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The Song Da magmatic suite revisited: A petrologic, geochemical and Sr–Ndisotopic study on picrites, flood basalts and silicic volcanic rocks

Tran Viet Anh a,b, Kwan-Nang Pang a, Sun-Lin Chung a,⇑, Huei-Min Lin a,c, Tran Trong Hoa b,Tran Tuan Anh b, Huai-Jen Yang c

a Department of Geosciences, National Taiwan University, Taipei P.O. Box 13-318, Taipei 10617, Taiwanb Institute of Geological Sciences, Vietnamese Academy of Science and Technology, Hanoi, Viet Namc Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan

a r t i c l e i n f o

Article history:Received 28 January 2011Received in revised form 7 July 2011Accepted 19 July 2011Available online 5 August 2011

Keywords:Emeishan large igneous provinceFlood basaltGeochemistrySilicic volcanic rocksSr–Nd isotopesVietnam

a b s t r a c t

Permian volcanic rocks, including picrites, flood basalts and silicic volcanic rocks, crop out in the Song Dadistrict, northern Vietnam. They occur as a �300-km-long belt emanating from the southernmost part ofthe Emeishan large igneous province, SW China. The belt represents a detached part from the provinceand was displaced �600 km southeastward along the Ailao Shan-Red River shear zone during the mid-Tertiary in response to the India–Eurasia collision. We report here geochemical and Sr–Nd isotopic dataon the Song Da volcanic rocks to decipher the petrogenetic processes responsible for their formation. Thepicrites show evidence of olivine fractionation and formed from high-Mg magmas (MgO = 20–25 wt.%)originated from depleted mantle peridotites from the Emeishan plume head. The plume head is charac-terized by eNd(t) value of +8. Some picrites and low-Ti basalts have low Nb/La, Nb/U and eNd(t), whichare indicative of plume–lithosphere interaction in various degrees. Contamination of continental crustalrocks (<20%) is able to produce the spectra of the observed trace element patterns and isotopic ratios. Thesilicic volcanic rocks are metaluminous or peraluminous (aluminium saturation index = 0.92–1.24) andare sufficiently different from those present elsewhere in the Emeishan large igneous province that aredominantly peralkaline. The Song Da silicic volcanic rocks have eNd(t) values of �0.1 to +0.6 and showstrong depletion in compatible trace elements (e.g. Ni = 0.8–3 ppm, V = 4–55 ppm). They most likelyformed by extensive fractional crystallization from the associated high-Ti basalts, possibly with somecrustal contamination.

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1. Introduction

The end-Guadalupian (�260 Ma) Emeishan large igneous prov-ince (ELIP) is a flood basalt province in SW China and its formationis attributed to a mantle plume (Chung and Jahn, 1995; Chunget al., 1998; Song et al., 2001; Xu et al., 2001, 2004; Ali et al.,2005). Its significance as a large igneous province is now widelyrecognized in international literature following the abundant stud-ies carried out over the past decade (Sheth, 2007; Bryan and Ernst,2008; Shellnutt et al., 2010). Despite the wealth of geochemicaldata obtained from the flood basalts and related rocks, many fun-damental aspects concerning their petrogenesis are still in hot de-bate: (i) What are the geochemical and isotopic features of theEmeishan plume head? (ii) What are the respective roles of sub-continental lithospheric mantle (SCLM) and continental crust in

the chemical variation of the lavas? (iii) What processes controlthe petrogenesis of rare silicic volcanic rocks in the ELIP?

Permian magmatic rocks in the Song Da district, northern Viet-nam encompass a variety of intrusive and extrusive rocks that arepart of the ELIP (Hanski et al., 2004; Wang et al., 2007). This suite(hereafter referred to as the Song Da suite) within the IndochinaBlock was displaced to the present location as a result of mid-Ter-tiary sinistral movement along the Ailao Shan-Red River shear zonefollowing the India–Eurasia collision (Chung et al., 1997, 1998).The volcanic facies of this suite include high-Mg volcanic rocks,flood basalts and silicic volcanic rocks. The high-Mg volcanic rockswere claimed to be examples of Phanerozoic komatiites (Hanskiet al., 2004), but this interpretation has been challenged by Wanget al. (2007) and Ali et al. (2010) due to the lack of olivine spinifextexture in these rocks. Some flood basalts exhibit lithospheric sig-natures that are attributed to crustal contamination (Wang et al.,2007). Elsewhere in the ELIP, however, contamination by SCLMwas used to explain similar geochemical features (Xiao et al.,2004; He et al., 2010). The silicic volcanic rocks have not been sub-jected to detailed studies and whether they can be attributed to

1367-9120/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.jseaes.2011.07.020

⇑ Corresponding author. Address: Department of Geosciences, National TaiwanUniversity, Taipei P.O. Box 13-318, Taipei 10699, Taiwan. Tel.: +886 2 3366 2934;fax: +886 2 2363 6095.

E-mail address: sunlin@ntu.edu.tw (S.-L. Chung).

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petrogenetic processes for similar rocks in the ELIP or not is uncer-tain (Xu et al., 2010). In this paper, we present major and trace ele-ment and Sr–Nd isotopic data for 24 samples including picrites,flood basalts and silicic volcanic rocks of the Song Da suite. Thesenew data, together with published data, provide valuable con-straints to the above problems.

2. Geological background

Southeast Asia contains a complex assembly of micro-continen-tal blocks, including the Yangtze Block, Indochina Block, Simao ter-rane, Sibumasu terrane and West Burma terrane, separated bymajor suture zones or fault zones (Metcalfe, 2002). The Song Dadistrict, northern Vietnam is situated between the Yangtze Blockand Indochina Block. It is bounded by the Ailao Shan-Red RiverFault to the northeast and by the Song Ma suture to the southwest(Fig. 1). It is a lozenge-shaped region with length and width of�430 km and �100 km, respectively. Both Archean and Proterzoicbasement complexes have been documented in the region (Lanet al., 2000, 2001; Nam et al., 2003). They are overlain by Carbon-iferous to Cenozoic strata.

Permian magmatic rocks in the Song Da district were investi-gated by many Russian workers (Polyakov et al. (1991, 1998) andreferences therein; Balykin and Petrova, 2000). The volcanic faciesof this suite, including picrites, flood basalts and silicic volcanicrocks, rest on early Permian limestones and are overlain uncon-formably by Triassic limestone and shale with coal seams. Theserocks crop out as a �300-km-long belt sub-parallel to the AilaoShan-Red River Fault, extending from the Jinping area near the bor-der between China and Vietnam southeastward to �100 km southof Hanoi. The Song Da suite was considered as a detached part ofthe end-Guadalupian ELIP displaced to the present locations bymid-Tertiary sinistral movement along the Ailao Shan-Red Rivershear zone (Chung et al., 1997, 1998; Hanski et al., 2004; Aliet al., 2010). Similar to the ELIP, the Song Da flood basalts were di-vided to low-Ti and high-Ti groups suggested to form from two dis-tinct mantle sources with different degrees of lithosphericcontamination (Wang et al., 2007).

Samples in this study were collected from four localities in theSong Da district: Chieng Ngam-Son La, Ta Khoa-Deo Chen, SuoiChat and Doi Bu (Fig. 1). The first two localities contain picrites,low-Ti basalts and high-Ti basalts whereas the other two containhigh-Ti basalts and silicic volcanic rocks (Fig. 2). Clinopyroxene spi-nifex texture is observed in some relatively MgO-rich low-Ti bas-alts (Fig. 3a) but absent in the picrites, which contain elongateand equant olivine phenocrysts (Fig. 3b and c). The phenocrystassemblages of the low-Ti and high-Ti basalts are generally similarto their counterparts elsewhere in the ELIP (Xu et al., 2001; Xiaoet al., 2004): low-Ti basalts contain clinopyroxene and olivinephenocrysts whereas high-Ti basalts contain phenocrysts of eitherplagioclase or clinopyroxene, or both. The silicic volcanic rockscontain quartz, plagioclase and biotite as do other Emeishan silicicvolcanic rocks (Xu et al., 2010).

