Sébastien Potel
email: s.potel@em.uni-frankfurt.de

R Ferreiro Mählmann

W Stern

J Mullis

M Frey

Mählmann R Ferreiro

Very Low-grade Metamorphic Evolution of

Keywords: low-temperature/high-pressure metapelites, illite crystallinity, coal rank, illite-muscovite b cell dimension, New Caledonia. Mineral

INTRODUCTION

Since the first study by [START_REF] Brothers | Lawsonite-Albite Schists from Northernmost New Caledonia[END_REF], New Caledonia has been well known for its Eocene high-pressure (HP) metamorphic belt located in the northern part of the island. In the central and northeast of the metamorphic belt, the Koumac and Diahot terranes [START_REF] Cluzel | Point de vue sur l'évolution tectonique et géodynamique de la Nouvelle-Calédonie (Pacifique, France)[END_REF] (Fig. 1), which are composed of Cretaceous to Eocene metasediments and metavolcanics, are characterised by diagenetic to blueschist facies assemblages. These terranes have been the subject of numerous petrological studies [START_REF] Brothers | Lawsonite-Albite Schists from Northernmost New Caledonia[END_REF][START_REF] Brothers | Tertiary plate tectonics and high-pressure metamorphism in New Caledonia[END_REF][START_REF] Black | Blueschist Ophiolites in the Melange Zone, Northern New Caledonia[END_REF][START_REF] Diessel | Coalification and Graphitization in High-Pressure Schists in New Caledonia[END_REF]Brothers & Yokohama, 1982;[START_REF] Ghent | Chloritoid-bearing rocks associated with blueschists and eclogites, northern New Caledonia[END_REF][START_REF] Black | Mg-Fe carpholites from aluminous schists in the Diahot region and implications for preservation of highpressure/low-temperature schists, northern New Caledonia[END_REF][START_REF] Cluzel | Point de vue sur l'évolution tectonique et géodynamique de la Nouvelle-Calédonie (Pacifique, France)[END_REF][START_REF] Clarke | Eclogites and blueschists of the Pam Peninsula, NE New Caledonia: a reappraisal[END_REF][START_REF] Carson | Hydration of eclogite, Pam Peninsula, New Caledonia[END_REF][START_REF] Fitzherbert | Lawsonite-omphacite-bearing metabasites of the Pam Peninsula, NE New Caledonia: Evidence for disrupted blueschist-to eclogitefacies conditions[END_REF], which focussed on the lawsonite-albite bearing schists in the SW of the Diahot terrane and omphacite-garnet schists in the Pouébo terrane. The very low-grade metamorphism has received limited attention, focussing on the determination of mineral assemblage stability fields and the mapping of mineral isograds [START_REF] Black | Mineralogy of New Caledonia Metamorphic Rocks. IV. Sheet silicates from Ouégoa District[END_REF][START_REF] Black | Regional High-Pressure metamorphism in New Caledonia: Phase equilibria in the Ouégoa District[END_REF][START_REF] Black | Blueschist Ophiolites in the Melange Zone, Northern New Caledonia[END_REF], and the evolution of organic material [START_REF] Diessel | Coalification and Graphitization in High-Pressure Schists in New Caledonia[END_REF].

In the study of very low-and low-grade metamorphic rocks, maturation of organic matter and reaction progress, and development of clay minerals, are useful tools to describe the thermal evolution during metamorphism. Vitrinite reflectance (VR) is an important maturity parameter that can be used to determine the transition from diagenesis to very low-grade metamorphism, because vitrinite reflectance increases irreversibly with temperature, and retains an indication of the maximum paleotemperature [START_REF] Teichmüller | Organic material and very low-grade metamorphism[END_REF][START_REF] Ferreiro Mählmann | Correlation of very low grade data to calibrate a thermal maturity model in a nappe tectonic setting, a case study from the Alps[END_REF]. Organic maturity is controlled by starting material, oxygen fugacity-water activity [START_REF] Ernst | Vitrinite alteration rate as a function of temperature, time, starting material, aqueous fluid pressure and oxygen fugacity-Laboratory corroboration of prior work[END_REF], temperature-time [START_REF] Teichmüller | Organic material and very low-grade metamorphism[END_REF], and is independent of host lithology. A review of the literature shows, however, that field observations and experimental data display inconsistencies when the effects of pressure on vitrinite maturation are considered. For example, in the Diablo Range (Franciscan Complex, California), Dalla [START_REF] Dalla Torre | Determination of white K-mica polytype ratios: comparison of different XRD methods[END_REF]Torre et al. ( , 1996a) ) demonstrated that the maturation of organic matter was lower than expected, compared to both the illite crystallinity and the metamorphic grade determined through the mineral assemblages. They attributed the abnormally low vitrinite reflectance values to the effect of pressure based on field evidence described by [START_REF] Goffé | Contrasted metamorphic evolution in thrusted cover units of the Briançonnais zone (Franch Alps): a model for the conservation of HP-LT metamorphic mineral assemblages[END_REF], [START_REF] Feldhoff | Ueber die Diagenese-/ Metamorphosebedingungen der Pindos-und Tripolitza-Serie auf der Insel Kreta (Griechenland). Sediment '90; 5. Treffen deutschsprachiger Sedimentologen[END_REF], [START_REF] Fang | The effect of oxidation-reduction conditions on the hydrocarbon potential and thermostability of organic matter[END_REF]Jiaynu (1992) and[START_REF] Ferreiro Mählmann | Zur Bestimmung von Diagenesehöhe und beginnender Metamorphose-Temperaturgeschichte und Tektogenese des Austroalpins und Süpenninikums in Vorarlberg und Mittelbünden[END_REF]. This model was subsequently corroborated by an experimental study on the pressure dependence of vitrinite maturation [START_REF] Dalla Torre | Experimental study on the pressure dependence of vitrinite maturation[END_REF]. Based on this work and new experiments, [START_REF] Ernst | Vitrinite alteration rate as a function of temperature, time, starting material, aqueous fluid pressure and oxygen fugacity-Laboratory corroboration of prior work[END_REF] proposed a thermo-barometric application of vitrinite reflectance based on a kinetic model. The problem in poly-metamorphic areas such as the Vanoise in the French Alps [START_REF] Goffé | Contrasted metamorphic evolution in thrusted cover units of the Briançonnais zone (Franch Alps): a model for the conservation of HP-LT metamorphic mineral assemblages[END_REF], the Tripolitza Series in Greece [START_REF] Feldhoff | Ueber die Diagenese-/ Metamorphosebedingungen der Pindos-und Tripolitza-Serie auf der Insel Kreta (Griechenland). Sediment '90; 5. Treffen deutschsprachiger Sedimentologen[END_REF], the Arosa Zone in the European Alps [START_REF] Ferreiro Mählmann | Zur Bestimmung von Diagenesehöhe und beginnender Metamorphose-Temperaturgeschichte und Tektogenese des Austroalpins und Süpenninikums in Vorarlberg und Mittelbünden[END_REF], and the Diablo Range in the Franciscan Complex [START_REF] Dalla Torre | Experimental study on the pressure dependence of vitrinite maturation[END_REF], is that no coherent relationship between organic and inorganic parameters for determining the metamorphic grade has been identified. This contribution summarizes the results of our investigation of the very low-grade metapelites of the Koumac and Diahot terranes in New Caledonia, using the illite crystallinity and b cell dimensions of K-white micas, together with thermodynamic modeling of Al-rich metapelites and fluid inclusion measurements. The determination of the b cell dimension of K-white mica and fluid inclusion barometry of synmetamorphic vein minerals, together with the use of vitrinite reflectance, provided an excellent opportunity to determine the effect of pressure on vitrinite reflectance in very low-to low-temperature domains. The results are compared with the existing vitrinite reflectance measurements for this area [START_REF] Diessel | Coalification and Graphitization in High-Pressure Schists in New Caledonia[END_REF] and oxygen thermometry studies [START_REF] Black | Blueschist Ophiolites in the Melange Zone, Northern New Caledonia[END_REF] in order to compare the evolution of illite crystallinity and vitrinite reflectance in a high-pressure (HP) metamorphic belt.

The maximum pressure and temperature metamorphic conditions in the Diahot terrane based on previous studies [START_REF] Black | Blueschist Ophiolites in the Melange Zone, Northern New Caledonia[END_REF]Bell & Brothers, 1985) are similar to those observed in the Tripolitza Series and the Diablo Range. This metamorphic belt provides a rare natural laboratory to test the experimentally observed pressure retarding effects on vitrinite reflectance recorded by Dalla [START_REF] Dalla Torre | Experimental study on the pressure dependence of vitrinite maturation[END_REF], by studying metapelites from a high-pressure metamorphic schist belt.

REGIONAL GEOLOGY

New Caledonia is a dispersed fragment of the eastern margin of Gondwana resulting from the opening of the Tasman Sea and rifting of New Caledonia and New Zealand from Australia. In Late Eocene time, the New Caledonian fragment collided with an intraoceanic island-arc system [START_REF] Cluzel | Point de vue sur l'évolution tectonique et géodynamique de la Nouvelle-Calédonie (Pacifique, France)[END_REF]Aitchison et al., 1995;[START_REF] Clarke | Eclogites and blueschists of the Pam Peninsula, NE New Caledonia: a reappraisal[END_REF][START_REF] Cluzel | Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications[END_REF], and a NE-facing subduction system was established beneath the Loyalty Basin. The collision, responsible for the main deformation of the continental basement of New Caledonia, generated a high-P schist belt coeval with the southwestwards thrusting of two nappes over the basement. The Poya nappe or "formation des basalts" [START_REF] Paris | Géologie de la Nouvelle-Calédonie, un essai de synthèse[END_REF]) is a mafic sheet overlying all the pre-Neogene tectonic units (Fig. 1), comprising dolerite, tectonized tholeiitic pillow basalts and abyssal argillites. The peridotite nappe is an ophiolite complex that initially covered much of the island; faulting and erosion has removed much of it, but it still dominates outcrops in the south of the island. The basement is composed of terranes of Palaeozoic to Mesozoic age of varying metamorphic grade, transgressively overlain by upper Cretaceous to Eocene sedimentary rocks deposited during Gondwana dispersal (Aitchison et al., 1995;[START_REF] Clarke | Eclogites and blueschists of the Pam Peninsula, NE New Caledonia: a reappraisal[END_REF].

