origin of low Î´26mg cenozoic basalts from south china block...

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Geochimica et Cosmochimica Acta 164 (2015) 298–317

Origin of low d26Mg Cenozoic basalts from South ChinaBlock and their geodynamic implications

Jian Huang a, Shu-Guang Li b,a,⇑, Yilin Xiao a,⇑, Shan Ke b, Wang-Ye Li a, Ye Tian a

a CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences,

University of Science and Technology of China, Hefei 230026, Chinab State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China

Received 13 November 2014; accepted in revised form 30 April 2015; available online 7 May 2015

Abstract

Origin of low d26Mg basalts is a controversial subject and has been attributed to interaction of isotopically light carbonatiticmelts derived from a subducted oceanic slab with the mantle (Yang et al., 2012), or alternatively, to accumulation of isotopicallylight ilmenite (FeTiO3) in their mantle source (Sedaghatpour et al., 2013). To study the origin of low d26Mg basalts and evaluatewhether Mg isotope ratios of basalts can be used to trace deeply recycled carbon, high-precision major and trace element and Mgisotopic analyses on the Cenozoic alkaline and tholeiitic basalts from the South China Block (SCB), eastern China have beencarried out in this study. The basalts show light Mg isotopic compositions, with d26Mg ranging from �0.60 to �0.35. The rel-atively low TiO2 contents (<2.7 wt.%) of our basalts, roughly positive correlations between d26Mg and Ti/Ti* and their constantNb/Ta ratios (16.4–20) irrespective of variable TiO2 contents indicate no significant amounts of isotopically light ilmenite accu-mulation in their mantle source. Notably, the basalts display negative correlations between d26Mg and the amounts of totalalkalis (i.e., Na2O + K2O) and incompatible trace elements (e.g., Ti, La, Nd, Nb, Th) and trace element abundance ratios(e.g., Sm/Yb, Nb/Y). Generally, with decrease of d26Mg values, their Hf/Hf* and Ti/Ti* ratios decrease, whereas Ca/Al andZr/Hf ratios increase. These features are consistent with incongruent partial melting of an isotopically light carbonated mantle,suggesting that large variations in Mg isotope ratios occurred during partial melting of such carbonated mantle under high tem-peratures. The isotopically light carbonated mantle were probably formed by interaction of the mantle with low d26Mg carbon-atitic melts derived from the deeply subducted low d26Mg carbonated eclogite transformed from carbonate-bearing oceaniccrust during plate subduction. As only the Pacific slab has an influence on both the North China Block (NCB) and SCB,our results together with the study of Yang et al. (2012) demonstrate that the recycled carbonatitic melts might have originatedfrom the stagnant Pacific slab beneath East Asia in the Cretaceous and Cenozoic and that a widespread carbonated upper man-tle exists beneath eastern China, which may serve as the main source for the <110 Ma basalts in this area. Thus, our studydemonstrates that Mg isotope ratios of basalts are a powerful tool to trace deeply recycled carbon.� 2015 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.gca.2015.04.054

0016-7037/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors at: State Key Laboratory of GeologicalProcesses and Mineral Resources, China University of Geo-sciences, Beijing 100083, China (S.-G. Li) and CAS Key Labora-tory of Crust-Mantle Materials and Environments, School of Earthand Space Sciences, University of Science and Technology ofChina, Hefei 230026, China (S.-G. Li, Y. Xiao).

E-mail addresses: [email protected] (S.-G. Li), [email protected] (Y. Xiao).

1. INTRODUCTION

Magnesium (Mg) is a major constituent of the mantle(MgO = 37.8 wt.%, McDonough and Sun, 1995) and thecontinental crust (MgO = 4.66 wt.%, Rudnick and Gao,2003) and the second most important cation in the seawater(MgO = �2.2 wt.%, Millero, 1974). It has three stable iso-topes, 24Mg, 25Mg and 26Mg, with natural abundances of


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J. Huang et al. / Geochimica et Cosmochimica Acta 164 (2015) 298–317 299

78.99%, 10.00% and 11.01%, respectively (Rosman andTaylor, 1998). The relatively mass difference between 24Mgand 26Mg is �8%, large enough to produce significant Mgisotope fractionation in cosmochemical, geochemical andbiological processes. Mg isotope fractionation has been usedto play constraints on a variety of scientific issues such as theevolution of the early solar system (e.g., Lee et al., 1976;Bizzarro et al., 2004), equilibrium temperatures of metamor-phic and mantle rocks (e.g., Li et al., 2011; Huang et al.,2013), continental weathering (e.g., Tipper et al., 2006;Teng et al., 2010a; Huang et al., 2012a; Liu et al., 2014;Wimpenny et al., 2014a), plant growth (e.g., Black et al.,2006; Bolou-Bi et al., 2010), enzyme synthesis(Buchachenko et al., 2008) and paleoclimate changes(Saenger and Wang, 2014).

With respect to the cosmochemistry and geochemistry ofMg isotopes, one of the most important findings is that somehigh-Ti lunar basalts and some terrestrial basalts have verylight Mg isotopic compositions with d26Mg as low as �0.60(Yang et al., 2012; Sedaghatpour et al., 2013). The interpre-tations for the origins of the low d26Mg basalts are inconsis-tent. Because carbonate rocks (e.g., dolostone and limestone)and minerals (e.g., calcite, aragonite, dolomite and magne-site) have extremely light Mg isotopic compositions, withd26Mg varying widely from �5.54 to �0.47 (e.g., Galyet al., 2002; Young and Galy, 2004; Tipper et al., 2006;Pogge von Strandmann, 2008; Higgins and Schrag, 2010;Jacobson et al., 2010; Ke et al., 2011; Pokrovsky et al.,2011; Wombacher et al., 2011), the low d26Mg terrestrialbasalts from the North China Block (NCB), eastern Chinahave been suggested to result from interaction of their mantlesource with isotopically light carbonatitic melts derived fromthe subducted oceanic slab (Yang et al., 2012). However,based on the large Mg isotope fractionation recorded inhigh-Ti and low-Ti lunar basalts, with the former generallyhaving much lower d26Mg values than the latter (�0.59 to�0.37 vs. �0.33 to �0.02, Sedaghatpour et al., 2013), thelow d26Mg high-Ti lunar basalts have been suggested to orig-inate from an isotopically light mantle source produced bycrystallization of ilmenite (FeTiO3) with low d26Mg valuesat the late stage in the lunar magma ocean (Sedaghatpouret al., 2013). To evaluate wheather Mg isotope ratios ofbasalts can be used to trace deeply recycled carbon, it is nec-essary to re-evaluate whether the low d26Mg basalts fromeastern China were caused by mixture of isotopically lightcarbonatitic melts into their mantle source as suggested byYang et al. (2012), or alternatively, by accumulation of iso-topically light ilmenite in their mantle source(Sedaghatpour et al., 2013), although it is not sure whetherilmenite has a light Mg isotopic composition, because sofar no Mg isotopic data for it has been reported.

Here, we present high-precision Mg isotopic analyses ona suite of well-characterized Cenozoic alkaline and tholei-itic basalts from the South China Block (SCB), easternChina. The basalts show a large variation in major andtrace element geochemistry as well as similar depletedSr-Nd isotopic compositions (e.g., Zou et al., 2000; Chenet al., 2009; Wang et al., 2011). Previous studies havedemonstrated that a carbonated mantle may be the mainsource for the alkaline basalts (Chen et al., 2009; Zeng

et al., 2010; Yang et al., 2012; Sakuyama et al., 2013).Our results show that these basalts have low d26Mg values.Interestingly, a negative correlation between d26Mg andTiO2 exists in the basalts studied here. Thus, the Mg iso-topic compositions of these basalts firstly allow us tore-evaluate whether the low d26Mg basalts were caused byrecycled carbonates through oceanic plate subduction oraccumulation of isotopically light ilmenite in their mantlesource. Secondly, we refer the geodynamic implicationsdeduced from the low d26Mg basalts from eastern China.

2. GEOLOGICAL SETTINGS AND SAMPLE

DESCRIPTIONS

In eastern China, the Cenozoic volcanic rocks are widelydistributed along the coastal provinces and adjacent off-shore shelf extending over 4000 km from HeilongjiangProvince in the north to Hainan island in the south in theeastern edge of the Eurasian continent (Fig. 1). They consti-tute an important part of the volcanic belt of the westerncircum-Pacific rim and are one of the world’s presentlyactive tectono-magmatic regions (e.g., Zhou andArmstrong, 1982). The Cenozoic volcanic rocks are mainlyalkaline basalts that are thought to represent melts derivedfrom the upper mantle, given their depleted Sr-Nd isotopiccompositions and OIB-like trace-element signatures in spi-dergram (e.g., enrichment in Nb, Ta and LREEs, and nega-tive K and Pb anomalies; e.g., Zhou and Armstrong, 1982;Peng et al., 1986; Liu et al., 1994; Zou et al., 2000; Xuet al., 2005; Tang et al., 2006; Chen et al., 2009; Zenget al., 2010, 2011; Wang et al., 2011).

