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25 June 2026

23 Pages

Interaction Between the Emeishan Mantle Plume and the Lithospheric Mantle: Evidence from Geochemistry and Sr-Nd-Hf-Pb Isotopes of Basalts from the Weng’an Area, Guizhou Province, China

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Hubei Key Laboratory of Planetary Geology and Deep-Space Exploration, School of Earth and Planetary Sciences, China University of Geosciences, Wuhan 430074, China
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Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration, College of Earth Sciences, Guilin University of Technology, Guilin 541004, China
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College of Physics and Electronic Information Engneering, Guilin University of Technology, Guilin 541004, China
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Author to whom correspondence should be addressed.
Minerals2026, 16(7), 672;https://doi.org/10.3390/min16070672
This article belongs to the Special Issue Geochronology and Geochemistry of Alkaline Rocks
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Abstract

The interaction between mantle plumes and the lithospheric mantle is critical for understanding the genesis of flood basalts. The Emeishan Large Igneous Province (ELIP), composed predominantly of basalts with minor picritic rocks and radiating mafic dyke swarms, offers an exceptional natural laboratory for studying this process. In this contribution, we present geochemical and Sr-Nd-Hf-Pb isotopic data for high-Ti basalts from Weng’an, at the easternmost margin of the ELIP. These basalts (TiO2 > 2.8 wt.%, Ti/Y > 500) are enriched in large ion lithophile elements (LILEs: Ba, Th, U) and slightly depleted in high field strength elements (HFSEs: Nb, Ta, Zr, Hf, Y). They exhibit initial (87Sr/86Sr)t ratios of 0.70594–0.70697, εNd(t) of +1.2 to +1.8, εHf(t) of +1.1 to +1.9, and (206Pb/204Pb)t of 18.11–18.51. Their geochemical signatures resemble those of other high-Ti ELIP basalts and ocean island basalts (OIB) but are distinct from those of depleted mantle sources. Trace element patterns and Pb–Pb isotope systematics indicate derivation from a garnet + spinel lherzolite source linked to the Emeishan mantle plume, with ~8–10% input from sub-continental lithospheric mantle (SCLM) metasomatized by slab-derived fluids during ascent. These results provide direct evidence for heterogeneous SCLM contributions to plume-derived magmas and highlight the role of lithospheric heterogeneity in shaping the composition of LIP magmatism.

1. Introduction

The Emeishan Large Igneous Province (ELIP) is located on the western margin of the Yangtze Craton [1] (Figure 1a). The ELIP offers an exceptional natural laboratory for investigating the genetic relationship between mantle plume activity and flood basalt volcanism. It also hosts significant Fe-Ti oxide and Cu-Ni-PGE (i.e., platinum-group elements) sulfide deposits [2,3,4,5,6,7]. The ELIP comprises diverse mafic and felsic lithologies, including continental flood basalts, mafic intrusions, picritic basalts, trachytes, and rhyolites [5,8,9,10], providing critical insights into eruption timing, magmatic evolution, and mantle source characteristics. Geochronological studies indicate that the ELIP formed at ~260 Ma [9,11,12,13,14], coincident with the end-Guadalupian mass extinction event (~256–259 Ma; [13,15,16]).
Figure 1. (a) Schematic map showing the location of the Emeishan Large Igneous Province and (b) distribution of the Emeishan basalts (modified after [17]).
Xu et al. (2001) [17] first proposed the Ti/Y = 500 threshold to divide Emeishan lavas into LT (Ti/Y < 500) and HT (Ti/Y > 500) suites. Integrated geochemical and stratigraphic studies further delineate the ELIP into inner, intermediate, and outer zones (Figure 1b) [16,18,19,20]. The inner and intermediate zones are dominated by thick low-Ti basalt sequences, whereas the outer zone comprises thin high-Ti basalts [5,20,21,22,23,24,25]. The eastern Guizhou region, located in the eastern part of the ELIP, hosts abundant mafic rocks [8,22,26,27,28], including basalts and diabase dikes from Zhijin–Nayong, Shuicheng, Xiongjiachang, Fuquan, Pu’an, and Hezhang. LA-ICP-MS and CA-TIMS zircon/titanite U-Pb dating yield ages of 260–249 Ma for these rocks [26,29,30], consistent with the main ELIP magmatic episode. These basalts provide crucial material for investigating the source characteristics of marginal magmatism and the extent of interaction between plume-derived magmas and the SCLM. Nevertheless, the petrogenesis of high-Ti basalts in the outer zone (Guizhou and Guangxi regions) remains debated. Most studies support a mantle plume origin [8,22,26,31,32], but the role of the SCLM is still controversial. Some authors propose mixing between plume-derived melts and the SCLM [8,26,29,30], whereas others advocate low-degree melting of SCLM enriched by plume activity [22,30].
Investigating the Weng’an basalts provides insights into the relationship between outer-margin basalts and the Emeishan mantle plume, while offering new constraints on the contribution and proportion of the SCLM to the genesis of Guizhou basalts. This study focuses on the outermost ELIP basalts in Weng’an, Guizhou. By integrating petrography, major and trace element geochemistry, and Sr-Nd-Hf-Pb isotopes, and by comparing with basalts from other Guizhou regions, we characterize their petrological assemblages, petrogenesis, and the interplay between the Emeishan mantle plume and SCLM.

