sign in sign up
<<
Home , Bushveld Igneous Complex, Hornfels, Igneous differentiation, Magaliesberg, R555 (South Africa)

Contrib Petrol (2015) 170:56 DOI 10.1007/s00410-015-1211-y

ORIGINAL PAPER

On the relationship between the Complex and its 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 Keywords Rooiberg Group · Bushveld Complex · into the 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 in modulating · 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 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 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 intrude at a Dufek Intrusion of Antarctica, and Bushveld Complex of deeper level in the north and become shallower to the . The Bushveld Complex is part of the massive south. Bushveld Igneous Province that also includes voluminous ferroan 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 and of the underlying Group 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 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- -, 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 (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 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 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 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 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 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 . 25°05′S to 25°19′S (near Tauteshoogte and ) 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. Large photograph shows the immediate roof zone above ite. Arrow in bottom left of large photograph indicates stratigraphic the contact with the Upper Zone. Small photos taken of the rocks up observed immediately above the contact including the small metased- and the overlying microgranite–hornfels roof is usually not 4. A fine-grained granophyre or cutting the horn- exposed because it is covered by stream gravel, but it can fels–microgranite. This is presumably the Stavoren be placed within about 10 m in the stream cut. The imme- Granophyre, which was defined by Walraven (1987) diate roof in this section is characterized by three distinct and is part of the Rashoop Granophyre Suite. To the lithologies: south in the Tauteshoogte area the Stavoren Grano- phyre exists as a locally discordant sheet up to 650 m 1. Enormous hornfels blocks meters to tens of meters thick. The maps of von Gruenewaldt (1972) and Moly- across. Most if not all of the blocks are angular on the neux (2008) show that the granophyre is present as thin meter scale, homogeneous, massive, and lithologically (<10 m) sill- or dike-like bodies intruded into the horn- identical to each other (see below). fels–microgranite along the entire Central Segment 2. Numerous small hornfels in the centimeter (e.g., von Gruenewaldt 1972). to tens of centimeter size range. Some xenoliths are clearly fragments of once larger ones, and many of the The contacts between the hornfels and microgranite small xenoliths are thinly laminated, some with appar- vary from sharp to gradational and may be straight, irregu- ent cross-bedding, suggesting sedimentary protoliths. lar, or sinuous. Some of the smaller xenoliths are partially 3. A matrix lithology of microgranite/granodiorite exists digested within the microgranite, so it is clear that the latter around the hornfels blocks (Fig. 2). Both the large and contains, at least locally, a hornfels component. In places small hornfels blocks are randomly distributed such where the hornfels and microgranite lithologies are inti- that the matrix microgranite is typically localized in mately mixed, the microgranite characteristically displays irregularly shaped masses on all scales from tens of considerable heterogeneity in both mode (see below) and centimeters to tens of meters across. grain size. In some cases, grain size in the microgranite

1 3 Contrib Mineral Petrol (2015) 170:56 Page 5 of 17 56

(a) (b)

Fig. 3 Field relationships in the Masekete Section of the Central granite and granophyre lithologies are dominant. Large photograph Segment. a Pristine outcrop exposure of hornfels–microgranite lithol- shows the outcrop exposure near the base of the Nebo Plateau scarp, ogy in the Masekete River immediately above the contact with the where both granite and granophyre are found in association with one Upper Zone. Hornfels appears as a very fine-grained light gray blocks another. Small photographs highlight the difference in grain size and with both rounded and sharp contacts to the microgranite. b Fur- contact relationships between the granite and granophyre. Arrow in ther up the Masekete River away from the Upper Zone contact, the bottom left of large photograph indicates stratigraphic up varies continuously from millimeter to centimeter size hornfels surrounded by a coarse-grained matrix of micro- away from the hornfels contacts. granite (Fig. 3), with the exception that the small metasedi- The excellent exposure provided the opportunity to mentary hornfels xenoliths are notably absent. determine the relative proportions of hornfels and micro- Further upsection along the Masekete River, the horn- granite and how those proportions change across strike by fels–microgranite roof rocks are overlain by a granophyre pacing along two segmented traverses in reasonably flat and fine-grained granite. Where granophyre–granite con- areas through the entire unit. Based on this exercise, the tacts are clearly exposed, the granite displays an intrusive rocks in the Droogehoek Section are estimated to comprise relationship with the granophyre but without chilled mar- 38 % microgranite and 62 % hornfels, with no systematic gins (Fig. 3), implying that the granophyre was hot when change in the proportion of lithologies away from the con- the granite was emplaced. Therefore, this granite may sim- tact with the Bushveld cumulate rocks. ply be another phase of the magma that gave rise to the granophyre. Masekete Section Mineralogy and of the Central Segment The Masekete Section (25° 11.979′S, 29° 46.398′E) is lithologies approximately 20 km south of the Droogehoek Section (Fig. 1). It is accessible along the Masekete River with Hornfels blocks permission of the local game lodge on the road R555. The immediate roof of the Bushveld Complex in the Masekete The rocks that make up the hornfels blocks in the immedi- Section is similar to that observed in the Droogehoek Sec- ate roof zone of the Central Segment are made of equant, tion, with large, homogeneous blocks of fine-grained rounded grains of quartz (diameters 100–250 µm), = 1 3 56 Page 6 of 17 Contrib Mineral Petrol (2015) 170:56

Fig. 4 Photomicrographs in plane and cross-polarized light of sam- same in all photomicrographs. *Geochemical analyses reported in ple B10-044, representative of a typical hornfels–microgranite lithol- Supplementary Tables 1, 2 for B10-044 are from the microgranite ogy. a Hornfels, b contact between hornfels and microgranite lithol- portion of the sample only ogy, c microgranite. Field of view and scale bar of 500 um are the hedenbergite, and in some samples variable but minor given sample is typically 10 An units. Average plagio- ± amounts of fayalite embedded in a matrix composed of clase compositions for the hornfels samples range from an equidimensional mosaic of and orthoclase An19–24; K-feldspar compositions range from Or79–86. In grains (400–600 µm). The texture, as noted, is classic the Masekete Section, the average plagioclase and ortho- granoblastic (Fig. 4). The rock also contains sparse plagio- clase of the hornfels samples are plagioclase An11–14 and clase up to 2 mm long. These display nor- orthoclase Or80–87. The plagioclase compositions measured mal zoning near the margins, which encase abundant quartz in the hornfels blocks are similar to those of grains. No other porphyroblasts are present. Typical modes from Damwal (An17–28; Buchanan et al. 2002) and the are estimated in thin section to be 40–50 % feldspar, 50 % Dullstroom high-magnesium felsite lavas (An11; Buchanan quartz, 8 % hedenbergite fayalite, and 2 % et al. 1999) (no data have been published on plagioclase + plus exsolved ilmenite. In two of three samples analyzed, compositions from the Dullstroom low-magne- the fayalite is nearly pure (>Fa98), so the assemblage and sium felsites). mineral compositions fix the fO2 at essentially the fayalite– magnetite–quartz (FMQ) buffer. The fayalite coexists with Microgranite hedenbergite of approximate composition En3Fs52Wo45. The third analyzed sample contains Fa93–94, which coexists The microgranite in the immediate roof of the Central Seg- with hedenbergite of En11Fs46Wo43. and ment consists of equant quartz grains (0.5–1.0 mm), equant are encountered only rarely and exist almost exclusively in to tabular, subhedral plagioclase grains ( 2 mm, usually ≤ rocks displaying mild sericitic alteration. Zircon and apatite partially sericitized), and perthitic orthoclase that is intersti- (30–60 µm) are the most abundant trace phases. A complete tial to both. The mafic phase is hornblende (ferropargasite), dataset of mineral compositions is provided in Supplemen- which is interstitial to plagioclase and quartz and may exist tary Table 1. as large, discrete grains up to 5 mm across poikilitically Plagioclase compositions in the hornfels blocks are enclosing those . Some rocks also contain traces of highly variable. Measured compositions from three ana- hedenbergite and fayalite. The textures suggest that these lyzed hornfels samples in the Droogehoek Section range minerals were originally more abundant but mostly altered from An1Ab98 to An39Ab59, though the range in any to . Biotite is present in minor amounts partially

1 3 Contrib Mineral Petrol (2015) 170:56 Page 7 of 17 56 replacing hornblende, and apatite and zircon are common (a) trace phases. The overall texture varies from poorly devel- oped to locally allotriomorphic granular (Fig. 4). Most of the microgranites are monzonitic, consisting of 20–30 % quartz and 8–10 % hornblende biotite (bulk + SiO 66–72 %), but some are distinctly more mafic, 2 = containing 15 % quartz and 20 % mafic minerals (bulk ≈ SiO 62–63 %). The more mafic samples are the ones 2 = that contain fayalite and hedenbergite. The microgranite lacks layering, and the mode, texture, and variability of each clearly distinguish it from the underlying Bushveld layered cumulate rocks. Average plagioclase compositions in the microgranites (b) are An5–25 in the Droogehoek Section and An14–17 in the Maskete Section. The large plagioclase compositional dif- ferences are among samples, but within individual samples the compositions are relatively homogenous. There is no correlation between plagioclase An content and distance away from the contact with the Bushveld Upper Zone. In nearly all cases where pairs of hornfels and adjacent microgranite were analyzed, the average plagioclase in the microgranite was 3–5 An units higher than in the horn- fels, implying a higher crystallization temperature or more likely a higher activity in the microgranite. Aside (c) from texture, the outstanding difference between the horn- fels and microgranite assemblages is that the mafic mineral in latter is almost exclusively hornblende, whereas hornfels contains hedenbergite and fayalite but little or no horn- blende. Therefore, the microgranite must have crystallized under a substantially greater aH2O.

Rock compositions

Bulk compositions of the Central Segment hornfels blocks and microgranite (Supplementary Table 2) are compared with those of the Rooiberg Group lavas and Stavoren granophyres in the eastern Bushveld in Fig. 5. Due to their small size and clear sedimentary nature, no bulk rock com- Fig. 5 Whole-rock major element geochemistry for the hornfels, positions were obtained for the small hornfels xenoliths microgranite, and Rashoop Granophyre for comparison with the Roo- present in the Droogehoek Section. iberg Group lavas. ASI is the Aluminum Saturation Index, MALI is Neither the hornfels blocks nor the microgranite has the Modified Alkali-Like Index, and Fe Index is the FeOtotal/FeOtotal MgO, according to Frost and Frost (2008). The hornfels, micro- exact compositional analogues among the Rooiberg Group granite,+ and granophyre all have compositions overlapping with the lavas, though they are remarkably similar in overall geo- upper Rooiberg Group lavas, the Damwal, Kwaggasnek, and Schrikk- chemical characteristics to some of the individual lava loof formations. Rooiberg lava and granophyre compositions are from formations. On average, the hornfels blocks have similar the compilation of Mathez et al. (2013) including data from Sch- weitzer (1998), Buchanan et al. (1999, 2002), Walraven (1987) SiO2 contents to the Kwaggasnek (~72 wt% SiO2) and are more silicic than either the Damwal or Dullstroom

LMF lavas (Fig. 5). The FeOtotal content of the hornfels blocks is also similar to that of the average Kwaggasnek lavas is typically between 0.3 and 1.00 %, leading to higher lava (ca. 5 wt% FeO) and slightly lower than those of the Fe/Mg ratios in the hornfels compared to the Dullstroom Damwal (ca. 6–7 wt%) or Dullstroom LMF lavas (ca. and Damwal (high Fe index, Fig. 5a). 5–7 wt%). The MgO content of the hornfels is typically Despite the offset to higher SiO2 and Fe/Mg, both of <0.10 %, whereas that of the Damwal and Dullstroom LMF which may be altered during thermal or hydrous alteration,

