JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 6
MGES 1385-HO8
1996
MARLINA A. ELBURG*
DEPARTMENT OF EARTH SCIENCES, MONASH UNIVERSITY, CLAYTON, VIC. 3168, AUSTRALIA
Genetic Significance of Multiple Enclave
Types in a Peraluminous Ignimbrite Suite,
Lachlan Fold Belt, Australia
The Violet Town Volcanics (Lachlan Fold Belt, Australia) are
an S-type ignimbrite suite containing microgranitoid enclaves,
basaltic andesite enclaves and enclaves of high-silica rhyolite.
The microgranitoid enclaves are similar to those in peraluminous granites. They typically have lower initial ^Sr/^Sr
and higher e^i than the host, and represent globules of a mafic,
mantle-derived magma, ivhich was hybridized by mixing and
diffusional exchange with the host magma. The basaltic
andesite enclaves were incorporated into the ignimbrite as
xenoliths, but their parental magma may have been similar to
that of the microgranitoid enclaves. They are isotopically less
depleted than other mantle-derived rocks from the Lachlan
Fold Belt, reflecting contamination by crustal material, or
derivation from less depleted mantle sources. The high-silica
rhyolite enclaves, previously interpreted to be related to the
ignimbrite by crystal fractionation, have Ejw values up to 3
units higher than their host, and cannot be related to their host
by crystal fractionation or assimilation-fractional crystallization (AFC) processes. The coexistence of S-type magmas
and mantle-derived magmas suggests that the latter may have
played a role in the Palaeozoic magmatism of the Lachlan
Fold Belt, acting as a heat source for melting and perhaps also
contributing chemical components to the crustally derived
magmas.
Poli & Tommasini, 1991; Blundy & Sparks, 1992;
Seaman & Ramsey, 1992). If enclaves have a restitic
origin, they can provide constraints on the composition and P—T conditions of the source rock
(Chappell et al., 1987; Chen et al., 1989, 1990;
Wyborn et al., 1991); an origin by magma mingling
implies that mafic magmas may have played a role
in the origin of the more felsic host magma (Vernon,
1984; Furman & Spera, 1985; Holden et al., 1987;
Barbarin, 1990; Moreno-Ventas et al., 1995); an
interpretation as cumulates or chilled margin gives
information about processes occurring in (high-level)
magma chambers (Phillips et al., 1981; Clemens &
Wall, 1984; Dodge & Kistler, 1990; Flood, 1993);
and if enclaves are interpreted to have a xenolithic
origin, they may tell us something about unexposed
rock types, and the interactions occurring between
xenoliths and host magma (Grout, 1937; Van
Bergen, 1984).
This paper presents data on three distinct types of
enclaves within an S-type ignimbrite suite, the Violet
Town Volcanics (VTV). These volcanic rocks are
part of the Lachlan Fold Belt, an area where
enclaves are often interpreted to have a restitic
origin (Chappell et al., 1987; Chen et al., 1989, 1990;
KEY WORDS: enclaves; magma mingling; magma mixing; S-type Wyborn et al., 1991). The data suggest that none of
the enclaves studied are directly related to the host
magma. At least one type has a magma mingling
origin, and is thought to be equivalent to the microINTRODUCTION
Enclaves within igneous rocks can potentially give us granitoid enclaves found in many intrusive rocks.
information on the origin and/or the evolution of the The other two types of enclaves are more likely to
magma in which they are found (Grout, 1937; have xenolithic origins, and they give us new inforKoyaguchi, 1985; Bacon, 1986; Holden et al, 1987; mation on the (isotopic) composition of unexposed
Zorpi et al., 1989; Barbarin, 1990; Srogi & Lutz, mafic and felsic extrusive rocks, which may have
1990; Chen et al., 1990; Eberz et al., 1990; Pin et al., played a role in the generation of S-type magmas in
1990; Stimac et al., 1990; Didier & Barbarin, 1991; the Lachlan Fold Belt.
•Present address: Department of Geology and Geophysics,
Univenity of Adelaide, Adelaide, S.A. 5005, Australia.
Telephone: +-61-8-3035973. Fax: +-61-8-3034347.
e-mail: [email protected]
© Oxford University Press 1996
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 6
DECEMBER 1996
& Wall, 1984). The VTV are intruded by the
younger Strathbogie Granodiorite (364 ± 6 Ma by
K/Ar; Richards & Singleton, 1981) to the south,
which has caused local recrystallization of the
groundmass of the volcanic rocks.
The quality of the outcrop of the VTV is reasonable, but all deposits have acquired a uniform
GEOLOGICAL SETTING
grey colour owing to superficial weathering and this
The VTV arc located in Central Victoria (Fig. 1) makes it very difficult to distinguish enclaves from
and belong to the Melbourne Basement Terrain, as host rocks. Nearly all sampling was therefore carried
defined by Chappell et al. (1988). The origin and out in two quarries, near the top of the volcanic pile
evolution of this rhyodacitic to rhyolitic ignimbrite (Fig. 1).
suite has been described by Clemens (1981) and
Six distinct groups of enclaves can be recognized
Clemens & Wall (1984). The age of the formation is in the VTV:
Late Devonian [373 ± 7 Ma by Rb-Sr (Clemens &
(1) Microgranitoid enclaves (1-10 cm diameter)
Wall, 1984); 379 ± 8 Ma, this study], and it uncon- are usually rounded, and rare concentric zonation
formably overlies the Early Devonian flysch can be observed. Some samples contain macrosequence of the Melbourne Trough (White, 1953). scopically recognizable plagioclase crystals. Clemens
The thickness of the volcanic pile is 400 m (Clemens & Wall (1984) referred to these enclaves as 'microtonalitic'.
VJ.10-60,65-741
•NEWSOUTH WALES
(2) High-silica rhyolites (7-15 cm) are lighter
coloured than the host and easily recognizable
0,
owing to high modal contents of bright white
feldspar phenocrysts. These enclaves are usually
angular. Clemens & Wall (1984) described similar
material in a plug near the southern margin of the
ignimbrite, and interpreted these enclaves as being
related to the rhyodacitic-rhyolitic host rocks by
fractional crystallization.
Violet
(3) Basaltic andesites (1-10 cm) are dark, finegrained enclaves with variable shapes.
(4) Hornfelses (2-5 cm) are light grey, angular
pieces offine-grainedquartz-rich metasediment.
Strathbogie \
(5) Biotite-rich enclaves (3-6 cm) often have
Granodiorite \
irregular outlines, sometimes interfingering with the
host rock. They are darker, and are generally finer
grained than the host rock.
(6) High-grade metamorphic enclaves are rare.
Clemens (1981) described a biotite schist, a garnetrutile-biotite-quartz-albite gneiss, and a quartzorthopyroxene-biotite—cordierite gneiss.
In this paper, the term 'mingling' applies to the
process in which two (or more) magmas remain distinguishable physical entities, whereas 'mixing' indicates that the process results in a single hybrid
magma.
Only the first three enclave types will be discussed
in this paper.
ANALYTICAL TECHNIQUES
Fig. 1. Location of the Violet Town Volcanics and the associated
Strathbogie Granodiorite. Arrowi indicate sample location!. The
dashed lines denote the approximate boundaries of the Melbourne
Trough. After Clemens (1981).
Major element analyses of minerals were obtained
using a CAMECA SX-50 electron microprobe in the
School of Earth Sciences, University of Melbourne.
All elements were analysed by wavelength-dispersive
spectrometry. Operating conditions were 15 keV
with a beam diameter of 5 fim for pyroxene, and 15
lim for amphiboles, micas and feldspars. Matrix
corrections were applied using the CAMECA PAP
1386
ELBURG
ENCLAVES IN AN IGNIMBRTTE, AUSTRALIA
program (Pouchou & Pichoir, 1984). International
mineral standards were analysed before each run to
check the accuracy of the analyses.
Whole-rock analyses were performed on an ARL
8420 XRF spectrometer also in the School of Earth
Sciences, University of Melbourne. A rhodium
source was used for all elements. Matrix calibration
and correction techniques have been outlined by
Haukka & Thomas (1977) and Thomas & Haukka
(1979). US Geological Survey (USGS) standard
AGV-1 was used as a normalizing standard for all
major elements, except K, for which granite CRPGGR was used. For each batch a least one sample was
fused in duplicate to monitor reproducibility, and at
least one international standard, generally granite
standard G2, was analysed to monitor accuracy.
Relative precision is better than 0-5% for the major
elements, except A12O3 and MgO, for which precision is better than 1-5%. Precision is better than
5% for Rb, Sr, Ba, Zr, Cr, Ni, V, Zn, Ga, Nb and Y,
except for levels near the detection limit, which is 2—
3 p.p.m. for these elements. Precision is better than
10% for Cu and Y. Relative accuracy is better than
1% for the major elements, except TiOj and CaO,
for which it is better than 2%, and P 2 O 5 , for which
it is better than 8%. Accuracy for Ba, Rb, Sr and Ga
is better than 5%; it is better than 10% for V, Cu,
Zn, Zr and Nb; and better than 20% for Cr and Ni
for the low concentrations of these elements in
granite standard G2.
Geology and Geochronology, LaTrobe University,
following procedures recently outlined by Elburg
& Nicholls (1995). The long-term average of the
La Jolla Nd standard for this machine is
1+3
Nd/ 144 Nd = 0-511858 ±10
(n = 60). Strontium
standard SRM987 gives "Sr/ 8 6 Sr = 0-71023 ± 4
(n = 50). Errors given are 1 SD. The reproducibility of the 87 Rb/ 86 Sr ratio is < 0 8 % (2 a), and
147
Srn/ 144 Nd<0-2% (2 a).
PETROGRAPHY AND
MINERAL CHEMISTRY
Host rock
The textures and mineralogy of the ignimbritic host
rocks have previously been described by Clemens &
Wall (1984). A brief description is included here to
allow comparison with the enclaves.
Most crystals in the host ignimbrite are broken or
kinked. The groundmass consists of microcrystalline
material and some recognizable pumice shards, but
near the contact with the Strathbogie granodiorite it
is recrystallized to a coarser grain-size.
Plagioclase phenocrysts ( < 5 mm diameter) are
normally zoned with superimposed fine oscillatory
and often convolute zoning. The cores can be as
anorthite rich as An^t, and rims are generally
around A1140 in the rhyodacitic rocks. K-feldspar
( < 3 mm) is rare in the rhyodacitic, but common in
the
rhyolitic units. It is often embayed, can display
For rare earth element (REE) analysis, ~ 8 0 mg of
finely ground sample was dissolved in pressurized simple twinning and contains ~ 1% An and 31—35%
Teflon bombs, and diluted 2000 times in 2-5 N Ab. Quartz ( < 4 mm) is always embayed and does
HNO3. The dissolved samples were analysed using a not contain inclusions. Euhedral biotite ( < 2 mm)
VG Elemental Plasmaquad PQ2+ inductively displays yellow to red—brown pleochroism and
coupled mass spectrometer at Monash University, locally has overgrown orthopyroxene. The mgusing calibration curves based on rock standard numbers [100 X Mg/(Mg+Fe tota j)] fall between 36
AGV1. Corrections for instrumental drift were per- and 47, and most crystals are unzoned, or have weak
formed by repeated analysis of dummy standards, normal zoning. Strongly pleochroic orthopyroxene
and the use of an internal In standard. Precision is ( ^ 4 mm) is more common in the rhyodacitic than
in the rhyolitic units. Orthopyroxene phenocrysts are
typically ~ 2 ~ 3 % .
