Xian-Hua Li

Michel Faure

Philippe Rossi

Wei Lin

Didier Lahondère

ÔØ Å ÒÙ× Ö ÔØ Age of Alpine Corsica ophiolites revisited: Insights from in situ zircon U-Pb age and O-Hf isotopes

Keywords: zircon U-Pb age, zircon O-Hf isotopes, ophiolite, Corsica, Alpine Tethys, Liguria-Piemonte oceanic basin Highlights

Knowledge of the age and timing of ophiolite sequences is essential for understanding the mechanisms of plate tectonics. The ophiolites in the Schistes Lustrés and the Upper nappes of Alpine Corsica represent remnants of the Liguria-Piemonte ocean basin that formed as a branch of the Central Atlantic basin during the opening of the Mesozoic Western Alpine Tethys. Despite numerous isotopic and paleontological studies, the age and timing of the ophiolites in the Schistes Lustrés nappe are still controversial. This study presents integrated in situ analyses of zircon U-Pb age and O-Hf isotopic data for ophiolitic gabbros and plagiogranites from three localities in the Schistes Lustrés nappe of Eastern Corsica. Our new results demonstrate that these rocks crystallized synchronously at ~159 Ma, approximately 10 m.y. younger than the ophiolites in the Balagne Upper nappe. Zircons from the gabbros and plagiogranites are characterized by highly positive εHf(t) (+15.0 to +15.9) and mantle-like δ 18 O (5.2-5.4‰) values. Thus, these ophiolitic rocks were cogenetic, and crystallized from magmas produced by partial melting of a depleted, N-MORB type mantle. By contrast, in the Balagne Upper nappe, the ~169 Ma ophiolites contain numerous xenocrystic zircons inherited from a continental crust. Our current knowledge of isotopic geochronology and geochemistry supports a paleogeographic reconstruction, in which the earliest ophiolites in the Balagne nappe were emplaced close to a continental margin at ~169 Ma, whilst the N-MORB type ophiolites in the Schistes Lustrés nappe

A C C E P T E D M A N U S C R I P T 1 Introduction

The age and timing of ophiolitic sequences provide important constraints on the opening and palinspastic reconstructions of ocean basins, thus, knowledge of these sequences is essential for understanding the mechanisms of plate tectonics. The ophiolites exposed in Corsica, Northern Apennines, Liguria, and Western Alps represent remnants of the Liguria-Piemonte ocean basin that formed as a branch of Central Atlantic basin during the opening of the Mesozoic Western Alpine Tethys separating Adria from Europe (e.g., [START_REF] Lemoine | Ultramafic and gabbroic ocean floor of the Ligurian Tethys (Alps, Corsica, Apennines): in search of a genetic model[END_REF]. Precise chronology of oceanic opening is crucial to understand the Mesozoic evolution of the Alpine Tethys, and, in a more general way, to decipher how continents break apart and new oceans form.

Alpine Corsica is characterized by two main ophiolite-bearing tectonic units, namely i) the "Schistes Lustrés" nappe, composed of dismembered Jurassic ophiolitic elements enclosed by carbonate-rich mudstone and siltstone, globally called "schistes lustrés" (i.e. glimmering schists), and ii) the "Upper nappes" of Balagne, Nebbio, Macinaggio, Pineto and Rio Magno units (Fig. 1; [START_REF] Durand-Delga | La Corse in Géologie de la France[END_REF][START_REF] Mattauer | Nouvelles données sur l'évolution structurale de la Corse Alpine[END_REF][START_REF] Caron | Géologie de la Corse, 26 th[END_REF][START_REF] Mattauer | Transverse lineation and large scale structures related to a high pressure-low temperature ophiolitic thrusting[END_REF][START_REF] Durand-Delga | Les massifs anciens de la France: la Corse[END_REF][START_REF] Fournier | Alpine Corsica metamorphic core complex[END_REF][START_REF] Egal | Structure and tectonic evolution of the external zone of Alpine Corsica[END_REF]Padoa et al., 2001;[START_REF] Saccani | Preliminary data on the Pineto gabbroic massif and Nebbio Basalts: progress toward the geochemical characterization of Alpine Corsica Ophiolites[END_REF][START_REF] Saccani | Corsica ophiolites: Geochemistry and petrogenesis of basaltic and metabasaltic rocks[END_REF]. The former experienced high-pressure/low-temperature (HP/LT) metamorphism, the latter only a weak prehnite-pumpellyite to low greenschist facies metamorphism, and in some cases no metamorphism [START_REF] Lahondère | Les schistes bleus et les éclogites à lawsonite des unités continentales et océaniques de la Corse alpine[END_REF] A C C E P T E D M A N U S C R I P T [START_REF] Saccani | Corsica ophiolites: Geochemistry and petrogenesis of basaltic and metabasaltic rocks[END_REF]Vitale Brovarone et al., 2013). Both isotopic and paleontological age determinations have been conducted on Corsican ophiolites belonging to different structural units. [START_REF] Ohnenstetter | Crystallization and age of zircon from Corsican ophiolitic albitites: consequences for oceanic expansion in Jurassic times[END_REF] first reported an ID-TIMS U-Pb zircon age of 161± 3 Ma for two ophiolitic albitites from Punta Corbara and Vezzani from the Liguria-Piemonte HP/LT metamorphic "Inzecca series" of the Schistes Lustrés nappe. [START_REF] Rossi | La marge européenne de la Téthys jurassique en Corse : datation de trondhjémites de Balagne et indices de croûte continentale sous le domaine Balano-Ligure[END_REF] reported a SHRIMP zircon U-Pb age of 169 ± 3 Ma for the plagiogranites from the weakly metamorphosed Balagne Upper nappe and recently a SHRIMP zircon U-Pb age of 152.5 ± 2.2 Ma for the trondhjemite of Rusio (15 km to the NE of Corte) on the same Liguria-Piemonte HP/LT unit from the metamorphic "Inzecca series" of the Schistes Lustrés nappe. [START_REF] Rampone | Isotopic equilibrium between mantle peridotite and melt: Evidence from the Corsica ophiolite[END_REF] obtained a Sm-Nd isochron age of 155 ± 6 Ma for a gabbro noritic veinlet from the Mt. Maggiore peridotite sliver in the Schistes Lustrés nappe.

Biostratigraphically, numerous micropalaeontological data suggest that the radiolarian sediments overlying the basalt of the Balagne Upper nappe range between Upper Bathonian and Upper Callovian in age [START_REF] Conti | Radiolarian cherts and ophiolites in the Northern Apennines and Corsica. Age, correlations and tectonic frame of siliceous deposition[END_REF][START_REF] Chiari | The age of the radiolarian cherts associated with the ophiolites in the Apennines (Italy)[END_REF]Peybernès et al., 2001a, b;[START_REF] Danelian | Revised radiolarian ages for the sedimentary cover of the Balagne ophiolite (Corsica, France). Implications for the palaeoenvironmental evolution of the Balano-Ligurian margin[END_REF], corresponding to a limited time span of 167-164 Ma according to the IGC Time Scale [START_REF] Gradstein | A new geologic time Scale with special reference to Precambrian and Neogene[END_REF][START_REF] Cohen | The ICS International Chronostratigraphic Chart[END_REF]. Unfortunately, away from this nappe that escaped the HP/LT metamorphic imprint, other cherts from the Schistes Lustrés nappe of Alpine Corsica never provided any consistent paleontological information. As radiometric ages are particularly rare, and an

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age discrepancy exceeding approximately 8 m.y. exists for some of the dated ophiolites from the Schistes Lustrés nappe, a need for new high precision dating appeared necessary.

