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

Very low-grade metamorphic study in the pre-Late Cretaceous terranes of New Caledonia (SW Pacific Ocean)

Keywords: phyllosilicates, illite crystallinity, New Caledonia, very low-grade metamorphism, hig-pressure/low-temperature

Very low-grade metamorphic study in the pre-Late Cretaceous terranes of New Caledonia (southwest Pacific Ocean)

INTRODUCTION

Since the first study by [START_REF] Brothers R | 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. This Eocene metamorphism has been the subject of numerous petrological studies [START_REF] Brothers R | Lawsonite-Albite Schists from Northernmost New Caledonia[END_REF][START_REF] Brothers R | 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 high-pressure/lowtemperature 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 eclogite-facies conditions[END_REF], showing an increase of grade toward the NE with an evolution from prehnite-pumpellyite-to eclogite-facies. A recent low-grade metamorphic study [START_REF] Potel | Very low-grade metamorphic evolution of pelitic rocks under high-pressure/low-temperature conditions, NW New Caledonia (SW Pacific)[END_REF] characterized through a multimethod investigation of phyllosilicates, fluid inclusion and organic matter the metamorphic phyllosilicate evolution in the Eocene HP metamorphic belt.

Less well studied are the pre-Late Cretaceous terranes in the central part of New Caledonia. [START_REF] Routhier P | Etude géologique du versant occidental de la Nouvelle-Calédonie entre le col de Boghen et la pointe d'Arama[END_REF] distinguished two events: one Pre-Permian (high pressure/low temperature) and one post Jurassic and pre-Senonian (greenschist facies). [START_REF] Guérangé | Données nouvelles concernant la stratigraphie, la sédimentologie, la pétrologie et la structure de la chaîne centrale de la Nouvelle-Calédonie[END_REF] demonstrated that the second metamorphic event locally reached blueschist facies. [START_REF] Paris | Géologie de la Nouvelle-Calédonie, un essai de synthèse[END_REF] reinterpreted the second event as being a sub-greenschist to greenschist facies and associated the presence of lawsonite to a local increase of the pressure in the vicinity of fault zones. A recent study by [START_REF] Cluzel | L'unité de la Boghen (Nouvelle-Calédonie, Pacifique sudouest) : un complexe d'accrétion jurassique. Données radiochronologiques preliminaries U-Pb sur les zircons détritiques[END_REF] demonstrated that the "pre-Permian basement" (pre-Late Cretaceous terranes) was of Jurassic age and only affected by a Late Jurassic HP metamorphism (ca 150 Ma). The main problem remains to differentiate and to characterize the metamorphism in the region due to the presence of the same index minerals in the Eocene belt and in the Central Chain, together with the scarcity of index minerals in the Central Chain.

In low-temperature/high-pressure (LT/HP) metamorphic belt, the characterization of metamorphic conditions is difficult. Only metabasites or Al-rich metapelites will develop the indicative index minerals. The common mineral assemblage in metapelites is essentially composed of quartz, illite/muscovite and chlorite. Therefore the study of reaction progress and development of clay minerals are useful tools to describe the thermal and barometric evolution during metamorphism. In the case of polymetamorphic areas, the characterization of phyllosilicates is the only method to decipher the metamorphic zonation.

The aim of this paper is to present the results of the first investigation of the very low-grade metapelites in the Central Chain, using the phyllosilicate crystallinities, chlorite thermometry and b cell dimensions of K-white micas. These data should help us to decipher the different metamorphic zonation and gradients in the central part of New Caledonia. Therefore, the results are compared with the existing data set obtained by [START_REF] Potel | Very low-grade metamorphic evolution of pelitic rocks under high-pressure/low-temperature conditions, NW New Caledonia (SW Pacific)[END_REF] in the Eocene HP metamorphic belt.

