Genesis of the Ke’eryin Two-Mica Monzogranite in the Ke’eryin Pegmatite-Type Lithium Ore Field, Songpan–Garze Orogenic Belt: Evidence from Lithium Isotopes

Abstract

Previous studies on the Ke’eryin pegmatite-type lithium ore field in the Songpan–Ganzi Orogenic Belt have explored the characteristics of the parent rock but have not precisely determined its magma source area. This uncertainty limits our understanding of the regularity of lithium ore formation in this region. In this study, to address the issue of the precise source area of the parent rock of lithium mineralization, a detailed analysis of the Li isotope composition of the ore-forming parent rock (Ke’eryin two-mica monzogranite) and its potential source rocks (Triassic Xikang Group metamorphic rocks) was conducted. The δ⁷Li values of the Ke’eryin two-mica monzogranite, Xikang Group metasandstone, and Xikang Group mica schist are −3.3–−0.7‰ (average: −1.43‰), +0.1–+6.9‰ (average: +3.83‰), and −9.1–0‰ (average: −5.00‰), respectively. The Li isotopic composition of the Ke’eryin two-mica monzogranite is notably different from the metasandstone and aligns more closely with the mica schist, suggesting that the mica schist is its primary source rock. The heavy Li isotopic composition of the two-mica monzogranite compared to the mica schist may have resulted from the separation of the peritectic garnet into the residual phase during the biotite dehydration melting process. Moreover, the low-temperature weathering of the source rocks may have been the main factor leading to the lighter lithium isotope composition of the Xikang Group mica schist compared to the metasandstone. Further analysis suggests that continental crust weathering and crustal folding and thickening play crucial roles in the enrichment of lithium during multi-cycle orogenies.

1. Introduction

Lithium, often termed “white petroleum”, has a pivotal role as an energy metal in the 21st century [1,2]. Following the launch of China’s geological survey of three rare (rare earth, rare metal, and rare dispersed metal) mineral resources in 2010, several pegmatite-type lithium ore fields have been identified in the Songpan–Ganzi Orogenic Belt (SGOB) of western China, including notable sites such as the Jiajika, Ke’eryin, and Zhawulong ore fields. These discoveries have made the SGOB a world-class lithium metallogenic belt [1,3,4]. Scholars have extensively studied the geological features [5,6,7,8,9,10], metallogenic epoch [9,11,12,13,14,15], ore-forming fluid evolution [6,16,17,18,19], and metallogenic mechanism [6,14,15,20,21] of these ore fields. The consensus among researchers is that the lithium-bearing pegmatites in these fields most likely originated from intense differentiation of spatially adjacent parent granites. Geochemical and Sr-Nd-Hf isotopic data [13,20,22,23,24,25] suggest that these parent granites may be derived from the Triassic Xikang Group sedimentary rocks. However, the composition of the Xikang Group is heterogeneous [26], and granitic magmas of almost all types in the SGOB bear the imprint of the Xikang Group [27]. The inability to precisely trace the origin of these ore-forming parent rocks constrains our understanding of the metallogenic regularity of these “endogenous” lithium deposits to a certain extent. This limitation also impedes the resolution of several scientific issues, including the metallogenic specificity of the granitic rock in the SGOB. Recent advancements in lithium isotope testing technology and theoretical insights offer new avenues for accurately tracing the magmatic sources of these granites.

Lithium has two isotopes, the lighter ⁶Li and the heavier ⁷Li, which are highly active [28] and exhibit a mass difference of approximately 17% in Earth samples [29,30,31]. These characteristics make lithium isotopes excellent tracers for various high-temperature and low-temperature geological processes such as partial melting, magmatic differentiation, hydrothermal alteration, fluid exsolution, and kinetic diffusion [31,32,33,34,35,36,37,38,39,40,41]. Variations in Li isotopic compositions between granites and various types of pegmatites [15,21,42], as well as between pegmatites and surrounding rocks [33,34,41], have been effectively utilized to elucidate the origins of pegmatites, their petrogenetic and metallogenic mechanisms, and diffusion effects. This highlights the significant research value of Li isotopes in the granite–pegmatite system. In contrast to traditional radiogenic isotopes like Sr-Nd-Hf, which broadly outline the characteristics of granite sources, Li isotopes can provide nuanced details that are essential for precise tracing [43,44,45]. This study analyzed the Li isotopic compositions of the ore-forming parent rocks, specifically the Ke’eryin pluton, as well as various end-members of the Xikang Group metasedimentary rocks within the Ke’eryin ore field. By integrating these results with published data, the precise magma source of the Ke’eryin pluton was constrained. This research contributes additional data that advance our understanding of the metallogenetic regularity of pegmatite-type lithium deposits within the Ke’eryin ore field and the broader SGOB.

2. Regional Geological Setting

The SGOB is situated in the eastern part of the Tibetan Plateau, comprising about one-fifth of the plateau. It represents an accretionary orogen that evolved from the passive continental margin on the western side of the Yangtze Craton due to the closure of the Paleo-Tethys Ocean [23,46,47,48]. The SGOB is bordered to the north by the North China Plate (East Kunlun–Qaidam–Qilian–Qinling block), to the west by the Qiangtang–Changdu block, and to the east by the Yangtze Plate. These borders are delineated by the East Kunlun–Animaqing suture zone, the Jinsha suture zone, and the Longmenshan fold and thrust belt, respectively (Figure 1a) [47,49,50,51,52,53]. The Neoproterozoic basement, primarily a crystalline complex aged between 1.0 Ga and 1.5 Ga, is sporadically exposed in the eastern part of the SGOB and is believed to share attributes with the Yangtze basement [47,51,54]. The main constituents of the SGOB’s sedimentary cover are the Triassic Xikang Group strata, a group which comprises a series of Middle to Upper Triassic flysch formations that developed across multiple depositional centers [47,55]. The Triassic Xikang Group predominantly comprises regionally metamorphosed gray-black quartz-sandstones, siltstones, slates, phyllites, schists, and minor amounts of marble [26,55]. During the late Indosinian orogeny (<ca. 230 Ma [56]), the closure of the Paleo-Tethys Ocean initiated the formation of the Songpan–Ganzi accretionary orogenic wedge through bidirectional subduction along the East Kunlun–Animaqing and Jinsha suture zones. Simultaneously, the Xikang Group experienced substantial thickening—ranging from 5 to 15 km—along with significant deformation and metamorphism, leading to the creation of NW- and NNW-oriented “Xikang-style” folds [4,46,56]. During the orogenic processes, granitic rocks intruded extensively between 227 and 196 Ma, manifesting a wide range of compositions including calc-alkaline I-type granites, peraluminous S-type granites, and alkaline A-type granites [13,47]. These granites were derived from varied sources, such as the Neoproterozoic metamorphic basement, Mesozoic metamorphic sedimentary rocks, and in some cases mantle materials [23,24,27,57,58,59,60]. Granite intrusions associated with pegmatites are typically peraluminous S-type granites and are often enveloped by metamorphic sedimentary rocks, forming gneiss domes such as Jiajika, Changzheng, Rongxuka, and Markam [4,60,61,62,63]. These domes frequently host pegmatite-type lithium deposits within their mantles. In recent years, significant lithium exploration breakthroughs, particularly in the Jiakika, Ke’eryin, and Zhawulong areas of the eastern belt, and the Bailongshan area in the western part, have positioned the SGOB as the largest concentration area for hard-rock-type lithium resources in China [4,8,64,65].

