Anne-Aziliz Pelleter
email: anne-aziliz.pelleter@univ-orleans.fr

Martial Caroff
email: caroff@univ-brest.fr

Carole Cordier

Patrick Bachelery

Pierre Nehlig

Delphine Debeuf

Nicolas Arnaud

Melilite-bearing lavas in Mayotte (France): an insight into the mantle source below the Comores

Keywords: Comores, ocean island basalts, olivine melilitite, HIMU, mantle heterogeneity, metasomatism

Introduction

Olivine melilitites are alkaline and ultracalcic basic igneous rocks rich in melilite, a calcic sorosilicate, and devoid of feldspar. They are thought to be produced by low degree of partial melting of a CO 2 -rich mantle source in both continental and oceanic contexts [START_REF] Gudfinnsson | Continuous gradiations among primary carbonatitic, kimberlitic, melilititic, basaltic, picritic and komatiitic melts in equilibrium with garnet lherzolite at 3-8 GPa[END_REF][START_REF] Dasgupta | Partial melting experiments of peridotite + CO 2 at 3 GPa and genesis of alkali ocean island basalts[END_REF]. In continental zones, they are found at or near rifting zones, often in association with carbonatites. In the African continent, (olivine) melilitites have been observed in Sahara [START_REF] Dautria | Carbonate metasomatism in the lithospheric mantle: peridotitic xenoliths from a melilititic district of the Sahara basin[END_REF], East African Rift [START_REF] Bailey | Melilitite at Fort Portal, Uganda. Another dimension of the carbonate volcanism[END_REF][START_REF] Dawson | Nephelinite-melilitite-carbonatite relationships: Evidence from Pleistocene-recent volcanism in northern Tanzania[END_REF][START_REF] Mattsson | Petrogenesis of the melititic and nephelinitic rock suites in the Lake Natron -Engaruka monogenetic volcanic field, northern Tanzania[END_REF], South Africa [START_REF] Rogers | Phlogopite in the generation of olivine-melilitites from Namaqualand, South Africa and implications for element fractionation processes in the upper mantle[END_REF][START_REF] Janney | A chemical and multiisotope study of the Western Cape olivine melilitite province, South Africa: implications for the sources of kimberlites and the origine of the HIMU signature in Africa[END_REF], and Madagascar [START_REF] Melluso | Petrogenesis and Nd-, Pb-, Sr-isotope geochemistry of the Cenozoïc olivine melilitites and olivine nephelinites ("ankaratrites") in Madagascar[END_REF]. In oceanic setting, the olivine melilitites are produced in intraplate volcanoes [START_REF] Maaløe | The Koloa volcanic suite of Kauai, Hawaii[END_REF][START_REF] Hoernle | The petrology of the tholeiites through the melilite nephelinites on Gran Canaria, Canary Islands: crystal fractionation, accumulation and depths of melting[END_REF]. They correspond generally to post-erosional, late units [START_REF] Brey | Origin of olivine melilititeschemical and experimental constraints[END_REF]. In the vicinity of the Africa continent, olivine melilitites have been collected in Gran Canaria [START_REF] Brey | Origin of olivine melilititeschemical and experimental constraints[END_REF][START_REF] Hoernle | The petrology of the tholeiites through the melilite nephelinites on Gran Canaria, Canary Islands: crystal fractionation, accumulation and depths of melting[END_REF] and Cape Verde Islands [START_REF] Brey | Origin of olivine melilititeschemical and experimental constraints[END_REF]. Several authors have highlighted the chemical resemblance, for incompatible trace element and isotopic ratios, between (olivine) melilitites and ocean island basalts (OIB), sometimes with a HIMU (or high μ, where μ = 238 U/ 204 Pb) affinity [START_REF] Rogers | Phlogopite in the generation of olivine-melilitites from Namaqualand, South Africa and implications for element fractionation processes in the upper mantle[END_REF][START_REF] Wilson | Melilitites: partial melts of the thermal boundary layer?[END_REF][START_REF] Janney | A chemical and multiisotope study of the Western Cape olivine melilitite province, South Africa: implications for the sources of kimberlites and the origine of the HIMU signature in Africa[END_REF].

The Comores Archipelago, composed of four volcanic islands (Mayotte, Anjouan, Moheli, and Grande Comore) is located in the Mozambique Channel between the continental blocks of Africa and Madagascar (Fig. 1). The initial volcanism period of the oldest island -Mayotteonto the ocean floor is estimated to be ca. 15-10 Ma [START_REF] Nougier | The Comores archipelago in the western Indian Ocean: volcanology, geochronology and geodynamic setting[END_REF], whereas the Karthala volcano of Grande Comore (the youngest island) is still active.

The mantle beneath the Comores is probably metasomatized, with modal amphibole and/or phlogopite [START_REF] Späth | The geochemistry of lavas from the Comores Archipelago, western Indian Ocean: petrogenesis and mantle source region characteristics[END_REF][START_REF] Class | Plume-lithosphere interactions in the ocean basins: constraints from the source mineralogy[END_REF]. In fact, [START_REF] Coltorti | Carbonatite metasomatism of the oceanic upper mantle: evidence from clinopyroxenes and glasses in ultramafic xenoliths of Grande Comore, Indian Ocean[END_REF] evidenced a CO 2 -related cryptic metasomatism episode beneath Grande Comore. The origin of the Comores volcanism is attributed by [START_REF] Hajash | Paleomagnetic and radiometric evidence for the age of the Comores Islands, West Central Indian Ocean[END_REF] and [START_REF] Emerick | Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics[END_REF] to the upwelling of a mantle plume. [START_REF] Nougier | The Comores archipelago in the western Indian Ocean: volcanology, geochronology and geodynamic setting[END_REF] proposed that the Comorian magmas derive from the upper mantle, beneath an ocean-continent transitional crust structured by old and deep lithospheric fractures, which were reactivated during late Tertiary. Four Comores lavasone sample from Moheli and the three analyzed samples from Anjouan [START_REF] Reisberg | Os isotope systematics in ocean island basalts[END_REF][START_REF] Späth | The geochemistry of lavas from the Comores Archipelago, western Indian Ocean: petrogenesis and mantle source region characteristics[END_REF][START_REF] Salters | Isotope constraints on mantle evolution[END_REF] have a more radiogenic Pb composition than the other Indian OIB. They display an isotopic signature close to that of the Atlantic islands near the western African coast, as Cape Verde and Canary Islands (Sushchevskaya et al., 2013).

The present study of Mayotte Island (Comores Archipelago), based on new petrological and geochemical data, reveals the coexistence of a large number of melilitebearing (including olivine melilitites) and HIMU-like lavas (with 206 Pb/ 204 Pb > 20). Mayotte Island is thus a good example for studying the genesis of melilite-bearing lavas and their relation with a specific mantle composition.

Geological setting

Mayotte is the oldest island of the Comores archipelago (Fig. 1), emplaced, while intraplate, in an active tectonic area. The present-day geography of the area is the consequence of the Permian-Trias Karoo NW-SE rifting, which resulted in the separation between Gondwana and Indian-Madagascan continental blocks [START_REF] Malod | Nouvelles contraintes sur la cinématique de Madagascar : les structures de la chaîne de Davie[END_REF][START_REF] Piqué | L'évolution géologique de Madagascar et la dislocation du Gondwana : une introduction[END_REF]. The oceanic basins of Somalia and Mozambique opened during the Jurassic, while the Madagascar Island drifted southwards along the Davie Ridge. The main part of the sedimentary products in the Mozambique Channel results from the erosion of the Davie Ridge surrected during an Eocene compression phase [START_REF] Leclaire | La ride de Davie dans le canal de Mozambique : approche stratigraphique et géodynamique[END_REF]. Since the middle Miocene, the tectonic regime in the Mozambique Channel is dominated by E-NE-W-SW extension, also identified in the East African Rift and in Madagascar [START_REF] Bertil | Seismotectonic of Madagascar[END_REF][START_REF] Piqué | L'évolution géologique de Madagascar et la dislocation du Gondwana : une introduction[END_REF].

