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

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 super-position of two distinct metamorphic events at 2·5 Ga and ∼750 Ma. The late Archaean event corresponds to an ultra-high-temperature metamorphism (1100° C, 10·5 kbar) characterized by a sapphirine-garnet-ortbopyroxene-quartz assemblage. Neoproterozoiv 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. © Oxford University Press 2004; all rights reserved.