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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

Keywords Rooiberg Group · Bushveld Complex · Driekop Dome · Mphanama · Masekete · Roof zone · Residual Zone · Upper Zone · Droogehoek · Stoffberg · Hornfels · Leptite

Introduction

Layered mafic intrusions represent the primary observa-tional record of igneous differentiation within a solidify-ing magma chamber. While the sequences of cumulate rocks provide information on the magmatic responses to solidification, it is the roofs of these intrusions that hold the key to understanding the mechanisms of heat loss and thermal evolution. Only six major layered mafic intrusions have both their roofs and floors preserved and exposed: the Skaergaard Intrusion of East Greenland, the Muskox, Sept Iles and Kiglapait Intrusions of Canada, the much larger Dufek Intrusion of Antarctica, and Bushveld Complex of South Africa. The Bushveld Complex is part of the massive 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 (250 × 350 km if a continuous sheet) belies the fact that it is generally poorly exposed, with the exception that parts of the eastern Bushveld are locally well exposed due to the rugged topography. These regions thus offer the unusual opportunity to understand how an enormous and long-lived body of mafic liquid interacted with its roof.

One model of emplacement holds that the Bushveld intruded along a regional unconformity between mainly shales and quartzites of the underlying Pretoria Group sediments and an overlying thick sequence of basaltic to rhyolitic lavas known as the Rooiberg Group, with the exception of one area in the southeast Bushveld where

Abstract Emplacement of large volumes of mafic magma into the crust undoubtedly causes significant thermal perturbation to the overlying crust. Despite the clear importance of the country rock in modulating 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 that make up the immediate roof of the intrusion in the Eastern Limb. In the Northern Segment of the eastern Bushveld, the roof is dominated by thermally metamor-phosed metapelites; in the Central Segment, the roof is dominated by highly metamorphosed meta-volcanic rocks and their partially molten equivalents; and in the Southern Segment, the roof is likely composed of mod-estly thermally metamorphosed felsic volcanic rocks. The variability of roof lithology is also reflected in the variability of floor rocks to the intrusion. A new model for the emplacement of the eastern Bushveld Complex is proposed in which the mafic magmas intrude at a deeper level in the north and become shallower to the south.

Communicated by Timothy L. Grove.

Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1211-y) contains supplementary material, which is available to authorized users.

* J. A. VanTongeren jvantongeren@eps.rutgers.edu

1 Department of Earth and Planetary Sciences, Rutgers University, 610 Taylor Rd., Piscataway, NJ 08854, USA

2 Department of Earth and Planetary Sciences, American Museum of Natural History, 79th and Central Park West, New York, NY 10024, USA

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the Dullstroom Formation, the lowest unit of the Roo-iberg Group, makes up the floor of the intrusion (Cheney and Twist 1991). The Rooiberg Group consists of a lower sequence of magnesian lavas and a petrologically distinct upper sequence of ferroan lavas (Twist and Harmer 1987; Mathez et al. 2013). In ascending stratigraphic order, these lavas have been subdivided into the Dullstroom, Damwal, Kwaggasnek, and Schrikkloof formations (SACS 1980). Some of the ferroan lavas may have been generated by fractional crystallization of the Bushveld mafic magmas (VanTongeren et al. 2010, see below).

Understanding the roof of the Bushveld Complex is complicated by the fact that the younger Lebowa Granite Suite granites intruded at various levels within the roof and lava sequences (Hill et al. 1996). The petrogenesis of the granites and their relationship to the Bushveld Com-plex and Rooiberg lavas have been debated (e.g., Hill et al. 1996; Schweitzer et al. 1997), but due to the younger age of the granites, their origin will not be considered here.

The Bushveld Complex is composed of the principle eastern, western, and northern limbs along with a num-ber of outliers. From east to west, the Bushveld extends over 350 km, and the eastern limb alone crops out for more than 150 km north–south. Due to the enormous size of the Bushveld, the roof of the intrusion is neither eve-rywhere laterally continuous nor exposed. In some places the mafic rocks are capped by quartzite and/or metapelite, and in others by a complicated lithology of leptite (see below), hornfels, microgranite, granophyre, felsite, and granite.

This paper describes the large-scale changes in contact relationships as they are observed between the rocks of the Upper Zone of the Bushveld Complex and its immediate roof in four locations from north to south in the eastern limb. The present report builds on the important works by Groeneveld (1970), von Gruenewaldt (1968, 1972), Lom-baard (1949), Molyneux (1970, 1974, 2008), and Walraven (1987). The goal of our study is to provide a systematic look at how the relationship between the final Bushveld Complex magmas and the roof zone evolves when there are different lithologies present in the overlying country rock.

Field relations

Throughout much of the eastern limb of the Bushveld, the immediate roof of the intrusion consists of a distinctive, 100- to 300-m-thick rock layer composed of a complex mixture of hornfels and microgranite. The term ‘leptite’ has been used by various authors (e.g., Von Gruenewaldt 1968, 1972; Molyneux 2008) to describe either the rock or the hornfels or both. Leptite originated as a nineteenth-century Swedish name for fine-grained, recrystallized,

quartz-feldspar, meta-volcanic or metasedimentary, supracrustal rocks, such as those found in the Protero-zoic Leptite Belt near Stockholm (e.g., Loberg 1980). The Glossary of Geology (4th) states that the term is now obsolete. Furthermore, where it exists in the roof of the Bushveld, leptite has come to mean different things to dif-ferent workers and does not actually describe the rock. For these reasons, we dispense with the term entirely and simply describe the rocks by listing the combination of specific lithologies present. We define hornfels as a fine- to very fine-grained, thermally metamorphosed rock with classic granoblastic texture. It is important to note that we use the term hornfels throughout the manuscript to describe the rock texture, with no implication for its protolith.

