Thermal-chemical conditions of the North China Mesozoic lithospheric mantle and implication for the lithospheric thinning of cratons

Earth and Planetary Science Letters 516 (2019) 1–11

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Earth and Planetary Science Letters www.elsevier.com/locate/epsl

Thermal-chemical conditions of the North China Mesozoic lithospheric mantle and implication for the lithospheric thinning of cratons Xianlei Geng a,b,∗ , Stephen F. Foley c , Yongsheng Liu b , Zaicong Wang b , Zhaochu Hu b , Lian Zhou b a

Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, China State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China c ARC Centre of Excellence for Core to Crust Fluid Systems, Dept. of Earth and Planetary Sciences, Macquarie University, North Ryde, New South Wales 2109, Australia b

a r t i c l e

i n f o

Article history: Received 11 September 2018 Received in revised form 12 February 2019 Accepted 7 March 2019 Available online xxxx Editor: T.A. Mather Keywords: thermal-chemical conditions lithospheric thinning mantle transition zone redox melting North China Craton primitive basalts

a b s t r a c t Cratons are the most ancient parts of continents that are underlain by thick, cold, old and refractory lithospheric roots. However, how cratonic roots remain stable for billions of years and become remobilized later is still not well understood. The eastern North China Craton (NCC) is the best region to illuminate this issue because of its well-known lithospheric thinning and decratonization during the Mesozoic–Cenozoic. The thinning mechanism is debated because of limited constraints on the thermal-chemical conditions (lithology and P –T –H2 O– f o2 ) of the Archean lithospheric mantle before and during its removal. Here, we provide constraints on these thermal-chemical conditions for the Archean lithospheric mantle beneath the eastern NCC during its extensive thinning in the form of wholerock chemical and Sr–Nd–Pb isotopic compositions and mineral (especially olivine) chemistry of the Early Cretaceous primitive basalts (MgO > 10 wt.%) from Yixian and Sihetun in the western Liaoning Province. Our data support a model in which the Yixian and Sihetun basalts were derived from metasomatized Archean lithospheric mantle under shallow (∼50–60 km), hot (∼1,290–1,350 ◦ C) conditions. This indicates the existence of a relict (∼25 km) of the Archean lithospheric mantle during the Early Cretaceous, supporting gradual or episodic erosion of the eastern NCC lithospheric mantle. Furthermore, the NCC lithospheric mantle was not only widely rehydrated (>1,000 ppm H2 O) but also highly oxidized (log f o2 FMQ = +1.5 ∼ +1.9 at 1.7–2.0 GPa) during its extensive thinning. Such rehydration and oxidization are demonstrated to be closely related to wet upwelling from the Mantle Transition Zone (MTZ) triggered by the deep subduction of the Paleo-Pacific oceanic slab in the period ∼200–125 Ma. We emphasize that the water released from the upwelling MTZ component and associated hydrous melt influx played a key role in the lithospheric thinning of the eastern NCC by oxidizing the lithospheric mantle and lowering its melting point, which led to redox melting, promoting the erosion of cratonic lithosphere. Our study provides key evidence for the role of deep volatile cycling from the MTZ in modifying thermal-chemical conditions and in the lithospheric thinning of cratons. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Cratons, the Archean cores of continents, are normally underlain by thick (>200 km), old and cold lithospheric roots that persist for billions of years. It is traditionally thought that the negative thermal buoyancy of the cold cratonic lithosphere is largely balanced by the positive chemical buoyancy due to the depletion in

*

Corresponding author at: Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, China. E-mail address: [email protected] (X. Geng). https://doi.org/10.1016/j.epsl.2019.03.012 0012-821X/© 2019 Elsevier B.V. All rights reserved.

iron by extensive melt extraction (Lee et al., 2011). In addition to the neutral to positive buoyancy, strong depletion of volatiles is also required to achieve high viscosities for the longevity and stability of cratons against convective erosion (Doin et al., 1997; Lee et al., 2011; Lenardic and Moresi, 1999; O’Neill et al., 2008). However, the dramatic influence of volatiles, especially water, on mantle rheology has been challenged by recent laboratory experiments (Fei et al., 2013), which suggest only a small effect of water on upper mantle rheology. Therefore, the preservation of cratonic roots for billions of years and the mechanisms of their later remobilization require more investigation.

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Fig. 1. (a) Surface tomography of East Asia and major tectonic divisions of the North China Craton (NCC, gray shaded area). Diamonds denote the Ordovician (∼470 Ma) kimberlites. (b) Vertical cross-section of P-wave tomography along the latitude 42◦ N profile (highlighted by the yellow dashed line in a) (Wei et al., 2012). (c) Geological map of the Yixian and Sihetun volcanic rocks and sampling sites. WB, TNCO and EB denote the three-fold division of the NCC into the Western Block, Trans-North China Orogen and Eastern Block, respectively (Zhao et al., 2001). (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.)

The North China Craton (NCC) is a unique example of lithospheric thinning and decratonization, which occurred during the Mesozoic-Cenozoic (Chen et al., 2006; Gao et al., 2002; Menzies et al., 1993), and is thus an ideal example to understand lithospheric thinning and destruction of cratons. The main mechanisms responsible for its lithospheric thinning have been suggested to include: (1) lithospheric delamination (e.g., Gao et al., 2008, 2004); (2) thermal and chemical erosion (e.g., Griffin et al., 1998; Menzies et al., 1993; Xu, 2001); (3) weakening of the lithosphere by hydration (Niu, 2005); and (4) melt-peridotite reaction (e.g., Zhang, 2005). The lack of unanimity lies in the limited constraints on the thermal-chemical conditions (lithology and P –T –H2 O) of the Archean lithospheric mantle before and during the removal (Xia et al., 2013). Furthermore, the redox conditions, which are important in cratonic lithosphere destruction (Foley, 2008; Tappe et al., 2007), are rarely considered. The Mesozoic, in particular the Early Cretaceous, is probably the most important period of lithospheric thinning and decratonizaton of the eastern NCC, reflected by extensive magmatism that includes A-type granites, mafic dyke swarms and basalts, metamorphic core complexes, metal (Au–Ag–Pb–Zn) mineralization and extensional sedimentary basins (Wu et al., 2005; Zhang et al., 2014). Unfortunately, due to the rarity of mantle xenoliths, direct information from the lithospheric mantle during this period is sparse. Mantle-derived primitive magmas, however, provide another important window to decipher the thermal-chemical conditions of the lithospheric mantle. Here, we present combined whole-rock chemical, Sr–Nd–Pb isotopic compositions and mineral chemistry of two suites of Early Cretaceous primitive basalts from the western Liaoning Province in the eastern NCC and use them to constrain the thermal-chemical conditions of the Mesozoic lithospheric mantle beneath the eastern NCC. We provide key evidence to help explain how the Archean lithospheric mantle of the eastern NCC has been removed.

