First SHRIMP U-Pb zircon dating of granulites from the Kontum massif (Vietnam) and tectonothermal implications

The Kontum massif in Central Vietnam represents the largest continuous expo sure of crystalline basement of the Indochina craton. The central Kontum ma ssif is chiefly made of orthopyroxene granulites (enderbite, charnockite) a nd associated rocks of the Kannack complex. Mineral assemblages and geother mobarometric studies have shown that the Kannack complex has severely metam orphosed under granulite facies corresponding to P-T conditions of 800-850 degreesC and 8 +/- 1 kbars. Twenty-three SHRIMP II U-Pb analyses of eightee n zircon grains separated from a granulite sample of the Kannack complex yi eld ca 254 Ma, and one analysis gives cn 1400 Ma concordant age for a zoned zircon core. This result shows that granulites of the Kannack complex in t he Kontum massif have formed from a high-grade granulite facies tectonother mal event of Indosinian age (Triassic). The cooling history and subsequent exhumation of the Kannack complex during Indosinian times ranged from simil ar to 850 degreesC at ca 254 Ma to similar to 300 degreesC at 242 Ma, with an average cooling rate of similar to 45 degreesC/Ma. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.