Geochronology and Zircon Hf Isotope of the Paleoproterozoic Gaixian Formation in the Southeastern Liaodong Peninsula: Implication for the Tectonic Evolution of the Jiao-Liao-Ji Belt

: The Jiao-Liao-Ji belt (JLJB), in the Eastern Block of the North China Craton, is a major Paleoproterozoic orogen and underwent a complicated tectonic evolution during 2.2–1.8 Ga. The Liaohe Group, an important stratigraphic unit in the JLJB, is key to understanding the complex evolution of this belt. In this paper, we present new detrital zircon U–Pb ages and Hf isotope data for meta-sedimentary rocks from the Gaixian Formation in different areas of the JLJB, in addition to compiled data for other formations of the Liaohe Group, to establish the depositional age and source of detrital materials of the group. U–Pb age results show that the age ranges of zircons from the different samples are broadly similar. The youngest zircon group is ca. 2.06 Ga, and the youngest single-grain age is ca. 2.0 Ga, constraining the depositional age of the Gaixian Formation to between 2.0 Ga and the metamorphic age of ca. 1.9 Ga. The zircon age data indicate that the provenance was primarily Archaean basement of the Nangrim Block and Paleoproterozoic volcanic rocks of the Li’eryu Formation. On the basis of the new geochronological data and results from previous studies, it is inferred that the JLJB underwent a successive process of rifting–subduction–collision, with the different formations of the Liaohe Group being deposited in different stages from rift to passive continental margin and then to active continental margin. Zircon Hf isotope data from the JLJB and adjoining Longgang and Nangrim blocks indicate that a major crustal growth event occurred at 2.9–2.5 Ga, followed by crustal growth and intense recycling of ancient crust at ca. 2.2 Ga.

The JLJB is characterized by the development of extremely thick Paleoproterozoic sediments. From north to south, these Paleoproterozoic sediments comprise the Ji'an and Laoling groups in southern Jilin Province, the South and North Liaohe groups on the Liaodong Peninsula, the Jinshan and Fenzishan groups on the Jiaodong Peninsula, and the Wuhe Group in Anhui Province [2,3,8,33,34]. Of these strata, the Liaohe Group is considered to be the best preserved and most representative, and it also contains the world's Traditionally, the JLJB has been regarded as an intracontinental rift that underwent a simple opening-and-closing process [8,[10][11][12][13][14][15][16][17][18]. The major lines of evidence for the intracontinental rift model include the occurrences of bimodal volcanic rocks, rift-related boron deposits, A-type granites, and early extensional deformation, as well as metamorphism with an anticlockwise P-T path [8,10,11]. More recently, an increasing number of studies have suggested that the JLJB was formed by arc/continent-continent collision [19][20][21][22][23][24][25][26][27][28][29][30][31] or underwent an integrated process of intracontinental rifting, emergence of a small ocean, subduction, and collision [2,[5][6][7]32]. Although progress has been made in understanding the metamorphism, magmatism, and structural deformation of the JLJB, the tectonic evolution of the belt is still not fully resolved.
The JLJB is characterized by the development of extremely thick Paleoproterozoic sediments. From north to south, these Paleoproterozoic sediments comprise the Ji'an and Laoling groups in southern Jilin Province, the South and North Liaohe groups on the Liaodong Peninsula, the Jinshan and Fenzishan groups on the Jiaodong Peninsula, and the Wuhe Group in Anhui Province [2,3,8,33,34]. Of these strata, the Liaohe Group is considered to be the best preserved and most representative, and it also contains the world's largest resources of magnesite ores [8,33]. During the last two decades, several studies using detrital zircon geochronology have constrained aspects of the timing of deposition, provenance, and sedimentary environment of the Liaohe Group [6,9,12,13,26,30,31,[35][36][37][38]. These previous studies revealed that the uppermost set of strata (i.e., the Gaixian Formation) show heterogeneous sediment sources and a variable degree of metamorphism depending on locality. However, there is still a need for a comprehensive study that focuses only on the Gaixian Formation in different areas to elucidate the variation in sedimentary source and depositional environment.
Systematic analyses involving U-Pb dating and Lu-Hf isotopes of detrital zircons from sedimentary rocks can enable determination of the maximum depositional age and the source of detrital materials [39,40]. In this paper, we report the results of analyses of U-Pb and Lu-Hf isotopes of zircons from meta-sedimentary rocks of the Gaixian Formation in the Xiuyan and Dandong areas and a compilation of the available detrital zircon U-Pb and Hf isotope data reported for the Gaixian Formation for different areas, as well as data for other formations of the Liaohe Group. We constrain the timing of deposition, sediment provenance, and tectonic setting of the Gaixian Formation. Combining our new results with information from previous studies, we also place constraints on the tectonic evolution and crustal growth of the JLJB.
Rocks of the Liaohe Group, which underwent greenschist-to granulite-facies metamorphism [33,[56][57][58], crop out extensively on the Liaodong Peninsula. The Liaohe Group is generally divided into two subgroups by the Qinglongshan-Zaoerling Shear Zone, referred to as the North Liaohe and South Liaohe subgroups, respectively ( Figure 1b). The North Liaohe subgroup is composed of the Langzishan, Li'eryu, Gaojiayu, Dashiqiao, and Gaixian formations from bottom to top [8] (Figure 2), and the South Liaohe subgroup contains these same formations except the Langzishan Formation. The Liaohe Group consists of boron-bearing meta-volcanic-sedimentary rocks (e.g., tourmaline-bearing fine-grained felsic gneiss, and amphibolite) in the Li'eryu Formation, a graphite-bearing meta-sedimentary rock assemblage composed of mica schist or fine-grained felsic gneiss in the Gaojiayu Formation, magnesite-bearing calcite marble in the Dashiqiao Formation, and a terrigenous clastic rock assemblage composed of meta-sandstones, mica schist, and quartzite in the Gaixian Formation ( Figure 2). In addition to the Liaohe Group, the Liaodong Peninsula contains thick deposits of extensively distributed Mesoproterozoic-Palaeozoic sedimentary rocks ( Figure 2). During the Mesozoic, the eastern NCC, including the Liaodong Peninsula, underwent intensive magmatism and deformation that were most likely associated with subduction of the paleo-Pacific oceanic plate.

