Timing, Provenance, and Tectonic Implications of Ore-Hosting Metasedimentary Rocks in the Giant Liba Gold Deposit, West Qinling Belt, China

: The closure time of the Shangdan Ocean is critical for understanding the tectonic evolution of the Proto-Tethys Ocean. However, the proposed closure time was prolonged from Ordovician to Devonian. In the present study, detrital zircon from the metasedimentary rocks of the Liba Group in the West Qinling Belt was analyzed to constrain the closure time of the Shangdan Ocean. The three youngest grains from the Liba Group yield a maximum deposition age of 418 ± 13 Ma, indicating the Middle Devonian deposition. Detrital zircon grains show two main U–Pb age peaks of 810 Ma and 440 Ma with ε Hf (t) values spanning from − 24.3 to +8.8 and − 6.3 to +4.1, respectively, suggesting that the sediments of the Liba Group were derived from both the North and South Qinling Belts. The Lower Devonian in the South Qinling Belt shows similar provenance to the Liba Group, whereas sediments from the North Qinling Belt are absent in the Silurian strata of the South Qinling Belt. From Late Silurian to Early Devonian, the tectonic setting changed from subduction to collision. This evidence consistently suggests the disappearance of the Shangdan Ocean. The noticeable decrease in magmatism from 510–420 Ma to 420–390 Ma and the shrinking of ε Hf (t) values from − 15.5–+12.8 to − 8.4–+4.2 reveal that the Shangdan Ocean, as the eastmost embranchment of the Proto-Tethys Ocean, was closed at ca. 420 Ma.


Introduction
The Qinling Belt, the eastmost branch of the Tethys domain, is divided into the North Qinling Belt and the South Qinling Belt by the Shangdan Suture Zone [1,2]. The Shangdan Ocean is the eastmost embranchment of the Proto-Tethys Ocean, which was closed during the Paleozoic and formed the Shangdan Suture Zone; therefore, the closure of the Shangdan Ocean recorded relevant information for understanding the tectonic evolution of the Proto-Tethys Ocean [3,4]. However, the closure time remains disputed, and several models have been proposed. Previous studies investigated S-and I-type granitoids, high Ba-Sr dioritic intrusions, and high-pressure and ultrahigh-pressure rocks, suggesting that the closure time of the Shangdan Ocean spanned from Ordovician to Silurian [5][6][7]. However, detrital zircon grains from the clastic rocks [8], adakitic granitoids [9], and mafic complex [10] indicate that the closure time can be constrained at Devonian.  [19]).
As the western segment of the Qinling Belt, the West Qinling Belt is contiguous to the East Qinling Belt and is bounded by the Huicheng basin or Foping dome [19,[28][29][30]. In the West Qinling Belt, the Precambrian basement is overlain by the Phanerozoic sedimentary rocks and all the strata are intruded by the Triassic granitoid ( Figure 1b) [31,32].  [19]).
As the western segment of the Qinling Belt, the West Qinling Belt is contiguous to the East Qinling Belt and is bounded by the Huicheng basin or Foping dome [19,[28][29][30]. In the West Qinling Belt, the Precambrian basement is overlain by the Phanerozoic sedimentary rocks and all the strata are intruded by the Triassic granitoid ( Figure 1b) [31,32]. The ages of the Phanerozoic sedimentary rocks span from Ordovician to Neogene, including the Cambrian-Silurian limestones and shales, Devonian-Carboniferous clastic rocks with limestone interlayers, and Permian and Lower Triassic sandstones [8,31,33]. The Triassic granitoid magmatism was mainly related to the continental collision between the South Qinling Belt and the South China Block with the formation of quartz diorites, quartz monzonites, granodiorites, and monzogranites [5,26,34,35]. Detailed geological mapping reveals that the Middle Devonian Liba Group was developed in the northern margin of the West Qinling Belt [32,36]. The Liba Group, also named the Shujiaba Group [32], is exposed between the Gaoqiao-Luoba fault to the south and the Shujiaba area to the north, which comprises marine metasedimentary rocks and hosts the giant Liba Gold deposit [32,36]. The Liba Group contains various rock types, including sericitic-chloritic slate, slate, metaquartz sandstone, and siltstone ( Figure 2) [37]. The major fault in the Liba gold deposit is the SW-dipping reverse F1 Fault ( Figure 2). Numerous secondary W-to WNW-trending faults related to F1 are presented in the SW domain of the F1 Fault, hosting the major gold resources of the Liba gold deposit (Figure 2). The Liba Group is intruded by the Triassic monzogranite ( Figure 2) [32]. The ages of the Phanerozoic sedimentary rocks span from Ordovician to Neogene, including the Cambrian-Silurian limestones and shales, Devonian-Carboniferous clastic rocks with limestone interlayers, and Permian and Lower Triassic sandstones [8,31,33]. The Triassic granitoid magmatism was mainly related to the continental collision between the South Qinling Belt and the South China Block with the formation of quartz diorites, quartz monzonites, granodiorites, and monzogranites [5,26,34,35]. Detailed geological mapping reveals that the Middle Devonian Liba Group was developed in the northern margin of the West Qinling Belt [32,36]. The Liba Group, also named the Shujiaba Group [32], is exposed between the Gaoqiao-Luoba fault to the south and the Shujiaba area to the north, which comprises marine metasedimentary rocks and hosts the giant Liba Gold deposit [32,36]. The Liba Group contains various rock types, including sericitic-chloritic slate, slate, meta-quartz sandstone, and siltstone ( Figure 2) [37]. The major fault in the Liba gold deposit is the SW-dipping reverse F1 Fault ( Figure 2). Numerous secondary W-to WNWtrending faults related to F1 are presented in the SW domain of the F1 Fault, hosting the major gold resources of the Liba gold deposit ( Figure 2). The Liba Group is intruded by the Triassic monzogranite ( Figure 2) [32].

