High oxygen fugacity magma: implication for the destruction of the North China Craton

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Abstract The mechanism of lithospheric removal and destruction of the North China Craton (NCC) has been hotly debated for decades.It is now generally accepted that the subduction of the (Paleo)-Pacific plate played an important role in this process. However, how the plate subduction contributed to the craton destruction remains unclear. Here we report high oxygen fugacity (fO2) characteristics of the Yunmengshan granite, e.g., hematite–magnetite intergrowth supported by zircon Ce4＋/Ce3＋ratios and apatite Mn oxygen fugacity indicator. High fO2 magmas are widely discovered in Late Mesozoic(160–130 Ma)adakitic rocks in central NCC.The origin of high fO2 magma is likely related to the input of the ‘‘oxidized mantle components’’, which shows a close connection between plate subduction and destruction of the craton.The research area is ～1500 km away from the current Pacific subduction zone. Considering the back-arc extension of Japan Sea since the Cretaceous, this distance may be shortened to ～800 km,which is still too far for normal plate subduction. Ridge subduction is the best candidate that was responsible for the large scale magmatism and the destruction of the NCC.Massive slab-derived fluids and/or melts were liberated into an overlying mantle wedge and modified the lithospheric mantle. Rollback of the subducting plate induced the large-scale upwelling of asthenospheric mantle and triggered the formation of extensive high fO2 intraplate magmas.

Keywords High oxygen fugacity·Decratonization·North China Craton · Plate subduction

1 Introduction

North China Craton(NCC),one of the oldest cratons in the world, has shown similar characteristics to those of other cratons before the Middle Ordovician,i.e.having lasted for billions of years(Menzies et al.1993,2007).However,the ancient, thick (＞200 km) and cold heat flow (40 mW/m2)cratonic mantle lithosphere was removed from the eastern NCC in the Paleozoic, and replaced by a young, thin(＜60 km) and high heat flow (80 mW/m2) lithospheric mantle during the Mesozoic. It was accompanied by widespread intraplate magmatism and stretching deformation, suggesting destruction of the NCC (i.e. decratonization) (Fan and Menzies 1992; Gao et al. 2004; Menzies et al.2007;Niu 2005;Wu et al.2002,2005a;Xu 2001;Xu et al. 2004; Yang et al. 2008; Zhang et al.2002, 2003, 2007; Zhu et al. 2011, 2012). There are currently several models for the destruction of the NCC, e.g.,delamination of the eclogitic lower continental crust(LCC)(Gao et al.2004;Wu et al.2002),thermal/chemical erosion of the lithospheric mantle (Menzies et al. 2007; Xu 2001;Zhang et al. 2003), decratonization triggered by subducted Triassic slab and continued in the Cretaceous (Yang et al.2008;Zhao et al.2016),hydration induced by Pacific Plate subduction(Niu 2005),the rollback of subducted flat slabs and melt–peridotite reactions (Kusky et al. 2014), slab rollback-induced unstable mantle flows (Zhu et al. 2011),the destruction induced by ridge subduction (Ling et al.2013; Wu et al. 2017). Anyhow, there is increasing evidence that the subduction of the (Paleo)-Pacific plate played a key role in the destruction of NCC (Ling et al.2013;Niu 2005;Wu et al.2005a,b;Zhu et al.2011,2012).

Oxygen fugacity is an intrinsic thermodynamic property that records the chemical activity of oxygen and controls the speciation of redox-sensitive elements in the solid Earth.More attentions have been paid to it in recent years,including the origin of high fO2in arc magmas, oxidized magmas associated with of porphyry Cu–Au–Mo deposits(Kelley and Cottrell 2009,2012;Lee et al.2005,2010;Sun et al. 2013, 2015, 2016). The fO2of intraplate settings,including abyssal peridotite, mid-ocean ridge basalt(MORB) and continental peridotite fall between ΔFMQ - 3 and 0 log units,whereas the fO2of subduction settings (arc magmas) range between ΔFMQ 0 and ＋3,indicating that intraplate magmas without influences from plate subduction are considerably reduced (Frost and McCammon 2008; Sun et al. 2013).

