The Genesis and Geological Significance of the Chaluo Granite in Yidun Magmatic Arc, Western Sichuan, China: Constraints from the Zircon U-Pb Chronology, Elemental Geochemistry and S-Pb-Hf Isotope

Abstract

The Chaluo granite is situated in the middle section of the Yidun magmatic arc in western Sichuan Province, China. It holds great significance for the study of the geological evolution of the Paleo-Neotethys tectonic belts. The Chaluo granite mainly consists of alkaline feldspar, quartz, and biotite, with a small amount of apatite. LA-ICP-MS zircon U-Pb dating yielded crystallization ages of (87 ± 3) Ma for the Chaluo granite, indicating its formation in the Late Cretaceous. Elemental geochemical testing results showed that the Chaluo granite exhibits I-type granite characteristics. It has undergone significant fractional crystallization processes, with high SiO₂ contents (72.83–76.63 wt%), K (K₂O/Na₂O = 1.33–1.53), Al₂O₃ (Al₂O₃ = 12.24–13.56 wt%, A/CNK = 0.91–1.08), and a high differentiation index (DI = 88.91–92.49). Notably, the MgO contents were low (0.10–0.26 wt%), and there were significant depletions of Nb, Sr, Ti, and Eu, while Rb, Pb, Th, U, Zr, and Hf were significantly enriched. The total rare earth element (REE) contents were relatively low (211–383 ppm), showing significant light REE (LREE) enrichment (LREE/HREE = 4.46–5.57) and a pronounced negative Eu anomaly (δEu = 0.09–0.17). In situ zircon Hf analyses, combined with ²⁰⁶Pb/²³⁸U ages, gave ε_Hf(t) values ranging from −3.8 to 1.72 and two-stage Hf ages (t_DM2) of 875–1160 Ma. Together with the S and Pb isotope compositions of the Chaluo granite, its magma likely originated from the partial melting of Middle–Neoproterozoic sedimentary rocks enriched in biogenic S. The tectonic-setting analysis indicates that the Chaluo granite formed in a post-orogenic intracontinental extensional environment. This environment was triggered by the northward subduction-collision of the Lhasa block, followed by slab break-off and the upwelling of the asthenosphere in the Neo-Tethys orogenic belt. We propose that the Paleo-Tethys tectonic belt was influenced by the Neo-Tethys tectonic activity, at least in the Yidun magmatic arc region during the Late Cretaceous.

1. Introduction

The world-renowned Qinghai-Tibet Plateau is located in the eastern part of the Tethys orogenic belt, whose uplift and geological evolution reflect the dynamic processes of the multi block-ocean basin system in the Tethys orogenic belt (Figure 1a). Based on the geological evolution sequence, the eastern segment of the Tethys orogenic belt in China is divided into three stages: (1) the Proterozoic-Paleozoic Proto-Tethys orogenic stage. The Proto-Tethys Ocean rifted and formed from the northern margin of Gondwana during the late Proterozoic. It continued to expand from the Cambrian to the Early Paleozoic, giving rise to typical ophiolite suites and abyssal sedimentary deposits. During the Late Ordovician to Silurian, the ocean basin initiated subduction and closure, ultimately leading to the collision of the North China Plate, South China Plate with the Siberian-Kazakhstani Plate [1,2,3,4,5,6,7,8]; (2) the Late Paleozoic-Triassic Paleo-Tethys orogenic stage. After the closure of the Proto-Tethys Ocean, the Paleo-Tethys Ocean rifted anew during the Late Paleozoic, forming a new oceanic basin. During the Triassic, subduction of this oceanic basin triggered island-arc magmatism and the amalgamation of continental blocks (including the Qiangtang Block, Changdu Block, and microcontinents on the northern margin of Gondwana), ultimately leading to its closure by the end of the Triassic [9,10,11,12]; and (3) the Triassic-Cenozoic Neo-Tethys orogenic stage. The Neo-Tethys Ocean rifted and formed from the northern margin of Gondwana during the Triassic. It continued to expand from the Jurassic to the Cretaceous, and closed in the Cenozoic due to the northward movement of the Indian Plate and its subduction beneath the Lhasa Block and Qiangtang Block [6]. To date, geologists lack sufficient evidence for studying the Proto-Tethys orogenic belt, and their understanding of its geological features remains incomplete. The Paleo-Tethys orogenic belt is situated on the southern side of the Proto-Tethys orogenic belt. During the period 440–200 Ma, the Paleo-Tethys Ocean developed between the Laurasia continent, the Pan-Huaxia continent, and the Cimmerian continent (then located on the northeastern edge of the Gondwana continent) [9]. As the Cimmerian continent gradually drifted northward and collided with the Laurasia continent at the end of the Triassic (~200 Ma), the Paleo-Tethys Ocean continuously shrank and eventually disappeared [10,11,12]. The subduction and collision of the Paleo-Tethys oceanic plate with its peripheral arc-basin systems eventually formed the Triassic Paleo-Tethys orogenic belt [13], with the Neo-Tethys orogenic belt situated to its south. The northern boundary of the Paleo-Tethys orogenic belt is defined by the Late Paleozoic Kangxiwa (South Kunlun)-Animaqing-Mianlue (South Qinling) suture zone, and its southern boundary by the Triassic Lancangjiang suture zone [14,15,16,17]. From north to south, the Paleo-Tethys orogenic belt comprises the Songpan-Ganzi Block, the Ganzi-Litang suture zone, the Zhongza Block (including the Yidun magmatic arc), the Jinshajiang suture zone, the Tianshuihai-North Qiangtang Block, and the Changdu-Simao Block. This division reflects that the Paleo-Tethys orogenic belt is a complex tectonic system consisting of multiple blocks and basins. Geologists have conducted in-depth research on the dynamic evolution of the Paleo-Tethys orogenic belt. Currently, most studies support that its geological evolution processes include: (1) the Jinshajiang oceanic basin expanded from the Early Carboniferous to the Early Permian [18,19,20]; (2) during the Late Early Permian, the Jinshajiang oceanic plate began to subduct eastward beneath the Zhongza block [21,22]; (3) in the Middle Triassic, the Jinshajiang oceanic basin closed and underwent arc–continent and continent–continent collision, forming the Jinshajiang suture zone [23,24,25,26]; (4) during the Early–Middle Triassic, the Ganzi-Litang oceanic basin, located east of the Zhongza Block, was in its peak expansion stage [27,28]; (5) in the Late Triassic, the Ganzi-Litang oceanic plate subducted westward beneath the Zhongza Block [29,30,31,32], leading to the formation of the Yidun magmatic arc on the eastern edge of the Zhongza Block; (6) at the end of the Late Triassic, the Songpan-Ganzi Block collided with the Zhongza Block, causing the closure and disappearance of the Ganzi-Litang oceanic basin and the formation of the Ganzi-Litang suture zone [23,33,34]. Triassic oceanic ridge-type tholeiitic basalts, picritic basalts, magnesian rocks, ultramafic cumulates, gabbros, diabase dikes, serpentinites (metamorphic peridotites), and radiolarian-bearing siliceous rocks are extensively developed in the Jinshajiang and Ganzi-Litang suture zones. Additionally, numerous Triassic island-arc-type basalts, andesites, dacites, rhyolites, and diorites occur in the Yidun magmatic arc [23,33,34,35,36]. These significant geological bodies are products of the geological evolution of the multi-block-oceanic basin system within the Paleo-Tethys orogenic belt.

