Canadian MineralogistVol. 19, pp.47-63 (1981)

PETROLOGY OF THE STHATHBOGIE BATHOLITH: A CORDIERITE-BEARING GRANITE

G. NEIL PHILLPS'!. VICTOR J. WALL AND JOHN D. CLEMENSDepartment of Earth Sciences, Monash University, Clayton 3168, Victoria, Australia

ABSTRACT

The Strathbogie batholith is a high-level. discor-dant, composite granitoid intrusion in southeasternAr:stralia. Granitic rocks containing magmatic gar-net, cordierite and biotite predominate in the batho-lith. Intrusion involved marginal faulting. rooflifting and "shouldering aside" of the countryrocks. which include the Violet Town volcanic com-plex of similar Late Devonian age. Textural evid-ence indicates early crystallization of biotite to-gether with garnet, plagioclase (- Anno) and somequartz. The bulk of the quartz, cordierite and K-feldspar crystallized later during emplacement. Earlymagmatic cfystallization occurred with P = 4 kbar,T > 800oC and water contents in the magma ofabout 4 tnl, Vo, Thus the Strathbogie magma wasrelatively hot and water-undersaturated. Emplace-ment occurred at P -0.5 kbar with initial temper-atures well in excess of the wet granite solidus.Oxygen fugacity was ( QFM throughout the mag-ma's history. The Strathbogie rocks have SiO, con-tents in the range 7A-76 wt. Vo with high K/(K -Na * Ca), normative corundum around 2 wt, Voand Me/(Me * Fe) - 0.4-0.5. CaO. MgO, TiO,and P2O5 decrease linearly as SiO: increases. Thesechemical trends cannot he fully explained by re-rnoval or variable incorporation of restite materials,as inclusions of any kind are scarc€ in all variants.Differentiation mechanisms involving separationand accumulation of early magmatic biotite. plagio-clase and accessory phases are favored over batchpartial-melting processes to model the chemicalvariation. Magma chemistry, together with inferredearly magmatic conditions, and the mineralogy ofinclusions in the granites suggest initial formationof the Strathbogie magma bv partial fusion ofbiotite-bearing quar?ofeldspathic and pelitic rocksunder granulite-facies conditions. These sourcerocks are apparently below the presentlv exposedlower Paleozoic sequences of southeastern Australia.The transitional orogenic-anorogenic setting of thebatholith is compatible with the partial-fusion eventhaving resulted from intrusion of mafic magmainto the deep crust.

Keywords: peraluminous granite, granite petro-genesis, granite geochemistry, granite emplace-ment, cordierite, garnet, Australia, Strathbogie.

*Now at the Department of Ceology, Universityof Western Australia, Nedlands 6009, W.A., Aus-tralia.

Sovrvernn

Le batholithe de Strathbogie (Sud-Est de l'Aus-tralie), dont le toit se trouve ir faible profondeur,est discordanl et composite. l.es roches granitiquesqui contiennent grenat, cordi6rite et biotite magma-tiques y sont dominantes. L'intrusion Jest produitepar glissement le long de failles marginales, sould-vement du toit et repoussement.lat6ral des rochesencaissantes. dont le complexe volcanique VioletTown. aussi d'Aee D6vonien tardif. D'aprEs lestextures, la biotite a cristallis€ t6t, avec grenat.pla-gioclase (- Anas) et un peu de guartz. Quartz,cordi6rite et feldspath potassique ont. en maieurepartie, cristallis€ plus tard, lors de la mise en place.Le stade initial de la cristallisation a eu lieu i unepression d'au moins 4 kbar, i une tempErature de800"C ou plus et i partir d'un magma contenant en-viron 4Vo d'eau (€n poids). Le magma 6tait doncrelativement chaud et sous-satur6 en eau. Pendantla mise en place, d une pression d'environ 0'5 kbar.la temp6rature initiale surpassait le solidus du sys-tdme granitique satur6 en eau. La fugacit6 d'oxy-gane est rest6e inferieure au tampon O.FM pendanttoute l'6volution du magma. Les granites contiennentde 70 i 767o de SiOr (en poids), et possBdent unrapport K/(K + Na * Ca) 6lev6. environ 2Vode corindon normatif et un rapport Me/(Mg +Fe) situ6 entre 0.4 et 0.5. Les concentrations deCaO, MgO, TiO: et PzOr d6croissent lin6airementquand SiO2 augmente. Ce comportement ne peuts'expliquer entierement par 6limination ou incorpo-ration variable de mat6riaux rdsiduels. vu la raret6des x6nolithes dans tous les facies. On pr6fdre lesm6canismes de diff6renciation qui impliquent las6paration et I'accumulation, dans le bain fondu, debiotite, plagioclase et phases accessoires. touteshitives, aux processus discontinus de fusion par-tielle pour expliquer la variation de composition.D'aprbs la chimisme du macma, les conditions mag-matiques initiales et la min6ralogie des x6nolithesdans les granites, une fusion partielle de rochesquartzofeldspathiques i biotite et de roches p6liti-ques, 6quilibr6es dans le facies granulite, serait iI'origine du magma. Ces roches forment le soclesous-jacent des s6ries du Pal6ozoique inf6rieur quiaffleurent i pr6sent dans le Sud-est de l'Australie.Le milieu de transition orog6nique-anorog6nique dubatholithe concorde avec la fusion partielle qui futle r6sultat de I'intrusion d'un magma mafique dansla cro0te profonde.

47

(Traduit par la R6daction)

48 THE CANADIAN MINERALOCIST

Mots-cles: granite peralumineux, p6trogendse gra-nitiQue, gEochimie du granite, niise en place desgranites, cordi6rite, grenat, Australie. Strathbogie.

INtnopucrroN

Peraluminous granitoid and volcanic rocksare widespread in the Tasman fold belt ofeastern Australia. These dominantly post-tectonicbatholiths and volcanic complexes contain oneor more of the peraluminous phases cordierite,garnet, biotite, andalusite, sillimanite and mus-

covite. On the basis of studies of granites insoutheastern New South Wales, Chappell &White ( 1974) have proposed the term "S-type"for these chemically and mineralogically dis-tinctive rocks, inferring their origin as a resultof partial melting of metasedimentary material.

Recent field and petrological studies (Birch &Gleadow 1974, Flood & Shaw 1975) have sug-gested that the peraluminous phases (cordieriteand garnet) are largely of residual metamorphiccharacter (i.e., restite) derived from the sourceregion of the partial melts. Here we describe a

Ftc. l. Regional geological setting of the Strathbogie batholith and otherfelsic igneous complexes of central Victoria.

PETROLOGY OF THE STRATHBOGIE BATHOLITH 49

contrasting situation in yhich textural featuresin a high-level pluton, subject to little solid-statereadjustment, indicate a magmatic origin forthe bulk of the cordierite, garnet and biotite.The implication is that moderately peraluminousmelts may be generated in the lower crust andevolve by processes largely independent of thepresence of restite phases.

FIELD RELATIoNS

General setting

The Strathbogie batholith is a large, discord-ant, composite body more than l5O0 km, in areasituated 150 km north-northeast of Melbourne(Fig. 1). The batholith, of Late Devonianage (365 -+ 5 Ma, K/Ar, J.R. Richards, writ-ten comm. 1975) intruded folded Siluro-De-vonian sedimentary rocks of the MelbourneTrough (Phillips & Wall 1980) and on its north-ern margin, the Violet Town volcanic complex(Birch er al. 1977) of Late Devonian age. TheBarjarg granite of White (1954) is here con-

sidered as part of the batholith but separatedfrom the rest of the Strathbogie mass by a nar-row zone without outcrop. The Strathbogiebatholith, which is markedly elongate in anE-W direction, cuts across major structures inthe Paleozoic sedimentary rocks.

A broad contact-metamorphic aureole sur-rounds the batholith and is particularly welldeveloped in the Paleozoic sedimentary rocks.At Bonnie Doon, spotted slate, biotite-musco-vite, cordierite-muscovite and cordierite-K-feld-spar zones have been delineated in the 3-km-wide aureole (Phillips & Wall 1980). Contact-metamorphic effects are present in the VioletTown volcanic complex up to several hundredmetres from the granite contact (Birch et al.1977, Clemens & Wall 1979).

Emplacement

The undeformed Violet Town volcanic com-plex postdates major folding, low-grade regionalmetamorphism and erosion of the Siluro-Devo-nian sedimentary rocks (Birch er al. 1977); allthese processes are presumably associated with

SIRATHBOGIE GRANITES +IEl

[T*l c*r."-g- ined sronite

Nl rorptyr;ric micrcgronite o locolion o{ onoly*d (XRf) sanples

€

tlKerrisdsle Cordieriie Mioqdonellite

Tolhngollook llomblende

Migqdarellite (l-tycc.)