Post Early Cretaceous tectonic units (Figs. 1 & 2) include the Poya and peridotite nappes and the Koumac, Diahot and Pouébo basement terranes. The Koumac terrane [START_REF] Cluzel | Point de vue sur l'évolution tectonique et géodynamique de la Nouvelle-Calédonie (Pacifique, France)[END_REF] is a part of the olistrostrome and collisional marginal plateau sedimentary terrane of [START_REF] Fitzherbert | Lawsonite-omphacite-bearing metabasites of the Pam Peninsula, NE New Caledonia: Evidence for disrupted blueschist-to eclogitefacies conditions[END_REF]. It consists of an intensively tectonized Late Cretaceous to Early Eocene sequence of flysch, chert, limestone and black shale.

The Diahot terrane contains an interbedded sequence of Cretaceous to Eocene metasediments, felsic volcanics metamorphosed under high-P/T conditions [START_REF] Fitzherbert | Lawsonite-omphacite-bearing metabasites of the Pam Peninsula, NE New Caledonia: Evidence for disrupted blueschist-to eclogitefacies conditions[END_REF]. The easternmost terrane is the Pouébo terrane; a mélange containing large mafic eclogite boulders enclosed in an argillaceous or serpentine-rich matrix [START_REF] Maurizot | Carte géologique à l'échelle du 1/50 000, feuille Pam-ouégoa[END_REF]. 40 Ar/ 39 Ar dating of white micas from the Diahot and Pouébo units yielded consistent Middle to Late Eocene cooling ages of 37 ± 1 Ma [START_REF] Ghent | 40 Ar/ 39 Ar dating of white micas from the epidote to the omphacite zones, northern New Caledonia: tectonic implications[END_REF]. Metamorphism is bracketed between Early Eocene (youngest Sediments) and Late Eocene (cooling ages).

This study is focused on the Koumac and Diahot terranes (Figs. 1,2 & 3). The Koumac terrane includes the lowest grade metasediments of the high-pressure belt as defined by the presence of pumpellyite and prehnite in basic igneous rocks [START_REF] Brothers | High-Pressure Schists in Northern New Caledonia[END_REF][START_REF] Black | Regional High-Pressure metamorphism in New Caledonia: Phase equilibria in the Ouégoa District[END_REF]. In this terrane vitrinite reflectance is 2.9 Rmax% and the maximum pressure and temperature were 0.3 GPa and 250°C [START_REF] Diessel | Coalification and Graphitization in High-Pressure Schists in New Caledonia[END_REF]. [START_REF] Fitzherbert | Lawsonite-omphacite-bearing metabasites of the Pam Peninsula, NE New Caledonia: Evidence for disrupted blueschist-to eclogitefacies conditions[END_REF] subdivided the Diahot terrane into four zones based on the metabasite mineral assemblage (Fig. 3): (1) lawsonite blueschists (P = 1.0 GPa and T = 400°C); (2) epidote blueschists (P = 1.4-1.5 GPa and T = 450-500°C); (3) almandine-hornblende blueschists (P = 1.4-1.6 GPa and T = 550-580°C); (4) paragonite-hornblende eclogites (P = 1.7 GPa and T = 600-620°C). Our study area in the Diahot unit is limited to the region to the southwest of the Gendarmerie fault (Fig. 2), equivalent to Zone 1 (lawsonite blueschist) of [START_REF] Fitzherbert | Lawsonite-omphacite-bearing metabasites of the Pam Peninsula, NE New Caledonia: Evidence for disrupted blueschist-to eclogitefacies conditions[END_REF].

MATERIAL AND METHODS

Eighty-six samples of metapelite and eleven meta-marls (Table 1) were collected from the Koumac and Diahot terranes (Fig. 3). In northern New Caledonia, outcrops are poor due to extensive tropical weathering, limiting the availability of fresh samples mainly to road cuts. Therefore, samples were only collected along two road profiles (Koumac-Ouégoa and road RM11) and along various small paths (Touho region) (Fig. 3). In many common rocks, such as marine pelites and carbonates rocks, no diagnostic minerals and mineral assemblages form in the very low-grade field. In these rocks, the transitions from non-metamorphic to very low-grade and from very low-grade to low-grade metamorphic domains take place through the diagenetic zone, the anchizone and the epizone, each zone being characterized by specific values of the illite Kübler Index (KI) (Árkai et al. 2003).

Clay mineral separation was conducted using techniques described by [START_REF] Schmidt | Very low grade metamorphism of the Taveyanne formation of western Switzerland[END_REF]. To perform the illite-muscovite polytype determination, randomly oriented samples were prepared using the technique described by Dalla [START_REF] Dalla Torre | Determination of white K-mica polytype ratios: comparison of different XRD methods[END_REF].

X-ray diffraction

Illite and chlorite crystallinity was measured on air-dried, glycolated and heated samples using a D5000 Bruker-AXS (Siemens) diffractometer, CuK radiation at 40 kV & 30 mA and automatic divergence slits (primary and secondary V20) with a secondary graphite monochromator.

Illite crystallinity was calculated using the software DIFFRAC Plus TOPAS (by ©Bruker AXS). The illite crystallinity index is defined as the full width at half maximum (FWHM) intensity of the first illite-muscovite basal reflection (Table 2).

Illite crystallinity values were transformed into Kübler index values using a correlation with the standard samples of [START_REF] Warr | Interlaboratory standardization and calibration of clay mineral crystallinity and crystallite size data[END_REF] (KICIS = 1.343 * ICBasel -0.003). The Kübler index was used to define the limits of metamorphic zones, and the transition values were chosen as follows: KI = 0.25 ∆°2 for the epizone to high anchizone boundary, KI = 0.30 ∆°2 for the high to low anchizone boundary and KI = 0.42 ∆°2 for the low anchizone to diagenetic zone. The same experimental conditions were also used to determine chlorite crystallinity on the (002) peak, where (ChC(002)) corresponds to the full width at half maximum intensity values of the second (7 Å) basal reflection of chlorite (Table 2). The ChC(002) measurements were calibrated with those of [START_REF] Warr | Interlaboratory standardization and calibration of clay mineral crystallinity and crystallite size data[END_REF] and expressed as the Árkai index (ÁI) [START_REF] Guggenheim | Report of the association internationale pour l'étude des argiles (AIPEA) nomenclature committee for 2001: order, disorder and crystallinity in phyllosilicates and the use of the "crystallinity index[END_REF]: ÁI = 0.766* ChC(002) + 0.117. The anchizone boundaries for the Árkai index were defined by correlation with the Kübler index and are given as 0.24 ∆°2 for the epizone to anchizone boundary and 0.30 ∆°2 for the anchizone to diagenetic zone.

The different types of mixed-layer clay minerals observed were classified using data from [START_REF] Moore | X-Ray Diffraction and the identification and analysis of clay minerals[END_REF]. The illite/smectite glycolated mixed-layer clay minerals were identified on the diffractogram by the presence of a reflection between 16 and 17.7 °2. After glycolation solvation, the chlorite/smectite was identified by expansion of the d(001)* to 31.1 Å. Heat treatment was necessary for identification of the kaolinite/smectite. Heating produces a shift from the 7.4 Å air-dried reflection to an increased d-value of 8.1 Å. K-white mica b cell dimensions (or b cell dimensions) were determined for samples free of paragonite and mixed-layered minerals. The b cell dimension is based on the d060,331 spacing and on the increasing celadonite substitution that occurs with pressure increase in white mica [START_REF] Ernst | Significance of phengitic micas from low grade schists[END_REF][START_REF] Guidotti | Compositional controls on the a and b cell dimensions of 2M1 Muscovites[END_REF]. [START_REF] Guidotti | Compositional controls on the a and b cell dimensions of 2M1 Muscovites[END_REF] presented linear regression equations that quantify the changes in the b cell dimensions of muscovite 2M1 that result from cation substitutions in the interlayer and octahedral sites. This b cell dimension value was determined by measurement of the (060) peak of the potassic white mica when present [START_REF] Sassi | The b0 of the potassic white micas as a barometric indicator in low-grade metamorphism of pelitic schists[END_REF], and using the program WIN-METRIC V.3.0.7 (by ©Bruker AXS), which is a cellrefinement program.

Electron microprobe and X-ray fluorescence data

Mineral compositions in the metapelites were determined by electron probe microanalysis using a JEOL JXA-8600 superprobe at the University of Basel with four wavelength dispersive spectrometers, equipped with Voyager software by NORAN instruments. A ZAF-type correction procedure was used for all data reduction and all Fe was assumed to be ferrous. In order to avoid volatilization of light elements, low-grade metamorphic minerals were analyzed using a 10 nA beam current, an accelerating voltage of 15 kV, an acquisition time of 10 or 20 seconds, rastered over an area of 26 µm 2 . The standards used for the different elements were as follows: olivine for Si and Mg; rutile for Ti; gehlenite for Al; graftonite for Mn and Fe; wollastonite for Ca; albite for Na; and orthoclase for K.

Two whole-rock chemical analyses were made at the Geochemistry Laboratories, University of Basel. Major oxide concentrations were measured on a fused glass bead (20 mm diameter) containing a mixture of 300 mg sample powder and 4700 mg Li-tetraborate. The two analyses (Table 3) were obtained with a Siemens SRS3000 Wavelength Dispersive Sequential X-Ray Spectrometer with a Rh end window tube (4 kV). The results were collected and evaluated using the Bruker AXS Spectra plus standardless evaluation program. Analytical precision varies from element to element, as does detection limits, but generally the major oxides have a detection limit of 0.01% and a relative accuracy of 0.5%.

Thermodynamic Modeling

Pressure and temperature conditions of equilibrium of the metamorphic rocks were determined by calculating equilibrium assemblages and equilibrium phase diagrams using the THERIAK-DOMINO software [START_REF] De Capitani | The computation of chemical equilibrium in complex systems containing non-ideal solutions[END_REF][START_REF] De Capitani | Gleichgewichts-Phasendiagramme: Theorie und Software[END_REF]. A modified internally consistent mineral database based on JUN92 of [START_REF] Berman | Internally-consistent thermodynamic data for minerals in the system Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-SiO2-TiO2-H2O-CO2[END_REF] was supplemented with thermodynamic data for ferro-and magnesiocarpholite, and magnesio-and ferro-chloritoid from Le Bayon (2002). Data for glaucophane and ferro-glaucophane [START_REF] El-Shazly | Glaucophane chloritoid-bearing assemblages from NE Oman: petrologic significance and a petrogenetic grid for high P metapelites[END_REF], daphnite and phengite [START_REF] Massonne | Thermodynamic properties of white micas on the basis of high-pressure experiments in the systems K2O-MgO-Al2O3-SiO2-H2O and K2O-FeO-Al2O3-SiO2-H2O[END_REF] were also added. The activity models are presented in Table 4. The main input consists of the bulk-rock composition of the rock studied.

