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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. 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.
- Ji’an Group
- geochronology
- magmatism
- metamorphism
- Jiao-Liao-Ji Belt
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][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][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][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][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][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.
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][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][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][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][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][27,28].Figure 1. Simplified geological maps of the North China Craton (NCC) and the Jiao-Liao-Ji Belt (JLJB). (a) Tectonic setting of the NCC; (b) regional Precambrian geological map of the Eastern Block in the NCC (modified after [6]).
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).
Figure 2. Distribution of the Paleoproterozoic Ji’an Group and Laoling Group and associated granites in the Tonghua area (modified after [22]).
Lithostratigraphic units of the Ji’an Group (modified after [33]).
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.
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 [30][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).Figure 4. Representative field photographs and photomicrographs of the metapelites. (a,b) Garnet–sillimanite–cordierite–biotite gneiss; (c–f) elongated porphyroblastic garnet associated with matrix sillimanite, biotite, feldspar and quartz. The fibrous sillimanite is enclosed in garnet, and degenerative cordierite appears in the matrix. Data from [31][35].
4. Metamorphic Evolution of the Ji’an Group
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 [32][40]; Bt + Pl + Qz → Grt + Melt [33][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 [33][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].
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 56a) [34][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 56b, the protoliths of the meta-igneous rocks consist mainly of calc-alkaline basalt, basaltic andesite, andesite, dacite and rhyolites.Figure 56. Classification diagrams for the metasedimentary and metavolcanic rocks. (a,c) (al + fm) − (c + alk) − Si, al = [Al2O3], fm = [FeO] + 2[Fe2O3] + [MnO] + [MgO], c = [CaO], alk = [K2O] + [Na2O] [35][44]; (b) Nb/Y versus Zr/TiO2*0.0001; (d) log (Fe2O3/K2O) versus log (SiO2/Al2O3) [36][45]. Data sources: [29][31][34][37][38][33,35,38,43,46].
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 FeOT, 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. In the (al + fm)–(c + alk) diagram (Figure 56c), 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 56d), 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][31][34][37][39][40][9,34,35,38,43,48], scholars 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 67c). The ages of metamorphic zircons are consistent with the metamorphic events at 1.90 and 1.85 Ga suggested by Meng et al. [34][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 67d). 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, scholars 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 67a,b).Figure 67. Age spectra (Ma) for zircons from the Ji’an Group and related granites. (a) Magmatic zircons of the porphyritic granite (Shuangcha pluton); (b) magmatic zircons of the gneissic granite (Qianzhuogou pluton); (c) metamorphic zircons of the Ji’an Group; (d) detrital zircons of the Ji’an Group. Data from [9][31][34][37][39][40][9,34,35,38,43,48].