Even for the best-preserved ophiolites, such as the Chenaillet massif in the Western Alps that escaped high-pressure metamorphism, zircons from the ophiolitic igneous rocks have been subjected to hydrothermal alteration resulting in elevated common Pb and altered O-isotopic values (e.g., Li et al., 2013a). Thus, a precise chronological knowledge is imperative for numerous Alpine Tethys ophiolites. Among the commonly-used radiogenic isotopic systems, zircon U-Pb and internal mineral Sm-Nd isochron have been widely used for dating ophiolites. For Mesozoic and younger ophiolitic rocks, Sm-Nd isotopic dating is rather imprecise due to the long half-life of 147 Sm (106 billion years), and limited fractionation in Sm/Nd ratios of major rock-forming minerals of ophiolitic rocks. Modern large radius magnetic sector SIMS (SHRIMP and Cameca IMS 1270/1280), in combination with cathodoluminescence (CL) images, have been shown to be successful in dating zircons from ophiolitic igneous rocks (e.g., [START_REF] Rossi | La marge européenne de la Téthys jurassique en Corse : datation de trondhjémites de Balagne et indices de croûte continentale sous le domaine Balano-Ligure[END_REF][START_REF] Rubatto | U-Pb dating of magmatic zircon and metamorphic baddeleyite in the Ligurian eclogites (Voltri Massif,Western Alps)[END_REF][START_REF] Rubatto | Dissolution-precipitation of zircon at low-temperature, high-pressure conditions (Lanzo Massif, Italy)[END_REF]Li et al., 2013a).

Aim of the study

In this study, we carried out integrated in situ zircon U-Pb age and O-Hf isotopic analyses for ophiolitic gabbros and plagiogranites (referred collectively

Geological background

Corsica consists of two major tectonic domains (e.g. [START_REF] Durand-Delga | La Corse in Géologie de la France[END_REF][START_REF] Durand-Delga | La Corse in Géologie de la France[END_REF]Rossi, 1991, Rossi et al., 2009) namely, Alpine Corsica exposed to the northeast, and Variscan Corsica in the rest of the island (Fig. 1A). The Variscan basement is made up of a Carboniferous granitic batholith including rare septa of Paleozoic and Neoproterozoic sedimentary and metamorphic rocks (cf. [START_REF] Rossi | A restored section of the ""southern Variscan realm"" across the Corsica-Sardinia microcontinent[END_REF][START_REF] Faure | Variscan orogeny in Corsica: New structural and geochronological insights, and its place in the Variscan geodynamic framework[END_REF][START_REF] Li | From crustal anatexis to mantle melting in the Variscan orogen of Corsica (France): SIMS U-Pb zircon age constraints[END_REF] for recent insights and references), and a Permian to Eocene sedimentary cover, partly reworked during the Alpine orogeny.

The Alpine belt of Corsica is a stack of west-directed nappes (Fig. 1B,C) (e.g. [START_REF] Durand-Delga | La Corse in Géologie de la France[END_REF]Mattauer and Proust, 1974;[START_REF] Mattauer | Transverse lineation and large scale structures related to a high pressure-low temperature ophiolitic thrusting[END_REF]; [START_REF] Ohnenstetter | Crystallization and age of zircon from Corsican ophiolitic albitites: consequences for oceanic expansion in Jurassic times[END_REF][START_REF] Faure | Etude structurale d'un cisaillement ductile. Le charriage ophiolitique corse dans la région de Bastia[END_REF][START_REF] Malavieille | Evolutionnary model for Alpine Corsica: mechanism for ophiolite emplacement[END_REF][START_REF] Fournier | Alpine Corsica metamorphic core complex[END_REF][START_REF] Molli | Orogenic processes and the Corsica/Apennines geodynamic evolution: insights from Taiwan[END_REF]Vitale Brovarone et al., 2013).

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It consists of three litho-tectonic units, from west to east, and from bottom to top: i) ductilely deformed Variscan basement, mostly exposed in the Tenda massif, and Vecchio area, in the north and south; ii) the "Schistes Lustrés" containing ophiolites; and iii) Upper undeformed units. Neogene sedimentary rocks unconformably overlie the nappe stack. A detail description of the structural and metamorphic evolution of Alpine Corsica is beyond the scope of this paper (see [START_REF] Molli | Orogenic processes and the Corsica/Apennines geodynamic evolution: insights from Taiwan[END_REF][START_REF] Molli | Orogenic processes and the Corsica/Apennines geodynamic evolution: insights from Taiwan[END_REF]Vitale Brovarone et al, 2013 for recent insights).

