Finding of Ca. 1.6 Ga Detrital Zircons from the Mesoproterozoic Dagushi Group, Northern Margin of the Yangtze Block

: The middle Mesoproterozoic is a crucial time period for understanding the Precambrian tectonic evolutionary history of the northern Yangtze Block and its relationship with the supercontinent Columbia. The Dagushi Group (Gp) is one of the Mesoproterozoic strata rarely found at the northern margin of the Yangtze Block. U–Pb geochronology and Lu–Hf isotopic analyses of detrital zircons were analyzed for three metamorphic quartz sandstone samples collected from the Luohanling and Dangpuling formations of the Dagushi Gp. These metasandstones yielded major zircon populations at ~2.65 Ga and ~1.60 Ga, respectively. The ~1.60 Ga ages ﬁrst discovered yield a narrow range of (cid:15) Hf (t) values from − 1.8 to +1.8, which lie above the old crust evolutionary line of the Yangtze Block, suggesting the addition of mantle material. Trace element data indicate that ~1.60 Ga detrital zircons share a basic provenance, whereby they have low Hf/Th and high Nb/Yb ratios. Zircon discrimination diagrams suggest that the ~1.60 Ga detrital zircon source rocks formed in an intra-plate rifting environment. Dagushi Gp provenance studies indicate that the ~1.60 Ga detrital zircon was most likely sourced from the interior Yangtze Block. Thus, we suggest that the late Paleoproterozoic to early Mesoproterozoic continental break-up occurred at the northern margin of the Yangtze Block.


Introduction
When compared with the supercontinents Pangea and Rodinia, the architecture of the supercontinent Columbia is ambiguous due to the lack of sufficient and reliable geological and paleomagnetic data [1][2][3][4][5]. There is a broad agreement that the fragmentation of Columbia commenced at ca. 1.60 Ga and formed a number of continental rift zones along the western margin of Laurentia [6,7], as marked by widespread~1.60-1.20 Ga continental rifting, anorogenic magmatism, and emplacement of mafic dyke swarms in almost all cratonic building blocks [1,4,8]. The scarcity of~1.60-1.50 Ga zircon sources worldwide [9] makes detrital zircons within this age range particularly useful for constraining the tectonic affinity between different continents.
Many~1. 75-1.50 Ga magmatic events in the southwestern Yangtze Block margin were reported in recent studies [8,[10][11][12], and the contemporaneous rift strata were formed, including the Dongchuan, Hekou, and Dahongshan groups [13][14][15][16], which were interpreted to be linked to the break-up of the Columbia supercontinent. However, this geological event has been rarely identified at the northern margin of the Yangtze Block. Previous studies have shown that the Shennongjia and Dagushi groups have undergone similar crustal evolution, and they are coeval successions deposited in similar environments [17][18][19][20]. Detrital zircon U-Pb ages and Lu-Hf isotopic compositions of the Shennongjia Gp were analyzed recently [18,[21][22][23]. Xiao [18] first identified~1.6 Ga detrital zircon from the Shennongjia Gp and contended that this time period corresponded to the breakup of the Yangtze Block from the Columbia supercontinent.
In this study, we report a new set of zircon U-Pb ages and Hf isotopic data for three meta-quartz sandstones from the Dagushi Gp. Combined with previous studies, our results shed new light on the regional tectonic evolution of the northern part of the Yangtze Block during the Late Paleo-to Mesoproterozoic, and they constrain a possible kinship between the Yangtze Block and the Columbia supercontinent.

