Canadian MineralogistVol. 17, pp. ll-23 (1979)

INTERGROWN CALCIC AND Fe-Mg AMPHIBOLESFROM THE WONGWIBINDA METAMORPHIC GOMPLEX, N.S.W., AUSTRALIA

N. C. N. STEPHENSONDeparlment ol Geology, University of New England,

Armidale, N.S.W. 2351, Australia

H. D. HENSEL

Departrnent of Geology, tlniversity ol Adelaide,G.P.O. Box 498, Adelnide, S.A. 5001' Australia

ABSTRACT

Hornblende-cummingtonite and actinolite-cum-mingtonite (or grunerite) pairs in sillimanite-or-thoclase-zone amphibolites and quartzites of theWongwibinda Complex occur as composite grainsshowing a variety of intergrowths. Coarse homo-axial intergrowths probably developed bysimultaneous primary crystallization of the am-phiboleo during prograde metamorphism. Finelamellar intergrowths are due to exsolution uponcooling. Irregu'lar patchy intergrofihs are probablyalso exsolution products, having formed eitherdirectly or by intragranular reorganization of ex-solved lamellae. A close approach to chemicalequilibrium between the coexisting amphiboles isindicated by relative uniformity in the arnphibolecompositions within domains of uniform host-rockchemistry, and by reasonable regularity in the dis-tributions of major elements. The pairs of inter-grown amphiboles clearly define the miscibility gapbetween the calcic and Fe-Mg amphibole seriesfor the PJ conditions of final exsolution andequilibration. There appears to have been littledifference in the solubilities of cummingtonite inhornblende and actinolite, whereas the solubilityof actinolite in cummingtonite was greater thanthat of hornblende. Calculated pre-exsolution ac-tinolite compositions suggest, by comparison withthe experimentally determined cummingtonite-ac-tinolite solvuS. that metamorphism peaked at about650"C and P(HrO) )3t2 kbat. This is consistentwith the conditions indicated by mineral assemblagesand evidence of incipient melting in pelitic andpsammitic schists, namely 650-700'C an'd 24kbar P(H,O). However, anomalies in present andpre-exsolution compositions of the coexisting cum-minglonites suggest that further study of the ef-fects of variables such as pressure and chemicalcomposition on the amphibole solvus will be neces-sary before coexisting calcic and Fe-Mg amphi-boles can be used as reliable geothermometers.

Sorvnvrerns

Dnns les amphibolites et quartzites de la zone

m6tamorphique i sillimanite et orthose du com-plexe Wongwibinda, on trouve des associationshornblende--cummingtonite et actinote--cummingto-nite (ou grunerite) sous forme de grains montrantune vari6t6 d'intercroissances' Des groupementshomoaxes en gros grains sont probablement d'ori'gine primaire, r6sultant d'une cristallisation simul-tan6o des amphiboles au cours d'un m6tamorphismeprogressif. Des groupements finement lamellairesindiquent une d6mixtion pendant le refroidissement.Des intercroissances err taches irr6gulidres sont pro'bablement aussi des produits de d6mixtion, soitdirectement, soit i la suite d'une r6organisationintragranulaire de fines lamelles exsolv6es. Lesamphiboles en prdsence se trouvaient dans un 6tatproche de l'6quilibre chimique, ainsi qu'en t6moi-gnent I'uniformit6 relative des compositions amphi-boliques ir l'int6rieur de domaines of le chimismede la roche-m6re est lui-meme uniforme et utrecertaine r6gularit6 dans le partage des 6l6mentsessentiels. I-es paires d'arnphiboles accoupl6es d6-finissent clairement la lacune de miscibilit6 entreles amphiboles calciques et ferromagn6siennes pourles conditions de pression et de temp6rature deI'exsolution et de l'6quilibration finales. La cum-mingtonite semble avoir 6tE presque 6galement so-lublo dans la hornblende et dans I'actinote, tandisgue I'actinote 6tait, plus que la hornblende, solubledans la cummingtonite. En regard du solvus cum-mingtonite-actinote d6termin6 exp6rimentalement,les compositions calcul6es de I'actinote avant lad6mixtion font penser que le m6tamorphisme aatteint son maximum d'intensit6 aux environs de650"C, avec P(HO) ) 3.5 kbar. Ces conditionssont aussi celles que I'on ddduit des assemblagesde min6raux et de la fusion incipiente observ6edan: les schistes cristallins p6litiques et psammiti-ques, i savoir: 65G-700'C et P(HgO) = 24 kbar,Toutefois, certaines anomalies observ6es dans le.scompositions, actuelle et d'avant la d6mixtion, descummingtonites indiquent qu'une 6tude plus approfondie des effets de Ia pression et de la com-position chimique sur le solws sera ndcessaireavant que les arnphiboles ferromagn6siennes et cal'ciques accoupl6es ne puisent servir de g6ottrermo-mitres.

(Traduit Par la R6daction)

11

t2 THE CANADIAN MINERALOGIST

INrnopucttoN

Chemical data on pairs of coexisting amphi-boles are important for the information theyprovide on the miscibility gaps between amphi-bole series, and for the possible use of elementdistributions in estimating physical conditionsoI crystallization. The association of hornblendeand cummingtonite or grunerite in metamorphicrocks, especially amphibolites and iron forma-tions, is not unusual; chemical analvses of co-existing pairs are fairly numerous (e.g., Vernon1962, Klein 1968, Bonnichson 1969, Kisch &Warnaars 1969, Robinson & Jaffe 1969, Rosset al. 1969, Leake 1972, Stout 1972, Hietanen1973, Im'mega & Klein 1976, Bostock 1977),The association of actinolite and cummingto-nite or grunerite ap,pears to be less co.mmon;most of the analyzed pairs are from the meta-morphosed iron-formations of North America(e.g., Mueller 196O, Klein 1968, Bonnichson1969, Ross et al.. 1,969, Simmons et al. 1974.Immega & Klein 1976).

Coexisting calcic and Fe-Mg amphibolescommonly form intergrowths for which variousorigins have been proposed. These include (i)simultaneous primary growth of the two am-phiboles, (ii) replacement of one amphibole!f tne other, (iii) overgrowth of one amphi-bole around the other, and (iv) exsolution ofthe intergrown phases from a primary homo-geneous amphibole. The exsolution origin ofamphibole intergrowths has been strongly em-phasized lately by Jaff.e et a/. (1968),' Rosset al. (1968, 1969'), Robinson et al, (1969),Papike et al. (1973), Gittos et al. (1974),Immega & Klein (1976) and others.

Ca,meron (1975) has investigated experi.mentally the actinolite--cummingtonite solvus

for compositions with Mg/Fe - 1 at 2 kbarP(H,O) and 1(Oz) defined by the FMQbuffer. However, quantitative experimental in-formation is lacking on the influence of varia-tions in P(H,O), l(Or), Mg/Fe ratio and othercomponents (e.9,, Al, Na, Mn, Fe"+) on theextent of solid solution between coexistingamphiboles. At present, the effects of factorssuch as these must be inferred from analvticaldata on natural amphibole pairs. There hasbeen no experimental study of the hornblende-cummingtonite solvus.

Mueller (1960, 1961, L962), Kisch & War-naars (1969) and Bostock (L977) have foundthe distribution of Mg and Fe between coex-isting calcic and Fe-Mg amphiboles to be non-ideal. Kisch & Warnaars suggested that theMg-Fe distribution may perhaps be used asan indicator of metamor'phic conditions withinthe limits of specific parageneses, but a usefulcorrelation between the distribution coefficientand P-T conditions has not been established.

This paper deals with intergrown hornblendrcummingtonite and actinolite-cummingtonite (orgrunerite) pairs from amphibolites and quart-zites of the Wongwibinda complex, northernNew South Wales, Australia. The Wongwibindacomplex, described by Binns (L966), is a rela-tively small north-northwest-trending belt ofmetamorphic rocks 27 km long and up to 11 kmwide, situated in the Nambucca Block about65 km northeast of Armidale (Fig. 1). Itprovides a progression, from west to east, fromincipiently metamorphosed greywackes and mud-stones through metagreywackes and phyllitesto high-grade schists and migmatites borderinga syntectonic granodiorite that probably formsthe core of the complex. The mineralogicalcharacteristics of the terrain suggest low-pres-

Frc. l. Geological sketch map of part of the Wongwibinda complex(after Binns 1966), showing sample locations.

Bo.olt Nsaatmontg

Orulodlorit l-lm6rmtit"

sctdsrs N*tff[.t'

\\\\\

2VD['l\:\-KY.(t1,t1ii ri'iffarr

?(:l]|;r)ffi':*t --'Xtlit,lli'l::;*7i/,ttt

TNTERGROWN AMPITIBOLES FROM TIIE WONGWIBINDA COMPLEX 13

sure, Abukuma-type metamorphism. Andalu-site is absent from the lower-grade schists,probably because they are not sufficientlyaluminous, but cordierite is common and al-mandine garnet is generally restricted to thehighest-grade schists. Staurolite and kyanite areabsent. The maximum grade attained was thatof the upper amphibolite facies. This is sug-gested by the occurrence of silli'manite *orthoclase and the absence of muscovite inpelitic schists, and by the absence oforthopyroxene in basic rocks. K-Ar biotite agessuggest 250 m.y. as the age of metamorphism(B1nns & Richards 1965); current Rb-Sr studiesindicate a complex meta,morphic history ex-tending over a considerable period of time.

Tns Hosr Rocrs

The amphibolites and quartzites in which theamphibole intergrowths occur are very minorcomponents of the Wongrvibinda com'plex.These rocks have been found in only one posi-tion in the metamorphic sequence, i.e., closeto, and on the high-grade side of, the silli-manite-orthoclase isograd. They occur as sev-eral thin (

t4 THE CANADIAN MINERALOGIST

detect growth ledges. (c1., Gittos et al. 1974).Measurements of the width and spacing oflamellae show that they may comprise upto about 15 vol. /p of. a grain, with no obviousrelationship between variations in la,mellaeabundance and the identity of the host phase.(ii) Patchy intergrowths. Both amphiboles mayenclose irregular patches of the other. Thesepatches are variable in their size, shape anddistribution within the host. A few show ioughlyplanar boundaries parallel to (T01) or (lO0).(iii) Coarse intergrowths. The coexisting am-phiboles may be coarsely intergrown in com-posite 'twin-like' crystals comprising two tofive broad bands, each about 0.05 to 0.25 mmwide. The interfaces between adjacent bandsare more or less planar, occasionally steppedplanar, and mostly parallel to (l0l), less com-monly parallel to (100), and rarely parallelto.(010). Interfaces parallel to (10O) commonlycoincide with a simple twin composition plane.The . coarse intergrowths are developed

-com-

monly between hornblende and cummingtonite,but very rarely between actinolite and cum-mingtonite.