2. Geological background and samples As one of the oldest Archean cratons in the world, the North China Craton preserves crustal remnants as old as 3.8 Ga (Liu et al., 1992) and can be divided into three parts: the Archean eastern and western blocks with a ∼100–300 km wide Proterozoic belt named the Trans-North China Orogen in the middle (Fig. 1a) (Zhao et al., 2001). Since the final cratonization at ∼1.8–1.9 Ga by amalgamation of the eastern and western blocks (Zhao et al., 2001), it remained undisturbed until the eruption of Ordovician (∼470 Ma) kimberlites (Chu et al., 2009; Zhang et al., 2008). From the early Paleozoic to the Permo-Triassic, the eastern NCC was affected by the southward subduction of the Paleo-Asian oceanic slab (Xiao et al., 2003) and the northward subduction of the PaleoTethys oceanic slab and the Yangtze Craton (Windley et al., 2010). Since the Late Triassic (∼200 Ma), the (Paleo-)Pacific slab has been subducting westwards along the Pacific margin of the NCC and is stagnated in the Mantle Transition Zone (MTZ) beneath eastern China (Fig. 1b). Geophysical tomography (Wei et al., 2012) and the widespread Mg isotope anomaly of the late MesozoicCenozoic basalts (≤106 Ma) from eastern China (Li et al., 2017; Li and Wang, 2018) reveal that the Paleo-Pacific oceanic slab probably began subducting into the MTZ at least 125 Ma ago. It has been demonstrated that the eastern NCC lost ∼120 km of its thick lithospheric root during the Mesozoic-Cenozoic (e.g., Chen et al., 2006; Gao et al., 2002; Menzies et al., 1993). This lithospheric thinning and decratonization peaked in the Early Cretaceous, which is reflected by extensive magmatism including A-type granites, alkaline rocks and related mafic-ultramafic rocks, metamorphic core complexes, metal (Au–Ag–Pb–Zn) mineralization and extensional sedimentary basins (Zhang et al., 2014). Volcanic rocks from the Early Cretaceous Yixian Formation in western Liaoning Province have a thickness of >2890 m and are representative of the most extensive Mesozoic magmatism in the eastern NCC (Fig. 1c). Our samples are picrites and high-Mg basalts (MgO > 10 wt.%) collected from the Mashenmiao type section

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State Key Laboratory of Continental Dynamics, Northwest University, China. Major element compositions of minerals were measured by electron probe microanalysis (EPMA) on a JEOL Superprobe JXA 8100 at the Department of Geology, Peking University, China and at the Key Laboratory of Submarine Geosciences, State Oceanic Administration, China. Major and trace element analyses of olivines were performed on polished sections (∼100 μm thick) by laser ablation ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan), China. Full details of analytical methods are provided in Appendix A.

4. Results Fig. 2. Primitive-mantle-normalized incompatible trace element patterns of the Yixian and Sihetun primitive basalts. Also shown are those of peridotite xenoliths carried by the Ordovician kimberlites from the NCC (Zhang et al., 2008; Zheng and Lu, 1999) for comparison.

(Yixian County) and the Xinkailing type section (Sihetun County) (Fig. 1c), respectively, at the base of the Yixian Formation. Both the Yixian picrites and Sihetun high-Mg basalts are highly porphyritic, with euhedral to subhedral olivine (∼10–20% and ∼5–8%, respectively) and clinopyroxene (∼5–10% and ∼1%) as the main phenocrysts, with minor Fe–Ti oxides (e.g., chromite and ilmenite) (∼2–4%) and very rare orthopyroxene and plagioclase. The Sihetun high-Mg basalts have been previously studied for olivine chemistry, whole-rock major and trace elements as well as Sr–Nd–Re–Os isotope compositions (Gao et al., 2008), which are presented here for the Yixian picrites. In addition, new whole-rock Pb isotope compositions and high-precision major and trace element data for minerals (olivine, clinopyroxene, and chromite phenocrysts and spinel inclusions) are provided for these two suites of Early Cretaceous primitive basalts. 3. Analytical methods Whole-rock major and trace element compositions were analyzed by X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometer (ICP-MS), respectively, at the State Key Laboratory of Continental Dynamics, Northwest University, China. Sr, Nd and Pb isotopic ratios were measured by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) at the

4.1. Whole rock analyses

The major, trace element and Sr–Nd–Pb isotopic compositions are provided in Appendix B. Both the Yixian and Sihetun primitive basalts have high MgO (10.8–14.6 wt.%), Mg# (72–76, molar Mg/(Mg + Fe)*100) and Na2 O + K2 O (4.1–5.4 wt.%), low CaO (7.0–8.5 wt.%), FeOT (7.5–8.6 wt.%), Al2 O3 (12.2–13.0 wt.%) and TiO2 (0.68–0.88 wt.%) with 48.9–50.5 wt.% SiO2 and 0.64–0.80 wt.% P2 O5 . Additionally, they have high Ni and Cr contents (up to 377 and 1,411 ppm, respectively). The Yixian and Sihetun primitive basalts are characterized by enrichments of large ion lithophile elements (LILEs, e.g., Rb, Ba, K, Pb and Sr) and light rare earth elements (LREEs), and depletions of high-field-strength elements (HFSEs, e.g., Nb, Ta, and Ti) and heavy REEs (HREEs), with high Sr/Y (31–57) and La/Yb ratios (12–37) (Fig. 2). However, they also display some differences in incompatible trace element signatures: the Sihetun high-Mg basalts exhibit much higher Rb, Ba, Th, U and LREEs contents with no or slight Pb and Sr peaks and striking Zr, Hf and Ti troughs when compared to the Yixian picrites (Fig. 2). Both the Yixian and Sihetun primitive basalts have moderate initial 87 Sr/86 Sr values varying in a narrow range from 0.706156 to 0.706773 and unradiogenic Pb isotope ratios (206 Pb/204 Pb ≤ 17.306, 207 Pb/204 Pb ≤ 15.399 and 208 Pb/204 Pb ≤ 37.374) (Fig. 3). However, the Sihetun basalts show distinctly higher ε Nd (t) values (−2.3 to −1.5) than the Yixian picrites (−11.8 to −10.3) (Fig. 3a).