Geological Setting
The Yangtze Block is sutured with the North China Craton along the Qinling-Dabie-Sulu orogenic belt to the north in the Triassic, which collided with the Cathaysia Block to the southeast in the early Neoproterozoic along the Sibao (or Jiangnan) orogeny (Figure 1a; [24]). The Kongling terrane is located at the northwestern part of the Yangtze Block (Figure 1a), which is exposed as a small dome of 360 km 2 and was intruded by the Paleoproterozoic Quanqitan granite and the huge Neoproterozoic Huangling complex (see [25] and references therein). There are three types of rock associations in this terrane: (1) dioritic, tonalitic, trondhjemitic, and granitic gneisses of intrusive origin (ca. 2.86-2.95 Ga), (2) metasedimentary rocks, and (3) amphibolite and locally preserved mafic granulite (see [25] and references therein). The study area is located in the Dahongshan region in the northern margin of the Yangtze Block,~200 km east of the Kongling terrane (Figure 1a,b). It is separated from the Tongbai orogen to the north by the Sanligang-Sanyang Fault (Figure 1b) that is part of the east-trending Xiangfan-Guangji Fault.
The Dagushi Gp is the oldest basement exposed in the study area, unconformably overlain by the Huashan Gp, which is in turn overlain by the Nanhua sedimentary units including the Liantuo and Nantuo formations ( Figure 1b). Lithologically, the Huashan Gp is divided into the Hongshansi Formation (Fm) at the bottom and the Liufangzui Fm at the top (Figure 1c) [26]. Recently, Du et al. [27] reported a zircon U-Pb age of 779 ± 12 Ma for a tuff layer from the Liantuo Fm. The five youngest zircons from the coarse-grained feldspathic sandstone in the Liufangzui Fm give a weighted average age of 816 ± 9 Ma [26], suggesting that initial deposition of the Huashan Gp occurred after ca. 816 Ma. The sedimentary sequences of the Dagushi Gp include, from the bottom up, the Taiyangsi, Hanjiawa, Luohanling, Chenjiachong, Lijiazui, and Dangpuling formations [29]. The Taiyangsi Fm consists of metaconglomerate, meta-pebbly sandstone, medium-coarseto fine-grained metasandstone intercalated with banded slate in the lower part, and metasiltstone and clay slate in the upper section with visible lenticular bedding of sand and gravel [29], pointing to a subaqueous alluvial fan depositional system. Sedimentary characteristics suggest that the water depth increased gradually from the lower to the upper part of the Taiyangsi  Subhorizontal beddings in this formation are formed in a tidal flat, shallow sea shelf environment. The Chenjiachong Fm contains micro-to fine-crystalline dolomite and stromatolitic dolomite with horizontal bedding. The Lijiazui Fm has silty-muddy slate with limestone lenses. The Dangpuling Fm consists of meta-quartz and meta-feldspar quartz sandstone (Figure 2f,g), with horizontal bedding and cross bedding, which is interpreted to have formed on a littoral shallow-water setting [29].  [24]); (b) simplified geological map of the Dahongshan area (modified after [28]); (c) 1:50,000 geological map of Guchengfan (modified after [29]; ages of the Luohanling Fm and the Liufangzui Fm are from [19,26], respectively); the red stars show the stratigraphic locations of the three metasandstone samples.
Tuff layers from the Luohanling Fm give two zircon U-Pb ages of~1.23 Ga and 1.24 Ga [19]. The two youngest zircons from the Taiyangsi Fm yield concordant dates of 919 ± 10 Ma and~912 ± 11 Ma (unpublished data). The youngest zircon date of~1.12 Ga for the Taiyangsi Fm was provided by [30]. Thus, the depositional ages of the Taiyangsi Fm and the Hanjiawa Fm are constrained to the Neoproterozoic or the latest Mesoproterozoic.

