Paleoproterozoic Adakitic Rocks in Qingchengzi District, Northeastern Jiao-Liao-Ji Belt: Implications for Petrogenesis and Tectonism

: Herein, zircon U-Pb geochronology, Lu-Hf isotopes, and whole-rock major and trace element geochemistry are presented for two Palaeoproterozoic granitic rocks in Qingchengzi district, northeastern Jiao-Liao-Ji Belt (JLJB). These new geochronological and geochemical data provide reference clues for exploring the petrogenesis and tectonic setting of Paleoproterozoic magmatic rocks in the Qingchengzi district, which further constrain the tectonic nature of the JLJB. Our zircon U-Pb dating denotes that the Paleoproterozoic magmatic events in the Qingchengzi district were emplaced at ~2163 Ma and ~1854 Ma, represented by granite porphyry and biotite granite, respectively. rocks wt.%), and Al 2 O 3 (15.53–16.78 wt.%) contents, low Y (2.1–9.0 ppm) and Yb (0.25–0.80 ppm) contents, which indicate an adakite a ﬃ nity. Combined with Hf isotopic composition ( ε Hf(t) = − 1.5~ + 4.8; T DM2 = 3109~2560 Ma), we believe that the Paleoproterozoic adakitic magma originated from partial melting of the thickened lower crust material in the Meso-Neoarchean. Moreover, these rocks are enriched in light rare earth elements and large ion lithophilic elements (e.g., K, Rb, and Cs), and depleted in heavy rare earth elements and high ﬁeld strength elements (e.g., Nb and Ta). These features are similar to magmatic rocks formed in an arc environment (either island arc or active continental margin) and are not consistent with an intraplate / intracontinental environment. According to this study and previous research results, we conclude that the arc–continent collision model is conducive to the Paleoproterozoic tectonic attribute of the JLJB, and the oceanic crust subduction between the Namgrim and Longgang blocks may have induced the widespread occurrence of magmatic events in the region.

In this contribution, we present petrological observations, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating, Lu-Hf isotope analysis, and whole-rock geochemical studies for the representative Paleoproterozoic granitoids in the Qingchengzi district, northeastern JLJB. These new datasets provide precise age, petrogenesis process, and magma source for the Paleoproterozoic "Liaoji granites," and yield new insights to further constrain the tectonic settings of the JLJB.

Tectonic Framework
The NCC is the oldest and largest (~1.5 million km 2 ) cratonic block in East Asia (Figure 1a,b), and the preserved metavolcanic/sedimentary rocks (e.g., orthogneisses and metavolcanic) could date back to the Eoarchean (ca. 3.8 Ga) [18,21,[39][40][41][42]. The craton extends from western Inner Mongolia in the west to the northern Korean peninsula in the east ( Figure 1) [3]. The NCC consists of two parts, the Eastern Block and Western Block, which belong to the Archean-Paleoproterozoic basement and collided with the Trans-North China Orogen (TNCO) at~1.85 Ga (Figure 1b,c) [3]. The Western Block consists of the Ordos and Yinshan blocks, which collided along the Khondalite Belt (KB) at 1.95 Ga. The Eastern Block also includes two blocks (Longgang and Namgrim), which are separated by the JLJB (~1.95 Ga; Figure 1b,c) [3,4]. The collage-aggregation of several microcontinent blocks was accompanied by the formation of important tectonic/orogenic belts (i.e., KB, JLJB, and TNCO) that marked the completion of cratonization, and then the craton was in a long-term stable stage until the occurrence of few magmatic events in Mesozoic [43].

Local Geology and Sample Descriptions
The Qingchengzi district is situated in the northeastern part of the JLJB (Figure 1c). The geological units exposed in this district are mainly the Paleoproterozoic Liaohe Group (metasedimentary rocks) and Quaternary sediments ( Figure 2). Within the district, the Liaohe Group can be divided into five formations, including the Gaixian, Dashiqiao, Gaojiayu, Li'eryu (missing in this district), and Langzishan (concealed) from top to bottom ( Figure 2). Detailed lithologic and structural features can be seen in Li et al. [44,45]. There are many magmatic events from Paleoproterozoic to Mesozoic in this district, which intruded into Precambrian strata ( Figure 2) [43][44][45]. In this study, the Paleoproterozoic magmatic rocks were collected from the central part of a dike and adjacent isolated large-scale batholith (southeastern) (Figure 2). The detailed petrography is described as follows.

