Redefinition of Early Mesoproterozoic (1800-1600 Ma) stratigraphy in the northern Kongling area, China: The nucleus of Yangtze Craton and its tectonic significance

Xio-ming Zho , Xio-fei Qiu Zhi-hui An , Nin-wen Wu Li Tin, Yun-xu Wei Tuo Jing

a Wuhan Center, China Geological Survey, Ministry of Natural Reources, Wuhan 430223, China

b Key Laboratory for Paleontology and Coevolution of Life and Environment, China Geological Survey, Ministry of Natural Reources, Wuhan 430223, China

c State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China

Keywords:

Zircon U-Pb

Wujiatai Formation

Pb-Pb isochron age

Columbia supercontinent

Northern Kongling area

A B S T R A C T

The Wujiatai Formation, which is well exposed in Huangjiatai-Xichahe region of the northern Kongling area of central Yangtze Craton, is a suite of epimetamorphic conglomerates to pebbly sandstones to fine sandstone-dolostones deposited in littoral-carbonate platform facies. The formation has angular unconformity contacts with both the overlying Neoproterozoic Nantuo Formation and the underlying Paleoproterozoic Huanglianghe Formation complex. Detrital zircons from metafine sandstones of the lower Wujiatai Formation have ages ranging from 3377-1828 Ma, with the youngest zircons dating to about 1828 Ma. In addition, whole-rock Pb-Pb isochron ages from dolostones in the upper Wujiatai Formation yield an age of 1718±230 Ma. These dates constrain the depositional age of the Wujiatai Formation between 1800 Ma and 1600 Ma. These are the earliest Mesoproterozoic sedimentary records reported in the Kongling region, and fill the gaps in Early Mesoproterozoic stratigraphy in Yangtze Craton. Histograms of detrital zircon ages for the Wujiatai Formation reveal four major peaks at 2039 Ma,2691 Ma, 2966 Ma and 3377 Ma, which is consistent with the ages of the basement rocks that underlie the center of Yangtze Craton, indicating that sediment provenance is mainly from the Kongling complex. The lower Wujiatai Formation mainly consists of clastic rocks, whereas the upper Wujiatai Formation consists of dolostones. This stratigraphic change implies a deepening sequence in an expanding basin with an initial cratonic rifting tectonic setting, corresponding to the initial breakup of the Columbia supercontinent in Yangtze Craton.

1. Introduction

The oldest rocks of Yangtze Craton, located in the Kongling area of western Hubei, China, are an important site used for Precambrian research. Recently, many important datasets have been obtained from Paleoarchean rock records,including characteristics of the Mesoarchean crust,mechanisms of Archean plate tectonics, and the locations of Paleoproterozoic and Neoproterozoic ophiolites, among others (Qiu YM et al., 2000; Zhang SB et al., 2006a, 2006b;Gao S et al., 2011; Peng SB et al., 2012; Guo JL et al., 2015;Han PY et al., 2017; Wan YS et al., 2018). Newly found rocks in this region with ages of 3450-1850 Ma are beneficial for understanding the geologic history of Yangtze Craton.

The northern Kongling area is dominated by high amphibolite facies to granulite facies metamorphic rocks and sedimentary rocks of the Cryogenian Nantuo Formation.Granitic gneisses, migmatites, Mesoproterozoic and Neoproterozoic granites, and sedimentary rocks of the Cryogenian Liantuo Formation are exposed mainly in the southern Kongling area (Fig. 1). Studies of late Paleoproterozoic to Mesoproterozoic rocks in the Kongling area are rare. The igneous and sedimentary rocks deposited between 1800-1000 Ma have not been well-studied, and their relationship with the Shennongjia Group are still uncertain. So far, the Mesoproterozoic strata observed in this region have only included the Miaowan Suite (Pt2m) in the southern Kongling area, the Xichahe Formation (Pt2x, Li FX et al., 1989), and the Wujiatai Formation (Pt2w, Hu ZX et al., 2012) in the northern Kongling area (Fig. 1). However, due to a lack of age data,the ages of the Xichahe and Wujiatai formations have only been constrained by the ages of the overlying and underlying strata. Thus, dating the Wujiatai Formation, which is wellpreserved in the Huangjiatai-Xichahe region of the northern Kongling area, is significant for understanding Mesoproterozoic stratigraphic subdivision and correlation in the northern Yangtze Craton, exploring the Paleoproterozoic-Mesoproterozoic geologic history of the Yangtze Craton, and comprehending the breakup of Columbia supercontinent in Archean.

