P.iVI. nOYEE. D.R, RELE ANI.) A.P. EE ROEX
Fine-grained pyroxenites from the Gansfontein kimberlite, South Africa: Evidence for megacryst magma - mantle interaction. p. M. Doyle Dfpanmcnl of Sticnccs, rni\'t'rsiry i)f Cafie '['own, RondL'hij.scli "700. .Soiiih Africa P.O. Box 31316, Tokai 7966. South Africa e-mail: gco-()dy.ssey@anan7,i.ct).za. D. R. Bell l)e[:)attnien: nf Gcoloi^ical SciL^nccs. rni\'crsily of Clape 'I'own, K()tn!cln.)sch 7700, SoLilh Alrica l)t-]")arriiient of Geological Sciences and Deparlment of CheiTiisti")' and Biochemistry, Arizona Slate University, lempc AZ 8S287-1604. L.ISA. e-mail: [email protected] A.P. le Roex Deparimenl of Geological Sciences. Uni\'er,sity of Cape Town. Kondebo.sch 7700. SoLitli Africa e-mail: aleroex@geol(\t;y.LKl.ai. .za
i 2004 Geological Society' of Soiuh AtVica
ABSTRACT Tin.' CT:!]^^ll"
Introduction periclotite anti occasional eclogile and pyt-oxenilc •fhe Gansfonlein kimberlite. situaiCLl in ihe Karoo north XL'noliths. as well as rare lowei' crtislal mafic garnet of Beaufori WeM, South Africa, is one of ihe southern granulitcs. (),s isotope studies ol ]X'rido!ites ga\e Re- most kimberliles in S<.)Lith Africa. "-t70km west of the depletion ages of -l.l'i to -1.2 Ga. confirming the [losl inferred boundary of the Kaapvaal cralon (Figure I). Archean (/.c. ••off-cr;ttiin"] age signatme of ihe manlle in 'I'hotigh a small anti historically little-studied pipe this region (Janne>' el clL. 1999: 20t)l). Tiie peridolile (Rogers, 1910: Wagner, 1914; Kickwood. 19691. it xenolilhs record evidence of a high temperaiure contains abundani inan[le-(.ieri\ ed xenoliths. Most perturbatitjn to a lypical shield geotherm (Janney cl dl.. prominent is a suite ol" variably-deformed, Fe- 1999; f5ell el ciL. 2003). richmegacrysts of oli\ine. ilmenite and jjhlogopite. and .^mong ihe Gansfonlein mantle xenoliths occur a LiniisLialK abLtntiant. coLUse-grained zircon (Doyle, number of samples v\ ith niinetal compositions similar to 1999). The kitnliedite also contains relatively abuTulant ihose of ihe Ct-poor megacryst suite (Eggler e! ill.. 19"'9:
AI-RICYW IIKRNAI. OV !.;F.OI,OG'I\ 21111-1. VOI.t.:Mi: 11.17. PAGt: 2HS-3(lil 286 FIKE-GRATNFD PYROXEMTES FROM THE GANSEONTEIN KliMBERLITE
Table 1. Mincmlogical composilion t>l .sanipk'S examined in thi.s study.
Sample No, Minerals and modal percentages Di.screle fii PMr>9y-(")48 opx (45%), phi (38%). ilm (17
.SOUTH AlklCAN JOt. RNAI. OT Cit'OLOGV P.M. OOM-E. D.R. BELL AND A.R LE ROEX 287 '^^^Wr^^\m.f^''-
Figure 2. Petrographic fS ol Clansfontc'in hyhrJcl k's (a) Low inagjiifiairion \ it-w of I' -{|^~, a cnarM,' (ili\ini.' sLirji>LinclfLl In A ZOML-CI, line-f^rained m 'I'IK' JniiLT tine <;r:iinc_-tl nmi.- lai.ljacL-nt lo o|j\inf) is mica-ixior. LLLHI iM.T jiriiinc-d, with Mil)L'(.|LKinl. blocky Drllmpyrnxenes. Thf oiin.-r /.tinv is niica-ricli and conUiins sht-al' like a^jrrej^aie.s ol' Hnc-j^rain onhopyroxL-ne. TIIL-SL" two coLTCspnntl u> lUc "jiianiilolihistic' antl 'fa.scitiikik'tr' textures described liy Boyd tV c;/, II9H-)1-)) frotn pyroxenites from the Mzonguana kirnherlite, liast (Trii[Li:ilani.i. (bl Low iiia^niticalion \ievv of .sample l'iMD99-ll7S, ;i line j^iained \ein Loinprisin.t' phlo,L;npite. ilnu-niie ani.i onhdpyro.Kene, lra\'er,sin,i; a coarse Iherzolile. (c) Biotky-textiired ("jiianoblastic") inner zone in .sample l'M!>99-i)'S"' showinj; siibroundetl oriliopyroxene (opx) aiul irregular shaped ilmenile (ilm), with interstitial, irregularly shapetl i.sotropic serpentine parches (,serp) .soiiietinies with calcite Ucl cores, (d) Magnified vit'w n{' re.sidiial \dlatile-rich mineral pati he.s lielween blotky orthopyioxene grains in mairix of tine grained pyroxcnite FMD99-057.