Here we divide the roof as it is exposed in the east-ern Bushveld into three segments, each with a different character. The roof of the most volumetrically significant ‘Central Segment’ (Fig. 1) is dominated by the distinctive hornfels + microgranite rock noted above. This rock type is well exposed in the Droogehoek and Masekete Sections, described below. In the ‘Northern Segment,’ outcrops of the Bushveld Upper Zone are limited and the map patterns and seismic profiles imply significant structural complexity. In this region the roof is dominated by metapelites and a sedimentary hornfels that is significantly different than the hornfels of the Central Segment. The roof lithologies char-acteristic of the Northern Segment are well exposed in the Mphanama Section, also described below (Fig. 1). Finally, least well exposed is the ‘Southern Segment,’ where Caw-thorn (2013) asserted that the Bushveld cumulate rocks are in immediately contact with the Rooiberg Group volcanics. However, our observations suggest that the roof in this area is similar to that in the Central Segment.

Central Segment

The Central Segment of the Bushveld roof extends from Magnet Heights south to the Tauteshoogte area (Fig. 1, 24°50′S to 25°19′S, ≈24°50′E). The area from about 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-neux (1974). In this region, while there is variability at the small scale, the same general lithologies are observed for ~60 km along strike. Along this entire strike length, the cumulate rocks of the Bushveld dip gently westward beneath the granites exposed on the Nebo Plateau, and the top of the Bushveld and the immediately overlying roof rocks are locally well exposed in incised stream beds along and below the steep eastern-facing escarpment of the Nebo Plateau. Two such streams are Droogehoek and Masekete (Fig. 1).

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Droogehoek Section

The Droogehoek Section (24° 51.767′S, 29° 54.384′E) fol-lows a streambed that provides nearly 100 percent exposure

of about a third of the section and perhaps 40 percent of the rest (Fig. 2). It is accessible with permission of the local community, nearest to the town of Ga-Maepa. The precise contact between the cumulate diorites of the Upper Zone

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

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and the overlying microgranite–hornfels roof is usually not exposed because it is covered by stream gravel, but it can be placed within about 10 m in the stream cut. The imme-diate roof in this section is characterized by three distinct lithologies:

1. Enormous hornfels blocks meters to tens of meters across. Most if not all of the blocks are angular on the meter scale, homogeneous, massive, and lithologically identical to each other (see below).

2. Numerous small hornfels xenoliths in the centimeter to tens of centimeter size range. Some xenoliths are clearly fragments of once larger ones, and many of the small xenoliths are thinly laminated, some with appar-ent cross-bedding, suggesting sedimentary protoliths.

3. A matrix lithology of microgranite/granodiorite exists around the hornfels blocks (Fig. 2). Both the large and small hornfels blocks are randomly distributed such that the matrix microgranite is typically localized in irregularly shaped masses on all scales from tens of centimeters to tens of meters across.

4. A fine-grained granophyre dike or sill cutting the horn-fels–microgranite. This is presumably the Stavoren Granophyre, which was defined by Walraven (1987) and is part of the Rashoop Granophyre Suite. To the south in the Tauteshoogte area the Stavoren Grano-phyre exists as a locally discordant sheet up to 650 m thick. The maps of von Gruenewaldt (1972) and Moly-neux (2008) show that the granophyre is present as thin (<10 m) sill- or dike-like bodies intruded into the horn-fels–microgranite along the entire Central Segment (e.g., von Gruenewaldt 1972).

The contacts between the hornfels and microgranite vary from sharp to gradational and may be straight, irregu-lar, or sinuous. Some of the smaller xenoliths are partially digested within the microgranite, so it is clear that the latter contains, at least locally, a hornfels component. In places where the hornfels and microgranite lithologies are inti-mately mixed, the microgranite characteristically displays considerable heterogeneity in both mode (see below) and grain size. In some cases, grain size in the microgranite

Fig. 2 Field relationships in the Droogehoek Section of the Central Segment. Large photograph shows the immediate roof zone above the contact with the Upper Zone. Small photos taken of the rocks observed immediately above the contact including the small metased-

imentary hornfels and the larger hornfels blocks with the microgran-ite. Arrow in bottom left of large photograph indicates stratigraphic up

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varies continuously from millimeter to centimeter size away from the hornfels contacts.

The excellent exposure provided the opportunity to determine the relative proportions of hornfels and micro-granite and how those proportions change across strike by pacing along two segmented traverses in reasonably flat areas through the entire unit. Based on this exercise, the rocks in the Droogehoek Section are estimated to comprise 38 % microgranite and 62 % hornfels, with no systematic change in the proportion of lithologies away from the con-tact with the Bushveld cumulate rocks.

Masekete Section

The Masekete Section (25° 11.979′S, 29° 46.398′E) is approximately 20 km south of the Droogehoek Section (Fig. 1). It is accessible along the Masekete River with permission of the local game lodge on the road R555. The immediate roof of the Bushveld Complex in the Masekete Section is similar to that observed in the Droogehoek Sec-tion, with large, homogeneous blocks of fine-grained

hornfels surrounded by a coarse-grained matrix of micro-granite (Fig. 3), with the exception that the small metasedi-mentary hornfels xenoliths are notably absent.

Further upsection along the Masekete River, the horn-fels–microgranite roof rocks are overlain by a granophyre and fine-grained granite. Where granophyre–granite con-tacts are clearly exposed, the granite displays an intrusive relationship with the granophyre but without chilled mar-gins (Fig. 3), implying that the granophyre was hot when the granite was emplaced. Therefore, this granite may sim-ply be another phase of the magma that gave rise to the granophyre.

