2.1 Regional geology

The SE Tibetan Plateau comprises a series of Gondwana-derived continental blocks that underwent heterogeneous collision during and after the closure of multiple Tethyan oceans (Li et al., 2018; Metcalfe, 2013; Sone & Metcalfe, 2008). The principal blocks include the South China, Indochina, Sibumasu, and West Burma blocks (Figure 1a). The Indochina Block is separated from the South China Block by the Ailao Shan–Red River Shear Zone to the east and from the Sibumasu Block by the Changning–Menglian–Chiang Main–Inthanon Suture Zone to the west (Figure 1a). The Sibumasu Block is further separated from the West Burma Block by the Myitkyina Suture Zone.

The Indochina block was extruded by the penetration of India into Asia along the Ailao Shan–Red River Shear Zone (e.g., Tapponnier et al., 1982; Wang et al., 2014b). The Simao–Lanping Basin, which is located in the northern Indochina Block (Ueno, 2003), developed on Paleo-Tethyan basement after collision between Indochina Block and South China Block and evolved from a remnant marine and marine–continental basin during the Triassic into a continental rift basin during the Jurassic–Cretaceous (Liu, 2013; Yang et al., 2014). The Jurassic–Cretaceous sedimentary fill consists of a thick sequence of continental red beds (YBGMR, 1990, 1996). The Sibumasu Block (an acronym for China–Siam–Burma–Malaysia–Sumatra) as defined by Metcalfe (2002, 2013) is a composite unit consisting of the Baoshan–Tengchong blocks, N and SW Thailand, the eastern Myanmar Block, and western Malaysia. The block is interpreted to have rifted from NW Australia (Gondwana) during the late Carboniferous–early Permian and collided and sutured with the Indochina Block during the Middle Triassic, as inferred from biogeographical (brachiopods and fusulinid foraminifera) and palaeomagnetic data (Kang et al., 2019; Metcalfe, 2013). To the west of the Sibumasu Block, the West Burma Block is delimited by the dextral Sagaing Fault in the east and by the Kabaw Fault in the west and comprises the central WMA and Tertiary Chindwin Basin (Mitchell et al., 2012). The WMA is a N–S-trending magmatic belt delineated by the Banmauk–Wuntho batholith in the north and by the Monywa and Popa Volcanics in the south (and is thus also referred to as the Wuntho–Popa Arc). The WMA is part of an Andean-type continental arc that formed along the South Asian margin during Neo-Tethyan oceanic plate subduction (Mitchell et al., 2012; Wang et al., 2014a; Li et al., 2019). The Chindwin Basin lies to the west of the WMA and is a fore-arc basin of the WMA (e.g., Wandrey, 2006). The basin is filled by upper Cretaceous–Eocene shallow-marine or deltaic clastic and carbonate rocks, as well as unconformably overlying Neogene fluvial sedimentary rocks (e.g., Wandrey, 2006).

The Lancangjiang tectonic zone and Changning–Menglian Suture Zone are positioned between the Simao Block and the Baoshan Block in western Yunnan, SW China (Metcalfe, 2002; Figure 1b). The Lancangjiang tectonic zone is a Paleo-Tethyan suture zone and comprises mainly the Paleo-Tethyan arc magmatic zone (i.e., the Lincang granite batholith; Peng et al., 2013; Cong et al., 2020) and low-grade metamorphosed sedimentary rocks intercalated with bimodal volcanic rocks (i.e., Damenglong and Lancang groups; YBGMR, 1990, 1996; Nie et al., 2015). Limited zircon U–Pb dating has suggested that the bimodal volcanic rocks were generated during the early Palaeozoic (480–450 Ma) in response to processes involved in the evolution of the Proto-Tethys (Nie et al., 2015). The Changning–Menglian belt represents the closure boundary of the main Paleo-Tethys Ocean in western Yunnan (e.g., Wang et al., 2018; Xie et al., 2021) and is correlated northwestward with the Longmu Co–Shuanghu Suture in Tibet and southeastward with the Chiang Mai–Inthanon Suture Zone in northern Thailand (Ueno, 2003; Metcalfe, 2013; Zhai et al., 2013; Figure 1a).

