Mantle Metasomatism and REE Enrichment in the Genetic Source of the Dalucao Carbonatite Complex (Sichuan, China): Insights from Elemental Geochemistry and In-Situ Sr Isotopes of Two Calcite Types

Rare earth elements (REEs) are a critical resource in high demand, primarily from high-technology industries. There is a growing global interest in the discovery and comprehension of REE resources. Carbonatites, igneous rocks comprising over 50% carbonate minerals by volume, are often found in temporal and spatial association with alkaline rocks [1, 2]. Geochemically, carbonatites exhibit the highest average concentration of REEs among all magmatic rocks, making them prime targets for REE exploration [3]. These rocks are considered characteristic mantle-derived products originating from carbonated lithospheric mantle sources [4, 5]. In recent decades, a global correlation between economically significant REE deposits and carbonatites has been widely recognized, such as Bayan Obo [6] and South Qinling in China [7-9] and Mountain Pass [10] and Wicheeda in North America [11]. The question of the mechanisms responsible for the enrichment of REEs in fertile carbonatites has drawn sustained attention from geologists [5, 12]. Traditional explanations suggest that REE enrichment in carbonatites is associated with liquid immiscibility [13] or fractional crystallization of alkaline silicate–carbonate melts [14], both of which are used to elucidate the origin of carbonatites. An increasing body of research indicates that slab subduction is a crucial geological process responsible for transferring REEs into Earth’s interior and controlling REE concentrations in mantle-derived carbonatitic magmas [15]. REE contents in carbonatites are generally believed to be primarily sourced from the subcontinental lithospheric mantle (SCLM), which has been previously metasomatized by subduction input of crustal materials [5, 16]. However, the processes by which subducted materials, especially marine sediments and/or altered oceanic crust, intrude and ultimately ascend into the SCLM, as well as the nature of the metasomatized SCLM capable of generating fertile carbonatitic magmas, remain enigmatic.

Calcite, as the predominant carbonate mineral in carbonatites, plays a pivotal role in systematic geochemical analysis to address the mentioned deficiencies. Several key factors support this choice: First, calcite is ubiquitous in all carbonatite types, including calcio-, magnesio-, and ferro-carbonatites. Importantly, its textural characteristics, particularly its paragenetic relationships with REE minerals like bastnäsite and monazite [17], provide crucial insights into understanding REE enrichment. Second, calcite acts as a significant carrier of REEs within carbonatites, as REEs can be incorporated into calcite through coupled substitution with Na [18]. Given the importance of REE information in calcite for assessing the extent of element incorporation into REE ores and understanding the controlling factors, comprehensive REE budgets of calcite, including total contents, standardized patterns, and the presence of Ce or Eu anomalies, can be used to trace their genetic source [19]. Lastly, specific isotopic systems, particularly in-situ strontium (Sr), prove beneficial for investigating contamination by other geological components, such as silicate rocks and subducted materials, and their roles in REE enrichment during the origin and evolution of carbonatitic magma [15]. In recent years, the fast-developing laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) technique has made it feasible to trace the petrogenesis of fertile carbonatites and related REE enrichment through systematic calcite studies.

The Dalucao carbonatite complex, situated in the Mianning–Dechang REE belt of Sichuan Province, Southwest China, hosts well-defined and traceable REE mineralized vein systems. Previous studies on the Dalucao carbonatite complex have primarily concentrated on geochronology [20, 21], fluid inclusions [22, 23], and the trace-element chemistry of fluorite and bastnäsite [24, 25]. However, some fundamental aspects of carbonatite petrogenesis, particularly pertaining to the nature of the mantle source area and the evolutionary process of the fertile carbonatitic magma, remain poorly understood. Additionally, the mechanism responsible for the extreme enrichment of REEs in this complex remains elusive. The Dalucao carbonatite complex stands as one of the most ideal study sites in China for investigating carbonatite petrogenesis through the analysis of calcite mineral chemistry, primarily due to two key factors: (1) the ubiquity of calcites within this complex, some of which are intimately associated with bastnäsite in paragenesis; and (2) the presence of distinct occurrences of calcite that can be readily identified, with many of these calcites featuring large crystals suitable for precise geochemical analyses. Nevertheless, systematic studies on calcite in this carbonatite complex have received relatively limited attention. In this present study, we first classified two types of calcite based on comprehensive field and petrographic investigations of the Dalucao carbonatite complex. Subsequently, we provided major and trace element compositions as well as in-situ Sr isotopic data for each type of calcite. This new dataset enables us to elucidate the nature of the mantle source area for the fertile carbonatitic magma and, ultimately, shed light on the mechanism responsible for the unique REE enrichment observed in this complex. Our findings offer valuable insights into comprehending the genesis of REE resources in a carbonatitic setting and hold significance for future prospecting endeavors.

