mineralogy and geochemistry of fenitized alkaline ultrabasic sills of the gifford creek complex ...

19 Pages·2006·1.99 MB·English

2Al
The Ca nadian M ine raln g i st
Vol.3 4,p p.20I-2r9( 1996)
MINERALOGYA ND GEOCHEMISTRY
OF FENITIZEDAL KALINEU LTRABASISCI LLSO FT HEG IFFORDC REEKC OMPLEX,
GASCOYNEP ROVINCEW, ESTERNA USTRALIA
JOANNAM. PEARSON
Key Centref or Mineral Deposi*, Universityo f WesternA wtrdia. Nedlands6 907,A ustralia
WAYNER. TAYLOR
ResearchS choolo f Earth SciencesA, ustralian Natianal University,C anberra,A CT 4200,A astralia
Ansrracr
The ProterozoicG ifford Creek complex in WestemA ustralia is a highJevel alkaline complexh ostedi n -1.8 Ga granitic
basemenLM agmatismo ccurredi n two episodes.D uring the fust phasea t -1.68 Ga with rphich this paper is conceme4 a
swarmo f ultrabasics ills (Lyons River sills) was emplacedT. his was accompaniedb y the developmento f an extensiveb elt of
fenite comparablein extentt o that surroundinga major alkaline intrusive complex.H owever,n o large bodieso f alkaline rock
are known from the area. suggesdngth at the sourceo f fluids is an unexposeda lkaline intrusive body. The later episodeo f
magmatisma t - 1.25G a wasc arbonatiticin characterA. mong the Lyons River minor intrusiveb odies,t wo forms mer ecoguized:
a swarmo f ultrabasics ills in which primary igneoust exturesa rep reserve4a nd a serieso f intenselyd eformedu ltrabasicl enses.
The degreeo fmetasomatismin tle fenite belt variesf rom pervasiveK -feldspathizationo fthe county rock in the outermostp art
of the aureolet o intenseg lowth of alknli amphibolea nd aegirinei n the innermosta ureole.h imary minerals in the ultrabasic
sills haveb eenr eplacedb y alkali arnphibolea nd aegirine,b ut basedo n the interpretationo f pseudomorphst,h e sills originally
consistedo f olivine macrocrystss et within a groundmasso f mica"p erovskite,t itanian magnetite,a nd carbonateT. he preserved
igneoust extures,e .9., cm-scalel ayering and gravity settling of macrocrystsa, re similar to thosed escribedfr om the Benfontein
kimberlite sills in SouthA frica. The deformedu lftabasicl ensesh aveb eene mplacedp lastically, at subsolidusle mperaturesin, to
overlying sedimentsp, ossiblyi n responseto movemento n a fault. No primaxyt exturesr emain,a nd the rock is now comprised
of deformedp hlogopitea nd potassianm agnesio-arfvedsonitsee t in a carbonatgm atrix. Major- and trace-elemengt eochemistry
of the Lyons River sills showss tronga ffinities to carbonate-richu ltrabasicr ocks sucha s the Benfonteins ills, Igwisi Hills lavas
or some exampleso f aillikite. Stable isotope compositionso f matrix carbonalesa re remarkably similar to those of the
Benfonteins ills: 613C= -5.5%0,0180* +l07oo,in dicating little disturbanced uring alkali metasomatismC. oexistingc arbonates
preservec ompositionsin dicative of temperatureso f fenitization above4 50oC.
Keywords:a lkali metasomatismf,e nite, riebeckite,a rfvedsonite,a patite,k imberlite, rare-earthe lement,c athodoluminescence,
fluorine, Benfonteins ills, GascoyneP rovince,W estemA ustralia.
