Thermal history of UHT metamorphism in the Napier Complex, East Antarctica: insights from zircon, monazite and garnet ages

High-grade gneisses from Mt. Riiser-Larsen, East Antarctica, have been dated by whole-rock-mineral Sm-Nd and SHRIMP zircon and monazite U-Pb to help define the thermal history of ultrahigh temperature (UHT) metamorphism in the Napier Complex. Both the monazite and youngest zircon yield a range of apparent ages (∼2.51-2.47 Ga), consistent with crystallization during an extended period of metamorphism. Some zircon also preserves an isotopic record of earlier events, placing an upper limit of a few million years on the duration of peak metamorphic conditions. The similarity of the monazite and zircon U-Pb ages implies rapid initial postpeak, cooling to below the blocking temperature of these minerals (∼900°C). Consistently lower Sm-Nd whole-rock-mineral isochron ages (∼2.38 Ga) indicate that cooling slowed before the temperature reached ∼650°C. The history of the UHT metamorphism is interpreted to be (1) protracted high-temperature (≥800°C) conditions ∼2.51-2.47 Ga, (2) peak conditions (up to 1100°C) for at most a few million years, (3) rapid cooling (10°-60°C/m.yr.) immediately after peak metamorphism, and (4) very slow cooling (≤4°C/m.yr.) at midcrustal levels (∼30-km depth) to a steady state geotherm by 2.38 Ga