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Journal of Petrology | Volume 43 | Number 3 | Pages 485-510 | 2002
© Oxford University Press 2002
Ductile Thrusting Recorded by the Garnet Isograd from Blueschist-Facies Metapelites of the Ile de Groix, Armorican Massif, France
1LABORATOIRE DE GÉOCHRONOLOGIE–GÉOCHIMIE, GÉOSCIENCES AZUR (UMR-CNRS 6526), UNIVERSITÉ DE NICE–SOPHIA ANTIPOLIS, PARC VALROSE, 06102 CEDEX NICE, FRANCE
2GÉOSCIENCES RENNES (UMR-CNRS 6118), UNIVERSITÉ DE RENNES 1, 35042 RENNES CEDEX, FRANCE
3LABORATOIRE DE GÉODYNAMIQUE DES CHAÎNES ALPINES (UMR-CNRS 5025), UNIVERSITÉ J. FOURIER, BP 53, 38041 GRENOBLE CEDEX 9, FRANCE
Received December 12, 2000; Revised typescript accepted September 12, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHY OF THE METAPELITESBULK-ROCK CHEMISTRYMINERAL CHEMISTRYREACTION HISTORY AND P-T...DISCUSSIONCONCLUSIONSREFERENCES |
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Mineral assemblages in the blueschist-facies metapelites from the Ile de Groix (Armorican Massif, France) permit the distinction of two main units. The Upper Unit is characterized by: (1) high modal proportions of garnet; (2) larger grain size; (3) the rarity of graphite-bearing layers; (4) a single, although composite, foliation S1. A Lower Unit is defined by: (1) low modal proportions of garnet; (2) smaller grain size; (3) an abundance of graphite-bearing layers; (4) a pervasive crenulation cleavage S2. In the Upper Unit, coexisting garnet and chloritoid are more magnesian and less manganiferous than in the Lower Unit. The differences in modal proportions and chemistry of coexisting minerals reflect different P–T conditions. The P–T history of the blueschist-facies metapelites is estimated using a simplified petrogenetic grid in the NFMASH system and thermodynamic calculations, which suggest peak P–T conditions at about P = 16–18 kbar, T = 450–500°C and P = 14–16 kbar, T = 400–450°C in the Upper and Lower Units, respectively. Peak P–T conditions were followed by a nearly isothermal decompression for both units at slightly different temperatures (of the order of 50°C). The contact between the two units, i.e. the garnet isograd, is interpreted as a greenschist-facies ductile thrust. Thrusting of the higher-grade unit, i.e. the Upper Unit, over the Lower Unit occurred after the high-pressure event, i.e. during the exhumation of both units. The observed superposition of higher-grade rocks over lower-grade rocks argues against models where the exhumation history is entirely controlled by crustal-scale vertical shortening (i.e. extension).
KEY WORDS: Armorican Massif; blueschist facies; Ile de Groix; metapelites; P–T path; garnet isograd
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHY OF THE METAPELITESBULK-ROCK CHEMISTRYMINERAL CHEMISTRYREACTION HISTORY AND P-T...DISCUSSIONCONCLUSIONSREFERENCES |
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Blueschist-facies metamorphism is usually interpreted to be the result of high-pressure/low-temperature (HP/LT) conditions consequent to oceanic subduction (e.g. Ernst, 1973
; Peacock, 1996). In most collision belts, such as the Alpine or the Himalayan belts, the early HP/LT event has been strongly overprinted by medium- to high-grade parageneses. In the rare situations where the collisional history has not been responsible for a major reworking of the HP/LT terrane, three main cases are observed. First, blueschist-facies rocks may occur in a ‘melange’ where metamorphic rocks from variable lithologies and metamorphic grades are hosted by a comparatively low-grade matrix (e.g. the Franciscan Complex of California, Cloos, 1986; Krogh et al., 1994). Second, blueschist- and eclogite-facies rocks with similar P–T histories may form coherent tectonic units, in some cases several tens of kilometres thick, in which case regional metamorphic zonations may be displayed. Notable examples include the northern part of New Caledonia (Brothers, 1974; Black, 1977) and the footwall rocks of the Oman ophiolite (Goffé et al., 1988; El-Shazly & Coleman, 1990). In both cases, the regional gradient in P–T conditions increases downwards, but metamorphic discontinuities reveal the existence of ductile to brittle faults cutting through the original crustal section (New Caledonia: Cluzel et al., 1995; Clarke et al., 1997; Oman: Michard et al., 1994; Searle et al., 1994; Miller et al., 1998). In the third case, the blueschist-facies terrane is overthrust by medium- to high-grade metamorphic rocks, which may induce an inverted metamorphic gradient, as in the Catalina schists (Platt, 1975) and the Pelona schists (Graham & England, 1976; Graham & Powell, 1984). According to thermal modelling, the inverted metamorphic sequences may result either from shear heating during emplacement of the hanging wall (Graham & England, 1976) or from underplating of metasedimentary rocks at the base of the hanging wall at falling temperature (Peacock, 1987).
The large-scale thermal structure of blueschist-facies terranes is thus of crucial importance to unravel their geodynamic setting. In particular, the preservation on a terrane scale of the prograde metamorphic zonation developed during subduction is exceptional. One potential example of such a preservation is the Ile de Groix (southern Brittany, France), a famous occurrence of late Palaeozoic blueschists (Barrois, 1883; Audren et al., 1993). A sequence of mineral zones bounded by isograds and progressive variations of the P–T conditions have been described within the island (Triboulet, 1974; Carpenter, 1976). The purpose of this paper is to investigate the significance of the garnet isograd (Carpenter, 1976), i.e. whether it represents a true reaction-isograd in an undisturbed metamorphic sequence or a discontinuity in the metamorphic sequence, thus indicating the presence of a (ductile) fault. This problem is emphasized by structural studies (Quinquis, 1980; Quinquis & Choukroune, 1981), which demonstrate that the higher-grade rocks lie structurally above lower-grade ones, allowing an interpretation in terms of an inverted metamorphic sequence (Barrientos, 1992). Our analysis is based on a detailed study of the metapelites, to determine the spatial variation of both peak P–T conditions and P–T paths.
