J. metamorphic Geol., 1997, 15, 253–266

Equilibration and reaction in Archaean quartz-sapphirine granulitexenoliths from the Lace kimberlite pipe, South AfricaJ . B . DAW SON,1 S. L . HARLEY ,1 R. L . RUDNICK 2 AND T. R . IRELAND 3

1 Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK2 Department of Earth and Planetary Science, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA3 Research School of Earth Sciences, The Australian National University, 1 Mills Road, Canberra ACT 0200, Australia

ABSTRACT Ultrahigh-temperature quartz-sapphirine granulite xenoliths in the post-Karoo Lace kimberlite, SouthAfrica, comprise mainly quartz, sapphirine, garnet and sillimanite, with rarer orthopyroxene, antiperthite,corundum and zinc-bearing spinel; constant accessories are rutile, graphite and sulphides. Comparisonwith assemblages in the experimentally determined FMAS and KFMASH grids indicates initial equili-bration at >1040 °C and 9–11 kbar. Corona assemblages involving garnet, sillimanite and minor cordier-ite developed on a near-isobaric cooling P–T path as both temperature and, to a lesser extent, pressuresdecreased. Garnet-orthopyroxene Fe-Mg exchange thermometers record temperatures of only 830–916 °C.These estimates do not indicate the peak metamorphic conditions but instead reflect the importance ofpost-peak Fe-Mg exchange during cooling. Correction of mineral Fe-Mg compositions for this exhangeusing a convergence approach of Fitzsimons & Harley (1994 ) leads to retrieved P–T estimates fromgarnet-orthopyroxene thermobarometry (c. 1000 °C and 10.5±0.7 kbar) that are consistent with the petro-genetic grid constraints. U-Pb dating of a single zircon grain gives an age of 2590±83 Ma, interpretedas the age of the metamorphic event. Protolith major and trace element chemistries of the xenoliths differfrom sapphirine-quartzites typical of the Napier Complex (Antarctica) but are comparable to less siliceous,high Cr and Ni, sapphirine granulites reported from several ultrahigh temperature granulite terranes.Key words: South Africa; xenoliths in kimberlite; Archaean; sapphirine granulite; ultrahigh-temperaturemetamorphism.

INTRODUCTION INITIAL MINERAL ASSEM BLAGES

Most xenoliths are rounded and, having been milledThe kimberlite forming the Lace or Crown Mine(27°27∞S, 27°06∞E) is one of a small group of kimberlite during the processing of the host kimberlite for

diamond, are usually <3 cm in diameter. They showintrusions, aged around 145 Ma (Skinner, 1989),penetrating the Archaean Kaapvaal craton and overly- variable grain-boundary alteration, and rare larger

blocks up to 30 cm have the same pervasive grain-ing Karoo sediments and lavas, 25–30 km NW of thetown of Kroonstadt, Orange Free State, South Africa. boundary alteration as found in the smaller xenoliths.

Most xenoliths comprise varying proportions of quartz,Although bearing abundant upper-mantle xenoliths,most kimberlites penetrating the Kaapvaal craton garnet, sillimanite and sapphirine together with minor

quantities of rutile, graphite and sulphides. Feldspar,contain no lower crustal xenoliths; but the Lacekimberlite is atypical because it contains abundant kyanite, corundum, spinel and ilmenite occur only in

some specimens (Table 1). Note (i) that the modesxenoliths of sapphirine-quartz and two-pyroxenegranulites. A study by Dawson & Smith (1987) on a should be regarded as approximations owing to the

small size of the specimens; and (ii ) that the modalreconnaissance collection established that the sapphir-ine granulites formed from a high-Mg, Al protolith percentages of individual phases include their marginal

alteration products. The xenoliths are too small tounder reducing conditions, and initially at hightemperatures (c. 900–1000 °C) and at >10 kbar; reac- ascertain whether any of the modal variations are as

a result of mineral banding. Bearing in mind thesetion rims around some primary phases indicatedsubsequent retrograde metamorphism. The present caveats, it is apparent that quartz, garnet and sapphir-

ine comprise the bulk of the xenoliths, although ininvestigation, based upon a more extensive collection,extends the mineralogy and mineral chemistry found rare specimens orthopyroxene and sillimanite may be

major constituents. The orange-brown spinel is morein the earlier study, thereby enabling more preciseestimates of the P–T conditions of initial equilibration abundant than in the suite previously studied by

Dawson & Smith (1987); and ilmenite is an additionallyand retrograde metamorphism, and also presentsgeochronological data on zircon from one xenolith. identified phase, though in only one specimen. Grain

253© Blackwell Science Inc., 0263-4929/97/$14.00Journal of Metamorphic Geology, Volume 15, Number 2, 1997, 253–266

254 J. B. DAW SO N E T AL .

Table 1. Modes of analysed granulite xenoliths.

Specimen Qtz Grt Spr Opx Sil Cor Rut Spl Ilm Fspr Graphite+Sulphide

7 28 27 35 – 2 X 2 – – 4 215 38 33 – – 6 – 2 2 – 18 123 7 77 8 – 1 – 2 1 – – 426 42 28 25 – X – 2 – – – 327 32 33 5 – 25 – 3 1 – – 133 51 24 12 8 – – 4 – – – 140 46 43 11 – 5 1 2 1 – – 156 49 35 7 5 – – 3 – – – 163 15 25 32 – 8 – 2 11 – 4 365 4 62 – – 2 1 1 8 – 20 166 6 41 51 – – – 1 – X – 175 11 34 28 22 – – 3 – – – 2

4183 38 45 10 X* 2 – 2 X* – X* 2

X=phase present but <0.5 vol.%; –=phase absent; *=phase present in reaction corona.