.SOUTH AFRICAN lOfRNAL OF GEOLOGY 2HH FINE-GRAINHD P^'ROXENITES FROM THE GAKSFONTEIN KIMHEKI.ITE
y. £• -i - r 5 - X
X ir- •<-, -I- * X -I ^ 1 ^1 I- O t- O
T3 f X r- - ir T C ir' c —
r- X 5^ ':j (T-, c; ir a "=•
"-, c^ X
= "•13— - C
>fl 00 ov -d >< ir o --1 t- 0 O ^ oio e- o t^ a
X C: X -f -c \r o o 3 X 3 X a
n-, -r c: X T, .y T^ 0 O o 5 >
•Si ;: •"• I r
r c o o
r (.0 2 (.0 2
X 5 O X C C: ri C
r- X -I- q ir r- r,i c o y a V -I- ^ "
•a J= -I "I --1 "I X X — 'VI ;§ ^ I & -yy • — -i. — -- I I =f C = != C _• ° ° ^ a C K il i ^ "a
SOI 111 Ai'KlCAN |UUR^A1. OV C,h:OLOG"i' P,M, DOVLE, r),R. HELL AND A,P, LH ROEX 2H9 cxc;i\ati<)ns on ihc kimliL't'lite, The satnples artd their Lhcrzolite ivilh pblogopile oi-tbopyroxciiilc rciit. inincral assemhiagcs arc listed in Table L Major element Sample PMI)99-078, a Iherz.olite, contains a thin (~icm) analyses were cletermineLl on -A sithsct of" llie samples \ein (Eigure 2b) thai is similar in mineralogy and texture using a C;tniec\i CamebLtx electron niicroprobe at the to the discrete pyroxenite xenoliths. This \ein contains r)e|"iarttnetiL of Geoloj^ical Sciences, Uni\'ersity of Cape phl<.)gopite, orthopyroxene and ilmenite, whereas Town, A lSk\' accelerating potential was usetl. with a the Iher/olite matrix consists of a coarse-grained iK-atn current of 40 iiA, Count times for CaO, MnO and (4 to 5mni), i^irotogranitlar assetnbLtge of oiivine (~80%), Cr>O-<, in garnet, and NiO and CaO in t>li\'ine were (.irthopyroxene (-lS*>i)) and clinopyroxene (~5'K)), .30 seconcLs |-)er element, while ;ill other elemental oxides Smaller, reciystallized oli\ine neoblasts are also found in had peak coittit times of 10 seconds. The data v\ere the Iherzolite matrix, mainly al(.)tig grain boundaries and reduced with the \\\P correction procedure (Poitchou close U) the vein, hi contrast to the lherzolite tnatrix. the and Fichoir, f991), Representati\ e electron microprobe vein is very ilmenite- and phlogopite-tich. ccMisisting of :uialy,'^es of the tiiineraLs are given in Table 2. -50% phlogopite, 5'^)"/" pyroxene, 20'Mi ilmenite and trace quantities of calcite, Orthopyroxene sometitiies forms Results lamellar intergrowths with ilmenite (Eigures 2b and 2e) Pelfography that are similar to the textLire seen in pyroxene-llmenile The suite e.\.amineLl in this study consists oi eight tiiegaciyst intergrowths (Boyd and Nixon, 1973: Gurney discrete, line-grained pyroxenites (i.e.. coniposetl of et ill.. PJ73), Similar graphic intergr<.)wth (.>f iiyroxene fine-grained pyroxenite alone), a coarse-grnined and ilmenite in a fine-grained pyroxenite was reported orthopyroxenite, two composite samples consi.sting cif fjy Rawiinson and Dawson (1979), Evenly dispersed coarse megaci'X'sts with fine-grained dotiiains sitnilar to sulphide of cLtbic habit is unusualK' abundant in this the tine-grained pyroxenite.s (Eigure 2:i), and one xenolith. peridotite xenolith with a fine-grained pyroxetiite vein {Eigure 2b), .'\ niieacecuts clinopyroxetiite with visual Composite pyro.xciiHc - niei>(icrvsl sample. similarity tii the MARIl) suite of micaceous xenoliths was Sample PMn99-057 consists of an oiivine core Liiscrystal and is dominated by large, ferroniagiiesian silicates to secondary hydrous ]">hases, coarse ateas with undulose extinction. Within this large commonly concentrated along fractures crystal are ,snialler grains with a range of grain sizes, showing variable degrees of strain and suggesting that FiiiL'-j>r(iiiiccl idscivti' xcnoUlbs. the oli\inc core was undergoing continuous These xenoliiiis occur both as rountietl nodules and as deformatitjn, annealing and recrystallization as it was angular rock fragments. The dominant mineral is sampled by the kimberlite. The oiivine core is ortho|")yroxene, witli ubic|uitoLis subordinate ilmenite surrounded by inner and oLiter /ones of fine-grained and phlogopite. Garnet may be locally abiindant ancl pyroxenite. with a textitre similar to that found in the trace oiivine is commonly present, Macroscopic discrete fine-grained pyroxenite satnples: P,MD-99-0S6. heterogeneiU' in minerLtl distribution i.s ctitiitnon. EMD-99-(}48 and PMn-99-099, The inner zone Orthop\"roxene exhibits a suliroLintletl, eciuant and (10 tolSmtii in thickness) is coarser-graineLl and sometimes blocky lutbil (1-igures 2a atid 2c), with relatively poor in phlogopite. contaitiing larger \'ariable grain sizes (0.2 to 2,()tnm), typically smaller than orrhopyroxenes (up to 2inm) with a sub-rounded, the minerals in peridotite xenoliths (comtn()nly 2 to ,slightly blocky appearance. This texture, illustrated in 10mm), Ilmenite occurs intergrown with, and interstitial Eigures 2a. c and d, cotresponds to that described as to, the .silicate phases, with rountled or lobate, amoeboid "granolilastic" by Boytl d al. (19H4b). The finer-grained grains that may coalesce into irregular, vein-like outer zone has higher phlogf)pite abundance, and structures (Figures 2e and I'i). It is normally evenly contains orthopyroxene in sheaf-like patches (Eigure distriliLited thixiughout the rock, with occurrence of 2a), Boyd el al. (I9H4b) termed this texture "fa,scicu!ate". s<.>tne coarse patches. The garnet-bearing sample The phlogopiti.' has bent cleavages and is poikilitic. with PMD99-099 contttins a megaciyst-like segregation of abundant inclusions of small (~O,lmtn) rounded to polycryslalline ilmefiiie, I cm in diameter, Phlogopite subhcclral orthopyroxene grains, Irregitlarly ,shaped Liccurs as tlark-bi'own, fine-grained, raiidomly-orienteil ilmenite is interspersed tliroughout the whole region, Liths and patches 10.2 to 2tmn) within polymineralic with a similar distribution of coarse and fine grain sizes, intergrowths. varying locally in abiindance to define a and appears interstitial to the orthopyroxene. Trace crude layering. Garnet, occurring as highly tracturcLl, amounts of oli\ine are dispersed within the turbid, brownish grains (0,-i to .5 mm), was ofisen^ed in orthopyroxene-phlogopite-ihnenile matrix. One patch of Iw^o .sainjiles. Some of the garnets are poikilitic, oiivine composed of reciystallised neoblasts is identical containing inclusions of orthopyroxene, SLiljihitle was to material at the margin of the oli\ine core (Eigure 2f), iiotetl in sample I'M1)99-O99, asstxiated wiih ilmenite. The outer zone of this sample contains irregular
SOL Til AI'KK:.'\N IOL'RNAL OF GFOtX)GY 290 F]NF:-GRAINI-D FYROXF.NITES FROM THE GAKSFONTETN KI.MBFRLTTE
Megactysl PMD99-I47 spinel harzbtifgite PMD99-O30 mica-pyroxenite GANS-2 garnet Iherzolite PMD99-078 Iherzolite with vein GANS-l garnet Iherzolite PMD99-154 harzburyite
n
n
70 75 80 85 90 95 100Mg/(Mg+Fe) - olivine
Figure 3- Hisruj^rani hhowiiiH tliMiiliuiiitii of olisinc^ fonipo.sjiions in C^nin.srDnlL'in XL-noliths. OMviiiL' nii.'g:ii.T\-.sl (.Litn aiv from [:)oylt- (1999) :iml i::ifh analysis represents a single .--ampk-', (Jlivinc data for GANSl and GANS2 from t'. Janney lunpiiblishfcH - .sec janney t'l al. 11999; 20(1] I. I'nr ilif ptTitiotite .samples, multiple analyses uve illu.slr;it«l to demonstralt IIIL- coniposilioiwl .spread. Sampk.s l'Mf)99-078 and GANS- 2 lia\t' [\\'spinels and perov.skite) suggests that the inclusion did exsoltition lamellae of possible elinop^roxene that rerain not result ftoni kimherlite infiltration near the surface, a cotistant orientation across grain btmndaries, and may represent an assemblage ciystallized from an SLiggesting that the polycrysialline nature may in part be eadier trapped liquid. In contra.st, sample PMDy9-09'S due to rectystallisLition of a larger, strained ciystal. is a cf>aise orthopyrnxene-ihnenite intergrowlh Iltnenite (25'M)) occurs throughotit the pyroxenite as with :t secondaiy (host kimherlite related) phlogopite- coarse (2 lo 5mm) blebs with irregular margins, the rich assemblage that includes small oli\'ine grains smaller blebs appearing to be interstitial and irregtilarly ek)ngate along the orthopyroxene grain boundaries.