The samples investigated in this study were collected fromPingmingshan, Anfengshan, Fangshan, Chongren, andLongyou from the SCB (Fig. 1). K-Ar dating results showthat basalts from Pingmingshan and Anfengshan have agesof 7.3–12.3 and 4.0–6.4 Ma, respectively (Chen and Peng,1988; Jin et al., 2003), whereas those from Fangshan,Chongren and Longyou have ages of 2.9–3.5, �26.4, and9.0–9.4 Ma, respectively (Chen and Peng, 1988; Ho et al.,2003). Mantle peridotite xenoliths are common in all locali-ties (e.g., Qi et al., 1995; Jin et al., 2003; Reisberg et al., 2005).

Twenty-three basaltic samples were selected for investi-gations, and all of them are of the porphyritic texture.Most of the studied samples are fresh and unaltered.Exceptions are samples 13AFS9-10 from Anfengshan,10LYSK11 from Longyou, which are altered with iddingsi-tization of olivine phenocrysts. The phenocrysts consist pre-dominantly of olivine in the Anfengshan basalts, of olivineand clinopyroxene in the Fangshan and Longyou basalts,and of olivine, clinopyroxene and plagioclase in thePingmingshan and Chongren basalts. The groundmass inthese basalts is variable and mainly consists of plagioclase,olivine, augite, nepheline, magnetite and glass.

3. ANALYTICAL METHODS

3.1. Major and trace element analysis

The samples were sawed into slices and only centralfresh parts were used for bulk-rock analyses. The pieces

Fig. 1. Simplified geological map of eastern China that mainly consists of the SCB, the NCB, and NE China (i.e., the Xing-Meng Block). TheNCB consists of the Western Block (WB), the Trans-North China Orogen (TNCO) and the Eastern Block (EB). The regions where basaltswere sampled for Mg isotope investigation in this study are marked as yellow triangles.

300 J. Huang et al. / Geochimica et Cosmochimica Acta 164 (2015) 298–317

were crushed in a corundum jaw crusher to 60 mesh, andthen �60 g of each crushed sample was powdered in anagate ring mill to <200 mesh in size. Bulk rock abundancesof major elements were determined using an X-ray fluores-cence spectrometer (XRF) on glass disks at the laboratoryof ALS minerals at Guangzhou. Pre-ignition was used todetermine the loss on ignition (LOI) prior to major elementanalyses. Accuracy and precision for major oxides are gen-erally better than 1% based on replicate analyses of certifiedUSGS rock standards. Bulk rock trace element data wereobtained by an ELAN DRCII inductively coupledplasma-mass spectrometry (ICP-MS) at the University ofScience and Technology of China (USTC) after ultrapureacid digestion (HNO3 + HF + HClO4) of sample powders

(�50 mg) in Teflon bombs. Analytical procedures weredescribed in detail by Huang et al. (2012b). The measuredvalues of international USGS standards (BHVO-2 andBIR-1) are in satisfactory agreement with the recommendedvalues within error, and the precision and accuracy formajority of trace elements analyzed are better than 6%(Table S1 in Supplementary Materials).

3.2. Mg isotope analysis

Magnesium isotopic analyses were performed at theState Key Laboratory of Geological Processes andMineral Resources, China University of Geosciences(Beijing) (CUGB), following the procedures very similar


to those established by Yang et al. (2009), Teng et al.(2010b) and Teng and Yang (2014). A brief description isgiven below.

All chemical procedures were conducted in a clean lab-oratory environment at CUGB. Sample powders were dis-solved in Savillex screw-up beakers in a mixture ofconcentrated HF–HCl–HNO3. Chemical separation ofMg was achieved by cation exchange chromatography withBio-Rad 200–400 mesh AG50W-X8 pre-cleaned resin in 1NHNO3 media. The same column procedure was processedtwice for all samples in order to obtain a pure Mg solutionfor mass spectrometry and to check the efficiency of ourcolumn to separate Mg from interference cations. Theeluted solutions were firstly evaporated to dryness and thenre-dissolved in 3% HNO3, ready for final dilution immedi-ately prior to analysis. Three USGS reference materials(i.e., BHVO-2, AGV-2 and GSP-2) were processed throughcolumn chemistry with each batch of the investigated sam-ples. The total procedural blanks during the course of thisstudy were �8 ng, comparable to that of Teng et al.(2010b).

Magnesium isotopic compositions were measured by thesample-standard bracketing method on a Neptune Plasma

MC-ICPMS in a low-resolution mode. The in-run precisionon the 26Mg/24Mg ratio for a single block of 40 ratios is< ±0.02& (2SD). The internal precision on the measured26Mg/24Mg ratio based on 4 repeated analyses of the samesample solution during analytical sessions of this study, is6 ±0.08& (2SD, Table. 2). The results are reported inthe conventional d notation that is defined asdXMg = [(XMg/24Mg)sample/(

XMg/24Mg)DSM3 � 1] � 1000,where X = 25 or 26, and DSM3 is an international refer-ence of solution made from pure Mg metal (Galy et al.,2003). Analyses of the well-characterized USGS Mg rockstandards in this study yielded d26Mg = �0.25 ± 0.05&

(2SD, n = 2) for BHVO-2, �0.15 ± 0.03& (2SD, n = 2)for AGV-2 and �0.03 ± 0.09& for GSP-2 (Table 2), whichare in excellent agreement with previously published valueswithin error (e.g., Teng et al., 2007, 2010a,b, 2015; Huanget al., 2009; An et al., 2014).

4. RESULTS

Results for bulk-rock major and trace element concen-trations and Mg isotopic compositions of the studiedbasalts are listed in Tables 1 and 2, respectively.

4.1. Major and trace elements

The investigated samples have a large compositionalrange of SiO2 (40.6–51.1 wt.%) and high contents of MgO(6.91–13.1 wt.%) and TiO2 (1.81–2.64 wt.%). They alsoshow high total alkalis (Na2O + K2O = 4.26–9.06 wt.%),with Na2O/K2O ratios ranging from 1.4 to 3.1, indicatingtheir alkali-rich and high-sodium nature. Following thenomenclature of Le Bas (1986), samples fromAnfengshan, Pingmingshan and Longyou are basanites,whereas those from Fangshan are trachybasalt, and thosefrom Chongren are normal basalts (Fig. 2). All basaltsare alkaline except for samples from Chongren, which are

tholeiites (Fig. 2). No clear correlation was observedbetween MgO and SiO2, TiO2, Fe2O3, CaO and K2O, whileAl2O3 displays a slightly negative correlation with MgO(Fig. 3).

In the chondrite-normalized REE diagram (Fig. 4a), allbasalts show enrichment of LREEs over HREEs([La/Yb]N = 6.0–32), with no significant Eu or Ce anomaly.In primitive mantle-normalized spidergram (Fig. 4b), allbut rocks from Chongren resemble many ocean islandbasalts in terms of enrichment in Nb and Ta and depletionin K and Pb relative to LREEs. Additionally, the basaltsgenerally show negative Zr, Hf and Ti (Hf/Hf* = 0.49–0.86, Ti/Ti* = 0.32–1.03, indexes for Hf and Ti anomalies,respectively) and high Ca/Al ratios (0.47–0.81) (Fig. 5).Nb/U and Ce/Pb ratios of the Cenozoic basalts range from36 to 52 and 13 to 34, respectively (Table 1), similar to thoseof MORBs and OIBs (Nb/U = 47 ± 10, Ce/Pb = 25 ± 5,Hofmann et al., 1986), but much higher than those of con-tinental crust (Nb/U = 6.2, Ce/Pb = 4, Rudnick and Gao,2003).