3. Geology of the Ke’eryin Ore Field

The primary strata exposed in the Ke’eryin ore field are part of the Triassic Xikang Group, including the Middle Triassic Zagunao Formation (T₂z), the Upper Triassic Zhuwo Formation (T₃zh), the Upper Triassic Xinduqiao Formation (T₃xd), and the Upper Triassic Luokongsongduo Formation (T₃lk) (Figure 1b). These formations have been subjected to the greenschist facies regional metamorphism and thermal contact metamorphism. The regional metamorphism is characterized by the widespread development of biotite, chlorite, and garnet, while the thermal contact metamorphism is marked by the development of a sillimanite–kyanite zone, a garnet–staurolite zone, and a biotite–andalusite zone radiating outward from the Ke’eryin pluton [61]. Additionally, near the pegmatite veins, there is notable formation of cordierite and extensive muscovitization and tourmalinization. The rocks within these formations can be divided into two end-members: those derived from clay-rich protoliths and those derived from clay-poor protoliths, which are represented by metamorphic sandstone and mica schist, respectively. The Ke’eryin pluton intrudes along the axis of the Ke’eryin anticline into the Triassic sedimentary strata, forming an irregular triangular outcrop that covers an area of approximately 250 km². The pluton comprises multiple lithofacies (Figure 1b), including biotite–K-feldspar granite [Qtz + Kfs + Bi ± Pl ± Ms ± Tur], two-mica monzogranite [Qtz + Kfs + Pl + Bi + Ms ± Tur ± Grt ± Chl], and muscovite–albite granite [Qtz + Pl (Ab) + Ms ± Tur ± Grt]. The primary lithofacies, the two-mica monzogranite, is further differentiated into fine-grained and medium-grained facies based on mineral composition and grain size (Figure 1b). The medium-grained facies, which contains a higher proportion of muscovite and less biotite, exhibits a greater degree of magmatic differentiation [15]. The muscovite albite granite is only locally exposed in the southeastern part of the Ke’eryin ore field and has not been found in other areas. Additionally, less than a kilometer to the southwest of the Ke’eryin pluton, the Taiyanghe pluton is exposed, consisting of quartz diorite and biotite monzogranite (Figure 1b). This pluton has been confirmed to have a magmatic source distinct from that of the Ke’eryin pluton [23,24]. Previous studies have categorized the pegmatites within the Ke’eryin ore field into approximately five types based on the compositions of the main rock-forming and rare metal minerals [7,68]. On a field scale, these different types of pegmatites form distinct regional zones that radiate outward from the Ke’eryin pluton. Within a 5 km radius from the pluton, the pegmatites are arranged sequentially as follows: microcline pegmatite [Mc + Qtz + Ms ± Ab ± Bi ± Tur], microcline–albite pegmatite [Mc + Ab + Qtz + Ms ± Tur] (MAP), albite pegmatite [Ab + Qtz + Ms ± Mc ± Spd ± Tur], albite–spodumene pegmatite [Ab + Qtz + Spd + Ms ± Mc ± Tur], and albite–lepidolite pegmatite [Ab + Qtz + Lpd ± Spd ± Mc ± Ms]. Typically, the pegmatite veins within the ore field occur as veins or lenses, ranging in length from tens to hundreds of meters, though a few extend over a kilometer. Most veins intersect the Triassic strata, while some align with the stratification. The pegmatites are often clustered, with groups of albite–spodumene pegmatite forming large to super-large lithium deposits at sites like Dangba, Lijiagou, Sizemu, Yelonggou, Guanyinqiao, and Jiada (Figure 1b). Based on their close spatio-temporal association [6,7,69] and the continuous geochemical and Li isotopic composition variations [6,15], the two-mica monzogranite of the Ke’eryin pluton is considered the parent rock of the pegmatites within the Ke’eryin ore field.

4. Samples and Analytical Method

Samples of the metasedimentary rocks of the Xikang Group were collected from its two end-members: metasandstone and mica schist. The metasandstone (Figure 2a,d,g,j;

Table 1. Mineralogical features of metasandstone and mica schist of the Triassic Xikang Group and the Ke’eryin two-mica monzogranite.

Lithology	Metasandstone		Mica Schist		Two-Mica Monzogranite
Major minerals	Quartz	60–80 vol%	Quartz	50–70 vol%	Quartz	30–40 vol%
	Biotite	10–15 vol%	Biotite	25–35 vol%	K-feldspar	20–25 vol%
	Plagioclase	3–5 vol%	Muscovite	3–5 vol%	Plagioclase	25–30 vol%
	Muscovite	1–3 vol%	Plagioclase	3–5 vol%	Biotite	5–8 vol%
	Staurolite	1–3 vol%	Staurolite	1–3 vol%	Muscovite	5–8 vol%
	Amphibole	1–2 vol%	Cordierite	1–3 vol%	Tourmaline	1–2 vol%
	Diopside	1–2 vol%	Andalusite	1–3 vol%
			Amphibole	1–2 vol%
			Diopside	1–2 vol%
Accessory minerals	Zircon, apatite, tremolite, wollastonite, sillimanite, chlorite, epidote		Zircon, apatite, tremolite, wollastonite, scapolite, idocrase, sillimanite, chlorite, epidote		Zircon, apatite, cassiterite, ilmenite, topaz
Texture	Granoblastic texture		Granoblastic texture		Inequigranular seriate hypidiomorphic texture

) samples were characterized by a dark gray color and a granoblastic texture, primarily consisting of quartz (60%–80%), biotite (10%–15%), plagioclase (3%–5%), muscovite (1%–3%), staurolite (1%–3%), amphibole (1%–2%), and diopside (1%–2%). In contrast, the mica schist samples (Figure 2b,e,h,k; Table 1), which were black to dark gray with a granoblastic texture, contained quartz (50%–70%), biotite (25%–35%), muscovite (3%–5%), plagioclase (3%–5%), staurolite (1%–3%), andalusite (1%–3%), cordierite (1%–3%), amphibole (1%–2%), and diopside (1%–2%). Notably, both the mica schist and metasandstone samples exhibited large quantities of tourmaline and muscovite near the pegmatite veins (Figure 2l). The two-mica monzogranite (Figure 2c,f,i; Table 1) exhibited a grayish-white color with an inequigranular seriate hypidiomorphic texture and was mainly composed of quartz (30%–40%), K-feldspar (20%–25%), plagioclase (25%–30%), biotite (5%–8%), muscovite (5%–8%), and tourmaline (1%–2%). To minimize chemical interference, these samples were intentionally collected at distances greater than 50 m from pegmatite veins.