Figure 1. Location of the study area (a) and Tectonic division of the eastern section of the Paleo-Tethys orogenic belt (b) (revised after Hou et al., 2003 [29]). AZ—Anzi, LT—Lata, QES—Queershan, QS—Qise, XS—Xiasai, CML—Cuomolong, JB—Junbei, GN—Genie, DC—Daocheng, SM—Shenmu, GK—Geka, XWC—Xiuwacu, ZJD—Zhujiding, HS—Hongshan, TCG—Tongchanggou, CL—Chaluo, HGL—Hagala, LCR—Luocuoren.

In recent years, geologists have discovered a large number of Late Cretaceous granites in the Yidun magmatic arc, located in the central part of the Paleo-Tethys orogenic belt (Figure 1b) such as the Chaluo granites and the Hagala granites [37,38,39]. These granites are distributed in the NNW-trending belt, consistent with the strike of the Jinshajiang and the Ganzi-Litang suture zones. However, they are clearly not products of the aforementioned geological evolution processes. It is still unclear what dynamic background they were formed under and what important geological events the Paleo-Tethys orogenic belt experienced during the Late Cretaceous. Therefore, this paper selected the Chaluo granites to conduct detailed studies on geochemistry, geochronology, and isotopes, aiming to unravel the aforementioned unsolved issues.

2. Regional Geological Background and Petrological Characteristics

The Yidun magmatic arc is located on the active continental margin of the Late Triassic at the eastern edge of the Zhongza Block, sandwiched between the Ganzi-Litang suture zone and the Jinshajiang suture zone [9,40,41,42]. It extends from Baiyu County in western Sichuan Province to Lijiang City in Yunnan Province, China, with a NNW arc-shaped distribution. The exposed strata mainly include the Early–Middle Triassic Lieyi Formation, the Dangen Formation, the Late Triassic Qugasi Formation, the Tumugou Formation, and a limited Silurian, Devonian, and Carboniferous strata. The Silurian, Devonian, and Carboniferous strata are mainly composed of carbonates. The Lieyi Formation and the Dangen Formation are mainly composed of fine clastic rocks in a continental slope turbidite environment [25,43], while the Qugasi and Tumugou Formations mainly comprise andesites, basalts, rhyolites interbedded with carbonates, and coarse clastic rocks [44,45]. Folds, faults, and other structures mainly extend in a NNW direction and were mainly formed during the subduction and compression between the Ganzi-Litang oceanic plate and the Zhongza Block in the Late Triassic. Acidic intrusions are widely exposed and divided into four stages (Figure 1b): (1) The Late Triassic (237–206 Ma) island-arc granites, including the Qise syenogranite-granodiorite (224 Ma) in Baiyu County [46], the Junbei monzogranite-granodiorite (213–218 Ma) in Xinlong County [26,46], the Daocheng monzogranite (215–224 Ma) in Daocheng County [47,48], the Shenmu granite (216–218 Ma) in Jiulong County [49], and the Geka monzogranite in Muli County, were formed within an island-arc setting. This setting was a consequence of the westward subduction of the Ganzi-Litang Ocean. Their magmatic activities are intricately associated with the partial melting of the mantle wedge, which was induced by the subduction of the oceanic crust; (2) The Jurassic-Early Cretaceous (206–138 Ma) syn-collisional granites, such as the Lata monzogranite-granodiorite in Ganzi County, were formed during the arc-continent collision stage subsequent to the closure of the Ganzi-Litang Ocean; (3) The Cretaceous (135–73 Ma) post-collisional granites, such as the Anzi monzogranite, Chaluo granite (83–85 Ma) in Baiyu County [50], and Hagala granite (78–98 Ma) in Batang County [48], were generated within a post-orogenic extensional regime. Their formation can be attributed to the asthenospheric upwelling and crustal extensional thinning triggered by the subduction-detachment of the Bangong-Nujiang Oceanic plate; and (4) Paleogene (65–15 Ma) post-orogenic granites such as the Genie monzogranite in Batang County [51]. These intrusions, which formed at different stages, might reflect the dynamic background of the Paleo-Tethys and Neo-Tethys orogenic belts in different periods.

The Chaluo granite is located in Chaluo Township, Batang County, Sichuan Province, China (Figure 2). It is exposed in an elongated lens-shaped or bead-like outcrop oriented northwest, with an exposed area of approximately 50 square kilometers. The intrusion cuts into the Late Triassic Tumugou Formation, while its southern portion is overlain by Quaternary sediments. The surface bedrock of the Chaluo granite is well-exposed, forming prominent outcrops (Figure 3a). Petrographically, it consists predominantly of biotite granite with a distinct porphyritic texture (Figure 3b). Near the contact zone with the Tumugou Formation, numerous angular xenoliths of Tumugou sandstones are embedded within the granite (Figure 3c). The contact area with sandstones and limestones of the Tumugou Formation exhibits intense hornfelsification and skarnization.

Figure 2. Geological map of the Chaluo granite. 1. Quaternary; 2. Late Cretaceous porphyritic biotite granite; 3. Late Triassic Lamaya Formation-fine sandstone, siltstone, carbonaceous slate; 4. Late Triassic Tumugou Formation-sandstone, conglomerate, intermediate acidic volcanic rock; 5. Late Triassic Qugasi Formation-crystalline limestone, composite conglomerate, intermediate basic volcanic rock; 6. Middle Triassic Lieyi Formation-sandstone, sericite shale, siliceous shale; 7. Early Triassic Dangen Formation-metamorphic sandstone, slate, phyllite, siliceous rock; 8. Late Permian Gangda Formation-metamorphic basic volcanic rocks interbedded with clastic rocks and limestone; 9. Sampling locations.

Figure 3. Field and microscopic photographs of the Chaluo granite. (a,b) Field photos of the Chaluo granite; (c) xenoliths of sandstones from the Tumugou Formation in the Chaluo granite; (d) alkaline-feldspar phenocryst in Chaluo granite; (e,f) Chaluo granite with hypidiomorphic crystal (crossed polarizers).

3. Sampling and Analytical Methods

3.1. Sampling

Sixteen samples were collected from fresh outcrops of the Chaluo granite to investigate their petrographic, geochemical, and geochronological characteristics. Among them, ten samples (R01, R02, R03, R04, R05, R06, R07, R08, R09, R10) were selected for petrographic and whole-rock geochemical analyses, one sample (U01) was collected for zircon U-Pb geochronology, in situ Hf isotope, and five samples (S01, S02, S03, S04, S05) were collected for whole-rock S and Pb isotopes. The sampling locations are shown in Figure 2.

