U-Pb Ages and Hf Isotopes of Detrital Zircon Grains from the Mesoproterozoic Chuanlinggou Formation in North China Craton : Implications for the Geochronology of Sedimentary Iron Deposits and Crustal Evolution

The Chuanlinggou Formation is the lower formation of the Changchengian System, and hosts sedimentary iron deposits (marine oolitic ironstones) of the North China Craton (NCC). To determine the age of the iron deposits, and provide insight into the crustal growth of the craton, laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)U-Pb and in situ Hf isotope analysis were performed on detrital zircon grains. Samples were taken from the roof sand-shale of the sedimentary iron deposits at Jiangjiazhai and Pangjiapbu. Overall, 186 detrital zircon grain U-Pb ages yield three major age populations, with weighted average ages of 2450 Ma, 1848 Ma, and 1765 Ma, respectively. Four younger ages from magmatic zircon grains were obtained, ranging from 1694 to 1657 Ma. Combined with observations from published studies, the results define the lower limit for the age of the Chuanlinggou Formation, and constrain the age of the sedimentary iron deposits (marine oolitic ironstone) close to 1650 Ma. The peak ages of 1848 Ma and 2450 Ma define the major collisional events of the NCC. The age of 1765 Ma can be linked to the age range of the widespread mafic dyke swarms that represent the rifting of the NCC within the Columbia supercontinent. Detrital zircon grains from the Chuanlinggou Formation form two obvious groups, with different εHf (t) values ranging from −1 to −8 and from +1 to +8, which correspond to the U-Pb age ranges of 1.7–1.9 Ga and 2.3–2.6 Ga, respectively. They have a similar two-stage Hf model age peak at 2.65–2.85 Ga, suggesting that the source rocks for each of these events were derived from the recycling of ancient crust. The source rocks of the older group of zircon grains might be derived from juvenile crust with a short reworking period. The critical crust–mantle differentiation event might happen during the period of 2.65–2.85 Ga, marking the most significant stage of the crustal growth in the NCC.


