Résumé
Nous avons réalisé une étude K-Ar et 39Ar-40Ar sur minéraux séparés provenant de roches magmatiques et métamorphiques des Vosges pour préciser l'histoire varisque de ce massif. À l'exception d'une amphibole provenant d'un pluton basique situé sur la bordure nord du massif des Ballons, qui donne un âge de 360 ± 6 Ma, les résultats 39Ar-40Ar sur les différents types de granitoïdes indiquent qu'ils se sont mis en place dans un laps de temps maximum de 10 Ma, entre 340 et 330 Ma. Il est à noter que l'on ne retrouve pas sur les analyses 39Ar-40Ar le hiatus d'âge montré par les résultats K-Ar entre la mise en place des granites subalcalins, 345-335 Ma, et celles des leucogranites 325-315 Ma. Cette convergence des résultats 39Ar-40Ar sur les granites des Vosges septentrionales et méridionales confirme l'hypothèse de P. Fluck (1980a) selon laquelle les Vosges forment un ensemble cohérent depuis le Carbonifère inférieur. L'apparition des bassins houillers au Carbonifère supérieur s'accompagne de manifestations effusives comme celle du Nideck, 294,2 ± 4,8 Ma (Lippolt et Hess, 1983) ou filoniennes, comme celle du Teufelsloch, 306,6 ± 6,2 Ma. La biotite d'un gneiss à grenat emballé dans la série volcano-sédimentaire d'Oderen et l'amphibole d'un gneiss de la série de Sainte-Marie-aux-Mines donnent respectivement des âges intégrés de 382 ± 10 Ma et 373 ± 11 Ma. Ces valeurs sont en accord avec celle de 386 ± 15 Ma obtenue par isochrone Rb-Sr sur roche totale (Bonhomme et Fluck, 1981). Elles marqueraient l'achèvement de la collision continentale au Dévonien moyen. Toutes les autres analyses 39Ar-40Ar sur les gneiss de Sainte-Marie-aux-Mines, quelle que soit la génération métamorphique des minéraux, donnent des âges entre 340 et 330 Ma. Un dilemme demeure sur leur signification géologique. Marquent-ils la fin d'une évolution continue de la série lors de sa remontée vers la surface ou proviennent-ils d'un réchauffement dû à la mise en place des granites ? La comparaison des résultats de cette étude avec ceux de travaux similaires effectués sur des formations analogues de Forêt-Noire et du Massif central montre que les Vosges s'inscrivent parfaitement dans les schémas géodynamiques de la branche sud de l'orogène varisque.
Abstract
A K-Ar and 39Ar-40Ar study has been carried out on mineral separates from magmatic and metamorphic rocks of the Vosges with the following aims: (1) to determine the age of the basement structure; (2) to establish the salient features of the thermochronology during the Variscan orogeny; (3) to place the Vosges massif within the context of the Variscan Belt of western Europe by comparing our resuits with geochronological data obtained on similar terranes situated in the French Massif Central and the Black Forest. The majority ofthe 39Ar-40Ar release spectra on mica from magmatic rocks reveals acceptable plateau ages which fall in the restricted 340-330 Ma range. These dates are viewed as closely approaching the time of granite emplacement. Even when some of the biotites fail to yield definite plateaus, we can state with confidence that the age of granite emplacement does not depart from the aforementioned range of 340-330 Ma. One should note that the 39Ar-40Ar results are somewhat at variance with the K-Ar conventional ages, which suggests a time span between the emplacement of subalkaline granites (345-335 Ma), and that of leucogranites (325-315 Ma). An exception to the pattern of the granite ages is displayed by a basic pluton located at the northern edge of the Massif de Ballons with an 39Ar-40Ar plateau age on amphibole of 360 ± 6 Ma. The absence of definite inherited argon in the mineral leads us to regard the later date as reflecting the time of the pluton emplacement. The striking similarity of the 39Ar-40Ar ages on granites of the northern and the southern Vosges reinforces the view (Fluck, 1980a) that the Vosges structure already existed in the Early Devonian. Magmatism in the Late Carboniferous is represented by huge volcanic outpourings such as the Nideck, well-dated at 294 ± 4.8 Ma (Lippolt and Hess, 1983), and by dike intrusions such as the Teufelloch (306.6 ± 6.2 Ma). Both accompanied the formation of Stephano-Permian basins. In contrast to the magmatic rocks, high-grade metarnorphics failed to yield any 39Ar-40Ar plateau age. Nevertheless it must be stressed that an 39Ar-40Ar integrated age of 382 ± 10 Ma and a conventional K-Ar age of 373 ± 11 Ma respectively measured on a biotite from a gneiss of the Oderen volcano-sedimentary); sequence and an amphibole from a gneiss of the Sainte-Marie-aux-Mines metamorphic unit are in agreement with a Rb-Sr age of 386 ± 15 Ma obtained by Bonhomme and Fluck (1981) on the latter formation. These dates might correspond to the termination of the continental collision in the Middle Devonian. The other results on minerals from the Sainte-Marie-aux-Mines gneisses indicate 340-330 Ma ages, irrespective of the metamorphic generation of the analysed minerals. The reconstruction of the thermochronology of this metamorphic core complex is still problematic. An age of 357 ± 4 Ma yielded by high-temperature steps of an amphibole and the moderate extent of the metamorphic aureole around granites lead us to prefer a progressive thermal evolution of gneisses during their ascent to the surface rather than an intense reheating caused by the granite emplacement. These dates correspond to the post-collisional extension stage demonstrated by structural analyses (Rey et al., 1991, 1992; Rey, 1992; Latouche et al., 1992). Examination of the geochronological data obtained on low-pressure high-temperature metamorphics and granites of the French Massif Central and the Black Forest shows that the three consecutive segments of the Variscan Belt have shared a common geological history since Silurian times. U-Pb analyses on zircon from eclogites in the French Massif Central and the Black Forest, for instance, yield discordia with upper intercept ages comprised between 460 and 415 Ma. They are tentatively seen to reflect the time of eclogite formation. The existence of a low-pressure high-temperature metamorphic event between 390 and 370 Ma is established in the Massif Central and to a less extent in the Vosges. The three massifs underwent a strong metamorphic event around 340 Ma, although with a probable slight diachronism, and an intense granitization at about the same time. These pieces of evidence demonstrate that the Vosges fits in with the geodynamic scheme of the southern branch of the Variscan Belt.
Dernière mise à jour le 28.07.2015