Science in China Series D: Earth Sciences

© 2008 SCIENCE IN CHINA PRESS

Springer

Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566

www.scichina.comearth.scichina.com

www.springerlink.com

Major element, trace element, and Sr, Nd and Pb iso-tope studies of Cenozoic basalts from the South China Sea

YAN QuanShu1,2, SHI XueFa1†, WANG KunShan1, BU WenRui1 & XIAO Long3 1 Key Lab of Marine Sedimentary and Environment Geology, The First Institute of Oceanography, State Oceanic Administration,

Qingdao 266061, China; 2 Institute of Oceanology, Chinese Academy of Sciences, Qingdao 26607, China; 3 Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China

The whole rock K-Ar ages of basalts from the South China Sea basin vary from 3.8 to 7.9 Ma, which suggest that intra-plate volcanism after the cessation of spreading of the South China Sea (SCS) is comparable to that in adjacent regions around the SCS, i.e., Leiqiong Peninsula, northern margin of the SCS, Indochina block, and so on. Based on detailed petrographic studies, we selected many fresh ba-saltic rocks and measured their major element, trace element, and Sr-Nd-Pb isotope compositions. Geochemical characteristics of major element and trace element show that these basaltic rocks belong to alkali basalt magma series, and are similar to OIB-type basalt. The extent of partial melting of mantle rock in source region is very low, and magma may experience crystallization differentiation and cu-mulation during the ascent to or storing in the high-level magma chamber. Sr-Nd-Pb isotopic data of these basaltic rocks imply an inhomogeneous mantle below the South China Sea. The nature of magma origin has a two end-member mixing model, one is EM2 (Enriched Mantle 2) which may be originated from mantle plume, the other is DMM (Depleted MORB Mantle). Pb isotopic characteristics show the Dupal anomaly in the South China Sea, and combined with newly found Dupal anomaly at Gakkel ridge in Arctic Ocean, this implies that Dupal anomaly is not only limited to South Hemisphere. In variation diagrams among Sr, Nd and Pb, the origin nature of mantle below the SCS is similar to those below Leiqiong peninsula, northern margin of the SCS and Indochina peninsula, and is different from those below north and northeast China. This study provides geochemical constraints on Hainan mantle plume.

Cenozoic alkali basalt, Dupal anomaly, Hainan mantle plume, the South China Sea, geochemistry.

1 Introduction

Primitive, mantle-derived rocks are traditionally used to constrain the composition of their mantle source regions. In contrast to mid-oceanic ridge basalts (MORB), whose origin is generally considered as the melting products of the depleted upper mantle, ocean island basalts (OIB) are believed to be derived from mantle plumes rooting in the lower mantle or even the D″ layer at the core/mantle boundary[1―7]. Therefore, study of mafic lava resulted from mantle fusion and melt migration is fundamental to

understand the dynamics evolution of the Earth. This information is crucial for understanding global cycling models and for assessing what material and how much is fed into the lower mantle at subduction zones and then returned back to the surface via deep mantle plumes[5].

After the cessation (15.5 Ma[8]) of seafloor spreading, Received October 30, 2007; accepted December 5, 2007 doi: 10.1007/s11430-008-0026-3 †Corresponding author (email: xfshi@fio.org.cn) Supported by Special Plan Project of Science and Technology Generalship in Qing-dao (Grant No.05-2-JC-79) and Special Project of Technical Foundational Work and Social Public Welfare Research (Grant No. 2003DIB3J114)

YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566 551

intraplate basaltic volcanism took place in or around the South China Sea (SCS), affecting a large-scale areas which include Indochina peninsula, Leiqiong peninsula, the ocean base of SCS and rifted microcontinental frag-ments in SCS. The distributional regions of Cenozoic basalt from intra-plate volcanism include the South China Sea, Leiqiong peninsula, Beibuwan Basin, Indo-china peninsula and so on, in which there appears a cer-tain extent of Cenozoic alkali basalts. Until recently, only several studies have been carried out on the Ceno-zoic basalt from the SCS (Figure 1). In earlier time, Wang et al.[9] first reported seamount basaltic rocks from the SCS, and subsequently Kudrass et al.[10] studied ba-saltic rocks from Reed bank in Nansha Islands. Several scholars reported some Cenozoic volcanic rocks from the SCS and Hainan Island[11―14], and suggested that mantle origin beneath the SCS is a binary mixing model including enriched and depleted mantle end-member and

further indicated that there may exist an endogenetic Dupal anomaly within SCS[11]. Hoang et al.[15] and Zhou et al.[16] also argued that mantle origin was consistent with a binary mixing model as Tu et al.[11] did, and fur-ther suggested that some basaltic rocks have continen-tal-contaminated characteristics[17]. However, the origin of enriched mantle end-member and deep geodynamical regime of Cenozoic basalts from the SCS and adjacent areas are still in debate. Some scholars believed that en-riched mantle end-member (EM) is from sub-continental lithospheric mantle (SCLM)[11,15 ― 17], and yet others suggest enriched end-member may be related to mantle plume and/or hotspots[18―21].

Data on post-spreading volcanism in the SCS is gen-erally limited, and previous work mainly focused on tholeiitic basalts from seamounts. In this study, we pre-sent data for major and trace elements, and strontium, neodymium, and lead isotopic compositions for late

Figure 1 Geological sketch of the South China Sea and its adjacent areas and sampling location (The approximate location of Hainan Mantle Plume is from ref. [7]).

552 YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566

Cenozoic alkali basalts from the SCS. Combined with latest geophysical data, this study attempts to discuss nature of mantle origin, develop a preliminary petroge-netic model, and provide geochemical evidence for the presence of Hainan mantle plume.

2 Geological setting

The SCS is one of the largest marginal basins in western Pacific Ocean. The SCS can be subdivided into three parts, i.e., northern continental margin, ocean basin and southern continental margin. Until now, the consensus about initial and terminal ages of seafloor spreading in the South China Sea has been that its initial spreading age is Late Eocene to Early Oligocene after the collision between Indian plate and Asian plate, and its terminal age is Middle Miocene due to the collision between the front side of Indo-Australian and Sunda conti-nent[8,11,22,23]. The remarkable morphological characteri- stics defining the SCS include East Vietnam fault in the west, the escarpment along the northern margin of Reed bank in the south, the Manila trench in the east, and a smoothly transition from continent to ocean in the north (Figure 1)[24]. Tectonically, the northern margin, which lies between south China fold belt and COT (continent to ocean transition belt) of the South China Sea, belongs to an extension region, the eastern and southern margin is mainly a compressive subduction zone, and the west-ern margin is a strike-slip and pull-apart zone. In addi-tion, there are several micro-continental blocks dis-persed in the South China Sea, which include Xisha- Zhongsha block, Nansha block and Reed-Northeastern Palawan block. In the ancient Tethyes, these blocks, together with Hainan block, were called “Qiongnan block” [23]. Due to the transformation from compression to extension in regional stress field, these blocks started to split from Indochina-south China ancient continent southeastward and southward since the Cretaceous[25―27]. However, debates still remain on the dynamical origin

about the split process, and the formation and evolution of the SCS[8,22,28,29].

3 Sampling, petrography and analytical methods

During national investigation program for marginal ba-sin, we carried out extensive rock sampling work and obtained significant amounts of alkali basaltic rocks from seven sampling stations, especially from location S04-12DG. The rock samples cover a large area from southern slope to northern slope of the SCS and are suf-ficient to represent the nature of Cenozoic alkali basalts from the SCS. Based on macroscopic and microscope observations, 16 fresh samples were selected for major and trace elemental analysis (Table 1). Therein 11 sam-ples were conducted Sr-Nd-Pb isotopic composition analysis. Whole rock K-Ar ages for these samples varied from 3.80 to 7.91 Ma (with average of 5.43 Ma), and the age difference among samples in the same location is very small. The dating results indicate that rock-forming age is late Miocene to early Pliocene, consistent with eruption event around the SCS, for example, 3.5 Ma for alkali basalts from the SCB ( K-Ar/Ar-Ar age)[11], 0.5―2.7 Ma for olivine basalts from Reed Bank Seamounts (K-Ar age)[10], 5.9―2.4 Ma for alkali basalts from Beibuwan basin (K-Ar age)[19] and Quaternary for alkali basalts from the Weizhou Island (14C age)[20] in the northern margin of the SCB, 0―13 Ma for alkali basalts in Vietnam (K-Ar/Ar-Ar age)[15], and 0.9 Ma for alkali basalts in Thailand (K-Ar age)[16].