The Schistes Lustrés nappe is a composite tectonic structure formed by dismembered ophiolitic rocks, including serpentinized ultramafics, gabbros, pillow lavas, radiolarian cherts, siliceous shales, and ophiolitic sediments such as greywackes or gabbroic sandstones with mafic detritus [START_REF] Lagabrielle | Alpine, Corsican and Apennine ophiolites: the slow-spreading ridge model[END_REF]Vitale Brovarone et al., 2011). These rocks are sedimentary and tectonically associated with calcareous siltstone and phengitic metapelite, called "Schistes Lustrés". Furthermore, slices of continental crust rocks made up of ortho-and paragneiss, amphibolites, and Mesozoic metasandstones and marbles crop out in the Schistes Lustrés nappe. [START_REF] Lahondère | Relations du « socle ancien» de la région de Bastia (Corse) avec les Schistes lustrés environnants[END_REF]. These rocks derived from the easternmost part of the Corsican continental crust have been tectonically inserted into the Schistes Lustrés metasediments (e.g. [START_REF] Mattauer | Transverse lineation and large scale structures related to a high pressure-low temperature ophiolitic thrusting[END_REF][START_REF] Faure | Etude structurale d'un cisaillement ductile. Le charriage ophiolitique corse dans la région de Bastia[END_REF][START_REF] Malavieille | Etude tectonique et microtectonique de la nappe de socle de Centuri (zone des schistes lustres de Corse), consequences pour la géométrie de la chaîne alpine[END_REF][START_REF] Malavieille | Evolutionnary model for Alpine Corsica: mechanism for ophiolite emplacement[END_REF], T 1998;Rossi et al. 2003;[START_REF] Meresse | A fossil Ocean-Continent Transition of the Mesozoic Tethys preserved in the Schistes Lustrés nappe of northern Corsica[END_REF]. The Pigno and Centuri units (Fig. 1) are among the largest examples of these continental slices. The Pigno area is composed of an assemblage of Variscan metagranite and metagabbro and diorite slivers sandwiched within alpine serpentinized mantle rocks and sediments [START_REF] Lahondère | Relations du « socle ancien» de la région de Bastia (Corse) avec les Schistes lustrés environnants[END_REF]Faure et Malavieille, 1981). According to [START_REF] Meresse | A fossil Ocean-Continent Transition of the Mesozoic Tethys preserved in the Schistes Lustrés nappe of northern Corsica[END_REF], the pre-alpine basement slivers were considered as former extensional allochthons of continental crust abandoned during breakup of a newly exhumed seafloor of peridotite composition and the metasedimentary sequences that do not show affinities with the typical cover series of the Piemonte margin domains were reported to a more internal domain (oceanward). Owing to the high variability of the sedimentary successions as well as the presence of a basement composed of both continental and upper-mantle rocks, [START_REF] Meresse | A fossil Ocean-Continent Transition of the Mesozoic Tethys preserved in the Schistes Lustrés nappe of northern Corsica[END_REF] proposed that this area was part of a Mesozoic Ocean-Continent Transition (OCT) of the Tethys basin. The ophiolitic slivers that occur as interleaved thrust slices within the complex stack of tectonic units of the Schistes Lustrés nappe, experienced HP/LT (blueschist and eclogites facies) metamorphism followed by a greenschist facies retrogression (e.g. Gruppo di Lavoro sulle Ofioliti Mediterranee, 1977;[START_REF] Ohnenstetter | Crystallization and age of zircon from Corsican ophiolitic albitites: consequences for oceanic expansion in Jurassic times[END_REF][START_REF] Fournier | Alpine Corsica metamorphic core complex[END_REF][START_REF] Caron | Metamorphism and deformation in Alpine Corsica[END_REF][START_REF] Lahondère | Les schistes bleus et les éclogites à lawsonite des unités continentales et océaniques de la Corse alpine[END_REF][START_REF] Miller | An O-isotope profile through the HP-LT Corsican ophiolite, France and its implications for fluid flow during subduction[END_REF][START_REF] Miller | Albite vein formation during exhumation of high-pressure terranes: a case study from alpine Corsica[END_REF][START_REF] Marroni | The architecture of an incipient oceanic basin: a tentative reconstruction of the Jurassic Liguria-Piemonte basin along the Northern Apennines -Alpine Corsica transect[END_REF]. The HT/LP metamorphism that is recorded in the Schistes Lustrés is one line of evidence that they were a part of a paleo-OCT.

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This situation was recognised elsewhere in the hyper extended Laurentian margin during the early stage of the Iapetus ocean [START_REF] Chew | New constraints on Grampian orogenesis and the development of blueschist-facies metamorphism in western Ireland[END_REF]van Staal et al., 2013). clinopyroxene as well as accessory minerals of apatite, magnetite and zircon.

Several
Plagioclase is altered to sericite, epidote and zoisite whilst clinopyroxene is largely altered to tremolite. Close to the previous sample, a metamorphosed quartz diorite sample (2BDL165) was collected to the south of Bevinco (N42°36'08", E09°25'45"). It consists of 70-75% plagioclase, 15-20% glaucophane and 5-10% quartz as well as accessory minerals apatite, magnetite and zircon. A trondhjemite (12BA10) was collected 500m to the SE of the village of Sampolo in the Inzecca canyon (N42°05'50", E09°16"33"). It experienced high-pressure metamorphism, consisting of 75-80% plagioclase, 15-20% stilpnomelane and 5% glaucophane as well as accessory minerals apatite, magnetite and zircon.

Zircon and baddeleyite U-Pb dating

Zircon concentrates were separated from ~5-kg of gabbro and ~2-kg of quartz diorite and trondhjemite using standard density and magnetic separation techniques. Baddeleyite was separated from gabbro sample 11CO73. Zircon and baddeleyite grains, together with standard zircons 91500, Temora, Penglai and Qinghu as well as a baddeleyite standard Phalaborwa, were mounted in an epoxy mount that was then ground and polished to expose the crystals for analysis. Prior to SIMS analyses, the mount was vacuum-coated with high-purity gold.

Measurements of U, Th and Pb isotopes for zircon and baddeleyite were conducted using a Cameca IMS 1280 SIMS at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGG-CAS) in Beijing, using analytical procedures of [START_REF] Li | Precise determination of Phanerozoic zircon Pb/Pb age by multi-collector SIMS without external standardization[END_REF]. For zircon and baddeleyite analyses, Pb/U fractionation and U and Th concentrations were calibrated against zircon standard 91500 [START_REF] Wiedenbeck | Three natural zircon standards for U-Th-Pb, Lu-Hf, trace-element and REE analyses[END_REF][START_REF] Wiedenbeck | Further characterisation of the 91500 zircon crystal[END_REF] and baddeleyite standard Phalaborwa [START_REF] Heaman | The application of U-Pb geochronology to mafic, ultramafic and alkaline rocks: An evaluation of three mineral standards[END_REF], respectively. Measured compositions were corrected for common Pb using non-radiogenic 204 Pb, apart from two plagiogranite (quartz diorite and trondhjemite) samples with variably high common Pb. An average of the present day crustal composition [START_REF] Stacey | Approximation of terrestrial lead isotope evolution by a two-stage model[END_REF] is used for the common Pb. Data reduction was carried out using the Isoplot/Ex v. 2.49 program [START_REF] Ludwig | Users manual for Isoplot/Ex rev. 2.49[END_REF]. In order to monitor the external uncertainties of SIMS U-Pb zircon measurements, an in-house 1 and2.

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Zircon oxygen isotopes

Zircon oxygen isotopes were measured from the same domains that were previously analysed for U-Pb age determinations by using the same Cameca IMS 1280 SIMS at IGG-CAS, following the standard procedures of Li et al. (2010b). After U-Pb dating, the sample mount was reground and repolished to ensure that any oxygen implanted in the zircon surface from the O 2  beam used for U-Pb determination was removed. A focused Cs + primary ion beam ("Gaussian beam") with an intensity of ~2 nA was accelerated at +10 kV. The spot size was about 10 μm in diameter. Oxygen isotopes were measured using multi-collection mode of two off-axis Faraday cups. Uncertainties on single analyses are normally better than 0.3‰ (2SE). The instrumental mass fractionation factor (IMF) was corrected using the zircon standard Penglai with a δ 18 O value of 5.3‰ (Li et al., 2010c). A second zircon standard Temora was analysed as an unknown to ascertain the veracity of the IMF. Twelve measurements of Temora zircon standard during the course of this study yield a weighted mean of δ 18 O = 8.24 ± 0.40‰ (2SD), which is consistent within

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errors with the reported value of 8.20‰ [START_REF] Black | Improved 206 Pb/ 238 U microprobe geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards[END_REF].

