Transformation of the Sub-Continental Lithospheric Mantle Beneath the North China Craton (NCC): Constraints from the Geochemical Characteristics of Olivine Websterite Xenoliths and Their Minerals in the Cenozoic Basalts from Hannuoba

: The sub-continental mantle beneath North China Craton (NCC) has attracted extensive attention in the past decades because of its dramatic transformation from an old, cold, thick, and refractory mantle to a juvenile, hot, thinner, and fertile mantle. However, the transformation mechanism remains largely controversial. The mantle xenoliths entrapped in basalts, as petrogenetic indicators, can provide an important window to reveal the evolution of the sub-continental lithospheric mantle. In this study, we present a systematical study on the geochemical characteristics of the olivine websterite xenoliths and their minerals in the Cenozoic basalts in the Hannuoba region located at the central orogenic belt of the NCC. The results, compared with the geochemical data of Paleozoic and Meosozic peridotites, the Cenozoic composite pyroxenites as well as the global cumulate pyroxenites, demonstrate that: (1) The source of the websterite is probably the lithospheric mantle, which is mainly newly accreted, but with small amounts of ancient mantle residues. The source may be contaminated by different degrees of crustal materials. The high Nb/Ta ratios (11.36–20.57) of the websterite indicate that the Ti-bearing minerals (such as rutile) are probably involved in the source; (2) The websterite is more likely to be produced as a result of interaction of mantle peridotite with the silica-rich melts that are mainly derived from the asthenospheric mantle and also contributed by the crustal materials; (3) The metasomatic crustal melts might be derived from the subducted Paleo-Paciﬁc plate. These melts interacted with the lithospheric mantle can signiﬁcantly transform the chemical composition of the lithospheric mantle, and consequently play an important role in the destruction of the NCC. An important implication for the destruction of the NCC is further discussed.


Introduction
Craton is a relatively stable structural unit on the Earth's surface, consisting of the continental crust and lower sub-continental lithospheric mantle. The sub-continental lithospheric mantle, which is an important link between the asthenospheric mantle and the crust, can not only provide important information on the crust-mantle interaction and lithosphere-asthenosphere mantle interaction, but determine the basic modes of various geological processes on the continent. Generally, the sub-continental lithospheric mantle beneath the craton (e.g., Kaapvaal and Siberian Cratons) is characterized by refractory, low geothermal gradient, old age, low density, thick lithospheric root, stable structure, and From the time of cratonization (~1.8 Ga) up to Paleozoic, the North China Craton remained cold and relatively stable, with a thick (>200 km) Archean lithospheric root preserved. The deep xenoliths in kimberlites are rich in peridotites, with some occurrence of phlogopites and pyroxenites [24]. These peridotites are mainly refractory harzburgites, which are widely distributed in Mengyin, Shandong Province, and Fuxian, Liaoning Province [5]. The Mg # (Mg # = 100 Mg 2+ /(Mg 2+ + Fe 2+ )) of olivines in these rocks are generally > 92 [25], and the lithospheric mantle is depleted in basaltic components (e.g., Al 2 O 3 and CaO), indicating highly refractory residue after melt extraction [25].
However, extensive magmatism and tectonism has occurred since the Mesozoic, producing a large amount of mineral resources [26]. The lithospheric mantle in Cenozoic became hot and thin, indicating a typical "oceanic" mantle [4,5]. The xenolith types which are dominated by lherzolites in the Cenozoic basalts are simpler than those in Paleozoic kimberlites [4]. The Mg # of olivines in these rocks are <90, much lower than those in the Paleozoic mantle peridotite. In addition, basaltic components such as Al 2 O 3 and CaO are relatively enriched in the Cenozoic lithospheric mantle [25]. These studies suggest that the lithospheric mantle of the North China Craton has transformed from cold, thick, and refractory in Paleozoic to hot, thin, and fertile in Cenozoic.
Pyroxenite, one usual rock type in mantle, is considered an important part of the sources of Oceanic Island Basalts (OIBs) [33,34], Mid-Oceanic Ridge Basalts (MORBs) [35], and Island Arc Basalts (IABs) [36], providing an important window to understand the magma source of basaltic melts, the formation of continental crust, and the composition and heterogeneity of mantle. In this sense, we choose the pyroxenites in the Cenozoic basalts from Hannuoba as the object of this study.
The sampling location is located in Jieshaba, Zhangjiakou city, Hebei Province (Figures 1 and 2) and the samples chosen in this study are olivine (Ol) websterite, with the mineral assemblage of Ol + Opx + Cpx +Sp± Grt. The major minerals are orthopyroxene (Opx) and clinopyroxene (Cpx) (Figure 3a-f), and the minor minerals are olivine (Ol) (Figure 3e,f). The accessory minerals are spinel (Sp) and (or) garnet (Grt). Olivines are typically olive green in hand specimens, while clinopyroxene is emerald green, and orthopyroxene is brown, with a small amount of opaque spinel. Most of the samples show residual porphyritic. Olivine displays plate or granular crystalloblastic, which are elongated in various degrees, with developed kinks; some parts between olivine and pyroxene particles show contact of triple points with balanced structure, with a straight boundary (Figure 3e