Zircon U-Pb Dating
We collected two samples of granite porphyry (sample TH) and biotite granite (sample DDZ) for LA-ICP-MS zircon U-Pb geochronology analysis. In Langfang Sincerity Geological Service Co. Ltd, the conventional heavy liquid and magnetic technologies were used to complete the separation of zircon grains. The cathodoluminescence (CL) images were performed by Quanta 200F ESEM

Zircon U-Pb Dating
We collected two samples of granite porphyry (sample TH) and biotite granite (sample DDZ) for LA-ICP-MS zircon U-Pb geochronology analysis. In Langfang Sincerity Geological Service Co. Ltd, the conventional heavy liquid and magnetic technologies were used to complete the separation of zircon grains. The cathodoluminescence (CL) images were performed by Quanta 200F ESEM (scanning electron microscope) at Sample Solution Analytical Technology Co. Ltd (Wuhan, China). Transparent and uncracked zircons, or those free of inclusions, were selected for age determination.
Using an Agilent 7500c quadrupole ICP-MS, automatic positioning system, and 193 nm ArF excimer laser (COMPexPro 102, Coherent, DE), zircon U-Pb geochronology data were collected in the Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Land and Resources (Changchun, China). The international standard zircon 91500 [46] was used as external standards to normalize isotopic fractionation and calculate the isotopic compositions. Australian Macquarie University standard zircon GJ-1 was used as a secondary standard to supervise the deviation of age measurements/calculations. The ratios of 206 Pb/ 238 U, 207 Pb/ 235 U, and 207 Pb/ 206 Pb were calculated with the ICPMSdatacal program (Version: 9.9) [47] and the plot of concordance and the weighted average age were made by Isoplot (Version: 3.0) [48]. The standard lead correction was based on the Andersen [49] method. For detailed experimental procedures and parameters, please see Li et al. [43,44].

Whole-Rock Major and Trace Element Analysis
Through petrographic observation, the samples were ground to 200 mesh using an agate mill, and the whole-rock major and trace geochemical analysis were completed at ALS Minerals-ALS Chemex (Guangzhou, China). The ME-XRF06 X instrument was used to determine the major elements; the trace element components were performed by ICP-AES and ICP-MS methods. During the test, we selected two international standard material samples (Canadian diorite gneiss SY-4 and kinzingite SARM-45) as calibration standards (listed in Supplementary Table S2). The analytical precision was better than 5% for the major elements and better than 10% for the trace elements [43].

Zircon Hf Isotopic Analysis
Using Neptune (MC) ICP-MS equipment equipped with a new generation up 213 laser ablation probe, the zircon in situ Hf isotopic analysis from the U-Pb dating point on the same crystal was completed in the State Key Laboratory of Isotope Geochronology and Geochemistry, Tianjin Institute of Geology and Mineral Resources (Tianjin, China). The standard zircon selected during the analysis was GJ-1, with an ablation rate of 10 Hz and a spot diameter of 45 µm. Helium was used as the carrier gas for the ablated aerosol. The 176 Hf/ 177 Hf ratio determined for the GJ-1 standard zircon was 0.282001 ± 0.000015 (n = 27). For detailed experimental operating conditions, procedures, and parameters, please refer to Geng et al. [50] and Li et al. [43].

Zircon U-Pb Ages
We separated and analyzed 23 zircons from sample TH and 20 zircons from sample DDZ. For the representative zircons, CL images are shown in Figure 4 and the U-Pb data (Supplementary Table  S1) are displayed graphically in Tera-Wasserburg diagrams ( Figure 5). All of the zircon grains show obvious oscillatory growth zone and a high Th/U ratio (0.11-1.79; Supplementary Table S1), suggesting that they are of magmatic origin [51].

