Geology of bastnaesite and monazite deposits in the Ambatofinandrahana area, central part of Madagascar: An overview

International audienceBastnaesite and monazite deposits of the Ambatofinandrahana region of Madagascar are closely associated to an alkaline to peralkaline complex of Panafrican age (similar to 540-570 Ma; Ambalavao suite), composed essentially of syenite and granite that intrude Paleoproterozoic Itremo Group metasediments, orthogneiss and metagabbro of the similar to 800 Ma Imorona-Itsindro suite. The peralkaline syenite is characterized by aegirine-augite and Na-amphiboles. Primary REE mineralization consists of monazite, allanite and chevkinite, however, economically exploitable mineralizations contain secondary bastnaesite and monazite that formed by late- to post-magmatic hydrothermal processes. Evidence includes silicification of microsyenite; chalcedony-barite-fluorine-bastnaesite and/or monazite veins; phlogopite-bastnaesite veins; alteration of chevkinite to bastnaesite; monazite-bearing marble. Bastnaesite, monazite and chevkinite have similar REE spectra, which are comparable to those obtained from other hydrothermal occurrences of these minerals and from carbonatites. Phlogopite associated to bastnaesite has a particular chemical signature (Si > 6 pfu and high X-F) that resembles that of phlogopite from the Bayan Obo REE deposit in China. These observations, and the similarities of these mineralizations with deposits such as Gakara (Western Rift Valley, Eastern Africa) and Mountain Pass (California), incite to better define the geodynamic and magmatic context of the alkaline magmatism in this region, and to prospect for the presence of carbonatite

Harmonized in situ datasets for agricultural land use mapping and monitoring in tropical countries.

The availability of crop type reference datasets for satellite image classification is very limited for complex agricultural systems as observed in developing and emerging countries. Indeed, agricultural land use is very dynamic, agricultural censuses are often poorly georeferenced and crop types are difficult to interpret directly from satellite imagery. In this paper, we present a database made of 24 datasets collected in a standardized manner over nine sites within the framework of the international JECAM (Joint Experiment for Crop Assessment and Monitoring) initiative; the sites were spread over seven countries of the tropical belt, and the number of data collection years depended on the site (from 1 to 7 years between 2013 and 2020). These quality-controlled datasets are distinguished by in situ data collected at the field scale by local experts, with precise geographic coordinates, and following a common protocol. Altogether, the datasets completed 27 074 polygons (20 257 crops and 6817 noncrops, ranging from 748 plots in 2013 (one site visited) to 5515 in 2015 (six sites visited)) documented by detailed keywords. These datasets can be used to produce and validate agricultural land use maps in the tropics. They can also be used to assess the performances and robustness of classification methods of cropland and crop types/practices in a large range of tropical farming systems. The dataset is available at https://doi.org/10.18167/DVN1/P7OLAP (Jolivot et al., 2021)