1 Introduction

The Songpan-Ganze Orogen (SGO) occupies a large area of the eastern Tibetan Plateau (Fig. 1a), and numerous isolated domes have been identified in its eastern margin. Among them, the Jianglang Dome is the most representative with the most complete tectonostratigraphic units (Yan et al. 1997, 2003a); however, its formation mechanism has been controversial. For instance, Hou (1996) and Hou and Fu (2002) proposed it as a structural dome, which was formed by overlapping of duplex compression and contraction. In contrast, Yan et al. (1997) assigned it as a metamorphic core complex associated with magma-induced uplift caused by lithospheric thermal anomalies. To date, direct evidence of magma-induced doming is still lacking in the core of the Jianglang Dome. During recent field geological mapping, a syenite intrusion was first discovered in this area (Fig. 1b) and was probably responsible for doming.

Fig. 1
figure 1

a Simplified tectonic division map of China, TXO = Tianshan-Xingmeng Orogen, TB. Tarim Block, NCB. North China Block, KQDO. Kunlun-Qinling-Dabie Orogen, SGO. Songpan-Ganze Orogen, YB. Yangtze Block, CB. Cathaysia Block. b Geological map of the Jianglang Dome. c The residual gravity anomaly map of the Jianglang Dome and its vicinity; note that warm colors indicate high gravity, while cool colors indicate low gravity

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Furthermore, a suite of high-grade (average 1.75%) stratiform Cu deposits occurs in the core of the Jianglang Dome, hosted by the Neoproterozoic Liwu Group (Fig. 1b), which is a metamorphosed volcano-sedimentary sequence. These deposits are one of the most important copper resource producers in southwest China (Dai et al. 2016). Due to overall paucity of ore-related minerals amenable to reliable isotopic dating, diverse syngenetic and epigenetic models have been suggested over the years, including: (1) a sedimentary exhalative origin overprinted by metamorphism (Yao 1990); (2) volcanogenic massive sulfide deposits superposed by deformation and metamorphism (Li et al. 2012); (3) a tectono-stratabound hydrothermal origin (Yan et al. 2003b); (4) a metamorphic hydrothermal origin (Ma et al. 2010); and (5) a (post-) magmatic hydrothermal origin (Chen et al. 2011; Zhou et al. 2017). These contrasting genetic models have hindered current mineral exploration, and establishing a correct mineralization model is crucial.

In this study, we employed integrated whole-rock geochemical and zircon U-Pb isotopic analyses of the newly discovered syenite intrusion as well as chalcopyrite Re–Os dating of the stratiform Cu deposits to determine genesis of the Jianglang Dome, genetic model of the stratiform Cu deposits, and rare metal mineralization potential of the syenite intrusion.

2 Regional geology

The SGO resulted from interactions between the Yangtze, North China, and Qiangtang Blocks (Fig. 1a) during the Middle to Late Triassic closure of the Paleo-Tethys Ocean, which induced accretionary orogeny (Roger et al. 2010; Yan et al. 2018a, b). It is exclusively filled by a 5–15-km-thick Triassic flysch that received materials from adjacent blocks (Weislogel et al. 2010). These flysch deposits underwent greenschist to amphibolite facies Barrovian metamorphism at 205–190 Ma (Huang et al. 2003a, b) and were intruded by widespread granitic plutons (Roger et al. 2010). Combined with tectonic evolution of the SGO, these granitoids were classified into syn- to late-orogenic/post-collisional (ca. 220–200 Ma) and post-orogenic (ca. 200–150 Ma) varieties (Roger et al. 2010). Among them, late- and post-orogenic granitoids were likely associated with thickened lower crust, lithospheric delamination, and asthenospheric upwelling, accompanied by 220–150 Ma extentional tectonics (Zhang et al. 2007; Roger et al. 2010).