I Bonnie Doon

20 KII,OMETBES

- / + + + + + ++ + + + a + + +. r * *\ '{+ + + + + + + + + +6 !*****\-#** * ***?***n*********"

F + + + | + + + + + + . + + + + + /' + + + + - + + + + + + a + + + + It i + + + | + + + + + +!+ + + + +' + + + + - + + + + + + a + + + +

o * + + + + + + + + + + + + + + tu f + + + + + + + + + + + + a + + / f

I f * * * * * * * * * * * * * * * * * * a l - t v p e C * * * / \I l t * * * * * * * * * * * * * * * * * *1 l - l ype ( t+a+

I t.l.:.:.l.l.l.l.l.l.lri.li.)+ + +

+ + +

$llil;> t = f ' T T a - T T ? ?+ + + + + + L a + + + +

+ + J\_+ + + la+ + + ++ + a - - + + + + + {+ t 7 - - - - ? ? ? ? ?

'/ R***a

Flc._2. D-istribution of granite types within the Strathbogie batholith. Cordierite and garnet are present inall variants except the I-type Tallairgallook hornblende microadamellite.

50

the Tabberabberan orogeny (Middle-Late De-vonian: Talent 1965). Similar volcanic rocksin the Cerberean cauldron in Victoria have beendated as post-Tabberabberan (McDougall et al,1966). Thus, at least the northern margin ofthe Strathbogie batholith, which intruded thesubaerial volcanic pile (present thickness 3O0m: Birch et al. 1,977), was probably emplacedat a very high level, i.e., pressures ( 1.0 kbar.The character of the contact-metamorphic as-semblages at Bonnie Doon also suggests veryIow pressures (O.5-1.0 kbar) and indicates peakmetamorphic temperatures of around 600oC(Phillips & Wall 1980). Ifence, taking into ac-count the temperature drop (approximately150oC) across the intrusive contact (Jaeger1957), the presently exposed granite was em-placed at shallow depth (ca. 3 km) and relative-ly high temperatures.

On a regional scale, the batholith is boundedby several nearly straight E-W and N-S con-tacts (Fig. 2). Major structures in the Paleozoicsedimentary rocks on the southern side of thegranite are truncated by the contact. However,the strike of the sedimentary rocks appears tobe deflected near the western end of the batholith(Vandenberg & Garratt 1976), with the diver-gence becoming apparent within 20 km of thegranite contact (Fig. 1). On a mesoscopic scale,abrupt right-angled steps occur along the con-tact. Between these steps the contact is straightfor distances of l0 to 100 m. The contacts aregenerally near-vertical, with marginal zones offiner-grained granite up to 10 m wide. Rareaplitic apophyses penetrate the country rock.

The development of high-grade contact meta-morphic rocks adjacent to the granite appearsto rule out major postemplacement faulting as

THE CANADIAN MINERALOCIST

an explanation of the geometry of the contact.Penetrative deformational fabrics are completelyabsent from the granite and hornfels. Faultingduring emplacement (brittle country-rock defor-mation) may, however, account for the steppednature of the contact. Marginal faulting is wellshown in the northern region where adjacentvolcanic rocks are block-faulted and tilted awayfrom the granite. In view of the paucity ofcounry-rock xenoliths, stoping is unlikely to bethe major mechanism of granite emplacement.The composite character of the batholith is alsoevidence against stoping (White et al. 1974).Fromthe above evidence we infer that emplacementwas dilational in character, with some "shoulder-ing aside" of the country rocks, marginal faultingand lifting of the roof of the magma chamber.This scheme differs from proposals of wrench-fault-conrolled emplacement of other discord-ant batholiths in southeastern Ausfralia.

In summary, the Strathbogie batholith is adiscordant, high-level, post-tectonic batholith ofcomposite character. The northern and easternmarginal portions (at least) represent near-roofzones, as they intruded the Violet Town volcaniccomplex. Clemens et al. (1979) suggested thatthe Violet Town volcanic complex and theStrathbogie batholith are comagmatic in viewof their close temporal and spatial associationand similar geochemistry.

Prtlocnepuv

Granitoid rocks

We have subdivided the Strathbogie batho-lith into four mappable variants on the basis

PETROGRAPHY

Rock rype "? Eiffiifi, Fabrlc Pherccrysts Gmundnass Accessorles

Porphyrl ticmicrcgmnlte

CoaEe-gralned

KerrlsdaleCordlerl teMlcmadmlI ite

Tal langall@kHornbleldeli l lcmad@lllte

porphjrit lc usually withL0 - 259 euhedral pheno-crys6

mstly equlgranular4 - 1 0 m

"Porphyritlc' - contlnuusrange ln grainslze .5 - 5m wlth flner sizeddoni mnt

porphyrltlc vlth - &euhedral phamcrysts

34.0

65.0

q u a r t z , 5 - 1 0 mplag ioc lase (Anqq- r r ) . 5 - 10 mo r t h o c l a s e , 4 - l 0 mcordlerlte, <6 mblotlte, <5 mgarnet (Type A)

sm orthoclase, 30 - 50 m

quarrzorthoclasep lag loc lase (An5a-2s)blotlte (mlnor)

quartz, 6 mp lag loc lase ,4 m

0 . 5 - 1 . 5 m l l r e n l t equartz,orthoclase pyrrhotiteplagloclase apatlt€ninor blotlte zlrmncordierite and gamet mmzlte(Type B) xenotlm

quartzplagioclase (An5q1 e)orthoclase dlttobiotltecordlerltegarret (Type A) nJnor

quartz, plagloclaseorthoclase, blotlte dlttocordlerl te

- 0.8 m sphenequartz,orthoclase mgnetltep laq loc lase(An, . ) l l ren l teirornblende, Sloilte apatlt!

zlrcon

< 0 . L

Average plagioclase composltions are quoted core to rln.

PETROLOGY OF THE STRATTIBOCIE BATHOLITH 5 l

of textural and mineralogical characteristics(Table 1). Although outcrop is good, contactsbetween the major variants have not been ob-served, and hence their temporal relations arenot clear. The distribution of the four variantsis shown in Figure 2, The map shows two dreasof porphyritic microgranite, one in the centreof the batholith and another smaller body inthe southwest. flhe terms "granite" and "ada-mellite" used in this papr are taken from thenormative classification of O'Connor (1965).1The latter microgranite body appears to haveintruded coarse grained granites. Such granitesoccnpy two areas in the east and the south..Thenorth-sotrth boundary between these two mainrock types is marked by a prominent scarp anda zone of no outcrop that is probably the ex-pression of a fault. Areas delineated as coarsegrained granite contain small (- 1 km'z) scat-tered bodies of porphyritic microgranite, parti-cularly in the eastern half of the batholith.

A small, separate phlton, the Kerrisdale cor-dierite microadamellite, granite in IUGSterms

'(Streckeisen 1973)" lies at the extreme

southwestern end of the batholith. The Tallan-gallook hornblende microadamellite, the onlymetahlminous variant observed in the Strath-bogie batholith, forms a small (< I km wide)stock intruded into coarse grained cordieritegranites in the east (Fig. 2).

Modal and chemical variation throughout thebatholith is limited. All outcrops (except theTallangallook hornblende microadamellite) ex-hibit cordierite and peraluminous biotite, al;though garnet is less commonly developed. TablesI and 2 and Figure 3 show the main modaland textural variations observed. All rocks aregranites according to the IUGS classification.In all the granites some of the quartz formsIarge euhedra, commonly with hexagonal-bipy-ramidal (6-quartz) habit. Inclusions of biotiteand plagioclase are found in this type of largephenocrystal quartz. Plagioclase has a tabnlarhabit and shows complex twinning' and slightnormal zoning with superimposed fine-scale,low-amplitude oscillatory zoning. Plagioclasecompositions are given in Table 1. Aggregatesof plagioclase suggest accumulation of this phaseduring magmatic evolution. Quartz and biotiteare the only inclusion phases found in the plagio-clase; Orthoclase microperthite forms large, sim-ply twinned subhedra that contain inclusionsof cordierite, biotite, quartz and plagioclase.Red-brown biotite forms large, discrete euhedralbooklets in all granite variants.

