Calc-alkaline andesite, dacite and rhyodacite rocks are distributed in two regions: the Song Da upstream and Dien Bien basin in the northwestern segment, and the Dak Lin and Chu Prong areas in the southeastern segment (Fig. 1).

In the northwestern segment, such volcanic rocks belong to the suite of the Permian Song Da formation [7,31], distributed along the sub-longitudinal Lai Chau-Dien Bien fault zone, and spatially correlated with calc-alkaline granitoids (Fig. 1). This volcanic suite comprises mainly andesitodacite and dacite; andesite and andesitic basalt are met. Their Permian ages were determined by the presence of a fauna in limestone, intercalated in terrigenous sediments and tuff of the Song Da Formation [7,31]. Ar–Ar dating of dacites in this region also shows their Permian–Triassic age (270–248 Ma) [11].

Intermediate–mafic volcanism is spread out in small areas, all over the southern part of Vietnam. A representative stratigraphy of this volcanism cropped out in the Dak Lin area (Fig. 1). In andesitic tuffs, Fusulinida (Pseudofusulina sp., Parafusulina, Bradyna and Schwagerina sp. (?) or Verbeekina sp.) are found, and Astartella lutungini Fedotov, Deltopecten sp. are present in siliceous limestone, which leads to the attribution of a Late Carboniferous–Permian age to this formation [19].

Andesites and andesitic basalts are abundant in the Dak Lin area. They are characterized by a porphyric texture with pilotaxitic and hyalopilitic groundmass. Weak flow alignments are found. Locally vesicular basalts are met. Plagioclase is a predominant phase (microlite and phenocrystal phase); locally pyroxenes are up to 10–15%. Chloritization, epidotization, and carbonation are common.

Explosive facies consist of tuffs of andesite, andesitic basalt, and a minority of dacite. Pyroclastic and brecciated rocks are common. Pyroclastic materials are glasses, andesitic and basaltic rock fragments, siliciclastic, sandstones, schists, crystal fragments (plagioclase, pyroxene, quartz). Major rock forming phases are clinopyroxene (augite, pigeonite), zoned plagioclase, and magnetite [42].

Geochemically, all rock types belong to the low K- to middle K- calc-alkaline series. The andesitic basalt and andesite have mainly medium silica contents (SiO₂ = 53–57 wt.%), low magnesium contents (MgO = 2.16–3.7 wt.%), medium to low titanium contents (TiO₂ = 0,8–1.7 wt.%), medium to high total alkaline species contents (K₂O + Na₂O = 3.9–6.1 wt.%), and are sodium rich (K₂O/Na₂O = 0.1–0.3 wt.%) (Table 1). The felsic rocks are dacite to rhyolite (Fig. 2, Table 1) with SiO₂ = 68.7–76 wt.%, (Na₂O + K₂O) = 5.6–7.4 wt.%, and are sodium rich, with K₂O/Na₂O = 0.34–0.75 for dacite and 1.5–4.7 for rhyolite (Table 1). Both types share the same fractionated differentiation trends, represented by an increase in the silica and potassium contents and a decrease in the Al, Ti, P, and Mg contents (Table 1). This feature is also shown by an increase of Zr and Nd, coupled with a decrease of Sr along with an increase of silica (Table 1).

The intermediate–mafic and felsic rocks have the same REE concentrations and REE distribution patterns (Table 1, Fig. 2-1). They are enriched with LREE and have flat HREE patterns, relative to C₁ chondrite [27], with 4.88 < (La/Yb)_N < 9.07, with a medium–weak negative Eu anomaly (Eu/Eu* = 0.57 to 0.67), which may indicate that plagioclase was not removed from the magma. Such patterns are found in the low- to medium-K calc-alkaline rocks of the active continental margin.

In primitive-mantle normalized spidergrams, all samples yield similar patterns, with depletion of high field strength elements (e.g., Nb, Ta, Zr, and Ti, Fig. 2-2). High concentrations of large ion lithophile elements (Cs, Rb, Th, U, and K) are coupled with pronounced depletion in Sr and P. Their trace-element patterns normalized to Ocean Ridge Granite (ORG, [23]) show generally similar shapes, marked by a variable enrichment in Rb and Th and a depletion of Ta, Nb, Hf, and Zr [42]. The andesitodacites have a smoother pattern, with a lower differentiation pattern, but represent negative anomalies of Ta, Nb, and Ce, which may indicate a strong contamination by pelagic sediments. Essentially, andesite and dacite of the Dak Lin area are similar as regards their Nb, La, and Ce contents, in comparison with similar rock types of the Dien Bien–Lai Chau area.

