text for explanation). (2002)Ðmodified from Guiraud et al. (1990)Ðwith results obtained with THERMOCALC (Holland & Powell, 1998) using our specific mineral compositions. the NFASH reaction Cld Gln Grt Chl Pg Qtz Vap: R4 Qualitative approach Four univariant reactions are shown (bold lines) in the P±T space considered (Fig. 15a): the two critical reactions limiting the lawsonite field toward the higher-temperature conditions in the FMASH system are experimentally determined: Lws Ab ˆ Pg Czo Qtz Vap Lws Jd ˆ Pg Czo Qtz Vap e:g: Heinrich & Althaus, 1988 the degenerate NASH equilibrium Ab ˆ Jd Qtz R1 R2 R3 whose location in P±T space is experimentally determined (e.g. Holland, 1980); The above reactions are assumed to emanate from one invariant point II in the NFASH subsystem. Glaucophane, epidote and jadeite (X Jd ˆ 09) are observed as inclusions in garnet and form the peak pressure assemblage. Equilibrium of glaucophane and epidote with the surrounding Type-2 garnet is strongly suggested by the correlation of their compositions in the vicinity of the inclusions: the oscillatory grossular content increases at the proximity of epidote inclusions (Fig. 9d), and the spessartine content is correlated with the composition of glaucophane (Fig. 8g) and epidote inclusions (Fig. 9d). The conditions of equilibrium for the assemblage glaucophane±epidote±jadeite±garnet are indicated by the grey area in Fig. 15b. It is bounded at high pressure by reactions (R2), which corresponds to the breakdown of clinozoisite into lawsonite (never observed in our samples), and (R4), which corresponds to the breakdown of garnet into chloritoid glaucophane. Additional constraints are provided by the absence of staurolite and the stability 1301

22 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 JULY 2003 Fig. 15. (a) Estimated and (b) calculated P±T path (white arrow) for the glaucophane-bearing micaschist (GBM) from the Clarea Nappe. (a) Schreinemakers analysis of the phase relations between garnet, glaucophane, chloritoid, paragonite, albite and quartz (with excess vapour) in the NFMASH system. AFM projections with the distribution of the saturating sodic phases (paragonite, albite or jadeite) have been performed by Bosse et al. (2002) using a method described by Thompson (1972). (See text for further explanation.) (b) The location of the NFMASH grid in P±T space follows Bosse et al. (2002), modified from Guiraud et al. (1990). The phengite isopleths (Massonne & Schreyer, 1987) and glaucophane stability curve (Maresch, 1977) are only indicative. The high-pressure peak of metamorphism (M 1 ) has been calculated using the THERMOCALC program (Holland & Powell, 1998). Each symbol (black diamonds) corresponds to one mineral assemblage. Boxes indicate peak of metamorphism recorded in the surrounding Schistes Lustres units from Agard et al. (2001) (1) and Coggon & Holland (2002) (2). limit of glaucophane, which suggests a maximum temperature of 550 C (Mahar et al., 1997). The lower-pressure limit of the peak pressure assemblage is constrained by reactions (R3) (breakdown of jadeite into albite) and (R6) (breakdown of garnet± glaucophane into chlorite±paragonite). According to Fig. 15, the peak pressure conditions are therefore 15 kbar at temperatures between 480 and 550 C. The growth zoning of garnet and its inclusions help in deciphering their prograde metamorphic history. In the glaucophane-bearing micaschists, the compositional zoning of Type-2 Alpine garnet is characterized by a decrease of Mn/Fe ratio from core to rim (Fig. 4c and d). The inclusions of sodic amphibole and epidote show the opposite variation, i.e. a decrease of Mn/Fe ratio from core to rim (Table 3). This correlation suggests that the reaction responsible for garnet growth involves sodic amphibole or/and epidote. According to the peak pressure conditions shown in