In all varieties of intergrowths the boundariesb€tween phases are optically and chemicallysharp. The two intergrown phases extinguislsimultaneously and, at first sight, cleavagetraces seem continuous across the interfaces.Ilowever, detailed examination of the coarseintergrowths suggests that (110) cleavagetraces and (100) lamellae may be deflected byup to 3" across interfaces parallel to (I0l),consistent with the observations of Jaff.e et al,(1968). Nevertheless, the intergrown phasesevidently share an approximately contintrouslattice and may be regarded as essentially co-herent.

The different types of intergrowths are be-lieved to have formed variously by simultane-ouo primary metamorphic growth of the twoamphiboles, by exsolution of one phase fromthe other during cooling, and perhaps by sub-sequent intragranular reorganization of the ex-solved phase.

_Fine regular lamellae oriented parallel to(101) and/or (1O0) in amphiboles are widelyinterpreted as the result of exsolution duringcooling (e.9., Jafte et al. 1968, Ross er al. L968,1969; Robinson et al. 1969, Papike et aI.1973,Gittos et al. 1974, Immega & Klein 1976, and.others); there is no evidence to suggest tlatthis interpretation is not applicable to the finelamellar amphibole intergrowths describedabove. It is not possible to deduce the exsolu-tion mechanism(s) from tle available evidence.The tapered la.mellae may have nucleated hete-

rogeneously at grain boundaries and twin com-position planes, but tapered lamellae are rela-tively rare and therefore should not be usedas a basis for general inferences; homogeneousnucleation of the exsolved phases cannot beruled out.

The irregular patches are probably also ex-solution products because (i) exsolution lamel-Iae are usually depleted or ab'sent in the hostin the vicinity of the patches, and (ii) thepatches themselves normally lack exsolutionlamellae. The ratio of inclusions (fine lamel-lae * patches) to the host phase seems tovary widely between individual glain$ evenwithin a single thin section, which is perhapsinconsistent with an origin for both the lamellaeand patches by unmixing of a homogeneousprecursor. However, this variation is incon-clusive as evidence against an exsolution originbecause the patches are irregularly distributedwithin their hosts; therefore, most sections ofindividual grains probably give a misleadingim'pression of the actual host-to-inclusion ratio.It is not certain whether the patches are thedirect product of exsolution or the result ofintragranular reorganization of exsolved la.mel-lae.

The origin of the coarse intergrowths isproblematical. Ross e, a/. (1969) have in-terpreted somewhat similar composite amphi-bole crystals as the result of intragranular re-organization of fine lamellar exsolution inter-growths. The present intergrowth texturesprovide some weak support for this interpreta-tion because the outlines of a few of the largerpatches (possibly reorganized exsolution lamel-lae) suggest that they may represent an inter-mediate stage in the development of coarseintergrowths. The relative proportions of thetwo amphiboles in the coarse intergrowths seemto vary widely, but.this is not conclusive evi-dence against an exsolution * reorganization ori-gin because random sections through such coarseintergrowths do not permit reliable estimationsof the relative abundances of the componentphases. However, an exsolution f reorganizationorigin for the coarse intergrowths seems un-likely because both components of these inter-gfowths commonly contain fine exsolutionlamellae of the other, whereas the irregularpatches very rarely contain fine lamellae. Rosset al. (1969) interpreted occasional fine lamel-lae in similar coarse intergrowths as relics oforiginal exsolution lamellae that escaped reor-ganization, but the lamellae are sufficientlynumerous and uniformly developed in thepresent coarse intergrowths to suggest thatunmixing occurred, or at least continued, 4lter

INTERGROWN AMPHIBOLES FROM TIIE WONGWIBINDA COMPLEX

TABTE I. ELECTROII IiIICROPROBE ANALYSES AIID CATTON PROPORTIOIIS FOR COEXISTING CAICIC AIID

F&I.Iq AIIPHIBOLES FRO!{ THE IIOIId{IBINDA COMPLEX' N.S.U.

15

HOft'IBLEIDES ACTINOLI'IEs Fe-ilg A'4PflIB0LES

F33 F33ar F25 F40 F60

43970 43970 43971 43972 43!'73

51.32 50.72 50.33 51.06 49.46

0.08

0.41 L43 0 .61 0 .35 0 .19

29.64 29.62 31.01 23,73 v,72

2.9 2 .24 3 .18 8 .53 6 .s )

12 .50 11 .74 9 .37 11 .43 5 .56

o.9o 1 .74 2 .24 l .9 l 1 .23

0.37 0 .45 0 .24 0 .20 0 .22

SalDla [o.

