Neoproterozoic Subduction Zone Fluids and Sediment Melt-Metasomatized Mantle Magmatism on the Northern Yangtze Block: Constraints from the Ca. 880 Ma Taoyuan Syenogranite

Abstract

The Yangtze Block, with its widespread Neoproterozoic mafic–felsic magmatic rock series and volcanic–sedimentary rock assemblages, is one of the key windows for reconstructing the assembly and fragmentation process of Rodinia. This study focuses on the Taoyuan syenogranite from the Micangshan Massif on the northern Yangtze Block, by conducting systematic chronology, mineralogy, and geochemistry analyses to investigate their source, petrogenesis, and tectonic setting. LA-ICP-MS U–Pb geochronology reveals that the medium- to coarse-grained and medium- to fine-grained syenogranites have crystallization ages of 878 ± 4.2 Ma and 880 ± 6.5 Ma, respectively. These syenogranites have aluminum saturation index (A/CNK) values ranging from 0.79 to 1.06, indicating quasi-aluminous to weakly peraluminous compositions, and are classified as calc-alkaline I-type granites. The geochemical indicators of these rocks, including Mg^# (44–48, mean 46), Zr/Hf (40.07), Nb/La (0.4), and zircon εHf(t) values (+9.2 to +10.9), collectively indicate a depleted lithospheric mantle source. The mantle source was metasomatized by subduction-derived fluids and sediment melts prior to partial melting as evidenced by their higher Mg^#, elevated Ba content, and distinctive ratios (Rb/Y, Nb/Y, Th/Yb, Th/Sm, Th/Ce, and Ba/La). Integrating regional data, this study confirms crust–mantle interaction along the northern Yangtze during the early Neoproterozoic, supporting a sustained subduction-related tectonic setting.

1. Introduction

As one of the core components of the Rodinia supercontinent, the Yangtze Block provides a critical geological window for understanding the dynamic processes of Rodinia’s assembly and breakup. The widespread Neoproterozoic mafic–felsic magmatic rock series and volcanic–sedimentary rock assemblages in this block comprehensively record the coupling relationship between deep tectonic processes and shallow geological responses during this period, making it an important carrier for deciphering the Neoproterozoic evolution history and dynamic mechanism of the Yangtze Block [1,2,3,4,5,6,7,8,9].

Regarding the Neoproterozoic dynamic mechanism of the Yangtze Block, three major academic disagreements currently exist: (1) Mantle plume rift model: Proposes that the Yangtze and Cathaysia Blocks had already coalesced during the Grenville orogeny (~1.1–1.0 Ga), with the entire region being in an intracontinental rift environment between 860 and 750 Ma [1,2,3,10]; (2) Plate subduction model: Argues that the western margin of the Yangtze Block evolved as an Andean-type active continental margin between 860 and 720 Ma, while its southeastern margin and the Cathaysia Block underwent convergence and accretion as late as 820 Ma [4,6,7,8,11,12,13]; (3) Plate rift model: Suggests that after 860 Ma, the Yangtze Block entered a post-orogenic collapse phase, entirely under an extensional tectonic setting [14,15].

The Micangshan Massif, located on the northern Yangtze Block, is a key area for understanding the tectonic evolution of the Yangtze Block (Figure 1). Recent breakthroughs in geochronology and geochemistry have made the tectonic magmatic evolution history of the Micangshan Block increasingly clear. Regarding the tectonic setting prior to 860 Ma, the only exception is the ijolites from the Pinghe area, which have been interpreted as evidence for an intracontinental rift setting [16]. Most rock assemblages exhibit island arc affinities, which are indicative of a subduction-related environment. These include the mafic rocks in the Hekou area, ijolites and urtites in the Pinghe area, cumulate gabbros in the Beibei area, and syenites in the Shuimo area [17,18]. Petrogenetic studies attribute these rocks to the partial melting of juvenile lower crust [17] or the mantle [17,19]. However, the role of mantle-derived magmatism—particularly its interaction with subduction-derived slab fluids and sediment melts—remains poorly constrained and warrants further investigation.

Therefore, this paper selects the syenogranite from the Taoyuan pluton on the northern margin of the Yangtze Block as the research object. Systematic field geology, petrology, zircon U–Pb geochronology, and geochemical studies have been conducted to precisely determine the formation age, material source, and genesis of the related magmatic rocks. By synthesizing regional geological data, this study aims to evaluate the role of slab-derived fluids and sediment melts in modifying mantle sources during Neoproterozoic subduction processes.

