Petrogenesis of the Dullstroom Formation, Bushveld Magmatic Province, South Africa |
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Authors: | Paul C Buchanan Christian Koeberl Wolf Uwe Reimold |
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Institution: | (1) Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria, AT;(2) Department of Geology, University of the Witwatersrand, Private Bag 3, Wits 2050 Johannesburg, South Africa, ZA |
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Abstract: | The Precambrian Dullstroom Formation of South Africa, which is predominantly composed of basaltic andesites interbedded with
subordinate sedimentary and felsic volcanic strata, represents the first phase of an extended period of magmatism that was
responsible for the Bushveld Magmatic Province, including the extrusive Rooiberg Group and the intrusive Bushveld Complex.
New geochemical and isotopic data for the Dullstroom Formation are presented in an effort to elucidate the petrogenetic processes
operative during the initiation of this magmatic episode. The volcanic units of the central portion of the Dullstroom Formation
have been subdivided into at least three interbedded compositional groups: low Ti mafic to intermediate units, high-Ti mafic
to intermediate units, and high Mg felsic units. High Ti and low Ti volcanic units are similar in some compositional characteristics
to basalts of the nearby northern and southern provinces, respectively, of the Mesozoic Karoo continental flood basalts. Isotopic
and compositional data for low Ti Dullstroom strata are consistent with bulk assimilation into a melt similar in composition
to a southern Karoo basalt of 20% upper continental crust accompanied by 20% fractional crystallization of pyroxene and plagioclase.
Isotopic and compositional data for high Ti Dullstroom strata are consistent with magma mixing of 30% northern Karoo K-rich
basalt and 70% southern Karoo basalt followed by 20% assimilation of upper continental crust and 20% fractional crystallization
of pyroxene and plagioclase. Compositions of high Mg felsic volcanic strata are consistent with 25% assimilation of a mixture
of silica-rich sedimentary rock and upper continental crust into a melt similar in composition to low Ti volcanic units with
25% fractional crystallization of pyroxene and plagioclase. However, it has been suggested that compositions of these high
Mg felsic strata may also be consistent with interaction of a crustal melt. Assimilation, fractional crystallization, and
magma mixing that apparently affected these Dullstroom Formation volcanic strata may have occurred in a series of shallow
magma chambers. These data are consistent with the suggestion that Dullstroom Formation volcanic rocks are the result of a
mantle plume. Mantle plume origin also is suggested by the large volume of intrusive and extrusive strata associated with
this magmatic episode. These data do not support the hypothesis that the Bushveld Complex and the Rooiberg Group formed by
impacts of a cluster of comets or asteroids.
Received: 14 October 1998 / Accepted: 17 May 1999 |
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