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 the
protocol of Jenner et al. 1990, with standard additions, pure elemental standards for external
calibration, and BHVO-1 as a reference material. Accuracies of the XRF analyses are estimated as
9
2 for major elements present in concentra- tions greater than 0.5wt and 9 5 for trace
elements. The ICP-MS analyses yield accuracy better than 9 5. Major oxides are normalized to
100 on a volatile-free basis and all data are listed in Table 1.
The spinifex rocks of the Jiepai and Hejia Flows have MgO ranging from 8.9 to 14.3 wt
Table 1 and Fig. 4. The spinifex rocks of the Zhongkui Flow have lower MgO 5.3 – 5.9 wt
than typical komatiitic basalts. This flow is highly fractionated to form a high-Mg cumulate zone in
the bottom with MgO contents ranging from 17.3 to 17.9 wt. Therefore, overall the rocks can be
called komatiitic basalts.
SiO
2
contents 49.1 – 57.8 wt are lower, but FeO
T
contents 9.4 – 13.6 wt are higher than typical boninites \ 53 wt SiO
2
and 6 – 7 wt FeO
T
at 10 – 12 wt MgO Crawford et al., 1989. All samples are low in TiO
2
0.44 – 0.90 wt, similar to low-Ti tholeiites Kerrich et al.,
1998, but higher than typical boninites Craw- ford et al., 1989. The Mg c ’s of all samples
range from 53 to 81 and correlate positively with Ni and Cr Fig. 4.
The komatiitic basalts have Al
2
O
3
TiO
2
close to 20, except the Zhongkui spinifex samples which
have higher ratios of ca. 33. CaOAl
2
O
3
ranges from 0.54 to 0.79. ZrY ratios range between 3.61
and 12.2, higher than the chondritic ratio ca. 2.5.
The spinifex-textured
samples in
the Zhongkui Flow have the highest SiO
2
and also the highest ThNb and LaSm values Fig. 4. ThNb
correlates positively with LaSm in samples from the Jiepai and Hejia Flows Fig. 4.
All rocks have REE
N
patterns enriched in LREE with flat HREE Fig. 5A. The primitive
mantle-normalized trace element plot Fig. 5B displays enrichment in large ion lithophile ele-
ments normally concentrated in the continental crust, especially Ba, Th, U, Pb, and Sr, and shows
strong negative Ti-, Y-, Nb-, P-, and Ta-anoma- lies. The rocks also have low concentrations of Sc
and V.
In a diagram of Ti versus Zr Fig. 6, the Sibao volcanic rocks are scattered. However, samples
from individual flows form noticeable trends. All samples have Ti contents similar to many other
komatiitic basalts and Siliceous High-Magnesian Basalts SHMB of Sun et al. 1989. High-Si
spinifex-textured rocks of the Zhongkui Flow and two 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 have
lower TiZr values than primary un-contami- nated komatiitic basalts ca.100 and MORB
100 – 200. In Fig. 7, the Hejia Flow falls in the MORB field because of its low V and Sc contents.
Again, all other samples plot in the SHMB field and have TiV and TiSc 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 least
mobile during hydrothermal alteration and green- schist facies metamorphism of mafic volcanic
rocks e.g. Ludden et al., 1982; Kerrich et al., 1998. Therefore, although the mineralogy of the
komatiitic basalts indicates that they have been metamorphosed to sub-greenschist facies, REE,
Zr, Y, Nb and Hf have coherent trends, likely reflecting characteristics of the primitive magma
and processes of magmatic differentiation either before, during or after eruption.
