The chemical composition of garnet in the high-pressure granulite shows almandine, pyrope and grossular components. A small amount of spessartine component (less than Sps₄) is observed in the outer thin rim of the garnet. The core of the garnets can be grouped into two types based on their chemical compositions: high-Mg and low-Ca (garnet 1: Alm_58–62Prp_16–21Grs_19–24Sps_0–2) and low-Mg and high-Ca contents (garnet 2: Alm_56–62Prp_9–13Grs_25–32Sps_0–2) (Fig. 3). The mantle of garnet 1 is at the mid-position of both garnet cores, or sometimes shows a composition similar to that of the core of garnet 2 (Fig. 3). On the other hand, garnet 2 has a homogeneous composition, except in its outer thin rim (Fig. 3). X-ray image mapping of garnet 1 shown in Fig. 4 indicates the presence of an Mg-rich and Ca-poor core surrounded by an Mg-poor and Ca-rich mantle to rim before breakdown of the garnet. The outer thin rim of both garnets is similar in composition, being characterised by medium to low Ca and the highest Fe contents (Fig. 3).

Clinopyroxene in the studied granulite occurs as a symplectitic intergrowth with plagioclase and a minor amount of orthopyroxene, hornblende, and quartz. Such symplectitic clinopyroxenes associated with plagioclase are generally observed in eclogites decompressed to granulite-facies condition (Zambezi Belt [5], Sveconorwegian Granulite Region [23], Snowbird Tectonic Zone [1], Bohemian Massif [2], and Kontum Massif [25]). These clinopyroxenes in the worldwide metamorphic terranes should contain high Na contents in high-pressure conditions, whereas they generally show augitic composition because their original jadeite components were replaced with the symplectitic sodic plagioclase. The shape of the symplectite (sometimes showing euhedral shape: Fig. 2B) in the studied granulite also suggests that the symplectite was formed from a single mineral phase, possibly corresponding to sodic clinopyroxene.

The symplectitic clinopyroxene in this study has augite composition with a few jadeite, aegyrine, and Ca-tschermak components (Jd_0–2Ae_0–3CaTs_0–5), and has a reasonably wide range of X_Mg values, from 0.52 to 0.59 (Fig. 5). The orthopyroxene + clinopyroxene + plagioclase ± hornblende ± quartz symplectite was analysed using a broad probe diameter (200 μm) to obtain the original clinopyroxene composition (labelled as whole symplectite in Fig. 5) stabilised under high-pressure conditions. The results show scattered Si content [1.87 to 2.01 pfu (per formula unit); Fig. 5] and do not agree with the stoichiometries of clinopyroxene (Table 2). Most are rich in SiO₂, and their total cations are below 4.0 pfu. The compositions are similar to non-stoichiometric supersilicic clinopyroxene, as suggested in [37]. Such clinopyroxene is generally observed in ultrahigh-pressure eclogites and mostly contains quartz rods [6,15,25]. However, no such evidence is observed in the studied sample. The variety of SiO₂ content in the original clinopyroxene depends on the modal amount of quartz in the intergrowth. It could indicate that quartz in the symplectite is originally an inclusion in initial omphacitic clinopyroxene under high-pressure conditions and is not formed by breakdown of the clinopyroxene. All analyses were recalculated using the volume proportions of quartz in the areas analysed and their composition. The modal amounts were obtained by the estimation procedure reported by [25], which uses brightness levels of individual pixels in backscattered images. The amounts of quartz in broad probe analyses vary from 0.0 to 6.2 vol%. The recalculated compositions (labelled as recalculated whole symplectite in Fig. 5) are in good agreement with clinopyroxene, and the Si contents vary from 1.87 to 1.96 pfu (Fig. 5). The compositions are regarded as original clinopyroxene composition, and contain relatively high jadeite and Ca–Tschermak components (Jd_12–22CaTs_7–16Aug_68–80).

Al^VI contents of orthopyroxene differ depending upon their occurrences. Orthopyroxene coronas surrounding garnets show relatively high Al^VI contents (0.01–0.04 pfu), whereas symplectitic orthopyroxene replacing original clinopyroxene contains low Al^VI contents (0.00–0.02 pfu; Fig. 5).

Amphibole occurring in the matrix shows homogeneous composition (Al in T-site = 1.0–1.5 pfu, Na in M4-site = 0.2–0.4 pfu and Na + K in A-site = 0.0–0.3 pfu), indicating hornblende composition. The inclusions in garnet show low Si and high Al and Na contents (Al in T-site = 1.5–2.2 pfu, Na in M4-site = 0.2–0.3 pfu and Na + K in A-site = 0.3–0.6 pfu), characterised by pargasite to tschermakite compositions. Symplectitic hornblende replacing original clinopyroxene contains high Si and low Al and Na contents (Al in T-site = 0.8–1.4 pfu, Na in M4-site = 0.1–0.3 pfu, and Na + K in A-site = 0.0–0.2 pfu), indicating tremolitic hornblende.

Anorthite components of plagioclase coronas around garnet are much higher (An_60–85) than those of symplectitic plagioclase replacing clinopyroxene (An_30–50).