size varies considerably up to 3 mm with garnet being (FMAS) system:constant in size and sapphirine very variable. The

garnet (Grt)+sapphirine (Spr)+quartz (Qtz)quartz grains are irregular in shape, in contrast toequant garnet and sapphirine. Orthopyroxene, present (Spl, Crd, Sil, Opx)in only a few samples, occurs as 0.5 mm grains usually

garnet+sapphirine+quartz+sillimanite (Sil )in or with coarse garnet. Rutile occurs in two forms(Spl, Crd, Opx)(i) equant grains up to 0.5 mm; and (ii) minute,

orientated acicular inclusions in quartz, garnet and garnet+sapphirine+quartz+orthopyroxene (Opx) .sapphirine. Graphite occurs in ragged acicicular grains (Spl, Crd, Sil )up to 0.3 mm. The 0.2 mm sulphide grains have not

where the absent phase notation is used to helpbeen studied in detail here but Dawson & Smithdistinguish the FMAS divariant and trivariant assem-(1987) identified pyrite, pyrrhotite, violarite and chalco-blages for later analysis. Rare examples lack sapphirinepyrite, sometimes intergrown.and instead consist of garnet and quartz coexistingOne further feature is worthy of note. Many quartzwith Fe-Mg-Zn-Cr spinel (Spl). Cordierite (Crd) isgrains comprise several subgrains with strained areasentirely absent as a primary phase in all samples.separated from unstrained areas by strongly crenulatedAlthough orthopyroxene and sillimanite occur inboundaries. In some quartz, the sub-grain boundariesdifferent parts of individual samples, they have notare offset by planar fractures or planar zones that arebeen observed together, so that Grt+Opx+Sil+Qtzdecorated by lines of aligned minute inclusions, whereasand Spr+Opx+Sil+Qtz are not stable assemblagesin simple quartz grains lacking subgrains, these planarin these rocks. The reaction textures locally developedfeatures cut across the entire width of the grainin these granulites will be described and interpreted(Fig. 1a). These features are superficially similar to thefollowing consideration of the mineral chemistry andplanar deformation features (PDFs) in quartz causedconstraints on the P–T conditions of formation of theby meteorite impact, but a transmitted electronpeak assemblages.microscope study by M. R. Lee (personal communi-

cation) of the Lace quartz grains has shown that theyare not as a result of the presence of the amorphous MINERAL CHEMISTRYsilica that decorates PDFs formed during experimentalhigh dynamic compression of quartz (Doukhan, 1994). Major and minor oxidesHowever, as we show later in this paper, the Lace

These were analysed by WDS on the Camebax electron microproberocks are very old, and an analogy might be drawnat the University of Edinburgh. Operating conditions were 20 nAwith the rocks of the nearby Vredefort Dome (#50 kmbeam current at 20 kV and the data were reduced using the PAPNE of Lace), in which PDFs formed during meteorite routine. The tabulated analyses are the means of at least five points

impact have degraded because of post-shock metamor- per grain. Ferric iron was calculated, where approporiate, using themethod of Droop (1987).phism (Schreyer, 1983). Currently we have no firm

Garnet is a low-Ca pyrope-almandine with XMg ranging mainlyexplanation for the origin of these features in thefrom 51 to 63 (Table 2). This range, and also the range for CaOLace rocks. (0.80–2.05 wt%) is wider than that found by Dawson & SmithThe samples can be divided into two groups based (1987 ). All garnet contains small but persistent amounts of MnO,

on the presence or absence of quartz. The quartz- TiO2 and Cr2O3. Stoichiometric calculations (assuming 8 cations)show that the Fe in the garnet is mainly Fe2+, withabsent assemblage garnet-spinel-corundum-sillimaniteFe2+/(Fe2++Fe3+) around 0.98 (Table 2). Garnet surrounding andis recognized from only one sample (49); the dominantreplacing spinel in sample 15 is much richer in Fe (XMg=40).quartz-bearing assemblages are considered in more Sapphirine shows a range of XMg from 72 to 86, with the more

detail here. These comprise three main assemblages iron-rich ones (e.g. in sample 7 ) being deeper green in colour andmore pleochroic. The range for most oxides (Table 3) is wider thanthat are essentially within the FeO-MgO-Al2O3-SiO2

AR CH AEAN QU ARTZ- SAPP HI RIN E G RAN UL IT E XE NO LI TH S 255

Fig. 1. Photomicrographs of textures in the Lace xenoliths (a) planar fractures cutting acoss the entire width of a quartz grain.Sample 26. Cross polars, width of field of view 0.55 mm. ( b) Grt+Sil coronal texture replacing sapphirine and quartz. Sample 12.Plane-polarized light, width of field of view 1.2 mm. (c) Garnet rind between sapphirine and orthopyroxene. Sample 76. Plane-polarized light, width of field of view 1.2 mm. (d) Grt+Sil+Spr with spinel ( brown). Spinel is surrounded by sapphirine which, inturn, is separated from quartz by Grt+Sil rinds. Sample 27. Plane-polarized light, width of field of view 1.2 mm. (e) Garnet andpinitized cordierite replacing a former grain boundary between spinel and quartz. Sample 65. Plane-polarized light, width of field ofview 1.2 mm. (f ) Garnet separating quartz from an aggregate involving coarse corundum that is rimmed by minor brown spineland major sillimanite. Sample 40. Plane-polarized light. width of field of view 1.2 mm.

