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Zoned olivine xenocrysts in a late Mesozoic gabbro from the southern Taihang Mountains: implications for old lithospheric mantle beneath the central North China Craton

Published online by Cambridge University Press:  20 August 2009

JI-FENG YING ,
HONG-FU ZHANG  and
YAN-JIE TANG
JI-FENG YING*
Affiliation:
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China
HONG-FU ZHANG
Affiliation:
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China
YAN-JIE TANG
Affiliation:
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China
*
*Author for correspondence: jfying@mail.iggcas.ac.cn
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Abstract

Zoned olivine grains are abundant in the late Mesozoic Shatuo gabbro (southern Taihang Mountains, central North China Craton). Olivine cores are rich in MgO and NiO, rims are rich in FeO and MnO, and both cores and rims have very low CaO contents. The cores invariably have a high Mg no. (92–94), similar to olivine xenocrysts from Palaeozoic kimberlites in eastern China. The compositional features of these olivines imply that they are xenocrysts rather than phenocrysts, namely, disaggregates of mantle peridotites at the time of intrusion. The compositional similarity of olivine cores to xenocrysts from Palaeozoic kimberlites suggests that the lithospheric mantle beneath the central North China Craton is ancient and refractory in nature, and quite different from eastern China, where the mantle is mainly composed of newly accreted materials resulting from large-scale lithospheric removal and replacement. The contrasting features of the lithospheric mantle beneath the eastern and central North China Craton imply that the large-scale lithospheric removal in Phanerozoic times was mainly confined to the eastern North China Craton.

Keywords

olivinexenocrystgabbrolithospheric mantleTaihang Mountains
Type
Original Article
Information
Geological Magazine , Volume 147 , Issue 2 , March 2010 , pp. 161 - 170
Copyright
Copyright © Cambridge University Press 2009

1. Introduction

Mantle xenoliths in volcanic rocks provide snapshots of the lithospheric mantle at the time of eruption, and direct evidence for the nature of the mantle. Thus, mantle xenoliths are of immense value in deciphering the nature and evolution of the lithosphere (Nixon, Reference Nixon1987; Pearson, Canil & Shirey, Reference Pearson, Canil, Shirey and Carlson2003). Mantle-derived xenocrysts in volcanic rocks, such as garnet, olivine, clinopyroxene and chromite, derived from disaggregated mantle xenoliths, also bear information about the protolith and can be used to extract information about the chemical nature and processes of the lithospheric mantle (Griffin et al. Reference Griffin, Fisher, Friedman, Ryan and O'Reilly1999; Scully, Canil & Schulze, Reference Scully, Canil and Schulze2004; Zhang, Reference Zhang2005; Zhang et al. Reference Zhang, Ying, Shimoda, Kita, Morishita, Shao and Tang2007), especially in regions where mantle xenoliths are not available. Here, we describe zoned olivine xenocrysts that are widely distributed in a gabbroic intrusion from the southern Taihang Mountains, north China, and present detailed compositions of these olivine xenocrysts. Further, we discuss the compositional characteristics and evolution of the sub-continental lithospheric mantle beneath the North China Craton.

