Petrogenetic modelling of strongly residual metapelitic xenoliths within the southern Platreef, Bushveld Complex, South Africa

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1 J. metamorphic Geol., 2010, 28, doi: /j x Petrogenetic modelling of strongly residual metapelitic xenoliths within the southern Platreef, Bushveld Complex, South Africa T. E. JOHNSON, 1 M. BROWN 2 ANDR.W.WHITE 1 1 Earth System Science Research Centre, Institute for Geosciences, University of Mainz, Becherweg 21, D Mainz, Germany (tjohnson@uni-mainz.de) 2 Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD 20742, USA ABSTRACT Xenoliths of quartz-absent Fe-rich aluminous metapelite are common within the platinum group element-rich mafic ultramafic magmatic rocks of the Platreef. Relative to well-characterized protoliths, the xenoliths are strongly depleted in K 2 O and H 2 O, and have lost a substantial amount of melt (>50 vol.%). Mineral equilibria calculations in the NCKFMASHTO system yield results that are consistent with observations in natural samples. Lower-grade rocks that lack staurolite constrain peak pressures to 2.5 kbar in the southern Platreef. Smaller xenoliths and the margins of larger xenoliths comprise micro-diatexite rich in coarse acicular corundum and spinel, which record evidence for the metastable persistence of lower-grade hydrous phases and rapid melting consequent on a temperature overstep of several hundred degrees following their incorporation in the mafic ultramafic magmas. In the cores of larger xenoliths, temperatures increased more slowly enabling progressive metamorphism by continuous prograde equilibration and the loss of H 2 O by subsolidus dehydration; the H 2 O migrated to xenolith margins where it may have promoted increased melting. According to variations in the original compositional layering, layers became aluminosilicate- and or cordierite-rich, commonly with spinel but only rarely with corundum. The differing mineralogical and microstructural evolution of the xenoliths depends on heating rates (governed by their size and, therefore, proximity to the Platreef magmas) and the pre-intrusive metamorphic grade of the protoliths. The presence or absence of certain phases, particularly corundum, is strongly influenced by the degree of metastable retention of lower-grade hydrates in otherwise identical protolith bulk compositions. The preservation of fine-scale compositional layering that is inferred to be relict bedding in xenolith cores implies that melt loss by compaction was extremely efficient. Key words: Bushveld Complex; corundum; melting; thermodynamic modelling; xenolith. INTRODUCTION The processes of partial melting and melt loss during high-grade metamorphism have fundamental implications for the bulk compositional, mineralogical, thermal and rheological evolution of the crust. During regional metamorphism, many of the important controls on melting and melt loss (principally heating rates and deformation) are complex and likely variable over the length- and timescale of the high-grade metamorphic event. In contact metamorphic environments, in which the temporal and spatial scales of metamorphism may be orders of magnitude smaller, the thermal evolution commonly can be accurately characterized and syn-intrusive deformation is more clearly understood. Advances in the thermodynamic models for phases of interest, including melt, have enabled quantitative forward and inverse modelling of the processes operating in partially molten siliciclastic rocks in increasingly sophisticated chemical systems (e.g. Holland & Powell, 2001; White et al., 2001, 2007; Johnson et al., 2008). In most regional migmatite terranes, it is problematic to forward model the melting and melt loss histories as unmelted protoliths to the migmatites rarely occur. In contrast, the steep thermal gradients that characterize contact metamorphic environments commonly result in the presence of both high-grade migmatites and their low-grade equivalents. The Platreef, part of the northern limb of the Bushveld Complex, is a world-class mafic ultramafic ore body rich in platinum group elements (PGE). There is abundant evidence for contamination of Platreef magmas with underlying country rocks (e.g. Harris & Chaumba, 2001; Harris et al., 2005; Sharman-Harris et al., 2005; Pronost et al., 2008). Contamination by partial melt derived from metapelitic rocks is suggested by sheets and veins of granite and the widespread occurrence of biotite within Platreef magmatic rocks (e.g. Kinnaird et al., 2005). Interaction with this granitic melt may have played a critical role in the petrogenesis of the PGE mineralization (e.g. Barton et al., 1986; Ruiz et al., 2004; Maier et al., 2008). This study focuses on xenoliths of highly residual (quartz-absent) Fe-rich aluminous metapelite of the 269

2 270 T. E. JOHNSON ET AL. Timeball Hill Formation that are abundant within the southern Platreef magmatic rocks. Mineralogically and texturally similar xenoliths occur elsewhere in the aureole of the eastern Bushveld Complex (Hall & Gardthausen, 1911; Willemse & Viljoen, 1970; see also Cameron, 1976) and are described from other basic intrusions (e.g. Preston et al., 1999; Markl, 2005). We integrate field, petrographic and bulk rock chemical data with mineral equilibria modelling to investigate the processes of metamorphism and partial melting. Quantitative constraints are placed on the P T conditions during metamorphism and on the amount of melt produced and subsequently lost as a function of temperature, bulk composition and pre-intrusive metamorphic grade of the protoliths. A particular focus is on the forward modelling of the varied aluminous assemblages using well-characterized protolith compositions, and on the constraints on the growth and preservation of corundum. GEOLOGICAL SETTING The Bushveld Complex, the northern limb and the Platreef Covering an area of some km 2, the Bushveld Complex in north-eastern South Africa is unique in its size and economic importance, hosting many important ores including much of the worldõs PGE. The Bushveld Complex was formed c Ma (Buick et al., 2001) by the intrusion of up to 8 km of layered mafic ultramafic rocks (the Rustenburg Layered Suite, RLS) and associated granite into rocks of the Transvaal sedimentary basin. The Bushveld Complex has four major sections, the eastern, western, northern (Mokopane or Potgietersrus) and southern (Bethal) limbs that are probably interconnected at depth (Hall, 1932; Cawthorn et al., 1998; Webb et al., 2004; Kruger, 2005; Clarke et al., 2009; Fig. 1, inset). The variability and cyclicity of mineral compositions and initial 87 Sr 86 Sr ratios of the RLS suggest formation via repetitive injections of mafic magma (e.g. Kruger & Marsh, 1982; Cawthorn & Walraven, 1998). Cawthorn & Walraven (1998) calculated that the RLS magmas were emplaced within years and had fully crystallized after c years. The composition of cumulate orthopyroxene implies magmatic temperatures of C (Cawthorn & Biggar, 1993). The northern limb of the Bushveld Complex has a maximum outcrop width of 15 km and extends northwards for over 100 km (Fig. 1). The rocks generally dip moderately to steeply towards the west Fig. 1. Simplified geological map of the southern and central sectors of the northern limb of the Bushveld Complex (after Nell, 1985). Core samples are from the farm Turfspruit located within the circle. The inset shows a simplified regional map of the Bushveld Complex.