Fig. 15 (and because no chloritoid inclusions have been observed in the garnet of the Clarea Nappe), the reaction responsible for the garnet growth is probably Pg Chl ˆ Alm Gln: R6 The almandine and grossular contents of garnet show strong variations close to the epidote inclusions (Fig. 9d). The X Alm and X Grs peaks are also correlated with the composition of epidote inclusions (Mn-poor epidote in the core and Mn-rich clinozoisite in the rim of garnet). These compositional changes probably result from the equilibrium Czo Ttn ˆ Grs Rt Qtz Vap: R9 Experimental data show that this reaction is strongly pressure dependent (Manning & Bohlen, 1991). Preliminary calculations in sample Ga-104 lead to pressures of 15 kbar at 515 C, which is in good agreement with the conditions estimated above (grey area in Fig. 15). 1302

23 GANNE et al. HIGH-PRESSURE METAMORPHISM, WESTERN ALPS Table 6: P±T calculations using THERMOCALC for 9 typical HP-metamorphic assemblages (Grt±Gln±Phe±Pg±Jd±Czo-Cld), thought to represent the D 1 peak pressure conditions Sample Primary D 1 assemblage T ( C) P (kbar) Cor, fit Ga-22 (Grt, Jd, Phe, Pg, Gln) Cor: 0.622, fit: 0.06 Ga-40b (Grt, Jd, Phe, Gln, Cld) Cor: 0.909, fit: 0.10 Ga-55 (Grt, Phe, Gln, Cld) Cor: 0.146, fit: 0.03 Ga-23 (Grt, Jd, Phe, Pg, Gln, Cld) Cor: 0.892, fit: 0.35 Ga-156 (Grt, Jd, Phe, Gln, Cld, Czo) Cor: 0.255, fit: 0.15 Ga-51 (Grt, Jd, Phe, Pg, Gln) Cor: 0.994, fit: 0.22 Ga-53 (Grt, Jd, Phe, Pg, Gln, Cld) Cor: 0.033, fit: 0.01 Ga-53b (Grt, Jd, Phe, Pg, Gln, Czo) Cor: 0.947, fit: 0.05 Ga-71 (Grt, Jd, Phe, Pg, Gln, Cld, Czo) Cor: 0.915, fit: 0.00 The error on each calculation is proportional to the value of the `fit' (Holland & Powell, 1998); fit is sigma(fit), i.e. the scatter of the residuals of the enthalpies and the activities normalized by their uncertainties; Cor is the correlation coefficient between calculated P and T. Quantitative approach The grid illustrated in Fig. 15 is mostly constrained by experimental data obtained for pure end-members. However, quantitative P±T estimates should take into account the compositional deviation of phases from the pure end-members, and the resulting decrease of their activity. For this reason, P±T estimates were calculated using THERMOCALC (Holland & Powell, 1998) with analysed compositions of the HP peak of metamorphism assemblages (Tables 5 and 6). Maximum pressure estimates were obtained using the Grt±Gln± Phe±Pg±Jd±Czo±Cld assemblages thought to represent the peak pressure conditions (see above). Chlorite inclusions were not considered, because preliminary thermobarometric estimates indicate that chlorite was not stable with glaucophane and epidote inclusions in garnet. Results of P±T estimates are reported in Fig. 15b. The pressure conditions range from 11 to 17 kbar with an average value of 15 kbar, and temperatures range from 462 to 520 C (T average ˆ 500 C). Such conditions are typical of epidote±blueschist close to eclogite-facies metamorphism (Evans, 1990). There is a good correlation between P±T estimates obtained from the GBM and from the ABM. However, the P±T estimates show a significant scatter, which might indicate varying P±T conditions during the prograde and retrograde path, or more probably uncertainties and errors resulting from (1) a lack of equilibrium between the selected phases used for the calculation, (2) the use of mineral compositions that do not correspond to the stable composition at peak pressure conditions (re-equilibration during the retrograde history, and (3) the poorly known composition±activity relations of epidote and glaucophane. IMPLICATIONS FOR THE ALPINE BELT Characterization of Alpine garnets Garnet in micaschists from the Ambin and South Vanoise massifs can be separated in two populations according to their large (Type-1 and Type-2) or small (Type-3) grain size, respectively. Zoning patterns