Although the volcano morphology in the Comores archipelago is rather consistent with a chronological progression of the volcanism northwestwards (freshest reliefs in Grande Comore; presence of a coral reef only in Mayotte and, to a less extend, in Moheli: Fig. 1), the K-Ar geochronological data of the archipelago suggest a more complex history (Fig. 1). With exception of Grande Comore, in which old lavas do not crop out, each island displays a Neogene volcanic activity (shield stage?) together with a more recent one. The oldest subaerial lavas of the archipelago are found in Mayotte, consistently with its eroded morphology and the presence of a large coral reef. Only one coarse-grained rock of Anjouan is older (11.1±0.5 Ma, [START_REF] Nougier | The Comores archipelago in the western Indian Ocean: volcanology, geochronology and geodynamic setting[END_REF], very different from the shield lavas (from 3.9±0.3 Ma to 1.5±0.2 Ma, [START_REF] Nougier | The Comores archipelago in the western Indian Ocean: volcanology, geochronology and geodynamic setting[END_REF]. However, no dating is available for the rocks constituting the submarine flanks of the islands. It is therefore very difficult to reasonably establish, or not, a chronological progression of volcanism throughout the archipelago.

Mayotte is located 300 km westward of Madagascar and 70 km southwestward of Anjouan (Fig. 1). It is constituted of two main volcanic islands (Grande Terre and Petite Terre: 348 km 2 and 16 km 2 in surface, respectively) and around twenty islets, within a lagoon of 1100 km 2 bordered by a 160 km-long coral reef (Fig. 2a). Grande Terre is a north-south lengthened island, culminating at 660 m (Bénara Mount). The emerged reliefs correspond to the superstructure of two coalescent eroded shield volcanoes, subsiding for 1.5 Ma [START_REF] Debeuf | Etude de l'évolution volcano-structurale et magmatique de Mayotte (Archipel des Comores, Océan Indien[END_REF][START_REF] Nehlig | Notice de la carte géologique de Mayotte à 1/25 000[END_REF]. Petite Terre is a 5 km-long island located 4 km eastward of Grande Terre (Fig. 2d).

Sampling and analytical techniques

Sampling

The rocks presented here were collected during three field trips, in 2001 (sampling: D.

Debeuf and P. Bachelery), 2010 and 2011 (sampling: P. Nehlig, F. Lacquement, and J.

Bernard, with the financial and logistic support of the French BRGM).

Mayotte Island is characterized by an intense lateritic weathering and an important plant cover. Consequently, the main part of the volcanic substratum cannot be sampled.

Although only the freshest rocks have been collected, a residual meteoric alteration induces relatively high loss on ignition (LOI) values in several samples. As all the samples having a LOI > 4.6 wt.% are affected by significant argillaceous / hydroxidic alteration, we have rejected them for the geochemical study.

Among about a hundred of samples collected in 2001, 24 lavas have been dated; 51 samples have been analyzed for major and trace elements, 17 for 87 Sr/ 86 Sr, 23 for 143 Nd/ 144 Nd isotopes, and 8 for Pb isotopes. Among the 35 samples collected in 2010 and 2011: 19 samples have been analyzed for major and trace elements, 10 for 87 Sr/ 86 Sr isotopes, and 14 for Nd and Pb isotopes. A selection of geochronological and chemical data is shown in Tables 1,2, and 3. The complete dataset is given in the supplementary Table S1.

40 Ar/ 39 Ar datations

Bulk rock samples (flows, dykes and domes) were dated by the 40 Ar/ 39 Ar method.

Analyses were performed on the 150-250 µm-size groundmass fraction. For each sample, one or more aliquots on different separates (typically feldspar and volcanic glass) were irradiated on January 2002 at the McMaster reactor, Ontario, in the 5C position for 26 h under a 10 18 neutrons cm -2 s -1 flux. The irradiation standard is a Fish Canyon sanidine standard with an age of 28.02 ± 0.16 Ma [START_REF] Renne | Intercalibration of standards, absolute ages and uncertainities in Ar 40 /Ar 39 dating[END_REF], with reproductive values within 0.6%.

Ages were obtained at the University of Montpellier 2, following a protocol similar to that described in [START_REF] Arnaud | Evidence for Mesozoic shear along the western Kunlun and Altyn-Tagh fault, northern Tibet (China)[END_REF] and [START_REF] Garcia | Rift jump process in northern Iceland since 10 Ma from 40 Ar/ 39 Ar geochronology[END_REF]. Details on the analytical conditions are available from the authors upon request. Age given from age spectra are weighted mean plateaus [START_REF] Fleck | Interpretation of discordant 40 Ar/ 39 Ar age spectra of Mesozoic tholeiites from Antarctica[END_REF] or simple mean ages. The latter case corresponds to the situation where either subsequent steps overlap only at the 1 level or total 39 Ar cumulated from these steps is insufficient to define a classical plateau. All errors from age spectra take the error on the J factor into account. Plateau criterions being rarely achieved, sometimes simple mean age is given. The isochron ages are obtained in an inverse isochron diagram of 36 Ar/ 40 Ar versus 39 Ar/ 40 Ar [START_REF] Roddick | The evolution of excess argon in alpine biotitesan 40 Ar-39 Ar analysis[END_REF], which allows homogeneous excess components to be individualized in many occasions. Errors on age and intercept age include individual errors on each point and linear regression by York's method [START_REF] York | Least square fitting on a straight line with correlated errors[END_REF].

The goodness of it relative to individual errors is measured by Mean Square Weighted Deviation (MSWD).

Mineral and whole rock analyses

Mineral compositions have been determined using a Cameca SX-100 electron microprobe (Microsonde Ouest, Brest, France) with an acceleration voltage of 15 kV and a beam current of 20 nA. Some high precision analyses of olivine were acquired using a JEOL JXA-8230 electron microprobe (ISTerre, Grenoble, France) with an acceleration voltage of 25 kV and a beam current of 900 nA.

Major and trace element analyses for the rocks sampled in 2001 were determined at the CRPG (Centre de Recherches Pétrographiques et Géochimiques, Nancy, France), whereas samples collected in 2010-2011 were analyzed at the PSO/IUEM (Pôle Spectrométrie Océan, Institut Universitaire Européen de la Mer, Brest, France). Whole-rock major elements were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) following the analytical method described by [START_REF] Cotten | Origin of anomalous rare-earth elements and yttrium enrichments in sub-aerially exposed basalts: evidence from French Polynesia[END_REF]. Relative standard deviations are < 2% (< 1% for SiO 2 ). Whole-rock trace elements were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) following the sample preparation and analysis method of [START_REF] Barrat | Determination of rare earth elements in sixteen silicate reference samples by ICP-MS after Tm addition and ion exchange separation[END_REF]. Relative standard deviations are < 5% for most of the trace elements and < 10% for Nb, Gd, Tb, Ta, Ni, Zn and Rb. The concentrations found in the Mayotte OIB are well above the detection limits for all the trace elements analyzed.

Sr and some Nd isotopic ratios were measured by thermal ionization mass spectrometry (TIMS, Laboratoire Magmas et Volcans, Clermont-Ferrand, France and PSO/IUEM/IFREMER, Brest, France), while Pb and some Nd isotopic ratios were measured on a multicollector ICP-MS (PSO/IFREMER, Brest, France). Samples were leached in 2N HCl for about one hour and rinsed three times in ultrapure water to remove weathering effect.

The protocol for element separation is described in [START_REF] Chauvel | Comprehensive chemical and isotopic analyses of basalt and sediment reference materials[END_REF]. Measured blanks are lower than 20 picograms (pg) for Nd, 60 pg for Sr and 100 pg for Pb (except one analysis session with 300 pg blank). Values used for mass fractionation corrections and relative standard deviations are given in Table 3.

Geochronology of Mayotte

Four volcanic domains have been recognized on Mayotte Island from geological and geomorphological observations, ages available in the literature [START_REF] Hajash | Paleomagnetic and radiometric evidence for the age of the Comores Islands, West Central Indian Ocean[END_REF][START_REF] Emerick | Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics[END_REF][START_REF] Nougier | The Comores archipelago in the western Indian Ocean: volcanology, geochronology and geodynamic setting[END_REF]McDougall unpublished in Debeuf, 2004), and new 40 Ar/ 39 Ar ages obtained on 24 samples (Table 1).