The spinifex textured rocks in the Sibao Group have lower SiO
2
contents but higher FeO
T
con- tents than typical boninites. Their LREE-enriched
REE patterns are also different from typical boninites. All these together with the lack of
olivine phenocrysts indicate that they are not products of a normal picritic magma. The enrich-
ment of LREE and other features of their geo-
Table 1 Chemical compositions of komatiitic basalts from the Sibao Group, Northern Guangxi Province, China
Sample Hejia flow
Jiepai flow location
Textures Spinifex-texture
GX-4 GX-5
GX-14 GX-16
GX-17 GX-18
GX-3 GX-1
Sample no. Major oxides
wt 52.4
52.1 50.7
52.1 55.8
53.7 53.9
54.0 SiO
2
0.51 0.67
TiO
2
0.90 0.70
0.74 0.73
0.74 0.64
11.9 14.4
15.0 15.0
14.1 15.0
Al
2
O
3
14.8 14.4
11.0 10.7
11.0 11.1
11.2 10.8
10.5 10.6
Fe
2
O
3 a
14.3 8.89
5.73 7.66
MgO 7.46
8.98 7.46
9.35 9.08
11.0 8.04
8.16 11.1
8.31 10.6
8.51 CaO
0.17 0.16
0.18 0.17
0.17 0.17
0.17 0.17
MnO 1.42
1.05 2.24
2.39 Na
2
O 2.60
1.20 2.34
1.20 0.66
0.38 0.70
1.20 0.27
1.16 0.74
1.26 K
2
O 0.06
0.07 0.05
0.07 0.12
0.08 0.07
0.08 P
2
O
5
3.36 2.78
2.48 2.60
2.44 2.24
LOI
b
2.83 3.18
Trace elements ppm
32 35
30 Sc
30 34
30 30
35 220
223 184
221 191
218 227
214 V
1358 453
224 Cr
338 445
321 311
507 359
103 24
34 111
31 Ni
29 101
76 88
71 93
92 15.0
9.7 13.9
Cu 99
92 91
163 Zn
99 94
96 98
37 61
131 85
55 89
65 92
Zr 11.6
18.1 24.6
11.9 11.3
21.2 23.5
25.8 Rb
57 105
129 125
Sr 158
105 144
120 12.4
10.0 16.7
13.9 13.2
15.9 Y
14.4 11.7
3.57 4.08
2.94 4.27
9.00 5.91
6.08 6.05
Nb 1.06
0.65 1.01
0.99 Cs
1.58 0.83
1.44 0.73
77 63
163 329
46 280
96 326
Ba 6.23
5.92 6.32
6.01 10.9
9.22 9.21
8.76 La
11.5 13.6
25.7 20.1
Ce 21.2
13.3 19.2
13.5 1.87
1.95 3.52
2.83 2.04
2.84 Pr
2.67 2.01
7.94 7.81
7.06 7.49
13.6 10.9
10.9 10.3
Nd 1.93
2.09 3.46
2.72 Sm
2.75 2.22
2.58 2.20
0.54 0.61
0.86 0.68
0.65 0.72
0.63 0.68
Eu 2.42
2.36 2.15
2.16 3.51
2.76 2.75
2.58 Gd
0.38 0.40
0.64 0.50
0.50 0.47
Tb 0.44
0.45 2.37
2.49 3.96
3.02 2.87
3.19 2.80
3.00 Dy
0.64 0.63
0.52 0.54
0.87 0.66
0.69 0.66
Ho 1.43
1.49 2.40
1.82 Er
2.00 1.77
1.92 1.83
0.22 0.22
0.36 0.28
0.28 0.31
0.26 0.29
Tm 1.84
1.76 1.34
1.39 2.34
1.78 2.08
1.88 Yb
0.20 0.20
0.34 0.27
0.31 0.29
Lu 0.26
0.28 0.98
1.58 3.57
2.34 1.62
2.35 Hf
2.37 1.81
0.24 0.27
0.19 0.26
0.62 0.40
0.40 0.39
Ta 5.37
8.34 12.5
8.88 9.53
8.73 Pb
11.9 10.4
0.96 1.07
3.00 2.40
1.08 2.42
0.91 2.14
Th 0.36
0.53 U
1.34 0.56
0.91 0.93
0.86 0.48
Table 1 Continued Hejia flow
Sample Jiepai flow
location Textures
Spinifex-texture GX-3
GX-1 GX-4
GX-5 GX-14
GX-16 GX-17
GX-18 Sample no.
72 61
50 58
63 58
c
Mg c 58
63 CaOAl
2
O
3
0.79 0.76
0.76 0.54
0.54 0.55
0.57 0.74
20.6 22.0
23.4 21.5
16.7 20.3
20.6 20.0
Al
2
O
3
TiO
2
4.37 12.2
9.79 11.3
8.96 9.48
ZrY 10.5
10.1 ThNb
0.33 0.22
0.25 0.33
0.41 0.40
0.35 0.30
Sample Zhongkui flow
location Cumulate
Spinifex-texture Textures
GX-20 GX-19
GX-21 GX-22
GX-23 GX-24
GX-25 GX-26
GX-27 Sample no.