Zircon U-Pb Dating
Zircons were separated by conventional magnetic and heavy liquid methods and then were selected, according to sizes, colors, shapes, and turbidity under a binocular microscope. The grains were then mounted on double-sided tape, cast in epoxy resin, and polished to expose even and intermediate surfaces. Zircon U-Pb dating was conducted by LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences Wuhan (SKLGPMR-CUG). Laser sampling was performed using a GeoLas 2005 excimer ArF laser-ablation system. An Agilent 7500a ICP-MS instrument was used to acquire ion signal intensities. Standard zircon 91,500 [31] ( Table A1, Appendix A) was employed as an external reference material to calibrate isotope fractionation, which was analyzed twice for every five analyses. Standard zircons GJ-1 were analyzed as unknowns [32] (Table A1, Appendix A). Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data processing are the same as described by [33]. The data were analyzed in the ISOPLOT program of Ludwig [34]. Due to lower intensity of the 207 Pb signal, 206 Pb/ 238 U ages are usually more precise than 207 Pb/ 235 U and 207 Pb/ 206 Pb ages especially for zircons younger than 1.0 Ga [35]. However, 207 Pb/ 206 Pb ages are less sensitive to Pb loss, which is more common in order zircons [36]. Therefore, in most cases, 207 Pb/ 206 Pb ages are employed for zircons of age ≥1.0 Ga and 206 Pb/ 238 U ages are employed for zircons of age <1.0 Ga.

Zircon Trace Element Analysis
The trace element analysis was performed simultaneously with the U-Pb analysis. Quantitative results for trace elements reported here were obtained through calibration of relative element sensitivities using the NIST-610 standard glass as the external calibration standard [37]. The precision and accuracy of the NIST-610 analyses is presented in Table A2 (Appendix A). Nb concentrations of 91,500 obtained in this study fall within the published variation ranges [38].

Zircon Lu-Hf Isotope Analysis
In situ Hf isotope analysis was carried out on a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany), in combination with the GeoLas 2005 in the SKLGPMR-CUG. Detailed operating conditions for the laser ablation system and the MC-ICP-MS instrument and data calibrating and processing were reported in [39]. We applied the directly obtained β Yb value from the zircon sample itself in real time [40]. 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 [41] and 173 Yb/ 171 Yb = 1.132685 [42] using an exponential correction for mass bias. Interference of 176 Yb on 176 Hf was corrected by measuring the interference-free 173 Yb isotope and using 176 Yb/ 173 Yb = 0.79639 to calculate 176 Yb/ 177 Hf [43]. Similarly, the relatively minor interference of 176 Lu on 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 [44] to calculate 176 Lu/ 177 Hf. Time-drift correction was performed using standard zircon 91,500. The interference and mass fractionationcorrected 176 Hf/ 177 Hf ratios of the samples were then calibrated against the standard using the recommended 176 Hf/ 177 Hf ratio of 0.282308 ± 6 (2σ) [45] (Table A3, Appendix A). Offline selection and integration of analytic signals, as well as mass bias calibrations, were performed using ICPMSDataCal [40]. To calculate the initial 176 Hf/ 177 Hf ratios, the decay constant of 1.867 × 10 −5 Ma −1 was used for 176 Lu [46]. The εHf (t) values were calculated with reference to a chondritic uniform reservoir (CHUR). The 176 Lu/ 177 Hf and 176 Hf/ 177 Hf ratios used for the CHUR were 0.0332 and 0.282772, respectively [44]. The single-stage model age (T DM1 ) was calculated relative to a depleted mantle with present-day 176 Lu/ 177 Hf and 176 Hf/ 177 Hf ratios of 0.0384 and 0.28325, respectively [47]. The two-stage Hf model age (T DM2 ), also interpreted as crust formation age, was calculated by projecting the zircon 176 Hf/ 177 Hf (t) back to the depleted-mantle model growth curve, assuming a mean crustal value for Lu/Hf ( 176 Lu/ 177 Hf = 0.015) [48].  (Table A4, Appendix A) and were colorless, transparent, and mostly anhedral, suggesting a longdistance transport. They were 80-150 µm long with length/width ratios of 1.2:1 to 3:1. The majority of zircons showed clear oscillatory or broad zoning ( Figure 3). Concentrations of U and Th were 50-843 ppm and 11-364 ppm, respectively. Th/U ratios ranged from 0.2 to 1.5. A major zircon population occurred in the range of~2.59-2.69 Ga defining a strong peak at ca. 2.65 Ga with two small age populations of~1.90 Ga and~2.80 Ga (Figure 4). The oldest detrital zircon was dated at~3.00 Ga.