Fig. 1. Geologic map of Mesoproterozoic rocks in the northern Kongling area, China.

2. Characteristic lithology and stratigraphic relationships of the Wujiatai Formation

Epimetamorphic siliceous-carbonates dominate sediments in the Wujitai Formation, unconformably truncated by the underlying Archean-Paleoproterozoic metamorphic Kongling complex, are well exposed in the Huangjiatai-Xichahe region in the northern Kongling area (Fig. 1b). These siliceouscarbonate dominated sediments were first described and named the Xichahe Formation and the Kongzihe Formation in Li FX et al. (1989), and are thought to belong to Mesopreoterozoic strata of the Xichahe Formation and the Huangjiatai Formation (Hu ZX et al., 2012). The Xichahe Formation is composed of quartze-chlorite-seritite schists,seritite-quartz schists, seritite phyllites, and fine-grained siltstones to sandstones. The Xichahe Formation was recently determined to be of Paleoproterozoic age by Wei YX, et al.(2016 ) rather than Mesoproterozoic age, and appears to be part of Huanglianghe the Formation-complex (Pt1h) with its associated underlying sedimentary rocks.

Based on recent field mapping and section measurements obtained from the Huangjiatai-Xichahe region in the northern Kongling area, we used the detrital zircon chronology method to constrain the depositional age of the Huangjiatai Formation by collecting five samples from the well-exposed PM012 section (Fig. 2; N31°22′00″; E111°11′59″).

The basal Wujiatai Formation is comprised of grey to reddish medium to thick bedded meta-anagenites, metaquartzites with quartzite-dominated conglomerates (1-6 cm in diameter) that are finely rounded (rounded to well-rounded in shape) with siliceous cements, truncated by the underlying seritite-quartz schists and seritite phyllites of the Huanglianghe Formation-complex (Fig. 2; Fig. 3a). The truncation surface is distinct for changes in lithology, attitude,and metamorphic grade (Fig. 3a). The lower Wujiatai Formation consists of grey to greyish white medium to thickly bedded meta-quartzites and grey to reddish medium bedded meta-quartzites with parallel beds and graded bedding planes(Fig. 3b, Fig. 3d). The middle Wujiatai Formation consists of grayish thin to medium bedded meta-quartzites and medium bedded sandy dolostones with parallel beds (Fig. 3c). The upper Wujiatai Formation is composed of white medium to thickly bedded dolostones containing algal mats and domeshaped stromatolites (Fig. 3e). The topmost dolostone is directly overlain by tillites of the Nantuo Formation (Zhao XM et al., 2018), with an angular unconformity contact surface (Fig. 3g).

Based on lithological compositions, the lower part of Wujiatai Formation was deposited in littoral facies, while the middle and upper parts of the formation were deposited in restricted carbonate platform facies, implying a deepening sequence in an expanding basin with an incipient rift type of tectonic setting.

3. Sampling and Age-dating Methods

3.1. Detrital zircon U-Pb dating

Detrital zircons are widely used to constrain the depositional age and provenance of rocks, and allow for the comparison of regional tectonic units and paleogeographic reconstructions (Berry RF et al., 2001; Leier AL et al., 2007;Wang LJ et al., 2012; Wang YJ et al., 2013; Li XH et al.,2014; Hu R et al., 2016; Xu DL et al., 2016; Zhang YL et al.,2016; Han PY et al., 2017). In this study, four meta-pebbled sandstones and meta-fine grained sandstones were collected from the PM012 section in the northern Kongling terrane.Laser ablation inductively-coupled mass spectrometer (LAICP-MS) analyses were conducted on these samples, with the aims of (1) discussing the depositional age and provenance of the Wujiatai Formation and (2) providing evidence for comparing regional tectonic units.