SOITH AFRICAN lOURNAL OF GEOt.OGV P.M. DOYLE. O.R. BELL AND A.P. 1,F ROEX 2') I
The ilniL-nite hits :i polyciy.sralline, mosaic itfxttire with 120'' graiti intcrsc'Ctions. 'I'hc ihnenite ;intl pvroxene arc no\ in tliix'ct tontact aiiLi it appears thai ilicir mutLKil boundaries are a conduit to late-stage IlLiids that cause Hem thin veins of hydrothermal alteration products alon,q the interface. A single large ((lmml, strainetl. primaiy phlogopite grain is present. GarLsfonlem meyacrvsts
PMD99-048 (fgd) Mineral Chemistry Olirine PMD99-099(fgd) Forsterite (Fo) conlents of [leridotilic and niegacryst PMD99-078(fgvein) olivines from Gansfontein arc shown in F'igLire 3. Tlic PMD99-030(mica-pyx)
coarse peridotite olivhies have a range in Fo wHitenls A Mzotigwana pyroxenites frotii 90 to 93.^. with the higlicr values occitrring in har^^liLirgitcs. In most xenolilhs the compositiotial heterogeneity is minor, however, two samples, GANS-2, a garnet Iherzolite studied by |anne\ c! a!. (1999) and -S PMD99-078, the Iherzolite with a jiyroxenitic \ein, show 1 Im 90 80 70 60 .SO 40 30 20 10 OcJk evidence for tiiore Fe-rich compo,siti(.)ns (Fo ~8S) in the small recrystallized ncohlasts. 'fhe mica-pyroxenite sample PMf)99-03O also contains I-V-rich olivine (Fo SO to 82), w'ilh a higher CaO content than [he mejjacrysts, ranging from (J.06 U) 0.10 weight %i. Tlie megacryst Figure 4. linit-nitL- compo.^ilions ploitcd in ihi.' I k'nianto- olivines are ihe most Fc-rich (Fo 7,^-78); one outlying CicikielirL'-Ilmcnili.' RTnary cli;i,iinim. tndividual Gansfonlein "niegaciyst" olivine with Fo 88 is aclually a fine-grained JliiR-nik' [n(.-L;aL'rysi LIIILIIV.SI'.S In 1111 t")nyk' ( 1999) and BL'II, seetindary reacticm proeluct formed in a h'acturc due to Linpubli.s!n.-i.l tlata. ^t7.onJ;\^:in;i p\'rn\(.'niu.- data troni [joyil c/ a/. kimherlitf reaction (,sec description of l'MD99-(-)9S { I98ih). Fgd = tine-j;raincd di.screle pyroxenites. mica-pyx = iiiua abo\e). clinoinroxL-nitL' "flin^ lint'-i^raiiu'il tlisfrcif xi.'n
sot lit Al KtCAN lOtRXAL C)t- Ci lUJI.Of. V 292 FINE-GRAINHD PYROXENITES FKO,\4 THE GANSFONTETN KIMBERLITF
X Gansfontein ineyacr\sls 1.6 - Ilmenite
1,4 - • PMD99-099(rgd)
1.2 - nPMD99-030(mica-py\) O O Mzongwana fasciculate pvx u 1,0 - AM/ongwana gratioblaslic p\x • o 0,8 - • 0.6 - X n 0,4 -
0,2 - Ax 0 0 - ,,,,,,,, 10 20 30 40 50 100Mg/(Mg+Fe'^)
Figure 5, lliiK'iiiic ss, Mi^', DatLi suiirics and ahlnvNiatinns :is for Figure i.
in FigLires "'a and . h, Oflhojiyroxcncs in liic pyroxenites Cliiiopyro.wnc frotn Mzongwana arc ,similarly hctcrogcncoLis (Tigurf 7), Sample PMI)-99-030 contains tlioji.siLle with a wide range of iVlg= honi 82,6 to 92.5, atiLl \ ery high Ti content Came! (0,6 to 1,2 weight % TiO^). similar tt) fasciculate Two pyroxenite sttitc xcnoliths contain garnet Ltnd only [lyroxenite from Mzongwana (l-ignre 'Ja), However, in iho,se ffotn PMf)99-099 were atialyzed, SLih,st:intial tnarked contrast to Cr-poor megacr^'st pyroxenes and heterogetieity was cncountet'ed in CaO. NLJIO :tnd 'fiO_- (he Mzungwana pyroxenites, it has a low Al content contents and Mg~ (Figure 8), Grains arc inLii\ idtuilly (<0,()'^ lu 1,2 weight % AhO^) that is similar to diopside zoned (Mg=f 67 tt) 72 in the most extreme case), with in MARIO xenolitlis (Dawson and Smith. 1977) and both symmetricLiI and asyninietrica! ztining being Grann\' Smith pyroxenes (Btjyd ct al.. I9K ta) (Figures 9b ob.served. Siniilar ccMiipositional heterogeneity is e\idctil and c). The range iti Ca^ from iT.9 to 50.6 is also similar in the Mzongwana anci Welte\redcn pyroxeiiites. lo these groups of xenoliths (Figtire 9c). The micaceotis A negative correlation between Mg* antl C^tO conteni of pyroxenite diopsides have low Cr contents (0,1 to 0.5 llic garnets (Figure He) is iiniistial for mantle-derived weight % CrjO^). garnets. particulatTy of the megacryst SLiite. which commonly have constant CaO contents, 'fhc CaO Geotbertnobarometry contents range from 2.2 to 3-^ wcighl %. which i.s lower Few opportLtnities for qtiantitati\ e theiinobaromctry of than tnegaciyst garnet.'^ in general and lower than the GatLsfotitein pyroxenite sttite xenolith.s exist because ob.ser\'ed for the Mzongwatia pyroxenite.s. bttt veiy of the lack o\' clinopyrcxxene atid the presence similar to ihc Wcl[e\"fedeti sample, Tlowever. ihe Mg- of garnet in only one sample. The toexistetice ni" verstts CaO trend proiccts tov\at"ds compositiotis of the garnet and orthopyroxene allows application of ihe tnost Fe-rich Monastery' niegacrysts (Figtne 8c) that arc thermobaronietty based on Fc-Mg-Al exchange between known to coexist with iltnenite IGiirney cf a/., 1979) aiul these two minerals. However, the large degree of in the case of TiOj (Figitre 8a), .sulxstantial o\'erlap with chetiiical diseqitilibrium ol)scr\cd in this sample the Fe-rich Monastery ctjmpositions I.s attaitied. For reqttires a careful application ofthe methods, With close Cr^O.^ - .Mg'^ relationships, ihis owrktfT is e,ssentially attention to petrographic context, we selected a closely cotiiplete (FigLire Mb), located pair of garnet atitl orthopyroxene analyses as
sot iii ,\iKic,'\N P.M. DOYLE, D.R. BT-M. .WO A.F, IE KORX
• J'_Ml:i99- X Mc^iicw
+ Mi.'^acr\' Is (MoiHsl XMc|jiii.'rvsi MlLiii,riiiilai)i m P\1| jyj. 111 raaciciilAli: P\'X A Phloizopite (a) J7 - p>r<,.c,K- (a) A D I.e. •