Microthermometry

Quartz-calcite veins were sampled along the road between Koumac and Ouégoa (Fig. 3), in order to substantiate the P-T estimations based on mineral equilibria and to reconstruct the fluid evolution history during metamorphism [START_REF] Frey | Very low-grade metamorphism in external parts of the Central Alps: Illite crystallinity, coal rank and fluid inclusion data[END_REF].

Microthermometry investigations on double polished thin sections were performed using a Chaixmeca heating-freezing stage at the University of Basel [START_REF] Poty | Un nouvel appareil pour la mesure des températures sous le microscope: l'installation de microthermométrie Chaixmeca[END_REF][START_REF] Mullis | Fluid regimes during late stage of a continental collision: Physical, chemical, and stable isotope measurements of fluid inclusions in fissure quartz from a geotraverse through the Central Alps, Switzerland[END_REF]. The uncertainty of the measurements is ± 0.1°C for -60 to 40°C and ± 1°C outside this range. Two-phase (consisting of vapor and liquid at room temperature) and one-phase fluid inclusions were identified. No dissolved volatile phase was observed, either by melting of CO2 at or below its triple point of -56.6°C, nor by formation or dissociation of a clathrate or liquid-vapor equilibrium of a volatile component such as higher hydrocarbons (HHC), CH4, CO2, N2 or H2S. Thus, microthermometry was restricted to measuring the eutectic temperature (Te), the melting temperature of ice (Tmice) and the bulk homogenization temperature of the fluid inclusions (ThI). The eutectic temperature provides evidence for the composition of the electrolytes dissolved in aqueous solutions [START_REF] Hollister | Short course in fluid inclusions: Applications to petrology[END_REF][START_REF] Shepherd | A practical guide to fluid inclusion studies[END_REF][START_REF] Mullis | Fluid inclusion studies during very low-grade metamorphism[END_REF]. As none of the investigated fluid inclusions contained any observable gas component, salinity was derived from the ice melting temperature in NaClequivalence after [START_REF] Potter | Freezing point depression of aqueous sodium solutions[END_REF]. Bulk homogenization temperatures of gas-free aqueous solutions were used for the determination of the volume fraction of the vapor and liquid phases after [START_REF] Zhang | Determination of the homogenization temperatures and densities of supercritical fluids in the system NaCl-KCl-CaCl 2 -H 2 O using synthetic fluid inclusions[END_REF].

From the salinity and volume fraction of vapor and liquid phases within the fluid inclusions, bulk density and inclusion composition were calculated according to the general equations published in [START_REF] Mullis | Fluid inclusion studies during very low-grade metamorphism[END_REF]. The isochors were calculated from the equation of state given by [START_REF] Zhang | Determination of the homogenization temperatures and densities of supercritical fluids in the system NaCl-KCl-CaCl 2 -H 2 O using synthetic fluid inclusions[END_REF].

RESULTS

Mineralogy

The mineralogy of the studied samples is given in Table 1. Minerals abbreviations are those of [START_REF] Kretz | Symbols for rock-forming minerals[END_REF]. Pelites, tuffs and metamarls were analyzed.

In the meta-marls, the mineral assemblages consist of Qtz + K-white mica + Kln and/or Chl with minor amounts of mixed-layer minerals (Illite/Smectite (I/S), and Chlorite/Smectite (C/S)), albite, and sporadically hematite. In their <2 µm grain-size fraction, illite-muscovite predominates in association with Chl or Kln, mixed-layer minerals and quartz. Albite is only present in minor amounts, and Vermiculite (Vm) and corrensite (Cor) are found in few samples.

In the metapelites, the mineral assemblages consist of Qtz, K-white mica, Chl and/or Kln with minor amounts of mixed-layer minerals (I/S, C/S and Kaolinite/Smectite (K/S)), Ab, and index minerals depending on the metamorphic grade (stilpnomelane, lawsonite, glaucophane, carpholite, epidote). In their <2 µm fractions, illite-muscovite or phengitic muscovite predominate in the higher-grade samples. Discrete paragonite is also found in minor quantities. Traces of smectite or kaolinite indicate oxidative weathering in some higher-grade samples. 2.

Structural characteristics of the phyllosilicates

The KI values range from low diagenetic to epizone. In the Koumac terrane, the KI data indicate low diagenetic to low anchizone values, ranging from 1.40 to 0.33 ∆°2. In the Diahot terrane, KI values vary between low anchizone and epizone. A trend of increasing grade from the diagenetic zone to the epizone is observed in a traverse from southwest to northeast along the 2 & 4a). An overall prograde sequence for the terranes is documented, with KI decreasing from southwest to northeast.

Significant positive linear correlation (r 2 = 0.65) is found between Kübler and Árkai indices (Fig. 5), giving greater reliability to the metamorphic grade estimate deduced from KI values, which can be hindered by broadening of the reflectance peak due to the presence of discrete paragonite.

The percentage of 2M1 illite-muscovite polytype relative to the KI along the Koumac-Ouégoa road shows a positive trend with increasing metamorphic grade (Fig. 4b). The complete conversion from 1Md to 2M1 is reached at KI values around 0.30 ∆°2 (limit of the low to high anchizone; Table 5).

The K-white mica b cell dimension for 20 K-white mica samples from the lawsonite and glaucophane zones all fall in the same range with a minimum 9.04 and a maximum 9.06 Å values. Eight samples from the pumpellyite-prehnite zone show lower values between 9.01 and 9.03 Å (Fig. 6). The b values from the glaucophanelawsonite zone are typical for HP-type metamorphism, whereas the prehnitepumpellyite values are more characteristic of Barrovian-type metamorphism [START_REF] Sassi | The tetrological and geological significance of the b0 values of potassic white micas in low-grade metamorphic rocks. An application to the Eastern Alps[END_REF][START_REF] Sassi | The b0 of the potassic white micas as a barometric indicator in low-grade metamorphism of pelitic schists[END_REF].

Mineral chemistry

Phyllosilicate chemistry

Due to the small grain size of the phyllosilicates, special care was taken to differentiate between detrital and newly-formed metamorphic grains. In certain cases (i.e. MF3003), detrital micas altered to K-white mica/chlorite or K-white mica/paragonite/chlorite stacks were observed. They could be distinguished from fine-grained, matrix-forming white mica and chlorite formed parallel to the metamorphic schistosity.

Representative compositions of K-white mica are listed in Table 6. Mica analyses showing more than 0.5% (MnO %+ TiO2 %) were rejected [START_REF] Vidal | Exhumation paths of high-pressure metapelites obtained from local equilibria for chlorite-phengite assemblages[END_REF]. In a Si-Al tot diagram (Fig. 7a), the analyses show a significant deviation from the Tschermak exchange line. Considering the analyses reported in Table 6, the total interlayer charge (t.i.c. = Ca+Na+2K) is between 1 and 0.90, except for sample MF3144, where a Prl substitution could be assumed (Figs. 7a and7d) in addition to the Tschermark's substitution. Additional substitutions are required to explain the excess of Fe and Mg (Figs. 7a and7b). The excess of Mg+Fe could be explained by a di/trioctahedral substitution common in dioctahedral micas [START_REF] Vidal | Exhumation paths of high-pressure metapelites obtained from local equilibria for chlorite-phengite assemblages[END_REF][START_REF] Parra | A thermodynamic model for Fe-Mg dioctahedral K white micas using data from phase-equilibrium experiments and natural peletic assemblages[END_REF]. Fig. 7c shows a correlation between the Fe and Mg contents in the individual samples (probably dependant on the bulk composition) except for samples PS56 and PS32. On the basis of model calculations supposing various mixtures (Fig. 7b), no contamination of the analyses by quartz or chlorite has occurred.

In Fig. 7d, the K-white mica analyses fall in a cluster between the muscovitephengite and the muscovite-illite lines, indicating a small deficit in t.i.c. (values range between 0.8 and 0.95 p.f.u.). Fig. 7e shows that the Na content of K-white mica is very low (except for samples MF3003 and PS89) and thus the b cell dimension is not influenced by K-Na exchange. Therefore use of the b cell dimension should be accurate [START_REF] Guidotti | Compositional controls on the a and b cell dimensions of 2M1 Muscovites[END_REF].

Although the number of data pairs is relatively low (n = 6), we investigated the relation between the metamorphic grade of our samples with the chemistry of the K-white micas. No real trend is observed between Na/(Na+K) ratio and metamorphic grade, as indicated by KI value (Fig. 7f), suggesting that subordinate amounts of discrete paragonite and/or mixed K/Na micas do not influence the KI. Fig. 7g reveals no obvious correlation between KI values and Si content. This indicates the occurrence of other substitutions in addition to the celadonitic one, probably the di/trioctahedral substitution previously discussed.

Compositions of chlorite are normalized to 28 oxygens (Table 7) and fulfill the criterion of non contamination, ∑Ca+Na+K < 0.2 [START_REF] Dalla Torre | Chlorites textures and composition from high pressure/low temperature metashales and metagraywackes, Franciscan complex, Diablo Range, California, U.S.A[END_REF]. Using the classification of [START_REF] Zane | A procedure for classifying rock-forming chlorites based on microprobe data[END_REF], chlorites in the studied samples are trioctahedral type I where (XMg + XFe ≥ XAl + Xvacancy). A Fe-Mg-Si diagram reveals intermediate compositions between chamosite and clinochlore (Fig. 8a). XMg varies between 0.431 and 0.955, and according to [START_REF] Hey | A new review of the chlorites[END_REF], the chlorites lie mainly in the ripidolite, pichnochlore and brunsvigite fields.

The low total interlayer charge of the chlorites (<0.2 p.f.u.) indicates that smectite or illite impurities are insignificant (Fig. 8b). The increase of (Al VI -Al IV ) could be attributed to increasing sudoitic substitution (di-trioctahedral) (Árkai et al., 2003).

