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 Al 2 O 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...

Origin of the granulite enclaves in Indo-Sinian peraluminous granites, South China and its implication for crustal anatexis

“…This age is consistent with the UHT metamorphism reported from the Kontum massif in the Indochina block (Nakano et al, , 2009Nam et al, 2001;Osanai et al, 2001Osanai et al, , 2004Owada et al, 2006Owada et al, , 2007. For instance, electron microprobe (EMP) dating of monazite showed that the timing of the metamorphism was ca.…”

“…This event exerted a strong effect on both of the SCB and the Indochina block (Chen et al, 2011;Trung et al, 2006). In the Indochina block, the Kontum massif in central Vietnam experienced ultrahightemperature metamorphism in the late Permian (Nakano et al, , 2009Nam et al, 2001;Osanai et al, 2001Osanai et al, , 2004Owada et al, 2006Owada et al, , 2007. As a counterpart of the Kontum massif in the SCB (Chen et al, 2011), contemporaneous HT granulite-facies metamorphism occurred in the DSGB.…”

Recent progress in lower crustal process:

“…5; , Osanai et al, 2001b Grt Opx Cpx Ol Cpx Spl Opx Cpx Ol Grt Cpx Qz Omp Pgt Cpx Opx Pgt Amp Prg Hbl Bt , Tsunogae et al, 2000Tsunogae et al, , 2003aOsanai et al, 2016a Po Scp Schmetzer and Bank, 1983 Fig. 6 Table S1 , Ishizuka et al, 1998;Osanai et al, 1999aTsunogae and van Reenen, 2006 , Baba, 1998Baba, , 1999 , Tsunogae et al, 2011a 2 ;, Yoshimura et al, 2008;Kawasaki et al, 2011Baba et al, 2010Nakano et al, 2011Hokada et al, 2016, Sajeev et al, 2001Tsunogae and Santosh, 2003, Satish-Kumar et al, 2002Morimoto et al, 2004Koshimoto et al, 2004Shimizu et al, 2009Sajeev and Osanai, 2004a 3 ;, Osanai et al, 2001a;Nakano et al, 2004a , Osanai et al, 1998Osanai et al, , 2006a Table S1 , Hensen and Osanai, 1994;Das et al, 2001 , Sato et al, 2004;Kawasaki and Osanai, 2008 Motoyoshi, 2000;Osanai et al, 2004a;Sajeev and Osanai, 2004a Al Grt Cpx Grt Opx Spr Spl Fe-Mg Opx Cpx , Osanai et al, 1998;Tsunogae et al, 1999;Nakano et al, 2004a;Koshimoto et al, 2004;…”

Late Permian plume–related magmatism and tectonothermal events in the Kontum Massif, central Vietnam

“…1a and 1b). Both of these metamorphic rock types experienced a similar clockwise P-T evolution during the Late Permian and Early Triassic periods (270-240 Ma: Osanai et al, 2004;Nakano et al, 2004Nakano et al, , 2007aNakano et al, , 2007bNakano et al, , 2008Osanai et al, 2008;Nakano et al, 2009Nakano et al, , 2010. This led Osanai et al (2008) to propose the name 'Trans Vietnam Oro-genic Belt' (TVOB) for the region containing metamorphic and igneous rocks of this age ( Fig.…”

“…Ultrahigh-pressure (UHP) and/or ultrahigh-temperature (UHT) metamorphic rocks, together with syn-to post-collisional igneous rocks, are locally exposed in several collision zones in Asia (Okay et al, 1989;Ree et al, 1996;Zhang et al, 1996;Osanai et al, 2004;Oh et al, 2005;Osanai et al, 2008;Nakano et al, 2008Nakano et al, , 2010. Late Permian to Early Triassic continental growth in Asia can be accounted for by collage tectonics, exemplified by events such as the collisions between the South China and Indochina cratons, and the North China and South China cratons (Metcalfe, 1999;Ishiwatari and Tsujimori, 2003).…”