2.2 Geology of the Huimin area, sampling, and petrography

The study area of Huimin is situated in the southern Lancangjiang tectonic zone of western Yunnan (Figure 1b). The concealed gabbroic unit of this area has been identified by 27 mine boreholes over a coverage area of >6.0 km² during exploration of the Huimin iron deposit (Figure 2a). Observations from the boreholes reveal that the gabbroic unit has a roof depth of at least 25 m and that the unit intruded metamorphosed volcaniclastic rocks (i.e., the early Palaeozoic Lancang Group). The gabbros show a slightly developed mylonitic fabric, contrasting with the wall rocks that show intense deformation and the development of pervasive foliation.

During our 2017 survey, we collected a total of 102 gabbro samples from the 27 boreholes. The samples are dark green and display massive structure and fine-grained texture (Figures 2b, 3). The primary mineral assemblage consists of orthopyroxene (5–10 vol%), clinopyroxene (30–35 vol%), plagioclase feldspar (45–50 vol%), and opaque minerals (<5 vol%), as well as subordinate serpentine, amphibole, chlorite, and epidote as secondary phases, which show intergranular texture (Figure 3a–e). The coexistence of equidimensional clinopyroxene, orthopyroxene, and plagioclase suggests a common magma parentage (Figure 3a,b). Orthopyroxenes are subhedral and display straight extinction with alteration to serpentine in rims (Figure 3b,c). Clinopyroxenes exhibit subhedral to anhedral shapes, with alteration to amphibole + chlorite ± epidote along cleavage planes and fractures (Figure 3b,d). The restricted occurrence of amphiboles is considered to be a product of uralitization (e.g., Azer, Gahlan, Asimow, & Alfaifi, 2018). Poikilitic texture is also observed, in which tiny plagioclases constitute the early-formed cumulus phase, followed by pyroxene, which appears as the intercumulus phase (Figure 3e). Plagioclase crystals are mostly anhedral and, in some places, show weak sericitization along their outer surfaces. Opaques with typical surrounding serpentine, amphibole, and chlorite rims support the inference of hydrothermal alteration. Small amounts of pyrite and chalcopyrite are also observed in the groundmass of pyroxene (Figure 3f).

4.1 Zircon U–Pb geochronology

Zircon grains from sample ZK20-7 are mostly transparent euhedral, long-prismatic shape and relatively large size with lengths of 80–130 μm and a width of 40–80 μm (Figure 4a). Some small-sized zircons represent fragments of euhedral, long-prismatic grains. All grains display sharp concentric, even, oscillatory zoning, which, together with their high Th/U ratios of >0.31 (Table 2), suggests a magmatic origin (Hoskin & Schaltegger, 2003). Of a total of 28 zircon geochronological analyses, 23 yielded a restricted range of ²⁰⁶Pb/²³⁸U ages of 143.0 ± 1.8 to 132.8 ± 1.7 Ma, with a weighted-mean age of 139.7 ± 1.3 Ma (1σ; MSWD = 4.9; Table 1; Figure 4a,b). The remaining five zircon grains likely represent inherited zircons and show older ages (ca. 211.2, 217.6, 232.2, 400.7, and 474.4 Ma). The crystallization age of the Huimin gabbroic unit is thus Early Cretaceous.

Trace-element analyses were performed as part of the dating procedure. Twenty-three zircons with uniform ²⁰⁶Pb/²³⁸U ages have similar chondrite-normalized REE patterns, characterized by heavy REE (HREE) enrichment, light REE (LREE) depletion ([La/Yb]_N < 0.00018), and negative Eu (Eu/Eu* = 2Eu_CN/[Sm_CN + Gd_CN] = 0.15–0.35; where CN is chondrite normalized) and positive Ce (Ce/Ce* = 2Ce_CN/[La_CN + Pr_CN] = 33–421) anomalies (Table 2; Figure 4c).