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GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHY OF THE METAPELITESBULK-ROCK CHEMISTRYMINERAL CHEMISTRYREACTION HISTORY AND P-T...DISCUSSIONCONCLUSIONSREFERENCES |
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Structural setting
Ile de Groix, located 10 km off the southern coast of Brittany, represents a small emerged part of a much larger NW–SE elongated blueschist unit (Fig. 1). Offshore geological and geophysical studies have shown that the blueschist unit is about 40 km long and 10 km wide (Delanoë et al., 1972
; Audren & Lefort, 1977). Blueschist-facies rocks also crop out further to the SE, in the Bois de Cené area (Vendée) (Guiraud et al., 1987; Triboulet, 1991). Geophysical and geological data suggest that the Ile de Groix blueschists represent a klippe of 
1 km thickness, which was thrust over low-grade volcanic rocks and schists (Lefort & Vigneresse, 1992). The volcanic rocks and schists overlie migmatitic gneisses, well exposed in the Golfe du Morbihan area (e.g. at Port-Navalo) (Jones & Brown, 1990; Brown & Dallmeyer, 1996). The contact between the migmatitic gneisses and the overlying low-grade schists is interpreted to be a major extensional fault (Gapais et al., 1993; Brown & Dallmeyer, 1996).
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Nature of the protoliths
The rocks of the Ile de Groix are made up of around 80% pelitic and 20% mafic rocks intercalated at all scales. Metabasites are fine- to medium-grained, schistose rocks that are adequately referred to as greenschists or blueschists. Mafic rocks occur either as centimetre- to metre-scale lenses with, sometimes, isoclinal folds or evidence of boudinage, or as layers up to 10 m thick. Geochemical data (Bernard-Griffiths et al., 1986) allow the characterization of two groups of mafic rocks (tholeiitic and alkaline basalt types) similar to those observed in present-day ocean islands. The metapelites have been interpreted by Bernard-Griffiths et al. (1986) as both continental and oceanic-derived sediments. As a result, the Ile de Groix blueschists may be considered as a melange between oceanic volcanic material and continentally derived sedimentary material (Bernard-Griffiths et al., 1986).
Deformation history
Two main domains have been distinguished on Ile de Groix (Cogné et al., 1966; Boudier & Nicolas, 1976; Carpenter, 1976; Quinquis, 1980) (Fig. 2). The eastern domain is characterized by flat-lying or gently dipping foliations and by a NNW–SSE-trending stretching lineation (Fig. 2). The western domain presents a major antiformal fold, whose axis is oriented NW–SE. In this area, the stretching lineation is parallel to the fold axis (Fig. 2).
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Structural data were initially interpreted in terms of superposed generations of folds, and foliations have been recognized by Cogné et al. (1966) and Boudier & Nicolas (1976). Progressive deformation is responsible for both development and subsequent folding of a tectono-metamorphic layering (D1 of Quinquis, 1980; Cobbold & Quinquis, 1980; Quinquis & Choukroune, 1981). Folds are isoclinal, in places sheath-like and trend in a north–south direction. This deformation started during the HP/LT event and continued during the greenschist-facies retrogression. The D1 deformation was strongly non-coaxial; the shear direction, given by the stretching lineation parallel to most fold axes, trends from north–south to NW–SE (Fig. 2). Pre-existing structures are reoriented by a post-metamorphic event, which is responsible for upright folds trending N120 associated with a late crenulation cleavage (D2 of Quinquis, 1980; Quinquis & Choukroune, 1981).
The interpretation of the deformation history of the Ile de Groix blueschists is still questionable. Because of the considerable amount of vertical shortening and the common occurrence of extensional shear bands, the flat-lying foliation could result either from bulk horizontal shortening during obduction (Quinquis & Choukroune, 1981) or from an extensional deformation during exhumation (Shelley & Bossière, 1999).
Metamorphic zonation of Ile de Groix
In his pioneering study, Triboulet (1974) was able to define three zones (Fig. 3a) with different P–T conditions during the HP event, i.e. 8·5 kbar at 530°C in zone I (garnet–clinopyroxene–glaucophane assemblages), 8 kbar and 500°C in zone II (garnet-bearing and clinopyroxene-absent assemblages) and 7·5 kbar at 470°C in zone 3 (garnet- and clinopyroxene-absent assemblages). All zones are retrogressed in the greenschist facies during a second metamorphic event (6·5 kbar and 470°C). Djro et al. (1989) have confirmed the zonation proposed by Triboulet (1974), with the following key mineral assemblages in the metapelites: garnet–chloritoid and garnet–biotite in zone I, garnet–chloritoid–chlorite in zone II and chloritoid–stilpnomelane ± chlorite in zone III. The boundaries between the three zones crosscut both D1 and D2 structures (Audren et al., 1993; Schulz et al., 2001).
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The zonation proposed by Carpenter (1976) and modified by Quinquis (1980) (Fig. 3b) is based on the occurrence of garnet in the metabasites, with a garnet-bearing zone (P = 8–9 kbar, T = 400°C) in the eastern part of the island, and a garnet-free zone (P = 6·5–8 kbar, T < 400°C) in the western part of the island. The boundary between the two zones, i.e. the ‘garnet isograd’, is parallel to S1, and has been folded during D2 (Quinquis, 1980; Quinquis & Choukroune, 1981). Structural data show that the garnet-bearing zone is located above the garnet-free zone (Quinquis, 1980; Figs 2 and 3b). Thus, the garnet isograd defined in the mafic rocks separates two units, which will be hereafter called Upper and Lower Units.
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PETROGRAPHY OF THE METAPELITES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHY OF THE METAPELITESBULK-ROCK CHEMISTRYMINERAL CHEMISTRYREACTION HISTORY AND P-T...DISCUSSIONCONCLUSIONSREFERENCES |
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Micaschists from the Upper and Lower Units show three main differences. First, their grain size is coarser in the Upper Unit. Conspicuous crystals of garnet (up to 1 cm) and/or chloritoid (up to 2 cm) are found between Port Mélin and Locmaria. Glaucophane needles up to 5 cm are occasionally seen south of Locmaria. By contrast, garnet and chloritoid grains in the Lower Unit are <1 mm long. Second, micaschists from the Lower Unit have a darker, greyish lustre as a result of the abundance of graphite, compared with the shiny appearance of the micaschists of the Upper Unit. Graphite is confined to a few layers (e.g. Port Mélite) in the Upper Unit. Third, garnet is much more abundant in the Upper Unit, where all micaschists contain garnet. In the Lower Unit, garnet, when present, occurs as small grains (from
500 µm to a few millimetres), preferentially located in certain layers (possibly sedimentary layers with appropriate bulk-rock composition). Early ductile deformation is well developed in all samples. The principal foliation S1 is parallel to the layering defined by alternating mica-rich and quartz-rich layers. The foliation is also defined by chloritoid, blue amphibole and epidote in the Upper Unit, and chlorite in the Lower Unit. In the Upper Unit, S1 is usually composite, some microlithons showing relicts of flattened, isoclinal, microfolds, whose axial plane is parallel to S1. Shear bands with high-pressure minerals (chloritoid and glaucophane) are common along the eastern coast between the Pointe des Chats area and Port Tudy. Abundant quartz rods usually define the stretching lineation. In the Lower Unit, S1 is deformed by open to tight microfolds associated with a crenulation cleavage S2.
