I Sagna

X Quidelleur

E Dioh

P Y Gillot

J C Lefèvre

L Baratoux

D P Diallo

Proterozoic magmatic events recorded in 40 Ar/ 39 Ar data from the northern part of the Kedougou Kenieba Inlier (eastern Senegal)

Keywords: 40 Ar/ 39 Ar thermochronology, Proterozoic, West African Craton, Senegal, Kedougou Kenieba Inlier

We present a thermal history of the Kedougou Kenieba Inlier (eastern Senegal), the westerncentral outcropping part of the West African Craton, based on 40 Ar/ 39 Ar analyses performed on hornblende, biotite, and K-feldspar minerals from Paleoproterozoic plutonic rocks. 40 Ar/ 39 Ar age spectra obtained for hornblendes and biotites range between 2055 ± 26 and 2028 ± 28 Ma, in very good consistency. They suggest that magmatism in the Kedougou Kenieba Inlier happened at the end of the Eburnean Orogeny (2.25-2.00 Ga). Taking into account previous zircon U/Pb ages, these new results imply that initial cooling from magmatic temperature to about 300°C (biotite closure temperature) occurred rapidly, and that these rocks did not experience any subsequent re-heating event above 300°C. On the other hand, 40 Ar/ 39 Ar age spectra obtained on K-feldspars display strongly disturbed patterns revealing that these minerals experienced argon loss due to re-heating events. Altogether, these data constrain a thermal history for the studied area involving two thermal events with temperatures around 275 and 225°C, at about 1750 and 1500 Ma, respectively. These thermal events, previously inferred from U/Pb ages of intruding mafic dykes, can be related to aborted attempts of continental breakup, as the West African Craton remained attached to Baltica and Amazonia cratons during the 1.8-1.3 Ga interval.

Content: Abstract: 206 words Text: 3843 words Figure 1: Map of studied area Figure 2a: Hornblende and biotite Ar/Ar age spectra Figure 2b: K-Feldspar Ar/Ar age spectra Figure 3: Thermal history and modelled K-Feldspar age spectra Table 1: Ar/Ar ages Supp. Mat.: Ar/Ar data Supp. Mat. 2: Monotonic cooling modelling of K-Feldspar age spectra Rev3 version -November 19 th , 2020

Introduction

Proterozoic plate reconstruction models (e.g., [START_REF] Johansson | Baltica, Amazonia and the SAMBA connection-1000 million years of neighbourhood during the Proterozoic?[END_REF][START_REF] Zhao | Review of global 2.1-1.8 Ga orogens: implications for a pre-Rodinia supercontinent[END_REF] suggest that the West African craton (WAC), the north-east of Central South America (Amazonia) craton, and the south-east of East European (Baltica) craton were connected for more than 1 Ga, until breakup of Rodinia (c.a. 0.8 Ga; [START_REF] Johansson | Baltica, Amazonia and the SAMBA connection-1000 million years of neighbourhood during the Proterozoic?[END_REF]2014;[START_REF] Wingate | Age and palaeomagnetism of the Mundine Well dyke swarm, Western Australia: implications for an Australia-Laurentia connection at 755 Ma[END_REF].

The Eburnean Orogeny completed the assemblage of the WAC at ca. 2.1 Ga [START_REF] Baratoux | Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny ( 2.2-2.0 Ga), western Burkina Faso[END_REF][START_REF] Begg | The lithospheric architecture of Africa: Seismic tomography, mantle petrology, and tectonic evolution[END_REF][START_REF] Feybesse | The paleoproterozoic Ghanaian province: Geodynamic model and ore controls, including regional stress modeling[END_REF]Wane et al., 2018, and references therein). The Trans-Amazonian Orogeny in South America led to the assembly of the Amazonian Craton at ca. 2.18-1.95 Ga [START_REF] Kroonenberg | Paleoproterozoic evolution of the Guiana Shield in Suriname: A revised model[END_REF][START_REF] Nomade | Thermal and tectonic evolution of the paleoproterozoic Transamazonian orogen as deduced from 40 Ar/ 39 Ar and AMS along the Oyapok river (French Guyana)[END_REF][START_REF] Hartmann | The Mesoproterozoic supercontinent Atlantica in the Brazilian shield-review of geological and U-Pb zircon and Sm-Nd isotopic evidence[END_REF][START_REF] Neves | Timing of crust formation, deposition of supracrustal sequences, and Transamazonian and Brasiliano metamorphism in the East Pernambuco belt (Borborema Province, NE Brazil): implications for western Gondwana assembly[END_REF]. The WAC was then separated from the Baltica at about 600-550 Ma [START_REF] Johansson | Baltica, Amazonia and the SAMBA connection-1000 million years of neighbourhood during the Proterozoic?[END_REF]2014), and from the Amazonia craton at about 200 Ma, when the Central Atlantic Ocean opened (e.g., [START_REF] Marzoli | Extensive 200-million-year-old continental flood basalts of the Central Atlantic Magmatic Province[END_REF][START_REF] Nomade | Chronology of the Central Atlantic Magmatic Province: implications for the Central Atlantic rifting processes and the Triassic-Jurassic biotic crisis[END_REF][START_REF] Merle | 40 Ar/ 39 Ar ages and Sr-Nd-Pb-Os geochemistry of CAMP tholeiites from Western Maranhão basin (NE Brazil)[END_REF]. The Mesoproterozoic period was considered as relatively quiet for the WAC, as no tectonic event was documented. However, recent studies have identified during this period large-scale magmatic events based on U-Pb ages obtained on dyke swarms [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF][START_REF] Youbi | The 1750 Ma Magmatic Event of the West African Craton (Anti-Atlas, Morocco)[END_REF]. 40 Ar/ 39 Ar analyses have been performed here on hornblende, biotite and K-feldspar minerals from mafic and felsic Paleoproteorozoic plutonic rocks intruding the Mako belt in order to reconstruct the thermal history of the Kedougou Kenieba Inlier (WAC). The aim of the present study is also to contribute to the identification of large-scale magmatic events having affected the WAC and to determine their thermal impact using 40 Ar/ 39 Ar thermochronology.

