Lithos 248–251 (2016) 469–477

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Letter

Hf isotopic evidence for a cogenetic magma source for the BushveldComplex and associated felsic magmas

J.A. VanTongeren a,b, N.A. Zirakparvar b, E.A. Mathez b

a Department of Earth and Planetary Sciences, Rutgers University, 610 Taylor Rd., Piscataway, NJ 08854 USAb Department of Earth and Planetary Sciences, American Museum of Natural History, 79th and Central Park West, New York City, NY 10024, USA

http://dx.doi.org/10.1016/j.lithos.2016.02.0070024-4937/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 December 2015Accepted 6 February 2016Available online 16 February 2016

Here, we test the hypothesis that the rhyolitic lavas of the Rooiberg Group and granophyres associated with theroof of the Bushveld Complex are differentiation products of Bushveld-agemafic liquids.We present Lu-Hf isoto-pic compositions in zircons from roof rocks that have been interpreted to represent thermally metamorphosedand remeltedRooibergGroup lavas and fromgranophyres interpreted to bedifferentiation products of the cumu-late rocks that make up the Bushveld Complex. All of these rocks were found to possess εHf (2.06 Ga) statisticallyindistinguishable from the intrusion-wide average εHf (2.06 Ga) value of−8.6 ± 1.2 of the Bushveld Complex. Ourresults, combined with chronologic and field relations, suggest that the felsic rocks were generated by fractionalcrystallization of Bushveld mafic liquids, including those that gave rise to the cumulate rocks of the BushveldComplex.

© 2016 Elsevier B.V. All rights reserved.

Keywords:Hf isotopesBushveld ComplexRooiberg GroupStavoren GranophyreRustenburg Layered Suite

1. Introduction

The Bushveld Complex of South Africa represents one of the largestoutpourings of magma recorded on the planet. Over a period of lessthan 6 million years, more than 1 million km3 of mafic and felsicmagmas were emplaced in the shallow crust over more than 400 kmalong strike and over 9 km vertically in what is present-day SouthAfrica. The Bushveld Complex, which itself is just one of the more than30 distinct intrusive bodies comprising the Bushveld MagmaticProvince (e.g. Rajesh et al., 2013), is composed of four main units: theRustenburg Layered Series, the Rooiberg Group lavas, the RashoopGranophyre Suite, and the Lebowa Granite Suite. The RustenburgLayered Suite (RLS) is the largest known layered mafic intrusionon the Earth, with conservative estimates of up to 600,000 km3

(Cawthorn and Walraven, 1998) of mafic–ultramafic cumulates thatproduced several important economic deposits. Estimates of the volumeof Rooiberg Group lavas reach up to 300,000 km3 (Twist and French,1983), and the later intrusive event forming the Lebowa Granite isestimated to have added another 120,000 to 200,000 km3 (Hill et al.,1996) to the Bushveld Magmatic Province. The Rashoop GranophyreSuite is not everywhere continuous, but is generally found in the centraland southern segments of the eastern Bushveld in variable thicknessesof up to ~300 m near the contact between the RLS and the overlyingRooiberg Group lavas (Walraven, 1985, 1987, 1997).

In this manuscript, we use the Lu-Hf isotope compositions of zirconsextracted from the rocks of the Bushveld Complex to test the hypothe-ses that portions of the Rooiberg Group lavas and/or Rashoop Grano-phyres are petrogenetically linked to the RLS (e.g. Mathez et al., 2013;VanTongeren and Mathez, 2015; VanTongeren et al., 2010). This is

achieved by comparison of new zircon Hf isotope data for the immedi-ate roof rocks of the RLS, the thermally metamorphosed RooibergGroup lavas, as well as the Rashoop Granophyres, with similar data forthe Hf isotope composition of the RLS (Zirakparvar et al., 2014).

2. Background/geologic context

Traditionally, themafic–ultramafic magmas of the RLS are consid-ered to have been emplaced along a regional unconformity betweenthe sediments of the Pretoria Group and the Rooiberg Group lavas(Cheney and Twist, 1991). The Rashoop Granophyre has beenthought to represent either the molten equivalent to the RooibergGroup lava (e.g. von Gruenewaldt, 1971; Walraven, 1987) or a resid-ual liquid from the crystallization of the RLS (e.g. Lombaard, 1949).The Lebowa Granite Suite shows clear cross-cutting relationshipswith all other units of the Bushveld Magmatic Province (BMP) (e.g.Hill et al., 1996) and is considered to be the final, late-stage magmaticevent.

