JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 11
PAGES 2053±2080
2003
DOI: 10.1093/petrology/egg070
Phonolitic Diatremes within the Dunedin
Volcano, South Island, New Zealand
RICHARD C. PRICE1*, ALAN F. COOPER2, JON D. WOODHEAD3
AND IAN CARTWRIGHT4
1
SCHOOL OF SCIENCE AND TECHNOLOGY, THE UNIVERSITY OF WAIKATO, PRIVATE BAG 3105, HAMILTON,
NEW ZEALAND
2
DEPARTMENT OF GEOLOGY, THE UNIVERSITY OF OTAGO, PO BOX 56, DUNEDIN, NEW ZEALAND
3
SCHOOL OF EARTH SCIENCES, UNIVERSITY OF MELBOURNE, VIC. 3010, AUSTRALIA
4
SCHOOL OF GEOSCIENCES, MONASH UNIVERSITY, CLAYTON, VIC. 3800, AUSTRALIA
RECEIVED SEPTEMBER 13, 2002; ACCEPTED MAY 19, 2003
The Port Chalmers Breccia is a vent-filling, clastic volcanic
unit exposed within the Miocene Dunedin Volcano of South
Island, New Zealand. Clasts (up to in excess of 1 m but
generally 520 cm) are supported in ash and fine lapilli of
phonolitic (ne-benmoreite or tephro-phonolite) composition
and the dominant clast type (55 to almost 100%) is also
phonolitic. Less abundant lithologies include ne-normative
basalt (basanite), hawaiite, mugearite and trachyandesite, syenites and microsyenites, coarse-grained mafic (gabbros) and
ultramafic rocks (pyroxenites, hornblendites), schists and sediments. The breccias were emplaced as diatremes associated with
localized, but highly explosive, eruptive events in which mantlederived CO2 was an important component. The syenitic and
ultramafic clasts could represent intrusive suites produced by
crystal fractionation acting on parental ne-benmoreite magmas
that may themselves have been derived by crystal fractionation
from basanitic precursors. An alternative variation on this model
is that the parental ne-benmoreites were generated through
partial melting of an alkalic igneous underplate. Sr, Nd and
Pb isotopic compositions are strikingly similar to those of
intraplate igneous rocks, ranging in age from 100 to less than
10 Ma, from elsewhere in the South Island, and New Zealand's
sub-Antarctic islands, the south Tasman Sea and the Ross Sea
region. This regional, HIMU-influenced, isotopic signature is
believed to be derived from within the lithospheric mantle.
INTRODUCTION
KEY WORDS: phonolite; diatreme; nepheline syenite; Dunedin
Volcano; alkalic rocks; fractional crystallization
The city of Dunedin on the SE coast of New Zealand's
South Island (Fig. 1) is located within a complex alkalic volcano of Miocene age. Coombs et al. (1986)
defined the Dunedin Volcanic Group to include rocks
of the volcano and other similarly aged alkalic eruptives and shallow intrusives in east Otago. Within the
Dunedin district, rocks of the Dunedin Volcanic Group
are exposed over 450---500 km2 with vertical relief of
around 700 m. Most of the Dunedin Volcano was
constructed between 13 and 10 Ma (McDougall &
Coombs, 1973) with the earliest activity being shallow
marine eruptions of silica-saturated trachytes and
basalts and the most recent events the subaerial emplacement of phonolitic domes and flow domes.
The vast majority of Dunedin volcanic rocks are
nepheline-normative (ne-) basalts and phonolites
(Benson, 1942, 1968; Coombs et al., 1960; Coombs &
Wilkinson, 1969; Allen, 1974; Price & Chappell, 1975).
The volcanic stratigraphy within the volcano was
divided by Benson (1959, 1968) into four phases
(Initial, First, Second and Third), based on inferred
regional correlations of inter-volcanic sedimentary
sequences interpreted as representing periods of erosion
and reduced volcanic activity.
The focus of this paper is a remarkable series of
volcanic breccias collectively referred to as the Port
Chalmers Breccia (PCB) (Fig. 2b). Compared with
the rest of the Dunedin Volcano the PCB contains a
*Corresponding author. E-mail: [email protected]
Journal of Petrology 44(11) # Oxford University Press 2003; all rights
reserved
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 11
NOVEMBER 2003
Fig. 1. Regional setting for Dunedin Volcano and distribution of volcanic and intrusive rocks of the Southern New Zealand--Tasman---Antarctic isotopic province. (a) Regional locality map showing location and ages of centres discussed in the text and track of
Balleny hotspot. SM, seamount. (b) Location and ages of Cretaceous---Tertiary igneous centres of South Island. Data from Coombs et al.
(1986), Cooper (1986), Gamble et al. (1986), Weaver & Smith (1989), Lanyon et al. (1993) and Baker et al. (1994).
wider variety of rock types in the form of fine- and
coarse-grained fragments comprising basalt, basanite,
ne-trachyandesite, ne-benmoreite and phonolite, hornblendites, gabbros and nepheline syenites, together
with fragments of schist and non-volcanic clastic sediments. The igneous clasts preserve an extended petrological history of processes that occurred in the
evolution of an undersaturated (basanite---phonolite)
magmatic series and permit fuller isotopic comparisons
with data for rocks from the distinctive Tasman--Balleny---New Zealand isotopic province (Lanyon
et al., 1993; Baker et al., 1994).
THE PORT CHALMERS BRECCIA
The PCB is exposed in vents aligned NW---SE from
Port Chalmers, across the central Otago Harbour
into Hoopers Inlet on the Otago Peninsula (Fig. 2a).
The most extensive exposure (13 km 25 km) occurs
at Port Chalmers on the northern side of Otago
Harbour (Fig. 2). Benson's 1968 map shows other
roughly circular vents containing PCB located on
Otago Peninsula and ranging in diameter from 160 to
650 m. Coombs (1965) noted that the explosive
eruptions that were associated with the emplacement
of the PCB were regarded by Benson (1942) as occurring at the end of the First Main Eruptive Phase,
preceding the main constructional event of the
Second Main Eruptive Phase (Coombs et al., 1960;
Benson, 1968).
Our primary focus has been on the most extensive
exposures of PCB around Port Chalmers (northern
locality; Fig. 2) where we have examined and
sampled two specific exposures (Leans Rock and
Scott Memorial; Fig. 2b) in considerable detail. We
have also collected data and samples from a locality on
the Otago Peninsula (southern locality; Fig. 2a).
In the vicinity of Port Chalmers, the PCB was
emplaced through microsyenites, and dolerites that
show hydrothermal alteration associated with abundant carbonate (calcite and ankerite) veins. Allen
(1974) described exposures in a railway tunnel cutting
the northern boundary of the Port Chalmers exposures,
which provide evidence that at least some of the
breccia was erupted onto an eroded surface. Dykes of
basalt and trachyandesite cut the dolerites and microsyenites underlying the PCB but not the PCB itself
(Fig. 2b), indicating that they were emplaced before
the breccia-forming eruptions. In turn, the PCB was
intruded by a later phase of basaltic dykes. At the
2054
PRICE et al.
PHONOLITIC DIATREMES OF DUNEDIN VOLCANO
Fig. 2. Maps showing location of outcrops of PCB discussed in this paper. (a) Dunedin District showing distribution of outcrops of PCB,
northern and southern localities, and area covered by Fig. 1b. H.In, Hoopers Inlet. (b) Map showing the geology of the northern locality.
Some data from Benson (1968) and Allen (1974).
southern locality, the emplacement occurred through
trachytic tuffs of the Initial Eruptive Phase (Benson,
1968; Allen, 1974).
The PCB shows a range of facies types from the
dominant massive, matrix-supported breccia to rare
bedded units in which layers of fine lapilli vary in
thickness from 2 cm to 1---2 m. Within the Port
Chalmers exposure bedding dips to the west, with the
angle of dip decreasing from around 40 at the margins
of the vent near inferred contacts with underlying
eroded flows to around 16 within the breccia pipe
(Allen, 1974, and this work). Neither cross-bedding
nor graded bedding have been observed in any of the
PCB exposures.
The clasts in the PCB are matrix supported, with
around 80% of a typical outcrop being matrix finer
than 1 cm and the largest clasts up to 16 m. At the
southern locality most of the clasts (almost 100%) are
phonolitic (ne-benmoreite or phonolite). Phonolitic
types are dominant (56%) at the northern locality
but clasts of basalt and dolerite (12%), trachyandesite
(9%), syenite (9%), gabbro and ultramafic cumulate
( 1%), schist (12%), and Tertiary or Cretaceous
sediment ( 1%) also occur. Most phonolitic clasts
show thin (1---5 mm) bleached rims and the matrix of
the PCB is cemented by carbonate, analcime,
rare alkali feldspar and kaolinite, suggesting that
hydrothermal alteration was pervasive following eruption and deposition. Clasts with the characteristics
expected of juvenile material (e.g. glassy rims, strong
vesiculation, jig-saw disaggregation) are rare. The
obvious candidates are pale coloured devitrified glassy
volcanic rocks containing abundant carbonate- and
analcime-filled vesicles and occurring either as discrete
clasts up to 1---2 cm in diameter or as occasional thin
discontinuous rims on syenite fragments.
The level of exposure of the PCB is close to the base of
the volcanic sequence, as inliers of the underlying sedimentary sequence are known in both localities (e.g.
Coombs et al., 1960). Also, close to the breccia outcrops
at Port Chalmers the country-rock trachytic tuffs are
cut by thin, fine-grained dykes of foraminiferal limestone, forced up from underlying sediments by lithostatic loading (P. Gurney, personal communication,
1992). The clasts in the PCB are, however, dominantly
phonolitic and rocks of this type are not a significant
component among the earliest eruptives of the
Dunedin Volcano, which are dominantly basalts and
quartz-normative trachytes. Because juvenile clasts are
rare, it is unlikely that the abundant phonolitic clasts
in the breccias represent disrupted juvenile magma. If
the phonolitic clasts are accidental then their
abundance suggests that they have come from above
the present levels of exposure in the PCB and this could
2055
JOURNAL OF PETROLOGY
VOLUME 44
only be the case if the breccias are significantly younger
than has previously been supposed. Rather than
being part of the First Eruptive Phase (Benson,
1968), the PCB must post-date at least the First and
possibly the Second Main Eruptive Phase and have
formed relatively late in the history of the Dunedin
Volcano.
The garnet---albite zone metamorphic grade represented by breccia schist clasts is the same upper greenschist facies assemblage as that observed in basement
exposures in the Dunedin district. Higher-grade assemblages are not observed.
PETROGRAPHY
Petrographic aspects of volcanic rocks of the Dunedin
Volcano and clasts from the PCB have been described
in detail by Benson (1942), Coombs & Wilkinson
(1969), Allen (1974) and Price & Chappell (1975).
The petrography of representative rock types is
summarized in Table 1. The unique feature of the
PCB, in contrast to other units of the Dunedin Volcano, is the presence of coarse-grained rocks showing
relatively extreme compositions. Ultramafic rocks have
textures indicating a cumulate origin and this is also
the case for some coarse syenites. Gabbroic rocks and
some syenites appear, however, to be coarse-grained
equivalents of basanitic and phonolitic volcanic rocks.
They do not show textures or chemical compositions
consistent with a cumulate origin. It should also be
emphasized that the modal compositions of ultramafic
cumulate rocks are dominated by pyroxene and amphibole. Olivine has not been observed in these rocks.
MINERAL CHEMISTRY
Methods
Minerals were analysed in carbon-coated polished thin
sections on the University of Otago's JEOL JXA-8600
electron microprobe using wavelength-dispersive techniques. Operating conditions used an accelerating voltage of 15 kV, a specimen current of 2 10 ÿ8 A, and a
beam diameter ranging from 1---2 mm for stable
minerals to broad beam for feldspathoids, feldspars
and glass. Pure compounds and natural minerals
were used as standards. Raw counts were corrected
by ZAF procedures.
