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Journal of Petrology Volume 41 Number 7 Pages 933-950 2000
© Oxford University Press 2000
On and Off the North China Craton: Where is the Archaean Keel?
1DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY UNIVERSITY OF LONDON, EGHAM TW20 OEX, UK
2CHANGSHA INSTITUTE OF GEOTECTONICS, ACADEMIA SINICA, CHANGSHA, HUNAN 410013, P.R. CHINA
3INSTITUTE OF GEOLOGY AND GEOPHYSICS, CHINESE ACADEMY OF SCIENCES, BEIJING 100029, P.R. CHINA
4DANISH LITHOSPHERE CENTRE, 10 ØSTER VOLDGADE, 1350 KØBENHAVN, DENMARK
5KINGSTON UNIVERSITY, PENRHYN ROAD, KINGSTON UPON THAMES KT1 2EE, UK
6VENING MEINESZ SCHOOL OF GEODYNAMICS, UTRECHT UNIVERSITY, 3508 TA UTRECHT, NETHERLANDS
Received June 11, 1999; Revised typescript accepted February 18, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Geophysical data indicate that the lithosphere beneath the North China Craton (NCC) is
80 km thick with high heat flow within the craton. This is somewhat in disagreement with the presence of Archaean crustal rocks and kimberlite-hosted xenoliths that point to the existence of garnet–diamond-facies mantle beneath the craton, as recently as the Ordovician. Basalt-hosted mantle xenoliths entrained during the Cenozoic may provide a clue to Phanerozoic changes. Of particular note is the predominance of spinel-facies peridotites (75–80 km) and the paucity of garnet-facies peridotites. The modal mineralogy of the spinel peridotites is similar to that observed in peridotite xenoliths from the lower oceanic lithosphere but distinct from that of abyssal peridotites. The orthopyroxene/olivine ratio is like that of peridotites from ocean basins and tectonically active continents, and the peridotites have ‘depleted’ Sr and Nd isotopic ratios similar to those of oceanic basalts. The basalt-hosted xenolith data from eastern China support geophysical data in revealing the presence of thin, hot lithosphere with a similarity, over distances of several thousand kilometres, to that found beneath tectonically active continents or ocean basins. These data do not, however, allow us to constrain which of the thermo-tectonic processes (i.e. plume, extension, delamination) was responsible for the loss of the cold, thick Archaean lithospheric root (200 km) in the last 400 my. What is clear is that the pre-existent, presumably heterogeneous, Archaean lithosphere has been very effectively replaced by ‘oceanic’-like mantle. The extent to which it was totally replaced is open to debate. KEY WORDS: North China Craton; mantle xenoliths; oceanic affinity
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Traditionally cratons are thought of as parts of the continental lithosphere in which Archaean crustal rocks are exposed at the Earth’s surface and where thermal and tectonic events occurred >2500 my ago. One of the best studied examples is the Kaapvaal craton, South Africa. Aspects of the petrology and geochemistry of kimberlite-hosted mantle xenoliths indicate that areas of Archaean crust are underlain by refractory peridotites (e.g. Cox et al., 1973
; Boyd & Nixon, 1978; Erlank et al., 1987). Geochronology and geochemistry have demonstrated the presence of ancient enriched mantle, coupled crust–mantle evolution, and tectonic and thermal stability for 3–4 by (Menzies & Murthy 1980; Walker et al., 1989; Hawkesworth et al., 1990; Carlson & Irving, 1994; Pearson et al., 1995a, 1995b). Silicate inclusions in megacrystic diamonds found in kimberlites reveal that a considerable thickness of lithosphere (200 km) formed in the first billion years of Earth’s history perhaps as a result of high thermal gradients and greater melt production (e.g. Richardson et al., 1984, 1993; Boyd et al., 1985). Seismic data indicate a high-velocity seismic lid imaged to depths of several hundred kilometres (i.e. 150–250 km) verifying the long-term stability and relative inertness of cratons (e.g. Jordan, 1988; Polet & Anderson, 1995; Van Der Lee & Nolet, 1997).
Until recently, petrological and chemical studies of on-craton peridotites revealed that cratonic mantle was chemically distinctive compared with peridotites from other tectonic settings such as the ocean basins and tectonically active continents (e.g. Boyd, 1989). For example, the petrology and geochemistry of xenoliths from the Siberian craton is similar to that of xenoliths from the Kaapvaal craton (Griffin et al., 1996; Boyd et al., 1997). However, craton–shield areas within Canada and Tanzania (Rudnick et al., 1994; Kelemen et al., 1998; Griffin et al., 1999a, 1999b; Lee & Rudnick, 2000) do not have the restricted range in olivine composition and the orthopyroxene/olivine ratio reported from the Kaapvaal craton (Boyd, 1989). Although Griffin et al. (1999a) cautioned against over-interpretation on the basis of limited samples, the most recent database for cratons such as Tanzania (e.g. Rudnick et al., 1994; Lee & Rudnick, 2000) indicates that the evolutionary history of some of the world’s cratons is different from that of Kaapvaal (Boyd, 1989). Griffin et al. (1999a, 1999b) proposed that the shallow sub-cratonic mantle beneath Canada was accreted above ancient subduction zones, hence the similarity with modern (<200 Ma) arc peridotites (Kelemen et al., 1998). Petrogenetic models founded on the belief that higher thermal gradients in the Archaean produced highly refractory residues (e.g. Hanson & Langmuir, 1978; Boyd, 1989; Walker et al., 1989) are now balanced by models invoking fluid processes as the mechanism for producing highly refractory residues (e.g. supra-subduction zone). Rudnick et al. (1994) and Kelemen et al. (1998) attributed the magnesian nature of cratonic peridotites to the interaction of silica-rich liquids with low orthopyroxene magnesian peridotites producing orthopyroxene-rich peridotites.
