Download petrology of the sthathbogie batholith: a cordierite-bearing

CanadianMineralogist
Vol. 19,pp.47-63(1981)
GRANITE
PETROLOGY
OF THESTHATHBOGIE
BATHOLITH:
A CORDIERITE-BEARING
G. NEIL PHILLPS'!. VICTOR J. WALL AND JOHN D. CLEMENS
Departmentof Earth Sciences,Monash University, Clayton 3168, Victoria, Australia
ABSTRACT
Sovrvernn
The Strathbogie batholith is a high-level. discorLe batholithe de Strathbogie (Sud-Est de l'Ausdant, composite granitoid intrusion in southeastern tralie), dont le toit se trouve ir faible profondeur,
Ar:stralia. Granitic rocks containing magmatic gar- est discordanl et composite. l.es roches granitiques
net, cordierite and biotite predominate in the batho- qui contiennent grenat, cordi6rite et biotite magmalith. Intrusion involved marginal faulting. roof
tiques y sont dominantes. L'intrusion Jest produite
par glissement le long de failles marginales, souldlifting and "shouldering aside" of the country
rocks. which include the Violet Town volcanic com- vement du toit et repoussement.lat6ral des roches
plex of similar Late Devonian age. Textural evid- encaissantes. dont le complexe volcanique Violet
ence indicates early crystallization of biotite to- Town. aussi d'Aee D6vonien tardif. D'aprEs les
gether with garnet, plagioclase (- Anno) and some textures, la biotite a cristallis€ t6t, avec grenat.
quartz. The bulk of the quartz, cordierite and Kpla-gioclase(- Anas) et un peu de guartz. Quartz,
feldspar crystallized later during emplacement. Early
cordi6rite et feldspath potassique ont. en maieure
magmatic cfystallization occurred with P = 4 kbar, partie, cristallis€ plus tard, lors de la mise en place.
T > 800oC and water contents in the magma of Le stade initial de la cristallisation a eu lieu i une
about 4 tnl, Vo, Thus the Strathbogie magma was pression d'au moins 4 kbar, i une tempErature de
relatively hot and water-undersaturated. Emplace800"C ou plus et i partir d'un magma contenant enment occurred at P -0.5 kbar with initial temper- viron 4Vo d'eau (€n poids). Le magma 6tait donc
atures well in excess of the wet granite solidus. relativement chaud et sous-satur6 en eau. Pendant
Oxygen fugacity was ( QFM throughout the mag- la mise en place, d une pression d'environ 0'5 kbar.
ma's history. The Strathbogie rocks have SiO, con- la temp6rature initiale surpassait le solidus du systdme granitique satur6 en eau. La fugacit6 d'oxytents in the range 7A-76 wt. Vo with high K/(K Na * Ca), normative corundum around 2 wt, Vo gane est rest6e inferieure au tampon O.FM pendant
toute l'6volution du magma. Les granites contiennent
and Me/(Me * Fe) - 0.4-0.5. CaO. MgO, TiO,
and P2O5 decrease linearly as SiO: increases. These de 70 i 767o de SiOr (en poids), et possBdent un
chemical trends cannot he fully explained by re- rapport K/(K + Na * Ca) 6lev6. environ 2Vo
rnoval or variable incorporation of restite materials, de corindon normatif et un rapport Me/(Mg +
as inclusions of any kind are scarc€ in all variants. Fe) situ6 entre 0.4 et 0.5. Les concentrations de
Differentiation mechanisms involving separation CaO, MgO, TiO: et PzOr d6croissent lin6airement
and accumulation of early magmatic biotite. plagio- quand SiO2 augmente. Ce comportement ne peut
clase and accessory phases are favored over batch s'expliquer entierement par 6limination ou incorpopartial-melting processes to model the chemical ration variable de mat6riaux rdsiduels. vu la raret6
variation. Magma chemistry, together with inferred des x6nolithes dans tous les facies. On pr6fdre les
m6canismes de diff6renciation qui impliquent la
early magmatic conditions, and the mineralogy of
s6paration et I'accumulation, dans le bain fondu, de
inclusions in the granites suggest initial formation
biotite, plagioclase et phases accessoires. toutes
of the Strathbogie magma bv partial fusion of
biotite-bearing quar?ofeldspathic and pelitic rocks hitives, aux processus discontinus de fusion parunder granulite-facies conditions. These source tielle pour expliquer la variation de composition.
rocks are apparently below the presentlv exposed D'aprbs la chimisme du macma, les conditions magmatiques initiales et la min6ralogie des x6nolithes
lower Paleozoic sequencesof southeastern Australia.
The transitional orogenic-anorogenic setting of the dans les granites, une fusion partielle de roches
batholith is compatible with the partial-fusion event quartzofeldspathiques i biotite et de roches p6litihaving resulted from intrusion of mafic magma ques, 6quilibr6es dans le facies granulite, serait i
I'origine du magma. Ces roches forment le socle
into the deep crust.
sous-jacent des s6ries du Pal6ozoique inf6rieur qui
Keywords: peraluminous granite, granite petro- affleurent i pr6sent dans le Sud-est de l'Australie.
genesis, granite geochemistry, granite emplace- Le milieu de transition orog6nique-anorog6nique du
ment, cordierite, garnet, Australia, Strathbogie.
batholithe concorde avec la fusion partielle qui fut
*Now at the Department of Ceology, University le r6sultat de I'intrusion d'un magma mafique dans
la cro0te profonde.
of Western Australia, Nedlands 6009, W.A., Australia.
(Traduit par la R6daction)
47
48
THE
CANADIAN
MINERALOCIST
Mots-cles: granite peralumineux,p6trogendsegra- covite. On the basis of studies of granites in
nitiQue,gEochimiedu granite, niise en place des southeastern New South Wales, Chappell &
granites,cordi6rite,grenat,Australie. Strathbogie. White ( 1974) have proposed the term "S-type"
for these chemically and mineralogically distinctive rocks, inferring their origin as a result
INtnopucrroN
of partial melting of metasedimentarymaterial.
Peraluminous granitoid and volcanic rocks
Recent field and petrological studies (Birch &
are widespread in the Tasman fold belt of Gleadow 1974, Flood & Shaw 1975) have sugeasternAustralia. These dominantly post-tectonic gested that the peraluminous phases (cordierite
batholiths and volcanic complexes contain one and garnet) are largely of residual metamorphic
or more of the peraluminousphasescordierite, character (i.e., restite) derived from the source
garnet, biotite, andalusite,sillimanite and mus- region of the partial melts. Here we describe a
Ftc. l. Regional geological setting of the Strathbogie batholith and other
felsic igneous complexes of central Victoria.
PETROLOGY OF THE STRATHBOGIE BATHOLITH
49
contrasting situation in yhich textural features
in a high-level pluton, subject to little solid-state
readjustment, indicate a magmatic origin for
the bulk of the cordierite, garnet and biotite.
The implication is that moderately peraluminous
melts may be generated in the lower crust and
evolve by processeslargely independent of the
presence of restite phases.
sidered as part of the batholith but separated
from the rest of the Strathbogie mass by a narrow zone without outcrop. The Strathbogie
batholith, which is markedly elongate in an
E-W direction, cuts across major structures in
the Paleozoic sedimentary rocks.
A broad contact-metamorphic aureole surrounds the batholith and is particularly well
developed in the Paleozoic sedimentary rocks.
At Bonnie Doon, spotted slate, biotite-muscoFIELD RELATIoNS
vite, cordierite-muscovite and cordierite-K-feldspar zones have been delineated in the 3-kmGeneral setting
wide aureole (Phillips & Wall 1980). Contactmetamorphic effects are present in the Violet
The Strathbogie batholith is a large, discord- Town volcanic complex up to several hundred
ant, compositebody more than l5O0 km, in area metres from the granite contact (Birch et al.
situated 150 km north-northeastof Melbourne 1977, Clemens& Wall 1979).