4.2 Whole-rock geochemistry

A degree of low-temperature alteration is observed throughout the rocks, identified by LOI values that range from 2.8 to 5.8 wt% (Table 3). All major elements were normalized on a volatile-free basis to reduce the effect of variable element dilution caused by alteration. The gabbros (LOI-free) have compositions of 45.6–49.5 wt% SiO₂, 15.3–18.4 wt% Al₂O₃, 7.88–16.1 wt% MgO, 9.89–11.9 wt% total Fe₂O₃ (Fe₂O₃^(t)), 0.76–1.24 wt% TiO₂, and 2.61–4.06 wt% total alkalis (Na₂O + K₂O), with Mg# values [100 × Mg/(Mg + Fe₂O₃^(t)) atomic ratio] of 58–74. All of the samples fall into the sub-alkaline field in a total-alkalis–silica (TAS) classification diagram (Middlemost, 1994; Figure 5a) and show a tholeiitic trend in an (Na₂O + K₂O) − Fe₂O₃^(t) – MgO diagram (Irvine & Baragar, 1971; Figure 5b).

The chondrite-normalized REE patterns display slight depletion in LREEs [(La/Yb)_CN = 0.73–0.92], with the one exception of ZK24-5-2 [(La/Yb)_CN = 1.12], and negligible Eu anomalies (Eu/Eu* = 0.99–1.19) (Figure 6a). All of the samples show high and variable enrichment in large-ion lithophile elements (LILEs; e.g., Rb, Ba, U, Pb, and Sr), and relatively flat patterns of high-field-strength elements (HFSEs), with slight depletion in Nb, Ta, and Ti in a primitive-mantle-normalized spider diagram (Figure 6b).

4.3 Whole-rock Sr–Nd isotopic compositions

The selected gabbro samples (n = 6) have initial ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios (t = 139.7 Ma) that range from 0.7079 to 0.7142 and from 0.51266 to 0.51271, respectively (Table 4). Their ε_Nd(t) values show a narrow range from +4.0 to +4.8, with uniform two-stage model ages (T_DM2) of 0.53–0.60 Ga (mean 0.58 Ga). It is noted that sample ZK6-3-4, which has a relatively high LOI value (5.7 wt%), has a very high Sr isotope ratio (0.7142), suggesting that Sr isotopes have been affected by alteration. Hence, these high Sr isotope ratios do not represent the characteristics of magma, and the below discussion of Sr–Nd isotopes therefore focuses on Nd.

5.1 Magmatic processes

It is important to assess whether the gabbro samples analysed during this study underwent low-temperature alteration and open-system processes before determining their mantle sources and what information they can provide about Neo-Tethyan tectonics (DePaolo, 1981). The low-temperature alteration for these samples inferred from their mineralogy (Figure 3) and LOI values (2.8–5.8 wt%; Table 3) is likely related to fluid infiltration after emplacement (e.g., Fan, Wang, Zhang, Zhang, & Zhang, 2010), which can also explain the highly variable contents of some mobile trace elements (e.g., Rb, Ba, Pb, and Sr). The lack of correlations between LOI values and Al₂O₃, Na₂O, La, Nb, and Zr contents, as well as the consistency of the REE and incompatible-element compositions, indicates that the majority of the major- and trace-element contents in our samples have not been affected by the alteration. Zirconium is a generally immobile element (e.g., Polat & Hofmann, 2003), and the fact that the Zr contents of the samples correlate with La, Th, Nb and Yb contents (not shown) indicate that the REE and HFSE contents of the samples have not been substantially affected by low-temperature alteration. Also, the consistency of Nd isotopic ratios and its irrelevance to the LOI values indicates that Nd isotopic ratios have not been affected by alteration.

