Chemical Geology 273 (2010) 35–45

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Chemical Geology

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Carbonated mantle sources for Cenozoic intra-plate alkaline basalts in Shandong,North China

Gang Zeng a, Li-Hui Chen a,⁎, Xi-Sheng Xu a, Shao-Yong Jiang a, Albrecht W. Hofmann b

a State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, Chinab Max-Planck-Institut für Chemie, Abteilung Geochemie Postfach 3060, D-55020 Mainz, Germany

⁎ Corresponding author.E-mail address: chenlh@nju.edu.cn (L.-H. Chen).

0009-2541/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.chemgeo.2010.02.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 October 2009Received in revised form 8 February 2010Accepted 9 February 2010

Editor: D.B. Dingwell

Keywords:Intra-plate alkaline basaltsCarbonatiteAsthenosphereNorth China

The genesis of intra-plate alkaline basalts remains controversial, and three sources have been proposed:silica-deficient eclogite–pyroxenite, hornblendite, and carbonated peridotite. Here, we assess these modelsby analyzing Cenozoic intra-continental alkaline basalts from Shandong province, North China. The Cenozoicbasalts of Shandong province consist of an early sequence of weakly alkaline rocks (alkali olivine basalts) anda late sequence of strongly alkaline rocks (basanites and nephelinites). In comparison with the weaklyalkaline rocks, the strongly alkaline rocks have lower concentrations of SiO2 (39.2–45.1 wt.%) and Al2O3

(10.3–13.8 wt.%), higher alkalis (Na2O+K2O=4.3–8.6 wt.%) and CaO (8.0–12.6 wt.%), higher concentrationsof most incompatible elements, and higher values of Ca/Al (0.7–1.3), La/Yb (39.4–65.7), and Sm/Yb (6.1–9.9).On the whole, primitive-mantle normalized spidergrams reveal that the strongly alkaline rocks havestronger negative K, Zr, Hf, and Ti anomalies (Hf/Hf*=0.59–0.77, Ti/Ti*=0.46–0.71) than do the weaklyalkaline rocks. All these rocks have superchondritic Zr/Hf ratios (N44). Inverse rare earth element (REE)modeling suggests that the strongly and weakly alkaline rocks represent melting amounts of b3% and 3–10%,respectively. Neither silica-deficient eclogite–pyroxenite melts nor hornblendite melts can satisfy all thefeatures mentioned above. Here we prefer a carbonated mantle source because the main characteristics ofthe strongly alkaline rocks resemble those of carbonatites (e.g., enrichment of most incompatible elements,high Ca/Al ratios, superchondritic Zr/Hf ratios, and negative K, Zr, Hf, and Ti anomalies). Since dry peridotitehas a higher solidus temperature than does carbonated peridotite in the mantle, an increased degree ofmelting under higher temperatures may result in the dilution of “carbonatitic fingerprints.” Althoughcontributions from silica-deficient eclogite–pyroxenite or hornblendite cannot be ruled out, our observationssuggest that carbonated peridotite is the main source for the Cenozoic strongly alkaline basalts of Shandong.

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1. Introduction

The genesis of intra-plate alkaline basalts remains a matter ofconsiderable debate. Such basalts display two distinctive ‘OIB-type’geochemical features: (1) enrichment of most of the incompatibleelements, both large ion lithophile elements (e.g., Ba, Rb, Cs, U, and Sr)and high field strength elements (e.g., Nb and Ta); and (2) negative Kand Pb anomalies in spidergrams. The following sources have beenproposed for these SiO2-unsaturated melts: (1) silica-deficienteclogite and garnet pyroxenite representing, for example, residualcrust in the asthenosphere (Hirschmann et al., 2003; Kogiso et al.,2003; Kogiso and Hirschmann, 2006), (2) hornblendite producedby hydrous metasomatism (McKenzie and O'Nions, 1995; Niuand O'Hara, 2003; Pilet et al., 2004, 2005, 2008), (3) carbonatedperidotite (Hirose, 1997; Dasgupta et al., 2007; Sisson et al., 2009),and (4) mixed sources (Prytulak and Elliott, 2007; Liu et al., 2008;Chen et al., 2009; Dasgupta et al., 2010).

Recent high-pressure experiments have produced ne-normativeliquids that are close to alkaline basaltic rocks in composition (Hirose,1997; Hirschmann et al., 2003; Kogiso et al., 2003; Kogiso andHirschmann, 2006; Dasgupta et al., 2007; Pilet et al., 2008). However,these experimental melts fail to match all the features of naturalexamples. For example, compared with natural rocks, experimentallyproduced melts from silica-deficient garnet pyroxenite have lowerK2O contents (Hirschmann et al., 2003; Kogiso et al., 2003), meltsfrom carbonated peridotite have lower TiO2 contents (Hirose, 1997;Dasgupta et al., 2007), and melts from hornblendite have higher TiO2

contents (Pilet et al., 2008). Pilet et al. (2008) claimed that alkalinemelts from their hornblendite and hornblendite plus peridotitemelting experiments reproduced not only key major-elementfeatures, but also key trace-element features of alkaline basalts.However, their work provided no explanation for Zr, Hf, and Tifractionation relative to REEs, which is commonly observed in intra-continental alkaline basaltic provinces; e.g., the Cenozoic of easternChina (Zou et al., 2000), eastern Australia (O'Reilly and Zhang, 1995),the Hyblean Plateau of Italy (Trua et al., 1998), and northwest Turkey(Aldanmaz et al., 2006). Apart from the work of Pilet et al. (2008), the

36 G. Zeng et al. / Chemical Geology 273 (2010) 35–45

other melting experiments provide no constraints on trace elements.Therefore, additional work is required before we can properly assessthe genesis of intra-plate alkaline basaltic rocks.

The Cenozoic alkaline basalts of eastern China is typical of intra-continental alkaline basalts produced in an extensional setting, and itprovides an ideal setting in which to study their genesis (Zhou andArmstrong, 1982; Peng et al., 1986; Song et al., 1990; Zhi et al., 1990;Basu et al., 1991; Liu et al., 1994; Zou et al., 2000; Xu et al., 2005; Tanget al., 2006; Liu et al., 2008; Chen et al., 2009). The basalts are thoughtto represent melts derived from the asthenosphere, given theirdepleted Sr–Nd isotopes and OIB-like trace-element signatures inspidergram (e.g., enrichment in Nb, Ta, and LREEs, and negative K andPb anomalies). Here, we present major and trace-element data for thebasanites and nephelinites of Shandong province in the North ChinaCraton, and compare these data with those from the less alkalinerocks, alkali olivine basalts in the same area. Based on these data, weargue that the asthenospheric source of the alkaline rocks wasenriched by carbonatitic liquids.

