The Dullstroom Formation

PETROGENETIC MODELIN

P. C. Buchanan(1), W. U. Reimold(2), and C. Koeberl(3).
(1) Mail Code: SN2, NASA Johnson Space Center, Houston, Texas 77058, USA; e-mail: pbuchana@ems.jsc.nasa.gov.
(2) Department of Geology, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa.
(3) Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria.

Introduction:
Similarities between the volcanic strata of the Proterozoic Dullstroom Formation of South Africa and the nearby Mesozoic Karoo continental flood basalts suggest that both provinces were derived from the same or similar source areas within the subcontinental lithosphere below southern Africa. The presence of high-Ti and low-Ti volcanic suites in both provinces apparently is the result of long-term compositional variations in this source area.

Evidence suggests that the melts that crystallized to form the Dullstroom volcanic strata resided in two or more shallow magma chambers in which magma mixing, assimilation of upper continental crust, and fractional crystallization occurred. These data are consistent with formation of the Dullstroom Formation by endogenic geological processes rather than by the impact of multiple asteroids or comets.

Data and Discussion:
The Proterozoic Dullstroom Formation of the Bushveld Magmatic Province (BMP) of South Africa represents the first phase of an extended period of magmatic activity that formed the intrusive Bushveld Complex and the extrusive Rooiberg Group. Formation of these units has previously been ascribed to three possible causes:
1) the impact of several comets or asteroids [e.g., 1],
2) a nearby subduction zone [2], or  3) a mantle plume [e.g., 3].

Bulk geochemical, mineralogical, and isotopic data for the Dullstroom Formation were recently reported [4]. The mafic volcanic units (predominantly basaltic andesites) of the Dullstroom Formation can be subdivided into two compositional groups of interbedded strata: low-Ti and high-Ti. These two suites apparently are not related by differences in proportions of partial melting or fractional crystallization.

Preliminary modeling [5] of the compositions of these two groups indicated that both experienced significant amounts of assimilation of upper continental crust and fractional crystallization of pyroxene and plagioclase. In an effort to further constrain the processes that affected these melts, we have undertaken additional modeling of these compositional data. There are several compositional similarities between the basaltic andesites of the Dullstroom Formation and the Mesozoic volcanic rocks of the nearby Karoo continental flood basalt province. On the basis of bulk compositional data, the Karoo rocks have been divided into northern and southern provinces by Cox et al. [6]. The boundary between these provinces trends east-southeast through the BMP [7].

Volcanic rocks of the northern Karoo province are strikingly enriched in Ti, K, and P relative to the volcanic rocks of the southern province. Hence, the compositional similarities and the close proximity of these Karoo continental flood basalts to the BMP suggest that they might provide reasonable approximations of the primary melts from which Dullstroom Formation strata were derived. A representative composition of southern Karoo basalts was taken as the average of Lesotho basalts [reported in several papers in 8] and a representative composition of northern Karoo basalts was taken as the average of the Sabie River basalts of the northern Lebombo [8].

These compositions were used to model the compositions of Dullstroom Formation basaltic andesites. For this modeling, the composition of the upper continental crust was taken from estimates in Taylor and McLennan [9].

Compositions of crystallizing feldspars were taken from electron microprobe analysis of phenocrysts in these rocks and compositions of crystallizing pyroxenes were calculated from bulk rock compositions using appropriate distribution coefficients. The average composition of the Dullstroom high- Ti suite is adequately modeled by a mixture of 30% northern Karoo basalt and 70% southern Karoo basalt with 20% assimilation of upper continental crust and 20% fractional crystallization of subequal proportions of plagioclase and pigeonite [4]. The average composition of the Dullstroom low-Ti suite is similar to southern Karoo basalt with 20% assimilation of upper continental crust and 20% fractional crystallization of subequal proportions of plagioclase and pigeonite [4].

Implications:
Similarities in compositions of basaltic andesites of the Proterozoic Dullstroom Formation and the nearby Mesozoic Karoo continental flood basalts suggest that differences between low-Ti and high-Ti volcanic suites in both provinces are the result of long-term compositional variations in their source areas in the subcontinental lithosphere below southern Africa [4]. The compositions of both suites in the Dullstroom Formation are consistent with magma mixing, assimilation, and fractional crystallization in two or more shallow magma chambers. These data and the voluminous character of the extrusive and intrusive phases of the BMP are consistent with formation of this province by a mantle plume. They do not indicate formation by the impact of several comets or asteroids.

Acknowledgments:
This work was partially supported by NASA grant RTOP 344-31-00-05 to D. J. Lindstrom. The Austrian FWF, project Y58-GEO (to C. K.), and the NRF of the Republic of South Africa (to W. U. R.) also provided support.

References:
[1] Rhodes R. C. (1975) Geology, 3, 549-554.
[2] Hatton C. J. (1988) Abstracts of the Congress of the Geological Society of South Africa, 22, 251-254.
[3] Hatton C. J. (1995) J. Afr. Earth Sci., 21, 571-577.
[4] Buchanan et al. (1999) Contrib. Mineral. Petrol., 137, 133-146.
[5] Buchanan P. C. et al. (1999) LPS XXX, 1290. [6] Cox K. G. et al. (1967) Am. Mineral., 52, 1451-1474.
[7] Duncan A. R. (1987) J. Volcanol. Geotherm. Res., 32, 13-34.
[8] Erlank A. J. (ed.) (1984) Geol. Soc. S. Afr. Spec. Publ. 13, p 395.
[9] Taylor S. R. and McLennan S. M. (1985) The continental crust: its composition and evolution. Blackwell Scientific Publications, Oxford, p.312.

Lunar and Planetary Science XXXI
Dullstroom Formation, South Africa: P. C. Buchanan, W. U. Reimold, and C. Koeberl