Magmatic Recharge Buffers the Isotopic Compositions Against Crustal Contamination in Formation of Continental Flood Basalts

Lithos 284–285 (2017) 1–10

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Magmatic recharge buffers the isotopic compositions against crustal contamination in formation of continental ﬂood basalts

Xun Yu, Li-Hui Chen ⁎,GangZeng

State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China article info abstract

Article history: Isotopic compositions of continental flood basalts are essential to understand their genesis and to constrain the Received 2 August 2016 character of their mantle sources. Because of potential crustal contamination, it needs to be evaluated if and to Accepted 27 March 2017 which degree these basalts record original isotopic signals of their mantle sources and/or crustal signatures. Available online 08 April 2017 This study examines the Sr, Nd, Hf, and Pb isotopic compositions of the late Cenozoic Xinchang–Shengzhou (XS) flood basalts, a small-scale continental flood basalt field in eastern China. The basalts show positive Keywords: 87 86 143 144 143 144 Continental flood basalts correlations between Sr/ Sr and Nd/ Nd, and negative correlations between Nd/ Nd and 176 177 Mantle heterogeneity Hf/ Hf, which deviate from compositional arrays of crustal contamination and instead highlight variations Magmatic recharge in magmatic recharge intensity and mantle source compositions. The lava samples formed by high-volume mag- Crustal contamination matic recharge recorded signals of recycled sediments in the mantle source, which are characterized by moderate Ba/Th (91.9–106.5), excess 208Pb/204Pb relative to 206Pb/204Pb, and excess 176Hf/177Hf relative to 143Nd/144Nd. Thus, we propose that magmatic recharge buffers the original isotopic compositions of magmas against crustal contamination. Identifying and utilizing the isotope systematics of continental flood basalts generated by high volumes of magmatic recharge are thus crucial to trace their mantle sources. © 2017 Elsevier B.V. All rights reserved.

1. Introduction and/or temporary storage in the crust (Arndt et al., 1993; Brandon et al., 1993; Meshesha and Shinjo, 2007; Yu et al., 2015b). Continental flood basalts are thought to be the volcanic manifesta- To recognize effects of crustal contamination during magma evolu- tions of large volumes of magma derived from Earth's mantle including tion, previous studies have examined correlations between SiO2 and mantle compositions with variable elemental and isotopic compositions incompatible element ratios (e.g., Ba/Th and Ce/Pb) and isotopic ratios (e.g., Hooper et al., 2007; Sobolev et al., 2009, 2011; Zhang et al., 2006). (e.g., 87Sr/86Sr and 143Nd/144Nd) (e.g., Arndt and Christensen, 1992; Understanding the compositional record of continental flood basalts Sharma et al., 1991; Xiao et al., 2004). This approach is based on the is thus crucial for a better understanding of mantle compositional principle assumption that continental flood basalt compositions are variations and ultimately the dynamics that lead to these variations. essentially mixtures of two end-members: mantle-derived melts and Radiogenic isotope systematics of melts derived from a mantle continental crust. Thus, ideally, mixing between these two end- source can help provide clues on the formation mechanisms members yields a mixing line indicating crustal contamination, other- (e.g., asthenosphere versus continental lithosphere, identifying the wise insignificant crustal contamination is suggested (e.g., Li et al., properties of the recycled crustal materials in mantle sources) of 2015; Xu et al., 2005). However, mixing or contamination by crustal continental flood basalts (e.g., Chung and Jahn, 1995; Hooper et al., materials in mantle sources can also form geochemical correlations as 2007; Sharma et al., 1992; Xiao et al., 2004). However, crustal contami- mentioned above. Besides of source mixing or crustal assimilation and nation commonly obscures the primary compositions and needs to be fractional crystallization (AFC), another kind of magmatic processes constrained (Carlson et al., 1981; Cox, 1980; Devey and Cox, 1987; for continental flood basalts is magmatic recharge in magma chambers Ramos et al., 2005; Silver et al., 2006). It remains uncertain and debated (e.g., Camp and Roobol, 1989; Devey and Cox, 1987; Kumar et al., 2010). whether the radiogenic isotopic compositions of continental flood Magmatic recharge means continuous or episodic magma replenish- basalts record elemental and isotopic heterogeneities of the mantle ment in the long-lived magma chambers, which are situated above a re- source or if this information is lost during their passage through gion undergoing relatively continuous magmatic fluxing (e.g., Bohrson and Spera, 2003; Davidson and Tepley, 1997; Lee et al., 2014; O'Hara and Mathews, 1981; Spera and Bohrson, 2002; Yu et al., 2015b). ⁎ Corresponding author. Although magmatic recharge commonly occurs in magma chambers E-mail address: [email protected] (L.-H. Chen). that produce flood basalts (Camp and Roobol, 1989; Devey and Cox,

