Download Rift-Related Transition from Andesite to Rhyolite

JOURNAL OF PETROLOGY
VOLUME 52
NUMBER 11
PAGES 2243^2263
2011
doi:10.1093/petrology/egr046
Rift-Related Transition from Andesite to
Rhyolite Volcanism in the Taupo Volcanic Zone
(New Zealand) Controlled by Crystal^melt
Dynamics in Mush Zones withVariable
Mineral Assemblages
C. D. DEERING1*, O. BACHMANN1, J. DUFEK2 AND D. M. GRAVLEY3
1
DEPARTMENT OF EARTH AND SPACE SCIENCES, UNIVERSITY OF WASHINGTON, MAILSTOP 351310, SEATTLE,
WA 98195-1310, USA
2
SCHOOL OF EARTH AND ATMOSPHERIC SCIENCES, GEORGIA INSTITUTE OF TECHNOLOGY, 311 FERST DRIVE,
ATLANTA, GA 30332, USA
3
DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CANTERBURY, PB 4800, CHRISTCHURCH 8020,
NEW ZEALAND
RECEIVED SEPTEMBER 2, 2011; ACCEPTED SEPTEMBER 9, 2011
The Taupo Volcanic Zone (TVZ), located in the North Island of
New Zealand, represents part of a magmatic arc that is at present
undergoing active extension. Around 0·9 Myr ago, an acceleration
in rifting was followed by a progressive transition in the composition of volcanic products (until 0·7 Ma) from typical arc-type
andesite into overwhelmingly large, caldera-forming rhyolitic eruptions with subordinate basalt and dacite in the Central TVZ.
Despite an obvious compositional gap in the erupted products in
the Central TVZ within the last 0·7 Myr (little to no erupted
products with SiO2 contents between 55 and 65 wt %), phenocryst
minerals (plagioclase, amphibole, pyroxene) show an uninterrupted
compositional record that suggests crystallization from a continuum
of melt compositions. Coupled with radiogenic isotope evidence, the
whole-rock and mineral chemistry data are consistent with magmatic differentiation controlled by crystal fractionation of primary
mantle-derived magmas accompanied by some assimilation of local
wall-rocks. In the Southern TVZ and in the early part of the
Central TVZ, magmatic differentiation was dominated by the
lower crustal evolution of relatively dry (1wt % H2O) arc basalts, crystallizing a pyroxene^plagioclase-dominated assemblage.
However, the conditions of crystallization in the lower crust appear
to have changed within the last million years in the Central TVZ,
with amphibole and oxides appearing earlier in the crystallization
sequence. In this framework and using numerical simulations coupling crystallization kinetics and multiphase fluid dynamics of
magma reservoirs, we show that melts extracted from crystal
mushes within an optimal ‘extraction window’ (50 and 80 vol.
% crystals) match those erupted at the surface. Lower crustal
mushes fed by basalt with 1wt % H2O (dominated by a pyroxene^plagioclase assemblage) release andesitic melts at the extraction window. These melts then erupt at the surface to form the
observed andesitic part of the arc. With a slightly higher water content (2 wt %) in the basalt, the melt composition at the extraction window from lower crustal mushes is dacitic rather than
andesitic. Although some dacitic melts will reach the surface, most
will be trapped in the upper crust and crystallize to form a silicic
mush. Extraction of the interstitial liquid after 450% crystallization from this upper crustal reservoir produces the large volumes of
rhyolitic magma erupted over the past 0·7 Myr (44000 km3 from
ignimbrite-forming eruptions).
*Corresponding author. E-mail: [email protected]
ß The Author 2011. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oup.com
JOURNAL OF PETROLOGY
KEY WORDS:
VOLUME 52
crystal mush; rifting; andesite; rhyolite; melt extraction;
arc volcanism
I N T RO D U C T I O N
For decades, igneous petrologists have recognized a transition in arc systems from typical andesitic volcanism
during the early stages of magmatism to silicic volcanism
as the system thermally matures (for reviews, see Lipman,
2007; Grunder et al., 2008). In some arcs, this transition appears strongly coupled with the beginning of extension
(e.g. Central America: Burkart & Self, 1985; Andes:
Coulon & Thorpe, 1981). One of the clearest examples of
such a petrological transition following the onset of extension is found along the onshore part of the Taupo Volcanic
Zone (TVZ), New Zealand, where it is both temporal and
spatial. Andesitic volcanism began at 2·0 Ma in the
Central TVZ and has become dominantly rhyolitic with
time (accompanied by some volumetrically subordinate
basalt and dacite eruptions). Modern volcanism within
the onshore TVZ is also geographically segmented into andesitic volcanism at the southern tip and dominantly rhyolitic volcanism in the north (Wilson et al., 1995; Fig. 1).
The cause of the transition from dominantly andesitic to
dominantly rhyolitic volcanism in the TVZ is still debated.
The presence of an obvious compositional gap in the
Central TVZ (most erupted magmas are rhyolitic, but
basalt and dacitic units also occur; Wilson et al., 1984) has
led some researchers to invoke magma generation processes dominated by widespread crustal anatexis of
Mesozoic greywackes (Reid, 1983) or previously emplaced
intermediate (andesite intrusive equivalent) igneous rocks
(Price et al., 2005) to explain the presence of the widespread rhyolitic volcanism, whereas the andesites have
been attributed to lower crustal fractionation processes.