3. Analytical techniques

Twenty-four samples (5 picrites, 16 basaltic rocks, 3 silicic vol-canic rocks) were collected in this study. They were carefully cutusing a brass saw blade to remove weathering rinds and pulverizedin agate mortars to minimize potential contamination during traceelement analysis. The samples were analyzed for major elementoxides with a Rigaku� RIX 2000 X-ray fluorescence (XRF) spec-trometer on fused glass disks at National Taiwan University, Tai-wan. Concentrations of trace elements were determined byinductively coupled plasma mass spectrometry (ICP-MS) using an

Agilent 7500s machine at the same university. Standard additionswere used to establish absolute abundance and pure elementalstandards were used for external calibration. Accuracies of XRFand ICP-MS analysis are estimated to be better than 5% and 3%,respectively. Detailed analytical procedures are described in Yanget al. (2005). Loss-on-ignition was obtained by routine methods.

Sr–Nd isotopic ratios were measured by a Finnigan MAT-262mass spectrometer, operated in static multi-collector mode, at Na-tional Cheng Kung University, Taiwan. Mass fractionation was cor-rected using 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219. Duringdata acquisition, the mean 87Sr/86Sr ratio of Sr standard NBS987was 0.710261 ± 0.000017 (n = 8) and the 143Nd/144Nd ratio of LaJolla standard JNdi-1 was 0.511830 ± 0.000180 (n = 12). Detailedanalytical procedures follow Liu et al. (2007).

4. Classification of the Song Da volcanic rocks

Hanski et al. (2004) considered Permian high-Mg volcanic rocksin the Song Da district as ‘komatiites’. This nomenclature has beenquestioned by Wang et al. (2007) and Ali et al. (2010), who notedthat olivine spinifex texture, which is typical of komatiites (Kerrand Arndt, 2001) is absent in these rocks. In this study, olivine inthe Song Da picrites is either elongate or equant, rather than spini-fex-textured (Fig. 3b and c). Thus, the use of the term ‘komatiite’ todescribe these rocks is misleading and it is more appropriate to usethe term ‘picrite’ instead.

The Song Da flood basalts have been divided into low-Ti andhigh-Ti groups by Wang et al. (2007), using a limit of Ti/Y = 500after Xu et al. (2001). Recently, the Ti-classification scheme hasbeen challenged by Shellnutt and Jahn (2011), who demonstratedthat the Emeishan flood basalts span a continuous range in TiO2

contents and Ti/Y. Our data on the Song Da flood basalts are in linewith their findings showing no major compositional gaps for theseparameters (Fig. 4a). In particular, four samples show dual charac-ter of Ti/Y < 500 and relatively high TiO2 contents (2.6–3.2 wt.%),and sample IGCP39 has Ti/Y of 506 and 1.52 wt.% TiO2. These high-light the potential problem of the conventional classificationscheme. He et al. (2010) proposed a new classification scheme ofthe Emeishan flood basalts by the parameter (0.08 xMgO + TiO2 � 2.91); positive values indicate affinity to high-Tigroup and negative values to low-Ti group. Using this scheme,three out of the four samples showing dual character belong tothe high-Ti group and sample IGCP39 to the low-Ti group(Fig. 4b). In this study, we retain the Ti-based nomenclature solelyfor descriptive purpose and recommend that its use should not beassociated with any genetic implications. On total alkalis–SiO2 andZr/TiO2–Nb/Y diagrams, low-Ti basalts are transitional betweensub-alkaline and alkaline fields whereas high-Ti basalts are domi-nantly alkaline (Fig. 5). Sample SL24-2 plots in the tephrite fieldbut its composition is different from other Emeishan tephrites doc-umented by Song et al. (2008) and Qi et al. (2008).

The Song Da silicic volcanic rocks have high Fe� [FeOT/(FeOT + MgO), Frost et al., 2001] ranging from 0.88 to 0.95 andare thus ferroan. Sample SL26 has an aluminium saturation index[ASI; molecular Al/(Ca � 1.67 x P + Na + K), Shand, 1943] of 0.92and molecular (Na + K)/Al of 0.80, and is thus metaluminous. Sam-ples H952 and H953 have ASI of 1.24 and 1.10 and molecular(Na + K)/Al of 0.73 and 0.75, features indicative of peraluminousnature. The Song Da silicic volcanic rocks are different from thosereported elsewhere in the ELIP that are dominantly peralkaline(Xu et al., 2010; Shellnutt and Jahn, 2010). To our knowledge, thisis the first documentation of metaluminous and peraluminous si-licic volcanic rocks in the ELIP. According to the modified alkali-lime index of Frost et al. (2001), samples SL26 and H952 belongto the alkalic series and sample H953 to the alkali-calcic series.

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5. Results

5.1. Major and minor elements

Concentrations of selected major, minor and trace elementsfor the Song Da volcanic rocks are listed in Table 1. Picritesare characterized by high and variable concentrations of MgO(13.8–30.2 wt.%), Ni (343–1410 ppm), Cr (1661–3187 ppm) andMg# [molar 100 �Mg/(Mg + Fe2+)] (72–85), consistent withvariable degrees of olivine and chrome spinel accumulation. Ingeneral, the low-Ti basalts have higher MgO concentrationsand Mg# than the high-Ti basalts, except a relatively primitive

high-Ti basalt (IGCP46) that has 10.3 wt.% MgO and Mg# = 60.Both types of basalts have lower contents of MgO, Ni and Cr,and higher contents of TiO2, Al2O3, Na2O, K2O and V than thepicrites. The silicic volcanic rocks are characterized by lowconcentrations of TiO2, Fe2O3, MgO, CaO and all transition ele-ments. The Al2O3 and alkalis contents of these rocks are highrelative to the basalts and picrites. Overall, concentrations ofAl2O3 and Zr in the rocks increase whereas those of MgO, CaOand Ni decrease with decreasing Mg# (Fig. 6). The TiO2, Fe2O3

and P2O5 contents of the rocks increase with decreasing Mg#,except for the silicic volcanic rocks that are low in theseelements (Fig. 6).

Fig. 1. Geological sketch map of the Song Da district, northern Vietnam showing the distribution of Permian volcanic rocks and sampling sites of this study. Inset shows thestudy area between South China and Indochina.

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5.2. Trace elements

Generally, the abundance of trace elements is very low in thepicrites, intermediate in the basalts and high in the silicic volcanicrocks. The Zr concentrations of the rocks correlate positively withmost trace elements, especially for rare earth elements (REE) andhigh field strength elements (HFSE) (Fig. 7a–d). This is consistentwith their incompatible behaviour and implies that the abundanceand ratios of these elements were not likely modified by deutericprocesses. The Sr and Ba trends are less well-defined comparedto those of REE and HFSE (Fig. 7e and f).