Several Archaean areas are not underlain by a clearly defined seismic lid reaching several hundred kilometres into the upper mantle (Anderson et al., 1992; Grand, 1994). For example, the Hebridean craton of NW Scotland (Menzies & Halliday, 1988) has a thickness of 80 km because it is part of the rifted volcanic margin of the North Atlantic Ocean and may have been thermally thinned by the Iceland plume. Similarly, the Arabian craton has exposures of Archaean crustal rocks (Whitehouse et al., 1994) with a lithosphere thickness of <100 km, perhaps because this is part of the rifted margin of the Red Sea–Gulf of Aden influenced by the Afar plume. Also, the seismic character of the mantle beneath the Slave Province, Canada, is different from that of the Kaapvaal and Siberian cratons, and there is some debate as to whether or not a high- or low-velocity structure exists beneath the craton (Hoffman, 1990).
In this paper we investigate the North China Craton and off-craton mobile belts of eastern China. The North China Craton (NCC) is characterized by late Archaean crustal rocks and has been intruded by Ordovician diamondiferous kimberlites and Cenozoic alkali basalts (Chi et al., 1992) both containing mantle xenoliths. Basalt-hosted spinel-facies mantle rocks are used to investigate the nature of the shallow mantle and these data are compared with data from other tectonic settings. This includes (1) Archaean cratons [e.g. Africa (Kaapvaal and Tanzania), North America, Greenland], (2) tectonically active continents (e.g. Basin and Range, North America), and (3) modern ocean basins (e.g. Atlantic Ocean). This study addresses the following questions. What is the nature of the uppermost mantle beneath eastern China? Does on- and off-craton provinciality exist as it does in other parts of the world? Which processes best account for the formation of the lithospheric mantle beneath eastern China? How does present-day lithosphere compare with the Archaean lithosphere known to have survived until the Ordovician?
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GEOLOGICAL BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The North China Craton (NCC) is composed of Archaean and Proterozoic metamorphic rocks with the oldest recorded crustal ages being
3·8 Ga (Liu et al., 1992). The cratonic nucleus is surrounded by three tectonically different regions from north to south, the NE China fold belts (Variscan), the Yangtze craton (Proterozoic) and the SE China fold system (Caledonian). The cratonic nucleus and the surrounding terranes have been intruded by kimberlites and basalts that have entrained fragments of the sub-crustal mantle at discrete periods during the Phanerozoic (Griffin et al., 1998; X. Xu et al., 1998). Several kimberlites (Chi et al., 1992) are reported from within the Archaean crust at Mengyin, Shandong province, and at Fuxian, Liaoning province (Fig. 1). The diamondiferous character of some of the kimberlite pipes and the predominance of P-type inclusions (i.e. peridotite) in diamonds (e.g. Chi et al., 1992) suggests that the early Phanerozoic lithosphere was thick and stable to depths within the diamond stability field (i.e. 180–200 km). Later eruption of post-Cretaceous rift-related basalts entrained shallow mantle rocks (e.g. Song & Frey, 1989; Tatsumoto et al., 1992) and thus provided two time windows on the nature of the mantle beneath the NCC (Menzies et al., 1993; Griffin et al., 1998). On- and off-craton lithosphere provinciality can best be investigated using Cenozoic xenolith-bearing basalts erupted along major lithospheric fault systems, through lithosphere of variable age, thickness and heat flow (e.g. Zhou & Armstrong, 1982; Peng et al., 1986; Zhi et al., 1990; Fan & Hooper, 1991; Zhang et al., 1991; Deng & Macdougall, 1992; Tatsumoto et al., 1992; Zhou & Zhu, 1992; Qi et al., 1995; Y. Xu et al., 1996, 1998; X. Xu et al., 1998).
