The Murchison Greenstone Belt, South Africa: Accreted slivers with contrasting metamorphic conditions

International audienceThis paper presents new petrological and geochronological data for the ∼3.09-2.92 Ga Murchison Greenstone Belt (MGB), located in South Africa's Kaapvaal Craton, and discusses their geotectonic implications. The MGB is made of three tectono-metamorphic units: the Silwana Amphibolites, the Murchison Unit and the La France Formation. They underwent contrasting clockwise pressure-temperature-deformation (P-T-D) histories, and are separated from each other by relatively narrow, high-strain shear zones, with a sinistral, transpressive top-to-the-south movement, consistent with the deformation patterns observed throughout the belt. These patterns are explained by a N-S compressional stress field, affecting the Murchison Belt between 2.97 and 2.92 Ga. Results of new petrological investigations indicate that ultramafic to felsic volcano-sedimentary rocks of the Murchison Unit underwent a greenschist- to lower-amphibolite-facies metamorphism at maximum P-T conditions of 5.6 ± 0.6 kbar at 570 °C, along a relatively hot, minimum apparent geotherm of ∼30 °C/km. In contrast, the Silwana Amphibolites and the La France Formation were metamorphosed at much higher peak metamorphic conditions of 8.7-10 kbar, 630-670 °C, and 8-9 kbar, 600-650 °C, respectively, and require a colder apparent geotherm of ∼20 °C/km. A retrograde, nearly isothermal-decompression P-T path followed by isobaric cooling is also inferred for the La France Formation. The timing of the structural-metamorphic overprint is bracketed between 2.97 and 2.90 Ga, which is constrained by U-Pb zircon ages of a syn-deformation granite within the Murchison Unit and the post-deformation Maranda granite, respectively. Monazite and xenotime from La France metapelites yield much younger ages of ca. 2.75 Ga, with few inherited components at 2.92 Ga. They point to a later activation of the MGB, perhaps related with tectono-thermal events in the Rooiwater Complex and the Pietersburg Greenstone Belt. The relatively cold apparent geotherms recorded in the Silwana and La France rocks, the contrasted peak P-T conditions between the different units, and the near isothermal decompression of the La France Formation indicate that the Kaapvaal craton crust must have been cold enough to enable significant crustal thickening and strain localisation along narrow shear zones and, as a consequence, fast tectonic juxtaposition of rocks metamorphosed at different crustal depths. These features are similar to those observed along Palaeozoic or modern day, oblique subduction-collision zones, but different to those of hot Archaean provinces. We therefore interpret the MGB as representing part of an oblique collision-zone between two terrains of the Kaapvaal craton: the Witwatersrand and Pietersburg terrains

Temporal relationships between Mg-K mafic magmatism and catastrophic melting of the Variscan crust in the southern part of Velay Complex (Massif Central, France)

Mg-K mafic intrusive rocks are commonly observed during the late stages of the evolution of orogenic belts. The Variscan French Massif Central has many outcrops of these rocks, locally called vaugnerites. Such magmas have a mantle-derived origin and therefore allow discussion of the role of mantle melting and crust-mantle interactions during late-orogenic processes. In the Southern Velay area of the French Massif Central, LA-ICPMS U-Pb dating on zircons and monazites from three vaugnerites and four coeval granites reveals that the two igneous suites formed simultaneously, at c. 305 Ma. This major igneous event followed after an early, protracted melting stage that lasted for 20-30 My and generated migmatites, but the melt was not extracted efficiently and therefore no granite plutons were formed. This demonstrates that widespread crustal anatexis, melt extraction and granite production were synchronous with the intrusion of vaugneritic mantle-derived melts in the crust. The rapid heating and subsequent melting of the crust led to upward flow of partially molten rocks, doming and collapse of the belt.JHS was financially supported by the Spanish grant CGL2008–02864 and the Andalusian grant RNM1595

Multilaboratory Evaluation of a Novel Lateral Flow Immunochromatographic Assay for Confirming Isolation of Mycobacterium bovis from Veterinary Diagnostic Specimens