n ).5 - 1—* 1 1 <• • • ).4 -
%*T +. ).: - + + X ).i • + HI - —1—,— -^—-— 7S 80 K2 84 86 KX 90 75 SO 17
16 -• A (b) (hi AJ_ A
14 - f -• -1 O 0 +• + ^ 12 X +J 4- *
10 1 •. •• - •• 1 L. 75 80 85 90 100Mg/(Mg+Fe)
Figure 6, i'lilu<;()pitL' onitiiivsitions: (al "I iOj \,s. AJg* iiiiJ (b) AljO;; • vs. Mjf=, "IliL' tTan.slonlL'in iiicgut-tysi fc>mp()^.i:ii)n is rlir mean of • • • —' 4 -—< 1—1 fitiet-n an;i]>^<.',s of .s:iniplr PMDW-H^J (Doyle, 19991. l'MD99-09=i < • • pliio^opiiL' i,s dark ivtl-htowii iiiiLii occLHTitnt; ;is .sccontkiry • rcplacvinL'nt of niL'^iKTysl ]ihlo,Liopili.' LIIOO.L; im'^ukir IVLICIHR'^. • Monii.sKTy mejiufnsi i,LiiorL- ( I'JWd). Myoi\n\v:m:i • liyroxenitL" diita from lioyi,! i-l
.sot. Til Al-RICAN lOt'RNAi, Oi- CKOLdGV FINK-CTRAINFI:> PYROXHNTTES FKOM 'I'llF GANSFONTF.IN KIMBERIJTF
csystLiilization deptlis of most cratonie megaciyst suites or the Fe-Ti rich sheared pcridotite xenoliihs with which ihey are thoughl to be associuletl (£'.,!,'., Bo\d antl Nixon. 1975: Hops da/., [9S9I. However, il overlaps with the lower end of ihe i':inge of pressures calculated by rhe O M/ongM ana tasciciitatc pys AM/ong\\una gniiiohlaslic py\ same methotls tor the high temiieraliire peridotile.s from ORD2(i27 Wc1lc\rcdcn the Kast Gri(.|LuilanLi off-craton kimberlites. 2,0 Garnet (a) Discussion Suiumaiy and significance of petrographic ob serrations 1.0 - The chemically-tiefined "megutjyst' suite at Garisfontein toniains both coarse and fine grained s:(mples thai exhibit a range of cleformation textures (Doyle, 1999). whereas the pyroxenite SLiite is distinguished liy the intergrowth of 0 0 minerals on a line (< 1mm to "^mm) scale, 'I'ltus, the fine grain size does not simply represent the recrystallisation of Lirge homogeneous domains, The sheaf like, : I) -- fasciculate pyroxenes, block)', gi'anobhistic jiyroxenes, and finely iniergrown, iobale ilmenite are also ciiLiraclerislic textures nol seen in the inegacrysi suite, R:iwlin,son aiul 1 )awson (1979) and Boyd cl (il. ( I9H4b) ascjibed similar fasciculate textures to rapid crystallization, amounting to quenching of a magma, possilily in ihin eiikes or bv inclusion of pyroxenite magma in kiinherlite. Textures resembling those in the IS 7i) nalur:il rocks, including silicate-ilmenite intergrowths. 7.0 \\ ere |Troducetl by melting and rLipidly crystallizing
f).!! -• sam|")les of ihe Welte\'reden sample Bl)2027 in the laboratory (Kawlinson and Dawson, i979). The \'ein-like feature in the Iher/olite (PiVlD99-(!7Sl supports a magmatic oiigin for tiiese xenolith.s, Ho\\e\'er, both 4.0 -• s:uiiple BD2()2" and PMD99-()99 contain later coarse- grained ilmenite, indic'Liting that interpretations other than c|uenching. that inv(jl\e longer time scales niList 2.0 -- (c) be [lerniissibie, becatise residence in the mantle has nol desirf)yeLl the finc-gi'ained ••(]uench-like" icxtui'es. —H- As discussed later, the composition of ihe 65 7n fine-grained pyroxenite is also inconsistent with know n magmatic liciuids, l-'or these I'easons, we consider an alternative to ihe niagnialic qiienching Figure 8. Garnci i.onipc),siii(jns. (a) 'I'iOj v&. Mfj«. (b) CijOi hypothesis. Mg- ;IIK1 (C) CaO vs. Mg-, Abbreviations nnci daia sourcc-s a.s The /.oned pyroxenitic margin lo the coLirse-gr:iined. tletormecl dunite/oli\ ine megacrysi suggests that these point is iilDtietl in Figure 1(1, alon,^ with piL'ssurc and textures could also arise from reaction between magma teiiipcratLircs of pcri(_l()(ilcs tri)in LfSdlho, l'ast and solid mantle. The textures of the fine grained Griqualand and llic nearby Heliron kimbcrlilt that we pyroxenite zones aroLinci the ccxtrse oli\ine sample liave c-alcukited from piiblislied daia using the same PMI)99-l)S7 look \ei-y similar to the textures of rhe < r'i[.i-FiiK\' ll:enn(.>har()nietric nie[hi)d,s. The preferred F- discrete pyroxenile samples PMD99-()4
soi.Tii ,'\i Ki(.:.\N I<)I.:RNAI. or RM. DOYLE. D.H. BFl.l. AKD A.P. LE ROEX 29=i
nicgaciy.'^i.s at Gansfonlcin atitl also to the abuntlanccs in iypicul peridotitf xcnolilhs. No garnet is obscn'ccl in the Gansloniciti i-iift^actysi sitile. wheivas tlic abiinclaticx- i)t orihopyr<.ixene and felalivc paittity of olivine in the + Mcmaslen,' mejiiiunsls i:>yroxcnilc sitite xenolitlis is opposite to the relali\'c AM/oii^wana Granoblustic ]'s\ • M/oni^wiiriiJ I'asi^itiilatc P\x aiiLinLlaiK-es in peritlolilc and the GanslVitilein o llast tjriqualand megiicrvsts tnegaeiysts. Trace ainoitnts ol' olivine in the pyfoxenitic X MARID • Gramiv Smith mepaciTsts rim of l'MD99-n57 and in PMD99-ai,S. -056 atid -09') may • PMDy^-fl.lO MiL'a-p\ro\ci)itc he relies ot reaction, preserved in the pyroxetiite reaction zone hetween a flLiiLl and olivine-doniinatcd 1 Clinop>roxctic la) 1.4 - ' ? ' lilhologies, Significanlly, the pyroxenitic vein in 6' 12 • • peridolile also contains lamellar otlhopyroxene-ilnietiite ~ t.o • • intergrowths, though these are not as well developed as =^. 0.8 • seen in typical megacr\st sitites sitch as those of ihe S 06 - " X • Monastery. Lesotho antl 1-rank Smilh kiniherlites (Nixon i].4 - and Boyd. l^r^: l^oyd. 1974: Gurney el al.. 1979) artd 0.2 - 0.0 - have not beeti observed among the Gansfontein 80 85 90 95 iTiegaft-\sts.