The oxygen isotopic compositions of whole-rock no more magnesian than m^-number 51 (Table 1),
samples were analysed following the procedure out- and they are often normally zoned down to mglined by Clayton & Mayeda (1963). In each batch of number 41 in the rims. AI2O3 varies from 1 to 3 % ;
10 samples one NCSU standard was run as well. All the wollastonite component is always lower than
samples were dried at 110°C for at least 12 h before 0-8% (Fig. 2). Orthopyroxene also occurs interanalysis. Extracted gas was analysed as CO2 on Fin- grown with cordierite as coronas on garnet. Clemens
nigan MAT Delta-E and 252 mass spectrometers. & Wall (1984) identified three types of garnet
The average composition obtained for the NCSU (mostly < 2 mm), two of which (A and C) are magstandard is 11 -59%o [relative to Vienna Standard matic phases (m^-number 7-25). The rare type D
Mean Ocean Water (VSMOW)], with a standard garnet has cores containing sillimanite inclusions,
and is thought to be a restitic or xenocrystic phase.
deviation of 0-19%o.
The same researchers distinguished several kinds of
Sr and Nd isotopes were measured using a
cordierite; those crystals containing sillimanite or
Finnigan-MAT 262 RPQ, thermal IR multigreen hercynitic spinel inclusions were interpreted as
spectral scanner in the VIEPS Centre for Isotope
1387
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 6
DECEMBER 1996
Table 1: Electron microprobe analysesfor plagioclase, pyroxene and biotite in host ignimbrite,
microgranitoid enclaves (ME) and basaltic andesite enclaves (BA)
Sample:
VT51H
VTQ18
VTQ18
VT18
VT18
VT14
VT61H
VT24
VT24
VTQ18
VT14
VT51H
Mineral:
PL2C
PLAG5c
PLAG5r
PLAG4c
PLAG4r
PLAG3
OPX12
PX3-1c
PX3-1r
PX4C
PX6
BT5R
BT6
Type:
host
ME
ME
ME
ME
BA
host
ME
ME
ME
BA
host
ME
S!O2
54-69
55-26
51-49
50-62
49-61
50-13
47-85
53-71
51-65
53-19
52-67
33 64
35-37
TI0 2
003
0-02
0-05
0-09
0-01
0-1
0-26
021
0-10
0-39
4-34
5-56
27-14
2786
30-51
30-93
31-01
31-66
1-2
2-57
1-23
2-65
2-35
15-69
16-18
FoO
0-08
0O4
0-16
0-64
0-13
032
30-75
11-39
20-75
10-93
3-48
22-37
16-11
FojO,
0
0
0
0
0
0
3-48
1-7
1-29
0
1-58
0
MnO
0
0-03
0
0
0
0-07
0-86
0-31
0-46
0-22
0-21
0-17
0-12
MgO
0
0
0
0-18
0
0
1407
28-14
22-32
29-67
1706
8-13
12-34
CaO
9-88
10-51
13-17
14-21
14-56
14-73
0-31
1-82
0-74
1-42
21-73
004
001
Na 2 O
5-81
5-27
3-78
3-01
3-01
3-12
0-04
0-05
0
0
0-29
0-32
0-28
K20
0-51
0-48
0-27
0-22
0-15
0-12
0-01
0-01
0
0
001
9-22
9-15
F
0
0-03
0-03
0
0-1
0
0
0
0
0
0
0-64
0-56
99-61
99-61
98-79
100-17
10000
94-70
94-74
AljO,
Total
9809
0
9988
98-70
100-12
98-67
98-24
VT18
0
Si
2-512
2-501
2349
2-307
229
2-285
1-918
1-916
1-948
1-907
1-924
5-292
Al
1-472
1-486
1-64
1-665
1-687
1-701
0-057
0-108
0-065
0-108
0-101
2-909
2-715
Fe 3 *
0
0
0
0
0
0
0-105
0-O46
0-037
0074
0044
0
0
5-368
Fe 2+
0-003
0-002
0-006
0-024
0005
0-O12
1-03
0-34
0-654
0-253
0-106
2-944
2-045
Mg
0
0
0
0-012
0
0
0-84
1-497
1-255
1-586
0-929
1-908
2-792
Ca
0-487
0-51
0-644
0-695
0-72
0-719
0-013
0-07
003
0055
0-851
0006
0002
Na
0-618
0-462
0-334
0-267
0-269
0-276
0-O03
0-O03
0
0
0021
0097
0082
K
003
0028
0-016
0-013
0-O09
0007
0O01
0
0
0
0
1-85
1-772
Tl
0-001
0001
0
0002
0-003
0
0-003
0-O07
0006
0-003
0011
0-513
0-635
Mn
0
0001
0
0
0
0-003
0-029
0-O09
0-015
0007
0007
0023
0015
F
0
0004
0-004
0
0O15
0
0
0
0
0
0
0-317
44-9
81-5
65-7
86-2
89-7
4
4
4
4
4
mg-no.
Tot cat.
5-025
4-996
4-994
4-986
4-997
5-005
Ab
5005
46-25
3364
27-35
26-98
27-54
An
4706
60-97
64-78
71-34
72-13
71-73
Or
2-90
2-77
1-58
1-32
0-89
0-72
En
44-61
7853
64-72
83 73
Fi
54-68
17-82
33-73
13-38
5-64
0-71
3-65
1-55
2-88
45-11
Wo
restite or xenocrysts. Most cordierite crystals are
altered to pinite along rims and cracks. Accessory
phases are apatite (euhedral, prismatic; ^0 - 2 mm in
diameter) and zircon (rounded, but sometimes
euhedral, <0-03 mm). Some large apatite crystals
contain zircon inclusions. Monazite is present in
minor amounts.
0-269
393
57-7
16-879
16-706
49-26
tures can occur in the same sample. The
microgranitoid enclaves contain plagioclase + Fe—Tioxides ± quartz ± sulphides ± biotite ± orthopyroxene. Apatite and zircon contents are variable,
but minor. Zoned microgranitoid enclaves contain
higher proportions of hydrous minerals (biotite,
actinolite) in their rims than their cores, and the
zoning is always concentric. Some microgranitoid
enclaves contain larger crystals (megacrysts) which
Microgranitoid enclaves
are mostly plagioclase, orthopyroxene and biotite.
Plagioclase megacrysts (<5 mm) are invariably
All microgranitoid enclaves are (sub)rounded. Textures vary from poikilitic to equigranular. Both tex- zoned, often with fine oscillations. They sometimes
1388
ENCLAVES IN AN IGNIMBRITE, AUSTRALIA
ELBURG
6- • opx host
O opx ME
540>
o
(A
JO
3D
2D
10
40
45
50
55
60
65
70
75
80
85
Enstatite (wt %)
Fig. 2. Diagram of enstatite vs wollastonite for orthopyroxene
from microgranitoid enclaves (ME; open squares) and host rocks
(filled diamonds). The two components are positively correlated.
Most orthopyroxene in the microgranitoid enclaves is more
magnesian than host orthopyroxene.
have sieve-textured cores with compositions similar
to plagioclase in the host rocks, and more An-rich
overgrowths (Fig. 3). Crystal shape is typically
tabular to slightly rounded. Small plagioclase
crystals (^0-8 mm) are often elongated, with aspect
ratios up to 6:1, and display oscillatory zoning. Their
compositions can be as calcic as An84, and these
anorthite-rich crystals can contain up to 0-8% FeO
(Table 1).
Biotite is pleochroic yellow to reddish brown,
euhedral to subhedral, but small crystals (<0-4 mm)
can be anhedral. The mg-number varies between 47
and 58, but systematic zoning is not observed.
Larger orthopyroxene crystals (usually ^1-5 mm)
display prominent red-to-green pleochroism. Small
crystals (^0-5 mm) are either elongate or tabular
and display normal zoning (Fig. 4). Alteration to
green-brown fibrous amphibole is widespread. The
cores of the small orthopyroxene crystals can have
bronzitic compositions (up to 84% enstatite),
whereas the rims of these crystals can reach En48,
which is indistinguishable from host rock orthopyroxene. Mg-rich cores contain up to 5% wollastonite
(Fig. 2) and 4% AI2O3, and rims generally contain
~ 1 % wollastonite, and 2% A12O3 (Table 1).
Quartz is interstitial, sometimes as poikilitic pools
(3 mm) in which the other crystals are set. Rounded,
inclusion-free quartz megacrysts (^ 10 mm) are rare.
Accessory minerals are elongated ilmenite, equidimensional sulphides, zircon and acicular apatite.
One microgranitoid enclave contains anhedral
crystals of garnet, rimmed by coronas of cordierite
and orthopyroxene, which are partially altered to
amphibole. Single crystals of cordierite occur in
some microgranitoid enclaves. In one sample, the
cordierite crystal (1-5 mm) contains inclusions of sillimanite and green spinel.
High-silica rhyolite enclaves
The high-silica rhyolite enclaves contain quartz and
K-feldspar as their major phenocrysts (2-3 mm). Kfeldspar is largely altered to clay. Quartz is always
embayed. Plagioclase crystals (< 1-5 mm) are scarce;
they are generally twinned and display subtle discontinuous zoning. Biotite is generally small (^0-4
mm) and elongated. Pinitized cordierite ( ^ 2 mm)
crystals are rare. Their outline is euhedral. Rare
irregular Fe-Ti-oxide crystals have been observed.
Subhedral garnet was found in one sample; it contains inclusions of Fe—Ti-oxide and apatite. The
groundmass is aphanitic.
iField for host rock pla9<pclase;;;;;»gg
40
0.0
0.2
0.4
0.6
0.8
: Field for host rock orthopyroxene
Distance from core (cm)
Fig. 3. Core-to-rim traverse for a plagioclase megacryst in microgranitoid enclave VTQ,18. The core falls within the field for host
rock plagioclase, but the thin rim is more An rich. The megacryst
is thought to have originated in the rhyodacitic host magma. After
transfer to the hotter, more mafic enclave magma it was first
dissolved, and then jacketed by more calcic plagioclase, in equilibrium with the (contaminated) mafic melt.
Fig. 4. Core-to-rim traverse for an orthopyroxene crystal from
microgranitoid enclave VT24. This crystal displays normal
zoning from a magnesium-rich core to a more iron-rich rim. This
can be the result of closed-system crystallization, or crystallization
from a progressively more contaminated melt. Even the rim of this
orthopyroxene is more magnesian than any orthopyroxene in the
host rhyodacite. The width of the core-rim traverse is ~0-5 mm.