256 J. B. DAW SO N E T AL .

Table 2. Representative analyses of garnet. is balanced by relatively high concentrations of Si and Fe.Stoichiometric calculations (assuming 7 cations) show that the Fe in

Specimen 7 15 26 33 40 56 65 4183 the sapphirine is mainly Fe2+, with Fe2+/(Fe2++Fe3+) varying from1.0 to 0.75 and calculated Fe3+ varying in the range 0–0.084 cations

SiO2 39.9 39.5 40.3 40.4 40.5 40.2 39.7 40.2 per formula unit. Correction of XMg for calculated Fe3+ yieldsTiO2 0.11 0.00 0.15 0.09 0.01 0.15 0.02 0.06 revised XMg in the range 73–89 (Table 3 ).Cr2O3 0.15 0.26 0.15 0.16 0.16 0.24 0.00 0.15Orthopyroxene, not found by Dawson & Smith (1987), wasAl2O3 22.4 21.7 22.7 22.7 22.6 22.8 22.3 22.7

identified as a primary phase in three specimens, and also occurs inFeO* 20.4 27.1 19.1 18.1 20.9 17.8 21.9 18.6MnO 0.29 0.22 0.18 0.13 0.32 0.16 0.25 0.18 reaction coronas surrounding garnet. TheXMg of primary orthopyrox-MgO 14.7 10.0 15.9 17.0 14.1 17.0 12.9 16.3 ene reported here (Table 4) is fairly constant at around 79–81; TiO2CaO 1.54 1.20 1.30 0.80 2.05 1.21 1.93 1.22 and Cr2O3 are present in small but fairly constant concentrations

(0.1–0.4 wt%). In some grains there is zoning from more-aluminousSum 99.49 99.98 99.78 99.38 100.64 99.56 99.00 99.41 cores to less-aluminous rims. For example, sample 75 shows a

rimwards change from cores with XAl of 0.18–0.19 (Al2O3=8.97 wt%)cations per 24 oxygensand XMg of 78.2 to a lower-Al rim (XAl=0.13; Al2O3=6.32 wt%) withhigher XMg (81.5). Cores of other grains retain XMg values similar toSi 5.954 6.050 5.954 5.962 6.000 5.934 5.998 5.950

Ti 0.012 0.000 0.009 0.010 0.000 0.016 0.002 0.006 these rims and hence may have been significantly affected by post-Cr 0.017 0.026 0.009 0.018 0.010 0.028 0.000 0.018 peak Fe-Mg exchangewith modally dominant garnet. OrthopyroxeneAl 3.941 3.932 3.944 3.940 3.946 3.960 3.982 3.944 intergrown with spinel and anorthite in a late corona surroundingFe 2.553 3.480 2.376 2.232 2.586 2.190 2.780 2.388 garnet contains higher Al, Fe and Ca, but lower Si and Mg than theMn 0.037 0.016 0.022 0.016 0.040 0.016 0.034 0.046 primary grains (Table 4 ). Stoichiometric calculations (assuming 6Mg 3.265 2.298 3.510 3.746 3.098 3.728 2.906 3.586 cations) show that the Fe in the orthopyroxenes is mainly Fe2+, withCa 0.246 0.200 0.206 0.126 0.324 0.190 0.314 0.194

Fe2+/(Fe2++Fe3+) varying from 0.96–0.90 and calculated Fe3+varying in the range 0.014–0.042 cations p.f.u. Correction of XMg forXMg 56.5 39.8 59.7 62.7 54.5 63.0 51.1 61.3calculated Fe3+ yields revised XMg values in the range 80–82.5 for

* All iron as FeO. Stoichiometric calculations (on basis of 24 oxygens) for specimen 7 give most samples (Table 4).FeO 20.1, Fe2O3 0.33, Fe2/(Fe2+Fe3 ) 0.985; and, for specimen 4193, FeO 18.3, Fe2O3 0.36, Sillimanite and corundum were previously recognized by DawsonFe2/(Fe2+Fe3 ) 0.984. V2O3 in concentrations up to 0.04 wt%. 7, 26, 40, 56 and 65 are & Smith (1987), both these aluminous phases contain significantcores of large homogeneous grains. 15. Surrounds spinel. 33. Occurs between quartz and concentrations of both Cr2O3 (up to 1.36 wt%) and Fe (up tosapphirine. 4183. Large grain surrounded by corona of spinel, Opx and anorthite. 2.1 wt% Fe2O3T) in solid solution (Table 5). Sillimanite and

corundum do not coexist with orthopyroxene, but occur with garnet,spinel and sapphirine in specimens 7 and 40.Table 3. Representative analyses of sapphirine.

Rutile and ilmenite has Cr and Fe as constant minor elements inSpecimen 7 33 40 56 75/1 75/2 4183 the rutile, and Al is also present in some grains (Table 5). Ilmenite

is a rare mineral; the one analysed example (Table 5 ) is magnesianSiO2 14.1 13.3 12.0 14.1 15.2 13.9 14.9 (6.85 wt% MgO) and contains c. 0.4 wt% of both Al2O3 and V2O3.Cr2O3 0.86 1.61 1.86 1.29 1.26 1.61 1.60 Spinel (a pleonaste) in one occurrence was found in the specimensAl2O3 56.1 61.4 61.7 59.4 58.0 60.6 58.1 previously studied by Dawson & Smith (1987). The primary spinelV2O3 0.14 n.a. n.a. n.a. 0.00 0.00 0.23FeO 10.8 5.24 6.64 6.93 7.57 6.25 7.37MnO 0.07 0.02 0.05 0.02 0.06 0.03 0.04

Table 4. Representative analyses of orthopyroxene.MgO 16.4 17.7 16.5 17.4 17.8 17.8 16.9

Specimen 33 56/1 56/2 75/1 75/2 4183Sum 98.27 99.27 98.75 99.14 99.89 100.19 99.14

