JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 1
PAGES 167–197
1999
Petrogenesis of High-K Arc Magmas:
Evidence from Egmont Volcano,
North Island, New Zealand
R. C. PRICE1∗, R. B. STEWART2, J. D. WOODHEAD3 AND I. E. M. SMITH4
1
SCHOOL OF SCIENCE AND TECHNOLOGY, THE UNIVERSITY OF WAIKATO, PRIVATE BAG 3105, HAMILTON,
NEW ZEALAND
2
DEPARTMENT OF SOIL SCIENCE, MASSEY UNIVERSITY, PALMERSTON NORTH, NEW ZEALAND
3
SCHOOL OF EARTH SCIENCES, UNIVERSITY OF MELBOURNE, PARKVILLE 3052, VIC., AUSTRALIA
4
DEPARTMENT OF GEOLOGY, UNIVERSITY OF AUCKLAND, AUCKLAND, NEW ZEALAND
RECEIVED MAY 27, 1997; REVISED TYPESCRIPT ACCEPTED JUNE 1, 1998
Egmont Volcano (Mt Taranaki) is located 140 km west of the
Taupo Volcanic Zone (TVZ), the principal locus of volcanic activity
in the North Island of New Zealand, and is one of four closely
associated Quaternary andesitic volcanoes in Taranaki province.
Taranaki eruptives are enriched in K and other large ion lithophile
elements compared with their counterparts at Ruapehu in the southern
TVZ, with the youngest Egmont andesites being the most K rich.
Egmont andesites are invariably fractionated but isotopic information
indicates that, unlike those at Ruapehu, they have not extensively
assimilated enriched crust. Ti/Zr, Ba/La, Ce/Pb, and K/Rb
ratios indicate that a more depleted mantle wedge and compositionally
different slab-derived fluids were involved in the generation of
Taranaki primary magmas. Magmas parental to Egmont eruptives
were relatively undersaturated, hydrous, high-Mg basalts generated
by low degrees of partial melting in a depleted mantle wedge fluxed
by deep slab fluids. Fractionation of these magmas at the base of the
crust produced basaltic andesite and extensive ultramafic cumulates.
Plagioclase fractionation was suppressed by high aH2O. Rising
geothermal gradients eventually resulted in partial anatexis of
amphibolitic underplated crust, and interaction of basaltic andesites
with these melts led to progressively more K-rich compositions.
KEY WORDS: cross-arc
variation; high-K andesite; subduction magmatism
∗Corresponding author. Telephone: 64 7 838 4520. Fax: 64 7 838
4218. e-mail: [email protected]
INTRODUCTION
Egmont Volcano is a 2518 m high stratovolcano (Mt
Taranaki) resting on an extensive ring plain of volcanic
debris in Taranaki province, western North Island, New
Zealand. It is the youngest and largest of four Taranaki
volcanoes (Fig. 1), which together form a NW–SE trending lineament along which activity has migrated southeastward with time (Neall et al., 1986). The Taranaki
volcanoes lie 140 km to the west of the Taupo Volcanic
Zone (TVZ), the principal locus of subduction-related
magmatism in the North Island.
There are significant petrological contrasts between
Egmont Volcano and the contemporaneous andesitic
stratovolcanoes of the southern TVZ, collectively referred
to as the Tongariro Volcanic Centre (TVC). For example,
Egmont andesites are commonly hornblende bearing
whereas those of the TVC are generally clinopyroxene
and orthopyroxene types. Egmont eruptives are also
relatively potassic (Neall et al., 1986; Price et al., 1992;
Stewart et al., 1996); they are high-K andesites according
to the classification of Gill (1981).
Both Taranaki and TVC are active volcanic systems
associated with the Tonga–Kermadec–New Zealand convergent plate margin. On the basis of isotopic data, Price
et al. (1992) suggested that both volcanic centres lie above
mantle wedge with broadly similar chemical characteristics. The volcanic systems are, however, located
on either side of a major terrane boundary and they
 Oxford University Press 1999
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 1
JANUARY 1999
Fig. 1. Geological and tectonic setting of Taranaki volcanoes. (a) Regional tectonic setting, with shaded area representing extent of continental
crust. (b) Details of geological setting in New Zealand’s North Island. Egmont Volcano or Mt Taranaki (E), Pouakai Volcano (P), and Kaitake
Volcano (K) define the Taranaki volcanic lineament, which may also include the Sugar Loaf Islands and Paritutu (S). The Tongariro Volcanic
Centre (TVC) and Ruapehu Volcano (R) lie to the east at the southern end of the Taupo Volcanic Zone. The Alexandra Volcanic Lineament
(Briggs et al., 1989) is also shown. Contours show depth to the Wadati–Benioff Zone (Adams & Ware, 1977).
therefore overlie different crustal sections. There are
other significant differences in their respective geological
settings; TVC volcanoes lie at the southern end of the
TVZ, which is a zone of attenuated crust (~15 km thick)
and anomalously high heat flow (Stern, 1987; Stern &
Davey, 1987; Hochstein et al., 1993) whereas Taranaki
volcanoes overlie ~25 km thick continental crust characterized by normal heat flow. There is therefore the
possibility that petrological differences between Taranaki
and TVC andesitic eruptives reflect differences in partial
melting processes and/or varying degrees of interaction
with the crust in different parts of the same subduction
system; one of the aims of this paper is to examine more
closely and explain the observed differences in chemistry
between Taranaki and TVC andesites.
Well-exposed flow sequences on Egmont Volcano provide an excellent opportunity to study temporal changes
in magma chemistry and petrography in a high-K andesite volcano. Consequently, an important objective of
our paper is to use new major and trace element, and Sr,
Nd, and Pb isotopic data to illustrate chemo-stratigraphic
change and examine possible mechanisms of magma
genesis and evolution at Taranaki. This goal is of considerable relevance to our broader aim of explaining
cross-arc variation in New Zealand andesite volcanoes.
GEOLOGIC SETTING AND ERUPTIVE
HISTORY
Regional tectonic setting
The Taranaki Volcanoes constitute the most westerly
(albeit isolated) manifestation of volcanism associated
with the Tonga–Kermadec subduction system in the
New Zealand region. A well-defined Wadati–Benioff
Zone dips westwards beneath the volcanoes (Adams &
Ware, 1977; Reyners, 1983) and underlies them at a
depth of 180 km; Egmont Volcano lies 180 km to the
west of the Hikurangi Trench (Fig. 1). The Taranaki
Volcanic lineament is located between the North and
South Taranaki Basins, bounded by the NE-trending
Cook–Turi Lineament to the north and the Taranaki
Fault to the south (Fig. 1). The Tongaporutu High, to
168
R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
the east of the Taranaki Fault, is a major structural
feature, which approximates the extension of the Median
Tectonic Zone (MTZ) north from the Nelson region of
the South Island. The MTZ marks the boundary between
the Palaeozoic Takaka terrane to the west and the
Mesozoic terranes to the east (Kimborough et al., 1993).
Mortimer et al. (1997) concluded that this major terrane
boundary occurs within the basement beneath Taranaki.
Mesozoic greywackes of the Waipapa and Murihiku
terranes occur to the east and are separated from Permian
sequences (Brook Street terrane) and MTZ to the west,
by the Taranaki fault. Egmont Volcano is located on
basement dominated by granitic and dioritic rocks of the
MTZ.
Geology of the Taranaki volcanic lineament
The volcanic centres that make up the Taranaki volcanic
lineament (Fig. 1b) are a sequence of remnant edifices that
progressively young to the south and show concomitant
decreases in the degree of erosion with time. The oldest
and most severely eroded centre is represented by a
group of nearshore islands (the Sugar Loaf Islands) and
onshore volcanic spires (including Paritutu on the outskirts of the city of New Plymouth) dated by the K/Ar
method at 1·7 Ma (Neall, 1979). These are plagioclasephyric, strongly porphyritic andesites, variously thought
to be part of a ring fracture or feeders to now eroded
vents (Grant-Taylor, 1964). The centre lies on the northeastern side of the lineament defined by the other three
Taranaki centres and, on these grounds and limited
geochemical data (see below), there must be some doubt
about its affinity with the other centres.
The next youngest centre, Kaitake, which is K–Ar
dated at 0·575 Ma [ J. J. Stipp, personal communication,
cited by Neall (1979)], is a deeply eroded stratovolcano
remnant, comprising a series of radial ridges about a
central plateau at 684 m altitude. Dykes occasionally
outcrop on the ridges but for the most part outcrop is
poor. Lithologies are predominantly hornblende andesite
with some diorite. A series of debris avalanche deposits
are exposed at the north Taranaki coast (Maitahi Lahars)
and these contain predominantly hornblende andesite
clasts. The Maitahi lahar deposits are the only remnants
of a previously extensive Kaitake ring plain.
The remnant cone of Pouakai lies 10 km SE of Kaitake
and is K–Ar dated at 0·25 Ma [ J. J. Stipp, personal
communication, cited by Neall (1979)]. The Pouakai
Ranges rise to 1399 m and cover an area about half to
two-thirds that covered by Egmont at its base. Pouakai
lavas are also hornblende andesites. Much of the northern
ring plain was protected by the Pouakai Ranges from
inundation by Egmont lahars and up to 30 m of tephra
from Pouakai is preserved across the north Taranaki
landscape. To the south, the younger Egmont Volcano
and its ring plain cover the Pouakai deposits.
The earliest identified activity at Egmont Volcano
occurred at ~115–120 ka  (Alloway et al., 1995) and
the evidence of this activity is preserved as lahar deposits
in the coastal cliffs of south Taranaki. The most recent
volcanic activity was an eruption in  1755 (Druce,
1966; Topping, 1974). The present cone has been constructed over the last 7 kyr (Neall et al., 1986; Stewart et
al., 1996).
Price et al. (1992) argued that magma generation along
the Taranaki lineament is directly related to fluid loss from
the slab, with fluid egress being structurally controlled by
propagating tears or fractures in the subducting slab.
Seismic data can be interpreted to indicate that the
subducting slab beneath the North Island is segmented
along a series of fractures that are orthogonal to the
Hikurangi Trench (Reyners, 1993), and the Taranaki
volcanic lineament may be a high-level expression of the
development of one of these segmenting fractures.
Stratigraphic summary
A stratigraphic summary is provided in Table 1 and a
geological map of the Egmont cone is presented in Fig. 2.
Numerous episodes of cone collapse during Egmont’s
history have resulted in the construction of an extensive
ring plain (Neall et al., 1986), which now contains the
only evidence of earlier events at the Egmont centre.
Most of the eruptive activity is therefore represented by
clasts in debris avalanche deposits, which form the ring
plain and range in age from ~120 ka to ~8 ka. Andesite
clasts within these deposits have been extensively sampled
and are regarded as representative of lavas that formed
earlier edifices constructed during previous cone-building
magmatic events. Although the ring plain deposits are
the products of single events, they homogenized preexisting lava flow sequences during cone collapse episodes
and the clast samples from these deposits can only be
crudely placed into a stratigraphic order. Consequently,
only two very broad stratigraphic units are used in the
discussion of the geochemistry of the ring plain deposits.
‘Old Ring Plain’ deposits are those which were emplaced
before 24 ka and ‘Young Ring Plain’ deposits were
emplaced between 24 and 8 ka.
Kahui Formation pyroclastic flows, which are radiocarbon dated at 8 ka  (Neall, 1979), form a base to
the present cone. Their emplacement was followed by
episodic construction of the present edifice by eruptions
of pyroclastic material and lavas. The Kahui Formation
separates lava flow sequences of the present cone from
laharic deposits of the ring plain. Among the post-Kahui
lava sequence, four groups of lavas of decreasing age
have been recognized on the basis of field relationships
169
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 1
JANUARY 1999
Table 1: Summary of stratigraphic groupings for Egmont Volcano lavas (Stewart et al.,
1996)
Stratigraphic group
Age (bp)
Summit
>0·7 ka
Flow morphology and composition
Relationships
Thin flows, domes and coulees
Includes time-equivalents of Newall Ash
Andesite to dacite
and lapilli (ad 1604), Burrell Lapilli (ad 1655)
and Tahurangi Ash (ad 1755)
Staircase
Fanthams Peak
0·7–2 ka
7–<1 ka
Thin flows
Possible correlative of Kaupokonui Tephra
Basaltic andesite to andesite
(1400 ka bp)
Complex parasitic cone of flows and scoria
Mangonui Tephra (3·3 ka bp) is a correlative
Basaltic andesite—some olivine bearing
Warwicks Castle
8–3 ka
Thick, ponded flows forming prominent bluffs
Includes ~8 ka olivine-bearing, low-silica
Basaltic andesite to andesite—abundant
andesite flows of Turehu Hill
amphibole
Young Egmont Ring Plain
Old Egmont Ring Plain
~30–15 ka
115–50 ka
Lahar and debris avalanche deposits
Base marked by Opunake Formation,
Dominantly andesite
top by Warea Formation
Older lahar and debris avalanche deposits
Deposits lie on 115 ka bp Inaha marine
Andesite, basaltic andesite, and basalt
bench; includes the Stratford lahars, ~50 ka bp
Pouakai volcano
250 ka
Dominantly andesite flows
Kaitake volcano
575 ka
Poorly exposed adesitic flows and dykes
Sugar Loaf Islands–Paritutu
1·7 Ma
Limited exposures of spines and domes
High-Si andesites
and palaeomagnetic data (Downey et al., 1994; Stewart
et al., 1996). In order of decreasing age of initial emplacement these stratigraphic units are: the Warwicks
Castle group, the Fanthams Peak group, the Staircase
sequence, and the Summit eruptives (Table 1).
