A P N A De Porto

A J J Lammers

R J Bennink

I J M Ten Berge

P Speelman

J B L Hoekstra

Keywords: Spleen function, Pitted erythrocytes, Howell Jolly Bodies, IgM Memory B cells, Radionuclide Scintigraphy

Hyposplenic patients are at risk for Overwhelming Post Splenectomy Infection (OPSI), which has a mortality of up to 70%. Therefore, preventive measures are warranted. However, patients with diminished splenic function are difficult to identify. In this review we discuss (i) immunological, (ii) haematological and (iii) scintigraphic parameters that can be used to measure splenic function.

(i) IgM memory B cells are a potential parameter to assess splenic function, however more studies are necessary for its validation. (ii) Detection of Howell Jolly bodies does not reflect splenic function accurately, whereas determining the percentage of pitted erythrocytes is a well evaluated method and seems a good first line investigation to assess splenic function. (iii) When assessing spleen function, 99m Tc labelled heat-altered autologous erythrocyte scintigraphy with a multimodality Single Photon Emission Computed Tomography (SPECT)-CT technology is the best approach, as all facets of splenic function are evaluated.

In conclusion, although scintigraphic methods are most reliable, they are not suitable for screening large populations. We therefore recommend using the percentage of pitted erythrocytes, albeit suboptimal, as a first line investigation and subsequently confirm abnormal readings by means of scintigraphy. More studies evaluating the value of potentially new markers are needed.

Introduction

The spleen is the largest lymphoid organ in the human body. Its rich and diverse population of immune cells and its ingenious anatomy that enables optimal surveillance and phagocytosis of circulating blood elements play an important role in the defence against pathogens. Table 1 summarizes the different aspects of splenic functions. After splenectomy, patients are at increased risk for Overwhelming Post Splenectomy Infection (OPSI), see Box [START_REF] Styrt | Infection associated with asplenia: risks, mechanisms, and prevention[END_REF][START_REF] Singer | Postsplenectomy sepsis[END_REF][START_REF] Holdsworth | Postsplenectomy sepsis and its mortality rate: actual versus perceived risks[END_REF][START_REF] Bisharat | Risk of infection and death among postsplenectomy patients[END_REF].

Apart from patients with a status after splenectomy, there is a much larger group of patients with diminished splenic function. Many diseases are associated with a dysfunctional spleen (Table 2) and the degree of splenic dysfunction varies between patients [START_REF] William | Hyposplenism: a comprehensive review. Part I: basic concepts and causes[END_REF]. For patients suspected to have a diminished functioning spleen, it is important to quantify their splenic function in order to assess the risk of developing OPSI. Subsequently, preventive measurements can be taken and, in case of infection, therapy can be started without delay. In this review we evaluate the available methods to measure splenic function.

After splenectomy, patients are at risk for overwhelming infection. This syndrome is called overwhelming post-splenectomy infection (OPSI) or post-splenectomy sepsis (PSS). Patients with functional asplenia are also at risk for this syndrome.

Symptoms: OPSI is characterized by a mild onset with flu-like symptoms such as low grade fever, chills, muscle aches and nausea. However, a subsequent fast deterioration may occur in hours rather than days leading to fulminant sepsis, disseminated intravascular coagulation and multi-organ failure [START_REF] Okabayashi | Overwhelming postsplenectomy infection syndrome in adults -a clinically preventable disease[END_REF].

Incidence: Incidence of OPSI is estimated to be low, 2-5 per 1000 asplenic patients per year [START_REF] Brigden | Pneumococcal vaccine administration associated with splenectomy: the need for improved education, documentation, and the use of a practical checklist[END_REF]. The lifetime risk for developing OPSI is estimated to be 5% [START_REF] Lynch | Overwhelming postsplenectomy infection[END_REF]. Although more than half of these infections occur within the first two years after splenectomy, the risk remains increased lifelong [START_REF] Styrt | Infection associated with asplenia: risks, mechanisms, and prevention[END_REF][START_REF] Cullingford | Severe late postsplenectomy infection[END_REF].

Mortality: Although incidence is low, mortality is high. Numbers in literature vary between 50 and 70% [START_REF] Styrt | Infection associated with asplenia: risks, mechanisms, and prevention[END_REF][START_REF] Holdsworth | Postsplenectomy sepsis and its mortality rate: actual versus perceived risks[END_REF]. Notably, 68% of patients die in the first 24 hours, and 80% within 48 hours after onset [START_REF] Holdsworth | Postsplenectomy sepsis and its mortality rate: actual versus perceived risks[END_REF][START_REF] Lutwick | Infections in Asplenic Patients[END_REF].

Micro-organisms: Encapsulated bacteria are important causative organisms of OPSI. S. pneumoniae causes 70% of bacteraemic episodes after splenectomy [START_REF] Holdsworth | Postsplenectomy sepsis and its mortality rate: actual versus perceived risks[END_REF]. Other pathogens, responsible for OPSI are H. influenzae, N. meningitidis, E. coli and Pseudomonas.

Guideline: To prevent OPSI several preventive measures should be taken, such as immunization against the encapsulated bacteria S. pneumoniae, H. influenzae B and N. meningitidis C. Furthermore, patients should use continuous prophylactic antibiotics during the first 2 years after splenectomy and have on-demand antibiotics to use in case of (suspected) infection [11][START_REF] Davies | Update of guidelines for the prevention and treatment of infection in patients with an absent or dysfunctional spleen[END_REF][START_REF] Mourtzoukou | Vaccination of asplenic or hyposplenic adults[END_REF] Box Overwhelming post-splenectomy infection.

Approaches to measure splenic function

Throughout the years, several methods have been developed to quantify the many different functions of the spleen. These methods are based on haematological, immunological and scintigraphic parameters.

Haematological parameters

Haematological methods reflect the capacity of the spleen to phagocytose deviant erythrocytes and to facilitate an environment wherein erythrocytes rid themselves of solid waste material [START_REF] Schnitzer | Erythrocytes: pits and vacuoles as seen with transmission and scanning electron microscopy[END_REF][START_REF] Holroyde | Acquisition of autophagic vacuoles by human erythrocytes. Physiological role of the spleen[END_REF]. In case of splenic dysfunction these capacities are impaired, which results in an increase of abnormal circulating red blood cells. Furthermore, large amounts of thrombocytes and leukocytes normally reside in the spleen. Circulating thrombocyte-and leukocyte counts can either be increased or decreased indicative of hyposplenism in a patient with a dysfunctional spleen (for example thrombocytosis in asplenia and thrombopenia associated with splenomegaly) [START_REF] William | Hyposplenism: a comprehensive review. Part I: basic concepts and causes[END_REF][START_REF] Doll | Functional hyposplenism[END_REF].

One of the first methods available to evaluate splenic function was the detection of erythrocytes containing Howell Jolly bodies, using a light microscope viewing a stained peripheral blood smear [START_REF] Lipson | The postsplenectomy blood picture[END_REF][START_REF] Damshek | Hypersplenism[END_REF]. Howell Jolly bodies are basophilic DNA remains from the nucleus of the erythrocyte precursor cell. Normally, upon leaving the bone marrow, the erythrocyte precursor cell expels its nucleus. In some erythrocytes however, a small portion of DNA remains. Normally the spleen clears the erythrocyte of these nuclear remnants or removes the erythrocytes from the circulation, but when the spleen is absent or has a decreased function these Howell Jolly body containing erythrocytes remain in the circulation. A recently developed method uses flow cytometry to quantify the amount of erythrocytes containing Howell Jolly bodies [START_REF] Harrod | Quantitative analysis of Howell-Jolly bodies in children with sickle cell disease[END_REF].

Other abnormalities that can be seen on peripheral blood smears of patients with absent or diminished splenic function are acanthocytes (spur cells), target cells (condocytes: erythrocytes with a pattern of central staining, a ring of pallor and an outer ring of staining), haemoglobin remnants (Heinz bodies), siderocytes and iron granulocytes (Pappenheimer bodies) [START_REF] William | Hyposplenism: a comprehensive review. Part I: basic concepts and causes[END_REF][START_REF] Doll | Functional hyposplenism[END_REF] .

In individuals with a dysfunctional or absent spleen the membrane of erythrocytes appears to contain so called "pits" when studied with interference phase microscopy [START_REF] Holroyde | The "pocked" erythrocyte. Red-cell surface alterations in reticuloendothelial immaturity of the neonate[END_REF]. With electron microscopy it was shown that these "pits" are in fact large vacuoles (about 300 nm in diameter) beneath or attached to the plasma membrane. These vacuoles have low optical density, due to contained waste material of the erythrocyte such as ferritine, haemoglobin and rest material of mitochondria and membranes [START_REF] Schnitzer | Erythrocytes: pits and vacuoles as seen with transmission and scanning electron microscopy[END_REF][START_REF] Holroyde | Acquisition of autophagic vacuoles by human erythrocytes. Physiological role of the spleen[END_REF][START_REF] Reinhart | Red cell vacuoles: their size and distribution under normal conditions and after splenectomy[END_REF]. In case of normal splenic function, pits are seen in 0-4% of the erythrocytes [START_REF] Holroyde | The "pocked" erythrocyte. Red-cell surface alterations in reticuloendothelial immaturity of the neonate[END_REF][START_REF] Corazza | Simple method of assessing splenic function in coeliac disease[END_REF][START_REF] Casper | A new method for studying splenic reticuloendothelial dysfunction in sickle cell disease patients and its clinical application: a brief report[END_REF]. A pit count above 4% has been associated with hyposplenism, although asplenia or clinically relevant hyposplenism is most often associated with much higher values, ranging from 15 to 70% [START_REF] Corazza | Simple method of assessing splenic function in coeliac disease[END_REF][START_REF] Pearson | Developmental pattern of splenic dysfunction in sickle cell disorders[END_REF][START_REF] Lane | Functional asplenia in hemoglobin SC disease[END_REF]. Casper et al. noted that in 5 patients with sickle cell disease who developed sepsis and/or meningitis, pit counts were higher than 15% and therefore the authors suggested this as a cut-off value for significant splenic dysfunction [START_REF] Casper | A new method for studying splenic reticuloendothelial dysfunction in sickle cell disease patients and its clinical application: a brief report[END_REF]. The same cut-off value was suggested by Corazza et al., who noted that patients that underwent splenectomy had functional residual splenic tissue when pits counts were beneath 16% [START_REF] Corazza | Return of splenic function after splenectomy: how much tissue is needed?[END_REF].