REGIONAL GEOLOGY

New Caledonia is a dispersed fragment of the eastern margin of Gondwana resulting from the opening of the Tasman Sea by 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][START_REF] Aitchison | Eocene arc-continent collision in New Caledonia and implications for regional southwest Pacific tectonic evolution[END_REF][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-dipping subduction system was established beneath the Loyalty Basin. The collision, responsible for the main deformation of the continental basement of New Caledonia, generated a HP 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]) (a mafic sheet comprising dolerite, tectonized tholeiitic pillow basalts and abyssal argillites) and the peridotite nappe (an ophiolite complex) (Fig. 1). The basement is composed of terranes of Paleozoic to Mesozoic age of varying metamorphic grade, transgressively overlain by upper Cretaceous to Eocene sedimentary rocks deposited during Gondwana dispersal [START_REF] Aitchison | Eocene arc-continent collision in New Caledonia and implications for regional southwest Pacific tectonic evolution[END_REF][START_REF] Clarke | Eclogites and blueschists of the Pam Peninsula, NE New Caledonia: a reappraisal[END_REF]. The basement is affected by Late Jurassic high-pressure metamorphism (ca 150 Ma, [START_REF] Cluzel | L'unité de la Boghen (Nouvelle-Calédonie, Pacifique sudouest) : un complexe d'accrétion jurassique. Données radiochronologiques preliminaries U-Pb sur les zircons détritiques[END_REF]. The late Eocene HP metamorphism observed in the northern part of New Caledonia overprinted the northern part of these terranes [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]. The Late pre-Cretaceous terranes (basement) comprised (Fig. 1): (1) the "Boghen terrane", an accretionary complex formed during the Jurassic period along the East-Gondwana active margin [START_REF] Cluzel | L'unité de la Boghen (Nouvelle-Calédonie, Pacifique sudouest) : un complexe d'accrétion jurassique. Données radiochronologiques preliminaries U-Pb sur les zircons détritiques[END_REF]) is a metamorphosed volcanosedimentary pile composed of pillow basalt, radiolarian chert, mafic and felsic tuffs and minor carbonatic sediments [START_REF] Cluzel D | Affinités intra-océaniques des métavolcanites de l'unité de la Boghen (ex-Anté-Permien de Nouvelle-Calédonie, Pacifiue sud-ouest). Conséquences paleogéographiques[END_REF]; (2) the "Central Chain terrane", volcanosedimentary arc terrane of mid-Triassic to late Jurassic age [START_REF] Meffre | The development of arc-related ophiolites and sedimentary sequences in New Caledonia[END_REF]; (3) the "Koh terrane ophiolites", a dismembered Late Carboniferous fore-arc ophiolite [START_REF] Aitchison | Regional implications of U/Pb SHRIMP age constraints on the tectonic evolution of New Caledonia[END_REF]; (4) the "Téremba-Moindou terrane" of Permian-to Late Jurassic age, composed of volcano-sedimentary rocks [START_REF] Aitchison | Regional implications of U/Pb SHRIMP age constraints on the tectonic evolution of New Caledonia[END_REF]. A transgressive sequence of Upper Cretaceous (Late Cenomanian to Late Eocene) coal measures and conglomerates ("formation à charbon"; [START_REF] Paris | Géologie de la Nouvelle-Calédonie, un essai de synthèse[END_REF] overlies the late pre-Cretaceous basement with angular unconformities. This unit provides a minimum age for the timing of amalgamation of New Caledonia basement terranes to Gondwana [START_REF] Aitchison | Regional implications of U/Pb SHRIMP age constraints on the tectonic evolution of New Caledonia[END_REF].

MATERIALS AND METHODS

This study is focused on the Central Chain and Boghen terranes and the "Formation à charbon" (Fig. 1). Samples for this study were collected from 86 localities (Table 1) in the Central Chain (53) and Boghen (16) terranes and in the "formation à charbon" (17).

In New Caledonia, outcrops are poor due to extensive tropical weathering, limiting the availability of fresh samples mainly to road cuts. 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 ANALYSIS

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]. For the XRD analyses of ≤2µm fractions, oriented samples, which were airdried, solvated with ethylene glycol and heated (550°C) were prepared. Illite and chlorite crystallinity was measured on air-dried and glycolated 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 (10 Å) and expressed in ∆°2 [START_REF] Kübler B | Evaluation quantitative du métamorphisme par la cristallinité de l'illite. Etat des progrès réalisés ces dernières années[END_REF]. Illite crystallinity values were transformed into Kübler index values using a correlation with the standard samples of Warr & Rice 8 (1994) (KI CIS = 1.343 * IC Basel -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. 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.

MINERAL CHEMISTRY

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.

RESULTS

MINERALOGY

The mineralogy of the studied samples is given in Table 1. Mineral abbreviations are those of [START_REF] Kretz | Symbols for rock-forming minerals[END_REF]. Pelites, tuffs and metamarls were analyzed. In Figure 2, the distribution of the index minerals lawsonite, glaucophane and stilpnomelane is represented.

The meta-marls were sampled in the three units. Generally, the mineral assemblages consist of Qtz + K-white mica + Chl + Ab + Cal. In the Boghen terrane, Ep and Gln are present in addition, and in the Central Chain terrane Stp is also observed (Fig. 2). In their <2 µm grain-size fraction, illite-muscovite predominates in association with Chl and quartz. Albite is only present in minor amounts.

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) (Fig. 2). In their <2 µm fractions, illite-muscovite or phengitic muscovite predominate in the higher-grade samples. Discrete paragonite is also found in minor quantities and margarite is observed in sample MF3100.

In the Central Chain terrane, the meta-tuffs were sampled in the region of Touho.

Their mineral assemblages are composed of Qtz + Ab + Chl + K-white mica ± Stp (Fig. 2). The <2µm grain-size fraction is dominated by illite-muscovite and chlorite.