1389
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 6
DECEMBER 1996
(1981) and Clemens & Wall (1984); their data have
been
included in the trends shown here. RepreThe basaltic andesite enclaves contain phenocrysts of
sentative
analyses are given in Table 2; the full set of
plagioclase, orthopyroxene, clinopyroxene and FeTi-oxide, which are set in afine-grainedto aphanitic analytical data can be obtained from the author
upon request. Special care has been taken to avoid
matrix.
inclusions
of xenoliths or enclaves in the host rocks
The plagioclase phenocrysts (<4 mm) are usually
analysed.
Even
though some scatter is present in the
euhedral, and display continuous or discontinuous
Harker
variation
diagrams owing to the slightly
zoning, sometimes oscillatory. Some have patchy
inhomogeneous
nature
of the ignimbritic host rocks,
cores or mantles. Compositions vary between An^
some
trends
can
clearly
be recognized.
and An75.
Only Na2O, K2O and SiO2 behave as incompaClinopyroxene phenocrysts (<1'5 mm) are
tible
elements (Fig. 5 and Table 2). The m^-number
euhedral and often display twinning. They have a
diopside to endiopside composition with m^-numbers decreases with increasing SiO2 content, which is in
as high as 90 (Table 1). Some phenocrysts are par- accordance with fractional crystallization. The contially replaced by green fibrous amphibole and centrations of most trace elements decrease with
biotite; this mostly occurs towards the rim of the increasing SiOj contents, except for Rb, Nb and Zn.
enclave. Some clinopyroxene crystals contain orthopyroxene cores. Orthopyroxene phenocrysts (<l - 5 Microgranitoid enclaves
mm) are generally euhedral, and can be strongly The array for the microgranitoid enclaves forms a
zoned. They are often altered to green fibrous (generally more diffuse) extension of the host rock
amphibole.
array. TiO2, MgO and CaO decrease monotonically
Phenocrysts of Fe—Ti-oxide (^2 mm) are with increasing SiO2 content, whereas both Na2O
anhedral to subhedral in shape. Some samples and K2O increase (Fig. 5). AJ2O3 is relatively concontain rounded clusters of quartz crystals in the stant at 15—17%, which is similar to the more mafic
matrix, which are most likely to be secondary.
host rock analyses. The most FeO*-rich sample,
The mineralogy of these samples is similar to that VTQ9 (55% SiO2), is not consistent with these
of the Torbreck Range Andesite (Douglas & Fer- trends: CaO, MgO and TiO 2 are lower, and the
guson, 1988), which occurs in the nearby Cerberean alkalis higher than expected. The mg-number of the
Cauldron. This andesite has an age similar to the microgranitoid enclaves is negatively correlated with
Violet Town Volcanics (373-358 Ma by K/Ar SiO2. VTQ_18 forms an exception, with a high mgdating on biotite; Richards & Singleton, 1981). The number of 56 at relatively high SiO2 contents. Alupossible relationship between the Torbreck Range minium saturation index (ASI) values [molecular
Andesite and the basaltic andesite enclaves will be Al2O3/(CaO+Na2O+K2O)] do not show a clear
discussed below.
trend and plot between 1-0 and 1-2, although the
Interaction between the enclaves and the host ASI of the more mafic samples can be < 1 (Fig. 5).
magma is exemplified by an outer zone in some
Scatter in trace elements is generally more proenclaves where biotite (together with plagioclase) is nounced for the microgranitoid enclaves than for the
a groundmass constituent. The grain-size of these host rocks. Sr appears to remain constant, but three
zones is somewhat coarser than that of the inner samples have distinctly elevated Sr contents. Ba, Rb
zone. Pyroxene phenocrysts in this outer zone are and Y increase with increasing SiO2 contents,
occasionally replaced by biotite or green amphibole. whereas Ni, Cr, V, Zr and Cu decrease. In spite of
The interior part of the enclave contains pyroxene, the general decrease in Ni and Cr concentrations
plagioclase and minor oxides as groundmass con- with increasing SiO2 contents, sample VTQJ8, at
stituents.
68% SiO2, contains more Ni than sample VT18EC,
with 55% SiO2 (Table 2). Cr and V are also higher
than expected in this enclave.
Basaltic andesite enclaves
WHOLE-ROCK
GEOCHEMISTRY
Major and trace elements
Host rocks and high-silica rhyolite enclaves
Whole-rock analyses for major elements, Rb, Sr, Ba
and Zr of host ignimbrite and high-silica rhyolite
enclaves have been reported previously by Clemens
Basaltic andesite enclaves
The basaltic andesite enclaves span approximately
the same SiO2 content as the microgranitoid
enclaves, and the behaviour of other major elements
is also similar. MgO, CaO and Na2O contents are
slightly higher and K2O lower than for the microgranitoid enclaves. It is unclear if the behaviour of
1390
Table 2: Whole-rock X-rayfluorescenceanalyses and 8 0 analyses for microgranitoid enclaves (ME), basaltic andesite enclaves (BA), highsilica rhyolite enclaves (HSR) and host ignimbrite
Ssmpto:
Type:
SiO 2
VT18EC
VT18ER
VTQ18
VT24E
VT1
VT17
VT2B
VTQ9
VT14E
VT66C
VT52
VT60
VT61
VT18H
ME
ME
ME
ME
ME
ME
ME
ME
BA
BA
HSR
HSR
host
host
52-87
77-9
78-7
55-21
68-73
68-18
57-96
60-42
67 04
61-55
54-87
66-27
68-13
72-31
TRA
56-47
TlOj
1-62
1-32
0-55
1-27
1-57
0-78
1-33
1-13
1-18
1-39
009
AI 2 Oj
16-72
16-72
15-33
16-84
15-56
15-64
16-41
15-94
16-27
14-82
11-89
FeO"
9-19
8-13
4-3
8-61
8-32
6-5
8-16
13-83
7-92
9-98
MnO
0-21
0-15
008
0-16
0-15
0-08
0-12
0-26
0-13
0-22
MgO
7-1
5-68
3-02
5-73
4-67
2 09
3-22
5-18
CaO
7-92
6-1
4-03
6-91
5-89
3-22
4-36
4-62
NBJO
1-15
1-73
2-76
1-46
2-19
2-62
303
2-75
K20
0-66
1-26
1-6
0-87
0-97
2-83
1-62
1-28
1-81
1-42
5-72
4-8
3-55
3-88
1-28
P2OB
0-2
0-17
0-14
0-18
0-25
0-19
0-2
0-14
0-37
0-31
0-18
0-17
0-19
0-17
0-19
LOI
0-59
0-79
0-76
0-92
0-22
1-6
0-76
0-10
1-32
1-91
0-34
0-47
0-66
1-32
2-45
100-25
10003
100-16
100-24
99-5
100-27
99-76
10001
99-66
99-95
99-82
10008
100-18
99-56
0-98
109
106
101
1-12
1-11
0-79
0-7
0-99
1-09
1-13
1-16
0-77
Total
ASI
mg-no.
100
1-13
1-18
0-72
0-47
0-26
11-7
16-58
14-34
14-82
0-61
1-1
4-42
307
802
0-01
001
008
005
0-18
5-6
7-97
0-1
007
1-62
0-98
8-77
8-29
9-13
0-52
0-4
2-98
204
7-74
2-16
1-9
2-99
2-92
2-73
2-7
2-26
68-7
009
58
55
56
54
50
40
41
40
56
23
10
40
36
Cr
233
199
81
69
67
22
84
162
197
448
1
0
35
23
Ba
300
574
738
394
715
831
658
204
859
616
572
78
1007
750
V
220
189
88
185
198
77
149
180
230
238
2
0
74
45
Cu
53
51
31
61
59
26
48
37
83
37
141
6
27
19
Zn
102
120
75
108
121
123
153
268
98
102
156
98
86
87
Ni
51
50
61
39
24
16
30
21
64
92
2
8
18
14
O
1
2
$
19
20
18
19
22
21
25
24
18
18
16
19
20
21
163
163
177
145
230
188
361
118
228
197
66
66
274
183
Y
36
31
25
31
41
41
28
23
26
30
28
23
47
36
Sr
190
190
318
204
197
178
207
194
660
367
57
12
185
135
345
Rb
64
78
102
94
66
135
119
89
77
78
250
238
153
174
30
9
10
13
9
13
16
20
21
10
6
16
20
16
15
10-55
9-33
7-27
11-15
9-78
All analyses have been normalized to 100% volatile-free, with all iron as FeO. TRA is a sample ofTorfareck Range Andesite, from Birch etal. (1970).
11-69
i
480
Ga
10-59
o
66
Z>
Nb
c
S
o
H
139
E
JOURNAL OF PETROLOGY
20
DECEMBER
NUMBER 6
VOLUME 37
1996
300
"18
%
i
200-
Q."
;i 6
A J(-
<
Q:
•
N100"
6>
121
<-s 8 -
O
4-
2
2
O150"
s;
o
VTQS °s
DVTQ18
.
A
D
50"
4
DVTQI 8
??D
400"
J300-
?6"
i.200i
VTQ9
o
ra
A a
100"
DVTQ18
6"
^ *4
VTQ9
^400"
3
:
to
200"
12001000^ 800^
600^400a
200-
1.2-
A
0.8-
aD
*cP(
gVTQ18
A o
a o
a
A
0.6
50
60
70
(wt %)
80
50
60
SiO2
70
(wt %)
80
Fig. 5. Selected Harker diagrams for host ignimbrite (filled diamonds), high-silica rhyolite enclaves (open circles), microgranitoid
enclaves (open squares) and basaltic andeiite enclaves (filled triangles), and one sample of Torbreck Range Andesite (cross) from
Birch it al. (1970). Data for host ignimbrite and high-silica rhyolite enclaves also from Clemens (1981).
1392
ELBURG
ENCLAVES IN AN IGNIMBRITE, AUSTRALIA
the alkalis is a primary feature, or is related to
alteration of the enclaves (see Petrography and
Mineral Chemistry). The m^-number can be higher
(up to 62), and ASI for the most mafic samples is
definitely lower than for the microgranitoid enclaves
(0-7-0-8).
The most obvious difference in trace element
content between the basaltic andesite enclaves and
the microgranitoid enclaves is the higher Sr content
of the former (500-600 p.p.m. vs 200-300 p.p.m.).
Cr and Ni are also higher, up to 450 and 150 p.p.m.
respectively (Table 2), values more typical of
mantle-derived melts than remelted sediments.
Table 2 also gives the composition of a sample of
Torbreck Range Andesite (Birch et al., 1970). The
composition is broadly similar to that of the basaltic
andesite enclaves, but an important difference exists
in TiC>2 content, which is significantly lower in the
Torbreck Range Andesite. It is therefore unclear if
the basaltic andesite enclaves can be considered to be
related to the Torbreck Range Andesites. However,
the only other mafic igneous rocks exposed in this
area of the Lachlan Fold Belt are Cambrian tholeiites, which are typically more mafic (SiC>2<50%),
and have flat REE patterns (Crawford & Keays,
1987), and thus show even less similarity to the
basaltic andesite enclaves (see below).
are broadly similar to those of the rhyodacitic host
rocks, but total REE contents are generally lower
(Fig. 6b). Samples which are characterized by high
proportions of orthopyroxene, such as microgranitoid enclaves VT24 and VT18EC, have the
lowest total REE contents, whereas VT17 and VT1,
which are mineralogically more similar to the host
rocks, have higher REE contents. Enclave VTQ18 is
characterized by high LREE/HREE ratios.