Others have suggested that the andesite and rhyolite
magmas follow two distinct liquid lines of descent (e.g.
Steiner, 1958; Ewart & Stipp, 1968; Graham et al., 1995). As
the isotopic compositions are inconsistent with melting of
old crustal lithologies (Graham et al., 1995) and thermal
models (Dufek & Bergantz, 2005; Annen et al., 2006) do
not support significant melting of older magmatic rocks at
shallow crustal depths (15^30 km), questions still remain
as to (1) the fundamental processes that explain the temporal and spatial transition from andesitic to rhyolitic volcanism in the TVZ and (2) the origin of the considerable
compositional gap and huge volumes of erupted rhyolite
in this part of the world (see Wilson et al., 1995).
The purpose of this study is to develop an integrated
petrogenetic model for the TVZ by coupling the physical
process of crystal^liquid separation with the available
bulk-rock and mineral chemical data that explicitly takes
into account the transition from normal, arc type andesitic
volcanism to predominantly rhyolitic volcanism. The
origin of the two types of eruptive products is shown to be
NUMBER 11
NOVEMBER 2011
a consequence of differences in the P^T conditions and
aH2O and/or absolute H2O contents of the starting basalt
intruded into the lower crust. These differences produce
two distinct liquid compositions (andesite or dacite) that
are extracted within a key crystal fraction interval between 50 and 80 vol. % (Dufek & Bachmann, 2010)
from lower crustal mafic magma bodies. The rhyolite, however, is produced in a second stage of magma evolution
through extraction (at the 50^80% crystallinity window)
from trapped dacite melt forming a silicic mush in the
upper crust.
T EC TON IC S ET T I NG
The tectonic regime of New Zealand’s North Island is
strongly linked to and influenced by the oblique (208)
convergence of the Pacific and Australian plates at c.
42 mm a1 (Fig. 1; DeMets et al., 1994) along a NE^
SW-trending subduction interface. The eastern part of the
North Island (the forearc to the Taupo Volcanic Zone) is
rotating in a clockwise direction as several discrete tectonic
blocks (within the North Island Fault System, NIFS)
(Reyners, 1998; Wallace et al., 2004). As a consequence, the
TVZ volcanic arc is actively rifting and the sub-arc crust is
thinning at the same time as magmatism is occurring. In
the Central TVZ, co-occurrence of rifting and magmatism
is variably expressed by tectono-magmatic faulting, dyking,
uplift, and subsidence all across the arc (Rowland et al.,
2010). At the surface, linear and subparallel NE^SWtrending normal fault structures (i.e. the Taupo Rift) are
organized into discrete segments separated by arcperpendicular accommodation zones (Rowland & Sibson,
2001). Geodetic, geophysical and paleoseismic fault studies
within theTaupo Rift all show that extension increases from
the Southern TVZ (3 mm a1; Villamor & Berryman,
2006) to the Northern TVZ (13 mm a1; Lamarche et al.,
2006). In plan view, a telescope-like pattern emerges with
rift segments of increasing width from south to north.
The evolution of the tectonic structure (i.e. rifting) in
the Central North Island is closely linked to the magmatic
activity (Manville & Wilson, 2003; Rowland & Wilson,
2009; Wilson et al., 2009; Rowland et al., 2010). Arc-parallel
rifting and volcanism have migrated from 16 Ma in a
stepwise en echelon pattern from the extinct Coromandel
Volcanic Zone (CVZ) to the TVZ (Rowland & Wilson,
2009). In the young (2 Ma) TVZ, the development of
the rifted arc is becoming more apparent as the temporal
and spatial resolution of preserved volcanic deposits (on
the surface and from geothermal drilling) across the
major fault segments improves (see Wilson et al., 2010). In
particular, the timing of accelerated rifting (i.e. 0·9 Ma;
Wilson et al., 1995) and southwesterly jumps in the locus of
rifting (i.e. 0·4 Ma; Gravley et al., 2009) directly precede
and/or coincide with periods of widespread silicic
magmatism.
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DEERING et al.
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177 E
176 E
oce
an
tine ic cru
s
nta
l cr t
ust
CV
Z
TV
Z
Australian
Plate
White
Island
Bay of Plenty
con
42 mm/yr
y
ar
100 km
d
un
Pacific
Plate
rn
g
un
o
Y VZ
T
bo
te
38 S
Z
d
Ol
s
we
TV
Ro
Ka
Ma
Central
TVZ
Rp
Ta
u
po
Ri
ft
Oh
Wh
Northern
TVZ
Ok
Tp
NIFS
Central
TVZ
39 S
Tongariro
Southern
TVZ
Ruapehu
0
20
40 km
N
Fig. 1. Regional map showing the location of the Taupo Volcanic Zone (TVZ) and its associated tectonic and volcanic structures; inset shows
the location of the CVZ and TVZ within the North Island of New Zealand and their relationship to subduction of the Pacific Plate beneath
the Australian Plate. The outline of the Old TVZ and the Young TVZ are fromWilson et al. (1995), as are the present-day compositional divisions
between the more andesitic Southern and Northern TVZ and the rhyolitic Central TVZ. Mapped faults in the TVZ delineate the Taupo Rift.
Rhyolitic calderas outlined are from Houghton et al. (1995) and Gravley et al. (2007): Ma, Mangakino; Ka, Kapenga; Ro, Rotorua; Rp,
Reporoa; Tp, Taupo; Wh, Whakamaru; Ok, Okataina; Oh, Ohakuri. CVZ, Coromandel Volcanic Zone; NIFS, North Island Fault System.
