The Role of Water Retention in the Anatexis of Metapelites in the Bushveld Complex Aureole, South Africa: an Experimental Study

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1 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 PAGES DOI: /petrology/egh033 The Role of Water Retention in the Anatexis of Metapelites in the Bushveld Complex Aureole, South Africa: an Experimental Study I. S. BUICK 1 *, G. STEVENS 2 AND R. L. GIBSON 3 1 DEPARTMENT OF EARTH SCIENCES, LA TROBE UNIVERSITY, BUNDOORA, VIC. 3086, AUSTRALIA 2 DEPARTMENT OF GEOLOGY, UNIVERSITY OF STELLENBOSCH, STELLENBOSCH, PRIVATE BAG X1, MATIELAND 7602, SOUTH AFRICA 3 SCHOOL OF GEOSCIENCES, UNIVERSITY OF THE WITWATERSRAND, PRIVATE BAG 3, WITS 2050, JOHANNESBURG, SOUTH AFRICA RECEIVED MAY 22, 2003; ACCEPTED MARCH 26, 2004 ADVANCE ACCESS PUBLICATION JULY 8, 2004 Highly restitic metapelites occur at the contact of the Rustenburg Layered Suite (Bushveld Complex). On the basis of previous experimental studies, the high ( 60%) degrees of melting required to form these restites via fluid-absent processes could not have occurred at the maximum temperatures estimated in the aureole ( 800 C). We have investigated their formation by partial melting experiments using a medium-grade and a low-grade metapelitic hornfels from the aureole that are isochemical except for water content ( 12 and 44 wt%h 2 O, respectively). The partial melting experiments ( C at 3 kbar) demonstrate strongly contrasting behaviour for the two samples. For the higher-grade starting composition only minor melting occurred up to 800 C(<13%); extensive melting (>50%) required a temperature of 1000 C. In contrast, for the lower-grade starting composition, extensive melting ( 50 65%) and the presence of a similar restite assemblage to that observed in the aureole rocks was achieved in all experiments run at temperatures 750 C. In the absence of fluid infiltration, the results suggest that during rapid heating in the innermost parts of high-temperature contact aureoles, extensive partial melting may occur at relatively low temperatures if the fluid from the low-grade protolith is still resident in the system. This locally renders metapelitic rocks less dehydrated and, hence, more fertile than if heating had been slower. KEY WORDS: low-pressure; melting experiments; metapelites INTRODUCTION The nature of partial melting of initially low-grade or unmetamorphosed pelitic rocks in high-temperature contact metamorphic aureoles around mafic intrusions may be expected to differ significantly from that produced during regional metamorphism in the middle and lower crust. In particular, heating to peak temperatures above the solidus will be much more rapid, and the duration of high-temperature metamorphism and anatexis much shorter, than during regional high-grade metamorphism. For example, conductive thermal models for heating adjacent to high-temperature intrusions predict that country rocks within tens to hundreds of metres from such intrusions reach peak temperatures above the wet solidus for quartzofeldspathic rocks on a time scale of years (Kerrick et al., 1991; Johnson et al., 2003; Dutrow et al., 2004). This suggests that processes such as reaction overstepping during heating, leading to the progress of metastable sub-solidus and melting reactions (Tracy & McLellan, 1985; Rubie & Brearley, 1987), may be important in high-temperature contact aureoles (Kerrick et al., 1991). The relationships between peak metamorphic temperature, heating rate and the duration of the thermal peak create potentially complex partial melting scenarios that may change as a function of distance from the intrusion. In the absence of fluid infiltration, the more slowly heated, outermost parts of anatectic zones around mafic intrusions would be expected to experience limited melting at the wet-pelite solidus, followed by incongruent melting of biotite-bearing assemblages. For low-pressure aureoles (<4 kbar), melting is unlikely *Corresponding author. Present address: School of Geosciences, Monash University, Melbourne, Vic. 3800, Australia. Fax: þ Ian.Buick@sci.monash.edu.au Journal of Petrology 45(9) # Oxford University Press 2004; all rights reserved

