In the past 2 decades, three Paleoproterozoic linear tectonic belts were identified in the western, central, and eastern parts of the North China Craton (NCC), respectively, named the Khondalite Belt (KB)”, “Trans-North China Orogen (TNCO)” and “Jiao-Liao-Ji Belt (JLJB)” (Figure 1; Zhao et al., 1998; Zhao et al., 2000; Zhao et al., 2001; Zhao et al., 2005; Zhang et al., 2007; Zhang et al., 2009; Zhang et al., 2012; Yin et al., 2009; 2011; Wang et al., 2023; Wang et al., 2024; Wang et al., 2025; Zhao et al., 2024). The Paleoproterozoic KB is generally considered as a collisional orogeny created by the amalgamation between the Yinshan Block and the Ordos Block at ca. 1.95 Ga, which has resulted in the formation of the Western Block in the NCC (Figure 1; Zhao et al., 2005). It is worth noting that the KB has been extensively studied in terms of structural investigations, metamorphism, geochemistry, and geochronology, however, the crustal evolution and tectonic setting remain unclear or debatable. Kusky and Li (2003) regarded the KB as part of a Paleoproterozoic orogen named the Inner Mongolia-North Hebei Orogen, which they assumed to have formed by collision between the northern margin of the NCC and an exotic arc terrane at ca. 2.3 Ga, and later collided with the Columbia supercontinent at 1.92–1.85 Ga. Zhai et al. (2010) suggested that the KB was similar to the JLJB and also underwent the initial rifting to form incipient oceanic basins followed by the processes of subduction and collision. Santosh et al. (2013) proposed that the high-grade metamorphism younging from ca. 1.95 Ga in the western segment to ca. 1.92 Ga in the eastern segment of the KB, indicates a scissor-like closure of oblique collision between the Yinshan and Ordos Blocks. In recent years, some researchers proposed that the KB underwent long-lived arc-continental accretion along the southern margin of the Yinshan Block during 2.55–2.30 Ga, through the formation resulting from a continent-continent collision the Yinshan and Ordos Block in the Western Block at 1.95–1.85 Ga (Liu J. H. et al., 2017; Wang et al., 2018; Wang et al., 2020; Wang et al., 2021; Wang et al., 2022). Li et al. (2022) established a prolonged magmatic-metamorphic evolution of the KB from subduction to collision, involving ca. 2.3–2.0 Ga subduction-related arc magmatism, ca. 1.95 Ga syn-collisional high-pressure high-temperature crustal anatexis, and ca. 1.92 Ga post-collisional magmatism and synchronous ultrahigh temperature metamorphism.

In the KB, the 2.47–2.39 Ga meta-mafic and felsic rock assemblages from the Daqingshan Complex have only been studied in geochronology (Wang et al., 2021). However, their Hf isotopic and trace element compositions are poorly constrained. It has hindered further understanding of the geodynamic process of the Paleoproterozoic magmatism, which may have played a significant role in the pre-collisional tectonic evolution of the KB. To resolve the crustal evolution and tectonic setting of the KB, we present detailed petrological, zircon Hf isotopic and trace element compositions on the meta-mafic and felsic rock assemblages recognized in the Daqingshan Complex in the northern margin of the KB. Combined with previous study of these assemblages, the results of this study will place important constraints on the crustal evolution and geodynamic process of the KB.

The Khondalite Belt, also named the Fengzhen Orogenic Belt or Inner Mongolia Suture Zone (Santosh et al., 2010), is a ∼1,000 km long east-west-trending belt that extends from the Helanshan-Qianlishan Complex in the west, through the Daqingshan-Wulashan Complex in the middle, to the Jining Complex in the east (Figure 2; Zhao et al., 2005; Guo et al., 2005; Wang et al., 2018; Wang et al., 2020; Wang et al., 2021; Wang et al., 2022).

Previous studies have shown that these complexes are mainly composed of low-to high grade metamorphosed supracrustal rocks (e.g., biotite plagioclase gneiss, garnet quartzite, marble, calc-silicate rock, and graphite gneiss) and TTG gneisses (tonalite-trondhjemite-granodiorite), which are collectively called “Khondalites” or “Khondalite Series” in the Chinese literature (Lu et al., 1996). In recent years, SHRIMP and LA-ICP-MS zircon U-Pb dating of the “Khondalite Series” indicates that they were deposited at 2.4–2.0 Ga, and underwent 1.95–1.85 Ga metamorphism (Zhao et al., 2002; Zhao et al., 2005; Xia et al., 2006; Xia et al., 2008; Xia et al., 2009; Dan et al., 2012; Jiao et al., 2013; Wang et al., 2018; Wang et al., 2020; Wang et al., 2021; Wang et al., 2022). Furthermore, many studies show that the protoliths of the “Khondalite Series” were deposited in the passive continental margin setting (Zhao et al., 2002; Zhao et al., 2005).

The Daqingshan Complex is in the central part of the KB (Figure 2). The Daqingshan Complex is mainly composed of Paleoproterozoic Erdaowa and Mesoproterozoic Majiadian Groups (Figure 3). The Erdaowa Group is dominated by greenschist to amphibolite facies metamorphosed volcanic-sedimentary rock series (Figure 3; Wang et al., 2018; Wang et al., 2020; Wang et al., 2021; Wang et al., 2022), and the Mesoproterozoic Majiadian Group is composed of low-grade metamorphic rocks such as slates, phyllites, and sandstones (Figure 3; Jiao et al., 2015). Recently, Wang et al. (2018), Wang et al. (2020) proposed that the low-grade metamorphic rocks of the Erdaowa Group formed at 2.47 Ga, with metamorphic ages of 1.95 to 1.85 Ga, and their igneous protoliths have an affinity to continental margin arcs. In addition, some late Archean-Paleoproterozoic TTG gneisses and monzogranites are in tectonic contact with the Erdaowa Group and experienced a metamorphic event at 1.95–1.85 Ga (Liu P. H. et al., 2017; Wang et al., 2020).

