Timing and Provenance Transition of the Neoproterozoic Wuling Unconformity and Xihuangshan Unconformity of the Yangtze Block: Responses to Peripheral Orogenic Events

: Middle Neoproterozoic sedimentary strata are widely distributed on the periphery of the Yangtze Block. In the western Jiangnan Orogen, they are divided into the Lengjiaxi and Banxi groups by the “Wuling unconformity”, and the Banxi Group is further divided into the Madiyi Formation and Wuqiangxi Formation by the “Xihuangshan unconformity”. However, the timing and tectonic significance of the Wuling and Xihuangshan unconformities remain unclear, which hampers our understanding of the Precambrian tectonic evolution of the Yangtze Block. Zircon U–Pb dating and Lu–Hf isotopic analysis were performed on the sedimentary rocks above and below the two unconformity boundaries in the western Jiangnan Orogen. These data were used to trace sedimentary provenance and provide new insights into the basin evolution and tectonic significance of the unconformities. Combined with previous studies, the Wuling unconformity is bracketed to have formed between ~830 and 813 Ma, and the provenance of the sediments above the unconformity remained unchanged. The detrital zircons from the upper parts of the Lengjiaxi Group and lower parts of the Banxi Group show the primary peak ages of 800–1000 Ma, 1.0–1.30 Ga, 1.40–1.90 Ga, and 2.30–2.60 Ga, and the provenance mainly derived from the southwestern margin of the Yangtze Block, Cathaysia Block, and Jiangnan Orogen. The provenance from the Cathaysia Block occurred in the upper part of the Lengjiaxi Group, indicating that the Yangtze Block and Cathaysia Block merged in the western Jiangnan Orogen earlier than the formation time of the Wuling unconformity (~830–813 Ma) and the collisional time in the eastern Jiangnan Orogen (~820–800 Ma). Thus, the collision between the Yangtze and Cathaysia blocks may have undergone a scissor-like closure process from west to east. The formation time of the Xihuangshan unconformity was at ~800–779 Ma. The field contact relationships changed from an angular unconformity to a disconformity and then to conformity, from north to south, indicating that the Xihuangshan unconformity was controlled by tectonic movement in the north. The provenance of the sedimentary strata changed above the Xihuangshan unconformity. The detrital zircon age peaks of the upper Banxi Group are 755–1000 Ma, 1.90–2.10 Ga, and 2.35–2.70 Ga, and the detritus were derived from the northern margin of the Yangtze Block and the Jiangnan Orogen. This unconformity is coeval with that of the ~800–780 Ma collisional orogeny at the northern and northwestern margins of the Yangtze Block. Thus, the Xihuangshan unconformity is likely a response to the collision orogeny in the northern and northwestern margins of the Yangtze Block and induces the transition of sedimentary provenance.

Over the past two decades, much attention has been focused on the petro genesis, and tectonic setting of Neoproterozoic igneous rocks in the Jiangnan Orogen (e.g., [6,7,13,15,21,22,[24][25][26][29][30][31][32]34,35,37]), whereas less attention has been given to the middle Neoproterozoic unconformities (i.e., Wuling and Xihuangshan unconformities) and associated sedimentary strata (e.g., [18,39,40]). The middle Neoproterozoic Wuling unconformity is generally considered a sign of the collision and merging of the Yangtze and Cathaysia blocks at the southeastern edge of the Yangtze Block (e.g., [23]). Above the "Wuling unconformity", there is another Neoproterozoic unconformity called the "Xihuangshan unconformity". The Neoproterozoic sedimentary strata (Lengjiaxi and Banxi groups) across these two unconformities record peripheral orogenic events and are crucial for understanding the Neoproterozoic tectonic evolution of the Yangtze Block. Some studies have found that the upper and lower strata across the Wuling unconformity have similar provenance [18,39,40], whereas others have proposed the existence of a provenance transition between the Banxi and Lengjiaxi groups across the Wuling unconformity in this region [41][42][43]. However, the formation age, distribution range, and tectonic significance of the Xihuangshan unconformity, as well as the provenance of the strata across the Xihuangshan unconformity, are still poorly understood.
In this study, middle Neoproterozoic clastic rocks above and below the Wuling and Xihuangshan unconformities in the Guzhang and Zhijiang areas of the Hunan Province in the western Jiangnan Orogen were observed. Detailed detrital zircon U-Pb dating and Hf isotopic analysis were performed to provide new insights into the tectonic significance of these two unconformities in the western Jiangnan Orogen and their implications for the Neoproterozoic orogenic event around the Yangtze Block.  [42,45]). (c) Simplified geological map of the Western Jiangnan Orogen (WJO) (revised after [46][47][48]).