Description of Samples
The Gaixian Formation has traditionally been considered to represent the formation of the Liaohe Group. This formation is widely distributed on the Lia insula, especially in the Xiuyan and Dandong areas of the JLJB. Lithologies o part of the formation are mainly schist and fine-grained gneiss, whereas those o are predominantly meta-sandstone and meta-siltstone. Five meta-sedimentary representative lithologies were collected from the Xiuyan and Dandong areas zircon laser-ablation-inductively coupled plasma-mass spectrometry (LA-IC Pb dating and Lu-Hf isotope analysis (Figure 1b).

Description of Samples
The Gaixian Formation has traditionally been considered to represent the uppermost formation of the Liaohe Group. This formation is widely distributed on the Liaodong Peninsula, especially in the Xiuyan and Dandong areas of the JLJB. Lithologies of the lower part of the formation are mainly schist and fine-grained gneiss, whereas those of the upper are predominantly meta-sandstone and meta-siltstone. Five meta-sedimentary samples of representative lithologies were collected from the Xiuyan and Dandong areas for detrital zircon laser-ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating and Lu-Hf isotope analysis (Figure 1b).

Analytical Methods
Separation of zircons using standard heavy-liquid and magnetic techniques was undertaken at the Regional Geological Survey Institute of Langfang, Hebei Province, Langfang, China. Zircon grains were mounted in epoxy resin and polished until their cores were exposed. Cathodoluminescence (CL) images of polished zircons were obtained using a Quanta 200 field-emission environmental scanning electron microscope (FE-SEM) to reveal their internal textures at Nanjing Hongchuang GeoAnalysis Company, Jiangsu Province, Nanjing, China. Zircon U-Pb dating and Lu-Hf isotope analysis were conducted at Yandu Zhongshi Geological Analysis Laboratories, Beijing, China. The U-Pb dating was conducted by LA-ICP-MS using a NWR193 laser-ablation microprobe (Elemental Scientific Lasers LLC) with a laser spot diameter of 30 µm, attached to a Analytikjena M90. Helium was used as the carrier gas. The trace element contents of zircon were quantified using SRM610 as external standard and Si as internal standard. Isotopic fractionation correction of U-Pb dating was carried out using zircon standard 91500 as an external standard and Plesovice as monitor. Offline data were processed using an open source ICPMSDataCal10.8 software version 10.8, created by Liu Yongsheng of State Key Laboratory of Geological Processes and Mineral Resources Faculty of Earth Sciences China University of Geosciences, Wuhan, China [59,60]. In situ zircon Lu-Hf isotope analysis was performed on the same analytical spots used for U-Pb dating, using a Neptune multi-collector (MC)-ICP-MS instrument with a laser spot diameter of 40 µm. Analytical procedures and interference corrections have been comprehensively described by Wu et al. [61]. ε Hf values were calculated using the unequilibrated chondrites 176 Hf/ 177 Hf ratio of 0.282785 and 176 Lu/ 177 Hf ratio of 0.0336, and the decay-constant value of 176 Lu is 1.867 × 10 −11 yr −1 [62].

Zircon U-Pb Dating