(b) .Siimnniiy and significance of mineral compos itions Mirieral.s c;ccLtrring iti ihe pyroxenite suite xenoliths generally have Mg-*^ for ilmenite and oithopyt"t>xene that s ^ overlap those of megacr\"sis IVoni Gatisfontein. Phlogopite has lower Mg= tliLiti a Gansfontein phlogfipite inegacrysi, although only a sitigle megactyst is available for fotnparis( m. The samples for wliicii there is \isi.ta! petrographic evitietice of peridotite inlet-action
H5 y lKt\c coni]-)ositions that are differcni tiotn the 100Mg/(Mg+Fc) inegacrysLs in terms of .Vlg= and Cr. lying at cot n posit ions intermediate to peridotite. The ilmenites frotii these roclvs have lower Ec"^ cotitents. • (c) Low Ee'^'*^/Ee was also noted for fin2027 (Rawlinson and DawsotT. 19~9). Together. these features are strong e\iden<,e for intei'action of the magtna with 4 - • V peridolite- 'ilie clieinical liem^ls lor oithop)roxenc, phlogopite. ilnietiite and garnet in the pyroxenilcs ail suggest that • X + n tliese minerals v^'ere formed from a melt that was similar X to that which precipitated the Cr-]ioor megaciysts ai • 0 Gansfontein. 'Lhis tneh was more Ke-rich than the 30 35 40 45 30 megacryst magma at other localities in southern Africa IO()Ca/(Ca+Mg) (fXiyle. 1999; Bell and Moore 200i. Hell, unpublished tlala). iticluding the relatively Ee-rich megactyst suite frotn the Monastery kimberlite iCiurney el ciL. 19"'9}. Garnets iti the line-graineLi pyroxenite .suite partially o\erlap the Motiasteiy megaciysl.s in tertns of Mg-L"e-'Li Figure 9. t^linopyroxi-'iK* romposiiiun^. la) lit'jjxs. Mg~. Ib) .WX')^ relationships, inilicating it: some cases an ec|Ltilibratioti \s, Mii". (c) Aht;< \h. C:i-. l"'.\irW9-U3:2 aiv :in:i!y.sc'.s of four with Monastety-like megacryst compositions for tlie.se i.:linopyroxL'nc grains trnm this Nt-nolitli MonahtLMv ami cotiiponents. The.se particular tncgacryst compositions Mz(ingv\'iin:i pyroxenitL' tkila a.s lor ['igurc ". E;i.si Gii(|Lialand are those observed to be iti equilibrium with ilmenile. incg:icr>"si data fron) lk:ll lunpuhl.l and Boytl and Nixon ( I*;KO1, However, the lower and \'ariable Ca cotitents indicate ihe Granny .Smitli mc^j^aciysl tlatu from Ikiyd t-l til. liyH-iLi), IJops lack of ecjuilibration of gartiet with orihopyroxene and (19H9) and Bell liinpiihlished data). M.A.RII:) data trnni Dawsnii and clitiopyroxctie that normally .should produce con.stant. Siiiilli ( \^r^). Waters (1986), Kon/cn cl nL 121K)(II. buffered (^a c'ontetits iti Cr-poor megaetist sLiites. In these fine-grained xenoliths. the theniical tiisec|uilibrium it]i|ilied by (he eomposiiional heterogeneity oceiirs oti \ery small s]')atial scales, itidicalitig crystallizaticjti shortly before en.ipti(jn ol the
SOUTH AFRICAN JOL'KNAL OF tlHOLOGV 296 FIKE-GKAINIiD PYROXEMTl'S FROM TIIT GANSFONTFTN KfMBFRllTE
T (deg. C) 400 600 800 1000 1200 1400 1600 0
50 o
00 a
OHehroii 50 o
+ L-esolho O 200 P\ID99-090
Figure 10. lYcssure-ieinpLTaiLMV diaj^ram indicatinj; eontliliniis ol" (inj;jii nf ^uriT-'l-bearin;^ lint.- Hi:iinL^d pyroxcnilc sample r'MrJ99-0yy. I-or cnnipari.sdn, rhc prL'ssurfs and Icnipt;raUircs ol" pcritloriR- xfnolJlhs I'rom ¥.iisi Ciricnialand kimhurlik-s (Boyd ani.1 Nixnn, 10791, thi_- ! Ifhron (I kiriehcL.'sltnntL'inl kinilx'ililL' isiiiiikn- lo (lunstnnlfin. located 5ilkiii lo llit- norihi (Kdhcy, 19>^l; K.R. HUVLI, iinptihlishctl i,lata) LMUI \ariuii.s Lesotliu kimlxTlilt's are pintteti for comparison. All F-T data have been ailctilaied usin^ tlie TMJ (Harley. 198-4] ihemujmeiLT in combination wiih the P]iKs 'lirey and Kolilcr, 1990) .yeobaroniclLT, Solid reference line i.s a mndel conductive f;eolherm calculatei.! for -A -fOniWni • .surface heat Ri.iw (Pollack antl Chapman. !9~"'l. Da.slu-d oLillinc lick! art' \'-T cuntlitionH of origin inferred for rapitlly ci-\^lalli/ei.l p\ni.\enilc xenoiilh.'i from .Mzonjiwana (Bin'd tV i^//.. 19H-fbl. kimberlite, as sLiiJgL'siccl l;)y Boyd d al. (1984b) lor llic with their sometimes ladiaiing arrangement of Mzongwanii .suite. 'IIK- !4iVLiCest degree of relative oilhopyro.xene cr\'stals that recall ciuencli textures, compositional heterogeneity occurs for Al in suggest the possibility of a ra|">iLlly-crystalli/e(.l magma. orihopyroxene, reflecting presumably the iov\- dilTusivity In (.)rder UJ examine this possibility we compLited of Al in pyroxene (Sautter and Harte. 199f)). Occurrence approximate bulk compositions by combining ihe of recrystallised oiivine neoblast,s with Fe-rich (~ Fo85) mineral compositi(.)ns with our estimates of iheir modal compositions in two peridotile xenoliths also indicates abundances. I'lie results are given in 'I'able 3. along with that the Fe-enriclimcnt in these samples occurred close analyses of similar fine grained pyroxenite suite nodules ro ihe time of eruption, before significant diffusion inlo from M;^ongwana (Bo>xl cl al.. l9Sth) and Welte\"reden larger, Lmreciysiallized <::)li\'inc.s occurred. In llic case (Kawlinson and l)aws(.)n, 19^9). of the veined sample, this may inLlicalc Uiat Two compositional lea ui res of the pyroxenite deformation accompanied vein formation, with xenolilhs preclude them iixnn being silicate mells infiltration of Fe-rich lit|uid from ihe win into tlie ciystaliized in a closed system: their extremeK' low CaO [leridotite matrix. contents (<1 weight"-!! t:aO) and their abnormally high 'i'iO_. contents (10-12 weight"'" TiOj). I'.ven if ihese Bulk compositions of fine-grained pyroxeniies features are exaggerated by incorrect modal estimates, it The presence of mcgaciyst-like mineral composilif>ns, is clear that the compositions are most unusual. We the vein-iike habit of t)ne sample, the internal layering, suggest, insreatl. tliat these xenoliths either represent presence of occasional idiobUisUc grains and the cLimLilales (where a re-sitkial liquid fractitin has presence of high temperatures are features that cfiuld escapetl). or they are the reaction products of melt with indicate an igneoLis origin (or these xenolilhs. soiiti mantle (with or without ihe loss of a resieiLial li(.|Liid In addition the fine graincLl nature ol" lliese sam|iles, fraction). One possiiiility is that a highly fluid, residuLiI
