Palaeoproterozoic tectonothermal evolution and deep crustal processes in the Jiao-Liao-Ji Belt, North China Craton: a review

Authors


Abstract

The Palaeoproterozoic Jiao-Liao-Ji Belt is located in the eastern margin of the Eastern Block of the North China Craton. In this paper, we synthesize the tectonothermal evolution and deep crustal processes in the Jiao-Liao-Ji Belt based on recent information. A mantle plume-related underplating from 2.53 to 2.36 Ga is envisaged which led to the emplacement of the 2.47–2.33 Ga alkali granite plutons and mafic dyke swarms, followed by the development of the Jiao-Liao-Ji Rift and bi-modal volcanism. The underplating resulted not only in different sedimentary environments in the upper crust, but also in a differentiation of the initial thermal structure in the rift. This controlled the metamorphism and style of P-T-t paths in the different parts of the rift. Subsequent underplating resulted in the emplacement of the A-type Liaoji granites of ca. 2.17 Ga in the lower crust, and the formation of associated pegmatites of 2.2 and 2.0 Ga, together with the development of a bedding-parallel extension. However, the main orogeny occurred between 1.93 and 1.88 Ga with closing of the rift, compressional deformation and high-pressure granulite metamorphism in the southern part of the orogen. Subsequently, lithospheric blocks were possibly delaminated at ∼1.85 Ga; anorogenic magmatic rocks such as rapakivi granite, alkaline granites and syenite were intruded, and pegmatite veins and mafic dyke swarms were emplaced cross-cutting all the earlier structural traces. We identify that the underplating styles, collision processes and delamination types in the deep lithosphere controlled the tectonothermal evolution of the crust in the Jiao-Liao-Ji region. Copyright © 2011 John Wiley & Sons, Ltd.

INTRODUCTION

The Palaeoproterozoic marks an important period in Earth history when the geodynamics and thermal history of the Earth changed dramatically. Several important geological events occurred during this period, such as the assembly of the first coherent supercontinent Columbia, associated surface environmental change and evolution of primitive life forms (Rogers and Santosh, 2002, 2009; Zhao et al., 2002a,b, 2004). This was followed in the late Palaeoprotero-zoic–Mesoproterozoic by the emplacement of massive mafic dyke swarms (Heaman, 1997; Halls and Zhang, 1998; Hou et al., 2008a, b), formation of thick sedimentary successions in passive continental margins (Windley, 1995; Partridge et al., 2008), rapid growth of continental crust (Reymer and Schubert, 1987; Rino et al., 2008), first appearance of ophiolites and manifestation of collisional orogeny similar to that of the Phanerozoic (Windley, 1995), a transition from planar to linear metamorphic belts, formation of continental roots (Polet and Anderson, 1995) and global anorogenic magmatism (Windley, 1995). At the same time, although the evolution of accretionary orogens and island arcs continued in the Palaeoproterozoic since their initiation in the Archaean (Windley, 1995), vertical and horizontal geodynamics and the related structures were characteristic of the Palaeoproterozoic to the Cenozoic. Both horizontal and vertical structures appeared coevally at this time, representing structures successive to Archaean and Phanerozoic times (Etheridge et al., 1987; Oliver et al., 1991; Windley, 1995; Trap et al., 2007, 2008). Thus, the Palaeoproterozoic history of the Earth provides an important geodynamic bridge to the preceding geological history of the Earth. Major global geological events are recorded during 2.4, 2.3, 2.2–2.1, 2.0–1.9, 1.8 and 1.7–1.6 Ga (Condie, 2001; Cawood and Korsch, 2008; Xia et al., 2008; Condie et al., 2009). For example, deposition of the ca. 2.48–2.46 Ga giant Hamersley banded iron formation (BIF) in Western Australia marks an important event in the Palaeoproterozoic (Partridge et al., 2008).

The Palaeoproterozoic framework of the North China Craton has been central to the recent debate on the evolution of the Columbia Supercontinent (e.g. Li et al., 2000, 2007; Kusky and Li, 2003; Zhao et al., 2005, 2007; Kusky et al., 2007; Santosh, 2010; Santosh et al., 2007a, 2010). The tectonic framework of the North China Craton is considered to have resulted from 1.85 Ga collision of the Western Block with the Eastern Block along the Trans-North China Orogen (TNCO) (Zhao et al., 1999, 2000a, b, 2001a, b, c, 2004, 2005; Zhao and Cawood, 1999; Kusky and Li, 2003; Kusky et al., 2007; Li et al., 2010; Santosh, 2010; Santosh et al., 2010). Considerable progress has been made in the recent years on the petrologic, geochemical, structural, geochronologic and tomography studies on the North China Craton and particularly on the Western Block and the TNCO (Luo et al., 2004, 2005; Zhao et al., 2004, 2005, 2006; Barley et al., 2005; Guo et al., 2005; Wilde et al., 2005; Zhai et al., 2005a,b; Kröner et al., 2005, 2006; Liu et al., 2005, 2006; Polat et al., 2005, 2006; Wan et al., 2005, 2006; Xia et al., 2006, 2008; Zhang et al., 2006; Kusky et al., 2007; Zhang et al., 2007, 2009; Santosh et al., 2007a, b, 2008; Hou et al., 2008a, b; Trap et al., 2008; Wu et al., 2008; Yang et al., 2008; Santosh, 2010; Santosh et al., 2010). However, the eastern margin of the Eastern Block of the North China Craton has not been investigated in detail, and several problems related to the tectonothermal evolution of this region remain unsolved (Hao et al., 2004; Faure et al., 2004; Li et al., 2004a, 2005, 2006; Lu et al., 2006).

Understanding the geodynamic history of the Palaeoproterozoic Earth requires detailed investigations on the crustal evolution including various processes in space and time, and the response of tectonothermal evolution in different layers of the lithosphere. The Jiao-Liao-Ji Belt is a well-preserved and vast Palaeoproterozoic tectonic belt in the eastern margin of the Eastern Block of the North China Craton, which shows a history of intense deformation, complex stratigraphic relationships, high-pressure (HP) metamorphism and formation of an array of large mineral deposits (Zhang et al., 1988; Jiang et al., 1997; Li et al., 2001a, b, 2004a, 2005, 2006; Hao et al., 2004; Faure et al., 2004; Lu et al., 2006; Li and Zhao, 2007; Figure 1). Therefore, the Jiao-Liao-Ji Belt offers an excellent terrane to investigate the late Archaean to Palaeoproterozoic tectonics in East Asia. In this study, we synthesize the relationship among sedimentation, structural fabrics, magmatism and metamorphism, deduce their responses to deep crustal processes in the Jiao-Liao-Ji Belt of the Eastern Block of North China Craton and integrate the results to propose a geodynamic model for the evolution of this region.

Figure 1.