Geological setting

West African Craton

Archean to Paleoproterozoic crusts from the WAC outcropp as the Reguibat Rise and the Leo-Man Rise, in its northern and southern parts, respectively (Fig. 1). Neoproterozoic to Paleozoic rocks from the Taoudeni Basin cover its central part. In the western part of the Reguibat Rise, the main phase of magmatism is dated at about 2.9 Ga [START_REF] Schofield | U-Pb dating and Sm-Nd isotopic analysis of granitic rocks from the Tiris Complex: New constaints on key events in the evolution of the Reguibat Shield, Mauritania[END_REF].

Then, metamorphism occurred at 2.7 Ga, as suggested by the age obtained there for granulite facies gneisses [START_REF] Cahen | The Geochronology and Evolution 14 of Africa[END_REF]. In the Man Domain of the Leo Rise, comparable ages have been measured for the Archean formation, but U-Pb ages obtained for gneiss reveal that magmatism started as early as 3.5 Ga [START_REF] Thiéblemont | Archean evolution of the Leo Rise and its Eburnean reworking[END_REF].

The WAC experienced an intense crustal growth between 2.2 and 2.1 Ga (e.g., [START_REF] Abouchami | A Major 2.1 Ga Event of Mafic Magmatism in West Africa -an Early Stage of Crustal Accretion[END_REF][START_REF] Dia | Isotopic constraints on the age and formation of a Palaeoproterozoic volcanic arc complex in the Kedougou Inlier, eastern Senegal, West Africa[END_REF][START_REF] Hirdes | U-Pb geochronology of paleoproterozoic rocks in the southern part of the Kedougou-Kenieba Inlier, Senegal, West Africa: evidence for diachronous accretionary development of the Eburnean province[END_REF]. It is characterized by emplacement of tonalite-trondhjemite-granodiorite (TTG) series, granite plutons, and intercalated greenstone belts. Based on its tholeiitic or calc-alkaline signature, the latter has been related to a metamorphosed volcanic arc environment [START_REF] Dia | Caractères et signification des complexes magmatiques et métamorphiques du secteur de Sandikounda-Laminia (Nord de la Boutonnière de Kédougou; Est du Sénégal): un modèle géodynamique du Birrimien de l'Afrique de l'Ouest[END_REF]. During the Eburnean Orogeny, several periods of deformation have been recognized with different interpretations proposed (e.g., [START_REF] Feybesse | The paleoproterozoic Ghanaian province: Geodynamic model and ore controls, including regional stress modeling[END_REF][START_REF] Perrouty | Revised Eburnean geodynamic evolution of the gold-rich southern Ashanti Belt, Ghana, with new field and geophysical evidence of pre-Tarkwaian deformations[END_REF]. First, a compression episode induced regional metamorphism due to thrusting and crustal thickening. Then, possibly following an extension phase, N to NE transcurrent shear zones developed during the second period. Overall, a voluminous plutonic activity occurred during the Eburnean Orogeny in the WAC. Calc-alkaline Na-rich plutonism of TTG-type was first emplaced and was followed by more alkaline syenitic magmatism [START_REF] Baratoux | Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny ( 2.2-2.0 Ga), western Burkina Faso[END_REF], from about 2250 to 2060 Ma (e.g., [START_REF] Goujou | Notice explicative Carte géologique à 1/200 000 du Sénégal, feuille Kossanto-Dalafi Est[END_REF][START_REF] Hirdes | U-Pb geochronology of paleoproterozoic rocks in the southern part of the Kedougou-Kenieba Inlier, Senegal, West Africa: evidence for diachronous accretionary development of the Eburnean province[END_REF][START_REF] Tshibubudze | Oldest U-Pb crystallisation age for the West African Craton From the Oudalan-Gorouol Belt of Burkina Faso[END_REF][START_REF] Wane | The onset of the Eburnean collision with the Kenema-Man craton evidenced by plutonic and volcanosedimentary rock record of the Masssigui region, southern Mali[END_REF]. At least 26 dyke swarms of variable orientations were mapped across the WAC using the airborne magnetic data [START_REF] Jessell | An updated map of West African mafic dykes[END_REF]. These dyke swarms were emplaced between ~2733 Ma [START_REF] Tait | The Ahmeyim Great Dyke of Mauritania: a newly dated Archaean intrusion[END_REF] and ~200 Ma [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF].