Mineralogy and geochemistry of the Central Segment lithologies

Hornfels blocks

The rocks that make up the hornfels blocks in the immedi-ate roof zone of the Central Segment are made of equant, rounded grains of quartz (diameters = 100–250 µm),

(b)(a)

Fig. 3 Field relationships in the Masekete Section of the Central Segment. a Pristine outcrop exposure of hornfels–microgranite lithol-ogy in the Masekete River immediately above the contact with the Upper Zone. Hornfels appears as a very fine-grained light gray blocks with both rounded and sharp contacts to the microgranite. b Fur-ther up the Masekete River away from the Upper Zone contact, the

granite and granophyre lithologies are dominant. Large photograph shows the outcrop exposure near the base of the Nebo Plateau scarp, where both granite and granophyre are found in association with one another. Small photographs highlight the difference in grain size and contact relationships between the granite and granophyre. Arrow in bottom left of large photograph indicates stratigraphic up

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hedenbergite, and in some samples variable but minor amounts of fayalite embedded in a matrix composed of an equidimensional mosaic of plagioclase and orthoclase grains (400–600 µm). The texture, as noted, is classic granoblastic (Fig. 4). The rock also contains sparse plagio-clase porphyroblasts up to 2 mm long. These display nor-mal zoning near the margins, which encase abundant quartz grains. No other porphyroblasts are present. Typical modes are estimated in thin section to be 40–50 % feldspar, 50 % quartz, 8 % hedenbergite + fayalite, and 2 % magnetite plus exsolved ilmenite. In two of three samples analyzed, the fayalite is nearly pure (>Fa98), so the assemblage and mineral compositions fix the fO2 at essentially the fayalite–magnetite–quartz (FMQ) buffer. The fayalite coexists with hedenbergite of approximate composition En3Fs52Wo45. The third analyzed sample contains Fa93–94, which coexists with hedenbergite of En11Fs46Wo43. Hornblende and biotite are encountered only rarely and exist almost exclusively in rocks displaying mild sericitic alteration. Zircon and apatite (30–60 µm) are the most abundant trace phases. A complete dataset of mineral compositions is provided in Supplemen-tary Table 1.

Plagioclase compositions in the hornfels blocks are highly variable. Measured compositions from three ana-lyzed hornfels samples in the Droogehoek Section range from An1Ab98 to An39Ab59, though the range in any

given sample is typically ± 10 An units. Average plagio-clase compositions for the hornfels samples range from An19–24; K-feldspar compositions range from Or79–86. In the Masekete Section, the average plagioclase and ortho-clase of the hornfels samples are plagioclase An11–14 and orthoclase Or80–87. The plagioclase compositions measured in the hornfels blocks are similar to those of phenocrysts from Damwal (An17–28; Buchanan et al. 2002) and the Dullstroom high-magnesium felsite lavas (An11; Buchanan et al. 1999) (no data have been published on plagioclase phenocryst compositions from the Dullstroom low-magne-sium felsites).

Microgranite

The microgranite in the immediate roof of the Central Seg-ment consists of equant quartz grains (0.5–1.0 mm), equant to tabular, subhedral plagioclase grains (≤2 mm, usually partially sericitized), and perthitic orthoclase that is intersti-tial to both. The mafic phase is hornblende (ferropargasite), which is interstitial to plagioclase and quartz and may exist as large, discrete grains up to 5 mm across poikilitically enclosing those minerals. Some rocks also contain traces of hedenbergite and fayalite. The textures suggest that these minerals were originally more abundant but mostly altered to amphibole. Biotite is present in minor amounts partially

Fig. 4 Photomicrographs in plane and cross-polarized light of sam-ple B10-044, representative of a typical hornfels–microgranite lithol-ogy. a Hornfels, b contact between hornfels and microgranite lithol-ogy, c microgranite. Field of view and scale bar of 500 um are the

same in all photomicrographs. *Geochemical analyses reported in Supplementary Tables 1, 2 for B10-044 are from the microgranite portion of the sample only

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replacing hornblende, and apatite and zircon are common trace phases. The overall texture varies from poorly devel-oped granitoid to locally allotriomorphic granular (Fig. 4). Most of the microgranites are monzonitic, consisting of 20–30 % quartz and 8–10 % hornblende + biotite (bulk SiO2 = 66–72 %), but some are distinctly more mafic, containing ≈15 % quartz and 20 % mafic minerals (bulk SiO2 = 62–63 %). The more mafic samples are the ones 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 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 water activity in the microgranite. Aside 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-positions were obtained for the small hornfels xenoliths present in the Droogehoek Section.

Neither the hornfels blocks nor the microgranite has exact compositional analogues among the Rooiberg Group lavas, though they are remarkably similar in overall geo-chemical characteristics to some of the individual lava formations. On average, the hornfels blocks have similar SiO2 contents to the Kwaggasnek rhyolites (~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 lava (ca. 5 wt% FeO) and slightly lower than those of the Damwal (ca. 6–7 wt%) or Dullstroom LMF lavas (ca. 5–7 wt%). The MgO content of the hornfels is typically <0.10 %, whereas that of the Damwal and Dullstroom LMF

lavas is typically between 0.3 and 1.00 %, leading to higher Fe/Mg ratios in the hornfels compared to the Dullstroom and Damwal (high Fe index, Fig. 5a).

Despite the offset to higher SiO2 and Fe/Mg, both of which may be altered during thermal or hydrous alteration,

(a)

(b)

(c)

Fig. 5 Whole-rock major element geochemistry for the hornfels, microgranite, and Rashoop Granophyre for comparison with the Roo-iberg Group lavas. ASI is the Aluminum Saturation Index, MALI is the Modified Alkali-Like Index, and Fe Index is the FeOtotal/FeOtotal + MgO, according to Frost and Frost (2008). The hornfels, micro-granite, and granophyre all have compositions overlapping with the upper Rooiberg Group lavas, the Damwal, Kwaggasnek, and Schrikk-loof formations. Rooiberg lava and granophyre compositions are from the compilation of Mathez et al. (2013) including data from Sch-weitzer (1998), Buchanan et al. (1999, 2002), Walraven (1987)

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the hornfels blocks bear striking resemblance in most other geochemical parameters to those of the Damwal and Dull-stroom LMF lavas. The modified alkali lime (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-granites in the Droogehoek Section. The majority of micro-granite samples have high SiO2 (66–70 wt%) similar to the hornfels and lavas, but two have distinctly lower SiO2 contents (62 wt%). The latter are also relatively FeO-rich (10 wt% compared to 5–7 wt% in the high SiO2 samples) and possess anomalously high Zr and Ba concentrations and high Zr/Y.