Zircon U-Pb Ages
We separated and analyzed 23 zircons from sample TH and 20 zircons from sample DDZ. For the representative zircons, CL images are shown in Figure 4 and the U-Pb data (Supplementary Table  S1) are displayed graphically in Tera-Wasserburg diagrams ( Figure 5). All of the zircon grains show obvious oscillatory growth zone and a high Th/U ratio (0.11-1.79; Supplementary Table S1), suggesting that they are of magmatic origin [51]. A total of 23 spots were analyzed on 23 zircon grains from sample TH (Supplementary Table  S1). Seven analyses were excluded because of high discordance (>5%). The remaining 16 spots had Th/U ratios and 207 Pb/ 206 Pb ages varying from 1.14 to 1.71 and 2190 to 2131 Ma, respectively (Supplementary Table S1). They plot on concordia with a weighted mean 207 Pb/ 206 Pb age of 2163 ± 9 Ma (n = 16; MSWD = 2.0) (Figure 5a), which is interpreted as the emplacement age of the granite porphyry.
Twenty analyses were made on 20 zircon grains from sample DDZ (Supplementary Table S1). Among them, four analyses were excluded because of high discordance (>5%). The remaining 16 spots had Th/U ratios and 207 Pb/ 206 Pb ages varying from 0.11 to 0.46 and 1923 to 1749 Ma, respectively (Supplementary Table S1). They plot on concordia with a weighted mean 207 Pb/ 206 Pb age of 1854 ± 21 Ma (n = 16; MSWD = 7.7) (Figure 5b), which is interpreted as the emplacement age of the biotite granite. A total of 23 spots were analyzed on 23 zircon grains from sample TH (Supplementary Table S1). Seven analyses were excluded because of high discordance (>5%). The remaining 16 spots had Th/U ratios and 207 Pb/ 206 Pb ages varying from 1.14 to 1.71 and 2190 to 2131 Ma, respectively (Supplementary Table S1). They plot on concordia with a weighted mean 207 Pb/ 206 Pb age of 2163 ± 9 Ma (n = 16; MSWD = 2.0) (Figure 5a), which is interpreted as the emplacement age of the granite porphyry.
Twenty analyses were made on 20 zircon grains from sample DDZ (Supplementary Table S1). Among them, four analyses were excluded because of high discordance (>5%). The remaining 16 spots had Th/U ratios and 207 Pb/ 206 Pb ages varying from 0.11 to 0.46 and 1923 to 1749 Ma, respectively (Supplementary Table S1). They plot on concordia with a weighted mean 207 Pb/ 206 Pb age of 1854 ± 21 Ma (n = 16; MSWD = 7.7) (Figure 5b), which is interpreted as the emplacement age of the biotite granite.

Whole-Rock Major and Trace Element Geochemistry
The whole-rock major and trace geochemical compositions of Qingchengzi Paleoproterozoic igneous rock are listed in Supplementary Table S2. As shown in the Q-A-P (quartz-alkali feldsparplagioclase) diagram [52], all the samples belong to the syenogranite/granite field ( Figure 6a); in the SiO2 vs. total alkali (Na2O + K2O; TAS) diagram [53], all fall within the granite fields (Figure 6b), which is consistent with the results of the petrographic observation.

Whole-Rock Major and Trace Element Geochemistry
The whole-rock major and trace geochemical compositions of Qingchengzi Paleoproterozoic igneous rock are listed in Supplementary Table S2. As shown in the Q-A-P (quartz-alkali feldspar-plagioclase) diagram [52], all the samples belong to the syenogranite/granite field ( Figure 6a); in the SiO 2 vs. total alkali (Na 2 O + K 2 O; TAS) diagram [53], all fall within the granite fields (Figure 6b), which is consistent with the results of the petrographic observation.   Table S2).

Zircon Hf Isotopic Compositions
In situ Hf isotopic analysis was carried out on samples that were zircon U-Pb dated, and the corresponding results are shown in Supplementary Table S3. The calculation of the two-stage model age (TDM2) assumes that the average value of the continental crust 176 Lu/ 177 Hf is 0.015, and back to the depleted mantle model growth curve by calculating the initial 176 Hf/ 177 Hf [57].
The granite porphyry has an initial 176 Hf/ 177 Hf ratio of 0.281403-0.281678, TDM2 varying from 3109 to 2560 Ma, and an εHf(t) value that has a small variation range (−1.5 to +4.8) (Figure 8). Song et al. [58] conducted Hf isotope analysis of the biotite granite. The results showed that the initial 176 Hf/ 177 Hf ratios are 0.281486-0.281691, the εHf(t) values range from −14.1 to +4.1, and the TDM2 model ages range from 3105 to 2453 Ma.  Table S2).