Map of the Palaeoproterozoic Jiao-Liao-Ji Belt in the Eastern Block of the North China Craton showing the distribution of the Fenzishan and Jingshan Groups in Eastern Shandong, South and North Liaohe Groups in Liaoning, Laoling and Ji'an Groups in Southern Jilin and Macheonayeong Group in North Korea (Zhao et al., 2005; Tam et al., 2010). The southern border of Central Asian Orogenic Belt (CAOB) is based on Xiao et al. (2003, Xiao et al., 2009a,b). CCO—Central China Orogen; TNCO—Trans-North China Orogen; WB—Western Block; EB—Eastern Block; LC—Longgang Complex; LNC—Liaonan-Nangnim Complex; WS—Western Shandong Province; ES—Eastern Shandong Province; EL—Eastern Liaoning Province; SJ—Southern Jilin Province. This figure is available in colour online at wileyonlinelibrary.com/journal/gj

REGIONAL GEOLOGY

The Precambrian basement of the North China Craton has been divided into three distinct geotectonic units: the Western Block, the TNCO and the Eastern Block (Figure 1, Zhao et al., 1998; 2000a; 2001a, c; 2002a, b; 2003; Zhao and Cawood, 1999; Guo et al., 2002; Guo and Zhai, 2001; Guan et al., 2002; Zhang et al., 2007; Zhao et al., 2007; Lu et al., 2008). The Eastern Block (also known as the Yanliao Block, Santosh, 2010) is subdivided into the Archaean Longgang Block to the north, the Liaonan-Nangnim Block to the south and the Palaeoproterozoic Jiao-Liao-Ji Belt or tectonic belt at the centre. The latter consists of the ENE-trending Liaohe Group (Figure 2), its equivalents and associated Liaoji granitoids (Yang et al., 1988; Zhang et al., 1988; Sun et al., 1993, 1996; Peng and Palmer, 1995; Li et al., 1996; Li et al., 2004a, b, 2005, 2006).

Figure 2.

Lithostratigraphic units of the North and South Liaohe Groups (after; samples seen in Luo et al. (2004, 2006, 2008)). This figure is available in colour online at wileyonlinelibrary.com/journal/gj

The Palaeoproterozoic Jiao-Liao-Ji Belt is considered as an important intraplate orogen (Zhang and Yang, 1988; Li et al., 1998a, 2006) of the Eastern Block in the North China Craton (Figure 1, Zhao et al., 1999, 2002). It is tectonically bound by the Palaeoarchaean to Neoarchaean Longgang Complex (LC in Figure 1) to the north and the northwest (Figure 1, Jiang, 1987; Song et al., 1998; Zhao et al., 2001a, 2005), the Liaonan-Nangnim Complex (LNC in Figure 1) to the east and the Triassic Dabie-Sulu Orogen and the Hongseong–Odesan belt in Korea to the south (the eastern segment of the Central China Orogen, abbreviated as CCO in Figure 1) (Sang, 1991; Oh, 2006; Tang et al., 2007; Li et al., 2007). The Palaeoproterozoic rocks in the orogen underwent intense deformation and are scattered in a wide area from the eastern Shandong Province (ES in Figure 1), through the eastern Liaoning Province (EL in Figure 1) to the southern Jilin Province (SJ in Figure 1) up to North Korea. The Jiao-Liao-Ji Belt constitutes the main part of the Jiao-Liao Massif (Zhao and Cawood, 1999; Zhao et al., 1999, 2000a, b, 2001a, b, c; Zhai and Bian, 2000; Jin, 2002; Lu et al., 2002, 2006; Li et al., 2005, 2006) and includes dominantly the Liaohe Group and of Liaoji granitoids, which extend mainly along a ENE-trending belt that is flanked by the late 2.45–2.55 Ga basement gneisses.

The Liaohe Group is conventionally divided into five formations, namely, the Langzishan, Li'eryu, Gaojiayu, Dashiqiao and Gaixian formations from bottom to top. They are usually assigned to two groups: the South and North Liaohe groups, which are transitional from a lower clastic-rich sequence through a carbonate-rich sequence to an upper clastic-rich sequence (Figure 2) (Zhang and Yang, 1988; Yang et al., 1988; Wang et al., 1995; Liu et al., 1997; Li et al., 1998a, b, 2001a, b, 2004b, 2005). Recent geochronological studies (Luo et al., 2004) show that the detrital zircons of the Langzishan Formation, the lowest sequence of the Liaohe Group, have concordant U–Pb ages ranging from 2.05 to 2.24 Ga, suggesting that the Liaohe Group was deposited at some time after 2.05 Ga. Although most of the overgrowth or recrystallization rims on the zircons are too narrow to be analysed, some of the rims of zircons from this formation yielded concordant or upper intercept ages around 1.93 Ga, interpreted as the peak metamorphic age of the Liaohe Group. Although the Yushulazi and Yongning formations overlapping the Liaohe Group were also traditionally considered as Palaeoproterozoic (Yang et al., 1988), the recent recognition of Mesoproterozoic (1.05–1.31 Ga) igneous zircons from the Yushulazi Formation indicates the existence of Grenvillian-aged magmatism in the eastern margin of the Eastern Block of the North China Craton (Luo et al., 2006). Therefore, the Yushulazi Formation must have been deposited at some time between 1.05 and 0.9 Ga, prior to the sedimentation of the 0.9–0.8 Ga Yongning Formation and, therefore, it does not belong to the upper part of the Palaeoproterozoic Liaohe Group, as previously interpreted by Luo et al. (2006) (Figure 2).

Many Palaeoproterozoic plutons are associated with the Liaohe Group, and are dominated by monzogranite and rapakivi granite/syenite (Figure 3). These intrusives have been investigated on the basis of lithology, geochemistry and structure/emplacement correlations (Zhang and Yang, 1988; Yang et al., 1988; Sun et al., 1993, 1996; Peng and Palmer, 1995; Li et al., 1997a, b, 1998a, b, 2001a, b; Hao et al., 2004). SHRIMP U–Pb zircon analyses reveal that the pre-tectonic magnetite-bearing monzogranitic gneisses were emplaced during the period 2176–2166 Ma and metamorphosed at 1914 Ma (Li and Zhao, 2007). The pre-tectonic hornblende/biotite-bearing monzogranitic gneisses were emplaced at 2150–2143 Ma (Li and Zhao, 2007). These data demonstrate that the pre-tectonic monzogranitic gneisses of the Liaoji granitoids are not younger than the deposition of the Liaohe Group, as previously considered (Lu et al., 2006; Li and Zhao, 2007). SHRIMP U–Pb zircon analyses also reveal that the post-tectonic porphyritic monzogranites and granites in the Jiao-Liao-Ji Belt were emplaced at 1875 ± 10 and 1856 ± 31 Ma, respectively, simultaneously with the emplacement of the alkaline syenites which have been dated as 1857 ± 20 and 1843 ± 23 Ma (Li and Zhao, 2007). In addition, some studies have identified large outcrops of Palaeoproterozoic monzogranites in the Bohai Sea and in western Shandong Province (Cao et al., 1990; Zhang et al., 2001), However, these Palaeoproterozoic monzogranites constitute the basement of the Jiao-Liao-Ji Belt (Figure 1).

Figure 3.

Map of the Palaeoproterozoic Jiao-Liao-Ji Belt in the Eastern Block of the North China Craton showing the locations of the samples of the Palaeoproterozoic plutons (revised from Li et al. (2007)). This figure is available in colour online at wileyonlinelibrary.com/journal/gj

The Jiao-Liao-Ji Belt runs generally parallel to the TNCO of the North China Craton (Figure 1, Li et al., 2010), where recent research has shed light on the tectonothermal evolution and geochronologic framework of the 1.85 Ga collision zone (Zhao et al., 2002a,b). However, no systematic discussion exists on how the widespread Palaeoproterozoic rocks in the Eastern Block reflect the general, pre- and post-collisional evolution and the relationship with the TNCO.

