HR: 0800h
AN: T11C-1273    [Abstracts]
TI: Metamorphic Evolution In The Northern Kontum Massif, Central Vietnam
AU: * Usuki, T
EM: usu@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, PO Box 1-55, Nankang, Taipei, 115 Taiwan
AU: Lan, C
EM: kyanite@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, PO Box 1-55, Nankang, Taipei, 115 Taiwan
AU: Tran, T
EM: phuongphg@hn.vnn.vn
AF: Department of Magmatism, Institute of Geological Sciences, National Center for Natural Sciences and Technology, Hanoi, 100 Viet Nam
AU: Yeh, M
EM: porphyloblast@yam.com
AF: Department of Earth Sciences, National Taiwan Normal University, No.162, HoPing E. Road, Sec. 1, Taipei, 106 Taiwan
AU: Iizuka, Y
EM: yiizuka@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, PO Box 1-55, Nankang, Taipei, 115 Taiwan
AB: Relict kyanite-bering rocks were found from the sillimanite zone of Kham Duc formation in the northern Kontum massif, central Vietnam. Kyanites occur as partially replaced by muscovite, margarite and quartz in garnet-muscovite (Grt-Mus) schist and included or surrounded by plagioclase in garnet-biotite-sillimanite (Grt-Bt-Sil) gneisses. Three metamorphic stages (M1-M3) are distinguished based on mineral paragenesis and compositions of these rocks. M1 stage is characterized by the presence of kyanite, high grossular (Xgrs = 0.21) and low pyrope (Xprp = 0.04) garnet and low anorthite content (Xan = 0.02-0.04) plagioclase. M2 stage is characterized by low grossular (Xgrs = 0.08) and high pyrope (Xprp = 0.14-0.18) garnet and high anorthite content (Xan = 0.36-0.45) plagioclase and appearance of sillimanite in gneisses. M3 stage is recorded in the rim of the garnet porphyroblast and resorbed garnet with coronal plagioclase. Pressure-temperature (P-T) conditions of M1-M3 stages calculated using geothermobarometers are M1: 11.8 kbar ($\sim$42 km) and 464 $\deg$C; M2: 6.8-7.0 kbar ($\sim$25 km) and 711-722 $\deg$C; M3: 4.0-4.6 kbar ($\sim$16 km) and 523-596$\deg$C. The P-T path of M1 to M2 indicates large temperature increase ($\sim$250$\deg$C) and pressure decrease (4.8 kbar). Although age and tectonic study are waited for detail studies, the preliminary result of 17 km exhumation with temperature increase may suggest significant lithospheric extension accompanying asthenospheric upwelling.
DE: 8109 Continental tectonics--extensional (0905)
DE: 3660 Metamorphic petrology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting