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
Archean komatiitic magmas are interpreted to have formed as part of ancient oceanic lithosphere
Desrochers et al., 1993 or primitive ocean is- lands Hoffman, 1990. However, the Belingwe
greenstone 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. Old
detrital zircons in some Archean komatiites confirm crustal contamination Compston et al.,
1986.
The Early Proterozoic komatiitic basalts of both the Cape Smith Belt Hoffman, 1990 and
the Vetreny Belt of the Baltic Shield Puchtel et al., 1997 formed in continental rifts, and the
komatiitic basalts in the Sibao Group are here considered to have been erupted in a similar
tectonic setting. A sub-continental mantle plume is a preferred source of very high temperature,
high-Mg magmas Nisbet et al., 1993.
The occurrence of the komatiitic basalts in the Sibao Group is consistent with the development
of 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 trace element pattern. Mantle-normalization values are from Sun and McDonough 1989.
Fig. 6. Ti vs. Zr for the komatiitic basalts from the Sibao Group. Boninite field from Poidevin 1994, Komatiitic basalts
from Arndt and Nesbitt 1982, Jahn et al. 1982, and SHMB from Sun et al. 1989.
later juxtaposed over the initial rift sequence Scott et al., 1992, rifting in the Northern Guangxi region
did not result in continental breakup and opening of a new ocean. The lack of any significant felsic
magmatism between 1.8 and 2.4 Ga in this region also implies that no large-scale arc system formed.
In northern Guangxi Province, uplift of the crust above the mantle plume and subsequent volcanism
explains why the Sibao Group is thinner than present day rift-related successions. Analogous suc-
cessions are found associated with flood basalts related to the Trindade plume in Western Brazil
Gibson et al., 1997 and the Tertiary plume-related volcanic rocks in Western Greenland Lightfoot et
al., 1997.
The extent of the Proterozoic rift environment is not clear. The Lengjiaxi Group, a flysch sequence in
NW Hunan Province, is considered the equivalent of the Sibao Group. Its lower part is represented by
a meta-volcanic sequence more than 2000 m thick, including mafic lavas and tuffs. Spinifex-textured
komatiites have also been reported Xiao, 1987. It is possible that this belt in NW Hunan Province
represents a separate, individual rift that formed simultaneously with that in Northern Guangxi
Province. The presence of a plume under the craton would have caused uplift and formed a series of
grabens in which sediments accumulated. One or more of them could be considered aulocogens.
This model for the formation of the komatiitic basalts requires that the Sibao Group was essen-
tially deposited on continental crust. The oldest rocks known in the Yangtze Block are the
Kangding and Kongling gneisses of 2400 – 2950 Ma Wang et al., 1985. The polymictic conglomerate in
the lower part of the Fanjingshang Group in Guizhou Province, an equivalent of the Sibao
Group, contains clasts of various metamorphic rocks and granites, apparently derived from the
underlying basement. SmNd isotopes from the granodiorite of the Sibao Group suggest that this
rock was a product of anatexis of older continental crust. The model age calculated is 2513 Ma Mao et
al., 1988. Older UPb ages of 2860–3289 Ma have been 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. TiV vs. TiSc for the komatiitic basalts from the Sibao Group. SHMB from Sun et al. 1989, MORB and komatiitic
basalt from Wolde et al. 1996, boninites from Hickey and Frey 1982.
erupted during deposition of terrigenous sediments. The early stages of this continental rifting are
marked by the deposition of conglomerates, quartzites, and arkoses in the lower part of the
Sibao Group. Subsequently, extensive eruption of high-Mg lavas was associated with the emplace-
ment of maficultramafic layered intrusions and followed by further rapid sedimentation.
Unlike the situation in the Chukotat Group of the Cape Smith Belt, where continental rift-type
volcanism gave way to the formation of transitional oceanic crust Hynes and Francis, 1982, which was
old granulitic crust. All of this is evidence of an old continental crust underlying the Sibao Group,
although no outcrops of this basement are exposed.
7. Conclusions