SOUTH .'\i RK:.\N IOIRNAL or Gt:oLoc;v P.M. nOVLK. i).R. BHi.L AA\ IJ- KOHX 297
Table 3- Wliolf rock L'
Sample PMD99-048 PMD99-099 PMD99-030 PHN3087A PHN3088 BD2027 Type fgd Mica pyx phl-kacrs-gar-ilm pyroxenitcs "(lucnch" pyx-ilm Ixtcalily Gansfontein Gansfontein Mztnigwana East Griqualatid Weltevreden .SiO2 M->.-i TiO2 10.2 12.3 5.36 10" Mil A12(.)3 6.1" 6.44 2.01 0.03 0.07 O.IH 20.1 6.96 14.1 13.1 14.6 Mill.) 0.12 O.IK 0.08 0.25 0.2" 0.16 19.4 1H.2 21.1 25.19 CaO n.2 5.02 2.27 0.25 0 10 0.48 0.42 0.43 K2O 1.47 0,4H l..-^2
70.-42 H2-30 74.5 71.2 75.5
i\i.'iiiU'.s will I prim.in kjcrMilin.- ,ind phli),i;iipin.') h\ XKh" Ironi Koul I'l ;il. ( IVSihi. D.ii.i liir liDJi).!" rrnm li:i\\|tnM>n .HKI I ),i\\Mjn ( IT'JI .Ml I'u i'.\|irL--.M'(.I
ciirbonatc fraciitin has escaped from the .sy.sicm- lr is Implicalioiis of hybrid pyroxenite xenolilbs for railiff tlifficLili to imagine how a higtiK' tiifTcrcntiatecl. niegacryst-forniing processes incompaTihle eleiiieni-iich nia^yma prohahly ivlaicci to The compositional similariiy of pyroxenilc-siiile minerals kiinbcrlilc could not be veiy rich in carbonate, yel ihcrc Ki megacrysts intiicalcs ihe in\'ol\emenl of a magma us only minor evidence for its presence in lliese compositionally similar to that whitli pivcipilaietl the Li.ssemblagcs. In this coniexl. and g'wcn the low aftinily Ciansfoniein megaerysts. The calculated liLilk of high field .strength elements iOv carbonalitie liqLilds, it compositions ot the pyroxenite xenoliths and, b\' is also difficLili to en\isage how a carbonate-rich inagtna implication, the pyroxenite hybrid \ein in peridolile, ww coLiid crysialli/.c lo an assemblage so lit-h in Ti. e\<,'n if not comjiLitible with known or plausible igneous liqiiitl a SLib.stanlial portion of it escapctl. compositions. Tims, ii is probable ihat the fine grained In contrast, the mica pyroxeniie .\cnolith has a btilk textures are not formed by (jiienching of a Ii(.]Liit.l (unless conifiosition lliai is \\ ithin a reasonable lange for a silica significant ciiiantilie.s of li(|LiiLl escaped from the system). Lindersaturated, Liltrapota.ssic silicate nieli. In addition, anL! iluu the resemblance to experimental and natural the bulk Mg« is close lo that expected for e(]Liilibriiiiii magmaiic (]uencli products is coincidental. Lack of with off-cralon mantle jieridotile (~Fo9l>. Thtis chemiL'al e(|iiilibrinm at high temperature is ne\'eitheless the miCLi-ctinopyroxenite xenolith cannot represcni ihe Linambigtiotis in implying relati\el) short timescales in magma that precipitated ihe Cr-i:)oor megacr\sts, or j;ave [he formalion of these rocks. rise U) ihe fine-grained jiyroxenite xenoliths. as il is loo Petrograc)liic obser\ations suggest tliai the fine Mg-rich. It coLild. ho\\e\XT. hv i.leri\e(.l from SLich LI grained textLiivs in the pyroxeniie xenoliihs ma\' result magma by subsec|Lienl ec|Liilibration with pcridolilic h"om reLiciion of magma with solitl mantle peridorite. mantle, thus exf^ilaining its higher Cr content and low This peridolite may inclLKle dtinite formed in pre\"ioiis Fe*. Il could have differentiated to lorni a magma megacrysl-forming e\ ents (sample I'MD^jy-O^^). parental to the inegat-'rysls. although ihe solid |")liasi.'s The mineral phases of the discrele p)roxeniles anti ihe that record si.ich an e\'olLilion are nol in ;.-\ iilence, so \ein in ihe peridolile sample are the same, bm their that we consitler this possibility less likely. The similarity t tie mica I signal LI res are some w hat clilterenl due of the micu p\roxenilc mineral assemblage and of the partial ec|Liilibration with pericknite. Ilo\\e\er. the pyroxene tomposilion lo (hose in MARID xenoliihs is compositions, e\'en in this thin vein, are siill much dosei' laken as e\'idence Inr A similar e\oli.ilionary path lor to ihe magmaiic pi'oLlucts than to ihe |')eridotitic wall these xenolilh types. I iowe\'er, the more Ti-rii, h mineral rock, suggesting thai if ihese roeks are reaction products, com|")ositions in the mi(.a-ehnop\'roxeniie suggest they are overwhelmingly tlominaicd by the magma and that its original compositions may ha\'e been tliffereni, iheiefore imply hij^h magma ID rotk ratios. This woultl and that il has an origin in the highly e\'ol\ed suggest that the pyroxenite xenoliths ma\' re|iresent the megaciyst magma thai appears to be a unitiue feature of loci of percolative magmatic flux through SLibcontinental the manile benealh Ciansfoniein at this time. This is mantk'. The macrosco|")ic layering may repieseni ihe SLipi^iorted by the otctirrence of zircon in this xenolilh - reaction fronts that rcsuU from succe.ssi\e episodic a relaiively rare occLirrence in MARID xenoliths liulses of magma through the system. aiiLi the otcasional elsewhere. owrgrowihs antf LMILISULII zoning patterns seen in