3.2. Analytical Techniques

3.2.1. Whole-Rock Geochemical Analyses

Whole-rock geochemical analyses were carried out at the laboratory of the Regional Geological and Mineral Survey and Research Institute of Hebei Province. A Zsx Primus II wavelength dispersive X-ray fluorescence spectrometer (XRF) produced by RIGAKU (Akishima, Japan), was used for the geochemical analysis in the whole rock. The X-ray tube was a 4.0 Kw end window Rh target and the test conditions were as follows: voltage—50 kV, current—60 mA, all major element analysis lines were kα. The relative standard deviation (RSD) was less than 2%.

3.2.2. LA-ICP-MS Zircon U-Pb Isotope Analyses

Zircon grains were isolated from the rock samples through a meticulous single mineral separation process, completed with assistance from the Hebei Provincial Institute of Regional Geology and Mineral Investigation. Subsequently, cathodoluminescence (CL) imaging was performed using a scanning electron microscope at the laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences.

Zircon U-Pb isotopic dating was conducted using LA-ICP-MS analysis at the Wuhan Sample solution Analytical Technology Co., Ltd. [52,53,54,55,56,57,58,59,60]. The laser ablation system employed was a UP193FX 193 nm ArF excimer system (NewWave, Washington, DC, USA), equipped with a laser from ATL (Munich, Germany) and an Agilent 7500a (Santa Clara, CA, USA) mass spectrometer for ICP-MS. The laser system operated at a wavelength of 193 nm with a pulse width of less than 4 ns. The laser spot diameter used for analysis was 35 μm. For external matrix correction, the internationally recognized Plesovice (²⁰⁶Pb/²³⁸U weighted average age (337.13 ± 0.37) Ma) and Qinghu standard zircons (²⁰⁶Pb/²³⁸U weighted average age (159.45 ± 0.16) Ma) were utilized. NISTSRM 612 with 29Si served as the internal standard element. Isotopic ratios and elemental content were calculated using the GLITER-ver 4.0 program (Macquarie University). Ordinary lead correction was performed using Anderson’s ComPbCorr # 3.17 correction program [61]. U-Pb concordia plots, age distribution frequency plots, and age weighted averages were generated using the IsoPlot/Ex_ver 3 program [62]. Detailed descriptions of the analytical methods and data processing procedures can be found in Liu et al. [63,64,65].

3.2.3. In Situ Zircon Hf Isotope Ratio Analysis

In situ Hf isotope ratio analysis was conducted using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Bremen, Germany) coupled with a Geolas HD excimer ArF laser ablation system (Coherent, Göttingen, Germany) that was hosted at Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. All data were acquired on zircon in single spot ablation mode at a spot size of 44 μm. Detailed operating conditions for the laser ablation system and the MC-ICP-MS instrument and analytical method may be found in Hu et al. [57,58].

3.2.4. Whole-Rock S and Pb Isotope Ratio Analysis

Whole-rock S analyses were conducted using a Neptune Plus MC-ICP-MS (Thermo, Waltham, MA, USA) in combination with a Conflo IV laser ablation system (Coherent, Göttingen, Germany), and Pb isotope ratio analyses were performed on a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Dreieich, Germany) equipped with seven fixed electron multiplier ICs and nine Faraday cups fitted with 10¹¹ Ω resistors at Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China with an accuracy better than 0.03 wt%. All data reduction for the MC-ICP-MS analysis of Pb isotope ratios was conducted using “Iso-Compass 1.0” software [66].

4. Results

4.1. Mineralogical Characteristics

Under the microscope, the Chaluo granite exhibited a medium- to fine-grained porphyritic texture (Figure 3b–d). Its microstructural characteristics are shown in Figure 3e,f. Phenocrysts were mainly composed of K-feldspar, plagioclase, and minor quartz, all of which displayed euhedral-subhedral habits. The K-feldspar phenocrysts measured approximately 1–2 cm in length and 0.5–1 cm in width, accounting for 70 vol% of the phenocryst assemblage. The matrix exhibited a medium- to fine-grained subhedral texture, with main minerals including K-feldspar, plagioclase, quartz, and biotite. K-feldspar occurred as subhedral platy crystals with well-developed twinning, accounting for approximately 45 vol% of the rock. Plagioclase appeared as subhedral platy crystals, featuring faintly visible zoning and well-developed twinning, and constituted about 25 vol% of the rock. Quartz was anhedral, occupying the interstices between feldspars and accounting for 20 vol% of the rock. Biotite occurred in sheet-like form and made up about 10 vol% of the rock. Opaque minerals were present as subhedral- to anhedral granular crystals, with an abundance of less than 1 vol% in the rock and were randomly distributed. The secondary minerals were mainly apatite and zircon.

4.2. Zircon U-Pb Geochronology

Zircon grains extracted from the Chaluo granite displayed a characteristic pink-yellow hue and well-defined morphologies, mostly taking the form of prismatic to short-prismatic crystals. The grains measured approximately 190–340 μm in length and 30–100 μm in width, with aspect ratios ranging from 2:1 to 3:1. Cathodoluminescence (CL) imaging revealed oscillatory zoning patterns within the zircon (Figure 4a), which is a strong indication of a magmatic origin. These patterns were characterized by distinct rhythmic zoning structures and minimal visible inherited cores. Secondary fractures were relatively scarce, and the Th/U ratios consistently exceeded 0.10 (Th/U = 0.10–0.59), further validating its magmatic origin. The results of the LA-ICP-MS zircon U-Pb dating for the Chaluo granite are presented in Table 1.

Figure 4. Cathodoluminescence (CL) images of representative zircon grains (a) and concordia plots (b) of zircon from the Chaluo granite. Red circles indicate the locations of LA-ICPMS U-Pb dating, while yellow circles indicate the positions of the LA-MC-ICPMS Hf isotope analyses.

Table 1. LA-ICP-MS zircon U-Pb dating of the Chaluo granite (U01).

