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Journal of Petrology Advance Access published online on August 19, 2004
Journal of Petrology, doi:10.1093/petrology/egh041
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Received February 4, 2003
Accepted April 14, 2004

Article

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

PHILIPPE GONCALVES 1*, CHRISTIAN NICOLLET 1, JEAN-MARC MONTEL 2

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

* To whom correspondence should be addressed. E-mail: philippe{at}geo.umass.edu.


  Abstract

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.

Keywords: Andriamena unit; Madagascar, ultrahigh-temperature metamorphism; electron microprobe dating of monazite; polymetamorphism; P-T-t path.
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