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 than the lavas, similar to that of the hornfels (Fig. 5a). Both the hornfels and microgranite are distinctly metaluminous and have MALI and ASI indices that overlap with the range of values observed in the Dullstroom LMF and Damwal lavas (Fig. 5b, c).

Trace element concentrations of the microgranites are nearly identical to coexisting hornfels, except in the case of Sr, Ba, and Zr, which are slightly higher on average in the microgranites relative to the adjacent hornfels (Fig. 6).

The microgranites have a distinctly different composi-tion from the Stavoren granophyre. While the microgranite has a similarly high Fe index to the granophyres, it is much lower in SiO2 and plots far from the granophyre field on the ASI and MALI indices (Fig. 5).

Northern Segment: Mphanama Section

Field relations

In the northern portion of the eastern Bushveld, from the towns of Jane Furse to Mohalatse (24°46′S, 29°54′E to 24°33′S, 29°46′E), the roof rocks are dominated by

metapelite and metasedimentary hornfels, which Molyneux (2008) mapped as ‘leptite-after-quartzite’ (Fig. 1). Good exposures of the Northern Section lithologies exist near the town of Mphanama (24°35.532′S, 29°48.751′E; Figs. 1, 7a, b).

A unit mapped as hornfels in this region consists of ther-mally metamorphosed mudstones and is thus radically dif-ferent from the hornfels of the Central Segment. Erosional surfaces on the mudstone lithologies preserve evidence of rip-up clasts and clear sedimentary structures, though rock interiors are fused (Fig. 7a). The most likely protolith to this lithology is the Vermont Formation of the Pretoria Group metasediments (P. Erikkson, personal comment).

A second unit mapped as ‘leptite-after-quartzite’ in the Northern Section is dominated by thermally metamor-phosed sandstones and contains large regions of metasedi-ments with structures indicative of previously unconsoli-dated fine-grained material similar to the mudstone protolith of the hornfels unit (Fig. 7b). The most likely protolith of this lithology is the Lakenvalei formation of the Pretoria Group metasediments (P. Eriksson, personal comment).

The ubiquitous microgranite that characterizes the Cen-tral Segment is nowhere present in the Northern Section.

Fig. 6 Whole-rock trace element geochemistry for the hornfels, microgranite, and granophyre for comparison with the Rooiberg Group lavas. The granophyre has a trace element composition very similar to that of the upper Rooiberg Group lavas and the Kwaggas-nek and Schrikkloof formations. The hornfels and microgranite have compositions distinct from the granophyre and upper Rooiberg Group lavas. Aside from a single sample with high Zr/Y, the hornfels blocks have very similar trace element ratios to those found in the Damwal formation lavas. The microgranite, however, has a much higher trace element compositional variability

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There is no evidence in either the metasedimentary horn-fels unit or the ‘leptite-after-quartzite’ unit of any partial melting during thermal metamorphism.

The patterns on Molyneux’s (2008) map reveal that the Upper Zone and roof in this region do not display the gentle westward dips that are typical of most of the rest of the eastern Bushveld. Rather, there exists a NW–SE trend-ing fold axis centered approximately 2 km to the east of the Upper Zone–roof contact (Molyneux 2008). The fold is also observed in the seismic reflection profile of Odgers and du Pleiss (1993) as a broad anticlinal structure within the Transvaal floor and Bushveld rocks, with the fold just to the east of the Malopo Dome and Wonderkop Fault. The fold results in shallow dips to the east near the Bushveld–roof contact, which limits the stratigraphic exposure of the Upper Zone cumulates (Molyneux 2008). Where the Upper Zone is observed, only the uppermost few hundred meters of stratigraphy from Magnetite Seam 17 (corresponding to ~375 m below the roof in the Magnet Heights type section of Molyneux 1970) to the roof are typically exposed. The expo-sures in this region seem to be limited to areas capped by a resistant layer of quartzite, such that the hills with outcrops display Upper Zone in contact with either hornfels or quartz-ite, overlain by massive quartzite (Fig. 1; Molyneux 2008).

The Malopo Dome, also known in the literature as the Driekop or Phepane Dome, has been interpreted to rep-resent a metasedimentary diapir from the floor of the

intrusion (Johnson et al. 2004; Uken and Watkeys 1997; Gerya et al. 2003) that penetrated partially molten Bush-veld rocks. However, the rocks observed along the flanks of the dome are similar to those observed in the roof expo-sures of the Mphanama section only ~4 km to the south-west, where floor diapirism is not inferred, suggesting that the Malopo Dome is normal Bushveld Upper Zone with a resistant quartzite cap. More work is required to understand the significant structural complexity and outcrop patterns of this region to understand the nature of the Malopo dome.

Mineralogy and geochemistry of the Northern Segment lithologies

Hornfels

In the Northern Segment, the unit mapped as ‘hornfels’ on the map of Molyneux (2008) makes up the immediate roof of the Upper Zone. From the field descriptions of the Mphanama Section above, outcrops are clearly dominated by thermally metamorphosed mudstones. Consistent with this, in thin section the rocks are very fine-grained metape-lites, with abundant orthoclase and only minor (<20 %) quartz. Plagioclase is largely absent. Nearly pure magnetite (Fe3O4) makes up an additional 10 % of the mode. Sam-ples have abundant small planar lathes of fibrolite and large elongated grains (0.5 mm) of sillimanite, which are also

(a) (b)

Fig. 7 Field relationships in the Mphanama Section of the North-ern Segment. a Large photograph shows the regional relationships between outcrops of quartzite, Upper Zone, and units mapped by Molyneux (2008) as leptite-after-quartzite and hornfels. Smaller pho-

tographs show typical rocks of the metasedimentary hornfels. b Large outcrop of unit mapped as ‘leptite-after-quartzite’ in the Mphanama section. Rocks are thermally metamorphosed mudstones and sand-stones and preserve their original sedimentary features

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present as inclusions in patches of biotite. Retrogressed muscovite is present in some samples.