2. Geological Background

Due to the development of a large number of rocks related to the Neoproterozoic orogenic belt, the Yangtze Block is listed as part of Rodinia [10,15]. The Yangtze Block is structurally bounded by: (1) the Qinling–Dabie–Sulu high- to ultrahigh-pressure metamorphic belt to the north, which separates it from the North China Block; (2) the Longmenshan Fault Zone to the west, marking its boundary with the Tibetan Plateau; (3) the Jiangnan orogenic belt to the southeast, forming the tectonic contact with the Cathaysia Block [38]. The crustal structure of the Yangtze Block exhibits distinct vertical stratification: (1) a lower layer consisting of Neoarchean to Paleoproterozoic highly metamorphosed crystalline basement, (2) a middle layer comprising Meso- to Neo-proterozoic greenschist facies-metamorphosed transitional basement, (3) an upper layer of unmetamorphosed Sinian to Mesozoic clastic and carbonate sedimentary cover [4].

The Micangshan tectonic belt is located on the northern Yangtze Block, at the junction of the Yangtze River drainage area, the South Qinling orogenic belt, and the northeastern Tibetan Plateau (Figure 1). The regional stratigraphy can be divided into three major structural layers. The crystalline basement (Houhe Group) comprises two principal lithological assemblages: (1) ~2.08 Ga high-grade metamorphic rocks including gneisses, migmatites, and metabasalts [39], (2) ~1.77 Ga intrusive suites dominated by K-feldspar granites and A-type granites [40,41]. The fold basement, represented by the Meso- to Neo-proterozoic Huodiya Group, unconformably overlies the crystalline basement and comprises three distinct lithostratigraphic units: (1) the Shangliang Formation, characterized by slate-dominated sequences with minor schist and siliceous intercalations, yielding a maximum depositional age of 837.6 ± 6.0 Ma [42]; (2) the Mawozi Formation, consisting predominantly of dolomitic carbonate deposits; (3) the Tiechuanshan Formation, composed mainly of metamorphosed volcaniclastic rocks, with interbedded rhyolite dated at 817.5 ± 5.0 Ma [26]. The sedimentary cover succession comprises a predominantly shallow marine sequence spanning from the Sinian to Middle Jurassic, characterized by alternating clastic and carbonate deposits with intermittent volcanic intercalations. This succession includes well-developed Sinian, Silurian–Permian, and Late Triassic–Middle Jurassic strata, while exhibiting a notable depositional hiatus marked by the absence of Devonian–Carboniferous units.

The Micangshan region records multiple Neoproterozoic magmatic intrusive events (Figure 1), which can be categorized geochemically into: subduction-related magmatism represented by alkaline rocks in the Pinghe pluton (872–860 Ma [19]); granitoids in the Tianpinghe pluton (863–860 Ma [17,27]); Guangwushan syenogranite (836–838 Ma [4,43]); intermediate–mafic rocks in the Beibei pluton (diorite, 869 Ma; gabbro, 814 Ma [17,29]); the Xihe gabbro–granite suite (829–824 Ma [4]); volcanic units comprising Xishenba granite (887 Ma [44]) and Shuimo syenite (869–860 Ma); metamorphic felsic rocks and amphibolites in the Hekoutou area (867–858 Ma); and Wangjiaping gneissic volcanic rocks (863–866 Ma) [18]. Back-arc extension-related magmatism includes the Tiechuanshan and Huangguan A-type granites and Taojiaba granodiorite [18]. Post-collisional magmatism is evidenced by the Xishenba monzogranite (706 Ma [4]). Continental rift-related magmatism comprises the Menzixiang quartz diorite–gabbro–norite–granite suite (774–745 Ma [45]).

3. Petrography

The study materials comprise two geochronological samples and nine geochemical samples collected from the Taoyuan pluton, adjacent to the northeast side of the Guangwushan pluton (Figure 2). The Taoyuan pluton shows distinct contact relationships: its northwestern margin is unconformably overlain by the Huodiya Group, while the southeastern portion is intruded by the Guangwushan granite (Figure 2).