(Fig. 1). The batholith, of Late Devonian
age (365 -+ 5 Ma, K/Ar, J.R. Richards, writ- Emplacement
ten comm. 1975) intruded folded Siluro-Devonian sedimentary rocks of the Melbourne
The undeformed Violet Town volcanic comTrough (Phillips & Wall 1980) and on its north- plex postdatesmajor folding, low-grade regional
ern margin, the Violet Town volcanic complex metamorphism and erosion of the Siluro-Devo(Birch er al. 1977) of Late Devonian age. The nian sedimentaryrocks (Birch er al. 1977); all
Barjarg granite of White (1954) is here con- these processesare presumably associatedwith
+
I
SIRATHBOGIE GRANITES
[T*l
c*r."-g-
ined sronite
Nl
rorptyr;ric micrcgronite
€
Kerrisdsle Cordieriie Mioqdonellite
tl
Tolhngollook llomblende
Migqdarellite (l-tycc.)
o
locolion
. r *
o{
*\
(XRf)
onoly*d
6 !*****\-#**
sanples
-/++++++
++++a+++
+ + + + + + + + +
'{+
*
El
***?***n*********"
t' i+ + + + | + + + + + + !. ++ + + + + /
F
+ + +-+ + + + + + a + + + + I
o*
+ + + + + + + + + + + + + + t
uf+
+ + + + + + + + + + + a+ +/f
+++
/\
a ll -- ltyv p
pe
e (Ct *+*a* +
+++
I lf t ** ** ******************************** 1
I t.l.:.:.l.l.l.l.l.l.lri.li.)
>t=f
$llil;
'
T
T
a-T
++++++La++++
+ + J\_+ + + la+
+t +7 -a- ----?+? +
+
? ?+? + + {
'/
I Bonnie Doon
T
?
?
+ + +
R***a
20 KII,OMETBES
Flc._2. D-istribution of granite types within the Strathbogie batholith. Cordierite and garnet are present in
all variants except the I-type Tallairgallook hornblende microadamellite.
50
THE
CANADIAN
MINERALOCIST
the Tabberabberan orogeny (Middle-Late Devonian: Talent 1965). Similar volcanic rocks
in the Cerberean cauldron in Victoria have been
dated as post-Tabberabberan(McDougall et al,
1966). Thus, at least the northern margin of
the Strathbogie batholith, which intruded the
subaerial volcanic pile (present thickness 3O0
m: Birch et al. 1,977),was probably emplaced
at a very high level, i.e., pressures( 1.0 kbar.
The character of the contact-metamorphic assemblages at Bonnie Doon also suggestsvery
Iow pressures(O.5-1.0 kbar) and indicatespeak
metamorphic temperatures of around 600oC
(Phillips & Wall 1980). Ifence, taking into account the temperature drop (approximately
150oC) across the intrusive contact (Jaeger
1957), the presently exposed granite was emplaced at shallow depth (ca. 3 km) and relatively high temperatures.
On a regional scale, the batholith is bounded
by several nearly straight E-W and N-S contacts (Fig. 2). Major structuresin the Paleozoic
sedimentary rocks on the southern side of the
granite are truncated by the contact. However,
the strike of the sedimentary rocks appears to
be deflected near the western end of the batholith
(Vandenberg & Garratt 1976), with the divergence becoming apparent within 20 km of the
granite contact (Fig. 1). On a mesoscopicscale,
abrupt right-angled steps occur along the contact. Between these steps the contact is straight
for distances of l0 to 100 m. The contacts are
generally near-vertical, with marginal zones of
finer-grained granite up to 10 m wide. Rare
aplitic apophysespenetrate the country rock.
The development of high-grade contact metamorphic rocks adjacent to the granite appears
to rule out major postemplacementfaulting as
an explanation of the geometry of the contact.
Penetrative deformational fabrics are completely
absent from the granite and hornfels. Faulting
during emplacement (brittle country-rock deformation) may, however, account for the stepped
nature of the contact. Marginal faulting is well
shown in the northern region where adjacent
volcanic rocks are block-faulted and tilted away
from the granite. In view of the paucity of
counry-rock xenoliths, stoping is unlikely to be
the major mechanism of granite emplacement.
The composite character of the batholith is also
evidenceagainststoping(White et al. 1974).From
the above evidence we infer that emplacement
was dilational in character, with some "shouldering aside" of the country rocks, marginal faulting
and lifting of the roof of the magma chamber.
This scheme differs from proposals of wrenchfault-conrolled emplacement of other discordant batholiths in southeasternAusfralia.
In summary, the Strathbogie batholith is a
discordant, high-level, post-tectonic batholith of
composite character. The northern and eastern
marginal portions (at least) representnear-roof
zones, as they intruded the Violet Town volcanic
complex. Clemens et al. (1979) suggestedthat
the Violet Town volcanic complex and the
Strathbogie batholith are comagmatic in view
of their close temporal and spatial association
and similar geochemistry.
Prtlocnepuv
Granitoid rocks
We have subdivided the Strathbogie batholith into four mappable variants on the basis
PETROGRAPHY
Rock
rype
Porphyrltic
micrcgmnlte
"?
Eiffiifi,
34.0
CoaEe-gralned
65.0
Kerrlsdale
Cordlerlte
MlcmadmlI ite
Tallangall@k
Hornblelde
lillcmad@lllte
<0.L
Fabrlc
Pherccrysts
Gmundnass
Accessorles
porphjritlc usually with
L0 - 259 euhedral phenocrys6
quartz,5-10m
p l a g i o c l a s e( A n q q - r r ) .5 - 1 0 m
orthoclase,4-l0m
cordlerlte, <6 m
blotlte, <5 m
garnet (Type A)
0.5-1.5m
quartz,orthoclase
plagloclase
ninor blotlte
cordierite and gamet
(TypeB)
llrenlte
pyrrhotite
apatlt€
zlrmn
mmzlte
xenotlm
mstly equlgranular
4-10m
sm orthoclase,30 - 50 m
quartz
plagioclase(An5q1e)
orthoclase
biotlte
cordlerlte
garret (Type A) nJnor
dltto
quarrz
orthoclase
p l a g l o c l a s e( A n 5 a - 2 s )
blotlte (mlnor)
quartz, plagloclase
orthoclase,blotlte
cordlerl te
dltto
quartz, 6 m
p l a g l o c l a s e , 4m
- 0.8 m
quartz,orthoclase
plaqloclase(An,.)
irornblende,
Sloilte
"Porphyritlc' - contlnuus
rangeln grainslze .5 - 5
m wlth flner sized
donimnt
porphyrltlc vlth - &
euhedral phamcrysts
Averageplagioclase composltionsare quotedcore to rln.
sphene
mgnetlte
llrenlte
apatlt!
zlrcon
5l
PETROLOGY OF THE STRATTIBOCIE BATHOLITH
of textural and mineralogical characteristics
(Table 1). Although outcrop is good, contacts
between the major variants have not been observed, and hence their temporal relations are
not clear. The distribution of the four variants
is shown in Figure 2, The map shows two dreas
of porphyritic microgranite, one in the centre
of the batholith and another smaller body in
the southwest. flhe terms "granite" and "adamellite" used in this papr are taken from the
normative classificationof O'Connor (1965).1
The latter microgranite body appears to have
intruded coarse grained granites. Such granites
occnpy two areas in the east and the south..The
north-sotrth boundary between these two main
rock types is marked by a prominent scarp and
a zone of no outcrop that is probably the expression of a fault. Areas delineated as coarse
grained granite contain small (- 1 km'z) scattered bodies of porphyritic microgranite, particularly in the eastern half of the batholith.
A small, separate phlton, the Kerrisdale cordierite microadamellite, granite in IUGS
'(Streckeisen
terms
1973)" lies at the extreme
southwestern end of the batholith. The Tallangallook hornblende microadamellite, the only
metahlminous variant observed in the Strathbogie batholith, forms a small (< I km wide)
stock intruded into coarse grained cordierite
granitesin the east (Fig. 2).
