Genesis and Magmatic Evolution of the Gejiu Complex in Southeastern Yunnan, China

Abstract

Gejiu, a prominent tin–polymetallic ore district, is distinguished by its diverse mineral complexes. However, the genesis of these complexes and their relationship with mineralization remain inadequately studied. This study utilized whole-rock geochemical analyses to investigate the magmatic sources and petrogenesis of different complex types, aiming to elucidate their implications for tin–polymetallic mineralization. The results indicate that gabbro, monzonite, diorite, and syenite are derived from enriched mantle-derived magmas and have undergone limited crustal contamination. Granites are formed by the mixing of mantle- and crust-derived magmas, involving both physical mixing and chemical diffusion. Major and trace element characteristics suggest that the Gejiu granites predominantly exhibit features of both A-type and I-type granites. Harker diagrams and whole-rock indicators, such as Nb/Ta and Zr/Hf, suggest that granites experienced a two-stage fractional crystallization process, ultimately forming highly evolved biotite monzogranite. Fractional crystallization is the dominant mechanism controlling magmatic evolution, while high-temperature melting and biotite decomposition reactions are critical for the formation of the world-class Gejiu tin deposit.

1. Introduction

Gejiu, which is situated in the southeastern region of Yunnan, is recognized as the world’s primary tin mining district [1]. The region has experienced widespread Mesozoic magmatism, predominantly characterized by granites and accompanied by mafic, intermediate, and alkaline rock bodies, collectively referred to as the Gejiu complex [2,3]. Since the 1980s, extensive research has been conducted by numerous scholars on the Gejiu tin–polymetallic deposits. Geochronological studies employing dating methods such as mica Ar-Ar, molybdenite Re-Os, and cassiterite U-Pb isotopic dating revealed the following ages for the Gejiu granites, gabbros, diorites, and alkaline rocks: 79.2~85.8 Ma, 82.89 ± 0.58 Ma, 81.35 ± 0.22 Ma, and 80.35 ± 0.72 Ma, respectively [4,5]. These findings indicate that these complexes are products of contemporaneous magmatism, having been primarily formed during the Late Cretaceous, with ages decreasing from granites to alkaline rocks [2,3,4,5]. Metallogenic age dating indicates that the mineralization ages of rocks in the Gejiu tin–polymetallic ore field range from 79.6 ± 0.5 Ma to 87.5 ± 0.6 Ma [6,7], consistent with the rock forming ages, suggesting that mineralization primarily occurred during the Late Cretaceous. The Gejiu tin–polymetallic deposit contains reserves of over 3 million tons of tin metal [1] and is both spatially and temporally linked to Late Cretaceous granites [1,8]. Prior research indicates that biotite granites exhibit significantly elevated tin concentrations (averaging 15 ppm), being approximately three times higher than the regional background levels [9]. Despite this observation, the underlying mechanisms remain poorly understood. Additionally, the thermal contributions from coeval mafic magmas, as well as the tin-rich hydrothermal fluids released by highly fractionated granite melts [10], may have played a critical role in facilitating hydrothermal tin mineralization.

Regarding the origin of the Gejiu complex, there are currently two main viewpoints: some scholars propose that these rock bodies formed during the Yanshanian period (77~98 Ma) and may be products of the same parental magma at different evolutionary stages [11,12]; others argue that these rocks exhibit distinct isotopic and geochemical characteristics, suggesting contemporaneous formation from different source regions [2,8]. Among the rocks that make up the Gejiu complex, the genetic classification of Gejiu granites has been a subject of ongoing debate. Traditional perspectives categorize them as S-type granites, analogous to the Yanshanian granites in South China [2,9], whereas others propose a hybrid origin involving contributions from both crustal and mantle sources [13]. Furthermore, certain studies highlight characteristics consistent with both A-type and highly fractionated I-type granites [14], while some classifications even assign them to the I-type category [3].

In terms of tectonic setting, some scholars suggest that the Gejiu region experienced intracontinental lithospheric extension and thinning during the Late Cretaceous [5,15], while others argue that magmatic activities in South China during the Cretaceous were dominated by the subduction of the Paleo–Pacific Plate [16,17,18,19]. Recently, some scholars have proposed a link between the magmatic activities in the Gejiu region and the northward subduction of the Neo-Tethys Plate [20,21,22].

Despite extensive research, the origin and tectonic setting of the Gejiu complex remain highly debated. Previous studies have primarily focused on individual rock bodies or rocks of similar composition. There is a lack of systematic comparative studies of different rock complexes. Therefore, this study aimed to systematically compare the major, trace, and rare earth element (REE) geochemistry of different complexes in the Gejiu tin mining district to understand their magmatic sources and petrogenesis and explore the evolutionary processes of the parental magmas.

2. Geological Settings

The study area is situated on the southwestern margin of the Yangtze Block, bordered by the Tethyan Sanjiang Fold Belt to the west, separated by the Red River Fault, and bordered by the Mile–Shizong Fault to the north, the Ailao Shan–Red River Fault to the west, and the Viet Bac Block to the south [8,23] (Figure 1a). The Gejiu area covers approximately 1600 square kilometers and has predominantly been a tectonic depression throughout its geological history [8]. The stratigraphic sequence from the Cambrian to the Quaternary is well-preserved and largely unexposed (Figure 1b). The Late Triassic-to-Cretaceous strata are preferentially exposed at the surface due to uplift associated with the Yanshanian (Mesozoic) tectonic movements. Consequently, the peripheral sediments of the Gejiu area include the Middle Triassic Gejiu Formation [5,14]. The Middle Triassic Gejiu Formation, composed of carbonate rocks, is 3000 m thick and interbedded with 1800~2800 m of mafic volcanic rocks, as well as the Middle and Upper Triassic Falang Formation, which consists of fine clastic rocks and carbonate rocks. The Middle and Upper Triassic Wuge Formation and Huobachong Formation, which are composed of fine clastic rocks, are interbedded with lenticular coal seams with a thickness ranging from 200 to 500 m. An unconformity separates the Lower Triassic and Upper Triassic, while the remaining strata are in conformable contact. The Gejiu area is predominantly characterized by east–west, northwest, north–northeast, and north–south trending faults. The east–west trending Songshujiao, Beiyinshan, Laoxiongdong, Xianrendong, and Bailong Faults divide the Gejiu ore field into five distinct ore deposits from north to south: Malage, Songshujiao, Gaosong, Laochang, and Kafang. The northwest-trending faults include the Baishachong Fault, while the north–south trending faults include the Gejiu Fault. The north–northeast trending faults include the Longchahe, Jiaodingshan, and Yangjiatian Faults. The north–south trending Gejiu Fault is the primary structure in the Gejiu area, dividing the area into eastern and western zones, with the main ore deposits concentrated in the eastern zone. Fold structures include the Wuzhishan Anticline and Jiasha Syncline, both of which strike NE at 30° and extend across the entire area. Parallel to the east–west-trending faults are the Jixinnao Anticline, Zhutoushan Syncline, and Dahua Shan Anticline (Figure 1b) [8,11]. These east–west-trending structures play a crucial role in the formation of the Gejiu tin–copper–polymetallic deposits.

Cretaceous igneous rocks are distributed in both the eastern and western zones of the Gejiu area. The western zone is characterized by Late Cretaceous gabbro, syenite, monzonite, and medium-to-fine-grained granite. These intrude into the Middle Triassic Falang Formation, including Jiasha gabbro, Longchahe granite, Shenxianshui granite, and Baiyunshan alkaline rocks. Triassic mafic volcanic rocks are predominantly distributed in the south. The eastern zone is dominated by Late Cretaceous gabbro, mafic microgranular enclaves, biotite granite, and syenite. The granites exhibit both porphyritic and equigranular textures. There is an intrusive contact relationship between the two types of granite (Figure 2a), both of which originate from the same granitic magma but experience different degrees of fractional crystallization. Granitic outcrops in the eastern Gejiu mining district are spatially limited, with surface exposures restricted to the Baishachong and Kafang localities north of the Malage ore field. These outcrops exhibit porphyritic textures (Figure 2b) (coarse phenocrysts in a fine-grained holocrystalline matrix), massive structures, and grayish-white coloration with sporadic black speckles. The majority of granites, however, are concealed within the lower Gejiu Formation at depths of 6–9 km, forming an NW-trending belt. Subsurface occurrences include the eastern Kafang, Laochang (Zhuyeshan, Wanzijie), Qibeishan, and Tangzi’ao mines. These granites are texturally homogeneous, with equigranular mineral distribution (Figure 2c), high compactness, and hues ranging from light gray to grayish white. Meanwhile, mafic, intermediate, and alkaline rocks are distributed throughout the mining area, mainly intruding into the Middle Triassic sandstone, shale, and carbonate rocks [11]. Granites near the ore bodies exhibit strong epidotization, albitization, and skarnization in the carbonate wall rocks [24].