Major oxides wt
56.3 56.9
56.3 49.4
57.8 49.1
56.9 49.2
49.4 SiO
2
0.44 0.45
0.47 0.45
0.48 0.52
0.52 0.51
0.52 TiO
2
15.2 15.1
15.2 10.2
Al
2
O
3
10.3 15.2
10.4 10.3
14.5 9.75
9.59 10.26
13.6 9.36
13.8 9.57
13.6 13.2
Fe
2
O
3 a
5.50 5.48
5.32 5.37
5.89 17.7
17.5 17.3
17.9 MgO
9.42 8.90
8.35 6.96
6.99 7.03
CaO 6.96
8.66 9.23
0.17 0.17
0.17 0.19
0.16 0.19
MnO 0.19
0.19 0.16
2.75 2.27
2.17 1.97
1.42 0.58
0.66 0.76
0.65 Na
2
O 1.07
1.40 1.74
0.55 0.59
0.69 K
2
O 0.57
1.26 0.20
0.08 0.07
0.08 0.06
0.06 0.06
0.07 0.06
0.06 P
2
O
5
LOI
b
1.37 2.20
1.60 1.71
4.57 4.82
4.40 4.83
1.80 Trace elements
ppm Sc
31.4 30
29.8 29.3
31.3 30.7
31.3 31.2
30.4 175
180 185
165 177
164 V
167 176
181 182
204 207
236 213
1390 1420
1429 1470
Cr 5.91
5.40 8.63
2311 Ni
1942 6.04
1748 1616
11.2 4.61
8.10 8.30
1327 7.73
1260 7.37
1085 1046
Cu 80
86 92
85 108
114 106
103 101
Zn 95
90 86
59 Zr
57 91
54 62
97 18.3
21.7 38.0
23.7 5.39
26.3 Rb
30.6 27.5
23.4 102
104 97
104 109
25.2 29.0
34.9 30.5
Sr 12.2
12.3 16.3
15.2 Y
14.4 12.9
14.9 16.0
13.5 8.50
8.63 9.13
4.12 8.25
4.19 8.81
3.94 4.53
Nb 0.39
1.24 0.72
1.84 3.81
1.21 1.38
1.47 1.44
Cs 160
Ba 243
217 245
119 144
175 153
52 11.6
11.3 12.9
8.50 10.4
8.84 11.39
9.04 9.31
La 23.3
24.3 25.3
23.6 29.6
16.7 17.5
17.5 17.8
Ce 3.35
3.27 3.79
2.47 Pr
2.42 3.21
2.47 2.47
3.27 11.8
11.6 13.5
9.13 11.7
8.84 11.3
9.11 9.29
Nd 2.70
2.49 2.67
2.61 3.06
2.24 2.22
2.31 2.27
Sm 0.62
Eu 0.58
0.57 0.68
0.73 0.71
0.71 0.74
0.63 2.39
2.34 2.87
2.32 2.46
2.33 Gd
2.33 2.38
2.22 0.44
0.40 0.43
0.41 0.50
0.40 0.40
0.40 0.42
Tb 2.68
Dy 2.65
2.56 3.31
2.53 2.49
2.50 2.62
2.70 0.60
0.59 0.73
0.54 0.54
0.55 0.56
0.60 Ho
0.56
Table 1 Continued Jiepai flow
Sample Hejia flow
location Textures
Spinifex-texture GX-3
GX-4 GX-5
GX-14 GX-16
GX-1 GX-17
Sample no. GX-18
Er 1.62
1.77 1.74
1.70 2.15
1.56 1.61
1.56 1.62
0.29 0.27
0.27 0.34
Tm 0.24
0.25 0.24
0.24 0.25
1.94 1.81
1.76 2.24
1.63 1.59
Yb 1.60
1.58 1.67
0.25 Lu
0.29 0.27
0.26 0.32
0.24 0.24
0.24 0.25
2.61 2.49
2.37 2.33
Hf 1.56
2.32 1.60
1.52 1.63
0.58 0.59
0.59 0.62
0.58 0.27
Ta 0.29
0.27 0.30
13.2 Pb
14.1 9.3
15.0 9.9
172 201
132 146
5.33 3.11
2.78 4.24
3.31 3.35
Th 3.43
3.48 3.47
1.72 1.65
1.60 1.67
0.67 1.55
0.69 U
0.64 0.68
c
Mg c 53
54 52
53 53
72 72
72 73
0.64 0.62
0.59 0.55
CaOAl
2
O
3
0.68 0.57
0.68 0.67
0.67 33.0
32.7 33.9
31.9 33.5
19.7 Al
2
O
3
TiO
2
20.0 20.3
20.0 8.54
ZrY 7.20
7.85 7.37
5.28 3.85
3.93 3.61
3.86 0.65
0.37 0.32
0.46 0.80
ThNb 0.80
0.40 0.87
0.77
a
Fe
2
O
3
as total iron.