The Dalucao carbonatite complex, along with three other fertile carbonatite complexes (Maoniuping, Muluozhai, and Lizhuang) located to its north, constitutes the Mianning–Dechang REE belt in the western margin of the Yangtze craton (Figure 1(a)–1(c)). This complex, controlled by the Daluxiang strike-slip fault, which formed in the eastern margin of the Qinghai–Tibet Plateau during the India–Asia continental collision, intrudes into the Proterozoic quartz diorites (Figure 2(a)). It is a 600–800 m-long and 400 m-wide rock complex consisting of carbonatites and syenites. Within this complex, carbonatites manifest as sills, dikes, and stocks within the syenite intrusions. In general, the carbonatites are medium- to coarse-grained, appearing massive to banded, and primarily composed of calcite, with varying amounts of other minerals, such as apatite, pyrite, barite, and fluorite. The syenites exhibit a gray, massive structure with fine-grained textures, comprising alkali feldspar, quartz, aegirine-augite, arfvedsonite, biotite, and accessory minerals.

The REE mineralized veins mainly develop along fractures within the carbonatite complex (Figure 3(a)) and give rise to the economically significant No.1 and No.3 REE orebodies hosted within breccia pipes (extending downward for more than 300 m; Figure 2(b) and 2(c)). Brecciated rocks are commonplace in the REE mineralized areas of the carbonatite complex (Figure 3(b)) and typically consist of angular wall-rock clasts set within a matrix of ferromagnesian silicates. Due to the influence of breccia pipes, the REE orebodies may undergo fragmentation, resulting in REE ores of varying sizes (Figure 3(c)). Fenitization, a distinctive alkaline alteration associated with REE mineralization, has produced an alteration halo surrounding the vein systems. Sensitive ion microprobe U‒Pb dating results for syenite-hosted zircon [20] and secondary ion mass spectrometry Th‒Pb dating data for bastnäsite [21] have yielded an age of approximately 12 Ma for the carbonatite complex.

Reddish-brown REE ores (Figure 3(d)) are prevalent within the mineralized veins, constituting the primary components of the orebodies (Figure 3(d)). Calcite predominates the ore mineralogy, accounting for approximately 30%–40% of the bulk mineral composition, followed by fluorite (20%–30%), barite (10%–25%), quartz (10%–20%), and bastnäsite (5%–10%). Light-yellow bastnäsite typically occurs as flaky or columnar grains, exhibiting vibrant interference colors under cross-polarized light (Figure 3(e)). Alkali feldspar is locally observed, while aegirine-augite is generally absent in these reddish-brown ores. Coarse-grained REE minerals are uncommon within these REE ores due to multistage brecciation; hence, these ores can also be designated as brecciated ores. Comprising angular to rounded clasts cemented by a hydrothermal matrix, these ores consist primarily of syenite and carbonatite clasts, with the hydrothermal matrix composed of calcite, quartz, fluorite, and barite. The presence of a substantial quantity of brecciated ores occurring as lenses in the complex suggests that the development of REE mineralization is accompanied by the release of high pressure, as demonstrated in our previous case study of fluid inclusions in gangue and REE minerals [22].

Based on mineral assemblages and textural characteristics, we have identified two varieties of calcite within the Dalucao carbonatite complex: coarse-grained (type I, ranging from 400 to 2000 µm in size; Figure 4(a)–4(c)) and fine-grained (type II, typically <400 μm; Figure 4(d)–4(f)) calcites. The key distinguishing feature between these calcite types lies in their paragenetic relationships with bastnäsite; type I calcites do not coexist with bastnäsite, whereas type II calcites exhibit a paragenesis where bastnäsite occurs interstitially with calcite.

Type I calcites are predominantly found within the REE-vein-free carbonatites, situated either far from the REE orebodies or in close proximity to syenite intrusions (Figure 4(a)). These carbonatites typically occur in deeper geological settings and host a mineral assemblage comprising coarse-grained calcite (type I calcite), green or colorless fluorite, loose barite, and silicate minerals (e.g., alkali feldspar, aegirine-augite, and phlogopite). Type I calcite grains exhibit an equant shape and display a euhedral granular texture with a homogeneous surface (Figure 4(b) and 4(c)). They typically maintain clear boundaries when in contact with fluorite and have rarely been broken, except for minor instances of cleavage or corrosion by anhedral barite. Some altered calcite grains in this category contain microscopic mineral inclusions, such as barite. On occasion, pyrite appears in the form of tiny veins or granular infillings within fractures between calcite grains (Figure 4(c)); however, its abundance remains low.