Sample Description
Two representative slate samples were collected from the Liba Group for zircon U-Pb and Lu-Hf analyses, namely, 20LB07 (34 •

Zircon U-Pb Dating
The separation of zircon grains was carried out at the Langfang Chengxin Geological Service Co., Hebei Province, China. The samples were first broken and crushed to 40-60

Zircon U-Pb Dating
The separation of zircon grains was carried out at the Langfang Chengxin Geological Service Co., Hebei Province, China. The samples were first broken and crushed to 40-60

Zircon U-Pb Dating
The separation of zircon grains was carried out at the Langfang Chengxin Geological Service Co., Hebei Province, China. The samples were first broken and crushed to 40-60 mesh, and then the standard heavy liquid and magnetic techniques were used to separate the zircon grains from the clastic grains. Detrital zircon grains without cracks and inclusions were handpicked under a binocular microscope and mounted in epoxy resin. The zircon-grain-mounted samples were subsequently polished to nearly half the original thickness in order to expose the internal structures. The samples were then cleaned in an ultrasonic washer containing a 5% HNO 3 bath. Before analysis, cathodoluminescence (CL) images were obtained using a JXA8800 electron microscope at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China, and all zircon grains were checked carefully to identify the internal structure.
Zircon U-Pb analyses were carried out using the LA-MC-ICP-MS at the Isotopic Laboratory, Tianjin Center, China Geological Survey. Laser sampling was performed using a Neptune double-focusing multiple-collector ICP-MS (Thermo Fisher Ltd., Waltham, MA, USA) attached to a NEW WAVE 193 nm-FX ArF Excimer laser ablation system (ESI Ltd., Fremont, CA, USA). The MC-ICP-MS is a double-focusing multi-collector ICP-MS. Two analyses of the standards followed every eight analyses to calibrate the data. The detailed analytical procedure of the laser ablation system, the ICP-MS instrument and the data reduction of zircon can be found in [38,39]. The common Pb corrections used the method of [40]. A laser beam diameter of 30 µm, a repetition rate of 8 Hz, and an energy density of 11 J/cm 2 were chosen for all U, Th and Pb analyses. Zircon 91500 [41] and zircon Plešovice [42] were used as standards. The NIST SRM 610 was used as external reference material and 29 Si as internal calibrant for calibration of U, Th and Pb concentrations. Probability density distribution curves and concordia diagrams of ages were plotted at the 2σ uncertainty level using the software DensityPlotter [43] and Isoplot software, respectively (version 3.75) [44]. The concordance was calculated for all data, with normal discordance <20% interpreted as geologically meaningful. Discordant data were excluded from the relative probability calculation. Ages of zircon grains inferior to 1.0 Ga are quoted using 206 Pb/ 238 U ages, whereas older grains are based upon their 207 Pb/ 206 Pb ages [45].

Zircon Lu-Hf Isotope Analysis
Zircon Lu-Hf isotopic analyses were conducted on the same spots or textural domains as analogous zircon U-Pb analysis. Hafnium isotopic compositions were determined with a Thermo Finnigan Neptune MC-ICP-MS system coupled to a New Wave UP193 nm laser ablation system at the Isotopic Laboratory, Tianjin Center, China Geological Survey. A laser beam diameter of 50 µm and a laser repetition rate of 11 Hz at 100 mJ were used for ablating zircon with helium as the carrier gas for the ablated aerosol. Details about the analytical procedures and the instrument operating conditions for Lu-Hf isotope analyses follow those described in [38]. Zircon GJ-1 was used as the reference standard with a weighted mean 176 Hf/ 177 Hf ratio of 0.282004 ± 24 (2σ) during routine analyses, which is significantly consistent with the recommended 176 Hf/ 177 Hf ratios of 0.282015 ± 19 (2σ) [46].