2. Geological setting and sample description

The North China Craton (NCC) is one of the world's oldest Archeancratons. It is located in the central part of eastern China, and can bedivided into three different tectonic regions based on the geology andP–T-t paths of its metamorphic rocks (Fig. 1) (Zhao et al., 2001).Cenozoic alkaline basalts in the NCC, as well as those in northeast andsoutheast China, represent the youngest magmatism in eastern China(Fig. 1). Major basaltic fields in the NCC include Hannuoba, Datong,Taihang, and Shandong. The Shandong province is located in the

Fig. 1. The Shandong Cenozoic alkaline basaltic province of the North China Craton. 1: Hannparts: the Western Block (WB), the Trans-North China Orogen (TNCO), and the Eastern BlocLaoheishan, Guandingshan, Niushan, Fangshan, and Dashan (closed triangles). The weakly aThe distribution of Cenozoic basalts in Shandong is after Shandong Provincial Bureau of Ge

southeast of the NCC. The Cenozoic alkaline magmatism in Shandongtook place during two periods: 24.0–10.3 Ma and 8.7–0.3 Ma (see thereview by Luo et al., 2009). The early magmatism was profuse andcharacterized by large volcanoes densely distributed in a narrow areanear the Tan-Lu Fault (Fig. 1); the rocks of this age are mainly alkaliolivine basalts. The later magmatism is characterized by small,isolated volcanoes, widely scattered in areas far from the Tan-LuFault (Fig. 1); the rocks of this group are dominated by basanites andnephelinites, and they commonly contain mantle xenoliths.

We chose five volcanoes for a study of the genesis of the morestrongly alkaline rocks: Laoheishan, Guandingshan, and Niushan inPenglai; Fangshan in Qixia; and Dashan in Wudi. Samples fromLaoheishan and Dashan are unaltered. Samples from Guandingshan,Niushan, and Fangshan are slightly altered, and olivine phenocrystsare partially altered to low-temperature iddingsite. All samples haveminor olivine (b15%) as phenocrysts set in a groundmass of olivine,Ti-magnetite, nepheline, and glass. Neither plagioclase nor pyroxenephenocrysts are observed in these samples. For comparison, we alsosampled alkali olivine basalts from Linqu, Changle, Weifang, andYishui in Shandong province; most of these samples are unaltered.

3. Methods

Measurements of whole-rock major elements and trace elementswere made at the Department of Geology, Northwest University,Xi'an, China. We used a RIX-2100 X-ray fluorescence spectrometer(XRF) to measure major elements. According to the measured valuesof standards (GSR-1 and GSR-3), the uncertainties are about ±1% forelements with concentrations N1.0 wt.%, and about ±10% for

uoba; 2: Datong; 3: Taihang; 4: Su-Wan. The North China Craton is divided into threek (EB) (Zhao et al., 2001). The strongly alkaline volcanoes of the Shandong area includelkaline olivine basalts (gray areas) are distributed in the area circled by the dotted line.ology and Mineral Resources (1991).

37G. Zeng et al. / Chemical Geology 273 (2010) 35–45

elements with concentrations b1.0 wt.%. Trace elements, includingrare earth elements (REEs), were determined using an ELAN 6100DRCinductively coupled plasma mass spectrometer (ICP-MS) after aciddigestion (HF+HNO3) of samples in Teflon bombs. Analyses of USGSrock standards (BHVO-2, AGV-1, BCR-2, and G-2) indicate precisionand accuracy better than 5% for Sc, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba,Pb, U, and REEs, and 10% for Hf, Ta, and Th. The results of the analysesof these standards are shown in Table 1.

4. Results

The analytical data for the major oxides and trace elements of thebasanites and nephelinites are shown in Table 2. These stronglyalkaline rocks have low contents of SiO2 (39.2–45.1 wt.%) and Al2O3

(10.3–13.8 wt.%), high contents of MgO (6.5–13.9 wt.%), CaO (8.0–12.6 wt.%), and alkalis (Na2O+K2O=4.3–8.6 wt.%), and high Ca/Alratios (0.7–1.3). According to the nomenclature of Le Bas et al. (1986),samples from Guandingshan and Niushan are basanites, and thosefrom Laoheishan, Fangshan, and Dashan are nephelinites (Fig. 2). Nocorrelation is observed between MgO and other oxides. In achondrite-normalized REE diagram (Fig. 3), these alkaline rocksshow a pattern of strong LREE enrichment (La/Yb=39.4–65.7) withno significant Eu or Ce anomalies. In a primitive-mantle normalizedspidergram (Fig. 4a), the rocks resemble many ocean island basalts interms of enrichment in Nb and Ta, and depletion in K and Pb, relative

Table 1Trace element compositions (10−9) of blank and international standards (10−6).

Blank BHVO-2 AGV-1 BCR-2 G-2

Thisstudy

Ref. Thisstudy

Ref. Thisstudy

Ref. Thisstudy

Ref.

Li 0.0001 4.58 4.80 10.9 10.7 10.2 9.00 34.3 34.0Be 0.0000 1.04 1.10 2.13 2.10 2.30 2.30 2.57 2.50Sc 0.0001 31.6 32.0 12.4 12.2 33.2 33.0 3.97 3.50V 0.0008 319 317 120 121 416 416 35.7 36.0Cr 0.0020 280 280 10.4 10.1 16.8 18.0 7.99 8.70Co 0.0001 44.7 45.0 15.2 15.3 37.5 37.0 4.59 4.60Ni 0.0013 119 119 15.6 16.0 13.3 13.0 2.52 5.00Cu 0.0004 128 127 57.3 58.0 23.7 19.0 10.8 11.0Zn 0.0007 103 103 86.8 88.0 132 127 84.3 86.0Ga 0.0001 20.9 21.7 20.6 20.0 22.2 23.0 24.6 23.0Ge 0.0000 1.66 1.60 1.21 1.25 1.65 1.16 1.14Rb 0.0005 10.7 9.80 67.8 66.6 48.5 48.0 166 170Sr 0.0018 395 389 670 660 345 346 473 478Y 0.0001 26.6 26.0 20.0 20.0 37.1 37.0 10.2 11.0Zr 0.0007 170 172 233 227 189 188 343 309Nb 0.0001 18.7 18.1 14.4 14.6 12.7 13.2 12.5 12.0Cs 0.0000 0.11 0.10 1.26 1.28 1.15 1.10 1.33 1.34Ba 0.0028 133 130 1217 1200 701 683 1875 1880La 0.0001 15.5 15.0 38.7 38.2 25.6 25.0 87.4 89.0Ce 0.0003 38.0 38.0 68.6 67.6 53.2 53.0 159 160Pr 0.0000 5.17 5.29 8.12 7.80 6.64 6.80 16.3 18.0Nd 0.0001 24.9 25.0 32.1 31.7 29.3 28.0 53.7 55.0Sm 0.0000 6.21 6.20 5.81 5.72 6.68 6.70 7.21 7.20Eu 0.0000 1.98 2.07 1.68 1.58 1.94 2.00 1.50 1.40Gd 0.0000 5.92 6.30 5.22 4.70 6.72 6.80 5.88 4.30Tb 0.0000 0.93 0.90 0.67 0.71 1.06 1.07 0.54 0.48Dy 0.0000 5.26 5.31 3.61 3.55 6.42 6.41 2.41 2.40Ho 0.0000 1.02 1.04 0.71 0.69 1.37 1.33 0.38 0.40Er 0.0000 2.50 2.54 1.85 1.82 3.68 3.66 0.99 0.92Tm 0.0000 0.33 0.33 0.26 0.28 0.53 0.54 0.12 0.12Yb 0.0000 2.02 2.00 1.69 1.63 3.47 3.50 0.75 0.80Lu 0.0000 0.28 0.28 0.24 0.24 0.51 0.51 0.11 0.11Hf 0.0000 4.28 4.10 4.93 5.10 4.84 4.80 8.00 7.90Ta 0.0000 1.20 1.14 0.87 0.90 0.81 0.78 0.82 0.88Pb 0.0002 1.83 2.60 35.1 36.0 10.6 11.0 30.9 30.0Th 0.0000 1.26 1.20 6.29 6.50 6.18 6.20 25.2 25.0U 0.0000 0.43 0.42 1.90 1.92 1.71 1.69 2.09 2.07

