Crustal residence history and garnet Sm–Nd ages of high-grade metamorphic rocks from the Windmill Islands area, East Antarctica

Abstract

Nd whole-rock data from the Windmill Islands area yield early Proterozoic to middle Archaean Nd model ages. These crustal residence times are consistent with regional correlations with other parts of Antarctica (Bunger Hills, Denman Glacier area) and the Albany-Fraser Orogen of south-western Australia during the Mid-Proterozoic and thus support reconstructions with a continuous Mid-Proterozoic orogen in these areas. The new Nd isotope data provide strong evidence that no age boundary exists between the higher- and lower-grade parts of the Windmill Islands area, and that the metamorphic complex represents a single terrane with a common crustal history. The data support the notion of a time-link between the occurrence of intrusive charnockites (C-type magmas) and high-grade metamorphism. The magmatic rocks and orthogneisses in the area are interpreted to have a mixed source consisting of older crustal components, i.e. older sediments (ca. 3.2–2.6 Ga) and a younger mafic component (ca. 1.9 Ga). Two garnet Sm–Nd isochrons yield ages of 1156±17 Ma and 1137±2.5 Ma and are identical to SHRIMP U–Pb results on monazite from these samples. A garnet Sm–Nd age of 1123±13 Ma for the Ford granite is significantly younger than the SHRIMP U–Pb zircon age for this sample. The difference relates to the different closure temperature of each isotopic system and is thus interpreted as initial cooling after granulite facies metamorphism.