3 MODELLING ALUMINOUS METAPELITIC XENOLITHS 271 or south-west. The mafic ultramafic rocks in the northern limb show an onlap relationship with the footwall rocks, in which the magmatic rocks thin northwards and eventually pinch out. The Platreef is a wide (up to 400 m) irregularly mineralized discordant facies at the base of the mafic ultramafic sequence that extends from just north of Mokopane for km northwards (Fig. 1). It comprises mafic units enriched in Ni Cu PGE and contains numerous footwall xenoliths. The variable lateral distribution of PGE in the Platreef suggests a complex pre-, syn- and post-magmatic history that appears to be related primarily to the lithology of the underlying footwall rocks. Studies have suggested a variety of triggers for sulphur saturation and precipitation ranging from strictly magmatic processes (Hulbert & von Gruenewaldt, 1982; Barton et al., 1986) to interactions with the various footwall lithologies transgressed by the Platreef (Buchanan et al., 1981; Buchanan & Rouse, 1982; Gain & Mostert, 1982; Cawthorn et al., 1985; Harris & Chaumba, 2001). Others have proposed that hydrothermal fluid flow was the dominant control on PGE distribution (Armitage et al., 2002). Although d 34 S values provide strong evidence for a contribution of crustal sulphur derived from footwall rocks in the southern Platreef (Buchanan & Rouse, 1982; Sharman- Harris et al., 2005), Penniston-Dorland et al. (2008) suggested the Platreef magmas were sulphur-saturated prior to any footwall interaction. The metamorphic aureole of the Bushveld Complex The RLS magmatic rocks underlie felsic volcanic rocks of the Rooiberg Group and overlie metasedimentary and metavolcanic rocks of the Ga Transvaal Supergroup and granite greenstone Archean basement (Armstrong et al., 1991; Catuneanu & Eriksson, 1999; Eriksson et al., 2001; Fig. 1). An extensive contact metamorphic aureole is developed in which economic deposits of iron, manganese, asbestos, andalusite, gold, fluorine, lead, zinc and tin occur (e.g. Eriksson et al., 1995). The aureole of the Bushveld Complex is unusual both in its scale (extending up to 4 km normal to the contact and up to 25 km in outcrop width) and in the fact that it is largely developed in footwall rocks underlying the intrusion. The thermal peak diminished and was reached at progressively later times with increasing depth below the contact (i.e. the thermal gradient was inverted relative to the common situation during regional metamorphism). In the eastern limb, where the aureole is widest and the full thickness of the RLS is present, the basal RLS country rock contact is generally bedding-parallel and within rocks of the upper Pretoria Group. At lower structural and stratigraphic levels, greenschist facies metapelitic rocks of the Lower Timeball Hill Formation (Lower Pretoria Group) contain abundant chloritoid and chlorite. Amphibolite facies rocks from the Upper Timeball Hill Formation contain assemblages rich in staurolite, andalusite and biotite that record pressures of 3 kbar (e.g. Waters & Lovegrove, 2002; Fig. 2a). In contrast, the RLS country rock contact in the northern limb is strongly transgressive from Upper Pretoria Group metasedimentary rocks in the southern section to basement Archean granitic orthogneiss in the north (Fig. 1). Nell (1984, 1985) studied in detail the metamorphic evolution of calcsilicate and metapelitic rocks in the northern limb up to their contact with the mafic ultramafic rocks. Lower-grade (amphibolite facies) rocks of the Timeball Hill Formation here contain abundant andalusite and biotite with or without muscovite, chlorite, chloritoid and cordierite, but lack staurolite. High-grade (pyroxene hornfels facies) equivalents traced to within a few metres of the contact generally lack quartz and variably contain aluminous assemblages rich in aluminosilicate, cordierite, spinel and corundum (Nell, 1984, 1985). Metapelitic rocks of the underlying Duitschland Formation are rich in orthopyroxene and cordierite and unusual parageneses such as cordierite + olivine + orthopyroxene + spinel + K-feldspar occur in meta-ironstones of the Penge Formation (Nell, 1984, 1985). The preservation of these minerals (e.g. White & Powell, 2002), their microstructural relations and the highly residual composition of the rocks relative to the lower-grade equivalents imply extreme degrees (>50%) of melting and subsequent melt loss (Nell, 1984, 1985). Throughout the aureole there is clear evidence for invasion and contamination of the contact proximal RLS by granitic melt in the form of sheets and veins (e.g. Willemse & Viljoen, 1970; Buchanan & Rouse, 1984; Cawthorn et al., 1985; Barton et al., 1986; Johnson et al., 2003). Calculations using the mineral compositions reported by Nell (1984) suggest peak temperatures of 800 C at the contact (Buick et al., 2004), consistent with estimates from the eastern limb (Johnson et al., 2003). Sample description At its southernmost extent, the Platreef is in direct or close contact with metapelitic rocks of the Duitschland and Timeball Hill Formations (Fig. 1). Drill cores from the farm Turfspruit (Fig. 1), made available by Ivanhoe Nickel and Platinum Ltd., intersect variably sized xenoliths of hornfels and, where stratigraphically appropriate, extend into the metapelitic footwall. Drill cores range from a few hundred to over 1000 m in length and transect the entire thickness of the Platreef. The xenoliths are commonly fine-to-medium grained and many are dark purplish grey reflecting an abundance of fresh cordierite. Cores commonly intersect sheets of graphic granite several tens of centimetres or more in thickness (e.g. Kinnaird et al., 2005; Hutchinson & Kinnaird, 2005; Fig. 2b).