Cordierite and garnet types in the granitesare listed in Tables 3 and 4 (also see Figs. 3d,

4). Nearly all the cordierite forms euhedralprisms (type A), rarely twinned and inclusion-poor, although some encloses small, euhedralbiotite flakes. In the coarse grained granites,cordierite is similar in size (4-10 mm; Fig. 3d)to plagioclase, biotite and quartz. In the finergrained Kerrisdale cordierite microadamellite,cordierite, like all other phases, has a grain sizeof 1-5 mm (Fig. 3b). Cordierite in the apliticrocks also has a grain size similar to that ofother phases present (0.5-1 mm).

Many of the large, corroded (type-A) gar-nets have been partly pseudomorphed by cordi-erite that preserves the shape of the originaleuhedral, inclusion-free garnet. Textural andchen'rical features of the type-B garnets are sum-marized in Tables 3 and 5.

The more leucocratic porphyritic microgran-ites, especially near the edges of the batholith,contain patches of tourmaline intergrown withthe groundmass phases. This feature is mostcommon in variants with a relatively coarsegrained g4oundmass. A late-stage magmaticorigin is favored for much of the tourmaline,as it rarely exhibits replacement textures. Insome porphyritic microgranites, the groundmasscontains small patches of microgranophyre, i.e.,eutectoid-like intergrowths of quartz and ortho-clase.

S-type granites hitherto described (Flood &Shaw 1975, White & Chappell 1977, Chappell1978, Hine et al. 1978) display textural featuresinterpreted by these authors as indicating thatmuch of the cordierite, garnet" biotite and, per-haps, plagioclase in these rocks is residual (i.e.,restite) material. The corroded. highly calciccores of plagioclase crystals in some granites areconsidered by White & Chappell (1977) to berestite grains mantled by magmatic feldspar.

TABLE 2. MODAL ANALYSES

l4l neral

orthoclase 25 29 25 2A 19 22

plagloclase 28- 19 16 24 32 34

c o r d i e r l t e l 6 5 3 4 '

b i o t i t e 9 5 L 7 9 8 3

garnet Tr I 4 Tr Tr

hofnbiende - 4

sphene - 1

1. typical porphyrl t lc nicrogranjt€ (864); 2. cordier i te-r ichp o r p h y r l t ' j c m J c r o g r a n i t e ( 7 5 6 ) ; 3 . g a r n e t - r J c h p o r p h y r l t i c n i c r o -qranite (889)i 4. coarse gralned gran' i te (average of 801 and 803)i5 . K e r r { s d a l e c o r d i e r i t e n i c r o d d a r e ' l l i t e ( 7 6 5 ) ; 6 . T a l l a n g a l l o o khornblende microadaml' l i te (861). Al l rcdes are based on >1000polnts. Both phenocryst and groundmss phases were counted.

52 THE CANADIAN MINERALOGIST

1,C?i.rnd

ffi#

PETROLOGY OF THE STRATHBOGIE BATHOLITH

Conposltlon (ilol. g) Relative Abundance

53

TABLE 3. GARTFT fiPES

granltic rccks rcunded, corrcded crystols (< 15 m)wlth cordier l te-blot i te react loncorcnasi i l renl te, apat l te, z lrconI nc lus 1ons

I - 2 m anhedro lntergmn r l th gmund-mss fels lc phases ln porphyrl t lc 'mrcroqranl res

ver:y smll embayed crystals lncludedln quartz and feldsDar

the mst comn type early mgmtlcscattered distributlon

comn laie mgmtlcor subsolldus

rare restlte,readJustedrestlte orxeml ithlc

1 6 7 5 5 4

5 8 0 1 3 2

lncluslons ofhigh-gradereglonal mta-mrphlc rck

Iype A an-d B garnets ln t ie Strdthbogle granltes are analogous to Blrch and cleadox's (1974)lyp€ z afr r gamets (respect iw' ly) fm upper lhvonlan acid volcanlc rccks ln the cer iereair cauldrcn, central v ictor ia.

Plagioclase in the Strathbogie granite does notcontain obviously distinctive cores. Indeed, theprogressive oscillalory zoning exhibited by theplagioclase (e.g., Fig. 3b) appean to be incom-patible with a metamorphic origin for this feld-spar. Features expected in restite micas (silli-manite inclusions and coarse, foliated aggre-gates) are entirely lacking in the Strathbogiebiotites. Cordierite from metamorphic terranesis commonly anhedral, ovoid in shape and in-clusion-rich. Such metamorphic cordierites area component of the rare gneissic inclusions inthe Strathbogie batholith. The euhedral, inclu-sion-free character of the type-A cordierite inthe Strathbogie rocks and the correlation be-tween grain size of the cordierite and the en-closing rock are compelling evidence for em-placement-stage magmatic crystallization of thecordierite. The initial euhedral shape and in-clusion-free character of the type-A garnetssuggest a magmatic origin for this phase also.The grain shape, composition and lack of reac-tion of type-B garnets with the magma arecompatible with a late-stage magmatic or sub-solidus origin. Thus, the textural evidence in-dicates that the great bulk of the garnet, cor-dierite, biotite and plagioclase has an entirelymagmatic origin in the Strathbogie granites.

Liquidus phase-relations of a particular mag-ma composition depend on variables such as P,T, l(O.) and the H:O content of the magma;hence, the inferred sequence of crystallizationof a rock can yield information on these para-meters. In deducing the sequence of crystalliza-tion in the Strathbogie granite (Fig. 5), in-clusion relations, reaction relations and grainshape are critical. We have assumed that smalleuhedral inclusions in the core of a large euhe-dral phenocryst are likely to have nucleated nolater than the enclosing phenocryst (Vernon1977). Inclusion relationships in the Strath-bogie rocks are shown in Table 6. It has furtherbeen assnmed that crystals growing suspendedin a magma are likely to grow as euhedranwhereas during the later stages of magmaticcrystallization crystals will mutually interfere,forming anhedral grains and interstitial fabrics.

In the Strathbogie rocks, quartz, plagioclaseand biotite are early-crystallizing phases thatform large euhedral crystals with few inclusions.Apatite, rarecarth phosphates, zircono ilmeniteand pyrrhotite found included in the biotitesuggest that these accessory phases also crystal-lized early. Cordierite began crystallization be-fore K-feldspar, which is commonly interstitialand anhedral. Quartz and alkali feldspar domi-

TAELE 4. CORDIERITE TYPES

0currence Cord l eriteType Fom and Paragenesls rZV^ l4gl(tlg+Fe) Relatlve Abundance 0r lg ln

gran'ltic rccks euhedra (< 6 m) wlth rare lncluslonsof blotlte, l lmnlte and zircon

lafge anhedra wlth blotlte and/orsil l lmnlte and green splnel incluslons

smll anhedra or, rore rarely, euhedralln aggregates conprlsing reactlon 116on Type A garnets

smll scattered anhedra derived bydlssaggregatlon of Type Cl

larle and smll anhedra wlth jnclusionsof blotlte and,/or sll l lnanlte andzincian hercynlt4

+ 60.

- 80"

- 80'

- 80.

0 .56

0.46

alrays present primry mgmtlc

rare readJustedrestlte

acconpanles Type A)garners ]

) mgmtlc)

of&n present )

ln all enclaves of readjusted restltehlgh-grade regloml or xerclithlcmtmrphic rcck

b

ct

0.34

'lncluslons ofhlgh-graderegioml mtn-mrphlc rcck

54 THE CANADIAN MINERALOGIST

TABLE 5. I{INERAL CHEfiTSTRYf: RESIILTS OF ELECTR0II-||ICR0PR0BE AiIALYSES

splnel ct(A) ct(A) nF{*) .16l Bt Br et or 1618}1rr rrn"?Lron,f(A) cd(A) cd(A) cd(A) cd(B) "l:l*!;.*

Rock lto. 872 846 1589. 846 u6 7r.4 1589 754 831 1597.- 846- 872 754 831 1597 846 1599 ,846 872

S.f0: - 0.21 37.08 37.46 36.08 35.88 35.43 11.79 34.m 35.26 n,d, 35.43 49.n 49.2e 49.13 49.20 48.44 49.29 48.5T lo2 - 0 .06 - 0 .1 , 4 .75 4 .47 4 ,41 4 .41 0 .71 4 .4941203 57.32 56.64 21.48 21.03 20.68 20.69 17.86 17.04 18.14 17.95 n.d. X6.58 33.42 33.49 32.59 32.49 32.69 9..27 32.9Feor 32,44 30.47 33.56 33.64 35.19 35.76 23.48 25.65 24.14 23,t7 22.34 24.45 10.24 9.46 9.97 9.90 11.85 13.09 U.anno 0.09 0.42 2.80 2.00 5.33 3.55 0.09 0.04 0.11 0.21 0.20 - 0.r3 0.18 O.47 0.37 0.50 0.75 0.c)Zno 8 .03 9 .86 n .d . n .d . n .d . n .d . n .d . n .d . n .d . n .d . n .d . n ,d . n .d . n .d . n .d . n .d . n .d . n .d . n .d .

nso z.LZ 1.37 3.82 4.41 1.69 1.37 6.33 5.02 5.76 6.51 6.72 7.sl 7.01 7.61 6.a} 6.58 5.90 4.61 5.11

Cao 1,27 1.35 1.02 0.62 0.14 0.04

Bao n .d . 0 .62 n .d . - n .d , n .d , n ,d . n .d . n .d . n .d . n .d .t{az0 n.d. O.42 - 0.05 0,42 - 0.09 - n.d.KuO 8.94 8.72 8.96 9.08 n.d. 9.66

n.d . n .d . n .d . n .d . n .d . n .d . n .d . n .d .

