Thermal State and Structure of the Lithosphere beneath Eastern China: A Synthesis on Basalt-Borne Xenoliths

INTRODUCTION

Estimation of equilibrium temperatures and pressures of mantle-derived xenoliths is pivotal to understanding geothermal gradients in the lithosphere and mechanisms of magma generation as well as the depth to important geological transitions such as the crust-mantle and lithosphere-asthenosphere boundaries. Xu Y G et al. (1995) have constructed a thermal gradient for the lithosphere mantle beneath eastern China using estimated equilibrium temperatures and pressures of both spinel and garnet peridotites. Although this geotherm is indicative of extensional setting that is consistent with other geological records of the same region, it also suffered the following shortcomings. (a) In their studies, equilibrium pressures for spinel-facies peridotites were obtained by application of the calibration of Ca^Ol-Cpx barometer by Köhler and Brey (1990). However, the application of this method to natural rocks is hampered by the common Ca-disequilibria between olivine and clinopyroxene due to later thermal event (Werling and Altherr, 1997; Köhler and Brey, 1990) and to lesser extent by the difficulty in precise measurement of Ca content in olivine with an electron microprobe. Moreover, there is a kink along the geotherm, separating the data collected for spinel and garnet peridotites. Given the different barometers used for spinel and garnet facies rocks, the reality of this kink remains unconstrained, because the consistency between different barometers has not been yet demonstrated. (b) Another potential problem associated with the approach used by Xu Y G et al. (1995) is that they tried to define a uniform geotherm for the lithosphere underlying different tectonic unites and large number of localities running from Heilongjiang in Northeast China to Hainan Island in South China (Fig. 1). However, different localities may have different thermal gradients. This is highlighted by recent reconstruction of geotherms (Fig. 2) using xenolith data of Nüshan (Huang et al., 2004; Xu X S et al., 1998), Hannuoba (Chen et al., 2001; Shi et al., 2000), Shanwang (Zheng et al., 2006), Kuandian (Fang and Ma, 1998), Jiaohe (Yu, 2008), Chao'erhe (Fan et al., 2008), Mingxi (Lin et al., 1999; Xu X S et al., 1998), Xinchang (Lin et al., 1995), Qilin (Xu Y G et al., 1999, 1996) and Leizhou Peninsula (Lin et al., 2003; Yu et al., 2003). Nevertheless, it remains unclear whether the observed difference in Fig. 2 is due to different thermobarometers used (e.g., Xu X S et al., 1999; Xu Y G et al., 1999) and/or data quality, or truly reflect different thermal gradients for different localities.

Figure  1.  Simplified diagram showing major tectonic units and xenolith location in eastern China. South China is subdivided to inland part and coast part (SE coastal gravity zone) by NNE Yuyao-Lishui (Zhejiang Province), Zhenghe-Dapu (Fujian Province) and Lianhuashan (Guangdong Province) (YLZDL) fault (Ma et al., 2002). Data sources for the ages of host rocks: Lixian (Yu et al., 2005), Leizhou Peninsula, Mingxi, Xinchang, Xilong (Ho et al., 2003), Anyuan (Ye et al., 2001), Nüshan, Shanwang, Hebi, Hannuoba, Kuandian (Liu et al., 1992), Jiaohe (Wang, 1996), Qilin (Huang et al., unpublished data).

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Figure  2.  The existing geotherms constructed on the basis of equilibrium pressures and temperatures of xenoliths from eastern China. Data source: Xu Y G et al.(1999, 1995); Xu X S et al.(1998, 1996); Chen et al. (2001); Lin et al.(2003, 1998); Huang et al. (2004) and Yu et al. (2003). The comparative geotherms (shield, oceanic, Spitsbergen RIFT and southeastern Australia) are after O'Reilly and Griffin (1996). The transition between spinel- and garnet-lherzolites is from O'Neill (1981), and the spinel pyroxenite-garnet pyroxenite boundary is from Herzberg (1978).

