JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 3
PAGES 449-170
1996
B. W. CHAPPELL
KEY CENTRE FOR THE GEOCHEMISTRY AND METALLOGENY OF CONTINENTS (GEMOC), DEPARTMENT OF GEOLOGY, THE AUSTRALIAN
NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
Magma Mixing and the Production of
Compositional Variation within Granite
Suites: Evidence from the Granites of
Southeastern Australia
Granite suites are groups of plutons possessing characteristic or a limited range in composition relative to the
features that are a result of their derivation from source material compositions of the source rocks of other suites.
of a specific composition. Variation within suites has been Differences between suites are a result of their genascribed to a variety of processes. Magma mixing or mingling is eration from source materials of different composia popular hypothesis, generally proposed in terms of blending tions. Variation within suites has been ascribed to
between a crustal melt and mafic materialfrom the mantle that several processes such as magma mixing or mingling,
caused that melting. When the compositions of pain of suites assimilation, fractional
crystallization, varying
from the Bega Batholith of southeastern Australia are com- degrees of separation of unmelted source material or
pared, any differences seen at either end of the range in compo- restite from a melt, and hydrothcrmal alteration,
sition are also seen at the other limit, so that both the most mafic and various combinations of those processes.
and mostfelsic rocks show similar relative abundances of parAn examination of recent publications indicates
ticular elements. Similar relationships are seen for other granites that for all but the most felsic granites, magma
in the region. These observations are not consistent with large- mixing or mingling is the mechanism most frescale magma mixing or mingling and, although those processes quently invoked to account for variation within
may operate on a small scale, they cannot have been responsible granite suites. For example, Wall et al. (1987) noted
for the major compositional variations. Likewise, assimilation that 'For some metaluminous granites particularly,
of country rocks had no significant role in producing variation in magma mixing or mingling seems to have been
the granites of southeastern Australia. The production of varia- important'. Cantagrel et al. (1984) stated that
tion by differential separation of melt from residual solid source 'magma mixing appears to be a frequent but not
material, or restite, must be favoured for many of the granite exclusive mechanism in the petrogenesis of granisuites of this region.
toids'.
The arguments used in support of magma mixing
below and have also
Pitcher (1993). This
paper presents evidence that bears on the question of
large-scale magma mixing or mingling, from some of
the granites of the mid-Palaeozoic Lachlan Fold Belt
INTRODUCTION
Individual bodies of granite that share similar pet- (LFB) and the late-Palaeozoic New England Bathorographic and compositional features can be grouped lith (NEB) of southeastern Australia. That evidence
into suites (Griffin et al., 1978; Hine et al., 1978; can be used to argue unequivocally against such
White & Chappell, 1983; White, 1995). Suites were mixing or mingling having been a significant process
derived from source rocks of a specific composition in the genesis of those granites, although mingling
KEY WORDS: assimilation; enclaves; granite suites; magma mixing;
or mingling are summarized
restite
recently been examined by
I Oxford University Preu 1996
JOURNAL OF PETROLOGY
VOLUME 37
took place on a small scale. The evidence presented
here shows conclusively that neither mixing nor
mingling occurred significantly on the scale required
to produce the overall patterns of chemical variation
within suites of these granites.
In a separate paper, fractional crystallization, as
an alternative model for the generation of variation
within granite suites of the LFB, will also be
examined. It can be shown that such a process
cannot have operated in producing most of the range
of compositional variation within suites of the Bega
Batholith, and except for the distinctive Boggy Plain
Supersuite (Wyborn et al., 1987) and a few other
very felsic granites, did not have a significant role in
I-type granite suites of the LFB. Likewise, fractional
crystallization was unimportant in causing variation
among the more mafic S-type granites, and also in
some S-type suites as they became more felsic. It
was, however, a more common mechanism in producing progressively more fractionated felsic S-type
granites than was the case for the I-type granites.
Unlike the processes of magma mixing and fractional crystallization, variation resulting from
separation of restite, the restite model of White &
Ghappell (1977) and Chappell et al. (1987), is consistent with the general patterns of chemical variation and other features of the bulk of the granites of
the LFB. It is therefore concluded that restite fractionation was the dominant mechanism that produced variation within the granite suites of
southeastern Australia, with an important supporting role being provided by fractional crystallization.
EVIDENCE CITED FOR
MAGMA MIXING OR
MINGLING
The term mixing refers to the complete combination
of two components such as two liquids, or, as Vernon
(1983) has thoughtfully expressed, the homogenization of melt phases and the conversion of any
pre-existing crystals to minerals stable in the hybrid
melt, or their armouring by stable minerals. Mingling is a combination of two components where they
retain some of their identity, such as the case of
basalt mingling with a granite magma to produce
mafic enclaves. However, the term mixing has frequently been used in an inclusive way in the literature. For example, the papers of Reid et al. (1983)
and Whalen & Currie (1984) have the term 'mixing'
in their titles but are concerned with mingling.
Didier (1984) wrote of microgranular enclaves and
their gTanitic host being the products of 'imperfect
450
NUMBER 3
JUNE 1996
mixing' in the magma mixing hypothesis. Mixing
has also been used in cases where the evidence, e.g.
chemical or isotopic, is not of a type that would discriminate between that process and mingling (e.g.
Arth et al., 1986). In this paper, in accord with such
general usage, the term mixing is generally used to
include all processes by which two components could
be combined together to produce variation within a
granite suite. This is done because the chemical data
that are used here cannot by their nature discriminate between the two processes. Where other
types of data that can make such a distinction are
available, it would seem worth while to distinguish
between magma mixing sensu stricto and magma
mingling.
Magma mixing or mingling is an attractive
hypothesis in explaining some features of granites,
and a variety of observations have been cited as evidence for such a process operating in the production
of granites. Some of these are summarized in the
following discussion.
Volcanic and subvolcanic rocks
Blake et al. (1965) discussed the occurrence of composite lava flows produced by the simultaneous
eruption of felsic and more mafic magmas at many
volcanic centres. They also pointed out that composite minor intrusions are characteristic of the Tertiary Hebridean Province and noted the possibility
that many of the mafic enclaves in granites may
have formed from liquid basalt. Sparks et al. (1977)
noted that tephra comprising an intimate mixture of
two or more contrasting compositions of magma are
a common feature of felsic pyroclastic deposits, again
pointing to the presence of liquids of two compositions occurring together, but not necessarily mixing.
Eichelberger (1978) argued that enclaves in andesite
and dacite are the result of magma mixing. Cantagrel et al. (1984), having examined evidence for
mixing in rocks from the Sancy Volcano, went on to
argue that plutonic rocks frequently show evidence
for the operation of similar processes. Bacon (1986)
discussed in detail the fine-grained enclaves that are
present in many intermediate to silicic lavas and
showed that they represent chilled blobs of magma.
He noted that the majority of mafic enclaves in
granites are interpreted by many workers in that
same way. Seaman et al. (1995) have ascribed flow
banding in volcanic rocks of the Aliso lava dome to
multistage magma mingling and have noted that
mingling of magmas in volcanic settings involves
many of the same characteristics as those described
in plutonic rocks.
CHAPPELL
GRANITE SUITE MAGMA MIXING
Mafic enclaves
There are innumerable references to mafic or microgranular enclaves in the literature, with a general
consensus that they represent the product of
quenching of basaltic liquid by an I-type granite
magma. A concise summary of the arguments supporting that view has been given by Zorpi et al.
(1989) in their introduction to a study of enclaves
from Sardinia. Pitcher (1993) stated that since the
observations of Harker (1904) many workers have
supported the general thesis that these enclaves
represent either cognate material or globules of
basaltic magma quenched in a granitic host. Clearly
in many cases these enclaves represent magma mingling, particularly when synplutonic dykes passing
into trains of enclaves are observed. There is,
however, a tendency to extrapolate more generally
from such occurrences, when there is no direct evidence. Pitcher (1993, p. 112) in discussing the I-type
granites of the LFB stated that 'the nature of the
dioritic, microgranular enclaves is in keeping with a
magma mixing/mingling model' but no specific evidence was presented for such a model applying to
those particular granites. Writing of metaluminous
(I-type) rocks, Wall et al. (1987) stated that textural,
geochemical, and isotopic characteristics favour
magma mixing or cognate magmatic origins for
essentially all microgranitic enclaves in such rocks;
linear variation plots and textural features should
distinguish those of magma mixing origin. Vernon
(1983) discussed several earlier accounts of mafic
enclaves and their implications for magma mixing or
mingling; he concluded that the enclaves show features that are most completely explained by
quenching of magma in the plutonic environment,
brought about by mingling of globules of more mafic
magma with felsic magma.
associated volcanic rocks, e.g. in the Kadoona Dacite
[Chappell et al. (1987) and below] shows that the
granite magma must have been nearly solid before
mixing to produce the K-feldspar crystals, as envisaged by Didier (1984), occurred.
