P-T-T PATHS AND POST-METAMORPHIC EXHUMATION OF THE DAY-NUI-CON-VOI SHEAR ZONE IN VIETNAM

The Day Nui Con Voi belt in Vietnam is the southeasternmost part of th e Red River shear zone in Asia. It is a narrow high-grade metamorphic core complex consisting of garnwet-sillimanite-biotite gneisses, mylon ite bands, amphibolite layers and migmatites. Geothermobarometric stud y of the complex revealed that the peak metamorphism took place under amphibolite-facies conditions of 690(-60)(+30)degrees C and 0.65 +/- 0 .15 GPa and the subsequent mylonitization occurred under greenschist-f acies conditions of similar to 480 degrees C and under 0.3 Gpa. Fiftee n synkinematic hornblende and biotite separates from gneisses, amphibo lites and mylonites were dated with the K/Ar method. Hornbiende separa tes from the Day Nui Con Voi give K-Ar ages of 26.4-28.5 Ma, and the b iotite separates do give 24.5-24.7Ma. Combination of thermobarometric and geochronological data yields the cooling history of 500 degrees C at 28 Ma and 300 degrees C at 24 Ma with a cooling rate of 70-110 degr ees C Ma(-1), and 23 km post-metamorphic exhumation of the core comple x. The first 16 km exhumation from the peak of metamorphism (at probab ly 31 Ma) to 28 Ma was triggered by the left-lateral strike-slip displ acement of the Red River shear zone. (C) 1998 Elsevier Science B.V. Al l rights reserved.