Fig. 3. Initial Sr, Nd and Pb isotope compositions of the Yixian and Sihetun primitive basalts. Also shown are peridotite xenoliths carried by the Ordovician kimberlites from the NCC (Zhang et al., 2008; Zheng and Lu, 1999) and potassic rocks from Northeast (NE) China (Wang et al., 2017) for comparison. MORB and OIB data are from http://georoc.mpch-mainz.gwdg.de/georoc/, and global subducting sediments (GLOSS) data are from Plank and Langmuir (1998). The thick solid curve in (b) represents the Pb isotopic growth line of global subducted sediments (GLOSS), assuming present-day 206 Pb/204 Pb = 18.913, 207 Pb/204 Pb = 15.673 (Plank and Langmuir, 1998) and a 238 U/204 Pb (μ) value of 9.74 (Asmerom and Jacobsen, 1993) for their source (upper continental crust). The thick green curve in (b) denotes the Pb isotopic growth line of MORB-source mantle, assuming present-day 206 Pb/204 Pb = 18.275, 207 Pb/204 Pb = 15.486 and μ = 10.8 (Workman and Hart, 2005). The green shaded field denotes the range of Pb isotopic compositions of subducting oceanic slab (MORB + GLOSS) during 0–450 Ma. The red dashed curves in (b) denote the Pb isotopic growth curve of sediments that subducted into the Mantle Transition Zone ca. 2.5 Ga ago with μ = 5.9.

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Fig. 4. Plots of Mg# versus Ca (a), Al (b), Ni (c), Mn (d), Cr (e) and V (f) in olivines of the Yixian and Sihetun primitive basalts analyzed by LA-ICP-MS. Also shown are data for olivines from MORB, komatiites, Hawaiian basalts in the Mauna Loa and Koolau (Sobolev et al., 2007), HIMU basalts (Weiss et al., 2016), mantle peridotites (De Hoog et al., 2010) and subduction zone basalts (Gavrilenko et al., 2016) for comparison. Filled red area in (c) denotes the calculated Ni contents in olivines that crystallize from primary melts of peridotite (0.37 wt.% NiO in the residue olivine) at near surface with a temperature difference ( T ) of 70–230 ◦ C between mantle melting and olivine crystallization (Matzen et al., 2013).

4.2. Mineral analyses Chemical compositions of olivines and their spinel inclusions are provided in Appendix B. The olivines (n = 174) show a large range of Mg# (77.3–92.5), which correlates positively with Al (50–305 ppm), Ni (945–4,370 ppm) and Cr (41–814 ppm) (Fig. 4), and negatively with Li (2.0–20.5 ppm), Mn (881–3,165 ppm), Co (133–176 ppm), Sc (2.6–6.3 ppm), and Zn (63 to 252 ppm) (Fig. 4 and Appendix A Fig. S2). No correlation is seen with Ca (121–1,739 ppm), Ti (6–53 ppm), P (110–726 ppm) and V (1.6–6.8 ppm) (Fig. 4 and Appendix A Fig. S2). Spinel inclusions (n = 24) hosted by olivines are characterized by high Cr# (45–70, molar Cr/(Cr + Al + Fe3+ )*100) and high Fe3+ /Σ Fe ratios (0.27–0.78). 5. Discussion 5.1. Magmatic phenocrysts vs. mantle xenocrysts Olivine is commonly the first abundant silicate phase to crystallize in primitive basaltic melts. In principle, it preserves nearprimary information about the melts, eliminating effects of latestage magmatic processes (e.g. degassing, charge-transfer reactions,

and alteration) (Mallmann and O’Neill, 2013). Trace elements in olivine phenocrysts can be used to indicate the lithological nature of mantle sources (Herzberg, 2006; Sobolev et al., 2007), recycling of crustal materials in the mantle (Ammannati et al., 2016; Foley et al., 2013; Prelevic´ et al., 2013), oxygen fugacity (Canil, 1997; Mallmann and O’Neill, 2013) and water content (Gavrilenko et al., 2016) of magmas. Before using them for these purposes, it is essential to distinguish them from mantle xenocrysts. The Ca content is widely used as a parameter to identify the origin (magmatic or mantle-sourced) of high-Mg# olivines; low CaO (<0.1 wt.% ) is typical for mantle xenocrysts (e.g., Gao et al., 2008; Qian and Hermann, 2010). Furthermore, Foley et al. (2013) have proposed that other trace elements, like Al, Ti, and Ni, are useful to distinguish magmatic olivines from mantle ones: most (85%) mantle olivines have low Al (<130 ppm) and Ti (<70 ppm) with a restricted range of Ni (2,200–3,400 ppm) compared to magmatic ones (Al up to 800 ppm, Ti up to 340 ppm and a large range of Ni). A large amount (∼21%) of olivines from our samples contain CaO < 0.1 wt.% (Fig. 4a) and the majority of olivines have Al < 200 ppm (Fig. 4b) and Ti < 40 ppm (Appendix A Fig. S2a), over-