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ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Representative spinel-facies peridotites were selected from several ‘on-craton’ (i.e. Hannuoba, Fansi, Linqu, Qixia, Penglai) and ‘off-craton’ (i.e. Yitong, Shangzhi, Shuangliao, Huinan, Ninghai, Xilong, Xuwen, Wenchang) (Fig. 1) localities. The modal mineralogy of the peridotites is based on spot counting of >1000 grains (Table 1; Fig. 2). Xenoliths were crushed to <30 mesh and clear olivines were carefully hand-picked under a binocular microscope. Clean olivines were mounted onto a glass holder and then surface-polished. Olivine major element compositions were determined at the Department of Geology, Birkbeck College, University of London, using a JEOL Superprobe equipped with an automated (AN 10000/55S with 2500 CPS) energy-dispersive system (EDS). Analyses were performed with a beam of 15 keV and 15 nA focused to a spot of
1 µm diameter. ZAF4 and a cobalt standard were used for the on-line correction and the spectrum calibration. Analytical uncertainties for major elements were estimated at the level of <1% with the majority between 0·2 and 0·5% (Table 2).
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Hand-picked clinopyroxene separates (200 mg), without visible inclusions, were leached with hot 6 M sub-boiled HCl for 30 min and dissolved in distilled HF–HNO3 and 6 M HCl in Savillex screwtop capsules at 150°C for 4–6 days. Each dissolved sample was split into two aliquots; one was used for trace element determination diluted to 5000 times before analysis and the other was used for the determination of Sr and Nd isotope composition. Trace elements in clinopyroxenes were measured at CARE Imperial College (Silwood Park, Ascot) by inductively coupled plasma-mass spectrometry (ICP-MS) (Table 3). A blank was processed with each set of 11 samples and blanks for most of the elements were <0·05 ppm. To monitor precision, three duplicates and three standards (i.e. BEN, NIMN, BHVO-1) were prepared using the same procedures. This standard and blank data are given in Table 4. The discrepancy between duplicates was <10% for the majority of elements and analyses of standards are very close to the recommended values with most elements <5% (Table 3).
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Clinopyroxene:orthopyroxene:olivine modes
The majority of the basalt-hosted xenoliths from eastern China are spinel-facies peridotites. On the basis of available samples both on-craton and off-craton localities (Fig. 2) are predominantly spinel lherzolites with essentially the same mineralogy and only slight variations in their clinopyroxene/orthopyroxene ratio. For the xenoliths from eastern China the modal compositions converge on an average composition of 64% olivine, 20% orthopyroxene, 14% clinopyroxene and 2% spinel. The modal mineralogy of the peridotites illustrates no real difference between on- and off-craton occurrences (Fig. 2).
Comparison of the mineral modes of the Chinese mantle rocks with a selection of on-craton data from elsewhere indicates some marked differences. On-craton (>2500 Ma), low-temperature peridotites from South Africa (Kaapvaal) tend to have a greater range in olivine/orthopyroxene ratio and to be clinopyroxene poor (Fig. 2a) relative to the Chinese sub-Moho peridotites. It should be noted that the range in modal mineralogy for the Matsoku ‘common’ peridotites defined by Cox et al. (1973) (i.e. ol:opx:cpx:gt = 45–75%:20–50%:0–5%:0–11%) lies within the on-craton range (Fig. 2a). Comparison of off-craton (<2500 Ma) data with the Chinese mantle rocks indicates some marked similarities. Off-craton, high-temperature peridotites from the western USA (e.g. Basin and Range) tend to be more olivine rich with a greater propensity toward dunite–harzburgite modes (Fig. 2c). Lherzolites from eastern China tend to be similar in their mode to lherzolites from the Basin and Range, western USA. Oceanic peridotites (<200 Ma) from abyssal and basalt-hosted peridotites display a considerable range in modal mineralogy (Fig. 2d) with differences between shallow and deep lithospheric mantle that may point to it being ‘stratified’. The shallow oceanic lithosphere, represented by abyssal, dredged and ophiolitic peridotites, tends to be clinopyroxene poor and dominated by harzburgites. In contrast, the deep oceanic lithosphere, represented by basalt-hosted xenoliths, tends to be more clinopyroxene rich and dominated by lherzolite. To complicate matters, the mode for shallow oceanic sub-Moho lithosphere overlaps with that for on-craton peridotites (see Kelemen et al., 1998). The mode for deep lithospheric ‘oceanic’ mantle has some of the variation observed in both the eastern China mantle rocks and those from the Basin and Range. Basalt-hosted spinel-facies mantle xenoliths from Hawaii and Tahiti are similar, in their modal abundance, to the lower lithosphere beneath eastern China. In other words, it appears that mantle processes that produced the deep oceanic lithosphere beneath Hawaii and Tahiti (100–150-my-old lithosphere) and tectonically active continental regions such as the Basin and Range may be similar to those processes responsible for the formation of part of the lithosphere beneath eastern China. In summary, the modal mineralogy of the Chinese xenoliths is very different from that of other Archaean cratons where garnet-facies harzburgites predominate. However, the Chinese xenoliths compare favourably with spinel-facies mantle xenoliths from ocean basins or tectonically active continents where spinel-facies lherzolites predominate (e.g. Stosch & Seck, 1980; Hauri et al., 1993; Sen et al., 1993).