2 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004 to involve muscovite because of its prior sub-solidus breakdown (Holland & Powell, 2001). Overall, the melting history would be similar to that experienced in regional low-p high-t environments. In contrast, in the innermost, very rapidly heated parts of the aureole: (1) overstepping of muscovite sub-solidus dehydration reactions and the persistence of muscovite above the wet-pelite solidus may allow metastable incongruent melting of muscovite (Rubie & Brearley, 1987; Brearley & Rubie, 1990); and/or (2) fluids released through prograde devolatilization reactions that had not had time to leave the system could flux extensive melting at the wetpelite solidus. In natural rocks there is likely to be an interplay between the possibilities discussed above. For example, rapidly heated rocks that start melting via fluid-present processes at the wet-pelite solidus may also contain metastable muscovite that ultimately melts via a metastable incongruent reaction. The composition of most metapelites will ensure that fluid from sub-solidus reactions will be consumed first by melting reactions before the rocks become quartz- or mica-absent above the wet-pelite solidus. This will ensure that further melting will occur through quartz-consuming, fluid-absent incongruent melting reactions of muscovite (metastable) or biotite. The variation in heating rates within high-temperature contact aureoles (Kerrick et al., 1991) might be expected to cause melting histories that differ significantly with distance from the contact. In this study, we present the results of an experimental investigation of low-pressure (3 kbar) melting in metapelitic rocks designed to investigate these potential variations, and to explain the development of relatively low-temperature ( 800 C), quartz-free, cordierite- and spinel-rich restitic metapelites developed at the contact of the Rustenburg Layered Suite, Bushveld Complex, South Africa. LOCAL GEOLOGY The 206 Ga Rustenburg Layered Suite (RLS) comprises a 6 8 km thick, shallow-dipping sheet-like body that intruded with a slight discordance into the late Archaean early Palaeoproterozoic Transvaal Supergroup. It was rapidly emplaced as a series of discrete pulses of ultramafic and mafic magma that ranged in temperature from 1300 C to 1160 C (Cawthorn & Walraven, 1998). Prior to emplacement, the Transvaal Supergroup sediments experienced lower greenschistfacies burial metamorphism (Engelbrecht, 1990). The combined effects of rapid intrusion and hot magmas have produced an extensive metamorphic aureole in the Transvaal Supergroup floor rocks beneath the RLS. Contact metamorphic effects are generally most pronounced in the rocks of the upper Transvaal Supergroup (Pretoria Group). However, in the NE lobe of the RLS, around Potgietersrus (Fig. 1a), the RLS has also strongly contact metamorphosed floor rocks of the lower Transvaal Supergroup (Chuniespoort Group). The intensity of syn-emplacement deformation and fabric development beneath the RLS varies considerably. Around the western and northeastern lobes of the RLS, contact metamorphic rocks are hornfels at all grades (Nell, 1985; Engelbrecht, 1990). In contrast, medium- to high-grade contact metamorphism in floor rocks beneath the eastern lobe of the RLS was accompanied by syn-emplacement deformation (Uken & Watkeys, 1997). Beneath the eastern lobe of the RLS, Johnson et al. (2003) modelled partial melting in floor rock metapelites of the Silverton Formation (Pretoria Group; Fig. 1b) as varying from minor (<5 vol. %) fluid-present melting at C in the lowermost part of the anatectic zone ( 350 m below the RLS) to modest (<19 25 vol. %) dehydration-melting of biotite-bearing assemblages at higher temperatures ( 750 C at 100 m below the RLS). Around the NE lobe (Fig. 1a) there is little evidence of partial melting in the floor rocks, except directly at the contact with the RLS. Here, Nell (1984, 1985) described the occurrence of quartz-absent, Al Fe Mg-rich and Ca Na K-poor rocks from the Timeball Hill Formation (Pretoria Group). These rocks typically contain the assemblage cordierite þ spinel þ sillimanite corundum K-feldspar. On the basis of comparisons with lowergrade, stratigraphic equivalents, Nell (1984, 1985) suggested that these rocks were Timeball Hill metapelites whose bulk composition had been modified by the extraction of 60 wt % of a silicate melt of peraluminous granitic composition. The fugitive granitic melt is not preserved at this locality, but elsewhere there is evidence of veining or contamination of the RLS by peraluminous granitic material (Engelbrecht, 1990; Johnson et al., 2003). The assemblages developed in the Timeball Hill restites are mineralogically distinct from quartz-absent, ultrahigh-temperature mullite- and/or sapphirinebearing metapelitic xenoliths in the RLS (Willemse & Viljoen, 1970). Metamorphic conditions in the aureole have been the subject of a number of studies. Pressure estimates are typically in the range 20 to 35 kbar (Nell, 1985; Engelbrecht, 1990; Johnson et al., 2003). Beneath the western lobe of the RLS, maximum temperatures of C were estimated by Engelbrecht (1990) from garnet cordierite orthopyroxene-bearing hornfelses using garnet cordierite Fe Mg exchange thermometry. Around the eastern lobe, Johnson et al. (2003) estimated similar temperatures of C (at 3 kbar) using garnet cordierite Fe Mg exchange thermometry from metapelites 100 m below the RLS. 1778

3 BUICK et al. MELTING EXPERIMENTS, BUSHVELD COMPLEX AUREOLE Fig. 1. (a) Geology of the northeastern lobe of the RLS and its country rocks. The open star shows the locality of Timeball Hill cordierite-rich restitic rocks at the RLS contact. (b) Geology of the eastern lobe of the RLS and its country rocks, showing the locality of starting compositions BV73 and BV77. Also shown are 500 C, 600 C and 700 C isotherms in the floor rocks (Sharpe & Chadwick, 1981). Beneath the NE lobe, Nell (1985) used an empirical calibration of the cordierite spinel Fe Mg exchange thermometer to estimate temperatures at the RLS contact from (1) the Timeball Hill cordierite spinel sillimanite corundum rocks (described above), and (2) Fe-rich hornfelses in the Penge Iron Formation (Chuniespoort Group), which contain the assemblage olivine orthopyroxene cordierite spinel (Nell, 1984). The former yielded temperatures of C, and the latter C (Nell, 1985). However, using the more recent experimental calibration of this thermometer (Nichols et al., 1992) and representative electron microprobe data from Nell (1984), we obtained temperatures of C (at 3 kbar) for five Timeball Hill cordierite spinel sillimanite corundum-bearing restites. We have analysed additional cordierite spinel pairs from one of these samples (PH169; Nell, 1984), as well as another sample (R495-69) collected for this study from the same locality. At 3 kbar, the averages of five cordierite and spinel compositions (analytical methods described below) for each of the two samples yield temperatures of 730 C and 740 C, respectively. Nell (1985) also estimated temperatures of 760 C from ferruginous Penge hornfelses that contain the assemblage quartz orthopyroxene garnet cordierite plagioclase using garnet cordierite Fe Mg exchange thermometry. Using representative mineral compositions from Nell (1984), we have calculated temperatures of C at 3 kbar for seven of these samples using garnet orthopyroxene thermometry corrected for late Fe Mg exchange (Pattison et al., 2003). In summary, a number of different geothermometers applied to the highest-grade aureole rocks, including 1779

