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Volume 30 Issue 6
Dec.  2019
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Fanxue Meng, Wenliang Xu, Qinglin Xu, Jingliang Guo, Yu Zhang. Decoupling of Lu-Hf and Sm-Nd Isotopic System in Deep-Seated Xenoliths from the Xuzhou-Suzhou Area, China: Differences in Element Mobility during Metamorphism. Journal of Earth Science, 2019, 30(6): 1266-1279. doi: 10.1007/s12583-019-1255-4
Citation: Fanxue Meng, Wenliang Xu, Qinglin Xu, Jingliang Guo, Yu Zhang. Decoupling of Lu-Hf and Sm-Nd Isotopic System in Deep-Seated Xenoliths from the Xuzhou-Suzhou Area, China: Differences in Element Mobility during Metamorphism. Journal of Earth Science, 2019, 30(6): 1266-1279. doi: 10.1007/s12583-019-1255-4

Decoupling of Lu-Hf and Sm-Nd Isotopic System in Deep-Seated Xenoliths from the Xuzhou-Suzhou Area, China: Differences in Element Mobility during Metamorphism

doi: 10.1007/s12583-019-1255-4
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  • This paper presents whole-rock Hf isotopic data for a suite of eclogite and garnet clinopyroxenite xenoliths hosted in the Early Cretaceous dioritic intrusions from the Xuzhou-Suzhou area along the southeastern margin of the Eastern Block of the North China Craton (NCC). Six of the eight studied xenolith samples plot significantly above the terrestrial Hf-Nd isotopic array and have εHf(0) value up to +60. All the samples define a well correlated 147Sm/144Nd-143Nd/144Nd age of 2 081 Ma, which is considered to record the granulite-facies metamorphism. In contrast, the Lu-Hf isotope system faithfully records the protolith information. The mineralogical assemblage, especially garnet and/or zircon (rutile to some extent) mainly controlled the decoupling of Hf-Nd isotope. The metamorphic modification on protolith characteristics and the differences in element mobility during metamorphism may also reinforce the observed decoupling between the Sm-Nd and Lu-Hf isotope systems; i.e., the absence of the correlations in εNd-εHf and also 87Sr/86Sr-143Nd/144Nd diagram. The Lu/Hf isochron age of 2 424 Ma is similar to the zircon age peak of the studied xenoliths and the dominant age of NCC basement, indicating that the igneous protolith has an affinity to the Archean basement of the NCC. Furthermore, the positive εHf(t) values at 2 500 Ma indicate a crustal growth event of 2 500 Ma in the NCC.
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Decoupling of Lu-Hf and Sm-Nd Isotopic System in Deep-Seated Xenoliths from the Xuzhou-Suzhou Area, China: Differences in Element Mobility during Metamorphism

doi: 10.1007/s12583-019-1255-4
    Corresponding author: Fanxue Meng;  Qinglin Xu

Abstract: This paper presents whole-rock Hf isotopic data for a suite of eclogite and garnet clinopyroxenite xenoliths hosted in the Early Cretaceous dioritic intrusions from the Xuzhou-Suzhou area along the southeastern margin of the Eastern Block of the North China Craton (NCC). Six of the eight studied xenolith samples plot significantly above the terrestrial Hf-Nd isotopic array and have εHf(0) value up to +60. All the samples define a well correlated 147Sm/144Nd-143Nd/144Nd age of 2 081 Ma, which is considered to record the granulite-facies metamorphism. In contrast, the Lu-Hf isotope system faithfully records the protolith information. The mineralogical assemblage, especially garnet and/or zircon (rutile to some extent) mainly controlled the decoupling of Hf-Nd isotope. The metamorphic modification on protolith characteristics and the differences in element mobility during metamorphism may also reinforce the observed decoupling between the Sm-Nd and Lu-Hf isotope systems; i.e., the absence of the correlations in εNd-εHf and also 87Sr/86Sr-143Nd/144Nd diagram. The Lu/Hf isochron age of 2 424 Ma is similar to the zircon age peak of the studied xenoliths and the dominant age of NCC basement, indicating that the igneous protolith has an affinity to the Archean basement of the NCC. Furthermore, the positive εHf(t) values at 2 500 Ma indicate a crustal growth event of 2 500 Ma in the NCC.

Fanxue Meng, Wenliang Xu, Qinglin Xu, Jingliang Guo, Yu Zhang. Decoupling of Lu-Hf and Sm-Nd Isotopic System in Deep-Seated Xenoliths from the Xuzhou-Suzhou Area, China: Differences in Element Mobility during Metamorphism. Journal of Earth Science, 2019, 30(6): 1266-1279. doi: 10.1007/s12583-019-1255-4
Citation: Fanxue Meng, Wenliang Xu, Qinglin Xu, Jingliang Guo, Yu Zhang. Decoupling of Lu-Hf and Sm-Nd Isotopic System in Deep-Seated Xenoliths from the Xuzhou-Suzhou Area, China: Differences in Element Mobility during Metamorphism. Journal of Earth Science, 2019, 30(6): 1266-1279. doi: 10.1007/s12583-019-1255-4
  • Deep-seated xenoliths brought to the surface by volcanism, especially the mafic and ultramafic xenoliths, are generally considered to be representative of the upper mantle and the lower crust (Montanini and Harlov, 2006), and can provide direct material evidences to the state and differentiation history of the lower crust and/or the upper mantle. Lower-crustal xenoliths are abundant in Paleozoic kimberlites, Mesozoic volcanic rocks and Cenozoic basalts in the North China Craton (NCC) (Jiang et al., 2013; Jiang and Guo, 2010; Zheng et al., 2009, 2005, 2004a, b, 2003, 2001; Huang et al., 2004; Liu et al., 2004, 2001) (Fig. 1). There are also abundant deep-seated xenoliths hosted in the Early Cretaceous dioritic intrusions from the studied Xuzhou-Suzhou area (Liu et al., 2013; Xu et al., 2009, 2006a, b, 2002). The P-T estimation results show that the xenoliths formed at pressure > 1.5 Ga, corresponding to crustal depth of > 45-50 km (Xu et al., 2006a), thus giving an opportunity to study the composition and evolution of the lower crust beneath this part of the NCC.