Figure 3 shows the distribution of the illite crystallinity (KI data). The FWHM values are presented in Table 1.

In the Central Chain terrane, metamorphic values range between high anchizone and epizone (Table 1, Fig. 4a). A peak in the distribution of KI-values is observed at epizonal values (Fig. 4a). The highest KI value (sample MF3110) is due to the presence of discrete paragonite in great amount, leading to a broad 10Å peak.

In the Boghen terrane the KI was determined on 15 samples (Fig. 4b). All of these samples yield epizonal metamorphic conditions (Fig. 4b) (Table 1).

The 16 KI values from the "formation à charbon" range from epizone to low diagenesis (Table 1 and Fig. 4c). Samples from the south are lower metamorphosed with only diagenetical conditions (Fig. 3). A trend of increasing grade from low anchizone to high anchizone (limit epizone, sample MF3080) is observed in the region between Poindimié and Ponérihouen (Fig. 3).

A significant positive linear correlation (r 2 = 0.65) is found between Kübler and Árkai indices (Fig. 5), rending 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. Overall, the pattern of very low-grade metamorphism revealed by the chlorite crystallinity (Fig. 6) agrees well with the pattern of the illite crystallinity (Fig. 3). No diagenetic ÁI values are observed due to the presence of kaolinite in samples where diagenetical KI values were measured.

In northern New Caledonia, [START_REF] Potel | Very low-grade metamorphic evolution of pelitic rocks under high-pressure/low-temperature conditions, NW New Caledonia (SW Pacific)[END_REF] deduced temperature limits for anchizone based on oxygen isotopic thermometry. The anchizone is bracketed between 230 ± 10°C and 295 ± 10°C, which is in good agreement with the approximate boundary found in the literature. Consequently, temperature estimates for the investigated samples ranged between ca. 230 and 350°C (0.16 < KI < 0.42 ∆°2). Only in south of the "formation à charbon", the temperatures were lower than 230°C (KI > 0.42 ∆°2).

Mineral chemistry

Due to the small grain size of the phyllosilicates, special care was taken to differentiate between detrital and newly formed metamorphic grains.

Representative compositions of K-white mica are listed in Table 2. Mica analyses showing more than 0.5% (MnO %+ TiO 2 %) 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 2, the total interlayer charge (t.i.c. = Ca+Na+2K) is between 1.00 and 0.94, except for sample PS46b, where a Prl substitution could be assumed (Figs. 7a & 7d) in addition to the Tschermark's substitution. Additional substitutions are required to explain the excess of Fe and Mg in samples PS54, PS56b, PS69, PS85 (Figs. 7a & 7b). 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]. Figure 7c shows a correlation between the Fe and Mg contents in the individual samples (probably dependant on the bulk composition) except for samples PS46b. On the basis of model calculations supposing various mixtures (Fig. 7b), no contamination of the analyses by quartz or chlorite has occurred.

In Figure 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 0.85 and 1.00 p.f.u.). Figure 7e shows that the Na content of K-white mica is very low 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 = 5), we investigated the relation between the metamorphic grade of our samples with the chemistry of the Kwhite micas. Figure 7f 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.

The Na/(Na+K) ratios are lower than 0.15 in the analyzed samples permitting to calculate K-white mica b cell dimension parameters using the regression equations from [START_REF] Guidotti | Compositional controls on the a and b cell dimensions of 2M1 Muscovites[END_REF]. [START_REF] Kisch | The b0 lattice parameter and chemistry of phengites from HP/LT metapelites[END_REF] outlined the possibility to overestimate or underestimate the b 0 in the case of Fe-free Phengite or Fe-rich celadonite. Here, the Fe 2+ /(Si-6) ratio is in the range where the regression equations can reasonably used and this up to ca. 3.55 Si p.f.u.. Compositions of chlorite are normalized to 28 oxygens (Table 3) and fulfill the criterion of non contamination, ∑Ca+Na+K < 0.2 [START_REF] Dalla | Chlorites textures and composition from high pressure/low temperature metashales and metagraywackes, Franciscan complex, Diablo Range, California[END_REF]. Using the classification of Zane & [START_REF] Weiss | A procedure for classifying rock-forming chlorites based on microprobe data[END_REF], chlorites in the studied samples are trioctahedral type I where (X Mg + X Fe ≥ X Al + X vacancy ). X Mg varies between 0.37 and 0.93, 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. 8a). 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. 8b) and apparent octahedral vacancies with sum of octahedral divalent cations (Fig. 8c) prove the sudoitic substitution in the analyzed chlorites. Figure 8d 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 266 and 327°C (Table 3). These temperatures are in good agreement with those deduced from the KI values.