Samples in locations S04-11DG, S04-12DG, S04-13DG,S04-14DG and S05-34DG are dredged from seamounts in ocean basin and mainly are alkali olivine basalt, and the whole petrographic characteristics of these basaltic samples are as follows: porphyritic texture (10% ― 25% phenocrysts) and vesicular structure, phenocryst minerals are olivine (Fo75.9−86.6), clinopy-roxene (Wo31−50En38−62Fs5−12) , plagioclase (An47−54) and

Table 1 Sampling locations for Cenozoic alkali basalts from the South China Sea

Location No. Sample No. Latitude (N) Longitude (E) Water depth (m) S04-11DG S04-11 16°20.6′ 116°05.9′ 2200 S04-12DG S04-12-8, S04-10―12, S04-18, S04-20―22 15°34.3′ 116°09.5′ 1290 S04-13DG S04-13-3B 15°07.9′ 116°30.0′ 1480 S04-14DG S04-14-1B, S04-14-2B 14°02.3′ 115°23.1′ 1470 S04-16DG S04-16 16°05.3′ 113°54.1′ 1500

S05-34ADG S05-34A 13°58.0′ 115°29.5′ 910 S08-69DG S08-69-1, 2 10°19.3′ 112°32.1′ 2280

YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566 553

no orthopyroxene, and there often appears plagioclase, clinopyroxene and olivine microlite, and a large amounts of volcanic glass in groundmass. Sample in location S04-16DG is dredged from northern slope and is olivine basalt, and the petrographic characteristics are porphyritic texture (20%―25% phenocrysts) and vesi- cular structure, phenocryst minerals are clinopyroxene (Wo32−35En58−61Fs6−7), olivine (Fo80.4−82.2), a small amount of hornblende, and no plagioclase and orthopy-roxene, and there often appear plagioclase, clinopyrox-ene,olivine microlite, and volcanic glass in groundmass. Samples in location S08-69DG are dredged from south-ern slope and are olivine basalt, and the petrographic characteristics are porphyritic texture (25% ― 30% phenocrysts) and vesicular structure, phenocryst miner-als are olivine (Fo83.3−84.6), clinopyroxene(Wo49En29Fs18), and no plagioclase and orthopyroxene, and there often appear plagioclase, clinopyroxene, olivine microlite, and volcanic glass in groundmass.

For sample preparation, whole-rock samples were crushed to > 1 cm size and pulverised in an agate disk mill after removal of vesicle-filling material. Major element and trace element abundance measurement was accomplished in National Research Center for Geoana-lysis(China). Sixteen whole-rock samples were prepared by removing weathered surfaces followed by pulveriza-tion in an agate shatterbox. In addition to FeO, all other major elements and the trace elements Ba, Cu, Sr, Ti, V, Zn were analyzed using Inductive Coupled Plasma - Atomic Emission Spectrometry (ICP-AES). Estimated precision is better than 2%―8% for major elements and better than 10% for trace elements. The analytical method of FeO is as follows: weigh 0.1000―0.5000g sample (sample amount weighed in terms of the content of FeO in sample) and put it into polytetrafluoro crucible, then add hydrofluotic acid and sulfate to decompose sample, finally titrate the content of FeO using potas-sium dichromate standard solution. Estimated precision is better than 10%. The calculating formulation of Fe2O3 is as follows: WFe2O3=WTFe2O3−WFeO×1.1113. The rare-earth elements (REE) and other trace element con-centrations were determined by Inductive Coupled Plasma-Mass Spectrometry (ICP-MS).The uncertainty of this method is better than 10%. Sr, Nd, Pb isotope ratios were measured on a Finnigan MAT-262 thermal ionization mass spectrometer (TIMS) in the Laboratory for Radiogenic Isotope Geochemistry, Institute of Geo-

logy and Geophysics, Chinese Academy of Sciences, Beijing. Technique details on chemical separation and measurement are described in Chen et al.[30]. 143Nd/144Nd ratios were normalized to 146Nd/144Nd=0.7219 and 87Sr/86Sr ratios to 86Sr/88Sr=0.1194. Procedural blanks were <200 pg for Sr and <50 pg for Pb and Nd. Pb standard NBS 981 was used to determine thermal frac-tionation and measured isotopic ratios of samples were corrected with a value of 0.1% per atomic mass unit.

4 Analytical results

4.1 Major elements

The major element compositions of the basaltic rocks from the SCS are given in Table 2. Alkali basalts are the dominant rock types; other rocks are trachybasalts and basalts (Figure 2). All rock sample data are plotted into alkali series field. Rittmann indices (σ = (Na2O+K2O)/ (SiO2―43) ( in weight percent )) of most of these rocks are higher than 9 (ranging from 8.3 to 65.0) (Table 2), suggesting they are alkaline magma series too. Chemical composition of these basaltic rocks are as follows: 40.08%―50.03% for SiO2 (average value is 45.85%); 2.13%―9.49% for MgO (average value is 4.97%). Ma-jor element compositions of these basaltic rocks are close to those of ocean island basalts (OIB), for example, alkali basalts from Kerguelen plateau[33].

Figure 2 TAS and alkaline discriminant diagram (after ref. [31]). Rock appellations represented by number in figure are as follows: 1, feld-spathoid; 2, picritic basalt; 3, basalt; 4, basaltic andesite; 5, tephrite- basanite ; 6, trachyte basalt; 7, basaltic trachyte; 8, phonolitic tephrite. The line dividing alkalic and tholeiitic fields is from ref. [32].

Figure 3 shows the variations of some major-element oxide concentrations versus weight MgO. As a whole, with decreasing MgO contents, Al2O3 and FeO take on

554 YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566

Figure 3 Variation diagram between MgO and other major elements.

increasing and decreasing trends, respectively, and variation trends of other major element compositions are not apparent, which indicates that parental magma ex-periences variable crystalline differentiation effects du- ring evolution. When MgO < 4%, there is no clear cor-relation between MgO and other major element oxides. In a word, the major elements correlation with MgO contents basically follows the common trends of Ol and Ol+Cpx fractional crystallization and/or phenocryst ac-cumulation. 4.2 Trace elements Trace element concentrations are also listed in Table 2. Nickel and Chromium concentrations decrease with de-creasing MgO from 232 to 13.6 ppm Ni (average value is 99.7 ppm) and from 248 to 5.6 ppm Cr (average value

is 125.8 ppm), which mostly is due to Ol and Cpx frac-tionation (Figure 4). Nickel, although being mostly con- trolled by olivine and clinopyroxene crystallization, scatters significantly in the lavas with MgO < 4wt.% (see Table 2; Figure 4(a), (c)), i.e., the variation range of Ni and Cr is 16.6―176 ppm and 5.59—194 ppm, re-spectively, which could be related to different degrees of Ol or Cpx accumulation in the individual rock samples. This is unlikely to be an analytical artifact since similar variations were obtained by both ICP-AES and ICP-MS methods, and could not be related to different degrees of olivine accumulation in the individual rock samples that result in Ni-MgO increasing coherently. We believe that the observed incompatible trace elements scattered in whole rocks represents true variations in the melt.