SiO2 50.7 49.6 50.8 49.9 52.6 45.2cations per 10 oxygensTiO2 0.32 0.15 0.13 0.36 0.12 0.45Cr2O3 0.32 0.41 0.35 0.33 0.26 0.08Si 0.880 0.792 0.728 0.847 0.905 0.823 0.897Al2O3 8.33 9.89 8.36 8.97 6.32 13.5Cr 0.042 0.076 0.089 0.061 0.059 0.076 0.076FeO 12.4 12.0 11.6 13.2 11.8 19.3Al 4.007 4.311 4.393 4.200 4.064 4.230 4.123MnO 0.04 0.05 0.06 0.05 0.04 0.18V 0.007 – – – – – 0.011MgO 27.3 27.0 28.0 26.7 29.3 19.9Fe3 0.000 0.037 0.084 0.060 0.000 0.064 0.000CaO 0.06 0.12 0.08 0.11 0.08 0.24Fe2 0.557 0.224 0.252 0.288 0.375 0.246 0.371Na2O 0.01 0.01 0.00 0.02 0.00 0.02Mn 0.004 0.001 0.003 0.001 0.003 0.001 0.002

Mg 1.482 1.572 1.482 1.559 1.576 1.576 1.517Sum 99.48 99.23 99.38 99.64 100.52 98.87

XMg 72.7 85.7 81.5 81.8 80.8 83.6 79.8cations per 6 oxygensXMg* 72.7 89.3 85.5 84.4 80.8 86.5 79.8

XMg*=Mg/(Total Fe–Fe3 )Si 1.808 1.772 1.809 1.776 1.850 1.681

The sapphirine can also contain small concentrations of TiO2 (<0.10 wt.%), CaO Ti 0.008 0.004 0.004 0.010 0.004 0.013(<0.05 wt.%) and Na2O (<0.04 wt.%). 7. Core of deep-green grain. 33. Grain between Cr 0.009 0.012 0.010 0.009 0.007 0.002quartz and garnet. 40, 56, 4183. Large homogeneous grains. 76/1, 76/2. Inclusion-rich Al 0.350 0.416 0.350 0.377 0.262 0.593core, and inclusion-free rim, respectively. Fe3+ 0.018 0.036 0.014 0.042 0.023 0.017

Fe2+ 0.353 0.322 0.332 0.353 0.326 0.583Mn 0.001 0.002 0.002 0.002 0.001 0.006

that reported by Dawson & Smith (1987 ). Cr2O3 is the only minor Mg 1.450 1.441 1.485 1.420 1.535 1.099Ca 0.002 0.005 0.003 0.004 0.003 0.010oxide of note (up to 1.86 wt%), though small amounts of V2O3 areNa 0.001 0.001 0.001 0.001 0.000 0.001present in some grains. Most grains are homogeneous but in

specimen 75 inclusion-rich cores are surrounded by inclusion-freeXMg 79.6 80.1 81.1 78.2 81.5 64.7rims that contain less Si and Fe, but more Al and Cr, than the cores.XMg* 80.4 81.6 81.7 80.1 82.5 65.3Most sapphirine compositions are well removed from the 25251XMg*=Mg/(Total Fe–Fe3 )end-member composition. They exhibit Al in excess of four cations

p.f.u. and Mg+Fe+Si less than three p.f.u. as a result of the 33. Large homogeneous grain. 56/1,56/2. Core and rim of grain, rim adjacent to garnet.tschermaks substitution (e.g. analyses 33 & 40, Table 3 ); sapphirine 76/1, 76/2. Core and rim of grain, rim adjacent to garnet. 4183 In corona round garnet,

intergrown with spinel and anorthite.in sample 7 appears to have a deficiency in tetrahedral Al but this

AR CH AEAN QU ARTZ- SAPP HI RIN E G RAN UL IT E XE NO LI TH S 257

Table 5. Representative analyses of sillimanite, corundum, rutile and ilmenite.

Sillimanite Corundum Rutile Ilmenite

Specimen 7 27 40/1 40/2 7 40 7 15 56 66

SiO2 36.9 36.2 36.3 36.2 0.01 0.02 0.02 0.01 0.01 0.02TiO2 0.07 0.03 0.00 0.00 0.04 0.00 98.2 99.4 99.3 55.0Cr2O3 1.01 0.51 0.73 0.39 1.07 1.36 0.29 0.48 0.33 0.03Al2O3 61.5 62.2 61.6 60.7 98.6 97.6 0.54 0.19 0.00 0.43Fe2O3 0.80* 0.06* 1.18* 2.10* 0.86* 0.48* 0.61* 0.32* 0.66* –V2O3 0.09 0.00 0.00 0.00 0.04 n.a. n.a. n.a. n.a. 0.48FeO – – – – – – – – – 37.0**MnO 0.02 – 0.01 0.00 0.02 0.00 0.00 0.00 0.00 0.33MgO 0.02 0.02 0.14 0.05 0.00 0.00 0.00 0.00 0.00 6.85

Sum 100.42 99.02 99.96 99.44 100.64 99.46 99.65 100.40 100.30 100.14

O 5 5 5 5 3 3 2 2 2 3Si 0.999 0.998 0.987 0.991 0.000 0.001 0.001 0.000 0.000 0.000Ti 0.001 0.001 0.000 0.000 0.001 0.000 0.979 0.991 0.987 0.990Cr 0.022 0.011 0.016 0.008 0.014 0.018 0.003 0.005 0.003 0.001Al 1.964 1.990 1.974 1.959 1.976 1.976 0.008 0.003 0.000 0.012Fe3 0.018 0.001 0.024 0.043 0.012 0.007 0.007 0.004 0.007 –V 0.001 0.000 0.000 0.000 0.001−0.000 n.a. n.a. n.a. 0.009Fe2 – – – – – – – – – 0.741Mn 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.001Mg 0.001 0.000 0.006 0.002 0.000 0.000 0.000 0.000 0.000 0.244