Type II calcites are commonly present in the reddish-brown REE ores within the REE-vein-bearing carbonatite complex (Figure 4(d)). Nearly all of these ores exhibit mineral assemblages consisting of fine-grained calcite (type II calcite), fluorite, barite, quartz, and bastnäsite (Figure 4(e) and 4(f)). Bastnäsite is locally observed as fine euhedral grains and often appears sporadically as finely automorphic crystals within the gangue minerals (Figure 4(f)). Type II calcite crystals exhibit an elongated or anhedral morphology with an equigranular texture, and they show subtle undulatory extinction under cross-polarized light. The most significant characteristic of type II calcite is its propensity to host numerous minuscule bastnäsite mineral inclusions within the leached areas. On rare occasions, bastnäsite manifests as columnar grains with lineations parallel to type II calcite, displaying homogeneous textures and uniform grain sizes (Figure 4(e)). Based on the described textural relationships, it is suggested that the deposition of bastnäsite predominantly occurred after, or in rare instances, simultaneously with the crystallization of type II calcite.

In this study, systematic sampling was conducted at the Dalucao carbonatite complex (locations depicted in Figure 2), with particular attention given to the collection of alteration-free samples. Polished thin sections were meticulously examined petrographically using a conventional optical microscope to characterize mineralogy and crosscutting textures. Among the thin sections subjected to petrographic analysis, least-altered calcite grains were chosen as the primary targets for subsequent geochemical analyses. Here, “least-altered” denotes that the minerals under scrutiny exhibit homogeneous textures devoid of any indistinct or contaminated surfaces.

Major element compositions of calcite grains were determined using a JEOL–JAX8230 electron microprobe (EMPA) at the Institute of Mineral Resources, Chinese Academy of Geological Sciences. The analyses were conducted under the following conditions: an accelerating voltage of 15 kV, a beam current of 12 nA, a beam spot size of 5 µm, and counting times ranging from 10 to 30 seconds. Backscattered electron images were acquired using a Nova NanoSEM 450 field emission scanning electron microscope located within the same laboratory.

In-situ trace element compositions of minerals were analyzed using the LA–ICP–MS method at Nanjing FocuMS Technology Co., Ltd., China. The Teledyne Cetac Technologies Analyte Excite laser ablation system and Agilent Technologies 7700× quadrupole ICP‒MS were employed for these experiments. The laser conditions for the 193 nm ArF excimer laser were set as follows: beam size, 33 µm; repetition rate, 5 Hz; and energy density, approximately 4 J/cm². A signal smoothing device (The Squid, Laurin Technic) was utilized to enhance the sample signal quality. Each spot analysis included a 20-second gas blank collection with the laser turned off, followed by a 30-second sample signal detection with the laser on. Calibration curves for each element were constructed by analyzing three reference glasses, namely BCR-2G, BHVO-2G, and GSD-1G. The external standard NIST SRM 610 was analyzed to correct for signal drift, and the Ca content of calcites, determined by previous electron microprobe analysis, was used as the internal standard. Raw data reduction was performed using the ICPMSDataCal software.

In-situ Sr isotope analysis of calcite was performed using the Resonetic 193 nm laser ablation system, coupled with a Nu Plasma II MC-ICP–MS at Wuhan SampleSolution Analytical Technology Co., Ltd., China. The samples were ablated in a mixture of helium (350 mL/min) and nitrogen (2 mL/min) gases, employing the following parameters: a 30-second baseline time, a 40-second ablation time, a spot size ranging from 32 to 104 µm, a repetition rate of 6 Hz, and an energy density of 6 J/cm². Every 5–7 sample analyses were bracketed by an analysis of an in-house coral standard, serving as the external standard to ensure analytical reliability and stability. The average ⁸⁷Sr/⁸⁶Sr isotopic composition obtained for the coral standard was 0.70915 ± 0.00004 (2σ, n = 30), consistent with the recommended value of 0.70910 ± 0.00002 [26].