3. Sampling and Analytical Methods

3.1. Sampling

The sampling locations of this study are illustrated in Figure 1, spanning the entire Gejiu mining area and encompassing both the eastern and western mining zones. A total of 36 fresh samples were collected from the Gejiu complex. These included 16 granite samples from the Falang, Laochang, Jiasha, Longchahe, Shenxianshui, and Gaosong areas; 3 granite enclave samples from Jiasha; 8 gabbro samples from Shenxianshui, Jiasha, and Baiyunshan; 4 syenite samples from Baiyunshan and Jiasha; 2 diorite samples from Jiasha and Shenxianshui; and 3 monzonite samples from the Shenxianshui area.

3.2. Petrographic Observations

The rock samples collected from Gejiu were cut into sizes suitable for thin-section preparation (typically 2–3 cm) using a diamond saw in the “Rock and Thin Section Preparation Laboratory” at Kunming University of Science and Technology. The samples were mounted on glass slides using epoxy resin and ground to a thickness of approximately 30 microns using a grinding machine until they became thin enough for light transmission. Final polishing was performed using a lapping machine to achieve a smooth surface suitable for detailed petrographic analysis under a polarizing microscope, enabling the observation of mineral composition, texture, and other petrographic features.

3.3. Whole-Rock Major, Trace, and Rare Earth Element Analysis

All 36 samples collected from the Gejiu complex were subjected to whole-rock major, trace, and rare earth element geochemical analyses. Sample preparation and analytical procedures were conducted at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. Whole-rock major element analysis was performed using a Rigaku 100e wavelength-dispersive X-ray fluorescence (WD-XRF) spectrometer (manufactured by Rigaku Corporation, Tokyo, Japan) with an analytical accuracy above 2%. Calibration was performed using silicate rock standards (GB/T 14506.32-2019 [25], GB/T 14506.14-2010 [26], and GB/T 14506.34-2010 [27]). The precision (1 σ) for most major elements was within ±1%, except for TiO₂ (~1.5%) and P₂O₅ (~2.0%). Samples were dried, crushed, and sieved to <75 μm (200 mesh). Approximately 0.7 g of the powdered sample was weighed and transferred to a 25 mL porcelain crucible. A flux mixture of 5.20 g of anhydrous lithium tetraborate, 0.40 g of lithium fluoride, and 0.30 g of ammonium nitrate was added and thoroughly mixed. The mixture was transferred to a platinum crucible, and 1 mL of lithium bromide solution was added. Fusion was carried out at 1150–1250 °C for 10–15 min using an automatic fusion machine, resulting in a glass disk, which was cooled and removed. The glass disk was labeled and analyzed using the WD-XRF spectrometer.

Trace and rare earth element concentrations were determined using an Agilent 7900 inductively coupled plasma mass spectrometer (ICP-MS) (manufactured by Agilent Technologies, Santa Clara, USA). The precision for most elements was above 5%, while the precision for Cu, Sc, and Zn was within 10%. Multi-element calibration standards were prepared from single-element standards provided by the National Iron and Steel Material Testing Center, Iron and Steel Research Institute. High-purity HNO₃ and HF were prepared via sub-boiling distillation, and ultrapure water (18.2 MΩ·cm) was obtained from a Millipore system. For detailed methods, refer to reference [28].

4. Results

4.1. Petrography

Based on field observations, structural characteristics, and petrographic features such as mineralogical and lithological compositions under the microscope (Figure 3), all samples were classified into four categories. The following is a summary of the petrological analysis. (1) Granite and granite enclaves: The granites within the study area were primarily classified into two types: equigranular and porphyritic varieties. Equigranular granites were characterized by medium-to-coarse-grained textures and hypidiomorphic equigranular fabrics. Porphyritic granites exhibited medium-to-coarse-grained textures with hypidiomorphic inequigranular or porphyritic fabrics. Additionally, medium-to-coarse-grained pinkish K-feldspar granites, located east of the Shenxianshui area, displayed extensive alteration features, including K-feldspathization, chloritization, and intense kaolinization. All analyzed granite samples shared similar mineralogical compositions, dominated by quartz (~30%), K-feldspar (10–25%), plagioclase (~10%), amphibole (~15%), and biotite (~10%), with accessory minerals (~10%) such as zircon, apatite, titanite, and magnetite (Figure 3a). Granitic enclaves, which are predominantly distributed in the Jiasha area, displayed coarse-grained textures and euhedral-to-subhedral fabrics. Their mineral assemblages consisted of quartz (~40%), plagioclase (~30%), amphibole (~15%), and biotite (~10%), with minor accessory phases including zircon, apatite, and monazite (Figure 3b).

(2) Gabbro: The gabbro samples were gray-black in color. The Shenxianshui gabbro was primarily composed of pyroxene (40%), plagioclase (40%), and biotite (20%), displaying a granular texture (Figure 3c). The Jiasha gabbro contained pyroxene (35%), plagioclase (40%), amphibole (15%), and biotite (10%). Amphibole exhibited fibrous textures, and plagioclase showed alteration features.

(3) Diorite and monzonite: The diorite samples were gray-black, predominantly exhibiting a medium-to-fine-grained texture and a massive structure. The main minerals were plagioclase (40%), amphibole (25%), biotite (15%), K-feldspar, and quartz (20%). Minor alteration minerals such as chlorite and sericite were also present. Plagioclase was hypidiomorphic granular, with some grains showing intense sericitization, and amphibole was partially chloritized (Figure 3d). Monzonite displayed a gray-black medium-grained texture and a massive structure, with quartz (20%), plagioclase (50%), K-feldspar (25%), and amphibole (5%). Occasionally, large feldspar grains were observed, along with tourmaline, calcite, and fluorite nodules (Figure 3e).

(4) Syenite: Nepheline syenite exhibited a gray-green medium-to-coarse-grained texture and massive structure. The primary minerals were nepheline (50%), alkaline amphibole (20%), K-feldspar (20%), alkaline pyroxene (5%), and biotite (5%). Accessory minerals included titanite, zircon, and ilmenite. Nepheline formed tabular anhedral crystals with cloudy surfaces and microfractures (Figure 3f). Amphibole was euhedral to subhedral, alkaline feldspar was subhedral granular, and biotite was euhedral to subhedral.

4.2. Geochemistry

The major, trace, and rare earth element (REE) concentrations of the samples from the Gejiu complex are presented in

Table 1. Whole-rock major element data analysis results of the Gejiu complex (wt%).

