Genesis and Formation Age of Albitite (Breccia) in the Eastern Segment of Qinling Orogen: Constraints from Accessory Mineral U–Pb Dating and Geochemistry

Abstract

There exists an east–west trending albitite (breccia) zone, approximately 400 km in length, closely related to gold mineralization, in Devonian strata in the South Qinling tectonic belt. The genesis and formation age of these albitite (breccia) are of great significance for understanding gold enrichment mechanisms and guiding future exploration. Past studies have mainly focused on the Fengxian–Taibai area in the western segment of the albitite (breccia) zone, whereas the eastern segment remains significantly understudied. In this study, a systematic field investigation, as well as petrology, geochemistry, and accessory-mineral geochronology studies were conducted on albitites and albitite breccias in the Shangnan area, the eastern segment of the albitite (breccia) zone. The results show that the albitites are interlayered with or occur as lenses within Devonian clastic rocks. The albitite breccias are mostly enclosed in albitite and Devonian strata, and the clasts within are subangular, uniform in type, and exhibit minimal displacement. Both albitites and albitite breccias exhibit similar trace-element characteristics and detrital zircon age spectra to those of Devonian clastic rocks. Abundant hydrothermal monazites with U–Pb ages ranging from 260 to 252 Ma are present in both albitites and albitite breccias but absent in Devonian clastic rocks. Collectively, these results indicate that the albitites in the Shangnan area are of hydrothermal metasomatic origin, while the albitite breccias record hydraulic fracturing and cementation, and both are products of the same fluid activity event in the Late Permian. We propose that albitite (breccia) zones in the South Qinling tectonic belt were formed under distinct tectonic settings during different evolution stages of the Late Paleozoic Mianlüe Ocean. Specifically, the albitites (breccias) in the Shangnan area are products of thorough metasomatism, local fracturing, and cementation of Devonian clastic rocks by mixed fluids, which ascended along the Fengzhen–Shanyang Fault coeval with the emplacement of magmatic rocks related to subduction of the Mianlüe Ocean. In contrast, the albitite breccias in the Fengxian–Taibai area are the result of fluid activity during the transition from regional compression to extension after the closure of the Mianlüe Ocean.

1. Introduction

Albitite is a specific type of rock mainly composed of albite (usually >50 vol.%). It can form through crystallization from Na-rich magmas or residual magma that has undergone a high degree of differentiation, and can also be produced via hydrothermal sedimentation, hydrothermal metasomatism, and medium-to high-grade metamorphism [1,2,3,4,5,6]. It is commonly developed in the margins of alkaline and intermediate-acidic intrusions, hydrothermally active areas, and the metamorphic basements of cratons. Albitite breccia, frequently associated with albitite, is characterized by clasts primarily consisting of albitite. Albitite and albitite breccia related to magmatic–hydrothermal activity are often associated with rare, precious, and radioactive mineral resources such as rare earth elements (REEs), niobium, tantalum, gold, and uranium [1,7,8,9,10,11,12,13]. Therefore, they have good mineralization prospects and are of great significance for geological exploration and ore prospecting.

The Devonian System in the South Qinling tectonic belt (SQTB) is the main ore-bearing rock series of the non-ferrous and precious metal metallogenic belts in the Qinling Orogenic Belt (QOB), central China. Numerous large-scale Pb-Zn and Au deposits have been discovered in it so far, including the Changba, Yinmusi, and Bafangshan Pb-Zn deposits, as well as the Shuangwang, Ertaizi, and Sanguanmiao gold deposits [14,15,16,17,18,19,20]. The Devonian strata in the SQTB is mainly a set of clastic–carbonate sedimentary formations, within which albitite (breccia) (short for albitite and albitite breccia) closely associated with gold is developed. These albitites (breccias) are distributed in the Devonian–Lower Carboniferous strata along both sides of the Fengzhen–Shanyang Fault, forming an albitite breccia zone with a length of approximately 400 km. In this zone, albitites (breccias) are best-exposed in the Fengxian–Taibai area (western segment) and Shangnan area (eastern segment). The discovery of the Shuangwang “albitite breccia-type gold deposit” in the Fengxian–Taibai area in the 1970s triggered a wave of attention and research on albitites, especially the albitite breccias closely associated with mineralization. Currently, most researchers suggest that the albitites in the Fengxian–Taibai area are products of either Devonian syngenetic hydrothermal sedimentation or late hydrothermal metasomatism [12,13,21,22], while albitite breccia results from the modification of albitites by tectono-magmatic–hydrothermal activities during the Late Triassic (ca. 220–214 Ma; [11,12,13,22,23,24]). The Shangnan area, located in the eastern segment of the SQTB, is another concentrated distribution area of albitite (breccia) in the QOB. Different types of albitite and albitite breccia occur in this area. In recent years, with the discovery of a series of gold deposits and occurrences in or nearby albitite (breccia), such as the Sanguanmiao gold deposit, the genesis of albitite and albitite breccia in the Shangnan area has gradually attracted attention. Some researchers have proposed that the albitites are formed by sodic magma crystallization and contemporaneous metasomatism of Devonian clastic rock, whereas the albitite breccias are the results of hydrothermal explosion during magma emplacement [19,25]. However, there are multiple viewpoints regarding their formation age, including the Late Devonian (ca. 365 Ma; [25]) and the Early Permian (ca. 280 Ma; [19]). So what is the genesis of the albitites and albitite breccias in the Shangnan area? Were they formed contemporaneously with those in the Fengxian–Taibai area? Does the albitite (breccia) zone in the SQTB have the same origin and is it formed concurrently under the same geological setting? The answers to these questions are of great significance for understanding the genesis of the albitite (breccia) zone in the SQTB and guiding subsequent mineral exploration.

In this paper, we conducted systematic field geological surveys, TESCAN Integrated Mineral Analyzer (TIMA) scanning, and geochemistry, as well as zircon and monazite U–Pb dating on albitites, albitite breccias, and their host clastic rocks in the Shangnan area. The results show that the albitites are formed by pervasive hydrothermal metasomatism, whereas the albitite breccias resulted from intense fluid-driven hydraulic fracturing and brecciation. Both of them are products of Late Permian (ca. 265–252 Ma) hydrothermal fluid activity probably triggered by arc magmatism associated with northward subduction of the Mianlüe Ocean. The albitite (breccia) zone in the SQTB was formed under distinct tectonic settings during different evolution stages of the Mianlüe Ocean evolution.

2. Geological Setting

The east–west trending QOB in central China is a composite orogenic belt that resulted from multiple episodes of subduction, accretion, and collision between the North China Block, Yangtze Block, and intervening microcontinents [26,27]. It can be subdivided into four major tectonic units from north to south: the southern margin of the North China Block, the North Qinling tectonic belt, the South Qinling tectonic belt (SQTB), and the northern margin of the Yangtze Block, which are separated by the Luonan–Luanchuan–Fangcheng Fault, Shangdan Suture Zone, Mianlüe Suture Zone, and Mianlüe–Bashan–Xiangguang Fault, respectively (Figure 1a).

The SQTB is a micro-block that rifted from the Yangtze Block during the expansion of the Mianlüe Ocean (a branch of the Paleo-Tethys Ocean) and re-amalgamated with the Yangtze Block after the closure of the Mianlüe Ocean in the Late Triassic (ca. 227–210 Ma; [27,29]). It comprises a Precambrian crystalline basement overlain by a Neoproterozoic–Mesozoic sedimentary cover sequence. The crystalline basement consists of the Neoarchean Douling Group, the Neoproterozoic Wudang Group, and the Yaolinghe Group. The Douling Group predominantly consists of TTG (tonalite–trondhjemite–granodiorite) gneiss, amphibolite, quartzite, and schist [30,31,32]. The Wudang Group comprises a thick metavolcano-sedimentary succession characterized by mafic to intermediate volcanic rocks in the lower part, overlain by intermediate to felsic volcanic rocks in the upper part [33]. The Yaolinghe Group is dominated by greenschist-facies metabasaltic volcanic rocks, with subordinate intermediate to felsic metavolcanics and metapelites interlayers [34]. The sedimentary cover comprises Ediacaran carbonates, Cambrian–Ordovician limestones, Silurian shales, Devonian–Carboniferous clastic rocks, and Late Permian–Early Triassic clastic rocks [26]. The Devonian strata of the southern belt occupies the region south of the Fengzhen–Shanyang Fault and north of the Ankang Fault. The stratigraphic sequence is subdivided into the Xichahe, Gongguan, Shijiagou, Dafenggou, Gudaoling, Xinghongpu, and Jiuliping Formations from bottom to top. The Xichahe Formation consists predominantly of diamictite, sandstone, and siltstone. The Gongguan Formation consists of dolomite in the lower part and calcarenite and argillaceous rocks in the upper part. The Shijiagou Formation comprises argillaceous carbonates and siltstones interbedded with dolomite. The Dafenggou Formation is dominated by carbonates and sandstones. The Gudaoling Formation is represented by polygenetic conglomerate and pebbly lithic sandstone at the base, overlain by sandstone and carbonate. The Xinghongpu Formation is mainly phyllites, sandstones, and slates intercalated with limestones. The Jiuliping Formation is dominated by graywacke, fine siltstone, and slate, with minor sandy marl interlayers [35,36].

Albitite and albitite breccia occur discontinuously as layered, vein-like, or irregular bodies in the Devonian strata near the Fengzhen–Shanyang Fault, forming an E–W trending albitite (breccia) zone about 400 km long, and they are well-developed in the Fengxian–Taibai area in the west and Shangnan area in the east. Albitite breccias are intimately associated with large-scale gold deposits, such as the Shuangwang gold deposit in the western segment, the Ertaizi copper–gold deposit in the middle segment, and the Sanguanmiao gold deposit in the eastern segment, forming an important ore-controlling zone. Since the albitite breccia in the Taibai area is the direct ore-host rock, previous researchers have conducted extensive studies on the albitite and albitite breccia in this area. Based on phenomena such as the interlayered occurrence of albitite and siltstone of the Xinghongpu Formation with straight-boundary, sedimentary structures including parallel and cross-bedding, and the similar rare earth element (REE) distribution patterns of albite to Devonian seawater, some scholars propose that the albitite in the Taibai area has a syngenetic hydrothermal sedimentary origin [13,21]. However, some scholars have proposed that albitite is the complete metasomatism product of siltstone by late-stage hydrothermal fluids, based on the observation that some albitites cut through the siltstone of the Xinghongpu Formation at a low angle [12,22]. Albitite breccia occurs as several lenticular bodies with obvious zonation. The core of the lenses contains clasts of diverse lithologies, of variable sizes, and predominantly subrounded. Towards the outer parts, the clasts are gradually dominated by country rock, with increasing size, angular shape, and showing a jigsaw-fit breccia structure. Therefore, most scholars hold the view that albitite breccia is formed via hydrothermal explosion or fluid-induced hydraulic fracturing [11,13,22,23,24]. The hydrothermal monazite in the cement of albitite breccia yielded a U–Pb age of 220.9 ± 1.8 Ma [24], which is consistent with the timing of widespread syn- to post-collisional magmatic activity in the SQTB, indicating that the albitite breccia in the Taibai area is likely a product of magmatic–hydrothermal activities during the transition from collisional compression to post-orogenic extension after the closure of the Mianlüe Ocean [11,37].

The albitite and albitite breccia in the Shangnan area are also known as “Dangjiang albitite”, and current research on their genesis remains insufficient. Li et al. [25] argued that the albitite has either a sharp intrusive or gradational metasomatic relationship with the surrounding Devonian clastic rocks, forming a dome structure. Additionally, Rb-Sr dating results of seven samples showed good linear correlations and yielded a well-defined isochron age of 364.9 ± 10.9 Ma. Therefore, they proposed that the albitites and albitite breccias in the Shangnan area are products of crystallization of sodic magma emplaced in the Late Devonian, which triggered metasomatism and hydrothermal explosion on surrounding rocks. Wang [19] proposes that the quartz albitite is of magmatic crystallization origin, while the remaining albitites resulted from the metasomatism of country rocks by magmatic fluids, and the albitite breccias are formed by hydrothermal explosion during magma decompression. However, he infers that these rocks may formed in the Early Permian (ca. 280 Ma) based on the youngest zircon ages obtained from albitite breccia.

3. Sample Description

Albitites and albitite breccias in the Shangnan area occur predominantly as lenticular, stratoid, or irregular bodies within the Xinghongpu and Jiuliping formations on the southern side of the Fengzhen–Shanyang Fault. The long axes of the lenticular bodies are generally parallel to the regional structure lineament (Figure 1b). In this study, systematic field investigation was carried out in the Xiwan, Liushuwan, and Boyugou sections, where albitites and albitite breccias are intensively exposed (Figure 1b). The result shows that there are five main rock types in this area, including silty phyllite, Na-altered phyllite, dolomite albitite, quartz albitite, and albitite breccia.