The REE patterns of the basaltic andesite enclaves
show a smooth decrease from LREE to HREE, and
their Eu anomalies are only slightly negative (Fig.
6c). They differ from the microgranitoid enclaves by
having a less negative Eu anomaly, but they show
similar LREE/HREE enrichment. The patterns
resemble those from recent island arc andesites
(Taylor & McLennan, 1985), with the slightly
negative Eu anomaly reflecting minor plagioclase
fractionation.
Oxygen isotopes
A small selection of samples of the Violet Town
Volcanics host ignimbrite, microgranitoid enclaves
and basaltic andesite enclaves were analysed for
oxygen isotopes. All analyses are expressed as per
mille difference relative to VSMOW. No samples of
the high-silica rhyolite were analysed, as the feldspars in these samples are moderately altered, and
Rare earth elements
this may influence the oxygen isotopic composition.
The host rock samples show a decrease in total REE
The 5 O values for the rhyodacitic ignimbrite
contents with increasing SiOj content of the samples vary between +10-4 and 116%o (Fig. 7), which is
(Table 3; Fig. 6a). Eu/Eu* also decreases, from 0-6 within the range for S-type rocks from the Lachlan
in the most mafic rhyodacite to 0-4 in sample Fold Belt (O'Neil et al., 1977). However, the
VT18H. Chondrite-normalized REE patterns tend observed variation is greater than analytical uncerto become flatter with increasing silica content, tainty (typically 0-2%o), and also more than expected
owing to a more rapid decrease in LREE than for a crystal fractionation sequence. It is likely that
HREE. Rhyodacitic samples show a smooth decrease the inhomogeneity in the samples reflects variability
from LREE to HREE, but the patterns for the high- in the composition of the magma, which may in turn
silica rhyolite enclaves show a break in slope at Ce reflect inhomogeneity of the source of the magma.
(Fig. 6a). Similar discontinuous REE patterns have
Most .microgranitoid enclaves do not differ greatly
been described previously for highly evolved granitic from the host rocks with respect to oxygen isotopic
rocks by Yurimoto et al. (1990), and were interpreted composition. They are on average slightly lower in
18
to result from the fractionation of monazite. The Eu/ <5 O, with sample VT1 being significantly lower at
Eu* values of the high-silica rhyolite enclaves can be +7-3%o, a value more typical for I-type rocks, and
as low as 0-07. The shape of the REE patterns is in near to those for mantle-derived rocks. However, this
agreement with the hypothesis that host rocks and may be an anomaly, as there is little geochemical
high-silica rhyolitic enclaves are related to each difference between VT1 and the other microother by crystal fractionation of plagioclase (to granitoid enclaves.
explain the increasing Eu anomaly), monazite
Sample VTQ18 can be considered to be depleted
(decreasing LREE contents) and zircon (decreasing in 18 O compared with the host rocks, as it has a
HREE). The REE pattern of the host rocks is similar whole-rock composition that is not much more mafic
in shape to that of Post-Archaean average Australian than the host, whereas it is at least l%o lower in
<518O. Although differences of the order of l%o are
shale (Taylor & McLennan, 1985).
The REE patterns for the microgranitoid enclaves also observed between microgranitoid enclave
1393
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 6
DECEMBER 1996
Table 3: Rare earth element analyses by ICP-MSfor host rocks, high-silica rhyolite enclaves (HSR),
basaltic andesite (BA) enclaves and microgranitoid enclaves
HSR
Host rocks
Sample:
VT63
VTQ16H VT18H
VT52
VT60
B A enclaves
Microgranrtoid enclaves
VT14E
VT65C
vn
36-3
23-7
78-9
53-5
9-7
70
7-3
6-6
370
27-8
33-1
30-2
VT17
VT24
29-6
286
20-4
190
223
28-3
62-6
57-2
42-4
42-2
47-1
55-1
6-2
5-4
5-8
22-2
25-1
24-9
25-2
VT18EC VT18ER VTQ18
La
380
39-1
25-7
12-3
Ce
76-0
78-2
50-5
28-6
Pr
8-6
85
6-1
Nd
38-0
360
26-7
Sm
8O
7-9
6-1
4-35
385
7-2
6-2
7-8
6-8
5-3
6-1
5-6
Eu
1-41
1-46
0 85
0-48
0-O9
2-32
1-88
1-50
1-31
1-18
1-09
1-20
1-35
Gd
7-7
6-9
5-9
4-52
3-99
7-3
6-4
7-3
6-3
4-49
6-2
5-4
487
323
12-8
7-20\
190
2-27
9-2
4-93
5-17
Tb
1-30
108
108
1-04
0-89
0-84
0-90
1-37
1-24
0-84
109
0 95
0-83
Dy
7-3
6-7
6-2
5-83
5-00
4-56
5-14
7-9
7-6
5-5
6-9
5-7
4-73
Ho
1-37
1-25
1-12
0-92
0-69
088
1-06
1-55
1-46
1-08
1-34
109
0-86
Er
3-49
2-61
2-68
2-12
1-24
2-34
2-80
3-65
3-49
2-45
3-59
285
2-29
Tm
0-69
0-52
0-42
0-30
0-18
0-34
0-41
0-69
0-67
0-44
0-55
0-44
0-34
Yb
3-57
282
2-79
1-95
1-24
2-23
2-60
3-83
3-85
2-58
3-60
305
2-37
Lu
059
0-49
0-41
0-31
0-15
0-34
0-40
0-66
0-65
0-43
0-53
0-43
0-35
VT18E and its host, this can be related to the more
mafic composition of the enclave sample compared
with the host (55% SiO2 vs 72% SiO2). The core
and rim of microgranitoid enclave VT18E have
identical oxygen isotopic compositions.
The basaltic andesite enclaves have relatively high
<518O values (+9-8%o) compared with mantle-derived
rocks, and this points towards the involvement of a
crustal source for some of the oxygen.
geochemistry; and, if this were the case, whether the
range in isotopic compositions would trend from
relatively unevolved (for the high-Mg orthopyroxene-rich samples) towards values more similar to
the host rock; a trend expected if interaction with
the host magma were responsible for the change in
mineralogy. All data are presented in Table 4. The
isochrons were calculated using the method of York
(1969).
Radiogenic isotopes
Thirteen samples were analysed for Sr and Nd isotopes. Three samples are host ignimbrite and two are
high-silica rhyolite enclaves. One of the six microgranitoid enclaves analysed (VT18E) was divided
into separate core (VT18EC) and rim (VT18ER)
before analysis. The cores of two basaltic andesite
enclaves were also analysed.
The seven analyses for the microgranitoid enclaves
encompass the analysed chemical and mineralogical
spectrum, from relatively mafic and/or orthopyroxene rich (core of zoned sample VT18E, sample
VTQ18, containing
high-Mg orthopyroxene
crystals) to more felsic, and biotite rich (biotite-only
sample VT17, biotite + low-Mg orthopyroxene
sample VT2B). This selection was designed to test
whether changes in mineralogy and whole-rock geochemistry were accompanied by changes in isotope
Rb—Sr isotopic system
Host rocks and high-silica rhyolite enclaves. T h e Rb—Sr
isochron for the host rocks and high-silica rhyolite
enclaves defines an age of 379 ±8 Ma, with an
87
Sr/86SrI- of 0-7092 (Fig. 8). This age is within error
of the age (373 ± 7) reported by Clemens & Wall
(1984), who also used an Rb-Sr whole-rock isochron. The mean square of weighted deviates
(MSWD) of this isochron is high (89), indicating
that scatter is more than can be explained from
analytical uncertainties. This means that the host
rocks and high-silica rhyolite enclaves are isotopically inhomogeneous, and that the isochron is
actually an errorchron. If the host rocks and rhyolite
enclaves are unrelated (see below) then the Rb—Sr
isochron could be a pseudochron, and not reflect a
geologically significant age. The only other constraint on the age of the Violet Town Volcanics is
1394
ELBURG
(a)
ENCLAVES IN AN IGNIMBRITE, AUSTRALIA
200
D
A
O
•
•
100:
0)
•D
C
O
VT63, host, 68.9% Si02
VTQ1 GH, host, 69.6% SiO2
VT18H, host, 72.3% SiO2
VT52, HSR, 77.9% S1O2
VT60, HSR, 78.7%
10:
o
o
La
(b)
Ce Pr
Nd
Sm Eu Gd Tb Dy Ho
Er Tm Yb Lu
200
100:
•D
c
o
D VT63, host, 68.9% SiO2
1 0 : + VT18EC, ME, 55.2% SiC>2
X
A
O
•
A
u
o
VT18ER, ME, 58.7% SiO2
VT24, ME, 58.0% SiO2
V T 1 , ME, 60.4% SiO2
VT17, ME, 67.0% SiCh
VTQ18, ME, 68.2% SiO2
La
Ce Pr
Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
(c) 200
D VT63, host, 68.9% SiO2
+
IOOH
0)
VT14e, BA, 56.3% SiO2
X VT65c, BA, 52.9% SiO2
T3
C
O
€1
10:
u
o
La
Ce Pr
Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 6. (a) Chondrite-normalized REE patterns for Violet Town Volcanics host rocks and high-silica rhyolitic enclaves. The irregular
patterns of the high-silica rhyolitic enclaves (HSR: VT52 and VT60) are thought to result from combined monazite and zircon-garnet
fractionation. (b) Chondrite-normsdized REE patterns for the microgranitoid enclaves (ME). The samples have lower REE contents
than the host rocks, but the patterns have a similar shape. Pattern for host rock VT63 shown for comparison, (c) Chondrite-normalized
REE patterns for the basaltic andesitic enclaves (BA). The Eu anomalies are less negative than for the other rock types studied. Pattern
for host rock VT63 shown for comparison.
1395
JOURNAL OF PETROLOGY
VOLUME 37
FeO
Fig. 7. Variation of <5'*O (relative to VSMOW) with iron content
of the samples. Filled diamonds, host rock; open squares, microgranitoid enclaves; filled triangles, basaltic andesitic enclaves.
Host rocks 5'BO signatures are typical for S-type rocks. The basaltic andesitic and microgranitoid enclaves have lower <5lflO values,
but most arc still higher than for juvenile, mantle-derived rocks.
This may reflect interaction with the host magma, or an origin as
hybrid rockj.
NUMBER 6
DECEMBER 1996
age does not seem to have geological significance, as
the MSWD for this errorchron is very high (73).
Recalculated to 373 Ma, the range in 8^Sr/b6Sr,ranges from 0-7056 (VT18EC) to 0-7097 (VT2B).
The core and rim parts of VT18 have different Sr
isotopic ratios, increasing from 0-7056 to 0-7071
from core to rim (Table 4). The 87 Sr/ 86 Sr, ratio for
the host rock of enclave VT18E (VT18H) is 0-710.