Zircon grains were separated by elution, and hand-picked under a binocular microscope. After polishing and carbon coating, their internal structures were examined using cathode luminescence (CL) for in-situ U-Pb isotopic analyses. The CL imaging was performed at Testing Center of Shandong Bureau of China Metallurgical Geology Bureau, using a JXA8230 environmental scanning electron microscope equipped with a JEOL XM-209013TP detector. Zircon U-Pb analyses for samples PM012-8, PM012-9, and PM012-10 were conducted using LA-ICP-MS techniques at the Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences. The analyses for sample PM012-7 were conducted at the State Key laboratory of Geological Processes and Mineral Resources, China University of Geosciences , using the same method. This facility utilizes an Agilent 7500 ICPMS and a Geo Las 2005 laser-ablation system equipped with a 193 nm ArF-excimer laser. The raw U-Pb data were processed using ICPMS Data Cal (ver. 8.3) software. Zircon 91500 was used as a U-Pb isotope standard and NIST 610 was used to calibrate trace element contents. Concordia diagrams and weighted-mean calculations were processed using the ISOPLOT program (Ludwig KR, 2003). In the discussion section,207Pb/206Pb ages are used, as their ages are all ＞1.0 Ga, and discordant zircons (concordance ＜95%) are not discussed in the following sections.

3.2. Whole-rock Pb-Pb isochron dating

Recent studies have indicated that precise stratigraphic ages of ancient carbonate strata can be obtained through Pb-Pb isochron dating techniques (Moorbath et al., 1987; Zhang QD et al., 2002; Gopalan K et al., 2013; Kuznetsov AB et al.,2013; Lu SS et al., 2016). The lower part of the Wujiatai Formation is comprised of meta-anagenite and meta-quartzite with quartz-dominated conglomerates, whereas the upper part is comprised of weakly-silicified dolostones. A continuous depositional boundary separates the lower and upper parts,providing the possibility for dating both sections using zircon U-Pb and whole-rock Pb-Pb methods. In this study, Pb-Pb isochron dating samples were collected from the upper part of PM012 section. Samples are grey to white medium to thickly bedded dolostones and dolostones containing stromatolites,which are the upper most strata that are observed in the Wujiatai Formation in this area. Whole-rock Pb isotope analyses were conducted at Zhongnan Mineral Resources Supervision and Test Center for Geoanalysis, Wuhan Center of China Geological Survey. The detailed analytical method was described elsewhere (Lu SS et al., 2016).

4. Results

4.1. Depositional age of the Wujiatai Formation

4.1.1. Zircon morphology

＞2000 zircon grains were obtained from the samples.Most of them were colorless or light yellow, rounded,euhedral to broken subhedral, in a size range of 60-100 μm in the length with aspect ratios of 1:1 to 2.5:1. The brightness of CL images was not equal, revealing different U and Th contents between distinct zircon grains. The zircons show clear concentric oscillatory zoning in CL images. Most of them have core-rim textures. These are typical features of magmatic zircons that have experienced metamorphism at a later date. Some of the zircons also have inherited cores,indicating their complicated provenance (Fig. 4).

4.1.2. U-Pb isotopes of detrital zircons

Sixty analyses were conducted on 60 zircon grains from sample PM012-7, of which 38 grains are concordant. Their Th/U ratios range from 0.35 to 1.10, which are the typical magmatic zircons. The results in Table 1 show that ages range from 2961 Ma to 1963 Ma. Two major periods of zircon crystallization can be resolved: Neoarchean (2627-2798 Ma)and Paleoproterozoic (1963-2110 Ma; Fig. 5a).

Table 1. LA ICP-MS U-Pb isotope data of Zircon in the Huangjiatai Formation in Huangjiatai-Xichahe region, China.

Table 1. (Continued)

Fig. 5. U-Pb Concordia diagram and probabilistic histogram for the Huangjiatai Formation in the northern Kongling area, China.

Fifty analyses were conducted on 50 grains from sample PM012-8. Only 24 passed the concordance test. Their Th/U ratios range from 0.22 to 2.32, which are the typical magmatic zircons. The U-Pb age stages are at 1921-2140 Ma,2628-2781 Ma, and 2846-2955 Ma (Fig. 5b).