Field observations reveal that the rocks generally exhibit a massive structure with relatively homogeneous petrographic features, though minor variations in quartz and plagioclase content, grain size, and biotite occurrence were noted. The dominant lithology is syenogranite, characterized by grayish-white to light gray fresh surfaces and a cataclastic texture (Figure 3a,b). Petrographic analysis reveals distinct mineralogical compositions between the two samples: Ty03 exhibits a modal composition of 46% potassium feldspar (dominant), 28% plagioclase, 22% quartz, and 4% muscovite; Ty04 shows a similar assemblage with 42% potassium feldspar, 30% plagioclase, 25% quartz, and 3% muscovite. Both samples contain accessory minerals including apatite, rhodochrosite, and opaque metallic minerals. Petrographic observations under a microscope (Figure 3c,d) show that (1) K-feldspars occur as perthite and microcline with anhedral tabular shapes, displaying typical gridiron twinning and strong argillic alteration, with occasional myrmekitic textures; (2) plagioclases show subhedral to anhedral tabular forms with well-developed polysynthetic twinning, exhibiting intensive sericitization and clay alteration; (3) quartz appears as anhedral grains with intracrystalline fractures and occasional undulose extinction; (4) muscovite occurs as anhedral flakes showing weak pleochroism and bright interference colors, occasionally containing accessory mineral inclusions such as apatite and zircon.

4. Analytical Methods

Zircon separation, mounting, and cathodoluminescence (CL) imaging were performed at Langfang Yantuo Geological Service Co., Ltd. (Langfang, China). Zircon U–Pb dating and Hf isotope analysis were conducted at Wuhan SampleSolution Analytical Technology Co., Ltd. (Wuhan, China).

Zircon U–Pb dating was performed using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The detailed instrument parameters and analytical procedures are described in Zong et al. (2017) [46]. The laser ablation system consisted of a GeolasPro equipped with a MicroLas optical system (incorporating a COMPexPro 102 ArF 193 nm excimer laser) Coherent Corp., Saxonburg, PA, USA, coupled to an Agilent 7700e ICP-MS instrument (Quadrupole Mass Spectrometer) Agilent Technologies, Santa Clara, CA, USA. Analyses employed a laser beam diameter of 32 µm and a repetition rate of 5 Hz. Analyses were performed in groups of ten spots, using standard zircon 91500 as the U–Pb external standard, NIST 610 as the composition standard, and ²⁹Si as the internal standard element [47,48]. Data processing was carried out using the ICPMSDataCal 10.9 software [49]. Zircon U–Pb concordia diagram plotting and weighted mean age calculations were performed using the Isoplot/Ex_ver. 3 software [50].

Zircon Hf isotope analysis was similarly conducted using a 193 nm ArF excimer laser ablation system (Geolas HD, Coherent, Dieburg, Germany) coupled to a Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS, Neptune Plus, Thermo Fisher Scientific, Bremen, Germany). The detailed analytical procedures and methods are provided in Hu et al. (2015) [51]. Analytical parameters included a laser spot size of 44 μm and an energy density output of ~7.0 J/cm². Isobaric interference corrections for ¹⁷⁶Lu and ¹⁷⁶Yb on ¹⁷⁶Hf were applied using ¹⁷⁶Lu/¹⁷⁵Lu = 0.02669 and ¹⁷⁶Yb/¹⁷²Yb = 0.5886. Standard zircons 91500, GJ-1, and SP-01 were used to monitor data quality during the analyses. Data processing, including the selection of sample and blank signals and mass bias correction for isotope ratios, was performed using the ICPMSDataCal 10.9 software [49].

Whole-rock major and trace element analysis samples were fresh rock powders ground to less than 200-mesh using an agate ball mill. Analyses were performed at Wuhan Shangpu Analytical Technology Co., Ltd. (Wuhan, China). Major elements were determined using a Rigaku Primus II X-ray Fluorescence Spectrometer (XRF, Rigaku Corporation, Tokyo, Japan), and trace elements were analyzed using an Agilent 7700e ICP-MS Agilent Technologies, Santa Clara, CA, USA. The detailed testing procedures are described in Liu et al. (2008) [52]. Data quality was monitored through replicate analyses, international rock standard AGV-2, and national rock standards GSR-1 and GRS-7, with analytical precision better than 4% and accuracy better than 3%.

