| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Journal of Petrology | Volume 44 | Number 7 | Pages 1281-1308 | 2003
© Oxford University Press 2003
Multi-stage Garnet in the Internal Briançonnais Basement (Ambin Massif, Savoy): New Petrological Constraints on the Blueschist-facies Metamorphism in the Western Alps and Tectonic Implications
1 LABORATOIRE DE GÉODYNAMIQUE DES CHAÎNES ALPINES, CNRS, UMR 5025, UNIVERSITÉ 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 GÉODYNAMIQUE DES CHAÎNES ALPINES, CNRS, UMR 5025, UNIVERSITÉ JOSEPH FOURIER, MAISON DES GÉOSCIENCES, B.P. 43, 38041 GRENOBLE, FRANCE
* Corresponding author. Telephone: (33) 4 79 75 81 33. E-mail: Jerome.ganne{at}univ-savoie.fr
RECEIVED FEBRUARY 25, 2002; ACCEPTED FEBRUARY 26, 2003
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTING OF THE...MINERAL ASSEMBLAGES AND...MINERAL CHEMISTRYDISCUSSIONIMPLICATIONS FOR THE ALPINE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
|---|
Three types of garnet have been distinguished in pelitic schists from an epidote–blueschist-facies unit of the Ambin and South Vanoise Briançonnais 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 D1 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 (Briançonnais and upper Schistes Lustrés units), which experienced metamorphism only in the stability field of carpholite–lawsonite (T < 400°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 Lustrés units and Briançonnais 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 Briançonnais basements; metamorphic gaps; multistage garnets; Western Alps
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...MINERAL ASSEMBLAGES AND...MINERAL CHEMISTRYDISCUSSIONIMPLICATIONS FOR THE ALPINE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
|---|
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 Briançonnais domain, have been regarded as relics of a pre-Alpine Barrovian metamorphism. Although a few workers suspected the existence of Alpine garnets (Ellenberger, 1958; Goffé, 1977; Caby, 1996), distinguishing pre-Alpine from Alpine garnets was an unresolved issue. 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), 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; Goffé & 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 BRIANÇONNAIS BASEMENTS |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...MINERAL ASSEMBLAGES AND...MINERAL CHEMISTRYDISCUSSIONIMPLICATIONS FOR THE ALPINE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
|---|
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, Briançonnais and Subbriançonnais) and the internal Penninic domain, the last being constituted by the so-called Liguro-Piemontais units (oceanic suture of the Schistes Lustrés) 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 Duchêne 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 (Goffé & 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’ (Goffé & 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 present-day 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 Briançonnais 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 (Briançonnais Mesozoic units, ocean-derived Liguria–Piemont zone units). The origin and pre-Alpine tectono-metamorphic evolution of these basement units are still poorly known. To simplify nomenclature and description, we will distinguish three main litho-tectonic 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 Lustrés’ 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) 1/50 000 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 (glaucophane + chlorite) and rare marbles. These lithologies may represent a dominantly pelitic, flysch-type 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), albite-bearing 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 D1, D2, D3 (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 Lustrés Nappes are those related to the D2 (+D3) retromorphic deformations. These ductile to brittle–ductile shear events overprint pre-existing fabrics such as S1 (D1 event), which is the earliest Alpine schistosity clearly distinguishable from pre-Alpine fabrics and linked to the HP metamorphic peak (M1; this study). The D1 event is well preserved in the Clarea Nappe, i.e. in the deeper part of the Ambin and South Vanoise massifs.
|
|
MINERAL ASSEMBLAGES AND MICROSTRUCTURAL RELATIONSHIPS |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...MINERAL ASSEMBLAGES AND...MINERAL CHEMISTRYDISCUSSIONIMPLICATIONS FOR THE ALPINE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
|---|
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 (0·2–1 cm, Fig. 3) and display a contrasting rim outlined at its inner contact by a variety of inclusions (= 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 (S1) 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 inclusion-rich rim outlines a major tectonic event, which clearly separates two generations of garnet.
|
Large Type-2 garnets (0·15–0·7 cm, Fig. 4) and small Type-3 garnets (d<0·5 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 S1 fabric as syn-kinematic crystals (Fig. 6a and b; GA-53); they preserve relicts of the S1 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 S1 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 (glaucophane-bearing micaschists), and scarce in others (albite-bearing micaschists). It occurs commonly within the S1 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 mica as inclusions in all garnet types (Fig. 8). In the Briançonnais 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 S1 fabric as an elongated syn-kinematic mineral, associated with titanite, blue amphibole and phengite. S1 may be folded within the internal Type-2 garnet (Figs 9 and 10b) during the Alpine D1 shearing event.