In the last 20 years, much progress has been made in the Jiao-Liao Massif via 1:50 000 scale geological mapping covering an area of 2000 km2 (Li et al., 1996; Liu et al., 1997; Li et al., 1998a, b, 2001a, b, 2005). Many studies have focused on the extensional and collisional deformation, division of the lithotectonic units, metamorphic P-T-t paths, laterally-progressive prograde metamorphic belts, HP granulites, kinematics of porphyroblasts, granitoid intrusion, underplating of basalt magmas, Neoarchean crustal breakup, geochronology and the relation between delamination and the switch from crustal extension to compression (Li et al., 1996; Sun et al., 1996; Li et al., 1998a, b, 2001a, b, 2005; Liu et al., 1997; Xu et al., 1998; Cui et al., 2001), as well as the occurrence of ophiolite and oceanic plagiogranite ranging from end Palaeoproterozoic to the beginning of middle Palaeoproterozoic (Ni et al., 2001a,b; Ni and Wang, 2001). Several investigations have addressed isotope geochemistry (Lu et al., 1996a, 1997; Jiang et al., 1997; Lu, 1998; Lu et al., 2006), formation of HP granulites (Li et al., 1997a; Zhai and Bian, 2000; Zhou et al., 2008) and the evolution of super-large ore deposits such as the boron mineral deposits (Bai, 1993; Qing et al., 1994; Lu et al., 1996a; Jiang et al., 1997; Peng and Palmer, 2002) in this area. Combining these results with some recent new geological and geochronological data (Li et al., 2003; Luo et al., 2004, 2006, 2007; Lu et al., 2006; Li et al., 2007), we synthesize below a series of pre-, syn- and post-orogenic deep lithospheric processes and related crustal responses in order to evaluate the tectono-thermal evolution of the Jiao-Liao-Ji Belt in the eastern margin of the Eastern Block, and to discuss the relationship between this belt and the Trans-North China Craton.

PRE-OROGENIC GEOLOGICAL RECORDS

Initial crust growth and formation of granitoid batholiths

Geochemical data on amphibolites in the Liaohe Group, Kuandian area show that the TDM age range of 2.46–2.75 Ga is related to partial melting of Nd depleted mantle source. However, the TDM age range of the Liaoji granitoids is 2.36–2.53 Ga, and TDM ≈ TSm–Nd > TU–Pb. According to the discrimination principle proposed by Lu et al. (1996a), the TDM of amphibolite represents the initial age of crustal growth (Sun et al., 1993).

Associated with this geological event is the Archaean alkaline granite belt, starting from Huzhuang Town in Dashiqiao City and extending along the northern margin of the Jiao-Liao-Ji Belt. In addition, a pegmatite emplacement event is dated as ca. 2.45 Ga (U–Pb zircon) in the north Archaean basement, outcropping at Dongxiaosi and Heiniuzhuang in Anshan, Xinbin in Liaoning Province, Fusong in Jilin Province (Wang, 1984; Zhang and Yang, 1988).

Zircon U–Pb age of the Mazhenkou granitic gneiss east of Qixia City, northeastern Shandong Province (called Jiaobei, north part of ES in Figure 1) shows 1.96 ± 0.07 Ga which represents the crystallization age of the granites, and has also a TDM of 2.41 Ga (Lu et al., 1997). The TDM of the amphibolites in the Jiaobei (such as Diaoyahou), however, is 2.41 Ga (Lu et al., 1997), indicating that the initial age of crustal growth might have manifested during 2.5–2.4 Ga in the Jiaobei as well (Lu et al., 1996a). The formation of the Palaeoproterozoic granites in the Jiaobei is similar to the granites in the Liaodong Peninsula (EL in Figure 1).

SHRIMP data show very few inherited Archaean zircons in the typical Liaoji granites (striated monzogranite) and the dominant age range falls between 2.17 and 2.15 Ga (Lu et al., 2006; Li and Zhao, 2007). Inherited zircons with ages between 3.8 and 2.7 Ga of the north Archaean Longgang Complex (the Liaobei Archaean Basement, Wu et al., 2008) are absent, either in samples from the central part or from the margin of the Jiao-Liao-Ji Belt (Li et al., 2003, Li et al., 2004a, 2006, 2007). This indicates that the magma source is not entirely related to the melting of Archaean basement in the Liaobei area (northeastern Liaoning Province). Geochemical characteristics of the striated monzogranite closely match with the intraplate characteristics of both S-type and A-type granites (Hao et al., 2004; Yang et al., 2007).

Geochemical data on the amphibolites in the Kuandian area show that K, Rb, Ba, Sr, Cr, Ni and Th are highly enriched. However, Nb and Ce are only slightly enriched, P, Zr, Hf, Sm, Ti, Y, Yb and Sc values are close to 1. Trace element pattern of the amphibolites ranges between volcanic arc, calc-alkalic and tholeitic basalts, i.e. flat rare earth elements (REE), light LREE enrichment and absence of Eu anomaly. Furthermore, the high K2O and total FeO, low Al2O3 are distinct from that of island arc-type low potassium basalt (Sun et al., 1993, 1996). Except for high Ba and V, low Th and Zn, the geochemical characteristics of the Liaoji granitoids in the Kuandian area are similar to that of A-type granites. They show enriched REE pattern, flat LREE and negative Eu anomaly and plot within the intraplate field in Rb-(Y + Nb) discrimination diagram (Sun et al., 1993, 1996). In addition, pegmatites with ages of 2.2 and 2.0 Ga were emplaced, and mostly distributed in the north Archaean Longgang Complex. At the same time, many 2.2–2.0 Ga, syn-D1 bedding-parallel pegmatite veins intruded widely in the Liaohe Group (Li et al., 2003).

The Jiaobei is located to the south of the Liaonan. Statistics of TDM data of Mesozoic granites in the Jiaobei show a high frequency between 2.1 and 1.9 Ga. Lu et al. (Lu et al., 1996b, 1998) suggested that the above-mentioned initial crustal growth in the Jiaobei lasted until 2.0 Ga. However, the TDM histograms show that another magmatic event might have occurred during the same time (Condie, 2001). The initial magmas were the probable source for the Mesozoic granites, in which case there was high velocity body until after Early Mesozoic and remained attached to the bottom of the lower crust (Li et al., 2001a, b).

