1. Introduction
The North China Craton (NCC), one of the oldest cratons on the Earth and the largest in China, contains rocks dating back to 3.8 Ga. Over time a series of tectonic, magmatic and metamorphic processes occurred. Recent geological investigations have delved into unravelling the Precambrian structure of the NCC, particularly elucidating a notable tectonothermal event that transpired during the Late Neoarchaean (2.55∼2.50 Ga) (Bao et al., Reference Bao, Liu, Wang, Teng and Sun2020; Kusky, Reference Kusky2011; Li et al., Reference Li, Guo, Guan and Liu2016; Wang et al., Reference Wang, Peng, Tong, Huang, Zheng, Zhang and Zhai2018; Zhao et al., Reference Zhao, Sun, Wilde and Sanzhong2005; Zhai, Reference Zhai2010, Reference Zhai2012; Zhao et al., Reference Zhao, Sun and Wilde2002). However, divergent perspectives exist regarding the cause of this event: some advocate for an island arc magmatism model within the framework of plate tectonics (Huang et al., Reference Huang, Kusky, Johnson, Wilde, Wang, Polat and Fu2020; Kröner et al., Reference Kröner, Wilde, Li and Wang2005; Wan et al., Reference Wan, Liu, Wang, Dong, Yang, Wang, Zhou, Ning, Du and Yin2010; Zhao et al., Reference Zhao, Cawood, Wilde, Sun and Lu2000; Zhao et al., Reference Zhao, Sun, Wilde and Sanzhong2005; Wan et al., Reference Wan, Song, Wang, Xie, Liu, Hou, Dong, Xie, Bai and Liu2017), while others propose vertical tectonic mechanism such as mantle plume (Geng et al., Reference Geng, Liu and Yang2006; Wu et al., Reference Wu, Lin, Wan, Gao and Stern2021; Yu et al., Reference Yu, Yang, Zhang, Zhao, Cawood, Yin, Qian, Gao and Zhao2022; Zhao and Zhai, Reference Zhao and Zhai2013; Zhai, Reference Zhai2010; Geng et al., Reference Geng, Shen and Ren2010). This ongoing debate centres on whether plate tectonics were active during the Neoarchaean, a fundamental question that has captivated researchers for decades.
Greenstone belts are extensively distributed worldwide. Currently, there are four predominant perspectives on the geodynamic origins of greenstone belt formation: (1) Mantle plume system, exemplified by the Onverwacht greenstone belt in the Barberton Craton (Stiegler et al., Reference Stiegler, Lowe and Byerly2008); (2) Island arc tectonic system, characterized by basic-intermediate-acidic volcanic rocks in greenstone belts, such as Limpopo greenstone belt in the Zimbabwe Craton (Khoza et al., Reference Khoza, Jones, Muller, Evans, Webb and Miensopust2013); (3) Mantle plume-island arc tectonic system, where ultrabasic rocks and basic volcanic rocks like komatiite are common, as seen in greenstone belts like the Kolar greenstone belt in the Dharwar Craton (Balakrishnan et al., Reference Balakrishnan, Hanson and Rajamani1991), Uchi greenstone belt (Hollings and Kerrich, Reference Hollings and Kerrich1999; Hollings et al., Reference Hollings, Wyman and Kerrich1999), Lumby Lake greenstone belt (Hollings and Kerrich, Reference Hollings and Kerrich1999; Hollings et al., Reference Hollings, Wyman and Kerrich1999), Abitibi belt greenstone belt (Kerrich et al., Reference Kerrich, Polat and Xie2008), Wawa greenstone belt (Polat et al., Reference Polat, Kerrich and Wyman1999) and Kidd-MunroAbitibi greenstone belt (Wyman and Kerrich, Reference Wyman and Kerrich2009; Wyman, Reference Wyman1999; Wyman et al., Reference Wyman, Kerrich and Polat2002) in the Superior Craton, Western Shandong Province granite-greenstone belt in the NCC (Wang, Reference Wang2010); and (4) Mantle plume system of continental rift margin, characterized by bimodal volcanic rocks and the coexistence high-Mg and low-Mg basalts in greenstone belts like the Penakacherla greenstone belt (Manikyamba, Reference Manikyamba2004) and Sandur greenstone belt (Manikyamba et al., Reference Manikyamba, Kerrich, Khanna, Keshav Krishna and Satyanarayanan2008) in the Dharwar Craton, Bulawayan greenstone belt in the Zimbabwe Craton (Prendergast, Reference Prendergast2004) and Kalgoorlie greenstone belt in the Yilgarn Craton (Said et al., Reference Said, Kerrich and Groves2010). Numerous greenstone belts have been identified in the NCC (Guo et al., Reference Guo, Li, Liu, Wang, Bao, Wang, Huang and Dou2022, Reference Guo, Liu, Gong, Wang, Wang, Fu and Qin2017; Lian et al., Reference Lian, Ren, Shi, Xu and Feng2023; Li and Qian, Reference Li and Qian1995; Li et al., Reference Li, Zhang, Dai, Wang and Li2012; Wang, Reference Wang2010; Zhu, Reference Zhu2016). Zhu (Reference Zhu2016) categorized the greenstone belts of the eastern and central continental block orogenic belts of the NCC into two belts: the western and eastern greenstone belts, with a microcontinental belt between them. The western belt comprises western Liaoning, western Jilin, Northern Liaoning, eastern Hebei, Wutai and Dengfeng, while the eastern belt includes eastern Jilin (Jiapigou, Helong and Banshigou), Anshan, Jiaodong, Western Shandong and Lushan.
The formation of the upper greenstone belt in the NCC occurred in approximately four distinct stages: Middle Archaean (2.9∼3.0 Ga); Early Neoarchaean (2.7∼2.9 Ga); Late Neoarchaean (2.5∼2.6 Ga) and Palaeoproterozoic, with the Neoarchaean (∼2.7 Ga) representing the peak of greenstone belt formation (Wang, Reference Wang2010). A typical greenstone belt consists of three layers: ultrabasic and basic volcanic rocks at the base, characterized by Komatiites; a central layer of calc-alkaline volcanic rock group, predominantly basalt, andesite, dacite and rhyolite, with chert; and sedimentary rocks at the top.