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lapping with olivines of mantle origin. However, the following observations provide strong evidence for a magmatic origin for these low-Ca olivines: (1) They are euhedral to subhedral in shape and lack deformation features (e.g., kink bands) (Appendix A Fig. S1a) in contrast to petrographic features of mantle olivine xenocrysts (anhedral or rounded in shape and containing undulose extinction or kink banding) (Giuliani, 2018); (2) Some low-Ca olivines contain melt, fluid, and euhedral spinel inclusions (Appendix A Fig. S1a), indicating magmatic origin; (3) Low Ca is a feature of not only the high-Mg# (up to 91) but also the low-Mg# olivines (down to 83) (Fig. 4a), which are uncommon in mantle peridotites; (4) The low-Ca olivines have much higher Ni, Mn, Cr, Li, P, Co, Sc, and Zn contents than mantle olivines (De Hoog et al., 2010) (Fig. 4 and Appendix A Fig. S2). We argue that the low Ca contents in the olivines can be ascribed to the inheritance of the low-Ca feature from the parental magmas and/or the effect of increasing H2 O suppressing the uptake of Ca in olivine (Feig et al., 2006; Gavrilenko et al., 2016). Therefore, all olivines from our samples are considered to be magmatic phenocrysts. Additionally, our data advocate for the prudent use of Ca content alone as an indicator for the origin of olivine, especially in low-CaO and hydrous magmas: other trace elements (e.g., Li, P, Mn, Cr, Co, Sc, Zn, Al) provide complementary evidence for the origin of olivines in basalts. 5.2. Water content, crystallization and melting temperature of the magmas One of the striking features of the Yixian and Sihetun olivine phenocrysts is their extremely low Ca contents that plot far below the locations of olivines from MORBs, komatiites, and OIBs at a given Mg# (Fig. 4a). Low Ca contents in olivine phenocrysts were also observed in basalts from Mauna Loa and Koolau, Hawaii (Sobolev et al., 2007), for which substantial pyroxenite materials have been proposed in the sources (Sobolev et al., 2007, 2005). However, this hypothesis cannot entirely account for the low Ca contents in the Yixian and Sihetun olivine phenocrysts, as they are substantially lower than those of the Hawaiian olivines (Fig. 4a). The Ca contents in the Yixian and Sihetun olivine phenocrysts are comparable to those of subduction zone olivines which crystallized from hydrous arc magmas (Fig. 4a). Therefore, the most likely explanation for the low Ca is that the magmas from which these olivine phenocrysts crystallized were enriched in water, which inhibits the partitioning of Ca into olivines (Feig et al., 2006; Gavrilenko et al., 2016). High water contents in the magmas are also supported by the unequivocal absence of plagioclase phenocrysts in these lavas – a common feature of hydrous magmas, which is caused by the suppression of plagioclase crystallization by water (Grove et al., 2012). The magma water contents can be estimated by the Ca-in-olivine geohygrometer (Gavrilenko et al., 2016) based on the compositions of high-Mg# (>89) olivines that have Kdol/L Fe/Mg values of 0.27–0.35 and are thought to be in equilibrium with their whole-rock hosts (Roeder and Emslie, 1970). These high-Mg# olivines yield water contents of 4.3 ± 1.3 wt.% (1σ , n = 15) for Yixian, and 3.7 ± 2.1 wt.% (1σ , n = 26) for Sihetun primitive basalts. They are similar to the water contents of the Early Cretaceous Feixian primitive basalts (3.4 ± 0.7 wt.%); they fall within the range of arc magmas (2.0–8.0 wt.%) and are much higher than those of MORBs (∼0.1–0.3 wt.%) and OIBs (∼0.3–1.0 wt.%) (Xia et al., 2013 and references therein). The water contents in these primitive basalts correspond to more than 1,000 ppm water in the mantle source rocks (Xia et al., 2013). The crystallization temperatures of olivine phenocrysts were calculated by two conventional geothermometers, the olivine/liquid equilibria thermometer (standard error of ±45 ◦ C) (Beattie, 1993; Putirka et al., 2007) and the Al-in-olivine/spinel thermometer (standard error of ±25 ◦ C) (Coogan et al., 2014), which are

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based on Fe/Mg partitioning between olivine and melt, and Al partitioning between coexisting olivine and Cr-spinel phenocrysts, respectively. The equation of Putirka et al. (2007) for hydrous magmas was used for the olivine/liquid thermometer and gives very similar crystallization temperature ranges of ∼1,189–1,299 ◦ C and ∼1,124–1,282 ◦ C for the Yixian and Sihetun high-Mg# olivine. The Al-in-olivine/spinel thermometer, which is largely independent of crystallization pressure, oxygen fugacity and melt composition (e.g., H2 O and CO2 ) (Coogan et al., 2014), yields ∼1,151–1,279 ◦ C for Yixian and ∼1,156–1,252 ◦ C for Sihetun based on the compositions of olivines (Mg# > 89) and their coexisting spinel inclusions. The thermobarometer of Lee et al. (2009) based on magma Si and Mg contents was used to obtain the temperatures and pressures of melting that generated the Yixian and Sihetun primitive basalts. Assuming a Kdol/L Fe/Mg value of 0.30, Mg# of 92 for olivine in mantle residue and water contents of ∼4.0 wt.%, the Yixian and Sihetun primary melts appear to have been produced at temperatures and pressures of ∼1,320–1,348 ◦ C (∼1.8–2.0 GPa) and ∼1,296–1,315 ◦ C (∼1.6–1.7 GPa), respectively. 5.3. The redox condition of the magmas and their mantle sources Vanadium can occur in multiple oxidation states (V2+ , V3+ , V4+ and V5+ ) in silicate and oxide systems at terrestrial oxygen fugacities (Canil, 1997; Mallmann and O’Neill, 2009). Previous experimental studies (Canil, 1997; Canil and Fedortchouk, 2001; Laubier et al., 2014; Mallmann and O’Neill, 2009, 2013; Ringwood, 1970; Shearer et al., 2006) delineated that V becomes more incompatible in major liquidus phases (e.g., olivine, clinopyroxene, orthopyroxene, spinel and garnet) with increasing valence state (from V2+ to V5+ ), and its partitioning behavior during mantle melting and magma crystallization is mainly controlled by the oxygen fugacity ( f o2 ) and is much less sensitive to the temperature and melt composition. As the partition coefficients of V between olivine and melt (D V ol/melt ) under different redox conditions are well characterized, D V ol/melt becomes, in principle, an intuitive oxybarometer of magmas and potentially of mantle sources (Canil, 1997; Mallmann and O’Neill, 2013). Here we use the D V ol/melt oxybarometer of Mallmann and O’Neill (2013) to estimate the oxygen fugacities of the Yixian and Sihetun primitive magmas. The D V ol/melt oxybarometer yields log f o2 values of FMQ + 1.7 (± 0.3, 1σ , n = 15) for Yixian and FMQ + 1.3 (± 0.3, 1σ , n = 26) for Sihetun basalts, based on the compositions of the high-Mg# (>89) olivines. These values are much more oxidized than those of MORB (FMQ + 0.1; (Berry et al., 2018)), but comparable to those of back-arc basin basalts (BABB; FMQ + 1.0 ∼ +1.5) and arc island basalts (AIB; FMQ + 1.4 ∼ +2.7) (Kelley and Cottrell, 2009). To test the reliability of the results inferred from the D V ol/melt oxybarometer, the redox states of these primitive magmas were also estimated independently from their whole-rock V/Yb ratios (Laubier et al., 2014) and using the olivine-spinel oxybarometer (Ballhaus et al., 1991), which uses the compositions of spinel inclusions and their host olivines in the primitive basalts. The Yixian and Sihetun primitive basalts have average V/Yb ratios of 163 ± 23 (1σ , n = 15) and 114 ± 6 (1σ , n = 15), respectively. These values are appreciably higher than those of MORB (88 ± 14) and comparable to those of primitive AIB (130–300) and BABB (116 ± 13) (Laubier et al., 2014). For the olivine-spinel oxybarometer, the temperature is given by the Al-in-olivine-spinel thermometer (Coogan et al., 2014) as discussed above, whereas the pressure is constrained by the maximum value (∼0.7 GPa) calculated by the cpx-liquid equilibria thermobarometer for hydrous systems (Putirka, 2008) based on the compositions of cpx (Mg# 85–90). The high-Mg# (>89) olivines give average log f o2 values of FMQ + 1.6 (±0.4, 1σ , n = 9) and FMQ + 1.3 (±0.2, 1σ , n = 11) for