1987; Kumar et al., 2010; Song et al., 2006; Yu et al., 2015a, 2015b)and contamination, thereby isotopic compositions of flood basalts with sig- layered intrusions (e.g., Bohrson and Spera, 2003; Howarth and Prevec, nificant volume recharge can provide constraints on the compositional 2013; Maydagan et al., 2014; Pang et al., 2009; Wark et al., 2007), the variations of the mantle. role of magmatic recharge has not been addressed adequately in previous studies. Magmatic recharge represents a plausible mechanism of di- 2. Background review luting the signal of crustal contamination and resulting in more primitive magma and the composition of the mantle-derived recharge Eastern China is composed, from north to south, of the Xingmeng magma may vary with time (Yu et al., 2015b). Therefore, it is important Block, the North China Craton, the Yangtze Craton, and the Cathysia to understand possible temporal variations in mantle-derived magma Block. Cenozoic basalts that have erupted from monogenetic volcanoes supply in addition to processes and compositional evolution in the crust. or as small-scale flood basalts are widely distributed in eastern China To quantify magmatic recharge and crustal contamination when an (e.g., Liu et al., 1992). According to the geophysical observations, isotopic study is conducted, radiogenic isotopic compositions of Sr, Nd, subducted paleo-Pacific oceanic slab is stagnant at the mantle transition Hf, and Pb of the Xinchang–Shengzhou (XS) flood basalts, a small- zone covering the distribution area of Cenozoic basalts of eastern China scale continental flood basalt field in Zhejiang Province, southeastern (Huang and Zhao, 2006)(Fig. 1a). Geochemical studies further China (Fig. 1a), were analyzed and evaluated. The XS flood basalts are suggested that the formation of these Cenozoic basalts has close known to have been influenced by magma chamber processes relationship with the stagnant slab which contributed recycled crust (i.e., crustal contamination and magmatic recharge) in the lower conti- and sediment materials (e.g., Li et al., 2015; Sakuyama et al., 2013; Xu nental crust (Yu et al., 2015a). Consequently, the XS flood basalts are et al., 2012; Xu, 2014). suitable for investigating the impact of magmatic recharge on the In the Cathysia Block, small-scale flood basalts occur in the coastal isotope systematics of continental flood basalts. Combined with major areas of Zhejiang, Fujian, Guangdong, and Hainan provinces. Among and trace elemental compositions (Yu et al., 2015a), the isotopic them, XS flood basalts crop out in Zhejiang Province of the northeastern compositions of the XS flood basalts give clues to the interplay of crustal Cathysia Block (Fig. 1a). The main body of the XS flood basalts is

‘contamination’, magmatic ‘recharge’, and mantle ‘heterogeneity’ (here- composed of high-SiO2 alkaline/tholeiitic basalts (here we deﬁned after termed the CRH model) and possible short-term variations. It is basalts with SiO2 N 47.5 wt.% and positive correlation between SiO2 and proposed that continuous and signiﬁcant recharge of the basaltic Na2O+K2O as high-SiO2 basalts while basalts with SiO2 b 47.5 wt.% magma chamber can suppress the isotopic variations by crustal and negative correlation between SiO2 and Na2O+K2O as low-SiO2

Fig. 1. Distribution of Cenozoic basalts in eastern China (a) and in the Zhejiang Province (b). Three major wrench faults in the Zhejiang Province are the Jiangshan–Shaoxing fault (A), the Lishui–Yuyao fault (B), and the Zhenhai–Wenzhou fault (C). The studied basalts crop out north and south of the surface trace of the Lishui–Yuyao fault (B). The sampling locations are those of Yu et al. (2015a). The indicated K–Ar and Ar–Ar ages of the basalts are from Ho et al. (2003) and Yu et al. (2015a). White symbols stand for low-SiO2 alkaline basalts, and blue and green symbols stand for high-SiO2 basalts sampled from south and north of the Lishui–Yuyao fault, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) The whole map is modified from Yu et al. (2015a). X. Yu et al. / Lithos 284–285 (2017) 1–10 3 basalts (Fig. 2)). Negative correlation of Ar–Ar ages versus latitudes of high-SiO2 basalts (Fig. 2BinYu et al., 2015a) suggests that the flood basalts were first emplaced in the south (Xinchang), and then moved northward continuously to the north (Shengzhou), across the Lishui– Yuyao trans-lithospheric fault from 9.4 Ma to 3.0 Ma (Fig. 1b). It indicates that the magmatism lasted more than six million years and the volcanic center was migrating northwards (Ho et al., 2003; Yu et al., 2015a).

Besides of low SiO2 contents and negative correlation of SiO2 versus Na2O+K2O(Fig. 2a), low-SiO2 alkaline basalts (including nephelinite, basanite and minor alkaline olivine basalt) are characterized with high

MgO contents (N8.5 wt.%), which are distinct from high-SiO2 alkaline/ tholeiitic basalts (Fig. 2b). These low-SiO2 basalts were erupted sporadically from 10.1 Ma to 3.0 Ma (Yu et al., 2015a) and marginally to the region of ﬂood basalt ﬁeld (Fig. 1b).

