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Precambrian Research 103 (2000) 175–189

Crustal-contaminated komatiitic basalts in Southern China:products of a Proterozoic mantle plume beneath the

Yangtze Block

Mei-Fu Zhou *, Tai-Ping Zhao 1, John Malpas, Min SunDepartment of Earth Sciences, Uni6ersity of Hong Kong, Pokfulam Road, Hong Kong

Received 21 October 1999; accepted 24 March 2000

Abstract

Abundant mafic and ultramafic rocks including basalts, komatiitic basalts, and peridotites occur in the ProterozoicSibao Group, northern Guangxi Province, China. Whereas the basalts are generally pillow lavas, the komatiiticbasalts are typically spinifex-textured and, in a few cases, show pyroxene accumulation associated with Ni�Cu�(PGE)sulfide deposits. The peridotites occur in the lower portions of differentiated sills, which contain gabbro and dioritein their upper parts. The sills are believed to be co-magmatic with the komatiitic basalts. The spinifex rocks of theJiepai and Hejia Flows have MgO ranging from 8.9 to 14.3 wt%. The Zhongkui Flow is highly fractionated to forma spinifex zone with lower MgO (5.3–5.9 wt%) and a cumulate zone with higher MgO (17.3–17.9 wt%). Overall therocks have TiO2=0.44–0.74 wt%. Relative to primitive mantle, they are enriched in Th and LREE, but exhibitnegative Ti-, Nb-, and P-anomalies. These features are consistent with their formation from a crustally-contaminatedkomatiitic magma. During this process of crustal contamination, the magmas assimilated sulfur from sediments,which caused sulfide-segregation resulting in the formation of Ni�Cu�(PGE) sulfide deposits. The occurrence of thekomatiitic basalts in the Sibao Group can be explained by the ascent of a mantle plume beneath a continental riftenvironment, and implies that the Yangtze Block may have had an Archean basement through which the Sibaokomatiitic basalts erupted. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Crustal contamination; Komatiitic basalt; Proterozoic; Southern China

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1. Introduction

Many Proterozoic terrains, including the CapeSmith Belt, Canada (Hynes and Francis, 1982)and the Baltic Shield (Park et al., 1984; Puchtel etal., 1997) contain komatiitic rocks that are inter-preted to represent products of high-Mg magmasderived by partial melting of the upper mantle.

* Corresponding author. Tel.: +852-28578251; fax: +852-25176912.

E-mail address: [email protected] (M.-F. Zhou).1 Present address: Henan Institute of Geology, Zhengzhou,

Henan, PR China.

0301-9268/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S 0301 -9268 (00 )00077 -2

M.-F. Zhou et al. / Precambrian Research 103 (2000) 175–189176

These rocks are of considerable importance instudies of the composition of the Proterozoic up-per mantle (Jahn et al., 1980; Nisbet, 1982; Sun,1987; Nisbet et al., 1993; Bickle et al., 1994) andaid in discerning the tectonic regimes of the hostterrains.

Although the tectonics of southern China havebeen widely discussed since Hsu et al. (1988, 1990)proposed a model involving complex overthrust-ing to explain the Mesozoic collision between theYangtze and Cathaysian Blocks, the presence ofkomatiitic basalts in northern Guangxi Provincehas not been discussed. Local geologists describedspinifex-textured rocks in the Middle ProterozoicSibao Group as komatiites (Mao et al., 1988;Yang, 1988), but this discovery has remainedobscure. Chen and Mao (1995) described someultramafic rocks as extrusive komatiites, but theseappear to be olivine cumulate rocks which haveresultant high Mg contents. Because their geo-chemistry is different from that of typical Archeankomatiites, Li (1996) suggested that the spinifexrocks are not komatiite, but normal basalt. It isclear that the geochemical signature of these rocksneeds to be re-examined and their significanceinterpreted.

This study provides a description of the distri-bution of the ultramafic and related rocks basedon a mapping programme carried out by Dong(1988, 1990) and during our own field work in thepast few years. This mapping has shown thatmany of the ultramafic and mafic rocks are, infact, high-level sills that are interpreted to beco-magmatic with lavas. Representative sampleshave been analyzed for major oxides and traceelements and an attempt has been made to iden-tify the composition of the parental magma. Con-sequently, some constraints can now be placed onthe tectonic setting of the magmatism and theProterozoic geological evolution of the region.

2. Regional geology

The area of northern Guangxi Province formsthe southern end of the Jiangnan Oldland, northof the Cathaysian continent (Huang, 1945;Chang, 1996) (Fig. 1). The Jiangnan Oldland con-

tains Precambrian rocks interpreted to representbasement to the Yangtze Block, and a coversequence of Sinian and Devonian sedimentarystrata of neritic and terrestrial facies, which ap-pear to deposit in a stable tectonic setting.

In northern Guangxi Province, the SibaoGroup forms the basal Proterozoic sequence andis uncomformably overlain by the Late Protero-zoic Danzhou Group (Figs. 1 and 2). The SibaoGroup includes the Wentong and Yuxi Forma-tions. The lower part of the Wentong Formationcomprises very mature terrigenous sandstonescontaining 40–86 modal % quartz. The upperpart contains turbidites displaying classic Boumasequences, cut by a number of sills and plugs ofultramafic and mafic lithologies (Fig. 2). The YuxiFormation is composed of intermediate to acidiclava flows, ignimbrites, tuffaceous sediments, andintercalated mudstones.

The exact thickness of the Sibao Group isdifficult to measure because the rocks are stronglyfolded and faulted, but is probably between 3 and5 km (Dong, 1988). The age of these supracrustalrocks is not precisely known. As expected, nofossils have been observed in the Sibao Group,but Li (1996) reported whole rock Sm�Nd datafor the mafic volcanic rocks yielding an age of19009100 Ma, revising a 2200 Ma Sm�Ndisochron obtained by Mao et al. (1990). TheSibao Group is intruded by the Sanfang andYuanbaoshan peraluminous S-type granites (Fig.1), which contain zircons dated by SHRIMP atca. 825 Ma (Li, 1999).

