Thermal History of UHT Metamorphism in the Napier Complex, East Antarctica: Insights from Zircon, Monazite, and Garnet Ages

Abstract

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.