Abstract
The North China craton (NCC) is one of oldest cratons in the world, with crust up to c. 3.8 Ga old, and has a complicated evolution. The main Early Precambrian geological events and key tectonic issues are as follows. (1) Old continental nuclei have been recognized in the NCC, and the oldest remnants of granitic gneiss and supracrustal rocks are 3.8 Ga old. The main crustal growth in the NCC took place at 2.9–2.7 Ga. The NCC can be divided into several microblocks, which are separated by Archaean greenstone belts that represent continental accretion surrounding the old continental nuclei. (2) By 2.5 Ga, the microblocks amalgamated to form a coherent craton by continent–continent, arc–continent or arc–arc collisions. The tectonic processes in Neoarchaean and modern times appear to differ more in degree than in principle. Extensive intrusion of K-granite sills and mafic dykes and regional upper amphibolite- to granulite-facies metamorphism occurred, and marked the beginning of cratonization in the NCC. Coeval ultramafic–mafic and syenitic dykes of c. 2500 Ma in Eastern Hebei indicate that the NCC became a stable, thick and huge continent at the end of the Archaean, and probably was a part of the Neoarchaean supercontinent that has been suggested by previous studies. (3) In the period between 2500 and 2350 Ma, the NCC was tectonically inactive, but the development of a Palaeoproterozoic volcanic and granitic rocks occurred between 2300 and 1950 Ma. The volcanic–sedimentary rocks are termed Palaeoproterozoic mobile belts; these have a linear distribution, and were affected by strong folding and metamorphism at 1900–1850 Ma, and intruded by granites and pegmatites at 1850–1800 Ma. The Palaeoproterozoic mobile belts formed and evolved within the craton or continental margin (epicontinental geosyncline). Some 2.30–1.95 Ga rift-margin, passive continental margin deposits, analogous arc or back-arc assemblages, as well as HP and HT–UHT metamorphic complexes seem to be comparable with many in the late Phanerozoic orogenic belts. Regarding Palaeoproterozoic orogeny in other cratons, it is possible that a global Palaeoproterozoic orogenic event occurred, existed and resulted in the formation of a pre-Rodinian supercontinent at c. 2.0–1.85 Ga. (4) In contrast, the c. 1800 Ma event is an extension–migmatization event, which includes uplift of the lower crust of the NCC as a whole, the emplacement of mafic dyke swarms, continental rifting, and intrusion of an orogenic magmatic association. This event has been considered to be related to the break-up of the pre-Rodinian supercontinent at 1.8 Ga, attributed to a Palaeoproterozoic plume. (5) As HP and HT–UHT metamorphic rocks occur widely in the NCC, their high pressure of 10–14 kbar has attracted attention from researchers, and several continental collisional models have been proposed. However, it is argued that these rocks have much higher geothermal gradient and much slower uplift rate than those in Phanerozoic orogenic belts. Moreover, HP and HT–UHT rocks commonly occur together and are not distributed in linear zones, suggesting that the geological and tectonic implications of these data should be reassessed.
The North China craton (NCC) covers over 300 000 km2, and is one of oldest cratons in the world with crust up to c. 3.8 Ga old, and has a complicated evolutionary history (Liu et al. 1992; Zhao, Z. P. 1993; Windley 1995; Wan et al. 2001; Zhai 2004; Kusky et al. 2007a). In recent years, its early tectonic evolution has attracted increasing attention from researchers. The key issues studied include early Precambrian high-pressure granulites, ultrahigh-temperature metamorphic rocks, Archaean ophiolite, Archaean coeval ultramafic–syenitic dykes, Palaeoproterozoic mafic dyke swarms, and Precambrian plate tectonics and plume tectonics (Zhai et al. 1992, 2002; Li, J. H. et al. 1997; Zhai 1997; Zhao, G. C. et al. 1999, 2007; Kusky et al. 2001, 2007a; Kröner et al. 2005; Peng et al. 2005; Wilde et al. 2005; Santosh et al. 2007; Lim, T. S. et al. 2010).
Pre-3.8 Ga rocks occur in the centre of the Archaean craton within Proterozoic belts, and it has been thought that these were Archaean cratons surrounded by Proterozoic belts. The Archaean cratons of the world are often quoted as containing two types of terrane: (1) gneiss-dominated belts metamorphosed largely to a high metamorphic grade; (2) well-preserved, low-grade, volcanic-dominated greenstone belts (Windley 1995). It is, in general, accepted that modern-style plate-tectonic processes were active by the Palaeo-Mesoproterozoic when large stable cratons had formed, against which trailing margins (Dann 1991; Loukola-Ruskeeniemi et al. 1991), Andean-type margins and collisional belts could develop (Windley 1995). Summarizing different models of Archaean continental evolution, Windley (1986, 1995) suggested that the most viable model basically combines a back-arc marginal basin setting for greenstone belt formation and the main arc (plutonic batholith) interpretation of the high-grade gneissic complex. Recently, Kusky (2004a, b) and Condie & Kröner (2008) summarized some geological indicators and suggested that modern plate tectonics were operational, at least in some places on Earth, by 3.0 Ga or even earlier, and that they became widespread by 2.7 Ga, not as a single global ‘event’ at a distinct time. The North China craton provides considerable positive and negative geological evidence for the above-mentioned models.
Some previous studies (Ouyang & Zhang 1998; Wu et al. 1998; Geng 1998; Zhai et al. 2005) reported several continental nuclei in the NCC greater than c. 3.0–3.8 Ga old, which are surrounded by late Archaean greenstone belts, although in these belts nearly all the rocks underwent amphibolite- to granulite-facies metamorphism. Nd, Pb and Hf isotopic geochemical and geochronological studies have shown that there are two discernible peaks at c. 2.7 and 2.9 Ga, representing main crustal growth epochs (Zhang et al. 1998). Zhai (2004) and Kusky et al. (2007a) also emphasized that c. 2.5–2.55 Ga is another important growth epoch.
Almost all Archaean rocks in high-grade regions or greenstone belts in the NCC underwent high-grade metamorphism at amphibolite or granulite facies, some at high-pressure granulite facies and ultrahigh-temperature granulite facies. They commonly have a multistage metamorphic history. For example, upper amphibolite- and granulite-facies rocks are commonly overprinted by low- to mid- amphibolite-facies assemblages. Two important metamorphic events in the NCC took place at 2600–2450 Ma and 1950–1800 Ma (Zhai et al. 2000a; Zhai & Liu 2001a, b; Zhao 2001), which indicate, respectively, that the united North China craton fundamentally formed by amalgamation of several microblocks at the end of the Archaean and the cratonization of a present-scale NCC was finally accomplished in the Palaeoproterozoic (Cheng 1994; Zhai 2008).
Palaeoproterozoic mobile belts with ages of 2300–1950 Ma, such as those of Jinyü and Jiaoliao, represent intracratonic orogenesis with some Phanerozoic orogenic characteristics. The evolution of the mobile belts includes basin rifting, subduction and final collision at c. 1900 Ma. After this, extensive migmatization and metamorphism occurred, termed the 1800 Ma event, followed by continental rift formation and the intrusion of mafic dyke swarms. This event led to the metamorphosed basement of the NCC being uplifted as a whole and becoming exhumed to the surface by a series of complicated detachment structures.
Oldest rocks and continental nucleus
Oldest rocks
Liu et al. (1992), Song et al. (1996) and Wan et al. (2001) reported zircon U–Pb isotopic data of c. 3.8 Ga from the NCC, indicating the presence of old continental crust at two localities, near Tiejiashan, NE China and near Caozhuang, eastern Hebei (Fig. 1).
Distribution of Early Precambrian rocks in the NCC.
The Neoarchaean Anshan complex is divided into three parts: the Tiejiashan gneiss, the Anshan supracrustal series and the Anshan gneiss (Zhai et al. 1990). The Anshan supracrustal series contain abundant banded iron formation rocks with Sm–Nd isochron ages of 2.7–2.66 Ga. The granitoid protoliths of the Anshan gneiss were intruded extensively into the Anshan supracrustal series at 2.55–2.47 Ga, based on their zircon U–Pb ages. The Tiejiashan gneiss constitutes the basement of the Anshan supracrustal series and includes a series of granitic and trondhjemitic orthogneisses and deformed felsic veins (Fig. 2b). Their zircon U–Pb ages range from c. 2960 Ma in the central part to c. 3306 Ma in the northern part (Liu et al. 1992). Two samples of sheared granitic gneiss were collected from Chentaigou village in the northwestern margin of the Tiejiashan gneiss. Liu et al. (1992) reported that ion microprobe U–Pb analyses show that the samples contain two generations of zircons, with ages of 3805±5 Ma and 3300 Ma. The zircon in a layered siliceous supracrustal rock near Chengtaigou yielded a sensitive high-mass resolution ion microprobe (SHRIMP) age of 3362±5 Ma that was interpreted as an age of detrital zircon (Song et al. 1996). Wan et al. (2001) further reported that the oldest Tiejiashan gneiss occurs as a large lens in the trondhjemitic country gneiss, and the latter yielded a 3.1 Ga SHRIMP zircon U–Pb age. In situ zircon Hf isotopic analyses indicate that these old granitoid rocks were derived from juvenile crust with age peaks of crustal growth at c. 3.4, 3.6 and 3.9 Ga (Wu et al. 2008). These data show that the Anshan complex has a very complicated geological history.
Sketch maps of (a) Caozhuang area and (b) Tiejiashan area.