4 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004 those from the RLS contact, yield maximum temperatures 800 C. Previous partial melting experiments at low to medium pressure predict only low to moderate degrees ( 10 30%; Pati~no Douce & Johnston, 1991; Pati~no Douce & Beard, 1995; Montel & Vielzeuf, 1997; Stevens et al., 1997) of melting of metapelites at these temperatures. Given this, it is unclear how the high degrees of partial melting required to produce the restitic Timeball Hill cordierite-rich rocks were attained. Although infiltration of water-rich fluids into the metapelites could have driven extensive water-saturated partial melting (Cartwright & Buick, 1998), there is no oxygen isotope evidence to support this in the aureole ( Johnson et al., 2003). Alternatively, extensive melting at the RLS contact may have resulted from geologically instantaneous heating of initially low-grade metapelites to anatectic temperatures, before either fluid evolved from sub-solidus reactions had escaped the system or low-temperature hydrous minerals had been able to break down through sub-solidus dehydration reactions. In this study we test these possibilities through partial melting experiments using two metapelitic rocks from the aureole that are isochemical other than for water content. One sample is a medium-grade rock equilibrated at temperatures of C that in the experiments should mimic the behaviour of metapelites that heat slowly enough to lose most of their prograde dehydration water prior to crossing the wet-pelite solidus. The other sample is a lower-grade equivalent, which will be used to assess the general potential for instantaneous melting at the contacts of mafic intrusions for generating local, high melt volumes. EXPERIMENTAL AND ANALYTICAL METHODS Experiments were conducted at the University of Stellenbosch in an internally heated, small-volume Hollowaytype gas vessel using high-purity argon as the pressure medium. The 5 W furnace used had a hot spot of 2 cm. When loaded with the 10 mm i.d. silica glass tubes used as sample carriers, the temperature variation within the hot spot was typically less than 5 C, at pressure and with the vessel mounted horizontally. Up to four capsules between 12 and 15 mm long were loaded into the glass tube sideby-side and two type-k Inconel sheathed thermocouples situated at either end of the capsules monitored the temperature gradient over the sample volume. Small degrees of vessel tilt were applied to keep the temperature gradient through the sample volume as low as possible during experiments. Temperature measurement of the primary thermocouple and control of the furnace was achieved using a Temperature Controls multifunction solid-state temperature controller with integral ice point acting through a 1 kw thyristor unit. The temperature of the second thermocouple was measured using a similar device. Temperatures were readily maintained within 1 C of the set point for the duration of an experiment. Confining pressures were monitored via a Heise Bourdon tube gauge and pressure was stable to 100 bar during the course of each experiment. Samples were ground for up to 2 h with a mechanical mortar and pestle in an attempt to produce average grain sizes of 5 mm in the powders. Despite the long grinding time, the existence of minerals of high hardness, as well as the lubricating effect of the substantial amount of mica in the samples ensured that some larger (20 50 mm) particles of hard minerals were always present. Sample powders were dried at 200 C overnight and kept under vacuum in a desiccator prior to loading into gold capsules. The capsules (typically 12 mm 5mm 5mm) were constructed from 02 mm thick high-purity annealed gold plate. Approximately mg of sample powder was sealed into the capsules using an arc welder. The capsules were then weighed and vacuum tested in a water bath for leaks. The duration of individual experiments varied between 138 and 232 h. On completion of the experiments, the run products were removed from the capsules and mounted in epoxy resin on glass slides that were then polished and carbon coated for scanning electron microscopy (SEM). Starting materials were analysed by X-ray fluorescence (XRF ) analysis at the University of Stellenbosch. Imaging of the run products and analysis of the phase compositions was accomplished using a LEO 1430VP scanning electron microscope at the University of Stellenbosch. Phase compositions were quantified by energy dispersive spectrometry (EDS) analysis using an Oxford Instruments 133 kev detector, Link Semquant software and natural mineral standards. Beam conditions during the analyses were 20 kv and 15 na, with a working distance of 13 mm. Pure Co, as well as Ti and Fe in ilmenite, were used periodically during the analyses to correct for detector drift. Glass analyses were conducted using a Hexland cryo-stage cooled by liquid nitrogen to minimize problems with migration of Na 2 O away from the beam. Mineral abbreviations follow Kretz (1983) apart from the following: Fe-Ts, Fe tschermakite component of muscovite; Gl, glass; Liq, silicate liquid; Ulv, ulv ospinel. STARTING MATERIALS The bulk compositions, mineral modes and estimated water contents of the two samples are given in Table 1. The higher-grade starting composition (BV73) is a finegrained, medium-grade cordierite andalusite biotite hornfels from the Silverton Formation (Pretoria Group) and was sampled from the contact aureole beneath the 1780

5 BUICK et al. MELTING EXPERIMENTS, BUSHVELD COMPLEX AUREOLE Table 1: Bulk composition of protoliths BV77 and BV73, and average compositions of Timeball Hill Formation low-grade metapelites and cordierite-rich restites from the RLS contact (Nell, 1985) Silverton Formation Timeball Hill Formation Low grade Medium grade Low grade Very high-grade Crd-rich restites BV77 BV73 (n ¼ 6) (n ¼ 5) wt % 1s 1s SiO TiO Al 2 O Fe 2 O 3Tot MnO MgO CaO Na 2 O K 2 O LOI Total Normalized to 100 wt % anhydrous (wt %) SiO TiO Al 2 O Fe 2 O 3Tot MnO MgO CaO Na 2 O K 2 O Total X Mg ASI Mineral modes (wt %) Qtz Bt 22.6 Ms Pl And 1.0 Crd 30.6 Chl 27.8 Ilm 1.0 trace r H 2 O (wt %) Mineral modes (wt %) for BV73 and BV77 were estimated using a least-squares mass balance approach; r 2 values are the sum of squares of residuals to the fit. Estimated water contents (wt %) were calculated from the mineral modes assuming ideal mineral compositions. 1781

