On the Relationship Between the Bushveld Complex and Its Felsic Roof Rocks, Part 2: the Immediate Roof

Contrib Mineral Petrol (2015) 170:56 DOI 10.1007/s00410-015-1211-y ORIGINAL PAPER On the relationship between the Bushveld Complex and its felsic roof rocks, part 2: the immediate roof J. A. VanTongeren1,2 · E. A. Mathez2 Received: 31 July 2015 / Accepted: 12 November 2015 © Springer-Verlag Berlin Heidelberg 2015 Abstract Emplacement of large volumes of mafic Keywords Rooiberg Group · Bushveld Complex · magma into the crust undoubtedly causes significant Driekop Dome · Mphanama · Masekete · Roof zone · thermal perturbation to the overlying crust. Despite Residual Zone · Upper Zone · Droogehoek · Stoffberg · the clear importance of the country rock in modulating Hornfels · Leptite the thermal evolution the Bushveld Complex, little is known about the nature and extent of its roof zone. This manuscript details the lateral variability of the rocks Introduction that make up the immediate roof of the intrusion in the Eastern Limb. In the Northern Segment of the eastern Layered mafic intrusions represent the primary observa- Bushveld, the roof is dominated by thermally metamor- tional record of igneous differentiation within a solidify- phosed metapelites; in the Central Segment, the roof ing magma chamber. While the sequences of cumulate is dominated by highly metamorphosed meta-volcanic rocks provide information on the magmatic responses to rocks and their partially molten equivalents; and in the solidification, it is the roofs of these intrusions that hold Southern Segment, the roof is likely composed of mod- the key to understanding the mechanisms of heat loss and estly thermally metamorphosed felsic volcanic rocks. thermal evolution. Only six major layered mafic intrusions The variability of roof lithology is also reflected in the have both their roofs and floors preserved and exposed: the variability of floor rocks to the intrusion. A new model Skaergaard Intrusion of East Greenland, the Muskox, Sept for the emplacement of the eastern Bushveld Complex Iles and Kiglapait Intrusions of Canada, the much larger is proposed in which the mafic magmas intrude at a Dufek Intrusion of Antarctica, and Bushveld Complex of deeper level in the north and become shallower to the South Africa. The Bushveld Complex is part of the massive south. Bushveld Igneous Province that also includes voluminous ferroan granites and other felsic rocks, all of similar age of 2.06 Ga. The enormous extent of the Bushveld Complex ≈ Communicated by Timothy L. Grove. (250 350 km if a continuous sheet) belies the fact that it × is generally poorly exposed, with the exception that parts Electronic supplementary material The online version of this of the eastern Bushveld are locally well exposed due to the article (doi:10.1007/s00410-015-1211-y) contains supplementary rugged topography. These regions thus offer the unusual material, which is available to authorized users. opportunity to understand how an enormous and long-lived * J. A. VanTongeren body of mafic liquid interacted with its roof. [email protected] One model of emplacement holds that the Bushveld intruded along a regional unconformity between mainly 1 Department of Earth and Planetary Sciences, Rutgers University, 610 Taylor Rd., Piscataway, NJ 08854, USA shales and quartzites of the underlying Pretoria Group sediments and an overlying thick sequence of basaltic 2 Department of Earth and Planetary Sciences, American Museum of Natural History, 79th and Central Park West, to rhyolitic lavas known as the Rooiberg Group, with the New York, NY 10024, USA exception of one area in the southeast Bushveld where 1 3 56 Page 2 of 17 Contrib Mineral Petrol (2015) 170:56 the Dullstroom Formation, the lowest unit of the Roo- quartz-feldspar, meta-volcanic or metasedimentary, iberg Group, makes up the floor of the intrusion (Cheney supracrustal rocks, such as those found in the Protero- and Twist 1991). The Rooiberg Group consists of a lower zoic Leptite Belt near Stockholm (e.g., Loberg 1980). sequence of magnesian lavas and a petrologically distinct The Glossary of Geology (4th) states that the term is now upper sequence of ferroan lavas (Twist and Harmer 1987; obsolete. Furthermore, where it exists in the roof of the Mathez et al. 2013). In ascending stratigraphic order, these Bushveld, leptite has come to mean different things to dif- lavas have been subdivided into the Dullstroom, Damwal, ferent workers and does not actually describe the rock. Kwaggasnek, and Schrikkloof formations (SACS 1980). For these reasons, we dispense with the term entirely Some of the ferroan lavas may have been generated by and simply describe the rocks by listing the combination fractional crystallization of the Bushveld mafic magmas of specific lithologies present. We define hornfels as a (VanTongeren et al. 2010, see below). fine- to very fine-grained, thermally metamorphosed rock Understanding the roof of the Bushveld Complex is with classic granoblastic texture. It is important