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These problems are elaborated in this article by applying the same pairs of thermometer and barometers on the data available for garnet-bearing peridotite, pyroxenite and granulite xenoliths from a number of localities in eastern China. This exercise is timely important because some new localities of garnet-bearing peridotite xenoliths (for which both reliable thermometer and barometer are available) are discovered in eastern China and also because increasing number of garnet peridotites and granulites are collected in recent years. On the other hand, the importance of garnet pyroxenites in the investigation of lithospheric geotherm has increasingly been recognized (Chen et al., 1996; Xu X S et al., 1996; Amundsen et al., 1987; O'Reilly and Griffin, 1985; Griffin et al., 1984). Given their common coexistence with spinel peridotites, garnet pyroxenites can yield information as to the thermal state for the lithospheric section occupied by spinel-facies peridotites, thus making it possible to overcome the aforementioned second problem associated with Xu Y G et al. (1995). The obtained data and inferred thermal state of the lithosphere will be evaluated in terms of major tectonic unites in eastern China (NE China, South China and North China) and major geological feature (e.g., north-south gravity lineament). These data, in combination with recent geophysical data, will be used to assess the nature of the stratigraphy and architecture of the lower crust and uppermantle beneath eastern China. We hope that this effort may provide a basic framework for the 4-D lithosphere mapping, one of key contents of the ongoing Cratonic Destruction Project in eastern China.

SAMPLE LOCALITIES AND DATA SCREEN

Xenoliths used in this study were collected from over twenty localities of Cenozoic basalts and/or mafic rocks/pipes in Northeast China, North China and South China (Fig. 2, Table 1). The locality in order from north to south include, Chao'erhe (Fan et al., 2008), Hannuoba (Chen et al., 2001; Shi et al., 2000), Nüshan (Xu X S et al., 1998), Xinchang (Lin et al., 1995; Xu Y G et al., 1995; Fan et al., 1989), Xilong (Yu et al., 2003), Mingxi (Lin et al., 1999a; Xu X S et al., 1998). The locality containing garnet pyroxenites include: Jiaohe (Yu, 2008), Kuandian (Fang and Ma, 1998), Hannuoba (Chen et al., 2001; Shi et al., 2000), Shanwang (Zheng et al., 2006), Nüshan (Huang et al., 2004; Xu X S et al., 1998), Qilin (Xu X S et al., 1996) and Leizhou (Huang et al., 2007; Lin et al., 2003; Yu et al., 2003). Granulite localities include Hannuoba (Chen et al., 2001; Huang et al., 2001), Nüshan (Huang et al., 2004) and Mingxi (Huang et al., unpublished data). These xenolith localities mostly occur to east of the DTGL. Specifically, Jiaohe is located in NE China, Nüshan, Shanwang and Kuandian in North China, and Leizhou Peninsula, Mingxi, Xinchang, Qilin in South China. Only two localities (Hannuoba, Chao'erhe) are situated to west of the DTGL (Fig. 2).

Table  1.  T-P estimates for granulites, garnet-bearing pyroxenite and lherzolite xenoliths from eastern China

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Mineral compositions are taken from literature (e.g., Fan et al., 2008; Huang et al., 2007, 2004, 2001; Zheng et al., 2006; Lin et al., 2003, 1999a, 1995; Yu et al., 2003; Chen et al., 2001; Fang and Ma, 1999; Xu X S et al., 1998, 1996; Xu Y G et al., 1998, 1993; Fan and Hooper, 1989). The data are screened before the calculation. Only electron microprobe (EPMA) data are used. The analyses with total less than 99% or greater than 101% are not used. Wet chemistry data collected in the eighties are not considered either. One of the most important pre-request for thermobarometric calculation is the thermal and compositional equilibrium between co-existing minerals. Because thermobarometry commonly involves Mg-Fe equilibrium between garnet and pyroxenes, we evaluate Mg-Fe partition between co-existing minerals for the purpose equilibrium assessment. There are excellent line correlations between Mg^# of orthopyroxene and clinopyroxene (Fig. 3a) and very well curve relations between Mg^# of garnet and those of orthopyroxene/clinopyroxene (Figs. 3b–3c), which strongly suggest that compositional equilibrium between co-existing minerals of garnet-bearing pyroxenites and peridotites is attained for the xenoliths selected for this study.

Figure  3.  Partitioning of Fe-Mg between co-existing garnet, clinopyroxene and orthopyroxene for peridotite and pyroxenite xenoliths selected for this study. Data from Xu X S et al.(1998, 1996), Lin et al.(1999, 1995), Fang and Ma (1999), Yu et al. (2003), Fan et al.(2008, 1989), Jin and Pan (1998), Chen et al. (2001), Yu (2001), Zheng et al.(2006, 2001), Huang et al.(2007, 2004), Su et al. (2007), Yu (2008).