Analytical Methods
The chemical compositions of minerals were determined by electron probe microanalyses (EMPA) using the JEOL JXA-8100 electron microprobe (Jeol Ltd., Tokyo, Japan). The accelerating voltage and current were 15 KV and 12 nA, respectively. The beam diameter was 5µm, and the counting time was 10-30 s. The standard minerals (e.g., pyroxene, albite, Cr 2 O 3 , and Fe 2 O 3 ) were used for calibration. The analytical accuracy is better than ±2% for major elements.
The major elements of the whole rock were analyzed by X-ray fluorescence spectrometry (XRF) after the mixture of whole rock powder (0.5 g) and Li 2 B 4 O 7 + LiBO 2 (5 g) in the glass disk. For the analysis of trace elements, the whole rock powder (40 mg) was added to a Teflon bottle, dropped into HF + HNO 3 , and then placed in a steel tank in the incubator at 200 • C for 5 days. The heated Teflon bottle was taken out and evaporated to dryness on the heating pot, and then digested with HNO 3 at 150 • C for one day. The final step was repeated. The dissolved sample was diluted with 49 mL 1% HNO 3 and 1 mL 500 ppb standard solution. Finally, the trace elements were determined by ICP-MS. The analytical accuracies of major and trace elements are within ± 2% and ± 5%, respectively. The elements, such as Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Fe 2+ , Fe 3+ , Rb, Ba, Th, U, Nb, Ta, La, Ce, Pb, Pr, Sr, Nd, Zr, Hf, Sm, Eu, Ti, Gd, Dy, Li, Ho, Er, Y, Yb, Lu, Cr, Tb, and Tm were determined.
All the analyses mentioned above were completed at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS).

Trace Elements of the Whole Rock
The trace elements of the websterites from Hannuoba are presented in Table 1. In the primitive mantle-normalized trace element patterns of the websterite ( Figure 5), most samples resemble the oceanic island basalt (OIB), indicating that the websterite might originate from the mantle source metasomatized by the asthenosphere melts. Some samples are rich in LILE (e.g., Rb, Ba, U, Pb, Sr) and relatively depleted in HFSE (e.g., Nb, Ta, Zr, Hf), suggesting that crustal materials may be involved in the source of the websterite. It is worth noting that some samples are depleted in Th, implying that the samples may be inherited from the geochemical characteristics of the depleted mantle. In addition, it can be also found that the trace element distribution patterns of most samples studied are similar to those of Cenozoic composite pyroxenites, but significantly distinct from those of Paleozoic garnet pyroxenite and Mesozoic wehrlites ( Figure 5) Figure 6) show that LREE are either enriched or slightly depleted in some samples. The Eu is weakly depleted in the websterite on the whole and does not show an upward inverted "U" type distribution pattern, suggesting the websterite is not cumulate origin. However, the distinction of REE distribution patterns may reflect the heterogeneity of the mantle source of the websterite, as can be seen from the comparison between the REE distribution patterns of websterite and other mantle-derived rocks (i.e., N-MORB, E-MORB and OIB) ( Figure 6).  (h): Plots of Mg # vs. TFe 2 O 3 . Note: the Paleozoic peridotite represents the remnant of Archean lithospheric mantle after partial melting [37]; the Mesozoic peridotite represents the residual Archean lithospheric mantle, which was later transformed by the mantle metasomatism involving the continental crust materials [37]; Cenozoic composite pyroxenites represents the product of peridotite-melt interaction [38]; the global cumulate pyroxenite represents pyroxenite with the cumulate origin [39]; Cr-pyroxenite of Hannuoba represents the residual residual-genetic pyroxenite through different degrees of melting of the primitive mantle [40].