Catalogue No.*sl02

Tl0a

Feofl1n0lilgo

ca0lla20Kz0Sm

Fee0s{fFe0t,

CATIOII PROPORTIOI

stAl lv

TIFss+ **

Fe2+ o{ilnl4g

CaNa(si+AllvAlvirTl+FeftihrlibCarl{a+(trts/(Ms+Fe"+)ih/(MgrFe2'+lh)Ca/ ( tlg+F€z++Ca)

F3 F68

43168 43969

47,68 47.39

0.60 0.85

7.05 10 .55

16.38 8 ,92

0 . 1 5 0 . 1 612.76 16.62

t0.44 10.52

l . l5 I .35. 0 . 1 9 0 . l l

96.41 96.47

2.37 1 .59't4.25 7.49

(23 orqygens)

7.080 6.798

0.920 l .m2

0.314 0,58:l

0.067 0.0920.265 0 .172't.770 0.s9

0.019 0 .019

2.&4 3.553'1,661 1.617

0.334 0.3760.036 0.020

8.000 8.0005.259 5.3182.031 2 .013

0.6't5 0.798

0.004 0.0040.266 0 .266

0.77 0 .8918.07 18 .97

5 . 5 J g . a q

l t . 8 l 1 1 , 6 9'10.52 8.78

0.m 0 .30- 0 .08

97.6'1 97.9

0.69 0.4917.45 18 .53

96.83 96.74 97.48

25.75 15.93 29.64

95.98 97.21 98.36

31.01 23.73 34.72

F:|i}43970,M,02

0.389.30

22,570.657.92

10.741.660.31

97.55

17.66

6.6761.324

0.3390.0430.6232.2&.0.08it1 .7901.745

0.4880,0608.@05,1202.83o.4440.0200.302

F25

4397149.63

0 . 1 32.71

r .068.91

10,350.540.09

96.96

2.9'l

20,92

7.566

0.4340.053

0.0150.334

0. '1372,0241 . 6 9 t0 .1600.0188.0@5,230'1.869

0.431

0,0280.265

l . 1 926.472.905.81

10.230.370 . 1 I

96 .81

o.72

6.22

7 .171

0.221

0.085

3.4540.3871.3641.727

0 . 1 1 30.0227.9925.29'l1 .8620.2830.0740.264

F3 F68

4:t968 43969

52.43 54.99

0.52 1 .39

25.75 15.93

0.61 0 .51't6.09 22.91

1 . 0 9 l . 0 l

0 ,24

F40 F40ai F60

43972 43972 43973

52.71 52.09 49.33

29.62

7.861u . t ! i

7.8400.158

0,077 0,0562.177 2,332

0.446 0.579

2..625 2.622

1.681 1 .4 t6

0.058 0.088- 0 .015

7.996 7.9985.325 5.5891.739 I .519

0.547 0,529

0.085 0 .1050.259 0.222

7.ffi2 7.835

0 . 1 1 0 0 . 1 6 5

: o:ou'

3.28 1 .898

0.077 0.062

3.596 4,865

0,175 0 .154

0.070

7.972 8.000

7.64 7,755

0.074 0.245

: c:0"

3.799 3.78'l

0.304 0.290

2.855 2.675

0. I48 0 .285

0 . 1 1 0 0 . 1 3 3

7.938 8.000

7.048 7,215

0.719 0 .4A 0 .414

0.009 0.044 0.043

0.022 0,022 0.042

7.877 7.477

0.113 0 .064

7,892

0.036

0.010

4.059 3.062 4.633

0 . 4 2 2 l . l l 5 0 . 9 1 9

2.185 2.628 1.346

0,376 0 .316 0 .210

0.073 0,060 0.068

7.990 7.941 7,928

7 .147

o.5270.01 10.025

7 . 1 1 4

0.350 0.462 0,225

0.063 0 .164 0 .133

0.057 0.0s3 0.03

F3: Anphlbollt€. Andsln€ + homblende + cmlngtonlte (+ llrenlte)

F68: Anphlbollte. Bytsnlt€ f homblend€ r ctsllngtonlte (+ llrenlte + K-feldspar + blotlte + quartz)

F$: quartzite. Quartz + gamt r homblende i cmlngtonit€ (r 'llrenlte + Mgnetlte t blotlte)

F25: Quartzlt€, quartz + gamet t actlnollte + cmingtonlte (+ llrenlte)

F4O: Quartzlte, Quaftz + ganet + actlnollte + cumlngtonlte (+ magnetlte * apatlte)

F6O: Quartzlte. Quartz t gamet + actinollte + grunerlte + ferrosaliter 0lscmte gralF deYold of vlslble exsolutlon lanEllae{ Refere to the @llectlon of the Geology Departilent' UnlYersiw of NeH England

0 Total Fe 6 Feogg Ferrcus and ferrlc lrcn cnlculatedi see text for splanatlon. The Fez0g contents of tie

Fe-tilg anphiboles are assui€d to be negllglbl€'

the development of the coarse intergrowths.Therefore, it is suggested that the coarse inter-growths may be tlte product of simultaneousprirnary homoaxial growth of the calcic andFe-Mg amphiboles during prograde metamor-phisrn. The (I01), (100) and (010) latticeplanes apparently provided coherent low-energyboundaries between the two phases.

