Petrogenesis of Jurassic Granite from the Shuitou Pluton in South Jiangxi Province, South China: Implications for Ion-Adsorption Rare Earth Element Enrichment

Abstract

Ion-adsorption rare earth deposits are mainly formed by the weathering and leaching of granite ore-forming parent rocks, and heavy rare earth elements (HREEs) are predominantly hosted in this type of deposit. In this study, we focused on the Late Jurassic REE mineralization parent rock, specifically the Shuitou pluton. We employed chronology, petrogeochemistry, and isotope geochemistry to elucidate the REE enrichment process in the granite. The results show that the zircon U–Pb age of the Shuitou pluton is ~150 Ma, and the monazite U–Pb age is ~145 Ma, suggesting that the pluton was formed in the Yanshan Stage. The rocks have high SiO₂ (72.85–75.55 wt%), Al₂O₃ (12.85–14.63 wt%), and K₂O (4.46–5.27 wt%) content, with A/CNK values of 1.05–1.19, differentiation index (DI) values of 87.48–95.59, zircon saturation temperature values of 689–746 °C, Nb/Ta ratios of 2.72–9.54, and Zr/Hf ratios of 7.12–26.11. In addition, the rocks also contain peraluminous minerals such as muscovite and garnet. These characteristics indicate that these rocks belong to highly fractionated S-type granite. The ε_Hf(t) values of zircon and monazite range from −10.04 to −6.78 and from −9.3 to −8.2, respectively, indicating that the magma was primarily derived from Proterozoic metamorphosed sedimentary rocks of crustal origin. In the extensional tectonic setting of South China, a high temperature promotes the melting of REE-enriched accessory minerals, and a higher content of F increases the solubility of REEs in the molten mass. The presence of heavy rare earth minerals, such as garnet, in these rocks contributes to a high content of heavy rare earth elements (HREEs). Additionally, REE-enriched minerals like titanite, bastnaesite, and allanite create the necessary material conditions for the formation of ion-adsorption REE deposits.

1. Introduction

The 17 rare earth elements, including lanthanide series elements and the transition metal elements Sc and Y, have been widely used in new energy technologies, semiconductor materials, defense, medicine, and other high-tech fields because of their unique atomic structures and excellent photoelectromagnetic properties. They have been designated as key or strategic resources by many countries [1]. The ion-adsorption-type rare earth element (REE) deposits in China are mainly distributed in South China, such as the Longnan deposit in Jiangxi Province and the Guposhan deposit in Guangxi Province. Up to now, more than 170 ion adsorption-type REE deposits have been found in China, providing 35% of the global rare earth resources and about 90% of the global heavy rare earth resources [2,3,4]. Ion-adsorption-type REE deposits are formed by the weathering and leaching of REE-enriched rocks such as volcanic rock, metamorphic rock, basaltic rock, and carbonate rock, among which granite is the most important ore-forming parent rock and the only parent rock type that can form heavy rare earth deposits [5]. The formation of ion-adsorption-type REE deposits can be categorized into two distinct stages: the pre-enrichment of rare earth elements in the parent rock, followed by secondary enrichment through weathering and leaching. The initial pre-enrichment of rare earth elements serves as a prerequisite for subsequent enrichment and mineralization via weathering and leaching. This latter stage is influenced by various factors, including pH levels, redox potential (Eh), intensity of weathering, and the content and characteristics of clay minerals, as well as the permeability coefficient associated with weathering. The weathering crust formed by the weathering of ion-adsorption-type rare earth ore-forming granites typically exhibits a thickness ranging from several meters to tens of meters, characterized by distinct stratification. From top to bottom, the layers are as follows: the humus layer (A layer), which has a thickness of less than 1 m; the fully weathered layer (B layer), with a thickness between 1 and 10 m; and the semi-weathered layer (C layer), with a thickness ranging from 3 to 20 m. During the weathering process of ore-forming granites, rock-forming minerals such as feldspar and mica undergo weathering and decomposition, resulting in the formation of significant quantities of clay minerals like kaolinite and halloysite. These clay minerals act as carriers for the adsorption and enrichment of rare earth ions within the weathering crust. The REE-enriched minerals in granite such as allanite, bastnaesite, and apatite break and are decomposed, releasing rare earth elements which infiltrate downward with rain water and migrate constantly toward the lower part of the weathering crust. As the result, the rare earth elements are adsorbed onto clay minerals such as kaolinite and halloysite as ion-adsorbed species, and are eventually enriched and mineralized in the lower part of the fully weathered layer and the upper part of the semi-weathered layer [6].

