Immiscibility in Magma Conduits: Evidence from Granitic Enclaves

Abstract

Many granitic enclaves are developed in the volcanic channel of the Xiangshan volcanic basin. To explore their genesis, this study examined the petrography, geochemistry, LA-ICP-MS zircon U–Pb chronology, and zircon Hf isotopes of the granitic enclaves and compared them with the porphyroclastic lavas. In general, the granitic enclaves and porphyroclastic lavas have similar structures, and the rock-forming minerals and accessory minerals have relatively close compositions. In terms of rock geochemical characteristics, the granitic enclaves are richer in silicon and alkalis but have lower abundances of aluminum, magnesium, iron, and calcium than the porphyroclastic lavas. Rb, Th, K, Sm, and other elements are more enriched, whereas Ba, Ti, Nb, P, and other elements are more depleted. The granitic enclaves have lower rare earth contents (195.53 × 10⁻⁶–271.06 × 10⁻⁶) than the porphyroclastic lavas (246.67 × 10⁻⁶–314.27 × 10⁻⁶). The rare earth element distribution curves of the two are generally consistent, both right-leaning, and enriched with light rare earth patterns. The weighted average zircon U–Pb ages of two granitic enclave samples were 135.45 ± 0.54 Ma (MSWD = 0.62, n = 17) and 135.81 ± 0.60 Ma (MSWD = 0.40, n = 20), respectively, which are consistent with the weighted average age of a single porphyroclastic lava sample of 134.01 ± 0.53 Ma (MSWD = 2.0, n = 20). The zircons of the two kinds of rocks crystallize at almost the same temperature. The consistent trend of the rare earth element distribution curve of zircons in the granitic enclaves and the porphyroclastic lava samples indicates that the zircons of the two samples were formed in the same stage. The formation process of granitic enclaves may be that the lower crustal melt is induced to rise, and the crystallization differentiation occurs in the magma reservoir and is stored in the form of crystal mush, forming a shallow crystal mush reservoir. The crystal mush reservoir is composed of a large number of rock-forming minerals such as quartz, feldspar, and biotite, as well as accessory mineral crystals such as zircon and flowable intergranular melt. In the later stage of magma high evolution, a small and short-time magmatic activity caused a large amount of crystalline granitic crystal mush to pour into the volcanic pipeline. In the closed system of volcanic pipeline, the pressure and temperature decreased rapidly, and the supercooling degree increased, and the immiscibility finally formed pale granitic enclaves.

1. Introduction

Enclaves are mineral clusters with distinct differences from the host rock [1]. They can be divided into dark enclaves and pale enclaves based on their coloration. Among these, mafic microgranular enclaves (MME), which have a deeper color than the host rock, are typically fine-grained intermediate–basic rock enclaves that are richer in iron and magnesium minerals than the host rock [2]. Pale enclaves exhibit a lighter color than the host rock and possess a clearly defined or transitional border with the host rock. Depending on their origin, enclaves can be divided into seven major categories: xenoliths, restites, residual shadow enclaves, autoliths, quenched enclaves, immiscible enclaves, and residual enclaves.

The various types of rock enclaves contain abundant information about rock formation, and studying the origin of enclaves is important to understanding crust–mantle interactions, the process of magma mixing, and even the evolution of magma [1,3]. For example, the processes of crust–mantle interaction can be revealed by using mineral thermometers and pressure gauges to study MMEs [4,5]. The mixing process of deep magma can often be reversed by analyzing dark enclaves using Sr–Nd, Fe, and Re–Os isotopic techniques, and information pertaining to magma crystallization history and magma flow can be obtained by studying MMEs [6,7,8,9,10].

Previous research has focused mainly on MMEs, and there is little research on pale enclaves. Therefore, on the basis of detailed field exploration, petrographic, rock geochemistry, zircon U–Pb geochronology, and Hf isotope studies were carried out on the porphyroclastic lava and pale granitic enclaves in the Ehuling Formation of Xiangshan, Jiangxi Province, China, and these data were utilized to explore the origin of the granitic enclaves. We hypothesize that the formation process of the granitic enclaves may be related to a small-scale and short-duration magmatic activity. During the upward movement of the crystal mush formed by the deep magmatic chamber evolution along the volcanic channel, under the conditions of reduced pressure and temperature, non-miscibility occurs and eventually leads to the formation of granitic enclaves. This procedure provides new information pertinent to the magmatic activity of Xiangshan.

2. Geological Setting

The Xiangshan volcanic basin is located at the junction of the Yangtze Plate and the Cathaysian Plate and includes the largest volcanic-type uranium ore field in China (Figure 1) [11]. The basin consists of two parts, the base and the caprock (Figure 2). The lithology of the basement is mainly the low greenschist facies metamorphic upper Qingbaikou series, which is followed by the Middle Devonian Yunshan Formation (D₂y) quartz sandstone and siltstone and the Upper Triassic Zijiachong Formation (T₃zj) glutenite and sandstone with carbonaceous shale, etc. The lithology of the cap is mainly intermediate–acid volcanic lava of the Lower Cretaceous Daguding Formation (K₁d) and Lower Cretaceous Ehuling Formation (K₁e) [12].

The volcanic activity had two obvious cycles. The first cycle was a fissure eruption, which formed the rhyodacite of the Daguding Formation; the second cycle was a more central eruption that formed the porphyroclastic lava of the Ehuling Formation [12]. The Daguding Formation can be divided into two lithologic sections. The first section (K₁d¹), which is in unconformity contact with the underlying strata, is mainly composed of sandstone, tuff, tuffaceous siltstone, conglomerate, and tuffaceous silty mudstone. The lithology of the second member (K₁d²) is mainly rhyodacite, which is mainly distributed in the northwestern part of the Xiangshan volcanic basin. The Ehuling Formation can also be divided into two lithologic sections. The first section (K₁e¹), which is in parallel unconformity contact with the underlying Daguding Formation, consists mainly of prunosus pebbly siltstone, tuffaceous siltstone, and tuff. The lithology of the second member (K₁e²) is mainly porphyroclastic lava, which can be divided into three subfacies. The marginal subfacies are porphyroclastic lava containing metamorphic breccia, the transitional subfacies are porphyroclastic lava, and the central subfacies are porphyroclastic lava containing granitic enclaves.

The volcanic basin is a collapsed basin with multiple craters, concealed volcanic channels, and lateral intrusion pipelines [13]. Based on geophysical, remote sensing, topographic, and other information, it is believed that the main chain crater of the rhyodacite of the Daguding Formation and the porphyroclastic lava of the Ehuling Formation is located near the main peak of Xiangshan [14]. Pale granitic enclaves are widely developed in the porphyroclastic lavas of the Ehuling Formation near the crater.

Figure 1. Brief tectonic map of Xiangshan volcanic basin. Figure 1 shows the location of the famous large volcanic uranium deposit field Xiangshan in the southern part of the junction area between the Yangtze Plate and the Huaxia Plate in China [15].

Figure 1. Brief tectonic map of Xiangshan volcanic basin. Figure 1 shows the location of the famous large volcanic uranium deposit field Xiangshan in the southern part of the junction area between the Yangtze Plate and the Huaxia Plate in China [15].

Figure 2. A schematic diagram showing the regional geological map of the Xiangshan volcanic basin in Jiangxi. The locations indicated in the figure refer to the approximate positions of the samples shown in Figure 3 [16,17].

Figure 2. A schematic diagram showing the regional geological map of the Xiangshan volcanic basin in Jiangxi. The locations indicated in the figure refer to the approximate positions of the samples shown in Figure 3 [16,17].

Figure 3. Field images of porphyroclastic lava and granitic enclaves. (a–d)—Field photographs of granitic enclaves; (e–h)—Field sketch map of granitic enclaves.

Figure 3. Field images of porphyroclastic lava and granitic enclaves. (a–d)—Field photographs of granitic enclaves; (e–h)—Field sketch map of granitic enclaves.

3. Sample Characteristics and Analysis Methods

Sample Characteristics

The granitic enclaves in the porphyroclastic lavas are off-white, vary in size, often have a spherical shape, and have a clear border with the porphyroclastic lavas (Figure 3a–c). Many tourmaline enclaves are often developed within the pale enclaves (Figure 3d–h).

The porphyroclastic lava has a typical porphyroclastic structure, and the phenocrysts show the characteristics of being crushed but not scattered, scattered but not separated, or separated by only a short distance. The broken particles do not disperse. The distances between the more dispersed particles are not far, and they resemble a mosaic pattern. The matrix has a microcrystalline texture. The phenocrysts are mainly K-feldspar, plagioclase, quartz, and biotite (Figure 4a–d). The K-feldspar phenocrysts are semi-automorphic and occasionally display zonal structure. They often occur as Carlsbad twinned crystals, and portions of K-feldspar cracks are developed with and filled by the matrix. The plagioclase phenocrysts are automorphic and plate-like, the polysynthetic twin is developed, and sericitization often occurs along cracks. Quartz phenocrysts mostly have an anhedral crystal structure, cracks are developed, and the edges are often bay-shaped and rounded. They are matrix-embedded and sometimes perforated. Biotite phenocrysts are flaky, strongly modified, and display substantial chloritization. In addition, some plagioclase and quartz phenocrysts have pearl–rim textures (Figure 4b).

The granitic enclaves are granite porphyries, with a massive porphyritic texture. The phenocrysts are large, mainly K-feldspar, plagioclase, quartz, and biotite, and the matrix has a microcrystalline texture. The K-feldspar phenocrysts are semi-automorphic, sometimes display a perthitic texture with plagioclase, and may exhibit Carlsbad twinning and partial sericitization. Plagioclase phenocrysts are automorphic–semi-automorphic crystals with well-developed polysynthetic twinning and strong sericitization along fractures. Quartz porphyritic crystals are anhedral crystal structures, and the edges of phenocrysts are often eroded into a bay shape. The biotite phenocrysts have a semi-automorphic, anhedral crystal structure and are strongly chloritized. Additionally, some K-feldspar and quartz porphyritic crystals have beaded structures (Figure 4g). Irregular polychromatic tourmaline fillings occur in quartz crystals and are often metasomatic with plagioclase (Figure 4e).

4. Analysis Methods

4.1. Mineral Research

An X-ray powder diffraction study was conducted using two samples of granitic enclaves containing tourmaline enclaves. The experiment was completed in the State Key Laboratory of Nuclear Resources and Environment, East China University of Technology. The experiment was performed using a D8 Advance multi-crystal X-ray diffractometer (Bruker, Germany) with a test temperature of 20 °C, Cu target X-ray tube, voltage of 40 kV, current of 40 mA, and scanning range of 5°–85°. The accuracy of the goniometer was 0.0001°, and the overall accuracy was ≤0.02°.

The accessory minerals were studied using two methods. First, accessory minerals were searched and located using α-track etching. Then, a scanning electron microscope was utilized to identify mineral types and acquire representative images.