Andesitic composite cones are shown as black triangles.
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Fig. 2. Maps illustrating the active eruptive centers during the pre-0·7 Ma and post-0·7 Ma TVZ. Rhyolitic calderas outlined are from
Houghton et al. (1995) and Gravley et al. (2007): Ma, Mangakino; Ka, Kapenga; Ro, Rotorua; Rp, Reporoa; Tp, Taupo; Wh, Whakamaru; Ok,
Okataina; Oh, Ohakuri. TVC, Tongariro Volcanic Complex. Mr, Maroa volcanic complex (from Leonard, 2003). Andesitic composite cones
and dome complexes: 1, Ruapehu (oldest dated lava 220 ka, Stipp, 1968); 2, Hauhungatahi (933 46 ka, Cameron et al., 2009); 3, Tongariro
(4340 ka, Grindley, 1960); 4, Maungakatote (5320 ka, Cole, 1978); 5, Kakaramea (220 ka, Stipp, 1968); 6, Tihia (undated); 7, Pihanga (130 ka,
Stipp, 1968); 8, Hauhungaroa (2·0 Ma, Stipp, 1968; identification of source cone, C. J. N. Wilson, unpublished data); 9, Rolles Peak (710 ka, B. F.
Houghton et al., unpublished data); 10, Rotokawa (41280 ka, Arehart et al., 2002); 11, Ngatamariki (41280 ka, Arehart et al., 2002); 12, Wairakei
(Browne et al., 1992); 13, Broadlands^Ohaaki (Browne et al., 1992); 14, Titiraupenga (1850 ka, Stipp, 1968); 15, Pureora (undated); 16, Waiotapu
(Browne et al., 1992); 17, Kawerau (Cole et al., 2010; C. J. N. Wilson, unpublished data); 18, postulated andesite cone engulfed by Mangakino rhyolite eruptions; 19, postulated andesite cone engulfed byTaupo rhyolite eruptions; 20, postulated andesite cone engulfed by Reporoa rhyolite eruptions; 21, Manawahe (430 ka, Broughton, 1988); 22, Edgecumbe andesite^dacite composite cone (3115 35 years BP, Carroll et al., 1997).
VO L C A N I C H I S T O RY O F T H E
TAU P O VO L C A N I C Z O N E
A number of publications have detailed the evolution of
volcanism in the TVZ (see Wilson et al., 1995, for an exhaustive summary). In this study, we simplify the volcanic
history by dividing it into two time periods (2·0^0·7 Ma
and 0·7 Ma^present; Fig. 2), the first being dominated by
andesite volcanism, whereas the second corresponds to the
time of rhyolite magmatism within the Central TVZ. We
highlight that the transition from andesite to rhyolite volcanism occurred over a significant timescale (200 kyr;
from 0·9 to 0·7 Ma), during which both magma compositions were erupting, but from spatially different regions
within the TVZ. It is important to note that the following
volcanic history is not inclusive of every eruption that has
occurred in the 2·0 Myr life of the TVZ, but focuses on
the major compositional trends that represent the dominant volume erupted.
First period: 2·0^0·7 Ma
This period is characterized by andesite volcanism and a
transition to rhyolite volcanism. The western boundary
of the TVZ is defined by three andesitic composite cones
(up to 2 Myr old; see Wilson et al., 1995) and the
Mangakino caldera (see Fig. 2), which became active at
1·6 Ma (Houghton & Wilson, 1995). The earliest
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eruptions from the Mangakino caldera include relatively
small rhyolitic and andesitic ignimbrite eruptions (Wilson,
1986). This period also includes the later eruption of two
large (4500 km3) rhyolitic magma bodies from the
Mangakino caldera (i.e. the Ongatiti and Kidnappers ignimbrites; Wilson, 1986). While this caldera volcanism was
occurring at Mangakino, at 1·2 Ma andesitic volcanism
was rejuvenated to the east (i.e. the Central TVZ; from
Wilson et al., 1995).
The locus of magmatism in the TVZ has progressively
migrated from the western margin of the TVZ eastwards
with time and by 0·9 Ma all volcanism had shifted east
of the Mangakino caldera, coincident with the onset of an
acceleration in rifting in the TVZ (i.e. the development of
the Taupo Rift; Wilson et al., 1995). The Taupo Rift is the
region of maximum extension and subsidence and lies
inboard from the western and eastern margins of the
TVZ. Here, andesitic lavas towards the axis of the rift are
in contact with the basement at 42 km depth, whereas
east and west of the rift andesite cones are still exposed at
the surface (see Fig. 2). Nearing the end of this period,
at 0·9 Ma, the locus of volcanism had completely shifted
eastwards and the Mangakino caldera became inactive
(Cole et al., 1981; Wilson et al., 1995).
Andesitic volcanism was prevalent in the Central
TVZ until 0·7 Ma. Evidence for widespread, andesitic,
composite cone-forming volcanism comes from (1) lithic
components in some later rhyolitic ignimbrites (e.g. the
Kaingaroa ignimbrite from the Reporoa caldera;
Beresford, 1997), (2) geothermal drillholes that have intercepted thick lava sequences (1^2 km thick in places) beneath Waiotapu, Ohaaki, Ngatamariki, Rotokawa and
Wairakei geothermal fields (Fig. 2; Browne et al., 1992;
Grindley et al., 1994; Arehart et al., 2002), and (3) exposed,
but eroded andesite cones, such as Rolles peak, that are
located on the eastern margin of the TVZ and represent
the waning stages of andesitic volcanism in the Central
TVZ at 0·7 Ma (Arehart et al., 2002).
ignimbrite, a pre-700 ka eruption of a quartz-, biotite-,
and hornblende-bearing rhyolite magma (Deering et al.,
2010).
