U-Th-Pb dating of the Brossasco ultrahigh-pressure metagranite, Dora-Mairamassif, western Alps

The Brossasco metagranite is part of the coherent ultrahigh-pressure metamo rphic terrane in the Dora-Maira massif and, although it has reached Alpine peak metamorphic conditions of about 725 degrees C/30 kbar, still locally p reserves an undeformed magmatic texture. In order to obtain new chronologic al constraints on this key area of the western Alps, we studied the behavio ur of the U-Th-Pb system in zircons and monazites from such an undeformed f acies. Only zircon fractions containing very small needle-shaped inclusions are concordant at 304 +/- 3 Ma. This result is interpreted as the emplacem ent age of the granite. All other zircons show an old inherited memory and the more U-rich of these crystals were affected by an episodic Pb loss with a poorly defined lower intercept at 60 +/- 45 Ma. Both TIMS (thermo-ionisa tion mass spectrometry) and electron-microprobe analysis of monazites show that this last event has also disturbed their U-Pb and Th-Pb isotopic syste ms, without resetting them completely. Comparison with cofacial rock-types which have been completely reworked under these extreme P-T conditions show s that deformation and mass transfer as well as the nature of the protolith s and the characteristics of their zircons have a much more pronounced infl uence on the behaviour of isotopic systems than the temperature and/or pres sure parameters. This evidence is an obvious limitation to the use of the b locking-temperature concept.