Tectonic Evolution of the JLJB, North China Craton, Revisited: Constraints from Metamorphism, Geochemistry and Geochronology of the Ji’an Group and Related Granites

Abstract

The Jiao-Liao-Ji Belt (JLJB) is the most representative Paleoproterozoic orogenic belt in the North China Craton (NCC). The sedimentation, metamorphism and magmatism of the Ji’an Group and associated granites provide significant insights into the tectonic evolution of the JLJB. In this study, we have synthesized published geochemistry and geochronology data on metasedimentary, metavolcanic and igneous rocks. According to the available data, the protoliths of the metasedimentary rocks are sets of shale, wacke, arkose, quartz sandstone and carbonate, while the protoliths of the metavolcanic rocks are calc-alkaline basalt, basaltic andesite, andesite, dacite and rhyolite. The rock assemblages indicate a transformation of the tectonic environment from a passive margin to an active continental margin following the onset of plate convergence and subduction. The A2-type gneissic granite (Qianzhuogou pluton) is formed in a subsequent back-arc basin extension setting at 2.20–2.14 Ga. The Ji’an Group was finally deposited in an active continental margin during the closure of a back-arc basin at 2.14–2.0 Ga. Then, the sediments were involved in a continent–arc–continent collision between the Longgang and Nangrim blocks at ~1.95 Ga. This process was accompanied by HP granulite-facies metamorphism at ~1.90 Ga. The subsequent exhumation and regional extension resulted in decompression melting during 1.90–1.86 Ga, producing metamorphism with an isothermal decompression clockwise P–T path. The resulting metapelites are characterized by perthite + sillimanite, and mafic granulites are characterized by orthopyroxene + clinopyroxene. The S-type porphyritic granite (Shuangcha pluton) is formed during the crustal anatexis. Meanwhile, extensive anatexis produced significant heating and triggered prograde to peak metamorphism with an anticlockwise P–T path. Cordierite-bearing symplectites around the garnet in the metapelites indicate a superposed isobaric cooling metamorphism. The ages of monazites and anatectic zircons suggest that the post-exhumation cooling occurred at 1.86–1.80 Ga. The Paleoproterozoic magmatism, sedimentation and metamorphism suggest a process of subduction back-arc basin extension and closure, collision and exhumation for the tectonic evolution of the JLJB.

1. Introduction

The supercontinent cycle of continental assembly and breakup plays a crucial role in governing mantle dynamics and crustal growth. The Columbia/Nuna, which formed in the Paleoproterozoic, is one of the oldest supercontinents. Orogens from 2.1 to 1.8 Ga have been recognized on almost all continents, including the Transamazonian Orogen of South America, the Eburnean Orogen of West Africa, the Capricorn Orogen of Western Australia, the Transantarctic Mountains Orogen of Antarctica, the Trans-North China Orogen in North China, etc. [1,2,3]. As one of the oldest existing cratons, the North China Craton (NCC) records multiple tectonic, magmatic and metamorphic events [4,5,6,7]. Since the discovery of 3.8 billion-year-old rocks in the craton [8], the NCC has been the focus of extensive research. An intensive study of the ancient orogenic belt (Neo-Archean–Paleoproterozoic) within the NCC is of great significance for elucidating the breakup and assembly of the continent [9,10]. The Jiao-Liao-Ji Belt (JLJB) is one of the most representative Paleoproterozoic orogenic belts in the NCC. While it is widely accepted that the JLJB was formed by the collision between the Longgang and Nangrim blocks, ongoing debates exist about the belt’s tectonic nature (rift versus collision models; e.g., [6,11,12]). A comprehensive understanding of the Paleoproterozoic sedimentation, metamorphism and magmatism is key to resolving this controversy [9,13,14,15,16,17,18,19,20]. The widely exposed meta-igneous and metasedimentary rocks (schist, gneiss, marble, felsic gneiss, granulite and amphibolite) of the Ji’an Group, as well as the related voluminous granites (gneissic granite and porphyritic granite), provide evidence of multiple metamorphic–deformational and magmatic–tectonic events, which could serve as strong constraints on the tectonic evolution of the JLJB.