Modal and chemical variation throughout the
batholith is limited. All outcrops (except the
Tallangallook hornblende microadamellite) exhibit cordierite and peraluminous biotite, al;
though garnet is lesscommonly developed.Tables
I and 2 and Figure 3 show the main modal
and textural variations observed. All rocks are
granites according to the IUGS classification.
In all the granites some of the quartz forms
Iarge euhedra, commonly with hexagonal-bipyramidal (6-quartz) habit. Inclusions of biotite
and plagioclase are found in this type of large
phenocrystal quartz. Plagioclasehas a tabnlar
habit and shows complex twinning' and slight
normal zoning with superimposedfine-scale,
low-amplitude oscillatory zoning. Plagioclase
compositionsare given in Table 1. Aggregates
of plagioclasesuggestaccumulation of this phase
during magmatic evolution. Quartz and biotite
are the only inclusion phasesfound in the plagioclase;Orthoclase microperthite forms large, simply twinned subhedra that contain inclusions
of cordierite, biotite, quartz and plagioclase.
Red-brown biotite forms large, discrete euhedral
booklets in all granite variants.
Cordierite and garnet types in the granites
are listed in Tables 3 and 4 (also see Figs. 3d,
4). Nearly all the cordierite forms euhedral
prisms (type A), rarely twinned and inclusionpoor, although some encloses small, euhedral
biotite flakes. In the coarse grained granites,
cordierite is similar in size (4-10 mm; Fig. 3d)
to plagioclase, biotite and quartz. In the finer
grained Kerrisdale cordierite microadamellite,
cordierite, like all other phases,has a grain size
of 1-5 mm (Fig. 3b). Cordierite in the aplitic
rocks also has a grain size similar to that of
other phasespresent (0.5-1 mm).
Many of the large, corroded (type-A) garnets have been partly pseudomorphedby cordierite that preserves the shape of the original
euhedral, inclusion-free garnet. Textural and
chen'ricalfeatures of the type-B garnets are summarized in Tables 3 and 5.
The more leucocratic porphyritic microgranites, especially near the edges of the batholith,
contain patches of tourmaline intergrown with
the groundmass phases. This feature is most
common in variants with a relatively coarse
grained g4oundmass. A late-stage magmatic
origin is favored for much of the tourmaline,
as it rarely exhibits replacement textures. In
some porphyritic microgranites, the groundmass
contains small patchesof microgranophyre,i.e.,
eutectoid-like intergrowths of quartz and orthoclase.
S-type granites hitherto described (Flood &
Shaw 1975, White & Chappell 1977, Chappell
1978, Hine et al. 1978) display textural features
interpreted by these authors as indicating that
much of the cordierite, garnet" biotite and, perhaps, plagioclasein theserocks is residual (i.e.,
restite) material. The corroded. highly calcic
cores of plagioclasecrystals in some granites are
consideredby White & Chappell (1977) to be
restite grains mantled by magmatic feldspar.
TABLE2.
MODAL
ANALYSES
l4l neral
orthoclase
25
29
25
2A
19
22
plagloclase
28-
19
16
24
32
34
I
4
Tr
Tr
cordierltel6534'
biotite95L7983
garnet
Tr
hofnbiende
-
4
sphene
-
1
1. typical porphyrltlc nicrogranjt€ (864); 2. cordierite-rich
porphyrlt'jc mJcrogranite (756); 3. garnet-rJch porphyrltic nicroqranite (889)i 4. coarse gralned gran'ite (average of 801 and 803)i
5. Kerr{sdale cordierite nicroddare'llite (765); 6. Tallangallook
(861). All rcdes are based on >1000
hornblende microadaml'lite
polnts. Both phenocryst and groundmss phases were counted.
52
THE
CANADIAN
MINERALOGIST
1,C?i.rn
d
ffi#
PETROLOGY OF THE STRATHBOGIE BATHOLITH
TABLE
3.
GARTFT
fiPES
Conposltlon (ilol. g)
granltic rccks
rcunded, corrcded crystols (< 15 m)
wlth cordierlte-blotite
reactlon
corcnasi ilrenlte,
apatlte, zlrcon
I nc lus 1ons
167554
I - 2 m anhedro lntergmn
rlth gmundmss felslc phases ln porphyrltlc'
mrcroqranl res
lncluslonsof
high-grade
reglonal mtamrphlc rck
ver:y smll embayedcrystals
ln quartz and feldsDar
53
5
8013
Relative
Abundance
the mst comn type
scattereddistributlon
2
comn
lncluded
early mgmtlc
laie mgmtlc
or subsolldus
rare
restlte,
readJusted
restlte or
xeml ithlc
Iype A an-dB garnets ln tie Strdthbogle granltes are analogous to Blrch and cleadox's (1974)
lyp€ z afr r gamets (respectiw'ly) fm
upper lhvonlan acid volcanlc rccks ln the ceriereair cauldrcn, central victoria.
Plagioclase in the Strathbogie granite does not
contain obviously distinctive cores. Indeed, the
progressive oscillalory zoning exhibited by the
plagioclase (e.g., Fig. 3b) appean to be incompatible with a metamorphic origin for this feldspar. Features expected in restite micas (sillimanite inclusions and coarse, foliated aggregates) are entirely lacking in the Strathbogie
biotites. Cordierite from metamorphic terranes
is commonly anhedral, ovoid in shape and inclusion-rich. Such metamorphic cordierites are
a component of the rare gneissic inclusions in
the Strathbogie batholith. The euhedral, inclusion-free character of the type-A cordierite in
the Strathbogie rocks and the correlation between grain size of the cordierite and the enclosing rock are compelling evidence for emplacement-stagemagmatic crystallization of the
cordierite. The initial euhedral shape and inclusion-free character of the type-A garnets
suggest a magmatic origin for this phase also.
The grain shape, composition and lack of reaction of type-B garnets with the magma are
compatible with a late-stage magmatic or subsolidus origin. Thus, the textural evidence indicates that the great bulk of the garnet, cordierite, biotite and plagioclasehas an entirely
magmatic origin in the Strathbogie granites.
TAELE4.
0currence
Cordl erite
Type
gran'ltic rccks
b
ct
Liquidus phase-relationsof a particular magma composition depend on variables such as P,
T, l(O.) and the H:O content of the magma;
hence, the inferred sequence of crystallization
of a rock can yield information on these parameters. In deducing the sequenceof crystallization in the Strathbogie granite (Fig. 5), inclusion relations, reaction relations and grain
shape are critical. We have assumed that small
euhedral inclusions in the core of a large euhedral phenocryst are likely to have nucleated no
later than the enclosing phenocryst (Vernon
1977). Inclusion relationships in the Strathbogie rocks are shown in Table 6. It has further
been assnmed that crystals growing suspended
in a magma are likely to grow as euhedran
whereas during the later stages of magmatic
crystallization crystals will mutually interfere,
forming anhedral grains and interstitial fabrics.
In the Strathbogie rocks, quartz, plagioclase
and biotite are early-crystallizing phases that
form large euhedral crystals with few inclusions.
Apatite, rarecarth phosphates,zircono ilmenite
and pyrrhotite found included in the biotite
suggestthat these accessoryphasesalso crystallized early. Cordierite began crystallization before K-feldspar, which is commonly interstitial
and anhedral. Quartz and alkali feldspar domi-
CORDIERITE
TYPES
Fom and Paragenesls
l4gl(tlg+Fe)
Relatlve
0 r l gl n
Abundance
euhedra(< 6 m) wlth rare lncluslons
of blotlte, llmnlte and zircon
+ 60.
0.56
alrays present
primry mgmtlc
lafge anhedrawlth blotlte and/or
silllmnlte and greensplnel incluslons
- 80"
0.46
rare
readJusted
restlte
smll anhedraor, rore rarely, euhedral
ln aggregatesconprlsingreactlon 116
on Type A garnets
smll scattered anhedraderived by
dlssaggregatlon of Type Cl
'lncluslonsof
hlgh-grade
regioml mtnmrphlc rcck
rZV^
larle and smll anhedrawlth jnclusions
of blotlte and,/orsllllnanlte and
zincian hercynlt4
- 80'
- 80.