Detrital Zircon U-Pb Geochronology
Detrital zircon morphology is illustrated in Figure 5. The two slate samples (20LB07 and 20LB29) show similar zircon morphologies. Cathodoluminescence (CL) images show that most zircon grains from the Liba Group have crystal faces and oscillatory zoning. Some grains exhibit irregular sector zoning and banded zoning textures. The rest of the grains are homogeneous with low to high luminescence. Zircon grains from sample 20LB07 have lengths of 30-120 µm; among them, 70% of the grains show small size (30-40 µm), while 30% have lengths of 60-120 µm (Figure 5a). Zircon grains from sample 20LB29 display lengths of 40-70 µm and length/width ratios ranging from 2:1 to 1:1 (Figure 5b). Detrital zircon U-Pb ages from the Liba Group are shown in Figure 6 and Table 1, the two samples display similar age spectra. Eighty detrital zircon grains from sample 20LB07 were analyzed, among which seventy show high concordance (concordance > 80%) (Figure 6a), yielding ages ranging from 2912 Ma to 256 Ma. The relative probability density curve plotted by the Density Plotter software [43] yields two major peaks at 803 Ma and 433 Ma and a minor peak at 1.2 Ga (Figure 6b). Forty zircon grains were analyzed from sample 20LB29, and most of them show high concordance (n = 34, concordance > 80%) (Figure 6c). The relative probability density curve shows a main peak at 826 Ma, followed by a second peak at 733 Ma and a minor peak at 1.8 Ga (Figure 6d). The U-Pb age distributions of the two samples are presented in Figure 7a   Detrital zircon U-Pb ages from the Liba Group are shown in Figure 6 and Table 1, the two samples display similar age spectra. Eighty detrital zircon grains from sample 20LB07 were analyzed, among which seventy show high concordance (concordance > 80%) (Figure 6a), yielding ages ranging from 2912 Ma to 256 Ma. The relative probability density curve plotted by the Density Plotter software [43] yields two major peaks at 803 Ma and 433 Ma and a minor peak at 1.2 Ga (Figure 6b). Forty zircon grains were analyzed from sample 20LB29, and most of them show high concordance (n = 34, concordance > 80%) ( Figure 6c). The relative probability density curve shows a main peak at 826 Ma, followed by a second peak at 733 Ma and a minor peak at 1.8 Ga (Figure 6d). The U-Pb age distributions of the two samples are presented in Figure 7a  Detrital zircon U-Pb ages from the Liba Group are shown in Figure 6 and Table 1, the two samples display similar age spectra. Eighty detrital zircon grains from sample 20LB07 were analyzed, among which seventy show high concordance (concordance > 80%) (Figure 6a), yielding ages ranging from 2912 Ma to 256 Ma. The relative probability density curve plotted by the Density Plotter software [43] yields two major peaks at 803 Ma and 433 Ma and a minor peak at 1.2 Ga (Figure 6b). Forty zircon grains were analyzed from sample 20LB29, and most of them show high concordance (n = 34, concordance > 80%) (Figure 6c). The relative probability density curve shows a main peak at 826 Ma, followed by a second peak at 733 Ma and a minor peak at 1.8 Ga (Figure 6d). The U-Pb age distributions of the two samples are presented in Figure 7a

Lu-Hf Isotopic Composition
Twenty-seven spots of the analyzed U-Pb domains or the same textural domains of the zircon grains from the two samples of the Liba Group were analyzed for Lu-Hf isotopic compositions, and the result is shown in Figure 7b and Table 2. Detrital zircon grains of the 2.5-1.0 Ga group are characterized by variable εHf (t) values spanning from −16.7 to +9.1 and T DM2 age of 3.8-1.7 Ga. Zircon grains clustering around 800 Ma show εHf (t) values from −24.3 to +10.4 and T DM2 age of 3.1-1.2 Ga. The 430 Ma population group has εHf (t) values between −6.3 and +5.5 with a median of +2.9 and T DM2 age of 1.8-1.1 Ga.

Lu-Hf isotopic composition
Twenty-seven spots of the analyzed U-Pb domains or the same textural domains of the zircon grains from the two samples of the Liba Group were analyzed for Lu-Hf isotopic compositions, and the result is shown in Figure 7b and Table 2. Detrital zircon grains of the 2.5-1.0 Ga group are characterized by variable εHf(t) values spanning from −16.7 to +9.1 and TDM2 age of 3.8-1.7 Ga. Zircon grains clustering around 800 Ma show εHf(t) values from −24.3 to +10.4 and TDM2 age of 3.1-1.2 Ga. The 430 Ma population group has εHf(t) values between −6.3 and +5.5 with a median of +2.9 and TDM2 age of 1.8-1.1 Ga.

Timing of Deposition of the Liba Group
Detrital zircon ages from the two metasedimentary rocks of the Liba Group are combined to discuss the depositional age owing to their similar age spectra (Figures 5 and 6). Most of the zircon grains show high concordance (>95%) and Th/U ratios of 0.10-1.98 (Figure 8), which, together with the well-developed oscillatory zoning in CL images (Figure 4), indicates the magmatic origin [47,48]. Except the three low-Th/U-ratio grains (21LB29-23: 0.02, 21LB29-29: 0.03, and 21LB07-44: 0.07) (Table 1), the high Th/U ratios of magmatic grains were further discussed. The present study follows the method of "mean age of the youngest two or more grains that overlap in age at 1σ", which yields a weighted mean age of 418 ± 13 Ma (n = 3, MSWD = 0.111), representing the maximum depositional age of the Liba Group [14]. The calculated youngest age indicates that the deposition of the Liba Group formed after ca. 418 Ma. Combined with the stratigraphic successions of the upper layers of the Late Devonian paleospore fossils and the bottom layers of the early Middle Devonian coral fossils [36,[49][50][51], the deposition of the Liba Group is constrained in the Middle Devonian.