Kedougou Kenieba Inlier (KKI)

The Birimian supergroup, mapped in detail by [START_REF] Theveniaut | Notice explicative de la carte géologique à 1/500 000 du Sénégal oriental[END_REF], makes up most of the Kedougou Kenieba Inlier (WAC, Eastern Senegal) where the present study is focused (Fig. 1). The western part of the Kedougou Kenieba Inlier is made of Paleoproterozoic volcano-plutonic and volcano-sedimentary rocks (Mako belt) crosscut by plutonic bodies of varying composition, from gabbroic to granitic [START_REF] Dioh | Diversity of the Palaeoproterozoic granitoids of the Kedougou inlier (eastern Senegal): Petrographical and geochemical constraints[END_REF] (Fig. 1).

They were emplaced in an arc-type setting [START_REF] Dia | Isotopic constraints on the age and formation of a Palaeoproterozoic volcanic arc complex in the Kedougou Inlier, eastern Senegal, West Africa[END_REF][START_REF] Sagna | K-Ar mineral ages and thermal history of magmatic and metamorphic Palaeoproterozoic units from the northern part of Kedougou Kenieba Inlier, West African Craton (Eastern Senegal)[END_REF] and accreted during the Eburnean Orogeny [START_REF] Baratoux | Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny ( 2.2-2.0 Ga), western Burkina Faso[END_REF][START_REF] Begg | The lithospheric architecture of Africa: Seismic tomography, mantle petrology, and tectonic evolution[END_REF][START_REF] Feybesse | The paleoproterozoic Ghanaian province: Geodynamic model and ore controls, including regional stress modeling[END_REF].

To the west of the KKI, the Mako Supergroup is mainly made of tholeiitic and calcalkaline lavas interbedded with volcano-detritic sediments, which are folded and have undergone a greenschist facies metamorphism [START_REF] Bassot | Etude géologique du Sénégal oriental et de ses confins Guinéo-Maliens[END_REF][START_REF] Bassot | Le complexe volcano-plutonique calco-alcalin de la rivière Daléma (Est du Sénégal): discussion de sa signification géodynamique dans le cadre de l'orogène éburnéenne (Protérozoïque inférieur)[END_REF][START_REF] Dia | Caractères et signification des complexes magmatiques et métamorphiques du secteur de Sandikounda-Laminia (Nord de la Boutonnière de Kédougou; Est du Sénégal): un modèle géodynamique du Birrimien de l'Afrique de l'Ouest[END_REF][START_REF] Dia | Isotopic constraints on the age and formation of a Palaeoproterozoic volcanic arc complex in the Kedougou Inlier, eastern Senegal, West Africa[END_REF]. Between 2158 and 2079 Ma, they were intruded by several plutonic bodies from the Badon-Kakadian Batholith [START_REF] Dia | Isotopic constraints on the age and formation of a Palaeoproterozoic volcanic arc complex in the Kedougou Inlier, eastern Senegal, West Africa[END_REF]. Amphibolite facies metamorphism of Paleo-Proterozoic metasediments, was documented around the northern edge of the Saraya batholith in the central part of the Kedougou Kenieba Inlier [START_REF] Ndiaye | Deformation and metamorphism of Birrimian formations, in relation to the emplacement of the Eburnean Saraya granite (eastern Senegal)[END_REF]. Within these metasediments, metamorphic monazites yielded LA-ICP-MS U-Pb ages of 2052 ± 7 and 2048 ± 8 Ma, and garnets neoblasts were dated using Sm-Nd at 2049.1 ± 3.1 Ma [START_REF] Kone | Goldschmidt Abstracts[END_REF].

The volcano-plutonic area investigated here corresponds to the northern part of the Mako Supergroup. Numerous granitoid bodies are present, as well as metabasites tholeites from the Konkoto and Mako areas. The metasediments and calc-alkaline metavolcanites of the Fouldé and Baniomba series are also found in this area [START_REF] Dia | Isotopic constraints on the age and formation of a Palaeoproterozoic volcanic arc complex in the Kedougou Inlier, eastern Senegal, West Africa[END_REF][START_REF] Dioh | Diversity of the Palaeoproterozoic granitoids of the Kedougou inlier (eastern Senegal): Petrographical and geochemical constraints[END_REF][START_REF] Goujou | Notice explicative Carte géologique à 1/200 000 du Sénégal, feuille Kossanto-Dalafi Est[END_REF]Gueye et al., 2007, and references therein). Granodiorite rocks have been sampled from the Diombalou (981M) and the Kaourou-Alinguel (984M and 9814M) plutons.