Fig. 8 illustrates the chondrite-normalized REE patterns for theSong Da volcanic rocks. The patterns for picrites display slight

IGCP19IGCP18LT

HT

IGCP17IGCP15IGCP16

IGCP21IGCP22

IGCP36IGCP37

LT

HT

IGCP38IGCP39IGCP40

IGCP42IGCP41-1

IGCP45IGCP44

IGCP43

SL24-1

SL24-2

HT

SL26

H951

H952

HT

H953Pic

Pic

Pic

Pic

Sic

Alk. bSic

CS

TD

SC DB

Carboniferous basement

IGCP46

Fig. 2. Simplified columnar sections showing the relative stratigraphic positions for individual samples at each sites. See Fig. 1 for abbreviations. LT = low-Ti basalts,HT = high-Ti basalts, Pic = picrite, Sic = silicic volcanic rocks, Alk. b. = alkaline basalt.

Cpx

0.5 mm

Ol

Cpx

Ol

Ol Ol

Ol

0.5 mm

0.5 mm

(a)

(b)

(c)

Fig. 3. Photomicrographs for the Song Da volcanic rocks. (a) Clinopyroxene spinifextexture in basalt. (b) Elongate olivine phenocrysts in basalt. (c) Elongate and equantolivine phenocrysts in picrite.

5

)%t

w(OiT

2

4

2

1

0

(a)

(b)

Ti/Y

2

OgM

80.019.2-

OiT+

x 2

400 6000 200 800

1

0

-2

-3

-1

Silicic rocksLow-Ti basaltHigh-Ti basaltPicrite

3

Y =

500

/iT

Dualcharacter

Fig. 4. Classification of the Song Da volcanic rocks. (a) Ti-classification after Xu et al.(2001); four samples have Ti/Y < 500 but relatively high TiO2 contents (dualcharacter). (b) Classification based on the parameter (0.08 x MgO + TiO2 � 2.91)after He et al. (2010); three out of four samples showing dual character wereclassified as high-Ti basalt using this parameter.

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enrichment or depletion in LREE and no significant fractionationbetween MREE and HREE (Fig. 8a). The patterns for high-Ti basaltsshow enrichment of LREE relative to HREE (i.e. CeN/YbN = 4.3–9.9)(Fig. 8b). The low-Ti basalts are also rich in LREE relative to HREE(i.e. CeN/YbN = 2.5–6.4) but the enrichment is less pronouncedcompared to the high-Ti basalts (Fig. 8c). Some low-Ti basalts shownegative Eu anomaly. The silicic volcanic rocks show enrichment ofLREE relative to HREE (i.e. CeN/YbN = 4.6–9.3) with or without neg-ative Eu anomaly (Fig. 8d). Fig. 9 illustrates the primitive mantle-normalized trace element patterns for the rocks. The picrites aredepleted in most trace elements relative to ocean island basalts(OIB) (Fig. 9a). In contrast, high-Ti basalts are characterized by pat-terns that are highly similar to OIB (Fig. 9b). Most low-Ti basaltsare depleted in most trace elements relative to OIB and have neg-ative Nb–Ta anomalies (Fig. 8c). However, the extent of depletion ismuch weaker than the picrites. The patterns for silicic volcanicrocks resemble the high-Ti basalts and OIB and are marked by neg-ative Sr and Ti anomalies (Fig. 9d).

5.3. Sr and Nd isotopes

The measured and age-corrected Sr–Nd isotopic ratios are listedin Table 2. The picrites are characterized by wide ranges of(87Sr/86Sr)i (0.7060–0.7116) and eNd(t) values (�7.6 to +6.7). Thehigh-Ti basalts have relatively restricted ranges of (87Sr/86Sr)i

(0.7048–0.7079) and eNd(t) values (�5.7 to +3.1). Most low-Ti

basalts have high (87Sr/86Sr)i (0.7079–0.7155) and eNd(t) values(�8.2 to �14.2), except two samples having Sr–Nd isotopic ratiosclose to bulk Earth (Table 2). Like the high-Ti basalts, the silicic vol-canic rocks have restricted ranges of (87Sr/86Sr)i (0.7041–0.7060)and eNd(t) values (�0.1 to +0.6). Fig. 10 illustrates the calculatedisotopic data in a eNd(t)–(87Sr/86Sr)i diagram. The Sr–Nd isotopiccompositions for low-Ti and high-Ti basalts are generally similarto their counterparts in the Song Da district or elsewhere in theELIP (Fig. 10).

6. Discussion

6.1. Pre-tertiary position of the Song Da district

Previous studies have established the genetic relation betweenthe Song Da magmatic suite and the end-Guadalupian ELIP (Hanskiet al., 2004; Wang et al., 2007). Its occurrence as a 300-km-longlimb emanating from the southern ELIP (present configuration)was attributed to large-scale sinistral movement along the AilaoShan-Red River shear zone following the mid-Tertiary India–Eur-asia collision (Chung et al., 1997, 1998). Reconstruction based onthe estimated offset of �600 km places the Song Da suite close toLijiang and Dali, SW China. Accordingly, the Song Da picrites wouldhave occurred adjacent to the picrite field, a sub-circular regionpresumably representing the fossil Emeishan plume head (seeFig. 2 in Ali et al., 2010). He et al. (2010) estimated the potentialsource temperatures from a limited number of Emeishan picritesand proposed that the Song Da district was located at the peripheryof the ELIP. We believe that this is unlikely and that the Song Dadistrict was situated at the central ELIP before Tertiary.

6.2. Origin of picrites and signature of the Emeishan plume head

The Emeishan picrites were generally considered to be mantle-derived high-Mg magmas formed under high source potential tem-peratures and, by implication, a mantle plume (Chung and Jahn,1995; Hanski et al., 2004, 2010; Zhang et al., 2006; Li et al.,2010; Ali et al., 2010). The picrites, like the flood basalts, are di-vided into low-Ti and high-Ti types and there is tendency for pi-crites and basalts of the same type to be associated in a lavasequence (Hanski et al., 2010). In particular, the Song Da picriteswere suggested to have formed from relatively high degrees of par-tial melting (�20%) of a depleted mantle source at relatively shal-low depths (Hanski et al., 2004; Wang et al., 2007).

Our new data for the Song Da picrites do not change the basicconclusions drawn from previous studies. Accordingly, high eNd(t)values (+5.7 to +6.7) and low Ce/Yb (1.3–2.3) of most picritespoints to a chemically and isotopically depleted mantle source.Highly variable MgO (13.8–30.2 wt.%) and Ni contents (343–1410 ppm) of the picrites are consistent with various degrees ofolivine accumulation, consistent with petrographic observations(Fig. 3). The lack of significant fractionation between MREE andHREE (e.g. Sm/Yb = 0.83–1.46) indicates partial melting withoutresidual garnet. This implies that either melting occurred in thespinel stability field or high degrees of melting that consumed allgarnet in the source. Calculations using the PRIMELT2 code ofHerzberg and Asimow (2008) indicate that the parental magmasof the picrites have MgO ranging from 20.9 to 24.4 wt.%, corre-sponding to eruption temperatures of 1463–1519 �C.

It was proposed that an eNd(t) value of +4 is indicative of theaverage Nd isotopic composition of the Emeishan plume head(Chung and Jahn, 1995). A wealth of subsequent studies on theELIP-related magmatic rocks indicates that the plume head mighthave had higher eNd(t) values than previously thought. Theseinclude: (i) an eNd(t) value of +8.0 was reported for the

10

8

6

4

2

040 50 60

Picro-basalt

70

Na 2

O +

K2O

(wt%

)

SiO2 (wt%)

RhyoliteTrachy-dacite

DaciteAndesite

Basaltic-andesite

Basalt

Trachy-basalt

Tephrite

Phono-tephrite

Trachy-andesiteBasaltic

trachy-andesite

100

10-1

10-2

10-3

(a)

(b)

Sub-alkaline

Alkaline

10-2 10-1 101

Nb/Y

Sub-alkaline basalt

100

Phonolite

Trachyte

Alkalibasalt

Trachy-andesite

Andesite/basalt

Andesite

Rhyolite

Comendite

Basanite

Rhyodacite/dacite

Silicic rocksLow-Ti basaltHigh-Ti basaltPicrite

Zr/T

iO2

Fig. 5. Discrimination diagrams for the Song Da volcanic rocks. (a) Total alkalis–SiO2 diagram (after Le Bas et al., 1986); the dashed line separating the sub-alkalineand alkaline fields was after Irvine and Baragar (1971). (b) Zr/TiO2–Nb/Y diagram(after Winchester and Floyd, 1977). Both diagrams show that the picrites and low-Tibasalts are dominantly sub-alkaline whereas the high-Ti basalts and silicic volcanicrocks are mildly alkaline to alkaline.