Metapelites were sampled along the coastal outcrops of the island (Fig. 4 and Table 1). In the Upper Unit, two main types of metapelites can be distinguished according to their primary assemblage; namely, the chloritoid-bearing micaschists and the chloritoid-absent micaschists.
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Chloritoid-bearing assemblages
Primary assemblages
In the Upper Unit, dominant assemblages are garnet–chloritoid–rutile ± blue amphibole. The foliation wraps around garnet crystals, which contain curved trails of microinclusions. Chloritoid occurs occasionally as large twinned porphyroblasts. Blue amphibole may be present as large prisms (sample 2) or as small grains preserved in the quartz (sample 3) and may be replaced by chlorite or biotite. White micas mainly consist of abundant phengite as well as some paragonite, the latter being smaller and intergrown with phengite. Epidote occurs in minor amounts either as a matrix phase (sample 3), or as inclusions within garnet (sample 2). Garnet inclusions are abundant and consist of rutile, chloritoid (samples 5* and 8b) and rare titanium-rich magnetite (sample 8b). In sample 2, chloritoid, together with epidote, rutile and titanite, occurs as inclusions in garnet; chloritoid is not present in the matrix, which consists of white micas, blue amphibole and quartz. Rutile is ubiquitous in the matrix as large brownish to yellowish grains.
In the Lower Unit, garnet grains are less common than in the Upper Unit and, in places, are largely pseudomorphed by chlorite or biotite (sample 15). Small prisms of chloritoid are parallel to S1, which is defined by phengite and some chlorite. Paragonite is rare (samples 15 and MEN 3). Rutile occurs as very small prisms or needles. Blue amphibole has not been observed in this part of the island. Garnet inclusions consist of very small prisms of rutile, graphite (samples 40, MEN 3 and PSN 1), chloritoid, titanite (PSN 7) and ilmenite (samples MEN 8 and PSN 7). Titanite is present in the matrix from sample PSN 7. Chlorite is ubiquitous. Although it is not easy to distinguish primary and secondary chlorite, grains aligned parallel to the foliation or in contact with idioblastic garnet suggest that chlorite may have coexisted in equilibrium with chloritoid and garnet during the high-pressure event.
Retrograde assemblages
Retrogression is characterized by the growth of albite, chlorite, paragonite, biotite and ilmenite. In the Upper Unit, retrogression is poorly developed except in the Saisies area and east of Port Tudy. Chlorite and biotite partially pseudomorph garnet, chloritoid and blue amphibole. Chlorite is always more abundant than biotite. Albite is rare in the chloritoid-bearing assemblages, where it crystallizes in the mica-rich layers usually associated with chlorite (sample 3). Chloritoid porphyroblasts are usually rimmed by fine-grained paragonite. Pseudomorphs consisting of chlorite and quartz are observed in some samples (6 and PSN 7) but their origin is debatable (blue amphibole, chloritoid or carpholite). Rutile is rimmed and in some cases totally replaced by ilmenite.
In the Lower Unit, the retrogression is more intense. Biotite is found in a few samples, where its modal proportion is very low (15, MEN 3 and PSN 7). Chlorite and ilmenite are ubiquitous.
Chloritoid-absent assemblages
In the chloritoid-free micaschists from the Upper Unit, garnet occurs as pinkish idioblastic grains. Garnet inclusions consist of titanite in the garnet core and epidote and rutile in the garnet rim. Blue amphibole is more frequent than in the chloritoid-bearing rocks, either as pale blue idioblastic needles (HEN 1 and 14) or as lozenge-shaped chlorite–quartz pseudomorphs (12 and 13). Epidote and phengite define the foliation together with some paragonite (12 and 14). Rutile is abundant in the matrix. Greenschist-facies assemblages are the same as in the chloritoid-bearing micaschists, except for albite, which is very abundant in some samples (12, 13 and 14).
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BULK-ROCK CHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHY OF THE METAPELITESBULK-ROCK CHEMISTRYMINERAL CHEMISTRYREACTION HISTORY AND P-T...DISCUSSIONCONCLUSIONSREFERENCES |
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Ten micaschists were analysed by inductively coupled plasma atomic emission spectrometry (ICP-AES) for major elements at the CRPG in Nancy (France) (Table 2 and Fig. 5). Major elements indicate that the Ile de Groix micaschists have relatively homogeneous compositions, close to mean shale composition [e.g. PAAS and NASC of Taylor & McLennan (1985)
]. Two samples (13 and 40) show higher SiO2 (>72%) and lower Al2O3 (<14%) contents compared with the others (SiO2 56–67% and Al2O3 16–22%). As a whole, the chloritoid-absent samples (12, 13 and 14) are more sodic and less aluminous than the chloritoid-bearing samples. The garnet–chloritoid–glaucophane micaschist (sample 2) is highly aluminous and slightly more sodic than the other chloritoid-bearing samples. Amongst the chloritoid-bearing samples, no chemical distinctions can be observed between the two units. Finally, all samples are poor in MnO (<0·3%).
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MINERAL CHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHY OF THE METAPELITESBULK-ROCK CHEMISTRYMINERAL CHEMISTRYREACTION HISTORY AND P-T...DISCUSSIONCONCLUSIONSREFERENCES |
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The chemical compositions of the phases in selected samples have been analysed by electron microprobe (Camebax SX 50, Microsonde Ouest, Brest, France) using the PAP correction supplied by the manufacturer. Analytical conditions were 15 kV accelerating voltage, 20 nA sample current and 20 s counting time. Standards were albite (Na), orthoclase (K), corundum (Al), wollastonite (Ca, Si), forsterite (Mg), MnTiO3 (Mn, Ti), Fe2O3 (Fe) and Cr2O3 (Cr).