METHODS
Sampling
Field samples (500–1000 g) were broken from flow outcrops; outside surfaces were removed and samples immediately double-bagged in press-seal plastic bags to
avoid contamination. A few samples were drilled from
fresh pavements with a coring drill. The ring plain
deposits were sampled by removing single large clasts
(generally >500 g) and trimming away outside surfaces
in the field before the samples were bagged. Overall
sampling is strongly biased towards fresh flows of the
young cone. Exposure in the Pouakai and Kaitake Ranges
is poor and many outcrops are weathered or altered.
Clast populations in laharic deposits generally represent
a broad cross-section of the volcanic edifice existing at a
particular time in the volcanic history and consequently
one might expect considerably more variability than is
observed in better constrained flow sequences of the
young cone.
Major and trace elements
All samples were crushed using a WC shatter box and
abundances of major and minor elements and selected
trace elements (Table 2) were determined at La Trobe
University by X-ray fluorescence (XRF) analysis. For
samples analysed by spark source mass spectrometry
(SSMS), a separate aliquot of sample was crushed in
agate. Major and minor elements (Si, Ti, Al, Fe, Mn,
Mg, Ca, Na, K, P, and S) were determined using methods
similar to those described by Norrish & Hutton (1969).
In general, precision for each major or minor element
is better than ±1% (1r) of the reported value. 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 on pressed powder pellets using methods similar
to those described by Norrish & Chappell (1977). For
most XRF trace elements, theoretical detection limits are
of the order of 1–2 ppm and reproducibility is better
than ±5% (1r).
For selected samples the rare earth elements (REE)
and other trace elements were analysed by inductively
coupled plasma mass spectrometry (ICPMS) at the VIEPS
Trace Element Laboratory at Monash University using
methods described by Price et al. (1997) or by SSMS
at the Australian National University using methods
described by Taylor (1965, 1971) and Taylor & Gorton
170
R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
Fig. 2. Geological map of young cone of Egmont Volcano (Mt
Taranaki) showing distribution of major stratigraphic units. Geographic
features mentioned in the text are also shown. ‘St’ is the Staircase and
‘Sh’ the Shark’s Tooth.
(1977). A few samples have been analysed by instrumental
neutron activation (INAA) at the Australian National
University using methods described by Chappell & Hergt
(1986). Precision for elements analysed by ICPMS,
SSMS, or INAA is typically better than 5% with accuracy,
based on analysis of BHVO-1, being, for most elements,
better than 5% at the 95% confidence level (Table 2).
Duplicate analyses of standard rocks and selected samples
by ICPMS, SSMS, and INAA indicate that the three
methods provide comparable data for the REE, Cs, Hf,
and Th. Analyses of the same sample by the different
techniques generally agree within 1–5% although for
one or two elements in one or two repeated samples,
differences of up to 10–12% were observed at low abundance levels. For Pb and U analyses the agreement between
ICPMS and SSMS data is generally in the range 1–5%.
Lead, strontium, and neodymium isotopes
Powders were leached in hot 6 M HCl for 30 min,
followed by sequential open beaker dissolution in HF–
HNO3 and HCl. Lead was purified using conventional
HBr–HCl column chemistry. Unless a separate dissolution was made for Sr–Nd, the HBr eluate from the
Pb column was collected, fumed with nitric acid and
taken up in 1 M HCl for further processing on standard
cation columns. Removal of Ba at this stage results in
significant improvement in ion currents and running
characteristics during Nd runs. Neodymium was further
purified using reverse phase ion exchange chromatography on HDEHP-coated Kel-F columns.
Lead was loaded onto single Re filaments using silica
gel–H3PO4. Procedural blanks vary between 30 and 50 pg
and are negligible relative to the sample sizes used.
Samples were run on a seven-collector Finnigan-MAT
262 in static mode at filament temperatures of
1250–1350°C, at 208Pb ion currents of (1–4) × 10–11
A. Typically, three blocks of 10 × 8 s scans were collected,
with in-run 2 SE of Ζ0·05%. Mass fractionation is
estimated to be 0·109% per mass unit, based on the
SRM981 Pb standard; appropriate correction factors
were applied to the data. Reproducibility for SRM981
(n = 78, 2r) is ±0·097% for 206Pb/204Pb, ±0·130% for
207
Pb/204Pb, and ±0·175% for 208Pb/204Pb. These error
ranges are consistent with repeat analyses for several
young volcanic rocks. Mass fractionation was checked
on selected samples using a 204Pb–207Pb double spike
(Woodhead et al., 1995). Five runs gave an average
fractionation factor of 0·091% (2r = ±0·034%), close
to our empirical value of 0·109%.
Strontium samples were loaded in H3PO4 onto single
Ta filaments. Mass fractionation was corrected by normalizing to 86Sr/88Sr = 0·1194. Typically, 5–7 blocks of
10 × 8 s integrations produced in-run precision (2r) of
±0·003%. 87Sr/86Sr (±2r) for SRM987 (n = 100) is
0·71023 ± 7 (0·01%), for BCR-1 (n = 6) 0·70500 ±
4, and for BHVO-1 (n = 19) 0·70348 ± 6.
Neodymium was loaded in H3PO4-doped 1 M HNO3
onto the Ta side of a Ta–Re double filament assembly.
Mass fractionation was corrected by normalizing to
146
Nd/144Nd = 0·7219. Typically, 5–7 blocks of 10 × 8 s
integrations produced in-run precisions (2r) of
±0·0025%. 143Nd/144Nd (±2r) for La Jolla (n = 100)
is 0·511860 ± 16, for BCR-1 (n = 7) 0·512634 ± 18,
and for BHVO-1 (n = 5) 0·512989 ± 13. Present-day
CHUR was taken as 0·512631.
PETROGRAPHY AND MINERAL
CHEMISTRY
Petrography
Taranaki lavas range from vesicular red and black scoria
through to non-vesicular to holocrystalline, porphyritic
lavas. Inclusions are abundant in many andesite flows.
Many of these are fragments of older andesitic eruptive
171
Table 2: Whole-rock major and trace element data for Taranaki eruptives
4
T89/19
A
Su
5
T89/16
A
Su
6
T90/2B
A
Su
7
T89/18Bb
A
Su
8
T90/28
A
St
9
T89/24
A
St
10
T90/21
A
St
11
T89/11
B
Fa
12
T89/14
B
Fa
13
T89/9
B
Fa
14
T89/8
BA
Fa
15
T90/27
A
Wa
16
T90/10
A
Wa
17
T90/4D
A
Wa
18
T90/4A
A
Wa
19
T89/22
A
Wa
20
T89/21
A
Wa
21
BR-6
A
Wa
SSMS
SSMS
ICPMS
ICPMS
SSMS
ICPMS
ICPMS
ICPMS
INAA
ICPMS
SSMS
INAA
SSMS
SSMS
ICPMS
ICPMS
INAA
INAA
INAA
SSMS
ICPMS
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
H2O+
H2O–
CO2
S
O=S
Total
53·92
0·98
17·44
4·66
3·22
0·17
3·75
8·36
3·90
2·42
0·34
0·44
0·38
0·05
0·05
0·025
100·06
54·57
0·98
17·78
4·13
3·65
0·17
3·55
8·19
3·69
2·50
0·39
0·20
0·29
0·04
0·01
0·005
100·14
54·59
0·94
17·46
4·68
3·17
0·16
3·87
8·39
3·41
2·51
0·38
0·25
0·11
0·07
56·89
0·78
17·78
3·58
3·10
0·15
3·07
7·30
3·96
2·83
0·29
0·25
0·21
0·01
0·04
0·020
100·22
54·77
0·85
17·36
4·41
3·26
0·16
3·68
8·39
3·68
2·23
0·28
0·40
0·33
0·13
49·54
1·17
16·39
4·82
5·51
0·17
6·40
11·09
2·99
1·53
0·26
0·23
0·21
49·93
1·14
17·68
6·18
4·07
0·19
5·07
10·71
3·20
1·56
0·28
0·19
0·20
52·14
1·01
18·37
4·38
4·40
0·19
3·67
9·46
3·40
1·84
0·31
0·28
0·35
53·17
1·14
17·68
4·13
4·53
0·17
4·06
8·68
3·95
2·18
0·33
0·13
0·05
0·01
53·45
0·97
18·28
3·22
4·93
0·15
3·76
9·08
3·25
2·07
0·34
0·25
0·12
0·09
53·89
0·95
17·40
3·36
4·67
0·16
5·15
8·99
3·27
1·72
0·29
0·08
0·06
0·03
54·22
0·96
17·63
3·08
4·87
0·15
4·81
8·91
3·35
1·77
0·29
0·08
0·03
0·01
55·51
0·78
18·37
3·52
3·87
0·18
3·01
8·28
3·65
1·99
0·35
0·28
0·03
56·00
0·84
18·00
3·31
3·81
0·16
3·15
8·33
3·58
2·21
0·31
0·22
0·12
0·06
99·99
55·20
0·76
17·27
3·44
3·25
0·16
3·65
7·76
3·71
2·66
0·30
0·62
0·34
0·88
0·03
0·015
100·02
50·33
13·05
52·44
14·61
54·74
16·53
4·0
981
60
719
12·1
7·6
5·0
20·4
43·4
5·2
23·5
5·2
1·36
4·46
0·75
4·19
0·84
2·48
3·4
914
67
724
12·3
6·6
1·7
17·7
36·1
4·4
22·0
5·1
1·36
4·26
0·70
3·99
0·77
1·96
0·31
2·10
0·32
22·5
129
3·6
0·42
5·0
17
209
20
8
109
68
20
6·06
4·1
977
72
595
20·6
6·3
1·8
18·7
36·8
4·7
18·8
4·1
1·35
3·92
0·58
3·34
0·70
1·89
0·29
1·97
0·30
20·3
129
3·6
0·58
5·1
17
171
33
9
96
69
19
6·81
mg-no.