The Anshan-Benxi area, situated in the northeast of the NCC and north of the Jiao-Liao-Ji Palaeoproterozoic active belt, is distinguished by extensive Archaean geological bodies and widely spread Archaean greenstone belts. This region has been a focal point of geological research. Over the past three decades, numerous scholars have conducted detailed studies in the Anshan-Benxi area, leading to significant geological discoveries. However, unresolved controversies persist, particularly regarding the formation environment of the greenstone belt in this area. Presently, two main perspectives exist on this issue. One viewpoint suggests that the greenstone belt in the Anshan-Benxi area formed in an island arc environment with mantle material added: For instance, Dai et al. (Reference Dai, Zhang, Wang, Liu, Cui, Zhu and Xiang2012) conducted a geochemical analysis on the surrounding rocks of the Chentaigou iron mines. By utilizing trace elements with stable geochemical properties, they were able to effectively determine the geotectonic environment of the rocks, concluding that the hidden greenstone belt in Chentaigou formed in an island arc environment (Dai et al., Reference Dai, Zhang, Wang, Liu, Cui, Zhu and Xiang2012). The other perspective posits that it formed in the back-arc basin. Wan (Reference Wan1992) collected an amphibolite from Gongchangling second mining area, systematically studying its petrography, petrochemistry and chronology and concluded that the greenstone belt originated in a tectonic environment of the back-arc marginal basin (Wan, Reference Wan1992); Wang et al. (Reference Wang, Xia, Zhao, Fu and Hou2013) conducted an extensive study on the geochemical characteristics of amphibolite and biotite granulite in Gongchangling mining area, concluding that Gongchangling greenstone belt was formed in the back-arc basin (Wang et al., Reference Wang, Xia, Zhao, Fu and Hou2013; Dai et al., Reference Dai, Zhang, Zhu, Wang and Liu2013a) performed a geochemical analysis on the surrounding rocks of Waitoushan iron mines. They utilized trace elements with stable geochemical properties to effectively trace the geotectonic environment of the rocks, leading to the conclusion that the Waitoushan greenstone belt formed in the back-arc basin environment (Dai et al., Reference Dai, Zhang, Zhu, Wang and Liu2013a,Reference Dai, Zhang, Zhu, Wang and Liub); Guo et al. (Reference Guo, Li, Liu, Wang, Bao, Wang, Huang and Dou2022) examined the geochemistry and zircon U-Pb ages of tholeiites, calc-alkaline basalts, andesites, dacites and rhyolites in the Waitoushan-Gangchangling-Benxi area. Their analysis indicated that the lithological assemblages of basalts in the back-arc basin were erupted around 2.55∼2.52 Ga, revealing that the Waitoushan-Gongchangling-Benxi area is positioned in the tectonic zone of the back-arc basin, with rocks in the tectonic belt experiencing consistent NE-SW compression during the back-arc closure (Guo et al., Reference Guo, Li, Liu, Wang, Bao, Wang, Huang and Dou2022); Tong et al. (Reference Tong, Wang, Peng, Huang, Zhang and Zhai2019) conducted in situ zircon U-Pb-Hf isotope, whole-rock geochemistry and Sm-Nd isotope analysis on the Dagushan metamorphic sedimentary rocks, suggesting that these sediments may have been deposited in the back-arc basin on the edge of an ancient continental crust (Tong et al., Reference Tong, Wang, Peng, Huang, Zhang and Zhai2019). Recent studies on the chronology of greenstone belts have determined a formation age of approximately 2.5Ga, with some ancient greenstone remnants, such as ≥3.35 Ga supracrustal rocks in Chentaigou (Song et al., Reference Song, Wu, Wan and Liu1994).
The formation age of the greenstone belt in the Anshan-Benxi area is estimated to be around 2.55 Ga based on previous dating data. However, ongoing debates persist concerning the stratigraphic sequence, basement tectonic style, tectonic environment and origin of the abundant iron ore within the greenstone belt. As a result, this study aims to analyse the petrography, chronology and petrochemistry of the greenstone belt in the Anshan-Benxi area to contribute evidence towards comprehending the formation and evolution of the greenstone belt.
2. Regional geological setting
The Anshan-Benxi area, situated in the Jiao-Liao ancient land at the northeastern fringe of the NCC (Fig. 1a), serves as a significant iron ore resource reservoir in China, boasting a multitude of large and extra-large Banded iron formation (BIF) type iron deposit. The oldest geological formations in this region comprise the Anshan Group and its associated granite intrusions, constituting the Archaean basement. The granite covers approximately 70% of the area, while the supracrustal rocks encompass approximately 30%, with the granitic rocks primarily situated in Tiejiashan and Gongchangling. Overlaying the basement are several younger strata, including Palaeoproterozoic Liaohe Group; Neoproterozoic Qingbaikou and Nanhua systems; Palaeozoic Cambrian; Ordovician; Carboniferous; Permian; Mesozoic Triassic and Cenozoic Quaternary unconsolidated sediments. The region exhibits a two-layer structure comprising a crystalline basement and cover layer of the North China platform.
Geological map of North China Craton and the Anshan-Benxi area Modified from Guo et al. (Reference Guo, Li, Liu, Wang, Bao, Wang, Huang and Dou2022). (a) Structural map of North China Craton; (b) Geological map of the Anshan-Benxi area and results of previous studies.
From the bottom up, the Anshan Group in the Anshan-Benxi area comprises the Cigou Formation, Dayugou Formation and Yingtaoyuan Formation. The Cigou Formation is predominantly distributed in locations such as Benxi Waitoushan, Maerling, Gongchangling and Sandaoling in Liaoyang. It consists of sericite chlorite schist, amphibolite, quartz hornblende schist, biotite quartz schist, magnetite quartzite, muscovite quartzite, among others. The Dayugou Formation is chiefly present in locations like Liaoyang Dayugou, Mianhuapuzi, Benxi East Dayugang and Sanjiazi. It is divided into two lithologic sequences: the initial part includes two-mica plagioclase gneiss, dolomitic plagioclase gneiss, biotite granite, two-mica sillimanite quartz schist, along with monzonitic granulite and biotite hornblende granulite. The latter segment comprises biotite leptite, biotite granulite, monzonite leptite, tourmaline-bearing leptite, mica schist, interspersed with amphibolite and magnetite quartzite. The Yingtaoyuan Formation is identified in locations like Anshan Yingtaoyuan, Hujiamiaozi, Yanqianshan, Dagushan, Donganshan and Xi’anshan, characterized by sericulite chlorite schist, dolomite chlorite schist, chlorite schist, two-mica schist, phyllite and thick layers of magnetite quartzite. The stratigraphic lithologic features of the Anshan Group in the Anshan-Benxi area are illustrated in a column diagram (Fig. 2). Additionally, four fault structures are oriented in the NNE, NE, NW and EW directions.
Column diagram of the Anshan group in the Anshan-Benxi area.
The ancient continental nuclei in the Anshan-Benxi area originated between 3.8 and 3.0 Ga. Towards the end of the Late Archaean period, significant quantities of basic-medium-acidic magma erupted, forming the primary Archaean rocks in the Anshan-Benxi area, constituting the base of regional evolution. Subsequently, during the transition from the Late Archaean to the Early Proterozoic period, the basic magma underwent greenschist facies and amphibolite facies metamorphisms. Simultaneously, large-scale granitic magma emplaced upward to form rock masses such as the Qidashan granite and Gongchangling granite (Bao et al., Reference Bao, Liu, Wang, Teng and Sun2020; Li et al., Reference Li, Guo, Guan and Liu2016; Wang et al., Reference Wang, Huang, Tong, Zheng, Peng, Nan, Zhang and Zhai2016; Wang et al., Reference Wang, Peng, Tong, Huang, Zheng, Zhang and Zhai2018; Zhou et al., Reference Zhou, Liu, Wan and Dong2008; Zhu, Reference Zhu2016). The Archaean crust in the Anshan-Benxi area underwent three significant tectonic deformation phases. The initial deformation phase occurred predominantly around 2.9 to 2.8 Ga, leading to the development of a dense and uniform gneiss in the Tiejiashan granite. This gneiss exhibits a NE-striking orientation with a steep dip angle, with predominant structural features observed in surface rocks including closed homoclinal folds and dense axial foliation. The subsequent stage of tectonic deformation was mainly formed at about 2.6 Ga (Dai et al., Reference Dai, Zhang, Zhu, Wang and Liu2013a, Reference Dai, Zhang, Zhu, Wang and Liu2013b; Guo, Reference Guo1994; Guo et al., Reference Guo, Liu, Gong, Wang, Wang, Fu and Qin2017; Liu et al., Reference Liu, Li, Liu, Li, Wen, Liang and Chang2017; Song et al., Reference Song, Nutman, Liu and Wu1996; Tong et al., Reference Tong, Wang, Peng, Huang, Zhang and Zhai2019; Wan et al., Reference Wan, Liu, Yin, Wilde, Xie, Yang, Zhou and Wu2007; Yang, Reference Yang2013; Yin, Reference Yin2006; Zhang et al., Reference Zhang, Jin, Zheng, Wang, Li, Cai and Wang2013; Zhu et al., Reference Zhu, Dai, Zhang, Wang and Liu2015, Reference Zhu, Liu, Xu and Wang2016). Prior to this, Gongchangling granite invaded, making the surface rock become the xenolith of Gongchangling granite. Characterized by NNE and EW trending ductile shear belts, Zhu (Reference Zhu2016) performed Muscovite Ar-Ar isotope dating on Donganshan granite situated in the vicinity of the ductile shear belt (Zhu, Reference Zhu2016), determining the formation age of muscovite to be 2545 ± 16 Ma, which coincides with the thermal disturbance period of deformation and metamorphism events on Donganshan granite. The third tectonic deformation phase, occurring around 2.0 Ga, led to the transformation of the NNW ductile shear belt of nappe into a strike-slip ductile shear belt and the EW-slip ductile shear belt into a nappe ductile shear belt, resulting in the formation of structural schist (Guo, Reference Guo1994). The intricate tectonic deformation history caused the supracrustal rocks in the Anshan-Benxi area to undergo complex internal deformation. Liu (2019) categorized the formation and evolution process of the granite-greenstone belt base in the Anshan-Benxi area into two primary stages: the initial BIF iron deposit phase and the emplacement of Qidashan granite in the Late Neoarchaean period. The accompanying metamorphic deformation distorted the original Archaean tectonic configuration and the distribution characteristics of the original iron-bearing structures (Liu et al., Reference Liu, Li, Liu, Li, Wen, Liang and Chang2017).