More than ten isolated domes were distributed along the eastern SGO, with medium- to high-grade metamorphic complexes in their cores (Yan et al. 1997). The Jianglang Dome consists of three tectonostratigraphic units, including: (1) a core complex known as the Liwu Group, (2) an overlying middle slab of the Paleozoic metamorphosed volcano-sedimentary sequences, and (3) a sedimentary cover of the Triassic Xikang Group (Yan et al. 1997, 2003a). These units are separated by ring detachment faults or ductile shear zones (Fig. 1b). The Liwu Group is a metamorphosed volcano-sedimentary sequence with a total thickness of > 3600 m and mainly comprises two-mica quartz schist, quartzite, and minor sandwiched metabasic rocks. Previous studies obtained a weighted mean 206Pb/238U age of 552.8 ± 5.7 Ma for the youngest magmatic zircons within quartzite, and interpreted it as the formation age of the Liwu Group (Li et al. 2016). The middle slab is composed of (1) the Ordovician Jianglang Formation of quartzite, (2) the Silurian Jiaba Formation of siliceous rock, carbonaceous slate, and minor metabasalt interlayer, and (3) the Late Permian Wulaxi Formation of marble with sandwiched metabasalt (Zhu et al. 2020).

Besides, the Wenjiaping and Wulaxi granitic plutons are developed in the north of the Jianglang Dome (Fig. 1b) and intrude the Liwu Group, Jiaba Formation, and Wulaxi Formation. Previous LA-ICP-MS zircon U-Pb dating indicates their emplacement ages of 164.5 ± 0.9 Ma and 164.3 ± 1.7 Ma. Geochemical and zircon Hf isotope data are suggestive of a post-orogenic extensional setting and a main derivation from ancient continental crust (Dai et al. 2017). According to the gravity data we have recently obtained, the core of the Jianglang Dome shows residual gravity low anomalies (Fig. 1c), indicating that there might be a large concealed granitic batholith in the deep (e.g., Mangkhemthong et al. 2020). During recent field geological mapping, a syenite intrusion was newly discovered in the northern core of the Jianglang Dome, near the Zhongzui deposits (Fig. 1b). It clearly intrudes the Liwu Group and shows typically massive structure (Fig. 2a, b).

Fig. 2
figure 2

a, b Field occurrence of syenite that intrudes the Liwu Group; c, d micrographs of syenite dating sample SYL-1; e, f first stage of coexisting silicic and biotitic alteration; g first stage of fine-grained garnet and second stage of megacrystic garnet; h second stage of megacrystic staurolite; i late stage of non-mineralized quartz veins crosscutting foliation; j, k hand specimens and micrographs of massive ores; l, m banded and disseminated ores

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3 Deposit geology

Several stratiform Cu deposits occur within the Liwu Group that occupies the core of the Jianglang Dome, typified by Liwu, Heiniudong, and Zhongzui (Fig. 1b). They are one of the most important copper resource producers in southwest China and have an average Cu grade of ~ 1.75% with erratic high grade up to ~ 20%. These deposits show uniformities in alteration type, mineralization style, and mineral assemblage and were collectively referred to as “Liwu-type” high-grade Cu deposits (Dai et al. 2016).

The alteration envelopes in these stratiform Cu deposits are bedding-parallel (Fig. 3) and are dominated by silicic, biotitic, sericitic, and chloritic alteration, with minor (total < 10 vol%) garnet, tourmaline, and staurolite. The hydrothermal alteration can be divided into three stages according to the paragenetic sequence of minerals: (1) first stage responsible for Cu mineralization, comprising coexisting silicic, biotitic, sericitic, and chloritic alteration as well as fine-grained tourmaline and garnet (Fig. 2e–g); (2) second stage characterized by megacrystic garnet, tourmaline, and staurolite (Fig. 2g, h), which were likely related to post-magmatic high-temperature solutions; (3) late stage represented by non-mineralized quartz and calcite veins crosscutting foliation (Fig. 2i).

Fig. 3
figure 3

Typical exploration lines of the Heiniudong a and Zhongzui b stratiform Cu deposits

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Three distinct styles of sulfide mineralization are identified in these stratiform Cu deposits: (1) massive with a high grade of > 5% Cu (Fig. 2j, k); (2) banded with a medium grade of 5–1% Cu (Fig. 2l); (3) disseminated with a low grade of < 1% Cu (Fig. 2m). The metal sulfides are dominated by chalcopyrite and pyrrhotite, with minor sphalerite and pyrite. The Cu orebodies in the Liwu, Heiniudong, and Zhongzui deposits all occur in hydrothermal alteration envelopes. Specifically, in the Heiniudong deposit, four orebodies were identified based on mineral exploration from top to bottom (Fig. 3a): II4 orebody with an extension of ~ 250 m, I3 main orebody extending > 900 m, I1 orebody with an extension of ~ 700 m, and III1 orebody extending > 350 m. In the Zhongzui deposit, two hydrothermal alteration envelopes were discovered based on geologic mapping and drill cores. They contain two layers of orebodies, including the upper Z1-1 orebody with an extension of > 3.50 km and the lower Z2-1 orebody extending ~ 1.0 km (Fig. 3b).