First, NaOH solution soaking was used to remove the film. Then, after the removal of the film, the probe piece was tightly attached and fixed with a long-tail hook. After a month, the film was taken off, placed in an etching solution at 65 °C, and maintained at a constant temperature. After one hour, it was washed clean with water. Then, after drying, it was placed under an optical microscope, the film etching points were found and marked, and quality imagery was acquired at the corresponding positions of the probe piece. Subsequently, a polished, thin section was sprayed with carbon, and scanning electroscopic research was performed. Back-scatter images of minerals were acquired, and energy spectra testing was performed. The scanning electron microscope work was completed in the State Key Laboratory of Nuclear Resources and Environment at East China University of Technology. The instrument used was an SEM450 field emission scanning electron microscope. The experimental conditions were a working voltage of 20 kV and a working current of 2.0 × 10⁻⁸ A. For more details about the process see [18].

4.2. Geochemical Analysis

First, the porphyroclastic lava and granitic enclaves were separated. Four fresh porphyroclastic lava samples and six granitic enclave samples without tourmaline enclaves were selected and crushed to a 200–mesh size without contamination. Major elements and trace elements were then analyzed and tested, respectively. The work was completed at the Aussie Analysis and Testing Company (Guangzhou).

An X-ray fluorescent spectrometer (ME-XRF26 protocol) was used to measure the major elements with a detection limit of 0.01%. Trace elements were measured using inductively coupled plasma mass spectrometry (ME-MS81). The determination of ferrous oxide was by titration. The environmental conditions of the above experiments were a temperature of 25 °C and a relative humidity of 50%.

4.3. Zircon U–Pb Dating and Hf Isotope Analysis

After the samples were crushed, zircon grains with few cracks and high transparency were selected under the binocular microscope. Each zircon was fixed to an epoxy resin target and polished and ground until the surface was exposed. Transmitted light, reflected light, and cathodoluminescence (CL) images were acquired. Avoiding inclusions and fissures, positions with smooth and uniform color were selected as test points for zircon U–Pb dating and Hf isotope analysis. The zircon U–Pb dating and Hf isotope analysis experiments were completed at the Nanjing Hongchuang Geological Exploration Technology Service Company. The analytical instruments used in the zircon U–Pb dating experiment were a ResolutionSE 193 nm deep ultraviolet laser ablation sampling system (Applied Spectra, Sacramento, CA, USA) and an Agilent 7900 inductively coupled plasma mass spectrometer (LA-ICP-MS) (Agilent Technologies International, Tokyo, Japan). The energy density of the laser ablation system was 2 J/cm², the beam spot diameter was 30 μm, the ablation frequency was 5 Hz, and the ablation depth was about 0.3 μm. In the test process, zircon 91500 was used as the calibration standard sample to correct the instrument quality discrimination and element fractionation. Standard zircon GJ-1 was used as the monitoring standard sample to test the quality of the U–Pb dating data. Deep fractionation correction was performed by an exponential equation [19]. To calculate the trace element contents of the sample, NIST610 was used as the external standard, and ⁹¹Zr was used as the internal standard. Iolite (version 3.71) software was used to plot the harmonic curve and calculate the weighted average age [19], Zircon trace element analysis was carried out in LA-ICP-MS zircon U-Pb age at the same time of age determination.

The zircon Hf isotope analysis was performed using a Neptune Plus laser ablation-multi-acceptance plasma mass spectrometer (LA-MC-ICPMS). The zircon U–Pb dating was performed using the same laser ablation system. Based on age determination, Hf isotope test points were arranged in the same or an adjacent position as the age test points, and the single point mode was used for analysis. The laser spot size was 50 μm, the laser ablation rate was 8 Hz, and the laser energy density was 6 J/cm². Using helium carrier gas, a small amount of nitrogen was introduced after the ablation cell to improve the sensitivity of the Hf element. The value ¹⁷⁶Yb/¹⁷³Yb = 0.79639 was used to deduct the same amount of ectopic interference from ¹⁷⁶Yb to ¹⁷⁶Hf, and ¹⁷⁶Lu/¹⁷⁵Lu = 0.02656 was used to deduct the equal amount of out-of-phase interference from the relatively small ¹⁷⁶Lu to ¹⁷⁶Hf. The mass fractionation coefficient β_Yb of Yb was used to correct the mass fractionation behavior of Lu. The Qinghu, 91500, and GJ-1 international zircon standards were used simultaneously with actual samples to ensure the reliability of the data. The data were consistent with the reference values within the error range and were processed offline using Iolite software.

5. Analysis Results

Features of the Mineral Composition

The results of the α-track etching of the porphyroclastic lava and granitic enclaves are shown in Figure 5a–f. The distribution of the etching points is mainly in cluster form, but a small portion occurs spotted in point form, with a better correspondence between the densely distributed points and most of the accessory minerals.

The XRD results (Figure 6) showed that all samples produced quartz, tourmaline, K-feldspar, plagioclase, and biotite diffraction peaks. These data, in combination with the observations under the microscope (Figure 4), were used to delineate the mineral assemblage characteristics of the granitic enclaves. Their mineral composition is mainly quartz, plagioclase, K-feldspar, biotite, and tourmaline.

The porphyroclastic lavas (Figure 7a–f) had an accessory mineral assemblage similar to that of the granitic enclaves (Figure 7g–l), without significant differences. Both included monazite, apatite, zircon, magnetite, and small amounts of rutile, xenotime, thorium, allanite, almandite, and so on. The monazite, which is mainly wrapped in feldspar, has a particle diameter concentrated in the range of 10–15 μm. The interior is often distinct due to a difference in Th content, and the back-scattered image shows a change in darkness. The mineral is often accompanied by apatite. Apatite consists mainly of semi-automorphic crystals with particle sizes concentrated within 50 μm–60 μm. It is often associated with monazite and zircon. Zircons are mostly automorphic to semi-automorphic grains with sizes concentrated within 60–70 μm. It is often associated with monazite and allanite. Magnetite is mostly automorphic to semi-automorphic, often wrapped in minerals such as monazite, and has particle sizes mainly concentrated within 30–110 μm. Other accessory minerals are rare, and their particle size is small.

6. Analysis of Rock Geochemical Results

6.1. Major Elements

Table 1. Analysis results and related parameters of major elements from Xiangshan porphyroclastic lava and granitic enclaves (%).

Number of Sample	xs-1-yuan	xs-3-yuan	xs-9-yuan	xs-10-yuan	xs-1-bu	xs-2-bu	xs-3-bu	xs-7-bu	xs-8-bu	xs-11-bu
lithologic characters	porphyroclastic lava				granitic enclave
SiO2	74.82	72.90	73.99	73.78	74.85	74.85	74.50	74.30	75.00	75.31
Al2O3	12.96	13.11	13.20	12.93	12.75	12.70	13.05	12.91	12.98	12.80
Fe2O3	0.54	0.41	0.69	0.53	0.64	0.80	0.60	0.50	0.59	0.80
FeO	1.36	1.88	1.35	1.67	1.69	1.11	1.39	1.57	0.98	0.88
TFeO	2.06	2.51	2.18	2.38	2.52	2.03	2.15	2.25	1.67	1.78
CaO	1.21	1.29	1.16	1.34	1.06	0.67	0.98	1.14	0.93	0.87
MgO	0.27	0.33	0.31	0.30	0.15	0.11	0.15	0.21	0.11	0.16
K2O	5.25	5.19	5.27	5.12	5.55	5.64	5.83	5.52	5.87	5.61
Na2O	2.76	2.84	2.85	2.71	2.69	2.33	2.79	2.69	2.80	2.79
TiO2	0.16	0.20	0.19	0.19	0.12	0.10	0.11	0.17	0.09	0.12
MnO	0.04	0.05	0.05	0.05	0.04	0.04	0.04	0.04	0.03	0.04
P2O5	0.04	0.04	0.04	0.04	0.02	0.01	0.02	0.04	0.02	0.02
LOI	0.72	0.71	0.69	0.57	0.36	0.74	0.38	0.46	0.41	0.51
Total	100.29	99.17	99.93	99.41	100.11	99.22	100.00	99.73	99.91	100.01
A/NK	1.27	1.27	1.27	1.29	1.22	1.28	1.20	1.24	1.18	1.20
A/CNK	1.04	1.04	1.05	1.04	1.03	1.14	1.03	1.03	1.02	1.04
ΔOx	0.71	0.82	0.66	0.76	0.72	0.58	0.70	0.76	0.63	0.52
Mg#	0.13	0.13	0.14	0.12	0.06	0.06	0.07	0.09	0.07	0.09

Note: A/NK = Al₂O₃/(NaO + K₂O); A/CNK = Al₂O₃/(CaO + NaO + K₂O); ΔOx = FeO/(Fe₂O₃ +FeO); Mg^# = w(MgO)/[w(MgO)/40 + w(FeO)/72 + w(Fe₂O₃)/160].

shows the major element analyses of the porphyroclastic lava and granitic enclaves. The host rock has an acid composition (SiO₂ content: 72.90%–74.80%), is rich in alkalis (Alk: 7.83%–8.12%), especially rich in potassium (K₂O: 5.12%–5.27%), rich in aluminum (Al₂O₃: 12.93%–13.20%), poor in magnesium (MgO: 0.27%–0.33%), iron deficient (^TFeO: 2.06%–2.51%), and low in calcium (CaO: 1.16%–1.34%).

Compared with the host rock, the granitic enclaves are more silicic (SiO₂: 74.85%–75.31%), rich in alkalis (Alk: 7.97%–8.67%), and rich in potassium (K₂O: 5.522%–5.87%), but they have low aluminum (Al₂O₃: 12.70%–13.05%), low magnesium (MgO: 0.11%–0.21%), low calcium (CaO: 0.67%–1.14%), and relatively less iron (^TFeO: 1.67%–22.52%).

The projection points of the porphyroclastic lava and granitic enclaves fell into the granite area of the TAS classification diagram (Figure 8a). The samples fell into the alkaline region of the SiO₂–AR diagram (Figure 8b), the high potassium calc-alkaline region of the SiO₂–K₂O diagram (Figure 8c), and the peraluminous region of the A/CNK–A/NK diagram (Figure 8d).

As shown by the Harker diagram, the ratios of the main element (Na₂O, P₂O₅, K₂O, and SiO₂) contents of the porphyroclastic lavas and granitic enclaves were not apparent, with TiO₂, Al₂O₃, ^TFeO, MgO, and CaO all decreasing with increasing SiO₂ content (Figure 9).

Figure 8. (a) TAS diagrams of porphyroclastic lava and granitic enclaves [20]. (b) SiO₂–AR diagrams of porphyroclastic lava and granitic enclaves [21]. (c) K₂O–SiO₂ diagrams of porphyroclastic lava and granitic enclaves [22]. (d) A/NK–A/CNK diagrams of porphyroclastic lava and granitic enclaves [23].

Figure 8. (a) TAS diagrams of porphyroclastic lava and granitic enclaves [20]. (b) SiO₂–AR diagrams of porphyroclastic lava and granitic enclaves [21]. (c) K₂O–SiO₂ diagrams of porphyroclastic lava and granitic enclaves [22]. (d) A/NK–A/CNK diagrams of porphyroclastic lava and granitic enclaves [23].