3. Sample collection and Petrology
This study focuses on actinolite schist, sericite quartz schist, chlorite schist, meta-rhyolite, meta-sandstone and amphibolite from the Anshan-Benxi area. The primary objective is to investigate their petrology, whole-rock geochemistry, zircon U–Pb and Lu–Hf isotopes. The sampling locations are illustrated in Fig. 1b, Fig. 2, Table 1.
Sample information of Anshan-Benxi area
Actinolite schist, sourced from the Yingtaoyuan Formation of the Anshan Group, displays a yellow-green weathered surface and a grey-green fresh surface with well-developed slaty cleavage. It exhibits a cylindrical granoblastic structure and schistose structure (Fig. 3a). Actinolite schist is primarily composed of quartz (45%), actinolite (35%), garnet (10%) and hornblende (10%) (Fig. 3b).
Photographs of supracrustal rocks of the Anshan Group in the Anshan-Benxi area. (a) Slaty cleavage in actinolite schist; (b) Actinolite schist; (c) Sericite quartz schist; (d) Augen structure in meta-rhyolite; (e) Meta-rhyolite; (f) Meta-sandstone; (g) Chlorite schist; (h) Amphibolite. Act-Actinolite, Amp-Amphibole, Bi-Biotite, Chl-Chlorite, Grt-Garnet, Mic-Microcline, Ms-Muscovite, Or-Orthoclase, Pl-Plagioclase, Q-Quartz, Ser-Sericite.
Sericite quartz schist, obtained from the Yingtaoyuan Formation of the Anshan Group, displays a soil-yellow weathered surface and a greyish-brown fresh surface with a lepido granoblastic texture and schistose structure. Sericite quartz schist is composed of sericite (10%), biotite (5%), plagioclase (15%), microcline (10%), quartz (60%) and a small amount of garnet (2%). Sericitization occurs in plagioclase, and microcline has a grid twin (Fig. 3c).
Meta-rhyolite, sourced from the Dayugou Formation of the Anshan Group, displays a yellowish-brown weathered surface and a greyish-white fresh surface with porphyritic texture and massive structure. It comprises matrix and porphyry. The porphyry is composed of quartz. The matrix is cryptocrystalline, consisting of microcrystalline perthite, mica, etc., displaying a flow structure. The observation of an augen structure indicates ductile deformation in this rock (Fig. 3d). Meta-rhyolite is composed of quartz (40%), feldspar (10%), sericite (28%), muscovite (10%), biotite (10%) and actinolite (2%). Muscovite is rose-red under single polarizing lens, containing Li, with actinolite acicular (Fig. 3e).
Meta-sandstone, extracted from the Dayugou Formation of the Anshan Group, exhibits a rust-brown weathered surface and earthy yellow fresh surface with a lepido granoblastic texture and schistose structure. It is composed of feldspar (48%), quartz (30%), biotite (20%) and muscovite (2%). The minerals exhibit a rounded shape with fine grain size, and sericitization occurs in plagioclase (Fig. 3f).
Chlorite schist, taken from the Cigou Formation of the Anshan Group, is yellowish-green on weathered surface and grey-green on fresh surface, displaying a lepido granoblastic texture and schistose structure. It consists of chlorite (70%) and quartz (30%). Chlorite displays abnormal interference colours such as indigo and rust, and it is both orientated and continuous (Fig. 3g).
Amphibolite, also from the Cigou Formation of the Anshan Group, is greyish-brown in weathering and greyish green on fresh surface, with a cylindrical granoblastic structure and massive structure. Amphibolite is composed of hornblende (60%), plagioclase (30%) and biotite (10%). Most of the hornblende is actinized, with plagioclase showing sericitized (Fig. 3h).
4 Analysis methods
4.a. Zircon U-Pb isotope dating
Zircon separation was completed at Langfang Keda Rock and Mineral Sorting Technology Service Co., Ltd. Zircon mount and image acquisition (including transmitted light, reflected light and cathode luminescence) were completed in Beijing Zirconia Linghang Technology Co., Ltd. Zircon U-Pb dating of samples G2303-1, G2304-1, G2305-1 and G2306-1 was completed by LA-ICP-MS analysis at the Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Natural Resources, Jilin University, China. The laser denudation system is a GeoLasPro 193nmArF excimer laser produced by COMPEx GMBH in Germany. The sample was analysed using a 32μm diameter laser beam with a frequency of 7Hz. The Agilent7900 type ICP-MS instrument is used in conjunction with the laser, and He is used as the carrier gas of the denudating material (Eggins et al., Reference Eggins, Kinsley and Shelley1998; Jackson et al., Reference Jackson, Pearson, Griffin and Belousova2004). The instrument optimization adopts the standard reference material NIST610 of synthetic silicate glass developed by the National Institute of Standards and Technology of the United States. In situ U-Pb analysis of zircon was carried out by using the 91500 standard zircon external correction method, please refer to Yuan et al. (Reference Yuan, Gao, Liu, Li, Günther and Wu2004) for the specific experimental test process. Isotope ratios and ages of 207Pb/206Pb、206Pb/238U and 207Pb/235U were calculated using Glitter software (Liu et al., Reference Liu, Zhao, Sun, Li, Liu, Chen, Zhang and Sun2010). The method of Andersen (Reference Andersen2002) was used to correct the results by ordinary lead, and the Isoplot programme was used to calculate the age. The error for a single data is 1σ and the weighted mean age error is 95% confidence. The 207Pb/206Pb age is used as the zircon age.
Zircon U-Pb dating of sample GCN-1 was analysed by SHRIMP II ion probe in Beijing SHRIMP Center. The dating principles and methods are described in Mckibben et al. (Reference Mckibben, Shanks and Ridley1998), and the dating process is detailed in Song et al. (Reference Song, Zhang, Wan and Jian2002) and Wan et al. (Reference Wan, Song, Yang and Liu2005). The primary ion flow O2 intensity is 4.5nA, and the beam spot size is ∼30μm. Standard zircon TEM and M257 are used for 206Pb/238U age and U and Th content calibration, respectively. The ratio of standard zircon (TEM) to the sample to be tested is 1:4, and each data point consists of 5 sets of scans, SHRIMP data are calculated by SQUID programme based on 204Pb contents subtracting common lead, ISOPLOT programme calculates the weighted average value and draws U-Pb concordia plot (Vermeesch, Reference Vermeesch2018). The individual data error is 1σ, and the weighted mean age error is 95% confidence.