The oldest supracrustal remnant in the NCC is the Caozhuang complex in Eastern Hebei Province (Fig. 2a). The Caozhuang complex can be divided into two parts; the Caozhuang Group and the Huangboyü banded grey gneiss. The Caozhuang Group is a slab 1.9 km long and 400–500 m wide that is a complicated synformal fold structure. The Huangboyü grey gneiss was intruded into the supracrustal rocks of the Caozhuang Group prior to intense multi-stage deformations. Zircons from the Huangboyü gneiss have zircon U–Pb ages of 3.0–3.3 Ga (Zhao, Z. P. 1993). The main supracrustal rocks include amphibolite, serpentinized marble, fuchsitic quartzite, metamorphosed calc-silicate rock, banded iron formation, biotite gneiss, and sillimanite–plagioclase gneiss. These rocks were metamorphosed to upper amphibolite facies to granulite facies (Yan et al. 1991). The amphibolites yielded Sm–Nb isochron ages of c. 3.5 Ga (Huang et al. 1983; Jahn & Zhang 1984). Zircons from the fuchsitic quartzite are colourless to lilac and the rounded shape of the grains is attributed to abrasion during sedimentary transport. Eighty-two analyses on 61 zircons using the SHRIMP technique yielded four age populations of 3.83–3.82, 3.8–3.78, 3.72–3.7 and 3.68–3.6 Ga. Higher U zircons rims were distinguished, which imply high-grade metamorphic events at 2.5 Ga and 1.89 Ga (Liu et al. 1992). Wu et al. (2005) reported in situ zircon Hf analyses from the fuchsitic quartzite. The data show that the Lu–Hf system remained closed during later thermal disturbances. Zircons with concordant ages have Hf isotopic model ages of about 3.8 Ga, suggesting a recycling of this ancient crust.
Continental nucleus and crustal growth
Besides these two oldest crust remnants described above, several additional old continental nuclei in the NCC have also been proposed; for example, the Huai'an complex in the western–central NCC, the Yishui complex in the eastern NCC, the Xinyang in the southwestern NCC and the Longgang complex in the northeastern NCC (Fig. 1a). They have isotopic ages 3.5–3.3 Ga (Kröner et al. 1987; Guo et al. 1991), 3.1–2.97 Ga (Wu et al. 1998) and 3.1–3.0 Ga (Zhai & Windley 1990), respectively.
The old continental nuclei, in general, are surrounded by late Archaean and Proterozoic supracrustal rocks as in other cratons (Bai et al. 1993). Zhai & Liu (2001a) reported that estimated volumetric crustal growth of the NCC was about 90% by 2.5–2.45 Ga, based on new geological maps, geophysical data and geochronological data. The Nd isotope, Hf isotope and trace element characteristics of early Precambrian rocks in the NCC and their implication for crustal growth have been discussed by various workers (Jahn 1990; Geng & Liu 1997; Zhang 1998; Liu et al. 2004; Wan et al. 2005; Wu et al. 2005; Zhai et al. 2007). The Sm–Nd TDM ages can roughly indicate the formation age of the crust. Figure 3b is a histogram of Nd TDM ages from mafic igneous rocks. The samples with ages >3.0 Ga account for c. 15% of the total, whereas those with ages <2.5 Ga account for only c. 7%, and samples with TDM ages between 3.0 and 2.5 Ga account for 78% of the total. There are two discernible peaks, at c. 2.7 and 2.9 Ga (Fig. 3b). The diagram of εNd(t) v. t/Ga (Fig. 3a) shows two characteristics: all values of εNd(t) are positive, and there is an obvious change of εNd(t) with the change of t. The values of εNd(t) deviate from the depleted mantle evolution curve at about 3.0 Ga, which is attributable to contamination of crustal materials, and indicates that a thick continental crust existed in the NCC during the Neoarchaean. Rare earth elements (REE) also demonstrate the same tendency; for example, the higher La/Nb ratios of pre-3.0 Ga mafic rocks indicate the presence of a considerable continent crust by that time (Jahn 1990). The most mafic granulites and amphibolites from the NCC display REE patterns similar to those of basalts from island arc, continental margin and within-continent settings, indicating that these rocks formed in different tectonic settings. However, few samples have mid-ocean ridge basalt (MORB) characteristics. The Hf isotopic model ages range from c. 1950 to 3800 Ma; the main range is 2600–3000 Ma with a peak at 2820 Ma (Fig. 3c). However, zircon U–Pb ages from magmatic rocks, mainly orthogneisss, demonstrate several ranges of 3600–3800, 3000–3200 and 2700–2900 Ma, with a peak value at 2500–2600 Ma, the latest of which indicates a extensively crust partial melting event in the NCC.
εNd(t)–t/Ga diagram (a), Sm–Nd tDM age histogram (b), and Hf isotopic model ages (c) of mafic igneous rocks from the NCC.
Archaean regions and cratonization
Archaean regions in the NCC
Figure 4 shows the distribution of greenstone belts in the NCC. According to rock associations, greenstone belts of amphibolite facies include the Yanlingguan in the central–eastern NCC, the Dengfeng in the southwestern NCC, the Dongwufenzi in the northwestern NCC, the Wutaishan in the central–western NCC and the Qingyuan (Hongtoushan) in the northeastern NCC. The Yanlingguan greenstone belt formed at c. 2700–2900 Ma, and is composed of bimodal volcanic rocks and sedimentary rocks metamorphosed to amphibolite facies. Mafic rocks are mainly amphibolites, which are of tholeiitic type. Most rocks exhibit slight light REE (LREE) enrichment and others have flat or LREE-depleted patterns (Wan et al. 1998; Zhai 2004). Some hornblendites and pyroxenites are similar to komatiites in chemistry, but typical spinifex texture is rare, possibly because of metamorphic recrystallization (Cheng & Xu 1991; Cheng & Kusky 2006). The metamorphosed sedimentary rocks include banded iron formations (BIFs), metapelites, and/or marble. The other four greenstone belts formed at the end of the Neoarchaean, although they contain some 2700–2900 Ma volcanic–sedimentary rocks and experienced a long evolution history (Zhai 1997; Geng 1998; Zhao et al. 1999; Geng et al. 2002). Their volcanic rocks have compositions ranging from basalt to andesite and to rhyolite, and show geochemical characteristics of an island arc association. The BIFs in the Wutaishan greenstone belt form industrial deposits of iron ore, and the Qingyuan greenstone belt contains Cu–Zn massive sulphide deposits with a few BIFs.
Distribution of the greenstone belts and microblocks in the NCC.
High-grade granulite–gneiss complexes are extensively developed in the NCC. The high-grade regions, in general, represent ancient crust, and are surrounded by greenstone belts. The main high-grade regions include the Baishanzhen and the Anshan in NE China, the Qian'an in Eastern Hebei, the Taishan in Shandong, the Huai'an at the junction of Hebei–Shanxi–Inner Mongolia, and the Lushan in central Henan. The greenstone belts and high-grade regions were generally intruded by granitic gneisses and charnockites, and underwent deformation together. The high-grade regions are composed of orthogneisses (80–90%), rootless metagabbros (5–10%) and slabs of supracrustal rocks (10–15%), associated with strong and complicated deformation. The supracrustal rocks comprise mafic granulites or amphibolites, BIFs, and medium-grained biotite gneisses with chemistries of slate–greywacke and intermediate–acid volcanic rocks (Sills et al. 1987). The Qian'an complex of granulite facies and the Anshan complex of amphibolite facies are characterized by abundant BIFs, although BIFs in high-grade regions are uncommon in the world. Field relations and geochemical characteristics show that these BIF-bearing supracrustal rocks and related intrusions were produced by subduction-related processes. Early arcs and back-arc basins that were engulfed by later plutonic rocks developed in the active continental margin (Zhai & Windley 1990; Windley 1995; Kröner et al. 2001).
Some previous studies (Deng et al. 1996; Ouyang & Zhang 1998; Wu et al. 1998; Zhai et al. 2005) proposed that the NCC can be divided into several microblocks with continental nuclei greater than c. 3.0–3.8 Ga old, which are surrounded by late Archaean and Proterozoic rocks. The greenstone belts are interpreted as arc–back-basin associations whereas the 3.0–2.5 Ga high-grade regions are thought to represent a continental margin or island arc association. The suggested microblocks are outlined by the greenstone belts (Fig. 4) and are, from east to west, the Jiaoliao (JL), Qianhuai (QH), Fuping (FP), Ji'ning (JN) and Alashan (ALS) blocks, with another two blocks, Xuhuai (XH) and Xuchang (XCH), in the south. Recent studies have led to a consensus that the basement of the NCC was composed of several blocks or terranes that were finally amalgamated to form a coherent craton (Zhao, Z. P. 1993; Wu et al. 1998; Zhai et al. 2000a; Kusky & Li 2003; Zhao, G. C. et al. 2005; Zhai & Peng. 2007).