Rock Type	Granite																												Granitic Enclave
Area	Falang								Laochang					Jiasha				Longchahe						Shenxianshui			Gao- Song		Jiasha
Sample	09-5		09-8		09-10		09-16		12-8		12-12		14-9	13-8		13-10		13-1		13-4		13-5		15-19	15-6	15-7	21-16		10-7		10-8		10-16
SiO2	73.30		74.95		73.88		70.34		74.51		75.41		74.44	76.58		75.35		67.59		68.56		64.21		71.04	74.47	74.42	77.96		62.60		59.84		56.78
Al2O3	13.20		12.69		13.93		14.04		12.72		12.75		12.82	12.04		13.45		15.28		15.39		15.58		12.94	12.56	13.29	12.67		16.32		16.01		16.29
Fe2O3	1.76		1.38		0.57		2.68		0.75		0.79		1.05	1.50		0.54		3.08		2.53		3.45		2.64	1.38	1.43	1.00		3.90		5.98		6.96
MgO	0.38		0.15		0.17		0.66		0.32		0.08		0.20	0.22		0.13		0.62		0.65		0.69		1.32	0.15	0.32	0.34		0.90		1.88		2.84
CaO	0.99		0.71		0.91		1.77		1.23		0.76		1.11	0.25		0.47		1.73		2.06		1.87		2.21	0.83	0.85	1.26		2.11		3.61		5.16
Na2O	3.44		3.25		3.61		3.08		3.07		3.70		3.00	2.77		3.20		3.39		2.73		2.58		3.25	3.57	3.33	0.19		2.59		3.22		2.78
K2O	5.15		5.33		5.43		5.83		4.49		4.50		5.30	4.94		5.67		6.28		6.54		9.47		4.78	5.36	5.21	3.57		8.80		6.43		5.19
MnO	0.05		0.02		0.02		0.06		0.03		0.03		0.02	0.02		0.02		0.06		0.04		0.06		0.11	0.06	0.05	0.02		0.06		0.11		0.13
P2O5	0.08		0.06		0.12		0.15		0.04		0.04		0.06	0.05		0.15		0.15		0.12		0.23		0.13	0.04	0.04	0.06		0.28		0.51		0.57
TiO2	0.14		0.09		0.15		0.24		0.06		0.07		0.06	0.23		0.01		0.29		0.29		0.52		0.24	0.16	0.16	0.10		0.54		0.66		1.05
SO3	0.02		0.00		0.01		0.01		0.13		0.05		0.02	0.01		0.00		0.02		0.01		0.02		0.07	0.03	0.01	0.01		0.02		0.15		0.10
LOI	0.91		0.71		0.64		0.43		2.29		1.03		1.13	1.16		0.81		0.67		0.39		0.57		0.91	0.48	0.52	3.07		1.30		0.69		1.50
SUM	99.41		99.34		99.44		99.29		99.64		99.21		99.21	99.77		99.79		99.16		99.31		99.25		99.64	99.09	99.63	100.3		99.41		99.07		99.35
K2O/Na2O	1.50		1.64		1.50		1.89		1.46		1.22		1.77	1.78		1.77		1.85		2.40		3.67		1.47	1.50	1.56	18.79		3.40		2.00		1.87
Na2O/K2O	0.67		0.61		0.66		0.53		0.68		0.82		0.57	0.56		0.56		0.54		0.42		0.27		0.68	0.67	0.64	0.05		0.29		0.50		0.54
Na2O + K2O	8.59		8.58		9.04		8.91		7.56		8.2		8.3	7.71		8.87		9.67		9.27		12.05		8.03	8.93	8.54	3.76		11.39		9.65		7.97
Rock Type	Gabbro														Syenite								Diorite					Monzonite
Area	Shenxianshui									Jiasha			Baiyun shan		Baiyunshan						Jiasha		Jiasha		Shenxian shui			Shenxianshui
Sample	10-19	10-20		10-20R		10-4		10-6		13-2		13-15	15-9		15-17		15-2		15-24		13-6		13-13		15-20			10-14		15-18		15-15
SiO2	46.66	45.82		46.01		54.41		55.97		49.39		46.62	44.21		56.48		58.26		54.16		63.73		40.62		48.64			51.68		51.76		51.84
Al2O3	12.57	14.82		14.78		17.68		18.42		16.77		14.77	11.3		21.39		18.87		21.87		17.23		11.77		14.83			16.33		22.00		21.90
Fe2O3	8.17	11.02		10.98		8.11		7.19		9.27		9.50	8.55		2.58		5.07		1.45		1.41		11.50		8.16			8.92		3.04		3.07
MgO	7.65	5.02		5.03		2.23		1.80		4.67		5.22	7.45		0.38		0.76		0.26		0.46		8.22		4.92			3.59		0.37		0.44
CaO	18.44	9.58		9.58		4.57		4.01		7.55		8.67	18.28		2.28		3.21		2.14		0.72		13.67		8.82			7.26		2.16		2.15
Na2O	1.43	2.49		2.58		3.43		3.34		2.68		2.12	1.65		4.00		4.79		4.54		1.58		1.12		1.99			2.56		6.94		6.95
K2O	0.94	5.04		5.05		5.64		6.51		4.58		4.87	2.08		8.34		6.73		12.61		12.13		3.87		7.15			4.64		7.41		7.37
MnO	0.11	0.18		0.18		0.14		0.11		0.16		0.16	0.13		0.09		0.16		0.05		0.04		0.16		0.17			0.16		0.11		0.11
P2O5	0.35	1.18		1.19		0.56		0.46		0.82		1.27	1.18		0.06		0.11		0.04		0.34		1.97		0.85			0.68		0.06		0.06
TiO2	1.27	1.78		1.80		1.22		1.09		1.27		1.78	1.22		0.23		0.42		0.07		0.29		1.78		1.07			1.12		0.40		0.44
SO3	2.06	0.11		0.10		0.00		0.01		0.03		0.26	0.06		0.19		0.10		0.00		0.01		0.26		0.00			0.10		0.07		0.07
LOI	0.07	0.99		0.99		1.14		0.64		1.43		4.16	3.66		3.18		1.14		1.97		1.24		4.16		3.95			2.18		4.43		4.43
SUM	99.70	98.02		98.27		99.12		99.55		98.63		99.4	99.76		99.19		99.61		99.16		99.18		99.11		100.55			99.22		98.75		98.83
K2O/Na2O	0.66	2.02		1.96		1.64		1.95		1.71		2.30	1.26		2.09		1.41		2.78		7.68		3.46		3.59			1.81		1.07		1.06
Na2O/K2O	1.52	0.49		0.51		0.61		0.51		0.59		0.44	0.79		0.48		0.71		0.36		0.13		0.29		0.28			0.55		0.94		0.94
Na2O + K2O	2.37	7.53		7.63		9.07		9.85		7.26		6.99	3.73		12.34		11.52		17.15		13.71		4.99		9.14			7.20		14.35		14.32

Note: LOI = loss on ignition. “R” stands for the same sample.

,

Table 2. Whole-rock trace element data analysis results of the Gejiu complex (ppm).