6.2. Trace Element Characteristics

The results of the trace element analysis of the porphyroclastic lava and granitic enclaves are shown in

Table 2. Analysis of trace elements and related parameters of porphyroclastic lava and granitic enclaves (×10⁻⁶).

Number of Sample	xs-1-yuan	xs-3-yuan	xs-9-yuan	xs-10-yuan	xs-1-bu	xs-2-bu	xs-3-bu	xs-7-bu	xs-8-bu	xs-11-bu
lithologic characters	porphyroclastic lava				granitic enclave
Ag	0.04	0.04	0.04	<0.01	0.05	0.06	0.03	0.04	0.07	0.04
As	3.10	3.70	2.90	2.90	3.30	4.40	5.40	3.40	4.20	2.80
Be	4.31	4.98	4.51	4.80	4.12	3.39	4.38	3.89	4.52	4.79
Bi	0.31	0.66	0.22	0.33	0.40	0.67	0.69	0.33	0.66	0.27
Cd	<0.02	0.03	0.02	0.04	0.02	0.05	0.03	0.03	0.03	0.03
Co	2.20	2.70	2.30	2.50	1.60	1.30	1.30	1.80	1.20	1.40
Cr	13.00	14.00	17.00	16.00	13.00	13.00	16.00	13.00	14.00	12.00
Ga	17.65	18.30	17.80	17.25	16.45	16.40	17.20	17.40	17.15	17.20
Ge	0.20	0.23	0.21	0.09	0.20	0.20	0.20	0.22	0.24	0.21
In	0.04	0.04	0.04	0.05	0.03	0.03	0.03	0.04	0.03	0.03
Li	65.40	59.80	44.90	54.20	44.70	35.70	31.80	53.70	33.40	27.90
Mo	1.17	1.09	1.35	1.68	1.50	1.97	2.14	1.57	2.02	1.52
Ni	2.20	2.90	2.40	2.70	2.50	1.20	1.20	1.40	0.90	1.40
Pb	29.80	30.60	29.80	28.60	31.50	30.80	31.90	31.80	31.20	32.10
Y	31.80	32.90	34.80	49.70	31.70	23.70	30.00	26.40	29.80	31.40
Rb	265.00	270.00	264.00	254.00	264.00	273.00	285.00	282.00	276.00	269.00
Sb	0.06	0.07	0.05	0.07	0.09	0.07	0.13	0.06	0.07	0.05
Sc	4.70	5.30	5.10	5.00	3.50	3.40	3.60	4.30	3.30	3.80
Ti	1.12	1.10	1.02	1.07	1.13	1.12	1.19	1.19	1.17	1.14
Zn	38.00	41.00	40.00	47.00	30.00	27.00	29.00	38.00	27.00	30.00
Ba	202.00	246.00	245.00	251.00	132.00	129.50	131.00	181.50	123.50	149.00
Cs	11.45	9.57	7.94	9.33	11.60	9.82	9.99	12.40	9.66	8.29
Hf	4.90	5.30	5.20	5.50	5.00	4.40	4.70	5.40	4.50	4.80
Nb	17.00	17.20	17.50	17.50	17.30	16.00	15.60	17.10	15.40	16.20
Sn	5.00	5.00	5.00	6.00	5.00	5.00	5.00	5.00	4.00	5.00
Sr	101.00	106.50	117.00	109.00	84.10	73.00	78.40	101.50	87.30	93.10
Ta	1.70	1.70	1.60	1.40	1.80	1.70	1.60	1.40	1.70	1.70
Th	23.70	23.20	23.80	25.90	25.90	26.60	26.00	24.20	25.00	24.60
U	6.53	4.14	4.85	4.96	5.43	4.26	4.72	4.76	4.56	4.35
V	15.00	22.00	18.00	5.00	12.00	6.00	10.00	16.00	30.00	9.00
W	2.00	2.00	1.00	2.00	2.00	4.00	4.00	2.00	5.00	2.00
Zr	167.00	181.00	184.00	177.00	157.00	126.00	139.00	179.00	132.00	147.00
Cl	0.02	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	<0.01

. The porphyroclastic lava and granitic enclaves are rich in Rb, Th, U, La, K, Pr, Nd, Sm, Dy, and other elements, with strong losses of Ba, Hf, Ti, P, Nb, and so on. Based on the mineralogical findings, the loss of Ba is mainly due to the crystallization effect of plagioclase, the loss in P is caused by the separation of phosphorus-rich minerals such as monazite, apatite, and yttrium phosphate, and the loss of Ti may be due to crystallization of ilmenite and rutile. There is a consistent trend between the porphyroclastic lava and granitic enclaves in the trace element spider diagram (Figure 10). Overall, the composition of the trace elements reflects the characteristic of “the rich are richer, and the losers are poorer”.

Figure 10. Primitive mantle-normalized trace element spider diagrams of porphyroclastic lava and granite enclaves [24]. The elemental analysis data from the host rock and the granitic enclaves are plotted in Figure 10, and the specific research values are listed in Table 2. During the testing process, the tourmaline enclaves were excluded.

Figure 10. Primitive mantle-normalized trace element spider diagrams of porphyroclastic lava and granite enclaves [24]. The elemental analysis data from the host rock and the granitic enclaves are plotted in Figure 10, and the specific research values are listed in Table 2. During the testing process, the tourmaline enclaves were excluded.

6.3. Characteristics of Rare Earth Elements

The results of the rare earth element analyses of the porphyroclastic lava and granitic enclaves are shown in

Table 3. Analysis of rare earth elements and related parameters of porphyroclastic lava and granitic enclaves in XiangShan (×10⁻⁶).

Number of Sample	xs-1-yuan	xs-3-yuan	xs-9-yuan	xs-10-yuan	xs-1-bu	xs-2-bu	xs-3-bu	xs-7-bu	xs-8-bu	xs-11-bu
lithologic characters	porphyroclastic lava				granitic enclave
La	188.19	208.44	237.13	242.19	176.37	142.62	173.42	222.78	158.23	190.30
Ce	152.29	165.03	178.92	184.64	147.39	126.47	143.14	178.10	129.74	135.95
Pr	109.47	117.89	130.00	127.89	106.32	87.05	104.95	124.74	95.16	111.58
Nd	75.25	81.09	88.73	89.34	76.06	60.16	73.04	86.92	66.00	77.67
Sm	49.74	50.52	54.31	55.49	50.33	39.22	48.82	52.55	44.51	51.24
Eu	11.72	13.28	14.48	15.34	10.00	8.79	10.52	12.76	10.34	14.14
Gd	30.51	32.26	33.28	38.30	31.05	23.02	29.25	29.98	27.40	31.19
Tb	26.47	27.81	28.88	33.69	27.01	20.05	24.60	24.87	22.99	27.54
Dy	22.01	22.13	24.09	29.45	21.89	16.54	20.31	20.00	19.92	23.27
Ho	19.96	19.96	21.02	27.39	19.43	14.66	18.37	17.49	17.84	20.14
Y	20.25	20.96	22.17	31.66	20.19	15.10	19.11	16.82	18.98	20.00
Er	18.73	18.25	19.70	28.22	18.67	13.66	16.92	15.83	17.22	18.97
Tm	18.04	18.04	18.82	25.88	18.43	14.12	16.86	15.29	16.86	19.22
Yb	17.53	16.94	17.65	24.18	18.29	14.35	16.12	14.53	16.24	18.82
Lu	18.11	17.32	18.11	25.20	18.50	14.96	16.54	15.35	17.72	19.29
ΣREE	246.67	264.52	288.53	314.27	241.07	195.53	232.57	271.06	215.00	239.37
ΣLREE	193.89	210.40	231.30	236.33	188.18	155.88	183.05	225.63	166.15	186.16
ΣHREE	52.78	54.12	57.23	77.94	52.89	39.65	49.52	45.43	48.85	53.21
LREE/HREE	3.67	3.89	4.04	3.03	3.56	3.93	3.70	4.97	3.40	3.50
δEu	0.30	0.33	0.34	0.33	0.25	0.29	0.28	0.32	0.30	0.35

Note: δEu = Eu_N/(Sm_N × Gd_N)^1/2.

. The total rare earth content of the porphyroclastic lava (246.67 × 10⁻⁶ to 314.27 × 10⁻⁶) is higher than that of the granitic enclaves (195.53 × 10⁻⁶ to 271.06 × 10⁻⁶). The heavy rare earth content in the porphyroclastic lava is 52.78 × 10⁻⁶ to 77.94 × 10⁻⁶, and in the granitic enclaves it is 39.65 × 10⁻⁶ to 53.21 × 10⁻⁶. The porphyroclastic lava has a higher heavy rare earth content, but there is only a small difference in light rare earth content. Both average values of δEu were 0.3, showing strong Eu loss, indicating that plagioclase fractional crystallization may have occurred in the source area during the formation of the porphyroclastic lava and granitic enclaves. The trends of the rare earth element distribution curves of the porphyroclastic lava and granitic enclaves are consistent (Figure 11), showing the characteristics of steep at the left and gentle on the right, with a rightward trend.

7. Zircon U–Pb Age Dating and Zircon Trace Element

7.1. Zircon U–Pb Age Feature

Most of the zircon crystals of the granitic enclaves and porphyroclastic lavas have a long columnar crystal shape and are intact. The cathodoluminescence images of the zircons revealed obvious oscillatory zoning (Figure 12). The Th/U ratios of zircons from the porphyroclastic lava and granitic enclave samples were all greater than 0.1 (

Table 4. LA-ICP-MS zircon U–Pb isotope analysis results for granite enclaves and porphyroclastic lava.