Geological Setting
It is believed that the South China Block was formed by the merger of the Yangtze Block in the northwest and the Cathaysia Block in the southeast during the Neoproterozoic (Figure 1b; [10,27,38]). It is separated from the North China Craton by the Qinling-Dabie Belt to the north and from Tibet by the Songpan-Ganzi Belt and Panxi Belt to the west [49]. The Precambrian basement of the Yangtze Block is mainly composed of Proterozoic rocks with scarce Archean outcrops [10]. The Archean to Paleoproterozoic rock assemblages of the Yangtze Block mainly contain the Dahongshan, Dongchuan, and Hekou groups on the southwestern margin of the Yangtze Block, the Kongling Complex, Yudongzi Group, and Houhe Complex on the northern margin of the Yangtze Block, and the Huangtuling Complex in the Qinling-Dabie Belt [10,42,50]. Mesoproterozoic rock outcrops such as the Tianli schist, Kunyang Group, and Huili Group are mainly distributed in the southeastern and southwestern margins of the Yangtze Block [50]. Contrary to the sporadic exposure of pre-Neoproterozoic rocks in the Yangtze Block, the early-middle Neoproterozoic (1000-720 Ma) metamorphosed volcanic sedimentary units and magmatic Orogen separated the Yangtze Block in the northwest from the Cathaysia Block in the southeast (modified after [42,45]). (c) Simplified geological map of the Western Jiangnan Orogen (WJO) (revised after [46][47][48]).

Geological Setting
It is believed that the South China Block was formed by the merger of the Yangtze Block in the northwest and the Cathaysia Block in the southeast during the Neoproterozoic (Figure 1b; [10,27,38]). It is separated from the North China Craton by the Qinling-Dabie Belt to the north and from Tibet by the Songpan-Ganzi Belt and Panxi Belt to the west [49]. The Precambrian basement of the Yangtze Block is mainly composed of Proterozoic rocks with scarce Archean outcrops [10]. The Archean to Paleoproterozoic rock assemblages of the Yangtze Block mainly contain the Dahongshan, Dongchuan, and Hekou groups on the southwestern margin of the Yangtze Block, the Kongling Complex, Yudongzi Group, and Houhe Complex on the northern margin of the Yangtze Block, and the Huangtuling Complex in the Qinling-Dabie Belt [10,42,50]. Mesoproterozoic rock outcrops such as the Tianli schist, Kunyang Group, and Huili Group are mainly distributed in the south-eastern and southwestern margins of the Yangtze Block [50]. Contrary to the sporadic exposure of pre-Neoproterozoic rocks in the Yangtze Block, the early-middle Neoproterozoic (1000-720 Ma) metamorphosed volcanic sedimentary units and magmatic rocks are widely distributed around the Yangtze Block [10,51]. Neoproterozoic metamorphic rocks are also widely exposed in the Cathaysia Block and are primarily affected by greenschist facies metamorphism [50].
The Jiangnan Orogen is located along the southeastern margin of the Yangtze Block, with a length of 1500 km in an east-west direction. It is mainly composed of Neoproterozoic sedimentary strata and magmatic rocks [23]. The Neoproterozoic sedimentary strata in the western Jiangnan Orogen are divided into two parts by the Wuling unconformity. The basement sequence below the Wuling unconformity includes phyllite, slate, sandstone, and siltstone with a small number of tholeiitic lavas and volcaniclastic interlayers, called the Lengjiaxi Group, Fanjingshan Group, and Sibao Group [46][47][48]. The overlying weakly metamorphosed Banxi, Xiajiang, and Danzhou groups are mainly composed of sandstone, slate, conglomerate, marl, carbonate, shale, and volcaniclastic rocks [16,23]. Available geochronological studies suggest that the Banxi Group was formed at~815-715 Ma [44,[52][53][54][55] and was separated into the Madiyi and Wuqiangxi formations from the bottom to the top by the Xihuangshan unconformity ( Figure 2; [56]). rocks are widely distributed around the Yangtze Block [10,51]. Neoproterozoic metamorphic rocks are also widely exposed in the Cathaysia Block and are primarily affected by greenschist facies metamorphism [50]. The Jiangnan Orogen is located along the southeastern margin of the Yangtze Block, with a length of 1500 km in an east-west direction. It is mainly composed of Neoproterozoic sedimentary strata and magmatic rocks [23]. The Neoproterozoic sedimentary strata in the western Jiangnan Orogen are divided into two parts by the Wuling unconformity. The basement sequence below the Wuling unconformity includes phyllite, slate, sandstone, and siltstone with a small number of tholeiitic lavas and volcaniclastic interlayers, called the Lengjiaxi Group, Fanjingshan Group, and Sibao Group [46][47][48]. The overlying weakly metamorphosed Banxi, Xiajiang, and Danzhou groups are mainly composed of sandstone, slate, conglomerate, marl, carbonate, shale, and volcaniclastic rocks [16,23]. Available geochronological studies suggest that the Banxi Group was formed at ~815-715 Ma [44,[52][53][54][55] and was separated into the Madiyi and Wuqiangxi formations from the bottom to the top by the Xihuangshan unconformity ( Figure 2; [56]). (revised after [48,57]). Abbreviations: Gr-Group; Fm-Formation.