Timing of deposition of the Liba Group
Detrital zircon ages from the two metasedimentary rocks of the Liba Group are combined to discuss the depositional age owing to their similar age spectra (Figures 5 and 6). Most of the zircon grains show high concordance (>95%) and Th/U ratios of 0.10-1.98 (Figure 8), which, together with the well-developed oscillatory zoning in CL images ( Figure  4), indicates the magmatic origin [47,48]. Except the three low-Th/U-ratio grains (21LB29-23: 0.02, 21LB29-29: 0.03, and 21LB07-44: 0.07) (Table 1), the high Th/U ratios of magmatic grains were further discussed. The present study follows the method of "mean age of the youngest two or more grains that overlap in age at 1σ", which yields a weighted mean age of 418 ± 13 Ma (n = 3, MSWD = 0.111), representing the maximum depositional age of the Liba Group [14]. The calculated youngest age indicates that the deposition of the Liba Group formed after ca. 418 Ma. Combined with the stratigraphic successions of the upper layers of the Late Devonian paleospore fossils and the bottom layers of the early Middle Devonian coral fossils [36,[49][50][51], the deposition of the Liba Group is constrained in the Middle Devonian.

Provenance of the Liba Group
The detrital zircon grains of the Liba Group were presumably sourced from the North Qinling Belt, South Qinling Belt, North China Block, and South China Block (Figure 1a). The U-Pb age and εHf(t) value of the detrital zircon grains from the Liba group and magmatic zircon from surrounding orogenic belts are compared (Figure 9) to trace the provenance of the Liba Group. Detrital zircon U-Pb ages indicate that the metasedimentary rocks of the Liba Group predominantly deposited during the Phanerozoic (440 Ma) and Neoproterozoic (810 Ma) and minimally were during the Archean-Mesoproterozoic (1.2 Ga, 1.8 Ga, and 2.5 Ga) (Figure 9a). The variable age populations of the Liba Group are similar to the zircon distributions of neighboring orogenic belts, suggesting mixed sources (Figure 9a).