By 0·34 Ma, rhyolitic volcanism was dominant in the
Central TVZ and was marked by a voluminous and widespread ignimbrite flare-up that erupted up to 3000 km3 of
rhyolitic magma from seven calderas over a period of 100
kyr (Gravley & Wilson, 2006). In total, almost 4000 km3
of rhyolitic magma has erupted in the Central TVZ from
0·34 Ma to the present (Wilson et al., 2009). In comparison,
andesitic volcanism in the Central TVZ over the same
period is extremely rare (e.g. the 3·4 ka Waimahia eruption; Blake et al., 1992). The beginning of the ‘ignimbrite
flare-up’at 0·34 Ma coincides with the emergence of andesitic volcanism to the south in what is now referred to as
the Southern TVZ (see Gamble et al., 2003). Andesitic volcanism was persistent in the Southern TVZ throughout
this period; however, it still represents 510% of the total
erupted volume of magma in the entire TVZ.
P E T RO L O G Y A N D I N T E N S I V E
PA R A M E T E R S
Bulk-rock geochemistry
The major and trace element geochemical characteristics
of the TVZ magmas have been discussed in a number of
studies (e.g. Cole et al., 1981; Reid, 1983; Graham et al.,
1992; Deering et al., 2008). The following two major observations are important.
Second period: from 0·7 to present
Rhyolitic ignimbrites erupted during times leading up to
and in the earlier portion of this period were sporadic
(e.g. the Akatarewa ignimbrite at 0·95 Ma, Wilson et al.,
2010; the Waiotapu ignimbrite at 0·71 Ma, Houghton et al.,
1995; the Utu ignimbrite at 0·55 Ma, Deering et al.,
2010) and caldera sources have not yet been confidently
delineated. There is evidence that more evolved and widespread magmatism was starting to occur at 0·55 Ma,
as revealed by (1) a 0·55 Ma peak in the zircon age
spectra for the 0·34 MaWhakamaru caldera-forming eruption (see Brown & Fletcher, 1999), (2) a shallow hornblende-bearing diorite (dacite) intrusive rock that was
intercepted by geothermal drilling and has an 0·55 Ma
age (Arehart et al., 2002), and (3) the 0·55 Ma Utu
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(1) In the post-0·7 Ma Central TVZ, the volcanism is
dominantly rhyolitic, but also clearly bimodal, showing a well-characterized compositional gap between
basalt and dacite (from 50^55 wt % SiO2 to
65 wt % SiO2; Fig. 3). There are a few units with
intermediate compositions; however, most of these
are mixed units and/or cumulate residues representing
both liquid þ crystal exchange and/or crystal accumulation (e.g. Gamble et al., 1993; Brown et al., 1998b;
Leonard et al., 2002; Wilson et al., 2006; Deering et al.,
2011).
(2) As first pointed out by Cole et al. (1981) and reinforced
by our newly expanded geochemical dataset (Fig. 4
and Electronic Appendix 1), the amphibole-bearing
dacite to rhyolite suite of the post-0·7 Ma Central
TVZ and the pyroxene-bearing basaltic andesite to
dacite suite from both the pre-0·7 Ma Central TVZ
and the Southern TVZ areas follow two different geochemical trends. The most obvious difference is
the higher K2O and Rb for a given SiO2 content in
the basaltic andesites to dacites compared with the
dacite to rhyolite suite (Fig. 4). This difference is most
clearly illustrated by the 10 ka eruption of both
two-pyroxene andesites and amphibole-bearing
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contain phenocrystic amphibole. Deering et al. (2008), however, showed that amphibole was present in varying
amounts in the parental magma that produced all the
Central TVZ rhyolite magmas, based on major and trace
element least-squares regression analysis, which indicated
cryptic amphibole fractionation (Davidson et al., 2007).
Intensive parameters
Fig. 3. Frequency distribution histogram of bulk-rock SiO2 for
Central TVZ basalts and rhyolites showing a compositional gap
(black) and buried andesite from the Central TVZ and exposed andesite volcanoes from the Southern TVZ (grey). Number of samples
roughly reflects the actual difference in the volume of erupted material. Data sources: Gamble et al. (1993), Karhunen (1993), Beresford
(1997), Brown et al. (1998b), Milner et al. (2003), Gravley (2004), Nairn
et al. (2004), Schmitz & Smith (2004), Wilson et al. (2006), Shane et al.
(2007), Deering et al. (2008). N, number of analyses.
dacites from the Tongariro volcano (Fig. 4; see also
Nakagawa et al., 1998).
Mineralogy
Andesites (and a few basaltic andesites) erupted from the
Southern TVZ and in the pre-0·7 Ma phase of the Central
TVZ are petrologically similar, although the modal proportions of minerals can vary. In general, the phenocryst
assemblage contains plagioclase þ clinopyroxene þ orthopyroxene olivine þ Fe^Ti oxides. The andesites have
been subdivided into a number of types to account for
subtle heterogeneities observed in their bulk-rock geochemistry and mineral assemblage (Graham et al., 1995). Dacite
eruptions are rare in the Southern TVZ, but at 10 ka a
crystal-poor amphibole-bearing dacite was erupted from
the Tongariro volcano along with a two-pyroxene andesite.