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SELECTION OF GEOTHERMOBAROMETERS IN TERMS OF MULTIPLE CRITERIA

A large number of thermometer and barometer are available in literature. Different thermobarometers are calibrated for different purposes and using different criteria. It is therefore of ultimate importance to choose appropriate thermobarometer(s) for the purpose of construction of thermal gradient for a given lithosphere. The criteria used in evaluating the reliability of a given thermometer or barometer is reiterated here (Xu Y G et al., 1999; Xu Y G, 1993): (1) A reliable thermobarometric calibration should perfectly reproduce the experimentally reversed data in natural systems under high temperature and pressure (Brey and Köhler, 1990; Carswell and Gibb, 1987; Bertrand and Mercier, 1985); (2) estimated P-T values should be consistent with known phase relationships (Finnerty and Boyd, 1984; Griffin et al., 1984); and (3) applications of geothermobarometric data to composite xenoliths should yield similar estimates for different parts of the composites. Another approach but more rarely used is to directly determine the stability limits of mineral assemblage by experiments on individual xenoliths (Adam et al., 1992; Irving, 1974).

The first criterion has been used by Bertrand and Mercier (1985) and Carswell and Gibb (1987) in the formulation of new geothermobarometers and the evaluation of existing ones. It has been shown that this may be the most reasonable approach in any thermobarometric evaluation (e.g., Brey and Köhler, 1990). From Figs. 4a and 4c it is apparent that the BM thermometer (Bertrand and Mercier, 1985) nicely fits the experimental temperatures, whereas the Ellis and Green (1979) thermometer underestimates the experimental temperatures for temperatures > 1 200 ℃ and overestimates for temperatures < 1 200 ℃ (Fig. 4c). When applied to the experimentally crystallized garnets and pyroxenes, the Ellis and Green (1979) calibration is found to systematically overestimate the temperature by about 150 ℃ for the pressure range of garnet pyroxenites (Green and Adam, 1991). A similar remark was also made by Ai (1994) who has accordingly revised the Fe-Mg exchange thermometer based on a comprehensive data set of both ultramafic and mafic compositions. The Sachtleben and Seck (1981) thermometer slightly underestimates the experimental temperatures for temperatures < 1 100 ℃ but the discrepancy becomes important for temperatures > 1 100 ℃ (Fig. 4d). Accordingly, this empirical thermometer has been improved by WittEickschen and Seck (1991).

Figure  4.  Comparison between the calculated temperatures from different thermometers and experimental temperatures (after Xu Y G et al., 1999). (a) Bertrand and Mercier (1985); (b) Brey and Köhler (1990), (c) Ellis and Green (1979); and (d) Sachtleben and Seck (1981). Filled diamond: data from Brey and Köhler (1990); open circle: data from Akella (1976), Mori and Green (1978).

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Figure 5 shows the results for barometers. Similar to the conclusion of Carswell and Gibb (1987), it is clear that the barometer of Wood (1974) underestimates the experimental pressures. By contrast, the barometer of Nickel and Green (1985) reasonably reproduces the experimental pressures.

Figure  5.  Comparison between the calculated pressures of Nickel and Green (1985)'s barometer and experimental pressures. Data source is the same as in Fig. 4; (b) comparison between P-T calculated by BK2P/NG and the graphite-diamond transition. Data of graphite- and diamond-bearing peridotites and coexisting multiphase inclusions in diamond are after Zheng et al. (2006), Pearson et al. (1994), Jaques et al. (1990), Shee et al. (1982), Tsai et al. (1979), Pokhilenko et al. (1977), Dawson and Smith (1975), and Nixon and Boyd (1973). The graphite-diamond transition is taken from Kennedy and Kennedy (1976).

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Three phase relationships have been used so far in thermobarometric evaluations: (1) graphitediamond transition (Finnerty and Boyd, 1984); (2) spinel to garnet pyroxenite transition (Griffin et al., 1984); and (3) spinel to garnet lherzolite transition (O'Reilly and Griffin, 1985). We performed a test for two pairs of thermometers/barometers listed above to see (1) whether they are consistent with the graphitediamond reaction using the available data on graphiteand diamond-bearing xenoliths (Zheng et al., 2006; Pearson et al., 1994; Jaques et al., 1990; Shee et al., 1982; Pokhilenko et al., 1977; Dawson and Smith, 1975; Nixon and Boyd, 1973), as well as multiphase inclusions in diamond (Tsai et al., 1979); (2) whether they are consistent with the spinel-garnet transitions using mineral chemistry data on garnet-bearing xenoliths (including pyroxenites and peridotites) from eastern China. It is clear from Figs. 4–5 that the BK2P/NG method (Brey and Köhler (1990)/Nickel and Green (1985)) yields results consistent with both the graphite-diamond transition of Kennedy and Kennedy (1976) and the spinel-garnet transition, whereas the estimates of the EG/W method are only consistent with the spinel-garnet transition but not with the graphite-diamond reaction (Xu Y G et al., 1999). These results strongly suggest that the BK2P/NG (as well as the T_BK2P/P_BKN method (Brey and Köhler (1990)/Brey and Köhler (1990)) which yields comparable results) is the first choice when estimating P-T of Mg-rich rocks.