Leptite‑after‑quartzite

The unit stratigraphically above the hornfels in this region is mapped as ‘leptite-after-quartzite,’ although quartzite is absent where the unit crops out in the Mphanama Sec-tion. Thin sections are dominated by large euhedral grains of quartz and orthoclase (Or98) with minor zircon, rutile, and apatite. Plagioclase feldspar is notably absent, except as exsolutions from the orthoclase (An0.2Ab99). Biotite and muscovite were found in a very minor area of a single thin section. In contrast to the hornfels unit, no sillimanite or fibrolite was found in any of the samples investigated. Dark nodules observed in hand sample were large (fist-sized or slightly smaller) regions with pyroxene poikilitically enclosing the quartz and orthoclase. The outcrop (Fig. 7b) and phase assemblage suggest a distinctly sedimentary protolith.

Upper Zone at contact

The diorite cumulates of the uppermost Upper Zone of the Bushveld are dominated by plagioclase (An40), clinopyrox-ene (En3Fs44Wo52), and olivine (Fa99–95). Quartz is absent and orthoclase is present only as antiperthite. Large euhe-dral grains of apatite are present mainly as inclusions in oli-vine and pyroxene. It is important to note that Fe–Ti oxides are rare (<1 %) in the uppermost samples of the Upper Zone in the Northern Section and where present are restricted to ilmenite, not the magnetite-ulvospinel common elsewhere in the Upper Zone. Grain sizes are similar to those through-out the Upper Zone and there is no evidence of a chilled margin or enhanced rate of crystallization.

Southeastern Segment: Stoffberg Section

Field relations

South of the Nebo Plateau, near the town of Stoffberg (from 25°16.541′S, 30° 2.674′E to 25° 44.970′S, 29° 55.773′E), the Lower and Critical Zones of the Bushveld Complex are absent such that the cumulates of the Main Zone rest directly on the Dullstroom volcanic rocks (Fig. 1).

In general exposure is very limited throughout the area, but a few isolated outcrops exist in a small hill (S25° 26.363, E029° 49.158) on a farm accessible by a private dirt road from road R33 with permission from the land own-ers. This section was previously described by Hall (1932) and Groeneveld (1970), and more recently reinterpreted by Cawthorn (2013). Several major features can be identified. Rocks near the base of the hill are medium-grained diorites

with no hornblende and likely correspond to the uppermost cumulates of the Upper Zone. Approximately two-thirds of the way up the hill, the rocks are amphibole-bearing dior-ites, termed hornblende quartz monzonites by Cawthorn (2013). Further up in the section, the rocks become signifi-cantly more felsic, with quartz, orthoclase, and plagioclase comprising >70 % of the total mineralogy. The uppermost lithologies resemble the granophyres observed at the top of the Masekete Section in the Central Segment. On a second ridge of the hill slightly to the south, a minor amount of thermally metamorphosed felsic rock forms a small cap at the top and is laterally continuous with those further to the NNW along the Bothasburg Plateau, according to Moly-neux (2008). The rocks exposed along the Stoffberg section were mapped as ‘leptite’ by Molyneux (2008); however, they lack the characteristic hornfels–microgranite lith-ologies typical of the ‘leptite’ unit mapped in the Central Segment. On the basis of trace elements, Cawthorn (2013) proposed that this felsite corresponds to the Damwal For-mation of the Rooiberg Group lavas.

A flat outcrop near the base of the hill reveals several features of the lithologies that are not possible to observe in the smaller outcrops elsewhere in the Stoffberg Section (Fig. 8). The rocks possess a chaotic fabric of leucocratic and melanocratic layers. There is a single small metasedi-mentary xenolith preserved (Fig. 8) within the medium-grained fabric. There is no clear contact relationship observed between the Bushveld cumulate rocks and the roof rocks in this section.

Mineralogy and geochemistry of the Southern Segment lithologies

Upper Zone near contact

The evolution of the uppermost Upper Zone in the Stoff-berg Section of the Southern Segment is difficult to deter-mine given the limited exposure. An Upper Zone cumulate sample interpreted to be the stratigraphically lowest out-crop exposed in the section contains plagioclase (An40), clinopyroxene (Mg#11), olivine (Fa95), minor (<1 %) Fe–Ti oxides, and no orthoclase or quartz. Grain sizes of the plagioclases are 1–3 mm and ~0.5 mm for the oxides and mafic phases. Approximately 30 m stratigraphically above the rocks also have plagioclase (An40), clinopyrox-ene (Mg# 20), and abundant magnetite. Olivine is notably absent from the assemblage, and magnetite grains are typi-cally rimmed by small quartz grains. Apatite and orthoclase (Or74) are present as minor phases.

These rocks likely represent the uppermost Upper Zone in this region. They possess cumulate textures in thin sec-tion as well as similar mineral compositions and grain sizes to the cumulate rocks at the roof contact in the Central and

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Northern Segments, and they resemble no other roof rocks exposed elsewhere.

Monzonite

A highly altered sample obtained from a location described as ‘monzonite’ by Cawthorn (2013) has significantly smaller grain size from the diorite cumulates found below. The rocks in this area contain, on average, 10–15 % quartz, 5–10 % orthoclase, 40 % plagioclase, 20 % fayalite and hedenbergite, 10 % alteration of the pyroxene to horn-blende, and 1–2 % Fe–Ti oxides (Cawthorn 2013). This sample has plagioclase An23Ab75Or3 and two distinct feldspar populations, one with An2Ab30Or67 and ortho-clase with An0Ab3Or97, likely corresponding to subsolidus reequilibration along the peristerite solvus. The monzonite sample analyzed in this study also contains olivine (Fo7.5) and clinopyroxene (Mg# 22), abundant magnetite and ilmenite as well as apatite.

Felsite

The composition of the ‘monzonite’ sample analyzed in our study is nearly identical to that of a nearby thermally metamorphosed felsite collected approximately 400 m to

the south. The felsite has a slightly smaller grain size than the ‘monzonite’ and is significantly less altered. The felsite sample contains the same three feldspar populations with An25Ab73Or2, An2Ab26Or72, and An1Ab3Or96 as the monzo-nite as well as abundant quartz and magnetite, and a trace of clinopyroxene.

