Delphine Battistelli
email: delphine.battistelli@paris-sorbonne.fr

Florence Amardeilh
email: florence.amardeilh@mondeca.com

Knowledge Claims in Scientific Literature, Uncertainty and Semantic Annotation: A Case Study in the Biological Domain

Keywords: H.3.1 [Content Analysis and Indexing]: Linguistic processing. I.2.4 [Knowledge Representation Formalisms and Methods] I.2.7 [Natural Language Processing]: Language model Design, Experimentation, Standardization, Languages, Theory Modality, hedging, ontology population, semantic annotation

Within the framework of a project which includes biologists, computer scientists and linguists, we aim at implementing a knowledge base on a recently discovered biological phenomenon. To ascertain the validity of the knowledge extracted from biological literature and to use it to semantically annotate the contents, we address the temporal, modal and/or enunciative characteristics of the identified information. This work answers a specific need expressed by the biologists regarding the quality of the information extracted and recombined for them in the knowledge base.

INTRODUCTION AND CONTEXT

Within the framework of the Microbio project, a collaboration between biologists, computer scientists and linguists, we aim at implementing a knowledge base on a recently discovered biological phenomenon which was awarded the Nobel Prize in 2006. This phenomenon concerns miRNA, their mutation and regulation and their impact on the development or inhibition of certain diseases. The knowledge base collects data from the existing distributed databases but also from the biomedical literature available on the PubMed portal. This knowledge base is intended to be consulted by biologists but also to contribute to the semi-automatic annotation of biomedical literature in this specific domain.

An initial domain ontology about miRNA was modeled by taking as input various existing resources (such as the databases, the Sequence Ontology) and a set of expert interviews with biologists from the Pasteur Institute in Montevideo. As described in [START_REF] Jilani | Enrichissement automatique d'une base de connaissances biologiques à l'aide des outils du Web sémantique[END_REF], an information extraction tool aiming at populating the miRNA ontology was configured based on a corpus of full text articles from Medline selected for their relevance and domain coverage by the biologists. It can automatically identify and tag sentences which state a miRNA and at least a Gene and/or a disease and/or a Mutation (SNP). The CA Manager infrastructure [START_REF] Amardeilh | Semantic Annotation and Ontology Population[END_REF] was used to configure and develop the service that enriches the knowledge base with new instances tagged by the information extraction tool. This tool was chosen because it already includes a set of consolidation algorithms that can automatically disambiguate some information, resolve URIs and check domain, range and restriction constraints based on the ontology model of the knowledge base.

However, to ascertain the validity of the new knowledge and to use it to semantically annotate content, the biologists pointed out that this is not enough. In fact, temporal, modal and/or enunciative features indicative of authorial commitment to the identified information in biological texts need to be explicitly tagged and included in the knowledge base. This aspect of our approach constitutes its originality and answers a specific need expressed by the biologists regarding the quality of the information extracted and recombined for them in the knowledge base: namely, the need to have access to the validation framework of the information (in vitro, in vivo, in silico … experimentations, by which team, when, the certainty or uncertainty status of the results, and so on). Within the biological domain, [START_REF] Wilbur | New directions in biomedical text annotation: definitions, guidelines and corpus construction[END_REF] suggest that "contradictions and speculations in the literature are likely to prove a fruitful source of new hypotheses. All of this is territory yet to be explored".

We address the issue by focusing on the explicit linguistic markers of epistemic qualification; that is to say by working directly on the textual units that express a certain degree of confidence about a new item of knowledge. These textual units are often classified in the linguistic literature under the term "modality" or "hedging" and have been widely explored in scientific corpora. None of these previous approaches, however, have attempted a formal representation of this linguistic category.

In this article, we present our preliminary work for establishing a conceptual linguistic framework by the means of an ontology of what we name "the linguistic context of validation" of an item of information. This ontology aims to capture the elements of the information perspectivization expressed by the author of a text when using certain modal markers (may, maybe, …), references to other enunciators' claims or tenses (e.g. continuous present). Thus, this ontology represents the relations between three linguistic categories involved in the characterization of validity conditions of information: enunciative conditions, aspectotemporal conditions and modal (or rhetorical) conditions. These categories are relevant for any domain. Thus the ontology is independent of any domain ontology and reusable for other applications. In this paper we particularly focus on the analysis and the modeling aspects of modal and/or enunciative characteristics of tagged sentences. This ontology can be used to semantically annotate the extracted information for a subsequent ontology population. In the next section, we discuss how this need for extra information about new knowledge also concerns the idea of trust in knowledge acquisition and semantic web fields. We then present the linguistic perspective, i.e. how modality is expressed in texts, and our model for an ontology of modality. In section 4, we illustrate our approach within the Microbio project on the miRNA phenomenon to explain how it can be used to semantically annotate textual information in order to enrich a knowledge base.

TRUST IN KNOWLEDGE ACQUISITION

The issue of qualifying the epistemic nature of the extracted knowledge concerns various domains, not only the biomedical one. It can be seen as related to the trustworthiness and the confidence given to any information found on the Web by humans.

Current knowledge acquisition tools aim to automatically or semiautomatically produce semantic annotations or new knowledge instances from textual documents, based on an ontology. If we look at the Semantic Web architecture proposed by Sir Tim Berners-Lee [START_REF] Berners-Lee | The Scientific American Magazine[END_REF] [START_REF] Berners-Lee | Artificial Intelligence and The Semantic Web[END_REF], it is composed of several layers that target specific needs for putting into practice this vision of a "semantic web", interpretable by both humans and machines. Among these layers, the trust layer is one of the top semantic web architecture layers still awaiting standards, or at least recommendations to the community. This layer proposes to answer the following questions: how trustworthy is information found on the semantic Web? How do I decide that it is trustworthy? In 2004, [START_REF] Carroll | Named graphs, provenance and trust[END_REF] [START_REF] Bizer | Using context-and contentbased trust policies on the semantic web[END_REF] defined three types of trust mechanisms:

• Reputation-based Trust Mechanisms • Context-based Trust Mechanisms • Content-based Trust Mechanisms
While the first two types refer more to security issues and have been studied since 2003 by the Semantic Web community in different working groups, the last one still needs to be addressed. This is particularly the case when semantically annotating a web resource and populating an ontology from the analysis provided by information extraction tools. How can the content itself be used to characterize the trustworthiness of the extracted information to be added to the knowledge base? This can be further redefined as addressing the following issues:

• aggregation of the same content published by different information providers • following the same event or knowledge evolution over a certain timeline (e.g. from rumors to announcements to facts in the competitive intelligence domain)

• fine-grained characterization of the semantics of the information itself: the possibility that an event may occur and how sure are we of its occurrence?

The first two issues can be addressed using consolidation algorithms based on inference rule or constraint validation from a reasoning engine plugged to the ontology repository [START_REF]Text Mining for Biology and Biomedicine[END_REF]. On the other hand, the last one requires more information to be extracted from the text in order to guide the end-user about the trustworthiness of the acquired knowledge. In fact we try to answer the following question: how can we identify in a textual document "valid" knowledge which can be used to annotate the resource and be stored in a knowledge base but also queried and trusted by the end-user. To address that question, we plan to look at the textual modalities that can be found in the text and that can be linked to the domain-related knowledge instance itself.

EXPLORING MODALITY OR HEDGING IN SCIENTIFIC TEXTS 3.1 Epistemic modality in texts: state of the art and presentation of our methodology

The named entity and relation identification tasks are common in the Text Mining field applied to biological textual corpora [START_REF]Text Mining for Biology and Biomedicine[END_REF]. The task that consists in identifying the modality occurrences and giving them, if possible, a semantic interpretation in their context of occurrence is less classical and represents a challenge for the Text Mining field in general, and no doubt in the biomedical domain as well. For [START_REF] Szarvas | The BioScope corpus: annotation for negation, uncertainty and their scope in biomedical texts[END_REF], "detecting uncertain and negative assertions is essential in most Text Mining tasks where in general, the aim is to derive factual knowledge from textual data. This is especially so for many tasks in the biomedical (medical and biological) domain, where these language forms are used extensively in textual documents and are intended to express impressions, hypothesised explanations of experimental results or negative findings." The first research attempt that aimed at exploring the area of modality in biomedical texts was [START_REF] Light | The language of bioscience: facts, speculations, and statements in between[END_REF]. It is now possible to distinguish major approaches depending on whether or not they refer to extensive linguistic analysis of modality in scientific texts as in [START_REF] Hyland | Hedging in scientific research articles[END_REF] 1 . This is the case for [START_REF] Mercer | The frequency of hedging cues in citation contexts in scientific writing[END_REF] and [START_REF] Rubin | Stating with Certainty or Stating with Doubt: Intercoder Reliability Results for Manual Annotation of Epistemically Modalized Statements[END_REF] but not for [START_REF] Light | The language of bioscience: facts, speculations, and statements in between[END_REF], [START_REF] Medlock | Weakly supervised learning for hedge classification in scientific literature[END_REF] or [START_REF] Thompson | Categorising modality in biomedical texts[END_REF]. The latter approach consists in defining manually a set of annotation schemas to be used mainly in machine learning algorithms2 .