AFKK.AN lOl HNAI. i.)F C.HOLOGV FINE-GRAJNED PYROXEKU'ES l-ROM THE GANSFONTEIN KIMBERLITE oithopyroxene and garnei cryst;ils, respectively. reflect composition is adjusted so that the garnet phase \x.)lume the compositional complexity' of these events. The close is intersected, as it would be for mtsre |:irimiti\e proximiLy to niegacrystalline olivine (PMD99-C)57) and inegacrysi magmas, A siiiiiltir sittiaiion arises lor ihe ilmenile (PMD99-099. 002027) is of particular interest in orthopyroxene-ilmenite intergrowths, which normally this regard, and may provide clues us to megacryst occur at compositions less evolved than those found in forming processes. One possibility is thai this fine the Gansfoniein megacryst suite. However, the grained i(xk represents grain boundary infiltration an^.! inlt'raclion with peridotite shifts liquid compositions to mctasomalic con\'ersitin oi the aureok' around a locus ol mimic a more primitive melt cfimposiiif)n, resulting in nicgacryst magma migraticni sucli LIS the coarse grained the intersection of reactions characteristic of less evolved \'t'in in BD2027. While the graphic pyroxene-ilmenite megacryst magmas. intergrowtlis represent cotectic ciystalliii^ation (Wyatt, The pyroxenites, in preserving u snapshot of the 1977: RawHn.son and Dawson. 1979). it is apparent that process of invasion of megaciyst magma into peridotite, this crystallization may OCCLIJ' during reaction between presumably represent processes at the margin of ihe melt and solid peridotite. megacryst-forming event. However, it is an interesting An unexc^lained obseiTatidn is ilic uniformly high questi(.;n as to whether these incompletely equilibrated orthopyroxene content of these xenolitlis \\hcre\cr ihc\' assemblages also represent a lime slice tluring the have been found thus far, Tliis high proportion is eon\'ersion t)f mantle into a megacr\'st-bearing zone by unexpected for the soiidificalion or reaction products ol" the infiltration and metasomatic action of a melt. a highly silica-undersatLirated mcll and is at odds with In sam]?le PMD99-099, megacryst-like gainet giows by the proporiions of olivine to orthopyr(.)xene seen in the interaction of le-rich megaciyst magma wilh whai are regarded as the true magmatic products, peridotite. It is not clear what the equilihi'ium finLil namely ihc Gansloniein megacrysts thumscKc'S. One assemblage for this particular mix of magma and possibility is thai the high Ti content of [he magma peridotile is. or how it might change wirh aciditioiial results in a reaction of the following type: input of melt. Howe\'er. in the mode! explored here, both garnet and oithopyroxene viith appn^ximately (Mg.Fe).SiO, + TiOj - (MgFe)TiO^ + (MgFe),SiOi megacryst like compositions can l">e produced by the olivine liquid ilmenite orthopyifixene reaction of megacryst magma with peridtnite. This ()bsei'\'ation ma\' have significant implications for the pelrokjgic motlels of megacryst compositional evolution, At Monastery, the onset of pyroxene-ilmenite namely thai megacrysts can be formed by the intergrowths coincides approximately with the metasomalic con\ersion of peridotite by melt at high disappearance of olivine from the megaciy.st me it rock ratios, such that compositional features of fraciionation sequence (Gurney et ciL. 1979: Moore peridtJtite are by and large erased, but ihal tlie megacr\'st 1986). This obseivation supports the proposed reaction phase assemblage is dictated by details of the reaction relationship beiween litaniterous megacryst magmas and between ihe two components. MegacrN'si-forming magnesian mantle olivine. We suggest that the unusuLiI processes have in the pasl been considered largely in textures of the lamellar pyroxene-ilmenile inlergrowths terms of magmatic evolution si.ich as mighl be expected could be linked to gmvAth under condilions ol' this in magma chambers or plating out of ciystals on the reaction relalionship and draw attention vo their walls of veins or other conduits. We envisage that occurrence in the pyroxenite vein in peridotite (sample megacryst formation may in\olve more solid state PMD99-078). With increased evolution of (he megacryst reaction than generally recognized. magmas, this reaction relationship would be required to cease, because the more Fe ridi. Ni-poor olivines. such Finally, the textures and composiiionai heterogeneity as those coexisting wilh zii-con. ticcur in stalile that argue strongly for the exi.stence of a megaciyst coexistence vv'ith ilmenite. magma ai the time of kimberlite eruption make it rather In a hybrid, melt-dominated system, mineral likeiy that the tw{j events are intimately related. assemblages are tiictated by lic|uid phase relations, Kimberlites are far from liquids, containing a great although the mineral proportions and their compositions deal ol solid mantle material, but the liquid component will reflect those of the bulk system. Therefore, in them may derive from this megacryst magma. megacryst minerals are expected to precipitate, bui in Reaction of megacryst magma with peridotite proportions different to [hose normally obser\'ed in to produce high-Mg* liquids that are highly enriched megacryst suites. We therefore propt)se that the in incompatible elements, as proposed for the origin of pyroxenite xenoliths record the interaction of megaciyst the Gansfoniein mica-clinopyroxenite, can accounl magmas wilh peridotite at relatively high integrated for n:iany geochemical features (jf kimberlites and Huid/roek raticjs, such that compositional features of the various metasomatic fluids (Jones. 