1 3 56 Page 8 of 17 Contrib Mineral Petrol (2015) 170:56 the hornfels blocks bear striking resemblance in most other geochemical parameters to those of the Damwal and Dull- stroom LMF lavas. The modified alkali (MALI) and aluminum saturation indices (ASI) of the hornfels blocks are strongly indicative of a volcanic protolith. Among the Rooiberg Group lavas present within the region, the horn- fels are most similar to the Damwal/Dullstroom LMF lavas (Fig. 5b, c) and are distinct from those in the Kwaggasnek and/or Schrikkloof formations. Fluid-immobile trace element compositions in the hornfels suggest that they are most closely related to the Damwal lavas. The average Zr concentration of the horn- fels samples is 386 11 ppm (1σ). This is nearly identical ± to the average of 376 ppm for the Damwal lava and dis- tinct from the average Kwaggasnek (475 ppm) or Schikk- loof (601 ppm) measured by Schweitzer et al. (1997). Rb, Sr, Y, and REE contents also suggest similarity between the average hornfels composition and the Damwal lavas (Fig. 6). There appears to be two distinct populations of micro- Fig. 6 Whole-rock trace element geochemistry for the hornfels, granites in the Droogehoek Section. The majority of micro- microgranite, and granophyre for comparison with the Rooiberg Group lavas. The granophyre has a trace element composition very granite samples have high SiO2 (66–70 wt%) similar to the hornfels and lavas, but two have distinctly lower SiO similar to that of the upper Rooiberg Group lavas and the Kwaggas- 2 nek and Schrikkloof formations. The hornfels and microgranite have contents (62 wt%). The latter are also relatively FeO-rich compositions distinct from the granophyre and upper Rooiberg Group (10 wt% compared to 5–7 wt% in the high SiO2 samples) lavas. Aside from a single sample with high Zr/Y, the hornfels blocks and possess anomalously high Zr and Ba concentrations have very similar trace element ratios to those found in the Damwal and high Zr/Y. formation lavas. The microgranite, however, has a much higher trace element compositional variability In contrast to the hornfels, the Si-rich microgranites have similar bulk rock SiO2 contents to the Dullstroom LMF and Damwal lavas (Fig. 5). The bulk rock Fe/Mg ratio (Fe index) of the microgranites, however, is higher metapelite and metasedimentary hornfels, which Molyneux than the lavas, similar to that of the hornfels (Fig. 5a). Both (2008) mapped as ‘leptite-after-quartzite’ (Fig. 1). Good the hornfels and microgranite are distinctly metaluminous exposures of the Northern Section lithologies exist near the and have MALI and ASI indices that overlap with the range town of Mphanama (24°35.532′S, 29°48.751′E; Figs. 1, 7a, of values observed in the Dullstroom LMF and Damwal b). lavas (Fig. 5b, c). A unit mapped as hornfels in this region consists of ther- Trace element concentrations of the microgranites are mally metamorphosed mudstones and is thus radically dif- nearly identical to coexisting hornfels, except in the case of ferent from the hornfels of the Central Segment. Erosional Sr, Ba, and Zr, which are slightly higher on average in the surfaces on the mudstone lithologies preserve evidence microgranites relative to the adjacent hornfels (Fig. 6). of rip-up clasts and clear sedimentary structures, though The microgranites have a distinctly different composi- rock interiors are fused (Fig. 7a). The most likely protolith tion from the Stavoren granophyre. While the microgranite to this lithology is the Vermont Formation of the Pretoria has a similarly high Fe index to the granophyres, it is much Group metasediments (P. Erikkson, personal comment). lower in SiO2 and plots far from the granophyre field on the A second unit mapped as ‘leptite-after-quartzite’ in the ASI and MALI indices (Fig. 5). Northern Section is dominated by thermally metamor- phosed and contains large regions of metasedi- Northern Segment: Mphanama Section ments with structures indicative of previously unconsoli- dated fine-grained material similar to the mudstone protolith Field relations of the hornfels unit (Fig. 7b). The most likely protolith of this lithology is the Lakenvalei formation of the Pretoria In the northern portion of the eastern Bushveld, from Group metasediments (P. Eriksson, personal comment). the towns of Jane Furse to Mohalatse (24°46′S, 29°54′E The ubiquitous microgranite that characterizes the Cen- to 24°33′S, 29°46′E), the roof rocks are dominated by tral Segment is nowhere present in the Northern Section.

1 3 Contrib Mineral Petrol (2015) 170:56 Page 9 of 17 56

(a) (b)

Fig. 7 Field relationships in the Mphanama Section of the North- tographs show typical rocks of the metasedimentary hornfels. b Large ern Segment. a Large photograph shows the regional relationships outcrop of unit mapped as ‘leptite-after-quartzite’ in the Mphanama between outcrops of quartzite, Upper Zone, and units mapped by section. Rocks are thermally metamorphosed mudstones and - Molyneux (2008) as leptite-after-quartzite and hornfels. Smaller pho- stones and preserve their original sedimentary features

There is no evidence in either the metasedimentary horn- intrusion (Johnson et al. 2004; Uken and Watkeys 1997; fels unit or the ‘leptite-after-quartzite’ unit of any partial Gerya et al. 2003) that penetrated partially molten Bush- melting during thermal . rocks. However, the rocks observed along the flanks The patterns on Molyneux’s (2008) map reveal that of the dome are similar to those observed in the roof expo- the Upper Zone and roof in this region do not display the sures of the Mphanama section only ~4 km to the south- gentle westward dips that are typical of most of the rest of west, where floor diapirism is not inferred, suggesting that the eastern Bushveld. Rather, there exists a NW–SE trend- the Malopo Dome is normal Bushveld Upper Zone with a ing fold axis centered approximately 2 km to the east of resistant quartzite cap. More work is required to understand the Upper Zone–roof contact (Molyneux 2008). The fold the significant structural complexity and outcrop patterns is also observed in the seismic reflection profile of Odgers of this region to understand the nature of the Malopo dome. and du Pleiss (1993) as a broad anticlinal structure within the Transvaal floor and Bushveld rocks, with the fold just Mineralogy and geochemistry of the Northern Segment to the east of the Malopo Dome and Wonderkop Fault. The lithologies fold results in shallow dips to the east near the Bushveld– roof contact, which limits the stratigraphic exposure of the Hornfels Upper Zone cumulates (Molyneux 2008). Where the Upper Zone is observed, only the uppermost few hundred meters In the Northern Segment, the unit mapped as ‘hornfels’ of stratigraphy from Magnetite Seam 17 (corresponding to on the map of Molyneux (2008) makes up the immediate ~375 m below the roof in the Magnet Heights type section of roof of the Upper Zone. From the field descriptions of the Molyneux 1970) to the roof are typically exposed. The expo- Mphanama Section above, outcrops are clearly dominated sures in this region seem to be limited to areas capped by a by thermally metamorphosed mudstones. Consistent with resistant layer of quartzite, such that the hills with outcrops this, in thin section the rocks are very fine-grained metape- display Upper Zone in contact with either hornfels or quartz- lites, with abundant orthoclase and only minor (<20 %) ite, overlain by massive quartzite (Fig. 1; Molyneux 2008). quartz. Plagioclase is largely absent. Nearly pure magnetite

The Malopo Dome, also known in the literature as the (Fe3O4) makes up an additional 10 % of the mode. Sam- Driekop or Phepane Dome, has been interpreted to rep- ples have abundant small planar lathes of fibrolite and large resent a metasedimentary diapir from the floor of the elongated grains (0.5 mm) of , which are also