ABSTRACT A novel lateral flow immunochromatographic device (LFD) was evaluated in several veterinary diagnostic laboratories. It was confirmed to be specific for Mycobacterium bovis and M. caprae cells. The performance of the novel LFD was assessed relative to the confirmatory tests routinely applied after culture (spoligotyping or quantitative PCR [qPCR]) in each laboratory; liquid (MGIT or BacT/Alert) and/or solid (Stonebrink, Coletsos, or Lowenstein-Jensen) cultures were tested. In comparison to spoligotyping of acid-fast-positive MGIT cultures, percent agreement between positive LFD and spoligotyping results was excellent in two United Kingdom laboratories (97.7 to 100%) but lower in the Spanish context (76%), where spoligotyping was applied to MGIT cultures previously confirmed to be positive for M. tuberculosis complex (MTBC) by qPCR. Certain spoligotypes of M. bovis and M. caprae were not detected by the LFD in Spanish MGIT cultures. Compared to qPCR confirmation, the agreement between positive LFD and qPCR results was 42.3% and 50% for BacT/Alert and MGIT liquid cultures, respectively, and for solid cultures, it ranged from 11.1 to 89.2%, depending on the solid medium employed (Coletsos, 11.1%; Lowenstein-Jensen, 55.6%; Stonebrinks, 89.2%). Correlation between the novel LFD and BD MGIT TBc Identification test results was excellent when 190 MGIT cultures were tested ( r = 0.9791; P &lt; 0.0001), with the added benefit that M. bovis was differentiated from another MTBC species in one MGIT culture by the novel LFD. This multilaboratory evaluation demonstrated the novel LFD's potential utility as a rapid test to confirm isolation of M. bovis and M. caprae from veterinary specimens following culture. </jats:p

LA-ICP-MS dating of zircons from Meso- and Neoarchean granitoids of the Pietersburg block (South Africa): Crustal evolution at the northern margin of the Kaapvaal craton

International audienceIn order to unravel the late-Archean magmatic evolution of the northern Kaapvaal craton, we performed LA-ICP-MS U-Pb isotopic analyses on zircon grains from 15 granitoid samples of the Pietersburg block, including tonalitic and granodioritic gneisses (TTG) as well as weakly to not deformed high-K monzogranites, granodiorites and diorites. Our new age data, coupled to already published results, show that the Pietersburg block is schematically made up of two lithologically and chronologically distinct domains, namely (1) gneiss units that cover most of the surface of this terrane and comprising the formerly defined Goudplaats, Hout River, Groot Letaba gneisses as well as the so-called "Duiwelskloof batholith", where crust formation and reworking took place between 3350 and 2840 Ma; and (2) late-stage high-K plutons and batholiths that emplaced at ~2780 Ma and ~2690 Ma. In details, the tectono-magmatic evolution of the Pietersburg block can be divided into five successive episodes: (1) The first crust-forming event is related to the emplacement of juvenile TTG magmas in the range 3150-3350 Ma. In our dataset, it is only represented by inherited zircon cores from younger granitoids; (2) Two TTG samples from the Goudplaats-Hout River gneiss unit and the Duiwelskloof area yielded emplacement ages of ~2950 Ma. This magmatic event is very widespread in the whole Pietersburg block, and we propose that it represents an important period of crustal growth linked with the accretion of the Pietersburg block to the northern edge of the older nucleus (ca. 3600-3100 Ma) of the Kaapvaal craton; (3) We obtained an intrusion age of ~2840 Ma for two samples of high-K biotite-bearing granites that are intimately associated with, and probably formed by melting of, the ~2950 Ma-old TTGs. Thus, this episode represents an important event of crust reworking that is likely associated with a long-lasting deformation event; (4) Further intracrustal melting led to the development of large batholiths of high-K biotite granites (e.g. Turfloop batholith) and their satellite intrusions, the latter being represented by one of our samples dated at ~2780 Ma; (5) The intrusion of high-K calc-alkaline granitoids of possible mixed crust-mantle origin (Mashashane, Matlala, Moletsi and Matok plutons) took place at ~2690 Ma. They are likely associated with collision between the Pietersburg block and the Central Zone of the Limpopo Belt, together with localized strain along shear zones (e.g. Hout River shear zone) and granulite-facies metamorphism that both occurred in the same time

Archean S-Type granites: petrology, geochemistry and geochronology of the Lekkersmaak and Willie plutons, Kaapvaal Craton, South Africa