2. Geological Background and Petrology

The ELIP spans Yunnan, Guizhou, and Sichuan, with minor exposures in western Guangxi and northern Vietnam (Figure 1b). In the eastern ELIP (Guizhou Province), the Permian basalts unconformably overlie the Lower Permian Maokou Formation limestone [8,22,33,34]. These volcanic rocks display a distinct wedge-shaped geometry, thickening towards the west and thinning towards the east. The thickness decreases from over 1200 m in the western region to only several tens of meters at the eastern margin, where they eventually pinch out [16].
The study area lies in western Weng’an County, within the ELIP’s outer zone. Exposed strata range from the Sinian Dengying Formation to the Cretaceous, with Carboniferous and Jurassic units absent (Figure 2). Sinian–Cambrian rocks consist of NE-trending medium- to thick-bedded dolomite and silty banded limestone. Permian strata form NNE–N–S belts and lack the Lower Series; Middle-Upper Permian units (Liangshan, Qixia, Maokou, Emeishan basalt, Longtan, Changxing, Heshan, and Dalong) are well preserved. Lithologies are uniform: grayish-yellow layered mudstone and gray to dark gray thick- to massive-bedded bioclastic micritic limestone and bioclastic mud-microcrystalline limestone.
Figure 2. Weng’an basalt distribution and sample collection location map (modified after [34]).
In the study area, Emeishan basalts are black, gray–black, gray–green, or blue–black massive tholeiites, locally amygdaloidal. Six discontinuous N–S-trending basalt outcrop belts aligned with regional structural axes are recognized [34]: one along the northern limb of the Longgang synclinorium (17.38 km); two on the western and one on the eastern limb of the Wenggong syncline (18.20 km, 29.61 km, 19.36 km); and one each on the western and eastern limbs of the Weng’an syncline (31.84 km, 13.92 km). Stratigraphically, the Emeishan basalts overlie the Middle Permian Maokou Limestone Formation and are in turn overlain by the late Permian Heshan and Longtan sedimentary Formations (Figure 3a–c).
Figure 3. Field photos of Weng’an basalts (a–c), hand specimen (d) and typical microstructure photos (c,d).
Eleven fresh basalt samples were collected from lava flows in Weng’an County. Hand specimens are gray-black and massive (Figure 3d). Petrographically, the basalts are composed mainly of plagioclase (65–70 vol.%), clinopyroxene (10–15 vol.%), and opaque minerals (3–5 vol.%), with olivine absent. Sparse plagioclase and clinopyroxene phenocrysts, up to 0.5 mm in length, are embedded in a microcrystalline to fine-grained groundmass consisting of plagioclase, clinopyroxene, and Fe–Ti oxides (Figure 4a,b). The groundmass displays a typical intergranular–intersertal texture. Plagioclase, the dominant mineral phase, occurs mainly as microlitic to fine-grained laths (<0.1 mm), whereas clinopyroxene is present as fine- to medium-grained anhedral crystals, some of which show weak chloritization.
Figure 4. Photomicrographs of Weng’an basalts (a,b). Cpx: clinopyroxene; Pl: plagioclase; Chl: chlorite; Fe-Ti: Fe-Ti oxides.