Basaltic andesitt enclaves. Recalculated to an age of
373 Ma, the 87 Sr/ 86 Sr ratio of the basaltic andesite
enclaves varies between 0-7043 and 0-7044 (Table
4). This is slightly higher than the 87 Sr/ 86 Sr ratio of
the Earth's uniform reservoir at 373 Ma (0-7041;
Faure, 1986).
Smr-Nd isotopic system
Host rocks and high-silica rhyolitt enclaves. T h e host
that they have been intruded by the Strathbogie
Granodiorite, the K-Ar biotite age of which is
364±6 Ma (Richards & Singleton, 1981). It is
therefore likely that the reported isochrons reflect the
true age of the Violet Town Volcanics. Because of
the limited number of samples on which the 379 Ma
errorchron was based, the 373 Ma age of Clemens &
Wall (1984) has been used for age corrections in the
following discussions.
Microgranitoid enclaves. The errorchron for the
microgranitoid enclaves is of poor statistical quality
owing to the small spread in 87 Rb/ 86 Sr for the
samples, and gives an age of 638 ±18 Ma (Fig. 8),
and an initial ^ S r / ^ S r ratio of 0-7033. The 638 Ma
1.0
>- 0.9to
oo
•
O
*
O
host rocks
microgranitoid enclaves
basaltic andestte enclaves
hiqh-SI enclaves
errorchron
host + highenclaves
rock and rhyolite enclaves together define an Sm-Nd
isochron of 714 ±24 Ma (Fig. 9), with an initial
143
Nd/ 144 Nd ratio of 0-511646 (e Nd = - l - 4 at 714
Ma). The MSWD of this isochron is low (0-82), and
this would suggest that this age has geological significance. However, it is certain that this age does
not represent the age of eruption of the Violet Town
Volcanics. It is unlikely that it represents the age of
the source from which the Violet Town Volcanics
were derived, because mantle extraction ages (7* )
for the host rocks are invariably older (1360-1430
Ma, Table 4), whereas those of the high-silica
rhyolite enclaves are meaningless, owing to fractionation of Nd from Sm as a result of monazite and
zircon fractionation. It appears to be most plausible
that the 714 Ma age has no geological significance,
and that the 'isochron' only reflects a mixing line.
Recalculated to 373 Ma, the rhyodacites have eNdlvalues of —4-15 to —4-8, a range which is slightly
errorchron
microgranitoio}
enclaves
U.3
1£CT
•
hostrockj
microgranitoid endaves
A basaltic and«slte endaves
O high-SI endaves
A
A
xs
a
0.5126-
to
a
0.5124-
a
a
^^
^ ^
^teochron 1
host + hgh-SI
endives
0.5122-
20
30
87Rb/86Sr
Fig. 8. "Rb/^Sr vs "'Sr/^Sr for host rocks, high-silica rhyolite
enclaves, microgranitoid enclaves and basaltic andcsite enclaves.
The host rock + high-silica rhyolite enclaves define an errorchron
of 379 ± 8 Ma, with an " S r / ^ r , of 0-7092 (MSWD = 89). The
errorchron for the micrograriitoid enclaves is poorly constrained
(MSWD = 73); it yields an age of 638 ± 18 Ma and an "Sr/^Sr,
of 0-7033. It is unlikely that this age has geological significance.
The age for the host rock is probably close to the age of eruption
of the Violet Town Volcanics.
0.51200.10
0.12
0.14
0.16 0.18 0.20
147Sm/144Nd
0.22
0.24
Fig. 9. l47 Srn/ l+4 Nd vs u 3 Nd/ 1 4 4 Nd for microgranitoid enclaves,
host rock, high-silica rhyolite enclaves and basaltic andesite
enclaves. The host + high-Si rhyolite enclaves define an isochron
of 714124 Ma (MSWD=0-8, ' 4S Nd/ 144 Nd, = 0-511646), of which
the geological significance is unclear (see text). The microgranitoid enclaves do not define any isochron.
1396
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ENCLAVES IN AN IGNIMBRTTE, AUSTRALIA
Table 4: Isotopic characteristics and mantle extraction ages for microgranitoid enclaves (ME),
basaltic andesite enclaves (BA), high-silica rhyolite enclaves (HSR) and host rocks
i7
Sample
Type
Rb
Sr
"Sr/^Sr
"Sr/St,
VT63
host
1460
191-1
0-721848(25)
0-710093
VTQ16H
host
163-1
200-4
0-720079 (27)
0-708325
2-21
VT18H
host
165-6
145-6
0-727783 (27)
0-710273
3-30
Rb/"Sr
221
VT52
HSR
259-9
59-5
0-775766 (32)
0-708227
12-72
VT60
HSR
246-8
16-2
0-949333 (47)
0-708929
45-17
VT18EC
ME core
60-5
195-5
0-710408(15)
0-706648
0-90
VT18ER
ME rim
82-1
196-4
0-713508(34)
0-707077
1-21
VT24
ME
98-9
207-5
0-713284(23)
0-705953
1-38
VTQ18
ME
97-3
320-6
0-711046(23)
0-706382
0-88
VT1
ME
63-5
206-4
0-712014(26)
0-708028
0-75
VT2B
ME
114-8
212-6
0-718047(26)
0-709737
1-57
VT17
ME
140-4
186-4
0-719940(17)
0-708351
2-18
VT14E
BA
79-0
672-9
0-706116(23)
0-704314
0-339
VT6BC
BA
80-5
365-7
0-707810(27)
0-704426
0-636
Sample
Type
Nd
Sm
143
VT63
host
37-4
7-84
0-512219(7)
0-611910
-4-84
0-1268
1433
VTQ16H
host
35-9
7-46
0-512245(13)
0-611939
-4-27
0-1254
1364
VT18H
host
35-3
7-51
0-512259(8)
0-611945
-4-15
0-1287
1399
VT52
HSR
11-1
3-74
0-512592(11)
0-512095
-1-23
0-2036
-1023
VT60
HSR
9-3
3-64
0-512687(6)
0-512110
-0-93
0-2361
190
VT18EC
ME core
23-9
6-84
0-512459(6)
0-612097
-1-17
0-1478
1347
VT18ER
ME rim
24-8
5-67
0-512283(11)
0-511946
-4-14
0-1382
1505
1375
1
Nd/ l44 Nd
147
"Nd/"Nd;
Sm/ 144 Nd
7-DM
VT24
ME
22-2
5-28
0-512409(12)
0-512059
-1-93
0-1437
VTQ18
ME
24-7
4-98
0-512450(7)
0 512152
-0-11
0-1221
1011
VT1
ME
330
7-48
0-512343(8)
0-512009
-2-90
0-1369
1381
VT2B
ME
45-5
8-91
0-512203(10)
0-511914
-4-76
0-1183
1334
VT17
ME
23-7
5-71
0-512318(6)
0-511963
-3-80
0-1454
1573
VT14E
BA
36-2
6-55
0-512516(10)
0-612242
+1-64
0-1124
832
VT65C
BA
27-4
5-84
0-512561 (8)
0 512246
+1-74
0-1288
905
Numbers in parentheses are the errors on the measurements (2 a). Mantle extraction ages for the rhyolite enclaves are meaningless owing
to a change in Sm/Nd ratio since extraction from the mantle. Those for the microgranitoid enclaves are also unlikely to be meaningful, as
the core of enclave sample VT18 appears to be younger than the rim. ICP-MS and isotope dilution analyses of Sm and Nd generally agree
(see Table 3), except for samples VT18H and VT17. The reason for this disagreement is not clear.
larger than analytical uncertainty. The two highsilica rhyolites have values of —0-9 and —1-2. All
these values are less negative than those of S-type
rocks from the eastern part of the Lachlan Fold Belt
(McCulloch & Chappell, 1982; McCulloch &
Woodhead, 1993; Fig. 10), but the values for the
rhyodacite arc similar to those measured for granites
of the Wilson's Promontory Batholith (Elburg &
Nicholls, 1995), also nearer to the western margin of
the Lachlan Fold Belt.
Microgranitoid enclaves. I t is not possible to con-
struct a meaningful Sm/Nd isochron for the microgranitoid enclaves (Fig. 9). Their eNd>- values range
from —0-1 to —4-8. The highest E^M value was measured in microgranitoid enclave VTQ.18, which is
characterized by the presence of high-Mg orthopyroxene crystals; the lowest £Nd,- was measured in
sample VT2B, which is similar in mineralogy to the
host rock. The £Ndi values decrease from —1-2 to -4-2
from core to rim of enclaves VT18E; the latter value
1397
JOURNAL OF PETROLOGY
2-
OJ
-6-
• host rock
O high-S) rhyollte enclaves
microgranftoid enclaves
A basaltic andesite enclaves
VOLUME 37
NUMBER 6
DECEMBER 1996
host rocks. The two basaltic andesite enclaves plot
within the I-type array, but their signature is more
'crustal' than that of the most depleted I-type rocks.
Taken together with their relatively high (518O
values, this suggests that crustal material may have
been involved in their petrogenesis.
DISCUSSION
Origin of the microgranitoid enclaves
The microgranitoid enclaves in the Violet Town
Volcanics bear a striking resemblance to those in
-10
plutonic rocks: both kinds have rounded shapes and
0.714
0.702
0.706
0.710
small grain-sizes, display poikilitic textures and
87
Sr/86Sri
mineralogical similarities to the host rock, and may
contain reversely zoned plagioclase megacrysts.
Fig. 10. Initial 87 Sr/* s Sr vi ENd at 373 Ma for the different rock
Microgranitoid enclaves in granites have variously
types analysed. The microgranitoid enclaves display a range in
been interpreted as fragments of restite, dislodged
initial isotope values, which are correlated with their mineralogical characteristics, and which can change from core to rim of a
pieces of cumulates or chilled margins belonging to
single enclave. The high-silica rhyolitc enclaves are displaced
the host magma, or globules of more mafic magma
towards higher «NJ values compared with the host rocks. The
which mingled with and chilled against the cooler,
basaltic andesitic enclaves display the most juvenile isotopic charmore felsic host magma. Microgranitoid enclaves
acteristics. The fields for S- and I-type rocks from the Lachlan
Fold BeJt are taken from McCulloch & Chappell (1982) and
within the Violet Town Volcanics have been interMcCulloch 4 Woodhead (1993).
preted as 'accumulations of early magmatic phases'
by Clemens & Wall (1984). This interpretation
is indistinguishable from the analysis of the agrees with the more magnesian composition of
immediate host rock (VT18H).
orthopyroxene in the microgranitoid enclaves comBasaltic andesite enclaves. The e^^i values for the pared with that in the host rock (Figs 2 and 4), but it
basaltic andesite enclaves vary between +1-64 and does not explain the more mantle-like isotopic com+ 1-74, which is within analytical uncertainty. These position of most enclaves (Fig. 10). This would argue
values are lower than those for melts derived from a against an origin as chilled margins or cumulates of
depleted mantle source at 373 Ma (which is the host magma. An origin as restite (sensu stricto) is
approximately +8). Considering that the basaltic also precluded by the differences in isotopic comandesite enclaves most closely resemble the Torbreck position between microgranitoid enclaves and host.