Pyroxene
Representative pyroxene compositions are listed in
Table 2 and pyroxene compositional variation, in
terms of Diopside---Hedenbergite---Acmite (Aegirine)
components is illustrated in Fig. 3a.
NUMBER 11
NOVEMBER 2003
The pyroxenes of the mafic clasts are reasonably
uniform in composition and, according to the classification scheme of Morimoto (1989), are augites,
diopsides, or their titanian or aluminous equivalents
(Wo48---50, En34---44, Fs6---17; TiO2 04---51 wt %, Al2O3
up to 77%). In contrast, phonolitic rocks (e.g. PCB20)
contain a bimodal pyroxene compositional population
(Fig. 3a), often with green and colourless microphenocrysts occurring side by side in a microcrystalline or
devitrified glassy groundmass. Green phenocrysts,
with patchy or concentric zoning, are acmitic (acm
18---22 mol %), low in Al and Ti, but contain appreciable Zr (ZrO2 042---052%). Coexisting acmite-poor
phenocrysts are sector and concentrically zoned, with
prism sectors characterized by an aluminium-bearing
diopside (e.g. PCB25 No. 3, Table 2) and basal sectors
by a low Al---Ti ferroan hedenbergite (e.g. PCB25
No. 2, Table 2). This type of disequilibrium crystallization feature has been reported in a wide variety of
igneous rocks (e.g. Nakamura & Coombs, 1973; Dowty,
1976; Cooper, 1986; Shearer & Larsen, 1994) and is
ascribed to control by crystallographic, proto-site
configurations.
In phonolites (e.g. PCB20) and syenites (e.g. PCB53)
some diopside grains carry a pale green, rounded,
Na---Fe-enriched core. In the analysed syenite, the
green core is a ferrian magnesian hedenbergite
(PCB53 No. 3, Table 2) rimmed by diopside (PCB53
No. 6) and then by the more acmitic `groundmass'
phase, which is a ferroan magnesian aegirine---augite
(PCB53 No. 7). These `green-cored' pyroxenes are
widely reported in nepheline-normative mafic and
intermediate rocks of southern New Zealand (Price,
1973; Cooper, 1979; Brodie & Cooper, 1989) and
from elsewhere (e.g. Brooks & Printzlau, 1978; Bedard
et al.,1988; Neumann et al., 1999).
Pyroxenes in feldspathoidal syenite have an irregular
or sub-ophitic texture, with colour zoning in places
truncated at grain boundaries. Compositions in the
feldspathoidal syenite clasts of the PCB show a much
broader range than has been observed in the rest of the
Dunedin Volcanic Group (Fig. 3b). They are typically
ferroan aegirine---augites (PCB1 No. 12, Table 2).
These cores are overgrown by oscillatory zones characterized by an overall chemical trend towards increasingly more sodic and Zr-rich compositions (e.g. PCB1
No. 16). The fractionation trend culminates in a
brown-coloured patchy overgrowth of a Mn-rich
aegirine (PCB1 No. 8, Table 2). Although Zr contents
correlate crudely with the abundance of the acmite
component, the maximum ZrO2 content of 248 wt %
is attained in a pyroxene with an acmite content of a
little over 80% (PCB48 No. 6, Table 2), and some of
the most acmitic pyroxenes are pale green and low in
ZrO2 (e.g. PCB48 No. 9, Table 2). Aegirines, although
2056
Table 1: Petrographic summary for clasts and matrix of the Port Chalmers Breccia
Clast types
Texture
Phenocrysts/mineralogy
Groundmass
Order of crystallization
Basalt and trachyandesite
Porphyritic/glomeroporphyritic,
plagioclase, olivine, Ti-augite, magnetite,
plagioclase, Ti-augite, magnetite,
olivine---magnetite---apatite, kaersutite,
Ne-benmoreite
Porphyritic
apatite, kaersutite
alkali feldspar, clinopyroxene (Ti-augite),
resorbed kaersutite, plagioclase,
apatite
alkali feldspar, aegirine, nepheline,
magnetite, apatite
Medium to coarse grained, equigranular
alkali feldspar, clinopyroxene
2057
and gabbros
aenigmatite, alkali feldspar,
aegirine---augite---plagioclase, alkali feld-
nepheline (phonolite only)
magnetite, apatite
spar---nepheline, sodalite
perthitic alkali feldspar, nepheline,
amphibole, plagioclase, alkali feld-
aegirine---augite, amphibole, biotite,
muscovite, and analcime as alteration
amphibole, biotite, plagioclase
plagioclase, magnetite, apatite,
spar---nepheline, aegirine, biotite,
zircon, eudialite, carbonate
alkali amphibole
Coarse-grained hypidiomorphic granular,
kaersutite, augite/Ti-augite with aegirine---
poikilitic to `loose' aggregate,
augite rims, biotite, apatite,
flow-textured (some gabbros); some
magnetite, plagioclase, olivine
Calcareous sandstone, pyritic mudstone,
coal derived from subvolcanic
Cretaceous---Tertiary sedimentary
sequence
Foliated, segregated, with rounded
Mineralogy: albite, quartz, muscovite,
porphyroblasts of albite; variable
epidote, biotite, chlorite, titanite,
alteration along carbonate-rich
oxides, graphite, apatite garnet
veinlets and intergranular films;
pronounced fenitization in some cases
Composed of finely broken (52 mm) lithic
clasts and mineral grains in a clay-rich or
oxide base; analcime and calcite occur
as patches of intergranular cement
intercumulus plagioclase and
alkali feldspar
ÐÐolivine---magnetite---apatite,
Ti-augite---plagioclase
ÐÐkaersutite---Ti- augite---aegirine---augite,
biotite, plagioclase, alkali feldspar
analcime
Matrix
aegirine---augite---magnetite---apatite---
sodalite, aegirine---augite/aegirine,
micro-amygdales infilled with
Schist
alkali feldspar, nepheline, sodalite,
or porphyritic, veined and/or layered;
have interstitial glass containing
Sedimentary rocks
Ti-augite---magnetite---apatite,
(aegirine---augite and/or Ti-augite),
phases; carbonate veins common
Mafic/ultramafic cumulates
nepheline, analcime, sodalite,
PHONOLITIC DIATREMES OF DUNEDIN VOLCANO
Feldspathoidal syenite
Pilotaxitic, porphyritic
Ti-augite, plagioclase, alkali feldspar
and biotite
olivine, biotite
Phonolite and Ne-trachyte
Ti-augite---plagioclase
olivine---magnetite---apatite, kaersutite,
PRICE et al.
pilotaxitic
Table 2: Representative pyroxene analyses from the Port Chalmers Breccia
Sample:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
PCB30
PCB57
PCB25
PCB25
PCB25
PCB20
PCB20
PCB48
PCB48
PCB53
PCB53
PCB53
PCB9
24273
PCB1
PCB1
PCB1
PCB5
1
1
2
3
17
30
41
6
9
3
6
7
35
2
16
8
12
10
Ucm
Gab
Ph
Ph
Ph
Ph
Ph
Nsy
Nsy
Nsy
Nsy
Nsy
Nsy
Nsy
Nsy
Nsy
Nsy
Nsy
SiO2
48.24
2.61
46.29
4.06
48.64
0.32
46.58
2.63
50.02
0.30
45.40
3.50
51.32
0.10
50.62
0.31
49.13
0.38
46.61
2.68
5.34
7.52
0.11
5.81
10.07
0.08
1.23
6.57
0.55
22.43
0.64
10.11
0.15
22.99
1.04
7.73
8.65
CaO
11.58
23.97
10.80
22.48
4.16
21.81
10.33
22.38
3.89
16.87
0.13
11.33
22.48
Na2O
0.45
0.65
0.77
0.01
0.64
2.81
0.03
0.65
0.02
TiO2
Al2O3
FeO
MnO
MgO
K2O
ZrO2
Total
2058
Calc. FeO
Corr. total
99.82
1.15
100.24
2.57
100.01
3.99
99.39
2.50
6.48
99.93
7.86
100.60
18.84
100.41
7.86
99.64
CationsÐassuming 6 oxygens
Si
1.809
1.743
5.92
17.66
99.61
99.89
3.80
5.23
100.27
1.00
0.78
26.17
0.82
29.88
0.24
0.84
29.90
0.57
1.87
16.11
0.41
6.23
7.74
1.75
9.67
0.00
1.80
0.15
2.18
7.79
22.16
11.99
23.01
7.42
11.66
0.01
11.61
0.00
1.11
0.01
0.54
0.75
98.79
2.48
98.27
0.55
96.73
0.07
99.04
0.06
99.04
17.09
10.79
26.25
6.26
29.16
3.66
5.39
11.26
3.45
4.64
100.50
100.90
99.65
99.58
99.39
0.18
0
51.97
1.07
51.45
0.31
50.95
0.22
51.92
0.42
48.12
0.07
50.18
1.66
0.86
25.78
0.41
0.91
1.43
0.51
0.77
28.24
0.03
1.08
29.43
0.28
0.66
17.03
0.74
29.10
0.91
28.25
1.86
27.85
1.18
7.21
1.11
1.60
0.18
0.93
0.19
4.43
0.00
1.89
12.08
0.00
12.71
10.37
0.01
12.04
0.00
0.09
97.62
0.29
96.66
1.24
98.08
27.95
3.09
31.33
1.24
24.79
6.79
100.42
99.80
100.56
19.32
2.78
0.02
0.36
99.44
8.08
9.76
100.25
96.89
28.81
2.33
99.78
0.00
0.06
99.28
7.97
20.68
100.08
2.29
14.20
4.36
0.02
0.02
99.78
8.05
18.54
100.59
1.701
0.099
0.341
1.979
0.013
0.046
1.988
0.003
0.036
1.972
0.009
0.039
1.908
0.011
0.086
0.031
0.064
1.982
0.009
0.049
1.983
0.006
0.030
1.954
0.076
0.277
1.939
0.021
0.041
2.008
0.009
0.026
0.012
0.023
0.002
0.037
1.978
0.049
0.040
0.032
0.203
0.073
0.247
0.119
0.626
0.071
0.249
0.177
0.587
0.107
0.164
0.502
0.352
0.765
0.203
0.855
0.119
0.158
0.366
0.098
0.146
0.234
0.314
0.802
0.098
0.908
0.040
0.726
0.221
0.839
0.075
0.243
0.702
0.239
0.611
0.035
Ca
0.673
0.929
0.024
0.414
0.797
0.063
0.065
0.009
0.010
0.038
0.030
0.011
0.185
0.041
0.633
0.903
0.013
0.451
0.922
0.061
0.231
0.719
0.019
0.009
0.091
0.006
0.584
0.909
0.027
0.102
0.404
0.008
0.606
0.907
0.022
0.246
0.928
0.005
0.647
0.963
0.078
0.033
0.792
0.014
0.135
0.600
Na
0.033
0.047
0.059
0.001
0.047
0.217
0.002
0.047
0.001
0.903
0.194
4.000
3.25
4.000
4.75
4.000
5.98
4.000
4.70
Al
Fe2 ‡
Mn
Mg
K
Zr
Total
Acmite
Enstatite
42.25
32.36
38.27
30.32
43.49
12.32
37.51
29.19
Ferrosilite
10.33
12.50
32.37
12.70
Wollastonite
0.010
4.000
17.92
36.34
11.66
31.47
4.000
4.82
35.12
31.64
8.40
0.561
0.075
0.876
0.000
0.877
0.084
0.039
0.208
0.001
0.893
0.949
0.783
0.000
0.014
4.000
0.047
4.000
0.010
4.000
0.001
4.000
0.001
4.000
0.007
4.000
0.002
4.000
0.005
4.000
0.024
4.000
50.67
20.41
81.11
3.96
85.50
4.13
8.41
42.13
3.94
38.06
20.85
38.18
81.82
3.33
90.80
1.72
74.10
9.43
5.14
19.14
0.00
11.16
0.44
6.90
22.55
18.96
33.66
7.59
20.69
16.92
3.22
5.07
0.52
2.45
0.56
12.82
Ucm, ultramafic cumulate; Gab, gabbro; Ph, phonolite; Nsy, nepheline syenite. Fe2O3 calculated assuming six oxygens and four cations.