Reference data for BHVO-2, AGV-1, BCR-2 and G-2 are taken from Govindaraju (1994).

to LREEs. However, they also show negative Zr, Hf, and Ti anomalies(Hf/Hf*=0.50–0.69, Ti/Ti*=0.40–0.52) and positive Sr anomalies.

Major and trace element data for the alkali olivine basalts fromShandong are listed in Supplementary Table 1. Compared with thestrongly alkaline rocks, these weakly alkaline rocks have relativelylow contents of alkalis (Na2O+K2O=3.0–5.5%), TiO2 (1.8=3.0 wt.%),and CaO (8.3–10.0 wt.%), higher contents of SiO2 (42.8–47.6 wt.%) andAl2O3 (12.3–14.7 wt.%) (Fig. 2), lower Ca/Al ratios (0.67–0.84), andlower concentrations of incompatible elements (Fig. 5). In achondrite-normalized REE diagram, the weakly alkaline rocks showless LREE/HREE and MREE/HREE differentiation (La/Yb=12.7–33.3,Sm/Yb=3.7–6.0) (Fig. 3). On a primitive-mantle normalized spider-gram, the patterns of the weakly alkaline rocks resemble those of thestrongly alkaline rocks, but with less pronounced Zr, Hf, and Tianomalies (Hf/Hf*=0.69–0.84, Ti/Ti*=0.91–1.26) (Fig. 4b).

To estimate the degree of melting represented by the alkalinebasalts in Shandong, we refer to a plot of La/Yb vs. Sm/Yb, followingthe inverse REE modeling reported in Feigenson et al. (2003) and Xuet al. (2005) (Fig. 6). The modeling indicates that variable degrees(1.5–10%) of batch melting of a hypothetical mantle source in thegarnet stability field can generate the La/Yb–Sm/Yb compositions ofthe Shandong rocks. The strongly alkaline rocks result from very lowdegrees of melting (1.5–3%), whereas the weakly alkaline rocks resultfrom higher amounts (3–10%).

All the alkaline rocks have superchondritic Zr/Hf ratios (Zr/Hf=38for chondrite–primitivemantle) (Fig. 7). The Zr/Hf ratios in the stronglyalkaline rocks and the weakly alkaline rocks are 44.4–50.8 and 44.7–49.3, respectively, higher than those in MORB and OIB (Fig. 7).

5. Discussion

5.1. Low degrees of melting in the asthenosphere

An asthenospheric origin with negligible crustal contamination hasbeen inferred from previous geochemical and Sr–Nd isotopic studies onrocks from the Cenozoic alkaline basaltic province of North China (ZhouandArmstrong, 1982; Penget al., 1986; Songet al., 1990; Zhi et al., 1990;Basu et al., 1991; Liu et al., 1994; Zou et al., 2000; Xu et al., 2005; Tanget al., 2006). This interpretation can also be applied to the alkalinebasalts fromShandongbecause of theirOIB-like features; e.g., negativeKand Pb anomalies and positive Nb and Ta anomalies (Fig. 4), anddepleted Sr–Nd isotopes (Peng et al., 1986). The occurrence of mantlexenoliths in these basalts indicates that the alkaline magma ascendedrapidly, with little chance for the magma to react with wall rocks.Furthermore, the enrichment of Nb also indicates negligible interactionswith lithospheric mantle, as such interactions would have lowered themagma's Nb/U ratios (Tang et al., 2006). In addition, the rocks of thisalkaline basaltic province have low SiO2 (39.2–47.6 wt.%) and highMgOcontents (6.5–13.9 wt.%), and there is no observable correlationbetween MgO and other oxides, suggesting negligible fractionalcrystallization of olivine and pyroxene. The lack of a negative Euanomaly (Fig. 3) suggests no significant removal of plagioclase.

Because these alkaline basalts experienced insignificant changesduring the magmatic process, the key differences in elementalgeochemistry between the strongly and weakly alkaline rocks ofShandong (Figs. 2–5) are likely to result from different mantle sourcesor from different degrees of melting of similar mantle sources.However, the Sr–Nd isotopic compositions of Cenozoic alkalinebasalts, not only from Shandong but from the entire NCC, are depletedand vary in a narrow range (see the review by Zhang et al., 2002),which suggests that both the strongly andweakly alkaline rocks sharesimilar sources. Fig. 6 shows that variable degrees (1.5–10%) of batchmelting of a hypothetical mantle source in the garnet stability fieldcan generate the La/Yb–Sm/Yb compositions of Shandong alkalinebasalts. The strongly alkaline rocks are a result of very low degrees ofmelting (1.5–3%), while the weakly alkaline rocks represent higher

Table 2Major element (wt.%) and trace element compositions (10−6) of strongly alkaline basalts from Shandong.