    Figure 1.  (a) Regional tectonic map of the North China Craton (NCC) (modified from Guo et al., 2014). Stars denote Early Cretaceous high-Mg adakitic dioritic intrusions in the Xuhuai (Xuzhou-Huaibei) area (Xu et al., 2006a, b). The xenoliths used in this study are from the Liguo and Jiagou intrusions. Solid circles represent the lower-crustal xenolith locations at Fuxian (Zheng et al., 2004a), Mengyin (Zheng et al., 2009), Xinyang (Zheng et al., 2005, 2003, 2001) and Nushan (Huang et al., 2004). (b) Tectonic map of Xuhuai area.

    Previous studies mainly focused on the zircon ages and mineralogy of the xenoliths (Xu et al., 2009, 2006a, b), indicating that the xenoliths, even the individual xenolith may form in different times, by different processes and from different sources. However, the characteristics and the tectonic environment of the protolith, the time and compositional variations of different xenoliths during metamorphism and magmatism are not well-constrained. The key questions on the timing of events of a high-pressure/ultra-high pressure (HP/UHP) metamorphic rock include: the age of the protolith, the ages of peak and retrograde metamorphism, the time of the later thermal disturbance, the episode of fluid-rock interaction (Jahn et al., 2003b). Different chronometers could be used to date different events. Unfortunately, the comprehensive geochronology study of the studied xenoliths is absent. We should also note that Lu-Hf dating may be superior to Sm-Nd isotope in some cases (Scherer et al., 2000; Duchêne et al., 1997), e.g., Lu-Hf chronometer has been applied to date eclogites and garnet pyroxenites for which the Sm-Nd ages were less precise or spurious (Blichert-Toft et al., 1999; Duchêne et al., 1997). However, studies on Hf isotopes are more limited and are practiced only with the development of MC-ICP-MS (Blichert-Toft et al., 1997). Until now only a few results have been published.

    On the other hand, before giving a reasonable interpretation, we should distinguish the original difference in the protolith and the gain-loss process of elements during the later magmatic and metamorphic events. The radiogenic isotopic data, combined with elemental characteristics, could provide an important tool to reveal the processes causing element fractionation (e.g., Hoffmann et al., 2011; Martin et al., 2010; Xia et al., 2008; Polat et al., 2003), and also provide reliable and detailed information on the timing of magmatic and metamorphic events (e.g., Wang S J et al., 2019; Li et al., 2018; Chen et al., 2017; Wang F et al., 2017). Especially, the Lu-Hf isotope, being less mobile in the presence of a fluid phase during metamorphism, might better preserve protolith characteristics (Liu et al., 2015; Turner et al., 2009; Woodhead et al., 2001), providing clues to the source of the xenoliths. Lu-Hf isotopes might be used as a protolith indicator in the high grade metamorphic rock but not the Sm-Nd system.

    In addition, the combination of the Nd-Hf isotopic systems is particularly useful in tracing the evolution of the lithosphere. Lu/Hf and Sm/Nd fractionations by terrestrial magmatic processes are normally strongly correlated (Vervoort et al., 2011, 1999, 1996; Blichert-Toft and Frei, 2001; Salters and Hart, 1991), but it is not always the case for fractionation caused by metamorphic disturbances (Albarède et al., 2000). Decoupling of Lu-Hf and Sm-Nd isotopic systems has frequently been observed in continental and oceanic lithospheric mantle (Yu et al., 2009; Choi et al., 2007; Ishikawa et al., 2007; Simon et al., 2007; Wittig et al., 2007; Ionov et al., 2006, 2005; Aulbach et al., 2004; Bedini et al., 2004; Bizimis et al., 2004; Carlson et al., 2004; Schmidberger et al., 2002), however, it is rare for crustal rocks and only been observed in several lower-crustal mafic granulites (Schmitz et al., 2004; Vervoort et al., 2000; Scherer et al., 1995). The origin of Hf-Nd isotope decoupling was still unclear, limiting the understanding of the process that control the fractionation of element and the geodynamic mechanisms. The decoupling of Nd-Hf isotope is an important key to probe the elemental variation during magmatic and metamorphic events, which might be controlled by mineralogical assemblages. The Hf-Nd isotope decoupling is observed in the deep-seated xenoliths from the Xuzhou-Suzhou area in the southern margin of the NCC, which offer preliminary chance to discuss the mineralogical controls and/or variable element mobility during magmatic and metamorphic events on Hf-Nd isotope evolution during crustal differentiation, and further identify the difference in elemental mobility and the characteristics of protoliths. Considering that the Lu-Hf isotope is hard to be modified and preserves the characters of the protolith and the Sm-Nd isotopes can not survive the later high grade metamorphism, decoupling of the Lu-Hf and Sm-Nd isotopes might be common in the high grade metamorphic rocks. Understanding the elemental mobility and its controlling factors has important geological significance, especially for reconstructing the evolution of metamorphic rocks and recovering the nature of protolith.

  • The NCC consists of Paleoarchean to Paleoproterozoic basement overlain by Mesoproterozoic to Cenozoic covers. Based on age, lithological assemblage, tectonic evolution and P-T-t paths, the NCC can be divided into the Western Block, the Eastern Block, and the intervening Trans-North China Orogen (central orogenic belt) (e.g., Zhao et al., 2005, 2001) (Fig. 1).

    The Xuzhou-Suzhou area, is located on the southeastern margin of the NCC, ~100 km west of the Tanlu fault zone and ~300 km north of the Dabie Orogen (Fig. 1). Several small Mesozoic intrusions, namely the Liguo, Banjing and Jiagou intrusions emplaced into the Late Permian strata (Fig. 1). Zircon U-Pb ages for the intrusions are ~130 Ma (Yang et al., 2008; Xu et al., 2004).