|
|
Chloritoid occurs in garnet–glaucophane–epidote–phengite aggregates, which possibly represent staurolite pseudomorphs (Fig. 6a and b). Sometimes it grows within the S1 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 S1 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 (Monié, 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 S1 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 S1 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 |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...MINERAL ASSEMBLAGES AND...MINERAL CHEMISTRYDISCUSSIONIMPLICATIONS FOR THE ALPINE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
|---|
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 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 http://www.petrology.oupjournals.org/.
|
|
|
|
|
Zoning patterns of garnet
Large garnets
Type-1 core garnets (Fig. 3) are a solid solution of almandine (XAlm = 0·58–0·70), grossular (XGrs = 0·08–0·15), spessartine (XSps = 0·09–0·30, exceptionally 0·60) and pyrope (XPrp = 0·04–0·10). These garnets also display a strong growth zoning characterized by FeMn-1 (±FeCa-1) exchange. XSps is always higher than XGrs. Close to the inclusion-rich rim, XSps, XGrs and XPrp contents converge toward a mean value of ±10%, whereas XAlm is maximum at 70%. Type-1 rims show (Figs 3 and 12) an abrupt increase in Ca at the expense of Mn + Fe (XGrs = 20–25%). Zoning is shown between only Mn and Ca.
|
Type-2 garnets (Fig. 4) are a solid solution of almandine (XAlm = 0·75–0·52), grossular (XGrs = 0·19–0·37), spessartine (XSps = 0·0–0·16) and pyrope (XPrp = 0·01–0·08). 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, XSps is always lower than XGrs.
Small garnets
Type-3 small garnets (Fig. 5) are a solid solution of almandine (XAlm = 0·64–0·84), grossular (XGrs = 0·18–0·30), spessartine (XSps = 0·014–0·14) and pyrope (XPrp = 0·01–0·08). 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 (D2 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 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 Fe-glaucophane 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 (1·8–5·68 mol %) and a higher Mn content (0·12–0·23 mol %) compared with matrix glaucophane. Na-clinopyroxenes occur as many small-grain assemblages associated with albitic plagioclase in the ABM-matrix (XJd 0·88) or with the blue amphibole in the GBM (XJd 0·55). 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 Fe3+/(Al + Fe3+) ratio ranging from 0·25 to 0·27 (Table 2).
Chloritoid
Chloritoid has an XMg ratio that varies between 0·08 and 0·14 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 Si4+ values between 3·35 and 3·55 (Table 2). Syn-kinematic phengite, elongated within the two Alpine schistosities (S1, S2) is sometimes interlayered with paragonite: Si4+ values are between 3·10 and 3·60. In contrast to Alpine phengites, colourless muscovites linked to the pre-Alpine events have very low Si contents (Si4+ <3·05) and high (Altotal + Na)/Si4+ ratios (
1·5).
Other minerals
Plagioclase is close to end-member albite in composition, with a maximum anorthite content of 0·01 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 (0·40–0·47) and AlVI content (between 0·48 and 0·65 atom p.f.u.) are similar both for the large pre-Alpine biotite (Bio1; Monié, 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.
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...MINERAL ASSEMBLAGES AND...MINERAL CHEMISTRYDISCUSSIONIMPLICATIONS FOR THE ALPINE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
|---|
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; Goffé, 1977; Caby, 1996), the multi-stage character of garnet has been systematically linked to pre-Alpine metamorphic events in the Briançonnais basement (Gay, 1971; Détraz & 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 unit—the Clarea Nappe—occurring 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 Briançonnais 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 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 AlIV 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 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, 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 (D1) 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; Monié, 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 FeMn-1 vector characterizes a growth zoning during gradual increase of P–T conditions (increase of Fe and ±Ca toward the edge of crystals)—this 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. XAlm 0·64–0·84, XPrp 0·01–0·08, XGrs 0·18–0·30, XSps 0·0–0·16).