Sotrnuanr
Le complexea lcalin prot6rozoiqued e Gifford Creek,e n Australie occidentale,a 6t6 mis en place en deux dpisodesd ansu n
socleg ranitiqued 'environ 1.8 milliard dann6es.U ne venuei nitiale, il y a environ 1.68 Ga" a produit la suite de filons-couches
ultrabasiquesd ite de Lyons River, et un cortbgei mposantd e f6nites, tout e fait comparablee n dtenduea ux manifestations
attenduesd 'un complexei ntrusif alcalin majeur.T outefois, il n'y a aucunm assif volumineux dansl a r6gion, ce qui nousm bne
i proposerq u'un tel complexe,d emeur6e nfoui, a assur6l a mobilisation des fluides ndcessaireh la f6nitisation. Ir deuxibme
6pisoded e magmatisme,h environ 1.3 Ga" avait un caractArec arbonatitique.P armi les filons-couchesu ltrabasiquesn, ous
reconnaissonus ne vari6t6 dansl aquelle les texturesi gn6ess ont conserv6ese, t une autre of nous ne trouvonsq ue desl entilles
ddform6esI.r degr6d e m6tasomatosaet teint dansl a ceinturef 6nitis6ee st variable, et mbned 'unep art i une feldspathisation
potassiquep ouss6ed esr ochese ncaissaatedsa nsl es partiesl es plus distales,e t d'autrep aft e une forrnationm assived 'amphibole
alcalineo u d'aegyrined ansl es partiesp roximalesd e la ceinture.l ,es min6rauxp rimairesd esf ilons-couchesu ltrabasiquesd, ont
des macrocristauxd 'olivine dans une pite d nica, p&ovskite, magn6titet itanifbre et carbonate,o nt 6te remplac6sp ar une
amphibolea lcalinee t I'aegyrine.k s texturesp rimairesp r6servdesp, ar exempleu n litage centim6triquee t une accumulationd es
macrocaistauxp ar gravit6, ressemblenit celles qui ont 6t6 ddcritesd ans les filons-couchesk imberlitiques de Benfontein,e n
Afrique du Sud. lrs lentilles d6form6est 6moignentd 'un remaniemenpt lastique,a une temp6ratures ousl e solidus, dansu ne
sdquenced e rochess 6dimentairess us-jacentesp, eut-ete suite au mouvementl e long d'une faille. Il ne rester ien des toxtures
primairesd ansc es lentilles; la roche contient mainlenantp hlogopite et magndsio-arfuedsonitpeo tassiqued 6formdesd a:rsu ne
pite carbonat6eD. 'aprbsl eur filiation g6ochimique( 6l6mentsm ajeurse t haces),l es filons-couchesd e Lyons River ressemblent
beaucoupa ux rochesu ltrabasiquesri chese n carbonatet,e lles que document6eds ansl es filons-couchesd e Benfontein,l es laves
de Igwisi Hills, et certainse xemplesd ai[ikite. [€s rapports d'isotopess tablesd es carbonat€s de la pdte, en particulier, se
rapprochenrt emarquablemendte sv ateursd 6critesp our Benfontein' 6r3C* -5.5Voo,6r8O= +lMoo. Cesr apportsn 'auraientd onc
pas 6t6 sensiblemenmt odifids au cours de la m6tasomatosea lcaline. l,a compositiond es carbonatesc oexistantsin dique une
temp6ratured e f6nitisation sup6rieurei 450oc.
(rraduit par la R.daction)
Mots-clds:m etasomatosael caline,f 6nite, riebeckite,a rfuedsonitea, patite,k imberlite, terresr ares,c athodoluminescenceflu, or,
filons-couchesd e Benfontein,p rovince de GascoyneA, ustralie occidentale.
202 TTIE CANADIAN MINERALOGIST
INIRoDucroN Frc. 2. a) Simplified geological map of the Gifford Creek
complex.b ) Enlargedm ap of the RadfordsR ise location,
showingd etail of deformedu ltrabasicl ens.
The Gascoyne Province comprises high-grade -+
metamorphic and granitic basement rocks of the
PaleoproterozoicC apricorn Orogen, located between
the Archean Pilbara and Yilgam Cratonsi n Western
Australia (Williams 1986, Fig. 1). The hovince is
overlain to the east by Mesoproterozoics edimentary
rocks of the BangemallB asin (Chuck 1984,M uhling & the Gifford Creek complex (GCC). The fenites of the
Brakel 1985).A lkaliner ocksh aven otpreviouslyb een GCC compriset he largest area of alkali metasomatic
describedf rbm this are4 althoughr are-earth-element- rocks presentlyr ecognizedin Australia.
ooironstones"
bearing (known informally as the Detailed field mapping of the GCC has established
Yangibana ironstones) of assumed carbonatitic various types of minor intrusive bodiesb asedo n style
affinity, werer ecognizedb y variousm ining companies of emplacementa nd mineralogical composition. In
(Gellatly 1975,N ewcrestM ining Ltd. 1989).R ecent this paper, we are mainly concerned with alkali
mapping( 1992-1994)b y the presenta utfiorsi ndicates amphibole- and sodic pyroxene-rich ultrabasic sills
that the ironstonesa re one of a number of high-level (the "Lyons River sills"), and associatedd eformed
alkaline intrusive bodies. They are exposed in a lenses.T hey form a swarmp arallel to the Lyons River
-700 km2 area that includes extensivez ones of valley and are associatedw ith an extensive belt of
fenitizedr ock. Thesee xposuresc ollectively makeu p a country-rock fenitization (Figs. 2a, 3). Other minor
newly recognizeda lkaline complex informally termed intrusive bodies include the Yansibanai ronstones.
Flc. 1. Geologicalm ap of the Capricom Orogen,s howingl ocation of the Gifford Creek complex.
FENTIZED IILTRABASIC SLI'.S. WESTERNA USTRALIA 203
2M
BangemalGl roup
t
sedimentary rocks
N
I
2 kilomemes
Fenile zones: A =Aureole fenite (A, =low
grade, An;medium grade, A6 high grade)
C=contact fenite
----. veinfenits
-----
riebeckite veins
ulraDasrc sills
b remobilised ultrabasic lens RR =
Radford's Rise, PBH =Pot Belly Hill
Ftc. 3. Sketchm ap of fenite zonesa ssociatedw ith the Lyons River ultrabasics ills and lenses.