Correlations observed for apparent octahedral vacancies with (Al VI -Al IV ) (Fig. 8c) and apparent octahedral vacancies with sum of octahedral divalent cations (Fig. 8d) prove the sudoitic substitution in the analyzed chlorites. Fig. 8e demonstrates that, in addition to the Tschermak substitution, the di-trioctahedral substitution plays an important role in chlorites.

Using the chlorite-Al IV thermometer of [START_REF] Cathelineau | Cation site occupancy in chlorites and illites as a function of temperature[END_REF], we calculated mean T values between 270 and 340°C (Table 7). Fig. 8f shows a negative trend between the metamorphic grade increases (KI values decrease) and the calculated T. This could be explained by the absence of smectite contamination (or other significant impurity) [START_REF] Schmidt | Very low grade metamorphism of the Taveyanne formation of western Switzerland[END_REF].

Index mineral chemistry

Lawsonite is very common in shales from the eastern coast. Representative analyses are presented in Table 8. Petrographic evidence indicates that lawsonite mainly grew during or after development of the main foliation. Crystals typically are elongated or have undeformed tabular forms in thin section.

Stilpnomelane is relatively widespread in the lawsonite zone, and it is typically associated with lawsonite. Microprobe analyses of the studied stilpnomelane show an XFe range from 0.70 to 0.80 corresponding to ferro-stilpnomelane (Table 8).

Sodic amphiboles in the Diahot terrane occur as fine-grained, thin, needle-like ellipsoidal fibres parallel to the main foliation, with pale blue to purple pleochroism.

Representative analyses of sodic amphibole from samples MF3031 and PS131 are presented in Table 8. The amphiboles are ferro-glaucophane with XMg ranging between 0.30 and 0.48 (after [START_REF] Leake | Nomenclature of amphiboles: Report of the subcommittee on amphiboles of the International Mineralogical Association, Commission on News Minerals and Mineral Names[END_REF].

Vein mineralogy, textural relationships and Microthermometry

We selected quartz-vein samples from four localities along the profile between Koumac and Ouégoa (Fig. 3). Oriented samples were collected from structurally controlled syntectonic fissures of a few millimetres in width. Rock sections were cut perpendicular to the bedding and retaining portions of the wall-rock. The chronology of the different fluid inclusion populations (Table 9) is defined with respect to their relative overgrowth sequence [START_REF] Mullis | Das Wachstumsmilieu der Quarzkristalle im Val d´Illiez (Wallis, Schweiz)[END_REF].

Veins are mainly composed of small quartz crystals, except sample MF3006, which contains in addition calcite and chlorite. Two types of fluid inclusions were identified with the microscope: one-phase inclusions and two-phase fluid inclusions (consisting of vapour and liquid at room temperature). Most of the fluid inclusions analyzed are secondary.

Different fluid inclusion assemblages can be defined (Table 9) based on their shape, salinity and relation between each other. We generally observed a first fluid inclusion assemblage stretched, partially or totally decrepitated (fluid inclusion assemblage 1). Fluid inclusions, with a high volatile ratio (≥80%) and well-shaped (negative quartz), are present in samples MF3022 and MF3027. Such fluid inclusions were formed during a heating event, postdating the deformation. In the case of sample MF3027, it was not possible to measure ThI due to the variation of the vapor/liquid ratios after repeated measurement. Fluid inclusions less stretched, with lower salinity are observed in samples MF3004 and MF3027 and define a third fluid assemblage in those samples.

P-T Estimates: Equilibrium Phase Diagrams

Sample MF3031 is from the lawsonite-glaucophane zone of the Diahot terrane, in the district of Ouégoa. Sample MF2994 was collected in the pumpellyite-prehnite zone (high diagenetic -low anchizone) of the Koumac terrane. Equilibrium phase diagrams were calculated using the THERIAK-DOMINO software. The main input consists of the modal bulk composition of the rock. To attain greater control on the model, bulk compositions without the minor elements were calculated in the systems NKFMASH (MF2994, where Ca and Ti are in minor amounts) and Ti-Ca-NKFMASH (MF3031). Calculations were performed in the temperature range between 150 and 500°C and for pressures between 0.1 and 2.0 GPa. We took also into account the effect of water activity on the stability of mineral assemblages. The water activity was evaluated to 0.9 for sample MF3031 on the basis of a previous study on the mineral and bulk-rock oxygen and carbon isotopic characteristics of the Ouégoa terrane [START_REF] Black | Regional High-Pressure metamorphism in New Caledonia: Phase equilibria in the Ouégoa District[END_REF].

The P-T section for sample MF3031 (Fig. 9) shows the observed mineral assemblage of the sample (quartz + phengite + glaucophane + chlorite + lawsonite + titanite). The assemblage is stable over a wide range of P and T, going from 0.8 to 1.8 GPa and from 300 to 475°C. In order to gain a better constraint, we calculated the isopleths of Fe-Gln and Ms (Fig. 9). These mineral compositions allow us to restrict the stability field to 1.3 to 1.8 GPa and 400 to 475°C. The b cell dimension of samples in the zone, 9.04 Å, reflects the high pressure character of metamorphism and points to pressure higher than 1.2 GPa in the temperature range of 400 to 475°C using the P-T diagram from Ramírez & Sassi (2001, Fig. 9).

Calculations for sample MF2994 from the pumpellyite-prehnite zone (high diagenetic -low anchizone) required accurate evaluation of the aH 2 O. [START_REF] Black | Regional High-Pressure metamorphism in New Caledonia: Phase equilibria in the Ouégoa District[END_REF] has shown that in this zone, aH 2 O ranges between 0.6 and 0.7. Calculations determined for a maximum aH 2 O equal to 0.7 (value for maximum aH 2 O of [START_REF] Black | Regional High-Pressure metamorphism in New Caledonia: Phase equilibria in the Ouégoa District[END_REF]) show a maximum temperature of 250°C for a pressure of 0.3 GPa for the assemblage Qtz + K-white mica + Kln + Chl (observed in the sample MF2994) (Fig. 10). Therefore, we can postulate P-T conditions below 250°C and 0.3 GPa in the very low-grade region, which is consistent with estimates by Bell & Brothers (1985) based on the appearance at a slightly higher grade of lawsonite and aragonite in the lawsonite zone.

DISCUSSION

Comparison of methods to determine the HP-LT event ("peak metamorphism")

In the northeast of New Caledonia, in the Oligocene HP-LT schist belt, the trends shown by KI values can be reproduced from the vitrinite reflectance measurements published by [START_REF] Diessel | Coalification and Graphitization in High-Pressure Schists in New Caledonia[END_REF]. Vitrinite reflectance data show an evolution from medium-volatile bituminite to meta-anthracite stage (1.4 to 6.5 %Rmax). In the HP epizone of the lawsonite-glaucophane zone, the maturation of vitrinite reaches the optical graphite stage. In Table 5, the high diagenesis and low anchizone (semianthracite stage) correlates with the appearance of 1Md mica, kaolinite, corrensite and pumpellyite. The high anchizone (anthracite stage) correlates with the occurrence of 2M1 mica, lawsonite, chlorite-smectite and pumpellyite. The epizone (high metaanthracite stage, including the incipient semi-graphite -optical graphite stage) correlates with the stability field of chlorite, lawsonite, glaucophane and carpholite.

The evolution of sheet silicates coherently reflects the coalification trend and the field gradient of increasing HP metamorphism.

In New Caledonia, Rmax values [START_REF] Diessel | Coalification and Graphitization in High-Pressure Schists in New Caledonia[END_REF] for the KI-anchizone boundaries are 2.95 ± 0.15% (high diagenetic zone-low anchizone), and 4.75 ± 0.7% (high anchizone-epizone). In other metamorphic belts, anchizone boundaries, in terms of vitrinite reflectance, were set to be 2.5 -3.1 % and 3.7 -5.5 % with a mean R0 (R0 = (2Rmax + Rmin)/3) for the lower and upper limit of the anchizone, respectively [START_REF] Kisch | Correlation between indicators of very low-grade metamorphism[END_REF]. A wider range of Rmax% values was subsequently recognized in different tectonic nappes of the Alps and in a literature revision by Ferreiro Mählmann (1994). More restricted ranges have been proposed by [START_REF] Underwood | Correlation among paleotemperature indicators within orogenic belts: Examples from pelitic rocks of the Franciscan complex (California), the Shimanto Belt (Japan), and the Kandik Basin (Alaska)[END_REF], but these authors do not take into account reaction kinetics of processes such as illitization and coalification, that are strongly controlled by the tectono-thermal and heat flow history [START_REF] Frey | Very low-grade metamorphism in external parts of the Central Alps: Illite crystallinity, coal rank and fluid inclusion data[END_REF][START_REF] Ferreiro Mählmann | Correlation of very low grade data to calibrate a thermal maturity model in a nappe tectonic setting, a case study from the Alps[END_REF]. A recent study by [START_REF] Ernst | Vitrinite alteration rate as a function of temperature, time, starting material, aqueous fluid pressure and oxygen fugacity-Laboratory corroboration of prior work[END_REF] has shown that oxygen fugacity and water activity have no influence on vitrinite reflectance. Thus, the following discussion is focused on the thermal history and geodynamic evolution.

Using the minimum pressure limit (1.2 GPa), deduced from the b cell dimension method for the low epizone (300°C, lawsonite zone), in the maturity kinetics model of Dalla Torre et al. (1997), which predicts that pressure lowers vitrinite reflectance, we tried to reproduce the measured maturity (VR) of 5.0 -5.5 %Rmax. The low epizone with a HP metamorphic facies was chosen, because at higher temperatures, diagnostic use of KI and vitrinite reflectance is not recommended [START_REF] Ferreiro Mählmann | Zur Bestimmung von Diagenesehöhe und beginnender Metamorphose-Temperaturgeschichte und Tektogenese des Austroalpins und Süpenninikums in Vorarlberg und Mittelbünden[END_REF][START_REF] Ferreiro Mählmann | Correlation of very low grade data to calibrate a thermal maturity model in a nappe tectonic setting, a case study from the Alps[END_REF]. Dependent on the time factor, the modeled vitrinite reflectance value is 2.8

R%max for 1 Ma and 3.4 R%max for 12 Ma. The maximum time range of 12 Ma is limited by sediment ages (Early Eocene) and cooling ages (Late Eocene; see [START_REF] Ghent | 40 Ar/ 39 Ar dating of white micas from the epidote to the omphacite zones, northern New Caledonia: tectonic implications[END_REF]. Hence, using the thermo-barometric model of vitrinite reflectance of Dalla [START_REF] Dalla Torre | Experimental study on the pressure dependence of vitrinite maturation[END_REF], it is not possible to reproduce the vitrinite reflectance observed in New Caledonia.