0.34
acconpanlesType A)
garners
]
)
of&n present
)
mgmtlc
)
ln all enclavesof readjustedrestlte
hlgh-graderegloml or xerclithlc
mtmrphic rcck
54
THE
CANADIAN
MINERALOGIST
AiIALYSES
TABLE5. I{INERALCHEfiTSTRYf:
RESIILTS
OF ELECTR0II-||ICR0PR0BE
splnel
Rocklto. 872
S.f0:
Tlo2
41203 57.32
Feor 32,44
nno
0.09
Zno
8.03
nso
z.LZ
Cao
Bao
n.d.
t{az0 n.d.
KuO
ct(A) ct(A) nF{*) .16l
846 1589.
846
0.21 37.08 37.46
0.06
56.64 21.48 21.03
30.47 33.56 33.64
0.42 2.80 2.00
9.86 n.d. n.d.
1.37 3.82 4.41
1,27 1.35
0.62 n.d.
O.42 0.05
u6
Bt
Br
et
7r.4 1589
754
831
35.43
4.75
17.86
23.48
0.09
n.d.
6.33
11.79
4.47
17.04
25.65
0.04
n.d.
5.02
34.m
4,41
18.14
24.14
0.11
n.d.
5.76
n,d.
0,42
8.94
n.d.
-
5.381
2.619
0.578
2.9s2
0.544
0.012
5.349
2.651
0.530
3.396
0.532
0.005
36.08 35.88
0.1,
20.68 20.69
35.19 35.76
5.33 3.55
n.d. n.d.
1.69 1.37
1.02 0.62
n.d, n.d,
8.72
or 1618}1rr
rrn"?Lron,f(A) cd(A) cd(A) cd(A) cd(B) "l:l*!;.*
1597.-
846-
35.26 n,d,
4.41 0.71
17.95 n.d.
23,t7 22.34
0.20
0.21
n.d.
n.d.
6.51
6.72
0.14
n.d.
n.d. n.d.
n.d.
0.09
n.d.
8.96 9.08
872
754
35.43
4.49
X6.58
24.45
-
49.n
872
831 1597 846 1599 ,846
49.2e 49.13 49.20 48.44 49.29 48.5
n,d.
7.sl
33.42
10.24
0.r3
n.d.
7.01
33.49
9.46
0.18
n.d.
7.61
n.d.
n.d.
n.d.
32.59
9.97
O.47
n.d.
6.a}
32.49
9.90
0.37
n.d.
6.58
0.04
n.d. n.d.
32.69
11.85
0.50
n.d.
5.90
9..27
13.09
0.75
n.d.
4.61
32.9
U.a
0.c)
n.d.
5.11
n.d.
n.d.
n.d.
9.66
88
Al(tv)
At(vl)
Fe
Tl
iln
Zn
ug
Ca
Ba
l{a
K
r.952
0,784
0.002
0.171
0.091
-
0.006 2.970 2.992
0.030 0.008
1.961 1.997 t.972
0,74A 2.245 2.248
0.004
0.011 0.190 0.135
0,2L4
0.060 0.455 0.525
0.109 0.116
2.945
0.055
L.934
2.402
-
2.939
0.051
1.936
2.449
0.007
0.368 0.385
5.ai2
2.668
0.635
3.119
0.512
0.014
5.375
2.625
0.601
2,954
0.523
0.027
0.040
0.206 0.167 1.433 1.184 x.326 1.502 1.718
0.090 0.054
0.027
1.732 L.76L L.766 t.766
l,lql(lilotFe)
0.10 O.o7 0.17 0.19
.total Fe expressedas Feo
5.404
2.596
0.915
3.n4
-
1.890
5.413 5.029 5.016 5.020 5.r08 4.987 5.125 5.017
0.013
2.587
0.400 4.020 4.015 4.U7 3.977 4.030 3.955 3.961
3.124 .874 .805 .852 .860 r.020 1.380 1.098
0.516
0.01r 0.016 0.041 0.033 0.044 0.066 0.044
I 596
1.066 1.X54 1.040 1.018 0.9M 0.714 0.880
1.883
0.34
0.55
0.35
0.08 0.06 0,33 0.26 0.30 0.34
S all analyses mmallzed to l00u ercept those of biotlte
0.59
0.55
0.54
'bW=='d
,
ffi,ffis
!::l.t
f:ild i"$
:iF
!#6ffi
ii,;,i
,
.: L:
.Tt*
,gs
:: !iE$ l
:l !:tirsi !:
airairi'\r
nii;,:
ll rnrn
0.47
0.34
0'45
PETROLOGY OF THE STRATHBOGIE BATHOLITH
EARLY
55
batholith, demonstratesour scheme of mineralogical evolution. This rock has a high phenocryst
content (5O7o) set in a fine grained groundmass. Relatively high concentrations of type-A
garnets and biotite crystals are present, and plagioclasephenocrystsare rather calcic (Anru-ro).
Cordierite occurs mostly in reaction rims on
garnets, although some euhedral phenocrystal
cordierite is also developed. K-feldspar phenocrysts are rare and the groundmass comprises
fine grained alkali feldspar and quartz. The
groundmass appears to represent chilled (pressure-quenched?)liquid. Thus K-feldspar did not
reach saturation in the 889 magma until the
emplacementstage, when this composition was
still more than SOVoliquid. The high concentration of garnet and biotite in 889 suggests
that these phases represent accumulations and
emphasizestheir importance in the differentiation of the Strathbogiemagma(s).
Inclttsiotts in the granitic rocks
Although generally scarce (- l%o) in the
tATE Strathbogie batholith, three types of inclusions
Ftc. 5. Schematic sequenceof crystallization for have been recognized. Micro-adamellite incluthe Strathbogiegranites.
sions are the most numerous. with those of
high-grade regional metamorphic rocks next in
nated the final stagesof magmatic crystallization. abundance. Near the margins of the batholith,
The role of garnet varies. The large, corroded the granitic rocks contain rare angular xenoliths
(type-A) garnets are apparently early and have of quartz-rich and pelitic hornfels. These acundergoneconsiderabletextural and some chem- cidental hornfels inclusions are derived from the
ical modification. The small, groundmass (type- surrounding country rock.
B) garnets are a late-crystallizing phase in the
The micro-adamellite inclusions are rounded.
porphyritic microgranites. The deduced sequence l0-lo0 mm in diameter, composedof large (5
of crystallization is portrayed in Figure 5.
mm) interlocking anhedra of quartz and orthoAlthough sequences of crystallization for clase, sieved with inclusions of plagioclase laths
granitic magmas based on the above criteria (Anrr) and small euhedral biotite flakes. Acare commonly equivocal, the grain shapes and cessory zircon, ilmenite, apatite and rare sorgrain-boundary relations of the Strathbogierocks dierite euhedra are also present. The microindicate little subsolidus readjustment. Further- adamellite inclusions are interpreted as accumumore, the crystallization sequence deduced for Iations of small, early-formed biotite, plagioclase
the Strathbogie magma closely matches that in- and cordierite crystals in interstitial liquid that
ferred for the related Violet Town volcanic suite crystallized as quartz and orthoclase.These rocks
(Clemenset al. 1979). Strathbogiespecimen889 probably represent a chilled phase of the granite
(Tables 2 and 7), the most mafic variant of the formed during the early stagesof crystallization
and later dislodged from the wall or roof of the
TABLE6. INCLUSION
RELATIONS
IN STRAIHBOGIE
!,IINERALS
magma chamber and reincorporated in the main
INCLUSION
PHASE
HOSTPHENOCRYSI
bulk of the magma (c1., Bateman & Chappell
Accessories Ct(A)
BicdPlQ
1979, pp. 474-475). The micro-adamelliteincluct(A)
sions resemble the Kerrisdale cordierite microBI
adamellitein both texture and composition.
x
cd(A)
The inclusions of high-grade regional metaPI
morphic rocks (10-100 mm in diameter) are
rounded in shape, fine- to medium-grained, and
q
always have a pronounced foliation (Fig. b).