Provenance of the Liba Group
The detrital zircon grains of the Liba Group were presumably sourced from the North Qinling Belt, South Qinling Belt, North China Block, and South China Block (Figure 1a). The U-Pb age and εHf (t) value of the detrital zircon grains from the Liba group and magmatic zircon from surrounding orogenic belts are compared (Figure 9) to trace the provenance of the Liba Group. Detrital zircon U-Pb ages indicate that the metasedimentary rocks of the Liba Group predominantly deposited during the Phanerozoic (440 Ma) and Neoproterozoic (810 Ma) and minimally were during the Archean-Mesoproterozoic (1.2 Ga, 1.8 Ga, and 2.5 Ga) (Figure 9a). The variable age populations of the Liba Group are similar to the zircon distributions of neighboring orogenic belts, suggesting mixed sources (Figure 9a).
The Phanerozoic detrital zircon grains are subhedral to angular ( Figure 5), whose ages range from 500 to 400 Ma with a dominant peak at 440 Ma (Figure 9a), implying a short transport distance [14,17]. The Phanerozoic ages are highly consistent with the early Paleozoic magmatism in the North Qinling Belt (Figure 9). The major peak of ca. 440 Ma coincides with the ca. 445 Ma magmatism of the Liuxiangping, Huichizi, and Zaoyuan granitic plutons [7]. The dominant positive and minor negative εHf (t) values from the Liba group generally overlap the available zircon εHf (t) values (−20 to +13) from granitoid and gabbro intrusions in the North Qinling Belt (Figure 9b) [27,53]. Consequently, the early Paleozoic magmatic rocks from the North Qinling Belt might be the main source of the Phanerozoic zircon grains of the Liba Group. The Phanerozoic detrital zircon grains are subhedral to angular ( Figure 5), whose ages range from 500 to 400 Ma with a dominant peak at 440 Ma (Figure 9a), implying a short transport distance [14,17]. The Phanerozoic ages are highly consistent with the early Paleozoic magmatism in the North Qinling Belt (Figure 9). The major peak of ca. 440 Ma coincides with the ca. 445 Ma magmatism of the Liuxiangping, Huichizi, and Zaoyuan granitic plutons [7]. The dominant positive and minor negative εHf(t) values from the Liba group generally overlap the available zircon εHf(t) values (−20 to +13) from granitoid and gabbro intrusions in the North Qinling Belt (Figure 9b) [27,53]. Consequently, the early Paleozoic magmatic rocks from the North Qinling Belt might be the main source of the Phanerozoic zircon grains of the Liba Group.
The dominant Neoproterozoic grains display ages ranging from 1000 Ma to 700 Ma with a major peak at 810 Ma and a minor peak at 985 Ma (Figure 9a). In the present study, the major peak of ca. 810 Ma closely matches with the age distribution of the South Qinling Belt and South China Block (Figure 9a). The South Qinling Belt is characterized by significant Neoproterozoic zircon ranging from 1000 to 700 Ma with an age peak at 820 Ma, which was defined by the Yaolinghe volcanic rocks (808-746 Ma), Mihunzhen plutons (885-737 Ma) and Liuba granitic intrusions (868-794 Ma) [27,52]. Additionally, the Neoproterozoic ages (ca. 900-700 Ma) are also widely reported in the South China Block, including the Bikou, Hannan, and Micanshan massif [27,53]. Furthermore, the εHf(t) values (−24.3 to +8.8, Figure 9b) from the ca. 900-700 Ma group of this study generally overlap the zircon εHf(t) values (−30.2 to +17.7) from the magmatic rocks in the South China Block (Figure 9b; [27]) and the negative εHf(t) values from the South Qinling Belt (Figure 10; [27]), which indicates that the two regions could be the potential provenance for the middle Neoproterozoic zircon grains. Moreover, considering that the South China Block and South Qinling Belt were separated by the Mianlue Ocean during the Devonian [3,56], the Neoproterozoic detrital zircon grains (ca. 810 Ma) were probably sourced from the South Qinling Belt. However, the age population of 1000-900 Ma shows different provenance from the ca. 810 Ma population. In contrast to the absence of the Neoproterozoic (ca. 1000-900 Ma) magmatic events in the South China Block or South Qinling Belt, the Neoproterozoic (ca. 1000-900 Ma) magmatism was reported in the North Qinling Belt, represented by the exposed Dehe granite (943-964 Ma), Niujiaoshan granite (929-959 Ma), and the Tianshui granitoid (915-978 Ma) [27,57]. Thus, the Neoproterozoic (1000-900 Ma) zircon grains of the Liba Group were likely sourced from the North Qinling Belt.
The ca. 1.5-1.0 Ga, ca. 1.9-1.7 Ga, and ca. 2.5 Ga age groups were also observed in the present study (Figure 9a). Mesoproterozoic detrital zircon grains whose ages range from The dominant Neoproterozoic grains display ages ranging from 1000 Ma to 700 Ma with a major peak at 810 Ma and a minor peak at 985 Ma (Figure 9a). In the present study, the major peak of ca. 810 Ma closely matches with the age distribution of the South Qinling Belt and South China Block (Figure 9a). The South Qinling Belt is characterized by significant Neoproterozoic zircon ranging from 1000 to 700 Ma with an age peak at 820 Ma, which was defined by the Yaolinghe volcanic rocks (808-746 Ma), Mihunzhen plutons (885-737 Ma) and Liuba granitic intrusions (868-794 Ma) [27,52]. Additionally, the Neoproterozoic ages (ca. 900-700 Ma) are also widely reported in the South China Block, including the Bikou, Hannan, and Micanshan massif [27,53]. Furthermore, the εHf (t) values (−24.3 to +8.8, Figure 9b) from the ca. 900-700 Ma group of this study generally overlap the zircon εHf (t) values (−30.2 to +17.7) from the magmatic rocks in the South China Block (Figure 9b; [27]) and the negative εHf (t) values from the South Qinling Belt (Figure 10; [27]), which indicates that the two regions could be the potential provenance for the middle Neoproterozoic zircon grains. Moreover, considering that the South China Block and South Qinling Belt were separated by the Mianlue Ocean during the Devonian [3,56], the Neoproterozoic detrital zircon grains (ca. 810 Ma) were probably sourced from the South Qinling Belt. However, the age population of 1000-900 Ma shows different provenance from the ca. 810 Ma population. In contrast to the absence of the Neoproterozoic (ca. 1000-900 Ma) magmatic events in the South China Block or South Qinling Belt, the Neoproterozoic (ca. 1000-900 Ma) magmatism was reported in the North Qinling Belt, represented by the exposed Dehe granite (943-964 Ma), Niujiaoshan granite (929-959 Ma), and the Tianshui granitoid (915-978 Ma) [27,57]. Thus, the Neoproterozoic (1000-900 Ma) zircon grains of the Liba Group were likely sourced from the North Qinling Belt.
The ca. 1.5-1.0 Ga, ca. 1.9-1.7 Ga, and ca. 2.5 Ga age groups were also observed in the present study (Figure 9a). Mesoproterozoic detrital zircon grains whose ages range from 1.5 to 1.0 Ga with a peak at 1.2 Ga are only reported in the Qinling Group and Kuanping Group in the North Qinling Belt [53], indicating that the Mesoproterozoic detrital zircon grains were sourced from the North Qinling Belt. The Archean-Paleoproterozoic gneiss and amphibolite from the North China Block show zircon ages of 3.5-3.1 Ga and 2.8-1.7 Ga with peaks at 2.6-2.4 Ga and 2.0-1.7 Ga [58][59][60], and their εHf (t) values scatter from −10 to +5 and −15 to +10, respectively [54,61]. The South China Block also displays age populations of 2.6-2.4 Ga and 2.0-1.8 Ga with εHf (t) values from −16 to +0 and −30 to +7, respectively [52]. The ages and εHf (t) values from the detrital zircon in this study closely match with those of zircon grains from the North China Block and South China Block ( Figure 9); thus, the Archean-Paleoproterozoic detrital zircon grains of the Liba Group mainly originated from the North China Block and South China Block.