Our modeling shows that the KI-VR mineral zone relationship was probably not established during maximum pressure metamorphism. The field of equilibrium between the different parameters that were used to determine metamorphic grade was probably reached in a later stage during retrogression. Fluid inclusions cannot be used to restore "peak conditions" of the HP-LT metamorphism because the first population is decrepitated. It is evident that parameters such as vitrinite reflectance, illite crystallinity, fluid inclusion data, mica b cell dimension data and mineral zoning define different points on a pressure-temperature path.

Pressure-temperature path evolution

Bell & Brothers (1985) demonstrated an increase of temperature during exhumation in the Lawsonite Zone. This P-T path was constrained by the transformation of lawsonite to zoisite ( Lws + Ab  Zo + Pg + Qtz + vapor). The reaction is controlled by the instability of lawsonite in the presence of CO2 generated by concurrent decarbonation, associated with conditions favorable for oxidation of Fe and growth of epidote. At the transition between lawsonite and epidote-blueschist zone, all organic matter changed to an increasingly ordered graphite structure [START_REF] Diessel | Coalification and Graphitization in High-Pressure Schists in New Caledonia[END_REF].

The concomitant appearance of optical graphite and epidote suggests that heat generated by decarbonation reactions may have enhanced the ordering of carbon in graphite, even though the pressure at this time was decreasing (Bell & Brothers, 1985). In the same way, [START_REF] Yokoyama | Regional eclogite facies in the highpressure metamorphic belt of New Caledonia[END_REF] presented in the higher metamorphic zones, a P-T path including a temperature-prograde phase of climax crystallisation that prevailed during a decline in pressure.

During or after decompression, an increase of temperature is also indicated by the maturity of organic matter, which is higher than expected in a blueschist facies metamorphic environment [START_REF] Dalla Torre | Experimental study on the pressure dependence of vitrinite maturation[END_REF]. We tried to reproduce the evolution of organic matter in New Caledonia using three different kinetic models. An increase in temperature and heat flow is the easiest way to elevate vitrinite reflectance in kinetic maturity models (Model 1). Hence, using the kinetic maturity model from Dalla Torre et al. (1997), but lowering the pressure conditions, it is also possible to increase the modeled vitrinite reflectance values to the measured values of 5.0 to 5.5 %Rmax with a small increase of the temperature. At 0.4 GPa and 350°C, the best fit is attained for a model in which the maximum temperature is attained at 1 Ma, corresponding to a vitrinite reflectance of 4.9 %Rmax; at 12 Ma the vitrinite reflectance increases to 5.8 %Rmax (outside the calibrated range of the model).

Following these observations, a Model 2 could be proposed. If cooling occurred at higher pressures, a long period of heating and slow decompression is required to inhibit or prevent suppression of vitrinite reflectance. If decompression was fast and heating took place mainly at much lower pressures, a short time span of heating is preferable (pressure retardation can be neglected). The latter Model 3 is supported by the significant Si decrease from the syn-kinematic mica (3.52 a.pf.u.) to the postkinematic mica (3.28 a.p.f.u.) (Fig. 7a). In that case, the HP minerals are pre-to synkinematic with respect to the ductile deformation (main foliation). The decrease of the celadonite content points to a decrease of pressure [START_REF] Ernst | Significance of phengitic micas from low grade schists[END_REF][START_REF] Massonne | Thermodynamic properties of white micas on the basis of high-pressure experiments in the systems K2O-MgO-Al2O3-SiO2-H2O and K2O-FeO-Al2O3-SiO2-H2O[END_REF] after the main deformation.

Support for an isothermal uplift model is provided by the first fluid inclusion assemblage, where the fluid inclusions are stretched (in the southwest) or decrepitated (towards the northeast). Once entrapped and enclosed, the pressure in any fluid inclusion is defined by the isochore valid for the fluid density and composition at the time of trapping. In the case of isothermal uplift, fluid inclusions have higher pressure than the ambient pressure. The inclusion can expand or lose a part of its content by leakage through microfissures and dislocations. This process is referred to as "explosion-decrepitation" (Van den [START_REF] Van Den Kerkhof | Fluid inclusion petrography[END_REF].

The second fluid inclusion assemblage is water-rich. In general, the ThI values cluster around 150°C and are similar at all locations irrespective of the orientation of the vein. In samples MF3022 and MF3027, fluid inclusions of heterogeneous vapor/liquid ratios (up to ≥ 0.9) and density are observed. Such fluid inclusions with small bulk densities are common for epithermal events (= boiling event) [START_REF] Hedenquist | Boiling and dilution in the shallow portion of the Waiotapu geothermal system[END_REF][START_REF] Valori | Early fluid migration in a deep part of the Larderello geothermal field: a fluid inclusion study of the granite sill from well Monteverdi 7[END_REF]. The age of this boiling event will postdate the adiabatic decompression event because the related fluid inclusions do not display any stretching or decrepitation features. No relationship between ThI and Rmax in the water-rich fluid inclusions is found. The vitrinite reflectance values, like the illite crystallinity measurements along the investigated cross section from Koumac to Ouégoa, indicate an increasing degree of metamorphism from southwest to northeast. This feature is not observed in the water-rich fluid population.

Maturation of the organic matter in New Caledonia

A comparison of the exhumation path in New Caledonia and in the Diablo Range (California) reveals contrasted trends (Fig. 11). In the Diablo Range, Dalla Torre et al.

(1996a) observed a retardation of the maturation of organic matter compared to the KI values, whereas vitrinite reflectance results from New Caledonia are consistent, with those found in LP-LT domains in the Central Alps [START_REF] Ferreiro Mählmann | The pattern of diagenesis and metamorphism by vitrinite reflectance and illite-"crystallinity" in Mittelbünden and in the Oberhalbstein Part1: The relationship to stockwerk tectonics[END_REF][START_REF] Ferreiro Mählmann | The pattern of diagenesis and metamorphism by vitrinite reflectance and illite-"crystallinity" in Mittelbünden and in the Oberhalbstein Part2: Correlation of coal petrographical and of mineralogical parameters[END_REF] (Figs. 12a &b). The strongest correlation is with those areas where the geodynamic conditions of Barrovian orogenic metamorphism predominate (heat flow 50 to 70 mW m -2 , thermal gradient 25 to 35°C km -2 , T < 400°C, pressure gradient 0.25 to 0.35 GPa km -2 , P < 4 GPa) and heating occurred over several million years (e.g. [START_REF] Frey | Very low-grade metamorphism in external parts of the Central Alps: Illite crystallinity, coal rank and fluid inclusion data[END_REF][START_REF] Ferreiro Mählmann | Zur Bestimmung von Diagenesehöhe und beginnender Metamorphose-Temperaturgeschichte und Tektogenese des Austroalpins und Süpenninikums in Vorarlberg und Mittelbünden[END_REF][START_REF] Ferreiro Mählmann | Correlation of very low grade data to calibrate a thermal maturity model in a nappe tectonic setting, a case study from the Alps[END_REF][START_REF] Schmidt | Very low grade metamorphism of the Taveyanne formation of western Switzerland[END_REF]. This, in contradiction to the HP mineral paragenesis found and the mica b cell dimensions measured. Both HP indicators are the unique petrologic remnants of the early collision during the Eocene of the New Caledonian fragment with an intra-oceanic island-arc system and subduction tectonics (e.g. [START_REF] Black | Blueschist Ophiolites in the Melange Zone, Northern New Caledonia[END_REF][START_REF] Clarke | Eclogites and blueschists of the Pam Peninsula, NE New Caledonia: a reappraisal[END_REF].

Slight differences in temperature between New Caledonia and the Diablo Range (Fig. 12a) are not enough to explain the wide differences in the evolution of organic matter. However, contrasts exist in the exhumation P-T paths of New Caledonia and the Diablo Range. In the latter, exhumation was along a cold path [START_REF] Ernst | Neoblastic jadeitic pyroxene in Franciscan metagreywackes from Pacheo Pass, central Diablo Range, California, and implications for the inferred metamorphic P-T trajectory[END_REF] reproducing the prograde pressure-temperature trajectory during retrogression.

In New Caledonia, the retrogression path was probably adiabatic (or with a slight increase of the temperature) with concomitant decrease of pressure (Fig. 11).

Prograde maturation of the organic matter during isothermal decompression appears to be a feasible explanation. In contrast, in the Diablo Range, maturity at near maximum temperature was limited to the HP-LT event. The vitrinite reflectance retardation effect by pressure was frozen due to subsequent cooling along a cold retrograde P-T path.

Comparison of K-white mica b cell dimensions in New Caledonia and the

Franciscan belt, in both cases, reveals values ranging between 9.01 and 9.07 Å, which are indicators of high-pressure conditions [START_REF] Sassi | The tetrological and geological significance of the b0 values of potassic white micas in low-grade metamorphic rocks. An application to the Eastern Alps[END_REF]. These K-white mica b cell dimensions suggest that they record the high-pressure stage, and not re-crystallization along the decompression path or a later event as suggested by the Kübler index and vitrinite reflectance values. It appears that even if the K-white mica becomes increasingly well crystallized as a response to an increase in temperature, the b cell dimension will still preserve the effect of the pressure (no modification of the b parameter). This is confirmed by comparing the calculated P-T conditions and those determined using the b cell dimensions [START_REF] Sassi | Metamorfismo[END_REF].

As shown above, maturation on an isothermal decompression path can be modeled using different time ranges of heating. The vitrinite reflectance-KI relationship, compared with the metamorphic mineral assemblages, can give another important indicator for the approximate time span of heating. As described by [START_REF] Teichmüller | Organic material and very low-grade metamorphism[END_REF], due to a higher reaction rate, coalification is more sensitive to the duration of near peak temperature heating than illite crystallinity [START_REF] Wolf | Inkolhungsuntersuchungen im Hunsrück, Rheinisches Schiefergebirge[END_REF].