Our approach is closer to the methodological and conceptual one of [START_REF] Mercer | The frequency of hedging cues in citation contexts in scientific writing[END_REF], [START_REF]Corpus Linguistics[END_REF] or [START_REF] Rubin | Stating with Certainty or Stating with Doubt: Intercoder Reliability Results for Manual Annotation of Epistemically Modalized Statements[END_REF] since we explicitly integrate in our modeling work the fact that the semantic -and/or pragmatic -interpretation of the modal markers still remains an open issue, unresolved in a systematic way. This is essentially due to the following three factors:

(i) no consensus has yet emerged regarding the types and numbers of semantic values to be retained (on a scale from 'totally certain' to 'totally uncertain') [START_REF] Rubin | Stating with Certainty or Stating with Doubt: Intercoder Reliability Results for Manual Annotation of Epistemically Modalized Statements[END_REF];

(ii) the problem of the ambiguity of these markers in the textual context is raised (i.e. their precise value or their domain of semantic coverage) [START_REF] Vold | Modalité épistémique et discours scientifique, Une étude contrastive des modalisateurs épistémiques dans des articles de recherche français, norvégiens et anglais[END_REF];

(iii) lastly, the main ambiguity resides in their semantic vs. pragmatic interpretation (these markers work as many clues for epistemic or for rhetorical functions) [START_REF] Hyland | Hedging in scientific research articles[END_REF] [START_REF] Latour | Le métier de chercheur[END_REF].

As we will see below, the fact that these factors are integrated from the outset in our modeling proposal allows us to consider an operative automatic annotation tool for modality just as it is expressed in the texts.

Several other points deserve to be stressed concerning our methodology and our modeling process for annotating modality in texts.

First of all, in the exploration and analysis of modal markers in a scientific textual corpus, our strategy involves selecting a set of sentences that are already relevant for the biologists (e.g. a set of sentences about relations between miRNA, genes and diseases for instance). It should be remembered that one of the methodological principles of our modeling task consists in starting from a need and its analysis (the one formulated by the Pasteur Institute biologists) and thus from a domain and a specific application objective. This methodological position is based on a prerequisite formulated by the Knowledge Engineering community when creating ontologies [START_REF] Bachimont | Engagement sémantique et engagement ontologique : conception et réalisation d'ontologies en ingénierie des connaissances[END_REF]. We thus propose a different view from studies interested in the expression of modality in texts, as these studies generally analyse all the sentences of a text (within the biomedical domain or not, cf. works cited above). Moreover, we contend that our approach simulates more accurately the reading process of scientific researchers in their disciplinary field], or, to put it differently, replicates the process whereby entities (and/or the relations between entities) are first identified and then the linguistic characteristics of validation modes are considered (information presented as certain or not, by who, when…).

Second, another divergence with the above cited works is that we envisage the definition of a linguistic ontology of modality, trying to take advantage of the inference rules that an ontology permits with a view to placing the information retrieval task of a biologist towards the possible formulation of new hypotheses or discoveries by displaying the textual fragments organised according to epistemic criteria. It should be mentioned here that although projects for creating linguistic ontologies already exist (see [START_REF] Goecke | GOLD and Discourse: Domain-and Community-Specific Extensions[END_REF][22] and [START_REF] Ciccarese | An Overview of the SWAN 1.0 Ontology of Scientific Discourse[END_REF] on the biomedical domain in particular), none address the issue of creating an ontology of modality.

Furthermore, these projects underline the specific difficulty in creating an ontology for categories that are not lexical but grammatical (such as time, aspect, modality, gender…). They handle the issue of creating linguistic ontologies referring not to a grammatical category but to the rhetorical structure of texts ( [20][21]). However, [START_REF] Ciccarese | An Overview of the SWAN 1.0 Ontology of Scientific Discourse[END_REF], which is closer to our approach, even if the authors do not mention that they are interested in the grammatical category of modality, questions the distribution of textual units such as 'research statements', 'research questions' and 'comments' in the texts.

Third, and this point distinguishes our work from studies conducted more generally in the field of modality (with the exception of the annotation standard TimeML [START_REF] Pustejovsky | The specification Language TimeML[END_REF]), we propose to link modality explicitly to the categories of enunciation and temporality. This choice appears in the definition of our ontology as the root concept is 'Linguistic_Context_of_Validation". Not only does this approach have a sounder theoretical basis in linguistic analysis (cf. general linguistics studies that mention the close interweaving between these categories), it also has a clear applicative interest in terms of activity for competitive intelligence tasks in science. It consists in being able to manage the evolution overtime in the validation status of the information units that are being monitored as well as their characteristics in terms of enunciative responsibility (by whom -either an individual or a collective scientific entity -a scientific fact is claimed in a text). That is why we speak about the "context of linguistic validation" of an occurrence of a fact reported in a text. This concept has three subtypes: "modal context", "enunciative context" and "temporal context". In the remainder of this paper, we focus mainly on the "modal context" and the "enunciative context" by analysing a relevant sentence from our biomedical corpus.

The ontology of Modality

The concept "Linguistic_Context_of_Validation" fits into a larger specification of a context of validation of an occurrence of an identified fact. Consequently, the root concept of our ontology is "Context_of_Validation" which has two sub-concepts, the Linguistic one and an Extra-linguistic one.

The "Linguistic_Context_of_Validation" deals with the knowledge that is relative to the context of validation as expressed by the author of the text, whereas the "Extra_Linguistic_Context_of_Validation" looks at external knowledge of a text (such as its Impact Factor). The latter concept will not be discussed here.

Taking into account the enunciative aspect enables us to provide one part of the linguistic context of validation of the propositional content (more prosaically called 'identified fact' here) 3 , presented with the following sub-concepts as shown later in Figure 2: • "Full authorial commitment", i.e. the author of the text accepts full responsibility for the claims made in the text4;

• "Indirect Speech", i.e. attributed to another enunciator, with a marker of agreement or on the contrary of distance from the author; • "Limited temporal universe", e.g. "X said in 2002 that…"; • "Limited experimental universe", e.g. "in the experimentation…, it appears that…."; • "Limited thematic universe", e.g. "Regarding…, X said that…";

Taking into account the modal (epistemic) aspect 5 enables us to provide another part of the linguistic context of validation of the propositional content, represented by the following sub-concepts:

• "No_Doubt", e.g. "X regulates Y"; • "Doubt", e.g. "X may regulate Y"; These sub-concepts can be extended with a more fine-grained scale of the degree of modality expressed in a text.

Taking into account the temporal aspect 6 enables us to provide yet another part of the context of validation of the propositional content, represented by the following sub-concepts:

• "Has happened", i.e. having occurred at a date or over a period of time (not necessarily explicitly mentioned in the text); • "In progress", i.e. being developed at a date or over a period of time (not necessarily explicitly mentioned in the text); • "Stable", e.g. established as a stable truth at a date or over a period of time (not necessarily explicitly mentioned in the text).

Figure 1. Hierarchy of constitutive operations of a statement according to Nuyts [27]

work within the French enunciative linguistics field (Bally, Benveniste or Culioli) and also in Anglo-Saxon computational linguistics, for instance in [START_REF] Pustejovsky | The specification Language TimeML[END_REF] and [START_REF] Wiebe | Recognizing and Organizing Opinions Expressed in the World Press[END_REF] looking at the embedded discourses or 'nested-sources' analysis. 5 In fact, following [START_REF] Sauri | Determining Modality and Factuality for Text Entailment[END_REF] for instance, we only handle the epistemic aspect of modality (the one relative to the degree of certainty expressed regarding the truth of a propositional content) which directly concerns our approach to analysing scientific texts. 6 As the categories of time and aspect are interrelated from the point of view of linguistic analysis, we have preferred to explicitly label the concept as such.

These three key aspects refer to part of the hierarchy of the constituent operations of a statement presented in [START_REF] Nuyts | Modality : overview and linguistic issues[END_REF] -cf. Figure 1. Many linguistic analyses postulate the existence of an ordered organization of the constituent operations of a statement, in which the enunciative aspect occurs more or less explicitly. While there is some debate about certain specific features of this organization, it reflects nevertheless a widely-accepted trend towards a hierarchy of temporal, aspect and modal-based operations in the construction of a statement. We note that the markers for these categories do not have the same impact and assume the hierarchical organization described in Figure 1. The category of "Evidentiality" that marks the source of information ("according to what I know", "as I see it"…) has the largest range.

In the current state of our work, the linguistic ontology of the linguistic context of validation of an occurrence of a fact is presented in Figure 2 7 .

Figure 2. Hierarchical concept tree of the ontology of modality that permits to provide the linguistic context of validation of an occurrence of a fact in scientific texts

Regarding point (i) (cf. section 3.1. above), experiments conducted to measure inter-annotator agreement on the basis of a fine-grained scale of certainty reveal the difficulty of specifying the analytical categories. Recent work by Rubin [START_REF] Rubin | Stating with Certainty or Stating with Doubt: Intercoder Reliability Results for Manual Annotation of Epistemically Modalized Statements[END_REF]

concludes that "it is not entirely clear that a five-level distinction of certainty [ABSOLUTE, HIGH, MODERATE, LOW CERTAINTY and UNCERTAINTY] is preferable to a simplistic distinction between statements with certainty and statements with doubt".

Regarding point (iii) (cf. section 3.1. above), it is worth recalling the difficulties underlined by several studies, among them those mentioned by Rubin [START_REF] Rubin | Stating with Certainty or Stating with Doubt: Intercoder Reliability Results for Manual Annotation of Epistemically Modalized Statements[END_REF]: "Little attempt, however, has been made in natural language computing literature to manually annotate and consequently automate identification of statements with an explicitly expressed certainty or doubt, or shades of epistemic qualifications in between. This lack is possibly due to the complexity of computing epistemic interpretations in different pragmatic contexts; and due to unreliability of variety of linguistic expressions in English that could explicitly qualify a statement. Another complication is a lack of agreed-upon and easily identifiable discrete categories on the continuum from certainty to doubt".