Number of Sample	Content (×10−6)			Th/U	Isotope Ratio						Age (Ma)
	Pb	Th	U		207Pb/206Pb	2σ	207Pb/235U	2σ	206Pb/238U	2σ	207Pb/235U	2σ	206Pb/238U	2σ	207Pb/206Pb	2σ
XS-7
1	7.7	136.4	316.1	0.4	0.0498	0.0026	0.1441	0.0074	0.0213	0.0003	136.1	6.5	135.6	1.9	150.0	110.0
2	13.9	226.6	581.7	0.4	0.0493	0.0021	0.1413	0.0059	0.0211	0.0002	133.8	5.2	134.9	1.5	152.0	90.0
3	26.2	285.3	1095.0	0.3	0.0504	0.0024	0.1427	0.0069	0.0210	0.0004	135.2	6.1	134.2	2.2	190.0	100.0
4	9.4	181.5	401.0	0.5	0.0477	0.0035	0.1370	0.0110	0.0214	0.0005	129.9	9.3	136.3	2.9	60.0	150.0
5	5.7	98.5	234.7	0.4	0.0507	0.0032	0.1446	0.0092	0.0212	0.0004	136.1	8.1	135.2	2.2	170.0	130.0
6	14.7	191.6	690.0	0.3	0.0522	0.0048	0.1450	0.0130	0.0214	0.0005	137.0	11.0	136.3	3.0	250.0	190.0
7	4.8	71.9	199.9	0.4	0.0487	0.0030	0.1405	0.0087	0.0214	0.0003	132.6	7.7	136.7	2.0	100.0	130.0
8	5.8	88.1	247.0	0.4	0.0484	0.0034	0.1460	0.0110	0.0215	0.0004	136.9	9.7	137.0	2.3	130.0	150.0
9	2.7	43.3	113.6	0.4	0.0489	0.0044	0.1390	0.0120	0.0213	0.0005	131.0	11.0	135.6	2.9	60.0	170.0
10	9.5	218.6	364.3	0.6	0.0519	0.0047	0.1480	0.0130	0.0211	0.0005	140.0	11.0	134.5	2.9	200.0	180.0
11	15.0	188.3	678.0	0.3	0.0517	0.0030	0.1454	0.0084	0.0212	0.0004	139.0	7.9	134.9	2.3	230.0	120.0
12	5.8	161.6	235.0	0.7	0.0516	0.0059	0.1460	0.0150	0.0213	0.0006	137.0	13.0	136.1	3.6	180.0	220.0
13	5.8	94.3	246.7	0.4	0.0489	0.0041	0.1410	0.0120	0.0212	0.0004	133.0	11.0	135.1	2.4	80.0	170.0
14	17.1	174.4	772.0	0.2	0.0503	0.0025	0.1447	0.0072	0.0212	0.0004	137.0	6.4	134.9	2.2	210.0	110.0
15	18.4	253.5	772.0	0.3	0.0496	0.0020	0.1451	0.0058	0.0214	0.0002	137.2	5.1	136.3	1.5	145.0	85.0
16	7.0	115.7	284.9	0.4	0.0488	0.0036	0.1420	0.0100	0.0211	0.0004	134.0	9.1	134.4	2.2	80.0	140.0
17	4.7	86.9	191.5	0.5	0.0473	0.0054	0.1380	0.0160	0.0210	0.0006	130.0	14.0	134.0	3.6	20.0	220.0
XS-11
1	13.1	195.6	582.0	0.3	0.0485	0.0020	0.1419	0.0059	0.0213	0.0002	134.3	5.2	135.8	1.5	96.0	85.0
2	7.2	143.4	291.3	0.5	0.0466	0.0059	0.1360	0.0170	0.0212	0.0006	128.0	15.0	135.0	3.8	−20.0	240.0
3	6.8	145.5	272.5	0.5	0.0488	0.0035	0.1398	0.0092	0.0213	0.0004	132.2	8.1	135.6	2.4	80.0	130.0
4	3.5	96.6	126.5	0.8	0.0491	0.0043	0.1380	0.0120	0.0211	0.0005	131.0	11.0	134.8	3.1	50.0	160.0
5	7.1	116.9	293.1	0.4	0.0483	0.0035	0.1400	0.0096	0.0214	0.0004	132.4	8.6	136.6	2.6	70.0	150.0
6	5.6	94.3	224.2	0.4	0.0483	0.0042	0.1390	0.0110	0.0213	0.0004	131.0	10.0	136.0	2.8	50.0	170.0
7	5.4	114.9	220.9	0.5	0.0478	0.0033	0.1373	0.0091	0.0212	0.0004	129.7	8.1	135.0	2.2	50.0	140.0
8	3.4	68.2	133.7	0.5	0.0468	0.0042	0.1360	0.0120	0.0212	0.0005	128.0	11.0	135.3	3.1	10.0	170.0
9	8.6	119.7	366.1	0.3	0.0488	0.0031	0.1411	0.0088	0.0214	0.0004	133.5	7.8	136.2	2.4	100.0	130.0
10	21.2	261.3	867.0	0.3	0.0486	0.0030	0.1403	0.0086	0.0212	0.0004	133.0	7.6	135.3	2.4	100.0	130.0
11	6.2	133.2	232.4	0.6	0.0500	0.0059	0.1550	0.0220	0.0214	0.0009	145.0	19.0	136.2	5.8	190.0	260.0
12	5.5	61.5	237.0	0.3	0.0481	0.0033	0.1440	0.0100	0.0214	0.0004	136.1	8.9	136.6	2.5	100.0	140.0
13	23.6	270.1	1071.0	0.3	0.0499	0.0035	0.1450	0.0095	0.0209	0.0005	137.3	8.4	133.1	3.3	170.0	150.0
14	12.2	178.0	487.9	0.4	0.0485	0.0034	0.1480	0.0110	0.0215	0.0004	139.4	9.3	137.1	2.7	120.0	150.0
15	2.2	55.6	86.1	0.7	0.0490	0.0066	0.1460	0.0190	0.0215	0.0006	136.0	17.0	137.4	4.0	10.0	250.0
16	8.7	190.5	348.5	0.6	0.0479	0.0031	0.1449	0.0095	0.0214	0.0004	136.9	8.3	136.7	2.4	90.0	130.0
17	6.8	157.1	270.7	0.6	0.0515	0.0043	0.1490	0.0150	0.0212	0.0005	143.0	12.0	135.0	3.2	220.0	170.0
18	2.0	52.8	77.8	0.7	0.0502	0.0050	0.1460	0.0140	0.0213	0.0006	136.0	13.0	135.9	3.5	60.0	190.0
19	9.5	108.6	412.0	0.3	0.0498	0.0026	0.1469	0.0073	0.0213	0.0003	138.6	6.4	135.9	2.1	160.0	110.0
20	1.4	55.3	51.0	1.1	0.0500	0.0110	0.1390	0.0280	0.0215	0.0008	134.0	26.0	137.1	5.3	−20.0	360.0
XSD-12
1	4.4	127.0	171.0	0.7	0.0487	0.0037	0.1460	0.0110	0.0213	0.0005	136.8	9.8	135.7	2.8	90.0	150.0
2	3.8	123.4	152.4	0.8	0.0459	0.0055	0.1330	0.0160	0.0211	0.0006	125.0	14.0	134.6	3.5	−80.0	220.0
3	6.9	125.8	292.9	0.4	0.0500	0.0047	0.1410	0.0140	0.0207	0.0005	133.0	12.0	132.1	3.2	100.0	180.0
4	3.7	104.6	144.7	0.7	0.0508	0.0053	0.1480	0.0150	0.0211	0.0006	139.0	13.0	134.5	4.0	160.0	210.0
5	5.5	135.8	235.2	0.6	0.0509	0.0031	0.1465	0.0086	0.0209	0.0004	139.1	7.8	133.5	2.3	180.0	120.0
6	4.6	67.8	205.5	0.3	0.0510	0.0037	0.1450	0.0100	0.0208	0.0003	137.5	9.4	132.4	2.1	140.0	140.0
7	6.8	180.9	270.9	0.7	0.0488	0.0036	0.1450	0.0110	0.0213	0.0004	135.9	9.6	135.8	2.2	70.0	140.0
8	11.2	192.9	486.0	0.4	0.0499	0.0027	0.1451	0.0083	0.0210	0.0003	137.0	7.3	133.7	2.1	150.0	110.0
9	11.5	153.3	517.0	0.3	0.0504	0.0031	0.1452	0.0090	0.0209	0.0004	137.3	8.0	133.3	2.6	230.0	130.0
10	9.8	183.1	412.0	0.4	0.0492	0.0025	0.1437	0.0071	0.0211	0.0003	135.8	6.3	134.7	2.0	120.0	100.0
11	9.8	153.2	411.4	0.4	0.0517	0.0040	0.1480	0.0110	0.0211	0.0004	139.6	9.8	134.6	2.4	200.0	150.0
12	7.4	238.2	289.8	0.8	0.0488	0.0033	0.1402	0.0094	0.0208	0.0004	132.2	8.4	132.6	2.3	70.0	130.0
13	7.8	221.6	313.4	0.7	0.0475	0.0028	0.1384	0.0081	0.0211	0.0003	130.9	7.3	134.7	2.2	30.0	120.0
14	7.1	144.3	294.0	0.5	0.0470	0.0028	0.1372	0.0079	0.0212	0.0003	129.9	7.1	135.3	2.2	30.0	120.0
15	6.6	130.4	277.8	0.5	0.0481	0.0031	0.1384	0.0085	0.0209	0.0004	130.8	7.5	133.0	2.4	50.0	120.0
16	6.8	149.1	279.8	0.5	0.0498	0.0038	0.1460	0.0110	0.0209	0.0003	136.9	9.9	133.2	2.1	130.0	150.0
17	4.8	89.3	202.6	0.4	0.0471	0.0047	0.1390	0.0130	0.0212	0.0005	131.0	12.0	135.1	2.9	−10.0	190.0
18	10.8	217.2	460.2	0.5	0.0487	0.0026	0.1409	0.0070	0.0209	0.0003	133.3	6.2	133.5	1.7	110.0	110.0
19	4.6	99.7	195.3	0.5	0.0486	0.0045	0.1390	0.0120	0.0207	0.0004	130.0	11.0	132.0	2.7	60.0	180.0
20	6.5	201.2	252.0	0.8	0.0495	0.0034	0.1457	0.0095	0.0213	0.0004	138.3	8.1	136.0	2.2	130.0	130.0

), indicating that these zircons are typical magmatic zircons [25]. Twenty reliable measurement data for the xs-D-12 porphyroclastic lava test sample ²⁰⁶Pb/²³⁸U surface age indicated a result between 132.0 Ma and 136.0 Ma, with a weighted average age of 134.0 ± 0.5 Ma (MSWD = 2.0) (Figure 13a,b). Seventeen reliable measuring point data for the xs-7 granitic enclave test sample ²⁰⁶Pb/³³⁸U surface age were between 134.0 Ma and 137.0 Ma, with a weighted mean age of 135.5 ± 0.5 Ma (MSWD = 0.62) (Figure 13c,d), and twenty reliable point measurement data of xs-11 granitic enclaves ranged from 133.10 Ma to 137.4 Ma, with a weighted average age of 135.8 ± 0.6 Ma (MSWD = 0.40) (Figure 13e,f).

7.2. Zircon Trace Element Characteristics

The content of trace elements in zircon was obtained simultaneously during LA-ICP-MS zircon U-Pb dating. The zircons from the samples of granitic enclaves and porphyroclastic lavas show basically overlapping trace element compositions with a small variation range (

Table 5. The trace element (×10⁻⁶) composition and t_Zr-Ti temperature of zircon in the porphyroclastic lava and granitic enclave from XiangShan (×10⁻⁶).