Rock Type	Granite																											Granitic Enclave
Area	Falang								Laochang						Jiasha			Longchahe			Shenxianshui					Gao- Song		Jiasha
Sample	09-5		09-8		09-10		09-16		12-8		12-12		14-9		13-8		13-10	13-1	13-4	13-5	15-19	15-6		15-7		21-16		10-7		10-8	10-16
Li	66.1		35.1		38.7		44.1		22.6		60.9		26.2		11.5		13.2	46.2	45.40	43.21	93.4	85.7		89.2		48.2		31.8		24.7	33.6
Be	13.8		7.54		9.06		7.83		19		11.9		18.9		5.92		4.73	7.78	6.90	7.49	14.6	12.1		13.6		7.81		4.32		6.37	4.48
Sc	2.14		1.17		2.83		3.46		2.24		3.97		6.24		2.52		1.82	4.21	4.76	3.88	3.76	3.77		3.85		3.39		9.3		9	14.4
V	9.90		3.78		4.34		22.3		0.62		0.83		15.3		7.56		3.83	16.6	15.79	16.15	16.7	16.32		16.11		5.74		59.7		62.7	93
Cr	7.36		1.55		2.62		18.7		6.64		2.18		492		662		7.34	5.25	6.30	5.56	8.81	6.73		6.89		13.1		7.17		7.47	12.6
Co	42.5		11.5		21.7		8.83		14.8		11.8		65		18.1		13.2	18.4	18.34	18.63	22.3	19.82		20.14		16.9		14.4		22.5	21
Ni	2.49		0.9		0.93		8.84		0.45		0.79		264		13.5		1.24	2.23	1.85	2.21	2.93	3.32		3.18		1.67		5.23		5.79	8.54
Cu	8.18		4.81		12.7		6.13		185		22		481		115		10.9	14.7	13.8	14.57	18.2	19.65		18.88		30.9		16.4		111	38
Zn	29.3		29.2		33		43.2		17.8		22.4		69.7		20.1		30.7	58.3	46.3	49.55	117	109		115		45.3		113		253	109
Ga	19.7		17.9		18.5		20.1		22.1		23.1		18.4		17.4		17.2	22.9	24.65	23.82	22.8	23.94		23.57		21.1		27.7		23.5	23.1
Ge	0.64		0.46		0.55		0.8		0.56		0.58		0.87		0.41		0.44	0.82	0.68	0.75	0.89	0.87		0.83		0.64		1.43		1.24	1.25
Rb	416		379		321		336		589		681		769		258		306	310	308	287	342	387		412		508		228		254	239
Sr	262		128		130		535		29.6		69.9		86		172		192	336	345	326	238	271		265		26.2		852		753	878
Y	16.1		16		12.8		28.1		69.9		83.5		61.6		9.21		6.33	25	30.44	28.9	14.9	15.1		15.5		42.7		39.2		38.7	32.9
Zr	171		181		55.8		276		65.9		77.3		103		113		29.1	332	320	318.5	219	229		217		97.1		765		411	374
Nb	29.4		31.2		16.9		30.8		33.3		38.7		40.5		17.6		15.2	30.3	27.5	29.8	20.7	25.3		24.7		29.4		53.7		31.4	34.3
Cs	15.3		8.91		12.8		21.9		20.8		26.1		19.5		7.11		10.9	9.73	9.52	9.13	24.1	27.2		27.6		28.7		3.93		6.6	8.64
Ba	324		152		227		1167		7.51		153		86		265		460	766	696	734	433	535		568		21		1950		1838	1935
Hf	5.01		5.31		2.1		7.18		6.05		5.05		4.12		4.05		1.14	8.13	7.15	8.06	5.98	6.05		6.11		4.51		16.1		9.63	8.54
Ta	3.17		3.57		2.67		3.41		7.97		11.7		8.53		1.61		2.3	2.07	2.45	2.09	2.0	2.32		2.09		6.74		2.17		1.55	1.73
Pb	67.4		57.5		91.7		48.5		78.4		51.7		146		35.6		61.1	59.2	60.53	60.25	97	85		87		35		51		60.2	44.7
Th	59.0		52.6		20.1		56.4		19.4		14.9		43.8		40.2		5.98	75.3	60.23	63.7	61.1	62.33		62.58		36.4		56.9		64.6	47.4
U	18.7		16.7		8.1		11.3		19.6		22.8		34.4		11.1		5.34	12	13.41	12.85	17.8	16.54		17.02		5.31		6.94		9.09	7.56
Rb/Sr	1.59		2.96		2.47		0.63		19.9		9.74		8.42		1.5		1.59	0.92	0.89	0.88	1.44	1.43		1.55		19.39		0.27		0.34	0.27
Nb/Ta	9.27		8.74		6.33		9.03		4.18		3.31		4.75		10.93		6.61	14.64	11.22	14.26	10.35	10.91		11.82		4.36		24.75		20.26	19.83
Zr/Hf	34.13		34.09		26.57		38.44		10.89		15.31		25		27.9		25.53	40.84	44.76	39.52	36.62	37.85		35.52		21.53		47.52		42.68	43.79
Sr/Y	16.27		8		10.16		19.04		0.42		0.84		1.4		18.68		30.33	13.44	11.33	11.28	15.97	17.95		17.10		0.61		21.73		19.46	26.69
Rock Type		Gabbro																Syenite					Diorite				Monzonite
Area		Shenxianshui										Jiasha				Baiyun shan		Baiyunshan			Jiasha		Jiasha		Shen xian shui		Shenxianshui
Sample		10-19		10-20		10-20R		10-4		10-6		13-2		13-15		15-9		15-17	15-2	15-24	13-6		13-13		15-20		10-14		15-18		15-15
Li		22.4		30.8		29.4		32.8		37.7		37.7		53.5		62.8		217	87.6	35.6	21.2		73.6		135		37.1		69		268
Be		3.03		2.74		2.65		5.95		5.5		4.19		3.88		8.23		7.16	9.44	2.33	2.33		3.39		10.1		4		6.09		3.55
Sc		23.9		21.5		21.1		5.11		10.5		20.7		31.5		2.39		0.9	1.81	0.21	5.46		32.1		17.9		16.6		1.4		2.43
V		410		219		208		24.6		95.9		171		199		31.9		29.9	22.1	11.8	11.5		246		168		110		25.5		25.4
Cr		144		2.69		2.57		8.4		4.07		208		196		9.14		1.38	4.07	2.72	9.69		120		59		23.8		142		16.4
Co		32.7		34		32		20.3		19.1		28.5		36.3		15.9		12	19.4	9.55	27.9		50.1		27.5		22.8		18.1		22.1
Ni		104		4.39		4.28		3.15		5.09		27.5		81.6		1.71		1.43	1.3	1.39	10.6		74.1		46.6		14		66.4		16
Cu		183		45.1		44.5		6.91		20.8		88.4		62.3		11.3		18.4	49.3	12.5	20.0		73.3		60.6		28		294		279
Zn		168		136		138		59.3		117		104		75.6		592		103	109	42.8	70.1		169		114		102		96.1		217
Ga		19		23.3		24.7		23.9		25.2		24.6		19.3		23.3		25.7	24.1	19.1	20.4		21.6		23.2		22.3		19.6		16.5
Ge		1.11		1.82		1.93		0.93		1.62		1.61		1.4		0.83		0.66	1.14	0.41	0.46		1.8		1.57		1.42		0.73		2.07
Rb		83.9		159		155		252		245		195		66.2		459		315	337	926	626		180		338		189		141		241
Sr		1677		1989		1973		440		926		1243		1540		1276		933	969	452	1217		1175		2015		1184		1316		834
Y		28.3		48.2		49.5		22.4		39.1		42.6		32.3		27.2		26.4	35.9	7.72	16.0		44.5		44.6		37.2		34.5		10.3
Zr		256		306		312		367		670		393		335		494		412	506	215	351		281		553		339		628		44.1
Nb		63.6		41.1		40.5		24		55.1		29.6		12		63.5		76.9	70.6	29.4	33.7		30.1		32.2		32		66.7		14.4
Cs		14.7		12.9		12.6		7.21		7.28		6.41		4.06		23.4		15	42	17.1	32.6		14.1		16		8.6		6.29		40.4
Ba		108		4657		4662		915		2138		2697		2242		2043		426	436	309	3387		4726		4483		2091		238		383
Hf		6.37		7.55		7.63		8.55		13.6		8.6		7.95		9.8		6.33	10.6	2.51	8.32		7.15		12.5		7.69		8.03		1.19
Ta		2.41		1.64		1.49		1.13		2.69		1.27		1.61		3.32		4.01	3.89	1.14	3.42		1.41		1.58		1.64		2.02		0.43
Pb		20.5		59.9		55.6		51.2		66.5		45.8		36.6		385		178	127	34.5	58.4		60		108		44.2		145		120
Th		10.7		20.1		20.7		67.6		57.2		44.9		33.8		89.2		116	81.2	26	25.8		30.1		66		39.4		112		17.8
U		5.36		2.98		2.85		9.05		7.88		6.44		7.73		14.5		22.8	15.1	15.4	7.05		5.6		12		6.46		24.6		4.15
Rb/Sr		0.05		0.08		0.08		0.57		0.26		0.15		0.04		0.36		0.34	0.35	2.05	0.51		0.15		0.17		0.16		0.11		0.29
Nb/Ta		26.39		25.06		27.18		21.24		20.48		23.31		19.67		19.13		19.18	18.15	25.79	9.85		21.35		20.38		19.51		33.02		33.49
Zr/Hf		40.19		40.53		40.89		42.92		49.26		45.7		42.14		50.41		65.09	47.74	85.66	42.19		39.3		44.24		44.08		78.21		37.06
Sr/Y		59.26		41.27		39.86		19.64		23.68		29.18		47.68		46.91		35.34	26.99	58.55	76.06		26.4		45.18		31.83		38.14		80.97

and

Table 3. Whole-rock rare earth element data analysis results of the Gejiu complex (ppm).

Rock Type	Granite																											Granitic Enclave
Area	Falang								Laochang						Jiasha			Longchahe			Shenxianshui					Gao- Song		Jiasha
Sample	09-5		09-8		09-10		09-16		12-8		12-12		14-9		13-8		13-10	13-1	13-4	13-5	15-19	15-6		15-7		21-16		10-7	10-8	10-16
La	58.4		44.7		22.8		104		18.5		8.48		31.4		24.7		10.8	139	140.4	162.3	77.7	78.56		67.64		28.9		220	175	128
Ce	107		85.3		42.4		197		43.6		23.2		66.9		45		12.9	247	223.6	278.9	144	139.7		110.3		70.9		417	301	244
Pr	9.49		8.04		4.02		18.6		5.13		3.36		8.46		4.96		1.98	23.1	23.36	30.98	13.2	11.85		9.85		7.37		37.5	31.9	25.2
Nd	28.9		25.5		13.5		61.4		19.8		15.1		33.9		17.2		6.77	72.7	65.03	97.26	41.7	36.97		29.77		25.9		122	107	89.0
Sm	4.38		4.27		2.68		10.1		7.39		7.25		9.92		2.8		1.28	10.8	7.69	14.32	6.19	5.75		4.59		5.7		17.9	16.6	14.2
Eu	0.68		0.49		0.45		1.51		0.04		0.10		0.32		0.51		0.37	1.67	1.44	2.43	0.96	0.69		0.57		0.1		3.38	2.97	3.22
Gd	3.99		3.65		2.56		8.67		7.70		8.37		8.99		2.4		1.19	9.56	4.98	10.37	5.35	4.36		3.98		5.84		15.6	14.2	12.2
Tb	0.46		0.45		0.40		1.03		1.51		1.76		1.60		0.28		0.19	1.01	0.73	1.57	0.55	0.62		0.47		1.05		1.60	1.52	1.32
Dy	2.48		2.42		2.28		5.22		10.1		12.1		10.2		1.44		1.08	4.76	3.65	8.34	2.60	3.77		3.36		6.88		7.57	7.24	6.36
Ho	0.48		0.47		0.39		0.94		2.00		2.42		2.00		0.27		0.19	0.85	0.58	1.62	0.46	0.75		0.52		1.35		1.36	1.28	1.13
Er	1.50		1.47		1.01		2.64		6.11		7.37		5.80		0.84		0.50	2.42	1.92	3.76	1.33	2.35		1.57		4.07		3.83	3.56	3.13
Tm	0.25		0.25		0.15		0.38		1.07		1.27		0.94		0.14		0.076	0.34	0.23	0.58	0.20	0.48		0.32		0.7		0.53	0.49	0.43
Yb	1.81		1.77		0.97		2.40		7.54		8.77		6.26		0.99		0.49	2.14	1.37	3.44	1.37	2.33		1.71		4.76		3.34	3.03	2.71
Lu	0.29		0.28		0.14		0.34		1.15		1.30		0.89		0.16		0.07	0.32	0.21	0.56	0.22	0.41		0.30		0.7		0.50	0.45	0.40
ΣREE	219.78		179.06		93.75		414.29		131.64		100.82		187.58		101.69		37.89	515.67	475.19	616.43	295.71	288.59		234.95		164.22		852.11	666.24	531.49
LaN/YbN	23.21		18.11		16.86		31.11		1.76		0.69		3.6		17.86		15.72	46.59	73.51	33.84	40.57	24.19		28.37		4.36		47.25	41.43	33.87
δEu	0.49		0.38		0.53		0.4		0.02		0.04		0.1		0.61		0.92	0.5	0.71	0.61	0.51	0.42		0.41		0.06		0.62	0.59	0.75
δCe	1.11		1.1		1.09		1.1		1.1		1.06		1.01		1		0.68	1.07	0.96	0.96	41.10	1.22		1.05		1.19		1.13	0.99	1.05
Rock Type		Gabbro																Syenite					Diorite				Monzonite
Area		Shenxianshui										Jiasha				Baiyun shan		Baiyunshan			Jiasha		Jiasha		Shen xian shui		Shenxianshui
Sample		10-19		10-20		10-20R		10-4		10-6		13-2		13-15		15-9		15-17	15-2	15-24	13-6		13-13		15-20		10-14		15-18	15-15
La		40.6		209		195		143		198		178		153		169		214	221	71.8	35.7		210		257		143		231	140
Ce		84		416		392		254		371		315		299		287		293	388	114	88.7		396		459		268		286	212
Pr		10.3		47.5		42.3		23.7		33.2		33.9		34.2		26.4		22.6	37.3	9.41	9.73		45.6		48.9		27.9		27.1	20.4
Nd		38.9		173		165		74.9		108		119		125		80.4		58.6	115	24.6	34.8		165		169		97.8		74.8	59.3
Sm		7.61		26.7		25.9		10.7		15.4		18.4		20		11.2		6.92	16.1	2.77	6.35		25.5		25.2		15.6		9.41	5.99
Eu		1.56		6.57		6.78		1.98		3.53		4.53		4.9		2.56		1.31	2.85	0.61	1.9		6.85		6.94		3.88		2.02	1.36
Gd		7.01		22.5		25.5		9.31		13.9		16.2		16.7		10.2		7.76	14.3	2.87	5.29		21.9		22.1		13.5		9.82	5.99
Tb		0.98		2.31		2.48		0.93		1.45		1.74		1.65		1.04		0.79	1.47	0.26	0.62		2.19		2.13		1.47		1.01	0.45
Dy		5.59		10.2		11.3		4.24		7.17		8.21		7.1		4.97		4.12	6.91	1.21	3		9.51		9.11		7.09		5.1	1.7
Ho		1.05		1.71		1.67		0.75		1.32		1.44		1.16		0.89		0.79	1.24	0.22	0.52		1.57		1.5		1.27		0.98	0.29
Er		2.89		4.53		4.58		2.12		3.8		3.95		3.04		2.61		2.4	3.55	0.69	1.49		4.14		3.99		3.53		2.97	0.93
Tm		0.42		0.56		0.59		0.29		0.54		0.53		0.37		0.38		0.37	0.5	0.1	0.22		0.5		0.49		0.49		0.44	0.12
Yb		2.68		3.39		3.56		1.82		3.52		3.33		2.21		2.48		2.49	3.18	0.68	1.44		2.92		2.97		3.08		2.93	0.78
Lu		0.41		0.51		0.57		0.28		0.53		0.49		0.33		0.37		0.36	0.48	0.1	0.21		0.41		0.43		0.45		0.41	0.11
ΣREE		204		924.48		877.23		528.02		761.36		704.72		668.66		599.5		615.51	811.88	229.32	189.97		892.09		1008.76		587.06		653.99	449.42
LaN/YbN		10.87		44.22		39.29		56.42		40.4		38.37		49.66		48.8		61.65	49.85	75.74	17.78		51.59		62.07		33.3		56.55	128.84
δEu		0.65		0.82		0.81		0.6		0.74		0.8		0.82		0.73		0.55	0.57	0.66	1.0		0.89		0.9		0.82		0.64	0.7
δCe		1.01		1.02		1.06		1.07		1.12		0.99		1.01		1.06		1.03	1.05	1.08	1.17		0.99		1.00		1.04		0.89	0.98