Fifty detrital zircons from sample PM012-9 were dated for U-Pb ages. Forty-two of 50 zircons have concordant U-Pb ages, with Th/U ratios of 0.10-1.58. Their ages range from 2975-1899 Ma and can be divided into two major stages:2028-2169 Ma and 2633-2789 Ma (Fig. 5c).

Fifty analyses were conducted on 50 grains from sample PM012-12. Of these, 32 zircon grains are concordant. Their Th/U ratios range from 0.23 to 1.54, which are the typical magmatic zircons. The results show ages ranging from 3377 Ma to 1828 Ma. Four major periods of zircon crystallization can be resolved: Mesoarchean (3377-3035 Ma), Neoarchean(2784-2624 Ma), and Paleoproterozoic (2165-2035 Ma and 1944-1828 Ma; Fig. 5d).

These four samples, collected in a relatively narrow layer(＜10 m), can be analyzed to constrain the depositional age of the bottom of the Wujiatai Formation. Two hundred detrital zircons were analyzed from these samples, only 136 passed the concordance test (＞95% concordant). Their ages range from 3377 Ma to 1828 Ma. The youngest concordant U-Pb age is 1828 Ma, indicating that Wujiatai Formation was likely deposited prior to 1828 Ma.

4.2. Provenance

Detrital zircons from the Wujiatai Formation can be generally divided into four stages: 3377-3306 Ma,3060-2806 Ma, 2798-2624 Ma and 2200-1828 Ma (Fig. 5e).The probabilistic histograms of detrital zircon ages reveal four major peaks at 2039 Ma, 2691 Ma, 2966 Ma, and 3377 Ma(Fig. 5f).

Zircons with ages ranging from 2200 Ma to 1828 Ma and an age peak at about 2039 Ma are consistent with the time of collision and rifting events in the Yangtze Craton, correlated with the evolution of Columbia supercontinent. In recent years, a series of Paleoproterozoic tectonic-thermal events were recognized in the Yangtze Craton, indicating that it might have been an important component of the Columbia supercontinent development (Zhao GC, 2012; Wu YB et al.,2012; Guo JL et al., 2015; Qiu XF et al., 2014, 2015). As the nucleus of the Yangtze Craton, Kongling complex contains many documented Paleoproterozoic magmatic and tectonicthermal rock types, including 1.85 Ga Quanyishang A-type granites (Peng M et al., 2012; Xiong XW and Chen YY,1991), 1.85 Ga Tandanghe mafic dykes (Peng M et al., 2009),and 1.8 Ga migmatites (Zhao FQ et al., 2006). Recently,Zhang SB et al. (2006a) reported 1970 Ma metamorphic ages in Kongling complex. Ling WL et al. (2001) reported that Kongling complex experienced regional metamorphism at 1958-1939 Ma. Wei YX et al. (2016) obtained 2183 Ma SHRIMP zircon U-Pb ages from volcanic rocks in the Huanglianghe Formation-complex from Kongling complex.

Zircons with ages ranging from 2798 Ma to 2624 Ma and an age peak at about 2691 Ma correspond to Neoarchean tectonic-magmatic events. Increasingly, massifs of similar age have been reported in Kongling complex. For example,Chen K et al. (2013) recognized 2.7-2.6 Ga granitic and trondjemitic gneisses from Kongling complex. Guo JL et al.(2015) obtained 2810-2636 Ma ages from two-mica granites and biotite granites in the northern Kongling area. Also, Han PY et al. (2017) obtained many 2.7-2.6 Ga detrital zircons from modern rivers in the northern Kongling area. These Neoarchean tectonic-magmatic events may record the early crustal evolution of the nucleus of the Yangtze Craton.

Zircons with ages ranging from 3060 Ma to 2806 Ma and an age peak at about 2966 Ma correspond to the generation of the nucleus of Yangtze Craton. These ages are common in Kongling complex. For example, Zhang SB et al. (2006a)reported 3242-2930 Ma from migmatites in Yemadong area in northern Kongling. Qiu YM et al. (2000) recognized trondjemitic gneisses in the Longtouping-Yemadong areas in the northern Kongling complex were formed at 2947-2903 Ma. Also, Wei JQ et al. (2009) suggested the age of amphibolite enclaves in the Kongling area are 3.0 Ga.