Zircons from sample 19LJ16-1 measure 40-100 µm in length and have aspect ratios of 1-2. The grains are subhedral and oval to columnar in shape. Most grains show clearly concentric oscillatory zoning in CL images (Figure 5a). Forty analytical spots yielded 207 Pb/ 206 Pb ages ranging from 2704 to 1998 Ma, with most Th/U ratios being larger than 0.1 (Supplementary Table S1 and Figure 6a). These ages can be divided into four age groups (Figure 6b Table S1). 207 Pb/ 206 Pb ages ranging from 2704 to 1998 Ma, with most Th/U ratios being larger than 0.1 (Supplementary Table S1 and Figure 6a). These ages can be divided into four age groups (Figure 6b): 2504-2415 Ma with a peak at 2477 Ma (n = 10), 2389-2309 Ma with a peak at 2331 Ma (n = 10), 2196-2152 Ma with a peak at 2174 Ma (n = 12), and 2075-2051 Ma with a peak at 2065 Ma (n = 6). The other three individual grains yielded ages of 1998, 2657, and 2704 Ma, respectively (Supplementary Table S1).   Table S1).
with only one Th/U ratio less than 0.1 (Supplementary Table S1 and Figure 6c). The 207 Pb/ 206 Pb age histogram shows a dominant age group of 2534-2437 Ma with a peak a 2480 Ma (n = 24; Figure 6d). The ages also yield two subordinate age groups of 2711-2681 Ma with a peak at 2692 Ma (n = 7) and 2123-2063 Ma with a peak at 2097 Ma (n = 4; Figure  6d). The three remaining single grains yielded older ages of 3003, 2989, and 2795 Ma, re spectively (Supplementary Table S1).   Table S1). Most Th/U ratios are greater than 0.4, with only one being less than 0.1 (Supplementary Table S1 and Figure 7a). The detrital zircon 207 Pb/ 206 Pb ages can be divided into one dominant age group of 2542-2468 Ma (n = 25), with a peak at 2491 Ma, and three subordinate age groups of 2224-2112 Ma (n = 6), 2371-2315 Ma (n = 5), and 2702-2687 Ma (n = 4) with peaks at 2137, 2350, and 2696 Ma (Figure 7b), respectively. 6) with a peak at 2192 Ma (Figure 6f). Four grains yielded older Pb/ Pb ages of 3328 2986, 2736, and 2687 Ma, respectively (Supplementary Table S1).
Zircons from sample 19LJ23-1 are small with lengths of <100 μm and have aspect ratios of 1-3. Under CL, zircons appear light or dark grey, some show clear or weak oscillatory zoning, and some display core-rim structures (Figure 5d). A total of 40 analyses yielded variable 207 Pb/ 206 Pb ages ranging from 2702 to 2112 Ma (Supplementary Table S1) Most Th/U ratios are greater than 0.4, with only one being less than 0.1 (Supplementary  Table S1 and Figure 7a). The detrital zircon 207 Pb/ 206 Pb ages can be divided into one dominant age group of 2542-2468 Ma (n = 25), with a peak at 2491 Ma, and three subordinate age groups of 2224-2112 Ma (n = 6), 2371-2315 Ma (n = 5), and 2702-2687 Ma (n = 4) with peaks at 2137, 2350, and 2696 Ma (Figure 7b), respectively. Zircons from sample 19LJ24-1 are subhedral with lengths mostly between 50 and 100 μm and aspect ratios of 1-2. Some grains are characterized by light-grey luminescence with obvious core-rim structure; others show homogeneous luminescence under CL (Figure 5e). Of the 40 analyses, 37 have concordance levels of >90% and yielded 207 Pb/ 206 Pb ages ranging from 2604 to 2133 Ma (Supplementary Table S1). These analyses have Th/U ratios of 0.13-1.57 (Supplementary Table S1 and Figure 7c). The 207 Pb/ 206 Pb age histogram  Table S1). These analyses have Th/U ratios of 0.13-1.57 (Supplementary Table S1 and Figure 7c). The 207 Pb/ 206 Pb age histogram shows a dominant age group of 2550-2466 Ma (n = 36) with a peak at 2500 Ma and a subordinate age group of 2169-2133 Ma (n = 7) with a peak at 2156 Ma (Figure 7d).