Corroborating the remarks made by [START_REF] Mercer | The frequency of hedging cues in citation contexts in scientific writing[END_REF], "Hedging is critical in scientific discourse because it helps gain communal acceptance for knowledge. Scientific 'truth' is as much a social as an intellectual category, and the distinction writers make between their subject matter and how they want readers to understand their relationship to is crucial to such a highly self-conscious form of discourse. Not only does it influence the effectiveness and credibility of argumentation, but helps define what it means to write science…".

THE MICROBIO PROJECT: AN ILLUSTRATION FOR THE USE OF THE ONTOLOGY OF MODALITY

Microbio ontology

The Microbio ontology aims at formalizing the recent discoveries related to miRNAs: their regulation with the genes, and especially the mRNA, and their mutations which can cause or on the contrary inhibit diseases. Although no ontologies specifically represent this domain, several terminological and ontological resources, such as the Gene ontology [START_REF] Ashburner | Gene ontology: tool for the unification of biology. The Gene Ontology Consortium[END_REF] or the Sequence Ontology [START_REF] Eilbeck | The Sequence Ontology: A tool for the unification of genome annotations[END_REF], propose a general overview of genes and their sequences. Some databases on mRNAs and miRNAs, such as Tarbase [START_REF] Sethupathy | TarBase: A comprehensive database of experimentally supported animal microRNA targets[END_REF] and miRBase [START_REF] Griffiths-Jones | The microRNA Registry[END_REF], have recently been created, testifying to the interest of biologists in this new issue, which was awarded the Nobel Prize in 2006.

For this project we needed a high-level ontology that would be the starting point for modelling a new ontology dedicated to the representation of miRNAs and their impact on gene regulation and mutation. We therefore decided to work with the Sequence Ontology (SO) as it already represents biological sequences in general and especially the concepts of "Gene", "mRNA" and "miRNA". Some interesting relations between concepts are also described such as « is_part_of » to represent the decomposition of a miRNA into its atomic parts (« loop », « stem »…) or "regulated_by" to model the phenomenon of regulation between a miRNA and a mRNA segment even if it has the status "deprecated" in the Sequence Ontology. Finally, we enriched and extended this subpart of the SO with current knowledge about miRNA thanks to a series of interviews with domain experts, namely the biologists at the Pasteur Institute in Montevideo.

We modelled the Microbio ontology in the OWL as it allows us to keep a reference to the SO concepts reused from the SO and therefore to ease the integration of our ontology in the SO if relevant. When a concept comes from the SO or is equivalent to an existing concept in the SO, then a semantic link between the Microbio concept and the SO one is created via the "owl:equivalentClass" construction. For instance, the concept "miRNA" from the SO is an equivalent class to the concept "miARN" of the Microbio ontology (in French so far). We apply the exact same principle for equivalences between properties thanks to the "owl:equivalentProperty" construction. By initially adopting a separate representation from the SO, the conceptual independence of our ontology can be maintained until it has been definitively validated by the biologists, while still keeping a link to the SO for the time when we will ask the SO working group to integrate our work in their ontology.

The goal of the Microbio ontology is to be automatically instantiated from a corpus of scientific articles extracted from the Medline portal and dealing with miRNAs. A semantic annotation platform named CA Manager [START_REF] Amardeilh | Semantic Annotation and Ontology Population[END_REF][33], based on both UIMA and Semantic Web standards, has been used to set up an ontology population workflow, including an information extraction tool dedicated to the identification and tagging of the miRNA, the mRNA, the genes, the disease and the mutations (SNP code) in the text [START_REF] Jilani | Enrichissement automatique d'une base de connaissances biologiques à l'aide des outils du Web sémantique[END_REF] with a Sesame server to store the discovered knowledge instances of both concepts and properties. Another benefit in using the OWL format is that some basic inference rules can be applied to the knowledge base in order to detect new relations between miRNAs and genes or even between miRNAs and diseases. For the biologists, however, even detecting new relations is not enough if they do not know the degree of certainty and therefore of confidence that they can attribute to the information so as to process it in their own biological analysis.

Applying modalities to Microbio ontology

To overcome this issue, we proposed the ontology of modality to the biologists and we integrated it with the Microbio ontology.

The ontology of Modality is also formalized in OWL and we modeled the following object property "modal:validates(modal:Linguistic_Context_of_Validation, owl:Class)" that allows us to describe any other class with an instance from the concept 'Context of Validation' or one of its sub-concepts. This kind of representation is flexible enough for the ontology of Modality to be used to annotate any other domain ontology. Other concepts and properties have been added to the ontology of Modality to represent the textual context, such as:

• modal:marks(modal:Textual_Unit, modal:Linguistic_Context_of_Validation), • modal:has_origin(modal:Indirect_Speech, modal:Source),…

To enrich the semantic graph produced by the CA Manager platform in order to automatically populate the Microbio ontology with the modality context, we adopted the following method:

i. First, in a corpus composed of scientific full texts obtained from PubMed with the query [miRNAS+human], we isolate the sentences that carry information about an explicit relation between instances of two different concepts: one (or several) miRNAs and one (or several) genes. This is done through the use of patterns such as $miRNAS+$Gènes. A later step will consist in adding a list of relational markers (verbal, nominal or adjectival) between these two entities, to be defined by the Pasteur Institute biologists.

ii. Second, we work on this set of sentences and analyze their modal markers. This sentence can be represented by the semantic network as illustrated in Figure 3.

Figure 3. An example of a semantic network including modality information

This semantic network is consolidated and disambiguated by the algorithms provided by the CA Manager. These algorithms automatically control the ontology constraints (domain, range, restrictions…) and resolve the reference to existing instances in order to avoid any redundancy within the knowledge base. A link to the original document is kept for each instance created in the knowledge base so that the scientific articles in which the information was found are easily retrieved for later use and validation. The whole OWL graph is then imported and stored in the repository, ready to be manually validated by the biologists if necessary and queried by both the biologists and the inference engine.

CONCLUSION AND PERSPECTIVES

According to [START_REF] Ananiadou | Text Mining for Biomedicine[END_REF], "key aspects are to discover unsuspected, new knowledge hidden in the vast scientific literature, to support data driven hypothesis discovery and to derive meaning from the rich language of specialists as expressed in the plethora of textual reports, articles, etc. With the overwhelming amount of information (~80%) in textual unstructured form and the growing number of publications, an estimate of about 2.5 million articles published per year [START_REF] Harnad | DOI= The Access/Impact Problem and the Green and Gold Roads to Open Access[END_REF] it is not surprising that valuable new sources of research data typically remain underexploited and nuggets of insight or new knowledge are often never discovered in the sea of literature. Scientists are unable to keep abreast of developments in their fields and to make connections between seemingly unrelated facts to generate new ideas and hypotheses". Following the experiment conducted by [START_REF] Blaschke | Automatic Extraction of Biological Information from Scientific Text: Protein-Protein Interactions[END_REF] and described in [START_REF] Mercer | The frequency of hedging cues in citation contexts in scientific writing[END_REF], we aim at providing a tool to support biologists in their daily tasks for annotating scientific articles and (semi-)automatically populating a knowledge base dedicated to a growing field of research. This tool collects textual units scattered in the scientific literature about a particular theme (here the relations between miRNAs and genes) according to an epistemic criterion linked to the presence of certain modal markers in the relevant sentences.

The next step for the project is to evaluate this ontology of Modality and the benefits that the biologists could actually get from. Then we will configure the semantic annotation tool with the patterns that will allow automatic identification of the enunciative, modal and aspecto-temporal concepts in the scientific articles. Then, the whole ontology population application can be implemented and evaluated by the biologists from the Pasteur Institute in Montevideo.

We would also like to participate in the construction of the Trust layer in the Semantic Web architecture and combine our approach with the use of named graph for the enrichment of the semantic network generated on the domain from the textual resources by the modality knowledge. We could then propose an annotation scheme to identify and implement a set of Content-based Trust Mechanisms.

  Let's take an example and look at the following sentence: "The earlier work of Anttila et al. (2003) has also suggested the role of DNA methylation in CYP1A1 regulation". The blue part of the sentence refers to the Microbio ontology whereas the red part refers to the ontology of Modality, expressing indirect speech, e.g. Anttila et al. (2003), and the uncertainty of this regulation by the use of the verb "suggest".

It should be mentioned that most of the analyses have been done on English. Two exceptions, however, are[START_REF] Vold | Modalité épistémique et discours scientifique, Une étude contrastive des modalisateurs épistémiques dans des articles de recherche français, norvégiens et anglais[END_REF], which provides a comparative epistemic modality analysis of scientific corpora in three different languages (French, English and Norwegian), and[START_REF] Latour | Le métier de chercheur[END_REF], which presents the problem of epistemic vs. rhetorical interpretation of modal markers in French scientific discourse.

See[START_REF]Text Mining for Biology and Biomedicine[END_REF] for a presentation of the limits of this type of approach (called 'corpus linguistics') in terms of linguistic semantic analysis.

The essential parameters of the state of enunciation are "by whom", "when", "where" and "to whom it was said". In this paper, we address only the parameters "by whom" and "when".