Two zircons in this study have ages of 3377 Ma and 3306 Ma, respectively, corresponding to Paleoarchean TTG magmatic events in the nucleus of the Yangtze Craton. These events might represent the earliest crustal generation in the Yangtze Craton (Qiu YM et al., 2000; Gao S et al., 2011; Guo JL et al., 2015).

In summary, the age ranges for detrital zircons from the Wujiatai Formation are consistent with the basement rocks at the nucleus of Yangtze Craton. Additionally, massifs with ages ＞3.3 Ga have only been recognized in northern Kongling complex so far. Therefore, we infer that the main provenance of the Wujiatai Formation was Kongling complex in the nucleus of Yangtze Craton.

4.3. Whole-rock Pb-Pb isochron ages

Eight dolostones were collected from the Wujiatai Formation. Pb isotope compositions are listed in Table 2. The206Pb/204Pb ratios of these dolostones show wide variations from 16.782-23.463. On contrary,207Pb/204Pb ratios and208Pb/204Pb ratios have narrow ranges from 15.358 to 16.009 and 37.092 to 38.606 respectively. Seven dolostone samples(excluding sample PM01206) show an obvious linear relationship between206Pb/204Pb and207Pb/204Pb, yielding a Pb-Pb isochron age of 1718±230 Ma (MSWD=37) (Fig. 6),which represents the depositional age of the upper Wujiatai Formation. This age is consistent with the youngest zircon age(1828 Ma) obtained from the lower Wujiatai Formation,further constraining the depositional age of the formation. In summary, we suggest that the depositional age range of the Wujiatai Formation is from 1800 Ma to 1600 Ma of Mesoproterozoic.

Table 2. Pb isotope compositions of the Wujiatai Formation.

Fig. 6. Pb-Pb isochron of the Wujiatai Formation in the northern Kongling area.

5. Mesoproterozoic stratigraphic correlation in the northern Yangtze Craton

The Mesoproterozoic strata of the north margin of Yangtze Craton are mainly exposed in the Shennongjia area(Shennongjia Group) and the Dahongshan area (Dagushi Group), including the Wujiatai Formation of northern Kongling area introduced in this study.

The Shennongjia Group is exposed in the northwestern Kongling area, covering about 1800 km2. The Shennongjia Group is composed of dolomites, siltstones and sandstones.breccias, and iron ores beds, and is made up of Yingwodong Formation, Dayanping Formation, Luanshigou Formation,Dawokeng Formation, Kuangshishan Formation, Taizi Formation, Yemahe Formation, Wenshuihe Formation,Shicaohe Formation, Songziyuan Formation, Wagangxi Formation, and Zhengjiaya Formation in ascending order (Qiu XF et al., 2011; Li HK et al., 2013). The Shennongjia Group seems to have not experienced strong metamorphism and is truncated by the overlying Neoproterozoic Macaoyuan Group or Cryogenian-Ediacaran strata, with an evident angular conformity contact surface. The Shennongjia Group was calibrated to 1.4-1.1 Ga by recent geochronological dating(Qiu XF et al., 2011, 2015; Li HK et al., 2013; Xu DL et al.,2016) and proposed to be the candidate section for the national standard of 1.4-1.1 Ga age sedimentary successions(Li HK et al., 2013; Geng YS and Lu SN, 2014).

The Dagushi Group located in the central of Dahongshan Anticline and is composed of Taiyangsi Formation, Hanjiawa Formation, Luohanling Formation, Chengjiachong Formation,Lijiazui Formation and Dangpuling Formation successively(Chen GX et al, 1996). Taiyangsi Formation and Hanjiawa Formation are comprised of breccias, sandstones, sandy slates, and few dolostones, while the Luohanling Formation and the Chengjiachong Formation are dominated by dolostones, with minor sandstones and sandy slates. The Lijiazui Formation and Dangpuling Formation are composed of sandstones, slates, and dolostones. The lithological composition of Dagushi Group is similar to Shennongjia Group, as are the sedimentary sequences and micro-plant fossil assemblages (Zhao YS et al., 1987; Xiong XW et al.,1991; Chen GX et al, 1996). The most recent detrital zircon U-Pb dating of the Taiyangsi Formation (basal the Dagushi Group) constrained its deposition to later than 1124 Ma (Liu H et al., 2017), with obvious variations in the age ranges of detrital zircons from Shennongjia Group, suggesting diachronous depositional ages. The ages of the detrital zircons from the Taiyangsi Formation correspond well to the volcanic ages of Yangtze Craton base, implying potential mineral sources from Kongling areas, or the unexposed igneous rocks at the base of Yangtze Craton, alternatively.