The third criterion is largely based on equilibrium composite xenoliths. The different parts of a composite are located at the same depths, therefore application of a geothermometer should yield similar temperature estimates for different parts of the xenoliths. However, this approach is frequently hampered because the lherzolite part in composites could be thermally affected by the magmatic intrusion represented by the pyroxenite vein (Xu X S et al., 1999).

The methods mentioned above are only associated with garnet-bearing xenoliths. For the P-T estimation for garnet-free granulites, McCarthy and Patiño Douce (1998)'s empirical geobarometer and Brey and Köhler (1990)'s two-pyroxene thermometer are used. The reasoning for this choice is provided by Huang et al. (2004).

RESULTS

A summary of estimated equilibrium temperatures and pressures are presented in Table 1, and results are plotted in Fig. 6. The majority of calculation results are consistent with experimental phase relationship. Specifically, the equilibrium P-T places garnet pyroxenites within the stability field of garnetfacies pyroxenite (O'Neill, 1981), and garnet peridotites within the stability field of garnet peridotite (Fig. 6). These demonstrate the robustness of the calculation results. Below, the results are described in terms of different tectonic units in eastern China, namely North China craton, northeastern China and South China.

Figure  6.  Equilibrium pressures and temperatures calculated for screened xenoliths from eastern China. (a) Eastern North China craton (NCC); (b) western NCC; (c) northeastern (NE) China; (d) South China. Data source: Chao'erhe (Fan et al., 2008), Hannuoba (Chen et al., 2001; Shi et al., 2000), Nüshan (Huang et al., 2004; Xu X S et al., 1998), Xinchang (Lin et al., 1995; Xu Y G et al., 1995; Fan and Hooper, 1989), Mingxi (Lin et al., 1999; Huang et al., unpublished data), Jiaohe (Yu, 2008), Kuandian (Fang and Ma, 1999), Shanwang (Zheng et al., 2006), Qilin (Xu X S et al., 1996), Leizhou (Huang et al., 2007; Lin et al., 2003; Yu et al., 2003), Lixian (Su et al., 2007; Yu et al., 2001), Anyuan (Zheng et al., 2004), Hebi (Fan et al., 2008). The comparative geotherms (shield, oceanic, Spitsbergen rift and southeastern Australia) are after O'Reilly and Griffin (1996). The transition between spinel- and garnet-lherzolites is from O'Neill (1981), and the spinel pyroxenite-garnet pyroxenite boundary is from Herzberg (1978).

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North China Craton

The typical xenolith locality in eastern North China craton (NCC) is Nüshan, where xenoliths include garnet and spinel peridotites, garnet pyroxenites and granulites covering a full range of the lithosphere (Huang et al., 2004; Xu X S et al., 1998). The estimated P and T values for the Nüshan xenoliths range from 0.4 to 2.4 MPa and from 600 to 1 200 ℃, thus defining tightly a geotherm. Data for garnet pyroxenites from Kuandian and garnet peridotites from Shanwang are also plotted proximal to this geotherm (Fig. 6a), suggesting that the Nüshan geotherm represent that for the lithosphere under the eastern NCC. This geotherm corresponds to a high thermal gradient (~ 80 mW/m²). It is noted that the geotherm is composed of two sections: the section with pressure less than 2 MPa is characterized by a slow slope, whereas the section with pressure greater than 2 MPa shows a roughly vertical pattern. Likely, the xenoliths from eastern NCC record two different mechanisms of heat transfer between lithosphere and asthenosphere (see DISCUSSION).

Thermal state for the lithosphere under western NCC is constrained by the xenoliths from Hannuoba. P-T data are restricted to granulites and garnet pyroxenites and fall into two groups (< 1.1 and > 1.5 MPa). Equilibrium P and T between them are not observed. These data nevertheless define a geotherm corresponding to ~70 mW/m² (Fig. 6b). This thermal gradient, slightly lower than that for eastern NCC, is characterized by a uniform slope, implying a single heat transfer mechanism for the lithosphere sampled by the Hannuoba xenoliths.