Here we report Late Mesozoic rocks in central NCC that are characterized by high fO2. The research area is distributed in central NCC and is ～1500 km away from the current Pacific subduction zone (Fig. 1). Generally, oxidized magmas are mainly distributed in subduction zones,why do these intraplate rocks show high fO2characteristics, and what was the mechanism of forming such large volumes of high fO2magmas? In this contribution, we present detailed studies on Yunmengshan and Fangshan batholiths, NCC. This together with a compilation of literature data indicate that high fO2characteristics of Late Mesozoic plutons in NCC may be widespread. This study aims to explore:(1)whether massive slab-derived material entered into the depth and recycled to the crust?(2)what is the redox state of the big mantle wedge beneath the eastern NCC? It may provide new insight on understanding the process of magma mixing,crust-mantle interaction,and the destruction of NCC.

2 Geological background and samples

The NCC formed in the Paleoproterozoic by the assembly of two Archean blocks, the Eastern and Western blocks,along the Trans-North China orogen (Zhao et al. 2001).The basement consists primarily of the Early to Late Archean (3.8–3.0 Ga) TTG (tonalite-trondhjemite-granodiorite) gneisses and ～2.5 Ga granitoids (Liu et al.1992). Subsequently, the NCC has been stable for～2.0 Ga and is covered by thick Proterozoic to Paleozoic sequence. Large-scale magmatic activities and metamorphic core complexes occurred in the NCC during the Late Mesozoic, indicating the reactivation of an ancient craton(Fan and Menzies 1992; Gao et al. 2004; Wu et al.2005a, b; Yang et al. 2008; Zhu et al. 2011). Geochronological studies indicate that the magmatic rocks mostly formed in the Late Jurassic (180–150 Ma) and Early Cretaceous (135–117 Ma) (Wu et al. 2005a, b). Among them,the Early Cretaceous magmatism is the strongest. Researches indicated that those rocks were mainly derived from partial melting of ancient continental crustal materials with the addition of mantle materials via magma mixing (Sun et al. 2010; Wu et al. 2005a).

In this study, we selected four famous batholiths in central NCC. The Yunmengshan batholith (Fig. 2a) is located north of Beijing in the Yanshan Fold and Thrust Belt (YFTB). The batholith is an NNE-elongate intrusion and is 25 km long and 12 km wide (with a total area of～270 km2). The Yunmengshan batholith contains Yunmengshan (YMS) granite and Shicheng (SC)monzodiorite intruding into the Archean-Paleoproterozoic metamorphic basement (Davis et al. 1996). The granite is medium-fine grained. The mineral assemblage consists of plagioclase(～30%), alkali-feldspar (～25%), quartz (～25%), biotite (～15%), and hornblende (2%). Accessory minerals include apatite, zircon, titanite, and Fe–Ti oxides. A mylonitic texture is commonly developed parallel to the boundary between granite and host rocks. Zircon U–Pb ages are 141–144 Ma(Davis et al.2001;Wang et al.2012;Zhao and Wang 2014; Zhu et al. 2015). Monzodiorite is distributed in the northeast corner of the batholith. The mineral assemblage consists of plagioclase (～55%),quartz (～5%), biotite (～25%), hornblende (10%), and pyroxene (～5%). Accessory minerals include apatite,zircon, titanite and Fe–Ti oxides. Zircon U–Pb dating revealed that the pluton intruded in 150–156 Ma.