Arvrpnrsore CnBnarcer ANALYSES

Six pairs of coexisting calcic and Fe-Mg

amphiboles, representing a fairly wide rangein Mg/Fe ratios and Mn contents, have beenanalyzed by electron microprobe. They com-priso three hornblende-cummingtonite 3ndthree actinolite-cummingtonite (or grunerite)pairs. The investigation covered all types ofinterrgrowth except exsolution lamellas nar-rower than about 3p. The analyses and cationproportions (based on 23 oxygens) are presentedin table 1, where each analysis is the meanof 4 to 8 fairly uniform microprobe determi-nations. The FezOs contents of the calcic am-phiboles were calculated, largely by the method

CATIoN PRoPoRT!oNS (23 orqygens)

t 6 THE CANADIAN MINERALOCIST

of Papike et aI. (1974). This method normallyprovides upper and lower limits for the FerOacontent; Papike (pers. comm.) recommendsusing the midpoint between these limits, a pro-cedure that was followed for actinolite F25and green hornblende F33. The calculation vieldsunique solutions for actinolites F40, F40a andF60. For the brown hornblendes F3 and F68the calculated midpoint FezO" values (4.72%and 4.96% respectively) are inconsistent withother data. The FerOg contents shown in TableI .for these two hornblendes were calculatedfrom the host-rock Feros contents and modalanalyses. This procedure yields FezOg valueswell within the limits indicated bv the calcula-tion method of Papike et al., ind consistentwith those obtained by wet-chemical analysisof three similar hornblendes separated fiomone-amphibole Wongwibinda amphibolites. TheFegOs contents of the Fe-Mg amphiboles wereassumed to be negligible.

The amphiboles were generally found to-bechemically uniform within domains of uniformhost-rock chemistry. Thus in the massive am-phibolites, the amphiboles show relatively littlechemical variation at least throughout the areaof a thin section, whereas in tle banded quart-zites they are uniform only within individualbands commonly less than 5 mm wide. Thisuniformity in amphibole compositions is usual-ly 'maintained within these specified domainsdespite variations in the textural occurrenceof the amphibolgs. This suggests that the am-phibole pairs have generally approached chem-ical equilibrium closely.

However, two significant instances of in-trasample variations in amphibole compositionshave been detected. In sample F33 two distinctcummingtonite compositions were found; cum-mingtonite intergrown with hornblende sliowslower Al and Ca contents than that occurringas discrete grains lacking visible exsolutionlamellae (analyses F33 and F33a respectively,Table 1). Similarly, in sample F40 actinoliieintergrown with cummingtonite has a higherCa content tlan that forming discrete grains(analyses F40 and F40a, Table 1). In-boththese instances it is assumed that the com-positions exhibited by the discrete grains (whichmay of course contain submicroscopic exsolu-tion lamellae) represent or approach fue originalcompositions of the amphiboles before unmix-ing, and that these compositions therefore didnot equilibrate with the compositions of theuumixed phases with which they coexist. Fortho purposes of element distritution studiesthe presumed disequilibrium cornpositions (anal-yses F33a and F40a) have been disregarded.

TABLE 2. DISTRISIJTION COEFFTCIENTS FOR EOEXISIING CALCICAND Fe-t'lg AIiIPHIBoLES FRoM THE IIoNGIIIBINDA CoMPLEX, N.S.t{.

KD.l,rg KD.M' KD.At l "

Homblend$cmingtonl te pairs

F3 1 .43 0 .40F68 1 .54 0 .48F33 1 .06 0 .45

Actinol I te-cmi ngtonl te pal rs

F25 | .41

F40 r I .4 lF60 | .36

0.43

0.47

0.52

ro.rn . lx/(t-x)Jca-il x [(t-x)/x]Fe-Mg-m, where

X. t t9 / (Mg+re2+) .

Slnllarly, h.Mn is based on x . Mn/(l, lg+Fe2++Mn),

and KD.A l iv on x . A l l v / (A t lv *s t ) .

ELsIvrENr DrsrnrsutroNs

Mg-Fd+ distibution

In all six amphibole pairs the Mg/(Mg*Fe'+)ratio is higher in the calcic amphibole than inthe coexisting cummingtonite or grunerite. TheMg distribution coefficients (Table 2) for fiveof these pairs are fairly uniform, ranging from1.36 to 1.54, whereas sample F33 gives asignificantly lower value of 1.06. With theexception of F33, the Ko.ue values show apositive correlation with the Mg/(Mg*Fe'*)ratios in both amphiboles (Fig. 3). A similartrend was found by Mueller (1960, 196I)

ro t2 14 t€Ko uc

Frc. 3. Relation between the distribution of Msbetween coexisting calcic and Fe-Mg amphiboleiand the Mg/(Mg *Fe'*) ratio in the Fe-Mgamphiboles. K2.y" is similarly related to thisratio in the calcic amphiboles. Solid circles re-fer to hornblende-

INTERCROWN AMPHIBOLES FROM THE WONCWIBINDA COMPLEX t7

in the data on actinolite-cumminglonite pairsfrom the metamorphosed iron-formations ofQu6bec. He attributed this feature to nonidealmixing in cummingtonite (c1., Mueller 1962).

Variations in Ku.rn values for coexistingcalcic and Fe--Mg amphiboles have been at-tributed by several authors to selective substi-tution of Al (and perhaps Ti and Fe'+) forMg in the calcic amphibole (e.9., Kisch &Warnaars 1969, Stout 1972, Bostock 1977),but there are no clear correlations in the presentdata to support this hypothesis. The cause ofthe relatively low Ko.*s value in F33 is notobvious. It may be related to the high Al'+Ti * Fet* content of the hornblende, or, be-cause of the possible influence of Al-Si sub-stitutions on Mg-Fe'+ substitutions (Ramberg1952), to the anomalously high Ka.lrt' valuefor this pair (Table 2). Less probably, it maysimply be the result of substantial underestima-tion of Fe'O. in the hornblende.

