Metamorphism in the West African Craton: geodynamic implications

National audienceIn the Paleoproterozoic domain of the West African Craton (WAC), most volcanic and sedimentary rocks were metamorphosed during the Eburnean orogeny between c. 2.13 and 1.9 Ga. Amphibolite facies conditions have been considered by some authors to be the result of contact metamorphism; however, a growing number of metamorphic studies provide evidence for the regional character of amphibolite-to-granulite facies metamorphism in Ghana, in Côte d'Ivoire, in Burkina Faso and in the Kédougou-Kéniéba Inlier (KKI) in Senegal. The P-T trajectories of the high-grade rocks indicate a clockwise character, starting with apparent geothermal gradients as cold as 10-20°C/km at early metamorphic stages reaching greenschist, blueschist, amphibolite and granulite facies. The peak-stage thermal gradients vary between 20 and 30°C/km and reach greenschist, amphibolite and granulite facies conditions. Subsequent metamorphic stages are characterized by decompression, sometimes accompanied by heating, leading in places to partial melting and the abundant presence of migmatites. The final stages are generally marked by further decompression and cooling under greenschist facies conditions. The oldest metamorphic U-Pb ages of monazite dating the peak of metamorphic conditions in northwestern Ghana yield 2137 ± 8 Ma for the early metamorphic stages and 2127 ± 7 Ma for the decompression/heating phase. A younger period of regional metamorphism has been documented in the KKI at c. 2095-2085 Ma for peak P stages and c. 2060-2050 Ma for decompression/heating stages (U-Pb on monazite, Sm-Nd on garnet). Similar metamorphic ages have been found in Ghana, ranging from 2095 ± 34 Ma to 2063 ± 9 Ma. The youngest U-Pb monazite metamorphic ages have been reported in western and southwestern Côte d'Ivoire, where they show several age peaks at 2050-2030 Ma, 2000 Ma and 1900-2000 Ma, corresponding to later prograde and retrograde metamorphic events. Metamorphic units from different locations in the craton show similar clockwise P-T paths. However, the timing of metamorphism demonstrates a diachronic character with ages spreading between c. 2130 and 1900 Ma. Such type of metamorphism is consistent with tectonic burial and differential exhumation during collisional stages of the Eburnean orogeny, which occurred at different time periods across the craton

From "ancient" to "modern" tectonics : West African craton persmective

International audienc

Metamorphism in the West African Craton: geodynamic implications

National audienceIn the Paleoproterozoic domain of the West African Craton (WAC), most volcanic and sedimentary rocks were metamorphosed during the Eburnean orogeny between c. 2.13 and 1.9 Ga. Amphibolite facies conditions have been considered by some authors to be the result of contact metamorphism; however, a growing number of metamorphic studies provide evidence for the regional character of amphibolite-to-granulite facies metamorphism in Ghana, in Côte d'Ivoire, in Burkina Faso and in the Kédougou-Kéniéba Inlier (KKI) in Senegal. The P-T trajectories of the high-grade rocks indicate a clockwise character, starting with apparent geothermal gradients as cold as 10-20°C/km at early metamorphic stages reaching greenschist, blueschist, amphibolite and granulite facies. The peak-stage thermal gradients vary between 20 and 30°C/km and reach greenschist, amphibolite and granulite facies conditions. Subsequent metamorphic stages are characterized by decompression, sometimes accompanied by heating, leading in places to partial melting and the abundant presence of migmatites. The final stages are generally marked by further decompression and cooling under greenschist facies conditions. The oldest metamorphic U-Pb ages of monazite dating the peak of metamorphic conditions in northwestern Ghana yield 2137 ± 8 Ma for the early metamorphic stages and 2127 ± 7 Ma for the decompression/heating phase. A younger period of regional metamorphism has been documented in the KKI at c. 2095-2085 Ma for peak P stages and c. 2060-2050 Ma for decompression/heating stages (U-Pb on monazite, Sm-Nd on garnet). Similar metamorphic ages have been found in Ghana, ranging from 2095 ± 34 Ma to 2063 ± 9 Ma. The youngest U-Pb monazite metamorphic ages have been reported in western and southwestern Côte d'Ivoire, where they show several age peaks at 2050-2030 Ma, 2000 Ma and 1900-2000 Ma, corresponding to later prograde and retrograde metamorphic events. Metamorphic units from different locations in the craton show similar clockwise P-T paths. However, the timing of metamorphism demonstrates a diachronic character with ages spreading between c. 2130 and 1900 Ma. Such type of metamorphism is consistent with tectonic burial and differential exhumation during collisional stages of the Eburnean orogeny, which occurred at different time periods across the craton

Size and shape constraints of (486958) Arrokoth from stellar occultations

We present the results from four stellar occultations by (486958) Arrokoth, the flyby target of the New Horizons extended mission. Three of the four efforts led to positive detections of the body, and all constrained the presence of rings and other debris, finding none. Twenty-five mobile stations were deployed for 2017 June 3 and augmented by fixed telescopes. There were no positive detections from this effort. The event on 2017 July 10 was observed by the Stratospheric Observatory for Infrared Astronomy with one very short chord. Twenty-four deployed stations on 2017 July 17 resulted in five chords that clearly showed a complicated shape consistent with a contact binary with rough dimensions of 20 by 30 km for the overall outline. A visible albedo of 10% was derived from these data. Twenty-two systems were deployed for the fourth event on 2018 August 4 and resulted in two chords. The combination of the occultation data and the flyby results provides a significant refinement of the rotation period, now estimated to be 15.9380 +/- 0.0005 hr. The occultation data also provided high-precision astrometric constraints on the position of the object that were crucial for supporting the navigation for the New Horizons flyby. This work demonstrates an effective method for obtaining detailed size and shape information and probing for rings and dust on distant Kuiper Belt objects as well as being an important source of positional data that can aid in spacecraft navigation that is particularly useful for small and distant bodies

FRIPON : a worldwide network to track incoming meteoroids

Context. Until recently, camera networks designed for monitoring fireballs worldwide were not fully automated, implying that in case of a meteorite fall, the recovery campaign was rarely immediate. This was an important limiting factor as the most fragile - hence precious - meteorites must be recovered rapidly to avoid their alteration. Aims. The Fireball Recovery and InterPlanetary Observation Network (FRIPON) scientific project was designed to overcome this limitation. This network comprises a fully automated camera and radio network deployed over a significant fraction of western Europe and a small fraction of Canada. As of today, it consists of 150 cameras and 25 European radio receivers and covers an area of about 1.5 x 10(6) km(2). Methods. The FRIPON network, fully operational since 2018, has been monitoring meteoroid entries since 2016, thereby allowing the characterization of their dynamical and physical properties. In addition, the level of automation of the network makes it possible to trigger a meteorite recovery campaign only a few hours after it reaches the surface of the Earth. Recovery campaigns are only organized for meteorites with final masses estimated of at least 500 g, which is about one event per year in France. No recovery campaign is organized in the case of smaller final masses on the order of 50 to 100 g, which happens about three times a year; instead, the information is delivered to the local media so that it can reach the inhabitants living in the vicinity of the fall. Results. Nearly 4000 meteoroids have been detected so far and characterized by FRIPON. The distribution of their orbits appears to be bimodal, with a cometary population and a main belt population. Sporadic meteors amount to about 55% of all meteors. A first estimate of the absolute meteoroid flux (mag =similar to 1 cm) amounts to 1250/yr/10(6) km(2). This value is compatible with previous estimates. Finally, the first meteorite was recovered in Italy (Cavezzo, January 2020) thanks to the PRISMA network, a component of the FRIPON science project