In contrast to the above andesite units, basaltic and andesitic quenched enclaves, gabbro^diorite, crystal-rich
dacite and andesite cumulate clasts entrained in dacite or
rhyolite from the Central TVZ typically contain phenocrystic amphibole (e.g. Graham & Worthington, 1988;
Beresford, 1997; Brown et al., 1998a; Wilson et al., 2006;
Deering, 2009). The more evolved rhyolites that dominate
the eruptive products, on the other hand, do not always
We used the clinopyroxene^liquid geobarometer of
Putirka (2008) to estimate the equilibration pressure of
the basaltic andesite to andesite lavas of the Southern
TVZ and pre-0·7 Ma Central TVZ eruptions at their
source. Clinopyroxenes with the highest Mg-number cores
were used in combination with the bulk-rock composition,
assuming that these were near-liquidus compositions.
Only clinopyroxene^liquid pairs that had KD(Fe^Mg)
values of 0·27 0·03 (Putirka, 2008), in accord with
temperature-dependent equilibrium ratios obtained from
experiments, were used for the calculations. The results of
these calculations indicate that these clinopyroxene crystals were in equilibrium with melt at pressures between
6·4 and 11·0 kbar (1·6 kbar), equivalent to crustal depths
between 21 and 33 km (assuming an average crustal density of 2·7 g cm3; Table 1 and Fig. 5). The empirical
method of Putirka (2008) also yields estimates of temperature, which range from 1200 to 10008C (458C) in accord
with estimates obtained for the 1995^1996 Ruapehu
andesites by Nakagawa et al. (1999) using the pyroxene
thermometer of Lindsley (1983).
Given the ubiquity of amphibole (and the near absence
of other suitable geothermobarometers that can be used to
compare all of these different rock types), we employed
the empirical amphibole geothermobarometer of Ridolfi
et al. (2010); the results are summarized in Table 2 with the
complete dataset available in Electronic Appendix 2
(http://www.petrology.oxfordjournals.org/). The results of
these calculations indicate a near-continuous range of temperatures from 7208C for the Central TVZ rhyolites up
to 10008C for the basalts at pressures ranging from 0·5
kbar to 4·2 kbar (0·1 kbar for the lowest pressures and
1·0 kbar for the highest pressures; Fig. 5). We note that
the lowest pressure estimates (0·5 kbar) for the rhyolites
are probably too low as other methods (e.g. CO2^H2O
melt inclusions; Taupo, Liu et al., 2006; Okataina, Smith
et al., 2010) indicate minimum pressures of 41·0 kbar.
Although these pressure estimates using the geobarometer
of Ridolfi et al. (2010) may be slightly too low, the relative
differences within our dataset indicate the approximate
range of pressure over which amphibole crystallized (up
to 7·0 kbar or 20 km of crustal thickness).
Within the post-0·7 Ma Central TVZ, there are, nonetheless, a few rock units that contain clinopyroxene, for
which pressure and temperature can be calculated and
compared with estimates for the basaltic andesites to andesites from the Southern TVZ using the Putirka (2008)
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Fig. 4. Variation of Rb (ppm) and K2O (wt %) vs SiO2 (wt %) for the TVZ basalts to rhyolites. Data sources: Graham & Hackett (1987),
Browne et al. (1992), Gamble et al. (1993), Karhunen (1993), Beresford (1997), Brown et al. (1998b), Nakagawa et al. (1998), Milner et al. (2003),
Gravley (2004), Nairn et al. (2004), Schmitz & Smith (2004), Shane et al. (2007), Deering et al. (2008). STVZ, Southern Taupo Volcanic Zone;
CTVZ, Central Taupo Volcanic Zone.
calibration. These units include (1) high-alumina basalt
sporadically erupted from monogenetic cones within the
Central TVZ and (2) rare, amphibole-bearing gabbro
and intermediate cumulates, erupted within rhyolitic
magmas, which also contain clinopyroxene (although typically rimmed by tschermakitic amphibole; Brown et al.,
1998a). As with the calculations for the basaltic andesites
to andesites from the Southern TVZ, we assume that the
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Table 1: Averages of clinopyroxene^liquid geothermobarometry forTVZ basalt or gabbro to andesite rocks
Eruptive unit
Rock type
Location
Sample(s)
P (kbar)
T (8C)
T (8C)
KD(Fe–Mg)
Eqn (31)
Eqn (33)
Eqn (34)
Eqn (35)
Atiamuri rhyolite
cognate hbl gabbro
Mr
SB5033
4·5
1166
1153
0·275
Kaharoa
HAB
Ok
H13
3·9
1126
1143
0·272
Tatua
HAB
Ka
10TAT1
5·6
1151
1141
0·274
Ruapehu
two-px andesite
TVC
Ru-1996-1; P57472A; 14855 MANG
9·0
1137
1127
0·281
Titiraupenga
two-px andesite
TVC
Type 1 and 2 (average); 3 (29571)
8·3
1155
1144
0·284
Ngauruhoe
two-px andesite
TVC
29250; DP16; TG001; TG020; TG192
5·8
1094
1112
0·270
Pukekaikiore
two-px andesite
TVC
DP13
9·2
1178
1148
0·287
Ohakune
two-px andesite
TVC
14798
6·4
1137
1152
0·281
Mr, Maroa volcanic complex; Ok, Okataina volcanic complex; Ka, Kapenga; TVC, Tongariro volcanic center. Data sources
for geothermobarometry calculations: Froude & Cole (1985), Hobden (1997), Brown et al. (1998a), Nakagawa et al.