8 8

0.006 2.970 2.992 2.945 2.939 5.381 5.349 5.ai2 5.375 5.404 5.413 5.029 5.016 5.020 5.r08 4.987 5.125 5.017

Al(tv) - - 0.030 0.008 0.055 0.051 2.619 2.651 2.668 2.625 2.596 2.587 0.013

At(vl) r.952 1.961 1.997 t.972 L.934 1.936 0.578 0.530 0.635 0.601 0.915 0.400 4.020 4.015 4.U7 3.977 4.030 3.955 3.961

Fe 0,784 0,74A 2.245 2.248 2.402 2.449 2.9s2 3.396 3.119 2,954 3.n4 3.124 .874 .805 .852 .860 r.020 1.380 1.098

Tl - 0.004 - 0.007 0.544 0.532 0.512 0.523 - 0.516iln 0.002 0.011 0.190 0.135 0.368 0.385 0.012 0.005 0.014 0.027 0.040 - 0.01r 0.016 0.041 0.033 0.044 0.066 0.044

Zn 0.171 0,2L4ug 0.091 0.060 0.455 0.525 0.206 0.167 1.433 1.184 x.326 1.502 1.718 I 596 1.066 1.X54 1.040 1.018 0.9M 0.714 0.880

Ca - - 0.109 0.116 0.090 0.054Bal{aK 1.732 L.76L L.766 t.766 1.890 1.883

l,lql(l i lotFe) 0.10 O.o7 0.17 0.19 0.08 0.06 0,33 0.26 0.30 0.34 0.35 0.34 0.55 0.59 0.55 0.54 0.47 0.34 0'45.total Fe expressed as Feo S all analyses mmallzed to l00u ercept those of biotlte

ffi,ffis , 'bW=='d

0.027

ll rnrn

i"$!::l.tf:ild:iF

!#6ffii i , ;, i

:: !iE$ l:l !:t irsi !:airairi'\r

nii ; , :

, . : L :

.Tt*,gs

EARLY

tATEFtc. 5. Schematic sequence of crystallization for

the Strathbogie granites.

nated the final stages of magmatic crystallization.The role of garnet varies. The large, corroded(type-A) garnets are apparently early and haveundergone considerable textural and some chem-ical modification. The small, groundmass (type-B) garnets are a late-crystallizing phase in theporphyritic microgranites. The deduced sequenceof crystallization is portrayed in Figure 5.

Although sequences of crystallization forgranitic magmas based on the above criteriaare commonly equivocal, the grain shapes andgrain-boundary relations of the Strathbogie rocksindicate little subsolidus readjustment. Further-more, the crystallization sequence deduced forthe Strathbogie magma closely matches that in-ferred for the related Violet Town volcanic suite(Clemens et al. 1979). Strathbogie specimen 889(Tables 2 and 7), the most mafic variant of the

TABLE 6. INCLUSION RELATIONS IN STRAIHBOGIE !,IINERALS

Accessories Ct(A)INCLUSION PHASE

B i c d P l Q HOST PHENOCRYSI

PETROLOGY OF THE STRATHBOGIE BATHOLITH

ct(A)

BI

cd(A)

PI

q

Kfs

batholith, demonstrates our scheme of mineral-ogical evolution. This rock has a high phenocrystcontent (5O7o) set in a fine grained ground-mass. Relatively high concentrations of type-Agarnets and biotite crystals are present, and pla-gioclase phenocrysts are rather calcic (Anru-ro).Cordierite occurs mostly in reaction rims ongarnets, although some euhedral phenocrystalcordierite is also developed. K-feldspar pheno-crysts are rare and the groundmass comprisesfine grained alkali feldspar and quartz. Thegroundmass appears to represent chilled (pres-sure-quenched?) liquid. Thus K-feldspar did notreach saturation in the 889 magma until theemplacement stage, when this composition wasstill more than SOVo liquid. The high concen-tration of garnet and biotite in 889 suggeststhat these phases represent accumulations andemphasizes their importance in the differentia-tion of the Strathbogie magma(s).

Inclttsiotts in the granitic rocks

Although generally scarce (- l%o) in theStrathbogie batholith, three types of inclusionshave been recognized. Micro-adamellite inclu-sions are the most numerous. with those ofhigh-grade regional metamorphic rocks next inabundance. Near the margins of the batholith,the granitic rocks contain rare angular xenolithsof quartz-rich and pelitic hornfels. These ac-cidental hornfels inclusions are derived from thesurrounding country rock.

The micro-adamellite inclusions are rounded.l0-lo0 mm in diameter, composed of large (5mm) interlocking anhedra of quartz and ortho-clase, sieved with inclusions of plagioclase laths(Anrr) and small euhedral biotite flakes. Ac-cessory zircon, ilmenite, apatite and rare sor-dierite euhedra are also present. The micro-adamellite inclusions are interpreted as accumu-Iations of small, early-formed biotite, plagioclaseand cordierite crystals in interstitial liquid thatcrystallized as quartz and orthoclase. These rocksprobably represent a chilled phase of the graniteformed during the early stages of crystallizationand later dislodged from the wall or roof of themagma chamber and reincorporated in the mainbulk of the magma (c1., Bateman & Chappell1979, pp. 474-475). The micro-adamellite inclu-sions resemble the Kerrisdale cordierite micro-adamellite in both texture and composition.

The inclusions of high-grade regional meta-morphic rocks (10-100 mm in diameter) arerounded in shape, fine- to medium-grained, andalways have a pronounced foliation (Fig. b).Biotite and cordierite are segregated into melano-

55

x

56 THE CANADIAN MINERALOGIST

si02 69.78 72.2r

T102 0.57 0.34

41203 14.56 L4.@

F e o * 3 . 6 6 , 2 . 4 2

ltno 0.01

ifgo 1.40 1.02

Cao 1.85 r.67

MzO 2.60 2.86

K20 4.74 4.50

P20S 0.19 0.22

sog 0 .10 0 .08

Loss * O.7 l 0 .47

TABLE 7. BULK-RoCK CHEMICAL 0ATA (XRF) At{D CIPH iloRtits Inclusions of country-rock hornfelses are rec-ognizable by their modal and mineralogical sim-ilarity to the Paleozoic rocks in the contactaureole (Phillips & Wall 1980). In contrast tothe regional metamorphic inclusions, the horn-fels inclusions lack well-developed foliations.The paucity of hornfels xenoliths and their re-striction to marginal variants in the Strathbogiebatholith imply that high-level cohtamination isof little importance in Strathbogie petrogenesisand that stoping of wall rocks is not a majorfactor in the emplacement of the Strathbogiemagmas.