4.b. In situ analysis of Hf isotopic composition
The Hf isotope analysis of zircon was carried out by LA-MC-ICP-MS in Wuhan Sample Solution Analytical Technology Co., Ltd. Experiments of in situ Hf isotope ratio analysis were conducted using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) in combination with a Geolas HD excimer ArF laser ablation system (Coherent, Göttingen, Germany) that was hosted at the Wuhan Sample Solution Analytical Technology Co., Ltd, Hubei, China. A ‘wire’ signal smoothing device is included in this laser ablation system, by which smooth signals are produced even at very low laser repetition rates down to 1 Hz (Hu et al., Reference Hu, Zhang, Liu, Gao, Li, Zong, Chen and Hu2015). Helium was used as the carrier gas within the ablation cell and was merged with argon (makeup gas) after the ablation cell. Small amounts of nitrogen were added to the argon makeup gas flow for the improvement of sensitivity of Hf isotopes (Hu et al. 2012). All data were acquired on zircon in single spot ablation mode at a spot size of 44 μm or 32 μm. Analytical methods are the same as described by Hu et al. (2012). The new high-performance cone combination designed by Neptune Plus was used in the analysis. The Hf isotope standard calibration of zircon was used with 91500, GJ-1 international zircon and SP-1 zircon to better monitor the quality of the measured isotope data. The external precision (2SD) of 91500 and GJ-1 is better than 0.000020 (Zhang et al., Reference Zhang, Hu and Spectroscopy2020).
4.c. Major and trace elements
The analysis of major elements and trace elements was carried out in Aoshi Analytical Testing (Guangzhou) Co., Ltd. After cleaning, grinding and crushing the fresh samples removed from the weathered surface, 200 mesh rock powder was obtained for the analysis of major and trace elements. The major elements were determined by X-ray fluorescence spectrometry (ME-XRF26d). The relative error of precision of ME-XRF26d method was controlled at <5 %, with the relative error of accuracy being controlled at < 5 %. Trace elements and rare earth elements were determined by inductively coupled plasma mass spectrometer using ICP-MS (ME-MS81). The relative error of precision of ME-MS81 method was controlled at < 10 %, with the relative error of accuracy being controlled at < 10 %.
5 Results
5.a. Zircon U–Pb dating
The zircons of amphibolite (GCN-1) are predominantly granular characteristics with an aspect-to-width ratio of approximately 1:1 and an average particle size of about 100μm. Zircons are mostly broken, the shape is not complete, along with many cracks on the surface. Zircons develop zoning, mainly broad zoning, with a small amount of zircons developing fine zoning. Some zircons display distinct bright metamorphic edges (Fig. 4). A total of 19 zircon grains were analysed in this sample, the data are listed in Table 2. Th contents of the zircon sample range from 79.78ppm to 1155.91ppm, while U contents range from 30.61ppm to 568.73ppm. The Th/U values vary from 0.51 to 23.58, with only one zircon exhibiting a value of 23.58, characteristic of magmatic zircons. The apparent age of the zircons falls within the range of 2577 Ma to 2543 Ma. On the zircon U-Pb age concordia diagram, some data points align with the 206Pb/238U-207Pb/235U concordia curve. The 207Pb/206Pb weighted average age of 11 data points (1.1,2.1,3.1,6.1,7.1,9.1,11.1,11.1,13.1,14.1,15.1,16.1) is 2571 ± 5.3Ma (MSWD = 1.5), interpreted as the formation age of the protolith of amphibolite. (Fig. 5a).
CL image of representative zircon (The red solid circle is the U-Pb spot, and the yellow dotted circle is the Lu-Hf isotope spot.).
Analysis results of Zircon U-Pb isotope
U-Pb concordia diagrams of zircons from supracrustal rocks in the Anshan-Benxi area.
The zircons from meta-rhyolite (G2304-1) exhibit cylindrical or equiaxed shapes. They are more idiomorphic with good crystal shape, with uniform size averaging around 100 μm. The length-width ratio falls between 1:2 and 1:3. In cathodoluminescence diagrams, most zircons exhibit irregular rhythmic bands with rings, suggesting recrystallization at varying ages. Metamorphic accretion edges on zircons are minimal (Fig. 4). A total of 30 zircon grains were analysed, all falling within the magmatic zircon compositional domain. Data for these zircons are detailed in Table 2. Results reveal Th contents ranging from 20.34 ppm to 367.07 ppm, U contents ranging from 62.78 ppm to 395.22 ppm. The Th/U value ranges from 0.25 to 1.35, with only two values below 0.4, indicative of magmatic zircon. The apparent ages of these zircons cluster around 2552 Ma to 2450 Ma. On the zircon U-Pb age concordia diagram, some data points lie on the 206Pb/238U-207Pb/235U concordia line (Fig. 5b). Points below the concordia curve form a discordant line with an upper intercept age of 2500 ± 40 Ma (MSWD=4.4). The 207Pb/206Pb weighted mean age of 16 data points (G2304-2-1, G2304-2-3, G2304-2-4, G2304-2-5, G2304-2-9, G2304-2-12, G2304-2-13, G2304-2-15, G2304-2-17, G2304-2-22, G2304-2-23, G2304-2-24, G2304-2-26, G2304-2-27, G2304-2-28, G2304-2-29) is 2470 ± 20 Ma (MSWD = 4.6). LA-ICP-MS zircon U-Pb dating was unfeasible due to the narrow or absent metamorphic accretionary margin in this sample.
The zircons of meta-sandstone (G2305-1) exhibit signs of degradation, damaging into the form of residual columns, fragments and subcircular shapes. These characteristics suggest weathering and transportation over a considerable distance, displaying typical detrital zircon features. Zircons grain sizes range from 50 μm to 150 μm, with an aspect ratio between 1:1 and 1:3. In the cathodoluminescence diagrams, most zircons develop annular bands and plate-like strips, indicating recrystallization at different times. Metamorphic and accretionary rims on these zircons are generally absent (Fig. 4). A total of 80 zircon grains were analysed, and the data are listed in Table 2. Th contents of zircon in this sample range from 54.05 ppm to 904.36ppm, U contents ranged from 89.53ppm to 1058.2ppm, with the value of Th/U ranging from 0.12 to 1.33. The majority of the zircon grains have Th/U ratios exceeding 0.4, and CL images reveal well-defined oscillatory ring structures, indicating magmatic origin. Validated data yielded zircon ages ranging from 2172 Ma to 2980 Ma (Fig. 5c), with a prominent peak at 2500 Ma (Fig. 5d).
The meta-sandstone (G2306-1) contains intact zircon crystals, predominantly cylindrical in shape, with rounded zircon grains ranging from 75 μm to 150 μm in size and aspect ratios between 1:1 and 1:3. Cathodoluminescence diagrams indicate that most zircons exhibit ring bands, suggesting multiple recrystallization events with narrow metamorphic accretion margins (Fig. 4). A total of 80 zircon grains were analysed, and the results are presented in Table 2. Th contents of zircon grains ranged from 15.18 ppm to 2756.36 ppm, while U contents varied from 79.47 ppm to 1421.02 ppm. The Th/U values range from 0.06 to 2.32, respectively. The majority of zircons have Th/U ratios above 0.4, displaying well-defined oscillatory ring structure in CL images, indicating a magmatic origin. The range of ages obtained from valid data varies from 1970 Ma to 3099 Ma (Fig. 5e), with a prominent peak at 2500 Ma (Fig. 5f).