The Archaean Eastern Hebei terrane: remnants of ancient oceanic crust or island arc
The Archaean Eastern Hebei region is a key area for understanding the early Precambrian evolution of the NCC (Fig. 5). Archaean rocks are well developed in the Eastern Hebei region, and include TTG and granitic gneisses, gabbroic intrusive rocks, and BIF-bearing supracrustal rocks metamorphosed to granulite and high-grade amphibolite facies (Geng 1998; Wu et al. 1998; Li, J. H. et al. 1999; Zhai & Liu 2003). These rocks constitute three lithological–tectonic units: the Paleoarchaean Caozhuang complex (CZC), the Meso-Neoarchaean Shuichang complex (SCC) and the Neoarchaean Zunhua complex (ZHC) (Fig. 5). The CZC represents the oldest sial crust of 3.8–3.5 Ga in the NCC, occupying only c. 4 km2. A Quaternary sequence covers the complex in the east, and the Palaeoproterozoic Changcheng System unconformably overlies the complex to the west and south. To the north, the CZC is in contact with the SCC by a ductile shear zone that is multiply deformed, and the CZC was finally thrust up on the SCC (Zhao, Z. P. 1993). The CZC includes metamorphosed supracrustal rocks and tonalitic gneisses named the Caozhuang Group and Huangboyü gneiss, respectively. The Caozhuang Group is composed of thin interlayered metavolcanic (30–53 vol.%) and sedimentary (65–70 vol.%) rocks, which include amphibolites, BIFs, serpentine marbles, fuchsite quartz schists and a few tremolitites and diopsidites. The protoliths of the amphibolites and tremolitites are tholeiitic or komatiitic. The amphibolites contain slightly high REE abundance with slightly enriched LREE patterns, and their contents of Cr (c. 226 ppm), Ni (c. 144 ppm) and MgO (up to 9.44%) are also slightly high. These basalts were proposed to be derived from larger-proportion mantle melting (Zhai et al. 2005). The Huangboyü tonalitic gneisses underwent very complicated deformation. Their zircon U–Pb ages are 3.4–3.2 Ga (Li, Z. Z. et al. 1980; Zhao, Z. P. 1993). The CZC is similar in its rock association and chemistry to other Meso-Neoarchaean high-grade complexes in the NCC (Li, Z. Z. et al. 1980; Wang, R. M. et al. 1985; Zhao, Z. P. 1993). Zhai & Liu (2003) suggested that this primary sial crust possibly was an old island arc related to subduction of intraoceanic crust. The SCC occurs as a dome with area of c. 700 km2, and is, therefore, traditionally termed the Qian'an migmatite–granitoid uplift terrane (Qian et al. 1985) or the Qian'an gneiss dome (Geng et al. 2006a). The U–Pb zircon ages of diorite and granodiorite of the Qian'an dome are 2499 Ma and 2494 Ma respectively (Liu et al. 1990). It is unconformably covered by the Palaeoproterozoic Changcheng System and Mesozoic sediments on its western and eastern margins respectively . Its northern boundary is a fault that separates it from the Neoarchaean ZHC. The meta-supracrustal rocks and some banded orthogneisses commonly occur as complicated folded slabs along the margins of the terrane, especially on the western and northern margins. The main supracrustal rocks are mafic granulites, pyroxene amphibolites, pyroxene-bearing biotite–plagioclase gneisses, and a few garnet–sillimanite gneisses and garnet-bearing fayalite peridotites (eulysite). Abundant BIFs constitute important iron deposits in China. The supracrustal rocks underwent metamorphism of granulite facies with moderate pressure. The supracrustal rocks and intrusive granitic sills have Sm–Nd and zircon U–Pb ages of 3280–3049 Ma (Geng 1998). The metamorphosed supracrustal rocks and banded gneisses of the SCC are considered to be a typical example of an old island arc complex, based on the rock association, deformation and geochemistry (Sills et al. 1987; Wang, R. M. et al. 1985; Windley 1995). However, granitic and charnockitic bodies occurring along the northern margin yield much younger zircon U–Pb ages (2647–2495 Ma) than those of supracrustal rocks (Liu et al. 1990; Zhao, Z. P. 1993).
Sketch map of Archaean outcrops of the Eastern Hebei region.
The ZHC (Li, T. S. 1999) can be divided into two parts: the linear Zunhua unit of high-grade amphibolite facies (partially granulite facies) and the dome-shaped occurrences of the Taipingzhai unit of granulite facies. The Zunhua unit occurs as strata trending NE–SW, and it mainly comprises supracrustal sequences with some granitoid sills. The Taipingzhai unit occurs as domes with complicated folds and underwent granulite-facies metamorphism; it mainly comprises TTG orthogneisses, gabbroic rocks and some small lenses of supracrustal rocks. The supracrustal rocks in the two units are almost the same in geochemistry, metamorphic history and geochronology. The main rock types include amphibolites or two-pyroxene granulites, biotite felsic gneisses or intermediate–acid granulites, and BIFs (He et al. 1991). The granitic gneisses of the Zunhua unit have zircon SHRIMP U–Pb ages of 2495–2536 Ma (Geng et al. 2006a). Metavolcanic rocks demonstrate an evolving trend from basaltic to andesitic and to rhyolitic, and have similar geochemistry to modern island arc volcanic rocks (Zhai et al. 1990; Wan et al. 1998; Li, J. H. et al. 1999). The Taipingzhai unit comprises tonalititc granulites and some metagabbros (mafic granulites). Their Rb–Sr and Sm–Nd isochron ages are 2470±70 Ma and 2480±125 Ma (Jahn & Zhang 1984; Jahn 1990), and they have single-grain zircon U–Pb ages of 2480–2530 Ma (Pidgeon 1980), and a zircon SHRIMP U–Pb age of 2564 Ma (Geng et al. 2006a). The Sm–Nd mineral–rock isochron ages for mafic rocks from Zunhua and Taipingzhai are 2591±142 Ma and 2644±112 Ma respectively (Li, T. S. 1999). The metamorphic history of mafic granulites is characterized by a mineral reaction texture in which clinopyroxene is surrounded by small garnets, indicating an anticlockwise PT path (Li, T. S. 1999). Therefore, Li, T. S. (1999) and Zhai & Liu (2001a) suggested that the Zunhua unit and the Taipingzhai unit jointly constitute a Neoarchaean island arc terrane, respectively representing the upper part and the root part. From the above, it can be seen that the eastern Hebei high-grade region is mainly composed of three ancient island arc terranes that formed during the Palaeoarchaean, Mesoarchaean and Neoarchaean periods respectively (Zhai & Liu 2003). This seems to indicate a tectonic process of island arc terrane accretion achieved by arc–arc or arc–micro continent collision (Zhai et al. 2005).
However, Wang, R. M. et al. (1985) suggested that the Zunhua unit is a high-grade metamorphosed greenstone belt. The geochemical characteristics of the metamorphosed mafic rocks have been modified by later metamorphism, and possibly they were oceanic tholeiitic. Kusky et al. (2001, 2004a, b), Li, J. H. et al. (2002), Huang et al. (2004), Kusky (2004a, b) and Huson et al. (2004) reported that ultramafic rocks occurring as lenses in Dongwanzi, in the northeastern part of the ZHC, were Archaean oceanic mantle. After opposition from Zhai et al. (2002), Zhang, Q. et al. (2003) and Zhao, G. C. et al. (2007) on the occurrence, geochemistry and rock association of the Dongwanzi ultramafic body, Kusky et al. (2007b) emphasized that the ophiolitic rocks had been intruded by five generations of magmatic rocks, and combined a series of ultramafic lenses from Dongwanzi, via Zunhua to Wutaishan and other basic metamorphic rocks, suggesting a Late Archaean ophiolite mélange zone. Li, J. H. & Kusky (2007) further suggested that the Wutaishan complex represents an island arc assemblage, and the Hutuo Complex represents a foreland complex. These two complexes and the Zunhua ophiolites together constitute an Archaean collisional zone in the central NCC, where finally the eastern Hebei terrane and western block amalgamated to form a coherent Archaean NCC. This model is different from those of Archaean arc–arc or arc–continent amalgamation (Zhai 1997, 2004) or Palaeoproterozoic continental collision (Zhao, G. C. et al. 1999, 2005).
Condie & Kröner (2008) proposed some tracking petrological assemblages as geological indicators in Early Precambrian geotectonic process for modern plate tectonics; these are ophiolites, arc or back-arc assemblages, accretionary prisms and ocean plate stratigraphy, a foreland basin, blueschists, a passive continental margin, and ultrahigh-pressure metamorphic rocks and paired metamorphic belts. Some of these petrological assemblages have been thought to occur in the Eastern Hebei region. For example, the BIFs and associated supracrustal rocks to some extent are similar to ocean plate stratigraphy. The TTG orthogneisses are similar to the Andean-type continental margin rock association, and the orthogneisses are converted by later deformation and associated high-grade metamorphism from the tonalities and granodiorites, although these magmatic and tectonic processes were probably more intense in the Archaean than in late Phanerozoic time (Windley 1986). However, ophiolites, blueschist and ultrahigh-pressure metamorphic rocks are difficult to find in the NCC. Voluminous granulites, TTG gneisses, komatiites, and the absence of UHP rocks and blueschists probably suggest that plume tectonics is important in the Archaean. In any case, it is reasonable that the processes of accretion and crustal growth in ancient and modern times may be different more in degree than in principle.
Cratonization and amalgamation event
Mainly based on early Precambrian geology of the Eastern Hebei region and combining other Archaean terranes, Cheng (1994) proposed a two-stage cratonization model of the NCC. The first stage took place in the Neoarchaean, when several microblocks were amalgamated and underwent granulite-facies metamorphism to form a present-scale NCC. The second stage took place at c. 2.5 Ga (the time of the boundary between the Archaean and Proterozoic), when the small ocean basins and continental rifts within the craton closed, with amphibolite-facies metamorphism and migmatization. Zhao, Z. P. (1993) suggested that the NCC fundamentally formed since c. 3.0 Ga and evolved to a platform-style craton at the end of the Neoarchaean, when it experienced sodium (TTG intrusive event), sodium–potassium (TTG and granitic intrusive event) and potassium (granitic intrusive event) cratonizations. The above-mentioned models are established on, respectively, the horizontal and vertical crustal growth.
Coeval ultramafic–syenitic dykes
Extensive granitic intrusive bodies or sills and mafic dyke swarms are marks of cratonization (Brown 1979; Zhao, Z. P. 1993; Windley 1995). Emplacement of ultramafic–mafic and alkaline dykes has long been considered to mark a post-orogenic or anorogenic event, and thus can be used to constrain the time of cratonization. The Neoarchaean granitoids and mafic dykes in the eastern Hebei region have been described in detail (Qian et al. 2005; Geng et al. 2006a).
Extremely rare Archaean coeval ultramafic–mafic and syenitic dykes have been found in the CZC, Eastern Hebei region (Li, T. S. et al. 2010). Figure 2a is a geological sketch map of the CZC. The>3.0–3.3 Ga rocks occupy half of the area of outcrop, and include BIF-bearing supracrustal rocks and TTG–granitic gneisses. The other half consists of Neoarchaean granitic rocks, including hypersthene granite with an age of 2.52–2.56 Ga, granite of 2.53 Ga and monzodiorite of 2.60 Ga (Zhao, Z. P. 1993). The country rocks of the ultramafic–mafic and syenitic dykes are gneissic monzodiorite and BIF-bearing supracrustal rocks. The intrusive boundaries are clear. The dykes cut the gneissosity of the country rocks and show typical spherical weathering in the field. They occur as subvertical intrusions with a NW310° strike, and are up to 300 m long and about 10 m wide.