which are magnetite-a nd hematite-richp ods (-40 to were emplaceda t approximately1 .68 Ga and L.25 Gu
150 m2) having marked radioactivity. The pods are respectively (J. Pearson, unpubl. results). Field
enclosedb y sinuousz oneso f K-feldspar - magnetite relationships suggest that the emplacemento f the
alterationa nd are traceablet o narrow ferrocarbonatite Lyons River sills and the developmento f the asso-
dykes at depth (Gellatly 1975, NewcrestM ining Ltd. ciated belt of fenitized rock occurred prior to
1989). Na:row flat-lying aegirine - apatite - pblogo- Bangemall Group sedimentation,a nd the remaining
pite - magnetite- pyrochlore-bearings heetso ccur in intrusive bodies,a nd deformationo f the sills, occurred
the centero f the complex( SpiderH ill, Fig.2a) and are after initiation of Bangemalls edimentation.
gradational into the magnetite pods. Limonitic, Our aim in this paper is to documentt he texture,
weatheredin trusive rocks occur within basal layers of mineralogy, nature of metasomatism,a nd geo-
an inlier of BangemallG roup sedimentaryr ocks @ald chemistry of the Lyons River ultrabasic sills and
Hill, Fig. 2a) andr are fluidization brecciasa re exposed deformedl enses,a nd to comparet he Lyons River sills
in the basemengt ranitoid rocks. with apparently similar intrusive bodies from
Preliminary SHRIMP dates determinedo n zircon Benfontein. South Africa. Such rocks have not
separatesin dicate that the Lyons River sills and the previously been documented from any Australian
magnetite-rich (ferrocarbonatitic) intrusive bodies locality.
FENITIZED ULTRASASIC SILIJ. WESTERNA USTRALIA 205
ME'rr{oDs ME"TASOMATTCZ ONATIoN
Identification of alteration and metasomatic The ultrabasics ills and lensesa re associaledw ith a
minerals was undertakenw ith standardp etrographic belt of alkali-metasomatica lteration (fenitization) of
techniques and by X-ray powder diffractometry of the country-rock granites and migmatites parallel to
mineral separatesC. athodoluminescenc(eC L) photo- the Lyons River @ig. 3). The metasomaticb elt is
micrographs and observationsw ere undertaken on asymmetricalf rom north to south, and about 5 h in
polished thin sectionsa nd rock slabsu sing a Nuclide width. The most intensely fenitized zone lies mostly
luminoscopea t University College London, operating under recent cover along the Lyons River. Following
at -0.8 mA and -12 kV acceleratingv oltage. Room the fenite classification scheme of Kresten (1988),
temperatureC L spectrao f zoned apatite grains were three styleso f fenite alterationa re recognized:
recordedo ver the range4 00 to 700 nm with an Odord Aureolef enites of -24 km width occupy most of
lnshumentsm onoCl gating spectrometear ttachedt o the belt. Furthestf rom the Lyons River, in the lowest-
a JEOL 6400 SEM at Oxford InstrumentsR esearch grade part of the aureole,t he country-rock granitoid
Laboratories,E ynsham, England. To avoid beam rocks show pervasives ericitizationo fplagioclase and
damaget o the specimen,t he spectraw ere obtaineda t incipient growth of K-feldspar, but primary textures
10 kV by signal averagingt he output of several2 0 pm are generally well preserved.C loser to the Lyons
by 20 pm areas. River, the granitoid rocks show progressiver eplace-
Mineral compositionsw ere determinedb y wave- ment of biotite, quartz and plagioclaseb y K-feldspar,
length-dispersiona nalysis (WDS) using a fully sericite and alkali amphibole( medium- to high-grade
automated CAMECA SX50 electron-probe micro- aureolef enites). The K-feldspar is too fine-grainedt o
analyzer at the Natural History Museum, London, optically determinet he structural state. Small stocks
operateda t an acceleratingv oltage of 20 kV and a of oosyenite"p, resumably arising from extreme, but
beam current of20 nA, and calibratedw ith a rangeo f localized, K-feldspathizationo fthe country rocks, are
synthetic and natural standards.A dditional analyses developedin the high-gradea ureole.
were performed by energy-dispersions pectometry Vein fenites cross-cutt he aureolef enites and vary
(EDS) using a LINK PC-XA systemo n a JEOL 6400 from K-feldspar-rich veins in the medium-gradep art
SEM. Reductiono f the EDS dataf ollowed the method of the aureole to alkali-amphibole-richv eins in the
of Ware( 1980). higher-gradep arts. The alkali amphibole veins may
Concentrationso f major elements ond Nb, 7n, be discontinuousa nd contain "clots" of riebeckite,
V, Zt, Y, Sr, Ba Rb, U, Cr, As, Pb, Mo and W, together with angular clasts of the feldspathized
obtained on pressedp owder pellets, were determined country-rock. The coarse-grainedK -feldspar in these
by X-ray fluorescence (XRF) spectrometry at veins is turbid, with a patchy pattern,a nd is devoid of
Analabs Analytical Laboratories, Perth. Con- grid twinning, but is similar in appearancet o the
centrations of rare-earth elements, and of Hf, Sc, description of microcline from other fenitic assem-
Ta and Th, were determined by instrumental blages( Siemiatkowska& Maftin 1975).