Sample Descriptions
Based on detailed geological fieldwork, a total of eight samples were selected across the Wuling and Xihuangshan unconformities in the Zhijiang and Guzhang regions (Hu-

Sample Descriptions
Based on detailed geological fieldwork, a total of eight samples were selected across the Wuling and Xihuangshan unconformities in the Zhijiang and Guzhang regions (Hunan Province, China) for zircon U−Pb dating and Hf isotopic analysis. Sample locations are shown in Figures 1c and 2  Samples GZ01 and ZJ05 were gray-green fine-grained lithic sandstone and mediumgrained feldspathic lithic sandstone, respectively, taken from the upper part of the Lengjixi Group (Figure 3a Table 1.

Zircon LA−ICP−MS U−Pb Dating
Zircon grains were obtained from the samples by heavy-liquid and magnetic separation techniques and then mounted in epoxy and polished. The external and internal struc-

Zircon LA−ICP−MS U−Pb Dating
Zircon grains were obtained from the samples by heavy-liquid and magnetic separation techniques and then mounted in epoxy and polished. The external and internal structures of the zircons were documented by transmitted and reflected light photomicrographs and Cathodoluminescence (CL) images, which were used to select target sites for U-Pb dating and Hf isotopic analysis. Zircon U-Pb dating analysis was performed by LA−ICP−MS at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan (SKLGPMR−CUG). A GeoLas 2005 excimer ArF laser-ablation system and Agilent 7500a ICP−MS instrument were combined for the experiments. The detailed instrumental setting, operating procedures, and offline data reduction were described by   [58]. The data were processed using the ISOPLOT program (ver. 3.0) of Ludwig (2003) [59]. According to the conventional rule, the measured 206 Pb/ 238 U (<1000 Ma) and 207 Pb/ 206 Pb (>1000 Ma) ages are presented in the figures and discussions.

Zircon Lu−Hf Isotope Analysis
In situ zircon Hf isotopic analysis was carried out on a Neptune Plus MC−ICP−MS (Thermo Fisher Scientific, Bremen, Germany), coupled with a GeoLas 2005 laser-ablation system at SKLGPMR−CUG, and the detailed operating conditions and the analytical procedures were the same as described by Hu et al. (2012) [60]. We applied the directly obtained β Yb value from the zircon sample itself in real time [61]. The 179 Hf/ 177 Hf and 173 Yb/ 171 Yb ratios were used to calculate the mass bias of Hf (β Hf ) and Yb (β Yb ), which were normalized to 179 Hf/ 177 Hf = 0.7325 and 173 Yb/ 171 Yb = 1.1248 [62] using an exponential correction for mass bias. Interference of 176 Yb in 176 Hf was corrected by measuring the interference-free 173 Yb isotope and using 176 Yb/ 173 Yb = 0.7876 [63] to calculate 176 Yb/ 177 Hf. Similarly, the relatively minor interference of 176 Lu in 176 Hf was corrected by measuring the intensity of the interference-free 175 Lu isotope and using the recommended 176 Lu/ 175 Lu = 0.02656 [64] to calculate 176 Lu/ 177 Hf. We used the mass bias of Yb (β Yb ) to calculate the mass fractionation of Lu because of their similar physicochemical properties. Offline selection and integration of analyte signals and mass bias calibrations were performed using ICPMSDat-aCal (ver. 9.0) written by Liu et al. (2010) [61].

Analytical Results
The results of zircon U−Pb dating are presented in Table S1 (Supplementary Materials) and the concordant diagrams ( Figure 5) with corresponding relative probability plots ( Figure 6). Detrital zircons with concordant U−Pb ages (concordance between 90 and 110%) were selected for in situ Hf isotopic analysis, and the results are given in Table S2 (Supplementary Materials) and plotted in Figure 7. Both a Probability Density Plot (PDP) and Kernel Density Estimation (KDE) were used to visualize the detrital age distribution patterns ( Figure 6; [65]). These two methods gave similar detrital zircon peaks in Figure 6; thus, the PDP is still useful as a probability density estimator in this study. Additionally, Spencer et al. (2016) [66] documented that the discordance of zircon ages may result in a meaningless age spectrum. In Figure 5, most of the Neoproterozoic detrital zircons overlapped the 1:1 concordance line, and no negatively skewed tail in the youngest zircon population is shown in Figure 6, indicating that the effect of lead loss is negligible.