Olivine and orthopyroxene modes and chemistry
Electron microprobe studies indicate that olivines in the eastern China xenoliths are basically homogeneous with no significant intra-grain and inter-grain variations. In terms of forsterite content, the maximum discrepancy between five analyses, within a single grain or between five discrete grains, is <0·4%. The olivine data (Table 2) represent average compositions of five analyses and their NiO content correlates with magnesium content. Olivine websterites have a much wider compositional range than the spinel lherzolites, especially in on-craton localities. Of the three samples studied, one from Hannuoba (92HD-02) has the highest MgO content (49·8%) and the highest NiO (0·6%) content of all the ‘on-craton’ xenoliths. Another from Qixia (92QX-03) has an extremely high iron content (FeO = 16%) and the lowest NiO (0·06%). On and off the craton, the olivines are similar in composition [(MgO)on-craton > 48 wt % and (MgO)off-craton > 46 wt %] (Fig. 3).
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, arc-peridotites from the Papuan ultramafic belt [Kelemen et al., (1998)
Orthopyroxene modal and olivine compositional data from on- and off- the North China Craton (Fig. 3a) are compared with data from other cratons and modern ocean basins (Fig. 3d). On-craton peridotites from Kaapvaal, South Africa (Fig. 3b) have a greater content of orthopyroxene than on-craton peridotites from Wiedemann Fjord–Ubekendt Island, Greenland, Tanzania and Lac de Gras, Canada (Fig. 3b) (Kelemen et al., 1998; Griffin et al., 1999a, 1999b; Lee & Rudnick, 2000). Except for a few scattered points, the combined cratonic datasets define a broad band of data with variable orthopyroxene mode and variable olivine composition. In the case of eastern China (Fig. 3a) both on- and off-craton spinel-facies peridotites have a modal orthopyroxene content similar to on-craton peridotites but the coexisting olivine is more iron rich. As such, the peridotites from eastern China are distinct from those found within the cratons of Canada, Greenland, Siberia, South Africa and Tanzania. However, the eastern China dataset occupies a similar field to high-temperature off-craton peridotites from South Africa (Boyd, 1989) (Fig. 3c), peridotites from tectonically active continents and peridotites from ocean basins (Fig. 3d). The important exception in any comparison of Chinese xenoliths with ocean basins is that peridotites from arc environments contain a high modal amount of magnesian olivines and as such are different. Indeed, several researchers have argued for a two-stage evolutionary process (e.g. Cox et al., 1987) in the formation of cratonic peridotites. Melt-rock reaction in arc environments has been recently invoked (Rudnick et al., 1994; Kelemen et al., 1998) as an explanation for the highly magnesian nature of cratonic peridotites. The Chinese peridotites are more similar to peridotites found in off-craton localities or peridotites from ocean basins than they are to low-temperature cratonic peridotites that lie on a shield geotherm.
Clinopyroxene trace element geochemistry
Trace element abundances in clinopyroxenes from eastern China spinel-facies mantle xenoliths are given in Table 3 and plotted in Fig. 4. Clinopyroxenes from on- and off-craton peridotites display the same overall variation in chondrite-normalized rare earth element (REE) pattern from light REE (LREE)-depleted compositions, through transitional LREE profiles to LREE-enriched profiles (Fig. 4). In turn, these groups are subdivided into moderate to strong depletions in the LREE, irregular or flat REE profile and moderate to strong enrichments in the LREE (Fig. 4).
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Moderately LREE-depleted clinopyroxenes from on- and off-craton localities have almost identical middle REE (MREE), heavy REE (HREE) and Y concentrations (10 x C1 chondrites) and display no REE fractionation from Eu to Lu. Strongly LREE-depleted clinopyroxenes display a slightly greater range in MREE and HREE but the HREE remain around 10 x C1 chondrites. These pyroxenes are similar to LREE-depleted pyroxenes (i.e. Type 1A) found in peridotites world-wide (McDonough & Frey, 1989) and there does not appear to be any systematic difference between on- and off-craton peridotites. Clinopyroxenes with irregular and flat chondrite-normalized REE profiles have a very restricted REE abundance between 4 x and 20 x C1 chondrite and show no significant LREE–HREE fractionation (Fig. 4). LREE-enriched clinopyroxenes show a considerable range in Ce/Yb very different from the other two groupings. Clinopyroxenes with an enrichment in the LREE have been reported from many other localities (e.g. Stosch & Seck, 1980; Johnson et al., 1990; Sen et al., 1993) but for eastern China there appear to be no systematic differences between on- and off-craton clinopyroxenes.
The style of LREE enrichment observed in clinopyroxenes from on-craton xenoliths in eastern China is more akin to that observed in spinel-facies basalt-hosted xenoliths from around the world and very different from on-craton garnet-facies kimberlite-hosted xenoliths from around the world [Menzies et al. (1987) and references therein]. Kimberlite-hosted cratonic peridotites contain clinopyroxenes with marked depletions in the HREE (<1 x chondrite) relative to the LREE (>10 x chondrite) with the exception of deformed, high-temperature peridotites. In contrast, clinopyroxenes from off-craton kimberlite- and basalt-hosted peridotites display a range in REE characteristics that includes LREE-depleted and LREE-enriched clinopyroxenes. Furthermore, clinopyroxene data from ancient and modern oceanic peridotites are dominated by LREE-depleted clinopyroxenes (e.g. Johnson et al., 1990) similar to the ‘strongly LREE-depleted’ pyroxenes from eastern China (Fig. 4). Also, orogenic–ophiolitic lherzolites contain pyroxenes with LREE-depleted profiles at 1 x chondrite-normalized LREE abundances and 10 x chondrite-normalized HREE abundances (e.g. Bodinier et al., 1991). These compare favourably with the ‘moderately LREE-depleted’ and ‘strongly LREE-depleted’ pyroxenes from eastern China (Fig. 4).
