Crust-formaticn processes in the Kontum Massif, Vietnam

点击翻译按钮获取中文摘要

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 calcsilicate rocks are also intercalated within the gneisses. 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 ℃<). On the other hand, … More 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. 1.2 GPa and ca. 1050 ℃ (M1 metamorphism). High-pressure M0 metamorphism (ca. 1.7 GPa<, ca. 1000 ℃) as part of the prograde metamorphism and low-pressure (but still ultrahigh-temperature) M2 metamorphism (0.9-1.0 GPa,ca. 1000 ℃) 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) during the continents collision in eastern Asia. The newly determined Sm-Nd internal isochron and SHRIMP ages from the Kannak and Ngoc Linh Complexes would also indicate that the Kontum massif had also undergone the Pan African metamorphic event (ca. 2.5,1.0 and 0.5 Ga). The present results of the metamorphic evolution of the Kontum massif including 41 journal papers indicate a strong constraint to consider the coninental evolution process in Eastern Asia. Less