Pre-Alpine garnets
Pre-Alpine garnets are systematically higher in Mn than in Ca. During their growth, under prograde 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 XAlm content of 70% for XPrp, XSps and XGrs contents around 10% (XAlm 0·58–0·70, XPrp 0·04–0·10, XGrs 0·08–0·15, XSps 0·09–0·6). The CaMn-1 exchange is not very significant, with XSps and XGrs 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 (M1). To assess the validity of our estimates, we have combined the P–T evaluations obtained with the NFMASH (Na2O–FeO–MgO–Al2O3–SiO2–H2O) petrogenetic grid of Bosse et al. (2002)—modified from Guiraud et al. (1990)—with results obtained with THERMOCALC (Holland & Powell, 1998) using our specific mineral compositions.
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:
(R1) 
(R2)
- the degenerate NASH equilibrium
whose location in P–T space is experimentally determined (e.g. Holland, 1980);
(R3)
- the NFASH reaction
(R4)
|
Glaucophane, epidote and jadeite (XJd = 0·9) 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 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
 | (R6) |
 | (R9) |
15 kbar at 515°C, which is in good agreement with the conditions estimated above (grey area in Fig. 15).
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 (Taverage = 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 |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...MINERAL ASSEMBLAGES AND...MINERAL CHEMISTRYDISCUSSIONIMPLICATIONS FOR THE ALPINE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
|---|
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 Briançonnais 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 = 500°C ± 20, P = 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<400°C, P = 12–15 kbar; Goffé, 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 facies: stability field of garnet) and the surrounding pelitic–carbonaceous covers (Briançonnais and upper Schistes Lustrés 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 Briançonnais domain, the ‘Zone Houillère Briançonnaise’ (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 contacts, now refolded, can be recognized within the monotonous pile of Carboniferous grits, conglomerates and black schists (Bertrand et al., 1996; Schmid & Kissling, 2000). Further east, in the Ambin–South Vanoise areas, similar early tectonic contacts affect both pre-Permian and Mesozoic formations (Ganne et al., 2003). If the synchronism of this major nappe stacking event and the subsequent large-scale refolding event are confirmed within the whole domain, then the axial planar schistosity related to the refolding would correspond to a reference datum surface. Up to now, the refolding event was classically related to a backthrusting and backfolding event by most workers. The best evidence favouring regional synchronism of a main Alpine schistosity [S2 schistosity according to Aillères et al. (1995)] is that it post-dates everywhere the major tectonic discontinuities between lithological units (the early ‘nappes’; Caron, 1977). Corresponding early structures are in turn displaced at low angle by these later small-scale to large-scale shear zones (Chavière tectonic zone, Rosoire–Echelle shear zone; the late nappes), operating under greenschist-facies conditions. At sample scale, the refolding event corresponds to the blueschist–greenschist transition (Ganne et al., 2003) and is manifested in the whole Penninic domain by the ubiquitous growth of syn- to late-kinematic albite poikiloblasts. Thus, among the major tectonic contacts historically mapped in the region, at least two completely distinct sets may be now identified, as previously proposed by Platt & Lister (1985).
- D1 stage: early thrust surfaces
1 and related S1 schistosity, the latter being ubiquitous in the Ambin–South Vanoise areas (in both basement and cover) but rarely observed in the ‘zone houillère briançonnaise’ (ZHB). Within the Ambin and South Vanoise basements, it corresponds to the main banding of the rocks and related epidote–blueschist-facies assemblages (this study). Previous observations (Platt et al., 1989) indicate that transport directions are most probably toward the north or NW.
- D2 ± D3 stages: late retrograde
2 shear zones at all scales and related S2 greenschist schistosity, cutting across lithological and early D1 tectono-metamorphic boundaries (Fig. 16).