Quartz albitite is a hard rock that exhibits a light-yellow color. In the Liushuwan section, quartz albitite occurs as medium- to thick-bedded (20–50 cm) stratoid bodies, intercalated with several thin layers (3–5 cm) of grayish-black silty phyllite (Figure 2a). In the Boyugou section, quartz albitite occurs as lenticular blocks. The core of the lenses consists of relatively pure quartz albitite (Figure 2b), with minor residual silty phyllite locally visible, and the rocks in outer parts of the lenses gradually transition into light-yellow to pale red Na-altered silty phyllite and bluish-gray silty phyllite (Figure 2c). In addition, in areas with intense deformation, quartz albitite can also be observed as small lensoid bodies or as intercalated bands in silty phyllite, which gradually pinch out or transition into Na-altered phyllite or grayish-black phyllite (Figure 2d).

Dolomite albitite occurs as light-yellow- to yellow-brown-colored thick-bedded layers interbedded with silty phyllite. The contact between the two lithologies is gradually transitional. Adjacent to the contacts, abundant grayish-black silty phyllite remnants and brownish-yellow Na-altered phyllite bands parallel to the bedding are present in the dolomite albitite layer, and these relics gradually disappear towards the interior of the layer (Figure 2e). A laminated structure characterized by alternating light-yellow and yellowish-brown laminae develops in the middle part of the dolomite albitite layer. The laminae are 0.5 to 2 cm thick, with clear and straight boundaries between them (Figure 2f). Numerous fractures that penetrate into the dolomite albitite are present in the grayish-black silty phyllite layer. The silty phyllite on both side of the fractures undergoes varying degrees of Na-alteration along the schistosity, resulting in the formation of brownish-yellow Na-altered phyllite (Figure 2g) and even dolomite albitite in the center of wide fractures (Figure 2f).

Albitite breccia is small in scale and mostly occurs in the marginal parts of albitite lenses. It is light-yellow in color, and contains 30 to 60 vol.% clasts, which are angular to subangular in shape and lithologically consistent with the surrounding rock. The clasts are mainly quartz albitite and dolomite albitite fragments in the albitite breccia adjacent to albitite (Figure 2h), but are phyllite fragments, with obvious Na-alteration and bleaching at their margins, in the albitite breccia near silty phyllite (Figure 2i).

To constrain the petrogenesis and formation age of albitite and albitite breccia in the Shangnan area, representative samples of the five rock types were systematically collected. Mineral abbreviations are from [38].

4. Analysis Methods

All analyses were carried out at the State Key Laboratory of Continental Evolution and Early Life, Xi’an, China, except for monazite U–Pb dating and trace-element analyses, which were performed at Wuhan SampleSolution Analytical Technology Co., Ltd (Wuhan, China).

Mineral Automated Quantitative Analysis (TIMA) was conducted using a TIMA3 X GHM instrument (TESCAN, Brno, Czech Republic). Analyses were performed in dot-mapping mode with an accelerating voltage of 25 kV, beam current of 10 nA, working distance of 15 mm, EDS pixel spacing of 9 μm, and >1000 X-ray counts per pixel.

Mineral compositions were determined using a JXA-8230 electron microprobe (JEOL, Tokyo, Japan). Operating conditions were 15 kV accelerating voltage, 10 nA beam current (50 nA for monazite), and beam diameters of 5 μm (feldspar and monazite) or 1 μm (other phases). Structure Probe, Inc. (SPI) standards were used for calibration, and data were reduced using ZAF correction procedures. Analytical precision was better than ±2%. The results are presented in

Table A1. Representative electron microprobe analyses of albite.

Lithology	Na-Altered Silty Phyllite		Quartz Albitite		Laminated Dolomite Albitite				Albitite Breccia
Texture					Yellowish-Brown Layer		Light-Yellow Layer		Clast		Cement
SiO2	69.13	68.44	68.21	67.41	67.83	67.23	67.98	68.03	68.06	68.52	68.02	68.27
TiO2	0.00	0.04	0.00	0.03	0.00	0.01	0.00	0.04	0.00	0.00	0.00	0.00
Al2O3	19.91	19.40	19.23	19.23	18.93	19.39	19.06	19.03	19.48	19.05	19.34	19.10
Cr2O3	0.04	0.00	0.00	0.00	0.00	0.03	0.00	0.01	0.01	0.02	0.01	0.00
FeO	0.12	0.12	0.03	0.53	0.06	0.02	0.02	0.04	0.05	0.17	0.15	0.04
MnO	0.02	0.05	0.00	0.00	0.03	0.02	0.03	0.02	0.04	0.01	0.02	0.00
MgO	0.00	0.00	0.00	0.01	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.01
CaO	0.04	0.21	0.14	0.13	0.20	0.00	0.47	0.24	0.21	0.05	0.15	0.17
Na2O	11.64	11.46	11.79	11.65	11.90	11.92	11.43	11.06	11.78	11.88	11.60	12.08
K2O	0.00	0.03	0.05	0.05	0.01	0.03	0.08	0.03	0.07	0.03	0.05	0.03
Total	100.91	99.75	99.45	99.04	98.97	98.65	99.09	98.50	99.69	99.73	99.32	99.70
O.N.	8	8	8	8	8	8	8	8	8	8	8	8
Si	2.989	2.996	2.998	2.984	2.999	2.982	2.999	3.010	2.986	3.004	2.993	2.996
Ti	0.000	0.001	0.000	0.001	0.000	0.000	0.000	0.001	0.000	0.000	0.000	0.000
Al	1.014	1.001	0.996	1.003	0.986	1.014	0.991	0.992	1.007	0.984	1.003	0.988
Cr	0.001	0.000	0.000	0.000	0.000	0.001	0.000	0.000	0.000	0.001	0.000	0.000
Fe2+	0.004	0.004	0.001	0.020	0.002	0.001	0.001	0.001	0.002	0.006	0.006	0.001
Mn	0.001	0.002	0.000	0.000	0.001	0.001	0.001	0.001	0.001	0.000	0.001	0.000
Mg	0.000	0.000	0.000	0.000	0.000	0.000	0.001	0.000	0.000	0.000	0.000	0.001
Ca	0.002	0.010	0.006	0.006	0.010	0.000	0.022	0.011	0.010	0.002	0.007	0.008
Na	0.976	0.973	1.005	1.000	1.020	1.025	0.977	0.949	1.002	1.010	0.989	1.028
K	0.000	0.001	0.003	0.003	0.001	0.001	0.005	0.002	0.004	0.001	0.003	0.002
SumCat	4.988	4.989	5.009	5.016	5.019	5.025	4.997	4.969	5.013	5.009	5.001	5.024

and

Table A2. Representative electron microprobe analyses of dolomite and calcite.

Lithology	Na-Altered Silty Phyllite		Quartz Albitite		Laminated Dolomite Albitite				Albitite Breccia
Mineral	Dol	Dol	Dol	Dol	Dol	Dol	Dol	Dol	Dol	Dol	Dol	Dol	Dol	Cal	Cal
Texture					Yellowish-Brown Layer		Light-Yellow Layer		Clast		Cement
SiO2	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
TiO2	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Al2O3	0.03	0.00	0.00	0.00	0.00	0.03	0.01	0.02	0.00	0.00	0.05	0.13	0.00	1.83	0.19
FeO	8.57	9.81	9.53	9.30	8.01	7.89	7.90	8.55	6.16	6.34	5.98	3.80	5.72	0.46	0.40
MnO	0.26	0.16	0.20	0.21	0.25	0.24	0.21	0.31	0.17	0.13	0.11	0.26	0.13	0.02	0.03
MgO	15.64	14.71	14.16	13.97	15.29	15.64	16.63	14.96	16.79	17.15	17.95	18.41	18.10	2.23	0.24
CaO	30.13	29.90	29.31	29.42	29.92	29.96	30.64	30.12	30.77	30.66	30.98	30.92	29.40	53.20	58.18
Na2O	0.07	0.01	0.00	0.02	0.02	0.05	0.02	0.03	0.00	0.02	0.02	0.01	0.00	0.12	0.01
K2O	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.00	0.02	0.00	0.00	0.01
SrO	0.00	0.00	0.03	0.00	0.08	0.00	0.00	0.05	0.04	0.00	0.00	0.00	0.00	0.00	0.12
Total	54.71	54.60	53.24	52.92	53.58	53.82	55.42	54.04	53.93	54.31	55.10	53.54	53.35	57.85	59.17
CO32− number	2	2	2	2	2	2	2	2	2	2	2	2	2	1	1
Factor	1.904	1.928	1.980	1.992	1.943	1.928	1.864	1.936	1.898	1.882	1.844	1.871	1.896	0.938	0.947
Si	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Ti	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Al	0.002	0.000	0.000	0.000	0.000	0.002	0.001	0.001	0.000	0.000	0.003	0.007	0.000	0.050	0.005
Fe2+	0.227	0.263	0.263	0.258	0.217	0.212	0.205	0.230	0.163	0.166	0.153	0.099	0.151	0.006	0.005
Mn	0.007	0.004	0.006	0.006	0.007	0.007	0.006	0.008	0.004	0.003	0.003	0.007	0.003	0.000	0.000
Mg	0.739	0.704	0.696	0.690	0.737	0.748	0.769	0.719	0.791	0.801	0.821	0.855	0.852	0.052	0.006
Ca	1.023	1.028	1.035	1.045	1.037	1.030	1.019	1.040	1.041	1.029	1.019	1.032	0.994	0.890	0.982
Na	0.002	0.000	0.000	0.001	0.001	0.002	0.001	0.001	0.000	0.001	0.001	0.000	0.000	0.002	0.000
K	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Sr	0.000	0.000	0.001	0.000	0.002	0.000	0.000	0.001	0.001	0.000	0.000	0.000	0.000	0.000	0.001
SumCat	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	1.000	1.000

.

Whole-rock trace-element concentrations were obtained using an Agilent 7500a inductively coupled plasma mass spectrometer (ICP–MS) (Agilent, Santa Clara, CA, USA). The analytical quality was controlled using USGS reference materials BHVO-2, AGV-2, BCR-2, GSP-2, and replicate samples, and the precision was better than 10% for most elements. The results are presented in

Table A3. Trace elements compositions (ppm) of albitite (breccia) and host rock in the Shangnan area.