Range Andesite, which has an age similar to the Although they could be argued to be pieces of more
VTV, the 8Nd values at 373 Ma are likely to be close refractory material, broadly similar to the source
to the true initial £Nd values for these enclaves.
rock for the ignimbrites (Chen et al., 1990), the
strong reversed zoning of the plagioclase megacrysts
87
(Fig. 3) within the enclaves would be hard to
Sr/ 86 Sr, vs e Nd variation
interpret within this scenario (see below). An origin
The microgranitoid enclaves display an overall by magma mingling would agree with their more
negative correlation between 87Sr/8 Sr,- and £Ndl (Fig. mafic character and fine grain-size. The difference in
10), and they overlap with the array for I-type rocks isotopic compositions between some microgranitoid
from the Lachlan Fold Belt as determined by enclaves and the host rocks suggests that this mafic
McCulloch & Chappell (1982) and McCulloch & 'enclave magma' was genetically unrelated to the
Woodhead (1993). The isotopic compositions of the rhyodacitic host magma.
microgranitoid enclaves which resemble the host
rocks in mineralogy overlap with those of the rhyoPlagioclase megacrysts in some microgranitoid
dacitic host rocks. The more mafic and orthopyr- enclaves, most notably sample VTQ.18, are similar
oxene-rich samples have higher ENJ,- and lower in size to plagioclase crystals in the host ignimbrite;
87
Sr/86Sr,- ratios than the more felsic or biotite-rich their cores have similar compositions to host plagiosamples.
clase, but their rims are more calcic (Fig. 3), similar
The high-silica rhyolites also have higher ENdl- to small plagioclase crystals within the enclaves. This
values, similar to those of the more mantle-like suggests that the megacrysts originated in the host
microgranitoid enclaves, but displaced towards magma, and became incorporated into the magma
87
Sr/ Sr,- ratios similar to those of the rhyodacitic that later formed the microgranitoid enclaves. As
1398
ELBURG
ENCLAVES IN AN IGNIMBRITE, AUSTRALIA
they were in thermal disequilibrium with the more
mafic magma, they started to dissolve and acquired
their rounded shape, then acted as nuclei for new
plagioclase growth during chilling of the magma
globule during mingling with the host magma. This
agrees with the interpretation of reversely zoned
plagioclase crystals in microgranitoid enclaves
within granitic rocks (Barbarin, 1990; Vernon,
1990).
The idea that this hybridization process is
responsible for the introduction of plagioclase megacrysts into enclave sample VTQJ8 agrees with the
high m^-number of this sample compared with its
rather silicic whole-rock composition. Mechanical
hybridization also explains why some enclaves
contain other megacrysts, such as quartz, garnet or
cordierite with sillimanite inclusions, similar to the
restitic-xcnocrystic cordierite in the host magma.
The strong normal zoning of some orthopyroxene
crystals within the enclaves (Fig. 4) can be interpreted to result from quick crystallization in a closed
system, or crystallization from a melt which was
progressively more contaminated by diffusional
exchange with the host magma, while the activity of
water was still low. It is not possible to make a
choice between the two models without additional
data on the isotopic composition of parts of individual crystals.
Microgranitoid enclave VTQ9, characterized by
its small size, a fine-grained equigranular texture
with strongly zoned orthopyroxene and very low
quartz contents, appears to be chemically different
from other enclaves (Fig. 5). This is most likely to be
the result of the expulsion of interstitial liquid,
leading to a cumulate composition for this sample.
Core-rim-host relationship for
microgranitoid enclave VT18
Microgranitoid enclave sample VT18E has been
divided into core (VT18EC) and rim (VT18ER)
portions for analysis. The core is dominated by
orthopyroxene + ilmenite + plagioclase in a quartz
matrix, whereas the rim contains more biotite, and
smaller amounts of orthopyroxene and ilmenite. The
whole-rock analysis of the enclave rim generally
plots between those of the core and the host rock.
Only the elements for which the observed differences
are well outside analytical uncertainty are discussed
here. SiO 2 , Na 2 O, K 2 O, Rb and Ba are higher in the
rim than in the core (Fig. 11), and the host rock
analysis is again higher than the enclave rim. No
difference between core and rim has been detected
with respect to A12O3, Zr and Sr; the host rock is
somewhat poorer in A12O3 and Sr, and richer in Zr.
(a)ioo
• VT18EC
E3 VT18ER
D VTI8H
.
s
8
.01
S»j
TK>2
AljOj
FeO*
MgO ClO
KjO
(b) 1000
•
7. ~
7
'g'lOO:
!
1
|
c
o
I 10|
Cr
Ba
Zn
Zr
Sr
Rb
Fig. 11. Concentrations of some oxidej (a) and trace elements (b)
in VT18EC-VT18ER-VT18H (core-rim-host of a microgTanitoid enclave). Most dementi either increase or decrease rystematically from enclave core to rim to host. An exception is Zn, and this
ij thought to reflect the change in mineralogy from orthopyroxene
to biotite from core to rim. {Note that the vertical axes have a log
icale.)
The constancy of A12O3 from core to rim is at odds
with the observed reaction of orthopyroxene to
biotite, a mineral far richer in alumina than orthopyroxene. The apparent lack of alumina enrichment
may, however, be related to a closure effect, as other
elements, most notably SiO 2 , may increase more
rapidly than AJ 2 O 3 (Cramer & Kwak, 1988). FeO*,
MgO, CaO, TiO 2 , V, Cr and to a lesser extent Ni
decrease from core to rim to host rock. Zn is higher
in the rim of the microgranitoid enclaves than in
either the core or the host rock.
REE patterns also change from core torimto host
(Fig. 12). VT18EC is the sample with the lowest
LREE contents of all microgranitoid enclaves, and
LREE contents of the rim arc intermediate between
those for the core and the host rock. From Sm to Ho,
the patterns are more complex. The rim has lower
1399
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 6
DECEMBER 1996
disregarded, as the thermal diffusion coefficient is
orders of magnitude higher than the coefficient for
(temperature driven) Soret diffusion (Sparks &
Marshall, 1986; Blundy & Sparks, 1992). Therefore,
any temperature gradient within the enclave is
•o
obliterated before Soret diffusion can take place. The
c
o
first option also does not seem to apply to the kind of
.c
u
core-rim relationship seen in VT18E, as it is unlikely
that residual liquids (which, in this model, migrate
u
towards the core of the enclave) would be enriched
o
in MgO, FeO and TiO2, and depleted in SiO2.
The gas filter pressing model requires that residual
liquid is expelled from the (core of) the enclave when
vapour-saturated crystallization in the enclaves produces high vapour pressures. Although this expulsion
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 12. Chondrite-normalized REE patterns for core, rim and of residual liquids is in accordance with the more
silicic compositions of the enclave rim compared
host rock of microgranitoid enclave VT18. The rim analysis does
not always plot between the core and the host rock. This may be with the core, there is no evidence for vapourthe result of different REE mineral—liquid distribution coefficients
saturated crystallization in the form of vesicles in the
for orthopyroxene (which is the main ferromagnesian mineral in
core of the enclave. Water pressures are likely to
the enclave core) and biotite (which is present in the rim).
have been fairly low, as the core of the enclave consists almost solely of water-free minerals. Moreover,
Sm, Gd, Tb, Dy and Ho than either microgranitoid isotopic differences between core and rim would not
enclave core or host rock; its Eu contents are, be expected if the core-rim differentiation were the
however, higher than those of the microgranitoid result of expulsion of residual liquids.
Therefore model 3, diffusional exchange between
enclave core or host.
The core of microgranitoid enclave VT18E has a microgranitoid enclave magma and host magma, is
lower 87Sr/86Sr,- and a higher ENdl- than the rim the most plausible mechanism to explain the chem(Table 4), which is intermediate between core and ical and mineralogical differences between enclave
host with respect to 87Sr/86Sr,-, and indistinguishable core and rim. The aberrant behaviour of Zn indifrom the host with respect to Nd isotopic composi- cates that the core-to-rim differences are not only the
effect of simple mixing between microgranitoid
tion.
enclave and host, because in that case all elements
should display a steady decrease or increase from
core torimto host. Orthopyroxene in the enclaves is
Mechanism of core-rim differentiation
Four mechanisms have been proposed to explain generally poorer in Zn than biotite (Elburg, 1995),
chemical differences between cores and rims of and it is therefore thought that the difference
microgranitoid enclaves in granites, assuming these between core andrimfor these elements is related to
formed by magma mingling, which is the preferred differences in modal mineralogy. The mineralogical
differences between core and rim are themselves
model for their origin:
related to the diffusional influx of elements from the
(1) Chilling of the rim of a magma globule against host magma, which stabilized biotite over orthopyrthe cooler host magma; residual liquid is displaced oxene. As Zn is more compatible in biotite than in
from this outer zone to the core of the enclave (Eberz orthopyroxene (Villemant et al., 1981; Bacon &
& Nicholls, 1990).
Druitt, 1988) the newly formed biotite has acted as a
(2) Differentiation in the liquid state owing to a sink for Zn, thereby promoting the diffusional influx
temperature gradient within the enclave (Soret dif- of this element into the rim of the enclave. Most of
fusion) (Eberz & Nicholls, 1990).
the REE systematics in the enclave core, rim and
(3) Diffusional exchange of elements between host rock can be explained by simple diffusional
enclaves and host magma (Barbarin, 1988; Cramer exchange between enclave and host. The cross-over
& Kwak, 1988; Eberz & Nicholls, 1990; Barbarin & between core and rim in the MREE may also be
Didier, 1992; Blundy & Sparks, 1992; Seaman & related to the higher modal amounts of biotite in the
rim, owing to the difference in mineral-melt parRamsey, 1992).
tition coefficients for the REE between biotite and
(4) Gas filter pressing (Bacon, 1986).
orthopyroxene (Hanson, 1980).
Of these proposed models the second one can be
100
a VT18EC
• VT18ER
• VT18H
1400
ELBURG
ENCLAVES IN AN IGNIMBRTTE, AUSTRALIA
It is remarkable that isotopic distinctions between
the rim of the enclave and the host rock are maintained for the Sr isotopic system, whereas the enclave
rim and host rock appear to be equilibrated with
respect to the Nd isotopic system. Several studies
have indicated that homogenization of Sr isotopes is
a relatively rapid process (Baker, 1989), which is
often observed between microgranitoid enclaves and
their host rocks (Allen, 1991; Holden et al., 1991);
equilibration of Nd isotopes is slower by at least a
factor of two (Lesher, 1990), and distinctions
between microgranitoid enclaves and host rocks are
often preserved within this isotopic system (Allen,
1991; Holden et al., 1991; Elburg & Nicholls, 1995).
This is contrary to the observations made here, and
this may be explained by the fact that the experimental studies refer to diffusion in a silicate liquid,
whereas it is likely that the enclaves already contained crystals when equilibration occurred. The
significance of this is explained below.
The fact that the mineralogical zoning of enclave
VT18E is roughly concentric with respect to its outer
rim suggests that this zoning was established after
the enclave had acquired its final shape, i.e. after
mingling with the host magma. Thermal diffusivity
for silicic melts is of the order of 8 x 10~7 m /s
(Huppert & Sparks, 1988), whereas the diffusivity
for fast-moving species such as the alkalis is 3 x 10"
m2/s at 900°C (Baker, 1991), and the self-diffusion
(isotopic diffusion) for Sr and Nd at this temperature
is 10~' 7 and 10"'^ m 2 /s, respectively (Lesher, 1994).
This means that thermal equilibration between
enclave and host would have been orders of magnitude faster than chemical or isotopic equilibration,
and as the more mafic microgranitoid enclave
magma is likely to have liquidus temperatures higher
than the temperature of the host with which it
mingled, most chemical and isotopic equilibration
will have occurred when the enclave was already
partially solid. Once the enclave was partially crystalline, Sr and Nd would have become incorporated
into solid phases such as plagioclase and apatite in
which these elements are compatible. If this is the
case, the rate-determining step for isotopic equilibration is probably not the self-diffusion of the elements in the silicic melt, but diffusion into and out of
the crystalline phases. The exact rate and extent of
equilibration are of course dependent on the
mineral—liquid distribution coefficients for Nd and
Sr, the modal proportions of the mineral phases
crystallizing in the enclave (and in the host), the
absolute concentrations of the elements in the two
(residual) magmas, the temperature of the system,
and the amount of time elapsed between mingling
and eruption of the felsic magma. However, if the
majority of the Sr in the microgranitoid enclave is
locked up in plagioclase, and Nd in apatite, then the
rates of equilibration for the two isotopic systems will
be dependent mainly upon the rate of diffusion of Sr
in plagioclase and Nd in apatite, as well as the size of
the plagioclase and apatite crystals. The diffusivity
of Sr in plagioclase (An^) at 800°C is of the order of
(2-5) x 10~21 m2/s (Giletti & Casserly, 1994). The
diffusivity of Nd in apatite does not seem to have
been studied widely, but that of Sm in apatite at
800°C is also of the order of 5 x 10~21 m2/s (Watson
et al., 1985), and, considering the similarities in size
and charge between Sm and Nd ions, it is likely that
the diffusion of Nd is equally rapid. As the diffusion
coefficients for Sr in plagioclase and Nd in apatite
are roughly equal, then the rates of equilibration will
depend only upon the size of the crystals. As apatite
crystals tend to be up to an order of magnitude
smaller (especially with respect to width) than plagioclase crystals in the microgranitoid enclaves,
equilibration will be more rapid for the Nd isotopic
system than for Sr. Moreover, the host magma
probably contained higher concentrations of Nd and
lower Sr (sec below) than the microgranitoid enclave
magma, and this is another factor that is likely to
promote Nd isotopic equilibration to a greater extent
than Sr isotopic equilibration. The fact that the Nd
isotopic system generally appears to preserve more
distinctions between microgranitoid enclaves and
host rocks than the Sr isotopic system could indicate
that hybridization of the 'enclave magma' often
occurs before the mingling event.
It is likely that diffusional exchange between
microgTanitoid enclave and host magma not only
explains the core-rim systematics of microgranitoid
enclave VT18E, but also the range of (isotopic)
compositions measured in the other enclaves analysed. It is probably significant that the most
'crustal' isotopic ratios are measured in enclaves
which also show mineralogical similarities to the host
rock, whereas those with higher proportions of
orthopyroxene display more juvenile isotopic characteristics. The fact that some microgranitoid
enclaves have attained isotopic compositions similar
to those of the host rocks could mean that they must
have spent an extended period of time in contact
with the host magma at elevated temperatures. It is
therefore likely that at least some of the mingling
occurred early in the evolution of the host magma.
The presence of megacrysts, similar in size to those
in the host ignimbrite, in some microgranitoid
enclaves indicates that some of the mingling must
have taken place later during the evolution of the
host magma. Mingling with mafic magma therefore
appears to have been a continuous process
1401
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 6
DECEMBER 1996
throughout the evolution of the felsic host magma.
An alternative interpretation of the isotopic, geochemical and mineralogical similarities between
some microgranitoid enclaves and their host is that
the magma that formed these enclaves was already
hybridized before the mingling event. This hybridization could have occurred when the enclave
magma was in contact with the felsic magma in a
stratified magma chamber (Barbarin, 1988; Poli &
Tommasini, 1991).
mantle-derived melt that has been contaminated by
crustal material. This could explain the intermediate
Sr and Nd isotopic ratio as well as the elevated 5iaO
signature. However, the high Cr content of the
basaltic andesite enclaves analysed suggests that
contamination by crustal material cannot have been
substantial. The isotopic signature of these samples
is, however, significantly less depleted than the
mantle-derived magmas which are thought to be
responsible for the magmatism in the eastern part of
the Lachlan Fold Belt (Collins, 1995). This may
reflect a fundamental difference in the nature of the
Interpretation of basaltic andesite enclaves mantle component involved in granitoid magmatism
The basaltic andesite enclaves are not S-type rocks, of the central part of the Lachlan Fold Belt, to which
as they contain clinopyroxene, a mineral which is the Violet Town Volcanics belong, compared with
unstable in a peraluminous environment, reacting the eastern part of this belt. A similar distinction in
with aluminous melt to form orthopyroxene and compositions of the granitoids from the central and
plagioclase (Clemens & Wall, 1988). The presence of eastern part of the Lachlan Fold Belt has previously
clinopyroxene, together with the generally angular been advocated by Gray (1990) on the basis of the
shape of the enclaves and the fact that parts of the age and Sr isotopic compositions of the plutons.
enclave rims are defined by fractured crystal However, the possibility that the Sr and Nd isotopic
margins, suggests that these rock bodies are xeno- compositions of the basaltic andesite enclaves have
liths, and are not genetically related to the magma been affected by interaction with crustal material
in which they are found. The presence of biotite-rich cannot be discarded, and more work on mafic rock
rims on the enclaves does, however, suggest that they types in the central Lachlan Fold Belt needs to be
done to test this hypothesis.
have interacted with the host magma.
The high <518O of the basaltic andesite enclaves
suggests that these enclaves are not purely mantlederived rocks, but carry a crustal component. This
crustal component may have been introduced by
interaction between the enclaves and the host ignimbrite, either after eruption (owing to fluids percolating through the volcanic pile) or before. The
87
Sr/ Sr,- and e^di ratios of these two samples may
also have been influenced by interaction with the host
magma, as was the case for oxygen isotopes. However,
equilibration with respect to oxygen isotopes is a
relatively rapid process (Cole & Ohmoto, 1986)
compared with that of Sr and Nd isotopes (Lesher,
1994). The high Sr contents of these samples (especially VT14) compared with the host magma makes
the 87Sr/86Sr,- ratios less susceptible to alteration.
Alteration or metasomatism does generally not affect
the ENJ values of rocks (Valbracht, 1991). Moreover,
the Sr and Nd isotopic compositions of the two analysed basaltic andesite enclaves are virtually indistinguishable, whereas those for the microgranitoid
enclaves, of which the variation in radiogenic isotopes
can be ascribed to interaction with the host magma,
are highly variable. It is therefore likely that the
87
Sr/86SrI- and ENJ,- ratios measured are close to their
primary values, and have suffered little effect from
interaction with the host ignimbrite.
The magma parental to the basaltic andesite
enclaves may have been a hybrid, consisting of a
Relationship between microgranitoid and
basaltic andesite enclaves
Proton microprobe analyses of orthopyroxene
crystals in some microgranitoid enclaves, most
notably sample VTQ18, show that they contain high
levels ofNi (s$1100 p.p.m.) and Cr (<3100 p.p.m.)
(Elburg et al., 1995; Table 5). This indicates that the
magma from which the orthopyroxene crystallized is
likely to also have had high contents of these elements. Using literature values for orthopyroxene—
liquid distribution coefficients the magma in equilibrium with the orthopyroxene crystals can be estimated to have contained at least 150 p.p.m. Ni and
300 p.p.m. Cr (Elburg et al., 1995). In a similar
manner, the Sr and Ba content of the magma can be
calculated from the Sr and Ba contents of the plagioclase. Using a plagioclase—liquid distribution coefficient of 2 for Sr and 1 for Ba [values from Dunn &
Sen (1994)] the magma parental to the microgranitoid enclaves is estimated to have contained
~500 p.p.m. Sr and 350 p.p.m. Ba (Elburg, 1995).
The high Ni and Cr contents of this 'recalculated'
enclave magma suggest that it was a juvenile,
mantle-derived magma. If the recalculated Sr and
Ba contents are correct, then the magma is likely to
have had calc-alkaline affinities. This mantle-derived
magma must afterwards have been contaminated
1402
ENCLAVES IN AN IGNTMBRITE, AUSTRALIA
ELBURG
Table 5: Selected trace elements in orthopynxene andplagioclase crystals in microgranitoid
enclave VTQ18, analysed by proton microprobe (Elburg et aL, 1995)
Sample
Mineral
Zn
Cr
Ni
1200
VTQ18
opx core
2700
VTQ18
small plag
—
Ga
Sr
Ba
110
27
27-6
991
371
with crustal melts, as the mineralogy of all micro- (a).
U.3 \CS'
simple mbdng model 1
granitoid enclaves studied is typical for S-type rocks:
* VTG5C
simple modng model 2
hornblende, clinopyroxene and magnetite are
mlcrognnttoW enclaves
a
0.5122- \
absent, ilmenite is present, and biotite has yellow to •o
red—brown pleochroism. The contamination may z
0.5121have resulted from assimilation of sediments or
\ \
mixing and/or difRisional exchange with an S-type
0.5120magma, most likely the host magma. It is therefore CO
probable that even the isotopic and chemical signatures of the 'least contaminated' enclaves, such as
^
\
0.5119VTV
^^^^^
Continentil
VT18EG or VTQ18, have been modified, and that
^^K^^crust
the primitive 'enclave magma' had more mantle-like
0.5118isotopic signatures.
0.704
0.708
0.712
0.716
0.720
87 S r /86 S r
If the 'enclave magma' was indeed characterized
by high Cr, Ni and Sr contents, combined with
higher £Ndl- and lower 87Sr/86Sr values, then this
magma broadly resembles the one that formed the
basaltic andesite enclaves. It is possible to model the
isotopic signature of the microgranitoid enclaves as a (b)
U.3 \C2m
simple mixture of the basaltic andesite magma
—»— simple mbdng model 1
A VT65C
— I — simple mbdng model 2
(sample VT65c) and crustal material (200 p.p.m.
D
mtcroflmntoid endaves
\
0.5122Sr, 37 p.p.m. Nd, 87Sr/86Sr = 0-718 and
143
T3
D
^.
Nd/144Nd = 0-5118), similar to the Cooma gneiss
Z
(McCulloch & Chappell, 1982), a sample which is
0.5121D
VV
•often considered to be the typical crustal component
D
\ \
T3
of Lachlan Fold Belt magmatism (Collins, 1995).
\
)
Z 0.5120"
\ !
CO
V
This gives a moderately good fit for the isotopic
composition of the enclaves (Fig. 13, model 1), con0.5119sidering their scatter and the fact that this model is a
\
Continental
gross oversimplification, as it does not take diffu>^ crust
0.5118sional exchange between enclaves and host magma
30
40
20
50
into account. A similar simple mixing model
Nd (pp.rn)
between VT65c and the VTV magma (Fig. 13,
13. Simple mixing models between basaltic andesite enclave
model 2) gives an even better fit for the isotopic Fig.
sample VT65C (350 p.p.m. Sr, 27 p.p.m. Nd, 87Sr/8SSr = 0-7044,
composition of the microgranitoid enclaves. l43 Nd/ 144 Nd = 0-5122) and continental crust, similar to the
However, the Nd concentrations of the micro- Cooma Gneiss (200 p.p.m. Sr, 37 p.p.m. Nd, 87Sr/e6Sr = 0-718,
and VTV
granitoid enclaves do not fit this model very well. If '•"Nd/""Nd = 0-5118) (model 1) and between VT65C
magma (200 p.p.m. Sr, 37 p.p.m. Nd, B7Sr/86Sr = 0-710,
the enclave magma was broadly similar to the l43 Nd/ 144 Nd = 0-5119) (model 2). Tick marks indicate the
magma parental to the basaltic andesite enclaves, amount of crustal material added to the basaltic andesite (steps
then the REE contents of the microgranitoid of 10%). The second model gives the most adequate reproduction
enclaves must have been reduced. Unusual enrich- of the isotopic composition of the microgranitoid enclaves (a), but
of the enclaves are generally lower than in
ments of certain elements in mixed mafic-felsic the Nd concentrations
the model (b). (See text for further discussion.)
1403
JOURNAL OF PETROLOGY
VOLUME 37
complexes have been reported previously (Marshall
& Sparks, 1984; Ayrton, 1991; Wiebe, 1993, 1994),
and perhaps similar processes of replenishment combined with crystal fractionation, or diffusion between
two chemically and thermally distinct magmas could
explain the REE depictions observed here.
Relationship between host rocks and highsilica rhyolite enclaves
The curvature of the trends for the Violet Town
Volcanics in some Harker variation diagrams (most
notably in the K2O diagram) was interpreted by
Clemens & Wall (1984) as reflecting fractional crystallization and was used as an argument against
restitc unmixing as an explanation for the chemical
variation in the ignimbrite. Clemens & Wall (1984)
included 'microtonalitic' (microgranitoid) enclaves
and high-silica rhyolite samples in their array. The
recognition of the curved trends (and thus of crystal
fractionation) relies heavily on these high-silica
rhyolite and microgranitoid enclaves.
It is very hard to argue for a direct relationship
between the ignimbrite host rocks and the high-silica
rhyolite enclaves by simple crystal fractionation
considering their disparate Nd isotopic characteristics. Another indication that they are not related
by simple fractionation is the higher contents of, for
example, Zn in the high-silica rhyolite samples compared with the most silicic samples of the host
ignimbrite. This element is highly compatible in
biotite (Nash & Crecraft, 1985; Giraud et al., 1986:
D biotite/liquid ^10) which constitutes 10% of the
modelled fractional crystallization assemblage
(Clemens & Wall, 1984). Zn is therefore expected to
decrease or stay approximately constant with
decreasing FeO* content, rather than to increase.
Although the high-silica rhyolite enclaves are Stype rocks, with garnet and cordierite occurring as
phenocrysts, their 14SNd/144Nd ratios are not nearly
as negative as for typical S-type rocks from the
Lachlan Fold Belt, whereas their 87Sr/86Sr ratio is
similar to that of other S-type rocks from this area.
It is unclear whether this means that the high-silica
rhyolite magma was originally a derivative of a
magma similar to that which formed the basaltic
andesite enclaves and microgranitoid enclaves, but
has been contaminated by the rhyodacitic host
magma; or that the high-silica rhyolite magma is
more closely related to the rhyodacitic host, but was
contaminated by the more mafic mantle-derived
melts. If the magma parental to the high-silica
rhyolite enclaves was originally more similar to a
mantle-derived magma, the decoupling of the Sr and
Nd isotopic systems may be explained by assim-
NUMBER 6
DECEMBER 1996
ilation combined with crystal fractionation (AFC).
This is demonstrated in Fig. 14 (model 1), which
shows AFC curves (DePaolo, 1981) for the contamination of a mantle-derived melt (470 p.p.m. Sr,
14 p.p.m. Nd, 87Sr/86Sr = 0-7035, 143Nd/1+4Nd =
0-512576) by assimilating continental crust (150
p.p.m. Sr, 26 p.p.m. Nd, 87Sr/86Sr = 0-716,
' 4 %d/ 144 Nd = 0-5118). Both the mantle and crustal
end-member arc similar to those used by McCulloch
(a)
O hfgh-S rfiyotlte endives
« - A F C model 1
0.5124VT65C
" ^ 0.5122•D
Z
CO
*
0.5120-
0.5118
0.700
Continental
cnut
0.705
0.710
0.715
0.720
D Hgh-Si rhyoBte endives
-«- AFC model 1
•AFCmode!2
VT65C
0.51180.5116
12
17
22
27
Nd (Rp.m)
Fig. 14. AFC models to explain the isotopic composition of the
high-silica, rhyolite enclaves. Model 1: assimilation of uppercrustal material (150 p.p.m. Sr, 26 p.p.m. Nd, 87 Sr/ S6 Sr = 0-716,
l43
Nd/ l 4 4 Nd = 0-5118) by a mantle-derived melt (470 p.p.m. Sr,
14 p.p.m. Nd, " S r / 1 * ^ 0-7035, M3 Nd/ 144 Nd = 0-512576). Both
components are roughly similar to those used by McCulloch &
Chappell (1982) for modelling magmatism in the Lachlan Fold
Belt. DSr = 3, £>Nd = 1-5, /? = 0-3. Model 2: assimilation of crustal
material (200 p.p.m. Sr, 27 p.p.m. Nd, B7 Sr/ M Sr = 0-718,
M3
Nd/ l 4 4 Nd = 0-5118) by basaltic andesite magma, as represented
by VT65C. Z)Sr = 3-5, £>Nd= 1-8, R = 0-3. Tick marks indicate the
amount of melt left (F). Both models reproduce the isotopic characteristics of the high-silica rhyolite enclaves (a), but only the first
one also reproduces their Nd content (b). (See text for further
discussion.)
1404
ELBURG
ENCLAVES IN AN IGNIMBRITE, AUSTRALIA
& Chappcll (1982) for their modelling of magmatism in the Lachlan Fold Belt. The bulk mineralliquid distribution coefficients for Sr and Nd were
taken to be 3 and 1-5, respectively, and the rate of
assimilation-crystallization (R) as 0-3. The process
modelled is a very simplified version of what is
thought to happen during AFC, as it does not allow
for changing distribution coefficients during the
process. Nevertheless, both the isotopic and trace
element characteristics of the high-silica rhyolitic
enclaves are adequately modelled. An AFC model in
which a magma similar to the basaltic andesite
enclaves assimilates crustal material (Fig. 14, model
2) gives less satisfactory results for the trace element
composition of the high-silica rhyolite enclaves. This
could reflect a period of fractional crystallization
without assimilation postdating the AFC process.
Town Volcanics magma. Hybridization of the mafic
magma occurred through magma mixing and diffusional exchange; these processes have obscured the
primary chemical, isotopic and mineralogical characteristics of the mafic magma. Trace element contents of plagioclase and orthopyroxene crystals in
relatively unequilibrated enclaves suggest that the
'enclave magma' was similar to the magma that
formed the basaltic andesite enclaves. These basaltic
andesite enclaves were incorporated into the ignimbrite suite as solid xenoliths. Their Sr and Nd isotopic characteristics are not nearly as depleted as
those of the juvenile magmatic component recognized in the eastern part of the Lachlan Fold Belt,
and this may point to the involvement of crustal
material in their petrogenesis, or derivation from a
less depleted mantle source.
The enclaves of high-silica rhyolite cannot be
Origin of the host ignimbrite
related to the host ignimbrite by fractional crystalAnalysed samples of host rock appear to have lization as they have eNdi values up to 3 units higher
variable isotopic compositions, and this may indicate than the host rocks. It is possible that they evolved
that the magma was derived from an inhomogeneous from a mantle-derived magma, perhaps similar to
(mixed) source, or that magma mixing played a role the one which formed the microgranitoid and
in its evolution. The presence of hybridized micro- basaltic andesite enclaves, by AFC processes.
granitoid enclaves within the host rock indicates that
The close association of the S-type Violet Town
processes such as magma mixing and mingling did Volcanics and juvenile mantle-derived melts suggests
take place. This mixing may also have influenced the that these mafic magmas may have played a role in
isotopic compositions of the host rocks, and this may the petrogenesis of the peraluminous magma, acting
explain why the Violet Town Volcanics have higher as a heat source for the partial melting event that
£
Ndi values than most previously analysed S-type produced the felsic magma. The relatively high £Ndlrocks from the Lachlan Fold Belt (at similar values values of the felsic magma suggest that they were
of SiOj). However, as most previous analyses have not produced by the melting of detrital sediments
been carried out on rocks from the eastern Lachlan only. It is possible that the mantle-derived magmas
Fold Belt, the atypical isotopic characteristics of the also contributed chemically to the Violet Town
Violet Town Volcanics could also be explained by a Volcanics, but an origin by the melting of a mixed
fundamental difference in source rocks for felsic source, or of very immature sediments cannot be
magmas in the eastern and central Lachlan Fold Belt ruled out.
(Gray, 1990). The question of whether the higher
fiNdi values for the Violet Town Volcanics and other
S-type rocks from the central Lachlan Fold Belt ACKNOWLEDGEMENTS
result from magma mixing, from partial melting of a The research for this paper was done while M.A.E.
source containing higher proportions of underplated received a Monash Graduate Scholarship and an
igneous material (Williams et al., 1990), or from Overseas Postgraduate Research Scholarship. The
partial melting of less mature detrital sediments majority of the paper was written while M.A.E. held
(Gray, 1990) is beyond the scope of this paper and a Monash Postgraduate Writing-Up Award. Maunnu
will be discussed elsewhere (Elburg, in preparation). Haukka, David Sewell, Roland Maas, Brady Bird,
Donna Korke and Nicola Fortune helped with the
chemical analyses. The help from Jeff Foster and
John Foden with calculating isochrons was much
CONCLUSIONS
Microgranitoid enclaves in the Violet Town Vol- appreciated. Paul Bons is acknowledged for his assiscanics are likely to represent globules of a more tance in the field. All analyses were funded by ARC
mafic, mantle-derived magma that mingled with the Small Grants to Ian Nicholls. Ian Nicholls and Alan
felsic host magma before eruption. Magma mingling Greig are thanked for their constructive criticism.
appears to have been a continuous process, occurring The manuscript benefited from detailed reviews by
both early and late in the evolution of the Violet Bill Collins and an anonymous reviewer.
1405
JOURNAL OF PETROLOGY
VOLUME 37
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