    There are abundant xenoliths in these intrusions (Liu et al., 2013, 2009; Xu et al., 2009, 2006a, b, 2002; Wang et al., 2005), which are metamorphosed and dominated by mafic rocks with minor felsic gneisses (Liu et al., 2013, 2009; Xu et al., 2002). Xu et al.(2009, 2006a, 2002) named the mafic xenoliths to be eclogitic xenoliths, and subdivided them into two types based on modal mineralogy and mineral chemistry (eclogite and garnet clinopyroxenite). All the samples in this study were described previously by Xu et al. (2009), thus, we adopt their nomenclature. The eclogite and garnet clinopyroxenite xenoliths are rounded or irregular in shape and are small in size (see Fig. 2 in Xu et al., 2009). The difference between them is that the clinopyroxene in the eclogites is omphacite, while it is augite in the garnet clinopyroxenites (Xu et al., 2006a, 2002). The zircon U-Pb ages of the eclogite and garnet-clinopyroxenite xenoliths show significant peak in Neoarchean (2 400-2 500 Ma) and Paleoproterozoic (1 700-1 800 Ma) (Xu et al., 2006a), similar to those of inherited zircons from the host rocks and gneiss xenoliths in the region and coincide with the important periods of crustal growth of the NCC (Xu et al., 2006b; Gao et al., 2004). The Triassic ages were also obtained (Xu et al., 2006a, 2002) and consistent with the peak eclogite-facies metamorphism in the Dabie-Sulu ultrahigh-pressure metamorphic belt (Guo et al., 2014; Ayers et al., 2002).

    Figure 2.  Zr/TiO2 vs. Nb/Y diagram (after Winchester and Floyd, 1976) showing the compositions of the studied xenoliths hosted in the Mesozoic dioritic porphyries with adakitic characteristics. Data are from Xu et al. (2009).

    The petrographic features, major and trace element compositions and some Sr-Nd isotope ratios of these samples (except for JG4-1) are documented in Xu et al.(2009, 2006a, b, 2002), hence, only the salient features are summarized here. All the xenoliths show basaltic compositions (SiO2=44.59 wt.%-50.82 wt.%) and mostly plot in the field of subalkaline basalt except for sample L-4 (Fig. 2). They show near-flat to slightly enriched LREE patterns with no Eu anomaly (Fig. 3a), except for sample JG4-1 has significant depletion in LREEs. These samples have large variations in K, Rb, Ba, Sr and depletion in HFSEs, such as Zr, Hf and Ti (Fig. 3b). It should be noted that the xenoliths have large variations in Nb/Ta ratios, with Ta ranging from negative to positive anomaly (Fig. 3b).

    Figure 3.  (a) Chondrite-normalized REE patterns and (b) primitive-mantle normalized trace-element patterns for the xenoliths. The trace elemental data are from Xu et al. (2009). Chondrite values are from Taylor and McLennan (1985), and primitive mantle values are from Sun and McDonough (1989).

    Here we mainly focus on the Hf isotopes (combined with the Sr-Nd isotopes and other geochemical characteristics) of eight xenoliths in Early Cretaceous Liguo and Jiagou dioritic intrusions (Fig. 1) including three eclogites (samples L-4, B1-10 and 603-2-2) and five garnet clinopyroxenites.

  • The altered surfaces of samples were removed and the fresh rocks were crushed to < 200 mesh for isotopic analysis. The Sr and Nd isotopic compositions were done in static mode at Northwest University, Xi'an using MC-ICP-MS (Nu Plasma HR, Nu Instruments, Wrexham, UK). The rock power was digested in sealed high-pressure Teflon bomb with a mixture of concentrated HNO3, HF and HClO4. The standard chromatographic columns of AG50W-X8 and HDEHP resins were used to separate Sr and Nd (and other REEs). Detailed analytical procedures were described by Meng F X et al. (2018) and Gao et al. (2004). The measured 143Nd/144Nd and 87Sr/86Sr ratios were normalized to 146Nd/144Nd= 0.721 9 and 86Sr/88Sr=0.119 4, respectively. Repeated analyses of international standards were required to monitor the external reproducibility of the isotopic measurement. The international Nd isotopic standard La Jolla gives an average 143Nd/144Nd of 0.511 833±0.000 008 (2σ, n=75), and the Sr standard NBS 987 gives 87Sr/86Sr=0.710 233±0.000 035 (2σ, n=28). The BCR-2 has 87Sr/86Sr=0.704 995±0.000 039 (2σ, n=3), 143Nd/144Nd=0.512 611± 0.000 013 (2σ, n=6) during measurement. All of them are identical to reference values (NBS 987: 87Sr/86Sr=0.710 248; La Jolla: 143Nd/144Nd=0.511 844; BCR-2: 87Sr/86Sr=0.704 958, 143Nd/144Nd= 0.512 635±0.000 029) within analytical error.

    The Hf isotopic analysis was carried out in static mode on the Nu Plasma MC-ICP-MS at Northwest University, Xi'an. About 75 mg of rock powder was decomposed in sealed bombs using a mixture acid of concentrated HNO3, HF and HClO4, and four copies of each sample were decomposed to improve the precision of the analysis. LN-Spec resins were used for Hf separation and purification following the methods of Yuan et al. (2007). Then, the four copies of collected solution for each sample were mixed and concentrated. The measured Hf isotopic ratios were calibrated for isobaric interference by monitoring 175Lu; the 176Lu interference was deducted by using a value of 176Lu/175Lu=37.699 69 (Rosman and Taylor, 1998). During the procedure of our analysis, the average 176Hf/177Hf ratio of international Hf standard JMC 475 is 0.282 189±0.000 026 (2σ, n=6). The measured data of samples are adjusted to correspond to the accepted value of 0.282 160 for Hf standard JMC-475 (Vervoort and Blichert-Toft, 1999). The measured in-house standard Alfa Hf gives an average 176Hf/177Hf of 0.282 210±0.000 021 (2σ, n=5), and BHVO-2 gives 176Hf/177Hf=0.283 090±0.000 036. The average 176Hf/177Hf ratios of BCR-2 and AGV-2 are 0.282 872± 0.000 023 and 0.282 983±0.000 014, respectively. The measured values of the standards are identical to their reference values within analytical errors.

  • The whole-rock Hf isotopic compositions and our new analyzed Sr-Nd isotopic data are presented in Table 1 together with previously published Sr-Nd isotope results from Xu et al. (2009). JG4-1 was duplicated for Hf isotope analysis and the results show no difference, confirming the high precision of the analytical data (Table 1).