Mantle-derived magmas are commonly contaminated by continental crust during their ascent (Rudnick & Gao, 2003). The presence of inherited zircons points to the development of contaminated interstitial melting during magmatic evolution (e.g., Wang et al., 2016). However, crustal contamination would induce, to a certain extent, variation in Nd isotopes, and also results in a positive correlation between Nd, MgO and ε_Nd(t) values (e.g., Liu et al., 2010). These features, however, are not observed in the studied gabbro (not shown; Tables 3, 4), ruling out the possibility of strong crustal contamination. Similarly, Nb/La ratios irrespective of SiO₂ content (not shown; Table 3) are inconsistent with crustal contamination (e.g., Fan et al., 2010). Furthermore, the involvement of at least 15% crustal materials (Rudnick & Gao, 2003) would be required in MORB-type magma to match the observed (Nb/La)_N ratios of 0.35–0.56 for these samples. Such a high proportion of crustal contamination into the primary mafic magma would lead to the formation of intermediate–felsic rocks and a marked decrease in the Nd isotopic composition, which is inconsistent with our geochemical data. This inferred lack of crustal contamination is further supported by the following lines of evidence: (a) incompatible-trace-element patterns are parallel, in spite of the presence of some mobile trace elements (Figure 6b); (b) sample ZK36-3-3 has the highest ε_Nd(t) value (+4.8; Table 4), but its SiO₂ (46.5 wt%) and MgO (9.3 wt%) contents are higher and lower than the mean values (45.9 and 9.8 wt%) of all samples, respectively (Table 3); and (c) sample ε_Nd(t) values of +4.0 to +4.8 suggest the influence of magma source region rather than significant crustal contamination en route. All of these arguments indicate negligible crustal contamination during magmatic evolution.

Fractional crystallization of magma also plays an important role in the magmatic evolution and petrogenesis of the resultant rocks. Our samples display variable MgO contents (7.88–16.1 wt%) and linear relationships between Mg# (58–74) and other major oxides (e.g., SiO₂, TiO₂, and CaO; Table 3), implying an evolved magmatic composition. Moreover, the variable contents of compatible elements such as Ni (111–335 ppm) and Cr (186–437 ppm) and generally positive correlations of Ni and Cr with Mg# (Table 3) further suggest that their parental magma underwent a degree of fractional crystallization. Owing to the differences in partition coefficients (Rollinson, 1993), diagrams of Ni versus Cr and V versus Cr can be used to distinguish between the fractional crystallization of olivine, clinopyroxene, and hornblende (Figure 7a,b). The resultant data pattern for the studied samples indicates that fractional crystallization of clinopyroxene, rather than olivine or hornblende, played a major role in magmatic evolution. Plagioclase fractionation was also minor, as inferred from the negligible Eu anomalies (Eu/Eu* = 0.99–1.19). We conclude, therefore, that the parental magma of the gabbroic unit underwent slight fractional crystallization involving mainly clinopyroxene during magmatic evolution.

5.2 Mantle source

Despite the known mobility of some LILEs during low-temperature alteration, the analysed gabbros generally have flat REEs and incompatible-element distribution patterns, with slight depletion in LREEs, Nb, Ta, and Ti in normalized-element diagrams (Figure 6a,b). Such characteristics are highly similar to those expected for N-MORB magma (Sun & McDonough, 1989). Values of Zr/Nb (41.4–50.2), Y/Nb (16.3–19.0), and Yb/Nb (1.6–2.0) for the samples also lie near the range of N-MORB compositions (Zr/Nb = 31.8, Y/Nb = 12.0, and Yb/Nb = 1.3), distinct from the compositions of E-MORB (Zr/Nb = 8.8, Y/Nb = 2.7, and Yb/Nb = 0.3) and ocean island basalt (OIB) (Zr/Nb = 5.8, Y/Nb = 0.6, and Yb/Nb = 0.05; Sun & McDonough, 1989). Furthermore, the relatively depleted ε_Nd(t) values (+3.96 to +4.84) for our samples are highly consistent with those of BABB-type basalts in the central Lhasa block (Early Jurassic Yeba Formation; Wei et al., 2017) and MORB-type ophiolites in the Indus–Tsangpo Suture Zone (Wang et al., 2016; Figure 8).