The in-situ major and trace element contents of the two calcite types are presented in online supplementary Tables S1 and S2. For type I calcites, the major element compositions are as follows: CaO (52.1–55.4 wt%), MgO (≤0.3 wt%), SrO (0.5–0.8 wt%), FeO (0.2–2.1 wt%), and MnO (0.2–1.4 wt%). Type II calcites exhibit the following major element compositions: CaO (52.9–55.3 wt%), MgO (≤0.4 wt%), SrO (0.8–1.2 wt%), FeO (0.3–1.0 wt%), and MnO (0.5–1.1 wt%). Type II calcite grains contain significantly higher Sr (6656–13,427 µg/g, with an average of 9865 µg/g) and display notably larger variations in Sr content compared with their type I counterparts (3646–7315 µg/g, averaging 5739 µg/g). Concerning other large ion lithophile elements (LILEs), both types of calcite exhibit concentrations several orders of magnitude lower than those of Sr. For instance, both calcite types contain much lower levels of Ba (≤38.6 µg/g in type I vs. ≤114 µg/g in type II) in comparison to Sr and are exceptionally depleted in Rb and U. Both types of calcite contain some amount of Pb, but there is no discernible distribution difference in their compositional ranges (57.1–113 µg/g in type I vs. 28.6–117 µg/g in type II). All calcite samples exhibit extremely low concentrations of high-field strength elements, such as Nb, Ta, Zr, Hf, and Th.

Trace element analyses also reveal a significant enrichment of total REEs (ΣREE) in both calcite types, with type II calcites showing substantially higher ΣREE concentrations (922–2567 µg/g, averaging 1544 µg/g) compared with type I species (226–627 µg/g, averaging 362 µg/g). Notably, distinct differences in the distribution of REE patterns between type I and type II calcites are observed in Figure 5(a), where the curves for all type II calcites consistently surpass those of their type I counterparts. In general, both types exhibit a pronounced enrichment in light rare earth elements (LREEs) relative to high rare earth elements (HREEs), with most calcite REE budgets dominated by Ce > Nd > La > Pr > Sm. Chondrite-normalized patterns for the two calcite types display a negative slope ([La/Yb]_cn = 1.45–5.15 in type I vs. 3.09–32.6 in type II) and appear relatively smooth (Figure 5(a)), characterized by the absence of significant Eu anomalies (δEu = 0.94–1.09 in type I vs. 0.89–1.04 in type II).

For comparison purposes, bastnäsite, which is intergrown with type II calcites in a paragenesis, was selected for REE measurements using the LA–ICP–MS method (online supplementary Table S3). This mineral crystal exhibits a strong preference for LREEs, with LREE contents being 518–747 times higher than HREE concentrations, as evidenced by the significantly high (La/Yb)_cn ratios (116,325–225,392 with an average of 152,991). Chondrite-normalized REE patterns are right-inclined flats and exhibit noticeable variations in slope (Figure 5(b)).

The in-situ Sr isotopic compositions of the two calcite types are detailed in online supplementary Table S4. Due to the notably high Sr (>5000 µg/g) and extremely low Rb concentrations (<1 µg/g) observed in the analyzed calcites, the measured ⁸⁷Sr/⁸⁶Sr ratios of the grains can be confidently regarded as their initial Sr isotopic signatures. Both types of calcite within the Dalucao carbonatite complex have high, radiogenic Sr isotopic compositions that exhibit minimal variation, with (⁸⁷Sr/⁸⁶Sr)_i ratios ranging from 0.7059 to 0.7060 (with an average of 0.7060) for type I calcites and 0.7059 to 0.7068 (averaging 0.7061) for type II calcites. The errors, represented as standard deviations in the isotopic ratios and determined at the 2σ level, do not exceed 0.0001 (specific error values are provided in online supplementary Table S4).

In this study, we initially compared the REE distribution patterns of calcites obtained from the Dalucao carbonatite complex to those originating from other mineralized (e.g., Maoniuping [27]) and barren carbonatite complexes (e.g., Bear Lodge and Magnet Cove in the USA, Aley in Canada, and Turiy Mys in Russia; all cited from Reference 19). Here, the classification of “mineralized” or “barren” is based on the presence or absence of well-defined, exploitable REE mineralized veins within the carbonatite complex. Some variations in the ratios of REEs that are sensitive to redox conditions in Dalucao calcites closely resemble those in Aley calcites (as observed in the plots of [La/Yb]_cn vs. Y/Ho, ΣREE vs. [La/Yb]_cn, and δCe vs. δEu, as shown in Figure 6(a)–6(c)), but they deviate from the compositional ranges of other carbonatite-hosted calcites. This suggests that certain elements, such as La, Ce, Yb, Eu, Y, and Ho, along with the derived parameters, are sensitive to crystallization oxidation and can serve as petrogenetic indicators of mineral systems. Chondrite-normalized patterns of calcites from the Dalucao and Maoniuping carbonatites display a negative slope and appear relatively smooth (Figure 6(d)), bearing visual similarities to those of igneous calcites from other barren carbonatites (with the exception of hydrothermal and supergene calcites in Bear Lodge). Having the highest ΣREE content is a significant diagnostic attribute of calcites from mineralized carbonatites when compared with those from barren counterparts (Figure 6(d)). This observation supports the hypothesis that carbonatitic systems forming discernible mineralized veins mobilize a greater quantity of REEs [12], resulting in REE enrichment within calcites.