- D2 ± D3 stages: late retrograde
|
According to this structural scheme, the Ambin and South Vanoise massifs are interpreted as slices of ‘eclogitized’ continental crust tectonically extruded within the Schistes Lustrés units and Briançonnais covers. The corresponding tectonic contacts (
2) with top-to-east movement directions (see Fig. 1c) are responsible for the juxtaposition of lower-grade metamorphic units above the Ambin massif (Fig. 16a). Close to the Gran Paradiso dome, the Schistes Lustrés complex (Fig. 1b) exhibits a similar geometry with a structural superposition of lower-grade over higher-grade rocks along D2-type shear zones. Their top-to-west dominant movement raises the question of whether they correspond to several discrete events or have formed together with the Ambin shear zones a regional-scale conjugate network as shown in Fig. 16b. At a regional scale, the exact role played by these east- or west-verging 2 kilometre-scale shear zones (or equivalent according to other studies; see Fig. 17) is not yet fully appreciated (Platt et al., 1989; Ballèvre et al., 1990; Wheeler & Butler, 1993; Ring, 1995; Caby, 1996; Rolland et al., 2000; Schmid & Kissling, 2000; Agard et al., 2001). In particular, these shear zones could explain some of the metamorphic gaps observed in the Penninic Briançonnais domain (Fig. 1b). To test and quantify this hypothesis, it will be necessary to complete a detailed mapping of the Briançonnais covers surrounding the crystalline basements, where the LT metamorphic assemblage lawsonite–carpholite occurs (Goffé & Velde, 1984; Goffé & Chopin, 1986). A mapping of precise metamorphic zonality with respect to observed thresholds and/or gaps in temperature should lead to better constraints for the exhumation mechanisms of HP rocks in the Alps [for review, see Platt (1993)].
|
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...MINERAL ASSEMBLAGES AND...MINERAL CHEMISTRYDISCUSSIONIMPLICATIONS FOR THE ALPINE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
|---|
The petrographic and chemical study of glaucophane-bearing micaschists from the pre-Permian basement of the Ambin and South Vanoise massifs has provided evidence for the Alpine age of their hosted garnets. In the absence of lawsonite, this mineral association is diagnostic of the epidote–blueschist metamorphic facies and implies P–T conditions (T = 500°C ± 20, P = 15 kbar ± 2) that are unusual in the Briançonnais domain. These new data confirm the heterogeneity of the metamorphic conditions observed in the innermost part of the Western Alps (Penninic domain). They are in agreement with structural investigations and allow us to interpret the Ambin and South Vanoise Briançonnais massifs as slices of continental crust ‘eclogitized’ during the Alpine collision and tectonically extruded within the less metamorphic Schistes Lustrés units and Briançonnais covers.
|
SUPPLEMENTARY DATA |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...MINERAL ASSEMBLAGES AND...MINERAL CHEMISTRYDISCUSSIONIMPLICATIONS FOR THE ALPINE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
|---|
For supplementary data, please refer to Journal of Petrology Online.
|
ACKNOWLEDGEMENTS |
|---|
The authors are grateful to J. M. Bertrand, S. Fudral and G. S. Lister for their encouraging advice and critical reading. S. Harley, M. Wilson and A. Lumsden are thanked for their constructive reviews and editorial handling. We also express grateful thanks to J. Desmons and B. Goffé for their constructive discussions. This study was funded by the GéoFrance3D program (INSU, BRGM, MNERT).
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...MINERAL ASSEMBLAGES AND...MINERAL CHEMISTRYDISCUSSIONIMPLICATIONS FOR THE ALPINE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
|---|
Agard, P., Jolivet, L. & Goffé, B. (2001). Tectonometamorphic evolution of the Schistes Lustrés complex: implications for the exhumation of HP and UHP rocks in the Western Alps. Bulletin de la Société Géologique de France 5, 617–636.
Aillères, L., Bertrand, J. M., Macaudière, J. & Champenois, M. (1995). Structure de la zone houillère briançonnaise (Alpes françaises), tectonique néoalpine et conséquences sur l'interprétation des Zones Penniques frontales. Comptes Rendus de l'Académie des Sciences, Série II 321, 247–254.
Ballèvre, M., Lagabrielle, Y. & Merle, O. (1990). Tertiary normal ductile faulting as a consequence of the lithospheric stacking in the Western Alps. Bulletin de la Société Géologique de France 156, 227–236.
Baudin, T. (1987). Étude géologique du massif du Ruitor (Alpes franco-italiennes): évolution structurale d'un socle briançonnais. Ph.D. thesis, Université de Grenoble, 259 pp.
Bearth, P. (1952). Géologie und Petrographie des Monte Rosa. Geological Karte Schweizerische, N.F. 96, 1–94.
Bertrand, J. M., Aillère, L., Gasquet, D. & Macaudière, J. (1996). The Pennine Front zone in Savoie (Western Alps), a review and new interpretation from the Zone Houillère Briançonnaise. Eclogae Geologicae Helvetiae 89, 1–24.
Bocquet (Desmons), J. (1974a). Le socle briançonnais de Vanoise (Savoie): arguments en faveur de son âge anté-alpin dans les Alpes françaises et de son polymétamorphisme. Comptes Rendus de l'Académie des Sciences, D 278, 2601–2604.