Studies have shown that rare earth elements are influenced by factors such as microbial activities, the adsorption/desorption of elements on/from iron manganese oxides and clay minerals, and the complexation of elements with ${CO}_3^{2-}$/${HCO}_3^{-}$ during the migration, fractionation, and enrichment of these elements in the weathering crust [7,8,9]. However, the pre-enrichment of rare earth elements in granite plays a decisive role in the formation of ion-adsorption rare earth deposits [5,7]. In the weathering crust, the REE assemblage pattern largely inherits the REE assemblage pattern of the parent rocks. Specifically, the parent rocks enriched with light rare earth elements form light rare earth deposits, while the parent rocks enriched with heavy rare earth elements form heavy rare earth deposits [6,10,11]. There are different viewpoints on the pre-enrichment mechanism of REEs in granite nowadays. Previous studies have shown that the pre-enrichment of REEs in granite is mainly related to magma–hydrothermal activities [11,12,13,14,15,16,17,18], but some others believe that it is related to the magmatic source region or to the replacement of HREE-enriched mantle-derived fluids [19,20]. The ion-adsorption rare earth ore-forming granites in South China are mostly formed in the Mesozoic strata. Geochemistry shows that the ore-forming granites are peraluminous, high-potassium calc-alkaline granites with higher initial Sr isotope (⁸⁷Sr/⁸⁶Sr) ratios and lower ε_Nd(t) values, formed by the strong differentiation evolution of the magma derived from the high-maturity crust [21]. The diagenetic age values are especially concentrated in 150–190 Ma, with the back-arc extensional tectonic setting caused by the Paleo–Pacific Plate subductions [22,23]. The ion-adsorption-type REE deposits formed during this period are numerous and have the richest types [24]. However, the relationship between the extensional tectonic setting and the enrichment of REEs in South China is still unclear. Therefore, detailed geochemical studies on the parent rocks of ion-adsorption-type REE deposits are crucial for understanding how source rock characteristics and magmatic processes influence rare earth mineralization, as well as the relationship between these factors and the Mesozoic extensional tectonic setting in South China.

Mesozoic granites associated with ion-adsorption rare earth mineralization are widely distributed across southern Jiangxi Province. The Shuitou pluton, traditionally considered to have formed during the Caledonian period, lacks supporting chronological data. In this study, we analyze the Shuitou pluton using petrography, zircon and monazite U–Pb geochronology, and zircon Lu–Hf and monazite Nd isotope geochemistry to clarify the magmatic source and evolution and to establish a Mesozoic tectonic–magmatic REE mineralization model.

2. Geological Background and Petrology

2.1. Geological Background

The South China Plate was formed by the amalgamation of the Cathaysia and Yangtze Blocks along the Jiangshan–Shaoxing Fault during the Neoproterozoic (Figure 1, [25]) and experienced three major tectonothermal events in the Paleozoic, Early Mesozoic, and Late Mesozoic, leading to extensive magmatism and the formation of rare and non-ferrous metal deposits [26]. Mesozoic granites and contemporaneous volcanic rocks are widely distributed in the South China Plate. Following the southwest-directed flat-slab subduction of the Jurassic Paleo–Pacific Plate, subsequent slab break-off and increasing subduction angle resulted in Jurassic–Cretaceous magmatic rocks that became progressively younger toward the southeast coast [22,23,27,28,29]. The Southern Jiangxi Region lies within the Cathaysia Block in the eastern South China Plate, with the Jiangshan–Shaoxing Fault Zone in the northwest and the Zhenghe–Dapu Fault Zone in the southeast of the Cathaysia Block. Magmatic rocks are widely developed throughout the region, with Yanshanian granites covering approximately 70% of southern Jiangxi (Figure 2a). A Precambrian crystalline basement has developed in this region, overlain by the Sinian–Cambrian sedimentary cover, and the Devonian, Carboniferous, and Cretaceous Series are in angular unconforming contact with the underlying Cambrian Series.

