Âge namurien du lamprophyre de Termes (massif de Mouthoumet, Aude) : sa signification dans l'édifice varisque pyrénéen

Namurian age of the Termes lamprophyre (Mouthoumet Massif, Aude): its place in the Variscan of the Pyrenees
M. Durand-Delga, R. Montigny, P. Rossi
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Dans la partie nord du massif de Mouthoumet, un filon de minette (lamprophyre) recoupe, près de Termes, les calcaires dévoniens de I'Autochtone relatif de ce massif à proximité immédiate du front de chevauchement de I'Allochtone. Dans la mesure où aucune déformation post-solidus, ni aucune rétromorphose de basse température n'affecte cette intrusion, sa mise en place est nécessairement postérieure à celle des nappes varisques. La datation 40Ar-39Ar selon la procédure du dégazage par paliers d'une biotite extraite de la minette donne un spectre d'âge irrégulie. Toutefois, en éliminant les deux premiers paliers, on obtient un âge moyen de 319 ± 5 Ma, pris comme base de discussion. L'examen systématique, tant du point de vue pétrographique que chimique, des séries alcalines des Pyrénées nous autorise à conclure que cette valeur représente probablement l'âge de l'intrusion. Ce résultat implique que la tectonique de nappes date du Namurien inférieur. Elle se situe entre le dépôt du flysch qui surmonte les calcaires dévoniens et la mise en place de la minette. Autrement dit, la tectonique de nappe est très vraisemblablement comprise dans l'intervalle de temps 325-314 Ma.


The Mouthoumet Massif, forming an inlier within the Mesozoic and Paleogene cover of the sub-Pyrenean foreland, consists, Early Ordovician to Middle Carboniferous rocks divided into several thrust units oriented WSW-ENE. The northwestern part of the massif corresponds to a parautochton that has been overthrust by several nappes forming the southeastern part of the massif. A Visean Namurian flysch ("Culm" is the young est rock implicated in the various units that in places are unconformably overlain by Stephanian volcaniclastic formations. In the northern part of the massif, near the village of Termes, parautochthonous Devonian limestones are intraded by a minette (lamprophyre) dike that crops out in the close vicinity of the allochthonous thrust front. The Termes minette is thus the only incontestable intrusive rock of the Mouthoumet Massif; no similar rock is known on the southern flank of the Montagne Noire. It is a blackish-grey dike, macroscopicalI homogeneous, with a well-defined N35°W strike extending over some 50 m. The Givetian (?) limestones are clearly intersected by the dike. Thin section studies of the country rock, over 10 m or so to either side of the dike, show that unlike the undeformed minette, the limestones underwent a series of deformation; only a late fracturing, masked by small calcite veinlets, aflects both the Devonian carbonates and the minette dike. Chemical analysis of the Termes minette (Table 1) confirms its ultra-potassic character with a K2O/Na2O ratio >3.5. Like all lamprophyres, this rock is characterized by an abundance of transition elements, HFSE and LREE. A strong La/Yb fractionation, absence of a negative Eu anomaly and high Cr, Co and Ni contents compared to K2O are also noted, reinforcing this affinity. Insofar as the minette is not affected by subsolidus deformation and low-temperature retrograde metamorphism, it must have been intruded after emplacement of the Variscan nappes. 40Ar-39Ar dating by the stepwise degassing technique of a biotite extracted from the minette yields an irregular age pattern; nevertheless, after discarding the first two steps, an average age of 319 ± 5 Ma is retained for discussion. A review of the petrography and chemistry of the alkaline series of the Pyrenees enables us to conclude that this age probably reflects the age of the dike intrusion. The 319 ± 5 Ma age assigned to the Termes lamprophyre dike corresponds to the Early Namurian, between the early Serpukhovian (325 Ma) and the middle Bashkirian (315 Ma). Apart from the ultra-potassic rocks, or "vaugnerites", related to the Ansignan charnockites (north Pyrenean zone), this is the first evidence of an intrusive rock of this age in the Pyrenean domain. The Termes dike also makes it possible to better confine the age of tectonism of the Variscan Haute Corbières nappes. Regardless of the origin of the nappes, the basal surface of the allochthon, before erosion, must have been a few hundred metres above the site of the dike. If the dike was pre-nappe, it would inevitably be deformed, which is not the case. No subsolidus deformation or low-tempeature retrograde metamorphism was noted in the petrographic analysis. The biotites used for radiometric dating are particularly well preserved. Nappe emplacement was thus earlier than the crystallization of the minette dike. Nappe emplacement was, howevev, later than deposition of the Early Carboniferous flysch ("Culm" dated at the Visean-Namurian boundary (approximately 325 Ma) in the Alet gorges. In taking the age data for the Alet fauna (pre-nappe) and those for the Termes dike (post-nappe), we conclude that the nappe tectonism occurred between 325 and 314 Ma, assuming the maximum age given by the uncertainty of the age determination on the dike, or between 325 and 324 Ma assuming the minimum age.

Dernière mise à jour le 28.07.2015