Northeastern China

Only two localities (Jiaohe and Chao'erhe) in northeastern China contain garnet-bearing xenoliths. In addition, the number of samples from these two localities is relatively small, making the geotherm for this region poorly constrained. The estimated P and T for garnet pyroxenites from Jiaohe vary from 0.8 to 1.8 MPa and from 820 to 1 030 ℃, respectively. They thus only provide thermal gradient for the uppermost upper mantle under the region to east of DTGL in NE China. The estimated P and T for garnet pyroxenites from Chao'erhe vary from 2.4 to 3 MPa and 1 120 to 1 340 ℃, respectively. The estimated pressures are considerably higher than those obtained for garnet peridotites from the NCC, but are similar to those for Qinling peridotites (Yu et al., 2001). It is noted that the thermal gradient defined by the Jiaohe xenoliths resembles the Nüshan geotherm, and equilibrium P-T data for the Chao'erhe xenoliths are plotted along the extended trend of the Hannuoba geotherm. This indicates the contrasting thermal gradient for the lithosphere across the DTGL.

South China

P-T data for the xenoliths from South China (Fig. 6c) do not define a single slope. Notably, the data for the Qilin xenoliths define a slope which is different from that for the Mingxi xenoliths. It is possible that the Mingxi and Xinchang xenoliths form one gradient with a kink at about 2 MPa. However, the slower slope for the deep lithosphere compared to shallow section is inconsistent with the heat transfer between lithosphere and asthenosphere (McKenzie and Bickle, 1988). Consequently, we believe that the Mingxi and Qilin xenoliths define two different thermal gradients. This is further supported by the Mingxi granulites whose equilibrium pressures and temperatures are distributed along the extended trend defined by Mingxi garnet peridotites (Fig. 6d). In spite of small number, the Xinchang samples seem to form a further different gradient.

DISCUSSION

Two Types of Geotherm for the Lithosphere beneath Eastern China

The data presented in this study apparently reveal two types of geotherm for the lithosphere beneath eastern China (Fig. 7). The first type, as exampled by the geotherms of Hannuoba, Mingxi and probably NE China, is characterized by constant slope of data in the P-T space (Fig. 7a). Noticeably, thermal gradients for three different regions are very similar. The second type, as exampled by the geotherms of Nüshan and probably Xinchang, is characterized by variable slopes in the P-T space (Fig. 7b). Specifically, the samples with pressure < 2 MPa define a slow slope, whereas the samples with pressure > 2 MPa define a virtually vertical slope. The following considerations make us believe that the two types of thermal gradient are real, rather than an artifact of thermobarometric calculation.

Figure  7.  Two types of xenolith-derived geotherms in eastern China.

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(a) The different slopes in the second type of geotherm may correspond to different heat transfer mechanisms, with conductive transfer for the shallow upper mantle and advective transfer for the deep mantle. This observed transition in thermal transfer mechanism is essentially similar to that predicted by theoretical modeling (McKenzie and Bickle, 1988), and the transition zone may correspond to the thermal boundary layer (TBL). We thus propose that the samples defining such a kind of geotherm must have been derived from all the range of the lithosphere and the samples with the highest P and T provide maximum estimates on the thickness of the lithosphere at the given region. Following this line of reasoning, the thickness of the lithosphere beneath eastern NCC is about 70 km, in good agreement with the recent seismic results (Chen et al., 2009).

(b) The higher thermal gradient for eastern NCC is consistent with higher equilibrium temperatures for spinel peridotites collected from this region, compared to those for other areas. For example, equilibrium temperatures for spinel peridotites from eastern NCC range from 800–1 200 ℃, with two peaks at 900 and 1 050 ℃. There are a significant number of samples with temperature of 1 000–1 100 ℃ in eastern NCC. In the regions (western NCC, NE China) characterized by the first type of geotherm, the equilibrium temperatures are relatively low, mostly concentrated between 800–1 000 ℃, despite a few > 1 000 ℃ (Fig. 8). If these spinel peridotites are equilibrated within the spinel facies stability, i.e., under same pressure range, it can be inferred that higher temperatures are indicative of a higher geotherm.

Figure  8.  Histograms of equilibrium temperatures for spinel peridotites from different regions in eastern China. Northeast China: Wangqing (Xu Y G et al., 1998) and Huinan (Xu Y G et al., 2003). Western NCC: Hannuoba (Chen et al., 2001; Shi et al., 2000) and Datong (Chen et al., 1997). Eastern NCC: Nüshan (Xu X S et al., 1998; Qi et al., 1995), Kuandian (Fang and Ma, 1998), Shanwang (Zheng et al., 1998), Hebi (Zheng et al., 2001) and Pangshishan (Liu and Yan, 2007; Chen et al., 1994). Inland South China: Mingxi (Lin et al., 1999; Qi et al., 1995), Anyuan (Zheng et al., 2004). Coast of South China: Xinchang (Lin et al., 1995; Xu Y G et al., 1995; Fan and Hooper, 1989), Qilin (Xu X S et al., 1996; Qi et al., 1995) and Leizhou (Yu et al., 2003).

