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2018-09-12 - Colloque/Présentation - poster - Anglais - page(s)

Cobert Corentin , Baele Jean-Marc , Decrée Sophie, Beyssac Olivier, "Upper stability limit of authigenic monazite in the Rocroi Inlier" in 6 th International Geologica Belgica Meeting 2018. Geology Serving Society, Leuven, Belgique, 2018

  • Codes CREF : Pétrologie (DI141A), Géologie et minéralogie (DI1410), Géochimie (DI1413)
  • Unités de recherche UMONS : Géologie fondamentale et appliquée (F401)
  • Instituts UMONS : Institut de Recherche en Science et Ingénierie des Matériaux (Matériaux)
  • Centres UMONS : Ingénierie des matériaux (CRIM)

Abstract(s) :

(Anglais) 1. Abstract Authigenic monazite (also called “grey” or “dark” monazite) is a Rare Earth Elements (REE) orthophosphate, which is Eu-rich and Th-poor compared to the more common “yellow” monazite of magmatic or high-grade metamorphic origin. This mineral is believed to form under late diagenesis to very low metamorphism conditions (250-400 °C and P~2-10 kbar) in carbonate-poor and organic matter(OM)-rich shales (e.g. Burnotte et al., 1989; Janots et al., 2006; Wan et al., 2007). Investigation on the potential economical interest of authigenic monazite is motivated by the worldwide distribution of this mineral and the occurrence of secondary concentration in (paleo )placer (e.g. Rosenblum and Mosier, 1983; Cobert et al., 2016). In Belgium, authigenic monazite has been reported in Lower Paleozoic rocks of the Rocroi inlier, Brabant massif and Stavelot-Venn inlier. In the Rocroi inlier, Nonnon (1984) firstly documented dark monazite in stream sediments in the Croix-Scaille massif before tracing to its source rock in shales of the Revin Group (middle Cambrian to Ordovician). Due to its structure, the Rocroi inlier offers an extensive outcrop of the Revin group. In addition, it is affected by a North to South prograde metamorphism (anchizone to epizone). Therefore, the Rocroi inlier is a good natural laboratory to carry out a petrographical and geochemical study on the behaviour of authigenic monazite during prograde metamorphism. Cathodoluminescence imaging (CL) and Scanning Electron Microscopy with Energy-Dispersive Spectroscopy (SEM-EDS) of the Revin Group shales reveal the presence of authigenic monazite in the northern and central part of the Rocroi inlier (Fig.1a), along the “Eau Noire” stream and at Revin, respectively. At Mairupt, in the southern part of the inlier, CL show images of nodules alike authigenic monazite with their typical irradiation haloes around but with different luminescence characteristics inside. SEM-EDS analysis indicate the presence of allanite (LREE-rich epidote), xenotime (HREE-rich phosphate) and LREE, Th-rich monazite assemblages in these nodules (Fig.1b). Raman Spectroscopy on Carbon Material (RSCM) showed that the local maximum temperature that was reached by the host rock is 500 ± 50 °C, while at Revin and in the “Eau Noire” valley, <300°C is reported in the literature (Beugnies, 1986). A few authors (e.g. Janots et al., 2008) have discussed the conditions for the destabilization of monazite and its transformation into allanite, which occur at 420–450 °C. Allanite can later be partially or totally destabilized in metamorphic monazite and xenotime at 550–575 °C and >585 °C, respectively, or by retrograde metamorphism only if the CaO/Na2O ratio < 0.54. Based on literature data, our results would indicate a breakdown of authigenic monazite at Mairupt and its transformation into allanite. Therefore, in the Rocroi inlier, the required conditions to form authigenic monazite have been reached and the upper stability limit of this mineral was crossed to the south, which would indicate a minimum temperature of 420°C if we use this limit as a geothermometer. 2. References Beugnies, A., 1986. Le métamorphisme de l’aire anticlinale de l’Ardenne. In Hercynica II (1), 17-33. Burnotte, E., Pirard, E., Michel, G.,1989. Genesis of gray monazites; evidence from the Paleozoic of Belgium. In Economic Geology, 84(5), 1417-1429. Cobert, C., Baele, J.M., Boulvais, Ph., Decrée, S., Dupont, N., Spagna, P., 2015. Grey Monazite Paleoplacers in Lower Cretaceous Continental Formations in the Mons Basin, Belgium. 13th SGA Biennial Meeting, Nancy, France, 2015. Abstract book. 703-706. Janots, E., Negro, F., Brunet, F., Goffé, B., Engi, M., Bouybaouene, M.L., 2006. Evolution of the REE mineralogy in HP–LT metapelites of the Sebtide complex, Rif, Morocco: monazite stability and geochronology. In Lithos, 87(3-4), 214-234. Janots, E., Engi, M., Berger, A., Allaz, J., Schwarz, J. O., Spandler, C., 2008. Prograde metamorphic sequence of REE minerals in pelitic rocks of the Central Alps: implications for allanite–monazite–xenotime phase relations from 250 to 610 C. In Journal of Metamorphic Geology, 26(5), 509-526. Nonnon, M., 1984. Découverte de monazite grise en nodules et d'or alluvionnaire dans le massif de la Croix-Scaille. In Bulletin de la société belge de géologie, 93(4), 307-314. Rosenblum, S., Mosier, E.L., 1983. Mineralogy and occurrence of europium-rich dark monazite. In U.S. Geological Survey Professional Paper 1181 67 pp. Wan, Y., Song, T., Liu, D., Yang, T., Yin, X., Chen, Z., Zhang, Q., 2007. Mesozoic monazite in Neoproterozoic metasediments: evidence for low-grade metamorphism of Sinian sediments during Triassic continental collision, Liaodong Peninsula, NE China. In Geochemical Journal 41, 47–55.