Reid et al. (1983) argued that most mafic enclaves
in plutons of the Sierra Nevada batholith represent
chilled pillows of mafic magmas, that dissolve (sic)
into their felsic host and contaminate it to granodioritic compositions. In their view, in any sample of
the granite other than the silicic end-member, nearly
all the disseminated hornblende, biotite, sphene and
apatite, and much of the magnetite, in the host
granite are products of earlier mafic injections, now
extensively disaggregated and recrystallized. Furman
& Spera (1985) described a dyke-like train of
enclaves in the Eagle Lake pluton of the Sierra
Nevada, formed when basalt intruded into a mostly
crystallized granite host. They proposed that many
of the Sierran enclaves represent the result of such a
process that occurred earlier after pluton emplacement and was followed by vigorous convection
that destroyed the conduits through which the mafic
magma travelled. Barbarin (1991) proposed that the
mafic enclaves of the Sierra Nevada batholith
represent mafic magma that was injected into and
subsequently mingled with already hybrid magmas
just before or during ascent. He considered that
those hybrid magmas had been produced by
thorough mixing between mantle-derived mafic
magma and crustal felsic magma whose production
was induced by the mafic component.
Holden et al. (1987) showed that the eNd values of
mafic enclaves from two Scottish granites are relatively high, which can be explained if the enclaves
represent synplutonic injections of mafic magma into
a granite magma. They stated that this implies that
mantle-derived magmas were intruded into the crust
at the time of granite production, so that the input of
melt and heat aided crustal melting and granite formation.
Cantagrel et al. (1984) stated that the very
common occurrence of mafic enclaves attests to a
mechanism of magma mixing in the petrogenesis of
granites. Didier (1984) noted that in the magma
mixing hypothesis microgranular enclaves and their
granitic host are products of the imperfect mixing of Local hybrids
mafic and felsic primary magmas. In his view, the Vernon (1983, pp. 91-92) summarized early
most obvious examples of mechanical mixing are accounts by various workers of hybrid rock develrepresented by the presence of K-feldspar and quartz opment, stating that cases in which granite was
in mafic enclaves and of pyroxenic or amphibolic inferred to have assimilated fragments of previously
clots in granites. An alternative view is that the clots solidified gabbro very probably represent the minformed by the recrystallization of earlier pyroxene gling of two magmas. With others (Vernon et al.,
crystals that could have either been precipitated or 1988), he has described an example of magma mindisaggregating restite, and that the large feldspars gling in the Tuross Head Tonalite, part of the
may have grown from a silicate melt within the Moruya Suite whose chemical variation will be conenclave, of similar composition to that of the granite. sidered below. Vernon et al. (1988) noted that rocks
The'general absence of large crystals of K-feldspar in of that suite show linear geochemical variations
451
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 3
JUNE 1996
which had been interpreted by Griffin et al. (1978) as dacite, with basaltic rocks absent or only a minor
a type example of restite unmixing, but they pointed component.
out that linear geochemical trends could also result
Whalen & Currie (1984) described evidence for
from mixing between two magmas. They later stated the coexistence of mafic and felsic magmas in comthat the widespread mingling at Tuross Head is posite dykes and hybrid rocks and net vein comstrong evidence against restite unmixing for pro- plexes of the Topsails complex. Chemical data
ducing the different magma batches at that locality. support the suggestion that these magmas mixed to
Vernon (1983) posed the question: 'Are examples produce limited volumes of hybrids. Those workers
like Tuross Head merely local indications of the kind stated that the Topsails terrane exhibits inconof process involved or are they the essence of the trovertible small-scale evidence for the hybridization
process?'.
and mixing of magmas.
One of the classic accounts of hybridization in a
granite complex is that by Deer (1935) of the
Cairnsmore of Carsphairn intrusion. That complex Patterns of chemical variation
has been re-examined by Tindle et al. (1988), who
have shown that hybridization (or mixing) alone Strong linear correlations for pairs of elements on
could not have produced the intermediate rocks, variation diagrams for granites are not uncommon.
although hybridization may have occurred locally These could be taken as evidence for magma mixing,
between the main rock types. Those workers just as they can alternatively be used as evidence
favoured fractional crystallization, mainly at depth, supporting the restite model (Chappell et al., 1987),
as the mechanism for generating the main composi- so that in themselves such observations are not conclusive. However, Wall et al. (1987) stated that
tional variations within the intrusion.
'mixing is the classic cause of linear variation in
The hybrid rock marscoite, described by Harker major and trace element Harker diagrams'.
(1904), is the classic case of mixing. Evidence
Reid et al. (1983) ascribed the linear variation
includes the presence of K-feldspar and plagioclase
diagrams
for major elements in plutons of the Sierra
phenocrysts in the marscoite, similar to those in
Nevada
batholith
to interaction between mafic
associated felsite and ferrodiorite, respectively. Conmagmas
that
intruded
into the lower crust and felsic
tacts between those other two rocks indicate that
magmas
produced
by
partial melting that resulted
they were emplaced together as liquids. The major
from
the
increased
temperatures.
element compositions of the rocks are consistent with
mixing about 26% of felsite with 74% ferrodiorite
(Wager et al., 1965). Vogel et al. (1984) showed that
chemical data fit a mixing model for the marscoite Isotope variations
remarkably well. They also argued that mixing did Gray (1984) argued that the variations in initial
not occur at the present level of exposure, but at 87Sr/86Sr values in granites of the LFB resulted from
greater depth, involving mechanical mixing followed mixing of basaltic material and granitic melt derived
by diffusional processes that homogenized the melt. from melting of the Ordovician sedimentary rocks,
Harker (1909, p. 357) described the development of so that all of the granitic rocks, from hornblende
marscoite as 'an exceptional incident'. He went on to tonalites to cordierite granodiorites, are a single
state: 'The British Tertiary Province offers, as has broad family, and products of variable mixing
been remarked, specially favourable conditions, between distinct batches of basaltic material and a
basic and acid rocks being found there in very uniform partial melt of the regional basement. Gray
intimate relation. Nevertheless there is, as far as is (1990) stated that 'the basaltic component is possibly
known, only one rock forming distinct intrusions intrusive into the source as dykes or their metamorwhich is so much modified by partly dissolved xeno- phosed equivalents, but more likely intrusive into the
lithic material as to be of medium acidity, and this granitic magma and dispersed into it by the
has only a restricted occurrence'.
mechanical breakdown of mafic xenoliths'. However,
There are later accounts of occurrences of hybrid metaluminous enclaves that could have been derived
rocks from the Hebridean Province that were not from such basaltic material are not known in the
known to Harker, but these still make up a small mafic S-type granites of the LFB (Wyborn et al.,
part of the total volume of rocks. Marshall & Sparks 1991; cf. Vernon, 1983). Also, there are features of
(1984) have pointed out that most ring complexes in the chemical compositions of both the I- and S-type
that region include at least one net-veined or mixed- granites of the LFB that are not consistent with the
magma ring dyke, and noted that the mafic com- widespread application of such a model; these will be
ponents typically range from basaltic andesite to discussed below.
452
CHAPPELL
GRANITE SUITE MAGMA MIXING
DePaolo (1981 A) showed that there are regular COMPOSITIONAL
variations in ENJ values across the Sierra Nevada and VARIATIONS IN THE
Peninsular Ranges batholiths in California and Baja
California, which correlate with initial 87Sr/86Sr and LACHLAN GRANITES
oxygen isotopic data. These data were interpreted as Features such as those listed above have led to the
the result of mixing between two relatively homo- widely held view that mixing or mingling of magmas
geneous end-members. One component comprised is an important process in generating compositional
magmas derived from the mantle or subducted variation within suites of granites. All of these feaoceanic crust and the other was derived from sedi- tures, except the presence of two magmas in volcanic
mentary and cratonic rocks, or from melts derived and subvolcanic rocks, are seen in granites of the
from those sources. DePaolo (19815) argued that LFB. Mafic enclaves are common (Vernon, 1983;
partial melting of mixed source materials, con- Chen et al., 1989, 1990), hybrid rocks are present
tamination of mantle-derived magmas, and mixing (Vernon et al., 1988; Keay & Collins, 1995), linear
of magmas derived from the end-member sources, correlations on variation diagrams are an important
are all possible, and all probably occurred to at least feature of most of the granite suites (Griffin et al.,
some extent, but that assimilation of crustal rocks 1978; Chappell et al., 1987), and mixing of basalt
could have been the major process involved.
and granite magmas has been invoked to account for
variations
in the isotopic compositions of the granites
Arth et al. (1986) ascribed variations in initial
87
Sr/86Sr over a distance of 7 km across a grada- (Gray, 1984, 1990). Because, in addition, there is a
tional contact between two contemporaneous large database of major and trace element composiplutons of the Pioneer batholith to mixing between tions for the granites of the LFB, this is an approisotopically distinct magmas; that process is sup- priate area in which to examine in detail the
ported by a correlation between the isotopic compo- variations in the composition of granite suites to see
if they are consistent with a magma mixing or minsitions and SiC>2 contents.
gling hypothesis.
