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
Basalts and komatiitic basalts in the Wentong Formation of the Sibao Group, occur as lava
flows, and peridotites and gabbros as layered, subvolcanic sills Fig. 2. The basaltic lavas ex-
hibit pillowed, spherulitic, and amygdaloidal structures, and thick flows display columnar joint-
ing e.g. near Jiepai. These volcanic rocks have compositions similar to andesitic basalts Mao et
al., 1988.
The komatiitic basalts are interbedded with sed- imentary rocks of the Wentong Formation and
occur as massive 5 – 100 m thick flows with brec- ciated and chilled margins. Actual thicknesses are
difficult to
estimate because
of widespread
polyphase 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 a quenched devitrified matrix Fig. 3A and B.
Both the Jiepai and Hejia Flows are spinifex-tex- tured throughout with well-developed chilled mar-
gin, whereas the Zhongkui Flow displays a spinifex-textured upper part and a pyroxene cu-
mulate lower parts. The pyroxene cumulates con- tain euhedral clinopyroxene, minor olivine and
intercumulate plagioclase, and variable amounts of magmatic sulfides Fig. 3C. Sulfide minerals
can 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 inset shows the location of the ‘Jiangnan Oldland’ Huang, 1945; Chang, 1996.
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