1 3 56 Page 10 of 17 Contrib Mineral Petrol (2015) 170:56 present as inclusions in patches of biotite. Retrogressed with no hornblende and likely correspond to the uppermost is present in some samples. cumulates of the Upper Zone. Approximately two-thirds of the way up the hill, the rocks are amphibole-bearing dior- Leptite‑after‑quartzite ites, termed hornblende quartz by Cawthorn (2013). Further up in the section, the rocks become signifi- The unit stratigraphically above the hornfels in this region cantly more felsic, with quartz, orthoclase, and plagioclase is mapped as ‘leptite-after-quartzite,’ although quartzite comprising >70 % of the total mineralogy. The uppermost is absent where the unit crops out in the Mphanama Sec- lithologies resemble the granophyres observed at the top of tion. Thin sections are dominated by large euhedral grains the Masekete Section in the Central Segment. On a second of quartz and orthoclase (Or98) with minor zircon, rutile, ridge of the hill slightly to the south, a minor amount of and apatite. Plagioclase feldspar is notably absent, except thermally metamorphosed felsic rock forms a small cap at as exsolutions from the orthoclase (An0.2Ab99). Biotite and the top and is laterally continuous with those further to the muscovite were found in a very minor area of a single thin NNW along the Bothasburg Plateau, according to Moly- section. In contrast to the hornfels unit, no sillimanite or neux (2008). The rocks exposed along the Stoffberg section fibrolite was found in any of the samples investigated. Dark were mapped as ‘leptite’ by Molyneux (2008); however, nodules observed in hand sample were large (fist-sized they lack the characteristic hornfels–microgranite lith- or slightly smaller) regions with poikilitically ologies typical of the ‘leptite’ unit mapped in the Central enclosing the quartz and orthoclase. The outcrop (Fig. 7b) Segment. On the basis of trace elements, Cawthorn (2013) and phase assemblage suggest a distinctly sedimentary proposed that this felsite corresponds to the Damwal For- protolith. mation of the Rooiberg Group lavas. A flat outcrop near the base of the hill reveals several Upper Zone at contact features of the lithologies that are not possible to observe in the smaller outcrops elsewhere in the Stoffberg Section The diorite cumulates of the uppermost Upper Zone of the (Fig. 8). The rocks possess a chaotic fabric of leucocratic

Bushveld are dominated by plagioclase (An40), clinopyrox- and melanocratic layers. There is a single small metasedi- ene (En3Fs44Wo52), and (Fa99–95). Quartz is absent mentary preserved (Fig. 8) within the medium- and orthoclase is present only as antiperthite. Large euhe- grained fabric. There is no clear contact relationship dral grains of apatite are present mainly as inclusions in oli- observed between the Bushveld cumulate rocks and the vine and pyroxene. It is important to note that Fe–Ti oxides roof rocks in this section. are rare (<1 %) in the uppermost samples of the Upper Zone in the Northern Section and where present are restricted to Mineralogy and geochemistry of the Southern Segment ilmenite, not the magnetite-ulvospinel common elsewhere lithologies in the Upper Zone. Grain sizes are similar to those through- out the Upper Zone and there is no evidence of a chilled Upper Zone near contact margin or enhanced rate of crystallization. The evolution of the uppermost Upper Zone in the Stoff- Southeastern Segment: Stoffberg Section berg Section of the Southern Segment is difficult to deter- mine given the limited exposure. An Upper Zone cumulate Field relations sample interpreted to be the stratigraphically lowest out-

crop exposed in the section contains plagioclase (An40), South of the Nebo Plateau, near the town of Stoffberg (from clinopyroxene (Mg#11), olivine (Fa95), minor (<1 %) 25°16.541′S, 30° 2.674′E to 25° 44.970′S, 29° 55.773′E), Fe–Ti oxides, and no orthoclase or quartz. Grain sizes of the Lower and Critical Zones of the Bushveld Complex the are 1–3 mm and ~0.5 mm for the oxides are absent such that the cumulates of the Main Zone rest and mafic phases. Approximately 30 m stratigraphically directly on the Dullstroom volcanic rocks (Fig. 1). above the rocks also have plagioclase (An40), clinopyrox- In general exposure is very limited throughout the ene (Mg# 20), and abundant magnetite. Olivine is notably area, but a few isolated outcrops exist in a small hill (S25° absent from the assemblage, and magnetite grains are typi- 26.363, E029° 49.158) on a farm accessible by a private dirt cally rimmed by small quartz grains. Apatite and orthoclase road from road with permission from the land own- (Or74) are present as minor phases. ers. This section was previously described by Hall (1932) These rocks likely represent the uppermost Upper Zone and Groeneveld (1970), and more recently reinterpreted by in this region. They possess cumulate textures in thin sec- Cawthorn (2013). Several major features can be identified. tion as well as similar mineral compositions and grain sizes Rocks near the base of the hill are medium-grained diorites to the cumulate rocks at the roof contact in the Central and

1 3 Contrib Mineral Petrol (2015) 170:56 Page 11 of 17 56

Fig. 8 Field relationships in the Stoffberg Section of the South- ern Segment. Large photograph demonstrates the generally poor outcrop exposure in this section and shows the inferred regional relationships between the rocks of the Upper Zone, granophyre, and felsite. Smaller photographs show the complicated layering found in diorite near the base of the hill. It is very difficult to determine exact petrogenetic relationships between the units in this region in the field. Arrow in bottom left of large photo- graph indicates stratigraphic up

Northern Segments, and they resemble no other roof rocks the south. The felsite has a slightly smaller grain size than exposed elsewhere. the ‘’ and is significantly less altered. The felsite sample contains the same three feldspar populations with