08GDS01 08GDS02 08GDS03 08GDS04 08GDS05 08QJFS01 08QJFS02 08QJFS03 08QJFS04 08QJFS05 08QJFS06 08QJFS07

N37°46′59.3″ N37°12′54.3″

E120°40′59.0″ E120°44′15.0″

SiO2 43.00 43.07 43.35 41.74 42.68 40.21 39.74 39.56 39.45 39.49 39.51 39.50TiO2 2.78 2.77 2.12 2.09 2.11 2.57 2.51 2.52 2.57 2.53 2.48 2.56Al2O3 12.57 12.58 12.45 12.23 12.30 11.41 11.30 11.33 11.19 11.27 11.21 11.32TFe2O3 14.47 14.41 13.31 12.75 13.12 15.06 14.65 14.76 14.79 14.74 14.50 14.78MnO 0.17 0.17 0.18 0.18 0.18 0.21 0.20 0.20 0.20 0.20 0.20 0.21MgO 7.81 7.87 9.43 11.30 10.36 8.99 9.20 9.99 10.25 10.28 10.86 10.07CaO 9.44 9.38 9.91 9.83 9.98 11.65 12.57 11.38 11.46 11.35 11.11 11.21Na2O 4.76 4.97 4.77 4.57 5.31 5.23 3.39 4.14 4.16 4.24 3.70 4.00K2O 2.17 2.20 1.25 1.52 1.56 2.79 1.99 2.41 2.33 2.33 1.97 2.20P2O5 1.38 1.35 1.23 1.18 1.21 0.79 0.85 0.89 0.87 0.87 0.89 0.89LOI 1.54 1.19 1.88 2.50 1.00 1.08 3.10 2.33 2.36 2.23 3.15 2.83Total 100.09 99.96 99.88 99.89 99.81 99.99 99.50 99.51 99.63 99.53 99.58 99.57mg# 0.52 0.52 0.58 0.64 0.61 0.54 0.55 0.57 0.58 0.58 0.60 0.57Li 11.17 11.65 11.82 10.98 11.14 15.51 15.39 14.80 13.40 15.70 14.41 16.27Be 3.55 3.73 3.30 3.67 3.61 6.26 6.30 6.38 6.49 6.67 6.64 6.86Sc 14.17 14.27 18.78 18.11 18.35 14.32 14.28 13.52 14.11 14.30 14.39 14.32V 174 175 193 180 184 185 164 180 185 180 181 182Cr 151 141 333 276 270 200 188 192 198 203 216 200Co 49.32 49.66 52.86 52.91 53.26 56.68 56.58 56.48 56.85 56.71 56.76 56.88Ni 147 137 252 237 239 174 172 174 171 181 185 176Cu 43.91 44.53 50.45 47.36 46.22 48.10 43.59 45.77 45.20 47.88 47.76 47.66Zn 160 162 129 127 127 174 168 172 173 170 167 171Ga 27.35 27.56 23.04 23.40 23.40 28.08 26.53 28.05 27.96 28.04 27.31 27.88Ge 1.62 1.61 1.60 1.55 1.57 1.48 1.48 1.45 1.48 1.49 1.44 1.46Rb 28.09 26.22 28.05 27.52 30.46 30.37 29.00 27.42 25.86 26.08 22.14 25.35Sr 1383 1359 1306 1334 1302 1270 2584 1483 1420 1515 1303 1336Y 31.32 30.91 28.02 27.98 27.89 32.16 31.79 31.28 31.34 31.57 32.01 31.78Zr 370 369 293 311 313 417 402 408 409 412 404 413Nb 110 112 96 106 108 150 144 146 147 146 144 149Cs 0.66 0.62 0.63 0.48 0.49 0.56 0.69 0.55 0.49 0.51 0.40 0.54Ba 799 779 725 483 563 275 871 388 291 515 284 265La 78.2 78.2 68.8 69.1 70.9 97.8 98.2 97.0 96.7 98.2 98.1 99.5Ce 146.1 146.3 128.5 127.5 130.8 179.8 178.8 179.2 177.6 181.6 179.4 182.0Pr 16.59 16.52 14.34 14.39 14.74 20.15 20.18 20.02 19.85 20.00 19.77 20.04Nd 66.21 66.35 57.48 58.12 59.23 80.34 80.43 79.74 80.05 81.04 80.71 82.07Sm 13.04 13.06 10.92 11.14 11.28 15.19 15.02 15.02 15.04 15.33 15.00 15.48Eu 3.93 3.93 3.29 3.29 3.37 4.52 4.58 4.50 4.52 4.58 4.55 4.62Gd 11.64 11.70 9.85 9.96 10.11 13.33 13.41 13.17 13.43 13.51 13.54 13.79Tb 1.50 1.49 1.25 1.25 1.27 1.68 1.66 1.66 1.68 1.70 1.70 1.71Dy 7.29 7.31 6.30 6.17 6.30 8.09 7.94 7.97 8.02 8.16 8.17 8.26Ho 1.21 1.21 1.09 1.06 1.08 1.31 1.31 1.29 1.32 1.33 1.33 1.33Er 2.58 2.61 2.49 2.43 2.48 2.79 2.76 2.77 2.81 2.83 2.87 2.85Tm 0.28 0.28 0.29 0.28 0.29 0.31 0.31 0.31 0.32 0.32 0.32 0.32Yb 1.58 1.56 1.67 1.65 1.69 1.64 1.63 1.66 1.69 1.67 1.72 1.68Lu 0.19 0.19 0.22 0.21 0.22 0.20 0.20 0.20 0.20 0.20 0.21 0.20Hf 7.78 7.74 5.96 6.35 6.43 9.14 8.91 8.93 9.15 9.22 9.10 9.29Ta 6.53 6.52 5.50 6.13 6.14 9.36 9.15 9.39 9.49 9.53 9.39 9.62Pb 5.57 5.56 5.25 5.00 5.06 6.64 6.68 6.62 6.60 7.06 6.69 6.99Th 10.64 10.71 9.20 9.46 9.59 15.72 15.66 15.62 16.00 16.31 16.54 16.72U 2.75 2.46 2.70 2.62 2.43 4.37 4.63 4.83 5.36 4.61 4.93 4.81Ca/Al 0.85 0.85 0.90 0.91 0.92 1.16 1.26 1.14 1.16 1.14 1.12 1.12La/Yb 49.62 50.13 41.07 41.81 42.06 59.47 60.24 58.58 57.03 58.95 57.19 59.38Sm/Yb 8.28 8.37 6.52 6.74 6.69 9.24 9.21 9.07 8.87 9.20 8.74 9.24Zr/Hf 47.59 47.72 49.15 48.99 48.75 45.69 45.06 45.75 44.65 44.62 44.42 44.52Na2O+K2O 6.93 7.17 6.02 6.09 6.87 8.02 5.38 6.55 6.49 6.57 5.67 6.20Ti/Ti⁎ 0.59 0.59 0.54 0.52 0.52 0.48 0.47 0.47 0.48 0.47 0.46 0.46Hf/Hf⁎ 0.66 0.65 0.59 0.62 0.62 0.65 0.64 0.64 0.65 0.65 0.65 0.65

TFe2O3, total iron as Fe2O3; mg#, molar Mg/(Mg+Fe)Hf/Hf*=HfN/(SmN×NdN)0.5; Ti/Ti*=TiN/(NdN

−0.055×SmN0.333×GdN0.722).

38 G. Zeng et al. / Chemical Geology 273 (2010) 35–45

degrees of melting (3–10%). These melting estimates are consistentwith the differing contents of incompatible elements such as Nb, Th,U, La, and Nd (Fig. 5), because the amount of incompatible elements inbasaltic melts increases with decreasing degree of melting.

The data and discussions above lead us to suggest that the Shandongalkaline basalts, especially the more strongly alkaline basanites andnephelinites, represent primitive melts derived from asthenosphericmantle. Their geochemical compositions closely reflect source composi-tions and degrees of melting. As shown in Figs. 2–5, with decreasing

degree ofmelting, the alkalinemelts show lower SiO2 andAl2O3 contents,higher total alkalis, CaO, and TiO2 contents, higher concentrations ofincompatible elements, increased La/Yb and Sm/Yb ratios, and morepronounced negative K, Zr, Hf, and Ti anomalies.