5.b. Zircon Hf isotopic data
The study presents the results of in situ Hf isotope analysis for one sample (G2304-1) of zircon, as shown in Table 3. The 176Hf/177Hf radios of 10 zircons from meta-rhyolite (G2304-1) range from 0.281175 to 0.281306, with corresponding εHf(t) values ranging from -1.19 to 1.47. The calculated Hf model age, expressed as tDM2 (Ma), falls within the range of 2922 Ma to 3132 Ma. The zircon age-εHf(t) diagram for meta-rhyolite in the Anshan-Benxi area is depicted in Fig. 6, along with the detailed results of zircon Hf isotope analysis detailed in Table 3.
Results of zircon Hf isotope analyses of meta-rhyolite from the Anshan-Benxi area
Zircon Age-εHf(t) diagram of meta-rhyolite in supracrustal rocks in the Anshan-Benxi area and supracrustal rocks in the Anshan-Benxi area of Zhu et al., Reference Zhu, Dai, Zhang, Wang and Liu2015, Dai et al., Reference Dai, Zhang, Zhu, Wang and Liu2013a and Wang et al., Reference Wang, Huang, Tong, Zheng, Peng, Nan, Zhang and Zhai2016.
5.c. Geochemistry
Sericite quartz schist: SiO2 contents range from 77.82 to 78.89wt.%, indicative of acidic rock. The Al2O3 contents range from 20.94 to 22.72wt.%; CaO contents from 0.66 to 0.71wt.%, Fe2O3T contents from 1.80 to 1.84wt.%, total alkali Na2O+K2O contents from 5.56 to 5.85wt.%, with minimal amounts of other oxides. The loss on ignition (LOI) ranges from 1.24 to 1.36wt.%, Mg# from 30.71 to 32.37, Rittmann Index (σ) from 0.86 to 0.98 (Table 4), indicating that the samples are mainly calc-alkaline rocks, as depicted in the diagram (Fig. 8f, h, i). In the TAS (Na2O+K2O vs. SiO2) and SiO2 vs. Zr/TiO2 diagrams, sericite quartz schist falls within the rhyolite region (Fig. 8d, e). Based on the comparison of Al2O3, Na2O+K2O and Na2O+K2O+CaO contents, the rock is classified as peraluminous, further affirmed by its position in the ANK-ACNK diagram within the peraluminous region (Fig. 8a).
Chondrite normalized REE (a,c: normalized values after Boynton Reference Boynton and Henderson1984), and primitive mantle normalized spider diagrams (b,d: normalized values after Sun and McDonough Reference Sun and McDonough1989) of sericite quartz schist, meta-rhyolite, chlorite schist, actinolite schist and amphibolite in the Anshan-Benxi area; PAAS-normalized REE patterns (e: normalized value after McLennan, Reference McLennan1989) and upper crust-normalized trace element spider diagram (f: normalized value after Taylor and McLennan, Reference Taylor and McLennan1985) of meta-sandstones in the Anshan-Benxi area. (a,b) Metamorphic basic volcanic rock; (c,d) Metamorphic acidic volcanic rock; (e,f) Meta-sandstone.
The rare earth element distribution pattern in sericite quartz schist shows a predominantly right-leaning, with an enrichment of light rare earth elements (Fig. 7c). The total rare earth elements (∑REE) range from 122.56ppm to 129.16ppm, with (La/Yb)N values ranging from 9.77 to 9.87, and (La/Sm)N values ranging from 4.68 to 4.84. The ratio of light to heavy rare earth elements (LREE/HREE) ranges from 9.97 to 10.15, with δEu values ranging from 0.54 to 0.55, indicating a negative anomaly of Eu. Among the trace elements, enrichment is primarily observed in Rb, K, Nd and Th, while Ba, Nb, P and Ti are notably deficient (Fig. 7d).
(a) A/CNK-A/NK diagram (Shand, Reference Shand1943); (b) MgO-CaO-FeOT diagram (Walker et al., Reference Walker, Joplin, Lovering and Green1959); (c) La/Yb-∑REE diagram (Gromet et al., Reference Gromet, Haskin, Korotev and Dymek1984); (d)TAS diagram (Bas et al., Reference Bas, Maitre, Streckeisen and Zanettin1986); (e) SiO2 vs. Zr/TiO2 diagram (Winchester and Floyd, Reference Winchester and Floyd1977); (f) SiO2 vs. AR diagram (Wright, Reference Wright1969); (g) TiO2 vs. SiO2 diagram (Tarney, J., Reference Tarney and Windley1976); (h) La vs. Yb diagram (Ross and Bédard, Reference Ross and Bédard2009); (i) FeOT/MgO vs. SiO2 diagram (Miyashiro, Reference Miyashiro1974) of supracrustal rocks in the Anshan-Benxi area.
Meta-rhyolite: SiO2 contents range from 64.69 to 76.28wt.%, typical of acidic rock. The Al2O3 contents range from 14.19 to 17.21wt.%; TiO2 contents from 0.40 to 0.61wt.%; MgO contents from 0.61 to 3.16wt.%; CaO contents from 0.27 to 1.08wt.%, Fe2O3T contents from 2.83 to 6.45wt.%, and the total alkali Na2O+K2O contents from 5.07 to 6.89 wt.%, with minimal amounts of other oxides. The LOI ranges from 1.96 to 2.81wt.%, Mg# from 30.03 to 49.98, Rittmann index (σ) from 0.77 to 2.19 (Table 4), indicating that meta-rhyolites are predominantly calc-alkaline rocks, as illustrated in the illustrations (Fig. 8f, h, i). In the TAS (Na2O+K2O vs. SiO2) and SiO2 vs. Zr/TiO2 diagrams it falls centrally within the rhyolite region (Fig. 8d, e). Upon comparison of the Al2O3, Na2O+K2O and Na2O+K2O+CaO contents, the rocks are identified as peraluminous, a classification reinforced by their positioning in the ANK-ACNK diagram within the peraluminous region.
The rare earth element partitioning pattern of the meta-rhyolite quarried at Jiajiagoucun shows a predominantly right-leaning, indicating a higher enrichment of light rare earth (Fig. 7c). The ∑REE of the meta-rhyolite ranges from 83.72 ppm to 138.63 ppm, with (La/Yb)N values ranging from 6.17 to 7.06 and (La/Sm)N values ranging from 3.65 to 4.20. The LREE/HREE ranges from 7.53 to 9.05, with δEu values ranging from 0.50 to 0.59, signifying a negative anomaly of Eu. Trace elements are primarily enriched in Zr and U and deficient in Ba, Nb, Ti and Sr (Fig. 7d). In the meta-rhyolite extracted from the east of Qianpaifangcun and the south of the Donggou, similar right-leaning patterns of rare earth element partitioning are observed. The ∑REE values range from 123.77ppm to 165.50ppm, (La/Yb)N values range from 15.16 to 17.24, with (La/Sm)N values ranging from 4.61 to 4.87. The LREE/HREE values range from 12.99 to 14.00, and δEu values range from 0.80 to 0.90, showing a weak Eu negative anomaly feature, and the trace elements are mainly enriched in Cs, U, K and Zr, alongside deficiencies in Ba, Nb, Sr and Ti (Fig. 7d).
Meta-sandstone: SiO2 contents range from 60.85 to 70.92wt.%. The Al2O3 contents range from 15.24 to 17.50wt.%; TiO2 contents from 0.34 to 0.53wt.%; CaO contents from 0.99 to 1.25wt.%, MgO content from 1.31 to 4.37wt.%, Fe2O3T contents from 3.17 to 7.60wt.%. The total alkali Na2O+K2O contents range from 7.29 to 7.83wt.% (Table 4), with relatively low amounts of other oxides present. The LOI ranges from 1.36 to 2.92wt.%, Mg# varies from 43.21 to 54.88, placing them within the sandstone region in the La/Yb vs.∑REE diagram (Fig. 8c).