    Lithology 147Sm/144Nd 143Nd/144Nd 2σ εNd(0) εNd(2 100 Ma) 87Rb/86Sr 87Sr/86Sr 2σ 176Lu/177Hf 176Hf/177Hf 2σ εHf(0) εHf(2 500 Ma)
    JG2-10 Garnet clinopyroxenite 0.198 4 0.512 737 ±14 2.0 3.3 0.416 0.712 275 ±15 0.048 0.283 532 ±22 26.9 1.6
    JG2-11 Garnet clinopyroxenite 0.167 5 0.512 643 ±12 0.1 9.0 0.661 0.716 009 ±20 0.061 0.284 400 ±29 57.6 9.1
    JG4-1 Garnet clinopyroxenite 0.25 0.512 282 ±8 -6.9 -21.5 0.018 0.710 702 ±11 0.057 0.282 606 ±8 -5.9 -47.8
    JG4-1 dup 0.057 0.282 605 ±6 -5.9 -47.9
    J3-52 Garnet clinopyroxenite 0.171 1 0.512 295 ±14 -6.7 -3.6 0.120 0.706 016 ±15 0.029 0.282 998 ±25 8.0 14.6
    L-4 Eclogite 0.137 2 0.511 895 ±5 -14.5 -0.1 0.124 0.704 469 ±13 0.012 0.281 977 ±33 -28.1 7.9
    B1-10 Eclogite 0.153 0.512 115 ±8 -10.2 1.8 0.244 0.707 782 ±34 0.038 0.283 115 ±32 12.1 2.5
    603-2-1 Garnet clinopyroxenite 0.180 1 0.512 713 ±8 1.5 3.6 0.315 0.707 668 ±16 0.046 0.283 620 ±15 30.0 8.1
    603-2-2 Eclogite 0.180 4 0.512 328 ±9 -6.0 -3.3 0.169 0.707 722 ±19 0.031 0.283 015 ±16 8.6 11.0
    dup. duplicate analysis. The Nd model age based on depleted mantle (DM) assumes a linear evolution of isotopic composition from εNd(t)=0 at 4.56 Ga to approximately +10 at the present time. Model ages (TDM) were calculated using equations: TDM=1/λ×ln{1+[(143Nd/144Nd)sample–0.513 15]/[(147Sm/144Nd)sample–0.213 7]}, where the decay constant λ of 147Sm used in model age caculation is 0.006 54 Ga-1. εNd(t) was calculated using equation: εNd(t)=10 000× [(143Nd/144Nd)sample–(143Nd/144Nd)chondrite)]/(143Nd/144Nd)chondrite, where suberscript sample and chrondrite denote values of sample and chondrite at the time of sample formation. The Hf model ages (TDM) were calculated using equations: TDM=1/λ×ln{1+[(176Hf/177Hf)sample–0.283 25]/[(176Lu/177Hf)sample–0.038 4]}, where the decay constant λ of 176Lu used in model age caculation is 0.018 67 Ga-1. εHf(t) was calculated using equation: εHf(t)=10 000×[(176Hf/177Hf)sample– (176Hf/177Hf)chondrite)]/(176Hf/177Hf)chondrite, where suberscript sample and chrondrite denote values of sample and chondrite at the time of sample formation. Sr-Nd isotopes (except JG4-1 and B1-10) are from Xu et al. (2009).

    Table 1.  Sr-Nd-Hf isotopic compositions of xenoliths from Xuhuai

    The xenoliths have variable Sr-Nd isotopic compositions that lack apparent correlations with each other, the present-day 87Sr/86Sr ratios and the εNd(0) values range from 0.704 469 to 0.716 009 and from -14.5 to 2.0, respectively (Fig. 4). All the xenoliths plot in the field that was constrained by DMM, EMI and GLOSS components (Fig. 4). The initial Sr and Nd isotopic compositions at 220, 2 100 and 2 500 Ma were also shown in Fig. 4 for comparison. Considering that Rb and Sr are fluid mobile elements, the calculated initial Sr isotopic composition might be meaningless. JG2-10 and JG2-11 have the highest Sr isotopic composition and 87Rb/86Sr ratios plotting towards the GLOSS (Figs. 4 and 6), however, their Nd isotopic composition is similar to the garnet clinopyroxenites (603-2-1) with εNd(0) > 0. The significantly negative Sr anomaly of JG2-10 and JG2-11 (Fig. 3) might account for their increased 87Rb/86Sr ratios and evolve to higher 87Sr/86Sr ratios. Furthermore, the correlation between 87Sr/86Sr and 87Rb/86Sr ratios gives an younger "isochron" age than Sm-Nd and Lu-Hf systems, also giving evidence to the disturbance in Rb-Sr system (see below) (Fig. 6). It should be noted that JG4-1 displays the lowest 87Rb/86Sr ratios, the highest 147Sm/144Nd ratios with intermediate Sr-Nd isotopic composition, and the enriched Nd isotope is similar to that of the lower crust (Fig. 6), combining with the positively inclined middle to heavy REE patterns and strong depletion in LREE, implying a recent melt extraction event.

    Figure 4.  εNd(t) vs. (86Sr/87Sr)i diagram showing the xenoliths from Xuhuai area. The Sr-Nd isotopic compositions at 2 500, 2 100 and 220 Ma have also been added for comparison. Data for DMM, GLOSS, and LCC are from Workman and Hart (2005), Plank and Langmuir (1998), and Jahn et al. (1999), respectively. The trends for EMI, EMII and mantle array are from Zindler and Hart (1986). The fields of lithospheric mantle are represented by peridotite xenoliths from Cenozoic basalts in Shanwang (Chu et al., 2009) and Beiyan (Xiao et al., 2010). The Yangtze Craton upper/middle and lower crustal data are from Gao et al. (1999), Ma et al. (2000), Chen and Jahn (1998). The field of the lower crust for the NCC is from Jiang et al. (2013), and the Nushan lower-crustal xenoliths are from Huang et al. (2004). As a whole, there is no correlation between Sr and Nd isotope. However, they all plot in the field constrained by DMM, EMI and GLOSS components (dashed line field). JG2-11 has the highest 87Sr/86Sr ratios, reflecting metamorphism mobilization of Rb-Sr isotope system and/or involvement of subducted sediment.