3. Analytical Methods

All whole-rock major and trace element, and Sr-Nd-Hf-Pb isotopic analyses were conducted at the Guangxi Key Laboratory of Hidden Metal Mineral Exploration, Guilin University of Technology. Fresh samples were crushed, and altered rinds, weathered surfaces, and veins were removed by hand. Cleaned fragments were leached in 4 N HCl for 30 min, dried, then ground to <200 mesh powder. Loss on ignition (LOI) was measured after heating at 1000 °C for 2 h. For XRF analysis, ~0.5 g of ignited powder was fused with 6 g Li2B4O7 and 0.2 g NH4Br in a Pt-Au crucible at 1200 °C for 10 min. Glass disks were analyzed on a Rigaku ZSX Primus II XRF (Rigaku Corporation, Tokyo, Japan). Calibration used certified reference materials (CRMs); concentrations were calculated from X-ray intensities. Accuracy and precision were monitored using W-2 and GSR-2: RSDs ≤ 2% for major oxides and ≤5% for trace elements. Full methods follow Zhang et al. [35] and Liu et al. [36].
For trace elements, ~40 mg of powder was digested in 1 mL HNO3 + 2 mL HF at 110 °C for 24 h, then re-dissolved and heated at 190 °C for 48 h in a Parr bomb. After evaporation, the residue was redissolved in 4 mL 25% HNO3 and heated at 150 °C for 4 h. The solution was diluted 2000× with 2% HNO3 and analyzed by Agilent 7500cx ICP-MS (Agilent Technologies, Santa Clara, CA, USA). Rh and Re corrected for drift; calibration used BHVO-2, BCR-2, and GSR-3. RSDs were <3%; biases were within ±5%. Sr-Nd-Hf-Pb isotope work was done in a Class 1000 clean lab. Blanks were <100 pg for Sr, Nd, Hf and <5 pg for Pb. Samples (50–100 mg) were digested in inverse aqua regia, then HF–HNO3–HCl to dissolve refractory phases. Sr and Pb were separated together on Sr Spec® resin: preconditioned with 7 N HCl and 0.3% HNO3 + 0.03% HF; loaded in 2 N HNO3; matrix eluted with graded HNO3; Sr collected in 0.3% HNO3; Pb in 7 N HCl. Nd was purified via AG50W-X8 and Ln Spec® (TrisKem International, Bruz, France) chromatography using HCl gradients. Hf was isolated on UTEVA® + LN® resins, eluted in 0.3 N HCl + 0.05 N HF after Zr/Ti removal with 3% H2O2-HF.
Isotope ratios were determined using a Thermo Scientific Neptune Plus multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS, Thermo Fisher Scientific, Waltham, MA, USA). Instrumental mass discrimination was corrected using standard-sample bracketing (SSB) combined with exponential mass bias correction. Instrument stability and performance were monitored using certified reference materials: NBS 987 for Sr, JNdi-1 for Nd, Alfa14374 for Hf, and NBS 981 for Pb. Internal normalization for mass fractionation was applied using 87Sr/86Sr = 0.1194 for Sr and 143Nd/144Nd = 0.7219 for Nd. Pb isotope ratios were corrected using an instrumental mass fractionation factor of 0.1% amu−1 based on repeated analyses of the NBS 981 standard.
Long-term reproducibility was monitored by repeated analyses of international standards during sample runs, yielding the following results: 87Sr/86Sr = 0.710282 ± 0.000010 (N = 12; 2σ) for NBS 987, 143Nd/144Nd = 0.512098 ± 0.000005 (N = 12; 2σ) for JNdi-1, 176Hf/177Hf = 0.282219 ± 0.000005 (N = 10; 2σ) for Alfa14374. The USGS rock reference material BCR-2 was processed as an unknown sample to monitor the entire analytical procedure (n = 4). The measured values are: 87Sr/86Sr = 0.705061 ± 9, 143Nd/144Nd = 0.512614 ± 6, 176Hf/177Hf = 0.282853 ± 5, 206Pb/204Pb = 18.744 ± 1, 207Pb/206Pb = 15.618 ± 1, 208Pb/206Pb = 38.706 ± 3 (2σ, n = 8).
All results are within the recommended uncertainties of the GeoREM reference database (https://georem.earth/), confirming high precision and accuracy of the isotopic data presented in this study.

4. Results

4.1. Major and Trace Element Characteristics

