Decompression process of mafic granulite from eclogite to granulite facies under ultrahigh-temperature condition in the Kontum Massif, Central Vietnam

Garnet-bearing mafic granulites from the Ngoc Linh Complex of the Kontum massif in central Vietnam occur as lenses or blocks within felsic gneisses. Some of these granulite blocks show some evidences for ultrahigh-temperature and high-pressure metamorphism. Samples studied here consist mainly of garnet (Alm34-48, Prp27-34, Grs18-24, Sps0-3), clinopyroxene (Mg# = 0.73-0.87) and quartz, which are relatively large grains. The garnet and clinopyroxene are surrounded by symplectites composed of orthopyroxene, plagioclase, magnetite and sometimes spinel. Therefore, the presumed equilibrium peak-pressure assemblage could be of garnet, clinopyroxene and quartz, which was stable at eclogite-facies conditions. Dual-stage generated symplectites involving orthopyroxene and plagioclase were identified among garnet, clinopyroxene and quartz. The first-generated symplectite is composed of Al- and Mg-poor orthopyroxene (Mg# = 0.61-0.68, Al ₂O ₃ = 1.5-2.6 wt%) and Na-rich plagioclase (An45-53). The second symplectite is composed of Al- and Mg-rich orthopyroxene (Mg# = 0.69-0.72, Al ₂O ₃ = 4.1-5.3 wt%), Na-poor plagioclase (An86-93), spinel (Mg# = 0.42-0.48) and magnetite. The compositional differences between the two symplectites can be interpreted to be due to the difference in metamorphic reaction stages. The first symplectite had been formed by the reaction between garnet, clinopyroxene and quartz, which occurred during the decompression process just after peak-pressure conditions. The second symplectite could be produced during the late breakdown of garnet. Temperature and pressure conditions around peak-pressure conditions are estimated from the garnet-clinopyroxene thermometries and from the reaction of jadeite + quartz = albite, respectively. The results indicated peak-pressure conditions as ca. 950°C and 1.6 GPa. The pressure-temperature conditions for the formations of first- and second-symplectites were 1000°C at 1.3 GPa and 850°C at 0.8 GPa, respectively. Whole mineral assemblages of the mafic granulites show that the metamorphic evolution took place under completely dry conditions. The textual observations as well as the pressure-temperature estimations indicate a near isothermal decompression process for this granulite. This evolution path is consistent with that of ultrahigh-temperature pelitic granulites from the Kannak Complex. Thus, the possibility of simultaneous ultrahigh-temperature metamorphic event in the Kontum massif can be argued.