In this study, we summarize the geochemical and geochronological data of the metasedimentary and meta-igneous rocks in the Ji’an Group, as well as associated granites (sample details are listed in Appendix A). The analytical results allow us to discuss the source property, protolith, stratigraphy, petrogenesis, metamorphic evolution, nature of the magma and geochronology outline of the JLJB. After reviewing the current state of research, we propose a complete tectonic cycle, including subduction, back-arc extension, the closure of the back-arc basin, collision, post-collisional extension and exhumation, which may provide crucial insights into unraveling the evolution of the JLJB.

2. Geological Background

The NCC can be subdivided into the Western Block and the Eastern Block, as well as three Paleoproterozoic orogenic belts known as the Jiao-Liao-Ji Belt, the Trans-North China Orogen belt and the Khondalite belt [5,7]. The JLJB is located between the Nangrim and Longgang blocks in the eastern NCC (Figure 1) [11]. It consists mainly of metasedimentary rocks, meta-igneous successions and igneous rocks, including Archean–Paleoproterozoic TTG gneisses, greenschist–amphibolite–granulite facies metasedimentary rocks, various granites (gneissic granite, alkaline granite, calc-alkaline granite, porphyritic granite, etc.), bimodal volcanic rocks, mafic dykes (veins) and andesitic–rhyolitic tuffs [9,21,22,23]. The metasedimentary and meta-igneous successions consist of the Ji’an Group and Laoling Group in southern Jilin, the North Liaohe Group and South Liaohe Group in eastern Liaoning, the Jingshan Group and Fenzishan Group in eastern Shandong, and the Wuhe Group and Fengyang Group in Anhui Province (Figure 1b; [6,10]). Some scientists argue for an intracontinental rift model for the evolution of the JLJB based on the occurrence of bimodal volcanic rocks and A-type granites, coupled with the anticlockwise P–T paths [24,25,26], while others suggest a continent–arc or continent–continent collision model based on the occurrence of 2.2–2.1 Ga mafic–felsic intrusions and clockwise P–T paths of Paleoproterozoic metamorphism [27,28]. Recent studies have provided two more compelling models involving a rift–subduction–collision cycle [10,12,29,30] and a back-arc basin or retro-arc foreland basin setting [9,31,32].

The Ji’an Group, distributed in the northeast of the Paleoproterozoic JLJB, is mainly exposed in Tonghua, Jilin Province (Figure 2). It is mainly composed of aluminous schist, pelitic gneiss, felsic gneiss, granulite, interlayered marble and thin-bedded quartzite [11]. Regionally, it can be compared with the South Liaohe Group, Jingshan Group and Motianling Group. The Ji’an Group is divided into Mayihe, Huangchagou and Dadongcha formations from the bottom upwards (Figure 3). The sediments are considered to have been deposited in an active continental margin environment at 2.2–1.9 Ga [33,34,35] and generally experienced greenschist to low-amphibolite facies metamorphism, with some parts of the sequence attaining high-amphibolite to granulite facies [11,35,36,37].

The Mayihe Formation is characterized by boron-bearing felsic gneisses and is mainly distributed in Jiayichuan, Huadianzi, Minshan and Wenzigou in the Ji’an area. The lower section of the formation is characterized by amphibolites and felsic gneisses. The middle section consists of serpentinized, dolomitic marbles and a few felsic gneisses and amphibolites. The upper section is dominated by tourmaline-bearing felsic gneisses, tourmaline-bearing quartzites and mica schists (Figure 3).

The Huangchagou Formation is characterized by graphitic rocks and is mainly distributed in Sanbanjiang, Quanyangou, Yaoyingzi, Toudao, Qinghe and Wenzigou in the Tonghua area. The lower section is characterized by graphitic felsic gneisses, interlayered graphitic garnet–biotite schist-gneisses, amphibolites, etc. The middle section is dominated by amphibolites, interlayered graphitic felsic gneisses, graphitic mica schist-gneisses and graphitic marbles. The upper section is mainly composed of graphitic felsic gneisses, graphitic marbles, interlayered graphite-mica schists, graphitic calcium–magnesium silicate rocks, mica schists and amphibolites.