4 Analytical methods

We collected five syenite intrusion samples as well as five massive ore samples from the Zhongzui and Heiniudong Cu deposits. Their sample locations are marked in Fig. 1b and listed in Table 4. The rock samples are mainly composed of ~ 35 vol% plagioclase, ~ 30 vol% K-feldspar, ~ 25 vol% quartz, ~ 10 vol% biotite, and < 2 vol% sericite (Fig. 2c–d). The massive ore samples chiefly comprise ~ 30 vol% chalcopyrite, ~ 40 vol% pyrrhotite, ~ 20 vol% quartz, and ~ 10 vol% sphalerite (Fig. 2j, k).

Rock samples were crushed for zircon separation (80 mesh) as well as whole-rock geochemical analysis (200 mesh). Ore samples were crushed into 80 mesh for chalcopyrite separation. Zircon and chalcopyrite were separated using conventional heavy liquid and magnetic techniques and then were hand-picked under a binocular microscope at the Regional Geological Survey Institute of Hebei Province (Geotourism Research Center of Hebei Province), Langfang, China.

Whole-rock major and trace elements were analyzed using the ME-XRF26 and ME-MS81 methods at the Analytical Laboratory of ALS Chemex (Guangzhou) Company Limited. Major oxides were measured using a PANalytical Axios-advance (Axios PW4400) x-ray fluorescence spectrometer, with analytical precision < 5%. Trace element measurements were performed using a Perkin-Elmer Elan 9000 ICP-MS, with precision > 10%.

Zircon cathodoluminescent images were taken to examine internal textures and morphology. U-Pb isotopes and trace elements were synchronously analyzed using an Agilent 7900 ICP-MS instrument equipped with an excimer laser ablation system at the Wuhan SampleSolution Analytical Technology Co., Ltd. Zircon standards GJ-1 (601.95 ± 0.4 Ma, Horstwood et al. 2016) and Plešovice (337.13 ± 0.13 Ma, Sláma et al. 2008) were used to monitor the precision and accuracy. They yielded mean 206Pb/238U ages of 601.8 ± 2.8 Ma (n = 4) and 338.1 ± 1.9 Ma (n = 4). Detailed operating conditions and procedures followed those in Liu et al. (2008).

Chalcopyrite Re-Os dating was performed at the National Research Center of Geoanalysis, Chinese Academy of Geological Sciences. Re and Os concentrations and isotope ratios were determined using negative thermal ionization mass spectrometry. A chalcopyrite standard GBW04477/JCBY was used to control reproducibility and instrument stability (Du et al. 2012), and detailed analytical procedures followed those in Du et al. (2004). Re-Os isochron ages were calculated using the online IsoplotR program (Vermeesch 2018).

5 Results

5.1 Major and trace elements

Whole-rock geochemical data of the investigated felsic intrusion are listed in Table 1. In detail, the samples yield SiO2 of 59.46 wt%–61.40 wt%, Al2O3 of 14.30 wt%–14.73 wt%, Fe2O3T of 9.27 wt%–10.96 wt%, Na2O of 4.67 wt%–5.24 wt%, and K2O of 5.04 wt%–5.48 wt%. They have A/CNK ratios [molar Al2O3/(CaO + Na2O + K2O)] of 0.76–0.79 and Mg# values [molar 100 × Mg/(Mg + ΣFe2+)] of 1.22–2.30. Specially, QAP, Q′ versus ANOR and (Na2O + K2O) versus SiO2 versus diagrams indicate that they are syenite in composition (Fig. 4a–c). Furthermore, A/CNK versus SiO2 and K2O versus SiO2 diagrams suggest their metaluminous and shoshonite series affinities (Fig. 4d, e).

