These rocks are of considerable importance in studies 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 and
aid in discerning the tectonic regimes of the host terrains.
Although the tectonics of southern China have been widely discussed since Hsu et al. 1988, 1990
proposed a model involving complex overthrust- ing to explain the Mesozoic collision between the
Yangtze and Cathaysian Blocks, the presence of komatiitic basalts in northern Guangxi Province
has not been discussed. Local geologists described spinifex-textured rocks in the Middle Proterozoic
Sibao Group as komatiites Mao et al., 1988; Yang, 1988, but this discovery has remained
obscure. Chen and Mao 1995 described some ultramafic rocks as extrusive komatiites, but these
appear to be olivine cumulate rocks which have resultant high Mg contents. Because their geo-
chemistry is different from that of typical Archean komatiites, Li 1996 suggested that the spinifex
rocks are not komatiite, but normal basalt. It is clear that the geochemical signature of these rocks
needs to be re-examined and their significance interpreted.
This study provides a description of the distri- bution of the ultramafic and related rocks based
on a mapping programme carried out by Dong 1988, 1990 and during our own field work in the
past few years. This mapping has shown that many of the ultramafic and mafic rocks are, in
fact, high-level sills that are interpreted to be co-magmatic with lavas. Representative samples
have been analyzed for major oxides and trace elements and an attempt has been made to iden-
tify the composition of the parental magma. Con- sequently, some constraints can now be placed on
the tectonic setting of the magmatism and the Proterozoic geological evolution of the region.
2. Regional geology
The area of northern Guangxi Province forms the southern end of the Jiangnan Oldland, north
of the
Cathaysian continent
Huang, 1945;
Chang, 1996 Fig. 1. The Jiangnan Oldland con- tains Precambrian rocks interpreted to represent
basement to the Yangtze Block, and a cover sequence of Sinian and Devonian sedimentary
strata of neritic and terrestrial facies, which ap- pear to deposit in a stable tectonic setting.
In northern Guangxi Province, the Sibao Group forms the basal Proterozoic sequence and
is uncomformably overlain by the Late Protero- zoic Danzhou Group Figs. 1 and 2. The Sibao
Group includes the Wentong and Yuxi Forma- tions. The lower part of the Wentong Formation
comprises very mature terrigenous sandstones containing 40 – 86 modal quartz. The upper
part contains turbidites displaying classic Bouma sequences, cut by a number of sills and plugs of
ultramafic and mafic lithologies Fig. 2. The Yuxi Formation is composed of intermediate to acidic
lava flows, ignimbrites, tuffaceous sediments, and intercalated mudstones.
The exact thickness of the Sibao Group is difficult to measure because the rocks are strongly
folded and faulted, but is probably between 3 and 5 km Dong, 1988. The age of these supracrustal
rocks is not precisely known. As expected, no fossils have been observed in the Sibao Group,
but Li 1996 reported whole rock SmNd data for the mafic volcanic rocks yielding an age of
1900 9 100 Ma, revising a 2200 Ma SmNd isochron obtained by Mao et al. 1990. The
Sibao Group is intruded by the Sanfang and Yuanbaoshan peraluminous S-type granites Fig.
1, which contain zircons dated by SHRIMP at ca. 825 Ma Li, 1999.
The Danzhou Group is equivalent to the Banxi Group of northern Jiangxi Province and the
Shangxi Group of southern Anhui Province. The base of the Danzhou Group is marked by con-
glomerates with granitic clasts and fragments derived from the Sibao Group. Slates and phyl-
lites represent continental shelf, neritic and semi- pelagic facies interpreted to be of back-arc basin
origin Guo et al., 1985. Ophiolites ca. 1000 Ma were emplaced onto these sedimentary rocks
along 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 as a Neoproterozoic collision between the Yangtze
and Cathaysian Blocks. However, Hsu et al.
1988, 1990 have suggested an alternative model in which Cathaysia was thrust on to the Yangtze
Block during the Triassic, with the Danzhou Group and its equivalents forming a tectonic
melange.
3. Mafic and ultramafic rocks in the Sibao Group