The Danzhou Group is equivalent to the BanxiGroup of northern Jiangxi Province and theShangxi Group of southern Anhui Province. Thebase of the Danzhou Group is marked by con-glomerates with granitic clasts and fragmentsderived from the Sibao Group. Slates and phyl-lites represent continental shelf, neritic and semi-pelagic facies interpreted to be of back-arc basinorigin (Guo et al., 1985). Ophiolites (ca. 1000 Ma)were emplaced onto these sedimentary rocksalong the southern margin of the Jiangnan Old-land (Zhou, 1989; Chen et al., 1991; Xing et al.,1992) (Fig. 1) during what is generally accepted asa Neoproterozoic collision between the Yangtzeand Cathaysian Blocks. However, Hsu et al.

M.-F. Zhou et al. / Precambrian Research 103 (2000) 175–189 177

(1988, 1990) have suggested an alternative modelin which Cathaysia was thrust on to the YangtzeBlock during the Triassic, with the DanzhouGroup and its equivalents forming a tectonicmelange.

3. Mafic and ultramafic rocks in the Sibao Group

Basalts and komatiitic basalts in the WentongFormation of the Sibao Group, occur as lavaflows, and peridotites and gabbros as layered,subvolcanic sills (Fig. 2). The basaltic lavas ex-hibit pillowed, spherulitic, and amygdaloidalstructures, and thick flows display columnar joint-ing (e.g. near Jiepai). These volcanic rocks havecompositions similar to andesitic basalts (Mao etal., 1988).

The komatiitic basalts are interbedded with sed-imentary rocks of the Wentong Formation andoccur as massive 5–100 m thick flows with brec-

ciated and chilled margins. Actual thicknesses aredifficult to estimate because of widespreadpolyphase folding. We studied three typical flows,i.e. the Jiepai, Hejia, and Zhongkui Flows (Fig.2). All three flows exhibit clinopyroxene spinifex-texture in which elongated and skeletal clinopy-roxenes (now replaced by tremolite) occur in aquenched devitrified matrix (Fig. 3(A) and (B)).Both the Jiepai and Hejia Flows are spinifex-tex-tured throughout with well-developed chilled mar-gin, whereas the Zhongkui Flow displays aspinifex-textured upper part and a pyroxene cu-mulate lower parts. The pyroxene cumulates con-tain euhedral clinopyroxene, minor olivine andintercumulate plagioclase, and variable amountsof magmatic sulfides (Fig. 3(C)). Sulfide mineralscan be highly concentrated in some occurrences,as reported by Mao et al. (1988).

The sills are subvolcanic intrusives injected par-allel to bedding planes in the Wentong Forma-tion. They vary in composition from peridotitic to

Fig. 1. A simplified geological map of the Northern Guangxi area, Southern China. Modified after Dong (1988). The upper-left insetshows the location of the ‘Jiangnan Oldland’ (Huang, 1945; Chang, 1996).


Fig. 2. Geological map of the Sibao area showing the distribution of mafic/ultramafic extrusives and intrusives in the Sibao Group.Drawn on the basis of geological mapping by Dong (1988, 1990) and this study.

dioritic (Fig. 2), and thicker sills are conspicu-ously layered. These layered sills have a thin lowerchilled margin overlain successively by (1) peri-dotite containing rounded olivine chadacrysts en-closed in large pyroxene oikocryts (Fig. 3(D)), (2)gabbro, and (3) diorite. This layering was formedby crystal settling of olivine, clinopyroxene andthen plagioclase within these subvolcanic bodies,which are interpreted to be co-magmatic with theextrusive komatiitic basalts.

4. Geochemistry of the komatiitic basalts

A number of komatiitic basalts from the Jiepai,Hejia, and Zhongkui Flows which display well-de-veloped spinifex or cumulate textures have beenselected for geochemical analysis. Major elementabundances were determined by X-ray fluores-cence spectrometry (XRF) on fused glass pellets.Trace elements Sc, V, Cr, Ni, Cu, and Zn weredetermined by XRF on pressed powder pellets.


Fig. 3. Mafic/ultramafic rocks in the Sibao Group. (A and B) Spinifex-textured komatiitic basalt from the Hejia Flow, showingelongate clinopyroxene (A) and skeletal clinopyroxene (B), Crossed nicols, field of view 2.87×4 mm. (C) Handspecimen of apyroxene cumulate from the Zhongkui Flow, showing sulfide mineral segregations, field of view 11.5×16 cm. (D) Peridotite froma sill near Jiepai with rounded olivine included in clinopyroxene, field of view 2.87×4 mm.


Other trace elements, including REE, were ana-lyzed on a VG Elemental PlasmaQuad 3 induc-tively coupled plasma-mass spectrometer (ICP-MS) at the University of Hong Kong. We use theprotocol of Jenner et al. (1990), with standardadditions, pure elemental standards for externalcalibration, and BHVO-1 as a reference material.Accuracies of the XRF analyses are estimated as92% for major elements present in concentra-tions greater than 0.5wt% and 95% for traceelements. The ICP-MS analyses yield accuracybetter than 95%. Major oxides are normalized to100% on a volatile-free basis and all data arelisted in Table 1.

The spinifex rocks of the Jiepai and HejiaFlows have MgO ranging from 8.9 to 14.3 wt%(Table 1 and Fig. 4). The spinifex rocks of theZhongkui Flow have lower MgO (5.3–5.9 wt%)than typical komatiitic basalts. This flow is highlyfractionated to form a high-Mg cumulate zone inthe bottom with MgO contents ranging from 17.3to 17.9 wt%. Therefore, overall the rocks can becalled komatiitic basalts.