Table 1 Whole-rock major (wt%) and trace element (ppm) data of syenite samples in the Jianglang Dome
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Fig. 4
figure 4

a Quartz-alkali feldspar-plagioclase (QAP) diagram (after Le Maitre 2002); b Q′ vs. ANOR diagram (after Streckeisen and Le Maitre 1979); c total alkali vs. silica (TAS) diagram (after Middlemost 1994), with the dashed line from Irvine and Baragar (1971); d molar Al2O3/(CaO + Na2O + K2O) vs. SiO2 diagram; e K2O vs. SiO2 diagram (after Peccerillo and Taylor 1976); f chondrite-normalized REE patterns; g primitive mantle-normalized trace element profiles. Values of chondrite, continental crust, and primitive mantle after Taylor and Mclennan (1985) and Sun and McDonough (1989). Data of ca. 207 Ma granitoids adjacent to deposits including ca. 207 Ma Majingzi two-mica granite (n = 21, Zhang et al. 2022; Zhao et al. 2022) and ca. 209 Ma Ke’eryin aplite (n = 14, Li et al. 2022; Fei et al. 2023). Data of ca. 207 Ma granitoids away from deposits including ca. 205 Ma Taiyanghe monzonite (n = 17, Yuan et al. 2010; Deschamps et al. 2017), ca. 209 Ma Tagong monzogranite (n = 15, Chen et al. 2017), ca. 210–205 Ma Jinsha suture granodiorite (n = 16, Liu et al. 2019), and ca. 207 Ma Riluku monzogranite (n = 19, Zhan et al. 2020)

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The rock samples have ΣREE contents of 216–281 ppm and Eu/Eu* values of 0.97–1.23 calculated using chondritic data. Several rare earth elements and trace elements are present in higher abundances than other elements, such as La (37.7–53.3 ppm), Ce (80.0–109 ppm), Nd (42.2–53.7 ppm), Rb (71.0–83.7 ppm), Ba (454–657 ppm), and Zr (258–395 ppm). Furthermore, all the rocks are LREE-enriched on chondrite-normalized diagram, with (La/Yb)N values of 5.5–7.0 calculated using chondritic data. Their primitive mantle-normalized trace element spidergram shows significant enrichment in large ion lithophile elements (e.g., Rb and K) and depletion in Sr and high-field-strength elements (e.g., Ti). All these geochemical signatures are broadly comparable to the continental crust and ca. 207 Ma granitoids in the SGO (Fig. 4f, g).

5.2 Zircon U-Pb dating

A total of 17 zircon grains were analyzed, and the dating results are provided in Tables 2 and 3. The zircons are angular and prismatic and show dull cathodoluminescent images without apparent oscillatory zoning (Fig. 5a). Their Th/U ratios range between 0.20 and 3.33 (mostly > 0.5), with an average of 1.10 (Fig. 5b). They have ΣREE contents of 1151–2428 ppm, Ce/Ce*values of 1.16–3.92, and Eu/Eu* values of 0.32–0.88, showing obvious HREE-enriched patterns (Fig. 5c).

Table 2 Trace and rare earth element data (ppm) of analyzed zircons within syenite sample SYL-1 in the Jianglang Dome
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Table 3 Zircon U-Pb dating results of syenite sample SYL-1 in the Jianglang Dome
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Fig. 5
figure 5

a Cathodoluminescent images of the dating sample SYL-1; b Th vs. U diagram; c chondrite-normalized REE patterns of the dated zircons, chondrite values after Taylor and Mclennan (1985)

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The zircons are plotted on or near the concordia curve (Fig. 6a), with 206Pb/238U ages of 214.0–198.7 Ma. They have a weighted mean age of 207.1 ± 2.0 Ma (MSWD = 4.3), and this is the best estimate for their crystallization age.

Fig. 6
figure 6

a zircon U-Pb concordia diagram and weighted mean age of the dated zircons; b chalcopyrite Re-Os isochron age for the stratiform Cu deposits

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5.3 Chalcopyrite Re-Os dating

Chalcopyrite Re-Os isotope data are shown in Table 4. Five chalcopyrite separates from the Zhongzui and Heiniudong deposits exhibit variable Re (2.364–6.684 ppb) and Os (0.0044–0.0302 ppb) concentrations. They have 187Re/188Os ratios of 423.8–2827.9 and 187Os/188Os ratios of 7.123–15.424. Their data points yield an isochron age of 207.1 ± 5.3 Ma and an initial 187Os/188Os of 5.766 ± 0.075 (MSWD = 2.5, Fig. 6b).