5. Results

5.1. LA-ICP-MS Zircon U–Pb Geochronology

Two representative granite samples (Ty03 and Ty04) from the Taoyuan pluton were selected for zircon U–Pb geochronology. The analytical results are provided in Table S1, with representative cathodoluminescence (CL) images and concordia diagrams shown in Figure 4 and Figure 5, respectively. The zircons from both samples were predominantly euhedral to subhedral prismatic crystals, with aspect ratios of ~1:1 to 4:1. CL imaging reveals oscillatory zoning and lengths of 80–250 μm (Figure 4), consistent with a magmatic origin. In sample Ty03, 10 reliable analysis points have varying Th (76.8–247 ppm) and U (114–336 ppm) contents, as well as Th/U (0.69–0.93) ratios. These 10 harmonic analysis points show that the age distribution of ²⁰⁶Pb/²³⁸U ranges from 866 ± 9.4 Ma to 903 ± 8.1 Ma, and the weighted average age of the obtained harmonic zircons is 878.8 ± 4.2 Ma (MSWD = 2.4 n = 10) (Figure 5a,b). For sample Ty04, 16 reliable analysis points have low Th (58.4–322 ppm) and U (83.8–452 ppm) content, as well as Th/U (0.55–0.96) ratios. These 16 analysis points obtained a harmonic weighted average age of 880 ± 6.5 Ma (MSWD = 2.7, n = 16) (Figure 5c,d). The consistent ages (~880 Ma) from both samples constrain the emplacement age of the Taoyuan pluton to the late Neoproterozoic (Tonian), correlating with regional magmatic events in the Micangshan Massif.

5.2. Hf Isotope Characteristics

During zircon U–Pb dating, 27 zircons were selected for in situ Lu–Hf isotope analysis. These zircons were co-located with the sites where U–Pb age determinations had been previously conducted, though the latter exhibited a broader age distribution (Figure 4). The measured ¹⁷⁶Lu/¹⁷⁷Hf ratios of zircons from the Jiangnan granite samples were predominantly below 0.002 (Table S2), suggesting minimal post-crystallization radiogenic Hf accumulation. Consequently, the initial ¹⁷⁶Hf/¹⁷⁷Hf ratios remained well preserved [53]. For the calculation of initial εHf(t) values and model ages (TDM2), the crystallization age of the syenogranite was applied as the correction reference. The results showed that the ¹⁷⁶Hf/¹⁷⁷ Hf at 27 test points ranged from 0.282507 to 0.282555, and the initial Hf isotope ratio εHf (t) was 8.8–10.9, with an average of 9.82. The TDM2 value was 1093–1159 Ma. On the ε Hf (t)–t diagram, all test points were located between the depleted mantle and the chondrite evolution line.

5.3. Geochemistry of Major Trace Elements in Whole Rocks

Samples Ty03 and Ty04 are characterized by a high SiO₂ content (63.29–68.73 wt.%), low K₂O content (0.94–2.19 wt.%), and elevated Na₂O content (5.63–7.34 wt.%). These samples exhibit a moderate Al₂O₃ content (15.51–16.57wt.%) and A/CNK ratios (molar Al₂O₃/(CaO + Na₂O + K₂O) = 0.79–1.06) indicative of quasi-aluminous characteristics and being part of the medium-potassium calc-alkaline series (Figure 6a,b). Additionally, they display a high MgO content (1.16–1.72 wt.%) and Mg# values (44–48, mean = 46), consistent with high-magnesium syenogranite. Notably, these samples are enriched in CaO (1.19–3.97 wt.%) and Fe₂O₃T (2.72–4.11 wt.%).

In the chondrite-normalized rare earth element (REE) diagram (Figure 7a), the Taoyuan syenogranite exhibits a total REE content (∑REE) of 99.1–120.5 ppm, with a coherent REE distribution pattern: slight enrichment in LREEs (La/Yb)N= 7.57–11.01); and relatively flat heavy rare earth elements (HREEs); alongside negligible Eu anomalies (Eu/Eu* = 0.80–0.98) (Figure 7b). On the primitive mantle-normalized trace element spider diagram (Figure 5b), the Taoyuan syenogranite demonstrates an enrichment in LILEs (e.g., Ba and K) and a depletion in HFSEs (e.g., Nb, Ta, Th, and U).

Furthermore, distinct from the typical trace element signatures of adakites [56,57,58,59,60,61], the Taoyuan syenogranite aligns with the normal andesite–dacite–rhyolite series, exhibiting variable Sr (116–349 ppm) and Y (11.4–14.1 ppm) concentrations, as well as Sr/Y ratios (8.32–30.51). These rocks also contain lower abundances of compatible elements (e.g., Cr = 8.64–14.33 ppm; Ni = 7.61–11.36 ppm).

Figure 7. Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element spidergram (b) for the Taoyuan syenogranite. The chondrite and primitive mantle data are from Sun and McDonough (1989) [62]. Data sources are in Table S3.