74.63 71.83

0.09 0 .36

13.66 14.25

1.05 2 .73

0.59 1 .04

L.29 2 ,14

2.94 3 .08

4.91 3 .93

0.22 0 .28

0 . 1 3 0 . 1 0

0.61 0 .50

72.78 7 ! .&

0.33 0 .30

14.0t 14.22

2,32 t .8 t

o .o1 0 .04

0.95 0 .71

! ,57 1 .85

2.65 4 ,25

4 , 5 2 4 , 6 9

0 . 1 9 0 . 0 9

0.06 0 .06

0.54 0 .33

Totn l 100.17 100.45 100.12 100.25

M9/(MgiFe) .40

ac0r

Ab

An

DI

Hy

I1

Ap

28.47 32.1,2

2 .24 2 .57

28.t9 26.60

22.17 24.20

7.99 6 .85

9.32 6 .42

1.08 0 .65

0.45 0 .52

l a t n ? 1 A 7

1, .7L r ,70

29.20 23.29

24.96 26 ,L4

4.96 8 .84

3.25 7 .03

0, r7 0 .68

0 . 5 2 0 . 6 6

* Total Fe expressed as Feo. * Fusion loss. l. Speciren 889, themst mflc variant of the Strathbogie granite; 2. specjren 717,representative coarse gralned granite, Strathbogle bathollth; 3.speciren 754, representatl ve porphyrit ic nicrogranlte, strathbogiebathollth; 4. speciren 765, Ksrisdale cordierlte nlcrcadarell ite;5. average of 2l S-type Strathbogie granltoid rocks; 6. average of2 analyses of Tallangallook hornblende adarell lte.

cratic layers separated by layers of felsic phases.Significant assemblages developed include(a) Or-Gt-Bi*Cd-Pl-Ilm-Ap-Po-Q and (b)Hc-Cd -f Sill. [Abbreviations are listed in Ap-pendix l.l

Descriptions of the garnet and cordierite typesfrom these inclusions are given in Tables 3 and4. Garnet is very rare, although at one time, itwas probably present in all the regional meta-morphic inclusions, as evidenced by pseudo-morphs of garnet by minerals of assemblage(b) above. Readjustment of the regional meta-morphic mineral assemblage by equilibrationwith the enclosing granitic magma has occurred,as indicated by the low-pressure assemblages.The alkali-poor, silica-depleted character ofthese inclusions is compatible with a restite or-igin, although their low-pressure mineralogyprecludes estimation of source-region conditions.

GEocHEMrsrRY

99.93 ee.es Mineral chemistry

34.12 23.46

26.72 27.72

22,42 35 .96

6.55 5 .88

- 2 .28

6 . 1 0 3 . 5 2

0.63 0 .57

0.45 0 .21

Ferromagnesian phases and plagioclase in theporphyritic microgranites and coarse grainedgranites were analyzed using the T.P.D. energy-dispersive electron microprobe at the ResearchSchool of Earth Sciences. Australian NationalUniversity. Some analyses were performed onthe Jeol JXA-SA and the ARL EMX micro-probes at the Departments of Geology, Mel-bourne and Monash universities, respectively.The compositions of the various garnet and cor-dierite types are of particular interest.

All Strathbogie garnets are almandine-richpyralspites (almandine: 74-80 mol. %). TheMg and Mn contents show the greatest varia-tion (Table 3). Garnet in the fine grainedgroundmass, free of reaction coronas (i.e., typeB), is relatively spessartine-enriched, with onlyminor . pyrope and grossular (e,g., specimen754). Large garnets with reaction rims (typeA) are relatively enriched in pyrope and gros-strlar, with only 4.57o spessartine. Rims of type-A garnets are Fe-Mn-enriched and Mg-depleted.The reason for the Mn enrichment is the strongpartitioning of this element into the garnet re-lative to the cordierite in the reaction rim.

Euhedral (type-A) cordierites generally haveMgl(Mg + Fe) in the range 0.54-0.59.(Throughout this paper Fe and FeO refer tototal iron in the sample. Structural formulae ofthe ferromagnesian phases indicate that Fe3+ isminor in most minerals.) The chemistry andoptical properties of type-A and type-B cor-dierites are quite different (Table 4). Type-Bcordierites and cordierites in gneissic inclusionsare more Fe-rich [average Mg/(Mg + Fe) -

0.461 and contain more Mn than type-A cor-dierites. Type-C cordierites are similar in opticalproperties to type B but have the lowest Mg,/

.41.42.50

PETROLOGY OF THE STRATHBOGIE BATHOLITH 57

(Mg * Fe) (average - O.34) of the cordieritetypes. The significance of these points will bediscussed in detail in the section on petrogenesis.

Biotites in the Strathbogie granites do not varygreatly in composition; they are all rather highin siderophyllite-annite, with an average com-position (K,Na) r.e(Fes.rMgr.rTio.sAle.6) (Alr.oSis.r)Ouo(OH,Cl,O,F). Semiquantitative crystal-spec-trometer analyses showed that fluor- and chlor-ine-biotite substitutions are minor ( < 5 mol.'To). Mg/(Me * Fe) is near 0.30, fallingbetween the values for type-A cordierite andgarnet and slightly lower than the bulk-rockvalues (Table 7). The biotite in the cordierite-biotite reaction rims surrounding type-A garnetis similar in composition to other biotites ex-cept for its very low Ti content. Normalizationof the Ti-rich (primary, magmatic) biotites to16 cations, assuming no octahedral vacancies(Bohlen et al. 198O), suggests extensive TiOz- (MgFe)(OH)z substitution, a feature in com-mon with other high-temperature biotites(Bohlen et al. l98O).

Green spinel (Tables 4, 5) contains consider-able amounts of the hercynite and gahnite com-ponents in solid solution: hercynite 72/o, gah-nite 2lVo. Some Mg and Mn are also present,but the magnetite component comprises ( 2mol. Vo. Similar zincian hercynites have beenreported in inclusions in acid volcanic rocks ofthe Cerberean cauldron and Violet Town vol-canic suite in Central Victoria (Birch et al.1977' t .

Rock geochemistry

Selected bulk-rock analyses (XRF) and CIPWnorms of the Strathbogie rocks are presented in

Table 7. Most of the batholith comprises corun-dum-normative (usually ) l%o C) S-type gran-ites. whereas less than O.lVo of. the batholithconsists of the diopside-normative Tallangallookhornblende microadamellite (I-type).

Compositions of all S-type granitoid rocksare highly silicic (70-76% SiOr) and moder-ately periluminous. All have rather high K/(K+ Na + Ca) ratios, reflected in the high pro-portion of modal alkali feldspar (Table 2).The ratio Mg/(Mg * Fe) is generally in therange 0.4G-0.50 and shows no distinct trendwith SiO: content. Two exceptionally Fe-richrocks [Mg/(MS * Fe) = 0.31 and 0.191 havesignificantly higher MnO and lower MgO andCaO.

Harker plots show strong correlations betweenseveral oxides and SiOs content (Fig. 6). Themore silicic variants are markedly depleted inP,O', TiOg, CaO, FeO and MgO. From7O7o to 76%io SiOt there is a 3OVo decrease inPrOs and about a 70Vo decrease in the otherfour oxides. No significant change in AlzO:r orK/Na is observed with increasing SiOr, althougha large scatter of molar K/Na ratios (0.7-1.4)is present.

Porphyritic microgranites are commonly moresilicic (average 73.4Vo SiO:) than the coarsegrained types (average72.3% SiOa). Spatial chem-ical variations within the batholith are alsoevident. The most silicic rocks are the porphy-ritic microgranites from the northwest (average74.6Vo SiOr). Coarse grained granites from theeast have much lower SiO: (average 71.4Vo)and have a different average composition fromthe coarse grained granites in the west (average73.O% SiO).

o.4

o.20 . 1

2.42.O

0.8o.4

1.6

1.20.8o.4

0.6

CaO

0'6 rl1-l rugfirvdF"r 'r---

74 75 76 69 70 71 72, 73 74 75 76SiO2 wt %

Ftc. 6. Harker variation diagrams for 23 analyzed specimens of Strathbogie granite. Large dots are thetwo analyzed l-type rocks.

7 1 . 7 2 7 3SlO2 wt %

70

58 THE CANADIAN MINERALOGIST

Although the Strathbogie I-type micro-adamel-lites are not well distinguished on the Harkerdiagrams of Figure 6, their higher NarO/KzOand lower (FeO * MgO) are distinctive. TheI-type granitoid rocks are diopside-normative.

PrrnoceNesls

Cottditions of emplacement

Pressure: Low confining pressure during emplace-ment of the exposed granites is inferred fromtwo lines of evidence. The granites intrude theViolet Town volcanic complex, which appearsnever to have had any significant cover (Birchet al. 1977). Hornfels developed on the southside of the batholith contains assemblages indi-cative of pressures € | khar (Phillips & Wallr980) .

Temperature' ancl HzO content of melt: E,m-placement temperatures san be limited by as-semblages developed in the contact aureole. Thehigh-grade cordierite-K-feldspar assemblagesin hornfels adjacent to the granite im-ply temperatures of the order 580-620'C (Phil-lips & Wall 1980). Using the methods of Jaeger(1957), this is equivalent to intrusion tempera-tures of 750oC (above the wet granite solidus)to 82O'C, if the sountry rocks are assumed tohave been at temperatures between 100 and200oC (Phillips & Wall 1980). Emplacementtemperatures can be further limited by utilizingthe crystal-melt equilibria determined experi-mentally by Clemens & Wall (1981).