neutron activation G,{AA) at BecquerelL aboratories, The contactfenite zonec ontainst he ultrabasics ills
Lucas Heights, Sydney. The concentration of and deformedl enses.F enitizationo f the host granitoid
fluorine was determined by NaOH fusion-specific rocks is extreme,w ith near-completer eplacemento f
ion electrode analysis at Acme Laboratories Ltd, the original mineralsb y K-feldspar, alkali amphibole,
Vancouvernw ith an accuracy of xJl%o or better. sodic pyroxenea nd carbonate.O nly trace amountso f
In all cases,a nalyticala ccuracyw as confinned against quartz,m ica, or plagioclaser emain.T he primary mafi.c
various internationalr eferencem aterials and inlernal minerals of the ultrabasic sills and defomed lenses
standardsX. RF determinationsa re accurateto *27o for haveb eenc ompletelyr eplacedb y alkali amphibolea nd
the major elementsa nd to better than +lOVof or the sodic pyroxene.
fraces.I NAA deterrninationsa rea ccurateto bettert han
y5%of or the light rare-earthe lements( LREE) Q,a to LvoNs Rrvnn Ulrnasasrc Str;.s
Eu) and to better than x.l1vo for the remaining AND DFORIMD I,ENSES
elements.
Carbon and oxygen isotope compositions were The ultrabasici ntrusive bodiesc omprisea swarmo f
determinedo n carbonatem ineral separates(m agnesite sills showing relatively undeformedp rimary igneous
and dolomite) from fio samFleso f the Lyons River textures.T hey are -0.5-2 m in width and aret raceable
ultabasic sills by phosphorica cid CO, extaction using in length for many tens of meters.T he ultrabasics ills
proceduresa nd correctionso utlined by Goldng et al. are confined to the basementa nd cross-cutt he rock
(1987). Analytical uncertaintiesa re up to t:D.syoofo r fabric of the host granitoid rocks as well as earlier
carbonate oxygen because of possible errors in K-feldspar metasomaticv eins. The deformed lenses,
estimation of the acid fractionation factor of the on the other hand, occur as bulbous pods within
dominantc arbonate. undeformedp orphyritic granite and as narrow lenses
206 TIIE CANADIAN MINERALOGIST
along the basal unconformity with sedimentaryr ocks by carbonate,a re set in a fine-grained mosaic of
of the Bangemall Group. The two types of intrusive carbonate,s odic pyroxene needles,a bundanta patite
bodies are distinctly different and will be described andb arite. Opaqueo xidesa rel ess abundantth an in the
separately. blue sills (Fig.4b).
Dark brown sills arer are. They weathert o a fine-
Pr imary- te xtured ultab asic sills grained, Fe-oxide-rich, brown porphyritic rock, with
abundantg listening -0.5 mm phenocrystso f mica and
Although the ultrabasic rocks have been meta- rounded -2 mm macrocrysts now replaced by
somatizeda nd the primary mineralsh ave beena lmost carbonatea nd Fe-oxides( hematitea nd goethite).T he
completely replaced,t he textures are preserved,w ith matrix minerals comprise largely fine-grained
very little deformation.T he texturesa reb est explained carbonates and Fe-oxide, with lesser fine-grained
in igneousp etrographict erms. The original minerals matrix mica that also mantles the mica phenocrysts
can be identified in most cases,a lthought he replacing (Fig.4c).
mineral phasev aries with sill type. The sills can be Green sills are fine grained and equigranular.
groupedi nto four main textural-mineralogicalt ypes: Macrocrysts are notably absent. Green sills are
Blue sills are the most common type. In hand carbonate-rich(> 60Voc ubonate),a nd consisto f a fine
specimen,t he rock has a distinctive blue color and is equigranularm osaico f carbonatew, ith abundantf ine-
generallyp orphyritic, w\th -2 mm macrocrystss et in grained apatite, and radiating needles of sodic
a fine-grained, carbonate-richm atrix. The pseudo- pyroxene( Fig. d).
morphsm ay have eudedralo utlines, or occur as more In all the types of sill, sodic pyroxene is not
rounded "blebs" consisting of a fine-grained mat of observedto replaceo livine, andw hereo livine hasb een
alkali amFhibole and carbonate.T hey are set in a replacedb y carbonateo nly, the groundmassin variably
mosaic of fine-grainedc arbonatet ogetherw ith either containss odic pyroxener atler than alkali amphibole.
coarser-grainedb lue amphibole or sodic pyroxene 41lali amFhibole and sodic pyroxene do not occw
needles, and fine-grained apatite, barite, monazite, togetheri n the groundmassa, nd where sodicp yroxene
zircon, phlogopite, and a trace of K-feldspar. is dominant in the groundmass,a patite is more
Cathodoluminescencset udiesr eveal that apatite may abundant,
make up to -5-l0%o of the rock. Rutile intergrown Field evidenceh as not establishedt he relative
with carbonate has replaced abundant -0.25 mm timing of the blue, light brown and dark brown sills, as
grainsa nd smaller -0.1 mm grainso f cubic shape; they are subparallel,a nd cross-cuttingr elations have
some of these were origrnally twimed or intergrown not beeno bservedH. owever,t he greens ills are late as
crystals. These features suggestt hat the original they cross-cutt he blue sills.
