A case study of a mantled gneiss antiform, the Hospitalet massif, Pyrenees (Andorra, France)

Abstract

The Hospitalet mantled gneiss antiform consists of monzo-granitic orthogneisses, which are overlain by a predominantly pelitic, at least 2.5 km thick succession of Cambro-Ordovician metasediments. During the Hercynian orogeny the rocks were polyphase deformed and plurifacially metamorphosed. Three stages in its tectonic evolution can be distinguished. Pre-antiform episode NE-SW trending (Dl) folds were formed and overprinted by upright EW trending (D2) folds and foliations, defining the suprastructure around the massif. Deformation occurred at low grade conditions and NS shortening is indicated, probably accompanied by thickening of the metasedimentary pile. Gneiss antiform formation Within the massif the D2 structures are overprinted by flat lying to gently inclined (D3) folds and foliations, defining the infrastructure. The geometry of S3 relative to the gneiss-cover contact limits antiform formation to vertical tectonics at the scale of the massif, accommodated by horizontal extension at the same scale. Subsequently, a low piT ratio metamorphism (650 C/km) took place in the infrastructure, associated with extensive, zonal growth of biotite, andalusite, cordierite, staurolite and sillimanite in highest grade. Structural and tectono-metamorphic considerations suggest that extension was largely accommodated by an extensional shear zone or shear zones of which the infrastructure represents the upper part. Post-antiform episode During isobaric cooling in lower amphibolite facies conditions the infrastructure has been deformed in (1) a 1 km wide zone of mylonitic strain at the gneiss-cover contacts, (2) a 1.5 km wide zone of (D4/5) refolding SW of the gneisses, (3a) a km scale recumbent fold in the eastern hinge zone of the gneiss antiform and (3b) small-scale recumbent (D6) folds throughout the massif. The antiformal gneiss core acted as a relatively rigid body and the metasediments and mineral zones were draped around it, first in NNE-SSW stretching, afterwards in WNW-ESE stretching, causing (a) seemingly high thermal gradients and (b) the antiformal pattern of the isograds. The three stages in the evolution of the Hospitalet mantled gneiss antiform are considered to represent early (Westphalian), middle (Westphalian- Stephanian) and late (Stephanian-early Permian) Hercynian tectonics. The structures mentioned have been cut by high angle faults, shear zones and folds, which are accompanied by low greenschist facies metamorphic conditions and which probably reflect Alpine movements.