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Table 1Major and trace element data of the Song Da volcanic rocks.

Picrite High-Ti basalt

Sample IGCP17 IGCP38 IGCP41-1 IGCP42 IGCP44 IGCP15 IGCP16 IGCP21 IGCP22 IGCP45 IGCP46Locationa 1 2 2 2 2 1 1 1 1 2 2

Major oxides (wt.%)SiO2 43.66 44.60 46.80 40.94 47.17 54.51 48.80 47.50 47.49 48.83 42.90TiO2 0.64 0.89 0.48 0.37 0.73 3.81 2.76 3.31 3.32 2.85 2.50Al2O3 11.58 10.73 6.33 5.06 11.97 12.18 13.91 14.05 13.62 13.61 14.47Fe2O3 12.21 12.37 9.56 10.76 10.85 13.08 13.61 14.42 14.28 16.51 13.47MnO 0.14 0.20 0.15 0.15 0.16 0.16 0.21 0.21 0.21 0.24 0.21MgO 16.03 14.09 22.48 30.18 13.86 2.59 6.02 4.60 5.29 4.11 10.27CaO 8.68 12.36 9.72 5.02 8.94 5.67 7.77 8.64 8.69 6.32 9.60Na2O 1.00 1.15 0.18 0.39 0.60 4.41 4.01 3.19 3.06 1.04 2.04K2O 0.00 0.00 0.00 0.05 1.47 1.81 0.28 0.80 1.13 2.66 0.63P2O5 0.02 0.19 0.02 0.05 0.07 0.53 0.29 0.55 0.56 0.40 0.28LOI 5.62 4.42 9.08 3.54 1.53 2.29 4.29 2.50 3.26 3.65

Total 99.58 101.00 95.72 102.05 99.36 100.28 99.95 101.56 100.15 99.83 100.02Mg#b 72.4 69.5 82.5 84.9 71.9 28.4 46.9 39.0 42.6 33.2 60.4

Trace elements (ppm)Sc 33.1 45.5 22.0 18.0 44.6 17.4 33.3 26.2 26.3 19.4 32.6V 147 283 143 127 263 260 316 332 347 261 241Cr 1661 828 2406 3187 1782 5.56 77.0 49.0 46.8 9.96 570Ni 803 343 1115 1409 386 27.4 70.6 58.1 55.2 20.9 245Rb 1.48 1.10 0.74 10.1 69.3 29.3 6.94 18.3 17.1 46.2 27.2Sr 50.4 60.3 9.27 9.51 35.4 146 343 550 541 313 350Y 16.9 15.9 10.0 6.60 17.6 40.2 33.9 27.5 28.7 52.6 26.1Zr 27.2 35.2 21.7 16.5 31.8 309 182 132 154 164 139Nb 0.61 6.55 0.67 2.36 5.09 40.9 15.5 18.2 22.7 19.2 22.1Cs 0.28 0.23 0.20 3.44 0.56 0.08 0.46 0.38 0.52 0.34 1.81Ba 8.15 20.1 2.67 20.4 265 519 128 1164 1089 517 172La 0.79 4.33 0.81 1.80 9.40 63.8 26.9 29.8 30.7 36.4 19.1Ce 2.10 8.92 2.20 3.60 18.4 120 59.2 66.4 68.4 71.5 41.8Pr 0.42 1.18 0.41 0.48 2.21 16.1 7.50 8.54 8.82 9.07 5.63Nd 2.73 5.47 2.40 2.19 9.15 65.3 32.4 37.8 38.9 38.5 26.1Sm 1.30 1.66 0.98 0.65 2.25 12.6 7.43 7.94 8.17 9.22 6.52Eu 0.52 0.43 0.30 0.23 0.46 3.37 2.08 3.03 3.26 2.56 2.16Gd 1.91 2.02 1.27 0.81 2.50 11.3 7.07 7.21 7.40 9.36 6.17Tb 0.41 0.40 0.26 0.16 0.46 1.63 1.16 1.08 1.11 1.61 0.99Dy 2.69 2.67 1.70 1.06 2.88 8.36 6.53 5.63 5.81 9.32 5.19Ho 0.61 0.59 0.37 0.24 0.63 1.54 1.30 1.06 1.10 1.91 0.98Er 1.71 1.67 1.03 0.71 1.75 4.03 3.48 2.73 2.85 5.11 2.44Tm 0.24 0.25 0.15 0.10 0.25 0.52 0.48 0.36 0.37 0.71 0.32Yb 1.58 1.58 0.97 0.71 1.55 3.20 3.00 2.17 2.30 4.32 1.91Lu 0.24 0.23 0.14 0.11 0.21 0.45 0.44 0.31 0.33 0.58 0.26Hf 0.81 1.11 0.62 0.44 1.04 7.97 4.70 3.41 3.86 4.19 3.47Ta 0.04 0.39 0.05 0.15 0.33 2.77 1.02 1.24 1.52 1.25 1.39Pb 0.13 0.09 0.13 0.07 0.76 6.43 3.03 2.08 1.81 9.92 2.29Th 0.07 1.16 0.12 0.42 3.08 9.03 3.67 2.64 2.80 9.61 2.35U 0.01 0.24 0.02 0.11 0.68 1.94 0.92 0.66 0.69 2.57 0.48

Low-Ti basalt

Sample SL24-1 H951 IGCP18 IGCP19 IGCP36 IGCP37 IGCP39 IGCP40 IGCP43 SL24-2Location 3 4 1 1 2 2 2 2 2 3

Major oxides (wt.%)SiO2 48.14 48.37 51.32 49.72 53.14 50.17 50.03 60.11 49.10 48.43TiO2 3.22 3.31 1.28 1.17 0.86 1.13 1.52 0.70 0.84 2.25Al2O3 13.85 13.24 13.86 16.85 13.97 13.93 12.45 15.22 15.65 15.97Fe2O3 17.31 14.00 12.86 11.08 9.04 10.42 11.37 7.31 10.37 13.09MnO 0.11 0.22 0.18 0.17 0.14 0.16 0.17 0.11 0.17 0.22MgO 3.36 5.29 5.63 6.92 8.03 6.58 7.81 4.50 7.52 3.28CaO 5.03 9.07 7.65 5.60 6.88 11.27 11.06 6.35 10.43 6.05Na2O 3.66 3.74 5.03 4.20 3.04 1.69 2.35 1.59 2.24 3.39K2O 1.05 0.33 0.00 0.74 1.85 1.74 1.01 1.54 0.76 4.15P2O5 0.81 0.47 0.12 0.10 0.06 0.19 0.35 0.06 0.07 0.75LOI 3.55 2.30 3.18 4.08 3.51 2.71 2.00 2.81 3.04 1.59Total 100.09 100.34 101.11 100.63 100.52 99.99 100.12 100.30 100.19 99.17Mg#b 28.0 43.0 46.7 55.5 64.0 55.8 57.9 55.2 59.2 33.4