minerals were probably perovskite and titanian
magnetite. Other minerals include fine irregular Deformcdl enses
Fe-oxides and rare grains of pyrite. Less common
minerals are rare ,-l mm phenocrystso f zoned mica The deformedl ensesa re composedp redominantly
replacedbys econdaryphaseasn, d- 1 mm lath-shaped of carbonate,a lkali amphibole and phlogopite, with
phenocrysts, now a mosaic of K-feldspar and lessera mountso f apatite,r utile, monazite,b arite, and
carbonatep, ossibly after primary carbonateo r mica, rare zircon. They are strongly deformed,w ithout relics
The blue sills commonly contain crystal-rich zones of their primary igneoust exture.W heret he lensesh ave
suggestiveo f gravity settling of macrocrysts,a nd fine beene mplacedin to the basalB angemallu nconformity,
centimeter-scalel ayering reminiscent of multiple- as at the Pot Belly Hill locality (Fig. 2), the host
magma-injectiont extures in the Benfontein sills sedimentaryr ocks have become deformed in a style
@awson & Has/thome 1973). Other inhusive bodies resembling soft-sedimentf olding, and intermixed
are composites ills with -10 to 50-cm thick layers within the marginso f the body. This suggeststh at the
in which carbonate-richl ayers are overlain by Bangemalls edimentaryr ocks were largely unconsoli-
macrocryst-rich layers. Sedimentary-style load dated at the time of emplacementT. he emplacement
structuresa nd a high concentrationo f Fe-Ti-oxides appearst o have occurred after the fenitization event
may be observeda longt he interfaceb etweent he layers and at relatively low temperaturesa, s there is no evi-
@e.4a). dencef or contactm etamorphismo r for fenitization of
Light brown sills arc less common.I n hand speci- the sedimentaryro cks; neitheri s theret extual evidence
men, the rock is generally porphyritic in appearance; for explosivev olcanic activity, as would be the casei f
however,t he 1-2 mm phenocrystsw eathero utol eaving the bodiesw ere emplaceda s a diatreme,f or example.
cavitiess imilar to thosef ound on weatheredu lhamafic Some lenses exhibit multiple phaseso f intrusion,
rocks. The phenocrystsw, hich are mostly of prismatic which can be distinguishedp rimarily on the color of
form and reminiscent of olivine and clinopyroxene, the weatheredo utcrop. At the RadfordsR ise locality
may be concentratedi n layers, again suggestiveo f (Fig. 2b), the earliestp hase,o ccurring on the margins
gravity settling. The phenocrysts,w hich are replaced ofthe lensesi,s a brown-coloredro ck. It is composed
FENrI]ZED IJLTRASASIC SILIJ" WESTERNA USTRALIA 207
Ftc. 4. Photomicrographso f representatives ampleso f Lyons River sills and deformedl enses.a ) Blue sill, showing replaced
olivine macrocrystsa nd lath-shapedm inerals,p ossibly after carbonatei,n a finer groundmasso f rutile pseudomorphsa fter
perovskitea nd carbonateF. ine needleso f aegirinea lso presenti n the groundmassc annotb e discernedo n the photograph.
b) Light brown sill, showing phenocrystp haser eplacedb y a fine-grained mosaic of carbonate,s et within a matrix of
carbonatea, patite,a nd aegirine.c ) Dark brown sill, showingi ndistinct macrocrystsa fter olivine, and mica phenocrystsi,n
a gxoundmasos f fine-grainedc arbonateF, e-oxides,a nd fine-grainedp blogopite.d ) Greens ill, showingr adiating needles
of aegirine within an aphanitic carbonateg roundmasse. ) Deformed lens, showing brittly deforned K-Mg-arfvedsonite
crystals set in a carbonate-richg roundmass0. Deformed lens. Photographo fpolished rock surface,s howing the intense
plastic deformationt hat is characteristico f this rock tvoe.