These experimental studies of the crystalliza-tion phase relations of the porphyritic micro-granite (specimen 889) were carried out at P- 11 kbar, T = 600-900oC, a(HzO) = 0.1-1.0 and log l(O,) - QFM to QFM-0.5. Thisrock was chosen for experimental study be-cause of its well-characterized sequence ofcrystallization and its apparently little-fraction-ated composition. However, it now appears tous that specimen 889 may have undergone en-richment in phenocrysts of garnet and biotite,although this would have little effect on theconclusions we reach below.

Specimen 889 was inferred (above) to havebeen at least 5OVo liquid with few if any K-feldspar crystals present when the magma wasintruded. Experiments at I kbar (for near-satu-ration H:O contents in the melt of 3.5-4.5 wt.7o ) show that the K-feldspar saturation bound-ary occurs at about 750'C. This suggests thatthe emplacement temp€rature was at least

750oC. The absence of orthopyroxene (or ob-vious pseudomorphs after this phase) indicatesthat the Strathbogie magmas were at temtr'era-tures below about 8O0oC at the level of em-placement (Clemens & Wall 1981, Fig. l).

Oxygen fugacity: The occurrences of coexistingilmenite and pyrrhotite as inclusions in theStrathbogie type-A garnets, the absence of mag-netite and the magmatic almandine-rich garnetsall suggest that l(Or) was low in the Strath-bogie magmas, probably around. QFM. Oxygenfugacity in S'type rocks cannot be calculatedr.rsing the Buddington & Lindsley (1964) ap-proach owing to the absence of coexisting Fe-Ti oxides.

Froese (1973) has tabulated thermochemicaldata for the reaction Alm * Oe = Cd * Mt* Q, which provides an upp€r limit for l(Or)(since magnetite is absent from the Strathbogierocks). This yields. at P = 2 kbar, log l(Or)< -16.8 at 700'C and ( -14.6 at 80O'C.

The low-pressure assemblage spinel-cd-sill iscommon as pseudomorphs, often around meta-morphic assemblages in some inclusions in theStrathbogie batholith. These phases can be re-lated by the reaction Cd + O, - Mt + Sill* Q. Assuming that these inclusions equili-brated with the enclosing magma at around 2kbar, this assemblage can be used to evaluatel(Ou) from the data of Froese (1973). Cal'culations using the analyzed cordierite and co-existing spinel compositions, and assuminga(SiO') = 1.0, yield log l(O,) - -18.3 at700'C and -15.5 at 8@oC. These figures arecompatible with the upper limits for l(Oz) setabove. The mole fraction of magnetite in theanalyzed spinel is small, and this constitutesthe major source of error in the calculation.However, an uncertainty of 3OVo in ar."nro'r cor-responds to a change in log l(Or) of only O.3.Thus a reasonable estimate of l(Or) for theStrathbogie magma (at the time of equilibrationof the cordierite-spinel-sillimanite assemblages)is abont an order of magnitude below the QFMbuffer.

Early crystallization

Experiments by Clemens & Wall (1981) alsogive an indication of the ranges of P, T andHzO content during early crystallization of theStrathbogie magmas. The sequence of earlycrystallization for rock 889 is Desl modeled bythe experimentallly determined phase relationsfor P between 4 and 7 kbar, T between 800and 850'C and water contents of the melt in

PETROLOGY OF THE STRATHBOCIE BATHOLITH 59

the range 3-5 wt. Vo. For HzO contents lessthan 3 wt. Vo, the biotite saturation boundaryis depressed to temperatures below the K-feld-spar saturation boundary. For water contentsgreater than SVo, the garnet saturation bound-ary is depressed to such an amount that theinferred sequence of crystallization cannot beduplicated with garnet as a nearJiquidus phase.

Summary: conditions of crystallizatiotr

The Strathbogie granite magma, prior to em-placement, was of relatively high temperature(750-850oC) and markedly water-undersatur-ated. Water saturation of the magma wasattained in the 11 kbar range, withless than 5O7o crystallization; by this stage somegarnet, plagioclase, biotite and quartz had de-veloped. Cordierite and K-feldspar formedIargely at emplacement levels.

Origitt of chemical variation in the Strathhogiebatholith

Several mechanisms have been proposed toexplain the chemical variation observed in theS-type batholiths (e.g.. White & Chippell 1977.Clemens et al. 1979). These mechanisms andtheir relevance to the Strathbosie batholith areexamined below.

"Unmixing" ol restite materiill: Chemical trendsin granites have been explained by a process ofprogressive "unmixing" of restite (refractoryphases) from a "minimum-melt" granitic liquid(Wyllie et al. 1976, Wyllie 1977, White & Chap-pell 1977, Chappell 1978). This process re-quires that the undifferentiated magma leave itsplace of origin carrying some residual solidmaterial. For the Strathbogie rocks, possiblerestite phases include biotite, garnet, ortho-pyroxene, quartz and plagioclase. Progressiveremoval of restite plagioclase and biotite duringmagma migration is essentially the same prosessdescribed below in the crystal fractionationmodel, except that the phases involved are con-sidered to be restite material rather than prod-ucts of niagmatic crystallization.

Within the Strathbogie batholith there is nofirm evidence for calcic plagioclase of restiteorigin (c1., White & Chappell 1977) or aggre-gates of restite biotite. The euhedral singlecrystals of biotite, in particular, are unlike thebiotite segregations in migmatites (Harme1965). Inclusions of any kind are rare and notmarkedly concentrated in the more mafic var-iants. The textural evidence has shown that the

bulk of the cordierite and garnet in the Strath-bogie batholith is magmatic in origin. Cordieritewith inclusions of spinel and sillimanite is easilyrecognized (texturally and chemically) as read-justed restite or xenocryst material, but com-prises less than l7o of the granite. In suchquantities, restite cordierite cannot have playeda major part in the chemical evoltttion. Restiteunmixing therefore seems unlikely to account forany significant chemical variation seen in theStrathbogie granite.

Crystal fractionation: The petrographic data forthe Strathbogie rocks indicate that biotite,plagioclase and quartz are the volumetricallysignificant early-crvstallizing phases. Numerousilmenite and apatite inclttsions are present inthe biotite. Based on the modal data, largeamounts of cordierite and garnet appear un-likely at this staqe.

Oualitatively. fractionation of plagioclase andbiotite with inclusions can explain the desreasein TiOg. P,O'. FeO. MgO and CaO as SiOrincreases (Fie. 6). Quantitative mixinq calcula-tions using rock and mineral compositions wereperformed with the least-squares computer pro-gram PETMIX III (Taylor et al. 1973).'TheStrathbogie primary magma was assumed tohave a composition similar to the average ofthe 2l analyzed S-type rocks (SiO, - 73'ttt. 7o,Table 7). This composition was chosen as anapproximation to the overall composition of themagma near its level of emplacement. Moremafic variants probably contain accumulationsof early-formed phenocrysts, as discttssed above.Separation of 8% plagioclase (An,u), 67o bio'tite, 3Eo quartz and O.7Vo each of ilmenite andapatite from the primary magma compositioncan account for the chemistry of the most felsic(SiO' = 76 wt. % ) Strathbogie granites.

Given the Fe-rich biotite compositions in therocks (Table 5), problems arise in explaining thelarge decrease in MgO with increased SiOrcontent. The liquidus biotite, however, was prob-ably more Mg-rich, but re-equilibration withthe magma at lower temperatures has cattseda drop in the ratio Mg/(Mg * Fe). As cor-dierite and garnet have substantial effects onAleOs, FeO and MgO contents, the role of thesephases has been minor. The lack of variationin MnO also suggests that garnet was not amajor participant in the fractionation processes.

The importance of fractionation processes inthe chemical evolution of the Strathbogie mag-mas is further supported by the distribution oftourmaline in the batholith. Tourmaline, andhence boron. are enriched in the leucocratic

60 THE CANADIAN

(high-SiOr) variants by at least an order ofmagnitude relative to the bulk of the batholith.Simple removal of even relatively large pro-portions of restite material could not have pro-duced sush strong enrichment in incompatibleelements such as boron. Ilowever, liquids mech-anically separated from univariant or invariantcrystalline mushes may exhibit exponential in-creases in incompatible elements, whereas themajor-element contents of the liquids may notvary much. Extreme fractionation processesmust therefore have been involved in the devel-opment of the highly silicic variants.