The most erodedand thus oldestarea is the southern part of the main island (Fig. 2b), where the hydrographic network is intensely erosive and most headlands correspond to inverted reliefs. The main building volcanic phase (> 10.6-~3.0 Ma) is mainly formed by alkali basalt, basanite and olivine nephelinite lava flows (Fig. 2c), strongly weathered to lateritic products. This phase ended with the intrusion of around fifteen phonolitic protrusions, associated with nephelinitic/phonolitic lava flows and pyroclastic density currents (3.9-3.1 Ma). The post-shield phase began with the emplacement of the tephriphonolitic/phonolitic Combani massif circa 2.4 Ma ago. The last events in the south domain are emission of pyroxene-bearing basanitic and nephelinitic lava flows (ca. 2.0-~1.5 Ma).

The northwest domain is characterized by sharp reliefs of moderately weathered basaltic-basanitic lava flow piles, contemporaneous to the southern main building phase (> 7.1-~4.0 Ma). These lavas constitute a second shield volcano, which would have emerged after the southern one (Stieljes, 1988). Lavas are cut by a basaltic intrusive network (sills and N-S-trending dykes). This volcanism was followed by an extension-related mafic magmatism, at about ~4.0-< 1.0 Ma, which renewed the reliefs. The two melilititic lava flows were probably emplaced during this event. Phonolitic domes (3.3 and 2.5 Ma) are especially rare (Fig. 2c).

The north central sector of Grande Terre is characterized by the least eroded, youngest reliefs (Fig. 2b). This is consistent with lava ages mainly comprised between 2.4 and 0.8 Ma, i.e. more recent than in both south and northwest domains. Two older samples have been dated at 3.4 Ma [START_REF] Hajash | Paleomagnetic and radiometric evidence for the age of the Comores Islands, West Central Indian Ocean[END_REF]) and 3.35 Ma [START_REF] Emerick | Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics[END_REF]), but they have not been analyzed for major and trace elements. From west to east, the north central sector is composed of: (1) the Digo massif, made of basaltic-tephritic lava flows; and (2) the Mtsapéré stratovolcano (basaltic-tephritic lava flows intruded by phonolitic protrusions, Fig. 2c), lying on the northwest shield formations.

The fourth volcanic zone is a recent explosive complex comprising small pyroclastic edifices (tuff rings and cones) built by phreatomagmatism and strombolian eruptions along the east coast of Grande Terre and in Petite Terre (Fig. 2c). The activity is estimated from 500 000 to 150 000 years in Grande Terre and up to 4 000 years in Petite Terre, where the volcanic edifices lay on a Holocene barrier reef younger than 9000 years [START_REF] Camoin | Late glacial to post glacial sea levels in the Western Ocean[END_REF]. The subaerial explosive volcanoes lay on lateritized lava flows in Grande Terre. The first strombolian phase built edifices with interbedded lava flows and pyroclastic levels (scoriaceous blocks and lapilli, and ash). Then, the volcanism became phreatoplinian, with pumice-bearing tuff ring structures. Two of them crop out in Grande Terre and four in Petite Terre (three in the eastern part and a fourth in the southernmost area, Fig. 2c).

Petrology

Lavas and hypabyssal rocks

The mineral assemblage of the studied lavas is presented in Fig. 3. The petrographic nomenclature is based on mineralogy, total alkali-silica (TAS) and basanite-nephelinitemelilitite classification schemes of Le [START_REF] Bas | A chemical classification of volcanic rocks based on the total alkali-silica diagram[END_REF] and Le [START_REF] Bas | Nephelinitic and basanitic rocks[END_REF], respectively (Figs. Tephritic and tephriphonolitic lavas occur in the south and north central domains.

They have a porphyric fluidal microlitic texture. Phenocrysts are zoned augite and kaersutite, whereas groundmass includes plagioclase, clinopyroxene, Fe-Ti oxides, hastingsite and apatite. Tephriphonolites can be distinguished from tephrites by the presence of both Kfeldspar as phenocrysts and arfvedsonite and aegirine-augite as microcrysts.

Phonolites occur as protusions or as derived lava flows, mainly in the south and north central domains and, sparsely, in the northwest one. They have a porphyritic microlitic texture, sometimes fluidal. Diopside, hornblende, K-feldspar, nepheline, and apatite occur as 1 1 phenocrysts and microcrysts. Moreover, the groundmass contains titanite, aegirine-augite, ferro-arfvedsonite, ferro-eckermannite and Fe-Ti oxides.

Pyroclastites and cumulates

Two types of pyroclastic formations have been identified: stombolian and phreatoplinian deposits, mainly located in the most recent domains of Mayotte (NE coast of Grande Terre and Petite Terre), and only sparsely represented in the older north central and south domains (Fig. 2c). Elements of the primary pyroclastic breccias are locally reworked as volcanogenic sedimentary deposits.

The strombolian edifices are small reddened scoria cones, composed of basaltic indurated lapilli-and bomb-sized fragments alternating with lava flows and ash levels.

Locally, blocks of sedimentary rock (sandstone, arkose) are embedded in the breccias. The fragments bear iddingsitized olivine, pyroxene, nepheline and Fe-Ti oxides.

The phreatoplinian deposits are constituted of meter-sized interbedded sequences of pumice levels and hyaloclastite layers. Pumices range in size from 5 to 50 cm. They have a vesicular hyaloporphyric texture, with euhedral phenocrysts of K-feldspar, hornblende, and Fe-Ti oxides. Among the xenoliths, fragments of coral, lavas and scoria, dunitic and pyroxenitic cumulates are present. The basanitic xenolith PN5b, cumulative in olivine and clinopyroxene, has been found in the pumice sample PN5a.

Cumulates have been collected as xenoliths or blocks. They are dunites, pyroxenites, essexites and ijolites.

Geochemistry

1 2

Major elements

The subsequent geochemical study focuses on lavas and hypabyssal rocks (dykes and protrusions) of Grande Terre, together with pumices of Petite Terre. Major and trace element compositions of representative samples from Mayotte are listed in Table 2. The TAS diagram of Le [START_REF] Bas | A chemical classification of volcanic rocks based on the total alkali-silica diagram[END_REF] (Fig. 4) can be used to evidence a large alkaline association in Mayotte from alkali basalts (only four analyzed samples), basanites and foidites to phonolites through intermediate lavas. Two trends are observed (Fig. 4): the first one, including mainly the south lavas, one basalt apart (M41), is highly silica-undersaturated; the second one, including the rocks from the three other domains, is moderately silica-undersaturated. Mafic rocks (MgO > 5 wt.%) are divided into three groups on the basis of silica-undersaturation, alkalinity and calcium content: basalts/basanites, olivine nephelinites and olivine melilitites (Fig. 5). Most of the nephelinites and olivine melilitites are gathered in the foidite field in the TAS diagram (Fig. 4). In general, silica-undersaturation increases from north central lavas to south shield lavas through northwest shield lavas (Fig. 4). Eleven olivine nephelinites and the two olivine melilitites contain CIPW normative larnite ( N larnite: Ca 2 SiO 4 ), with contents higher than 10 % normative for olivine melilitites (Fig. 5), consistently with the criterion proposed by [START_REF] Wooley | Classification of lamprophyres, lamproites, kimberlites and th kalsitic, melilitic and leucitic rocks[END_REF] to define this type of lavas. According to [START_REF] Médard | Liquidus surfaces of ultracalcic primitive melts: formation conditions and sources[END_REF], olivine melilitites can be considered as ultracalcic lavas (CaO > 13.5 wt.% and CaO/Al 2 O 3 > 1 wt.%).

Plots of CaO

Trace elements

We investigate in Figs. 6e-h (4) Amongst the south domain samples, the figure 6 diagrams can be used to discriminate four lavas, all of them located in the post-shield Combani massif. They plot apart in both Nb and Hf vs. Th diagrams (Fig. 6e andf), together with the Petite Terre pumices,.

(5) The northwest domain, mainly constituted of shield mafic lavas, cannot be easily distinguished in the figure 6 trace diagrams. The corresponding lavas plot either in the north central trend (Fig. 6a andf) or with the southern samples (Fig. 6e andg). Post-shield olivine melilitites M66 and FL386, presented above, belong to this domain.