Closure time of the Shangdan Ocean
The widely distributed Middle Devonian Liba Group in the South Qinling Belt received abundant Phanerozoic sediments from the North Qinling Belt, suggesting that the Shangdan Ocean between the North and South Qinling Belts had closed in the Middle Devonian. The difference between the crystallization and deposition ages (CA-DA) of the detrital zircon grains can be demonstrated (Figure 10) by the method of [17]. Overall, a significant proportion of the youngest detrital zircon (CA-DA < 150 Ma at 5% and CA-DA > 100 Ma at 30% of the zircon population), combined with the overlapping feature of the cumulative proportional curves of the Liba Group in the B (blue) area (Figure 10), consistently supports that the Liba group was formed in a collisional setting. Consequently, the Shangdan Ocean presumably closed in the Middle Devonian during the deposition of the Liba Group. Figure 10. Detrital zircon U-Pb age cumulative probability distributions. Summary plot of the general fields for convergent (A: orange field), collisional (B: blue field), and extensional basins (C: green field). Extensional setting shows CA-DA > 150 Ma in 5% of the youngest zircon grains (step 1), convergent setting shows CA-DA < 100 Ma in 30% of the youngest zircon grains (step 2), and collisional Figure 10. Detrital zircon U-Pb age cumulative probability distributions. Summary plot of the general fields for convergent (A: orange field), collisional (B: blue field), and extensional basins (C: green field). Extensional setting shows CA-DA > 150 Ma in 5% of the youngest zircon grains (step 1), convergent setting shows CA-DA < 100 Ma in 30% of the youngest zircon grains (step 2), and collisional setting shows CA-DA < 150 Ma and CA-DA > 100 Ma in 5% and 30% of the youngest zircon population, respectively (modified after [17]).

Closure Time of the Shangdan Ocean
The widely distributed Middle Devonian Liba Group in the South Qinling Belt received abundant Phanerozoic sediments from the North Qinling Belt, suggesting that the Shangdan Ocean between the North and South Qinling Belts had closed in the Middle Devonian. The difference between the crystallization and deposition ages (CA-DA) of the detrital zircon grains can be demonstrated ( Figure 10) by the method of [17]. Overall, a significant proportion of the youngest detrital zircon (CA-DA < 150 Ma at 5% and CA-DA > 100 Ma at 30% of the zircon population), combined with the overlapping feature of the cumulative proportional curves of the Liba Group in the B (blue) area (Figure 10), consistently supports that the Liba group was formed in a collisional setting. Consequently, the Shangdan Ocean presumably closed in the Middle Devonian during the deposition of the Liba Group.
Based on the early Paleozoic (ca. 450 Ma) and Neoproterozoic (900-700 Ma) age groups and their negative εHf (t) values, previous studies suggest that the Silurian strata in South Qinling Belt received sediments from the South China Block and Ordovician detritus from the South Qinling Belt without the contribution of the North Qinling Belt and the North China Block (Figure 11a,c) [52,62,63]. However, the Lower Devonian in the South Qinling Belt dominantly shows early Paleozoic ages (ca. 450 Ma) with positive εHf (t) values and new Neoproterozoic zircon ages (1000-900 Ma) (Figure 11b,d), suggesting that the North Qinling Belt is the major source [62,64,65]. Furthermore, the detrital zircon grains from the Middle Devonian [66] and Middle-Upper Devonian strata [8,27,[65][66][67][68][69][70] show similar provenance to the Lower Devonian (Figure 11b,d), indicating that the tectonic setting remains unchanged. The apparent provenance difference between the Silurian and Lower Devonian strata might be the result of the collision between the North and South Qinling Belts [2]. The cumulative proportion curves also show tectonic settings changed from convergent to collision settings from Upper Silurian to Lower Devonian ( Figure 10). Among the youngest zircon grains, the Upper Silurian samples present 42% (more than 30%) detrital grains that have CA-DA < 100 Ma, while lower Devonian samples present 5% zircon grains that show CA-DA < 150 Ma and 30% zircon grains that have CA-DA > 100 Ma, suggesting that the Upper Silurian and Lower Devonian strata formed in convergent and collision settings, respectively ( Figure 12). In the South Qinling Belt, the Silurian strata did not originate from the North Qinling Belt, while abundant Lower Devonian sediments were sourced from the North Qinling Belt. Combined with the change of the tectonic setting from subduction to collision, the closure time of the Shangdan Ocean is constrained between Late Silurian and Early Devonian.
Based on the early Paleozoic (ca. 450 Ma) and Neoproterozoic (900-700 Ma) age groups and their negative εHf(t) values, previous studies suggest that the Silurian strata in South Qinling Belt received sediments from the South China Block and Ordovician detritus from the South Qinling Belt without the contribution of the North Qinling Belt and the North China Block (Figure 11a,c) [52,62,63]. However, the Lower Devonian in the South Qinling Belt dominantly shows early Paleozoic ages (ca. 450 Ma) with positive εHf(t) values and new Neoproterozoic zircon ages (1000-900 Ma) (Figure 11b,d), suggesting that the North Qinling Belt is the major source [62,64,65]. Furthermore, the detrital zircon grains from the Middle Devonian [66] and Middle-Upper Devonian strata [8,27,[65][66][67][68][69][70] show similar provenance to the Lower Devonian (Figure 11b,d), indicating that the tectonic setting remains unchanged. The apparent provenance difference between the Silurian and Lower Devonian strata might be the result of the collision between the North and South Qinling Belts [2]. The cumulative proportion curves also show tectonic settings changed from convergent to collision settings from Upper Silurian to Lower Devonian ( Figure 10). Among the youngest zircon grains, the Upper Silurian samples present 42% (more than 30%) detrital grains that have CA-DA < 100 Ma, while lower Devonian samples present 5% zircon grains that show CA-DA < 150 Ma and 30% zircon grains that have CA-DA > 100 Ma, suggesting that the Upper Silurian and Lower Devonian strata formed in convergent and collision settings, respectively ( Figure 12). In the South Qinling Belt, the Silurian strata did not originate from the North Qinling Belt, while abundant Lower Devonian sediments were sourced from the North Qinling Belt. Combined with the change of the tectonic setting from subduction to collision, the closure time of the Shangdan Ocean is constrained between Late Silurian and Early Devonian.  . Detrital zircon U-Pb age spectra and εHf (t) values versus U-Pb age diagram from the Phanerozoic strata in the South Qinling Belt and the surrounding potential sources. (a,c) Silurian [52,62,63]; (b,d) Devonian (this study, [8,27,62,[64][65][66][67][68][69][70][71][72] 12). A remarkable magmatism decrease that appears at ca. 420 Ma (Figure 13a) is due to the collision between the North and South Qinling Belts [17]. The notable shrinking of εHf(t) values from −15.5-+12.8 to −8.4-+4.2 related to zircon ages ranging from 510-420 Ma to 420-390 Ma, respectively, reflects the change in the relative contributions of the mantle and crustal sources in their genesis [73]. The significant change in the εHf(t) value at ca. 420 Ma is correlated with plate subduction and collision [16,[74][75][76]. Therefore, the closure of the Shangdan Ocean might have occurred at ca. 420 Ma.