By default, the author of a text takes responsibility for the entire set of statements of a text, that is to say that he is the primary enunciative source. This conception is explicitly adopted in

As in[START_REF] Sauri | Determining Modality and Factuality for Text Entailment[END_REF], we chose to separate evidential and modal categories. This is an important theoretical option to point out and it distinguishes our approach from that of[START_REF] Wiebe | Recognizing and Organizing Opinions Expressed in the World Press[END_REF] for instance, where, as in[START_REF] Nuyts | Modality : overview and linguistic issues[END_REF], the category of 'evidentiality' is considered as a subcategory of 'modality'.

ACKNOWLEDGEMENTS

The Microbio project is partially funded by the Stic-Amsud collaborative research program.


Paul Turrillot
email: p.turrillot@brgm.frou

Romain Augier

Michel Faure

The top-to-the-southeast Sarzeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica

Keywords: Late-orogenic extension, extensional shear-zone, monazite U-Th/Pb chemical dating, Armorican Massif, Variscan Belt

published or not. The documents may come

yielded by the monazite in the dykes comply with the interpretation of a synkinematic magmatism. In the Golfe du Morbihan, the geometric relationships between the SSZ and the migmatitic host rocks do not support a previous interpretation as a Metamorphic Core Complex. Regionally, the SSZ kinematics is consistent with the Late Carboniferous orogen-

INTRODUCTION

It is widely acknowledged that in collisional orogens, the thickened continental crust commonly experiences thinning driven by gravitational instability. Removal of the thick orogenic root is thus accommodated by normal faulting coeval with the opening of intramontane sedimentary basins and by partial melting. Such a process has been documented in the French Variscan belt. In the French Central Massif, several ductile normal faults, metamorphic domes and syn-kinematic plutons are described [e.g. Malavielle et al., 1990;Van Den Driesche and Brun, 1991-92;Faure, 1995 and enclosed references]. Numerous studies have demonstrated the relationships between pluton emplacement and the regional extensional setting [e.g. [START_REF] Faure | Crustal thinning recorded by the shape of the Namurian-Westphalian leucogranite in the Variscan Belt of the Northwest Massif Central, France[END_REF][START_REF] Roig | Folding and granite emplacement inferred from structural, strain, TEM, and gravimetric analyses : the case study of the Tulle antiform, SW French Massif Central[END_REF][START_REF] Chen | Pluton-dykes relationships from AMS and microstuctural studies in a Variscan granite from French Massif Central[END_REF][START_REF] Chen | A multidisciplinary study of a syntectonic pluton close to a major lithospheric-scale fault: relationships between the Montmarault granitic massif and the Sillon Houiller Fault in the Variscan French Massif Central. Part I: Geochronology, mineral fabrics and tectonic implications[END_REF]. Deformation-assisted thinning mechanisms have been also invoked in the southern part of the Armorican Massif [START_REF] Gapais | La zone de cisaillement de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF]Brown and Dallmeyer, 1996;[START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF] that presents numerous lithological and structural similarities with the Central Massif. South of Vannes, migmatites and peraluminous granitoids are well exposed along the Golfe du Morbihan sea coast. This area has been interpreted as a diapiric dome [START_REF] Audren | Evolution structurale de la Bretagne méridionale au Paléozoïque[END_REF][START_REF] Angelier | Inversion of field data in fault tectonics to obtain the regional stress-III. A new rapid direct inversion method by analytical means[END_REF]Le Métour, 1978] or a Metamorphic Core Complex (MCC) [Brown and Dallmeyer, 1996]. However, the structural and radiometric data supporting these models are rare. This paper aims to present a microtectonic analysis of the newly defined here "Sarzeau Shear Zone" (SSZ) that will be interpreted as an extensional ductile shear zone. Moreover, monazite U-Th/Pb chemical datings of syn-kinematic leucogranite are also provided for the first time. The relationships between migmatites, plutonism and ductile shearing are discussed and replaced in the Late Carboniferous extensional framework of Southern Armorica.

GEOLOGICAL SETTING

The French Armorican Massif belongs to the European Variscan belt. It forms the northern part of the Ibero-Armorican orocline, a major syntaxis of the Variscan belt of Western Europe. The geodynamic evolution of the Armorican Massif is still discussed nevertheless most of authors agree with the following tectonic stages [e.g., [START_REF] Pin | Age des épisodes de métamorphisme paléozoïques dans le Massif central et le Massif armoricain[END_REF]Ballèvre et al., 1992;[START_REF] Matte | -The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of the Armorica microplate: a review[END_REF][START_REF] Faure | Paleozoic tectonic evolution of medio-Europa from the example of the French Massif Central and Massif Armoricain[END_REF]. Since Early Ordovician, the Armorica microcontinent was rifted from the northern margin of Gondwana and the Medio-European (or Galicia-Central Massif) Ocean opened. This ocean closed in Late Silurian through a north-directed subduction. In Late Silurian or Early Devonian, the North-Gondwana margin, represented by the Southern part of the Armorican Massif and the entire Central Massif, experienced continental subduction responsible for nappe stacking and high-pressure metamorphism. This Eo-Variscan collision was followed in Early Devonian, around 390 Ma, by a pervasive crustal melting and the exhumation of the HP rocks [e.g. [START_REF] Cocherie | -Electron microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites[END_REF][START_REF] Faure | -Devonian geodynamic evolution of the Variscan Belt, insights from the French Massif Central and Massif Armoricain[END_REF]. The Carboniferous evolution of the Armorican Massif is characterised by an intra-continental deformation coeval with granitic plutons emplacement.

The Armorican Massif is subdivided into several tectono-metamorphic domains by two dextral shear zones, namely the North Armorican Shear Zone (NASZ) and South Armorican shear zone (SASZ) that split into a WNW-ESZ northern branch and a NW-SE southern branch [Jégouzo, 1980] (fig. 1b). However, these Carboniferous shear zones which developed late in the tectonic evolution do not represent suture zones or plate boundaries.

When dealing with the Early Variscan collisional events, the Nort-sur-Erdre fault represents the suture zone (i.e. the remnant of the Medio-European Ocean) between Central Armorica (or Armorica microcontinent) and South Armorica. In the following, only an outline of South Armorica geology will be described, readers unfamiliar with the geology of the French Armorican Massif are referred to recent publication [e.g. [START_REF] Faure | Paleozoic tectonic evolution of medio-Europa from the example of the French Massif Central and Massif Armoricain[END_REF].

South of the southern branch of the SASZ, several litho-tectonic units are described [e.g. Cogné, 1974;[START_REF] Le | Le massif Armoricain[END_REF]. From top-to bottom, they are: i) the blueschist facies uppermost klippes of Ile de Groix and Bois de Cené, ii) the Upper Unit composed of intermediate pressure/temperature metamorphic rocks, iii) a Lower Unit made of high temperature metamorphic rocks and migmatites. Moreover, several generations of peraluminous plutons intrude the base of this nappe-stack (fig. 1a).

In the study area, around the Golfe du Morbihan, only the later two units are recognised (fig. 2a,b). There, the intermediate unit (Upper Unit), called the Vilaine and Belle Ile groups [START_REF] Audren | Notice explicative[END_REF]] consists of metapelites, quartzites, or rare amphibolites and metarhyolites [START_REF] Triboulet | Controls on P-T-t-deformation path from amphibole zonation during progressive metamorphism of basic rocks (estuary of the river Vilaine, south Brittany, France)[END_REF]. Structurally, these rocks, that belongs to the geometrically highest unit located to the SE and E of the Golfe du Morbihan are characterised by a sub-horizontal foliation and a NW-SE trending pervasive stretching lineation. The underlying unit consists of WNW-ESE trending sub-vertical to vertical migmatites that were interpreted as a diapiric dome [Audren and Le Métour, 1976;[START_REF] Audren | Evolution structurale de la Bretagne méridionale au Paléozoïque[END_REF][START_REF] Angelier | Inversion of field data in fault tectonics to obtain the regional stress-III. A new rapid direct inversion method by analytical means[END_REF] developed at the expense of paragneiss, orthogneiss, amphibolite and rare pyroxene bearing gneiss.

These rocks are dated at 376 ±19Ma and 372 ±24Ma by whole rock Rb/Sr and zircon U/Pb methods, respectively [START_REF] Pin | Age des épisodes de métamorphisme paléozoïques dans le Massif central et le Massif armoricain[END_REF]. However, on the basis of recent monazite U-Th/Pb electron probe microanalyser (EPMA) analyses, these dates must be revised as ). This granitic dyke swarm is collectively called "the Sarzeau granite" [START_REF] Gapais | La zone de cisaillement de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF]. To the west of the study area crops out the Carnac granitic pluton (fig. 2a) whose genetic relationships with the Sarzeau leucogranite will be discussed later.

West of the Golfe du Morbihan, the Quiberon Shear Zone is a kilometre-scale ductile extensional shear zone with top-to-the-W kinematics coeval with the emplacement of the Quiberon leucogranitic pluton. This shear zone has been interpreted as the structure responsible for the exhumation of the Golfe du Morbihan migmatites (fig. 1a) [START_REF] Gapais | La zone de cisaillement de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF]Brown and Dallmeyer, 1996]. Yet, the systematic spatial association of ductile extensional shear zones and leucrogranitic plutons in South Armorica has been generalised as a regional scale flat-lying detachment fault extending from Quiberon to the Sables d'Olonne in Vendée [START_REF] Gapais | La zone de cisaillement de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF].

STRUCTURAL ANALYSIS

The "Golfe du Morbihan" experienced a polyphase tectono-metamorphic evolution as illustrated by several generations of ductile and brittle structures. The field-based structural analysis aims to 1) describe the first order structural framework of the study area, 2) define the main shear zone kinematics and 3) insure an accurate relation between structures and magmatic bodies.