Bocquet (Desmons), J. (1974b). Étude minéralogique et pétrologique sur les métamorphismes d'âge alpin dans les Alpes françaises. Thèse d'Etat, Université de Grenoble, 489 pp.
Borghi, A., Caddoppi, P., Porro, A. & Sacchi, R. (1985). Metamorphism in the north part of the Dora-Maira Massif (Cottian Alps). Bollettino del Museo Regionale di Scienze Nationali di Torino 3, 369–380.
Borghi, A., Compagnoni, R. & Sandrone, R. (1994). Evoluzione termo-tettonica alpina del settore settentrionale del massico del Gran Paradiso (Alpi Occidentali). Atti Ticinensi di Scienze della Terra (special issue) 1, 137–152.
Borghi, A., Gattiglio, M., Mondino, F. & Zaccone, G. (1999). Structural and metamorphic evidences of pre-Alpine basement in the Ambin nappe (Cottian Alps, Italy). Memorie de Scienze Geologiche Italiana, Padova 51, 205–220.
Bosse, V., Ballèvre, M. & Vidal, O. (2002). The garnet isograd in the blueschist-facies metapelites of the Île de Groix (Armorican Massif, France): a record of ductile thrusting during exhumation. Journal of Petrology 43, 485–510.
Caby, R. (1996). Low-angle extrusion of high-pressure rocks and the balance between outward and inward displacement of middle Penninic units in the western Alps. Eclogae Geologicae Helvetiae 89, 229–268.[Web of Science]
Callegari, E., Sacchi, R., Bovo, S. & Torassa, G. (1980). Osservazioni strutturali sul versante italiano del massico di Ambin (Alpi Graie). Bolletino delle Società Geologica Italiana, 99, 395–404.
Caron, J. M. (1977). Lithostratigraphie et tectonique des Schistes Lustrés dans les Alpes cottiennes septentrionales (France et Italie). Bulletin de la Société Géologique de France 16, 255–268.
Coggon, R. & Holland, T. J. B. (2002). Mixing properties of phengitic micas and revised garnet–phengite thermobarometers. Journal of Metamorphic Geology 20, 683–696.[CrossRef][Web of Science]
Dal Piaz, G. V., Hunziker, J. C. & Martinotti, G. (1972). La Zona Sesia–Lanzo e l'evoluzione tettonico-metamorfica delle Alpi nord occidentali interne. Memorie de Scienze Geologiche Italiana 11, 433–466.
Debelmas, J. & Lemoine, M. (1970). The Western Alps: paleogeography and structure. Earth-Science Reviews 6, 221–256.
Debelmas, J., Desmons, J., Ellenberger, F., Goffé, B. & Fabre, J. (1989). Carte géologique de France (1/50 000), feuille Modane (775). Notice par J. Debelmas et al. BRGM: Orléans, 53 pp.
Debelmas, J., Desmons, J., Deville, E., Fudral, S., Goffé, B., Guillot, F. & Jaillard, E. (1998). Géologie de la Vanoise. Mémoire du BRGM 266, 183 pp.
Desmons, J. (1992). The Briançon basement (Pennine Western Alps): mineral composition and polymetamorphic evolution. Schweizerische Mineralogische und Petrographische Mitteilungen 72, 37–55.
Desmons, J. & Ghent, E. D. (1977). Chemistry, zonation and distribution coefficient of elements in eclogitic minerals for the eastern Sesia unit, Italian western Alps. Schweizerische Mineralogische und Petrographische Mitteilungen 57, 397–411.
Desmons, J., Aprahamian, J., Compagnoni, R., Cortesogno, L. & Frey, M., with the collaboration of Gaggero, L., Dallagiovanna, G. & Seno, S. (1999a). Alpine metamorphism of the Western and Southern Alps. Schweizerische Mineralogische und Petrographische Mitteilungen 79, 89–110.
Desmons, J., Compagnoni, R., Cortesogno, L., Frey, M. & Gaggero, L. (1999b). Pre-alpine metamorphism of the Internal zones of the Western Alps. Schweizerische Mineralogische und Petrographische Mitteilungen 79, 23–39.
Détraz, G. & Loubat, H. (1984). Facies à disthène, staurotide et grenat dans un micaschiste appartenant à l'unité des 
gneiss du Sapey
(Vanoise, alpes française). Géologie Alpine, Université de Grenoble 60, 5–12.