The Fangshan pluton (Fig. 2b) is located to the southwest of Beijing in the YFTB, and intruded into Archean gneiss, with an outer annulus of quartz diorite surrounding a central core of granodiorite with abundant mafic microgranular enclaves (MME) and mafic dyke. Previous U–Pb dating yielded formation ages of 129–134 Ma for the porphyritic core and 130–134 Ma for the outer annulus quartz diorite,the ～134 Ma for MME and ～134 Ma for mafic dyke (Sun et al. 2010; Xu et al. 2012). The finegrained quartz diorites are mainly composed of finegrained plagioclase (40–45%), K-feldspar (25–30%),quartz (～10%) and amphibole (10–15%), with a minor amount (＜5%) of apatite, zircon, titanite and Fe–Ti oxides. The granodiorite is composed of plagioclase(45–50%), alkali-feldspar (15–20%), quartz (20–25%),biotite (5%) and hornblende (5–10%). Accessory minerals include apatite,zircon,titanite and Fe–Ti oxides.The mafic enclaves are mainly dioritic in composition, containing plagioclase (45–55%), hornblende (30–35%), biotite(5–10%), and small amounts of quartz, K-feldspar,pyroxene, and accessory titanomagnetite, titanite, apatite and zircon.The mafic dyke swarms are about 2–3 km long and 0.1–3 m wide,including some fresh diabases,gabbros,and diorites.

The Dabie Orogen is extensively distributed by largescale Cretaceous (143–129 Ma) granitic and minor mafic–ultramafic intrusions (130–123 Ma). The Tiantangzhai(TTZ) intrusion is the biggest of the early granites with an outcrop area of more than 400 km2and is characterized by the high Sr/Y and (La/Yb)Nratios, showing characteristics of adakitic rocks. Previous researches have shown that the granites plausibly originated from the thickened LCC(＞40–50 km) (Ling et al. 2011; Liu et al. 2010; Wang et al. 2007). The Liguo (LG) intrusive in the Xu-Huai region,consisting of granodiorites and hornblende diorites,were formed in Early Cretaceous (～130 Ma). Liguo pluton has high SiO2(64–71.6 wt%), high Sr(502–655 ppm), low Y (3.0–9.1 ppm), with high Sr/Y ratios (55.9–178.3), and (La/Yb)Nratios. The rocks contained abundant inherited zircon (～2.5 Ga), corresponding to the age of the NCC basement.Previous studies have shown that those rocks were derived from the partial melting of subduction oceanic crust (Sun et al. 2019).

3 Methods

3.1 Whole-rock major and trace elements analyses

The major elements of the bulk rock samples were analyzed at the State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry, Chinese Academy of Sciences(GIGCAS).The bulk rock major elements were conducted using X-ray fluorescence spectrometry (Rigaku 100e) with analytical precisions better than 1%. The trace element analysis of the whole rock was conducted on an Agilent 7700e ICP-MS at the Wuhan Sample Solution Analytical Technology Co. Ltd. Wuhan, China (Sun et al.2018a).

The whole rocks Fe2＋(FeO wt%) contents were analyzed by redox titrations. Firstly, the samples were dissolved in sulfuric acid and hydrofluoric acid.Secondly,the sample solutions were transferred to dilute sulfuric acid.Finally, titration is done with potassium dichromate. This analysis was done in ALS Chemex Laboratories(Guangzhou). The detection limit of FeO is 0.01–100%.All results of major and trace element analyses for the Yunmengshan and Fangshan plutons are listed in Tables S1 and S2.

3.2 Trace element analysis of zircon

Fresh samples were broken into small pieces, then washed and crushed to 200-mesh for zircon separation. Trace element of zircon was obtained using Agilent 7900 ICP-MS coupled with a Resonetics RESOlustion S-155 ArF 193 nm laser-ablation system at the Key Laboratory of Mineralogy and Metallogeny, GIGCAS. A laser spot of 29 μm in diameter was operated with an energy density and a repetition frequency of 8 Hz. The NIST SRM 610 glass and

TEMORA were used as an external standard and91Zr as an internal standard for trace elements analyses (Sun et al.2018b).The calculations of zircon isotope ratios and zircon trace elements were performed by ICPMSDataCal 8.3(Liu et al. 2008). All the results are listed in Table S3.