Spot	Content (×10−6)			Isotope Ratio							Age (Ma)
U01	Pb	Th	U	207Pb/ 206Pb	1σ	207Pb/ 235U	1σ	206Pb/ 238U	1σ	Th/U	206Pb/ 238U	1σ	207Pb/ 235U	1σ	207Pb/ 206Pb	1σ
1	6.86	194	328	0.0454	0.0020	0.0859	0.0038	0.0138	0.0002	0.59	88	2	84	1	93	3
2	24.1	719	1217	0.0512	0.0013	0.0986	0.0026	0.0139	0.0001	0.59	89	2	96	1	92	2
3	189.9	3128	13017	0.0620	0.0018	0.1067	0.0031	0.0125	0.0001	0.24	80	3	103	1	104	4
4	397	1624	15719	0.1254	0.0055	0.2208	0.0086	0.0130	0.0001	0.10	83	1	203	1	639	36
5	171	2577	9098	0.0954	0.0017	0.1655	0.0025	0.0126	0.0001	0.28	81	2	156	1	157	6
7	266	2811	15516	0.0823	0.0016	0.1481	0.0032	0.0130	0.0001	0.18	83	3	140	1	188	6
10	10.81	256	653	0.0479	0.0019	0.0924	0.0037	0.0140	0.0002	0.39	90	3	90	1	88	3
14	88.0	2792	4700	0.0512	0.0008	0.0912	0.0018	0.0129	0.0002	0.59	83	2	89	1	95	2
15	189	762	5384	0.1372	0.0094	0.2843	0.0230	0.0144	0.0002	0.14	92	3	254	1	687	77
17	37.9	684	2686	0.0495	0.0010	0.0913	0.0018	0.0134	0.0001	0.25	86	2	89	1	88	2
18	36.4	776	2139	0.0535	0.0019	0.1064	0.0040	0.0144	0.0002	0.36	92	4	103	1	95	2
24	201.9	1496	15478	0.0535	0.0010	0.1017	0.0022	0.0137	0.0001	0.10	88	2	98	1	138	5
25	27.3	595	2875	0.0483	0.0011	0.0852	0.0016	0.0128	0.0001	0.21	82	2	84	1	102	2
26	41.3	585	3182	0.0482	0.0009	0.0864	0.0018	0.0130	0.0002	0.18	83	2	84	1	103	2
27	67.7	2461	4200	0.0733	0.0012	0.1277	0.0025	0.0126	0.0001	0.59	81	2	122	1	58	1
28	109.5	971	7699	0.0624	0.0015	0.1097	0.0022	0.0129	0.0001	0.13	82	2	106	1	170	7
30	41.0	674	3107	0.0477	0.0009	0.0829	0.0016	0.0126	0.0001	0.22	81	2	81	1	99	2

The Chaluo granite displayed a consistent ²⁰⁶Pb/²³⁸U age distribution across all zircon grains. The ²⁰⁶Pb/²³⁸U age values ranged from 80 to 92 Ma, clustering around the concordia line (Figure 4b). The weighted average age of the 17 ²⁰⁶Pb/²³⁸U measurements was (87 ± 3) Ma (MSWD = 1.5), indicating that the Chaluo granite was crystallized during the Late Cretaceous.

4.3. Zircon In Situ Hf Isotope

A total of 16 zircon grains from the Chaluo granite were analyzed for Hf isotope analysis, and the results are presented in Table 2. The ratios of ¹⁷⁶Yb/¹⁷⁷Hf, ¹⁷⁶Lu/¹⁷⁷Hf, and ¹⁷⁶Hf/¹⁷⁷Hf ranged from 0.020399 to 0.204525, 0.000585 to 0.001775, and 0.282622 to 0.282833, respectively. All calculated ¹⁷⁶Lu/¹⁷⁷Hf ratios were less than 0.002. As established in previous studies [67], such low ratios indicate that there has been no significant accumulation of radioactive Hf isotopes subsequent to the formation of zircon. Therefore, the measured ¹⁷⁶Lu/¹⁷⁷Hf ratios can be considered as representative of the ratios at the time of zircon crystallization.

Table 2. LA-ICP-MS zircon in situ Hf isotopic dating of the Chaluo granite (U01).

Point	Age (Ma)	176Yb/177Hf	176Lu/177Hf	176Hf/177Hf	±2σ	εHf (0)	εHf (t)	±2σ	TDM1 (Hf) (Ma)	±2σ	TDM2 (Hf) (Ma)	±2σ	fLu/Hf
1	88	0.039009	0.001092	0.282645	0.000030	−4.5	−2.60	0.00	861	0	1101	0.0001	−0.97
2	89	0.060036	0.001628	0.282768	0.000030	−0.1	1.72	0.00	698	0	875	0.0001	−0.95
3	80	0.077677	0.001436	0.282656	0.000021	−4.1	−2.42	0.00	854	0	1085	0.0001	−0.96
4	83	0.153702	0.001261	0.282673	0.000024	−3.5	−1.76	0.00	826	0	1053	0.0001	−0.96
5	81	0.157196	0.001243	0.282730	0.000025	−1.5	0.21	0.00	745	0	948	0.0001	−0.96
7	83	0.129135	0.001665	0.282663	0.000026	−3.9	−2.14	0.00	850	0	1073	0.0001	−0.95
10	90	0.020399	0.000585	0.282665	0.000025	−3.8	−1.84	0.00	822	0	1062	0.0001	−0.98
14	83	0.113589	0.001048	0.282689	0.000028	−3.0	−1.20	0.00	799	0	1023	0.0001	−0.97
15	92	0.140473	0.001775	0.282633	0.000026	−4.9	−3.03	0.00	896	0	1126	0.0001	−0.95
17	86	0.045561	0.001256	0.282668	0.000023	−3.7	−1.87	0.00	833	0	1060	0.0001	−0.96
18	92	0.043819	0.001250	0.282722	0.000030	−1.8	0.17	0.00	756	0	959	0.0001	−0.96
24	88	0.113438	0.001348	0.282726	0.000022	−1.6	0.23	0.00	752	0	952	0.0001	−0.96
25	82	0.048762	0.001363	0.282628	0.000025	−5.1	−3.36	0.00	892	0	1135	0.0001	−0.96
26	83	0.039832	0.001138	0.282615	0.000024	−5.6	−3.80	0.00	905	0	1160	0.0001	−0.97
28	82	0.204525	0.001132	0.282833	0.000030	2.2	3.92	0.00	595	0	754	0.0001	−0.97
30	81	0.046690	0.001321	0.282622	0.000023	−5.3	−3.59	0.00	899	0	1147	0.0001	−0.96

ε_Hf(t) = {[((¹⁷⁶Hf/¹⁷⁷Hf)_Sample − (¹⁷⁶Lu/¹⁷⁷Hf)_Sample × (e^λT−1))/((¹⁷⁶Hf/¹⁷⁷Hf)_CHUR − (¹⁷⁶Lu/¹⁷⁷Hf)_CHUR × (e^λt − 1))] − 1} × 100,000; T_DM1 = 1/λ × ln{1 + [((¹⁷⁶Hf/¹⁷⁷Hf)_Sample − (¹⁷⁶Hf/¹⁷⁷Hf)_DM)/((¹⁷⁶Lu/¹⁷⁷Hf)_Sample − (¹⁷⁶Lu/¹⁷⁷Hf)_DM)]}; T^C_DM2 = T_DM1 − (T_DM1 − t) × [(f_CC − f_S)/(f_CC − f_DM)], f_CC, f_S, and f_DM represent the f_Lu/Hf values of continental crust, zircon samples and depleted mantle, respectively. fcc = −0.55, f_DM = 0.16, fs = (¹⁷⁶Hf/¹⁷⁷Hf)_sample/(¹⁷⁶Lu/¹⁷⁷Hf)_CHUR − 1; (¹⁷⁶Hf/¹⁷⁷Hf)_CHUR = 0.282772, (¹⁷⁶Lu/¹⁷⁷Hf)_CHUR = 0.0332, (¹⁷⁶Hf/¹⁷⁷Hf)_DM = 0.28325, (¹⁷⁶Lu/¹⁷⁷Hf)_DM = 0.0384. λ = 1.867 × 10⁻¹¹a⁻¹, t = ²⁰⁶Pb/²³⁸Uage.