Duchêne, S., Blichert-Toft, J., Luais, B., Telouk, P., Lardeaux, J. M. & Albarède, F. (1997). The Lu–Hf of garnets and the ages of the Alpine high-pressure metamorphism. Nature 387, 586–588.[CrossRef]
Ellenberger, F. (1958). Études géologiques du pays de Vanoise (Savoie). Mémoire Explicatif de la Carte Géologique de France, 561 pp.
Escher, A., Hunziker, J. C., Marthaler, M., Masson, H., Sartori, M. & Steck, A. (1997). Geologic framework and structural evolution of the western Swiss–Italian Alps. In: Pfiffner, O. A., Lehner, P., Heitzmann, P., Mueller, S. & Steck, A. (eds) Deep Structure of the Swiss Alps. Basel: Birkhäuser, pp. 205–221.
Evans, B. W. (1990). Phase relation of epidote-blueschists. Lithos 25, 3–23.[CrossRef][Web of Science]
Fudral, S., Deville, E., Pognante, U., Gay, M., Fregolent, G., Lorenzoni, S., Robert, D., Nicoud, G., Black, C., Jayko, A., Jaillard, E., Bertrand, J. M., Forno, M. G. & Massazza, G. (1994). Carte géologique de la France à 1:50 000, feuille 776 Lanslebourg Mont-d'Ambin. Orléans: Ministère de l'industrie–BRGM, 98 pp.
Ganne, J. (1999). Evolution tectono-métamorphique de la bordure NW du massif d'Ambin (Alpes occidentales, France). Diplôme d'Etude Approfondie, Université de Savoie, 35 pp.
Ganne, J., Bertrand, J. M. & Fudral, S. (2003). Structural and metamorphic evolution of the Ambin massif (western Alps): toward a new exhumation model for the Briançonnais domain. Tectonophysics (in press).
Gay, M. (1971). Le massif d'Ambin et son cadre de Schistes Lustrés (Alpes franco-italiennes). Thèse d'État, Université de Lyon, 296 pp.
Ghent, E. D. (1986). A review of chemical zoning in eclogite garnets. In: Smith, D. C. (ed.) Eclogite and Eclogite-facies Rocks. Amsterdam: Elsevier, pp. 207–236.
Goffé, B. (1977). Succession de subfacies métamorphiques en Vanoise méridionale (Savoie). Contributions to Mineralogy and Petrology 62, 23–41.[CrossRef][Web of Science]
Goffé, B. & Chopin, C. (1986). High-pressure metamorphism in the Western Alps: zoneography of metapelites, chronology and consequence. Schweizerische Mineralogische und Petrographische Mitteilungen 66, 41–52.
Goffé, B. & Velde, B. (1984). Contrasted metamorphic in thrusted cover units of the Briançonnais zone (French Alps): a model for the conservation of HP–LT metamorphic mineral assemblages. Earth and Planetary Science Letters 68, 351–360.[CrossRef][Web of Science]
Guiraud, M., Holland, T. J. B. & Powell, R. (1990). Calculated mineral equilibria in the greenschist–blueschist–eclogite facies in Na2O–FeO–MgO–Al2O3–SiO2–H2O. Contributions to Mineralogy and Petrology 104, 85–98.[CrossRef][Web of Science]
Heinrich, H. & Althaus, E. (1988). Experimental determination of the reaction 4 lawsonite + 1 albite = 1 paragonite + 2 quartz + 6 H2O. Neues Jahrbuch füur Mineralogie, Monatshefte 11, 516–528.
Hollister, L. S. (1966). Garnet zoning: an interpretation based on the Rayleigh-fractionation model. Science 154, 1647–1651.
Holland, T. J. B. (1980). The reaction albite = jadeite + quartz determined experimentally in the range 600–120°C. American Mineralogist 65, 129–134.[Abstract]
Holland, T. J. B. & Powell, R. (1998). An internally consistent thermodynamic dataset for phases of petrological interest. Journal of Metamorphic Geology 16, 309–343.[CrossRef][Web of Science]
Kienast, J. R., Lombardo, B., Biino, G. & Pinardon, J. L. (1991). Petrology of very-high-pressure eclogitic rocks from the Brossasco–Isasca Complex, Dora-Maira Massif, Italian Western Alps. Journal of Metamorphic Geology 9, 19–34.[Web of Science]
Kretz, R. (1973). Kinematics and crystallization of garnet at two localities near Yellowknife. Canadian Mineralogist 12, 1–20.
