Résumé
L'âge de 345 ± 10 Ma (Viséen inférieur), obtenu sur le microgranite de Picampoix (Nièvre) par la méthode de datation Pb/Pb par évaporation sur zircons, s'inscrit dans l'évolution magmatique de la chaîne varisque dans le Morvan durant le Carbonifère. Cette datation ainsi que celles obtenues antérieurement, permettent de reconsidérer la chronologie des diverses expressions du magmatisme. des volcanites, des granites et plus particulièrement des roches filoniennes microgranitiques rencontrées en abondance dans les diverses unités du Paléozoïque du Morvan. Cinq générations de roches microgranitiques ont été distinguées selon leurs caractères pétrochimiques et leurs modes de gisement. Chacune de ces générations peut être reliée soit à un épisode volcanique, soit à une phase d'intrusions granitiques. La génération 1 de trondhjémite est associée au volcanisme orogénique calco-alcalin du Famennien-Tournaisien. La génération 2, représentée par le microgranite de Picampoix, est contemporaine des intrusions de granites subalcalins monzonitiques (Gien-sur-Cure, Grury) qui succèdent au volcanisme orogénique acide du Tournaisien-Viséen inférieur: La génération 3 des filons de microgranites, orientés NE-SW est contemporaine des volcanites acides et aériennes du Viséen supérieur dont la mise en place est guidée par un régime d'extension NW-SE. La génération 4 des fiIons de microgranites à deux micas, orientés NW-SE, recoupe les intrusions de granites alumino-potassiques datées à 327 ± 5 Ma (Pierre-Qui-Vire). La génération 5 est représentée par les microgranites qui accompagnent le volcanisme acide d'âge stéphanien de la caldeira de Blismes-Montreuillon.
Abstract
Periodic magmatic activity, related to the evolution of the Variscan belt from the Middle Devonian to the Permian, is indicated in the northeast of the French Massif Central by: a) volcanic extrusives (basalt to soda-rhyolite and associated tuffs) and their hypovolcanic suites intercalated in the Morvan marine sedimentary succession, which accumulated continuously from the Famennian to the Early Vïsean, and subaerial acid volcanics accompanied by hypovolcanic rocks in the Late Visean; b) the Settons and Luzy batholiths, with intrusion of monzonitic subalkaline granite and associated microgranite during the Early Visean, followed by alumino-potassic granite during the Early Namurian; to the northwest and southeast, these batholiths bound and cut the Morvan Devonian-Dinantian volcano-sedimentav succession; c) acid volcanic-plutonic caldera activity in the Blismes-Montreuillon basin from the Stephanian to Saxonian. The dating of the Picampoix microgranite (Nièvre} at 345 ± 10 Ma (Early Visean), using the Pb/Pb evaporation method on zircons, combined with the ages previously obtained on the granites and volcanic rocks, have made it possible to distinguish five generations of microgranitic rock according to their petrochemical characteristics and mode of occurrence. Generation 1, represented by trondhjemite veins and stocks, is associated with the Famennian to Early Visean talc-alkaline volcanic activity. The intrusions are located in the southern part of the Morvan and are both concordant and discordant with the general direction of the volcano-sedimentary layers. Volcanic activity began in the Middle Devonian with the Chizeuil soda-rhyolites and soda-dacites dated at 318 ± 2 Ma (U/Pb, Calvez et al., 1985), continued with basalt and andesite lava flows and thick layers of basic tuff during the Famennian, and ended with dacites, commonly ignimbritic, from the Tournaisian to the Early Vïsean. The geochemical signatures (major and trace elements [Lemière, 1982; Delfour 1989] and rare earths [Pin et al., 1982]) indicate that the volcanic rocks belong to a talc-alkaline suite emplaced in a mature island-arc type setting with a thick continental crust, associated with a subduction zone (Behr et al., 1984). Generation 2, represented by the Picampoix microgranite (345 ± 10 Ma), is associated with the major Early Visean magmatic phase which led to the generation of the monzonitic subalkaline biotite granites at Grury (348 ± 6 Ma, U/Pb, Supply, 1985) Settons and Gien-sur-Cure (about 345 Ma, Rb/Sr Rolin and Stussi, 1991). Generation 3 is represented by NE- SW-oriented microgranite dykes cutting the Devonian-Dinantian succession and the biotite granites. Controlled by a NW- SE extensional regime, this generation was contemporaneous with the acid ignimbritic volcanic activity associated with Late Vïsean crustal melting and which shows the same talc-alkaline characteristics. The Vausségré granophyre dated at 331 + 10/-8 Ma (U/Pb, Supply, 1985) which intrudes the northern part of the Château-Chinon granite, belongs to this generation. Generation 4 consists of pink musçovile-biotite microgranite dykes trending NW-SE to sub N-S. The dykes, commonly forming sets several kilometres long, are narrow and subvertical, banded along the edges and have Sharp contacts with the country rock. They cut the Folin and Pierre-Qui-Vire alumino- potassic granites (327 ± 5 Ma, U/Pb, Supply, 1985, and 323 ± 4 Ma, Rb/Sr Duthou et al., 1984). Their orientation suggests that they were associated with the Stephanian-Autunian NE-SW extension (Faure and Becq-Giraudon, 1993). Generation 5 is represented by a microgranite and granophyre dyke suite which accompanied the subalkaline votcanic activity at Blismes from the Late Stephanian to the Autunian (Carpena et al., 1984). The dykes, although abundant in the Stephanian B and C tuffs and rhyolites (291 ± 7 Ma), are less common in the Early-Late Atunian volcano-sedimentary succession, and absent in the Montreuillon ignimbritic nappe dated at 267 ± 5 Ma and assigned to the Saxonian. The NW-SE- and ENE-WSW- oriented veins were controlled by the tectonic constraints prevailing during the formation of the Blismes-Montreuillon caldera. During the Middle Visean (340-330 Ma), the intra-Visean epeirogenic phase caused emergence of the basement, marking the boundary between the two periods of volcanic activity which characterize the Devonian-Dinantian evolution of the Morvan (Michel-Lévy 1908; Bébien and Gagny, 1980). The first period occurred from 380 to 340 Ma, with calc-alkaline volcanic activity from the Middle Devonian to Tournaisian/Early Visean, and ended with the emplacement of the Picampoix microgranite and the large Grury, Gien- sur-Cure and Settons subalkaline granite intrusions (including the Château-Chinon and Dommartin granites) during the Early-Midde Visean. These intrusions cut marine sediments deposited in an environment with distant or no relief. The second shorter period (330 to 325 Ma) consisted in a) subaerial acid volcanic activity during the Late Visean and the emplacement of NE-SW-oriented microgranite veins and nappes (Vausségré type) and b) intrusion of alumino-potassic granite. The peraluminous material, derived from crustal melting, was emplaced as volcanic rocks in NE-SW-oriented volcano-plutonic troughs, also present elsewhere in the northeastern Massif central (Leistel and Gagny, 1984), and as granite and leucogranite intrusions in the WNW-ESE extensionaI zones between the crustal ENE-WSW sinistral shear zones (Rolin and Stussi, 1991). Afier a gap of 25 Ma, during which time the Devonian-Dinantian volcano-sedimentary succession was folded by the Sudetic orogeny, the Middle Stephanian to Saxonian magmatic activity (300- 260 Ma) was restricted to NW-SE- oriented muscovite-biotite microgranite intrusions (generation 4) and to volcanic-plutonic caldera activity with emplacement being essentiallv controlled by NE-SW extension (Faure and Becg- Giraudon, 1993).
Dernière mise à jour le 28.07.2015