Sixteen analyses with ²⁰⁶Pb/²³⁸U ages of ca. 87 Ma from sample U01 yielded ε_Hf(t) values from −3.8 to 3.7 (with average of −1.3), which correspond to crustal model ages (T_DM2 (Hf)) of 0.754–1.160 Ga. The wide spread of ε_Hf(t) values and Hf isotope crustal model ages for these samples indicates a heterogeneous composition of zircon Hf isotopes.

4.4. Whole-Rock Geochemistry

Major and trace elements, along with the calculated normative mineral abundances (CIPW-norm) and selected geochemical ratios for the Chaluo granite, are shown in Table 3.

Table 3. Contents of the major elements (wt%), trace elements (ppm), and CIPW-norm of the Chaluo granite.

Sample	Chaluo Granite
	R01 (U01)	R02 (S01)	R03 (S02)	R04	R05 (S03)	R06 (S04)	R07	R08	R09	R10 (S05)
SiO2	73.74	74.07	74.21	73.69	74.80	72.83	75.51	76.63	74.81	75.26
TiO2	0.15	0.16	0.15	0.20	0.13	0.24	0.14	0.13	0.12	0.13
Al2O3	13.05	13.18	13.05	13.16	13.56	13.19	12.53	12.24	12.56	12.49
Fe2O3	0.52	0.87	0.24	0.40	0.63	0.63	0.45	0.25	0.06	0.17
FeO	1.31	0.94	1.56	1.63	0.71	2.56	1.48	1.33	1.48	2.02
FeO*	1.78	1.72	1.78	1.99	1.28	3.13	1.88	1.55	1.53	2.17
MnO	0.034	0.036	0.033	0.042	0.029	0.055	0.036	0.031	0.029	0.034
MgO	0.15	0.15	0.13	0.20	0.10	0.26	0.14	0.12	0.13	0.13
CaO	1.68	1.05	0.86	1.09	0.90	0.95	0.89	0.93	1.52	0.92
Na2O	3.30	3.36	3.36	3.49	3.54	3.42	3.34	3.29	3.47	3.29
K2O	4.73	4.85	5.15	4.92	4.70	4.94	4.86	4.71	4.87	4.91
P2O5	0.049	0.043	0.038	0.050	0.044	0.063	0.038	0.035	0.035	0.035
LOI	0.93	0.56	0.47	0.25	0.41	0.45	0.11	0.45	0.39	0.29
Total	99.64	99.27	99.25	99.12	99.55	99.59	99.52	100.15	99.47	99.68
K2O/Na2O	1.43	1.44	1.53	1.41	1.33	1.44	1.46	1.43	1.40	1.49
K2O + Na2O	8.03	8.21	8.51	8.41	8.24	8.36	8.20	8.00	8.34	8.20
A/CNK	0.96	1.04	1.03	1.01	1.08	1.04	1.01	1.00	0.91	1.01
A/NK	1.24	1.22	1.17	1.19	1.24	1.20	1.16	1.16	1.14	1.16
Ti	904	868	825	1166	718	1466	825	698	665	634
Y	58.6	66.9	52.7	68.6	50.1	89.0	62.9	57.8	50.0	53.5
V	9.79	7.46	7.95	8.50	6.32	8.12	7.07	6.99	6.62	6.50
Cr	17.5	0.84	1.00	0.14	0.55	3.33	2.84	0.76	0.85	3.66
Co	1.64	1.55	1.30	1.70	1.00	2.31	1.57	1.27	1.12	1.40
Ga	19.8	20.4	20.3	21.2	20.4	21.7	20.1	19.0	17.9	18.9
Rb	409	421	425	423	384	423	408	385	377	392
Sr	52.7	51.2	56.4	73.4	59.0	50.6	50.2	47.0	62.7	50.3
Zr	145.00	145.00	124.00	179.00	108.00	221.00	134.00	121.00	120.00	101.00
Nb	45.1	45.7	39.4	58.6	30.3	62.6	49.4	34.4	36.7	41.7
Hf	4.36	4.03	3.42	4.12	3.12	6.85	3.84	3.76	3.28	2.60
Ta	4.60	3.97	4.22	6.87	3.18	9.49	6.17	4.42	5.33	4.79
Pb	53.8	54.1	54.0	52.9	53.1	51.3	52.8	51.4	51.9	52.0
Th	55.3	63.4	53.7	67.9	56.6	85.8	64.2	52.1	51.3	47.2
U	19.6	18.7	13.0	19.0	14.1	27.6	18.5	15.8	14.2	16.4
La	51.1	56.2	51.7	65.9	48.4	82.0	55.4	42.1	45.2	39.8
Ce	94.4	103	93.7	122	88.5	149	102	78.7	81.7	72.5
Pr	11.2	12.3	11.0	14.4	10.4	17.7	12.1	9.37	9.84	8.66
Nd	38.2	42.2	37.4	49.3	35.5	61.3	41.2	32.7	33.7	30.0
Sm	8.23	9.19	7.90	10.4	7.56	13.2	8.87	7.43	7.19	6.64
Eu	0.38	0.38	0.41	0.38	0.41	0.36	0.37	0.34	0.31	0.34
Gd	8.00	8.92	7.56	10.03	7.13	12.34	8.61	7.27	6.80	6.60
Tb	1.57	1.78	1.45	1.92	1.37	2.38	1.70	1.47	1.34	1.36
Dy	10.6	12.1	9.75	13.0	9.04	15.9	11.5	9.92	8.89	9.44
Ho	2.19	2.45	1.94	2.63	1.79	3.20	2.34	2.02	1.80	1.95
Er	7.12	7.95	6.27	8.50	5.82	10.37	7.67	6.57	5.83	6.35
Tm	1.28	1.42	1.12	1.49	1.04	1.88	1.35	1.17	1.03	1.13
Yb	8.63	9.53	7.54	9.99	7.05	12.61	9.00	8.05	6.96	7.51
Lu	1.20	1.33	1.06	1.38	0.99	1.78	1.25	1.14	0.96	1.06
δEu	0.14	0.13	0.16	0.11	0.17	0.09	0.13	0.14	0.13	0.16
(La/Yb)N	4.25	4.23	4.91	4.73	4.93	4.66	4.42	3.75	4.66	3.80
ΣLREE	203.50	223.41	202.07	262.57	190.80	323.34	219.88	170.61	177.99	157.94
ΣHREE	40.61	45.45	36.70	48.95	34.23	60.46	43.46	37.61	33.60	35.39
ΣLREE/ΣHREE	5.01	4.92	5.51	5.36	5.57	5.35	5.06	4.54	5.30	4.46
ΣREE	244.10	268.86	238.77	311.52	225.04	383.80	263.33	208.23	211.59	193.33
TZr (°C)	771	780	766	794	759	815	772	764	751	748
10,000 × Ga/Al	1.51	1.55	1.55	1.61	1.50	1.65	1.60	1.55	1.43	1.51
Zr + Nb + Ce + Y	343.16	360.67	309.76	428.32	276.80	521.34	348.28	291.94	288.46	268.70
Quartz (Q)	32.62	33.63	32.29	31.32	34.27	30.28	34.68	36.56	32.43	33.92
Anorthite (An)	6.91	4.99	4.07	5.14	4.21	4.34	4.19	4.4	4.35	4.36
Albite (Ab)	28.29	28.8	28.78	29.87	30.21	29.19	28.43	27.92	29.63	28.01
K-feldspar (FK)	28.32	29.04	30.81	29.41	28.02	29.45	28.89	27.92	29.05	29.19
Diopside (Di)	1.05								2.65
Hyersthene (Hy)	1.65	1.3	2.84	2.94	0.91	4.57	2.55	2.38	1.49	3.76
Ilmenite (Il)	0.29	0.31	0.29	0.38	0.25	0.46	0.27	0.25	0.23	0.25
Magnetite (Mt)	0.76	1.22	0.35	0.59	0.9	0.92	0.66	0.36	0.09	0.25
Apatite (Ap)	0.11	0.1	0.09	0.12	0.1	0.15	0.09	0.08	0.08	0.08
Corundum (C)		0.6	0.48	0.23	1.13	0.65	0.25	0.12		0.17
R1	2650	2615	2566	2531	2629	2480	2707	2833	2631	2705
R2	449	383	359	389	370	377	350	347	419	352
Differentiation index (DI)	89.22	91.48	91.88	90.60	92.49	88.91	92.00	92.4	91.11	91.12