Zircon Hf isotopes

Hafnium isotopes were measured on the same zircon grains that were previously analyzed for U-Pb and O isotopes, using a Neptune multi-collector ICP-MS equipped with the Geolas-193 laser-ablation system at IGG-CAS.

Analytical procedures and isobaric interference corrections are similar to those described by Wu et al. (2006) [START_REF] Woodhead | Preliminary appraisal of seven natural zircon reference materials for in situ Hf isotope determination[END_REF].

In situ zircon O and Hf isotopic results are listed in Appendix Table 3.

Analytical results

Zircon and baddeleyite U-Pb ages (1) The Lento ferrogabbros

Zircons from three Lento ferrogabbro samples (11CO72, 11CO73 and

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11CO75) have similar morphological and textural features. They are mostly transparent, 50-100 μm in length, and have aspect ratios of ~1:1 to 2:1. In CL images they are quite homogeneous, showing faint and broad zonation (Fig. 2).

Twelve analyses were conducted on 12 zircons from sample 11CO72 during a single analytical session (Appendix Table 1). These zircons are quite low in U (7-69 ppm) and Th (3-96 ppm); Th/U ratios vary between 0.29 and 1.40. (2) The Ortale gabbro

Zircons from Ortale gabbro sample (11CO78) are transparent, 50-150 μm in length, and have aspect ratios of ~1:1 to 2:1. In CL images they are quite

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homogeneous, with faint and wide zones (Fig. 2). Sixteen analyses were conducted on 16 zircons during a single analytical session (Appendix Table 1).

U and Th contents range from 24 to 331 ppm, and 12-795 ppm, respectively;

Th/U ratios vary between 0.34 and 3.75. (3) The Bevinco quartz diorite and Inzecca trondhjemite

Zircons from Bevinco quartz diorite sample (2BDL165) are transparent to semi-transparent, brownish in colour, 100-200 μm in length, and have aspect ratios of ~1:1 to 2:1. Zoning parallel to crystal faces is common under CL (Fig.

2); microfractures are often visible. Analyses were performed on 19 zircon grains during a single analytical session (Appendix Table 2). These zircons are characteristically low in U (14-53 ppm) and Th (6-38 ppm); Th/U ratios are between 0.30 and 0.80.

In situ zircon O-Hf isotopes (1) The Lento ferrogabbros

In situ O-Hf isotopic measurements were conducted on 41 zircons from three Lento ferrogabbro samples (11CO72, 11CO73 and 11CO75). Values of the measured δ 18 O zir are between 5.01‰ to 5.67‰ (Appendix Table 3), forming a Gaussian distribution. Thus, these Lento ferrogabbros exhibit homogeneous zircon oxygen isotopic compositions, with a mean δ 18 O zir value of 5.38 ± 0.35‰ (2SD) (Fig. 6).

These zircons yield 176 Yb/ 177 Hf ratio between 0.011 and 0.224 (Appendix Table 3), which are reasonably low for accurate isobaric interference correction (Wu et al., 2006). All the analyses give a small range of 176 Hf/ 177 Hf ratios between 0.283030 and 0.283184 (Appendix Table 3), corresponding to an average εHf(t) = +15.0 ± 3.0 (2SD).

(

2) The Ortale gabbro

In situ O-Hf isotopic measurements were conducted on 18 zircons from the Ortale gabbro sample (11CO78). The measured δ 18 O zir values range from 5.08‰ to 5.64‰, forming a Gaussian distribution. Thus, these zircons have homogeneous oxygen isotopic compositions, with a mean δ 18 O zir value of 5.35 ± 0.33‰ (2SD) (Fig. 6). They have low 176 Yb/ 177 Hf ratio (<0.10); the 176 Hf/ 177 Hf ratios are between 0.283040 and 0.283194 (Appendix Table 3), corresponding to an average εHf(t) = +15.9 ± 3.0 (2SD).

(

3) The Bevinco quartz diorite and Inzecca trondhjemite

Nineteen zircons from the Bevinco quartz diorite sample 2BDL165 were analyzed for O-Hf isotopes. They have a relatively wide range of the measured δ 18 O zir values varying from 4.20‰ to 5.40‰ (Appendix Table 3), forming a "skewed" distribution pattern (Fig. 6). It is noted that zircons with high common Pb (f 206 = 50.4 -9.8%) are low in δ 18 O zir values between 4.20‰ and 4.94‰.

The origin of the low δ 18 O zircons is discussed in detail later. Zircons with relatively low common Pb (f 206 = 5.26 -0.02%) have relatively high δ 18 O zir values between 5.01‰ and 5.40.‰, with the probability density function becoming more "Gaussian" in appearance (Fig. 6). These zircons give an average of 5.20 ± 0.22‰ (2SD) which is indistinguishable within analytical errors from the average value of 5.35 ± 0.33‰ for the Ortale gabbro. In contrast to the heterogeneous oxygen isotopes, all these zircons are fairly homogenous in Hf isotopes. They have slightly higher 176 Yb/ 177 Hf ratio (0.095 -0.224) than the gabbro zircons. The measured 176 Hf/ 177 Hf ratios are between 0.283073 and 0.283189 (Appendix Table 3), corresponding to an average εHf(t) = +15.8 ± 2.7 (2SD). (2SD). These zircons yield 176 Yb/ 177 Hf ratio of 0.087-0.127, which are in between those of gabbro zircons (mostly <0.10) and quartz diorite zircons (mostly >0.12). They have fairly homogenous 176 Hf/ 177 Hf ratios between 0.283073 and 0.283189 (Appendix Table 3), corresponding to an average εHf(t) = +15.5 ± 2.2 (2SD).

Overall, all analysed zircons from 4 gabbroic and 2 felsic samples have consistent εHf(t) values between +15.0 and +15.9 (Fig. 7).

Discussion

Formation age for ophiolites in the Schistes Lustrés nappe

Ophiolites from the Inzecca series of the Schistes Lustrés nappe were previously dated at 161± 3 Ma by ID-TIMS U-Pb zircon method for two ophiolitic albitites from Punta Corbara and Vezzani [START_REF] Ohnenstetter | Crystallization and age of zircon from Corsican ophiolitic albitites: consequences for oceanic expansion in Jurassic times[END_REF], and at 153 ± 2 Ma on zircon of trondhjemite from Rusio [START_REF] Rossi | Notice explicative[END_REF]. A 155 ± 6 Ma age was obtained from an internal (plagioclase-whole rock-clinopyroxene) Sm-Nd isochron for a gabbronoritic veinlet from Mt.