Zircon Hf Isotopes
Analysis of Lu-Hf isotopes of zircons can reveal the age of crustal growth and trace the material sources of igneous rocks, leading to a better understanding of the growth and evolution of continental crust [63][64][65][66]. To reveal more information regarding the genesis of the dated detrital zircons, we selected 70 zircon grains with concordant or nearly concordant U-Pb ages and that also covered the dominant and subordinate age groups for Lu-Hf isotope analysis. The Lu-Hf analytical results are presented in Supplementary Table S2 Table S2 and Figure 8).
of the dated detrital zircons, we selected 70 zircon grains with conco cordant U-Pb ages and that also covered the dominant and subordi Lu-Hf isotope analysis.  Figure 8).

Depositional Age of the Gaixian Formation
It is widely acknowledged that the depositional age of a sedim younger than the youngest detrital zircon within that unit [79]. Our results therefore provide constraints on the depositional age of the The peak age of the youngest zircon group and the youngest single zi and ca. 2.0 Ga, respectively (Figures 6 and 7). Thus, the depositiona Formation in the Xiuyan-Dandong area is younger than ca. 2.0 Ga. Pr obtained similar constraints from detrital zircon ages; for example, ported a youngest zircon age for the Gaixian Formation in the Dand Ga, and a metamorphic age of 1.91-1. 86

Depositional Age of the Gaixian Formation
It is widely acknowledged that the depositional age of a sedimentary unit must be younger than the youngest detrital zircon within that unit [79]. Our zircon U-Pb dating results therefore provide constraints on the depositional age of the Gaixian Formation. The peak age of the youngest zircon group and the youngest single zircon age are ca. 2.06 and ca. 2.0 Ga, respectively (Figures 6 and 7). Thus, the depositional age of the Gaixian Formation in the Xiuyan-Dandong area is younger than ca. 2.0 Ga. Previous studies have obtained similar constraints from detrital zircon ages; for example, Zhang et al. [80] reported a youngest zircon age for the Gaixian Formation in the Dandong area of ca. 2.01 Ga, and a metamorphic age of 1.91-1. 86 Ga. Samples from the Gaixian Formation in the Xiuyan-Dandong area have a youngest zircon age of ca. 2.0 Ga, with a metamorphic age of ca. 1.85 Ga [36,37].
Several previous studies have constrained the timing of deposition of the Gaixian Formation in other areas of the Liaodong Peninsula, such as the Haicheng, Kuandian, and Caohekou areas. Wan et al. [81] reported a youngest zircon age from the Gaixian Formation in the Haicheng area of ca. 2.02 Ga. The youngest zircon age obtained from the Gaixian Formation in the Caohekou area is ca. 1.98 Ga, with a metamorphic age of ca. 1.9 Ga [26,35,82]. The Gaixian Formation in the Kuandian area has a youngest zircon age of ca. 2.02 Ga, with a metamorphic age of 1.91-1. 86 Ga [83].
In conclusion, the youngest detrital zircon ages and the regional metamorphic ages reported above constrain the deposition age of the Gaixian Formation in different parts of the Liaodong Peninsula to a consistent 2.0-1.9 Ga.