Figure 7. Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element spidergram (b) for the Taoyuan syenogranite. The chondrite and primitive mantle data are from Sun and McDonough (1989) [62]. Data sources are in Table S3.

6. Discussion

6.1. Classification

According to the source rock properties of granite, it is traditionally classified into three types: Type I, Type S, and Type A. The syenogranite exhibits distinct non-A-type granite characteristics, as evidenced by the following: (1) In the thin sections, no characteristic alkaline dark minerals such as sodium amphibole, iron amphibole, aegirine pyroxene, and iron olivine were observed, which is significantly different from A-type granite [63]. (2) The sample has the characteristics of low total alkali mass fraction (mean 8.33%) and low TFeO/MgO (mean 2.11), which is significantly different from the high total alkali (>8.5%) and high TFeO/MgO values (>10) of A-type granite [64,65]. (3) Taoyuan syenogranite exhibits relatively low concentrations of Ga and high-field-strength elements (HFSEs), including Zr, Nb, and Ta, which contrasts sharply with the enrichment patterns typical of A-type granites [66,67]. (4) The saturation temperature of whole rock zircon is 726–767 °C, which is significantly lower than the average temperature of A-type granite at 833 °C [68]. (5) In the classification map of granite rock types, all samples of granodiorite porphyry are located in non-A-type granite areas (Figure 8; [64]).

In addition, the geochemical characteristics of Taoyuan syenogranite exclude the possibility of S-type granite: it has a lower A/CNK value (0.79~1.06 < 1.10) and the P₂O₅ content (0.07%~0. 12% < 0.20%) obviously does not conform to the characteristic of S-type granite of a strong aluminum content [63]. Syenogranite has a high Na₂O content (>5.6%), which is completely different from the typical S-type granite characteristics [69]. In addition, in mineralogy, only a small amount of muscovite is observed, and no aluminum-rich minerals such as corundum, tourmaline, cordierite, garnet, etc., are found, which is significantly different from typical S-type granite (such as [70]).

Instead, the Taoyuan syenogranite exhibits geochemical signatures consistent with I-type granites. It is characterized by high Na₂O and CaO contents and a metaluminous to weakly peraluminous composition which is consistent with typical Type I granite [71]. In the Harker diagram, there is a negative correlation between SiO₂ and P₂O₅ (Figure 10f), which is consistent with the evolution trend of type I granite [63].

6.2. Source Nature

The determination of magma source characteristics is crucial for understanding magmatic rock petrogenesis and tectonic settings. The syenogranite exhibits an average Rb/Sr ratio of 0.19, approximating the continental crustal average (0.24) and markedly exceeding the upper mantle value (0.025). The mean Nb/Ta ratio (12.68) is intermediate between mantle-derived rocks (17.5 ± 2 [72,73]) and continental crust (~11; [73,74]). Zr/Hf ratios (38.86–41.69, mean = 40.07) significantly surpass typical crustal values (~33; [73,74]). These systematic geochemical variations demonstrate that the syenogranite was not generated solely through lower crustal partial melting but involved substantial mantle-derived contributions.

Zircon Hf isotopes provide robust constraints on magma sources due to their high closure temperature and resistance to post-crystallization alteration [75,76]. The εHf(t) values and ¹⁷⁶Hf/¹⁷⁷Hf ratios plot of the syenogranite is located between the depleted mantle (DM) and chondritic uniform reservoir (CHUR) evolution lines (Figure 9a,b), indicating derivation from either depleted mantle or juvenile crustal sources. However, experimental petrology demonstrates that partial melts from mafic lower crust typically yield low MgO (<3 wt.%) and Mg# (<45) values [77]. In contrast, the syenogranite exhibits elevated MgO (1.16–1.72 wt.%) and Mg# (44–48, mean = 46) values (Figure 9c), which are inconsistent with pure crustal melting but indicative of a mantle source. This interpretation is further supported by high Zr/Sm and Hf/Sm ratios, precluding carbonate metasomatism [78] (Figure 9d), and Nb/La ratios (0.4) matching lithospheric mantle values (0.3–0.4) rather than OIB-type asthenosphere (>1; [79]).