Introduction
The North China Craton (NCC) covers an area of about 1.5 million km 2 and is one of the oldest cratons, with age components as old as ~3.8 Ga [1][2][3].The craton is bounded by the early Palaeozoic Qilianshan Orogen and late Palaeozoic Central Asian Orogenic Belt to the west and north, and the Mesozoic Qinling-Dabie and Su-Lu ultrahigh-pressure metamorphic belts to the south and east, Qilianshan Orogen and late Palaeozoic Central Asian Orogenic Belt to the west and north, and the Mesozoic Qinling-Dabie and Su-Lu ultrahigh-pressure metamorphic belts to the south and east, respectively (Figure 1).From the Archean, the craton experienced a geological history containing many recorded tectonic and metamorphic events [2,[4][5][6][7][8][9][10][11][12].Three main models for the formation of the NCC have been proposed using stratigraphic, structural, metamorphic, geochemical, and geochronological studies.(1) The NCC was originally divided into several micro-blocks (Jiaoliao, Qianhuai, Fuping, Jining, Xuchang, and Alashan), which formed the final craton during the late Neoarchean (~2. 5 Ga) [3,6,[13][14][15][16][17]; (2) Zhao et al. (2001,2005,2012) [1,2,7] suggested a model for the evolution of the NCC, which divided the NCC into three parts: the Western Block, Eastern Block, and Trans-North China Orogeny (TNCO) (Figure 1).Prior to the collision of the Western and Eastern blocks along the TNCO at ~1. 85 Ga, the Western Block was formed by the amalgamation of the Ordos Block in the south and the Yinshan Block in the north, along the east-west-trending Khondalite Belt, and the Eastern Block underwent a Paleoproterozoic rifting event along its eastern continental margin in the period 2.2-1.9Ga, forming the Jiao-Liao-Ji Belt [18][19][20][21][22][23][24]; (3) Another model suggests that the Eastern and Western blocks were amalgamated at ca. 2. 5 Ga, and that at ca. 1. 85 Ga the Central Orogenic Belt experienced a metamorphic reworking/overprinting by the collision of the northern margin of the NCC with another continental block in the Columbia (Nuna) Supercontinent [25][26][27][28].The North China Craton is rich in mineral resources; the Precambrian is one of the critical mineralization periods, which is marked by the appearances of large iron, rare earth elements, leadzinc, and magnetite deposits [29,30].Precambrian iron deposits are the most important mineral resource in the North China Craton, of which the banded iron formation (BIF) type is predominant, accounting for ~80% of metamorphosed sedimentary iron ores in China, with a peak formation age of 2.5-2.6 Ga [31][32][33][34][35][36][37].Marine oolitic ironstone is another type of sedimentary hosted iron ore in the NCC, referred to as the Xuanlong type deposit in the Chinese literatures.The oolitic iron deposit is a distinctive subset of iron deposits, characterized by spherical grains composed of concentric layers The North China Craton is rich in mineral resources; the Precambrian is one of the critical mineralization periods, which is marked by the appearances of large iron, rare earth elements, lead-zinc, and magnetite deposits [29,30].Precambrian iron deposits are the most important mineral resource in the North China Craton, of which the banded iron formation (BIF) type is predominant, accounting for ~80% of metamorphosed sedimentary iron ores in China, with a peak formation age of 2.5-2.6 Ga [31][32][33][34][35][36][37].Marine oolitic ironstone is another type of sedimentary hosted iron ore in the NCC, referred to as the Xuanlong type deposit in the Chinese literatures.The oolitic iron deposit is a distinctive subset of iron deposits, characterized by spherical grains composed of concentric layers containing hematite and goethite.Different to BIF, most deposits of this type have a biogenic origin.As a result of its special genesis environment, oolitic iron ores also provide much significant information about the evolution of palaeogeographic facies and marine chemical conditions [38][39][40].
Minerals 2018, 8, 547 3 of 30 Oolitic iron ores (ironstones) occur worldwide and were formed throughout the geological time from the Proterozoic Eon to the Cenozoic Era.In the NCC, it is considered to have occurred in the Mesoproterozoic, hosted in the Chuanlinggou Formation.However, the geochronology of the Chuanlinggou Formation with the oolitic iron deposit is still poorly understood.
In this study, we present U-Pb ages and Hf isotope data from detrital zircon grains from sand-shale samples collected from the roof of the sedimentary iron ores at Jiangjiazhai and Pangjiabu (Figure 1), in order to determine the chronology of the iron deposition, supplying a piece of the picture of the time frame of the distribution of oolitic iron deposits in the global context, and provide further information on the crustal growth and evolution of the NCC.

Regional Geology
In the NCC, the marine oolitic ironstones are located in the Trans-North China Orogeny (TNCO), which was defined by Zhao et al. (2001Zhao et al. ( , 2005) ) [1,2], with boundaries of the Xinyang-Kaifeng-Shijiazhaung-Jianping fault zone and the Huashan-Lishi-Datong-Duolun fault zone.During the late Archean to early Paleoproterozoic, the western margin of the Eastern Block faced a major ocean, and east-dipping subduction beneath the western margin of the Eastern Block led to the formation of magmatic arcs that were subsequently incorporated into the TNCO [7].Continued subduction resulted in a major continental-continental collision, leading to extensive thrusting and high-pressure metamorphism.During the period 2.56-1.85 Ga, the Huai'an, Hengshan, Wutai, Fuping, Zanhuang, Lvliang, Zhongtiao, Dengfeng, Taihua, and other metamorphic complexes formed in the TNCO [41,42], providing records of the tectonic activity.The available age data for metamorphism and deformation in the Trans-North China Orogen indicate that this collisional event occurred at about 1. 85 Ga ago, resulting in the formation of the TNCO and final amalgamation of the North China Craton [7,43].After 1.80 Ga, the NCC experienced extensional tectonism, marked by the development of large mafic and alkaline intrusions [9,16,44].The North China Craton experienced large-scale sedimentation in the Mesoproterozoic, forming the Changchengian System and the Jixianian System.The Changchengian System (~3000 m thick) contains four units from its base to top; the Changzhougou Formation (~860 m thick), Chuanlinggou Formation (~890 m thick), Tuanshanzi Formation (~520 m thick) and Dahongyu Formation (~480 m thick) (Zhang et al., 2015 [45] and references therein).