Discrimination of the genesis of various carbonatite-hosted calcite varieties necessitates an approach that combines petrographic characteristics and compositional information. This is due to the possibility of textural re-equilibration and hydrothermal overprinting in their crystals [14, 19], and it is a prerequisite for interpreting the origin of the carbonatitic system and assessing its relationship to REE enrichment. In geochemical systems, the fractionation between Y (1.019 Å) and Ho (1.015 Å) rarely interferes with magmatic processes, as these two elements have similar ionic radii and electronegativities [28]. Therefore, the Y/Ho ratio is widely considered a useful proxy, which can be used to determine whether different types of calcites are cogenetic [29]. The Dalucao calcites and bastnäsites exhibit chondritic Y/Ho ratios of approximately 28, and their Y versus Y/Ho plots predominantly fall within the field of the primitive mantle (Figure 7(a)), implying charge-and-radius-controlled processes [30]. This information strongly suggests a calcite origin related to the carbonatitic magmatic-hydrothermal system and significantly differs from that of supergene and metasomatic calcites, both of which exhibit a wide Y/Ho range of 15–55 [14, 29, 30]. Therefore, it is proposed that these two types of calcite originate from different stages of carbonatitic evolution.

The first type of calcites represents early-crystallizing phases that originate from the initial carbonatitic melt due to the following key characteristics: First, they exhibit a euhedral granular texture with a homogeneous surface and are commonly associated with coarse-grained quartz and fluorite (Figure 4(b) and 4(c)). These features are consistent with the mineral crystallization conditions during the early stages of carbonatite magmatism [31]. Second, they possess relatively low concentrations of Sr (3646–7315 µg/g), Ba (8.72–38.5 µg/g), and ΣREE (226–627 µg/g), which align with the composition of early-crystallizing calcites [19]. Third, they display gently sloping chondrite-normalized REE patterns (Figure 5(a)) with the lowest (La/Yb)_cn ratios ranging from 1.45 to 5.15. Lastly, they yield near-unity δCe values (0.98–1.04) and exhibit slight Eu deficiency (δEu = 0.89–0.93). In contrast, we suggest that the crystallization of type II calcites is related to the overprinting of fluids derived from evolved carbonatitic magma. Type II calcites typically exhibit elongated shapes with subtle undulatory extinction and show close contact with the mineral surface of bastnäsite (manifested by overlapping, filling, and cutting relationships; Figure 4(b) and 4(c)). Furthermore, type II calcites are characterized by higher concentrations of Sr (6566–13,427 µg/g), Ba (28.8–114 µg/g), and ΣREE (922–2567 µg/g) compared with their type I counterparts. The high levels of these elements in type II calcites can be reasonably explained by the exposure of calcite mineral surfaces to REE-mineralizing fluids. The two types of calcite exhibit significant variations in La/Ho, exceeding the variations observed in Y/Ho by a factor of approximately 4 (Figure 7(b)). The nearly horizontal correlation between Y/Ho and La/Ho (Figure 7(b)), which is consistent with observations in the Huayangchuan (an REE-mineralized carbonatite complex in North Qinling [32]) calcites, suggests a gradual evolutionary process of the carbonatitic system from the crystallization of type I to type II calcites. This evolutionary process is characterized by the concurrent increase in ΣREE and Sr or Ba from type I to type II calcites, as indicated by the positive correlations in ΣREE versus Sr or ΣREE versus Ba (shown as shadow arrows in Figure 7(c) and 7(d)).