Table 4 Chalcopyrite Re-Os isotope data for stratiform Cu deposits in the Jianglang Dome
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6 Discussion

6.1 Petrogenesis of the newly discovered syenite intrusion

Zircon commonly contains various trace elements that are sensitive to its formation condition. Grimes et al. (2007) introduced U/Yb versus Hf and U/Yb versus Y diagrams to distinguishing zircon provenance of continental or ocean crust environments. These discrimination diagrams provide a continental derivation for our dated zircons (Fig. 7a, b). They have high Th/U ratios, positive Ce anomalies, negative Eu anomalies, and HREE-enriched patterns (Fig. 5b, c), indicating a dominantly magmatic origin (Hoskin and Schaltegger 2003). Noticeably, their dull cathodoluminescent images are atypical, without oscillatory zoning, which is likely attributed to hydrothermal alteration. Hoskin (2005) adopted (Sm/La)N versus La diagram to discriminate hydrothermal and magmatic zircons. However, many natural zircons were plotted between these two categories (Fig. 7c), and geochemical criteria for distinguishing hydrothermal and magmatic varieties were questioned as insufficient (Bell et al. 2019). More recently, Bell et al. (2016) investigated Jack Hills zircons of Western Australia and defined quantitative criteria of Light Rare Earth Element Index (LREE-I = Dy/Nd + Dy/Sm) for recognizing hydrothermal alteration. They proposed that hydrothermally altered zircons have low LREE-I values of < 30, while unaltered magmatic zircon compositions show high LREE-I values of > 30 (Bell et al. 2016, 2019). In this contribution, the dated zircons have LREE-I values of 367–28 with an average of 123 (Fig. 7d), implying insignificant hydrothermal impacts on these zircons. This is supported by low LOI (loss on ignition) values of 0.60–0.16 according to bulk geochemical data (Table 1). Taken together, our dated zircons within the studied syenite intrusion are dominated by magmatic origin, with negligible hydrothermal fingerprints.

Fig. 7
figure 7

Discriminant diagrams for the dated zircons. a, b U/Yb vs. Hf and Y diagrams (after Grimes et al. 2007); c (Sm/La)N vs. La diagram (after Hoskin 2005); d LREE-I vs. Th/U diagram

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Furthermore, medium-pressure Barrovian-type metamorphism was initiated at ca. 210–205 Ma because of crustal thickening and shortening, and peak metamorphism (600–410 °C and 8–3 kbar) throughout the whole SGO was recorded at ca. 204–190 Ma according to monazite U-Pb and garnet Sm-Nd ages (Huang et al. 2003a, b). In this article, we proposed that ca. 204–190 Ma peak metamorphism probably fingerprinted the morphology of magmatic zircons within the syenite intrusion and was responsible for their atypical cathodoluminescent images (Fig. 5a). However, due to extremely high closure temperature of zircons (974 °C, Cherniak and Watson 2001), their U-Pb isotope systematics remained undisturbed and thus yielded robust dating results. This is exemplified by anatectic zircons, which were formed by anatexis during regional peak metamorphism and show relatively weak luminescence and indistinct oscillatory zoning (e.g., Grant et al. 2009; Lei et al. 2024). Furthermore, the short time interval between magmatism and peak metamorphism signifies that they were related to the same tectonothermal event (Grant et al. 2009).

The Songpan-Ganze Ocean, a branch of the Paleo-Tethys Ocean, was finally closed during the Late Triassic (Yan et al. 2018a, b). This induced the Triassic orogeny, which involved crustal shortening and thickening and widespread emplacement of syn- to late-orogenic/post-collisional granites with ages of 220–200 Ma (Roger et al. 2010). The Late Triassic igneous rocks marked the last episode of orogenic magmatism in the SGO, after which this region entered into a post-orogenic setting (ca. 200–150 Ma, Roger et al. 2010). In this contribution, zircon U-Pb dating results reveal that the newly discovered syenite intrusion was formed at ca. 207 Ma (Fig. 6a), indubitably associated with 220–200 Ma syn- to late-orogenic/post-collisional tectonic regime in the SGO (Roger et al. 2010). This is also confirmed by the Rb versus (Y + Nb) discriminant diagram, which shows that all the rock samples are seated in the field of post-collisional setting (Fig. 8a). Moreover, Patiňo Douce (1999) carried out experimental studies and established pressure discrimination diagrams, which show low-pressure formation conditions for the studied syenite rocks (Fig. 8b). This is in accord with their late-orogenic/post-collisional extension regime.