4 272 T. E. JOHNSON ET AL. (a) (b) (c) (d) (e) (f) Fig. 2. Photomicrographs of core samples. (a) Photomicrograph of a typical finely layered sample of Timeball Hill Formation andalusite staurolite biotite metapelite from Annesley andalusite mine in the lower-grade part of the aureole in the eastern limb. (b e) Aluminous assemblages within a 10-m thick xenolith of Timeball Hill Formation metapelite within core ITS041: (b) graphic granite sheet at the top of the xenolith bounded by Platreef ultramafic rocks rich in biotite and sulphides (top). Beneath the granite sheet the Platreef rocks contain abundant feldspar patches; (c) decussate corundum- and spinel-rich micro-diatexite horizon 2 m from the top of the xenolith. The dark layer at the top left of the central core comprises >90% spinel with feldspar. The majority comprises subequal proportions of corundum and spinel with abundant feldspar patches; (d) the central and lower parts of the xenolith are finely layered and comprise paler horizons rich in cordierite + aluminosilicate and darker horizons rich in spinel (±cordierite, aluminosilicate, corundum). Note the close spatial association of pale feldspar surrounding euhedral spinel and the rare corundum porphyroblasts; (e) finely laminated horizon 3 m from the base of the xenolith rich in spinel porphyroblasts (left) and cordierite + aluminosilicate + spinel (right). Thin dark-coloured spinel-rich layers on the far right additionally contain corundum. Sample ITS041 H4, discussed in the text, is from the area indicated. (f) Small xenolith comprising spinel within abundant feldspar. The acicular habits of many spinel grains suggest they are replacing corundum. Core diameters for all samples are mm.

5 MODELLING ALUMINOUS METAPELITIC XENOLITHS 273 Xenoliths of Timeball Hill Formation metapelite range from a few centimetres to several metres in thickness. A 10 m thick xenolith within core ITS041 contains the complete range of observed aluminous assemblages (Fig. 2b e). The xenolith is layered on a millimetre to decimetre scale and many horizons contain conspicuous porphyroblasts of spinel and or corundum. At the margins of the xenolith, corundum is abundant as variably oriented acicular glassy-grey porphyroblasts, up to 30 mm in length, within layers rich in leucocratic patches of feldspar and which lack internal compositional layering (Fig. 2c). These layers contain abundant spinel that may comprise >50 vol.% of individual layers (Fig. 2c). Similar assemblages are preserved in the smallest xenoliths that comprise abundant feldspar with acicular spinel grains that are inferred to have replaced corundum (Fig. 2f). In all cases, contacts between the metasedimentary xenoliths and the host Platreef are sharp. In the centre and further towards the base of the same 10m thick xenolith, layers commonly contain equant porphyroblasts of dark-green spinel, up to a few millimetres in diameter, which are surrounded by pale feldspar patches (Fig. 2d,e). These layers contain abundant aluminosilicate and cordierite with rare equant porphyroblasts of corundum (Fig. 2d). Where porphyroblasts are rare or absent, the rocks preserve fine-scale millimetric layering with distinct horizons rich in cordierite, aluminosilicate or both (± finegrained spinel; Fig. 2d,e). Biotite is abundant, with sulphide minerals, as coarse plates within the enclosing Platreef mafic rocks (Fig. 2b, top). Rafts of Duitschland Formation metapelite are up to 70 m in thickness and comprise less aluminous cordierite-rich metapelite containing conspicuous porphyroblasts of orthopyroxene, cordierite and, rarely, garnet. Although the abundance of phases is highly variable, assemblages are much less varied than those within xenoliths of the Timeball Hill Formation and are not described in detail here. Petrography Xenoliths and or layers within xenoliths of Timeball Hill Formation metapelite are here subdivided into: (i) those rich in aluminosilicate, cordierite or both (usually with spinel), and (ii) those rich in corundum and or spinel, although all these phases may co-exist in individual samples. All samples contain fine-grained ilmenite and lack quartz and prograde biotite (red brown pleochroic biotite is rarely present as a retrograde product). All samples also contain patches of anhedral coarse-grained plagioclase (colourless) and or alkali feldspar (pale brown) that are commonly intergrown. In many cases, single optically continuous grains of feldspar many millimetres or tens of millimetres in size enclose porphyroblast and or matrix phases. The abundance of feldspar varies greatly from sample to sample, from a few vol.% to >50 vol.%. Aluminosilicate- and cordierite-rich layers The boundary between laterally continuous layers richer in aluminosilicate and those richer in cordierite may be gradational but is commonly sharp (Fig. 3a,b). Aluminosilicate-rich layers may contain more than 50 vol.