U-Pb chronology on accessory minerals of the Velay anatectic dome (French Massif Central)

The thermal history of the Velay anatectic dome, which extends over 10000 km(2) in the southern part of the French Massif Central, is still poorly constrained, especially as to its duration. We studied by U-Pb chronology the accessory minerals (zircon, monazite, apatite) of three samples collected on the southern edge, which represent the main types recognized in the dome and the best candidates to constrain the age of the two migmatization stages (M3 and M4) already described. The M3 stage is constrained at 314+/-5 Ma by the U-Pb data on abraded monazites from the leucosome part of a paragneiss. The M4 stage has been dated with monazites from the cordierite-bearing Velay granite at 301+/-5 Ma. Monazites from the paragneiss and the Velay granite even after an abrasion process are still discordant in the concordia diagram, and monazites from the third sample, an orthogneiss linked to the M3 stage, reveal an unusual pattern with a reverse discordia. Zircons in each case demonstrate inheritance, presenting any precise age determination. Even in the case of the Velay granite where zircon fractions define a normal discordia pattern, single grain analysis reveal the presence of inherited lead. The apatites in all three samples indicate an age at 289+/-5 Ma, which is interpreted as a cooling age during the uplift of the Velay region.