(1999), Hiess et al. (2007). Geothermobarometry equations from Putirka (2008). HAB: High-alumina basalt.
Fig. 5. Pressure and temperature estimates for basalts to rhyolites using two methods: (1) clinopyroxene^liquid geothermobarometry
(Putirka, 2008); (2) empirical amphibole geothermobarometry (Ridolfi et al., 2010). Sources of mineral data used for the calculations: Patterson
& Graham (1988), Brown et al. (1998a), Nakagawa et al. (1999), Hiess et al. (2007). Vertical depth scale is estimated based on a crustal density of
2·7 g cm3. Description of crustal cross-section is based on interpretations of seismic velocity profiles from Harrison & White (2004) and
Reyners et al. (2006), and 2D inverse modeling of magnetotelluric data by Heise et al. (2007).
clinopyroxene is a near-liquidus phase so that the clinopyroxene cores and the bulk-rock compositions can be used to
calculate the temperature and pressure. These conditions
should provide an estimate of the ‘initial’ P^T conditions
at which the magmas began to crystallize within the
crust. These estimates (P ¼ 5·4^3·7 1·6 kbar and
T ¼11508C) indicate higher pressures and temperatures
than those obtained from the amphibole pseudomorphs
(P ¼ 2·5 0·05 kbar and T ¼ 9258C; Fig. 5). In summary,
the pressures estimated for the post-0·7 Ma Central TVZ
are lower than those obtained for the andesites of the
Southern TVZ and the pre-0·7 Ma Central TVZ, indicating that these magmas ascended and equilibrated at shallower depths.
2250
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Table 2: Average P^T^H2Omelt^fO2 of representativeTVZ basalt or gabbro to rhyolite rocks determined from amphibole
compositions
Eruption unit
Rock type
T (8C)
Uncertainty
P (MPa)
Uncertainty
Depth
NNO
Uncertainty
(km)
H2Omelt
Uncertainty
(wt %)
10 ka Tongariro
dacite
894
22
261
65
9·9
1·1
0·4
6·3
0·8
Atiamuri
cognate gabbroic inclusion
859
22
136
34
5·1
1·2
0·4
3·9
0·6
Whakamaru
mixed dacite
916
22
216
54
8·2
1·6
0·4
5·7
0·9
Whakamaru
rhyolite
827
22
124
31
4·7
1·2
0·4
4·9
0·5
Matahina
cumulate andesite/mixed
852
22
155
39
5·8
1·2
0·4
5·4
0·5
Matahina
rhyolite
813
22
118
30
4·5
1·2
0·4
5·1
0·5
Matahina
fine-grained basalt
803
22
109
27
4·1
1·5
0·4
5·4
0·4
Matahina
mixed basaltic andesite
902
22
201
50
7·6
1·0
0·4
4·9
0·7
Oruanui
mixed andesite
896
22
236
59
8·9
0·5
0·4
6·1
0·8
Oruanui
mixed basaltic andesite
919
22
305
76
11·5
0·3
0·4
6·6
1·0
Oruanui
rhyolite
833
22
158
39
6·0
0·8
0·4
5·7
0·5
Rotoiti
rhyolite
825
22
103
26
3·9
1·2
0·4
6·2
0·4
Young Mangaone
rhyolite
850
22
115
29
4·3
1·7
0·4
4·9
0·4
Utu
rhyolite
797
22
95
24
3·6
1·4
0·4
5·2
0·4
Chimp
rhyolite
802
22
94
23
3·5
1·3
0·4
4·9
0·4
P–T–H2Omelt–fO2 determined using the geothermobarometer of Ridolfi et al. (2010). Full dataset and calculations are given
in Electronic Appendix. Data sources: Brown (1994), Brown et al. (1998a), Hobden (1997), Schmitz & Smith (2004),
Wilson et al. (2006), Deering et al. (2008), Deering (2009).
OR IG I N OF T H E GEOC H E M IC A L
VA R I A B I L I T Y
The origin of the difference in the K2O and Rb trends
between the two-pyroxene basaltic andesites to dacites
and the amphibole dacites to rhyolites has not previously
been explored in detail. Instead, earlier studies focused on
other similarities and differences in the observed bulk-rock
geochemistry and radiogenic isotope compositions
(Graham et al., 1995; Price et al., 2005). One potential explanation for this difference in K2O and Rb could be that
the two-pyroxene basaltic andesite to dacite trend and the
Central TVZ dacite to rhyolite trend reflect different
amounts of assimilation of Mesozoic crustal lithologies.
If this model is correct, there should be a correlation between K2O/Rb and the Nd^Sr isotope composition (crustal lithologies have high K2O, high Rb, high 87Sr/86Sr and
low 143Nd/144Nd compared with the young volcanic
rocks). However, there is a lack of a correlation between
Sr and Nd isotope composition and K2O for samples
from the two distinct trends (Fig. 6), which precludes a
dominant control by assimilation. Therefore, the isotopic
overlap observed between the dacites to rhyolites
(low-K2O type) in the Central TVZ and the two-pyroxene
andesites in the Southern TVZ and buried within the
Central TVZ (high-K2O type) is probably the result of
similar degrees of assimilation in the lower crust (as proposed by several other studies; e.g. Graham et al., 1995;
Price et al., 2005; Deering et al., 2008).
