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

Geochronological data, combined with field and petrological evidence, const rain the timing and rate of near-isothermal decompression at granulite faci es temperatures in rocks from the Lutzow-Holm Complex of East Antarctica. G ranulite facies gneisses from Rundvagshetta in Lutzow-Holm Bay experienced a peak metamorphic temperature of over 900 degrees C at c. 11 kbar, as evid enced by primary orthopyroxene-sillimanite-bearing assemblages, and seconda ry cordierite-sapphirine-bearing assemblages in metapelites. Peak metamorph ic assemblages show strong preferred mineral orientation, interpreted to ha ve developed synchronously with pervasive ductile deformation. Zircon from a syndeformational leucosome has a U-Pb age of 517 +/- 9 Ma, which is inter preted as a melt crystallization age. This age provides the best estimate o f the time of peak metamorphic conditions. The post-peak metamorphic histor y is characterized by near-isothermal decompression, recorded by mineral te xtures in a variety of rock compositions. Field and textural relations indi cate that decompression post-dated pervasive ductile deformation. K/Ar and Ar-40/Ar-39 ages from hornblende and biotite represent closure ages during cooling subsequent to decompression, and indicate cooling to temperatures b etween c. 350 and 300 degrees C by c. 500 Ma, thus placing a lower time lim it on the duration of the high-temperature isothermal decompression episode . The combination of the zircon age from a syndeformational melt with K/Ar and Ar-40/Ar-39 closure ages indicates that near-isothermal decompression f rom c. 11 to c. 4 kbar at granulite facies temperatures, followed by coolin g to c. 300 degrees C, took place within a time interval of 20 +/- 10 Myr. Simple one-dimensional models for exhumation-controlled cooling indicate th at 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.