Zircon Hf Isotopic Compositions
In situ Hf isotopic analysis was carried out on samples that were zircon U-Pb dated, and the corresponding results are shown in Supplementary Table S3. The calculation of the two-stage model age (T DM2 ) assumes that the average value of the continental crust 176 Lu/ 177 Hf is 0.015, and back to the depleted mantle model growth curve by calculating the initial 176 Hf/ 177 Hf [57].
The granite porphyry has an initial 176 Hf/ 177 Hf ratio of 0.281403-0.281678, T DM2 varying from 3109 to 2560 Ma, and an εHf(t) value that has a small variation range (−1.5 to +4.8) (Figure 8). Song et al. [58] conducted Hf isotope analysis of the biotite granite. The results showed that the initial

Paleoproterozoic Magmatic Events of the Northeastern Jiao-Liao-Ji Belt
Paleoproterozoic magmatism is one of the more prominent events in the geological history of Earth, and the geodynamics and thermal state have changed globally [60]. The northeast part of the JLJB is one of the most intense areas of Paleoproterozoic magmatic-tectonism in eastern China. As shown in Figure 9, Paleoproterozoic granitoids are widely distributed in the northeastern JLJB. Therefore, the geochronological information and emplacement histories of these granites can constrain the evolution and nature of magmatism affecting the area.
In this study, the CL image demonstrates that the zircon crystals are euhedral-subhedral, with obvious oscillation growth zones ( Figure 4). Furthermore, combined with the high Th/U ratios (0.11-1.79; Supplementary Table S1), the results indicate that these zircons have a magmatic origin [51]. The weighted average 207 Pb/ 206 Pb ages of these two granitoids are 2163 ± 9 and 1854 ± 21 Ma, respectively, suggesting that these granitoids were emplaced during the Paleoproterozoic. To better understand the spatial and temporal distribution characteristic of magmatic rocks in the Qingchengzi district (including adjacent areas), we collected geochronological data of the northeastern JLJB, which are plotted/listed in Figure 9 and Supplementary Table S4. There are mainly three major phases of magmatic events identified in the northeastern JLJB, i.e., (1) early stage of Paleoproterozoic (2213-2205 Ma), when the granitic gneiss, granite porphyry, and monzogranitic gneiss were emplaced; (2) early-middle stage of Paleoproterozoic (2189-2119 Ma), when abundant magmatic rocks (peak), including monzogranite, granitic gneiss, granodiorite, (biotite) granite, and syenogranite were emplaced; and (3) late stage of Paleoproterozoic (1995-1740 Ma), mainly including (quartz) diorite, (porphyritic) monzogranite, porphyritic granite, and syenite.