SiO2 contents (49.1–57.8 wt%) are lower, butFeOT contents (9.4–13.6 wt%) are higher thantypical boninites (\53 wt% SiO2 and 6–7 wt%FeOT at 10–12 wt% MgO) (Crawford et al.,1989). All samples are low in TiO2 (0.44–0.90wt%), similar to low-Ti tholeiites (Kerrich et al.,1998), but higher than typical boninites (Craw-ford et al., 1989). The Mgc ’s of all samplesrange from 53 to 81 and correlate positively withNi and Cr (Fig. 4).

The komatiitic basalts have Al2O3/TiO2 close to20, except the Zhongkui spinifex samples whichhave higher ratios of ca. 33. CaO/Al2O3 rangesfrom 0.54 to 0.79. Zr/Y ratios range between 3.61and 12.2, higher than the chondritic ratio (ca.2.5). The spinifex-textured samples in theZhongkui Flow have the highest SiO2 and also thehighest Th/Nb and La/Sm values (Fig. 4). Th/Nbcorrelates positively with La/Sm in samples fromthe Jiepai and Hejia Flows (Fig. 4).

All rocks have REEN patterns enriched inLREE with flat HREE (Fig. 5(A)). The primitivemantle-normalized trace element plot (Fig. 5(B))displays enrichment in large ion lithophile ele-ments normally concentrated in the continental

crust, especially Ba, Th, U, Pb, and Sr, and showsstrong negative Ti-, Y-, Nb-, P-, and Ta-anoma-lies. The rocks also have low concentrations of Scand V.

In a diagram of Ti versus Zr (Fig. 6), the Sibaovolcanic rocks are scattered. However, samplesfrom individual flows form noticeable trends. Allsamples have Ti contents similar to many otherkomatiitic basalts and Siliceous High-MagnesianBasalts (SHMB) of Sun et al. (1989). High-Sispinifex-textured rocks of the Zhongkui Flow andtwo samples from the Hejia Flow have Zr con-tents higher than SHMB, whereas all other sam-ples plot in the SHMB field (Fig. 6). They havelower Ti/Zr values than primary (un-contami-nated) komatiitic basalts (ca.100) and MORB(100–200). In Fig. 7, the Hejia Flow falls in theMORB field because of its low V and Sc contents.Again, all other samples plot in the SHMB fieldand have Ti/V and Ti/Sc ratios similar to komati-ites and komatiitic basalts from other locations.They are, however, distinct from boninitic rocks(Fig. 7).

5. Crustal contamination in the komatiitic basalts

The elements Al, Ti, REE, HFSE (Th, Nb, Ta,Zr, Hf), Y, Sc and V are considered the leastmobile during hydrothermal alteration and green-schist facies metamorphism of mafic volcanicrocks (e.g. Ludden et al., 1982; Kerrich et al.,1998). Therefore, although the mineralogy of thekomatiitic basalts indicates that they have beenmetamorphosed to sub-greenschist facies, REE,Zr, Y, Nb and Hf have coherent trends, likelyreflecting characteristics of the primitive magmaand processes of magmatic differentiation eitherbefore, during or after eruption.

The spinifex textured rocks in the Sibao Grouphave lower SiO2 contents but higher FeOT con-tents than typical boninites. Their LREE-enrichedREE patterns are also different from typicalboninites. All these together with the lack ofolivine phenocrysts indicate that they are notproducts of a normal picritic magma. The enrich-ment of LREE and other features of their geo-


Table 1Chemical compositions of komatiitic basalts from the Sibao Group, Northern Guangxi Province, China

Sample Hejia flowJiepai flowlocationTextures Spinifex-texture

GX-4 GX-5 GX-14 GX-16 GX-17 GX-18GX-3GX-1Sample no.

Major oxides (wt%)52.4 52.1 50.7 52.1 55.8 53.7 53.9 54.0SiO2

0.51 0.67TiO2 0.900.70 0.74 0.73 0.740.6411.9 14.4 15.0 15.014.1 15.0Al2O3 14.814.4

11.010.7 11.0 11.1 11.2 10.8 10.5 10.6Fe2O3a

14.3 8.89 5.73 7.66MgO 7.468.98 7.469.359.08 11.0 8.04 8.1611.1 8.3110.6 8.51CaO

0.170.16 0.18 0.17 0.17 0.17 0.17 0.17MnO1.42 1.05 2.24 2.39Na2O 2.601.20 2.341.200.66 0.38 0.70 1.200.27 1.160.74 1.26K2O

0.060.07 0.05 0.07 0.12 0.08 0.07 0.08P2O5

3.36 2.78 2.48 2.60 2.44 2.24LOIb 2.83 3.18

Trace elements (ppm)32 35 30Sc 3034 30 3035

220 223 184 221 191 218 227 214V1358 453 224Cr 338445 321 311507359 103 24 34111 31Ni 29101

7688 71 93 92 15.0 9.7 13.9Cu99 92 91 163Zn 9994 969837 61 131 8555 8965 92Zr

11.618.1 24.6 11.9 11.3 21.2 23.5 25.8Rb57 105 129 125Sr 158105 14412012.4 10.0 16.7 13.913.2 15.9Y 14.411.7

3.574.08 2.94 4.27 9.00 5.91 6.08 6.05Nb1.06 0.65 1.01 0.99Cs 1.580.83 1.440.73

77 63 163 32946 28096 326Ba6.235.92 6.32 6.01 10.9 9.22 9.21 8.76La

11.5 13.6 25.7 20.1Ce 21.213.3 19.213.51.87 1.95 3.52 2.832.04 2.84Pr 2.672.01

7.947.81 7.06 7.49 13.6 10.9 10.9 10.3Nd1.93 2.09 3.46 2.72Sm 2.752.22 2.582.200.54 0.61 0.86 0.680.65 0.720.63 0.68Eu

2.422.36 2.15 2.16 3.51 2.76 2.75 2.58Gd0.38 0.40 0.64 0.50 0.50 0.47Tb 0.44 0.452.37 2.49 3.96 3.022.87 3.192.80 3.00Dy