Research indicates that young mantle components primarily participate in the formation of granitic rocks through two mechanisms: one is the mixing of mantle-derived magma and magma induced by the partial melting of crustal material in the deep crust to form crust–mantle mixed-source magma [93,94]. Another type is that mantle magma first invades the crustal bedrock to form the primary crust, and then undergoes partial melting under the influence of later thermal events [95]. Generally, positive εHf(t) values suggest a source derived from juvenile lower crust or depleted mantle [96], while negative εHf(t) values indicate ancient crustal sources. The Hf isotopic values of the Taoyuan syenogranite range from 8.8 to 10.9 (Figure 9a,b), exhibiting homogeneity distinct from the scattered Hf signatures typical of magma mixing. This suggests that the Taoyuan syenogranite did not undergo significant magma mixing. In terms of mineralogy, no magnesian dark inclusions or quenched characteristic mineral needle-shaped apatite were found in the Taoyuan syenogranite, which differs from the mineral characteristics of magma mixing, thus ruling out the magma mixing model.

Collectively, these lines of evidence establish that the Taoyuan syenogranite originated from depleted mantle sources, with its evolved geochemical features attributable to crust–mantle interaction during magma generation and ascent.

6.3. Origin of Taoyuan Syenogranite

6.3.1. Alteration, Crustal Contamination, and Fractional Crystallization

The effectiveness of geochemical data is crucial for determining the source area, genesis, and tectonic background of magmatic rocks. Usually, four geological processes will transform geochemical data, including post-emplacement alteration, crustal contamination, fractional crystallization, and partial melting (e.g., [97,98,99]). The loss on ignition (LOI) values of the syenogranite samples range from 1.54% to 3.73%, suggesting that the samples may have undergone a certain degree of post-magmatic alteration. However, HFSEs such as Th, Ti, Nb, Ta, Zr, Hf, Y, and rare earth elements (REEs) typically remain relatively stable during alteration and weathering processes [100,101]. Zr is commonly used to quantify the mobility of other incompatible elements [102]. In bivariate diagrams, Nb, Ta, Th, and U exhibit significant positive correlations with Zr, and the samples display subparallel REE and multi-element patterns (Figure 7), indicating that these elements were affected minimally by alteration and can thus represent the primary composition [103].

The Taoyuan syenogranites exhibit significant enrichment in LILEs (especially Ba) and LREEs (Table S3), with concentrations exceeding those typical of continental crust [104]. This pronounced enrichment, coupled with their relatively uniform zircon εHf(t) values (9.2–10.6), effectively precludes significant crustal contamination or magma mixing during their formation. Several lines of geochemical evidence support this interpretation: (1) the absence of pronounced positive Zr–Hf anomalies in trace element patterns contradicts the expected signatures of crustal contamination, given the crustal affinity of these elements; (2) elevated Nb/Th (1.2–1.6) and Th/Yb (3.3–5.7) ratios are inconsistent with substantial crustal input; (3) the homogeneous zircon U–Pb ages and lack of ancient inherited zircons further confirm minimal interaction with crustal materials during magma evolution. These collective observations demonstrate that despite their emplacement within the continental crust, the syenogranites preserve primary magmatic signatures that are largely unaffected by crustal contamination processes.

The syenogranite exhibits distinct fractional crystallization trends. In the Harker diagrams (Figure 10a–i), with increasing SiO₂ content, the concentrations of CaO, MgO, FeOt, Cr, and Ni decrease while the total alkali content increases, suggesting the possible fractional crystallization of olivine, pyroxene, amphibole and Ti–Fe oxides. During magma differentiation, plagioclase fractionation can produce negative Sr and Eu anomalies, whereas K-feldspar removal leads to negative Eu and Ba anomalies. Although sample Ty03 shows only a slight negative Eu anomaly, both Sr and Ba display varying degrees of negative anomalies (Figure 6), indicating possible feldspar fractionation. In the Ba–Sr diagram, the compositional trend of Ty03 closely follows the K-feldspar fractionation line. The trace element modeling further demonstrates that the combined fractionation of plagioclase and biotite in different proportions can produce similar results (Figure 11a). However, the H₂O-rich nature of the primary magma (4% biotite content) may have delayed K-feldspar crystallization [105]. The positive correlation between Ba and SiO₂ (Figure 10h) suggests that K-feldspar likely crystallized at later stages. Sample Ty04 may not have experienced significant feldspar fractionation. Although feldspar phenocrysts are present in alkaline rocks (Figure 3c,d), the decreasing Al₂O₃ with increasing SiO₂ (Figure 10a) indicates that plagioclase was not an early fractionating phase. This interpretation is supported by the negligible negative Eu anomaly (Figure 5a) and relatively elevated Sr content (Figure 5b and Figure 10g).