4

Mafic samples (alkali basalts, basanites, olivine nephelinites and olivine melilitites) from Mayotte are enriched in light and middle REE (LREE and MREE, respectively) with respect to heavy REE (HREE), consistently with their OIB characteristics (Fig. 7a). Slope of the REE patterns increases from basalts/basanites to olivine nephelinites and to olivine melilitites. These latter are also slightly more enriched in middle MREE. In the primitive mantle-normalized multi-element diagram (Fig. 7b), patterns of the mafic lavas are bell shaped, as expected for OIB, and show strong negative anomalies in Rb, K and Pb, and, to a lesser extent, in U and Ti. Olivine melitititesespecially FL 386display high values in U, Sr, P, Nd, Sm, Eu and Gd. Olivine-free nephelinites have, on average, higher REE contents than the mafic rocks (Fig. 7c). Phonolites have characteristic concave-up REE patterns, with low MREE values, and LREE and HREE compositions close to those of mafic and intermediate rocks. The pumices of Petite Terre, which have the highest REE contents amongst the phonolites, show a negative anomaly in Eu. Olivine-free nephelinites and all evolved rocks display negative anomalies in P and Ti (Fig. 7d). Moreover, phonolites (lavas and pumices) are depleted in Ba and phonolitic pumices also in Sr.

Isotopes

A selection of Sr and Nd isotopic ratios and all the Pb data are given in Table 3. In the show isotope ratios between these two groups but closer to north central lavas. This isotopic distribution seems to roughly reflect the volcanological history of the island. In general, lavas showing the most radiogenic Pb ratios have been sampled in the oldest parts of the island (> 3.0 Ma), whereas the other samples belong to formations having an age < ~2.5 Ma. The only exceptions are the HIMU-flavored M25 and DMM-like M137 lavas, samples respectively dated to 2.0 and 5.0 Ma but for which no plateau age could be calculated (Table 1). There is also an uncertainty about the olivine melilitites (M66 and FL386), the age of which ranges from 4.0 to < 1.0 Ma. Even if the variations in the 143 Nd/ 144 Nd vs. 206 Pb/ 204 Pb diagram are less contrasted, similar trends can be observed. The 143 Nd/ 144 Nd ratios of the rocks from the shield volcanoes, highly radiogenic in Pb, are more scattered and in average lower than those of the other samples. The 87 Sr/ 86 Sr ratio is less discriminative (Table 3).

When compared to other Comores Islands and mantle end-member (Depleted MORB-Mantle: DMM; high 238 U/ 204 Pb:HIMU; Enriched Mantle: EM1 and 2, of [START_REF] Zindler | Chemical geodynamics[END_REF] isotope compositions, the most recent lavas from Mayotte tend towards the depleted mantle, such as all the lavas produced by the La Grille volcano in Grande Comore and two samples from Moheli. In contrast, samples collected in the south and northwest shield volcanoes have a clear HIMU affinity, such as one sample from Moheli and all the lavas from Anjouan that have been analyzed for Pb isotopes.

The south alkali basalt M41, that spatially belongs to south group but that shows less silica-undersaturated composition, displays a Sr-Nd composition close to that of the Karthala volcano in Grande Comore, which tends toward the EM end-members (Table 3): its 143 Nd/ 144 Nd ratio is low (0.51272) and its 87 Sr/ 86 Sr ratio especially high (0.70383). M41 is relatively fresh (LOI=1.1) and it has low Rb/Sr ratio (0.5) suggesting that its high 87 Sr/ 86 Sr ratio is a pristine characteristic. M41 sample has not been analyzed for Pb isotopes. As EM-1 6 like composition is restricted to this sample in Mayotte, sampling date and location have been thoroughly checked and confirm the expression of EM mantle component in Mayotte.

Spatial and temporal distribution of the geochemical characteristics

The lavas from the main building phase of the shield volcanoes (south and northwest domains) have more radiogenic Pb compositions than the post-shield rocks (Combani massif, north central domains and Petite Terre).

Shield units

The lavas from the south shield are among the highest silica-undersaturated of the island (Fig. 4) including olivine-bearing and olivine-free nephelinites, basanites and phonolites. They have the highest CaO contents together with the northwestern melilitites (Fig. 6a), and include most of the N larnite samples (Table 2). The south shield lavas have a HIMU affinity, with high Pb isotope ratios (Fig. 8).

Most of the analyzed samples from the northwest shield are moderately silicaundersaturated mafic lavas, mainly basanites, with characteristic low incompatible trace element contents. They show a HIMU affinity (Fig. 8). The basanitic xenolith PN5b from Petite Terre (Table 2) shares the trace and isotopic signature of the northwest shield lavas.

These features suggest that this sample could correspond to a lava from the northwest shield substratum, extracted during an explosive event and incorporated within the pumices.

Post-shield units

The only olivine melilitites of Mayotte are emplaced in the northwest domain during a post-shield extensional phase.

The post-shield lavas from the north central domain are moderately silicaundersaturated. They follow linear evolutionary trends in major and trace element diagrams 1 7 from basalts/basanites to phonolites through tephrite/tephriphonolites (Fig. 6). All the petrological types have a similar isotopic composition (e.g., Nd isotope ratios ranging from 0.51288 to 0.51290, Fig. 8b). Such features suggest that chemical variations in north central lavas reflect differentiation processes from melts generated from a rather homogeneous mantle source, more depleted than for the other lavas.

The Combani evolved samples follow the highly silica-understurated trend in the TAS diagram (Fig. 4). They have, however, lower CaO with respect to highly silica-undersaturated intermediate and felsic lavas of the south domain (Fig. 6a). Both groups have also similar behavior in HREE vs. Th diagrams (Fig. 6g and Table 2), reflecting probably comparable modalities of crystal fractionation. However, the Combani lavas are close to those of the north central domain in Hf vs. Th (Fig. 6f) and they have specific high Nb contents (Fig. 6e).

Amongst the post-shield lavas, they have the most radiogenic 206 Pb/ 204 Pb values (Fig. 8).

The pumices of Petite Terre are phonolitic in composition. They belong to the moderately silica-undersaturated trend in the TAS diagram (Fig. 4) and plot in a restricted area of the trace element diagrams (Fig. 6e-g), away from all the other Mayotte samples.

Discussion

Olivine melilitite genesis

The olivine melilitites are ultracalcic mafic lavas, characterized by high contents in CaO and MgO; high silica-undersaturation, with SiO 2 contents < 40 wt.% [START_REF] Wilson | Melilitites: partial melts of the thermal boundary layer?[END_REF]; low values in Al 2 O 3 , and, to a lesser extent, alkalis; and straight slopes in chondritenormalized REE diagrams [START_REF] Brey | Origin of olivine melilititeschemical and experimental constraints[END_REF][START_REF] Rogers | Phlogopite in the generation of olivine-melilitites from Namaqualand, South Africa and implications for element fractionation processes in the upper mantle[END_REF][START_REF] Wilson | Melilitites: partial melts of the thermal boundary layer?[END_REF]. Based on such chemical features, [START_REF] Brey | Origin of olivine melilititeschemical and experimental constraints[END_REF] and [START_REF] Rogers | Phlogopite in the generation of olivine-melilitites from Namaqualand, South Africa and implications for element fractionation processes in the upper mantle[END_REF] have proposed that melilititic 1 8 primary magmas are generated by very low degree of partial melting of a garnet-(and phlogopite-?) bearing CO 2 -rich mantle source. Such low partial melting models for melilititic melts are consistent with the experimental works of [START_REF] Gudfinnsson | Continuous gradiations among primary carbonatitic, kimberlitic, melilititic, basaltic, picritic and komatiitic melts in equilibrium with garnet lherzolite at 3-8 GPa[END_REF]. [START_REF] Wilson | Melilitites: partial melts of the thermal boundary layer?[END_REF] and [START_REF] Melluso | Petrogenesis and Nd-, Pb-, Sr-isotope geochemistry of the Cenozoïc olivine melilitites and olivine nephelinites ("ankaratrites") in Madagascar[END_REF] have suggested that carbonates (probably dolomite) were involved in the melting process. From data of experimental petrology, [START_REF] Gudfinnsson | Continuous gradiations among primary carbonatitic, kimberlitic, melilititic, basaltic, picritic and komatiitic melts in equilibrium with garnet lherzolite at 3-8 GPa[END_REF] have positioned the field of the mantle at the origin of melilititic primary melts in the domain of garnet stability, in presence of CO 2 , at pressures ranging from 3 and 4 GPa. Partial melting experiments of carbonated peridotite at 3 GPa generate melt compositions close to melilititic lavas [START_REF] Hirose | Partial melt compositions of carbonated peridotite at 3 GPa and role of CO 2 in alkali-basalt magma generation[END_REF][START_REF] Dasgupta | Partial melting experiments of peridotite + CO 2 at 3 GPa and genesis of alkali ocean island basalts[END_REF], except for TiO 2 and FeO T , lower in these experimental melts than in natural olivine melilitites. To account for their geochemistry, [START_REF] Dasgupta | Immiscible transition from carbonaterich to silicate-rich melts in the 3 GPa melting interval of eclogite + CO 2 and genesis of silica-undersaturated ocean island lavas[END_REF][START_REF] Dasgupta | Partial melting experiments of peridotite + CO 2 at 3 GPa and genesis of alkali ocean island basalts[END_REF] propose as melting source a carbonated peridotite metasomatized by pyroxenitic melts.