Recently, however, the unconformity model of Cheney and Twist(1991) has been called into question, and the relationships betweenthe various lithologies present in the Bushveld Complex have beenrevisited (VanTongeren and Mathez, 2015). The unconformity modelimplies that the Rooiberg Group lavas are older than the RLS maficmagmas and are laterally continuous throughout the roof of theintrusion. Indeed, several authors have considered the Rooiberg Grouplavas as a “floating carapace” above the RLS magma chamber (e.g.Cawthorn, 2013; Kruger, 2005). However, building on the previousmapping efforts of several earlier Bushveld researchers (Groeneveld,1970; Lombaard, 1949; Molyneux, 1970, 1974; Molyneux, 2008; von

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Gruenewaldt, 1968, 1972; Walraven, 1987), VanTongeren and Mathez(2015) showed that the immediate roof of the RLS is highly variablealong strike. They documented changes in the immediate roof rocksthroughout the eastern limb of the Bushveld Complex and concludedthat the RLS in the eastern limb intruded between Pretoria Groupsediments in the Northern Segment, whereas in the Central Segmentthe roof is metavolcanic and the floor is sedimentary, and in the South-ern Segment both the floor and roof are metavolcanic. The fieldevidence clearly shows that the Rooiberg Group lavas could not haveformed the “floating carapace” for the entire emplacement of the RLS.

Furthermore, VanTongeren et al. (2010) showed that the upperunits of the Rooiberg Group lavas, the Kwaggasnek and Schrikkloofformations, as well as the Stavoren Granophyre of the Rashoop Grano-phyre Suite, are similar to the mafic rocks of the RLS in both Sr and Ndisotopic composition as well as age, and have the requisite major andtrace element compositions to represent the missing magma from theRLS previously postulated by Cawthorn and Walraven (1998).VanTongeren et al. (2010) concluded that portions of the RooibergGroup lavas and/or Rashoop Granophyres were erupted from theuppermost portion of the RLS after significant fractionation hadoccurred. This conclusion was amended by Mathez et al. (2013), whoshowed that the metaluminous character of the Rashoop Granophyremore closelymatched thatwhichwould be expected as a residual liquidfrom crystallization of the RLS, whereas the ferroan Kwaggasnek andSchrikkloof formations were slightly more peraluminous. However,

Fig. 1. Geologic map of the Eastern Limb of the Bushveld Complex with special reference to thknown distribution of the Bushveld Magmatic Province, adapted from Hatton and Schweitzer

Mathez et al. (2013)were unable to rule out the Rooiberg compositionsas potential candidates for the missing liquid.

2.1. General stratigraphy

Due to the locally rugged topography, the eastern limb of theBushveld Complex provides the best exposures of the complete stratig-raphy of the RLS and roof.While there is considerable variability in boththe roof and floor, it is generally accepted that the Dullstroom volcanics,a series of basaltic andesites and minor magnesian felsites, wereerupted prior to the emplacement of the RLS and form the floor in theSouthern Segment of the eastern limb (Fig. 1). Some authors considerthe Dullstroom volcanics to be the earliest stage of the Rooiberg Groupvolcanics (e.g. Buchanan et al., 1999; Schweitzer, 1998; Schweitzeret al., 1995). However, the Dullstroom volcanics are, despite theirwide range in composition, all magnesian and calcic and in theserespects are distinctly different from the dominantly ferroan and morealkalic upper members of the Rooiberg Group (e.g. Buchanan et al.,1999; Buchanan et al., 2002; Mathez et al., 2013). In fact, it is conceiv-able that the two groups represent unrelated magmatic events.

The intrusive rocks of the RLS are divided into four principal zones:the Lower Zone of ultramafic orthopyroxenites and harzburgites; theCritical Zone containing pyroxenites and norites with abundantchromitite layers, the Main Zone of gabbros, and the Upper Zone ofgabbros, magnetite gabbros, and diorites (Figs. 1, 2).

e roof lithologies, adapted from VanTongeren and Mathez (2015). Inset shows the entire(1995).

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Generalized Stratigraphy of the Central Segment - Eastern Limb

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Fig. 2. Generalized stratigraphic column of the RLS mafic cumulates and roof rocks in theCentral Segment of the Eastern Limb, adapted from VanTongeren and Mathez (2015).Sample numbers, corresponding to those in Figs. 4, 5 and Table 1, are shown near thelithology from which they were derived.