Another method to evaluate spleen function is counting erythrocytes containing argyrophilic inclusions, where normal values range from 0-3%. This method uses a silver stain and in comparison with a normal Wrights stain, the argyrophilic inclusions show to be Howell Jolly bodies, Pappenheimer bodies and other inclusions visible in patients with a decreased or absent splenic function [START_REF] Tham | A simple splenic reticuloendothelial function test: counting erythrocytes with argyrophilic inclusions[END_REF].

Immunological parameters

The spleen contains a large amount of immune cells [START_REF] Mebius | Structure and function of the spleen[END_REF]. In comparison to the peripheral blood lymphocyte compartment, the spleen percentually contains more B-cells and less CD4 + and CD8 + T cells. The percentage CD8 + T cells is higher in the spleen, leading to an inverse CD4/CD8 ratio. Both splenic CD4 + and CD8 + T cell populations show a higher number of activated cells and splenic CD8 + T cells show a more differentiated cytotoxic CD27 -CD45RA + memory phenotype. Thus, the distribution of the different lymphocyte subsets is markedly different between spleen and peripheral blood, inferring an important and distinct role for the spleen in CD4 + and CD8 + T cell activation [START_REF] Langeveld | T-lymphocyte subset distribution in human spleen[END_REF]. After splenectomy, some immunological functions of the spleen can be taken over by other organs such as liver, bone marrow and peripheral lymph nodes. Therefore these functions are not suitable as a reliable parameter for measuring spleen function.

However, the spleen has a specific role in the defence against encapsulated bacteria [START_REF] Styrt | Infection associated with asplenia: risks, mechanisms, and prevention[END_REF][START_REF] Singer | Postsplenectomy sepsis[END_REF][START_REF] Holdsworth | Postsplenectomy sepsis and its mortality rate: actual versus perceived risks[END_REF][START_REF] Bisharat | Risk of infection and death among postsplenectomy patients[END_REF]. This is mainly related to the marginal zone (MZ) containing marginal zone B cells (MZ B cells) and macrophages. Marginal zone macrophages are able to capture whole encapsulated bacteria from the circulation and subsequently initiate a humoral immune response [START_REF] Koppel | Specific ICAM-3 grabbing nonintegrin-related 1 (SIGNR1) expressed by marginal zone macrophages is essential for defense against pulmonary Streptococcus pneumoniae infection[END_REF]. MZ B cells are a distinct B cell lineage that, unlike other B cell lineages, develop and mutate Immunoglobin (Ig) receptors during the first years of life without being engaged in any immune response. Upon stimulation with thymus independent type 2 (TI-2) antigens expressed by encapsulated bacteria, the prediversified MZ B cells can rapidly proliferate and differentiate into antigen presenting cells or into IgM-, IgG-, and IgA-secreting plasma cells, circulating for several months. MZ B cells do not differentiate into memory cells and are therefore part of the (immediate) innate immunity against invading pathogens [START_REF] Kruetzmann | Human immunoglobulin M memory B cells controlling Streptococcus pneumoniae infections are generated in the spleen[END_REF][START_REF] Weller | Human blood IgM "memory" B cells are circulating splenic marginal zone B cells harboring a prediversified immunoglobulin repertoire[END_REF][START_REF] Weller | Vaccination against encapsulated bacteria in humans: paradoxes[END_REF]. MZ B cells do not only reside in the MZ but are also present in the circulation and in other lymphoid tissue [START_REF] Dono | Heterogeneity of tonsillar subepithelial B lymphocytes, the splenic marginal zone equivalents[END_REF][START_REF] Spencer | Human marginal-zone B cells[END_REF][START_REF] Liu | Memory B cells from human tonsils colonize mucosal epithelium and directly present antigen to T cells by rapid up-regulation of B7-1 and B7-2[END_REF]. The spleen is however essential for the maintenance of the MZ B cell population, as appears from a decrease in MZ B cell counts after splenectomy. In contrast with one report [START_REF] Kruetzmann | Human immunoglobulin M memory B cells controlling Streptococcus pneumoniae infections are generated in the spleen[END_REF], other studies have shown that young patients with congenital asplenia have a normal blood MZ B cell population whereas this circulating MZ B cell subset fails to expand in older asplenic individuals [START_REF] Weller | Human blood IgM "memory" B cells are circulating splenic marginal zone B cells harboring a prediversified immunoglobulin repertoire[END_REF][START_REF] Sabatino | Depletion of immunoglobulin M memory B cells is associated with splenic hypofunction in inflammatory bowel disease[END_REF]. Therefore, the amount of circulating MZ B cells may be an indication of immunological function of the spleen. The effect of diminished spleen function on the composition of naïve-, memory-and effector (antigen specific) T cells in the circulation is not yet known. Decreased numbers of circulating memory B cells have been described in patients with diminished splenic function [START_REF] Kruetzmann | Human immunoglobulin M memory B cells controlling Streptococcus pneumoniae infections are generated in the spleen[END_REF][START_REF] Weller | Human blood IgM "memory" B cells are circulating splenic marginal zone B cells harboring a prediversified immunoglobulin repertoire[END_REF][START_REF] Sabatino | Depletion of immunoglobulin M memory B cells is associated with splenic hypofunction in inflammatory bowel disease[END_REF] although this might be due to only a decrease in IgM memory B cells rather than other B memory cells [START_REF] Kruetzmann | Human immunoglobulin M memory B cells controlling Streptococcus pneumoniae infections are generated in the spleen[END_REF][START_REF] Sabatino | Depletion of immunoglobulin M memory B cells is associated with splenic hypofunction in inflammatory bowel disease[END_REF]. Some studies have described tuftsin as a potential marker for immunological spleen function, since production of this peptide is mainly depending on the spleen [START_REF] Spirer | Decreased tuftsin concentrations in patients who have undergone splenectomy[END_REF][START_REF] Constantopoulos | Phagocytic activity of the leukemic cell and its response to the phagocytosis-stimulating tetrapeptide, tuftsin[END_REF]. Tuftsin is a tetrapeptide with protective bactericidal characteristics, since it has been shown to stimulate phagocytosis by neutrophils and macrophages [START_REF] Najjar | Tuftsin, a natural activator of phagocyte cells: an overview[END_REF]. Decreased serum levels of tuftsin are seen in splenectomised patients [START_REF] Spirer | Decreased tuftsin concentrations in patients who have undergone splenectomy[END_REF][START_REF] Constantopoulos | Phagocytic activity of the leukemic cell and its response to the phagocytosis-stimulating tetrapeptide, tuftsin[END_REF], patients suffering from sickle cell disease [START_REF] Spirer | Decreased serum tuftsin concentrations in sickle cell disease[END_REF] and celiac disease [START_REF] Corazza | A reassessment of splenic hypofunction in celiac disease[END_REF].

Scintigraphic parameters

Like haematological parameters, scintigraphic parameters use the capacity of the spleen to filter the blood of deviant cells and particles to measure its activity. The radiopharmaceutical most commonly used for this purpose is technetium-99m ( 99m Tc) labelled heat-altered autologous erythrocytes, which has replaced the previously commonly used 99m Tc labelled sulphur colloids [START_REF] Ambriz | Accessory spleen compromising response to splenectomy for idiopathic thrombocytopenic purpura[END_REF][START_REF] Massey | Residual spleen found on denatured red blood cell scan following negative colloid scans[END_REF][START_REF] Van Nostrand | Value of selective spleen scintigraphy when liver/spleen image shows equivocal spleen defects: concise communication[END_REF][START_REF] Ramchandran | Spleen scanning in humans with Tc-99m-labeled erythrocytes: concise communication[END_REF][START_REF] Armas | A simplified method of selective spleen scintigraphy with Tc-99m-labeled erythrocytes: clinical applications. Concise communication[END_REF][START_REF] Ehrlich | Splenic scintigraphy using Tc-99m-labeled heat-denatured red blood cells in pediatric patients: concise communication[END_REF]. 99m Tc-labelled sulphur colloid scintigraphy has been used for visualisation of liver and spleen phagocytic function and was once a common study to evaluate for the presence or absence of neoplastic disease, cirrhosis or portal hypertension, being largely supplanted by other modalities like ultrasonography, (PET)-CT or MRI to date [START_REF] Hladik | Radiopharmaceuticals for liver and spleen imaging[END_REF]. For the assessment of spleen function or presence of an accessory spleen, 99m Tc-labelled heat-altered autologous erythrocyte scintigraphy is now recommended, because in contrast to sulphur colloid scintigraphy sensitivity is not hampered by the relatively high liver uptake [START_REF] Ambriz | Accessory spleen compromising response to splenectomy for idiopathic thrombocytopenic purpura[END_REF][START_REF] Massey | Residual spleen found on denatured red blood cell scan following negative colloid scans[END_REF][START_REF] Van Nostrand | Value of selective spleen scintigraphy when liver/spleen image shows equivocal spleen defects: concise communication[END_REF][START_REF] Ramchandran | Spleen scanning in humans with Tc-99m-labeled erythrocytes: concise communication[END_REF][START_REF] Armas | A simplified method of selective spleen scintigraphy with Tc-99m-labeled erythrocytes: clinical applications. Concise communication[END_REF][START_REF] Ehrlich | Splenic scintigraphy using Tc-99m-labeled heat-denatured red blood cells in pediatric patients: concise communication[END_REF][START_REF] Armas | Clinical studies with spleen-specific radiolabeled agents[END_REF]. Sulphur colloids are captured by phagocytosis, whereas autologous heat-altered erythrocytes are sequestrated by the normal spleen [START_REF] Armas | Clinical studies with spleen-specific radiolabeled agents[END_REF][START_REF] Owunwanne | Radionuclide imaging of the spleen with heat denatured technetium-99m RBC when the splenic reticuloendothelial system seems impaired[END_REF]. The normal spleen accumulates about 90% of injected autologous heat-altered erythrocytes, as compared to 10% of sulphur-colloids which are mainly phagocytosed by the liver [START_REF] Armas | Clinical studies with spleen-specific radiolabeled agents[END_REF]. After intravenous re-injection of these cells, splenic function can be determined by (i) measuring the clearance rate of the injected cells from the circulation, by analysing blood samples using a gamma well-counter, or (ii) by determining the splenic uptake either solely or by determining the spleen-to-liver uptake ratio using a gamma probe or -camera. Besides quantitative information on splenic function, planar or dynamic scintigraphy enables visualisation of organ function. In addition to planar scintigraphy, modern multi-modality Single Photon Emission Computed Tomography (SPECT)-CT gamma camera's enable assessment of both function and anatomy (organ volume and structure) within a single investigation potentially introducing clinically useful parameters like organ specific functional volumes [START_REF] Schmitz | Measurement of splenic function with heat-damaged red blood cells[END_REF].