5.2. Sources of alkaline basalts

Although the asthenosphere is dominantly garnet peridotite,dry garnet peridotite is not a suitable source for the Shandong

08NIUS01 08NIUS02 08NIUS03 08LHS01 08LHS02 08LHS03 08LHS04 08LHS05 08LHS06 08LHS07 07JSS01 07JSS02 07JSS03 07JSS04 07JSS05

N37°44′31.2″ N37°58′14.9″ N38°0′44.3″

E120°43′17.1″ E120°35′53.8″ E117°40′49.2″

44.43 45.02 45.14 39.38 39.49 39.42 39.15 39.36 39.52 40.37 40.94 40.66 41.09 40.89 40.912.85 2.93 2.91 2.96 2.98 2.97 2.98 2.94 2.80 2.88 2.91 2.78 2.91 2.93 2.9413.26 13.80 13.70 11.37 11.34 11.37 11.30 11.35 11.41 11.23 10.72 10.27 10.75 10.74 10.7014.37 14.42 14.03 15.97 16.00 15.98 15.99 16.06 15.31 16.03 15.41 15.31 15.45 15.62 15.520.16 0.16 0.15 0.20 0.20 0.20 0.20 0.20 0.19 0.20 0.20 0.20 0.20 0.20 0.207.32 6.52 6.46 8.82 8.87 8.87 9.02 8.81 9.16 8.00 11.96 13.87 12.03 11.96 12.068.15 8.03 8.22 11.02 11.08 11.06 11.38 11.31 11.49 11.65 9.92 9.52 9.88 9.87 9.865.62 5.47 5.54 5.45 5.51 5.67 4.68 4.86 3.43 2.94 4.71 4.47 4.65 4.75 4.731.93 1.82 1.56 2.70 2.71 2.89 2.60 2.66 1.59 1.32 2.45 2.19 2.35 2.39 2.421.16 1.16 1.09 1.29 1.39 1.30 1.17 1.13 1.25 1.03 1.23 1.17 1.16 1.15 1.220.63 0.78 1.18 0.63 0.45 0.24 1.34 0.83 3.85 4.34 −0.33 −0.27 −0.36 −0.28 −0.4099.88 100.11 99.98 99.79 100.02 99.97 99.81 99.51 100.00 99.99 100.12 100.17 100.11 100.22 100.160.50 0.47 0.48 0.52 0.52 0.52 0.53 0.52 0.54 0.50 0.61 0.64 0.61 0.60 0.6110.63 9.35 9.87 13.29 14.30 14.16 11.15 9.95 13.89 16.58 11.54 10.45 10.78 11.16 11.063.59 3.58 3.45 4.95 5.25 5.14 5.13 5.39 4.60 5.12 3.73 3.55 3.66 3.62 3.6913.45 14.07 14.41 14.32 14.68 15.04 14.61 15.11 18.23 16.99 15.07 15.79 16.88 16.75 16.76150 199 173 223 238 233 236 234 242 242 211 195 206 210 207134 125 122 123 126 125 120 126 173 174 346 483 348 352 34647.30 46.73 48.54 56.77 59.79 59.51 58.42 58.38 59.84 58.69 59.91 62.58 60.25 60.02 60.22116 110 111 107 112 108 106 106 131 124 290 398 290 281 28438.25 45.47 43.02 51.67 58.07 64.26 57.04 54.73 55.62 61.04 50.97 56.35 52.55 44.77 46.82163 163 160 174 184 176 178 181 169 183 154 149 153 154 15328.44 28.95 28.82 30.13 31.38 30.96 31.43 31.43 27.76 28.97 24.22 22.87 24.07 23.99 23.881.51 1.53 1.54 1.66 1.75 1.71 1.70 1.72 1.70 1.76 1.46 1.44 1.43 1.49 1.4636.74 39.55 46.19 29.88 31.83 34.33 33.09 31.98 26.98 17.78 30.87 32.46 30.55 30.77 31.021254 1271 1196 1681 1780 1851 1607 1493 1680 1312 1135 1105 1086 1087 114529.66 29.39 29.26 32.67 33.18 33.07 32.71 33.96 32.15 33.14 32.69 31.15 32.36 32.64 32.44377 371 360 441 452 438 439 452 396 415 359 340 351 351 353102 101 98 147 151 148 148 153 132 144 124 116 120 119 1210.54 0.55 0.60 0.18 0.18 0.32 0.57 0.56 0.89 0.31 0.55 0.49 0.54 0.55 0.55523 569 551 429 522 668 544 293 293 360 520 575 402 332 50465.1 64.9 62.0 94.6 98.2 97.3 98.8 99.8 88.3 96.7 70.1 68.2 69.4 69.4 69.2123.9 122.5 117.6 176.3 180.5 177.2 181.7 183.6 162.3 179.3 135.2 131.0 133.8 134.7 133.514.27 14.07 13.36 19.79 20.34 20.00 20.42 20.77 18.34 20.24 15.72 15.26 15.57 15.66 15.5158.04 57.34 55.43 77.50 79.40 78.43 79.39 80.81 73.02 78.49 62.65 60.92 62.56 62.38 62.3411.68 11.56 11.20 14.75 14.97 14.76 14.89 15.18 13.85 14.80 12.19 11.80 12.29 12.24 12.143.52 3.53 3.41 4.25 4.37 4.32 4.36 4.38 4.01 4.28 3.82 3.71 3.78 3.82 3.7910.55 10.35 10.17 12.79 13.10 12.91 12.89 13.17 12.07 12.85 10.33 9.99 10.34 10.27 10.261.37 1.36 1.32 1.57 1.60 1.57 1.58 1.61 1.48 1.57 1.57 1.50 1.54 1.55 1.546.70 6.75 6.55 7.44 7.60 7.46 7.50 7.63 7.06 7.50 7.22 6.89 7.13 7.18 7.081.12 1.13 1.10 1.21 1.22 1.21 1.21 1.23 1.16 1.23 1.21 1.16 1.19 1.22 1.212.43 2.45 2.39 2.61 2.64 2.60 2.60 2.65 2.51 2.66 2.69 2.56 2.67 2.69 2.660.27 0.27 0.27 0.28 0.28 0.28 0.28 0.29 0.28 0.30 0.29 0.28 0.28 0.29 0.281.47 1.49 1.47 1.51 1.54 1.55 1.50 1.56 1.56 1.63 1.66 1.58 1.62 1.67 1.660.18 0.19 0.18 0.18 0.19 0.18 0.18 0.19 0.19 0.20 0.21 0.19 0.20 0.20 0.207.87 7.81 7.70 8.88 9.12 8.67 8.77 8.89 8.06 8.19 7.42 7.08 7.29 7.26 7.366.11 6.08 5.91 8.63 8.73 8.63 8.68 8.83 7.47 8.30 7.65 7.28 7.49 7.43 7.485.11 4.74 5.10 1.06 1.50 1.35 4.64 6.15 5.63 6.04 4.83 6.20 5.45 6.00 5.489.29 9.08 8.65 13.60 13.78 13.61 13.71 13.94 11.90 13.12 10.17 9.70 10.01 9.76 10.132.48 2.54 2.35 3.96 4.02 3.93 4.06 4.19 1.96 3.76 3.03 2.43 2.92 2.79 3.030.70 0.66 0.68 1.10 1.11 1.10 1.14 1.13 1.14 1.18 1.05 1.05 1.04 1.04 1.0444.16 43.55 42.31 62.75 63.79 62.77 65.72 64.18 56.41 59.53 42.25 43.07 42.83 41.64 41.707.92 7.76 7.64 9.78 9.72 9.52 9.91 9.76 8.85 9.11 7.35 7.45 7.59 7.34 7.3147.82 47.45 46.81 49.69 49.53 50.56 50.12 50.81 49.15 50.70 48.41 47.94 48.20 48.30 47.987.55 7.29 7.10 8.15 8.22 8.56 7.28 7.52 5.02 4.26 7.16 6.66 7.00 7.14 7.150.67 0.70 0.71 0.57 0.56 0.57 0.57 0.55 0.57 0.55 0.69 0.68 0.69 0.70 0.700.75 0.75 0.77 0.65 0.66 0.63 0.63 0.63 0.63 0.60 0.67 0.65 0.65 0.65 0.66