* All Fe as Fe2O3; ** all Fe as FeO. Sillimanites: 7 isolated grain; 27 surrounding sapphirine; 40/1 surrounding spinel; 40/2 adjacent to corundum. Ilmenite in specimen 66 surrounds(?replaces) rutile.

reported here has a wide compositional range (Table 6), with XMg primary spinel. Secondary spinel occurring in veins that cut garnetand in reaction coronas around both sapphirine and garnet areranging from 48 to 65. Stoichiometric calculations (assuming 24

cations) show that the spinel contains only minor Fe3+, with relatively high in FeO (up to 22.3 wt%) compared with the primaryspinel, reflecting the higher Fe content of the minerals that areFe2+/(Fe2++Fe3+) varying from 1 to 0.903. Extraction of calculated

Fe3+ leads to corrected XMg in the range 48–67 for primary coarse reacting out; this secondary spinel also has lower Cr/Al ratiosbut, like the primary spinel, the Fe is mainly reduced withspinel. The spinel is aluminous with Al2O3 50.0–62.3 wt%, though

Cr2O3 is also an important oxide (4.63–15.6 wt%) and ZnO is Fe2+/(Fe2++Fe3+) 0.91–0.95. Overall, as a consequence of thevariable but important amounts of other components (Cr, V, Zn), itpresent in unusually high concentrations (4.14–6.64 wt%). V2O3 is

an important minor oxide (0.33–1.05 wt%). Resulting XAl (= is not appropriate to consider the spinel as an FMAS phase in thesegranulites as its stability will be enhanced to higher pressures relativeAl/(Al+Cr+V+Fe3+)) is always greater than 0.77 and typically in

the range 0.81–0.93, and (Fe+Mg)/(Fe+Mg+Zn) is 0.87–0.92 for to FMAS spinel of similar XMg (Shulters & Bohlen, 1989; Nichols

Table 6. Representative analyses of spinel.

Specimen 7 15 23 27 40 65 66 4183/1 4183/2

SiO2 0.03 0.02 0.02 0.03 0.05 0.03 0.10 0.02 0.08Cr2O3 6.53 8.05 15.6 5.08 12.6 4.63 0.39 17.2 0.88Al2O3 55.9 53.3 50.2 56.9 50.0 59.3 62.3 44.8 62.0V2O3 0.33 0.79 0.58 0.38 1.05 0.72 0.04 1.25 0.10Fe2O3 0.00 0.00 1.06 1.74 1.63 1.13 2.34 1.32 1.98FeO 15.3 20.5 12.6 14.6 16.3 14.6 21.7 22.3 21.2MnO 0.02 0.03 0.10 0.04 0.10 0.05 0.14 0.09 0.09MgO 13.7 10.4 14.2 13.8 11.7 14.7 12.7 8.49 13.2ZnO 6.64 5.77 5.60 5.88 6.33 4.14 0.11 3.12 0.05

Sum 98.45 98.86 99.96 98.45 99.76 99.30 99.86 98.59 99.58

cations per 32 oxygens

Si 0.006 0.008 0.005 0.008 0.012 0.008 0.024 0.008 0.016Cr 1.138 1.440 2.735 0.856 2.242 0.792 0.064 3.224 0.144Al 14.535 14.192 13.077 14.768 13.292 14.896 15.480 12.248 15.440V 0.058 0.144 0.102 0.088 0.189 0.128 0.001 0.240 0.016Fe3+ 0.000 0.000 0.176 0.288 0.277 0.184 0.376 0.240 0.304Fe2+ 2.827 3.856 2.311 2.688 3.076 2.648 3.848 4.408 3.800Mn 0.003 0.024 0.019 0.008 0.019 0.008 0.024 0.016 0.016Mg 4.480 3.512 4.690 4.472 3.936 4.736 4.896 3.000 4.200Zn 1.081 0.960 0.914 0.856 1.054 0.664 0.016 0.568 0.008

XMg 61.31 47.67 65.34 60.04 54.00 62.57 53.10 39.23 50.57XMg* 61.31 47.67 66.99 62.45 56.13 64.13 55.99 40.50 52.50XMg*=Mg/(Total Fe–Fe3 )

7. Large grain. 15. Spinel surrounded by garnet. 23. Core of deep-orange grain. 27. Grain embedded in sillimanite. 40. Large grain. 65. Large grain; variations in Cr and Fe. 66. In alterationvein in garnet c.f. 4183/2. 4183/1. In alteration rim round sapphirine, with sillimanite; sum includes 0.05 wt% TiO2. 4183/2. In alteration corona round garnet, intergrown with opx and anorthite.

258 J. B. DAW SO N E T AL .

et al., 1992 ). For example, Cr/(Cr+Al) in spinel (0.15–0.07) is Table 8. Mineral trace-element concentrations.approximately seven times that ratio in coexisting sapphirine

Garnet Plagioclase Sapphirine Sillimanite(0.02–0.01) or sillimanite, and garnet has a significantly lowerCr/(Cr+Al) ratio still (0.002–0.007).