Concluding statement
It can be seen from the above that there has been
much support for a magma mixing mechanism in
producing compositional variation in granites.
However, much of the evidence refers to phenomena
that have been documented only on a small scale
(mixed volcanic rocks and hybrid plutonic rocks), or
can be interpreted according to alternative models
(enclaves and linear chemical variation), or have
been presented generally without supporting evidence from chemical variations (isotopic changes).
After discussing the evidence for magma mingling
and mixing, Pitcher (1993, p. 136) stated: 'For all
their eye-catching display in outcrop, mingling and
mixing in the higher levels of the crust represent but
second-order processes in the diversification of the
granitic rocks. Although outcrop evidence provides
important clues to the nature of these processes we
have to accept, however reluctantly, that the mixing
of magmas only takes place in bulk within the deep
crust, even more probably at the crust-mantle
interface where true melts are produced'.
Chemical evidence will now be presented that
bears on whether or not such large-scale magma
mixing occurs, and whether it is responsible for
variation within granite suites.
In arguing for the importance of separation of
restite in generating many of the compositional
changes observed within granite suites of the LFB,
Chappell et al. (1987) concentrated on arguing the
merits of that case against one of fractional crystallization. They noted that some of the features that
they used in arguing for the compositions of granites
being controlled by varying degrees of separation of
melt and restite would also result from the mixing of
mafic and felsic magmas, but they did not regard
magma mixing models as particularly appropriate
for these granites. For the more than 10 I-typc
plutons of the LFB with an area in excess of 500 km2,
Chappell et al. (1987) stated that fairly uniform
mixing of volumes in excess of 1000 km3 would be
required. They noted that even if such large-scale
mixing were physically possible, other observations
made on granites of the LFB would argue against its
application in that region. Those observations
included: (1) the variation lines for different suites
would pass through different end-members at both
the mafic and felsic ends, so that basalt-granite
mixing would involve both different basalts and different granites; (2) in the case of S-type granite
suites, the basalt end-member would have to be
peraluminous, because such suites become more peraluminous as they become more mafic; (3) in some Itype granite suites, elements such as Ni and Cr are
too low in abundance if the linear trend is extrapolated to basaltic compositions, although the data
453
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 3
JUNE 1996
the relative compositions of possible end-members.
For many elements, correlations in composition
within suites are essentially linear and, except in a
few extremely felsic granites some of which are discussed below, there are no cases of chemical variations showing inflexions that would point
unequivocally to processes of fractional crystallization. These I-type granites show some striking
chemical and isotopic regional asymmetries. From
east to west, in general for rocks of all suites of comparable SiC>2 content, the most significant changes
are in Na and Sr, which decrease markedly, whereas
Al and P decrease to a lesser extent. Ca and Sc
SUITES OF THE BEGA
increase significantly towards the west, whereas Rb
and V increase slightly (Beams, 1980). There are
BATHOLITH
The Bega Batholith is the largest dominantly I-type distinct variations in the isotopic compositions of Sr,
granite complex in the LFB, with an area of 8940 Nd and O, which take place rather systematically
km2 if the physically separate but compositionally across the batholith and correlate with chemical
coherent Moruya Batholith (265 km2) is included compositions (McCulloch et al., 1982; Chappell et al.,
(Fig. 1). This total complex contains several small A- 1990). The striking transverse patterns for some
type granite plutons (179 km2) and minor S-type chemical elements in the Bega Batholith must be the
granite bodies (70 km ) and is located in the south- result of similar patterns in source materials, as they
eastern corner of the LFB, east and south-east of correlate strongly with isotopic patterns. The uniCanberra, extending meridionally through a dis- formity in composition of Pb isotopes in the Bega
tance of > 300 km parallel to the present continental Batholith (McCulloch & Woodhead, 1993) contrasts
margin, with a maximum width of 75 km (White & with the significant changes in the compositions of
Chappell, 1983). The locations of the > 130 plutons other isotope systems.
that make up the batholith are shown on the map of
Rocks of the Bega Batholith range from monzoChappell et al. (1991). The usefulness of the concept granite through dominant granodiorite to tonalite.
of granite suites is illustrated by such a large Tonalites are mainly from the Moruya Suite. The
complex, in which some 50 suites are recognized; in Cobargo Suite is distinctive in its low quartz content,
discussing many aspects of the genesis of the rocks and its quartz monzonite and quartz monzodiorite
those suites can be utilized, rather than the much composition. The more mafic rocks of the batholith
larger number of plutons. Suites of the Bega Bath- always contain hornblende, and only one of the
olith that share broadly similar features but differ in seven suites examined here, Kameruka, is both too
detail are grouped more loosely into seven super- felsic and otherwise of inappropriate composition to
suites (Beams, 1980). Supersuites, and suites when contain that mineral.
made up of several plutons, generally extend parallel
Wall et al. (1987) noted that the field evidence for
to the axis of the batholith. Whitten et al. (1987) magma mixing is mainly the occurrence of chilled
carried out a cluster analysis of chemical data from
mafic fragments in a silicic matrix in volcanic rocks,
this batholith, and they stated that they were not
but that in the intrusive environment these quench
able to find a wholly objective set of criteria for
identifying suites in that way. They concluded that textures are partly obscured by recrystallization.
these suites should be abandoned, or defined differ- They stated that if a granitic pluton has these field
ently. Inspection of Figs 2 and 3, and the discussion features, and shows linear variation in Harker diabelow, will show that suites can be clearly separated grams and isotopic data plotting as pseudo-isochrons
among the granites of the batholith; if this cannot be or mixing lines, then 'it is reasonable to conclude
done using cluster analysis it is perhaps more an that magma mixing has played a major role in its
argument against applying that technique in such a chemical evolution'. As these chemical and isotopic
way, than against the suite concept as it is used in features are shown by plutons of the Bega Batholith,
and in addition the Sr and Nd isotopic compositions
the LFB.
range from rather primitive to fairly evolved, and
The Bega Batholith provides an ideal case for because small-scale mixing has been described by
examining a model of large-scale magma mixing Keay & Collins (1995), this batholith clearly probecause it contains several granite suites in which the vides a suitable area for a more detailed assessment
composition range is sufficiently large to characterize of the role of large-scale magma mixing.
would be consistent with an andesite or basaltic
andesite as one end-member. It is the first of those
earlier observations, initially made by White &
Chappell (1985), that leads to the strongest arguments against magma mixing as a mechanism generating the principal compositional variations within
granite suites of the LFB. This will now be considered in greater detail, with reference initially to
the granites of the Bega Batholith.
454
CHAPPELL
GRANITE SUITE MAGMA MIXING
Fig. 1. Map ihowing the location of selected granite suites of the Lachlan Fold Belt. For the Bega Batholith, suites discussed here are
shown in black, with others from that batholith in dark grey. S-type granites that are discussed are shown as crosses. Other granites in
light grey. Gabbro bodies are shown with a stippled pattern. The numbers refer to the following granite suites: 1, Moruya; 2, Cobargo;
3, Kameruka; 4, Candelo; 5, Bemboka; 6, Glenbog; 7, Tonghi; 8, Shannons Flat; 9, Dalgety; 10, Cooma; 11, Bullenbalong; 12,
Ingebyrah.
Bega Batholith and show a very wide and continuous
range in composition. Relative to other granites in
The seven granite suites of the Bega Batholith that the LFB, rocks of the Moruya Supersuite are partialso give their names to the seven supersuites of the cularly high in Na and Sr and low in Rb (Fig. 2).
batholith were selected for examination; they are Griffin etal. (1978) ascribed the compositional range,
listed in Table 1 and their locations are shown on the from 60-1 to 74-9% SiC^, to varying degrees of
map (Fig. 1).
retention of material residual from partial melting,
The Moruya Suite (195 km2) consists of seven or restite, in a felsic melt, so that the more felsic
separate plutons ranging in area from 1-3 to 69 km2 rocks are relatively restite free, and the more mafic
that are located to the east of the main body of the rocks relatively restite rich. White & Chappell
Some selected suites of the Bega Batholith
455
JOURNAL OF PETROLOGY
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•
,
30
0.3
o
% P2O5
o
°oo n
08
0.2
.
1
1
.
I
.
1
.
•
20
V
.
1
.
1
.
p.p.m. Ni
•
o
•
•
•
o
0.1
1
••
10
•
' . ^
0
0
58
62
66
70
74
1
58
1
•
1
62
•.