Overall, low-SiO2 alkaline basalts are more enriched in highly incompatible elements than high-SiO2 alkaline/tholeiitic basalts (Fig. 3). In a primitive mantle-normalized multi-element plot, low-

SiO2 alkaline basalts are characterized by pronounced negative K, Pb, Zr, Hf, and Ti anomalies and positive Nb, Ta, and Sr anomalies. In com- Fig. 3. Primitive mantle-normalized multi-element diagram of Xinchang–Shengzhou ﬂood basalts. parison, high-SiO2 alkaline/tholeiitic basalts show variable K anomaly, The primitive mantle values are from McDonough and Sun (1995). The trace elemental data less degree of negative Pb, Zr, and Hf and positive Sr anomalies than of basalts are from Yu et al. (2015a). low-SiO2 alkaline basalts (Fig. 3). Among high-SiO2 basalts, high-SiO2 alkaline basalts are more enriched in highly strong incompatible elements and light rare earth elements (LREEs) than high-SiO2 tholeiitic continental crust (Yu et al., 2003, 2015a). All high-SiO2 basalts, have basalts. evolved compositions (e.g., MgO b 8%) and the later erupted basalts

According to the correlations of MgO versus Ni, CaO/Al2O3 are always more evolved than the former erupted. For example, the and Rayleigh fractional calculations for SiO2 versus TiO2 (Fig. S1 of Shengzhou high-SiO2 alkaline/tholeiitic basalts have lower MgO Yu et al., 2015a), the low-SiO2 alkaline basalts are characterized by contents and higher Differentiation Index than the Xinchang high-SiO2 minor fractionation of olivine while the high-SiO2 alkaline/tholeiitic alkaline/tholeiitic basalts in Fig. 2. The above observations conﬁrm that basalts are characterized by 10% to 30% fractionation of olivine and magma chamber processes are necessary in the formation of XS ﬂood clinopyroxene. As most of the basalts are located on the mixing trend basalts (Yu et al., 2015a). formed by nephelinite and the local juvenile lower continental crust in According to the variations of fractional crystallization sensitive the plot of Sm/Nd versus Ce/Pb, both kinds of basalts are proposed to major elemental ratios (e.g., (Na2O+K2O)/MgO; Fig. 2C and D of Yu be geochemically affected by crustal contamination in the lower et al., 2015a) and Differentiation Index (Fig. 2c; Thornton and Tuttle,

Fig. 2. Plots of SiO2 versus Na2O+K2O (a) and MgO (b), latitude versus Differentiation Index and age (c) for the Xinchang–Shengzhou ﬂood basalts. (a) Trend one (1) stands for the low

SiO2 alkaline basalts, and trend two (2) and trend three (3) highlight high-SiO2 alkaline and tholeiitic trends. (c) The axis of age is only valid for high-SiO2 alkaline/tholeiitic basalts which is estimated from the relationship between latitude and age (Age = −15.303 × latitude + 457.2) for high-SiO2 alkaline/tholeiitic basalts in Fig. 2B from Yu et al. (2015a). White circles stand for low-SiO2 alkaline basalts. The blue and green symbols stand for high-SiO2 ﬂood basalts from Xinchang and Shengzhou. Among them, triangle and square symbols stand for alkaline and tholeiitic basalts. Closed symbols represent data of the present study while open symbols represent published data for high-SiO2 alkaline/tholeiitic basalts (Ho et al., 2003). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) 4 X. Yu et al. / Lithos 284–285 (2017) 1–10

1960) relative to the latitude (and the basalt age), we found that elements other than Pb. Detailed chemical separation procedures are the differentiation degree of magma stagnated for about six million given by Kuritani and Nakamura (2002). Thallium was added to correct years from 9.4 Ma to 3.5 Ma. Subsequently, the differentiation mass bias during the measurement. Measured Pb isotopic ratios were re-accelerated until the final eruption of this flood basalt field, which corrected for instrumental mass fractionation by reference to replicate has been explained as response to the decrease of recharge intensity analyses of the standard NIST-981. Detailed measurement procedures and magma temperature (Yu et al., 2015a). In addition, the low-SiO2 are given by White et al. (2000). Measured values for the NIST-981 Pb alkaline basalts erupted sporadically around the margin of the flood standard are 16.9318 ± 0.0003 for 206Pb/204Pb, 15.4858 ± 0.0003 for 207 204 208 204 basalt body among the eruption interval of high-SiO2 alkaline/tholeiitic Pb/ Pb, and 36.6819 ± 0.0008 for Pb/ Pb, respectively. In basalts verifying the continuous partial melting of the mantle source. addition, international standards BHVO-2 and BCR-2 were also tested Thus a magmatic recharge and magma chamber growth model was in this method as unknown samples. Measured values for the BHVO-2 proposed to explain the northward-directed formation and younging Pb standard are 16.6315 ± 0.0010 for 206Pb/204Pb, 15.4878 ± 0.0009 207 204 208 204 of the high-SiO2 alkaline/tholeiitic basalts of XS flood basalts (Yu et al., for Pb/ Pb, and 36.1756 ± 0.0023 for Pb/ Pb. Measured values 2015a), where magmatic recharge plays an important role in holding for the BCR-2 Pb standard are 18.7881 ± 0.0005 for 206Pb/204Pb, the degree of magma evolution unchanged and controlling the eruption 15.6196 ± 0.0004 for 207Pb/204Pb, and 38.8005 ± 0.0010 for 208Pb/ center migration. 204Pb (reference values can be referred to Weis et al., 2006).