Combining our KI data with the organic matter studies from [START_REF] Diessel | Coalification and Graphitization in High-Pressure Schists in New Caledonia[END_REF], the same VR-KI-graphitization trends as described above are found in the same temperature range between 250 to 400°C in the Alps [START_REF] Ferreiro Mählmann | Correlation of very low grade data to calibrate a thermal maturity model in a nappe tectonic setting, a case study from the Alps[END_REF][START_REF] Ferreiro Mählmann | Transmission electron microscopy study of carbonaceous material in a metamorphic profile from diagenesis to amphibolite facies (Bünderschiefer, Eastern Switzerland)[END_REF]. These areas in the Alps are defined by low pressures (< 0.4 GPa) and a long duration of metamorphic thermal conditions (5 to 20 Ma) or by greenschist facies metamorphism overprinting an older blueschist facies event. A long period of heating during slow exhumation will promote mineral reaction, leading illite to equilibrate with organic material. Long duration of metamorphism at constant temperature, close to steady-state thermal conditions, in an orogenic metamorphic setting [START_REF] Ferreiro Mählmann | Correlation of very low grade data to calibrate a thermal maturity model in a nappe tectonic setting, a case study from the Alps[END_REF], or during a nearly isothermal exhumation path (this study) will lead to the same KI-VR-T°C relationship. Therefore, the relationship described by Ferreiro Mählmann ( 2001) is probably generally valid for estimating the state of advancement of different methods at a specific temperature during very lowand low-grade metamorphism. In contrast to the conditions in the Alps (around 20 Ma), the period for reaching equilibrium in New Caledonia (< 12 Ma) was much shorter.

Another way to explain the correlation between KI-VR-T°C would be to document the effect of tectonic strain on the illite crystallinity. As illustrated by [START_REF] Merriman | Strain-related differences in the crystal growth of white mica and chlorite: a TEM and XRD study of the development of metapelitic microfabrics in the Southern Uplands thrust terrane, Scotland[END_REF], tectonic strain can promote dislocations and subgrain development (i.e. a decrease in KI) as stored elastic strain promotes dislocation creep. Coalification and graphitization can also be enhanced by strain, resulting in elevated vitrinite reflectance values [START_REF] Teichmüller | Organic material and very low-grade metamorphism[END_REF]Árkai et al., 2002;[START_REF] Ferreiro Mählmann | Transmission electron microscopy study of carbonaceous material in a metamorphic profile from diagenesis to amphibolite facies (Bünderschiefer, Eastern Switzerland)[END_REF]. These phenomena are evident in rocks with a tight penetrative foliation and higher amounts of deformation than that observed in our samples. Contrasting trends between mylonitic and less deformed rocks were not detected (Árkai et al., 2002).

CONCLUSIONS

The pre-, syn-and post-metamorphic/kinematic mineral assemblages, K-white mica evolution, fluid inclusion evolution and vitrinite maturity documented in the rocks from New Caledonia are compatible with a model that involves slow isothermal decompression after HP-LT metamorphism. Such P-T evolution is probably due to thermal rebound in the upper crust after stacking of nappes during subduction and collision, followed by obduction of the Poya nappe together with the ophiolite nappe on top of all other units. After transportation to deep crustal levels during initial subduction, and continuous thrusting of the HP nappes on the accretional nappe stack, exhumation in a subduction channel as described by [START_REF] Chemenda | Continental subduction and a mechanism for exhumation of high-pressure metamorphic rocks: new modelling and field data from Oman[END_REF] permitted isothermal decompression. Due to higher temperatures at greater depth during subduction, the lower viscosity of the rocks allows for fast transportation in the initial stage of exhumation. Later at lower pressure the exhumation slows down [START_REF] Burov | A thermomechanical model of exhumation of HP and UHP metamorphic rocks in Alpine mountain belts[END_REF] causing a relaxation of the geotherm. During accretion of the slices, greater heat flow resulted from emplacement of igneous intrusions and kept temperatures elevated. As the rate of exhumation slowed, re-equilibration of organic matter maturation to a typical LT-LP level was achieved. Neither pressure retardation of vitrinite reflectance, nor an enhancement of vitrinite reflectance versus Kübler index resulted. Therefore, to reproduce the maturity observed a combination of the three presented models is required. Possibilities include Model 1, lowering of the pressure of peak metamorphism; Model 2, involves a long period of heating and slow decompression, whereas Model 3 involves fast decompression and the heating occurs at lower pressure. The conditions required for Model 3 best explain the field and petrologic observations and the P-T evolution from blueschist to greenschist facies. Furthermore, this model is in agreement with the observations of [START_REF] Maurizot | Carte géologique à l'échelle du 1/50 000, feuille Pam-ouégoa[END_REF], and more recently, [START_REF] Fitzherbert | Lawsonite-omphacite-bearing metabasites of the Pam Peninsula, NE New Caledonia: Evidence for disrupted blueschist-to eclogitefacies conditions[END_REF], for higher grade metamorphosed pelites in the area. They show that the pelites of the Diahot and Pouébo terranes underwent two metamorphic events: the first a (classic) prograde pressure and temperature event, and the second at decreasing pressure and increasing temperature. KI values are in good agreement with temperatures generally proposed for the boundaries of the anchizone in metamorphic belts of medium to low-pressure gradients (Fig. 12b). Correlation between vitrinite reflectance and Kübler index values show a trend similar to that commonly observed in other low pressure metamorphic belts (Figs. 12a &b). An adiabatic decompression path during exhumation explains the concordance between the evolution of illite crystallinity and coal rank in New Caledonia; hence organic matter matures during decompression, and no effect of pressure on organic matter is preserved (Fig. 11).

This study shows that the evolution of organic matter and illite crystallinity is not only a function of the temperature and pressure gradient or the maximum temperature and pressure reached, but that it also depends on the P-T path evolution through time. [START_REF] Cluzel | Point de vue sur l'évolution tectonique et géodynamique de la Nouvelle-Calédonie (Pacifique, France)[END_REF][START_REF] Fitzherbert | Lawsonite-omphacite-bearing metabasites of the Pam Peninsula, NE New Caledonia: Evidence for disrupted blueschist-to eclogitefacies conditions[END_REF]. The inset shows the location of New Caledonia in the southwest Pacific. The Koumac-Ouégoa and RM11 road profiles are indicated (pale grey solid lines). indicates the presence of diabase. Mineral abbreviations are those of [START_REF] Kretz | Symbols for rock-forming minerals[END_REF]. New Caledonia. (This study and [START_REF] Diessel | Coalification and Graphitization in High-Pressure Schists in New Caledonia[END_REF] Eastern Alps units. [START_REF] Ferreiro Mählmann | The pattern of diagenesis and metamorphism by vitrinite reflectance and illite-"crystallinity" in Mittelbünden and in the Oberhalbstein Part1: The relationship to stockwerk tectonics[END_REF] Diablo Range, Franciscan Complex. [START_REF] Dalla Torre | Very-low temperature metamorphism of shales from the Diablo Range, Franciscan complex, California: New constraints on the exhumation path[END_REF] Vichuquén basin, Chile. (Belmar, 2000) Figure 12 Mineral abbreviations are from [START_REF] Kretz | Symbols for rock-forming minerals[END_REF]; except: KWM = K-white micas (illite, muscovite), I/S = illite/smectite, C/S = chlorite/smectite, K/S = kaolinite/smectite, Vm = vermiculite, Cor = corrensite. [START_REF] Diessel | Coalification and Graphitization in High-Pressure Schists in New Caledonia[END_REF], oxygen isotopic thermometry (after [START_REF] Black | Oxygen isotope study of metamorphic rocks from Ouégoa District, New Caledonia[END_REF]. Mineral abbreviations are after [START_REF] Kretz | Symbols for rock-forming minerals[END_REF]. Calculations are based on 11 oxygens (anhydrous basis). SD: standard deviation. b0 values calculated after [START_REF] Guidotti | Compositional controls on the a and b cell dimensions of 2M1 Muscovites[END_REF]. (1) syn-kinematic K-white micas; (2) post-kinematic K-white micas. t.i.c = total interlayer cations (Ca+Na+K). Calculations are based on 28 oxygens (anhydrous basis). SD: standard deviation. All Fe is assumed to be Fe 2+ . Temperatures are determined using the chlorite thermometer of [START_REF] Cathelineau | Cation site occupancy in chlorites and illites as a function of temperature[END_REF]. (*) Samples MF3155 and MF3023 are metabasites, others are pelites.

Figs. 2

 2 Figs. 2 & 3 show the distribution of the illite crystallinity (KI data) and the FWHM

  It is shown that the b cell dimension is a good pressure indicator surviving polyphase and prograde temperature overprints and therefore a robust method to determine pressure in LT domains. electron microprobe analyses, C. de Capitani for the introduction to the program THERIAK-DOMINO, and Ronan le Bayon for the thermodynamic data and the discussion about thermodynamic modeling. The manuscript has benefited greatly from thoughtful early help of W. G. Ernst and C. J. Hetherington for English and scientific comments. We also thank P. Black, R. Bevins, R. J. Merriman, G. L. Clarke and O. Vidal for their constructive and helpful reviews. This research was supported by grant 50-506842.99 of the Swiss National Foundation. REFERENCES Aitchison, J. C., Clarke, G. L., Meffre, S. & Cluzel, D. (1995). Eocene arc-continent collision in New Caledonia and implications for regional southwest Pacific tectonic evolution. Geology 23, 161-164. Árkai, P., Ferreiro Mählmann, R., Suchy, V., Balogh, K., Sykorová, J. & Frey, M. (2002). Possible effects of tectonic strain on phyllosilicates: a case study from the Kandersteg area, Helvetic Domain, Central Alps, Switzerland. Schweizerische Mineralogische und Petrographische Mitteilungen 82, 273-290. Árkai, P., Faryad, S.W., Vidal, O. & Balogh, K. (2003). Very low-grade metamorphism of sedimentary rocks of the Meliata unit, Western Carpathians, Slovakia: implications of phyllosilicate characteristics. International Journal of Earth Sciences 92, 68-85. Árkai, P., Sassi, F.P. & Desmons J. (2004). A systematic nomenclature for metamorphic rocks: Very low-to low-grade metamorphic rocks. Recommendations by the IUGS subcommission on the systematcics of metamorphic rocks, 11 pp. Bell, T. H. & Brothers, R. N. (1985). Development of P-T prograde and P-retrograde, in New Caledonia, as indicated by progressively developed porphyroblast microstructures. Journal of Metamorphic Geology 3, 59-78.