4.000
86.53
4.04
0.00
7.03
0.001
4.000
19.44
37.46
1.66
37.14
0.333
0.001
4.000
24.91
31.29
7.02
32.61
NOVEMBER 2003
1.989
0.075
0.294
NUMBER 11
1.932
0.010
0.058
1.766
0.115
0.258
Fe3 ‡
1.981
0
0.55
18.21
2.47
0.074
0.236
Ti
1.756
50.36
0.71
VOLUME 44
Calc. Fe2O3
0.52
99.02
50.75
0.46
JOURNAL OF PETROLOGY
Analysis:
Rock type:
PRICE et al.
PHONOLITIC DIATREMES OF DUNEDIN VOLCANO
Fig. 3. Clinopyroxene compositions for samples of PCB (a) compared with (b) Dunedin Volcanics (Price, 1973) and pyroxenes from alkaline
intrusions from East Africa (Malawi), the Arkansas alkaline province and Greenland. NNAÐNorthern Nyasa alkaline syenites, Malawi (Eby
et al., 1998); QÐQ^oroq, Greenland (Stephenson, 1972); MÐMagnet Cove, Arkansas Alkaline Province (Flohr & Ross, 1990); IÐIlõÂ maussaq,
Greenland (Larsen, 1976; Marks & Markl, 2001).
generally low in Ti, can contain appreciable TiO2 (up
to 215 wt %; e.g. PCB9 No. 35, Table 2).
The pyroxenes of the clasts in the PCB collectively
define an extended trend, similar to that observed in
peralkaline undersaturated intrusive suites in south
Greenland (Q^oroqÐStephenson, 1972; IlõÂ maussaqÐ
Larsen, 1976; Marks & Markl, 2001) and at Chinduzi,
Chilwa (Woolley & Platt, 1988).
Amphibole
Representative amphibole compositions are listed in
Table 3. From mafic to felsic clast types there is a
progressive change in amphibole chemistry from calcic
to sodic. Within the mafic cumulates and gabbros the
amphiboles are generally compositionally homogeneous, varying with rock type from kaersutites to
ferro-kaersutites and titanian ferroan pargasites [classification of Leake et al. (1997)]. TiO2 abundance
ranges from 271 to 578 wt %, Na2O is typically
290% and K2O 1%. Mg number [ˆ Mg/(Mg ‡
Fe)] ranges from 0670 to 0390. NaB values are low,
with a maximum content of 022 cations per formula
unit (p.f.u.).
In the phonolites, amphiboles are kaersutites, titanian ferroan pargasites and potassian titanian hastingsites. Some of the ultramafic cumulates contain
amphibole with similar compositions to that in phonolite, suggesting cumulates may relate to crystallization
from evolved magmas. However, the extended range in
phonolite amphiboles, principally towards lower Mg
number and TiO2, probably reflects continued fractionation beyond the stage recorded in amphiboles from
analysed cumulates.
Syenite amphiboles range from ferro-kaersutites,
hastingsites and pargasites in PCB5, PCB42 and OU
24259, through kataphorites in PCB51 and PCB53, to
ferric-ferronyobite in OU 242273. The spectrum is
marked by a decrease in Mg number from 0477 to
0092 and TiO2 from 513 to 025 wt %, and increases
2059
Table 3: Representative amphibole analyses from the Port Chalmers Breccia
3
4
5
6
7
8
9
10
11
12
13
14
15
PCB30
PCB36
PCB3
PCB11
PCB25
PCB20
PCB20
PCB51
PCB53
24273
24259
PCB5
PCB42
PCB42
Analysis:
14
6
36
3
3
16
17
20
5
16
22
3
14
5
14
Rock type:
Ucm
Ucm
Gab
Gab
Gab
Ph
Ph
Ph
Nsy
Nsy
Nsy
Nsy
Nsy
Nsy
Nsy
SiO2
39.47
4.36
40.39
4.68
39.86
5.17
39.22
2.71
41.46
4.76
38.82
5.63
38.46
1.83
39.89
5.92
41.47
2.05
49.66
1.12
46.55
1.64
39.71
4.98
39.42
2.65
40.81
5.13
37.55
1.72
12.40
15.44
12.18
11.12
11.77
17.09
10.05
25.67
10.17
15.72
13.33
14.22
10.72
27.78
12.10
13.04
5.59
27.60
3.24
17.16
2.47
28.54
10.45
19.15
10.36
24.98
10.49
17.02
10.94
29.73
0.20
9.84
0.03
12.64
0.16
8.57
0.38
4.57
0.13
10.90
0.15
10.08
0.56
3.70
0.12
10.94
1.35
3.70
0.74
11.77
3.42
2.33
0.30
7.90
0.49
4.83
0.22
8.70
0.72
3.08
10.87
2.99
12.25
2.52
11.45
2.79
10.58
2.79
11.32
2.84
11.78
2.63
10.63
2.46
11.79
2.66
6.86
3.96
6.00
5.68
0.33
8.68
11.20
2.44
10.97
2.69
11.07
2.58
10.48
2.38
0.89
0.00
1.48
0.03
0.93
0.00
1.07
0.00
0.97
0.00
0.95
1.45
0.08
1.00
1.37
0.67
1.46
0.44
1.60
0.14
1.33
1.57
0.26
1.06
1.92
0.04
96.46
0.80
97.32
0.00
97.79
0.00
97.04
2.51
98.27
0.07
97.59
0.00
97.67
5.28
97.46
0.00
94.62
7.08
97.27
4.05
95.70
7.52
97.46
0.00
98.22
0.78
97.08
0.00
98.56
7.64
Calc. H2O
1.96
1.96
99.75
1.88
99.18
1.99
1.88
2.00
1.89
99.58
100.08
99.46
2.00
99.68
1.94
100.27
1.83
97.16
1.87
98.50
1.99
99.31
2.00
Corr. total
98.32
99.40
100.19
1.96
99.04
101.22
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
ZrO2
Total
2060
Calc. Fe2O3
Aliv
Alvi
Ti
Fe2 ‡
Mn
Mg
Ca
Na
K
OH*
Total
6.109
1.891
0.235
6.240
1.760
0.125
6.229
1.771
0.030
5.855
2.145
0.224
5.987
2.013
0.128
6.795
1.080
0.000
7.434
0.566
0.005
6.144
6.240
1.880
0.131
6.120
0.468
0.000
1.856
0.050
1.760
0.172
6.245
1.755
0.136
2.033
0.017
5.968
0.501
0.092
0.530
0.000
0.596
0.000
0.324
0.301
0.538
0.008
0.639
0.000
0.219
0.633
0.668
0.000
0.253
0.873
0.126
0.457
0.198
1.037
0.580
0.000
0.316
0.094
0.591
0.000
0.206
0.913
1.881
0.026
1.399
0.004
2.191
0.021
3.115
0.051
1.968
0.017
1.794
0.019
3.064
0.076
1.637
0.015
2.909
0.187
1.691
0.094
2.797
0.465
2.478
0.039
3.213
0.066
2.178
0.029
3.038
0.097
2.240
1.779
2.833
1.974
1.958
1.880
1.084
1.804
2.441
1.822
2.266
1.904
0.878
1.812
2.448
1.896
0.904
1.204
2.627
0.962
0.558
0.057
1.822
1.857
1.140
1.860
1.985
1.815
0.730
1.784
0.886
0.173
0.735
0.284
0.829
0.182
0.861
0.217
0.827
0.186
0.769
0.183
0.759
0.294
0.774
0.192
1.258
0.286
1.649
0.279
2.703
0.328
0.732
0.263
0.826
0.317
0.766
0.207
0.733
0.389
2.000
17.838
2.000
17.993
2.000
17.891
2.000
17.882
2.000
17.836
2.000
17.798
2.000
17.866
2.000
17.757
2.000
17.749
2.000
17.89
2.000
18.088
2.000
17.82
2.000
18.003
2.000
17.705
2.000
17.907
NOVEMBER 2003
Fe3 ‡
6.075
1.925
0.235
7.477
1.971
0.261
NUMBER 11
CationsÐassuming 23 oxygens
Si
6.029
1.89
VOLUME 44
2
PCB55
JOURNAL OF PETROLOGY
1
Sample:
PRICE et al.
2
3
4
5
6
7
8
9
10
11
12
13
14
15
PCB55
PCB30
PCB36
PCB3
PCB11
PCB25
PCB20
PCB20
PCB51
PCB53
24273
24259
PCB5
PCB42
PCB42
Analysis:
14
6
36
3
3
16
17
20
5
16
22
3
14
5
14
Rock type:
Ucm
Ucm
Gab
Gab
Gab
Ph
Ph
Ph
Nsy
Nsy
Nsy
Nsy
Nsy
Nsy
Nsy
Amphib. group
Ca
2.000
Ca
2.000
Ca
2.000
Ca
2.000
Ca
Ca
2.000
Ca
Ca
2.000
Na-Ca
2.000
Na-Ca
2.000
Alkali
2.088
Ca
2.000
Ca
Ca
2.000
Ca
0.221
0.838
0.026
0.993
0.120
0.891
0.196
0.882
0.796
0.749
1.038
0.890
2.031
1.000
0.143
0.851
0.544
0.260
0.669
0.000
0.472
0.000
0.258
0.706
0.866
0.223
0.829
0.104
0.861
0.599
0.000
0.237
1.000
0.608
0.988
0.166
1.000
0.424
0.000
Sum of S2
13.000
13.000
13.000
13.000
13.000
12.896
13.000
13.000
13.000
12.969
Classification
Kaer
Kaer
Fe-Kaer
Fe-Par
Kaer
Katop
Katop
Mn-Arf
Kaer
(Ca ‡ Na) (B)
Na (B)
2061
(Na ‡ K) (A)
Mg/(Mg ‡ Fe2 ‡ )
Fe3 ‡ /(Fe3 ‡ ‡ Alvi )
2.000
0.178
0.836
0.096
0.855
0.554
0.204
0.558
0.000
13.000
12.942
Kaer
Kaer
2.000
0.188
Hast
2.003
0.143
0.185
0.787
1.000
0.262
0.352
0.477
0.000
13.000
12.918
Par
Kaer
2.000
0.216
0.907
0.194
0.982
13.000
Hast
*Calculated assuming 2(OH) p.f.u.
Ucm, ultramafic cumulate; Gab, gabbro; Ph, phonolite; Nsy, nepheline syenite; Kaer, kaersutite; Par, pargasite; Hast, hastingsite; Katop, katophorite; Mn-Arf, Mn-rich
arfvedsonite; Fe is ferroan.