Eleven fresh basalt samples were analyzed (Table 1). The Weng’an basalts exhibit low SiO2 (45.91–50.81 wt.%) and Al2O3 (12.83–13.46 wt.%), but high MgO (4.28–5.16 wt.%), Na2O (2.05–3.97 wt.%) and TiO2 (3.12–4.49 wt.%). In the Nb/Y vs. Zr/TiO2 × 0.0001 diagram, they plot within the alkali basalt field (Figure 5a). However, most basalt samples are silica-saturated and fall in the subalkaline basalt field in the TAS diagram (Figure 5b), and the AFM discrimination (Figure 5c) confirms that these basalts exhibit typical tholeiitic affinities. As shown in Figure 5d, the Ti/Y ratios exceed 500 in nine of the eleven analyzed samples, ranging from 471 to 630 with a mean value of ~550 (Table 1). Meanwhile, their TiO2 concentrations (3.12–4.49 wt.%) plot adjacent to the boundary of the documented high-Ti basalt domain of the ELIP.
Table 1. Analysis results of major elements (wt.%) and trace elements (ppm) of Weng’an basalts.
Figure 5. (a) Nb/Y vs. Zr/TiO2 × 0.0001 diagram (, modified after [37]), (b) SiO2 vs. Na2O + K2O (TAS) diagram (modified after [38]), (c) Na2O +K2O vs. FeOT vs. MgO (AFM) diagram (modified after [39]) and (d) TiO2 vs. Ti/Y diagram (modified after [12]). ELIP high Ti basalt data source: [17,22,23,24,40]; ELIP low Ti basalt data source: [17,24]; Guizhou mafic rock data source: [22,26,27,28,30,33].
The Harker diagram presents variations in Ni (ppm), Fe2O3T(wt.%), CaO (wt.%), CaO/Al2O3, Al2O3 (wt.%), and TiO2 (wt.%) against MgO (wt.%) as the differentiation index (Figure 6a–e). In the Weng’an basalts, Ni (Figure 6a), CaO (Figure 6c), CaO/Al2O3 ratio (Figure 6d) and Al2O3 (Figure 6e) all show very weak positive correlations with MgO. In contrast, MgO exhibits a strong positive correlation with Fe2O3T (Figure 6b), while its correlation with TiO2 is weakly negative (Figure 6e).
Figure 6. Harker diagram of (a) MgO vs. Ni, (b) MgO vs. Fe2O3T, (c) MgO vs. CaO, (d) MgO vs. CaO/Al2O3, (e) MgO vs. Al2O3 and (f) MgO vs. TiO2 for the Weng’an basalts. The data sources and data symbols are the same as in Figure 5.
Chondrite-normalized REE patterns show marked LREE enrichment relative to HREE [(La/Yb)N = 8.0–11.8, (La/Sm)N = 2.2–2.6, δEu(δEu = EuN/[SmN × GdN]1/2) = 0.9–1.1], similar to OIB (Figure 7a). The primitive mantle-normalized incompatible element spider diagram reveals enrichment in Ba and LILE (Th, U), with slight HFSE depletion (Nb = 20.1–40.3 ppm, Ta = 1.12–2.07 ppm, Zr = 158–355 ppm, Y = 21.9–41.4 ppm; Figure 7b). Their Nb/La (0.79–1.08) and Th/Ta (2.03–3.56, mean 2.79) ratios approximate primitive mantle values (Nb/La = 1, Th/Ta = 2.3; [41,42]). The slightly lower Nb/La compared to OIB is attributed to weak slab-fluid metasomatism in the lithospheric mantle source. Ce/Pb (12.7–21.5) and Nb/U (23.8–32.2), deviating from typical MORB/OIB mantle values, are controlled by crust/SCLM material input featured by a positive Pb anomaly, which proves lithospheric component addition.
Figure 7. Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element patterns (b) for the basalt from Weng’an. The chondrites, primitive mantle, OIB and E-MORB values are reported by [43]; the data sources for the Emeishan high-Ti and low-Ti basalts are the same as in Figure 4.
Although fractional crystallization of olivine and Fe-Ti oxides has lowered MgO contents and may have slightly modified absolute TiO2 concentrations, the consistently high Ti/Y ratios and characteristic OIB-like incompatible element patterns securely classify the Weng’an basalts within the high-Ti suite of the ELIP. The minor deviations in Ti/Y and HFSEs from inner-zone high-Ti basalts are interpreted as resulting from a higher degree of fractional crystallization and enhanced SCLM input in the outer zone. Therefore, following Xu et al. [17], we classified them as high-Ti basalts.