0.640.63 0.52 0.54 0.87 0.66 0.69 0.66Ho1.43 1.49 2.40 1.82Er 2.001.77 1.921.830.22 0.22 0.36 0.280.28 0.310.26 0.29Tm

1.841.76 1.34 1.39 2.34 1.78 2.08 1.88Yb0.20 0.20 0.34 0.27 0.31 0.29Lu 0.26 0.280.98 1.58 3.57 2.341.62 2.35Hf 2.371.81

0.240.27 0.19 0.26 0.62 0.40 0.40 0.39Ta5.37 8.34 12.5 8.88 9.53 8.73Pb 11.9 10.40.96 1.07 3.00 2.401.08 2.420.91 2.14Th0.36 0.53U 1.340.56 0.91 0.93 0.860.48


Table 1 (Continued)

Hejia flowSample Jiepai flowlocationTextures Spinifex-texture

GX-3GX-1 GX-4 GX-5 GX-14 GX-16 GX-17 GX-18Sample no.

72 61 50 5863 58cMgc 5863CaO/Al2O3 0.79 0.76 0.76 0.54 0.54 0.55 0.570.74

20.6 22.0 23.4 21.5 16.7 20.3 20.6 20.0Al2O3/TiO2

4.37 12.2 9.79 11.38.96 9.48Zr/Y 10.510.1Th/Nb 0.330.22 0.25 0.33 0.41 0.40 0.350.30

Sample Zhongkui flowlocation

CumulateSpinifex-textureTextures

GX-20GX-19 GX-21 GX-22 GX-23 GX-24 GX-25 GX-26 GX-27Sample no.

Major oxides (wt%)56.3 56.9 56.3 49.457.8 49.156.9 49.2 49.4SiO2

0.440.45 0.47 0.45 0.48 0.52 0.52 0.51 0.52TiO2

15.2 15.1 15.2 10.2Al2O3 10.315.2 10.4 10.314.59.75 9.59 10.26 13.69.36 13.89.57 13.6 13.2Fe2O3

a

5.505.48 5.32 5.37 5.89 17.7 17.5 17.3 17.9MgO9.42 8.90 8.35 6.96 6.99 7.03CaO 6.968.66 9.230.17 0.17 0.17 0.190.16 0.19MnO 0.19 0.190.16

2.752.27 2.17 1.97 1.42 0.58 0.66 0.76 0.65Na2O1.07 1.40 1.74 0.55 0.59 0.69K2O 0.571.26 0.200.08 0.07 0.08 0.060.06 0.060.07 0.06 0.06P2O5

LOIb 1.372.20 1.60 1.71 4.57 4.82 4.40 4.831.80

Trace elements (ppm)Sc 31.430 29.8 29.3 31.3 30.7 31.3 31.230.4

175 180 185 165177 164V 167 176181182204 207 236 213 1390 1420 1429 1470Cr

5.91 5.40 8.63 2311Ni 19426.04 1748 161611.24.61 8.10 8.30 13277.73 12607.37 1085 1046Cu

8086 92 85 108 114 106 103 101Zn95 90 86 59Zr 5791 54 629718.3 21.7 38.0 23.75.39 26.3Rb 30.6 27.523.4

102104 97 104 109 25.2 29.0 34.9 30.5Sr12.2 12.3 16.3 15.2Y 14.412.9 14.9 16.013.5

8.50 8.63 9.13 4.128.25 4.198.81 3.94 4.53Nb0.391.24 0.72 1.84 3.81 1.21 1.38 1.47 1.44Cs

160Ba 243217 245 119 144 175 1535211.6 11.3 12.9 8.5010.4 8.8411.39 9.04 9.31La

23.324.3 25.3 23.6 29.6 16.7 17.5 17.5 17.8Ce3.35 3.27 3.79 2.47Pr 2.423.21 2.47 2.473.27

11.8 11.6 13.5 9.1311.7 8.8411.3 9.11 9.29Nd2.702.49 2.67 2.61 3.06 2.24 2.22 2.31 2.27Sm

0.62Eu 0.580.57 0.68 0.73 0.71 0.71 0.740.632.39 2.34 2.87 2.322.46 2.33Gd 2.33 2.382.22

0.440.40 0.43 0.41 0.50 0.40 0.40 0.40 0.42Tb2.68Dy 2.652.56 3.31 2.53 2.49 2.50 2.622.700.60 0.59 0.73 0.54 0.54 0.55 0.560.60Ho 0.56


Table 1 (Continued)

Jiepai flowSample Hejia flowlocationTextures Spinifex-texture

GX-3 GX-4 GX-5 GX-14 GX-16GX-1 GX-17Sample no. GX-18

Er 1.62 1.77 1.74 1.70 2.15 1.56 1.61 1.56 1.620.29 0.27 0.27 0.34Tm 0.240.25 0.24 0.24 0.251.94 1.81 1.76 2.241.63 1.59Yb 1.60 1.58 1.67

0.25Lu 0.29 0.27 0.26 0.32 0.24 0.24 0.24 0.252.61 2.49 2.37 2.33Hf 1.562.32 1.60 1.52 1.630.58 0.59 0.59 0.620.58 0.27Ta 0.29 0.27 0.30

13.2Pb 14.1 9.3 15.0 9.9 172 201 132 1465.33 3.11 2.78 4.24 3.31 3.35Th 3.433.48 3.471.72 1.65 1.60 1.67 0.671.55 0.69U 0.64 0.68

cMgc 53 54 52 53 53 72 72 72 730.64 0.62 0.59 0.55CaO/Al2O3 0.680.57 0.68 0.67 0.67

33.0 32.7 33.9 31.933.5 19.7Al2O3/TiO2 20.0 20.3 20.08.54Zr/Y 7.20 7.85 7.37 5.28 3.85 3.93 3.61 3.86

0.65 0.37 0.32 0.46 0.80Th/Nb 0.800.40 0.87 0.77

a Fe2O3 as total iron.b LOI, loss on ignition.c Mgc =100×Mg2+/(Mg2++Fe2+), Fe2+ is calculated from total FeO. Major oxides were recalculated to 100% on a

volatile-free basis.

chemistry also distinguish them from most otherkomatiitic basalts (Arndt et al., 1977; Hynes andFrancis, 1982) which are high Mg, LREE-de-pleted lavas. Nor can the REE signatures of thespinifex-textured rocks in the Sibao Group beexplained by fractionation of olivine and pyroxe-nes from a komatiitic magma depleted in LREE,as these minerals do not fractionate REE effi-ciently enough to generate the observed LREE-enriched patterns.