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Above the RLS in the eastern Bushveld, the immediate roof islaterally variable, from metasedimentary in the north to metavolcanicin the south (VanTongeren and Mathez, 2015). For simplicity, in thismanuscript, we will refer only to the generalized type-section fromthe Central Segment, where exposures are nearly complete and

Fig. 3. Field relationship between themetavolcanic hornfels andmicrogranite at the top of the Rmetamorphosed Rooiberg Group lava and the microgranite to be its molten equivalent. Photos

show the range of magmatic rocks present in the region (Fig. 1). Inthis region, the immediate roof is a complicated lithology consisting ofmetavolcanic hornfels and their partially molten equivalents (Figs. 2,3). Previous workers have termed this layer ‘leptite’; however, due toconflicted use of the term in the literature, VanTongeren and Mathez(2015) referred to the rocks by their constituent lithologies: hornfels-microgranite. Finally, above and intruding the immediate roof of theRLS intrusion, there exist several granophyres that have been lumpedtogether in the Rashoop Granophyre Suite (see below).

In the Central and Southern Segments, the upper Rooiberg Grouplavas are found in several stratigraphic locations above the RLS. Theupper Rooiberg Group is divided into three formations; from bottomto top they are the Damwal Formation, the Kwaggasnek Formation,and the Schrikkloof Formation. It is possible that the Damwal formationhas some equivalent in the floor of the intrusion; however, it isgeochemically similar to the low magnesium felsite member of theDullstroom Formation so the distinction between the two can beobscure (Mathez et al., 2013). The upper Rooiberg Group lavas appearto follow a continuous liquid line of descent from the dacitic composi-tions of the Damwal to the rhyolitic compositions of the Kwaggasnekand Schrikkloof formations (Buchanan et al., 2002). In the CentralSegment, massive flows of the Kwaggasnek and Schrikkloof lavas aretypically found stratigraphically above the Stavoren Granophyre.VanTongeren and Mathez (2015) showed that the immediate roof ofthe RLS contains a significant volume of metavolcanic hornfels andmicrogranite that they interpreted to be highly metamorphosed andremelted Damwal lava (Fig. 3).

Granites of the Lebowa Granite Suite are found throughout the roofof the eastern limb of the Bushveld Complex. The oldest and mostvoluminous phase of the granites is the Nebo Granite, which displayscrosscutting relationships with both the RLS and the Rooiberg Groupfelsites. Subsequent granitic magmas, including the Makhutso,Bobbejaankop, Lease, and Klipkloof granites, are either far removedfrom the RLS and Rooiberg or intrude and crosscut the main NeboGranite (Hill et al., 1996). It is for these reasons that the LebowaGranitesare thought to represent the final magmatic event in the emplacementof the greater Bushveld Complex.

The stratigraphic relationships of the various above lithologies aregeneralized in Fig. 1. It is important to note that the illustration doesnot represent a single stratigraphic section, but a composite of theknown relationships in the eastern Bushveld.

3. Hypotheses for the origin of the Bushveld Magmatic Province

3.1. Origin of the RLS

It is not the intention of this manuscript to review the literature onthe source of the RLS, but brief mention is warranted to place theanalyses into context. Several theories have been put forth includingimpact-induced volcanism or plume-source magmatism. It is clear

microgranite

hornfels

LS. VanTongeren andMathez (2015) interpreted the hornfels to represent highly thermallyby J. VanTongeren.

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from the petrology and isotope geochemistry of the RLS that multiplepulses of magma contributed to the construction of the RLS. Feederdikes from the region suggestmagma sourcesmay range in compositionfrom siliceous high magnesium basalts (Barnes, 1989) to tholeiiticbasalts. Some authors have proposed that the source magmas to theRLS underwent significant crustal assimilation in the lower to mid-crust prior to emplacement (Harris et al., 2005; Prevec et al., 2005),while others (e.g. Richardson and Shirey, 2008; Schoenberg et al.,1999) have concluded that it would be difficult to envision an assimila-tion scenario wherein the RLS would have acquired its current traceelemental and isotopic characteristics while still maintaining its overallmafic bulk composition.While numerous stable isotope (e.g. Harris andChaumba, 2001; Harris et al., 2005; Penniston-Dorland et al., 2012;Schiffries and Rye, 1989; Schiffries and Rye, 1990) and radiogenicisotope (e.g. Harmer et al., 1995; Hart and Kinloch, 1989; Kruger et al.,1987; Maier et al., 2000; McCandless and Ruiz, 1991; Richardson andShirey, 2008; Schoenberg et al., 1999; Sharpe, 1985) systems havebeen applied to understanding the origin of the RLS parent magmasand the crystallization history of the RLS magma chamber, there iscurrently no consensus in the literature on the volume, nature, andmechanisms of crustal assimilation experienced by the Bushveldmagmatic system. The results of these earlier studies are summarizedin Zirakparvar et al. (2014), but there are two important points thatbear on the results of this study. First, zircon εHf(2.06 Ga) values areuniform throughout vertical and lateral extent of the RLS, with anintrusion-wide average εHf(2.06 Ga) = −8.6 ± 1.2. Secondly, this valueis within error of the intrusion-wide average value displayed bythe 100-km distant Phalaborwa Complex, which is another Bushveld-age mafic–ultramafic intrusion (Wu et al., 2011). Zirakparvar et al.(2014) argued that neither of these features are consistent with signifi-cant crustal contamination and instead suggest that the negativeεHf(2.06 Ga) signature is that of the regional metasomatically enrichedsubcontinental lithospheric mantle (see further discussion below).