Mineral Compositions
The major element compositions of the minerals (i.e., Ol, Cpx, Opx, Sp) are shown in Tables 2-5 and Figures 7-12, compared with those of the peridotites from Hebi [15] that represent the residual old mantle, and the peridotites from Shanwang [41] that represent the juvenile mantle ( Figure 7).  The Shanwang peridotite is characterized by a fertile mantle, which represents the newly-accreted lithospheric mantle [41]; the Hebi peridotite represents the remnant of the old cratonic lithospheric mantle [15].          (Figure 8). In addition, Mg # is positively correlated with SiO 2 , but negatively correlated with MnO, Al 2 O 3 , CaO, similar to the trend represented by with the oxides of lherzolite. Figure 7 shows that most of the core and mantle of Opx studied fall in the field of the peridotites from Shanwang [41]. However, no samples fall in the field of the peridotite from Hebi [15] (Figure 7). Similar to the case of Opx, the core and mantle of Cpx have the same compositions, but their compositions are distinct from those of the rim, indicting obvious core-rim zoning. Generally, Mg # in the core of Cpx (Mg # = 86.3-91.8) are higher than those in the rim of Cpx (Mg # = 80.5-91.8) (Figure 8). Mg # is positively correlated with Cr # , SiO 2 and CaO, but negatively with Al 2 O 3 (Figures 9 and 10). Figure 10 shows small variation in Mg # and Cr # of the core and mantle, but wide ranges of Mg # and Cr # in the rim. The large variation in Cr # of Cpx from Hannuoba peridotites implies that the lithospheric mantle beneath Hannuoba may have experienced different degrees of melting extraction [45].

Olivine (Ol)
As the Ol with a magmatic origin has CaO > 0.1 wt%, and the post-melting mantle residual Ol shows CaO < 0.1 wt%, the content of CaO in Ol can well indicate the genesis of olivine. The compositions of the Ol cores studied almost fall in the field of mantle peridotite, indicating that olivine may be remnants from mantle peridotite ( Figure 11). Ol also shows obvious core-rim reaction structures, with exception of only a few samples characterized by the same core and rim composition. The Mg # of Ol is relatively low (<90), indicating that the fertile lithospheric mantle represented by websterite. Mg # has a weak positive correlation with SiO 2 and a good positive correlation with NiO, but has a negative correlation with MnO and CaO, implying the occurrence of mantle metasomatism. The comparison of the Shanwang and Hebi peridotite data shows that most of the data studied fall in the field of Shanwang peridotite (Figure 11), suggesting that the lithospheric mantle represented by websterite may be newly accreted.

Spinel (Sp)
Sp in the websterite have no obvious core-rim zoning but shows wide ranges of Mg # (65.89-75.96) and Cr # values (11.29-46.97) (Figure 12). In the figure of Mg # vs. Cr # , most of the samples fall in the field of juvenile mantle represented by the peridotite from Shanwang region, whereas a few samples fall in the field of ancient mantle represented by the peridotite from Hebi (Figure 12).

Equilibrium Temperature and Pressure of Websterite
The calculated results of the equilibrium temperature and pressure of the websterites studied are shown in Table 6, suggesting that the formation temperature and pressure of the websterite from Hannuoba fall in the range of 1000-1100 • C, and 13-26 kbar (corresponding to the depth of 43-82 km, [46]), respectively.

Mantle Source of the Websterite
The equilibrium temperature and pressure data ( Table 6) of the websterite from Hannuoba indicate that the websterite originated from the mantle. As shown in the Harker diagram of major element composition (Figure 4), most of the samples studied fall in the field of the Cenozoic composite pyroxenite, suggesting that the websterite is derived from a newly accreted fertile lithospheric mantle. It is worth noting that some samples fall in the field of the residual Archean lithospheric mantle, indicating that the ancient and juvenile lithospheric mantle coexists beneath Hannuoba. The REE distribution pattern diagram of Hannuoba websterite ( Figure 6) shows that there is a large variation in the total content of REE (∑ REE = 9.66-67.28 ppm), and considerable enrichment of LREE in some samples. Furthermore, as shown in the mantle-normalized trace element diagram ( Figure 5), most samples are relatively depleted in high field strength elements (HFSEs). These, altogether, suggest that the mantle source of websterite may be contaminated by different degrees of materials from the crust. On the other hand, the websterites have high Nb/Ta ratio (11.36-20.57), implying that the Ti minerals (e.g., rutile) may be involved in the source of the websterite.

