Discovery of high-pressure granulite-facies metamorphism in northern Vietnam: Constraints on the Permo-Triassic Indochinese continental collision tectonics

Abstract

High-pressure mafic granulites containing granoblastic garnet, quartz, and minor hornblende have been found from the Song Ma Suture zone in northern Vietnam, regarded as a microcontinental boundary between the South China and Indochina blocks. Finegrained symplectite formed during the decompression stage is developed in the granulite and is divided into orthopyroxene +plagioclase and orthopyroxene + clinopyroxene + plagioclase AE hornblende. The former replaces garnet and the latter is regarded as a breakdown of sodic clinopyroxene. Detailed observation and careful data selection revealed that the high-Mg and low-Ca garnet should be in equilibrium with the precursor sodic clinopyroxene, and the pair indicates high-temperature and -pressure conditions (910-930 8C at 1.9-2.0 GPa). Although we could not obtain quantitative age data from the high-pressure granulite, the U-Th-Pb age (233 AE 5 Ma) of pelitic gneiss strongly suggests a Middle to Early Triassic metamorphic event. If the age indicates the timing of the high-pressure granulite-facies metamorphism, it might be related to a continental collision setting by following crustal subduction. According to the metamorphic signatures, north to central Vietnam may be regarded as an orogenic belt formed by the micro-continental collision between the South China and Indochina cratons. To cite this article: N.

Collision zone metamorphism in Vietnam and adjacent South-eastern Asia: Proposition for Trans Vietnam Orogenic Belt

“…Estimated peak -P metamorphic conditions of the eclogite are ~ 25 kbar and ~ 650 °C (stage M0 SM ) with clockwise P -T evolution and decrease to 20 kbar, at peak thermal conditions of 700 -750 °C . Another high -P garnet granulite, which indicates very high -T peak metamorphic conditions (~ 20 kbar, 910 -930 °C), is also newly recognized from the Song Ma suture zone (Nakano et al, 2008) (Fig. 7).…”

“…8) (e.g., Tran Ngoc Nam, 1998Maluski et al, 1999, Collision zone metamorphism in Trans Vietnam Orogenic Belt 2002Nagy et al, 2001;Carter et al, 2001;Osanai et al, 2001Osanai et al, , 2003Osanai et al, , 2004aNakano et al, 2003Nakano et al, , 2004aNakano et al, , 2004bNakano et al, , 2006aNakano et al, , 2006bNakano et al, , 2007aNakano et al, , 2008Lan et al, 2003;Lepvrier et al, 2004Lepvrier et al, , 2008Miyamoto et al, 2006;Roger et al, 2007). These works showed Permo -Triassic metamorphic ages (230 -270 Ma) that were determined using SHRIMP, CHIME, Sm -Nd and Rb -Sr internal isochrones, and K -Ar mineral dating, but there is no distinct spatial or temporal relationship with metamorphic grade.…”

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).…”