I. Description of first-order structures

The major ductile structure, called the Sarzeau Shear Zone (SSZ hereafter), corresponds to the contact between the Upper and Lower Units (fig. 2a,b). Furthermore, metre to decametrescale low to moderately dipping faults and discrete ductile shear zones are scattered in the core of the Lower Unit. These discrete ductile shear bands are particularly concentrated near the top of the Lower unit. In the Lower Unit, low-strain domains preserve older structures.

The 1/50 000 scale geological mapping (R. Augier, work in progress) allows us to accurately outline the SSZ. In map view, the SSZ shows a low to moderate angle of dip. It displays a 300 to 600m thick zone of S-C mylonites and locally ultramylonites (e.g. near Sarzeau village, fig. 2a) embracing both the Upper Unit and the Lower Unit. However, strain is mostly concentrated in the footwall where mylonites reach locally 400m thick (fig. 2a,b). Mylonites exhibit a sub-horizontal to gently (10° to 20°) eastward dipping foliation carrying a pervasive NW-SE trending stretching lineation often accompanied by unambiguous shear criteria.

Below the SSZ, scattered metre-scale shear bands and low-angle faults occur. The first ones are large-scale heterogeneities within the migmatites which foliation strikes about N 130°E with a vertical attitude. The best exposures are located on the cliffs at the Rohu cape (fg.3B and D). There, few decimetres to metre-scale shear zones concentrate into aplitic or pegmatitic dykes cutting across a migmatitic orthogneiss devoid of any post-migmatitic ductile deformation (fig. 3B andD). In the dykes, the deformation evolves from an S-C fabric to an ultramylonitic fabric from border to core, respectively (fig. 3B).

Shear deformation is also accommodated by flat or gently dipping faults ranging from about 5 to 25° (fig. 3E) that often overprint earlier ductile features; some of which are in their present geometry, gently dipping reverse faults (fig. 4a). High-temperature striations, marked by quartz rods or fibres and preferred orientation of micas are often observed on low-angle fault planes. Quartz and muscovite crystallisations conspicuously present on these fault planes, argue for fluid circulation coeval with the faulting. The above described ductile structures are commonly reactivated during a late brittle stage of deformation as attested by cataclasitic zones and fault breccias. Generally, the ductile or ductile-brittle gently-dipping structures are reworked by highly-dipping faults (fig. 3E).

II. SSZ kinematics

In the study area, a total of 52 structural sites, mainly located in the Lower Unit, were visited.

A structural site is defined as a continuous outcrop of approximately 5 to 30m long, in which ductile structures and gently dipping fault planes can be observed. Distinction and sequence of ductile structures is the result of the confrontation of criteria such as (1) published pressuretemperature conditions of the deformation [i.e. Brown and Dallmeyer, 1996;[START_REF] Brown | Quantitative Constraints on Metamorphism in the Variscides of Southern Brittany -a Complementary Pseudosection Approach[END_REF], (2) cross-cutting field relations between successive deformational events and

(3) reactivation of pre-existing defects.

At outcrop scale, the foliations and lineations have routinely been measured within the SSZ and at its vicinity. The sense of shear has been determined by well developed kinematic criteria such as S-C fabric, fold asymmetry or sigma or delta-type porphyroclast systems (fig. 3C) [Paschier and Trouw, 1996]. The S-C fabric, particularly abundant in the Sarzeau granite, consistently indicates a top-to-the-SE shearing (fig. 3A). In its present geometry, the SSZ appears as an extensional shear zone with a top-to-the-SE kinematics. Meso-scale shear zones also display unambiguous kinematic indicators that also argue for a top-to-the-SE shearing in agreement with the SSZ (fig. 3C).

Only the structural sites where ductile features such as shear bands (fig. 3A) are not subsequently overprinted by brittle faults are presented in figure 4a. An average of 25 ductile shear planes and the associated lineation have been measured (Table 1) and plotted with the Tector software [START_REF] Angelier | Inversion of field data in fault tectonics to obtain the regional stress-III. A new rapid direct inversion method by analytical means[END_REF] (fig. 4a). The two-mica Sarzeau granite has been preferred as it is likely to be a material devoid of inherited structures. GPS locations as well as detail results are given in Table 1.

The lineation direction is well defined in most of structural sites and argue for a N 120°E stretching, the X finite strain axis over the study area. This stretching direction remains unchanged whatever the position of the sites along the strike of the SSZ. Nevertheless, it is worth to note that site 25 shows a rather N 177°E stretching that can be explained by a dextral rigid rotation around a vertical axis (fig. 4a). Associated smaller-scale structures such as shear bands and low-angle faults (noted "S" on figure 4a) yield consistent N 120°E trending lineation associated with a top-to-the-SE sense of shear (fig. 4a). Thus, the SSZ appears as a gently dipping extensional shear zone in its western part and admits in its eastern part a relative sinistral strike-slip motion (fig. 2b, cross-section A), as shown in sites 21 and 42 (fig. 4a). Therefore, the eastern part of the SSZ behaves as a transfer fault that accommodates the top-to-the-SE normal shearing. It is worth noting that a sinistral displacement along a NW-SE trending vertical fault is kinematically in contradiction with the dextral strike-slip displacement along the SASZ.

III. Typology and structure of the late granite dykes

As mentioned in the geological setting section, the Lower Unit is made of high-grade metamorphic rocks, migmatites and several generations of granitic bodies. The fine-grained two-mica Sarzeau granite is widespread in the study area, particularly within the SSZ where these rocks are deformed by a single set of shear bands (fig. 2a, fig. 4a). Field work in the SSZ footwall shows that the Sarzeau granite does not form a single pluton but rather consists of a dense network of meter-scale to kilometre-scale aplitic dykes (fig. 2a, fig. 4a). Two main types of dyke are recognised. The first one, which is volumetrically the most abundant, consists of isotropic fine-grained two-mica granite with sub-vertical sharp boundaries with respect to the surrounding rocks (fig. 3F). The second type which is less abundant corresponds to systematically gently-west dipping dykes (i.e. 20 to 40°; fig. 3G andH) showing sometimes a bending upward. The best exposures of this type of dyke are located in the western end of the Ile aux Moines (fig. 2a). These dykes display an internal penetrative fabric defined by the alternation of quartz-feldspar-rich mica-poor, and biotite-rich layers lying parallel to the dyke borders which is interpreted here as a magmatic layering. Whatever their types the dykes cross-cut at right angle the host-rock migmatites (fig. 3I andJ).

The dyke preferred direction is clearly visible on the geological map (fig. 2a,R. Augier work in progress, J. Le Métour work in progress). A statistical investigation, based on direct measurements of dyke walls, reveals an N 30°E regional consistency (fig. 4b) that stands at right-angle from the NW-SE stretching direction inferred from the mineral lineation in the SSZ (fig. 4a). It is worth noting that the eastern border of the Carnac pluton exhibits also several NNW-SSE trending apexes (fig. 2a,J. Le Métour work in progress). On the basis of petrology and preliminary structural investigations, we propose that the Carnac pluton and the Sarzeau dyke swarm formed during the same magmatic event. The Carnac pluton might be considered as the feeder pluton of the Sarzeau dykes. Such an interpretation requires further studies but seems to be in agreement with geological mapping and recent radiometric dating (A. Cocherie and J. Le Métour, personal communication).

TIME CONSTRAINTS

The EPMA is a very efficient tool for determining in situ ages. Due to the lack of common lead, and high U and Th contents, monazite is the most convenient mineral to apply the chemical U-Th/Pb geochronological method [START_REF] Suzuki | The chemical Th-U-total Pb isochron ages of zircon and monazite from the gray granite of the Hida terranne, Japan[END_REF]Cocherie and Albarède, 2001]. This method is suitable to determine the age of high temperature events such as magma crystallisation [e.g. Be et al., 2006] or high temperature or fluid assisted recrystallisation. In this section, we aim to assign accurate time constraints on the S te -Anne d'Auray type anatectic granite and two-mica Sarzeau granite.

I. Sampling strategy

Seven samples have been dated from the magmatic bodies of the Lower Unit of the Golfe du Morbihan area (see location in fig. 2a andTable 2). Sample V.403 is from a S te -Anne d'Auray type anatectic granite body interlayered within host-rocks migmatites. This rock V.403 consists in a biotite-rich granite where mylonitisation and metamorphic reactions are responsible for secondary muscovite blastesis coeval with progressive disappearance of biotite and grain-size reduction (quartz-feldspar grain size is about 0.5mm).

The other samples are from Sarzeau granite dykes intruding migmatites: samples V.102, V.104, V.108 and V.404 are undeformed granite picked structurally far below the SSZ (ca 800m to 3km). Conversely, samples V.402 and V.405 are from mylonitized granite within the SSZ exhibiting an averaged grain-size of about 0.3mm or even less. Petrologically, all these samples derive from above-mentioned quartz-plagioclase two-mica Sarzeau granite; the most deformed samples being devoid of biotite. The lack of biotite seems to be correlated to an intense shearing as shown in figure 3A (V.405). All dated samples contain accessoryphases such as zircon, monazite, apatite and undifferentiated polymetallic oxides.