The Dadongcha Formation is characterized by aluminum-bearing gneisses and is mainly distributed in the towns of Toudao and Qinghe. The lower section is characterized by garnet felsic gneisses, interlayered quartzites, garnet–mica schists and garnet–sillimanite–plagioclase gneisses. The upper section is dominated by garnet–sillimanite–plagioclase gneisses, garnet–cordierite–plagioclase gneisses and interlayered felsic gneisses.

In addition, voluminous Palaeoproterozoic igneous rocks are associated with the metasedimentary and meta-igneous successions in the Ji’an Group, mainly including the Qianzhuogou pluton and Shuangcha pluton [38]. The Qianzhuogou pluton is mostly in contact with the Huangchagou Formation. It is mainly composed of gneissic monzonitic granite and syenogranite and displays geochemical signatures of A2-type granite [38]. The deformed A2-type granite of 2.2–2.1 Ga is usually interpreted as being emplaced during an oceanic plate subduction environment [28,39]. The Shuangcha pluton mostly intruded into the Dadongcha Formation. It is characterized by abundant potassium feldspar and garnet phenocrysts and is known as porphyritic granite. The undeformed porphyritic granite was predominantly emplaced at ca. 1.88–1.85 Ga, suggesting a post-collisional or post-orogenic extensional setting [38,39].

3. Representative Petrography and Microstructures

The greenschist–amphibolite–granulite facies metasedimentary and meta-igneous successions are widespread in the Ji’an Group. Within these successions, the pelitic garnet–sillimanite–cordierite–biotite gneisses and mafic clinopyroxene–orthopyroxene granulites preserve mineral assemblages consistent with granulite-facies metamorphism. The garnet–sillimanite–cordierite–biotite gneisses consist mainly of garnet (10%–15%), plagioclase (10%–15%), potassium feldspar (15%–25%), quartz (15%–20%), cordierite (20–25%), biotite (8%–10%), sillimanite (5%–8%) and small amounts of magnetite and ilmenite (1%–2%). Most garnet porphyroblasts are sieve-shaped, elongated or rounded, with a grain size of about 0.5–4 mm. Fibrous sillimanite, fine-grained biotite, quartz and ilmenite/magnetite inclusions can be found in garnet. In the matrix, acicular sillimanite, biotite, potassium feldspar and plagioclase are arranged discontinuously forming a gneissic structure. Symplectic cordierites form rims around the garnets (Figure 4c–f). The Cpx-Opx granulite is characterized by a mineral assemblage of coarse-grained clinopyroxene, orthopyroxene, garnet, amphibole, biotite, plagioclase and quartz. Clinopyroxene is dominated by diopside. Amphiboles form rims around the clinopyroxene. Inclusions of biotite grains can be observed within the clinopyroxene [37]. In addition, field observations show that felsic melts of varying sizes, irregular veinlets, reticulate veins and lenses of group distribution in the metapelites are associated with anatexis (Figure 4a,b).

4. Metamorphic Evolution of the Ji’an Group

The metamorphic evolution of the JLJB is now reasonably well known. Previous investigations have shown that the the Jingshan, Fenzishan and Wuhe Groups in the Jiaobei area underwent granulite-facies metamorphism. In contrast, the metamorphism of the Liaohe and Ji’an Groups in the Liaoji area only reached greenschist–amphibolite facies [11,22]. However, in recent years, granulite facies mafic rocks and metapelites have been identified in the Ji’an Group [37] and the South Liaohe Group [17].

The process of metamorphism can be divided into three stages: prograde, peak and retrograde. The mineral assemblages and the metamorphic reactions differ in each stage, but the prograde reactions are often overprinted by the retrograde reactions. Based on the petrography, mineral chemistry and phase equilibria modeling, we present a careful review of the metamorphic evolution of the felsic–mafic granulites in the Ji’an Group [36,37].

4.1. Peak Stage

The Cpx-Opx mafic granulite is characterized by a mineral assemblage of coarse-grained clinopyroxene, orthopyroxene, garnet, amphibole, biotite, plagioclase and quartz. Possible reactions include the following: Hb + Pl → Cpx + Opx + Hb + Pl + H₂O; Hb + Qtz → Cpx + Opx + Pl + H₂O; Opx + Pl → Cpx + Grt + Qtz.