3.2. Origin of the Rashoop Granophyre Suite

In his comprehensive study of the field and geochemical relation-ships of the Rashoop Granophyre Suite, Walraven (1985, 1987)documented three distinctive phases of the Rashoop Granophyres,namely, the Stavoren Granophyre, the Diepkloof Granophyre, and theZwartbank Pseudogranophyre. The Stavoren Granophyre is mostvoluminous in that it exists as sills and dikes of varying thicknesses inthe roof rocks throughout the eastern Bushveld, at least where theroof is exposed. For example, where it was mapped by vonGruenewaldt (1972) in the Paardekop area of the eastern Bushveld(≈25°05′S, 29°45′E), the Stavoren forms a prominent sheet severalhundred meters thick. Based on field relations, he hypothesizedthat the granophyre represents the partially molten Rooiberg Grouplava formed at the contact with the RLS magmas, while Walraven(1985, 1987) suggested that the Stavoren Granophyre is an intrusiveequivalent to the Rooiberg lavas. The Zwartbank pseudogranophyre,which is present only in the northern segment of the eastern Bushveld,is described by Walraven (1985) as a partially molten and thermallymetamorphosed metasedimentary rock, and is therefore notpetrogenetically related to the RLS or Rooiberg Group. The DiepkloofGranophyre occurs where the RLS strikes E–W near the Kruis River.Here, Lombaard (1949) documented a gradual transition in mineralogyand texture over about 150 m stratigraphically from olivine dioritecumulate of the RLS into granophyre, suggesting that the granophyrewas formed by differentiation of RLS magma.

Recently, the relationships between the RLS, the Rooiberg Grouplavas, and the Rashoop Granophyre have been reevaluated. Mathezet al. (2013) showed that because of its generally metaluminous,ferroan character, the Stavoren Granophyre is themost likely candidateto represent the missing magma of the RLS derived by extensivefractional crystallization. In addition, VanTongeren and Mathez (2015)

documented the field relationships between the RLS and its immediateroof zone, a complicated mixture of microgranite and metavolcanichornfels previously referred to as ‘leptite’ (Fig. 3). They showed thatboth the microgranites and hornfels present in the immediate roof arecompositionally similar to the Damwal formation and concluded thatthey represent thermally metamorphosed (hornfels) and remelted(microgranite) Rooiberg lava.

3.3. Origin of the Rooiberg Group lavas

There are two hypotheses for the formation of the Rooiberg Grouplavas: partial melting of the Kaapvaal lower crust or fractional crystalli-zation of the RLS. Hatton and Schweitzer (1995) envisioned theRooiberg Group lavas formed by partial melting of the continentalcrust during passage of the mafic, plume-sourced, magmas of the RLS.Buchanan et al. (2002) followed up on this hypothesis by presenting ageochemical model of fractionation to generate the Kwaggasnek andSchrikkloof lavas by fractionation and mixing of the Damwal and RLSmagmas. Mathez et al. (2013) showed, however, that this hypothesisis inconsistent with the observed compositional trends. They furtherdemonstrated that the Damwal, Kwaggasnek, and Schrikkloof lavasare all ferroan and essentially identical in composition to ferroan dacitesand rhyolites associated with mafic rocks in other settings, such asIceland, and based on this concluded that ferroan members aredominantly products of fractional crystallization of mafic Bushveldliquids.