Table 2 Major (wt.%) and trace element (ppm) concentrations in basalts from the South China Sea

Sample No. S04-11 S04-12-8 S04-12-10 S04-12-11 S04-12-12 S04-12-18 S04-12-20 S04-12-21 S04-12-22 S04-13-3B S04-14-1B S04-14-2B S04-16 S05-34A S08-69-1 S08-69-2

Rock type (TAS) Tephrite Tephrite Tephrite Tephrite Basalt Tephrite Tephrite Tephrite Tephrite Feldspathoid Tephrite Tephrite Tephrite Tephrite Trachyte

basalt Trachyte basalt

SiO2 44.8 48.75 48.28 47.57 45.43 50.03 46.97 48.48 42.35 40.08 43.6 44.9 44.42 46.8 45.72 45.46 TiO2 2.97 2.33 2.36 2.07 2.59 2.04 2.2 2.18 2.54 3.51 2.84 2.79 2.88 3.14 1.79 1.79 Al2O3 16.23 19.14 19.04 17.12 18.12 19.15 16.95 17.5 17.41 16.06 17 17.3 15.65 17.51 15.02 15.02 Fe2O3 3.29 3.21 3.34 4.42 8.72 4.33 5.33 2.61 13 8.7 8.03 6.31 4.37 3.85 3.6 5.41 FeO 6.48 4.28 4.48 5.2 2.73 2.7 4.85 6.84 1.47 2.79 1.95 2.87 5.68 5.17 6.31 5.83 MnO 0.19 0.14 0.15 0.23 0.23 0.11 0.33 0.2 0.44 0.22 0.56 0.12 0.53 0.28 0.16 0.2 MgO 7.79 2.42 2.6 5.74 3.81 2.13 5.91 5.61 3.8 3.58 2.7 2.9 7.89 4.17 9.49 8.93 CaO 9 8.52 8.83 8.1 9.87 8.45 7.96 8.11 8.19 12.26 10.45 10.5 9.05 7.44 9.79 9.78 Na2O 4.33 5.13 5.13 4.58 3.21 5.1 4.05 4.89 2.87 3.23 3.9 3.92 4.18 4.75 2.9 2.78 K2O 2.74 2.78 2.74 2.32 1.28 2.83 2.04 2.61 1.01 1.38 2.35 2.44 2.55 3.35 2.1 2.22 P2O5 0.67 0.77 0.83 0.85 0.95 0.94 0.7 0.68 1.17 3.42 2.81 2.77 0.68 1.24 0.47 0.47 H2O+ 2.08 2.24 1.87 1.93 2.9 2.1 2.77 0.54 4.13 2.65 1.85 2.2 1.65 1.7 1.54 1.7 CO2 0.09 0.09 0.09 0.05 0.13 0.05 0.05 0.13 0.13 0.5 0.5 0.36 0.13 0.05 0.09 0.05 Total 100.7 99.8 99.7 100.2 100 100 100.1 100.4 98.5 98.4 98.5 99.4 99.7 99.5 99 99.6 Mg# 59.5 37.6 38.3 52.7 39.1 36.6 52.2 52.1 34 37.6 34.4 37.7 59.4 46.3 63.9 59.8 σ 27.71 10.89 11.71 10.42 8.31 8.95 9.33 10.27 0 0 65 21.29 31.86 17.28 9.21 10.17 La 61.3 66.9 64.6 49.1 48.8 70.2 60.2 57 65.8 48.1 81.2 57 67.8 99.3 49.1 50.8 Ce 103 108 158 86.9 87.4 111 96.6 97.1 111 96.3 130 104 126 196 79.5 79.8 Pr 11.4 12.4 12 9.26 9.64 12.3 10.5 10.6 13 10.6 15.5 11.4 12.1 19 8.21 8.41 Nd 43.4 46.6 45.8 34.5 39.4 46 40.1 40.3 52.1 45.7 61.7 45.4 47.6 69.6 29.7 31.1 Sm 8.64 8.78 8.58 6.61 7.66 8.4 7.74 7.64 10.2 9.07 12.4 8.98 9.89 12.9 5.52 5.68 Eu 2.82 2.82 2.81 2.19 2.52 2.7 2.52 2.45 3.18 3.02 3.75 2.91 3.03 3.81 1.84 1.87 Gd 7.42 7.48 7.58 5.68 6.69 7.49 6.71 6.67 9.49 7.66 12 7.42 8.54 10.4 4.79 4.94 Tb 1.08 1.16 1.16 0.86 0.99 1.12 1.03 0.95 1.37 1.13 1.77 1.08 1.28 1.45 0.74 0.75 Dy 6.06 6.32 6.37 4.85 5.59 6.54 5.9 5.32 7.79 6.35 11.1 6.12 7.41 7.61 4.23 4.22 Ho 1.15 1.18 1.24 0.92 1.05 1.21 1.11 1.02 1.46 1.17 2.22 1.09 1.33 1.39 0.78 0.81 Er 3.16 3.37 3.48 2.85 3.01 3.54 3.15 2.85 4.16 3.24 6.58 3.02 3.8 3.81 2.15 2.21 Tm 0.43 0.45 0.48 0.36 0.39 0.47 0.45 0.38 0.53 0.43 0.9 0.38 0.49 0.49 0.27 0.3 Yb 2.67 2.88 2.95 2.24 2.43 3.05 2.65 2.39 3.2 2.56 5.63 2.3 3.04 2.98 1.8 1.8 Lu 0.43 0.44 0.46 0.35 0.37 0.48 0.41 0.36 0.48 0.39 0.92 0.35 0.48 0.45 0.26 0.27 δEu 1.05 1.03 1.04 1.06 1.05 1.01 1.04 1.02 0.97 1.07 0.92 1.05 0.98 0.97 1.06 1.05 δCe 0.88 0.84 1.28 0.92 0.92 0.84 0.86 0.89 0.86 0.99 0.83 0.93 0.98 1.02 0.87 0.85 (La/Yb)N 15.6 15.8 14.9 14.9 13.6 15.6 15.4 16.2 14 12.8 9.8 16.8 15.2 22.6 18.5 19.2 ΣREE 253 268.8 314.9 206.6 215.9 274.5 239.1 235 283.8 235.7 345.7 251.5 292.8 429.2 188.9 193 LREE/HREE 10.3 10.5 12.3 10.4 9.5 10.5 10.2 10.8 9 4.7 9.3 5.8 7.4 10.6 10.1 14

(To be continued on the next page)

(Continued)

Sample No. S04-11 S04-12-8 S04-12-10 S04-12-11 S04-12-12 S04-12-18 S04-12-20 S04-12-21 S04-12-22 S04-13-3B S04-14-1B S04-14-2B S04-16 S05-34A S08-69-1 S08-69-2

Y 31.4 34 35.1 24.2 30.2 37.1 31 26.5 45.6 34.9 83.9 28.6 35.7 35.1 20.8 22.1 Zr 250 316 303 250 232 336 275 279 211 205 238 233 229 436 87.7 90.7 Hf 5.72 6.14 6.17 5.28 5.17 6.42 5.92 5.61 5.16 5.12 5.99 5.72 4.95 8.06 2.67 2.77 Nb 99.3 82.8 81.5 69.3 58.8 87.7 71.6 74.5 56.1 54 69.4 66.3 106 126 69.8 72.6 Ta 5.79 5.03 4.89 4.49 3.57 5.15 4.63 4.33 3.66 3.07 4.12 3.98 5.67 7.36 3.62 3.65 Ba 973 625 650 538 485 603 492 592 348 376 790 783 995 768 967 985 Cu 38.9 42.4 41.8 39.8 42.7 43.4 43.3 42 38.5 33.3 35.5 26.2 50 32.8 51.8 55.8 Sr 773 721 737 652 597 685 631 661 1929 748 754 738 714 801 539 551 V 231 150 163 148 219 141 146 152 201 206 191 181 224 161 201 211 Zn 98.7 96 101 102 129 84.2 107 91.3 141 117 100 103 107 93.6 98.1 107 Li 7.84 8.34 8.69 11.6 16.9 9.88 14.4 8.36 19.4 16.7 13.6 14.6 7.89 8.71 5.02 7.28 Be 2.27 3.02 3.03 2.56 1.38 3.37 2.47 2.96 1.6 1.34 1.34 1.35 2.14 2.65 1.12 1.19 Sc 23.9 18.7 20.3 18 23.7 18.5 18.7 18.8 23.7 20.8 16.4 16.1 24.5 12.8 27 28.6 Cr 110 111 124 149 195 155 140 121 161 58.1 47.5 47.4 104 5.59 248 236 Co 54.3 30.1 32.4 46.4 44.1 28.3 57.2 44.1 76.6 38.8 120 29.6 97.8 43.7 59.8 63.6 Ni 135 13.6 16.6 100 67.6 22.4 112 77.2 171 50.3 107 35.8 193 37.6 232 224 Ga 21.1 23.2 24 20.9 21.6 24.4 20.3 23 17.8 17 19.8 18.5 22.2 22.3 18.5 19.7 Rb 97 83.2 82.9 66.8 16.9 85.5 54.4 78.8 16.4 16.9 29 31 91.1 95.3 61.7 69.1 Mo 3.94 5.12 5.38 4.51 2.84 5.46 4.21 5.28 5.6 1.41 8.02 1.23 10.2 6.51 1.69 1.92 Cd 0.12 0.09 0.2 0.16 0.11 0.12 0.14 0.23 0.35 0.26 0.27 0.14 0.22 0.15 0.22 0.13 In 0.09 0.07 0.08 0.08 0.08 0.07 0.08 0.08 0.09 0.08 0.08 0.08 0.09 0.08 0.07 0.07 Cs 1.21 1.25 1.14 0.92 0.34 1.26 0.77 1.07 0.53 0.16 0.12 0.11 1.15 1.25 0.65 0.92 W 17.8 17 16.8 20.7 9.18 14.1 18.4 17.9 1.4 5.43 10.1 26.9 45.6 19.3 17.4 34.6 Tl 0.2 0.27 0.26 0.28 0.38 0.57 0.39 0.49 1.1 0.49 1.82 0.3 1.54 0.7 0.24 0.26 Pb 7.76 7.44 7.42 6.82 7.9 7.16 9.75 7.26 36.5 9.86 53.6 6.13 49 20 6.7 5.96 Bi 0.15 0.12 0.12 0.13 0.14 0.13 0.15 0.14 0.49 0.21 0.71 0.1 0.71 0.31 0.16 0.14 Th 11.1 9.85 9.78 8.92 6.66 10.8 8.59 9.07 8.19 5 9.91 7.26 13.1 12.6 9.59 9.64 U 2.06 2.24 2.27 1.98 1.27 2.65 1.69 2.02 1.68 2.33 2.36 2.04 2.24 2.65 1.52 1.38 Nb/U 48.2 36.96 35.9 35 46.3 33.09 42.37 36.88 33.39 23.18 29.41 32.5 47.32 47.55 45.92 52.61 Ce/Pb 13.27 14.52 21.23 12.74 11.06 15.5 9.91 13.37 3.04 9.77 2.43 16.97 2.57 9.8 11.87 13.39 Nb/Ta 17.15 16.46 16.68 15.43 16.47 17.03 15.46 17.21 15.33 17.59 16.84 16.66 18.69 17.12 19.28 19.89 Zr/Hf 43.65 51.42 49.06 47.29 44.82 52.29 46.4 49.68 40.83 39.98 39.68 40.68 46.2 54.06 32.85 32.74

YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566 557

Figure 4 Variation diagram between MgO and other major elements.

The concentrations of most of incompatible trace

elements such as Rb, Sr, Y, Zr, Nb increase with de-creasing MgO and are largely controlled by fractional crystallization (Figure 4). As is the case with Ni and Cr, the plotted points for these elements scatter significantly in the lavas when MgO < 4 wt.%, which indicates that more evolved lava may be affected by accumulation of earlier fractional crystallization minerals. Ba, as one of incompatible trace elements, is an exception; it displays positive correlation with MgO, which could indicate that Ba occupies the position of K in earlier crystallization minerals; thereby Ba tends to enrich in crystallization solid phase and deplete in residual melt during fractional crystallization.

In primitive mantle-normalized trace-element distri-bution diagram (Figure 5), all samples show similar dis-tributional pattern. As a whole, large-ion lithophile ele-ments (LILE) and high field strength elements (HFSE) such as Nb, Ta, Ti and Y show enriched characteristics, and Yb, Sc, Sr and heat-producing elements such as K, U and Th show relative depleted characteristics, which

is obviously different from those of mid-ocean ridge basalts (MORB) and island-arc basalts (IAB). Although the concentrations of most LILE anf HFSE are slightly higher than OIB, as a whole, the basaltic rocks from the South China Sea have the general characteristics of intra-plate OIB trace element[35―37]. The Ce/Pb (2.4―21.2, its average value is 11.3) and Nb/U (23.2―52.6, its average value is 39.16) ratios of these basaltic rocks from the South China Sea are close to those for primi-tive mantle(Nb/U≈30, Ce/Pb≈9)[38], and are obviously higher than the average values of upper crust (Nb/U≈9, Ce/Pb≈3)[39].

4.3 Rare earth elements

Table 2 also lists rare earth element (REE) concentra-tions. The total content of REE (ΣREE) for these basal-tic rocks is relatively high, 188.9―429.2 ppm (its ave- rage value is 264.3 ppm), which is slightly higher than average OIB values (198.9)[35]. In a chondrite-normali- zed rare-earth element (REE) abundance patterns dia- gram (Figure 6), all samples exhibit strong enrichment

558 YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566

Figure 5 Spider diagrams showing primitive mantle-normalized trace element abundances in basalts from the South China Sea. Trace element abun-dances of the primitive mantle are from ref. [34]. OIB average value is from ref. [35].

Figure 6 C1 chondrite-normalized REE abundance patterns for the Cenozoic basalts from the South China Sea. C1 chondrite abundances are from ref. [34]. OIB average value is from ref. [35].

of LREE, combined with LREE/HREE ratio (9.8―22.6, with average value of 15.7) and (La/Yb)N ratio (4.7―14.0, its average value is 9.7), which suggests that the extent of partial melting of mantle rocks is relatively low. No significant negative Eu anomalies (δ Eu=0.92―1.07, with average value of 1.02) are observed, indicating that plagioclase fractionation was insignificant during the generation and evolution of these magmas. This is con-sistent with petrographic observation that plagioclase phenocrysts are extremely rare in these basalts. No sig-nificant Ce anomalies (δ Ce = 0.83―1.28, with average value of 0.92) are observed for most of these samples, and slightly negative Ce anomalies for individual sam-ples may suggest that they underwent low extent subma-rine weathering or are the expression of inherent char-acteristics of source material (e.g., Ce anomalies for lavas from Gough island in south Atlantic[40]). Small

differences exist among samples in ΣREE, which might indicate that magmas have undergoes a different extent of fractional crystallization during the ascent to the sur-face. As a whole, the Cenozoic alkali basalts from the South China Sea have the common characteristics of intra-plate OIBs[35―37,41].

4.4 Sr, Nd and Pb isotopes

Isotopes in igneous rocks have been proved to be a use-ful tool for discussing or discriminating origin charac-teristics, because only fractional crystallization is diffi-cult to carry out isotope fractionation. Table 3 lists all the isotopic data which are also summarized graphically in Figures 7 and 8 (end-member data and other regions data from references[11―13, 15,16, 18,20, 42―50].

Sr, Nd isotope ratios of Cenozoic alkali basalts from the South China Sea have relatively limited ranges, 87Sr/86Sr=0.703418―0.705174 and 143Nd/144Nd = 0.512663―0.512965, respectively. In the 143Nd/144Nd vs. 87Sr/86Sr correlation plot (Figure 7), a clear negative correlation is shown. They are plotted in the OIB fields, which is consistent with plotting regions of basaltic rocks from Leiqiong peninsula[12,13], Northern margin of the SCS [18,20], Indochina block [15,16] and southeast China[45―48]. The regressive line for data points is appro- ximately parallel to the linking line between EM2 and DMM, which suggests a binary mixing model in mantle component. One is moderately depleted Indian Ocean mid-ocean ridge basalts (MORB)-like or DMM end-member, the other is EM2 end-member. However, the plotting field of north China and northeastern China

YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566 559

Table 3 Nd-Sr-Pb isotopic compositions in basalts from the South China Sea Sample No. 87Sr/86Sr Error (2σ) 143Nd/144Nd Error (2σ) 206Pb/204Pb Error (2σ) 207Pb/204Pb Error (2σ) 208Pb/204Pb Error (2σ)

S04-11 0.703418 12 0.512965 10 18.692 0.009 15.599 0.009 38.831 0.009 S04-12-12 0.704038 11 0.512863 12 18.622 0.009 15.596 0.009 38.767 0.009 S04-12-18 0.703961 11 0.512894 12 18.372 0.005 15.534 0.006 38.443 0.006 S04-12-20 0.704137 12 0.512855 11 18.501 0.008 15.596 0.008 38.665 0.007 S04-12-8 0.703963 13 0.512901 11 18.409 0.01 15.558 0.01 38.523 0.011 S04-13-3B 0.70494 10 0.512847 13 18.705 0.008 15.634 0.008 38.891 0.008 S04-14-1B 0.705174 12 0.512675 11 18.648 0.012 15.718 0.013 39.057 0.015 S04-16 0.70366 11 0.512906 12 18.697 0.009 15.711 0.011 39.085 0.014 S05-34A 0.703771 11 0.51287 13 18.703 0.01 15.645 0.014 38.926 0.017 S08-69-1 0.705049 11 0.512687 10 18.783 0.007 15.674 0.007 39.211 0.008 S08-69-2 0.705041 13 0.512663 12 18.807 0.007 15.667 0..008 39.24 0.008

Figure 7 143Nd/144Nd vs. 87Sr/86Sr diagram for the basaltic rocks studied. The approximate fields for DMM, HIMU, EM1 and EM2 are from ref. [42]; The field for OIB is from ref. [43]; The field for Indian Ocean-type MORB is from ref. [44].

shows mixing model between EM1 and DMM, which may be related to lithospheric thinning process of north China during Mesozoic and Cenozoic[51,52].