PHONOLITIC DIATREMES OF DUNEDIN VOLCANO
1
Sample:
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 11
NOVEMBER 2003
Table 4: Representative biotite analyses from the Port Chalmers Breccia
Sample:
1
2
3
4
5
6
7
PCB11
PCB20
PCB53
24259
PCB48
PCB5
PCB9
Analysis:
5
23
20
11
27
16
41
Rock type:
Gab
Ph
Nsy
Nsy
Nsy
Nsy
Nsy
SiO2
34.86
6.91
35.98
3.28
39.18
1.08
33.12
5.01
32.08
1.45
34.30
2.87
34.24
2.55
12.77
22.00
9.18
31.82
10.85
18.58
13.28
29.10
8.16
40.55
11.60
33.63
9.43
35.69
0.05
9.46
0.97
5.29
0.54
15.77
0.40
5.27
2.29
0.68
0.72
2.29
1.02
3.77
0.00
0.73
0.01
0.53
0.00
0.67
0.00
0.48
0.03
0.30
0.02
0.11
0.00
0.50
8.54
0.00
95.32
8.79
0.00
95.85
9.30
9.44
7.87
0.00
95.97
0.00
96.10
9.10
0.27
94.91
8.30
0.03
95.53
3.85
99.17
3.67
99.52
3.96
99.93
3.71
99.81
3.58
98.49
99.10
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
ZrO2
Total
Calc. H2O
Calc. total
Cations assuming 22 oxygens
5.4242
Si
Aliv
Alvi
Ti
Fe2
Mn
Mg
Ca
Na
2.3421
0.0000
0.8088
2.8629
0.0066
2.1937
0.0000
0.2202
96.76
3.57
5.8819
1.7689
5.9344
1.9371
5.3469
2.5270
5.7389
1.7206
5.7380
2.2873
5.7459
1.8652
0.0000
0.4034
0.0000
0.1231
0.0000
0.6084
0.0000
0.1951
0.0000
0.3612
0.0000
0.3219
4.3505
0.1343
2.3536
0.0693
3.9290
0.0547
6.0668
0.3470
4.7051
0.1020
5.0089
0.1450
1.2888
0.0018
3.5598
0.0000
1.2679
0.0000
0.1813
0.0058
0.5709
0.0036
0.9429
0.0000
0.1680
1.8333
0.1968
1.7971
0.1503
1.9443
0.1041
1.7962
0.0357
1.9422
0.1627
1.7770
Mg number
1.6953
43.38
22.85
60.20
24.40
Classification
Ti-Bio
Bio
Bio
T-Bio
K
93.41
3.35
2.90
Ann
10.82
15.84
Ann
Ann
Gab, gabbro; Ph, phonolite; Nsy, nepheline syenite; Ti-Bio, titanian biotite; Bio, biotite; Ann, annite.
in SiIV (from 6027 to 7505 cations p.f.u.), Na2O (up
to 868 wt %), K2O (up to 186 wt %), and NaB (from
0274 to 1000). Weak chemical zoning in individual
grains of the most evolved ferric-ferronyobites of
OU 24273 mimics the general trend described above,
with the exception that there is a slight increase in Mg
number from core to rim. MnO is markedly enriched
in these evolved ferric-ferronyobites, with contents
reaching 357 wt %, reflecting the behaviour of Mn
across the amphibole spectrum, and resulting in an
analogous enrichment in Mn to that described for
late-stage acmitic pyroxenes in evolved Dunedin
nepheline syenites.
The compositional trends defined by amphibole
compositions of the Dunedin Volcano are very similar
to those in undersaturated plutons of the Gardar
Province, Greenland (see Mitchell, 1990).
Biotite
A trioctahedral biotite mica occurs as a minor phase in
mafic and phonolitic clasts and is abundant in some of
the nepheline syenites. Compositionally, micas are
phlogopite---annite solid solutions, there being no octahedral Al to form an eastonite or siderophyllite component (Table 4). Many of the biotites show marked
deficiencies in the tetrahedral site (up to 04 cations
p.f.u., Table 4) and significant Fe3 ‡ is presumably
required to fill the site (Rieder et al., 1998). This is a
characteristic of biotites from undersaturated rocks
(e.g. Cooper, 1979). As with pyroxenes and amphiboles, there is a general decrease in the Mg number of
biotite from the gabbros (0423) to nepheline syenites
(0029). There is a correlation between Ti
(cations p.f.u.) and Mg number, with Ti being most
2062
PRICE et al.
PHONOLITIC DIATREMES OF DUNEDIN VOLCANO
Fig. 4. Compositional variation in biotites from clasts of the
PCB. (a) Cations of Ti vs Mg number [100 Mg/(Mg ‡ Fe)].
(b) Compositions projected onto a Mg---Al---Fe2 ‡ ternary diagram
and comparisons with biotites from syenitic rocks from elsewhere.
J is compositional trend for biotites from Junguni, Chilwa alkaline
complex, Malawi (Woolley & Platt, 1988); MC is trend for biotites
from Magnet Cove, Arkansas alkaline province (Flohr & Ross,
1990); IU and KC are trends defined by biotite compositions from
Kasunga---Chipala and Ilomba and Ulindi nepheline syenites,
respectively, from the north Nyasa alkaline province, Malawi (Eby
et al., 1998).
abundant (up to 684 wt % TiO2) in biotites of the
mafic rocks (Fig. 4a).
In Fig. 4b, the compositions of PCB biotites have
been compared in terms of Al, Mg and Fe2 ‡ contents
with those from syenites from elsewhere. Biotites from
Port Chalmers nepheline syenite clasts are relatively
iron rich but they extend the trends defined by biotite
compositions from syenites of Malawi (Fig. 4b). Al
enrichment trends observed in biotites of the Ilomba
and Ulindi nepheline syenites of the North Nyasa alkaline province are not present in the biotites of the PCB
nepheline syenite clasts.
Fe---Ti oxides
Representative oxide compositions are shown in
Table 5. Magnetite is ubiquitous in clasts of the PCB
and in most rocks it is homogeneous and does not show
exsolution. Phonolites and nepheline syenite clasts
show a similar range of magnetite compositions from
Usp63Mt37 to Usp10Mt90 (Fig. 5). Magnetites in the
mafic rocks show a much more limited compositional
range and they tend to be more Ti rich (Usp44Mt56 to
Usp94Mt6). Chromium and MgO contents are generally low. MnO abundances are consistently higher
than MgO contents, with magnetites in nepheline syenites containing up to 44 wt % and those in ultramafic
cumulates up to 23 wt % MnO. The pattern of compositional variation in magnetites from clasts in the
PCB reflects that shown within the Dunedin Volcanic
Group. Dunedin phonolites contain magnetites that
are generally less Ti rich than those observed in the
basanites and basalts (Price, 1973).
Exsolved ilmenite in the nepheline syenites is Ti
rich (Ilm90---96) and application of the Ghiorso & Sack
(1991) oxide geothermometer and oxygen geobarometer to a coexisting spinel---ilmenite pair in syenite
PCB42 gives oxygen conditions close to the FMQ
(fayalite---magnetite---quartz) buffer (918 C and
log f O2 ˆ ÿ122). This estimate is consistent with the
more oxidized end of the range for nepheline syenites
from elsewhere (e.g. IlõÂ maussaq, Larsen, 1976; Marks
& Markl, 2001), with conditions from FMQ to
more reducing being commonly estimated. The presence of aenigmatite in phonolitic clasts of the PCB is
also an indication of relatively low f O2 (Thompson &
Chisholm, 1969; Lindsley, 1970; Hodges & Barker,
1973).
Feldspar
Feldspars occur in all clasts of the PCB except a few of
the more extreme ultramafic cumulate compositions.
Compositional variation in the feldspars is illustrated
in Fig. 6. Plagioclase in a dolerite (PCB57) ranges in
composition from calcic cores (An65Ab34Or1) to sodic
rims (An11Ab78Or21). Alkali feldspar (An1Ab39Or60) is
present in the outermost rim of zoned grains and in
the groundmass. The feldspars in phonolitic rocks are
dominantly anorthoclase (e.g. PCB25: An3Ab54Or43;
PCB20: An4Ab63Or33) but a few intermediate plagioclase phenocrysts (e.g. PCB20: An28Ab68Or4) are present in most phonolites and are reasonably common in
ne-benmoreites. The compositional range among feldspars in phonolite clasts is wider than is the case for
feldspars from phonolitic rocks of the Dunedin Volcanic Group (Fig. 6), with some extremely potassic
(PCB20: An0Ab2Or98) and sodic feldspars (PCB20:
An5Ab95Or0) resulting from perthitic exsolution.
Such compositions do not occur in rocks of the
Dunedin Volcanic Group (Fig. 6).
The feldspars of the syenitic rocks show much the
same range of variation as that observed among the
feldspars of the phonolitic clasts, with alkali feldspars
2063
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 11
NOVEMBER 2003
Table 5: Representative Fe---Ti oxide analyses from the Port Chalmers Breccia
Fe---Ti oxide:
1
2
3
4
5
6
7
8
Mt
Mt
Mt
Mt
Mt
Il
Mt
Il
Sample:
PCB55
PCB20
PCB25
PCB51
PCB42
PCB42
PCB5
PCB53
Analysis:
10
16
15
19
18
17
18
27
Rock type:
Ucm
Ph
Ph
Nsy
Nsy
Nsy
Nsy
Nsy
SiO2
0.08
TiO2
17.92
2.70
Al2O3
FeO
MnO
MgO
CaO
Cr2O3
Total
Calc. Fe2O3
Calc. FeO
Calc. total
0.14
9.57
0.03
0.05
0.00
0.04
0.03
0.09
20.55
3.28
12.03
0.13
17.23
1.05
47.24
0.24
19.67
3.68
49.19
0.05
42.84
4.41
72.80
0.74
1.61
83.24
0.51
70.78
1.14
78.32
3.91
76.69
0.77
48.10
3.05
72.58
0.92
1.38
0.06
0.00
0.19
0.02
0.00
0.97
0.00
0.00
0.04
0.00
0.19
0.41
0.07
0.63
0.43
0.03
0.00
0.01
95.68
30.93
95.28
48.48
96.75
25.42
94.48
44.82
95.96
34.16
99.15
10.52
97.51
27.11
97.02
4.15
44.96
98.77
39.62
47.90
99.29
37.99
98.97
45.95
99.38
38.63
100.14
100.20
48.19
97.23
39.11
97.44
Cations assuming 4 oxygens (magnetite) or 3 oxygens (ilmenite)
0.0030
0.0053
0.0011
.
.
Al
0 1188
0 0715
0.1433
.
.
Ti
0 5032
0 2713
0.5729
0.0019
0.0059
0.0000
0.0468
0.0010
0.0071
0.0011
0.1595
0.0023
0.0015
0.3475
1.2953
0.4903
0.9726
0.8956
0.1996
0.5441
0.7501
0.9566
0.0807
1.2200
0.1272
1.4538
0.0247
0.8142
0.0651
1.4821
0.0287
0.8455
0.0966
0.0023
0.0000
0.0107
0.0012
0.0154
0.0019
0.0345
0.0000
0.0166
0.0003
0.0000
39.96
0.0000
50.83
0.0000
0.0000
59.20
0.0000
62.94
37.06
60.04
49.17
Si
Fe
3‡
Fe2 ‡
Mn
Mg
Ca
Cr
0.8690
1.4036
1.3753
1.2489
0.7088
1.4846
0.0234
0.0768
0.0163
0.0107
0.0358
0.0536
0.0024
0.0000
0.0008
0.0000
0.0000
0.0000
Ulvospinel
56.01
Magnetite
43.99
28.30
71.70
40.80
Ilmenite
89.96
Haematite
10.04
95.95
4.05
Mt, magnetite; Il, ilmenite; Ucm, ultramafic cumulate; Ph, phonolite; Nsy, nepheline syenite.
dominating (PCB53: An0Ab72Or28 to OU 24273:
An0Ab8Or91) and plagioclase rare (PCB42: An15
Ab82Or3 to PCB5: An34Ab60Or6). Compositional
zoning is subdued and trends are often inconsistent;
some grains become enriched in Na from core to rim
(e.g. PCB9: core An0Ab57Or43, rim An0Ab61Or39),
whereas in other rocks zoning trends are obscured by
the effects of perthite exsolution.
Nepheline, sodalite and analcime
Representative nepheline, sodalite and analcime compositions are given in Table 6. Nepheline occurs as a
groundmass and phenocryst phase in phonolite and
ne-trachyandesite, and is common in syenite clasts.
Nepheline compositions are plotted in the system
Q---Ne---Ks in Fig. 7, along with temperature limits on
nepheline solid solution determined by Hamilton
(1961). In nepheline---sodalite syenite, PCB9, nephelines are consistently zoned, with phenocryst core
compositions (Na2O 1745%, K2O 420%) evolving
through rims (Na2O 1652%, K2O 519%) to a more
potassic groundmass phase (Na2O 1652%, K2O
587%). Similar potassic enrichment with fractionation is observed in phonolite PCB20. The nepheline
compositions in phonolitic rocks are consistent with
temperatures in the range 1000 to 700 C, with syenitic
rocks showing a wider compositional range that is
perhaps indicative of a wider temperature range
(41068 C to 500 C).