VanTongeren et al. (2010) noted that the upper Rooiberg Grouplavas have Sr and Nd isotopic compositions that overlap with themafic magmas of the RLS, thus questioning the crustal partial meltinghypothesis of Hatton and Schweitzer (1995) and Buchanan et al.(2002). VanTongeren et al. (2010) showed that adding the compositionof the upper Rooiberg Group lavas to the RLS Upper Zone bulk composi-tion is necessary to complete themajor and trace elementmass balanceof the RLS. They suggested that the upper Rooiberg Group lavas wereerupted from the shallow RLS magma chamber after protractedfractional crystallization. The eruption of the upper Rooiberg Grouplavas from the RLS magma chamber is further supported by the traceelement crystal-liquid equilibrium between the lavas and the upper-most ~300 m of RLS cumulates (e.g. VanTongeren and Mathez, 2012).

3.4. Petrogenesis of the mafic and felsic lithologies

The relations between the source magmas of the RLS, the RooibergGroup lavas, and the Stavoren Granophyre are investigated here usingHf isotopic compositions in magmatic zircons. Zircon has an extremelylow 176Lu/177Hf ratio, which mitigates uncertainty in the calculatedinitial 176Hf/177Hf value resulting from uncertainty in the sample'spresent-day 176Lu/177Hf ratio. This makes the Lu-Hf isotope system inzircon ideally suited for understanding how a magma relates to themajor terrestrial Hf isotope reservoirs (e.g. Belousova et al., 2010).Therefore, the two dominant hypotheses for the origin of the RooibergGroup lavas (partial melting of the continental crust or fractionalcrystallization of the magmas represented by the RLS) should be easilydistinguished by fingerprinting the isotopic composition of the sourceusing Hf isotopes in magmatic zircons.

4. Methods

4.1. Samples

Samples of the roof rocks analyzed for this study are from the CentralSegment of the Eastern Limb of the Bushveld Complex (Figs. 1, 2).Samples from the RLS were previously analyzed for zircon Hf isotopiccomposition by Zirakparvar et al. (2014), and are shown for referencein Figs. 4–6 Two samples corresponding to the granophyre, B07-053and B07-054, were chosen for zircon separation. An additional sample

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-13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4

Fig. 4. Zircon εHf (2.06 Ga) values for each individual analysis of the samples considered in this study: (a) Previous results fromzirconHf analyses of the RLS cumulate rocks from Zirakparvaret al. (2014). Red line indicates the Gaussian distribution of εHf (2.06 Ga) with a mean of−8.6 ± 2.6. (b) Results from individual samples of granophyre; red lines are identical to the RLSdistribution for comparison. (c) Results for individual samples ofmicrogranite; red lines are identical to the RLS distribution for comparison. Some samples (B07-051 and B07-042) did notyield a suitable quantity of zircons for analysis.

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of granophyre, B10-054, was previously analyzed by Zirakparvar et al.(2014). Four samples represent the microgranite lithology of theimmediate roof, B07-042, B07-049, B07-051, and B10-048. SampleB10-048 is from the Nebo Plateau in an area previously mapped byvonGruenewaldt (1968, 1971, 1972). No robust analyses of the hornfelsunit were obtained due to the very small grain size of zircons separated.Likewise, due to the aphyric nature of the Rooiberg Group lavas in theeastern Bushveld, it has not been possible to recover zircons fromthem. Following the conclusions of VanTongeren and Mathez (2015),the hornfels is likely highly metamorphosed Dullstroom/Damwal lava.If this conclusion is correct, the Hf isotopic composition of the hornfelsand its molten equivalent, the microgranite, is representative of thatof the original lavas.

4.2. Zircon Hf isotope analysis

Zircons were successfully separated from the seven samplesdescribed above. All of the samples are part of the collection at the

American Museum of Natural History (AMNH) and available to theacademic community for further study. Zircons were extractedfrom their host rocks using standard gravimetric and magnetic tech-niques at the AMNH and then selected for analysis under a binocularmicroscope based on adequacy of size and lack of inclusions. Grainswere then mounted in epoxy, polished to expose their interiors, andcleaned in dilute HCl prior to analysis by laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) atthe Lamont Doherty Earth Observatory of Columbia University-AMNHICP-MS laboratory using a New Wave® UP 193 FX excimer laserablation system coupled to a Thermo-Finnigan NeptunePlus® MC-ICP-MS with a spot-size of 50 μm and following the protocols specific tothis instrument (Zirakparvar, 2015; Zirakparvar et al., 2014).