I
66
70
74
% SiO 2
% SiO 2
Fig. 2. Variation diagram] for various elements in granites of suites within the Bega Batholith. The suites represented are Moruya (open
circles), Cobargo (filled circles), Candelo (filled squares), Bemboka (open squares) and Glenbog (filled diamonds).
(1977) again discussed these rocks in proposing the
restite model [see also Chappell et al. (1987)]. Alternatively, the patterns of compositional variation
could be regarded as strong evidence for a mixing
process, and that will be examined here.
The Cobargo Suite (210 km2) comprises four
plutons with areas between 1-4 and 159 km2 at the
eastern edge of the Bega Batholith, which range in
composition from 589 to 730% SiO2. Rocks of this
suite are generally similar in composition to those of
the Moruya Suite, but can be separated by distinctly
lower Na abundances (Fig. 2) and slightly lower Ti,
P and Nb, and higher K, Rb, Th and Ni contents.
The Kameruka Suite (630 km ) consists of two
plutons near the eastern side of the batholith, of
Table 1: Supersuites of the Bega
Batholith in sequence east to west
Name
Supersulte
Suite
area (km2)
area (km2)
%s,o;i in suite
Minimum
Maximum
Moruya
265
195
60-13
74-91
Cobargo
370
210
58-91
72-99
Kameruka
1350
630
66-95
71-83
Candelo
1200
290
59-27
6704
Bemboka
1800
1050
6752
74-39
Glenbog
1750
1510
6552
73-79
570
465
65-98
74-76
Tonghi
456
GRANITE SUITE MAGMA MIXING
CHAPPELL
0.8
TiO 2
0.6
4
R • R
A
A * A
A
0.4
V-.
0.2
% Na2O
0.25
U
%CaO
•
n
»«, d
:
P2O5
0.20
0.15
R •"
J
R
_ D
a a 0CD CUD
aa Q
0.10
-
•
ouoa
•
H
aa
°
«
a <
0.05
1
1
1
1
1
1
1
i
25
%CaO
i
i
i
i
p.p.m. Sc
20
A
A
15
R * %
• • •
" " « • • •
M
R
R
R RRM
R
10
A
R R
R
5
•
'65
1
i
I
67
i
1
i
1
69
i
0
1
71
73
75
I
65
i
67
i i
69
i I i I i i
71
i I i
73
75
SiO2
SiO2
Fig. 3. Variation diagrams for various elements in suites of the Bega Batholith. The suites represented are Kameruka (crosses), Bembolca
(open squares), Glenbog (filled diamonds) and Tonghi (open triangles).
which the Kameruka Granodiorite (570 km ) is
dominant. Relative to other granites of comparable
SiC>2 range in the Bega Batholith, this suite is lower
in Ca (Fig. 3) and higher in K and Al. It is also
higher in Na than all the other granites except
members of the Moruya and Cobargo supersuites
(Figs 2 and 3). Ga is higher than any other I-type
granites of similar SiO 2 content in the batholith. It is
more restricted in range of SiC>2 than any of the
other suites being considered (Table 1).
The two members of the Candelo Suite (290 km2)
form a zoned pluton with the more mafic Candelo
Tonalite (45 km2) being marginal and gradational
to the Yurammie Granodiorite (245 km 2 ). The
Candelo Suite is distinguished from other Bega
Batholith suites by a combination of relatively high
Sr, light rare earth elements (LREE), Cu, K (Fig.
2), Ba, Rb, Th and U, moderate Na (Fig. 2), Sc, Zn,
V, Mn and Cr, and low FeO, Ca, Ca/Al and Rb/Sr
(Beams, 1980).
The major component of the Bemboka Suite (1050
km2) is the Bemboka Granodiorite (970 km 2 ), which,
if it is in fact a single pluton, would be the largest in '
the LFB; there is one other pluton in this suite.
Rocks of the Bemboka Suite can be distinguished
from others in the Bega Batholith by their overall
high Fe, Ca (Fig. 3), Sc, V, Mn, Co and Zn, and
their low Na at higher SiO 2 contents (Fig. 2) and Sr
(Fig. 2). Samples from the Bemboka Suite produce a
'tight' linear trend on Harker diagrams for most
457
JOURNAL OF PETROLOGY
VOLUME 37
elements, with continuous variation from 67-5 to
74-4% SiO 2 .
The Glenbog Supersuite extends for 300 km along
the western edge of the Bega Batholith. It is dominated by the Glenbog Suite, which makes up 1510
km 2 of the total area of 1752 km 2 . That suite comprises 12 plutons which individually range in area
from 8 to 335 km 2 ; as a group they vary in S1O2
between 65-52 and 7379%, but with each pluton
having a narrower range in composition (Fig. 2;
Chappell et al., 1987). This supersuite is distinguished from all others in the batholith by its low
Na and Sr contents (Figs 2 and 3), with the latter
element here having its lowest abundance in the
batholith.
The Tonghi Suite (465 km2) consists of four
plutons, with areas from 37 to 245 km 2 , and SiO2
between 65-98 and 74-76%. The suite extends for
100 km along the southwestern edge of the batholith.
Rocks of this suite and the associated Nalbaugh
Suite have the lowest Na contents in the Bega
Batholith (Fig. 3).
NUMBER 3
JUNE 1996
Cobargo suites converge at their mafic end (Fig. 2)
and the Kameruka and Glenbog suites do so at their
most felsic compositions (Fig. 3).
The effect of feldspar fractionation
The Sr variation diagram shown for the Moruya,
Bemboka and Glenbog suites is more complex near
its felsic end. Over most of the range in composition,
the three suites are clearly separated for Sr, but the
contents of that element for the two most felsic
samples of the Moruya Suite and of three from the
Bemboka Suite fall to lower levels than would be
expected by projecting the variation in the less felsic
rocks, so that all three suites have Sr concentrations
in the range from 100 to 125 p.p.m. at 74-75%
SiO2. For the Moruya Suite, both low-Sr samples
are from the Bodalla Monzogranite, for which there
is also a third analysis with a higher Sr content; the
three analysed rocks have Sr contents of 255, 123
and 119 p.p.m. and SiO 2 values of 71-88, 74-91 and
74-65%, respectively. This abrupt drop in Sr content
is ascribed to the fractional crystallization of feldspars before formation of the two most felsic rocks.
This is supported by data for Y (values of 14, 25 and
23 p.p.m. in those three rocks), as Y increases
CHEMICAL VARIATION
sharply in amount in I-type granites as the rocks
WITHIN SUITES OF THE
evolve by fractional crystallization of dominantly
BEGA BATHOLITH
feldspars and quartz at felsic compositions (Chappell
Harker diagrams for various elements from the suites & White, 1992). The three most felsic samples of the
listed above are plotted in Figs 2 and 3. The ele- Bemboka Suite show less marked effects of feldspar
ments plotted are mainly those that vary system- fractionation, and such effects have not been
atically in abundance across the batholith, as those observed in the Glenbog Suite, the third suite in the
are generally also the elements that clearly separate Sr variation diagram of Fig. 2.
different suites. Six of the eight elements listed above
as varying systematically across the batholith (Ca,
Na, P, Sc, Rb and Sr) are plotted at least once. CHEMICAL VARIATION
Other elements that have been included in the
WITHIN SUITES OF THE NEW
figures are Ti, K and Ni.
Figures 2 and 3 show that strong correlations exist ENGLAND BATHOLITH
between the plotted elements and SiO2, individually It could be claimed that the patterns of variation
of the kind that could be generated either by magma exhibited by the I-type suites of the Bega Batholith
mixing or restite separation. The most striking are a feature only of that batholith, or of the LFB in
feature, however, is that a relative separation in any general, and a product of some unusual feature of
one part of the compositional range is carried that region. It is therefore useful to examine I-type
throughout that range, or, expressed in another way, granites from elsewhere in eastern Australia, using
the relative order of abundance of an element in two rocks of different compositional character and age.
suites at either felsic or mafic compositions is gen- Data for two suites occurring near the southern end
erally also shown at the other extreme of com- of the NEB are plotted in Fig. 4. Granites of that
position. None of the variations observed in pairs of area are late Permian in age in contrast to the
suites plotted in Figs 2 and 3 cross over during the Devonian age of the Bega Batholith, but more
variation, and no cases are known from the Bega important, they are members of a high-K suite
Batholith where this happens for any element. In which shows other very distinctive compositional
some instances, the compositions converge at one features, such as high Sr, Ba, Pb and Th, and low Y/
end for a particular element, and two cases are illu- Ce relative to all granites of the LFB (Chappell,
strated where that occurs; for Na the Moruya and 1978). In contrast with the LFB, in which there are
458
CHAPPELL
t.