Based on [START_REF] Sassi R | The baric character of the Patagonian basement as deduced from the muscovite d 060,331 spacing: a first contribution from Eastern Andean Metamorphic Complex (Andes, Chile)[END_REF], the temperature derived from the chlorite and the K-white b cell dimension values are used to estimate the P conditions in samples PS56b and PS46b. In Figure 9, pressure can be estimated around 1.1 GPa for PS46b and 1.3 GPa for PS56b. In comparison, we give the P and T conditions obtained in the glaucophane zone of the Eocene HP metamorphic belt (up to 1.5 GPa and 410°C; [START_REF] Potel | Very low-grade metamorphic evolution of pelitic rocks under high-pressure/low-temperature conditions, NW New Caledonia (SW Pacific)[END_REF].

DISCUSSION

Mineral assemblages observed in the studied samples are compatible with the mineral boundary drawn by [START_REF] Paris | Géologie de la Nouvelle-Calédonie, un essai de synthèse[END_REF] (Fig. 2). Lawsonite and stilpnomelane are observed in the Central Chain terrane, in the region of Poindimié and in the southern part of the studied area. In the region of Poindimié, lawsonite is also present in the "formation à charbon". Glaucophane minerals are found in samples from the Boghen terrane in the region of the "col des Roussettes". The metamorphic conditions observed by the illite and chlorite crystallinity are very similar to those indicated by the mineral facies. In the lawsonite Eocene zone (Figs. 3 & 6), the metamorphic conditions indicated by the phyllosilicates crystallinity (KI and ÁI) are in good agreement with those observed in the Eocene HP belt to the north [START_REF] Potel | Very low-grade metamorphic evolution of pelitic rocks under high-pressure/low-temperature conditions, NW New Caledonia (SW Pacific)[END_REF]. The KI values indicate epizonal conditions, except in the "formation à charbon" where values are anchizonal (but still compatible with the lawsonite occurrence). There, an increasing metamorphic gradient from south to north is observed (Fig. 3).

In the south of the studied area, the "formation à charbon" reaches only diagenetical conditions (IC ≥ 0.42 ∆°2) and kaolinite is present in all samples, contrasting with higher metamorphic conditions in the Boghen and Central Chain terranes. In the profile 2 (Figs. 2 & 10), metamorphic contrast between "formation à charbon" and Central Chain terrane is observed with the presence of diagenetical samples in the proximity of epizonal ones. An increasing metamorphic gradient can be observed with a decrease of the KI and ÁI values in the Boghen terrane from NE towards SW (profile 1, Figs. 2 & 10). Due to the lack of samples in the Central Chain this gradient cannot clearly observed. An increase of the values in the middle of the Boghen terrane could be explained by the presence of a thrust fault. In profile 2 (Fig. 10) due to the low density of the sample (only two samples), the trend in each terrane is not so clear with an apparent metamorphic inversion in the Chain Central terrane and an iso-metamorphism in the Boghen terrane. However, the phyllosilicate crystallinities are in good agreement with the previous metamorphic studies based on mineral assemblages, indicating an increasing of metamorphism from NE toward SW [START_REF] Cluzel | L'unité de la Boghen (Nouvelle-Calédonie, Pacifique sudouest) : un complexe d'accrétion jurassique. Données radiochronologiques preliminaries U-Pb sur les zircons détritiques[END_REF]. In the north of the studied area (Central Chain terrane), metamorphic conditions reach anchizone (Fig. 3) with the presence of stilpnomelane (no lawsonite was observed, Fig. 2).

Overall in the studied area, K-white b cell dimensions (Table 2) range between 9.04 and 9.05 Å indicating high-pressure conditions [START_REF] Merriman | Very low-grade metapelites: mineralogy, microfabrics and measuring reaction progress[END_REF].

However, different metamorphic evolution can be observed. In the Eocene lawsonite zone, it is observed that the temperature (chlorite thermometry) is higher along the eastern coast as to the west. Moreover, in the "formation à charbon" another gradient is observed with increase of metamorphic conditions from S to N as indicated by illite crystallinity. These observations lead to the definition in the Eocene lawsonite zone of a metamorphic gradient increasing from SW towards NE (Fig. 11), like in the northern part of New Caledonia [START_REF] Potel | Very low-grade metamorphic evolution of pelitic rocks under high-pressure/low-temperature conditions, NW New Caledonia (SW Pacific)[END_REF]. In the Nimbaye Valley (Fig. 11), south of the Eocene lawsonite zone, the chlorite thermometry indicates higher temperature (T = 312 ± 12°C, PS46b) than northern in Eocene (T = 298 ± 8°C, MF3100). In this Valley, we observed an increase of metamorphic grade (decrease of the KI and ÁI values) from NE towards SW, i.e. inverse to the gradient observed in the Eocene Zone. Based on all these results, an approximate limit between the Eocene metamorphism and the Late Jurassic metamorphism can be drawn (Fig. 11), similar to that drawn by [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].