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(c) As will be discussed in the following sections, the higher thermal gradient is, the thinner lithosphere is. In the case of eastern NCC, the lithosphere is inferred to be ~70 km thick. Given the experimental determined spinel-garnet phase transition at 70–80 km (Robinson and Wood, 1998), lithospheric xenoliths sampled by asthenosphere-derived melts will be essentially spinel-facies peridotites. The same reasoning leads to the suggestion that both spinel and garnet facies peridotites can be found in the regions with low thermal gradient, because the depth to the lithosphereasthenosphere boundary in these areas is > 90–100 km, well below the depth at which spinel-garnet phase transition occurs. This prediction is matched by observation that so far garnet peridotites are exclusively found in the areas with low thermal gradient (west of the DTGL, South China), whereas so far no garnet peridotite xenoliths are observed in eastern NCC (high thermal gradient).

The two types of geotherm are not mutually exclusive. The second type may represent a geotherm for whole lithospheric section including both mechanical boundary layer (MBL) and TBL, while the first type may only depict the MBL. In particular, the peculiar geotherm for Xinchang may only depict the TBL. The reason why different types of geotherm, or different sectors are sampled by xenoliths remains unclear. It may be related to entrainment mechanism of xenoliths by basalts, a phenomenon not well understood so far.

It should be noted the peculiarity of the geotherm for Qilin and Leizhou, which crosses the first type of geotherm. It is also characterized by a weak convexupward pattern, probably reflecting a transitional feature between conductive and advective heat transfer. Qilin and Leizhou are situated to the northern margin of South China Sea, an area intruded by upwelled seismically low velocity materials (Lei et al., 2009). It is therefore possible that the thermal perturbation associated with Qilin and Leizhou may reflect the effect of impregnation of mantle plume on the base of the lithosphere.

Heterogeneous Lithospheric Structure in Eastern China

Thermal gradient, in comparison with equilibrium temperature of spinel peridotites and seismic data, can be used to characterize the nature of the crust-mantle boundary (CMB) and the lithosphereasthenosphere boundary (LAB) for a given region. The information as to these important boundaries is pivotal to understanding deep lithospheric processes and magma genesis.

Depth to Moho

The "seismic Moho" is defined as the base of the crust, across which the compressional wave velocity increases rapidly to ~8 km/s, whereas the crust-mantle boundary (CMB) is the transition between felsic-mafic crustal rocks and a dominantly ultramafic upper mantle (Griffin and O'Reilly, 1987). They are not always consistent, particularly in continental regions of high heat flow where the seismic Moho may lie deeper than the CMB (Griffin and O'Reilly, 1987). The depth to CMB can be estimated by comparing the lowest temperature estimates of the spinel lherzolites and xenolith-based geotherm (O'Reilly and Griffin, 1985). If the relative proportion of mantle xenoliths at the earth surface can be taken as indicative of the lithology of the upper mantle, then spinel peridotite is the dominant rock of the shallow mantle. It is thus reasonable to infer that the shallowest depth from which spinel peridotites were sampled by the host basalts defines the maximum thickness of the crust. If the depth to CMB is greater than the seismic Moho, a crust-mantle transition zone can be inferred (O'Reilly and Griffin, 1996), probably indicative of magmatic underplating which took place previously. To investigate the depth to CMB, we compare histograms of equilibrium temperature with geotherm for the NCC, coast of South China and inland of South China, respectively.

North China

In eastern NCC, the equilibrium temperatures for the granulite xenoliths are all below 800 ℃, whereas mantle-derived xenoliths are mostly equilibrated at temperatures higher than 820 ℃ (Figs. 6a, 7). 820 ℃ is therefore considered as the temperature at the base of the lower crust. By reference of the geotherm in this area, this suggests that the depth to the CMB is ca. 30 km, consistent with the depth to Moho as revealed by seismic refraction data (Chen, 1988). In contrast, the depth to the CMB at Hannuoba (~28 km) is significantly shallower than the seismic Moho (~42 km, Table 2), implying a thick (~14 km) crust-mantle transition zone. The different crust-mantle structures in the NCC may have resulted from different degrees of magma underplating in the western and eastern NCC (Xu Y G, 2007; Huang et al., 2004). Magmatic underplating must have been important in western NCC, because geochronologic studies (Wilde et al., 2003; Fan et al., 1998) reveal that magma underplating occurred during the Late Mesozoic, coeval with the initiation of widespread thermo-tectonic reactivation of the NCC (Xu Y G, 2001; Griffin et al., 1998). In contrast, underplating was minor in eastern NCC given the pre-dominant "Proterozoic age" of the granulite xenoliths and the coincidence of the "petrologic" and "seismic" Mohos (Huang et al., 2004). Alternatively, the contrast in crust structures in the regions separated by the DTGL may be related to crustal foundering processes (Gao et al., 2004; Xu Y G, 2002).