6 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004 eastern lobe of the RLS (Fig. 1b). This sample was taken at a metamorphic grade interpreted to be below the first occurrence of partial melting in the inner aureole (assumed to be approximated by the 700 CisotherminFig.1b).The lower-grade starting composition (BV77) is a very finegrained muscovite chlorite hornfels that was sampled 30 km south along strike from BV73 (Fig. 1b). On an anhydrous basis, the bulk compositions of BV73 and BV77 are nearly identical (Table 1), with similar ASI values [molar Al/(Ca þ Na þ K); ] and X Mg [molar Mg/(Mg þ Fe); ]. For the experiments, rocks from the Silverton Formation were preferred to those from the Timeball Hill Formation because the latter is much more compositionally variable than the former (Nell, 1984). Notwithstanding this, the chemical composition of BV77 is similar to that of average lowgrade Timeball Hill metapelites (Nell, 1984; Table 1). In thin section BV77 contains Qtz þ Ms (Ms Pg Fe-Ts 2 4 ) þ Chl [X Mg ¼ Mg/(Mg þ Fe) ¼ ] þ Pl (Ab 62 An 37 Or 1 ) þ Ilm [X Mn ¼ Mn/(Mn þ Mg þ Fe 2þ ) ¼ ; Table 2]. It contains randomly oriented laths of fine-grained ( 10 mm long) muscovite and chlorite, and coarser-grained (20 25 mm long) porphyroblasts of ilmenite. Apatite and zircon occur at trace levels. BV73 contains the assemblage Qtz þ Ms (Ms Pg Fe-Ts 2 3 ) þ Bt [X Mg ¼ Mg/(Mg þ Fe) ¼ ; Ti cations per 22 oxygen formula unit] þ And þ Crd (X Mg ¼ 051; cations Na per 18 oxygen formula unit) þ Pl (Ab An Or 0 1 ;Table2)þ Ilm. Two compositionally distinct types of ilmenite occur; relatively Mn-rich as in BV77 (X Mn ¼ ) and Mn-poor (X Mn ¼ 004). The sample contains millimetre-diameter poikiloblasts of cordierite and subordinate andalusite, set in a matrix of randomly oriented muscovite and biotite, plagioclase and quartz. Both cordierite and andalusite contain inclusions of biotite, ilmenite and quartz. Monazite, zircon and apatite occur in trace abundance. RESULTS The results of 3 kbar melting experiments undertaken at 700 C, 750 C, 800 C, 900 C and 1000 C for starting materials BV73 and BV77 are described below, and are summarized in Table 3. Textural relationships are shown in Figs 2 and 3, and the compositions and proportions of experimentally produced silicate glasses, and mineral reactants and products are shown in Tables 4 7 and Figs Phase and textural relationships BV73 (12 wt%h 2 O) The 700 C experiment produced no glass and no new minerals. In the 750 C experiment, very minor amounts of glass were present. This suggests that the solidus for the sample is located at slightly below 750 C at 3 kbar. In the 750 C experiment, glass occurs as thin (<1 3 mm wide) films around quartz, andalusite, biotite, plagioclase and poikiloblastic cordierite that were inherited from the hornfels precursor. These minerals occur as anhedral grains draped by films of glass, but with little evidence of embayment, recrystallization or new mineral growth. Muscovite is present in the sample as slightly embayed anhedral grains rimmed by thin (1 3 mm) melt films (Fig. 2a). In general, the largest melt accumulations are in the vicinity of muscovite grains. Muscovite has been observed in the run products in direct contact with quartz, as well as separated from quartz by only a thin glass film. In the 800 C experiment, muscovite is absent and (10 20 mm wide) films of glass are developed around quartz, andalusite, plagioclase, biotite and poikiloblastic cordierite (Fig. 2b d). Biotite occurs as anhedral grains that are studded with typically sub-micron grains of Fe 3þ -rich spinel. Although most original biotite grains were 5 20 mm long, a number of coarser grains ( 50 mm long) managed to survive the grinding process. Within pools of glass, relict plagioclase is commonly overgrown by a shell of K-feldspar. Euhedral, inclusion-free K-feldspar (Fig. 2b), as well as a compositionally distinct new generation of plagioclase also occur. Very locally, relatively large pseudomorphs were observed. They are complex intergrowths of euhedral, mm long K-feldspar, <5 mm long needles of sillimanite, and interstitial pockets of silicate glass (Fig. 2b). These phases are inferred to have replaced coarse-grained ( 50 mm long) muscovite flakes. New cordierite occurs as euhedral ( 7 12 mm) crystals (Fig. 2d) within silicate glass. Ilmenite and rare grains of Al-rich hercynitic spinel also occur locally within the melt pools. The 900 C run product (Table 3) is similar to that at 800 C, with the exception that neither acicular sillimanite nor K-feldspar is observed. Both relict poikiloblastic and finer-grained, new, euhedral cordierite occur, with many grains of new cordierite containing older, compositionally distinct, cores (Fig. 2e). Both ragged plagioclase inherited from the hornfels, and euhedral, new, compositionally distinct plagioclase also occur (Fig. 2e). Compared with the 800 C experiment the proportion of new cordierite and glass appear to have increased, and the proportion and grain size of relict cordierite, biotite, plagioclase and quartz appear to have decreased. In the 1000 C experiment abundant new euhedral cordierite (15 20 mm) occurs together with Al-rich spinel (2 15 mm), embayed quartz (<10 15 mm) and rare, acicular orthopyroxene (2 4 mm mm) in a glass matrix. Relics of reactant cordierite were also observed as irregular, Fe-rich cores to the new cordierite (Fig. 2f ). Locally, 30 mm 50 mm aggregates of relatively coarse-grained (<7 15 mm diameter) spinel and glass occur (Fig. 2f ). 1782