Sr 0.38 2580 0.60 9.2Feldspar is mostly antiperthite in which the plagioclase component Ba 0.092±0.014 1950 1.34 10.0varies from Ab62 to Ab78 and the alkali feldspar lamellae vary from Y 409 0.43 0.24 0.19Or63 to Or80 (denoted as Msp in Fig. 1e); Ba is an appreciable Zr 1906 0.29±0.06 0.40 0.81±0.16component in the alkali feldspar lamellae, comprising 1–1.2 mol% Nb 0.27 0.036±0.009 0.25 0.025±0.008

V 442 14.3 1640 2233celsian (Table 7). The overall low modal amounts and low anorthiteLa 0.008±0.001 65.2±0.6 0.026±0.004 0.079±0.008content of the feldspar, together with the low Ca contents of theCe 0.496±0.037 103.8±0.8 0.047±0.008 0.188±0.016garnet, are all indications of low Ca concentrations in the bulkPr 0.294±0.024 9.22±0.23 0.006±0.001 0.018±0.003rocks. Occasional homogeneous plagioclase grains (analysis 1,Nd 4.44±0.14 25.5±0.4 0.030±0.003 0.067±0.008Table 7) are andesine. Feldspar intergrown with spinel and orthopy- Sm 5.83±0.27 1.36±0.14 0.002±0.003 0.031±0.007roxene in a reaction corona around garnet is very calcic – An96 and Eu 0.237±0.095 5.24±0.60 <0.006 0.003±0.011

contains high amounts of iron (0.96 wt% FeO). Tb 2.19±0.14 0.052±0.048 0.001±0.001 <0.004Dy 14.6±0.5 0.334±0.045 0.006±0.003 0.009±0.005Ho 3.39±0.16 0.221±0.027 0.002±0.001 0.004±0.002Er 10.37±0.45 0.330±0.049 0.006±0.002 0.007±0.004Mineral trace element contentsTm 1.470±0.088 <0.038 0.002±0.001 <0.002

Trace element abundances in garnet, sapphirine, plagioclase andSc, Hf, Gd, Yb and Lu were not determined. All concentrations in p.p.m. All errors forsillimanite in a granulite from the earlier collection (BD3603I) weretrace elements are <10% unless otherwise stated. Errors from counting statistics are ±1s,measured on a Cameca IMS-3F ion microprobe at Washington upper limits 2s.University, St Louis, following the techniques described by Zinner

& Crozaz (1986). The silicate sensitivity factors determined byIreland et al. (1991 ) were used for all the minerals. Trace elementabundances are given in Table 8; major and minor oxide analysesare given in Dawson & Smith (1987 ).

The most significant aspects of the trace element analyses are thehigh V contents in sapphirine (1640 p.p.m.) and sillimanite (2233p.p.m.) and high Zr, Y and V in the garnet. Sr and Ba are both highin plagioclase. REE patterns for garnet and plagioclase (Fig. 2 ) aretypical for these phases, although the Sm/Nd of garnet (1.3) is lowerthan that obtained for many granulite garnets. REE contents ofboth sapphirine and sillimanite (Fig. 2) are very low and flat(0.1–0.3×chondrite), and contribute little to the overall REEinventory of the rock.

BULK ROCK COMPOSIT IONS & COMPARISONSFig. 2. Chondrite-normalized REE plot for mineral phases inW ITH OTHER AREASLace sapphirine granulite xenolith.

The phase compositions associated with the Grt+Spr+Qtz assem-blages, are depicted in the AFM diagram of Fig. 3. Those samples containing Sil ( i.e. the assemblage (Spl, Opx, Crd)) retain garnet

with the lowest XMg of the three assemblages, whereas the highestXMg garnet is generally preserved in the Opx-bearing assemblage. ItTable 7. Representative analyses of feldspar.is apparent that the presence or absence of orthopyroxene reflects asubtle compositional variation in either bulk XMg or A/AFM, withSpecimen 7 15A 15B 63A 63B 4183the Opx-bearing assemblages occurring in samples with either

SiO2 56.1 65.3 63.3 64.6 58.5 47.1 greater XMg or lower A/AFM. As garnet dominates the modesAl2O3 27.7 18.9 22.9 18.8 25.4 32.9 relative to sapphirine, the bulk compositions of the granulites plotFe2O3 0.04 0.01 0.03 0.05 0.08 1.07 near or above garnet on the AFM diagram, and the bulk XMg valuesCaO 9.80 0.39 4.17 0.22 7.62 17.9 of the samples are in the range 65–70. Using mineral modes averagedBaO 0.01 0.46 0.06 0.42 0.03 n.a. from the data presented by Dawson & Smith (1987; Table 1 ) andNa2O 5.94 3.77 9.30 1.87 7.11 0.32

determined approximately here, we obtain an A/AFM of c. 40±6K2O 0.11 10.6 0.34 13.3 0.22 0.08and XMg of 67.5±3 for the Opx-absent granulites, and SiO2 contents

Sum 99.70 99.43 100.10 99.26 98.96 99.27 in the range 45–53 wt%.The mineral analyses for Cr and V when combined in proportion

cations per 8 oxygens to the mineral modal abundances yield elevated whole rock Cr andV contents of 3000±1500 and 600±200 p.p.m., respectively. Mineral

Si 2.524 2.984 2.800 2.998 2.640 2.181 REE analyses combined in proportion to their approximate modalAl 1.473 1.017 1.193 1.002 1.351 1.783 abundances yield a whole rock REE pattern that is relatively flat atFe3+ 0.001 0.000 0.001 0.002 0.001 0.040c. 40–10×chondrite but with a small negative Eu anomaly, largelyCa 0.473 0.019 0.198 0.021 0.368 0.878controlled by the actual modal amount of plagioclase present.Ba 0.000 0.009 0.001 0.011 0.000 0.000