Table  2.  Inferred CMB and LAB comparing with seismic Moho and LAB

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Inland of South China (Mingxi)

Comparison of the lowest equilibrium temperatures of spinel peridotites from Mingxi yields the depth to the CMB for these areas is ca. 32 km (Fig. 6d). This estimate is close to the seismic Moho (Wang et al., 1993; Table 2). The consistency between the petrologic Moho and seismic Moho suggests an insignificant crust-mantle transition zone in this area. However, care should be taken because the equilibrium distribution pattern is irregular (Fig. 6d). Notably, there lack samples with temperature of 850–1 000 ℃, suggesting biased sampling of the lithospheric mantle by host rocks.

Coast of South China (Leizhou and Qilin)

Comparison of the lowest equilibrium temperatures of spinel peridotites from Leizhou and Qilin yields the depth to the CMB for these areas is ca. 31 and 35 km, respectively (Fig. 6d). These estimates are greater than the seismic Moho (28, 31 km) (Yao et al., 2006; Table 2). We don't know whether this contradiction is related to the error associated with thermobarometry and seismic investigation or to the representativeness of xenolith population. As noted above, the geotherm in this area is peculiar, probably reflecting heating of the lithosphere by impregnating plume. Alternatively, the anomalously high geotherm at the crust-mantle boundary may be related to lateral heat transport from the underplating materials in South China basin, where the seismic velocity structure (V_P=6.6–7.6 km/s) is suggestive of a thick crust-mantle transition zone.

Depth to LAB

According to McKenzie and Bickle (1988), the lithosphere includes TBL and overlying MBL. With the MBL, heat transport is via conduction, whereas in the TBL, both conduction and advection play role in heat transport. Compared to the MBL, the TBL is characterized by a steeper slope in the P-T space. The depth the transition between MBL and TBL provides an estimate to the lithospheric thickness. For the region where no MBL-TBL transition is observed, the greatest depth of garnet peridotites represents the minimum thickness of brittle lithosphere (O'Reilly and Griffin, 1996). Given the lack of evidence of a mantle plume for eastern China, potential temperature of the asthenosphere (T_P=1 280 ℃) (McKenzie and Bickle, 1988) is taken to reflect the maximum temperature at the base of the lithosphere. Projecting this temperature along a given geotherm yields a maximum estimation for the depth to the LAB. This is a reasonable assumption because the highest temperatures for spinel- and garnet-peridotites (1 150 to 1 200 ℃, 1 200 to 1 280 ℃, respectively; Table 1; Fig. 8) approach T_P (1 280 ℃).

Using this approach, the depth to LAB in eastern NCC is ca. 70 km, in remarkably good agreement with recent seismic investigation results (Chen, 1988; Table 2). In western NCC, the lithospheric thickness is greater than 90–100 km, again revealing the contrasting lithospheric structure across the DTGL. The thickness of the lithosphere under North China is less well constrained, but is likely greater than that for eastern NCC (Table 2). Given the similarity of thermal gradient in western NCC and northeastern China, the lithosphere in NE China may be similar to that in Hannuoba.

There is a variation in lithospheric thickness from coastline to inland in South China. Lithosphere in Xinchang is about ~80 km thick, whereas it becomes > 90 km. This spatial variation is broadly consistent with geophysical data of Ma et al. (1991), also with increasing depth of magmatic generation as suggested by Chung et al. (1994). The depth to LAB in Leizhou and Qilin is less well constrained, due to possible plume imprint in this area.

Deep Process Inferred from Variation in Thermal Gradient

Despite the difference in detail for different geologic parts, the Cenozoic lithosphere beneath eastern China is characterized by a high geothermal gradient (70–80 mW/m²), as inferred from the geothermo-barometry of garnet-bearing peridotites and pyroxenites brought by alkali basalts to the surface (Fig. 6). The high thermal gradient is in accordance with extensional tectonics in eastern China during the Cenozoic. This gradient is significantly higher than that for the lithosphere beneath North China in the Ordovician (Xu Y G, 2001). Such a contrast in thermal state for the same lithosphere at different times formed the basis of the current widely accepted concept that there was a dramatic change in lithospheric architecture during the Phanerozoic (Menzies et al., 1993; Fan and Menzies, 1992).