Lithology	Silty Phyllite		Na-Altered Silty Phyllite				Quartz Albitite				Laminated Dolomite Albitite				Albitite Breccia
Sample	XW-1	BYG-11	XW-4	BYG-6	BYG-3	BYG-1	BYG-9	BYG-8(2)	BYG-8(1)	LSW-8	LSW-6	LSW-5-(1)	LSW-5-(2)	LSW-4	LSW-3(1)	LSW-3(2)	BYG-5(1)	BYG-5(2)
Sc	12.5	10.4	13.1	14.2	10.8	12.4	7.96	11.7	12.1	9.19	10.0	10.6	10.7	9.01	32.8	25.0	7.96	8.02
V	80.8	67.6	82.1	146	88.2	69.7	73.6	87.8	93.8	34.7	57.5	48.3	48.6	51.3	23.6	20.7	70.55	75.58
Cr	77.4	63.7	80.9	116	82.6	63.2	68.3	83.9	85.5	89.7	67.3	65.0	64.3	74.8	71.1	62.0	71.67	73.26
Co	38.2	16.4	18.6	25.2	55.0	36.3	42.6	48.6	18.5	24.0	42.0	39.1	38.9	44.5	37.5	22.9	23.08	18.33
Ni	39.2	27.7	30.0	41.8	23.0	18.3	13.9	30.0	30.1	12.1	20.4	17.6	17.6	15.8	41.0	30.6	11.21	11.74
Cu	28.2	521	2.67	2.44	5.37	3.95	2.41	9.06	1.95	2.56	3.04	3.73	2.68	3.02	3.93	2.14	3.54	2.57
Zn	17.4	3.43	2.49	2.70	2.47	2.12	1.96	3.56	1.80	2.25	2.34	2.69	2.36	3.04	6.81	5.04	2.19	1.91
Rb	166	80.7	117	29.9	6.71	7.91	2.74	19.8	19.6	4.00	49.2	9.11	9.16	9.89	5.56	4.63	2.19	2.23
Sr	44.1	47.1	34.0	40.2	29.9	221	25.8	24.9	25.2	81.5	144	113	115	105	97.0	104	28.23	27.54
Y	25.2	35.8	25.3	26.6	19.7	33.6	16.3	21.4	21.2	16.0	26.0	27.5	26.5	19.8	18.2	15.4	17.00	17.52
Zr	187	168	190	189	171	313	270	142	142	199	140	204	187	472	115	119	179.15	193.96
Nb	14.3	12.8	14.6	13.1	14.1	9.13	14.3	14.3	14.0	14.7	13.5	12.8	13.0	14.4	9.82	9.20	14.47	14.96
Cs	3.49	1.24	1.68	0.24	0.15	0.17	0.040	0.36	0.39	0.069	0.63	0.14	0.14	0.16	0.19	0.15	0.04	0.04
Ba	306	186	224	37.1	44.2	19.9	5.43	12.3	13.4	5.96	141	20.0	20.2	16.4	16.7	13.5	5.46	4.74
La	34.1	42.1	41.7	43.0	14.5	27.2	43.6	35.9	30.1	31.0	24.7	15.0	14.8	42.1	17.1	22.1	28.02	45.14
Ce	67.2	84.6	83.5	88.1	28.5	55.7	88.0	74.9	63.1	64.6	50.2	27.1	27.1	83.6	37.5	48.8	58.96	93.62
Pr	7.47	9.13	9.21	9.43	3.36	6.76	9.67	8.05	7.22	7.07	5.92	3.00	3.01	9.22	4.51	5.89	6.80	10.12
Nd	28.8	35.5	35.1	35.9	13.1	25.4	37.1	30.9	26.1	26.0	22.2	10.9	10.9	35.5	16.8	21.7	25.08	38.55
Sm	5.57	6.94	6.64	6.46	2.90	5.07	6.44	5.47	4.72	4.39	4.75	2.22	2.22	6.42	3.30	4.00	4.78	6.89
Eu	1.06	1.00	1.40	0.99	0.59	0.87	0.93	0.87	0.78	0.83	0.95	0.39	0.40	1.19	0.62	0.70	0.77	1.07
Gd	4.98	6.35	5.49	5.51	3.08	4.66	5.17	4.63	4.11	3.58	4.76	2.57	2.54	5.24	3.06	3.36	4.01	5.45
Tb	0.73	0.94	0.78	0.76	0.51	0.74	0.63	0.61	0.58	0.45	0.76	0.50	0.49	0.68	0.46	0.45	0.55	0.67
Dy	4.27	5.75	4.42	4.44	3.21	4.76	3.20	3.52	3.40	2.55	4.55	3.63	3.56	3.66	2.62	2.38	3.04	3.44
Ho	0.84	1.19	0.86	0.89	0.66	1.08	0.56	0.70	0.70	0.51	0.89	0.87	0.83	0.68	0.53	0.45	0.58	0.61
Er	2.43	3.59	2.49	2.61	1.93	3.50	1.56	2.12	2.09	1.55	2.38	2.66	2.54	1.95	1.57	1.30	1.68	1.72
Tm	0.36	0.57	0.37	0.40	0.30	0.59	0.23	0.33	0.33	0.25	0.34	0.42	0.40	0.29	0.25	0.20	0.26	0.26
Yb	2.39	3.84	2.40	2.72	2.06	4.20	1.62	2.30	2.30	1.75	2.18	2.72	2.61	2.01	1.70	1.39	1.84	1.82
Lu	0.35	0.58	0.36	0.41	0.32	0.65	0.26	0.35	0.35	0.27	0.32	0.41	0.39	0.32	0.27	0.21	0.29	0.29
Hf	4.98	4.47	5.06	4.99	4.46	7.86	6.81	3.87	3.82	5.14	3.64	5.23	4.83	11.8	3.01	3.13	4.73	5.04
Ta	1.16	0.90	1.06	1.03	1.17	0.80	1.05	1.23	1.09	1.08	1.05	1.02	1.03	1.26	0.73	0.62	1.05	1.08
Pb	0.83	0.84	0.50	0.97	1.33	0.97	1.65	0.91	1.11	0.95	0.89	0.52	0.54	0.58	1.90	1.20	0.85	0.89
Th	11.6	10.9	12.5	14.9	12.2	11.0	13.3	16.8	15.9	12.5	9.48	9.12	9.13	14.1	7.45	8.12	11.52	13.11
U	2.04	2.30	1.87	3.34	3.80	3.36	2.42	3.09	3.07	3.60	1.80	2.47	2.41	2.75	2.44	2.29	2.63	2.55

.

Zircon and monazite grains were separated by heavy-liquid and magnetic separation techniques, and then embedded in epoxy and polished to their half-thickness. Zircon cathodoluminescence (CL) imaging was conducted using a Mono CL3+ detector (Gatan, Pleasanton, CA, USA) mounted on a scanning electron microscope. Zircon U–Pb isotopic and trace element compositions were analyzed using an Agilent 7500a ICP–MS (Agilent, Santa Clara, CA, USA) coupled with a ComPex102 Excimer laser (Lambda Physik, Göttingen, Germany) and a GeoLas 200M optical system (MicroLas, Göttingen, Germany) with a spot size of 35 μm. Natural reference material 91500 was used as the external age standard. Natural reference material GJ-1 was used to monitor data quality and yielded a concordia age of 606.5 ± 2.7 Ma (n = 26), which is consistent with the recommended age of 599.8 ± 1.7 Ma [39]. National Institute of Standards and Technology (NIST) standard reference material 610 was used as the external reference material and ²⁹Si as the internal standard element to calibrate trace element concentrations. Age calculations and concordia diagram construction were performed using Isoplot (ver. 4.15) [40]. The results are presented in

Table A4. LA–ICP–MS Zircon U–Pb data of the albitites and host rock in the Shangnan area.