Details of the analytical and data processing parameters (includingthe determination of mass bias factors for Hf, Lu, and Yb as well as theapplication of blank and interference corrections) are provided in fullin this paper's electronic appendix. The zircon standards Plešovice(Slama et al., 2008), Mud Tank, R33, 91500, and Penglai (Li et al.,

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-13.5 -13 -12.5 -12 -11.5 -11 -10.5 -10 -9.5 -9 -8.5 -8 -7.5 -7 -6.5 -6 -5.5 -5 -4.5 -3.5-4

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Fig. 5. Zircon εHf (2.06 Ga) values for each individual analysis grouped by lithology. Red lines are the RLS distribution.

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2010) were measured throughout this study, and the following long-term (~9 months) averages were obtained (where n = # of analyses,followed by the average of 176Hf/177 Hf values with 2σ errors(population standard deviation) and the corresponding εHf): Plešovice:n=265, 176Hf/177Hf = 0.282451 ± 38, εHf =−11.8 ± 1.4; Mud Tank:n=211, 176Hf/177Hf=0.282482±28, εHf=−10.7±1.0; R33: n=41,176Hf/177Hf = 0.282698 ± 89, εHf = −3.1 ± 3.2; 91500: n = 25,176Hf/177Hf = 0.282313 ± 93, εHf = −16.7 ± 3.3; Penglai: n = 7,176Hf/177Hf = 0.282901 ± 22, εHf = +4.1 ± 0.8. All these values arewithin error of the accepted values for these standards (refer to summa-ry of Table 1 in Fisher et al. (2014)). The values obtained for thePlešovice standard were used in applying a final session normalizationfactor (see Supplemental Material) to the unknowns relative topublished 176Hf/177Hf = 0.282482 and 178Hf/177Hf = 1.4672 forPlešovice (Slama et al., 2008).

5. Results

In Table 1, the Lu-Hf isotope data for each sample are summarized asaverages of individual analyses from that sample whereas the fullanalytical data set is provided in the electronic appendix. Summariza-tion of the data from each sample is possible because most samplesdisplay internally homogeneous zircon Hf isotopic compositions(Fig. 4). This is reflected in the low 2σ (in most cases) uncertainty onthe average 176Hf/177Hf values of each sample. For the averages inTable 1, the quoted uncertainties are 2σ of the population mean,whereas for the individual analyses (data provided in the electronicappendix) it is 2σ of the mean for the 60 cycles.

For the individual analyses (data provided in the electronic appen-dix), the initial 176Hf/177Hf values are corrected for ingrowth usingthat analyses' measured 176Lu/177Hf ratio. However, the uncertainty onthese initial values only reflects uncertainty in the present-day176Hf/177Hf value and does not account for analytical error in the176Lu/177Hf or any age-related uncertainty. In all of the tables and fig-ures, εHf (2.06 Ga) is calculated using present-day CHUR values of176Hf/177Hf= 0.282785 and 176Lu/177Hf=0.0336 (Bouvier et al., 2008).

Zircons from themafic cumulates of the RLS have a normally distrib-uted range in εHf (2.06 Ga) = −8.6 ± 1.2 (Fig. 5; data from Zirakparvaret al., 2014). Zircons from all microgranites analyzed and by association,the Rooiberg Group lavas, overlap in range with those of the RLS cumu-lates, with a peak at εHf (2.06 Ga) =−8.3 ± 1.7 (Fig. 5). Zircons isolatedfrom all samples of the Stavoren Granophyre also overlap with the RLSand Rooiberg samples, and have their peak at εHf (2.06 Ga) = −9.4 ±1.6 (Fig. 5).

6. Discussion

6.1. How are the different magma types related?

Despite minor variability in the average, the Hf isotopiccompositions of all of the roof lithologies investigated here arestatistically indistinguishable from that of the RLS mafic magmas atεHf (2.06 Ga) = −8.6 ± 1.2 (Figs. 4, 5, 6). The commonality betweenthe microgranite, representing partially molten Rooiberg Group lavas,the Stavoren Granophyre, and the RLS mafic magmas clearly rules outthe hypothesis of Schweitzer and Hatton (1995) that the Rooiberg

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LZ

UZ

no zircon availablefrom the MZ

Fig. 6. Summary of all average zircon εHf (2.06 Ga) data for each sample reported in thismanuscript as well as data from Zirakparvar et al. (2014) for the RLS. Y-axis delineatesdata from different lithologies (RLS, microgranite, granophyre) and is not to scale. RLSdata from Zirakparvar et al. are shown in stratigraphic order. LZ= Lower Zone; CZ=Crit-ical Zone; MR = Merensky Reef; MZ = Main Zone; UZ = Upper Zone. Gray-shaded re-gions correspond to the range of RLS values; dark gray = range of RLS sample averages,light gray = range of RLS sample uncertainties.