Upper Lengjiaxi Group
Sample GZ01 was collected from the upper part of the Lengjiaxi Group in Guzhang County. Sixty-five zircon grains were analyzed for U−Pb ages, although one was discarded because of discordance of more than 10% (Table S1, Supplementary Materials). Among these ages, Neoproterozoic (802−998 Ma) subhedral and oscillatory-zoned (Figure 5a) zircon grains are the dominant population and have one major age peak and a minor peak at ca. 837 Ma and ca. 913 Ma, respectively (Figure 6a). Twenty pre-Neoproterozoic zircon grains have 207 Pb/ 206 Pb ages ranging from 1.03 Ga to 3.68 Ga. The pre-Neoproterozoic zircon grains are subhedral to rounded and show partially oscillatory zoning (Figure 5a). Thirteen Neoproterozoic zircons were selected for the Hf isotopic analysis (Table S2, Supplementary Materials). Eleven Neoproterozoic (828-915 Ma) zircon grains have positive ε Hf (t) values (1.7 to 13.9), except for two zircon grains with negative ε Hf (t) values (−3.6 and −3.9) (Figure 7a).
Sample ZJ05 was collected from the upper part of the Lengjiaxi Group in Zhijiang County. Sixty-five zircon grains were analyzed for U-Pb ages, although one was discarded due to a discordance of more than 10% (Table S1, Supplementary Materials). These zircons yield a predominant age population of 822-962 Ma with two major age peaks at ca. 830 and 854 Ma (Figure 6b). Pre-Neoproterozoic zircon grains cluster at groups of 1.01-1.39 Ga, 1.54-1.91 Ga, and 2.34-2.53 Ga, and one Paleoarchean zircon grain has a 207 Pb/ 206 Pb age of 2.85 Ga. The Neoproterozoic zircon grains are subhedral and have well-preserved oscillatory zoning. The pre-Neoproterozoic zircon grains are subhedral to rounded and display partly oscillatory-zoned grains (Figure 5b). Twenty-two zircons were selected for the Hf isotopic analysis (Table S2,

Lowermost Madiyi Formation
Sample GZ03 was collected from the lowermost part of the Madiyi Formation in Guzhang County. Sixty-five zircon grains were analyzed for U−Pb ages, although one was discarded due to a discordance of more than 10% (Table S1, Supplementary Materials). Neoproterozoic zircon grains yield an age population of 803-948 Ma, with one major age peak at ca. 846 Ma and one minor peak at ca. 914 Ma (Figure 6c). These zircons are generally euhedral to subhedral and have well preserved oscillatory zoning (Figure 5c). Pre-Neoproterozoic zircon grains are rare, with ages ranging from 1.19 Ga to 2.68 Ga. Thirteen Neoproterozoic (817−926 Ma) zircon grains were selected for the Hf isotopic analysis (Table S2, Supplementary Materials) and have variable εHf(t) values (−9.7 to 15.5) and TDM2 ages (816-2137 Ma) (Figure 7c).
Sample ZJ11 was collected from the lowermost part of the Madiyi Formation in Zhijiang County. Sixty zircon grains were analyzed for U−Pb age dating, and 27 of them were selected for the Hf isotopic analysis (Tables S1 and S2, Supplementary Materials). Among these ages, zircon grains yield two major age peaks at ca. 856 Ma and 880 Ma and three minor peaks at ca. 937 Ma, 1.59 Ga, and 1.90 Ga (Figure 6d). These zircon grains are generally subhedral to rounded with partly preserved oscillatory zoning (Figure 5d

Uppermost Madiyi Formation
Sample GZT11 was collected from the uppermost part of the Madiyi Formation in Guzhang County. Sixty zircon grains were analyzed for U−Pb ages, and 23 of them were selected for the Hf isotopic analysis (Tables S1 and S2 Sample ZJ14 was obtained from the uppermost part of the Madiyi Formation in Zhijiang County. Sixty zircon grains were analyzed for U−Pb ages, although two were discarded due to a discordance of more than 10%, and 28 of them were selected for the Hf isotopic analysis (Tables S1 and S2

Lowermost Wuqiangxi Formation
Detrital zircons of sample GZT12 were obtained from the lowermost part of the Wuqiangxi Formation in Guzhang County. Sixty zircon grains were analyzed for U−Pb ages, and 22 of them were selected for the Hf isotopic analysis (Tables S1 and S2  From north to south of Hunan Province, the composition of the Madiyi Formation in the lower part of the Banxi Group changes from coarse to fine grains, the sedimentary strata become thicker, and the color of the rocks changes from purple-red to gray-green. Additionally, its contact with the underlying Lengjiaxi Group changes from a high-angular unconformity to a low-angular unconformity and disconformity, and the degree of deformation and metamorphism of the underlying Lengjiaxi Group gradually weakens in the south (Figures 8a and 9C-G; [47,48,77]). However, in northern Guangxi, sediments from the lower part of the Danzhou Group (equivalent strata of the Madiyi Formation) change from fine to coarse grains from northeast to southwest, the sedimentary strata become thinner towards the southwest, and their contact with the underlying Sibao Group changes from a disconformity to an angular unconformity (Figure 8a; [46,77]).