The ultramafic–mafic dykes are dark grey coloured. They consist mainly of clinopyroxene, olivine, and minor orthopyroxene, hornblende, biotite, Fe–Ti oxides and spinel, and may be with or without plagioclase. Depending on the existence of the plagioclase, the dykes can be subdivided into olivine pyroxenite and olivine gabbro, which have medium granular texture and gabbroic texture, respectively. Reaction rims of orthopyroxene and hornblende replacing olivine are common. Also observed in the dykes is exsolution of orthopyroxene along cleavages of clinopyroxene. The syenitic dykes (syenitite) consist predominantly of orthoclase and biotite, with minor plagioclase (<7%), quartz (<5%) and Fe–Ti oxides, without pyroxene. The olivine gabbro sample 04QA09 and syenitite sample 04QA08 for zircon SHRIMP U–Pb dating were collected in the Caozhuang–Naoyumen area at 39°55′55″N, 118°33′16″E. The Th/U ratios are 0.83–1.90 and 0.91–1.78 for zircons from the olivine gabbro and syenitite dykes, respectively. Because of low zircon amounts, only six zircon grains were determined from the olivine gabbro, and they yielded a weighted mean 207Pb/206Pb age of 2516±26 Ma (Fig. 6a), interpreted as the crystallization age. Fifteen zircon grains from the syenitite dyke yielded a weighted mean 207Pb/206Pb age of 2504±11 Ma (Fig. 6b), also interpreted as the crystallization age of the syenitite. Zircons from the olivine gabbro have 176Hf/177Hf ratios varying from 0.281286 to 0.281368, yielding single-stage Hf model ages between 2668 and 2740 Ma, with a mean age of 2705 Ma, and εHf(t) values between 1.9 and 4.2, with a weighted mean value of 3.13±0.40. Zircons from the syenitite have 176Hf/177Hf ratios varying from 0.281280 to 0.281382, yielding single-stage Hf model ages between 2646 and 2705 Ma, with a mean value of 2677 Ma, and εHf(t) values between 2.7 and 4.4, with a weighted mean value of 3.56±0.23.
Concordia plots of zircon SHRIMP U–Pb analytical data and diagrams of Th/U age (Ma): (a) for olivine gabbro (04QA09; (b) for syenitite (04QA08).
The olivine gabbros with Mg-numbers values of 59–63 are similar to high-magnesian tholeiitic basalt. They demonstrate relatively LREE-enriched patterns without Eu anomalies (La/YbN=9.28–9.78, Gd/YbN=2.8, δEu=0.96–1.00), with enrichment in large ion lithophile elements (LILE) and depletion in high field strength elements (HFSE). They also have high Cr and Ni contents, and have Zr/Hf and Nb/Ta ratios similar to those of the primitive mantle. The syenitites are alkaline in composition, with 8.67–8.88 wt% Na2O+K2O, and show high total REE contents (543–854 ppm) and strongly LREE-enriched patterns with minor Eu negative anomalies (La/YbN=50–101, Gd/YbN=5.2–5.3, δEu =0.80–0.82). Spider diagrams of trace elements show a strong LILE enrichment relative to HFSE. Zr/Hf and Nb/Ta ratios are 47–49 and 26–76, respectively, and are much higher than those of primitive mantle and higher than the average Nb/Ta ratio of post-Archaean continental crust. The petrological and geochemical features indicate that these dykes were derived from a deep subcontinental lithospheric mantle source, which implies that the NCC probably had a large-scale continental crust with a considerable thickness. Although it is difficult to estimate the depth of the magma source, we may confirm that the coeval ultramafic–mafic and syenitic dykes should be derived from a deep mantle based on their petrology and geochemistry.
Archaean–Proterozoic boundary and cratonization
The coeval olivine gabbro and syenitite dykes yield c. 2.5 Ga zircon SHRIMP U–Pb ages. It is note worthy that 2.5 Ga is the boundary age between the Archaean and Proterozoic. In the period between c. 2500 and 2350 Ma, the NCC was tectonically inactive, but the development of Palaeoproterozoic volcanic and granitic rocks occurred between 2350 and 1950 Ma. The Archaean rocks and Proterozoic rocks demonstrate an enormous difference in rock association and geochemistry. The connotation of the Archaean–Proterozoic boundary is global cratonization and basically formation of continents at their present scale. Windley (1995), Condie et al. (2001) and Rogers & Santosh (2004) suggested that there was a c. 2.5 Ga Neoarchaean supercontinent, followed by a continental break-up in the Palaeoproterozoic.
Extensive granitic intrusive bodies or sills and mafic dyke swarms are marks of cratonization (Zhao, Z. P. 1993; Windley 1995). Emplacement of ultramafic–mafic and alkaline dykes has long been considered to mark a post-orogenic or anorogenic event, and thus can be used to constrain the time of cratonization. The Neoarchaean granitoids and mafic dykes in the eastern Hebei region have been described in detail (Qian et al. 2005; Geng et al. 2006a). Figure 5 shows the locations of the Late Neoarchaean granites with SHRIMP zircon U–Pb ages, which are widely distributed in the Eastern Hebei region and intruded into various metamorphic supracrustal rocks and magmatic rocks (Geng et al. 2006a; J. H. Yang, pers. comm.). The granites occur as sills and small bodies, accompanied by strong migmatization and pegmatite intrusion. As the youngest magmatic intrusion in Archaean, the coeval ultramafic–mafic and syenitic dykes were intruded into the c. 2.5–2.6 Ga granite sills and migmatite and pegmatite, indicating that the Archaean cratonization was finally accomplished.
The other microcontinental blocks in the NCC are the JL, QH, FP, JN, XCH, XH and ALS microblocks (Fig. 4). The rock types and distribution in these microblocks display distinct differences. For example, old rocks with ages up to 3.8 Ga and abundant Mesoarchaean BIFs are present only in the QH block. Rocks older than Neoarchaean are not exposed in the JN and FP blocks, although they may exist in the deep crust, based on geophysical data. Neoarchaean volcanism and magmatism in these blocks took place at 2.9–2.7 Ga and 2.6–2.45 Ga, but their occurrence in different blocks varies greatly. Volcanic activity from 2.9 to 2.7 Ga was, in general, strong in all blocks, especially in the JL, QH and XCH blocks, associated with abundant BIFs. However, BIFs are not common in the ALS block (Geng et al. 2006b). Volcanic activity at 2.5 Ga was weak in the JN block; however, it was rather intense in the JL, FP and XCH blocks. Basic–intermediate–acid volcanic rocks in the FP and XCH areas are closely associated with the BIFs. In the JL block, however, volcanic rocks contain massive sulphide Cu–Zn ores. All these differences indicate that these microblocks possibly developed in different tectonic settings; that is, they had not been amalgamated into a coherent craton until at least c. 2.5 Ga. It is noteworthy that nearly all Archaean rocks underwent metamorphism at c. 2.5–2.55 Ga, and voluminous granitic sills with an age of c. 2.5 Ga were intruded into neighbouring blocks, suggesting amalgamation prior to intrusion. For example, a series of granite bodies are located along the contact zone between the JL and QH blocks (Wu et al. 1998), indicating that the microblocks were assembled and constituted a combined NCC by the end of the Neoarchaean (Li, J. H. et al. 1999). Recent published data reveal that the granite sills and pegmatites with 2457–2570 Ma zircon U–Pb ages are extensively developed in all six microblocks (Zhang, W. J. et al. 2000; Guan et al. 2002; Luo et al. 2004; Shen et al. 2004; Wang, Z. J. et al. 2004; Yang et al. 2004; Gao et al. 2005; Jian et al. 2005; Kröner et al. 2005; Li, J.-H. et al. 2005; Wan et al. 2005; Wilde et al. 2005; Lu, X. P. et al. 2006; Zhang, H. F. et al. 2006 Liu, S. W. et al. 2007a, b). For example, the Molihong high-potassium granite and Caiyu hypersthene-bearing granite in the JL block, the Angou granite and Shipaihe granodiorite in the XCH block, the Wanzi granite and Pingshan granite in the FP block, the Guyang granite in the JN block, and the Shanhaiguan granite and Qianan granite in the QH block are representative intrusive bodies with c. 2.5 Ga zircon U–Pb ages, and clearly marked the Neoarchaean cratonization of the NCC (Zhao, Z. P. 1993; Wang, Y. J. et al. 2004; Li, J. H. et al. 2006; Zhai & Peng 2007). Therefore, it is reasonable that the NCC at the present scale formed at the end of the Archaean. Qian et al. (1985) proposed an important tectonic unconformity event between the Archaean basement and Palaeoproterozoic metamorphic sedimentary rocks (khondalite series). Although both of them underwent high-grade metamorphism and complicated deformation, they show substantial differences in rock association, geochemistry, structural style and other features. The khondalite series are, as a common Precambrian rock association, distributed throughout the NCC, and are called the Fengzhen Formation in the JN block, the Lüliang Formation in the FP block, the Fengzishan Formation in the JL block, the Huoshan Formation in the XH block, the Louzishan Formation in the QH block, and the Lushan Formation in the XCH block. Detailed geological and geochemical studies support that the khondalite series were mainly formed at c. 2.30 Ga; this series is mostly composed of argillo-arenaceous rocks, and therefore requires a relatively large-scale Archaean cratonic basement (Wan et al. 2000a, b).