Whole-rock trace element testing was conducted at the Southwest Metallurgical Geology Testing Center using an XSeries II Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The analytical precision for these tests was reported to be better than 5%. For detailed procedures regarding sample analysis and data processing, refer to Qi et al. [70]. Whole-rock lithium isotope analysis was performed at the MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS. Lithium was extracted using three cation-exchange columns filled with AG50W-X8 200-400-mesh resin (Bio-Rad, Berkeley, CA, USA). The lithium isotope ratios were measured using a Neptune plus Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) (Thermofisher, Waltham, MA, USA). Mass fractionation was calibrated using the standard sample-blank (SSB) method. The analytical process was thoroughly detailed in Tian et al. [71]. The lithium isotope composition was expressed using delta notation as δ⁷Li (‰) and was calculated using the following formula: δ⁷Li (‰) = [(⁷Li/⁶Li)_sample /(⁷Li/⁶Li)_IRMM-016 − 1] × 1000. To optimize the analysis quality, the δ⁷Li values of two international standards, basalt sample BHVO-2 and andesite sample AGV-2, were measured, resulting in δ⁷Li values of +4.3 ± 0.9‰ (2SD, n = 9) for BHVO-2 and +6.2 ± 0.6‰ (2SD, n = 9) for AGV-2. These results were consistent with values that were previously published by Moriguti and Nakamura [72] and Pennston-Dorland et al. [73], fitting within the expected analytical errors.

5. Results

The trace elements and Li isotopic compositions of the two-mica monzogranite, metasandstone, and mica schist samples from the Ke’eryin ore field are detailed in Table 2 and Table 3. Compared to the upper continental crust (UCC [74]), the two-mica monzogranite samples were notably enriched with rare metal elements such as Li, Be, Rb, Ta, and Cs, and depleted of Ba, Sr, Zr, and Ti (Figure 3a). The metasandstone and mica schist samples, relative to the UCC, were enriched with certain rare metal elements like Li, Be, and Cs but exhibited slight depletions of Rb and Ba (Figure 3a). Regarding the Li contents, a progressive increase was observed from metasandstone (average: 84.30 ppm) to mica schist (average: 127.60 ppm) and two-mica monzogranite (average: 163.54 ppm). The two-mica monzogranite samples displayed LREE enrichment distribution patterns (Figure 3b), aligning with observations of the metasandstone and mica schist samples. However, the fractionation of LREEs and HREEs in the two-mica monzogranite samples was more pronounced than that in the metasandstone and mica schist samples. The δ⁷Li values of the two-mica monzogranite samples ranged from −3.3‰ to −0.7‰ (average: −1.43‰). In comparison, the metasandstone samples had a heavier Li isotopic composition (the δ⁷Li values ranged from +0.1‰ to +6.9‰, with average of +3.83‰), while the mica schist samples had a lighter Li isotopic composition (the δ⁷Li values ranged from −9.1‰ to 0‰, with average of −5.00‰) (Figure 4).

Table 2. Trace element compositions of metasandstone and mica schist of the Triassic Xikang Group and the Ke’eryin two-mica monzogranite.