Isotopic data are listed in Table 5. The isotopic ratios of mafic and felsic rocks are relatively different. The mafic series are characterized by homogenous isotopic ratios with ⁸⁷Sr/⁸⁶Sr(i) = 0.70563–0.70731; ¹⁴³Nd/¹⁴⁴Nd(i) = 0.51288–0.51296, similar to andesitic basalts of active continental margins (Table 5, Fig. 3). Dacite and rhyolite display ⁸⁷Sr/⁸⁶Sr(i) = 0.70643–0.70706; ¹⁴³Nd/¹⁴⁴Nd(i) = 0.51251–0.51214 with ɛ_Nd(T) varying from –5.27 to 2.06 (Table 5, Fig. 3). The volcanic rocks are characterized by relatively homogeneous lead isotopic ratios: ²⁰⁶Pb/²⁰⁴Pb = 18.52–18.91, ²⁰⁷Pb/²⁰⁴Pb = 15.52–15.63, ²⁰⁸Pb/²⁰⁴Pb = 38.38–39.15 (Table 5, Fig. 4), which are lower than those encountered in typical arc-volcanic rocks, but similar to those from the active continental margin. The T_DM age of the rock varies from 0.58 to 1.40 Ga. The similarity of their geochemical features implies that they were derived from similar mantle wedge–derived sources, but with various degrees of differentiation.

The high LREE, low HREE concentrations, and relatively flat REE distribution patterns in all samples imply that they were derived from a shallower source, in which garnet is absent.

Source contamination by sediment-derived fluxes from the subducting slab at the mantle wedge was also important. Positive LILE spikes are shown in Fig. 2. Moreover, negative anomalies of HFS elements such as Ta, Nb, Ti, Hf, and Zr represent the contribution of pelagic sediment to the subduction environment [29,30].

The Nb/U ratios of the rocks (2–7.5) are much lower than those of the continental crust are, implying that the enrichment in U relative to Nb could not be produced by any normal mixing process between the mantle and crustal sources. It can only be explained by a contribution of a subducted slab-derived fluid. Coupled with the U concentration, high Th/Yb (2–10) and medium Ta/Yb (0.1–0.4) ratios strongly exhibit that the surveyed rocks are of active continental margin environment. This feature is also indicated by higher Zr and Hf concentrations than in MORB, showing a difference with arc-derived rocks. Their variations also indicate the predominant fractionated differentiation in the formation of the rocks.

Calc-alkaline granitoids are distributed all over the three segments of the Truong Son belt, but they are more abundant in the northwestern and southeastern segments.

In the northwestern segment, granitoid intrusions (Dien Bien complex) are exposed along the Dien Bien–Lai Chau fault zone (Fig. 1). The rock types are diorite, granodiorite, amphibole (hornblende)–biotite granite, in which diorites are considered as the earliest stages of the intrusions. Their Permian–Triassic ages were determined by their stratigraphic correlation with the Song Da formation and overlain by Late Triassic terrigenous sediments [7,45]. The ages of granodiorite and amphibole–biotite granite are 266–252 Ma (Rb–Sr) [7] and 272 Ma (U–Pb) [33]. According to [33], several individual diorite bodies in this region yield ages from 415 to 430 Ma, which are considered of an older magmatism stage, and they are not included in this paper.

Diorite, granodiorite, and granite intrusions of the southeastern segments are distributed in the Kontum Uplift and in its northern and western margins (the Ben Giang–Que Son complex). At the northern margin of the Kontum Uplift, calc-alkaline granitoids form large massifs, along the Permo-Triassic latitudinal ductile deformation zones [14]. Due to the Indosinian tectonic activities, the rocks are locally deformed. The SHRIMP age of zircon from Tien Phuoc granite (Ben Giang–Que Son complex) is 260 ± 4 Ma, corresponding to the Late Permian (our unpublished data). At the western margin, in the Sa Thay area of the Kontum Uplift, similar granitoids are found. However, in this region, porphyritic granites are more abundant [38]. Noteworthy features are the widespread calc-alkaline mafic lamprophyres (kersantite, sperssartite) and porphyrite coupled with granitoids. This is a typical feature of active continental margin magmatism.