Trace elements (ppm)Sc 23.7 24.5 36.5 37.8 33.9 30.7 36.2 29.0 38.0 8.7V 246 299 315 221 207 250 240 172 212 152Cr 36.1 67.1 60.9 269 471 202 269 87.8 281 1.05Ni 30.7 66.3 45.4 7.00 58.9 31.4 74.0 6.85 28.4 11.5Rb 35.9 12.8 1.44 31.0 45.5 33.7 14.7 36.4 17.0 140

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Table 1 (continued)

Low-Ti basalt

Sample SL24-1 H951 IGCP18 IGCP19 IGCP36 IGCP37 IGCP39 IGCP40 IGCP43 SL24-2Location 3 4 1 1 2 2 2 2 2 3

Sr 209 481 59.1 146 113 400 404 102 110 436Y 43.1 35.4 27.6 26.8 23.7 19.0 18.0 31.2 24.1 37.4Zr 128 156 105 112 105 93.8 62.3 123 51.7 164Nb 34.7 24.5 7.04 7.98 7.47 11.5 17.6 9.42 6.62 68.9Cs 0.28 1.02 0.19 0.35 0.07 0.11 0.10 0.18 0.25 1.38Ba 457 128 13.5 94.4 442 494 369 196 213 1381La 47.5 30.2 14.7 18.6 21.1 16.6 16.3 26.6 14.6 65.7Ce 88.5 65.6 27.5 38.8 40.4 32.4 33.5 53.9 27.8 115Pr 11.1 8.36 3.31 4.52 4.48 3.85 4.17 6.12 3.37 13.4Nd 45.8 36.7 13.7 18.2 17.2 15.9 18.2 23.8 13.6 49.9Sm 9.74 8.24 3.46 4.22 3.75 3.65 4.35 5.09 3.10 9.14Eu 2.86 2.70 0.89 0.98 0.91 1.21 1.34 0.96 0.73 2.61Gd 9.53 8.03 3.86 4.39 3.92 3.65 4.17 4.97 3.35 8.79Tb 1.51 1.28 0.73 0.79 0.68 0.60 0.66 0.85 0.61 1.31Dy 8.34 6.87 4.66 4.72 4.12 3.44 3.58 5.15 3.81 7.11Ho 1.62 1.35 1.04 1.01 0.88 0.69 0.69 1.10 0.85 1.37Er 4.25 3.49 3.00 2.85 2.49 1.89 1.75 3.18 2.41 3.64Tm 0.57 0.46 0.44 0.42 0.37 0.26 0.23 0.47 0.36 0.48Yb 3.45 2.81 2.91 2.76 2.38 1.65 1.38 3.10 2.29 2.98Lu 0.45 0.40 0.44 0.42 0.36 0.23 0.19 0.44 0.32 0.39Hf 3.18 4.36 2.83 3.08 2.69 2.45 1.90 3.18 1.58 3.53Ta 2.09 1.75 0.45 0.58 0.48 0.70 1.03 0.71 0.47 4.09Pb 3.82 3.64 2.02 2.17 3.90 4.67 0.98 4.89 2.84 5.99Th 6.27 3.65 4.11 6.84 8.14 3.02 1.76 9.16 4.61 12.01U 1.22 0.90 0.75 1.23 1.49 0.77 0.50 1.90 0.95 2.21

Silicic volcanic rocks

Sample SL26 H952 H953Location 3 4 4

Major oxides (wt.%)SiO2 58.45 62.67 71.26TiO2 1.35 0.76 0.53Al2O3 16.10 18.41 12.45Fe2O3 8.00 5.27 4.18MnO 0.13 0.07 0.06MgO 1.06 0.37 0.20CaO 2.73 0.77 1.13Na2O 5.67 6.31 1.76K2O 3.34 2.91 5.99P2O5 0.26 0.06 0.06LOI 2.55 2.53 2.95

Total 99.64 100.13 100.57Mg#b 20.9 12.3 8.7

Trace elements (ppm)Sc 5.59 5.21 3.09V 55.0 9.56 3.99Cr 0.38 1.81 1.79Ni 2.86 1.39 0.82Rb 85.4 133 149Sr 343 310 74.2Y 50.3 108 55.6Zr 417 1187 555Nb 78.1 127 74.2Cs 1.46 8.15 1.87Ba 983 500 865La 71.5 140 94.8Ce 136 260 200Pr 17.0 36.4 27.7Nd 63.9 137.0 110.0Sm 12.4 27.7 24.3Eu 2.96 4.24 4.19Gd 11.5 25.4 19.0Tb 1.82 4.04 2.55Dy 10.0 22.5 12.7Ho 1.96 4.56 2.38Er 5.33 12.9 6.50Tm 0.75 1.89 0.90Yb 4.73 12.1 5.66Lu 0.67 1.81 0.81Hf 10.3 29.3 13.7Ta 5.21 9.64 4.94

(continued on next page)

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least-contaminated Song Da picrite (Hanski et al., 2004), (ii) aneNd(t) range of +5.0 to +7.8 for the majority of Muli high-Ti picrites(Li et al., 2010), and (iii) an eNd(t) value up to +5.1 for the �262 Ma

mafic–ultramafic dykes in the Yanyuan area (Zi et al., 2008). Thus,we propose a revised eNd(t) value of +8.0 for the average Nd isoto-pic composition of the Emeishan plume head.

Table 1 (continued)

Silicic volcanic rocks

Sample SL26 H952 H953Location 3 4 4

Pb 11.9 17.1 2.97Th 16.7 26.7 17.4U 2.19 6.56 3.52

a See Fig. 1 for sampling locations.b Mg# = [molar 100 �Mg/(Mg + Fe2+)], assuming 10% of total iron oxide is ferric.

15

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20

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(a)

(c) (d)

(b)

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(f)(e)0.8

15

CaO

(wt%

) 10

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0.2

(h)(g)

Mg# Mg#

Zr (p

pm)

104

Ni (

ppm

)

40 800 60

104

0 20 60 8020 100

103

102

101

100

10-1

103

102

10140 100

Al2O

3 (w

t%)

MgO

(wt%

)P 2

O5

(wt%

)

Fe2O

3T (w

t%)

TiO

2 (w

t%)

Fig. 6. Concentrations of selected major and trace elements versus Mg# for the Song Da volcanic rocks. (a) TiO2, (b) Al2O3, (c) Fe2O3T (total Fe as Fe2O3), (d) MgO, (e) CaO, (f)P2O5, (g), Ni, (h) Zr.

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6.3. Lithospheric contamination

Most proposed petrogenetic models for the Emeishan picritesand flood basalts emphasize the role of plume–lithosphere interac-tion whereby plume-derived magmas are interacted with SCLMand/or continental crust through which they pass before eruption(Chung et al., 1998; Xu et al., 2001; Xiao et al., 2004; Wang et al.,2007; Zhou et al., 2008; He et al., 2010). This section examinesthe roles of SCLM and continental crust in the formation of theSong Da picrites and flood basalts.

Large variations of Sr–Nd isotopic compositions for the Song Dapicrites and flood basalts are indicative of substantial lithosphericinputs to magmas derived from the Emeishan plume (Fig. 11a andb). For example, the picrites are characterized by an eNd(t) rangefrom �7.8 to +8.0 (�16 epsilon units) and the low-Ti basalts from�14.2 to +6.5 (�20 epsilon units). The high-Ti basalts are charac-terized by an eNd(t) range from �5.7 to +3.1 (�9 epsilon units). Po-sitive trends on the Nb/La–eNd(t) and Nb/U–eNd(t) diagrams(Fig. 11c and d) demonstrate that the rocks contain a lithosphericcomponent, because many continental crustal rocks and probablyalso those from the SCLM have low Nb/La, Nb/U and eNd(t) (Rud-nick and Gao, 2003).