4.2. Sr-Nd-Hf-Pb Isotopic Characteristics

Isotopic data, corrected to 260 Ma, are presented in Table 2. The Weng’an basalts display narrow ranges: initial 87Sr/86Sr = 0.70594–0.70697, 143Nd/144Nd = 0.51237–0.51240, εNd(t) = +1.2 to +1.8, 176Hf/177Hf = 0.28265–0.28268, εHf(t) = +1.1 to +1.9, 206Pb/204Pb = 18.11–18.51, 207Pb/204Pb = 15.58–15.60, and 208Pb/204Pb = 38.31–38.72. They exhibit DUPAL-type Pb isotopic signatures (Δ7/4 = 8.8–11.5, Δ8/4 = 59.2–70.6; [44]).
Table 2. Sr-Nd-Hf-Pb isotope analysis results of Weng’an basalts.
In the 87Sr/86Sr vs. εNd(t) diagram (Figure 8a), Weng’an basalts plot near the OIB field, overlapping with ELIP high-Ti basalts and Guizhou mafic rocks, distinct from depleted mantle (DM). In the 206Pb/204Pb vs. 87Sr/86Sr diagram (Figure 8b), they fall between EMI and EMII. In the εNd(t) vs. εHf(t) diagram (Figure 8c), they plot within the OIB field.
Figure 8. (87Sr/86Sr)i vs. εNd(t) diagram of Weng’an basalt ((a), modified after [45]), (206Pb/204Pb)t vs. (87Sr/86Sr)i diagram ((b), modified from [21]) and εNd(t) vs. εHf(t) diagram (c) Mantle components fields are from [46]; data sources and data symbols are the same as in Figure 4.

5. Discussion

5.1. Fractional Crystallization and Crustal Contamination

Compared to primitive mantle-derived magmas (MgO > 9 wt.%, Cr > 1000 ppm; [47]), the Weng’an basalts have lower MgO (4.28–5.16 wt.%) and Cr (23.4–55.7 ppm), indicating evolved compositions. Weak positive correlations between Ni, CaO, CaO/Al2O3 and MgO suggest limited clinopyroxene fractionation, whereas the positive Al2O3–MgO correlation and weak Sr negative anomalies reflect plagioclase fractionation (Figure 6). Positive TiO2-MgO and Fe2O3T-MgO correlations are consistent with Fe–Ti oxide crystallization, as observed petrographically.
The Th/Ta ratios of the Weng’an basalts (2.03–3.56, mean 2.79) are close to primitive mantle values (~2.3; [41]) and significantly lower than upper-crustal values (>10), implying minimal crustal contamination. Their stable Nb/La ratios (Figure 9a,b), along with elevated Mg# and narrowly constrained εNd(t) and initial 87Sr/86Sr values across the Mg# range (Figure 9c,d), strongly argue against significant crustal contamination. The isotopic homogeneity further supports a primary mantle-derived source, modified by subduction-related metasomatism rather than assimilation of upper-crustal material.
Figure 9. (a) Zr/Nb vs. Nb/La diagram, ((b) Nb vs. Nb/La diagram (modified by Xia et al. [48]), (c) Mg# vs. 87Sr/86Sr(t), and (d) Mg# vs. εNd(t) for Weng’an basalts. The data sources and data symbols are the same as in Figure 5.