Number of Sample	Th	U	Ta	Ce	P	Pr	Nd	Hf	Sm	Eu	Ti	Dy	Y	Ho	Yb	Lu	T (°C)
XS-7
7-1	136.40	970.00	1.85	13.11	221.00	0.04	1.08	11,320.00	2.78	0.27	4.74	76.20	827.00	28.65	242.00	46.41	679.34
7-2	226.60	1784.00	3.01	11.77	237.00	0.03	0.90	12,180.00	3.04	0.24	2.31	89.40	1022.00	34.09	313.80	61.33	626.69
7-3	285.30	3294.00	6.15	7.62	256.00	0.02	0.60	14,340.00	2.32	0.09	1.79	104.10	1261.00	42.05	386.00	77.50	609.38
7-4	181.50	1195.00	2.15	11.83	463.00	0.08	1.73	10,610.00	3.99	0.48	5.90	117.60	1331.00	44.20	397.70	74.70	696.62
7-5	98.50	701.00	1.85	10.34	185.00	0.02	0.77	11,680.00	2.07	0.21	3.13	67.00	755.50	25.88	221.70	43.32	648.22
7-6	191.60	2140.00	4.42	8.82	201.00	0.02	0.40	12,700.00	2.32	0.15	3.30	90.80	1043.00	36.60	344.00	63.50	652.07
7-7	71.90	609.00	1.19	8.72	138.00	0.02	0.39	11,840.00	1.81	0.16	3.15	51.50	593.00	20.16	180.00	34.66	648.68
7-8	88.10	761.00	1.54	8.67	219.00	0.04	0.81	11,930.00	2.19	0.28	4.07	63.80	726.00	24.60	230.10	43.80	667.67
7-9	43.33	350.00	0.93	5.12	288.00	0.06	1.03	9720.00	2.60	0.55	11.80	61.90	691.00	23.20	211.80	42.37	755.73
7-10	218.60	1130.00	1.21	12.12	193.00	0.17	2.38	10,900.00	5.92	0.70	5.60	126.50	1333.00	46.01	346.00	66.10	692.45
7-11	188.30	2044.00	4.35	9.72	233.00	0.02	0.68	12,330.00	2.65	0.17	2.39	102.10	1249.00	39.94	405.00	78.20	629.05
7-12	161.60	697.00	0.67	10.10	199.00	0.24	4.36	9490.00	9.59	1.65	9.00	160.50	1552.00	54.90	397.00	71.90	731.80
7-13	94.30	773.00	1.99	8.86	183.00	0.02	0.56	11,480.00	1.84	0.19	2.62	63.30	712.00	24.55	216.50	42.09	635.49
7-14	174.40	2275.00	4.79	6.20	231.00	0.03	0.83	13,160.00	2.23	0.21	3.19	93.40	1130.00	36.80	360.10	71.20	649.59
7-15	253.50	2360.00	3.90	11.64	244.00	0.04	0.78	12,150.00	2.80	0.20	2.15	93.00	1104.00	36.48	340.90	66.70	621.75
7-16	115.70	880.00	1.67	10.97	200.00	0.05	0.88	11,010.00	2.33	0.30	4.43	69.90	766.90	26.13	226.10	44.66	674.13
7-17	86.90	583.00	0.98	6.68	494.00	0.08	2.08	9790.00	4.54	0.47	9.80	102.60	1164.00	38.50	338.50	67.20	739.20
XS-11
11-1	195.60	1774.00	2.97	11.20	196.00	0.01	0.83	11,940.00	2.40	0.25	2.94	83.90	966.00	32.48	304.50	57.67	643.69
11-2	143.40	878.00	1.54	12.48	234.00	0.06	1.38	10,510.00	3.92	0.39	7.00	80.80	870.00	29.82	248.50	48.20	710.56
11-3	145.50	833.00	0.94	9.68	202.00	0.11	2.63	10,530.00	6.12	0.89	5.18	121.80	1253.00	43.30	321.50	60.70	686.27
11-4	96.60	391.00	0.47	8.69	281.00	0.58	6.14	8990.00	8.60	1.87	11.90	119.90	1176.00	41.40	288.10	55.80	756.50
11-5	116.90	880.00	2.01	10.98	188.00	0.02	0.71	11,260.00	2.60	0.24	3.10	73.10	841.00	28.17	254.70	49.30	647.52
11-6	94.30	689.00	1.25	8.95	309.00	0.06	1.04	10,530.00	2.77	0.38	6.70	82.20	922.00	31.02	280.40	55.26	706.95
11-7	114.90	682.00	0.85	8.15	365.00	0.17	2.61	9420.00	5.54	1.07	10.90	114.40	1210.00	40.99	337.40	65.59	748.60
11-8	68.20	400.00	0.80	6.82	256.00	0.07	1.40	9640.00	2.66	0.57	7.60	71.80	773.00	26.11	223.00	44.50	717.41
11-9	119.70	1105.00	2.65	7.69	187.00	0.04	0.59	12,440.00	2.04	0.17	2.70	70.30	827.00	27.86	258.50	51.18	637.62
11-10	261.30	2654.00	4.35	10.90	238.00	0.02	0.72	12,820.00	2.45	0.18	2.80	94.10	1138.00	37.24	358.90	69.80	640.20
11-11	133.20	694.00	0.83	9.23	321.00	0.08	2.30	10,870.00	5.90	0.76	6.50	127.30	1339.00	45.60	342.20	66.70	704.47
11-12	61.50	694.00	1.83	6.05	161.00	0.01	0.27	12,240.00	0.95	0.13	2.58	38.60	477.00	15.22	159.40	31.50	634.41
11-13	270.10	3170.00	5.50	10.05	271.00	0.04	0.77	12,890.00	2.85	0.17	2.70	115.70	1399.00	43.40	438.00	81.60	637.62
11-14	178.00	1481.00	2.58	11.33	189.00	0.03	0.85	12,040.00	2.66	0.23	2.77	79.30	921.00	30.76	272.90	54.70	639.43
11-15	55.60	246.00	0.36	3.58	320.00	0.13	2.74	9880.00	6.14	1.17	8.50	104.00	1063.00	36.60	270.90	50.50	726.89
11-16	190.50	1035.00	1.40	12.93	288.00	0.11	2.68	10,730.00	6.53	0.60	3.90	135.20	1466.00	49.10	373.20	70.00	664.45
11-17	157.10	815.00	1.26	11.81	241.00	0.11	1.92	9760.00	3.94	0.56	8.20	82.80	883.00	29.43	238.90	46.70	723.82
11-18	52.80	233.70	0.36	3.18	301.00	0.14	2.31	9050.00	3.91	0.85	14.80	74.20	772.00	25.75	203.00	40.68	776.64
11-19	108.60	1239.00	2.45	7.68	216.00	0.02	0.44	11,980.00	1.77	0.09	2.55	64.20	762.00	25.40	238.90	47.32	633.58
11-20	55.30	164.00	0.44	6.62	369.00	0.15	2.52	10,470.00	4.62	1.01	10.00	86.20	887.00	30.43	238.10	45.50	740.97
XSD-12
12-1	127.00	505.00	0.70	9.67	211.00	0.22	3.90	9320.00	7.78	1.46	7.30	125.20	1226.00	42.90	304.00	59.50	714.04
12-2	123.40	430.00	0.56	7.82	168.00	0.45	6.93	8750.00	9.94	2.08	9.20	146.70	1386.00	48.90	353.90	66.00	733.70
12-3	125.80	858.00	1.78	11.42	182.00	0.03	0.76	11,800.00	2.11	0.20	3.80	60.10	675.00	22.56	192.90	38.98	662.51
12-4	104.60	412.00	0.63	10.67	177.00	0.09	1.64	9960.00	4.81	0.69	5.40	101.80	1042.00	36.50	257.70	48.20	689.56
12-5	135.80	710.00	0.77	8.41	209.00	0.16	3.46	9470.00	7.34	1.07	7.00	132.60	1323.00	45.90	345.10	64.10	710.56
12-6	67.80	620.00	1.28	7.07	95.00	0.01	0.33	11,820.00	1.16	0.13	2.53	52.50	594.00	19.90	181.00	36.35	633.03
12-7	180.90	810.00	0.84	10.05	201.00	0.31	5.38	9160.00	10.35	1.80	7.70	184.20	1777.00	61.30	438.30	81.20	718.51
12-8	192.90	1467.00	2.48	10.31	170.00	0.04	0.94	11,870.00	3.55	0.27	2.13	118.30	1333.00	45.00	379.00	73.20	621.11
12-9	153.30	1538.00	3.99	8.14	212.00	0.02	0.67	12,150.00	2.15	0.19	1.89	90.80	1063.00	34.40	340.70	67.90	613.01
12-10	183.10	1226.00	2.07	11.70	222.00	0.06	1.63	11,300.00	4.24	0.33	3.49	122.40	1323.00	45.21	372.60	73.10	656.18
12-11	153.20	1221.00	2.76	10.55	260.00	0.07	1.02	11,620.00	2.38	0.16	3.49	81.10	934.00	30.36	275.40	55.80	656.18
12-12	238.20	890.00	1.13	20.40	199.00	0.34	5.00	10,360.00	10.30	0.97	5.29	168.50	1634.00	56.90	392.50	75.20	687.93
12-13	221.60	952.00	1.10	16.52	216.00	0.13	3.87	10,580.00	8.50	0.92	4.88	173.40	1666.00	58.00	408.00	75.20	681.60
12-14	144.30	900.00	1.45	10.91	233.00	0.08	1.84	10,360.00	4.89	0.60	4.94	111.90	1186.00	40.90	331.60	64.90	682.56
12-15	130.40	848.00	1.72	11.52	166.00	0.06	1.05	11,310.00	2.83	0.29	3.33	79.10	839.00	28.64	233.40	45.07	652.73
12-16	149.10	848.00	1.09	11.68	152.00	0.07	1.43	11,360.00	4.08	0.50	4.20	116.60	1201.00	41.30	307.00	59.50	670.05
12-17	89.30	617.00	1.52	9.64	174.00	0.02	0.73	10,670.00	2.20	0.26	4.68	69.50	781.00	25.98	230.20	45.14	678.35
12-18	217.20	1411.00	2.34	16.42	235.00	0.08	1.44	10,990.00	3.68	0.40	5.52	94.10	1025.00	34.80	292.10	56.55	691.30
12-19	99.70	588.00	1.37	11.94	218.00	0.07	1.58	9750.00	3.79	0.47	7.90	81.40	882.00	30.10	256.50	51.20	720.67
12-20	201.20	761.00	0.85	11.88	190.00	0.44	6.88	9440.00	11.86	1.65	7.50	184.60	1781.00	63.10	430.00	80.30	716.30

).

The trace elements of zircons generally show the characteristics of high Th/U ratio, enrichment of heavy rare earth elements instead of light rare earth elements, obviously Ce positive anomaly, and Eu negative anomaly (

Table 6. Analysis of rare earth elements and related parameters of zircon in the porphyroclastic lava and granitic enclave from XiangShan (×10⁻⁶).