primarily of carbonate,p hlogopite,a nd ilmenite, with phlogopite.T he lens is brecciateda nd redisfributedb y
ootrails"
of fine-grained blue amphibole,a nd contains a blue-coloredl ens. Within the blue phase,t here are
-l-2 cm angular clasts of very fine-grainedm assive poorly defined -0.5-2 m long lenses of sodic-
208 THE CANADIAN MINERALOGIST
pyroxene-richr ock in which the pyroxene forms TSE I. MMNAM OMSNONS OFMMMEROM I]WAIC
coarse-grainedo oknots"w ithin a sodic pyroxene - m.msNffi
c-0a.r5b onmamte m craytsrtiax.ls T ohf ea blklaulei alemnps hciobnotlea ainnsd d gisrtainincsti voef MsmEE Ptb6b8 52 --mbt1@0 t9 !0bt0t@t9 Pt740f Pl7€1 Pt7& ml@3
mica that are attenuateda ndb roken.a ndi n somec ases SlO2 55.05 $.4 9.4 34.n 55.16 t4.51 9,v
rotated@ g. 4e). They are seti n a matrix composedo f TAq:Q 00.J343 00%.6 00..0417 00..00'3/ 00..0s 8 00,.6x oo,,3m2
a fine-grained, equigranular carbonate that shows cr2q 0.0, 0.09 0.08 0.t2 0.t4 0.m M
lI8 1321 16.& 10.38 9.9 ll.m I 1.07
evidenceo fplastic flow (Fig.4f). In the centero fthe &o 7.37 l.16 5.9 3J0 4.4 3.74 4,9
mo MbdM bu M o.& 0.6
outcrop is an apparentlyl ate, poorly exposd buff- Mso 11.98 l4J9 t0.15 15.68 l5.m 14,90 13.tr
m 0.58 0.18 0J1 0.14 032 0.69 O,B
colored lens in which clastso f country rock or earlier N&o 6.91 73t 7,12 6,24 6.70 3.93 631
KrO 1.16 4.J0 0.13 5.36 5.24 1.@ 5,tr
intrusive phasesa re absent. F 0.36 2.21 0.m t.14 t.62 l.l5 1.83
Inte veins TOoEelF qo7..l4J2 v0.X,9 6 903..0n3 q0 ..B6 1 908..3698 90.4.423 q0.6n
ffi: 8.@ 8,000 8.@ &m 8.000 7.w 8.m
At the Radford's Rise deformed lens locality, late &k 0o.00@@ 00..m0@ 00..mm 00..mm 00..mm 00..001807 00..0@)0
veins consisting of coarse-graineda lkali amphibole, n", 00..003596 00..4011 0 00,.mut1 00..@8 7 00..000184 00.m002 7 00..001323
rhombs of dolomite and coarse-grainedm asseso f H. 0t..307006 01..0403 9 0l..&@30 0t..mt4 l 0l..m0@ 0l.,lml33 l%
ilrnenite, in some cases> 30 cm in size, cross-cutt he Xg- 20..85s% 03..1144 8 20%.& 3oA.ry\1 3o..2ff9i 30%.45 3 30..060124
bcooadrys.e -Eglrsaeinwehded roelo, -mLi-tcem a-nwdi daep avtietein, sc,r ocsosn-ctauitn tihneg H o0.o0r0s0 i0..w6 1 0o..0m1o5 00..@m 00,.mm 000...@f1r304 7 000...0mmtr,
ultrabasic sills. Late quartz - Fe-oxide veins with F 00t...%9ot02 0z0m..0m0 0 0z0m,.mm0 o01.,.g07n@6 8 0or..m&0o4 9 00l...gm6 001...009101306
ftehled uslptraatbhaizseicdse inllsv.e lopesc ross-cuta nd locally disrupt MK 00..033254 00..080301 00..0m141 00.909@9 00..nm0 0r..m0m 0o.9@4
r5.ffi t5.a4 15.m8 t'.w r5.8J8 16.m 15.70
88, 74.3 95.1 13.1 88.9 8.2 81.7 83.0
MTNERACLm msrnv
3.m0 8.@ 8.m 8.m 8.O0 8.0@ 8.@
Ml-3 5.m 4.UX 5.0@ 4.93 4.9t 5.m0 4.896
M4 2.W 2m 2.m nq l$3 Lw l.w
Alkali amphibok 0.sl 0.831 0.88 0.w 0.90 l.mo 0.948
bU = blow led of d@id
Alkali amphiboleh asb een aralyzedf rom the ultra-
basic sills and deformedl enses,a nd from incorporated
clasts of fenitized granite. The amphiboles vary in
compositionf rom potassianm agnesio-arfvedsonitteo
magnesio-riebeckite( IMA classification). Repre-
sentativec ompositionsa re glven in Table 1. Because
the amphibolesc ontain only minor amountso f Al, the the groundmass.A mphibole cores in the ultrabasic
proportion of femic ion in the amphiboles was sills, and amphibole grains in the remobilized lenses
calculatedf rom the microprobea nalysesb y setting Si and fenite clasts,a rer ich in the KNqMgaFe3+Si8O22F2
to 8 cations, following the scheme of Ha*'liorne (potassian magnesio-fluor-arfuedsonite)c omFonent.
(1981).T his procedurey ields, in most cases,a valid Amphibole rims and unzonedg rains in the sills grade
crystal-chemicals olution such that for 23 atoms of toward magnesio-riebeckite-riccho mpositionst hat are
oxygen, cation totals fall between 15 and 16, and the poor in K and F (Ftg. 5b). Two amphibolec omposi-
Ml-3 and M4 sites are full or have minor vacancies tions have extremeK contents( -7.5 wt%oK rO) that
(337o). Catton-sitev acancieso f this order in alkali appeaxf rom a literature search to be significantly
amphiboles are a phenomenont hat was noted by higher than previouslyr eportedv aluesf or arfvedsonite
Hawthorne (1976). In some alkali amphibole @or instanceM, an & Le Bas (1986)r eportedu p to
compositionsn, ormalizationo f Si to 8 cationsc reatesa 3.27 wt%o& O in magnesio-arfvedsonitfero m the Loe
crystal-chemicallyu nacceptables olution (total cations Shitnan carbonatites)C. onventionals chemeso f cation
>16); for thesec ompositionst,h e proportiono f ferric allocation to the 7" M1,-3. M4 and A sites are not
ion was estimatedb y normalizing total cations to 16 applicable for these arfvedsonitic compositions.