The metapelite is characterized by a mineral assemblage of sillimanite, biotite, plagioclase, K-feldspar, perthite, quartz and garnet. The garnet rims are replaced by large biotite, potassium feldspar, sillimanite and quartz grains. Garnet may grow continuously via the consumption of sillimanite and biotite. Possible reactions include the following: Bt + Sil + Qtz ± Pl→Grt ± Kfs ± Melt [40]; Bt + Pl + Qz → Grt + Melt [41].

4.2. Retrograde Stage

The Cpx-Opx mafic granulite is characterized by a mineral assemblage of orthopyroxene, clinopyroxene, biotite, plagioclase and quartz. The coarse-grained orthopyroxene is present in the garnet relict. The possible reactions include the following: Grt + Qz → Opx + Pl [41]; Bt + Pl + Qz → Opx + Grt + Melt.

The metapelites are characterized by a mineral assemblage of cordierite, sillimanite, biotite, plagioclase, quartz and garnet (rim). Garnets are rimmed by the coarse-grained cordierite and a symplectic texture (cordierite + sillimanite + quartz). Possible reactions include the following: Grt + Sil + Qz → Crd; Grt + Sil + Melt → Crd + Bt + Fe-Oxide.

4.3. P–T Paths

Systematic petrographic observations, geothermobarometry (Grt-Bt and Grt-Crd) and pseudosection thermobarometry (Thermocalc and Perplex) have been used to estimate the P–T conditions of different metamorphic stages of the Ji’an Group. Conventional geothermobarometry suggests that the P–T condition of the garnet–cordierite–biotite gneiss is ~750–700 °C and ~0. 65–0.52 GPa, which was previously attributed to amphibolite facies metamorphism with anticlockwise P–T paths [24]. However, recent phase equilibria models of some metapelites in the Ji’an Group limit the the P–T conditions of the peak stages to ~1.0–0.7 GPa/890–820 °C and the retrograde stages to ~0.7–0.5 GPa/760–620 °C, which have been attributed to granulite facies metamorphism with clockwise P–T paths (Figure 5, [18,35,36,37]).

5. Geochemistry of the Ji’an Group

5.1. Meta-Igneous Rocks

A large number of meta-igneous rocks are found in the lower section of the Ji’an Group, including pyroxene amphibolites, amphibole–plagioclase gneisses, biotite–plagioclase gneisses and felsic gneisses (Figure 6a) [43]. They display medium- to fine-grained granoblastic textures with subhedral–anhedral pyroxene, biotite and feldspar. Considering that the samples may have undergone dehydration and metamorphism, the mobile components cannot be used to determine the properties of the original rocks. The Nb/Y–Zr/TiO₂*0.0001 diagram is effective in evaluating the original properties of the meta-igneous rocks. As shown in Figure 6b, the protoliths of the meta-igneous rocks consist mainly of calc-alkaline basalt, basaltic andesite, andesite, dacite and rhyolites.

5.2. Metasedimentary Rocks

The Ji’an Group is characterized by thick successions of metasedimentary rocks, including aluminous schist, pelitic gneiss, felsic gneiss, granulite, interlayered marble and thin-bedded quartzite. The metasedimentary rocks of the Ji’an Group are generally enriched in Al₂O₃, depleted in CaO and FeO_T, have K₂O/Na₂O values of >1 and contain garnet and cordierite, all of which are consistent with a metasedimentary origin. The chemical composition of sedimentary rocks depends on the composition of its source rocks. Here, we reconstruct the source properties by some specific discrimination diagrams [47] based on the geochemical data in the literature [33,35,46]. In the (al + fm)–(c + alk) diagram (Figure 6c), the data fall in the area of the pelitic sedimentary rocks. Combined with the sediment assemblage and information from the log (Fe₂O₃/K₂O)–log (SiO₂/Al₂O₃) diagram (Figure 6d), the protoliths of metasedimentary rocks are mainly shale, wacke, arkose, quartz sandstone and carbonate.