Note: “R” stands for the same sample. REE values normalized to chondrite (Boynton, 1984) [29]; The (La/Yb)ₙ ratio is normalized to chondritic meteorite values, (La/Yb)ₙ = [(La_sample/Yb_sample)/(La_chondrite/Yb_chondrite)] (McDonough, W.F., and Sun, S.S. (1995)) [30]; Eu/Eu* = [Eu_N/(Sm_N + Gd_N)/2], Ce/Ce* = Ceₙ/Ceₙ*(Ceₙ* = (2La_n + Nd_n)/3).

. These data provide a comprehensive geochemical characterization of the various lithologies within the complex, including granites, gabbros, diorites, monzonites, and syenites.

4.2.1. Major Element Geochemistry

The major element compositions of the samples are presented in Table 1. The Gejiu granites exhibited significant spatial heterogeneity in their major element compositions. In the western sector, the Falang, Jiasha, and Shenxianshui granites displayed relatively homogeneous geochemical characteristics: the Falang granite showed Na₂O/K₂O ratios of 0.53–0.67, Rittmann index (σ = (K₂O + Na₂O)²/(SiO₂ − 43)) values of 2.30–2.90, and aluminum saturation indices (A/CNK = Al₂O₃/(K₂O + Na₂O + CaO)) of 0.96–1.04; the Jiasha granite had a Na₂O/K₂O ratio of 0.56, σ values of 1.77–2.43, and A/CNK values of 1.10–1.16; and the Shenxianshui granite was characterized by Na₂O/K₂O ratios of 0.64–0.68, σ values of 2.30–2.53, and A/CNK values of 0.89–1.05. These consistent geochemical signatures suggest a common magmatic source and evolution dominated by fractional crystallization. In contrast, the Longchahe granite in the same sector exhibited broader compositional variations, with Na₂O/K₂O ratios ranging from 0.27 to 0.54, σ values of 3.36–6.85, and A/CNK values of 0.87–1.01, indicating complex differentiation processes.

In the eastern sector, the Laochang and Gaosong granites showed distinct alteration features. The Laochang granite was marked by high loss on ignition (LOI = 1.03–2.29 wt%), Na₂O/K₂O ratios of 0.57–0.82, σ values of 1.81–2.19, and A/CNK values of 1.01–1.05. Notably, its low TiO₂ content (0.06–0.07 wt%) may reflect titanium depletion during chloritization. The Gaosong granite, which was affected by intense chloritization, exhibited an extremely low alkali content (total alkalis = 3.76 wt%), with a Na₂O/K₂O ratio of 0.05, σ value of 0.4, and a high A/CNK value of 1.96. Its significantly high LOI (3.07 wt%) further corroborates pervasive hydrothermal alteration. These spatial variations highlight the interplay between primary magmatic processes and post-magmatic fluid interactions in shaping the geochemical diversity of the Gejiu granites.

In the A/CNK-A/NK diagram, the samples predominantly display characteristics of metaluminous rocks, typically showing a positive correlation with SiO₂ (Figure 4b). On the TAS diagram, these samples are shown to be mainly distributed within the granite field (Figure 4a). In Harker variation diagrams, Al₂O₃, TiO₂, Fe₂O₃, MgO, MnO, CaO, and P₂O₅ can be seen to decrease with increasing SiO₂, while Na₂O remained relatively constant (Figure 5), indicating the fractional crystallization of ferromagnesian minerals, plagioclase, Ti-Fe oxides, and apatite.

The gabbro samples exhibited a wide range of total alkali (ALK = K₂O + Na₂O) contents, varying from 2.37 to 9.85 wt%. The Na₂O/K₂O ratios ranged from 0.44 to 1.52, with most values below 1. The A/NK values ranged from 1.87 to 5.30, and the aluminum saturation index (A/CNK = Al₂O₃/(CaO + Na₂O + K₂O)) ranged from 0.51 to 1.33. The Rittmann index (σ) was relatively high, ranging from 1.53 to 20.11. In the A/CNK-A/NK diagram, the samples exhibit characteristics of metaluminous-to-peraluminous rocks (Figure 4b).

The monzonite samples had low Na₂O/K₂O ratios (0.55–0.94) and a wide range of Rittmann indices (5.97–23.51). The diorite samples exhibited relatively low SiO₂ contents (40.62–48.64 wt%), which is attributed to silica depletion caused by intense alteration processes such as chloritization (Figure 3e). Concurrently, the samples contained high concentrations of Fe₂O₃ (8.16–11.50 wt%), MgO (4.92–8.22 wt%), and K₂O (3.87–7.15 wt%).

The syenite samples had low Na₂O/K₂O ratios (0.36–0.71) and high Rittmann indices (8.70–26.36), likely due to argillic alteration, which resulted in elevated K₂O contents (6.73–12.61 wt%). On the TAS diagram, the syenite samples are plotted within the syenite field (Figure 4a).

4.2.2. Trace Element Geochemistry

The trace element geochemistry of the samples is presented in Table 2. The trace element geochemical characteristics of granites, gabbros, syenites, diorites, and monzonites collectively showed depletion in specific high-field-strength elements (HFSEs: P, Ti) and K from the large-ion lithophile element (LILE) group, while exhibiting selective enrichment in U (LILE) and Th (HFSE). In contrast, compared to other rocks, granites exhibited negative anomalies in Ba (LILE) and Nb (HFSE) but positive anomalies in Ta (HFSE) (Figure 5a). Gabbros and diorites/monzonites showed enrichments (LILEs, e.g., Ba), indicating geochemical signatures typical of arc-related igneous rocks (Figure 5b,c). The significant depletions in Ba, Nb, P, and Ti (Figure 5a) indicate magmatic differentiation during granite formation [14].