Chuanlinggou Formation
The Chuanlinggou Formation is widely distributed in North China and can be divided into three depositional regions.The eastern area, including Jixian, Zunhua, Xinglong, and Kuancheng, is characterized by the development of thick black shale.The middle area is located in the northern part of Miyun-Huairou and comprises lacustrine dolomite and lagoon facies depositions.The western area is located in the Xuanhua-Zhangjiakou area, where the thickness of the formation is significantly thinner, and is characterized by the development of the sedimentary iron rocks.Xuanlong type iron deposits occurred in this typical area.
The type profile of the Chuanlinggou Formation is divided into three members [46] (Figure 2a).Member one contains yellowish brown sandy shale with yellow-green siltstone and iron-rich sandstone.Clay minerals are mainly illite.Sandstone, siltstone, and shale exhibit obvious rhythmic sedimentary characteristics, with horizontal bedding and microwave bedding, mud cracks, and ripple level structures.The maximum thickness of one single rhythm is 127 mm; member two is mainly composed of black and dark grey silty shale, locally intercalated with thin silty bars and dolomite, with horizontal bedding.The silty shale is the basic rhythmic layer and is less than 1 cm thick; and member three is composed of black silty shale and greyish white thin siltstone, with 2-3 layers of lightly colored microcrystalline carbonaceous dolomite at the top, with wavy and horizontal bedding.Some bedding planes show mud cracks and underwater sliding load curling structures.layers of lightly colored microcrystalline carbonaceous dolomite at the top, with wavy and horizontal bedding.Some bedding planes show mud cracks and underwater sliding load curling structures.Zhao, 1994 [48]).
The Chuanlinggou Formation contains three ironstone layers: the lower one is a stromatolite hematite, the middle is a pisolitic and oolitic hematite, and the upper part is an oolitic hematite (Figure 2b).Between the layers is a sandy shale unit.There are commonly siderite intercalations or thin lenses in the transitional parts of the shale and hematite layers.The stromatolite hematite is purplish red, usually upright without bending, developing a single tubular or bell-shaped  Zhao, 1994 [48]).
The Chuanlinggou Formation contains three ironstone layers: the lower one is a stromatolite hematite, the middle is a pisolitic and oolitic hematite, and the upper part is an oolitic hematite (Figure 2b).Between the layers is a sandy shale unit.There are commonly siderite intercalations or thin lenses in the transitional parts of the shale and hematite layers.The stromatolite hematite is purplish red, usually upright without bending, developing a single tubular or bell-shaped aggregate structure.Quartz grains are deposited among the stromatolite hematite and are cemented by siderite or hematite.The pillar shaped stromatolite hematite has a diameter of 0.5-2 cm and a height of about 5 cm.The columnar bedding is fine and regular and has good symmetry.Oolitic hematite is dark red, of which 50-70% is oolitic.Ooids vary in size, have a particle size of 0.5-3 mm, and are mostly spherical or sub-spherical.Most of the terrigenous clastic material, primarily quartz, is deposited between the oolitic grains.Most of the oolitic cores are single or clustered quartz grains, and sometimes they are also composed of iron debris and clay mineral mixtures, feldspar clasts and apatite fragments.The ooid shells have a transparent-translucent concentric ring structure and are composed mainly of hematite and siderite (Figure 3).aggregate structure.Quartz grains are deposited among the stromatolite hematite and are cemented by siderite or hematite.The pillar shaped stromatolite hematite has a diameter of 0.5-2 cm and a height of about 5 cm.The columnar bedding is fine and regular and has good symmetry.Oolitic hematite is dark red, of which 50-70% is oolitic.Ooids vary in size, have a particle size of 0.5-3 mm, and are mostly spherical or sub-spherical.Most of the terrigenous clastic material, primarily quartz, is deposited between the oolitic grains.Most of the oolitic cores are single or clustered quartz grains, and sometimes they are also composed of iron debris and clay mineral mixtures, feldspar clasts and apatite fragments.The ooid shells have a transparent-translucent concentric ring structure and are composed mainly of hematite and siderite (Figure 3).The sedimentary iron deposits are mainly distributed in the Zhangjiakou area, the western part of the Yanshan, in the south of Huai'an-Chicheng and north of Huaxiaoying-Xiahuayuan-Xinglinbao.This covers an area of 3900 km 2 , with a length of 130 km oriented east-west and a width of 154 km running north-south.There are 23 Xuanlong type iron deposits in the belt, containing 304 million metric tons (Mt) of iron [49].In this study, the samples analyzed were collected from the roof sand-shale (Figures 2b and 3d) overlying the first hematite layer of the Chuanlinggou Formation in representative iron deposits at Jiangjiazhai (JJZ12-08, E 115°34'55" N 40°42'25") and Pangjiabu (PJB12-11, E 115°27'31" N 40°37'39") (Figure 1).The sedimentary iron deposits are mainly distributed in the Zhangjiakou area, the western part of the Yanshan, in the south of Huai'an-Chicheng and north of Huaxiaoying-Xiahuayuan-Xinglinbao.This covers an area of 3900 km 2 , with a length of 130 km oriented east-west and a width of 154 km running north-south.There are 23 Xuanlong type iron deposits in the belt, containing 304 million metric tons (Mt) of iron [49].In this study, the samples analyzed were collected from the roof sand-shale (Figures 2 and 3) overlying the first hematite layer of the Chuanlinggou Formation in representative iron deposits at Jiangjiazhai (JJZ12-08, E 115 • 34 55 N 40 • 42 25 ) and Pangjiabu (PJB12-11, E 115 • 27 31 N 40 • 37 39 ) (Figure 1).