II. Analytical procedure

The analysed monazite grains were extracted by crushing and sieving of an average of 5 kilograms of rocks per sample, followed by heavy liquids separation by P. Jézéquel. For this study, the minerals were mounted in resin and polished. The monazite grains are then observed through a scanning electron microscope (SEM) in back-scattered electron (BSE) mode in order to choose the larger and less fractured grains for dating. The chemical analyses were carried out using a Cameca SX 50 EPMA operated jointly by the BRGM-CNRS -Orléans University. The accelerating-voltage and beam-current-intensity conditions were 20kV and 200nA, respectively. The counting time (peak, background) for monazite was 240s for Pb, 200s for U and 40s for all the other elements. According to this procedure, the detection limit (2σ) is 110ppm for Pb, 105ppm for U, and 130ppm for Th [Cocherie and Albarède, 2001]. The average grain U, Th and Pb contents are given in Table 2.

III. Monazite description

SEM-BSE observation allows us to recognise three monazite populations that are present in all dated samples. A representative grain for each population is shown in figure 5. The first population consists of grains with a patchy zoning related to a relatively large variation in Th and U contents (fig. 5C). This kind of zoning is very common in monazite but does not reflect any growth zoning [Zhu and O'Nions, 1999, Cocherie et al., 2005[START_REF] Santosh | The timing of ultrahigh-temperature metamorphism in Southern India: U-Th-Pb electron microprobe ages from zircon and monazite in sapphirine-bearing granulites[END_REF]]. The second monazite population shows homogenous grains (fig. 5A), and the third one represents zoned grains with a clear core and rim structure (fig. 5B). This kind of zoning is characteristic of a polyphase crystallisation [Be et al., 2006]. All the grains selected for dating are unaltered and have a rather large size ranging from about 50 to 300µm (Table 2).

IV. Results

Table 2 gives for each sample, the number of grains analysed, the number of analysis, the average size of the dated monazite, the Pb, U, Th contents, and Th/Pb and U/Pb ratios and the U-Th/Pb ages. The EPMA allows an age calculation from a small analytic spot (2µm diameter) of U, Th and Pb concentrations. But, the multiplication of a large number of analyses in different grains from the same rock provides a statistical determination of an average age, assuming that all the analysed grains formed during the same stage of crystallisation. Details on the calculation method are given in [START_REF] Legendre | Potential minerals for determining U-Th-Pb chemical age using electron microprobe[END_REF].

Analytical data are plotted in a U/Pb vs. Th/Pb diagram [Cocherie and Albarède, 2001]. The statistical analysis and the graphic projection have been realised with the software EPMA dating.2 and Isoplot.3, respectively [START_REF] Pommier | EPMA Dating User's manual: Age calculation from electron probe microanalyser measurements of U-Th-Pb[END_REF][START_REF] Ludwig | User's manual for ISOPLOT/EX, version2. A geochronological toolkit for Microsoft Excel (version 3.1)[END_REF]. The rather large U concentrations (Table 2) and the significant variation in Th/U in the analysed monazite lead to a large spread of the data in the U/Pb vs. Th/Pb diagram. Consequently, a well-constrained regression line, close to the theoretical isochron can be defined (fig. 6). Therefore, a statistical age can confidently be calculated at the centroid of the population. The theoretical Th-Pb and U-Pb ages calculated at the intercepts of the regression line with the axis are similar within the errors (fig. 6), attesting to the quality of data processing.

To explore chemical and age significance of monazite core and rim zoning (fig. 5B as a representative example), as discussed in Be et al. (2006), age scatter for a single monazite grain has been studied. Despite a large spread, the individual ages within cores and rims, plotted in figure 5d, yield statistically identical mean ages within uncertainty. All of the dated core and rim zoned monazites yield internally consistent ages, irrespective of this chemical zoning, and argues for a single crystallisation event.

The S te -Anne d'Auray type anatectic granite (Sample V.403) yields an age of 322 ±3Ma, (fig. 6). Whatever their deformation state (i.e. mylonitised or not), the samples of Sarzeau granite give two age populations (fig. 6) (i.e. Late Carboniferous and Late Devonian-Early Carboniferous). Samples V.402 and V.404 yield ages of 321 ±3Ma and 316 ±3Ma, respectively, which are similar within the errors. Conversely, samples V.104, V.108, and V.405 yield ages of 377 ±3Ma, 382 ±3Ma, and 379 ±4Ma respectively. Finally, sample V.102 gives an age of 365 ±3Ma. The geological significance of these Late Devonian-Early Carboniferous ages will be discussed in the next section.

DATA INTERPRETATION AND DISCUSSION

I. The Sarzeau granite as a strain and time marker for the extensional deformation

The field study highlighted that the Sarzeau granite consists in a dense network of finegrained two-mica granite dykes. Preferred dyke orientations strikes N 30°E and then stands at right-angle with respect to the bulk N 120°E stretching deduced from the structural analysis.

Based upon these geometric relations, it is then tempting to interpret this dyke population as tension gashes of various volumes coevally infilled by the Sarzeau granite. Traces of planar magmatic flow are the main characteristic of the second type of dyke. Furthermore this type of dyke with its upward bent geometry, suggest a progressive eastward shearing. Thus, the Sarzeau granite dykes are regarded as reliable strain markers consistent with the regional N 120°E stretching and deformed by the top-the-the-SE non-coaxial ductile shearing.

In high-strain zones where mylonites develop, dating the deformation is commonly achieved using the 40 Ar/ 39 Ar method on syn-kinematic micas [e.g. [START_REF] Muller | Dating fault-generated pseudotachylytes: comparison of 40 Ar/ 39 Ar stepwise-heating, laser ablation and Rb-Sr microsampling analysis[END_REF][START_REF] Hancock | A new approach to dating slate: Ar-Ar Laser probe analysis of strain fringes in the Welsh Basin slate belt, UK[END_REF]. In this context, the 40 Ar/ 39 Ar age significance is dominantly controlled by recrystallisation processes rather than overall cooling [START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40 Ar/ 39 Ar constraints and implications for the Western Alps[END_REF]Augier et al., 2005]. Thus, the ages are interpreted to record the last increments of deformation. Commonly the early stages of deformation are overprinted and the duration of deformation remains poorly constrained. In our study, we are able to place constraints on the age of the early stage of deformation before mylonitic stage of the SSZ. The U-Th/Pb chemical method of monazite dating allows us to determine the age of granite emplacement [e.g. Be et al., 2006;[START_REF] Cocherie | -Electron microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites[END_REF].

Monazites from the S te -Anne d'Auray type anatectic granite yield an age of 322 ±3Ma which is considered as representative for the partial-melting event responsible for the development of the Golfe du Morbihan migmatites and the S te -Anne d'Auray pluton. It is noteworthy that this age complies with those obtained by the 207 Pb/ 206 Pb method on monazite [i.e. 323 ±4 and 314 ±10Ma;[START_REF]Géochronologie des roches métamorphiques (Rb-Sr et U-Pb). Exemples choisis au Groënland, en Laponie, dans le Massif armoricain et en Grande Kabylie[END_REF]. Available ages for metamorphic peak-conditions are rare and controversial since they are based upon Rb/Sr whole rocks age (379 ±19 Ma) and U/Pb zircon populations ones (381 ±10 Ma and 384 ±24 Ma) [Vidal, 1980;[START_REF]Géochronologie des roches métamorphiques (Rb-Sr et U-Pb). Exemples choisis au Groënland, en Laponie, dans le Massif armoricain et en Grande Kabylie[END_REF].

Monazites from the Sarzeau granite dykes yield several distinctive age clusters: 316 ±3 -321 ±3 Ma and 377 ±3 -382 ±3 Ma ages and 365 ±3 Ma age. A straightforward conclusion is that the youngest age cluster will be considered as the Sarzeau granite emplacement age, in regard with field observation. The older ages are discussed below.

Comparison of the new monazite U-Th/Pb ages with available time-constraints is proposed in figure 7a. Closure temperatures for the different minerals and methods are given in the figure 7 caption. The last part of this chart could be divided in two steps of exhumation (from about 325 to 300Ma). Linear regression for the first segment, between c. 750 ±50°C and 550 ±50 °C yield a cooling rate of the order of 11 ±2°C /Ma. A second step, during which exhumation from c. 550 ±50°C to near surface conditions is achieved, is characterised by an infinitely fast cooling rate due to the age clustering at ca. 300Ma. Because of the importance of error brackets for a given age and the sensitivity of closure temperature to various factors [Villa, 1998], an average cooling rate of 33 ±4°C /Ma from 700 ±50°C down to 85 ±50°C can be computed. Whatever the possible exhumation paths (i.e. paths 1 and 2, figure 7a), our results point to fast cooling rates in agreement with Brown and Dallmeyer (1996) conclusions.

Furthermore, these cooling rates are an order of magnitude faster than those expected for erosional processes [e.g. [START_REF] Thompson A.-B | Pressure-temperature-time paths of regional metamorphism, I. Heat transfer during the evolution of regions of thickened continental crust[END_REF][START_REF] Pinet | -Continental erosion and large scale relief[END_REF]Ring et al., 1999 and enclosed references] and comparable to cooling rates obtained from various tectonic settings such as slab-retreat or lithosphere delamination [e.g. [START_REF] Thompson A.-B | Pressure-temperature-time paths of regional metamorphism, I. Heat transfer during the evolution of regions of thickened continental crust[END_REF][START_REF] England | -A physical model for Cenozoic extension of western North America[END_REF][START_REF] Soto | Thermal evolution, rate of exhumation, and tectonic signification of metamorphic rocks from the floor of the Alboran extensional basin, western Mediterranean[END_REF]Augier et al., 2005]. This result is well in line with published near-isothermal retrograde P-T path for the Golfe du Morbihan migmatites [START_REF] Dallmeyer | -Chronology of Caledonian high-pressure granulite-facies metamorphism, uplift, and deformation within northern parts of the Western Gneiss Region, Norway[END_REF]Brown and Dallmeyer, 1996] and the activity of major extensional shear zones roofing regionally the Lower Unit.