In this study, we further subdivided the high-SiO2 alkaline/tholeiitic Hf isotopic data were obtained using a Neptune plus (Thermo basalts of the XS basalt field into the Xinchang high-SiO2 basalts in the Finnigan) multi-collector inductively coupled plasma mass spectrome- south and the Shengzhou high-SiO2 basalts in the north, relative to the ter (MC-ICP-MS). 100 mg basaltic powder was leached for 12 h in position of the Lishui–Yuyao trans-lithospheric fault (Fig. 1), as the warm 2.5 N HCl and dissolved in 15 ml Teflon beakers in an HF–HClO4 Shengzhou high-SiO2 basalts are more evolved than the Xinchang acid mixture at 120 °C for more than 5 days. After evaporation to high-SiO2 basalts (Fig. 2). dryness, all samples were dried at 200 °C in order to break CaF bonds. Finally, the samples were dissolved in 3 N HCl. Hafnium was separated 3. Methods from the rock matrix by ion exchange procedures using an Eichrom® Ln-Spec resin. The detailed analytical procedure for the Hf isotopic Here, newly measured Sr, Nd, Pb, and Hf isotopic compositions are measurement can be seen elsewhere (Yang et al., 2010). Hf isotopic reported for 26 samples. All data are shown in Table S1. Isotopic analy- compositions were normalized to 179Hf/177Hf = 0.7325. The sample ses were performed at the State Key Laboratory for Mineral Deposits results were then normalized to the 176Hf/177Hf value of 0.282160 Research, School of Earth Sciences and Engineering at Nanjing University. (Vervoort et al., 1999) using the daily average of the JMC 475 Hf Basaltic samples were first crushed into gravel-size chips. Clean chips standard. The JMC 475 Hf standard analyzed during the session of the were then pulverized in a corundum mill for isotopic analysis. All chem- sample analyses gave an average value of 176Hf/177Hf = 0.282157 ± ical digestion and separation were carried out in a Class 100 ultra-clean 0.000005. In addition, international standards BHVO-2 and BCR-2 laboratory and the mass spectrometric analyses were performed in a were also used as reference standards. Measured values for BHVO-2 Class 1000 clean laboratories. All reagents used for leaching, dissolution and BCR-2 were 0.283082 ± 0.000004 and 0.282857 ± 0.000006 and separation were twice-distilled, extra-pure grade. All dilutions (reference values can be referred to Weis et al., 2007). were made using ≥18.2 MΩ cm−1 de-ionized water, and all labware The total procedure blanks for Sr, Nd, Hf, and Pb isotopes were less was acid-washed prior to use. than 100 pg, 60 pg, 50 pg, and 50 pg, respectively. Isotope ratios of Sr, Nd were measured on a TRITON (Thermo Finnigan) thermal ionization mass spectrometer (TIMS) in static mode 4. Results with relay matrix rotation on single W (Sr) or double Re (Nd) filaments.

100 mg basaltic powder was leached for 12 h in warm 2.5 N HCl, The low-SiO2 alkaline basalts have variable isotopic compositions for 87 86 and dissolved in a hot HF–HNO3 mixture followed by ion exchange Sr, Nd, Hf and Pb, and the Sr/ Sr ratio ranges from 0.70328 to 0.70435, 207 204 procedures. Sr and Nd isotopic compositions were normalized to εNd from +5.2 to +7.0, εHf from+7.7to+9.2,andthe Pb/ Pb ratio 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219. Detailed analytical pro- from 15.520 to 15.589. The basalts show positive correlations for Nd–Hf cedures for Sr and Nd isotopes can be seen elsewhere (Pu et al., 2005). and Pb–Pb isotope systematics, and a negative correlation for Sr–Nd

During the analytical session for our samples, the measured values for isotope systematics (Fig. 4). In comparison, the high-SiO2 alkaline/ 87 86 the NBS987 Sr standard and JNdi-1 Nd standard were 0.710239 ± tholeiitic basalts show lower Sr/ Sr ratios at a given εNd value and 87 86 143 144 87 86 0.000002 for Sr/ Sr and 0.512128 ± 0.000004 for Nd/ Nd, lower εNd at a given εHf value ( Sr/ Sr = 0.70359–0.70434, εNd = 87 86 143 144 206 204 respectively. Measured Sr/ Sr and Nd/ Nd values for the basaltic +3.5 to +5.4, εHf = +6.2 to +9.1; Fig. 4a, b). For a given Pb/ Pb 207 204 standard BCR-2 were 0.705018 ± 0.000003 and 0.512624 ± 0.000004, ratio, high-SiO2 alkaline/tholeiitic basalts show higher Pb/ Pb and respectively (reference values from Weis et al. (2006) are 0.705019 ± 208Pb/204Pb (207Pb/204Pb = 15.560–15.619; Fig. 4c, d). Within the