2. Geological setting and petrography

The North China Craton, with its Archaean to early Proterozoic basement, is the largest and oldest craton in China (Jahn et al. Reference Jahn, Auvray, Cornichet, Bai, Shen and Liu1987; Liu et al. Reference Liu, Nutman, Compston, Wu and Shen1992). Geological and geophysical observations reveal that the North China Craton is traversed by two large-scale N–S-trending linear zones: the Tan-Lu Fault in the east, and the Daxing'anling–Taihang gravity lineament in the west (Ma, Reference Ma1989; Menzies & Xu, Reference Menzies, Xu, Flower, Chung, Lo and Lee1998) (Fig. 1). The Daxing'anling–Taihang gravity lineament was traditionally regarded as the boundary between the eastern and western parts of the North China Craton. The regions to the west of the Daxing'anling–Taihang gravity lineament are characterized by thick crust (> 40 km) and lithosphere (> 100 km), large negative Bouguer gravity anomalies and low heat flow. In contrast, the regions to the east of the Daxing'anling–Taihang gravity lineament have much thinner crust (< 35 km) and lithosphere (60–80 km), Bouguer gravity anomalies are weakly negative to positive and heat flow is relatively high (Ma, Reference Ma1989; Hu, He & Wang, Reference Hu, He and Wang2000; Chen et al. Reference Chen, Tao, Zhao and Zheng2008). The North China Craton can also be divided into three parts, according to lithological and geochemical studies, and metamorphic P–T–t paths of the basement rocks, namely the Eastern and Western blocks and the Trans-North China Orogen (also called the Central Zone) (Zhao et al. Reference Zhao, Wilde, Cawood and Sun2001) (Fig. 1). The Eastern Block is composed of early to late Archaean orthogneisses intruded by 2.5 Ga syntectonic granitoids. The Western Block consists of Archaean basement with overlying Archaean to Palaeoproterozoic metasediments (Li et al. Reference Li, Qian, Huang and Liu2000; Zhao et al. Reference Zhao, Cawood, Wilde, Sun and Lu2000). The Trans-North China Orogen is composed of late Archaean amphibolites, granulites and greenstones overlain by bimodal volcanic rocks and terrigenous sedimentary rocks. It was generally considered that the Eastern and Western blocks evolved independently from late Archaean to early Palaeoproterozoic times before colliding into a coherent craton along the Trans-North China Orogen at c. 1.85 Ga (Zhao et al. Reference Zhao, Cawood, Wilde, Sun and Lu2000, Reference Zhao, Wilde, Cawood and Sun2001).

Figure 1. Simplified geological map showing the major tectonic units in eastern China and the location of the Shatuo gabbro; the locations of Palaeozoic kimberlites (open squares) and Mesozoic (open triangles) and Cenozoic (open circles) xenolith-bearing basalts are also marked. 1 – Mengyin kimberlites; 2 – Fuxian kimberlites; 3 – Junan basalts; 4 – Jiaozhou basalts; 5 – Qingdao basalts; 6 – Penglai basalts; 7 – Qixia basalts; 8 – Changle basalts; 9 – Hebi basalts; 10 – Fansi basalts; 11 – Yangyuan basalts; 12 – Hanuoba basalts. Note that the North China Craton is traversed by two large-scale N–S-trending linear zones, namely, the Tan-Lu fault zone (TLFZ) to the east and the Daxinganling–Taihang gravity lineament (DTGL) to the west. Two shaded dashed lines outline the three-fold tectonic division of the North China Craton (after Zhao et al. Reference Zhao, Wilde, Cawood and Sun2001). Inset shows location of the North China Craton relative to other cratonic blocks and intervening fold belts.

The southern Taihang Mountain is a part of the Trans-North China Orogen of the North China Craton. Mesozoic intrusive complexes are widely distributed in this region, with a range of rock types including gabbros, hornblende diorites, syenites and monzonites. The gabbro outcrops are generally small (usually less than 0.5 km2) and occur as knobs or xenoliths hosted by hornblende diorites (Shanxi Bureau of Geology and Mineral Resources, 1982). The samples in this study were collected from the Shatuo gabbro in Huguan county, Shanxi Province. Gabbroic samples are fresh, dark grey and medium- to coarse-grained rocks, composed of plagioclase (40–50%), clinopyroxene (20%), olivine (5–10%), biotite (10%), amphibole (5%) and alkali feldspar (5%). Accessory phases include zircon, sphene, apatite and Fe–Ti oxides. Analyses of zircons extracted from gabbro using a Cameca 1280 secondary ion mass spectrometer yielded a concordant U–Pb age of 128.4 ± 1.2 Ma (Ying et al. unpub. data). Olivines are generally rounded and have varied grain size (from < 1–8 mm). All olivine grains show compositional zonation in backscattered electron images (BSE), with darker Mg-rich cores and lighter Fe-rich rims (Fig. 2).

Figure 2. Backscattered electron images (BSE) of zoned olivine xenocrysts; traverse lines analysed by electron probe micro-analysis are also shown. (a) SG03.1, (b) SG03.2, (c) SG02, (d) 02ST-5.