The metamorphic pattern revealed by the study of the phyllosilicate is compatible with the model given for the evolution of the central New Caledonia. The diagenetic values observed in the "formation à charbon" (Senonian age?) in the south confirmed the Pre-Late Cretaceous age of the metamorphism in this region (150 Ma, K-Ar on Glaucophane, Blake & Brothers (1977)). This metamorphism is of highpressure type (K-white b cell dimensions higher than 9.04 Å) with an increasing gradient from NE towards SW. The later Eocene metamorphism affected only the north of the central area, leading to the drawn of an approximate limit based on the metamorphism observed in the "formation à charbon" and the temperature values calculated on chlorite. The increase in metamorphic grade from SW to NE within this region is similar to that observed in the north of New Caledonia [START_REF] Potel | Very low-grade metamorphic evolution of pelitic rocks under high-pressure/low-temperature conditions, NW New Caledonia (SW Pacific)[END_REF]. The Touho fault (a dextral strike slip fault) separates this region from the late Eocene formation (Fig. 11, [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 moved it in its actual position.

AKNOWLEDGMENENTS

This research was supported by grant 50-506842.99 of the Swiss National Foundation.

Prof. W. Franke and M. P. Doublier are thanked for their help in the correction of the manuscript. et al., 1994[START_REF] Fitzherbert | Lawsonite-omphacite-bearing metabasites of the Pam Peninsula, NE New Caledonia: Evidence for disrupted blueschist-to eclogite-facies conditions[END_REF]. The inset shows the location of New Caledonia in the southwest Pacific. 1988). In comparison, the stability field of sample MF3031 (cross hatched area) of the HP Eocene belt is represented [START_REF] Potel | Very low-grade metamorphic evolution of pelitic rocks under high-pressure/low-temperature conditions, NW New Caledonia (SW Pacific)[END_REF]. Iso K-white micas b cell dimension curves (dashed lines) are taken from [START_REF] Sassi R | The baric character of the Patagonian basement as deduced from the muscovite d 060,331 spacing: a first contribution from Eastern Andean Metamorphic Complex (Andes, Chile)[END_REF]. Mineral abbreviations are those of [START_REF] Kretz | Symbols for rock-forming minerals[END_REF]. 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]. (*) Sample PS81 is a metabasite, others are pelites.

Fig. 1 :

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

Fig. 2 :

 2 Fig. 2: Distribution of index minerals (Glaucophane, lawsonite and Fe-stilpnomelane)

Fig. 3 :

 3 Fig. 3: Distribution of Kübler index (KI) values in the studied area. The Peridotites and

Fig. 4 :

 4 Fig. 4: Histograms of illite crystallinity (KI) in the different terranes. (a) Central Chain

Fig. 5 :Fig. 6 :

 56 Fig. 5: Correlation between KI (10Å) and ÁI (7Å) values. n = 46.

Fig. 7 :

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

Fig. 8 :

 8 Fig. 8: (a-d) Compositional variation of chlorite. Cation numbers and ratios are

Fig. 9 :

 9 Fig. 9: Pressure-temperature conditions for samples PS46b and PS56b using K-white

Fig. 10 :

 10 Fig. 10: Schematic cross-sections along two profiles (Fig. 2). Average values of Kübler

Fig. 11 :

 11 Fig. 11: Schematic geological map, the Peridotites and Poya Nappes are coloured in

Table 1 :