Garnet
Garnet is an almandine–spessartine–pyrope–grossular solid solution (Tables 1 and 3) with a low pyrope content (maximum 10–11 mol % in the Pointe des Chats area). The grossular content [XGrs = Ca/(Fe* + Mn + Mg + Ca)] is higher in the chloritoid-absent micaschists (core: 18–29 mol %; rim: 17–32 mol %) than in the chloritoid-bearing micaschists (core: 11–20 mol %; rim: 8–20 mol %). The spessartine content of garnet rims is higher in samples from the Lower Unit (8–32 mol %) compared with the Upper Unit (1–6 mol %). In all the analysed micaschists, garnet grains display a pronounced chemical zonation from core to rim (Figs 6 and 7).
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All samples from the Upper Unit show common features (Fig. 6): (1) an increasing almandine and pyrope content from core to rim; (2) a strong decrease in spessartine content from core (maximum values of 35 mol % in sample 2) to rim (between 1 and 6 mol %); (3) an increasing XMg value [where XMg = Mg/(Fe2+ + Mg)]. The grossular content is much more variable (Tables 1 and 3), reflecting slight differences in bulk-rock chemistry (Table 2). In samples where epidote is absent from the matrix, the grossular content either decreases from core to rim (samples 2 and HEN 1) or is nearly constant (samples 5* and 8b). In both cases, the grossular content of garnet rims is similar ( 15 mol %). Because the former samples contain epidote as inclusions in garnet cores, whereas the latter samples do not, the patterns record consumption of epidote and/or titanite during garnet growth. In samples where epidote is observed in the matrix (3, 12, 13 and 14), the grossular content slightly decreases from core to rim (Table 1 and 3 ) but is higher than in the other samples.
In the Lower Unit, chemical zonations are smoother than in the Upper Unit (Fig. 7). In garnet grains from samples 15 and MEN 3, the almandine content increases from core to rim. The pyrope content is nearly constant in sample MEN 3 and shows a slight decrease from core to rim in sample 15. The grossular content shows an almost flat pattern in both samples. Finally, the spessartine content decreases from core to rim. Consequently, the zoning pattern of these two samples is similar to the chloritoid-bearing micaschists from the Upper Unit, except that the rim compositions show a higher spessartine content (around 8 and 18 mol % in samples 15 and MEN 3, respectively). In samples MEN 8 and PSN 7, the zoning pattern is more complex (Fig. 7), mainly characterized by a late decrease in pyrope content and increase in spessartine content, associated with a change in buffering assemblage. In sample MEN 8, garnet cores contain rutile inclusions, whereas ilmenite is present in garnet rims. In sample PSN 7, chloritoid inclusions are abundant in garnet cores, but chloritoid is absent from the matrix. The grossular content in sample PSN 7 increases slightly towards the rim, and is much higher than in sample MEN 8, which is consistent with the abundance of epidote and titanite in sample PSN 7. In both samples, the spessartine-rich rims display two parts, namely, a sharp increase then a slight decrease in spessartine content.
Chloritoid, glaucophane and chlorite
In the Upper Unit, matrix chloritoid has a XMg value that varies between 0·19 and 0·22 with no significant variations from core to rim (Tables 1 and 4). Chloritoid inclusions in garnet show a lower XMg and higher manganese content compared with matrix chloritoid. In the Lower Unit, the XMg value of matrix chloritoid varies between 0·10 and 0·14 (Table 1). Blue amphiboles plot in the glaucophane and ferro-glaucophane fields of the IMA classification (Leake et al., 1997). Their XMg value shows much larger variations between samples (from 0·38 to 0·65) than inside grains, where a slight decrease of the XMg value from core to rim is observed (Tables 1 and 5). Chlorites from the Upper Unit have XMg values ranging from 0·41 and 0·56 (Tables 1 and 6). No chemical zonation has been observed, except in sample 12, where chlorite grains show a range of XMg values between 0·29 and 0·37 (Table 1). In the Lower Unit, chlorite is Mg poor (XMg values between 0·29 and 0·45) (Table 1). In both units, no significant variations of the Si content of chlorite are observed.
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White micas
The maximum celadonite content of phengite lies around Si 6·8–6·9 a.p.f.u. with XMg values ranging from 0·55 to 0·65 (Table 7). No significant variations of the maximum Si content are observed between chloritoid-bearing and chloritoid-absent assemblages (Tables 1 and 7). A core-to-rim decrease of the Si content is observed in most samples, but its amplitude varies from sample to sample. Phengite contains a small amount of paragonite [Na/(Na + K) = 0·03–0·10], whereas paragonite contains small amounts of phengite [K/(Na + K) = 0·04–0·07].
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Other minerals
Epidote grains do not show significant compositional zoning. The Fe3+/(Al + Fe3+) ratio is lower in sample PSN 7 (0·18–0·19) compared with the other samples (0·26–0·29) (Table 8). Albite is close to end-member composition, with a maximum anorthite content of 0·01 mol %. Biotite and stilpnomelane are potentially present as retrograde phases, being described as rare minerals in the Ile de Groix (Triboulet, 1971; Makanjuola & Howie, 1972; Barrois, 1883) and Bois de Cené blueschists (Guiraud et al., 1987). Nevertheless, the poor quality of the microprobe analyses does not allow the distinction between oxychlorite, chloritized biotite and intergrown stilpnomelane and chlorite. Titanite is nearly pure (Table 9).
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The AFM projection
Because the studied rocks invariably contain quartz and phengite (and paragonite in the Upper Unit), rim compositions from the coexisting phases are reported in an AFM projection (Thompson, 1957), assuming excess water. In the Upper Unit (Fig. 8a), the garnet–chloritoid samples show similar XMg values irrespective of the location of the sample in the Upper Unit. When present, chloritoid inclusions within garnet are more iron rich than matrix chloritoid (samples 2 and 5*). Garnet–glaucophane assemblages are distinctly more iron rich than the garnet–chloritoid assemblages and exhibit a larger spread of XMg values. In the rare assemblage garnet–chloritoid–glaucophane (sample 3), garnet and chloritoid have similar XMg values to garnet–chloritoid assemblages but glaucophane has the highest XMg value (0·65). Late chlorite is also reported in the AFM diagram.