3.3 Zircon Ce4+/Ce3+ ratio

Zircon is a common accessory mineral in most intermediate to felsic igneous rocks and is resistant to post crystallization disturbances. Zircon partitions Ce4＋in strong preference to Ce3＋, which means the Ce4＋/Ce3＋ratio can be a sensitive indicator of magma oxygen fugacities. Zircon-melt partition coefficients for Ce3＋and Ce4＋are estimated using the lattice strain model(Blundy and Wood 1994):

where Di= Dzircon/rock. The zircon Ce4＋/Ce3＋ratios are estimated using the equation (Ballard et al. 2002; Liang et al. 2006).

Whole rock trace element data and zircon trace elements combined with zircon-melt partition coefficients for Ce3＋and Ce4＋are used to calculate the zircon Ce4＋/Ce3＋ratio.The standard error of the Ce4＋/Ce3＋ratio is from the linear fitting between trace element partition coefficients of zircon and whole rock (Dzircon/rock) and ionic radius. In this study, the error of the zircon Ce4＋/Ce3＋ratio is less than 35 (Zhang et al. 2013, 2017). There are some differences between the method described by Ballard et al.(2002)and Zhang et al.(2017).Zhang et al.(2017)exclude REEs such as La, Pr, and Eu for Ce3＋estimation, and also the tetravalent cation U for Ce4＋estimation because of the low concentrations of LREEs cannot be accurately measured,and also can be disturbed by inclusions. In this study, we stick to the method of Zhang et al. (2017) to calculate zircon Ce4＋/Ce3＋ratios.The results are listed in Table S3.The area of ore barren and ore-bearing rocks in Chile is from Ballard et al.(2002);the area of ore-bearing rocks in Dexing is from Zhang et al. (2013, 2017).

3.4 EMPA analysis of apatite

Major element compositions of apatite grains in situ were analyzed using a JEOL JXA 8230 electron microprobe(EMPA) at the Key Laboratory of Mineralogy and Metallogeny, GIGCAS. The operating conditions were as follows: 15 kV accelerating voltage, 20 nA beam current,and 3 μm beam diameter.The results are listed in Table S4.

3.5 Oxygen fugacity estimation by apatite

Apatite is an important accessory mineral widely grew in intermediate and silicic igneous rocks. Research showed that the Mn content of apatite seems to be largely independent of Mn concentration in the melt, and there is an apparent negative correlation between the oxygen fugacities of magma and Mn concentrations in apatite from a range of intermediate to silicic volcanic rocks (Miles et al.2014), such that:

Therefore we can estimate oxygen fugacities by Mn concentration of apatite.

ΔFMQ = log fO2(sample) - log fO2(FMQ buffer),and the log fO2(sample)were estimated based on compositions of apatite (Miles et al. 2014), and FMQ (fayalite-magnetite-quartz buffer curve) referenced to O’Neill (1987).The apatite saturation temperatures were obtained using the methods of Harrison and Watson (1984).

4 Results

Magnetite—hematite intergrowths are commonly found in YMS granite (Fig. 3). The samples are fresh without weathering and mineralization, and some magnetite and hematite grains are included in other rock-forming minerals (e.g., feldspar), suggesting very high fO2of the magmas.

The samples of YMS and FS batholiths range from diorite to granite (52–75 wt.% SiO2), and both show the higher bulk rock Fe3＋/ΣFe ratios (between 0.3 and 0.6).The Fe3＋/ΣFe of low silica (SiO2＜65 wt.%) samples of Dabie are consistent with YMS and FS, but the high SiO2samples show the low ratios(between 0.1 and 0.3)(Fig. 4a,Table S1). Compared with MORB, all the samples show high fO2.

The majority of zircons Ce4＋/Ce3＋and Eu/Eu* ratios are mostly in the range of 100–1000, and consistent with ore-bearing porphyries in Chile and China (e.g., Dexing)(Zhang et al. 2017), indicating the high fO2. Conversely,the inherited zircon(～2.5 Ga)Ce4＋/Ce3＋ratios of Liguo and FS plutons range from 10 to 200,similar to those of ore barren porphyries in Chile, i.e. low fO2(Fig. 5a, Table.S3).