Besides the wide range of geochemical features listed above, the olivine melilitites of Mayotte present specific characteristics, namely high P 2 O 5 and Sr contents (Figs. 6d and7b, and Table 2), and relative enrichments in MREE with respect to the other mafic rocks (Fig. 7a). The sole other mention of P-rich melilitites has been made by [START_REF] Bailey | Melilitite at Fort Portal, Uganda. Another dimension of the carbonate volcanism[END_REF] for Ugandan rocks. In the figure 6d diagram, it is shown that all the N larnite samples are enriched in P 2 O 5 . Consequently, to explore the melilititic character of the P-rich N larnite lavas, we have reported them in the P 2 O 5 vs. CaO/(SiO 2 +Al 2 O 3 ) diagram of figure 9: the higher this ratio is, the more silica-undersaturated, CaO-rich and Al 2 O 3 -poor the rock is. The positive correlation demonstrates that a high P 2 O 5 content is also a melilititic feature in Mayotte. The P 2 O 5 enrichment of the N larnite lavas (Figs. 6d and9), together with the relatively high Sr and MREE contents of both olivine melilitites (Table 2 and Fig. 7), is consistent with the hypothesis of an apatite contribution in the genesis of these lavas. Apatite can be involved through two mechanisms: (1) melting at very low degree of apatite-bearing pyroxenite veins 1 9 in lithospheric metasomatized mantle [START_REF] Foley | Vein-plus-wall-rock melting mechanisms in the lithosphere and the origin of potassic alkaline magmas[END_REF]; ( 2) assimilation of apatite-bearing veins during the ascent of primary melts formed at a greater depth [START_REF] Bailey | Melilitite at Fort Portal, Uganda. Another dimension of the carbonate volcanism[END_REF]. Scenario [START_REF] Nougier | The Comores archipelago in the western Indian Ocean: volcanology, geochronology and geodynamic setting[END_REF], implying small melting degrees, is consistent with the high silica-undersaturation of the N larnite lavas, especially olivine melilitites. Both models imply a metasomatism episode, in which CO 2 -rich fluids or carbonatitic melts are probably involved [START_REF] Hirschmann | Alkalic magmas generated by partial melting of garnet pyroxenite[END_REF]. This is consistent with the classic models of melilitite genesis (see references above) and with the occurrence in Grande Comore lavas of mantle xenoliths metasomatized by alkalicarbonatitic melt [START_REF] Coltorti | Carbonatite metasomatism of the oceanic upper mantle: evidence from clinopyroxenes and glasses in ultramafic xenoliths of Grande Comore, Indian Ocean[END_REF].

HIMU-flavored lavas

In order to minimize the effects of melt differentiation (crystallization, contamination) and to discuss the contribution of the mantle sources, this section focuses on relatively mafic lavas (MgO > 5 wt.%). As evidenced in Figure 8, the isotopic composition of Mayotte lavas is consistent with mixing between depleted mantle and HIMU components.

The HIMU-flavored mafic lavas, characterized by high Pb isotopic ratios, are reputed to be enriched in Nb and Ta and depleted in alkali/earth-alkali elements relative to the other incompatible trace elements [START_REF] Dupuy | Subducted and recycled lithosphere as the mantle source of ocean island basalts from southern Polynesia, central Pacific[END_REF][START_REF] Willbold | Trace element composition of mantle end-members : Implications for recycling of oceanic and upper and lower continental crust[END_REF]. These features are noticeable in the HIMU-like olivine nephelinites collected in the south shield of Mayotte (Table 2 and Fig. 7b). Nevertheless, rocks from the northwest shield, which also have a clear HIMU isotopic signature, do not show these trace element features.

The HIMU mantle end-member is generally considered as the signature of a long-lived mantle reservoir (> 1 Ga). It has been ascribed to (1) a metasomatized mantle that has undergone a removal of Rb and Pb [START_REF] Zindler | Chemical geodynamics[END_REF]) and/or an U enrichment through CO 2 -rich fluids [START_REF] Nakamura | Pb, Nd and Sr isotopic evidence for a multi-component source for rocks of the Cook-Austral Islands and heterogeneities of mantle plumes[END_REF]; ( 2) an ancient subduction-modified 0 oceanic lithosphere, depleted in LILE as the result of dehydration [START_REF] White | Sr and Nd isotope geochemistry of oceanic basalts and mantle evolution[END_REF][START_REF] Dupuy | Subducted and recycled lithosphere as the mantle source of ocean island basalts from southern Polynesia, central Pacific[END_REF]; or (3) combination of both models, i.e. a peridotite metasomatized with carbonated silicate melts, carbonatites derived from a recycled oceanic crust [START_REF] Jackson | Compositions of HIMU, EM1 and EM2 from global trends between radiogenic isotopes and major elements in ocean island basalts[END_REF] or carbonated eclogites [START_REF] Dasgupta | Major element chemistry of ocean island basalts -Conditions of mantle melting and heterogeneity of mantle source[END_REF]. The fact that the lavas from the northwest shield are not enriched in Ta and Nb (Table 2) is not really consistent with the model [START_REF] Suschevskaya | Geochemical evolution of Indian Ocean basaltic magmatism[END_REF].

In Mayotte, only the oldest rocks show a HIMU signature. Contrary to some Marquesas examples as Ua Pou [START_REF] Duncan | Plume vs lithospheric melts at Ua Pou, Marquesas Islands[END_REF][START_REF] Legendre | Origin of exceptionally abundant phonolites on Ua Pou Island (Marquesas, French Polynesia): Partial melting of basanites followed by crustal contamination[END_REF] and Eiao [START_REF] Caroff | Rapid temporal changes in Ocean Island Basalt composition: evidence from a 800 m-deep drill hole in Eiao shield (Marquesas)[END_REF], where the HIMU-like lavas have distinct petrographic and geochemical features compared to the other rocks, here, the isotopic composition is disconnected from the silicaundersaturation and elemental (major and trace elements) composition of the lavas, which precludes any connection between source composition and partial melting modalities.

The isotopic composition of Mayotte lavas is consistent with a mixing between DMM and HIMU end-members (Fig. 8). This feature has already been stated for the Canary Archipelago by [START_REF] Gurenko | Enriched, HIMU-type peridotite and depleted recycled pyroxenite in the Canary plume: A mixedup mantle[END_REF] and [START_REF] Aulinas | Small-scale mantle heterogeneity on the source of the Gran Canaria (Canary Islands) Pliocene-Quaternary magmas[END_REF]. However, the HIMU-like lavas in Gran Canaria correspond to a post-shield phase [START_REF] Aulinas | Small-scale mantle heterogeneity on the source of the Gran Canaria (Canary Islands) Pliocene-Quaternary magmas[END_REF], unlike Mayotte. Isotopic data for Canaries and Mayotte are both consistent with the assumption of small-scale HIMU heterogeneities in a depleted mantle (see section 7.3).