Another possible scenario to explain the two geochemical trends is the development of two liquid lines of descent
governed by variable pressure, temperature, oxygen fugacity or water content, in crustal magma reservoirs, which
led to different crystallizing assemblages. Experiments by
Kawamoto (1996) that have evaluated magma evolution
starting from a basaltic composition at crustal depths,
similar to that observed in the North Island, New
Zealand, have shown slight differences in K2O content for
a given SiO2 content for melts derived from basalt crystallization with H2O contents of 1 or 2 wt %. The small difference in H2O content in the initial melt conditions was
enough to change the phase proportions (higher percentage of amphibole and oxides in the wetter experiment),
inducing noticeable changes in the composition of the liquids at a given degree of crystallinity. In experiments
with higher initial H2O contents, amphibole is stabilized
early and, as a result, the compatibility of K2O in amphibole produces a diverging trend with a shallower slope
than that of the experiments with lower initial H2O contents [Fig. 7; similar results were shown by Beard &
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Fig. 6. Variation of 143Nd/144Nd and 87Sr/86Sr vs K2O (wt %) for the TVZ basalts to rhyolites. Data sources: Graham & Hackett (1987),
Graham & Worthington (1988), Patterson & Graham (1988), Browne et al. (1992), Gamble et al. (1993), Karhunen (1993), Sutton et al. (1995),
Beresford (1997), Brown et al. (1998b), Milner et al. (2003), Gravley (2004), Schmitz & Smith (2004), Deering et al. (2008).
Lofgren (1991)]. It is, therefore, reasonable that the differences in K2O (and Rb) are related to the aH2O or
amount of H2O in the primitive magmas, which is also
qualitatively supported by the presence of amphibole in
the low-K2O dacite and its absence in high-K2O dacite.
For the purpose of evaluating the origin of the diversity
between the basaltic andesites to andesites and dacites to
rhyolites in the TVZ, we use, in multiphase simulations,
the two liquid lines of descent defined by the experiments
of Kawamoto (1996), but with the most primitive TVZ
2252
DEERING et al.
ANDESITE TO RHYOLITE VOLCANISM
Fig. 7. Comparison of experimental results on calc-alkaline basalt from Kawamoto (1996) under different starting P^aH2O conditions. Shaded
areas represent melt compositions within the 50^70% crystal fraction window.‘Hot-andesite’ refers to the two-pyroxene andesite magma type
produced under relatively dry conditions at high temperature, whereas ‘cold-dacite’ refers to the amphibole-bearing dacite magma type produced under wetter conditions at slightly lower temperatures.
basalt as the starting composition (Kakuki; Gamble et al.,
1993).
N U M ER ICA L MODELS
Physical process of crystal fractionation
To cause the chemical differentiation observed in the TVZ
magmas by crystal fractionation, viscous silicate melt
must be efficiently separated from millimeter-sized crystals. In a recent contribution, Dufek & Bachmann (2010)
argued that crystal^melt separation is optimized within a
crystallinity window that extends between 50 and 80 vol.
% crystals, depending on the ambient conditions. This optimal melt extraction window arises because (1) at
near-liquidus conditions, magmas cool rapidly, leaving
little time for phase separation (Marsh, 1981; Huber et al.,
2009), and convection currents maintain close physical
coupling between melt and crystals (Burgisser et al., 2005),
and (2) at near-solidus conditions, silicic melt extraction
by compaction of the crystal framework is extremely slow
(McKenzie, 1985; Bachmann & Bergantz, 2004) despite
the longer time the magma spends at high crystal fractions
(Koyaguchi & Kaneko, 1999; Huber et al., 2009).
To determine the composition of the magmas most likely
to erupt in the TVZ, we performed multiphase simulations
using the numerical model developed by Dufek &
Bachmann (2010) with two liquid lines of descent (one dry
and one slightly wetter; see below). The simulations consider a 100 m thick sill that intrudes at its liquidus temperature at various levels in the crust (30, 20 and 10 km below
the surface; see justification of crystallization depth
below) and is permitted to cool against the surrounding
crust. The initial wall-rock temperature of the sills is determined by their position relative to a steady-state geothermal gradient (calculated using a surface heat flow of 100
mW m2). Although the heat flux certainly must have
varied in space and time in the TVZ and the magma
bodies may be of larger or smaller extent than those modeled here, the general trend of preferential melt extraction
in a narrow crystallization window remains robust to variations in wall-rock temperature and intrusion size (Dufek
& Bachmann, 2010). We use a 2 m resolution in the 2D
simulations near the intrusion, and the simulations are
coupled to 1D thermal models of the entire crustal section.
A summary of the simulation conditions is provided in
Table 3 and further details on the numerical approach
have been given by Dufek & Bachmann (2010). The simulations do not consider the presence of a volatile phase.
Buoyant volatiles may help to speed up melt extraction,
but only in the upper crust; volatile exsolution is generally
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Table 3: Summary of conditions in numerical simulations
Initial comp.