The fO2of YMS and FS batholiths was estimated by apatite, as shown in Fig. 6 and Table S4. The fO2of SC monzodiorite ranges from ΔFMQ ＋3 to ΔFMQ ＋4, and the fO2of the enclaves contained in FS pluton ranges from ΔFMQ ＋1.5 to ΔFMQ ＋2.5, showing very high fO2characteristics. However, compared with the low silicon rocks (SiO2＜60 wt.%), the YMS granite and FS granodiorite show relatively low fO2.

5 Discussion

5.1 High oxygen fugacity magma of the NCC

The Fe–Ti oxides are important accessory minerals of granitic rocks, their abundance and proportions mainly depend on temperature (T) and fO2of the magma (Krishnamurthy 2015).Primary hematite in close association with magnetite of the samples in this study (Fig. 3) strongly suggests very high fO2, reaching the magnetite-hematite buffer,which is about ΔFMQ ＋4(Sun et al.2013,2015).The detailed study of the fO2from our samples and literature data, including zircon Ce4＋/Ce3＋, and fO2estimated by apatite and the whole rock Fe3＋/ΣFe ratios (Figs. 3, 4,5, 6) indicate that these rocks have very high fO2.Furthermore,data compilation of＞40 plutons in the NCC show Fe3＋/ΣFe ratios ranging from 0.3 to 0.6 (Fig. 4a),indicating that the high fO2characteristics in the NCC during the Late Mesozoic may be widespread.

Fig. 3 Reflected light microphotographs and Raman spectra of hematite and magnetite intergrowths from YMS granite

Fig. 4 a Compiled Fe3＋/ΣFe ratios plotted as function of SiO2. The data of MORB is after Cottrell and Kelley (2011). b The FeOT (wt.%)versus SiO2 diagram. The red circle (n ＞400) are data in NCC from published literatures and GEOROC

Fig. 5 a Zircon Ce4＋/Ce3＋ratios versus Eu anomalies diagrams for intrusive rocks of NCC. The area of ore barren and ore bearing rocks in Chile is from Ballard et al.(2002);the area of ore bearing rocks in Dexing from Zhang et al.(2017)and Zhang et al.(2013).b Sr/Y versus(La/Yb)N diagram discriminating adakites of different origin, after Ling et al. (2013) and Liu et al. (2010). The (Eu/Eu*)N ratios and (La/Yb)N are chondrite normalized values(Sun and McDonough 1989).The Mujicun deposit is a large porphyry type Cu–Mo deposit,after Gao et al.(2012).More detailed data is provided in the appendix: supplementary data S5

Convergent margins (arc magmas) have higher fO2compared to intraplate settings (Brounce et al. 2015; Frost and McCammon 2008; Grocke et al. 2016; Kelley and Cottrell 2009;Sun et al.2013).However,the origin of high fO2arc magmas has been hotly debated(Kelley and Cottrell 2009,2012;Lee et al.2005,2010).It has been proposed that the mantle wedge above the subduction zone is apparently more oxidized as a result of metasomatism by slab-derived fluid and/or melt (Brounce et al. 2015; Kelley and Cottrell 2009).Lee et al.(2010)argued that the redox state of initial arc magmas is not significantly distinguishable from those of MORB, and high fO2of arc magma may result from shallow-level magma differentiation processes.

We compiled the whole-rocks major compositions from more than 40 adakitic rocks (nearly 400 data) from NCC.In the plot of Fe3＋/ΣFe and FeOTversus SiO2, FeOTdecreases while SiO2increases, indicating the magma differentiation process, which is not shown by Fe3＋/ΣFe(Fig. 4). Only when the SiO2content rises above～68 wt.%, the Fe3＋/ΣFe ratios slightly decrease, which may be the result of magnetite separation. This is well supported by Grocke et al. (2016), in which the samples from Tequila(Mexico)and Pinatubo(Philippines)show an insignificant change in Fe3＋/ΣFe ratios from basalt to rhyolite. Hence, we propose that the impacts of the shallow-level magma differentiation processes on the fO2of our samples may be insignificant.