5.2. Petrogenesis and Source Characteristics

Previous studies have demonstrated that the mafic rocks in the Guizhou region represent products of outer-zone magmatism associated with the ELIP [8,27,40,45]. The Weng’an basalts exhibit enrichment in LILE and LREE (Figure 5a,b), coupled with low initial 87Sr/86Sr ratios and high εNd(t) values (Figure 7a)—isotopic signatures analogous to ELIP high-Ti basalts. These features suggest a shared mantle source. However, subtle depletions in Nb, Ta, Zr, and Hf relative to ELIP high-Ti basalts (Figure 7b) indicate potential contributions from enriched components, such as lithospheric mantle or subduction-related materials.
Slab fluids represent the primary Pb source, whereas enrichment in immobile elements (e.g., Th, Nd, Nb, Yb, Y) characterizes melts derived from oceanic crust sediments and igneous rocks [49]. Elevated Th and LREE abundances, together with high Th/Ce ratios, are diagnostic of oceanic sediment-derived melts, whereas oceanic igneous melts typically display lower Th/La, Th/Nb, and Th/Yb ratios [49,50]. The Weng’an high-Ti basalts show elevated Th/Sm (0.53–0.63), Th/Ce (0.06–0.09), and Th/Yb (1.42–2.22) ratios, implying subducted sediment input into their source. Furthermore, the invariant Th/Yb and Th/Nb ratios with respect to Ba/La and U/Th (Figure 10a,b) reflect slab fluid signatures, consistent with previous reports of subduction-modified mantle sources for ELIP basalts [19,51,52,53]. Collectively, these observations indicate that the source of the Weng’an basalts underwent slab fluid metasomatism.
Figure 10. Th/Yb vs. Ba/La diagram (a) and Th/Nb vs. U/Th diagram (b) of Weng’an basalt. The global subduction sediments (GLOSS) and bulk continental crust (BCC) data are derived from [54]. The data sources and data symbols are the same as in Figure 5.
Weaver. 1991 [40] identified three OIB-enriched end-members based on source characteristics: HIMU (recycled dehydrated oceanic crust with Ta-Nb-rich residues), EMI (HIMU mixed with minor deep-sea sediments, enriched in Ba), and EMII (HIMU with minor terrigenous sediments). Geochemically, mantle plume-derived melts typically display OIB-like signatures with moderately radiogenic Pb isotopic compositions, low radiogenic 87Sr/86Sr, and high positive εNd(t) values near the HIMU end-members. In contrast, metasomatized subcontinental lithospheric mantle (SCLM) is distinguished by more radiogenic Pb isotopic ratios, elevated 87Sr/86Sr, and low εNd(t) values, plotting between the EMI and EMII end-members, as a result of long-term modification by subduction fluids and melts [55]. In Pb-isotope space, the Weng’an basalts plot within the overlapping field of SCLM and Emeishan basalts, between the EMI and EMII end-members, suggesting SCLM contribution to their source (Figure 11a–d). Thus, the Weng’an basalts likely originated from an EMI–EMII mantle source contaminated with sediments, with mixing of SCLM during ascent. To quantify the SCLM contribution, we applied a binary mixing model using initial (87Sr/86Sr)i and εNd(t) values. Picritic magmas represent parental melts for flood basalts; therefore, ELIP picrites (Sr = 28.75 ppm, (87Sr/86Sr)i = 0.70356, Nd = 9.56 ppm, εNd(t) = 4.10; [56]) were selected as the mantle plume end-member. The SCLM end-member (metasomatized, with enriched Nd isotopic composition) was defined as Sr = 541 ppm, (87Sr/86Sr)i = 0.70849, Nd = 20.5 ppm, εNd(t) = −10.87 [18]. Published Sr-Nd data of Emeishan outer-zone basalts in Guizhou Province [8] show that high-Ti basalts have initial (87Sr/86Sr)i ranging from 0.70425 to 0.70595 and εNd (t) of +0.4~+3.6 (local data from Shuicheng, Luodian, and western Guangxi adjacent to southern Guizhou). Our minimum (87Sr/86Sr)i = 0.7059 falls exactly within the regional range, which is markedly higher than typical mantle-plume OIB (~0.7030) and represents a highly radiogenic Sr signature rather than a primitive plume composition. Two factors account for the elevated Sr ratios of our evolved basalts: (1) contamination from enriched ancient SCLM metasomatized by pre-Permian subduction fluids, analogous to HT2 basalts in Ethiopia [55]; and (2) minor upper crustal contamination during magma ascent plus post-eruption low-temperature alteration in Guizhou outcrops. Therefore, the elevated Sr signature is consistent with regional ELIP geochemical features rather than a pure mantle plume origin. Meanwhile, the modeling result indicates that the Weng’an magmas formed by mixing 8–10% SCLM melt with Emeishan plume-derived melts (Figure 12). It should be noted that not all SCLM domains yield high Sr isotope ratios; the radiogenic signature observed here is specific to the metasomatized lithospheric mantle beneath the eastern Yangtze Block, which records a long history of subduction modification.
Figure 11. (a) (206Pb/204Pb)t vs. (208Pb/204Pb)t diagram, (b) (206Pb/204Pb)t vs. (207Pb/204Pb)t diagram, (c) 208Pb*/206Pb* vs. 207Pb*/206Pb* diagram and (d) (208Pb*/206Pb*)t vs. (206Pb/204Pb)t diagram of Weng’an basalt. SCLM, Pacific and Indian Ocean MORB, EMI, EMII data are derived from the PetDB database (http://www.earthchem.org), Emeishan basalt data from the Georoc database (http://georoc.mpch-mainz.gwdg.de/georoc/Entry.html, accessed on 15 March 2026); Northern Hemisphere Reference Line (NHRL) from [44].
Figure 12. (87Sr/86Sr)i and εNd(t) constitute a binary simple mixed model. The Emeishan mantle plume: Sr = 28.75 ppm, (87Sr/86Sr)i = 0.70356, Nd = 9.56 ppm, εNd(t) = 4.10 ([56]); SCLM: Sr = 541 ppm, (87Sr/86Sr)i = 0.70849, Nd = 20.5 ppm, εNd(t) = −10.87 ([18]); symbols are the same as in Figure 4.
Elemental ratios (Th/Yb, Nb/Yb, TiO2/Yb) remain relatively constant during partial melting and fractional crystallization, and thus effectively trace mantle sources. Thorium is fluid-immobile, making Th/Yb a robust proxy for source enrichment or depletion [57]. In the Ta/Yb–Th/Yb diagram, the Weng’an basalts plot within the OIB field, overlapping with ELIP and Guizhou mafic rocks, confirming a shared mantle source (Figure 13a). The degree of partial melting correlates with initial melting depth [58,59]; ELIP high-Ti basalts are interpreted to result from low-degree melting of garnet lherzolite [7,24,45,60]. REE patterns and Sm/Yb ratios are minimally affected by fractional crystallization and constrain melting degree and source enrichment [20,61,62]. In the Sm–Sm/Yb diagram, the Weng’an basalts plot between the melting curves of garnet + spinel lherzolite and garnet lherzolite, indicating a transitional source lithology and 5–15% partial melting (Figure 13b), followed by variable fractional crystallization to produce the high-Ti melts.
In summary, the Weng’an basalts share geochemical affinities with ELIP high-Ti basalts but exhibit subtle Nb-Ta-Zr-Hf depletions indicative of lithospheric mantle or subducted material involvement. Pb-isotope systematics reveal an EMI–EMII source with SCLM mixing and sediment contamination, supported by high Th/Sm, Th/Ce, and Th/Yb ratios as well as enriched εNd(t) values reflecting slab fluid modification. Sm–Sm/Yb modeling indicates a transitional source from garnet + spinel lherzolite to garnet lherzolite with 5–15% partial melting. We propose that the Weng’an basalts formed through low-degree melting of the Emeishan plume mantle, subsequently mixed with 8–10% subduction-metasomatized SCLM melt. This interpretation is further supported by the recent work of Li et al. [63], who documented the internal anatomy of the Emeishan mantle plume head and recognized radial fingering structures and small plumelets developed at the plume margin. Such marginal plumelets facilitated enhanced interaction between plume melts and the overlying heterogeneous lithospheric mantle, consistent with the geochemical and isotopic characteristics observed in the Weng’an high-Ti basalts.
Figure 13. (a) Ta/Yb vs. Th/Yb diagram (, modified after [57]) and (b) Sm vs. Sm/Yb diagram (modified after [64]) of Weng’an basalts. The data sources and data symbols are the same as in Figure 4.