51·54
(Hy+Q)-Ne 7·44
172
Trace elements (ppm)
Cs
3·8
Ba
946
Rb
60
Sr
696
13·0
Pb
Th
8·4
U
3·7
La
20·3
Ce
46·6
Pr
5·5
Nd
24·1
Sm
4·8
Eu
1·37
Gd
4·09
Tb
0·65
Dy
4·11
Ho
0·75
Er
2·02
Tm
Yb
2·06
Lu
Y
23·4
Zr
109
3·6
Hf
Ta
Nb
4·6
Sc
18
V
229
Cr
19
Ni
6
Cu
124
Zn
84
Ga
23
7·07
La/Ybn
2·34
25·6
114
4·2
4·6
17
210
11
4
118
80
23
6·25
99·96
58·27
0·72
17·35
2·87
3·01
0·15
2·70
6·72
3·87
3·09
0·29
0·77
0·25
0·13
0·03
0·015
100·21
50·52
13·21
50·04
15·48
5·8
1026
79
627
11·9
9·8
4·0
22·3
42·7
4·9
21·7
4·4
1·23
3·98
0·67
3·62
0·71
2·06
4·0
1137
81
661
19·0
5·3
1·3
20·8
40·9
5·1
21·3
4·7
1·56
4·29
0·60
3·45
0·71
1·84
0·30
1·86
0·28
20·5
142
2·8
0·44
5·5
10
148
13
6
132
61
21
8·03
2·23
25·1
132
4·3
5·0
14
145
12
7
81
71
21
7·17
57·60
0·76
17·91
3·26
2·84
0·14
2·75
7·03
3·69
3·01
0·32
0·39
0·21
0·05
54·33
0·92
17·57
3·62
4·30
0·16
3·72
8·27
3·86
2·25
0·30
0·33
0·27
0·12
0·000
100·02
54·70
0·94
17·48
3·94
4·10
0·17
3·87
8·49
3·90
2·22
0·29
0·14
0·03
0·06
0·02
0·010
100·34
0·000
99·93
0·02
0·010
100·32
50·38
15·31
50·86
12·53
51·56
11·14
51·70
13·22
57·75
-1·78
5·0
1133
85
585
23·1
7·8
2·3
20·5
39·2
4·8
19·1
4·2
1·48
3·91
0·55
3·29
0·69
1·87
0·31
1·82
0·30
20·2
149
4·3
0·39
5·6
13
198
38
10
87
74
19
8·08
3·3
843
54
618
15·3
4·9
1·5
15·7
32·8
4·3
18·0
4·3
1·48
4·29
0·62
3·67
0·76
1·97
0·33
2·04
0·32
22·2
106
3·3
0·49
4·5
18
229
16
7
147
79
20
5·52
3·2
821
54
598
13·6
5·9
1·5
15·3
31·2
3·8
18·4
4·3
1·18
5·54
0·61
3·62
0·71
1·87
0·29
2·03
0·31
20·8
93
3·1
0·57
4·6
18
212
32
10
96
76
19
5·41
1·3
665
34
552
6·6
4·2
6·1
11·6
24·7
3·1
14·6
4·2
1·21
3·95
0·64
3·97
0·75
2·18
1·7
958
54
635
16
5
1·4
15·6
34
19
4·1
1·26
0·72
0·7
2·1
0·31
22
103
2·6
1
18
207
9
6
161
73
22
5·33
2·05
0·03
0·015
100·42
51·49
1·05
18·31
4·56
4·63
0·19
4·14
10·11
3·46
1·70
0·31
0·17
0·10
0·09
0·03
0·015
100·33
0·02
0·010
99·81
0·000
100·21
0·000
99·96
0·000
100·02
52·54
-1·21
49·92
2·42
48·06
9·34
50·87
2·36
50·25
14·84
58·46
17·50
2·4
766
34
660
5·9
4·2
0·8
14·6
30·7
4·2
19·7
4·5
1·30
4·10
0·72
4·19
0·82
2·07
1·8
797
40
656
7·6
5·0
1·0
12·5
27·2
3·7
16·9
4·0
1·31
3·93
0·59
4·00
0·75
2·06
1·7
879
51
663
13·2
4·6
1·4
17·8
37·8
5·0
21·4
5·1
1·71
5·20
0·74
4·29
0·89
2·35
0·38
2·25
0·35
25·8
112
3·3
0·40
5·1
18
235
10
7
169
69
22
5·67
1·0
889
48
664
12·0
4·5
1·4
15·3
31·7
4·2
17·9
4·4
1·48
4·39
0·66
3·67
0·80
2·00
0·34
2·04
0·31
22·6
97
3·1
0·35
4·2
15
237
13
6
91
70
21
5·38
1·6
724
32
619
5
2·8
0·8
10·8
24·5
15·6
3·9
1·25
0·68
0·75
22·9
73
3·0
1·95
0·30
23
65
1·8
22·5
72
2·8
2·20
23·2
82
2·9
2·17
2·8
30
308
92
25
197
78
21
4·06
2
25
277
26
10
144
82
20
3·97
3·3
19
245
15
6
127
84
22
4·76
2·6
17
220
9
3
68
88
23
4·13
2·0
668
41
629
14
3·7
1·1
12·8
29·0
0·000
100·16
55·07
0·91
18·53
3·40
4·00
0·17
2·97
8·45
4·15
2·07
0·33
0·21
0·10
0·02
0·02
0·010
100·39
0·04
0·020
99·84
0·000
100·10
56·96
16·90
46·94
10·76
47·35
15·57
49·39
15·20
1·8
856
53
725
7·5
6·5
2·3
22·2
47·2
5·5
25·5
5·1
1·45
4·48
0·71
4·65
0·89
2·62
2·0
803
58
635
24·3
5·0
1·4
16·8
35·4
4·8
19·7
4·5
1·41
4·43
0·64
3·83
0·81
2·18
0·32
2·26
0·35
23·5
121
3·5
0·56
4·9
14
173
16
8
85
74
21
5·33
0·9
705
43
648
11
3·8
1·1
13·0
29·0
1·1
789
52
733
15
4·6
1·4
16·0
35·5
16·6
3·9
1·23
16·8
3·8
1·21
20·5
4·1
1·28
0·67
0·66
0·68
0·80
0·70
0·80
1·90
0·29
19
91
2·3
1·90
0·28
20
92
2·3
2·00
0·29
22
115
2·8
31·8
118
4·5
7
16
187
128
50
89
72
21
4·83
7
15
190
100
41
98
69
21
4·91
3
12
163
14
5
65
73
23
5·74
6·4
12
131
9
5
87
75
23
5·90
2·7
JANUARY 1999
3
T90/13
A
Su
NUMBER 1
2
T89/15
A
Su
VOLUME 40
1
T89/10
A
Su
JOURNAL OF PETROLOGY
Sample:
Rock type
Strat.
group:
Method:
0·000
99·96
0·000
100·01
173
2·8
827
50
616
16·8
5·0
1·5
17·3
36·2
4·7
19·5
4·5
1·61
4·57
0·68
3·88
0·85
2·21
0·35
2·27
0·35
23·7
109
3·2
0·66
5·2
14
180
4
5
60
77
20
5·47
6·3
13
176
11
7
43
80
20
4·49
25·4
115
3·2
2·97
2·6
824
51
605
12·8
4·2
1·1
18·6
43·2
5·4
21·1
5·8
1·72
5·59
0·82
4·42
0·93
2·68
47·88
14·71
0·000
99·99
55·51
0·89
18·24
3·31
3·98
0·18
3·04
8·26
3·78
2·02
0·32
0·22
0·15
0·09
SSMS
24
T90/46
A
YP
1·1
805
50
620
8·2
5·0
1·2
18·3
38·0
4·5
21·9
4·9
1·26
4·07
0·68
4·00
0·79
2·10
0·34
2·30
0·37
24·1
130
3·1
0·53
5·2
12
189
7
6
81
75
20
5·71
46·35
14·97
56·48
0·79
18·16
3·53
3·25
0·16
2·64
7·81
4·00
2·03
0·32
0·43
0·27
0·13
0·01
0·005
100·01
ICPMS
25
T90/32C
A
YP
15·1
29
293
113
119
166
80
19
6·26
21·3
281
2·0
1·49
0·2
409
18
576
2·7
1·6
0·3
13·0
28·3
3·5
16·6
4·1
1·40
4·01
0·63
3·38
0·66
1·60
58·30
2·42
0·000
100·08
48·92
1·58
16·21
4·76
5·87
0·18
6·75
10·57
2·87
1·20
0·35
0·44
0·32
0·06
SSMS
26
T90/42A
B
OP
2·0
543
47
453
11·3
4·2
1·2
16·0
33·2
4·3
17·6
4·0
1·35
4·20
0·65
3·77
0·81
2·16
0·35
2·20
0·34
24·0
104
3·0
0·56
5·6
17
194
43
17
101
77
20
5·22
52·09
21·88
0·000
100·04
55·70
0·82
18·14
3·28
4·36
0·18
3·78
8·14
3·19
1·58
0·25
0·34
0·23
0·05
ICPMS
27
T90/42C
A
OP
1·7
1096
52
628
10·6
4·8
1·5
19·1
36·7
4·9
20·3
4·5
1·49
4·17
0·59
3·47
0·73
1·88
0·29
1·95
0·30
20·9
102
3·2
0·57
4·3
17
203
2
4
83
72
24
7·03
51·51
14·44
54·92
0·83
18·38
4·20
2·97
0·16
3·41
7·69
3·93
2·16
0·31
0·60
0·40
0·04
0·05
0·025
100·03
ICPMS
28
T89/33A
A
Po
1·5
986
58
654
11·2
5·5
1·6
22·5
39·6
5·1
19·5
4·0
1·33
3·58
0·52
2·88
0·60
1·67
0·27
1·71
0·27
18·3
109
3·2
0·56
4·8
11
159
2
4
299
65
23
9·45
50·94
19·14
57·31
0·65
18·12
3·69
2·43
0·15
2·84
6·65
3·94
2·16
0·30
1·11
0·54
0·12
0·03
0·015
100·03
ICPMS
29
T89/36
A
Po
0·9
616
50
462
11·0
4·4
1·4
19·3
35·4
4·9
20·3
4·4
1·42
4·72
0·72
4·16
0·93
2·42
0·40
2·46
0·39
28·3
118
3·3
0·54
5·5
13
169
5
7
135
82
20
5·62
47·70
20·68
0·000
100·15
57·60
0·78
18·36
2·92
3·92
0·17
2·84
7·75
3·47
1·78
0·24
0·16
0·11
0·05
ICPMS
30
T90/41
A
Po
0·5
914
14
455
6·3
4·6
1·1
11·1
23·9
2·7
10·9
2·5
1·06
2·53
0·41
2·42
0·53
1·46
0·22
1·62
0·26
15·4
69
2·1
0·47
3·2
18
168
22
10
37
56
23
4·91
56·86
24·56
53·98
0·63
19·76
3·84
2·67
0·16
3·84
6·71
3·52
1·09
0·19
2·53
1·36
0·01
0·01
0·005
100·30
ICPMS
31
T89/6A
A
K
1·1
2043
60
864
4·5
16·2
3·7
45·3
79·1
9·0
31·6
5·3
2·02
4·57
0·61
3·20
0·63
1·67
0·25
1·73
0·27
20·0
132
2·5
0·99
7·7
9
109
2
3
7
40
24
18·76
41·03
19·84
59·25
0·57
17·98
3·80
1·90
0·29
1·76
6·51
4·32
1·80
0·31
0·77
0·35
0·28
0·03
0·015
99·91
ICPMS
32
T89/3
A
Pa
2·9
326
47
252
10·6
4·0
1·1
12·0
25·8
3·5
14·1
3·2
0·95
3·44
0·57
3·37
0·71
2·00
0·31
2·10
0·33
21·0
105
3·4
0·80
4·3
27
186
101
36
48
68
16
4·10
55·69
27·15
58·06
0·69
16·64
3·07
4·95
0·12
4·61
7·20
3·26
1·38
0·11
ICPMS
33
Average
Ruapehu
Andesite
0·13
131
9
402
2·6
1·1
0·4
15·8
38·9
5·6
25·2
6·2
2·06
6·37
0·96
5·17
0·99
2·49
0·33
2·01
0·29
28·3
185
4·4
1·23
19·1
30
320
287
110
163
104
20
50·08
2·78
13·69
2·87
8·49
0·18
7·37
11·48
2·31
0·52
0·30
ICPMS
34
BHVO1
Data in italics were obtained by inductively coupled plasma mass spectrometry (ICPMS), instrumental neutron activation analysis (INAA) and spark source mass
spectrometry (SSMS) (see ‘Method’ row). All other trace element data are from X-ray fluorescence analysis. Stratigraphic groups: Su, Summit; St, Staircase; Fa,
Fanthams; Wa, Warwicks Castle; YP, Young Ring Plain; OP, Old Ring Plain; Po, Pouakai Ranges; K, Kaitake Ranges; Pa, Paritutu. Rock types: B, basalt; BA, basaltic
andesite; A, andesite. Data for an average Ruapehu andesite and for standard rock BHVO-1 are also shown. (Hy+Q)-Ne, normative hypersthene + quartznepheline. mg-numbers calculated assuming Fe2+/Fe3+ = 0·2.
Trace elements (ppm)
Cs
0·8
Ba
756
Rb
39
Sr
571
9·0
Pb
Th
4·5
U
1·1
La
13·7
Ce
28·4
Pr
3·5
Nd
17·9
Sm
4·5
Eu
1·22
Gd
3·87
Tb
6·73
Dy
3·99
Ho
0·80
Er
2·04
Tm
0·31
Yb
2·10
Lu
0·32
Y
22·3
Zr
96
2·7
Hf
Ta
0·48
Nb
4·7
Sc
22
V
304
Cr
21
Ni
10
Cu
216
Zn
84
Ga
21
4·68
La/Ybn
47·50
16·38
55·49
0·90
18·28
3·36
3·88
0·17
2·97
8·02
3·63
2·03
0·30
0·50
0·32
0·11
ICPMS
ICPMS
51·44
1·14
18·01
3·97
5·14
0·17
4·51
9·93
3·10
1·89
0·30
0·20
0·13
0·08
23
T90/45A
A
YP
22
T90/32A
B
YP
mg-no.
52·12
(Hy+Q)-Ne 7·36
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
H2O+
H2O–
CO2
S
O=S
Total
Sample:
Rock type:
Strat.
group:
Method:
Table 2: continued
R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 1
JANUARY 1999
Table 3: Summary of mineralogy and mineral chemistry of Taranaki basaltic andesites and andesites (see
Stewart et al., 1996)
Mineral
Olivine
Orthopyroxene
Composition
Comments
core: Fo82–87
Relatively uncommon (<5% of rock)
rim: Fo71–77
Appears to be xenocrystic
cpx-mantled: En74–79
Relatively less common than clinopyroxene
groundmass: En60–66
Clinopyroxene
mg-no. 0·72–0·82
Most common phenocryst phase after plagioclase (5–25% of phenocrysts)
Amphibole
mg-no. 0·72–0·92
Commonly partially or completely resorbed
Also in groundmass
AlIV 1·7–2
Biotite
Plagioclase
Titanomagnetite
Only in high-Si andesites
glomerocrysts: An91–74
Most common phase as phenocryst (15–45%) or groundmass
cores: An89–55
Phenocrysts are complexly zoned, resorbed, sieve-textured
K-rich rims An6Or27
Many reverse zoned phenocrysts
glomerocryst: Usp 0·204
Common in groundmass and as microphenocrysts (2–10% of rock)
groundmass: Usp 0·403
glomerocryst: f O = 2·5–3·0 DFMQ; T = 1150°C
2
groundmass: f O2 = 0·8–1·2 DFMQ; T = 860–965°C
Ilmenite
groundmass: Hem 0.269
Relatively rare
Chromite
cr-no. 0·80–0·70
As inclusions in olivine xenocrysts
mg-no. 0·23–0·29
fe3+-no. 0·24–0·31
material entrained in the host flow during eruption.
However, inclusions of diorite and gabbro (inclusions up
to 1 m in diameter), hornblendite, and Tertiary sediments
are also common. Meta-amphibolite and schist xenoliths
are relatively uncommon and inclusions of troctolite,
dunite and pyroxenite are rare. Inclusions are particularly
common and the inclusion population is most varied in
andesite clasts of the laharic deposits of the ring plain.