GRANITE SUITE MAGMA MIXING
1000
MgO
%
p.p.m. Sr
4
800
3
600
*•
•
•
•i
2
400
-
1
200
••
•
1
60
1
1
1
1
a D
i ,
1
•
•
i
i
i
2000
p.p. m. Ni
p.p.m. Ba
1500
40
1000 ;
20
Ten
-
d
500 -
•
4
a
i
56
60
i
i
i
i
i
64
i
i
i
68
i
i
Bo
. 1 , 1 , 1
i
72
76
SiO 2
60
,
i
,
64
i
68
72
76
% SiO 2
Fig. 4. Variation diagrami for various elements in the Inlet Suite (filled circles) and Moonbi Suite (open squares) of the New England
Batholith. These units belong to the I-type Moonbi Supenuite.
no igneous, sedimentary or metamorphic rocks that
can be directly related to a process of subduction
through comparison with modern analogues, the
NEB developed in an environment that matched
closely many of the features of modern active continental margins (Chappell, 1994).
The Inlet Suite consists of a single small pluton
(12-5 km2), the Inlet Quartz Monzonite, which is
zoned from marginal quartz monzodiorite to central
monzogranite with a range in composition from 57'5
to 67-0% SiC^. The Moonbi Suite comprises two
larger units of monzogranite, Moonbi (248 km2) and
Bendemeer (54 km ), which are more felsic (SiOj
from 65-2 to 710% and from 65-7 to 75-3%,
respectively). The distinctive compositional features
of these suites have been noted above. These are part
of the Moonbi Supersuite, with an area of 4000—4500
km2, which is a major component of the NEB.
Harker diagrams for MgO, Ni, Sr and Ba in the
Inlet and Moonbi suites are shown in Fig. 4. As for
the earlier examples from the Bega Batholith, the
variations are essentially linear, with differing
degrees of scatter. As with suites of the Bega Batholith, when the compositions are extrapolated to felsic
and mafic compositions the relative abundances of
the elements in those extremes of composition are
correlated.
459
CHEMICAL COVARIATION
AND THE MAGMA MIXING
HYPOTHESIS
White & Chappell (1985) and Chappell et al. (1987)
pointed out that the variation lines for different
suites of the LFB show that a magma mixing process
for those suites would have involved both different
felsic and different mafic end-members. That a
change in the mafic component should always be
associated with a change in the felsic end-member,
or conversely, is highly unlikely. That observation
therefore established that a general magma mixing
process for the production of the compositional variation in those suites is an event of rather low probability. The additional observation made here, that
the relative abundances of different elements in the
two end-member magmas would generally be correlated, effectively reduces that probability to zero.
Two felsic granite melts containing different relative
amounts of a range of elements would have to know
that they had to mix with mafic components showing
the same general relative abundances. The correlations that exist between the mafic and felsic compositions of individual rock suites must result from
some other process through which their compositions
are linked in some way.
JOURNAL OF PETROLOGY
VOLUME 37
The possibility that the mafic and felsic endmembers of an I-type suite resulting from magma
mixing or mingling could be related by an earlier
process of fractional crystallization should be mentioned, as that process would lead to correlations in
at least some element abundances between the mafic
and felsic components. As the mafic and intermediate members of a fractionation series would be
solid before the production of felsic melt, mixing of
mafic and felsic magmas is no longer an issue.
However, mingling between solidified mafic components and the felsic melt might be possible.
Nockolds (1934) proposed such a hypothesis of'contrasted differentiation' to account for the production
of hybrid rocks formed by reaction between felsic
melt and solid mafic material. However, such a
process of differentiation should produce various
intermediate compositions, and not just a granitic
composition separated in bulk from a crystalline
gabbroic phase, as pointed out by Holmes (1936). If
the granites of intermediate composition in the Bega
Batholith were the result of magma mingling
between two components related by an earlier
episode of fractional crystallization, then other rocks
should also exist that are earlier-formed intermediate
members of that fractionation series. It is difficult to
have it both ways.
Magma mixing can therefore be excluded as a
mechanism for generating the major compositional
variations in the Bega Batholith. It should be noted
that this batholith is a significant complex in terms
of size as it has an area of 8940 km2, which makes up
14% of the area of exposed granites in the LFB and
is a little more than the total area of 8300 km2 for
the Caledonian Newer Granites of Britain and
Ireland. Support for a wider application of that
conclusion comes from the similar deductions drawn
from an examination of some of the granites of the
.NEB.
NUMBER 3
JUNE 1996
independently support a restricted application of the
magma mixing model.
The S-type granites
S-type granites make up a little more than half of the
granites of the LFB and their distinctive features
have been discussed by Chappell & White (1992). As
with I-type granites, these gTanites can also be
grouped into suites. The mafic cordierite-bearing Stype granite suites of the LFB will always present
difficulties for magma mixing models because, as
White & Chappell (1988) pointed out, within such
suites the compositions become more peraluminous
as they become more mafic (Fig. 5). Clearly, these
rocks cannot be products of simple magma mixing,
as the mafic end-member would be strongly peraluminous and more complex mixing scenarios
would be required. Gray (1990) proposed such a
scenario, whereby the mixing or mingling of basalt
and crust-derived granite melt produced a magma
which fractionated to yield the more felsic and less
peraluminous rocks. The Cooma Granodiorite composition nominated by Gray (1984) as his crustal
melt end-member contains 39-9% normative quartz,
6-7% normative corundum, and only 12'3% normative albite, taking an average of sample Cl of
White & Chappell (1988) and five analyses of
Munksgaard (1988). Such a quartz-rich and albitepoor composition would require an extremely high
temperature to completely melt, even at high H2O
contents, and it would be difficult to dissolve the
excess AI2O3 in the fraction of the rock that melted,
or even in a total melt. Because of problems such as
these, a degree of consensus seems to have developed
that the S-type granites of the LFB were produced
by the partial melting of sedimentary source rocks
E
3
OTHER ARGUMENTS AGAINST
MAGMA MIXING
I
O
o
The above arguments against assigning a major role
>
to magma mixing in the origin of the granites of the
LFB are conclusive for those plutons or suites of that
belt where there is a sufficiently wide range in como
position and for which the corresponding chemical
c
data are available. Furthermore, these are the suites
for which the strongest argument would also be
made for magma mixing on the basis of extended
near-linear variations. However, there are additional
% total FeO
lines of evidence, including those cited by Chappell et Fig. 5. Plot of normative corundum VJ total FeO for S-type granal. (1987) that have been repeated above, which
ilei of the Bullenbalong Suite.
460
CHAPPELL
GRANITE SUITE MAGMA MIXING
Elements with relatively poor correlations
within suites
with the major compositional variations within a
suite generally resulting from varying degrees of
retention of unmelted source material, according to
the restite model of Chappell et al. (1987). Pitcher
(1993, p. 112) stated that there are various features
of the S-type granites of the LFB which 'when taken
together, offer strong support for the restite model as
an explanation for the origin of the Lachlan S-type
granites'.
If the highly correlated variations that are seen for
many elements in I-type suites of the LFB, such as
those shown in Figs 2 and 3, resulted from magma
mixing, then one would expect to see comparable
degrees of correlation for all elements. In fact, many
pairs of elements in suites of the Bega Batholith show
poorer correlations. Such differences in the degree of
correlation are much more likely to result from
varying amounts of restite separation, rather than
from magma mixing, because in the latter case the
two separate magmas, which would each have been
liquid at some time, would each have had the
opportunity to homogenize to a degree before
Chemical variation in S-type granite suites
S-type granite suites of the LFB individually show
more scatter in element concentrations about the
dominant trend for a suite than do I-type suites. This
is consistent with a view that sedimentary or supracrustal source rocks would be more heterogeneous
than I-type or infracrustal sources, with that variation being reflected within the derived magmas,
and ultimately in the compositions of individual
granite bodies and suites. Another factor may be
that S-type magmas, coming from shallower levels
than I-type magmas, did not have the same opportunity to homogenize. In contrast, the total range of
S-type compositions, particularly for trace elements,
is more restricted than for I-type granites, because
those sedimentary rocks that contain the haplogranite components, and are therefore 'fertile' in
terms of granite production, are themselves restricted
in composition. Put another way, the 'first-order'
variation among the granites of the LFB, that is
between suites, is greater for the I-type granites,
whereas the 'second-order' variation, within suites, is
larger for the S-types. It follows that examples of the
clear separation of suites among the S-type granites,
analogous to those shown for the I-type granites in
Figs 2 and 3, are uncommon, but two examples are
given in Figs 6 and 7. Figure 6 shows the distinctly
different trends for the BuUenbalong and Ingebyrah
suites of the Kosciusko Batholith (Hine et al., 1978),
both part of the BuUenbalong Supersuite. In Fig. 7,
the variation for TiC>2 is shown for the Dalgety and
Shannons Flat granites from the Berridale and Murrumbidgee batholiths, respectively. Rocks of these
two units are grouped in the Dalgety Suite because
they have compositions that plot on a very similar
trend for all elements except Ti and Cr, although
Shannons Flat is generally more felsic. Figures 6 and
7 therefore show the same general relationships seen
for the I-type suites, with a correlation between
relative abundances of pairs of elements in mafic and
felsic rocks.