Spot	Th	U	Th/U	Isotopic Ratios						Age (Ma)
	ppm	ppm		207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ	207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ
BYG-11 Silty phyllite
1	869.3	971.5	0.89	0.0759	0.0009	1.8712	0.0236	0.1787	0.0020	1093	24	1071	8	1060	11
2	97.9	90.3	1.08	0.0983	0.0019	3.7264	0.0732	0.2748	0.0034	1593	36	1577	16	1565	17
3	77.1	73.5	1.05	0.0671	0.0021	1.0973	0.0343	0.1185	0.0016	842	65	752	17	722	9
4	562.7	542.4	1.04	0.0581	0.0010	0.5694	0.0104	0.0711	0.0008	532	39	458	7	443	5
5	261.5	452.9	0.58	0.0757	0.0013	1.4776	0.0265	0.1415	0.0017	1087	35	921	11	853	9
6	172.8	233.5	0.74	0.0554	0.0016	0.5524	0.0161	0.0724	0.0009	426	64	447	11	450	6
7	233.0	535.5	0.44	0.0682	0.0010	1.3515	0.0196	0.1437	0.0016	875	29	868	8	865	9
8	152.8	297.4	0.51	0.0757	0.0012	1.7135	0.0284	0.1642	0.0019	1086	32	1014	11	980	11
9	108.3	146.4	0.74	0.0575	0.0018	0.5830	0.0177	0.0735	0.0010	510	66	466	11	457	6
10	274.1	467.5	0.59	0.0595	0.0013	0.5576	0.0119	0.0679	0.0008	586	45	450	8	424	5
11	73.0	198.5	0.37	0.0683	0.0013	1.5208	0.0290	0.1616	0.0019	876	39	939	12	966	11
12	214.4	437.0	0.49	0.1618	0.0018	10.7698	0.1272	0.4826	0.0054	2475	18	2504	11	2539	23
13	50.7	88.1	0.58	0.0675	0.0019	1.2588	0.0350	0.1353	0.0018	852	57	827	16	818	10
14	225.9	217.4	1.04	0.1621	0.0019	10.6691	0.1325	0.4774	0.0054	2477	20	2495	12	2516	24
15	73.1	170.1	0.43	0.0628	0.0016	0.8761	0.0217	0.1011	0.0013	702	52	639	12	621	7
16	323.1	303.8	1.06	0.0674	0.0012	1.4008	0.0246	0.1508	0.0018	849	35	889	10	906	10
17	316.0	389.4	0.81	0.0557	0.0013	0.5387	0.0123	0.0702	0.0008	440	50	438	8	437	5
18	218.8	287.8	0.76	0.0667	0.0011	1.2616	0.0218	0.1372	0.0016	828	35	829	10	829	9
19	324.2	398.0	0.81	0.1072	0.0013	4.3751	0.0550	0.2960	0.0033	1752	21	1708	10	1672	17
20	465.4	437.9	1.06	0.0562	0.0011	0.5614	0.0113	0.0724	0.0009	461	44	452	7	451	5
21	807.9	952.3	0.85	0.0586	0.0010	0.5115	0.0092	0.0633	0.0007	551	38	420	6	396	4
22	134.7	289.0	0.47	0.0712	0.0011	1.6376	0.0264	0.1669	0.0019	962	32	985	10	995	11
23	151.3	194.1	0.78	0.0893	0.0013	2.8278	0.0434	0.2295	0.0027	1411	28	1363	12	1332	14
24	398.5	441.3	0.90	0.0668	0.0010	1.3156	0.0199	0.1429	0.0016	830	30	853	9	861	9
25	327.0	510.9	0.64	0.0910	0.0011	3.2541	0.0419	0.2593	0.0029	1447	23	1470	10	1486	15
26	389.1	1540.7	0.25	0.0941	0.0010	3.3197	0.0383	0.2558	0.0028	1510	20	1486	9	1469	15
27	40.1	76.0	0.53	0.0581	0.0026	0.5526	0.0239	0.0690	0.0011	533	94	447	16	430	6
28	219.5	216.6	1.01	0.0688	0.0012	1.4786	0.0265	0.1560	0.0018	891	36	922	11	934	10
29	90.5	180.2	0.50	0.1127	0.0016	5.0193	0.0729	0.3231	0.0038	1843	25	1823	12	1805	18
30	265.7	604.1	0.44	0.0562	0.0011	0.5528	0.0105	0.0713	0.0008	460	41	447	7	444	5
31	292.4	306.9	0.95	0.0562	0.0013	0.5356	0.0122	0.0691	0.0008	459	50	436	8	431	5
32	177.5	268.9	0.66	0.0669	0.0012	1.2441	0.0223	0.1350	0.0016	833	36	821	10	816	9
33	114.1	112.3	1.02	0.0659	0.0016	1.2618	0.0303	0.1388	0.0017	804	50	829	14	838	10
34	315.5	360.5	0.88	0.0544	0.0013	0.5124	0.0118	0.0684	0.0008	386	51	420	8	426	5
35	274.4	347.4	0.79	0.0734	0.0013	1.2967	0.0228	0.1281	0.0015	1026	35	844	10	777	9
36	348.2	866.5	0.40	0.0558	0.0011	0.5443	0.0105	0.0707	0.0008	445	41	441	7	441	5
37	312.4	521.9	0.60	0.0723	0.0011	1.3380	0.0209	0.1342	0.0015	994	31	862	9	812	9
38	376.2	561.0	0.67	0.0663	0.0010	1.2566	0.0188	0.1375	0.0016	815	30	826	8	831	9
39	192.2	417.0	0.46	0.0726	0.0011	1.6484	0.0250	0.1647	0.0019	1002	30	989	10	983	10
40	296.5	268.4	1.10	0.0809	0.0013	2.2601	0.0377	0.2026	0.0024	1219	32	1200	12	1189	13
41	168.3	205.1	0.82	0.0980	0.0015	3.9192	0.0597	0.2899	0.0034	1587	27	1618	12	1641	17
42	536.4	879.9	0.61	0.0597	0.0009	0.6696	0.0104	0.0813	0.0009	594	32	521	6	504	5
43	278.3	486.3	0.57	0.0718	0.0012	1.1685	0.0191	0.1180	0.0014	980	32	786	9	719	8
XW-4 Na-altered silty phyllite
1	121.5	143.7	0.85	0.0978	0.0016	3.8657	0.0632	0.2866	0.0033	1583	30	1607	13	1624	17
2	62.9	173.5	0.36	0.0719	0.0014	1.6131	0.0311	0.1627	0.0019	983	39	975	12	972	11
3	157.3	227.9	0.69	0.0574	0.0015	0.5538	0.0142	0.0700	0.0009	505	56	448	9	436	5
4	955.6	586.8	1.63	0.0750	0.0010	1.9553	0.0260	0.1890	0.0021	1069	26	1100	9	1116	11
5	339.1	528.9	0.64	0.1070	0.0012	4.4530	0.0525	0.3019	0.0033	1748	20	1722	10	1701	16
6	86.6	141.9	0.61	0.0557	0.0017	0.5229	0.0159	0.0681	0.0009	439	67	427	11	425	5
7	102.4	160.0	0.64	0.0671	0.0013	1.2644	0.0247	0.1366	0.0016	842	40	830	11	825	9
8	96.5	271.7	0.35	0.0604	0.0013	0.6499	0.0141	0.0780	0.0009	619	46	508	9	484	5
9	524.5	491.2	1.07	0.0660	0.0009	1.1516	0.0166	0.1265	0.0014	807	29	778	8	768	8
10	1720.8	941.5	1.83	0.0560	0.0009	0.4932	0.0077	0.0639	0.0007	452	33	407	5	399	4
11	164.4	462.5	0.36	0.1622	0.0017	9.9574	0.1122	0.4452	0.0048	2479	18	2431	10	2374	22
12	247.0	438.8	0.56	0.0646	0.0010	1.2000	0.0183	0.1348	0.0015	761	31	801	8	815	8
13	630.0	840.8	0.75	0.1536	0.0016	9.3491	0.1048	0.4415	0.0048	2386	18	2373	10	2357	21
14	282.8	303.3	0.93	0.0571	0.0012	0.5213	0.0110	0.0662	0.0008	495	46	426	7	413	5
15	75.4	80.9	0.93	0.0749	0.0019	1.6818	0.0417	0.1628	0.0021	1066	50	1002	16	972	11
16	387.7	349.8	1.11	0.0666	0.0011	1.1347	0.0188	0.1236	0.0014	824	34	770	9	752	8
17	95.2	224.8	0.42	0.0861	0.0012	2.6944	0.0384	0.2269	0.0025	1340	27	1327	11	1319	13
18	451.7	1144.2	0.39	0.0565	0.0009	0.6362	0.0105	0.0816	0.0009	472	36	500	7	506	5
19	221.3	184.7	1.20	0.0528	0.0023	0.4908	0.0206	0.0675	0.0010	319	94	406	14	421	6
20	31.7	29.0	1.09	0.1058	0.0024	4.8184	0.1098	0.3304	0.0044	1728	41	1788	19	1840	21
21	126.4	220.9	0.57	0.0586	0.0015	0.5320	0.0136	0.0659	0.0008	551	55	433	9	411	5
22	175.9	204.2	0.86	0.1046	0.0014	4.2452	0.0564	0.2943	0.0033	1708	23	1683	11	1663	16
23	605.9	645.5	0.94	0.0744	0.0012	1.4336	0.0227	0.1398	0.0016	1052	31	903	9	843	9
24	248.1	203.4	1.22	0.0666	0.0013	1.2332	0.0236	0.1342	0.0016	827	39	816	11	812	9
25	120.2	148.2	0.81	0.0646	0.0015	1.2181	0.0276	0.1367	0.0017	762	47	809	13	826	9
26	159.8	193.5	0.83	0.0656	0.0013	1.2424	0.0240	0.1374	0.0016	793	40	820	11	830	9
27	139.4	921.7	0.15	0.1632	0.0017	9.6555	0.1058	0.4290	0.0046	2489	17	2403	10	2301	21
28	129.7	86.7	1.50	0.0596	0.0017	0.9638	0.0264	0.1173	0.0015	589	59	685	14	715	9
29	63.2	134.2	0.47	0.0904	0.0014	3.0312	0.0489	0.2433	0.0028	1433	30	1416	12	1404	15
30	135.4	318.8	0.42	0.1435	0.0016	8.4556	0.0979	0.4275	0.0047	2269	19	2281	11	2294	21
31	128.6	207.5	0.62	0.0527	0.0014	0.4911	0.0125	0.0676	0.0008	316	57	406	8	422	5
32	20.1	36.6	0.55	0.0641	0.0030	1.1861	0.0551	0.1343	0.0022	744	97	794	26	812	13
33	151.2	991.1	0.15	0.0705	0.0009	1.6181	0.0222	0.1665	0.0018	942	27	977	9	993	10
34	76.4	127.8	0.60	0.0521	0.0016	0.4916	0.0152	0.0684	0.0009	289	70	406	10	427	5
35	219.7	236.6	0.93	0.0593	0.0016	0.5583	0.0146	0.0682	0.0008	579	56	450	9	426	5
36	419.0	718.8	0.58	0.0956	0.0011	3.7268	0.0437	0.2826	0.0031	1541	21	1577	9	1605	15
37	167.7	196.6	0.85	0.0949	0.0013	3.6103	0.0509	0.2759	0.0031	1526	26	1552	11	1571	16
38	165.2	451.4	0.37	0.0515	0.0013	0.4907	0.0123	0.0691	0.0008	262	57	405	8	431	5
39	389.5	476.1	0.82	0.0546	0.0010	0.5497	0.0100	0.0730	0.0008	396	40	445	7	454	5
40	367.2	922.1	0.40	0.0663	0.0009	1.0968	0.0155	0.1199	0.0013	817	29	752	8	730	8
41	192.8	237.1	0.81	0.0645	0.0013	1.2350	0.0250	0.1388	0.0016	759	42	817	11	838	9
42	161.0	144.8	1.11	0.0567	0.0018	0.5218	0.0161	0.0667	0.0009	480	68	426	11	416	5
43	106.5	147.0	0.72	0.0744	0.0046	0.7161	0.0434	0.0698	0.0014	1051	121	548	26	435	9
44	102.5	110.7	0.93	0.0772	0.0016	1.7817	0.0373	0.1674	0.0020	1127	41	1039	14	998	11
45	449.6	400.4	1.12	0.0542	0.0019	0.5237	0.0185	0.0701	0.0010	379	78	428	12	437	6
46	202.4	202.5	1.00	0.0603	0.0014	0.7820	0.0184	0.0941	0.0011	613	50	587	10	580	7
47	280.2	476.1	0.59	0.0553	0.0010	0.6055	0.0107	0.0795	0.0009	423	38	481	7	493	5
48	250.7	356.3	0.70	0.0670	0.0011	1.1321	0.0183	0.1225	0.0014	838	33	769	9	745	8
49	199.6	362.1	0.55	0.0728	0.0011	1.6956	0.0253	0.1689	0.0019	1009	29	1007	10	1006	10
LSW-8 Laminated dolomite albitite
1	223.0	258.5	0.86	0.0550	0.0014	0.5624	0.0142	0.0742	0.0010	411	55	453	9	462	6
2	41.9	73.0	0.57	0.0598	0.0028	0.5958	0.0277	0.0722	0.0012	598	99	475	18	450	7
3	116.6	218.2	0.53	0.0872	0.0012	3.0125	0.0454	0.2505	0.0030	1366	27	1411	11	1441	16
4	197.9	181.1	1.09	0.1998	0.0023	15.4531	0.1965	0.5612	0.0068	2824	18	2844	12	2872	28
5	393.5	570.3	0.69	0.0642	0.0010	1.3482	0.0214	0.1524	0.0018	748	32	867	9	914	10
6	140.2	191.6	0.73	0.0815	0.0013	2.6840	0.0455	0.2390	0.0029	1233	31	1324	13	1381	15
7	72.3	148.2	0.49	0.0654	0.0014	1.2520	0.0276	0.1389	0.0018	787	45	824	12	838	10
8	82.2	358.4	0.23	0.0713	0.0012	1.6399	0.0280	0.1669	0.0020	965	33	986	11	995	11