Clinopyroxenes from off- and on-craton xenoliths in eastern China (Type I and II) display similar Sr abundances but a much larger variation in Th [Sr = 3–438 ppm; (Th)N = 0·2–360; Table 3] relative to kimberlite-hosted on-craton xenoliths [Sr = 15–400 ppm; (Th)N = 0·6–27ppm]. The abundance of Sr and Th in off-craton clinopyroxenes correlates with the LREE [Nd vs Sr; (Ce/Yb)N vs (Sr)N or (Th)N]. Strongly LREE-depleted and extremely LREE-enriched pyroxenes from off-craton localities have a range in Sr/Nd ratio whereas most xenoliths from on-craton localities have a near-constant Sr/Nd ratio of 15 ± 5, similar to the chondritic ratio (McDonough & McCulloch, 1987). Some samples with Sr enrichment relative to Nd also have rather irregular REE profiles.
Sr and Nd isotopes in clinopyroxenes
Representative Sr and Nd isotope ratios in clinopyroxenes from eastern China spinel-facies mantle xenoliths are given in Table 4 and shown in Fig. 5. Apart from one sample, clinopyroxenes from on-craton localities in eastern China (Fig. 5a) are characterized by a similar range in Sr and Nd isotopes to off-craton localities. Compared with on-craton xenoliths from eastern China, a greater range in 87Sr/86Sr and 143Nd/144Nd ratios is observed in other on-craton xenoliths. The on- and off-craton basalt-hosted xenoliths from eastern China have greater similarities with off-craton xenoliths from around the world and with oceanic peridotites (Fig. 5c). The latter are characterized by a restricted range in Sr and Nd isotopes with the majority of samples located in the top left-hand quadrant, a location consistent with a history of melt extraction. Some clinopyroxenes from eastern China have Sr and Nd isotopic affinities to on-craton peridotites from elsewhere. The bulk of the isotope data points to an affinity with off-craton–oceanic processes as did the modal mineralogy of the eastern China peridotites.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Pioneering work by Zhou & Armstrong (1982)
indicated that the Cenozoic lithospheric mantle beneath eastern China was laterally and vertically heterogeneous, a conclusion supported by later work (e.g. Zhou & Zhu, 1992). Cenozoic basalts from eastern China were shown to have isotopic characteristics similar to those of oceanic volcanic rocks and continental rift basalts, with the highest 87Sr/86Sr ratios occurring at Wudalianchi. These isotopic ratios, as well as incompatible trace elements in Cenozoic alkali basalts, required an ‘enriched’ component in their mantle source and it was proposed by several workers that this enriched source was shallow and located within the sub-lithospheric mantle (Zhang et al., 1991, 1995, 1998; Zhou & Zhu, 1992). The xenolith data presented in this study indicate that in tectonically active eastern China there is an overall lack of enriched reservoirs, much of the sub-Moho lithosphere beneath eastern China being chemically and isotopically depleted. Basalt-hosted spinel peridotites from eastern China have similarities to shallow mantle occurring beneath tectonically active continents (e.g. Basin and Range) or modern ocean basins. The similarities include: a predominance of mantle peridotites that formed in the spinel lherzolite stability field; an olivine:clinopyroxene:orthopyroxene mode that is similar to that found beneath tectonically active continents or the lower oceanic lithosphere; a forsterite content like that found in off-craton peridotites or in abyssal, ophiolitic and orogenic peridotites; and an ‘oceanic’ partial-melting trend commensurate with the modal variation in residual peridotites formed by extraction of basaltic melts from a primordial mantle source.
The broad similarities with an ‘oceanic’ provenance, despite the age of the overlying crust (i.e. 3·3 Ga), must argue for petrogenetic processes beneath the NCC similar to those that led to accretion of lower lithosphere beneath ocean basins and tectonically active continents. Several independent sources of information support the presence of relatively thin (spinel facies <75 km) lithosphere beneath eastern China. These include seismic, heat flow and thermobarometric data. Seismic tomography (Chen et al., 1991; Polet & Anderson, 1995) indicates that the present-day lithosphere beneath eastern China is <100 km thick and may be 80 km thick, with greatly thinned on-craton lithosphere around the Bohai Sea. Heat flow studies in eastern China (Teng et al., 1983) support the presence of thin lithosphere and a shallow low-velocity structure similar to an ocean ridge. Indeed, a region of very high heat flow exists on the NCC in the vicinity of the Bohai Sea and Beijing (Fig 1). The measured heat flow corresponds to geotherms observed in tectonically active continents or ocean basins (50–105 mW/m2). The eastern China geotherm, reconstructed from the geothermobarometry of garnet lherzolites, garnet websterites and spinel lherzolites, has a high thermal gradient similar to that of ocean basins or tectonically active continental regions such as SE Australia (Griffin et al., 1998; X. Xu et al., 1998).