Granophyre

A single sample from the Stoffberg Section was identified in outcrop as granophyre due to its slightly coarse grain size and granophyric texture. In thin section, however, the sample looks nearly identical to the ‘monzonite’ sample. It contains abundant quartz, orthoclase, and magnetite, and minor zircon and apatite. As in the monzonite and felsite samples, the granophyre contains three feldspar popula-tions, An16Ab82Or2, An4Ab95Or1, and An1Ab13Or86. The compositions of the feldspars are more evolved than in the monzonite and felsite samples and may have re-equil-ibrated to lower temperatures. Minor pyroxene has Mg# of 24, and no olivine was found.

The similarity in grain size, phase assemblage, and min-eral compositions of the monzonite, felsites, and grano-phyre described here suggest that the ‘monzonite’ from this region does not correspond to the uppermost Upper Zone,

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

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as proposed by Cawthorn (2013), but is in fact thermally metamorphosed and remelted roof material. If this is the case, the cumulates at the top of the Upper Zone are similar in composition and mineralogy to those present at the con-tact in the Central and Northern Segments. However, expo-sures are isolated and so severely limited in this region that it remains unclear how the various lithologies are petroge-netically related.

Discussion

Due to the excellent exposure and large geographic range, the Central Segment of the eastern Bushveld has received the most attention in previous studies of the Bushveld Com-plex roof, and the lithologies present there have become synonymous with ‘the roof.’ As described above, however, the roof is laterally variable, with the Northern Segment dominated by metapelite; the Central Segment dominated by granophyre, granite, and felsite; and the Southern Seg-ment dominated by felsite (Fig. 1, 9).

From the above descriptions two outstanding questions remain:

1. How are the various lithological units petrogenetically related to one another? In particular, what is the origin of the microgranite/granodiorite present in the immedi-ate roof of the intrusion, and how does it relate to the other rocks in its immediate vicinity?

2. How were the mafic magmas of the RLS emplaced within the shallow crust?

Northern Segment Hornfels

The hornfels roof zone in the Northern Segment is domi-nated by thermally metamorphosed mudstones and sand-stones. There are no blocks of felsic hornfels anywhere present in the outcrops exposed in the Mphanama Section. There does not appear to be any evidence for partial melt-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 areas stratigraphically close to the contact with the Bush-veld Complex (Fig. 7). For these reasons, we conclude that the immediate roof of the Bushveld Complex in the North-ern Section is not petrogenetically related to the hornfels–microgranite lithologies of the Central Segment, nor related to the Rooiberg Group lavas or Stavoren granophyres in the region.

Felsic Hornfels

Von Gruenewaldt (1972) mapped the immediate roof rocks in the Central Segment in the Tauteshoogte and Paardekop areas. There he reported that it consists of blocks of ‘lep-tite’ (i.e., hornfels) of variable size chaotically mixed with and veined by ‘microgranite’ and locally by ‘micrographic felsite,’ identical to the rocks observed in the Droogehoek and Masekete Sections. Uncertain about the origins of these rocks, he identified three possibilities, namely that (a) the hornfels protolith is Rooiberg lava and the micro-granite its molten equivalent, (b) the hornfels protolith is Pretoria Group sandstone 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

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equivalent, or (c) the hornfels protolith includes both Roo-iberg lava and sedimentary rocks, and the microgranite mainly represents a residual melt from fractional crystalli-zation of the Bushveld mafic magmas locally mixed with melt derived by partial digestion of the hornfels. The first of these was von Gruenewaldt’s (1968, 1972) preferred choice.

On the basis of bulk rock major and trace element com-position (Figs. 5, 6), mineral compositions, and field rela-tions, we conclude that the hornfels blocks in the immediate roof of the Central Segment are thermally metamorphosed equivalents to the Dullstroom LMF and/or Damwal lavas. Due to the similarity in composition between the Dull-stroom LMF and the Damwal lavas, it is not possible to distinguish between them. The small sedimentary hornfels clasts that are present within the roof zone in the Central Segment are consistent with the known occurrences of minor intercalated sediments within the Rooiberg Group (e.g., Twist 1985). Alternatively, the presence of the sedi-mentary hornfels in the Droogehoek Section, and absence in the Masekete Section, could be due to the close proxim-ity of the Droogehoek Section to the predominantly meta-sedimentary roof of the Northern Section.

Microgranite

All samples of microgranite analyzed in this study have trace element patterns similar to those of the hornfels blocks. We conclude that the microgranite present in the hornfels–microgranite lithology of the Central Segment was formed by partial melting of the adjacent hornfels due to thermal metamorphism with the underlying RLS. This conclusion supports possibility (a) of von Gruenewaldt

(1972) that the microgranite is partially melted Dullstroom LMF or Damwal lava. Walraven (1987) modeled the ther-mal effect of the emplacement of the Bushveld mafic magma on the overlying roof rocks and concluded that the amount of heat available from the crystallization of the Bushveld was capable of partially melting up to 1000 m of overlying material. While this is likely a maximum estimate, given the necessary assumptions regarding the emplacement depth, emplacement rate, and lithology, there is no doubt that mafic magmas of the Bushveld Complex were capable of partially melting significant volumes of Damwal and/or Dullstroom lava to form the microgranite.

The lack of microgranite observed in contact with the thermally metamorphosed felsite in the Southern Segment is due to lower peak metamorphic temperatures in the Southern Segment, where the RLS is considerably thinner and was likely emplaced at a shallower stratigraphic level as compared with the Central and Northern Segments (see below). Lundgaard et al. (2006) documented a larger por-tion of trapped melt in the cumulate of the Main Zone near the Southern Segment, which they attributed to faster cool-ing rates in this region. The inference of faster cooling rates in this segment supports our hypothesis of lower peak met-amorphic temperatures, as the heat from the emplacement of the intrusion must has been more readily dissipated.