The rare earth element partitioning pattern of meta-sandstones closely resembles that of Post-Archaean Australian average shale (PAAS), the ∑REE range from 142.12 ppm to 146.59 ppm, with (La/Yb)N ranging from 14.00 to 15.65, (La/Sm)N ranging from 4.90 to 4.99 (Fig. 7e). The LREE/ HREE falls within the range of 11.71 to 12.67, δEu values range from 0.79 to 0.94, indicating a weak negative anomaly of Eu. Furthermore, trace elements in these meta-sandstones are primarily enriched in Ba and Sm while showing deficits in Nb, P, Zr and Tb (Fig. 7f).
Chlorite schist: SiO2 contents range from 50.21 to 52.99 wt.%, indicating they are of medium basic rock. The Al2O3 contents range from 14.30 to 14.64wt.%; TiO2 contents from 1.21 to 1.32wt.%; CaO contents from 0.16 to 0.19wt.%, MgO contents from 9.94 to 10.29wt.%, Fe2O3T contents from 21.08 to 23.06 wt.%. The total alkali Na2O +K2O contents are very slight at about 0.05wt.%, with low levels of other oxides present. Mg# ranges from 47.16 to 48.53 (Table 4). In the TAS (Na2O+K2O vs. SiO2) and SiO2 vs. Zr/TiO2 diagrams, the chlorite schist falls centrally within the basalt region (Fig. 8d,e). By comparing the magnitude of the Al2O3, Na2O+K2O and Na2O+K2O+CaO contents, the rock is peraluminous, with all of them in the Tholeiitic series (Fig. 8f, h, i).
The rare earth element partitioning pattern of the chlorite schist is mainly flat, with insignificant enrichment of light rare earth (Fig. 7a). The ∑REE ranges from 86.29ppm to 90.58ppm, with (La/Yb)N values ranging from1.97 to 2.34 and (La/Sm)N values ranging from 1.35 to 1.50. The LREE/HREE ranges from 2.34 to 2.65, and δEu values range from 0.77 to 0.79, indicating a weak negative Eu anomaly. Furthermore, trace elements in these chlorite schist samples are mainly enriched in Rb, U, La, Nd, Sm and Dy, while showing deficits in Ba, K, Sr, Zr and Ti (Fig. 7b).
Actinolite schist: SiO2 contents range from 40.38 to 49.92 wt.%, indicating they are within the range of basic-ultrabasic rocks. The Al2O3 contents range from 11.09 to 15.72 wt.%; CaO contents from 0.07 to 0.17wt.%, Fe2O3T contents from 27.90 to 29.13wt.%, total alkali Na2O+K2O contents from 0.05 to 0.30wt.%, TiO2 contents from 0.55 to 1.03 wt.%; MgO contents from 9.60 to 13.65wt.%, with other oxides contents are minimal. Mg# ranges from 47.16 to 48.53 (Table 4). In the TAS (Na2O+K2O-SiO2) and SiO2 vs. Zr/TiO2 diagrams, the actinolite schist falls centrally within the tholeiitic region (Fig. 8d,e). All of them show a series of tholeiitic (Fig. 8f, h, i), which are determined to be peraluminous through a comparison of the magnitude of Al2O3, Na2O+K2O and Na2O+K2O+CaO contents.
The rare earth element partitioning pattern of actinolite schist is mainly flat, with insignificant enrichment of light rare earth (Fig. 7a). The samples exhibit obvious U-positive anomalies, Rb-positive anomalies, Sr-negative anomalies and Ti-negative anomalies (Fig. 7b). The ∑REE ranges from 23.16ppm to 41.55ppm, with (La/Yb)N values ranging from 0.59 to 1.21 and (La/Sm)N values ranging from 0.73 to 1.13. The LREE/HREE ranges from 1.45 to 1.98.
Amphibolite: SiO2 contents range from 40.82 to 53.61wt.%, which places them in the category of basic rocks. The Al2O3 contents range from 9.6 to 15.25wt.%; TiO2 contents from 0.39 to 0.52wt.%; CaO contents from 0.23 to 8.22wt.%, MgO contents from 5.68 to 10.89wt.%, Fe2O3T contents from 13.29 to 34.19wt.%, total alkali Na2O+K2O contents from 0.3 to 4.59wt.%, while other oxide contents are low. Mg# contents range from 38.91 to 58.09 (Table 4). The samples fall into the region of amphibolite in the La/Yb-∑REE diagram (Fig. 8c). The rock is identified as peraluminous, in addition, tholeiitic basalt series by comparing the magnitude of Al2O3, Na2O+K2O and Na2O+K2O+CaO contents (Fig. 8f, h, i). In the MgO-CaO-FeOT diagram, amphibolite falls into ortho-amphibolite, indicating that the protolith of amphibolite is igneous rock, which is further supported in the TiO2 vs. SiO2 diagram (Fig .8b, g).
The rare earth element partitioning pattern of amphibolite is mainly flat, with insignificant enrichment of light rare earth elements (Fig. 7a). The ∑REE ranges from 25.44ppm to 56.23ppm, with (La/Yb)N values ranging from 0.72 to 3.05, (La/Sm)N values ranging from 0.86 to 2.42. The LREE/HREE ranges from 1.16 to 3.44, and δEu values range from 0.61 to 1.12, indicating a weak negative Eu anomaly. The trace elements are primarily enriched in Rb and K, while deficient in Ba, Sr and Ti (Fig. 7b).
6. Discussion
6.a. Distribution and formation age of supracrustal rocks
Archaean supracrustal rocks are extensively exposed in the NCC. Towards the end of the Neoarchaean period, the magmatic activity in the NCC intensified, accompanied by metamorphism. In the Jidong area, Xiang et al. (Reference Xiang, Cui, Wu, Zhang and Zhang2012) conducted zircon U-Pb dating on amphibolite, the surrounding rock of the Zhoutaizi iron ore mine in Luanping County. The results revealed that the upper intercept age of magmatic zircon was 2512 ± 21 Ma, representing the volcanic eruption and precipitation of the BIF in the Zhoutaizi iron ore deposit. Additionally, the weighted average age of metamorphic zircon was determined to be 2394 ± 21 Ma, suggesting the timing of metamorphic event in the Zhoutaizi iron ore deposit, which led to the alteration of the protoliths and ores. Additionally, εHf(t) values and two- stage depleted mantle model age indicated that the magma source region was subjected to mingling by ancient crustal materials; Zhang et al. (Reference Zhang, Zhang, Xiang, Wan and Pirajno2011) carried out zircon U-Pb dating on amphibolite gneisses intercalated with the Shuichang iron ore deposit. The findings revealed volcanic activity at 2547 ± 7 Ma and metamorphism at 2513 ± 4 Ma in the area. Based on the geochemical characteristics of the surrounding rocks, it was inferred that it formed a back-arc basin associated with subduction.
In the Fushun and Qingyuan regions of Liaoning, SHRIMP zircon U-Pb age determinations conducted by Wan et al. (Reference Wan, Li, Wilde, Liu, Chen, Yan, Song and Yin2005) of the Archaean amphibolite yielded a magmatic zircon weighted average age of 2515 ± 6 Ma for the hornblende metamorphic granulites of the Xiaolaihe Iron Ore Mine Area, 2510 ± 7 Ma for the hornblende metamorphic granulites of the Fushun Tangtu and 2479 ± 5 Ma for the hornblende metamorphic granulite metasedimentary zircon age of the Qingyuan Group in the northern part of Qingyuan. These results suggest a significant tectonic-thermal event during the Late Palaeozoic, likely occurring in an island arc setting. Further LA-ICP-MS zircon U-Pb isotope analyses of amphibolite in the eastern Tangtu area by Bai et al. (Reference Bai, Liu, Yan, Zhang, Wang, Guo and Guo2014) revealed a minimum age of magma crystallization of 2530 ± 5 Ma (Bai et al., Reference Bai, Liu, Yan, Zhang, Wang, Guo and Guo2014). In the Banshigou area of Baishan, Li et al. (Reference Li, Guo, Guan and Liu2016) conducted zircon U-Pb dating on amphibolite and hornblende schist in the Banshigou crustal rocks. The results showed that amphibolite magmatic zircons were formed at 2548 ± 11 Ma, while epidote hornblende schistic magmatic zircons were formed at 2548 ± 23 Ma, indicating the age of crustal rock formation in an island arc environment. In the Guyang area, Liu et al. (Reference Liu, Zhang, Dai, Wang and Li2012) carried out SIMS U-Pb dating on amphibolite in the surrounding rock of the Sanheming iron deposit, revealing an upper intersection age of the zircon core is 2562 ± 14 Ma, suggesting formation in a back-arc basin with a tectonic environment of mantle plume superposition.