9	102.6	288.5	0.36	0.0693	0.0011	1.6255	0.0261	0.1702	0.0020	907	31	980	10	1013	11
10	102.2	123.9	0.82	0.1130	0.0017	5.4397	0.0855	0.3492	0.0043	1848	27	1891	13	1931	21
11	344.2	196.4	1.75	0.0557	0.0014	0.6495	0.0165	0.0846	0.0011	439	55	508	10	524	6
12	245.6	242.2	1.01	0.1212	0.0015	6.6611	0.0896	0.3987	0.0048	1974	22	2068	12	2163	22
13	62.5	96.0	0.65	0.0533	0.0028	0.5054	0.0263	0.0687	0.0012	343	115	415	18	429	7
14	374.4	832.2	0.45	0.0574	0.0010	0.5302	0.0095	0.0670	0.0008	505	38	432	6	418	5
15	80.6	444.1	0.18	0.0687	0.0009	1.5785	0.0230	0.1666	0.0020	891	28	962	9	993	11
16	261.7	373.7	0.70	0.0545	0.0011	0.5523	0.0116	0.0735	0.0009	391	45	447	8	457	5
17	128.1	377.1	0.34	0.0654	0.0010	1.2157	0.0199	0.1348	0.0016	788	33	808	9	815	9
18	188.9	494.2	0.38	0.1444	0.0015	8.4560	0.1003	0.4246	0.0049	2281	18	2281	11	2281	22
19	269.0	473.3	0.57	0.0559	0.0010	0.5694	0.0108	0.0738	0.0009	449	40	458	7	459	5
20	28.8	122.6	0.23	0.0658	0.0017	1.3670	0.0355	0.1508	0.0021	799	53	875	15	906	11
21	1316.1	1033.7	1.27	0.0607	0.0012	0.5306	0.0108	0.0634	0.0008	628	43	432	7	396	5
22	95.1	194.2	0.49	0.1194	0.0015	6.2643	0.0863	0.3806	0.0045	1947	23	2014	12	2079	21
23	190.3	855.7	0.22	0.0752	0.0009	1.9877	0.0257	0.1918	0.0022	1073	24	1111	9	1131	12
24	72.9	79.9	0.91	0.1134	0.0018	5.2887	0.0892	0.3383	0.0043	1854	29	1867	14	1878	21
25	205.9	585.2	0.35	0.0734	0.0011	1.7105	0.0274	0.1690	0.0020	1025	30	1013	10	1007	11
26	377.6	413.3	0.91	0.0586	0.0010	0.7647	0.0137	0.0946	0.0011	552	38	577	8	583	7
27	38.4	89.5	0.43	0.0750	0.0019	1.8498	0.0470	0.1789	0.0024	1068	50	1063	17	1061	13
28	397.8	234.6	1.70	0.0655	0.0013	1.1953	0.0237	0.1322	0.0016	792	40	798	11	801	9
29	141.7	208.3	0.68	0.1537	0.0018	9.1189	0.1160	0.4301	0.0050	2388	20	2350	12	2306	23
30	60.6	113.6	0.53	0.0639	0.0016	1.1568	0.0285	0.1312	0.0017	739	51	780	13	795	10
31	37.9	44.9	0.84	0.0631	0.0023	1.2396	0.0447	0.1425	0.0022	713	75	819	20	859	12
32	433.2	590.5	0.73	0.0540	0.0010	0.5595	0.0105	0.0751	0.0009	372	41	451	7	467	5
33	165.4	256.2	0.65	0.0746	0.0013	1.6034	0.0276	0.1560	0.0019	1056	34	972	11	934	10
34	77.4	534.6	0.14	0.0690	0.0010	1.3486	0.0205	0.1418	0.0017	897	30	867	9	855	9
35	84.6	240.9	0.35	0.0635	0.0013	1.0691	0.0214	0.1221	0.0015	725	41	738	10	743	9
36	218.5	251.2	0.87	0.1584	0.0018	9.4750	0.1175	0.4338	0.0050	2438	19	2385	11	2323	23
37	136.2	265.5	0.51	0.1844	0.0020	12.7171	0.1544	0.5002	0.0058	2692	18	2659	11	2615	25
38	57.8	99.7	0.58	0.1633	0.0022	10.5589	0.1522	0.4689	0.0058	2490	22	2485	13	2479	25
39	224.3	477.7	0.47	0.1595	0.0017	9.5139	0.1132	0.4324	0.0049	2451	18	2389	11	2317	22
40	294.9	279.3	1.06	0.1644	0.0018	10.0551	0.1229	0.4433	0.0051	2502	19	2440	11	2366	23
41	183.3	101.4	1.81	0.1559	0.0020	10.7152	0.1496	0.4985	0.0062	2412	21	2499	13	2608	27
42	179.4	330.8	0.54	0.0565	0.0012	0.6297	0.0135	0.0808	0.0010	471	47	496	8	501	6
43	154.7	127.3	1.22	0.0672	0.0016	1.1884	0.0279	0.1283	0.0016	842	48	795	13	778	9
44	138.1	83.2	1.66	0.0570	0.0023	0.5045	0.0203	0.0642	0.0010	489	88	415	14	401	6
45	21.2	49.1	0.43	0.1308	0.0031	6.7123	0.1604	0.3721	0.0057	2109	40	2074	21	2039	27
46	51.5	106.4	0.48	0.0762	0.0020	1.7299	0.0442	0.1645	0.0022	1101	50	1020	16	982	12
47	196.5	461.8	0.43	0.0748	0.0013	1.9722	0.0348	0.1912	0.0023	1063	34	1106	12	1128	12
48	61.4	87.8	0.70	0.1511	0.0020	9.1512	0.1313	0.4392	0.0053	2358	23	2353	13	2347	24
49	87.1	112.3	0.78	0.0689	0.0015	1.6437	0.0366	0.1730	0.0023	897	45	987	14	1029	12
50	94.2	100.5	0.94	0.0654	0.0017	1.2156	0.0315	0.1349	0.0018	786	53	808	14	816	10
51	62.5	156.2	0.40	0.1209	0.0016	7.1375	0.1047	0.4281	0.0053	1970	24	2129	13	2297	24
52	376.5	408.5	0.92	0.1575	0.0017	9.4575	0.1144	0.4357	0.0051	2429	18	2383	11	2331	23
LSW-4 Quartz albitite
1	351.1	625.9	0.56	0.0610	0.0011	0.5318	0.0098	0.0632	0.0008	639	37	433	7	395	5
2	280.4	609.4	0.46	0.0592	0.0010	0.5168	0.0095	0.0633	0.0008	574	37	423	6	396	5
3	759.0	1364.3	0.56	0.0554	0.0012	0.4883	0.0109	0.0639	0.0008	429	47	404	7	399	5
4	381.8	571.1	0.67	0.0582	0.0012	0.5261	0.0109	0.0656	0.0009	535	43	429	7	410	5
5	98.7	158.1	0.62	0.0525	0.0021	0.4797	0.0188	0.0662	0.0010	308	87	398	13	413	6
6	207.9	325.6	0.64	0.0576	0.0016	0.5260	0.0145	0.0662	0.0009	514	59	429	10	413	6
7	343.4	398.0	0.86	0.0588	0.0012	0.5468	0.0118	0.0674	0.0009	560	45	443	8	421	5
8	406.4	551.6	0.74	0.0559	0.0010	0.5392	0.0102	0.0700	0.0009	447	39	438	7	436	5
9	179.2	258.5	0.69	0.0617	0.0014	0.6022	0.0141	0.0708	0.0010	663	48	479	9	441	6
10	1297.4	721.9	1.80	0.0588	0.0010	0.5746	0.0103	0.0709	0.0009	560	36	461	7	441	5
11	211.1	251.4	0.84	0.0588	0.0013	0.5805	0.0135	0.0716	0.0010	559	49	465	9	446	6
12	330.1	357.3	0.92	0.0619	0.0016	0.6170	0.0159	0.0723	0.0010	671	53	488	10	450	6
13	113.6	208.6	0.54	0.0548	0.0017	0.5483	0.0167	0.0725	0.0010	406	66	444	11	451	6
14	79.7	166.2	0.48	0.0540	0.0016	0.5424	0.0166	0.0729	0.0010	369	67	440	11	454	6
15	460.9	351.7	1.31	0.0567	0.0013	0.5720	0.0131	0.0732	0.0010	478	49	459	8	455	6
16	118.6	214.2	0.55	0.0607	0.0013	0.6183	0.0138	0.0739	0.0010	628	46	489	9	460	6
17	94.7	179.0	0.53	0.0590	0.0015	0.6017	0.0155	0.0740	0.0010	567	54	478	10	460	6
18	536.1	590.7	0.91	0.0541	0.0010	0.5631	0.0108	0.0755	0.0010	375	40	454	7	469	6
19	484.7	468.2	1.04	0.0556	0.0011	0.5911	0.0121	0.0771	0.0010	437	43	472	8	479	6
20	310.4	471.4	0.66	0.0559	0.0013	0.5950	0.0140	0.0772	0.0010	446	50	474	9	480	6
21	252.6	227.2	1.11	0.0562	0.0013	0.6827	0.0161	0.0881	0.0012	459	51	528	10	544	7
22	743.4	1070.1	0.69	0.0655	0.0009	0.8207	0.0125	0.0909	0.0011	790	28	608	7	561	7
23	299.3	348.9	0.86	0.0599	0.0013	0.7869	0.0170	0.0952	0.0013	601	44	589	10	586	7
24	418.3	415.4	1.01	0.0634	0.0012	0.8705	0.0173	0.0996	0.0013	721	40	636	9	612	8
25	462.1	1057.3	0.44	0.0665	0.0009	0.9276	0.0146	0.1011	0.0013	823	29	666	8	621	7
26	87.7	1085.6	0.08	0.0659	0.0008	1.0641	0.0148	0.1170	0.0015	804	25	736	7	714	8
27	442.3	589.4	0.75	0.0662	0.0010	1.1103	0.0186	0.1216	0.0015	812	32	758	9	740	9
28	367.2	608.7	0.60	0.0663	0.0009	1.1223	0.0174	0.1227	0.0016	817	29	764	8	746	9
29	40.3	346.7	0.12	0.0665	0.0011	1.1382	0.0206	0.1242	0.0016	822	35	772	10	754	9
30	120.8	165.3	0.73	0.0645	0.0015	1.1198	0.0272	0.1258	0.0017	759	49	763	13	764	10
31	34.6	34.9	0.99	0.0621	0.0026	1.0897	0.0455	0.1272	0.0021	679	87	748	22	772	12
32	71.8	147.1	0.49	0.0614	0.0021	1.0823	0.0362	0.1279	0.0019	653	70	745	18	776	11
33	160.4	284.5	0.56	0.0685	0.0011	1.2105	0.0211	0.1281	0.0016	884	33	805	10	777	9
34	101.3	118.1	0.86	0.0637	0.0015	1.1497	0.0269	0.1310	0.0018	730	48	777	13	793	10
35	266.0	408.6	0.65	0.0645	0.0010	1.1689	0.0194	0.1315	0.0017	757	32	786	9	796	9
36	507.2	450.2	1.13	0.0664	0.0011	1.2074	0.0206	0.1320	0.0017	817	33	804	9	799	10
37	137.9	180.9	0.76	0.0667	0.0014	1.2191	0.0269	0.1326	0.0018	828	44	809	12	802	10
38	29.4	101.9	0.29	0.0670	0.0016	1.2578	0.0314	0.1362	0.0019	838	50	827	14	823	11
39	314.1	716.3	0.44	0.0690	0.0009	1.1448	0.0170	0.1202	0.0015	900	27	775	8	732	9
40	73.9	412.4	0.18	0.0681	0.0011	1.4090	0.0238	0.1501	0.0019	871	32	893	10	901	11
41	597.1	1636.7	0.36	0.0693	0.0008	1.3318	0.0179	0.1394	0.0017	907	23	860	8	841	10
42	94.1	246.9	0.38	0.0699	0.0011	1.5062	0.0253	0.1564	0.0020	924	32	933	10	937	11
43	420.1	486.2	0.86	0.0704	0.0012	1.1969	0.0217	0.1233	0.0016	939	34	799	10	750	9
44	109.8	273.0	0.40	0.0705	0.0011	1.3866	0.0235	0.1426	0.0018	943	32	883	10	859	10
45	117.4	198.7	0.59	0.0708	0.0012	1.5829	0.0287	0.1622	0.0021	951	34	964	11	969	12
46	210.6	329.6	0.64	0.0716	0.0014	1.1746	0.0234	0.1189	0.0016	975	38	789	11	724	9
47	197.3	441.9	0.45	0.0728	0.0011	1.4427	0.0230	0.1437	0.0018	1008	29	907	10	866	10
48	106.8	328.8	0.32	0.0730	0.0011	1.7218	0.0275	0.1711	0.0022	1014	29	1017	10	1018	12
49	272.9	617.2	0.44	0.0758	0.0011	1.7661	0.0280	0.1689	0.0021	1091	29	1033	10	1006	12
50	127.7	177.0	0.72	0.0761	0.0013	1.7719	0.0323	0.1688	0.0022	1099	34	1035	12	1005	12
51	219.8	896.6	0.25	0.0775	0.0009	1.7768	0.0244	0.1662	0.0021	1134	24	1037	9	991	11
52	547.7	1882.6	0.29	0.0814	0.0009	2.1598	0.0281	0.1924	0.0024	1231	21	1168	9	1134	13
53	140.7	180.5	0.78	0.0975	0.0015	3.8216	0.0628	0.2843	0.0037	1576	28	1597	13	1613	18
54	236.6	211.5	1.12	0.0999	0.0015	4.2952	0.0691	0.3118	0.0040	1622	27	1692	13	1750	20
55	113.8	453.6	0.25	0.1331	0.0016	6.6192	0.0913	0.3606	0.0045	2139	21	2062	12	1985	22
56	346.6	454.1	0.76	0.1529	0.0017	7.3507	0.0960	0.3487	0.0044	2378	19	2155	12	1928	21
57	357.4	332.4	1.07	0.1656	0.0018	10.4855	0.1362	0.4593	0.0057	2513	18	2479	12	2436	25
58	207.0	384.2	0.54	0.3679	0.0038	38.0965	0.4763	0.7510	0.0094	3783	16	3722	12	3611	34