Based on trace element abundances, we can rule out pos-sibility (c) of von Gruenewaldt (1972) that the microgran-ite represents the residual liquid of the RLS. VanTongeren and Mathez (2012) calculated the rare earth element (REE) concentration of the residual melt in equilibrium with apa-tites present at the top of the RLS. According to their esti-mate, the REE contents of the microgranite are too low to represent the residual melt from the RLS (Fig. 10). Instead,

Fig. 10 Whole-rock rare earth element concentrations of the micro-granite, granophyre, and Rooiberg Group lavas for comparison with the estimated residual liquid composition from the Bushveld Com-plex mafic magmas. Residual liquid compositions for the Bushveld Complex calculated from apatite–liquid trace element partitioning by VanTongeren and Mathez (2012). Gray bar indicates the range of

compositions in equilibrium with the top of the Bushveld Complex. The microgranite lithology found in the immediate roof zone is not a match for the residual liquid from the Bushveld Complex. The grano-phyre and/or Kwaggasnek and Schrikkloof lavas are likely to repre-sent the residual liquid erupted from the Bushveld (e.g., VanTongeren et al. 2010; VanTongeren and Mathez 2012; Mathez et al. 2013)

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the Rashoop granophyre has nearly identical REE concen-tration to that predicted for the RLS residual liquid and is more likely to represent the missing magma (Fig. 10).

Granophyre

In the southern part of the Central Segment west of Rooss-enekal, the Stavoren Granophyre comprises a several hundred-meter-thick sheet between underlying hornfels–microgranite and overlying Rooiberg lavas, with which it displays an intrusive relationship (von Gruenewaldt 1972; Walraven 1987). Von Gruenewaldt (1972) interpreted the granophyre to represent Rooiberg rhyolite completely remelted by the RLS magmas, and Walraven (1987) inter-preted it to represent the intrusive phase of those rhyolites. Our results suggest that it is the microgranite that repre-sents re-melted Rooiberg felsites (specifically Damwal or Dullstroom formations) and that the compositional differ-ence between the Stavoren Granophyre and the microgran-ite suggests that the Stavoren Granophyre does not repre-sent partially remelted Rooiberg rhyolite. Furthermore, although the major element compositions of the Stavoren Granophyre and the Kwaggasnek and Schrikkloof mem-bers of the Rooiberg Group rhyolites are nearly identical, Mathez et al. (2013) pointed out that the generally meta-luminous character of the former distinguishes it from the rhyolites, which tend to be weakly peraluminous. As noted above, the granophyre also has REE concentrations nearly identical to those expected of the residual liquids of 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 Granophyre an ideal candidate to represent the residual liq-uid from the crystallization of the Upper and Upper Main Zones of the RLS (e.g., VanTongeren et al. 2010). The gen-eral compositional similarity between the Stavoren Grano-phyre and Kwaggasnek and Schrikkloof rhyolites, however, makes it difficult to definitively rule out the possibility that these lavas may have been generated by fractional crystal-lization of the RLS.

For about 14 km along the east–west striking RLS con-tact south of the Kruis River (25°23′S, 29°37′E to 25°22′S, 29°44′E) (Fig. 1), the contact is marked by a <200-m-thick sheet of granophyre. Here Lombaard (1949) documented a progressive change over about 150 m stratigraphi-cally in the rock mode from (45 % plagioclase + 18 % hornblende + 14 % clinopyroxene + 8 % fayalite + 4 % quartz + 1 % K-feldspar) to (59 % K-feldspar + 28 % quartz + 13 % hornblende), illustrating that the granophyre displays a gradational contact with the underlying olivine diorite cumulate. Based on this, he interpreted the grano-phyre to represent the differentiated liquid from which the cumulates formed. Walraven (1987) regarded the rock unit

as distinct from the Stavoren Granophyre and named it the Diepkloof Granophyre for the farm on which it is well exposed. The Diepkloof Granophyre appears on both Lom-baard’s (1949, Plate XIX) and Walraven’s (1987, p. 35) maps. These are the only areas known to us where the field evidence alone indicates that a granophyre may have been produced by differentiation of RLS magmas.

Thin dikes and sills of granophyre are found associated with the distinctive hornfels–microgranite unit within the roof rocks throughout the eastern Bushveld (e.g., Moly-neux 2008). In the Southern Segment along the edge of the Bothasberg plateau, for example, the immediate roof of the RLS is composed of layers of both rocks, which under-lie the Rooiberg lavas (Groeneveld 1970). In the Central Segment, the various granophyre bodies display consider-able variability in the coarseness and development of their characteristic micrographic texture (e.g., von Gruenewaldt 1968, 1972). The immediate roof rocks also host similarly small bodies of relatively fine-grained granite, as observed in the Masekete Section (Fig. 6). The relationship between these small granite bodies and the Nebo Granite, which forms the massive cliff face along the Nebo Plateau scarp, is unknown. In fact, the intimate association and textural variability of these small granophyre and granite bodies render a clear distinction difficult, and it is conceivable that some of these rocks are merely local variants of each other. It is also not clear how the small granophyre bodies relate to the large Stavoren Granophyre sill mapped by von Grue-newaldt (1972).

Emplacement model

Based on the above relationships, we propose a new model for the emplacement of the RLS in the eastern Bush-veld and its relationship to the roof and floor rocks. It is clear from the map (e.g., Molyneux 2008) and the above descriptions that the RLS did not simply intrude along a single unconformity between the Rooiberg and the Pre-toria Group, as previously proposed (Cheney and Twist 1991; Twist and French 1983). Over roughly 50 % of the eastern Bushveld, the roof and floor rocks are composed of mudstones and quartzites of the Pretoria Group. In the regions to the north of Magnet Heights, there are no fel-sic meta-volcanic hornfels blocks, microgranite or felsic granophyre anywhere present (Fig. 9). Immediately to the south of Magnet Heights, the roof is dominated by felsic meta-volcanic hornfels and its molten microgranite equiva-lent (plus minor metasedimentary xenoliths), with a greater proportion of meta-volcanic hornfels further to the south (Fig. 9). This is mirrored in the floor rocks that crop out in this region, which grade from Pretoria Group sediments in the Central region to Dullstroom volcanics in the south

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(Fig. 1). Further to the south, meta-volcanic rocks and their recrystallized equivalents dominate the roof of the intru-sion, and metasedimentary rocks are entirely absent.