Tectonic implications
The Proto-Tethys Ocean was formed during the breakup of Rodinia at ca. 750 Ma [77]. Different parts that split up from Rodinia led to the formation of the South China, North China, Alex, Qaidam, and Tarim Blocks before 550 Ma [1,2]. Several branches of the Proto-Tethys Ocean were formed, including the Shangdan Ocean between the North and South China blocks, the Northern and Southern Qilian Oceans between the Alxa, Central Qilian Terranes, and Qaidam Blocks, and the Kunlun Ocean between the Qaidam, Qiangtang, and Tarim Blocks [2]. Those embranchments opened and subducted during ca. 541-485 Ma [2] and gradually closed in the early Paleozoic (500-420 Ma) [1,2].
The Shangdan Ocean, as the eastmost embranchment of the Proto-Tethys Ocean, is critical for reconstructing the evolution of the Proto-Tethys Ocean [4,56,78]. The subduction-related ophiolitic rocks show that the formation time of the Shangdan Ocean can be constrained at ca. 534-470 Ma [28]. Widely distributed ca. 510-420 Ma arc-magmatism and the formation of the gabbroic and granitic intrusions in the North Qinling Belt suggest northward subduction of the Shangdan Ocean underneath the North Qinling Belt during As a result of Shangdan Ocean closure, the clastic sediments from the Devonian should record the tectonic evolution. Therefore, detrital zircon grains that formed during Late Silurian to Early Devonian were selected to investigate the closure process, and the ages and εHf (t) values are illustrated in Figure 12. These grains show three main populations of 510-475 Ma, 475-420 Ma, and 420-400 Ma (Figure 12a), with εHf (t) values ranging from −11.6 to +13.2, −17.7 to +14.7, and −14.1 to +4.2, respectively (Figure 12b). The different age populations are genetically related to the northward subduction of the Shangdan Ocean and the subsequent collision between the North and the South Qinling Belts [3,4,57]. Since the closure time of the Shangdan Ocean spans over a wide period ranging from Late Silurian to Early Devonian, the 430-410 Ma group is further discussed (Figure 12). A remarkable magmatism decrease that appears at ca. 420 Ma (Figure 13a) is due to the collision between the North and South Qinling Belts [17]. The notable shrinking of εHf (t) values from −15.5-+12.8 to −8.4-+4.2 related to zircon ages ranging from 510-420 Ma to 420-390 Ma, respectively, reflects the change in the relative contributions of the mantle and crustal sources in their genesis [73]. The significant change in the εHf (t) value at ca. 420 Ma is correlated with plate subduction and collision [16,[74][75][76]. Therefore, the closure of the Shangdan Ocean might have occurred at ca. 420 Ma.