Alternatively, unaltered autologous or donor erythrocytes or platelets can be radio-labelled for assessment of pathological sequestration in the spleen in patients with low peripheral cell counts such as idiopathic thrombocytopenic purpura or auto-immune anaemia [START_REF] Ambriz | Accessory spleen compromising response to splenectomy for idiopathic thrombocytopenic purpura[END_REF][START_REF] Armas | Clinical studies with spleen-specific radiolabeled agents[END_REF][START_REF] Demar | Plasmodium falciparum malaria in splenectomized patients: two case reports in French Guiana and a literature review[END_REF][START_REF] Pettit | Spleen function[END_REF][START_REF] Phom | Comparative evaluation of Tc-99m-heat-denatured RBC and Tc-99m-anti-D IgG opsonized RBC spleen planar and SPECT scintigraphy in the detection of accessory spleen in postsplenectomy patients with chronic idiopathic thrombocytopenic purpura[END_REF]

Different approaches compared

As there are many approaches to assess splenic function, the question arises what method is most reliable and what method is best for clinical use. Knowledge about correlation and functionality of the different available methods is required before a deliberate decision on which method to use can be made. In the next paragraphs we give an overview of studies comparing the haematological, immunological and scintigraphic parameters to measure splenic function.

Scintigraphic parameters compared

Although 99m Tc-labelled heat-altered autologous erythrocytes as well as 99m Tc-labelled sulphur colloids have been used in studies on splenic function, not much recent data can be found on their correlation when determining the amount of functional splenic tissue. When computing splenic volumes based on planar scintigraphy in two groups of celiac patients, splenic volumes derived from 99m Tc-labelled sulphur colloid scintigraphy correlated well with those from 99m Tc-labelled heat-altered erythrocyte scintigraphy [START_REF] Robinson | Splenic size and function in adult coeliac disease[END_REF]. Furthermore, there was a good correlation between volume of functional splenic tissue and splenic function measured using 99m Tc-labelled heataltered erythrocyte clearance rates from the circulation. Another publication by Smart et al. in patients with mainly Inflammatory Bowel Disease (IBD) showed a strong correlation between clearance of the cells from the circulation and functional spleen volume, with a large variation about the regression line leading to the conclusion that functional spleen size determination was not able to replace measurement of the rate of heat-altered erythrocyte clearance from the circulation in the assessment of hyposplenism [START_REF] Smart | Relationship between splenic size and splenic function[END_REF]. However, these studies were performed in the pre-tomographic and ultrasonographic era using planar imaging for volume calculation, making it less reliable. Furthermore, only functioning spleen was visualized and eligible for volume calculation, implying a direct correlation between function and size [START_REF] Smart | Relationship between splenic size and splenic function[END_REF][START_REF] Robertson | Blood film appearances in the hyposplenism of coeliac disease[END_REF] A more recent study by Gotthardt et al. showed that spleen-liver ratios as soon as 10 min after reinjection of 99m Tc-labelled heat-altered erythrocytes reliably predict spleen function in IBD patients when compared to rate of clearance of the cells from the circulation. The spleen-liver ratio measured with 99m Tc-labelled sulphur colloids showed no correlation with the clearance of the 99m Tc-labelled heat-altered erythrocytes [START_REF] Gotthardt | Scintigraphy with 99mTc-labeled heat-altered erythrocytes in diagnosing hyposplenia: prospective comparison to 99mTc-labeled colloids and colourcoded duplex ultrasonography[END_REF].

Scintigraphic and haematological parameters compared

The correlation between haematological parameters and scintigraphic parameters has been studied more accurately. In patients with sickle cell disease (SCD), a correlation was found between the uptake of 99m Tc-labelled sulphur colloid by functional splenic tissue and the percentage of pitted erythrocytes [START_REF] Casper | A new method for studying splenic reticuloendothelial dysfunction in sickle cell disease patients and its clinical application: a brief report[END_REF][START_REF] Pearson | Developmental pattern of splenic dysfunction in sickle cell disorders[END_REF][START_REF] Lane | Functional asplenia in hemoglobin SC disease[END_REF]. In a study by Pearson et al. amongst 64 children with homozygous SCD between 8 and 13 months of age it was found that sensitivity, specificity and predictive values were all between 90% and 98% when correlating uptake with a percentage of pitted erythrocytes less then 3,5% [START_REF] Pearson | Developmental pattern of splenic dysfunction in sickle cell disorders[END_REF]. Another study by Lane et al. described patients with heterozygous SCD (HbSC), where it was demonstrated that pit counts of more than 20% were indicative for functional asplenia, whereas pit counts lower than 20% were associated with normal or near normal splenic function [START_REF] Lane | Functional asplenia in hemoglobin SC disease[END_REF]. Furthermore, in a study of patients with celiac disease and dermatitis herpetiformis, a correlation was found between the percentage of pitted erythrocytes and the size of functioning splenic tissue, as measured by using 99m Tc-labelled autologous heat-altered erythrocytes rather than sulphur colloids [START_REF] Corazza | Simple method of assessing splenic function in coeliac disease[END_REF]. In this same group of patients, a significant correlation was found between the percentage of pitted erythrocytes and the clearance rate of 99m Tc-labelled heat-altered erythrocytes. However, another study describing patients with megaloblastic anaemia and iron deficient anaemia, which are rare causes for hyposplenia, no correlation was found between the percentages of pitted erythrocytes and the blood clearance rate, splenic uptake values and splenic volumes [START_REF] Zago | Discrepancy between pit counting and spleen function tests in nutritional anemias and hemoglobinopathy C[END_REF]. An explanation for these results could not be given by the authors, however they state that erythrocyte pits may be heterogeneous in origin, composition, or removal kinetics and may be different in individuals that are hyposplenic for various reasons.

The presence of the Howell Jolly bodies has historically been associated with diminished splenic function. However, Howell Jolly bodies have been shown not to correlate with blood clearance of the 99m Tc-labelled heat-altered erythrocytes [START_REF] Robertson | Blood film appearances in the hyposplenism of coeliac disease[END_REF][START_REF] Gotthardt | Scintigraphy with 99mTc-labeled heat-altered erythrocytes in diagnosing hyposplenia: prospective comparison to 99mTc-labeled colloids and colourcoded duplex ultrasonography[END_REF]. Similar results were obtained using 51 Chromium-labelled heat-altered erythrocytes [START_REF] Ryan | Hyposplenism in inflammatory bowel disease[END_REF]. The presence of Howell Jolly bodies did also not correlate with the spleen-liver activity ratio measured with either 99m Tc-labelled heat-altered erythrocytes or 99m Tc-labelled sulphur colloids [START_REF] Gotthardt | Scintigraphy with 99mTc-labeled heat-altered erythrocytes in diagnosing hyposplenia: prospective comparison to 99mTc-labeled colloids and colourcoded duplex ultrasonography[END_REF].

Haematological parameters compared

Although there is discussion in literature, it was found that the percentage of erythrocytes containing Howell Jolly bodies correlated with the percentage of pitted erythrocytes [START_REF] Casper | A new method for studying splenic reticuloendothelial dysfunction in sickle cell disease patients and its clinical application: a brief report[END_REF][START_REF] Corazza | Howell-Jolly body counting as a measure of splenic function. A reassessment[END_REF]. This correlation however, was only present at pit counts higher than 8% and when at least 10.000 erythrocytes were examined. Mild cases of hyposplenism could not be detected by determining percentages of erythrocytes with Howell Jolly bodies, since a pit count above 4% is indicative for hyposplenism. No notice was made of what percentage Howell Jolly Bodies indicates hyposplenism [START_REF] Corazza | Howell-Jolly body counting as a measure of splenic function. A reassessment[END_REF].

The argyrophilic inclusion positive erythrocyte count has a sensitivity of 88,9% and a specificity of 97,1% for splenic dysfunction when using the percentage of pitted erythrocytes as a golden standard [START_REF] Tham | A simple splenic reticuloendothelial function test: counting erythrocytes with argyrophilic inclusions[END_REF].