II. Significance of inherited ages within the Variscan framework

The ages of 377 ±3Ma, 379 ±4Ma and 382 ±3Ma, which are similar within the error and the 365±3 Ma age, are obtained from monazite grains that are devoid of any evidence of recrystallisation or growth zoning. Therefore, these old ages can be interpreted as those of refractory grains inherited from earlier crystallisation stages during the Variscan orogeny. During Early to Middle Devonian, a D1 event is responsible for top-to-the-SW ductile shearing coeval with crustal melting and exhumation of the high-pressure rocks. In the Monts du Lyonnais, the migmatites yield a Rb/Sr whole rock age of 384 ±16Ma [START_REF] Chenevoy | -Age Rb-Sr, Dévonien moyen des migmatites à cordiérite du Lyonnais (Massif central français)[END_REF]. In the Limousin, the partial melting of orthogneiss has been dated at 375 ±6Ma, 383 ±5Ma, 378 ±5Ma by Rb/Sr whole rock, zircon U/Pb and monazite U-Th/Pb methods, respectively [START_REF]Chronologie Rb-Sr et géochimie des granitoïdes d'un segment de la chaine Varisque[END_REF][START_REF] Lafon | Géochronologie U-Pb appliqué à deux segments du Massif Central Français[END_REF][START_REF] Faure | -Devonian geodynamic evolution of the Variscan Belt, insights from the French Massif Central and Massif Armoricain[END_REF]. Closer to the study area, the Champtoceaux migmatites, East of the SASZ, and south of Nort-sur-Erdre fault yield a monazite U-Th/Pb age of 387±6 Ma [START_REF] Cocherie | -Electron microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites[END_REF]. Moreover, migmatites from South Armorica yield a Rb/Sr whole rock and zircon U/Pb ages of 376 ±19Ma and 372 ±24Ma, respectively [Vidal, 1980;[START_REF]Géochronologie des roches métamorphiques (Rb-Sr et U-Pb). Exemples choisis au Groënland, en Laponie, dans le Massif armoricain et en Grande Kabylie[END_REF]. This D1 event is followed by a Late Devonian-Early Carboniferous D2 event coeval with an intermediate pressure-temperature metamorphism. At the end of the D2 event, per-aluminous biotite-cordierite plutons dated around 360-350Ma, and called Guéret-type granitoids, emplaced in several places of the Central Massif [Berthier et al., 1979;[START_REF] Cartannaz | Characterisation of wrench tectonics from dating syn-to postmagmatism in the north-western French Massif Central[END_REF][START_REF] Faure | -Devonian geodynamic evolution of the Variscan Belt, insights from the French Massif Central and Massif Armoricain[END_REF].

However, such magmatism event is not recognised in the southern part of the Armorican Massif. Nevertheless, it is reasonable to link up the two sets of inherited ages pointed out by our geochronological study with the tectonic, metamorphic and plutonic D1 and D2 events. The 377-380Ma and 365Ma monazite grains extracted from the Sarzeau granite could have crystallised during the D1 migmatisation or late D2 plutonism, respectively. Consequently, it might be argued that the Late Carboniferous Sarzeau granite formed by melting of underlying Early-Middle Devonian migmatites or Late Devonian-Early Carboniferous peraluminous granites.

In the Temperature-time plot (fig. 7a), our Late Devonian-Early Carboniferous ages are in agreement with previous zircon U/Pb ones [START_REF]Géochronologie des roches métamorphiques (Rb-Sr et U-Pb). Exemples choisis au Groënland, en Laponie, dans le Massif armoricain et en Grande Kabylie[END_REF]. These ages cluster in the high temperature field about 800°C-700°C. In the present state of knowledge, the link between these inherited Devonian ages and the Late Carboniferous cooling remains speculative (path 3, fig. 7a). This straightforward path such as proposed by Brown and Dallmeyer (1996), although possible, is not documented yet; a two-stepped evolution stands as an alternative explanation (path 4, fig. 7a).

III. Is the Lower Unit a metamorphic-core-complex?

Structural analyses conducted within the SSZ and on associated smaller-scale structures distributed toward the core of the Lower Unit show a consistently NW-SE oriented maximum principal finite strain axis and a top-to-the-SE sense of shear (fig. 4a). The SSZ thus appears as a major extensional shear zone accommodating a top-to-the-SE normal shearing. Moreover, the ductile structures, particularly toward the top of the Lower Unit, display a clear reactivation under brittle conditions but with the same NW-SE maximum stretching direction (fig. 3E, Turrillot work in progress). These kinematic patterns suggest that deformation is coeval with a bulk cooling when the Lower Unit reaches the surface [ca. 300Ma, [START_REF] Carpena | Fission Track Geochronology of the Hercynian Platform in France[END_REF]. Our observations emphasize the synkinematic injection of the Sarzeau granite as tension gashes subsequently penetratively sheared within the SSZ.

It has been argued that the Lower Unit is a "metamorphic-core-complex" (MCC) [Brown and Dallmeyer, 1996]. However, MCC refers to a precise geometry and a peculiar behaviour of the continental crust [e.g. [START_REF] Davis | Geological development of Cordilleran metamorphic core complexes[END_REF]Crittenden et al., 1980]. The syn-tectonic injection of Sarzeau and Quiberon granites and strain localisation along gently dipping ductile shear zones are in agreement with the MCC model. Nevertheless, two main points argue that the Golfe du Morbihan differs from a classical MCC as describe above.

Migmatites foliation is consistently vertical, and a dome shape cannot be documented. Furthermore, the SSZ cuts at right angle the migmatites foliation. These lines of evidence do not allow us to establish an unequivocal relationship between syn-plutonism extensional tectonics, anatexis and MCC development.

IV. Regional scale implications

Late-orogenic post-thickening thermal re-equilibration is often regarded as a key condition for large-scale pervasive extension and a way to exhume deeply buried metamorphic rocks [START_REF] Thompson A.-B | Pressure-temperature-time paths of regional metamorphism, I. Heat transfer during the evolution of regions of thickened continental crust[END_REF][START_REF] England | -A physical model for Cenozoic extension of western North America[END_REF]. In South Armorica, a widespread Late-Carboniferous post-thickening extension characterised by crustal-scale extensional shear zones, migmatites domes and large amounts of granite has been described [START_REF] Gapais | La zone de cisaillement de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF]. In agreement with previous studies [START_REF] Gapais | La zone de cisaillement de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF] our observations confirm that the Quiberon shear zone is a top-to-the-W mylonitic zone. The regional trend of the stretching lineation complies well with the interpretation of these extensional shear zones as orogen-parallel late-orogenic structures. Recently [START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF] explored the different behaviour of the Variscan crust developed during and partly in response to late-orogenic extensional tectonics. Two "end-members" were defined. The first one is illustrated by the localised Quiberon extensional shear zone on which most of the strain is concentrated. The other one corresponds to a presumed migmatitic dome, such as the Sables d'Olonne area, which exhibits high thermal gradients of 70-80°C/km [START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique Varisque sur l'encaissant épi à mésozonal de Vendée[END_REF] as well as a consistent flat-lying foliation. In this case, these observations therefore point to a pervasive crustal thinning at the roof of the migmatites [START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF]. In the light of our new structural data the Golfe du Morbihan area is rather similar to the Quiberon model with strain localisation within the SSZ. However, the kinematics deduced from the Quiberon and Sarzeau shear zones, as well as the Sables d'Olonne area, are conflicting as they are top-to-the-W (locally SW), to-the-SE and to-the-WNW respectively. Consequently, a single 200 km long detachment fault separating the Upper and Lower Units, as proposed by [START_REF] Gapais | La zone de cisaillement de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF], can hardly account for the regional structure of southern Armorica. In contrast, it is therefore tempting to propose that late-orogenic extension was accommodated by a set of extensional structures, localised or not whose kinematics reflect local dynamics such as the inception of domes and granite bodies overprinted on the regional NW-SE crustal stretching.

CONCLUSION

The above reported new field structural investigation results, combined with the new monazite U-Th/Pb chemical dating from the Sarzeau two-mica granite allows us to constrain a part of the late-Variscan geodynamic evolution of the Golfe du Morbihan area. The extensional Sarzeau Shear Zone and the related ductile structures observed below both document a regional-scale N 120°E trending stretching coeval with emplacement of N 30°E trending leucogranitic dykes interpreted as kilometre-scale tension gashes. The monazite EPMA chemical dating yields a Late Carboniferous ages, ca. 316-321Ma for granite emplacement.