We compare the Nd/Yb vs. SiO 2 and Nd/Yb vs. Nb/Zr diagrams (Fig. 10) and the isotopic ratios (Fig. 8) in order to discriminate source composition, apatite contribution and melting degree. Apatite contribution during a deep metasomatism episode increases Nb/Yb ratio at constant Nb/Zr ratio and SiO 2 content. Decrease of melting degree induces a decrease of SiO 2 content and an increase of Nd/Yb and Nb/Zr ratios. From the data distribution in Figure 10, N larnite-bearing lavas would result from low partial melting degree of a metasomatized apatite-bearing mantle source, with ( N larnite-bearing nephelinites) or without 1 (olivine melilitites) HIMU affinity (Fig. 8). South and northwest shield lavas would derive from a similar HIMU-like mantle source (Fig. 8) and their elemental distinction results from increase of the partial melting degree from south to northwest lavas. North central post-shield lavas derive from a more depleted mantle (Fig. 8), through higher melting degrees than lavas of the main building stage (Fig. 10).

Genesis of Comores lavas

Two groups of mantle source models below Comores are proposed, which imply interactions between melt issued (1) from an EM1-HIMU heterogeneous plume and a DMM metasomatized lithosphere [START_REF] Deniel | Geochemical and isotopic (Sr, Nd, Pb) evidence for the plume-lithosphere interactions in the genesis of Grande Comore magmas (Indian Ocean)[END_REF]; and (2) from a homogeneous plume with a dominant EM1 signature and a DMM-HIMU metasomatized lithosphere (Class andGoldstein, 1994, 1997;[START_REF] Späth | The geochemistry of lavas from the Comores Archipelago, western Indian Ocean: petrogenesis and mantle source region characteristics[END_REF][START_REF] Class | The process of Plume -Lithosphere interactions in the Ocean Basins -The case of Grande Comore[END_REF]. In the latter models, the HIMU component would be introduced in the Comorian mantle reservoir through delamination or thermal erosion of a continental lithosphere during the Gondwana break-up. This group of models is consistent with the geographic position of the Comores Archipelago between two HIMU-bearing continental blocks: Africa and Madagascar [START_REF] Bell | Nd, Pb and Sr isotopic compositions of East African carbonatites: evidence for mantle mixing and plume inhomogeneity[END_REF][START_REF] Melluso | Petrogenesis and Nd-, Pb-, Sr-isotope geochemistry of the Cenozoïc olivine melilitites and olivine nephelinites ("ankaratrites") in Madagascar[END_REF].

Our data are rather consistent with the second type of models for the mantle source below Mayotte (Fig. 11): clear DMM-HIMU mixing and quasi absence of EM1 compositions (except M41, Table 3). During the main building stage, partial melting of a DMM-HIMU CO 2 -metasomatized lithosphere produced the south and northwest shield melts, with lower partial melting degrees for the south lavas (Fig. 10). The melting processes occurred in the garnet stability field, as suggested by the presence of N larnite-bearing ultracalcic melts [START_REF] Brey | Systematic study of liquidus phase relations in olivine melilitite + H 2 O + CO 2 at high pressures and petrogenesis of an olivine melilitite magma[END_REF]. During the post-shield phase, the more isotopically depleted homogeneous north central melts were formed through higher melting degrees. The other post-shield rocks have Nd and Pb isotopic features intermediate between this north central group and the lavas of the main building phase (Fig. 8). The progressive increase of the depleted component influence through time (Fig. 8), especially for the north central rocks (Fig. 10), can be related to a lower proportion of HIMU component in the mantle source or to a progressive dilution of the melts issued from fusible and small-sized HIMU heterogeneities when melt productivity increases. However, HIMU-flavored northwest lavas are produced by similar melting degrees as the depleted north central lavas. We thus propose that the isotopic signature of the postshield north central lavas is related to a high partial melting of a source where the HIMU component would have been partially consumed during the main building stage. The olivine melilitites would be generated from a source isotopically equivalent, but enriched in CO 2 and P 2 O 5 (probably with modal dolomite ± apatite) and with a low degree of partial melting. The most plausible lithologies for generating the silica-undersatured primary melts of Mayotte lavas are carbonated garnet peridotite [START_REF] Hirose | Partial melt compositions of carbonated peridotite at 3 GPa and role of CO 2 in alkali-basalt magma generation[END_REF][START_REF] Dasgupta | Partial melting experiments of peridotite + CO 2 at 3 GPa and genesis of alkali ocean island basalts[END_REF] and/or silicadeficient garnet pyroxenite/eclogite [START_REF] Hirschmann | Alkalic magmas generated by partial melting of garnet pyroxenite[END_REF][START_REF] Dasgupta | Major element chemistry of ocean island basalts -Conditions of mantle melting and heterogeneity of mantle source[END_REF][START_REF] Le Roux | Mineralogical heterogeneities in the Earth's mantle: Constraints from Mn, Co, Ni and Zn partitioning during partial melting[END_REF]. Considering that the HIMU signature of the rocks is disconnected from partial melting modalities and that the Mayotte lavas, more or less fractionated, are not primary magmas, it would be difficult to go further in characterization of the source lithology.

The model of a depleted lithospheric mantle with HIMU small-scale heterogeneities can be extended to the other islands of the Comores archipelago. Indeed, the mixing trend DMM-HIMU is clear for Moheli, Anjouan and "La Grille"-type lavas from Grande Comore (Fig. 8). Nevertheless, the "Karthala"-type recent lavas from Grande Comore show a specific EM1 signature ( 87 Sr/ 86 Sr > 0.70350 and 143 Nd/ 144 Nd < 0.51275). [START_REF] Class | The process of Plume -Lithosphere interactions in the Ocean Basins -The case of Grande Comore[END_REF] argued that the "Karthala"-type lavas result from mixtures between plume and lithosphere melts. Thus, we propose that Comores lavas are mainly generated by partial melting of a DMM-HIMU CO 2 -metasomatized lithospheric mantle. The EM1 Comorian mantle plume would just have a thermal effect, except for the "Karthala"-type lavasand for the Mayotte basalt M41, exhibiting an equivalent Sr-Nd isotopic compositionwhich probably resulted from plume/lithosphere interactions (Fig. 11).

Concluding remarks

The new set of geochronological and geochemical data presented in this study is used to discuss the chemistry and the mineralogy of the mantle source below Mayotte. The two main results are: (1) the characterization for the first time in the Comores of melilite-bearing lavas, including two olivine melilitites; and ( 2) the clear evidence of a HIMU signature in the shield lavas. Both features may or may not concern same samples.

Without doubt, the melting mantle source involved in the genesis of the Mayotte lavas is chemically and mineralogically heterogeneous at small scale. The HIMU affinity could find its origin in lithospheric components metasomatized by CO 2 -rich fluids, whereas melilitebearing lavas would derive from partial melting at very low degree of a mantle source, isotopically similar or not, bearing modal carbonate and apatite. Time-increasing partial melting during the main building phase of the shield volcanoes would favor the expression in the post-shield lavas of a more depleted mantle signature, as the consequence of the dilution of the HIMU heterogeneities in the mantle, maybe coupled with their partial consumption during the shield phase.