Maggiore peridotite [START_REF] Rampone | Isotopic equilibrium between mantle peridotite and melt: Evidence from the Corsica ophiolite[END_REF]. It is noteworthy that common Pb is high for zircons from the Punta Corbara and Vezzani albitites, as shown by the low measured 206 Pb/ 204 Pb ratios, between 500 and 870 [START_REF] Ohnenstetter | Crystallization and age of zircon from Corsican ophiolitic albitites: consequences for oceanic expansion in Jurassic times[END_REF]

A C C E P T E D M A N U S C R I P T

Ohnenstetter et al., 1981;[START_REF] Borsi | Age, origin and geodynamic significance of plagiogranites in lherzolites and gabbros of the Piedmont-Ligurian ocean basin[END_REF][START_REF] Costa | Evolution of the Ligurian Tethys in the Western Alps: Sm/Nd and U/Pb geochronology and rare-earth element geochemistry of the Montgenève ophiolite (France)[END_REF][START_REF] Rossi | La marge européenne de la Téthys jurassique en Corse : datation de trondhjémites de Balagne et indices de croûte continentale sous le domaine Balano-Ligure[END_REF][START_REF] Schaltegger | The transition from rifting to seafloor spreading within a magma-poor rifted margin: field and isotopic constraints[END_REF][START_REF] Rubatto | U-Pb dating of magmatic zircon and metamorphic baddeleyite in the Ligurian eclogites (Voltri Massif,Western Alps)[END_REF][START_REF] Stucki | Mesozoic ophiolite relics in the Southern Steep Belt of the Central Alps[END_REF][START_REF] Rubatto | Dissolution-precipitation of zircon at low-temperature, high-pressure conditions (Lanzo Massif, Italy)[END_REF]Li et al., 2013a; this study), because some of the plagiogranite zircons experienced hydrothermal alteration. The limited fractionation in Sm/Nd ratios of the analyzed minerals from ophiolitic gabbronorite [START_REF] Rampone | Isotopic equilibrium between mantle peridotite and melt: Evidence from the Corsica ophiolite[END_REF] et al., 1998, 2005;[START_REF] Valley | Oxygen isotopes in zircon[END_REF]. Thus, high common Pb in some zircons is attributed to post-magmatic hydrothermal alteration (see discussion below). By Ma. Taking into account the uncertainties, the age of 158±3 Ma for the Bevinco quartz diorite marginally overlaps with the age of 153±2 Ma for Rusio trondhjemite [START_REF] Rossi | Notice explicative[END_REF]. Ophiolites from the Schistes Lustrés nappe are some 10 m.y. younger than the plagiogranites of the Balagne nappe dated at 169 ± 3 Ma by SHRIMP zircon U-Pb method (Rossi et al., 2002, Fig. 8).

Thus, our new data support the paleogeographic reconstruction suggesting the more proximal position of the Balagne ophiolites with respect to a continental margin than the ophiolitic remnants enclosed in the Schistes Lustrés nappe [START_REF] Beccaluva | The Trace Element Geochemistry of Corsican Ophiolites[END_REF]Venturelli et al.,1979;[START_REF] Lahondère | Les schistes bleus et les éclogites à lawsonite des unités continentales et océaniques de la Corse alpine[END_REF][START_REF] Durand-Delga | Arguments en faveur de la position au Jurassique, des ophiolites de Balagne (Haute Corse) au voisinage de la marge continentale européenne[END_REF][START_REF] Principi | The Pre-orogenic volcano-sedimentary covers of the western Tethys oceanic basin: A review[END_REF][START_REF] Saccani | Corsica ophiolites: Geochemistry and petrogenesis of basaltic and metabasaltic rocks[END_REF]. for zircons from the Lento and Ortale gabbros (~5.36‰) and zircons in high-temperature equilibrium with juvenile mantle-derived magmas (~5.3‰) [START_REF] Valley | Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts. segments of subducted lithosphere in a high-pressure wedge: The example of Alpine Corsica (France)[END_REF](Valley et al., , 2005;;[START_REF] Valley | Oxygen isotopes in zircon[END_REF]. Three mechanisms may account for the genesis of the low-18 O zircons in the ophiolitic diorites. (1) These zircons were crystallized from low-18 O magmas that were produced by partial melting of oceanic gabbros that have been subjected to hydrothermal alteration at temperature above 200-250 °C, (2) they were reworked by high-pressure metamorphism during Alpine orogeny, and (3) the zircons were altered by subsolidus hydrothermal alteration.

Isotopic constraints on genesis of ophiolitic igneous rocks

The first mechanism is unlikely for the following reasons:

hydrothermally-altered oceanic gabbros are characterized by δ 18 O values lower than 5‰ (e.g., [START_REF] Muehlenbachs | Alteration of the ocean crust and the 18 O history of seawater[END_REF][START_REF] Bindeman | Oxygen isotope evidence for slab-derived in modern and ancient subduction zones[END_REF][START_REF] Alt | Oxygen isotope composition of a section of lower oceanic crust, ODP Hole 735[END_REF]. Thus, if the plagiogranites were derived by partial melting of these altered, low-18 O oceanic gabbros, they should be uniquely low in δ 18 O values. However, the majority of the plagiogranite zircons have δ 18 O values higher than 5‰. Zircons with low common-Pb from the Bevinco quartz diorite (e.g. [START_REF] Corfu | Atlas of Zircon Te xtures[END_REF][START_REF] Booth | Oxygen isotopic composition and U-Pb discordance in zircon[END_REF]. Hydrothermal alteration is thought

A C C E P T E D M A N U S C R I P T
to be the major cause for many low-δ 18 O zircons in altered ocean-crust [START_REF] Grimes | Uniformly mantle-like δ 18 O in zircons from oceanic plagiogranites and gabbros[END_REF] and the Chenaillet albitites in the Western Alps (Li et al., 2013a). Thus, we argue that the low-δ 18 O zircons in the Bevinco quartz diorite and the Inzecca trondhjemite were attributed to subsolidus hydrothermal alteration. Despite modifying the O-isotopes, hydrothermal alteration has little impact on Hf-isotopes recorded by these low-δ 18 O zircons. Moreover, the majority of low common-Pb zircons still preserve their primary oxygen isotopic compositions. Thus, the plagiogranites show similar zircon U-Pb age and O-Hf isotopic signatures to the gabbros, suggesting that the all these rocks were likely cogenetic, with the plagiogranites being formed by extreme fractional crystallization from basaltic magma.

Implications for opening and palinspastic reconstruction of the

Liguria-Piemonte ocean basin

High-precision geochronology of ophiolites can be used to constrain the timing of opening and palinspastic reconstructions of ancient oceanic domains.