Zircon LA-MC-ICP-MS U-Pb Dating
The samples were crushed and individual zircon grains were separated using conventional heavy liquid and magnetic techniques.Grains were handpicked under a binocular microscope, mounted in epoxy resin discs, and then polished.Zircon grains were examined under transmitted and reflected light, and then imaged by cathodoluminescence (CL) using a HITACHI S3000-N microprobe in the Institute of Mineral Resources, Geology Chinese Academy of Geological Sciences.The zircon grains, which were euhedral or subhedral, with a striped cathodoluminescence pattern and oscillatory zoning rims, were selected for U-Pb and Hf isotope dating.
U-Pb dating analyses were conducted by laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing.Detailed operating conditions for the laser ablation system and the MC-ICP-MS instrument and data reduction are the same as described by Hou et al. (2009) [50] and Liu et al. (2010) [51].Laser sampling was performed using a New Wave UP 213 laser ablation system, with a beam diameter of 25 µm.A Thermo Finnigan Neptune MC-ICP-MS instrument was used to acquire ion-signal intensities.The array of four multi-ion-counters and three Faraday cups allows for the simultaneous detection of 202 Hg (on IC5), 204 Hg, 204 Pb (on IC4), 206 Pb (on IC3), 207 Pb (on IC2), 208 Pb (on L4), 232 Th (on H2), and 238 U (on H4) ion signals.Helium was used as a carrier gas.Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP.Each analysis incorporated a background acquisition of approximately 20-30 s (gas blank) followed by 30 s of data acquisition from the sample.Off-line raw data selection and the integration of background and analyzed signals, as well as time-drift correction and quantitative calibration for U-Pb dating, were performed by ICPMSDataCal program [51].Zircon GJ-1 was used as a reference material for U-Pb normalization, and was analyzed twice every 5-10 analyses.Time-dependent drifts of U-Th-Pb isotopic ratios were corrected using a linear interpolation (with time) for every 5-10 analyses according to the variations of GJ-1 (i.e., 2 GJ-1 zircon grains + 5-10 samples + 2 GJ-1 zircon grains) [51].The preferred U-Th-Pb isotopic ratios used for GJ-1 are from Jackson et al. (2004) [52].Uncertainty of preferred values for the external standard GJ-1 was propagated to the ultimate results of the samples.In all analyzed zircon grains, the common Pb correction has not been made due to the low signal of common 204 Pb and high 206 Pb/ 204 Pb.U, Th, and Pb concentrations were calibrated by zircon GJ-1 (with U:315 ppm and Th: 9.33 ppm; Liu et al., 2010 [51]).Concordia diagrams and weighted mean calculations were made using Isoplot 3.0 [53].The reference zircon Plešovice was treated as an unknown and yielded the weighted mean 206 Pb/ 238 U age of 336.6 ± 2.5 Ma (2σ, n = 20), which is in good agreement with the recommended 206 Pb/ 238 U age of 337.13 ± 0.37 Ma (2σ) [54].