Sample 983M is from a fine-grained amphibole-bearing rock within the Badon pluton, while 9813M is from a lower gabbroic unit from the Sansankhoto pluton (Fig. 1). The Badon batholith (981M) was recently dated at 2102 ± 8 Ma (U-Pb, zircon; Théveniaut et al., 2010) and the Sansankhoto batholith (9813M) yields 2058 ± 8 Ma (U-Pb zircon; [START_REF] Goujou | Notice explicative Carte géologique à 1/200 000 du Sénégal, feuille Kossanto-Dalafi Est[END_REF].

Recent K-Ar ages, performed on hornblende, biotite and K-feldspar minerals from the same samples as those investigated in the present study, have shown that plutonism in the Kedougou Kenieba Inlier occurred between 2.0 and 2.1 Ga, and suggested either a slow cooling rate for hundreds of Ma, or that a thermal event partially re-opened the K-Ar system of K-feldspars from this area [START_REF] Sagna | K-Ar mineral ages and thermal history of magmatic and metamorphic Palaeoproterozoic units from the northern part of Kedougou Kenieba Inlier, West African Craton (Eastern Senegal)[END_REF].

In the KKI, two dyke swarms were identified using the airborne magnetic data [START_REF] Jessell | An updated map of West African mafic dykes[END_REF]: a NNE to NE-trending Kedougou swarm (1764 ± 4 Ma) and an E-W oriented Sambarabougou swarm (1521 ± 3 Ma), both dated by TIMS U-Pb method on baddeleyite grains; [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF] (Fig. 1b).

Methods

40 Ar/ 39 Ar dating

The 40 Ar/ 39 Ar analyses were performed at Orsay (France) using the multi-collector mass-spectrometer and procedure presented in [START_REF] Coulié | Exploring the multicollection approach for the 40 Ar/ 39 Ar dating technique[END_REF]. During 60 h, samples and flux monitors were irradiated together in the Corvallis Triga reactor (Radiation center, Oregon state University, USA), using the CLICIT facility. Gas extraction of unkowns was realised by step-heating with a high frequency furnace. As their behavior during heating is supposingly ideal, a Nd-YAG laser (1064 nm) was used to fuse 20 mg aliquots of sanidine separated from the Fish Canyon tuff in a single step. From these analyses, the J factors were calculated based on the age of 28.201 Ma [START_REF] Kuiper | Synchronizing Rock Clocks of Earth History[END_REF]. From repeated analyses of air aliquots measured with the same 40 Ar signal than that measured for a typical step, the mass discrimination was corrected using a linear law. 40 K decay constants and the K isotopic ratio of [START_REF] Steiger | Subcommission on Geochronology: convention on the use of decay constants in Geo and Cosmochronology[END_REF] have been used.

Precise and accurate measurements are mandatory to constrain the age and duration of geological events. We have shown that 40 Ar/ 39 Ar ages with an analytical relative precision as low as 0.1% can be reached with our instrument (Ricci et al., 2013). However, as 40 Ar/ 39 Ar is a relative dating technique, standard ages and decay constant uncertainties are the main limiting factor for the accuracy of this technique [START_REF] Kuiper | Synchronizing Rock Clocks of Earth History[END_REF]. Recent studies have proposed ages for the widely used Fish Canyon Tuff sanidine ranging from 27.89 [START_REF] Westerhold | Timescale contro-versy: accurate orbital calibration of the early Paleogene[END_REF] to 28.305 ± 0.072 Ma [START_REF] Renne | Joint determination of 40 K decay constants and 40 Ar/ 40 K for the Fish Canyon sanidine standard, and improved accuracy for 40 Ar/ 39 Ar geochronology[END_REF]. Such difference of about 1% can be explained by inter-laboratory biases, or by the presence of previously un-detected 39 ArK recoil [START_REF] Phillips | Astronomical calibration of 40 Ar/ 39 Ar reference minerals using high-precision, multi-collector (ARGUSVI) mass spectrometry[END_REF]. Consequently, we conservatively use a systematic error of 1% for the J-Factor determination involved in the 40 Ar/ 39 Ar age calculation presented here. Uncertainties are quoted at the 2σ level throughout this manuscript.