Sample	Lithology	Li	Be	Sn	Nb	Ta	Rb	Cs	Ba	Bi	Cd	Co	Cr	Ga	Ge	Hf
		μg/g
23RMS2	Meta- sandstone	77.99	9.05	73.45	15.85	1.26	201.75	40.08	711.57	0.18	3.15	13.42	68.73	10.45	4.41	7.68
23RMS3		50.16	6.49	22.66	9.79	0.72	7.27	9.47	20.62	0.12	1.40	20.92	43.05	17.93	2.50	3.08
23RMS4		41.94	3.51	14.20	12.12	0.90	2.59	1.45	49.49	0.12	0.86	8.97	53.29	14.05	2.60	6.20
23RMS6		80.47	4.09	35.12	13.93	1.02	63.89	52.98	181.48	0.13	1.55	9.25	71.99	14.26	3.24	9.70
23RMS9		177.92	2.61	4.03	13.75	1.07	67.12	73.15	176.39	0.27	0.47	11.75	61.40	20.06	2.70	7.78
23RMS10		77.30	11.25	36.43	13.35	1.12	73.81	35.43	176.04	0.14	1.60	10.96	62.11	18.58	3.30	16.60
23RMS1	Mica schist	173.47	4.11	6.22	19.64	1.33	172.59	26.75	479.37	0.62	0.56	18.23	104.29	28.91	2.63	4.65
23RMS5		80.54	2.69	4.71	17.00	1.53	141.73	17.72	364.21	0.22	0.41	78.39	77.29	22.63	2.15	6.62
23RMS7		165.24	2.73	20.45	16.98	1.13	67.90	9.63	245.39	0.13	1.28	15.81	77.57	20.25	2.60	7.24
23RMS8		109.31	3.50	24.20	14.07	0.97	66.23	18.50	152.84	0.24	1.42	11.58	71.65	19.80	3.07	7.76
23RMS11		140.50	4.56	13.80	15.07	0.95	7.75	11.32	18.34	0.83	1.19	16.82	66.10	20.93	4.84	7.84
21FTG-1	Two-mica monzo granite	337	9.9	-	15.7	2.28	420	63.3	250	1.26	0.025	1.05	3.05	21.8	-	3.49
21FTG-2		178	6.57	-	15.3	2.1	337	19	416	1.07	0.098	1.42	4.67	22.4	-	3.4
21FTG-3		110	2.84	-	14	1.71	250	12	201	0.325	0.037	1.3	2.52	16.3	-	3.58
21FTG-4		95.3	3.11	-	16.2	1.27	339	11.6	254	0.243	0.059	1.32	4.76	22	-	4.03
21FTG-5		93.4	7.82	-	15.7	2.07	357	19.5	208	0.793	0.178	0.858	7.14	22.9	-	2.96
21FTG-6		115	3.86	-	18.2	1.81	312	14.3	272	0.488	0.05	1.34	3.73	21	-	3.58
21MTG-1		256	8.31	-	19.7	3.2	377	35.1	105	2.26	0.121	0.705	4.08	21.7	-	2.42
21MTG-2		236	11.8	-	22	4.16	414	51.7	135	1.62	0.075	1.09	3.43	22.3	-	2.56
21MTG-3		97.7	3.68	-	18	1.71	302	10.2	127	4.14	0.064	0.847	5.66	22.5	-	2.39
21MTG-4		117	3.72	-	20.5	2.01	328	16.6	145	1.22	0.09	1.1	8.42	23.2	-	2.61
Sample	Litho- logy	Mn	Mo	Ni	Pb	Sb	Sc	Sr	Th	Ti	Tl	U	V	W	Zn	Zr
		μg/g
23RMS2	Meta- sandstone	1251.59	0.52	13.42	30.40	6.90	14.03	283.78	17.10	4187.41	0.90	3.66	82.13	2.44	84.38	276.59
23RMS3		2584.99	0.38	20.92	19.60	15.13	11.76	679.00	8.35	2653.14	0.03	2.31	51.02	77.43	54.76	110.11
23RMS4		1808.64	0.19	8.97	10.67	12.45	11.83	538.83	13.13	3548.75	0.01	3.05	58.98	2.02	55.52	217.82
23RMS6		809.37	1.21	9.25	19.39	10.48	11.96	372.38	17.94	4032.09	0.32	4.06	62.06	4.14	57.16	355.45
23RMS9		2422.87	0.70	11.75	16.43	8.42	14.54	499.38	15.38	4066.77	0.38	3.94	69.16	2.14	73.58	270.88
23RMS10		915.55	0.29	10.96	19.47	13.66	11.53	575.56	20.90	3751.11	0.36	5.19	57.77	3.79	69.96	618.95
23RMS1	Mica schist	1739.57	0.31	18.23	32.42	11.74	23.99	435.90	19.81	5109.92	0.81	4.94	150.66	2.14	125.86	155.40
23RMS5		653.60	0.59	78.39	25.73	9.94	17.75	342.50	17.50	4964.95	0.68	3.94	90.52	251.45	96.45	240.87
23RMS7		4432.06	0.44	15.81	32.03	14.98	16.26	797.56	16.94	4927.16	0.29	3.96	82.82	2.05	87.74	265.74
23RMS8		1570.70	1.02	11.58	22.58	12.87	14.04	382.96	16.29	3879.00	0.29	3.98	66.67	2.61	93.11	284.86
23RMS11		2804.22	1.37	16.82	10.69	12.15	14.41	320.46	14.76	4287.73	0.03	3.92	75.46	1.42	106.77	281.79
21FTG-1	Two-mica monzo granite	271.06	0.201	1.27	39.1	0.04	1.9	84.2	20.2	1042.85	2.61	3.51	5.09	-	65.8	99.9
21FTG-2		456.93	0.495	1.47	37.6	0.06	1.71	117	18.7	1060.83	2.09	2.02	6.78	-	62.2	100
21FTG-3		333.02	0.346	1.74	46.1	0.07	2.29	81.3	19.8	803.12	1.34	4.44	5.07	-	34.4	90
21FTG-4		348.51	0.161	1.16	38.6	0.04	2.63	69.2	25.5	1072.82	1.9	2.57	5.8	-	58	111
21FTG-5		302.04	0.124	0.981	41.3	0.03	1.91	70.4	13	815.10	2.06	2.81	4.14	-	63.7	78.6
21FTG-6		402.72	0.275	0.863	43.9	0.09	2.33	91.2	19.8	1000.90	1.8	3.6	5.1	-	60.6	103
21MTG-1		286.55	0.171	0.818	45.4	0.05	2.17	54.1	7.3	485.47	2.36	2.68	3.35	-	54.1	54.1
21MTG-2		379.48	0.215	1.37	36.2	0.05	1.9	49.6	6.4	641.30	2.42	2.55	3.84	-	59.2	64.6
21MTG-3		325.27	0.177	0.979	35	0.06	1.83	46.4	5.94	539.41	1.7	2.95	3.51	-	57.5	62.7
21MTG-4		333.02	0.178	1.38	41.5	0.06	2.67	56.6	8.05	707.22	2	3.72	4.48	-	56.6	72.3
Sample	Litho- logy	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Y
		μg/g
23RMS2	Meta sandstone	35.56	104.00	11.25	37.03	7.82	1.86	6.74	1.09	5.14	1.18	3.02	0.49	2.73	0.50	36.30
23RMS3		23.31	67.44	7.49	25.59	6.02	1.61	5.41	0.99	4.90	1.13	2.83	0.45	2.51	0.44	37.47
23RMS4		30.65	86.64	9.45	30.11	5.94	1.48	5.28	0.85	3.91	0.90	2.34	0.38	2.16	0.39	28.19
23RMS6		41.21	117.79	12.51	39.52	7.68	1.54	6.82	1.07	4.80	1.09	2.80	0.46	2.63	0.50	32.53
23RMS9		34.32	98.27	10.82	35.23	7.24	1.81	6.34	1.02	4.76	1.09	2.87	0.49	2.86	0.52	33.60
23RMS10		38.47	109.31	12.08	38.87	7.67	1.50	6.81	1.07	4.95	1.16	3.06	0.51	2.95	0.56	38.09
23RMS1	Mica schist	42.41	122.86	13.00	41.39	8.31	1.86	7.48	1.22	5.79	1.37	3.58	0.60	3.44	0.61	42.65
23RMS5		38.83	113.16	12.07	38.78	7.77	1.80	6.96	1.10	4.99	1.14	2.93	0.48	2.71	0.49	35.18
23RMS7		38.80	110.12	11.64	37.34	7.50	1.88	6.94	1.13	5.27	1.20	3.15	0.52	2.97	0.53	40.26
23RMS8		37.03	104.34	11.10	35.09	7.04	1.66	6.30	0.99	4.50	1.01	2.61	0.42	2.44	0.43	32.00
23RMS11		31.44	91.35	9.96	32.72	7.22	1.75	6.55	1.07	4.80	1.07	2.69	0.43	2.39	0.44	34.38
21FTG-1	Two-mica monzo granite	33.60	63.90	7.03	25.90	5.16	0.44	3.40	0.55	2.22	0.30	0.78	0.10	0.59	0.08	9.95
21FTG-2		34.00	59.40	6.16	22.00	4.18	0.57	2.87	0.45	1.94	0.29	0.79	0.11	0.64	0.09	9.1
21FTG-3		24.20	45.30	5.09	18.30	4.44	0.40	3.37	0.79	4.10	0.75	2.05	0.33	1.94	0.27	25.9
21FTG-4		37.60	75.80	8.65	32.30	6.41	0.43	4.21	0.72	2.91	0.43	0.99	0.14	0.83	0.11	12.3
21FTG-5		23.50	45.00	5.17	17.50	4.26	0.44	2.97	0.56	2.44	0.31	0.72	0.10	0.58	0.08	10.7
21FTG-6		32.50	60.00	6.70	24.40	5.27	0.54	3.88	0.68	2.90	0.41	1.05	0.15	0.86	0.11	13.3
21MTG-1		12.00	24.00	2.63	9.43	2.53	0.27	2.04	0.48	2.62	0.47	1.00	0.16	0.94	0.12	14.3
21MTG-2		10.80	23.80	2.43	9.01	2.59	0.27	2.04	0.50	2.50	0.42	0.89	0.13	0.84	0.09	13.5
21MTG-3		11.70	21.50	2.30	8.67	2.29	0.24	1.73	0.41	2.04	0.38	0.87	0.15	0.84	0.11	12.3
21MTG-4		12.90	24.20	2.90	11.00	3.24	0.27	2.60	0.68	3.32	0.55	1.22	0.19	0.92	0.12	19.5

Note: data for the metasandstone are taken from [75]; data for the two-mica monzogranite are taken from [15].

Table 2. Trace element compositions of metasandstone and mica schist of the Triassic Xikang Group and the Ke’eryin two-mica monzogranite.

