Abstract:

Up to now, both the basement and the cover sequences of the Larsemann Hills in East Antarctica have been considered involved in the high-grade deformation and metamorphism of the Pan-African event, which peaked at 550—530 Ma and was responsible for the widespread granitic magmatism at 525—500 Ma. The late Neoproterozoic-Paleozoic (Pan-African) orogenesis was thus constrained to the period of 550—500 Ma. Our studies demonstrated that the zircons of the garnet-biotite-plagioclase gneiss were severely disturbed and reset in the Pan-African high-grade metamorphism. The Pan-African metamorphism and granites can obliterate and reset the older magmatic or metamorphic zircons, and the zircons produce complex inner textures; thus, the true age of metamorphism is hard to obtain. However, the actual metamorphic rims are seldom present and the reset zircons give an average age of 522.7±6.6 Ma. Granite postdating the peak metamorphism places the crystallizing time at 545 ± 9 Ma, while some younger granites suggest a date of c.500 Ma. Peak metamorphism should be older than the oldest granite of the Pan-African event. The Pan-African metamorphism and associated granites seem to have been derived from a special intracontinental reactivation mechanism of older continental materials, not from intercontinental subduction-collision as conventionally considered.

Key words: Pan-African, zircon age, metamorphism time, granite, Larsemann Hills