The ~23 kyr old Pungarehu Formation, which comprises
extensive laharic, fluvial and debris avalanche deposits
across the western ring plain of the volcano (Ui et al.,
1986), is a major source for a wide variety of xenolith
types.
In thin section, Egmont lavas are seen to range from
holocrystalline to hypocrystalline. Some are seriate textured but most are porphyritic. Phenocryst proportions
range from 25 to 55% [Table 3; see also Neall et al.
(1986)]. Glomerocrysts are common and comprise clinopyroxene ± titanomagnetite ± plagioclase ± olivine
with rare orthopyroxene and amphibole. Occasional very
large (>10 cm) hornblende crystals are found. All lavas
examined contained ‘phenocrysts’ of (in order of abundance) plagioclase, clinopyroxene, titanomagnetite and
hornblende. Olivine is present in small amounts in most
lavas whereas orthopyroxene is rare, consistent with the
high aH2O and low aSiO2 inferred for these high-K rocks
(Stewart et al., 1996). Groundmass assemblages include
glass, plagioclase, titanomagnetite and clinopyroxene,
with rare olivine, orthopyroxene, amphibole and ilmenite.
Apatite and zircon are common accessories. Mineralogy
and mineral chemistry are summarized in Table 3.
MAJOR AND TRACE ELEMENT
CHEMISTRY
Our database of chemical analyses from the Taranaki
volcanoes comprises 141 major and trace element analyses representing all phases of eruptive activity in the
region. The representation is, however, uneven. Early
periods of activity are only poorly represented by samples
from the Sugar Loaf centre and the Kaitake and Pouakai
Ranges because outcrop in these areas is limited and
alteration common. Samples of early Egmont eruptives
are clasts from debris avalanche deposits, and age relationships are only generally known. In contrast, samples
of the present lava cone (<8 ka ) provide a detailed
picture of younger magmatic evolution. The chemical
analyses reported here are mostly from lava flow sequences and consequently pyroclastics and tephras, which
generally have higher SiO2 contents, are poorly represented. Only from the most recent phase of activity
(the Summit sequence) have we analysed samples of
174
R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
Major elements
Fig. 3. K2O vs SiO2 variation diagrams for Taranaki eruptives and
comparisons with Ruapehu andesites and dacites. (a) Details of variation
among Taranaki eruptives. (b) Comparison with Ruapehu and classification of Gill (1981).
pyroclastic flow deposits and these are the samples showing the highest SiO2 abundances.
Analyses of representative whole-rock samples from
Egmont Volcano are presented in Table 2 along with
an average andesite composition for Ruapehu volcano
in the TVC. For all the variation diagrams, major element
data have been recalculated on an anhydrous basis and
assuming an Fe2O3/FeO ratio of 0·2. Egmont eruptives
range in composition from medium-K and high-K basalts
and basaltic andesites through to high-K, high-Si andesites (Fig. 3). Using Gill’s (1981) definitions, the majority
of samples are high-K, low-Si andesites, and young
Taranaki eruptives are generally significantly more potassic than their equivalents in the TVC [Fig. 3; see also
Price et al. (1992)]. Consequently, they tend to be relatively
more alkalic; basalts and basaltic andesites tend to be
weakly hypersthene or even nepheline normative
(Table 2).
Major element variation is illustrated in Fig. 4 using
silica variation diagrams. TiO2, FeOtotal, MgO, and CaO
abundances decrease systematically and linearly with
increasing SiO2 abundance, whereas Na2O and K2O
contents increase (Figs 3 and 4). Al2O3 variation is complex; the stratigraphic groups are to some extent distinguished in the Al2O3 vs SiO2 diagram. Pouakai and
Kaitake data are moderately scattered, with one, probably
altered, sample having a very high Al2O3 content (>20%)
and other samples having Al2O3 contents similar to those
observed in young Egmont cone lavas. Old Ring Plain
material forms a scattered linear array within which SiO2
and Al2O3 abundances are correlated. Some of the Young
Ring Plain samples have compositions overlapping with
data for Fanthams Peak samples, which cluster along a
well-defined linear array at higher Al2O3 abundance for
a given SiO2 value. The lavas of the Warwicks Castle
group are distributed into two distinct sub-groupings;
one shows relatively high and constant Al2O3 abundances
and the other, which includes the Turehu Hill samples,
has lower Al2O3 and SiO2 contents. Data for some of
the Young Ring Plain eruptives overlap in the Al2O3 vs
SiO2 diagram with the higher Al2O3 Warwicks Castle
group. Staircase group samples form a tight cluster of
data points with relatively low Al2O3 abundances, and
the Summit group defines a scattered linear distribution
with relatively low and constant Al2O3 abundances.
Some of these distinctions are also evident in other
variation diagrams. Fanthams Peak data form a distinctive linear array in the MgO vs SiO2 diagram and
some of the Young Ring Plain data overlap this field. In
the same diagram, the two Warwicks Castle groups are
separated. The Old Ring Plain data form a crude linearly
distributed field at relatively higher MgO for a given
SiO2 value. The Pouakai and Kaitake data overlap with
the field of Egmont cone and Young Ring Plain data.
K2O vs SiO2
The data presented in Fig. 3 confirm earlier observations
(Dickinson & Hatherton, 1967; Neall et al., 1986; Price
et al., 1992) that young Egmont lavas are distinctly more
potassic than equivalent andesitic eruptives in the TVC,
but our new, more comprehensive data set for Egmont
allows a closer examination of this feature. In detail
(Fig. 3b), K2O abundance varies with time in Egmont
Volcano and within the Taranaki volcanoes in general
(Fig. 5). The highest K2O contents are observed in
the Summit group and progressively older stratigraphic
groupings show a systematic shift to lower K2O contents
at a given SiO2 value. In the K2O vs SiO2 diagram,
Summit, Fanthams, Staircase, and Warwicks Castle
groups each define separate linear arrays showing positive
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Fig. 4. Major element variation as a function of SiO2 abundance for Taranaki eruptives.
correlation between K2O and SiO2 abundances. Within
the Summit sequence the most potassic and siliceous
eruptives are also the youngest.
Some of the Old Ring Plain samples are distinctly less
potassic than other Egmont samples and have K2O contents that are very similar to those of some Ruapehu andesites with similar SiO2 contents. Samples from the Pouakai
and Kaitake Ranges have K2O contents scattering between the elevated levels seen in Warwicks Castle group
samples and the Ruapehu medium-K values.
Trace elements
Trace element abundances for representative Taranaki
samples are listed in Table 2 and variation for some
elements is illustrated in Fig. 6 using SiO2 variation
diagrams. Rubidium, Ba, and Zr abundances increase
systematically with increasing SiO2 content and the variation in these elements is similar to that shown by K2O
(Fig. 3). Plots of K content vs abundance of Rb and Ba
(Fig. 7) further illustrate this point, with abundances of
these elements being strongly correlated with K abundance. Zr abundances are elevated in some of the Old
Ring Plain samples, but for most data there is a correlation
between K and Zr contents.
Strontium abundances show much less coherent variation than is observed for K, Ba, Rb, and Zr. The Sr vs
SiO2 variation diagram shows some of the distinctions
between different groups that have been recognized in the
Al2O3–SiO2 variation diagram. Fanthams Peak samples
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R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
with only five samples having Ni contents >12 ppm.
Among the samples from the Old Ring Plain sequence
four have Ni contents >20 ppm, ranging up to 49 ppm.
Most samples of the cone flows have Cr contents <25
ppm and Cr variation is similar to that observed for Ni.
Normalized trace element plots (Fig. 8) are characterized by features considered to be distinctive of
subduction-related magmas from elsewhere (e.g. Pearce,
1982; McCulloch & Gamble, 1991). Relative to normal
mid-ocean ridge basalts (N-MORB), Taranaki lavas
show: enrichment in strongly incompatible large ion
lithophile elements (LILE) such as Rb, Ba, and K; strong
depletion in Nb relative to K and Th; and enrichment
in Pb over Ce. The light rare earth elements (LREE; La
and Ce) are enriched [(La/Yb)n = 7·0–16·1] over heavy
rare earth elements (HREE) and Y, which show abundances similar to those for N-MORB. One basaltic clast
from the Old Ring Plain sequence (T90/42A) has a
normalized pattern showing a significantly more subdued
arc signature.
Comparisons with Ruapehu (TVC andesite)
trace element behaviour
Fig. 5. Temporal variations in K abundance (a) and 87Sr/86Sr isotopic
ratios (b) for Taranaki eruptives. In (a), the K content at 55% SiO2 is
indicated for each group. Su, Summit group; St, Staircase group; Fa,
Fanthams Peak; Wa, Warwicks Castle group; YP, Young Ring Plain;
OP, Old Ring Plain. Po, Pouakai data; K, Kaitake data. Boxes show
total ranges of age, isotopic ratio and K for each group.
form a coherent group with Sr concentration showing a
positive linear correlation with SiO2 content, in contrast
to the Summit group, within which there appears to be
a slight decrease in Sr abundance with increasing SiO2
content. The Warwicks Castle group is distributed into
two overlapping sub-groups (high and low Sr) and, within
each, Sr and SiO2 abundances are not correlated.
Samples from the Old Ring Plain sequence show a spread
in Sr abundances to lower values and this is also the case
for samples from the Pouakai and Kaitake centres.
Abundance variation for Ni, Cr, V, and Sc is similar
to that shown by MgO and FeO; abundances decrease
systematically as SiO2 contents rise (Table 2). This is
illustrated in Fig. 6 using V and Sc as examples of this
type of behaviour. There is a suggestion that some of
the structure identified in the Al2O3 vs SiO2 diagram (see
above) can be distinguished in the Sc vs SiO2 diagram;
the Fanthams Peak and Summit groups can be distinguished, as can two sub-groups in the Warwicks Castle
group. Vanadium abundances are negatively correlated
with SiO2 contents.
Abundances of Ni and Cr are generally low (Table 2).
Nickel abundance is <10 ppm for most samples from
the cone sequence (Warwicks Castle to Summit groups),
Data for andesites from Ruapehu Volcano in the TVC
are compared with Taranaki data in Figs 6, 7, 8 and 9.
As is the case for Egmont data, Ruapehu analyses define
strong positive correlations between K content and Ba,
Rb and Zr abundances (Fig. 7), but the two volcanoes
show distinctly different trends in the diagrams; Ruapehu
eruptives have mean K/Ba = 33·2, K/Rb = 286, and
K/Zr = 110·6 whereas Egmont lavas (excluding Kaitake
and Pouakai data) have mean ratios of 21·4, 347, and
162·9, respectively. A few Pouakai and Kaitake samples
tend to have Ba contents above those observed in other
Taranaki eruptives and this may indicate that the samples
in question are altered.
In the Sr vs K diagram (Fig. 7), data for the two
volcanoes define two very different fields. Egmont eruptives have higher Sr contents with Old Ring Plain data
scattering down to lower Sr values approaching those
of Ruapehu lavas. Ruapehu eruptives have relatively
constant Sr contents across a range in K abundance.
The Pouakai and Kaitake centres show Sr contents
similar to the range observed in the Old Ring Plain
eruptives and distinctly higher than those observed at
Ruapehu.
The comparison between Taranaki and Ruapehu trace
element patterns has been examined further using normalized trace element plots in which an average Ruapehu
andesite has been used as the normalizing composition
(Fig. 9). Ruapehu eruptives show the same distinctive arc
signature as Egmont eruptives, but when Taranaki data
are normalized to a Ruapehu average, distinct differences
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Fig. 6. Trace element variation as a function of SiO2 abundance for Taranaki eruptives. Ruapehu andesites and dacites (shown as diagonal
crosses) are plotted for comparison in the Ni and Cr diagrams (g and h) to illustrate the relatively more fractionated nature of Taranaki andesites
and mixed or hybrid character of Ruapehu lavas.
178
R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
Fig. 7. Trace element variation as a function of K abundance for Taranaki eruptives and comparisons with variation for Ruapehu andesites
and dacites. Ruapehu data from Graham & Hackett (1987) and R. C. Price (unpublished data, 1997).
emerge. Younger Egmont eruptives (from the young cone
and Young Ring Plain) have Rb, Th, U, Nb, Zr, Ti,
and Y abundances that are similar to those of the average
Ruapehu andesite, but Ba, K, Sr, and P abundances are
distinctly higher in the Egmont eruptives. The LREE
show higher abundances in the younger Egmont eruptives
but the middle rare earth element (MREE) and HREE
abundances are comparable. Younger Egmont eruptives
show significantly higher K/Nb (or La/Nb) and K/Ta
ratios, and generally have higher Nb/Ta and Ce/Pb
ratios.
Among the Old Ring Plain samples the trace element
patterns are variable. A basaltic clast from the Inaha
lahars (T90/42A) shows a pattern that is distinctly different from others. Relative to the Ruapehu reference
average, it is depleted in Rb, Th, U and Pb, but enriched
in Nb and Ti. Like other Egmont samples it is enriched
in Sr and P relative to the average Ruapehu andesite.
The other Old Ring Plain samples show patterns similar
to those of other Egmont samples.