The calc-alkaline granitoids are mainly diorite, granodiorite and amphibole–biotite granite, in which granodiorite is predominant. They are porphyritic rocks with large plagioclase and/or K-feldspar megacrysts (up to 2–4 cm in size). Major minerals are plagioclase + quartz + K-feldspar + biotite + amphibole, with different amounts. Accessory minerals are titanite, apatite, zircon, locally orthite. Leucogranites are rare. Porphyritic granites form small intrusions and dykes that are distributed mainly at the western margin of the Kontum Uplift (Sa Thay area).

Biotites in granitoids are high in Ti (TiO₂ = 2.5–3.5 wt.%), Mg (MgO = 7.5 wt.% in diorite, 12.5 wt.% in granodiorite), and low in Al (Al₂O₃ = 14.9–15.3 wt.%). It is a typical feature of biotite in calc-alkaline and high-K calc-alkaline granitoids [34]. After these authors, amphiboles are high-magnesium hornblendes, tschemakitic hornblende and magnesium hastingsite with various MgO contents (from 5.83 to 12.30 wt.%). Contrary to the MgO content, the FeO contents of amphibole and biotite tend to increase from diorite to granite, indicating magmatic differentiation.

Calc-alkaline granitoids have low to medium alkalinity, are sodium-rich or K-Na rich; some are potassium-rich monzogranites (Table 2). Diorite, granodiorite and granite of the Dien Bien and Ben Giang–Que Son complexes are identical in concentrations and distribution patterns of trace elements such as Rb, Sr, Ba, Zr, Nb, Ta, Th, U, as well as REE (Table 2, Fig. 2-B1-2). Gabbrodiorite and diorite are characterized by flat REE patterns with 8.35 < (La/Yb)_N < 9.41, weak negative Eu anomalies (Eu/Eu* = 0.99–1.03, Fig. 2-B-1), whilst quartz diorite and granodiorite present steep patterns, with higher concentration of LREE (5.06 < (La/Yb)_N < 19.28), neat Eu negative anomalies (Eu/Eu* = 0.1–0.71). The differences in element concentrations between intermediate and felsic rocks might represent a magmatic differentiation. Their trace elements and REE patterns, especially depletions of Nb, Ta, and Zr (Fig. 2-B1-2), are similar to active continental margin granitoids.

The Permo-Triassic granitoids are characterized by various Sr isotopic ratios (⁸⁶Sr/⁸⁷Sr = 0.7058 to 0.7101), coupled with homogenous Nd isotopic ratio (¹⁴³Nd/¹⁴⁴Nd = 0.512304–0.512489, ɛ_Nd = 0.48 to –4.45), showing that they were strongly contaminated by crustal material (Table 5, Fig. 3). Lead isotopic ratios are relatively homogenous (²⁰⁶Pb/²⁰⁴Pb = 18.678–19.138, ²⁰⁷Pb/²⁰⁴Pb = 15.678–15.684, ²⁰⁸Pb/²⁰⁴Pb = 38.978–39.145) (Table 5, Fig. 4), and similar to EM2; they also indicate strongly crustal contamination. Their T_DM modal ages give a crustal resident age of 1.03–1.50 Ga. Sr, Nd, and Pb isotopic significances of the Permo-Triassic granitoids are relatively close to the above-mentioned Permian volcanic rocks, implying that they were both derived from similar mantle wedge sources above the subduction slab. However, crustal signatures are identical in Permo-Triassic granitoids. Decreasing Nb/U ratios from gabbrodiorite (9.3) to granite (2.9) also indicate the increase of crustal contamination from intermediate to felsic rocks.

Permo-Triassic peraluminous granites are scattered in all three segments of the Truong Son belt, especially in the central segment. They consist of large intrusions, such as Muong Lat, Tuong Duong to the north, Kim Cuong, Dong Hoi, and Hai Van to the south of the central segment. Small intrusions of similar peraluminous granite are also found in the Kontum Uplift. Among these intrusions, the Hai Van and Ba Na massifs in the North of Kontum Uplift are investigated in detail. In previous researches, these granites were grouped into different complexes: for instance the Pu Si Lung, Muong Lat, Kim Cuong, Dong Hoi intrusions were grouped into the Late Triassic Phia Bioc granite complex [7], or the Carboniferous biotite–muscovite granites of the Truong Son complex [45]. In the Geological Map of Vietnam, scale 1:500,000, the Kim Cuong and Dong Hoi intrusions belong to the Truong Son complex, whilst other intrusions are associated with the Phia Bioc complex to the north, and the Hai Van complex to the south [5,32]. The U–Pb, Rb–Sr and Ar–Ar isotopic ages of biotite granite and of two-mica granite of the Ba Na intrusions are 248–245 Ma [21,22]. Biotite granite of the Pu Si Lung intrusion in the northwestern segment was dated from the Late Permian (259 Ma, Rb–Sr, after [18]).