Kfs
Biotite and cordierite are segregatedinto melano-
56
THE
TABLE7.
CANADIAN
BULK-RoCK
CHEMICAL
0ATA(XRF)At{DCIPHiloRtits
si02
69.78
72.2r
74.63
71.83
72.78
T102
0.57
0.34
0.09
0.36
0.33
0.30
41203
14.56
L4.@
13.66
14.25
14.0t
14.22
2.42
1.05
2.73
2,32
t.8t
o.o1
0.04
0.71
7!.&
Feo*
3.66,
ltno
0.01
ifgo
1.40
1.02
0.59
1.04
0.95
Cao
1.85
r.67
L.29
2,14
!,57
1.85
MzO
2.60
2.86
2.94
3.08
2.65
4,25
K20
4.74
4.50
4.91
3.93
4,52
4,69
P20S
0.19
0.22
0.22
0.28
0.19
0.09
sog
0.10
0.08
0.13
0.10
0.06
0.06
L o s s*
O.7l
0.47
0.61
0.50
0.54
0.33
Totn l
100.17
100.45
100.12
100.25
99.93
M9/(MgiFe)
.50
.40
.42
.41
34.12
23.46
28.47
32.1,2
2.24
2.57
0r
28.t9
26.60
29.20
23.29
26.72
27.72
Ab
22.17
24.20
24.96
26,L4
22,42
35.96
An
7.99
6.85
4.96
8.84
6.55
5.88
tn
1,.7L
?1 A7
r,70
-
DI
Inclusions of country-rock hornfelses are recognizableby their modal and mineralogicalsimilarity to the Paleozoic rocks in the contact
aureole (Phillips & Wall 1980). In contrast to
the regional metamorphic inclusions, the hornfels inclusions lack well-developed foliations.
The paucity of hornfels xenoliths and their restriction to marginal variants in the Strathbogie
batholith imply that high-level cohtamination is
of little importance in Strathbogie petrogenesis
and that stoping of wall rocks is not a major
factor in the emplacement of the Strathbogie
magmas.
GEocHEMrsrRY
ee.es Mineral chemistry
a
c
la
MINERALOGIST
2.28
Hy
9.32
6.42
3.25
7.03
6.10
3.52
I1
1.08
0.65
0,r7
0.68
0.63
0.57
Ap
0.45
0.52
0.52
0.66
0.45
0.21
* Total Fe expressedas Feo. * Fusion loss. l. Speciren889, the
mst mflc variant of the Strathbogie granite; 2. specjren 717,
representativecoarse gralned granite, Strathboglebathollth; 3.
speciren 754, representatlve porphyritic nicrogranlte, strathbogie
bathollth; 4. speciren 765, Ksrisdale cordierlte nlcrcadarellite;
5. averageof 2l S-type Strathbogie granltoid rocks; 6. averageof
2 analysesof Tallangallook hornblendeadarelllte.
cratic layers separatedby 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 Appendix l.l
Descriptions of the garnet and cordierite types
from these inclusions are given in Tables 3 and
4. Garnet is very rare, although at one time, it
was probably present in all the regional metamorphic inclusions, as evidenced by pseudomorphs of garnet by minerals of assemblage
(b) above. Readjustment of the regional metamorphic mineral assemblage by equilibration
with the enclosing granitic magma has occurred,
as indicated by the low-pressure assemblages.
The alkali-poor, silica-depleted character of
these inclusions is compatible with a restite origin, although their low-pressure mineralogy
precludes estimation of source-regionconditions.
Ferromagnesianphasesand plagioclase in the
porphyritic microgranites and coarse grained
granites were analyzed using the T.P.D. energydispersive electron microprobe at the Research
School of Earth Sciences. Australian National
University. Some analyses were performed on
the Jeol JXA-SA and the ARL EMX microprobes at the Departments of Geology, Melbourne and Monash universities, respectively.
The compositionsof the various garnet and cordierite types are of particular interest.
All Strathbogie garnets are almandine-rich
pyralspites (almandine: 74-80 mol. %). The
Mg and Mn contents show the greatest variation (Table 3). Garnet in the fine grained
groundmass, free of reaction coronas (i.e., type
B), is relatively spessartine-enriched,with only
minor . pyrope and grossular (e,g., specimen
754). Large garnets with reaction rims (type
A) are relatively enriched in pyrope and grosstrlar, with only 4.57o spessartine.Rims of typeA garnetsare Fe-Mn-enriched and Mg-depleted.
The reason for the Mn enrichment is the strong
partitioning of this element into the garnet relative to the cordierite in the reaction rim.
Euhedral (type-A) cordierites generally have
Mgl(Mg
+ Fe) in the range 0.54-0.59.
(Throughout this paper Fe and FeO refer to
total iron in the sample. Structural formulae of
the ferromagnesian phasesindicate that Fe3+ is
minor in most minerals.) The chemistry and
optical properties of type-A and type-B cordierites are quite different (Table 4). Type-B
cordierites and cordierites in gneissic inclusions
are more Fe-rich [average Mg/(Mg + Fe) 0.461 and contain more Mn than type-A cordierites. Type-C cordierites are similar in optical
properties to type B but have the lowest Mg,/
PETROLOGY OF THE STRATHBOGIE BATHOLITH
57
(Mg * Fe) (average- O.34) of the cordierite Table 7. Most of the batholith comprises coruntypes. The significance of these points will be dum-normative (usually ) l%o C) S-type grandiscussedin detail in the section on petrogenesis. ites. whereas less than O.lVo of. the batholith
Biotites in the Strathbogiegranites do not vary consists of the diopside-normative Tallangallook
greatly in composition; they are all rather high hornblende microadamellite (I-type).
in siderophyllite-annite, with an average comCompositions of all S-type granitoid rocks
position (K,Na) r.e(Fes.rMgr.rTio.sAle.6)
(Alr.oSis.r) are highly silicic (70-76% SiOr) and moderOuo(OH,Cl,O,F). Semiquantitativecrystal-spec- ately periluminous. All have rather high K/(K
trometer analysesshowed that fluor- and chlor- + Na + Ca) ratios, reflected in the high proine-biotite substitutions are minor ( < 5 mol. portion of modal alkali feldspar (Table 2).
'To). Mg/(Me * Fe) is near 0.30, falling The ratio Mg/(Mg
* Fe) is generally in the
between the values for type-A cordierite and range 0.4G-0.50 and shows no distinct trend
garnet and slightly lower than the bulk-rock with SiO: content. Two exceptionally Fe-rich
values (Table 7). The biotite in the cordierite- rocks [Mg/(MS * Fe) = 0.31 and 0.191have
biotite reaction rims surrounding type-A garnet significantly higher MnO and lower MgO and
is similar in composition to other biotites ex- CaO.
cept for its very low Ti content. Normalization
Harker plots show strong correlations between
of the Ti-rich (primary, magmatic) biotites to several oxides and SiOs content (Fig. 6). The
16 cations, assuming no octahedral vacancies more silicic variants are markedly depleted in
(Bohlen et al. 198O), suggestsextensiveTiOz P,O', TiOg, CaO, FeO and MgO. From
- (MgFe)(OH)z substitution,a featurein com- 7O7o to 76%ioSiOt there is a 3OVo decreasein
mon with other high-temperature biotites PrOs and about a 70Vo decreasein the other
(Bohlen et al. l98O).
four oxides. No significant change in AlzO:r or
Green spinel (Tables 4, 5) contains consider- K/Na is observedwith increasing SiOr, although
able amounts of the hercynite and gahnite com- a large scatter of molar K/Na ratios (0.7-1.4)
ponents in solid solution: hercynite 72/o, gah- is present.
nite 2lVo. Some Mg and Mn are also present,
Porphyritic microgranites are commonly more
but the magnetite component comprises ( 2 silicic (average 73.4Vo SiO:) than the coarse
mol. Vo. Similar zincian hercynites have been grainedtypes(average72.3%SiOa).Spatialchemreported in inclusions in acid volcanic rocks of ical variations within the batholith are also
the Cerberean cauldron and Violet Town vol- evident. The most silicic rocks are the porphycanic suite in Central Victoria (Birch et al. ritic microgranites from the northwest (average
1977't.