Sample	Lithology	Li	Be	Sn	Nb	Ta	Rb	Cs	Ba	Bi	Cd	Co	Cr	Ga	Ge	Hf
		μg/g
23RMS2	Meta- sandstone	77.99	9.05	73.45	15.85	1.26	201.75	40.08	711.57	0.18	3.15	13.42	68.73	10.45	4.41	7.68
23RMS3		50.16	6.49	22.66	9.79	0.72	7.27	9.47	20.62	0.12	1.40	20.92	43.05	17.93	2.50	3.08
23RMS4		41.94	3.51	14.20	12.12	0.90	2.59	1.45	49.49	0.12	0.86	8.97	53.29	14.05	2.60	6.20
23RMS6		80.47	4.09	35.12	13.93	1.02	63.89	52.98	181.48	0.13	1.55	9.25	71.99	14.26	3.24	9.70
23RMS9		177.92	2.61	4.03	13.75	1.07	67.12	73.15	176.39	0.27	0.47	11.75	61.40	20.06	2.70	7.78
23RMS10		77.30	11.25	36.43	13.35	1.12	73.81	35.43	176.04	0.14	1.60	10.96	62.11	18.58	3.30	16.60
23RMS1	Mica schist	173.47	4.11	6.22	19.64	1.33	172.59	26.75	479.37	0.62	0.56	18.23	104.29	28.91	2.63	4.65
23RMS5		80.54	2.69	4.71	17.00	1.53	141.73	17.72	364.21	0.22	0.41	78.39	77.29	22.63	2.15	6.62
23RMS7		165.24	2.73	20.45	16.98	1.13	67.90	9.63	245.39	0.13	1.28	15.81	77.57	20.25	2.60	7.24
23RMS8		109.31	3.50	24.20	14.07	0.97	66.23	18.50	152.84	0.24	1.42	11.58	71.65	19.80	3.07	7.76
23RMS11		140.50	4.56	13.80	15.07	0.95	7.75	11.32	18.34	0.83	1.19	16.82	66.10	20.93	4.84	7.84
21FTG-1	Two-mica monzo granite	337	9.9	-	15.7	2.28	420	63.3	250	1.26	0.025	1.05	3.05	21.8	-	3.49
21FTG-2		178	6.57	-	15.3	2.1	337	19	416	1.07	0.098	1.42	4.67	22.4	-	3.4
21FTG-3		110	2.84	-	14	1.71	250	12	201	0.325	0.037	1.3	2.52	16.3	-	3.58
21FTG-4		95.3	3.11	-	16.2	1.27	339	11.6	254	0.243	0.059	1.32	4.76	22	-	4.03
21FTG-5		93.4	7.82	-	15.7	2.07	357	19.5	208	0.793	0.178	0.858	7.14	22.9	-	2.96
21FTG-6		115	3.86	-	18.2	1.81	312	14.3	272	0.488	0.05	1.34	3.73	21	-	3.58
21MTG-1		256	8.31	-	19.7	3.2	377	35.1	105	2.26	0.121	0.705	4.08	21.7	-	2.42
21MTG-2		236	11.8	-	22	4.16	414	51.7	135	1.62	0.075	1.09	3.43	22.3	-	2.56
21MTG-3		97.7	3.68	-	18	1.71	302	10.2	127	4.14	0.064	0.847	5.66	22.5	-	2.39
21MTG-4		117	3.72	-	20.5	2.01	328	16.6	145	1.22	0.09	1.1	8.42	23.2	-	2.61
Sample	Litho- logy	Mn	Mo	Ni	Pb	Sb	Sc	Sr	Th	Ti	Tl	U	V	W	Zn	Zr
		μg/g
23RMS2	Meta- sandstone	1251.59	0.52	13.42	30.40	6.90	14.03	283.78	17.10	4187.41	0.90	3.66	82.13	2.44	84.38	276.59
23RMS3		2584.99	0.38	20.92	19.60	15.13	11.76	679.00	8.35	2653.14	0.03	2.31	51.02	77.43	54.76	110.11
23RMS4		1808.64	0.19	8.97	10.67	12.45	11.83	538.83	13.13	3548.75	0.01	3.05	58.98	2.02	55.52	217.82
23RMS6		809.37	1.21	9.25	19.39	10.48	11.96	372.38	17.94	4032.09	0.32	4.06	62.06	4.14	57.16	355.45
23RMS9		2422.87	0.70	11.75	16.43	8.42	14.54	499.38	15.38	4066.77	0.38	3.94	69.16	2.14	73.58	270.88
23RMS10		915.55	0.29	10.96	19.47	13.66	11.53	575.56	20.90	3751.11	0.36	5.19	57.77	3.79	69.96	618.95
23RMS1	Mica schist	1739.57	0.31	18.23	32.42	11.74	23.99	435.90	19.81	5109.92	0.81	4.94	150.66	2.14	125.86	155.40
23RMS5		653.60	0.59	78.39	25.73	9.94	17.75	342.50	17.50	4964.95	0.68	3.94	90.52	251.45	96.45	240.87
23RMS7		4432.06	0.44	15.81	32.03	14.98	16.26	797.56	16.94	4927.16	0.29	3.96	82.82	2.05	87.74	265.74
23RMS8		1570.70	1.02	11.58	22.58	12.87	14.04	382.96	16.29	3879.00	0.29	3.98	66.67	2.61	93.11	284.86
23RMS11		2804.22	1.37	16.82	10.69	12.15	14.41	320.46	14.76	4287.73	0.03	3.92	75.46	1.42	106.77	281.79
21FTG-1	Two-mica monzo granite	271.06	0.201	1.27	39.1	0.04	1.9	84.2	20.2	1042.85	2.61	3.51	5.09	-	65.8	99.9
21FTG-2		456.93	0.495	1.47	37.6	0.06	1.71	117	18.7	1060.83	2.09	2.02	6.78	-	62.2	100
21FTG-3		333.02	0.346	1.74	46.1	0.07	2.29	81.3	19.8	803.12	1.34	4.44	5.07	-	34.4	90
21FTG-4		348.51	0.161	1.16	38.6	0.04	2.63	69.2	25.5	1072.82	1.9	2.57	5.8	-	58	111
21FTG-5		302.04	0.124	0.981	41.3	0.03	1.91	70.4	13	815.10	2.06	2.81	4.14	-	63.7	78.6
21FTG-6		402.72	0.275	0.863	43.9	0.09	2.33	91.2	19.8	1000.90	1.8	3.6	5.1	-	60.6	103
21MTG-1		286.55	0.171	0.818	45.4	0.05	2.17	54.1	7.3	485.47	2.36	2.68	3.35	-	54.1	54.1
21MTG-2		379.48	0.215	1.37	36.2	0.05	1.9	49.6	6.4	641.30	2.42	2.55	3.84	-	59.2	64.6
21MTG-3		325.27	0.177	0.979	35	0.06	1.83	46.4	5.94	539.41	1.7	2.95	3.51	-	57.5	62.7
21MTG-4		333.02	0.178	1.38	41.5	0.06	2.67	56.6	8.05	707.22	2	3.72	4.48	-	56.6	72.3
Sample	Litho- logy	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Y
		μg/g
23RMS2	Meta sandstone	35.56	104.00	11.25	37.03	7.82	1.86	6.74	1.09	5.14	1.18	3.02	0.49	2.73	0.50	36.30
23RMS3		23.31	67.44	7.49	25.59	6.02	1.61	5.41	0.99	4.90	1.13	2.83	0.45	2.51	0.44	37.47
23RMS4		30.65	86.64	9.45	30.11	5.94	1.48	5.28	0.85	3.91	0.90	2.34	0.38	2.16	0.39	28.19
23RMS6		41.21	117.79	12.51	39.52	7.68	1.54	6.82	1.07	4.80	1.09	2.80	0.46	2.63	0.50	32.53
23RMS9		34.32	98.27	10.82	35.23	7.24	1.81	6.34	1.02	4.76	1.09	2.87	0.49	2.86	0.52	33.60
23RMS10		38.47	109.31	12.08	38.87	7.67	1.50	6.81	1.07	4.95	1.16	3.06	0.51	2.95	0.56	38.09
23RMS1	Mica schist	42.41	122.86	13.00	41.39	8.31	1.86	7.48	1.22	5.79	1.37	3.58	0.60	3.44	0.61	42.65
23RMS5		38.83	113.16	12.07	38.78	7.77	1.80	6.96	1.10	4.99	1.14	2.93	0.48	2.71	0.49	35.18
23RMS7		38.80	110.12	11.64	37.34	7.50	1.88	6.94	1.13	5.27	1.20	3.15	0.52	2.97	0.53	40.26
23RMS8		37.03	104.34	11.10	35.09	7.04	1.66	6.30	0.99	4.50	1.01	2.61	0.42	2.44	0.43	32.00
23RMS11		31.44	91.35	9.96	32.72	7.22	1.75	6.55	1.07	4.80	1.07	2.69	0.43	2.39	0.44	34.38
21FTG-1	Two-mica monzo granite	33.60	63.90	7.03	25.90	5.16	0.44	3.40	0.55	2.22	0.30	0.78	0.10	0.59	0.08	9.95
21FTG-2		34.00	59.40	6.16	22.00	4.18	0.57	2.87	0.45	1.94	0.29	0.79	0.11	0.64	0.09	9.1
21FTG-3		24.20	45.30	5.09	18.30	4.44	0.40	3.37	0.79	4.10	0.75	2.05	0.33	1.94	0.27	25.9
21FTG-4		37.60	75.80	8.65	32.30	6.41	0.43	4.21	0.72	2.91	0.43	0.99	0.14	0.83	0.11	12.3
21FTG-5		23.50	45.00	5.17	17.50	4.26	0.44	2.97	0.56	2.44	0.31	0.72	0.10	0.58	0.08	10.7
21FTG-6		32.50	60.00	6.70	24.40	5.27	0.54	3.88	0.68	2.90	0.41	1.05	0.15	0.86	0.11	13.3
21MTG-1		12.00	24.00	2.63	9.43	2.53	0.27	2.04	0.48	2.62	0.47	1.00	0.16	0.94	0.12	14.3
21MTG-2		10.80	23.80	2.43	9.01	2.59	0.27	2.04	0.50	2.50	0.42	0.89	0.13	0.84	0.09	13.5
21MTG-3		11.70	21.50	2.30	8.67	2.29	0.24	1.73	0.41	2.04	0.38	0.87	0.15	0.84	0.11	12.3
21MTG-4		12.90	24.20	2.90	11.00	3.24	0.27	2.60	0.68	3.32	0.55	1.22	0.19	0.92	0.12	19.5