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Fig. 5. (a) log f o2 FMO vs. pressure (GPa) for the Mesozoic lithosphere mantle of the North China Craton. Also shown are those of other cratons, oceanic mantle and supra-subduction mantle (Foley, 2011; Frost and McCammon, 2008) for comparison. The lines with a slope of 0.7 FMQ units per GPa indicate the pressure effect on f o2 (Frost and McCammon, 2008). (b) The stability fields of the C–O–H components in the mantle in pressure- f o2 space (Foley, 2011). Redox melting occurs when passing from the yellow to the pink area during ascent. This erodes and removes the deeper parts of the cratonic mantle. Later pulses of oxidized melting give rise to the primitive basalts at shallower depths.

Yixian and Sihetun, which is an extremely good fit to the estimates from the D V ol/melt oxybarometer. As these results are obtained from the most forsteritic olivine phenocrysts (Mg# > 89, and up to 91.5), the resulting f o2 values could indicate the redox conditions of the primary melts. However, FMQ is slightly pressure dependent (∼0.17 log units per GPa; (Kress and Carmichael, 1991)), so that average log f o2 values of FMQ +1.9 and +1.5 are obtained for the f o2 of the Yixian and Sihetun melts as they left their mantle sources (at ∼2.0 and ∼1.7 GPa, respectively). As shown in Fig. 5, our data reveal that the lateMesozoic lithospheric mantle beneath the eastern NCC was strikingly oxidized when compared to other typical Archean cratonic lithospheric mantles (e.g., Slave, Kaapvaal) (Frost and McCammon, 2008). 5.4. Peridotitic rather than pyroxenitic sources The most widely used discriminators for the source lithology of mantle-derived melts are the Ni content and Fe/Mn ratio in olivine phenocrysts (e.g. Gao et al., 2008; Herzberg, 2011; Howarth and Harris, 2017; Sobolev et al., 2007, 2005; Weiss et al., 2016). These two lithology discriminators are mainly based on the assumptions that the olivine-melt partitioning for Ni and Fe/Mn does not strongly vary with temperature and pressure (Herzberg and Zhang, 1996; Sobolev et al., 2005; Weiss et al., 2016) and their partition coefficients between cpx, opx, gt and melt are much lower than those for olivine (Le Roux et al., 2011; Sobolev et al., 2007). However, more recent high-quality experimental results (Balta et al., 2011; Li and Ripley, 2010; Matzen et al., 2013, 2017) suggest that the olivine-melt partitioning for Ni and Mn are more sensitive to the temperature and pressure than previously thought, and even a small change in mineral modal abundance (especially garnet) can lead to a large variation of Fe/Mn ratio in mantlederived melts and consequently in olivine phenocrysts that crystallize from them. Furthermore, a study of the Karoo meimechites (Heinonen and Fusswinkel, 2017) demonstrated that high Ni and Fe/Mn in olivine phenocrysts do not always reflect pyroxenitic mantle sources. Therefore, other discriminators, including Zn/Fe, Zn/Mn and Co/Fe ratios in olivine (Howarth and Harris, 2017; Le Roux et al., 2011), whole-rock CaO/MgO ratios (Herzberg, 2006; Tappe et al., 2016), and the FC3MS value (FeOT /CaO–3*MgO/SiO2 ; all in wt.%) (Yang et al., 2016) should be combined to provide more robust evidence for the source lithology of mantle-derived melts. Both the Yixian and Sihetun olivines have Ni contents that are systematically higher than those of MORBs and komatiites at a given Mg# value, but comparable to those in basalts from the