0.000016 and 0.512634 ± 0.000012). high-SiO2 alkaline/tholeiitic basalts, the Xinchang high-SiO2 basalts Pb isotopic compositions were obtained using a Neptune plus and the Shengzhou high-SiO2 basalts form two distinct trends in Pb– (Thermo Finnigan) multi-collector inductively coupled plasma mass Pb isotope systematics, characterized by higher 207Pb/204Pb and spectrometer (MC-ICP-MS). 200 mg basaltic powder was leached for 208Pb/204Pb at a given 206Pb/204Pb ratio (Fig. 4c, d). The XS ﬂood basalts

12 h in warm 2.5 N HCl before dissolved in HF–HNO3 acid mixture at thus form three groups with distinct major and trace element composi- 120 °C for more than 36 h. After evaporation to dryness, all samples tions (Yu et al., 2015a) and with distinct Sr, Nd, Hf, and Pb isotopic were repeatedly taken up and dried with concentrated HNO3 with compositions (Figs. 2 and 4). traces of HF in order to break CaF bonds. Finally, the samples were dissolved in HBr–HNO3 acid mixture. Pb was separated from the rock 5. Discussion matrix by chromatographic extraction using an anion exchange resin (200–400 mesh Bio-RAD resin). The chemical procedure of Pb separa- 5.1. Crustal contamination in magma chambers tion from natural rock samples consists of two steps. In the first step, a large column was used to remove most of the elements other than Pb. For the XS flood basalts, a trend of mixing between a MORB&OIB-like In the second step, a small column was used to remove trace contami- mantle component and a lower continental crust component has been nants of elements in the Pb fraction and organic materials derived inferred on the basis of Ce/Pb versus Sm/Nd (Fig. 3 in Yu et al., 2015a). from resin in the first column. A HBr–HNO3 mixture is used to remove The lower continental crust component is represented by the granulitic X. Yu et al. / Lithos 284–285 (2017) 1–10 5

87 86 206 204 207 204 208 204 Fig. 4. Plots of Sr/ Sr versus εNd and εNd versus εHf,and Pb/ Pb versus Pb/ Pb, Pb/ Pb for the Xinchang–Shengzhou flood basalts. In (a) and (b), crustal contamination trends are modeled by mixing between nephelinite and lower crustal granulite from southeastern China (Yu et al., 2003, 2007). 2 sigma standard errors for Sr, Nd, and Hf isotopic compositions are shown. Parameters for simulation can be seen in Table S2. In (c) and (d), the 2 sigma standard error of each samples is smaller than the size of the symbols. xenoliths reported in the basalts from southeastern China, which are (Hf/Hf* = 0.58–0.76 and Ti/Ti* = 0.80–0.97); and super-chondritic characterized by low Ce/Pb b 15.0 (Yu et al., 2003). The high-SiO2 Zr/Hf ratios (41.3–48.1). A carbonated mantle source was proposed to alkaline/tholeiitic basalts have low ratios of Ce/Pb (13.8–22.1) and explain such carbonatite-like geochemical features of nephelinites high Sm/Nd ratios (0.24–0.29), which are comparable to the juvenile and basanites elsewhere in eastern China (e.g., Huang et al., 2015; lower continental crust component as represented by the granulitic Zeng et al., 2010). Addition of carbonatitic components can enrich the xenoliths with Ce/Pb b 15.0, and Sm/Nd N 0.30 (Yu et al., 2003). The depleted mantle in incompatible elements besides of Zr, Hf, and Ti. low-SiO2 alkaline basalts, in contrast, show moderate Ce/Pb ratios Thus melting of such carbonated source generate melts with geochem- between 20.7 and 33.1, possibly suggesting that these basalts were ical features similar to carbonatites (e.g., Dasgupta et al., 2007), which not as contaminated as the high-SiO2 basalts (Yu et al., 2015a). explains the key geochemical features of low-SiO2 alkaline basalts The isotopes (Sr, Nd, and Hf) also indicate that both low-SiO2 from the XS volcanic field. Recently, this proposal has been supported 26 66 alkaline basalts and high-SiO2 alkaline/tholeiitic basalts were affected by prevalent low δ Mg and high δ Zn values of Cenozoic alkaline by crustal contamination (Fig. 4a, b). In the study area, the juvenile basalts from eastern China, because carbonated mantle only can be lower continental crust as represented by granulitic xenoliths and generated by the contribution of recycled carbonates (Li et al., 2016; 87 86 granulitic batholith is characterized by high Sr/ Sr and low εNd,as S.-C. Liu et al., 2016; S.-A. Liu et al., 2016). Thus, the above observations well as low εHf values (Table S2; Xu et al., 2007; Yu et al., 2003, 2007) suggest a carbonated component with depleted isotopic compositions (Fig. 4a, b). Thus contamination by lower continental crust yields a in the asthenosphere source of the studied flood basalts (Yu et al., negative correlation of the Sr–Nd and a positive correlation of the 2015a). Nd–Hf isotope systematics of the basalts (Fig. 4a, b). According to their different isotopic arrays, high-SiO2 alkaline/tholeiitic basalts are less 5.2.2. Crustal contamination or source heterogeneity consistent with such crustal contamination than the low-SiO2 alkaline The isotopic compositions of the Xinchang high-SiO2 alkaline/ basalts (Fig. 4a, b). tholeiitic basalts deviate from the mixing trend of crustal contamination