All of the calcites within the Dalucao carbonatite complex exhibit a narrow range of (⁸⁷Sr/⁸⁶Sr)_i values, with type I calcites ranging from 0.7059 to 0.7060 and type II calcites ranging from 0.7059 to 0.7068. This observation suggests a remarkable consistency in Sr–Nd isotopic compositions between the two calcite types. While hydrothermal reworking of post-magmatic fluids is commonly considered a significant factor leading to isotopic variations in evolving diagenetic systems [26], its influence on Sr isotopic variation in the Dalucao carbonatitic system appears to be less pronounced due to specific characteristics of carbonatitic magma: (1) its rapid ascent toward the surface owing to the low density and viscosity [4] and (2) its distinct mantle source signature characterized by Sr enrichment [13]. These magmatic attributes effectively mitigate or buffer the impact of hydrothermal reworking on Sr isotopic systems in carbonatitic settings [5, 12]. Consequently, we conclude that the close Sr isotopic compositions observed in both calcite types are inherited rather than mutated, providing valuable insights into their mantle source area. These calcites exhibit highly radiogenic (⁸⁷Sr/⁸⁶Sr)_i ratios (>0.7059 for all), with both types falling within the range between enriched mantle I (EM I) and enriched mantle II (EM II) on a (⁸⁷Sr/⁸⁶Sr)_i versus Sr binary diagram (Figure 8(a)). Typically, the high Sr isotopic ratios in carbonatite-hosted calcites have been ascribed to crustal assimilation [33] or sedimentary contamination [34]. However, the crustal assimilation hypothesis for the origin of Dalucao calcites can be challenged on two grounds: (1) crustal assimilation fails to account for the high ΣREE (>5000 µg/g) in these calcites, as the high Sr concentrations can effectively buffer the effect of crustal assimilation on ⁸⁷Sr/⁸⁶Sr values [4]; and (2) Pb isotopes, the most sensitive indicators of crustal assimilation, have been studied previously [5], revealing relatively uniform Pb isotopic compositions in Dalucao carbonatite-hosted calcites (with ²⁰⁶Pb/²⁰⁴Pb ratios ranging from 18.23 to 18.27).

In contrast, the highly radiogenic Sr signatures observed in the Dalucao calcites are indicative of sedimentary contamination within their magma source. Considering that fluids or melts dissolved from subducted materials are the most important agents for mantle metasomatism [15], we propose that the contamination contributors are most likely sourced from marine sediments during the subduction of slabs. First, the Dalucao calcites contain a significant amount of REEs, with a notable enrichment of LREEs compared with HREEs (online supplementary Table S2). Recent studies have demonstrated that marine sediments are characterized by REE enrichment, with REE possibly coming from the remains or bodies of marine organisms [35]; therefore, their sedimentary contamination can produce primary carbonatitic melts rich in REEs. Second, subducted marine sediments are rich in LILEs, and their Sr/Ba ratio (as opposed to the Rb/Sr ratio, which is typically low due to the extremely high Sr content in marine sediments) can serve as a first-order approximation to reflect the amount of REEs within the sediments [36, 37]. Due to the similar geochemical behavior of Sr and Ba, carbonatitic melts derived from mantle metasomatized by marine sediments should theoretically exhibit Sr/Ba ratios similar to those of subducted sediments. Both types of calcite display a clear negative correlation between Sr/Ba ratios and ΣREE (Figure 8(b)), consistent with that of separated calcites from global fertile carbonatites [5]. This suggests that the proportion of carbonate/hydrothermal phases within subducted marine sediments influences the REE levels in carbonatitic melts. Third, subducted marine sediments are known for their highly radiogenic ⁸⁷Sr/⁸⁶Sr ratios [38]. Such Sr isotopic signatures are related to pelagic clays, metalliferous sediments, Fe oxyhydroxides, and phillipsite sedimentary muds that can persist in subducted marine sediments over extended geological periods [36, 39, 40]. Therefore, contamination by subducted marine sediments has the potential to significantly elevate Sr isotopic values.

In summary, the mantle components responsible for generating the fertile magma of the Dalucao carbonatites have experienced mantle metasomatism by subducted marine sediments. This argument is further supported by a wide range of Li isotopic values (–4.5 to +10.8 ‰ for δ⁷Li [16]) for the Dalucao carbonatites and separated calcites. These values reflect the mixture of mantle components with subducted oceanic crust and marine sediments. Evidently, the recycling of marine sediments plays a pivotal role in enhancing the fertility of carbonatite magma, as this process can introduce abundant REEs into mantle components during the subduction of slabs.

Traditionally, extensive fractional crystallization from mantle-derived alkaline silicate magma has been considered a potential mechanism for the origin of carbonatites [14, 41]. This viewpoint is rooted in the fact that crystal fractionation leads to a compositional trend within carbonatites, transitioning from early calcitic to late dolomite-ankeritic or sideritic compositions [42]. However, two observations from the Dalucao carbonatite complex challenge the crystal fractionation hypothesis: (1) the predominant carbonate mineral recognized in the Dalucao carbonatites is calcite, with minimal quantities of magnesium carbonate minerals like dolomite and ankerite; and (2) olivinite, typically considered a key indicator of the crystal fractionation model [42, 43], is conspicuously absent in this complex. In most instances, carbonatites, as representative mantle-derived rocks, are believed to originate from low-degree partial melting of the upper mantle (commonly <1% melting proportion) [4]. Carbonatites with such an origin typically exhibit a high silicon component [44, 45]. The presence of silicate minerals in the Dalucao carbonatites provides mineralogical evidence supporting the notion that these carbonatites are silicon-rich, thereby corroborating the hypothesis of mantle partial melting.