Mauna Loa and Koolau, Hawaii (Fig. 4c). However, the wholerock Ni contents (218–377 ppm) are consistent with those of peridotite-derived melts with MgO of ∼13–14 wt.% (250–420 ppm Ni; (Herzberg et al., 2013)). The higher Ni contents in the Yixian and Sihetun olivines can be largely explained by the temperature difference ( T , 70–230 ◦ C) between mantle melting and olivine crystallization as estimated above, although minor pyroxenite might be in the sources. Assuming 0.37 wt.% NiO (the mean value for global peridotite olivines) in residual mantle olivine and  T of 70–230 ◦ C, the Ni content in near-surface olivine from the peridotite-derived primary melts is calculated to be ∼3,200–4,100 ppm (Matzen et al., 2013), which is consistent with the Ni contents in the most forsteritic olivines of the Yixian and Sihetun primitive basalts (Fig. 4c). Yixian olivines with Mg# > 87 have relatively constant Fe/Mn ratios that are comparable to those of peridotite melts (e.g., MORBs and komatiites), whereas the Sihetun olivines have much higher Fe/Mn ratios that are comparable to those of basalts from the Mauna Loa and Koolau (Hawaii) for which substantial amounts of pyroxenite have been proposed in the sources (Sobolev et al., 2007, 2005) (Fig. 6a). However, such high Fe/Mn ratios in the Sihetun olivines can be explained by fractional crystallization of chromite, which is supported by abundant chromites (Fe/Mn of 34–63 with an average of 47) in the Sihetun lavas. Peridotitic rather than pyroxenitic sources for these basalts are also supported by other first-row-transition-element (FRTE) ratios (e.g., Zn/Fe, Zn/Mn and Co/Fe) in olivine phenocrysts. The Zn/Fe and Zn/Mn ratios negatively correlate with the Mg# (Figs. 6b and 6c), whereas the Co/Fe ratio positively correlates with the Mg# for the Yixian and Sihetun high-Mg# (>87) olivine phenocrysts (Fig. 6d). These correlations reflect fractional crystallization of olivine, clinopyroxene, and Fe–Ti oxides. Nevertheless, the most forsteritic olivines (Mg# > 89) show Zn/Fe, Zn/Mn and Co/Fe ratios characteristic for peridotite-derived melts. Therefore, the FRTE ratios of the olivine phenocrysts suggest dominantly peridotitic sources for the Yixian and Sihetun primitive lavas. Low CaO/MgO has been widely used to indicate melts of pyroxenite relative to peridotite melts (e.g., Gao et al., 2008; Herzberg, 2006; Tappe et al., 2016). However, Yang et al. (2016) highlighted that high-Na2 O + K2 O peridotite melts can also have low CaO/MgO (CaO − 13.81 + 0.274 ∗ MgO < 0), which is consistent with the high Na2 O + K2 O contents of the Yixian and Sihetun primitive basalts (Figs. 7a and 7b). In contrast, the FC3MS value of high-MgO melts (>7.5 wt.%) is a more robust parameter for distinguishing melts of pyroxenite and peridotite (Yang et al., 2016). As shown in Fig. 7c, the Yixian and Sihetun primitive basalts have uniformly low FC3MS values (0.27–0.37) within the field of peridotite

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Fig. 6. Plots of Mg# versus the first-row transition-element ratios in olivines of the Yixian and Sihetun primitive basalts analyzed by LA-ICP-MS. (a) Fe/Mn, (b) Zn/Fe∗104 , (c) Zn/Mn, and (d) Co/Fe∗104 . Solid curves with arrows denote the liquid lines of descent for the magmas. Dashed lines denote the potential boundaries between peridotite and pyroxenite melts (Le Roux et al., 2011). Sources of other olivine data are as in Fig. 4.

melts. Thus, the whole-rock major compositions support peridotitic sources for the Yixian and Sihetun primitive basalts. Collectively, the combined evidence from whole-rock and olivine chemical compositions reveals predominantly peridotitic sources with little or no contribution from pyroxenite for the Yixian and Sihetun primitive basalts. The high Rb/La (1.0–1.7) and Ba/Th (210–429) ratios as well as high water contents indicate the presence of phlogopite and/or amphibole in the mantle sources. Because these phases are unstable at asthenospheric temperatures, involvement of subcontinental lithospheric mantle (SCLM) is required for these primitive basalts. A refractory (cpx-depleted) SCLM source is also supported by the low CaO (<8.5 wt.%) and TiO2 (<1.0 wt.%) contents as well as high Fo values (up to 92.5) of olivines. After the thinned SCLM was heated by the underlying convective mantle, the phlogopite and/or amphibole were no longer stable and broke down, leading to water-flux melting of the SCLM and the generation of the Yixian and Sihetun primitive basalts. 5.5. Wet upwelling of an MTZ component into the lithospheric mantle As discussed above, our data reveal that the lithospheric mantle (∼50–65 km) beneath the eastern NCC was hot (∼1,290–1,350 ◦ C), water-rich (>1,000 ppm H2 O) and oxidized (log f o2 FQM + 1.5 ∼ +1.9) during the peak of lithospheric thinning (Early Cretaceous). These features are distinctly different from stable Archean cratonic lithospheric mantle (e.g., Slave, Kaapvaal) (Fig. 5) (Frost and McCammon, 2008; Grant et al., 2007; Hasterok and Chapman, 2011). Therefore, in order to understand the lithospheric thinning and destruction it is critical to understand how the eastern NCC lithospheric mantle was hydrated and oxidized. The Yixian and Sihetun primitive basalts are characterized by enrichments of fluid-affinitive trace elements (Rb, Ba, K, Pb, Sr) (Fig. 2), EM1-like Sr–Nd and unradiogenic Pb isotopic compositions (Fig. 3). These signatures are distinctly different from those of the mantle xenoliths carried by Ordovician (∼470 Ma) kimberlites from the eastern NCC (Chu et al., 2009; Zhang et al., 2008;