39G. Zeng et al. / Chemical Geology 273 (2010) 35–45

alkaline basalts, for several reasons. First, high-pressure experi-ments on dry garnet peridotite do not produce melts of suitablecomposition (e.g., Hirose and Kushiro, 1993; Walter, 1998).Second, significant depletion of Zr, Hf, and Ti (Fig. 4a) cannotbe explained by partial melting of dry garnet peridotite becausethe bulk partition coefficients for Zr, Hf, and Ti between garnetperidotite and silicate melt are similar to those of middle REEs(Sm, Eu, and Gd) (Salters et al., 2002). Third, the superchondriticZr/Hf ratios in the alkaline basalts (Fig. 7) cannot be produced by

melting dry garnet peridotite because the peridotite has similarpartitioning coefficients for Zr and Hf (Salters et al., 2002).Previous experiments have shown that three kinds of source(silica-deficient eclogite–garnet pyroxenite, hornblendite, andcarbonated peridotite) can produce alkaline melts in shallowupper mantle environments (see the review by Pilet et al., 2008).A successful source candidate for the Shandong alkaline basaltsshould explain not only the key major and trace element features,but also the geochemical variations produced by different degrees

Fig. 2. Variations in Na2O+K2O, TiO2, CaO, and Al2O3 vs. SiO2 for alkaline basalts from Shandong. The strongly alkaline rocks and the weakly alkaline olivine basalts are shown asfilled circles and filled squares, respectively.

40 G. Zeng et al. / Chemical Geology 273 (2010) 35–45

of melting. In the following sections, we discuss the Shandongrocks in terms of these possible sources.

5.2.1. A hornblendite source?Sun and Hanson (1975) were the first to propose metasomatized

mantle with amphibole-rich veins as a possible source for alkalinebasalts. Recently, this proposal has been revived based on the resultsof studies of natural materials (Niu and O'Hara, 2003; Pilet et al., 2004,2005) and high-pressure experiments (Pilet et al., 2008). High-pressure melting experiments on hornblendite, clinopyroxene horn-blendite, and hornblendite plus peridotite convincingly demonstratethat such veined mantle can produce alkaline melts with composi-tions identical to those of alkaline basaltic rocks (Pilet et al., 2008).

Fig. 3. Chondrite-normalized REE patterns of Cenozoic alkaline basalts from Shandong.Carbonatite data are average values for oceanic magnesio-carbonatites and calcio-carbonatites from Cape Verdes (Hoernle et al., 2002), oceanic calico-carbonatites fromFuerteventura (Hoernle et al., 2002), and continental calico-carbonatites from Africaand Canada (Bizimis et al., 2003). The chondrite values are from Anders and Grevesse(1989).

Based on experimental results, Pilet et al. (2008) demonstrated thatgeochemical variations from nephelinite to alkali olivine basalt (suchas increasing SiO2 and Al2O3 and decreasing total alkalis, TiO2, andCaO) are induced by reactions between alkaline melts and peridotiticmantle.

On the whole, the partial melting of hornblendite satisfies keygeochemical features of the Shandong alkaline basalts, includingenrichment in strongly incompatible elements and negative K, Pb, Zr,and Hf anomalies (Fig. 4). The trends shown in the plots of SiO2

against other oxides, total alkalis, TiO2, CaO, and Al2O3 (Fig. 2) are alsoconsistent with those expected from reactions between hornblenditemelts and peridotite (see Fig. 1 in Pilet et al., 2008). In addition, anegative Ti anomaly and superchondritic Zr/Hf ratios can be observedin melts where the degree of melting is low and where the residuesare Ti-rich amphibole and ilmenite (Figs. 4c and 7).

However, in a plot of total alkalis against TiO2 (Fig. 8), the variationtrend of the hornblendite melts is different from that of the Shandongalkaline basalts and other experimentally produced melts. TheShandong alkaline basalts plot in the zone of (K2O+Na2O)/TiO2N1,whereas the hornblendite melts plot in the zone of (K2O+Na2O)/TiO2b1. For a given total alkalis content, the TiO2 content of thehornblendite melts is clearly higher than that in the Shandong rocks.The TiO2 contents are controlled by two factors. First, the hornblenditesource is composed mainly of Ti-rich amphibole, with excess Tirelative to alkalis. Second, a decreasing degree of melting of such a Ti-rich source does not result in significantly increased (K2O+Na2O)/TiO2 ratios in the melts produced. Therefore, it seems clear thatmelting of hornblendite cannot explain the alkalis–TiO2 relationshipsin the Shandong alkaline basalts (Fig. 8).

5.2.2. A silica-deficient eclogite–garnet pyroxenite source?The role of recycled oceanic crust (with or without sediment) in the

production of intra-plate alkaline basaltic rocks has been reviewed, forexample, by Hofmann (1997). Melting of recycled crust produces amagmaoversaturated in silica (PertermannandHirschmann, 2003) and

Fig. 4. Traceelement spidergram for (a) strongly alkaline rocks fromShandong, (b)weaklyalkaline rocks from Shandong, (c) melts from hornblendite and hornblendite plusperidotite, and (d) average carbonatites. Data for hornblendite and data from “sandwich”experiments are fromPilet et al. (2008). Data sources for average carbonatites are the sameas for Fig. 3. The primitive mantle values are from McDonough and Sun (1995).

41G. Zeng et al. / Chemical Geology 273 (2010) 35–45

thus cannot produce alkaline magmas directly. To resolve this problem,it has been suggested that recycled crust could be transformed intosilica-deficient garnet pyroxenites (Hirschmann et al., 2003; Kogisoet al., 2003) or eclogites (Kogiso and Hirschmann, 2006) by theextraction of silica-rich liquids during subduction.