Immunological and haematological parameters compared

Because the amount of circulating IgM memory B cells was first described in 2005 as a method to quantify splenic hypofunction, research on this subject is still limited. Two studies describe a correlation between the amount of circulating IgM memory B cells and the percentage of pitted erythrocytes in treated patients with celiac disease and IBD [START_REF] Sabatino | Depletion of immunoglobulin M memory B cells is associated with splenic hypofunction in inflammatory bowel disease[END_REF][START_REF] Sabatino | Splenic hypofunction and the spectrum of autoimmune and malignant complications in celiac disease[END_REF]. In one study, patients with IBD were divided into either having a decreased splenic function (>4% pitted erythrocytes) or having a normal splenic function (<4% pitted erythrocytes) and both were compared to a control group [START_REF] Sabatino | Depletion of immunoglobulin M memory B cells is associated with splenic hypofunction in inflammatory bowel disease[END_REF]. Patients with decreased splenic function were shown to have lower amounts of circulating memory B cells, mainly IgM memory B cells, as compared to healthy controls as well as individuals classified to have normal splenic function. Furthermore, IgM memory B cells were shown to be completely absent in the peripheral blood of splenectomised patients. As described above, serum tuftsin might be indicative for splenic function although not much research on the subject has been published. This potential marker was studied in 52 untreated patients with celiac disease [START_REF] Corazza | A reassessment of splenic hypofunction in celiac disease[END_REF]. In accordance with the study on IgM memory B cells, patients were divided into groups based on pit count. It was found that hyposplenic as well as eusplenic celiac patients had significantly lower tuftsin activity than healthy controls, but significantly higher than splenectomised patients. There was less tuftsin activity in hyposplenic patients than in eusplenic patients. Furthermore, a correlation was found between serum tuftsin activity and the percentage of pitted erythrocytes.

Discussion

Knowledge about splenic function is important since patients with an absent spleen or decreased splenic function are at risk to develop severe infections with a high mortality rate. Quantification of spleen function could become an important tool for physicians in their decision-making about the need for preventive measures. However, when assessing splenic function in a clinical setting, physicians should be aware of the multiple facets of spleen function (as described in Table 1) and thus the different possible approaches to determining splenic function.

In many diseases associated with splenic hypofunction such as sickle cell disease, celiac disease, IBD and Systemic Lupus Erythematosis, splenic function changes as the underlying disease activity alters [START_REF] Robinson | Splenic size and function in adult coeliac disease[END_REF][START_REF] Smart | Relationship between splenic size and splenic function[END_REF][START_REF] Ryan | Hyposplenism in inflammatory bowel disease[END_REF][START_REF] Pearson | Transfusion-reversible functional asplenia in young children with sickle-cell anemia[END_REF][START_REF] Ferster | Bone marrow transplantation corrects the splenic reticuloendothelial dysfunction in sickle cell anemia[END_REF][START_REF] Palmer | Further experience of hyposplenism in inflammatory bowel disease[END_REF][START_REF] Dillon | Splenic atrophy in systemic lupus erythematosus[END_REF]. It has been suggested that these changes in splenic function are due to two components of splenic hypofunction in active disease; (i) impaired splenic function that may deteriorate during high disease activity but may improve with treatment, and (ii) splenic atrophy that may lead to irreversible loss of volume and therefore also irreversible loss of function. Illustrating this phenomenon, two patients are described in whom the size of the functional splenic tissue did not alter during relapse of the disease causing the hyposplenia, while the clearance rate of heat damaged autologous erythrocytes was prolonged [START_REF] Robinson | Splenic size and function in adult coeliac disease[END_REF][START_REF] Smart | Relationship between splenic size and splenic function[END_REF]. Furthermore, shifts in the splenic volume-function relation can also occur otherwise. For example, splenomegaly is frequently observed in hyposplenic heterozygote sickle-cell patients [START_REF] Nagel | The paradox of hemoglobin SC disease[END_REF]. Also, hypersplenism with homogenous organ function, splenic infarction, splenomas (regenerating nodules) [START_REF] Mestre-Fusco | Sickle cell anemia and functioning splenic tissue: correlation of scintigraphic findings and CT[END_REF] or transition to autosplenectomy can shift the splenic volume-function relation [START_REF] Zago | Splenic function in sickle-cell diseases[END_REF].

Because functional splenic tissue can be temporarily impaired during increased disease activity, whereas splenic atrophy is permanent, it is important to be informed about function as well as the actual volume of the organ. To measure the activity of the functional compartment of the spleen, 99m Tc-labelled heat-altered autologous erythrocyte scintigraphy with quantification of spleen uptake seems the most appropriate technique. This method is well evaluated, especially in comparison with other methods [START_REF] Gotthardt | Scintigraphy with 99mTc-labeled heat-altered erythrocytes in diagnosing hyposplenia: prospective comparison to 99mTc-labeled colloids and colourcoded duplex ultrasonography[END_REF]. Clearance rates of 99m Tc-labelled heat-altered autologous erythrocytes from the circulation should be considered carefully since this is not solely dependant on splenic sequestration as the liver also partially participates in this process. Although liver uptake of 99m Tc-labelled heat-altered erythrocytes is low in controls, absolute liver uptake can vary considerably, potentially affecting secondary parameters like the spleen-liver ratio [START_REF] Schmitz | Measurement of splenic function with heat-damaged red blood cells[END_REF].

Consequently, as the spleen is not the unique sequestering organ, with variability of liver uptake that possibly increases when splenic function is diminishing, this phenomenon may affect the axioma that measured blood clearance of cells reflects pure spleen function. Therefore, assessment of pure splenic uptake in function of the administered dose might be a better strategy.

Performing 99m Tc-labelled heat-altered erythrocyte scintigraphy on state-of-the-art SPECT-CT gamma cameras will enable combination of both function and anatomy (volume) within a single investigation with the possibility of accounting for the exact organ volume and the volume of functional organ tissue within the organ.

The large amount of potential hyposplenic patients (Table 2) makes it almost impossible to evaluate splenic function by means of scintigraphy in every patient. Laborious preparation (cell isolation, denaturation and labelling), gamma (SPECT/CT) camera availability and even the radiation burden -albeit low-requires selection of patients eligible for this advanced technique.

To screen a large group of potential hyposplenic patients, a more economical, simple and easily accessible method without radiation burden is needed. An alternative is counting the percentage of pitted erythrocytes, which is also well evaluated [START_REF] Corazza | Simple method of assessing splenic function in coeliac disease[END_REF][START_REF] Casper | A new method for studying splenic reticuloendothelial dysfunction in sickle cell disease patients and its clinical application: a brief report[END_REF][START_REF] Pearson | Developmental pattern of splenic dysfunction in sickle cell disorders[END_REF][START_REF] Lane | Functional asplenia in hemoglobin SC disease[END_REF][START_REF] Zago | Discrepancy between pit counting and spleen function tests in nutritional anemias and hemoglobinopathy C[END_REF]. It is quick, cheap and non invasive. However, interference phase microscopy needs to be available as well as trained personnel. It should also be considered that erythrocyte pits may be heterogeneous as to their origin, composition, or removal kinetics [START_REF] Zago | Discrepancy between pit counting and spleen function tests in nutritional anemias and hemoglobinopathy C[END_REF]. Percentages indicating hyposplenism may therefore be different in individuals who are hyposplenic for various reasons. Detection of Howell Jolly bodies does not seem to be a reliable method for evaluating splenic function, as correlation with other methods is poor [START_REF] Gotthardt | Scintigraphy with 99mTc-labeled heat-altered erythrocytes in diagnosing hyposplenia: prospective comparison to 99mTc-labeled colloids and colourcoded duplex ultrasonography[END_REF]. However, measuring the percentage of Howell Jolly bodies via flow cytometry is a potentially more reliable parameter as large amounts of erythrocytes can be screened [START_REF] Harrod | Quantitative analysis of Howell-Jolly bodies in children with sickle cell disease[END_REF]. The percentage of argyrophilic inclusion positive erythrocytes is a parameter that is simple and seems reliable as well [START_REF] Tham | A simple splenic reticuloendothelial function test: counting erythrocytes with argyrophilic inclusions[END_REF]. Measuring the percentages of both Howell Jolly bodies by flow cytometry as well as argyrophilic inclusion positive erythrocytes do not require special equipment. Both methods however require extensive validation. More studies evaluating the value of potentially new (immunological) markers are needed. Measuring the amount of IgM memory B cells seems a promising method giving the opportunity to measure the susceptibility to infection in a more direct way [START_REF] Kruetzmann | Human immunoglobulin M memory B cells controlling Streptococcus pneumoniae infections are generated in the spleen[END_REF][START_REF] Sabatino | Depletion of immunoglobulin M memory B cells is associated with splenic hypofunction in inflammatory bowel disease[END_REF][START_REF] Sabatino | Splenic hypofunction and the spectrum of autoimmune and malignant complications in celiac disease[END_REF]. Until these new methods have been validated, quantification of the percentages of pitted erythrocytes seems most reliable to screen for potential hyposplenic patients. Abnormal readings can subsequently be confirmed by scintigraphy.

Conclusion and recommendations

Large studies comparing all available methods in various patient populations with splenic hypofunction are missing, and data on sensitivity and specificity are scarce. To measure splenic function accurately it is important to have knowledge about the volume and function of the active splenic tissue as well as the volume of the organ itself. Function in splenic tissue can temporarily be decreased due to increased disease activity while the spleen might actually still be partially functioning and is not in state of atrophy. Assessment of spleen function using 99m Tc labelled heat-altered autologous erythrocyte scintigraphy combined with a multimodality SPECT-CT approach seems best for this purpose as all facets of splenic function are evaluated. Measuring the clearance rates of 99m Tc-labelled heat-altered autologous erythrocytes from the circulation should be considered carefully as a method to assess splenic function since this is not solely dependent on spleen activity. The population of hyposplenic patients is too large to screen by the use of scintigraphy as a first line investigation. Therefore a cheaper, simpler, more accessible method is necessary. At present, we recommend to use the percentage of pitted erythrocytes for this purpose, and refer patients with abnormal percentages for scintigraphy. Finally, more studies evaluating the value of potentially new (immunological) markers are needed. Table 1: Functions of the spleen The red pulp  Extramedullary hematopoiesis if necessary  Facilitating an environment wherein erythrocytes rid themselves of solid waste material  Blood filter for foreign material and damaged and senescent blood cells  Storage site for iron, erythrocytes, platelets, plasmablasts and plasma cells  Rapid release of antigen specific antibodies into the circulation produced by red pulp plasma cells  Defence against bacteria using the iron metabolism of its macrophages