74.6Vo SiOr). Coarse grained granites from the
east have much lower SiO: (average 71.4Vo)
Rock geochemistry
and have a different average composition from
Selectedbulk-rock analyses(XRF) and CIPW the coarse grained granites in the west (average
norms of the Strathbogie rocks are presented in 73.O% SiO).
2.4
2.O
0.6
o.4
CaO
0.8
o.4
o.2
0.1
1.6
1.2
0.8
0 ' 6r
'r--l1-l rugfirvdF"r
o.4
70
71. 72
73
SlO2 wt %
74
75
76
69
70
71
72,
73
74
75
76
SiO2 wt %
Ftc. 6. Harker variation diagrams for 23 analyzed specimens of Strathbogie granite. Large dots are the
two analyzed l-type rocks.
58
THE
CANADIAN
Although the Strathbogie I-type micro-adamellites are not well distinguished on the Harker
diagrams of Figure 6, their higher NarO/KzO
and lower (FeO * MgO) are distinctive. The
I-type granitoid rocks are diopside-normative.
PrrnoceNesls
Cottditions of emplacement
Pressure:Low confining pressureduring emplacement of the exposed granites is inferred from
two lines of evidence. The granites intrude the
Violet Town volcanic complex, which appears
never to have had any significant cover (Birch
et al. 1977). Hornfels developed on the south
side of the batholith contains assemblagesindicative of pressures€ | khar (Phillips & Wall
r980).
Temperature' ancl HzO content of melt: E,mplacement temperatures san be limited by assemblagesdevelopedin the contact aureole. The
high-grade cordierite-K-feldspar assemblages
in hornfels adjacent to the granite imply temperaturesof the order 580-620'C (Phillips & Wall 1980). Using the methods of Jaeger
(1957), this is equivalent to intrusion temperatures of 750oC (above the wet granite solidus)
to 82O'C, if the sountry rocks are assumed to
have been at temperatures between 100 and
200oC (Phillips & Wall 1980). Emplacement
temperatures can be further limited by utilizing
the crystal-melt equilibria determined experimentally by Clemens& Wall (1981).
These experimental studies of the crystallization phase relations of the porphyritic microgranite (specimen 889) were carried out at P
- 11 kbar, T = 600-900oC, a(HzO) = 0.11.0 and log l(O,) - QFM to QFM-0.5. This
rock was chosen for experimental study because of its well-characterized sequence of
crystallization and its apparently little-fractionated composition. However, it now appears to
us that specimen 889 may have undergone enrichment in phenocrysts of garnet and biotite,
although this would have little effect on the
conclusions we reach below.
Specimen 889 was inferred (above) to have
been at least 5OVo liquid with few if any Kfeldspar crystals present when the magma was
intruded. Experiments at I kbar (for near-saturation H:O contents in the melt of 3.5-4.5 wt.
7o) show that the K-feldspar saturation boundary occurs at about 750'C. This suggeststhat
the emplacement temp€rature was at least
MINERALOGIST
750oC. The absence of orthopyroxene (or obvious pseudomorphs after this phase) indicates
that the Strathbogie magmas were at temtr'eratures below about 8O0oC at the level of emplacement(Clemens& Wall 1981, Fig. l).
Oxygen fugacity: The occurrencesof coexisting
ilmenite and pyrrhotite as inclusions in the
Strathbogie type-A garnets, the absenceof magnetite and the magmatic almandine-rich garnets
all suggest that l(Or) was low in the Strathbogie magmas, probably around. QFM. Oxygen
fugacity in S'type rocks cannot be calculated
r.rsing the Buddington & Lindsley (1964) approach owing to the absence of coexisting FeTi oxides.
Froese (1973) has tabulated thermochemical
data for the reaction Alm * Oe = Cd * Mt
* Q, which provides an upp€r limit for l(Or)
(since magnetite is absent from the Strathbogie
rocks). This yields. at P = 2 kbar, log l(Or)
< -16.8 at 700'C and ( -14.6 at 80O'C.
The low-pressure assemblagespinel-cd-sill is
common as pseudomorphs, often around metamorphic assemblagesin some inclusions in the
Strathbogie batholith. These phases can be related by the reaction Cd + O, - Mt + Sill
* Q. Assuming that these inclusions equilibrated with the enclosing magma at around 2
kbar, this assemblagecan be used to evaluate
l(Ou) from the data of Froese (1973). Cal'
culations using the analyzed cordierite and coexisting spinel compositions, and assuming
a(SiO') = 1.0, yield log l(O,) - -18.3 at
700'C and -15.5 at 8@oC. These figures are
compatible with the upper limits for l(Oz) set
above. The mole fraction of magnetite in the
analyzed spinel is small, and this constitutes
the major source of error in the calculation.
However, an uncertainty of 3OVoin ar."nro'rcorrespondsto a changein log l(Or) of only O.3.
Thus a reasonable estimate of l(Or) for the
Strathbogie magma (at the time of equilibration
of the cordierite-spinel-sillimanite assemblages)
is abont an order of magnitude below the QFM
buffer.
Early crystallization
Experimentsby Clemens& Wall (1981) also
give an indication of the ranges of P, T and
HzO content during early crystallization of the
Strathbogie magmas. The sequence of early
crystallization for rock 889 is Desl modeled by
the experimentallly determined phase relations
for P between 4 and 7 kbar, T between 800
and 850'C and water contents of the melt in
PETROLOGY OF THE STRATHBOCIE BATHOLITH
the range 3-5 wt. Vo. For HzO contents less
than 3 wt. Vo, the biotite saturation boundary
is depressedto temperatures below the K-feldspar saturation boundary. For water contents
greater than SVo, the garnet saturation boundary is depressed to such an amount that the
inferred sequence of crystallization cannot be
duplicatedwith garnet as a nearJiquidus phase.
Summary: conditions of crystallizatiotr
The Strathbogie granite magma, prior to emplacement, was of relatively high temperature
(750-850oC) and markedly water-undersaturated. Water saturation of the magma was
attained in the 11
kbar range, with
less than 5O7o crystallization; by this stagesome
garnet, plagioclase, biotite and quartz had developed. Cordierite and K-feldspar formed
Iargely at emplacementlevels.
Origitt of chemical variation in the Strathhogie
batholith
Several mechanisms have been proposed to
explain the chemical variation observed in the
S-type batholiths (e.g.. White & Chippell 1977.
Clemens et al. 1979). These mechanismsand
their relevance to the Strathbosie batholith are
examined below.
"Unmixing" ol restite materiill: Chemical trends
in granites have been explained by a processof
progressive "unmixing" of restite (refractory
phases) from a "minimum-melt" granitic liquid
(Wyllie et al. 1976,Wyllie 1977,White & Chappell 1977, Chappell 1978). This process requires that the undifferentiated magma leave its
place of origin carrying some residual solid
material. For the Strathbogie rocks, possible
restite phases include biotite, garnet, orthopyroxene, quartz and plagioclase. Progressive
removal of restite plagioclaseand biotite during
magma migration is essentiallythe same prosess
described below in the crystal fractionation
model, except that the phasesinvolved are considered to be restite material rather than products of niagmatic crystallization.
Within the Strathbogie batholith there is no
firm evidence for calcic plagioclase of restite
origin (c1., White & Chappell 1977) or aggregates of restite biotite. The euhedral single
crystals of biotite, in particular, are unlike the
biotite segregations in migmatites (Harme
1965). Inclusions of any kind are rare and not
markedly concentrated in the more mafic variants. The textural evidence has shown that the
59
bulk of the cordierite and garnet in the Strathbogie batholith is magmatic in origin. Cordierite
with inclusions of spinel and sillimanite is easily
recognized (texturally and chemically) as readjusted restite or xenocryst material, but comprises less than l7o of the granite. In such
quantities, restite cordierite cannot have played
a major part in the chemical evoltttion. Restite
unmixing thereforeseemsunlikely to accountfor
any significant chemical variation seen in the
Strathbogie granite.