    Figure 6.  Correlation between isotopes and their respective parent/daughter ratios (i.e., P/D). JG4-1 display the lowest 87Rb/86Sr ratios, highest 176Lu/177Hf and 147Sm/144Nd ratios with intermediate Sr-Nd-Hf isotopic composition, leading to the deviation of JG4-1 from the trend. The different trace element and isotopic composition implying a different origin was excluded in calculating "isochron" age. The weak correlations between Sr isotope and 87Rb/86Sr and the co-linear distributed in the 147Sm/144Nd-143Nd/144Nd plot, irrespective of constituent mineral assemblages and chemical compositions, give the ages of 1 013 and 2 081 Ma, respectively. The Sm-Nd age of 2 081 Ma is considered to record the granulite-facies metamorphism. The xenoliths give a whole-rock Lu-Hf isochron age of 2 424 Ma, indicating the dominance of protolith characteristics on Lu-Hf isotope system.

    The present-day εHf values for the xenoliths examined in this study vary widely from -28.1 to 57.6. In the εHf vs. εNd diagram, the xenoliths plot above the terrestrial Hf-Nd isotopic array (with the exception of L-4 and JG4-1) (Fig. 5), and they are characterized by the deviations from the igneous trends in the εNd-εHf, and 147Sm/144Nd-176Lu/177Hf plots (Figs. 5 and 7). They still plot above the terrestrial εHf-εNd array when back-calculated to the emplacement time of the intrusions (i.e., 130 Ma). When further back-calculated to the formation age of protolith at 2 500 Ma (Guo and Li, 2009; Liu et al., 2009; Xu et al., 2009, 2006a, 2002), the initial Hf isotopic composition scattered from 1.6 to 14.6 if JG4-1 was excluded. Again, JG4-1 displays the highest 176Lu/177Hf ratios (0.057) with enriched 176Hf/177Hf composition (0.282 606±0.000 007, an average from duplicates, Table 1) (Fig. 5), which is consistent with the Sr-Nd isotope.

    Figure 5.  εHf vs. εNd diagram for the xenoliths. The field for the Hf-Nd crustal array is from Vervoort et al. (1999). Open squares are data from the literature on lower-crustal granulites worldwide with coupling Hf-Nd isotopes (Vervoort et al., 2000), i.e., plot in the terrestrial array. Inverted open triangles (dashed line region) are the globally available lower-crustal data showing displacement from the mantle-crustal array (decoupled) (Schmitz et al., 2004; Vervoort et al., 2000). The red triangles represent present-day compositions of the xenoliths we studied. Field of pelagic sediments is from Vervoort et al. (1999). The Hf-Nd isotopic composition of host intrusions (our unpublished data) had also been labeled (yellow circle), which plot within the terrestrial Hf-Nd isotopic array. The Nd-Hf isotopic compositions at 2 500, 2 100 and 220 Ma have also been added for comparison. This means that the xenoliths are not the residues of the partial melting process that generated the dioritic intrusions.

    Figure 7.  147Sm/144Nd versus 176Lu/177Hf plot showing that three xenoliths deviate from the correlation trend (dashed line region) defined by rocks derived from continental magmatism and arc magmatism. The field is modified after Liu et al. (2015) and references therein. The high Lu/Hf ratios were possibly increased during metamorphism.

  • A significant characteristic of the studied xenoliths is that most of the analyzed samples plot above the terrestrial Hf-Nd isotopic array with εHf value up to +60 (Fig. 5). Furthermore, the xenoliths are deviated from the igneous trends in 147Sm/144Nd vs. 176Lu/177Hf plots and are scattered in 87Sr/86Sr vs. εNd diagram (Figs. 4 and 7). It suggests that the decoupling of the Lu-Hf, Sm-Nd, Rb-Sr isotopic system occurred in the studied xenoliths.

    Before giving a reasonable interpretation on the origin of Hf-Nd isotopic decoupling, we should remember that the xenoliths were deep-seated (depth > 40 km on basis of the P-T estimation by Xu et al. (2009) and underwent multi-stage melt extraction or metamorphic events deduced from the zircon age data of the xenoliths showing significant peaks at 2 400-2 500, 1 700-1 800, ~750, ~213, and 129 Ma (Xu et al., 2009, 2006a, b). The addition and/or runout of melt/fluid phase during high pressure and temperature metamorphism and the melt extraction events might cause redistribution of different elements and reset the isotopic system. However, the xenoliths might have different sources and undergone different events at various time, and the age of different events for individual xenolith is unclear, it is difficult to recover the evolutionary history and constrain the exactly elemental variations at different stages. To be simple, we assume that the elemental variations are caused by a single-stage event, and if it is the case, different isotopic systems might record this event and give similar ages. Conversely, different isotopic systems might give different information on events leading to elemental variations.

    Firstly, the correlations between isotopes and their parent/ daughter (P/D) elemental ratios of xenoliths (Fig. 6) suggest that the decoupling of the Lu-Hf, Sm-Nd, Rb-Sr isotopic system can not be the result of recent events at ~130 or 220 Ma due to the following reasons. The host intrusions have been suggested to be formed by partial melting of the Archean metabasalts and these metabasalts were the protoliths of the Xuhuai (Xuzhou-Huaibei) eclogite and garnet clinopyroxenite xenoliths on the basis of the complementary major-trace element compositions of the host intrusions and the xenoliths (Xu et al., 2006a). The host intrusions and the xenoliths should have the same or similar Hf-Nd isotopic compositions at the time of emplacement of the intrusions (i.e., ca. 130 Ma) (Xu et al., 2004). However, the xenoliths (except JG4-1) still plot above the εHf-εNd array when back-calculated to 130 Ma, while the host intrusions plot within the Hf-Nd isotopic array (Fig. 5). Meanwhile, the quantitative modeling shows that extremely high degrees of melting (> 60%) were needed if the melting occurred 100 Ma ago (Meng, 2011), which seems to be unrealistic. And at least 500 Ma were needed to evolve to the observed Hf-Nd isotopic characteristics that plot above the Hf-Nd isotopic array. Similarly, the xenoliths also plot above the εHf-εNd array when back-calculated to 220 Ma. The eclogitization is a medium-high temperature (> 500 ℃), ultra-high pressure metamorphism, considering that the closure temperature of Lu-Hf and Sm-Nd isotopic system is estimated to be higher than 700 ℃ (Scherer et al., 2000), the 220 Ma event might have insignificant effect on modification of Lu-Hf and Sm-Nd isotopic system of the xenoliths. Thus, the disturbance in the elemental and isotope systems must be ancient.