Number of Sample	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Y	Ho	Er	Tm	Lu	ΣRee
XS-7
7-1	0.00	13.11	0.04	1.08	2.78	0.27	17.98	6.02	76.20	827.00	28.65	134.50	27.28	46.41	1423.321
7-2	0.02	11.77	0.03	0.90	3.04	0.24	18.49	6.74	89.40	1022.00	34.09	167.30	34.98	61.33	1764.126
7-3	0.00	7.62	0.02	0.60	2.32	0.09	19.10	7.65	104.10	1261.00	42.05	208.30	43.33	77.50	2159.687
7-4	0.00	11.83	0.08	1.73	3.99	0.48	26.30	9.38	117.60	1331.00	44.20	212.20	43.94	74.70	2275.124
7-5	0.00	10.34	0.02	0.77	2.07	0.21	15.60	5.24	67.00	755.50	25.88	123.20	25.16	43.32	1296.009
7-6	0.00	8.82	0.02	0.40	2.32	0.15	17.00	6.67	90.80	1043.00	36.60	174.80	37.60	63.50	1825.673
7-7	0.00	8.72	0.02	0.39	1.81	0.16	11.50	4.05	51.50	593.00	20.16	98.40	19.97	34.66	1024.345
7-8	0.00	8.67	0.04	0.81	2.19	0.28	13.34	5.00	63.80	726.00	24.60	119.90	25.21	43.80	1263.738
7-9	0.00	5.12	0.06	1.03	2.60	0.55	15.25	4.99	61.90	691.00	23.20	110.30	23.03	42.37	1193.203
7-10	0.09	12.12	0.17	2.38	5.92	0.70	34.00	10.77	126.50	1333.00	46.01	208.80	40.92	66.10	2233.475
7-11	0.00	9.72	0.02	0.68	2.65	0.17	19.17	7.70	102.10	1249.00	39.94	199.60	43.71	78.20	2157.657
7-12	0.00	10.10	0.24	4.36	9.59	1.65	45.70	14.25	160.50	1552.00	54.90	236.70	46.20	71.90	2605.092
7-13	0.00	8.86	0.02	0.56	1.84	0.19	13.12	4.91	63.30	712.00	24.55	116.50	24.28	42.09	1228.723
7-14	0.00	6.20	0.03	0.83	2.23	0.21	16.73	6.69	93.40	1130.00	36.80	185.40	39.80	71.20	1949.627
7-15	0.00	11.64	0.04	0.78	2.80	0.20	18.79	6.96	93.00	1104.00	36.48	178.80	38.00	66.70	1899.092
7-16	0.03	10.97	0.05	0.88	2.33	0.30	15.42	5.45	69.90	766.90	26.13	123.90	25.54	44.66	1318.552
7-17	0.00	6.68	0.08	2.08	4.54	0.47	24.10	8.22	102.60	1164.00	38.50	183.20	37.80	67.20	1977.974
XS-11
11-1	0.00	11.20	0.01	0.83	2.40	0.25	16.56	6.50	83.90	966.00	32.48	158.50	33.62	57.67	1674.412
11-2	0.00	12.48	0.06	1.38	3.92	0.39	21.30	6.67	80.80	870.00	29.82	139.30	28.02	48.20	1490.845
11-3	0.00	9.68	0.11	2.63	6.12	0.89	33.50	10.52	121.80	1253.00	43.30	194.20	37.58	60.70	2095.534
11-4	0.88	8.69	0.58	6.14	8.60	1.87	38.80	11.02	119.90	1176.00	41.40	180.80	33.90	55.80	1972.48
11-5	0.00	10.98	0.02	0.71	2.60	0.24	15.51	5.84	73.10	841.00	28.17	135.70	28.09	49.30	1445.964
11-6	0.00	8.95	0.06	1.04	2.77	0.38	18.79	6.25	82.20	922.00	31.02	147.00	30.86	55.26	1586.983
11-7	0.02	8.15	0.17	2.61	5.54	1.07	29.87	9.43	114.40	1210.00	40.99	189.20	37.79	65.59	2052.226
11-8	0.00	6.82	0.07	1.40	2.66	0.57	18.05	5.78	71.80	773.00	26.11	123.90	25.02	44.50	1322.679
11-9	0.00	7.69	0.04	0.59	2.04	0.17	13.96	5.23	70.30	827.00	27.86	136.10	28.60	51.18	1429.258
11-10	0.00	10.90	0.02	0.72	2.45	0.18	18.70	7.02	94.10	1138.00	37.24	186.30	39.10	69.80	1963.43
11-11	0.01	9.23	0.08	2.30	5.90	0.76	33.80	10.45	127.30	1339.00	45.60	204.50	40.50	66.70	2228.332
11-12	0.00	6.05	0.01	0.27	0.95	0.13	7.48	2.82	38.60	477.00	15.22	77.60	16.96	31.50	833.998
11-13	0.00	10.05	0.04	0.77	2.85	0.17	20.40	8.86	115.70	1399.00	43.40	219.20	46.50	81.60	2386.537
11-14	0.00	11.33	0.03	0.85	2.66	0.23	16.90	6.22	79.30	921.00	30.76	149.20	30.18	54.70	1576.253
11-15	0.00	3.58	0.13	2.74	6.14	1.17	30.90	9.43	104.00	1063.00	36.60	162.90	30.81	50.50	1772.803
11-16	0.01	12.93	0.11	2.68	6.53	0.60	34.60	11.46	135.20	1466.00	49.10	224.80	44.00	70.00	2431.225
11-17	0.02	11.81	0.11	1.92	3.94	0.56	21.50	6.99	82.80	883.00	29.43	136.30	27.22	46.70	1491.195
11-18	0.01	3.18	0.14	2.31	3.91	0.85	21.07	6.38	74.20	772.00	25.75	117.20	23.24	40.68	1293.921
11-19	0.00	7.68	0.02	0.44	1.77	0.09	12.46	4.73	64.20	762.00	25.40	123.50	26.28	47.32	1314.792
11-20	0.00	6.62	0.15	2.52	4.62	1.01	24.30	7.34	86.20	887.00	30.43	134.50	26.86	45.50	1495.148
XSD-12
12-1	0.01	9.67	0.22	3.90	7.78	1.46	36.00	11.29	125.20	1226.00	42.90	188.00	36.10	59.50	2052.038
12-2	0.03	7.82	0.45	6.93	9.94	2.08	46.20	13.40	146.70	1386.00	48.90	213.10	40.50	66.00	2341.948
12-3	0.06	11.42	0.03	0.76	2.11	0.20	14.32	4.63	60.10	675.00	22.56	107.20	22.36	38.98	1152.625
12-4	0.00	10.67	0.09	1.64	4.81	0.69	30.30	9.08	101.80	1042.00	36.50	156.90	30.01	48.20	1730.392
12-5	0.00	8.41	0.16	3.46	7.34	1.07	37.60	11.47	132.60	1323.00	45.90	209.70	40.50	64.10	2230.412
12-6	0.00	7.07	0.01	0.33	1.16	0.13	10.74	3.88	52.50	594.00	19.90	98.00	20.12	36.35	1025.188
12-7	0.02	10.05	0.31	5.38	10.35	1.80	53.00	16.33	184.20	1777.00	61.30	272.50	52.40	81.20	2964.135
12-8	0.00	10.31	0.04	0.94	3.55	0.27	27.30	9.47	118.30	1333.00	45.00	213.50	43.10	73.20	2256.975
12-9	0.00	8.14	0.02	0.67	2.15	0.19	16.77	6.92	90.80	1063.00	34.40	174.40	38.20	67.90	1844.26
12-10	0.00	11.70	0.06	1.63	4.24	0.33	28.60	9.90	122.40	1323.00	45.21	215.40	43.28	73.10	2251.455
12-11	0.10	10.55	0.07	1.02	2.38	0.16	17.30	6.47	81.10	934.00	30.36	152.40	31.43	55.80	1598.538
12-12	0.23	20.40	0.34	5.00	10.30	0.97	52.10	15.19	168.50	1634.00	56.90	250.20	47.90	75.20	2729.731
12-13	0.00	16.52	0.13	3.87	8.50	0.92	48.10	14.95	173.40	1666.00	58.00	262.20	49.40	75.20	2785.194
12-14	0.00	10.91	0.08	1.84	4.89	0.60	27.50	9.18	111.90	1186.00	40.90	191.50	38.85	64.90	2020.644
12-15	0.00	11.52	0.06	1.05	2.83	0.29	20.60	6.47	79.10	839.00	28.64	131.50	26.67	45.07	1426.205
12-16	0.00	11.68	0.07	1.43	4.08	0.50	29.10	9.92	116.60	1201.00	41.30	191.60	37.00	59.50	2010.775
12-17	0.00	9.64	0.02	0.73	2.20	0.26	15.31	5.53	69.50	781.00	25.98	126.80	26.30	45.14	1338.603
12-18	0.03	16.42	0.08	1.44	3.68	0.40	22.83	7.66	94.10	1025.00	34.80	165.10	33.29	56.55	1753.478
12-19	0.03	11.94	0.07	1.58	3.79	0.47	21.10	6.77	81.40	882.00	30.10	142.00	28.40	51.20	1517.347
12-20	0.00	11.88	0.44	6.88	11.86	1.65	55.80	16.52	184.60	1781.00	63.10	274.50	52.30	80.30	2970.834

). In the primitive mantle normalized trace element spider diagram of zircon (Figure 14), porphyroclastic lava and granitic enclaves have the characteristics of overlapping distribution, and the trend of zircon rare earth element distribution curve in granitic enclaves and porphyroclastic lava samples is also consistent (Figure 15, Table 6).

The formation age of zircons in granitic enclaves and porphyroclastic lava samples is consistent, with a consistent trend of rare earth element distribution curve.

7.3. Hf Isotopes

In situ Hf isotopic analysis (

Table 7. LA-ICP-MS zircon Hf isotope analysis results for granite enclaves and porphyroclastic lava.