Regional Geological Features and Timing of the Wuling Unconformity
From north to south of Hunan Province, the composition of the Madiyi Formation in the lower part of the Banxi Group changes from coarse to fine grains, the sedimentary strata become thicker, and the color of the rocks changes from purple-red to gray-green. Additionally, its contact with the underlying Lengjiaxi Group changes from a high-angular unconformity to a low-angular unconformity and disconformity, and the degree of deformation and metamorphism of the underlying Lengjiaxi Group gradually weakens in the south (Figures 8a and 9C-G; [47,48,77]). However, in northern Guangxi, sediments from the lower part of the Danzhou Group (equivalent strata of the Madiyi Formation) change from fine to coarse grains from northeast to southwest, the sedimentary strata become thinner towards the southwest, and their contact with the underlying Sibao Group changes from a disconformity to an angular unconformity (Figure 8a; [46,77]). Dickinson and Gehrels (2009) [84] suggested that the youngest age peak is more consistent with the depositional ages of the strata than the youngest single-grain age. In this study, the youngest age peaks of detrital zircons from sandstone samples of the upper Lengjiaxi Group are ~830 Ma and ~837 Ma (Figure 6a [85] reported a tuff at the bottom of the Banxi Group in the Zhijiang area with an age of 813.5 ± 9.6 Ma. Therefore, the timing of the Wuling unconformity in the study area could be constrained to ~830-813 Ma. Numerous studies have been conducted on the timing of the Wuling unconformity from north to south as follows. (1) The youngest detrital Dickinson and Gehrels (2009) [84] suggested that the youngest age peak is more consistent with the depositional ages of the strata than the youngest single-grain age. In this study, the youngest age peaks of detrital zircons from sandstone samples of the upper Lengjiaxi Group are~830 Ma and~837 Ma (Figure 6a [85] reported a tuff at the bottom of the Banxi Group in the Zhijiang area with an age of 813.5 ± 9.6 Ma. Therefore, the timing of the Wuling unconformity in the study area could be constrained to~830-813 Ma. Numerous studies have been conducted on the timing of the Wuling unconformity from north to south as follows. (1) The youngest detrital zircons suggest a maximum deposition age of~830 Ma for the Lengjiaxi Group in the Yangjiaping section, Shimen County, northwestern Hunan [41]. The tuff of the Laoshanya Formation above the Wuling unconformity was formed at 809 ± 16 Ma [83]. The timing of the Wuling unconformity was limited to~830-809 Ma ( Figure 9C). (2) The timing of the Wuling unconformity in the Lucheng section of Linxiang County in northeastern Hunan was constrained to~822-802 Ma [86]. (3) The timing of the Wuling unconformity in Changsha, northern Hunan, has been suggested to be~827-798 Ma [69]. (4) In the Fanjingshan region, the youngest peak age of the Fanjingshan Group below the Wuling unconformity is~816 Ma [49], whereas the U−Pb age of the tuff above the unconformity is 814 ± 6 Ma [87], and the timing of the Wuling unconformity is limited to~816-814 Ma. (5) The timing of the Wuling unconformity in northern Guangxi was limited to~835-795 Ma or~832-803 Ma ( Figure 9I; [39,40]). The findings of all of the above studies are consistent with our data, thus supporting the formation of the Wuling unconformity at~830-813 Ma in the western Jiangnan Orogen.  [41]. The tuff of the Laoshanya Formation above the Wuling unconformity was formed at 809 ± 16 Ma [83]. The timing of the Wuling unconformity was limited to ~830-809 Ma ( Figure 9C). (2) The timing of the Wuling unconformity in the Lucheng section of Linxiang County in northeastern Hunan was constrained to ~822-802 Ma [86]. (3) The timing of the Wuling unconformity in Changsha, northern Hunan, has been suggested to be ~827-798 Ma [69]. (4) In the Fanjingshan region, the youngest peak age of the Fanjingshan Group below the Wuling unconformity is ~816 Ma [49], whereas the U−Pb age of the tuff above the unconformity is 814 ± 6 Ma [87], and the timing of the Wuling unconformity is limited to ~816-814 Ma. (5) The timing of the Wuling unconformity in northern Guangxi was limited to ~835-795 Ma or ~832-803 Ma ( Figure 9I; [39,40]). The findings of all of the above studies are consistent with our data, thus supporting the formation of the Wuling unconformity at ~830-813 Ma in the western Jiangnan Orogen.