The lithospheric signatures of the rocks mentioned above indi-cate either contamination of plume-derived magmas by SCLM and/or continental crust, or the presence of low-eNd(t) material in thesource, or both. The SCLM is thought to be some one to several

hundred degrees below the dry solidus of mantle peridotites dueto its refractory nature, and is unlikely to melt without undergoingmetasomatism (Arndt and Christensen, 1992). However, metaso-matized peridotites worldwide exhibit substantial compositionalvariability; for example, their Nb/La values span a broad rangefrom 0 to 10 (Erlank et al., 1987; Menzies et al., 1987; McDonough,1990). The Emeishan tephrites, suggested to be low-degree partialmelts from the SCLM, serve as an estimate of its metasomatizedcomponents (Song et al., 2008). These rocks have low eNd(t)(�10.6 to �11.1) but high Nb/La (2.06–4.00) and Nb/U (30.2–92.5). Thus, contamination of plume-derived magmas by SCLMcannot explain the low Nb/La and Nb/U exhibited by some SongDa picrites and low-Ti basalts (Fig. 11c and d). This is also illus-trated by the mixing calculations (Table 3), which show insteadthat continental crustal rocks are plausible contaminants. The re-sults indicate that the majority of low-eNd(t) picrites and low-Tibasalts can be modelled by less than 10% and less than 20% crustalcontamination, respectively (Table 3). Similar degrees of crustalcontamination of high-Ti basalt only result in slightly negativeeNd(t). As for the picrites, the unrealistically high degree of con-tamination (�50%) suggested by Wang et al. (2007) is only validif the picrites and crustal rocks have comparable Nd concentra-tions, an assumption that does not hold in this case. Based on theabove calculations, therefore, we suggest that crustal contamina-tion (<20%) explains the lithospheric signatures of the Song Da pi-crites and flood basalts, although existence of low-eNd(t) material,

Eu (p

pm)

La (p

pm)

(a)

(c) (d)

(b)

Nb

(ppm

)

Th (p

pm)

(f)(e)

Zr (ppm) Zr (ppm)

Ba (p

pm)

Sr (p

pm)

104103102101104103102101 100

101

102

103

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10-1

101

100

10-1

101

102

103

100

102

101

100

10-1

10-2

101

102

103

100

10-1

Fig. 7. Concentrations of selected trace elements versus Zr for the Song Da volcanic rocks. (a) La, (b), Eu, (c), Th, (d), Nb, (e), Sr, (f) Ba.

T.V. Anh et al. / Journal of Asian Earth Sciences 42 (2011) 1341–1355 1349

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(a) Picrite

(d) Silicic rocks

Sam

ple/

Cho

ndrit

e

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

(b) High-Ti basalt

101

102

103

100

(c) Low-Ti basalt

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

IGCP39 (tuff)

101

102

103

100

101

102

103

100

Sam

ple/

Cho

ndrit

e

101

102

103

100

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 8. Chondrite-normalized REE variation diagrams for the Song Da volcanic rocks. Normalizing values after Sun and McDonough (1989). (a) Picrite, (b), high-Ti basalt, (c),low-Ti basalt, (d) silicic volcanic rocks.

(b) High-Ti basalt

(d) Silicic rocks(c) Low-Ti basalt

(a) Picrite

TiRb Th Nb Ta Ce Pb Sr Hf Sm Gd Dy Y LuBa U La Nd Zr Eu Tb Er YbK

Sam

ple/

Prim

itive

man

tle

OIB

IGCP39 (tuff)

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itive

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TiRb Th Nb Ta Ce Pb Sr Hf Sm Gd Dy Y LuBa U La Nd Zr Eu Tb Er YbK TiRb Th Nb Ta Ce Pb Sr Hf Sm Gd Dy Y LuBa U La Nd Zr Eu Tb Er YbK

TiRb Th Nb Ta Ce Pb Sr Hf Sm Gd Dy Y LuBa U La Nd Zr Eu Tb Er YbK

Fig. 9. Primitive mantle-normalized diagrams for the Song Da volcanic rocks. Normalizing values after McDonough and Sun (1995). Elemental concentrations for ocean islandbasalt after Sun and McDonough (1989). (a) Picrite, (b), high-Ti basalt, (c) low-Ti basalt, (d), silicic volcanic rocks.

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such as subducted sediments in the source cannot be completelyruled out (Zhang et al., 2006; Shellnutt and Jahn, 2011).

6.4. Origin of silicic volcanic rocks

Rare Emeishan silicic volcanic rocks were documented by Xuet al. (2010) and Shellnutt and Jahn (2010); both groups of authorssuggest that these rocks represent products after extensive frac-tional crystallization of basaltic magmas in the ELIP. The Song Da

silicic volcanic rocks are metaluminous or peraluminous whereasthose occurring elsewhere in the ELIP are peralkaline and it isuncertain if they share a common origin. This section examinesvarious petrogenetic processes responsible for the formation ofthe Song Da silicic volcanic rocks.

Crustal melting is able to produce silicic magmas in two ways:melting of felsic crustal rocks and partial melting of underplatedmafic rocks. Precambrian crust crops out in and forms the base-ment of the Song Da district. One example is the �1.35 Ga PosenComplex, northern Vietnam characterized by eNd(t) values from�15.8 to �17.0 (Lan et al., 2000). However, the Song Da silicic vol-canic rocks have much higher eNd(t) values ranging from �0.1 to0.6, ruling out the Precambrian basement as possible source rocks.This is supported by the distinct incompatible trace element ratiosbetween the silicic volcanic rocks (Th/Ta = 2.78–3.52, Rb/Nb = 1.09–2.00) and the basement rocks (Th/Ta = 7.40–8.82, Rb/Nb = 6.04–8.57). Partial melting of underplated mafic rocks, suchas cumulates related to the Emeishan LIP, should lead to liquidsrelatively rich in Al2O3 and CaO, features that are absent fromthe Song Da silicic volcanic rocks (Fig. 12a and b). Further, the si-licic volcanic rocks are so poor in compatible trace elements, suchas Ni and V, to fit the trajectories deduced from partial meltingmodels (Fig. 12c and d). Thus, crustal melting cannot solely explainthe major and trace element characteristics of the Song Da silicicvolcanic rocks.

Extreme fractionation of basaltic magmas is able to generate si-licic magmas. Certain geochemical features of the Song Da silicicvolcanic rocks are consistent with formation involving dominantlyfractional crystallization of the associated high-Ti basalts, includ-ing (i) the mantle Sr–Nd isotopic signatures of the silicic volcanicrocks, (ii) the similar, OIB-like trace element patterns betweenthe silicic volcanic rocks and the basalts (Fig. 9), and (iii) the lowNi and V contents of the silicic volcanic rocks close to those

Table 2Rb–Sr and Sm–Nd isotopic data of the Song Da volcanic rocks.