Komatiitic basalts from the Sibao Group havepronounced negative Nb and Ti anomalies withcorresponding LREE enrichment, similar to theSHMB of the Yilgarn Craton, Australia (Sun etal., 1989) and the komatiitic basalts from theVetreny Belt, Southern Baltic Shield (Puchtel etal., 1997). These are geochemical features consid-ered indicative of a komatiitic magma contami-nated by continental crustal material (Arndt andJenner, 1986; Juteau et al., 1988; Sun et al., 1989;Puchtel et al., 1997; Kerrich et al., 1998).

Average subcontinental lithospheric mantle andcontinental crust are both depleted in Nb and Ta

relative to Th and La (McDonough, 1990;Jochum et al., 1991). Contamination of astheno-spheric melts occurs during their ascent throughthe lithosphere and/or crust, resulting in a markedincrease in Ba, Th, Zr, U, and LREE, but littlechange in Ta, Nb, HREE, and Ti concentrations.This results in negative Ta, Nb, and Ti anomalieson mantle or chondrite-normalized trace elementvariation diagrams. The Sibao komatiitic basaltsexhibit such negative Nb- and Ta-anomalies, butthe rocks with the lowest Nb/Th and Nb/ La alsohave the highest SiO2 and Zr contents. This canonly be explained if the magmas parental to thelavas and intrusions were contaminated by theassimilation of felsic crustal rocks. Positive corre-lations between La/Sm, Th/Nb, and Zr/Y resultwhen a magma from a relatively depleted mantlesource (low Th/Nb, Zr/Y, and La/Sm) is mixedwith an enriched component (high Th/Nb, Zr/Y,and La/Sm) (e.g. Puchtel et al., 1997). Followingassimilation of crustal material, the magma waseither erupted as lava flows, or intruded in asubvolcanic environment, where crystal settling


Fig. 4. Compositional and elemental ratio plots of the komatiitic basalts from the Sibao Group. Mgc =100×Mg/(Mg+Fe2+);Fe2+ is calculated from total FeO.

formed layered sills. The thick lower peridotitezones indicate accumulation of olivine and repre-sent sheet sills like those in the Cape Smith Belt,Canada (Hoffman, 1990).

The occurrence of Ni�Cu�(PGE) sulfide accu-mulations in the pyroxenite cumulates of somekomatiitic basalt flows from the Sibao Group canalso be explained by a process of crustal assimila-tion. The parental high-Mg magmas were likelyS-undersaturated (Keays, 1995). Melting of crustal

sulfur-bearing sediments generated immis ciblesulfide melts. Indeed, the cumulate pyroxenitescontaining massive sulfides in the Zhongkui Flowhave the highest Th/Nb and La/Sm (Fig. 4), inter-preted to result from the most extensive contami-nation. These stratiform layers of massive andnet-textured sulfides in the Zhongkui Flow indi-cate that the magma reached sulfide saturation atan early stage in the crystallization history of thehost unit (e.g. Lesher and Groves, 1986; Lesher


and Stone, 1996). A similar situation occurs atKambalda, Australia, in response to incorpora-tion of sulfide-rich sediments during the flow ofkomatiitic lava over unconsolidated sediments(Huppert et al., 1984; Lesher et al., 1984; Lesherand Campbell, 1993).

6. Tectonic setting and implications for the geologyof southern China

Archean komatiitic magmas are interpreted tohave formed as part of ancient oceanic lithosphere(Desrochers et al., 1993) or primitive ocean is-lands (Hoffman, 1990). However, the Belingwegreenstone belt of Zimbabwe provides a well-ar-gued case that certain komatiites formed in a

continental extensional setting (Nisbet et al., 1977;Bickle et al., 1994; Hunter et al., 1998). Olddetrital zircons in some Archean komatiitesconfirm crustal contamination (Compston et al.,1986).

The Early Proterozoic komatiitic basalts ofboth the Cape Smith Belt (Hoffman, 1990) andthe Vetreny Belt of the Baltic Shield (Puchtel etal., 1997) formed in continental rifts, and thekomatiitic basalts in the Sibao Group are hereconsidered to have been erupted in a similartectonic setting. A sub-continental mantle plumeis a preferred source of very high temperature,high-Mg magmas (Nisbet et al., 1993).

The occurrence of the komatiitic basalts in theSibao Group is consistent with the developmentof a continental rift in which high-Mg lavas

Fig. 5. (A) Chondrite-normalized REE patterns of the komatiitic basalts from the Sibao Group. (B) Mantle-normalized traceelement pattern. Mantle-normalization values are from Sun and McDonough (1989).


Fig. 6. Ti vs. Zr for the komatiitic basalts from the SibaoGroup. Boninite field from Poidevin (1994), Komatiitic basaltsfrom Arndt and Nesbitt (1982), Jahn et al. (1982), and SHMBfrom Sun et al. (1989).

later juxtaposed over the initial rift sequence (Scottet al., 1992), rifting in the Northern Guangxi regiondid not result in continental breakup and openingof a new ocean. The lack of any significant felsicmagmatism between 1.8 and 2.4 Ga in this regionalso implies that no large-scale arc system formed.In northern Guangxi Province, uplift of the crustabove the mantle plume and subsequent volcanismexplains why the Sibao Group is thinner thanpresent day rift-related successions. Analogous suc-cessions are found associated with flood basaltsrelated to the Trindade plume in Western Brazil(Gibson et al., 1997) and the Tertiary plume-relatedvolcanic rocks in Western Greenland (Lightfoot etal., 1997).