Regional Geological Features and Timing of the Xihuangshan Unconformity
A typical section of the Xihuangshan unconformity is located in Zhijiang County with the occurrence of bottom conglomerate and weathering crust (Figure 3f; [56]). Based on a regional comparison, it was established that the Xihuangshan unconformity is widely

Regional Geological Features and Timing of the Xihuangshan Unconformity
A typical section of the Xihuangshan unconformity is located in Zhijiang County with the occurrence of bottom conglomerate and weathering crust (Figure 3f; [56]). Based on a regional comparison, it was established that the Xihuangshan unconformity is widely distributed in Hunan Province [52,55,81], and from north to south, it changes from an angular unconformity to a disconformity and then to a conformable contact relationship (Figures 8b and 9A-I, and references therein), indicating that the Xihuangshan unconformity was formed by the uneven uplift of the block from north to south [56].
The tuff of the upper Laoshanya Formation below the Xihuangshan disconformity in the Yangjiaping section of northwestern Hunan formed at 809 ± 16 Ma [83]. Thus, the initial time of the Xihuangshan unconformity should have been later than 809 Ma. In this study, samples were collected from the top of the Madiyi Formation below the unconformity and from the bottom of the Wuqiangxi Formation above the unconformity in Guzhang and Zhijiang. The youngest detrital zircon 206Pb/238U age peaks of samples from the top of the Madiyi Formation are 803 Ma and 800 Ma (Figure 6e-f), which limits the maximum age of the Xihuangshan unconformity to~800 Ma. In addition, the tuff age of the lower Liantuo Formation (equivalent to the Wuqiangxi Formation) in Yichang and Dahongshan areas, Hubei Province, above the Xihuangshan unconformity, are 776.6 ± 3.8 Ma and 779 ± 12 Ma, respectively [82,88]. These results are consistent with the~780 Ma peak age of the clastic rock sample GZT12 from the bottom of the Wuqiangxi Formation (Figure 6g). Therefore, the timing of the Xihuangshan unconformity was constrained to~800-779 Ma.
The Neoproterozoic (800-860 Ma) detrital zircons constitute the largest and most important age group. Most zircons exhibited euhedral to subhedral morphology (Figure 5a-d), suggesting a proximal source. Many 860-760 Ma magmatic rocks are exposed in the western Jiangnan Orogen [6,7,15,22,34,95,98,99], implying that the rapid erosion of these rock assemblages may provide a potential source [40]. In this study, most of the measured zircon εHf(t) values for this age group were positive (Figure 7a,c), indicating mantle-derived magma input and juvenile crust generation. These zircons can be compared with those of 854 Ma mafic rocks from the Yuanbaoshan area in northwest Guangxi and~832-837 Ma granodiorite from the Dongma and Longyou areas [34,49]. Therefore, the 800-860 Ma detrital zircons mainly arose from the rapid erosion of adjacent magmatic rocks in the western Jiangnan Orogen.
The 860-1000 Ma zircons showed both positive and negative εHf(t) values (Figure 7a,c). The positive εHf(t) values of zircons correspond well to those of the magmatic rocks in the eastern Jiangnan Orogen, such as ophiolites in northeast Jiangxi Province, volcanic rocks in the Shuangxiwu arc, and~905 Ma granitoids that intruded into the Pingshui Formation in the Shaoxing area [24,26,35,37,100]. However, 900−1000 Ma magmatic rocks with negative ε Hf (t) values have only been reported in the Cathaysia Block ([29] and references therein). These zircons exhibit euhedral to subhedral morphologies, suggesting relatively short transport distances. Additionally, the~1.0-1.3 Ga,~1.85 Ga, and~2.50 Ga zircons are consistent with the age of the Cathaysia Block (Nanling-Yunkai region) [71,96]. Furthermore, paleocurrent analysis showed that the primary sedimentary detrital of the upper Fanjingshan Group (equivalent strata of the Lengjiaxi Group) came from the south (current coordinates) [101]. Hence, the Cathaysian Block may have been an important source of these strata. Notably, the 870-985 Ma inherited zircons reported in the 835−800 Ma S-type granite plutons in Guangxi suggest that there may be undiscovered early Neoproterozoic magmatic activity in the western Jiangnan Orogen, in which unexposed basement rocks may be the source of these sediments [40,98].
The detrital zircons of 1.5-1.90 Ga and 2.40-2.60 Ga exhibit complex morphological characteristics (Figure 5a-d), indicating that they could have diverse sources. These age groups are similar to those from the Kunyang, Huili, Dongchuan, and Dahongshan groups [102][103][104] in the southwestern margin of the Yangtze Block, implying that the sediments are partly sourced from the southwestern margin of the Yangtze Block [43].
In summary, the Lengjianxi Group and lower Banxi Group (and their equivalent strata) in the western Jiangnan Orogen have similar detrital zircon age peaks, with a mixture of sources from the southwestern margin of the Yangtze Block, Cathaysia Block, and Jiangnan Orogen.

