Permo-triassic ultrahigh-temperature metamorphism in the Kontum Massif, Central Vietnam

Yasuhito Osanai, Nobuhiko Nakano, Masaaki Owada, Tran Ngoc Nam, Tsuyoshi Toyoshima, Toshiaki Tsunogae, Pham Binh

Research output: Contribution to journalArticle

48 Citations (Scopus)

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-Al 2O 3 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.

Original languageEnglish
Pages (from-to)225-241
Number of pages17
JournalJournal of Mineralogical and Petrological Sciences
Volume99
Issue number4
DOIs
Publication statusPublished - Dec 1 2004

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ultrahigh temperature metamorphism
Vietnam
garnets
garnet
Triassic
orthopyroxene
metamorphic rocks
metamorphic rock
gneiss
temperature
cordierite
clinopyroxene
metamorphism
grade
schist
sillimanite
decompression
granulite facies
amphibolite
pressure reduction

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geology

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Permo-triassic ultrahigh-temperature metamorphism in the Kontum Massif, Central Vietnam. / Osanai, Yasuhito; Nakano, Nobuhiko; Owada, Masaaki; Nam, Tran Ngoc; Toyoshima, Tsuyoshi; Tsunogae, Toshiaki; Binh, Pham.

In: Journal of Mineralogical and Petrological Sciences, Vol. 99, No. 4, 01.12.2004, p. 225-241.

Research output: Contribution to journalArticle

Osanai, Yasuhito ; Nakano, Nobuhiko ; Owada, Masaaki ; Nam, Tran Ngoc ; Toyoshima, Tsuyoshi ; Tsunogae, Toshiaki ; Binh, Pham. / Permo-triassic ultrahigh-temperature metamorphism in the Kontum Massif, Central Vietnam. In: Journal of Mineralogical and Petrological Sciences. 2004 ; Vol. 99, No. 4. pp. 225-241.
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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-Al 2O 3 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.",
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T1 - Permo-triassic ultrahigh-temperature metamorphism in the Kontum Massif, Central Vietnam

AU - Osanai, Yasuhito

AU - Nakano, Nobuhiko

AU - Owada, Masaaki

AU - Nam, Tran Ngoc

AU - Toyoshima, Tsuyoshi

AU - Tsunogae, Toshiaki

AU - Binh, Pham

PY - 2004/12/1

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N2 - 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-Al 2O 3 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.

AB - 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-Al 2O 3 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.

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