7 BUICK et al. MELTING EXPERIMENTS, BUSHVELD COMPLEX AUREOLE Table 2: Composition of minerals in the starting bulk compositions BV73 and BV77, expressed as structural formulae BV73 Mineral: Bt Bt Ms Ms Pl Pl Crd Crd And And No. Ox: Si Ti Al Mg Fe 2þ Ca Mn Na K Total X Mg An Ab Or BV77 Mineral: Chl Chl Ms Ms Pl Pl Ilm Ilm No. Ox: Si Ti Al Mg Fe 2þ Ca Mn Na K Fe 3þ Total X Mg An Ab Or X Hem X Mn BV77 (44 wt%h 2 O) Melting experiments on the muscovite chlorite hornfels produced a very small proportion of glass at 700 C (Fig. 3a), suggesting that the solidus for this sample was just below 700 C at 3 kbar. In contrast, large proportions of glass formed in all experiments from 750 C to 1000 C (Fig. 3b e). Only the 700 C experiment contained muscovite. It additionally contained pools of glass of 1 3 mm diameter (Fig. 3a) in association with rare, new, very finegrained (1 2 mm long) biotite, euhedral, 2 5 mm diameter 1783

8 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004 Table 3: Summary of the 3 kbar experiments undertaken using the BV73 and BV77 starting compositions Experiment P (kbar) T ( C) Duration (h) Assemblage Qtz Crd Ms Bt Pl Sil Kfs And Gl Opx Ilm Spl BV73 medium-grade metapelite R R,N N N N N R R,N R,N R,N R N N N R R,N R,N R,N N N R N N N R R,N N R,N R,N N R N N R R R R R R R BV77 low-grade metapelite N N N N R N N N N R N N N N R N N N N N R N R N N N N R Qtz, quartz; Bt, biotite; Crd, corderite; Pl, plagioclase; And, andalusite; Sil, sillimanite; Kfs, K-feldspar; Opx, orthopyroxene; Ilm, ilmenite; Spl, spinel; Gl, glass; R, relict from protolith; N, newly formed or recrystallized. 1784

9 BUICK et al. MELTING EXPERIMENTS, BUSHVELD COMPLEX AUREOLE Fig. 2. BSE images of 3 kbar partial melting experiments using the BV73 bulk composition at (a) 750 C, (b) (d) 800 C, (e) 900 C and (f ) 1000 C. Textural relationships are described in the text. Subscripts 1 and 2, where present, refer to minerals that occur as both relict grains from the hornfels protolith (1) and as newly formed products of the experiments (2). new cordierite and K-feldspar, partially resorbed 4 7 mm quartz and both relict and compositionally distinct new plagioclase (see below). All higher-temperature experiments (Fig. 3b e) contained abundant, small (<5 8 mm), euhedral cordierite crystals, and less common grains of poikiloblastic ilmenite (5 15 mm; Fig. 3d) that are texturally (but not compositionally) similar to that in the starting bulk composition. These experiments also contained euhedral, 1 5 mm grains of Fe 3þ Ti Al spinel (Fig. 3c e), whereas 1 2 mm diameter crystals of Al-rich hercynitic spinel occurred only in the 1000 C experiment. Irregular, globular and corroded grains of quartz (Fig. 3c) occurred in the C experiments, and needles of sillimanite (<4 6 mm long) were found in glass-rich domains only in the 750 C experiment (Fig. 3b). Glass chemistry Mean compositions of silicate glasses from the run products over the temperature interval C for BV77, and C for BV73, are listed in Table 4. Small amounts of glass also occurred in the 700 C BV77 and 750 C BV73 experiments, but could not be analysed. Care was taken to avoid glass analyses contaminated by minerals, and the mean values are typically averages of 5 10 analyses. Absolute 1s standard deviations about the means are generally small; typically 025 wt % for 1785