Provenance of the Upper Banxi Group
The detrital zircon age groups of the upper Banxi Group above the Xihuangshan unconformity mainly include 755-1000 Ma, 1.90-2.10 Ga, and 2.35-2.70 Ga, which are substantially different from those in the lower Banxi Group (Figures 10 and 11), indicating a change in provenance. Most Neoproterozoic zircons exhibited euhedral to subhedral morphologies (Figure 5g,h), suggesting proximal sources. The ratios of negative εHf(t) values of detrital zircons <840 Ma are significantly higher than those of 950-840 Ma (Figure 7e,g). This is similar to the distribution characteristics of detrital zircon εHf(t) values in the Nanhua Formation of the Yangtze Gorges area and the entire northern Yangtze Block [73], indicating that Neoproterozoic detrital zircons may have been derived from the northern part of the Yangtze Block.

Implications for the Tectonic Evolution
As mentioned above, the Lengjianxi Group below the Wuling unconform lower Banxi Group above the Wuling unconformity have similar detrital zircon a tra, indicating that the provenance did not change across this unconformity. Pro analysis suggests that these strata have received materials from the Cathaysia B plying that the Yangtze Block and the Cathaysia Block probably collided in the Jiangnan Orogen earlier than the formation of the upper Lengjixi Group and ear the timing of the Wuling unconformity. In the eastern Jiangnan Orogen, the Fuch tinental margin arc in southern Anhui may have lasted until 820 Ma, and the Figure 11. A comparison of age probability histograms of representative zircon age distributions for the lower Banxi Group and its equivalents (a), the upper Banxi Group and its equivalents (b), and the Lengjiaxi Group and its equivalents (c) along the WJO, respectively. Data from: [18,23,36,39,43,44,52,55,[68][69][70][71][72][73][74][75][76][95][96][97][122][123][124] and this study. 207 Pb/ 206 Pb ages are used for zircons with ages >1000 Ma, 206 Pb/ 238 U ages are used for zircons younger than 1000 Ma. In conclusion, the provenance of the upper and lower parts of the Banxi Group in the western Jiangnan Orogen changed above the Xihuangshan unconformity, and the provenance of the upper Banxi Group mainly came from the northern Yangtze Block and partly from the Jiangnan Orogen.