Boron mineralization and basin formation

In general, the boron content of the lower continental crust and mantle are extremely low (Peng and Palmer, 2002). Therefore, boron enrichment is uncommon in continental rifts such as the African rift valley. However, boron ore deposits are commonly present in the extensional basins formed in a convergent background. Boron enrichment requires subduction of a boron-rich oceanic crust releasing boron-rich fluids to metasomatize the lithospheric mantle in the mantle wedge zone, which would then be erupted and deposited in a basin (Palmer, 1991; Peng and Palmer, 2002). However, the pre-metamorphic origin of the Houxianyu borate deposit in the Jiao-Liao-Ji Belt is similar to that of the Tertiary non-marine evaporite borate deposits of western Turkey (Palmer, 1991; Peng and Palmer, 2002). There are also similarities with other Proterozoic early rifting environments (e.g. the Neoproterozoic Damaran Orogen of South Africa) that have implications for the structural evolution and mineral potential of the region (Jiang et al., 1997). Of particular interest is the large difference in δ11B-values of marine and non-marine evaporite borates. Hence, δ11B-values can be used to trace the origin of boron in different geological environments such as marine evaporite-hosted (Red Sea); non-marine evaporite-hosted (Salton Sea, California); clastic sediment-hosted (Escanaba Trough and Guaymas Basin); back-arc sediment-starved spreading centre (Mariana back-arc basin) and sediment-starved midocean ridge spreading centres (21° and 11–13°N East Pacific Rise, Juan de Fuca Ridge, and 23° and 26°N Mid-Atlantic Ridge) (Palmer, 1991; Jiang et al., 1997). Jiang et al. (1997) and Peng and Palmer (2002) reported that the δ11B-values of the tourmaline and borates from Houxianyu range from +0.8 to +11.1‰, overlapping with the δ11B-values between marine and non-marine evaporites (+18.2 to +3 1.7‰ and −30.1 to +7‰, respectively) (Swihart et al., 1986). The data are consistent with a model of depositional environment of the boron-bearing sequence being overlapped with the transition from non-marine to marine conditions in the Jiao-Liao-Ji Belt. In addition, zircons from both the North and South Liaohe groups have similar Hf model age peaks (equation image peaks at 2.66–2.75 and 2.80–3.00 Ga) and εHf values (−5 to +9 and −7 to +5) (Luo et al., 2008). This is also in support of the inference that the South and North Liaohe groups formed at one rift basin with the same Archaean Basement.

Extension and sedimentary framework

The spatial distribution of Palaeoproterozoic strata in the Jiao-Liao-Ji Belt records the basin pattern at the initial phase of rifting. Sedimentary sequence, mineralization, metamorphism and ages of the North Liaohe and Laoling groups in the Liaoning and Jilin provinces, respectively, are comparable with those of the Fenzishan Group in the Jiaobei (Figures 1 and 4). These groups are spatially aligned in en echelon pattern. They are controlled by NE-striking border faults, and these border faults are, therefore, en echelon. Their present-day sedimentary sequence, facies and spatial distribution suggest that the early extensional basin is a half-graben with a series of faults developed in the north border of the basins and their stratigraphic overlapping in the south border of the basins (Li et al., 2001b). Therefore, we consider its prototype of the basin as en echelon rifts or half-grabens. In each graben (Figure 4), the northwest part shows passive marginal sedimentary facies; however, the southeast part shows more active volcano-sedimentary formation (Figure 5).

Figure 4.

The spatial distribution of Palaeoproterozoic strata in the Jiao-Liao-Ji Belt showing basin pattern at the initial phase of rifting. Profile A–B is shown in Figure 7. Abbreviations in the inset map: WB—Western Block; EB—Eastern Block; TNCO—Trans-North China Orogen; KB—Khondanite Belt. This figure is available in colour online at wileyonlinelibrary.com/journal/gj

Figure 5.

Schematic diagram showing the relationship between intracrustal responses and deep processes in the Jiao-Liao Massif during Palaeoproterozoic. Profile A–B is located in Figure 4. 1—igneous rock; 2—carbonate; 3—perlite; 4—sandstone; 5—Palaeoproterozoic granite or gneiss; 6—Archaean basement; 7—oceanic crust; 8—underplated intrusives; 9—lithospheric mantle; 10—delaminated slab; 11—folds; 12—thrust fault.

In addition, there is direct evidence that the lower crust was thicker northward represented by the high density anomaly across the Donggou-Dongwu Global Geological Transect (GGT). The thickness of the Liaoji granitoids become thicker northward than southward revealed by 1:200 000-scale high-resolution gravity anomaly data.

Metamorphism and P-T-t paths

The metamorphic characteristics and P-T-t paths show that the North Liaohe, Laoling and Fenzishan groups (Figure 1) in the Jiao-Liao-Ji Belt underwent moderate P-T metamorphism, with heating and pressure increase at the early metamorphic stages (including M1 and M2) (Figure 6). However, the South Liaohe, Ji'an and Jingshan groups underwent low pressure, high temperature metamorphism with an isobaric rapid-heating at the early stage of metamorphism. The early stages of ∼1.93 Ga peak metamorphism in the northern and southern parts are coeval and consistent with 2.2–1.93 Ga felsic underplating of monzogranite between the lower Palaeoproterozoic cover and the Archaean basement. Therefore, the segments of P-T-t paths related to M1 and M2 stages reflected the initial spatial variation of the thermal architecture of this rift. Several 2.2–2.0 Ga bedding-parallel pegmatites, mainly parallel to schistosity (S1), are widely distributed in the Liaohe Group, which are closely associated with the 2.2–2.0 Ga extension and M1 and M2 extensional metamorphism (Li et al., 1998b, 2005).

Figure 6.

P-T-t paths of the Liaohe Group and its equivalents. equation image, equation image after Li et al. (2001a); equation image, equation image after He and Ye (1998); equation image, equation image modified after Lu et al. (1996a) 1—Archaean basement; 2—Palaeoproterozoic granite; 3—Neoproterozoic strata; 4—Liaohe Group and equivalents; 5—major faults; 6—Mesozoic plutons; 7—Mesozoic strata; Ky—Kyanite; And—Andalusite; Sill—Sillimanite.

SYN-OROGENIC GEOLOGICAL RECORDS

Collisional granites and high-pressure metamorphism

The protolith of khondalites is often pelitic rocks developed within stable passive continental margin, although the tectonic setting of the Jiao-Liao-Ji Belt has also been considered as volcanic-type passive continental margin (Cui et al., 2001). A NE-trending Palaeoproterozoic belt of HP granulite-ultramafic rocks, dated as 1895–1750 Ma and tectonically emplaced, has been reported from within the Archaean gneiss in the Laixi-Qixia area of the Jiaobei (Li et al., 1997a; Zhou et al., 2008; Tam et al., 2010). Palaeoproterozoic medium-low pressure granulites are also reported in the Laixi-Pingdu area of the Jiaobei (Lu et al., 1996a), and a possible Palaeoproterozoic ophiolite relic has been discovered in Haiyang, Jiaodong peninsula (Wang et al., 1995; Ni et al., 2001a, b; Ni and Wang, 2001). Although there is considerable debate surrounding these findings, the other major Palaeoproterozoic rocks such as island-arc-type metavolcanics, and HP metapelitic and mafic granulites from the Jiao-Liao-Ji Belt suggests that the southern part of the Jiao-Liao-Ji Belt underwent subduction, collision and exhumation of the root of a thickened orogen (Li et al., 2001a; Lu et al., 2006; Zhou et al., 2008).

Some I-type granites, related to the Palaeoproterozoic collision, occur in the South Jilin Province such as the Shanchengzi plagiogranite in Ji'an City with a 1.88 Ga Rb–Sr whole-rock isochron age (Bai, 1993). Field relationship shows that they cut through gneissic monzogranites. Partial melting of mantle is considered as the source of the I-type granites because the total REE in these granites is close to REE-enriched mantle (10.18 ppm). The average Sm/Nd ratio of 0.28 is also close to that of mantle value (0.31), and 87Sr/86Sr ratio (0.704) matches that of I-types granites (<0.708) derived from mantle sources (Bai, 1993).