Paleoproterozoic Magmatic Events of the Northeastern Jiao-Liao-Ji Belt
Paleoproterozoic magmatism is one of the more prominent events in the geological history of Earth, and the geodynamics and thermal state have changed globally [60]. The northeast part of the JLJB is one of the most intense areas of Paleoproterozoic magmatic-tectonism in eastern China. As shown in Figure 9, Paleoproterozoic granitoids are widely distributed in the northeastern JLJB. Therefore, the geochronological information and emplacement histories of these granites can constrain the evolution and nature of magmatism affecting the area.
In this study, the CL image demonstrates that the zircon crystals are euhedral-subhedral, with obvious oscillation growth zones ( Figure 4). Furthermore, combined with the high Th/U ratios (0.11-1.79; Supplementary Table S1), the results indicate that these zircons have a magmatic origin [51]. The weighted average 207 Pb/ 206 Pb ages of these two granitoids are 2163 ± 9 and 1854 ± 21 Ma, respectively, suggesting that these granitoids were emplaced during the Paleoproterozoic. To better understand the spatial and temporal distribution characteristic of magmatic rocks in the Qingchengzi district (including adjacent areas), we collected geochronological data of the northeastern JLJB, which are plotted/listed in Figure 9 and Supplementary Table S4. There are mainly three major phases of magmatic events identified in the northeastern JLJB, i.e., (1) early stage of Paleoproterozoic
The Paleoproterozoic intrusions in the district have low-medium Mg # values and incompatible element content (e.g., Cr, Co, and Ni), among them, the Mg # values (48)(49)(50)(51) and Cr (26-27 ppm), Co (3.4-4.2 ppm), and Ni (10.8-13.2 ppm) contents in granite porphyry are slightly higher than those in biotite granite (Mg # = 38-40; Cr = 11-12 ppm; Co = 1.9-2.0 ppm; Ni = 2.2-2.5 ppm), which show that the Paleoproterozoic magma may not be formed by partial melting of a delaminated lower crust or subducting oceanic slab. During these processes, mantle peridotites are inevitably entrained with magma ascending through the mantle wedge with concurrent metasomatism, and increased the magma Mg # and Cr-Co-Ni contents [61,63,66]. The adakite produced by basaltic magma AFC processes usually requires a large amount of basaltic or dacitic rocks [69,70]. However, there are no mixed textures, mafic microgranular enclaves, and contemporaneous mafic rocks, which rules out the possibility of mixing from a basaltic magma or AFC process.
This indicates that the Paleoproterozoic intrusions in the district originated from the partial melting of thickened lower crust material. In the genetic discrimination diagram, these samples belong to the average crust and thickened lower crust (Figure 10b-d). According to the Hf isotope composition, the Paleoproterozoic adakitic rocks are considered to be a product of partial melting of the thickened lower crust material in the Meso-Neoarchean.
The Paleoproterozoic intrusions in the district have low-medium Mg # values and incompatible element content (e.g., Cr, Co, and Ni), among them, the Mg # values (48)(49)(50)(51) and Cr (26-27 ppm), Co (3.4-4.2 ppm), and Ni (10.8-13.2 ppm) contents in granite porphyry are slightly higher than those in biotite granite (Mg # = 38-40; Cr = 11-12 ppm; Co = 1.9-2.0 ppm; Ni = 2.2-2.5 ppm), which show that the Paleoproterozoic magma may not be formed by partial melting of a delaminated lower crust or subducting oceanic slab. During these processes, mantle peridotites are inevitably entrained with magma ascending through the mantle wedge with concurrent metasomatism, and increased the magma Mg # and Cr-Co-Ni contents [61,63,66]. The adakite produced by basaltic magma AFC processes usually requires a large amount of basaltic or dacitic rocks [69,70]. However, there are no mixed textures, mafic microgranular enclaves, and contemporaneous mafic rocks, which rules out the possibility of mixing from a basaltic magma or AFC process.
This indicates that the Paleoproterozoic intrusions in the district originated from the partial melting of thickened lower crust material. In the genetic discrimination diagram, these samples belong to the average crust and thickened lower crust (Figure 10b-d). According to the Hf isotope composition, the Paleoproterozoic adakitic rocks are considered to be a product of partial melting of the thickened lower crust material in the Meso-Neoarchean.