to note complicated by the fact that the younger Lebowa Granite that we use the term hornfels throughout the manuscript Suite granites intruded at various levels within the roof to describe the rock texture, with no implication for its and lava sequences (Hill et al. 1996). The petrogenesis of protolith. the granites and their relationship to the Bushveld Com- Here we divide the roof as it is exposed in the east- plex and Rooiberg lavas have been debated (e.g., Hill et al. ern Bushveld into three segments, each with a different 1996; Schweitzer et al. 1997), but due to the younger age of character. The roof of the most volumetrically significant the granites, their origin will not be considered here. ‘Central Segment’ (Fig. 1) is dominated by the distinctive The Bushveld Complex is composed of the principle hornfels microgranite rock noted above. This rock type + eastern, western, and northern limbs along with a num- is well exposed in the Droogehoek and Masekete Sections, ber of outliers. From east to west, the Bushveld extends described below. In the ‘Northern Segment,’ outcrops of the over 350 km, and the eastern limb alone crops out for Bushveld Upper Zone are limited and the map patterns and more than 150 km north–south. Due to the enormous size seismic profiles imply significant structural complexity. of the Bushveld, the roof of the intrusion is neither eve- In this region the roof is dominated by metapelites and a rywhere laterally continuous nor exposed. In some places sedimentary hornfels that is significantly different than the the mafic rocks are capped by quartzite and/or metapelite, hornfels of the Central Segment. The roof lithologies char- and in others by a complicated lithology of leptite (see acteristic of the Northern Segment are well exposed in the below), hornfels, microgranite, granophyre, felsite, and Mphanama Section, also described below (Fig. 1). Finally, granite. least well exposed is the ‘Southern Segment,’ where Caw- This paper describes the large-scale changes in contact thorn (2013) asserted that the Bushveld cumulate rocks are relationships as they are observed between the rocks of the in immediately contact with the Rooiberg Group volcanics. Upper Zone of the Bushveld Complex and its immediate However, our observations suggest that the roof in this area roof in four locations from north to south in the eastern is similar to that in the Central Segment. limb. The present report builds on the important works by Groeneveld (1970), von Gruenewaldt (1968, 1972), Lom- Central Segment baard (1949), Molyneux (1970, 1974, 2008), and Walraven (1987). The goal of our study is to provide a systematic The Central Segment of the Bushveld roof extends from look at how the relationship between the final Bushveld Magnet Heights south to the Tauteshoogte area (Fig. 1, Complex magmas and the roof zone evolves when there are 24°50′S to 25°19′S, 24°50′E). The area from about ≈ different lithologies present in the overlying country rock. 25°05′S to 25°19′S (near Tauteshoogte and Roossenekal) was mapped by Von Gruenewaldt (1972) and the area north of that to Magnet Heights (25°19′S–24°50′S) by Moly- Field relations neux (1974). In this region, while there is variability at the small scale, the same general lithologies are observed Throughout much of the eastern limb of the Bushveld, the for ~60 km along strike. Along this entire strike length, immediate roof of the intrusion consists of a distinctive, the cumulate rocks of the Bushveld dip gently westward 100- to 300-m-thick rock layer composed of a complex beneath the granites exposed on the Nebo Plateau, and the mixture of hornfels and microgranite. The term ‘leptite’ top of the Bushveld and the immediately overlying roof has been used by various authors (e.g., Von Gruenewaldt rocks are locally well exposed in incised stream beds along 1968, 1972; Molyneux 2008) to describe either the rock and below the steep eastern-facing escarpment of the Nebo or the hornfels or both. Leptite originated as a nineteenth- Plateau. Two such streams are Droogehoek and Masekete century Swedish name for fine-grained, recrystallized, (Fig. 1). 1 3 Contrib Mineral Petrol (2015) 170:56 Page 3 of 17 56 Fig. 1 General map of the Eastern Limb of the Bushveld Complex, adapted from Molyneux (2008) showing the geographic locations of all the towns, sections, and segments mentioned in the text as well as geologic context Droogehoek Section of about a third of the section and perhaps 40 percent of the rest (Fig. 2). It is accessible with permission of the local The Droogehoek Section (24° 51.767′S, 29° 54.384′E) fol- community, nearest to the town of Ga-Maepa. The precise lows a streambed that provides nearly 100 percent exposure contact between the cumulate diorites of the Upper Zone 1 3 56 Page 4 of 17 Contrib Mineral Petrol (2015) 170:56 Fig. 2 Field relationships in the Droogehoek Section of the Central imentary hornfels and the larger hornfels blocks with the microgran- Segment.

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