4.2. Magnesium isotopes

In the plot of d25Mg vs. d26Mg (Fig. 6a), the basalts andthe USGS standards fall along the terrestrial equilibriummass fractionation line with a slope of 0.521, similar to pre-vious studies of natural and synthetic samples (e.g., Youngand Galy, 2004; Yang et al., 2009, 2012; Li et al., 2010;Teng et al., 2010a,b). The D25Mg0 (D25Mg0 = d25Mg0 � 0.521d26Mg0, where dXMg0 = 1000 � ln[(dXMg + 1000)/1000], X = 25 or 26, Young and Galy, 2004) values forall basalts range from �0.01 to 0.04, very close to zero(Table 2). In general, our Cenozoic basalts have highly vari-able but overall light Mg isotopic compositions, with d26Mgvalues ranging from �0.60 to �0.35 (Fig. 6). These d26Mgvalues are similar to those of the <110 Ma basalts fromthe NCB (�0.60 to �0.42, Yang et al., 2012) but obviouslylighter than that of the normal mantle (� �0.25, Teng et al.,2007, 2010b; Handler et al., 2009; Bourdon et al., 2010) andthe >120 Ma basaltic rocks from the NCB (�0.27 ± 0.05,n = 5, 2SD, Yang et al., 2012) (Fig. 6b). Interestingly, theMg isotopic compositions of the basalts are negatively cor-related with the abundance of total alkalis (Na2O + K2O)and incompatible elements (e.g., La, Nd, Ti, Nb, Th) as wellas trace element abundance ratios of Sm/Yb and Nb/Y(Figs. 7 and 8).

5. DISCUSSION

5.1. Effects of shallow-level processes

5.1.1. Post-magmatic alteration

Loss on ignition (LOI) of our basalts varies from �0.08to 4.94 wt.%, suggesting that some samples have experi-enced alterations, manifested by the transformation of oli-vine phenocrysts to low-T iddingsite in samples13AFS9-10 and 10LYSK11. However, good correlationsbetween abundances of the fluid-mobile elements (e.g.,Ba, Sr, Pb, Th, U, La, Nd) and the fluid-immobile elementNb (Fig. 9) suggest that the effect of alteration, which

Table 1Major and trace element concentrations of the Cenozoic basalts from the South China Block.

Sample a 13PMS1 13PMS1R 13PMS2 13PMS3 13PMS4 13PMS5 13PMS6 13PMS7

Location Pingmingshan

Major element (wt.%)

SiO2 41.02 40.86 41.08 41.05 41.14 41.07 40.91TiO2 2.43 2.39 2.43 2.41 2.41 2.41 2.39Al2O3 11.71 11.70 11.74 11.86 11.73 11.66 11.58Fe2O3 13.34 13.18 13.23 13.00 13.20 13.35 13.24MnO 0.20 0.20 0.20 0.20 0.20 0.20 0.20MgO 10.03 9.93 10.00 9.71 10.11 10.10 10.28CaO 9.52 9.36 9.43 9.58 9.47 9.50 9.53Na2O 4.73 4.50 4.63 4.29 4.43 4.60 4.22K2O 2.23 2.38 2.32 2.46 2.27 2.33 2.18P2O5 1.25 1.25 1.29 1.23 1.26 1.28 1.26LOI 3.05 3.22 3.24 3.36 3.35 3.00 3.61Total 99.51 98.97 99.59 99.15 99.57 99.50 99.40Na2O + K2O 6.96 6.88 6.95 6.75 6.70 6.93 6.40Na2O/K2O 2.12 1.89 2.00 1.74 1.95 1.97 1.94Mg#b 0.64 0.64 0.64 0.64 0.64 0.64 0.65Ca/Alc 0.74 0.73 0.73 0.74 0.74 0.74 0.75

Trace element (ppm)

Li 10.4 11.2 11.9 9.76 11.7 11.7 12.2 12.2Sc 12.9 12.8 12.6 14.4 12.5 12.7 13.0 13.2V 119 122 130 136 125 130 133 139Cr 197 203 217 225 198 216 223 232Ni 193 197 202 217 197 206 211 220Cu 56.1 57.9 51.8 45.1 54.5 51.6 53.5 53.7Rb 34.7 35.9 38.5 39.3 45.9 37.9 38.9 41.7Sr 1211 1275 1490 1557 1583 1348 1417 1420Y 30.2 31.8 32.9 33.6 33.6 32.7 33.3 33.3Zr 307 316 335 312 336 328 333 334Nb 112 113 124 116 126 119 120 119Cs 1.49 1.41 1.49 1.32 1.49 1.47 1.50 1.36Ba 642 613 672 592 697 648 634 634La 77.6 73.0 78.7 71.4 79.1 75.9 75.2 75.5Ce 139 131 141 129 127 137 127 135Pr 15.6 14.8 15.9 14.1 16.0 15.6 15.4 15.3Nd 64.3 60.5 65.1 61.8 65.5 63.9 62.9 62.6Sm 14.7 13.7 14.9 13.8 15.0 14.6 14.3 14.2Eu 4.58 4.28 4.68 4.43 4.67 4.57 4.46 4.49Gd 13.1 12.0 13.0 14.6 13.1 12.8 12.5 12.6Tb 1.80 1.65 1.79 1.62 1.80 1.75 1.70 1.73Dy 8.67 8.00 8.65 7.51 8.66 8.38 8.24 8.28Ho 1.27 1.17 1.28 1.16 1.26 1.22 1.20 1.20Er 2.85 2.64 2.87 2.60 2.91 2.78 2.73 2.75Tm 0.32 0.29 0.32 0.28 0.32 0.31 0.30 0.31Yb 1.75 1.60 1.75 1.74 1.79 1.70 1.67 1.69Lu 0.22 0.20 0.21 0.26 0.22 0.21 0.21 0.21Hf 6.59 6.14 6.71 6.14 6.65 6.53 6.45 6.43Ta 6.46 5.99 6.71 5.96 6.68 6.44 6.28 6.27Pb 5.86 5.57 5.47 5.71 5.58 5.23 5.34 5.31Th 11.1 10.8 11.6 11.0 11.7 11.3 11.1 11.2U 2.81 2.73 3.13 3.18 3.09 3.14 3.09 3.06Nb/U 39.9 41.5 39.7 36.4 40.8 37.9 38.9 38.9Ce/Pb 23.7 23.5 25.8 22.6 22.8 26.2 23.8 25.4La/Yb 44.3 45.6 45.0 41.0 44.2 44.6 45.0 44.7Sm/Yb 8.4 8.6 8.5 7.9 8.4 8.6 8.6 8.4Nb/Ta 17.3 18.9 18.5 19.4 18.9 18.5 19.1 19.0Zr/Hf 46.6 51.5 49.9 50.9 50.5 50.2 51.6 51.9Ti/Ti*d 0.46 0.45 0.43 0.45 0.46 0.47 0.47Hf/Hf*e 0.54 0.54 0.53 0.53 0.54 0.54 0.54

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Table 1 (continued)

Sample 13PMS8 13AFS1 13AFS2 13AFS3 13AFS4 13AFS9 13AFS10 10FS6

Location Pingmingshan Anfengshan Fangshan

Major element (wt.%)SiO2 41.17 44.46 44.41 42.41 42.36 40.86 40.65 46.56TiO2 2.64 2.36 2.37 2.49 2.49 2.34 2.32 2.12Al2O3 12.48 12.05 12.04 11.89 11.86 11.73 11.51 13.89Fe2O3 14.58 12.48 12.50 13.06 13.04 12.57 12.55 11.35MnO 0.22 0.20 0.20 0.21 0.21 0.20 0.20 0.15MgO 8.03 7.26 7.24 8.44 8.36 10.54 10.13 9.72CaO 9.24 8.77 8.74 9.08 9.08 9.61 9.54 7.43Na2O 6.44 6.19 6.22 5.59 5.61 3.73 4.20 3.66K2O 2.46 2.87 2.78 2.15 2.17 1.88 2.01 2.60P2O5 1.13 1.43 1.42 1.27 1.27 1.35 1.44 0.54LOI 1.39 1.63 1.60 2.90 2.88 4.66 4.11 1.60Total 99.78 99.70 99.52 99.49 99.33 99.47 98.66 99.62Na2O + K2O 8.90 9.06 9.00 7.74 7.78 5.61 6.21 6.26Na2O/K2O 2.62 2.16 2.24 2.60 2.59 1.98 2.09 1.41Mg# 0.56 0.58 0.58 0.60 0.60 0.66 0.66 0.67Ca/Al 0.67 0.66 0.66 0.70 0.70 0.75 0.75 0.49

