Journal of Petrology Advance Access originally published online on August 19, 2004
Journal of Petrology 2004 45(10):1923-1957; doi:10.1093/petrology/egh041
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Journal of Petrology 45(10) © Oxford University Press 2004; all rights reserved
Petrology and in situ U–Th–Pb Monazite Geochronology of Ultrahigh-Temperature Metamorphism from the Andriamena Mafic Unit, North–Central Madagascar. Significance of a Petrographical P–T Path in a Polymetamorphic Context
1 LABORATOIRE MAGMAS ET VOLCANS, UNIVERSITÉ BLAISE PASCAL–CNRS, 5, RUE KESSLER, 63038 CLERMONT-FERRAND CEDEX, FRANCE
2 LMTG, UNIVERSITÉ PAUL SABATIER–CNRS, 14, AVENUE EDOUARD BELIN, 31400
* Corresponding author. Present address: Department of Geosciences, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003-9297, USA. Telephone: 413-545-0745. Fax: 413-545-1200. E-mail: philippe{at}geo.umass.edu
Petrological studies and electron microprobe dating of monazite from the mafic Andriamena unit, north–central Madagascar, indicate that an apparently continuous P–T path inferred for Mg-granulites is actually discontinuous, resulting from the superposition of two distinct metamorphic events at 2·5 Ga and 
750 Ma. The late Archaean event corresponds to an ultrahigh-temperature metamorphism (1000°C, 10·5 kbar) characterized by a sapphirine–garnet–orthopyroxene–quartz assemblage. Neoproterozoic ages are associated with the development of a sapphirine–cordierite-bearing assemblage, symplectites of orthopyroxene–sillimanite and partial melting at 850°C and 7 kbar. This sequence of reactions and mineral assemblages could be interpreted as the result of near-isothermal decompression to about 4 kbar followed by isobaric cooling to 650°C. However, geodynamic constraints suggest that the granulites underwent a phase of cooling to the stable geotherm following the ultrahigh-temperature metamorphism at 2·5 Ga. Consequently, we suggest that the ‘petrographical path’ inferred from the Mg-granulites is not representative of the actual P–T–t path. The decompression, in particular, is an artefact of the P–T path with no geological meaning; it results from the equilibration of the refractory late Archaean ultrahigh-temperature assemblages at a lower pressure during the middle Neoproterozoic event.
KEY WORDS: Andriamena unit; Madagascar, ultrahigh-temperature metamorphism; electron microprobe dating of monazite; polymetamorphism; P–T–t path
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