Palaeoproterozoic mobile belts and high-temperature–high-pressure metamorphism
Palaeoproterozoic mobile belts
The NCC behaved as a stable continent block without tectonic–thermal action during the period c. 2500–2350 Ma, similar to other cratons in the world (Condie et al. 2001). A Palaeoproterozoic volcanic and granitic event with an age of 2300–1970 Ma is widespread in the NCC. These Palaeoproterozoic volcanic–sedimentary rocks typically occur as linear fold belts, commonly unconformable over Archaean basement, and underwent low-grade metamorphism and intrusion by granites. Their occurrences are different from the Archaean greenstone–granite belts and demonstrate Phanerozoic orogenic characteristics in some places; therefore Zhai & Peng (2007) termed them Palaeoproterozoic mobile belts. Three Palaeoproterozoic mobile belts in the NCC are shown in Figure 7, which are located, respectively, in northeastern China, central–western China and northwestern China. The representative rock sequences are the Liaohe Group and the Fengzishan Group in the Liaoji Mobile Belt, the Lüliang Group, Hutuo Group and Zhongtiao Group in the Jinyü Mobile Belt, and the Fengzhen Group in the Fengzhen Mobile Belt.
Distribution of Palaeoproterozoic mobile belts in the NCC.
The volcanic–sedimentary rocks in the Liaoji Mobile Belt are called the Liaohe Group in NE China and the Fenzishan Group in Eastern Shandong Peninsula, and are, respectively, metamorphosed to greenschist–amphibolite facies and amphibolite–granulite facies. The rocks are divided into two formations. The lower formation is composed of basic–acid volcanic rocks and sedimentary rocks. The volcanic rocks have bimodal petrological and geochemical characteristics. The basic rocks have the chemistry of continental tholeiitic basalt and the felsic volcanic rocks show Si oversaturation and are enriched in Na and K. The sodium-felsic rocks contain thick boron formations with tourmalinization. The sedimentary rocks include argillaceous schists, siltstones and marbles, in which graphites commonly occur. The upper formation is layered detrital–carbonate sedimentary rocks containing a major magnesite mineral deposit. Zhang, Q. S. (1984) suggested that these rocks formed in a rift environment. However, Bai et al. (1993) emphasized that basic volcanic rocks in the Liaohe Group have the characteristics of marine basalts and sedimentary rocks are turbiditic. Three metamorphism stages have been described by Li, S. Z. et al. (1998): vertically progressive, laterally progressive and extensional. Deformed Palaeoproterozoic granite and undeformed granites yield zircon U–Pb ages of 2160 Ma and 1850 Ma, respectively, and the later age shows that the latest deformation was earlier than 1850 Ma. The Liaoji Mobile Belt has been suggested to be a continental rift basin or a small old ocean basin located between the Longgang massif in NE China and the Nangrim massif in the northern Korea peninsula. With a change in tectonic regime from extension to compression, the formation of the Liaohe Mobile Belt is related to a change from subduction to collision (Bai et al. 1993; Chen et al. 2003; Lu, X. P. et al. 2006).
The Jinyü Mobile Belt includes several Palaeoproterozoic formations, which are developed in the region 34°15′–39°N and 110°45′–114°50′E. The rocks have similar protoliths, tectonic style and metamorphic history, and are named the Hutuo Group, Lüliang Group and Zhongtiao Group in central–northern Shanxi, western Shanxi, and southwestern Shanxi–northwestern Henan Provinces. The Hutuo Group is composed of conglomerates, sandstones, pelites, basic volcanic rocks, and basic and felsic tuffs. Some basic volcanic rocks or basic dykes have three groups of SHRIMP zircon U–Pb ages of c. 2450–2390 c. 2200–2000 and c. 1900–1800 Ma (J. S. Wu et al. pers. comm., 2008; Z. J. Wang, pers. comm., 2008). We prefer to interpret them, respectively, as residual, magmatic and metamorphic ages (Zhai & Peng 2007), although other researchers have different opinions. On the basis of the rock associations and geochemistry, these rocks have been interpreted to have been deposited in a continental rift (Zhao, Z. P. 1993; Miao et al. 1999) or foreland basin (Li, J. H. et al. 2000, 2006). The sedimentary rocks in the Zhongtiao Group are mainly coarse- to medium-grained clastic rocks, pelites and carbonates, and the volcanic rocks are basalts, basaltic andesites and minor dacites. Sun & Hu (1993) reported that the geochemical characteristics of basic and intermediate–acid volcanic rocks are similar to those of island arc volcanic rocks. Their metamorphic grade is greenschist or amphibolite facies. The geochronological data for the metamorphosed volcanic rocks, granites and related Cu-ores are consistent, and can be divided into three rock-forming periods at 2.36–2.32 Ga, 2.16–2.01 Ga and 2.09–2.06 Ga, and a metamorphic period at 1.9–1.83 Ga. The geochronological data are similar to those of the Hutuo Group. Basaltic rocks in the Lüliang Group show geochemical characteristics of continental basalt with εNd(tDM) of +3.0. The felsic volcanic rocks are rich in LILE and ΣREE have high La/Yb values (Geng et al. 2003). Zircon U–Pb ages are 2360–2031 Ma for basic volcanic rocks, and 2050–2031 Ma for granites (Yu et al. 1996, 1997; Wan et al. 2000a, b). The rocks of the Lüliang Group also record a 1866–1850 Ma metamorphic event, associated with charnockite intrusion at 1800 Ma (Yu et al. 1997; Geng et al. 2003).
The Fengzhen Group in the Fengzhen Mobile Belt is a thick sequence of metasedimentary rocks rich in aluminium, containing little volcanic material, which underwent granulite-facies metamorphism (Qian et al. 1985; Shen et al. 1992; Wu et al. 2000). The two formations of the Fengzhen Group are a lower metamorphosed detrital–pelite formation and an upper carbonate formation. The lower formation includes mainly quartzite with or without garnet, graphite gneiss, mica–quartz schist and garnet–sillimanite gneiss, which are called khondalite sequences. The upper formation is mainly composed of marbles with a few mica schists and calc-silicate rocks. Although some geologists believe that the Fengzhen Group is Archaean in age based on geological features (Zhao, Z. P. 1993; Lu, L. Z. et al. 1995; Qian 1996), zircon U–Pb ages of the khondalites support an interpretation of their being Palaeoproterozoic (Wan et al. 2000a; Wu et al. 2000; Zhao, G. C. et al. 2005). The protoliths of these rocks were deposited either in a cratonic basin (Condie et al. 1992; Qian 1996; Wan et al. 2000a; Xu et al. 2005) or in a marginal sea (Lu, L. Z. et al. 1995). Another opinion worth noting is that the clockwise PT path of the khondalites from moderate-pressure granulite facies to amphibolite facies may be considered to represent a continental collision (Wu et al. 2000). Recently, high-pressure and ultrahigh-temperature granulites have also been reported in the Fengzhen Group (Lu, L. Z. et al. 1995; Guo et al. 2006; Santosh et al. 2007). The ages of ultrahigh-temperature metamorphism and retrograde high-temperature metamorphism of sapphirine-bearing Mg–Al granulites are 1910–1900 Ma and 1850–1870 Ma. The garnet granites are commonly associated with the khondalites, showing intrusive or transitional contact. The garnet granites and khondalitic rocks have similar geochemical characteristics. The zircon U–Pb ages for garnet granites are 1912–1892 Ma (Guo et al. 1999a, 2005). Zhai et al. (2003a) suggested that the garnet granites are crustal partial melting granites related to an ultrahigh- or high-temperature metamorphic event, the protolith rocks are khondalitic rocks, and the estimated depth of anatexis is 35–40 km.
In short, the Palaeoproterozoic mobile belts in the NCC formed and evolved within a craton or continental margin at c. 2350–1950 Ma. Some 2350–1900 Ma rift-margin and passive continental margin deposits (St-Onge & Lucas 1990; Windley 1995), ophiolites (Kontinen 1987; Helmstaedt & Scott 1992; Kusky 2004a; Furnes et al. 2007), orogens (Hoffman 1988; Condie 2007) and BIF-bearing foreland basins (Hoffman 1987; Giles et al. 2002) in other cratons seem to be comparable with many in the late Phanerozoic (Windley 1995). Compared with other Palaeoproterozoic basins and belts in the world, we agree that these rocks herald a new major stage in Earth history, and their geotectonic environment is different from comparable Archaean examples (Windley 1986). The Palaeoproterozoic mobile belts in the NCC have Phanerozoic orogenic characteristics, indicating that plate tectonics was operative in the NCC at a much smaller scale.
High-pressure granulites and retrogressed eclogites
High-pressure mafic granulites and retrogressed eclogites were discovered in 1992 and 1995, respectively (Zhai et al. 1992, 1995). The first locations are situated at the junction of the QH, JN and FP blocks, in the western–central NCC. The high-pressure rocks occur within granitoid gneisses that commonly underwent strong deformation and mylonitization, associated with granite sills and pegmatite dykes. Therefore, the high-pressure granulites, retrogressed eclogites and their country rocks were named the Sanggan–Chengde structural belt or central zone, which has been proposed to be an Early Precambrian continent–continent collision belt between the QH block and FP–JN blocks, or a western block and an eastern block, similar to a modern high-pressure orogenic belt (Zhai et al. 1992; Guo et al. 1993; Zhai 1997; Zhao et al. 1999). The high-pressure rocks have two types of occurrence, as follows. (1) Long flat tectonic slabs occur at the northern boundary of the Sanggan–Chengde structural belt (Guo et al. 1993; Li et al. 1997). The high-pressure granulite layers developed together with two-pyroxene mafic granulites and intermediate to acid (tonalitic) granulites; for example, at Manjinggou, NW Hebei. They are locally sheared and mylonitized. (2) Lenses within tonalitic gneisses, migmatites and granites are distributed throughout the Sanggan–Chengde structural belt. The lenses are normally several metres to more than 10 m long, and commonly occur in groups. Locally, tens of lenses different sizes occur in narrow belts, for example at Baimashi in northern Shanxi, or as deformed or broken dykes in garnet-bearing felsic orthogneiss, for example at Mashikou in eastern Shandong (Fig. 8).
