Timing and rate of isothermal decompression in Pan‐African granulites from Rundvågshetta, East Antarctica

• Published in: Journal of Metamorphic Geology
• Main Authors: Fraser, G., McDougall, I., Ellis, D. J., Williams, I. S.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2000
• Subjects: East Antarctica; Lützow-Holm Bay; Antarc*; Antarctica
• Online Access: http://dx.doi.org/10.1046/j.1525-1314.2000.00270.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1046%2Fj.1525-1314.2000.00270.x; https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1525-1314.2000.00270.x

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 Lützow‐Holm Complex of East Antarctica. Granulite facies gneisses from Rundvågshetta in Lützow‐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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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.