Elemental and isotopic data for Chinese mantle xenoliths provide the most direct evidence for the degree of mantle heterogeneity. It is not known, however, to what extent the mantle sampled by spinel-facies xenoliths is related, in time and space, to that involved in the genesis of the volcanic rocks. Mantle xenoliths from eastern China have a magnesium-rich character (Fo > 89) with minor iron-rich samples (Fo < 89, Table 2). Their REE profiles vary from strongly REE depleted (Fig. 4) to extremely REE enriched. This may point to modification of a protolith (LREE depleted) by a melt (LREE enriched). High field strength element (HFSE) and large ion lithophile element (LILE) heterogeneity is also apparent, as is a large range in Sr and Nd isotopic composition (Figs 5 and 6). Eastern China peridotites have a range in isotopes from a depleted mantle composition (i.e. 87Sr/86Sr = 0·7035, 143Nd/144Nd = 0·5135) toward an enriched mantle composition (87Sr/86Sr > 0·7045, 143Nd/144Nd < 0·5122) (Fig. 5). Such an isotopic range straddles that observed in modern ocean basins and tectonically active continental regions with some of the characteristics of on-craton peridotites. Within a single locality such as Qixia and Shangliao, xenoliths exhibit variable REE profiles from extremely LREE depleted to LREE enriched (Table 3) as well as other trace elements, and single localities also exhibit large variations in Sr and Nd isotopes (Table 4). The variations in olivine composition, (Ce/Yb)N ratio and Sr–Nd isotopes for mantle xenoliths from different localities, and within single localities, demonstrate that the lithospheric mantle beneath the NCC and the NE China fold belts is locally and regionally heterogeneous (see Zhou & Armstrong, 1982).
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Mantle history
Several aspects of the eastern China xenoliths indicate a role for partial melting processes in their origin. The magnesium-rich nature (Fo = 89·4–91·3%) in the dominant mineral, olivine, as well as a broadly positive correlation between MgO and NiO, suggests that the peridotites may represent the refractory residues formed by melt extraction (e.g. McDonough, 1990). The Chinese ‘on-craton’ xenoliths are petrographically distinct from the Kaapvaal ‘on-craton’ xenoliths, particularly with regard to their cpx/opx ratio (Fig. 2) and forsterite content (Fig. 3). Both on-craton and off-craton xenoliths from eastern China plot within the field of off-craton xenoliths from Kaapvaal or the field occupied by oceanic, abyssal or ophiolitic peridotites. One can argue that they have an ‘oceanic’ affinity. This indicates that the lithosphere beneath the NCC contrasts with typical continental lithospheric mantle beneath cratons such as Kaapvaal. The similarity with oceanic lithosphere, accreted at ridges by the extraction of basaltic melt from the asthenosphere, may indicate that recent accretion of mantle produced part of the lithospheric mantle during periods of Cenozoic volcanism. However, the existence of relatively undifferentiated garnet lherzolites from Mingxi may indicate that undifferentiated asthenosphere may be part of that accretionary process.
The depleted chondrite-normalized REE profiles of the spinel lherzolites from eastern China are similar to those of depleted mantle rocks occurring elsewhere as spinel-facies peridotites [e.g. abyssal peridotites, ophiolitic metamorphic peridotites, orogenic (non-ophiolitic) metamorphic peridotites and spinel-facies basalt-hosted xenoliths] (Fig. 4). Such peridotites are believed to have been involved in melt extraction (and transfer) processes at relatively shallow depths (i.e. 75–80 km) in the absence of garnet. Melt transfer preferentially removes the LREE and Rb. When integrated over geological time, such elemental changes are recorded as low Sr and high Nd isotopic ratios (Fig. 5). This is consistent with the presence of depleted Sr and Nd isotopes in xenoliths from the NCC. The very low 143Nd/144Nd ratios and high 87Sr/86Sr ratios for several peridotites from Penglai, Linqu and Hannuoba reveal a mantle component perhaps isolated from the convecting upper mantle for billions of years. Such material has not been reported from modern ocean basins and is more commonly reported from on-craton regions. However, in the case of eastern China, it occurs both on and off the craton and, as such, cannot be used as a unique piece of evidence for the presence of ancient lithosphere.
