Abstract
Si content in Cpx is considered as an important indicator of UHP metamorphic condition. Typical UHP a mineral such as Coe or micro-Dim coexists with supersilicic Cpx within Zrn grains. Cpx in the matrix also forms Qtz-exsolution lamellae during retrograde metamorphism. Then it is suggested that formation of Qtz-lamellae in Cpx would be suggesting UHP conditions (e.g. Liou et al., 1998). During the detailed observation, we found the Grt-Opx-Cpx granulite from the Kontum massif, which contains Cpx with Qtz-exsolution lamellae.
The granulite is normally found as blocks or lenses enclosed within mylonitic felsic gneisses. These granulite blocks are considered to be originally high-temperature eclogites. However, various retrograde minerals have also been formed during isothermal decompression through the following reactions;
Grt+ Si, Na-rich Cpx1 (CaEs10-13) + Qtz (UHT-UHP condition)
Grt+Cpx1+ Qtz= Al-poor Opx1+ Na-rich Pl1+ Si, Na-poor Cpx2 (UHT-HP condition)
Grt= Al-rich Opx2+ Na-poor Pl2+ Spl (HT/ HP condition)
Cpx1= Qtz+ Na-rich Cpx3
Cpx1= Na-rich Pl3+ Cpx4
Cpx2= Opx3+ Hbl+ Pl4+ Cpx5
We assume that the peak metamorphic condition of these granulites would have been up to UHT-UHP condition due to the presence of supersilicic Cpx and estimated P-T condition. After the peak metamorphism, a near isothermal decompression had taken place with forming the Grt breakdown assemblage.