3. Analytical methods

The mineral chemistry of olivine was obtained with a JEOL Superprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences. The analyses were operated at 15 kv accelerating voltage, 10 nA beam current and 2 μm beam diameter. The counting time varied between 10 and 30 seconds for different elements. Natural mineral standards were used for calibration.

4. Olivine chemistry

As shown in backscattered electron images, the olivine grains exhibit apparent zonation and span a wide compositional range (Table 1). The cores invariably have higher MgO and NiO contents and lower FeO and MnO contents than the rims, and the compositional change towards the rims is gradual (Fig. 3). The Mg nos of cores are generally higher than 91 and the highest can reach to 94 (sample 02ST-9). The rims have variable compositions, with Mg no. ranging from 79 to 89 (Table 1). The cores and the rims show comparable and low CaO concentrations (< 0.1 wt%).

Table 1. Representative analyses of olivine xenocrysts from Mesozoic Shatuo gabbros

Figure 3. Compositional traverses for four representative olivine xenocrysts.

5. Discussion

5.a. Are zoned olivines phenocrysts or xenocrysts?

Both olivine phenocrysts and xenocrysts are common in volcanic rocks (Hirano et al. Reference Hirano, Yamamoto, Kagi and Ishii2004; Zhang et al. Reference Zhang, Ying, Xu and Ma2004b; Zhang, Reference Zhang2005). Phenocrysts are genetically related to the host magma and usually crystallized in the magma chamber prior to magma eruption; in contrast, no genetic relationship exists between the host magma and xenocrysts, and the latter were considered to be entrained in the host magma during the volcanic eruption or magmatic intrusion.

It has been observed that olivines of magmatic origin usually have higher CaO contents (> 0.2%) than those in mantle xenoliths (Gurenko, Hansteen & Schmincke, Reference Gurenko, Hansteen and Schmincke1996; Thompson & Gibson, Reference Thompson and Gibson2000), though some low-Ca olivine phenocrysts were found recently in some subduction-related magmas, and such low-Ca olivines were usually characterized by the presence of melt inclusions (Kamenetsky et al. Reference Kamenetsky, Elburg, Arculus and Thomas2006; Elburg & Kamenetsky, Reference Elburg and Kamenetsky2008). The olivine grains in this study show very low CaO contents regardless of cores or rims (Fig. 4); in addition, melt inclusions were not observed in all olivine grains, so it can be ruled out that these olivines are crystallizing phases of host magma, namely phenocrysts. Moreover, a simple calculation shows that the forsterite content of olivines in equilibrium with the host magma should be no more than 91 (Fig. 5), far lower than that of the olivine cores; therefore, these olivines are disaggregated from mantle xenoliths rather than magmatic phenocrysts. As a result, these olivine xenocrysts can be used to constrain the nature of the lithospheric mantle beneath that region at the time of the gabbroic intrusion.

Figure 4. Olivine CaO content versus Mg no. (% forsterite) plot showing the discrimination between phenocrysts crystallized from magma and xenocrysts from disaggregated mantle (Gurenko, Hansteen & Schmincke, Reference Gurenko, Hansteen and Schmincke1996; Thompson & Gibson, Reference Thompson and Gibson2000). Shaded region: olivines from mantle xenoliths included in Palaeozoic kimberlites, Mesozoic and Cenozoic basalts in eastern China (Zheng, Reference Zheng1999; Fan et al. Reference Fan, Zhang, Baker, Jarvis, Mason and Menzies2000; Zheng et al. Reference Zheng, O'Reilly, Griffin, Lu, Zhang and Pearson2001; Yan, Chen & Xie, Reference Yan, Chen and Xie2003; Ying et al. Reference Ying, Zhang, Kita, Morishita and Shimoda2006). Olivine phenocrysts in Cenozoic basalts in eastern China (E & Zhao, Reference E and Zhao1987; Tang, Zhang & Ying, Reference Tang, Zhang and Ying2004).