40 Ar/ 39 Ar thermochronology

The multi-diffusion domain (MDD) approach has been used here in order to recover the continuous thermal history recorded in K-feldspars [START_REF] Lovera | 40 Ar/ 39 Ar geothermometry for slowly cooled samples having a distribution of diffusion domain sizes[END_REF][START_REF] Lovera | Computer programs to model 40 Ar/ 39 Ar diffusion data from multi-domain samples[END_REF]. It relies on the hypothesis that diffusion of Ar occurs in nature by the same mechanics as in the laboratory, and that diffusion of K-feldspar argon can be modeled by a discrete distribution of non-interacting domains. Using the 40 Ar/ 39 Ar step-heating measurements, diffusion parameters (i.e., activation energy and frequency factor) are retrieved from the Arrhenius diagram obtained from the degassing systematics of reactor-produced 39 ArK (Loveral et al., 1989). Monotonic cooling thermal histories can be obtained through inverse modeling, hence without a-priori thermal history, while forward modeling approach is necessary when reheating is imposed in the input thermal history. (981M hornblende), when considering only the three well-defined plateaus including more than 90% of total 39 Ar (Fig. 2). Note that the other 3 plateaus, which include only between 50 and 70% or 39 Ar, hence satisfying plateau criteria [START_REF] Fleck | Interpretation of discordant 40 Ar/ 39 Ar agespectra of Mesozoic tholeiites from Antarctica[END_REF], also fall within clusters.

Results

4

Because argon is released in a short temperature interval as dehydration and decomposition of hornblende occur [START_REF] Wartho | Mechanisms of Ar release from Himalayan metamorphic hornblende[END_REF], plateau ages are defined here only by few steps.

However, they cover a large proportion of the total 39 Ar released of 93 and 98%, for 981M and 983M, respectively. Age spectrum for 984M hornblende shows distinct behavior, with increasing ages from about 1800 Ma before reaching a rather poorly defined plateau (58% of total 39 Ar) at 2055 ± 26 Ma (Figure 2a). Similarly, 984M and 9813M biotite spectra show slightly disturbed patterns, but yield plateau ages of 2053 ± 26 (66% of total 39 Ar) and 2028 ± 28 Ma (52% of total 39 Ar), respectively. Besides younger ages obtained for low temperature steps, 9814M biotite displays a plateau over 93% of 39 Ar released with an age of 2034 ± 26 Ma (Fig. 2a).

K-feldspars age spectra display more disturbed patterns than biotite and amphibole samples (Fig. 2b). Only 981M spectrum can be used to calculate a plateau age of 1793 ± 24

Ma between 800 and 1025°C (63% of total 39 Ar), although, as it displays overall increasing ages, it should be regarded with caution. However, excluding the first step, apparent ages overall increase from about 1550 to 1900 Ma. The three other feldspar samples do not display any plateau age s.s. and have increasing ages from low to high temperature steps. Sample 983M K-feldspar display increasing ages up to 1778 Ma, with a very large step of 1740 Ma obtained between 1175 and 1295°C. Note that it is poorly defined due to the very large amount of gas released. Indeed, it has necessitated the use of the highest measurement range, which lacked a precise calibration. Apparent ages of 984M K-feldspar show a steep increase at low temperature up to about 1500 Ma, followed by a shallower increase of apparent ages up to about 1750 Ma at high temperature. Finally, Sample 9814M K-feldspar displays similar but more pronounced features than the latter, with two pseudo-plateaus at about 1530 Ma between 765 and 945°C, and one at about 1730 Ma for the high temperature steps.

40 Ar/ 39 Ar thermochronology

Thermal histories from K-feldspars were first retrieved under the assumption of monotonic cooling (Supp. Mat. 2). Note that modeling for 981M K-feldspar was not possible as the age spectrum was obtained from a too low number of temperature steps. For samples 984M and 9814M, MDD modeled and measured age spectra match, while for 9814M, they are noticeably different. The three thermal histories obtained here display clear differences, which seems incompatible with a common regional geodynamic history. Furthermore, they suggest cooling through the 300°C isotherm (i.e., the closure temperature of biotite; [START_REF] Hodges | Pressure-temperature-time paths[END_REF] at 1700 Ma for 983M, and at 1400 Ma for 984M and 9814M. This is in strong disagreement with the well-constrained biotite 40 Ar/ 39 Ar ages, ranging from 2028 ± 28 to 2053 ± 26 Ma (Fig. 2a and Table 1). Hence, monotonic cooling below 650°C, the closure temperature of hornblende [START_REF] Harrison | Investigations of an intrusive contact, northwest Nelson, New Zealand-II. Diffusion of radiogenic and excess 40 Ar in hornblende revealed by 40 Ar/ 39 Ar age spectrum analysis[END_REF], since 2050 Ma cannot account for disturbances observed in K-feldspar 40 Ar/ 39 Ar spectra shown in Fig. 2b. Consequently, it is necessary to invoke one or several re-heating events having affected the study area in order to explain the re-opening of the Ar system recorded in K-feldspars.