Pb isotope compositions of Cenozoic alkali basalts from the SCS are as follows: 206Pb/204Pb=18.3715―18.8069, 207Pb/204Pb=15.5343―15.7180, 208Pb/204Pb= 38.4434―39.2402，which are similar to Kueguelen plateau[50,53]. In general, the plotting trend of the SCS is consistent with those of Leiqiong peninsula, Northern margin of the SCS, Indochina block and southeast China (Figure 8), and is inconsistent with those of north China and northeastern China, which is similar to the case in 143Nd/144Nd vs. 87Sr/86Sr correlation plot (Figure 7). In 87Sr/86Sr vs. 206Pb/204Pb correlation plot (Figure 8(a)), all data are plotted in the OIB fields, and the elongated line for data points is approximately parallel to the linking

line between EM2 and DMM. In 207Pb/204Pb, 208Pb/204Pb vs. 206Pb/204Pb correlation plots (Figure 8(b), (c)), all samples lie above northern hemisphere reference line (NHRL), and the slope of linking line of plotting data points is higher than that of NHRL. Note that the plot-ting point from the SCS is the closest to EM2 end member in 207Pb/204Pb vs. 206Pb/204Pb correlation plot (Figure 8(b)). In addition, they generally show higher 207Pb/204Pb and 208Pb/204Pb at a given 206Pb/204Pb, ex-tending towards EM2 component, which is similar to Dupal anomaly illustrated by Hart[50]. In diagram 8c, all samples lie in the fields of Dupal-type OIB. However, although mixing trend of Pb isotope from southeast China is the same as those of the SCS and its adjacent areas, its radiogenic Pb content is relatively low, which is different from the general characteristics of Dupal

560 YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566

Figure 8 87Sr/86Sr, 207Pb/206Pb, 208Pb/206Pb vs. 206Pb/204Pb diagram for the basaltic rocks studied. Data for the approximate fields of DMM, HIMU, EM1, EM2 and Indian Ocean-type MORB is the same as that of Figure 7. NHRL is North Hemisphere reference line[50]. The field for Dupal anomaly is from ref. [49].

anomaly [50]. New Sr, Nd and Pb isotope data strengthen the previously identified Dupal signature of SCS rocks[11].

As a whole, Sr, Nd and Pb isotope characteristics show that there exists mantle heterogeneity beneath the South China Sea. A binary mixing model in mantle component exists: one is DMM end-member, the other is EM2 end-member. And the mantle component of the South China Sea is similar to Dupal anomaly[50,54,55].

5 Discussion

In Ti-Zr-Y discriminant diagram for tectonic setting[56] (Figure 9), most of Cenozoic basalts data points from the SCS are plotted into the field of intra-plate basalt. In Th/Hf vs. Th/Ta discriminant diagram[57] (Figure 10), data points mainly lie in the field for basalts from mantle plume. The cases mentioned above suggest that Ceno-zoic basalts from the SCS formed in intra-plate tectonic setting, and may be related to mantle plume genetically.

YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566 561

Figure 9 Ti-Zr-Y discriminant diagram for the basaltic rocks studied ( after ref. [56]). (a) Island arc tholeiite; (b) MORB+ Island arc tholeiite+ calc-alkaline basalt; (c) calc-alkaline basalt; (d) intra-plate basalt.

Figure 10 Th/Hf vs. Ta/Hf discriminant diagram of tectonic setting of Cenozoic basalts from the SCS (after ref. [57]. I, NMORB in divergent margin; II, Convergent margin: II-1, Oceanic island-arc basalt; II-2, Con-tinental margin island-arc and volcanic-arc basalt; III, Oceanic intra-plate island and seamount basalt, TMORB and EMORB; IV, Continental in-tra-plate setting: IV-1, Tholeiite in intracontinental and continental margin rift; IV-2, Alkaline basalt in continental rift; IV-3, Basalt in continental extension or incipient rift; V, Basalt in thermal mantle plume.

The main trace element ratios, for example, Nb/Ta ratio (15.33―19.89, with average value of 17.08), Zr/Hf ratio (32.74―54.06, with average value of 44.48), are similar to those of primitive mantle[38] (Nb/Ta and Zr/Hf ratio are 17.5±0.5 and 36.27, respectively.), together with Nb/U and Ce/Pb ratio as well (see section 4.2 ), which implies that basaltic magmas almost do not undergo crust contamination during its ascent to the surface, and the result is consistent with the case that basaltic rocks mainly erupted above the base of oceanic crust. At the same time, trace element ratios of three samples situated on the continental slope’s bottom (i.e., S04-16, S08-69-1 and S08-69-2) are slightly higher than or close to those of primitive mantle (Table 2) and different from crust average value, which also suggests that they are not contaminated by continental crust. Therefore, they can reflect the components of magma origin, which is con-sistent with characteristics of basaltic rocks from north-ern margin of the SCS[19,20] and southeast China[46].

5.1 Mantle source compositions

Our data only show the local variations in the SCS. To study the isotope regional variations in the SCS and its adjacent areas and even in eastern China, we collect the data from Indochina peninsula, Leiqiong peninsula, the northern margin of the SCS and eastern China. The iso-topic variation diagrams (Figures 7 and 8) show that the source of basaltic rocks from north China and northeast China is mainly a mixing of EM1 and DMM[45―47], which may be related to lithospheric thinning process of north China during Mesozoic and Cenozoic[51], and the basaltic rock’s source with added lithospheric mantle (i.e., SCLM[47]) is the result of crust-mantle interaction induced by thermo-mechanic and chemical erosion in continental lithospheric thinning[52]. In contrast, the mantle origin of Cenozoic basaltic rock from southeast China is a binary mixing model between EM2 and DMM, and has little EM1 component[46]. The mantle source of Cenozoic alkali basalt from the SCS (this study), Leiqiong peninsula and the northern margin of the SCS can also be explained by using binary mixing model. The negative correlation between 143Nd/144Nd and 87Sr/86Sr (Figure 7) and the positive relationship between 87Sr/86Sr and 206Pb/204Pb (Figure 8) suggest a binary mixing of EM2 and DMM; EM2 represents en-riched end-member from mantle plume[7] material in the South China Sea, which is similar to southeast China and different from north China and northeast China. In

562 YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566

addition, there may be involvement of EM1 in Indochina peninsula[15].