.

For the quartz albitite, monazite grains were separated, mounted in epoxy, and polished for analysis; for all other samples, in situ monazite analysis was performed directly on polished thin sections. Backscattered electron (BSE) imaging of monazite was performed using a JXA-8230 electron microprobe (JEOL, Tokyo, Japan). Simultaneous U–Pb isotopic dating and trace element analysis were carried out utilizing an Agilent 7900 ICP–MS system with a spot size of 16 μm and repetition rate of 2 Hz. Monazite 44069 [41] and glass reference material NIST standard reference material 610 were used as external standards for isotope and trace element fractionation corrections, respectively. Monazite Trebilcock served as a monitoring standard and yielded a concordia age of 274.9 ± 0.78 Ma (n = 6), which is consistent with the recommended age of 272 ± 2 Ma [42]. Age calculations and concordia diagram construction were performed using Isoplot (ver. 4.15) [40]. The results are listed in

Table A5. CaO (wt%) and rare earth element (ppm) contents in monazite.

Spot	CaO *	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	ΣREO
XW-4 Na-altered silty phyllite
1	0.25	142,971	249,365	27,512	104,453	17,283	3097	10,233	974	3004	246	246	12	33	2	65.93
2	0.80	135,021	245,537	27,332	105,097	16,822	3038	8822	787	2472	210	227	11	31	2	64.27
3	0.73	139,313	243,424	26,711	103,807	17,957	3569	10,469	982	2997	239	229	12	29	2	64.81
4	0.46	131,990	234,665	26,838	105,570	18,336	3672	10,672	961	2760	202	172	7	18	1	63.08
5	0.72	136,728	239,805	27,438	106,803	17,996	3425	10,793	1011	3047	236	225	12	32	2	64.54
6	0.60	139,677	244,728	27,068	101,232	15,753	2876	8371	746	2369	211	226	13	32	2	64.02
7	0.76	135,381	266,347	28,204	97,517	12,344	1794	5648	470	1295	108	112	6	15	1	64.27
8	0.94	135,165	244,390	27,649	102,940	16,376	2767	7941	731	2169	200	225	13	41	3	63.68
9	0.82	136,966	256,469	31,086	111,303	15,421	2161	7566	602	1694	138	152	8	17	1	65.93
LSW-8 Laminated dolomite albitite
1	0.62	130,908	254,265	27,928	105,042	14,927	2845	7281	548	1572	141	150	8	19	1	64.15
2	0.68	145,338	258,655	27,559	98,214	13,111	2121	6369	484	1343	128	132	7	16	1	65.09
3	0.58	124,361	253,011	29,001	110,886	17,886	3018	10,265	853	2479	230	251	14	29	2	65.13
4	0.59	121,644	255,609	30,009	115,182	18,487	3123	10,446	849	2484	238	256	14	36	2	65.90
5	0.43	131,838	251,937	28,860	108,505	16,531	2775	8919	706	2023	193	207	11	30	2	65.10
6	0.40	142,971	249,365	27,512	104,453	17,283	3097	10,233	974	3004	246	246	12	33	2	65.97
7	0.80	139,313	243,424	26,711	103,807	17,957	3569	10,469	982	2997	239	229	12	29	2	64.80
LSW-4 Quartz albitite
1	0.93	136,983	263,343	29,029	109,273	17,666	2897	8833	749	2077	181	231	16	48	4	66.83
2	0.44	136,966	256,469	31,086	111,303	15,421	2161	7566	602	1694	138	152	8	17	1	66.78
3	0.84	132,893	254,172	28,247	102,804	15,693	2167	9709	948	3228	351	490	34	107	7	65.30
4	0.60	142,934	253,497	27,428	97,985	14,844	2067	9016	855	3046	355	510	35	116	8	65.61
5	0.84	142,562	252,424	29,586	115,849	21,366	4464	13,082	1067	2650	191	180	8	23	2	68.63
6	0.60	139,064	254,690	28,002	101,888	15,901	2167	9839	968	3385	387	563	40	132	10	65.96
7	0.38	142,562	252,424	29,586	115,849	21,366	4464	13,082	1067	2650	191	180	8	23	2	68.24
8	0.72	139,064	254,690	28,002	101,888	15,901	2167	9839	968	3385	387	563	40	132	10	65.16
9	0.79	130,865	249,306	27,165	101,610	15,814	1835	10,868	1088	3890	477	697	50	160	11	64.81
10	0.48	124,941	249,842	28,778	114,436	24,910	4228	20,951	1992	4701	254	175	7	21	2	68.33
11	0.42	128,690	247,300	28,004	108,924	17,969	2355	11,836	1222	4209	462	612	39	118	8	65.40
12	0.88	129,497	245,885	27,343	101,100	16,336	2017	10,852	1094	4023	474	686	48	157	11	63.66
13	0.83	150,739	252,762	27,177	97,472	14,324	2007	8695	805	2847	333	468	32	101	7	66.69
14	0.54	136,983	263,343	29,029	109,273	17,666	2897	8833	749	2077	181	231	16	48	4	67.80
15	0.74	151,691	266,295	29,642	110,984	19,049	4006	11,279	920	2426	186	200	10	8	1	70.73
16	0.99	132,183	248,367	27,906	111,108	17,284	2517	12,200	1062	3224	302	381	26	90	7	65.56
17	0.37	143,622	255,476	27,423	96,474	14,339	2011	8433	790	2814	319	466	31	99	7	66.06
LSW-3 Albitite breccia
1	0.28	128,311	262,471	28,825	99,775	11,774	1588	4845	482	1600	156	174	11	29	2	63.55
2	0.74	109,311	248,759	28,810	119,020	18,815	3279	11,627	994	2741	229	233	11	25	1	64.08
3	0.70	116,618	257,179	30,625	116,911	15,733	2334	7716	740	2276	206	217	12	29	2	64.87
4	0.87	134,619	257,996	29,157	103,748	14,942	2226	7690	637	1778	149	150	8	22	2	65.06
5	0.35	122,005	247,921	28,860	115,610	18,130	3058	10,623	935	2442	203	203	10	23	1	64.80
6	0.95	126,991	257,064	30,997	113,387	15,799	2624	8854	791	2307	176	195	10	25	1	65.83
7	0.54	123,807	254,182	30,402	113,051	16,425	2559	9035	788	2206	196	205	11	27	2	65.12
8	0.57	119,402	255,437	29,268	113,202	15,608	2329	7634	715	2272	201	228	12	32	2	64.38
9	0.83	115,775	255,849	29,749	116,175	15,374	2433	7968	628	1917	164	146	8	19	1	64.23
10	0.26	128,176	251,623	30,162	121,573	14,666	2327	7674	627	1996	172	199	11	29	2	65.82
11	0.57	116,700	242,957	29,386	113,934	16,912	2434	8548	683	1916	164	186	9	22	1	62.86

* Measured by EPMA.

and

Table A6. LA–ICP–MS Monazite U–Pb data of the albitite (breccia) and host rock in the Shangnan area.

Spot	Th	U	Th/U	Isotopic Ratios						Age (Ma)
	ppm	ppm		207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ	207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ
XW-4 Na-altered silty phyllite
1	4044.8	41.3	98.0	0.4329	0.0740	3.9344	0.5876	0.0901	0.0092	4027	258	1621	121	556	55
2	5542.8	6.0	928.2	0.1858	0.0504	3.5115	0.6416	0.0708	0.0051	2705	462	1530	144	441	31
3	8704.6	20.6	422.4	0.1793	0.0491	1.6794	0.6249	0.0571	0.0053	2646	471	1001	237	358	33
4	5529.9	8.8	626.8	0.1594	0.0453	2.2050	0.5157	0.0559	0.0052	2449	499	1183	163	351	32
5	7130.0	7.6	938.8	0.2712	0.0704	2.4738	0.4812	0.0588	0.0049	3313	419	1264	141	369	30
6	8604.7	10.7	805.1	0.1829	0.0387	2.1250	0.3122	0.0527	0.0049	2679	356	1157	101	331	30
7	5079.1	10.1	504.6	0.1452	0.0414	4.3101	0.6638	0.0668	0.0092	2300	510	1695	127	417	56
8	5423.6	6.4	843.9	0.2304	0.0513	5.8191	1.4710	0.0901	0.0084	3055	358	1949	219	556	50
9	8252.2	15.9	517.9	0.0906	0.0249	0.9045	0.1990	0.0483	0.0039	1439	544	654	106	304	24
LSW-8 Laminated dolomite albitite
1	8527.6	18.6	459.0	0.0936	0.0320	0.4776	0.1208	0.0423	0.0032	1500	688	396	83	267	20
2	2653.9	3.6	728.8	0.1607	0.0457	4.5762	1.0438	0.0856	0.0122	2463	498	1745	190	530	72
3	1773.2	16.8	105.7	0.2025	0.0459	1.2319	0.2217	0.0497	0.0037	2847	377	815	101	313	23
4	6317.0	9.7	651.4	0.1033	0.0321	1.3250	0.2293	0.0475	0.0047	1684	603	857	100	299	29
5	3256.3	8.6	377.6	0.1023	0.0308	5.1502	2.9177	0.0817	0.0274	1678	584	1844	482	507	163
6	2847.1	7.7	370.6	0.1779	0.0453	3.4023	0.8181	0.0668	0.0062	2635	436	1505	189	417	37
LSW-4 Quartz albitite
1	17,156.1	22.5	763.4	0.0809	0.0116	0.4199	0.0670	0.0407	0.0012	1218	287	356	48	257	8
2	30,550.8	292.8	104.3	0.0539	0.0034	0.2954	0.0177	0.0402	0.0006	365	143	263	14	254	3
3	36,135.1	372.4	97.0	0.0491	0.0034	0.2784	0.0183	0.0412	0.0006	150	159	249	15	260	4
4	29,657.6	336.4	88.2	0.0525	0.0032	0.2907	0.0161	0.0410	0.0007	306	139	259	13	259	4
5	4371.1	25.3	172.8	0.1230	0.0210	0.6794	0.0869	0.0442	0.0020	2000	307	526	53	279	13
6	31,462.0	166.8	188.6	0.0568	0.0037	0.3126	0.0183	0.0407	0.0008	483	146	276	14	257	5
7	3817.4	24.7	154.6	0.0604	0.0034	0.3538	0.0210	0.0424	0.0006	617	122	308	16	267	4
8	28,632.1	253.4	113.0	0.1348	0.0138	0.7760	0.0726	0.0442	0.0013	2161	180	583	41	279	8
9	58,920.0	341.5	172.5	0.0602	0.0070	0.3156	0.0286	0.0405	0.0011	613	258	279	22	256	7
10	23,549.6	46.9	502.0	0.2317	0.0291	1.3333	0.1535	0.0459	0.0024	3064	202	860	67	289	14
11	32,129.1	329.2	97.6	0.0647	0.0078	0.3402	0.0341	0.0414	0.0013	765	262	297	26	261	8
12	11,677.2	18.1	645.0	0.0675	0.0071	0.3703	0.0367	0.0408	0.0009	854	220	320	27	258	6
13	49,930.5	32.7	1525.5	0.0578	0.0039	0.3219	0.0207	0.0411	0.0006	520	149	283	16	259	4
14	30,699.3	336.1	91.3	0.4155	0.0499	2.8774	0.2018	0.0616	0.0026	3966	181	1376	53	385	16
15	45,981.7	69.8	658.5	0.2548	0.0430	1.2797	0.1939	0.0478	0.0029	3215	269	837	86	301	18
16	7017.2	19.6	357.8	0.0603	0.0052	0.3231	0.0221	0.0405	0.0009	617	189	284	17	256	5
17	42,659.0	112.9	377.8	0.0508	0.0030	0.2764	0.0153	0.0401	0.0006	232	137	248	12	254	4
LSW-3 Albitite breccia
1	8657.0	13.7	630.6	0.1277	0.0340	0.8330	0.1958	0.0429	0.0029	2078	487	615	108	271	18
2	5146.2	19.1	269.3	0.2213	0.0672	0.8505	0.1300	0.0460	0.0032	2990	509	625	71	290	20
3	11,208.6	13.9	807.7	0.2136	0.0590	1.0775	0.2310	0.0436	0.0035	2933	462	742	113	275	22
4	3176.8	8.0	398.3	0.1303	0.0374	3.8756	1.0237	0.0676	0.0106	2103	524	1609	213	422	64
5	4008.9	8.7	461.0	0.3519	0.0739	14.0561	2.8259	0.1716	0.0182	3715	325	2753	191	1021	100
6	3044.4	11.3	268.6	0.1327	0.0299	1.1804	0.1750	0.0563	0.0039	2200	404	792	82	353	24
7	8501.4	12.9	656.8	0.0653	0.0210	0.6097	0.2362	0.0504	0.0049	783	563	483	149	317	30
8	12,318.4	12.2	1009.9	0.1462	0.0317	1.5777	0.3428	0.0568	0.0046	2302	380	961	135	356	28
9	4497.5	12.3	364.7	0.1966	0.0403	1.4615	0.2425	0.0521	0.0045	2798	342	915	100	327	28
10	12,222.8	24.7	495.8	0.1035	0.0212	0.4783	0.0668	0.0380	0.0023	1687	390	397	46	241	14
11	4729.8	9.2	512.3	0.3837	0.0773	2.9399	0.3482	0.0631	0.0059	3847	309	1392	90	394	36

.

5. Results

5.1. Petrography

The silty phyllite (XW-1) is grayish-black in color and exhibits a fine-grained lepidoblastic texture, and a phyllitic structure. It consists of quartz (50–55 vol.%), sericite (30–35 vol.%), biotite (5–10 vol.%), and dolomite (~1 vol.%), with minor amounts of apatite, ilmenite, rutile, and pyrite. Albite was not observed (Figure 3a). Mineral grains are 0.1–0.3 mm in size and show a preferred orientation parallel to the phyllitic foliation.

The Na-altered silty phyllite (XW-4) is brownish-yellow to pale red in color. It is composed of quartz (40–45 vol.%), sericite (25–30 vol.%), albite (20–30 vol.%), and dolomite (3–5 vol.%), with minor amounts of biotite, apatite, rutile, zircon, monazite, and limonite (Figure 3b,c). Compared with the silty phyllite, the contents of quartz and sericite in this rock are significantly lower, albite appears in large quantities, and all ilmenite has been transformed into rutile.

Quartz albitite (LSW-4) is pale yellow in color, and exhibits an anhedral granular texture and massive structure. It primarily consists of albite (50–55 vol.%), quartz (35–40 vol.%), dolomite (5–10 vol.%), and sericite (~1 vol.%), with minor amounts of schorl, apatite, rutile, and pyrite (Figure 4a,b). Albite occurs as anhedral grains of 1.0–1.2 mm, showing no preferred orientation and it has an Ab component content of 99%. Quartz is also present as anhedral grains of 0.02–0.05 mm. Dolomite fills interstices between quartz and albite in anhedral, irregular shapes. Its MgO content ranges from 13.97 wt% to 15.72 wt%, FeO content from 9.16 wt% to 9.53 wt%, and Mg# [= Mg/(Mg + Fe²⁺) × 100] is 72.6–75.3, belonging to ferroan dolomite.

The laminated dolomite albitite (LSW-8) displays an obvious laminated structure with alternating light-yellow and yellowish-brown laminae. The mineral assemblages are the same, but their grain size and modal proportions differ in different laminae. The yellowish-brown laminae have relatively coarser grains (0.05–0.2 mm), dominated by albite (50–55 vol.%), dolomite (20–30 vol.%), quartz (15–20 vol.%), muscovite (1–2 vol.%), and apatite (1–2 vol.%), with minor schorl, rutile, pyrite, and limonite. The light-yellow laminae are finer-grained (0.02–0.10 mm), and characterized by significantly higher albite content (70–75 vol.%) but markedly lower proportions of dolomite (10–15 vol.%) and quartz (8–12 vol.%) (Figure 4c,d). There is no obvious difference in mineral compositions among different laminates. The Ab endmember content in albite ranges from 97.3% to 99.9%. Dolomite has MgO contents of 14.96–16.63 wt%, and FeO contents of 7.89–8.55 wt%, with Mg# of 75.7–79.0 belonging to ferroan dolomite.