Genesis of the Websterite
The websterite, as an important component of mantle rocks, can provide an important window for unraveling the formation and evolution of lithospheric mantle. There are mainly several models to interpret the genesis of websterite shown as follows: (1) formed by high-pressure fractional crystallization of basaltic magmas during magma ascent through the mantle channel. The resulted rocks were considered to be characterized by upward inverted "U" curve of REE, with obvious enrichment of Eu [39,40,47,48]; (2) originated from reaction of peridotite with silica-rich melts derived from the recycled oceanic crust or subducted lower continental crust, which results in the formation of minerals such as Opx and even Cpx [32,34,49,50]; (3) derived from the solidification of infiltrating magmas or in-situ crystallization of peridotitic melts [28,47,51,52]; (4) produced as a part of the sub-continental lithospheric mantle by high-pressure metamorphism of the gabbros from subducted oceanic crust or the basaltic cumulate rocks. These kinds of rocks produced are mainly characterized by distinctly positive Eu anomaly [53][54][55]; (5) directly sourced from residual mantle [56,57].
The major elemental compositions of the websterites studied are obviously different from those of the pyroxenites with the global cumulate origin (Figure 4). On the one hand, the Mg # , TiO 2 and Na 2 O contents of most samples studied are significantly higher than those of the pyroxenite. On the other hand, the REE distribution patterns of the websterites almost show weak depletion rather than upward-inverted "U" enrichment ( Figure 6) pattern. Therefore, the cumulate origin of the websterites can be excluded. This can be also evidenced by the formation pressure of the websterites (12-26 kbar) ( Table 6). Depletion of the HFSEs (e.g., Zr, Hf) in the websterite indicates that the pyroxenite are not formed by the high-pressure fractional crystallization of the melts during the magma ascent through the mantle channel. The Eu in the websterites do not show distinctly positive anomaly, suggesting that pyroxenite is also not metamorphosed from subducted oceanic crust. The trace element distribution patterns of the websterites are distinct from those of the Paleozoic garnet pyroxenites that represent the melt crystallization. Some of the major elements in the websterites fall into the field of the residual Archean lithospheric mantle, and some data fall in the field of Hannuoba Cr-pyroxenite that represents the residual mantle through different degrees of melting; the basaltic components (e.g., Al 2 O 3 and CaO) are negatively correlated with Mg # . These altogether suggest that the websterites inherit part of the geochemical characteristics of residual mantle. This is also demonstrated by the major element covariance diagram of Mg # vs. oxides of Opx and Cpx in the websterites (Figures 8 and 9). However, the trace element characteristics of the websterites cannot be produced from a single residual mantle source. Therefore, Hannuoba websterite is more likely to be produced by the interaction between the ancient residual mantle peridotite and the late-stage melts, as evidenced by the following points: (1) The CaO content of the Ol in the websterite is less than 0.1%, and that in the core almost fall into the field of mantle peridotite, indicating that the Ol is probably the residual Ol of mantle peridotite; in addition, most of the whole rock data fall into the field of the Cenozoic composite pyroxenite formed by interaction, suggesting that the formation of the websterite is closely related to peridotite-melt interaction; (2) The typical minerals of websterite xenoliths, such as olivine, orthopyroxene, and clinopyroxene, mostly have core-rim reaction structures, demonstrating that the metasomatic melts might be involved in the source. Positive correlation of Mg # with SiO 2 and NiO, but negative correlation with MnO and CaO may suggest the metasomatism. It is worth noting that Mg # and NiO in the rim of Ol lower than those in the core, but MnO in the former higher than the latter may be related to the imbalance of peridotite-melt reaction or the impact of post-metasomatism; (3) The Al 2 O 3 and CaO of the websterites decrease with the MgO implies that the formation of the websterites may be the incorporated effects of partial melting of the ancient lithospheric mantle and peridotite-melt reaction [11], which is also demonstrated by the good correlation of Mg # vs. MnO, Al 2 O 3 , CaO and NiO of Opx, Cpx, and Ol.