In this study, we conduct a comprehensive study of representative meta-mafic and felsic rock assemblages from the Daqingshan Complex. The meta-mafic and felsic rock assemblages are composed of two types of rock units. The meta-mafic rock is mainly composed of plagioclase amphibolite, which shows a fine-medium grain crystalloblastic texture and massive structure (Figure 4a). Petrographic observations indicate that plagioclase amphibolite is mainly composed of amphibole (35%–45%), plagioclase (30%–35%), biotite (10%–15%), and other accessory minerals, such as epidote, titanite and pyrite (∼5% in total) (Figures 4b,c). The felsic rock mainly consists of biotite plagiogneiss, which shows a medium or fine-grained crystalloblastic structure and gneissic structure (Figure 4d). The biotite plagiogneiss is mainly composed of plagioclase (±40%), hornblende (±25%), quartz (±20%), and biotite (±15%) (Figures 4e,f).

Cathodoluminescence (CL) images and LA-ICP-MS U-Pb dating of the studied zircons have been published in Wang et al. (2021). Based on the zircon dating results, the same sites for Lu-Hf isotope analyses on the cores of the zircons were selected using a Neptune Plus (MC-ICP-MS) instrument equipped with a New Wave UP21 LA system at the Chengpu Geological Testing Co. Ltd, Langfang, China. The beam diameter of the spot was 44 μm, and the laser repetition rate was 8 Hz at 6 J/cm² for 27 s. The detailed analytical procedures and interference corrections used were similar to those described by Wu et al. (2007). The ε _Hf(t) values were calculated based on the chondrite values of ¹⁷⁶Hf/¹⁷⁷Hf (0.282772) and ¹⁷⁶Lu/¹⁷⁷Hf (0.0332) ratios.

In cathodoluminescence (CL) images, most zircon grains from the meta-mafic and felsic rock assemblages (17DQ7-5, 17DQ32-1, 17DQ55-1, 17DQ67-1) a typical core-rim structure, characterized by oscillating zoning in the core and homogeneous structures in the rim with bright luminescence (Figure 5). Zircon U-Pb dating results showed that the protoliths of meta-mafic and felsic rock assemblages yield crystallization ages ranging from 2.47 to 2.39 Ga and metamorphic ages of 1.91 to 1.83 Ga (Supplementary Table S1; Figure 6). Lu-Hf isotopic compositions by the zircons from the meta-mafic and felsic rock assemblages have been tested in this study (Supplementary Table S2).

Sample 17DQ7-5, represented of the biotite plagiogneiss, was chosen for Lu-Hf isotopic analysis. Twenty-one zircons yield ¹⁷⁶Hf/¹⁷⁷Hf isotopic compositions ranging from 0.281305 to 0.281446, initial ¹⁷⁶Hf/¹⁷⁷Hf ratios between 0.281269 and 0.281424. The results yield ε _Hf(t) values ranging from +2.4 to +7.8 (Figure 7a) and depleted-mantle two-stage zircon Hf model ages (T_DM ^C) of 2,835 to 2,503 Ma (Figure 7b). Twenty-one zircons from the biotite plagiogneiss sample (17DQ32-1) have ¹⁷⁶Hf/¹⁷⁷Hf isotopic compositions dominantly between 0.281311 and 0.281474, initial ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.281284 to 0.281462 (Supplementary Table S2). Their ε _Hf(t) values and depleted-mantle two-stage zircon Hf model ages (T_DM ^C) are of +2.7 to +9.1 (Figure 7a) and 2,808 to 2,419 Ma (Figure 7b), respectively. Thirty zircons from the plagioclase amphibolite sample (17DQ55-1) yield ¹⁷⁶Hf/¹⁷⁷Hf isotopic compositions ranging from 0.281285 to 0.281536, initial ¹⁷⁶Hf/¹⁷⁷Hf ratios between 0.281263 and 0.281527 (Supplementary Table S2). The data show ε _Hf(t) values in the range of +0.3 to +9.7 (Figure 7a) and depleted-mantle two-stage zircon Hf model ages (T_DM ^C) of 2,899 to 2,428 Ma (Figure 7b). The ¹⁷⁶Hf/¹⁷⁷Hf isotopic compositions of thirty zircon grains from the plagioclase amphibolite (17DQ67-1) are mainly concentrated between 0.281277 and 0.281432 (Supplementary Table S2). The initial ¹⁷⁶Hf/¹⁷⁷Hf ratios scatter between a range of 0.281234–0.281395 (Supplementary Table S2). The data yield ε _Hf(t) values ranging from −0.3 to +5.4 (Figure 7a) and the depleted-mantle two-stage zircon Hf model ages (T_DM ^C) of 2,951 to 2,616 Ma (Figure 7b).

(1) The zircon Lu-Hf isotopic composition indicates that the protoliths of the 2.47–2.39 Ga meta-mafic and felsic rock assemblages in the Daqingshan Complex originate from depleted lithospheric mantle and/or juvenile continental crust.