Photographs showing the gabbroic dykes of high-pressure granulite facies and their country rocks. (a–c) HP granulite dyke in orthogneiss; (d) garnet porphyroblast with symplectite of HP granulite; (e) country gneiss with folded dykes; (f) country augen gneiss; (g) folded country gneiss; (h) HP granulite lens in deformed country gneiss.
Early Precambrian high-pressure granulites in the NCC, which were first defined by Zhai et al. (1992), are based on garnet coexisting with plagioclase and pyroxene in the mafic granulites (Carney et al. 1991; Harley 1988), indicating a metamorphic mineral reaction of garnet in quartz tholeiite. Carswell & O'Brien (1993) defined these metamorphic mineral assemblages as garnet granulite facies. The typical mineral assemblages of the high-pressure mafic rocks in the NCC are clinopyroxene (Cpx)+Grt±Plg+Qtz+rutile, Cpx+Grt+Hb+Plg±Qtz and Cpx±orthopyroxene (Opx)+ Grt+Plg±Qtz. The garnets are surrounded by a symplectite of fine-grained Opx+Cpx+Plg, indicating decompression. The high-pressure rocks also underwent a strong metamorphic overprint in the amphibolite facies and, as a result, some mafic granulites have been completely or partially retrogressed to amphibolites. Liu et al. (1996) suggested two metamorphic episodes of high-pressure granulite facies in the NCC with and without orthopyroxene, whereas O'Brien et al. (2005) proposed that these garnet granulites may have formed in the same metamorphic dynamic process but were situated at different crustal depths. A minor amount of eclogites were retrogressed to garnet granulites, which are found in Baimashi, Hengshan, Shanxi Province only. Mineral assemblages and reaction textures reveal three metamorphic stages that are successively of eclogite, high-pressure granulite and amphibolite facies. The mineral assemblage of the first eclogite stage is composed of Ca-rich garnet+pseudomorphed omphacite+rutile+quartz, which are preserved as inclusion minerals in garnets. Pseudomorph omphacite is composed of a very fine-grained aggregation of hypersthene, diopside and albite, which retains the crystal form and whole chemical composition of omphacite. Some matrix clinopyroxenes have been broken down to a vermiform albite and Na-poor clinopyroxene symplectite, indicating previous omphacite. The estimated P–T conditions of the three metamorphic stages for the high-pressure granulites are 1.2–1.4 GPa and c. 800 °C, 0.7–0.9 GPa and c. 820 °C and 0.5–0.7 GPa and c. 600 °C, with a decompressional PT path (Fig. 9), revealing that the granulites were uplifted from depths of 45–50 km to <20 km (Zhai et al. 1992; Guo et al. 1999b, 2005; Zhai & Liu 2001b). Protoliths of the mafic high-pressure granulites and retrogressed eclogites are gabbroic dykes and small bodies, as found by studies on petrology and geochemistry (Zhai & Liu 2001b; Kröner et al. 2005; Zhao, G. C. et al. 2005). The retrogressed eclogites have similar metamorphic P–T path; the conditions are c. 1.6–1.7 GPa and c. 700–800 °C, 0.8–1.1 GPa and c. 820 °C, 0.5–0.7 GPa and c. 600 °C (Zhai et al. 1995, 2000b).
P–T path of HP granulites from Manjinggou (a) and Hengshan (b).
Several geochronological methods have been used to determine the age of these events. The SHRIMP zircon U–Pb data concentrate in three ranges: 2000–1900 Ma, 1860–1830 Ma and 1810–1760 Ma (Guo et al. 2005; Kröner et al. 2005; Peng et al. 2005; Zhang, H. F. et al. 2006), which, respectively, represent high-pressure granulite-facies, moderate-pressure granulite-facies and amphibolite-facies metamorphic stages. The whole-rock Nd tDM ages are 2640–2510 Ma, and the mineral Sm–Nd isochron ages are 1860–1820 Ma, which have been interpreted by Zhai & Liu (2003) and Guo et al. (2005) to be ages of magma source and granulite-facies metamorphism. However, the timing of the high-pressure metamorphic stage is still controversial. For example, one interpretation is that the age of high-pressure metamorphism is c. 2.5 Ga and the age of moderate-pressure granulite-facies metamorphism is c. 1.83 Ga, as indicated by symplectite minerals (Zhang, G. H. et al. 1998; Hu et al. 1999; Mao et al. 1999; Li, J. H. et al. 2000), and another is that the age of high-pressure metamorphism is c. 1.82–1.86 Ga and that the protolith age is c. 2.5 Ga (Zhao, G. C. et al. 1999, 2000; Guo et al. 2001, 2005; Kröner et al. 2001).
Recently, more locations of high-pressure granulites have been reported (Ma & Wang 1994; Li, J. H. et al. 1998; Liu et al. 1998; Zhai et al. 2000b; Tang et al. 2003), in Sifangdong, Miyun, Chengde and Jianping in the northern NCC from west to east; in Yangyuan, Fuping, Zanhuang and Xingtai in the central NCC; in Lushan, Xinyang in the southern NCC; and in Qixia, Laixi, Laiyang and Pingdu in the eastern NCC. Therefore, high-pressure granulites are developed almost throughout the area where ancient metamorphic basement rocks crop out in the NCC, not only along the Sanggan structural belt or in the central zone (Fig. 10).
Distribution of high-pressure granulites and high–ultrahigh-temperature granulites in the NCC.
High- and ultrahigh-temperature granulite-facies rocks
High-temperature (HT) granulite-facies rocks are extensively distributed in the NCC (Fig. 8). The typical rocks are khondalites, garnet–pyroxene granulites and hypersthene granites. The mineral assemblages of Grt+sillimanite (Sill)+cordierite (Cord)+Plg+Qtz±rutile±spinel (Spl), Grt+Cpx+Opx±graphite+Plg+Qtz, biotite (Bi)+ Plg+K-feldspar+Opx±Cord are marks of high-temperature metamorphism. Hypersthene granites are commonly associated with granulite-facies rocks, and used to be called charnockite or enderbite. They were derived from partial melting from old crust and closely followed high-temperature metamorphism (Shen et al. 1992; Bai et al. 1993; Lu, L. Z. et al. 1995; Zhai et al. 2003a). Ultrahigh-temperature (UHT) granulites have been reported by Guo et al. (2006) and Santosh et al. (2007), which are sapphirine (Spr)bearing khondalites. Two localities are Tuguiwula and Wuchuan in the Fengzhen Mobile Belt, Inner Mongolia. The representative UHT mineral assemblages are Spr+Grt+Spl, Grt+Spl+Sill, Spr+Qtz+Sill+Spl, and the estimated PT conditions for this assemblage are c. 1.0 GPa and >1050 °C. Secondary mineral assemblages are Bi (high-Ti)+Spr+Sill+Spl, Sil+Cord+Qtz+Grt and Spl+Opx+Qtz, and the estimated PT conditions for these are c. 1.0 GPa and 900–1000 °C. The third mineral assemblage is Spl+Plg±Sill+magnetite (c. 0.7–0.8 GPa, 850–900 °C), and the occurrence of chlorite and high-Fe biotite may represent a later mineral assemblage. HT and UHT rocks have a complicated metamorphic history in three or four stages, and commonly underwent initial isobaric cooling followed by isothermal decompression (Fig. 11; Guo et al. 2006; Santosh et al. 2007). However, the metamorphic process from LT–LP middle crustal rocks to hot lower crustal granulites was not been recorded because of overprinting by a late HT metamorphism. Lu, L. Z. et al (1995) suggested that khondalites in the Fengzhen Mobile Belt also underwent a high-pressure metamorphic stage before the HT–UHT stage with remnants of kyanite (Ky), but the Ky remnants are very difficult to find. Zhou et al. (2004) reported HP granulite-facies metamorphosed pelite with Grt+Ky+perthite in khondalitic rocks of the Fengzishan Group in the Liaoji Mobile Belt, eastern NCC. Wan et al. (2000a, b, 2003) summarized that the khondalitic rocks were deposited at 2400–2200 Ma and metamorphosed at 1880–1820 Ma, and Zhang, H. F. et al. (2006) and Santosh et al. (2007) further suggested that metamorphic ages can be subdivided into a HT–UHT metamorphic stage at 1930–1900 Ma and a secondary HT–MT metamorphic stage at 1850–1820 Ma.
P–T path of UHT rocks from Wuchan (a) and Tuguiwula (b).
Relationship of HP and HT–UHT metamorphic rocks and basement uplift
The times of metamorphism of the HP granulites and HT–UHT rocks are similar, as mentioned above, and these two kinds of granulite-facies rocks commonly occur in the same area. For example, in Laiyang and Laixi in Shandong, Huangtuyao in Inner Mongolia, Manjinggou in northern Hebei, and Gushan in northeastern Shanxi, the mafic garnet granulites are next to the khondalites. Liu & Li (2008) emphasized this distribution of HP granulites and HT–UHT metamorphic rocks, and suggested a Palaeoproterozoic paired zone in the southern part of Central Inner Mongolia. Moreover, the possible transitional rocks from HP to HT–UHT may be found; for example, garnet–two-pyroxene granulites with or without graphite are located between HP granulites and khondalites in Songtan and Haojiatan, eastern Shandong, and their PT condition is 1.0–1.1 GPa and c. 750-850 °C, between those of the HP and HT–UHT rocks (Zhai et al. 2000b). The HT–HP khondalites with Ky+Grt+K-feldspar in the HT khondalite sequence (Lu, L. Z. 1995; Zhou et al. 2004) indicate that the HP and HT–UHT rocks underwent a continuously changing range of P–T conditions. Both rocks have a peak metamorphic age at c. 1900 Ma followed by a c. 1850 Ma retrograde metamorphism, and underwent deformation with a foliation striking ENE–WSW and dipping NNW at c. 70°.