6. Geochronology of the Ji’an Group and Related Granites

The Paleoproterozoic JLJB has a complex origin and underwent multistage evolution. The geochronology of the metasedimentary rocks and related granites can provide significant constraints on the formation of the JLJB. Based on the published zircon isotope geochronology data, zircon trace element data (e.g., Th and U) and the geochronological outline of the metamorphism and magmatism in the Ji’an Group [9,34,35,38,43,48], we provide constraints in the petrogenic age of the protoliths (detrital zircons with Th/U > 0.4), the metamorphic age of the metasedimentary rocks (metamorphic zircons with Th/U < 0.1) and the Paleoproterozoic magmatism (magmatic zircons with Th/U > 0.4). The metamorphic zircons in the Ji’an Group suggest that the metamorphism can be divided into two periods of 1950–1870 Ma and 1870–1800 Ma, with peak ages of 1901 Ma and 1860 Ma, respectively (Figure 7c). The ages of metamorphic zircons are consistent with the metamorphic events at 1.90 and 1.85 Ga suggested by Meng et al. [43]. Additionally, the younger ages down to 1800 Ma may indicate a cooling stage. The detrital zircons in the Ji’an Group show four statistical ages of 2191–2138, 2120–2084, 2048–1995 and 1887–1852 Ma, with peak ages of 2670 and 2460 Ma (Figure 7d). These data suggest four periods of magmatism in the Paleoproterozoic JLJB. The magmatic zircons of the porphyritic granite (Shuangcha pluton) are mainly concentrated in 1887–1852 Ma with ages of ~2175 Ma and ~2625 Ma. The magmatic zircons of the gneissic granite (Qianzhuogou pluton) yield ages of 2200–1800 Ma with peaks at 2191–2138 Ma. Consequently, we conclude that the gneissic granite was formed at 2191–2138 Ma, with a few zircon records of later magmatic events. The porphyritic granite was formed at 1887–1852 Ma and preserved inherited zircons with ages of ~2175 Ma and ~2625 Ma (Figure 7a,b).

In this study, the maximum peak age of metamorphic zircons in the metasedimentary rocks was used to represent the minimum depositional age (>1901 Ma). Combined with the minimum peak ages of detrital zircons, the deposition ages for the Mayihe, Huangchagou and Dadongcha formations can be constrained to be 2141–1946, 2117–1917 and 2017–1917 Ma, respectively (Figure 8). These ages are similar to the depositional ages suggested by other authors [33,34,35].

7. Discussion

7.1. Magmatism

The detrital zircons (Th/U > 0.4) in the metasedimentary and meta-igneous rocks, as well as the magmatic zircons in the granites, have preserved consistent ages ranging from 2200 Ma to 1800 Ma, with four periods of magmatism at 2191–2138 Ma, 2120–2084 Ma, 2048–1995 and 1887–1852 Ma. Surprisingly, these magmatic events were also recorded in Paleoproterozoic igneous rocks in the JLJB, as follows: ~2190–2160 Ma: calc-alkaline andesitic–rhyolitic tuffs, A2-type gneissic monzogranite, syenogranite and albite granite [9,38,49]; ~2160–2110 Ma: tholeiitic mafic rocks, metagabbro/diabase and amphibolite [31,32,50]; ~2110–2080 Ma: K-feldspar granite, albite granite and monzogranite [51,52,53,54]; ~2000 Ma granites [9]; and ~1870 Ma: granitoids, porphyritic granite and intermediate alkaline rocks [38,51,55]. The composition of these igneous rocks and metavolcanic rocks can be used to assess potential tectonic environments. The calc-alkaline igneous associations (andesitic–rhyolitic tuffs) can be associated with an earlier subduction event [9]. The metavolcanic rocks from the lower part of the Ji’an Group exhibit continuously varying compositions from basalt and andesite to rhyolite, which are typical assemblages of continental volcanic arcs [56]. The gneissic granite (Qianzhuogou pluton) is a highly fractionated, aluminous A2-type granite, which is probably produced in an extensional setting caused by slab rollback during the early stage of subduction. The tholeiitic mafic rocks are related to a back-arc extension [31]. The porphyritic granite (Shuangcha pluton) is an S-type granite formed in a post-collisional extension [9]. To further confirm the tectonic setting of the igneous rocks, the chemical compositions of granites were plotted on the tectonic discrimination diagrams suggested by Pearce et al. [57]. As shown in the Nb–Y tectonic-setting discrimination diagram (Figure 9a), the porphyritic granite (Shuangcha pluton) and gneissic granite (Qianzhuogou pluton) are plotted in the volcanic arc and within plate granite fields. In the Th/Yb–Ta/Yb diagram, metavolcanic rocks fall mainly in the active continental margin and within-plate volcanic zones (Figure 9b). It is speculated that the regional igneous rocks are productions of magmatism under a subduction–back-arc extension–collision system that occurred between 2200 and 1800 Ma.