10 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004 Fig. 3. Back-scattered electron (BSE) images of 3 kbar partial melting experiments using the BV77 bulk composition at (a) 700 C, (b) 750 C, (c) 800 C, (d) 900 C and (e) 1000 C. Textural relationships are described in the text. major elements other than SiO 2 (e.g. Al 2 O 3 and K 2 O), and 015 wt % for elements with oxide abundances of 0 2 wt %. Mean SiO 2 contents have 1s uncertainties of wt %, with most being 05 wt %. In some experiments, MgO contents were below the detection limits of the EDS system ( 02 wt % in typical glass compositions from this study). All the glasses are granitic (Fig. 4) and peraluminous (Fig. 5a) in composition. All except for the BV C experiment have aluminium saturation index [ASI; molar Al 2 O 3 /(CaO þ Na 2 O þ K 2 O)] values >11 and could be classed as having S-type granite characteristics (Chappell & White, 1974). Both sets of glasses are corundum normative (BV73 normative corundum 08 21%; BV77 normative corundum 25 61%; Table 4). Normative corundum contents for BV73 show no consistent variation with rising temperature; those from BV77 increase with rising temperature (Table 4). Even though the low-temperature experiments for the two starting compositions contained very different proportions of minerals and glass, glass compositions are similar (Fig. 5). Glasses from BV73, which contained quartz at all temperatures, have ASI values that are constant within error (ASI ¼ ; Fig. 5a). Glasses from BV77, which became quartz-absent in the highest-temperature experiment, show a considerably wider range (ASI ¼ ), with the ASI value low and approximately constant in the C experiments, and increasing with rising temperature from 800 C to 1000 C (Fig. 5a). With rising temperature, glasses from BV73 show general trends of decreasing SiO 2, Al 2 O 3 and K 2 O, nearconstant Na 2 O, and increasing MgO, FeO and TiO 2 contents (Fig. 5b d). Glasses from BV77 show similar trends (Fig. 5b d), with the exception that SiO 2 contents remain almost constant from 700 C to 900 C before decreasing at 1000 C (Fig. 5b). Mineral chemistry Feldspars Relict plagioclase (Ab An Or ) from the hornfels precursor occurs in all the run products for BV73 up to 900 C. New plagioclase is more anorthitic, and becomes more anorthitic and orthoclase-rich with rising temperature: Ab An Or (800 C) to Ab An Or (900 C; Table 5). New plagioclase compositions are similar to those in experiments in the An Ab Or ternary system at comparable temperatures (Fuhrmann & Lindsley, 1988; Fig. 6). New K-feldspar in the same experiments is Ab An Or (800 C; Table 5). In the 700 C BV77 experiment, both plagioclase identical to that in the precursor and new, more Ca-rich plagioclase (Ab An Or ; Table 6) are present. The new K-feldspar is similar in composition to that in the 800 C BV73 experiment (Ab 23 An 3 Or 74 ; Table 6). The compositions of new plagioclase and K-feldspar are also consistent with equilibration at elevated temperatures (Fig. 6). 1786

11 BUICK et al. MELTING EXPERIMENTS, BUSHVELD COMPLEX AUREOLE Table 4: Average glass compositions, recalculated on an anhydrous basis 800 C 1s 900 C 1s 1000 C 1s BV73 (normalized to 100 wt % anhydrous) SiO TiO Al 2 O MgO b.d b.d b.d FeO MnO b.d CaO Na 2 O K 2 O Total ASI CIPW norm Q Or Ab An Crn Hy Ilm Pl (X An ) C 1s 800 C 1s 900 C 1s 1000 C 1s BV77 (normalized to 100 wt % anhydrous) SiO TiO Al 2 O MgO b.d FeO MnO CaO Na 2 O K 2 O Total ASI CIPW norm Q Or Ab An Crn Hy Ilm Pl (X An ) X Mg b.d., below detection limit. 1787

12 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004 Table 5: Representative compositions for new minerals formed in the BV73 experiments (expressed as structural formulae) T ( C): Mineral: Pl Kfs Pl Crd Crd Spl Spl Spl Opx Ms Bt Bt Bt Ilm Ilm Ilm No. Ox: Si Ti Al Mg Fe 2þ Fe 3þ Mn Ca Na K Total An Ab Or XMg SplþHc X Ulv X Mag X Hem X

13 BUICK et al. MELTING EXPERIMENTS, BUSHVELD COMPLEX AUREOLE Table 6: Representative compositions for new minerals formed in the BV77 experiments (expressed as structural formulae). T ( C): Mineral: Crd Crd Crd Crd Crd Bt Ms Kfs Pl Spl Spl Spl Spl Spl Ilm Ilm Ilm Ilm Ilm No. Ox: Si Ti Al Mg Fe 2þ Fe 3þ Mn Ca Na K Total Ab An 3 29 Or 74 4 XMg SplþHc X Ulv X Mag X Hem X