Samples from the Kaitake and Pouakai centres have
normalized trace element characteristics similar to those
observed in other Taranaki eruptives (Fig. 9f ), but the
pattern for a single sample from Paritutu is different,
with abundances of LILE and high field strength elements
(HFSE) being significantly higher than those observed
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Fig. 8. MORB-normalized plots of trace element data for Taranaki eruptives. Data for (a) representative samples from the young cone of
Egmont Volcano, (b) the ring plain, and (c) the older Taranaki centres. (d) An average abundance pattern for Ruapehu andesites. Normalizing
values are from Sun & McDonough (1989).
in other Taranaki samples. Relative to the Ruapehu
reference, the Paritutu sample does, however, show the
same relative depletions (Nb and Pb) and enrichments
(Sr and P) as are observed in other Taranaki samples.
The higher LILE abundances in the Paritutu sample
relative to other Taranaki samples could indicate that
the former is more fractionated; this is consistent with
the fact that this sample has the highest SiO2 content
(60·34%) of any Taranaki sample.
ISOTOPE GEOCHEMISTRY
Lead, Sr, and Nd isotopic data for Taranaki eruptives
are presented in Table 4. Given the young age of all
rocks, the measured isotopic ratios approximate closely
to the initial ratios at the time of emplacement; age
corrections are negligible.
Lead isotopic composition
In 207Pb/206Pb vs 206Pb/204Pb and 208Pb/204Pb vs 206Pb/
204
Pb diagrams (Fig. 10), the Pb isotopic data for Egmont
samples define a linear array above and parallel to the
Northern Hemisphere Reference Line (NHRL) of Hart
(1984). Egmont samples have Pb isotopic compositions
that overlap with those of TVZ basalts and andesites,
but the Egmont Pb isotopic ratios cover a more restricted
range.
Strontium and neodymium isotopic
composition
In a 143Nd/144Nd vs 87Sr/86Sr isotopic ratio plot (Fig. 11),
data for Egmont Volcano define a roughly linear field
displaced above Bulk Earth. 87Sr/86Sr ratios range from
0·70378 to 0·70504 (mean = 0·70457 ± 22 at 1r) and
143
Nd/144Nd ratios from 0·51276 to 0·51291 (mean =
0·51285 ± 3 at 1r). In Fig. 11 the field defined by data
for Egmont eruptives lies within the field of TVZ ratios
and overlaps with the Kermadec field. As is the case for
the Pb isotopic data, Ruapehu data extend to more
radiogenic Sr and Nd isotopic compositions than are
observed in the Taranaki data (Fig. 11).
There is no apparent systematic change in 143Nd/144Nd
isotopic ratio with stratigraphic position but, within the
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R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
Fig. 9. Ruapehu andesite-normalized (see Table 2 for normalizing data) plots of trace element data for Taranaki eruptives.
cone, 87Sr/86Sr shows a slight but significant progressive
change as the samples become younger (Figs 5 and 11).
The Summit eruptives tend to have higher 87Sr/86Sr
isotopic ratios than Fanthams, Staircase, or Warwicks
Castle samples. Data for the samples from the Old Ring
Plain deposits, with 87Sr/86Sr ratios from 0·703781 to
0·704775 and 143Nd/144Nd from 0·51284 to 0·512877,
show considerable scatter beyond the range for the young
cone.
A single Kaitake sample has an Sr and Nd isotopic
composition which lies at the high 143Nd/144Nd and low
87
Sr/86Sr end of the array defined by the Egmont data
in the Sr/Nd isotopic diagram (Fig. 11). Three Pouakai
samples together show a spread in isotopic ratios similar
to that shown by the Summit group; one sample has a
relatively high 87Sr/86Sr ratio and a low 143Nd/144Nd ratio
and the other two samples have virtually identical and
intermediate isotopic compositions (Fig. 11). A single
Paritutu sample plots at the low 87Sr/86Sr ratio end of
the Egmont array in the Sr/Nd plot.
181
Oxygen isotope compositions
A detailed oxygen isotope study of Egmont carried out
by R. B. Stewart et al. (unpublished data, 1997) showed
Table 4: Whole-rock isotopic compositions for Taranaki eruptives
Sample:
1
2
3
4
5
6
7
8
9
10
T89/10
T89/15
T90/13
T89/19
T89/16
T90/2B
T89/18Bb
T90/28
T89/24
T90/21
A
A
A
A
A
A
A
A
A
A
Strat. group:
Su
Su
Su
Su
Su
Su
Su
St
St
St
dO18:
+6·3
+5·5
87
Sr/86Sr
+5·4
+5·8
0·704621±10
0·704630±9
0·704735±22
0·704699±22
0·704735±10
0·704738±21
0·704698±32
0·704649±18
0·704362±10
0·512858±13
0·512852±7
0·512847±10
0·512854±9
0·512850±7
0·512858±11
0·512836±15
0·512862±10
0·512865±13
143
Nd/144Nd
206
Pb/204Pb
18·766
18·773
18·78
18·795
207
Pb/204Pb
15·62
15·614
15·615
208
Pb/204Pb
38·636
38·628
38·639
Sample:
+5·4
0·704606±24
0·512838±9
18·772
18·786
18·779
18·815
18·765
18·749
15·63
15·612
15·618
15·614
15·679
15·595
15·579
38·703
38·63
38·646
38·628
38·879
38·567
38·513
11
12
13
14
15
16
17
18
19
20
21
T89/11
T89/14
T89/9
T89/8
T90/27
T90/10
T90/4D
T90/4A
T89/22
T89/21
BR-6
B
B
B
BA
A
A
A
A
A
A
A
Strat. group:
Fa
Fa
Fa
Fa
Wa
Wa
Wa
Wa
Wa
Wa
Wa
dO18:
+6·1
86
Sr/ Sr
+4·8
+6·1
+5·5
0·704668±10
0·704603±12
0·704589±22
0·704590±10
0·704941±15
0·704563±22
0·704357±13
0·704395±29
0·704379±11
0·704593±12
0·704617±26
0·512793±8
0·512876±19
0·512864±14
0·512856±9
0·512803±8
0·512873±10
0·512893±11
0·512858±9
0·512877±7
0·512814±6
0·512826±9
Nd/144Nd
206
Pb/204Pb
18·772
18·779
18·745
18·76
18·774
18·749
18·757
18·757
18·744
18·763
207
Pb/204Pb
15·618
15·618
15·615
15·617
15·598
15·584
15·604
15·604
15·604
15·609
208
Pb/204Pb
38·636
38·65
38·61
38·626
38·611
38·574
38·569
38·569
38·57
38·618
22
23
24
25
26
27
28
29
30
31
32
33
T90/32A
T90/45A
T90/46
T90/32C
T90/42A
T90/42C
T89/33A
T89/36
T90/41
T89/6A
T89/3
Average
B
A
A
A
B
A
A
A
A
A
A
Ruapehu
YP
YP
YP
YP
OP
OP
Po
Po
Po
K
Pa
Andesite
dO18:
87
Sr/86Sr
0·704701±24
0·704749±26
0·705041±11
0·704760±23
0·703781±10
0·704775±15
0·704636±17
0·704626±20
0·704954±18
0·704356±16
0·704354±20
0·512841±10
0·512799±8
0·512800±11
0·512821±9
0·512977±10
0·512814±15
0·512861±12
0·512861±12
0·512770±13
0·512910±12
0·512839±11
0·70527
143
Nd/144Nd
206
Pb/204Pb
18·781
18·775
18·774
18·774
18·729
18·731
18·748
18·761
18·747
18·785
18·889
18·806
207
Pb/204Pb
15·615
15·639
15·637
15·617
15·575
15·598
15·587
15·602
15·604
15·599
15·655
15·605
208
Pb/204Pb
38·655
38·704
38·727
38·653
38·508
38·624
38·572
38·626
38·658
38·612
38·836
38·639
0·51276
Stratigraphic groups: Su, Summit; St, Staircase; Fa, Fanthams; Wa, Warwicks Castle; YP, Young Ring Plain; OP, Old Ring Plain; Po, Pouakai Ranges; K, Kaitake
Ranges; Pa, Paritutu. Rock types: B, basalt; BA, basaltic andesite; A, andesite.
JANUARY 1999
Rock type:
Strat. group:
NUMBER 1
143
Sample:
VOLUME 40
182
Rock type:
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JOURNAL OF PETROLOGY
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HIGH-K ANDESITES FROM EGMONT VOLCANO
Fig. 10. Lead isotope data for Taranaki eruptives. (a), (b) Comparisons of Taranaki data (Ε) with regional data and Hart’s (1984) Northern
Hemisphere Reference Line (NHRL). (c), (d) Details of data for Egmont Volcano. Circles labelled T, W and M represent average data for
Torlesse and Waipapa terranes and Median Tectonic Zone, respectively. Ke, Kermadecs; B/A, basalts and andesites of the Taupo Volcanic
Zone (TVZ); R/D, field for rhyolites and dacites of the TVZ; Alex., field for the Alexandra volcanics. Data sources: Ewart & Hawkesworth
(1987), Briggs & McDonough (1990), Graham et al. (1992), McCulloch et al. (1994), and unpublished data compilations of A. Tulloch (1996),
J. A. Gamble (1997), and R. C. Price (1997).
that d18O values range from 4·8 to 6·3‰ for whole rocks
(Table 4). This is within the range exhibited by MORB
and island arcs rather than extending to higher values
found in continental arcs where interaction has occurred
between ascending magmas and continental crust. The
lowest d18O value observed (+4·8 in sample T90/10;
see Table 4) in the suite of analysed samples can be taken
to indicate interaction of the magma represented by this
sample with a strongly depleted fluid, perhaps as a
consequence of hydrothermal circulation in the middle
to upper crust (e.g. Muehlenbachs, 1986).
Oxygen isotope disequilibrium clearly existed between
some minerals and this is attributed to: (1) interactions
with fluids in the mantle in the case of olivine; (2)
re-equilibration (for titanomagnetite); and (3) high-level
interactions between magma and water which modified
groundmass d18O composition (R. B. Stewart et al., unpublished data, 1997).
Mineral pair d18O geothermometry indicates crystallization temperatures of between 1000°C and 1200°C,
compared with 860–965°C derived from oxide geothermometry on groundmass magnetite–ilmenite pairs
(Stewart et al., 1996). This is consistent with the crystallization and cooling sequence inferred for the magma,
with the phases giving higher temperatures crystallizing
at depth and the oxide temperatures representing closure
in the oxides during or immediately after lava extrusion.
DISCUSSION
Available evidence is consistent with the view that the
basalts of island arcs have an ultimate origin in the
mantle (e.g. Grove & Kinzler, 1986; Crawford et al.,
1987; Hawkesworth et al., 1991; McCulloch & Gamble,
1991; Tatsumi & Eggins, 1995), most probably within
the mantle wedge above the subducting slab. Melting is
initiated in the mantle wedge by the release of fluids
from the slab through dehydration reactions (Nicholls &
Ringwood, 1973; Ringwood, 1974; Hawkesworth et al.,
1979; Arculus & Powell, 1986). Subduction-related volcanics are characterized by a distinctive ‘arc’ signature
observed in the trace element abundance patterns, and
this is argued to arise from partitioning of trace elements
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be noted that each of these general components may
be geochemically complex and individually represent a
broad range of subtly different compositions (e.g. Whitford et al., 1979; Foden & Varne, 1980).
It is generally argued that evolved rock types such as
andesites or dacites are generated from more primitive
basaltic precursors by complex crystal fractionation processes (e.g. Foden, 1983; Ewart & Hawkesworth, 1987;
Woodhead, 1988) and in continental settings it is highly
likely that these involve assimilation of crustal materials
and/or mixing of crustally derived melts with fractionated, mantle-derived melts (e.g. McBirney, 1977;
DePaolo, 1981; Grove et al., 1982; Graham & Hackett,
1987).
The discussion to follow will emphasize two aspects of
magma genesis at Egmont Volcano with the objective of
obtaining a better understanding of magma genesis in
subduction systems in general: (1) petrological variation
within and between the magmatic series identified in
Egmont Volcano will be used to develop a model for the
generation and evolution of high-K andesitic magmas
in a continental setting; and (2) the issue of cross-arc
geochemical variation will be considered in the light of
similarities and contrasts between the geochemistry of
Ruapehu and Egmont eruptives and variations in magma
geochemistry in the Taranaki volcanoes.
Fig. 11. 87Sr/86Sr vs 143Nd/144Nd diagrams for Taranaki sample suite
and comparisons with regional data. (a) Comparison of Taranaki data
(Ε) with data for Ruapehu andesites and dacites (Φ). Fields are shown
for Kermadecs, Taupo Volcanic Zone (TVZ) basalts, dacites and
rhyolites, and Alexandra Volcanics (Alex.). T, W and M represent
average data for Torlesse and Waipapa terranes and Median Tectonic
Zone. Bulk Earth composition is from Faure (1986). Data sources:
Ewart & Hawkesworth (1987), Graham & Hackett (1987), Briggs &
McDonough (1990), McCulloch et al. (1994), Gamble et al. (1995), and
unpublished data compilations of A. Tulloch (1996) and R. C. Price
(1997). (b) Details of data for Taranaki volcanics.
between fluid and residual phases (Perfit et al., 1980;
Saunders et al., 1980; McCulloch & Perfit, 1981; McCulloch & Gamble, 1991). Relative depletion of the HFSE
(Nb, Ta) is considered to be an intrinsic feature of the
mantle wedge (e.g. Perfit et al., 1980; Gill, 1981; Pearce,
1982; Vukadinovic & Nicholls, 1989; McCulloch &
Gamble, 1991; Saunders et al., 1991) and a consequence
of the immobility of these elements in fluids (Tatsumi et
al., 1986; Brenan et al., 1995; Tatsumi & Eggins, 1995;
Keppler, 1996).
