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. Reproduc ibility of the garnet data is outside analytical error, but this does not affect the consistency of ages obtained. The garnet ages are consi derably younger than the inferred initial crystallization age, based o n petrographical and fabric analysis, structural, and whole-rock and z ircon isotopic data which support initial growth during the Grenvillia n orogeny(similar to 1200-1100 Ma). The garnet ages, however, are indi stinguishable from a large number of Rb-Sr mineral ages. The possibili ty 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 p ossibility that the garnets crystallized during the Pan-African event cannot be discounted. In this case, the data imply that the Pan-Africa n orogeny attained lower granulite- to amphibolite-facies metamorphic conditions, as indicated by geothermometry and geobarometry calculatio ns involving garnet. In either case, the H.U. Sverdupfjella area was s ubjected to significant Pan-African heating to temperatures of the ord er of 600-650 degrees C.