Zheng and Lu, 1999), which are featured by depletions of Rb, Ba, K, Pb, Sr (Fig. 2), EM2-like Sr–Nd and radiogenic Pb isotopic compositions (Fig. 3). This suggests that a more recent (<470 Ma) fluid-rich event rehydrated and oxidized the eastern NCC lithospheric mantle. Considering the tectonic evolution of the NCC, the water-rich metasomatic agent could potentially be derived from shallow subduction zones of the Paleo-Asia oceanic slabs (∼450–250 Ma; (Xiao et al., 2003)) or deeply subducted (Paleo-) Pacific oceanic slab (∼200–0 Ma; (Windley et al., 2010)). However, the fluids/melts from recently (<450 Ma) subducted oceanic slabs (e.g., Paleo-Asia and (Paleo-) Pacific oceanic slabs) are expected to have MORB or EM2-like Sr–Nd and radiogenic Pb isotopic compositions (Fig. 3) that are inconsistent with the isotopic signatures of these basalts. Alternatively, we propose that the fluids/hydrous melts that hydrated and oxidized the Mesozoic NCC lithospheric mantle could have been derived from the MTZ, which has been considered as a gigantic water reservoir in deep Earth (e.g., Fukao et al., 2009; Karato, 2011; Pearson et al., 2014) due to the capacity of wadsleyite and ringwoodite to store enormous amounts of water at depths between 410 and 660 km and multiple deep subductions of oceanic slabs over time. Upwelling of the MTZ component would lead to the upward release of water due to the decompression transformation of water-enriched wadsleyite and ringwoodite to water-free/poor forsterite, promoting water-fluxed melting of the mantle at shallow depths and ultimate hydration and oxidization of the overlying lithospheric mantle. The MTZ has also been thought to be an important EM1-type mantle reservoir with unradiogenic Pb isotopic compositions produced by long-term preservation of K-hollandite-bearing subducted sediments (low U/Pb, Th/Pb, Sm/Nd but moderate Rb/Sr), which was applied to interpret the isotopic signatures of ultrapotassic rocks from Northeast China (Wang et al., 2017). For example, our modeling suggests that the ancient (∼2.5 Ga) sediments in the MTZ with μ (238 U/204 Pb) = ∼5.9 could have unradiogenic Pb isotopic composition similar to those of the Early Cretaceous primitive basalts studied (Fig. 3b). The involvement of subducted sediments would also explain the high Li contents in olivines (up to 20 ppm in

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Fig. 7. Major element plots of whole-rock compositions for identifying peridotite and pyroxenite melts (Yang et al., 2016). (a) CaO versus MgO, (b) CaO–13.81 + 0.274∗MgO versus Na2 O + K2 O, and (c) FC3MS (FeOT /CaO–3∗MgO/SiO2 ) versus MgO. The field of experimental peridotite melts is from Yang et al. (2016). MORB data are from http://georoc.mpch-mainz.gwdg.de/georoc/.

Yixian basalt olivines), which is used as an indicator for crustal involvement in the source (Foley et al., 2013). The wet upwelling of the MTZ component would be triggered by the penetration of the deeply subducted Paleo-Pacific slab into the MTZ beneath the eastern China >125 Ma ago, which has been revealed by tomography studies of East Asia (Wei et al., 2012) and the widespread Mg isotope anomaly of the late Mesozoic–Cenozoic basalts with a maximum age of ∼106 Ma from the eastern NCC (Li et al., 2017; Li and Wang, 2018). Therefore, we suggest that the Mesozoic lithospheric mantle of the eastern NCC was hydrated and oxidized by fluids/hydrous melts from the upwelling MTZ components. 5.6. Insights into the lithospheric thinning of cratons The generation of the Yixian and Sihetun primitive basalts attests to the survival of the Archean lithospheric mantle at shallow mantle depths beneath the northern margin of the eastern NCC up until Early Cretaceous times. The P –T conditions for melting of the lithospheric mantle estimated from these primitive basalts are ∼1.7–2.0 GPa and ∼1,290–1,350 ◦ C, indicating a very high

Fig. 8. (a) Mantle melting conditions for the generation of the Yixian Sihetun primitive basalts and peridotite solidus under diverse conditions (Dasgupta et al., 2013; Foley and Pintér, 2018). Blue and green areas indicate carbonatitic melts and carbonate-rich silicate melts, respectively (Foley, 2011). The grey dashed lines denote cratonic geotherms (Hasterok and Chapman, 2011). (b) and (c) Cartoon sections showing lithospheric rehydration and oxidization caused by wet upwelling of the Mantle Transition Zone (MTZ) component during the late-Mesozoic and initiation of the lithospheric thinning and decratonization of the eastern North China Craton.

geotherm in the lithosphere (∼90 mW/m2 ; Fig. 8a) during the Early Cretaceous. Such a high geotherm implies that the relict Archean lithospheric mantle was very thin (∼25 km below the crust) in the Early Cretaceous, providing an adiabat with potential temperatures of 1,300–1,400 ◦ C (Fig. 8a), supporting a progressive or episodic erosion model for lithospheric thinning of the eastern NCC. However, the mechanism of initiation of the lithospheric thinning is not well known yet.