Collisional deformation and P-T-t paths

Two stages of folding (D2 and D3) that overprinted the rocks in the region and a series of small-scale thrusts have been identified. The D2 stage of folding produced mainly tight, ENE-trending upright horizontal folds, whereas D3 stage of folding produced mainly open, WNW-trending upright or local overturned folds. They were superimposed to form a series of Ramsay's type 1 (dome and basin) or Ramsay's type 2 (mushroom) refolded folds (Li et al., 2005). During this fold superposition, early monzogranitic gneisses were also folded to form the cores of domes, locally seen as a non-conformity in the Liaohe Group. 40Ar/39Ar age of orientated biotite taken from schistosity parallel to D2 fold axial plane shows an age of 1896 ± 7 Ma (Yin and Nie, 1996).

Zhou et al. (2008) applied EPMA U–Th–Pb monazite and SHRIMP U–Pb zircon dating techniques to determine the metamorphic ages of the HP pelitic granulites in the Jiaobei. CL images and Th/U ratios show that all monazite and zircon grains in the analysed samples are of metamorphic origin. Two samples yielded three age groups at 1895–1882, 1814–1813 and 1706–1692 Ma, and the SHRIMP ages of one sample range from 1864 to 1803 Ma. The oldest ages of 1895–1882 Ma were obtained from high-Y monazite grains enclosed in garnet and sillimanite pseudomorph after kyanite, and thus interpreted as the time of the HP (M2) granulite-facies metamorphism. The EPMA monazite ages of 1814–1813 Ma and SHRIMP zircon ages of 1864–1803 Ma are interpreted as the approximate ages of the post-HP granulite-facies metamorphism (M3), probably resulting from the exhumation of the HP granulites to the medium-pressure granulite-facies levels. The youngest age group of 1706–1692 Ma obtained from low-Y monazite grains in the matrix which can be interpreted as the age of late cooling and retrograde (Barrovian) metamorphism (M4) coinciding with the exhumation of the HP pelitic granulites to the upper crustal level. These results imply that the southern part witnessed a prolonged metamorphic evolution history as compared to the northern part of the Jiao-Liao-Ji Belt (Tang et al., 2007).

The syn-collisional and post-collisional segments of the P-T-t paths of the North Liaohe, Laoling and Fenzishan groups can be distinguished by the growth-sequence of different porphyroblasts (Li et al., 1998a, b, 2005). They show a fast heating and pressure increase during collision, simultaneously reaching the metamorphic peak, followed by rapid decompression and slow cooling (Figure 6). This indicates that these areas underwent fast crustal thickening and, later, pronounced exhumation. However, the same P-T-t trajectory segments of the P-T-t paths of the South Liaohe, Ji'an and Jingshan groups also show a smaller range of pressure increase at relatively constant temperature, followed by isobaric cooling, and thus do not correlate with crustal thickening. Evidence for neither fast exhumation nor rapid erosion and weathering after collision exists in this region.

POST-OROGENIC GEOLOGICAL RECORDS

Anorogenic magmatism

Coeval emplacement of rapakivi granites (1.88 Ga), alkaline syenites (1.87 Ga) and moyites (1.85 Ga, Lu et al., 2002, 2006) has been recorded in the Jiao-Liao-Ji Belt. According to the age data on anorogenic granites from this domain, the time range is between 1.88 and 1.66 Ga, earlier than the anorogenic magmatism in other areas of the Eastern Block (Yu et al., 1996), and earlier than the main rifting (1.85–1.60 Ga) of the North China Craton.

Differential uplifting and pegmatite veining within the orogen

Pegmatite veins are widespread in the Jiao-Liao-Ji Belt. Four Palaeoproterozoic pegmatitic events can be distinguished. K–Ar age data show that these events had a cycle of approximate 200 myr and occurred at 2.2, 2.0, 1.8 and 1.6 Ga (Wang, 1984). Among these, the 1.8 and 1.6 Ga events are commonly represented in the Liaohe Group and their equivalents. The 1.8 Ga event is relatively later than the M3 metamorphic stage of 1.911–1.896 Ga (Yin and Nie, 1996; Li et al., 2003; Luo et al., 2004), and might be related to heating or a discrete thermal event. The 1.77 Ga mafic dykes and 1.6 Ga pegmatite veins cut all structural traces in the Liaohe Group related with M4 regional retrogression (Wang, 1984; Wang et al., 2008). This marks the final consolidation of the crystalline basement of the North China Craton.

The contrasting processes at depth are probably reflected in the successive uplift in different locations on the surface. The K–Ar age isolines of the Liaohe Group extend from east to west and their values decrease gradually southward, suggesting that cooling in the northern domain is earlier than that in the south (Wang, 1984). However, the whole Jiaodong and Liaodong Peninsula shows absence of Mesoproterozoic sediments indicating that the late Palaeoproterozoic basement was already exhumed to the surface, cooled and consolidated. However, we do not exclude the possibility that the K–Ar systems may have been disturbed by local heating at a late stage such as during the Grenvillian (Mesoproterozoic), Jinningian (late Neoproterozoic), Indosinian (late Triassic) or Yanshanian (middle Jurassic to Cretaceous) movements, especially in the southern part of the Eastern Block of the North China Craton.

Post-orogenic collapse

In the northwest part of the Jiao-Liao-Ji Belt, rifting events are well-recorded as the formation of an aulacogen and of a marginal rift valley, emplacement of a mafic dyke swarm (1769 ± 2.5 Ma, U–Pb age of single zircon), anorthosites (Rb–Sr age of 1683 ± 193 Ma and Sm–Nd age of 1735 ± 239 Ma), rapakivi granites (1716 ± 31 Ma, U–Pb age) and peralkaline volcanic rocks in the upper Changcheng System (1683 ± 67 and 1625 ± 6 Ma) (Wang et al., 2007; Lu et al., 2008). Post-orogenic rifting in the Eastern Block of the North China Craton, therefore, occurred mainly during 1.72–1.62 Ga (Lu et al., 2002).

The above data show that around 1.7 Ga, the southern margin of the Eastern Block should have been part of an active continental margin (lower part of Figure 5) and that a collision of an island-arc to the continent and a closure of back-arc rift or basin might have occurred.

TECTONIC INTERPRETATION AND EVOLUTION OF THE EASTERN MARGIN OF THE EASTERN BLOCK

Discussion and interpretation on deep geological processes

Mafic magma underplating

Underplating of basaltic magma refers to a process where basaltic magma produced by partial melting of the upper mantle intruded or was added into the base of the lower crust (Jin and Gao, 1996). This process also includes upwelling and intrusion of magma into the upper-middle crust, resulting from partial melting of the lower crust related to heating and influx of CO2-rich fluids provided by the underplated basaltic magmas (Jin and Gao, 1996; Santosh and Omori, 2008; Santosh et al., 2008). Magmatic underplating may occur in several different tectonic settings, such as continent–continent collision zone, active and, passive continental margin, continental rift valley, regions of hot spot or mantle plume and stable platform. Neutral buoyancy can convincingly support this kind of vertical material differentiation within the lithosphere. Some of the recent studies have revealed basaltic magma underplating in the Jiao-Liao-Ji Rift (Lu et al., 1996a; Li et al., 2001a).

Statistical analysis of isotopic parameters such as TDM and εmath image for rocks of the Jiao-Liao-Ji Belt (Lu et al., 1996a; Li et al., 2001a) shows that a major basaltic magma underplating at depth occurred in the Jiao-Liao-Ji Belt during early Palaeoproterozoic.