7.2. Sedimentation

The Ji’an Group consists of thick metasedimentary and meta-igneous successions. Special immobile major and trace elements can be used to discuss protolith composition, provenance and tectonic settings [47]. In the SiO₂/Al₂O₃–K₂O/Na₂O diagram, most samples are plotted in an active continental margin (ACM), and a small number are plotted in the passive margin (PM) and the evolved island arc (A2) area (Figure 9a [47]). In the Th–Sc–Zr/10 discrimination diagrams [47], the samples are mainly plotted in the continental arc (CA) and ACM areas (Figure 9b). The protoliths of the metasedimentary rocks are sets of shale, wacke, arkose, quartz sandstone and carbonate (Figure 6d). The terrigenous sediments may have formed along a passive margin. Moreover, the La/Th-Hf source rock discrimination diagram suggests that the materials were mainly from an arc source with the addition of ancient sediments (Figure 10; [58]). Combined with information from the tectonic setting diagrams, the sediments in the JLJB may have undergone a transformation of the tectonic environment from the passive margin to an active continental margin.

The U–Pb ages of detrital zircons are potential indicators of sedimentary provenance and crustal evolution [59]. The detrital zircons in the metasedimentary rocks sampled in the JLJB yield ages of 2191–1995 Ma (Figure 7d). Considering that there are 2.2–2.06 Ga A-type granites and high-K calc-alkaline granites in the Ji’an Group [10], we speculate that the Paleoproterozoic granites were the main material source. According to records of the Neo-Archean metamorphic basement in the Longgang block [11], a few ages of 2.5 Ga, 2.7 Ga, 3.1 Ga and 3.5 Ga may indicate the contribution of fragments of the Neo-Archean metamorphic granitic gneiss to the Ji’an Group. These ages could also result from sediment recycling. The recycling of detrital zircons does not alter the U–Pb age spectra of detrital zircon populations [60]. However, the relative probability diagrams for detrital zircon age data obtained from the Ji’an Group and sedimentary rocks in the JLJB [9] display distinct age spectra.

In addition, zircon Hf isotopes can effectively reflect the source properties (juvenile crust, ancient crust or depleted mantle; [61]). This paper summarizes the zircon Hf isotopes of miscellaneous metamorphic rocks and granites in the Ji’an Group. According to the εHf (t)-T diagram, the metasedimentary rocks of the Ji’an Group have εHf values between −10 and 5, which mainly originate from the recycling of an ancient crust, including Paleoproterozoic granites, coeval differentiated volcanic rocks and small amounts of Archean granites. The metavolcanic rocks of the Ji’an Group have positive εHf values of 2–7 and show low SiO₂ and high contents of FeO^T, MgO, CaO, Cr, Co and Ni, which were predominantly derived from the depleted mantle [33,42,43]. The gneissic granites (Qianzhuogou pluton) have εHf values of between −3 and 3, similarly to some of the metamorphic rocks. We infer that the gneissic granites are derived from both Archean TTG gneisses and the metamorphic basement of the ancient upper crust. The porphyritic granites show similar εHf values to metasedimentary rocks and are more likely to be derived from productions of the partial melting of metapelites (Figure 11). The resulting partial melting of the mantle and crust may reflect a convergent setting.