Note: data for the metasandstone are taken from [75]; data for the two-mica monzogranite are taken from [15].

Table 3. Lithium isotopic compositions of metasandstone and mica schist of the Triassic Xikang Group and the Ke’eryin two-mica monzogranite.

Lithology	Sample	Li (μg/g)	δ7Li (‰)	2SD
Metasandstone	23RMS2	77.99	0.1	0.5
	23RMS3	50.16	6.9	0.5
	23RMS4	41.94	5.0	0.5
	23RMS6	80.47	3.7	0.5
	23RMS9	177.92	3.7	0.5
	23RMS10	77.30	3.6	0.5
Mica schist	23RMS1	173.47	−8.2	0.5
	23RMS5	80.54	−8.6	0.5
	23RMS7	165.24	−1.2	0.5
	23RMS8	109.31	0	0.5
	23RMS11	140.50	−1.6	0.5
	GYQ07-1 *	116	−6.3	0.5
	KRY11-1 *	108	−9.1	0.5
Two-mica monzo granite	21FTG-1	337	−1.0	0.5
	21FTG-2	178	−1.7	0.5
	21FTG-4	95.3	−3.3	0.5
	21FTG-5	93.4	−1.1	0.5
	21MTG-1	256	−0.8	0.5
	21MTG-2	236	−1.4	0.5
	21MTG-3	97.7	−0.7	0.5

Note: data for the metasandstone and two-mica monzogranite are taken from [75]; “*” denotes data from [76].

Table 3. Lithium isotopic compositions of metasandstone and mica schist of the Triassic Xikang Group and the Ke’eryin two-mica monzogranite.

Lithology	Sample	Li (μg/g)	δ7Li (‰)	2SD
Metasandstone	23RMS2	77.99	0.1	0.5
	23RMS3	50.16	6.9	0.5
	23RMS4	41.94	5.0	0.5
	23RMS6	80.47	3.7	0.5
	23RMS9	177.92	3.7	0.5
	23RMS10	77.30	3.6	0.5
Mica schist	23RMS1	173.47	−8.2	0.5
	23RMS5	80.54	−8.6	0.5
	23RMS7	165.24	−1.2	0.5
	23RMS8	109.31	0	0.5
	23RMS11	140.50	−1.6	0.5
	GYQ07-1 *	116	−6.3	0.5
	KRY11-1 *	108	−9.1	0.5
Two-mica monzo granite	21FTG-1	337	−1.0	0.5
	21FTG-2	178	−1.7	0.5
	21FTG-4	95.3	−3.3	0.5
	21FTG-5	93.4	−1.1	0.5
	21MTG-1	256	−0.8	0.5
	21MTG-2	236	−1.4	0.5
	21MTG-3	97.7	−0.7	0.5

Note: data for the metasandstone and two-mica monzogranite are taken from [75]; “*” denotes data from [76].

6. Discussion

6.1. Formation Conditions of the Ke’eryin Two-Mica Monzogranite

Recent studies have indicated that the temperature range for muscovite dehydration melting reactions of pelitic and felsic rocks is between 650 and 700 ℃ [85,86,87,88]. Previous studies did not include quantitative calculations of the melting temperature and pressure of granite in the Ke’eryin area. Published biotite data [15] suggest that the crystallization temperature of the Ke’eryin two-mica monzogranite is approximately 700–730 ℃ (Ti in biotite geothermometer [89]), which is significantly higher than the typical range for muscovite dehydration melting reactions. The Ke’eryin two-mica granite has low Sr and Ba contents, and its tectonic location is far from the subduction zone, ruling out the possibility of water-induced muscovite/biotite melting [90]. Zhao et al. [61] reported the peak metamorphic conditions for the Markam gneiss dome, estimating the formation pressures of the Ke’eryin two-mica monzogranite at 0.6–0.8 GPa. Additionally, published biotite data [15] indicate a crystallization pressure of 0.49 GPa (Al in biotite geobarometer [91]). Therefore, it is most likely that the magma melt of the Ke’eryin two-mica monzogranite formed via biotite dehydration melting reactions (biotite + K-feldspar + quartz = garnet + melt, biotite + quartz + plagioclase = garnet + melt; 700–850 °C, 0.5 GPa–1.0 GPa; [90,92,93,94,95]), with peritectic garnet being the primary residual mineral.