In Situ Zircon Lu-Hf Isotope Analyses
The zircon Hf analyses of two samples were performed on the same grains as used for U-Pb dating.Zircon Hf isotope analysis was carried out in situ using a New Wave UP 213 laser-ablation microprobe attached to a Neptune multi-collector ICP-MS at the Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing.The same positions of zircon grains were used for the present analyses, with a beam diameter of 55 µm.Helium was used as the carrier gas to transport the ablated sample from the laser-ablation cell to the ICP-MS torch, via a mixing chamber where it was mixed with argon.To correct the isobaric interferences of 176 Lu and 176 Yb on 176 Hf, the 176 Lu/ 175 Lu and 176 Yb/ 173 Yb values were determined (0.02658 and 0.796218, respectively) [55].For instrumental mass bias correction, Yb and Hf isotope ratios were normalized to 1.35274 for 172 Yb/ 173 Yb [55] and to 0.7325 for 179 Hf/ 177 Hf using an exponential law.The mass bias behavior of Lu was assumed to follow that of Yb.The mass bias correction protocols details are described in Wu et al. (2006) [56] and Hou et al. (2007) [57].Zircon GJ-1 was used as the reference standards during our routine analyses, with a weighted mean 176 Hf/ 177 Hf value of 0.282017 ± 0.000021 (2σ, n = 13).This is not distinguishable from the weighted mean 176 Hf/ 177 Hf value of 0.282000 ± 0.000005 (2σ) using a solution analysis method by Morel et al. (2008) [58].Also, a Temora zircon standard was measured daily, giving a mean 176 Hf/ 177 Hf value of 0.282681 ± 33 (2σ, n =4), which is in good agreement with the recommended mean 176 Hf/ 177 Hf value of 0.282680 ± 31 [56].

Analytical Results
The 200 detrital zircon grains have been subjected to U-Pb age dating, and the most concordant (>95%) results are listed in Table 1.The 186 concordant (>5%) detrital zircon grains have been analyzed for Hf isotopic data.The Lu-Hf isotope data are listed in Table 2. Representative zircon grain CL images and Th/U values are shown in Figures 4 and 5.The results are presented as a series of concordia diagrams (Figure 6).The probability density maps of ages are given in Figure 7. Hf isotope characteristics and U-Pb ages of detrital zircon grains are shown in Figures 8 and 9.

Sample JJZ12-08
The zircon grains collected from sample JJZ12-08 were mostly well rounded to short columnar in shape, with grain sizes ranging from 100 μm to 500 μm (Figure 4).One hundred zircon grain U-Pb

Sample JJZ12-08
The zircon grains collected from sample JJZ12-08 were mostly well rounded to short columnar in shape, with grain sizes ranging from 100 μm to 500 μm (Figure 4).One hundred zircon grain U-Pb

Sample JJZ12-08
The zircon grains collected from sample JJZ12-08 were mostly well rounded to short columnar in shape, with grain sizes ranging from 100 µm to 500 µm (Figure 4).One hundred zircon grain U-Pb ages were obtained, and 6 more discordant (>5%) zircon grains were discarded.Uranium concentrations ranged from 8 to 220 ppm, and thorium from 5 to 561 ppm.The Th/U values are mostly greater than 0.4 and are clustered at 0.6 to 1.5 (Figure 5), which, combined with the oscillatory growth zoning in CL images, indicates their magmatic origin [62][63][64][65]
Lu-Hf isotopic analyses were performed on the 92 samples already used for zircon U-Pb age dating.εHf (t) values from these zircon grains cluster into two main populations of −0.5 to −9 and 1 to 8, corresponding to age ranges of 1.7 Ga to 1.9 Ga and 2.3 Ga to 2.6 Ga, similar to sample JZZ12-08 (Figure 8).

