Triassic eclogite from northern Vietnam: inferences and geological significance

Abstract

The first finding of low-temperature eclogites from the Indochina region is reported. The eclogites occur along the Song Ma Suture zone in northern Vietnam, which is widely regarded as the boundary between the South China and Indochina cratons. The major lithology of the area is pelitic schist that contains garnet and phengite with or without biotite, chloritoid, staurolite and kyanite, and which encloses blocks and lenses of eclogite and amphibolite. The eclogites commonly consist of garnet, omphacite, phengite, rutile, quartz and ⁄ or epidote with secondary barroisite. Omphacite is commonly surrounded by a symplectite of Na-poor omphacite and Na-rich plagioclase. In highly retrograded domains, diopside + tremolite + plagioclase symplectites replace the primary phases. Estimated peakpressure metamorphic conditions based on isochemical phase diagrams for the eclogites are 2.1-2.2 GPa and 600-620°C, even though thermobarometric results yield higher pressure and temperature conditions (2.6-2.8 GPa and 620-680°C). The eclogites underwent a clockwise P-T trajectory with a post-peak-pressure increase of temperature to a maximum of >750°C at 1.7 GPa and a subsequent cooling during decompression to 650°C and 1.3 GPa, which was followed by additional cooling before close-to-isothermal decompression to 530°C at 0.5 GPa. The surrounding pelitic schist (garnetchloritoid-phengite) records similar metamorphic conditions (580-600°C at 1.9-2.3 GPa) and a monazite chemical age of 243 ± 4 Ma. A few monazite inclusions within garnet and the cores of some zoned monazite in garnet-phengite schist record an older thermal event (424 ± 15 Ma). The present results indicate that the Indochina craton was deeply (>70 km) subducted beneath the South China craton in the Triassic. The Silurian cores of monazite grains may relate to an older non-collisional event in the Indochina craton.

Petrogenesis and zircon U–Pb ages of the Thien Ke granitic pluton in the Tam Dao region: Implications for early Paleozoic tectonic evolution in NE Vietnam

“…The magmatic rocks are mainly distributed in the central and NE parts of the study area and were formally divided into the Pia Oac (Da Lien and Thien Ke plutons) and Nui Dieng (Nui Dieng and Nui Phao plutons) complexes, which were considered to be products of the Yanshanian and Indosinian tectonothermal events, respectively. In northern Vietnam, evidence of Mesozoic magmatism was reported by many geologists (e.g., Anh et al, 2002;Lepvrier et al, 2008;Nakano et al, 2008Nakano et al, , 2010Hieu et al, 2013;Thuy and Chung 2013), but Caledonian igneous activity has not yet been recognized. In recent years, some Caledonian granites were reported in the Song Chay area, NE Vietnam (e.g., Roger et al, 2000;Yan et al, 2006), in NW Vietnam (Nguyen et al, 2005) and in central Vietnam (Nagy et al, 2001;Nakano et al, 2013).…”

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).…”

Petrology, geochemistry, and origin of metamorphosed mafic rocks of the Trans Vietnam Orogenic Belt, Southeast Asia

“…The protoliths of these metamorphic rocks are diverse, and include pelitic, felsic, mafic, and ultramafic rocks, as well as carbonate and calc -silicate rocks, and they were metamorphosed in various crustal condition ranging from greenschist to granulite facies, as well as eclogite facies. Although the peak pressure -temperature (P -T) conditions of metamorphism preserved in each of these six terranes are different, both the P -T path trajectories and the timing of metamorphism are similar among all terranes (i.e., Permo -Triassic with a clockwise P -T path; Osanai et al, 2001Osanai et al, , 2004aOsanai et al, , 2004bOsanai et al, , 2008Nakano et al, 2004Nakano et al, , 2007aNakano et al, , 2007bNakano et al, , 2008Nakano et al, , 2009aNakano et al, , 2010. Osanai et al (2008) suggested that all of these terranes were formed by a single tectonic event, and accordingly they proposed the term Trans Vietnam Orogenic Belt (TVOB) to collectively describe all of the metamorphic terranes sandwiched between the Indochina and South China cratons.…”

“…However, the tectonic settings of protolith formation for these various metamorphic terranes prior to collision in the TVOB orogeny remain poorly understood. With regards to the ages of the protoliths of these metamorphic rocks, some papers have obtained preliminary ages that are much older than the timing of the TVOB orogeny (~ 430 Ma: e.g., Osanai et al, 2003;Roger et al, 2007;Nakano et al, 2010; ~ 680 Ma: Nakano et al, 2003;Osanai et al, 2003), which were interpreted as being related to the so -called Pan -African Orogeny and the formation of the Gondwana supercontinent (e.g., Lan et al, 2003;Osanai et al, 2004aOsanai et al, , 2008Nakano et al, 2007aNakano et al, , 2008Nakano et al, , 2010. A few recent studies have also employed Sr and Nd isotopes and trace element geochemistry of amphibolites to constrain the origin of their protoliths.…”