Ab. Moyes et Pb. Groenewald, ISOTOPIC CONSTRAINTS ON PAN-AFRICAN METAMORPHISM IN DRONNING-MAUD-LAND, ANTARCTICA, Chemical geology, 129(3-4), 1996, pp. 247-256
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.