% aluminosilicate, which occurs as prismatic grains generally less than 0.5 mm in length and which commonly define a weak to moderate preferred orientation that is generally parallel or subparallel to compositional layering (Fig. 3b). Cordierite-rich horizons have a typical granoblastic microstructure (Fig. 3b). Isolated, intergranular patches or larger optically continuous patches of anhedral cuspate feldspar enclose matrix phases, but the proportion of feldspar in these layers is generally low. Porphyroblasts of spinel are common within cordierite- and aluminosilicate-rich layers (Figs 2d,e & 3a f). Where conspicuous patches of feldspar are absent, individual spinel grains are resorbed and partially replaced by lobate cordierite (Fig. 3c). In contrast, large, unresorbed euhedral porphyroblasts of spinel several millimetres across are located within single grains of feldspar that also enclose matrix minerals at their margins (Fig. 3d). Corundum occurs only rarely as stubby euhedral hexagonal porphyroblasts of a similar size to those of spinel, where it is commonly partially replaced by fine-grained hydrous phases, mostly muscovite (see Fig. 2d). Spinel- and corundum-rich layers Spinel- and corundum-rich layers lack internal compositional layering and may be volumetrically dominated by feldspar. In finely layered samples, individual horizons several millimetres thick may comprise more than 50 vol.% euhedral spinel that is commonly characterized by thin overgrowths of cordierite and partly altered to diaspore (Fig. 3e). In these layers, spinel grains are contained within a single optically continuous feldspar grain that extends across the width of the layer and may extend into neighbouring layers. Corundum occurs in two microstructural variants. First, corundum occurs as rare stubby, euhedral porphyroblasts within thin spinel- and feldspar-rich layers described above (Fig. 3f). Most commonly, however, corundum occurs as large (up to 30 mm long) prismatic, skeletal porphyroblasts at the margins of xenoliths. These form spectacular randomly oriented aggregates within patches of coarse-grained plagioclase and pale brown alkali feldspar, in which feldspar may constitute >50 vol.% (Fig. 3g,i m). Porphyroblasts of spinel are ubiquitous in the layers, although spinel is largely absent from feldspar patches immediately surrounding corundum (Fig. 3j). In some instances, corundum forms subhedral cores of composite grains with spinel (Fig. 3h). More commonly, acicular corundum is partially to completely replaced by spinel (Fig. 3i).

6 274 T. E. JOHNSON ET AL. 1 mm 0.5 mm 10 mm (a) (b) (c) 2 mm 0.5 mm 1 mm (d) (e) (f) 2 mm 0.5 mm 0.5 mm (g) (h) (i) 1 mm 1 mm 2 mm (j) (k) (l) Fig. 3. Photomicrographs showing the variety of assemblages and microstructures within Timeball Hill Formation xenoliths. (a) Examples of fine-scale layering. The sample at the top is from the right-hand side of Fig. 2e and comprises alternating spinel-, aluminosilicate- and cordierite-rich layers. The lower coarser-grained sample comprises aluminosilicate- and cordierite-rich horizons. Feldspar contents in both samples are very low. (b) Close-up of part of the lower sample shown in (a) showing sharp contact between aluminosilicate- and cordierite-rich layers. (c) Lobate replacement of spinel by cordierite in an aluminosilicate- and cordierite-rich layer. (d) Euhedral porphyroblasts of spinel within optically continuous alkali feldspar grain that extends into the aluminosilicate and cordierite-dominated matrix. Corundum porphyroblasts of a similar size to those of spinel occur 3 mm to the left of the larger spinel. (e) Euhedral spinel with rims of cordierite within optically continuous feldpar patch [from area within white circle in (a)]. The cores of many spinel grains are altered to diaspore + magnetite ilmenite. (f) Thin spinel-rich layer containing skeletal corundum porphyroblasts and associated with optically continuous feldspar [from area within black circle in (a)]. Layers either side are feldspar-deficient and rich in aluminosilicate. (g) Decussate corundum-bearing micro-diatexite layers comprise porphyroblasts of corundum and spinel within coarse-grained feldspar patches that comprises 50 vol.%. These rocks retain no internal layering. (h) Porphyroblasts of spinel within coarse feldspar rarely contain cores of corundum. (i) Replacement of acicular corundum by spinel in feldspar-rich rocks. (j) Skeletal acicular corundum surrounded by feldspar that is in optical continuity with matrix of tightly packed euhedral cordierite and spinel. The feldspar patches comprise intergrown plagioclase (colourless) and pale brown alkali feldspar [also seen in (k)]. (k) Rare assemblage containing corundum, aluminosilicate, cordierite and spinel (not in field of view). (l) Highly resorbed angular fragment of granoblastic cordierite- and aluminosilicate-rich matrix surrounded by feldspar and corundum. Spinel is also abundant. All photomicrographs in plane polarized light except (l).