The surface heat flow ranging from 70–80 mW/m² to which the xenolith-derived Cenozoic geotherm corresponds is higher than the present-day measured values in North China (~65 mW/m²; Wang and Wang, 1992). Such a difference may imply that the lithosphere under eastern China experienced cooling from a hotter temperature regime (Menzies and Xu, 1998), and thermal relaxation at surface is faster than in the lithospheric interior. Thermal relaxation is also recorded in basin analysis. Based on apatite fission track analyses and thermal modeling, Hu et al. (2000) showed that the heat flow in Bohai basin during the Eocene was as high as 90 mW/m², corresponding to the rifting stage in this region. Since the Miocene, the basin experienced a period of thermal subsidence and relaxation, resulting in the low current heat flow (64 mW/m²).

Thermal gradient is variable in different areas in eastern China. The highest thermal gradient is observed in eastern NCC, consistent with the thinnest lithosphere in this area. A similar high geotherm can be inferred for Xinchang, along the coast of South China. Western NCC, NE China and the interior of South China are characterized by a relatively "cool" geotherm, indicative of relatively thick lithospheres under these regions. The contrast in thermal gradient and inferred lithospheric thickness in both sides of the DTGL can be attributed to the diachronous lithospheric thinning, with that in the eastern NCC significantly earlier than the Cenozoic thinning in the western NCC (Xu and Bodinier, 2004). This process eventually gave birth to the DTGL (Xu Y G, 2007).

The thermal state in South China is complex. Nevertheless, there is a decrease in thermal gradient and in lithospheric thickness from the coast to the inland. Such a variation is collaborated with westward increase in basalt's alkalinity, degree of silica undersaturation and abundance of incompatible elements and decrease in Pb isotopic ratios of Cenozoic basalt from an NE-trending extension axis in the western Taiwan Strait (Chung et al., 1994). Such a spatial chemical and isotopic variation in the basalts can be explained by different degrees of decompression melting of convecting asthenosphere under the lithosphere with different thickness (Chung et al., 1994). In this model, the spatial variation in lithosphere in South China likely resulted from lithospheric extension, probably related to the opening of the South China Sea. The greatest lithosphere thinning occurred during the Miocene beneath the axial zone at Taiwan Strait.

It appears that Cenozoic lithospheric extension only resulted in thinning of the lithosphere under the coastal regions of South China, leaving vast continental interior less affected. This is supported by restricted distribution of Cenozoic magmatism along the coastline and by thick lithosphere (> 150 km) revealed by geophysical investigation. Asthenospherederived intraplate-type basalts were erupted during Middle Jurassic–Cretaceous in Hunan and Jiangxi provinces and some of them carried spinel facies peridotite xenoliths (Chen et al., 2009). This indicates a relatively thin lithosphere (< 80 km) in the interior of the South China continent until the Cretaceous. Therefore, there was an event leading to the thickening of the lithosphere underneath this region since the Jurassic, although the nature of this event is unclear and deserves further investigation.

CONCLUSIONS

Application of reliable thermobarometer on garnet-bearing mantle xenoliths and granulite xenoliths entrained by Cenozoic basalts in eastern China leads to the following conclusions.

(1) Two types of geotherm exist for the lithosphere beneath eastern China. The first type, as exampled by Hannuoba, Mingxi and probably NE China, is characterized by constant slope of data in the P-T space. The second type, as exampled by the high geotherms of Nüshan and probably Xinchang, shows variable slopes, with conductive transfer for the shallow upper mantle and advective transfer for the deep mantle.

(2) Eastern North China craton (NCC) shows a second-type geotherm, indicating a thin lithosphere (~70 km) and a thin crust (~30 km). In contrast, western NCC possesses a relatively low thermal gradient, indicative of a thick lithosphere (> 90–100 km) and a thick crust-mantle transition zone. The dramatic change in crustal and mantle structure across the DTGL, also revealed by seismic studies, may have resulted from diachronous lithospheric thinning processes.

(b) There is a decrease in thermal gradient and in lithospheric thickness from the coast to the inland in South China (from ~80 km to > 90 km), which is collaborated with westward variation in basalt geochemistry. This likely resulted from lithospheric extension related to the opening of the South China Sea, with the greatest lithosphere thinning during the Miocene beneath the axial zone at Taiwan Strait.

(c) The peculiar geotherm in Qilin and Leizhou peninsula is transitional between conductive and advective heat transfer. This characteristic may be related to impregnation of mantle plume on the base of the lithosphere, but further studies are needed.