We propose that the Dalucao carbonatite complex was derived from the mantle stability field of garnet based on the following evidence: (1) Phlogopite, a characteristic mineral of melts derived from the mantle stability field of garnet [46], is abundant in the Dalucao carbonatite complex and commonly occurs alongside aegirine–augite. (2) Melts produced by partial melting of the mantle stability field of garnet typically exhibit a high Dy/Yb ratio (>2.5), whereas those from the mantle stability field of spinel yield a low Dy/Yb ratio of <1.5 [47]; previously reported literature provided a Dy/Yb ratio of 3.0–3.4 for the Dalucao carbonatite complex [13], thus suggesting a carbonatitic source within the garnet stability field. Such a mantle source implies a pressure of approximately 2.0 GPa (corresponding to depths of 70–80 km), which can significantly lower the mantle solidus temperature (by at least 300°C) and, consequently, generate fertile carbonatitic melts [4]. This hypothesis is further supported by a compilation of published C–O isotopic data for the Dalucao carbonatites (Figure 9; detailed data in online supplementary Table S5). The δ¹³C_V-PDB values of the Dalucao carbonatites exhibit a remarkably narrow range and differ significantly from those of carbonatites lacking REE mineralization (e.g., carbonatites from Greenland and North America; see Reference 48 and references therein). This C–O isotopic signature resembles that of other REE-mineralized carbonatites, such as the Huayangchuan carbonatite, which is also believed to originate from the partial melting of metasomatized lithospheric mantle [32].

Based on the integration of the above discussions, we have developed a comprehensive conceptual model (Figure 10) to elegantly illustrate the enrichment of REEs in the Dalucao carbonatite complex. As previously mentioned, the Dalucao carbonatite complex has been fertilized by the recycling of subducted marine sediments. This recycling is likely associated with the subduction of ancient oceanic lithosphere beneath the Yangtze Craton during the Neoproterozoic era. This is supported by the calculated Nd model ages for the carbonatites in the Mianning–Dechang area, which ranges from 0.69 to 1.63 Ga with a peak at 0.91 Ga [49]. The Nd model ages for the carbonatites coincide with the onset of initial subduction of the Neoproterozoic ancient oceanic plate [27, 50], and this subduction event is corroborated by a 1000-km-long Neoproterozoic magmatic arc (1000–740 Ma) along the western margin of the Yangtze Craton [51]. The question of whether the subducted metasomatic processes in the mantle source area of the Dalucao carbonatite complex occurred in multiple stages cannot be definitively answered at this time (a thorough investigation involving nontraditional isotopes such as Ca and Mg will be necessary). A substantial quantity of REE-rich marine sediments could have been introduced into the mantle through slab subduction, ultimately leading to the fertilization of the SCLM. Similar metasomatic enrichment processes have been documented in the Weishan fertile carbonatite complex, where the mantle was metasomatized by the subduction of the Paleo-Pacific Plate since the Late Jurassic [52]. Accompanying the eventual closure of the Neo-Tethys Ocean and the collision/post-collision of the Indian Plate beneath the Asian continent, the tectonic regime of the Mianning–Dechang area in the western Yangtze Craton entered an extension/transtensional setting during the Cenozoic era. The orogenic activity, resulting from the India–Asia continental collision, led to crustal shortening and thickening, inducing the formation of a series of large-scale strike-slip faults. Partial melting of the mantle, which had been previously metasomatized by subducted marine sediments, was likely triggered by upwelling of the asthenosphere. This process generated fertile carbonatite parental magma that ascended along translithospheric faults near cratonic edges. In the Dalucao area, the volatile-rich carbonatitic magma experienced continuous differentiation during its emplacement into the crustal level. Highly oxidized fluids were exsolved as temperatures decreased and stress relaxed, resulting in intense fenitization that occurred around the Proterozoic quartz diorites.