The Permo-Triassic peraluminous granites are mainly biotite and two-mica granites. However, granodiorite and rarely diorite are locally met. The rocks are characterized by a porphyric texture with megacrysts of plagioclase and/or K-feldspar. Biotite is a major mineral phase in all intrusions. The biotites have high Al (Al₂O₃ = 16.77–17.32 wt.%) and Ti (TiO₂ = 2.94–3.88 wt.%) contents, low Mg contents (MgO = 4.2–7.8 wt.%), relatively high F contents (0.77–0.94 wt.%), and low Cl contents (0.03–0.08 wt.%) [34]. Other aluminous mineral phases are muscovite, silimanite, and cordierite. Locally, in leucogranite, garnets are met.

The chemical compositions of the Hai Van and Ba Na peraluminous granites (Table 3) show that the rock vary from intermediate type (SiO₂ = 62.9–67.68 wt.%) to typical granite (SiO₂ = 71.35–74.66 wt.%) and leucogranite (SiO₂ = 75.22–76.07 wt.%). The rocks are characterized by medium alkalinity, some samples from the Ba Na intrusion have high alkalinity (up to 11 wt.%), probably due to feldspathization associated with greisenization.

Biotite granites are enriched with Rb, Th, U, LREE, depleted in Ba, Sr, Zr, Nb, Ta. Two-mica granites are enriched with Li, Be, Sn (Table 3). Chondrite-normalized REE patterns show strongly negative Eu anomalies, coupled with flat HREE (Fig. 2-C1). On the primitive mantle normalized spidergrams (Fig. 2-C2), the rocks present negative anomalies of Ba, Nb, Ta, Sr, and Zr, which are typical features of the Permo-Triassic magmatism in the Truong Son belt [38]. Coupled with them are peaks of U, Th, and Pb. In comparison to Permian calc-alkaline granites that were described previously, they share the same distribution pattern, in spite of different concentrations.

Isotopic data of peraluminous granites are listed in Table 5. The ⁸⁷Sr/⁸⁶Sr ratios of the Hai Van, Ba Na, and Tra My intrusions are high (0.75026–0.80728), implying they were melted from old crystalline crust that has high Rb/Sr ratio. Initial ¹⁴³Nd/¹⁴⁴Nd ratios are relatively homogeneous (0.511958–0.512116), with ɛ_Nd(0) = –13.3 for the Hải Vân intrusion and –10.2 for the Ba Na and Tra My intrusions (Table 5). Their crustal resident ages (1.8–2.23 Ga) imply that they were melted from a mature continental crust [25,38]. These characteristics are quite different from those of the Late Mesozoic peraluminous granites of the Da Lat volcano-plutonic belt, located to the south-southeast of the Kontum Uplift, which have lower ⁸⁷Sr/⁸⁶Sr ratios (0.7055–0.7072) and higher ɛ_Nd(T) values (–2.02 and –2.06) [38].

Middle Triassic post-collision volcano-plutonic associations are widely distributed in the central and southeastern segments. They are mainly volcanic to hypabyssal (sub-volcanic) formations; intrusions are merely met. The boundaries between hypabyssal and overlain extrusive layers are undistinguishable. At the northern part of the central segment, volcano-plutonic associations form NW–SE-oriented narrow bands, from Sam Nua (Laos), crossing Song Ma (along the Song Ma ophiolite belt) to Deo Ngang–Hoanh Son (Fig. 1), which were named the Sam Nua–Hoanh Son belt [20,45]. The volcanic rocks were grouped into the Dong Trau Formation, while hypabyssal and intrusive rocks were grouped in the Song Ma Complex [5]. Volcano-plutonic rocks of the southeastern segment were formed in wide fields or bands in the Kontum Uplift, in which volcanic rocks belong to the Mang Yang Formation, whilst hypabyssal and intrusive rocks belong to the Van Canh Complex [5,19]. Middle Triassic ages of volcanic and associated granite–granophyres are based on the isotope dating Ma after different authors (see below).

Middle Triassic volcanic rocks are mainly porphyric dacite, rhyodacite, and rhyolite with plagioclase, K-feldspar, and quartz phenocrysts, felsite, and/or glassy groundmass. Extrusive rocks are predominant. Associated hypabyssal and intrusive rocks are granophyre granite with ‘graphic’ texture, minor granodiorite and diorite, especially in the Van Canh complex. Biotite and amphibole are common. Typical accessory mineral phase of the Van Canh formation is fluorite.