The initial crustal growth corresponds to the time of basaltic underplating (Sun et al., 1993). However, the TDM of granitoids represent the age of crustal reworking. Based on these data, it was deduced that there was a major event of basaltic underplating which occurred in the Liaodong Peninsula during 2.5–2.4 Ga in the early Palaeoproterozoic, and caused the rejuvenation of the lower crust close to the Moho which generated the ca. 2.47–2.33 Ga pre-Palaeoproterozoic, post-orogenic alkaline granites and 2.5–2.2 Ga sedimentation of part of the Laoling Group and its equivalents (Peng and Palmer, 2002; Li et al., 2003; Lu et al., 2006).

All of these resulted from deep crustal reworking linked to subduction-related basaltic magma underplating rather than plume-related underplating as no plume-related, radiating mafic dykes have been found in this region.

Alternatively, the magmas of the Liaoji granites with S- and A-type affinities were possibly derived from a mixed source resulting mainly from the reworking of 2.5 Ga crustal rocks under extensional environment to a 2.47–2.36 Ga basaltic underplating (Lu et al., 2006). Thus, it is possible that there was an important 2.17–2.15 Ga heat source located towards Liaonan (southern Liaoning Province, south part of EL in Figure 1).

Therefore, we interpret that vertical basaltic magma accretion through magmatic underplating was an important process that contributed to rapid crustal growth in the Early Palaeoproterozoic based on the large population of early Palaeoproterozoic zircons, markedly higher than the populations of zircons with other age values (Lu et al., 2006).

As underplating could supply heat, the source material in the lower crust that produced the S-type Liaoji granites was reactivated, reworked and melted during 2.17–2.15 Ga. In an extensional setting related to the wide rifting and local back-arc spreading of a continental margin, the formation of new magma results during active rifting leading to secondary underplating in a larger scale, after the initial basaltic magma was underplated. Furthermore, SHRIMP data show abundant 2.17–2.10 Ga inherited zircons in Mesozoic granites in the Liaonan and the Jiaobei, implying that a large part of the Palaeoproterozoic intrusion was not yet exhumed (Li et al., 2003).

All the characters of the amphibolites in the Kuandian area indicate that underplating-related rifting had occurred possibly above a rising mantle plume and their ages are consistent with the 2.45 and 2.0–1.9 Ga worldwide Palaeoproterozoic plume activity (Condie, 2001; Condie et al., 2009).

Subduction and extension

The magmatic underplating processes discussed in the previous section probably led to the formation of the Jiao-Liao-Ji Rift at ca. 2.16–1.93 Ga (Lu et al., 2006). Asymmetry in the geometry and kinematics is generally a reflection of the asymmetry in geodynamics. Studies of the geometry and kinematics of extensional structures of the Liaohe Group (Qing et al., 1994; Li et al., 1998a, 2005) shows that the early Palaeoproterozoic magmatic underplating in the Jiao-Liao-Ji Belt was asymmetric. The asymmetry of the underplated Liaodong felsic magma is revealed by the distribution of the Liaoji granitoids which become thicker northward, based on 1:200 000-scale high-resolution gravity anomaly data. In general, ages of the mafic and felsic underplated rocks match the ages of the sedimentary associations. This further implies that underplating, extensional tectonics and their sedimentary associations are closely related in space and time (Li et al., 2001a). Different patterns of deep basaltic magma underplating may lead to variable extensional basins in the upper crust, determining the spatial and temporal framework of the sedimentary system and filling pattern (Table 1, Figures 4 and 5).

Table 1. Geological events in a chronological order of Palaeoproterozoic deep processes and their intracrustal responses in the Jiao-Liao-Ji Belt (revised after Li et al., 2003)
Time range2.5–2.4 and 2.16–1.913 Ga1.913–1.875 Ga1.875–1.6 Ga
Intracrustal response(1) Early underplating arises from oblique rifting of Archaean crust, mafic dyke (2.462 Ga ± ), alkaline granite emplacement(2.47 Ga ± ), and sedimentation of part of the Laoling Group (2.5–2.2 Ga)(1) Two stages of compressive folding-thrusting (1.913–1.875 Ga)(1) Rapakivi granite emplacement (1.875–1.79 Ga)
(2) Formation and emplacement of large-scale early monzogranites (2.17–2.09 Ga) and event of bedding-parallel pegmatite vein of 2.1–2.0 Ga(2) Formation of (M2) horizontal progressive prograde metamorphic belt (1.896–1.875 Ga)(2) Alkali syenite and moyite emplacement (after 1.866 Ga) and M3 metamorphism (1.864–1.803 Ga)
(3) Late underplating (2.2–2.0 Ga) led to bimodel volcanic activity (2.12–1.98 Ga) and other sedimentation of Liaohe (2.05–1.93 Ga) and Ji'an (2.12–1.98) groups(3) HP (M2) granulite-facies metamorphism (1.895–1.882 Ga)(3) 1.8 Ga Pegmatite vein events and 1.77 Ga mafic dykes, and exhumation of high-pressure granulites (1.77 Ga?)
(4) Late underplating led to fast extension and accumulation, followed by extensional deformation of sedimentary cover (2.16–1.93 Ga)(4) Collisional granites emplacement (1.885 Ga ± )(4) M4 retrogression (∼1.706 to 1.692 Ga)
(5) Extensional metamorphism (M1) and formation of vertical progressive prograde metamorphic belt (1.93–1.913 Ga) (5) 1.6 Ga Pegmatite vein events
   (6) Sedimentation of Yushulazi Group and Yongning Formation (post-1.05 Ga)
Deep processUnderplating (2.53–2.33 Ga)Underplating (2.17–1.913 Ga)∼1.90 Ga Slab deep subductionGravity isostatic readjustment
Mantle plume (?)Deep thermal doming (?)∼1.85 Ga Delamination

Based on these striking similarities, we propose that the protoliths of the North and South Liaohe groups may have formed simultaneously, and that the basement rocks underneath the two groups belong to the same Archaean continental block (Li et al., 2006), rather than two different blocks and back-arc basin as previously considered (He and Ye, 1998a,b; Faure et al., 2004). Therefore, the Jiao-Liao-Ji Rift, predecessor of the Jiao-Liao-Ji Belt, should have formed on the basement of an Archaean active continental margin. This basement consists of 2.52 Ga granites which experienced 2.49 Ga granulite facies metamorphism. Subsequently, a Neoarchaean collision/subduction-related post-orogenic evolution occurred, followed by the 2.47 Ga post-orogenic alkaline granite event (Zhang and Yang, 1988; Peng and Palmer, 2002). However, the boron ores were deposited in the Palaeoproterozoic Liaohe Group, implying that this is associated with Palaeoproterozoic orogeny rather than reactivation of the Archaean back-arc-like basement. The intracrustal geologic processes and their related deep lithospheric history are important for understanding the tectonic evolution of the Jiao-Liao-Ji Rift and related ore deposits.

Magmatic underplating not only leads to variation in sedimentary sequence and basin filling pattern, but also results in a difference of thermal architecture within the rift, thus defining a spatial change in the style of metamorphism within a basin. P-T-t paths can reveal spatial variation of initial thermal architecture in a rift basin. This implies that the heat source of metamorphism is closely associated with coeval magmatic underplating (upper profile in Figure 5). The distribution of different metamorphic types in space reflects an asymmetry of magmatic underplating. The underplated material is thicker in the south than in the north, or the upper crust above the underplated magma is thicker in the north than in the south, inducing anomalous heating in the south.