7 MODELLING ALUMINOUS METAPELITIC XENOLITHS 275 In most corundum-rich samples, cordierite is rare or absent. However, the margins of feldspar patches that surround acicular corundum may be characterized by a granoblastic matrix of euhedral cordierite and spinel with cuspate interstitial feldspar that is in optical continuity with the feldspar surrounding corundum (e.g. Fig. 3j). Only rarely do samples contain both coarse-grained acicular corundum and aluminosilicate, although the two may occur in contact (Fig. 3k). Some samples contain small patches of granoblastic cordierite aluminosilicate-dominated matrix that is highly resorbed in proximity to corundum and spinel (Fig. 3l). The full range of assemblages developed in aluminous metapelites, all of which co-exist with ilmenite and feldspar, is: cordierite, cordierite + aluminosilicate, cordierite + spinel, cordierite + spinel + aluminosilicate, corundum + cordierite + spinel + aluminosilicate, corundum + cordierite + spinel, corundum + spinel and spinel. All occur within a single-layered thin section [ITS041 H4; Figs 2e, 3a(top) & 4]. a b c d e f Mineral chemistry Minerals were analysed using the JEOL JXA 8900 RL electron probe microanalyser at the University of Mainz with an accelerating voltage of 15 kv, a beam current of 12 na and a beam diameter of 2 lm (5lm for feldspar). Both natural and synthetic standards were used for calibration. Table 1 shows selected mineral analyses from the finely layered sample ITS041 H4 [Figs 2e, 3a(top) & 4], in which the variation in mineral chemistry is broadly representative of the aluminous xenoliths as a whole. Minor elements in spinel were measured separately with modified operating conditions of 20 kv and 20 na and appropriate standards. In all aluminous samples, spinel, cordierite, aluminosilicate and feldspar may exhibit significant major element variability. No minerals exhibit any significant intragranular compositional zoning. Whereas molar Fe (Fe + Mg) proportions (X Fe ) vary, those of spinel are consistently higher than those of neighbouring cordierite. Where fine-scale layering is preserved (e.g. Fig. 4), the X Fe of spinel and cordierite is constant within individual layers but may vary significantly from layer to layer. In corundum-rich samples that lack internal compositional layering and contain both cordierite and spinel (e.g. Fig. 3k,l), these phases may exhibit significant compositional variation on a thin-section scale. However, in every case the most Ferich cordierite is always spatially associated with the most Fe-rich spinel. Cordierite has X Fe in the range , although values are commonly The most Fe-rich cordierite co-exists with the most Fe-rich spinel (X Fe = ) in the finely layered sample shown in Fig. 3a (bottom). The compositional variability of minerals from layer to layer in this sample is small. The g h i j Fig. 4. Photomicrograph of sample ITS041 H4 showing the various compositional layers, representative mineral analyses from which are shown in Table 1. The dark-coloured layers b,d,f,h and j are rich in spinel and feldspar and most contain rare corundum (circled). Paler layers are rich in aluminosilicate and cordierite and deficient in feldspar (width of thin section is 22 mm). most Mg-rich cordierite co-exists with aluminosilicate in isolated fragments within feldspar in the corundumrich rock shown in Fig. 3l, in which nearby spinel has X Fe of 0.58, the most Mg-rich measured. Na in cordierite is in the range cations per formula unit (based on 18 O), but is variable within individual layers. MnO in cordierite is commonly 0.10 wt% and wt% in spinel. Minor element concentrations (in wt%) of 98 analyses of spinel from sample ITS041 H4 (Fig. 4) are in the ranges: V 2 O 3 = , NiO = , ZnO = , Cr 2 O 3 = and CoO = Feldspar compositions are highly variable. In many rocks (e.g. Fig. 3d,g), feldspar is almost exclusively albite containing <5 mol.% anorthite and 1 2 mol.% orthoclase. In some samples (e.g. Fig. 3j), feldspar is dominantly mol.% orthoclase with rare

8 276 T. E. JOHNSON ET AL. Table 1. Selected representative EPMA composition of phases within sample ITS041 H4. Phase Biotite Cordierite Corundum Feldspar (in leucosome) Ilmenite Al-silicate Spinel Analysis Layer a a c e g h a b b j j j c a a b c d e f g h j SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K2O Cr 2 O Total O Si Ti Al Fe Mn Mg Ca Na K Cr Sum X Fe Xan X or

9 MODELLING ALUMINOUS METAPELITIC XENOLITHS 277 albite-rich grains. Other samples (e.g. Fig. 3k,l) contain relatively Ca-rich plagioclase (40 50 mol.% anorthite) that co-exists with albite. The full spectrum of feldspar compositions occurs within ITS046 H4 (Fig. 4). Aluminosilicate in all xenoliths is a solid solution between mullite and sillimanite (Fig. 5). Based on 24 analyses from five separate samples, more mullite-rich compositions are more abundant than sillimanite-rich compositions, although there is no clear systematic variation. The full range of compositions shown in Fig. 5 occurs within sample ITS041 H4. TiO 2 contents are wt% in mullite-rich and <0.1 wt% in sillimanite-rich compositions, with Fe-contents (assumed to be all ferric) of wt%. Ferric iron contents in corundum are up to 0.5 wt%. Rare grains of retrograde biotite containing 2 4 wt% TiO 2 are most commonly spatially associated with corundum porphyroblasts. Bulk compositional data Figure 6 shows bulk compositional data plotted in ternary projections in which bulk compositions were first reduced to the KFMASH subsystem by accounting for the Ca associated with P in apatite and then the Si and Al associated with Na and Ca in plagioclase. Compositions of low-grade protoliths are taken from the literature (Bo hmer, 1977; Nell, 1984, 1985; Schreiber et al., 1992; Eriksson et al., 1994; Reczko, 1994; Eriksson & Reczko, 1995; Coetzee, 2001; Catuneanu & Eriksson, 