Dacite, rhyolite, and granophyre granite of Song Ma, Hoanh Son and Mang Yang belong to medium to low alkaline series with fluctuated K₂O/Na₂O ratios, from a sodium-rich variety (Song Ma) to a potassium-rich one (Hoanh Son). In the Mang Yang formation, sodium-rich varieties are concentrated on the western margin of the Kontum Uplift, while potassium-rich varieties are found in its centre. However, dacites, rhyolites, and granophyre granites of different areas are quite similar in trace elements and REE characteristics (Table 4, Fig. 2-D1-2). Their major differences are depletions of Eu, Ba, Ti, and enrichments in HREE of the rocks from the Mang Yang formation in comparison with those from the Song Ma and Hoanh Son areas. Their concentrations and distribution patterns are similar to those of Early Triassic granites (Fig. 2-C and D). Enrichment in K, Rb, Th, U, as well as depletion of Ba, Sr, Nb, Hf, indicates genetic similarities between those intrusive rocks and volcanic ones. The rocks from the Mang Yang formation show relatively low ⁸⁷Sr/⁸⁶Sr ratios (0.7054, [19]), similar to the above-mentioned calc-alkaline granitoids. However, the hypabyssal granite of the Hoanh Son area presents higher ⁸⁷Sr/⁸⁶Sr ratios (0.713, [17]), indicating crustal contamination (Table 5)

Geochemical and isotopic significances of the Late Palaeozoic–Early Mesozoic magmatic associations of the Truong Son belt indicate that they represent a full magmatic evolution trend of an active continental margin in relation to the Indochina (IC)/North Vietnam–South China (NV–SC) amalgamation. Presence of andesite in the Dak Lin and Song Da formations is a strong indication of the onset of a subduction process [42–44]. Such formations were considered as volcanic-arc andesites [5]. According to their geochemical and isotopic significance, which were described above, the andesites and calc-alkaline granitoids of the Dien Bien and Ben Giang–Que Son complexes are products of an active continental margin magmatism [40,42,43]. The duration of the calc-alkaline magmatism is relative long, from the Late Permian to the Middle Triassic (270–245 Ma). The similarities between calc-alkaline granitoids and two remote segments (Dien Bien complex in the northeastern segment and Ben Giang–Que Son complex in the northern Kontum Uplift) strongly support the conclusion that they are both products of the same magmatism process in relation to the (IC)/(NV–SC) amalgamation. It is noteworthy that, during the Cenozoic, the IC block was displaced to the southeast and rotated 60–90° clockwise [8,28], so that the reconstructed subduction zone might connect three segments of the Truong Son belt.

In addition, Permo-Triassic granitoids are not only present to the west of the Po Ko sub-longitudinal fault zone, but also to the east of this fault zone (northern margin and on the Kontum Uplift). Similar distributions of Middle Triassic volcano-plutonic felsic rocks (the Mang Yang Formation and Van Canh complex), as well as post-collision lamprophyre dykes, are recorded in both sides of the Po Ko fault zone (Fig. 1; III) [38,44]. Thus, the presence of a west-impinged subduction zone along the Po Ko fault zone [12] in Permian–Triassic times is hard to prove. Moreover, detailed researches on mafic-ultramafic rocks along the Po Ko zone (Plei Wek formation, after [19]) could exclude the ophiolitic affinity of this formation [24].

The collision between (IC) and (NV-SC) due to the closure of Palaeotethys had led to the formation of peraluminous granite (Truong Son and Hai Van complexes). Compositionally, these granites are similar to syn-collision granites. Their ages of formation are still discussed. In the central segment, peraluminous granite intrusions such as Kim Cuong and Dong Hoi were considered of Carboniferous age [5,44]. The ages of the Ba Na intrusion are 245–248 Ma (U–Pb) [21,22]. The age of the Pu Si Lung peraluminous granites of the northeastern segment is 259 Ma (Rb–Sr, [18]). The similarities of rock types, mineral composition, as well as geochemical and isotopic characteristics of the peraluminous granites along the Truong Son belt, indicate that the intrusions are all related to the same Early Triassic magmatism. Coeval with the peraluminous granites, a number of gabbro-pyroxenite intrusions (Cha Van Complex, not in this article) exhibit calc-alkaline affinity [1] and an age of 243 Ma [19], like the monzogabbros and alkaline lamprophyres [38]. These igneous rocks are scattered all over the Kontum Uplift and its northern and western margins. The formation of mafic-ultramafic intrusions under a compressional tectonic regime is a typical feature of the Permian–Triassic magmatic activities in eastern Indochina. In previous researches, intrusions of gabbro-pyroxenite, monzogabbro and peraluminous granites are considered as collision bimodal gabbro–granite association [5,10]. However, the mafic-ultramafic intrusions are spatially associated with calc-alkaline granitoids (Ben Giang–Que Son complex). The association of mafic intrusions with volcanic arc/continental margin igneous rocks are recorded in different folding belts, such as the Central Asian fold belt [2]. The formation of peraluminous granites along the Truong Son belt is correlated with the collision between the IC and NV-SC blocks.