Provenance of the Gaixian Formation
U-Pb ages and Hf isotopes of detrital zircons record information about the tectonothermal history of the source rocks and can therefore be used to trace sedimentary provenance [39,40,84]. Although the Gaixian Formation on the Liaodong Peninsula shows a consistent depositional age across different areas, these areas record different and complicated provenance signals (Figure 9a-e). Age frequency histograms show that the Gaixian Formation in areas close to the southern margin of the LGB (e.g., the Haicheng and Gaizhou areas) are characterized by unimodal Paleoproterozoic ages (Figure 9a,b). In comparison, Neoarchaean detrital zircon ages of the Gaixian Formation increase in proportion towards the northern margin of the NRB (e.g., the Xiuyan, Dandong, and Kuandian areas; Figure 9c-e), where the detrital zircon ages show a distinct bimodal pattern, with peaks at ca. 2.1 and ca. 2.5 Ga (Figure 9c-e).
Previous studies have shown that the JLJB underwent intensive magmatism during 2.2-2.1 Ga, including the Liaoji granites [6,7,14,15,[85][86][87][88][89][90] and the coeval meta-volcanic rocks of the Li'eryu Formation [6,21,83,91]. However, despite their suitable age, the 2.2-2.1 Ga Liaoji granites were unlikely to have been the source of the Paleoproterozoic sedimentary rocks of the Gaixian Formation, as the JLJB was under continuous extension before the collision occurred at ca. 1.9 Ga [8], meaning that the intrusive Liaoji granites could not have been exhumed and eroded before ca. 2.0 Ga. Conversely, the 2.2-2.1 Ga volcanic rocks of the Li'eryu Formation are a more likely source for the Paleoproterozoic sedimentary rocks. It should be noted that the Gaixian Formation at locations near the LGB contains minor Neoarchaean detrital zircon ages (Figure 9a,b), suggesting that the Archaean basement of the LGB might have been covered by Paleoproterozoic rift-related volcanic rocks and subsequent strata during the deposition of the Gaixian Formation.
The LGB and NRB are both characterized by widespread Neoarchaean rocks, including TTG, granite, and diorite [13,14,51,52,54,55,76]. In contrast to the NRB, the LGB preserves considerable pre-Neoarchaean rocks in the Anshan-Benxi area [1,41,42,[46][47][48][49][50], which is adjacent to the northern boundary of the JLJB ( Figure 1). As mentioned above, the Gaixian Formation in the Xiuyan, Dandong, and Kuandian areas yields high proportions of Neoarchaean detrital zircon ages but only minor pre-Neoarchaean ages (Figure 9c-e). Considering that the LGB was unlikely to have been a source of sediment for the Gaixian Formation in the northern JLJB, it therefore also may not have supplied sediment to the southern JLJB. In addition, the scarcity of pre-Neoarchaean detrital zircons in the Gaixian Formation in the Xiuyan, Dandong, and Kuandian areas suggests that the NRB rather than the LGB provided the Neoarchaean clastic grains.
Detrital zircon ages of the lowest part of the Langzishan Formation display a unimodal shape and yield a single Neoarchaean age group with a peak of ca. 2.53 Ga in the age histogram ( Figure 9j). The minor ca. 2.2 Ga zircons are thought to have been sourced from early volcanic rocks [106]. This age spectrum suggests that the Langzishan Formation might represent the earliest intracontinental rift-related sediments [107]. Subsequently, the intracontinental rift underwent strong magmatism at 2.20-2.16 Ga (Figure 9i), forming resultant widespread volcanic rocks of the Li'eryu Formation and intrusive A-type granites [6,7,88,90,95,97]. Under continuing extension, the rift probably evolved into a stable passive continental margin along its northern boundary, with deposition of a suite of turbidite (i.e., the Gaojiayu Formation [108]) and carbonate rocks (i.e., the Dashiqiao Formation). During the depositional of the Gaojiayu and Dashiqiao formations, the interior Neoarchaean basement of the LGB and the 2.20-2. 