Luohanling Fm
JS14-1: Seventy-three zircon grains were separated from sample JS14-1 of the Luohanling Fm and were analyzed for U-Pb ages (Table A4, Appendix A). Thirty-three representative dated zircons were selected for the Hf isotope analysis (Table A5, Appendix A). Zircons were colorless, khaki, and anhedral to subhedral. They were 100-200 µm long with length/width ratios of 1.1:1 to 2.5:1. Most zircons showed oscillatory zoning ( Figure 3). Concentrations of U and Th were 155-1436 ppm and 78-2674 ppm, respectively. Th/U ratios ranged from 0.2 to 2.2. The significant U-Pb age population was at~1.45-1.68 Ga with two small age populations of~1.85-2.23 Ga and~2.62-2.86 Ga, and the dominant peak appeared at ca. 1.60 Ga (Figure 4).   JS14-2: Thirty zircon grains were separated from sample JS14-2 from the Luohanling Fm and were analyzed for U-Pb ages (Table A4, Appendix A). The morphological features of detrital zircons were similar to those of sample JS14-1. The major age population was 1.55-1.68 Ga with two small age populations of~1.78-2.32 Ga and~2.62-2.79 Ga, and the major peak was at ca. 1.60 Ga (Figure 4).

Provenance of the Dagushi Gp
The detrital zircon ages from the Dagushi Gp in our study yielded two major peaks at 2.65 Ga and~1.60 Ga (Figures 4 and 5a). The Luohanling Fm had two age peaks (~1.60 Ga and~2.65 Ga), while the Dangpuling Fm had only one (~2.65 Ga), demonstrating significant discrepancy in the age spectrum among different samples. In addition, Kong et al. [49] reported ages from the Lijiazui and Luohanling formations, which are distinctly different from U-Pb ages presented above, reflecting changes in provenance for individual Dagushi Gp samples.
Magmatism and metamorphism at ca. 1.60 Ga have not been documented in the northern margin of the Yangtze Block except for the detrital zircon ages obtained from the Shennongjia Gp ( Figure 5c) [18]. This episode of magmatism and contemporaneous detrital zircon U-Pb ages have been confirmed in the southwestern [11,14,[67][68][69] and southeastern margins of the Yangtze Block (Figure 5b,d). Thus, the major contributor to these~1.60 Ga detrital zircons may lie in the Yangtze Block itself rather than an external continent. At ca. 1.60 Ga the Qinling Belt was part of the South China Craton, and the proto-Yangtze Block may have been located in the position at the nexus of north Laurentia, southwest Siberia, and north Austria [70]. Thus, they could also have served as sources for~1.60 Ga detrital zircons. Li et al. [19] proposed that the Dagushi Gp correlates well with the Shennongjia Gp in terms of sedimentology and geochronology. The newly acquired~1.60 Ga detrital zircon ages and corresponding εHf (t) values reported in this study ( Figure 6a) further corroborate this correlation. Both groups are interpreted to have formed in a continental rift setting [17], as suggested by the occurrence of intra-plate alkali and tholeiitic basalts in the Shennongjia Gp [71]. Wang et al. [72] suggested that the Shennongjia Gp was deposited during a transgressive system tract in a carbonate ramp steepening toward the west. The clastic rocks are widely distributed in the Shennongjia strata, suggesting continued terrestrial material input mainly from the ancient Yangtze land to the south. These lines of evidence suggest that the~1.60 Ga detrital zircons of the Dagushi Gp may have also been derived from the interior of the Yangtze Block. In addition, the narrow age spectrum and εHf (t) values of these zircons favor a single source.   [26] and references therein, as well as this study).
The Hf (t) values of~1.80-1.35 Ga zircons in the northern and western margin of the Yangtze Block overlap (Figure 6a), indicating the similarity of their sources. The Hf (t) values are significantly elevated at~1.60-1.40 Ga, as illustrated by the evolution curve of ancient rocks in the interior of the Yangtze Block ( Figure 6b). Additionally, Xiao [18] reported zircon δ 18 O values between +5.41 and +5.67, suggesting that mantle-derived materials were added to the crust during this period. Trace element compositions of zircon are useful for delineating its source rock types [86], and those of~1.60 Ga detrital zircons from the Dagushi Gp are shown in Figure 7 and in Table A6   Broadly speaking, trace element characteristics of magmatic zircon could also reflect the compositions of its source magma and the crystalline environment [86,87]. The ca. 1.60 Ga detrital zircons from the Dagushi Gp have a median Nb concentration of 17 ppm, significantly higher than that of continental arcs (1.7 ppm) and MOR (1.6 ppm) [88]. In addition, they have U/Yb ratios (mostly 0.1-1.3) intermediate between those of MOR and typical continental crustal zircons, but higher than Icelandic basalt [89]. On the Hf/Th-Th/Nb and Nb/Hf × 10,000-Th/U discrimination diagrams (Figure 8a,b), zircons from the Dagushi Gp mostly fall in the field of within-plate/anorogenic environments. On the U/Yb-Hf and U/Yb-Nb/Yb × 10,000 discrimination diagrams (Figure 8c,d), they mostly plot in the Hawaii-Iceland field. A subset of zircons with high U/Yb and Hf/Th ratios fall outside the within-plate area (Figure 8a,d), which can be attributed to the fractional crystallization of minerals such as titanite, ilmenite, and zircon. These minerals exert a dominant control on the U, Yb, Nb, and Hf elements of the remaining melt, and they enrich U and Hf in later-formed grains [88]. We interpret the source rocks to have been formed in an intra-plate rifting environment, according to the Hf and O isotope and zircon trace element compositions.
Xiong and Chen [17] argued that the Shennongjia Gp and Dagushi Gp were deposited in the Mesoproterozoic aulacogen at the northern margin of the Yangtze Block. Li et al. [21] believed that the upper subgroup of the Shennongjia Gp and the Dagushi Gp represented a shallow marine environment at the margin of the initial rift basin. Liu et al. [71] reported a set of metabasalt (alkaline basalt series)-tuff (tholeiite basalt series) assemblages from the Shicaohe Fm of the Shennongjia Gp, and they attributed their geochemical characteristics to within-plate rift basalts. These lines of evidence may indicate a long-term rifting history in the northern margin of the Yangtze Block during the Mesoproterozoic.