Fig. 8
figure 8

a Rb vs. (Y + Nb) tectonic discrimination diagram (after Pearce et al. 1984); b Molar (Al2O3 + CaO + Na2O + K2O) vs. molar Al2O3/(CaO + Na2O + K2O) pressure discrimination diagrams (after Patiňo Douce 1999); c, d Nb and Ce vs. 10000×Ga/Al diagram (after Whalen et al. 1987); e Ca/Na2O vs. Al2O3/TiO2 diagram (after Patiňo Douce and Harris 1998); f Rb/Ba vs. Rb/Sr diagram (after Patiňo Douce 1999). Data sources as in Fig. 4

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The rock samples have significantly high (Zr + Nb + Ce + Y) concentrations of 619–436 ppm (Table 1), comparable to those of typical A-type granitoids (> 350 ppm, Whalen et al. 1987). This is also illuminated by 10000×Ga/Al versus Nb and Ce diagrams (Fig. 8c, d). According to zircon saturation thermometry proposed by Boehnke et al. (2013), our rock samples yield temperatures of 740–684 °C with an average of 707 °C (Table 1). Besides, estimated based on the calibration of Watson et al. (2006), the dated zircons show Ti-in-zircon temperatures of 1001–708 °C with an average of 827 °C (Table 3). These data are compatible with high-temperature formation conditions of A-type granitoids (King et al. 1997). Previous studies indicated that A-type granitoids can be formed by: (1) fractional crystallization of mantle-derived mafic magmas (Eby 1992), (2) partial melting of a crustal source (Patiňo Douce 1997), or (3) mixing of the two end members (Yang et al. 2008). On chondrite-normalized diagram and primitive mantle-normalized trace element spidergram (Fig. 4f, g), the syenite samples resemble the patterns of continental crust and thus indicate a dominantly crustal origin. Besides, our dated zircons were all derived from a continental provenance (Fig. 7a, b). These geochemical signatures are compatible with their extremely low Mg# values of 2.30–1.22 (Table 1), which could be significantly elevated by mantle components (Rapp and Watson 1995). Although there is a lack of isotopic evidence, CaO/Na2O versus Al2O3/TiO2 and Rb/Ba versus Rb/Sr diagrams collectively indicate a mixing of basalt-derived (about 20% – 40%) and pelite-derived melts for their magma source (Fig. 8e, f).

As mentioned above, previous works uncovered that the Triassic SGO involves significant shortening and subsequent crustal thickening of the orogenic wedge, and such thickened crust induced the emplacement of ca. 220–200 Ma syn- to late-orogenic granitoids (Roger et al. 2010). Together, the studied syenite intrusion was emplaced at ca. 207 Ma and exhibits a crustal derivation in the Late Triassic SGO, with negligible mantle components. This is in favor of the viewpoint that A-type granitoids formed by partial melting of a crustal source (e.g., Patiňo Douce 1997).

6.2 Multistage epigenetic Cu mineralization in the Jianglang Dome

In the core of the Jianglang Dome, a suite of high-grade stratiform Cu deposits is hosted by the Liwu Group, which was assigned a latest Neoproterozoic age of ca. 553 Ma according to zircon U-Pb dating (Li et al. 2016). In the past few decades, contrasting syngenetic (Yao 1990; Li et al. 2012) and epigenetic (Yan et al. 2003b; Ma et al. 2010; Chen et al. 2011; Zhou et al. 2017) models were proposed for stratiform Cu deposits in this area. Indeed, previous sulfur isotope data of metal sulfides (δ34SV-CDT = 8.7‰–5.6‰, Yan et al. 1997) and boron isotope data of ore-associated tourmalines (δ11B = − 15.47‰ ± 0.83‰ to − 5.91‰ ± 0.67‰, Zhou et al. 2017) suggested a magmatic-hydrothermal affinity for regional stratiform Cu deposits. Furthermore, these deposits are closely associated with widespread hydrothermal alteration (Fig. 2e–g), probably indicating their epigenetic origin. However, robust age determinations are lacking because of the overall paucity of suitable minerals for isotopic dating.