The Gejiu granites are characterized by high concentrations of Rb (average = 385.79 ppm), Y (average = 30.89 ppm), and Nb (average = 29.51 ppm), indicating a highly differentiated magmatic environment. As an incompatible element, Rb preferentially concentrates in late-stage melts during prolonged magmatic evolution. The high Rb content suggests extensive fractional crystallization and significant late-stage fluid activity, facilitating Rb enrichment in the residual melt. The elevated Y abundances may be attributed to either a Y-rich source mineralogy (e.g., amphibole) or the incorporation of Y-enriched materials during magma generation and evolution. Additionally, interactions with garnet and zircon, which selectively sequester HREEs, could further enhance Y retention in the melt. The pronounced Nb enrichment likely reflects a high degree of partial melting in the magma source. As a high-field-strength element (HFSE), Nb exhibits relatively high partition coefficients during partial melting, favoring its concentration in the melt phase. This geochemical signature aligns with magmatic systems derived from metasomatized mantle or crustal sources undergoing advanced differentiation. Collectively, the Rb-Y-Nb systematics underscore the roles of prolonged fractional crystallization, fluid–melt interactions, and source characteristics in shaping the geochemical evolution of the Gejiu granites.

The ratios of mobile versus immobile elements indicate magma source characteristics [3]. The Gejiu complex exhibits significant variations in trace element ratios among its lithologies. Granites displayed elevated Rb/Sr ratios (0.26–19.90, average 4.68) significantly higher than or comparable to the primitive mantle value (0.88) [32], indicating that these rocks experienced significant magmatic differentiation. In contrast, gabbros, diorites, monzonites, and syenites exhibited lower Rb/Sr ratios (0.04–0.57, 0.11–0.84, and 0.34–2.05, with averages of 0.23, 0.29, and 0.91, respectively). Granites displayed the lowest Nb/Taratios (3.31–24.15, average 11.07), comparable to highly evolved mineralized granites [33]. The Nb/Ta ratios of gabbros, diorites, monzonites, and syenites were comparable (19.13–27.18, 20.38–21.35, 19.51–33.49, and 9.85–25.79, with averages of 22.81, 20.87, 28.67, and 18.24, respectively), being significantly higher than the mantle Nb/Ta ratio (17.5 ± 2) [34,35]. Granites exhibited Zr/Hf ratios ranging from 10.89 to 47.52, with an average of 33.80, which are lower than the primitive mantle and chondritic values (34–36) [36]. In contrast, gabbros and diorites/monzonites exhibited Zr/Hf ratios higher than the primitive mantle and chondritic values (40.19–50.39 and 35.27–78.21, with averages of 45.26 and 46.36, respectively). Syenites displayed Zr/Hf ratios ranging from 47.74 to 85.66, with an average of 66.16, which are significantly higher than the primitive mantle and chondritic values. Therefore, the gabbros, diorites, monzonites, and syenites in the Gejiu complex likely originated from a similar magma source, while the source of granites is distinct from that of these rocks.

Figure 5. Spider diagram of trace elements for (a) granite, (b) gabbro, (c) diorite and monzonite, and (d) syenite.

Figure 5. Spider diagram of trace elements for (a) granite, (b) gabbro, (c) diorite and monzonite, and (d) syenite.

4.2.3. Rare Earth Element (REE) Geochemistry

The rare earth element geochemistry of the samples is presented in Table 3. The granites exhibited a wide range of total rare earth element (ΣREE) contents, but there were systematic differences among different rock groups (Figure 6a). Porphyritic granites from the Longchahe and Jiasha areas showed the highest ΣREE values (average = 609.52 ppm) and weak negative Eu anomalies (δEu = 0.50–0.75, average = 0.63). Their steep (La/Yb)ₙ ratios (33.84–73.51) indicate pronounced fractionation between light (LREEs) and heavy rare earth elements (HREEs), suggesting prolonged magmatic evolution dominated by garnet and zircon fractionation [23]. In contrast, alkali feldspar granites from the Falang and Shenxianshui areas exhibited lower ΣREE (average = 249.90 ppm) and moderate Eu anomalies (δEu = 0.38–0.53, average = 0.45). Their moderately high (La/Yb)ₙ ratios (16.86–40.57) imply continued magmatic differentiation, where plagioclase fractionation altered Eu partitioning between minerals and melt, intensifying the Eu depletion. Equigranular biotite granites from the Laochang and Gaosong areas displayed the lowest ΣREE value (average = 140.07 ppm) and the most pronounced Eu anomalies (δEu = 0.02–0.1, average = 0.055). Their flat (La/Yb)ₙ ratios (0.69–4.36) reflect limited LREE-HREE fractionation, likely resulting from the late-stage crystallization of plagioclase and K-feldspar, combined with fluid-mediated HREE mobilization during hydrothermal activity. The systematic transition from right-sloping REE patterns (Figure 6a) in porphyritic granites to flattened trends in equigranular types highlights a co-genetic magmatic differentiation sequence. Progressive plagioclase fractionation reduced δEu values, while residual melts accumulated LREEs, ultimately generating distinct REE signatures across the granite suite. These variations underscore the interplay between mineral–melt partitioning, fluid interactions, and magmatic evolution in shaping REE distribution.

The rare earth element (REE) characteristics of gabbros, diorites, monzonites, and syenites are summarized below.

All three rock types had high total REE contents (gabbros: ΣREE = 204.00–924.48 ppm, average 625.51 ppm; diorites and monzonites: ΣREE = 69.55–1008.76 ppm, average 610.16 ppm; syenites: ΣREE = 229.32–811.8 ppm, average 552.24 ppm). Strong fractionation between LREEs and HREEs was observed in all three rock types (gabbros: (La/Yb)_N = 10.87–56.42, average 40.56; intermediate rocks: (La/Yb)_N = 4.91–128.84, average 56.24; syenites: (La/Yb)_N = 49.85–75.74, average 62.41). Eu anomalies were weakly negative (gabbros: δEu = 0.60–0.82, average 0.74; intermediate rocks: δEu = 0.64–1.04, average 0.83; syenites: δEu = 0.55–0.66, average 0.59). The chondrite-normalized REE distribution patterns of all three rock types displayed right-sloping trends with pronounced LREE enrichment and concavity in intermediate-to-heavy REEs, indicating significant amphibole differentiation (Figure 6b–d). Gabbros exhibited the most pronounced enrichment in total rare earth elements (ΣREE) and light rare earth elements (LREEs), a feature that is likely linked to delayed saturation of rare earth element (REE)-bearing mineral phases (e.g., apatite, zircon) within the gabbroic system, which prolonged the enrichment of REEs in the residual melt and resulted in elevated total REE concentrations (ΣREE). Concurrently, fractional crystallization during magmatic evolution led to the crystallization and removal of heavy rare earth element (HREE)-rich minerals (e.g., garnet, amphibole), leaving the residual magma significantly enriched in light rare earth elements (LREEs).

5. Discussion

5.1. Fractional Crystallization and Mineralogical Evolution of the Gejiu Granites

In the Harker variation diagrams, the granites exhibit distinct linear trends (Figure 7). These trends may result from various processes, including closed-system behavior, fractional crystallization, partial melting, and magma mixing [37]. Closed-system behavior refers to a geological system that is chemically isolated from its surroundings, meaning that elements are neither lost nor gained within the system. The linear decreases in TiO₂, CaO, P₂O₅, Fe₂O₃T, and Al₂O₃ with increasing SiO₂ content rule out the influence of closed-system behavior [38].

Additionally, the flat REE distribution patterns, strong negative Eu anomalies, and low total rare earth element (REE) abundances in the syenogranites suggest plagioclase crystallization in the magma source region. As the geochemical characteristics of the rocks in the previous text show, the significant depletion of Ba, Nb, P, and Ti indicates that magmatic differentiation played a dominant role in the formation of the granite. Further crystallization vector studies suggest that plagioclase, potassium feldspar, amphibole, hornblende, apatite, and monazite played important roles in the magmatic evolution process. Therefore, fractional crystallization is likely the primary petrogenetic process governing magma evolution, and the granites likely formed through fractional crystallization during magmatic evolution.

The variations in major elements can be attributed to a two-stage fractional crystallization process in the Gejiu granite. Whole-rock geochemical indicators (e.g., Nb/Ta, Zr/Hf) (Figure 8b,c) reveal that the magmatic evolution progressed from lowly evolved porphyritic granite to moderately evolved syenogranite, K-feldspar granite, and equigranular biotite granite, culminating in highly evolved equigranular biotite monzogranite (Figure 7). The Sr/Y ratio initially increased and then sharply decreased, confirming a two-stage fractional crystallization process dominated by amphibole and feldspar (Figure 8a). In the Harker diagrams, Al₂O₃, TiO₂, Fe₂O₃, MgO, MnO, CaO, and P₂O₅ can be seen to decrease with increasing SiO₂, while Na₂O remains nearly constant, indicating intense fractional crystallization or metasomatic processes. This suggests the fractional crystallization of mafic minerals, plagioclase, Ti-Fe oxides, and apatite. The presence of varying negative Eu anomalies in the chondrite-normalized REE patterns further supports this interpretation (Figure 6a). Li et al. [17] suggested that these features reflect the highly fractionated nature of I-type granite, resulting from the differentiation of apatite, monazite, and Ti-bearing minerals.