Trace element (ppm)Li 13.5 13.8 17.8 14.3 14.4 14.7 16.6 9.28Sc 11.4 10.4 12.8 11.0 10.9 14.1 14.3 15.9V 119 112 120 164 209 131 132 145Cr 104 107 114 111 113 186 172 323Ni 113 97.9 108 106 103 168 160 296Cu 55.2 43.0 36.2 47.5 47.5 60.9 51.5 54.8Rb 29.7 42.0 39.3 37.4 40.0 32.0 50.5 45.4Sr 1468 1429 1632 1596 1566 1518 1823 585Y 36.8 42.4 43.7 42.4 41.3 39.5 40.5 20.4Zr 363 372 423 385 371 372 343 197Nb 154 138 139 148 141 130 133 56.2Cs 1.60 1.91 1.86 1.66 1.81 1.28 1.66 0.82Ba 585 728 701 699 675 633 701 437La 82.2 92.6 92.1 92.6 92.4 84.3 87.4 28.8Ce 125 140 169 141 140 125 132 54.6Pr 17.1 19.4 18.5 19.5 19.1 17.3 18.0 6.25Nd 71.4 79.3 82.1 79.7 78.8 71.0 73.3 25.1Sm 16.3 17.8 17.7 18.3 18.1 16.3 17.0 5.76Eu 5.10 5.08 5.09 5.39 5.35 5.01 5.23 1.92Gd 14.2 15.3 19.4 15.9 15.6 14.0 14.8 5.39Tb 1.95 2.14 2.10 2.18 2.14 1.94 2.02 0.82Dy 9.36 10.30 9.57 10.40 10.30 9.30 9.66 4.34Ho 1.35 1.54 1.49 1.51 1.49 1.36 1.41 0.71Er 3.01 3.51 3.35 3.45 3.39 3.08 3.20 1.78Tm 0.33 0.42 0.39 0.39 0.38 0.35 0.36 0.23Yb 1.75 2.35 2.45 2.19 2.15 1.97 2.04 1.39Lu 0.21 0.31 0.35 0.28 0.27 0.25 0.26 0.19Hf 8.30 8.50 8.11 8.47 8.36 6.99 7.20 4.04Ta 8.56 7.19 7.08 7.76 7.53 6.48 6.70 3.29Pb 5.66 7.72 7.55 7.28 7.38 6.16 6.27 3.09Th 12.2 14.4 13.8 13.9 13.6 13.1 13.3 4.0U 4.12 3.10 3.27 2.87 2.89 3.45 3.34 1.18Nb/U 37.4 44.6 42.4 51.6 48.9 37.7 39.9 47.8Ce/Pb 22.1 18.1 22.4 19.4 19.0 20.3 21.1 17.7La/Yb 47.0 39.4 37.6 42.3 43.0 42.8 42.8 20.7Sm/Yb 9.3 7.6 7.2 8.4 8.4 8.3 8.3 4.1Nb/Ta 18.0 19.2 19.6 19.1 18.7 20.1 19.9 17.1Zr/Hf 43.7 43.8 52.1 45.5 44.4 53.2 47.6 48.8Ti/Ti* 0.46 0.38 0.32 0.38 0.39 0.41 0.38 0.98Hf/Hf* 0.61 0.57 0.54 0.56 0.56 0.52 0.51 0.85

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Table 1 (continued)

Sample 10FS8 10FS9 10FS10 10FS11 10CR1 10CR2 10LYSK11 10LYSK13

Location Fangshan Fangshan Chongren Longyou

Major element (wt.%)SiO2 46.66 46.73 46.48 46.44 51.10 51.13 43.4 42.8TiO2 2.10 2.12 2.10 2.13 2.29 2.14 1.81 2.14Al2O3 13.72 13.84 13.72 13.88 11.87 13.47 10.1 11.4Fe2O3 11.12 11.22 11.13 11.33 13.08 12.23 10.78 12.2MnO 0.15 0.15 0.15 0.16 0.18 0.17 0.16 0.18MgO 10.08 9.99 9.99 10.11 6.91 7.49 13.05 12CaO 7.20 7.29 7.27 7.56 6.07 8.54 9.02 9.87Na2O 3.90 3.94 3.71 3.67 3.27 3.22 2.99 4.01K2O 1.68 1.61 1.74 2.12 1.46 1.04 2.07 1.44P2O5 0.53 0.53 0.53 0.53 0.58 0.34 0.91 1.11LOI 2.23 2.31 2.48 1.89 3.06 -0.08 4.94 2.15Total 99.37 99.73 99.30 99.82 99.87 99.69 99.23 99.30Na2O + K2O 5.58 5.55 5.45 5.79 4.73 4.26 5.06 5.45Na2O/K2O 2.32 2.45 2.13 1.73 2.24 3.10 1.44 2.78Mg# 0.68 0.68 0.68 0.68 0.55 0.59 0.74 0.70Ca/Al 0.48 0.48 0.48 0.50 0.47 0.58 0.81 0.79

Trace element (ppm)Li 9.13 9.38 9.49 8.55 7.10 6.47 16.9 9.47Sc 15.6 15.8 15.7 15.7 13.4 19.0 19.3 19.4V 130 145 144 140 135 161 143 152Cr 347 355 351 397 108 168 504 375Ni 307 304 303 303 170 157 451 349Cu 60.9 62.4 63.4 58.3 112 79.7 55.7 56.5Rb 32.6 32.9 32.5 41.6 33.9 20.6 64.4 38.3Sr 608 609 630 557 298 344 974 1194Y 19.9 19.8 19.9 19.3 33.4 20.7 25.1 29.2Zr 191 192 190 184 221 125 219 244Nb 55.0 55.6 55.1 53.2 30.0 20.2 88.8 107Cs 2.10 1.73 1.51 1.10 1.03 0.41 1.1 1.06Ba 421 425 427 455 544 320 648 915La 27.6 27.9 28.1 30.4 27.8 15.2 57.1 68.9Ce 52.9 53.3 53.6 58.3 55.7 31.0 106 126Pr 6.03 6.07 6.09 6.66 7.01 3.89 11.7 14.2Nd 24.3 24.4 24.5 27.2 31.9 18.2 48.0 57.8Sm 5.53 5.56 5.67 6.23 9.15 5.35 9.43 11.4Eu 1.83 1.84 1.84 2.08 2.76 1.89 2.82 3.44Gd 5.08 5.15 5.20 5.74 8.78 5.48 8.13 9.55Tb 0.77 0.78 0.78 0.87 1.43 0.91 1.08 1.27Dy 4.11 4.15 4.13 4.69 7.72 4.99 5.41 6.28Ho 0.68 0.68 0.68 0.75 1.30 0.86 0.87 1.02Er 1.68 1.69 1.69 1.88 3.24 2.11 2.15 2.46Tm 0.22 0.22 0.22 0.25 0.43 0.28 0.27 0.3Yb 1.30 1.33 1.35 1.46 2.61 1.73 1.55 1.74Lu 0.18 0.18 0.18 0.20 0.35 0.24 0.21 0.23Hf 3.92 3.93 3.99 4.06 5.47 3.36 4.56 4.99Ta 3.20 3.21 3.20 3.23 1.62 1.23 4.65 5.54Pb 2.69 2.69 2.84 3.09 4.10 2.46 4.34 3.75Th 3.9 4.0 4.0 3.8 3.7 2.1 9.0 10.0U 1.15 1.15 1.16 1.09 0.78 0.49 1.98 2.18Nb/U 48.0 48.5 47.7 49.0 38.7 41.6 44.8 49.1Ce/Pb 19.7 19.8 18.9 18.9 13.6 12.6 24.4 33.6La/Yb 21.2 21.0 20.8 20.8 10.7 8.8 36.8 39.6Sm/Yb 4.3 4.2 4.2 4.3 3.5 3.1 6.1 6.6Nb/Ta 17.2 17.3 17.2 16.5 18.5 16.4 19.1 19.3Zr/Hf 48.7 48.9 47.6 45.3 40.4 37.2 48.0 48.9Ti/Ti* 1.03 1.02 1.00 0.92 0.65 0.99 0.53 0.47Hf/Hf* 0.85 0.85 0.85 0.79 0.81 0.86 0.54 0.49

a 13PMS1R is the replicated analysis of sample 13PMS1 for trace element concentrations.b Mg# = Mg/(Mg + 0.85Fetot).c Ca/Al = ([CaO]wt.%/56)/(2*[Al2O3]wt.%/102).d Ti/Ti* = TiN/(NdN

�0.055 � SmN0.333 � GdN

0.722).e Hf/Hf* = HfN/(SmN � NdN)0.5.


Fig. 2. Na2O + K2O vs. SiO2 diagram (Le Bas, 1986) for the SCBCenozoic basalts.


would markedly disturb such correlations, is not significant.In addition, residual products of basalts (e.g., saprolite andsoil) usually have heavier Mg isotopic compositions com-pared to their protoliths due to preferential incorporationof 26Mg into the secondary Mg-bearing clay minerals(e.g., Pogge von Strandmann et al., 2008; Teng et al.,2010a; Huang et al., 2012a; Liu et al., 2014; Wimpennyet al., 2014b). However, the studied basalts have lighterMg isotopic compositions relative to normalmantle-derived magmatic rocks (e.g., MORBs and OIBs,�0.25 ± 0.07, Teng et al., 2010b) (Figs. 6–8), opposite tothe expected results of alteration. Therefore, the lowd26Mg values of our basalts are unlikely to result frompost-magmatic alteration.