5.3. Heterogeneous Lithospheric Mantle Contamination

Geochemical variations among ELIP basalts have generated considerable debate regarding their petrogenesis, particularly concerning the origin of high-Ti and low-Ti basaltic suites. Extensive research has identified heterogeneous mantle sources [2,11,18,20,24,25,65,66,67], variable lithospheric thickness, crustal contamination, and melting parameters (pressure and degree of partial melting; [12,17,18,21,22,24,29,60,68,69,70,71]), as well as magma mixing processes [29], as the primary controls on the compositional dichotomy between high-Ti and low-Ti basalts.
Previous studies have documented substantial geochemical variability among mafic rocks in Guizhou Province. Hezhang basalts and southern Guizhou mafic dikes are enriched in LILEs (Rb, Ba, Th, U) but depleted in HFSEs (Nb, Ta, Zr, Yb; [27,30]). Conversely, Zhijin and Nayong basalts exhibit HFSE (Nb, Ta) enrichment [22], whereas Shuicheng, Xiongjiachang, and Fuquan basalts show Ta enrichment coupled with Rb depletion [8]. These regional variations extend to isotopic signatures: Shuicheng, Xiongjiachang, and Fuquan basalts display low Sr and high Nd isotopic compositions ((87Sr/86Sr)i = 0.70459–0.70503, εNd(t) = +1.3 to +2.7; [33]), whereas Weng’an and Xiongjiachang basalts analyzed by Yang and Liu (2019) [28] exhibit elevated Sr and depleted Nd signatures ((87Sr/86Sr)i = 0.70571–0.70648, εNd(t) = +0.8 to +1.2). Furthermore, distinct enriched mantle components are identified regionally: an EMI-type source for Shuicheng–Xiongjiachang–Fuquan [8] versus an EMII-type source for Zhijin–Nayong [22]. Critically, these basalts underwent comparable fractional crystallization (predominantly plagioclase with minor clinopyroxene) and show negligible to minimal crustal contamination. Consequently, this geochemical diversity fundamentally reflects source heterogeneity rather than differentiation processes. The relative isotopic homogeneity of plume-derived magmas implies that regional magmatism involved variable interaction with heterogeneous SCLM. Analogous scenarios are documented within the ELIP inner zone: Xu et al. [66] demonstrated high-Ti magma generation from SCLM-contaminated plume sources, while He et al. [68] proposed that low-Ti basalts formed through mixing of voluminous hot plume melts with minor SCLM melts.
The South China Block has experienced multiple Proterozoic-Phanerozoic tectonothermal events that potentially modified its lithospheric mantle. Li et al. [72] identified four episodes of inherited zircons in Beisi Formation dacites from Pingxiang, southwestern Guangxi, corresponding to: (1) Neoproterozoic subduction-accretion-rifting-reaccretion of the South China Ocean between the Yangtze and Cathaysia blocks; (2) Nanhuan rifting of the amalgamated Yangtze-Cathaysia Block; (3) Early Paleozoic Caledonian orogeny; and (4) Paleotethyan ocean closure. These data confirm recurrent lithospheric modification. During subduction, incompatible element- and water-rich sediments are incorporated into the SCLM [73], inducing metasomatic enrichment that manifests as low εNd(t) values. Additionally, SCLM subjected to melt extraction exhibits low Sm/Nd ratios and elevated Nd concentrations, evolving toward low 143Nd/144Nd ratios (εNd(t) < 0) over time. Subsequent metasomatism can generate Rb/Sr, Sm/Nd, and Lu/Hf ratios comparable to those of crustal lithologies [74]. Guizhou Province lies at the junction between the Yangtze Block and the South China Fold Belt, where the SCLM likely records the effects of these multiphase tectonic events, resulting in pronounced compositional heterogeneity. Numerous studies confirm SCLM involvement in Guizhou high-Ti and low-Ti mafic magmatism [8,22,26,29,30], representing a plausible mechanism for the observed geochemical diversity.
Recent studies have revealed that the Emeishan plume head has an asymmetric, fingering structure, and small plumelets were formed at the periphery, which controlled the magma origin and eruption in the outer zone of the ELIP [63]. Our interpretation is consistent with the new model proposed by [63], who documented radial fingering structures of the Emeishan plume head allowed small plumelets to be trapped beneath the thinned lithosphere in the outer zone, resulting in distinctive magma compositions through plume–lithosphere interaction. The geochemical and isotopic signatures of the Weng’an basalts thus record the fingerprint of such marginal plumelets and their interaction with the heterogeneous subcontinental lithospheric mantle.
As previously established, the Weng’an basalts originated from mixing between the Emeishan plume and SCLM-derived melts. The plume end-member resembles OIB sources, which may contain enriched recycled oceanic crustal components. Given that Sm is more concentrated in oceanic crust and sediments than in enriched mantle, incorporation of such materials elevates the source Sm/Nd ratio, generating radiogenic Nd isotopic compositions over time [73]. Integrating regional geological constraints, we propose the following petrogenetic model shown in Figure 14. The study area occupies the eastern margin of the ELIP (Figure 1), where stratigraphic and geophysical data indicate thickened lithosphere [16,20]. During the Middle–Late Permian, the ascent of the Emeishan mantle plume triggered partial melting of the heterogeneous Yangtze Block SCLM. Mixing between plume-derived and SCLM-derived melts subsequently produced the parental magmas of the Weng’an high-Ti basalts.
Figure 14. Genetic model for the Weng’an high-Ti basalt and related Emeishan mantle plume (modified after [22]).