The Shuitou pluton is located at the junction of Youshui Town (Huichang County) and Tianxing Town (Anyuan County), Ganzhou City. The main pluton body lies in Huichang County, with an exposed area of about 70 km². Recent studies by the authors’ team indicate that the granites in the area mainly comprise the Yanshanian Shitouping pluton and the Caledonian Chengkeng and Sunwu plutons (Figure 2b), rather than the previously identified Caledonian Shuitou and Sanbiao plutons [17,31,32]. The diagenetic ages of the Shitouping, Chengkeng, and Sunwu plutons are ~140 Ma [18], ~450 Ma, and ~450 Ma [32], respectively. As the oldest exposed strata in the region, the Neoproterozoic Taoxi Formation is mainly exposed on both sides of the near-north–south-trending Huichang Basin. The exposed Nanhuan to Cambrian strata are relatively continuous, consisting of medium- to thick-bedded, weakly metamorphosed greywacke interbedded with thin slate layers and minor siliceous rocks. The Jurassic system is mainly exposed in the northern part of the study area as a set of miscellaneous, terrigenous clastic rocks. The Early Cretaceous strata comprise intermediate to acidic volcanoclastic rocks and lavas, primarily distributed along the phacolith and within the Caifang volcanic basin, formed during the early stage of the Early Cretaceous [33,34]. The distribution of Late Cretaceous volcanic rocks is clearly controlled by regional NNE-trending fault zones (Figure 2b).

Figure 2. (a) Distribution of granites in southern Jiangxi region (according to Sun et al., 2006 [35]) and (b) simplified geological map of Shuitou pluton. CGBG = coarse-grained biotite syenogranite; FGTG = fine-grained two-mica monzogranite.

Figure 2. (a) Distribution of granites in southern Jiangxi region (according to Sun et al., 2006 [35]) and (b) simplified geological map of Shuitou pluton. CGBG = coarse-grained biotite syenogranite; FGTG = fine-grained two-mica monzogranite.

2.2. Petrology

The coarse-grained biotite syenogranite (CGBG) sample (ST-5) is flesh-red (Figure 3a), exhibiting a granitic texture and blocky structure, and is primarily composed of plagioclase, K-feldspar, quartz, and minor biotite. The plagioclase is in a hypidiomorphic tabular shape, with well-developed polysynthetic twins (Figure 3b). Some of the plagioclase (32 vol%) has developed annular structures and is clayified within the annular structures (Figure 3c,d). The surface of potassium feldspar (28 vol%) experiences weak clayification (Figure 3e). The quartz (35 vol%) is heteromorphic granular in shape, with a small part displaying wavy extinction (Figure 3b). The biotite (5 vol%) is completely chloritized, retaining only its original flaky morphology (Figure 3f). Abundant mafic components exsolve along cleavage planes and grain boundaries, forming opaque metallic minerals.

The fine-grained two-mica monzogranite (FTMG) sample (ST-12) is gray in color (Figure 3g), with a granitic texture and a blocky structure, and is mainly composed of plagioclase, potassium feldspar, quartz, and a small amount of biotite and muscovite. The plagioclase (29 vol%) is in a hypidiomorphic tabular shape, with well-developed polysynthetic twins (Figure 3h). The potassium feldspar (34 vol%) occurs as anhedral tabular crystals, predominantly perthitic with well-developed exsolution textures and surface clay alteration (Figure 3i,j). The quartz (30 vol%) is heteromorphically granular in shape, with a small part displaying wavy extinction (Figure 3h). The biotite (3 vol%) has a hypidiomorphic–heteromorphic flaky shape (Figure 3k) and has obvious pleochroism. Some mafic components exsolve along the cleavage cracks and the edges and form opaque metallic minerals. The muscovite (4 vol%) appears as anhedral flakes and displays bright interference colors under cross-polarized light (Figure 3l).

Figure 3. Hand specimens and microscopic microphotographs of the Shuitou pluton. Bt—biotite; Chl—chlorite; Kfs—potassic feldspar; Ms—muscovite; Pl—plagioclase; Qtz—quartz; Zrn—zircon.

Figure 3. Hand specimens and microscopic microphotographs of the Shuitou pluton. Bt—biotite; Chl—chlorite; Kfs—potassic feldspar; Ms—muscovite; Pl—plagioclase; Qtz—quartz; Zrn—zircon.

The rare earth minerals contained in the bedrock include apatite, allanite, titanite, bastnaesite, xenotime, monazite, zircon, and garnet (Figure 4a), whereas the rare earth minerals derived from the magma crystallization include apatite, zircon, and thorite (Figure 4a,e). Apatite and allanite are hydrothermally altered to xenotime and monazite, respectively (Figure 4a,b); metasomatic pores in allanite are infilled with bastnaesite (Figure 4c–e); titanite and allanite exhibit co-crystallization (Figure 4e); and garnet occurs as a filling phase within feldspar (Figure 4f).