 1 Geographical location, rock type and mineralogy of the studied samples. FWHM and ChC(002) data of the studied samples.

	Sample

Latitude Longitude Alt in m. Rock type Qtz Ab Ms Chl Pg Kln Stp ML others

	MF3125 21°29'32" 165°47'50"	140	pelite	* *	* *			I/S	Senonian	0.52 -
	MF3126 21°17'53" 165°34'48"	30	pelite	*	*	* *		I/S	Senonian	0.93 Kln
	MF3127 21°17'28" 165°33'10"	40	pelite	* *	*	*			Cent. Chain 0.17 Kln
	MF3128 21°17'19" 165°32'53"	20	pelite	* *	* *				Terrane Cent. Chain 0.19 0.22 IC ÁI
	MF3077 20°56'10" 165°21'10" MF3129 21°17'38" 165°31'56"	20 40	pelite pelite	* * *	* * *	*		C/S Lws	F. charbon -Cent. Chain 0.20 0.20 0.17
	MF3078 20°56'10" 165°21'00" MF3130 21°18'00" 165°28'41"	20 40	pelite pelite	* * *	* * * *	* *			F. charbon 0.20 -Cent. Chain 0.24 0.18
	MF3080 21°00'15" 165°24'00" MF3131 21°18'30" 165°27'31"	40 40	pelite pelite	* * *	* * * *	*		C/S Lws	F. charbon 0.20 -Cent. Chain 0.20 0.26
	MF3081 21°00'50" 165°22'50" MF3132 21°19'42" 165°26'48"	10 50	pelite marl	* * * *	* * * *	*		Lws Ep+Cal	F. charbon 0.26 0.16 Boghen 0.23 0.21
	MF3082 21°00'21" 165°20'51" MF3133 21°22'49" 165°26'11"	40 180	pelite marl	* * *	* * * *	*		Cent. Chain 0.24 0.21 Ep+Cal+Gln Boghen 0.22 0.18
	MF3083 21°01'32" 165°23'10" MF3135 21°23'15" 165°24'24"	20 120	pelite pelite	* * * *	* * *	* *			F. charbon 0.23 Kln Boghen 0.21 0.21
	MF3085 21°02'29" 165°23'52" MF3136 21°01'32" 165°22'12"	20 40	pelite pelite	* * * *	* * *	*	*	C/S Lws	F. charbon 0.23 -Cent. Chain 0.20 -
	MF3086 21°02'45" 165°23'19" MF3137 21°01'38" 165°21'55"	20 60	marl pelite	* * * *	* * * *			Cal Lws	F. charbon 0.13 0.19 Cent. Chain 0.17 0.15
	MF3087 21°02'45" 165°23'59" MF3138 21°02'43" 165°17'42"	20 5	marl pelite	* *	* *			Cal	F. charbon 0.10 0.14
	MF3088 21°04'20" 165°24'50"	5	pelite	* *	*	* *			F. charbon 0.27 Kln
	MF3089 21°05'12" 165°25'40"	5	pelite	* *	*	* *		K/S	F. charbon 0.27 Kln
	MF3090 21°08'51" 165°28'11"	5	pelite	* *	* *				Cent. Chain 0.11 0.18
	MF3098 20°58'38" 165°11'21"	40	pelite	* *	* *	*		C/S Lws	Cent. Chain 0.18 -
	MF3099 20°58'20" 165°11'53"	30	pelite	* *	* *				Cent. Chain 0.17 0.16
	MF3100 20°57'45" 165°13'48"	20	pelite	*	*			Mrg+Ep	Cent. Chain -	0.17
	MF3101 20°58'22" 165°10'51"	30	pelite	*	*	* *		I/S	Cent. Chain 0.22 Kln
	MF3102 20°58'12" 165°11'49"	20	pelite	*	* *	*			Cent. Chain 0.19 0.18
	MF3103 20°58'00" 165°11'38"	20	pelite	* *	* *			Lws	Cent. Chain 0.19 0.16
	MF3105 20°52'10" 165°14'40"	60	pelite	* *	* *		*		Cent. Chain 0.14 0.21
	MF3108 20°52'36" 165°14'18"	20	pelite	*	* *	*			Cent. Chain 0.29 0.16
	MF3110 20°53'18" 165°13'58"	20	pelite	* *	*	* *			Cent. Chain 0.49 Kln
	MF3115 20°53'18" 165°10'12"	10	pelite	* *	* *			Lws+Ep	Boghen	0.16 0.14
	MF3118 20°55'12" 165°04'52"	110	pelite	* *	* *	*		Lws+Ep	Boghen	0.18 0.17
	MF3119 21°19'59" 165°38'51"	20	pelite	*	*	*			Senonian	0.49 -
	MF3120 21°20'18" 165°39'03"	40	pelite	*	*	*			Senonian	1.13 Kln
	MF3121 21°20'28" 165°39'23"	40	pelite	* *	*	* *			Senonian	0.74 Kln
	MF3122 21°20'52" 165°40'02"	140	pelite	*	* *	*		C/S	Senonian	0.43 Kln
	MF3123 21°21'32" 165°40'32"	340	pelite	* *	* *	*			Senonian	0.43 Kln

Table 1 :

 1 (continued)