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In the Lower Unit (Fig. 8b), garnet and chloritoid XMg values are very similar whatever the sample and its location. Chlorite shows a much larger spread in composition (XMg = 0·29–0·45) compared with garnet and chloritoid. If chlorite was in equilibrium with garnet and chloritoid, this should not be observed. The spread in composition could thus indicate either (1) re-equilibration of chlorite at a later stage or (2) the existence of several generations of chlorite within some samples.
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REACTION HISTORY AND P–T PATH |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHY OF THE METAPELITESBULK-ROCK CHEMISTRYMINERAL CHEMISTRYREACTION HISTORY AND P-T...DISCUSSIONCONCLUSIONSREFERENCES |
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An appropriate system for investigating the reaction history of the studied samples is the Na2O–FeO–MgO–Al2O3–SiO2–H2O (NFMASH). K2O is not considered because there is only one potassic phase (i.e. phengite) stable during most of the history. Biotite and/or stilpnomelane developed late during the retrograde history. Phases of interest in the NFMASH system are garnet, chloritoid, chlorite, glaucophane, paragonite, albite, quartz and vapour. The graphical representation of phase relations (Fig. 9) assumes that quartz and vapour are in excess. In addition, the distribution of saturating sodic phases (i.e. paragonite, albite and/or jadeite) is diagrammatically shown on the AFM projection, using a method similar to that of Thompson (1972)
. In divariant assemblages where glaucophane coexists with two other Fe–Mg phases, only one saturating Na-bearing phase can be present. In trivariant assemblages where glaucophane coexists with one other Fe–Mg phase, two saturating Na-bearing phases may be simultaneously present, and the glaucophane composition in the projected phase diagram occupies two points (Fig. 9).
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Phase relations in the NFMASH system are still poorly known, either because of unreliable thermodynamic data for some key end-member phases (especially Fe-glaucophane) or because of the effect of minor components, especially ferric iron, resulting in major differences between calculated grids. This leads us to simplify the reaction topology. In the portion of the P–T space considered, three univariant reactions are shown, namely, the degenerate NASH equilibrium
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), and two NFMASH reactions:
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and El-Shazly & Liou (1991), who considered it to be located in the stability field of albite. Considerable uncertainty exists on the stability and location of the invariant point II. According to Guiraud et al. (1990), it is not stable in the NFASH system but is stabilized if Fe3+ enters glaucophane and chloritoid. The higher the amount of ferric iron, the lower the pressure and temperature of invariant point II. Calculations by El-Shazly & Liou (1991) show that the invariant point II is located in the albite stability field. Natural assemblages studied by Okay & Kelley (1994) show that the glaucophane–chloritoid assemblage is stable in the jadeite stability field in Fe-rich metapelites (XFe 
0·85 for chloritoid), supporting the location of invariant point II within the jadeite + quartz stability field (Fig. 9).
Reaction history during garnet growth
In the chloritoid-bearing micaschists from the Upper Unit, garnet growth could involve two divariant NFMASH reactions, namely,
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and our sample 3] indicates that peak P–T conditions were located above the univariant NFMASH reaction (R2). Jadeite has not been observed in the blueschist-facies rocks from the Ile de Groix, suggesting that the divariant reaction
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To sum up, garnet growth in the Upper Unit took place at rising temperature (increasing XMg of garnet from core to rim and increasing XMg of chloritoid inclusions compared with matrix chloritoid, steep slope of the equilibrium Cld + Chl = Grt) and pressure (transition from titanite to rutile in epidote-bearing samples, flat slope of the equilibrium Czo + Ttn = Grs + Rt). Garnet rims thus record peak P–T conditions, in the stability field of the garnet–chloritoid–glaucophane assemblage (Fig. 10).
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In the Lower Unit, reaction (R4) can explain garnet growth. With increasing MnO content, the equilibrium curve is displaced towards lower temperatures (Fig. 10). The low modal abundance of garnet, for bulk-rock composition with low amounts of MnO (Table 2), and its high spessartine and very low pyrope contents suggest that temperatures in the Lower Unit did not exceed those of reaction (R4). Garnet zonations in samples 15 and MEN 3 are consistent with rising temperature. The interpretation of the prograde evolution in samples MEN 8 and PSN 7 is less straightforward. To summarize, metapelites from the Lower Unit record lower temperatures than in the Upper Unit (Fig. 10) because (1) coexisting garnet and chloritoid have a lower XMg, and (2) the MnO content of garnet rims is higher.
Reaction history during retrogression
In the chloritoid-bearing micaschists from the Upper Unit, the divariant reaction (R4) was responsible for chlorite growth. Albite is very rare in the chloritoid-bearing micaschists where paragonite is the only stable sodic phase at low pressure (Fig. 9). In glaucophane-bearing micaschists, chlorite growth occurs either because of the univariant NFMASH reaction (R2) or as a result of the divariant reaction
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Albite growth could result from several reactions [(R1), (R3) and associated divariant reactions] (Fig. 9). Late biotite growth results from phengite reacting with garnet, glaucophane and/or chloritoid, suggesting continuous reactions such as
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The importance of the above reactions in controlling late biotite growth in high-pressure metapelites has been recognized earlier (e.g. Konopasek, 1998). Because reaction (R9) is H2O-conserving, it has a flat slope, indicating that biotite growth at the expense of garnet records decreasing pressure. In the Lower Unit, decompression is also recorded by chlorite growth at the expense of garnet and chloritoid, suggesting back-reaction through equilibrium (R4) (Fig. 10). To sum up, retrograde reactions in both units essentially record decompression at nearly constant temperature.
Calculated P–T path
Metamorphic P–T conditions were estimated with a multi-equilibrium approach using the program TWEEQU (Version 1.02; Berman, 1991) with the Jun92.rgb updated database and the thermodynamic data and corresponding solid solution models from Berman (1990) for garnet, from Evans (1990) for glaucophane and clinozoisite, and from Vidal et al. (1994), Vidal & Parra (2000) and Vidal et al. (2001) for Mg- and Fe-Cld, daphnite (Daph), Mg-amesite (Mg-Am), and Mg–Al-celadonite (cel) (Fig. 11). Ideal solid solution models were adopted for titanite and rutile. The pressure and temperature conditions were determined simultaneously by the intersection of three or more independent reactions. The use of several chlorite and muscovite end-members, according to the Tschermak, the FeMg-1, the di/trioctrahedral and the pyrophyllitic substitutions, makes P–T estimates possible for high-variance parageneses (Vidal & Parra, 2000; Vidal et al., 2001). In particular, pressure estimates can be made in garnet and/or chloritoid-bearing metapelites where phengite and/or chlorite are present. The advantage of the multi-equilibrium technique (Berman, 1991) is that it provides information on the equilibration state of a specific mineralogical assemblage. A good convergence of the intersection points suggests equilibrium, whereas a large scatter suggests that one or more phases is not equilibrated with the other phases of the considered paragenesis.