Crystal fractionation: The petrographic data for
the Strathbogie rocks indicate that biotite,
plagioclase and quartz are the volumetrically
significant early-crvstallizing phases. Numerous
ilmenite and apatite inclttsions are present in
the biotite. Based on the modal data, large
amounts of cordierite and garnet appear unlikely at this staqe.
Oualitatively.fractionation of plagioclaseand
biotite with inclusions can explain the desrease
in TiOg. P,O'. FeO. MgO and CaO as SiOr
increases(Fie. 6). Quantitative mixinq calculations using rock and mineral compositionswere
performed with the least-squarescomputer program PETMIX III (Taylor et al. 1973).'The
Strathbogie primary magma was assumed to
have a composition similar to the average of
the 2l analyzedS-typerocks (SiO, - 73'ttt. 7o,
Table 7). This composition was chosen as an
approximationto the overall compositionof the
magma near its level of emplacement. More
mafic variants probably contain accumulations
of early-formed phenocrysts,as discttssedabove.
Separationof 8% plagioclase(An,u), 67o bio'
tite, 3Eo quartz and O.7Voeach of ilmenite and
apatite from the primary magma composition
can account for the chemistry of the most felsic
(SiO' = 76 wt. % ) Strathbogie granites.
Given the Fe-rich biotite compositions in the
rocks (Table 5), problems arise in explaining the
large decrease in MgO with increased SiOr
content.The liquidus biotite, however,was probably more Mg-rich, but re-equilibration with
the magma at lower temperatures has cattsed
a drop in the ratio Mg/(Mg * Fe). As cordierite and garnet have substantial effects on
AleOs, FeO and MgO contents, the role of these
phases has been minor. The lack of variation
in MnO also suggests that garnet was not a
major participant in the fractionation processes.
The importance of fractionation processesin
the chemical evolution of the Strathbogie magmas is further supported by the distribution of
tourmaline in the batholith. Tourmaline, and
hence boron. are enriched in the leucocratic
60
THE
CANADIAN
(high-SiOr) variants by at least an order of
magnitude relative to the bulk of the batholith.
Simple removal of even relatively large proportions of restite material could not have produced sush strong enrichment in incompatible
elementssuch as boron. Ilowever, liquids mechanically separated from univariant or invariant
crystalline mushes may exhibit exponential increases in incompatible elements, whereas the
major-element contents of the liquids may not
vary much. Extreme fractionation processes
must therefore have been involved in the development of the highly silicic variants.
Fractional partial melting: Progressive partial
melting of a particular region of crustal material
leads to the production of successively more
mafic, less silicic magmas. Thus, removal and
emplacement .of successivemelt fractions may
lead to a compositionally zoned or composite
batholith. Ideally, unless the site of melting
shifts, this process produces a granitic suite in
which the younger plutons are the more mafic.
Melting and crystallization experiments on
granitic compositions (Winkler 1974, Winkler
et al. 1975,Wyllie 1977, Clemens& Wall 1981)
provide some idea of the compositionsexpected
in a fractional partial melting sequence.Near the
solidus, liquids are rich in K-feldspar, plagioclase and quartz components and are Fe-MgTi poor. With increasing temperature, Fe-Mg
phases become more soluble in the melt, and
initially low Mg/ (Mg * Fe) ratios are expected.
Only at high temperaturesdo more mafic melts,
with ahigher Mg/(MB f Fe) ratio, form. Since
garnet crystallization is favored by higher Fe,
and cordierite crystallization by higher Mg,
such trends would lead to more garnet in the
silicic and more cordierite in the more mafic
variants. The opposite trend is observed in the
Strathbogie batholith. The separate fractions of
partial melt should crystallize as plutons in
which the mafic minerals have differing Mg/
(Mg + Fe) valuescharacteristicof each pluton.
The major variants of the Strathbogie batholith
all have the same suite of early magmatic
phases.
Suntmary
Although re-equilibrated restite materials
exist in the Strathbogie batholith, their low concentration, even in the mafic granites, suggests
that restite unmixing was not a major process
involved in the magmatic evolution. Fractional
partial melting would produce chemical trends
that run counter to those observed. Thus. to.
explain the variation in the Strathbogie batholith, we prefer a differentiation mechanism involving separation of early crystallized biotite
and 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) indicate
that initial 8?5r/865risotopic ratios of eastern
Anstralian S-type granites and volcanic rocks
have a mean value in excess of 0.710. These
high 875r/865rratios, together with the peraluminous nature of the granitoid suite, suggest
that magma formation involved partial melting
of old metasedimentaryrocks (Chappell & White
1974"White & Chappell 1977'). As pointed out
by Flood & Shaw (1977), S-type rocks with
lower initial ratios (around 0.706) suggestthe
involvement of vottnger or less-evolved crustal
sources for the magmas. Experiments carried
otrt bv Green (1976. 1977). and Kilinc (1969.
1972\ show that liquids formed by partial
melting of sillimanite-bearingpelitic materials
must crvstallize peraluminous minerals' Green's
inferred granitic liquids have, in general, more
normative corundum than the Strathbogie granites (Table 7). Experimentalresults reported bv
Clemens & Wall (1981) for a natural granitic
composition from the Strathbogie batholith
(specimen 889) seededwith sillimanite show
rhat a liquid of this composition is too poor
in alumina for equilibrium with sillimanite.This
suggests that sillimanite-bearing pelitic rocks
are unlikely to be the dominant litholoey in the
source region of the Strathbogie magma. A muscovite-bearingsollrce region is ruled out bv the
high temperaturesand torv water contents inferred for the magma.
The most direct evidence concerning the
nature of the source region of the gr4nitic magmas comes from the granites themselves.Apart
from hiehJevel hornfels and the magmatic micro-adamellite, the most common inclusions
encountered in Strathbogie granites are hlghgrade pelitic gneisses.Provided that these inclusions represent either source-region material
(Chappell & White 1974) or readjusted restite
material from the source region, these inclusions
indicate some of the probable assemblagesin
the source area during the extraction of melt.
In the Strathbogie batholith the rare gneissicinchrsionscontain quartz, biotite, sillimanite"plagioclase, minor orthoclase, cordierite and hercynite. and rare garnet. The cordierite-hercynite
PETROLOGY OF THE STRATHBOGIE BATHOLITH
6l
assemblagerepresents a low-pressure readjust- postulate an anatectic source-region, characterment of an original garnet-sillimanite assem- ized by quartzofeldspathic to p"l-itic,'high-grade
blage by reactions similar to
regional metamorphit rocks. Such rocks-are not
presently exposed in central Victoria. The min:
FeaAlzSi.rOrz
2Al2SiO6
FeAlsO+
'r8
+
* FezAlaSioo.eralogy of these source rocks would most likely
Alm
Sill
Hc
Cd
involve quartz, orthoclase, plagioclase, almanIndeed, sorne cordierite-hercynite aggregatesap- dine-rich garnet, Fe-rich biotite and orthopypear to pseudomorphoriginal idioblastic garnets. roxene. Rocks containing sillimanite, cordierite
Accessory phases include ilmenite, pyrrhotite, and hercynite may also have been present but
apatite and zircon. Thus, the mineralogy of in minor proportions. Accessoriesin the source
these inclusions suggeststhat some of the source rocks would include ilmenite, pyrrhotite, aparocks for the Strathbogie magma contained as- tite and zircon. Magnetite and hornblende are
semblagesinvolving combinations of plagioclase, unlikely. Although the actual mineral assemquartz, garnet, K-feldspar, biotite and silliman- blages are unknown, this range of minerite along with ilmenite and pyrrhotite.
alogy, the relatively high temperatures and the
calculations based on reac- HrO-undersaturated nature of the granitic melts
- Thermodynamic
tions An
Gr + Sill + Q (Ghent 1976) and produced are consistentwith granulite-faciesreAn * Fs - Gr + Alm * Q (Phillips 1978) gional metamorphic conditions in the source
indicate that garnet with a composition similar region. Consideiing ( I ) the high geothermal
to the early, type-A garnetsfrom the Strathbogie gradient required to generate the high temperagranites could have been in equilibrium with tures at the moderate crustal depths proposed
plagioclase(- An'o) - sillimanite - quartz or pla- for the source region and (2) the transiiional
gioclase(Anro) - orthopyroxene(Enso) - quartz orogenic-anorogenic setting of the Strathbogie
at n 5 kbar and 800"C.'Any pyroxene formerly batholith, we suggest that this granulite-facies
present in the magma or its restite inclusions partial-fusion event was brought about by emhas reacted out and is no longer found in the placement of mantle-derived material in the
assemblages.
lower crust (c1., Wells 1980).