The formation mechanism of enriched end-member of mantle source beneath east China, the SCS and its adja-cent areas is still in debate. Due to the scarcity of geo-physical data for deep parts of the Earth, several scho- lars ascribe the formation of EM to the addition of SCLM in extensional setting, and slab subduction is the main prerequisite for causing the enrichment of SCLM[11,47]. The model may be reasonable in explaining EM1 end-member in basalts from north China and northeast China; however, it is difficult to explain EM2 end-member in basalts from southeast China, the SCS and its adjacent areas. The latest geophysical data show that intraplate volcanism in Changbai Mountains may be related to plate subduction[58,59]. Pacific plate is sub-ducted beneath Asia plate in northeast China and stag-nant in 670 km discontinuity. The dehydration of plate itself (i.e., the dehydration of aqueous minerals) de-grades liquidus and induces partial melting, then as-thenospheric mantle in which fluids from subducted plate is added metasomatizes lithospheric mantle during its ascent. The metasomatism is the possible mechanism for the formation of EM1 in basalts from northeast China and north China[47] and may be one of thinning mechanism for lithosphere, which implies that intraplate volcanism in these areas do not need the participation of deeper process (material and heat). Montelli et al.[7]

identified mantle plume around Hainan island, and low-velocity anomaly can extend 1900 km beneath the Earth’s surface (i.e., mid-mantle depth), which may re-flect a dying mantle plume that is rooted in the base of lower mantle. At the same time, Zhao[60] also pointed out that there exists mantle plume near Hainan island, and indicated that Hainan plume is one of twelve mantle plumes originated from mantle-core boundary. The geophysical data mentioned above provide us a new cognition about material source of EM2 in basalts from the SCS and its adjacent areas. Tu et al.[11] considered that the enrichment of sub-continental lithospheric man-tle (SCLM) caused by shallow subduction process is the formation mechanism of enriched end-member, when explaining enriched end-member of Cenozoic basalts from the SCS. Although the model is reasonable to a certain extent, the authors[11] also pointed out the ab-sence of geophysical evidences from mantle upwelling or mantle plume. This study provides another explana-tion for EM2, i.e., the EM2 component in Cenozoic ba-

salts from the SCS is originated from Hainan plume[6,7,60] as indicated by the geophysical data. The material source of Hainan plume may be continental lithospheric subducted slab after the collision between Yangtze Cra-ton and North China Craton prior to 200 Ma[61], or oce-anic lithospheric subducted slab[37] before the collision between continents, which is consistent with material source of most oceanic island basalts (OIBs) in the In-dian Ocean (e.g., Kerguelen[33]). The analysis and dis-cussion above may indicate that the geodynamical set-tings of North China and Northeast China may be obvi-ously different from those of the South China and its adjacent areas as well as Southeast China. The former may be related to plate tectonics, and the latter (at least for the South China and its adjacent areas) may be mainly related to intraplate deep process, i.e., mantle plume[62].

5.2 Partial melting and fractional crystallization

Compared to normal mid-ocean ridge basalt (NMORB), large ion lithophile elements (LILE) of ocean island ba-salt, especially for alkali basalt, are relatively enriched, and the concentration of these elements is controlled by source composition, residual mineral phases, the degree of partial melting and subsequent fractional crystalliza-tion[41]. The studies indicate that low degree of partial melting of mantle rocks generally results in strongly enrichment of incompatible elements in melt, and with increasing the extent of partial melting, the enrichment of incompatible elements gradually became no obvious and even depleted, e.g., NMORB. Highly incompatible elements and light rare earth elements (LREEs) are mainly enriched in silica-poor, alkali-rich basalts (i.e., basanite and alkali basalt), caused by low degree (2%―

10%) partial melting of mantle source with the enrich-ment of incompatible elements under 1.5 GPa or more[16,63,64]. The characteristics of trace elements in Cenozoic alkali basalts from the SCS are similar to those of OIB (Figures 5 and 6), and the strong enrichment of highly incompatible elements and LREE suggest that the degree of partial melting of mantle rock in this study area is relatively low (usually less than 10%); the dif-ference of enrichment extent among rock samples may reflect different extent of partial melting. The Hainan plume may provide heat to partial melting, and syn-chronous increase of the concentration of SiO2 and in-compatible elements and alkalinity (Table 1) may reflect that the Hainan plume is a great material contributor to

YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566 563

the melts. In the variation diagrams of MgO vs. major and trace

elements in basaltic rocks (Figures 3 and 4), many ele-ments take on a whole increasing or decreasing trend when MgO content is decreased. With decreasing MgO, the concentrations of nickel and chromium correspond-ingly decrease, which indicates that melts experienced extensive olivine and/or clinopyroxene fractional crys-tallization during its ascent or storing in high level magma chambers. This is consistent with petrographic observations, e.g., there appeared olivine, Ti-rich clino-pyroxene and plagioclase phenocrysts. The chemical compositions (the author’s data, discussed in another paper) of olivine phenocrysts indicate gradual fractional crystallization of magma, and earlier phenocrysts will further react with melt and show zoning texture. How-ever, weak zoning texture shows no extensive reaction and suggests high ascending rate of magma. The ab-sence of a negative Eu anomaly indicates that plagio-clase fractionation has not taken place extensively in the melts (Figure 6). Although it appears that olivine is the main phase removed from the melts during the fractional crystallization, low Sc concentration (12.8―28.6 ppm) might suggest a small amount of clinopyroxene frac-tionation as well. The MgO contents of some basaltic rocks are relatively low (Table 2), which may show ex-tensive fractional crystallization. However, the plotted points of these basaltic rocks with low MgO content are scattered and do not take on linear relationship (Figures 3 and 4), which may also indicate that they experienced accumulation of earlier phenocrysts. Accumulation min-erals would remain in magma chamber at different depth, thereby extensively decreasing MgO contents in evolved magma, and to certain extent reflecting mantle hetero-geneity in the South China Sea.

5.3 Dupal anomaly

Dupal anomaly was proposed by Hart[50] in terms of Pb-Sr isotopic variations which were found by Dupre and Allegre [65] in studying Indian Ocean basalts[50]. In comparison with most MORBs, Dupal anomaly has ele-vated 87Sr/86Sr, 207Pb/206Pb and 208Pb/206Pb ratios with given 206Pb/204Pb ratio. The author[50] further built a Dupal anomaly zone between 0° and 60°S and pointed out that it was in correlation with topography. Pb iso-topic data of this study and Tu et al.[11], suggest that there exist a Pb isotopic anomaly zone with Dupal char-acteristics in the SCS, however, which raises several

questions as follows: (1) Is Dupal anomaly only limited to south Hemisphere? Is Dupal anomaly in the SCS af-filiated with that in south Hemisphere ? (2) Is the source of Dupal anomaly the deep lower mantle[54] or shallow SCLM [66] ? In 2006 interRidge association convened a workshop, entitled “polar ridge” in Italy, and Goldstein et al. point out that isotopic geochemistry of basalts from Gakkel ridge in Arctic Ocean shows Dupal ano- maly[67], which to certain extent suggests that Dupal anomaly lacks regional exclusivity and could appear in other areas, e.g., the SCS and the Arctic Ocean. Paleon-tologic, paleogeomagnetic and paleoclimatic data show that most areas in south China once may be a part of Gongwana supercontinent[68―70], which seems to in-dicate that mantle source of Cenozoic basalts in the SCS can be affiliated with Dupal anomaly in the South Hemisphere. However, plate process or supercontinent disassembly commonly is only limited to lithospheric level and it cannot cause identical movement of large scale asthenospheric mantle. In isotopic variation dia-grams, it seems that DMM end-member of Cenozoic basalts from the SCS can be linked with Indian Ocean MORB-like; however, this linkage needs a large scale mantle convection as preconditions. The Southeast and southwest part of the SCS are collision zones of subduc-tion of Indo-Australian plate along Asian plate which potentially obstructs asthenospheric mantle flow from the Indian Ocean to the South China, and Indian MORB is just a regional mantle component and is not one of four fundamental mantle end-member[42]. Therefore, based on the discussions above, we suggest that the Dupal anomaly of the SCS may have no relation with that of the South Hemisphere. The latest geophysical data show that there exist Hainan plume[11,60],which may have a great contributor to enriched mantle component (EM2)(see section 4.1). In addition, Tu et al.[11] sug-gested that EM2 end member may be originated from SCLM. However, based on its sampling location and dating results, Cenozoic basalt in this study was erupted above extremely thinned lithosphere/crust in which thinning process took place during the period (32―15.5 Ma)[8] of seafloor spreading in the SCS, and it is difficult to imagine that there still remained old SCLM during intra-plate volcanism in the SCS. Therefore, the source of Dupal anomaly in the SCS may not be originated from SCLM [11,66], but from the base of lower mantle at core/mantle boundary[54]. In other words, Dupal ano-

564 YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566

maly does not appear only in south Hemisphere, and Dupal anomaly in the SCS may be an endogenetic anomaly[11] which may have no linkage with that in south Hemisphere. The formation of Dupal anomaly in the SCS is tightly related with Hainan plume.