Petrological and geochemical data can thus be used to draw a composite picture of the mantle below Mayotte, which probably changes with time. Regardless of what lavas are concerned, the melting lithosphere below Mayotte presents a general characteristic feature, which reconciles both isotopic and mineralogical data: it has undergone CO 2 -metasomatism. [START_REF] Hajash | Paleomagnetic and radiometric evidence for the age of the Comores Islands, West Central Indian Ocean[END_REF][START_REF] Hajash | Paleomagnetic and radiometric evidence for the age of the Comores Islands, West Central Indian Ocean[END_REF][START_REF] Emerick | Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics[END_REF][START_REF] Emerick | Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics[END_REF]; Moheli (12 data by [START_REF] Emerick | Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics[END_REF][START_REF] Emerick | Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics[END_REF][START_REF] Nougier | The Comores archipelago in the western Indian Ocean: volcanology, geochronology and geodynamic setting[END_REF]; Anjouan (8 data by [START_REF] Hajash | Paleomagnetic and radiometric evidence for the age of the Comores Islands, West Central Indian Ocean[END_REF][START_REF] Emerick | Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics[END_REF][START_REF] Nougier | The Comores archipelago in the western Indian Ocean: volcanology, geochronology and geodynamic setting[END_REF]; Mayotte (34 data by [START_REF] Hajash | Paleomagnetic and radiometric evidence for the age of the Comores Islands, West Central Indian Ocean[END_REF][START_REF] Emerick | Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics[END_REF][START_REF] Nougier | The Comores archipelago in the western Indian Ocean: volcanology, geochronology and geodynamic setting[END_REF] showing Mayotte data compared with previously published data (Grande Comore: [START_REF] Dupré | Pb-Sr isotope variation in Indian Ocean basalts and mixing phenomena[END_REF][START_REF] Reisberg | Os isotope systematics in ocean island basalts[END_REF][START_REF] Späth | The geochemistry of lavas from the Comores Archipelago, western Indian Ocean: petrogenesis and mantle source region characteristics[END_REF][START_REF] Class | Plume-lithosphere interactions in the ocean basins: constraints from the source mineralogy[END_REF][START_REF] Class | The process of Plume -Lithosphere interactions in the Ocean Basins -The case of Grande Comore[END_REF][START_REF] Deniel | Geochemical and isotopic (Sr, Nd, Pb) evidence for the plume-lithosphere interactions in the genesis of Grande Comore magmas (Indian Ocean)[END_REF][START_REF] Class | Grande Comore island: a well-constrained low 3He/4He mantle plume[END_REF][START_REF] Class | Osmium isotopes in Grande Comore lavas: a new extreme among a spectrum of EM-type mantle endmembers[END_REF]Anjouan: Reisberg et al, 1993;[START_REF] Salters | Isotope constraints on mantle evolution[END_REF]Moheli: Reisberg et al., 1993;[START_REF] Späth | The geochemistry of lavas from the Comores Archipelago, western Indian Ocean: petrogenesis and mantle source region characteristics[END_REF]. The position of the DMM, EM I and II, and HIMU end-members are from [START_REF] Zindler | Chemical geodynamics[END_REF]. Symbols as in figure 4. M54 (1) PN5a (2) PN15 (2) M25 (1) M41 (1) M75 (1) M35 (1) M60 (1) M82 (1) M97 (1) M108 (1) PN3 (2) PN5b (2) FL386 (2) M66 (1) Olivine nephelinites Sample FL468 (2) J2 (2) M19 (1) M50 (1) M88 (1) M89 (1) M112 (1) M129 (1) M132 (1) M133 (1) M135 (1) M137 (1) M138 (1) PN2 (2) PN12f ( 2 2) Samples collected during two field trips in 2010 and 2011, major elements analyzed by ICP-AES (PSO/IUEM, Brest, France) following the analytical method described by [START_REF] Cotten | Origin of anomalous rare-earth elements and yttrium enrichments in sub-aerially exposed basalts: evidence from French Polynesia[END_REF], trace elements analyzed by ICP-MS (PSO/IUEM, Brest, France) following the sample preparation and analysis method of [START_REF] Barrat | Determination of rare earth elements in sixteen silicate reference samples by ICP-MS after Tm addition and ion exchange separation[END_REF].

Olivine-free nephelinites Tephrites Tephri-phonolites Phonolites Sample M140 (1) PN8 (2) J7 (2) M4 (1) FL478 (2) M91 (1) M123 (1) M127 (1) FL426 (2) FL442 (2) M58 (1) M103 (1) M131 (1) PN7 (2) PN11 (2)

4

  and 5). Most of the lava flows of south and northwest domains are alkali basalts and basanites. Lavas are usually porphyritic, sometimes cumulative (> 35 vol.% phenocrysts), with 1 0 clinopyroxene and/or olivine (Fo 79-85 ) phenocrysts. Groundmass generally has a microlitic texture. It is constituted of clinopyroxene (fassaite-diopside), olivine (Fo 62-81 ), plagioclase (An 47-64 ), nepheline, Fe-Ti oxides and apatite. Perovskite crystals have been identified as inclusions in clinopyroxene phenocrysts of one basanite from the south domain. Two olivine melilitite lavas were collected near Mtsangamouji in the northwest domain. They are porphyritic with zoned olivine phenocrysts (Fo 84-88 ) immersed in a microlitic groundmass constituted of melilite, olivine (Fo 79-84 ), titanomagnetite, apatite, clinopyroxene, and tiny nepheline. One sample displays mingling features. Olivine-free and olivine-bearing nephelinites occur as lava flows or dykes in the south domain and in a few areas of the northwest domain. They have a porphyritic texture with phenocrysts of clinopyroxene (fassaite-diopside) and nepheline. Groundmass crystals include nepheline, clinopyroxene, Fe-Ti oxides, and apatite. Olivine phenocrysts (Fo 74-87 ) and microcrysts (Fo 69-78 ) are only present in the olivine nephelinites, whereas hornblende, aegirine-augite, perovskite and titanite are detected only in olivine-free nephelinites. Microcrystals of melilite (< 10 vol.%) have been identified in two olivine nephelinites (PN2 and PN12f).

1 3

 1 , Al 2 O 3 , K 2 O, and P 2 O 5 against MgO are shown in figure 6. CaO is positively correlated with MgO for lavas with MgO < 8 wt%. The N larnite samples and the mafic xenolith PN5b of Petite Terre have the highest CaO contents (> 12 wt. %). The (tephri)phonolites of the Combani massif form a low-CaO trend together with the lavas of the north central zone. Al 2 O 3 and K 2 O display remarkable negative correlations with MgO. Except for the P 2 O 5 -rich N larnite-bearing lavas, P 2 O 5 increases with decreasing MgO up to MgO ~ 5 wt.%, then decreases.

  the chemical distinction of the volcanic domains using incompatible trace element variations against Th. (1) The post-shield lavas of the north central domain form linear trends versus Th. They are distributed in two groups with different Th concentrations and variable contents on Nb, Hf, and Yb. These samples have the lowest Nb/Zr ratios of the collection, displaying a decreasing trend with increasing Th. (2) The recent phonolitic pumices of Petite Terre have the highest Nb, Hf, and Yb contents. Their Nb/Zr values range from 0.25 to 0.27. (3) Most of the south domain samples plot linearly on the trace variation diagrams. Most of the corresponding lavas are contemporaneous with the shield phase. The trend has a slope especially low in the Hf versus Th diagram of figure 6f. The south domain samples are scattered in the Nb/Zr versus Th diagram of figure 6h.

  207 Pb/ 204 Pb vs.206 Pb/ 204 Pb diagram of Figure8a, there are evident relationships between the isotopic compositions of the samples and their geographic location and age. In fact, most of the lavas from the south and northwest shield volcanoes exhibit the most radiogenic 207 Pb/ 204 Pb (> 15.64) and 206 Pb/ 204 Pb (> 19.87), whereas the post-shield north central domain lavas display the lowest ratios ( 207 Pb/ 204 Pb < 15.6 and 206 Pb/ 204 Pb < 19.35). The post-shield Combani lavas, northwest olivine melititites, and Petite Terre samples (xenolith PN5b apart) 1 5

Fig. 1 .

 1 Fig. 1. The Comores archipelago. K-Ar age ranges are indicated for each island: Grande

Fig. 2 .

 2 Fig. 2. Geophysical and geological maps of Mayotte. (a) Bathymetry (from Audru et al.,

Fig. 3 .

 3 Fig. 3. Summary of petrographic data for Mayotte lavas: mineral occurrence and their

Fig. 4 .

 4 Fig. 4. Total alkali-silica (TAS) discrimination diagram for lavas and pumices from Mayotte

Fig. 5 .

 5 Fig. 5. Nephelinite-melilitite-basanite classification diagram of Le Bas (1989) for mafic lavas

Fig. 6 .

 6 Fig. 6. Variation of CaO, Al 2 O 3 , K 2 O and P 2 O 5 vs. MgO (weight percent) and Nb, Hf, Yb and

Fig. 7 .

 7 Fig. 7. Trace element patterns of representative Mayotte samples. (a) and (b): mafic lavas

Fig. 8 .

 8 Fig. 8. (a) 143 Nd/ 144 Nd vs. 206 Pb/ 204 Pb and (b) 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb isotope diagrams

Fig. 9 .

 9 Fig. 9. P 2 O 5 vs. CaO/SiO 2 (wt. %) diagram for the N larnite lavas. Symbols as in figure 4.

Fig. 10 .