	Sample

Latitude Longitude Alt in m. Rock type Qtz Ab Ms Chl Pg Kln Stp ML others

	PS66	21°23'29" 165°39'40"	60	marl	* *	* *		Cal	Cent. Chain 0.18 0.19
	PS67	21°23'28" 165°39'28"	70	pelite	* *	* *		Lws	Cent. Chain 0.18 0.20
	PS68	21°23'28" 165°39'28"	70	pelite	* *	* *		Lws	Cent. Chain 0.17 0.19
									Terrane	IC	ÁI
	PS69	21°22'48" 165°38'23"	110	pelite	* *	* *	*		Cent. Chain 0.16 0.17
	PS28	20°57'20'' 165°13'50"	10	pelite	* *	* *			Cent. Chain 0.18 0.15
	PS70	21°22'58" 165°38'41"	95	pelite	* *	* *			Cent. Chain 0.21 0.18
	PS35	20°56'42'' 165°19'32"	20	pelite	* *	* *			Cent. Chain 0.19 0.17
	PS71	21°23'49" 165°39'00"	80	pelite	* *	* *		Ep+Cal	Boghen	0.15 0.14
	PS36	20°57'09" 165°19'48"	10	pelite	* *	* *		Lws	Cent. Chain 0.18 0.27
	PS72	21°24'25" 165°38'41"	80	pelite	*	* *			Boghen	-	0.28
	PS37	20°58'30" 165°20'52"	80	pelite	* *	* *	*		Cent. Chain 0.14 0.14
	PS73	21°24'41" 165°38'19"	90	pelite	* *	* *		Lws	Boghen	0.21 0.17
	PS38	21°04'41" 165°22'48"	20	pelite	* *	* *			Cent. Chain 0.22 0.18
	PS74	21°25'30" 165°38'12"	100	pelite	* *	* *			Boghen	0.15 0.15
	PS39	21°04'59" 165°20'42"	5	tuff	* *	* *			Cent. Chain 0.20 0.15
	PS75	21°25'52" 165°38'08"	100	pelite	* *	* *		Cal	Boghen	0.15 0.14
	PS40	21°04'59" 165°20'42"	5	tuff	* *	* *			Cent. Chain 0.26 0.17
	PS77	21°20'20" 165°26'25"	40	pelite	* *	* *		Ep+Cal	Boghen	0.14 0.15
	PS41	21°04'48" 165°20'28"	40	tuff	* *	* *	*		Cent. Chain 0.19 0.16
	PS78	21°20'50" 165°25'32"	80	pelite	* *	* *		Ep	Boghen	0.13 0.13
	PS44	21°06'10" 165°19'26"	10	tuff	* *	* *	*		Cent. Chain 0.19 0.16
	PS79	21°21'08" 165°25'31"	85	pelite	* *	* *		Ep+Cal	Boghen	0.20 0.17
	PS45	21°06'19" 165°18'20"	40	tuff	* *	* *	*		Cent. Chain 0.24 0.16
	PS82	21°23'21" 165°24'20"	160	pelite	* *	* *			Boghen	0.15 0.13
	PS46b	21°06'03" 165°17'51"	80	tuff	* *	* *			Cent. Chain 0.19 0.16
	PS84	21°22'40" 165°25'59"	180	pelite	* *	* *		Gln+Cal	Boghen	0.12 0.14
	PS47	21°05'55" 165°17'48"	90	pelite	*	* *	*		Cent. Chain -	0.15
	PS85	21°22'42" 165°26'00"	200	pelite	* *	* *		Gln+Ep	Boghen	0.23 0.15
	PS48	21°08'32" 165°27'00"	5	pelite	* *	* *			Cent. Chain 0.23 0.33
	PS49	21°08'45" 165°25'56"	25	pelite	* *	* *	*		Cent. Chain 0.17 0.20
	PS50	21°09'20" 165°25'38"	3	marl	* *	* *	*	Cal	Cent. Chain 0.28 0.24
	PS51	21°09'20" 165°25'38"	3	marl	* *	* *	*	Cal	Cent. Chain 0.22 0.21
	PS52	21°11'32" 165°26'38"	40	pelite	* *	* *			Cent. Chain 0.23 0.29
	PS53	21°10'23" 165°23'38"	20	pelite	* *	* *			Cent. Chain 0.23 0.19
	PS54	21°02'08" 165°21'02"	5	pelite	*	* *			Cent. Chain 0.19 0.17
	PS55	21°02'28" 165°20'54"	20	pelite	* *	* *			Cent. Chain 0.15 0.15
	PS56b	21°02'33" 165°21'56"	5	pelite	* *	* *			Cent. Chain 0.20 0.19
	PS57	21°00'02" 165°19'58"	20	pelite	* *	* *	*		Cent. Chain 0.23 0.19
	PS58	21°09'52" 165°27'53"	20	pelite	* *	* *			Cent. Chain 0.20 0.20
	PS59	21°10'11" 165°27'39"	25	pelite	* *	* *			Cent. Chain 0.23 0.24
	PS60	21°10'23" 165°27'22"	20	pelite	* *	* *			Cent. Chain 0.22 0.24
	PS61	21°10'58" 165°26'41"	20	pelite	* *	* *			Cent. Chain 0.19 0.20
	PS62	21°11'31" 165°26'38"	20	pelite

Mineral abbreviations are from

[START_REF] Kretz | Symbols for rock-forming minerals[END_REF]

; except: KWM = K-white micas (illite-muscovite). ML = mixed-layer; I/S = illite/smectite. C/S = chlorite/smectite. K/S = kaolinite/smectite.