    However, we should also note that sample JG4-1 has the lowest 87Rb/86Sr ratios, highest 176Lu/177Hf and 147Sm/144Nd ratios, that are unsupported by its intermediate Sr-Nd-Hf isotopic composition as discussed above, leading to the deviation of JG4-1 from the trend in Sr, Nd, Hf isotopes vs. their respective parent/ daughter (P/D) ratios diagram (Fig. 6). This sample also displays the positively inclined middle to heavy REE patterns and strong depletion in LREE (Fig. 3), and the REE pattern is consistent with calculated melt residues of the lower continental crust (Kemp and Hawkesworth, 2004) where the degree of melting is 30%. Its Sr-Nd-Hf isotopic composition is similar to those of the host intrusions at 130 Ma. These characteristics are consistent with a recent melt extraction event, and there is insufficient time to produce enough radiogenic ingrowth, thereby giving rise to trace element-isotope decoupling. It is also evidenced by the SHRIMP U-Pb dating on four zircons from JG4-1 that gives the 206Pb/238U ages of 161.3-192.8 Ma.

    Further analysis of the decoupling of the Lu-Hf and Sm-Nd isotope systems requires knowledge of the compositional and mineralogical controls on REE and HFSE partitioning. The Hf-Nd isotope decoupling is mainly attributed to the presence of garnet (DLu≫1 and DHf < 1) (Schmitz et al., 2004; Johnson et al., 1996; Vervoort and Patchett, 1996). As a result of the different geochemical behaviors between a REE and a high field strength element, more fractionation is generally expected in the Lu/Hf ratios in garnet than in the Sm/Nd ratios. Garnet strongly fractionates Lu over Hf, even more than it fractionates Sm over Nd (Cheng, 2019), which evolves isotopically to high 176Hf/177Hf relative to 143Nd/144Nd (e.g., Schmitz et al., 2004; Schmidberger et al., 2002; Vervoort and Patchett, 1996; Scherer et al., 1995; Salters and Hart, 1991). Garnets have the highest known 176Lu/177Hf ratios of the major rock-forming minerals (Scherer et al., 2000). The xenoliths studied here mostly have superchondritic Lu/Hf ratios, similar to the garnet pyroxenites from Beni Bousera, Morocco (Blichert-Toft et al., 1999). These rocks might represent either cumulates or residues from melting events in the presence of garnet. Given that the voluminous granitoids derived from the melting of the lower crust, it is not surprising that the lower crust represents residual material. This view is supported by the presence of widespread trodjemite-tonalite-granodiorite (TTG) magmatism associated with the episodic crustal growth in the southern NCC during the Neoarchean to Early Paleoproterozoic (Huang et al., 2013, 2012, 2010), which were generally derived from partial melting of crustal materials at great depth where garnet is stable as a residual phase of the source. On the other hand, the Hf-rich, low Lu/Hf minerals are potentially important for HFSE-REE decoupling (Schmitz et al., 2004). Zircons in particular, have high Hf content due to that Hf is geochemically identical to Zr, which can strongly influence the Lu-Hf system of rock matrix. As the granitoids derived from melting of continental crust contain abundant zircons, it might further argument the original Lu/Hf ratio in residues. It is consistent with the negative Zr and Hf anomalies in the studied xenoliths. The residual zircons during formation of basaltic protolith might also give a negative Zr-Hf anomalies and high Lu/Hf ratios. The residual zircons were used to explain the negative Zr and Hf anomalies in basaltic arc magmas (Tollstrup and Gill, 2005; Wade et al., 2005; Rubatto and Hermann, 2003). Thus, the mineralogical assemblages might contribute to the decoupling of Hf-Nd isotope. However, we should remember that, the elemental variations and the associated variations in isotopic system cannot occur if the whole-rock is a closed system, i.e., the elemental variations controlled by garnet and/or other minerals need the involvement of melt/fluid phase during melt extraction and/or metasomatism or metamorphic events. Meanwhile, the different isotopic systems would record the events.

    Seen from Fig. 6, all of the xenoliths correlate well in Sr, Nd, Hf isotope vs. their parent/daughter (P/D) ratios diagram (Figs. 6a, 6b, 6c). However, different isotopic systems give different "isochron" age. The samples give a whole-rock Lu-Hf isochron age of 2 424 Ma (Fig. 6c), which is consistent with the age peak of zircons at 2 400-2 500 Ma (Xu et al., 2009, 2006a, b). Eight analyses of igneous zircons from sample 07JG34 (a mafic garnet amphibolite collected from Jiagou intrusions near Suzhou as described by Liu et al.(2013, 2009) also recorded 207Pb/206Pb concordant ages ranging from 2 389 to 2 567 Ma with a weighted mean age of 2 480±49 Ma (MSWD=0.59). And the 2 480±49 Ma age was interpreted to record the formation age of the igneous precursor (Liu et al., 2013). The consistency between these ages leads us to believe that the Lu-Hf "isochron" age of 2 424 Ma records the formation age of igneous protolith. It is not surprising considering that Lu and Hf are immobile or significantly less mobile during metamorphism as suggested previously (Liu et al., 2015; Turner et al., 2009; Woodhead et al., 2001).

    Meanwhile, all the xenoliths are co-linear distributed in the 147Sm/144Nd-143Nd/144Nd plot, irrespective that these samples differ in constituent mineral assemblages and chemical compositions. And there is a relatively weak correlation between Sr isotope and 87Rb/86Sr. The correlations between Sr, Nd isotope and their respective parent/daughter (P/D) ratios give the ages of ~1 300 and 2 081 Ma, respectively. The younger ages shown by Sm-Nd and Rb-Sr radioactive decay systems are non-conformable with the Lu-Hf system, suggesting that the "ages" might be reset by later thermo-tectonic events. Rb and Sr are fluid-mobilized elements during progressive dehydration of prograde metamorphism and/or rehydration of retrograde metamorphism (Bea et al., 2017; Rumble et al., 2005). Previous studies on eclogites from Western Norway (Griffin and Brueckner, 1985) and from the Dabie and Sulu terranes (Jahn, 1998; Ames et al., 1996) indicated that many eclogites have highly radiogenic 87Sr/86Sr ratios that are "unsupported" by their Rb/Sr ratios. The large variations in LILEs (K, Rb, Sr, and Ba) of xenoliths indicate the mobilization of these elements. Numerous petrological studies have established that the protoliths of eclogites underwent progressive dehydration during prograde metamorphism, and many of them were later rehydrated to some extent during retrograde metamorphism when the rocks were exhumed (e.g., Hirajima and Nakamura, 2003; Zheng et al., 1999; Liou et al., 1998). LILEs (K, Rb, Cs, Sr, Ba), U and to some extent, La and Ce, could be mobilized along with volatile species such as H2O and CO2 (Jahn et al., 2003b). In fact, the tectono-thermal events at ~1.3 Ga have not been identified in the southeastern margin of the North China Craton, nor found in previous studies on zircons collected in this region (Liu et al., 2013; Xu et al., 2009, 2006a). The positive correlation in 87Rb/86Sr-87Sr/86Sr plot may simply reflect a two-component mixing line considering the existence of an 87Sr/86Sr-1/Sr correlation (Fig. 8). The regression line could be geologically meaningless.