(Hawthornel9 8l). Specifically,F e3+is requiredi n the 7 site, K is required
Compositionalv ariation of the alkali amphibolesi s n the M4 site. and Ca and a small amounto f Na must
shown in Figures5 a and 5b. Amphibolesi n the ulna- be allocatedt o the ML-3 sites to achievea cceptable
basic sills show strongc ompositionalz onationi n both site totals.
mg# and Na/(Na+K) ratio, whereas amphibole
zonationi n the remobilizedl ensesis weaka ndi nvolves Sodicp yroxene
only slight decreasei n mg# (Table 1). There is no
noticeable difference in composition between tle The sodic pyroxene in all the rock types
amphiboles in the replaced macrocrysts and in investigated( sills, lenses and fenites) is aegirine
FEMITZED I]LTRABASIC SLLS. WESTERNA USTRALIA 249
Arfvedsonite
+ Fenites
t Deformedultrabasicl enses
o Deformedultrabasicl ensesQ Iigh-K)
o llluabasicsills(Cores)
Riebeskiie
o Ultrabasics ills (Rims)
Both
' Ultrabasics ills (Unzoned)
Mg Fe2+
Magaesio- Magnesio-riebeckitgg
riebeckits - Riebeckite5g
FI
arfvedsonite riebeckite
^ 1.5
N
E
qir
potassian-
arfvedsonite
Nai(Na+K) (atomic)
Ftc. 5. Compositionalv ariation of alkali amphibolef rom the Lyons River ultrabasics ills
lensesa nd fenites plotted on a) a K-Mg-Fe2+,ror.,d iagram to show discrimination
betweenp otassianm agnesio-arfuedsonitaen dm agnesianri ebeckitef or the amphibole
coresi n the remobilizedl enses.b ) F (wt%) rrersasN a/(Na + K) (atomic) to show the
desease in F content from core of samplet o rim, and the high fluorine content of
the remobilizedl enses.
with a high (mostly >90 mol.Vo) NaFe3+Si2O6-4 wt%oT iO2. There are no significant differences
component( Table 2). Ti is the only other significant in composition of the sodic pyroxene among rock
minor element" with a maximum content of types.
2t0
TI{E CANADIAN MINERALOCIST
TABLE 2. REPRF.SENTATIVEC OMPOSITIONSO F PYROXENB TABLE 3. REPRESENTATTVEC OMPOSMONS OF MICA FROM
FROMUL1RABASrc SILLS AND FEMTE ULTRABASIC SLLS AND DBT'ONMEDL PNSTS
DEFORMP
LBNS
-
TYPB bhs btB bt@
SAMPLB 761A/ 767rEt ',167tEl m1093t P01093P4 0l093rP 0t@31 sAwE 76716 ml0l9 P0l0l9 P16852 bPmr7e!l(l8p) bPrclm7l(lm8 ) F01090
T7sjii<oot,,, [email protected] 8 [email protected] u2 5t2 @,.6925 il3.bB.r4dr8 s2oz.:.o7n8 5oor...mrEr 7 5o3r,..or83'r8 fsAnfiolio:ro'r 4093,.,9r78 4 4092...9976 7 4094.,.831196 4t0l1..0.6rl54 31553..4.08973 31r43..7.9024 4at1.20.3579
AClrz2oOr , 00..5305 0b.l1d6 0b,3U1 oo..oo7e oo..rol2l oo..oi?o oo..oo53 eA b0t.d0 4 b0l.d0 4 b0t.d0 5 b0l.d0 4 00..0273 o0..l0t9 00..0084
VzQ@E@ 0.24 0.06 0.34 0.06 F6rq0 7.32 EJ6 1,16 10.93 e 18.@ 15.05
Fe2OJ. 29,55 n,85 29,4 26XO X22 32.6 n 3r [email protected]
FeG 4,U 3.26 2.24 2.t9 1.80 0.94 2.06 Mno bld bld btd 0.03 bld bld bld
MlO 0,14 bld bld o.g2 bh bld o.@ Mgo 21.86 2t.N 2r,53 20.41 13.m r4J2 t8.28
MsO bld 0.67 0.22 0.89 0.52 0.04 O.Eo CoO bld bld bld 0.03 0.11 0.13 0.16
CsO 0,16 0.28 0.u 0.09 O.O7 O.O9 O.O7 Na2O 0.07 btd 0.06 0.u 0.06 0.08 02s
Na2O lZ/2. 13.29 13.28 13.17 13.33 13.13 l32O Kzo 10.73 10.52 I0.8 9,74 8.61 9.70 9.:N
KrO bld btd btd 0.06 0.06 O.M O.O3 FBso 03,.M89 b3u. 81 b3ld.9 9 b2ld.@ 00..3614 01,.1034 02..0q5l
Totsl 98.11 101.84 100.68 98.73 9.47 9.t2 98.71 FBO t.64 1,6 1,68 0.84 0.27 0.4 \.23
o{ Totd 96.X 96,58 96.10 95.25 95.22 97.86 95.?8
si 2,(Dl 2.W 2.013 1.999 2.N7 2.012 2.@3
Ti 0.0rl 0.088 0.046 0.101 0.080 0.003 0.091 si 6.2a7 6.11X 6,357 6.028 5.421 5.5n 6.M7
7At o.iz: - o.ora o.iol 00..000011 0.m3 00..w0201 TA 01..1J05?5 0t..615047 01.5@496 0t3.i3l02 20..459833 02..128068 0t.4.122t8
0.011 0.@5 - 0.003 . 0.009 0.001 - 0.005 0006 " 0.030 0.010 0.@9