2064
PRICE et al.
PHONOLITIC DIATREMES OF DUNEDIN VOLCANO
Fig. 5. Compositions of Fe---Ti oxide minerals in samples from the PCB plotted in terms of TiO2---FeO---Fe2O3. Dashed line connects coexisting
rhombohedral oxide and spinel.
Fig. 6. Feldspar compositions for clasts of PCB (ultramafic and mafic, phonolitic and syenitic clasts) and Dunedin Volcanic Group (Dunedin
Volcanics) plotted in terms of An, Ab and Or components. *, pyroxene compositions from gabbros. Dunedin data from Price (1973).
Sodalite occurs in many of the syenite clasts. In
PCB9 it forms primary, euhedral, octahedral phenocrysts that crystallized along with feldspar and nepheline. Analysed sodalites are unzoned; all are low in
calcium and there is no detectable solid solution with
a potassic end-member (Table 6).
Analcime is also common in syenites, forming interstitial patches and cross-cutting veinlets associated
with calcite and white mica. In syenite, PCB9,
analcime forms a marginal replacement of sodalite.
In some ultramafic cumulates analcime occurs filling
vesicles in interstitial glass, and in a similar, petrogenetically late mode, as a cement in the PCB.
Glass
Spherical glass inclusions (001---005 mm diameter)
occur within pyroxenes, amphiboles and apatites of
2065
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 11
NOVEMBER 2003
Table 6: Representative nepheline, sodalite and analcime analyses
1
2
3
4
5
6
Sample:
PCB48
PCB48
PCB51
PCB51
PCB51
PCB9
24273
Analysis:
22 (core)
21 (rim)
10 (core)
11 (rim)
13 (gmass)
1 (rim)
7
Rock type:
Nsy
Nsy
Nsy
Nsy
Nsy
Nsy
Nsy
SiO2
46.64
31.01
45.52
31.34
45.59
32.46
45.50
32.38
44.73
31.56
37.90
31.71
50.89
24.07
1.32
0.00
1.32
0.00
0.68
0.04
0.79
0.06
0.74
0.03
0.47
0.09
0.09
0.36
17.22
3.61
16.51
3.64
17.85
3.85
16.89
5.38
16.77
5.44
26.15
13.64
0.61
Al2O3
Fe2O3
CaO
Na2O
K2O
5.56
Cl
7.92
H2O calc.
Total*
7
99.80
98.33
100.47
101.00
99.27
99.26
97.58
Cationsy
Si
Al
Fe3 ‡
Ca
Na
K
1.1066
0.8672
1.0951
0.8887
1.0781
0.9048
1.0768
0.9033
1.0788
0.8972
6.0750
5.9000
1.9272
1.0744
0.0236
0.0000
0.0239
0.0000
0.0121
0.0010
0.0141
0.0015
0.0134
0.0008
0.0454
0.0187
0.0025
0.0146
0.7922
0.1093
0.7701
0.1117
0.8185
0.1162
0.7751
0.1624
0.7842
0.1674
0.0000
8.0140
1.0016
0.0295
1.4895
Cl
1.0000
H2O
Ne
12.16
79.17
12.63
78.21
Q
8.67
9.16
Ks
12.82
17.82
81.16
6.02
76.37
5.81
18.19
76.56
5.25
1---5, analyses of nepheline; 6, sodalite; 7, analcime.
*Totals include correction for O equivalent to Cl and calculated H2O.
yNepheline calculated on basis of four oxygens; sodalite on basis of 21 oxygens in 3Al2O36SiO2 framework, analcime on basis
of six oxygens and one H2O.
Fig. 7. Nepheline compositions for PCB phonolites and syenites plotted in terms of Q---Ne---Ks components. Continuous-line solution
boundaries are from Hamilton (1961).
mafic cumulate rocks, and interstitial and devitrified
glass patches are common in gabbro and clinopyroxenite clasts. Average glass compositions of
the spherical inclusions, obtained using electron
microprobe analysis, are shown in Table 7. Glasses in
the ultramafic cumulates and gabbros are strongly
undersaturated and phonolitic (ne-benmoreite or
tephro-phonolite).
2066
PRICE et al.
PHONOLITIC DIATREMES OF DUNEDIN VOLCANO
Table 7: Analyses of representative glasses
from clasts in the Port Chalmers Breccia
1
2
3
Sample:
PCB55
PCB36
PCB3
Rock type:
Ucm
Gab
Gab
Average
Range
Average
Major and trace element variations
Range
n:
5
SiO2
57.12
0.25
56.93---57.86
0.19---0.34
58.74
0.43
57.38---61.77
0.15---0.72
55.58
0.17
20.75
2.87
20.60---20.96
2.58---3.06
20.57
3.62
19.60---21.39
4.98---5.10
19.29
3.72
MnO
0.10
0.16
1.18
0.06---0.13
0.12---0.19
0.98---1.48
0.10
MgO
0.27
1.29
0.04---0.14
0.07---0.53
0.59---2.48
0.22
0.16
1.12
8.29
5.01
7.55---8.75
4.82---5.30
8.39
4.70
5.18---8.86
3.84---5.33
9.89
2.58
0.19
0.15---0.22
0.24
0.19---0.28
TiO2
Al2O3
FeO
CaO
Na2O
K2O
7
1
95.92
0.04
98.35
0.05
0.23
92.96
0.05
Ne
95.88
16.86
98.3
14.29
92.91
19.16
Diff. index
89.88
88.51
90.41
Cl
Total
OˆCl
Total
at Monash University using methods described by
Price et al. (1997). Precision for these elements is typically better than 5% and accuracy, based on analysis
of BHVO-1, better than 5% at the 95% confidence
level.
Normative data calculated on anhydrous basis and assuming
Fe2O3/FeO ratio of 0.2. Ucm, ultramafic cumulate; Gab,
gabbro; Nsy, nepheline syenite. n, number of glass analyses
included in the average. Ranges are shown for averages
1 and 2.
WHOLE-ROCK MAJOR AND TRACE
ELEMENT GEOCHEMISTRY
Methods
Major and minor elements (Si, Ti, Al, Fe, Mn, Mg,
Ca, Na, K, P and S) were determined by X-ray fluorescence (XRF) at La Trobe University using methods
similar to those described by Norrish & Hutton (1969).
For these elements, precision is generally better than
1% (1s). FeO abundances were measured by direct
titration using a standardized CeSO4 solution, and
H2O and CO2 by a gravimetric method. Selected
trace elements were determined by XRF on pressed
powder pellets (Norrish & Chappell, 1977) and, for
these elements, theoretical detection limits are of the
order of 1---2 ppm and reproducibility is better than
5% (1s).
For selected samples the rare earth elements (REE)
and a selection of other trace elements were analysed
by inductively coupled plasma mass spectrometry
(ICPMS) at the VIEPS Trace Element Laboratory
Representative major and trace element data for samples from the PCB are presented in Table 8. All analysed rocks are nepheline-normative (ne-) and, using
the total alkalis vs SiO2 classification scheme of Le
Maitre et al. (1989), the fine-grained clasts range from
basanitic and basaltic through trachyandesitic to
phonolitic (Fig. 8). Phonolitic clasts [tephri-phonolites
and phonolites under the scheme of Le Maitre et al.
(1989)] have been classified on the basis of normative
compositions and differentiation index (Coombs &
Wilkinson, 1969; Price & Chappell, 1975) as nebenmoreites and phonolites.
The bulk composition of the matrix of the PCB is
ne-benmoreite. The major element variations defined
by clasts from the PCB mirror those observed in the
Dunedin Volcanic Group (Fig. 8). Most of the syenitic
clasts are chemically similar to the phonolites but
several have compositions resembling those of Dunedin
trachyandesites. The majority of the syenite clasts
do, however, have lower K2O contents than is the
case for phonolites of the Dunedin Volcanic Group
including the PCB (Fig. 8). Trace element variations
among PCB clasts are broadly similar to those observed
in the Dunedin Volcanic Group (Fig. 9) but syenitic
clasts tend to have higher Sr contents and a few
have significantly higher Zr abundances (up to
5236 ppm). As expected, ultramafic rocks and gabbros
show higher abundances of MgO, FeO, TiO2, CaO,
Cr, Ni and V, and lower Al2O3, Na2O, K2O, Ba, Rb,
Sr, Nb and Zr contents. Two ultramafic rocks have
higher Al2O3, Ba and Sr abundances than other cumulates and they also show relatively elevated Rb
(10---12 ppm compared with 4---7 ppm for other cumulates) and Nb (160---376 ppm compared with
29---72 ppm) abundances. In both of these clasts interstitial glass is common.