    Figure 8.  87Sr/86Sr versus 1/Sr plot showing the correlations of the samples. The existence of a 87Sr/86Sr-1/Sr correlation may simply reflect a two-component mixing line.

    The Sm-Nd age of 2 081 Ma is similar to the weighted mean age of 2 167±58 Ma (MSWD=0.14) of four metamorphic rims of zircons from a mafic garnet amphibole collected from Jiagou intrusions (07JG34), which is consistent with the upper intercept age of 2 165±68 Ma defined by 6 spot analyses of rim domains within error. And the age of 2 167±58 Ma was proposed to record the time of granulite-facies metamorphism (Liu et al., 2013). In fact, the 1.8-2.1 Ga granulite-facies metamorphic events have also been recorded in lower-crustal xenoliths in the Cenozoic Nushan basalt (Huang et al., 2004). And the 1.8-1.9 Ga metamorphic event is widespread in the NCC (Zhai and Santosh, 2011; Zhai et al., 2011, 2010; Zhai, 2009), particularly in the southern part of the NCC (Huang et al., 2013, 2004). These lower crustal samples were metamorphosed to granulite facies in the Early Proterozoic. The HREE Lu and the HFSE Hf are immobile or significantly less mobile than Sm and Nd in the presence of a fluid phase during metamorphism (Turner et al., 2009; Woodhead et al., 2001). The studies on eclogite from Sulu ultra-high pressure metamorphic terrane also proposed that peak metamorphism had a strong control on the Sm-Nd isotope system (Liu et al., 2015). Meanwhile, the closure temperatures of Lu-Hf are proposed to be higher than or equal to that of Sm-Nd, leading to the older Lu-Hf ages, or within error of the Sm-Nd ages, consistent with our data. It is also supported by garnet clinopyroxenite from Bearpaw Mountains giving a younger Sm-Nd age than the Lu-Hf age (Scherer et al., 2000). During the high-temperature granulite-facies metamorphism, the Sm-Nd age might be reset during high-grade metamorphism, whereas the Lu-Hf system records the protolith characteristics faithfully. The differences in element mobility (e.g., Kessel et al., 2005; John et al., 2004; Jahn et al., 2003a, b; Polat et al., 2003) during high-grade metamorphism may reinforce the decoupling between the Rb-Sr, Sm-Nd, and Lu-Hf isotope systems; i.e., the absence of the correlations in εNd-εHf and 87Sr/86Sr-143Nd/144Nd diagram (Figs. 4 and 5). This kind of decoupling in Nd-Hf isotopes has been also reported from the Archean metabasalts and metagabbros from southern West Greenland (Hoffmann et al., 2011). In summary, the mineralogical assemblages, especially the garnet and zircon, and their modes and differences in elemental behavior during partial melting or metamorphic events impart the different P/D ratios and the resulting decoupling of Hf-Nd isotope.

  • As discussed above, the elemental behavior is strongly various during high-grade metamorphism. Large ion lithophile elements (e.g., K, Rb, Cs, Sr, Ba) were more easily to be mobilized, and the LREEs had also been altered (Fig. 3) suggested by the REE pattern (Fig. 3) and the younger "isochron" age of Sm-Nd isotope system. Thus, the relatively immobile high field strength elements such as Zr, Hf, Nb, Ta, Y, Ti, and heavy rare earth elements (HREEs) are used to reflect the geochemical characteristics of the protoliths. The xenoliths studied here are characterized by the absence of aluminous phases such as kyanite and sillimanite, suggesting that the protoliths were of igneous rather than sedimentary origin (Dessai et al., 2004). They all have a basaltic composition with SiO2=44.59 wt.%-50.82 wt.% (Xu et al., 2009). The xenoliths from the Xuzhou-Suzhou area are characterized by depletion in Zr, Hf, Nb, Ta, Ti (Fig. 3). Compared with rocks from mantle derived magmas in all geological environments, Nb-Ta-Ti depletion is characteristic of volcanic arc rocks or the "arc signature" shared by continental crust. Previous studies suggested that the protolith should be an arc origin (Xu et al., 2009). However, as discussed above, the crystallization of zircon during melting of continental crust to produce granitoids might also produce the negative Zr and Hf anomalies. Meanwhile, presence of accessory minerals such as rutile as a residual phase also causes depletion in Hf due to that rutile is the titanian oxide with DLu/DHf≪1, and DHf≫1 (Foley et al., 2000; Green and Pearson, 1986). Then, the presence of rutile as a residual phase might also play a role in the decoupling of Hf-Nd isotope. In addition, it can also interpret the Nb-Ta-Ti depletion of basaltic rock as observed here.

    On the other hand, the Lu-Hf isotope system of the xenoliths provided in this paper was not affected by the high-grade metamorphism and gave a Lu-Hf isochron age of 2 424 Ma. It is consistent with the compiled zircon ages of xenoliths in the study area with significant peaks at 2 400-2 500, 1 700-1 800, and 700-800 Ma (Liu et al., 2013; Xu et al., 2009, 2006a), and also consistent with the dominant age of the NCC Archean basement (e.g., Zhang, 2012; Liu et al., 2011; Zhai and Santosh, 2011; Hou et al., 2006; Guo et al., 2005; Kröner et al., 2005; Zhai et al., 2005, 2000; Gao et al., 2004; Kusky and Li, 2003; Zhai and Liu, 2003; Zhao et al., 2000). The NCC is one of the oldest cratons in the world and zircon ages of the Precambrian basements of the North China Craton mainly range from 2.8 to 2.5 Ga with a peak at 2.5 Ga (Meng et al., 2018; Gao et al., 2004; Zhao et al., 2001), except for minor volume of > 3.6 Ga components (Wu et al., 2008; Zheng et al., 2004a; Liu et al., 1992). Previous studies had proposed that majority of the eclogite and garnet-clinopyroxenite xenoliths should be attributed to the NCC basement owing to their ages of 2 400-2 500 Ma (Zheng et al., 2006; Zheng, 2003). Thus, the protoliths were formed in Archean at 2.4-2.5 Ga, and have an affinity to the NCC basement.