Ib3 o.izr o.irr 0.-8 47 00..@n57 00..0E0126 00..0913I 6 00..070924 00..@195 0.925 0n3 0l..t090l 5 00..0@@ 02..001131 01..m6650
P"2 0.133 o.ta 0.071 0.67 0.058 0.031 0.066 0.000 0000 0.000 0.000 2.2& 0.m 0.m
Mn 0,005 - 0.001 0,c01 Mn - 0.c04
MlCsag 00..-90 3047 000...90063u88 000..."c00 711928 0000....9000805053t4 00o0....90@i90r3o63 o000....090-080.w482,0000....0@@9483268 rMK&ag 4001....60t9?U{6D. 08 401J..08E242 83 40r-,. .g60tn1J 4? 400L-. ..14@0911E,51 00301.....00006n3118 2880 00301....,m1007J2287320 3000t.....79000392m7s521
mCd8io#n Tolrl 40. 000 246..0800 t44..@9 443..03@ 343..m80 46..0700 440..@8 Cfrdgi#on Tolal 1t050.4.02 .0 11m5..04 03 [email protected] 79 11050..300 3 51373.37 2 [email protected] 1lm5,.t031
t .EDS analysis
I .WDS analysi3 b! ld== a[b eiMlo v cleiltwtrls otd@FeioOn of Fe2O3
cb=lde =l oblegltow lev€l of ddim ((pm)) == p'mbdtlqes sple@ple
Mica contentG ig. 6). Mica phenocrystsfr om the dark brown
sills plot ash igh-Ti, low-F biotite on Figures6 a-+; they
Representativec ompositionso f metasomaticm ica arec hfiacteized by higher Ba andC r contentst han the
from the blue and dark brown ultrabasics ills. and from metasomatic mica compositions, although these
the deformedl enses,t ogetherw ith the compositiono f abundanceasr el ower than found for phenocrysticm ica
a primary phenocrysticm ica from a dark brown sill, are from most ultrabasic lamprophyres and kimberlites
given in Table 3. For the purposeso f comparisonF, e in (Rock 1990). The mica phenocrystsd o not yield
the metasomaticm ica hasb eenr ecalculateda s total Fe crystal-chemically valid structural formulae if
= FezO: in Table 3. This procedure yields crystal- recalculatedu sing total Fe = F%Ol, suggestingt hey
chemically valid cation totals and site distributions, have relatively low Fe3+lFe2v+a lues.
and is consistent with the high estimated Fe2O3
contentso f coexistinga lkali amphiboles.T otal Fe has Apatite
been used to construct the classification diagram in
Figure 6a; on this basis,t he metasomaticm ica compo- The ultrabasics ills" remobilized lensesa nd fenites
sitions from the blue sills and remobilized lensesp lot contain mostly zoned crystals of fluorapatite, with F
as Al-deficient fluorphlogopitea nd biotite that in most contentso f. -34 wt7o.S r andt he /,REE showv ariable
casesr equire Fe3+t o completely fill the T-site, i.e., abundancesw, ith rangesf rom -0.5 to 2 wt%ofo r SrO
they trend toward the "tetrafemiphlogopite"e nd- and -0.1 to 0.5 wtvof or Ce2OrC. athodoluminescence
member( Fig. 6a). The mica is A1- and Ti-poor and is (CL) studieso f the apatitei n the sills and remobilized
similar in composition to mica in kimberlites and lensesr evealsc omplexp atternso fzonation attributable
ultrabasic (or ultramafic) lamFrophlnesG .ock 1990; to variationsi n concentrationo fCL activatori ons (e.g.,
Fig. 6b). Fluorine contentsv ary from L.5 to 4.5 wtEo Fu2+,E u3*, sm3*, Dy3* and Tb3+)i ncorporatedd uring
@ig. 6c); the higherl evels ares ignificantly greatert han growth of the zones (Mariano 1989). The pattem of
normally encounteredin primary mica from ultrabasic zonation is generally consistentw ithin any particular
lamprophyres( Rock 1990). sample,a lthough it varies betweenf fierent layers in
The mica that rims primary phenocrystsf rom the the composites ills. Zonationt exture$a nd the natureo f
dark brown sills and that from fenitized basement inclusions suggest that most crystals grew in
sranite consist of nondistinctiveb iotite with a low F successives tages,f rom waves of metasomaticf luid