Based on the recent compilation of reliable U-Pb zircon ages (Li et al., 2013a and references therein), and the new U-Pb age data in this study, the majority of the Alpine Tethys ophiolitic rocks were formed at ~158-169 Ma, indicating a fairly short life-span of ~11 m.y., rather than a ~30 m.y. life span as previously thought (e.g. [START_REF] Costa | Evolution of the Ligurian Tethys in the Western Alps: Sm/Nd and U/Pb geochronology and rare-earth element geochemistry of the Montgenève ophiolite (France)[END_REF]. Among them, the Corsican Alpine ophiolites dated at ~158-160 Ma in the Schistes Lustrés nappe are the A C C E P T E D M A N U S C R I P T youngest, whilst those dated at ~169 Ma [START_REF] Rossi | La marge européenne de la Téthys jurassique en Corse : datation de trondhjémites de Balagne et indices de croûte continentale sous le domaine Balano-Ligure[END_REF] in the Balagne Nappe are the oldest (Fig. 8). Thus, they might represent two distinct portions of the Liguria-Piemonte oceanic basin [START_REF] Beccaluva | The Trace Element Geochemistry of Corsican Ophiolites[END_REF]Venturelli et al.,1979;[START_REF] Principi | The Pre-orogenic volcano-sedimentary covers of the western Tethys oceanic basin: A review[END_REF][START_REF] Saccani | Corsica ophiolites: Geochemistry and petrogenesis of basaltic and metabasaltic rocks[END_REF].

Zircon Hf isotopic data, despite being less abundant than radiometric ages, have also been reported for other ophiolites from the Chenaillet nappe in the Western Alps, and the Platta and Err Nappes in the Eastern Central Alps. The Chenaillet troctolites and albitites dated at ~165 Ma have εHf(t) = +13.0 to +13.5 (Li et al., 2013a); the Platta and Err gabbros and albitites dated at ~161

Ma have εHf(t) = +14.4 to +14.9 [START_REF] Schaltegger | The transition from rifting to seafloor spreading within a magma-poor rifted margin: field and isotopic constraints[END_REF]. Our new analyses indicate that the gabbros and plagiogranites dated at ~159 Ma in the Schistes Lustrés nappe of Corsica have εHf(t) = +15.0 to +15.9. These highly positive εHf(t) values point to their derivation from highly depleted, MORB-like mantle sources. It is interesting to note that these εHf(t) values, despite indistinguishable within analytical errors, increase slightly with decreasing age.

If this correlation is true, the most depleted mantle source would be invoked for the younger ophiolitic rocks as previously proposed by [START_REF] Saccani | Corsica ophiolites: Geochemistry and petrogenesis of basaltic and metabasaltic rocks[END_REF].

In contrast to derivation from a highly depleted MORB-like depleted mantle source for the ~159 Ma ophiolites in the Schistes Lustrés nappe, the ~169 Ma ophiolites in the Balagne nappe contain numerous ancient xenocrystic zircons derived from a continental crust [START_REF] Rossi | La marge européenne de la Téthys jurassique en Corse : datation de trondhjémites de Balagne et indices de croûte continentale sous le domaine Balano-Ligure[END_REF]. It is suggested that the E-MORB type ophiolites in the Balagne nappe were emplaced within the OCT the Schistes Lustrés nappe (Fig. 1C). Thus the questions are: 1) to which margin does the Balagne nappe belong, and 2) how to reconstruct the paleogeography of the Liguria-Piemonte ocean? These questions were debated for a long time since [START_REF] Kober | Beiträge zur Geologie von Korsika[END_REF], [START_REF] Mattauer | Nouvelles données sur l'évolution structurale de la Corse Alpine[END_REF] Durand [START_REF] Durand-Delga | Arguments en faveur de la position au Jurassique, des ophiolites de Balagne (Haute Corse) au voisinage de la marge continentale européenne[END_REF], [START_REF] Lahondère | Les schistes bleus et les éclogites à lawsonite des unités continentales et océaniques de la Corse alpine[END_REF] and still more recently (e.g. [START_REF] Rosenbaum | The Western Alps from the Jurassic to Oligocene: spatio-temporal constraints and evolutionary reconstructions[END_REF][START_REF] Marroni | The architecture of an incipient oceanic basin: a tentative reconstruction of the Jurassic Liguria-Piemonte basin along the Northern Apennines -Alpine Corsica transect[END_REF][START_REF] Handy | Reconciling plate-tectonic reconstructions of Alpine Tethys with the geologicalgeophysical record of spreading and subduction in the Alps[END_REF][START_REF] Molli | Orogenic processes and the Corsica/Apennines geodynamic evolution: insights from Taiwan[END_REF].

According to [START_REF] Meresse | A fossil Ocean-Continent Transition of the Mesozoic Tethys preserved in the Schistes Lustrés nappe of northern Corsica[END_REF], the continental units of Centuri and upper plate during the Alpine orogeny (Fig. 9A), or a marginal basin in its western margin (Fig. 9B). Constrained by its Apenninic affinity, the paleolocation of the E-MORB Rio Magno Unit (Padoa et al., 2001) was reported (Padoa et Durand Delga, 2001) to the internal "igurides" Ligurides that escaped from deep involvement during the "Apenninic" subduction beneath the European continental margin. , 1997). This is similar to the opening of the South China Sea, a hyper extended margin where a complex OCT was developed [START_REF] Savva | Different expressions of rifting on the South China Sea margins[END_REF]. In this location, granitic blocks are enclosed by exhumed mantle areas as a result of boudinage. This is a possible proxy of the model suggested for the European Liguria-Piemonte OCT at the margin of the Corsica-Sardinia batholith [START_REF] Rossi | Genesis of a Variscan batholith: field, mineralogical and geochemical evidence from the Corsica-Sardinia batholith[END_REF]. This alternative palinspastic reconstruction model, however, needs to explain the absence of ductile shearing and syntectonic metamorphism in the Balagne nappe. If this unit is interpreted as a "proximal" and thus close to the Corsican continental margin, it would have experienced an intense synmetamorphic ductile deformation both during continental subduction and exhumation, however such features are presently lacking.

Conclusions

Regardless [START_REF] Rossi | La marge européenne de la Téthys jurassique en Corse : datation de trondhjémites de Balagne et indices de croûte continentale sous le domaine Balano-Ligure[END_REF]. See text for discussion.

A C C E P T E D M A N U S C R I P T

  rocks including diorite, quartz diorite, tonalite, trondjhemite and albitite) from three localities in the HP/LT Schistes Lustrés nappe located in the same group of structural units. The aim of the study was to: (1) provide new high-precision chronology for the Corsican Alpine ophiolites in order to better estimate the timing of spreading of the basin, (2) place new isotopic constraints on the genesis of these ophiolitic igneous rocks, particularly evidence for continental crust contamination through the presence of possible inherited zircons, and major variations in the Hf isotope ratio, and (3) discuss the paleoposition of the Corsican ophiolites within the Liguria-Piemonte ocean basin.