Some mantle rocks from eastern China show an enrichment in LREE and LILE (Fig. 4) that is inconsistent with them being a simple melt residue. Thus melt ingress or metasomatic enrichment is invoked and is best illustrated by the existence of very rare hydrous minerals in spinel lherzolites and/or pyroxenites (Tatsumoto et al., 1992; Zhou & Zhu, 1992). In addition, the presence of irregular chondrite-normalized REE profiles (Fig. 4) illustrates processes that relate to melt migration (X. Xu et al., 1998). One important aspect is that the LREE-enriched samples have a lower abundance of the HREE than the LREE-depleted samples. Clearly, the diversity of the REE patterns (Fig. 4) in clinopyroxenes from eastern China requires more than one enrichment mechanism. For example, the spoon-shaped patterns in clinopyroxenes from peridotites can be best accounted for by chromatographic metasomatism. For our database, we prefer a percolation-reaction model, which takes into account both chromatographic and melt-rock reaction effects and shows some similarities between the modelled REE patterns and the measured REE profiles [X. Xu et al. (1998) and references therein]. The spoon-shaped REE pattern is believed to be the product of very early stages of small-volume melt infiltration where percolation-controlled metasomatism was terminated as a result of incorporation into basaltic magma. The enrichment of La and Ce in small-volume melts leads to their enrichment in clinopyroxene relative to other LREE (Pr, Nd, Sm). On the other hand, the convex profile, apparent in strongly LREE-enriched samples, may reflect metasomatic recrystallization of previous clinopyroxene or direct crystallization from melts in a closed system. The latter suggestion is supported by the extremely high LREE in clinopyroxene and the iron content of olivine from an on-craton websterite.
The relationship between trace elements and isotopes is presented in Fig. 6. Most of the Chinese xenoliths are isotopically depleted but many of these depleted xenoliths are LREE enriched (Figs 5 and 6). This indicates ‘recent’ decoupling of trace elements and Nd isotopes as a result of addition of the LREE by a fluid. In contrast to such ‘young’ processes that may relate to the periods of volcanism there is an indication of possibly ‘older’ enrichments in the mantle. A few LREE-enriched xenoliths, mainly pyroxenites or megacrysts, have high 87Sr/86Sr and low 143Nd/144Nd ratios, which indicate a coupling of trace elements and isotopes that may reflect the presence of ‘old’ enrichment processes in the lithosphere (e.g. >2·5 Ga). However, as their origin is so complex the ‘enriched’ signature may also indicate input from crustal reservoirs.
Some insights into what type of melts may have been responsible for changes to the post-Archaean lithosphere are forthcoming from the application of published partition coefficient data to trace element data. Clinopyroxenes with convex REE profiles may be the products of equilibration with LREE-depleted silicate or non-silicate melts, or the products of direct crystallization from such LREE-depleted melts. These hypothetical melts are distinctly different from the compositions of oceanic island and mid-ocean ridge basalts. This indicates that the host alkaline silicate melts were not the melts that produced the trace element enrichment in these mantle xenoliths, a suggestion supported by isotope data. The large isotope variation in the mantle xenoliths is very different from the restricted range of isotopes in the host basalts. Partition coefficient data for clinopyroxene–carbonatite melts (Green et al., 1992) permits the calculation of coexisting hypothetical equilibrium melts for LREE-enriched clinopyroxenes. This reveals a melt with elemental characteristics similar to those of natural carbonatites (Nelson et al., 1988), which could account for the nature of the enriched xenoliths. It is very difficult to infer the composition of a hypothetical coexisting melt on the basis of trace elements in clinopyroxene without trace element data on coexisting minerals. We are certain that the metasomatic melt responsible for the trace element enrichment in some of these mantle xenoliths was LREE rich and Fe poor. The extent of the enrichment or depletion of Ti and Zr in such melts relative to the LREE is less certain and awaits more detailed study of the trace element geochemistry of coexisting phases.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The petrological and geochemical data presented in this paper allow us to answer the questions posed in the Introduction.
What is the nature of the sub-Moho mantle beneath eastern China?
Sub-crustal peridotites from eastern China are predominantly shallow-facies mantle rocks (i.e. spinel-facies 75–80 km). Their lherzolitic nature, olivine/orthopyroxene mode and the composition of olivine reveals remarkable similarities to the lithosphere found beneath tectonically active continents or modern ocean basins (200 Ma).
Does on- and off-craton provinciality exists as it does in other parts of the world?
On- and off-craton provinciality is not clearly defined in eastern China. Although a small number of megacrysts and pyroxenites from the NCC have enriched Sr and Nd isotopes like on-craton peridotites from Kaapvaal, the majority of peridotites on and off the NCC are depleted in Sr and Nd isotopes.
What processes best account for the formation of lithospheric mantle beneath eastern China?
Most of the on- and off-craton peridotites from eastern China have evolved in response to processes similar to those responsible for the formation of the lower lithosphere in ocean basins or tectonically active continents. These processes are thought to relate to the extraction of basaltic magmas from the asthenosphere (1300°C) with subsequent incorporation and accretion of residual peridotites. It is proposed that the spinel-facies peridotites represent shallow accretion of mantle during asthenospheric upwelling in the Phanerozoic.
How does present-day lithosphere compare with the Archaean lithosphere known to have survived until the Ordovician?