Diagrams of Yb versus La/Yb and Sm/Yb versus La/Sm based on the nonmodal batch melting equation with REE simulations can be used to constrain the characteristics of the mantle source and the degree of partial melting (Aldanmaz et al., 2000; McKenzie & O'Nions, 1991). Our sample data plot along the spinel lherzolite melting trend on a straight line nearly parallel to the Y-axis (Yb content) in a Yb versus La/Yb diagram (Figure 9a), indicating that the rocks were formed from spinel lherzolite in the mantle with a degree of partial melting of approximately 15–25%. The ratios of La/Sm and La/Yb in a melt that is derived from spinel lherzolite gradually decrease with an increasing degree of partial melting, but the Sm/Yb and Dy/Yb ratios remain almost constant (Aldanmaz et al., 2000). In contrast, mantle melting involving garnet lherzolite material (at depths of >~80 km) generates melts that are relatively depleted in HREEs (e.g., McKenzie & O'Nions, 1991), leading to substantially higher Sm/Yb and Dy/Yb ratios in the melts. Our sample data plot along the spinel lherzolite melting curve in a diagram of Sm/Yb versus La/Sm (Figure 9b), indicating that they originated from 8 to 15% partial melting of a shallow and depleted MORB-type mantle (McKenzie & O'Nions, 1991). This interpretation is further supported by the relatively low Ce/Y ratios (0.31–0.39) in a Ce/Y versus Zr/Nb diagram (Figure 9c) and (Dy/Yb)_CN ratios of 1.09–1.20 in a (Dy/Yb)_CN versus (Gd/Lu)_CN diagram (Figure 9d).

All of these above-mentioned data patterns indicate that the parental melt of the gabbroic unit was generated by relatively high-degree partial melting of a shallow and depleted mantle source (spinel lherzolite), similar to the mantle source of N-MORB.

5.3 Tectonic setting

The studied gabbros have geochemical features similar to those of typical N-MORB, as outlined above. They also fall in a small area between the domains of fluid-related metasomatism and depleted mantle in a (Hf/Sm)_N versus (Ta/La)_N diagram (Figure 10a). Such characteristics can be interpreted as a result of modification by subduction-related melts/fluids (e.g., Fan et al., 2010). However, the slight negative Nb and Ti anomalies and the relatively high (Nb/La)_N values (0.35–0.56) of the samples differ slightly from those of typical arc basalts, which are characterized by marked depletion in Nb and Ti and by (Nb/La)_N values of ~0.27 (e.g., Hawkesworth, Hergt, Ellam, & McDermott, 1991). In addition, our samples plot mostly in the overlapping fields of island-arc basalt (IAB) and MORB in diagrams of Zr/Y versus Zr (Pearce & Norry, 1979; Figure 10b), La/Nb versus La (Furnes et al., 2007; Figure 10c), and Nb × 2 – Zr/4 – Y (Meschede, 1986; Figure 10d). All of these features suggest that the geochemical signature of the studied gabbros is distinct, lying between IAB and MORB.