Figure 5. Olivine Mg no. versus MgO contents of whole rocks of the Shatuo gabbro. Fe–Mg partition coefficient of 0.33 between olivine and melt (Ulmer, Reference Ulmer1989) was used in calculation. The MgO and FeO contents of the Shatuo gabbro are as follows: SG02: 8.43 wt%, 5.50 wt%; SG03: 8.41 wt%, 5.55 wt%; 02ST-5: 7.20 wt%, 5.10 wt%; 02ST-9: 11.81 wt%, 5.75 wt%; 02ST-11: 9.66 wt%, 5.60 wt%.

5.b. Formation of the zoned olivine xenocrysts

Olivine xenocrysts, which were almost neglected in previous studies, have been widely observed recently in the late Mesozoic and Cenozoic basalts from both eastern and western parts of the North China Craton, and all xenocrysts found invariably show zoned texture (Pei et al. Reference Pei, Xu, Wang, Wang and Lin2004; Tang, Zhang & Ying, Reference Tang, Zhang and Ying2004; Zhang et al. Reference Zhang, Ying, Xu and Ma2004b; Shao et al. Reference Shao, Lu, Zhang and Yang2005; Zhang, Reference Zhang2005). Because other mineral phases in the gabbros are not zoned, the zonation texture induced by late-stage magmatic fluid infiltration or post-emplacement processes can be ruled out, and we attribute the chemical zonation texture in olivines to re-equilibration through diffusional exchange between the olivine and host magma (Maaløe & Hansen, Reference Maaløe and Hansen1982; Nakamura, Reference Nakamura1995; Larsen & Pedersen, Reference Larsen and Pedersen2000; Klugel, Reference Klugel2001). When olivine crystals are entrapped in high-temperature basaltic magma, the chemical disequilibrium between olivine and host magma will cause a reaction and chemical exchange, and the degree of re-equilibration depends on the size of the olivine and the duration of the reaction. The bigger the olivine xenocrysts are, the wider the compositional gradients that exist between the cores and rims. For example, the olivine in sample 02ST-9 is the biggest grain in this study (with diameter up to 8 mm) (Table 1), and it exhibits the largest core–rim variations in Mg no. (94–87). As shown by the line analyses of olivine xenocrysts, the chemical variation between the cores and rims is mainly manifested in the Fe and Mg differences, since Mg and Fe diffusion in olivine is rapid (Mg diffusion coefficient in olivine is around 1.4 × 10−8 cm2 s−1: Brearley & Scarfe, Reference Brearley and Scarfe1986; Redfern et al. Reference Redfern, Henderson, Wood, Harrison and Knight1996; Chakraborty, Reference Chakraborty1997). As a result, preservation of the zoned olivine xenocrysts requires the reaction duration to be short. Since Mn has a comparable diffusion coefficient to Mg–Fe, while the diffusion coefficient of Ca is about 1 order of magnitude slower than that for Mg–Fe diffusion in olivine (Chakraborty, Reference Chakraborty1997; Coogan et al. Reference Coogan, Hain, Stahl and Chakraborty2005), the compositional zonation is also apparent in terms of Mn, while there is no measurable zonation of Ca on such a short timescale. It is also worth noting that the Mg no. of the olivine xenocryst rims should be equal or close to that of calculated olivine equilibrating with gabbro, however, this is not what we observed; the rims are much less Mg-rich than expected for bulk equilibrium (Fig. 5). The likely explanation is that when we calculated the equilibrating olivine using whole rock compositions, the olivine xenocrysts were not excluded, which definitely increased the Mg no. of the whole rocks and the subsequent calculated olivine values. Furthermore, olivine xenocrysts described in basalts elsewhere (Tang, Zhang & Ying, Reference Tang, Zhang and Ying2004; Zhang et al. Reference Zhang, Ying, Xu and Ma2004b; Shao et al. Reference Shao, Lu, Zhang and Yang2005; Zhang, Reference Zhang2005) show much narrower rims than those in the Shatuo gabbro because the reaction durations between olivine xenocrysts and extrusive basalts are considerably shorter than those of intrusive gabbros.