Geochronology of the Sedimentary Iron Deposit
During the Mesoproterozoic, the North China Craton underwent a long period of sedimentation.In the geological time scale of China (GTSC), the lower to upper Mesoproterozoic consists of the Changchengian and Jixianian systems.Within the older Changchengian System, the lowermost Changzhougou and Chuanlinggou Formations (Figure 2) unconformably overlie the

Geochronology of the Sedimentary Iron Deposit
During the Mesoproterozoic, the North China Craton underwent a long period of sedimentation.In the geological time scale of China (GTSC), the lower to upper Mesoproterozoic consists of the Changchengian and Jixianian systems.Within the older Changchengian System, the lowermost Changzhougou and Chuanlinggou Formations (Figure 2) unconformably overlie the Archean to Paleoproterozoic crystalline basement sequence.Gao et al. (2009) [66] obtained an emplacement age of 1638 ± 14 Ma by SHRIMP U-Pb dating method for a diabase dike crosscutting the Chuanlinggou Formation at Jixian.Zhang et al. (2013) [67] obtained an emplacement age of 1634 ± 9 Ma by LA-ICP-MS U-Pb dating method for a dioritic porphyrite dyke emplaced into the Chuanlinggou Formation at Jixian.Sun et al. (2013) [68] obtained an age for the tuff from the upper most part of the Chuanlinggou Formation of 1621 ± 12 Ma by SHRIMP U-Pb dating method, which is within the error range of the intrusion ages in this formation.These geochronological ages constrain the Chuanlinggou Formation deposition age to be not later than 1638 Ma.  7).There is a distinctly young age in each of the two samples: 1565 Ma (JJZ12-08-14) and 1391 Ma (PJB12-11-95) with Th/U values of 0.24 and 0.07, respectively, and the reason for these clear outlier ages is uncertain.Minimum depositional ages are interpreted from populations of multiple grains.There are four zircon grain U-Pb ages in the youngest zircon age group (JJZ12-08-2, 1657 ± 17 Ma; JJZ12-08-1, 1661 ± 12 Ma; JJZ12-08-84, 1683 ± 20 Ma; and JJZ12-08-17, 1694 ± 30 Ma), with high Th/U values (0.75-1.49) and oscillatory zoning.Conventionally, the maximum depositional age of the sediments can be constrained by the age of the youngest zircon grains, which has not been altered since deposition.Therefore, the zircon U-Pb age constrains the depositional age of the Xuanlong type iron deposit to be not older than 1657 Ma.Combining this information with previous studies, and considering the considerable thickness of this formation, this type of iron deposit was deposited at 1634-1657 Ma, close to 1650 Ma.
Detrital zircon ages from the Chuanlinggou Formation contain zircon populations defining peaks similar to the two major events at 1848 Ma and 2450 Ma, which align with the ages mentioned above.These two major tectonic-thermal event ages (ca.1.85 Ga and ca.2.5 Ga) have been widely recognized [7,16,[19][20][21]24,93,[95][96][97][98][99].However, there is still debate surrounding the processes of the crustal amalgamation and cratonisation.In this work, another age population of 1.74-1.80Ga from the Chuanlinggou Formation has been discovered.This is consistent with the age of the mafic dyke swarms which must have caused crustal melting, which recorded the extension, uplift, crust-mantle interaction and mantle upwelling events after 1.8 Ga in the central part of the NCC.