Chalcopyrite Re-Os chronometers are widely used to date mineralization ages of sulfide deposits (e.g., Zhu and Sun et al. 2013). Zhou et al. (2017) first obtained a chalcopyrite Re-Os isochron age of 151.1 ± 4.8 Ma (2σ, n = 5, MSWD = 5.8) for the Liwu and Zhongzui deposits. This is likely suggestive of a post-magmatic hydrothermal origin (Zhou et al. 2017), associated with ca. 164 Ma Wenjiaping and Wulaxi granites (Dai et al. 2017). In this paper, five chalcopyrite separates yield an isochron age of 207.1 ± 5.3 Ma (MSWD = 2.5, Fig. 6b), which markedly postdates the formation age of their ore-hosting rocks (ca. 553 Ma, Li et al. 2016) and places a best estimate for another epigenetic mineralization age. The initial 187Os/188Os ratio of 5.766 ± 0.075 is higher than that of the upper continental crust (1.9–1.4, Peucker-Ehrenbrink and Jahn 2001) and indicates significant crustal contributions (e.g., Mathur et al. 2000; Chen et al. 2016; Soares et al. 2021). Our determinations thus suggest a crustal origin of metals, which is identical with the studied syenite intrusion. Noticeably, our dating result (ca. 207.1 Ma) contrasts the previously published Re-Os isochron age (ca. 151.1 Ma, Zhou et al. 2017). This is probably because of partial disturbance or resetting of Re-Os isotope systems (e.g., Nozaki et al. 2014; Muchez et al. 2015), induced by ca. 164 Ma granitic plutons in this region (Fig. 1b).

Perfectly, our new Re-Os age determination matches the emplacement age of the newly discovered syenite intrusion (207.1 ± 2.0 Ma, Fig. 6a) near the Zhongzui deposit (Fig. 1b), indicating ca. 207 Ma magmatic-hydrothermal mineralization. Considering chalcopyrite Re-Os age of ca. 151 Ma (Zhou et al. 2017), we propose multistage epigenetic mineralization for stratiform Cu deposits in the Jianglang Dome and advocate that ca. 207 Ma Cu mineralization event was overprinted by ca. 151 Ma category. Combined with age results as well as sulfide sulfur isotope (δ34SV-CDT = 8.7‰–5.6‰, Yan et al. 1997) and tourmaline boron isotope (δ11B = − 15.47‰ to − 5.91‰, Zhou et al. 2017) data, these two mineralization events clearly show magmatic-hydrothermal affinities. According to the gravity data we have recently obtained in the core of the Jianglang Dome, this region shows residual gravity low anomalies (Fig. 1c). This probably indicates a large deep-seated granitic batholith (e.g., Mangkhemthong et al. 2020) and further supports magmatic-hydrothermal mineralization in this area.

6.3 Regional comparison and rare metal mineralization potential

In recent decades, several giant rare metal deposits have been discovered in the SGO, e.g., Jiajika and Ke’eryin. These pegmatite-type deposits are probably associated with ca. 209–207 Ma granitoids in their vicinities (e.g., Li et al. 2022; Zhang et al. 2022). In this contribution, based on zircon U-Pb dating, we assign a Late Triassic age (ca. 207 Ma, Fig. 6a) to the newly discovered syenite intrusion in the Jianglang Dome. This age is coeval with ca. 209–207 Ma granitoids related to rare metal mineralization in the SGO. Moreover, field investigation shows abundant granitic pegmatites intruded ancient strata in this region (Fig. 9a), likely indicating certain rare metal mineralization potential. For regional comparison, we select whole-rock geochemical data of (1) granitoids adjacent to rare metal deposits, including ca. 207 Ma Majingzi two-mica granite near Jiajika (n = 21, Zhang et al. 2022; Zhao et al. 2022) and ca. 209 Ma Ke’eryin aplite (n = 14, Li et al. 2022; Fei et al. 2023); (2) granitoids away from deposits, including ca. 205 Ma Taiyanghe monzonite (n = 17, Yuan et al. 2010; Deschamps et al. 2017), ca. 209 Ma Tagong monzogranite (n = 15, Chen et al. 2017), ca. 210–205 Ma Jinsha suture granodiorite (n = 16, Liu et al. 2019), and ca. 207 Ma Riluku monzogranite (n = 19, Zhan et al. 2020). These compiled granitoids (210–205 Ma, n = 102), together with our syenite samples (n = 5), show broadly consistent REE and trace element patterns (Fig. 4f–g) and a common post-collisional regime (Fig. 8a) as well as compositional heterogeneity (Fig. 4a–e and Fig. 8b–f).