2002). Compositions of high-grade xenoliths were measured by XRF using the Philips Magix Pro at the University of Mainz for which major elements 6.0 Al pfu Mullite Sillimanite Si pfu Fig. 5. Aluminosilicate compositions (n = 24) from five separate samples based on 13 O pfu. concentrations are accurate to <1% relative (except Na 2 O, accurate to <1.5% relative). Protoliths Figure 6a shows protolith bulk compositions projected onto AFM from muscovite, quartz and H 2 O, together with end-member compositions of appropriate greenschist and lower-amphibolite facies minerals (Thompson, 1957). The two tie-lines show the composition of biotite co-existing with andalusite in two samples of amphibolite facies Timeball Hill metapelite reported by Nell (1984). In general protoliths are Al- and Fe-rich and Ca- and Mg-poor compared to average shales worldwide, including those from other stratigraphic levels within the Transvaal sedimentary basin, and contain highly variable quantities of organic carbon (0 12%; Bo hmer, 1977; Eriksson et al., 1994). Molar FeO (FeO + MgO) values are 0.80 ± 0.24 (2r). All but five of the Timeball Hill compositions are aluminous metapelite that project above (i.e. at higher Al 2 O 3 ) the garnet chlorite join; a significant number plot above the chloritoid cordierite join (Fig. 6a). Figure 6b shows bulk compositions projected onto AS(F + M) from muscovite and H 2 O. With the compound apex FeO + MgO, assemblages within projected tie triangles are trivariant rather than divariant. Although there are significant variations in the Fe Mg ratio of the rocks (Fig. 6a), the Timeball Hill metapelites define a broad compositional trend plotting away from the S-apex on either side of the quartz chloritoid join (Fig. 6b). This trend suggests variable SiO 2 was a first-order control on the compositional heterogeneity of the protoliths, presumably due to a variable supply of fine sand or silt. Comparing xenoliths and protoliths Figure 6c shows xenolith and protolith bulk compositions projected onto AFM from K-feldspar, quartz and H 2 O (Thompson, 1957). Figure 6d shows these compositions projected onto AS(F + M) from K-feldspar and H 2 O (the black star is the projected composition of a granite sheet from the top of the xenolith in core ITS041; Fig. 2b). Although these projections are not appropriate for assemblages within high-grade xenoliths that lack primary quartz and commonly lack K-feldspar and that would not be expected to develop a free H 2 O volatile phase on cooling (e.g. White & Powell, 2002), they are nonetheless instructive in examining the change in bulk composition of xenoliths relative to protoliths. Projected from K-feldspar, the vast majority of the protolith compositions plot at higher or much higher Al 2 O 3 than the spinel cordierite join in AFM (Fig. 6c), consistent with the commonly observed association of aluminosilicate, cordierite and spinel. Relative to the protoliths, the high-grade xenoliths generally plot at higher Al 2 O 3 and MgO (Fig. 6c) and at much lower

10 278 T. E. JOHNSON ET AL. Fig. 6. KFMASH projections of Timeball Hill Formation bulk rock compositions: (a) AFM projection from muscovite, quartz and H 2 O; (b) AS(FM) projection from muscovite and H 2 O; (c) AFM projection from K-feldspar, quartz and H 2 O; (d) AS(FM) projection from K-feldspar and H 2 O. NellÕs (1984) data are five greenschist to lower-amphibolite facies metapelite samples from the southern part of the northern limb, rocks that can be traced along strike to their contact with the Platreef further north (Fig. map); ReczkoÕs (1994) data are from the eastern limb (i.e. the north-eastern Transvaal basin); Bo hmerõs (1977) data are from black shales at the base of the Timeball Hill Formation in the south of the basin. CoetzeeÕs (2001) data are from the western limb. The white star shows the position of the average Timeball Hill metapelite used in several pseudosections. The black star in (d) is the composition of a granite sheet. SiO 2 (Fig. 6d). These bulk compositional changes are consistent with extensive loss of melt as documented by Nell (1984, 1985). MINERAL EQUILIBRIA MODELLING Mineral equilibria calculations in the NCKFMASHTO (Na 2 O CaO K 2 O FeO MgO Al 2 O 3 SiO 2 H 2 O TiO 2 O) system using THERMOCALC 3.33i (Powell & Holland, 1988) with the ds55 data set (Holland & Powell, 1998; November 2003 update) use the following a x models: biotite and melt (White et al., 2007), garnet (Diener et al., 2008a; modification after White et al., 2007), orthopyroxene, spinel and magnetite (White et al., 2002), amphibole (Diener et al., 2007), cordierite (Holland & Powell, 1998), K-feldspar and plagioclase (Holland & Powell, 2003), white mica (Coggon & Holland, 2002) and ilmenite hematite (White et al., 2000). Mn is not considered for the reasons given in White et al. (2007). The modelling uses the thermodynamic data of endmember sillimanite for the aluminosilicate phase. Modelling aluminosilicate as a sillimanite mullite solid solution is currently not possible as end-member data for mullite are not included within the Holland & Powell (1998) data set. While this has implications for the absolute thermal stability of the aluminosilicate phase at the highest temperatures, this effect is probably small. Additional limitations exist for this and other modelled phases, particularly with respect to minor elements that may occur in significant concentrations at the high temperatures experienced by the xenoliths (e.g. Fe 3+ and Ti in corundum, aluminosilicate and quartz). Where given, the modelled proportions of phases are on a molar basis normalized to one oxide to approximate volume proportion. Concentrations of ferric iron in the protoliths are unknown and fixed (to 5% of total Fe) such that hematite is not stable in the pseudosections for amphibolite facies subsolidus rocks (Fig. 6), consistent with the lack of reported hematite (Nell, 1984, 1985). The primary effect of varying the amount of O is to alter the Fe 2+ (Fe 2+ + Mg) ratio (see White et al., 2000; Johnson et al., 2008). Phase abbreviations are as follows: opx = orthopyroxene; g = garnet; cd = cordierite; sp = spinel; ged = gedrite; bi = biotite; mu = muscovite; pa = paragonite; and = andalusite; ky = kyanite; sill = sillimanite; ksp = alkali feldspar; pl = plagioclase feldspar; ilm = ilmenite; mt = magnetite; q = quartz; cor = corundum; liq = silicate melt. Calculations use several bulk compositions detailed in Table 2. Lower-grade metamorphism and pressure constraints A consideration of the lower-grade (subsolidus) metamorphism is important to: (a) constrain postemplacement pressures enabling subsequent modelling