Monzonite An25Ab73Or2, An2Ab26Or72, and An1Ab3Or96 as the monzo- nite as well as abundant quartz and magnetite, and a trace A highly altered sample obtained from a location described of clinopyroxene. as ‘monzonite’ by Cawthorn (2013) has significantly smaller grain size from the diorite cumulates found below. Granophyre The rocks in this area contain, on average, 10–15 % quartz, 5–10 % orthoclase, 40 % plagioclase, 20 % fayalite and A single sample from the Stoffberg Section was identified hedenbergite, 10 % alteration of the pyroxene to horn- in outcrop as granophyre due to its slightly coarse grain blende, and 1–2 % Fe–Ti oxides (Cawthorn 2013). This size and granophyric texture. In thin section, however, the sample has plagioclase An23Ab75Or3 and two distinct sample looks nearly identical to the ‘monzonite’ sample. It feldspar populations, one with An2Ab30Or67 and ortho- contains abundant quartz, orthoclase, and magnetite, and clase with An0Ab3Or97, likely corresponding to subsolidus minor zircon and apatite. As in the monzonite and felsite reequilibration along the peristerite solvus. The monzonite samples, the granophyre contains three feldspar popula- sample analyzed in this study also contains olivine (Fo7.5) tions, An16Ab82Or2, An4Ab95Or1, and An1Ab13Or86. The and clinopyroxene (Mg# 22), abundant magnetite and compositions of the are more evolved than in ilmenite as well as apatite. the monzonite and felsite samples and may have re-equil- ibrated to lower temperatures. Minor pyroxene has Mg# of Felsite 24, and no olivine was found. The similarity in grain size, phase assemblage, and min- The composition of the ‘monzonite’ sample analyzed in eral compositions of the monzonite, felsites, and grano- our study is nearly identical to that of a nearby thermally phyre described here suggest that the ‘monzonite’ from this metamorphosed felsite collected approximately 400 m to region does not correspond to the uppermost Upper Zone,

1 3 56 Page 12 of 17 Contrib Mineral Petrol (2015) 170:56 as proposed by Cawthorn (2013), but is in fact thermally Northern Segment Hornfels metamorphosed and remelted roof material. If this is the case, the cumulates at the top of the Upper Zone are similar The hornfels roof zone in the Northern Segment is domi- in composition and mineralogy to those present at the con- nated by thermally metamorphosed mudstones and sand- tact in the Central and Northern Segments. However, expo- stones. There are no blocks of felsic hornfels anywhere sures are isolated and so severely limited in this region that present in the outcrops exposed in the Mphanama Section. it remains unclear how the various lithologies are petroge- There does not appear to be any evidence for partial melt- netically related. ing of the sedimentary lithologies during thermal meta- morphism. In fact, original sedimentary structures, such as cross-bedding and rip-up clasts, remain preserved, even in Discussion areas stratigraphically close to the contact with the Bush- veld Complex (Fig. 7). For these reasons, we conclude that Due to the excellent exposure and large geographic range, the immediate roof of the Bushveld Complex in the North- the Central Segment of the eastern Bushveld has received ern Section is not petrogenetically related to the hornfels– the most attention in previous studies of the Bushveld Com- microgranite lithologies of the Central Segment, nor related plex roof, and the lithologies present there have become to the Rooiberg Group lavas or Stavoren granophyres in the synonymous with ‘the roof.’ As described above, however, region. the roof is laterally variable, with the Northern Segment dominated by metapelite; the Central Segment dominated Felsic Hornfels by granophyre, granite, and felsite; and the Southern Seg- ment dominated by felsite (Fig. 1, 9). Von Gruenewaldt (1972) mapped the immediate roof rocks From the above descriptions two outstanding questions in the Central Segment in the Tauteshoogte and Paardekop remain: areas. There he reported that it consists of blocks of ‘lep- tite’ (i.e., hornfels) of variable size chaotically mixed with 1. How are the various lithological units petrogenetically and veined by ‘microgranite’ and locally by ‘micrographic related to one another? In particular, what is the origin felsite,’ identical to the rocks observed in the Droogehoek of the microgranite/granodiorite present in the immedi- and Masekete Sections. Uncertain about the origins of ate roof of the intrusion, and how does it relate to the these rocks, he identified three possibilities, namely that other rocks in its immediate vicinity? (a) the hornfels protolith is Rooiberg lava and the micro- 2. How were the mafic magmas of the RLS emplaced granite its molten equivalent, (b) the hornfels protolith is within the shallow crust? Pretoria Group and the microgranite its molten

Fig. 9 Illustration of the change in lithology present in the immediate roof zone of the Bushveld Complex Upper Zone from N to S in the east- ern limb. Thicknesses of units is not to scale, as it is not possible to pinpoint unit boundaries in many of the sections

1 3 Contrib Mineral Petrol (2015) 170:56 Page 13 of 17 56 equivalent, or (c) the hornfels protolith includes both Roo- (1972) that the microgranite is partially melted Dullstroom iberg lava and sedimentary rocks, and the microgranite LMF or Damwal lava. Walraven (1987) modeled the ther- mainly represents a residual melt from fractional crystalli- mal effect of the emplacement of the Bushveld mafic zation of the Bushveld mafic magmas locally mixed with magma on the overlying roof rocks and concluded that the melt derived by partial digestion of the hornfels. The first amount of heat available from the crystallization of the of these was von Gruenewaldt’s (1968, 1972) preferred Bushveld was capable of partially melting up to 1000 m choice. of overlying material. While this is likely a maximum On the basis of bulk rock major and trace element com- estimate, given the necessary assumptions regarding the position (Figs. 5, 6), mineral compositions, and field rela- emplacement depth, emplacement rate, and lithology, there tions, we conclude that the hornfels blocks in the immediate is no doubt that mafic magmas of the Bushveld Complex roof of the Central Segment are thermally metamorphosed were capable of partially melting significant volumes of equivalents to the Dullstroom LMF and/or Damwal lavas. Damwal and/or Dullstroom lava to form the microgranite. Due to the similarity in composition between the Dull- The lack of microgranite observed in contact with the stroom LMF and the Damwal lavas, it is not possible to thermally metamorphosed felsite in the Southern Segment distinguish between them. The small sedimentary hornfels is due to lower peak metamorphic temperatures in the clasts that are present within the roof zone in the Central Southern Segment, where the RLS is considerably thinner Segment are consistent with the known occurrences of and was likely emplaced at a shallower stratigraphic level minor intercalated sediments within the Rooiberg Group as compared with the Central and Northern Segments (see (e.g., Twist 1985). Alternatively, the presence of the sedi- below). Lundgaard et al. (2006) documented a larger por- mentary hornfels in the Droogehoek Section, and absence tion of trapped melt in the cumulate of the Main Zone near in the Masekete Section, could be due to the close proxim- the Southern Segment, which they attributed to faster cool- ity of the Droogehoek Section to the predominantly meta- ing rates in this region. The inference of faster cooling rates sedimentary roof of the Northern Section. in this segment supports our hypothesis of lower peak met- amorphic temperatures, as the heat from the emplacement Microgranite of the intrusion must has been more readily dissipated. Based on trace element abundances, we can rule out pos- All samples of microgranite analyzed in this study have sibility (c) of von Gruenewaldt (1972) that the microgran- trace element patterns similar to those of the hornfels ite represents the residual liquid of the RLS. VanTongeren blocks. We conclude that the microgranite present in the and Mathez (2012) calculated the rare earth element (REE) hornfels–microgranite lithology of the Central Segment concentration of the residual melt in equilibrium with apa- was formed by of the adjacent hornfels due tites present at the top of the RLS. According to their esti- to thermal metamorphism with the underlying RLS. This mate, the REE contents of the microgranite are too low to conclusion supports possibility (a) of von Gruenewaldt represent the residual melt from the RLS (Fig. 10). Instead,