Fig. 6 ΔFMQ versus silica content of whole rock diagrams. Where ΔFMQ = log fO2 (sample) - log fO2 (FMQ buffer), and the log fO2(sample) were estimated based on compositions of apatite (Miles et al. 2014), and FMQ (fayalite-magnetite-quartz buffer curve)referenced to (O’Neill 1987)

Many inherited zircons (～2.5 Ga) are present in FS and Liguo plutons,and the zircon Ce4＋/Ce3＋ratios and Eu/Eu* are very low. This is consistent with ore barren porphyries in Chile, showing low fO2(Fig. 5). In addition,the Fe3＋/ΣFe ratios of adakitic rocks (SiO2＞65 wt.%)from Dabie is far lower than the YMS and FS plutons(Fig. 4a), also revealing the relatively low fO2. Previous studies have shown the Dabie adakitic rocks originated from the partial melting of thickened LCC (Fig. 5b) (Ling et al. 2013; Liu et al. 2010). Thus, the ancient Archean basement is likely to be reductive, and the high fO2characteristics of YMS and FS should not inherit from the partial melting of LCC or the contamination of the surrounding rocks.

Yunmengshan granite and FS granodiorite are characterized by the high Sr/Y (65–200) and (La/Yb)N(20–100)ratios, old TDM2(Hf) model age (～2.5 Ga). Using the discrimination diagram of adakites, if the adakites originated from partial melting of LCC (e.g. Dabie complex),the slope of(Sr/Y)/(La/Yb)Nis very low,but many samples of YMS and FS deviate from the region(partial melting of LCC)and are close to the region of slab melting(Fig. 5b),indicating that magmas mixed with some subduction related material(Ling et al.2013;Liu et al.2010).The oxygen fugacity estimated by apatite also show well mixture characteristics (Fig. 6) such as FS pluton. The enclaves show the high fO2,the granodiorite shows the low fO2,and the quartz diorite show the medium fO2, revealing the characteristic of a mixture of two components. Similarly,the YMS pluton shows the high fO2as a result of the input of mafic melts.On the contrary,the adakitic rocks of Dabie show the low fO2without the contamination of ‘‘mantle component’’. In addition, Late Mesozoic granitoids contains abundant MMEs and mafic dykes, widely distributed in central NCC, which are important markers of mafic magma underplating and crust-mantle interaction. Therefore, the origin of high fO2is more possibly related to the addition of ‘‘mantle components’’.

Iron is an element with variable valences and sensitive to redox reaction.Which also has very high abundances in the Earth, iron-bearing minerals are the important oxygen fugacity buffer, controlling the change of fO2in different geological processes. So the whole rocks Fe3＋/ΣFe can usually be used to indicate the redox state of magma.

In order to understand how much of the impact can the mafic magma adds into the felsic magma, we perform a simulation calculation. We ignored the effect on fO2of other factors (including magmatic degassing and differentiation).Underplating basic magma was selected as a mafic member, we presumed the primary composition of melt,the SiO2(～50 wt.%), Fe3＋/ΣFe (0.4), and the FeOT(10 wt.%).The high silicon adakitic rocks were selected as felsic members. We also presumed the composition, the SiO2(～75 wt.%), Fe3＋/ΣFe (0.20), and the FeOT(2 wt.%).Mixing results of the two members with different proportions are shown in Fig. 4a. Only if the 10% mafic magma was added into felsic magma was the change of Fe3＋/ΣFe ratios more than 0.07. Obviously, the mixing ratios are more than 10% in nature. Therefore, we believe the Late Mesozoic large-scale underplating of basic magma may be the important factor that increases the fO2of adakitic rocks.

