Ultrahigh-temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long-lived high-T orogen

We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO_2 relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (<29 ± 8 Ma) suggests that a thin mantle lithosphere (<80 km) or advective heating may have also been important in the formation of this high‐T, low‐P terrane

Ultrahigh-temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long-lived high-T orogen

We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO_2 relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (<29 ± 8 Ma) suggests that a thin mantle lithosphere (<80 km) or advective heating may have also been important in the formation of this high‐T, low‐P terrane

Carbone des sols en Afrique

Les sols sont une ressource essentielle à préserver pour la production d’aliments, de fibres, de biomasse, pour la filtration de l’eau, la préservation de la biodiversité et le stockage du carbone. En tant que réservoirs de carbone, les sols sont par ailleurs appelés à jouer un rôle primordial dans la lutte contre l’augmentation de la concentration de gaz à effet de serre. Ils sont ainsi au centre des objectifs de développement durable (ODD) des Nations unies, notamment les ODD 2 « Faim zéro », 13 « Lutte contre le changement climatique », 15 « Vie terrestre », 12 « Consommation et production responsables » ou encore 1 « Pas de pauvreté ». Cet ouvrage présente un état des lieux des sols africains dans toute leur diversité, mais au-delà, il documente les capacités de stockage de carbone selon les types de sols et leurs usages en Afrique. Il propose également des recommandations autour de l’acquisition et de l’interprétation des données, ainsi que des options pour préserver, voire augmenter les stocks de carbone dans les sols. Tous les chercheurs et acteurs du développement impliqués dans les recherches sur le rôle du carbone des sols sont concernés par cette synthèse collective. Fruit d’une collaboration entre chercheurs africains et européens, ce livre insiste sur la nécessité de prendre en compte la grande variété des contextes agricoles et forestiers africains pour améliorer nos connaissances sur les capacités de stockage de carbone des sols et lutter contre le changement climatique

Les corindons gemmes dans les basaltes alcalins et à leurs enclaves à Madagascar (Signification pétrologique et métallogénique)