The strong positive correlation between Sc + V and TiO₂ (Figure 11b) confirms biotite as a fractionating phase. Decreasing P₂O₅ with SiO₂ along with nearly constant Nd/P ratios suggest apatite removal (Figure 11c). The wide data scatter in the LaN/YbN–La diagram (Figure 11d) likely reflects the variable fractionation of REE-rich accessory minerals (e.g., allanite, apatite, and sphene).

In summary, the syenite shows no significant crustal contamination. The feldspars were not early-fractionating phases in the granite. The magmatic evolution was dominantly controlled by the fractionation of biotite and REE-rich minerals (allanite, apatite, and monazite).

6.3.2. Mantle Metasomatism by Subduction-Derived Fluids and Sediment Melts

The Taoyuan syenogranite exhibited enriched LREEs and LILEs, along with depletions in Nb, Ta, P, and Ti (Figure 7), displaying a “crust-like” enriched signature [107]. This enriched geochemical signature is primarily controlled by two magmatic processes: (1) significant crustal contamination during late-stage magmatic evolution [108,109,110], (2) mantle source metasomatism prior to the generation of parental magmas [108,111,112]. The following evidence opposes crustal contamination during magma evolution, indicating that the “crustal like” enrichment characteristics of the Taoyuan syenogranite may reflect pre-magmatic mantle metasomatism. Firstly, previous studies have demonstrated that significant crustal contamination during magmatic evolution would result in abundant ancient zircon age populations during U–Pb geochronological analysis [111]. However, the Taoyuan syenogranite yields entirely concordant U–Pb ages (Figure 5), with no evidence of ancient age populations. Secondly, crustal contamination leads to negative Nb, Ta, and Ti anomalies along with slightly positive Zr and Hf anomalies [12,62,113]. Although the Taoyuan syenogranite exhibits marked depletions in Nb, Ta, and Ti, it lacks significant Zr and Hf enrichments relative to Nd and Sm (Figure 7). Furthermore, the samples display homogeneous zircon Hf isotopic compositions (εHf(t) = +8.8 to +10.9, ~2 εHf(t) units; Figure 9a,b), and the absence of mafic enclaves or disequilibrium mineral textures (Figure 2) precluding magma mixing between mafic and felsic melts [94,111].

Extensive studies have shown that in arc magmatic systems, depleted mantle sources can be modified by enriched components, such as LILE-rich subduction-derived fluids, sediment melts, or slab melts [7,111,114,115,116,117,118,119,120]. The following combination of evidence excludes the contribution of slab melt. Generally, Y behavior is controlled by slab melts [118,121,122]. Slab melts, which are Y-depleted due to garnet retention in the slab residue, elevate the Nb/Y ratios of metasomatized mantle-derived melts [56,77]. In contrast, fluid-related metasomatism produces melts with high Rb/Y ratios and Ba contents [122]. Similar to the Golovin and Belaya arc volcanic rocks in Kamchatka [122], the Taoyuan syenogranite exhibits variable Rb/Y (1.24–4.01) and Ba (219–800 ppm) values but limited Nb/Y (0.45–0.70) ratios (Figure 12a,b), indicating that their mantle source was metasomatized primarily by subduction-derived fluids rather than slab melts. In addition, the Taoyuan syenogranite has low ΣREE (99.1–120.5 ppm) and HFSE (e.g., Nb = 6.25–8.62 ppm, Zr = 115–157 ppm, Hf = 2.96–3.79 ppm) contents, which differ significantly from slab melt-modified arc lavas (e.g., [122,123]), suggesting a negligible influence of slab melts on their mantle source.

The Taoyuan syenogranite is deeply modified by subducting fluids. Firstly, its high Ba/Th and Sr/Nd ratios, along with low Nb/La and Nb/U values (compared with the primitive mantle values of Nb/La ≈ 1 and Nb/U ≈ 47), indicate the modification of the mantle source by subduction-derived fluids. Secondly, experimental petrology further demonstrates that the hydrous melting of upper mantle peridotite is a plausible mechanism for generating high-Mg magmas [120]. The calc-alkaline affinity (Figure 6) and presence of hydrous minerals (e.g., biotite and muscovite; Figure 3) in the Taoyuan syenogranite further support a potential hydrous mantle source [118,124,125].