Rocks with mixed IAB and MORB compositions are commonly considered to have formed in a supra-subduction zone such as a fore-arc location (Macpherson & Hall, 2001; Reagan et al., 2010) or back-arc basin (Shinjo et al., 1999; Shuto et al., 2006; Taylor & Martinez, 2003). Fore-arc volcanic rocks generally have compound rock associations varying from boninitic and high-Mg andesitic to andesitic and felsic components (Macpherson & Hall, 2001; Reagan et al., 2010). This clearly differs from the studied gabbroic unit, which has a rather uniform rock association. More importantly, there is no evidence to suggest the existence of a former fore-arc environment in the Huimin study area, especially considering the fact that the Simao–Indochina and Baoshan (Sibumasu) blocks, on both sides of this area, became completely spliced together after closure of the Paleo-Tethyan ocean basin during the early Mesozoic (Sone & Metcalfe, 2008; Yin & Harrison, 2000). Therefore, a more suitable explanation is a back-arc basin regime exhibiting the geochemical characteristics of both MORB- and arc-like end-members, as suggested for BABBs from Mariana (Gribble et al., 1998; Shinjo et al., 1999; Shuto et al., 2006), Lesser Caucasus (Rolland et al., 2009), NW Hearne (Sandeman et al., 2006), and South Atlantic Ocean (Fretzdorff et al., 2002). The tectonic setting of a back-arc basin can induce the production of widespread magmatic rocks such as IAB, BABB, and MORB-like basalts (e.g., Taylor & Martinez, 2003). BABB has mixed geochemical features that differ from those of IAB and MORB (Shinjo et al., 1999). Our samples plot in the BABB field in diagrams of Zr/Y versus Zr (Figure 10b), indicating that they have a BABB affinity. Moreover, the REE and incompatible-element distribution patterns of the studied gabbro samples are subparallel to those of well-studied BABBs, such as the Okinawa BABB (Shinjo et al., 1999; Figure 6a,b).

A diagram of V versus Ti/1,000 can be used to evaluate the influence of subduction-related melts (Shervais, 1982). MORB-like basalts, which have intermediate Ti/V ratios, show a reduction in Ti/V ratios and exhibit an arc-type basalt trend when they are affected by subduction-related fluids (Pearce, 2014). Our samples have moderate Ti/V ratios (0.001 × Ti/V = 24.3–32.2), falling into the MORB and BABB fields and showing affinity to the Okinawa BABB (Figure 11a). Similarly, the data pattern in a diagram of La/Nb versus Y (Winchester & Floyd, 1977; Figure 11b) provides further support for a BABB-like tectonic setting for the formation of the studied gabbros.

The Th–Nb–Yb system is one of the most useful proxies for identifying subduction-related material and discriminating between different geochemical processes within subduction zone settings (Pearce, 2008). This attribute reflects the fact that Nb and Yb contents remain generally constant within subduction zone environments, whereas Th can be mobilized by subduction-related components, such as slab-derived fluids and sediment-derived melts. This means that supra-subduction zone-related magmas are typically characterized by high Th/Yb and low Nb/Yb ratios (Pearce, 2014). The studied samples show moderately high Th/Yb (0.18–0.42) and low Nb/Yb (0.50–0.62) ratios and plot above the MORB–OIB array (Figure 11c), suggesting the involvement of a subduction-related component. However, these samples still belong to the tholeiitic basalt series and plot far from the volcanic arc array (Figure 11c). Importantly, our samples fall in or near the Okinawa BABB field (Shinjo et al., 1999) rather than in the field of fore-arc basalts from the Izu–Bonin–Mariana arc (Reagan et al., 2010; Yogodzinski et al., 2018) in diagrams of Th/Yb versus Nb/Yb and Ce/Nb versus Th/Nb (Figure 11c,d). It is thus reasonable to conclude that the gabbroic unit in the Huimin study area shows strong BABB-type geochemical affinities and was thus generated in a back-arc rifting (basin) environment.

5.4 Geodynamic implications

The back-arc rifting (basin) environment inferred for the formation of the studied gabbroic unit fits well with the development of a continental rift basin (the Simao–Lanping Basin) during the Jurassic–Cretaceous (Liu, 2013; Yang et al., 2014). The Jurassic–Cretaceous sediments within the basin consist mainly of thick graben deposits, including the J₂ Hepingxiang (Huakaizuo), J₃ Bazhulu, and K₁ Jinxing, Mangang, and Bashahe formations (YBGMR, 1990, 1996), all of which were derived from continental margins (Chen et al., 2004; Yang et al., 2014). The formation mechanism of a typical back-arc rifting (basin) is generally controlled by the geometrical relationships of the subducted slab and the overriding plate, as well as the effect of lateral mantle flow on the mantle wedge (Sdrolias & Müller, 2006). Combining our results with these geodynamic considerations (e.g., YBGMR, 1990, 1996; Pan et al., 2012; Zhu et al., 2013; Wang et al., 2014a; Li et al., 2018), we propose that east-directed subduction (in current orientation) of the Neo-Tethyan oceanic slab was the main cause of this Early Cretaceous BABB-type magmatism and back-arc rifting (basin) environment.