6.2. Magma Source

Numerous studies have investigated the magma source of the Ke’eryin two-mica monzogranite, reaching a consensus that it predominantly originates from the crust [22,23,24]. This study measured the Li content and δ⁷Li values of the two-mica monzogranite to be 93.4–337 ppm and –3.3‰ to −0.7‰, respectively, which align with the typical values of upper continental crust rocks (continental shales, loess, granite, and other upper crust rocks; 8–187 ppm, −5.2‰–+5.2‰ [83,84]) and distinctly differ from those of mantle-derived rocks (<10 ppm, −0.2‰–+6‰ [30,78,79,80,81]) and mid–lower crust rocks (5–33 ppm, +1.7‰–+7.5‰ [82]) (Figure 4), further indicating an upper crustal source. Additionally, this testing, along with published data [45], showed that the overall δ⁷Li values for the Triassic Xikang Group range from −14.1‰ to +7.9‰. Although this range is broad, it markedly contrasts with the δ⁷Li values of mantle-derived rocks and is close to that of middle to upper crust rocks. Similar to other parent rocks in pegmatite-type lithium ore fields within the SGOB, such as the Jiajika two-mica granite and Zhaulong muscovite granite, published Sr-Nd (Figure 5) and zircon Hf (Figure 6) isotope data suggest that the Ke’eryin two-mica monzogranite originated from partial melting of the Triassic Xikang Group [13,22,23,24,96]). However, these traditional methods cannot trace the source area precisely. Recent advancements in lithium isotope testing technology offer new opportunities to accurately determine the magma source (e.g., [45]). In this study, the Triassic Xikang Group metasedimentary rocks in the Ke’eryin ore field were divided into two groups based on their protolith lithology: metasandstone, representing the end-member lacking clay components in its protolith, and mica schist, representing the end-member with a protolith rich in clay components. The Li isotopic compositions of these two end-members and the Ke’eryin two-mica monzogranite were analyzed to accurately constrain the magmatic origins.

The δ⁷Li value of the Ke’eryin two-mica monzogranite falls within the Li isotopic composition range of mica schist but is distinctly different from that of metasandstone (Figure 4). Although the δ⁷Li range of the two-mica monzogranite aligns with that of the mica schist, its average δ⁷Li value is approximately 3.6‰ higher. Generally, Li isotopes do not fractionate significantly during high-temperature partial melting, and unaltered granitic rocks are expected to retain the same Li isotopic compositions as their source rocks [79,105,106,107,108,109]. However, investigations into leucogranites and their residual enclaves have revealed notable Li isotope fractionation when Li-rich minerals are incorporated into residual phases [40,44,110]. According to studies by Sun et al. [44] and Zhang et al. [45], the formation of Ke’eryin two-mica monzogranite required 40% to 50% partial melting of a crustal source rock, with the mineral composition of the residual phase estimated to be 50% garnet + 0.5% titanite + 19.5% plagioclase + 20% biotite + 10% quartz [44,111]. The biotite in the residual phase, which did not participate in the reaction, does not exhibit alterations in its Li isotopic composition [44]. Garnet, as a peritectic mineral formed in the residual phase, is traditionally believed to incorporate minimal Li, with low garnet/fluid and garnet/melt partition coefficients of only 0.005– 0.008 [112,113] and 0.018 [114], respectively. However, more recent findings suggest that during partial melting of basaltic rocks, Li⁺ can couple with HREE³⁺ to substitute for octahedral Mg²⁺ and Fe²⁺ [115,116], leading to Li enrichment within garnet. In the Ke’eryin ore field, garnet is prevalent in the mica schist and metasandstone, while it is virtually absent in the two-mica monzogranite, except where pollucite formed from fluid is present at the contact with pegmatite. Additionally, the two-mica monzogranite exhibits notable depletion of Heavy Rare Earth Elements (HREEs) compared to the mica schist and metasandstone (Figure 3b), a pattern consistent with garnet retention in the residual phase during biotite dehydration melting processes. The equilibrium Li isotopic fractionation between minerals and silicate melts at a constant temperature depends on the energy of the chemical bonds involved [117,118,119], where ⁷Li typically favors low coordinations with high bond energy and ⁶Li prefers higher coordinations with lower bond energy. Mineral phases (such as mica and garnet) and silicate melts predominantly comprise octahedral and tetrahedral groups, respectively [115,120,121,122,123]. Therefore, during biotite dehydration melting reactions, the separation of peritectic garnet could result in a heavier isotopic composition for a magma melt compared to its source rock. Additionally, due to the large fractionation factor between the fluid and mineral phases for lithium isotopes (α= (⁷Li/⁶Li)_fluid / (⁷Li/⁶Li)_mineral>1; [117,124,125]), Rayleigh distillation induced by the dehydration of hydrous minerals during dehydration melting could also result in a heavier Li isotopic composition for a new melt compared to its source rock. Thus, the significantly higher δ⁷Li values in the metasandstone compared to the two-mica monzogranite suggest that the metasandstone cannot be the sole source rock for the monzogranite. Considering the Li isotope variations and previously published Sr-Nd-Hf isotopic data, it is proposed that the mica schist in the Ke’eryin ore field was the dominant component of the magma source of the two-mica monzogranite.

6.3. Implications on Lithium Mineralization

In orogenic belts, clay-rich pelitic rocks deposited in the deep ocean are particularly effective at absorbing Li [126] and serve as principal material sources for rare metal pegmatites [127]. During orogenic processes, these clay-rich sediments undergo regional metamorphism, transforming into schists, phyllites, etc., with clay components metamorphosing into fine-grained mica minerals. These mica minerals are important carriers of rare metals such as Li, as well as volatile components like F, B, P, and H₂O [128,129]. Subsequently, the partial melting of these metamorphic rocks releases these elements into granitic melts [130]. In the Ke’eryin ore field, the mica schist—compared to the metasandstone—is rich in fine-grained mica minerals and contains high concentrations of rare metal elements (6951 ppm of Li, 27 ppm of Be, 2668 ppm of Rb, 2498 ppm of Cs, 281 ppm of Nb, and 46 ppm of Ta [131]), providing a robust material foundation for lithium mineralization in this region. The abundance of Li and volatiles in the mica minerals can significantly lower the solidus–liquidus temperature of the metamorphic strata [132,133], facilitating large-scale crustal melting and the formation of granitic plutons. Moreover, the presence of these elements can also effectively reduce the viscosity of granitic magma and the solidus–liquidus temperature [134,135,136], promoting extreme fractional crystallization. This process is generally a key factor in the formation of pegmatitic melts and the ultra-normal enrichment of lithium [137]. Recent studies show that beyond the SGOB, schist, phyllite, and other rocks originally rich in clay are also widely exposed in granite–pegmatite-type lithium deposits across the Western Kunlun [138], Altai [139], Altyn [140], and South China [141] orogenic belts. Similarly, mica-rich metamorphic sedimentary rocks have been identified as the source materials for the granite–pegmatite systems in notable ancient pegmatite-type lithium deposits globally, such as Bikita, Tanco, and Greenbushes [142]. Therefore, we further conclude that the mica schist of the Xikang Group served as a critical material source for the parent rock of the pegmatite-type lithium deposits within the SGOB. This inference is supported by lithium isotope data from the Jiajika ore field, where the Li isotopic characteristics of schist and the parent pluton of pegmatites (i.e., the Majingzi two-mica monzogranite) are consistent with those observed in this study [45] (Figure 4). Moreover, although some other granitic rocks also display the distinct signature of the Triassic Xikang Group [27], they may lack the capacity to generate pegmatite-type lithium deposits, potentially due to differences in their source rocks compared to the Ke’eryin and Majingzi two-mica monzogranites, such as a dominance of sandy or intermediate–mafic, clay-poor components [143]. Additionally, regions with intense magmatic activity and distributions of schist, phyllite, and clay-rich rocks should be the focus of prospecting for pegmatite-type lithium deposits.