Fig. 9
figure 9

a Granitic pegmatite veins intruding the Triassic strata in the Jianglang Dome; b K/Rb vs. SiO2 diagram (after Blevin 2004); c Nb/Ta vs. K/Rb diagram (after Li et al. 2017); d Rb-Ba-Sr diagram (after Elbouseily and Elsokkary 1975); e Nb/Ta vs. Zr/Hf diagram (after Ballouard et al. 2016). Data sources as in Fig. 4

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Previous studies advocated that granitic pegmatites related to rare metal deposits were derived from evolved granitic systems via extreme differentiation (Roda-Robles et al. 2012; Hulsbosch et al. 2014). Blevin (2004) investigated metallogenic classification parameters for granitoid and related rocks of eastern Australia in detail and proposed that those granitoids with K/Rb ratios > 400 can be regarded as being compositionally unevolved. On K/Rb versus SiO2 diagram, ca. 207 Ma granitoids adjacent to Jiajika and Ke’eryin rare metal deposits show strongly evolved signatures, while most granitoids away from deposits as well as our syenite samples are moderately evolved and unevolved (Fig. 9b). This is further illuminated by Nb/Ta versus K/Rb and Rb-Ba-Sr diagrams, which indicate strongly differentiated features for mineralization-related granitoids; however, a purely magmatic system without strong differentiation is suggested for another category (Fig. 9c, d). Using a compilation of published whole-rock geochemical data, Ballouard et al. (2016) uncovered that some immobile element ratios are good geochemical indicators of the fertility of granitic rocks. In this paper, the studied syenite samples show much higher Nb/Ta and Zr/Hf ratios, in contrast to fertile granitoids associated with Sn-W-U and rare metal mineralization (Fig. 9e). Taken together, the newly discovered syenite intrusion in the Jianglang Dome, with unevolved and barren affinities, exhibits negligible rare metal mineralization potential.

6.4 Implications for the nature of the Jianglang Dome

More than ten isolated domes were distributed in the eastern SGO, and they should be formed under a unified geodynamic background (Yan et al. 1997). To date, copper deposits with a certain scale have only been discovered in the core of the Jianglang Dome. Therefore, the genesis of this dome is critical to understanding the formation mechanism of stratiform Cu deposits in its core and even domal metamorphic bodies in the eastern SGO. However, the nature of the Jianglang Dome remains a matter of debate, including (1) a structural dome associated with overlapping of duplex compression and contraction (Hou 1996; Hou and Fu 2002) and (2) a metamorphic core complex related to magma-induced uplift caused by lithospheric thermal anomalies (Yan et al. 1997, 2003a).

Geophysical data indicate residual gravity low anomalies in the core of the Jianglang Dome (Fig. 1c), which is probably attributed to a large concealed granitic batholith in the deep (e.g., Mangkhemthong et al. 2020); however, direct evidence of igneous intrusions is lacking. Fortunately, the Late Triassic syenite intrusion was first discovered during our field geological mapping in this region (Fig. 2a–d). This probably suggests deep-seated granitoid batholith, which was responsible for thermal doming and multistage epigenetic Cu mineralization with magmatic-hydrothermal affinities in the Jianglang Dome (Yan et al. 1997; Zhou et al. 2017). Essentially, these geological events were all attributed to the Late Triassic post-collisional extension in the SGO (Roger et al. 2010; Huang et al. 2003a, b).

7 Conclusions

In this study, a syenite intrusion was first discovered in the core of the Jianglang Dome. It has an emplacement age of 207.1 ± 2.0 Ma, with A-type granite affinities and a crustal origin. This intrusion was attributed to the Late Triassic post-collisional extension in the SGO, and ca. 204–190 Ma peak metamorphism induced atypical morphological features of magmatic zircons. Selected from massive ores of regional stratiform Cu deposits, chalcopyrite Re–Os dating yields an isochron age of 207.1 ± 5.3 Ma and an elevated initial 187Os/188Os ratio of 5.766 ± 0.075, indicating epigenetic mineralization associated with the Late Triassic syenite intrusion.

Compared with ca. 209–207 Ma fertile granitoids in the SGO, the studied syenite intrusion shows unevolved and barren affinities and thus has insignificant rare metal mineralization potential. Synthesized with residual gravity low anomalies in the core of the Jianglang Dome, we prefer thermal doming as a result of magma-induced uplift for its formation mechanism.