The extent of the Proterozoic rift environment isnot clear. The Lengjiaxi Group, a flysch sequence inNW Hunan Province, is considered the equivalentof the Sibao Group. Its lower part is represented bya meta-volcanic sequence more than 2000 m thick,including mafic lavas and tuffs. Spinifex-texturedkomatiites have also been reported (Xiao, 1987). Itis possible that this belt in NW Hunan Provincerepresents a separate, individual rift that formedsimultaneously with that in Northern GuangxiProvince. The presence of a plume under the cratonwould have caused uplift and formed a series ofgrabens in which sediments accumulated. One ormore of them could be considered aulocogens.

This model for the formation of the komatiiticbasalts requires that the Sibao Group was essen-tially deposited on continental crust. The oldestrocks known in the Yangtze Block are theKangding and Kongling gneisses of 2400–2950 Ma(Wang et al., 1985). The polymictic conglomerate inthe lower part of the Fanjingshang Group inGuizhou Province, an equivalent of the SibaoGroup, contains clasts of various metamorphicrocks and granites, apparently derived from theunderlying basement. Sm�Nd isotopes from thegranodiorite of the Sibao Group suggest that thisrock was a product of anatexis of older continentalcrust. The model age calculated is 2513 Ma (Mao etal., 1988). Older U�Pb ages of 2860–3289 Ma havebeen obtained from zircons in the Sanfang Granite,which intrudes the Sibao Group (Huang, 1998).These zircons are interpreted as refractory xeno-crysts trapped in granitic magmas derived from the

Fig. 7. Ti/V vs. Ti/Sc for the komatiitic basalts from the SibaoGroup. SHMB from Sun et al. (1989), MORB and komatiiticbasalt from Wolde et al. (1996), boninites from Hickey andFrey (1982).

erupted during deposition of terrigenous sediments.The early stages of this continental rifting aremarked by the deposition of conglomerates,quartzites, and arkoses in the lower part of theSibao Group. Subsequently, extensive eruption ofhigh-Mg lavas was associated with the emplace-ment of mafic/ultramafic layered intrusions andfollowed by further rapid sedimentation.

Unlike the situation in the Chukotat Group ofthe Cape Smith Belt, where continental rift-typevolcanism gave way to the formation of transitionaloceanic crust (Hynes and Francis, 1982), which was


old granulitic crust. All of this is evidence of an oldcontinental crust underlying the Sibao Group,although no outcrops of this basement are exposed.

7. Conclusions

The komatiitic basalts of the Sibao Group ex-hibit clinopyroxene spinifex texture and REE pat-terns enriched in LREE. They are depleted in Ti,Nb, and P and enriched in Th and Si. These featuressuggest that they represent the quench crystalliza-tion of a high-Mg magma contaminated by assim-ilation of crustal sediments, which erupted in acontinental rifting environment. The study furtherindicates that there was an old continental crustbeneath the Sibao Group at the time of its forma-tion and that the magmatic event resulted in asubstantial addition of new juvenile mantle mate-rial to the Yangtze continent. Crustal contamina-tion of this high-Mg magma locally resulted inS-saturation and we suggest that the area hassignificant potential for the discovery of economicNi�Cu�(PGE) sulfide deposits.

Acknowledgements

This study was fully supported by a grant fromthe Research Grant Council of the Hong KongSpecial Administrative Region, China (HKU7120/97P). Three field seasons investigating the SibaoGroup were assisted by senior geologists DongBaolin, Li Guotao, and Liang Guobao, all at theGeological Survey of Guangxi Province, China. Wethank Yang Lizhen for informative discussion andDr Li Jianwei for assistance with the preparationof this manuscript. We appreciate constructivecomments made by Professor C.M. Lesher and ananonymous reviewer that helped to improve anearlier version.

References

Arndt, N.T., Nesbitt, R.W., 1982. Geochemistry of the MunroTownship basalts. In: Arndt, N.T., Nisbet, E.G. (Eds.),Komatiites. Allen & Unwin, London, pp. 309–329.

Arndt, N.T., Jenner, G.A., 1986. Crustally-contaminated ko-matiites and basalts from Kambalda, Western Australia.Chem. Geol. 56, 229–255.

Arndt, N.T., Naldrett, A.J., Pyke, D.R., 1977. Komatiitic andiron-rich tholeiitic lavas of Munro Township, NortheastOntario. J. Petrol. 18, 319–369.

Bickle, M.J., Nisbet, E.G., Martin, A., 1994. Archean green-stone belts are not oceanic crust. J. Geol. 102, 121–138.

Chang, E.Z., 1996. Collisional orogen between North andSouth China and its eastern extension in the KoreanPenisula. J. SE Asian Earth Sci. 13, 267–277.

Chen, T., Mao, J., 1995. Metallogenic Series of Ore Depositsand Metallogenic Evolution Through Geological Historyin Northern Guangxi (in Chinese). Science and TechnologyPress of Guangxi, Guangxi, China, p. 433.

Chen, J.F., Foland, K.A., Xing, F.M., Xu, X., Zhou, T.X.,1991. Magmatism along the southeast margin of theYangtze block: Precambrian collision of the Yangtze andCathaysian Blocks of China. Geology 19, 815–818.

Compston, W., Williams, I.S., Campbell, I.H., Gresham, J.J.,1986. Zircon xenocrystals from the Kambalda volcanics:age constraints and direct evidence for older continentalcrust below the Kambalda-Norseman Greenstones. EarthPlanet. Sci. lett. 76, 299–311.

