Precambrian Research 103 2000 175 – 189
Crustal-contaminated komatiitic basalts in Southern China: products of a Proterozoic mantle plume beneath the
Yangtze Block
Mei-Fu Zhou , Tai-Ping Zhao
1
, John Malpas, Min Sun
Department of Earth Sciences, Uni6ersity of Hong Kong, Pokfulam Road, Hong Kong Received 21 October 1999; accepted 24 March 2000
Abstract
Abundant mafic and ultramafic rocks including basalts, komatiitic basalts, and peridotites occur in the Proterozoic Sibao Group, northern Guangxi Province, China. Whereas the basalts are generally pillow lavas, the komatiitic
basalts are typically spinifex-textured and, in a few cases, show pyroxene accumulation associated with NiCuPGE sulfide deposits. The peridotites occur in the lower portions of differentiated sills, which contain gabbro and diorite
in their upper parts. The sills are believed to be co-magmatic with the komatiitic basalts. The spinifex rocks of the Jiepai and Hejia Flows have MgO ranging from 8.9 to 14.3 wt. The Zhongkui Flow is highly fractionated to form
a spinifex zone with lower MgO 5.3 – 5.9 wt and a cumulate zone with higher MgO 17.3 – 17.9 wt. Overall the rocks have TiO
2
= 0.44 – 0.74 wt. Relative to primitive mantle, they are enriched in Th and LREE, but exhibit
negative Ti-, Nb-, and P-anomalies. These features are consistent with their formation from a crustally-contaminated komatiitic magma. During this process of crustal contamination, the magmas assimilated sulfur from sediments,
which caused sulfide-segregation resulting in the formation of NiCuPGE sulfide deposits. The occurrence of the komatiitic basalts in the Sibao Group can be explained by the ascent of a mantle plume beneath a continental rift
environment, and implies that the Yangtze Block may have had an Archean basement through which the Sibao komatiitic basalts erupted. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Crustal contamination; Komatiitic basalt; Proterozoic; Southern China www.elsevier.comlocateprecamres
1. Introduction
Many Proterozoic terrains, including the Cape Smith Belt, Canada Hynes and Francis, 1982
and the Baltic Shield Park et al., 1984; Puchtel et al., 1997 contain komatiitic rocks that are inter-
preted to represent products of high-Mg magmas derived by partial melting of the upper mantle.
Corresponding author. Tel.: + 852-28578251; fax: + 852- 25176912.
E-mail address
:
mfzhouhkucc.hku.hk M.-F. Zhou.
1
Present address: Henan Institute of Geology, Zhengzhou, Henan, PR China.
0301-926800 - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 0 0 0 0 0 7 7 - 2
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