Tables

  The white pulp T cell zone (periarterial lymphatic sheath) & B cell zone (follicles) Storage site for B and T lymphocytes  Development of B and T lymphocytes upon antigenic challenge  Release of immunoglobulins upon antigenic challenge by B lymphocytes  Production of immune mediators involved in clearance of bacteria such as complement, opsonins, properdin and tufstin Mariginal zone (MZ)  Phagocytosis of circulating microorganisms and immune complexes by MZ macrophages  Development of marginal zone B lymphocytes upon TI-2 antigenic challenge  Blood trafficking of B and T lymphocytes  Release of immunoglobulins upon antigenic challenge by splenic B lymhocytes Tables one and two

Table 2 :

 2 Causes of hyposplenism (Adapted from William B.M. et al, Table 1 [5])

	Congenital disorders	Haematologic/Neoplastic disorders
	 Congenital asplenia (isolated)	 Bone Marrow transplantation
	 Ivemark's syndrome	 Graft versus host disease
	 Stormorken's syndrome	 Acute leukemias
	 APECED syndrome	 Chronic lymphocytic leukemia
	 Fetal hydantion syndrome	 Non-Hodgkin's lymphoma
	 Congenital cyanotic heart disease	 Essential thrombocythemia
	 Normal and premature neonates	 Systemic mastocytosis
	Sickle hemoglobinopathies	 Sezary syndrome
	 SS	 Pure red cell asplenia
	 SC	 Fanconi syndrome
	 S/B-thalassemia	 Advanced breast cancer
	 SE	 Hemangiosarcoma of the spleen
	 SO-Arab	 Hemangioendothelioma of the spleen
	 SD-Los Angeles	 Malignant histiocytosis
	Gastrointestinal diseases	Sepsis/infectious diseases
	 Celiac disease	 Disseminated meningeococcemia
	 Ulcerative Colitis	 Acquired immunodeficiency
	 Crohn's disease	syndrome
	 Dermatitis herpetiformis	Circulatory disorders
	 Tropical sprue	 Spenic artery thrombosis
	 Whipple's disease	 Splenic vein thrombosis
	 Idiopathic ulcerative enteritis	 Celiac artery thrombosis
	 Intestinal Lymphangiectasie	Miscellaneous
	Hepatic disorders	 Old age
	 Alcoholic liver disease	 Alcoholism
	 Chronic active hepatitis	 Sarcoidosis
	 Liver cirrhosis and portal hypertension	 Amyloidosis
	 Primary biliary cirrhosis	 Methyldopa administration
	Autoimmune disorders	 Hypopituitarism
	 Systemic lupus erythematosis	 Selective IgA deficiency
	 Discoid lupus	 Primary pulmonary hypertension
	 Antiphospholipid syndrome	 Splenic irradiation
	 Vasculitis	 Thorotrast exposure
	 Rheumatoid arthritis	 Total parenteral nutrition
	 Glomerulonephritis	 ? high-dose corticosteroids
	 Sjögren's syndrome	Surgical Splenectomy
	 Mixed connective tissue disease
	 Graves'disease
	 Hashimoto's thyreoiditis
	 Multiple sclerosis


Romain Tartèse
email: romain.tartese@univ-rennes1.fr

Marc Poujol

Gilles Ruffet

Philippe Boulvais

Philippe Yamato

Jan Košler

Keywords: 40 Ar/ 39 Ar dating, Armorican Massif, Lizio granite, South Armorican Shear Zone, U-Pb dating Datation 40 Ar/ 39 Ar, Massif Armoricain, Granite de Lizio, Cisaillement

LA-ICP-MS U-Pb analyses performed on zircon grains from the Lizio granite yielded an emplacement age of 316 ± 6 Ma. Typical S-C structures show that the Lizio granite was emplaced contemporaneously with dextral shearing along the northern branch of the South Armorican Shear Zone and that it was therefore active at that time. 40 Ar/ 39 Ar analyses performed on muscovite grains yielded plateau dates ranging between 311.5 and 308.2 Ma.

Muscovite chemistry is typical of primary magmatic muscovite, which precludes a late fluidsinduced resetting of the K-Ar isotopic system. 40 Ar/ 39 Ar dates thus likely correspond to the cooling ages below the argon closure temperature. Considering the uncertainties on the measured ages, we can propose that either the Lizio granite cooled down quickly in less than a million of years or that it remained in a hot environment for several millions of years after its emplacement. This latter scenario could have been sustained by shear heating during dextral shearing along the northern branch of the South Armorican Shear Zone.

Résumé

Les analyses U-Pb réalisées sur des zircons du granite de Lizio par LA-ICP-MS ont livré un âge de mise en place de 316 ± 6 Ma. Des structures C-S caractéristiques démontrent que le granite de Lizio s'est mis en place pendant le cisaillement dextre le long de la branche nord du Cisaillement Sud Armoricain, qui était donc toujours actif à cette époque. Les analyses 40 Ar/ 39 Ar réalisées sur les muscovites ont donné des âges plateaux allant de 311.5 à 308.2 Ma.

La chimie des muscovites est typique de celle d'une muscovite magmatique primaire, ce qui écarte une remise à zéro tardive, induite par des fluides, du système isotopique K-Ar. Ces dates 40 Ar/ 39 Ar correspondent donc probablement à des âges de refroidissement sous la température de fermeture de l'argon. En tenant compte des incertitudes sur les âges obtenus, cela implique soit un refroidissement rapide du granite de Lizio en moins d'un million d'années soit qu'il est resté dans un environnement chaud pendant plusieurs millions d'années après sa mise en place, grâce à la chaleur frictionnelle produite pendant le cisaillement dextre le long du Cisaillement Sud Armoricain.

Introduction

In the continental crust, granitic plutons emplace at different stages during the evolution of an orogen. In the Armorican Variscan belt, numerous peraluminous granites were emplaced during the Carboniferous (e.g. [START_REF] Bernard-Griffiths | Petrogenesis of Hercynian leucogranites from the Southern Armorican Massif: contribution of REE and isotopic (Sr, Nd, Pb and O) geochemical data to the study of source rock characteristics and ages[END_REF]. Most of these granites are spatially associated with the dextral lithospheric-scale South Armorican Shear Zone.

Moreover, emplacement of these granites is often coeval with shearing as indicated by the S-C structures they display [START_REF] Berthé | Orthogneiss, mylonite and non coaxial deformation of granites -Example of the South Armorican Shear Zone[END_REF][START_REF] Gapais | Shear structures within deformed granites: mechanical and thermal indicators[END_REF]. The precise dating of these synkinematic granites and of their cooling history is therefore an essential tool to place time constraints on the different tectono-thermal events that occur during the life of the Variscan orogen.

So far, available geochronological data on the emplacement of these granites are solely provided by whole rock Rb-Sr isochron ages [START_REF] Bernard-Griffiths | Petrogenesis of Hercynian leucogranites from the Southern Armorican Massif: contribution of REE and isotopic (Sr, Nd, Pb and O) geochemical data to the study of source rock characteristics and ages[END_REF][START_REF] Peucat | Définition géochronologique de la phase bretonne en Bretagne centrale. Etude Rb/Sr de granites du domaine centre armoricain[END_REF]. Along the northern branch of the South Armorican Shear Zone, three granitic massifs are assumed to have emplaced synkinematically around 344-337 ± 13-8 Ma, which implies that the shear zone was active during the Lower Visean. In order to better constrain dextral shearing along the northern branch of the South Armorican Shear Zone, we have performed new zircon U-Pb and muscovite 40 Ar/ 39 Ar datings.

Geological framework

During Variscan times, the Armorican Massif has undergone deformation related to a major continental collision between Gondwana and Laurussia (e.g. [START_REF] Ballèvre | Palaeozoic history of the Armorican Massif: Models for the tectonic evolution of the suture zones[END_REF]. This episode was followed by the development of dextral shear zones that can be traced over a distance of a couple of hundred of kilometres, namely the North Armorican Shear Zone (NASZ) and the South Armorican Shear Zone (SASZ) (Fig. 1a). The SASZ separates two distinct domains within the Armorican Massif that are characterized by contrasted metamorphic and structural histories. To the South, the south Armorican domain (SAD) is mainly composed of deep crustal units (medium to high-grade micaschists, migmatitic gneisses and anatectic granites) that have been exhumed during the extension associated with the chain collapse (e.g. [START_REF] Brown | Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France[END_REF][START_REF] Gapais | La zone de cisaillement de Quiberon : témoin d'extension de la chaîne varisque en Bretagne méridionale au Carbonifère[END_REF]Turrillot et al., 2009).To the North, the central Armorican domain (CAD) is made of a late Proterozoicearly Palaeozoic sedimentary succession, which has been affected by low-grade metamorphism (e.g. Le [START_REF] Le Corre | Le Massif Armoricain[END_REF]. In the CAD, both strain intensity [START_REF] Gumiaux | Tectonic history of the Hercynian Armorican Shear belt (Brittany, France)[END_REF], metamorphic degree [START_REF] Le Corre | Le Massif Armoricain[END_REF] and evidence of fluid flow [START_REF] Gloaguen | Palaeozoic oolitic ironstone of the French Armorican Massif: a chemical and structural trap for orogenic base metal-As-Sb-Au mineralisation during Hercynian strikeslip deformation[END_REF][START_REF] Lemarchand | Giant quartz vein formation and high elevation meteoric fluid infiltration into the South Armorican Shear Zone: geological, fluid inclusion and stable isotope evidence[END_REF] increase southward reaching a maximum on the SASZ itself.