19 •SOI Til AI'KICAN RNAl. Ol- GEOLOGV P.M. nOYLK, D.I?. BI'LI. AND A.P. LE ROIZX c()mp<),sition.s [liat ;irc similiir to thiisc u\' niegLicrysts I'lipuhlishL-d H.Sc llouiiiiis ihcsis, riiiiviyily nf (Mfic Tain:. Vi'//^/i Afrlcii. h'om Gansfontein. hiil LlitTcr in ^c\cral .significant H,i;,^ii,'t, 1). !!.. .MeCnllLiiii. .M.I-:, and Siuiilt. C.IS. ( I'r'J). .Mej;:ii.-r\'st a.-iseiiihlayc-.'^ respects. It is ]->rop{)secl rhar they formed liy ihc in kinilxTliIf li'inii nnrllicm Cnlnr.idu and M>LUliL-rn Wyoming: petrology. intcracUcjn of inegaciyst magniLi with solid manile in ilie Heniliermonietry-hLiroiiH'ln . anil are;il diMri bill ion. In- F R. lioyil ,ind II. foi'ni of pcridolilc. and previously formeel me<4acryst O. A. .Mt-yer ( Kdiior.sl Tile mande ^aniplei indusions in ki^lberlite^ and \eins or aggregates. Hie niincrLiI lexRires. proportion.s D.L.. V.S..\ . Lind composiliuns obser\ed in these fine grained pyi'oxenites jirovide e\ idence foi' [iroeesse.s of melt-solid Criirin. W I... (J'lieilh. S. ^"., .\aiapo\'. I.. \1. and ilyjn, C. G. {211(1.^). HIL- fsniijiiori of llic- lilhospliL-fie m:niUe ht.-iK'a:li llie KaLiliLiii Craton and its reaction in the mantle .nid suggest (hat these ina>' be iiiai-);i[i>. /./7/iitt. 71. 2l';-2il. impoiiant in generation of the Cr-p(.)or inegacry.st suite Guniey. J. I.. W'^f.\. H. \X\ anti Kalile. \'.. \. I"). (J9~.^t. c:iiniipyroxfne-ilnK-nit(; in kinihei'lite.s. iiuerj;ro\vrhs tVom kiinl'erliic: a re-jpprai.sal. In: I'. H. Ni^nn (l-'dilnr) l.tMiiho kiiiiherliles. Uvolho \alit)iud Ik-tvhipiiivnt Cciponilidii Must'iii. Acknowledgments 2.W-2^.-^. Cnirncy. J j .\\K\ ILirle. IV lI'WDI ChL-iiiical varialion^ in upper mantle We are grateful ru jock Robey of De Beers and .Vlichele nnvliile.s hum -.outht-rn Ahican kiniberlite>. I'I'i/iisiipbkiil 7'riiiisi/Lllfiii.\ '•/ Francis for assistance in the field and thank Pliil janne\' I/H' h'(mil Socivly (ifL'Oidun. A297. 2"3-2<).-^. for sharing iiis unpublished analy.ses of Gansfontein (airnc.-\, J. J.. Jjkob, K. K.. I'Javvson. j. H. (lV~';i .\ie,i;a(.-rysL.v Irnui llie peridotiies. We grateliilly acknowledge the support oi' Moiia.siery kiiiiberlut.-. Soinh Alricii. /*;. V. K. linyd and H. (J. A..\U'yt-r De Beers for mantle and kimberlite .stLidies at I'C'r. l^Ml) I I'xlitnr.s) '11 le mantle sample: iiK'ki.->ioii> in kiinberlites and other viilcanics Aiui'n(.ini (k'o/jhYsiciil I'liio)!. lt"mV.i/«,(if(i». D.C.. IKS.A.. 2i7-i-4.-S. was supported by a South African government NRF llark'y. ^ I. ( I'JKi). .\ti e\perimenul >[Lidy of ihe partitkmiiij; of t-e and Wn honours bursary. We are grateful to .Mr. 1, Potgieter ol' belwt-i'n garnci and orlhopyroxent.-. Oiiilrihuliuii.' I" Miin-nt/nfiv iiiiil Leeiiklixjf for access \o the kimberlite on his /\'ln,liinv. 86. .lS9-r.^. properly anti l(ir his supi^ort and interest in the project. Fl.irif. H., Hunter. K. M and Kinny. R 1). l!99.1i, Mt-tr f>erimetrj'. niovfintnl MLinuscript re\ iews by Sle\e Shirey, Deon De Bruin and .mil tiysiLiIMiration, in relaiinii lo nianLlc dyke.s, veins and niftustiiiiiili.sm. (.•ditorial assistance from Lew Asliwal are appreciated. l'hik,s(i!:h:fnl '/hiiiMU-lio'is iiflhf Rnyiil SiKU'ly of bnuhin. A342, 1-21. llop^, J. J. I 19W) Some Li>pefl> ol [he i;eiicliL'nii>uy o! high-tempewte perkloIiU's and [nej;jinsi.s Irotn llie jagersfontein kimheHiie pipe. Soulh References .MrifLi. I ii/iiihlisbi'il i'h.ll. rbi:^h, t'/iiiviyily af Ciifii.' Tnivii. Soiiih Afru'ti. Hrll. I) K .iiicl MuiirL'. H. O. (iuo-n. \h\-p diciiijail MrLiiUiru iiflht- M)ijUu-r[i iUHpp. Alric.in iiunLk- trom kinilx'tiilt.- iiii.-gjtr\">i.s. Snulh Afyh'an jiiiiniat I'f Ho|i.s. |. |.. Gttrnt-y.J. J.. ll.inf, B. :uid Wimerburn, R (I9W1. Megaenst^ LUid 107. hij^h IfmpLTLilLire nodules from thu iajjersfoiiieiti kimberlite pipe. /'/.• Bell. I). K iii;^. M. 1).. Janncv. !'. i-;. (JOd.-i) MI.'MJ/OIC iht'rniLiI L'voliitmn .1. Kn^^. A. L..kn|Lies.J Ferjiti.Miri. 11. M. l".reen. S. V. (.rKeiliy, R. \ Danclitn (i| i!iL -.ouilii-Tii MVifan [iiaiiili- li:lni^plifri.v Lilhtis. 71. r\'l^~ and .\. J. A. Janse. l Tdilors). Kimlierlites am! Ki-l;ik'd rofk.s. VnluDie 2. Tlieir Hoyd. \\ K. I iVil. 1 illE'triialic notkilL's liiim ihr l-r:tnk ^mlll^ kimlKTlilc pipt'. mantle •jrw-l selling. i.li:mi(inds ami diLiiiiond ex|>loralinn (k'ulii^kiil Cjiniiyjc li'slilniitiN uf Wiishfiinimi )l-iirhi,iilj. 73. iH^-i'>i. Suciny (if Aii^tnilki Sf'cciiil /'iih/iciiii SOL 111 Ai'KIC.'XN JOtR.XAl, OH CiHOI.OGV 300 FINn-GRAINKD i'^"ROXF.NTT^.S FROM THE GANSFOKTFIX KIMBI-RIJTF H. [inti SuitT, I.;. F. (19921. Trace ck'Hicnl geinht-nii.sm of ilniniiii' S( linl/c. i). I. ( i')H4]. c:r-poor mt'gacrysis trom ihc Hamilton lirandi mc-gairysi.s lioni ihf ,Mon:t,sti'r>' kimlxrlilf. South Afric-a. LHhns. 29, !IK. ki[iihi:-rliit-. 1 Iliuii County. Kt'niucky. Ill:}. Kornjirolist iKditnr) Kimlic'rlitc-s Nfc-al. t;. li. :IIKI [l;i\'klMin. |. I'. I WH^)). An iinmt'tasomatizcd soiiri'ij lor ihf II. *l'lit Mamie and Crust-MLUILIC Kt'lalioiisliips Di'ivhipnii'iils in Fclrtilof^y. \lal:iil;in aliioiit- {SnliiiiKiii l.sl;inii.s) ; l'L'lrni;L'[ii.'sis invoMng /iint- rc--liiiini;, /•/sfi'k'i: AuistcriUiiu. ibf .W-lbi-rkiiuh. llB. 9~-l(i,s, tnci;:ii.TVSl friiclionalinn, and iissmiilation iif occanii." lilliosplu-rL'. Stltnl/i-. 1)J. (IW). .Vk'^jaciysts from alkalic vok\inic rucks, hi: I'.li. MNOII Ck'i'L'himk'ii I'/ (jismiiuhiinica Ada. 53. 19~^-l'WiJ. ll'dilur). Manik-Xcnulillis.yo/)/! \S'ilL'y iiiiil ^iiiis. Cl'iJ'v.'^h'r. I .K . i3.-^-n2. \iMin. I'. II. iind lio\(.l. KK (IT,-;i. rhf disccctL' nixiiik' ass .SOUTH l,