The absence of blueschists and ultrahigh-pressure metamorphic terranes make it difficult to support the operation of modern plate tectonics in the Early Precambrian, although various researchers have suggested a continental collision model based on HP granulites (Zhai et al. 1992, 1995; Zhao, G. C. et al. 1999; Li, J. H. et al. 2000; Kusky et al. 2001). The high geothermal gradient seems to indicate a shallow subduction. The estimated geothermal gradients are c. 16 °C km−1 for UP rocks and c. 22–25 °C km−1 for HT–UHT rocks, but c. 6 °C km−1 for UHP rocks in the Dabieshan orogenic belt. The estimated uplift rates are 0.33–0.5 mm a−1 for UP and UT–UHT rocks in the NCC, but 3–5 mm a−1 for rocks in the Dabieshan orogenic belt. Therefore, the uplift rates of early Precambrian high-grade metamorphic rocks in the NCC are much slower than and very different from the rates in Phanerozoic orogenic belts. The uplift rates of early Precambrian high-grade metamorphic rocks are estimated from three main metamorphic epochs of HP granulite, MP granulite and amphibolite facies, representing uplift from lower, middle–lower and middle crustal levels. The time for uplift to the surface is ascertained from 1780 Ma unmetamorphosed mafic dykes and 1780–1760 Ma unmetamorphosed volcanic rocks in the Palaeoproterozoic Xiong'er–Yanshan aulacogens that unconformably covered the high-grade metamorphic basements. The fact that HT–UHT rocks are associated with HP granulites indicates a more complicated tectonic process than modern plate tectonics. Various workers (Zhai & Liu 2003; Zhai et al. 2005; Zhai & Peng 2007) gave attention to the occurrence and metamorphic ages of HP and HT–UHT rocks in the NCC, and indicated the further possibility that these rocks represent the lowermost–lower crustal rocks with a high geothermal gradient, and were probably distributed in a broad area rather than in a narrow zone. These lower crustal rocks were rather slowly uplifted to the surface relative to the rate for Phanerozoic orogenic belts by an unknown tectonic mechanism.
Mafic dyke swarms and Palaeo-Mesoproterozoic continental rifting
Mafic dyke swarms
The Palaeoproterozoic mafic dyke swarm, extending over 1000 km in the NCC, plays an important role in understanding Palaeoproterozoic evolution of the NCC (Fig. 12; Chen & Shi 1983; Qian & Chen 1987; Zhang, J. S. et al. 1994; Hou et al. 1998, 2001; Halls et al. 2000; Li, J. H. et al. 2001; Peng et al. 2005). The dykes are vertical to subvertical with chilled margins. Single dykes are up to 60 km long, and 0.5–100 m wide, and the density of the dykes is several to tens of dykes per kilometre. The dykes have NW–SE and east–west orientations in the western–central part and NE–SW and NW–SE orientations in the eastern part. They consist of clinopyroxene and plagioclase, with Fe–Ti oxides, biotite, alkali-feldspar, apatite and quartz as accessory phases. Olivine phenocrysts can be also observed in the dykes. The dykes vary from alkaline to tholeiitic and a few from basalt to dacite in composition. All of the dykes have a high content of total rare earth elements (ΣREE) and are characterized by slight to moderate LREE enrichment. They are relatively enriched in LILE (except Sr) and depleted in HFSE. Three groups of dykes have been identified by Peng et al. (2005) on the basis of their chemistry. (1) high-Mg to high-Fe tholeiite basalt, with a low FeO(total)–TiO2–P2O5 content; (2) high-Fe basalt, with a high FeO(total)–TiO2–P2O5 content; (3) high-Fe tholeiite basalt to andesite. These three groups derived from enriched mantle, showing different differentiation trends with different degrees of crustal assimilation.
Sketch map showing distribution of Palaeoproterozoic mafic dykes and rifts.
Published ages for dykes include a single-zircon dilution U–Pb age of 1769±3 Ma (Halls et al. 2000; Li et al. 2001), and a SHRIMP zircon U–Pb age of 1778±3 Ma for a sample (SX020) from the central part (Peng et al. 2005). The zircon grains of sample SX020 are brownish and translucent, and some are prismatic, mostly shorter than 150 µm. BSE images of the zircon grains clearly show oscillatory zoning (Fig. 13). The sample dyke yields a baddeleyite isotopic dilution U–Pb age of 1777.6±3.4 Ma (Fig. 14; Peng et al. 2006). Another dyke sample yields a baddeleyite isotopic dilution U–Pb age of 1789±28 Ma. K–Ar, Rb–Sr and Sm–Nd ages are centralized on the Late Palaeoproterozoic (Qian & Chen 1987; Hou et al. 1998, 2001), and 40Ar–39Ar ages at about 1780 Ma have also been reported (Wang, Y. J. et al. 2004). These dates suggest that the mafic dykes formed no later than 1.78 Ga.
Zircon CL images and analysed spots of sample SX020.
Baddeleyite isotopic dilution U–Pb age of 1777.6±3.4 Ma for dyke (SX020) (after Peng et al. 2006).
Rifting event
An important rifting event occurred within the NCC at c. 1780–1640 Ma closely followed basement uplift and mafic dyke swarm intrusion. The Xiong'er (XER) and Yanshan (YS) aulacogen rifts are distributed in the northern and southern NCC (Fig. 12), and the northern marginal rifts and northern Qinling marginal rift are distributed along the northern and southern margins of the NCC, respectively (Kusky & Li 2003; Zhai et al. 2003b). The Palaeoproterozoic continental rifting probably represents a limited break-up event of the rigid continent.
Aulacogen rifts
The volcanic rocks in the XER aulacogen are widely developed in the southern part of the NCC, overlying the early Precambrian metamorphic basement, and are overlain by Meso-Neoproterozoic and Palaeozoic terrigenous detrital rocks, carbonate and tillite. The volcanic rocks crop out over an area of >6000 km2 with a thickness ranging from 3000 to 7000 m, and are composed of basalt, andesite, trachyandesite, dacite and volcanic clastic rocks. The volcanic rocks define two magma cycles: (1) from mafic to intermediate, and intermediate to felsic; (2) from mafic–intermediate to intermediate. The volcanic rocks typically exhibit thick and continuous flow layers with thin sedimentary layers in some places, and are overlain by a conglomerate–sandstone–mudstone sequence containing volcanic clastic rocks. Geochemically, most of the volcanic rocks are high in LILE but exhibit negative anomalies in HFSE. The εNd(t) values calculated at t=1780 Ma range from −5.4 to −9.7. Nd TDM ages range from 2.4 to 3.2 Ga, whereas the 87Sr/86Srt values show large variations (Peng et al. 2008). Zircons from five volcanic rocks were dated by the U–Pb SHRIMP method (Zhao, T. P. et al. 2004a). The results indicate that the Xiong'er Group formed at 1.80–1.75 Ga. Although geochemical characteristics of the Xiong'er volcanic rocks are partly similar to island arc affinity (e.g. Chen et al. 1992), more geologists prefer to consider that these rocks formed in an intracontinental setting, based on the sedimentary analyses and rock associations (Sun et al. 1985; Bai et al. 1993; Zhao, Z. P. 1993; Pirajno & Chen 2005). Zhao, T. P. et al. (2001) suggested that the volcanic eruption centre of XER was in western Henan Province, and the aulacogen rift extended to the west, east and north, forming a triple point. Finally, dioritic intrusions indicate magmatic activity at the end of rifting. Recently, Zhao, T. P. et al. (2002), Peng et al. (2006, 2008) and Kusky & Santosh (2010) proposed that the Xiong'er volcanic rocks could be linked to a break-up event of the NCC, associated with the disassembly of the supercontinent Columbia.
The YS aulacogen mainly trends NE–SW to east–west, and branches into the Taihang Mountains to the south. The sedimentary–volcanic sequence is termed the Changcheng System, and is unconformably covered by the Mesoproterozoic Jixian System, Neoproterozoic Qingbaikou System and Palaeozoic sediments of platform-type sedimentary basins. The Changcheng System is subdivided into five groups, which are, from bottom to top, the Changchenggou, Chuanlinggou, Tuanshanzi, Dahongyu and Gaoyuzhuang Groups. The dominant rocks are thick layered conglomerates, quartz sandstones, greywackes, fine-grained sandstones, mudstones, shales and carbonates. The Tuanshan Group and the Dahongyu Group consist of volcanic lavas and clastic rocks. The volcanic rocks are alkali basalts with a small amount of acid rocks, showing the characteristics of a continental bimodal volcanic series (Yu et al. 1994, 1996). SHRIMP U–Pb ages of detrital zircons for feldspathic quartzites from the lower part of the Changzhougou Group and the Chuanlinggou Group are 2580–2360 Ma with a peak of 2500 Ma, and 2600–2350 Ma and 1900–1800 Ma, respectively (Wan et al. 2003). Therefore Wan et al. suggested that the depositional time of the Changcheng System was younger than c. 1800 Ma. Volcanic rocks of the Tuanshanzi Group and Dahongyu Group yielded zircon U–Pb ages of 1720–1620 Ma (Li, H. K. et al. 1995; Lu, S. N. et al. 1995, 2002), which are younger than the Xiong'er volcanic rocks.
Continental margin rift basins
Several continental rift basins are developed along the northern margin of the NCC: the Bayan Obo, Langshan–Zhaertai and Huade basins (Fig. 12). Their sedimentary successions are similar, with multi-depositional cycles of greywacke, sandstone, mudstone, shale, carbonate and evaporite, termed the Bayan Obo Formation, Langshan–Zhaertai Formation and Huade Formation. The Bayan Obo Formation and the Langshan–Zhaertai Formation contain a minor amount of alkali volcanic rocks (Li, Q. L. et al. 2007; Peng & Zhai 2004), whereas the Huade Formation contains no volcanic rock (Hu et al. 2009). Most of the rocks underwent low-grade metamorphism and deformation, as a result of which some of the mudstones and shales changed to garnet- or andalusite-bearing slates and mica–quartz schists.