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An initial set of 60 mineralogical assemblages corresponding to the following parageneses have been identified in 13 thin sections, based on petrological observations (classic ‘equilibrium’ textures, e.g. Barker, 1990; Vidal and Parra, 2000): Grt–Ph–Chl–Cld–Qtz, Grt–Ph–Chl–Qtz, Cld–Ph–Chl–Qtz, Grt–Cld–Gln–Ph–Qtz, Zo–Ttn–Rt–Grt–Ph–Qtz and Chl–Ab–Ph–Pg–Qtz. One-third of the assemblages were discarded on the basis of mineral composition criteria (Vidal & Parra, 2000). The equilibria were calculated using the Alm and Prp end-members for the Zo–Ttn–Rt–Grt–Ph–Qtz assemblages, and the Fe– and Mg–Cld, Clin, Daph, Mg–Am, Mus and Mg–Al–Cel end-members. The paragonite end-member was added to the phengite solid solution model to calculate the equilibria involving glaucophane or albite and paragonite. Water with unit activity was also included in all the calculations. The results of average P–T estimates corresponding to the INTERSX output (after one iteration of the exclusion analysis) are reported in Fig. 11. The error bars correspond to the 1
standard deviations. The temperature standard deviations lie within the ±0–40°C range suggested by Berman (1991) as a reasonable criterion for satisfaction of the equilibrium assumption. On the other hand, the pressure standard deviations are generally >0·5 kbar, which has been proposed as the upper permissible value for water-absent parageneses in the 4–10 kbar range (e.g. Berman, 1991). However, it is emphasized that many of the equilibria calculated for the blueschist-facies parageneses listed above are dehydration or Fe–Mg exchange equilibria with steep slopes. It is therefore not surprising that the P–T calculations lead to larger P values than those generally accepted. Moreover, the same parageneses involving different mineral composition sets can lead to similar P–T conditions but very different P and T (e.g. open circles in Fig. 11). It is thus believed that the standard deviations obtained in the present study are compatible with the assumption of equilibrium.
As a whole, calculated results (Fig. 11) are in a good agreement with the estimated conditions based on the NFMASH grid (Fig. 10), and also suggest a P–T path characterized by a near-isothermal decompression followed by cooling at decreasing pressure. Maximum P–T estimates in the Upper Unit are at around 16–18 kbar and 450–500°C whereas in the Lower Unit maximum values are 14–16 kbar and 400–450°C. Temperature estimates are slightly lower than those proposed in Fig. 10 and strongly decrease in both units at 7 kbar, from 480°C to 320°C at 3 kbar.
The temperature difference between the two units is not easily established using thermodynamic modelling and is within error, i.e. between 50 and 100°C. Maximum pressure estimates in the Upper Unit are obtained using the Grt–Cld–Zo–Ttn–Rt assemblages, thought to represent a prograde stage during garnet growth. Unexpectedly, Grt–Cld–Gln assemblages yield lower pressure estimates compared with Grt–Cld–Zo–Ttn–Rt assemblages. This discrepancy may indicate the following: (1) lack of equilibrium between the selected phases used for the calculation; (2) available thermodynamic data are poorly constrained for the mineral assemblages, especially for glaucophane; (iii) the mineral compositions used for the calculation do not record the stable composition at peak pressure because the minerals have been later re-equilibrated by diffusion. P–T conditions become similar in both units at 10 kbar. This can be the result of either similar but diachronous P–T paths, or similar and synchronous P–T paths.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHY OF THE METAPELITESBULK-ROCK CHEMISTRYMINERAL CHEMISTRYREACTION HISTORY AND P-T...DISCUSSIONCONCLUSIONSREFERENCES |
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P–T conditions during the blueschist facies
In the study of Triboulet (1974)
, pressure and temperature estimates were based on the jadeite content of clinopyroxene (30%), and the FeMg-1 partitioning between garnet and clinopyroxene in zone I (P = 8·5 kbar, T = 530°C). The Cld + Gln = Pg + Chl equilibrium from Kiénast & Triboulet (1972) limits the stability field of the early assemblages towards lower pressures. P–T conditions in zones II and III were extrapolated from those obtained in zone I, because the small size of the island suggests minor departures from the conditions calculated in zone I. In his garnet-bearing zone II, Carpenter (1976) estimated P–T conditions at 8–9 kbar, 400°C based on the jadeite content of the clinopyroxene, the FeMg-1 partitioning between garnet and clinopyroxene and the lawsonite stability field. In zone I where garnet is absent, estimations are 6·5–8 kbar, <400°C.
The above estimations were made with geothermobarometers that are now superseded but, more importantly, rely on a wrong assumption: that albite coexisted with omphacite in the metabasites. In the absence of albite, only minimum pressures are obtained. Consequently, Dudek & Kiénast (1989) calculated minimum pressures of 10·5 kbar at 480 ± 50°C for an eclogite from the northern coast of the Ile de Groix. Our results are in good agreement with the P–T estimates proposed by Barrientos (1992) for the blueschist-facies event in the metabasites; namely, at 16–18 kbar and 450–550°C (Pointe des Chats, i.e. Upper Unit) and at 10–14 kbar and 450–500°C (Vallon du Lavoir, i.e. Lower Unit).
P–T path during decompression
The blueschist-facies rocks were to a variable extent overprinted during a second event, ascribed to the greenschist facies (Makanjuola & Howie, 1972; Triboulet, 1974; Carpenter, 1976). On the basis of the study of mineral assemblages in the micaschists, Djro et al. (1989) proposed a late temperature rise to 650°C at P 8 kbar. They calculated P–T paths combining pressure and temperature estimates based on the Si content in the phengite (Velde, 1967b) and the FeMg-1 partitioning between garnet and phengite (Krogh & Raheim, 1978). The major drawback of their approach is that they need to assume coexisting phengite, K-feldspar and biotite in the Ile de Groix micaschists, which is not the case. Although this was recognized by Djro et al. (1989), their P–T path is drawn assuming that the Si content of the phengite does indicate the exact (and not minimum) pressure.