International audienceS-type granites correspond to reworking of pre-existing continental material and form by partial melting of (meta)sediments. Early-to-mid Archean S-type granites are rare and are more frequently found, usually as relatively small intrusions, during the Neoarchean and the Archean-Proterozoic transition. In the context of Archean geology, their paucity is therefore significant, in that it matches the uncommon nature of processes during this period. In this study, we focus on the Late Mesoarchean Willie pluton and Neoarchean Lekkersmaak pluton, which crop out to the south of the Murchison Greenstone Belt in the Northern Kaapvaal Craton, South Africa. These intrusions represent the oldest S-type plutons known so far in the region. We constrain their petrogenesis by documenting their petrology, mineralogy (mineral composition), geochemistry (major and trace elements, Sr, Nd and O isotopes) and zircon U-Pb geochronology. The large Lekkersmaak and smaller Willie plutons represent two granites sharing many geochemical characteristics; they are both peraluminous, Ms-bearing S-type granites although they are more sodic and less potassic than typical S-type granites. The Willie granite was emplaced 2816 ± 9 Ma ago while the Lekkersmaak yielded an emplacement age of 2771 ± 2 Ma. Therefore, S-Type magmatism in the area occurred twice at 2.82 Ga and then 40 Myr later at 2.77 Ga, involving comparable immature metasedimentary sources and conditions of partial melting. Finally, a sample from the Lekkersmaak pluton devoided of S-type features as it is muscovite-free and aluminum poor was emplaced 2734 ± 11 Ma ago. This confirms that the magmatic activity south of the Murchison Greenstone Belt was episodic as stated in previous studies

Geochemistry and petrogenesis of high-K "sanukitoids" from the Bulai pluton, Central Limpopo Belt, South Africa: Implications for geodynamic changes at the Archaean-Proterozoic boundary

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The history of a successful diagnostic tool for bovine tuberculosis detection and control in France: the ifn-γ test

International audienceIntroduction: Since 2004, just three years after having obtained the disease-free status, the number of bovine tuberculosis (bTB) breakdowns disclosed at the slaughterhouse started to increase in extensive cattle rearing regions in France, indicating that the current skin testing-cull strategy was no longer a useful means no only to eradicate residual infection but even to control the disease.Methods: The use of the interferon gamma release assay (IGRA) was introduced at first in an experimental manner in the most affected regions –Camargue, Dordogne and Côte d’Or- allowing us to adapt the test as best as possible to their inherent variable epidemiological situation for result interpretation but also introducing technical changes to render it as robust and specific as possible. Among these adaptations was the introduction of an interpretation formula including the positive control value in a ratio, which allows a decreasing variability of result and inter-comparability, the systematic use of a mitogen for controlling false negative animals/samples and also the use of Mycobacterium tuberculosis complex specific antigens for increasing specificity, the use of adapted cut-off values taking into consideration the degree of animal stress and a result-interpretation chart to adapt the test to the epidemiological context of the herd.Results: Once the test was settled, a network of authorised laboratories, trained and evaluated by the national reference laboratory was set up at a national level to make its use wider and to reinforce surveillance. Nowadays, the test is used in parallel with or in serial after the skin test to increase detection of positive predictive value depending on the regional epidemiological situation.After almost 20 years, the test proved it usefulness as the disease was almost completely eliminated from Camargue, showed a spectacular prevalence decrease in Côte d’Or. However, besides helping the authorities to realise the real difficult epidemiological situation on south west France, the reinforced control programme which includes the use of this test does not seem to improve the difficult context in this region, explained by several present risk factors, including human borne- noncompliance with regulatory recommendations regarding the skin test or sanitary management-, that can explain its failure.Conclusions: As any current bTB ante-mortem test, this test is not perfect, but from our point of view it is the closet to provide results as reliable as possible both at the individual and at the herd level. Improvements can still be introduced to improve technical specificity. Thus, we consider that IGRA is a valuable tool for a bTB eradication programme provided that a good communication between committed stakeholders exists and that the employed screening tests are adapted to the local epidemiological context

Long term melt-generation in the Archaean crust: a case study from the Northern Kaapvaal craton, South Africa

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