Minor sediment melts have been confirmed to have been added to the source area of the Taoyuan syenogranite. Firstly, in primitive mantle-normalized trace element diagrams (Figure 7), unlike the Neoproterozoic fluid-modified mantle-derived magmatism in the western Yangtze Block (e.g., the Guandaoshan mafic–intermediate rocks and Dajianshan gabbrodiorites [113,126]), the Taoyuan syenogranite lacks pronounced Th enrichment. However, its Th/Ce ratios (0.09–0.17, avg. 0.14) are higher than those of N-MORB (0.016 [62]) and comparable to global subducted sediments (0.12 [121]), suggesting minor sediment input into the mantle source. Its elevated Th/Yb ratios (3.34–5.74, avg. 4.50) relative to N-MORB (0.04; [62]) further support sediment involvement [127]. Moreover, the variable Th/Ce (0.09–0.17) and Th/Sm (1.10–2.28) ratios (Figure 12c) also attest to weak sediment-derived metasomatism [112,116]. In addition, the co-variations in Th/Yb (3.34–5.74) and Ba/La (11.19–40.78) in the Taoyuan syenogranite (Figure 12d) thus reflect joint metasomatism by subduction-derived fluids and minor sediment melts [118,127].

Therefore, we conclude that the Neoproterozoic Taoyuan syenogranite along the northern Yangtze Block margin was derived from the partial melting of a mantle source metasomatized by subduction-derived fluids and sediment melts.

6.4. Tectonic Implications

Extensive geological evidence indicates that the northwestern Yangtze Block was situated in a subduction-related tectonic environment for an extended period during the Neoproterozoic. Petrologically, the Neoproterozoic magmatic rock assemblage in this region is dominated by intermediate to felsic granitoids, accompanied by minor mafic–ultramafic rocks. Examples include basaltic andesites, ophiolites, and andesites dated at ~985–930 Ma [82,83,89,90,128], plagiogranites at 905 ± 8 Ma [129], mafic intrusions at 890 Ma [13,16,82], gabbros at 885 Ma [17,92], granites at 887 Ma [16,44], and diorites at 880 Ma [92]. This lithological assemblage distinctly differs from mantle plume-related large-scale continental flood basalts [130,131], ruling out a mantle plume-dominated magmatic model. These rocks exhibit typical arc-like trace element patterns with relatively low concentrations of incompatible elements, resembling intra-oceanic arc magmas (e.g., [132]). In the Y vs. Sr/Y and (Yb)N vs. (La/Yb)N diagrams, the values plot within the normal andesite–dacite–rhyolite series alongside the Guangwushan granites (Figure 13a,b), strongly supporting their formation in a subduction environment.

Considering the high Rb/Y, Nb/Y, Th/Yb, Th/Sm, Th/Ce, and Ba/La ratios, we propose that the primitive mantle source of the Taoyuan syenogranite underwent metasomatism by both subduction-derived fluids and sediment melts (Figure 12a–d). Regionally, contemporaneous plutonic rocks (e.g., 880–876 Ma gabbros, diorites, and gabbro–diorites in the Bikou Terrane [89,90,91], and 879 Ma gabbros in the Hannan Complex [91]) also originated from mantle sources metasomatized by both subduction-derived fluids and sediment melts (Figure 12a–d). The extensive mantle metasomatism related to subduction melts along the northern Yangtze margin supports a Neoproterozoic subduction-related tectonic setting (Figure 14).

Furthermore, the >200 Myr duration of Neoproterozoic magmatism in the northwestern Yangtze Block matches the prolonged arc magmatism characteristic of Andean-type active continental margins [133], contrasting sharply with the short-lived (1–5 Myr) pulses typical of mantle plume events [131].

7. Conclusions

The LA-ICP-MS zircon U–Pb geochronological results indicate that the Taoyuan syenogranite were emplaced at ca. 880 Ma. The geochemical characteristics demonstrate that these rocks are metaluminous and calc-alkaline in composition. Their MgO contents (1.16–1.72 wt.%), Mg# values (46–48, mean 46), and positive zircon εHf(t) values (+9.2 to +10.9) collectively suggest derivation from a depleted lithospheric mantle source. Considering the negligible crustal contamination and relatively enriched geochemical signatures (e.g., elevated Ba and Th contents, high Rb/Y, Nb/Y, Th/Yb, Th/Sm, Th/Ce, and Ba/La ratios), we propose that the primitive mantle source of the Taoyuan syenogranite underwent metasomatism by both subduction-derived fluids and sediment melts. The extensive crust–mantle interaction documented here provides robust evidence for a subduction-related tectonic setting along the northwestern margin of the Yangtze Block during the early Neoproterozoic.