Discussion

Cooling temperatures of the granitoids

New 40 Ar/ 39 Ar ages obtained here on hornblendes and biotites range within 2028 ± 14 and 2055 ± 10 Ma (Table 1), with a mean age of about 2040 Ma. Comparison with previous K-Ar ages realized on the same samples [START_REF] Sagna | K-Ar mineral ages and thermal history of magmatic and metamorphic Palaeoproterozoic units from the northern part of Kedougou Kenieba Inlier, West African Craton (Eastern Senegal)[END_REF] shows that the 40 Ar/ 39 Ar ages obtained here are more clustered and some are significantly older. This could be explained by the fact that the K-Ar technique is unable to detect loss of radiogenic argon from the less retentive sites during thermal events (e.g., [START_REF] Mcdougall | Geochronology and thermochronology by the 40 Ar/ 39 Ar method[END_REF]. On the other hand, such loss seems to have been recorded by the 40 Ar/ 39 Ar technique, as shown by the younger ages obtained here from low temperature steps of biotite and hornblende samples (Fig. 2a). Previously published 40 Ar/ 39 Ar hornblende, biotite and muscovite ages from the whole KKI range between 2212 and 2020 Ma [START_REF] Gueye | New evidences for an early Birimian evolution in the West African Craton: An example from the Kedougou-Kenieba inlier, southeast Senegal[END_REF], in agreement with the present study. However, as samples come from different units than those investigated here, no direct comparison can be performed.

We have calculated cooling rates using a closure temperature of 900°C for zircon U-Pb ages [START_REF] Lee | Pb, U and Th diffusion in natural zircon[END_REF], 550°C for hornblende 40 Ar/ 39 Ar ages [START_REF] Mcdougall | Geochronology and thermochronology by the 40 Ar/ 39 Ar method[END_REF], and 300°C for biotite 40 Ar/ 39 Ar ages [START_REF] Hodges | Pressure-temperature-time paths[END_REF]. For the Badon batolith, based on the U-Pb zircon age of 2102 ± 8 Ma (Théveniaut et al., 2010), and the 40 Ar/ 39 Ar hornblende ages of 2053 ± 12 and 2055 ± 10 Ma for 981M and 984M, respectively (Table 1), we have calculated cooling rates of 7°C.Myr -1 . An even lower rate of 5°C.Myr -1 is obtained for sample 983M, which displays and 40 Ar/ 39 Ar hornblende ages of 2032 ± 12 Ma (Table 1). Within the range 900°C-300°C, cooling rates of about 10°C.Myr -1 are calculated when considering biotite ages of 2053 ± 10 and 2034 ± 8 Ma, for 984M and 9814M samples, respectively (Table 1). A significantly higher rate of 20°C.Myr -1 is obtained when considering only the Sansankhoto pluton dated at 2058 ± 8 Ma with U-Pb on zircon [START_REF] Goujou | Notice explicative Carte géologique à 1/200 000 du Sénégal, feuille Kossanto-Dalafi Est[END_REF], and at 2028 ± 28 Ma using 40 Ar/ 39 Ar on biotite (Table 1).

Cooling rates of batholiths emplaced within other more recent convergent geodynamic settings show values on the order of those reported above for the KKI batholith from about 900 to 300°C (e.g., [START_REF] Maurel | U/ Pb emplacement and 40 Ar/ 39 Ar cooling ages of the eastern Mont-Louis granite massif (Eastern Pyrenees, France)[END_REF][START_REF] Quidelleur | Thermal evolution and slip history of the Renbu Zedong Thrust, southeastern Tibet[END_REF].

Re-opening of the 40 Ar/ 39 Ar system during younger thermal events

The flat shape of the biotite age patterns (Fig. 2a) suggest that there was no further thermal event reaching a temperature higher than 300°C after emplacement of the batholith.

Regarding K-feldspars, Figure 2b shows that the apparent age spectra of all three samples display disturbed patterns suggesting that they experienced one or several moderate thermal perturbations younger than 1800 Ma. As continuous cooling cannot account for 40 Ar/ 39 Ar age spectra from K-feldspar, we have attempted a modeling of the thermal history including re-heating events. The thermal history used to compute the K-feldspar age spectra using forward MDD modelling [START_REF] Lovera | Computer programs to model 40 Ar/ 39 Ar diffusion data from multi-domain samples[END_REF]) is shown in Figure 3. This thermal history has been constructed using previous zircon U-Pb ages [START_REF] Goujou | Notice explicative Carte géologique à 1/200 000 du Sénégal, feuille Kossanto-Dalafi Est[END_REF]Théveniaut et al., 2010), hornblende and biotite ages from this study (Table 1), and their associated closure temperature (see setion 5.1). In addition, we have included two 10 Ma long re-heating events at 1750 and 1500 Ma, of 275 and 225°C, respectively, suggested by the K-feldspar 40 Ar/ 39 Ar spectra described in the results section. Using this input thermal history, all modelled age spectra display 40 Ar/ 39 Ar age spectra that are rather similar to measured spectra (Fig. 3 inserts).