The petrographical and mineral chemical features suggest that sodic clinopyroxene has been stable prior to the exhumation of the granulite. The chemical composition of garnet 1 drastically changes from Mg-rich and Ca-poor to Mg-poor and Ca-rich compositions (Fig. 4), indicating later external influences such as penetration of calcic melt or fluid. Hence, the high-Mg and low-Ca garnet (core of garnet 1) was stable in peak metamorphic conditions. The original sodic clinopyroxene, which is commonly stable at eclogite-facies, was probably in equilibrium with the garnet under such high-pressure/high-temperature metamorphic conditions.

The pressure/temperature conditions were estimated by garnet–clinopyroxene geothermometry [7,34,35] and activity jadeite geobarometry [12] using the core of garnet 1 and recalculating original clinopyroxene. Some recalculated original clinopyroxenes showing low Mg contents (X_Mg < 0.5) may be due to the relation to the melt (or fluid) or uncertainties of wide prove analyses, and thus they were ignored. An average value obtained from 27 recalculated clinopyroxenes (X_Mg = 0.53, X_Jd (Jd/(Jd + Aug) = 0.18) was used in this calculation. The result yielded 910–930 °C at 1.9–2.0 GPa (Table 3). Because some low-Mg clinopyroxenes were ignored in this calculation, the condition can be regarded as the minimum peak metamorphic condition for the granulite, which corresponds to high- to ultrahigh-temperature-type high-pressure granulite observed in the world [27]. After this condition, the granulite was uplifted and formed orthopyroxene- and plagioclase-bearing symplectites replacing sodic clinopyroxene and garnet during the decompression.

High-grade metamorphic rocks including granulites and eclogites have mainly been observed in past or present plate convergence zones [21,27], and therefore, such rocks are one of the best indicators of the location of the microcontinental boundary in Asia. In the well-defined continental boundary between the North and South China cratons, such rocks have been reported: granulites from the North Dabie complex, China [39] (see inset map in Fig. 7), Imjingang belt, South Korea [36] and Higo terrane, Japan [30] and eclogites from Dabie-Sulu belt, China [8] and Gyeonggi massif, South Korea [28], some of which indicate ultrahigh-temperature [30] and ultrahigh-pressure metamorphic conditions [29]. Recently, granulites and eclogites have also been reported from the Kontum massif in central Vietnam [24–26,32] (Fig. 7). In addition, ultrahigh-temperature [24,32] and possible ultrahigh-pressure [25] conditions have been estimated from these rocks. The similarities of these rocks occurring in East to Southeast Asia are not only the metamorphic grade, but also the metamorphic age indicating the Late Permian to the Early Triassic [8,18,26,28,30,31,36] (Fig. 7). Although some older ages (∼400 Ma) were also reported, their quantitative pressure/temperature conditions indicate low-pressure types (see eastern Kontum in Fig. 7), suggesting uncollisional orogeny.

The first finding of high-pressure granulites in this region strongly implies that the Song Ma Suture zone can be regarded as one of the continental boundaries between the South China and Indochina blocks. The pressure/temperature conditions (910–930 °C at 1.9–2.0 GPa) estimated from the high-pressure granulite correspond to high-temperature eclogite-facies metamorphic conditions, indicating a depth of more than 60 km. These conditions are similar to the minimum pressure conditions of eclogite overprinted by ultrahigh-temperature granulite-facies metamorphism from the Kontum massif in central Vietnam (ca. 950 °C at 1.6 GPa; [24]), though ultrahigh-pressure conditions are expected from the eclogite [25]. These rocks observed from both northern and central Vietnam commonly preserve evidence of eclogite-facies metamorphic conditions, as complicated decompression textures may indicate similar rapid exhumation from deep crustal levels. Even though we could not obtain any quantitative ages directly from the high-pressure granulite, the one of 233 ± 5 Ma, detected from pelitic gneiss, is consistent with eclogite from the Kontum massif (240 ± 16 Ma; [26]). If we assume that the age indicates a high-grade metamorphic event in northern Vietnam, these metamorphic signatures involving pressure/temperature/time conditions are quite similar to each other.

Although no evidence for granulite- and eclogite-facies metamorphism has been observed between the Song Ma Suture zone and the Kontum massif, amphibolite-facies metamorphic rocks from the Truong Son belt located between both areas have Late Permian to Early Triassic ages [16,17] (Fig. 7). In the case of the Dabie-Sulu belt (ca. 2000 km in length) in central China, well established as a continental boundary between the North and South China blocks, eclogites and granulites are only exhumed in the Dabie and Sulu areas. Therefore, in the present study, we preliminarily conclude that North to central Vietnam forms a Late Permian to Early Triassic orogenic belt (ca. 1500 km in length), due to the collision of the Indochina and South China cratons, and crustal materials subducted to deep levels are now distributed only in northern and central Vietnam (as the Song Ma metamorphic belt and the Kontum massif, respectively). Although first finding of high-pressure granulite in northern Vietnam is from a boulder sample and the age is not obtained directly from the granulite, this study strongly proposes that other evidences of the Permo-Triassic high-grade metamorphism, including eclogite and granulite, should be sought to further constrain the tectonic setting in the Indochinese collision zone.