Résumé
Neuf coupes à travers le massif de l'Aston sont présentées ; ces coupes sont localisées sur une carte du massif, révisée. Deux coupes E-W débutent dans les paragneiss migmatitiques et intéressent essentiellement les micaschistes paléozoïques de couverture du massif ; ces coupes montrent l'importance des phases tardives NE et NW qui affectent ces terrains. Sept coupes de direction S-N intéressent les gneiss œillés de Riète, les gneiss clairs de leurs bordures nord et sud et les paragneiss migmatitiques sous jacents ; ces formations sont également plissées, les plis majeurs étant à vergence nord pour les orthogneiss. Les phases tardives NE et NW apparaissent localement à l'origine des foliations des gneiss œillés. Un liseré de granitoïdes clairs, gneiss de Peyregrand, granites clairs, leptynites, gneiss clairs, jalonne le pourtour des gneiss œillés de Riete et, dans la terminaison est du massif, l'ensemble des gneiss œillés et des paragneiss. Certains de ces granitoïdes clairs peuvent représenter des diflérenciats des granitoïdes porphyroïdes ayant abouti à la formation des gneiss œillés ; toutefois les relalions mutuelles entre ces granitoïdes clairs ne sont pas élucidées, pas plus que les relations éventuelles de ces roches avec les puissants bancs de granites clairs que l'on cartographie à l'intérieur du massif de gneiss œillés. L'ensemble des gneiss œillés appa- raît comme un laccolite (ou une lame) granitique plissé et faillé frangé de granitoïdes leucocrates ; la puissance de la lame de gneiss œillés diminue d'est en ouest alors que la puissance des granitoïdes clairs sous-jacents augmente. La bordure nord de ce laccolite apparaît systématiquement faillée, en contact avec les micaschistes, ce contact étant jalonné de mylonites et envahi de granites dont le feuillet de granite d'Ax est le plus développé ; a l'ouest, le contact grossièrement N-S entre les ortho et paragneiss et les micaschistes du Cambrien apparaît systématiquement jalonné de mylonites et d'ortho-amphiholites. La comparaison géochimique entre les paragneiss pélitiques migmatitiques et les shales des micaschistes cambriens affleurant à l'ouest confirme des différences de régime de sédimentation perceptibles sur le terrain et suggère que le laccolite de l'Aston s'est mis en place au voisinage du contact entre terrains caumbriens et infra ou pré-cambriens.
Abstract
The Aston massif, one of the large gneisses (ortho- and paragneisses) and micaschists massifs of the Pyrenean axiul zone, crops out to the north of the Merens fault zone where it covers some 36 km in an E-W direction with a maximum N-S width of about 15 km. To the south of the Merens fault zone it is continued by the Hospitalet orthogneisses massif. Nine sections (two E-W and seven S-N) through the gneisses and surrounding micaschists of the Aston massif are described in the present note. The location of the sections is shown on a revised map of the massif, which displays some noticeable additions with respect to previous publications. The two E-W sections, one on the Andorran side and the other one on the French side of the French-Andorran border, begin in the migmatitic paragneisses of the so-called Quioules and Port de Siguer formations and continue in the Mounicou micaschists which probably covers the gneisses. The two sections mainly cross micaschists and reveal the importance of the NW and NE folding phases which deformed these rocks: four folding phases of unequal importance have been recognized. The section on the Andorran side shows, from its eastern end to the Mounicou valley, (1) a recumbent thrust fold including plagioclasic gneisses and micaschists, (2) a west-verging overturned anticline with an almost horizontal axial plane (the anticlinal hinge being marked by Tristanya's marble), and (3) an underlying short syncline leading to (4) the monoclinal formation of the Mounicou valley. The seven S-N sections cross the Riete augengneisses, the leucocratic orthogneisses and foliated granites of its northern and southern margins, and the underlying migmatitic paragneisses. The rocks are folded, with north-trending vergence for the major folding phases. The foliations of the augengneisses, at least in the centre of the massif appear to be related to late NE and NW folding phases. A fringe of leucocratic granitoids (Peyregrand gneisses, leucocratic granites, leptynites and leucocratic gneisses) surrounds the Riete augengneisses on both its southern and northern borders; locally, the orthogneisses are also,folded. Two S-N sections to the east and west of the Ariege valley cross the sheet of Ax-les-Thermes granite. This granite, which formed in two episodes, is from 350 to 750 m wide and separates the augengneisses into two unit. Leucocratic granites and leptynites in the eastern part of the massif occur both around the augengneisses and on the southern border of the migmatitic paragneisses where the rocks of the Aston massif are invaded by tonalitic blastomylonites. These blastomylonites are probably associated with the intrusions of the Querigut massif to the east. The augengneisses are invaded by numerous dykes or lenses of leucocratic granites or fine-grained orthogneisses. Three of these, a few hundred metres wide and several kilometres long, reflect the preferential NE and NW structural trends: i.e. the NW-SE-trending Jasse des Galis composite intrusion, the NE-SW-trending Sirbal torrent composite intrusion and the NE-SW-trending Calviere valley composite intrusion. The genetic relationships between the granitic rocks of the intrusions and the leucocratic granitoids surrounding the augengneisses are as yet unknown. The particular structure observed in the Rieutort des Gascous valley, where leucocratic orthogneisses occurs below, above and around the augengneisses, can be interpreted as due either to granitic intrusions in the augengneisses, or to folds insolving the augengneisses and their leucocratic orthogneisses cover, or, which is more probable, to a combination of the two phenomena. The leucocratic granitoids surrounding the augengneisses probably has a diverse origin; part may have originated as differentiates or porphyritic grani- toids now transformed to augengneisses. Geochemical studies have not yet suceeded in elucidating the mutual relationships between these rocks. The complex consisting of Riete augengneisses and part of the surrounding granitoids appears to have been emplaced as a laccolith or a sheet of now-transformed porphyritic granites that have been folded and faulted since their formation. The thickness of the augengneisses decreases from the eastern end to the western end of the massif, whereas the thickness of the underlying orthogneisses increases. The northern border of the laccolith appears to be systematically faulted along its contact with the mica schists; this contact is marked by mylonites and granitic sheets, the Ax granite being the best developed. The roughly N-S contact between the ortho- and paragneisses and the micaschists at the western border appears to be systematically marked by mylonites and orthoamphibolites. Geochemical comparisons between the migmatitic pelitic or plagioclasic paragneisses of the Quioules Formation and the metashales or metagraywackes of the Cambrian micaschists exposed at the western border of the gneisses, confirm the differences noted in the fleld. These may be ascribed to different sedimentary regimes, i.e. before their métamorphism. It is suggested that the sheet or laccolith which gave rise to the Aston augengneisses was emplaced near the contact between Cambrian and Infra or Precambrian terranes during an orogenic phase at the beginnirig of the Caledonian cycle. Some remobilization may have possibly affected the rocks of this complex during the Ordovician and later during the Hercynian metamorphism.
Dernière mise à jour le 28.07.2015