In the KKI, two dyke swarms were dated using TIMS U-Pb on baddeleyites [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF]: a NE oriented Kedougou dyke swarm at 1764 ± 4 Ma, and an E-W oriented Sambarabougou swarm at 1521 ± 3 Ma [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF]. The two dated swarms occur in the study area, even though not in the immediate vicinity of the granitoids studied here. The closest NE oriented dyke is located ca 10 km far north from the granitoids while the E-W dykes are even farther, about 50 km to the north. There is also a possibility that the dykes are closer to the study area, but they are too small or too deep to be mapped by airborne magnetic data. In any case, the two ages at ca 1750 and 1500 Ma, inferred from K-felsdpars age spectra, are similar to the ages obtained by U-Pb method on the two dyke swarms [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF] and, therefore, we suggest that reopening of the 40 Ar/ 39 Ar system may be related to these two regional thermal events.

Chronology of the thermal events in the WAC

Regional dyke swarms are often considered as the arrival of a mantle plume at the base of the lithosphere (e.g., [START_REF] Park | A proposed giant radiating dyke swarm fragmented by the separation of Laurentia and Australia based on paleomagnetism of ca. 780 Ma mafic intrusions in western North America[END_REF][START_REF] Li | The breakup of Rodinia: did it start with a mantle plume beneath South China?[END_REF][START_REF] Ernst | Large Igneous Provinces[END_REF], or emplaced from a shallow mantle source related to lithospheric extension (e.g., [START_REF] Jourdan | Major and trace element and Sr, Nd, Hf, and Pb isotope compositions of the Karoo Large Igneous Province, Bostwana-Zimbabwe : Lithosphere vs Mantle plume Contribution[END_REF]Hastie etal., 2014;[START_REF] Merle | Sr, Nd, Pb and Os isotope systematics of CAMP tholeiites from Eastern North America (ENA): evidence of a subduction-enriched mantle source[END_REF]. In both cases, such intrusion of regional dyke swarms can induce large-scale thermal effects.

Within the WAC, the Libiri and Kedougou thermal events, dated at 1790 and 1750 Ma, respectively [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF], and the 1750 Ma Tagragra of Akka event [START_REF] Youbi | The 1750 Ma Magmatic Event of the West African Craton (Anti-Atlas, Morocco)[END_REF] have been identified based on the U-Pb ages of dyke swarms. As these dykes are converging towards the northeast of the WAC [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF], it can be assumed that this area was the locus of an intense magmatic activity which is manifested by several pulses around 1750 Ma. [START_REF] Youbi | The 1750 Ma Magmatic Event of the West African Craton (Anti-Atlas, Morocco)[END_REF] suggest that the mafic dykes and felsic dykes of four Anti-Atlas Inliers are derived from a 1.75 Ga bimodal felsic-mafic magmatism emplaced in a continental rifting setting. All together, these ages can be interpreted as the occurrence of two thermal events [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF]. The first one at 1790 Ma, from the west to the center of the Amazonia, Libiri in WAC, and some of the Sarmatia dykes in Baltica. The second one at 1765 Ma, from the youngest swarm of Sarmatia dykes, and within the WAC, the Kedougou dykes dated at 1764 ± 4 Ma [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF] and the dykes of the Anti-Atlas dated at 1750 Ma [START_REF] Youbi | The 1750 Ma Magmatic Event of the West African Craton (Anti-Atlas, Morocco)[END_REF].

At about 1520 Ma, another major thermal event having affected the WAC can be inferred from the similar ages around 1519-1528 Ma recorded for the dyke swarms of Essakane in the NE of Burkina Faso (1519 to 1525 Ma), of Sambarabougou in the KKI, Senegal and Mali (1521 Ma) and of Käyser, Suriname, [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF]. These authors suggested that the Essakane-Sambarabougou-Käyser events might be a huge swarm of dykes of ca. 1519-1528 Ma, associated with a LIP emplacement (Essakane LIP; [START_REF] Ernst | How Large Igneous Provinces affect global climate, sometimes cause mass extinctions, and represent natural markers in the geological record[END_REF] within the Leo Man Shield, to the SW of the WAC and formerly adjacent to the Amazonia Craton. Note that within the same crustal block, coeval LIP magmatism dated at ca 1500 Ma has been reported in São Francisco (South America) craton [START_REF] Silveira | First precise U-Pb baddeleyite ages of 1500 Ma mafic dykes from the São Francisco Craton, Brazil, and tectonic implications[END_REF].