Analysis Point	176Hf/177Hf	2σ	176Lu/177Hf	2σ	176Yb/177Hf	2σ	t (Ma)	(176Hf/177Hf)i	tDM1 (Ma)	tDM2 (Ma)	εHf(t)
XS-D12-1	0.282478	0.000022	0.000837	0.000017	0.025320	0.000570	135.7	0.282476	1091	1662	−7.5
XS-D12-2	0.282510	0.000023	0.001067	0.000014	0.032340	0.000460	134.6	0.282507	1052	1593	−6.4
XS-D12-3	0.282455	0.000024	0.000465	0.000001	0.014353	0.000064	132.1	0.282454	1112	1714	−8.4
XS-D12-4	0.282498	0.000022	0.000799	0.000006	0.024390	0.000200	134.5	0.282496	1062	1618	−6.8
XS-D12-5	0.282511	0.000023	0.001092	0.000005	0.034160	0.000230	133.5	0.282508	1052	1591	−6.4
XS-D12-6	0.282508	0.000019	0.000638	0.000026	0.019240	0.000780	132.4	0.282506	1043	1596	−6.5
XS-D12-7	0.282518	0.000022	0.001251	0.000013	0.039390	0.000310	135.8	0.282515	1046	1575	−6.1
XS-D12-8	0.282491	0.000018	0.000867	0.000014	0.025890	0.000560	133.7	0.282489	1073	1635	−7.1
XS-D12-9	0.282495	0.000019	0.000739	0.000002	0.021500	0.000160	133.3	0.282493	1064	1625	−6.9
XS-D12-10	0.282493	0.000018	0.001035	0.000011	0.031800	0.000360	134.7	0.282490	1075	1630	−7.0
XS-D12-11	0.282442	0.000018	0.000537	0.000006	0.016010	0.000270	134.6	0.282441	1132	1742	−8.8
XS-D12-12	0.282488	0.000020	0.000842	0.000014	0.026090	0.000620	132.6	0.282486	1077	1642	−7.2
XS-D12-13	0.282512	0.000019	0.000846	0.000019	0.026460	0.000680	134.7	0.282510	1043	1587	−6.3
XS-D12-14	0.282488	0.000017	0.000882	0.000004	0.026370	0.000160	135.3	0.282486	1078	1640	−7.2
XS-D12-15	0.282451	0.000019	0.000668	0.000011	0.020450	0.000390	133.0	0.282449	1124	1723	−8.5
XS-D12-16	0.282472	0.000021	0.000764	0.000012	0.023190	0.000270	133.2	0.282470	1097	1677	−7.8
XS-D12-17	0.282475	0.000020	0.000663	0.000003	0.019940	0.000150	135.1	0.282473	1090	1668	−7.6
XS-D12-18	0.282470	0.000020	0.000628	0.000002	0.018960	0.000100	133.5	0.282468	1096	1680	−7.8
XS-D12-19	0.282541	0.000026	0.000991	0.000030	0.030500	0.001100	132.0	0.282539	1007	1524	−5.4
XS-D12-20	0.282488	0.000022	0.000978	0.000020	0.030670	0.000780	136.0	0.282486	1081	1640	−7.1
XS-7-bu-1	0.282466	0.000018	0.000677	0.000003	0.019220	0.000120	135.6	0.282464	1103	1688	−7.9
XS-7-bu-2	0.282462	0.000019	0.000694	0.000006	0.020202	0.000092	134.9	0.282460	1109	1698	−8.1
XS-7-bu-3	0.282438	0.000020	0.000895	0.000002	0.023395	0.000066	134.2	0.282436	1149	1753	−8.9
XS-7-bu-4	0.282472	0.000019	0.000970	0.000006	0.029600	0.000200	136.3	0.282470	1103	1676	−7.7
XS-7-bu-5	0.282480	0.000019	0.000663	0.000004	0.018650	0.000140	135.2	0.282478	1083	1657	−7.4
XS-7-bu-6	0.282466	0.000018	0.000729	0.000004	0.021626	0.000033	136.3	0.282464	1104	1688	−7.9
XS-7-bu-7	0.282494	0.000022	0.000768	0.000021	0.023310	0.000560	136.7	0.282492	1066	1626	−6.9
XS-7-bu-8	0.282483	0.000020	0.000800	0.000010	0.024110	0.000290	137.0	0.282481	1083	1650	−7.3
XS-7-bu-9	0.282447	0.000021	0.000700	0.000003	0.020190	0.000053	135.6	0.282445	1130	1731	−8.6
XS-7-bu-10	0.282518	0.000021	0.001023	0.000018	0.031590	0.000410	134.5	0.282515	1040	1575	−6.1
XS-7-bu-11	0.282466	0.000018	0.000726	0.000011	0.020510	0.000340	134.9	0.282464	1104	1689	−7.9
XS-7-bu-12	0.282501	0.000022	0.001015	0.000006	0.031500	0.000260	136.1	0.282498	1064	1612	−6.7
XS-7-bu-13	0.282463	0.000021	0.000555	0.000002	0.016640	0.000160	135.1	0.282462	1104	1695	−8.0
XS-7-bu-14	0.282489	0.000018	0.000988	0.000007	0.029680	0.000300	134.9	0.282487	1080	1639	−7.1
XS-7-bu-15	0.282493	0.000018	0.001012	0.000002	0.029174	0.000084	136.3	0.282490	1075	1629	−7.0
XS-7-bu-16	0.282469	0.000021	0.000613	0.000006	0.017870	0.000220	134.4	0.282467	1097	1682	−7.8
XS-7-bu-17	0.282493	0.000021	0.001033	0.000009	0.030590	0.000370	134.0	0.282490	1075	1631	−7.0

) was carried out on the porphyroclastic lava and granitic enclaves. The zircon ε_Hf(t) value and the two-stage model age were calculated using the U–Pb weighted average age of the sample where the analysis point was located. The value of ¹⁷⁶Lu/¹⁷⁷Hf was less than 0.002. This indicates that the radiogenic accumulation of Hf isotope in the zircon is very small. This low value can be used to discuss the origin of the rock by using the ratio of ¹⁷⁶Lu/¹⁷⁷Hf in the rock mass [26]. The ratio of ¹⁷⁶Hf/¹⁷⁷Hf represents the Hf isotope composition of the zircon crystallization system. In the porphyroclastic lava and granitic enclaves, the ratio varied within the ¹⁷⁶Lu/¹⁷⁷Hf range of 0.000465 to 0.001092 and 0.000700 to 0.001033, respectively. The variation ranges of zircon ¹⁷⁶Hf/¹⁷⁷Hf in the porphyroclastic lava and granitic enclaves were 0.282442 to 0.282541 and 0.282438 to 0.282518, respectively. The ranges of ε_Hf(t) were −8.9 to −5.4 and −8.9 to −6.1, respectively. The ε_Hf(t) values of the two types of sample were very close, and the range was small. The single-stage Hf model ages (t_DM1) of the porphyroclastic lavas and granitic enclaves were between 1043 Ma and 1112 Ma and between 1040 Ma and 1149 Ma, and the two-stage Hf model ages (t_DM2) were 1524 Ma to 1742 Ma and 1575 Ma to 1753 Ma, respectively.

8. Discussion

8.1. Formation Age of Granitic Enclaves

The volcanic intrusive complex in the Xiangshan uranium ore field includes mainly rhyodacite, porphyroclastic lava, granite porphyry, and some other lithologies. A previous study obtained a large number of chronological results from the different lithologies. These results indicated that the age of the rhyodacite is mainly within 137 Ma [27], the age of the porphyroclastic lava is mainly within 134 Ma [28,29], the age of the granite porphyry is mainly within 136 Ma [28]. These ages within their error range appear to be fairly consistent, but the field occurrences indicate that there is a more obvious sequential relationship.

Academics used the single-grain zircon U–Pb dating method to determine the age of the granitic enclaves and obtained a result of 134.2 ± 1.9 Ma [12]. In this study, LA-ICP-MS zircon U–Pb dating was carried out on the granitic enclaves and porphyroclastic lavas of the central phase of the porphyroclastic lavas in the Ehuling Formation of the Xiangshan uranium ore field. The age data of the two granitic enclave samples were 135.5 ± 0.5 Ma (MSWD = 0.62) and 135.8 ± 0.6 Ma (MSWD = 0.4), and the age of the single porphyroclastic lava sample was 134.0 ± 0.5 Ma (MSWD = 2.0). Thus, the granitic enclaves and porphyroclastic lava formed at the same time (in the Early Cretaceous). This result is basically consistent with the diagenetic age of the main ore-forming rock mass of the intrusive complex in the Xiangshan volcanic basin obtained by predecessors.

8.2. Material Source of Granitic Enclaves

The zircons have a high Hf isotope system closure temperature, which can indicate the characteristics of different source rocks in the magma source area. It is an important tool for evaluating the origin of a magma [30,31,32,33,34]. Thus, the material sources of the porphyroclastic lavas and granitic enclaves can be evaluated by using the Hf isotope composition characteristics. In this study, zircon Hf isotope tests and analyses were carried out on porphyroclastic lava and granitic enclave samples. The ε_Hf(t) values were −8.9 to −5.4 and −8.9 to −6.1, respectively. The ε_Hf(t) values of the two lithologies were close, and their range of variation was small (Figure 16). In the ε_Hf(t)-t diagram (Figure 17), the test points of the porphyroclastic lava and granitic enclave samples concentrated between the chondrite evolution line and the lower crust evolution line, indicating that the porphyroclastic lava and granitic enclaves were derived from the partial melting of crustal material.

Through extensive petrological and geochemical studies, previous research demonstrated that the Xiangshan volcanic intrusive complex formed mainly by the partial melting of silicon–aluminum crustal material [35]. Previous studies on the Sr–Nd–Hf isotopic compositions of the rhyodacite, porphyroclastic lava, granitic enclaves, and granite porphyry in the Xiangshan area (Table 8) produced similar I_Sr, ε_Nd(t), and ε_Hf(t) values, also indicating that the Xiangshan volcanic-intrusive complex originated from the partial melting of the crust.

Table 8. Summary of Sr–Nd–Hf isotopic compositions of the Xiangshan volcanic intrusive complex.

Lithologic	ISr	εNd(t)	εHf(t)	t (Ma)	Reference
Rhyodacite	0.7087~0.7098	−8.03~−7.48		135 Ma	[36]
	0.710108	−7.52		135 Ma	[35]
	0.7103~0.7115	−9.70~−8.46		135 Ma	[15]
	0.711915	−7.36	−8.5~−5.7	135 Ma	[37]
Porphyroclastic lava	0.7091~0.7113	−8.33~−7.74		135 Ma	[36]
	0.7098	−7.68	−10.3~−6.3	135 Ma	[37]
	0.7110~0.7114	−9.15~−8.49		135 Ma	[15]
	0.7075~0.7087	−8.3~−7.7		135 Ma	[35]
			−8.9~−5.4	134 Ma	body of the work
Granitic enclaves	0.7102	−7.76		134 Ma	[36]
			−8.9~−6.1	135 Ma	body of the work
Granite porphyry	0.7108	−7.43		135 Ma	[35]

Table 8. Summary of Sr–Nd–Hf isotopic compositions of the Xiangshan volcanic intrusive complex.

Lithologic	ISr	εNd(t)	εHf(t)	t (Ma)	Reference
Rhyodacite	0.7087~0.7098	−8.03~−7.48		135 Ma	[36]
	0.710108	−7.52		135 Ma	[35]
	0.7103~0.7115	−9.70~−8.46		135 Ma	[15]
	0.711915	−7.36	−8.5~−5.7	135 Ma	[37]
Porphyroclastic lava	0.7091~0.7113	−8.33~−7.74		135 Ma	[36]
	0.7098	−7.68	−10.3~−6.3	135 Ma	[37]
	0.7110~0.7114	−9.15~−8.49		135 Ma	[15]
	0.7075~0.7087	−8.3~−7.7		135 Ma	[35]
			−8.9~−5.4	134 Ma	body of the work
Granitic enclaves	0.7102	−7.76		134 Ma	[36]
			−8.9~−6.1	135 Ma	body of the work
Granite porphyry	0.7108	−7.43		135 Ma	[35]

Figure 16. ε_Hf(t) histogram of porphyroclastic lava and granitic enclaves [26].

Figure 16. ε_Hf(t) histogram of porphyroclastic lava and granitic enclaves [26].

Figure 17. Hf zircon isotopic diagram of Xiangshan porphyroclastic lava and granitic enclaves.

Figure 17. Hf zircon isotopic diagram of Xiangshan porphyroclastic lava and granitic enclaves.

