Isotopic constraints on Pan-African metamorphism in Dronning Maud Land, Antarctica

Garnet Sm-Nd isotopic data are presented for selected samples from the metamorphic terrane of the H.U. Sverdrupfjella, western Dronning Maud Land, and yield Pan-African ages ranging from 680 to 415 Ma. Reproducibility of the garnet data is outside analytical error, but this does not affect the consistency of ages obtained. The garnet ages are considerably younger than the inferred initial crystallization age, based on petrographical and fabric analysis, structural, and whole-rock and zircon isotopic data which support initial growth during the Grenvillian orogeny(similar to 1200-1100 Ma). The garnet ages, however, are indistinguishable from a large number of Rb-Sr mineral ages. The possibility that the garnets remained above the blocking temperature until Pan-African exhumation is discounted. Polymetamorphism of the metamorphic terrane associated with both the Grenvillian and Pan-African orogenies is favoured. The garnet ages are interpreted as reset ages, but the possibility that the garnets crystallized during the Pan-African event cannot be discounted. In this case, the data imply that the Pan-African orogeny attained lower granulite- to amphibolite-facies metamorphic conditions, as indicated by geothermometry and geobarometry calculations involving garnet. In either case, the H.U. Sverdupfjella area was subjected to significant Pan-African heating to temperatures of the order of 600-650 degrees C.