formed by the low-SiO2 alkaline basalts (Fig. 4a, b). Their unusual 87 86 5.2. Mantle heterogeneity positive correlation between Sr/ Sr and εNd, and negative correlation between εNd and εHf which are inconsistent with the crustal contamina- 5.2.1. Carbonated source with depleted isotopic compositions tion need to be explained (Fig. 4a, b). The decoupling relationship

As the isotopic variations in low-SiO2 alkaline basalts indicate crustal between different isotopic systems has been attributed to recycled contamination, the most depleted isotopic values, represented by two oceanic crust (Lassiter et al., 2003; Simonsen et al., 2000; Xu et al., nephelinite samples, may approach or record the primary isotopic 2012), recycled pelagic sediments (Blichert-Toft et al., 1999; Chauvel compositions of their mantle sources, i.e., a source with low 87Sr/86Sr et al., 2008; Kuritani et al., 2011) or recycled lower continental crust ratios as well as high εNd and εHf values (Fig. 4a, b). Low-SiO2 alkaline (Chen et al., 2009; Zeng et al., 2011). basalts are characterized by enriched in incompatible elements, high To better understand the origin of the isotopic co-variations of high- 87 86 CaO/Al2O3 ratios (N0.8); strongly negative Zr, Hf, and Ti anomalies SiO2 alkaline/tholeiitic basalts, Δ Sr/ Sr and ΔεHf for the XS ﬂood 6 X. Yu et al. / Lithos 284–285 (2017) 1–10

87 86 87 86 Fig. 5. Plots of latitude versus Δ Sr/ Sr and ΔεHf for the Xinchang–Shengzhou ﬂood basalts. Δ Sr/ Sr refers to the relative deviation from the mixing trend formed by low-SiO2 alkaline 87 86 87 86 basalts in Sr–Nd systematics as calculated by Δ Sr/ Sr = ( Sr/ Sr − (895.35 − εNd) / 1263.5) × 1000. ΔεHf refers to the relative deviation from the mantle array in Nd–Hf systematics 87 86 (ΔεHf = εHf − 1.55εNd − 1.21; Vervoort et al., 2011). 2SE for Δ Sr/ Sr and ΔεHf are also presented. Symbols are the same as in Fig. 2. basalts were calculated, estimating the relative deviation from the et al., 2011) or recycled pelagic sediments (Chauvel et al., 2008; Choi mixing trends formed by low-SiO2 alkaline basalts in Sr–Nd systematics et al., 2014) in mantle sources. Here, a contribution of recycled oceanic and the mantle array in Nd–Hf systematics (Vervoort et al., 2011) sediments, rather than recycled lower continental crust, may explain 87 86 87 86 (Δ Sr/ Sr = ( Sr/ Sr − (895.35 − εNd) / 1263.5) × 1000; ΔεHf = the elemental and isotopic signatures of Xinchang high-SiO2 basalts. εHf − 1.55εNd − 1.21). Here the mixing trend formed by low-SiO2 Sediments are usually characterized by moderate Ba/Th and high alkaline basalts in Sr–Nd systematics represents the trend of crustal Th/La ratios (Plank and Langmuir, 1998; Plank et al., 2007) (GLOSS: contamination while the ΔεHf values are calculated relative to the Nd– Ba/Th = 112.3, Th/La = 0.24; average sediments from the west Paciﬁc Hf trend formed by global samples (the mantle array; Vervoort et al., Ocean: Ba/Th = 105.6, Th/La = 0.15). As Ba/Th and Th/La ratios are 87 86 2011). The Xinchang high-SiO2 basalts show Δ Sr/ Sr values between not commonly affected by partial melting and fractional crystallization −0.85 and −1.91, and ΔεHf between −1.19 and +1.38, showing (Kuritani et al., 2011; Plank, 2005), they are considered as ideal tools higher degrees of deviation than the Shengzhou high-SiO2 basalts, to trace the source differences besides identifying the signal of crustal 87 86 which show Δ Sr/ Sr between −0.54 and −1.34, and ΔεHf between contamination. As lower continental crust in the study area is character- −3.04 and −1.32 (Fig. 5). It indicates, besides crustal contamination, ized by high Ba/Th and low Th/La ratios (Yu et al., 2003)(Ba/ThN 470, an alternative component, e.g. a kind of melts with distinct isotopic Th/La b 0.08), and low εHf, contamination of lower continental crust compositions from the carbonated mantle endmember, is needed to can generate negative εHf–Ba/Th (Fig. 6a) and positive εHf–Th/La corre- explain the isotopic variations of high-SiO2 basalts. Therefore, Xinchang lations (Fig. 6b). Accordingly, contamination by lower continental crust high-SiO2 basalts record not only crustal contamination, but also the can explain the geochemical trends observed for the low-SiO2 alkaline source heterogeneity. basalts and the Shengzhou high-SiO2 basalts. However, the Xinchang high-SiO2 basalts show largely invariant Ba/Th (91.9–106.5) and Th/La 5.2.3. Recycled sediments in the source (0.145–0.150) ratios, which are comparable to those of average