The SASZ is geometrically defined by ca. 100-1000m wide zone of highly strained mylonitic rocks [START_REF] Jégouzo | The South Armorican Shear Zone[END_REF]Tartèse et al., 2011a). In details, the SASZ is divided into two main branches (Fig. 1a). In the southern branch network (SBSASZ), the mylonitic foliation bears a 5-10° eastward dipping stretching lineation. Along this branch, a dextral displacement of ca. 150 to 200 km has been proposed based on the width of the mylonitized rocks [START_REF] Jégouzo | La Branche Nord du Cisaillement Sud-Armoricain (France): un essai d'évaluation du déplacement par l'analyse des mylonites[END_REF]. The northern branch (NBSASZ) is sub-linear and extends for about 300 km. It displays a subvertical mylonitic foliation also bearing a stretching lineation dipping at ca. 10° eastward. Along this branch, a 40 km minimal dextral offset has been estimated from geometrical reconstructions [START_REF] Jégouzo | La Branche Nord du Cisaillement Sud-Armoricain (France): un essai d'évaluation du déplacement par l'analyse des mylonites[END_REF].

The Lizio two-mica granite was emplaced along the NBSASZ (Fig. 1b). This granitic massif is highly emblematic because it represents the locus typicus where S-C structures were first described as resulting from a single deformational event [START_REF] Berthé | Orthogneiss, mylonite and non coaxial deformation of granites -Example of the South Armorican Shear Zone[END_REF]. These structures indicate that the Lizio granite was emplaced during shearing along the NBSASZ. Indeed, "S-planes" correspond to shearing planes which localized the dextral deformation.

These "S-planes" are vertical and their orientation mimics that of the NBSASZ. "C-planes" correspond to cleavage planes. These "C-planes" rotate during non-coaxial shearing. They define an angle of around 45° with "S-planes" in slightly deformed samples, angle which decreases with increasing strain until parallelisation of "S-planes" and "C-planes" in ultramylonitized samples [START_REF] Berthé | Orthogneiss, mylonite and non coaxial deformation of granites -Example of the South Armorican Shear Zone[END_REF]. According to [START_REF] Gapais | Shear structures within deformed granites: mechanical and thermal indicators[END_REF], these S-C structures developped during cooling of intrusives at temperatures around 550°C. The synkinematic characteristics of the Lizio granite emplacement is also evidenced by the triplepoint defined by the foliation around the intrusion (Fig. 2). This triple-point formed by the combination of the regional stress and the stress related to the granite emplacement.

Whole rock Rb-Sr data obtained on the Lizio granite define a 338  13 Ma isochron age (2) interpreted as the granite emplacement age [START_REF] Peucat | Définition géochronologique de la phase bretonne en Bretagne centrale. Etude Rb/Sr de granites du domaine centre armoricain[END_REF]. This age is similar within error to the 344 ± 8 Ma and 337 ± 13 Ma ages (2) obtained by these authors on the Pontivy and Bignan synkinematic massifs, both located a couple of kilometres westward and also rooted into the NBSASZ (Fig. 1b). More recently, a zircon U-Pb emplacement age of 316 +5/-3 Ma (ID-TIMS, [START_REF] Béchennec | Rosporden 1/50000 geological map manual (347[END_REF] has been obtained on the St-Thurien metagranite ca. 250 m south of the NBSASZ (3°40'23''W; 47°57'50''N). This St-Thurien metagranite is highly sheared and displays characteristics S-C structures, with subvertical "S-planes" and "C-planes" oriented N105-110° and ca. N080°, respectively [START_REF] Béchennec | Rosporden 1/50000 geological map manual (347[END_REF]. The St-Thurien metagranite is thus synkinematic with dextral shearing along the NBSASZ, which indicates that shearing continued after 330-325 Ma.

The geochemistry of the Lizio two-mica granite has been studied in details by [START_REF] Tartèse | Differentiation of peraluminous leucogranites "en route" to the surface[END_REF]. This study provided mineral and whole rock geochemical data as well as new Rb-Sr, Sm-Nd and O isotope data which are consistent with the few data published by [START_REF] Bernard-Griffiths | Petrogenesis of Hercynian leucogranites from the Southern Armorican Massif: contribution of REE and isotopic (Sr, Nd, Pb and O) geochemical data to the study of source rock characteristics and ages[END_REF]. The Lizio granite is highly silicic, poor in ferro-magnesian and was formed by partial melting of a metasedimentary source. Geochemical data show that a subtle magmatic differentiation occurred by crystal fractionation from the melt [START_REF] Tartèse | Differentiation of peraluminous leucogranites "en route" to the surface[END_REF]. Finally the Lizio granite intruded Brioverian metapelites of the CAD. During its emplacement in the Brioverian metapellites, a syntectonic paragenesis of Bt + Ms + Chl + St + Grt ± And developed [START_REF] Berthé | Le Cisaillement Sud Armoricain dans la région de St. Jean Brévelay (Morbihan). Analyse de la déformation cisaillante[END_REF]mineral abbreviations are after Whitney and Evans, 2010), corresponding to the facies 2b of [START_REF] Pattison | Phase equilibria and thermobarometry of metapelites[END_REF] and to a pressure of around 4 kbar [START_REF] Pattison | Polymetamorphic origin of muscovite + cordierite + staurolite + biotite assemblages: implications for the metapelitic petrogenetic grid and for P-T paths[END_REF].

Samples description

Petrography

A full petrographic description of the samples presented in this study, as well as mineral and whole-rock geochemical data, and oxygen isotope data are given in [START_REF] Tartèse | Differentiation of peraluminous leucogranites "en route" to the surface[END_REF]. To summarize, in the Lizio two-mica granite, quartz and feldspar (orthoclase, microcline and plagioclase) represent around90% of the mineral assemblage (Fig. 3) while biotite and muscovite represent the remaining 10%, biotite being generally more abundant than muscovite. Accessory minerals are apatite, zircon and monazite.

In undeformed samples, quartz is large and often euhedral (Fig. 3a-b) whereas it is recrystallized in ribbons in deformed samples (Fig. 3c-d). Plagioclase is euhedral to subeuhedral and shows thin polysynthetic twinning. Alkali feldspar is generally sub-euhedral.

Microcline and orthoclase generally displays well-expressed tartan and Carlsbad twinning, respectively. Pleochroic biotite forms euhedral brown-reddish flakes (Fig. 3a-b) and hosts most of the accessory minerals (Fig. 3b). Muscovite usually forms large flakes, some of them having a fish-like habit due to shearing (Fig. 3c). In deformed samples, shear bands are usually localized in small biotite and muscovite grains aggregates (Fig. 3c-d).

Mineral chemistry

Analytical procedure for electron microprobe analysis of minerals are detailed in [START_REF] Pitra | Wagnerite in a cordierite-gedrite gneiss: witness of long-term fluid-rock interaction in the continental crust (Ile d'Yeu, Armorican Massif, France)[END_REF] and [START_REF] Tartèse | Differentiation of peraluminous leucogranites "en route" to the surface[END_REF]. K-feldspar is close to the orthose end-member (Or = 91-92 mol.%) and plagioclase is albite on average (Ab = 91 mol.%). Biotite composition is homogeneous in the investigated samples and typical of peraluminous granites [START_REF] Tartèse | Differentiation of peraluminous leucogranites "en route" to the surface[END_REF]. Concerning muscovite, complementary electron microprobe data have been aquired, compared to the dataset published in [START_REF] Tartèse | Differentiation of peraluminous leucogranites "en route" to the surface[END_REF], as muscovite is the mineral chosen for 40 Ar/ 39 Ar dating in this study. Only data obtained on large muscovite phenocrysts are reported in Table 1 and displayed in Fig. 4. Muscovite grains have a homogeneous chemical composition with FeO = 1.1-1.4 wt.%, MgO = 0.8-0.9 wt.% and Na 2 O = 0.5-0.7 wt.% (Table 1). They also have a high TiO 2 content (0.6-1.1 wt.% on average, Table 1) and consistently fall in the primary magmatic muscovite field in the Mg-Ti-Na ternary diagram [START_REF] Miller | Composition of plutonic muscovite; genetic implications[END_REF]Fig. 4a). In the Mg + Fe against Si plot (Fig. 4b), muscovite analyses are clustered showing that limited phengitic substitution occurred in the studied samples.

Geochronological results

Zircon U-Pb dating

Zircon U-Pb data were obtained in-situ by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) performed at the Centre for Element and Isotope Analysis, University of Bergen, Norway. A classical mineral separation procedure has been followed to concentrate zircon grains suitable for U-Pb dating from samples LRT10, LRT12 and LRT15, these three samples representing undeformed, deformed and mylonitized end-members in the rock collection. Rocks were crushed and the powder fractions with a diameter < 250 µm have been isolated. Heavy minerals were successively concentrated by Wilfley table and heavy liquids procedures. Magnetic minerals were then removed with an isodynamic Frantz separator. Zircon grains were carefully examined under a binocular microscope, selected, and embedded in epoxy mounts. Then grains were hand-grounded and polished on a lap wheel with 6 μm and 1 μm diamond suspension successively. They were imaged in transmitted and reflected light, and their internal structure has been revealed using SEM BSE imaging at the LaSalle Institute, Beauvais, France.