Fractional partial melting: Progressive partialmelting of a particular region of crustal materialleads to the production of successively moremafic, less silicic magmas. Thus, removal andemplacement .of successive melt fractions maylead to a compositionally zoned or compositebatholith. Ideally, unless the site of meltingshifts, this process produces a granitic suite inwhich the younger plutons are the more mafic.

Melting and crystallization experiments ongranitic compositions (Winkler 1974, Winkleret al. 1975, Wyllie 1977, Clemens & Wall 1981)provide some idea of the compositions expectedin a fractional partial melting sequence. Near thesolidus, liquids are rich in K-feldspar, plagio-clase and quartz components and are Fe-Mg-Ti poor. With increasing temperature, Fe-Mgphases become more soluble in the melt, andinitially low Mg/ (Mg * Fe) ratios are expected.Only at high temperatures do more mafic melts,with ahigher Mg/(MB f Fe) ratio, form. Sincegarnet crystallization is favored by higher Fe,and cordierite crystallization by higher Mg,such trends would lead to more garnet in thesilicic and more cordierite in the more maficvariants. The opposite trend is observed in theStrathbogie batholith. The separate fractions ofpartial melt should crystallize as plutons inwhich the mafic minerals have differing Mg/(Mg + Fe) values characteristic of each pluton.The major variants of the Strathbogie batholithall have the same suite of early magmaticphases.

Suntmary

Although re-equilibrated restite materialsexist in the Strathbogie batholith, their low con-centration, even in the mafic granites, suggeststhat restite unmixing was not a major processinvolved in the magmatic evolution. Fractionalpartial melting would produce chemical trendsthat run counter to those observed. Thus. to.

explain the variation in the Strathbogie batho-lith, we prefer a differentiation mechanism in-volving separation of early crystallized biotiteand plagioclase with minor garnet, cordierite,quafrz and accessory Phases.

Nature ol the source

The available Sr isotope data (Pidgeon &Compston 1965, McDougall el al. 1966, Brooks& Leggo 1973, Flood & Shaw 1977) indicatethat initial 8?5r/865r isotopic ratios of easternAnstralian S-type granites and volcanic rockshave a mean value in excess of 0.710. Thesehigh 875r/865r ratios, together with the pera-luminous nature of the granitoid suite, suggestthat magma formation involved partial meltingof old metasedimentary rocks (Chappell & White1974" White & Chappell 1977'). As pointed outby Flood & Shaw (1977), S-type rocks withlower initial ratios (around 0.706) suggest theinvolvement of vottnger or less-evolved crustalsources for the magmas. Experiments carriedotrt bv Green (1976. 1977). and Kil inc (1969.1972\ show that liquids formed by partialmelting of sillimanite-bearing pelitic materialsmust crvstallize peraluminous minerals' Green'sinferred granitic liquids have, in general, morenormative corundum than the Strathbogie gran-ites (Table 7). Experimental results reported bvClemens & Wall (1981) for a natural graniticcomposition from the Strathbogie batholith(specimen 889) seeded with sillimanite showrhat a liquid of this composition is too poorin alumina for equilibrium with sillimanite. Thissuggests that sillimanite-bearing pelitic rocksare unlikely to be the dominant litholoey in thesource region of the Strathbogie magma. A mus-covite-bearing sollrce region is ruled out bv thehigh temperatures and torv water contents in-ferred for the magma.

The most direct evidence concerning thenature of the source region of the gr4nitic mag-mas comes from the granites themselves. Apartfrom hiehJevel hornfels and the magmatic mi-cro-adamellite, the most common inclusionsencountered in Strathbogie granites are hlgh-grade pelitic gneisses. Provided that these in-clusions represent either source-region material(Chappell & White 1974) or readjusted restitematerial from the source region, these inclusionsindicate some of the probable assemblages inthe source area during the extraction of melt.In the Strathbogie batholith the rare gneissic in-chrsions contain quartz, biotite, sillimanite" pla-gioclase, minor orthoclase, cordierite and her-cynite. and rare garnet. The cordierite-hercynite

PETROLOGY OF THE STRATHBOGIE BATHOLITH

assemblage represents a low-pressure readjust- postulate an anatectic source-region, character-ment of an original garnet-sillimanite assem- ized by quartzofeldspathic to p"l-itic,'high-gradeblage by reactions similar to regional metamorphit rocks. Such rocks-are notFeaAlzSi.rOrz + 2Al2SiO6 : FeAlsO+ * FezAlaSioo.-

presently exposed in central Victoria. The min-

Alm Sill Hc Cd 'r8

eralogy of these source rocks would most likelyinvolve quartz, orthoclase, plagioclase, alman-

Indeed, sorne cordierite-hercynite aggregates ap- dine-rich garnet, Fe-rich biotite and orthopy-pear to pseudomorph original idioblastic garnets. roxene. Rocks containing sillimanite, cordieriteAccessory phases include ilmenite, pyrrhotite, and hercynite may also have been present butapatite and zircon. Thus, the mineralogy of in minor proportions. Accessories in the sourcethese inclusions suggests that some of the source rocks would include ilmenite, pyrrhotite, apa-rocks for the Strathbogie magma contained as- tite and zircon. Magnetite and hornblende aresemblages involving combinations of plagioclase, unlikely. Although the actual mineral assem-quartz, garnet, K-feldspar, biotite and silliman- blages are unknown, this range of miner-ite along with ilmenite and pyrrhotite. alogy, the relatively high temperatures and the- Thermodynamic calculations based on reac- HrO-undersaturated nature of the granitic melts

tions An - Gr + Sill + Q (Ghent 1976) and produced are consistent with granulite-facies re-An * Fs - Gr + Alm * Q (Phillips 1978) gional metamorphic conditions in the sourceindicate that garnet with a composition similar region. Consideiing ( I ) the high geothermalto the early, type-A garnets from the Strathbogie gradient required to generate the high tempera-granites could have been in equilibrium with tures at the moderate crustal depths proposedplagioclase(- An'o) - sillimanite - quartz or pla- for the source region and (2) the transiiionalgioclase(Anro) - orthopyroxene(Enso) - quartz orogenic-anorogenic setting of the Strathbogieat n 5 kbar and 800"C.'Any pyroxene formerly batholith, we suggest that this granulite-faciespresent in the magma or its restite inclusions partial-fusion event was brought about by em-has reacted out and is no longer found in the placement of mantle-derived material in theassemblages. lower crust (c1., Wells 1980).

The l(O,) during early crystallization of theSrathbogie magma was calculated to be aboutan order of magnitude below the QFM buffer.Together with the inferred low oxidation ratioof the magma, this suggests anatexis of highlyreduced source rocks that may have containedgraphite during part pf their nretamorphichistory.

Partial melting of a metapelitic rockwill commonly yield K-feldspar-bearing restitesowing to the high K/(Na * Ca) rarios of thesecompositions. Thus, magmas derived from suchhighly aluminous source rocks will usually besaturated with K-feldspar all the way from thesource region (Clemens & Wall l98l). How-ever, K-feldspar crystallizes late from the Srath-bogie magmas and plagioclase is a near-liquidusphase. We would therefore infer that the bulkof the Strathbogie source region was under-saturated with respect to K-feldspar at the timeof magma extraction. Appropriate source rocksfor the Stratbbogie magmas would includeweakly peraluminous metasedimentary rocks orother quartzofeldspathic rocks poor in K-feld-spar. Pelitic inclusions in the Strathbogie gran-ites thus probably represent highly refractory,melt-depleted materials not typical of the bulkcomposition of the source region of the magma.

In snmmary, for the Strathbogie and othercenftal Victorian S-type granitic magmas, we

6 l

AcKNowLEDGEMENTS

We are grateful for suggestions and criticismsmade by I.A. Nicholls, A.J.R. White. W.D.Birch. and D.B. Clarke. R.H. Flood and R.A.Binns reviewed earlier manuscripts. A.R. Milnesgave valuable advice during the bulk-rock anal-ysis. Ms. G.E. Earl drafted the diagrams.

RrrenENcrs

BarrvlN, P.C. & CHeppELL, B.W. (1979): Crystal-lization, fractionation, and solidification of theTuolumne intrusive series. Yosemite NationalPark. California, Geol, Soc, Amer. Bull. 90.465-482.

Btncs, W.D., Crn*rNs, J.D. & Pmr,rrps, G.N.(1977): Dev<inian acid igneous complexes incentral Victoria. Second Aust. Geol. Conv. Ex-cursion Guide C.