When the compressional regime that was formed by collision between the IC and NV-SC blocks was changed into an extensional regime on the continental margins, the igneous activity is represented by hypabyssal felsic volcano-plutonic association spreading out over the Truong Son belt. Their petrography, as well as geochemical and isotopic compositions, implies that they are typical post-collisional associations [11,38]. The differences in potassium contents from the north to the south of the central segment might represent local variations of the magma in relation to their distance to the Kontum ancient continental core. The formation ages of volcano-plutonic rocks of this association are not well determined. On the geological map scale 1:500,000, they are grouped into a Middle to Late Triassic formation [32]. Nevertheless, our Ar–Ar age of phlogopite in the minette dyke that intersects the Mang Yang trachyrhyolite in the Dak Long area (west of the Kontum Uplift) yields 246–240 Ma [44], implying that the Indosinian Orogeny might end earlier, probably in Early to Middle Triassic times. This age is coeval with deformation activities (248–233 Ma) along the Po Ko fault zone to the west, Tam Ky–Phuoc Son, and Song Ma to the north, as well as in the ‘core’ of the Kontum Uplift [12,14].

There are number of hypothesis about the plunged direction of the subduction in relation to the IC/NV–SC amalgamation: southward [9,15], northwestward [45], or declining northward [12]. Based on the distribution of calc-alkaline granitoids (Dien Bien complex), as well as of post-collision volcanic and hypabyssal rocks (Dong Trau Formation and Song Ma complex) in the South of the Song Ma ophiolite belt, we agree with the interpretation of Lan et al. [11] that the subduction of the Palaeotethys under the northern margin of Indochina occurs in Permian–Triassic times. This model seems to be not adapted to the distribution of the Permo-Triassic magmatic rocks in the southeastern segment of the Truong Son belt, and it needs more detailed research.

Firstly, the Indosinian magmatism is characterized by widespread peraluminous granites; it was derived from crustal sources. The second characteristic in the Indosinian orogeny is characterized by the presence of alkaline mafic intrusions at the early stage (monzogabbro, not included in this paper) and ultrapotassic lamprophyres (minette) at the latest stage in the Kontum Uplift, as well as in its northern and western margins. In the formation of potassic and ultrapotassic lamprophyres, the metasomatism of the lithospheric mantle and subduction-derived fluid play an important role [41]. In the Central Asia Fold Belt (Tianshan–Altai), similar Triassic alkaline lamprophyres (245–236 Ma) result from the interaction between the plumes of the lithospheric mantle and/or of the lower crust [48].

The Permo-Triassic magmatism in northern Vietnam is widespread. Its coeval activities with those from the Truong Son belt (Indosinian) have put forward a question: are these formations also products of the Indosinian magmatism? The first example is the Song Da rift, northwestern Vietnam. Some authors considered that mafic–ultramafic formations of Song Da are relics of a newly built oceanic crust [13], or ophiolite constituents [46,47], and that closing of the Song Da rift took place in Late Triassic [26] due to the Indosinian orogeny. With the northward [12] or northeastward [45] subducting direction of the Palaeotethys, the Song Da formation is interpreted as resulting from an intraplate or back-arc extension [16]. Detailed investigation on geochemistry and isotopic characteristics of the volcano-plutonic mafic–ultramafic associations of the Song Da shows that they are products of intraplate magmatism that were closely related to the Emeishan Large Igneous Province (LIP) [3,35,38]. In addition, volcano-plutonic mafic–ultramafic associations in folding structures of East Bac Bo are products of mantle plume (Emeishan) impacts on a distinct lithospheric mantle [36–39]. Thus, according to this point of view, the Indosinian magmatism is only present along the Truong Son fold belt.