According to kimberlite-hosted xenolith data from the NCC, Ordovician lithosphere had an architecture similar to that of Archaean lithosphere. It was stable to garnet- and diamond-facies, cold (shield geotherm) and it reached a thickness of 180–200 km. This is in marked contrast to Cenozoic lithosphere, which is ‘oceanic’ with some indication of an ‘older’ pre-existent lithosphere. It has been proposed that the Archaean lithospheric keel was affected by thermo-tectonic processes (Menzies et al., 1993; Griffin et al., 1998) in the form of plume activity, delamination or rifting, or a combination of such processes.
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ACKNOWLEDGEMENTS |
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The late Keith Cox was an inspiration to us all, a great companion in the field or on a long plane flight. He will never be forgotten for his friendship, his support, his dry wit and those memorable ‘stories’. Keith always believed that we should understand the petrology and mineralogy of our rocks before tackling geochemistry, particularly isotopes.
The Royal Society and the Chinese Academy of Sciences are thanked for supporting a link with Changsha University (Dr Fan Weiming), the catalyst for this project. The Overseas Research Student Scheme (UK) and the K. C. Wong Education Foundation (Hong Kong) are thanked for financial support in a form of a pre-doctoral year and a 3-year doctoral studentship to Hongfu Zhang (Northwestern–Beijing) tenable at Royal Holloway College, University of London. Andy Beard is thanked for his technical assistance with electron microprobe and Sr–Nd isotope analyses. The authors appreciate the considerable efforts of Graham Pearson, both as a doctoral examiner of H.Z. and as a reviewer. The detailed and constructive comments of Roberta Rudnick have tightened up this manuscript considerably. Roberta is thanked for her time and effort.
This study was supported by grants from the Chinese Academy of Sciences (KZ951-A1-401-01) and from the National Sciences Foundation of China (49733110).
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FOOTNOTES |
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*Corresponding author. e-mail: menzies{at}gl.rhbnc.ac.uk
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 H.-F. Zhang Temporal and spatial distribution of Mesozoic mafic magmatism in the North China Craton and implications for secular lithospheric evolution Geological Society, London, Special Publications, January 1, 2007; 280(1): 35 - 54. [Abstract] [Full Text] [PDF]
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W. Fan, F. Guo, Y. Wang, and H. Zhang Late Mesozoic mafic magmatism from the North China Block: constraints on chemical and isotopic heterogeneity of the subcontinental lithospheric mantle Geological Society, London, Special Publications, January 1, 2007; 280(1): 77 - 100. [Abstract] [Full Text] [PDF]
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T. D. Cope and S. A. Graham Upper crustal response to Mesozoic tectonism in western Liaoning, North China, and implications for lithospheric delamination Geological Society, London, Special Publications, January 1, 2007; 280(1): 201 - 222. [Abstract] [Full Text] [PDF]
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S. Hu, M. Fu, S. Yang, Y. Yuan, and J. Wang Palaeogeothermal response and record of Late Mesozoic lithospheric thinning in the eastern North China Craton Geological Society, London, Special Publications, January 1, 2007; 280(1): 267 - 280. [Abstract] [Full Text] [PDF]
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X. Chang, Y. Liu, M.-G. Zhai, and Y. Wang Crustal P-wave velocity distributions and metallotectonics around the North China Craton Geological Society, London, Special Publications, January 1, 2007; 280(1): 293 - 302. [Abstract] [Full Text] [PDF]
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S. OKAMURA, R. J. ARCULUS, and Y. A. MARTYNOV Cenozoic Magmatism of the North-Eastern Eurasian Margin: The Role of Lithosphere Versus Asthenosphere J. Petrology, February 1, 2005; 46(2): 221 - 253. [Abstract] [Full Text] [PDF]
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Y.-G. XU and J.-L. BODINIER Contrasting Enrichments in High- and Low-Temperature Mantle Xenoliths from Nushan, Eastern China: Results of a Single Metasomatic Event during Lithospheric Accretion? J. Petrology, February 1, 2004; 45(2): 321 - 341. [Abstract] [Full Text] [PDF]
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null Zhang Hong-Fu, Z. HONG-FU, S. MIN, Z. MEI-FU, F. WEI-MING, Z. XIN-HUA, and Z. MING-GUO Highly heterogeneous Late Mesozoic lithospheric mantle beneath the North China Craton: evidence from Sr-Nd-Pb isotopic systematics of mafic igneous rocks Geological Magazine, January 1, 2004; 141(1): 55 - 62. [Abstract] [Full Text] [PDF]
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null Chen Bin, B. Chen, B.-m. Jahn, and M. Zhai Sr-Nd isotopic characteristics of the Mesozoic magmatism in the Taihang-Yanshan orogen, North China craton, and implications for Archaean lithosphere thinning Journal of the Geological Society, December 1, 2003; 160(6): 963 - 970. [Abstract] [Full Text] [PDF]
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B. Chen, B. CHEN, and M. ZHAI Geochemistry of late Mesozoic lamprophyre dykes from the Taihang Mountains, north China, and implications for the sub-continental lithospheric mantle Geological Magazine, January 1, 2003; 140(1): 87 - 93. [Abstract] [Full Text] [PDF]
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