The convergent settings have a high proportion of detrital zircons (statistically generally greater than 50%) with ages close to the age of the sediment. Combined with stratigraphy, tectonics and geochemistry, high-quality detrital zircon spectra can reflect the tectonic setting [62]. Stratigraphically, the Ji’an Group can be associated with the Laoling Group. The Ji’an Group is located in the south of the Tonghua area and is mainly composed of aluminous schist, pelitic gneiss, felsic gneiss, granulite and interlayered marble and quartzite. The Laoling Group is mainly located in the north of the Tonghua area. The rock types of the Dataishan and Zhenzhumen formations in the Laoling group mainly include quartzite, felsic granulite and marble. We have compiled detrital zircon data from the metasedimentary rocks in the Ji’an Group and plotted them with depositional ages of 2140, 2120 and 2020 Ma from the Mayihe, Huangchagou and Dadongcha formations [33,34,35]. The dataset suggests that the Ji’an Group was formed in a convergent setting (CA − DA < 100 Ma at 30% of the zircon population). Furthermore, detrital zircon ages closely resemble the Mt Isa basin (Figure 12), which is a typical back-arc basin setting [63]. Detrital zircon data from the Dataishan and Zhenzhumen formations reflect an extensional setting (CA − DA > 150 Ma at 5% of the zircon population), such as a passive margin (Figure 12). Combined with the stratigraphic rock assemblage and detrital zircon, the Ji’an and Laoling Groups were deposited at the same time in different areas during the early stage and later switched to deposit in layers with each other. During the early stage, the Ji’an Group was deposited on the continental arc side, and the Laoling Group was deposited on the passive margin side. During the late stage, both groups were deposited in a back-arc basin setting [20].

7.3. Metamorphism and Anatexis

Some studies on the metamorphic evolution of the Ji’an and South Liaohe Groups obtained low-pressure near-isobaric cooling (IBC) anticlockwise P–T paths, which were previously attributed to a post-orogenic thermal event related to the underplating of mantle magmas [24], whereas recent studies on the Al-rich gneisses showed that they experienced granulite-facies metamorphism and display a near-isothermal decompression (ITD) clockwise P–T path, which is better explained by a subduction/collision model rather than a single rift model [37,64,65]. Whether the metamorphic evolution of the JLJB is a clockwise or an anticlockwise P–T path (an isobaric cooling or an isothermal decompression) remains controversial.

As the metamorphic zircons in the Ji’an Group show (Figure 7c), there may have been two periods of metamorphic events during the formation of the JLJB. Four periods of magmatism and the 1.90–1.80 Ga syn-collisional granites also indicate the possibility of more than one metamorphic event. In addition, the garnet on the prograde clockwise path and cordierite-bearing assemblages on the post-exhumation cooling anticlockwise path fit better with the petrography and growth of cordierite after garnet. These results suggest the existence of superposed clockwise and anticlockwise P–T paths in the JLJB due to the temporal and spatial disparities in an orogenic system [66]. During 1.95–1.90 Ga, the collisional assembly of the Longgang and Nangrim blocks within the NCC was associated with an HP granulite-facies metamorphism. The subsequent exhumation of the orogenic root and regional extension resulted in decompression melting during 1.90–1.86 Ga [67], producing metamorphism with an isothermal decompression clockwise P–T path. Meanwhile, the ascent of the orogenic root was compensated by the contemporaneous descent of part of the upper–middle crustal materials [66]. These downward-moving materials were heated by synchronous extensive anatexis, hot orogenic root and shear heating, triggering the significant heating with a pressure increase and producing metamorphism with an isobaric cooling anticlockwise P–T path at 1.86–1.80 Ga.

Anatexis is a common consequence of high-grade metamorphism. With the discovery of 1.90–1.86 Ga granulite-facies metamorphism in the Ji’an Group, regional anatexis is also found throughout the Ji’an Group [11]. The ages of the anatectic zircons in the Ji’an Group and Jiaobei area suggest that anatectic melt crystallization (cooling stage) occurred at 1.86–1.84 Ga [11,35]. In addition, the U–Pb dating of monazites yielded a peak age of 1.85 Ga [36], which is also interpreted as the timing of melt crystallization during isobaric cooling. All of these findings suggest that the metamorphic history can be reduced to a granulite-facies peak metamorphism, followed by a near-isothermal decompression, and finally a post-exhumation isobaric cooling to amphibolite-facies retrograde metamorphism.