& GLEADow, AJ.W. (1974): The genesisof garnet and cordierite in acid volcanic rocks:Evidence from the Cerberean cauldon. centralVictoria, Australia. Cont. Mineral. Petrolosv45. t-13.

BoHLEN, S.R., Peacon, D.R. & EssrNb, EJ. (1980):Crystal chemistry of a metamorphic biotite andits significance in water barometry. Amer. Min-eral. 65, 55-62.

62 THE CANADIAN MINERALOGIST

Bnoors, C. & Lrcco, M.D. (1973): The local chro-nology and regional implications of a Rb-Sr in-vestigation of granitic rocks from the Corryongdistrict, southeastern Australia. Geol. Soc. Aust.

"r. 19, 1-19.BUDDTNGToN, A.F. & LrNlslrv, D.H. (1964): Iron-

titanium oxide minerals and synthetic equivalents.J. Petrology 5, 310-357.

CHernELr, B.W. (1978): Granitoids from the Moon-bi district, New England batholith, eastern Aus-tralia. Geol. Soc. Aust. l. 25, 267-284.

& Wntrs, A.J.R. (1974): Two contrastinggranite types. Pacilic Geol.8, 173-174.

CLEMENs, J.D., PHutps, G.N. & WnI,r, V.J.(1979): Petrogenesis of an S-type volcanic-plu-tonic association - the Strathbogie igneous com-plex, central Victoria. /rr Symposium on Crustand Upper Mantle of Southeastern AustraliaBur. Mineral Rer. Rec. L979/2, 8-9 (abstr.).

& Weu, V.J. (1979): Crystallization andorigin of some "$type" granitic magmas. /nSymposium on Crust and Upper Mantle of South-eastern Australia. Bur. Mineral Res. Rec. L97g/2'l0-11 (abstr.).

& - (1981): Origin and crystalliza-tion of some peraluminous ($type) graniticmagmas. Can. Mfueral. 19, 111-131.

Frooo, R.H. & Snew, S.E. (1975): A cordierite-bearing granite suite from the New England ba-tholith, N.S.W., Australia. Contr. Mineral. Pe-trology 52, 151-164.

& - (1977): Two ..S-type,l granitesuites with low initial 8?Sr/80Sr ratios from theNew England Batholith, Aushalia. Contr. Min-eral. Petrology 6t, 163-173.

FRoEsE, E. (1973): The oxidation of almandineand iron cordierite. Can, Mineral, 11, 991-1002.

Gnrxr, E.D. (1976): Plagioclase-garnet-Al2Si0r-quartz: a potential geobarometer-geothermo-meter. Amer. Mineral. 6L, 710-714.

Glners, T.H. (1976): Experimental generation ofcordierite- or garnet-bearing granitic liquids froma pelitic composition. Geology 4, 85-88.

(l97Xl: Garnet in silicic liquids and itspossible use as a P-T indicator, Contr. Mineral.

"Petrology 65, 59-67.

Henue, M. (1965): On the potassium migmatitesof southern Finland. Comm. G6ol, Finlande Bull.219.

HINr, R., Wrr-r,rnvs, I.S., Cuerrnr.r., B.W. & Wruru,A.J.R. (1978): Contrasb between I- and $typegranitoids of the Kosciusko batholith. Geol. Soc.Aust, J,25, 219-234.

Jercqn, J.C. (1957): The temperature in the neigh-borhood of a cooling intrusive she.et, Amer. J.Jci. 255, 306-318.

KILtNc, I.A. (1969) : Experimental Metantorphismand Anatexis of Shales and Graywac&es. Ph.D.thesis, Pennsylvania State Univ., University Park,Pa.

(1972): Experimental study of partial melt-ing of crustal rocks and formation of migmatites.Int. Geol. Cong. 24th, Proc.2, 109-113.

McDouclr-1. I.. Coupsror, W. & BortNcEn, V.M.( 1966): Isotopic age determinations on UpperDevonian rocks from Victoria, Australia: a re-vised estimate for the age of the Devonian-Carboniferous boundary. Geol. Soc. Amer. Bull.77. 1075-1088.

O'CoNNoR, J.T. ( 1965): A classification for quartz-rich igneous rocks based on feldspar ratios' U.^t.Geol. Surv. Prof . Pap. 5258,79-84.

Purt-t-rps, G.N. (1978): Metutnorphisnt and Geo-c'lretnistry of the Willyanru Contplex, Broken Hill.Ph.D. thesis. .Monash Univ., Clayton, Australia.

& Wer-1, V.J. (1980): The geologY andmetamorphism of the Bonnie Doon area, Victo-ria. Roy. Soc. Yict. Proc.9L, 33-42.

PTDGEoN, R.T. & CovrpstoN, W. (1965): The ageand origin of the Cooma granite and its asso-ciated metamorphic zones, New South Wales.J. Petrology 6, 193-222.

Srrr.crr,rsrN, A.L. (1973): Plutonic rocks: classi-fication and nomenclature recommended by theIUGS Subcommission on the Systematics of lgne-ous Rocks. Geotimes 18(10), 26-30.

TelrNr, J.A. (1965): The stratigraphic and dia-strophic evolution of central and eastern Victoriain middle Palaeozoic times. Roy. Soc. Vict. Proc.79 ,197- t95 .

TAyLoR, S.R.. Gonror*, M.P., Murn, P., N"wcr,W.B." RunowsKl, R. & Wenr, N. (1973): Com-position of the Descartes region, Lunar Highlands.Geochim. Cosmochim. Acta 37, 2665-2683.

VANDENBERc, A.H.M. & Gennerr, MJ. (1976):The M.elbourne Trough. In Geology of Victoria(J.G. Doucr.ns & J.A. FrncusoN, eds.) Geol.Soc. Atrst. Spec. Publ. E, 45-62.

VrnNoN, R.H. (1977): Relationships between mi-crostructures and metamorphic assemblages, Tec-rottophys. 39, 439-452.

Wrtls, P.R.A. ( 1980): Thermal models for theaccretion and subsequent metamorphism of con-tinentaf crust. Earth Planet. Sci. Lett. 46, 253-265.

WHrrE, AJ.R. & CHerPEr.r, B.W. (1977): Ultra-metamorphism and granitoid genesis. Tectonopltys.43, 7-22.-, - & Crrenv, J.R. (1974): Geologicalsetting and emplacement of some Australian Pa-laeozoic batholiths and implications for intrusivemechanisms. Pacilic Geol. 8, 159-171.

63

WHITE, D.A. (1954): The geology of the Strath-bogie igneous complex, Victoria, Australia. Roy.Soc. Yict, Proc. ffi,25-52.

WtNrrrn, H.C.F. (1974): Petrogenesis of Meta-m or ph ic .Roc&s (3rd ed. ) . Springer-Verlag, Berlin.

-, BoEsE, M. & MARcopour.os, T. (1975):Low temperature granitic melts. Neues lahrb.M ineral. Monatsh' 245-268.

Wvrrrr, PJ. (1977): Crustal anatexis: an experi-mental review. Tectottophys. 48, 4l-71.

-, HUANG, Wuu-Lrexc, SrrnN, C.R. &MAAL@E, S. (1976): Granitic magmas: possibleand impossible sources, water contents, andcrystallization sequences. Can. l. Earth Sci.18,1007-1019.

Zscr, H.P. (1970): An erupted migmatite fromCerro del Hoyazo, SE Spain. Contr, Mineral.Petrolotrly 26, 225-246.

Received October 1980, revised manuscript acceptedDecember 1980.

Gr grossularEn enstatiteFs ferrosiliteC corundumCd cordieriteSill sillimaniteHc hercyniteMt magnetiteIlrn ilrneniteAp apatitePo pyrrhotite

Pnvsrcocuprutcer. Syrrnors

pressure in kilobarstemperature in degrees Celsiuslogro oxYSen fugacity in barslrater content of meltsupercritical fluid phaseoxygen fugacity equivalent to that oltlre qtartz-fayalite-magnetite bufferactivity of species imagmatic silicate liquid

MrueRALs

qtJartzpotassium feldsparorthoclaseplagioclaseanortlitemuscovitebiotitegarnetalmandinepitropespessartine

P kbarT'Clog/(Oz)X(H29;'atFIQFIUI

aQ)Liq

PETROLOGY OF THE STRATHBOCIE BATHOLITH

AppnNorx l. Tenrn oF ABBREVTATToNS USED

aKfsOrPIAnMuscBiGtAlrnPySp