5.1.2. Crustal contamination

The Cenozoic alkaline basalts from eastern China havebeen suggested to originate from the upper mantle withnegligible crustal contamination based on previous geo-chemical and Sr-Nd isotopic studies (e.g., Zhou andArmstrong, 1982; Peng et al., 1986; Liu et al., 1994; Zouet al., 2000; Xu et al., 2005; Tang et al., 2006; Chen et al.,2009; Zeng et al., 2010, 2011; Wang et al., 2011). This inter-pretation can also be applied to the alkaline basalts investi-gated here because their OIB-like features (e.g., positive Nband Ta anomalies and negative K and Pb anomalies,Fig. 4b) and depleted Sr-Nd isotopic compositions (Zouet al., 2000; Chen et al., 2009; Wang et al., 2011) are incon-sistent with crustal contamination. The occurrence of man-tle xenoliths indicates that the basaltic magmas ascendedrapidly, which would leave little time for magma evolutionor wall-rock assimilations. OIB-like Ce/Pb and Nb/U ratiosof our basalts (Table 1) further indicate negligible crustalcontamination that would lower these ratios of the basalticmagmas (Hofmann et al., 1986). Finally, the negative corre-lation between SiO2 and Na2O + K2O contents (Fig. 2) isalso inconsistent with crustal contamination that wouldresult in an opposite trend.

5.1.3. Crystal fractionation

Most of the investigated basalts have relatively highMg# (0.64–0.74, molar Mg/[Mg + Fe+2], Table 1),

suggesting that their compositions are close to those ofthe primary magmas (Langmuir et al., 1977) and thatinsignificant fractional crystallization of olivine and pyrox-ene occurred. A few basalts (i.e., 13AFS1-4 and 10CR1-2)with relatively low Mg# (0.55–0.60) and Ni (<170 ppm)and Cr (<170 ppm) contents (Table 1) might have experi-enced olivine and pyroxene fractionation. However, previ-ous studies show that fractional crystallization of olivineand pyroxene cause no detectable Mg isotope fractionationduring basalt differentiation (e.g., Teng et al., 2007), but canresult in increases in incompatible element contents ofbasalts (Allegre et al., 1977). Thus, if fractional crystalliza-tion of these two minerals is significant, no correlationsbetween d26Mg values and incompatible element contentsshould be observed in basalts. Our basalts display obvi-ously negative correlations between d26Mg values and con-tents of incompatible elements, such as Ti, La, Nd, Nb andTh (Fig. 7), suggesting negligible fractional crystallizationof olivine and pyroxene. Furthermore, since fractional crys-tallization of minerals such as olivine and pyroxene has avery limited effect on two incompatible element abundanceratios (e.g., Sun and Hanson, 1975; Minster and Allegre,1978), the negative correlations between d26Mg values andSm/Yb and Nb/Y ratios in our basalts (Fig. 8) must recordthe original features of the primary magmas. Finally, nonegative Eu anomalies (Fig. 4a) are indicative of negligibleremoval of plagioclase. Thus, the observed variations ingeochemistry and d26Mg values of our basalts are largelyrelated to mantle processes.

5.2. Geochemical variations with degree of partial melting in

the upper mantle

Since shallow-level processes (including post-magmaticalteration, crustal contamination and crystal fractionation)have insignificant effects, the compositions of the studiedbasalts are close to the primary magmas. Thus, the differentfeatures in geochemistry are likely to result from differentmantle sources or from different degrees of partial meltingof similar mantle sources. Given that the Sr-Nd isotopiccompositions of these basalts are depleted and vary in anarrow range (Zou et al., 2000; Chen et al., 2009; Wanget al., 2011), their LILE and LREE enrichments suggestthat they were probably derived from similar sources, whichwere recently enriched by mantle metasomatism after along-term depletion. Since La and Sm are more incompat-ible than Yb during partial melting of the mantle rocks,La/Yb and Sm/Yb ratios are sensitive to partial meltingdegree. As illustrated in Fig. 10, variable degrees (2–20%)of batch melting of a hypothetical light REE-enriched man-tle source ([La/Yb]N > 1) in the garnet stability field canproduce the La/Yb vs. Sm/Yb systematics of the alkalineand tholeiitic basalts investigated here. Specifically, alkalinebasalts with high La/Yb and Sm/Yb ratios fromPingmingshan have the lowest degree of partial melting(�2%), while tholeiitic basalts with lower La/Yb andSm/Yb ratios from Chongren have the highest degree ofmelting (�20%). These estimates are consistent with the dif-ferent contents of incompatible elements such as Ba, Th, U,La, Sr, Nd and Nb (Fig. 9), because the amount of

Fig. 3. MgO vs. other oxides diagrams for the SCB Cenozoic basalts.


incompatible elements in basaltic melts increases withdecreasing degree of partial melting. In addition, theCenozoic basalts investigated here have highly variableNb/Y ratios that are also strongly dependent on the degreeof partial melting, because Nb is much more incompatiblethan Y during partial melting of the mantle rocks (Fig. 8b).

Therefore, the data and discussions above lead us tosuggest that the basalts studied here originated from theupper mantle and their geochemical differences closelyreflect variable degrees of partial melting. With decreasingdegree of partial melting, the basalts display higher totalalkalis and TiO2 contents, higher concentrations of incom-patible elements (e.g., Ba, Sr, La, Nd, Nb, U, Th), increasedLa/Yb, Sm/Yb and Nb/Y ratios, more pronounced nega-tive K, Zr, Hf and Ti anomalies in primitive mantle(PM)-normalized trace element spidergram, and muchlighter Mg isotopic compositions, as shown inFigs. 4 and 7–10.

5.3. Origin of the low d26Mg Cenozoic basalts from the SCB

Previous studies have shown that no significant Mg iso-tope fractionation occurs during partial melting of the man-tle and subsequent basalt differentiation as well as granite

differentiation (Teng et al., 2007, 2010b; Handler et al.,2009; Bourdon et al., 2010; Li et al., 2010; Liu et al.,2010; Telus et al., 2012), implying that basalts and granitestheoretically should have a mantle-like Mg isotopic compo-sition if no isotopically distinct components (such as sedi-mentary rocks, including carbonate rocks, shale, loess andsoil, which have d26Mg ranging from �5.57 to 1.8, e.g.,Galy et al., 2002; Young and Galy, 2004; Tipper et al.,2006; Pogge von Strandmann, 2008; Higgins and Schrag,2010; Jacobson et al., 2010; Li et al., 2010; Teng et al.,2010a; Wombacher et al., 2011; Liu et al., 2014) areinvolved during their genesis. The heavy Mg isotopic com-positions (d26Mg = up to 0.44) of I-type granites fromsouthern California were attributed to the recycled highd26Mg sedimentary rocks in their source (Shen et al.,2009). Yang et al. (2012) has identified a suite of lowd26Mg continental basalts from the NCB and interpretedsuch a feature as due to interactions between the mantleand the isotopically light carbonatitic melts derived fromthe subducted oceanic slab. On the other hand,Sedaghatpour et al. (2013) more recently reported thathigh-Ti lunar basalts also display light Mg isotopic compo-sitions (as low as �0.60). Based on the negative correlationbetween d26Mg and TiO2 in high-Ti basalts, they suggested

Fig. 4. Chondrite-normalized REE patterns (a) and primitivemantle (PM)-normalized trace element distribution patterns (b) ofthe SCB Cenozoic basalts. Normalized values are fromMcDonough and Sun (1995), and data for N-MORB and OIBare from Sun and McDonough (1989). The average values formagnesio- and calico-carbonatites are taken from references(Hoernle et al., 2002; Bizimis et al., 2003).

Fig. 5. Ti/Ti* vs. Ca/Al (a) and Hf/Hf* (b) in the SCB Cenozoicbasalts. Yellow stars represent the average ratios of Ca/Al (248.4),Ti/Ti* (0.106) and Hf/Hf* (0.016) for magnesio- and calico-carbonatites (Hoernle et al., 2002; Bizimis et al., 2003). Thoseratios for N-MORB are calculated based on major and traceelement data presented by Hofmann (1988).


that ilmenite has light Mg isotopic compositions and theaccumulation of ilmenite in the mantle source at the latestage in the lunar magma ocean shifts high-Ti basalts tolow d26Mg values (Sedaghatpour et al., 2013). Similarly, anegative correlation between d26Mg and TiO2 has also beenobserved in our basalts (Fig. 7b). Therefore, we have toevaluate the possibility that the light Mg isotopic composi-tions of our basalts might have resulted from the ilmeniteaccumulation in their mantle source.