Madagascar est l'un des plus grands pays producteurs de corindons gemmes provenant des environnements de basaltes alcalins. Les gisements, les plus connus, sont les gisements secondaires situés à Ambondromifehy au Nord du pays (paléoplacer et placer à saphir), à Soamiakatra - Mandrosohasina au Centre (gisement de rubis primaire, placer et paléoplacer à rubis et saphir), et à Vatomandry dans partie orientale de l'île (paléoplacer et placer à rubis et à saphir). Soamiakatra est le seul gisement primaire où les rubis se rencontrent dans des xénolithes (métagabbros et pyroxénites) qui ont été remontés jusqu'à la surface par le volcanisme basaltique de l'Ankaratra. La pétrographie a permis de mettre en évidence deux conditions de formation du rubis à la limite du domaine de l'éclogite (HT ~ 1100C, HP ~ 20 Kb) et du faciès granulitique (HT ~ 1100C, BP < 15Kb). Par contre, les saphirs proviennent d'une chambre magmatique mafique alcaline à la limite croûte continentale inférieure-manteau supérieur infra-continental. Ces saphirs se rencontrent dans et/ou associés à des xénolithes de syénite et d'anorthoclasite enclavés dans les basaltes. Les basaltes alcalins transportent ces corindons vers la surface, et ils sont liés à la remontée asthénosphérique et à l'amincissement lithosphérique Oligocène-Quaternaire qui se situe sous le plateau de l'Ankaratra, et marqués par des extensions E-W et N-S. Les valeurs isotopiques de l'oxygène des rubis issus des placers (?18O moyen = 3,1 +- 1,1 ) s'intègrent dans l'intervalle des valeurs isotopiques des rubis associés aux roches mafiques et ultramafiques (1,25 < ?18O < 6,8 ), alors que celles des saphirs issus des paléoplacers et placers ont une valeur moyenne de ?18O de 4,5 +- 0,5 qui se superpose à l'intervalle isotopique des roches syénitiques à saphir (4,4 < ?18O < 8,3 ). Les inclusions solides identifiées tels que pyrochlore, samarskite, uraninite et anorthoclasite dans les saphirs confirment une source syénitique. Les zircons qui sont associés aux saphirs donnent des âges U-Pb inférieurs à 35 Ma, contemporains du volcanisme, alors que les zircons inclus dans les rubis d'origine métamorphique et associés aux xénolithes mafiques et ultramafiques, donnent un âge U-Pb Pan-Africain à environ 750 Ma. On peut toutefois noter que les zircons datés in-situ des xénolithes d'anorthoclasite montrent deux âges de cristallisation, 35 et 17 Ma, légèrement antérieurs à contemporains aux premières manifestations volcaniques de l'Ankaratra. La typologie des corindons, leur analyse isotopique et les datations obtenues mettent en évidence au moins deux sources majeures pour les corindons associés aux champs basaltiques malgaches : les rubis associés aux métagabbros et pyroxénites à grenat seraient liés à des complexes mafiques-ultramafiques de base de croûte métamorphisés dans le faciès éclogitique, et rétromorphosés dans le faciès granulitique au Pan-Africain ; alors que les saphirs remontés durant l'Oligo-Plio-Quaternaire seraient contemporains au volcanisme et générés par des liquides différenciés d'un magmatisme alcalin comme le suggère leur composition isotopique en oxygène à signature syénitique.Madagascar is one of the principal producers of gem corundums recovered from continental basaltic fields. The main deposits are the secondary deposits of Ambondromifehy in the northern part of the country (sapphire-bearing palaeoplacer and placer), Soamiakatra - Mandrosohasina in the central part (primary ruby deposit, and ruby and sapphire-bearing placer and palaeoplacer) and Vatomandry in the eastern part of the island (ruby and sapphire-bearing paleoplacer). Soamiakatra is the only known primary deposit where ruby is found in metagabbro and pyroxenite xenoliths, which were entrained and brought up to the upper crust by the Ankaratra volcanic event. Petrographic studies have demonstrated the existence of two different conditions of ruby formation at the boundary of the eclogite domain (HT ~ 1100C, HP ~ 20Kb) and granulite facies (HT ~ 1100C, BP < 15Kb). In contrast, the sapphires originated from alkaline mafic magmatic chambers at the lower continental crust-mantle boundary. They occur within or associated with syenite and anorthoclasite xenoliths in the basalts. These alkali basalts transported corundum to the surface ; they are linked with asthenosphere upwelling and thinning of the lithosphere underneath the Ankaratra Plateau during the Oligocene-Quaternary, and to some E-W and N-S tectonic extension structures. Oxygen isotopic compositions of the rubies from placer deposits with ?18O = 3.1 +- 1.1 are typical of ruby in mafic and ultramafic rocks (1.25 < ?18O < 6.8 ). The sapphires from palcer and palaeoplacer deposits have ?18O values of 4.5 +- 0.5 , which coincides with the range in sapphire-bearing syenitic rocks (4.4 < ?18O < 8.3 ). Solid inclusions such as pyrochlore, samarksite, uraninite and anorthoclasite in the sapphires confirm their syenitic origin. Zircons associated with the sapphires gave U-Pb ages younger than 35 Ma, coeval with the volcanic event. The zircon inclusions in rubies of metamorphic origin associated with the mafic and ultramafic xenoliths rocks gave a Pan-African U-Pb age around 750 Ma. Nevertheless, dating of zircon crystals in situ within the anorthoclasite xenoliths has revealed two different ages, 35 and 17 Ma, which are slightly older than or contemporaneous with the onset of the volcanic events. The characteristics of the corundum, their isotopic compositions as well as their ages demonstrate the existence of two distinct sources of corundum associated with basalts in Madagascar. On one hand, the rubies associated with metagabbros and garnet-bearing pyroxenites are linked to mafic and ultramafic complexes of eclogite facies in the lower crust, retrograded to granulite facies during the Pan African event. On the other hand, the sapphires brought up to the upper crust during the Oligo-Plio-Quaternary are interpreted to be coeval with a volcanic event involving differentiated alkaline magma as revealed by their oxygen isotopic composition with syenitic signatures.NANCY1-Bib. numérique (543959902) / SudocSudocFranceF

Fluorine-controlled composition of biotite in granulites of Madagascar: the effect of fluorine on thermometry of biotite-garnet gneisses

International audienceBecause of the strong preference of fluorine (F) for "hydrous" magnesian silicate minerals, the temperature T(0F) given by the reaction (Bi-Gt) of Fe-Mg exchange between biotite and garnet [phlogopite (Phl) + almandine (Alm) = annite (Ann) + pyrope (Py)] can be dramatically underestimated when the reciprocal effects between octahedral and hydroxyl site occupancies in biotite are ignored. In the granulites of southeastern Madagascar, widespread Mg metasomatism was associated with F-rich fluids. It is mainly manifested by the regional occurrence of diopside skarns hosting F-phlogopite and thorianite mineralisation. At first, biotite- and garnet-bearing granites and leucogneisses appear unaffected by this event. However, their biotites are rich in F [up to 6 wt%; X F = X F/(F + OH) = 0.69) and in Mg ( X Mg = Mg/(Mg + Fe) up to 0.8], which shows the tendency of equilibrium with the regional fluids. This equilibration leads to low values of Fe/Mg, which in turn cause strong underestimation of the temperature: T(0F) as low as 500 °C, whereas the temperature T(Ti) given by the TiO2 content of biotite is approximately 750 °C. The slightly modified thermodynamic model of Zhu and Sverjensky (Geochim Cosmochim Acta 56:3435-3467, 1992) for reciprocal solid solution is applied to a set of 19 samples to correct this underestimation and to quantify the effect of F on thermometry. The reciprocal activity coefficients for phlogopite ( λ (Phl)) and annite ( λ (Ann)) calculated by the model are added to the winTWQ software to obtain corrected values T(F) of the temperature. Similar values of Δ T = T(F) - T(0F) are obtained by (1) the linear fit of the whole set of the samples and (2) RTln K λ = RTln λ (Ann) - RTln λ (Phl) = 33.417 X F - 0.1092 ( r 2 = 0.9994) equation applied to a median sample: Δ T = 30-40, 60-70, 95-100, 160, and 200 °C for X F = 0.1, 0.2, 0.3, 0.5, and 0.7, respectively. The same calculations based on the Thermocalc thermodynamic data set lead to values of Δ T 1.5 times higher than those given by winTWQ. Thus, the results of these calculations confirm (for the first time from a set of natural samples) the validity of the thermodynamic model of reciprocal solid solution and the necessity of accounting for F in thermometry based on Fe-Mg exchanges involving biotite