Structural analysis of fold superposition shows that the Fenzishan Group is possibly a folded equivalent of the North Liaohe Group as proposed by Bai (1993). However, this topic is still debated. Although both groups have similar associations of sedimentary strata, the possibility that they formed in two different half-graben basins controlled by two syn-sedimentary listric faults cannot be excluded (Li et al., 2001b). Similarly, sedimentary associations, mineralization, metamorphism and ages of the South Liaohe and the Ji'an groups in the Liaoning and Jilin provinces are also comparable with those of the Jingshan Group in the Jiaobei, located south of the North Liaohe, Liaoling and Fenzishan groups (Figure 1), respectively. Their present-day sedimentary sequence and spatial distribution suggest that the early pattern of the extensional basin represents a series of faults developed in the north border of the basins with a stratigraphic overlap in the south border of the basins (Figure 5, Li et al., 2001b).

Collision and high-pressure metamorphism

Studies on the Palaeoproterozoic khondalites suggest that the peak age for the orogeny in the North China Craton is 2.0–1.8 Ga (Jin, 2002; Santosh et al., 2007a, b). However, Lu et al. (2002) and Zhao et al. (2005) considered the timing to be 2.0–1.85 Ga (with peak at 1.85 Ga). Lu et al. (1996a) pointed out that the South Liaohe Group in the Jiao-Liao-Ji Belt and its equivalent strata consist of khondalite series. Recent evidence shows that the time duration of metamorphism of these khondalites is relatively short between 1.93 and 1.88 Ga (Yin and Nie, 1996; He and Ye, 1998a,b; Li et al., 2003; Luo et al., 2004; Li and Zhao, 2007). However, during 2.16–1.93 Ga, the Jiao-Liao-Ji Rift was still under extension as the anorogenic Liaoji granitoids were emplaced at this time (Zhang et al., 1988; Li et al., 2003; Lu et al., 2006; Li and Zhao, 2007). In the late Palaeoproterozoic, the Jiao-Liao-Ji Belt underwent collisional orogeny and preserve intracrustal physical and chemical records of this movement in the remnant crust.

In the early Palaeoproterozoic, the Jiao-Liao Massif underwent mainly vertical accretion and extensional deformation caused by gravity instability (Qing et al., 1994; Li et al., 1998a). However, during the change in subduction direction, there was a switch from vertical extension to horizontal compression and the basin could have been reactivated along a series of listric faults (lower profile of Figure 5).

The P-T-t paths (Figure 6) of the Jiao-Liao-Ji Belt associated with the different structures (Bai, 1993; Lu et al., 1996a, b, 1997; Li et al., 1998b; He and Ye, 1998a,b) show that the relation between collision and exhumation is rather complex.

Delamination and exhumation of high-pressure rocks

Delamination generally refers to lithospheric mantle, continental lower crust or oceanic crust foundering into the asthenosphere or deep mantle due to gravitational instability (Gao and Jin, 1996). There are many styles of delamination, such as slab breakoff, removal and thinning of lithospheric mantle by convection, magma underplating-related delamination, slab roll-back and slab delamination under single or pure shearing. The different types of delamination can produce different physical and chemical effects on the overlaying crust. The precise style of delamination in the Jiao-Liao-Ji Belt is still debated, i.e. small-scale slab breakoff (Li et al., 2001b) or removal and thinning of thickened lithospheric mantle by convection (Lu et al., 1996a). Due to faster cooling of the northern part as compared to the southern part of the Jiao-Liao-Ji Belt, we interpret that the delamination should have been caused by breakoff of a small faulted block with a northward asymmetry (lower profile of Figure 5).

Collision normally leads to fast crustal thickening followed by rapid exhumation because of isostatic rebound. Based on the previous interpretation, a fast exhumation and consequent isothermal decompression occurred after collision in the North Liaohe, Laoling and Fenzishan groups (Figures 1 and 6). Thus it is deduced that the lithospheric mantle did not break off and that the asthenospheric temperature remained unchanged. The thickened crust might have resulted due to thrusting and folding and resultant uplift by isostasy. According to the interpretation of Jamieson and Beaumont(1988), unloading or unloading with a combination of erosion and extension, leads to fast adiabatic decompression. However, extensional tectonic unloading will not result in fast uplift.

Li et al. (Li et al., 2001a, b, 2003) proposed that the differentiated initial crustal thermal architecture of the basin was replaced by late basin inversion, which should be taken into account to analyse the contrasting P-T-t paths in different parts of the orogen (Figure 6). This is important, since so far, there has been no convincing interpretation of the deep geodynamic process to explain the isobaric cooling history. Isobaric cooling in anticlockwise P-T-t path is generally interpreted to result from slow cooling of an underplated mass. Nevertheless, the evidence for magmatic underplating in the study area is pre-collisional, and the crust above the underplated mafic body must have been thickened during the compressional orogeny. Another possibility is that below the underplated mafic body, tectonic underplating of A-type subducted crust might have resulted in decompression and fast uplift in the hanging wall rather than isobaric cooling due to isostasy. It is not feasible to assume that only delamination results in fast heating because delamination often leads to density decrease and doming (Li et al., 2001a, b). In the South Liaohe Group there is evidence only for isobaric cooling, so no rapid uplift and decompression occurred. Further investigations on the complex processes in the deep lithosphere after collision are necessary to solve this problem.

Structurally, folding was the only major event in the South Liaohe, Jingshan and Ji'an groups during this time. After collision, the lenticular or net-like lithospheric mantle separated into a series of lithospheric ductile shear zones making it easy for the blocks to be pulled down into the asthenosphere and then delaminated. This drag force at the stage of initial delamination led to crustal thickening, and coeval emplacement of rapakivi granites and syenites, as well as moyites. According to the age data on anorogenic granites from this domain, the time range is between 1.88 and 1.66 Ga, earlier than the anorogenic magmatism in other areas of the Eastern Block (Yu et al., 1996), and earlier than the main rifting (1.85–1.60 Ga) of the North China Craton. Moreover, probably due to the small scale of delamination in this area, no widespread heating is recorded. Furthermore, post-orogenic shortening of the lithosphere which was caused by delamination resulted in metamorphism and folding of the Yushulazi Formation and subsidence of the asthenosphere (lower profile of Figure 5).

The ∼2.0 Ga underplated mass was not delaminated and, therefore, it is possible that this served as the source for the Mesozoic granites in the Jiaobei (Lu et al., 1996a).

Delamination under the Jiao-Liao-Ji Belt and post-orogenic rifting in this area may have induced a back-arc extension due to a southeastward retrogressive island arc of 1.7–1.6 Ga age. The late Mesoproterozoic to Neoproterozoic (1.05–1.31 Ga) igneous zircons from the Yushulazi Formation also demonstrate the existence of Grenvillian-aged magmatism in the eastern margin of the Eastern Block of the North China Craton (Luo et al., 2006). This might have resulted in a series of reconstruction events from Grenvillian to Jinningian with distinct characteristics within the Jiao-Liao-Ji Belt.

Tectonic evolution of the eastern margin of the Eastern Block

Combining the results from isotope chronology, isotope and trace element geochemistry, deformation, metamorphism, magmatic evolution and sedimentation, together with an evaluation of the recent models on continental geodynamics, we propose that the complex deep process and related intracrustal response in the Jiao-Liao-Ji Belt might have lasted for nearly 900 myr during the Palaeoproterozoic (Table 1, part of the SHRIMP data are from Li et al., 2003; Li et al., 2006).