% total FeO
Fig. 6. Variation diagram for Sr in the S-type BuUenbalong (filled
circles) and Ingebyrah (open squares) suites. Both luitcj are part
of the BuUenbalong Supermite.
)
1
2
3
4
% total FeO
Fig. 7. Variation diagram for TiO 2 in the S-type Shannons Flat
(filled squares) and Dalgety (open circles) granitej. These two
units make up the Dalgety Suite.
461
JOURNAL OF PETROLOGY
VOLUME 37
mixing occurred. In contrast, granite compositions
in the restite unmixing scenario reflect the overall
variation in composition of solid source rocks that
were partially melted to produce a magma of high
bulk viscosity that was subsequently fractionated
into components with differing proportions of melt.
A strong correlation between two elements within a
granite where variations were produced by restite
separation must reflect a source rock in which those
two elements were previously of rather uniform concentration, and conversely. That converse situation
is demonstrated very clearly by the S-type granites of
the LFB, for which the scatter in variation diagrams
is generally much greater than for the I-type
granites, as seen above.
JUNE 1996
NUMBER 3
1200
45
55
65
75
% SiO2
Fig. 8. Variation diagram for Ba in the Boggy Plain pluton.
The porphyritic Kadoona Dacite
Apart from the compositional patterns discussed
above, specific arguments against magma mixing are
difficult to make for the I-type granites of the LFB.
However, there is one volcanic rock for which such
arguments can be made, the I-type Kadoona Dacite,
which is comagmatic with granites of the Bega
Batholith. It is of intermediate composition (SiO2
~ 65%) and contains phenocrysts of quartz, plagioclase and pyroxenes. Chappell et al. (1987) pointed
out that for it to have formed by magma mixing, the
mafic component must have contributed the crystals
of pyroxenes and plagioclase and die felsic component must have contributed the phenocrysts of
quartz, but not K-feldspar and more albitic plagioclase. Hence in a mixing model, quartz would have
been the sole crystalline phase in the felsic endmember whereas all of the other granitic components
were present only in the melt, which is most unlikely.
granite complexes can be produced by processes
other than magma mixing.
It is noteworthy that within plutons of the Boggy
Plain Supersuite, the variation of elements in Harker
diagrams is normally very much more regular than
for plutons of the Bega Batholidi. Examples are Ba
(Fig. 8) and, notably, Zr [seefig.3 of Chappell et al.
(1987)]. Although this must to an extent reflect the
nature of the fractional crystallization process, it also
implies that the source magma was rather uniform in
composition, as would be expected in a molten or
largely molten magma of relatively high temperature
and low viscosity. This contrasts with die greater
heterogeneity of originally solid source rocks of the
Bega Badiolith, discussed in the previous section.
The Burnham model
Burnham (1992) calculated the temperature at
which plagioclase was in equilibrium with quartz on
the liquidus for the Moruya and three other suites of
Suites produced by fractional
the LFB at 500 MPa and X* = 0-30. He stated that
crystallization
his findings virtually preclude accumulation of
There is one group of granites within the LFB in minerals precipitated earlier at higher temperatures
which the chemical variation clearly resulted from Uian the calculated liquidus, or mixing of magmas
fractional crystallization. Rocks of this Boggy Plain from different sources, as processes by which the bulk
Supersuite show textural and compositional features compositional variations within each of the suites
that unequivocally point to such a process (Chappell investigated could have been produced. Burnham
et al., 1987; Wyborn et al., 1987). An example of the (1992) noted that, throughout the suites he
type of chemical variation that supports the examined, plagioclase of fixed composition was in
operation of fractional crystallization is that for Ba equilibrium with melt of fixed composition, with
in the Boggy Plain pluton, shown in Fig. 8. In that rocks of different compositions containing different
case, Ba dropped rapidly in abundance after biotite proportions of the liquid and solid components. In
appeared as a crystallizing phase. The largest unit of each suite, plagioclase and hypersdiene of constant
that supersuite, the composite Yeoval complex, has composition were in equilibrium with melt of conan area of 1430 km2 and ranges in composition from stant composition at the same temperature,
gabbro (47-0% SiO2) to felsic monzogranite (765% regardless of total crystal content and hence bulk
SiO2). This shows that large and chemically diverse composition; the fact that they were in equilibrium
462
CHAPPELL
GRANITE SUITE MAGMA MIXING
implies that if mingling of mafic enclaves occurred,
then they were from the same source as the melt
(C.W. Burnham, personal communication, 1995).
there is differential separation of those two components.
Rheological difficulties
ASSIMILATION OF
SEDIMENTARY MATERIAL IN
THE LACHLAN GRANITES
On the basis of their rheological modelling, Frost &
Mahood (1987) stated that simple mixing between
basalt and granite is not capable of yielding large
volumes of magmas having compositions as silicic as
dacite or rhyodacite, and that the maximum predicted SiOj content of a hybrid formed in that way
under reasonable crustal conditions does not exceed
~ 6 3 % . They suggested that more silicic magmas are
produced dominantly by fractional crystallization.
In the LFB, 93-5% of 2047 analysed granites and
closely associated more mafic rocks contain > 6 3 %
SiC>2 (see also Figs 2, 3 and 8), with an average SiO 2
content close to 70%. In the tonalitic batholiths of
the Cordillera, a majority of granites contain > 6 3 %
SiO 2) with Silver & Chappell (1988) reporting an
average SiC>2 content of 65-5% for 323 analysed
rocks of the Peninsular Ranges batholith. For some
other Cordilleran batholiths, such as the Sierra
Nevada, the proportion of more felsic granites is
higher.
It has been noted above that a feature of the cordierite-bearing S-type granite suites of the LFB is
that their normative corundum content increases as
they pass from felsic 'minimum temperature' to more
mafic compositions (Fig. 5). Therefore the possible
role of contamination, or assimilation of pelitic sedimentary material, in producing both that feature
and also those I-type granites of the LFB that have
more evolved isotopic compositions, needs to be
examined. Although such processes would be
accompanied by fractional crystallization (Taylor,
1980; DePaolo, 1981a) and therefore be more
complex than changes associated with simple mixing
or mingling of two components, they would produce
large-scale within-suite correlations between chemical and isotopic compositions, which have not
been observed.
The role of assimilation in S-type
granite suites
THE MAGMA MIXING
The presence of cordierite and aluminosilicate
SCENARIO
minerals in the granites of the LFB that are now
It has been suggested by several workers that both called S-type, has long been recognized, most
mafic enclaves and granite hybrids result from min- notably by Baker (1940), who favoured a pyrgling and mixing of a mantle-derived mafic magma ogenetic origin resulting from enrichment of the
and a felsic magma produced when that mafic granite magma in Al by assimilation of argillaceous
magma transfers heat to the crust (e.g. Eichelberger, country rock. Tattam (1925) had earlier suggested
1978; Reid et al., 1983; Gray, 1984; Barbarin, 1991). that cordierite in the Bulla granite near Melbourne
According to that scenario, the felsic granite melt, resulted from contamination. Snelling (1960) subdespite its high viscosity, must separate from its divided granites of the Murrumbidgee Batholith of
restite, with which it must initially be intimately the LFB into 'contaminated' and 'uncontaminated'
mixed. After separation, it must then mix or mingle types, and suggested that the 'contaminated' granites
with a mafic component that is generally presumed were derived from a parental magma akin to the
to have introduced the necessary heat and that is 'uncontaminated' Shannons Flat Granodiorite in
unrelated to the restite. In addition, the mafic composition, which incorporated and assimilated
material must share chemical features with the felsic country rocks at depth. Both of Snelling's groups are
melt compared with the mafic and felsic source S-type. In a manner analogous to Chappell (1966),
components of other granite suites. This rapidly who argued that the more mafic hornblende-bearing
develops into a scenario that is too complex to granites of the Moonbi district contained larger
account for the widespread association of mafic amounts of refractory source material, Joyce (1973),
enclaves with more mafic granites, and for the gen- in considering the peraluminous granites of the
eration of compositional variation in such a common Murrumbidgee batholith, suggested that the more
type of rock. The complexities of such a series of mafic cordierite and biotite-rich 'contaminated'
processes should be contrasted with the simple granites were produced when parental magma
mechanisms of the restite model (Chappell et al., reacted with relict solid material at or near its
1987), in which, first, a mixture of crystals and source. Chappell & White (1974) proposed that the
liquid is produced by partial melting, and second, distinctive features of the S-type granites of the LFB
463
JOURNAL OF PETROLOGY
VOLUME 37
were derived from the source rocks, ruling out contamination.