We propose that the mafic magmas of the RLS were emplaced within the Pretoria Group sediments, now quartzites, in the north whereas further to the south, where both the floor and roof rocks are volcanic, the RLS was emplaced within the lower portions of the Rooiberg Group felsites (Dullstroom and Damwal formations) (Fig. 11). The Central Segment just to the south of Magnet Heights represents a transition between these two, in that the floor rocks are dominated by sedimentary lithologies, whereas the roof rocks are clearly meta-volcanic.

Our proposed emplacement scenario (Fig. 11) is sup-ported by the presence of a large xenolith of meta-vol-canic, metamorphosed Dullstroom lavas nearly entirely transformed to microgranite within the cumulates of the Main Zone and Upper Zone in the southeastern Bushveld (Fig. 1). The microgranite/Dullstroom raft is present near the Main Zone–Upper Zone boundary and appears to be concordant with the igneous layering. It extends for nearly 40 km along strike from ~25° 20′S, 29°53′E to 25° 45′S, 29°50′E and possibly further to the south beyond the map of Molyneux (2008). In our model, the microgranite/Dull-stroom volcanic raft represents a portion of the pre-Upper Zone roof that was intruded by the growing RLS magma chamber, rather than a stoped block of material that fell coherently from the roof, as was previously proposed by Cawthorn (2013). A stoped block of felsic material, with

significantly lower density as compared with the mafic magma, would not be likely to sink more than two kilo-meters vertically in a convecting magma chamber. The microgranite raft is everywhere located at or above the Main Zone–Upper Zone boundary, lending further support to the hypothesis that the Upper Zone marks a major new influx of magma (e.g., Cawthorn et al. 1991; VanTongeren and Mathez 2013), which led to vertical and lateral magma chamber growth (Kruger 2005). Our model is consistent with the hypothesis of Clarke et al. (2009) suggesting that the mafic magmas propagated as magmatic fingers in a general NW–SE direction away from the Thabazimbi-Mur-chison Lineament.

The volcanic rocks that are present stratigraphically above the RLS in the Southern Segment are not observed in contact with the uppermost cumulates. Where the felsite is present in the roof of the Stoffberg Section, it is not par-tially remelted and therefore likely did not experience the same degree of thermal metamorphism, as the hornfels–microgranite pairs seen in the Central Segment. One pos-sibility is that the RLS in this region is significantly thinner, and therefore the total thermal perturbation to the country rocks was not as high as in the Central Segment where sig-nificant volumes of partial melt were created. This is con-sistent with the lack of Critical or Lower Zone in the South-ern Segment. Another possibility is that Rooiberg volcanic rocks in this region were erupted after significant crystalli-zation of the RLS (e.g., VanTongeren et al. 2010) and were not present during the majority of RLS emplacement, as

Fig. 11 New model for the emplacement of the Bushveld Complex layered mafic magmas within the Transvaal Basin prior to emplace-ment of the later Nebo and Lebowa Granites. In the Northern Seg-ment of the eastern limb, the roof and floor are defined by metasedi-mentary rocks of the Transvaal Supergroup. In the Central Segment, the floor is metasedimentary, but the roof is meta-volcanic—likely the Dullstroom and/or Damwal lavas of the Rooiberg Group. In the

Southern Segment, both the floor and roof are defined by meta-vol-canic rocks of the Rooiberg Group, and there is a 40-km-long meta-volcanic raft that is stratigraphically conformable along strike near the geochemical boundary between the Upper Zone and the Main Zone. We propose that the mafic magmas of the Bushveld Complex intruded the crust shallowly and at different stratigraphic levels from north to south

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is commonly considered (e.g., Cawthorn 2013). We sug-gest that the field observations are most consistent with extremely shallow emplacement of the RLS in the south.

Conclusions

We have described in detail the various lithologies pre-sent in the immediate roof zone of the Bushveld within three segments from north to south in the eastern limb. The immediate roof of the intrusion grades from dominantly metasedimentary in the north to dominantly meta-volcanic in the Central and Southern Segments. In the Central Seg-ment, the roof of the intrusion is dominated by a complex lithology consisting of hornfels blocks embedded in a matrix of microgranite. On the basis of bulk rock and min-eral compositions, we conclude that the hornfels blocks are thermally metamorphosed Dullstroom LMF and/or Dam-wal lava. The microgranite does not bear any geochemi-cal relationship to the ubiquitous Stavoren granophyre and does not have the requisite REE concentration to be the residual liquid from the Bushveld mafic magma. Instead, the microgranite that forms the matrix of the hornfels–microgranite lithology likely formed due to partial melting of Dullstroom LMF or Damwal hornfels at extreme tem-peratures such as found in the pyroxene-hornfels facies.

The changing roof and floor lithologies present in the eastern Bushveld implies that, instead of intruding along a regional unconformity between sediments of the Pretoria Group and the Rooiberg Group lavas (Cheney and Twist 1991), the Bushveld magmas were emplaced within units of the Pretoria Group sediments in the north, most likely Magaliesberg sandstone and quartzite in the floor and Houtenbek, Lakenvalei, or Vermont formation in the roof. In the Central Segment, the intrusion assumes the canoni-cal relationship of sedimentary rocks of the Magaliesberg quartzite in the floor and Dullstroom and/or Damwal lava in the roof zone. Further to the south, however, Dullstroom volcanics make up the immediate floor of the intrusion and the top of the RLS may grade directly into granophyre with a felsite roof. On the basis of these relationships, we infer that the RLS was emplaced as an enormous hypabyssal sill. The sill occupied varying structural levels, being deeper in the north than in the Central and Southern Segments, where it may have come very near to the surface (Fig. 11).

More detailed mapping of each of these segments will be required in order to understand the nature of these emplace-ment and petrogenetic relationships as well as the thermal evolution of the Bushveld magmas and its roof.

Acknowledgments This work was supported by NSF-EAR 0947247 awarded to E. A. Mathez.

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