Our K-feldspar 40 Ar/ 39 Ar results having recorded two thermal events, together with the U-Pb ages obtained for dykes in the same area, they can be tentatively interpreted as imprints of successive arrival of large magmatic systems in the KKI area. However, the WAC and adjacent cratons did not experience breakup at ~1750 nor ~1500 Ma. The NW of the Amazonia was linked to the SW of the Baltica from at least 1.8 Ga to at least 0.8 Ga, and the WAC was probably linked to the NE of the Amazonia and to the SE of the Baltica during the same period [START_REF] Johansson | Baltica, Amazonia and the SAMBA connection-1000 million years of neighbourhood during the Proterozoic?[END_REF]. The Amazonia and the WAC remained linked from about 2.0

Ga until Gondwana breakup in connection with the opening of the Southern Atlantic Ocean at about 120 Ma. As breakup did not occur within the WAC in the 1800-1500 Ma interval, it is tempting to propose that the two thermal events identified here in K-feldspar disturbed age spectra at ~1750 and ~1500 Ma, and coeval dyke swarm emplacements, might instead only correspond to several aborted attempts of continental breakup.

Conclusion

New 40 Ar/ 39 Ar ages obtained for hornblendes and biotites from the Kedougou Kenieba Inlier (eastern Senegal), the western-central outcropping part of the West African Craton, range between 2055 ± 26 and 2028 ± 28 Ma, in agreement with previous results from rocks asociated with the 2.2 -2.0 Ga Eburnean Orogeny [START_REF] Hirdes | U-Pb geochronology of paleoproterozoic rocks in the southern part of the Kedougou-Kenieba Inlier, Senegal, West Africa: evidence for diachronous accretionary development of the Eburnean province[END_REF].

Together with the published U-Pb ages, our new 40 Ar/ 39 Ar ages show that the Kedougou Kenieba Inlier was affected by two moderate thermal events reported at the scale of the WAC at ~1750 and ~1500 Ma. Our well-constrained 40 Ar/ 39 Ar biotite and hornblende ages argue for a relatively fast initial cooling between 5 and 20°C.Myr -1 and rule out the hypothesis of a significant re-heating event above 300°C after emplacement of the batholith.

On the other hand, disturbance of 40 Ar/ 39 Ar K-feldspars age spectra can be explained by invoking two re-heating events at ~1750 and ~1500 Ma, with temperatures of about 275 and 225°C, respectively.

Mafic dyke intrusions within Kedougou Kenieba Inlier, previously reported at 1764 ± 4 Ma and 1521 ± 3 Ma from U-Pb ages [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF], could be related to these reheating events. According to their structural orientation, the first thermal event (~1750 Ma) can be correlated with the dyke swarm of Tagragra of Akka in Morocco, and the second one (~1500 Ma) with the dyke swarm of Käyser in Suriname [START_REF] Baratoux | New U-Pb Baddeleyite Ages of Mafic Dyke Swarms of the West African and Amazonian Cratons: Implication for Their Configuration in Supercontinents Through Time[END_REF]. As the WAC remained attached to Baltica and Amazonia cratons during the 1.8-1.3 Ga interval [START_REF] Johansson | Baltica, Amazonia and the SAMBA connection-1000 million years of neighbourhood during the Proterozoic?[END_REF], only failed continental rifting events can be inferred from these thermal events recorded in 40 Ar/ 39 Ar K-feldspars age spectra at about 1750 and 1500 Ma. b) Same as a) for K-feldspars samples.

Figure 3

Thermal history of the studied area based on K-feldspar 40 Ar/ 39 Ar ages (shown as inserts), and hornblende and biotite 40 Ar/ 39 Ar ages (dashed square). The two thermal events inferred are shown as 1 and 2. The blue line in each insert shows the modelled age spectra based on this thermal history. The same activation energy value of 42 kcal.mol -1 , the lowermost value obtained from monotonic cooling modeling, was used for all K-feldspars. Note that it is close to the average value of 46 kcal.mol -1 derived from a large K-feldspars dataset [START_REF] Lovera | Systematic analysis of K feldspar 40 Ar/ 39 Ar step-heating experiments I: Significance of activation energy determinations[END_REF].

  Figure captions

Figure 2 a

 2 Figure 2a)40 Ar/ 39 Ar apparent age spectra determined from Ar isotope data for hornblende and biotite samples. The plateau age is calculated for steps shown by the arrow. Analytical uncertainty, and total (i.e. including J-factor error) uncertainty (in parentheses) are given at the two-sigma level.

  .1.40 Ar/ 39 Ar ages Plateau and integrated 40 Ar/ 39 Ar ages obtained here from the Kedougou Kenieba Inlier are given in Table 1, and full dataset can be found in Supp. Mat. 1. Apparent 40 Ar/ 39 Ar age spectra for hornblendes and biotites yield ages ranging between 2055 ± 26 and 2028 ± 28 Ma, with two clusters at 2033 ± 10 Ma (983M hornblende and 9814M biotite) and 2053 ± 12 Ma

Acknowledgements

We are grateful to IFAN Cheikh Anta Diop from University Cheikh Anta Diop (Dakar, Se ńe ǵal) for I. Sagna's visiting grants at University Paris-Sud. French Centre National de la Recherche Scientifique (CNRS) and University Paris Sud supported this work. We thank Renaud Merle and Jean-Paul Liégeois for their detailed reviews, which contributed to significantly improve this manuscript. This is LGMT contribution number 163.