Na 0.519 0.334 0.796 0.168 0.622 0.028 CeN/TmN is calculated to be only 2–6 for realistic ranges inK 0.006 0.616 0.019 0.786 0.013 0.005 garnet:plagioclase ratios, and there is no Eu anomaly for c. 10 modalAb 52.0 34.1 78.5 17.0 62.0 3.1 percent plagioclase. LREE abundances (5–10×chondrite) wereAn 47.4 1.9 19.5 2.1 36.7 96.4 calculated assuming contributions from the major phases only, butOr 0.6 63.0 1.9 79.7 1.3 0.5 will be higher given the presence of minor amounts of accessoryCs 0.0 1.0 0.1 1.2 0.0 0.0 monazite. A mafic or ultramafic source component in the protolith

is strongly suggested by these REE calculations, together with the7. Large homogeneous grain. 15A,15B K-feldspar and plagioclase lamellae of antiperthite.high V and the high Cr contents in all the phases, and the high bulk16A, 16B K-feldspar and plagioclase lamellae of antiperthite. 4183 In corona round garnet,

intergrown with spinel and orthopyroxene; total includes 0.30 wt.% MgO. rock XMg.

AR CH AEAN QU ARTZ- SAPP HI RIN E G RAN UL IT E XE NO LI TH S 259

et al., 1987 ) that contains 47.5% SiO2, has an XMg of 67, and hasvery high Cr (1020 p.p.m.) and V (243 p.p.m.) would match the Lacebulk compositions well. Two samples from Forefinger Point,Antarctica (Harley et al., 1990) have comparable SiO2 (45–49 wt%)and A/AFM (30 ) values but have high Rb/Sr (5 ) and low Cr (30–35p.p.m.), Ni (33 p.p.m.) and V (100–170 p.p.m.) in relation to Lace.A suite of sapphirine granulites from the Palni Hill Ranges, southernIndia, includes a garnet-sapphirine granulite (PA3–3) that has abulk chemistry broadly comparable to the Lace samples in both itsmajor element features (SiO2=53.3 wt%; A/AFM=28.5; XMg=71.5 )and, significantly, its elevated Cr (442 p.p.m.), Ni (190 p.p.m.) and V(180 p.p.m.) trace element contents (Raith et al., 1997).

CONDITIONS OF METAMORPHISM

Constraints from FMAS and KFMASH grids

It was noted in the section on mineral assemblagesthat the Grt+Spr+Sil association was stable underpeak conditions with either Opx or Sil, and thatOpx+Sil+Qtz was not identified as a stable assem-blage. These observations, assuming that the xenolithsFig. 3. AFM diagram projected from quartz, showing the main

features of the mineral assemblages in the Lace xenoliths. all crystallized under the same P–T conditions, con-Shaded fields correspond to three phase assemblages, and strain the peak P–T field for these granulites to lie inarrows indicate the direction of translation of these fields with the shaded area of Fig. 4, constructed following Hensendecreasing temperature or increasing pressure.

& Green (1973) and Bertrand et al. (1991a). In thisNote that the Grt+Sil+Spr+Qtz assemblages occur at lowerXMg than the Grt+Spr+Opx+Qtz assemblages. Low modal grid, the peak P–T field is bounded by the FMASabundance of Opx in the latter assemblage indicates that the invariant points [Spl] and [Sil], their linking univari-relevant bulk composition is near the Grt+Spr tieline. ant reaction (Spl, Sil ) and the (Crd) reactions emanat-

ing from each point.The stability of Grt+Crd relative to the higher

The mineral mode, bulk rock and trace element data may be pressure assemblage Opx+Sil+Qtz largely controlscompared with similar data for other relatively reduced (i.e. Fe-oxidethe pressure position on the grid in Fig. 4. Hensen &poor) sapphirine-quartz granulites worldwide. Dawson & Smith

(1987) suggested that the Lace granulites may correlate with the Green (1973) placed the invariant points [Spl] andsapphirine-quartz granulites of the Napier Complex on the basis of [Opx] at P–T conditions of 9.5 kbar, 1030 °C andsimilar P-T conditions and broad compositional comparisons. 8.1 kbar, 1042 °C, respectively, in an approximateHowever, the more comprehensive data presented here show FMAS system in which cordierite was probably stabil-that the Lace and typical Napier Complex sapphirine-quartz

ized by the presence of H2O and CO2, and garnet bygranulites differ in detail. The Lace samples are dominatedby garnet+sapphirine whereas in the Napier Complex the presence of minor Ca. Bertrand et al. (1991a) foundorthopyroxene+sapphirine is the main assemblage with K-feldspar little difference in the pressure stability of cordieriteor osumilite as common additional phases (Ellis et al., 1980; Grew, under both H2O- and CO2-bearing conditions, and1980; Sheraton et al., 1987; S. L. Harley, unpublished data).

placed the equivalent FMAS invariant points atCompilations of bulk compositions of these Napier Complexsapphirine-quartzites (Ellis et al., 1980; Grew, 1980; Sheraton et al., 10.8 kbar, 1040 °C and 9.6 kbar, 1060 °C (Fig. 4). Based1987 ) also indicate significantly higher SiO2 (60–88 wt%) and lower on comparisons of these experiments with KFMASHCr and V contents than the Lace samples. melt-bearing experiments by Audibert et al., (1995) andOther examples of sapphirine-quartzites that appear to differ Carrington & Harley (1995), Carrington (1995) con-considerably in bulk composition from the Lace samples include

cluded that the pressures at which volatile undersatur-those reported from Sipiwesk Lake, Manitoba (Arima & Barnett,1984 ) and Wilson Lake, Labrador (Arima et al., 1986; Currie & ated cordierite and garnet react to form Opx+SilGittins, 1988). These vary in their SiO2 contents from 67 wt% to bearing assemblages (with melt) are probably lowervalues as low as 48 wt%, but generally have low Cr, V and Ni than those defined by Hensen & Green (1973) but still(20–50 p.p.m. Cr; 35–90 p.p.m. V; 7–30 p.p.m. Ni) and high A/AFM

