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2016-01-27 - Colloque/Présentation - communication orale - Anglais - 38 page(s)

Cobert Corentin , Baele Jean-Marc , Boulvais Philippe, Poujol Marc, Beyssac Olivier, Decrée Sophie, "Upper Silurian magmatism in the Rocroi Massif, Ardennes : new constraints from geochronology and geochemistry of microgranite dyke at Mairupt" in 5th International Geologica Belgica Meeting, Jan 26-29, 1, 186, University of Mons, Belgium, 2016

  • Codes CREF : Géologie et minéralogie (DI1410), Géochimie (DI1413), Géochronologie (DI1437)
  • Unités de recherche UMONS : Géologie fondamentale et appliquée (F401)
  • Instituts UMONS : Institut de Recherche en Science et Ingénierie des Matériaux (Matériaux)

Abstract(s) :

(Anglais) The Rocroi Massif (Ardennes) is essentially composed of Cambrian to Early Ordovician low-grade metamorphic pelites and quartzites. It is intruded in its southern part by WSW-ENE-orientated, mafic and felsic dykes, such as the ones in Mairupt and Grande Commune. At Willerzie (eastern edge of the Rocroi Massif), occurrences of volcanites are found in Upper Silurian to Lower Devonian sediments, which locally contain abundant quartz phenocrysts and rhyolitic clasts associated with brecciated horizons in the Haybes Formation (Meilliez, 2006). A cogenetic origin for the Willerzie volcanites and the bimodal magmatism in the Rocroi Massif was first proposed by Beugnies (1969). This proposition was invalidated by zircon U-Pb geochronology on the Mairupt and Grande Commune igneous rocks, which yielded a lower intercept age of 373+8/-9 Ma (i.e. Upper Devonian; Goffette et al., 1991). Ongoing investigations of Cambrian metapelites of Mairupt required a re-evaluation of the age of the microgranite. In situ U-Pb geochronology was conducted by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at Géosciences Rennes on zircon grains from microgranite samples from two different outcrops in Mairupt. Upper Silurian (Pridoli) concordia ages were obtained for the two sample sets, i.e. 420.5±2.9 Ma (2σ) and 421.1±4.0 Ma (2σ). These results reinforce the hypothesis of a genetic link between dyke magmatism in the southern part of the Rocroi Massif and the volcanites of Willerzie. Raman spectrometry of the carbon material and mineralogy of REE-bearing phases in the metapelites are consistent with a sub-surface intrusion in the pelites before they were buried at monazite stability conditions. The Mairupt microgranite displays some variations in SiO2, K2O, Na2O, Zr/Hf and Nb/Ta. Together with petrographic and cathodoluminescence observations, these geochemical data suggest that this peraluminous microgranite has undergone two types of secondary alteration. The first type is silicification+albitization, which, based on textural observation, developed prior to the Variscan orogeny. The second type is a greenschist facies alteration, marked by chloritization and carbonation, and developed during the regional deformation. Strontium and neodymium isotope geochemistry (initial 87Sr/86Sr isotopic ratios from 0.7116 to 0.7161, εNd(t) between -6.3 and -3.7) indicates a crustal source for the Mairupt microgranite. The Pridoli age found for the bimodal magmatism points to a Silurian magmatic event in an extensional context (passive margin), which occur at the onset of the Devonian sedimentation in the Ardennes area.