Before analysis, grain mounts were cleaned using a weak HNO 3 solution and rinsed several times using Millipore water and ethanol in an ultrasonic bath. They were then mounted in the laser ablation cell together with several standards for quality control. A 213 Nd-YAG laser (New Wave UP-213) was used coupled to a single-collector, double focusing magnetic sector ICP-MS system (Thermo Finnigan Element 2). Data have been plotted in Wetherill concordia diagrams using the software Isoplot/Ex [START_REF] Ludwig | Isoplot/Ex version 3.70: A geochronological toolkit for Microsoft Excel[END_REF]. In Fig. 5, data are plotted with their uncertainties at 1. Results are reported in the Table 2 where uncertainties are listed at 1. When ages are calculated, all errors are reported at the 2confidence level Zircon grains were ablated using a laser beam of ca. 15 µm in diameter that was rastered over the sample surface to create a line of ca. 50 µm in length depending on the grain size. The laser energy was set at 5 J.cm -2 with a repetition rate of 10 Hz. Together with the sample introduction, the system allowed the simultaneous nebulization of an internal standard tracer solution following the technique described by [START_REF] Košler | U-Pb dating of detrital zircons for sediments provenance studiesa comparison of laser ablation ICPMS and SIMS techniques[END_REF] and [START_REF] Košler | Present trends and the future of zircon in geochronology: laser ablation ICPMS[END_REF]. Zircon data were collected on unknown zircon grains and three zircon standards, 91500 (1065 Ma; Wiedenbeck et al., 1995), Plešovice (337 Ma; [START_REF] Sláma | Plešovice zircon -a new natural reference material for U-Pb and Hf isotopic microanalysis[END_REF] and GJ-1 (609 Ma; [START_REF] Jackson | The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U-Pb zircon geochronology[END_REF]. Typically 10-15 unknowns were collected for every 7 analyses of the standards. Analysing and treating the zircon standards as unknowns is a robust quality control procedure that allows assessment of the accuracy and precision of the technique during an analytical session. Analyses done on the Plešovice and GJ-1 zircon standards yielded concordant ages of 335 ± 4 Ma (n = 12; MSWD = 1.13) and 603 ± 11 Ma (n = 9; MSWD = 0.30). Raw data were processed off-line using an Excel spreadsheet-based program (Lamdate; [START_REF] Košler | Laser Ablation ICP-MS in the Earth Sciences: Current Practices and Outstanding Issues[END_REF]. BSE images of zircon grains from sample LRT12 revealed unzoned grey cores surrounded by thin darker rims, without evidence of inherited cores (Fig. 5d). In sample LRT15, zircon grains display a more pronounced oscillatory zoning where dark cores are surrounded by brighter rims (Fig. 5d). Some zircon grains also contain small monazite inclusions (for example in LRT15-Zirc3 in Fig. 5d). In sample LRT10, two analyses are largely discordant and yield therefore meaningless dates (Fig. 5a). From the remaining concordant analyses, one yields a date of ca. 950 Ma and another five dates are grouped around 600-500 Ma (Fig. 5a). In sample LRT15, the complete dataset of 11 analyses plot in a concordant to sub-concordant position (Fig. 5b). Among them, six define an imprecise concordia age of 319 ± 15 Ma (MSWD = 0.23). The five remaining datapoints define dates between ca. 600 and 450 Ma. Finally, in sample LRT12, eleven analyses have been performed (Table 2). When plotted in a Wetherill concordia diagram (Fig. 5c), nine datapoints define a concordia age of 316.4 ± 5.6 Ma (MSWD = 0.64), identical within error with the age recorded by sample LRT15 but more precise.

Muscovite 40 Ar/ 39 Ar dating

Euhedral to subeuhedral single grains of muscovite, with variably deformed shapes, were handpicked from the 0.25-1.50 mm fraction. Irradiation was performed at the McMaster reactor (Hamilton, Canada) and lasted 43.33 hr (total fluence of 2.6  10 18 n.cm -2 ). It was monitored with Taylor Creek Rhyolite (TCR-2) sanidine (28.34 Ma, [START_REF] Renne | Intercalibration of standards, absolute ages and uncertainties in 40 Ar/ 39 Ar dating[END_REF].

Muscovite single grains were analyzed by step-heating with a 40 Ar/ 39 Ar laser probe, following the procedure described in [START_REF] Ruffet | Comparison of 40 Ar/ 39 Ar conventional and laser dating of biotites from the North Tregor batholiths[END_REF]1995). Blanks were performed routinely each first or third step, and subtracted from subsequent sample gas fractions. A plateau age is obtained when apparent ages of at least three consecutive steps, representing a minimum of 70% of the 39 Ar released, agree within 2 error bars with the integrated age of the plateau segment. The 40 Ar/ 39 Ar analytical data are portrayed as age spectra in Fig. 6. All reported uncertainties both in the Fig. 6 and in the text are at the 2 confidence level.

Single muscovite grains have been analyzed in four samples from the Lizio granite (LRT10, LRT11, LRT13 and LRT15). These samples yielded plateau dates ranging between 311.5 ± 0.4 Ma down to 308.2 ± 0.6 Ma (Fig. 6). They display slight saddle-shaped 40 Ar/ 39 Ar age spectra that might reflect slight perturbations of the K-Ar isotopic system (e.g. [START_REF] Alexandrov | Muscovite recrystallization and saddle-shaped 40 Ar/ 39 Ar age spectra: Example from the Blond granite (Massif Central, France)[END_REF][START_REF] Cheilletz | 40 Ar/ 39 Ar dating of shear zones in the Variscan basement of the Greater Kabylia (Algeria). Evidence of an Eo-Alpine event at 128 Ma (Hauterivian-Barremian boundary): geodynamic consequences[END_REF]. An important point is that there is no relationship between the obtained dates and the shearing gradient observed throughout the Lizio granite.

Interpretation and discussion

Considering the high closure temperature for the U-Pb radiogenic system in zircon in excess of 800°C [START_REF] Cherniak | Pb diffusion in zircon[END_REF], we interpret the age of 316 ± 6 Ma obtained on sample LRT12 from the synkinematic Lizio granite as its emplacement age along the NBSASZ. The age of 319 ± 15 Ma obtained on sample LRT15 is imprecise but consistent with this emplacement age. Older dates obtained on zircon grains from samples LRT10 and LRT15 illustrate the classical phenomenon of inheritance which often characterizes granitic rocks (e.g. [START_REF] Bea | Zircon inheritance reveals exceptionally fast crustal magma generation processes in Central Iberia during the Cambro-Ordocvician[END_REF][START_REF] Harrison | Observations and controls on the occurrence of inherited zircon in Concord-type granitoids, New Hampshire[END_REF][START_REF] Miller | Hot and cold granites? Implications of zircon saturation temperatures and preservation of inheritance[END_REF][START_REF] Roddick | U-Pb dating of granites with inherited zircon: Conventional and ion microprobe results from two Paleozoic plutons, Canadian Appalachians[END_REF]. Here, inherited dates mainly range between Neo-Proterozoic and Upper Ordovician times. Concerning the emplacement age of 316 ± 6 Ma, it is identical to the zircon U-Pb emplacement age of 316 +5/-3 Ma of the St-Thurien metagranite found by [START_REF] Béchennec | Rosporden 1/50000 geological map manual (347[END_REF] and contemporaneous with dextral shearing along the NBSASZ. It is also identical to the U-Pb zircon emplacement age obtained recently on the southward Questembert granite by Tartèse et al. (2011b), which is contemporaneous with dextral shearing along the SBSASZ. This new U-Pb emplacement age obtained on the Lizio granite thus indicates that both branches of the SASZ were active at the same time around 315-320 Ma. However, it is younger and does not overlap with the whole rock Rb-Sr isochron age of 337 ± 13 Ma (2), previously considered as the emplacement age of the granite (Whole rock Rb-Sr isochron, recalculated from the six data of [START_REF] Peucat | Définition géochronologique de la phase bretonne en Bretagne centrale. Etude Rb/Sr de granites du domaine centre armoricain[END_REF] and the three data of [START_REF] Tartèse | Differentiation of peraluminous leucogranites "en route" to the surface[END_REF]. So far, all the available geochronological data imply that the NBSASZ has been active from ca. 344 Ma (whole rock Rb-Sr isochron age of the Pontivy granite, [START_REF] Peucat | Définition géochronologique de la phase bretonne en Bretagne centrale. Etude Rb/Sr de granites du domaine centre armoricain[END_REF] down to ca. 315 Ma. However, it would be interesting to get new zircon U-Pb data on the other granitic massifs emplaced along the NBSASZ. Indeed, the existing Rb-Sr isochron ages may be too old for these granites. Such old inherited "isochrons" have been described for example by [START_REF] Roddick | Strontium isotopic equilibration: A solution to a paradox[END_REF] for the crustally-derived Murrumbidgee Batholith, Australia. Moreover, whole rock-mineral isochrons on two samples from the Pontivy granite (whole rock Rb-Sr isochron age of 344 ± 8 Ma) yielded ages identical within error at 311 ± 9 and 310 ± 9 Ma respectively [START_REF] Peucat | Définition géochronologique de la phase bretonne en Bretagne centrale. Etude Rb/Sr de granites du domaine centre armoricain[END_REF], ages that are consistent with a Late Carboniferous emplacement for the Pontivy granite. We thus believe that the ca. 340 Ma old emplacement ages obtained through whole rock Rb-Sr dating of synkinematic granites along the NBSASZ are actually not emplacement ages of these granites but rather represent inherited "isochrons" in the sense of [START_REF] Roddick | Strontium isotopic equilibration: A solution to a paradox[END_REF]. In that case, initiation of dextral shearing along the NBSASZ may not be as old as the ca. 340 Ma age that has been considered so far. Indeed, our data on the Lizio granite give a maximum age of 322 Ma for the activity along the NBSASZ. 40 Ar/ 39 Ar dates obtained on muscovite grains from four samples of the Lizio granite range between 311.5 and 308.2 Ma. The two oldest dates are identical within error with the zircon U-Pb emplacement age of 316 ± 6 Ma and involve a sub-instantaneous cooling of the intrusion, and the two other dates are slightly younger, considering the uncertainties associated with both techniques. This implies that either the younger muscovite grains have been affected by fluid-induced recrystallization processes after their magmatic isotopic closure (e.g. Villa, 2010) which would have reset the 40 Ar/ 39 Ar dates, or that the Lizio granite remained sufficiently warm during 2-3 Ma after its emplacement, above the argon closure temperature of muscovite. The muscovite chemistry shows that all the muscovite grains have a typical primary magmatic composition, which likely precludes the first hypothesis of a fluids-induced recrystallization and resetting process. The closure temperature for argon in muscovite has for a long time been considered to be around 350°C based in part on calibration of obtained ages vs. metamorphic grade (e.g. [START_REF] Purdy | K-Ar ages on rock-forming minerals from the Central Alps[END_REF]. However, [START_REF] Harrison | Diffusion of 40 Ar in muscovite[END_REF] recently published the first experimental study of argon diffusion in muscovite and showed that Ar retentivity in muscovite is greater than previously assumed. Based on their diffusion parameters (E a = 264 kJ.mol -1 ; D 0 = 2.3E -04 m 2 .s -1 ), and taking 500 and 1000 µm for the diffusion radius, we have computed the different closure temperature as a function of cooling rates (Fig. 7a): the closure temperature ranges between ca. 450 and 550°C, for the diffusion dimensions and cooling rates investigated. In a Temperature-Time diagram (Fig. 7b), the minimum cooling rate calculated considering U-Pb and 40 Ar/ 39 Ar ages and their associated uncertainties is 10.4°C/Ma.