Generally, the trace element information and isotopic
data available for arc basalts and andesites are only
reconciled through complex models. For example, Kay
et al. (1978) explained Pb and Sr isotopic data for Aleutian
volcanics in terms of three components: subducted sediment, subducted oceanic crust, and the mantle wedge.
A general model developed by Ellam & Hawkesworth
(1988) also involves these three components, but it should
Petrological variation in eruptive sequences
of Egmont Volcano
The young Egmont cone consists of clearly defined
sequences of lava flows, which we have grouped stratigraphically and geochemically into packages (Warwicks,
Fanthams, Staircase and Summit). Available data providing geochronological control on the probable ages of
each group suggest that magmas have been emplaced
during eruptive cycles with a duration of the order of
1–2 kyr (Downey et al., 1994).
Contrasting geochemical variation between the different stratigraphic groups is exemplified by the K2O vs
SiO2 plot (Fig. 3). Within each group SiO2 content and
incompatible element abundances increase and compatible element abundances decrease. Each group occupies a distinct compositional space. A general
observation is that within groups SiO2 abundance increases and between groups K2O increases.
Modelling of the geochemical variation in the Egmont
eruptives is problematic because of the complexity of the
processes involved and the nature of the material sampled.
Phenocrysts record very complex histories suggesting
either that rocks could represent mixed magmas (e.g.
Eichelberger, 1975; Tsuchiyama, 1985) or that phenocryst fractionation could have been counteracted in some
cases by resorption of phenocrysts during decompression
184
R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
as the magmas ascended (e.g. Nelson & Montana, 1992).
Most of the samples from Egmont Volcano are from
porphyritic flows in which phenocryst abundances vary
between 30 and 60 vol. %, and consequently samples do
not necessarily represent melts on simple liquid lines of
descent. The design of quantitative treatments that adjust
lava compositions for phenocryst accumulation is virtually
impossible because of all the variables involved and the
difficulty of constraining these. For example, much of
the olivine is xenocrystal (Stewart et al., 1996) and at
least some of the phenocrysts of other phases could be
as well. Modal abundance of plagioclase does not correlate with Al2O3 abundance nor is modal clinopyroxene
content correlated with Sc concentration, and it is therefore not possible to identify samples in which phenocryst
accumulation has taken place.
For all these reasons, quantitative modelling can provide only a very broad approximation to the processes
actually involved, even for individual stratigraphic flow
groups. Consequently, the following discussion aims to
use data and quantitative models in a more general way
to evaluate the various processes that may have influenced
the evolution of Egmont magmas.
Crystal fractionation in Egmont lavas
Within the whole sample suite and within individual
stratigraphic groups, trends in major element variation
are consistent with a broad control by crystal fractionation
processes (e.g. Bowen, 1928; Gill, 1981; Gamble et al.,
1990; Graham et al., 1995); Mg, Fe, and Ca abundances
decrease systematically as Si content increases, and K
concentration increases. The common occurrence of
cumulate-textured mafic and ultramafic inclusions in the
lavas is also evidence that crystal fractionation has been
an important process controlling major element variation.
Inclusions of this type range in composition from dunite,
through wehrlite, to gabbro and diorite; cumulate mineralogy is dominated by clinopyroxene, olivine, plagioclase, orthopyroxene, titanomagnetite, and amphibole
(Stewart et al., 1996). All these minerals also occur as
phenocrysts, xenocrysts, or in glomerocrysts in the lavas.
The problems involved in quantitative modelling
of major and trace element data for Egmont andesites
can be illustrated with an example. Table 5 shows
a least-squares mixing model (Wright & Doherty,
1970) for the major element chemistry of one of the
evolved summit series lavas (T89/16 from the
summit dome); one of the less evolved summit series
lavas (T89/10) has been assumed to be parental.
Models involving either plagioclase–clinopyroxene–
titanomagnetite–amphibole or plagioclase–clinopyroxene–titanomagnetite–orthopyroxene give reasonable
fits for the major element chemistry for most components,
although there is a high residual for K2O. If, however,
the least-squares solution for the major elements is used
as the basis for trace element modelling, the fit between
model and actual abundances is not particularly good
(Table 5). The explanation for the poor fit between major
and trace element modelling is in part explained by a
closer consideration of plagioclase fractionation.
The major element model calculates a bulk composition
for the model parent composition by combining the
supposed daughter composition with the compositions of
the fractionating phases. The outcome does not depend
upon the path by which the daughter evolved; all that
matters is the bulk extract. Let us suppose, however, that
plagioclase has been added back into fractionated melts;
this could be related to sidewall accumulation in magma
conduits or a magma chamber, or simply arise through
inefficient segregation (flotation) of plagioclase within
derivative magmas. The major element modelling will
still provide a reasonable approximation to the bulk
process, but the direct translation of the major element
least-squares model to a trace element fractionation
model is not necessarily appropriate.
A check on whether or not plagioclase has been removed by fractionation or added back into fractionated
magmas can be provided by plotting SiO2 (as a fractionation index) against Eu anomaly expressed as Eu/
Eu∗, where Eu is the measured chondrite-normalized
Eu abundance and Eu∗ is the normalized abundance
obtained by extrapolation between normalized Sm and
Gd abundances. Because plagioclase is a dominant component in the fractionation process, it is expected that,
provided f O2 remains reasonably constant (Drake &
Weill, 1975), simple fractionation will result in a negative
correlation between Eu/Eu∗ and SiO2; greater fractionation causes more plagioclase to be removed and a
negative Eu anomaly should become more pronounced.
When data for Summit series lavas are plotted in an Eu/
Eu∗ vs SiO2 diagram (Fig. 12), they define a crude
positive trend indicating that something more complex
than simple crystal fractionation has taken place. At
least some Summit series eruptives represent zones of
plagioclase accumulation in crustal magma chambers
and this could explain the poor fit between major and
trace element models for these eruptives.
Although the example illustrates the difficulty of quantitatively modelling the complex processes by which andesite magmas evolve, it does provide a crude demonstration
that crystal fractionation processes were important in
controlling geochemical variation within Egmont magma
batches.
185
Contrasts between Egmont and Ruapehu
volcanoes
Egmont and Ruapehu volcanoes share a convergent
margin setting and their products originate in the same
JOURNAL OF PETROLOGY
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NUMBER 1
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Table 5: Major and trace element models for Summit eruptives of Egmont
Model 1—T89/10 to T89/16: plagioclase, clinopyroxene, amphibole, magnetite fractionation
Parent
Observed
Major elements
SiO2
54·87
TiO2
1·00
Al2O3
17·75
FeO∗
7·45
MnO
0·17
MgO
3·81
CaO
8·51
Na2O
3·97
K 2O
2·46
Rr 2 = 0·0611
Parent
Calc.
Daughter
54·90
57·19
1·04
0·78
17·77
7·45
0·16
17·87
6·32
0·15
3·81
8·46
3·09
7·34
3·78
3·98
2·33
2·84
Solution
Fraction of liquid: 0·79.
Proportions of phases: Cpx = 0·22, Pl = 0·50, Mt = 0·09, Amph = 0·19
Ba
Rb
Trace elements; daughter
Estimated
1185
1066
Observed
1026
(T89/16)
76
71
79
La
Ce
Sr
Nd
Sm
Eu
Y
Yb
24·0
23·6
57·7
48·6
747
387
29·4
23·3
5·7
3·4
1·59
0·38
24·2
16·7
2·43
1·46
22·3
42·7
627
21·7
4·4
1·23
25·1
2·23
Model 2—T89/10 to T89/16: plagioclase, clinopyroxene, orthopyroxene, magnetite fractionation
Parent
Observed
Major elements
SiO2
54·87
TiO2
1·00
Al2O3
17·75
FeO∗
7·45
MnO
0·17
MgO
3·81
CaO
8·51
Na2O
3·97
K 2O
2·46
Rr2 = 0·1036
Parent
Calc.
Daughter
54·91
57·19
0·95
0·78
17·73
7·46
17·87
6·32
0·16
3·73
0·15
3·09
8·48
7·34
3·73
3·98
2·28
2·84
Solution
Fraction of liquid: 0·78.
Proportions of phases: Cpx = 0·29, Pl = 0·56, Mt = 0·11, Opx = 0·05
Ba
Rb
Trace elements; daughter (T89/16)
Estimated
1202
77
1066
72
Observed
(as above)
La
Ce
Sr
Nd
Sm
Eu
Y
Yb
24·0
24·3
58·2
50·7
741
348
29·6
27·8
5·8
4·5
1·60
0·37
27·3
20·6
2·45
2·04
Partition coefficients from Feeley & Davidson (1994) and Ewart & Hawkesworth (1987). Concentration ranges are defined
by the range of bulk partition coefficients possible for each element.
186
R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
Fig. 12. Europium anomaly expressed as Eu/Eu∗ vs SiO2 abundance
for Summit (triangles), Staircase (diamonds), and Fanthams Peak
(crosses) groups. Eu is the measured Eu abundance normalized to
chondritic abundance and Eu∗ is the normalized abundance obtained
by linear extrapolation between Sm and Gd.
subduction system fluxing the same mantle wedge.
Differences between them are therefore most likely to be
due to: (1) progressive, cross-arc changes in the nature
of the mantle wedge and/or the geochemistry of slab
input; (2) variation in the thickness and/or composition
of the mantle wedge involved in each case; (3) contrasts
in the thickness and composition of the crust involved
beneath each volcano; and/or (4) different degrees of
partial melting taking place in the mantle wedge to
produce different primary magmas.
Fig. 13. 87Sr/86Sr isotopic ratio vs SiO2 abundance for Taranaki
eruptives and comparison with Ruapehu andesites and dacites. Ruapehu
data from Graham & Hackett (1987) and R. C. Price (unpublished
data, 1997).
no apparent correlation between SiO2 abundance and
isotopic composition (Fig. 13), consistent with the conclusion that assimilation of older crust has not played a
major role in the generation of Taranaki magmas. Oxygen isotope data (R. B. Stewart et al., unpublished data,
1997) for Egmont Volcano fall within the range of data
for oceanic island arcs and MORB and show no evidence
for extensive interaction with material rich in 18O.
Mineralogical and petrographic considerations
Isotopic comparisons
Lead isotopic data (Fig. 10) are not consistent with
extensive involvement of old crust in the genesis of
Taranaki magmas, and differences in crustal composition
are unlikely to provide an explanation for the contrasts
between Ruapehu and Taranaki eruptives. Data for the
two volcanoes have a common trend in the Pb isotope
diagrams, with Taranaki Pb isotopic ratios extending to
less radiogenic compositions. Basement beneath Taranaki
is probably Waipapa terrane and intrusive rocks of the
MTZ, and these have Pb isotopic compositions that
overlap with those of Torlesse terrane greywackes presumed to form the basement beneath Ruapehu (Fig. 10).
The Pb isotope data are consistent with the conclusion
that crustal contamination has played a significant role
in the evolution of Ruapehu lavas, but has been of lesser
importance at Taranaki.
Graham & Hackett (1987) pointed out that 87Sr/86Sr
isotopic ratios in Ruapehu eruptives show a broad positive
correlation with SiO2 abundance; a feature they attributed to assimilation of a crustal component during
crystal fractionation. The Taranaki data show a very
limited range in 87Sr/86Sr isotopic ratios and there is
Amphibole is rare in Ruapehu eruptives (Cole et al.,
1986; Graham & Hackett, 1987) but is common in
Taranaki lavas, indicating that magmas parental to many
Egmont lavas were relatively hydrous. Stewart et al. (1996)
also demonstrated that Taranaki magmas were relatively
oxidized.
An apparent paradox is immediately obvious. The
subducted slab beneath Egmont Volcano has presumably
already lost fluid beneath the TVZ to the east and would
be relatively dehydrated; intuitively one might expect
magmas generated further from the trench to be less
hydrous. An explanation may be that lower degrees
of melting are involved in the mantle wedge beneath
Taranaki, so that, even though a lower fluid flux is
involved, the water contents of parental magmas are
higher. Such an interpretation would also be consistent
with the relatively undersaturated nature of Taranaki
eruptives. An additional consideration is that fluid may
be lost progressively and continuously from the descending slab, with dehydration reactions involving phases
such as phengitic mica generating hydrous fluid at different depths deep into the mantle and well below amphibole
breakdown (Sorensen et al., 1997).
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Xenoliths of crustal metamorphic rock are common
in many Ruapehu lavas and they include meta-quartzites,
pyroxene hornfels, pyroxene granulites and schists (Graham & Hackett, 1987). Such xenoliths are relatively less
common in Taranaki eruptives, and cumulate-textured
diorite, gabbro and hornblendite inclusions are much
more common and reach large sizes. The xenolith population at Taranaki provides evidence for an extensive
suite of cumulate rocks at depth, confirms the importance
of amphibole in the evolution of Taranaki magmas,
and supports the conclusion drawn from the isotopic
information that, in contrast to Ruapehu, older crust has
not been extensively assimilated.
Trace and minor element considerations
It has been recognized for some time (Dickinson &
Hatherton, 1967; Neall et al., 1986) that young Egmont
lavas are more potassic than Ruapehu lavas. The higher
K is associated with higher abundances of other LILE
(Ba and LREE) but the ratios of these elements to K (or
to each other) are not the same in the two volcanoes.