11 MODELLING ALUMINOUS METAPELITIC XENOLITHS 279 Table 2. Normalized NCKFMASHTO compositions used in the calculation of Figs 6 10 (in mol.%). Figure H 2 O SiO 2 Al 2 O 3 CaO MgO FeO K 2 O Na 2 O TiO 2 O lhs rhs a lhs a rhs b lhs b rhs a lhs a rhs b lhs b rhs lhs rhs Fig. 7. Subsolidus NCKFMASHTO P T pseudosection based on NellÕs (1984) sample PH-122. The stability field of staurolite (highlighted) provides an upper pressure limit for the level of the Timeball Hill Formation in the northern limb. The dotted line shows the limit of staurolite for an assemblage in equilibrium with graphite and a C O H fluid phase as calculated by adding 1 mol.% carbon to the bulk given in Table 2. The star shows the position of the calculated assemblage most consistent with that reported by Nell (1984) in sample PH-122. in T X space; and (b) investigate the potential role of H 2 O-retention in protoliths (Buick et al., 2004). With respect to the latter, the abundance of structurally bound H 2 O in lower-grade protolith assemblages is dependent on the pre-intrusive temperature of the country rocks. The presence of a significant volume of pre-bushveld sills (Buchanan et al., 1981; Cawthorn et al., 1981; Sharpe & Hulbert, 1985; Engelbrecht, 1990) will have raised the ambient temperature in the footwall rocks prior to intrusion of the main body of the RLS and partially dehydrated them. However, constraining the precise pre-intrusive metamorphic grade of the protoliths (and by inference their bulk H 2 O contents) is problematic. Figure 7 shows a P T pseudosection for the composition of sample PH-122 (Nell, 1984; Fig. 6; Table 2). This sample is a lower amphibolite facies Timeball Hill metapelite from the southern part of the northern limb, close to the average composition of five samples reported by Nell (1984), which contains cordierite, chloritoid, andalusite, biotite, chlorite, white mica, plagioclase, quartz and opaque minerals. Calculated low-grade assemblages for PH-122 are rich in chloritoid, chlorite and muscovite, consistent with

12 280 T. E. JOHNSON ET AL. stratigraphically equivalent low-grade rocks elsewhere in the aureole (e.g. Nell, 1984, 1985; Waters & Lovegrove, 2002). Prograde paths to higher temperature cross a region of low-variance fields that record the progressive consumption of the low-grade hydrous phases chloritoid, chlorite and white mica and the growth of biotite and andalusite (or sillimanite) with cordierite (at lower pressure), staurolite (at higher pressure) or both over a temperature interval of C (Fig. 7). This region of low variance results in significant dehydration of the rocks. The lower-p limit of assemblages containing staurolite provides a maximum pressure constraint for the contact metamorphism. The staurolite stability field is shown in Fig. 7 both for rocks in equilibrium with pure H 2 O and for those in equilibrium with graphite and a mixed H 2 O CO 2 CH 4 volatile phase with 1 mol.% carbon added to the bulk composition (Table 2; R. Powell, pers. comm.). Nell (1984) did not report staurolite from any rocks in the northern limb, although this phase is abundant in Timeball Hill metapelitic rocks in the aureole of the eastern limb (Fig. 1a), where assemblages are consistent with pressures of 3 kbar (Waters & Lovegrove, 2002; Johnson et al., 2003). Although not known precisely, the RLS must be at least 4 5 km thick in the area of the southern Platreef (Kinnaird et al., 2005). The combined thickness of the RLS and overlying Rooiberg Group Transvaal Supergroup rocks and the lack of staurolite in metapelitic rocks together imply postemplacement pressures of 2.5 kbar at the stratigraphic level of the Timeball Hill Formation for this part of the northern limb. The low-variance assemblage reported by Nell for sample PH-122 (invariant when reduced to KFMASH) cannot be reconciled with Fig. 7. In particular, the stable co-existence of chloritoid and cordierite is not predicted, even at very low pressure. Metastability is supported by the microstructures reported by Nell (1984), in which chloritoid is strongly resorbed in contact with cordierite. However, it is likely that metastable persistence and or delayed nucleation of particular phases was controlled by the sluggish reaction kinetics consequent on the relatively low temperatures and slow heating rates distal to the contact with the mafic ultramafic rocks (Waters & Lovegrove, 2002). Omitting chloritoid, the assemblage reported for PH-122 (at 2.5 kbar) is stable at the temperature shown by the star in Fig. 7, suggesting peak temperatures of 540 C. Figure 7 shows that, with relatively weak greenschist facies metamorphism (<450 C) related to the pre-bc sills, assemblages in Timeball Hill protoliths immediately prior to intrusion of the RLS (i.e. at lower pressure) would have been rich in the hydrous phases chlorite, muscovite and chloritoid. Higher-T amphibolite facies metamorphism (>500 C) would have resulted in (relatively) dehydrated assemblages rich in cordierite, andalusite, biotite and, at higher temperature, K-feldspar (Fig. 7). For example, the assemblage chloritoid chlorite muscovite in PH-122 requires mol.