5.c. Implications for the lithospheric mantle evolution of North China Craton

It has been well documented that the North China Craton experienced extensive lithospheric extension during Late Mesozoic to Cenozoic times, which resulted in large-scale removal of lithospheric mantle and significant compositional change of the mantle (Menzies, Fan & Zhang, Reference Menzies, Fan, Zhang, Prichard, Alabaster, Harris and Neary1993; Griffin et al. Reference Griffin, Zhang, O'Reilly, Ryan, Flower, Chung, Lo and Lee1998; Xu, Reference Xu2001; Gao et al. Reference Gao, Rudnick, Carlson, Mcdonough and Liu2002; Zhang et al. Reference Zhang, Sun, Zhou, Fan, Zhai and Yin2002). The North China Craton had a thick, cold and highly refractory lithospheric mantle, at least prior to mid-Ordovician times, as indicated by mantle xenoliths, xenocrysts and diamond inclusions entrapped in Palaeozoic kimberlites in Mengyin and Fuxian counties. The Mg nos of olivine xenocrysts in those kimberlites are high (92–95) and comparable to those of olivines from kimberlite-borne peridotitic xenoliths (Zheng et al. Reference Zheng, O'Reilly, Griffin, Lu, Zhang and Pearson2001). Such highly refractory mantle peridotites have been widely replaced by fertile mantle peridotites since Mesozoic times, and olivines in those peridotites are less magnesian (Mg no. < 91) (Menzies, Fan & Zhang, Reference Menzies, Fan, Zhang, Prichard, Alabaster, Harris and Neary1993; Griffin et al. Reference Griffin, Zhang, O'Reilly, Ryan, Flower, Chung, Lo and Lee1998; Fan et al. Reference Fan, Zhang, Baker, Jarvis, Mason and Menzies2000; Xu, Reference Xu2001). It is worth noting, however, that these observations were mostly confined to the eastern part of the North China Craton; the nature and evolution of lithosphere in the central and western parts of the North China Craton were not well constrained.

As mentioned above, the olivine xenocrysts in the Shatuo gabbro represent disaggregated minerals from the mantle peridotites at the time of gabbroic intrusion, so the olivine xenocrysts can be used to discern the nature of the lithospheric mantle beneath the central North China Craton. The olivine cores from the Shatuo gabbro have a very high Mg no. (> 92), which is similar to those from Palaeozoic kimberlites in eastern China and the olivines from high-Mg peridotites entrained in Mesozoic and Cenozoic basalts (Fig. 6). Re–Os isotopic studies of the refractory mantle xenoliths entrained in Palaeozoic kimberlites have revealed that the lithospheric mantle is Archaean in age (Gao et al. Reference Gao, Rudnick, Carlson, Mcdonough and Liu2002; Wu et al. Reference Wu, Walker, Yang, Yuan and Yang2006; Zhang et al. Reference Zhang, Goldstein, Zhou, Sun, Zheng and Cai2008). The high-Mg peridotite xenoliths in Cenozoic Hebi basalts were interpreted as relics of Archaean lithospheric mantle, confirmed by Re–Os isotopic dating of sulphide inclusions in olivines (Zheng et al. Reference Zheng, O'Reilly, Griffin, Lu, Zhang and Pearson2001, Reference Zheng, Lu, Griffin, Yu, Zhang, Yuan and Wu2006).

Figure 6. Olivine Mg no. versus emplacement ages of mantle xenolith-bearing volcanic rocks in eastern China. Data sources: E & Zhao, Reference E and Zhao1987; Zheng, Reference Zheng1999; Fan et al. Reference Fan, Zhang, Baker, Jarvis, Mason and Menzies2000; Zheng et al. Reference Zheng, O'Reilly, Griffin, Lu, Zhang and Pearson2001; Yan, Chen & Xie, Reference Yan, Chen and Xie2003; Ying et al. Reference Ying, Zhang, Kita, Morishita and Shimoda2006; J. Zhang, unpub. Ph.D. thesis, Chinese Acad. Sciences, 2007.