Early hypotheses proposed to explain potassium variation
across subduction-related magmatic systems included
pressure-dependent variation in partition coefficients for
K (Dickinson, 1968), pressure dependence of K concentrations in eclogitic (slab) melts (Marsh & Carmichael,
1974) or variation in degree of slab melting with depth
( Jakes & White, 1972), and enrichment of K in fluids
or melts by wall rock reaction in the mantle wedge (Best,
1975). Recent models have concentrated on the influence
of crustal thickness (e.g. Meen, 1987), the thickness of the
melting mantle column in the wedge (Plank & Langmuir,
1988), and variation in degree of melting within the
mantle wedge (e.g. Stern et al., 1993), although some
models for generating high-K magmas in subductionrelated arcs involve a complex interplay between crust
and magmas derived from lithospheric and asthenospheric mantle sources (Edwards et al., 1991). The
generally higher abundances of LILE and HFSE in
Taranaki lavas may, in part, reflect greater degrees of
crystal fractionation, but they may also indicate intrinsically higher abundances in parental magmas, which
would be consistent with the proposal that lower degrees
of melting were involved, at source, in the mantle wedge.
The isotopic data appear to preclude more complex
models involving crust or magmas derived from lithospheric mantle.
Although both Taranaki and Ruapehu andesites have
normalized trace element patterns characterized by the
‘arc’ signature (e.g. low Ce/Pb ratios and high Ba/Nb
ratios relative to N-MORB), there are contrasts in the
extent to which the arc signature is developed. Ba/Nb
ratios in Taranaki eruptives are generally higher than in
Ruapehu equivalents although some Older Ring Plain
NUMBER 1
JANUARY 1999
samples and one Pouakai sample have Ba/Nb ratios
similar to or lower than those observed in Ruapehu
eruptives (Fig. 14). This would suggest that slab fluid
influence is higher in later Taranaki eruptives than at
Ruapehu.
Ce/Pb ratios are commonly interpreted to indicate
sediment influence in the slab fluid component. A comparison of Taranaki and Ruapehu Ce/Pb ratios shows
a clear contrast in behaviour with the Ba/Nb variation
(Fig. 14). Ce/Pb ratios are similar for most Taranaki and
Ruapehu samples but are elevated in some Fanthams
Peak, Warwicks Castle, and ring plain samples. If the
Ba/Nb ratios can be used to suggest a more significant
slab fluid influence in Taranaki magmas, then the Ce/
Pb data could be taken to mean that the fluids involved
in magma genesis beneath Taranaki are compositionally
different from those entering the wedge beneath Ruapehu. In particular, the influence of sediment on slabderived fluids declines at deeper levels within the subduction system.
Woodhead et al. (1993) argued that Ti/Zr ratios of arc
lavas can be used to draw inferences about the degree
of depletion in mantle sources from which magmas
parental to the lavas were derived. Taranaki lavas are
clearly fractionated and the Ti/Zr ratios have been
influenced by processes as well as source composition,
but a comparison of trends for Ruapehu and Taranaki
volcanoes provides an indication of the nature of their
respective sources. In a Ti/Zr ratio vs Zr abundance
plot (Fig. 14), Ruapehu and Taranaki samples define two
distinctly separate but converging trends. Least evolved
Taranaki samples have significantly higher Ti/Zr ratios
at low Zr abundance and this might imply that magmas
parental to the Taranaki suite eruptives have derived
from a mantle wedge source that is distinctly more
depleted than is the case beneath Ruapehu. The convergence of the trends is probably related to more significant magnetite fractionation influencing evolved
compositions at Taranaki. K/Rb and Cs/Rb (Fig. 14)
ratios are relatively elevated in most Taranaki eruptives,
which is also consistent with derivation of parental
magmas from a more depleted mantle source. There
may be other factors affecting K/Rb ratios, and one
possibility, interaction between parental magmas and the
lower crust, is discussed in a later section.
The Nb/Ta ratios for Taranaki eruptives and particularly Ruapehu lavas are low relative to MORB and,
because samples analysed by ICPMS methods were
crushed in WC, the possibility that they have been
contaminated with Ta, with consequent artificially
lowered Nb/Ta ratios, cannot be precluded. Comparisons of Nb/Ta data with those for other subductionrelated volcanics are limited, because not many analytical
programs routinely analyse for Ta. Feeley & Davidson
(1994) reported Nb/Ta ratios ranging from 6 to 16 for
188
R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
Fig. 14. Trace element ratios plotted against Zr abundance for Taranaki eruptives and comparisons with Ruapehu andesites and dacites.
MORB composition is from Sun & McDonough (1989). Ruapehu data from Graham & Hackett (1987) and R. C. Price (unpublished data,
1997).
Volcán Ollagüe calc-alkaline andesites (average of 11·4),
but in their study, the Nb abundances were determined
by relatively imprecise X-ray fluorescence analysis and
the authors did not report the methods used for crushing
samples. Peate et al. (1997) presented ICPMS data for
Vanuatu arc lavas and volcanogenic sediments that were
apparently crushed in agate, and they obtained Nb/Ta
ratios ranging from 9·8 to 23·4 with an average value of
16·7. These values overlap with the range we have
obtained for Taranaki samples.
Nb/Ta ratios reported in other work on New Zealand–
Kermadec subduction-related volcanics are similar to
those we have obtained. For example, Gamble et al.
(1993) obtained Nb/Ta ratios ranging from 2·8 to 16
(average 11·6) for Taupo Volcanic Zone and Havre
Trough basalts, and Briggs & McDonough (1990) obtained ratios of 6·7–20 (average 13·4) for samples from
the Alexandra Volcanics. Unfortunately, in both these
studies, all samples were crushed in WC and Nb analyses
were carried out by X-ray fluorescence spectrometry.
Regardless of the possible effects of contamination on
the absolute values for Nb/Ta ratios, it should be valid
to make a relative comparison of Ruapehu and Taranaki
data presented here; samples from both volcanoes cover
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JOURNAL OF PETROLOGY
VOLUME 40
a similar range in major element composition and all
have been prepared for analysis in an identical manner.
On this basis, it could be concluded that Ruapehu
eruptives show relatively lower Nb/Ta ratios than their
Taranaki counterparts.
Clearly, Egmont lavas lack the characteristics of primitive arc magmas (e.g. Tatsumi & Eggins, 1995), with
even the lowest Si compositions being evolved. Among
the few samples showing more elevated Mg, Ni, and Cr
there is evidence for contamination by xenocrystic olivine
(Stewart et al., 1996). In contrast, although low-Si (basaltic)
compositions are not common at Ruapehu, there is a
higher proportion of comparatively magnesian basaltic
andesites and low-Si andesites, and Ruapehu andesites
show a spread in composition to higher Ni and Cr
abundances (Fig. 6 and Table 2). In general terms,
Ruapehu andesites are less evolved than those at Taranaki
and this may reflect differences in crustal structure beneath the two volcanoes. The crust beneath Egmont
Volcano is of the order of 25 km thick whereas the
crustal thickness beneath the TVZ is estimated to be
15 km (Stern & Davey, 1987). Heat flow is much higher
in the TVZ (Hochstein et al., 1993), which is considered
to be a zone of pervasive extension and normal faulting
(e.g. Stern, 1987; Cole, 1990). Consequently, magmas
ascending beneath Taranaki are more likely to become
trapped and fractionated beneath the crust than magmas
generated beneath the TVZ.
Strontium is enriched in Taranaki relative to Ruapehu
samples. The abundance of Sr remains relatively constant
with increasing K and Si in the Ruapehu suite whereas
Sr contents are elevated in older Taranaki eruptives.
They become progressively higher in the cone sequences
of Egmont Volcano, reaching maximum values in some
of the Summit flows before decreasing slightly in the
most evolved Summit group eruptives. One of the effects
of higher water contents in andesitic magmas is contraction of the liquidus field of plagioclase (e.g. Gaetani
et al., 1993). Later crystallization of plagioclase in some
Egmont andesites may partially account for the difference
in Sr behaviour between Ruapehu and Egmont suites.
NUMBER 1
JANUARY 1999
(Table 2) is an example of this unusual rock type. It has
relatively low SiO2 abundance (~49%), and, although
still fractionated, relatively high MgO (6·75%), Ni (119)
and Cr (113) abundances. Normative Hy is low (2·42%)
so that the rock is relatively undersaturated. The sample
still manifests the arc trace element signature but, relative
to other Taranaki samples, this is somewhat more subdued, with lower La/Nb ratios and higher Ce/Pb ratios;
Nb abundances are 2–3 times higher than normal (15
ppm). Isotopically this sample is distinctive. It has the
lowest 87Sr/86Sr isotopic ratio and a relatively unradiogenic Pb isotopic composition. These clasts appear
to represent relatively low-degree melts from a depleted
mantle source.
Data available for Pouakai Volcano, to the northwest
of Egmont along the Taranaki lineament, indicate that
most of the eroded remnant of this large volcano is
similar to the older (relatively lower K) eruptives of
Egmont, although higher-K material similar to the
younger Egmont eruptives has also been recovered from
laharic deposits associated with Pouakai Volcano.
The data can be interpreted to indicate that the earliest
eruptives of the Taranaki volcanoes included andesitic
magmas not dissimilar to those that characterize TVZ
andesitic volcanoes to the east. The potassic character is
manifested most strongly in the later stages of construction
of the Taranaki volcanoes.
Trends in K2O abundances with time at
Taranaki
The early magmatic history of Egmont Volcano, predating the construction of the present-day cone, is recorded in the andesite clast assemblages of laharic deposits
within the ring plain of the volcano. Data from this
succession show that the strongest K enrichments are
developed in the youngest Egmont eruptives.
Some of the andesites from the oldest Egmont laharic
deposits have distinctive geochemical features that are
atypical of the younger eruptives. Sample T90/42A
190
The origin of Taranaki magmas
Contrasts between Taranaki and Ruapehu: source influences
on primary magmas
Higher K, Ba, and LREE abundances in eruptives of
Egmont Volcano relative to their counterparts at Ruapehu to the east may reflect intrinsically higher abundances in parental magmas and this could ultimately be
explained in terms of lower degrees of melting in the
mantle source beneath Taranaki. Woodhead & Johnson
(1993) suggested that variations in HFSE abundances
across the New Britain arc could be related to decreasing
degrees of melting in the mantle wedge. The amount
of melt in the mantle wedge should be approximately
proportional to the amount of water fluxed from the slab
(Stolper & Newman, 1994), although it will also depend
on temperature and on the composition of the mantle
wedge. Stern et al. (1993) explained variation in magma
composition across the Marianas arc in terms of decreasing degrees of melting as a consequence of diminishing slab fluid flux across the arc.
Some of the isotopic and trace element differences
between Taranaki and Ruapehu eruptives appear to arise
because crustal contamination has been much more
significant at Ruapehu than beneath Taranaki. This may
R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
reflect contrasts in crustal structure and heat flow beneath
the two volcanoes. Regional heat flow is significantly
lower beneath Taranaki and this probably reflects thicker
crust and emplacement of smaller volumes of magma
over a shorter period of time. It is therefore likely that
parental magmas beneath Egmont were intrinsically less
capable of assimilating crustal material. Thinner crust
and higher heat flow beneath Ruapehu also mean that
amphibole is unlikely to be stable at the crust–mantle
boundary and consequently amphibolitization and underplating of the crust has not been a factor in petrogenesis.
At Ruapehu, hotter, drier and less evolved magmas
have risen and ponded at higher levels so that crustal
contamination involving basement meta-sediments and
early TVZ andesitic eruptives has been an important
factor controlling magma chemistry.
The mantle source involved in the generation of primary magmas beneath Taranaki appears to have been
more depleted than is the case beneath Ruapehu. This
is reflected, for example, in the K/Rb, Ti/Zr, and Rb/
Cs ratios and it may arise because the mantle being
carried down above the descending slab has already been
through a partial melting event beneath the frontal arc.
Furthermore, the Ba/La and Ce/Pb ratios indicate that
the slab fluid component involved in the melting process
changes composition across the arc. It should be emphasized that our model for the origin of magmas parental
to the Egmont eruptives involves an interplay between
several changing parameters; the mantle wedge becomes
progressively depleted as it is carried down above the
slab, diminishing fluid fluxes result in decreasing degrees
of partial melting, and the slab fluids change composition
with depth. An important feature that differs from other
models is that the mantle wedge component is depleted.
For example, Stern et al. (1993) proposed a similar model
for the Kasuga cross-arc chain in the Marianas, but
appealed to progressive involvement of an ocean island
basalt (OIB) type component as fluid flux and consequently degree of partial melting declined across the
arc. Hochstaedter et al. (1996) also argued that magmas
in the back-arc region of the Kamchatka arc derived
from a more enriched mantle than those in the frontal
arc; melting in the back arc depleted the mantle in the
wedge and this material was carried by convection into
the frontal arc region.
Magma evolution in the Taranaki volcanoes
The arguments presented earlier [see also Stewart et al.