The more mafic cordierite-rich S-type granites of
the LFB contain an assemblage of metasedimentary
enclaves that could be interpreted as representative
of any assimilated material. The pelitic enclaves are
of higher grade than the contact metamorphic rocks;
sillimanite and less commonly almandine-rich garnet
are present in the pelitic enclaves but not in the
contact aureoles, as first noted by Stevens (1952) for
the Cowra Granodiorite. That granite was intruded
into comagmatic volcanic rocks and both it and the
pelitic enclaves contain garnet that formed at depths
of ~ 1 5 km. Chappell et al. (1987) also pointed out
that these higher-grade enclaves do not occur in
adjacent I-type granites, which would be expected if
the enclaves were of deep accidental origin. Chen et
al. (1989) noted that for the S-type Jillamatong
Granodiorite, the distinctive needle-like rutile
crystals seen in the quartz of that granite (and not in
other nearby S-type plutons) are present also in the
quartz of the enclaves, implying that the enclaves
are not accidental and that in fact both they and the
host granite came from the same source.
The more peraluminous character that S-type
granites of the LFB generally possess as they become
more mafic (Fig. 5) is in detail not one that would
result from contamination with the observed country
rocks, which contain very little feldspar and consequently have low abundances of Na and Ca
(Wyborn & Chappell, 1983). If they had been
assimilated to any extent, very distinctive compositions would have resulted, unlike those of the
abundant more mafic S-type granites but similar to
those of the Cooma Supersuite. Hence chemical data
also show that any significant assimilation must have
occurred at depth and of material compositionally
different from the country rocks exposed at the
surface. This conclusion is supported by the limited
amount of isotopic data that are available
(McCulloch & Chappell, 1982).
An additional argument against contamination
producing the compositional variation in the S-type
granite suites of the LFB is the correlation in the
relative abundance of some elements between the
most felsic and mafic rocks, as discussed above. Differences in compositions of the type shown in Figs 6
and 7 are much greater if S-type suites of generally
different character are examined, rather than ones
that are similar for most elements. The data in Fig. 9
show such a separation for Sr between the Dalgety
and Bullenbalong suites, with a correlation in
relative abundances between the felsic and mafic
compositions. Both suites have low Ni contents at
their most felsic near minimum-temperature com-
NUMBER 3
JUNE 1996
225
% total FeO
Fig. 9. Variation diagrams for Sr and Ni in the S-type Bullenbalong (open circles) and Dalgety (filled circles) suites. Regression
lines are shown.
positions but the Ni contents diverge as the two
groups become more mafic. If the more mafic compositions resulted from contamination, then clearly
the contaminants were different in composition. This
is unlikely, as the two suites were intruded into the
same sedimentary lithologies with <10 km between
their nearest points at the present level of exposure.
The role of assimilation in I-type granite
suites
Mixing of material derived from sedimentary and
mantle sources was invoked by Gray (1984, 1990) to
account for patterns of variation in initial 87 Sr/ Sr
in granites of the LFB. However, it is not in accord
with other features of those granites. It has already
been shown that the amount of mantle-derived
material that can be present in the mafic S-type
granites is limited by their strongly peraluminous
nature. Likewise, there are chemical features of the
I-type granites that are not consistent with the presence of a significant amount of a sedimentary component. Prior to the Gray (1984) model, McCulloch
& Chappell (1982) had shown that incorporation of
464
CHAPPELL
GRANITE SUITE MAGMA MIXING
the necessary amount of a sedimentary component
into the I-type Jindabyne Tonalite to completely
account for the isotopic composition (initial 87Sr/86Sr
~0-708, eNd ~ —5), is not consistent with the chemical composition of that rock. For example, the
abundances of K, Rb and Cr in the Jindabyne
Tonalite are much too low for its source material to
have contained the ~60% sedimentary component
required by isotopic compositions alone, and the
alternative hypothesis of old and chemically primitive source rocks must be preferred (Compston &
Chappell, 1979). Hence, although the Gray (1984)
model has attractions in terms of providing a simple
explanation for the isotopic compositions, that in
itself is not an unique argument in its favour, and it
breaks down when the chemical compositions of the
rocks are also considered.
The Glenbog Suite is the most isotopically evolved
(initial 87Sr/86Sr = 0-7083-0-7108, eNd
72 to
-5-5) of the I-type suites of the Bega Batholith
(Chappell & McCulloch, 1990). Relative to other
suites of that batholith it also contains relatively
small amounts of Na and Sr (Figs 2 and 3). Hence
the isotopic compositions and at least some of the
chemical features are consistent with a greater component of sedimentary material in the Source
material of that suite, compared with the rest of the
batholith. However, that sedimentary component
must have been present at the source and was not
introduced by assimilation because there is no correlation between the chemical and isotopic compositions within rocks of the suite as they change in
composition from felsic to mafic (Chappell &
McCulloch, 1990).
A small amount of assimilation of country rock
material is observed for the normally zoned Boggy
Plain pluton (34 km2), where initial 87Sr/86Sr ratios
increase from an average of 0-70441 in the marginal
diorites and the granodiorites (29% of area; mean
Sr = 645 p.p.m.), to an average of 0-70479 in the
monzogranites (70%; 362 p.p.m.), to 0-70554 in one
sample from the central aplitic rocks (0-9%; 204
p.p.m.) (Wyborn, 1983). This pluton is representative of the hottest magmas of the LFB, as discussed above, which were completely or largely
molten when emplaced, yet the change in the
87
Sr/86Sr ratio through most of the pluton is very
slight. However, although high temperatures and
continuing fractional crystallization would favour
assimilation, the high Sr content of that body as
shown by the above data would make such a process
more difficult to detect. Moreover, the country rock
sedimentary rocks are low in Sr, with unpublished
data of 38 samples of that type from the LFB having
a mean of 64 p.p.m. Sr. They are, however, fairly
465
radiogenic; Munksgaard (1988) reported a mean
value for 87Sr/86Sr of 0-7173 for metasedimentary
samples from the Cooma region, in similar lithologies 60 km from the Boggy Plain intrusion.
THE MAFIC ENCLAVES
The more mafic I-type granites of the Lachlan and
New England belts generally contain mafic or
microgranular enclaves, with tie distinctive rocks of
the Boggy Plain Supersuite again being a notable
exception. Chen et al. (1990) studied such enclaves
from the Glenbog and Blue Gum suites of the
Glenbog Supersuite in some detail and their fig. 6
shows very clearly that there are correlations
between the abundances of elements in the enclaves
and the host granites, so that, for example, the
enclaves in the Glenbog Suite contain less Sr than
those of the Moruya Suite (see also Fig. 2). Hence,
the chemical asymmetries that are seen in granites of
the Bega Batholith are also present, at least to a
degree, in enclaves from those granites. Such correlations between the compositions of granites and
enclaves for Na, P, Sr, Rb, Sc and V in the Moruya,
Kameruka and Glenbog suites are shown in variation diagrams in Fig. 10. The enclaves plotted in
that figure generally show more scatter than host
granites, but the same relative abundances for the
elements in the enclaves as for the granites are generally seen. In most cases, the enclave compositions
lie about an extrapolation of the granite compositions to more mafic values, but for Sr in Moruya and
Kameruka, and to a lesser extent Na2O in Glenbog,
this is not so; for those three cases, mixing or mingling of a felsic magma with the observed enclave
compositions could not have produced the variations
observed in the granites. For the Moruya and
Glenbog suites, located at opposite sides of the Bega
Batholith, both the granites and their enclaves are
always clearly separated for the six elements plotted
in Fig. 10. For P2O5 and Sr, the Kameruka granites
lie close to the trend for the Moruya granites, near
the felsic end, and the enclaves likewise generally
plot within the field of the Moruya enclaves. In the
case of Rb, both the Kameruka granites and
enclaves plot towards the lower side or below the
trends for Moruya. For Na2O, Sc and V, Kameruka
plots between the two other trends for both granites
and enclaves. Chappell (1966) observed similar features in I-type granites of the NEB, where both
granites and enclaves of the Attunga Creek Suite
contain less Mg and Ca, and more Al, Mn and Na,
than granites and enclaves of the Moonbi Suite.
Zorpi et al. (1989) have likewise reported that the
enclaves from each of four plutons in northern
JOURNAL OF PETROLOGY
VOLUME 37
4.5
200
<?°oV °°o °°
A
3.5
JUNE 1996
NUMBER 3
'•CD *
150
°
•
•
•
A
Tk
A*
100
M
A
D
2.5
50
% Na 2 O
p.p.m. Rb
i
0.5
'.