The albitite breccia (LSW-3) contains 30–40 vol.% angular clasts, which are mainly quartz albitite and dolomite albitite fragments (Figure 2h). The cement comprises coarse-grained albite (45–50 vol.%), dolomite (30–40 vol.%), calcite (8–13 vol.%), quartz (1–5 vol.%), and pyrite (3–5 vol.%), with minor amounts of limonite, muscovite, rutile, apatite, schorl, and analcime (Figure 4e,f). The albite in the clasts and cement is similar in composition, with Ab endmember content of 97.8%–99.6%. Dolomite in the clasts has an Mg# of 82.8–82.9, classified as ferroan dolomite. The dolomite in the cement is also ferroan dolomite but exhibits obvious compositional zoning, with Mg# decreasing from 89.6 to 84.3 from core to rim.

5.2. Whole-Rock Trace Elements Composition

The silty phyllite has REE concentrations of 161–202 ppm. In the Post-Archean Australian Shale (PAAS)-normalized REE diagram, it exhibits flat patterns plotting near a value of 1 (Figure 5a), with slight negative Eu anomalies (δEu = 0.70–0.93) and no Ce anomalies (δCe = 0.97–0.99). Except for lower Ba, Pb, and Sr contents, the concentrations of other trace elements are comparable to the upper continental crust (UCC). Consequently, it displays nearly flat patterns on the UCC-normalized trace elements spider diagram (Figure 5b).

The REE concentration in the Na-altered silty phyllite (75.1–202 ppm) is equivalent to that in the silty phyllite. In the PAAS-normalized REE diagram, it displays flat patterns similar to those of the silty phyllite (Figure 5a), with weak negative Eu anomalies (δEu = 0.78–1.08) and no Ce anomalies (δCe = 0.94–1.00). In the UCC-normalized trace elements spider diagram, it shows nearly flat distribution patterns that basically overlap with those of the silty phyllite, except for lower Rb and Ba concentrations and reduced negative Sr anomalies, which correspond to the decrease in mica content and increase in dolomite during the Na-alteration process (Figure 5b).

The quartz albitite and laminated dolomite albitite have REE concentrations of 71.8–193 ppm and 145–199 ppm, respectively. In the PAAS-normalized REE diagram, they both exhibit flat patterns that overlap with those of silty phyllite and Na-altered silty phyllite, and show weak negative Eu anomalies (δEu = 0.76–0.98) and no Ce anomalies (δCe = 0.93–1.01) (Figure 5a). Similarly, on the UCC-normalized trace element spider diagram, they display nearly flat patterns comparable to those of the Na-altered phyllite and silty phyllite (Figure 5b).

The albitite breccia has REE concentrations of 90.3–210 ppm. After PAAS-normalization, it displays flat REE patterns with weak negative Eu anomalies (δEu = 0.82–0.90) and no Ce anomalies (δCe = 0.98–1.01) (Figure 5a). Except for the obvious negative anomalies of Ba, Pb, and Sr, it also exhibits nearly flat patterns around the ratio of 1 in a UCC-normalized spider diagram, which completely overlap with those of the silty phyllite (Figure 5b).

5.3. Zircon U–Pb Dating

Following the method of Anderson [49], over 45 detrital zircon grains from each sample were randomly selected for U–Pb analyses. Only data with concordance between 90% and 110% were retained. For spots with ages of <1.0 Ga, the ²⁰⁶Pb/²³⁸U ages were adopted, otherwise the ²⁰⁷Pb/²⁰⁶Pb ages were used. The final results are presented in Table A4.

Zircons from the silty phyllite range from 50 to 100 μm in size, mostly are subrounded or short-prismatic, and they display varying degrees of rounding, characteristic of detrital origin. Most of them show obvious oscillatory zoning in CL images (Figure 6a). Of 53 analyzed spots, 43 are nearly concordant, and the obtained youngest detrital zircon age is 395.9 Ma. The major peaks in age spectrum are at ~438 Ma, ~837 Ma, and ~978 Ma, with subordinate peaks at ~1581 Ma and ~2473 Ma (Figure 6b,c).

Zircons from the Na-altered silty phyllite are 50–100 μm in size, and show clear evidence of rounding. Their CL characteristics are consistent with those from the silty phyllite (Figure 6d). A total of 51 zircon grains were selected for U–Pb dating, of which 49 spots yielded concordant ages. The youngest age obtained is 399.1 Ma. The age spectrum is characterized by major peaks at ~422 Ma, ~821 Ma, and ~1048 Ma, and subordinate peaks at ~1542 Ma, ~1729 Ma, and ~2473 Ma (Figure 6e,f).

Zircon grains from the quartz albitite exhibit short-prismatic to subrounded morphologies, with sizes ranging from 50 to 100 μm. Most grains show varying degrees of rounding. Their CL characteristics are also similar to those of the silty phyllite (Figure 6g). Analyses on 80 zircon grains yielded 58 nearly concordant ages, the youngest of which is 395.1 Ma (Figure 6h,i). The age spectrum reveals major peaks at ~438 Ma, ~781 Ma, and ~932 Ma, and subordinate peaks at ~1598 Ma, ~2140 Ma, and ~2513 Ma.

Zircon grains from the laminated dolomite albitite are predominantly subrounded, 50–100 μm in size, and show varying degrees of rounding, indicative of detrital origin. Clear oscillatory zoning can be observed in most grains in CL images (Figure 6j). Among 56 analyses, 52 spots yielded concordant ages, giving a youngest zircon age of 396.2 Ma (Figure 6k,l). The age spectrum shows major peaks at ~446 Ma, ~814 Ma, and ~1056 Ma, and subordinate peaks at ~1964 Ma, ~2434 Ma, and ~2747 Ma.

5.4. Monazite U–Pb Dating and Trace Elements

TIMA phase mapping shows that there is no monazite in silty phyllite, but abundant monazite grains are uniformly distributed in Na-altered silty phyllite, particularly in coarser-grained domains (Figure 3c). These monazites are subhedral to anhedral 20–50 μm grains, and homogeneous in BSE images (Figure 7). In situ analyses of monazites in thin sections show that they have low Th contents (2638.8–8704.6 ppm) and very low U concentrations (5.1–20.6 ppm), with extremely high Th/U ratios of 422.4–938.8. They have total REE oxides (ΣREO) and CaO contents ranging from 63.08 wt% to 65.93 wt% and 0.25 wt% to 0.94 wt%, respectively. In the ΣREO-CaO diagram, nearly all spots are plotted in the field of hydrothermal and low-temperature metamorphic monazite (Figure 8b). In the chondrite-normalized REE diagram, all analyses display strongly right-sloping patterns with minor negative Eu anomalies (δEu = 0.61–0.80), similar to those of hydrothermal and low-temperature metamorphic monazites but distinct from those of magmatic ones (Figure 8a). Since the U-Pb analyses were conducted in situ in thin sections, the analytical error for individual analysis is relatively large. Nine spots define a discordia line on the Tera–Wasserburg diagram, yielding a lower intercept age of 260 ± 45 Ma (MSWD = 2.0) (Figure 9a).

Monazite is found in both laminae in laminated dolomite albitite, but monazite in the yellowish-brown laminae is slightly larger and higher in modal proportion (Figure 4b). Most monazite grains are subhedral or short-prismatic, 40–70 μm in size, and homogeneous (Figure 7b). In situ analyses on monazites in thin sections reveal that it have low Th (1773–8528 ppm) and very low U (3.6–288 ppm), corresponding to high Th/U ratios of 12–729, low CaO (0.43–0.80 wt%) content, and high ΣREO (64.15–65.97 wt% concentrations. In the ΣREO-CaO discrimination diagram, all analyses plot in the hydrothermal and low-temperature metamorphic monazite field (Figure 8b). These gains all exhibit right-sloping REE patterns on the chondrite-normalized diagram (Figure 8a). Six analyses define a discordia line with a lower intercept age of 252 ± 34 Ma on the Tera–Wasserburg diagram (MSWD = 0.2) (Figure 9b).

Monazites in quartz albitite are homogeneous, subhedral grains of 50–70 μm in size (Figure 7c) and are evenly distributed (Figure 4b). The separated grains have high Th (3817.4–58,920.0 ppm) and ΣREO (64.15–65.97 wt%) and low U (18.1–372.4 ppm) and CaO (0.43–0.80 wt%) content, resulting in high Th/U ratios of 88.2–1525.5. Almost all analyses are plotted in the hydrothermal and low-temperature metamorphic monazite field in the ΣREO-CaO discrimination diagram (Figure 8b). They display right-sloping REE patterns with slight negative Eu anomalies (δEu = 0.46–0.84) in the chondrite-normalized diagram (Figure 8a), similar to those of monazites in Na-altered silty phyllite. Seventeen analyses on separated grains give more precise results and form a well-defined discordia line in the Tera–Wasserburg diagram, giving a lower intercept age of 256.6 ± 2.6 Ma (MSWD = 0.71) (Figure 9c).

In albitite breccia, monazites are present in both clast and cement (Figure 4f), but only the grains in the cement were in situ analyzed in the section in order to obtain the age of brecciation. The analyzed monazites are granular or short-prismatic, homogeneous, and 50–100 μm in size (Figure 7d). They have high ΣREO (62.86–65.83 wt%) concentrations, but low Th (3044.4–12,318.4 ppm), U (8.0–24.7 ppm), and CaO (0.28–0.95 wt%) content. All analyses exhibit right-sloping REE patterns with slight negative Eu anomalies (δEu = 0.62–0.68) (Figure 8a), and fall in the hydrothermal and low-temperature metamorphic monazite field in the ΣREO-CaO discrimination diagram (Figure 8b). On the Tera–Wasserburg diagram, eleven analyses form a discordia line and give a lower intercept age of 256 ± 20 Ma (MSWD = 1.8) (Figure 9d).

6. Discussion

6.1. Genesis and Formation Age of the Albitites in Shangnan

Albitite (breccia) in the SQTB is discontinuously exposed within Devonian strata along the Fengzhen–Shanyang Fault, forming an albitite (breccia) zone extending about 400 km from Baoji in the west to the border between Shaanxi and Henan provinces in the east. They are most extensively developed and best exposed in the Fengxian–Taibai area in the western segment and the Shangnan area in the eastern segment. Numerous gold deposits, including the Shuangwang, Ertaizi, Sanguanmiao, and Jiuchaigou deposits, and gold occurrences, such as the Taipingzhuang and Baiyugou, have been discovered within this albitite (breccia) zone. Consequently, the genesis of this zone has attracted intense research interest.

Previous work mainly focused on Fengxian–Taibai area in the western segment of the albitite (breccia) zone. Although some scholars proposed that the albitite in the Fengxian–Taibai area was formed by hydrothermal metasomatism of Devonian clastic rock, this view has not gained widespread acceptance, since timing of the metasomatism was not established. Most researchers hold the opinion that albitite in the Fengxian–Taibai area has a syngenetic hydrothermal sedimentary origin, based on the following lines of evidence: (1) albitites primarily occur as conformable layers within the Devonian clastic rocks with distinct and straight boundaries [53]; (2) albitites are spatiotemporally associated with hydrothermally sedimentary siliceous rocks and barite [21,54]; (3) sedimentary structures including parallel bedding, graded bedding, rhythmic bedding, and convolute bedding, can be observed [13,21]; and (4) albite exhibits seawater-like, left-sloping chondrite-normalized REE patterns with pronounced negative Ce anomalies [13].

Compared with the Fengxian–Taibai area, albitites in the Shangnan area have not been thoroughly studied. Existing work has reported that albitites in this area occur as domes or lenses along fault zones, showing intrusive contacts with Devonian strata, and has suggested that these albitites are formed during Devonian Na-rich magma emplacement according to the obtained Rb-Sr isochron age of 364.9 ± 10.9 Ma [25].