Kretz, R. (1983). Symbols for rock-forming minerals. American Mineralogist 68, 269–279.
Lardeaux, J. M. & Spalla, M. I. (1991). From granulites to eclogites in the Sesia zone (Italian Western Alps): a record of the opening and closure of the Piedmont ocean. Journal of Metamorphic Geology 9, 35–59.[Web of Science]
Leake, B. E. (1978). Nomenclature of amphiboles. American Mineralogist 63, 1023–1052.[Abstract]
Lemoine, M. & de Graciansky, P.C. (1988). Histoire d'une marge continentale passive: les Alpes occidentales au Mésozoïque—Introduction. Bulletin de la Société Géologique de France 8, 597–600.
Mahar, E. M., Baker, J. M., Powell, R., Holland, T. J. B. & Howell, N. (1997). The effect of Mn on mineral stability in metapelites. Journal of Metamorphic Geology 15, 223–228.[CrossRef][Web of Science]
Manning, C. E. & Bohlen, S. R. (1991). The reaction titanite + kyanite = anorthite + rutile and titanite–rutile barometry in eclogites. Contributions to Mineralogy and Petrology 109, 1–9.[CrossRef][Web of Science]
Maresch, W. V. (1977). Experimental study on glaucophane: an analysis of present knowledge. Tectonophysics 43, 109–125.[CrossRef][Web of Science]
Massone, H. J. & Schreyer, W. (1987). Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz. Contributions to Mineralogy and Petrology 96, 212–224.[CrossRef][Web of Science]
Matsumoto, N. & Hirajima, T. (2000). Garnet in pelitic schists from a quartz-eclogite unit of the southern Dora-Maira massif, Western Alps. Schweizerische Mineralogische und Petrographische Mitteilungen 80, 53–62.
Michel, R. (1957). Les facies à glaucophane dans le massif d'Ambin (Alpes franco-italiennes). Comptes Rendus de la Société Géologique de France 6(VII), 130–131.
Monié, P. (1990). Preservation of Hercynian 40Ar/39Ar ages through high-pressure low-temperature alpine metamorphism in the western Alps. European Journal of Mineralogy 2, 343–361.
Platt, J. P. (1993). Exhumation of high-pressure low-temperature metamorphic rocks. Geological Society of America Bulletin 86, 133–147.
Platt, J. P. & Lister, G. S. (1985). Structural history of high-pressure metamorphic rocks in the southern Vanoise massif, French Alps, and their relation to Alpine tectonic event. Journal of Structural Geology 7, 19–35.[CrossRef][Web of Science]
Platt, J. P., Lister, G. S., Cunningham, P., Weston, P., Peel, F., Baudin, T. & Dondey, H. (1989). Thrusting and backthrusting in the Briançonnais domain of the Western Alps. In: Coward, M. P., Dietrich, D. & Park, R. G. (eds) Alpine Tectonics. Geological Society, London, Special Publications 45, 135–152.
Pognante, U. (1991). Petrological constraints on the eclogite- and blueschist-facies metamorphism and P–T–t paths in the Western Alps. Journal of Metamorphic Geology 9, 5–17.[Web of Science]
Pognante, U., Castelli, D., Bogliotti, C. & Callegari, E. (1984). Carattere petrografici petrochemici di alcuni metagabbri ed ortogneiss tardi-paleozoici del massico d'Ambin, Zona Brianzonese interna (Alpi occidentali). Società Italiana di Mineralogica e di Petrologia 39, 275–780.
Polino, R., Delapierre, F., Borghi, A., Carraro, F., Fioraso, G. & Giardino, M. (1999). Carta geological d'Italia alla scala 1:50 000, foglia Bardonecchia—Note illustrative. Servizio Geologico d'Italia, Torino, 118 pp.
Ring, U. (1995). Horizontal contraction or horizontal extension? Heterogeneous Late Eocene and Early Oligocene general shearing during blueschist and greenschist facies metamorphism at the Pennine–Austroalpine boundary zone in the Western Alps. International Journal of Earth Sciences 84, 843–859.