Geological research commonly identifies mica schist, which is rich in mica and felsic minerals and depleted of dark minerals, as an indicator of high crustal maturity [144]. The formation of such high-maturity crust typically requires multiple orogenic cycles [145]. During the uplift stage of multi-cycle orogenesis, low-temperature weathering on the continental surface leads to the breakdown of rare-metal-bearing minerals, while weather-resistant and clay components are preserved. Through successive cycles of weathering and sedimentation, crustal maturity increases, and rare metal elements such as Li are continuously “purified”. The average lithium content in the mica schist samples from the Ke’eryin ore field was approximately 128 ppm, which was significantly higher than those found in the source regions (Proterozoic metamorphic sedimentary rocks from the South Qinling (~30 ppm) and Kunlun (~25 ppm) orogenic belts [145]), indicating notably higher crustal maturity for the mica schist. Additionally, the mica schists from the Ke’eryin ore field exhibited a noticeably lighter Li isotopic composition compared to the metasandstone (Figure 4), a characteristic linked to the described weathering processes: during the breakdown of primary minerals under low-temperature weathering, ⁷Li preferentially enters the fluid phase (such as river water, seawater, etc.) as part of the hydrate [Li(H₂O)⁴]⁺, while ⁶Li substitutes for Mg²⁺, Fe²⁺, and Al³⁺ in the lattice vacancies of secondary minerals like clay [146,147]. Furthermore, clay minerals with lower surface charges, such as gibbsite, can selectively adsorb ⁶Li into the octahedral vacancies in their structures [148], resulting in the enrichment of ⁶Li. Therefore, mica schist, due to the higher clay contents in its protolith, exhibits a lighter Li isotopic composition. Accordingly, we found that the highly differentiated two-mica granites from the Qinling area (δ⁷Li = +3.2; [149]) have significantly higher δ⁷Li values than Ke’eryin and Majingzi two-mica monzogranites. This may be precisely because the source rocks for the Ke’eryin and Majingzi plutons in the SGFB have undergone additional tectonic cycling (i.e., the Indosinian cycle) compared to those of the Qinling two-mica granites, during which ⁶Li was incorporated into the clay-rich sediments, and ⁷Li was distributed into clay-poor sediments or Paleo-Tethys seawater.

The above discussion does not consider the input of Li from submarine hydrothermal vents, primarily because its contribution is considered too low [150], and it tends to be absorbed by the altered oceanic crust [151,152,153]. The mechanisms by which clay-rich sediments are incorporated into the continental crust and participate in subsequent tectonic cycles are critical to further enrichment of lithium. Studies show that the Li contents, Li/Y ratios, and Li isotopic compositions of initial arc magmas across various global regions are almost identical to those of MORB [154], indicating that Li from deep-ocean sediments rarely re-enters the continental crust through subduction. This may be due to several factors: the mobile Li elements in deep-ocean sediments may have already been released at fore-arc locations [155,156], and Li can be “intercepted” by the overlying Mg-rich mantle peridotite [157,158]. The folding and exhumation during orogenic processes appear to be the primary pathways for Li to re-enter the continental crust. Significant crustal thickening during orogenic processes can expand the spatial extent of crustal melting, promote vertical magmatic differentiation, and facilitate further lithium enrichment [154]. Accordingly, the SGOB underwent significant folding and thickening during the late Indosinian orogeny. Therefore, this paper highlights the crucial roles of continental crust weathering and folding exhumation in lithium enrichment during multi-cycle orogenies. Furthermore, we believe that the Li isotopic fractionation between the Xikang Group metasedimentary rocks and the parent granite for Li mineralization in the Ke’eryin ore field underscores the intrinsic connection between cyclic orogeny and the formation of the source rocks for lithium-rich granite–pegmatite systems. Additionally, variations in the lithium isotopic compositions of granitic rocks from different geological periods also appear to enable comparisons of crustal maturity during those times. For instance, globally, from the Archean Tanco pegmatite [119], Proterozoic Black Hills pegmatite [33], and Late Paleozoic Qinghe pegmatite [40] to the Mesozoic Jiajika and Jiada pegmatites in the SGOB [21,75] and the Cenozoic Himalayan pegmatites [159], there is a trend towards lighter Li isotopic compositions in pegmatites corresponding to higher crustal maturity, and the intensity of pegmatite formation and rare metal mineralization is increasing globally [145,160,161]. However, given the varied tectonic cycles experienced in various global tectonic locations, this may not consistently apply over short periods or on a global scale. Nevertheless, in orogenic belts that have undergone multiple orogenic cycles, such as China’s Altai and South China orogenic belts [162,163], further studies on Li isotopes are warranted to confirm this correlation.

Combining the findings of previous studies with the results of this research, the formation process of the parent rock of the pegmatites and the associated lithium mineralization in the Ke’eryin ore field can be summarized as follows (Figure 7): Prior to the convergence of the North China Plate, Qiangtang–Changdu Block, and Yangtze Plate, weathering processes broke down Li-bearing minerals in the source area. Lithium was absorbed by clay components, transported by surface water, and eventually deposited at the Yangtze passive margin. During transport and deposition, significant Li isotopic fractionation occurred, with ⁶Li being incorporated into clay-rich sediments and ⁷Li entering seawater and clay-poor sediments. During the later stages of the Indosinian orogeny, intense collisional dynamics triggered regional metamorphism, transforming clay-rich sediments into mica schist and clay-poor sediments into metasandstone. At the end of the Indosinian orogeny, delamination of the lower continental crust facilitated the upwelling of deep heat [57,58,59], which in turn triggered large-scale crustal remelting. Deeply buried mica schist underwent biotite dehydration melting, and the separation of peritectic minerals such as garnet during melting resulted in granite melts with a heavier Li isotopic composition than the mica schist. Subsequently, intense magmatic fractionation led to the formation of pegmatitic melts. Further fractional crystallization, coupled with the enrichment of exsolved fluids, ultimately led to the formation of lithium-rich pegmatites [15,96].

7. Conclusions

This study analyzed the Li isotopic compositions of the mica schist and metasandstone of the Triassic Xikang Group, and the parent pluton, i.e., the Ke’eryin two-mica monzogranite, within the Ke’eryin pegmatite-type lithium ore field. On the basis of the results and the published geochemical and isotopic data, the following conclusions were drawn: (1) The mica schist of the Triassic Xikang Group is most likely the source rock for the Ke’eryin two-mica monzogranite; (2) the Ke’eryin two-mica monzogranite most likely formed through a biotite dehydration melting process, and the separation of the peritectic garnet into the residual phase is a potential reason why the lithium isotopic composition of the two-mica monzogranite is heavier than that of its source rock; and (3) low-temperature weathering led to the Li isotopic fractionation between the protoliths of mica schist and metasandstone.