Fig. 2. Geological map of the Sibao area showing the distribution of maficultramafic 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 lower
chilled margin overlain successively by 1 peri- dotite containing rounded olivine chadacrysts en-
closed in large pyroxene oikocryts Fig. 3D, 2 gabbro, and 3 diorite. This layering was formed
by crystal settling of olivine, clinopyroxene and then plagioclase within these subvolcanic bodies,
which are interpreted to be co-magmatic with the extrusive 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 been selected for geochemical analysis. Major element
abundances were determined by X-ray fluores- cence spectrometry XRF on fused glass pellets.
Trace elements Sc, V, Cr, Ni, Cu, and Zn were determined by XRF on pressed powder pellets.
Fig. 3. Maficultramafic rocks in the Sibao Group. A and B Spinifex-textured komatiitic basalt from the Hejia Flow, showing elongate clinopyroxene A and skeletal clinopyroxene B, Crossed nicols, field of view 2.87 × 4 mm. C Handspecimen of a
pyroxene cumulate from the Zhongkui Flow, showing sulfide mineral segregations, field of view 11.5 × 16 cm. D Peridotite from a 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 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