near or above 8 kbar. This implies that the equivalent(60–70) compared with the Lace samples. Sapphirine-quartz granu-lites from In Ouzzal (Bertrand et al., 1991b) appear from their of the [Spl] point, involving Grt, Opx, Sil, Qtz, Sprmineral compositions and assemblages to be more magnesian and and Crd coexisting with a KFMASH silicate melt, willsiliceous than the Lace samples, whereas oxidized sapphirine-quartz- occur at pressures lower than those given in Fig. 4.magnetite-hematite granulites such as those from Labwor Hills, However, as the samples considered here lack discreteUganda (Sandiford et al., 1987) and Wilson Lake (Currie & Gittins,

K-feldspar (though it does occur as a very minor1988 ) contain significantly higher total Fe than the Lace xenoliths.Sapphirine granulites broadly comparable in bulk composition to constiutent in the form of exsolution lamellae in the

the Lace xenoliths have been reported from both the Napier rare grains of antiperthite), the key assemblages in theComplex and the Eastern Ghats, although as noted above most of approximately FMAS grid are inferred to have beenthe sapphirine-quartzites in these areas and for which data are

stable at at least 8.1–9.5 kbar (Hensen & Green, 1973)available are compositionally distinct. A garnet-sapphirine gneissfrom the Napier Complex (76283350: Ellis et al., 1980; Sheraton or 10.8–9.6 kbar (Bertrand et al., 1991a) (Fig. 4 ).

260 J. B. DAW SO N E T AL .

Fig. 4. Partial FMASpetrogenetic grid for the phasesGrt, Spr, Spl, Opx, Sil, Crd andQtz, modified after Hensen &Green (1973 ) and Bertrand et al.(1991a), and with approximateP–T conditions indicated.Ornamented field designates theP–T conditions constrained bythe two peak assemblages (Spl,Opx, Crd ) and (Spl, Sil, Crd ) (seetext for details). The arrowedpath crossing the lines (Spl, Sil )and (Spl, Opx) is a retrogradeP–T path (dP/dT=5 bar/°C)consistent with the principalreaction textures seen in thesegranulites.

Peak temperatures of c. 1050 °C are indicated from which leads to XAl not being equivalent to Al/2.Pressures calculated using Al/2 as the measure ofthe FMAS grid based on Hensen & Green (1973).

However, these estimates are likely to be too high alumina solubility are listed in Table 8 for variouscalibrations and an assumed temperature of 1000 °C,because Cr is partitioned preferentially into sapphirine

within the spinel-absent assemblages, leading to selected for compatibility with the FMAS mineralassemblage constraints and in view of the likelihoodmigration of the [Spl] point down-temperature with

increasing Cr-contents. The magnitude of this effect is of Fe-Mg reequilibration on cooling. The barometersof Wood (1974) and Harley & Green (1982), whichnot known for sapphirines with 1.2–1.9 wt% Cr2O3,but is not expected to be greater than 50–100 °C. employ an empirical correction for the dependenceof XAl on XMg, yield similar pressure estimates of10–11 kbar (10.77±0.67 and 10.49±0.67 kbar, respect-

Thermobarometry ively) at 1000 °C; calculated pressures would be c.3.1–3.3 kbar lower at 900 °C (i.e. 7.6–7.2 kbar, seeThe coexistence of aluminous orthopyroxene and

garnet in four sapphirine+quartz granulites allows Table 8 and Fig. 5). The barometer developed byHarley (1984b), which explicitly accounts for variationscalculation of P–T conditions based upon the partition-

ing of Fe and Mg between garnet and orthopyroxene in both garnet and orthopyroxene XMg along with XAl,yields lower pressure estimates (8.18±0.72 kbar). All(Harley, 1984a; Sen & Bhattacharya, 1984; Lee &Ganguly, 1988; Carswell & Harley, 1990; Bhattacharya pressures would be raised by c. 1 kbar if calculated

using M1 site Al corrected for Fe3+.et al., 1991) and the solubility of alumina in orthopy-roxene (Wood, 1974; Harley & Green, 1982; Harley,1984b). Results of these calculations for core phase

Retrieval calculations and convergence of P–T estimatescompositions are presented in Table 9.It is unlikely that orthopyroxene will have retained itsinitial (peak) composition with respect to XMg in these

Garnet-orthopyroxene thermometry granulites. This contention is based upon the evidencefrom reaction textures for cooling from high-T con-The minimum Kd values of 2.27–2.35 yield calculated

temperatures that are lower than those inferred from ditions combined with the observation that orthopy-roxene occurs in low modal abundance and smallFMAS petrogenetic grids by 84–170 °C, depending on

the calibration used. The thermometer of Harley (1984a) grainsize relative to garnet. Because of its low modalabundance, Fe-Mg reequilibration on exchange withyields an average 10 kbar temperature estimate of only

829 °C, in contrast to the 916 °C estimate obtained from cooling will preferentially affect the orthopyroxene anddrive its composition to higher XMg (e.g. Fitzsimons &Sen & Bhattacharya (1984), and 908 °C from Lee &

Ganguly (1988). The calibration of Bhattacharya et al. Harley, 1994). The effects of Fe-Mg exchange operatingsubsequent to the peak metamorphic conditions has(1991), corrected for errors in their published equations,

yields a temperature estimate of 829 °C. been assessed here using the technique of Fitzsimons& Harley (1994), which allows retrieval of thetemperatures at which the