Rather than obtaining a late temperature rise during a single, clockwise P–T loop, Schulz et al. (2001) suggested that two clockwise cycles (M1 and M2) are recorded in the metabasites, based on the chemical zonation of adjacent Na- and Na–Ca-amphiboles (Triboulet, 1992). This model does not agree with our observations in the micaschists for a number of reasons. First, the P–T conditions estimated for the peak M2 event in the western part of the island are at P 9 kbar, T 700°C. At such pressures and temperatures, most experimental data and calculated grids for the KFMASH system (Fig. 10) would predict that: (1) chloritoid and chlorite are no longer stable in metapelitic rocks; (2) staurolite and biotite should be present, the stable assemblage being garnet–biotite ± staurolite or kyanite/sillimanite. Second, the two cycles should have been recorded not only by a change in mineral assemblages but also by a significant break in the garnet zonation (e.g. Karabinos, 1984). The observed zoning patterns in the Upper Unit (Fig. 6) are not compatible with the superposition of two clockwise cycles. Zoning patterns in the Lower Unit (Fig. 7) also record a single stage of garnet growth in samples 15 and MEN 3. The complex patterns shown by two samples (MEN 8 and PSN 7) from the Lower Unit cannot be related to a superposition of two cycles, because the Mn-rich overgrowths are associated with a decrease in XMg. This could not result from a modification of rim compositions by diffusion during garnet resorption (e.g. de Béthune et al., 1975), in which case we would expect a continuous increase of spessartine with maximum values close to the rim. Third, no evidence for diffusion during peak M2 conditions has been observed (Figs 6 and 7), which leaves open only two possibilities, either a very short M2 event at high temperature (700°C) or no significant rise in temperature during M2.
For all these reasons, the superposition of two clockwise P–T paths is considered to be unlikely. The retrogression mainly records decreasing pressures at constant or slightly falling temperatures, as proposed by Barrientos (1992).
The ‘garnet isograd’: a ductile fault
An isograd is defined as the surface marking the first appearance (or disappearance) of a specific mineral (or mineral assemblage) (Tilley, 1925). To identify an isograd in a metamorphic terrane, one has to assume (1) that there is no change (or mappable changes) in the bulk-rock chemistry, and (2) that the metamorphic sequence has not been later disrupted by ductile or brittle shear zones. In the Ile de Groix, stratigraphic markers have not been identified at the scale of the island, but bulk-rock chemical analyses show that the two main types of metapelites are equally widespread throughout the island. Changes in mineral assemblages could not result from changes of the bulk-rock composition, but indicate P–T variations at the scale of the island.
An additional difficulty with the exact definition of an isograd results from the fact that most garnet-producing reactions are continuous, allowing garnet appearance in a large volume (ideally planar) rather than along a surface. In the Ile de Groix, garnet is present throughout the island but microprobe analyses, e.g. the MnO (Fig. 4) and XMg (Fig. 8) of garnet rims, are consistent with higher temperature in the eastern part of the island than in the western part. A garnet isograd could thus be defined in the Ile de Groix, based on the distribution of the Mn-poor garnet in metapelites, consistent with the observations based on the metabasites (Carpenter, 1976). Nevertheless, two sets of data can be used to show that the garnet isograd represents a ductile fault zone rather than an arrested prograde reaction.
No progressive variation of rim composition of garnet (or matrix composition for the other phases) has been observed with respect to the location of the samples within each unit (Fig. 4). It follows that a transitional zone is either absent, or reduced to a zone thinner than the distance between the nearest samples with the same bulk-rock chemistry, hence mineral paragenesis. Comparing garnet–chloritoid assemblages, the garnet isograd along the southern coast should be located somewhere between samples 8b and 15. A closer location is not possible because the micaschists on the western side of the Locmaria Bay are of a different chemistry, with garnet–glaucophane parageneses.
The garnet isograd is located in an area where there is a rapid change in structural style (S1 dominant east of the isograd and development of a pervasive crenulation S2 to the west) and metamorphic grade (no progressive change in the chemistry of coexisting garnet and chloritoid on both sides of the isograd). There is no obvious brittle fault that could juxtapose the two domains, but a diffuse zone of high strain developed at greenschist-facies conditions. Indeed, a narrow (400 m in thickness) ductile shear zone (the Saisies Shear Zone) is observed between Locmaria and the Porh Roëd beach (Fig. 2). In this area, the greenschist-facies retrogression is largely developed in the micaschists with abundant albite and chlorite. Microscopic studies reveal that albite and chlorite growth is synkinematic, the high-pressure minerals being either highly strained (phengite) or preserved in quartz layers (glaucophane). Within the shear zone, the foliation dips moderately to the east, and the stretching lineation slightly plunges to the SE. Unfortunately, shear criteria are inconclusive. The present-day attitude of the structures is the result of the late, D2, folding. This suggests that the garnet isograd coincides with a ductile fault.
We thus interpret the greenschist-facies shear zone (i.e. the Saisies Shear Zone) as a ductile contact separating two units that have distinct P–T evolutions (Fig. 10). In this interpretation, the ‘garnet isograd’ is a tectonic contact. Because the Upper Unit, i.e. the higher-grade unit, is located structurally above the Lower Unit (Quinquis & Choukroune, 1981), the shear zone is interpreted as a ductile thrust. Coincidence of mineral isograds with structural boundaries, especially ductile shear zones, can be difficult to decipher, but has already been reported in classic examples such as New Caledonia (Cluzel et al., 1995), the Oman Mountains (Michard et al., 1994) and New Zealand (Craw, 1998). In the case of the Ile de Groix, Cannat (1985) postulated the existence of a thrust, but its location was not precise enough to permit a correlation with the metamorphic zonation of the island.
Tectonic significance of the ‘garnet isograd’
The blueschist-facies rocks from the Ile de Groix cannot be considered as a ‘melange’ such as the Franciscan (e.g. Cloos, 1986; Krogh et al., 1994) because the metabasites and the metapelites share similar P–T histories. A model of subduction-related inverted metamorphism (Peacock, 1987) cannot be applied to the Ile de Groix, because the blueschists belong to the upper plate (Fig. 1) and because the stacked units share a high-pressure metamorphic event. Finally, the metamorphic zonation in the Ile de Groix is different from those observed in the Oman Mountains (e.g. Michard et al., 1994; Searle et al., 1994; Miller et al., 1998) and in New Caledonia (e.g. Cluzel et al., 1995; Clarke et al., 1997), where the metamorphic grade is decreasing upwards.