 10 Fig. 10. (a) Nd/Yb (ppm) vs. SiO 2 (wt. %) and (b) Nd/Yb vs. Nb/Zr (ppm) diagrams for the

Fig. 11 .

 11 Fig. 11. Schematic model showing the mantle structure below Mayotte during its main

  and one relative age determined by Zinke et al., 2003, for a pumice layer interbedded in the lagoon stratigraphic succession). The value of 11.1 Ma of Anjouan corresponds to a remote value measured on a syenitic xenolith. The Mayotte new age range of Table 1 is indicated in italic.

Inset: Mayotte in its geodynamical context.

Table 1 .

 1 Dating results from 40 Ar-39 Ar geochronology for Mayotte samples For each sample, the plateau age and the isochron age are given when available; the preferred age is indicated in the last column. MSWD: mean square weighted deviation. SMA: simple mean age, WMP: weighed plateau age. Type of statistical approach: NO: no plateau or isochron could be calculated; SMA: simple mean age; WMP: weighed mean plateau age

			Sample			Plateau age				Inverse isochron age		Integrated age	Preferred age	Remark
	Unit Number Location	Petrological type	Type	Age (Ma)	Steps Type	Age (Ma)	40 Ar/ 36 Ar i MSWD Steps Age (Ma)	Age (Ma)
		M104	Lava flow	Basanite	SMA 6.1 ± 0.3 1-3		4.3 ± 0.1 311 ± 10 0,1	1-6	5.0 ± 0.1	6.1 ± 0.3	SMA is maximum age Data too clustered on isochron
	Northwest	M112 M108 M25 M20	Dyke Dyke in M104 Basanite Ol.-nephelinite Lava flow Alkali basalt Dyke Basanite	SMA 5.2 ± 0.3 1-3 SMA 3.8 ± 0.1 1-4 NO --SMA 0.6 ± 1.25 4-5	4.4 ± 1.0 305 ± 10 2,4 4.2 ± 0.8 291 ± 10 7,1 2.0 ± 0.9 382 ± 46 26,8 NO ---	1-5 1-6, 8 3.2 ± 0.1 5.3 ± 0.1 1-4 3.2 ± 0.1 -8.9 ± 0.2	4.4 ± 1.0 3.8 ± 0.1 2.0 ± 0.9 Maximum age < 1 Ma Maximum age
		M59	Lava flow	Basanite	WMP 0.8 ± 0.2 1-5		0.8 ± 0.1 303 ± 150 0,1	1-6	0.8 ± 0.2	0.8 ± 0.2 Data too clustered on isochron
		M97	Lava flow	Basanite	SMA 1.9 ± 0.1 1-5		2.4 ± 0.3 291 ± 6	4,8	1-5	1.8 ± 0.1	2.4 ± 0.3
		M4	Lava flow	Tephrite	NO	-	-		2.3 ± 0.3 240 ± 30 12,7	1-3, 5-7	3.5 ± 0.6	2.3 ± 0.3 Maximum age
	North central	M29 M123 M56	Dome Lava flow Lava flow	Trachyte Tephriphonolite Basaltic trachy-andesite	NO SMA 1.5 ± 0.6 1-3 --1.2 ± 0.1 / 1.4 ± 0.1 -	NO -NO -1.4 ± 0.1 311 ± 11 2,6 ----	--1-4	2.9 ± 0.6 1.2 ± 0.1 / 1.4 ± 0.1 1.4 ± 0.1	< 3 Ma 1.4 ± 0.1 1.4 ± 0.1	Maximum age
		M82	Lava flow	Basanite	SMA 1.4 ± 0.1 1-4		2.1 ± 0.1 265 ± 10 0,9	1-4	1.1 ± 0.1	1.1 ± 0.1
		M58	Lava flow	Phonolite	SMA 0.2 ± 0.1 1-3		0.8 ± 0.1 345 ± 18 4,3	1-6	0.6 ± 0.1	0.8 ± 0.1 Nice isochron
	Petite Terre	M53	Strombolian cone	Alkali basalt	NO	-	-	NO -	-	-	-	0.5 ±0.1	0.2 ± 0.1 All individual steps below 0.2 Ma
		M137	Lava flow	Ol.-nephelinite	NO	-	-		5.0 ± 0.3 319 ± 2	1,1	1-6	13.1 ± 0.2	5.0 ± 0.3
		M41	Lava flow	Alkali basalt	NO	-	-		10.6 ± 0.5	270 ± 20 5,9	2-6	6.2 ± 0.1	10.6 ± 0.5 Steps used are the richest in 39 Ar
		M140	Lava flow	Ol.-free neph.	WMP 5.9 ± 0.1 1-4		6.1 ± 0.1 306 ± 5	2,2	1-5	5.8 ± 0.1	5.9 ± 0.1
		M93	Lava flow	Ol.-nephelinite	NO	-	-		3.4 ± 0.2 308 ± 6	4,8	1-4	4.4 ± 0.1	3.4 ± 0.2
	South	M132 M38	Lava flow Lava flow	Ol.-nephelinite Ol.-nephelinite	SMA 3.8 ± 0.1 2-4 SMA 4.6 ± 0.1 1-2	4.0 ± 0.3 302 ± 7 NO --	5,5 -	1-6 -	4.7 ± 0.1 3.9 ± 0.1	4.0 ± 0.3 3.9 ± 0.1	The integrated age is preferred to badly defined plateau
		M39A	Dome	Phonolite	WMP 3.9 ± 0.4 2-7		4.7 ± 0.9 263 ± 49 0,8	2-7	4.3 ± 0.1	3.9 ± 0.4 Good plateau
		M55	Lava flow	Ol.-free neph.	NO	-	-		3.6 ± 0.4 291 ± 42 5,9	1-4, 7 3.8 ± 0.5	3.6 ± 0.4
		M115	Lava flow	Ol.-nephelinite	NO	-	-		1.9 ± 0.2 327 ± 11 6,4	1-7	2.1 ± 0.4	1.9 ± 0.2
		M95	Lava flow	Ol.-nephelinite	SMA 2.5 ± 0.2 1-2	NO -	-	-	-	1.5 ± 0.8	< 2.5 Ma

Table 2 .

 2 Major and trace elements data for Mayotte

	Phonolitic pumices	Alkali basalts	Basanites

6

Acknowledgements

Field studies were conducted in 2001 with the financial support of the Conseil Régional de La Réunion and the Laboratoire GéoSciences Réunion, and in 2010 and 2011 with the financial and logistic support of the BRGM (Bureau de Recherches Géologiques et Minières) and with the participation of F. Lacquement and J. Bernard. ISterre is part of Labex OSUG@2020 (ANR10 LABX56). The authors are grateful to J. Langlade and V. Batanova (microprobe analyses), C. Bollinger (ICP-MS Brest), Ph. Nonnotte (TI-MS), C. Liorzou (ICP-AES), Y. Germain (MC-ICP-MS Ifremer), and P. Tiffenbach (thin sections). Detailed and constructive remarks by Drs Meritxell Aulinas and Hannes B. Mattsson helped us a lot to improve the manuscript. We also thank Dr Nelson Eby for his editorial assistance.

Appendix. Supplementary Table

 S1.

Supplementary data to this article can be found online at doi: Table 3. Sr, Nd, Pb isotopic data for Mayotte.

(1) S: south domain; S C : Combani Massif; NW: northwest domain; NC: north central domain; PT: Petite Terre domain (see Fig. 2c for domain location).

(2) Sr and Nd isotopic ratios measured in 2003 on a TI-MS (Laboratoire Magmas et Volcans, Clermont-Ferrand, France) - [START_REF] Debeuf | Etude de l'évolution volcano-structurale et magmatique de Mayotte (Archipel des Comores, Océan Indien[END_REF]. Average value of NBS 987 (standard for Sr) is 87 Sr/ 86 Sr = 0.710253 ± 0.000020 (1σ ; n=39) and average value of Rennes-AMES (internal standard for Nd) is 143 Nd/ 144 Nd = 0.511960 ± 0.000010 (1σ ; n=41).

(3) Sr and Nd isotopic ratios measured in 2013 on a TI-MS (PSO/IUEM, Brest, France). Sr and Nd isotopic values are corrected for mass fractionation to 88