14 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004 Table 7: Modal mineral and melt proportions in partial melting experiments using bulk compositions BV73 and BV77 Temperature ( C) BV73 wt % Glass Qtz Pl old Pl new Kfs 1.5 Bt Sil/And Crd old Crd new Opx 4.4 Al Spl Complex Spl tr 1.4 tr Ilm Total r BV77 wt % Glass Qtz Sil 0.2 Crd Al Spl Complex Spl Ilm Total r tr, trace amount. Least-squares mixing calculations used measured bulk compositions (XRF) and mineral melt compositions (EDS). r 2 is the sum of squares of deviation of the calculated bulk compositions from the measured bulk compositions. Muscovite Muscovite is present in the BV73 experiments at 700 and 750 C and in the BV77 experiment at 700 C. As the metamorphic grade of the starting rocks is very different, the composition of the muscovite in BV73 and BV77 differs significantly. Muscovite in BV73 is significantly more Tirich and has lower Si þ Al vi than that in the lower-grade BV77 (Fig. 7). Total Fe þ Mg þ Mn for both starting muscovites is similar, at 01 p.f.u., and is higher for all experimental muscovite (Fig. 7a). No obvious systematic Fig. 4. Normative Ab An Or contents of silicate glasses produced in the melting experiments compared with the classification of Barker (1979). correlation exists between total Fe þ Mg þ Mn and Ti, such as might be expected if Ti and the divalent cations were being incorporated into muscovite via a coupled substitution such as Ti þ Fe ¼ Al vi (Guidotti, 1978). Rather, Ti appears to be incorporated through the substitution Ti þ Al iv ¼ Al vi þ Si (Guidotti, 1978) (Fig. 7b). In Fig. 7b, the 700 C muscovite compositions from the low- and mediumgrade compositions converge, suggesting that they have (at least) partially equilibrated at this temperature. Biotite Biotite occurs in the 700 C, 750 C, 800 C and 900 C BV73 experiments as relict grains. In the 800 C and 900 C experiments it also occurs as new, euhedral and very small crystals (<1 mm wide; 5 mm long; Fig. 2c) that were too small to analyse by SEM-EDS. Relict biotite grains have, however, adjusted their compositions, and these change systematically with rising temperature of the experiments. Overall, with rising temperature of the experiment, biotite becomes more Mg-rich and Al-poor (Table 5; Fig. 8), and we assume that the most Mg-rich and Al-poor biotite grains, which are typically the smallest, are the most compositionally equilibrated for any given experiment (see Pati~no Douce et al., 1993; Montel & Vielzeuf, 1997; Stevens et al., 1997). Given this, the best estimates for the X Mg of biotite in equilibrium with melt at 750 C, 800 C and 900 Care 048, 051 and 060, respectively (Fig. 8). For any given experiment, biotite shows a relatively large range in octahedral Al (Al VI ), and a somewhat smaller range in Ti contents (typically 035 and cations p.f.u., respectively; Table 5; Fig. 8). Although with rising temperature there is a general trend of increasing Ti and decreasing Al VI contents, respectively, there is considerable overlap between data from experiments at different temperatures. For all experiments, biotite compositions 1790

15 BUICK et al. MELTING EXPERIMENTS, BUSHVELD COMPLEX AUREOLE Fig. 5. Plot of (a) aluminium saturation index (ASI) vs temperature ( C), and (b) (d) major element composition of glasses produced during partial melting of BV73 and BV77 vs temperature ( C). Error bars are 1s uncertainties on average values; if not indicated, they are smaller than the size of the plot symbol. show the same linear relationship between Fe þ Mg and Ti þ Al VI cations p.f.u. (Fig. 8). A similar result was noted by Stevens et al. (1997), who ascribed the compositional variation to a coupled substitution involving vacancies (u)ofthe form 05Ti þ 066Al VI þ 083u ¼ 2(Fe,Mg). Only two acceptable analyses of biotite could be obtained from the very small biotite crystals produced in the BV C experiment; their composition varies significantly (X Mg ¼ 031 and 039; 013 and 018 Ti atoms p.f.u.). Cordierite Relict poikiloblastic cordierite in all BV73 experiments has a similar composition (X Mg ¼ ) to that from the hornfels precursor. Analysing newly formed cordierite in both BV73 and BV77 was difficult because of the small size of overgrowths (BV73) or new grains (BV77). In general, cation totals for all cordierite crystals analysed were above the ideal value of 11 per 18 oxygen formula unit. The degree to which the cation total exceeds 11 cations appears to be a function of the total alkali (Na þ K) content; cation totals of >11 p.f.u. were obtained even from coarse-grained cordierite in the BV73 hornfels precursor. This suggests that alkali elements are not bound to the cordierite silicate framework, but instead occur in channelways, where they are charge balanced by volatile species (Vry et al., 1990). New cordierite overgrowths in the BV C experiment have similar X Mg to the poikiloblastic cordierite (X Mg ¼ ); those in the 900 C and 1000 C experiments have similar compositions (X Mg ¼ ; Table 5) that are 1791