The intracontinental rift model was originally proposed by Zhang [22]. During the early stage of continental breakup, granitic magma was emplaced and formed a large number of Paleoproterozoic granites. With the increased degree of rift evolution, a large amount of mantle material upwelling formed a series of ultrabasic-basic rock bodies. This process also formed a largescale boron-bearing rock series in the area, and the rift eventually closed between the Longgang and Namgrim blocks [22,23]. According to the systematic summary of previous studies, Li et al. [77] and Zhao et al. [3] further improved the theory of the intracontinental rift evolution based on the following pieces of evidence. (a) The Liaohe Group (e.g., the Gaixian and Li'eryu Formation) consists of a large number of greenschist to lower-amphibolite facies metamorphic basic rocks and metamorphic rhyolites, forming a bimodal volcanic assemblage [24,25,78]. In addition, A-type granites and rapakivi granites were also developed in the area [24,25,78]. (b) There are contemporaneous Tonalite-Trondhjemite-Granodiorite (TTG) gneiss basement (~2.5 Ga) and mafic rock wall (~2.46 Ga) on both sides of the JLJB [78]. (c) Li et al. [77] identified and established
The intracontinental rift model was originally proposed by Zhang [22]. During the early stage of continental breakup, granitic magma was emplaced and formed a large number of Paleoproterozoic granites. With the increased degree of rift evolution, a large amount of mantle material upwelling formed a series of ultrabasic-basic rock bodies. This process also formed a large-scale boron-bearing rock series in the area, and the rift eventually closed between the Longgang and Namgrim blocks [22,23]. According to the systematic summary of previous studies, Li et al. [77] and Zhao et al. [3] further improved the theory of the intracontinental rift evolution based on the following pieces of evidence. (a) The Liaohe Group (e.g., the Gaixian and Li'eryu Formation) consists of a large number of greenschist to lower-amphibolite facies metamorphic basic rocks and metamorphic rhyolites, forming a bimodal volcanic assemblage [24,25,78]. In addition, A-type granites and rapakivi granites were also developed in the area [24,25,78]. (b) There are contemporaneous Tonalite-Trondhjemite-Granodiorite (TTG) gneiss basement (~2.5 Ga) and mafic rock wall (~2.46 Ga) on both sides of the JLJB [78]. (c) Li et al. [77] identified and established deformation patterns related to extension events. (d) The geochemical characteristics of the talc deposits in the JLJB show nonmarine origin, similar to those in the Neoproterozoic Damaran Rift, South Africa [3,79,80].
Bai [28] and Faure et al. [29] suggested that the JLJB is in an active continental marginal environment, either at the northern margin of the Namgrim Block or the southern margin of the Longgang Block. In recent years, some scholars proposed that the JLJB may be a continental arc magmatic belt, based on the study of~2.2-2.1 Ga magmatic rocks (mainly mafic and granitic intrusions), which further supports the arc-continent collision model [31,32].
The geochronological information in the two Archean-Palaeoproterozoic blocks of Longgang and Namgrim is of great importance for understanding these arguments [35]. The basement of the Longgang Block was formed at~3.8-2.47 Ga [1,39,40,81,82], including the oldest geological unit (Anshan Group;~3.8 Ga) [39] in China. However, no geochronological data as old as 3.8 Ga has been reported on the basement rocks of Namgrim Block [8]. Moreover, the basement rocks of the Namgrim Block show amphibolite facies metamorphism [8,41], while the Longgang Block shows amphibolite-granulite facies metamorphism [39,40]. Therefore, the geochronology and metamorphic characteristics on both sides of the JLJB are quite different, and these geological facts suggest that the intracontinental rift model is unjustified.
In summary, the whole-rock geochemical composition reported in this study shows that the Paleoproterozoic adakitic rocks in the Qingchengzi district are characterized by high Al 2 O 3 and SiO 2 contents and Sr/Y ratios, LREEs and LILEs enrichment, and HREEs and HFSEs depletion (Supplementary Table S2). Combining the Hf isotopic characteristics, these granitoids are derived from the partial melting of the Meso-Neoarchean thickened lower crust material, consistent with the magmatic rocks formed in the tectonic setting of an active continental margin or island arc (e.g., [83][84][85][86]). In addition, previous studies have shown that the JLJB Paleoproterozoic volcanic rocks (e.g., Liaohe Group) are calc-alkaline series and show the characteristics of I-type granites, and they were formed in an volcanic arc environment [34]. Therefore, we suggest that the Paleoproterozoic granitoids of the JLJB were formed in an active continental margin setting, which may be related to the subduction of oceanic crust between the Namgrim and Longgang blocks.

1.
Our LA-ICP-MS zircon U-Pb dating determined that the Paleoproterozoic magmatism in the Qingchengzi district occurred during two periods:~2163 Ma (granite porphyry) and~1854 Ma (biotite granite).

2.
The Paleoproterozoic granitoids in Qingchengzi district have the affinity of adakitic rocks, which originated by partial melting of the thickened lower crust material in the Meso-Neoarchean.

3.
These granitoids were formed in the tectonic setting of an active continental margin that may be related to the subduction of oceanic crust between the Namgrim and Longgang blocks.