However, this proposal cannot reproduce the negative Ti anomalydisplayed by the strongly alkaline rocks (Fig. 4a) unless residual rutilefeatures in the mantle source. Unfortunately, residual rutile would notonly induce the depletion of Ti, but also the depletion of Nb and Ta in themelts (Foley et al., 2000), and this is inconsistent with the observed Ti–Nb (Ta) decoupling in the strongly alkaline rocks (Fig. 4a).

In addition, this proposal is at odds with the observed negative Zrand Hf anomalies (Fig. 4a) and superchondritic Zr/Hf ratios (Fig. 7).Zircon exerts a strong influence on the concentrations of Zr and Hf, and

is certainly the most important phase for their fractionation. However,zircon cannot survive in such alkaline melts because of its highsolubility, and even if zircon did crystallize, it would not significantlyfractionate Zr from Hf (Linnen and Keppler, 2002). Therefore, thestrongly elevated Zr/Hf ratios (N44) of the alkaline basalts cannot beinducedby zircon. Inmantlemelting, thepartition coefficients for Zr andHf between residual rocks and silicate melts are functions of theclinopyroxene/garnet ratio (Klemme et al., 2002). DZr, DHf, and DZr/DHf

all increase as a function of a reduced clinopyroxene/garnet ratio(Klemme et al., 2002), which means that garnet clinopyroxenite (cpx/grtN1) can producemeltswith high Zr/Hf ratios butwithout negative Zrand Hf anomalies, and eclogite (cpx/grtb1) can produce melts withnegative Zr and Hf anomalies but with low Zr/Hf ratios. Therefore,pyroxenite–eclogite cannot produce melts with both superchondriticZr/Hf ratios and negative Zr–Hf anomalies.

In conclusion, a source of silica-deficient eclogite–garnet pyroxe-nite cannot explain the fractionation phenomena of Zr, Hf, and Ti inthe melts, and therefore cannot be the main contributor in theproduction of the Shandong alkaline basalts.

5.2.3. A carbonated peridotite source?A carbonated mantle source for alkaline basaltic rocks was proposed

originally as a result of early experimental work (e.g., Wyllie and Huang,1976; Wendlandt and Mysen, 1980). Carbonatite is one of the mostenriched rocks in the world, and is enriched in incompatible elementsexcept for K, Zr, Hf, and Ti. It is also characterized by extremely high Zr/Hfand Ca/Al ratios, and strongly negative K, Zr, Hf, and Ti anomalies inspidergram (with extremely lowTi/Ti* andHf/Hf* ratios), as observed forrocks such as oceanic carbonatites from Cape Verdes, Fuerteventura, andcontinental carbonatites from Africa and Canada (Figs. 3, 4d, 7, and 9)(Hoernle et al., 2002; Bizimis et al., 2003). Carbonatitic liquids areexpected to show these anomalies because the bulk partition coefficients(Dgarnet lherzolite/carbonatite liquids) for Zr, Hf, and Ti are much higher thanthose for REEs under the conditions of the deep uppermantle (Dasguptaet al., 2009). The addition of a carbonatitic component may, of course,enrich thedepletedmantle in incompatible elements (except for K, Zr,Hf,and Ti), as observed in mantle xenoliths (Yaxley et al., 1991, 1998;Dautria et al., 1992; Ionovet al., 1993, 1996;Rudnicket al., 1993;Norman,1998; Gorring and Kay, 2000). Melting of such carbonated mantle willproduce enrichedmelts with negative K, Zr, Hf, and Ti anomalies, similarto natural alkaline basaltic rocks (Hirose, 1997; Dasgupta et al., 2007).This explains the key geochemical features of the strongly alkalinebasaltic rocks from Shandong, including the negative K, Zr, Hf and Tianomalies (low Ti/Ti* and Hf/Hf* ratios) and high Zr/Hf and Ca/Al ratios(Figs. 3, 4d, 7, and 9); we suggest that these distinctive “carbonatiticfingerprints” would be evident even in the case of a small degree ofmelting.

One major criticism of the carbonated source model is thatmelts produced experimentally from carbonated peridotite showmuch lower TiO2 contents than do natural alkaline basalts (Piletet al., 2008) (Fig. 8). However, the lowest degree of melting in theexperiments performed by Dasgupta et al. (2007) was 5.6%, whichis higher than the degrees of melting calculated for the stronglyalkaline rocks from Shandong (1.5–3%). As shown in a plot of totalalkalis against TiO2 (Fig. 8), experimental melts from carbonatedperidotite define a well-correlated positive array, and both Ti andtotal alkalis contents increase with decreasing degrees of melting.All the Shandong rocks plot along the extension of the line definedby the experimental array. If the degrees of melting are lowenough, carbonated peridotite can produce alkaline liquids withTiO2 and alkali contents as high as those of the strongly alkalinerocks from Shandong (Fig. 8). Therefore, very low degrees ofmelting of carbonated peridotite can reproduce all the key majorand trace element signatures of the strongly alkaline rocks ofShandong.

Fig. 5. Variations in Th, U, La, and Nd vs. Nb for Cenozoic alkaline basalts from Shandong. Symbols for various Shandong rocks are the same as in Fig. 2.

42 G. Zeng et al. / Chemical Geology 273 (2010) 35–45

5.3. Genesis of Cenozoic alkaline basalts in Shandong

To evaluate whether the key trace-element features of the stronglyalkaline rocks from Shandong (including the negative Zr, Hf, and Tianomalies) are inherited from carbonated mantle, we performedtrace-element modeling of mantle melting. As shown in Fig. 10, a lowdegree (1.5–3%) of batch melting of carbonated peridotite (depletedMORB mantle+0.3–1% carbonatite) can reproduce these “carbonati-tic fingerprints,” including the enrichment of incompatible elementsexcept for Zr, Hf, and Ti. With an increased contribution of the

Fig. 6. Variations in Sm/Yb vs. La/Yb for Cenozoic alkaline basalts from Shandong. Datafor Anfengshan basalts (open triangles) are from Chen et al. (2009); other alkalinebasaltic rocks from the NCC (filled gray squares) include examples from Hannuoba (Liuet al., 2008), Datong (Xu et al., 2005), Taihang (Tang et al., 2006), and Su-Wan (Zhanget al., 2009). Also shown are batch melting curves calculated for garnet peridotite.Partition coefficients are taken from Johnson et al. (1990). The inverse modeling usedhere follows Feigenson et al. (2003). Symbols for the Shandong rocks are the same as inFig. 2.

carbonatitic component or a decreased degree of melting, these keygeochemical features gradually become evident.