Fig. 1 :

 1 Fig. 1: Structural map of New Caledonia showing the major tectonic units (after

Fig. 2 :

 2 Fig. 2: Schematic cross-section between Koumac and Ouégoa modified after Rawling

Fig. 3 :

 3 Fig. 3: Distribution of Kübler index (KI) values in the area. Metamorphic

Fig. 4 :

 4 Fig. 4: (a) Distribution patterns of KI (air-dried and glycolated) as a function of

Fig. 5 :

 5 Fig. 5: Correlation between KI (10Å) and ÁI (7Å) values.

Fig. 6 :

 6 Fig. 6: Cumulative curves of K-white mica b cell dimensions for 20 samples from the

Fig. 7 :

 7 Fig. 7: (a-e) Compositional relations of metamorphic dioctahedral white micas.

Fig. 8 :

 8 Fig. 8: (a-e) Compositional variation of chlorite. Cation numbers and ratios are calculated from electron microprobe analyses. (a) Fe 2+ -Mg-Si diagram of chlorite

Fig. 9 :Fig. 10 :Fig. 11 :

 91011 Fig. 9: Pressure-temperature conditions for sample MF3031 (glaucophane zone near

Fig. 12 :

 12 Fig. 12: Comparisons of New Caledonian data with other metamorphic belts of

Table 1 :

 1 Geographical location, rock type and mineralogy of the studied samples.

	Samples Latitude Longitude Alt. (m) Rock Type	Qtz	AbKWM Chl Pg Kln Stp I/S C/S K/S Lws Gln others Terrane
	Profile between Koumac-Ouégoa
	MF2989 20°30'15'' 164°16'26''	48	marl	*	*	*			*			Hem Koumac
	MF2992 20°27'59'' 164°19'58''	60	marl	*		*			*			Hem Koumac
	MF2994 20°27'49'' 164°20'02"	80	marl	*		*	*		*			Hem Koumac
	MF2995 20°27'50'' 164°20'36"	100	pelite	*		*			*			Hem Koumac
	MF2996 20°27'42" 164°21'08"	100	pelite	*		*	*					Koumac
	MF3008 20°27'48" 164°21'12''	100	marl	*		*	*					Cor	Koumac
	MF3009 20°27'48" 164°21'12''	100	marl	*	*	*	*					Vm	Koumac
	MF2997 20°27'38" 164°21'14"	90	marl	*	*	*	*					Cor	Koumac
	MF2998 20°26'59" 164°21'34"	110	pelite	*		*	*	*	*			Koumac
	MF2999 20°26'40" 164°21'46"	120	pelite	*		*			*	*		Koumac
	MF3000 20°26'29" 164°21'44"	120	pelite	*		*			*	*		Koumac
	MF3001 20°26'38" 164°22'06"	120	pelite	*	*	*	*	*				Koumac
	MF3002 20°26'23'' 164°22'12"	140	pelite	*		*			*			Koumac
	MF3003 20°26'10" 164°22'22"	180	pelite	*	*	*	*		*			Koumac
	MF3005 20°26'08" 164°22'28"	200	pelite	*	*	*	*		*		*	Diahot
	MF3007 20°25'58" 164°22'49"	205	pelite	*	*	*			*			Diahot
	MF3014 20°25'58" 164°22'49"	205	pelite	*	*	*	*					Diahot
	MF3017 20°25'55" 164°22'51"	180	pelite	*	*	*	*	*	*			Diahot
	MF3018 20°25'55" 164°22'51"	180	pelite	*	*	*	*	*				Diahot
	MF3020 20°25'40" 164°23'02"	130	pelite	*	*	*		*	*			Diahot
	MF3024 20°25'37" 164°23'20"	100	pelite	*	*	*	*	*	*	*	*	Diahot
	MF3025 20°25'34" 164°23'24"	80	pelite	*	*	*		*		*		Diahot
	MF3026 20°25'53" 164°23'29"	60	marl	*	*	*	*				*	Diahot
	MF3027 20°25'33" 164°23'29"	60	pelite	*	*	*	*				*	Diahot
	MF3028 20°25'12" 164°24'03"	60	pelite	*	*	*				*	*	Diahot
	PS89	20°25'12" 164°24'03"	60	pelite	*		*	*	*				Carp	Diahot
	MF3029 20°24'51" 164°24'07"	80	pelite	*	*	*				*	*	*	Diahot
	MF3030 20°23'49" 164°24'29"	20	pelite	*	*	*				*	*	*	Diahot
	MF3031 20°23'33" 164°24'35"	20	pelite	*	*	*	*				*	*	Diahot
	PS131	20°22'11" 164°25'02"	20	pelite	*	*	*	*				*	*	Diahot
	MF3070 20°21'49" 164°25'13"	10	pelite	*	*	*			*	*		*	Diahot
	MF3071 20°20'31" 164°25'15"	50	pelite	*		*		*	*	*		Diahot
	Touho region
	PS11	20°43'51" 165°01'29"	20	pelite	*	*	*	*					Diahot
	PS12	20°44'11" 165°00'52"	20	pelite	*	*	*	*				*	Diahot
	PS13	20°44'26" 165°00'15"	25	pelite	*	*	*	*		*			Diahot
	PS14	20°46'50" 165°06'20"	40	pelite	*	*	*	*					Diahot
	PS15	20°47'02" 165°06'01"	40	pelite	*	*	*	*					Diahot
	PS16	20°41'50" 164°44'14"	400	pelite	*	*	*	*					Ep	Diahot
	PS29	20°47'36" 165°08'32"	20	pelite	*	*	*	*				*	Diahot
	PS30	20°47'40" 165°08'05"	10	pelite	*	*	*	*		*		*	Diahot
	PS31	20°46'57" 165°10'41"	2	pelite	*	*	*	*				*	Diahot
	PS32	20°47'55" 165°13'59"	460	pelite	*	*	*	*		*			Diahot
	PS33	20°48'03" 165°13'30"	350	pelite	*	*	*	*		*			Diahot
	PS36	20°57'09" 165°19'48"	10	pelite	*	*	*	*		*		*	Diahot
	PS37	20°58'30" 165°20'52"	80	pelite	*	*	*	*		*			Diahot
	PS54	21°02'08" 165°21'02"	5	pelite	*		*	*					Diahot
	PS55	21°02'28" 165°20'54"	20	pelite	*	*	*	*					Diahot
	PS56	21°02'33" 165°21'56"	5	pelite	*	*	*	*					Diahot
	PS57	21°00'02" 165°19'58"	20	pelite	*	*	*	*		*			Diahot

Table 2 :

 2 FWHM and ChC(002) data of the studied samples.

	Samples	FWHM	ChC(002)
	Profile between Koumac-Ouégoa
	MF2989	1.74	-
	MF2992	0.40	-
	MF2994	0.33	-
	MF2995	0.39	-
	MF2996	0.31	0.21
	MF3008	0.34	0.22
	MF3009	0.39	Weat.
	MF2997	0.30	0.18
	MF2998	0.32	-
	MF2999	0.54	-
	MF3000	0.25	-
	MF3001	Int.	0.31
	MF3002	0.24	-
	MF3003	0.35	0.30
	MF3005	0.27	0.23
	MF3007	0.23	Weat.
	MF3014	0.20	0.18
	MF3017	0.25	0.18
	MF3018	0.22	0.17
	MF3020	0.21	Weat.
	MF3024	0.20	Weat.
	MF3025	0.21	Weat.
	MF3026	0.23	0.19
	MF3027	0.17	0.16
	MF3028	0.21	Weat.
	PS89	-	-
	MF3029	0.19	Weat.
	MF3030	0.20	Weat.
	MF3031	0.17	Weat.
	PS131	-	-
	MF3070	0.16	Weat.
	MF3071	0.15	Weat.
	Touho region
	PS11	0.17	Weat.
	PS12	0.15	Weat.
	PS13	0.17	0.17
	PS14	0.14	0.16
	PS15	0.18	0.17
	PS16	0.18	0.19
	PS29	0.14	0.14
	PS30	0.16	0.16
	PS31	0.17	0.16
	PS32	0.18	0.16
	PS33	0.18	0.16
	PS36	0.18	Weat
	PS37	0.14	0.14
	PS54	0.19	0.17
	PS55	0.15	0.15
	PS56	0.20	0.18
	PS57	0.22	0.19
	FWHM = full width at half maximum intensity of the (001)
	illite-muscovite reflection. Weat. = weathering and Int =
	interference with another peak.

FWHM = full width at half maximum intensity of the (001) illite-muscovite reflection. Weat. = weathering and Int = interference with another peak.

Table 3 :

 3 Whole-rock compositions of samples used for thermodynamic modeling.

	Oxides wt% MF3031	MF2994
	SiO2	50.74	90.02
	TiO2	1.08	0.20
	Al2O3	21.03	5.00
	FeO	5.52	1.54
	MnO	0.35	0.00
	MgO	2.34	1.02
	CaO	0.84	0.00
	Na2O	2.70	0.00
	K2O	7.15	0.85
	P2O5	0.43	0.10
	L.O.I.	7.56	1.19
	Fe/(Fe+Mg)	0.70	0.60

Table 4 :

 4 Solution model used for thermodynamic modeling.

	Epidote-Clinozoisite: ideal one site mixing
	aEp = XFe M
	aCzo = XAl M
	Chlorite: ideal one site mixing
	aDaph = XFe 5
	aClin = XMg 5
	Glaucophane: ideal one site mixing
	aFe-Gln = XFe 3
	aMg-Gln = XMg 3

Table 5 :

 5 Correlation between KI, illite-muscovite polytypes transformations, vitrinite re ectance, temperature and mineral parageneses.