5.4 A model for petrogenesis of Cenozoic alkali ba-salts from the South China Sea

Isotope data of Cenozoic alkali basalts from the SCS suggest that there exist binary mixing models in mantle component with Dupal Pb anomaly: one is DMM end-member, and the other is EM2 end-member which may be originated from Hainan plume at core/mantle boundary. While Hainan plume provides heat energy necessitated for partial melting of lithospheric mantle, it also is a great material contributor. These basalts under-went different extents of partial melting and fractional crystallization during its ascent or high level magma chamber, and possible accumulation also took place. During the ascent to the surface, the basaltic magma did not undergo contamination of continental crust, which is consistent with the fact of erupting on the oceanic crust. The geochemical characteristics of rocks samples situ-ated on continental slope’s bottom also reflect no ap-parent contamination with continental materials. There is one important implication from our petrogenetic model, i.e., the introduction of Hainan plume indicated by geophysical means perfectly explains geodynamical process of intra-plate volcanism and the mantle source of enriched end-member of basaltic rocks in the SCS.

6 Conclusions

(1) The Cenozoic alkali basalts from the SCS are products of intra-plate volcanism since late Miocene. Alkali basalts are the dominant rock types, and other rocks are trachybasalts and basalts. All these rocks be-long to alkali basaltic magma series, and have enriched isotopic characteristics and relatively homogeneous trace element concentrations. Major, trace element and Sr-Nd-Pb isotope compositions of these basaltic rocks are similar to those of OIBs.

(2) Isotopic characteristics of Cenozoic alkali basalts from the SCS suggest that there exists a binary mixing model in mantle component with Dupal Pb anomaly: one is DMM end-member, and the other is EM2 end-member which may be originated from Hainan plume at core/mantle boundary. While Hainan plume provides heat energy necessitated for partial melting of lithospheric mantle, it also is a great material contributor. Hainan plume is satisfied to explain the EM2 end-member of basaltic rocks in the SCS. The Cenozoic basalts from the SCS in turn provide geochemical con-straints on Hainan plume.

(3) Dupal anomaly does not occur only in south Hemisphere, and Dupal anomaly in the SCS may be an endogenetic anomaly which may have no linkage with that in south Hemisphere.

We thank several anonymous reviewers for thorough reviews and com-ments.

1 Williams Q, Garnero E J. Seismic evidence for partial melt at the base

of the Earth's mantle. Science, 1996, 273: 1528―1530[DOI] 2 Williams Q, Revenaugh J, Garnero E A. Correlation between ul-

tra-low basal velocities in the mantle and hot spots. Science, 1998, 281: 546―549[DOI]

3 Revenaugh J, Meyer R. Seismic evidence of partial melt within a possibly ubiquitous low-velocity layer at the base of the mantle. Sci-ence, 1997, 277: 670―673[DOI]

4 Van der Hilst R, Widiyantoro R S, Engdahl R. Evidence for deep mantle circulation from global tomography. Nature, 1997, 386: 578―584 [DOI]

5 Gurenko A A, Hoernle K A, Hauff F, et al. Major, trace element and Nd-Sr-Pb-O-He-Ar isotope signatures of shield stage lavas from the central and western Canary Islands: Insights into mantle and crustal processes. Chem Geol, 2006, 233: 75―112[DOI]

6 Montelli R, Nolet G, Dahlen F A, et al. Finite-frequency tomography reveals a variety of plumes in the mantle. Science, 2004, 303: 338―343[DOI]

7 Montelli R, Nolet G, Dahlen F A, et al. A catalogue of deep mantle

plumes: New results from finitefrequency tomography. Geochem Geophys Geosyst, 2006, 7(1): 1―69[DOI]

8 Briais A, Patriat P, Tapponier P. Updated interpretation of magnetic anomalies and seafloor spreading stages in the South China Sea: Im-plications for the Tertiary tectonics of southeast Asia. J Geophys Res, 1993, 98(B4): 6299―6328

9 Wang X J, Wu M Q, Liang D H, et al. Some geochemical character-istics of basalts from the South China Sea. Geochemica (in Chinese), 1984, 4: 332―340

10 Kudrass H R, Wiedicke M, Cepek P, et al. Mesozoic and Cenozoic rocks dredged from the South China Sea (Reed Bank area) and Sulu Sea and their significance for plate-tectonic reconstructions. Mar Petrol Geol, 1986, 3: 9―30[DOI]

11 Tu K, Flower F J, Carlson R W, et al. Magmatism in the South China Basin 1. Isotopic and trace-element evidence for an endogenous Dupal mantle component. Chem Geol, 1992, 97(1-2): 47―63[DOI]

12 Zhang M, Tu K, Xie G H, et al. Trace element and isotope geochem-istry of Cenozoic basalts fron Hainan island. In Liu R X, ed. Geo-chronology and Geochemistry of Cenozoic Volcanic Rocks in China

YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566 565

(in Chinese). Beijing: Seismic Publishing House, 1992. 246―268 13 Zhu B Q, Wang H F. Nd-Sr-Pb isotopic evidence for MORB- OIB

transitional type mantle-source volcanism in Leiqiong region. Geo-chemica (in Chinese), 1989, 3: 193―201

14 Fan W M, Menzies M A. Lithospheric mantle nature under volcanism in extension geodynamics setting: Geochemical evidence from Ce-nozoic basaltic rocks in Leiqiong region. In Liu R X, ed. Geochro-nology and Geochemistry of Cenozoic Volcanic Rocks in China (in Chinese). Beijing: Seismic Publishing House, 1992. 320―329

15 Hoang N, Flower M, Carlson R W. Major-, trace element, and isotopic compositions of Vietnamese basalts: Interaction of hydrous EMl-rich asthenosphere with thinned Eurasian lithosphere. Geochim Cosmo-chim Acta, 1996, 60 (22): 4329―4351[DOI]

16 Zhou P B, Mukasa S B. Nb-Sr-Pb isotopic, and major- and trace-element geochemistry of Cenozoic lavas from the Khorat Pla-teau, Thailand: Sources and petrogenesis. Chem Geol, 1997, 137: 175―193

17 Hoang N, Flower M. Petrogenesis of Cenozoic Basalts from Vietnam: Implication for Origins of a ‘Diffuse Igneous Province’. J Petrol, 1998, 39(3): 369―395 [DOI]

18 Zou H P, Li P L, Rao C T. Geochemistry of Cenozoic volcanic rocks in Zhujiangkou Basin and its geodynamic significance. Geochemica (in Chinese), 1995, 24(Suppl): 33―45

19 Jia D C, Qiu X L, Hu R Z, et al. Geochemical nature of mantle res-ervoirs and tectonic setting of basalts in Beibu gulf and its adjacent region. J Trop Oceanogr (in Chinese), 2003, 22(2): 30―39

20 Li C N, Wang F Z, Zhong C S. Geochemistry of Quaternary basaltic volcanic rocks of Weizhou island in Beihai city of Guangxi and a discussion on characteristics of their source. Acta Petrol Mineral (in Chinese), 2005, 24(1): 1―11

21 Xiao L, Zhou H M, Dong Y X, et al. Geochemistry and petrogenesis of Cenozoic volcanic rocks from Sanshui basin: Implications for spatial and temporal variation of rock types and constraints on the formation of South China Sea. Geotect Metallo (in Chinese), 2006, 30(1): 72―81

22 Taylor B, Hayers D E. The tectonic evolution of the South China Sea. In: Hayers D E, ed. The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, 1. Am Geophys Union, Washington, DC, 1983. 23―56

23 Tapponnier P, Peltzer G, Armijo R. On the mechanics of the collision between India and Asia. In: Coward M P, Rics A C, eds. Collision Tectonoics. Geol Soc Spe Publ, 1986, 19: 115―157

24 Yan P, Zhou D, Liu Z S. A crustal structure profile across the northern continental margin of the South China Sea. Tectonophysics, 2001, 338: 1―21[DOI]

25 Yao B C, Wang L, Wu N Y. Cenozoic plate tectonic activities in the Great South China Sea area. Geol China (in Chinese), 2004, 31(2): 113―122

26 Holloway N H. North Palawan block, Philippine-its relation to Asian mainland and role in evolution of South China Sea. Am Assoc Petrol Geol, 1982, 66: 1355―1383

27 Schluter H U, Hinz K, Block M. Tectono-stratigraphic terranes and detachment faulting of the South China Sea and Sulu Sea. Mar Geol, 1996, 130: 39―78[DOI]

28 Karig D E. Origin and development of marginal basin in the western

Pacific. J Geophys Res, 1971, 76: 2543―2561 29 Li S T, Lin C S, Zhang Q M, et al. Dynamic process of episodic rifting

in continental marginal basin and fectonic events since 10 Ma in South China Sea. Chin Sci Bull (in Chinese), 1998, 43(8): 797―810