b
LOI, loss on ignition.
c
Mg c = 100×Mg
2+
Mg
2+
+ Fe
2+
, Fe
2+
is calculated from total FeO. Major oxides were recalculated to 100 on a volatile-free basis.
chemistry also distinguish them from most other komatiitic basalts Arndt et al., 1977; Hynes and
Francis, 1982 which are high Mg, LREE-de- pleted lavas. Nor can the REE signatures of the
spinifex-textured rocks in the Sibao Group be explained 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 have pronounced negative Nb and Ti anomalies with
corresponding LREE enrichment, similar to the SHMB of the Yilgarn Craton, Australia Sun et
al., 1989 and the komatiitic basalts from the Vetreny Belt, Southern Baltic Shield Puchtel et
al., 1997. These are geochemical features consid- ered indicative of a komatiitic magma contami-
nated by continental crustal material Arndt and Jenner, 1986; Juteau et al., 1988; Sun et al., 1989;
Puchtel et al., 1997; Kerrich et al., 1998.
Average subcontinental lithospheric mantle and continental 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 through the lithosphere andor crust, resulting in a marked
increase in Ba, Th, Zr, U, and LREE, but little change in Ta, Nb, HREE, and Ti concentrations.
This results in negative Ta, Nb, and Ti anomalies on mantle or chondrite-normalized trace element
variation diagrams. The Sibao komatiitic basalts exhibit such negative Nb- and Ta-anomalies, but
the rocks with the lowest NbTh and Nb La also have the highest SiO
2
and Zr contents. This can only be explained if the magmas parental to the
lavas and intrusions were contaminated by the assimilation of felsic crustal rocks. Positive corre-
lations between LaSm, ThNb, and ZrY result when a magma from a relatively depleted mantle
source low ThNb, ZrY, and LaSm is mixed with an enriched component high ThNb, ZrY,
and LaSm e.g. Puchtel et al., 1997. Following assimilation of crustal material, the magma was
either erupted as lava flows, or intruded in a subvolcanic environment, where crystal settling
Fig. 4. Compositional and elemental ratio plots of the komatiitic basalts from the Sibao Group. Mg c = 100 × MgMg + Fe
2 +
; Fe
2 +
is calculated from total FeO.
formed layered sills. The thick lower peridotite zones indicate accumulation of olivine and repre-
sent sheet sills like those in the Cape Smith Belt, Canada Hoffman, 1990.
The occurrence of NiCuPGE sulfide accu- mulations in the pyroxenite cumulates of some
komatiitic basalt flows from the Sibao Group can also be explained by a process of crustal assimila-
tion. The parental high-Mg magmas were likely S-undersaturated Keays, 1995. Melting of crustal
sulfur-bearing sediments generated immis cible sulfide melts. Indeed, the cumulate pyroxenites
containing massive sulfides in the Zhongkui Flow have the highest ThNb and LaSm Fig. 4, inter-
preted to result from the most extensive contami- nation. These stratiform layers of massive and
net-textured sulfides in the Zhongkui Flow indi- cate that the magma reached sulfide saturation at
an early stage in the crystallization history of the host unit e.g. Lesher and Groves, 1986; Lesher
and Stone, 1996. A similar situation occurs at Kambalda, Australia, in response to incorpora-
tion of sulfide-rich sediments during the flow of komatiitic lava over unconsolidated sediments
Huppert et al., 1984; Lesher et al., 1984; Lesher and Campbell, 1993.
6. Tectonic setting and implications for the geology of southern China