Although the Mg no. of olivine actually only reflects the degree of partial melting that the mantle experienced rather than age, there is a correlation between the ages of lithospheric mantle and olivine Mg no. based on the studies of sub-continental peridotite xenoliths entrained in kimberlites, lamproites and basalts from different tectonic regimes. The Mg no. of olivine of Archaean lithospheric mantle is usually greater than 92, those of Proterozoic lithospheric mantle between 91 and 92 and those of the Phanerozoic less than 91 (Griffin, O'Reilly & Ryan, Reference Griffin, O'Reilly, Ryan, Fei, Bertka and Mysen1999). Such a relationship can be accounted for by the relatively higher degree of partial melting (25–30%) of the mantle peridotites in the Archaean and Proterozoic due to the higher geothermal gradient at that time. The geothermal gradient in the Phanerozoic was low and the mantle underwent a lower degree of partial melting, which would form less magnesian olivines if no thermal anomaly such as a hot spot or mantle plume was present. Though some highly magnesian olivines in young dunites have been reported, the occurrence of such dunites was only restricted to ophiolites and was small in volume (veins or pods) (Suhr et al. Reference Suhr, Hellebrand, Snow, Seck and Hofmann2003), and it is unlikely that the above dunitic olivines are widely distributed in a cratonic lithospheric mantle. Consequently, it is reasonable to infer that the lithospheric mantle beneath the central North China Craton during late Mesozoic times, as reflected by the olivine xenocrysts, was still refractory and ancient in nature. In addition, all late Mesozoic mafic igneous rocks in the central North China Craton demonstrated the EM I isotopic feature (Chen & Zhai, Reference Chen and Zhai2003; Zhang et al. Reference Zhang, Sun, Zhou and Fan2004a; Wang et al. Reference Wang, Fan, Zhang and Peng2006), a character shared by cratonic ancient mantle worldwide (Menzies, Reference Menzies1990). Recent Sr, Nd and Os isotopic studies of the peridotite xenoliths from the Cenozoic Yangyuan and Fansi alkali basalts in the central North China Craton provided direct evidence that the underlying lithospheric mantle is Archaean or very early Proterozoic (Xu et al. Reference Xu, Blusztajn, Ma, Suzuki, Liu and Hart2008).

In contrast to the widespread existence of Archaean lithospheric mantle beneath the central North China Craton in the late Mesozoic and Cenozoic, the lithospheric mantle underneath the eastern North China Craton is predominantly composed of young and newly accreted material (Fan et al. Reference Fan, Zhang, Baker, Jarvis, Mason and Menzies2000; Gao et al. Reference Gao, Rudnick, Carlson, Mcdonough and Liu2002; Wu et al. Reference Wu, Walker, Ren, Sun and Zhou2003), although in some regions ancient mantle residues are still present as shown by the existence of high-Mg peridotite xenoliths (Zheng et al. Reference Zheng, O'Reilly, Griffin, Lu, Zhang and Pearson2001; Ying et al. Reference Ying, Zhang, Kita, Morishita and Shimoda2006) and Re–Os isotopic data (Gao et al. Reference Gao, Rudnick, Carlson, Mcdonough and Liu2002; Wu et al. Reference Wu, Walker, Yang, Yuan and Yang2006). In addition, geophysical data, such as low heat flow (Hu, He & Wang, Reference Hu, He and Wang2000) and a negative Bouguer gravity anomaly (Ma, Reference Ma1989), have also implied a thick, cold lithosphere in the western North China Craton. The contrasting nature of the lithospheric mantle between the eastern and central North China Craton suggests that they had a different evolutionary history in Phanerozoic times; the extensive lithospheric removal which resulted in replacement of the ancient and refractory lithospheric mantle by a young fertile one was mainly confined to the eastern North China Craton, while the lithospheric mantle beneath the central and western North China Craton was weakly affected.