Note: FeO* = FeO + 0.899 × Fe₂O₃; δEu = [2 × Eu/(Sm + Gd)]_N; R1 = 4Si − 11(Na + K) − 2(Fe + Ti); R2 = 6Ca + 2Mg + Al.

4.4.1. Major Elements

The Chaluo granite exhibited SiO₂ ranging from 72.83 to 76.63 wt% (Table 3). Other major element concentrations included Al₂O₃ (12.24–13.56 wt%), K₂O (4.70–5.15 wt%), Na₂O (3.29–3.54 wt%), TiO₂ (0.12–0.24 wt%), MnO (0.029–0.055 wt%), MgO (0.10–0.26 wt%), CaO (0.86–1.68 wt%), and P₂O₅ (0.035–0.050 wt%). The K₂O + Na₂O content varied between 8.00 and 8.51 wt%, with a K₂O/Na₂O ratio of 1.33 to 1.53. The A/CNK and A/NK ratios fell within the ranges of 0.91–1.08 and 1.14–1.24, respectively, exhibiting metaluminous to weakly peraluminous characteristics (Figure 5a) [68]. Normative mineral calculations based on the CIPW norm revealed a quartz content ranging from 30.28 to 36.56 wt%, an apatite content of 0.08 to 0.15 wt%, and a corundum content of 0.12 to 1.13 wt%. The Chaluo granite was characterized by high potassium, high sodium, and low calcium content, along with relatively low concentrations of TiO₂, MnO, and P₂O₅. This geochemical signature is consistent with magma that has undergone significant fractional crystallization. The K₂O–SiO₂ diagram (Figure 5b) [69] confirms that the granite belongs to the high-K calc-alkalic series.

Figure 5. (K₂O + Na₂O) wt% versus (SiO₂) wt% and (K₂O) wt% versus (SiO₂) wt% diagram of the Chaluo granite ((a) after Maniar P.D., 1989 [68]; (b) after Irvine T.N., et al. 1971 [69]).

4.4.2. Trace Elements

The trace element abundances of the Chaluo granite are detailed in Table 3 and Figure 6. The total rare earth element (REE) content (∑REE) ranged from 193.33 to 383.80 ppm, which was relatively low. Chondrite-normalized REE patterns showed a significant enrichment of light REEs (LREEs) over heavy REEs (HREEs), with limited fractionation among HREEs [70]. This is reflected by (La/Yb)N values of 3.75–4.93, indicating pronounced LREE-HREE fractionation. The prominent negative Eu anomaly (δEu = 0.09–0.17) strongly suggests plagioclase fractionation during magma evolution.

Figure 6. (a) Chondrite-normalized REE distribution patterns (after Boynton, 1984 [70]) and (b) primitive mantle-normalized trace element distribution patterns (after Sun and McDonough, 1989 [71]) for the Chaluo granite.

The Chaluo granite exhibited pronounced enrichment of incompatible trace elements (e.g., Rb, Th, U, Pb, Nd) relative to the primitive mantle [71], while displaying significant depletion in Nb, Ta, Sr, Eu, and Ti. This geochemical signature is characteristic of highly fractionated felsic magmas derived from crustal sources, likely formed under extensional tectonic conditions.

4.4.3. S-Pb Isotope

The whole-rock S, Pb isotope results of the analysis of five whole-rock samples from the Chaluo granite are given in Table 4 and Table 5.

Table 4. Contents of S isotope ratios of the Chaluo granite.

Sample	δ34SV-CDT‰
S01 S02 S03 S04 S05	−21.38 −26.35 −22.41 −25.63 −23.17

Table 5. Contents of the Pb isotope ratios of the Chaluo granite.

Sample	206Pb/204Pb	2σ	207Pb/204Pb	2σ	208Pb/204Pb	2σ
S01 S02 S03 S04 S05	18.7592 18.7705 18.7684 18.7624 18.7631	0.0005 0.0005 0.0004 0.0004 0.0004	15.7322 15.7326 15.7333 15.7329 15.7332	0.0005 0.0005 0.0004 0.0004 0.0004	39.1680 39.1733 39.1956 39.1849 39.1861	0.0013 0.0014 0.0012 0.0010 0.0012

The δ³⁴S values spanned from −26.35‰ to −21.38‰. These highly negative values strongly suggest that the magma source of the Chaluo granite likely originated from the remelting of sedimentary materials, as proposed in previous studies [72].

The Pb isotope ratios of ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb were 18.7592–18.7705, 15.7322–15.7333 and 39.1680–39.1956, respectively, indicating characteristics of upper crustal source [73,74,75].