The deep processes of crust–mantle interaction in the Jiao-Liao-Ji Belt attest to the notion that the Palaeoproterozoic was an important transitional period of geodynamics and Earth's thermal and tectonic history. The deformation is controlled coevally by vertical and horizontal tectonic regimes related to deep magma underplating, delamination and crust–mantle recycling mechanism. As an intracrustal response, early extensional metamorphism is also translated into a contraction-related metamorphic event with a switch over from vertical to laterally prograde metamorphic belts. Felsic magmatism spans a transition from anorogenic monzogranites to collisional granites and anorogenic rapakivi granites.

The Jiao-Liao-Ji Belt and the TNCO are proximal and located in the eastern and western margins of the Eastern Block of the North China Craton, respectively, and could share the history of a coeval evolution (Li et al., 2010).

During the period 2520–2480 Ma in the western margin of the Eastern Block, subduction beneath the Hengshan-Wutai island arc caused partial melting of the lower crust to form the Hengshan tonalitic-trondhjemitic-granodioritic (TTG) suites, whereas eastward-directed subduction of the marginal sea led to the reactivation of the Fuping relict arc, where the Fuping TTG suite was emplaced (Zhao et al., 2007). At the same time, in the eastern margin of Eastern Block, the northern Archaean Longgang Block and the southern Nangrim Block were welded together with the formation of anorogenic alkali granites along the southern margin of the north Archaean Longgang Block (Zhang and Yang, 1988).

Between 2360 and 2000 Ma, several phases of granitoid magmatism occurred in both the Hengshan-Wutai island arc and the Fuping reactivated arc (Figure 7A) (Kröner et al., 2005, 2006; Zhao et al., 2007), forming 2360, ∼2250 and 2000–2100 Ma granitoids in the Hengshan Complex, the ∼2100 Ma Wangjiahui and Dawaliang granites in the Wutai Complex and the 2100–2000 Ma Nanying granitoids in the Fuping Complex (Zhao et al., 2007). At the same time, the Jiao-Liao-Ji Rift opened because of the western far-field stress inducing back-arc oblique rifting.

Figure 7.

Schematic cartoons showing the geological evolution of the Trans-North China Orogen and the Jiao-Liao-Ji Belt (after Li et al., 2010). (A) At 2520–2000 Ma, the tectonic emplacement and crystallization of the Hengshan, Yixingzhai and Fuping TTG gneisses in the period 2520–2475 Ma. At the same time, in the eastern margin of the Eastern Block, the north Archaean Longgang Block and the south Nangrim Block merged together followed by the emplacement of anorgenic alkaline granites along the southern margin of the north Archaean Longgang Block. Subsequently, a Japanese-type arc formed at the western margin of the Eastern Block, with emplacement of 2.3–2.0 Ga arc-related granitoids, and an Andean-type continental margin at the southern margin of the Yinshan Block. At the same time, the Jiao-Liao-Ji Rift was opened because of the western far-field stress inducing back-arc oblique rifting. (B) The collision of the Yinshan Block and the Ordos Block to form the Khondalite Belt in the Western Block at 1925 Ma, intrusion of mafic dykes in the Hengshan and Fuping Complexes, opening of the Jiao-Liao-Ji Rift in the period of 2000–1915 Ma are also identified. (C) Scissor-type shrinking from north to south of the ocean between 1920–1900 Ma, northeastward subduction of the oceanic crust, closure of the Jiao-Liao-Ji Rift to form the Jiao-Liao-Ji Belt in the Eastern Block, the D1 deformation in the TNCO. This NW-SE-directed far-field driving force led to the final closure of the Jiao-Liao-Ji Rift system and the D1–D2 deformation of the Jiao-Liao-Ji Belt at 1914–1896 Ma. (D) Events at 1900–1880 Ma include the formation of the Hutuo Group in the TNCO. The far-field effect led to D3 folding of the Jiao-Liao-Ji Belt, where deformation and metamorphism occurred before 1875 ± 10 Ma. This figure is available in colour online at wileyonlinelibrary.com/journal/gj

At about 1915 Ma, mafic dykes, the precursors of the Hengshan HP granulites, intruded in the active continental margin of the western part of the Eastern Block (Kröner et al., 2006). This was possibly driven by subduction of a mid-ocean ridge, leading to the tectonic emplacement of a major mafic dike swarm (Figure 7B) (Zhao et al., 2007). Between 1920 and 1900 Ma, the back-arc basin completely closed due to arc-continent collision (Figure 7C). This NW-SE-directed far-field driving force led to the final closure of the Jiao-Liao-Ji Rift system and the D1–D2 deformation of the Jiao-Liao-Ji Belt at 1911–1896 Ma (Yin and Nie, 1993; Zhao et al., 2005; Li et al., 2005, 2007).

Between 1900 and 1880 Ma in the western margin of the Eastern Block, the mafic dykes and associated TTG gneisses underwent HP granulite facies metamorphism at ∼1888–1848 Ma and was subjected to the D1 deformation (Zhao et al., 2000a; Kröner et al., 2006). The driving force for the D1 deformation in the Hengshan–Wutai–Fuping Complexes was directed NE-SW during the initial collision (Figure 7D). The far-field effect led to D3 folding of the Jiao-Liao-Ji Belt, where deformation and metamorphism occurred before 1875 ± 10 Ma (Li et al., 2005, 2007, 2010).

At present, there is no adequate information on the geodynamic processes and intracrustal responses, and the continental geodynamics in the North China Craton requires further attention. Many aspects remain ambiguous such as the difference between the deep processes and intracrustal responses in the Jiao-Liao-Ji Belt and the Palaeoproterozoic evolution of the TNCO, and its relationship with the formation of the 9193 m thick Changcheng–Jixian–Qingbaikou System of 1.8–0.85 Ga age (Lu et al., 2002, 2008). Debate also surrounds the configuration of the Palaeoproterozoic supercontinent Columbia, particularly with regard to the location and affinity of the Jiao-Liao-Ji Belt. Wilde et al. (2002) considers the North China Craton as connected with the Baltica blocks, whereas Zhao et al. (2003, 2004) proposed that the Eastern Block including the Jiao-Liao-Ji Belt should be correlated with the Southern Block of the Indian Craton. Recent alternative models propose further different configurations (e.g. Kusky et al., 2007; Hou et al., 2008a; Rogers and Santosh, 2009). A more detailed evaluation of the processes of continental geodynamics and associated tectonic models requires more precise geochronological constraints as well as the nature of metamorphic P-T-t paths. The Jiao-Liao-Ji Belt could provide important clues to understand the crustal evolution and geodynamic history of the North China Craton.

ACKNOWLEDGEMENTS

This research was funded by No. 41072152, 90814011 and 40776038 of China NSFC, program for New Century Excellent Talents in University (NCET-06–0595) of Ministry of Education to Li Sanzhong, S863 key project of 2009AA093401 and Hong Kong RGC Grants No. HKU7057/08P. The authors thank Mr Hao Defeng, Dr Luo Yan and Profs Sun Min and Wu Fuyuan for their help. The authors appreciate the constructive reviews from Profs. Guo Jinghui and Xiao Wenjiao and the editorial comments from Prof. Nick Timms.

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