Crustal melting plays a major role in the late-orogenic exhumation of the ductile crust of southern Armorica. However, timing and detailed tectonic setting of the Golfe du Morbihan migmatites stay poorly documented as well as the late stage of their exhumation. projection [START_REF] Angelier | Inversion of field data in fault tectonics to obtain the regional stress-III. A new rapid direct inversion method by analytical means[END_REF]. Are also represented, for some tectonic sites the average stretching lineation direction carried by the foliation planes (solid lines). In the most deformed tectonic sites (especially within the SSZ) the lineation carried by S planes is overprinted by penetrative shear planes, such as in ultramylonite bodies (fig. 3C Explanations are given in the text. c. 800 ±50°C [START_REF] Kamber | Pitfalls and new approaches in granulite chronometry. An example from the Limpopo Belt, Zimbabwe[END_REF]; monazite, U-Th/Pb, c. 700 ±50°C [START_REF] Cocherie | -Electron microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites[END_REF];

Fig.5 -Image au Microscope Electronique à Balayage (MEB) en électrons rétrodiffusés montrant les textures internes des populations de monazite des échantillons (A, B et C). (A)

U-Pb zircon, c. 800 ±50°C [START_REF] Cherniak | -Pb diffusion in zircon[END_REF]; RT, Rb/Sr, c. 700 ±50°C [Kagami et al., 2003]; biotite, Rb/Sr, c. 325 ±25°C [START_REF] Jenkin | An investigation of closure temperature of the biotite Rb-Sr system: The importance of cation exchange[END_REF]; muscovite, Ar/Ar, c. 425 ±50°C [Villa, 1998]; hornblende, Ar/Ar, c. 550 ±50°C [Villa, 1998]; and apatite, fission-tracks (FT) c. 85 ±25°C [Gunnell, 2000].

  parallel extension, already recognised in other areas of Southern Armorica, but does not support the 200km -long flat detachment fault model. La zone de cisaillement ductile vers l'Est de Sarzeau et sa place dans la tectonique tardi-orogénique extensive de l'Armorique méridionale Mots clés.-Extension tardi-orogénique, zone de cisaillement extensive, datation chimique U-Th/Pb sur Monazite, Massif Armoricain, Chaîne Varisque Résumé.-Cette étude apporte de nouvelles contraintes structurales et géochronologiques (méthode chimique) U-Th/Pb pour les roches magmatiques de la zone du Golfe du Morbihan situé en Armorique méridionale, et plus précisément au sud du Cisaillement Sud Armoricain (CSA). Une structure ductile extensive majeure définie ici comme la Zone de Cisaillement de Sarzeau (ZCS), constitue la limite entre deux unités de degrés métamorphiques différents. L'unité inférieure est constituée de migmatites et du granite d'anatexie de S te -Anne d'Auray daté du Carbonifère, alors que l'unité supérieure est constituée de micaschistes fortement rétromorphosés. L'unité inférieure est recoupée à angle droit par des filons de leucogranite datés ici du Carbonifère supérieur. Ces filons sont progressivement cisaillés par le jeu de la ZCS. Cette dernière est caractérisée par une foliation mylonitique à faible pendage vers le SE et une linéation d'étirement orientée NW-SE. Les critères cinématiques indiquent un cisaillement vers le SE. Dans l'unité inférieure, des filons de leucogranite orientés

  Carboniferous (A. Cocherie, personal communication). North of Vannes, a biotite granite, called the S te -Anne d'Auray massif develops widely (fig.2a, b). Meter to kilometre-scale bodies of the S te -Anne d'Auray type granitic massif crop out within the migmatites foliation (fig.2a, b (cross section A)). Field and petrology observations show that the Golfe du Morbihan migmatites and the S te -Anne d'Auray anatectic granite result from a same crustalmelting event. Moreover, the Golfe du Morbihan migmatites are intruded by numerous, NE-SW trending, fine-grained leucogranite dykes. The largest ones crop out in the Arradon cap or the Ile aux Moines (fig.2a, b cross section B

  Indeed, such Late Devonian to Early Carboniferous ages can be easily interpreted in the geological frame of the Variscan orogeny. The southern part of the Armorican Massif and the entire Central Massif belong to the internal zone of the Variscan Belt that experienced polyphase ductile and synmetamorphic deformation related to continental subduction of the North Gondwana margin below Armorica microcontinent. Several tectonic and metamorphic events are responsible for the built up of the Southern Armorican Massif and Central Massif stack of nappes [e.g. Faure et al., 2005 and enclosed references].

Furthermore

  , the relationships between the belt-parallel extension and the dextral shearing along the South Armorican Shear Zone during the late-orogenic evolution of the Variscan belt remains unsettled yet. AUDREN C. (1990). -Evolution tectonique et métamorphique de la chaîne Varisque en Bretagne méridionale. -Schweizerische Mineralogische Petrographische Mitteilungen, 70, 17-34. AUGIER R., JOLIVET L. & ROBIN C. (2005). -Late orogenic doming in the Eastern Betics : final exhumation of the Nevado-Filabride complex and its relation to basin genesis. -Tectonics, 24, 19p. BALLEVRE M., PARIS F. & ROBARDET M. (1992). -Corrélation ibero-armoricaines C. R. Acad. Sci. Paris, 315, 1783-1789. BE MEZEME E., COCHERIE A., FAURE M., LEGENDRE O. & ROSSI P. (2006). -Electron microprobe monazite geochronology of magmatic events: examples from Variscan migmatites and granitoids, Massif Central, France. -Lithos, 87, 276-288. BERTHIER F., DUTHOU J.-L. & ROQUES M. (1979). -Datation géochronologique Rb/Sr sur roches totales du granite de Guéret (Massif Central). Age fini-Dévonien de mise en place de l'un de ces faciès types. -Bull. BRGM. Fr., I, 31-42 BROWN M. & DALLMEYER D. (1996). -Rapid Variscan exhumation and the role of magma in core complex formation: Southern Brittany Metamorphic Belt, France. -J. Metam. Geol., 14, 361-379. VAN DEN DRISSCHE J. &. BRUN J.-P. (1991-92). -Tectonic evolution of the Montagne Noire (French Massif Central) : a model of extensional gneiss dome. -Geodinamica Acta, 5, 85-99. VIDAL P. (1980). -L'évolution polyorogénique du Massif Armoricain : apport de la géochronologie et de la géochimie isotopique du strontium. -Mém. Soc. Géol. Minéral. Bretagne, 21, 162p. VILLA I.M. (1998). -Isotopic closure. -Terra Nova, 10, 42-47. ZHU X. K. & O'NIONS R. K. (1999). -Zonation of monazite in metamorphic rocks and its implication for high temperature thermochronology: a case study from the Lewisian terrain. -Earth and Planetary Science Letters, 171, 209-220.

Fig. 1 -

 1 FIGURE CAPTIONS:

Fig. 1 -

 1 Fig.1 -Aperçu géologique du Massif Armoricain et du domaine Sud Armoricain [modifié

Fig. 2

 2 Fig.2 -(a) Geological map of the Golfe du Morbihan study area, showing the main

Fig. 2 Fig. 3 -

 23 Fig.2 -(a) Carte géologique de la région du Golfe du Morbihan (zone d'étude) montrant les

Fig. 4 -

 4 Fig.4 -(a) Geological map restricted to the SSZ. Are located on the map, the 16 tectonic

Fig. 5 -

 5 Fig.5 -SEM BSE pictures showing representative internal structure of monazite grains from

Fig. 7 -

 7 Fig.7 -(a) Temperature-time (T-t) plot for the Golfe du Morbihan migmatites, the S te -Anne

  (b) WNW-ESE representative orogen-parallel cross-section through the Quiberon and the Golfe du Morbihan area. Main tectono-metamorphic subdivisions are presented with the same legend and formalism than in figure4a.

Fig. 7

 7 Fig.7 -(a) Diagramme Température-temps (T-t) pour les migmatites du Golfe du Morbihan, le granite anatectique de type S te -Anne d'Auray et le granite de Sarzeau (Unité Inférieure,

  Fig.3

  Pb vs. Th/Pb isochron diagrams. All standard deviations are quoted at 95% confidence level and error ellipses are plotted as 2σ. Are given on each diagram the name and the magmatic body (S te -Anne d'Auray type anatectic and two-mica Sarzeau granites), the number of analyzed monazite grain, the isochron calculated age, the number of analysis (n) and the value of the MSWD parameter; (a) The significance of the lines is given on a theoretical chart at the bottom-right of the figure.

	signification n'est pas clairement établie; (D) Représentation de tous les âges provenant des
	analyses réalisées sur le grain de monazite M23 (échantillon V.404) tant sur le coeur que la
	bordure, projetés dans un diagramme de type âges moyens. Les explications sont données
	dans le texte.
	Fig.6 -U/
	Monazite montrant une texture interne homogène; (B) Monazite montrant une zonation de
	type « coeur-bordure ». Ce type de zonation est normalement issu d'une cristallisation en deux
	étapes. Les points blancs sont les positions des analyses chimiques ponctuelles réalisées à la
	microsonde; (C) Monazite montrant une zonation de type « patchy zoning » dont la

Fig.6 -Diagrammes isochrones U/Pb vs. Th/Pb. Les erreurs sont établies avec un degré de confiance de 95% et les ellipses d'erreur avec une incertitude à 2σ. A chaque diagramme y est associé le nom de l'échantillon ainsi que le nom du corps magmatique dans lequel il a été prélevé (granites anatectique de type S te -Anne d'Auray et granite de Sarzeau), le nombre de grains de monazite analysés, l'âge isochrone calculé, le nombre d'analyses (n) et la valeur du paramètre statistique MSWD ; (a) La signification des courbes et des lignes constitutives de ces diagrammes est donnée sur un diagramme théorique et fictif en bas à droite de la figure.

Acknowledgements

This work was supported by the BRGM (CGF, French geological mapping programme) and the contribution of the ISTO (CNRS UMR 6113). We gratefully acknowledge P. Rossi A.

Cocherie and J. Le Métour for fruitful discussions on geochronology and regional geology; P.

Jézéquel for monazite extraction.