3. Analytical Methods

3.1. Zircon and Monazite U–Pb Dating

Zircon and monazite grains selected from the coarse-grained biotite syenogranite (HC-2) and fine-grained two-mica monzogranite (HC-3) were subjected to target identification and imaging by Langfang City Chenchang Rock and Mineral Testing Technology Service Co., Ltd. (Langfang, China) U–Pb dating of zircon and monazite was conducted at the State Key Laboratory of Nuclear Resources and Environment, East China University of Science and Technology. An Agilent 7900 ICP–MS (Agilent Technologies, Santa Clara, CA, USA) coupled with a GeoLasHD laser ablation system (Coherent Inc, Santa Clara, CA, USA) was used, with spot sizes of 32 µm for zircon and 16 µm for monazite. Zircon U–Pb dating employed zircon 91500 as the external standard, with one standard analyzed after every five samples. NIST610 glass was used to calibrate trace element concentrations [36]. Monazite U–Pb dating used monazite 44069 as the external standard, with one standard analyzed after every five samples [36]. NIST610 glass was also used for trace element calibration. Offline data processing was performed using ICPMSDataCal (V9.5) software [36]. Concordia ages and weighted mean ages of zircon and monazite were calculated using Isoplot/Exver 3 [37].

3.2. Zircon Hf and Monazite Nd In Situ Analyses

In situ Lu–Hf isotope analyses of U–Pb-dated zircon spots were carried out using a Nu Plasma MC-ICP-MS (Nu Ins, North Wales, UK) coupled with a RESONICS S-155 (Australian Scientific Instrument, NSW, Australia) ArF excimer laser ablation system. This work was completed by the Nanjing Hongchuang Geological Exploration Technology Service Co., Ltd. (Nanjing, China). The excimer laser operated at an energy density of 3.5 J/cm², with a spot size of 50 μm and a frequency of 9 Hz. One standard zircon was analyzed after every five sample analyses. ε_Hf(t) values were calculated using chondritic ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf ratios reported by Blichert-Toft et al. (1997) [38]. The present-day depleted mantle values of ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf from Griffin et al. (2000) [39], along with a crustal ¹⁷⁶Lu/¹⁷⁷Hf ratio of 0.015 [40], were used to calculate two-stage Hf model ages.

In situ Nd isotope analyses of monazite were conducted using LA–MC–ICP–MS (RESOlution SE 193nm + Neptune plus (Thermo Fisher Scientific, Waltham, MA, USA)) at Kehui Testing (Tianjin) Technology Co., Ltd. (Tianjin, China). The laser ablation spot diameter used was 20 μm, the frequency was 6 Hz, and the energy density was 5 J/cm². One standard monazite sample was tested after every five monazite samples had been tested. Nd isotope data were processed using Iso-Compass software [41]. Refer to Xu et al. (2015) [42] for detailed analytical procedures.

3.3. Whole-Rock Major and Trace Element Analyses

Fresh samples were ground to <200 mesh for analysis. The whole-rock major and trace element analysis was completed by Aoshi Analysis and Testing (Guangzhou) Co., Ltd. (Guangzhou, China). The main element analysis was conducted by using an X-ray fluorescence spectrometer (XRF), with the instrument being PANalytical PW2424 (Malvern, Worcestershire, UK), which has a relative deviation of less than 5% (RD < 5%). The trace element analysis was conducted by using an inductively coupled plasma mass spectrometer (ICP–MS), with the instrument being Agilent 7900, which has a relative deviation of less than 10% (RD < 10%). The standard samples used were GSR3 and GSR5 [18].

4. Analysis Results

4.1. Zircon U–Pb Dating

Zircon U–Pb age data for the coarse-grained biotite syenogranite (HC-2) and fine-grained two-mica monzogranite (HC-3) from the Shuitou pluton are presented in Supplementary Table S1. The zircon grains are predominantly colorless to light yellow and exhibit subhedral columnar shapes. Grain lengths range from 80 to 220 μm, with aspect ratios of 1:1 to 3:1, and they exhibit distinct magmatic oscillatory zoning (Figure 5a,c). In HC-2 samples, Th and U concentrations range from 60 to 598 ppm and 99 to 1549 ppm, respectively, with Th/U ratios of 0.30–0.90 (Figure 5f), a positive Ce anomaly, and enrichment in heavy rare earth elements (Figure 5e), all indicating magmatic zircon characteristics. All 25 analyses show concordance above 90%, with the points clustered on or near the concordia line (Figure 5a). The ²⁰⁶Pb/²³⁸U ages range from 146 to 165 Ma, with a weighted mean age of 151.2 ± 1.7 Ma (MSWD = 1.08, n = 25), representing the crystallization age of the HC-2 rock (Figure 5b). In HC-3 samples, Th and U concentrations range from 64 to 302 ppm and 111 to 923 ppm, respectively, with the Th/U ratios being 0.28–0.76 (Figure 5f), similar to those of the HC-2 samples (Figure 5e), further confirming the magmatic origin of the zircon (Figure 5e), also indicating the presence of magmatic zircon. All 20 analyses show concordance above 90%, clustering on or near the concordia line (Figure 5c). The ²⁰⁶Pb/²³⁸U ages range from 146 to 154 Ma, with a weighted mean age of 150.1 ± 2.9 Ma (MSWD = 0.09, n = 20), representing the crystallization age of the HC-3 rock (Figure 5d).