5.2 Subduction of the (Paleo)-Pacific plate and an oxidized big mantle wedge

The increasing evidence studies show that the NCC experienced continuous subduction of the (Paleo)-Pacific plate from Jurassic (～170 Ma) (Wu et al. 2005a, b), then the rollback of the subducting plate during Early Cretaceous(～130 Ma)(Fig. 7)(Ling et al.2013;Sun et al.2007;Wu et al. 2005a, b; Zhu et al. 2012). However, we know very little about the link between the subduction plate and the deep mantle under NCC.

Plate tectonics drives the recycling of surface material into the Earth’s interior,introducing hydrated and oxidized oceanic lithosphere into the mantle at subduction zones(Evans 2012; Evans et al. 2017). Fluids liberated during dehydration of subducted crust trigger partial melting of the overlying mantle leading to the formation of volcanic arcs, dominated by oxidized rocks (Ballhaus 1993). The enrichment of LILE and the strong depletion of HFSE are generally taken as significant characteristics of arc magmas(Kelemen et al. 2003). A lot of alkali basalts erupted in Eastern China during Late Mesozoic,including Fangcheng,Feixian, Chengde, Jinling, Fuxin, Qujiatun, Laohutai, etc.(Zhang et al. 2003). There is a geochemical contrast between＞108 Ma and＜108 Ma basalts(Wu et al.2017).

Fig. 7 a The subduction of Paleo-Pacific may have started in the Early Jurassic.Northwestward flat subductions started in the Late Jurassic to Early Cretaceous, which may have reached the inner land and was responsible to the destruction of the North China Craton (Ling et al. 2009, 2013).Much slab-derived fluid and/or melt releases into overlying mantle wedge.b The rollback of the (Paleo)-Pacific plate,resulted in upwelling of asthenospheric mantle, and triggered the formation of largescale high fO2 intraplate magmas

The ＜108 Ma alkali basalts are characterized by oceanic island basalts (OIB)-like geochemical compositions. However,the＞108 Ma alkali basalts are an enrichment of much fluid mobile elements and depletion of HFSE,showing arclike features. The flux of fluid mobile elements of alkali basalts show a close link with the(Paleo)-Pacific plate drift direction (Wu et al. 2017). Previous researches discovered that the water content(＞1000 ppm)of lithospheric mantle source beneath NCC(～120 Ma)is much higher than other cratons,and the water content of primitive basaltic magma can even reach the level of modern island arc basalts (Xia et al.2013).In addition,the δ18O values of clinopyroxene in the OIB-type mafic rocks in the east NCC, are dominantly higher than that of the clinopyroxene from normal MORB(5.4–5.8 ‰), which confirms the role of recycled oceanic crust in their mantle sources (Liu et al. 2017). The Liguo pluton shows adakitic characteristi and high oxygen fugacity, and contains high F and Cl contents (Sun et al.2019). All these indicate that the ancient lithospheric mantle in eastern China may be significant modified by the subducted slab during the Late Mesozoic.

Oceanic sediments may contain Fe3＋/∑Fe ratios up to 0.82 and altered basalts up to 0.19–0.24 (Le´cuyer and Ricard 1999). The subducting oceanic lithospheric mantle may also become oxidized through the formation of serpentine and brucite,which release Fe from their olivine that then forms magnetite (and H＋, which escapes the system)at the expense of H2O (Berndt 1996). That excess Fe3＋could be transported directly from the slab into the mantle wedge by dissolving in hypersaline brines, supercritical fluids, or silicate melts (Kelley and Cottrell 2012). Recent research has shown that the oxidized materials (including sulfur and carbon) carried by the subduction plate may be very important, which could oxidize the mantle wedge of subduction zones without requiring direct transport of Fe3＋.More Fe2＋is oxidized to form Fe3＋by sulfur and carbon reduction in the mantle wedge beneath arc (Evans 2012;Rielli et al.2017).Brounce et al.(2015)reported that fO2of the mantle wedge in Mariana was raised by ～1.3 orders of magnitude as the addition of oxidizing slab fluids,reaching conditions essentially equivalent to the modern arc in just 2–4 Ma. The NCC suffered nearly 50 Ma of subduction,we propose that the mantle wedge beneath NCC gradually become oxidizing with the input of oxidized fluids and/or melts.This is also well supported by abundant water of the mantle wedge beneath NCC in the Late Mesozoic.