Fifteen

  zircons from the Inzecca trondhjemite sample 12BA10 were analysed for O-Hf isotopes. The measured δ 18 O zir values range from 4.58‰ to 5.39‰ (Fig. 6). Two zircons with high-common Pb give relatively lower δ 18 O zir values of 4.58-4.81‰. The remaining zircons with low-common Pb have fairly homogeneous δ 18 O zir values of 5.06-5.39‰, with an average of 5.18 ± 0.24‰ A C C E P T E D M A N U S C R I P T

  using the SIMS microanalytical technique, we have been able to target zircon domains at a scale of 2030 μm to avoid visible alteration and microfracture. Despite some analyses showing variably high common Pb, the lower-intercept A C C E P T E D M A N U S C R I P T ages are well defined at 158 ± 3 Ma and 159 ± 2 Ma for sample 2BDL165 and 12BA10, respectively, obtained from low-common Pb (least-altered) zircon domains. Thus, these two ages can be interpreted as the timing of crystallization age of the diorite and trondhjemite, which are in good agreement with the crystallization ages of the ophiolitic gabbros. To sum up, our new SIMS zircon and baddeleyite U-Pb data indicate that the ophiolitic fragments in the Schistes Lustrés nappe crystallized at ca. 159

  All analysed zircons from Lento and Ortale ophiolitic gabbros provide typical mantle-like δ 18 O zir value of 5.36 ± 0.34‰ (2SD, n = 59) and highly positive zircon εHf(t) value of +15.5 ± 3.0 (2 SD, n = 59), suggesting their A C C E P T E D M A N U S C R I P T crystallization from magmas that were produced by partial melting of a depleted, MORB-like mantle without any assimilation of continental crustal materials. Zircons from the Bevinco quartz diorite and the Inzecca trondhjemite show variable δ 18 O zir values ranging from 4.2‰ to 5.4‰. Ten of 34 measured plagiogranite zircons yield δ 18 O zir values of <4.9‰, which are lower than those

  trondhjemite have δ 18 O values of 5.20 ± 0.22‰ (2SD) and 5.18 ± 0.24‰ (2SD), respectively, which are indistinguishable within error from those of the gabbroic zircons. In addition, the Bevinco quartz diorite, the Inzecca trondhjemite, and other ophiolitic gabbros in the Schistes Lustrés nappe were crystallized synchronously at ~159 Ma, inconsistent with the formation of the felsic rocks by re-melting of the gabbroic rocks. High-pressure metamorphism could affect zircon O-isotopes to varying degrees (e.g.,[START_REF] Fu | O-Hf isotope constraints on the origin of zircon in high-pressure mélange blocks and associated matrix rocks from Tinos and Syros, Greece[END_REF]. While all the gabbros and plagiogranites in the Schistes Lustrés nappe were overprinted by the Alpine high-pressure metamorphism, only those zircons with high common-Pb in plagiogranites are low in δ 18 O values. In contrast, all the gabbro zircons preserve the O-isotope compositions of juvenile mantle-derived magmas. Therefore, an overprint by the Alpine high-pressure metamorphism seems unlikely as the major cause for the genesis of low-δ 18 O zircons in plagiogranites, though this mechanism cannot be entirely excluded.Low-δ 18 O values are uniquely found in the high common Pb zircons in the plagiogranites, implying that low-18 O signatures might be intrinsically associated with the involvement of non-radiogenic Pb components. It is noted that microfracture is often visible in plagiogranite zircons. Microfractures usually provide a passageway for subsolidus hydrothermal fluids and/or metamictization, resulting in common-Pb addition and/or radiogenic Pb-loss

  position (i.e. close to continental margin) of the Balagne ophiolite than the Schistes Lustrés ophiolites. It is also well documented that both the Schistes Lustrés nappe and the Upper units (Balagne, Nebbio, Maccinagio) were displaced from east to west. Moreover, from a simple geometric point of view, mapping shows that the Upper units tectonically overly

  Pigno (Fig 1), enclosed in the Schistes Lustrés nappe, belong to an OCT. Therefore, two OCT"s were documented in Corsica. On one hand, the "Pigno OCT" is intensely sheared and metamorphosed and crops out in the Schistes Lustrés nappe in contrast to the less deformed and weakly metamorphosed Balagne Upper unit. Assuming that there was only one Liguria-Piemonte oceanic basin, the Pigno OCT would likely represents the easternmost highly extended continental margin of the Corsica continent. The weakly deformed and metamorphosed Balagne OCT on the other hand might either represent the eastern margin of the Liguria-Piemonte Ocean, belonging to the future A C C E P T E D M A N U S C R I P T

  of the discussion related to continental margin position of the Balagne ophiolite within the Liguria-Piemonte ocean, our new SIMS U-Pb zircon and baddeleyite dating demonstrates that in Alpine Corsica, the ophiolitic magmatic rocks (gabbro, quartz diorite and trondhjemite) found in the Schistes Lustrés nappe crystallized synchronously at ~159 Ma, that is approximately 10 m.y. younger than the ophiolites of the Balagne Nappe. Zircons from the gabbros and plagiogranites are characterized by highly positive εHf(t) (+15.0 to +15.9) and mantle-like δ 18 O (5.2-5.4‰) values, indicating they were cogenetic, and crystallized from magmas that were produced by partial melting of a depleted, MORB-like mantle. The ophiolites in the Schistes Lustrés nappe, characterized by the most depleted Hf isotopic compositions, represent the youngest remnants of the western part of the Liguria-Piemonte ocean basin. By contrast, the ~169 Ma ophiolites in the Balagne nappe, having numerous xenocrystic zircons inherited from a continental crust, represent the oldest, and most marginal remnants of the Liguria-Piemonte oceanic basin emplaced upon the Corsica continental margin. .Y., Yang, Y.H., Xie, L.W., Yang, J.H., Xu, P., 2006. Hf isotopic compositions of the standard zircons and baddeleyites used in U-Pb geochronology. Chemical Geology 234, 105-126.

Figure captions Fig. 1 A

 captions1 Figure captions

Fig. 2

 2 Fig. 2 Cathodoluminescence (CL) images for representative zircons from

Fig. 3

 3 Fig. 3 Conventional zircon U-Pb concordia plots for the Lento ferrogabbros

Fig. 4

 4 Fig. 4 Conventional baddeleyite U-Pb concordia plot for the Lento

Fig. 5

 5 Fig. 5 Inverse zircon U-Pb concordia plots for the Bevinco quartz diorite

Fig. 6

 6 Fig. 6 Probabilistic histograms of zircon δ 18 O values for the ophiolites in the

Fig. 8

 8 Fig. 8 Synoptic plot of available radiometric and biostratigraphic constraints

Fig. 9

 9 Fig. 9 Two alternative palinspastic reconstructions of the Liguria-Piemonte

Figure 1 A

 1 Figure 1

Figure 5 A

 5 Figure 5

Figure 7 A

 7 Figure 7

  Common Pb is low; values of f 206 are lower than 0.5%. Ratios of 206 Pb/ 238 U and 207 Pb/ 235 Pb agree internally within the analytical precision (Fig. 3). A weighted mean 206 Pb/ 238 U age is calculated at 159 ± 1 Ma (2SE, MSWD = 0.71), which is interpreted as crystallization age of this gabbro.