5.2. Genesis of the Gejiu Complex

5.2.1. Genesis of Granites and Granitic Enclaves

(1) Genesis of Granites

The first modern geochemical classification scheme for granitic rocks was proposed by Chappell and White [39], distinguishing I-type and S-type granites. I-type granites are metaluminous to weakly peraluminous, with relatively high sodium and a wide range of SiO₂ contents (55–77%), suggesting their formation from mafic metamorphic igneous rocks. In contrast, S-type granites are strongly peraluminous, with relatively high potassium and higher SiO₂ contents (64–77%), indicating their derivation from the partial melting of metasedimentary rocks. This classification assumes that granitic rocks can be easily distinguished by their source materials. However, some granites may form from the partial melting of multiple sources, including both crustal and mantle materials. TheGejiu granites exhibited Nb/Ta ratios similar to those of highly evolved, mineralized granites [33]. A-type granites are a kind of granitic rocks that are rich in alkali (K₂O + Na₂O) and have a high Fe/Mg ratio and low water content. The geochemical characteristics of A-type granites are usually characterized by high silicon content: SiO₂ is typically >70%, but lower than highly differentiated I/S-type granites (such as haplogranite); low aluminum saturation: A/CNK is mostly <1.1, belonging to sub-aluminous to weakly peraluminous, which is significantly different from S-type (peraluminous) granites; rich in alkali (Na + K): K₂O + Na₂O > 8%, and the K₂O/Na₂O ratio is variable (can be >1 or <1). The study area is composed of a shoshonite series (gabbro, diorite, syenite, and monzonite) and high-silica granites (with aplitic granite—highly differentiated characteristics, such as the Laocang and Gaosong rock bodies). It should be noted that the major element composition and mineral assemblage of highly differentiated alkali feldspar granites tend to approach haplogranite [40], which may interfere with traditional genetic models.

The Gejiu granites exhibited a wide range of acidity (SiO₂: 64.21–77.96 wt%, with an average of 72.94%) and displayed characteristics typical of I-type granites. For instance, in the P₂O₅ vs. SiO₂ diagram, they show a negative correlation (correlation coeff r = −0.915), consistent with the evolutionary trend of I-type granites. This is because apatite is a preferentially crystallizing mineral in metaluminous-to-peraluminous and peralkaline granitic magmas, and the P content of the residual magma decreases with apatite fractional crystallization. Consequently, the P₂O₅ content of I-type granites decreases with increasing SiO₂ content [41,42,43]. However, these granites had high total alkali contents (average K₂O + Na₂O value of 8.50, greater than 7.5%). The A/CNK values ranged from 0.87 to 1.96 (average: 1.07). They were metaluminous to weakly peraluminous (except for two samples from the Jiasha and Gaosong areas, which were strongly peraluminous). According to the classification (I-type/A-type granites A/CNK < 1.1, S-type granites A/CNK > 1.1 [42]), The Gejiu granites mainly exhibit the characteristics of A-type and I-type. Moreover, most Gejiu granites (except those from Longchahe and Shenshui areas) exhibited low contents of Zr + Nb + Y + Ce (<350 ppm) and relatively low 10,000 × Ga/Al ratios. Therefore, it suggests a type A and type I hybrid composition for the granites of the Gejiu complex. The TsatZr values of the Laochang and Gaosong area granites are relatively low (with an average of 748.35 degrees), which is related to the highly differentiated characteristics of the granites in the two regions. The average values of FeOt/MgO and (Na₂O + K₂O)/CaO in the two regions are 5.24, 2.65, and 8.14, 2.98, respectively. It is worth noting that, when viewed comprehensively, the 13–10 sample from the Jiasha area (with an A/CNK value of 1.16, a FeOT/(FeOT + MgO) value of 0.86, and a δEu value of 0.61) and the sample from the Gaosong area (with an A/CNK value of 1.16, a FeOT/(FeOT + MgO) value of 0.73, and a δEu value of 0.06), respectively, also exhibit characteristics of S-type and strongly altered S-type granites.

As mentioned earlier, the contemporaneous occurrence of abundant mantle-derived magmas highlights the significant contribution of mantle materials to the Gejiu granites [8]. However, high concentrations of trace elements such as Th (43.92, >10 ppm), Pb (78.72, >20 ppm), and U (12.37, >2 ppm) suggest that the formation of these granites involved crustal components, indicating a crustal signature [44]. Previous studies have used the Ce/Yb vs. Eu/Yb discrimination diagram to identify the genetic types of granites, proposing that a linear relationship indicates significant mixing characteristics [45]. The Gejiu granites exhibited an excellent linear relationship in this diagram (Figure 9a). Additionally, the presence of numerous mafic enclaves in the granites further supports the occurrence of magma mixing during their evolution [46]. Sisson (2005) [47] proposed three petrogenetic models for I-type granites: (1) the mixing of mantle-derived and crust-derived magmas; (2) the contamination of mantle-derived magmas by crustal materials [48]; and (3) the simultaneous melting of pre-existing amphibolites (with basaltic protoliths) and metasedimentary rocks in the basement [43]. Considering the evidence for the occurrence of magma mixing during the evolution of the Gejiu granites, this study suggests that their formation involved the mixing of mantle-derived and crust-derived magmas.

The formation of supergiant granitic bodies requires not only a substantial supply of materials but also significant heat. The additional heat generated by the melting of the lithospheric mantle is sufficient to cause partial melting of crustal rocks [10]. Studies [49,50,51] have shown that the initial magma temperatures of the Late Cretaceous granites in the Gejiu area ranged from 806 °C to 835 °C, indicating that the primary magmas of the Gejiu granites were products of high-temperature partial melting. The dehydration melting of biotite, i.e., the decomposition reaction of biotite, often generates substantial heat (~800 °C or higher). In the Pb-Ba discrimination diagram (Figure 9b), Pb is an order of magnitude lower than Ba, reflecting melts derived from biotite decomposition rather than low-temperature muscovite decomposition, as Pb is more likely to enter muscovite than biotite [52,53]. Therefore, the primary magmas of the Gejiu granites were likely generated by the decomposition reaction of biotite.

Figure 9. (a) Ce/YB-EU/Yb diagram of Gejiu [10,54]. (b) Whole-rock Ba-Pb diagram of Gejiu (The legend is consistent with that of Figure 8).

Figure 9. (a) Ce/YB-EU/Yb diagram of Gejiu [10,54]. (b) Whole-rock Ba-Pb diagram of Gejiu (The legend is consistent with that of Figure 8).

(2) Genesis of Granitic Enclaves

The composition of the granitic enclaves in the Gejiu granites ranged between gabbro and granite (Figure 10). Various trends suggest that these enclaves result from the mixing of two magma components: gabbro-related mafic magmas and granitic magmas represented by other granite samples. Specifically, the Longchahe and Jiasha granitic enclaves were formed by the mixing of mantle-derived mafic magmas and peralkaline granitic magmas. The granitic enclaves exhibited relative enrichment of incompatible trace elements, such as Nb, which was present in higher concentrations in the enclaves compared to gabbro samples (Figure 10). This enrichment may be attributed to the lower degree of partial melting of the mantle-derived mafic magmas within the enclaves.

Watson and Jurewicz [55] proposed that large-scale interdiffusion of elements can occur between crustal felsic and mantle-derived mafic melts. During the mixing process, certain elements may be sequestered into minerals due to their slower diffusion rates, leading to the formation of minerals such as amphibole, ilmenite, and apatite [56]. These minerals are abundant in the Gejiu granites and their enclaves. Therefore, this study proposes a two-stage magmatic evolution model to explain the formation of the Gejiu granites and their enclaves: (1) In the early stage, mafic magmas derived from the lithospheric mantle mixed with crustal melts to form granitic magmas. This stage involved both bulk physical mixing and interdiffusion of elements. (2) In the later stage, the mixing of mafic magmas with peralkaline granitic magmas resulted in the formation of the granitic enclaves in the Gejiu area.

5.2.2. Genesis of Gabbro

The Gejiu gabbro exhibited high total rare earth element (∑REE) contents and was strongly enriched in light rare earth elements (LREEs). Its Nb/Ta ratios (average: 21.77) were close to those of primitive mantle and chondrites, indicating a mantle-derived source for the magmas. The La/Nb (0.64–12.75, average: 5.19) and La/Ta (16.85–148.94, average: 118.94) ratios were consistent with basaltic magmas (La/Nb > 1.5, La/Ta > 22), ruling out the possibility of crustal partial melting [57]. Li et al. [58] emphasized that the formation of intermediate-mafic rocks and alkaline rocks is closely related to mantle-derived materials. Furthermore, the high content of hydrous minerals (e.g., amphibole and biotite) in the Gejiu gabbro (Figure 3c) and its trace element characteristics, including enrichment in large-ion lithophile elements (LILEs) and LREEs and depletion in high-field-strength elements (HFSEs, e.g., Nb, Ta, Ti, P) (Figure 5), further suggest that the gabbro originated from an enriched lithospheric mantle rather than the asthenospheric mantle. Therefore, this study concludes that the Gejiu gabbro is a product of fractional crystallization of basaltic parental magmas derived from the lithospheric mantle.