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It has been suggested that lithospheric rehydration could play a key role in the effective removal of cratonic lithospheric roots by lowering the viscosity (Lee et al., 2011; Peslier et al., 2010; Xia et al., 2013). However, if the hydration of the NCC lithospheric mantle were caused by wet upwelling from the MTZ triggered by deep subduction of the Paleo-Pacific slab as we propose, the time scale of the hydrated state of the NCC lithospheric mantle would be less than ∼75 Ma, which is constrained by the time of initiation of subduction of the Paleo-Pacific plate (∼200 Ma; (Windley et al., 2010)) and by the eruption date (∼125 Ma) of the primitive basalts studied here. Within such a short period, it appears impossible to achieve the successful thinning over 120 km (from >200 km to <80 km) of the eastern NCC lithosphere merely by physical erosion of the convective mantle. For example, geodynamical models (Wang et al., 2014) suggested that it requires over 2.0 Ga for eroding buoyant cratonic lithosphere from 240 km to 160 km provided a different strengthening factor η = 1 (∼1,000 ppm H2 O in the lithospheric mantle (Xia et al., 2013), notwithstanding the large uncertainty of the effect of water on mantle rheology (Fei et al., 2013). However, our data reveal that the NCC cratonic lithospheric mantle was not only largely rehydrated but also highly oxidized (log f o2 FQM + 1.5 ∼ +1.9 at ∼1.6–2.0 GPa) during its extensive thinning. We explain this as being controlled by water-rich MTZ-derived melts which caused widespread erosion of the original lower cratonic lithosphere (∼200 km) by redox melting. The water released from the upwelling MTZ is crucial in these processes, and is of especial importance for the lithospheric thinning of the eastern NCC, where there is no evidence for the existence of a deep-sourced plume during the Phanerozoic. The high water content could lead to the water-flux melting of the upwelling mantle before the base of the lithospheric mantle is reached (Fig. 8a), despite the relatively low potential temperatures (e.g., ∼1,400–1,450 ◦ C) for MTZ-sourced upwelling mantle relative to of deeply sourced (e.g., the core-mantle boundary) plumes (>1,500–1,600 ◦ C). The rising partially molten upwelling region severely affects the melting behavior of the base of the cratonic lithosphere when it reaches it. Although the stable cratonic lithosphere was probably in a reduced state due to geochemical depletion during its formation in the Archean (Foley, 2011; Frost and McCammon, 2008), it will have been gradually impregnated with carbon-rich melts over more than 2 billion years (Fig. 8b; Foley and Fischer, 2017). Metasomatism of this type has been shown to hydrate and oxidize the cratonic lithospheric root by up to four orders of magnitude f o2 (Yaxley et al., 2017). However, the metasomatized root remains unmelted under low-water conditions because of the low geotherms (35–44 mW/m2 ; Hasterok and Chapman, 2011) and insufficiently low melting point of dry carbonated peridotite (+CO2 ) (Dasgupta et al., 2013) (Fig. 8a). However, the addition of water into previously carbonated lithospheric mantle can significantly lower the melting point (H2 O + CO2 ) (Fig. 8a), causing the prodigious generation of carbonate-rich melts which then rapidly erode the cratonic roots. This is the carbonated redox melting mechanism (Foley, 2011): rising H2 O-rich material from the “barren” region (yellow in Fig. 5b) pass upwards into the oxidized realm, causing melting due to the drop of the solidus temperature by more than 120 ◦ C at 200 km depth (Foley and Pintér, 2018). Under such H2 O + CO2 -rich conditions, the carbonate-rich melts produced by redox melting of the lower lithosphere move upwards and progressively corrode the cratonic roots (Figs. 8b and 8c) (Foley, 2008, 2011; Tappe et al., 2007, 2017), leading to the rapid thinning of the lithospheric mantle. The Yixian and Sihetun basalts result from a later pulse of melting during the late stages of craton erosion. Our study thus provides new insights into the role of deep volatile cycling from the MTZ in the lithospheric thinning and de-

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struction of Archean cratons. The trigger for the rapid thinning of cratonic lithosphere in this area appears to be the interaction of the deeply subducted oceanic plate with an anomalous MTZ mantle containing an older enriched component (Fig. 8b). 6. Conclusions Our detailed studies of the whole-rock chemical and Sr– Nd–Pb isotopic compositions combined with mineral (especially olivine) chemistry reveal that the Early Cretaceous Yixian and Sihetun primitive basalts were derived from shallow (∼50–60 km) Archean refractory lithospheric mantle that was melted in hot (∼1,290–1,350 ◦ C), oxidized (log f o2 FQM = +1.5 ∼ +1.9), and hydrous (>1,000 ppm H2 O) conditions. The existence of relict Archean lithospheric mantle during the Early Cretaceous, although probably very thin (∼25 km), suggests that the lithospheric thinning of the eastern NCC proceeded gradually, perhaps episodically. Combined with the tectonic evolution of the NCC, the high water contents, EM1-like Sr–Nd and unradiogenic Pb isotopic compositions of these primitive basalts suggest that the Mesozoic NCC lithospheric mantle was strikingly rehydrated and oxidized by fluids/hydrous melts from the upwelling MTZ. Such wet upwelling of an MTZ-derived component would be triggered by the penetration of the deeply subducted Paleo-Pacific slab. Our study provides new insights into the role of deep volatile cycling from the MTZ in the lithospheric thinning and destruction of Archean cratons. Acknowledgements This research was supported by the National Natural Science Foundation of China (grant nos. 41503015, 41173016 and 41373026) and Chinese Ministry of Education (B07039), the Fundamental Research Funds for National Universities, special funds from the State Key laboratory of Geological Processes and Mineral Resources (MSFGPMR01) and from the State Key Laboratory of Continental Dynamics. SF is supported by ARC grant FL180100134. We acknowledge Cees-Jan De Hoog and Sebastian Tappe for constructive reviews, which have improved this paper significantly. We finally thank Tamsin Mather for comments and editorial handling. Appendix. Supplementary material Supplementary material related to this article can be found online at https://doi.org/10.1016/j.epsl.2019.03.012. References Ammannati, E., Jacob, D.E., Avanzinelli, R., Foley, S.F., Conticelli, S., 2016. Low Ni olivine in silica-undersaturated ultrapotassic igneous rocks as evidence for carbonate metasomatism in the mantle. Earth Planet. Sci. Lett. 444, 64–74. Asmerom, Y., Jacobsen, S.B., 1993. The Pb isotopic evolution of the Earth: inferences from river water suspended loads. Earth Planet. Sci. Lett. 115, 245–256. Ballhaus, C., Berry, R.F., Green, D.H., 1991. High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle. Contrib. Mineral. Petrol. 107, 27–40. Balta, J.B., Asimow, P.D., Mosenfelder, J.L., 2011. Manganese partitioning during hydrous melting of peridotite. Geochim. Cosmochim. Acta 75, 5819–5833. Beattie, P., 1993. Olivine-melt and orthopyroxene-melt equilibria. Contrib. Mineral. Petrol. 115, 103–111. Berry, A.J., Stewart, G.A., O’Neill, H.S.C., Mallmann, G., Mosselmans, J.F.W., 2018. A re-assessment of the oxidation state of iron in MORB glasses. Earth Planet. Sci. Lett. 483, 114–123. Canil, D., 1997. Vanadium partitioning and the oxidation state of Archaean komatiite magmas. Nature 389, 842. Canil, D., Fedortchouk, Y., 2001. Olivine liquid partitioning of vanadium and other trace elements, with applications to modern and ancient picrites. Can. Mineral. 39, 319–330. Chen, L., Zheng, T.Y., Xu, W.W., 2006. A thinned lithospheric image of the Tanlu Fault Zone, eastern China: constructed from wave equation based receiver function migration. J. Geophys. Res., Solid Earth 111, B09312.

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