(1996)] lead to the conclusion that magmas parental to
Taranaki eruptives were relatively hydrous, oxidized,
high-Mg basalts generated in the mantle wedge by fluxing
of fluid from the subducting slab; none of these primary
magmas are represented in the sample set. Hydrous
primitive magma ponded at the crust–mantle boundary,
191
Table 6: Analysis of amphibolite
xenolith from Taranaki andesite
Sample
SiO2
T95-10X1
37·85
TiO2
2·20
Al2O3
12·49
Fe2O3
FeO
7·22
13·57
MnO
0·18
MgO
11·02
CaO
11·87
Na2O
1·87
K2 O
0·75
P2O5
0·03
CO2
0·07
H2O+
0·47
H2O–
0·07
Total
99·66
Cs
Ba
0·13
236
Rb
7
Sr
262
Pb
1·8
Th
0·37
U
0·12
La
3·0
Ce
8·0
Pr
1·5
Nd
8·5
Sm
2·9
Eu
0·97
Gd
3·29
Tb
0·51
Dy
3·01
Ho
0·57
Er
1·45
Tm
0·19
Yb
1·15
Lu
0·17
Y
16·1
Zr
42
Hf
1·7
Ta
0·25
Nb
1·3
Sc
72
V
867
Cr
41
Ni
35
Cu
47
Zn
98
Ga
21
Trace elements in italics by ICPMS. Analytical methods as
described in Table 2 and in text.
JOURNAL OF PETROLOGY
VOLUME 40
Table 7: Trace element models for melting of
amphibolite
(a) Calculation of bulk distribution coefficients
DCpx
DAmph
D25
D50
D77
K
0·003
0·081
0·073
0·058
0·003
Ba
0·005
0·150
0·136
0·107
0·005
Rb
0·004
0·050
0·045
0·036
0·004
Sr
0·086
0·420
0·387
0·320
0·086
La
0·020
0·500
0·452
0·356
0·020
Sm
0·140
4·300
3·884
3·052
0·140
Yb
0·200
4·720
4·268
3·364
0·200
Partition coefficients for clinopyroxene (Cpx) are from Ewart
& Hawkesworth (1987). Those for amphibole (Amph) are
from Feeley & Davidson (1994) and Nagasawa & Schnetzler
(1971). Bulk distribution coefficients are shown for 25%, 50%,
and 77% melting of amphibole. Stoichiometry of melting is
from Holloway & Burnham (1972).
(b) Calculation of melt compositions
Model 1
Model 2
Model 3
% Melt:
77
50
25
Residue:
Cpx
Amph+Cpx Amph+Cpx Amph
K
8079
11774
20421
6226
Ba
306
427
671
236
Rb
9
14
25
7
Sr
332
397
485
262
La
3·82
4·4
5·0
3·0
Sm
3·62
1·4
0·9
2·9
Yb
1·41
0·53
0·33
1·15
0
Trace element compositions for melting of amphibolite composition shown in Table 6. Melting stoichiometry and bulk
distribution coefficients from Table 7a. These melt compositions have been used in Fig. 15 to illustrate effect of
mixing a Ruapehu-type basaltic andesite with melts derived
from amphibolite underplate.
evolving to high-Al basalt by olivine–pyroxene–spinel
fractionation. P–T conditions at the crust–mantle boundary were such that amphibole began to crystallize from
the magmas and within anhydrous lower-crustal and
mantle wall rocks, buffering melt compositions to basaltic
andesite (Foden & Green, 1992) and underplating the
crust with amphibole, olivine, and pyroxene cumulates.
Not all high-Al basalt magma was ponded and trapped
at the crust–mantle boundary. Some rose to the surface,
fractionating olivine and clinopyroxene and eventually
plagioclase, and evolving to fractionated basaltic andesites
and low-Si andesites. Much of the earlier formed amphibole was partially or completely resorbed as these
magmas ascended higher in the crust, moving, in P–T
space, away from the amphibole stability field. Xenoliths
and xenocrysts of olivine, pyroxene, amphibole, and
192
NUMBER 1
JANUARY 1999
plagioclase from wall rocks and earlier cumulates were
entrained by the ascending magmas.
Eruptions are probably fed from small, high-level
magma chambers immediately beneath the volcano.
These are sites in which there is a complex interplay
between magma egress and recharge, crystal fractionation, magma mingling and mixing, and plagioclase
accumulation (Smith et al., 1996). Segregation of felsic
magma and crystal cumulates resulted in the formation of
relatively hydrous magmas, which cooled until amphibole
was again stable. The most evolved samples we have
analysed from Taranaki are pumices from the Burrell
Ash containing pale green amphibole and abundant
complexly zoned plagioclase phenocrysts. Many clasts
and bombs from this unit are complexly layered, mixed
pumices.
The temporal geochemical changes within Taranaki
eruptives could be simply a consequence of progressive
variation in the degree of partial melting in the mantle
source. This could also be associated with changes in the
composition of the mantle sources being progressively
melted and progressive changes in the nature of slabderived fluids. With time, declining levels of partial
melting at source could lead to progressively rising K
contents, a more significant fluid trace element signature,
and a dilution of depleted mantle wedge signature. The
temporal changes could wholly reflect changes in the
nature of the primary magmas.
An alternative explanation, based on a model proposed
by Foden & Green (1992), has been offered by Stewart
et al. (1996), who suggested that progressive underplating
and intrusion of magmas into the lower crust could raise
the geothermal gradient so that eventually P–T conditions
at the crust–mantle boundary rise above the amphibole
stability field. Amphibolites generated by the passage of
earlier more hydrous magmas begin to melt incongruently
and produce small amounts of relatively K-rich melt. The
model provides a possible explanation for the increase in
K with time and is also consistent with the petrography
and mineral chemistry of lava samples, xenocrysts, glomerocrysts and xenoliths. The xenoliths are cumulates
and fragments of amphibolitized upper mantle and lower
crust incorporated as the magmas were tapped from
the crust–mantle boundary. Strontium, Nd, Pb, and O
isotopic data indicate that crustal contamination was
limited and this is also consistent with the model; the
only contamination is from small quantities of lowercrustal melt at the more advanced stage of development
of the system. One of the implications of the model is that
magmatism associated with subduction systems should
become more potassic with time because progressive
underplating of the arc thickens the crust and raises
geothermal gradients. Amphibolitization of the lower
crust means that with time, a thick hydrous underplate
develops and accumulation of magma and heat from melt
R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
Fig. 15. Variations in K/Rb, K/Sr, Ba/La, and K/Ba ratios with K abundance for Ruapehu and Taranaki eruptives and comparison with
trends produced by mixing between a low-K Ruapehu basaltic andesite and model melts produced by breakdown of amphibolite. The models
are explained in the text and in Table 7. Models 2 and 3 involve both residual clinopyroxene and amphibole during amphibolite melting. Model
1 represents more extreme melting where the only residual phase is clinopyroxene. Numbers alongside symbols for melting model in (a) indicate
per cent mixing between the basaltic andesite and the model melt. Vectors labelled C and M indicate general trends produced by crystal
fractionation, or crystal fractionation coupled with crustal assimilation (C), and mixing of basaltic andesite and melts produced by partial melting
of amphibolite (M).
crystallization eventually raises the geothermal gradient to
a point where the underplate begins to melt incongruently
and contaminate magmas with potassic melt.
Some of the effects of amphibole melt contamination
on trace element behaviour have been tested using simple
numerical models. In the models the minor and trace
element composition of an amphibolitic lower-crustal
component has been assumed to be similar to that of
amphibolite xenoliths contained in some of the lavas. An
example of one of these compositions is shown in Table 6.
This particular xenolith sample (T95/10X1), which was
originally 25 cm in diameter, is composed almost exclusively (>95%) of large (>10 cm) bladed hornblende
crystals with minor plagioclase and clinopyroxene (<5%).
In the models it has been assumed that amphibole with
a trace element composition similar to that of xenolith
T95/10X1 (Table 6), has melted by the simple reaction
Hornblende → Melt + Clinopyroxene (Holloway &
Burnham, 1972); the models are not affected very strongly
if spinel is assumed to be an additional phase produced
in the melting reaction.
Table 7 shows calculations of bulk distribution coefficients and melt compositions for 25, 50, and 77%
melting, assuming the melting stoichiometry of Holloway
& Burnham (1972); at 77% melting, amphibole is no
longer a residual phase and the melts are in equilibrium
with clinopyroxene. Figure 15 shows the effect on K/
Rb, K/Sr, Ba/La, and Ba/K ratios and K abundance,
when model melt compositions are mixed with a relatively
low-K basaltic andesite from Ruapehu.
Clearly, the models are poorly constrained; the calculations involve very broad assumptions about the trace
element composition of the amphibolite component that
has melted, partition coefficients, and the nature of the
melting reaction. None the less, the calculated variations
illustrated in Fig. 15 can be used to argue that progressive
contamination of andesitic magmas by melts derived
from underplated amphibolitic crust, particularly at low
193
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 1
JANUARY 1999
Fig. 16. Schematic summary of processes involved in generation and evolution of Egmont high-K eruptives. (a) Processes involved in evolution
of high-K magmas. Phase relationships at left are from Foden & Green (1992). Stage 1: (A) high-Al basalt (HAB) magma is derived from hydrous
high-Mg magma by crystal fractionation involving olivine, clinopyroxene, and spinel; (B) magmas pond at base of crust where amphibole
crystallizes, cumulates underplate crust and basaltic andesites form; (C) ascent and fractionation (olivine–pyroxene–plagioclase–magnetite) of
these magmas forms andesites; some amphibole is resorbed. Stage 2: rising geotherms as a consequence of continued underplating. (A) As in
stage 1; (B) interaction with magmas and melts produced by anatexis in amphibolitic underplate; (C) as in stage 1; (D) fractionation in highlevel magma chambers produces high-Si andesites which crystallize plagioclase–pyroxene–amphibole. (b) Schematic illustration of processes
envisaged to influence magma generation across the subduction system in New Zealand’s North Island. AFC, Assimilation and fractional
crystallization. Crustal structure and details of subduction system are based on data from Stern (1987) and Reyners (1983), the mantle flow
regime is from Davies & Stevenson (1992), and proposals for slab mantle interaction largely follow Pearce & Peate (1995).
degrees of partial melting, could produce geochemical
variation broadly similar to the temporal geochemical
variation observed at Taranaki, and this mechanism
could also explain some of the geochemical differences
between Ruapehu and Taranaki eruptives.
The models can be used to explain differences in K/
Rb and Ba/La ratios and K abundance data between
Taranaki and Ruapehu, but this explanation is less convincing for Ba/K and K/Sr data, which may indicate
that other factors such as plagioclase fractionation and/
or differences in source component contribution may
also be influencing the overall variation.
CONCLUSIONS
A model for the origin of high-K Taranaki eruptives is
summarized schematically in Fig. 16.
194
Contrasts between the geochemistry of eruptives at
Ruapehu and Taranaki reflect a complex interplay between processes and sources across the subduction system.
Egmont lavas are generally more fractionated than their
Ruapehu counterparts but show relatively little evidence
for crustal contamination. They are also relatively more
potassic and comparatively enriched in Ba, U, Th, La,
Ce, and Sr. Ba/La and Ce/Pb ratios can be used to
infer that the slab contribution beneath Taranaki is
compositionally different from that involved at Ruapehu,
and comparisons of Rb/Cs and Ti/Zr and, possibly, K/
Rb ratios suggest that the mantle wedge component
involved at Taranaki is relatively more depleted. Variation across the arc from Ruapehu to Taranaki in part
reflects contrasts in the composition of parental magmas
because of (1) variation in the degree of depletion of the
mantle wedge, (2) contrasts in the composition of fluid
fluxed from the subducting slab, and (3) declining degrees
of partial melting across the subduction system. Ad-
R. C. PRICE et al.
HIGH-K ANDESITES FROM EGMONT VOLCANO
ditionally, differences in crustal thickness and heat flow
have caused magmas to evolve in different ways at the
two volcanic centres.
Magmas parental to lavas of Egmont Volcano were
relatively undersaturated, hydrous, and LILE-enriched
high-Mg basalts. Fractionation of these at the crust–
mantle boundary produced high-Al basalts and basaltic
andesites and complementary mafic cumulates including
amphibolites and pyroxenites. Ascent of these magmas
resulted in further fractionation and formation of gabbroic cumulates. The youngest eruptives are complex
magmas produced by a combination of high-level crystal
fractionation, crystal accumulation, and magma mixing
and/or mingling.
Two possible explanations are offered for temporal
change in K content of magma batches at Taranaki.
The trends could reflect variation at source; declining
degrees of partial melting, changes in fluid flux, and/or
mantle source heterogeneity could result in the formation
of progressively more K-rich magmas. Alternatively, rising geothermal gradients at the base of the crust could
lead to interaction between evolving magmas and newly
underplated crust. Partial anatexis of amphibolitic underplated material formed potassic melts and assimilation of
this material by fractionating high-Al basalt and basaltic
andesite magmas might lead to the formation of successively more potassic eruptives.
ACKNOWLEDGEMENTS
Technical support for this project was provided by
R. Maas, D. Korke, J. Metz, I. McCabe, and Allan
Jacka. B. W. Chappell and R. Rudnick provided INAA
and SSMS analyses, and W. S. Downey, C. M. Gray,
and R. Kellett were involved in field mapping and
sampling on Taranaki. Discussion and comment provided
by J. A. Gamble and M. T. McCulloch were of considerable value. R. J. Arculus and J. D. Foden are thanked
for their constructive reviews. Funding by the Australian
Research Council is gratefully acknowledged.
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