Based on the information provided, it can be inferred that most of the supracrustal rocks in the NCC were formed around 2500 Ma, underwent late-stage metamorphism and originated in island arc environments. The age determinations indicate that amphibolite (GCN-1) of this study was dated at the age of 2571 ± 18 Ma, meta-rhyolite (G2304-1) at the age of 2470 ± 20 Ma, additionally, two meta-sandstone samples (G2305-1, G2306-1) display a prominent peak at 2500 Ma, indicating that the supracrustal rocks in the Anshan-Benxi area were formed during the Late Neoarchaean. Furthermore, an early detrital zircon with an age of 3099 ± 60 Ma was found in the meta-sandstone (G2306-1), indicating the presence of older rocks in the source region. The contemporaneous formation of supracrustal rocks in the Anshan-Benxi area as well as in Jidong, Fushun, Liaoning, Qingyuan, Baishan Banshigou and Guyang, implies a significant magmatic event during the Late Neoarchaean in the NCC (2550 Ma∼2500 Ma).
6.b. Assessments of element mobility
The volcanic rocks in the Anshan-Benxi area have experienced multiple stages of deformation until greenschist metamorphism. This process can alter the activity of certain elements, rendering them unreliable as diagenetic indicators of the protoliths. Therefore, it is crucial to assess the element activity of these metavolcanic rocks to comprehend their petrogenesis and the tectonic context of the Anshan-Benxi area. Polat and Hofmann (Reference Polat and Hofmann2003) discussed the element activity of rocks that have experienced intermediate and high-level metamorphism, in case where the LOI of the sample exceeds 6% and Ce exhibits significant anomalies (Ce*>1.1 or &<0.9), which indicates considerable impact from later alteration or other thermal events. Additionally, when the correlation coefficient (R) with immobile components Al2O3/TiO2 and Zr is above 0.75, the element is considered relatively stable and unaffected by subsequent thermal events.
The LOI of metamorphic acidic rocks ranges from 1.24 to 2.81, all less than 6%, with Ce* values ranging from 0.91 to 1.03 (except for G2303-2-1 and G2303-2-2, whose Ce* are 1.29 and 1.22 respectively). The chondrite normalized REE spider diagrams of chondrites show no discernible Ce anomalies, suggesting that the original chemical composition of metamorphic acidic rocks has not been significantly modified by later events (Polat and Hofmann, Reference Polat and Hofmann2003). Element covariation diagrams with immobile components Al2O3/TiO2 and Zr demonstrate a strong linear trend for SiO2, TiO2, Al2O3, CaO, Na2O, K2O, as well as Rb, Nb, Yb, Hf and Sr in metamorphic acidic rocks (Fig. 9), with correlation coefficients close to 1, indicating that these elements are relatively immobile during post-magmatic alteration. These elements, along with REEs and HFSE, are chosen to represent the original magma components for analysing the origin of metamorphic acidic rocks and the geodynamic process.
Al2O3/TiO2 and Zr vs. selected elements variation diagrams for supracrustal rocks in the Anshan-Benxi area.
The LOI of metamorphic basic rocks is relatively high, ranging from 1.94 to 7.33, with most exceeding 6%. Covariation diagrams of elements with immobile components Al2O3/TiO2 and Zr reveal a strong linear trend for TiO2, K2O and Hf in metamorphic basic rocks (Fig. 9), with a correlation coefficient close to 1. Due to the protoliths above formed by the metamorphism of basic rocks, unstable elements (such as K, Na, Rb, Sr, etc.) are prone to migrate during alteration and metamorphism, while high-field-strength elements (such as Zr, Hf, Nb, Ta, Ti, Y, etc.) and rare earth elements can remain stable during alteration and metamorphism. This study therefore focuses on elements with high stability to discuss the geological background and origin of the rocks.
6.c. Tectonic environment and petrogenesis of supracrustal rocks in the Anshan-Benxi area
6.c.1. Metamorphic acidic rocks
The metamorphic rhyolite and sericite quartz schist found in the Anshan-Benxi area exhibit relatively high SiO2 (64.69∼78.89wt.%) and Al2O3 (12.38∼17.21wt.%) and lower MgO (0.4∼3.16wt.%) and Mg# (30.03∼49.48). Its protolith is equivalent to calc-alkaline rhyolite (Fig. 8d, e). This group of samples has a high (La/Yb)N ratio (6.17∼17.24), and the normalized rare earth element distribution pattern of chondrites is right-leaning. The total amount of rare earth elements is high (122.56 ppm∼165.50 ppm), enriched in light rare earth elements, developing obvious negative anomalies of Nb and Ti. These characteristics align closely with typical island arc volcanic rocks, indicating a probable origin associated with an island arc magma environment.
To further corroborate this interpretation, tectonic setting discrimination diagrams were employed. In the Rb/30-Hf-3Ta, R1-R2, Rb-Y+Nb, Nb-Y, Rb-Ta+Yb, Ta-Yb tectonic setting discrimination diagrams (Fig. 10), a significant portion of metamorphic acidic rocks were classified within the categories of volcanic arc granite and syn-collision granite. It was judged that the protolith of this type of rock likely formed within an island arc environment linked to subduction processes.
Tectonic setting discrimination diagrams of metamorphic acidic volcanic rocks in the Anshan-Benxi area. (a) Rb/30-Hf-3Ta, after Harris et al., Reference Harris, Pearce and Tindle1986; (b) R1-R2, R1 = 4Si-11(Na+K)-2(Fe+Ti), R2 = 6Ca+2Mg+Al, after Batchelor + Bowden, 1985; (c) Rb-Y+Nb; (d)Nb-Y; (e) Rb-Ta+Yb; (f) Ta-Yb, after Pearce et al., Reference Pearce, Harris and Tindle1984.
Zircon Lu-Hf isotopes serve as a relatively valuable tool for tracing magma sources. The Hf isotope test results of metamorphic rhyolite (G2304-1) reveal that εHf(t) ranges from -1.19 to 1.47, intersecting the CHUR evolution line and extending to the DM line on the age-εHf(t) diagram. The two- stage depleted mantle model age, tDM2(Ma), falls within the range of 2922 to 3132Ma, surpassing the zircon formation age of 2500 Ma. The Na/Yb value of metamorphic acidic rocks ranges from 9.92 to 15.22 (with an average of 13.30), which closely aligns with the typical Earth’s crust (11∼12, Green, Reference Green1994); the Zr/Hf ratio ranges from 32.29 to 40.56, with an average value of is 37.31, indicating values higher than the average Zr/Hf value of crust-derived magma (33, R. G, 1985; Zhang et al., Reference Zhang, Shi and Shi2021), but lower than that of mantle-source magma (39, Zhang et al., Reference Zhang, Hu, Zhang, Xiong, Zhu, Jia and Gong2020). These findings suggest that the metamorphic acidic protolith magma in the study area likely originated from the partial melting of the 3.0 Ga basaltic crust. The observation of high Mg# in metamorphic acidic rocks implies that the addition of mantle components to the protolith during its diagenesis, corroborated by its extension towards the mantle end-member on the age-εHf(t) diagram. In conclusion, it is speculated that the metamorphic acidic rocks in the Anshan-Benxi area may have originated from the partial melting of 3.0Ga basaltic crust, supplemented with lithospheric mantle contributions.