% H 2 O for saturation at 450 C and 1.0 kbar (an estimate of the pre-intrusive pressure). Up temperature isobaric equilibration to the assemblage cordierite andalusite biotite at 550 C involves the release of more than half of this structurally bound H 2 O. High-grade metamorphism and anatexis Given the good pressure constraints and the quantity of major element data for the Timeball Hill Formation protoliths, much of the subsequent suprasolidus modelling of the aluminous xenoliths uses isobaric (pressure = 2.5 kbar) T X pseudosections to model compositional variations about an average Timeball Hill composition (Table 2; Fig. 6). Varying H 2 O Figure 8 shows a T X pseudosection for the average Timeball Hill protolith with varying H 2 O, from no excess H 2 O on the left-hand side, increasing to a quantity based on the average loss on ignition (LOI) on the right-hand side (Table 2). The two arrows at the base of Fig. 8 show H 2 O-contents appropriate to higher-grade amphibolite facies protoliths (i.e. low H 2 O) and lower-grade greenschist facies protoliths (i.e. high H 2 O). At temperatures above those estimated for the contact (800 C), the average Timeball Hill composition contains no biotite and develops assemblages rich in melt, cordierite, spinel and aluminosilicate (with minor K-feldspar, quartz and or magnetite; Fig. 8). With increasing H 2 O content, assemblages are characterized by the successively lower-t consumption of plagioclase, K-feldspar and quartz, which react congruently with H 2 O to produce melt (Fig. 8). Once the melt is H 2 O saturated at any particular temperature, increasing the quantity of H 2 O further has little additional effect on melt volumes. The low-variance suprasolidus field containing co-existing biotite, spinel, cordierite, aluminosilicate, K-feldspar, quartz and melt (emboldened on Fig. 8) records the NCKFMASHTO equivalent of the H 2 O- absent univariant KFMASH melting reaction: bi þ sill þ q! cd þ sp þ ksp þ liq: ð1þ This reaction, which occurs at C, results in the first prograde consumption of biotite and the production of spinel. At temperatures of 900 C and assuming no melt loss, the average Timeball Hill Formation metapelite would have contained mol.% melt depending on the H 2 O content (Fig. 8). Variations in AFM Figure 9 (a) and (b) show T X pseudosections for the average Timeball Hill protolith with varying X Al

13 MODELLING ALUMINOUS METAPELITIC XENOLITHS 281 Fig. 8. NCKFMASHTO T X pseudosection based on the average Timeball Hill composition with varying H 2 O (Table 2). The left-hand side contains an amount of H 2 O just sufficient to saturate the solidus; the right-hand side is an average of the loss on ignition from XRF analyses of the weakly or unmetamorphosed protoliths. Contours of melt content (in mol.%) are shown. The emboldened field records reaction (1) detailed in the text. The black arrows at the base represent low and high H 2 O contents appropriate to (relatively) high- and low-grade protoliths respectively. [molar Al 2 O 3 (Al 2 O 3 + FeO + MgO)] and X Fe [molar FeO (FeO + MgO)] respectively, in which the amount of H 2 O is fixed to an arbitrary (conservative) value reflecting low excess H 2 O in the protolith (lefthand arrow at the base of Fig. 8). The compositional ranges shown reflect those in the protoliths (Fig. 6). The labels for the low-variance fields equivalent to reaction (1) are emboldened. Almost all of the Timeball Hill protolith compositions plot at high X Al and on the right-half of Fig. 9a (see Fig. 6c). At temperatures at or above 800 C, such compositions are rich in aluminosilicate with spinel and cordierite. Compositions with intermediate X Al are rich in cordierite with minor spinel and or magnetite. Low X Al compositions contain orthopyroxene and cordierite. Although this association is abundant in the complete xenolith population, most are from much larger rafts of Duitschland Formation metapelite. The upper-t stability of orthopyroxene or aluminosilicate is strongly dependent on X Al, whereas that of quartz is only weakly so. The complete prograde consumption of biotite occurs at less than 720 C in aluminous compositions but is extended to close to 800 C in less aluminous compositions (Fig. 9a). Even though the protoliths are generally Fe-rich, they exhibit a significant range in X Fe (Fig. 6a,c). At >800 C, compositions close to, or more Fe-rich than the average contain abundant spinel with cordierite; relatively Mg-rich compositions contain abundant cordierite but lack spinel (Fig. 8b). Aluminosilicate is abundant in all compositions across the observed range at lower-t, but is completely consumed upgrade at temperatures close to 950 C (for the bulk H 2 O used). The upper-t stability of biotite is weakly dependent on X Fe, but is nowhere stable at >725 C. In both diagrams, assuming no melt loss, melt fractions at 900 C would be mol.% for the bulk H 2 O-content used (Fig. 9a,b). Variations in SiO 2 content Figure 10 shows T X pseudosections for the average Timeball Hill protolith with varying X Si, in which the amount of SiO 2 decreases towards the right-hand side. The compositional range shown in Fig. 10 corresponds to that recorded in the protoliths. Figure 10a is calculated for the low-h 2 O composition on Fig. 7. At temperatures above 800 C, most compositions contain assemblages rich in melt, cordierite, aluminosilicate and spinel. At the lowest observed values of X Si, assemblages containing corundum are predicted. Corundum appears in the SiO 2 -poorest rocks at 760 C in the narrow low-variance (NCKFMASHTO quadrivariant) field containing corundum, cordierite, spinel, biotite, aluminosilicate, K-feldspar and melt (Fig. 10a). Prograde passage through this field corresponds to the KFMASH univariant melting reaction: