Geochemistry of intermediate to siliceous volcanic rocks of the Rooiberg Group, Bushveld Magmatic Province, South Africa

The volcanic Rooiberg Group represents the earliest phase of Bushveld-related magmatism and comprises, in some areas, the floor and roof rocks of the mafic–ultramafic intrusive units of the Bushveld Complex. The lower to middle Dullstroom Formation is composed of two interbedded series of low Ti and high Ti volcanic strata, which are predominantly basaltic andesites. Volcanic units above these strata range from andesites to dacites in the upper Dullstroom Formation and to predominantly rhyolites in the overlying Damwal and Kwaggasnek Formations. Compositional data suggest that these intermediate to siliceous volcanic rocks are petrogenetically related to the low Ti volcanic suite and suggest that the low Ti magmas resided in a shallow magma chamber where they experienced fractional crystallization and assimilation of crustal material. In contrast, the high Ti volcanic suite is petrogenetically unrelated. These data confirm previous suggestions that Bushveld-related magmas experienced significant amounts of assimilation of continental crust. Introduction The Rooiberg Group, which includes, in ascending order, the Dullstroom, Damwal, Kwaggasnek, and Schrikkloof Formations (Schweitzer et al. 1995), is located in the eastern part of South Africa (Fig. 1) and represents a voluminous ( 6-km thick) series of predominantly volcanic rocks with minor interbedded sediments (Fig. 2). The original areal extent of this sequence was large, as the present outcrop exposures extend over parts of an area of 65,000 km (Tankard et al. 1982; Schweitzer et al. 1995). In some areas, these volcanic units make up the floor and roof rocks of the Rustenburg Layered Suite (RLS), the mafic– ultramafic phase of the intrusive Bushveld Complex. The Lebowa Granite Suite, the granitic phase of the Bushveld Complex, intrudes the Rooiberg Group in other areas. Most of the isotopic age dates acquired for samples of the Rooiberg Group and the Bushveld Complex range from 2061±2 to 2052±48 Ma (e.g.; Walraven et al. 1987, 1990; Walraven 1997; Harmer, personal communication 2000; Buick et al. 2001). The close spatial relationships of these intrusive and extrusive rocks and the similarity of their crystallization ages suggest that they were deposited during the same magmatic event. Irvine (1982) suggested that all of these rocks should be included in the Bushveld Magmatic Province. The intrusive relationship of the Bushveld Complex with the Rooiberg Group indicates that these volcanic rocks represent the initiation and early development of Bushveld magmatic activity. Three hypotheses have been proposed for the origin of the Rooiberg Group. First, several authors (e.g., Rhodes 1975; Elston 1992) attributed these strata and the Bushveld Complex to the simultaneous impact of several comets or asteroids. However, Buchanan and Reimold (1998) discounted this hypothesis. Second, Hatton (1988) suggested that subduction-related processes associated with a nearby plate margin formed Contrib Mineral Petrol (2002) 144: 131–143 DOI 10.1007/s00410-002-0386-1 P.C. Buchanan (&) Æ W.U. Reimold Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050 Johannesburg, South Africa E-mail: [email protected] C. Koeberl Institute of Geochemistry, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria F.J. Kruger Economic Geology Research Institute-Hugh Allsopp Laboratory, School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa Present address: P.C. Buchanan Antarctic Meteorite Research Center, National Institute of Polar Research, 1-9-10 Kaga Itabashi-ku, Tokyo 173-8515, Japan Editorial responsibility: T.L. Grove these units. Third, Harmer and von Gruenewaldt (1991), Hatton (1995), and Hatton and Schweitzer (1995) attributed the Rooiberg Group and the Bushveld Complex to partial melting of subcontinental lithosphere and lower crust by a mantle plume. Recently, Buchanan et al. (1999) studied the high Ti and low Ti andesites and basaltic andesites of the Dullstroom Formation and suggested that they represent magmas that were derived by partial melting of compositionally distinct source areas and that they resided in at least two magma chambers in which magma mixing, fractional crystallization, and assimilation of crustal material occurred. The goal of this study is to elucidate the petrogenesis of the overlying intermediate to siliceous volcanic rocks. Bulk geochemical compositions should help to trace the development of these early stages of Bushveld-related magmatism and to determine whether these magmas were derived by crustal melting or by differentiation and contamination of the same melts represented by the more mafic volcanic rocks of the lower to middle Dullstroom Formation. Samples and analytical techniques Most of the samples analyzed in this study were collected in 1996– 1997, as described in Buchanan et al. (1999). Supplementary samples representative of strata from the upper Dullstroom Formation were collected in 2000 in the northeastern part of the Bushveld Magmatic Province. Sampling areas are noted in Fig. 1 and in Buchanan et al. (1999). Preliminary compositional data for these units suggested that Rooiberg Group strata in the western and central parts of the province, close to outcrops of the Lebowa Granite Suite, were pervasively altered. To avoid this alteration as much as possible, most of the samples considered in this study were obtained from the southeastern part of the province. This proved possible for the Dullstroom, Damwal, and Kwaggasnek Formations, but outcrops of the Schrikkloof Formation were more difficult to find in this area and, hence, this unit was excluded from the present study. Samples were processed and analyzed using the same techniques described in Buchanan et al. (1999). Fig. 1. Generalized geologic map of the southeastern portion of the Bushveld Complex after Council of Geosciences (1978). The inserts at the bottom indicate the regional position of the study area. Sampling areas for rocks analyzed in this study are noted here and in Buchanan et al. (1999) 132