5. Discussion

5.1. Petrogenesis of the Chaluo Granite

5.1.1. The I-Type Affinity of the Chaluo Granite

Based on the characteristics of the magma sources, the genesis of granites can be mainly categorized into I-type, S-type, A-type, and M-type, with the M-type being relatively scarce [76,77]. Typical A-type granites generally contain alkaline ferromagnesian minerals. In terms of their chemical composition, they are typically rich in Si, K, and Fe as well as high-field-strength elements such as Ga, Zr, Nb, Ce, and Y [78]. A-type granites are characterized by high crystallization temperatures and thus fall into the category of high-temperature granites [79,80,81]. Although the Chaluo granite exhibited a high silica content (SiO₂ ranging from 72.81 wt% to 76.63 wt%), high alkali content (K₂O + Na₂O: 8.00–8.51 wt%), and potassium enrichment (K₂O/Na₂O ratio ranging from 1.33 to 1.53), detailed mineral identification did not detect the presence of alkaline ferromagnesian minerals. According to the zircon saturation temperature calculation formula T_Zr = 12,900/(2.95 + 0.85 M + lnD^{Zircon/solution}) [82], the crystallization temperature of the Chaluo granite ranges from 748 °C to 815 °C (with an average of 772 °C) (Table 3), which is significantly lower than the average temperature of 833 °C for A-type granites [83]. Therefore, the Chaluo granite does not exhibit the mineralogical and geochemical characteristics of A-type granites and should be classified as either I-type or S-type granite. The mineral composition of the Chaluo granite contained no aluminum-rich minerals such as muscovite or garnet. Its A/CNK (molar Al₂O₃/(CaO + Na₂O + K₂O)) values ranged from 0.91 to 1.08, with an average of 1.06, and the corundum (C) content in CIPW normative minerals ranged from 0.12% to 1.13%. These characteristics indicate that the Chaluo granite should be classified as a weakly peraluminous rock [84]. In addition, the Ga/Al ratio (expressed as 10,000 × Ga/Al) of the Chaluo granite was notably lower than the well-established threshold value of 2.6 for A-type granites [84]. Moreover, the sum of Zr + Nb + Ce + Y in the Chaluo granite ranged from 268 ppm to 521 ppm, with an average of 343 ppm, which was below the lower limit of 350 ppm for A-type granites [84] (Table 3). These geochemical characteristics further confirm that the Chaluo granite does not display the defining petrological and geochemical features of A-type granites. Instead, its properties are more consistent with those of I-type or S-type granites, as evidenced by the relevant discrimination diagrams (Figure 7a,b) [85].

Figure 7. Geochemical discrimination diagrams for the Chaluo granite samples. (a) (K₂O + Na₂O)/CaO versus Zr + Nb + Ce + Y (ppm) (after Whalen et al., 1987 [84]); (b) (K₂O + Na₂O)/CaO versus 10,000*Ga/Al (after Whalen et al., 1987 [85]); (c) P₂O₅ versus SiO₂ (after Collins et al., 1982 [86]); and (d) Th versus Rb (after Chappell, 1999 [78]).

Previous studies have established that apatite solubility decreases with increasing SiO₂ content during magma differentiation in metaluminous to weakly peraluminous magmas [83,86]. The Chaluo granite displayed an extremely low P₂O₅ content (0.035–0.063 wt%) that systematically decreased with increasing SiO₂ concentration (Figure 7c) [86], which is consistent with the well-documented evolutionary trend of I-type granites. Furthermore, the samples of the Chaluo granite plotted within the I-type granite field on the Th-Rb diagram (Figure 7d) [78]. Therefore, it can be preliminarily inferred that the Chaluo granite differs significantly from S-type granite in terms of mineralogical and geochemical characteristics and should be classified as an I-type granite.

5.1.2. Magma Source of the Chaluo Granite

The Chaluo granite was relatively enriched in Rb, Pb, Th, U, Zr, Hf, and LREE, but deficient in Sr, Eu, Nb, Ta, Ti, and HREE. It had a low Mg# (Mg# = 10–17) and high Th/U (Th/U = 2.82–4.13). At the same time, the Sr/Y of the Chaluo granite ranged from 0.57 to 1.25 (with an average of 0.94), the Nb/Ta ranged from 6.60 to 11.51 (with an average of 8.67), and the Zr/Hf ranged from 32.18 to 43.45 (with an average of 35.83). All of these ratios were significantly lower than their respective average crustal values, namely 12.22 for Sr/Y [87], 12–13 for Nb/Ta, and 37 for Zr/Hf [88]. The Rb/Sr ranged from 5.76 to 8.36, with an average of 7.43, which was higher than the global average of the upper crustal value (0.32) [87]. These features indicate that continental crust materials are involved in petrogenesis. The Chaluo granite is weakly peraluminous, with a low CaO/Na₂O ratio (0.25–0.51) and a high Al₂O₃/TiO₂ ratio (54.96–104.67). In the discrimination diagram of Al₂O₃/TiO₂–CaO/Na₂O (Figure 8) [89], all samples plotted in the melt region derived from mudstone.

Figure 8. The Al₂O₃/TiO₂ versus CaO/Na₂O discrimination diagram for the Chaluo granite samples (after Sylvester, 1998 [89]).

The ratios of S and Pb isotopes can be effectively used to trace the magma source. The extremely low δ³⁴S values (−26.35–−21.38) show that the magma source might come from sedimentary rocks rich in biogenic sulfur [72]. The high Pb isotope ratios also reflect a magma source related to the partial melting of upper crustal materials [73,74,75]. The diagrams of ²⁰⁷Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb (Figure 9a) and ²⁰⁸Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb (Figure 9b) show that the samples are close to the evolution lines of Pb in the upper crust and orogenic belt, both exhibiting crustal source properties [90]. The ε_Hf (t) values from −3.8 to 3.7 (with an average of −1.3) and T_DM2 (Hf) ranged from 0.754 Ga to 1.160 Ga. All of the aforementioned characteristics indicate that the magma source of the Chaluo granite might originate from the partial melting of Middle-Neoproterozoic sedimentary rocks rich in biogenic sulfur.

Figure 9. The ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb (a) and ²⁰⁸Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb (b) discrimination diagram for the Chaluo granite samples (after Zartman,1981 [90]).

The standardized trace element diagram after the primitive mantle showed that the samples were significantly deficient in high-field strength elements such as Nb, Ta, and Ti, which may be related to the crystallization of rutile. The chondrite-normalized REE (rare earth element) pattern showed a smooth right-skewed distribution, with enrichment in LREEs (light rare earth elements) and depletion in HREEs (heavy rare earth elements). An Eu anomaly was observable on the chondrite-normalized REE diagram, while the Sr anomaly was visible on the primitive mantle normalized diagram. These features suggest that plagioclase underwent fractional crystallization during magma evolution. Additionally, the slight fractionation of HREEs indicates the presence of garnet residues in the magma source. In the discrimination diagrams of Zr/Hf–Nb/Ta and 100 × (MgO + FeO* + TiO₂)/SiO₂ − (Al₂O₃ + CaO)/(FeO* + K₂O + Na₂O) (Figure 10a,b) [89,91], the samples of the Chaluo granite plot were within the highly differentiated granite region. This further indicates that the Chaluo granite underwent a strong differentiation evolution during the intrusion process.