Taking the minimum zircon U-Pb age and the oldest muscovite dates corresponds to a sub-instantaneous cooling of the granitic body. A simple 2D thermal numerical model (details in Appendix A) shows that a Lizio-type granitic pluton emplaced at mid-crustal depths cools down in less than 1 Ma (Fig. 8), consistently with this scenario. Conversely, a slow cooling rate of ca. 10°C/Ma would imply that the Lizio granite remained at a temperature above ca. 480-500°C for a long time after its emplacement. The Lizio granite was emplaced around 4 kbar, which corresponds to a depth of 15 km considering only the lithostatic pressure and a granite density of 2.7. At this depth, temperatures of around 550°C can be reached and maintained along continental strike-slip shear zones due to heat production by shear heating [START_REF] Leloup | Shear heating in continental strikeslip shear zones: model and field examples[END_REF]. The cooling below the closure temperature of ca. 500°C for such a slow cooling rate would have occurred when heat advection due to shear heating and therefore shearing along the NBSASZ had stopped. Moreover, one cannot totally rule out the influence of some fluid-rock interactions which would have disturbed 40 Ar/ 39 Ar age spectra without being recorded by the whole rock and muscovite chemistry. Indeed, the analyzed muscovite grains display subtle saddle-shaped 40 Ar/ 39 Ar age spectra that may indicate fluid-induced disturbance (e.g. [START_REF] Alexandrov | Muscovite recrystallization and saddle-shaped 40 Ar/ 39 Ar age spectra: Example from the Blond granite (Massif Central, France)[END_REF][START_REF] Cheilletz | 40 Ar/ 39 Ar dating of shear zones in the Variscan basement of the Greater Kabylia (Algeria). Evidence of an Eo-Alpine event at 128 Ma (Hauterivian-Barremian boundary): geodynamic consequences[END_REF]. Finally, to a first order, our new data are consistent with a simple cooling of the intrusion, with possible, but limited, influence of shear heating and fluid-rock interaction. A surprising result is the absence of consistency between the muscovite 40 Ar/ 39 Ar dates and the shearing gradients observe throughout the Lizio granite. This could result from the choice we made to date large muscovite phenocrysts in all the samples. It would deserve further studies, notably dating increasingly deformed micas toward the NBSASZ to better evaluate if potential postemplacement events have affected the K-Ar system in micas.

Conclusion

Zircon U-Pb dating carried out on the Lizio granite yielded an age of 316 ± 6 Ma that is interpreted as the emplacement age of the granite, and therefore also of dextral shearing along the northern branch of the South Armorican Shear Zone. This is significantly younger than the previous ca. 340 Ma whole rock Rb-Sr age interpreted as dating the emplacement of the Lizio granite. The significiance of this older age is not yet well understood and it should therefore be treated with caution. 40 Ar/ 39 Ar analyses performed on various muscovite grains from the Lizio granite yielded dates of 311.5-308.2 Ma. As geochemical evidence precludes late fluids-induced large perturbations of the K-Ar isotopic system in muscovite, these dates likely reflect cooling ages below the argon closure temperature. Two extreme scenarios may explain the obtained ages, either a sub-instantaneous cooling of the granite or conversely a very slow cooling which lasted several millions of years. This last scenario requires that the Lizio granite remained in a hot environment for a long time after its emplacement. This could have been caused by shear heating due to dextral shearing along the northern branch of the South Armorican Shear Zone.

Tartèse, R., Boulvais, P., Poujol, M., Gloaguen, E., 2011b X (km)

Host-rock ρ h = 2700 kg.m -3 k h = 2 W.m -1 .K -1 Cp h = 750 J.kg -1 .K -1 Intrusion ρ g = 2400 kg.m -3 k g = 3 W.m -1 .K -1 Cp g = 1250 J.kg -1 .K -1

  . The late Carboniferous Variscan evolution of the Armorican Massif (France): magmatism, hydrothermalism and metallogenic consequences. Geophys. Res. Abstr. 13, 2422. Turrillot, P., Augier, R., Faure, M., 2009. The top-to-the-southeast Sarzeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica. Bull. Soc. Géol. Fr. 180, 247-261. Villa, I.M., 2010. Disequilibrium textures versus equilibrium modelling: geochronology at the crossroads. In: Spalla, M.I., Marotta, A.M., Grosso, G. (Eds.), Advances in Interpretation of Geological Processes: Refinement of Multi-scale Data and Integration in Numerical Modelling. Geol. Soc. London Spec. Pub. 332, 1-15.Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., von Quadt, A.,Roddick, J.C., Spiegel, W., 1995. Three natural zircon standards for U-Th-Pb, Lu-Hf, trace element and REE analyses. Geostandard. Newslett. 19, 1-23. Whitney, D.L., Evans, B.W., 2010. Abbreviations for names of rock-forming minerals. Am.Mineral. 95,[185][186][187]

Figure caption Fig

 caption Figure caption

Fig. 2 :

 2 Fig.2: Simplified geological map of the Lizio granite (after[START_REF] Berthé | Le Cisaillement Sud Armoricain dans la région de St. Jean Brévelay (Morbihan). Analyse de la déformation cisaillante[END_REF].

Fig. 3 :

 3 Fig. 3: Photomicrographs showing the typical structures of the Lizio granite samples, from

Fig. 4

 4 Fig. 4: a) Muscovite chemical compositions plotted in the ternary Mg-Ti-Na diagram of

Fig. 5

 5 Fig. 5: a) Wetherill concordia diagram for zircon data from the sample LRT10. b) Wetherill

Fig. 6 :

 6 Fig. 6: 40 Ar/ 39 Ar age spectra of analyzed muscovites. The age error bars for each temperature

Fig. 8 :

 8 Fig. 8: 2D thermal modelling of the cooling of the Lizio granite during 1 Ma (see details in

Fig. A1 :

 A1 Fig. A1: Initial setup used for the 2D thermal modelling.

Table caption Table 1 :

 caption1 Average chemical compostions of muscovite.

Table 2 :

 2 Zircon U-Pb isotopic data. All uncertainties are at 1.

Table 1 :

 1 Average chemical compositions of muscovite.

		LRT15 (n=16)	sd	LRT10 (n=19)	sd	LRT11 (n=10)	sd	LRT12 (n=14)	sd	LRT14 (n=15)	sd
	SiO 2	46.5 0.3	45.7 0.4	46.4 0.2	45.7 0.5	46.1 0.3
	TiO 2	1.1 0.2	0.7 0.2	0.6 0.2	1.1 0.2	1.0 0.2
	Al 2 O 3	34.5 0.2	35.8 0.3	35.0 0.3	34.7 0.5	34.9 0.4
	FeO	1.4 0.1	1.1 0.1	1.2 0.2	1.3 0.1	1.4 0.1
	MgO	0.9 0.1	0.8 0.1	0.8 0.1	0.8 0.1	0.8 0.1
	Na 2 O	0.5 0.1	0.7 0.1	0.6 0.1	0.5 0.1	0.5 0.1
	K 2 O	11.0 0.2	10.8 0.2	10.7 0.6	11.2 0.3	11.1 0.2
	SUM	95.8		95.6		95.3		95.4		95.8
			Structural formula based on 22 oxygen atoms
	Si	6.17 0.02	6.08 0.02	6.17 0.03	6.11 0.04	6.13 0.02
	Ti	0.11 0.02	0.07 0.02	0.06 0.02	0.11 0.02	0.10 0.02
	Al	5.40 0.03	5.61 0.03	5.49 0.03	5.48 0.06	5.47 0.05
	Fe	0.16 0.01	0.13 0.01	0.14 0.02	0.15 0.01	0.15 0.01
	Mg	0.18 0.01	0.15 0.01	0.15 0.02	0.16 0.02	0.17 0.02
	Na	0.12 0.02	0.19 0.03	0.15 0.04	0.14 0.03	0.14 0.03
	K	1.86 0.04	1.84 0.04	1.83 0.09	1.91 0.04	1.88 0.03
	SUM	14.00		14.06		13.99		14.06		14.04

MUSCOVITE

Oxide contents in wt.% and cationic contents in apfu. sd = Standard deviation

Table 2 :

 2 Zircon U-Pb isotopic data. All uncertainties are at 1σ

	Rho

Appendix A: 2D thermal numerical model

The 2D thermal code used in this study solves the heat diffusion equation (A.1):

where , C p and k are material density, specific heat capacity and thermal conductivity, respectively and Hr corresponds to the radiogenic heat production. This equation is solved numerically in the implicit formulation using the finite difference method. In this study, the heat production (Hr) is set to zero. Indeed, preliminary tests have been done with values from 1 to 5 W.m -3 and the results show that results are not sgnificantly differents after 1Myr of experiment. The initial setup is displayed in the Fig. A1. The granitic intrusion is modelled as an ellipse (5 km high and 10 km long) located at 15 km depth in a host rocks medium with a linear thermal gradient of 30°C.km -1 . The resolution used is 151 x 151 nodes (i.e., 200 m).

Aknowledgments

We are grateful to J. Tudri and O. Pourret (LaSalle Insitute, Beauvais, France) for providing us access to the secondary electron microprobe and to M. Bohn (IFREMER, Brest, France) for his assistance during electron microprobe analyses. This work was funded by grants from the CNRS-INSU ( "3F" and "Action incitative" programs).