Fluids exsolved from the Dalucao carbonatitic magma are characterized by significant enrichment of REEs due to the following factors: (1) Previous studies have demonstrated that REEs preferentially partition into the fluid phase during their expulsion from carbonatitic melts [31]. (2) Fenitization plays a crucial role in REE extraction and transfer through fluid-mineral interactions, effectively leaching REEs from preexisting igneous minerals into the hydrothermal system [53]. (3) During the fenitization process, none of the crystallizing minerals, mainly alkaline silicate minerals such as alkali feldspar, aegirine–augite, and phlogopite, have the significant capacity to incorporate REEs into their crystal structures; consequently, these elements become concentrated in the late-stage residual fluids. REEs in these fluids are expected to be transported in the form of complexing ligands, such as fluorine, sulfate, and chlorine complexes. This is consistent with mineralogical evidence that fluorite, barite, calcite, and bastnäsite typically form stable mineral assemblages in the Dalucao REE ores. As the ascending fluids cooled, they reached the saturation limits for CaF₂, BaSO₄, and CaCO₃, triggering the deposition of fluorite, barite, and calcite in fractures that had not been previously filled with crystallized material [54]. Our research reveals that both fluorite and type II calcite have much lower (La/Yb)_cn ratios than bastnäsite, even several orders of magnitude lower. Since the δEu values of bastnäsite (averaging 0.96; online supplementary Table S2) are higher than the chondritic value of approximately 0.8 [55], its deposition would have resulted in progressive Eu depletion in the evolving fluids. We hypothesize that extensive deposition of fluorite and type II calcite (possibly involving barite) removed the complexing ligands from the fluids (i.e., F⁻, [CO₃]²⁻, and [SO₄]²⁻), consequently raising the concentration of light lanthanides. The loss of complexing ligands, coupled with changes in physical-chemical conditions such as the cooling of hydrothermal fluids and the influx of meteoric water (as demonstrated in one of our previous fluid inclusion studies [22]), diminished the transport capacity of the fluids. This, in turn, triggered the development of large-scale REE mineralization.

In this study, we emphasize the crucial role of tectonism in the formation of the Dalucao carbonatite complex, which is evident in several key aspects. First, the geological setting “located in the western margin of the Yangtze craton” is an excellent geological configuration for forming the Dalucao carbonatite complex. This is because the carbonatites were previously fertilized by subducted marine sediments and must have occurred in areas along cratonic edges. Second, the emplacement duration of fertile carbonate magma was controlled by regional-scale strike-slip faults formed during the India–Asia continental collision. The emplacement of carbonatite magma, beginning at the Eocene epoch, was triggered by an upwelling asthenosphere and slid from south to north along the right plate of the Yalongjiang fracture. As a result, the early magma (approximately 28–25 Ma) migrated further northward and from the formation of the Maoniuping, Muluozhai, and Lizhuang carbonatite complexes, whereas the late magma (around 12 Ma) migrated closer and gave rise to the Dalucao carbonatite complex. This temporal variation in magma migration explains the progressive diagenetic ages of carbonatites in the Mianning–Dechang area, with aging occurring from south to north. Third, local structural fissures within the Dalucao area have controlled the development of REE mineralized veins. In the course of fluid evolution, the intrusion of meteoric water through open fissures into the hydrothermal system led to rapid cooling, triggering the destabilization of REE complexes. Lastly, the presence of frequent and intense brecciations, as evidenced by the breccia pipes (Figure 2(a)), facilitated efficient fluid circulation and accumulation within and around the hydrothermal system. This ultimately promoted the formation of high-grade REE ores in the Dalucao area. Based on the integration of the above information, we propose that areas exhibiting pronounced and frequent tectonic activity along cratonic edges should be prioritized as priority targets for REE exploration. This perspective finds solid support in recent publications [54, 56], which emphasize that tectonism is one of the key factors controlling the diversity of carbonatite-related REE deposits.

1. Two distinct types of calcite are identified in the Dalucao carbonatite complex. Type I calcites are the early-crystallizing mineral phases that formed within the initial carbonatitic melt, while type II calcites, enriched in REEs, Sr, and Ba, are genetically associated with the overprinting of fluids derived from carbonatitic magma.

2. The Dalucao carbonatite complex likely originates from low-degree partial melting of the continental lithospheric mantle. Mantle components were previously metasomatized by subducted marine sediments during slab subduction, thus resulting in the enrichment of REEs in the generated carbonatite magma.

3. Large-scale REE mineralization primarily occurred at the hydrothermal processes, driven by the widespread exsolution of fertile carbonatite-derived fluids. Tectonism played a pivotal role in governing magma emplacement and promoting fluid accumulation, contributing significantly to the development of carbonatite-related REE veins.

This study was financially supported by the National Natural Sciences Foundation of China (No. 42302076), China Postdoctoral Science Foundation (No. 2022M723380), National Key Research and Development Program of China (No. 2023YFE0104000), and Fundamental Research Funds for the Central Universities (No. 2022QN1057).

The authors declare that they have no conflicts of interest.

The data are provided in Supplementary Files.