    Furthermore, the initial Sr-Nd isotopic compositions can not be used to deduce the characteristic of protolith due to the isotopic system is reset during high-grade metamorphism or other events. The large variations in Sr isotope and extremely lower 87Sr/86Sr ratios (Fig. 4) are the results of "erroneous" Rb/Sr ratios that had been altered. In contrast, the Hf isotope could provide the nature of protolith. The εHf(2 500 Ma) of the xenoliths (except for JG4-1) range from +1.6 to +14.6, overlapping largely with the initial εHf values of the ~2 500 Ma magmatic zircon grains (+1.0- +12.1) in sample 07JG34 (Liu et al., 2013). The positive εHf(2 500 Ma) values of the samples (Table 1) indicate that the protolith might be juvenile mantle-derived component. It is evidenced by the initial of the regression line, corresponding to εHf(2 500 Ma)=10.7. The addition of juvenile mantle-derived compositions at 2 500 Ma is consistent with the widespread Late Archean tectonothermal event in the NCC (Gao et al., 2004). In summary, the protolith of the xenolith is the basaltic rock derived from juvenile mantle-derived melt of Archean basement of the NCC.

  • As described above, the whole-rock Lu-Hf isotope system, being less mobile in the presence of a fluid phase during metamorphism, might better preserve protolith characteristics and record earlier melting events. The combination of various isotopic systems, ages, and geochemical characteristics provide more information on the evolution of the lower crust. The xenoliths studied here record the ~2.5 Ga arc magmatism and ~2.1 Ga granulite-facies metamorphism. The metamorphic event may be associated with basaltic underplating at the base of the lower crust, as evidenced by widespread extension, rifting and related emplacement of mafic magma in the NCC during this period (Zhai and Santosh, 2011). This study further reinforces that the Lu-Hf isotope might be a useful tool to trace the protolith characteristics of metamorphic rocks. The initial Hf isotopic compositions of xenoliths at 2 500 Ma range widely from 1.6 to 14.6, indicating the juvenile mantle-derived component during protolith generation and emplacement, i.e., the ~2.5 Ga event might represent a crustal growth event in this area. In fact, the 2.5 Ga was considered to be the major crustal growth period of the NCC (e.g., Gao et al., 2004). In addition, the Archean peak also agrees well with the worldwide compilations (e.g., Condie et al., 2009; Pietranik et al., 2008). The continental crust can grow by lateral accretion of arc complexes in subduction zones and vertical addition of underplating in crust-mantle interface (Rudnick, 1990). The underplated basaltic melt provides heat flux to the melting of the thickened lower-crust during subduction. This view is further supported by the presence of widespread trondhjemite-tonalite-granodiorite (TTG) magmatism associated with the episodic crustal growth in the southern NCC during the Neoarchean to Early Paleoproterozoic (Huang et al., 2013, 2012, 2010). The ~2.5 Ga granitic xenoliths (personal communication with Pengyuan Han) with protolith characteristics of TTG also give evidence to the crustal growth event in this area.

    Furthermore, the recycling of the crust (which might also include the sub-continental lithospheric mantle) with decoupled Nd-Hf isotope might contribute to the observed large variations in Hf isotopic composition at a given Nd isotopic composition of mid-ocean ridge basalts (MORBs) on a global scale (e.g., Zhang et al., 2016; Debaille et al., 2006; Blichert-Toft et al., 2005; Salters and White, 1998; Salters and Zindler, 1995; Johnson and Beard, 1993; Salters and Hart, 1991; Patchett, 1983; Patchett and Tatsumoto, 1980). Recent studies have recognized that distinct ancient depletion signals may exist in refractory mantle domains (Hamelin et al., 2013; Salters et al., 2011; Stracke et al., 2011). This observation adds a new level of complexity to the interpretation of heterogeneous nature of the depleted mantle. More Lu-Hf and Sm-Nd isotopic data might be useful in tracking the signatures of ancient mantle depletion.

  • The Hf-Nd isotope studies on deep-seated eclogite and garnet clinopyroxenite xenoliths from the Xuzhou-Suzhou area along the southeastern margin of the Eastern Block of the NCC yield the following conclusions.

    (1) The xenoliths plot above the terrestrial εHf-εNd array with εHf values up to +60, i.e., they show decoupling of Lu-Hf and Sm-Nd isotopic systems.

    (2) The mineralogical assemblage, especially garnet and/or zircon (rutile to some extent) might interpret the decoupling of Hf-Nd isotope if there is addition or remove of melt/fluid during melting or metamorphic events.

    (3) Metamorphism can significantly affect the Rb-Sr isotope system. Meanwhile, the Sm-Nd isotopic system was also "reset" by ~2.1 Ga granulite-facies metamorphism due to the lower closure temperature than Lu-Hf during high-grade metamorphism. However, the Hf isotope system can faithfully record the protolith characteristics. The differences in element mobility during metamorphism may reinforce the observed decoupling between the Rb-Sr, Sm-Nd, and Lu-Hf isotope systems.

    (4) The Lu-Hf isochron age of 2 424 Ma, combined with the previously published zircon ages suggest that the xenoliths have an affinity to the NCC.

    (5) The positive εHf(t) values at 2 500 Ma indicate a crustal growth event in the southeastern margin of the North China Craton.

  • This study was supported by the National Natural Science Foundation of China (Nos. 41876037, 41273013), the SDUST Research Fund (No. 2015TDJH101), the Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (Nos. 2016RCJJ008 and 2015RCJJ012), and the Shandong Provincial Natural Science Foundation of China (No. ZR2019PD017). The first author would like to express heartfelt thanks to late Prof. Shan Gao, the supervisor for her PhD study, for his guidance. The authors would like to thank the reviewers and the editors for their helpful advices. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1255-4.

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