4.2. Monazite U–Pb Dating

The monazite U–Pb isotopic data of the fine-grained two-mica monzogranite (FGTG) sample from the Shuitou pluton are listed in Supplementary Table S2. The monazite grains are predominantly irregular in shape, with the length ranging from 50 to 120 μm and the aspect ratios being 1:1–3:1. Most monazite samples are dark gray and have cracks, with distinct features of melt alteration. A few grains exhibit banded structures with alternating light and dark zones (Figure 6a), which may reflect heterogeneous distributions of U, Th, and Pb during crystal growth. Th and U concentrations range from 63,417 to 264,274 ppm and from 5836 to 14,384 ppm, respectively, in the monazite samples, with the Th/U ratios being 7.32–25.81. Of the 22 analyses, point 7 yielded an anomalously low age and point 8 had an anomalously high age; thus, both were excluded from further interpretation. The remaining 20 analyses cluster on or near the concordia line (Figure 6a), yielding a weighted mean age of 145.3 ± 1.4 Ma (MSWD = 0.58, n = 20), which is interpreted as the crystallization age of the HC-3 monazite (Figure 6b).

4.3. Whole-Rock Geochemical Characteristics

4.3.1. Major Elements

The whole-rock major and trace element data of the coarse-grained biotite syenogranite (CGBG) and the fine-grained two-mica monzogranite (FGTG) samples from the Shuitou pluton are listed in Supplementary Table S3, indicating similar geochemical characteristics between the CGBG and FTMG samples. SiO₂, Al₂O₃, K₂O, Na₂O, and total alkali (K₂O + Na₂O) contents are 72.85–75.55 wt%, 12.85–14.63 wt%, 4.46–5.27 wt%, 2.72–4.50 wt%, and 7.99–9.04 wt%, respectively (Figure 7a,b). The ratios of K₂O/Na₂O are 1.01–1.94 (avg. 1.43), and the differentiation index (DI) values are 87.48–95.59 (avg. 92.87). Compared with FTMG, CGBG samples have relatively higher FeOt, MgO, and CaO contents (Figure 8). The sample points are projected into the sub-alkaline granite region in the SiO₂ vs. (K₂O+Na₂O) diagram (Figure 7a), whereas the sample points fall in the range of the high-potassium calc-alkaline series to shoshonite series region (Figure 7b). The aluminum saturation index (A/CNK) ranges from 1.05 to 1.19, placing all samples in the peraluminous field of the A/CNK vs. A/NK diagram (Figure 7c). In the SiO₂ vs. FeOt/(FeOt+MgO) diagram, all samples exhibit ferroan affinities (Figure 7d).

4.3.2. REE and Trace Elements

The total rare earth element (REE) content in the coarse-grained biotite syenogranite (CGBG) and fine-grained two-mica monzogranite (FGTG) samples from the Shuitou pluton ranges from 166 to 236 ppm. Chondrite-normalized REE patterns indicate a relative enrichment of heavy rare earth elements (HREEs) compared to light rare earth elements (LREEs), with (La/Yb)_N = 0.22–0.83 and obvious negative Eu anomalies (Eu/Eu* = 0.03–0.09), suggesting that extensive fractional crystallization of plagioclase and K-feldspar occurred during magma evolution (Figure 9a). The primitive mantle-normalized trace element diagram shows that elements such as Rb, Th, U, and Nd are relatively enriched, whereas elements such as Ba, Nb, Sr, P, and Ti are relatively depleted (Figure 9b). The values of Sr and Yb are 5.63–19.70 and 6.77–23.45 ppm, respectively, indicating the presence of low-Sr and high-Yb-type granite, suggesting that the Shuitou pluton formed in a crustal thinning tectonic setting under low pressure (<0.8 Gpa) and shallow depth (<30 km) [48].

