Permo-Triassic ultrahigh-temperature metamorphism in the Kontum massif, central Vietnam

Abstract

The Kontum massif in central Vietnam consists of low-grade schists and amphibolite- to granulite-facies metamorphic rocks, that have been intruded by S-type and I-type granites. This terrane was formerly considered to be composed mainly of Archean granulites (the Kannak Complex), Proterozoic amphibolite-facies metamorphic rocks (the Ngoc Linh Complex) and low-grade schists (the Kham Duc Complex). They were thought to be the basement of the Indochina Craton in south-east Asia.
    The Kannak Complex is dominated by pelitic-semipelitic gneisses metamorphosed under high- to ultrahigh-temperature (UHT) conditions into granulite-facies. A minor amount of mafic and calc-silicate rocks are also intercalated within the gneisses. The major types of ultrahigh-temperature pelitic metamorphic rocks in this complex are garnet-orthopyroxene-sillimanite-cordierite gneiss, orthopyroxene-bearing garnet-cordierite-silliamnite-biotite gneiss and garnet-orthopyroxene charnockitic gneiss. The highest-grade metamorphic condition is determined from garnet-orthopyroxene-sillimanite-cordierite gneiss, which indicates that multi-stage symplectite formation during retrograde stage started from isothermal decompression in UHT condition (1000°C<). Because of the high amount of pyrope (up to 59 mole%) in garnet and high-Al₂O₃ in orthopyroxene (up to 10 wt%) , these minerals were unstable in the P-T conditions during the retrograde stage.
    On the other hand, the newly found garnet-clinopyroxene-orthopyroxene granulites (eclogitic ultrahigh-temperature mafic granulite) from the Ngoc Linh Complex shows a series of changes in divariant assemblages from garnet-clinopyroxene-quartz to hornblende-quartz through clinopyroxene-orthopyroxene-plagioclase-(garnet). It was identified that these rocks were formed as a result of their metamorphic evolution of isothermal decompression followed by nearly isobaric cooling.
    The ultrahigh-temperature metamorphic rocks in the Kontum massif, which are exposed along the Dac To Kan shear zone, show a clockwise pressure-temperature path with the peak metamorphic condition of ca. 12 kbar and ca. 1050°C (M1 metamorphism). High-pressure M0 metamorphism (ca. 17 kbar<, ca. 1000°C) as part of the prograde metamorphism and low-pressure (but still ultrahigh-temperature) M2 metamorphism (9-10 kbar, ca. 1000°C) as part of the retrograde metamorphism during the clockwise pressure-temperature evolution are also recognized. The widely reported Permo-Triassic metamorphic event (ca. 240-260 Ma) from the ultrahigh-temperature metamorphic rocks would indicate a rapid metamorphic evolution from M0 stage to low-pressure and low-temperature retrograde stage (later M2 stage). The recently determined Sm-Nd internal isochron age of 670 Ma for the garnet amphibolite and monazite-CHIME age of ca. 480-500 Ma for the pelitic granulites from the Kannak Complex would also indicate that the Kontum massif had also undergone the Pan-African metamorphic event. The present results indicate that ultrahigh-temperature metamorphic rocks from the Kontum massif are remnants of previously metamorphosed rocks that might be derived from the Gondwana super continent and then re-metamorphosed by the Permo-Triassic metamorphism during the continents collision in eastern Asia.