Intraplate basalts with positive ΔεHf values can be interpreted to sediments from the west Paciﬁc, and we thus infer that they comprise indicate recycled old lower continental crust (Chen et al., 2009; Zeng asigniﬁcant component of low Ba/Th and high Th/La sediments

Fig. 6. Plots of εHf versus Ba/Th and Th/La ratios for the Xinchang–Shengzhou flood basalts. Data for the lower continental crust from southeastern China are referred to Yu et al. (2003, 2007). Data for GLOSS and average sediments of the west Pacificaremodified from Plank and Langmuir (1998). Trends for primitive mantle (PM) and depleted mantle (DM) are estimated from Sun and McDonough (1989) and Workman and Hart (2005). Symbols are the same as in Fig. 2. X. Yu et al. / Lithos 284–285 (2017) 1–10 7

and Langmuir, 1998; Plank et al., 2007; Vervoort et al., 2011). Direct mixing between depleted mantle and recycled sediments, however, cannot generate the observed isotopic co-variations for Xinchang

high-SiO2 basalts (Fig. 4a, b). Here, we propose instead a two-stage mixing model that explains the isotopic characteristics of the Xinchang

high-SiO2 basalts (Fig. 8). During the ﬁrst stage, recycled oceanic sediments (~2%) mixed with the depleted asthenospheric mantle (~98%) and produced mantle sources with relatively enriched isotopic

signatures (lower εNd but higher εHf values than carbonated components in mantle sources) (“enriched asthenosphere” in Fig. 8). During the second stage, melts derived from these enriched isotopic mantle

sources mixed with 20% to 30% crustally contaminated low-SiO2 melts represented by the low-SiO2 alkaline basalts (Fig. 8). The high-SiO2 Xinchang basalts represent the ﬁnal products of this two-stage mixing. Fig. 8 and Table S2 present the inferred mixing trends and parameters in detail.

5.2.4. Isotopic response for the drop off of magmatic recharge

Compared to Xinchang high-SiO2 basalts, Shengzhou high-SiO2 basalts show lower 206Pb/204Pb ratios (Fig. 4) and lower Δ87Sr/86Sr Fig. 7. Simpliﬁed three endmembers mixing model for Pb isotopes. Endmembers of Δε ancient (~1.5 Ga) and recent sediments are derived from Kuritani et al. (2011) (ancient and Hf values (Fig. 5). Since Xinchang high-SiO2 basalts and 206 204 207 204 sediments: Pb/ Pb = 16.757, Pb/ Pb = 15.478, Pb = 18 ppm; recent Shengzhou high-SiO2 basalts are distributed along the two sides of the sediments: 206Pb/204Pb = 18.827, 207Pb/204Pb = 15.672, Pb = 18 ppm). The starting Lishui–Yuyao fault, reasonably, their difference in isotopic compositions endmember is the depleted mantle material (DMM) from Workman and Hart (2005) can be generated by the chemical heterogeneity of the lithosphere, with assumed Pb content (206Pb/204Pb = 18.275, 207Pb/204Pb = 15.486, Pb = 1 ppm). Symbols are the same as in Fig. 2. e.g. lower continental crust as the contaminants. However, such a proposal is not supported by the mixing trend of the Shengzhou high-

SiO2 basalts on the plots of εHf versus Ba/Th and Th/La ratios (Fig. 6). On the contrary, they can be generated by mixing between Xinchang (Fig. 6). This inference is consistent with the deduction that recycled high-SiO2 basalts and lower crustal materials from study area. This is 206 204 crustal material forms part of the mantle source of alkaline basalts consistent with their lower Pb/ Pb ratios because old lower from the study area as was inferred on the basis of Os isotopes continental crust is characterized by their unradiogeneic Pb isotopic (187Os/188Os = 0.15–0.90) (Li et al., 2015). Here Pb isotopic systematics features (e.g., Liu et al., 2008). Therefore, Shengzhou high-SiO2 basalts ﬂ are used to discover the nature of the sediments qualitatively (Fig. 7). re ect a more crustal contaminated feature than Xinchang high-SiO2 We cited the ancient (~1.5 Ga) and recent sediments endmembers basalts due to the deceleration of magmatic recharge (Yu et al., 2015a). from Kuritani et al. (2011) (ancient sediments: 206Pb/204Pb = 16.757, In summary, a heterogeneous mantle source has been recorded in 207Pb/204Pb = 15.478, Pb = 18 ppm; recent sediments: the XS basalts. The Xinchang high-SiO2 basalts indicate a mantle source 206Pb/204Pb = 18.827, 207Pb/204Pb = 15.672, Pb = 18 ppm). The with more enriched isotopic signatures than that of the low-SiO2 DMM (depleted MORB mantle) is regarded as the starting endmember alkaline basalts (Fig. 8). In addition to crustal contamination, they thus record local and small-scale mantle heterogeneity. for high-SiO2 alkaline/tholeiitic basalts. Therefore, three endmembers mixing lines could be modeled (Fig. 7), where we can see that the recycled crustal materials are mainly composed of recent sediments. 5.3. Magmatic recharge buffers isotope systematics against crustal Sediments characteristically have higher Rb/Sr, and lower Sm/Nd contamination and Lu/Hf ratios than depleted mantle (Plank and Langmuir, 1998; Workman and Hart, 2005). Accordingly, sediments represent reservoirs To better understand the relationship between isotope systematics, with high 87Sr/86Sr, and low 143Nd/144Nd and 176Hf/177Hf ratios (Plank and the spatial and temporal distribution of the XS ﬂood basalts, a