Figure 10. The Zr/Hf versus Nb/Ta (a) (after Wu et al. 2017) [91] and 100 × (MgO + FeO* + TiO₂)/SiO₂ versus (Al₂O₃ + CaO)/(FeO* + K₂O + Na₂O) (b) (after Sylvester, 1989) [89] discrimination diagrams of the Chaluo granite.

5.2. Geological Significance

A large number of Cretaceous intrusions have been discovered in the Yidun magmatic arc, which are distributed along the NNW tectonic belt. The zircon LA-ICP-MS U-Pb ages of Queershan granite, Xiasai granite, Hongshan porphyritic monzogranite, Tongchanggou granodiorite porphyry are (94–102) Ma, 103 Ma, (75–81) Ma, and 84 Ma [46,92,93,94,95,96], and the biotite K-Ar isochron ages of Anzi monzogranite, Hagala granite, Cuomolong granite, Luocuoren granite, Zhujiding granite porphyry, Xiuwacu porphyritic granite are 87 Ma, (77–78) Ma, 90 Ma, 86 Ma, and (85–88) Ma, respectively, [44,46,93,95,96,97]. There are three controversies regarding the dynamic background of the formation of the intrusions above-mentioned. The first view supports that they were formed in the extensional tectonic environment after the collision between the Yidun magmatic arc and the Songpan-Ganzi Block [46,49]. The second view suggests that their formation was related to the inland extension after the collision between the Lhasa Block and Qiangtang Block [40,41], while the third view suggests that they formed in the extensional tectonic environment caused by the northward subduction of the Neo-Tethys in the Yidun magmatic arc [26]. It is evident that all three aforementioned viewpoints agree that the formation of Late Cretaceous intrusions in the Yidun magmatic arc was related to intracontinental extension.

In the discrimination diagrams of Nb-Y (Figure 11a) [98], Rb-(Y + Nb) (Figure 11b) [98], R2-R1 (Figure 11c) [99], and SiO₂-TFeO/(TFeO + MgO) (Figure 11d) [99], all samples of the Chaluo granite plotted within the POG or Late POG and WPG. This geochemical evidence, combined with the detailed petrogenesis analysis, supports the formation of the Chaluo granite in a post-orogenic intracontinental extensional tectonic environment. The research results not only determine the tectonic setting of the formation of the Chaluo granite, but also further confirm the existence of the intracontinental extensional tectonic dynamic background within the Yidun magmatic arc in the Late Cretaceous. However, the Songpan-Ganzi Block and the Zhongza Block as well as the Qiangtang Block and the Zhongza Block had already undergone continent–continent collision in the late Triassic and middle Triassic, respectively. There was an interval of about 100–115 Ma between the continent–continent collision event and the Late Cretaceous magmatic activity, while the interval between continent–continent collision and post-collisional magmatic activity in many large Mesozoic-Cenozoic orogenic belts in the world is about 20–35 Ma [47,100]. Therefore, the dynamic background of the formation of the Late Cretaceous intrusion belt in the Yidun magmatic arc might not be closely related to the extensional tectonic background after the aforementioned continent–continent collision. Instead, it might be related to the northward subduction-collision of the Neo-Tethys orogenic belt (including the Bangong-Nujing oceanic plate) in the Cretaceous, followed by slab break-off and asthenosphere upwelling (Figure 12). We suggest that the Paleo-Tethys tectonic belt was influenced by the Neo-Tethys tectonic activity at least in the Yidun magmatic arc region during the Late Cretaceous.

Figure 11. Discrimination diagrams for the Chaluo granite. (a) Y versus Nb and (b) (Y + Nb) versus Rb (after Pearce et al., 1984 [98]); (c) R1 versus R2; and (d) SiO₂ versus TFeO/(TFeO + MgO) (after Batchelor et al., 1985) [99]. Syn-COLG (syn-collision granite); WPG (within plate granite); VAG (volcanic arc granite); POG (post-orogeny granite), and ORG (mid ocean ridge granite).

Figure 12. One possible schematic model depicts the dynamic background of the Late Cretaceous magmatic belt in the Yidun magmatic arc. Abbreviations: BNSZ: Bangong-Nujiang suture zone, JSJSZ: Jinshajiang suture zone, LCIB: Late Cretaceous intrusions belt; SCLM: sub-continental lithospheric mantle.

6. Conclusions

This study comprehensively investigated the Chaluo granite in the Yidun magmatic arc, yielding several significant and detailed conclusions.

Regarding the geochemical nature of the Chaluo granite, in-depth geochemical data analysis has provided clear evidence. The Chaluo granite falls into the category of weakly peraluminous, high-K calc-alkaline I-type. The weakly peraluminous characteristic is determined by its chemical composition, specifically the ratio of aluminum-related components. The high-K calc-alkaline nature implies its specific geochemical evolution path during magma formation. Moreover, the Chaluo granite has undergone a highly fractionated process, which is manifested in its elemental distribution. For instance, during the fractionation process, elements with different chemical activities have been separated and concentrated to varying degrees. Trace element variations and the fractionation of major elements are clear indicators of this highly fractionated process, which has significantly influenced the final mineral assemblage and chemical composition of the Chaluo granite.

Concerning the origin of the Chaluo granite, it is proposed that the 87 Ma Chaluo granite likely originated mainly from the partial melting of Middle-Neoproterozoic sedimentary rocks, so the Chaluo granite formed in a post-orogenic intracontinental extensional tectonic environment. In this tectonic setting, the crust underwent stretching and thinning, which led to decompression melting in the mantle or lower crust. The extensional environment provided the necessary space and energy for the generation and ascent of magma, ultimately resulting in the formation of the Chaluo granite.

Regarding the dynamic background of the Late Cretaceous intrusions in the Yidun magmatic arc, it is hypothesized that the formation of the Late Cretaceous intrusion belt in the Yidun magmatic arc might be related to the complex tectonic evolution of the Neo-Tethys orogenic belt in the Cretaceous. Specifically, it is likely associated with the northward subduction-collision of the Neo-Tethys oceanic plate, followed by slab break-off. When the oceanic slab subducted northward, it caused intense tectonic stress and magma generation in the overriding plate. Subsequently, slab break-off occurred, which led to the upwelling of the asthenosphere. The hot asthenosphere provided additional heat and material sources for magma formation, promoting the large-scale intrusion of magmas and the formation of the Late Cretaceous intrusion belt in the Yidun magmatic arc. This complex tectonic process was the key factor driving the magmatism in this region during the Late Cretaceous.