Crawford, A.J., Falloon, T.J., Green, D.H., 1989. Classifica-tion, petrogenesis and tectonic setting of boninites. In:Crawford, A.J. (Ed.), Boninites and Related Rocks. UnwinHyman, London, pp. 1–49.

Desrochers, J.P., Hubert, C., Ludden, J.N., Pilote, P., 1993.Accretion of Archean oceanic plateau fragment in theAbitibi greenstone belt. Can. Geol. 21, 451–454.

Dong, B.-L., 1988. A discussion on the several problems of thePrecambrian tectonics in Northern Guangxi (in Chinese).Geol. Jiangxi 2, 176–181.

Dong, B.-L., 1990. The Sibao Group of Guangxi Province andits metallogeny (in Chinese). Guangxi Reg. Geol. 3, 53–58.

Gibson, S.A., Thompson, R.N., Weska, R.K., Dickin, A.P.,Leonardos, O.H., 1997. Late cretaceous rifting-related up-welling and melting of the Trindade Starting mantle plumehead beneath Western Brazil. Contrib. Miner. Petrol. 126,303–314.

Guo, L.Z., Shi, Y.S., Ma, R.S., Lu, H.F., Ye, S.F., 1985. Platemovement and crustal evolution of the Jiangnan Protero-zoic mobile belt, SE China. Earth Sci. J. Geol. Soc. Jpn. 2,156–166.

Hickey, R.L., Frey, F.A., 1982. Geochemical characteristics ofboninite series volcanics: implications for their sources.Geochim. Cosmochim. Acta 46, 2099–2115.

Hoffman P.F., 1990. On accretion of granite–greenstone ter-ranes. In: Robert, F. (Ed.), Greenstone Gold and CrustalEvolution, Val D’Or, Canada, Geological Association ofCanada Mineral Deposits Division, Ottawa, pp. 32–45.

Hsu, K.L., Sun, S., Li, J.L., Chen, H.H., Pen, H.P., Sengor,A.M.C., 1988. Mesozoic overthrust tectonics in SouthChina. Geology 16, 418–421.

Hsu, K.J., Li, J., Chen, H., Wang, Q., Sun, S., Senger,A.M.C., 1990. Tectonics of south China: key to under-standing west Pacific geology. Tectonics 183, 9–39.


Huang, T.K., 1945. On the major tectonic forms of China. Geol.Mem. Ser. A 20, 165.

Huang, X., 1998. Mid–Late Proterozoic (pre-Sinian) crust. In:Ma, X., Bai, J. (Eds.), Precambrian Crustal Evolution ofChina. Geolical Publishing House, Beijing, pp. 214–222.

Hunter, M.A., Bickle, M.J., Nisbet, E.G., Martin, A., Chap-man, H.J., 1998. Continental extensional setting for thearchean Belingwe greenstone belt. Zimb. Geol. 26, 883–886.

Huppert, H.E., Sparks, R.S.J., Turner, J.S., Arndt, N.T., 1984.Emplacement and cooling of komatiite lava. Nature 309,19–22.

Hynes, A., Francis, D., 1982. Komatiitic basalts of the CapeSmith Fold Belt, New Quebec, Canada. In: Arndt, N.T.,Nisbet, E.G. (Eds.), Komatiites. Allen & Unwin, London,pp. 159–170.

Jahn, B.-M., Gruau, G., Glikson, A.Y., 1982. Komatiites of theOnverwacht Group, S. Africa: REE geochemistry, Sm/Ndage and mantle evolution. Contrib. Miner. Petrol. 80, 25–40.

Jahn, B.-M., Auvray, B., Blais, S., Capdevila, R., Cornichet, J.,Vidal, F., Hameurt, J., 1980. Trace element geochemistryand petrogenesis of Finnish greenstone belts. J. Petrol. 21,201–244.

Jenner, G.A., Longerich, H.P., Jackson, S.E., Fryer, B.J., 1990.ICP-MS—a powerful tool for high precision trace elementanalysis in earth sciences: evidence from analysis of selectedUSGS reference samples. Chem. Geol. 83, 133–148.

Jochum, K.P., Arndt, N.T., Hofmann, A.W., 1991. Nb�Th�Lain komatiites and basalts: constraints on komatiite petrogen-esis and mantle evolution. Earth Planet. Sci. Lett. 107,272–289.

Juteau, M., Pagel, M., Michard, A., Avobarede, F., 1988.Assimilation of continental crust by komatiites in thePrecambrian basement of the Carswell structure(Saskatchewan, Canada). Contrib. Miner. Petrol. 99, 219–225.

Keays, R.R., 1995. The role of komatiitic and picritic magma-tism and S-saturation in the formation of the ore deposits.Lithos 34, 1–18.

Kerrich, R.W., Wyman, D., Fan, J., Bleeker, W., 1998. Boniniteseries: low-Ti tholeiite associations from the 2.7 Ga Abitibigreenstone belt. Earth Planet. Sci. Lett. 164, 303–316.

Lesher, C.M., Campbell, I.H., 1993. Geochemical and fluiddynamic modelling of compositional variations in Archeankomatiite-hosted nickel sulfide ores in Western Australia.Econ. Geol. 88, 804–816.

Lesher, C.M., Groves, D.I., 1986. Controls on the formation ofkomatiite-associated nickel�copper sulfide deposits. In:Friedrich, G., Genkin, A.D., Naldrett, A.J., Ridge, J.D.,Sillitoe, R.H., Vokes, F.M. (Eds.), Geology and Metal-logeny of Copper Deposits. Springer-Verlag, Berlin, pp.43–62.

Lesher, C.M., Stone, W.E., 1996. Exploration geochemistry ofkomatiites. In: Wyman, D.A. (Ed.) Trace Element Geo-chemistry of Volcanic Rocks, Short Course Notes, vol. 12,Geological Association of Canada, Winnipeg, pp. 153–204.