16 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004 differ considerably between crystals. Variable, nonequilibrium Al 2 O 3 contents appear to be a general feature of orthopyroxenes produced in experimental studies of this type (see Montel & Vielzeuf, 1997; Stevens et al., 1997). All orthopyroxene grains analysed are Ca- and Na-free and contain wt%tio 2 and wt%mno. Fig. 6. Projection of new feldspar compositions on the Ab Or An plane. The projections of the feldspar solvus at C are from Fuhrmann & Lindsley (1988). considerably more Mg-rich than relict poikiloblastic cordierite. Newly formed cordierite in the BV77 experiments was very difficult to analyse because of its extremely small grain size, with the result that most cordierite analyses had greater than the ideal five Si atoms per 18 oxygen formula unit, and correspondingly lower Al contents than ideal. Given the lack of other high-si/low-al phases in the experimental charges, such anomalous Si and Al values could have been caused by sub-micron-scale inclusions of glass, which could not be observed by back-scattered electron (BSE) imaging (see Montel & Vielzeuf, 1997). Regardless of the cause of the high Si and low Al values, relatively large departures from ideal structural formulae (i.e. deviation of 02 cations Si above the nominal five per 18 oxygen formula unit) were not accompanied by significant differences in X Mg (DX Mg ) suggesting that, even for these analyses, the calculated X Mg values are fairly robust. Least contaminated analyses are relatively Fe-rich at 700 C (X Mg ¼ 043) and Mg-rich at 1000 C(X Mg ¼ ); X Mg values vary little from 750 C( ) to 900 C( ; Table 6). This lack of compositional variation is mirrored by almost constant melt proportions between 750 C and 900 C (see below). Orthopyroxene Orthopyroxene occurred only in the 1000 C BV73 experiment, and has a lower X Mg ( ) than coexisting cordierite ( ; Table 5). Orthopyroxene crystals have variable Al 2 O 3 contents (28 64 wt%).al contents are homogeneous on an intra-grain scale, but Ilmenite All ilmenite analyses contain very small amounts of SiO 2 (typically <1 wt %). Ilmenite in the BV77 precursor muscovite chlorite hornfels has X Hem [Fe 3þ /(Fe 3þ þ Fe 2þ )] ¼ 00 and is Mn-rich. In contrast, ilmenite in the BV C and 750 C experiments has a range of compositions (X Hem ¼ and , respectively), and is Mn-poor (Table 6). Ilmenite in highertemperature experiments shows higher and more uniform X Hem values, i.e (800 C), (900 C) and (1000 C). It also becomes slightly more Mg-rich with rising temperature, from X Mg ¼ at 700 Cto at 1000 C (Table 6). Ilmenite also occurs in the C BV73 experiments, where it increases in X Mg (from 005 to 012) and X Hem (from 021 to 044) with rising temperature (Table 5). In the BV C experiment, ilmenite has a moderate haematite component (X Hem ¼ 036) and is relatively Mg-rich (X Mg ¼ 011). Spinel group minerals Two types of spinel occur and, in some experiments, coexist. Analysing both of the spinels was particularly difficult because of their very small grain size. As a result, all analyses show small Na, K and Si contents that cannot be accounted for in the structural formulae, and the data discussed are for least contaminated analyses. Complex spinels are solid solutions between magnetite, ulv ospinel and hercynite spinel components, and occur in all BV77 experiments, where they coexist with ilmenite and, at 1000 C, additionally with Al-rich spinel (hercynite spinel solid solutions). Complex spinel in the BV77 experiments becomes more Al-rich with rising temperature, i.e. from Mag Ulv 5 6 SplþHc at 750 C to Mag Ulv SplþHc at 1000 C (Fig. 9; Table 6). Al-rich spinel in the 1000 C experiment has the composition Mag Ulv 2 SplþHc and X Mg ¼ (Table 6). Similar relationships between Al-poor and Al-rich spinel, and ilmenite occur in BV73. Complex spinel occurs in the 800 and 900 C experiments, whereas Al-rich spinel occurs in the C experiments. Al-rich spinel was too small to be successfully analysed in the 800 C experiment, but in the 900 C and 1000 C experiments it has a similar composition (Mag Ulv 0 1 SplþHc ; X Mg ¼ ; Table 5). 1792

17 BUICK et al. MELTING EXPERIMENTS, BUSHVELD COMPLEX AUREOLE Fig. 7. Muscovite compositional variation in terms of (a) Fe þ Mg þ Mn vs Ti and (b) Ti þ Al iv vs Si þ Al vi cations per 22 oxygen formula unit. b Fig. 8. Biotite compositional variation in terms of (a) X Mg vs temperature and (b) Ti þ Al VI vs Fe þ Mg cation contents per 22 oxygen formula unit. 1793

18 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004 Fig. 9. The composition of complex and Al-rich spinels expressed within the Fe 3þ Al Ti ternary system. Wt% Melt BV77 BV Temperature ( C) T Max in aureole Fig. 10. Variation in the abundance of silicate glass (quenched melt) expressed in wt % as a function of temperature for partial melting experiments using protoliths BV73 and BV77. Curved lines fitted to the data for both experiments are hypothetical melt production fits based on the assumption that melt productivities are primarily controlled by water availability. The shaded region shows maximum temperature estimates for contact metamorphism in the aureole obtained from Engelbrecht (1990) and Johnson et al. (2003), and recalculated using the compositional data of Nell (1984, 1985) as discussed in the text. DISCUSSION Approach to equilibrium The mineral and melt compositions described above result solely from partial melting experiments and hence the melting behaviour of the two bulk compositions was not reversed. However, the following lines of evidence suggest that the experimental results represent an approach to equilibrium: (1) the analysed glass compositions for any given experiment are very uniform and granitic (Table 4), as expected; (2) the glass compositions show systematic and predictable changes as a function of temperature and buffering mineral assemblage; (3) newly formed minerals are euhedral and compositionally unzoned; (4) the compositions of minerals in the experiments differ from those of the same minerals in the starting compositions (e.g. plagioclase, biotite and cordierite in BV73; ilmenite in BV77), and change systematically with rising temperature; (5) the proportion of melt and product and reactant minerals changes with rising temperature in both sets of experiments in a systematic manner (see below). Melt and mineral proportions Mineral and melt proportions (in wt %) for all the experiments in which melt compositions could be determined were calculated by a constrained least-squares mass balance approach, with the goodness-of-fit monitored through the sum of squared residuals (r 2 ). For the two experiments in which glass was observed but could not be analysed (700 C BV77 and 750 C BV73), melt proportions were constrained by image analysis of BSE images and found to be very small (<5%). The results of mass balance calculations for glass proportions are shown in Fig. 10 and summarized in Table 7. For experiments involving BV77, from 750 C to 900 C the experiments have very similar modal mineral and glass proportions, with glass contents between 48 and 50 wt %. The 1000 C experiment had an appreciably different mode; it was characterized by significantly higher proportions of glass ( 64 wt %), lacked quartz and had correspondingly lower cordierite contents than in the lower-temperature experiments. In the BV73 experiments, mass balance calculations were hampered by the mineralogical complexity of the samples (e.g. the occurrence of new and old generations of plagioclase and cordierite) and because in some cases new minerals observed by BSE imaging could not be analysed because of their extremely small grain size. To facilitate calculations, the following assumptions were made: (1) in the 800 C experiment, new cordierite appeared to have essentially the same composition as that of the precursor poikiloblastic cordierite; therefore, the composition of this cordierite was used in the calculations; 1794