Mn distribution

The Mn distribution coefficients are fairlyuniform, ranging from 0.40 to 0.52 (Table 2).The distribution clearly illustrates the prefer-enco of Mn for the Fe-Mg amphibole struc-ture. Bancroft et al. (1967) and Papike er a/.(1969) have shown that Mn in cummingtoniteis preferentially accommodated in the Mr sites.In the calcic amphiboles the Mt sites are large-ly occupied by Ca and Na, and Mn is forcedinto the less favorable M4 Mz and Mt positions.

Conclusions

The reasonable uniformity in the Mg-Fe2*and Mn distributions supports the conclusionthat a close approach to chemical equilibriumbetween the coexisting amphiboles has beenachieved within the relatively small domainsconsidered. It also seems that Mg-Fe'* dis-tributions should not be used as indicators ofcrystallization temperatures without allowingfor the influence of compositional factors onthe distribution coefficients.

Tne Mrscrsrlny Gep BprwesN THE CALcrclwo Fr-Mc ArupHrson SEnrns

The analyzed hornblendes and actinolitescoexisting with Fe-Mg amphiboles show rela-tively low Ca contents (5.23 cations per 23 oxygens, ex-cept F33) (Table 1). The large excess ofAl"+Ti+Fe*Mnayg cations over the num-ber required to fill the 5.O available Mu Mz

ar'd Mz positions, implies substantial occupancyof. the Mn sites by some bf these cations (prin-cipally Mn, Fe'+ and perhaps Mg), i.e., thereic significant solid solution of the cumming-tonite component in these calcic amphiboles.Nine hornblendes and actinolites from similar,closely associated one-amphibole assemblagesfrom the amphibolites and quartzites of theWongwibinda complex have been analyzed forcomparison. These show higher Ca contents(1.77 - 1.93 cations per 23 oxygens), andlower Ald*Ti+Fe+Mn*Mg contents (5.00 -

5.24 cations per 23 oxygens). This indicateslower cummingtonite contents in the singlecalcic amphiboles than in those coexisting withFe-Mg amphibole.

The Fe-Mg amphiboles coexisting with calcicamphiboles contain 0.15 - 0.38 Ca cations,0.00 - O.13 Na cations, and 0.O4 - 0.26 Alcations per 23 oxygens (Table 1), suggestingsome solid solution of the calcic amphibolecomponent. Four analyzed Fe-Mg amphibolesfrom similar one-amphibole assemblages fromthe Wongwibinda complex differ consistentlyonly in their lower Ca contents (

l 8 THE CANADIAN IVTINERALOGIST

perimentally by Cameron (1975) and inferredfrom natural amphibole pairs from other re-gions (e.9., Klein 1968). The width of themiscibility gap in Figure 4 provides a rneasureof the maximum amount of solid solution pos-sible between coexisting calcic and Fe-Mgamphiboles at the P-T conditions of exsolutionand equilibration. The following discussion ofthe amphibole solvus is based essentially on thequadrilateral components (i.e. on Ca:Mg:Fea'proportions), though the possible influence ofother cations is considered.

Five of the six calcic amphiboles coexistingwith Fe-Mg amphibole contain 8+-.1.3 mol Vocummingtonite in solid solution, irrespectiveof large variations in Mg/Fe ratio and contentsof Al and Mn. Hornblende F33 apparentlycontains little or no cummingtonite component.The lack of consistent differences betweenthe hornblendes and actinolites from two-am-phibolo assemblages in their present and ex-solved cummingtonite contents suggests thatthe solubility of cummingtonite in calcic am-phibole was either little affected by large varia-tions in the Al content of the host phase (c1.,Papike et al. 1973, Immega & Klein 1976), orthat the influence of the Al content was nul-Iified by other factors, e.9., variations in theMg/Fe ratio or Mn content.

The F*Mg amphiboles coexisting withhornblende contain 8:t0.7 mol 4o calcic am-phibole in solid solution, whereas those co-existing with actinolite contain 12--20 nol %calcic amphibole. Therefore, the solubility ofactinolite in Fe-Mg amphibole appears to begreater than that of hornblende at the P-Tconditions of final equilibration in the Wongwi-binda rocks.

The analyzed amphiboles from one-amphiboleassemblages plot in the one-amphibole fieldsaway from the pro.iection of the inferred solvusin Figure 4, suggesting that these single am-phiboles were undersaturated in the other am-phibole component at the P{ conditions offinal equilibration. This explains the absenceof exsolution textures in the single amphiboles.

Cummingtonite F33a and actinolite F4Oa,which occur in two-amphibole assemblages asdiscrete grains devoid of visible exsolutionla,mellae, plot in the two-amphibole field inFigure 4. This suggests that these two discreteamphiboles may be relics of the original pre-exsolution amphiboles.

Cameron (1975) has studied experimentallythe actinolite-cummingtonite solvus for composi-tions with Mg/Fe = 1 at P(HIO) = 2 kbarand l(Or) defined by the FMQ buffer. Hesuggested that the compositions of actinolites

with Mg/Fe - 1 that coexist with cummingto-nites can be used to estimate or put lorverlimits on metamorphic temperatures, providedthe actinolite limb of the solvus is not sig-nificantly affected by smdl variations in pres-sure, Mg/Fe ratio and the content of othercations (e,g., Na, Mn, Al, Fet*).