Age constraints on the tectonic evolution of the Itremo region in Central Madagascar

The Itremo region in Central Madagascar comprises a deformed metasedimentary sequence (Itremo Group) that has undergone greenschist to lower amphibolite facies metamorphism. During a first phase of deformation (D1) Itremo Group sediments were deformed into a fold-and-thrust belt and transported toward the E to NE on top of migmatitic gneisses rocks of Anatananarivo block. A second phase of deformation (D2) affected both the fold-and-thrust belt and structurally underlying units, and formed large-scale N-S trending folds with steeply dipping axial planes. A Late Neoproterozoic Th–U–Pb XRF monazite age (565±17 Ma) dates the emplacement of a granite that truncates first-phase structures in the Itremo Group, and indicates that the fold-and-thrust belt formed prior to ≈565 Ma. Th–U–Pb electron microprobe dating was applied to elongated monazites that lie within the first-phase foliation of Itremo Group metapelites. The detrital cores of zoned monazites reveal two distinct age populations at ∼2000 and 1700 Ma, the latter age giving a maximum depositional age for the Itremo Group. Statistical analysis of ages determined from the rims of zoned monazites and from unzoned monazites indicates three Late Proterozoic–Early Paleozoic monazite growth events at about 565–540, 500 and 430 Ma. The oldest age population is contemporaneous within error, with the intrusion of the dated granite. The two younger age populations are found both in the Th–U–Pb and Ar–Ar data; together with the perturbation of the Rb–Sr system we interpret both ages as due to alteration related to fluid circulation events, possibly connected to the emplacement of pegmatite fields in Central Madagascar. Syn-D1 tectonic growth of contact metamorphism minerals such as andalusite has been observed locally in metapelites along the margin of Middle Neoproterozoic (≈800 Ma) granites, suggesting that D1 in the Itremo Group is contemporaneous with the intrusion of granites at ≈800 Ma. The N-S trending D2 folds are associated with ≈E-W shortening during the final assembly of Gondwana in Late Neoproterozoic–Early Cambrian times

Paleomagnetism, geochronology and tectonic implications of the Cambrian-age Carion granite, Central Madagascar

International audienceThe Carion granitic pluton in central Madagascar was intruded into warm continental crust following orogenic events related to the final amalgamation of Gondwana. U-Pb SHRIMP dating of the pluton yields an emplacement age of 532.1±5.2 Ma followed by relatively slow cooling as constrained by 40Ar/ 39Ar ages on hornblende and biotite. Four hornblende samples yielded a mean 40Ar/ 39Ar age of 512.7±1.3 Ma. A biotite sample yielded an age of 478.9±1.0 Ma. Paleomagnetic samples from the pluton and surrounding country rocks exhibit either SE-upwardly directed magnetizations (mean Dec=113°, Inc=-56°, k=106, α95=12°) or NW-downwardly directed magnetizations (mean Dec=270°, Inc=+64°, k=30, α95=11°) that pass a reversal test with a classification of 'C' and an angular difference of 14.4°. The 'normal' (negative inclinations) and 'reverse' (positive inclinations) directions also show a spatial bias within the pluton, suggesting a field transition from reverse to normal during cooling. The paleomagnetic pole calculated from the mean direction falls at 6.8°S, 001°E (dp=13°, dm=17°). Estimates of the blocking temperature for the magnetization are compared to the cooling history of the pluton and an age of 508.5±11.5 Ma is assigned to the pole. The Carion pole falls near similar-age poles from elsewhere in Gondwana, supporting the idea that the major orogenic events during Gondwana assembly were complete. A slight revision of the Gondwana apparent polar wander path (APWP) is proposed with rapid APW from 540 to 520 Ma; however, the proposed mechanisms to explain this rapid APW (including intertial-interchange true polar wander (TPW) or enhanced mantle driving forces) cannot fully explain all the data