475J.A. VanTongeren et al. / Lithos 248–251 (2016) 469–477

Group lavas represent partial melts of the lower to middle crust due toheat from the rising RLS plume, and also rules out a crustal partial melt-ing origin for the Stavoren Granophyres. Partial melts at ~2.06 Ga of thecontinental crust currently exposed in the vicinity of the Bushveld Com-plex would be expected to have muchmore negative εHf (2.06 Ga) values(e.g. εHf (2.06 Ga) b −25), as discussed by Zirakparvar et al. (2014). Anyfelsic roof rock derived solely by partial melting of the regional crustwould therefore display a much more negative εHf (2.06 Ga) value thanis observed in our data set.

Table 1Average zircon Lu-Hf compositionSummary of sample-average zircon εHf (2.06 Ga) values obtained in this study. The full analyticalevel. Uncertainty on the initial 176Hf/177Hf values only takes into the consideration uncertaint

Sample ID Number ofanalyses

176Hf/177Hf Present day 176Lu/177Hf

± εHf ± ±

B07-042 n = 5 0.281233 168 −54.9 5.9 0.00065 29B07-049 n = 21 0.281271 44 −53.6 1.5 0.00076 140B07-051 n = 9 0.281262 114 −53.9 4.0 0.00104 46B07-053 n = 15 0.281264 106 −53.8 3.8 0.00158 165B07-054 n = 13 0.281259 94 −54 3.3 0.00144 90B10-048 n = 16 0.281282 75 −53.1 2.7 0.00109 58

*All errors are reported at 2 standard deviations of all the analyses within a sample for the ave

Instead, the overlapping Hf isotopic compositions between the felsicroof rocks and the mafic RLS magmas give rise to two possibilities:(1) the Rooiberg Group and Stavoren Granophyre were derived fromthe RLS magma chamber after significant fractional crystallization; or(2) all of the Bushveld Complex rocks, including the RLS, the RooibergGroup, and the Stavoren Granophyre were derived by variable degreesof melting from the same mantle source with little to no subsequentcrustal contamination.

In the case of the Stavoren Granophyre, the similarity in major andtrace element geochemistry between the RLS and the granophyresupports the conclusion of Mathez et al. (2013) that the granophyrewas generated by fractional crystallization of the mafic magma in theRLS chamber. This conclusion is also supported by the fact that theStavoren Granophyre crops out along the immediate roof of RLS and isnot found elsewhere.

In the case of the similarity in Hf isotopic composition betweenthe RLS and the Rooiberg Group lavas, there are two possibilities:(1) fractionation of the RLSmagmas and the Rooiberg Group in a similarshallow to mid-crustal staging chamber(s), and (2) fractional crystalli-zation of the RLS magma to yield the Rooiberg Group rhyolites. Thefirst possibility, fractionation of both mafic and felsic magmas in adeeper crustal ‘staging chamber’ is qualitatively similar to the hypothe-sis of Buchanan et al. (2002) for the generation of the upper RooibergGroup lavas, but without any crustal contamination. The idea of adeeper crustal staging chamber for the RLS has been invoked by severalresearchers to explain entrained crystal populations in themafic cumu-lates (e.g. Mondal and Mathez, 2007; among others). While a deeperstaging chamber may have been present during the emplacement anddifferentiation of the RLS, the major element compositions of the lavasindicate a low-pressure origin. In particular, the Kwaggasnek andSchrikkloof rhyolites are calc-alkalic to alkali-calcic and also plot nearthe granite Ab-Qz-Or ternary minimum at pressures between 0.1 and0.2 GPa (Mathez et al., 2013).

There are several additional reasons why we prefer a residual RLSliquid origin to explain the similarity in Hf isotopic composition of theRooiberg Group lavas and RLS. The major and trace element composi-tional similarity between the Kwaggasnek and Schrikkloof formationsand the missing magma from the RLS led VanTongeren et al. (2010)and VanTongeren and Mathez (2012) to conclude that the lavas mayalso represent the residual liquid from the RLS. In addition, the upperRooiberg lavas also overlap the RLS rocks in age (see Table 1 ofYudovskaya et al., 2013), and in initial Sr, Nd (e.g. Buchanan et al.,2004), and O isotopic compositions (e.g. Fourie and Harris, 2011).