Several lines of evidence are against this possibility.First, our basalts have TiO2 contents (<2.7 wt.%) muchlower than those of high-Ti lunar basalts (>6 wt.%,Sedaghatpour et al., 2013), implying that no significantaccumulation of ilmenite in the mantle source for ourbasalts. Second, as ilmenite generally displays an enrich-ment of Nb-Ta and preferentially incorporates Ta relativeto Nb with DTa/DNb = �1.3 between ilmenite and maficmelts (Dygert et al., 2013), accumulation of ilmenite inthe mantle source would cause a negative correlationbetween Nb/Ta and TiO2 rather other constant Nb/Taratios in our basalts irrespective of variable TiO2

(Fig. 11). Third, the roughly positive correlation betweend26Mg and Ti/Ti* (Fig. 13d) in our basalts also doesn’tstand for this interpretation, because if large amounts of

isotopically light and Ti-rich ilmenites are present in themantle source, an opposite trend should be observed.Thus, the light Mg isotopic compositions of the investigatedbasalts cannot be resulted from ilmenite accumulation intheir mantle source.

In addition to the negative correlation between d26Mgand TiO2, the d26Mg values of our basalts also decreasewith the amounts of other incompatible elements (e.g.,La, Nd, Nb, Th) and trace element abundance ratios(e.g., Sm/Yb, Nb/Y) (Figs. 7 and 8) that are sensitive topartial melting. This suggests that large variations in Mgisotope ratios have occurred during partial melting of themantle under high temperatures and pressures, with meltsproduced by low degrees of melting having lighter Mg iso-topic compositions relative to melts produced by highdegrees of melting. A dissolution experiment on BoulderCreek Granodiorite has shown that concomitant variationsin d26Mg values of reactive fluids reflect conservative mix-ing of Mg released from isotopically distinct minerals(e.g., chlorite, biotite and hornblende) rather than Mg iso-tope fractionation. It is experimentally determined that dur-ing partial melting of carbonated peridotite, the firstincipient melts near solidus are carbonatitic melts, whichevolve to carbonated silicate melts (625 wt.% CO2 in melts)with increasing degree of melting (Dasgupta et al., 2007,2013). Hence, it is expected that during this evolution path,

Fig. 6. (a) d26Mg vs. d25Mg in the SCB Cenozoic basalts and theUSGS standards. It is noted that all data distribute along theterrestrial equilibrium mass fractionation line with a slope of 0.521(Young and Galy, 2004); (b) d26Mg vs. Mg# in the SCB Cenozoicbasalts. Also shown for comparison are Mg isotopic data of freshbasaltic lavas from the NCB (Yang et al., 2012). Gray barrepresents the widely accepted d26Mg of the normal mantle(�0.25 ± 0.07, Teng et al., 2010b).

Table 2Magnesium isotopic compositions of the Cenozoic basalts from theSouth China Block.

Sample d26Mga 2SDb d25Mg 2SDb N D25Mg0c

13PMS1 �0.52 0.07 �0.25 0.09 4 0.0313PMS2 �0.53 0.04 �0.26 0.03 4 0.0213PMS3 �0.52 0.05 �0.24 0.03 4 0.03Replicated �0.52 0.05 �0.24 0.03 4 0.0313PMS6 �0.52 0.05 �0.26 0.03 4 0.0113PMS8 �0.60 0.05 �0.31 0.05 4 0.0113AFS1 �0.52 0.04 �0.26 0.02 4 0.01Replicate �0.51 0.08 �0.26 0.05 4 0.0013AFS2 �0.52 0.08 �0.25 0.07 4 0.0213AFS3 �0.59 0.04 �0.29 0.04 4 0.02Replicate �0.58 0.05 �0.29 0.04 4 0.0213AFS4 �0.56 0.04 �0.28 0.02 4 0.0210FS6 �0.41 0.07 �0.22 0.03 4 0.00Replicate �0.42 0.06 �0.21 0.02 4 0.0110FS8 �0.42 0.05 �0.20 0.03 4 0.0110FS9 �0.43 0.05 �0.23 0.05 4 �0.0110FS10 �0.42 0.04 �0.22 0.04 4 �0.0110FS11 �0.40 0.03 �0.20 0.05 4 0.0010CR1 �0.37 0.05 �0.19 0.04 4 0.0010CR2 �0.35 0.06 �0.18 0.05 4 0.0010LYSK13 �0.41 0.05 �0.21 0.05 4 0.01BHVO-2 �0.26 0.04 �0.14 0.06 4 0.00Replicate �0.24 0.06 �0.12 0.02 4 0.01AGV-2 �0.16 0.03 �0.10 0.06 4 �0.02Replicate �0.15 0.02 �0.06 0.04 4 0.01GSP-2 0.03 0.09 0.00 0.06 4 �0.01

a dXMg = {(XMg/24Mg)sample/(XMg/24Mg)DSM3 � 1)} � 1000,

where X = 25 or 26 and DSM3 is solution made from pure Mgmetal (Galy et al., 2003).

b 2SD indicates twice the standard deviation of the population of4 repeat measurements of a sample solution.

c D25Mg = d25Mg � 0.521d26Mg0, where dXMg = 1000 � ln[(dXMg + 1000)/1000] (Young and Galy, 2004). It is reportedlargely as a quality control on the data, with values that should beclose to zero.

d Replicate denotes repeating sample dissolution, column chem-istry and instrumental analysis.


there should exist large variations in Mg isotopic composi-tions because of different ratios of isotopically light carbon-atitic melts/isotopically heavy silicate melts at differentdegrees of partial melting of carbonated peridotite.

Teng et al. (2007, 2010b) demonstrated that Mg isotopefractionation is insignificant during partial melting of dryperidotite and subsequent basalt differentiation. In the stud-ies of Teng et al. (2007, 2010b), the Mg-rich mineralsinvolved during magmatic processes are olivine and pyrox-ene which usually have mantle-like Mg isotopic composi-tions (e.g., Handler et al., 2009; Yang et al., 2009; Huanget al., 2011; Liu et al., 2011; Pogge von Strandmannet al., 2011; Xiao et al., 2013). This may explain why no sig-nificant Mg isotope fractionation was observed in theirstudies. The basalts in the present study overall have muchlighter Mg isotopic compositions compared to globalmid-ocean ridges basalts (MORBs, Figs. 7, 8 and 13),suggesting that the mantle source for the studied basaltsis not a dry garnet peridotite that has an average Mgisotopic composition identical to that of MORBs

(d26Mg = �0.26 ± 0.07 vs. �0.25 ± 0.04, Teng et al.,2010b). The following observations also suggest that drygarnet peridotite is not a suitable source for our alkalinebasalts. First, alkaline basalts usually have lower SiO2

and Al2O3, and higher TiO2, Fe2O3t and CaO at a givenMgO content than high-pressure experimentally-derivedpartial melts of dry garnet peridotite (e.g., Hirose andKushiro, 1993; Walter, 1998; Dasgupta et al., 2007).Second, partial melting of dry garnet peridotite cannot pro-duce the superchondritic Zr/Hf ratios observed in ourbasalts (Table 1) because Zr and Hf have similar partitioncoefficients between dry peridotite and basaltic melts(Salters et al., 2002). Third, because the bulk partition coef-ficients for Zr, Hf and Ti between garnet peridotite andbasaltic melts are similar to those of middle REEs (Sm,Eu and Gd) (Salters et al., 2002), the negative anomaliesof Zr, Hf and Ti relative to neighboring REEs cannot beexplained by partial melting of dry garnet peridotite butare consistent with the features of carbonatites as shownin Fig. 4b.

Fig. 7. Variations of d26Mg with abundances of incompatible elements in the SCB Cenozoic basalts. The d26Mg of N-MORB is cited as�0.25 ± 0.07 (Teng et al., 2010b) and the values for incompatible elements are taken from Hofmann (1988).