The cummingtonite content of the analyzedWongwibinda actinolites with intermediate Mg/Fe ratios and intergrown with cummingloniteis roughly 8 mol /6, suggesting, by com-parison with Cameron's data (1975, Fig. 6)'a temperature of exsolution and final equilibra'tion of about 550oC. The higher cummingtonitecontent (22 mol %) of actinolite grains lack-ing visible exsolution lamellae (and hence in-terpreted as relics of the original actinolite)in sample F40 suggests that a temperaturearound 650"C was attained during metamor-phism. Calculation of the original actinolitecomposition from the relative amounts of acti-nolite host and cummingtonite exsolution lamel-lae indicates an original cummingtonite con-tent of about 15-20 mol /o, and hence, a tem-perature of 60G-650'C. This calculation isbased on the observed host-toJamellae ratiobetween 85:15 and 90:10 in grains showinglittle or no evidence of post-exsolution reor-ganization, and on the assumption that lamellaetoo fine for microprobe analysis are similar incomposition to the analyzed lamellae. Similarly'catculation of the original cornposition ofthe cummingtonites containing actinolite exso-lution lamellae (with a host-to-lamellae ratioof 85:15 to 90:10) suggests an original acti-nofite content of about 27-30 mol Vo.

Although the temperatures indicated by theactinolite compositions are not unreasonable,such extensive original solid solution betweencoexisting actinolites ant cummingtonites (withintermediate Mg/Fe ratios) in pyroxene-freeasse.mblages is inconsistent with the phase rela-tions determined by Carneron (1975) (see Fig.5). This discrepancy is presumably due to dif-ferences between the experimental systemstudied by Cameron and the natural systemsprovided by the Wongwibinda amphibolites andquartzites. The most obvious differences ofIikely significance are (i) components suchas Al, Na and Mn, present in the naturalsamples but absent in the experimental system,and perhaps (ii) differences in pres$ure.

Papike et al. (1973) and Immega & Klein(1976) suggested that the solubility of cudn-minglonite in calcic amphibole decreases wilhincreasing Al content in the latter phase. Al-though the present study has not confirmedthis conclusion, the available evidence suggests

lll'*'or, cPx+oPrtQtz

Ast+Gpx+Qtz

Act+ Cum

INTERGROWN AMPHIBOLES FROM THE WONGWIBINDA COMPLEX t9

800

oo

JrcoL=gteooEtg

500

o so looCuMMrNcrorm AcnNoLm

Mole percent octinoliteFro. 5. Experimental phase diagram for the pseudo-

binary join Mgs.5Fe3.5SisOgz(OH)z - CazMgz.sFeg.5-SisOrr(OH), + excess HzO at P(fluid) - 2 kbarand l(Og) defined by the FMQ buffer (afterCameron 1975).

that components such as Al, Na and Mn arenot likely the cause of the observed increasedsolid solution (relative to Cameron's results)between the coexisting amphiboles.

Hence, the relatively narrow pre-exsolutionmiscibility gap inferred from the analyticaldata on the Wongwibinda amphibole pairs maybe due to metamorphism at P(H,O) higherthan that employed by Cameron (i.e., ) 2 kbar).Papike et al. (L973) and Cameron (L975)reached a similar conclusion regarding thel'arge amounts (up to 50 vol. %) of exsolvedcummingtonite found in calcic amphiboles inmetamorphosed iron-formations of Montana.Their conclusion is consistent with the P-Tconditions of metamorphism inferred for theMontana iron-formations from associated peliticassemblages, namely 65O-750"C and 4-6 kbarP(HrO) in the Tobacco Root Mountains area,and slightly lower ternperature or higher pres-sure in the Carter Creek area of the RubyMountains (Immega & Klein 1976).

However, the precise explanation for thisproposed influence of increased P(HIO) onthe miscibility gap between the calcic andFe-Mg amphibole series is not perfectly clear.Cameron (L975) assumed that variations inpressure have no effect on the solvus itself.He suggested that increased P(HrO) should raisethe upper boundary of the two-amphibole fieldin Figure 5 to higher temperatures, thereby ex-posing more of the solvus and allowing in-creased solid solution of cummingtonite in calcicamphibole.

This explanation adequately accounts for theestimated original compositions and paragenesis

of the Wongwibinda actinolites. If the upperboundary of Ca'meron's two-amphibole, pyrox'ene-free field is raised by only 30'C, projectionof the actinolite limb of the solvus to higbertemperatures suggests that actinolite (Mg/Fe -1) with up to 20 mal % cummingtonite insolid solution would be stable in pyroxene-freeassemblages (as in samples F25 and F40) atabout 65'0oC. An increase of 30oC in theupper boundary of the two-amphibole, pyrox-ene-free field requires an increase in P(HzO)of ly2-3 kbar above that employed byCameron (i.e., P(H:O) ) 3/z kbar) assuming'by analogy with the experimental data ol am-phibole stability summarized by Ernst (1968),a p(g,O)-f gradient of. I kbat/ lA-2OoC forthe actinolite breakdown curve above 2 kbatP(H:O). Thus, the estimated original contentof up to 20 mol % cum'mingtonite in theWongwibinda actinolites in pyroxene-free as-semblages is consistent with peak metamorphismat about 650'C and P(H:O) ) 3Vz kbat.