30 Chen F, Siebel W, Satir M, et al. Geochronology of the Karadere basement (NW Turkey) and implications for the geological evolution of the Istanbul zone. Int J Earth Sci, 2002, 91: 469―481[DOI]

31 Le Maitre R W, Bateman P, Dudek A, et al. A Classification of Igneous Rocks of Glossary of Terms. Oxford: Blackwell, 1989

32 Irvine T N, Baragar W R A. A guide to the chemical classification of the common volcanic rocks. Can J Earth Sci, 1971, 8: 523―548

33 Wallace P J. Volatiles in submarine basaltic glasses from the northern Kerguelen plateau (ODP site 1140): Implications for source region compositions, magmatic processes, and plateau subsidence. J Petrol, 2002, 43: 1311―1326[DOI]

34 McDonough W F, Sun S S. The composition of the Earth. Chem Geol, 1995, 120: 223―253[DOI]

35 Sun S S, McDonough W F. Chemical and isotopic systematics of ocean basalt: Implications for mantle composition and processes. In: Saunders A D, Norry M, eds. Magmatism in the Ocean Basins. Geol Soc Spe Publ, 1989, 42: 323―345

36 Halliday A N, Lee D C, Tommasini S, et al. Incompatible trace ele-ments in OIB and MORB and source enrichment in the sub-oceanic mantle. Earth Planet Sci Lett, 1995, 133: 379―395[DOI]

37 Niu Y L, O’Hara M J. Origin of ocean island basalts: A new perspec-tive from petrology, geochemistry, and mineral physics considerations. J Geophys Res, 2003, 108(B4): 5―19

38 Hofmann A W. Chemical differentiation of the Earth: the relationship between mantle, continental crust and oceanic crust. Earth Planet Sci Lett, 1988, 90: 297―314[DOI]

39 Taylor S R, McLennan S M. The Continental Crust: Its Composition and Evolution. Oxford: Blackwell, 1985

40 Class C, le Roex A P. Ce anomalies in Gough Island lavas―Trace element characteristics of a recycled sediment component. Earth Planet Sci Lett, 2007, doi: 10.1016/j.epsl.2007.10.030

41 Wilson M. Igneous Petrogenesis. Amsterdam: Kluwer Academic Pulishers, 1989. 245―286

42 Zindler A, Hart S R. Chemical geodynamics. Annu Rev Earth Planet Sci Lett, 1986, 14: 493―571[DOI]

43 Staudigel P, Zindler A, Hart S R, et al. The isotope systematics of a juvenile intra-plate volcano: Pb, Nd and Sr isotope ratios of basalts from loihi Seamount, Hawaii. Earth Planet Sci Lett, 1984, 69: 13―29[DOI]

44 Mahoney J J, Natland J H, White W M, et al. Isotopic and geochemical provinces of western Indian Ocean spreading centers. J Geophys Res, 1989, 94: 4033―4052

45 Chung S L. Trace element and isotope characteristics of Cenozoic basalts around the Tanlu Fault with implication for the eastern plate boundary between north and south China. J Geol, 1999, 107: 301―312[DOI]

46 Zou H B, Zindler A, Xu X S, et al. Major, trace element, and Nd, Sr and Pb isotope studies of Cenozoic basalts in SE China: mantle sources, regional variations, and tectonic significance. Chem Geol, 2000, 171: 33―47[DOI]

47 Liu C Q, Xie G H, Masuda A. Geochemistry of Cenozoic basalts from

566 YAN QuanShu et al. Sci China Ser D-Earth Sci | Apr. 2008 | vol. 51 | no. 4 | 550-566

eastern China: (II) Sr, Nd, and Ce isotopic compositions. Geochemica (in Chinese), 1995, 24(3): 203―214

48 Zhou X H, Zhu B Q. Isotopic system and mantle geochemical regions of Cenozoic basalts in eastern China. In Liu R X, eds. Geochronology and Geochemistry of Cenozoic Volcanic Rocks in China (in Chinese). Beijing: Seismic Publishing House, 1992. 366―391

49 Hamelin B, Allègre C J. Large scale regional units in the depleted upper mantle revealed by an isotopic study of the south-west Indian ridge. Nature, 1985, 315: 196―198[DOI]

50 Hart S R. A large-scale isotope anomaly in the southern hemisphere mantle. Nature, 1984, 309: 753―757[DOI]

51 Zhai M G, Zhu R X, Liu J M, et al. Critical timing of transformation of Mesozoic tectonic regime in east part of north China. Sci China Ser D-Earth Sci, 2003, 33(10): 913―920

52 Xu Y G. Roles of thermo-mechanic and chemical erosion in conti-nental lithospheric thinning. Bull Mineral Petrol Geochem (in Chi-nese), 1999, 18(1): 1―5

53 Frey F A, Coffin M F, Wallace P, et al. Origin and evolution of a submarine Large Igneous Province: The Kerguelen Plateau and Bro-ken Ridge, Southern Indian Ocean. Earth Planet Sci Lett, 2000, 176: 73―89[DOI]

54 Castillo P. The Dupal anomaly as a trace of the upwelling lower mantle. Nature, 1988, 336: 667―670[DOI]

55 Wen L X. A compositional anomaly at the Earth's core–mantle boundary as an anchor to the relatively slowly moving surface hot-spots and as source to the DUPAL anomaly. Earth Planet Sci Lett, 2006, 246: 138―148[DOI]

56 Pearce J R, Cann J R. Tectonic setting of basic volcanic rocks deter-mined using trace element analysis. Earth Planet Sci Lett, 1973, 19: 290―300[DOI]

57 Wang Y N, Zhang C J, Xiu S Z. Th/Hf-Ta/Hf identification of tectonic setting of basalts. Acta Petrol Sin (in Chinese), 2001, 17(3): 413―421

58 Zhao D P, Lei J S, Tang R Y. Origin of the Changbai intra-plate vol-canism in Northeast China: Evidence from seismic tomography. Chin

Sci Bull, 2004, 49(13): 1401―1408 59 Zhao D P, Maruyama S, Omori S. Mantle dynamics of Western Pacific

and East Asia: Insight from seismic tomography and mineral physics. Gondwana Res, 2007, 11: 120―131[DOI]

60 Zhao D. Seismic images under 60 hotspots: Search for mantle plumes. Gondwana Res, 2007, 12: 335―355[DOI]

61 Lebedev S, Nolet G. Upper mantle beneath Southeast Asia from S velocity tomography. J Geophys Res, 2003, 108(B1): 20―48

62 Yan Q S, Shi X F. Hainan mantle plume and the formation and evo-lution of the South China Sea. Geol J Chin Univ (in Chinese), 2007, 13(2): 311―322

63 Jaques A L, Green D H. Anhydrous melting of peridotite at 0-15 kb pressure and the genesis of tholeiitic basahs. Contrib Mineral Petrol, 1980, 73: 287―310[DOI]

64 Takahashi E, Kushiro I. Melting of a dry peridotite at high pressures and basalt magma genesis. Am Mineral, 1983, 68: 859―879

65 Dupre B, Allegre C J. Pb-Sr isotope variation in Indian Ocean basalts and mixing phenomena. Nature, 1983, 303: 142―146[DOI]

66 Hawkesworth C J, Mantovani P, Taylor N, et al. Evidence from the parana of south Brazil for a continental contribution to Dupal basalts. Nature, 1986, 322: 356―358[DOI]

67 Goldstein S L, Soffer G, Langmuir C H. Isotope geochemistry of gakkel ridge basalts and origin of a “Dupal” signature. Ofioliti, 2006, 31(1): 59―60

68 Ridd M F. South-East Asia as a part of Gondwanaland. Nature, 1971, 234: 531―534[DOI]

69 Lin J, Fullel M, Zhang W. Preliminary Phanerozoic polar wander paths for the North and South China blocks. Nature, 1985, 313: 444―449[DOI]

70 Nie S Y, Rowley D B, Ziegler A M. Constraints on the locations of Asian microcontinents in palaeo-Tethys during the late Paleozoic. In: Mckerrow W S, Scotese C R, eds. Paleozoic Paleogeography and Biogeography. Geol Soc Memoir, 1990, 12: 397―409