The low d26Mg values (as low as �0.60, Figs. 6–8) ofalkaline basalts from Anfengshan and Pingmingshan,which were generated by low degrees (2–3%, Fig. 10) of par-tial melting of the mantle, suggest that the mantle sourcefor our basalts has much lighter Mg isotopic compositionsrelative to the normal mantle (�0.25 ± 0.07, Teng et al.,2010b). This indicates that the mantle source for our basaltshas been metasomatized by isotopically light carbonatiticmelts, because the deeply recycled carbonates and carbon-ated eclogites have light Mg isotopic compositions, withd26Mg of �2.51 to �0.53 (Wang et al., 2014).Additionally, the decrease of the total alkalis (i.e.,Na2O + K2O) and TiO2 with increasing degree of melting(Fig. 7a, b) is consistent with the compositional trendsobserved in carbonated silicate melts produced by partialmelting of fertile natural peridotite KLB-1 + 1�2.5 wt.%CO2 (Dasgupta et al., 2007, 2013). This further indicatesthat our basalts were probably sourced from a carbonated

mantle. Furthermore, the presence of carbonatitic melts inthe mantle source is also suggested by Zeng et al. (2010)based on the relationship between total alkalis(Na2O + K2O) and TiO2. In the plot of total alkalis vs.TiO2 (Fig. 12), the Cenozoic basalts from eastern China fallalong the trend defined by experimentally-derived melts ofcarbonated peridotite, implying that carbonated peridotiteis probably the main source for them.

As carbonation significantly lowers the solidus of mantleperidotite, carbonated mantle may melt before anythingelse and contribute more when the degrees of melting arelow. It has been experimentally determined that the incipi-ent melts from carbonated mantle near solidus are carbon-atitic melts (Dasgupta et al., 2007, 2013). Carbonatiticmelts will evolve to carbonated silicate melts with increas-ing temperature and degree of melting as the dissolutionof clinopyroxene and/or olivine into carbonatitic meltsbecomes significant, melt fraction increases and the

Fig. 8. d26Mg vs. Sm/Yb (a) and Nb/Y (b) in the SCB Cenozoicbasalts. The negative correlations between d26Mg and Sm/Yb andNb/Y, trace element abundance ratios that are sensitive to partialmelting, suggest that the observed d26Mg variation in the SCBCenozoic basalts is caused by partial melting of a carbonatedmantle (See text for details).


concentration of CO2 in melts gets diluted (Dasgupta et al.,2007, 2013). Therefore, under high degrees of melting, theso-called “carbonatitic fingerprints” (e.g., high Ca/Al andZr/Hf ratios, extremely low Ti/Ti* and Hf/Hf* ratios, andstrongly negative anomalies of Zr, Hf and Ti in spidergram,Hoernle et al., 2002; Bizimis et al., 2003) will be diluted asshown in the studied basalts from Fangshan and Chongren(Figs. 3 and 4). The features that the d26Mg values in ourbasalts increase with increasing degree of melting couldbe attributed to the incongruent melting of the carbonatedmantle. The basalts produced by low degrees of meltingwere contributed more from isotopically light carbonatiticmelts and thus show much lighter Mg isotopic composi-tions, as observed in basalts from Anfengshan andPingmingshan; while the basalts produced by higherdegrees of melting were contributed largely from isotopi-cally heavy silicate melts and thus display heavy Mg iso-topic compositions, as observed in basalts fromChongren. As illustrated in Fig. 13, the basalts with lowerd26Mg values generally have higher Ca/Al and Zr/Hf ratios,and lower Hf/Hf* and Ti/Ti* ratios. Because carbonatiticmelts have high ratios of Ca/Al and Zr/Hf and low ratios

of Hf/Hf* and Ti/Ti* (Hoernle et al., 2002; Bizimis et al.,2003), these features also imply that our basalts probablyrecord the compositional evolution trend from carbonatiticmelts with light Mg isotopic compositions to silicate meltswith heavy isotopic compositions (represented byN-MORB in Fig. 13) as observed in partial melting exper-iments on carbonated peridotite (e.g., Dasgupta et al., 2007,2013). Thus, the mantle source for our basalts from theSCB is a carbonated mantle with light Mg isotopic compo-sitions that formed by incorporation of isotopically lightcarbonatitic melts into the upper mantle. As the studiedbasalts are enriched in LILE and LREE and have depletedSr-Nd isotopic compositions (e.g., Zou et al., 2000; Chenet al., 2009; Wang et al., 2011), the incorporation of carbon-atitic components into the upper mantle must take placerecently without a long time-integrated ingrowth of Sr-Ndisotopic systems.

5.4. Did the carbonatitic components come from the

subducted oceanic slab?

The carbonatitic melts that metasomatized the uppermantle to form carbonated peridotite for generating thealkaline basalts might come from the deep mantle at greatdepths or the subducted slabs (Dasgupta et al., 2007,2013). It is experimentally shown that the carbonatitic meltsfrom the deep mantle can be generated by partial melting orredox melting of the primitive carbon-bearing peridotite atdepths of greater than 200 km (Dasgupta and Hirschmann,2006; Dasgupta et al., 2013; Stagno et al., 2013), where car-bon is stored chiefly in carbonates (e.g., dolomite and mag-nesite), graphite/diamond and carbides (e.g., Luth, 1999;Dasgupta and Hirschmann, 2010). The graphite or dia-mond in the deep mantle would react with silicates to formdolomite or magnesite during their ascent through the fol-lowing redox reactions proposed by Eggler and Baker(1982) and Luth (1993):

2Cgraphite=diamond

þ2O2 þ 2Mg2SiO4olivine

¼Mg2Si2O6enstatite

þ 2MgCO3magnesite

ð1Þ

and

CaMgSi2O6dioposide

þ 2Mg2SiO4olivine

þ 2Cgraphite=diamond

þ2O2

¼ 2Mg2Si2O6enstatite

þCaMgðCO3Þ2dolomite

ð2Þ

Both reactions are involved with free O2 as oxidants, whichcould originate from the mantle-derived fluids throughreduction of oxidized species, such as CO2, H2O and sul-fates (e.g., Luth, 1993). These oxidized species are usuallyabundant in mantle-derived fluids, manifested by the studyof fluid inclusions in diamond and minerals of spinel andgarnet-peridotite xenoliths (e.g., Navon et al., 1988;Frezzotti et al., 2012). According to these reactions, the ele-ment Mg in these carbonates is mainly derived from olivineand pyroxene that have mantle-like Mg isotopic composi-tions in the normal mantle (e.g., Handler et al., 2009;Yang et al., 2009; Huang et al., 2011; Liu et al., 2011;Pogge von Strandmann et al., 2011; Xiao et al., 2013). A

Fig. 9. Variations of selected elements versus Nb in the SCB Cenozoic basalts.


recent experimental study shows that Mg isotope fraction-ation between olivine and carbonate (e.g., magnesite) is lim-ited at temperatures of P800 �C (d26Mgolivine-magnesite 6 0.04 ± 0.04, Macris et al., 2013), suggesting that primitivecarbonate and olivine have similar Mg isotopic composi-tions under mantle temperatures. Therefore, the deepmantle-derived carbonatitic melts are inferred to have nor-mal mantle-like Mg isotopic compositions and cannot pro-duce the low d26Mg values of our basalts.

Sedimentary carbonates so far reported have the lightestMg isotopic compositions, with d26Mg of �5.54 to �0.47(e.g., Galy et al., 2002; Young and Galy, 2004; Tipperet al., 2006; Pogge von Strandmann, 2008; Higgins andSchrag, 2010; Ke et al., 2011; Pokrovsky et al., 2011;

Wombacher et al., 2011). A recent study revealed differen-tial isotopic exchange between the eclogites and carbonates(e.g., limestone to dolostone) during subduction and foundthat the deeply recycled carbonates and carbonated eclog-ites have light Mg isotopic compositions, with d26Mg of�2.51 to �0.53 (Wang et al., 2014). Thus, partial meltingof rocks containing low d26Mg carbonates will generatelow d26Mg carbonatitic melts. Metasomatism of the mantleby isotopically light carbonatitic melts from the subductedslab could form carbonated peridotite and shift the Mg iso-topic compositions of the mantle to light values (Yanget al., 2012; Xiao et al., 2013). The carbonatitic melts wereprobably derived from partial melting of the recycled car-bonated eclogite transformed from the subducted

Fig. 10. La/Yb vs. Sm/Yb in the SCB Cenozoic basalts. Alsoshown is the batch melting curve calculated for garnet peridotite.Partition coefficients are taken from Johnson et al. (1990). Thestarting material is olivine, 55%; orthopyroxene, 20%; clinopyrox-ene, 15%; garnet, 10%; melting reaction in garnet field (Walter,1998): olivine, 3%; orthopyroxene, 3%; clinopyroxene, 70%; garnet,24%. The inverse modeling used here follows Feigenson et al.(2003) and Xu et al. (2005).