A
% P2O5
f
i
i
i
t
p.p.m. Sc
40
0.4
30
0.3
O
O
• 1
0.1
• •
•
i
•
10
^
i
i
A
i
i
o
o
p.p.m. Sr
p.p.m. V
200
o
450
300
20
m
0.2
"•S a.
*
.
•
.
•
•
:
••• r
*
150
0Q
C
o
4
A 4* 9
100
AAA
A
150
50
•
i
55
i
i
.
i
60
.
t
i
i
i
65
i
i
i
70
•
i
•
i
•
0
75
55
% SiO 2
SiO 2
Fig. 10. Variation diagrams for Na, P, Sr, Rb, Sc and V for the Moruya (circles), Kameruka (triangles) and Glenbog (squarei) suites of
the Bega Batholith. Open symbols are for granites and filled symbols are for enclaves. One Moruya enclave at 5436% SiOj and 0-77%
P 2 Oj, and Glenbog enclaves at 59-31% SiO2 and 48 p.p.m. Sc, 60-39% SiO2 and 55 p.p.m. Sc, 56-71% SiOj and 272 p.p.m. V, and
58-62% SiO 2 and 246 p.p.m. V are not plotted.
Sardinia have a narrow range of FeO t /MgO which
relate to that ratio in the host granites. Those
workers attributed that relationship to the granites
having been modified by incorporation of mafic
magmas of different FeO t /MgO ratios, now represented in a modified form by the mafic enclaves
The origin of microgranular enclaves remains in
dispute, partly because these rocks can undoubtedly
form in a variety of ways, as discussed by Chen et al.
(1990) for enclaves from the Glenbog Supersuite.
The observation that the correlations found between
the abundance of elements in the more felsic and
mafic granites also extend to the enclaves places
some limitations on the origin of these enclaves. It is
possible that the compositions of enclaves within a
suite share distinctive features with the host rocks
because their composition was modified by surrounding magma. If that is so, then their present
compositions are of little assistance in understanding
their origin, and it is also likely that earlier textures
of the enclaves were changed and would likewise be
of little value in considerations of origin, as Vernon
(1991) and others have used them.
Alternatively, the distinctive compositional features of the enclaves that are shared with the host
granite might have been a primary feature of the
enclave, and not be a result of chemical exchange
with the magma. If that is so, then the enclaves must
be products of mafic material closely related to the
granites in some way. If the enclaves represent
466
CHAPPELL
GRANITE SUITE MAGMA MIXING
fragments of solidified mafic magma, then the compositional similarities between the enclaves and the
granites would imply that such a magma also contributed to the composition of the host granite.
However, it has been shown that magma mixing or
mingling was not responsible for the large-scale
variations in composition within suites of the Bega
Batholith, and it is therefore unlikely that these particular enclaves represent solidified pieces of such
magma, although it is likely that such enclaves do
occur elsewhere (e.g. Hill, 1988), consistent with
their multiple origins. In any case, the zircon age
inheritance data of Chen & Williams (1990) have
shown that the enclaves in the Glenbog Suite were
derived from material much older than the age of
crystallization of the granites.
The compositional similarities between the
enclaves and granites of different suites could result if
the enclaves formed as chilled margins or cumulate
rocks. Chen et al. (1990) argued against both of those
origins, pointing out that no rocks that could be
interpreted as chilled margins are seen at the present
level of exposure. Furthermore, the mafic enclaves
are more or less evenly distributed throughout
plutons, rather than being concentrated near the
margins, as is generally the case for accidental xenoliths. If the enclaves were cumulate rocks then Cr,
Ni, Sr and Eu should be relatively high in the
enclaves, which is not so (note, in particular, the
abundance of Sr in enclaves of the Glenbog Suite in
Fig. 10). Sr is particularly important as this is one of
those elements whose systematic fall in abundance
westward across the Bega Batholith is reflected by
the enclaves (Fig. 10). Also, as Didier (1984) has
pointed out, the much finer grainsize of enclaves
compared with the host granites would not be consistent with a cumulate origin. Also again, both of
these possible origins are inconsistent with the
pattern of the zircon inheritance found in four mafic
enclaves from the Glenbog and Blue Gum suites by
Chen & Williams (1990).
A final possibility is that the enclaves represent
restite, and this would conform with the nature of
the zircon inheritance. Chen & Williams (1990)
have shown that the enclaves contain inherited
zircons with ages from 630 to 430 Ma, which are
similar to those in the host granites except that the
~ 1000-Ma ages that are common in the granites are
apparently absent from the enclaves. The presence of
older zircons would not conform with an origin of
the enclaves from mafic material introduced into the
crust at the time of the magmatic event nor, in the
absence of the 1000-Ma component in the enclaves,
a blending of such material with the granite magma.
The age data are consistent with a restite derivation
467
from 630-Ma source material, where other rocks
containing 1000-Ma zircons were also present and
were incorporated as a restite component in the
granites, but not in the enclaves. Such an origin
could also provide a simple explanation of the compositional similarities with the host rocks, as both the
granites and the enclaves would have been derived
ultimately from parts of the same source materials.
The more mafic S-type granites of the LFB
contain a variety of enclaves that are clearly restites
(White et al., 1991), as well as a distinctive more
mafic type of enclave to which Vernon (1983) has
ascribed an igneous origin, on the basis of textures.
However, the chemical compositions of those particular enclaves are not those of igneous rocks with, for
example, eight from the Cowra Granodiorite having
an average of 135% Na 2 O, 62% SiO2 and 0 6 2 %
normative corundum. Those enclaves are most likely
also of restite origin (Wyborn et al., 1991). That is
also the only origin that could be applied generally
to both I- and S-type enclaves, which, because of
their analogous occurrences and consanguinity with
the host rocks in both cases, might both be inferred
to have had similar origins. Cantagrel et al. (1984)
stated that 'such an interpretation does not explain
the typical igneous textures of these enclaves suggestive of a purely magmatic origin'. However,
many 'igneous' textures in enclaves should be taken
only as evidence for the presence of a melt, and not
necessarily for their initial derivation from a melt.
In part because of the uncertainties about the
extent to which the primary textures and compositions of enclaves are or are not modified in the magmatic environment, the origin of mafic enclaves in
I-type granites remains obscure, and the data presented here are offered as a contribution towards the
ultimate solution of a difficult problem.
MIXED SOURCE ROCKS
The arguments used here against the hypothesis of
magma mixing apply only to such a process producing the variations within granite suites. Mixing of
components may still be a viable model for the generation of the source rocks or source magmas that
later fractionated in some way to produce the variation observed within suites. Indeed, sometimes
such mixing seems to be required. The common
occurrence of older cores in zircons from all suites of
the Bega Batholith apart from the Moruya Suite
(Williams, 1995) means that it is necessary to postulate mixing of some sedimentary material into the
source materials of those granites. Such mixing is not
simple, and is therefore difficult to quantify in terms
of end-member compositions and proportions. For
JOURNAL OF PETROLOGY
VOLUME 37
example, Na and Sr are both relatively low in
amount in sedimentary rocks that contain a pelitic
component, and both elements decrease in amount
westward across the Bega Batholith. However, Na
has its lowest abundance in the Tonghi Suite (Fig. 3)
and Sr in the Glenbog Suite (Fig. 2). Ca, which also
has a low abundance in such sedimentary rocks, and
particularly in the Ordovician rocks of the LFB
(Wyborn & Chappell, 1983), increases significantly
in abundance between suites (Fig. 3) as Na and Sr
decrease (Fig. 2). These patterns can only be
accounted for if the dominant igneous component of
those western suites was itself different from that of
the Moruya Suite. Hence the whole question of the
number of source components and their proportions
in the source material for each suite is unresolved.
What is certain is that the mixing that did occur
took place before commencement of the fractionation processes that produced variation within
individual granite suites, and not afterwards.
NUMBER 3
JUNE 1996
varieties. In short, like other hybrids, these hybrid
rocks are barren.'
ACKNOWLEDGEMENTS
I thank Susan Keay for making me think seriously
about magma mixing as a possible factor in the
origin of the granites of the Lachlan Fold Belt.
Charlotte Allen, Peter Brown, Ed Stephens, Allan
White, Ian Williams, Doone Wyborn and E-an Zen
commented on the manuscript. Two reviewers and
Tony Ewart as editor provided many helpful comments. Figure 1 was drawn by Mirra Huber at the
Cartographic Services Unit of the Australian Geological Survey. Colleen Bryant prepared the other final
figures and plotted a large number of variation diagrams. Financial support from the Australian
Research Council Grant A39232908 is acknowledged, for the study of granites formed at active
continental margins. This is Publication 31 of the
Key Centre for the Geochemical Evolution and
Metallogeny of Continents.
CONCLUSIONS
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