Reconstruction of the Supercontinent Columbia
Active continental margin magmatism occurred along the margin of the supercontinent Columbia at ca. 1.80-1.40 Ga, with rift-type magmatic pulses in its interior [93]. Detrital zircons dated at ca. 1.70-1.50 Ga are documented from the western, southwestern, and northern margins of the Yangtze Block, as well as in minor amounts from the southeastern margin of the Yangtze Block; in contrast, these Paleo-to Mesoproterozoic detrital zircons are rare in the interior of the Yangtze Block [20]. As discussed above, ca. 1.60 Ga detrital zircons of the Dagushi Gp and Shennongjia Gp have nearly chondritic Hf isotope composition, mantle-source oxygen isotope composition, and primitive properties of basalts, which are trace element characteristics of intra-plate magma, indicating a derivation from typical intra-plate magmatic events. Thus, we suggest that extensive break-up may have occurred at the periphery of the Yangtze Block, responding to the fragmentation of the supercontinent Columbia. Meanwhile, widespread~1.60-1.30 Ga Mesoproterozoic post-orogenic and anorogenic magmatism has been identified in many Precambrian cratons, including the~1.60 Ga Dalma magmatic rocks in northeast India [94], the~1.59 Ga tholeiitic dyke swarms in South America [95], the~1.50 Ga mafic dykes in South America [96], and the ca. 1.50 Ga mafic magmatism in South China [8]. The marked contrast with arc magmatism at the margin of the supercontinent Columbia [97][98][99] further suggests that the Yangtze Block may have been located at the center of the supercontinent Columbia ( Figure 9).

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the research process of this project.

Conflicts of Interest:
The authors declare no conflict of interest.