  Common Pb is highly variable; values of f 206 range from 0.02% to 50.5%. For analyses with very high common Pb, common-Pb correction will result in very large uncertainty for radiogenic Pb isotopic ratios. Pb-correction (Appendix Table2). Thus, we interpret the age of 159 ± 2 Ma as the best estimate of the timing of crystallization of the Inzecca trondhjemite sample 12BA10. This ageagrees with the previous TIMS U-Pb zircon age of 161± 3 Ma determined by[START_REF] Ohnenstetter | Crystallization and age of zircon from Corsican ophiolitic albitites: consequences for oceanic expansion in Jurassic times[END_REF].

	5). This age is indistinguishable within errors from the weighted mean
	206 Pb/ 238 U age of 159 ± 2 Ma (MSWD = 1.9) based on 207 Pb-correction
	(Appendix Table 2), which is interpreted as the timing of crystallization of this quartz diorite sample. Zircons from Inzecca trondhjemite sample (12BA10) are transparent to semi-transparent, 100-200 μm in length, and have aspect ratios of ~1:1 to 2:1. M A N U S C R I P T Microfractures are visible in some zircons. In CL images they are relatively
	A C C E P T E D

Thus, we plot the common-Pb uncorrected isotopic ratios on the Tera-Wasserburg inverse U-Pb concordia diagram. All measured data define a discordia line with a lower-intercept age of 158 ± 3 Ma (2SE, MSWD = 2.6) (Fig. homogeneous and show faint zonation (Fig. 2). Analyses were made on 15 zircon grains during a single analytical session (Appendix Table 2). Similar to zircons from Ortale gabbro and Bevinco quartz diorite, the Inzecca trondhjemite zircons are also very low in U (11-34 ppm) and Th (3-11 ppm); Th/U ratios are fairly constant, between 0.26 and 0.44. Common Pb is variably high, values of f 206 are between 1.07% and 36.9%. On the Tera-Wasserburg inverse U-Pb concordia diagram, all the common-Pb uncorrected data give a lower-intercept age of 159 ± 2 Ma (MSWD = 1.4), identical within errors with the weighted mean 206 Pb/ 238 U age of 159 ± 2 Ma (MSWD = 1.9) based on 207

  , resulting in highly discordant 206 Pb/ 238 U (143-163 Ma) and

207

Pb/ 206 Pb (165-205 Ma) age results. In fact, high-common Pb is often observed in zircons from many Alpine Tethysian ophiolitic plagiogranites (e.g.

  make precise Sm-Nd isochron dating difficult.

	Zircons from the Bevinco quartz diorite (sample 2BDL165) and Inzecca
	trondhjemite (sample 12BA10) with variably high proportion (up to 50%) of
	common Pb often show microfracture. These zircons are characterized by low
	δ 18 O zir values (4.2-4.9‰), lower than that (5.3 ± 0.3‰, 1SD) for zircons in
	high-temperature equilibrium with juvenile mantle-derived magmas (e.g. Valley

Our new SIMS U-Pb zircon dating results give consistent crystallization ages of ~158-160 Ma for the ophiolitic gabbros from the Schistes Lustrés nappe. In addition, SIMS analysis gives a baddeleyite U-Pb age of 159 ± 2 Ma for the Lento ferrogabbro sample 11CO73, indistinguishable from the U-Pb age of 158 ± 2 Ma obtained from coexisting zircon.

  The determination of which continent it might be, either Adria or AlKaPeCa, is beyond the scope of this paper (see[START_REF] Molli | Orogenic processes and the Corsica/Apennines geodynamic evolution: insights from Taiwan[END_REF] for discussion). It is worth noting that geological arguments related to the provenance of detrital elements as well as geochronological and geochemical criteria to solve this question are quite difficult to use since the (European plate) such as Mg-K or hypersolvus granites (Durand-Delga et al.

	M A N U S C R I P T
	Liguria-Piemonte ocean opened in late Middle Jurassic within a continental crust formed in the Variscan orogeny. Thus, similar Paleozoic magmatic and metamorphic rocks, and Triassic to Early Jurassic sedimentary sequences should be encountered on both continental margins if the continental breakup A C C E P T E D
	was not superimposed to (or inherited from) a previous paleogeographical
	boundary.
	The Balagne nappe may alternatively belong to the European margin (Fig.
	9B, after Rossi et al. 2002) because the oceanic basin was fed from the
	Middle-Upper Jurassic, by detrital sediments and zircons which have been
	derived from specific rock types restricted to the Western Corsica basement

Acknowledgements

We thank J. Li, C. Yang and X.X. Ling for their assistance in zircon U-Pb age and O-Hf isotopic analyses. We also would like to thank two reviewers, Luigi Beccaluva and David Chew and Andrew Kerr (editor-in-chief) for their valuable suggestions and comments which have greatly improved the manuscript. Robin Offler is thanked for proofreading the paper. This work is supported by the National Natural Science foundation of China (grant 41273070) and the State Key Laboratory of Lithospheric Evolution, IGG-CAS.

in the very marginal part of the Liguria-Piemonte oceanic basin, and the N-MORB type ophiolites in the Schistes Lustrés nappe were formed approximately 10 m.y. later in the most central part of the ocean [START_REF] Rossi | La marge européenne de la Téthys jurassique en Corse : datation de trondhjémites de Balagne et indices de croûte continentale sous le domaine Balano-Ligure[END_REF].

The division between (depleted) N-MORB and (enriched) E-MORB (or T-MORB) has been discussed [START_REF] Arevalo | N-MORB + E-MORB = Global MORB AGU Fall Meeting[END_REF] and was re-examined for ophiolites from the Western Tethyan realm in External Liguride, Alpine, and Corsica. Many of the previously interpreted N-or E-MORB ophiolite were re-interpreted as N-MORB-type with a marked garnet signature resulting from partial melting of a depleted N-MORB type source bearing garnet-pyroxenite relics left by the delamination of the sub-continental mantle at the OCT zone [START_REF] Montanini | Petrogenesis of basalts and gabbros from an ancient continent-ocean transition (External Liguride ophiolites, Northern Italy)[END_REF][START_REF] Piccardo | The Jurassic Ligurian Tethys, a fossil ultraslow-spreading ocean: the mantle perspective[END_REF][START_REF] Saccani | Corsica ophiolites: Geochemistry and petrogenesis of basaltic and metabasaltic rocks[END_REF].

Basalts with garnet signature or G-MORB (Garnet-influenced MORB; [START_REF] Saccani | A new method of discriminating different types of post-Archean ophiolitic basalts and their tectonic significance using Th-Nb and Ce-Dy-Yb systematics[END_REF] corresponding to C-MORB [START_REF] Dilek | Ophiolite genesis and global tectonics: Geochemical and tectonic fingerprinting of ancient oceanic lithosphere[END_REF]) characterize the OCT.

The integration of high-precision U-Pb zircon geochronology, trace element composition, zircon Hf isotopic compositions reflective of the nature of mantle sources, and other aspects of geological observations by previous authors enable us to discuss the paleoposition of the Corsican ophiolites in the Liguria-Piemonte ocean basin (Fig. 9).

Presently, available radiometric, bulk geochemical and isotopic data, as well as biostratigraphy and sedimentology (summarized in section 3), argue for