Lesher, C.M., Arndt, N.T., Groves, D.I., 1984. Genesis ofkomatiite-associated nickel sulfide deposits at Kambalda,

Western Australia: a distal volcanic model. In: Buchanan,D.L., Jones, M.J. (Eds.), Sulfide Deposits in Mafic andUltramafic Rocks. Institute of Mining and Metallurgy,London, pp. 70–80.

Li, X.-H., 1996. Sm�Nd isotopic system of the Sibao Group inthe southern margin of the Yangtze Block and significancefor the crustal evolution (in Chinese). Sci. Geol. Sin. 31,218–227.

Li, X.-H., 1999. U�Pb zircon ages of granites from northernGuangxi and their tectonic significance (in Chinese).Geochimica 28, 1–9.

Lightfoot, P.C., Hawkesworth, C.J., Olshefsky, K., Green, T.,Doherty, W., Keays, R.R., 1997. Geochemistry of Tertiarytholeiites and picrites from Queqertarssuq (Disko Island)and Nuussauq, West Greenland, with implications for themineral potential of comagmatic intrusions. Contrib. Miner.Petrol. 128, 139–163.

Ludden, J., Gelinas, L., Trudel, P., 1982. Archaean metavol-canics from the Rouyn-Noranda district, Abitibi greenstonebelt, Quebec, Part 2. Mobility of trace elements and petroge-netic constraints. Can. J. Earth Sci. 19, 2276–2287.

Mao, J., Sun, S., Chen, T., 1988. Igenous Rock and Tin Depositsin Guibei (in Chinese). Beijing Scientific and TechnologicalPress, Beijing.

Mao, J., Zhang, Z., Dong, B., 1990. A new Sm�Nd isotopicchronology of the Sibao Group in Southern margin of theYangtze massif (in Chinese). Geol. Rev. 36, 264–268.

McDonough, W.F., 1990. Constraints on the composition of thecontinental lithospheric mantle. Earth Planet. Sci. Lett. 101,1–18.

Nisbet, E.G., 1982. The tectonic setting and petrrogenesis ofkomatiites. In: Arndt, N.T., Nisbet, E.G. (Eds.), Komatiites.Allen & Unwin, London, pp. 501–518.

Nisbet, E.G., Bickle, M.J., Martin, A., 1977. The mafic andultramafic lavas of the Belingwe greenstone belt, Rhodesia.J. Petrol. 18, 521–566.

Nisbet, E.G., Cheadle, M.J., Arndt, N.T., Bickle, M.J., 1993.Constraining the potential temperature of the Archeanmantle: a review of the evidence from komatiites. Lithos 30,291–307.

Park, A.F., Bowes, D.R., Halden, N.M., Koistinen, T.J., 1984.Tectonic evolution at an Early Proterzoic continental mar-gin: the Svecokarelides of eastern Finland. J. Geodyn. 1,359–386.

Poidevin, J.-L., 1994. Boninite-like rocks from thePalaeoproterozoic greenstone belt of Bogoin, CentralAfrican Republic: geochemistry and petrogenesis. Precam.Res. 68, 97–113.

Puchtel, I.S., Haase, K.M., Hofmann, A.W., Chauvel, C.,Kulikov, V.S., Garbe-Schonberg, C.D., Newchin, A.A.,1997. Petrology and geochemistry of crustally contaminatedkomatiitic basalts from the Vetreny Belt, Southeatern BalticShield: evidence for an early Proterozoic mantle plumebeneath rifting Archean continental lithosphere. Geochim.Cosmochim. Acta 61, 1205–1222.

Scott, D.J., Helmstaedt, H., Bickle, M.J., 1992. Purtuniqophiolite, Cape Smith belt, Northern Quebec, Canada: areconstructed section of Early Proterozoic oceanic crust.Geology 20, 173–176.


Sun, S.-S., 1987. Chemical composition of Archaean komatiites:implications for early history of the earth and mantleevolution. J. Volcan. Geotherm. Res. 32, 67–82.

Sun, S.-S., McDonough, W.F., 1989. Chemical and isotopicsystematics of oceanic basalts: implications for mantlecomposition and processes. In: Saunders, A.D., Norry, M.J.(Eds.), Magmatism in the Ocean Basins, Geol. Soc. Spec.Publ. 42, 313–345.

Sun, S.-S., Nesbitt, R.W., McCulloch, M.T., 1989. Geochem-istry and petrogenesis of Archean and early Proterozoicsiliceous high-magnesian basalts. In: Crawford, A.J. (Ed.),Boninites and Realted Rocks. Unwin Hyman, London, pp.148–173.

Xing, F., Xu, X., Chen, J., Zhou, T., Foland, K.A., 1992.Sm�Nd isotopic age of Proterozoic ophiolites in NE Jiangxiand its geological significance (in Chinese). Acta Petrol.Miner. 11, 120–124.

Wang, Y.Y., Xie, J.B., Chen, Y.L., Qin, S.Y., Zhu, S.C., 1985.Evolution of upper Precambrian volcanic–sedimentary se-quences in the western part of Jiangnan stratigraphicprovince, China. Precam. Res. 29, 109–119.

Wolde, B., Asres, Z., Desta, Z., Gonzalez, J., 1996. Neoprotero-zoic zirconium-depleted boninite and tholeiitic series rocksfrom Adola, Southern Ethiopia. Precam. Res. 80, 261–279.

Xiao, X., 1987. Basic characteristics of spinifex-textured komati-ites in Hunan Province. J. South-Central College of Mineral.and Metall. 18, 498–493. (In Chinese)

Yang, L.Z., 1988. Spinifex textured komatiites in the SibaoGroup, Guangxi Province (in Chinese). Guangxi Reg. Geol.16, 9–12.

Zhou, G.Q., 1989. The discovery and significance of thenortheastern Jiangxi province ophiolite, its metamorphicperidotite and associated high-temperature pressure meta-morphic rocks. J. SE Asian Earth Sci. 3, 237–247.

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