The lithosphere in a back-arc weakens in rheological strength and tends to thin owing to ongoing mantle flow migration and decompression melting (Taylor & Martinez, 2003). Moreover, roll-back of the subducted slab will further intensify these deep processes and provide sufficient space for spreading in the developing back-arc basin (Nakakuki & Mura, 2013). During this study, the inferred slight crustal contamination of the parental magma, the occurrence of fractional crystallization (Figure 7a,b), and the presence of captured zircons (Table 1; Figure 4a), together indicate the existence of a thin slice of continental crust above the mantle source of this unit (e.g., Lin et al., 2020). Moreover, the study area and adjacent areas have no evidence of marine sediments and oceanic-related magmatism since the late Mesozoic (YBGMR, 1990, 1996). All of them indicate that the back-arc rifting (basin) may not have evolved into a wider-scale continental breakup and seafloor spreading. We thus propose a Cretaceous tectonic model of the SE Tibetan Plateau in Figure 12 after referencing the schematics of the Andean margin by Dill (1988) and back-arc rifting zone in southern Tibet by Wei et al. (2017). The model clearly illustrates the causal relationships between the subducting Neo-Tethyan oceanic slab and decompression melting of the overriding lithosphere, development of the back-arc rift basin, and occurrence of BABB-type magmatism in the SE Tibetan Plateau.

In terms of timing and mechanism, a similar back-arc rifting environment to that inferred in this study has also been identified in southern Tibet, as interpreted from the Shiquan River–Nam Tso mélange zone (SNMZ) and Early Jurassic volcanism (Yeba Formation) within the Lhasa Block. The SNMZ extends for more than ~2,000 km and is a tectonic boundary separating a fragment of juvenile crust to the north from the Lhasa microcontinent to the south (Zhu et al., 2011). The BABB characteristics of the basalts associated with the ophiolites along the SNMZ indicate that this zone is a back-arc basin that developed during the earliest Early Cretaceous in response to subduction of the Neo-Tethys oceanic slab (Girardeau et al., 1985; Zhang et al., 2004; Zhu et al., 2013). In addition, Early Jurassic Yeba Formation volcanic rocks in the southern Lhasa Block show the characteristics of transitional I–S type granites, which are understood to have been caused by the initiation of back-arc rifting (Wei et al., 2017). Combining our results with previous findings, it is reasonable to propose a continuous back-arc rifting zone extending from the Lhasa Block to the east of the Sibumasu Block and corresponding to subduction of the Neo-Tethyan oceanic slab beneath the Lhasa and Sibumasu blocks.

A question remains as to whether earlier (e.g., Jurassic) arc magmatism occurred to the west of the Sibumasu Block. The oldest arc magmatic activity currently recorded by the WMA is Early Cretaceous (130–105 Ma; Mitchell et al., 2012; Li et al., 2019). However, initiation of Neo-Tethyan oceanic slab subduction is widely considered to have been at least Early Jurassic (ca. 180 Ma; Chu et al., 2006; Ji et al., 2009; Wen et al., 2008; Zhu et al., 2013). The development of back-arc rifting generally occurs after the initiation of oceanic plate subduction as well as the associated arc magmatism (Nakakuki & Mura, 2013; Sdrolias & Müller, 2006; Taylor & Martinez, 2003). The formation age of ca. 139.7 Ma obtained in this study, combined with the inferred Jurassic–Cretaceous Simao–Lanping rift basin, indicates that earlier (i.e., Jurassic) arc magmatism may have occurred in the WMA or adjacent areas.