Tectonic Implications
The Proto-Tethys Ocean was formed during the breakup of Rodinia at ca. 750 Ma [77]. Different parts that split up from Rodinia led to the formation of the South China, North China, Alex, Qaidam, and Tarim Blocks before 550 Ma [1,2]. Several branches of the Proto-Tethys Ocean were formed, including the Shangdan Ocean between the North and South China blocks, the Northern and Southern Qilian Oceans between the Alxa, Central Qilian Terranes, and Qaidam Blocks, and the Kunlun Ocean between the Qaidam, Qiangtang, and Tarim Blocks [2]. Those embranchments opened and subducted during ca. 541-485 Ma [2] and gradually closed in the early Paleozoic (500-420 Ma) [1,2].
The Shangdan Ocean, as the eastmost embranchment of the Proto-Tethys Ocean, is critical for reconstructing the evolution of the Proto-Tethys Ocean [4,56,78]. The subductionrelated ophiolitic rocks show that the formation time of the Shangdan Ocean can be constrained at ca. 534-470 Ma [28]. Widely distributed ca. 510-420 Ma arc-magmatism and the formation of the gabbroic and granitic intrusions in the North Qinling Belt suggest northward subduction of the Shangdan Ocean underneath the North Qinling Belt during early Paleozoic (Figure 13a) [3]. In contrast to Silurian strata, these magmatic ages are only found in the Devonian strata of the South Qinling (Figures 11 and 13). Furthermore, a large number of detrital zircon grains at 510-420 Ma also supports this hypothesis (Figure 12). At ca. 420 Ma, the noticeable reduction in magmatism and the change in εHf (t) values of the detrital zircons from the Devonian strata in the South Qinling Belt unveil the closure of the Shangdan Ocean as well as the collision between the North and South Qinling Belts (Figure 13b). This is also supported by the metamorphic ages of metamorphic rocks. The amphibolite facies metamorphism of the Qinling Complex is constrained at 420 to 400 Ma, which is correlated to the collision between the North and South Qinling Belts [28,79,80]. The Wushan ductile shear zone as a branch of the Shangdan Suture Zone was intruded by a granitic dyke with the zircon U-Pb age of 403 ± 3.5 Ma, which is regarded as the minimum formation age of the Shangdan Suture Zone [81]. Therefore, the Shangdan Ocean as the eastmost embranchment of the Proto-Tethys Ocean was closed at ca. 420 Ma, and the Shangdan Suture Zone was formed by the collision between the North and the South Qinling Belts (Figure 13b). early Paleozoic (Figure 13a) [3]. In contrast to Silurian strata, these magmatic ages are only found in the Devonian strata of the South Qinling (Figures 11 and 13). Furthermore, a large number of detrital zircon grains at 510-420 Ma also supports this hypothesis ( Figure  12). At ca. 420 Ma, the noticeable reduction in magmatism and the change in εHf(t) values of the detrital zircons from the Devonian strata in the South Qinling Belt unveil the closure of the Shangdan Ocean as well as the collision between the North and South Qinling Belts (Figure 13b). This is also supported by the metamorphic ages of metamorphic rocks. The amphibolite facies metamorphism of the Qinling Complex is constrained at 420 to 400 Ma, which is correlated to the collision between the North and South Qinling Belts [28,79,80]. The Wushan ductile shear zone as a branch of the Shangdan Suture Zone was intruded by a granitic dyke with the zircon U-Pb age of 403 ± 3.5 Ma, which is regarded as the minimum formation age of the Shangdan Suture Zone [81]. Therefore, the Shangdan Ocean as the eastmost embranchment of the Proto-Tethys Ocean was closed at ca. 420 Ma, and the Shangdan Suture Zone was formed by the collision between the North and the South Qinling Belts (Figure 13b).

Conclusions
The maximum deposition age of the Liba Group is constrained at 418 ± 13 Ma, indicating that the strata were deposited during the Middle Devonian. Three age groups and their sources are recognized in the Liba Group, the Phanerozoic detrital zircon grains were sourced from the North Qinling Belt, and the main population represented by Neoproterozoic grains that originated from both the North and South Qinling Belts and the minor Archean-Mesoproterozoic grains came from the North Qinling Belt and the North and South China blocks. Integrated zircon εHf(t) values, sedimentary provenance, magmatism,

Conclusions
The maximum deposition age of the Liba Group is constrained at 418 ± 13 Ma, indicating that the strata were deposited during the Middle Devonian. Three age groups and their sources are recognized in the Liba Group, the Phanerozoic detrital zircon grains were sourced from the North Qinling Belt, and the main population represented by Neoproterozoic grains that originated from both the North and South Qinling Belts and the minor Archean-Mesoproterozoic grains came from the North Qinling Belt and the North and South China blocks. Integrated zircon εHf (t) values, sedimentary provenance, magmatism, and tectonic setting changes indicate that the Shangdan Ocean as the eastmost embranchment of the Proto-Tethys was closed at ca. 420 Ma.