4.4. Zircon Hf Isotopic Results

The zircon Hf isotope data of the coarse-grained biotite syenogranite (CGBG) samples from the Shuitou pluton are shown in Supplementary Table S4. The high closure temperature of the zircon Lu–Hf isotopic system provides valuable constraints on the genetic evolution of zircon [47]. The ratios of the zircon ¹⁷⁶Yb/¹⁷⁷Hf and ¹⁷⁶Lu/1⁷⁷Hf are 0.022590–0.044477 and 0.000740–0.001446, respectively, and the ¹⁷⁶Lu/¹⁷⁷Hf ratios are all less than 0.02, indicating a low accumulation of the radioactive Hf element. Therefore, the initial ¹⁷⁶Hf/¹⁷⁷Hf ratio likely reflects the isotopic composition at the time of zircon crystallization [50]. The zircon f_Lu/Hf values range from −0.96 to −0.98, significantly lower than the f_Lu/Hf value (−0.34) of the ferromagnesian crust [51] and the f_Lu/Hf value (−0.72) of the salic crust [52]. Thus, the two-stage model ages reflect the timing of source material extraction from the depleted mantle.

The zircon ¹⁷⁶Hf/¹⁷⁷Hf ratios range from 0.282401 to 0.282488, showing relatively uniform Hf isotope compositions and a weighted average value of 0.282442; the corresponding ε_Hf(t) values are −10.04 to −6.78, with an average value of −8.50 (Figure 10a). The two-stage model age values range from 1633 to 1832 Ma, with an average of 1738 Ma (Figure 10b).

4.5. Monazite Nd Isotopic Results

Monazite Nd isotopic data for the fine-grained two-mica monzogranite (FGTG) samples from the Shuitou pluton are presented in Supplementary Table S5. The monazite ¹⁴³Nd/¹⁴⁴Nd ratios range from 0.512096 ± 0.000016 to 0.512143 ± 0.00018; the corresponding ε_Nd(t) values are −9.27 to −8.26, with an average value of −8.61, and the corresponding two-stage model ages range from 1684 to 1613 Ma, with an average of 1645 Ma (Figure 11).

5. Discussion

5.1. Petrogenesis of Shuitou Pluton

Granites are commonly classified into I-, S-, A-, and M-types based on their petrogenetic origins [54]. M-type granite is formed by the separation and crystallization of the mantle-derived basic magma and is rarely found in nature [55]. I-type granites are typically derived from igneous source rocks, characterized by hornblende as a diagnostic mineral and relatively depleted Sr, Nd, and Hf isotopic compositions. Additionally, they exhibit a low aluminum saturation index (A/CNK < 1.1) and FeOt content, along with a negative correlation between SiO₂ and P₂O₅ concentrations [55,56]. S-type granites originate primarily from metasedimentary source rocks and are characterized by the presence of diagnostic minerals such as garnet, muscovite, and cordierite. The aluminum saturation index A/CNK is greater than 1.1, and the content value of P₂O₅ is greater than 0.2 wt% [56,57]. A-type granite forms at a relatively high temperature [58,59] and is characterized by a non-orogenic, alkaline, and relatively anhydrous environment in its source region [60], with the mineral combinations mainly including quartz, dark ferromagnesian minerals, and alkaline feldspar. The values of Zr+Nb+Ce+Y > 350 ppm and 10,000 Ga/Al > 2.6 are taken as the discriminant indicators of A-type granite.

Samples from the Shuitou pluton exhibit relatively low 10,000 Ga/Al values (2.56–3.01), with most data points plotted within the I-type or S-type granite fields in both the (Zr+Nb+Ce+Y) vs. 10,000 Ga/Al and the (Zr+Nb+Ce+Y) vs. (K₂O+Na₂O)/CaO diagrams (Figure 12a,b). The whole-rock zircon saturation temperatures range from 689 to 746 °C (average 729 °C) (Figure 13b), notably lower than the typical crystallization temperatures of A-type granites (>800 °C) [61]. The Shuitou pluton has Rb/Sr ratio values of 32.08–111.48 (avg. 46.01) and relatively high A/CNK (1.05–1.19) and K₂O/Na₂O (1.01–1.94) ratio values; meanwhile, it contains muscovite and garnet, which are the diagnostic minerals of S-type granite. Therefore, the Shuitou pluton is best classified as an S-type granite.