The l(O,) during early crystallization of the
Srathbogie magma was calculated to be about
AcKNowLEDGEMENTS
an order of magnitude below the QFM buffer.
Together with the inferred low oxidation ratio
We are grateful for suggestionsand criticisms
of the magma, this suggestsanatexis of highly made by I.A. Nicholls, A.J.R. White. W.D.
reduced source rocks that may have contained Birch. and D.B. Clarke. R.H. Flood and R.A.
graphite during part pf their nretamorphic Binns reviewed earlier manuscripts. A.R. Milnes
history.
gave valuable advice during the bulk-rock analPartial melting of a metapelitic rock ysis. Ms. G.E. Earl drafted the diagrams.
will commonly yield K-feldspar-bearing restites
owing to the high K/(Na * Ca) rarios of these
RrrenENcrs
compositions. Thus, magmas derived from such
highly aluminous source rocks will usually be BarrvlN, P.C. & CHeppELL,
B.W. (1979): Crystalsaturated with K-feldspar all the way from the
lization, fractionation, and solidification of the
source region (Clemens & Wall l98l). HowTuolumne intrusive series. Yosemite National
ever, K-feldspar crystallizes late from the SrathPark. California, Geol, Soc, Amer. Bull. 90.
465-482.
bogie magmas and plagioclaseis a near-liquidus
phase. We would therefore infer that the bulk Btncs,
W.D., Crn*rNs, J.D. & Pmr,rrps, G.N.
of the Strathbogie source region was under(1977): Dev<inian acid igneous complexes in
saturated with respect to K-feldspar at the time
central Victoria. SecondAust. Geol. Conv. Exof magma extraction. Appropriate source rocks
cursion Guide C.
for the Stratbbogie magmas would include
& GLEADow, AJ.W. (1974): The genesis
weakly peraluminous metasedimentary rocks or
of garnet and cordierite in acid volcanic rocks:
other quartzofeldspathic rocks poor in K-feldEvidence from the Cerberean cauldon. central
spar. Pelitic inclusions in the Strathbogie granVictoria, Australia. Cont. Mineral. Petrolosv
ites thus probably represent highly refractory,
45. t-13.
melt-depleted materials not typical of the bulk BoHLEN,
S.R., Peacon, D.R. & EssrNb, EJ. (1980):
composition of the source region of the magma.
Crystal chemistry of a metamorphic biotite and
In snmmary, for the Strathbogie and other
its significance in water barometry. Amer. Mincenftal Victorian S-type granitic magmas, we
eral. 65, 55-62.
62
THE
CANADIAN
Bnoors, C. & Lrcco, M.D. (1973): The local chronology and regional implications of a Rb-Sr investigation of granitic rocks from the Corryong
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Bur. Mineral Rer. Rec. L979/2, 8-9 (abstr.).
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MINERALOGIST
KILtNc, I.A. (1969) : Experimental Metantorphism
and Anatexis of Shales and Graywac&es. Ph.D.
thesis, Pennsylvania State Univ., University Park,
Pa.
(1972): Experimental study of partial melting 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 Upper
Devonian rocks from Victoria, Australia: a revised estimate for the age of the DevonianCarboniferous boundary. Geol. Soc. Amer. Bull.
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O'CoNNoR,J.T. ( 1965): A classificationfor quartzrich igneous rocks based on feldspar ratios' U.^t.
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Purt-t-rps,G.N. (1978): Metutnorphisnt and Geoc'lretnistryof the Willyanru Contplex, Broken Hill.
Ph.D. thesis. .Monash Univ., Clayton, Australia.
& Wer-1, V.J. (1980): The geologY and
metamorphism of the Bonnie Doon area, Victoria. Roy. Soc. Yict. Proc.9L, 33-42.
R.T. & CovrpstoN, W. (1965): The age
PTDGEoN,
and origin of the Cooma granite and its associated metamorphic zones, New South Wales.
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(1981): Origin and crystalliza& granitic
tion of some peraluminous ($type)
magmas. Can. Mfueral. 19, 111-131.
Srrr.crr,rsrN, A.L. (1973): Plutonic rocks: classiFrooo, R.H. & Snew, S.E. (1975): A cordieritefication and nomenclature recommended by the
bearing granite suite from the New England baIUGS Subcommission on the Systematicsof lgnetholith, N.S.W., Australia. Contr. Mineral. Peous Rocks. Geotimes 18(10), 26-30.
trology 52, 151-164.
TelrNr, J.A. (1965): The stratigraphic and dia(1977): Two ..S-type,l granite
& strophic evolution of central and eastern Victoria
8?Sr/80Sr
ratios from the
suites with low initial
in middle Palaeozoictimes. Roy. Soc. Vict. Proc.
New England Batholith, Aushalia. Contr. Min79,197-t95.
eral. Petrology 6t, 163-173.
TAyLoR, S.R.. Gonror*, M.P., Murn, P., N"wcr,
FRoEsE, E. (1973): The oxidation of almandine
W.B." RunowsKl, R. & Wenr, N. (1973): Comand iron cordierite. Can, Mineral, 11, 991-1002.
position of the Descartes region, Lunar Highlands.
Geochim. Cosmochim. Acta 37, 2665-2683.
Gnrxr, E.D. (1976): Plagioclase-garnet-Al2Si0rquartz: a potential geobarometer-geothermoA.H.M. & Gennerr, MJ. (1976):
VANDENBERc,
meter. Amer. Mineral. 6L, 710-714.
The M.elbourne Trough. In Geology of Victoria
(J.G. Doucr.ns & J.A. FrncusoN, eds.) Geol.
Glners, T.H. (1976): Experimental generation of
cordierite- or garnet-bearing granitic liquids from
Soc. Atrst. Spec. Publ. E, 45-62.
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VrnNoN, R.H. (1977): Relationships between mi(l97Xl: Garnet in silicic liquids and its
crostructures and metamorphic assemblages,Tecpossible use as a P-T indicator, Contr. Mineral.
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(1965):
Henue, M.
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63
PETROLOGY OF THE STRATHBOCIE BATHOLITH
WHITE, D.A. (1954): The geology of the Strathbogie igneous complex, Victoria, Australia. Roy.
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a
Kfs
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oF ABBREVTATToNSUSED
MrueRALs
Or
PI
An
Musc
Bi
Gt
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(1970):
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X(H29;'at
Petrolotrly 26, 225-246.
FI
QFIUI
Received October 1980, revised manuscript accepted
December 1980.
l.
aQ)
Liq
qtJartz
potassium feldspar
orthoclase
plagioclase
anortlite
muscovite
biotite
garnet
almandine
pitrope
spessartine
Gr
En
Fs
C
Cd
Sill
Hc
Mt
Ilrn
Ap
Po
grossular
enstatite
ferrosilite
corundum
cordierite
sillimanite
hercynite
magnetite
ilrnenite
apatite
pyrrhotite
Pnvsrcocuprutcer.Syrrnors
pressurein kilobars
temperaturein degreesCelsius
logrooxYSenfugacity in bars
lrater content of melt
supercritical fluid phase
oxygen fugacity equivalent to that ol
tlre qtartz-fayalite-magnetite buffer
activity of species
i
magmaticsilicateliquid