Because carbonation lowers the solidus of mantle peridotite,carbonated mantle may melt before anything else, and contributemore where the degree of melting is low. Near-solidus melts fromcarbonated mantle are carbonatites (Hirose, 1997; Dasgupta et al.,2007). Given that the degree of melting increases with elevatedtemperature, contributions from carbonated mantle will decrease inthe less alkaline basalts (Fig. 8), as dry peridotite (and/or quartz-deficient eclogite–garnet pyroxenite) would also start to melt underthese conditions. Therefore, under high degrees of melting and higher

Fig. 7. Variations in Zr/Hf vs. Zr for Cenozoic alkaline basalts from Shandong. The Zr/Hfratio (79) and Zr content (54 ppm) for carbonatite are average values for continentaland oceanic carbonatites (Hoernle et al., 2002; Bizimis et al., 2003). MORB and OIB dataare from Munker et al. (2003). Also shown are hornblendite melts (Pilet et al., 2008).The chondrite values are after Anders and Grevesse (1989). Symbols for Shandongrocks are the same as in Fig. 2.

Fig. 8. Variations in Na2O+K2O vs. TiO2 for Cenozoic alkaline basalts from Shandong and experimental alkaline melts. High-pressure experimental melts are those from carbonatedperidotite (Dasgupta et al., 2007), carbonated eclogite (Dasgupta et al., 2006), garnet pyroxenite (Hirschmann et al., 2003; Kogiso et al., 2003), bimineralic eclogite (Kogiso et al.,2006), and hornblendite (Pilet et al., 2008), respectively. Symbols for Shandong rocks are the same as in Fig. 2.

43G. Zeng et al. / Chemical Geology 273 (2010) 35–45

temperatures, the so-called “carbonatitic fingerprints”will be diluted,as shown in the weakly alkaline basalts of Shandong.

Although we suggest that carbonated mantle is the main source ofthe Shandong alkaline basalts, we cannot exclude the influence of aminor component of recycled crust (quartz-deficient eclogite–garnetpyroxenite), mechanically mixed in the mantle source. This mayexplain the subchondritic Nb/Ta ratios of the Shandong alkalinebasalts (Liu et al., 2008) and the negative correlations of 87Sr/86Sr,143Nd/144Nd, and 206Pb/204Pb ratios vs. ΔεHf values observed inAnfengshan basalts, a single Miocene volcano located in the Sulu UHPbelt (Chen et al., 2009). In a plot of La/Yb vs. Sm/Yb, all the NCC

Fig. 9. Variations in Ca/Al and Hf/Hf* vs. Ti/Ti* for Cenozoic alkaline basalts fromShandong. The values of Ca/Al (313), Hf/Hf* (0.015), and Ti/Ti* (0.013) for carbonatiteare average values for continental and oceanic carbonatites (Hoernle et al., 2002;Bizimis et al., 2003). Element anomalies are calculated as follows: Hf/Hf*=HfN/(SmN×NdN)0.5; Ti/Ti*=TiN/(NdN

−0.055×SmN0.333×GdN0.722). Symbols for Shandong rocks

are the same as in Fig. 2.

alkaline basalts plot on themelting curvemodeled from the Shandongrocks (Fig. 6). For a given La/Yb ratio, the Anfengshan basalts havehigher Sm/Yb ratios than other basalts, and deviatemarkedly from theShandong melting curve. Such a “garnet signature” suggests eclogitecomponents in the source (Fig. 6). Therefore, we deduce that (1) thecontribution of residual crust (eclogite) in the shallowmantle beneaththe NCC is very limited, or (2) the residual crust is too refractory to bethe source of alkaline basalts. In addition, these observations suggestthat the relations between La/Yb and Sm/Yb ratios might provide analternativemethod of quantifying the contribution of recycled crust inthe mantle to the production of alkaline basalts.

Intra-plate alkaline basalts worldwide show a wide range ofgeochemistry, and some lack the characteristic features of the stronglyalkaline rocks from Shandong. For example, some alkaline provinces haveexcessTiO2 contents (PrytulakandElliott, 2007)orexcessTFe2O3 contents(Dasgupta et al., 2010), and in these cases we cannot exclude the

Fig. 10. Results of trace element modeling of the melting of carbonated mantle.Partition coefficients for Ba, Th, U, Pb, and Ti between ol, cpx, opx, gt, and melt are takenfrom Adam and Green (2006); partition coefficients for Nb, Zr, Hf, Y, and REEs betweenol and melt are taken from Zanetti et al. (2004); partition coefficients for Nb, Zr, Hf, Y,and REEs between cpx, opx, gt, andmelt are taken from Green et al. (2000). The startingmaterials are as follows: ol, 62%; opx, 20%; cpx, 15%; and gt, 3%; melting reaction in thegarnet field (Walter, 1998): ol, 3%; opx, 3%; cpx, 70%; gt, 24%. The data for depletedupper mantle (DMM) are from Workman et al. (2005). The average values forcarbonatites are based on data for oceanic magnesio-carbonatite from Cape Verdes(Hoernle et al., 2002). The primitive mantle values are from McDonough and Sun(1995).

44 G. Zeng et al. / Chemical Geology 273 (2010) 35–45

possibility of a source comprising quartz-deficient (and/or carbonated)eclogite–garnet pyroxenite or hornblendite. According to the composi-tions of those experimental melts, for a given Na2O+K2O, alkalinebasalts frommixed sources (carbonated peridotite+carbonated eclogite/hornblendite)may have increased TiO2 content (Fig. 8). Indeed, given thewide variations in chemistry, it is unlikely that a single source type isresponsible for all alkaline basalts worldwide, and more work is clearlyneeded. Anomalies in the conductivity of the asthenospherehave recentlybeen interpreted to indicate the presence of small amounts of carbonatemelt (0.1 vol.% on average) in the peridotite (Gaillard et al., 2008). Thisfinding leads us to suggest that carbonate might play a more importantrole in magmatism than previously thought.

6. Conclusions

The formation of the Cenozoic alkaline basalts in Shandong, Chinainvolved two stages. The first involved a profuse outpouring of weaklyalkaline rocks (alkali olivine basalts) in association with a major faultline, and the second was marked by the production of more stronglyalkaline rocks (basanites and nephelinites) in the form of scatteredsmaller volcanoes. Thestrongly alkaline rocks represent lowerdegreesofmelting at the source compared with the weakly alkaline rocks, asestimated fromtheir La/YbandSm/Yb ratios. Those rocks that formedviaa low degree of melting possess geochemical signatures similar to thoseof carbonatites, including high Ca/Al and Zr/Hf ratios, and negative K, Zr,Hf, and Ti anomalies. We suggest that the carbonatitic “fingerprints” ofthese rocks are inherited from an asthenospheric source that hadundergone enrichment with carbonatitic liquids, and our observationsindicate that thesource rocks of the strongly alkalinebasalts in Shandongwere mainly carbonated peridotite.

Acknowledgements

We are grateful to Y. Liu for her technical support. J.-Q. Liu, S.-L. Huand D. Luo attended the field investigations of this study.We appreciatethe thoughtful and constructive reviews provided by Rajdeep Dasguptaand an anonymous reviewer. This study was supported by the NationalNatural Science Foundation of China (Grant 40772035) and ChineseMinistry of Science and Technology (Grants 2006CB403508).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.chemgeo.2010.02.009.

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