Source and Characteristics of Metasomatic Melt
As mentioned above, mantle metasomatism [58] (or melt-peridotite interaction) is the key point for the formation of Hannuoba websterites. Mantle metasomatism can be generally divided into three types: (1) Concealed metasomatism: the mineral composition changes with the formation of new minerals during the metasomatism, e.g., Ol reacts with melt 1 to form Cpx and melt 2, with Mg # decrease in the residual olivine [59]; (2) Apparent metasomatism: typical volatile-bearing minerals such as phlogopite and hornblende are formed during this metasomatism [60]; (3) Cryptic metasomatism: no characteristic metasomatic minerals are formed, with only change in the compositions of the minerals or whole rocks during metasomatism [61]. In this study, the mineral assemblage of websterite is Ol + Opx + Cpx + Sp ± Grt, with the main minerals of Cpx and Opx, and the minor minerals as Ol, indicating that the mantle metasomatism beneath Hannuoba belongs to the concealed metasomatism.
The melts involved in the mantle metasomatism are divided into silicic melts and carbonatic melts [62]. Carbonatite melts are characterized by the depletion of Si, Al, Ti, Ga, and Zr, enrichment of REEs, and high (La/Yb) N [63]. Their reaction with peridotites, shown as carbonatite melts + Opx = Cpx + Ol + melts, can transform harzburgites or lherzolites into Cpx-rich peridotites or websterite [64], consequently resulting in the transformation of the refractory and depleted (depleted in Cpx and LREEs) mantle into fertile and enriched mantle (rich in Cpx and LREEs). However, carbonatite melts that react with metasomatic mantle peridotite cannot produce the petrographic characteristics of the websterite observed in this study. Furthermore, all the websterites studied fall into the metasomatic field of silicate melts (Figure 13), consistent with the case of Cenozoic composite pyroxenites [38]. Therefore, it is suggested that the websterite from Hannuoba is formed by the interaction between mantle peridotite and silica-rich melts. Silica-rich melts include the following source of melts: (1) Asthenosphere-derived melts whose interaction with the peridotite can result in enrichment of mantle peridotites, with a decrease in refractory components and an increase in fusible components; (2) The melts originated from subducted crustal plate, whose interaction with peridotites and melts can form harzburgites or websterite and cause the enrichment of the refractory and depleted lithospheric mantle.
In the Harker diagram of major elements in the whole rocks (Figure 4), the fusible basaltic components such as Al 2 O 3 and CaO decrease with Mg # , which can be interpreted as a result of a two-stage process, that is, the websterite may be formed following the incorporated trends of partial melting (residual mantle peridotite formed by high-degree melting) and interaction (interaction of asthenosphere-derived basaltic melt with peridotite) [11], suggesting that the metasomatic melts may be mainly asthenosphere-derived basaltic melts, as also evidenced by the OIB-type geochemical characteristics of the websterites ( Figure 5). This is consistent with the point suggested by the previous studies [65,66]. The REE and trace element patterns of the websterite show a large variation in ΣREE, LREEs, and HREEs (ΣREE = 9.66-67.28 ppm, LREE = 5.1-57.28 ppm, HREE = 2.37-16.79 ppm), and enrichment in LREEs, LILEs (e.g., Rb, Ba, U, Pb, Sr), and depletion in HFSE (e.g., Zr, Hf, Nb, Ta) in some samples (Table 1, Figure 6), suggesting that the crustal materials are probably involved in the source.

Implication for Destruction of North China Craton
The North China Craton remained stable up to Paleozoic. However, strong activation that took place in the North China Craton (especially in the eastern part) during Mesozoic and Cenozoic not only thinned the ancient and thick lithospheric mantle by more than 100 km, but also drastically transformed the physical and chemical properties of the lithospheric mantle beneath the North China Craton. This process is also known as "destruction". Pyroxenite, though less in the mantle, plays an important role in the formation and evolution of the mantle. The websterites from Hannuoba are the products of the interaction between the silica-rich melts and the lherzolites formed by the partial melting of the ancient residual mantle. These silica-rich melts are mainly derived from the asthenospheric mantle, with involvement of crustal materials. The interaction between silica-rich melts and mantle peridotite can transform an ancient and refractory mantle into a newly accreted and fertile lithospheric mantle, thus resulting in the destruction of the North China Craton.
Previous studies suggested that the Paleo-Pacific plate subducted toward the lithospheric mantle beneath eastern China since the Jurassic [67][68][69]. This implies that the crustal melts that interact with the mantle may be derived from the subducted Paleo-Pacific plate. The interaction between the lithospheric mantle and the melts released from the subducted oceanic plate can significantly transform the physic-chemical properties of the lithospheric mantle. This implies that the subduction of the Paleo-Pacific plate may play an important role in the destruction of the North China Craton.

Conclusions
Based on the petrological, mineralogical, and geochemical studies of the Ol websterite xenoliths entrapped in the Cenozoic basalts from Hannuoba, NCC, the following conclusions can be drawn: (1) The source of the websterite is mainly the newly accreted lithospheric mantle, which may be contaminated by different degrees of crustal materials; in addition, a small amount of residual ancient lithospheric mantle may be involved in the source. The high Nb/Ta ratios of the websterites indicate that the source may be affected by Ti-bearing minerals (e.g., rutile); (2) The websterite is mainly produced by the interaction between mantle peridotite and silica-rich melts. These melts are mainly derived from the asthenospheric mantle, with small involvement of crustal materials; (3) The metasomatic crustal melts may be derived from the subducted Paleo-Pacific plate. These melts interacted with the lithospheric mantle can significantly change the chemical composition of the lithospheric mantle, and consequently result in the destruction of the North China Craton.