In the Ile de Groix, the inverted gradient is due to the stacking of slices that have similar protoliths and, to a first approximation, metamorphic histories (i.e. clockwise P–T paths with peak P–T conditions along a low geothermal gradient followed by near-isothermal decompression) but differ in detail by the P–T values achieved during the metamorphic cycle (higher P–T conditions in the Upper Unit). This situation presents similarities to some of the islands from the Cycladic belt, especially Sifnos, where greenschists with rare relicts of high-pressure parageneses (Avigad et al., 1992) are structurally overlain by well-preserved blueschists and eclogites (Okrusch et al., 1978; Schliestedt, 1986). The transition from blueschists to greenschists involved significant hydration (e.g. Schliestedt & Matthews, 1987), as in the Ile de Groix (Barrientos & Selverstone, 1993). As peak P–T conditions are assumed to be similar in the structurally higher blueschist unit and the structurally lower greenschist unit, there is no need for a tectonic contact between the two units, although Avigad (1993) argued that a ductile normal fault separates the two units. In the Ile de Groix, lower peak P–T conditions in the Lower Unit indicates the existence of an inverted gradient, well illustrated by the ‘garnet isograd’.
Figure 13 shows four models for the metamorphic zonation of the Ile de Groix. In model (a) (Triboulet, 1974; Djro et al. 1989; Audren et al., 1993), the HP/LT event induces a continuous metamorphism from the eastern part to the western part of the island. The isograds bounding the different zones are thought to be complexly folded during or shortly after the peak metamorphism by sheath folds, which are responsible for the present-day pattern of the metamorphic zonation (Fig. 12). Whereas in model (a) the metamorphic gradient is normal, the three other models have in common an inverted metamorphic gradient.
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In model (b), following Carpenter (1976) and Quinquis (1980), the garnet isograd coincides with the eastern flank of the late antiform (D2) observed in the western part of the island (Fig. 12). Quinquis (1980) also pointed out that the higher-grade zone, located structurally above the lower-grade zone, implies the existence of an inverted metamorphic gradient. According to Quinquis (1980), this inverted metamorphism may represent ‘fossil’ inverted gradients that form in the oceanic lithosphere during the early stages of subduction.
Model (c) (Barrientos, 1992) is based on a combination of the Triboulet (1974) and Carpenter (1976) zonation, and also relies on the structures established by Quinquis (1980). Accordingly, the higher-grade zone [Porh Morvil area, or zone I of Triboulet (1974)] overthrusts the lower-grade zones, possibly inducing an inverted gradient below the thrust. In this hypothesis, the garnet isograd would document rising temperature in the footwall towards the thrust.
Model (d) relies on our observations. In common with previous studies is the general zonation of the blueschist-facies event, with the eastern part of the island being the higher-grade zone. Following Quinquis (1980), this domain is structurally located above the lower-grade zone, hence the major subdivision of the island in two units. The thrust boundary between the Upper and the Lower Units coincides with the garnet isograd (Fig. 12). This latter point is of importance when considering the possible exhumation mechanisms of high-pressure rocks. Thrusting of the higher-grade unit, i.e. the Upper Unit, onto the Lower Unit necessarily occurred after the HP event, thus during the exhumation of both units, implying bulk horizontal shortening (i.e. compression). This model is at variance with those assuming bulk horizontal stretching (i.e. extension) during exhumation of the blueschists, including the Ile de Groix (Shelley & Bossière, 1999). The deformation history of the Ile de Groix will be re-examined in detail in a subsequent study.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHY OF THE METAPELITESBULK-ROCK CHEMISTRYMINERAL CHEMISTRYREACTION HISTORY AND P-T...DISCUSSIONCONCLUSIONSREFERENCES |
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- Mineral assemblages and mineral compositions from the Ile de Groix blueschist-facies metapelites define two units. In the higher-grade unit, key assemblages include garnet–chloritoid–glaucophane, garnet–chloritoid and garnet–glaucophane. In the lower-grade unit garnet–chloritoid–chlorite and chloritoid–chlorite assemblages are observed. Garnets from the Upper Unit are less manganiferous and more magnesian than in the Lower Unit. A ‘garnet isograd’ separates the two units.
- In both units, garnet zonation records the prograde part of a single metamorphic cycle. Retrograde phases are unequally developed, mainly consisting of chlorite, biotite and albite in the Upper Unit, and chlorite and ilmenite in the Lower Unit.
- Estimated and calculated P–T conditions are at about 16–20 kbar, 500°C and 12–16 kbar, 450°C for the blueschist-facies event in the Upper Unit and Lower Unit, respectively. Decompression is near-isothermal.
- Structural data show that the higher-grade zone is located above the lower-grade zone. The inverted gradient results from the superposition of two blueschist-facies units that have experienced different prograde P–T paths. Thrusting of the higher-grade unit, i.e. the Upper Unit, onto the Lower Unit necessarily occurred after the HP event, thus during the exhumation of both units.
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ACKNOWLEDGEMENTS |
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This study is part of Géofrance 3D–Armor 2 program (project leaders J.-P. Brun and P. Guennoc). Financial support from this programme is gratefully acknowledged, as are discussions with members of the research team, especially D. Gapais. Sampling in the Mineralogical Reserve has been allowed by its Scientific Committee, and was carried out under the supervision of C. Robert. We are grateful to X. Le Coz, who made thin sections with variably weathered samples, and to M. Bohn, whose expert advice on the microprobe helped investigation of the garnet zonations. Preliminary results of this study have been presented during a field excursion of the Metamorphic Studies Group (1999). Thanks also for helpful reviews are due to G. Clarke and J. Schumacher.
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FOOTNOTES |
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*Corresponding author. Present address: Laboratoire Magmas Volcans – CNRS, Université Blaise Pascal, 5 rue Kessler, 63038 Clermont Ferrand, France. Telephone: (33)-04-73-34-67-05. Fax: (33)-04-73-34-67-44. E-mail: V.Bosse{at}opgc.univ-bpclermont.fr
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHY OF THE METAPELITESBULK-ROCK CHEMISTRYMINERAL CHEMISTRYREACTION HISTORY AND P-T...DISCUSSIONCONCLUSIONSREFERENCES |
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