In this study, a more detailed field investigation was conducted on the occurrence of albitite in the Shangnan area and its contact relationship with Devonian clastic rocks, and a systematic investigation, including whole-rock geochemistry and the distribution and U–Pb dating of accessory minerals, was performed. In terms of field occurrence, quartz albitite in the Liushuwan section occurs as layers, intercalated with grayish-black silty phyllite (Figure 2a). At the Boyugou section, quartz albitite forms lenticular bodies, but pure quartz albitite is only present in the center of the lenses (Figure 2b), and gradually transitions to light-yellow Na-altered silty phyllite and bluish-gray phyllite towards the periphery of the lenses (Figure 2c), which is also inconsistent with magma emplacement. Although dolomite albitite is interlayered with Devonian bluish-gray silty phyllite and preserves a laminated structure (Figure 2f), it differs from albitite in the Fengxian–Taibai area in containing abundant bluish-gray phyllite remnants (Figure 2e). Moreover, there are fractures in the phyllite layers extending into the dolomite albitite layer, and obvious albitization and bleaching have occurred along both sides of the fractures, as well as schistosity, with local formation of albitite (Figure 2f). These features indicate that the dolomite albitite is not of hydrothermal sedimentary origin but is more likely the result of hydrothermal metasomatism.

Geochemically, hydrothermal sedimentary rocks are characterized by left-inclined PAAS-normalized REE patterns with obvious negative Ce anomalies, similar to those of seawater and marine chemical sedimentary rocks (Figure 5a). However, quartz albitite and laminated dolomite albitite in the Shangnan area exhibit flat PAAS-normalized REE patterns distributed around ratios of 1, without Ce anomalies, which completely overlap with those of the Devonian silty phyllite and Na-altered silty phyllite, arguing against a syngenetic hydrothermal sedimentary origin (Figure 5a). In the UCC-normalized spider diagram, albitites also display flat patterns around ratios of 1, which are fully consistent with those of silty phyllite (Figure 5b), and are markedly depleted in elements enriched in hydrothermal sediments, such as Ba and Sr. These signatures indicate that the protoliths of the Shangnan albitites were normal clastic rocks.

In terms of zircon genesis and dating results, acidic and alkaline magmatic rocks may contain minor amounts of captured or inherited zircons, but the zircons in them should be dominated by magmatic ones [55,56]. Hydrothermal sedimentary rocks generally form in deep-water settings with limited terrigenous clastic input, resulting in a low content of detrital zircons. However, both quartz albitite and laminated dolomite albitite contain abundant zircons, which all exhibit varying degrees of rounding (Figure 6g,j), indicating a detrital origin rather than magmatic origin. The youngest age and age spectra of the detrital zircons in both quartz albitite and laminated dolomite albitite are consistent with those of the regional Devonian clastic rocks (Figure 6 and Figure 10), providing further evidence that protoliths of the Shangnan albitites are Devonian clastic rocks.

Monazite is a widespread accessory phosphate mineral in intermediate to felsic igneous, metamorphic, and hydrothermal–genetic rocks. It has high contents of Th and U, but extremely low initial common Pb, making it one of the most suitable minerals for U–Pb geochronology. Compared to zircon, monazite can easily undergo recrystallization and dissolution–reprecipitation processes during metamorphism and fluid-rock interaction, thereby serving as a more robust recorder of metamorphic or fluid events [50,52]. Moreover, monazites of different origins exhibit distinct textural properties and vary in composition. Magmatic monazite typically exhibits oscillatory, sector zoning, or homogeneous texture, has relative high ThO₂ (typically >3 wt%) and CaO (0.5–3.0 wt%) but low ΣREO (mostly <62 wt%) contents, and shows a pronounced negative Eu anomaly (δEu = 0.01–0.20) in a chondrite-normalized diagram (Figure 8). Hydrothermal monazite commonly show concentric oscillatory, sector zoning, or homogeneous texture, whereas low-temperature (greenschist-facies) metamorphic monazite typically shows a skeletal structure or occurs as overgrowth rims on inherited cores [50,52,60,61,62,63]. These two types of monazite have similar trace elements composition of low ThO₂ (typically <3 wt%), CaO (0.03–0.9 wt%) but high ΣREO (62–70 wt%) content, and both show weak negative Eu anomalies (Figure 8). TIMA phase mapping reveals that monazite is completely absent in silty phyllite, but appears in both Na-altered silty phyllite and albitites, which clearly demonstrates that monazite is not a detrital phase but formed during the albitization of Devonian strata. All the monazites from Na-altered silty phyllite and albitites are homogeneous (Figure 7) and exhibit right-sloping REE patterns with a weak negative Eu anomaly (Figure 8a), consistent with typical hydrothermal monazites. In the ΣREO-CaO diagram, all analyzed spots fall into the hydrothermal and low-temperature metamorphic monazite field (Figure 8b). These characteristics collectively indicate that the monazites in the Na-altered silty phyllite and albitites are of hydrothermal origin. In situ dating on these monazites yielded three lower intercept ages of 260 ± 45 Ma, 256.6 ± 2.6 Ma, and 252 ± 34 Ma, which are consistent within analytical uncertainties. These results are significantly younger than the previously reported Rb-Sr isochron age of 364.9 Ma [25]. However, considering that the protolith of albitites is Devonian clastic rocks, shown by geochemical data and detrital zircon spectra, and Rb is mainly hosted in K-rich detrital phases (such as muscovite), the Rb-Sr isochron age is likely a mixed age of the detrital provenance.

In summary, based on the comprehensive analyses of field occurrence, geochemical characteristics, and zircon and monazite dating results, the Shangnan albitites differ from those in the Fengxian–Taibai area. They are neither hydrothermal sedimentary rocks nor magmatic intrusions, but formed by extensive albitization of Devonian clast rocks at ca. 260–252 Ma.

6.2. Genesis and Formation Age of the Albitite Breccia in Shangnan

The albitite breccias in the Fengxian–Taibai area occur as eight lenticular bodies, which exhibit a low-angle oblique cutting relationship with Devonian strata [11,12]. Deep drilling at the Shuangwang gold deposit reveals that these breccia bodies extend in tabular shape at depth and exhibit systematic internal zonation in clast composition, morphology, and size, from the core towards the margins. The core zone is characterized by a relatively low clast content, smaller clast sizes (predominantly subangular or rounded), complex clast types, and the presence of clasts derived from deep-seated strata. Toward the margin, the clast content gradually increases, clasts become larger, and displacement decreases progressively, displaying a jigsaw-fit structure, and the clast types gradually transition to country rock clasts. Furthermore, the cement contains abundant fluid inclusions with high vapor-to-liquid ratios (20%–90%), and some three-phase inclusions even contain liquid CO₂. The homogenization temperatures of these inclusions reach 320–340 °C. Based on these characteristics, previous studies suggested that the central part of albitite breccia in the Fengxian–Taibai area is the product of vapor-driven hydrothermal explosion [22,23], whereas the outer zones are the result of hydraulic fracturing induced by episodic fluid overpressuring [13,24]. Fan et al. [23] obtained a youngest detrital zircon age of ca. 278 Ma from the albitite breccias, and Cheng [64] yielded a zircon U–Pb age of ca. 214 Ma for a lamprophyre that cut through the albitite breccia, indirectly constraining the formation age of the breccia to between ca. 278 and 214 Ma. Recently, He [24] performed U–Pb dating on hydrothermal monazite in the cement of albitite breccia, directly constraining its formation age to 220.9 ± 1.8 Ma.

Compared with the Fengxian–Taibai area, the albitite breccias in Shangnan are smaller in scale and are mostly observed locally within the albitite lenses, especially in the mantle and marginal parts of the lenses. The clasts are predominantly angular to subangular. The lithological types of clasts are similar to the surrounding host rocks, and no clasts from deeper stratigraphic units are observed (Figure 2h,i), indicating limited displacement of clasts during brecciation. Furthermore, studies on the Sanguanmiao gold deposit reveal that the ore-forming fluid is dominated by liquid-rich fluid inclusions with vapor–liquid ratios of only 5%–30%, and the homogenization temperatures are 180–220 °C [19]. These features indicate that both the fluid pressure and temperature during brecciation in the Shangnan area were lower than those in the Fengxian–Taibai area. In situ dating of hydrothermal monazite in the cement yields a lower intercept age of 256 ± 20 Ma, consistent with uncertainty regarding the formation age of albitites (Figure 9). Collectively, these results suggest that albitite breccias in Shangnan are likely the product of hydraulic fracturing. They formed by localized rock fragmentation and subsequent cementation due to concentrated fluid overpressure during the Late Permian hydrothermal albitization of Devonian clastic rocks.

6.3. Formation Model of the Albitite (Breccia) Zone in SQTB

The QOB is a composite orogenic belt with a protracted evolutionary history. It recorded multiple tectonic cycles, including the closure of the Meso-Neoproterozoic Kuanping Ocean, the Early Paleozoic Shangdan Ocean, and the Late Paleozoic Mianlüe Ocean, as well as Cenozoic intracontinental orogenesis [27]. The South Qinling tectonic belt is widely regarded as a microcontinent that rifted from the Yangtze Block during the Devonian–Carboniferous, and it documented the subduction and closure processes of the Mianlüe Ocean [27,29]. Extensive studies show that the Mianlüe Ocean subducted northward beneath South Qinling between 300 and 220 Ma and generated numerous 250–220 Ma subduction-related I-type granitoids. Its closure and subsequent continental collision occurred at 220–210 Ma, leading to the formation of syn-collisional granites (220–210 Ma) and post-collisional rapakivi granites at 210–200 Ma in SQTB [27,28,65].

Hydrogen and oxygen isotope analyses of ores closely associated with the albitite breccia in the Sanguanmiao gold deposit indicate that the ore-forming fluids were dominated by magmatic water [19]. Carbon and oxygen isotope analyses of carbonates in Sanguanmiao gold ores and Shangnan albitite (breccia) reveal that the hydrothermal fluids have δ¹³C_PDB values ranging from −0.17‰ to −0.58‰ and δ¹⁸O_SMOW values from 1.46‰ to 2.06‰. These data plot between the fields of marine carbonate and granite in the δ¹⁸O versus δ¹³C diagram, with the majority distributed along the dissolution trend of marine carbonate (Figure 11; ref. [19]; our unpublished data). Considering that the Devonian Gudaoling Formation and the underlying Sinian strata are dominated by carbonate rocks, the fluids involved in the formation of the albitites and albitite breccias in the Shangnan area may have been a mixture of magmatic water and regional formation fluids. The formation age of Shangnan albitites and albitite breccias (ca. 260–252 Ma) is essentially coeval with the northward subduction period of the Mianlüe Ocean. Therefore, we propose that the Shangnan albitites and albitite breccias may have been formed during the Permian subduction of the Mianlüe Ocean. Mixed fluids, derived from magmatic water associated with arc magmatism and regional formation fluids generated by dehydration and decarbonation of local strata, ascended along the Fengzhen–Shanyang Fault and caused extensive albitization of Devonian clastic rocks, accompanied by local fracturing and cementation, ultimately leading to the formation of the albitites and albitite breccias.

Extensive studies on the albitites and albitite breccias in the Fengxian–Taibai area suggested that the albitites were formed by Devonian syngenetic hydrothermal sedimentary processes [13,21]. However, the formation age of the albitite breccias is consistent with the widespread syn- to post-collisional magmatic activity in SQTB [24,27,37,65]. Thus, it can be inferred that the albitite breccia in the Fengxian–Taibai area formed as a result of hydrothermal explosion, fracturing, and cementation of albitites by hydrothermal fluids generated during the transition from continental collision to post-orogenic extension after the closure of the Mianlüe Ocean [11,13,24].

In summary, the formation of the albitites (breccia) zone in the SQTB is closely related to fluid activities during the subduction of the Mianlüe Ocean and the subsequent orogenic processes after its closure. The albitites (breccia) zone in the SQTB were formed under distinct tectonic settings at different stages of the Mianlüe Ocean evolution.

7. Conclusions

(1)

The albitites in the Shangnan area occur as stratiform layers or lenticular bodies within the silty phyllite of the Xinghongpu or Jiuliping Formations. They are not of magmatic or syngenetic hydrothermal sedimentary origin, but rather products of complete albitization of Devonian clastic rocks at ca. 260–252 Ma.

(2)

The Shangnan albitite breccias contain angular to subangular clasts, which have similar lithology to hosting rock and show limited displacement. The 256 ± 20 Ma age obtained on hydrothermal monazites in the cement indicates that the albitite and albitite breccias in the Shangnan area are products of the same hydrothermal activity.

(3)

During the Permian subduction of the Mianlüe Ocean, mixed fluids, derived from magmatic water associated with arc magmatism and regional formation fluids generated by dehydration and decarbonation of local strata, ascended along the Fengzhen–Shanyang Fault and caused extensive metasomatism of Devonian clastic rocks, accompanied by local fracturing and cementation, ultimately leading to the formation of the albitites and albitite breccias.

(4)

The albitites (breccia) zone in the SQTB were formed under distinct tectonic settings at different stages of the Mianlüe Ocean’s evolution.