Rolland, Y., Lardeaux, J. M., Guillot, S. & Nicollet, C. (2000). Extension syn-convergence, poinçonnement vertical et unités métamorphiques contrastées en bordure Ouest du Grand Paradis (Alpes Franco-Italiennes). Geodinamica Acta (special issue) 13, 133–148.[CrossRef]
Sandrone, R. & Borghi, A. (1992). Zoned garnets in the northern Dora-Maira Massif and the contribution to a reconstruction of the regional metamorphic evolution. European Journal of Mineralogy 4, 465–474.
Scambelluri, M., Hoogerduijn, E. H., Piccardo, G. B., Visser, R. L. M. & Rampone, E. (1991). Alpine olivine and titanian clinohumite bearing assemblages in the Erro Tobbio peridotite (Voltri massif, NW Italy). Journal of Metamorphic Geology 9, 79–92.[Web of Science]
Schmid, S. & Kissling, E. (2000). The arc of the western Alps in the light of geophysical data on deep crustal structure. Tectonics 19, 62–85.[CrossRef][Web of Science]
Schwartz, S., Lardeaux, J. M., Guillot, S. & Tricart, P. (2000). Diversité du métamorphisme éclogitique dans le massif ophiolitique du Monviso (Alpes occidentales, Italie). Geodinamica Acta 13, 179–186.
Spalla, M. I., Lardeaux, J. M., Dal Piaz, G. V., Gosso, G. & Messiga, B. (1996). Tectonic significance of alpine eclogites. Journal of Geodynamics 21, 257–285.[CrossRef][Web of Science]
Spear, F. S. (1993). Metamorphic Phase Equilibria and Pressure–Temperature–Time Path. Mineralogical Society of America, Monograph, 799 pp.
Spear, F. S. & Cheney, T. J. (1989). A petrogenetic grid for pelitic schists in the system SiO2–Al2O3–FeO–MgO–K2O–H2O. Contributions to Mineralogy and Petrology 101, 149–164.[CrossRef][Web of Science]
Stampfli, G. & Marchant, R. (1995). Geodynamic evolution of the Tethyan margins of the western Alps. In: Deep Structure of Switzerland. Result from NFP 20. Basel: Birkhäuser.
Steck, A. & Hunziker, J. (1994). The Tertiary structure and thermal evolution of the Central Alps—compressional and extensional structures in an orogenic belt. Tectonophysics 238, 229–254.[CrossRef][Web of Science]
Thompson, J. B., Jr (1957). The graphical analysis of mineral assemblages in pelitic schists. American Mineralogist 42, 842–858.[Web of Science]
Thompson, J. B., Jr (1972). Oxides and sulfides in regional metamorphic minerals. In: International Geological Congress, 24th, 10, pp. 27–35.
Todd, C. S. & Engi, M. (1997). Metamorphic field gradients in the Central Alps. Journal of Metamorphic Geology 15(4), 513–530.[CrossRef][Web of Science]
Tracy, R. J. (1982). Compositional zoning and inclusions in metamorphic minerals. In: Ferry, J. M. (ed.) Characterization of Metamorphism through Mineral Equilibria. Mineralogical Society of America, Reviews in Mineralogy 10, 355–397.
Tracy, R. J. (1994). Rocks and minerals; metamorphic petrology. Geotimes 39, 31–32.[Web of Science]
Tracy, R. J., Robinson, P. & Thompson, A. B. (1976). Garnet composition and zoning in the determination of temperature and pressure of metamorphism in central Massachusetts. American Mineralogist 61, 762–775.[Abstract]
Wheeler, J. & Butler, W. H. (1993). Evidence for extension in the western Alpine orogen: the contact between the oceanic Piemonte and overlying continental Sesia units. Earth and Planetary Science Letters 117, 457–474.[CrossRef][Web of Science]
Yardley, B. W. D. (1977). An empirical study of diffusion in garnet. American Mineralogist 62, 793–800.[Abstract]
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  M.A. FORSTER and G.S. LISTER Tectonic sequence diagrams and the structural evolution of schists and gneisses in multiply deformed terranes Journal of the Geological Society, September 1, 2008; 165(5): 923 - 939. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Ganne, J.-M. Bertrand, S. Fudral, D. Marquer, and O. Vidal Structural and metamorphic evolution of the Ambin massif (western Alps): toward a new alternative exhumation model for the Brianconnais domain Bulletin de la Societe Geologique de France, November 1, 2007; 178(6): 437 - 458. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||