DECONVOLVING ULTRAHIGH TEMPERATURE METAMORPHISM USING LARGE ZIRCON GEOCHEMICAL DATASETS, AN EXAMPLE FROM THE NAPIER COMPLEX, EAST ANTARCTICA

There remain a number of outstanding questions in regard to the ultrahigh temperature (UHT) metamorphic record contained in the rocks of the Napier Complex in East Antarctica, these include. Were there one or two metamorphic events in the Napier Complex? What were the P–T conditions of the events? What were the heat sources for two separate UHT events? In order to further our understanding of the evolution of this system we undertook a laser ablation split stream petrochronological investigation of zircon from 20 samples across the Napier Complex. Zircon U-Pb geochronology, REE and Ti geochemistry provide evidence of two metamorphic events. The first, at 2.8 Ga, is a low-pressure, UHT event with peak pressures below the stability of garnet. The second event also occurred under UHT conditions but at higher pressure with garnet stable. Zircons from a number of metasedimentary rocks in the Napier Complex range in age up to 3.5 Ga. Most have T_DM Hf ages between 3.8 and 3.4 Ga, but some are as old as 4.2 Ga. The observed large vertical arrays in eHf space at 2.8 and 2.5 Ga are consistent with two scenarios. The vertical arrays are both related to juvenile/mantle inputs coincident with metamorphism, something that is not obvious in the field relationships and is absent in most UHT terrains in general. Thus the eHf is points to the heat source – mantle magmatism. A second interpretation of the dataset is that the vertical array in eHf in the 2.5 Ga zircon is a mixture between totally reset juvenile 2.8 Ga zircons and the Hf compositions from the older crustal components. The new dataset points to a 2.8 Ga event that was a magma-driven low-pressure event, whereas the higher pressure 2.5 Ga event is a result of thickening; because the orogen had been pre-conditioned by a previous UHT event it didn’t take as much thermal energy to get back up to 1000 °C for a second time.