Fig. 10 Whole-rock rare earth element concentrations of the micro- compositions in equilibrium with the top of the Bushveld Complex. granite, granophyre, and Rooiberg Group lavas for comparison with The microgranite lithology found in the immediate roof zone is not a the estimated residual liquid composition from the Bushveld Com- match for the residual liquid from the Bushveld Complex. The grano- plex mafic magmas. Residual liquid compositions for the Bushveld phyre and/or Kwaggasnek and Schrikkloof lavas are likely to repre- Complex calculated from apatite–liquid trace element partitioning sent the residual liquid erupted from the Bushveld (e.g., VanTongeren by VanTongeren and Mathez (2012). Gray bar indicates the range of et al. 2010; VanTongeren and Mathez 2012; Mathez et al. 2013)

1 3 56 Page 14 of 17 Contrib Mineral Petrol (2015) 170:56 the Rashoop granophyre has nearly identical REE concen- as distinct from the Stavoren Granophyre and named it tration to that predicted for the RLS residual liquid and is the Diepkloof Granophyre for the farm on which it is well more likely to represent the missing magma (Fig. 10). exposed. The Diepkloof Granophyre appears on both Lom- baard’s (1949, Plate XIX) and Walraven’s (1987, p. 35) Granophyre maps. These are the only areas known to us where the field evidence alone indicates that a granophyre may have been In the southern part of the Central Segment west of Rooss- produced by differentiation of RLS magmas. enekal, the Stavoren Granophyre comprises a several Thin dikes and sills of granophyre are found associated hundred-meter-thick sheet between underlying hornfels– with the distinctive hornfels–microgranite unit within the microgranite and overlying Rooiberg lavas, with which it roof rocks throughout the eastern Bushveld (e.g., Moly- displays an intrusive relationship (von Gruenewaldt 1972; neux 2008). In the Southern Segment along the edge of Walraven 1987). Von Gruenewaldt (1972) interpreted the the Bothasberg plateau, for example, the immediate roof of granophyre to represent Rooiberg completely the RLS is composed of layers of both rocks, which under- remelted by the RLS magmas, and Walraven (1987) inter- lie the Rooiberg lavas (Groeneveld 1970). In the Central preted it to represent the intrusive phase of those rhyolites. Segment, the various granophyre bodies display consider- Our results suggest that it is the microgranite that repre- able variability in the coarseness and development of their sents re-melted Rooiberg felsites (specifically Damwal or characteristic micrographic texture (e.g., von Gruenewaldt Dullstroom formations) and that the compositional differ- 1968, 1972). The immediate roof rocks also host similarly ence between the Stavoren Granophyre and the microgran- small bodies of relatively fine-grained granite, as observed ite suggests that the Stavoren Granophyre does not repre- in the Masekete Section (Fig. 6). The relationship between sent partially remelted Rooiberg rhyolite. Furthermore, these small granite bodies and the Nebo Granite, which although the major element compositions of the Stavoren forms the massive cliff face along the Nebo Plateau scarp, Granophyre and the Kwaggasnek and Schrikkloof mem- is unknown. In fact, the intimate association and textural bers of the Rooiberg Group rhyolites are nearly identical, variability of these small granophyre and granite bodies Mathez et al. (2013) pointed out that the generally meta- render a clear distinction difficult, and it is conceivable that luminous character of the former distinguishes it from some of these rocks are merely local variants of each other. the rhyolites, which tend to be weakly peraluminous. As It is also not clear how the small granophyre bodies relate noted above, the granophyre also has REE concentrations to the large Stavoren Granophyre sill mapped by von Grue- nearly identical to those expected of the residual liquids of newaldt (1972). the RLS (e.g., VanTongeren and Mathez 2012). This along with its highly ferroan and metaluminous major element signature (e.g., Mathez et al. 2013) makes the Stavoren Emplacement model Granophyre an ideal candidate to represent the residual liq- uid from the crystallization of the Upper and Upper Main Based on the above relationships, we propose a new model Zones of the RLS (e.g., VanTongeren et al. 2010). The gen- for the emplacement of the RLS in the eastern Bush- eral compositional similarity between the Stavoren Grano- veld and its relationship to the roof and floor rocks. It is phyre and Kwaggasnek and Schrikkloof rhyolites, however, clear from the map (e.g., Molyneux 2008) and the above makes it difficult to definitively rule out the possibility that descriptions that the RLS did not simply intrude along a these lavas may have been generated by fractional - single unconformity between the Rooiberg and the Pre- lization of the RLS. toria Group, as previously proposed (Cheney and Twist For about 14 km along the east–west striking RLS con- 1991; Twist and French 1983). Over roughly 50 % of the tact south of the Kruis River (25°23′S, 29°37′E to 25°22′S, eastern Bushveld, the roof and floor rocks are composed 29°44′E) (Fig. 1), the contact is marked by a <200-m-thick of mudstones and quartzites of the Pretoria Group. In the sheet of granophyre. Here Lombaard (1949) documented regions to the north of Magnet Heights, there are no fel- a progressive change over about 150 m stratigraphi- sic meta-volcanic hornfels blocks, microgranite or felsic cally in the rock mode from (45 % plagioclase 18 % granophyre anywhere present (Fig. 9). Immediately to the + hornblende 14 % clinopyroxene 8 % fayalite 4 % south of Magnet Heights, the roof is dominated by felsic + + + quartz 1 % K-feldspar) to (59 % K-feldspar 28 % meta-volcanic hornfels and its molten microgranite equiva- + + quartz 13 % hornblende), illustrating that the granophyre lent (plus minor metasedimentary xenoliths), with a greater + displays a gradational contact with the underlying olivine proportion of meta-volcanic hornfels further to the south diorite cumulate. Based on this, he interpreted the grano- (Fig. 9). This is mirrored in the floor rocks that crop out phyre to represent the differentiated liquid from which the in this region, which grade from Pretoria Group sediments cumulates formed. Walraven (1987) regarded the rock unit in the Central region to Dullstroom volcanics in the south