Table 2 :

 2 Representative chemical analyses of K-white micas.

	Sample N°	PS46b		PS54		PS56b	PS69	PS85
	n	11	SD	18	SD	17	SD	12	SD	7	SD
	SiO 2	50.33 0.57	50.48 0.98	49.72 0.74	51.90 0.55	52.55 0.63
	TiO 2	0.10 0.09	0.06 0.06	0.05 0.06	0.13 0.14	0.14 0.18
	Al 2 O 3	28.14 1.09	23.10 0.68	24.56 1.05	23.75 0.69	23.48 0.41
	FeO	3.78 0.42	2.80 0.81	4.82 0.68	4.63 0.47	4.20 0.41
	MnO	0.09 0.08	0.05 0.06	0.07 0.05	0.05 0.06	0.06 0.05
	MgO	2.06 0.37	4.49 0.48	2.78 0.47	4.63 0.23	4.31 0.24
	CaO	0.06 0.07	0.02 0.02	0.04 0.03	0.05 0.04	0.09 0.10
	Na 2 O	0.10 0.06	0.06 0.02	0.11 0.28	0.12 0.16	0.09 0.03
	K 2 O	9.86 0.10	10.41 0.56	10.77 0.26	11.03 0.27	11.44 0.38
	Total	94.52 0.42	91.47 0.73	92.91 0.66	95.56 0.77	96.35 0.56
	Si	3.39 0.04	3.52 0.03	3.46 0.05	3.50 0.03	3.52 0.03
	Ti	0.00 0.00	0.00 0.00	0.00 0.00	0.01 0.01	0.01 0.01
	Al IV	0.61 0.04	0.48 0.03	0.54 0.05	0.50 0.03	0.48 0.03
	Al VI	1.62 0.04	1.42 0.05	1.47 0.04	1.39 0.03	1.37 0.03
	Fe 2+	0.21 0.02	0.16 0.05	0.28 0.04	0.26 0.03	0.23 0.02
	Mn	0.00 0.00	0.00 0.00	0.00 0.00	0.00 0.00	0.00 0.02
	Mg	0.21 0.04	0.47 0.05	0.29 0.05	0.39 0.02	0.43 0.03
	Ca	0.00 0.01	0.00 0.00	0.00 0.00	0.00 0.00	0.01 0.01
	Na	0.01 0.01	0.01 0.00	0.01 0.01	0.02 0.02	0.01 0.00
	K	0.85 0.01	0.93 0.05	0.96 0.02	0.95 0.02	0.98 0.03
	t.i.c.	0.86 0.01	0.94 0.05	0.97 0.02	0.97 0.02	1.00 0.02
	b 0	9.04		9.05		9.05		9.05		9.06
	KI	0.25		0.25		0.26		0.21		0.30

Calculations are based on 11 oxygens (anhydrous basis). SD: standard deviation. b 0 values calculated after

[START_REF] Guidotti | Compositional controls on the a and b cell dimensions of 2M1 Muscovites[END_REF]

. t.i.c = total interlayer cations (Ca+Na+K).

Table 3 :

 3 Representative chemical analyses of chlorite.

	Sample N°	MF3100	MF3128	PS46b		PS54		PS56b		PS81*
	n	13	SD	17	SD	8	SD	7	SD	7	SD	5	SD
	SiO 2	29.29	0.43	25.68	0.41	26.09	0.42	28.42	0.65	25.06	0.49	26.16	0.55
	TiO 2	0.01	0.03	0.04	0.06	0.06	0.05	0.06	0.03	0.11	0.11	0.09	0.13
	Al 2 O 3	20.54	0.20	17.33	0.39	19.41	0.14	17.20	0.35	19.17	0.49	18.19	0.57
	FeO	3.79	1.01	31.76	0.60	30.41	0.43	19.96	0.32	32.19	0.53	28.29	1.24
	MnO	0.27	0.04	0.64	0.06	0.43	0.08	0.57	0.11	0.50	0.09	0.49	0.07
	MgO	29.45	0.74	9.98	0.36	10.87	0.31	19.35	0.37	9.39	0.22	13.82	0.51
	CaO	0.05	0.02	0.13	0.04	0.08	0.16	0.03	0.02	0.05	0.04	0.18	0.11
	Na 2 O	0.01	0.01	0.01	0.01	0.03	0.03	0.01	0.01	0.04	0.03	0.05	0.04
	K 2 O	0.02	0.02	0.05	0.02	0.03	0.03	0.03	0.05	0.14	0.07	0.02	0.02
	Total	83.42	0.76	85.63	0.77	87.40	0.81	85.62	0.73	86.66	0.78	87.29	1.12
	Si	5.76	0.05	5.79	0.07	5.68	0.04	5.96	0.09	5.58	0.10	5.65	0.09
	Ti	0.00	0.00	0.01	0.01	0.01	0.01	0.01	0.01	0.02	0.02	0.01	0.01
	Al IV	2.24	0.05	2.21	0.07	2.32	0.04	2.04	0.09	2.42	0.10	2.35	0.09
	Al VI	2.53	0.03	2.39	0.06	2.65	0.03	2.22	0.07	2.62	0.05	2.29	0.08
	Fe 2+	0.62	0.17	5.99	0.12	5.53	0.09	3.50	0.05	5.99	0.08	5.11	0.22