Fig. 8. Modeling results for mantle source mixing and crustal contamination using Sr, Nd, and Hf isotope systematics. The source mixing trend is modeled by mixing between depleted mantle material and recycled sediment from the west Paciﬁc Ocean (Plank and Langmuir, 1998; Vervoort et al., 2011; Workman and Hart, 2005). The crustal contamination trend is simulated as in Fig. 4. Symbols are the same as in Fig. 2. 8 X. Yu et al. / Lithos 284–285 (2017) 1–10

Fig. 9. Cartoon illustrates the magma chamber processes to explain the formation of XS ﬂood basalts. (a) Primitive low-SiO2 melts were trapped in the lower crust and formed small magma reservoirs which were contaminated by the crust. The erupted magmas produced the low-SiO2 alkaline basalts. (b) The small magma reservoir was recharged by melts from mantle sources which were enriched by recycled sediments, resulting into an increase in chamber size. The erupted magmas produced the Xinchang high-SiO2 basalts. (c) With the decrease of the magmatic recharge, the strengthening of the crustal contamination renewed. The erupted magmas then produced the Shengzhou high-SiO2 basalts.

three-stage-evolution model is proposed: infancy (Eq. (5-1)), growth ‐ þ contamination ‐ Xinchang high SiO2 melts Crust → Shengzhou high SiO2 basalts (Eq. (5-2)), and decline (Eq. (5-3)), as follows. ðÞEnlarged magma chambers ðÞDeclined magma chambers ð5 3Þ

contamination During the ﬁrst stage, partial melting of carbonated mantle generat- Low‐SiO2 alkaline melts þCrust Low‐SiO2 alkaline basalts ðÞ→ ðÞ Carbonated mantle Small magma chambers ed low-SiO2 alkaline melts. Some of these primitive low-SiO2 melts ð5 1Þ were trapped in the lower continental crust, where they formed small magma reservoirs and were contaminated by crust (Fig. 9a). The

erupted magmas produced the low-SiO2 alkaline basalts with negative ‐ þ ‐ recharge ‐ correlations in Sr and Nd isotopic compositions and positive correla- Low SiO2 alkaline melts High SiO2 melts → Xinchang high SiO2 basalts ðÞSmall magma chambers ðÞEnriched mantle ðÞEnlarged magma chambers tions in Nd and Hf isotopic compositions (Eq. (5-1)). In the second ð5 2Þ stage, the small magma chambers were recharged from the mantle

Fig. 10. Continuously erupted sequence from the Emeishan large igneous province divided into upper and lower parts (data from Song et al. (2006)). MgO, Ni and (Na2O+K2O)/MgO were selected to infer the role of magmatic recharge. Nb/La and εNd were selected to detect the signal of crustal contamination. The indicated average lower continental crust composition is from Rudnick and Gao (2003). X. Yu et al. / Lithos 284–285 (2017) 1–10 9 source beneath by continuously partial melting (Fig. 9b), resulting in an location of his samples. Two anonymous reviewers are acknowledged increase in chamber size via melt supply from mantle sources which for their constructive and helpful comments. This study was supported were enriched by recycled sediments (Eq. (5-2)). Subsequently, the by NSFC (grants 41430208, 41688103 and 41202039). erupting magmas produced the Xinchang high-SiO2 basalts (Figs. 5, 6 and 8), while the residual magma chambers were growing northwards as evidenced by coupled basalt distributions, age correlations and ele- Appendix A. Supplementary data mental variations (Yu et al., 2015a). In this stage, the unusual Sr–Nd and Nd–Hf istopic correlations were generated (Fig. 4). The final stage Supplementary data to this article can be found online at http://dx. is characterized by waning magmatic recharge and renewed strength- doi.org/10.1016/j.lithos.2017.03.027. ening of the signal of crustal contamination in the magma chambers (Fig. 9c). The magmas with stronger signal of crustal contamination References erupted as the Shengzhou high-SiO2 basalts (Eq. (5-3)). fl We therefore suggest that mixing of variable volumes of juvenile Arndt, N.T., Christensen, U., 1992. 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