1 3 Contrib Mineral Petrol (2015) 170:56 Page 15 of 17 56

(Fig. 1). Further to the south, meta-volcanic rocks and their significantly lower density as compared with the mafic recrystallized equivalents dominate the roof of the intru- magma, would not be likely to sink more than two kilo- sion, and metasedimentary rocks are entirely absent. meters vertically in a convecting magma chamber. The We propose that the mafic magmas of the RLS were microgranite raft is everywhere located at or above the emplaced within the Pretoria Group sediments, now Main Zone–Upper Zone boundary, lending further support quartzites, in the north whereas further to the south, where to the hypothesis that the Upper Zone marks a major new both the floor and roof rocks are volcanic, the RLS was influx of magma (e.g., Cawthorn et al. 1991; VanTongeren emplaced within the lower portions of the Rooiberg Group and Mathez 2013), which led to vertical and lateral magma felsites (Dullstroom and Damwal formations) (Fig. 11). chamber growth (Kruger 2005). Our model is consistent The Central Segment just to the south of Magnet Heights with the hypothesis of Clarke et al. (2009) suggesting that represents a transition between these two, in that the floor the mafic magmas propagated as magmatic fingers in a rocks are dominated by sedimentary lithologies, whereas general NW–SE direction away from the Thabazimbi-Mur- the roof rocks are clearly meta-volcanic. chison Lineament. Our proposed emplacement scenario (Fig. 11) is sup- The volcanic rocks that are present stratigraphically ported by the presence of a large xenolith of meta-vol- above the RLS in the Southern Segment are not observed canic, metamorphosed Dullstroom lavas nearly entirely in contact with the uppermost cumulates. Where the felsite transformed to microgranite within the cumulates of the is present in the roof of the Stoffberg Section, it is not par- Main Zone and Upper Zone in the southeastern Bushveld tially remelted and therefore likely did not experience the (Fig. 1). The microgranite/Dullstroom raft is present near same degree of thermal metamorphism, as the hornfels– the Main Zone–Upper Zone boundary and appears to be microgranite pairs seen in the Central Segment. One pos- concordant with the igneous layering. It extends for nearly sibility is that the RLS in this region is significantly thinner, 40 km along strike from ~25° 20′S, 29°53′E to 25° 45′S, and therefore the total thermal perturbation to the country 29°50′E and possibly further to the south beyond the map rocks was not as high as in the Central Segment where sig- of Molyneux (2008). In our model, the microgranite/Dull- nificant volumes of partial melt were created. This is con- stroom volcanic raft represents a portion of the pre-Upper sistent with the lack of Critical or Lower Zone in the South- Zone roof that was intruded by the growing RLS magma ern Segment. Another possibility is that Rooiberg volcanic chamber, rather than a stoped block of material that fell rocks in this region were erupted after significant crystalli- coherently from the roof, as was previously proposed by zation of the RLS (e.g., VanTongeren et al. 2010) and were Cawthorn (2013). A stoped block of felsic material, with not present during the majority of RLS emplacement, as

Fig. 11 New model for the emplacement of the Bushveld Complex Southern Segment, both the floor and roof are defined by meta-vol- layered mafic magmas within the Transvaal Basin prior to emplace- canic rocks of the Rooiberg Group, and there is a 40-km-long meta- ment of the later Nebo and Lebowa Granites. In the Northern Seg- volcanic raft that is stratigraphically conformable along strike near ment of the eastern limb, the roof and floor are defined by metasedi- the geochemical boundary between the Upper Zone and the Main mentary rocks of the . In the Central Segment, Zone. We propose that the mafic magmas of the Bushveld Complex the floor is metasedimentary, but the roof is meta-volcanic—likely intruded the crust shallowly and at different stratigraphic levels from the Dullstroom and/or Damwal lavas of the Rooiberg Group. In the north to south