Crustal Growth
Detrital zircon U-Pb age spectra coupled with Hf isotope data can be used to provide insight into crustal growth and recycling, as well as provenance [100][101][102][103]. Hf isotope data from the Chuanlinggou Formation show two obvious groups, with different εHf (t) values.Most of the detrital zircon grains with U-Pb age ranges of 1.7-1.9Ga have modest negative εHf (t) values, clustering at −1 to −8, suggesting that the source rocks of these zircon grains were derived from the recycling of somewhat more ancient crust.Most of the detrital zircons with age ranges of 2.3-2.6 Ga have εHf (t) values, clustering at +1 to +8.However, these older zircon grains' Hf two-stage model ages do not show the expected ca.2.5 Ga peak, but instead indicate a peak at 2.6-2.9Ga, which is slightly older than the apparent age and single stage Hf model peak age (Figure 9), suggesting that the rocks were not directly derived from the differentiation of the depleted mantle; i.e., from basalt.The source rocks of these zircon grains might be derived from juvenile crust with a short period of reworking while residing in the crust [104].It is obvious that there is a trend of decreasing εHf (t) values with ages (Figure 8), which also suggests that, within this period, the younger zircon grains are being derived from greater amounts of reworking of an older crust, or the reworking of recently formed crust; i.e., 2.6 Ga crust has been reworked to form 2.3 Ga crust.In addition, the two ancient grains found hint at the presence of truly ancient crust in this reworking period.These features are earmarks of the crustal evolution of the North China Craton and are consistent with previous studies in the TNCO (Zhang et al., 2013 [105] and references therein) and similar to those in the East Block and West Block [104,[106][107][108][109][110][111][112].Furthermore, detrital zircon grains from all of these studies have a similar two-stage Hf model age (T DM2 ) ranges of 2.4-3.0Ga, with a peak age of 2.65-2.85Ga, implying the possibility of crust-mantle differentiation during this period, marking the most significant stage of crustal growth of the NCC.This is similar to the Nd model age peak at 2.8-2.4Ga obtained by Wu et al. (2005) [113].

Conclusions
The following are the conclusions of this study.

Figure 3 .
Figure 3. (a) Outcrop photo of the sandy shale; (b) photo of the stromatolith hematite ore with oolitic hematite appearing at the top; (c) photo of the oolitic hematite ore; (d) micrograph of the sandy shale.

Figure 3 .
Figure 3. (a) Outcrop photo of the sandy shale; (b) photo of the stromatolith hematite ore with oolitic hematite appearing at the top; (c) photo of the oolitic hematite ore; (d) micrograph of the sandy shale.

Figure 4 .
Figure 4. Representative zircon grain cathodoluminescence (CL) images with 207 Pb/ 206 Pb ages, and εHf (t) values (within parentheses).The analysis positions are shown, with solid lines for 207 Pb/ 206 Pb ages and dashed lines for Hf isotope values.

Figure 4 .
Figure 4. Representative zircon grain cathodoluminescence (CL) images with 207 Pb/ 206 Pb ages, and εHf (t) values (within parentheses).The analysis positions are shown, with solid lines for 207 Pb/ 206 Pb ages and dashed lines for Hf isotope values.

Figure 4 .
Figure 4. Representative zircon grain cathodoluminescence (CL) images with 207 Pb/ 206 Pb ages, and εHf (t) values (within parentheses).The analysis positions are shown, with solid lines for 207 Pb/ 206 Pb ages and dashed lines for Hf isotope values.

Figure 6 .
Figure 6.U-Pb concordia and peak age distribution diagrams of zircon grains from samples JJZ12-08 (a-d) and PJB12-11 (e-h) collected from the Chuanlinggou Formation.

Figure 6 .
Figure 6.U-Pb concordia and peak age distribution diagrams of zircon grains from samples JJZ12-08 (a-d) and PJB12-11 (e-h) collected from the Chuanlinggou Formation.

Figure 7 .
Figure 7. Relative probability of the detrital zircon grains' U-Pb ages.

Figure 9 .
Figure 9. (a) Histogram of the one-stage Hf model ages of detrital zircon grains from JJZ12-08 and PJB12-11; (b) Histogram of the two-stage Hf model ages of detrital zircon grains from JJZ12-08 and PJB12-11.

Table 1 .
LA-MC-ICP-MS U-Pb dating data for detrital zircons from the Chuanlinggou Formation.