6. Conclusions

The zoned olivine crystals from the late Mesozoic Shatuo gabbro are xenocrysts that disaggregated from mantle peridotites and can be used to discern the nature of the lithospheric mantle at the time of the gabbro intrusion. The zoned texture was formed by diffusional exchange between olivine and the host melt due to their compositional disequilibria. The compositions of olivine cores represented those of mantle peridotites. The similarities in composition of the olivine cores to those from Palaeozoic kimberlites and high-Mg peridotites imply that the lithospheric mantle beneath the central North China Craton is ancient and refractory, which is in sharp contrast to the prevalent young and newly accreted features of the mantle beneath the eastern North China Craton. The different natures of lithospheric mantle of the eastern and western North China Craton suggest that the extensive lithospheric removal was mainly confined to the eastern North China Craton, while the lithospheric mantle beneath the central North China Craton was weakly affected.

Acknowledgements

We are grateful to Y. G. Ma and Q. Mao for their help with arrangements and assistance in electron microprobe analyses. Dr David Pyle is thanked for his thoughtful suggestions and constructive comments. Mrs J. Holland is also thanked for her helpful editorial efforts. This work was financially supported by the Chinese Academy of Sciences (KZCX2-YW-103).

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Figure 0

Figure 1. Simplified geological map showing the major tectonic units in eastern China and the location of the Shatuo gabbro; the locations of Palaeozoic kimberlites (open squares) and Mesozoic (open triangles) and Cenozoic (open circles) xenolith-bearing basalts are also marked. 1 – Mengyin kimberlites; 2 – Fuxian kimberlites; 3 – Junan basalts; 4 – Jiaozhou basalts; 5 – Qingdao basalts; 6 – Penglai basalts; 7 – Qixia basalts; 8 – Changle basalts; 9 – Hebi basalts; 10 – Fansi basalts; 11 – Yangyuan basalts; 12 – Hanuoba basalts. Note that the North China Craton is traversed by two large-scale N–S-trending linear zones, namely, the Tan-Lu fault zone (TLFZ) to the east and the Daxinganling–Taihang gravity lineament (DTGL) to the west. Two shaded dashed lines outline the three-fold tectonic division of the North China Craton (after Zhao et al. 2001). Inset shows location of the North China Craton relative to other cratonic blocks and intervening fold belts.

Figure 1

Figure 2. Backscattered electron images (BSE) of zoned olivine xenocrysts; traverse lines analysed by electron probe micro-analysis are also shown. (a) SG03.1, (b) SG03.2, (c) SG02, (d) 02ST-5.

Figure 2

Table 1. Representative analyses of olivine xenocrysts from Mesozoic Shatuo gabbros

Figure 3

Figure 3. Compositional traverses for four representative olivine xenocrysts.

Figure 4

Figure 4. Olivine CaO content versus Mg no. (% forsterite) plot showing the discrimination between phenocrysts crystallized from magma and xenocrysts from disaggregated mantle (Gurenko, Hansteen & Schmincke, 1996; Thompson & Gibson, 2000). Shaded region: olivines from mantle xenoliths included in Palaeozoic kimberlites, Mesozoic and Cenozoic basalts in eastern China (Zheng, 1999; Fan et al. 2000; Zheng et al. 2001; Yan, Chen & Xie, 2003; Ying et al. 2006). Olivine phenocrysts in Cenozoic basalts in eastern China (E & Zhao, 1987; Tang, Zhang & Ying, 2004).

Figure 5

Figure 5. Olivine Mg no. versus MgO contents of whole rocks of the Shatuo gabbro. Fe–Mg partition coefficient of 0.33 between olivine and melt (Ulmer, 1989) was used in calculation. The MgO and FeO contents of the Shatuo gabbro are as follows: SG02: 8.43 wt%, 5.50 wt%; SG03: 8.41 wt%, 5.55 wt%; 02ST-5: 7.20 wt%, 5.10 wt%; 02ST-9: 11.81 wt%, 5.75 wt%; 02ST-11: 9.66 wt%, 5.60 wt%.

Figure 6

Figure 6. Olivine Mg no. versus emplacement ages of mantle xenolith-bearing volcanic rocks in eastern China. Data sources: E & Zhao, 1987; Zheng, 1999; Fan et al. 2000; Zheng et al. 2001; Yan, Chen & Xie, 2003; Ying et al. 2006; J. Zhang, unpub. Ph.D. thesis, Chinese Acad. Sciences, 2007.