6.2. What is the source?

Regardless of how the Rooiberg Group and Stavoren Granophyresare precisely related to the RLS, the uniform and unradiogenic(εHf (2.06 Ga) = −8.6) Hf composition is not entirely unique to

l data set is provided in the electronic appendix. Uncertainties are reported at the 2sigmay in the present-day 176Hf/177Hf and not uncertainty in the 176Lu/177Hf ratios.

176Yb/177Hf 178Hf/177Hf Meanintensities (V)

Calculated at 2.06 Ga

± ± 176 171 176Hf/177Hf εHf

0.0227 96 1.4674 22 0.7 0.03 0.281208 −9.20.0332 595 1.46727 8 1.6 0.06 0.281241 −8.10.0434 206 1.46724 11 0.3 0.03 0.281221 −8.80.0559 584 1.46733 11 0.9 0.12 0.281202 −9.40.0516 318 1.46737 13 0.8 0.1 0.281203 −9.40.0429 249 1.46724 8 0.6 0.07 0.28124 −8.1

rages.

476 J.A. VanTongeren et al. / Lithos 248–251 (2016) 469–477

the RLS and overlying Rooiberg Group and Stavoren Granophyres.For example, Zirakparvar et al. (2014) showed that the RLS has anεHf (2.06 Ga) that is nearly identical to another mafic–ultramafic intru-sion (the Phalaborwa Complex) that also intruded the northernmarginof the Kaapvaal Craton at ~2.06 Ga. However, the εHf (2.06 Ga) = −8.6value is distinct from the regional sediments (εHf(2.06 Ga) = −35 to−25) and upper continental crust (εHf(2.06 Ga)=−30 to−15) inferredto be the regional basement and country rocks present at the time ofmagmatism. By forward modeling the isotopic and trace elementcharacteristics of different reservoirs possibly involved in mixing,Zirakparvar et al. (2014) showed that it is not possible to produce theHf isotopic and trace element composition of the RLS magmas fromany amount of mixing between a depletedmantle melt and continentalcrust. Instead, they developed a model wherein the magmas of the RLSwere derived from a mafic–ultramafic metasomatically enriched reser-voir residing in the subcontinental lithospheric mantle beneath theKaapvaal Craton. They went on to show that the enrichment neededto produce such a reservoir could have happened as a result ofsubduction-related processes prior to the collision between theZimbabwe and Kaapvaal cratons, or could be related to an episode ofplume-related magmatism that affected the region ~600 m.y. beforethe emplacement of the RLS and related rocks. A similar conclusionwas reached by Laurent and Zeh (2015) for the Proterozoic evolutionof the Pietersburg Block in the northern Kaapvaal craton. Both of thesestudies are consistent with the observed Os isotopic compositions insulphides of the RLS Merensky Reef, indicating a significant role forenriched SCLM material (Richardson and Shirey, 2008) in the sourceof the Bushveld Complex.

7. Conclusion

Our data show that the RLS and the felsic roof rocks, both RooibergGroup lavas and Stavoren Granophyre, contain zircon with εHf (2.06 Ga)

values that are within error of each other (Fig. 6). We conclude thatboth the mafic and felsic magmas that contributed to the BushveldComplex were derived from the same magma source. Our data supportthe hypothesis of Mathez et al. (2013) and VanTongeren and Mathez(2015) that the Rashoop Granophyres are the likely missing magmafrom the RLS. Additionally, due to their isotopic similarity with theRLS, we cannot rule out the hypothesis of VanTongeren et al. (2010)that the upper Rooiberg Group lavas also represent themissingmagma.

We suggest that the upper Rooiberg Group lavas are likely derivedby fractional crystallization of the shallow crustal RLS chamber forseveral reasons: (1) they overlap in age with the RLS; (2) they havetrace element concentrations nearly identical to the estimated equilibri-um liquids at the top of the Upper Zone; (3) partialmelting of continen-tal crust (e.g. Schweitzer andHatton, 1995) is ruled out by the similarityin Hf isotopic composition; (4) it is difficult to explain the extreme,ferroan nature of these lavas by a process other than fractional crystalli-zation; but (5) at the same time fractional crystallization in a deepcrustal RLS staging chamber (e.g. Buchanan et al., 2002) is ruled outby the major element compositions of the lavas (Mathez et al., 2013).

Acknowledgements

This work was supported by NSF-EAR 0947247 awarded to E.A.Mathez and institutional funding from theAmericanMuseumofNaturalHistory to Alex Zirakparvar. We thank Louise Bolge of the LDEO ICP-MSfacility for her analytical expertise.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2016.02.007.

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