7.4. Evolution of the JLJB

The JLJB has experienced multiple stages of metamorphic–deformational and magmatic–tectonic events [9,14,15,16,17,19,20,34,36,37,49,54]. These events have made reconstructing the tectonic evolution of the JLJB extremely challenging. However, recent intensive studies have allowed for the creation of a broad summary of the tectonic evolution model of the JLJB, consisting of the following four main models: (1) the intracontinental rift opening and closing model based on the occurrence of A-type granites, bimodal volcanics, basic intrusions and metamorphism with anticlockwise P–T paths [25,26,39,49,52,53,54,68,69,70,71]; (2) the continent–arc–continent collision model supported by high-pressure granulite, volcanic arc rocks, basic dikes and clockwise P–T paths [27,28,31,52,71,72]; (3) the rift–subduction–collision cycle model based on the HP pelitic granulites, clockwise P–T paths and a basin-controlling boundary fault [10,12,29,73,74,75]; (4) the back-arc basin opening and closing model supported by the geochemical studies of ca. 2.2–2.1 Ga mafic–granitic intrusions and detrital zircons of sedimentary rocks within the JLJB [31,32,50,55,76,77].

To resolve the controversy, we discuss these proposed constraints by contrasting them with those possibilities based on the geochronology and tectonic setting of the sedimentation, metamorphism and magmatism in the JLJB. The ~2190 Ma calc-alkaline basalt–andesite–rhyolites (protoliths of the meta-igneous rocks) formed a continuous magmatic sequence rather than bimodal volcanic rocks formed in an intracontinental rift. The ~2160 Ma gneissic granites were A2-type granites formed in a back-arc extensional environment rather than A1-type granites formed in a rift environment. The ~2140–1950 Ma sedimentary rocks were deposited in a passive margin-back-arc basin setting rather than in a rift. The ~1900–1800 Ma metamorphic events show granulite-facies metamorphism with a clockwise isothermal decompression P–T path, followed by amphibolite-facies metamorphism with an anticlockwise isobaric cooling P–T path, implying that the JLJB underwent a subduction–collision–extension rather than a single rift model.

Combining these observations with the sedimentation, metamorphism and magmatism identified in the JLJB, we suggest a subduction–back-arc basin extension–collision–exhumation model for the evolution of the JLJB. The gneissic granite (Qianzhuogou pluton) and protoliths of the meta-igneous rocks (e.g., calc-alkaline basalt, andesite, dacite and rhyolite) in the Ji’an Group were formed in a subduction–back-arc basin extensional environment at ~2.2–2.16 Ga. Following this, a sequence of shales, wakes and arkoses were deposited in a continental back-arc basin at ~2.14 Ga, related to the protoliths of the metasedimentary rocks of the Ji’an Group. Later, these sediments were involved in the continent–arc–continent collision between the Longgang and Nangrim blocks at ~1.95 Ga, at which point the peak metamorphism occurred at 1.90 Ga, and isothermal decompression melting occurred during the post-collisional extension at 1.86 Ga. Meanwhile, the porphyritic granite (Shuangcha pluton) was formed in the resulting extensional environment. Finally, the post-exhumation isobaric cooling stage occurred at 1.86–1.80 Ga (Figure 13).

8. Conclusions

Based on a systematic investigation of the previous sources, this paper presents an overview of the formation and evolution of the Ji’an Group and the JLJB. The protoliths of the metasedimentary rocks of the Ji’an Group consist of shale, wacke, arkose, quartzite and carbonate, while the meta-igneous rocks consist of continuous calc-alkaline basalt–andesite–dacite–rhyolite. These sediments were formed in an active continental margin and back-arc environment at 2.2–2.0 Ga and were involved in a continent–arc–continent collision between the Longgang and Nangrim blocks in the NCC at ~1.95 Ga. The collision coincidentally led to the closure of the back-arc basin and resulted in regional HP metamorphism at ~1.90 Ga, followed by isothermal decompression melting. The crustal anatexis triggered the 1.86 Ga MP metamorphism and subsequent post-exhumation isobaric cooling at 1.86–1.80 Ga.