Table 6 :

 6 Representative chemical analyses of K-white micas.

	Sample N°	MF3031	MF3144	MF3003	PS22(1)	PS22(2)	PS32	PS89	PS54	PS56
	n	15	SD	11	SD	6	SD	19	SD	4	SD	14	SD	8	SD	21	SD	17	SD
	SiO2	50.10 0.69	50.72 0.84	45.50 0.96	52.38 0.87	48.91 1.30	51.96 0.74	47.60 1.61	50.01 1.46	49.69 0.72
	TiO2	0.10 0.12	0.09 0.07	0.25 0.19	0.08 0.08	0.15 0.22	0.07 0.07	0.10 0.16	0.06 0.05	0.04 0.06
	Al2O3	26.56 0.75	24.42 0.47	35.36 0.77	24.04 0.68	28.94 2.01	24.31 0.87	33.25 2.84	23.61 2.08	24.54 1.03
	FeO	3.87 0.35	4.49 0.67	1.75 1.03	4.37 0.42	5.39 1.71	4.50 0.57	1.25 1.65	2.79 0.80	4.83 0.66
	MnO	0.12 0.06	0.03 0.03	0.03 0.03	0.03 0.04	0.02 0.03	0.11 0.07	0.03 0.04	0.05 0.06	0.07 0.05
	MgO	2.64 0.08	3.34 0.12	0.78 0.32	3.69 0.29	2.30 0.66	3.14 0.41	2.04 0.73	4.37 0.81	2.82 0.49
	CaO	0.02 0.03	0.05 0.06	0.02 0.02	0.05 0.05	0.03 0.02	0.04 0.05	0.01 0.01	0.02 0.03	0.04 0.03
	Na2O	0.29 0.07	0.09 0.03	0.73 0.27	0.24 0.24	0.20 0.21	0.10 0.05	0.88 0.44	0.08 0.09	0.11 0.07
	K2O	10.45 0.44	9.37 0.31	9.58 0.84	10.88 0.31	10.34 1.06	10.94 0.21	9.49 0.96	10.41 0.39	10.78 0.26
	Total	94.16 0.99	92.60 0.40	94.19 1.13	95.77 0.71	96.28 0.24	95.17 0.69	94.62 1.35	91.41 0.69	92.92 0.64
	Si	3.41 0.03	3.50 0.03	3.06 0.03	3.52 0.03	3.28 0.08	3.51 0.03	3.17 0.08	3.49 0.09	3.46 0.05
	Ti	0.01 0.00	0.00 0.00	0.01 0.01	0.00 0.00	0.01 0.01	0.00 0.00	0.00 0.01	0.00 0.00	0.00 0.00
	Al IV	0.59 0.03	0.50 0.03	0.94 0.03	0.48 0.03	0.72 0.08	0.49 0.03	0.83 0.08	0.51 0.09	0.54 0.05
	Al VI	1.54 0.02	1.48 0.03	1.87 0.05	1.42 0.03	1.57 0.08	1.45 0.04	1.78 0.13	1.44 0.10	1.47 0.04
	Fe 2+	0.22 0.02	0.26 0.04	0.10 0.06	0.25 0.02	0.30 0.10	0.25 0.03	0.07 0.10	0.16 0.05	0.28 0.04
	Mn	0.01 0.00	0.00 0.00	0.00 0.00	0.00 0.00	0.00 0.00	0.01 0.00	0.00 0.00	0.00 0.00	0.00 0.00
	Mg	0.27 0.01	0.34 0.01	0.08 0.03	0.37 0.03	0.23 0.27	0.32 0.04	0.20 0.08	0.46 0.09	0.29 0.05
	Ca	0.00 0.00	0.00 0.00	0.00 0.00	0.00 0.00	0.00 0.00	0.00 0.00	0.00 0.00	0.00 0.00	0.00 0.00
	Na	0.04 0.01	0.01 0.00	0.09 0.03	0.03 0.03	0.03 0.03	0.01 0.01	0.11 0.06	0.01 0.01	0.01 0.01
	K	0.91 0.04	0.82 0.03	0.82 0.07	0.93 0.03	0.88 0.09	0.94 0.02	0.81 0.10	0.93 0.04	0.96 0.02
	t.i.c.	0.95 0.03	0.83 0.03	0.91 0.05	0.96 0.02	0.91 0.11	0.95 0.02	0.92 0.06	0.94 0.03	0.97 0.02
	b0	9.04		9.05		9.01		9.05		9.04		9.05		9.02		9.05		9.05
	KI	0.17		0.14		0.35		0.17		-		0.18		-		0.19		0.20

Table 7 :

 7 Representative chemical analyses of chlorite.

	Sample N°	MF3144	MF3155 *	MF3023 *	PS22		PS32		PS89		PS131
	n	6	SD	11	SD	15	SD	12	SD	7	SD	5	SD	4	SD
	SiO2	25.48	0.39	27.86	0.70	30.42	0.44	26.65	0.50	26.01	0.66	28.19	0.89	24.93	0.31
	TiO2	0.08	0.07	0.01	0.02	0.03	0.04	0.03	0.02	0.03	0.03	0.00	0.09	0.03	0.04
	Al2O3	17.19	0.33	17.95	0.52	16.52	0.26	18.82	0.47	18.75	0.54	23.65	0.68	20.25	0.14
	FeO	32.12	0.58	26.04	0.79	12.58	0.63	30.52	0.58	32.82	0.35	14.38	0.64	29.15	0.38
	MnO	0.45	0.04	0.31	0.06	0.14	0.03	0.28	0.09	0.43	0.08	0.14	0.07	0.72	0.10
	MgO	10.72	0.11	15.54	0.47	25.71	0.58	11.75	0.19	9.76	0.15	20.74	0.18	10.88	0.24
	CaO	0.02	0.02	0.25	0.12	0.15	0.04	0.02	0.04	0.03	0.03	0.00	0.01	0.09	0.04
	Na2O	0.02	0.02	0.01	0.02	0.03	0.02	0.05	0.08	0.04	0.02	0.03	0.02	0.02	0.01
	K2O	0.07	0.07	0.03	0.02	0.02	0.02	0.03	0.02	0.03	0.03	0.23	0.18	0.04	0.01
	Total	86.14	0.57	88.00	0.45	85.60	0.84	88.14	0.65	87.90	1.15	87.41	1.06	86.09	0.50
	Si	5.72	0.04	5.87	0.12	6.12	0.08	5.74	0.09	5.70	0.09	5.58	0.11	5.49	0.03
	Ti	0.01	0.01	0.00	0.00	0.00	0.01	0.01	0.01	0.00	0.00	0.00	0.01	0.00	0.01
	Al IV	2.28	0.04	2.13	0.12	1.88	0.08	2.26	0.09	2.30	0.09	2.42	0.11	2.51	0.03
	Al VI	2.27	0.09	2.32	0.07	2.03	0.05	2.52	0.07	2.55	0.06	3.09	0.13	2.75	0.03
	Fe 2+	6.03	0.13	4.59	0.16	2.11	0.10	5.50	0.11	6.02	0.06	2.38	0.13	5.37	0.08
	Mn	0.01	0.01	0.06	0.01	0.02	0.00	0.05	0.02	0.08	0.01	0.02	0.01	0.13	0.02
	Mg	3.59	0.05	4.88	0.14	7.71	0.14	3.77	0.06	3.19	0.04	6.12	0.14	3.57	0.07
	Ca	0.01	0.01	0.06	0.03	0.03	0.01	0.01	0.01	0.01	0.01	0.00	0.00	0.02	0.01
	Na	0.01	0.01	0.01	0.01	0.01	0.01	0.02	0.03	0.02	0.01	0.01	0.01	0.01	0.01
	K	0.02	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.06	0.05	0.01	0.00
	Fe 2+ /(Fe 2+ +Mg) 0.63	0.00	0.48	0.01	0.21	0.01	0.59	0.01	0.65	0.00	0.28	0.01	0.60	0.01
	T°C	305	7	281	19	241	12	302	14	312	9	328	17	341	5

Table 8 :

 8 Representative chemical analyses ofFe-stilpnomelane (Fe-Stp), lawsonite (Lws),Fe-glaucophane (Fe-Gln) andMg-carpholite (Mg-Carph).

	Mineral		Fe-Gln	Fe-Gln		Lws		Lws		Lws		MgCarph	Fe-Stp	Fe-Stp	Fe-Stp
	Sample N°		MF3031	PS131		MF3031	PS131	MF3144		PS89	MF3144	PS32	PS22
	n	23	SD	6	SD	4	SD	4	SD	15	SD	15	SD	19	SD	9	SD	7	SD
	SiO2	54.51 1.28	55.63 0.64	37.74 0.96	37.70 1.06	38.93 1.50	38.64 0.78	45.25 1.47	45.18 1.61	46.92 0.85
	TiO2	0.09 0.08	0.11 0.08	0.17 0.17	0.23 0.18	0.11 0.14	0.18 0.16	0.04 0.05	0.07 0.07	0.10 0.11
	Al2O3	10.24 0.51	11.98 0.29	31.45 1.46	32.03 0.42	28.99 1.33	32.84 0.54	6.14 0.52	6.57 0.40	6.59 0.41
	FeO	18.45 1.01	16.21 0.28	0.44 0.05	0.40 0.09	0.37 0.17	6.45 0.62	28.62 0.83	29.41 0.84	27.45 1.64
	MnO	0.38 0.13	0.29 0.12	0.04 0.04	0.03 0.03	0.04 0.04	0.12 0.08	1.17 0.09	1.10 0.11	0.62 0.06
	MgO	4.48 0.24	5.55 0.16	0.01 0.01	0.02 0.02	0.05 0.07	9.16 0.39	5.19 0.34	4.85 0.26	6.62 0.31
	CaO	0.41 0.18	0.32 0.17	16.92 0.30	16.79 0.30	16.39 0.55	0.01 0.01	0.03 0.02	0.03 0.02	0.12 0.08
	Na2O	6.74 0.19	7.13 0.07	0.02 0.01	0.03 0.01	0.26 0.10	0.02 0.01	0.31 0.34	0.23 0.13	0.26 0.34
	K2O	0.09 0.17	0.02 0.02	0.05 0.04	0.07 0.08	0.09 0.13	0.03 0.04	4.30 1.37	3.47 1.35	2.54 1.48
	Total	95.39 0.71	97.24 0.48	86.92 2.09	87.28 1.04	85.04 1.55	87.44 0.63	91.06 0.95	90.90 0.72	91.21 1.53
	Si	7.95 0.14	7.87 0.05	2.02 0.03	2.00 0.04	2.13 0.07	2.00 0.03	8.00	-	8.00	-	8.00	-
	Ti	0.01 0.01	0.01 0.01	0.01 0.01	0.01 0.01	0.00 0.01	0.01 0.01	0.01 0.01	0.01 0.01	0.01 0.01
	Altot	1.76 0.09	2.00 0.05	1.98 0.04	2.01 0.03	1.87 0.06	2.00 0.03	1.28 0.11	1.37 0.11	1.33 0.09
	Al IV	0.11 0.05	0.13 0.05	-

ACKNOWLEDGEMENTS

This manuscript benefits greatly from the comments of M. Wilson and N.T. Arndt. We thanked S. Th. Schmidt for her helpful assistance and discussion concerning