Implications for the Tectonic Evolution
As mentioned above, the Lengjianxi Group below the Wuling unconformity and lower Banxi Group above the Wuling unconformity have similar detrital zircon age spectra, indicating that the provenance did not change across this unconformity. Provenance analysis suggests that these strata have received materials from the Cathaysia Block, implying that the Yangtze Block and the Cathaysia Block probably collided in the western Jiangnan Orogen earlier than the formation of the upper Lengjixi Group and earlier than the timing of the Wuling unconformity. In the eastern Jiangnan Orogen, the Fuchuan continental margin arc in southern Anhui may have lasted until 820 Ma, and the Fuchuan ophiolite (FCO complex) of 840-824 Ma represents the final amalgamation between the Yangtze and Cathaysia [125]. In addition, the~820 Ma cordierite-bearing granodiorites are characterized by high ASI values (peraluminous), positive ε Hf (t) values, and high δ 18 O values and record the arc-continent collision orogeny in southern Anhui [126]. The~800 Ma composite dikes intruded into these S-type granitoids, indicating that postorogenic extension occurred shortly after the Neoproterozoic orogeny [127]. Thus, arc-continent collision in the eastern Jiangnan Orogen may have occurred mainly during the period 820-800 Ma [128]. Therefore, the Yangtze and Cathaysia blocks collided earlier in the western Jiangnan Orogen than in the eastern Jiangnan Orogen, exhibiting a scissor-like closure process (Figure 12a).
In this study, the timing of the Xihuangshan unconformity was constrained to ~800-779 Ma. The contact relationship of the Xihuangshan unconformity transitioned from an angular unconformity to a disconformity and conformity from north to south (Figure 8b and references therein), indicating that the Xihuangshan unconformity spread from north to south. Additionally, a change in provenance occurred between the upper and lower Banxi groups. The detrital zircons in the lower Banxi Group were mainly derived from the southwestern margin of the Yangtze Block, Cathaysia Block, and Jiangnan Orogen, whereas the detrital zircons in the upper Banxi Group mainly came from the northern margin of the Yangtze Block. Moreover, the proportion of ~2.0 Ga and ~2.5 Ga detrital zircons is larger near the northern margin of the Yangtze Block and gradually decreases or even disappears to the south, indicating that the transmission path is from the north to south ( Figure 10). Therefore, we propose that the main driver of the Xihuangshan unconformity comes from the northern margin of the Yangtze Block. Previous studies have reported abundant Neoproterozoic magmatism and long-term subduction (870-740 Ma) at the northwestern and northern margins of the Yangtze Block [51,129]. Based on the changes in εHf(t) values, Yang et al. (2018) [42] suggested that a collision may have occurred in the northern Yangtze Block at ~790 Ma. In the Dahongshan area, northern Yangtze Block, the Liantuo Formation overlies the Huashan Group with an angular unconformity formed at 810-780 Ma [42,130,131], which may have been caused by collision orogeny. Moreover, 800-780 Ma high amphibolite facies metamorphism has  [17,125]).
In this study, the timing of the Xihuangshan unconformity was constrained tõ 800-779 Ma. The contact relationship of the Xihuangshan unconformity transitioned from an angular unconformity to a disconformity and conformity from north to south (Figure 8b and references therein), indicating that the Xihuangshan unconformity spread from north to south. Additionally, a change in provenance occurred between the upper and lower Banxi groups. The detrital zircons in the lower Banxi Group were mainly derived from the southwestern margin of the Yangtze Block, Cathaysia Block, and Jiangnan Orogen, whereas the detrital zircons in the upper Banxi Group mainly came from the northern margin of the Yangtze Block. Moreover, the proportion of~2.0 Ga and~2.5 Ga detrital zircons is larger near the northern margin of the Yangtze Block and gradually decreases or even disappears to the south, indicating that the transmission path is from the north to south ( Figure 10). Therefore, we propose that the main driver of the Xihuangshan unconformity comes from the northern margin of the Yangtze Block.
Previous studies have reported abundant Neoproterozoic magmatism and longterm subduction (870-740 Ma) at the northwestern and northern margins of the Yangtze Block [51,129]. Based on the changes in ε Hf (t) values, Yang et al. (2018) [42] suggested that a collision may have occurred in the northern Yangtze Block at~790 Ma. In the Dahongshan area, northern Yangtze Block, the Liantuo Formation overlies the Huashan Group with an angular unconformity formed at 810-780 Ma [42,130,131], which may have been caused by collision orogeny. Moreover, 800-780 Ma high amphibolite facies metamorphism has been reported in South Qinling, representing the collision between the Douling Block and the Yangtze Block [115,132]. The contemporary~800 Ma upper amphibolite to granulitefacies metamorphism and peraluminous granite in the northwestern part of the Yangzte Block [133,134] were proposed to have been generated at the syncollision stage (Figure 12b).
These studies indicate a collisional orogeny of~800-780 Ma in the northern and northwestern margins of the Yangtze Block. It is suggested that the Xihuangshan unconformity may be induced by collisional events in the northern and northwestern margins of the Yangtze Block, combined with provenance transition and unconformity distribution characteristics.

Conclusions
The formation time of the Wuling unconformity is~830-813 Ma, whereas that of the Xihuangshan unconformity is~800-779 Ma; Provenance analysis shows that the upper Lengjiaxi Group and lower Banxi Group in the research areas have similar detrital zircon age spectra with age groups of 800-1000 Ma, 1.50-1.90 Ga, and 2.40-2.60 Ga, indicating that the provenance has not changed. The detrital zircon U−Pb chronology and Hf isotope characteristics suggest that the sediments were sourced from the southwestern margin of the Yangtze Block, Cathaysia Block, and Jiangnan Orogen. The appearance of the provenance of the Cathaysia Block in the upper Lengjiaxi Group implies that the collision of the Yangtze Block and Cathaysia Block in the western Jiangnan Orogen occurred much earlier than the Wuling unconformity (~830-813 Ma) and earlier than that in the eastern Jiangnan Orogen. Thus, the collision between the Yangtze and Cathaysia blocks may have undergone a scissor-like closure process from west to east; Above the Xihuangshan unconformity, the provenance of the sedimentary strata changed. The contact relationship and provenance transition demonstrate that the Xihuangshan unconformity is a tectonic unconformity spreading from north to south, coeval with the collision orogeny in the northern and northwestern margins of the Yangtze Block at approximately 800-780 Ma. This unconformity may have been driven by a collision orogeny in the northern and northwestern margins of the Yangtze Block.