16 Ga rift volcanoes were the main provenances (Figure 9h,g). In addition, the ca. 1.9 Ga high-pressure pelitic granulite [109] and ca. 1.95 Ga subduction-related adakitic granite [5] recently identified in the JLJB suggest that the rift might have evolved into the formation of a limited oceanic crust that separated the LGB and NRB and allowed the ensuing subduction to occur [2]. During the deposition of the Gaixian Formation (i.e., 2.0-1.9 Ga), the extensional basin shrank under a convergent margin setting as a result of the subduction [26], with mainly terrigenous clastic sediments being deposited, which were provided by Archaean basement of the NRB and Paleoproterozoic volcanic rocks of the Li'eryu Formation (Figure 9f). Considering the spatial variability in the provenance of the Gaixian Formation, the southeastern region (including the Xiuyan-Dandong area) of the Liaodong Peninsula might be part of the NRB. The Xiuyan-Dandong area also lacks the typical carbonate-dominant Dashiqiao Formation and the Gaojiayu Formation, which were deposited in the passive continental margin of the LGB (Figure 1b). Subsequently, the LGB and NRB collided at 1.90-1. 85 Ga to form the JLJB, with accompanying greenschist-to granulite-facies metamorphism and subsequent post-collision magmatism [33,[56][57][58][110][111][112].
In summary, evidence from magmatism, metamorphism, and sediments suggests that the JLJB likely underwent a successive process of rifting-subduction-collision during 2.2-1.8 Ga.
The compiled data for detrital zircons of the Liaohe Group display two age peaks at ca. 2.5 and ca. 2.17 Ga, and their Hf isotopes yield T DM2 model ages varying from ca. 4.0 to ca. 2.2 Ga, with a peak at 3.0-2.5 Ga (Figure 10a). In comparison, 2.2-2.1 Ga zircons of the Liaoji granitoids have T DM2 model ages varying from ca. 2.9 to ca. 2.2 Ga with a peak at 2.7-2.5 Ga (Figure 10b). Zircons of the 2.15-2.10 Ga mafic rocks on the Liaodong Peninsula yield T DM1 model ages of 2.4-2.1 Ga with a peak at ca. 2.2 Ga (Figure 10c). We infer from the combined data for the Liaohe Group, 2.2-2.1 granites, and 2.15-2.10 Ga mafic rocks of the JLJB that the JLJB underwent a degree of juvenile crustal growth and substantial crustal recycling during the rifting stage at 2.2-2.1 Ga (Figures 8 and 10d).
The LGB and NRB are both composed mainly of ca. 2.5 Ga TTG rocks and may share a similar history of crustal growth. Neoarchaean zircons from both the LGB and NRB display variable and mostly positive ε Hf (t) values ( Figure 8). Zircon T DM2 model ages of the LGB and NRB both range from 4.0 to 2.5 Ga with a peak at 2.9-2.5 Ga (Figure 10e,f). Therefore, the LGB and NRB are inferred to have undergone substantial crustal growth at 2.9-2.5 Ga, as well as intensive crustal recycling at the end of the Neoarchaean (Figure 8). The Hf isotopic information recorded in the JLJB is consistent with those of the LGB and NRB. In conclusion, the LGB and NRB underwent pronounced crustal growth at 2.9-2.5 Ga and extensive crustal recycling at ca. 2.5 Ga, and the JLJB underwent a degree of crustal growth and strong recycling of ancient crust at ca. 2.2 Ga.

Conclusions
(1) The timing of deposition of the Gaixian Formation in the JLJB on the Liaodong Peninsula, eastern North China Craton, can be constrained to 2.0-1. 9 Ga on the basis of detrital zircon ages and the timing of regional metamorphism.