Chondrite-normalized REE abundance patterns for
representative PCB clasts are shown in Fig. 10, along
with patterns for representative samples from the
Dunedin Volcanic Group. Phonolitic and syenitic
patterns are all broadly similar with relatively flat
heavy REE (HREE) to middle REE (MREE), enrichment of light REE (LREE) over MREE and HREE
[(La/Yb)n ˆ 22---60] and moderate depletions in Eu
relative to Sm and Gd (Eu/Eu* ˆ 053---075). PCB
syenites and phonolites show patterns that are similar
to those observed in Dunedin Volcano ne-benmoreites
2067
Table 8: Representative major and trace element data for Port Chalmers Breccia and Dunedin Volcano
4
5
6
7
12
13
14
15
16
17
18
19
Sample:
PCB55
1
PCB30
PCB36
PCB57
PCB54
PCB10
PCB19
PCB24
PCB25
PCB20
PCB5
PCB16
PCB1
PCB51
PCB56
30449
30428
PCB40
PCB61
Rock type:
Ucm
Ucm
Gab
Gab
Ba
Nha
Nmu
Nbe
Ph
Ph
Nsy
Nsy
Nsy
Nsy
Brec
Nbe
Ph
Sch
Fen
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
2068
H2O --CO2
10
11
41.79
4.22
40.25
3.99
45.57
2.09
45.34
2.93
50.93
1.96
47.95
1.80
51.10
1.02
53.68
0.58
56.07
0.25
50.21
1.02
56.56
0.54
54.87
0.07
55.83
0.32
51.91
0.99
50.99
1.06
56.97
0.20
67.48
0.60
45.06
0.70
2.80
21.30
8.73
4.34
13.74
3.80
13.54
2.38
15.07
4.86
17.81
4.76
16.65
4.19
19.01
3.07
19.63
3.12
19.65
2.25
18.58
3.66
19.07
2.69
22.04
3.63
19.30
2.43
17.02
4.69
18.75
3.46
19.06
2.89
14.73
1.49
16.40
0.44
8.75
0.38
7.29
0.16
10.05
0.26
8.46
0.18
7.20
0.19
4.46
0.19
5.24
0.20
4.39
0.21
3.08
0.19
2.95
0.18
4.03
0.20
3.15
0.15
0.92
0.14
2.99
0.19
2.05
0.15
4.46
0.21
2.85
0.21
3.02
0.08
4.24
0.34
6.95
15.86
12.07
16.37
6.21
12.25
8.63
11.03
5.12
10.05
2.69
5.66
5.30
5.85
1.76
4.30
0.79
2.49
0.63
1.51
1.55
4.23
0.90
2.57
0.22
1.15
0.71
2.03
2.32
4.19
2.35
4.71
0.34
1.63
1.79
2.39
2.04
12.01
0.66
0.11
1.55
0.69
3.35
0.89
3.27
1.15
3.59
1.63
5.25
3.01
7.45
3.32
7.89
3.69
8.91
4.30
9.27
3.59
6.68
3.98
6.92
4.60
10.29
2.25
8.07
3.93
5.88
3.70
8.29
3.55
8.59
5.23
3.12
1.62
4.71
3.10
2.70
2.93
0.10
1.29
2.08
2.93
0.37
1.60
0.58
2.45
0.54
2.26
0.39
1.17
0.26
2.61
0.16
2.36
0.07
2.40
0.26
2.84
0.21
1.79
0.02
4.30
0.09
2.90
0.26
3.98
0.50
0.64
0.07
1.10
0.14
2.53
0.17
1.20
0.18
0.25
0.37
0.35
0.09
0.01
0.18
0.87
0.67
0.65
0.69
0.34
0.20
0.18
0.32
0.16
0.34
0.08
0.62
0.71
0.46
2.42
0.37
0.37
0.41
0.36
0.32
0.27
0.83
2.68
0.39
0.07
0.42
0.13
0.12
0.22
0.20
8.72
0.01
99.33
0.02
99.91
0.02
99.33
0.02
100.34
0.01
100.56
0.04
99.91
0.01
99.80
0.02
99.72
0.34
100.32
0.02
100.13
0.01
99.90
0.01
100.68
0.04
99.40
0.01
100.66
0.01
99.44
0.02
99.70
99.33
99.33
7.32
9.88
7.39
6.78
6.23
28.01
23.57
25.33
19.49
17.89
9.92
21.76
14.70
9.07
26.00
20.02
100.12
3.18
0.07
Cs
0.11
0.7
1.67
1.92
2.21
5.19
2.51
3.2
3.79
9.15
2.44
50
232
584
369
458
735
624
929
763
316
905
840
134
422
484
751
108
388
590
Rb
4
7
12
35
48
71
94
117
135
117
120
187
98
174
116
116
274
71
128
518
2399
626
793
889
885
1048
680
529
852
628
450
786
585
1118
97
288
419
11
9
15
12
31
20
20
14
16
23
22
54
12
13
167
196
Sr
925
Pb
51
Th
4
U
3
Zr
179
0.46
0.48
0.21
172
Nb
72
6.8
29.7
Y
62
17
La
156
Ce
238
Hf
Pr
Nd
1.19
3.53
0.89
218
7.8
177.9
70
2.99
4.96
1.5
163
4 .1
6.87
1.62
232
4.6
51.6
5 .7
73.8
25
30
14.9
42.5
119.9
259.2
33.6
65.4
45.1
6.3
34.3
30.4
141.9
7.1
32.1
9 .6
43.5
88
10.48
17.67
4.15
10.89
21.91
4.55
11.86
15.01
6.92
4
5
365
429
462
542
104
171
8.6
163.1
9.3
178.5
16.6
260.8
34
34
33
32
28
76
103
95.3
99
121
147
164.6
15.5
165.8
48.6
82.9
59.3
55.6
15
1101
7.5
27.2
9.45
8.65
3.26
17.14
26.35
4.05
439
622
3
11
153.6
32
6
13
6
19.44
20.41
30.37
5
5
17
577
610
1159
769
740
148.9
168
13.9
188.6
141
117
241
9
12
32
14
35
36
32
56
17
26
18.1
8.9
88.9
90.2
39
105.1
101
134.5
85.1
17
28
153.7
14.6
148.6
13.2
55
176.1
15.7
138
233.9
22.7
126.8
50
58
55
47.7
65.5
45.6
57.6
15
NOVEMBER 2003
Ba
NUMBER 11
Norm. Ne
9
30.70
6.55
S
Total*
8
VOLUME 44
H2O
‡
3
JOURNAL OF PETROLOGY
SiO2
2
1
6
7
12
13
14
15
16
17
18
19
PCB30
2
PCB36
PCB57
PCB54
PCB10
PCB19
PCB24
PCB25
PCB20
PCB5
PCB16
PCB1
PCB51
PCB56
30449
30428
PCB40
PCB61
Rock type:
Ucm
Ucm
Gab
Gab
Ba
Nha
Nmu
Nbe
Ph
Ph
Nsy
Nsy
Nsy
Nsy
Brec
Nbe
Ph
Sch
Fen
102
Sm
8
Eu
1.75
6.39
0.95
4.41
Ho
0.68
Er
1.29
0.15
Gd
Tb
Dy
Tm
2069
Yb
0.86
Lu
0.12
3
8
10
11
8.7
1.91
7.55
4.7
0.94
4.66
8.4
2.02
7.78
8.9
2.19
8.92
8.9
1.93
8.58
7.4
1.58
7.52
1.57
22.21
6.6
1.49
5.91
3
14.83
0.87
4.72
1.11
5.78
1.2
6.28
1.16
6.13
0.72
4.31
0.89
1
2.46
5.51
0.67
0.84
2.03
0.28
1.03
2.39
0.33
1.14
0.86
2.45
0.43
5.21
0.98
1.18
2.76
0.4
1.12
2.82
0.4
4
0.64
1.42
0.17
2.59
0.43
4.24
0.6
1.82
0.26
2.13
0.3
0.37
2.63
0.37
3.33
0.49
1.05
0.16
26
5.09
9.6
9
2.64
11.5
1.4
8 .7
2 .7
9 .9
10.8
1.5
1 .1
8.6
2.1
6 .6
1 .2
3.04
0.47
6.1
3 .4
2.82
3.36
6.4
3 .1
0.41
0.49
9
1.21
6.41
Sc
29
55
25
30
24
11
12
4
2
4
7
2
1
1
V
29
442
110
224
260
109
132
46
17
19
49
7
1
16
71
50
3
103
Cr
362
727
110
503
158
97
244
69
44
161
97
48
50
86
73
28
51
48
76
Ni
109
85
32
179
71
14
95
22
3
11
14
2
51
10
38
24
51
15
23
Cu
49
33
22
52
64
17
16
5
51
4
6
1
51
51
7
24
3
17
15
Zn
239
65
86
89
105
100
116
110
134
152
128
106
74
147
116
122
219
67
96
Ga
21
20
22
22
26
28
29
29
30
38
29
30
29
37
27
26
34
12.41
0.723
20.27
0.631
13.28
0.714
15.22
0.704
25.94
0.714
27.02
0.668
10.46
0.607
60.49
0.750
22.94
0.641
15.07
0.378
19.69
0.886
(La/Yb)n
Eu/Eu*
22.41
0.530
10
For trace elements, italics indicate analyses by ICPMS (all PCB samples) or spark source mass spectrometry [analyses 16 and 17, from Price & Taylor (1973)]. All other
data by XRF. Rock types (1---15): Ucm, ultramafic cumulate; Gab, gabbro; Ba, basanite; Nha, ne-hawaiite; Nmu, ne-mugearite; Nbe, ne-benmoreite; Ph, phonolite; Nsy,
nepheline syenite. 15 (Brec) is a bulk sample of breccia matrix; 16 and 17 are samples from Dunedin Volcano; 18 and 19 are a schist (Sch) and fenite (Fen) from the Port
Chalmers Breccia.
PHONOLITIC DIATREMES OF DUNEDIN VOLCANO
5
PCB55
PRICE et al.
4
Sample:
JOURNAL OF PETROLOGY
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Fig. 8. Abundances of selected major elements plotted against SiO2 (wt %) for PCB samples. Most of the ultramafic samples have been
omitted to ensure a scale selection that gives sufficient detail. The arrows indicate the trend defined by ultramafic (cumulate) samples that
have not been plotted. Dunedin data are from Price (1973). Classification boundaries on the total alkali vs silica diagram are from Le Maitre
et al. (1989). U1, basanite---tephrite field; U2, phono-tephrite; U3, tephro-phonolite; Ph, phonolite field; S1, trachybasalt field; S2, basaltic
trachyandesite field; S3, trachyandesite field. `PCB matrix' is the composition of a bulk sample of matrix (free of clasts 41 cm) of the Port
Chalmers Breccia (No. 15 in Table 8).
but they lack the more extreme Eu depletions of the
most felsic Dunedin phonolites (Fig. 10). Gabbro and
basanite clasts show very similar normalized rare
earth patterns with enrichment of LREE over HREE
[(La/Yb)n ˆ 13---15] and weak negative Eu anomalies
(Eu/Eu* ˆ 07). They are in many respects compositionally similar to basanites of the Dunedin Volcanic
Group. Ultramafic cumulate clasts show more
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PRICE et al.
PHONOLITIC DIATREMES OF DUNEDIN VOLCANO
Fig. 9. Abundances of selected trace elements (ppm) plotted against SiO2 (wt %) for PCB samples. Dunedin data are from Price (1973). `PCB
matrix' is the composition of a bulk sample of matrix (free of clasts 41 cm) of the Port Chalmers Breccia (No. 15 in Table 8).
variability than the gabbro clasts. PCB30 has MREE
abundances similar to those observed in other ultramafic and mafic rocks but shows depletion of LREE
relative to MREE and lower abundances of HREE
(Fig. 10).
In major, trace and minor element terms, syenite
and phonolite clasts are very similar to ne-benmoreite
compositions from the Dunedin Volcanic Group. Relative to basanite, these rocks are enriched in large ion
lithophile elements such as Rb, K and the REE, and
some high field strength elements such as Zr and Nb,
but Ba, Sr, Eu and Ti are relatively depleted. Ultramafic rocks generally have trace and minor element
abundance patterns complementary to those of the
felsic rocks. None of the phonolitic or syenitic rocks of
the PCB shows the more extreme depletions in Ba, Sr,
Eu and Ti observed in some of the phonolites of the
Dunedin Volcanic Group.
ISOTOPE GEOCHEMISTRY
Methods
Strontium, Nd and Pb isotopic data were obtained in
the VIEPS isotope laboratory at La Trobe University
using a seven-collector Finnigan-MAT 262 spectrometer and methods described in detail by Price et al.
(1999). Reproducibility on Pb isotopic compositions
2071
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Fig. 10. Chondrite-normalized rare earth element patterns for clast samples from the PCB and Dunedin Volcanic Group. Ucm, ultramafic
cumulate; Gab, gabbro; Bas, basanite; Ba, basalt; Nbe, ne-benmoreite; Ph, phonolite; Nsy, nepheline syenite. Dunedin data are from Price &
Taylor (1973). Normalizing values are from Sun & McDonough (1989).
for SRM981 (n ˆ 78, 2s) is 0097% for 206 Pb/204 Pb,
0130% for 207 Pb/204 Pb, and 0175% for 208 Pb/
204
Pb. The average fractionation factor during runs
was 0091% (2s ˆ 0034%), close to the empirical
value of 0109%.
For Sr, instrumental mass fractionation was corrected by normalizing to 86 Sr/88 Sr ˆ 01194. Typically,
5---7 blocks of 10 8 s integrations produced in-run
precision (2s) of 0003%. 87 Sr/86 Sr (2s) for
SRM987 (n ˆ 100) is 071023 7 (001%), for
BCR-1 (n ˆ 6) 070500 4, and for BHVO-1 (n ˆ
19) 070348 6.
For Nd, fractionation was corrected by normalizing
to 146 Nd/144 Nd ˆ 07219. Typically, 5---7 blocks of
10 8 s integrations produced in-run precisions (2s)
of 00025%. 143 Nd/144 Nd (2s) for La Jolla (n ˆ
100) is 0511860 16, for BCR-1 (n ˆ 7) 0512634 18, and for BHVO-1 (n ˆ 5) 0512989 13. Presentday CHUR was taken as 0512631.
Stable isotope ratios were measured at the VIEPS
stable isotope facility at Monash University using a
Finnigan MAT 252 mass spectrometer. CO2 was
extracted from calcite by reaction with H3PO4 at 25 C
for 12---18 h in sealed vessels (McCrea, 1950). d18 O and
d13 C values are expressed relative to V-SMOW and
V-PDB, respectively. Internal calcite standard ISACC
analysed at the same time as the samples yielded d13 C
and d18 O values within 01% of its long-term average.
This standard was calibrated using IAEA-CO-1 and
its long-term average d13 C and d18 O values are 637 006% and 1268 013%. Based on replicate analyses, reproducibility is estimated as 01% for both O
and C.
Pb, Sr and Nd isotopic data
Pb, Sr and Nd isotopic data for PCB samples are presented in Table 9. The Sr data have been age corrected
but, given the relatively young ages (10---13 Ma), the
differences between initial and present-day ratios are
minor.
In 207 Pb/204 Pb vs 206 Pb/204 Pb and 208 Pb/204 Pb vs
206
Pb/204 Pb diagrams (Fig. 11), data for PCB clasts
form a tight cluster on the Northern Hemisphere
Reference Line (Hart, 1984), with 207 Pb/204 Pb ratios
ranging from 15624 to 15711 and 208 Pb/204 Pb ratios
from 39155 to 39547. Sr and Nd isotopic compositions
also show very little variation (Fig. 11), with presentday 87 Sr/86 Sr ratios ranging from 0702794 to 0703410
and 143 Nd/144 Nd from 0512866 to 0512926. Lead
isotopic data are not available for other rocks from
Dunedin Volcano, but Sr and Nd isotopic data (Price
& Compston, 1973; Coombs et al., 1986; McDonough
et al., 1986) indicate that the clasts of the PCB are
2072
PRICE et al.
PHONOLITIC DIATREMES OF DUNEDIN VOLCANO
Table 9: Isotopic data for selected samples from the Port Chalmers Breccia
2s*
Rb/Sr
87
0.702794
0.702824
17
0.0135
0.0050
0.703010
0.703410
26
0.703056
0.702968
26
87
Sr/86 Sr
Rb/86 Sr
Nd/144 Nd
2s*
Sri(12 Ma)
143
0.0390
0.0145
0.70279
0.70282
0.512926
0.512913
0.0605
0.2212
0.1749
0.6392
0.70298
0.70330
0.512897
0.512866
13
0.1985
0.1116
0.5738
0.3226
0.70296
0.70291
0.512901
0.512876
21
Pb/204 Pb
208
Pb/204 Pb
39.420
39.504
19.578
19.922
15.625
15.646
39.155
39.324
20.016
19.855
15.640
15.638
39.539
39.380
9
19.532
19.580
15.653
15.624
39.547
39.182
9
19.901
15.643
39.345
Amphibole gabbro
PCB54
Basanite
PCB20
Phonolite
PCB25
Phonolite
PCB24
Ne-benmoreite
PCB5
Nepheline syenite
18
0.1409
0.2978
0.4071
0.8606
0.70286
0.70319
0.512906
0.512890
11
Nepheline syenite
0.702925
0.703331
21
PCB16
PCB51
Nepheline syenite
0.703401
20
0.2214
0.6398
0.70329
0.512869
23
207
15.612
15.711
Ultramafic cumulate
PCB36
23
Pb/204 Pb
19.967
19.786
PCB30
16
206
8
12
12
10
*2s indicates last two of six decimal places.
Fig. 11. Isotopic compositions of PCB clast samples. (a) and (b) are 208 Pb/204 Pb and 207 Pb/204 Pb vs 206 Pb/204 Pb diagrams and (c) shows
143
Nd/144 Nd vs 87 Sr/86 Sr. (d) shows details for a portion of (c). Comparisons are made with mid-ocean ridge basalt (MORB), end-member
mantle components, Samoa, the Marquesas and Mangaia, and with data for Balleny Islands, Tasmantid seamounts and Tasmanian Tertiary
basalts, Tapuaenuku in north Canterbury, lamprophyre dykes of the South Island west coast, the Auckland Islands, East Otago (Dunedin
Volcanic Group) and Banks Peninsula Tertiary volcanic rocks. NHRL is the Northern Hemisphere Reference Line of Hart (1984). DM,
HIMU, EM1 and EM2 are depleted, high m, and enriched (1 and 2) mantle components from Zindler & Hart (1986). Data sources:
McDonough et al. (1986); Barriero & Cooper (1987); Wright & White (1987); Dupuy et al. (1988); Eggins et al. (1991); Lanyon et al. (1993);
Baker et al. (1994); Lanyon (1994); Woodhead (1996); and unpublished data of J. A. Gamble (2002), S. D. Weaver (2002) and J. A. Baker
(2002).
isotopically indistinguishable from Tertiary East
Otago and Banks Peninsula volcanics (Fig. 11; see
Fig. 1 for geographical distribution).
Regional comparisons illustrate that Dunedin volcanic rocks are isotopically very similar to volcanic and
intrusive rocks of various ages from elsewhere in southern New Zealand, the Tasman Sea and the subAntarctic region to the south (Fig. 11; see Fig. 1 for
geographical distribution) and these similarities have
been recognized for some time. McDonough et al. (1986)
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Table 10: Carbon and oxygen isotopic
compositions of Port Chalmers Breccia
d13 C (%)
d18 Osmow (%)
Sample
Description
PCB58
Calcite in open vug in breccia
17
134
PCB59
Calcite in vein in dolerite
ÿ58
72
PCB60
Calcite in vein within a syenite clast
ÿ60
75
PCB61
Calcite from fenitized schist clast
ÿ72
84
NUMBER 11
NOVEMBER 2003
primitive compositions with d18 Osmow in the range
72---75% and d13 C between ÿ58 and ÿ60%. The
fenitized schist carbonate sample also shows primitive
values (84 and ÿ72%). The third vein sample is significantly different (d18 O ˆ 134% and d13 C ˆ 17%)
and has a composition that is more like that expected
for a sedimentary rock. The more primitive isotopic
compositions have carbonatitic affinity (e.g. Deines,
1989; Reid & Cooper, 1992) and they indicate that
magmatic CO2 was a significant volatile during the
crystallization and emplacement of the PCB.
PCB58, -59 and -60 are from section behind container wharf
at Koputai Bay. PCB61 is from Leans Rock.
reported that east Otago volcanic rocks have 87 Sr/86 Sr
ratios ranging from 07029 to 07032 and eNd from
‡47 to ‡63, and noted that this limited isotopic
range was very similar to those observed among rocks
of the New Zealand sub-Antarctic islands and from
Banks Peninsula. Tertiary lamprophyric dykes of the
West Coast of New Zealand's South Island (the Alpine
dyke swarmÐCooper, 1986; Barriero & Cooper, 1987)
also show a very close isotopic similarity to Dunedin
volcanic rocks (Fig. 11). Lanyon et al. (1993) ascribed
the isotopic characteristics of the southern New
Zealand igneous rocks to the regional influence of a
mantle plume and broadened the region showing these
isotopic features to include Tertiary basalts in
Tasmania, seamounts of the southern Tasman Sea,
and volcanics of the Balleny Islands and Marie Byrd
Land in Antarctica (Fig. 1). Most recently, Baker et al.
(1994) noted the similarity between the isotopic composition of the Alpine dyke swarm and the Tapuaenuku
igneous complex, a layered alkalic intrusive complex of
Cretaceous age (90---100 Ma) in the Inland Kaikoura
Ranges of north Canterbury (Fig. 1), thereby extending the isotopic regional province into the northern
South Island and back in time to the late Mesozoic.
The New Zealand---Tasman---Antarctic isotopic province has Pb, Sr and Nd compositions that partially
overlap with those observed in the Marquesas and
Samoan seamount chains (Fig. 11), but the Pb isotopic
compositions extend towards more radiogenic values
and Lanyon et al. (1993) and Baker et al. (1994) argued
that an enriched high U/Pb (high m or HIMU) component was required in the mantle source.
Stable isotope data
Four samples were analysed for carbon and oxygen
isotopic composition. Three of these are carbonate
veins and the fourth is a sample of carbonate separated
from a fenitized schist clast. Results are presented in
Table 10. Two of the vein samples have relatively
DISCUSSION
Emplacement of the Port Chalmers Breccia
The outcrops of PCB across the central Otago
Harbour (Fig. 2) have all the characteristics of diatremes (Lorenz, 1986) and appear to represent feeder
structures to what were probably small maar
volcanoes.
The Laacher See volcano in the East Eifel district of
Germany could provide an analogue for the volcanoes
that lay above the Port Chalmers diatremes. An estimated 53 km3 of phonolitic pyroclastic material
(Worner & Schminke, 1984a; Freundt & Schminke,
1986) was erupted at Laacher See to form a dispersed
tuff-ring around a central crater that has a present-day
diameter of 2---3 km. Worner & Schminke (1984a,
1984b) concluded that eruptions tapped a shallow
(3---6 km) zoned magma chamber and they attributed
the explosive nature of the eruptive activity to interaction between groundwater and magma. The PCB pipes
also have similarities to phonolitic breccia pipes of the
Cripple Creek district of Colorado. Kelley et al. (1998)
proposed that Cripple Creek phonolitic magmas
evolved in the lower crust or upper mantle and then
rose rapidly to the surface along faults. Some of the
Cripple Creek Breccia pipes appear to have been generated when magmas encountered groundwater, but
others may have formed during explosive emplacement
arising from vapour saturation (H2O and CO2) in
rapidly rising magma.
In cases where diatreme and maar formation is
argued to involve interaction between magma and
groundwater (e.g. Worner & Schminke, 1984a;
Lorenz, 1986), depth of diatreme growth is generally
related to water-table depth or, very commonly, to
the intersection of rising magma with specific aquifers.
For the PCB, the deepest potential aquifer is the basal
unit of the Cretaceous---Tertiary cover sequence, the
Taratu Formation (McKellar, 1966) at a depth of
800---900 m (Coombs, 1965). Clasts from this unit are
extremely rare but schist clasts are common, indicating that brecciation occurred in the basement below
2074
PRICE et al.
PHONOLITIC DIATREMES OF DUNEDIN VOLCANO
the cover sequence. Brecciation of cumulate and syenitic blocks must have also taken place during explosive disruption of a melt cumulate zone but the
metamorphic grade of the schists places a relatively
shallow limit (2---3 km?) on the depth at which this
took place.
If the explosive emplacement of the PCB was not
driven by interaction with groundwater it is likely
that exsolution of juvenile gas was involved. The
abundance of amphibole and biotite indicates that
magmas were water bearing but there is geological
and petrological evidence that CO2 was also a significant volatile component (e.g. carbonate veins and fenitization of schist clasts). Stable isotope data indicate
that the CO2 involved was largely of magmatic
origin. CO2 solubility is generally higher in silicaundersaturated than in silica-saturated or -oversaturated magmas (e.g. Mysen et al., 1975; Spera &
Bergman, 1980; Holloway & Blank, 1994) and decreasing pressure favours decreased CO2 and H2O solubility
(e.g. Burnham, 1967; Spera & Bergman, 1980; Stolper
et al., 1987; Blank & Brooker, 1994). H2O and CO2 also
lower melt viscosity and melt density (Burnham, 1967;
Blank & Brooker, 1994). The bulk chemistry and the
volatile contents of PCB clasts are therefore consistent
with a magmatic system in which parental melts were
relatively low density, low viscosity, and CO2 and H2O
rich. Conceivably, fracturing along a NW---SE fault
could have released such magmas rapidly and explosively from shallow crustal sources.
The generation and evolution of the
magmas represented by the clasts of
Port Chalmers Breccia
The only clasts found in the PCB that can be interpreted as representing juvenile material are small,
originally highly vesicular, analcime- and calciteimpregnated, devitrified glassy fragments found within
the breccia matrix. Other clasts show evidence of being
transported within, or physically infiltrated by a very
similar gas-charged magma. Compositions of fresh
glassy inclusions in minerals indicate that this magma
was phonolitic (ne-benmoreite or phono-tephrite).
Price & Chappell (1975) argued that Dunedin nebenmoreites were derived from basanitic parental
magmas by crystal fractionation involving olivine,
pyroxene, amphibole and magnetite (see also Coombs
& Wilkinson, 1969). Similar hypotheses have been put
forward to explain geochemical variation in basanite--phonolite associations elsewhere (e.g. Baker, 1969;
Nash et al., 1969; W
orner & Schmincke, 1984b; Price
et al., 1985; Le Roex et al., 1990).
The ne-benmoreite clasts of the PCB are relatively
mafic when compared with the more felsic phonolites
flows and domes of the Dunedin Volcanic Group and
they do not exhibit the extreme relative depletions in
Mg, Ca, Sr and Eu commonly observed in the felsic
phonolites. The differences could reflect the levels at
which crystal fractionation has taken place, with the
ne-benmoreites generated by crystal fractionation
in the deep crust or upper mantle (Irving & Price,
1981) and more strongly fractionated phonolites, with
their relatively low Sr contents and Eu-depleted REE
patterns, by shallow-level, feldspar-dominated crystal
fractionation.
An alternative, crustal anatexis model has been suggested for the origin of phonolitic rocks in the Kenya
Rift (e.g. Bailey, 1964; Williams, 1970; Hay &
Wendlandt, 1995). Proponents of a crustal melting
origin for phonolites argue that volume considerations,
the uniformity of plateau-type flood phonolites and the
paucity of intermediate compositions are all factors
that present problems for a fractional crystallization
origin for these rocks. Hay & Wendlandt (1995) used
high-pressure experiments to demonstrate that
Kenya rift flood phonolites have equilibrated under
lower-crustal conditions and that it is feasible to
generate phonolites by partial melting of an alkali
basaltic composition in the lower crust. They supported their experimental conclusions with an analysis
of geochemical data for Kenya rift phonolites
(Hay et al., 1995).
An analogous two-stage model could apply to the nebenmoreites of the PCB (Fig. 12). During the first
stage, mantle melting under near water-saturated conditions could have generated basanitic magmas that
underplated and intruded the lower crust. Some of this
material could have crystallized in the lower crust to
form alkali gabbroic assemblages consisting of plagioclase, clinopyroxene, amphibole, biotite and magnetite.
Other batches of magma moved upwards towards the
surface, undergoing varying degrees of crystal fractionation and forming small temporary magma reservoirs
or crystallizing to form sills, dykes and small intrusions
distributed throughout the crust (Fig. 12). Reilly
(1971) interpreted gravity data for the Dunedin Volcano to indicate that an extensive volume of the crust
underlying the volcano is composed of dense intrusive
rocks. At the second stage, rising geotherms associated
with continued injection of mafic magma could cause
partial melting of lower-crustal mafic intrusives to produce ne-benmoreite magmas.
The textures, mineralogy and chemistry of the
coarse-grained clasts in the PCB indicate that they
constitute a disrupted assemblage of coarse-grained
basanitic rocks (alkali gabbros), ultramafic and felsic
cumulates, and differentiated nepheline syenites.
Amphiboles in some ultramafic cumulates in the
PCB are very similar compositionally to those in the
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Fig. 12. Model for the origin of the PCB and for ne-benmoreites and phonolites of the Dunedin Volcanic Group. Early stages of the
development (a) were marked by emplacement of basaltic rocks and quartz-normative trachytes. Mantle-derived magmas intruded and
underplated the lower crust or migrated towards the surface, fractionating along the way. Continued emplacement of mafic magma led to
partial melting of the underplate and formation of ne-benmoreite magma. Basaltic and ne-benmoreite magma and derivative magmas and
cumulates from these accumulated in a plexus of dykes and sills and small magma reservoirs, distributed throughout the crust. Fracturing of
the crust (b) on NW---SE faults tapped into the higher-level parts of this system, explosively releasing volatile-saturated magma to form maar
volcanoes at the surface and diatremes of breccia at depth. The subsequent history (c) saw continued emplacement of mafic magma into the
lower crust and passage of some of this material to the surface, with crystal fractionation taking place during transit. Partial melting of earlier
mafic underplate continued to produce ne-benmoreite magma. Crystal fractionation of this material within the crust produced large volumes
of highly fractionated felsic phonolite.
ne-benmoreites and some of the syenites, and this might
indicate that at least some amphibole cumulates have
crystallized from relatively evolved magmas. Olivine is
not present in ultramafic cumulate rocks, which have
modal compositions dominated by amphibole and pyroxene. It would therefore appear feasible that much of
the ultramafic and possibly some of the syenitic cumulates in PCB were derived from crystal fractionation of
relatively felsic magmas (e.g. ne-benmoreite).
The processes of transfer of magmas from the mantle,
melting of mafic crust and differentiation of mantleand crust-derived magmas are envisaged to have continued throughout much of the history of the Dunedin
Volcano. Emplacement of the Port Chalmers diatremes appears, however, to have been a unique
event triggered by faulting, which caused the rapid
ascent and degassing of ne-benmoreite magma with
consequent brecciation and entrainment of a wide
range of lithologies between source and surface. The
presence of clasts of Cretaceous sediment means that
the depth of the source has to be at least 1 km but the
metamorphic grade of the schist clasts indicates that it
was located in the upper (1---3 km?) rather than middle
or lower crust. We envisage that, before disruption, the
source region for the PCB consisted of a mush of near
volatile-saturated ne-benmoreite melt within a complex
of gabbros, syenites, and ultramafic and felsic cumulates.
SiO2-undersaturated basaltic and phonolitic magmas were continuously emplaced throughout the history of the Dunedin Volcano but the more extremely
fractionated felsic phonolites become progressively
more common among the younger eruptives. We interpret these patterns to indicate that, both before and
after the PCB event, mantle- and lower-crustal-derived
magmas continued to feed into a dispersed magmatic
system in which crystal fractionation continued to take
place in dykes, sills and other small magma reservoirs
throughout the crust. Virtually all the mafic rocks in
the Dunedin volcano have compositions indicating
that they have undergone crystal fractionation. The
felsic low-Mg, low-Sr phonolitic rocks were derived
from ne-benmoreite precursors by more extreme
fractionation at relatively shallow levels (Price &
Chappell, 1975).
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PRICE et al.
PHONOLITIC DIATREMES OF DUNEDIN VOLCANO
Mantle sources and tectonic setting
The Pb, Sr and Nd isotopic data indicate that magmas
represented by the rocks of the PCB were generated
from mantle sources with compositions between
HIMU and a depleted mantle component. This particular mantle source appears to be very widespread
over a region that includes the South Island of New
Zealand, the South Tasman Sea, Tasmania, the New
Zealand sub-Antarctic islands, the Balleny Islands and
parts of mainland Antarctica (Lanyon et al., 1993).
Tertiary volcanics in eastern Australia (McDonough
et al., 1985) and seamounts of the central and north
Tasman Sea (Eggins et al., 1991) are isotopically different and appear to have derived from mantle sources
with compositions involving depleted and enriched
(EM1) components.
Lanyon et al. (1993) noted that the isotopic similarities of mantle source compositions across the region
were supported by trace and minor element data, and
argued that these regional HIMU signatures arise from
the effects of two separate plume sources that are now
located in the Balleny Islands and Marie Byrd Land,
respectively. They concluded that all intraplate association igneous rocks in the region showing the influence of a HIMU mantle source are related directly to
plume activity, which was also the driving force
initiating continental rifting and separation before
83 Ma BP. The work of Baker et al. (1994) on the
Tapuaenuku igneous complex of Cretaceous age (90--100 Ma) in the Inland Kaikoura Ranges of the South
Island has, however, illustrated that the HIMU
influence on regional mantle isotopic signatures was
already present around 20 Myr before the initial rifting
associated with Tasman Sea opening.
The problem for the plume model is that a specific
isotopic signature is manifested over a vast area
(Fig. 1), in a wide range of intraplate basaltic rocks of
various ages (from virtually the present day to
100 Ma). The seamounts of the South Tasman Sea
and the Balleny Islands show systematic progression in
age that can be reconciled with a hotspot trace but
across the rest of the region there appears to be no
systematic spatial---temporal relationship (see Adams,
1981). One possibility is that the influence of the
Balleny and Marie Byrd plumes was very widespread
before continental separation so that the lithospheric
mantle across the region was chemically preconditioned. Magmas produced during any subsequent
thermal events in the region are either derived directly
from the lithospheric mantle or are contaminated by it.
Such a model would require a substantial lead time
(420 Myr) between the arrival of a plume at the base of
the lithosphere and the first significant rifting in the
Tasman Sea. An alternative to the plume model would
be along the lines proposed by Baker et al. (1994),
whereby the characteristic isotopic signature of the
post-Mesozoic intraplate igneous rocks reflects a shallow regional mantle reservoir that was generated during prolonged subduction throughout the Mesozoic
and before continental separation. The implication is
that the Dunedin Volcanic Group magmas contain a
significant component that originated in the subcontinental, lithospheric mantle.
CONCLUSIONS
Construction of the Dunedin Volcano began around
13 Myr ago with the submarine emplacement of basalts
and quartz-normative trachytes, and continued over a
period of 3 Myr with subaerial eruption of dominantly
silica-undersaturated basaltic and phonolitic magmas.
Outcrops of PCB are found only in contact with older
units of the volcanic sequence but the clast assemblages
in the breccias indicate that they were emplaced after
significant volumes of phonolitic magma had been
erupted and a substantial subaerial volcanic complex
constructed. The PCB outcrops represent diatremes
and proximal pyroclastic deposits associated with
maar volcanoes aligned along a NW---SE fault system.
They were emplaced when faulting released volatilerich ne-benmoreite magma from melt---cumulate zones
located 1---3 km beneath the volcano.
The PCB is phonolitic in composition (nebenmoreite or tephro-phonolite) and consists of
abundant clasts of ne-benmoreite, phonolite, basanite,
ne-trachyandesite,
syenite,
gabbro,
ultramafic
cumulate, altered schist and Cretaceous or Tertiary
sediment in a sand and silt matrix with a bulk composition of ne-benmoreite.
Mineralogically, the syenites of the PCB show variations similar to those observed in nepheline syenites
from elsewhere (e.g. South Greenland, East Africa)
but some of the mineralogical compositional variation
is more extreme in the PCB syenite clasts. Pyroxenes
are dominantly aegirine and aegirine---augite, and
biotite is abundant and Ti rich. Alkali amphibole is
present within syenite clasts but Ti-rich calcic amphiboles are abundant in all coarse-grained rocks and
dominate ultramafic cumulates and gabbros. Nepheline is ubiquitous in syenitic blocks and sodalite is
common.
Major and trace element variations defined by the
clast suite from the PCB are similar to those observed in
the Dunedin Volcanic Group, although the extreme
depletions in Mg, Ca and Sr and strong negative Eu
anomalies shown by the most fractionated Dunedin
phonolites do not feature among the phonolite clasts
of the PCB and are uncommon among the syenite
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JOURNAL OF PETROLOGY
VOLUME 44
clasts. The clast assemblage of the PCB could be interpreted to represent a consanguineous suite related by
crystal fractionation of a basanitic parent in a crustal
magmatic system. Ne-benmoreites could be crystal
fractionation products from basanite but we believe it
is more likely that they were generated in a two-stage
process involving partial melting of deep crustal intrusive rocks. Highly fractionated phonolites of the
Dunedin Volcanic Group were derived by feldspardominated crystal fractionation from ne-benmoreite
precursors.
Parental magmas of the Dunedin Volcanic Group
magmas came from an isotopically distinct mantle
source with a Sr, Nd and Pb isotopic composition
between HIMU and depleted mantle. The composition matches closely those of Mesozoic and Tertiary
intraplate igneous rocks across southern New Zealand,
the South Tasman Sea, Tasmania, the New Zealand
sub-Antarctic islands, the Balleny Islands and Marie
Byrd Land. The pattern indicates the presence of a
distinctive, long-lived and extensive lithospheric mantle reservoir in the region.
ACKNOWLEDGEMENTS
The technical support of Ian McCabe, Jorg Metz and
Allen Jacka is gratefully acknowledged. John Gamble,
Steve Weaver and Joel Baker very generously gave
access to unpublished isotopic data for Auckland
Islands, Banks Peninsula and Tapuaenuku samples.
The development of the project benefited significantly
from discussions with Professor Doug Coombs. Constructive and thorough reviews of an earlier version of
the paper by Anton Le Roex and Brian Upton are
gratefully acknowledged, as is the editorial assistance
of Richard Arculus. This research was funded by
research grants from La Trobe University and the
University of Otago.
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