5.2. Magma Source

Previous studies have demonstrated that S-type granite magmas are derived from the partial melting of metasedimentary rocks [63]. The source rock compositions can be determined through the CaO/Na₂O ratio. Granites derived from metapelites typically exhibit CaO/Na₂O ratios below 0.3, and the granite formed from the partial melting of metagraywackes has values greater than 0.3 [64]. The coarse-grained biotite syenogranite in the Shuitou pluton has CaO/Na₂O values of 0.26–0.56 (avg. 0.38), indicating metagraywackes in its source region, whereas the fine-grained two-mica monzogranite shows CaO/Na₂O ratios ranging from 0.10 to 0.50 (average 0.12), indicating a metapelitic source. These findings are generally consistent with those inferred from the granite source discrimination diagram (Figure 13a). Trace elements are important indicators used to distinguish the evolution of granite source regions [65]. The Shuitou pluton has Nb/Ta ratios of 2.72–9.54 (avg. 5.82), much lower than those of the chondrites (19.9) and the continental crust (13.4) [66], and has Zr/Hf ratios of 7.12–26.11 (avg. 16.86), also much lower than those of the chondrites (34.3) and the continental crust (36.7). These data suggest that the Shuitou pluton has undergone significant magmatic fractionation, consistent with the patterns observed in Figure 12. The Mg^# value is a useful indicator for assessing whether mantle-derived material contributed to the magma source [67]. The values of Mg^# in the rocks being 2–28 (<40) indicate that no mantle materials are mixed in the source region. In the Shuitou pluton samples, both zircon Lu–Hf and monazite Sm–Nd isotopic data indicate that the granites originated from the anatectic melting or re-melting of ancient crustal material [68]. The zircon U–Pb ages of the Sunwu and Xiekeng plutons, situated approximately 20 km south of the Shuitou pluton, are determined to be 450 Ma and 235 Ma, respectively. Microscopic examination reveals a significant presence of muscovite within these rocks. Both the Sunwu and Xiekeng granites are classified as S-type granites which originated from Proterozoic metamorphic sedimentary rocks [69,70]. Therefore, it can be concluded that the source rocks of the Shuitou pluton are crustal metasedimentary in origin.

Figure 13. (a) Na₂O+K₂O+FeO+MgO+TiO₂ vs. (Na₂O+K₂O)/(FeO+MgO+TiO₂) [71] and (b) SiO₂ vs. T_Zr diagrams for the Shuitou pluton. MP = metapelite; MGW = metagraywacke; AMP = amphibolite. The data sources are the same as for Figure 12. T_Zr (°C) is calculated according to Watson and Harrison (1983) [72].

Figure 13. (a) Na₂O+K₂O+FeO+MgO+TiO₂ vs. (Na₂O+K₂O)/(FeO+MgO+TiO₂) [71] and (b) SiO₂ vs. T_Zr diagrams for the Shuitou pluton. MP = metapelite; MGW = metagraywacke; AMP = amphibolite. The data sources are the same as for Figure 12. T_Zr (°C) is calculated according to Watson and Harrison (1983) [72].

5.3. Geodynamic Setting

The Yanshanian tectonic setting in South China has attracted considerable scholarly attention, and multiple hypotheses have been proposed, including reverse thrusting and crustal overturning [73,74], continent extending and rifting [75,76], the upwelling of mantle plumes [77,78,79], the multi-phase subducting and retreating model [80,81], and the back-arc extension setting [76,82,83]. The current mainstream viewpoint is that the subduction action of the Paleo–Pacific Plate is the fundamental dynamic mechanism for the formation of the Yanshanian granitic rocks, i.e., volcanic rocks [22,26,84,85,86,87,88], but there are still controversies about the precise subduction process. For instance, Li et al. (2007) [84] proposed that the Paleo–Pacific Plate initially subducted as a flat plate, followed by processes of plate fracturing, delamination, and retreat. Additionally, Zhou et al. (2006) [22] noted that the Yanshanian magmatic rocks exhibit a progressive age gradient from inland to coastal areas. They established a model that integrates lithospheric subduction with the upward intrusion of basaltic magma into the lower crust and suggested a gradual steepening of the subduction angle over time. The third stage (140–125 Ma) of magmatic activity in eastern South China took place in an extensional tectonic regime [76,89,90]. The extensively exposed A-type granites and bimodal volcanic rocks provide compelling evidence that South China was situated in a back-arc extensional environment during this period.

The CGBG and FGTG of the Shuitou pluton, formed during the Late Jurassic, display relatively high Nb, Y, and Rb contents, and samples of these two types of rocks all fall into the post-collisional tectonic setting in the Y vs. Nb diagram (Figure 14a) and the (Y+Nb) vs. Rb diagram (Figure 14b), aligning with the tectonic environment of most Late Jurassic granites in South China [84,91]. This is consistent with the fact that Cretaceous A-type granites (i.e., Shitouping) occurred in the southern part of the Shuitou pluton, which are considered to have been formed in an extensional setting associated with the break-off of the Paleo–Pacific oceanic slab.