Our data indicates that the Wujiatai Formation was deposited from 1800 Ma to 1600 Ma. In this case, the stratigraphic sequence of Mesoproterozoic rocks in the northern Yangtze Craton from the bottom upward are the Wujiatai Formation, Shennongjia Group, and Dagushi Group,respectively. The well-sorted and rounded conglomerates and pebbly sandstones in the lower part of Wujiatai Formation represent the initial break-up stage of Columbia supercontinent. The dolostones in middle part of Wujiatai Formation indicate there was a growth of rift basin and a rise in sea level. Other studies’ results show that Yangtze Craton had high pressure metamorphic events during 2.0-1.9 Ga (Wu YB et al., 2012; Qiu XF et al., 2014, 2015), and post-collision magmatism at about 1.85 Ga (Peng M et al., 2012; Qiu XF et al., 2015). In this study, the data suggest that the Wujiatai Formation represents sedimentary records of the preliminary stage of breakup of Columbia super-continent in South China Block. A new rift basin grew after further breakup in the northern Yangtze Block occurred at 1600 Ma. In this scenario,the sources supplying the two sides of the rift basin would be different. In southern part, the provenance is still Kongling complex. Whereas in the northern part, the sources are the residual basement of rift block and igneous rock, which are both re-deposited. In this case, the detrital zircon age peak of 1600 Ma was present in Shennongjia Group, but absent in the Kongling area (Qiu XF et al., 2014). The collision of Shennongjia area and the nucleus of Yangtze Craton occurred at 1100-900 Ma, coincident with the collision in the western and south-eastern Yangtze Craton (Wang LJ et al., 2010; Qiu XF et al., 2011, 2014; Peng SB et al., 2012; Wu YB et al.,2012; Xu DL et al., 2016). These collisions caused hyperplasia of the plate, and are attributed to the basement of Yangtze Craton.

6. Conclusions

(i) The youngest detrital zircon age from meta-fine sandstone of the lower Wujiatai Formation is 1828 Ma.Whole-rock Pb-Pb isochron ages from the upper dolostones in the Wujiatai Formation yield an age of 1718±230 Ma. These ages constrain the Wujiatai Formation deposition between 1800-1600 Ma.

(ii) The detrital zircon ages from the lower Wujiatai Formation are consistent with those from the basement rocks of the Kongling complex. In addition, ＞3.3 Ga zircons, only discovered in the Kongling complex, were observed in the lower Wujiatai Formation. These zircon ages indicate that the provenance of Wujiatai Formation was mainly the igneous rocks of the Kongling complex.

(iii) The Mesoproterozoic strata in the northern Yangtze Craton are almost complete. They are the Wujiatai Formation,Shennongjia Group, and Dagushi Group, in ascending order.The Wujaitai Formation documents the initial and incipient stage of the rift basin between 1.8 Ga and 1.6 Ga. The mature stage of the rift basin is deposited in the lower-middle Shennongjia Group between 1.6 Ga and 1.1 Ga, while the upper Shennongjia Group and Dagushi Group represent the deposition of the decline of rift basin. In this case,Mesoproterozoic strata in the northern Yangtze Craton document the breakup process of Columbia super-continent in Yangtze Craton.

Acknowledgement

This work was supported by the National Science Foundation of China (41530104, 41303026) and the projects of China Geological Survey (121201009000161427,DD20160029, DD20160030). The authors would like to thank the following people: Dr. Hou Kejun for help in testing and analysis, Prof. Tong Jinnan and Prof. Zhang Quanxu for discussions, and anonymous reviewers for their detailed comments and valuable suggestions that improved this paper.