and mineral inclusion distribution indicate that Type-2 and Type-3 garnets, as well as the rim of the Type-1 garnets, grew during an HP Alpine stage. Only cores of the large Type-1 garnets are inherited from pre-alpine metamorphic rocks. Alpine HP metamorphism in the BriancËonnais basement The Alpine HP metamorphic peak recorded in the Ambin and South Vanoise massifs corresponds to the development of a stable assemblage with (Ca,Fe)- garnet, Fe-glaucophane, phengite, Fe-chloritoid, paragonite, clinozoisite and jadeitic pyroxene. Whatever the reliability of the P±T values obtained by petrogenetic grid (Thompson, 1957) and traditional geothermobarometry (THERMOCALC: T ˆ 500 C 20, P ˆ 15 kbar 2; Fig. 15b), this metamorphic assemblage, in absence of lawsonite, characterizes the epidote±blueschist facies close to eclogitic conditions (Evans, 1990). The estimated P±T conditions are higher than previously thought (T C, P ˆ 12± 15 kbar; Goffe, 1977; Platt & Lister, 1985; Desmons et al., 1999a) and serve to demonstrate the distinct difference in metamorphism between the Ambin±South Vanoise basements (epidote±blueschist 1303

24 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 7 JULY 2003 Fig. 16. East±west structural cross-section through the Ambin massif (see Fig. 1c for location) showing the relationships between Clarea, Ambin and Schistes Lustres Nappes. The upper part of the Ambin massif is affected by a pervasive D 2 shearing event with top-to-the east movement direction. This D 2 shearing is well expressed in the Ambin Nappe, where oceanic and BriancË onnais cover are strongly deformed with slices of basement. The deeper part of the massif (i.e. the Clarea Nappe), consisting exclusively of basement, preserves early HP structures (D 1 ). The P±T paths corresponding to the Clarea Nappe (epidote±blueschist facies: stability field of garnet) and to the Schistes Lustres Nappe (lawsonite±blueschist facies: stability field of carpholite, Agard et al., 2001) overlying the massif meet at the end of D 2 shearing (Ganne et al., 2003). This is direct evidence of the link between the general D 2 shearing and the gap of metamorphism observed between the basement and oceanic covers. Because the upper nappe of the Schistes Lustres, i.e. the lower-grade unit, is lying directly above the Clarea Nappe, the corresponding metamorphic gap suggests that the large-scale, F 2 shear zones acted as detachment faults. (b) A geodynamic model to be tested for the Western Alps. At the scale of the Penninic domain, the D 2 shearing event results in a partitioning of the deformation between domains in which a simple-shear regime prevails, located at the edge of the domes, and domains in which a pure-shear regime prevails, located between these domes. Assuming that all these large-scale (east±west) shear zones are synchronous, a great part of the exhumation of the HP±LT rocks occurs during a generalized thinning event (Oligocene?) of the Penninic edifice. facies: stability field of garnet) and the surrounding pelitic±carbonaceous covers (BriancË onnais and upper Schistes Lustres units; lawsonite±blueschist facies: stability field of carpholite; Agard et al., 2001). This apparent `gap' in metamorphic P±T conditions, in terms of both P and T, may be an artefact of differences in bulk-rock composition (e.g. lawsonite in carbonaceous vs garnet in Fe,Al-rich rocks) or a real `gap' caused by tectonic juxtaposition. Significance of metamorphic gaps across the western Penninic domain Structural mapping carried out during the last decade in the most external unit of the BriancË onnais domain, the `Zone Houillere BriancË onnaise' (ZHB) has provided a wealth of new observations, which may be extrapolated toward the easternmost, more metamorphic, Ambin±South Vanoise and Gran Paradiso regions. The critical observation is that early tectonic 1304