Tang Liwei [5] demonstrated that the gabbro exhibits heterogeneous Sr-Nd isotopic characteristics (initial ⁸⁷Sr/⁸⁶Sr = 0.7096–0.7111, εNd = −7.9 to −5.3), indicating crustal contamination during magma evolution. Combined with the gabbro’s high aluminum saturation index (ASI), it is inferred that the mantle-derived magmas were contaminated by crustal materials during their ascent, leading to an increase in aluminum content. Thus, the Gejiu gabbro experienced crustal contamination during its evolution.

5.2.3. Genesis of Intermediate Rocks (Diorite and Monzonite)

The genetic model for intermediate rocks is similar to that of gabbro, with two primary mechanisms proposed in previous studies [59,60,61]: (1) partial melting of mafic-to-intermediate rocks in the lower crust, and (2) crustal contamination of mantle-derived mafic magmas. Research has shown that magmas derived from the partial melting of felsic crust typically exhibit high SiO₂ and low MgO contents [57]. In contrast, the Gejiu intermediate rocks were characterized by relatively low SiO₂, high MgO, and enriched Cr and Ni. Their Nb/Ta ratios (average: 25.55) were close to those of primitive mantle and chondrites (17.5 ± 2.0), significantly differing from rocks formed by crustal partial melting, indicating a mantle-derived source. Additionally, the enrichment of Ba and light rare earth elements (LREEs), as well as the crystallization of K-feldspar and plagioclase (Figure 3), suggests the incorporation of crustal components. Therefore, the Gejiu intermediate rocks are proposed to have originated from mantle-derived mafic magmas that experienced significant crustal contamination.

Bergantz (1989) [62] proposed that basaltic mafic magmatism can trigger the generation of felsic magmas in the lower crust, accompanied by assimilation and mixing processes. Castro and Gerya [63] demonstrated that the interaction of felsic melts with peridotite can form orthopyroxene-rich reaction zones. Dorfler’s study [57,61] of the Cortlandt complex further confirmed that even at temperatures below 900 °C, mantle-derived basaltic magmas can induce partial melting of lower crustal rocks, forming hybrid monzodiorite. Therefore, it is inferred that the Gejiu intermediate rocks resulted from the selective contamination of mantle-derived mafic magmas by crustal felsic magmas. During the Early Paleozoic, the subduction of oceanic slabs led to the formation of juvenile lower crust, which reacted with basaltic magmas, causing widespread partial melting of lower crustal rocks. This process ultimately formed the Gejiu diorite and monzonite, a genesis consistent with that of the monzonite in the Cortlandt complex of New York [60,61].

5.2.4. Genesis of Alkaline Rocks (Syenite)

The formation mechanisms of alkaline rocks primarily include (1) partial melting of felsic crustal materials under high pressure; (2) contamination of mantle-derived basaltic magmas by crustal granitic magmas; and (3) fractional crystallization of mantle-derived alkaline magmas [64]. The Gejiu syenite was characterized by low SiO₂, Fe₂O₃T, TiO₂, Cr, and Ni contents, strong enrichment in light rare earth elements (LREEs), and high Nb/Ta ratios (18.15–25.79, average: 18.24). These characteristics rule out the possibility of partial melting of felsic crustal materials [65], suggesting a likely origin from an enriched lithospheric mantle. Additionally, the enrichment of Ba and LREEs indicates the incorporation of crustal components. Therefore, similar to the mafic gabbro and intermediate monzodiorite, the Gejiu alkaline syenite is proposed to be a hybrid crust–mantle magma derived from enriched mantle parental magmas through fractional crystallization and subjected to limited crustal contamination.

5.3. Integrated Model of Magmatic Evolution in the Gejiu Region

The widespread magmatic activity in the Gejiu region necessitates the occurrence of an active tectonic event. From the perspective of regional tectonic history, this magmatic activity cannot be attributed to continental collision orogeny, as the collision between the Indian Plate and the Eurasian continent only began around 65 Ma [66,67]. The presence of the Laojunshan metamorphic core complex [68], rift basins [69,70], emplacement of the Kunlun Pass alkaline granite in Guangxi [71], and the discovery of late Yanshanian-to-early Himalayan mafic–alkaline rocks near the Gejiu granites are inconsistent with a compressional environment typical of intracontinental collisional orogeny. In the Y-Ce-Nb discrimination diagram (Figure 11), A1 represents magmas related to mantle plumes or rifts, while A2 indicates magmas associated with crustal extension at continental margins [38,72]. All samples from the Gejiu complex plot are located within the A1 field, suggesting that the Gejiu complex formed through the delamination of the thickened crust and lithosphere. This process involved the upwelling of magmas due to the intrusion of the asthenosphere into the lithospheric mantle, with the thickened crust likely being juvenile lower crust formed by earlier oceanic plate subduction. Combined with the earlier discussion on the non-compressional tectonic environment, these findings indicate that the lithosphere in this region experienced extension and thinning due to asthenospheric upwelling. Therefore, the Gejiu complex formed in a dynamic setting of lithospheric extension and thinning.

Lithospheric extension triggered the partial melting of the enriched lithospheric mantle, generating alkaline mafic magmas through mixing with the lower crust [73]. These mantle-derived magmas ascended, providing the heat necessary for partial melting of the lower crust [57,62]. The mixing of these two magma types formed the Gejiu granitic bodies, which is consistent with the genetic studies discussed earlier. The granitic melts were emplaced as plutons in the upper crust. The gabbro and diorite–monzonite originated from early mantle-derived basaltic magmas, which were subsequently contaminated by hydrous crustal magmas, forming diorite–monzonite similar to that found in the Cortlandt complex [59]. The granitic enclaves represent intermediate products of the mixing between basaltic mafic melts and granitic magmas formed during the emplacement of upper crustal plutons. Through mechanisms such as physical mixing, mineral crystallization, and elemental diffusion, the diverse rock types of the Gejiu complex were ultimately formed (Figure 12).

5.4. Influence of Magmatic Sources and Partial Melting Conditions on Initial Tin Enrichment

The heat from the upwelling asthenosphere triggered extensive melting of the lithospheric mantle and lower crust, leading to crust–mantle interactions. This process not only provided the thermal energy necessary for crustal partial melting but also introduced minor mantle-derived materials into the granitic melts, forming a bimodal magmatic association [74]. In addition to granites, coeval igneous rocks such as gabbro, diorite, monzonite, and alkaline rocks are widely distributed in the western Cathaysia Block [14]. These Late Cretaceous mafic, intermediate, and alkaline rocks are aligned with extensional structures, forming a volcanic rock assemblage controlled by lithospheric extension and asthenospheric upwelling, together with the granitic bodies [75].

Quantitative partial melting models by Zhao et al. [50] demonstrate that magmas generated by biotite dehydration–melting under high temperatures (>800 °C) can significantly enrich Sn, as seen in the Sn-bearing granites in the South China tungsten–tin metallogenic province and other global W-Sn metallogenic belts. As shown in Figure 9b, the primary magmas of the Gejiu granites were likely produced by biotite decomposition reactions, which are closely associated with biotite breakdown, as observed in Sn-bearing granites worldwide. The input of mantle-derived heat promoted biotite decomposition, releasing Sn into the magmas and resulting in the high enrichment of Sn in the Gejiu melts, laying the foundation for tin mineralization. Therefore, high-temperature melting and biotite decomposition reactions are critical for the formation of initial Sn-enriched granitic melts.

Furthermore, the delayed crystallization of biotite may further enhance tin enrichment. The final product of fractional crystallization in the Gejiu granites is equigranular biotite granite, indicating that biotite crystallized last. Biotite crystallization tends to incorporate Sn, significantly reducing the Sn content in the residual melt and weakening the ore-forming potential of the magma. Thus, the delayed crystallization of biotite prevented the premature sequestration of Sn, providing favorable conditions for hydrothermal tin mineralization and ultimately forming world-class tin deposits [76].

6. Conclusions

(1) The granites underwent a two-stage fractional crystallization process, with the magmatic evolution progressing from lowly evolved porphyritic granite to moderately evolved syenogranite, K-feldspar granite, and equigranular biotite granite, culminating in highly evolved equigranular biotite monzogranite. The final product of this evolutionary sequence is the equigranular biotite monzogranite.

(2) The basic gabbro, intermediate diorite, monzonite, and alkaline syenite in the Gejiu complex come from the same magmatic source region, while the source region of acid granite is different from that of the above rocks.

(3) The Gejiu granites mainly exhibit characteristics of both I-type and A-type granites. The gabbro, monzonite, diorite, and syenite were formed from mantle-derived parental magmas that experienced limited crustal contamination. The granites originated from the mixing of mantle-derived and crust-derived magmas, involving both physical mixing and interdiffusion of elements.

(4) The Gejiu complex formed in a dynamic setting of lithospheric extension and thinning. High-temperature melting and biotite decomposition reactions likely played a significant role in the hydrothermal tin mineralization process, ultimately contributing to the formation of world-class tin deposits.