Timing and rate of isothermal decompression in Pan-African granulites from Rundvagshetta, East Antarctica

Geochronological data, combined with field and petrological evidence, constrain the timing and rate of near-isothermal decompression at granulite facies temperatures in rocks from the Lutzow-Holm Complex of East Antarctica. Granulite facies gneisses from Rundvagshetta in Lutzow-Holm Bay experienced a peak metamorphic temperature of over 900 °C at c. 11 kbar, as evidenced by primary orthopyroxene-sillimanite-bearing assemblages, and secondary cordierite-sapphirine-bearing assemblages in metapelites. Peak metamorphic assemblages show strong preferred mineral orientation, interpreted to have developed synchronously with pervasive ductile deformation. Zircon from a syndeformational leucosome has a U-Pb age of 517±9 Ma, which is interpreted as a melt crystallization age. This age provides the best estimate of the time of peak metamorphic conditions. The post-peak metamorphic history is characterized by near-isothermal decompression, recorded by mineral textures in a variety of rock compositions. Field and textural relations indicate that decompression post-dated pervasive ductile deformation. K/Ar and ⁴⁰Ar/³⁹Ar ages from hornblende and biotite represent closure ages during cooling subsequent to decompression, and indicate cooling to temperatures between c. 350 and 300 °C by c. 500 Ma, thus placing a lower time limit on the duration of the high-temperature isothermal decompression episode. The combination of the zircon age from a syndeformational melt with K/Ar and ⁴⁰Ar/³⁹Ar closure ages indicates that near-isothermal decompression from c. 11 to c. 4 kbar at granulite facies temperatures, followed by cooling to c. 300°C, took place within a time interval of 20±10 Myr. Simple one-dimensional models for exhumation-controlled cooling indicate that these data require exhumation rates of the order of c. 3 km Myr⁻¹ for several million years, then cessation of exhumation followed by relatively isobaric cooling during thermal re-equilibration.