6.c.2. Metamorphic basic rocks
The chondrite normalized rare earth element distribution pattern of the chlorite schist, actinolite schist and amphibolite in the Anshan-Benxi area of metamorphic basic rocks exhibits flat, lacking significant enrichment in light rare earth elements but showing enrichment in Cs and depletion in Sr, Ti and K. Utilizing tectonic setting discrimination diagrams to identify them, they fall into the island arc basalt and mid-ocean ridge tholeiitic basalt in Fig. 11. In the tectonic setting discrimination diagrams of Hf/3-Th-Nb/16, Zr-Ti, 10MnO-TiO2-10P2O5 and Zr/4-2Nb-Y, metamorphic basic rocks fall into island arc tholeiitic basalt, island arc calc-alkaline basalt, depleted mid-ocean ridge tholeiitic basalt and the transitional zone between depleted mid-ocean ridge basalt and island arc basalt (Fig .12). Further geochemical analysis reveals that the Th (0.51∼3.32) of metamorphic basic rocks surpass Ta (0.1∼0.6), while the TiO2 content remains relatively low (0.39∼1.32%), which is consistent with the characteristics of island arc basalt (Yang et al., Reference Yang, Wang, Zhang, Chen, Pan, Du, Jiao and Wang2016).
Tectonic setting discrimination diagrams of metamorphic basic volcanic rocks in the Anshan-Benxi area (Agrawal et al., Reference Agrawal, Guevara and Verma2008).
Tectonic setting discrimination diagrams of metamorphic basic volcanic rocks in the Anshan-Benxi area. (a) Hf/3-Th-Nb/16, after Wood, Reference Wood1980; (b) Zr-Ti, after Pearce, Reference Pearce1982; (c) 10MnO-TiO2-10P2O5, after Mullen, Reference Mullen1983; (d) Zr/4-2Nb-Y, after Meschede, Reference Meschede1986.
The geochemical characteristics of metamorphic basic rocks in the Anshan-Benxi area exhibit elevated Mg# values ranging from 40.77 to 58.09, closely resembling the Mg# value of mid-ocean ridge tholeiitic basalt (60±, BEARD and LOFGREN, Reference Beard and Lofgren1991). Additionally, the average Nb/Ta radio is 14.17, and the average Zr/Hf radio is 33.24, comparable to the primitive mantle values (Nb/Ta = 17.5, Zr/Hf = 36.27) and higher than the average continental crust values (Nb/Ta = 11, Zr/Hf = 33) (Stolz et al., Reference Stolz, Jochum, Spettel and Hofmann1996; Taylor and McLennan, Reference Taylor and McLennan1985). The average Th/La ratio stands at 0.19, significantly lower than the average continental crust ratio of 0.28 but marginally higher than the Th/La ratio of the lower crust (0.15), indicating that the basic rock magma was less heavily contaminated by the crust during its ascending emplacement process. Therefore, it can be judged that the protolith of metamorphic basic rock in the Anshan-Benxi area is basaltic volcanic rock, primarily derived from the mantle with limited crustal contamination.
6.c.3. Metamorphic sandstone
The geochemical composition of meta-sandstone plays a crucial role in deciphering the tectonic setting of its formation. By scrutinizing certain trace element contents and ratios, researchers can deduce the tectonic setting of the original sedimentary rock formation. Bhatia (Reference Bhatia1983), Bhatia and Crook (Reference Bhatia and Crook1986) and McLennan and Taylor (Reference McLennan and Taylor1991) utilized this information to categorize the geotectonic environment into four distinct tectonic settings: oceanic island arcs, continental island arcs, active continental margins and passive continental margins. Through a comparative analysis of meta-sandstones from the Anshan Group with those from diverse tectonic environments using the aforementioned theory, it becomes apparent that the La, Ce and ∑REE contents in the meta-sandstones of the Anshan Group closely resemble those found in the continental island arc. Additionally, the trace elements and ratios exhibit a remarked similarity or consistency with those characteristic of the continental island arc (Table 5). The key distinguishing parameters in the meta-sandstones of the Anshan Group overwhelmingly resemble the distinctive characteristic parameters of the continental island arc, as evidenced by the Zr-Th and La-Th tectonic diagrams (Fig. 13) Therefore, it can be inferred that the protolith of the meta-sandstones in the Anshan Group were deposited by an island arc.
Analysis results of major and trace elements
Geochemical parameters (all in ppm) of the Anshan Group meta-sandstones versus tectonic setting (after (Bhatia, Reference Bhatia1983; Bhatia and Crook, Reference Bhatia and Crook1986))
Tectonic setting discrimination diagrams of meta-sandstones. (a) after Pearce J A, 1973; (b) after M. R. Bhatia, 1988; A, oceanic island arc; B, continental island arc; C, active continental margin; D, passive continental margin.
In recent years, the existence of plate movement in the Archaean has become a pivotal scientific inquiry and a subject of considerable debate within Precambrian research. Some scholars posited that plate movement commenced in the Neoarchaean, with the magmatic activity towards the end of the Neoarchaean originating from the base intrusion of the mantle column. Conversely, recent studies offer a contrasting viewpoint. They propose that plate tectonics were already in operation at the end of the Archaean, and the magmatic occurrences at the end of the Neoarchaean magmatic period were the result of arc-continent and continent-continent collisions. Drawing from previous opinions and the findings of this study, it is postulated that the metamorphic acidic rocks in the Anshan-Benxi area may have originated from the partial melting of 3.0Ga basaltic crust, possibly contaminated with lithospheric mantle materials. The protoliths of the metamorphic basic rocks are believed to be basaltic volcanic rocks, deriving from the mantle, without obvious crustal contamination. Tectonic setting discrimination diagrams indicate that the supracrustal rocks in the Anshan-Benxi area originated in island arc and mid-ocean ridge environments, comprising a blend of structural melange through plate subduction, thus bolstering the argument for plate tectonics during the Late Neoarchaean period. In conclusion, the Late Neoarchaean magmatism events in the Anshan-Benxi area of the northeastern NCC formed in an arc tectonic setting associated with plate subduction.
7. Conclusions
Based on zircon U-Pb geochronology, geochemistry and Hf isotope analysis of the supracrustal rocks in the Anshan-Benxi area of northeastern NCC, the following conclusions can be drawn:
(1) Magmatic zircons in the amphibolite of the Anshan Group in the Anshan-Benxi area formed at 2571 ± 18 Ma and the meta-rhyolite magmatic zircons formed at 2470 ± 20 Ma. Additionally, the two meta-sandstone samples show a peak at 2500 Ma, respectively, indicating that the supracrustal rocks in the Anshan-Benxi area were formed in the Late Neoarchaean.
(2) The protolith compositions of sericite quartz schist and metamorphic rhyolite are acidic rhyolite volcanic rock, with magma originating from the partial melting of 3.0Ga basaltic crust. In contrast, the protoliths of actinolite schist, chlorite schist and amphibolite are basaltic volcanic rocks, primarily deriving from mantle sources.
(3) Supracrustal rocks in the Anshan-Benxi area were formed within an island arc and mid-ocean ridge environment, representing a structural melange formed by plate subduction. The Late Neoarchaean magmatism observed in the Anshan-Benxi area in the northeastern part of the NCC occurred within an arc tectonic environment associated with plate subduction.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 42272224).
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