5.3 Geodynamic implications

The research area is ～1500 km away from current subduction zones, however, the high fO2characteristics were inherited from oxidized mantle sources.This is too far even for flat subduction (Fig. 1). The reconstruction of tectonic evolution history of the Japan Sea, show that the distance of back-arc spreading are over ～700 km since the Cretaceous(Jolivet et al.1994),we restore the Japan island arc to its original position and find that the ancient subduction zone (～130 Ma) close to the south edge of the Korean peninsula. This distance may be shortened to 800 km,which is still too far for normal plate subduction.However,this is within the distance of flat plate subduction during ridge subduction (Ling et al. 2009, 2013; Wu et al. 2017).In addition, high-resolution mantle tomography of NCC revealed the Pacific slab is stagnant in the mantle transition zone under eastern China, and the western edge of the stagnant slab is roughly coincident with eastern of the gravity gradient zone (Fig. 7) (Zhao et al. 2007). Previous studies have confirmed that flat-lying slabs in the mantle transition zone are generated largely by the rollback of the subduction zone, not lateral penetration of the slabs, and dehydration reactions from slabs can significantly hydrate the overlying mantle (Kusky et al. 2014).

This model also offers a good explanation for the high fO2intraplate magmas. During the northwestward plate subduction, slab-released fluids and/or melts continuous modified the overlying mantle wedge (Fig. 7a), and then the rollback of subducting Pacific plate, resulting in backarc extension and the upwelling of convective asthenosphere mantle, which have triggered the large-scale partial melting of the metasomatized mantle and thickened LCC,the high fO2mafic melts that mixed with those melts derived from LCC (Fig. 7b).

Widespread Late Mesozoic high fO2granitoids formed by partial melting of ancient low crust, with significant input of the oxidized mantle component via magma mixing.This is associated with the removal of lithospheric and the destruction of NCC.The ridge subduction of the Pacific and Izanagi plates have significant influence on this process, including the following aspects: (1) the flat subduction during ridge subduction resulted in intense physical erosion on the thick lithosphere root(Ling et al.2013);(2)voluminous slab-released fluid and/or melt weakened the strength of lithosphere mantle (Niu 2005; Wu et al. 2017);(3) the rollback of the flat plate strongly disturbed asthenosphere, resulting in large scale magmatism. Moreover, the timing and scale of destruction of the NCC are also highly consistent with the ridge subduction. We propose that ridge subduction in the Late Mesozoic was responsible for the destruction of the NCC.

6 Conclusions

That adakitic rocks with high oxygen fugacity are very widespread in the NCC.Those magmas were derived from partial melting of thickened lower continental crust with the mixing of mantle materials, and the high fO2characteristic inherited from an oxidized mantle source that has been modified by fluids and/or melt derived from (Paleo)-Pacific plate. The research of magmas fO2provides new insight into understanding the process of magma mixing and the crust-mantle interaction in NCC during the Late Mesozoic. Ridge subduction provides a good explanation for such high oxygen fugacity. These results also support that ridge subduction was the main control factor that induced the destruction of the NCC.

AcknowledgementsThis study was supported by National Key R&D Program of China (2016YFC0600408), Strategic Priority Research Program (B) of the Chinese Academy of Sciences(XDB18020102), Guangdong Natural Science Funds(2014A030306032 and 2015TQ01Z611), and Youth Innovation Promotion Association CAS(2016315).We thank Drs.Rongqing Zhang and Liuyi Zhang for their assistance of field work. No. IS-2794 from GIGCAS has a contribution in this work.