Table 2 .
In situ Lu-Hf isotopes analytical data for detrital zircons from the Chuanlinggou Formation.
. The 94 grains yield three age populations: 1739-1799 Ma, with a weighted average age of 1774 ± 8 Ma (n = 22, MSWD = 0.73); 1839-1857 Ma, with a weighted average age of 1849 ± 8 Ma (n = 22, MSWD = 0.08); and 2436-2465 Ma with a weighted average age of 2453 ± 8 Ma (n = 10, MSWD = 0.71) (Figures 6 and 7).There are two smaller populations with ages in the ranges of 1872-1907 Ma and 1945-2000 Ma, and a few ages are plotted in the ranges 2068-2394 Ma and 2526-2611 Ma.Four younger magmatic detrital zircon ages are obtained; the youngest is 1657 ± 17 Ma (JJZ12-08-2).Lu-Hf isotopic analyses were performed on the 94 grains already used for zircon U-Pb age dating.εHf (t) values from these zircon grains cluster into two main populations of −1 to −8 and +1 to +8, corresponding to age ranges of 1.7-1.9Ga and 2.3-2.6 Ga, respectively (Figure 8).The zircon population with an age of 1739-1799 Ma has one zircon grain with a positive εHf (t) value of 2.38, whereas the other grains have negative εHf (t) values from −0.08 to −9.20, 176 Hf/ 177 Hf values of 0.281420 to 0.281678, and two-stage Hf model ages (T DM2 ) of 2429-2993 Ma.The zircon population with age of 1839-1857 Ma has five zircon grains with positive εHf (t) values of 0.80 to 5.08, whereas the other grains have negative εHf (t) values from −1.20 to −8.07, 176 Hf/ 177 Hf values of 0.281409 to 0.281605, and T DM2 of 2579-2998 Ma.The zircon population with an age of 2436-2465 Ma has one zircon grain with a high positive εHf (t) value of 14.05, whereas the other grains have positive εHf (t) values from 1.11 to 5.13, 176 Hf/ 177 Hf values of 0.281288 to 0.281432, and T DM2 of 2657-2894 Ma.
The zircon grain population with an age range of 1743-1766 Ma has zircon grains with negative εHf (t) values from −4.46 to −11.16, 176 Hf/ 177 Hf values of 0.281367 to 0.281566, and two-stage Hf model ages (T DM2 ) of 2694-3113 Ma.The zircon grain population with an age of 1831-1863 Ma has zircon grains with negative εHf (t) values from −0.52 to −9.37, 176 Hf/ 177 Hf values of 0.281364 to 0.281627, and two-stage Hf model ages (T DM2 ) of 2536-3072 Ma.The zircon population with an age of 2435-2476 Ma has two zircon grains with negative εHf (t) values of −8.92 and −4.28, whereas the other grains have εHf (t) values from 0.23 to 5.24, with 176 Hf/ 177 Hf values of 0.281247 to 0.281388 and T DM2 of 2638-2951 Ma.
480,71]l.(2011Lietal.(,2013) )eathered mantle clastic rock detrital zircon U-Pb ages of 1682 ± 20 Ma and 1708 ± 6 Ma by SHRIMP and LA-ICP-MS dating methods.These rocks are covered directly by the sandstones of the Changzhougou Formation, suggesting that the age of the base of the Changzhougou Formation should be younger than 1682 Ma.Li et al. (2011Li et al. ( , 2013) )[70,71]obtained granite-porphyry dike ages of 1673 ± 10 Ma and 1669 ± 20 Ma by LA-MC-ICP-MS and SHRIMP U-Pb dating methods, and these dykes are unconformably overlain by the conglomerates and sandstones of the Changzhougou Formation, further suggesting that the base age of the Changchengian System in the NCC should be younger than 1670 Ma.This also implies that the Chuanlinggou Formation deposition age is not older than 1670 Ma.In this study, a total of 186 U-Pb analyses of detrital zircon grains from two samples of the Chuanlinggou Formation in the Xuanhua area yield apparent 207 Pb/ 206 Pb ages (Th/U value > 0.4) ranging from 1657-3109 Ma.The two samples display similar age population ranges.The detrital zircon age spectra can be broadly subdivided into three major age populations: 1.74-1.8Ga,withapeakage of 1765 Ma; 1.84-1.86Ga,withapeak age of 1848 Ma; and 2.44-2.48Ga,with a peak age of 2450 Ma (Figure