8.3. Genesis of Granitic Enclaves

Academics pointed out that the granitic enclaves are the pre-stage material of the magma that produced the porphyroclastic lava [12]. However, in the process of later-stage magma upwelling, the early phase material would likely be removed from the magma channel and would be difficult to preserve, which is inconsistent with the geological fact that granitic enclaves are widely developed in the ancient volcanic channel. Therefore, the authors suggest that the granitic enclaves are unlikely to be pre-stage material of the magma that produced the porphyroclastic lava.

Furthermore, the granitic enclaves have a typical porphyritic structure and no obvious residual structure. In addition, the granitic enclaves and porphyroclastic lava are products of the same magmatic event, rather than representing some relationship between the source area and the molten magma, so they do not have a restite or residual shadow origin [38,39].

Autoliths are the edge phase of an early rock mass or the cumulate phase formed by crystal separation. The granitic enclaves do not have a cumulate structure and are not typical autoliths [40,41]. The back-scattered images revealed that the apatite in the granitic enclaves does not have a needle-like shape and that the enclaves do not have the characteristics of typical quenched enclaves, such as chilled borders and reverse veins [42,43]. Residual enclaves are the product of the aggregation and crystallization of the residual, silicon-rich, alkali-rich, low-melting components in the late stage of magmatic crystallization. However, the age of the granitic enclaves is consistent with the age of the host rock, and there is no typical embedded crystal structure and micro-image structure, which indicates that the enclaves are not residual enclaves. Furthermore, because the granitic enclaves are mostly round and have an age that is consistent with the age of the host rock, they are obviously different from any xenoliths from the irregular massive surrounding rock [8,43].

In summary, the granitic enclaves in the porphyroclastic lavas are not restites, residual shadow enclaves, autoliths, quenched enclaves, residual magma enclaves, or rock xenoliths, but rather must be immiscible enclaves.

The geochemical characteristics of porphyroclastic lava and granitic enclaves can well prove that they are the products of an immiscibility process. Although there is no significant difference in the main elements, granitic enclaves (SiO₂ = 74.30%~75.31%, K₂O = 5.52%~5.83%) are significantly enriched in Si and K elements compared with porphyroclastic lavas (SiO₂ = 72.90%~74.82%, K₂O = 5.12%~5.27%), while porphyroclastic lavas are more enriched in Fe, Mg, Ca, Ti, P compared with granitic enclaves. The results are basically consistent with the previous studies that the immiscibility of silicate melt makes the net-forming elements (Si, Al, Na, K) enter the silicon-rich phase, and the net-changing elements (Fe, Mg, Ca, Ti, P) enter the relatively low silicon liquid, that is, the iron-rich phase [44,45,46,47]. The above-mentioned distribution characteristics of major elements show that SiO₂, K₂O, and Na₂O in the two phases separated by immiscibility tend to migrate and enrich in the granitic enclaves, which makes the enclaves evolve towards the acidic direction. Al₂O₃, TiO₂, MgO, and ^TFe₂O₃ tend to be enriched in porphyroclastic lava and evolve towards the basic direction. Therefore, the porphyroclastic lava magma can be regarded as the iron-rich phase in the two liquid phases, while the granitic enclave’s magma can be regarded as the silicon-rich phase in the two liquid phases.

In the process of magma immiscibility, the distribution of trace elements between two immiscible liquid phases is strictly controlled by their ion characteristics and melt structure [47,48]. High-valent ions are often strongly enriched in the immiscible iron-rich phase, while low-valent ions are mostly enriched in the silicon-rich phase due to their low field strength. Sr and Ba between the two are often not affected by the melt structure due to their medium field strength [49]. It can be seen from Figure 9 that the high-valent ions Y, U, Th, Zr, and Hf are strongly concentrated in the porphyroclastic lava, the iron-rich phase (relative to the basic melt phase), compared with the granitic enclaves.

Some believe that in the experiment that REE are the most obvious elements [47], which are strongly enriched in the iron-rich phase. From Figure 10, it can be seen that REE are also relatively enriched in the iron-rich phase. This distribution law also follows the results of previous studies [50,51,52].

The magma chamber is mainly composed of crust mush and melt, which is connected by multiple deep and shallow magma chambers with different composition and physical properties through magma pipelines to form a magma system [53]. When the melt temperature drops to the critical temperature of immiscibility, melt separation occurs and two liquid phases are formed. Therefore, the large change in supercooling and turbulent environmental conditions are conducive to the immiscibility of magma [54].

The beaded structure is a structure formed by the rapid movement of mineral phenocrysts from the magma chamber into the ultra-shallow environment, temperature and pressure changes, and the phenocrysts are crystallized to adapt to changes in the physical and chemical conditions of the environment. The mineral phenocrysts in both granitic enclaves and porphyroclastic lavas have a beaded structure (Figure 3), indicating that the mineral phenocrysts have undergone a sharp decline in temperature and pressure. The pressure drop process of magma emplacement is the trigger factor of immiscibility.

At present, there are relatively few instances where the enclaves formed by magma immiscibility have minor differences in chemical composition compared to the host rock [55,56]. The enclaves in the ilílymaussaq alkaline complex in South Greenland have similar major and trace element characteristics to the host rocks. The spherical bodies in the host rock of this location can account for more than 25% of the total rock volume. These spherical bodies vary in shape from spherical to various elliptical shapes, and their lengths range from a few centimeters to several tens of centimeters. The host rock and the mineral composition of the spherical bodies are almost the same, and each part is composed of different mineral combinations. Host rock (e.g., SiO₂ = 54.79%~56.02%; K₂O = 1.62%~2.13%; Na₂O = 9.94%~11.34%; Al₂O₃ = 11.18%~13.66%) and the spherical bodies (e.g., SiO₂ = 50.34%~50.54%; K₂O = 1.28%~1.75%; Na₂O = 10.72%~11.71%; Al₂O₃ = 11.18%~14.65%) exhibit similar characteristics of major element composition. These enclaves may be formed by immiscibility between hydrogen-rich magma and host magma in the later stage of intergranular melt crystallization [57].

The mineral assemblages of the enclaves in the alkaline syenite near Trivandrum in southern India are similar to those of the host rocks. The host rock (e.g., SiO₂ = 54.79%~56.02%; K₂O = 1.62%~2.13%; Na₂O = 9.94%~11.34%; Al₂O₃ = 11.18%~13.66%) and the spherical bodies (e.g., SiO₂ = 50.34%~50.54%;K₂O = 1.28%~1.75%; Na₂O = 10.72%~11.71%; Al₂O₃ = 11.18%~14.65%) have little difference in the chemical composition of major elements. The distribution trends of trace elements are consistent, and they have similar REE patterns, similar initial Sr isotopic compositions and the same diagenetic age. It is considered that the formation of enclaves is related to the immiscibility of magma [58].

Due to the limitation of conditions, it is impossible to directly observe the direct evidence left by the growth process of enclaves and the evolution process of immiscibility. Magma immiscibility often overlaps with crystallization, and even the immiscibility is mixed in the process of mineral crystallization. Crystallization will affect and cover up the immiscibility.

Therefore, the two-phase separation process caused by immiscibility and the genesis of spherulite enclaves are all experimental results and theoretical deduction. In addition, the composition difference between silicon-rich and iron-rich two-phases studied by previous researchers is large. Whether there is immiscibility for acidic magma with small composition difference between two liquid phases has rarely been concluded so far, so whether the conclusion is correct has great uncertainty [53].

As mentioned previously, there are many tourmaline cysts with a large amount of quartz in granitic enclaves. Two-phase separation occurred before the formation of tourmaline. The formation of late tourmaline nodules is a transitional stage from granitic enclave magma to hydrothermal fluid in the late stage of evolution. The tourmaline and quartz are crystallized from the boron-bearing magmatic hydrothermal fluid separated from the magma [59,60].

The tourmaline cysts in granitic enclaves were removed during the geochemical test of granitic enclaves, which is one of the reasons for the small difference in composition between granitic enclaves and host rocks.

Moreover, before the immiscibility, a large number of mineral phenocrysts have been formed. It is the immiscibility of a small part of the intergranular melt that forms two liquid phases. This is the second reason the composition level gap between the granitic enclaves and the host rock is very small.

The granitic enclaves have similar structures and mineral assemblages with the host rocks, forming a consistent age, similar Sr-Nd-Hf variation range, and initial magma temperature. The trace element composition of zircon is difficult to distinguish, with similar trends and crystallization temperatures, and both plagioclase phenocrysts crystallize at the same time. However, the main elements and trace elements of the two have the characteristics of immiscibility.

As far as the formation process of granitic enclaves is concerned, the author speculates that the lower crust melt is induced to rise and stored in the form of crust mush after crystallization differentiation in the magma reservoir, forming a shallow crust mush reservoir. The crust mush reservoir is composed of a large number of quartz, feldspar, biotite, and other rock-forming minerals, as well as accessory mineral crystals such as zircon and flowing intergranular melt (Figure 18). In the later stage of magma high evolution, a small and short-time magmatic activity caused a large amount of crystalline granitic crystal mush to flow into the volcanic pipeline.

In the closed system of the volcanic pipeline, the pressure and temperature decreased rapidly, and the supercooling degree increased, resulting in immiscibility. Because the large complex anion groups possess high viscosity, are difficult to flow, and are complex, they are more likely to aggregate into bead-drop nuclei. The droplet nuclei of the granitic enclaves continuously accumulate and migrate, forming spherules. These spherules then collide and aggregate, causing the spherules to continuously grow. The silicon-rich phase collides and agglomerates in the relatively basic iron-rich phase [54] and continuously grows into a semi-solidified mass. Under the action of buoyancy, it is gradually wrapped by the iron-rich phase, maintaining its stability to present the forms we observed in the field (such as round shapes, clumpy shapes, etc.), and form the phenomenon where granitic enclaves are encapsulated within brecciated lava, and finally forming pale granitic enclaves.

9. Conclusions

Granitic enclaves and host rocks have similar porphyritic structures and the same mineral assemblages. Compared with host rocks, the granitic enclaves are relatively enriched in silicon and alkalis but relatively depleted in aluminum, magnesium, iron, and calcium. Both of them are enriched in Rb, Th, U, La, K, Pr, Nd, Sm, Dy, and other elements, and strongly depleted in Ba, Hf, Ti, P, Nb, and other elements. The total rare earth content of porphyroclastic lava is higher than that of granitic enclaves, and both have a consistent trend of a rare earth element distribution curve. The diagenetic age of granitic enclaves is consistent with that of porphyroclastic lava. Both are about 135 Ma. The crystallization temperatures of zircons in granitic enclaves and porphyroclastic lavas are almost the same. The initial temperature of the two magmas is also the same, and the trend of the zircon rare earth element distribution curves of the two is consistent. Granitic enclaves and porphyroclastic lavas are mainly derived from partial melting of crustal materials.

The formation process of granitic enclaves may be that in the later stage of magma high evolution, a small and short-time magmatic activity caused a large amount of crystalline granitic crystal mush to pour into the volcanic pipeline. The pressure and temperature decreased rapidly, and the supercooling degree increased, and the immiscibility finally forms pale granitic enclaves.