Original paper

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

Paquette, Jean-Louis; Montel, Jean-Marc; Chopin, Christian

European Journal of Mineralogy Volume 11 Number 1 (1999), p. 69 - 78

40 references

published: Feb 11, 1999
manuscript accepted: Aug 19, 2019
manuscript received: Jun 2, 2099

DOI: 10.1127/ejm/11/1/0069

BibTeX file

ArtNo. ESP147051101001, Price: 29.00 €

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Abstract The Brossasco metagranite is part of the coherent ultrahigh-pressure metamorphic terrane in the Dora-Maira massif and, although it has reached Alpine peak metamorphic conditions of about 725 °C/30 kbar, still locally preserves an undeformed magmatic texture. In order to obtain new chronological constraints on this key area of the western Alps, we studied the behaviour of the U-Th-Pb system in zircons and monazites from such an undeformed fades. Only zircon fractions containing very small needle-shaped inclusions are concordant at 304 ± 3 Ma. This result is interpreted as the emplacement 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-ionisation mass spectrometry) and electron-microprobe analysis of monazites show that this last event has also disturbed their U-Pb and Th-Pb isotopic systems, without resetting them completely. Comparison with cofacial rock-types which have been completely reworked under these extreme P-T conditions shows that deformation and mass transfer as well as the nature of the protoliths and the characteristics of their zircons have a much more pronounced influence on the behaviour of isotopic systems than the temperature and/or pressure parameters. This evidence is an obvious limitation to the use of the blocking-temperature concept


metagraniteDora-Maira massifultrahigh-pressure metamorphismU-Pb zirconmonaziteelectron- microprobe dating