However, the relatively high original actinolitecontents Q710 mol %) of the cummingto-nites coexisting with actinolite are not so easilyexplained by simply raising the upper boundaryof the two-amphibole field within geologicallyreasonable limits of temperature and watervapor pressure, unless the slope of the cum-mingtonite limb of the solvus on Cameron'sT-composilion diagram starts to decrease mar-kedly at temperatures not much above 700"C(Fig. 5). Furthermore, even if the solubilityof actinolite in cummingtonite does increasemore rapidly with increasing temperatureabove 700oC, the present and original actinolitecontents of the Wongwibinda cummingtonitesare inconsistently high compared with thecummingtonite contents of the coexisting acti-nolites, if the amphibole solvus determined byCameron (1975) is applicable to the Wongwi-binda rocks. This suggests that the solvus itselfmay be affected by increased P(HIO), althoughthii proposal is inconsistent with the rninimalinfluence of pressure on the analogous pyroxenesolvus at temperatures up to 900'C (Mori &Green L975). Ifowever, the possibility thatthe solvus may be affected by variations inpressure (or, for that mattero by variations inComposition) casts doubt on the F{ condi-tions of metamorphism inferred above fromthe actinolite compositions; an independent as-sessment of these conditions is necessary.

P{ CotqorrloNs oF MereuonPrnsrvt

Pelitic sc.hists associated with the two-am-

20 THE CANADIAN MINERALOGIST

phibole-bearing rocks of the Wongwibinda com-plex locally contain garnet, cordierite and silli-manite which appear, on textural evidence, tobe related by the divariant reaction: cordierite

INTERGROWN AMPHIBOLES FROM THE WONGWIBINDA COMPLEX 2l

The maximum possible temperature of meta-morphism may be estimated from the absenceof orthopyroxene in cummingtonite-bearingrocks. Orthopyroxene occurs rarely in the quart-zites,-but is restricted to those bearing gruneritewith Mg/ (Mg * Fe) < 0.30, rather thancummingtonite. At 2 kbar P(HIO), FrMgamphiboles with intermediate Mg/Fe ratios be-gin to break down to orthopyroxene * quartzat about 72O"C where P(H:O) = P (total)and l(O,) is defined by the FMQ buffer(Popp et aL L977). The breakdown tempera-tures at higher pressures are not known, buta value exceeding 800oC at 6 kbar P(HzO) isperhaps likely (cune 3, Fig. 6).

The pelitic and psammitic schists associatedwith the two-amphibole-bearing amphibolitesand quartzites locally contain very small graniticsegregations, their first appearance, with in-creasing metamorphic grade, in the Wongwi-binda complex. Although these segregationshave not been analyzed, they seem to approxi-mate minimum melting compositions, andtherefore suggest that the relevant minimummelting temperature may have been reached,but not significantly exceeded, at this locality.If these segregations are in fact the productof incipient melting, their appearance here,slightly on the high-grade side of the sillimanite-orthoclase isograd, permits a more precise es-timate of the P-T conditions of metamorphismin the associated two-amphibole-bearing rocks.Comparison (Fig. 6) of the minimum meltingcurve for metagreywackes similar to theWongwibinda schists (Winkler 1976, p. 316)with the muscovite + quartz breakdown curve(Althaus et al. 197O) suggests a pressure of2-3/z kbar and a temperature of roughly 65G-700oC, where P(H,O) - P(total).

If P(HIO) < P (total), displacement of themelting curve to higher temperatures, and ofthe muscovite + quartz breakdown curve tolower temperatures means that a higher totalpressure may be inferred. Ilowever there is noreason to suppose that 'P(H:O) was signifi-cantly less than P (total); therefore, meta-mophism of the two-amphibole-bearing rocksand related schists probably occurred at about24 kbar and 650-700'C.

CoNcrusroNs

P-T conditions of metamorphism (650'Cand P(H,O) > 31/2 kbar), inferred from acomparison of the original composition of acti-nolites coexisting with cummingtonite withCameron's (1975) experimental data, are

broadly consistent with those indicated by min-eral assemblages and evidence of incipient melt-ing in pelitic and psammitic schists (650-700'C and P(H'O) - 24 kbar). However,the present and original compositions of thecoexisting cummingtonites seem anomalous, sug-gesting that the effects of variables such asP(HzO), Mg/Fe ratio and contents of othercations (e,g., Al, Na, Mn) on the miscibilitygap between calcic and Fe-Mg amphiboles arenot yet well understood. Further work on syn'thetic and natural systems is necessary beforethe compositions of coexisting amphiboles canbe used as reliable indicators of P-T condi-tions of crystallization.

AcKNowLEDGEMENTS

The electron microprobe analyses were carriedout in the Research School of Earth Sciences,Australian National University, with assistancefrom N. G. Ware. J. S. Cook and N. Petraszprepared the thin sections, M. R. Bone draftedthe diagrams and Mrs. R. K. Vivian typedthe manuscript. The paper was substantiallyimproved by constructive criticism from R. A'Binns, A. F. Cooper and J. J. PaPike.

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Received lane 1978; revised manuscript acceptedOcrober 1978.