Rb(ppm)

Sr(ppm)

87Rb/86Sr 87Sr/86Sr ±2rm (87Sr/86Sr)i Sm(ppm)

Nd(ppm)

147Sm/144Nd 143Nd/144Nd ±2rm (143Nd/144Nd)i eNd(t)

PicriteIGCP17 1.48 50.4 0.085 0.704255 0.000010 0.7039 1.30 2.73 0.288 0.513138 0.000010 0.5126 6.7IGCP38 1.10 60.3 0.053 0.706164 0.000010 0.7060 1.66 5.47 0.184 0.512498 0.000011 0.5122 �2.3IGCP41-

1– – – – – – 0.98 2.40 0.247 0.513018 0.000010 0.5126 5.7

IGCP42 10.1 9.51 3.06 0.713223 0.000020 0.7019 0.65 2.19 0.179 0.512475 0.000011 0.5122 �2.6IGCP44 69.3 35.4 5.67 0.732528 0.000011 0.7116 2.25 9.15 0.149 0.512168 0.000010 0.5119 �7.6

High-Ti basaltIGCP15 29.3 146 0.58 0.707910 0.000013 0.7058 12.6 65.3 0.117 0.512395 0.000010 0.5122 �2.1IGCP16 6.94 343 0.059 0.706155 0.000011 0.7059 7.43 32.4 0.139 0.512522 0.000010 0.5123 �0.3IGCP21 18.3 550 0.10 0.705737 0.000011 0.7054 7.94 37.8 0.127 0.512563 0.000011 0.5123 0.8IGCP22 17.1 541 0.091 0.705574 0.000010 0.7052 8.17 38.9 0.127 0.512526 0.000011 0.5123 0.1IGCP45 46.2 313 0.43 0.709485 0.000010 0.7079 9.22 38.5 0.145 0.512257 0.000010 0.5120 �5.7IGCP46 27.2 350 0.23 0.705614 0.000011 0.7048 6.52 26.1 0.151 0.512720 0.000010 0.5125 3.1SL24-1 35.9 209 0.50 0.709201 0.000012 0.7074 9.74 45.8 0.129 0.512408 0.000010 0.5122 �2.2H951 12.8 481 0.077 0.706668 0.000011 0.7064 8.24 36.6 0.136 0.512506 0.000010 0.5123 �0.6

Low-Ti basaltIGCP18 1.44 59.1 0.070 0.708228 0.000011 0.7080 3.46 13.7 0.152 0.512113 0.000010 0.5119 �8.8IGCP19 31.0 146 0.62 0.714369 0.000010 0.7121 4.22 18.2 0.140 0.512005 0.000011 0.5118 �10.5IGCP36 45.5 113 1.17 0.717542 0.000014 0.7132 3.75 17.2 0.132 0.511800 0.000011 0.5116 �14.2IGCP37 33.7 400 0.24 0.708800 0.000010 0.7079 3.65 15.9 0.138 0.512119 0.000011 0.5119 �8.2IGCP39 14.7 404 0.11 0.705111 0.000010 0.7047 4.35 18.2 0.145 0.512507 0.000011 0.5123 �0.8IGCP40 36.4 102 1.04 0.719308 0.000010 0.7155 5.09 23.8 0.129 0.512067 0.000012 0.5118 �8.9IGCP43 17.0 110 0.45 0.713391 0.000011 0.7117 3.10 13.6 0.138 0.512050 0.000010 0.5118 �9.5SL24-2 140 436 0.93 0.709201 0.000012 0.7058 9.14 49.9 0.111 0.512431 0.000010 0.5122 �1.2

Silicic volcanic rocksSL26 85.4 343 0.72 0.707443 0.000013 0.7048 12.4 64.0 0.117 0.512497 0.000011 0.5123 �0.1H952 133 310 1.24 0.710528 0.000011 0.7060 27.7 137 0.122 0.512542 0.000012 0.5123 0.6H953 149 74.2 5.80 0.725460 0.000011 0.7041 24.3 110 0.134 0.512534 0.000011 0.5123 0.1

(87Sr/86Sr)i and eNd(t) values were calculated based on an age of 260 Ma, k(87Rb) = 1.42 � 10�11 year�1, k(147Sm) = 6.54 � 10�12 year�1, and present day chondritic values of143Nd/144Nd = 0.512638, 147Sm/144Nd = 0.1967, 87Sr/86Sr = 0.7045 and 87Rb/86Sr = 0.0827 (Faure and Mensing, 2005).

8

4

0

-8

-12

-16617.0807.0407.0007.0

)(dN

t

( Sr/ Sr)87 86i

-4

0.712

yarr

aelt

naM

Bulk Earth

htraE

kluB

1

2

Silicic rocksLow-Ti basaltHigh-Ti basaltPicrite

Fig. 10. Binary plot of eNd(t) values versus (87Sr/86Sr)i for the Song Da volcanicrocks. The isotopic data were calculated at t = 260 Ma. Reference fields of (1) high-Tibasalts and (2) low-Ti basalts of ELIP are also outlined, after Xu et al. (2001), Xiaoet al. (2004), Zhang et al. (2006), Wang et al. (2007), Qi and Zhou (2008) and Qi et al.(2008).

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predicted by fractional crystallization (Fig. 12c and d). The frac-tional crystallization hypothesis was tested by least-squaresapproximations for major elements using the GCDkit program(Janoušek et al., 2006). We choose sample IGCP22 to represent

the starting high-Ti basalt and sample H952 to represent thederivative silicic liquid Table 4. Mineral compositions in Xu et al.(2010) were used, except olivine composition that is taken fromPang et al. (2009) because it agrees better with the Mg# of sample

6

Freq

uenc

y

Freq

uenc

y12

8

4

0

8

2

(a)

(c)

(b)

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0

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-5 5 10-10-15

0.700 0.704 0.708 0.712 0.716

Silicicr ocksLow-Ti basaltHigh-Ti basaltPicrite

Nb/

La

1.5

0

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-16 -12 -8 -4 0 4 8

2

130

Nb/

U

40

-5 0-15 -10 105

50

0

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20

Upper crust (0.39)

Primitive mantle (1.04)Primitive mantle (30.8)

Upper crust (4.44)

Silicic rocksLow-Ti basaltHigh-Ti basaltPicrite

Nd (t)ε

Nd (t)ε(87Sr/86Sr)i

Nd (t)ε

Fig. 11. Frequency distribution diagrams and binary plots related to magma–lithosphere interaction for the Song Da volcanic rocks. (a) Distribution of (87Sr/86Sr)i values, (b)distribution of eNd(t) values, (c) Nb/La–eNd(t) diagram (after Peate, 1997), (d) Nb/U–eNd(t) diagram. Data sources include Hanski et al. (2004), Wang et al. (2007) and thisstudy. Primitive mantle and upper crust values after McDonough and Sun (1995) and Rudnick and Gao (2003), respectively.

Table 3Mixing calculations related to lithospheric contamination for the Song Da volcanic rocks.

Xa SiO2 MgO Nb La Nb/La Nd eNd(t)

End-member compositionsb

(1) Picrite 44.7 22.1 0.64 0.64 1.00 2.37 8.0(2) Low-Ti basalt 47.2 8.87 0.37 0.60 0.62 2.28 6.4(3) High-Ti basalt 42.9 10.3 22.1 19.1 1.16 26.1 3.1(4) Continental crust 66.1 1.25 11.1 54.1 0.21 35.9 �17.0(5) SCLM 41.7 11.5 56.4 18.8 3.00 17.6 �10.8

Mixing calculationsPicrite + crust: (1) + (4) 10% 46.8 20.0 1.69 5.99 0.28 5.72 �7.7

20% 49.0 17.9 2.73 11.3 0.24 9.07 �11.730% 51.1 15.8 3.78 16.7 0.23 12.4 �13.6

Picrite + SCLM: (1) + (5) 10% 44.4 21.0 6.22 2.46 2.53 3.89 �0.520% 44.1 20.0 11.8 4.27 2.76 5.42 �4.230% 43.8 18.9 17.4 6.09 2.85 6.94 �6.3