Fig. 11. TiO2 vs. Nb/Ta in the SCB Cenozoic basalts.


carbonate-bearing oceanic crust during plate subduction.This can be inferred from the following observations.First, the OIB-like trace element distribution patterns(Fig. 4) and key element ratios (e.g., U/Pb, Th/Pb, Nb/U,Ce/Pb, Table 1) for most of the basalts imply the involve-ment of a few percent of recycled oceanic crust in theirmantle source (e.g., Wang et al., 2011; Xu et al., 2012;Sakuyama et al., 2013). Second, carbonated eclogite hasbeen reported to have light Mg isotopic compositions, withd26Mg as low as �1.93 (Wang et al., 2012, 2014).

5.5. Geodynamic implications

The light Mg isotopic compositions of the <110 Mabasalts from the NCB have been attributed to the interactionof mantle peridotite with isotopically light carbonatitic meltsderived from the subducted oceanic slab (Yang et al., 2012).

However, the NCB suffered from three circum-craton ocea-nic subductions since the Paleozoic era, including thePaleo-Tethys oceanic subduction from south, the Mongoliaoceanic subduction from north and the Pacific oceanic sub-duction from East (Windley et al., 2010). It is thus difficultto judge which oceanic subduction supplied recycled carbon-ated eclogite in the upper mantle of the NCB. Previous stud-ies show that on the northern side of the NCB, southwardssubduction of the Mongolia oceanic plate started in theOrdovician and ceased in the Permo-Triassic (Xiao et al.,2003), while northwards subduction of the Paleo-Tethysoceanic plate below the southern margin of the NCB beganin the Paleozoic and ceased in the Triassic (Li et al., 1993,2000, 2001). On the basis of plate reconstruction, westwardssubduction of the Izanaghi-Pacific plate beneath the easternAsian continent was suggested to start as early as earlyCretaceous (Muller et al., 2008). Meanwhile, themantle-like Mg isotopic compositions of the >120 Mabasalts in the NCB (Fig. 6b) suggest that the isotopically lightmantle source didn’t form till 120 Ma (Yang et al., 2012).Considering only the Pacific plate was subducting beneaththe NCB in the Mesozoic-Cenozoic, Yang et al. (2012) fur-ther pointed out that subduction of Pacific plate plays animportant role in generating the light Mg isotopic composi-tions of the <110 Ma basalts of the NCB. In this study, thelight Mg isotopic compositions of the Cenozoic basalts fromthe SCB provide a convincing evidence to support that therecycled carbonated eclogite in the upper mantle of theNCB were derived from the Pacific slab, because only thePacific slab has an influence on both blocks of North andSouth China. In addition, our results combined withYang’s study also suggest that the upper mantle of easternChina may be significantly metasomatized by carbonatiticmelts formed by partial melting of carbonated eclogite trans-formed from the subducted Pacific slab. Such carbonatiticmelts might be responsible for the abrupt changes of Mgand Nd isotopic compositions between the >120 and<110 Ma continental basalts from eastern China (Yanget al., 2012).

High-resolution seismic tomography revealed that thePacific slab is subducting beneath the Japan Islands andbecomes stagnant in the mantle transition zone (410–660 km) beneath eastern China, with its western edge�2000 km away from the Japan Trench (e.g., Zhao et al.,2011). The stagnant Pacific slab might bring large amountsof carbonated eclogites into the mantle transition zone asthey can survive from subduction-zone dehydration andmelting at modern subduction zones (Dasgupta, 2013 andreferences therein). Thus, we propose that the carbonatiticmelts for creating the light Mg isotopic compositions ofthe upper mantle of eastern China probably originate fromthe stagnant Pacific slab. Partial molten carbonatitic meltswould metasomatize the upper mantle and lead to the for-mation of a carbonated mantle above the stagnant Pacificslab beneath East Asia, which have formed a big mantlewedge (e.g., Zhao et al., 2011). Carbonated peridotite withlight Mg isotopic compositions existed in the upper mantlewould melt first and generate the isotopically light basalts,because carbonation lowers the solidus of peridotite (e.g.,Dasgupta et al., 2007, 2013).

Fig. 12. Na2O + K2O vs. TiO2 for the Cenozoic basalts in eastern China. Data sources for the Cenozoic basalts in eastern China are from thereferences (Zhi et al., 1990; Zou et al., 2000; Xu et al., 2005; Tang et al., 2006; Liu et al., 2008; Chen et al., 2009; Zeng et al., 2010, 2011; Wanget al., 2011; Xu et al., 2012). Also shown for comparison are experimentally-derived melts from carbonated peridotite (Hirose, 1997; Dasguptaet al., 2007), carbonated pyroxenite (Gerbode and Dasgupta, 2010), pyroxenite or eclogite (Hirschmann et al., 2003; Kogiso et al., 2003;Pertermann and Hirschmann, 2003; Kogiso and Hirschmann, 2006), hornblendite (Pilet et al., 2008), and carbonated eclogite (Dasgupta et al.,2006).

Fig. 13. d26Mg vs. Ca/Al (a), Zf/Hf (b), Hf/Hf* (c) and Ti/Ti* (d) in the SCB Cenozoic basalts.


6. CONCLUSIONS

High-precision major and trace element data and Mgisotopic analyses on the Cenozoic alkaline and tholeiiticbasalts from the SCB, eastern China lead us to make con-clusions as follows:

(1) The Mg isotopic compositions of the studied basaltsare much lighter relative to the normal mantle, withd26Mg values ranging from �0.60 to �0.35. The pos-sibility of isotopically light ilmenite accumulation in

their mantle source for causing the light Mg isotopiccompositions of our basalts can be ruled out, because(i) their relatively lower TiO2 contents (<2.5 wt.%)compared to high-Ti lunar basalts (>6.5 wt.%) inves-tigated by Sedaghatpour et al. (2013) suggest no sig-nificant abundance of ilmenite in the mantle sourceof our basalts; and (ii) the roughly positive correla-tion between their d26Mg values and Ti/Ti* as wellas their constant Nb/Ta ratios irrespective of variableTiO2 contents suggests no isotopically light ilmeniteaccumulation in their mantle source, which would


result in negative correlations between Nb/Ta andTiO2, d26Mg and Ti/Ti*. Thus, the low d26Mg basaltsfrom the SCB were probably sourced from a carbon-ated mantle that formed by interaction of the mantlewith isotopically light carbonatitic melts, and ourresults confirm that Mg isotope ratios can be usedas a powerful tool to trace recycled carbonates.

(2) The d26Mg values of our basalts decrease with theamounts of incompatible elements (e.g., Ti, La, Nd,Nb, Th) and trace element abundance ratios (e.g.,Sm/Yb, Nb/Y) that are sensitive to partial melting,suggesting that large variations in Mg isotope ratiosoccurred during partial melting of the mantle underhigh temperatures and pressures. Additionally, theirHf/Hf* and Ti/Ti* ratios increase, and Ca/Al andZr/Hf ratios decrease with increasing degrees of par-tial melting. These features can be ascribed to theincongruent partial melting of the carbonated man-tle. At low degrees of melting, the partial melts arecontributed more from isotopically light carbonatiticmelts that have high Ca/Al and Zr/Hf ratios, lowHf/Hf* and Ti/Ti* ratios, while at higher degrees ofpartial melting, the partial melts are contributedmore from isotopically heavy silicate melts that havelow Ca/Al and Zr/Hf ratios, high Hf/Hf* and Ti/Ti*

ratios. Thus, the large Mg isotopic variations in ourbasalts represent conservative mixing of isotopicallydistinct materials rather than isotope fractionationat mantle pressures and temperatures.

(3) The carbonatitic melts probably originate from thestagnant Pacific slab beneath East Asia, which is con-sistent with the results of seismic tomography (e.g.,Zhao et al., 2011). Thus, our results combined withthe study of Yang et al. (2012) demonstrate that thesubducted Pacific slab provides the recycled carbon-ates and that there exists a widespread carbonatedupper mantle beneath eastern China, which servesas the main source for the <110 Ma alkaline basalts.

ACKNOWLEDGEMENT

This work is financially supported by grants from the NationalScience Foundation of China (Nos. 41090372, 41230209, 41328004and 41273037) and the Fundamental Research Funds for theCentral Universities (WK2080000068). We are grateful toZhen-Hui Hou and Hai-Yang Liu for assistance during analysesof trace elements and to Ting Gao, Zi-Jian Li, Ze-Zhou Wangfor Mg isotopic analyses. Grateful thanks are due to four anony-mous reviewers whose constructive and critical reviews greatlyimprove this contribution. We acknowledge the executive editorDr. Marc Norman and the associated editor Dr. Weidong Sunfor their punctuality and dedication during the editing work.

APPENDIX A. SUPPLEMENTARY DATA

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.gca.2015.04.054.

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origin of low Î´26mg cenozoic basalts from south china block...

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