5.4. Implications for REE Enrichment

The ΣREE values of the coarse-grained biotite syenogranite (CGBG) and the fine-grained two-mica monzogranite (FGTG) samples from the Shuitou pluton are 166–236 ppm. These values exceed the average ΣREE contents of the upper continental crust (170 ppm) [93] and are similar to the ΣREE contents of granites in the Nanling area in South China (229 ppm) [94]. The abundance of granitic parent rocks of ion-adsorption-type REE deposits is relatively high, generally ranging from 200 to 500 ppm, with some rock bodies containing up to 500 to 800 ppm. Research shows that the threshold of parent rock for ion-adsorption-type rare earth deposits is 150 ppm [95]. Generally, the higher the abundance of the ore-forming parent rock, the more favorable it is for the formation of ion-adsorption-type rare earth deposits. However, the abundance of REEs in the parent rock is not a prerequisite and may not be the decisive factor. In the parent rocks of ion-adsorption-type REE deposits, rare earth minerals predominantly exist in both independent rare earth minerals and accessory minerals that contain rare earth elements. The weathering resistance of these (rare earth) minerals plays a crucial role in controlling the formation of the deposit.

The petrogenetic type does not play a decisive role in determining the REE content in granite, and I-type, S-type, and A-type granites can all form ion-adsorption rare earth deposits after undergoing natural weathering [18,19,96,97]. Statistical analyses of the geochemical characteristics of the light and heavy rare earth ore-forming parent rocks in South China indicate that the light rare earth ore-forming parent rocks are dominantly A-type granite (Figure 12), with greater A/CNK ratios and higher formation temperatures, and are mainly derived from the partial melting of metagraywackes. Meanwhile, the heavy rare earth ore-forming parent rocks are dominantly highly fractionated I-type or S-type granite, with lower formation temperatures, and primarily originate from the partial melting of metapelites (Figure 13a). The formation of ion-adsorption rare earth deposits in South China is influenced by factors such as climate, topography, hydrodynamics, and microorganisms, and it also relates to the special tectonic setting [97]. The parent rocks of ion-adsorption REE deposits in South China mainly formed between 150 and 190 Ma, with the Mesozoic granite formed in an extensional tectonic setting. The high-temperature conditions induced by mantle-derived fluids favor the formation of A-type and S-type granites (Figure 15) and promote the partial melting of REE-rich accessory minerals, which is confirmed by the positive correlation between zirconium saturation temperature and REE content [97]. F plays a significant role in the formation processes of the parent rocks associated with ion-adsorption-type REE deposits. Yan et al. (2021) [98] conducted a statistical analysis of the ages of fluorite deposits in South China and discovered that a significant number of fluorite mineralization ages coincide with the peak formation ages of the parent rocks associated with ion-adsorption-type REE deposits in this region. Furthermore, the spatial distribution characteristics of ion-adsorption-type REE deposits exhibit a relatively close relationship with F minerals. The subduction of the Mesozoic slab in South China caused the decomposition of some minerals such as the polysilicic muscovite and released a large amount of F-enriched fluids. The higher content of F can increase the solubility of REEs in the molten mass [99,100,101], resulting in a higher REE content in the Shuitou pluton. In addition, REE-enriched minerals such as garnet, titanite, bastnaesite, and allanite in the Shuitou pluton serve as a crucial material basis for the formation of the ion-adsorption REE deposit.

6. Conclusions

• The Shuitou pluton was formed at about 150 Ma and it contains Al-enriched minerals such as muscovite and garnet, has a high A/CNK value, and is classified as S-type granite.
• The ε_Hf(t) values of zircon in the coarse-grained biotite syenogranite (CGBG) are −10.04 to −6.78, and the ε_Hf(t) values of monazite in the fine-grained two-mica monzogranite (FGTG) are −9.27 to −8.26, indicating that the Shuitou pluton was derived from the partial melting of lower crustal sedimentary rocks.
• The extensional tectonic setting is conducive to the initial enrichment of rare earth elements in the granite, and the higher content of F increases the solubility of REEs in the molten mass. Furthermore, REE-enriched minerals such as garnet, titanite, bastnaesite, and allanite in the Shuitou pluton also provide essential material conditions for the formation of the ion-adsorption REE deposit.