LTB + crust: (2) + (4) 10% 49.1 8.11 1.44 5.95 0.24 5.64 �8.520% 51.0 7.35 2.52 11.3 0.22 8.99 �12.230% 52.9 6.58 3.59 16.7 0.22 12.4 �14.0

LTB + SCLM: (2) + (5) 10% 46.7 9.13 5.97 2.42 2.47 3.81 �1.520% 46.1 9.40 11.6 4.24 2.73 5.34 �4.930% 45.6 9.66 17.2 6.06 2.83 6.88 �6.8

HTB + crust: (3) + (4) 10% 45.2 9.37 21.0 22.6 0.93 27.1 0.520% 47.5 8.47 19.9 26.1 0.76 28.1 �2.030% 49.9 7.56 18.8 29.6 0.64 29.0 �4.3

HTB + SCLM: (3) + (5) 10% 42.8 10.4 25.5 19.1 1.34 25.3 2.120% 42.7 10.5 29.0 19.0 1.52 24.4 1.130% 42.5 10.6 32.4 19.0 1.70 23.6 0.0

a X = proportion of contaminant; SiO2 and MgO in wt.%, Nb, La and Nd in ppm.b The least-contaminated volcanic rocks in the Song Da district are selected for the calculations, including sample B6889 (picrite, Hanski et al., 2004), HK-17 (low-Ti basalt,

Wang et al., 2007) and IGCP46 (high-Ti basalt, this study). Data for continental crust after Rudnick and Gao (2003) (elemental concentrations) and Lan et al. (2000) (Ndisotopes of the Posen Complex, northern Vietnam). Data for SCLM are taken from Emeishan tephrites suggested to be low-degree partial melts from the SCLM (Song et al.,2008).

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IGCP22 given an appropriate olivine-liquid Fe–Mg exchange coeffi-cient of 0.3. The results show that the silicic liquid can be gener-ated after �64% fractionation of a magnetite gabbroic assemblagefrom the high-Ti basalt. The very low sum of residuals in the calcu-lations (0.032) suggests an excellent fit. In summary, the Song Dasilicic volcanic rocks likely formed from extreme fractionation ofthe associated high-Ti Emeishan basalts.

Our results imply that fractionation of basaltic magmas in theELIP was capable in producing not only peralkaline but also meta-luminous and peraluminous silicic magmas. Frost and Frost (2011)suggested that both peralkaline and metaluminous granitoids canbe generated by extreme differentiation of basaltic melts; tholeiiticparental magma and high pressure of differentiation favour theformation of the latter. Further, the peraluminous feature exhibitedby some Song Da silicic volcanic rocks may be related to contami-

nation of small amount of crustal melts (Shellnutt and Zhou, 2007;Frost and Frost, 2011) or extensive fractionation of hornblende(ASI < 0.5, Zen, 1986). For example, Jahn et al. (2001) noted that amagmatic suite can evolve from metaluminous to peraluminousvia hornblende and pyroxene fractionation with small amount ofpelite contamination. Unfortunately, these processes cannot betested rigorously with our limited data on the metaluminous andperaluminous rocks to date.

7. Concluding remarks

� The Song Da picrites represent high-temperature mantle-derived magmas, variably affected by olivine fractionation andaccumulation, from the Emeishan plume head. It is character-ized by an average eNd(t) value of +8.

(a)5

CaO

(wt%

)

7

1

(b)

0

4

55 60 65 70 75 80

(d)

800

1600

Zr (p

pm)

1200

400

0

V (ppm)200 300 4001000

H953H952

SL26

H2 O-saturated

stlemlatnemirepxe

2

3

6

22

18

14

10

H952

SL26

H953

12

3

Emeishansilicic rocks

55 60 65 70 75SiO2 (wt%)

80

800

Ni (ppm)

1600

0 200100 300

1200

400

0

(c)20

30

40

60

80

30

40

6080

20

30

40

60 80

8060

40

30

Zr (p

pm)

SiO2 (wt%)

Al2O

3 (w

t%)

Fig. 12. Binary plots related to the origin of the Song Da silicic volcanic rocks. (a) Al2O3–SiO2 diagram; reference fields are experimental melts produced at different waterpressure (after Thy et al., 1990): (1) PTotal ¼ PH2 O ¼ 3:5 kb, (2) PTotal = 1–2.5 kb; PH2 O ¼ 1� 1:7 kb, (3) PTotal = 1–3 kb; PH2 O ¼< 1 kb. (b) CaO–SiO2 diagram; reference fielddenotes experimental melt compositions under H2O-saturated condition (after Beard and Lofgren, 1991), (c) Zr–Ni diagram, (d) Zr–V diagram. Grey field denotescompositions of silicic volcanic rocks elsewhere in the ELIP (Xu et al., 2010). Solid line and dashed lines in c and d denote trends for fractional crystallization and batchmelting, respectively, using the most primitive high-Ti basalt (IGCP46) as a starting composition and bulk partition coefficients of DZr = 0.5, DNi = 4 and DV = 4.

Table 4Modeling of fractional crystallization of Song Da high-Ti basalt in the formation of silicic volcanic rocks by least-squares approximation.

Mineral compositionsa Rock compositionsb

Olivine Clinopyroxene Plagioclase Magnetite Ilmenite Apatite HTB Sic Modelled HTB

SiO2 37.43 51.50 52.40 – – – 47.49 62.67 47.50 (�0.009)TiO2 – 0.80 0.10 13.26 49.93 – 3.32 0.76 3.34 (�0.02)Al2O3 – 3.13 29.60 0.07 – – 13.62 18.41 13.63 (�0.009)Fe2O3 25.99 8.13 0.30 78.58 44.74 – 14.28 5.27 14.30 (�0.021)MnO 0.36 0.20 – – 4.00 – 0.21 0.07 0.21 (0.004)MgO 35.93 15.65 0.10 0.07 – – 5.29 0.37 5.32 (�0.029)CaO 0.07 19.73 13.00 – – 51.40 8.69 0.77 8.73 (�0.037)Na2O – 0.37 4.20 – – – 3.06 6.31 3.23 (�0.171)K2O – – 0.40 – – – 1.13 2.91 1.12 (0.009)

Result: HTB = 0.3559 Sic + 0.0363 olivine + 0.2463 clinopyroxene + 0.2128 plagioclase + 0.1029 magnetite + 0.0298 ilmenite + 0.0160 apatite (R2 = 0.032).a Olivine composition after Pang et al. (2009) (JS85), other mineral compositions after Xu et al. (2010).b HTB = high-Ti basalt (IGCP22), Sic = silicic volcanic rocks (H952), numbers in brackets denote residuals (R) of the modelled HTB composition.

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� Some Song Da picrites and low-Ti basalts show low Nb/La, Nb/Uand eNd(t) relative to mantle values, consistent with contami-nation by lithospheric material. Mixing calculations show thatcontinental crustal rocks are likely contaminants, rather thanSCLM.� The Song Da silicic volcanic rocks are metaluminous or peralu-

minous. The geochemical and Sr–Nd isotopic data of them areconsistent with extreme fractionation of the associated high-Ti basalts, a model consistent with silicic volcanic rocks else-where in the ELIP.

Acknowledgements

This study is part of the VAST-NTU joint research project oninvestigating the magmatic and tectonic evolution in northernVietnam, financially supported by the National Science Council(NSC), Taiwan, ROC. The senior author acknowledges the fellow-ship provided by the NSC and VAST that supported his PhD studyat NTU-Geosciences. Journal reviews by Christina Yan Wang andGregory Shellnutt, and useful comments by the editor Bor-MingJahn, are gratefully acknowledged.

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