5.4. Comparison with Other LIPs

The Emeishan LIP is not unique in showing a spatial partitioning between low-Ti and high-Ti basalts. Similar patterns have been documented in the Ethiopian LIP [74,75,76,77] and the Parana–Etendeka LIP [78,79,80,81,82,83,84], and a range of petrogenetic interpretations have been presented for each province.
In the Ethiopian LIP, basaltic lavas are zonally arranged: the inner zone (near the inferred plume axis) is characterized by very high-Ti transitional basalts and picrites (HT2) with higher Nb/La ratios and positive εNd(t), the middle zone by high-Ti tholeiitic basalts (HT1) enriched in incompatible elements and characterized by intermediate εNd(t) and 87Sr/86Sr, and the outer zone is dominated by low-Ti tholeiitic basalts (LT) with lower Nb/La ratios and εNd(t), but higher 87Sr/86Sr [75,76]. This zonation has been attributed to variable degrees of partial melting of the plume head and increasing contribution from metasomatized SCLM toward the periphery [74,75,76]. The Weng’an high-Ti basalts share many of these geochemical features with the HT1 series of the Ethiopian LIP, including intermediate TiO2 contents (3.12–4.49 wt.%), elevated LILE and LREE, negative Nb–Ta anomalies, and EMI–EMII-like Pb isotopes, suggesting a similar petrogenetic model: low-degree melting of the plume head followed by 8–10% mixing with SCLM melts.
The Parana–Etendeka LIP exhibits a more complex pattern. High-Ti basalts (e.g., Urubici, Pitanga, Paranapanema) are not strictly confined to the outer zone; instead, they occur in several distinct geographic provinces and are linked to specific enriched mantle sources (EMI, EMII, or mixtures) that are interpreted to have been inherited from ancient subduction-modified lithosphere [82]. Low-Ti basalts (Gramado, Esmeralda) are generally more contaminated by the upper crust and require higher degrees of partial melting [81,85]. Importantly, some high-Ti basalts in Paraná-Etendeka (e.g., Urubici) are considered to be direct low-degree melts of a heterogeneous OIB-type plume with only minor SCLM contribution [82], in contrast to the Weng’an basalts, which require a measurable SCLM component.
What explains these inter-LIP differences? The “inverse” zonation (high-Ti outside, low-Ti inside) observed in both Emeishan and Ethiopia, but not in Paraná-Etendeka, likely reflects the following controlling factors. (1) Lithospheric thickness and thermal structure: A thinner lithosphere at the margin promotes lower-degree melting of the plume and allows more time for magma–lithosphere interaction. (2) Pre-existing SCLM heterogeneity: The Yangtze Craton has experienced multiple Proterozoic–Paleozoic subduction events that metasomatized its lithospheric mantle, creating high−87Sr/86Sr, low-εNd(t) domains that are preferentially sampled in the outer zone. (3) Plume head geometry: Recent geophysical and geochemical studies suggest that the Emeishan plume head is asymmetric and likely develops radial fingering structures and small plumelets at its periphery [63]. These inferred plumelets facilitate enhanced interaction between plume-derived melts and the overlying heterogeneous SCLM, resulting in the geochemical signatures observed in Weng’an. The lack of well-developed marginal plumelets in Paraná-Etendeka may explain why its high-Ti basalts are not systematically associated with SCLM mixing.
Thus, while all three LIPs share the fundamental process of plume–lithosphere interaction, the specific expression of HT/LT zonation is controlled by regional lithospheric architecture, pre-existing mantle enrichment, and the detailed dynamics of the plume head. The Weng’an basalts provide a clear example of how outer-zone magmatism can record the geochemical fingerprint of inferred marginal plumelets interacting with a metasomatized SCLM.

6. Conclusions

  • The Weng’an basalts are massive tholeiites without olivine, dominated by plagioclase and clinopyroxene with minor Fe–Ti oxides, and show an intergranular–intersertal texture. These rocks are high-Ti basalts (TiO2 > 2.8 wt.%, Ti/Y > 500), sharing major and trace element as well as Sr-Nd-Hf-Pb isotopic characteristics with ELIP high-Ti basalts, confirming their genetic affinity with the Emeishan mantle plume.
  • The Weng’an basalts were derived from garnet + spinel lherzolite source rocks that underwent 5–15% partial melting. The magmas experienced fractional crystallization of plagioclase, Fe–Ti oxides, and minor clinopyroxene, with limited crustal contamination.
  • The Weng’an basalts formed by mixing Emeishan plume-derived melts with ~8–10% SCLM components that had been metasomatized by slab fluids. The heterogeneous SCLM reflects multiple tectono-thermal events in the South China Block.

Author Contributions

Conceptualization, B.-H.W. and Y.L.; Methodology, B.-H.W., Y.L. and X.-J.L.; software, G.-D.H.; validation, L.X., Z.-L.L., X.-Z.J. and X.L.; formal analysis, B.-H.W. and Y.L.; investigation, X.-J.L., W.-M.H. and G.-D.H.; resources, X.-J.L. and Z.-L.L.; data curation, J.-H.L.; writing—original draft preparation, B.-H.W., Y.L. and Z.-L.L.; writing—review and editing, L.X. and X.-J.L.; visualization, B.-H.W., Y.L., Z.-L.L. and X.-J.L.; supervision, L.X. and X.-J.L.; project administration, Z.-L.L. and X.-J.L.; funding acquisition, X.-J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Natural Science Foundation of China (Grants 42473063), Guangxi Science and Technology Program (Grants Guike LT2600640017), the Seventh Guangxi Zhuang Autonomous Region Bagui Scholars Program (to Liu Xi-Jun) and Guilin University of Technology Scientific Research Startup Fund Project (to Li Zheng-Lin).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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