The Bayan Obo Formation is well known for its abundant REE–Nb–Fe mineral deposits (Ren & Wang 2000). The Bayan Obo rift basin is located north of the Zhaertai Basin and NW of the Huade Basin; its length is c. 800 km and thickness c. 10 000 m. Clastic rocks and argillaceous rocks are dominant, occupying over 90 vol.% of all sediment sequences. Other rocks are carbonatites and volcanic rocks. Volcanic activity was concentrated in two intervals in the lower part and upper middle part of the formation. The volcanic rocks are characterized by high abundances of K and Na, and their contents of LiO2, Nb2O5 and BaO are much higher than in normal igneous rocks. The abnormal REE enrichment is related to carbonatites. The carbonatites commonly contain schohartite, aegirite–augite, zircon, monazite, bastnasite, apatite and spinel, and their contents of FeO, P2O5 and K2O are high. They are also enriched in Nb, Ta, Ce, Ti, Th, Ba and Zr. The δ13C values range from −6.57‰ to +0.36‰, and δ18O values range from +8.28‰ to +19.36‰ (Chen and Shao 1987; Wang & Li 1987). Their LREE contents are high, similar to those of alkaline continental basalts. These characteristics indicate that the carbonatites came from a deep source.
LA-ICP-MS U–Pb ages of detrital zircons for sandstones in the lower part of the Bayan Obo Group are concentrated at 1850–2000 Ma and c. 2500 Ma, and basalts yield a zircon U–Pb age of 1852±4 Ma (Wang, Y. X. et al. 2002; Fan, H. R. pers. comm.). Recently, Li, Q. L. et al. (2007) and Hu et al. (2009) reported LA-ICP-MS U–Pb ages of detrital zircons from sandstones in the Zhaertai Formation and the Huade Formation. The detrital zircon ages of the Zhaertai Formation are concentrated at 2550–2400 Ma, and a basalt age is 1743 Ma. The ages of the Huade Formation show two main peaks at 1800±50 Ma and 1850±50 Ma, as well as two minor age peaks at c. 2500 Ma and c. 2000 Ma. The youngest concordant age of the detrital zircons from meta-pebbly arkose at the bottom of the Huade Group is 1758±7 Ma, which constrains the oldest depositional age of this sequence. All of the above-mentioned ages are similar to those of the Yanshan and Xiong'er aulacogen rifts, and correlated to main crustal growth events of the NCC. Therefore, we suggest that the Bayan Obo, Langshan–Zhaertai and Huade formations represent Palaeo-Mesoproterozoic stable shallow–hypabyssal sedimentary basins in the NCC. These three basins commonly constitute a passive continental margin rift system at the northern margin of the NCC. This rift initiated at about the same time as the Yanshan and Xiong'er aulacogen rifts.
Anorogenic magmatic intrusion
In the North China craton, a rapakivi–anorthosite anorogenic magma association is exposed in the Archaean–Palaeoproterozoic metamorphic basement rocks in the northern NCC. Rapakivi granites mainly occur in Miyun, Chicheng, Luanping and Kuandian and through the Beijing, northern Hebei and Liaoning provinces. Their country rocks are Archaean metamorphic rocks of 2521 Ma and Palaeoproterozoic granitic and migmatitic gneisses (Liu et al. 2007; Yang et al. 2008). The representative rapakivi granite body is the Miyun pluton, with an exposed area of 25 km2. It was intruded into the Archaean Miyun complex, which includes orthogneiss, metabasites of granulite facies and BIFs. The pluton has characteristically porphyritic and mega-porphyritic textures with ovoid alkali feldspars distributed homogeneously throughout the granite. These alkali feldspars range from 10–40 mm to 600 mm in diameter and most of them are mantled by plagioclase, with sharp irregular contacts. The margin of the pluton is changed to fine-grained granite. The main mafic minerals are hornblende and biotite. Yang et al. (2005) and Liu et al. (2007) reported zircon LA-ICPMS U–Pb ages of 1681±10 Ma and 1679±10 Ma, respectively, and a zircon SHRIMP U–Pb age of 1685±15 Ma for the Miyun pluton. The zircons have an εHf(t) value of −5.0, indicating that the rapakivi pluton was derived from a crustal source. The two-stage model ages (TDM2) of rapakivi granite are about 2.6–2.8 Ga, similar those of the host Archaean gneiss, indicating that the rapakivi granite was derived from partial melting of the crust during the Neoarchaean.
The Damiao anorthosite complex Hebei Province, is the only massif-type intrusion in the NCC. This complex is composed of dominant anorthosite, and leucogabbro, gabbro, norite and mangerite. The host rocks are the Archaean–early Palaeoproterozoic Dantazi complex, consisting of supracrustal rocks and orthogneisses metamorphosed to granulite–amphibolite facies. The mafic granulites have a ‘white eye-socket’ feature, as a result of the decompressional corona texture of fine-grained Plg+Cpx+Opx±Amp surrounding garnet, and a ‘red eye-socket’ feature, as a result of the corona growth of garnet, indicating high–moderate metamorphic pressure. Zhao, T. P. et al. (2004b) reported single-zircon U–Pb ages of 1693±7 Ma and 1715±6 Ma; the zircons were separated from norite and mangerite, respectively.
The crystallization ages of the rapakivi granites and the anorthosite complex are very consistent with each other, showing that the emplacement of the anorogenic magma association occurred at 1715–1685 Ma in the NCC. Some researchers have paid attention to the relationship of Palaeoproterozoic events in the NCC: the c. 1800 Ma basement uplift, the 1780 Ma mafic dyke swarm, c. 1800–1650 Ma XER and YS (Changcheng System) rifting with alkali volcanic rocks at 1720–1620 Ma, and the 1715–1685 Ma anorogenic magma association. The anorogenic magma is genetically related to those of the mafic dyke swarm and the volcanic rocks in the Palaeoproterozoic YS and XER aulacogen rifts (Xie & Wang 1988; Zhao, Z. P. 1993; Yu et al. 1994; Rämö et al. 1995; Zhai & Liu, 2003; Zhai et al. 2003b; Peng et al. 2008). Therefore, it is possible that the Palaeoproterozoic events were caused by an upwelling mantle and are an indicator of the break-up of a Palaeoproterozoic continent.
Discussion and conclusion
(1) Old continental nuclei are recognized in the NCC, and the oldest remnants of granitic gneiss and supracrustal rocks are 3.8 Ga old. The main crustal growth in the NCC took place at 2.9–2.7 Ga. The NCC can be divided into seven microblocks, which are separated by Archaean greenstone belts that represent continental accretion surrounding the old continental nuclei.
(2) By 2.5 Ga, the microblocks amalgamated to form a coherent craton by continent–continent, arc–continent or arc–arc collisions. The tectonic processes in Neoarchaean and modern times appear to be different in degree rather than in principle. Extensive intrusion of K-granite sills and mafic dykes and regional metamorphism at upper amphibolite–granulite facies occurred, and marked the realization of cratonization in the NCC. Coeval ultramafic–mafic and syenitic dykes of c. 2500 Ma in eastern Hebei indicate that the NCC was a stable, thick and huge continent at the end of the Archaean, and probably was a part of the Neoarchaean supercontinent that has been suggested by previous studies (Condie et al. 2001; Rogers & Santosh 2004; Kusky & Santosh 2010).
(3) In the period between c. 2500 Ma and 2350 Ma, the NCC was tectonically inactive, but the development of Palaeoproterozoic volcanic and granitic rocks occurred between 2300 and 1950 Ma. Volcanic–sedimentary rocks have been termed Palaeozoic mobile belts by Zhai & Liu (2003), and occur in a linear distribution with strong folding and metamorphism at 1900–1850 Ma, and intrusion by granites and pegmatites at 1850–1800 Ma. The Palaeoproterozoic mobile belts formed and evolved within a craton or continental margin (epicontinental geosyncline). Some 2.30–1.95 Ga rift-margin, passive continental margin deposits, analogous to arc or back-arc assemblages, as well as HP and HT–UHT metamorphic complexes, seem to be comparable with many in the late Phanerozoic orogenic belts (Fig. 15). Regarding Palaeoproterozoic orogeny in other cratons (Zhao, G. C. et al. 2002), a global Palaeoproterozoic orogenic event possibly existed and resulted in the formation of a pre-Rodinian supercontinent at c. 2.0–1.85 Ga.
Tectonic model showing Neoarchaean–Palaeoproterozoic evolution of the North China craton.
(4) In contrast, the c. 1800 Ma event is an extension–migmatization event, which includes uplift of the lower crust of the NCC as a whole, mafic dyke swarms, continental rifting and intrusion of an orogenic magmatic association. This event has been considered to be related to the break-up of a pre-Rodinian supercontinent at 1.8 Ga, attributed to a Palaeoproterozoic plume.
(5) HP and HT–UHT metamorphic rocks occur widely in the NCC. Their high pressure of 10–14 kbar has attracted the attention of researchers, and several continental collisional models have been proposed. However, it is still argued that these rocks have a much higher geothermal gradient c. 16 °C km−1 and much slower uplift rate (0.33–0.5 mm a−1 than those in Phanerozoic orogenic belts. Moreover, HP and HT–UHT rocks commonly occur together and their distributions are not in linear zone c. All these observations suggest that we should rethink geological and tectonic models for the evolution of the North China craton.
Acknowledgments
This study represents the research results of a project (Grant 2006CB403504) supported by the Ministry of Science & Technology, and projects (Grants c. 40672128, 90714003 and 40721062) supported by the National Nature Science Foundation of China. We thank J. Guo, N. Jiang, G. Zhao, J. Li, S. Wilde, T. Kusky and A. Kröner for their co-operation, help and discussion, and especially T. Kusky for polishing the English. I expressly thank my old friend and teacher Brian Windley; we started our co-operation in 1981 and I have learnt a lot from him. I offer this paper for his outstanding contribution to Earth Science.
- © The Geological Society of London 2010