1 3 56 Page 16 of 17 Contrib Mineral Petrol (2015) 170:56 is commonly considered (e.g., Cawthorn 2013). We sug- References gest that the field observations are most consistent with extremely shallow emplacement of the RLS in the south. Buchanan PC, Koeberl C, Reimold WU (1999) Petrogenesis of the Dullstroom formation, Bushveld Magmatic Province, South Africa. Contrib Miner Petrol 137:133–146 Buchanan PC, Reimold WU, Koeberl C, Kruger FJ (2002) Geochem- Conclusions istry of intermediate to siliceous volcanic rocks of the Rooiberg Group, Bushveld Magmatic Province, South Africa. Contrib We have described in detail the various lithologies pre- Miner Petrol 144:131–143 Cawthorn RG (2013) The Residual or Roof Zone of the Bushveld sent in the immediate roof zone of the Bushveld within Complex, South Africa. J Petrol 54:1875–1900 three segments from north to south in the eastern limb. The Cawthorn RG, Meyer PS, Kruger FJ (1991) Major addition of magma immediate roof of the intrusion grades from dominantly at the Marker in the Western Bushveld Complex, metasedimentary in the north to dominantly meta-volcanic South-Africa. J Petrol 32:739–763 Cheney ES, Twist D (1991) The conformable emplacement of the in the Central and Southern Segments. In the Central Seg- Bushveld Mafic rocks along a regional unconformity in the ment, the roof of the intrusion is dominated by a complex Transvaal Succession of South-Africa. Precambr Res 52:115–132 lithology consisting of hornfels blocks embedded in a Clarke B, Uken R, Reinhardt J (2009) Structural and compositional matrix of microgranite. On the basis of bulk rock and min- constraints on the emplacement of the Bushveld Complex, South Africa. Lithos 111:21–36 eral compositions, we conclude that the hornfels blocks are Frost BR, Frost CD (2008) A geochemical classification for felds- thermally metamorphosed Dullstroom LMF and/or Dam- pathic igneous rocks. J Petrol 49:1955–1969 wal lava. The microgranite does not bear any geochemi- Gerya TV, Uken R, Reinhardt J, Watkeys MK, Maresch WV, Clarke cal relationship to the ubiquitous Stavoren granophyre and BM (2003) Cold fingers in a hot magma: numerical modeling of country-rock diapirs in the Bushveld Complex, South Africa. does not have the requisite REE concentration to be the Geology 31:753–756 residual liquid from the Bushveld mafic magma. Instead, Groeneveld D (1970) The structural features and the petrography the microgranite that forms the matrix of the hornfels– of the Bushveld Complex in the vicinity of Stoffberg, Eastern microgranite lithology likely formed due to partial melting Transvaal. Geol Soc S Afr Spec Publ 1:36–45 Hall AL (1932) The of the central Trans- of Dullstroom LMF or Damwal hornfels at extreme tem- vaal. S Afr Geol Surv Mem 28:510 peratures such as found in the pyroxene-hornfels facies. Hill M, Barker F, Hunter D, Knight R (1996) Geochemical character- The changing roof and floor lithologies present in the istics and origin of the Lebowa Granite Suite, Bushveld Com- eastern Bushveld implies that, instead of intruding along a plex. Int Geol Rev 38:195–227 Johnson T, Brown M, Gibson R, Wing B (2004) – regional unconformity between sediments of the Pretoria symplectites replacing : evidence for melt-assisted Group and the Rooiberg Group lavas (Cheney and Twist diapirism in the Bushveld Complex, South Africa. J Metamorph 1991), the Bushveld magmas were emplaced within units Geol 22:529–545 of the Pretoria Group sediments in the north, most likely Kruger FJ (2005) Filling the Bushveld Complex magma chamber: lateral expansion, roof and floor interaction, magmatic uncon- sandstone and quartzite in the floor and formities, and the formation of giant , PGE and Ti-V- Houtenbek, Lakenvalei, or Vermont formation in the roof. magnetite deposits. Miner Depos 40:451–472 In the Central Segment, the intrusion assumes the canoni- Loberg BE (1980) A Proterozoic zone in southern Swe- cal relationship of sedimentary rocks of the Magaliesberg den. Earth Planet Sci Lett 46:287–294 Lombaard AF (1949) Die geologie van die Bosveldkompleks langs quartzite in the floor and Dullstroom and/or Damwal lava Bloedrivier (The geology of the Bushveld Complex along Blood in the roof zone. Further to the south, however, Dullstroom River). Geol Soc S Afr Trans 52:343–376 volcanics make up the immediate floor of the intrusion and Lundgaard KL, Tegner C, Cawthorn RG, Kruger FJ, Wilson JR the top of the RLS may grade directly into granophyre with (2006) Trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex, South Africa. Contrib Mineral Pet a felsite roof. On the basis of these relationships, we infer 151(3):352–369 that the RLS was emplaced as an enormous hypabyssal sill. Mathez EA, VanTongeren JA, Schweitzer J (2013) On the relation- The sill occupied varying structural levels, being deeper in ships between the Bushveld Complex and its felsic roof rocks, the north than in the Central and Southern Segments, where part 1: petrogenesis of Rooiberg and related felsites. Contrib Miner Petrol 166:435–449 it may have come very near to the surface (Fig. 11). Molyneux TG (1970) The geology of the area in the vicinity of Mag- More detailed mapping of each of these segments will be net Heights, eastern Transvaal, with special reference to the mag- required in order to understand the nature of these emplace- netic . Geological Society of South Africa Special Publi- ment and petrogenetic relationships as well as the thermal cation 1:228–241 Molyneux TG (1974) A geological investigation of the Bushveld evolution of the Bushveld magmas and its roof. Complex in Sekhukhuneland and part of the Valley. Trans Geol Soc S Afr 77:329–338 Acknowledgments This work was supported by NSF-EAR Molyneux T (2008) Compilation on a scale of 1:50000 of the geology 0947247 awarded to E. A. Mathez. of the eastern compartment of the Bushveld Complex. Johannes

1 3 Contrib Mineral Petrol (2015) 170:56 Page 17 of 17 56

Willemse Bushveld Intelligence Center, Dept of Geology, Uni- implications for tectonic discrimination diagrams. J Volcanol versity of Pretoria, Pretoria Geoth Res 32:83–98 Odgers A, du Plessis A (1993) Interpretation of a regional reflec- Uken R, Watkeys MK (1997) Diapirism initiated by the Bushveld tion seismic survey in the north-eastern Bushveld Complex. 3rd complex, South Africa. Geology 25:723–726 SAGA Biennial conference and exhibition VanTongeren JA, Mathez EA (2012) Large-scale liquid immiscibil- SACS (1980) Lithostratigraphy of the Republic of South Africa, ity at the top of the Bushveld Complex, South Africa. Geology South West Africa/Namibia and the Republics of Botswana, 40:491–494 Transkei, and Venda. S Afr Geol Surv Handb 8:690 VanTongeren JA, Mathez EA (2013) Incoming magma composi- Schweitzer JK (1998) The Dullstroom Formation and the Rooiberg tion and style of recharge below the pyroxenite marker, Eastern Group, volcanic rocks associated with the Bushveld Complex. Bushveld Complex, South Africa. J Petrol 54:1585–1605 Ph.D. Thesis, University of Pretoria VanTongeren JA, Mathez EA, Kelemen PB (2010) A felsic end to Schweitzer JK, Hatton CJ, DeWaal SA (1997) Link between the gra- Bushveld differentiation. J Petrol 51:1891–1912 nitic and volcanic rocks of the Bushveld Complex, South Africa. von Gruenewaldt G (1968) The Rooiberg Felsite north of Middleburg J Afr Earth Sci 24:95–104 and its relations to the Layered Series of the Bushveld Complex. Twist D (1985) Geochemical evolution of the Rooiberg Silicic Lavas Trans Geol Soc S Afr 71:153–172 in the Loskop Dam Area, Southeastern Bushveld. Econ Geol von Gruenewaldt G (1972) The origin of the roof-rocks of the Bush- 80:1153–1165 veld Complex between Tauteshoogte and Paardekop in the east- Twist D, French BM (1983) Voluminous acid in the Bush- ern Transvaal. Trans Geol Soc S Afr 76:207–227 veld Complex: a review of the Rooiberg Felsite. Bull Volcanol Walraven F (1987) Textural, geochemical, and genetic aspects of the 46:225–242 granophyric rocks of the Bushveld Complex, Geological Sur- Twist D, Harmer REJ (1987) Geochemistry of contrasting siliceous vey of South Africa: Memoir, vol 72. Republic of South Africa, magmatic suites in the Bushveld Complex: genetic-aspects and Department of Mineral and Energy Affairs, Geological Survey

1 3