Résumé
Les filons de microgranite qui recoupent le dôme granitique du Velay sont classiquement considérés comme représentant les ultimes manifestations anatectiques dans cette partie du Massif central. Deux de ces filons ont été étudiés sur le plan pétrologique et géochronologique. Tous les deux sont peralumineux et potassiques, et présentent de fortes analogies avec les granites tardimigmatitiques, mais montrent des signes de refroidissement rapide et d’altération hydrothermale. Les datations des monazites par microsonde électronique montrent dans les deux cas un âge Stéphanien qui correspondrait à la mise en place, et un âge Permien supérieur (Thuringien), qui daterait l’épisode hydrothermal.
Mots-clefs : Microgranite, Datation, U-Th-Pb, Stéphanien, Permien sup., Haute-Loire, Ardèche, Velay.
Abstract
The Velay granitic dome is characterised by three Late Hercynian anatectic events (Montel et al., 1992). The first, dated at 314±5 Ma (Mougeot et al., 1997), was responsible for a major development of layered migmatites. The second, dated at 300 Ma (Mougeot et al., 1997; Williamson et al., 1992), caused major fluid-absent melting that produced the various Velay granites. The third and final event was an emplacement of dykes of porphyritic granite and granite porphyry. The age of the end of this major thermal event is poorly constrained; apatite U-Pb dating suggests a general cooling at about 290 Ma (Mougeot et al., 1997), whereas Rb-Sr whole-rock isochrons on late granites yield Permian ages. The present paper provides new data for two microgranites - the Montasset and Charron microgranites. The Montasset microgranite, which is described in detail elsewhere (Bouloton et al., 2000), is situated in the southern part of the dome (Fig. 1). It displays a typical porphyritic texture, with phenocrysts of corroded quartz, altered plagioclase and K-feldspar, chloritized biotite, pinitized cordierite, and large garnets. The matrix is mainly aphyric, with spherulitic and granophyric textures in places. Secondary minerals include muscovite, calcite, and fluorite. The Charron microgranite, situated in the core of the Velay granite (Fig. 1), has a porphyritic texture with phenocrysts of centimetre-long K-feldspar, corroded quartz, plagioclase, biotite, and cordierite. Skeletal K-feldspar and acicular biotite form microphenocrysts and the matrix is aphyric with numerous spherulitic structures, indicating a fast cooling rate for this facies. Geochemically, both facies are peraluminous, leucocratic, and K-rich and are very similar to other late granites in the Velay dome (Montel and Abdelghaffar, 1993). The source temperature estimated by monazite and zircon thermometry (Montel, 1993; Harrison and Watson, 1983) is 820-850°C, which is identical to source temperatures of other late granites in this area. Electron-probe dating of monazite was carried out using the method described in Montel et al. (1996). The results, which are presented in Table 1 and Figure 2, are identical for the two studied samples. The age population is complex, and the statistical analysis shows that a double-age model must be considered for both samples. The ages of the two events are 257±8 Ma and 306±12 Ma (MSWD=1.55) for the Montasset sample, and 252±11 Ma and 291±9 Ma for the Charron sample. We consider that the Permian ages represent a hydrothermal event and that the Stephanian ages reflect melting and emplacement.
Key-words: Microgranite, Dating, U-Th-Pb, Stephanian, Upper Permian, Haute-Loire France, Ardèche France, Velay
Dernière mise à jour le 01.07.2015