Lead inheritance phenomena related to zircon grain size in the Variscan anatectic granite of Ax-les-Thermes (Pyrenees, France)

U-Pb geochronology on zircon fractions and by the direct evaporation single-zircon-grain method for this leucogranite intrusive into gneisses in the axial zone of the Pyrenees gave two arrays, depending on the size of the zircon grains: a reverse discordia for the coarse zircons and a normal discordia for fine- grained ones. The ages obtained from these discordias (350 and 370 m.y.) are, however, inconsistent with previous Rb-Sr dating at 300 m.y. and with field evidence for a syn- to late-Variscan intrusion (approx 300 m.y.). This exceptional pattern may be attributable to a combination of different processes, including the coexistence of relict and newly crystallized zircon and the inheritance of old radiogenic Pb and various stages of Pb loss.