P-T-t constraints on the metamorphic history of the Lower Gneiss Unit. The P-T-t paths of orthogneisses and paragneisses of the LGU indicate first an increase in temperature followed by isobaric cooling. This is particularly well illustrated by the P-T-t path of the south Velay which is characterized by a HT/LP gradient. The PT path is depicted by the white arrows and numbers correspond to radiometric ages.