H2O (wt %)
Depth of
Aspect ratio
Melt fraction
Liquidus (injection)
intrusion (km)
(height/width)
relation
temperature (8C)
Case 1
Basalt*
1·0
30
0·25
MELTS
1308
Case 2
Basalt*
2·0
20
0·25
Kawamoto, 1996
1200
Case 3
Dacite
4·0
10
0·25
MELTS
986
*Starting composition used was the most primitive TVZ basalt (Kakuki; Gamble et al., 1993).
Density, heat capacity and viscosity are derived from parameterizations from MELTS simulations (Ghiorso & Sack, 1995).
considered to have a minor effect on melt extraction rates
(Sisson & Bacon, 1999; Bachmann & Bergantz, 2004).
The different modal proportions of minerals, as well as
the major-element chemistry, viscosity and density of
melts at a given crystallinity were estimated using
MELTS simulations (Ghiorso & Sack, 1995) for the ‘dry’
(1wt % H2O) liquid line of descent and the experiments
of Kawamoto (1996) for the wetter (2 wt % H2O) liquid
line of descent. The MELTS program could not be used
for the ‘wet’ melt evolution path because it is not calibrated
for amphibole saturation. However, it does accurately reproduce the mineral assemblage for the ‘dry’ magma evolution case at a given temperature and pressure and also the
melt characteristics (SiO2 content, viscosity, density).
These melt evolution paths were used to assess the behavior of crystal^melt separation in crystal mushes following
the two liquid lines of descent in the TVZ.
Production of andesite from a deep
(20^40 km depth) dry mush zone
There are several independent lines of evidence that suggest that the depth of differentiation during andesite volcanism in the Southern TVZ and the pre-0·7 Ma Central
TVZ is at or near the base of the crust (30^40 km).
Pressure estimates from clinopyroxene^liquid compositions are between 6 and 12 kbar, equivalent to 20^40 km
depth. These estimates are consistent with seismic imaging
that currently places a melt-bearing zone at or near the
crust^mantle boundary beneath the Southern TVZ
(40 km depth, Reyners, 2010).
Basalt crystallization experiments under fairly dry conditions (1wt % H2O) and at the pressures expected for
the lower crust in the Southern TVZ (6^12 kbar) produce,
after 50^80% crystallization, magmatic liquids with temperatures and SiO2 contents similar to those observed in
the andesitic units of the arc (e.g. Wolf & Wyllie, 1994;
also see Fig. 4). For dry, lower crustal conditions, the incipient crystallizing phases at high temperature in the basalt
(mostly pyroxenes and to a lesser extent plagioclase) do
not allow the liquids to increase in SiO2 (e.g. Wolf &
Wyllie, 1994; Kawamoto, 1996). However, as temperatures
decline to 1100^10008C, Fe^Ti oxides start crystallizing,
and as the crystal fraction increases to 50^80%, the melt
in equilibrium with the phase assemblage may reach 58%
SiO2.
Using the multi-phase numerical modeling procedure of
Dufek & Bachmann (2010) for 100 m thick sills and a dry
basalt phase diagram at high pressure (derived from
MELTS; Ghiorso & Sack, 1995), we show that the liquid
with the greatest probability of being extracted from such
a magma reservoir, and therefore erupting at the surface,
is an 1100^10008C pyroxene-bearing andesite (Fig. 8a
and b). As a result of rapid cooling and efficient stirring at
low crystal fractions, basalt rarely reaches the surface
(51% of the total erupted volume is basaltic). Dacitic eruptions are also very rare (55% of the erupted volume in
the Southern TVZ) as dacitic melts are not formed until
very high crystal fractions (480 vol. %), at which point
the effectiveness of melt extraction becomes severely hindered by the decrease in permeability. Importantly, the andesite and dacite isotopic compositions overlap (e.g. Fig.
6), implying that the dacitic eruptions are truly higher
SiO2 liquids extracted at higher crystal fractions in the
mush, rather than being just the products of an anomalously higher degree of assimilation. We note that for thinner
sills the preferential extraction window typically peaks at
5^10% fewer crystals, owing to the diminished time the
sill remains above its solidus (Dufek & Bachmann, 2010).
However, this would result in only a 1^2% reduction in
magmatic SiO2 wt %.
Production of dacite at intermediate
pressure (15^25 km depth) and wetter
conditions
The distinct differences in bulk-rock geochemistry (high
and low K2O; Fig. 4) between the dacites and the
pyroxene-bearing andesites suggest different P^T^aH2O
conditions of crystallization. The temperature interval of
the Central TVZ dacitic eruptions is 1000^9008C and
most contain amphibole (e.g. Mt Edgecumbe: Carroll,
2254
DEERING et al.
ANDESITE TO RHYOLITE VOLCANISM
Fig. 8. Multiphase numerical models showing the probability of extracting melt from different initial magma compositions as they crystallize at
different depths. All compositions have been normalized so that the integrated probability for extracting a melt is equal to 100%. (a, b)
Probability of extracting basaltic andesite to dacitic interstitial melts from a ‘dry’ basaltic crystalline residue in the lower crust. These numerical
results considered sill thicknesses of 100 m intruded at a reference depth of 30 km. (c, d) Probability of extracting dacitic to rhyolitic interstitial
melts from a ‘wet’ basaltic crystalline residue in the lower crust. These numerical results considered sill thicknesses of 100 m intruded at a reference depth of 20 km. (e, f) Probability of extracting rhyolitic interstitial melts from a‘wet’ dacitic crystalline residue in the upper crust. These numerical results considered sill thicknesses of 100 m intruded at a reference depth of 10 km. The grey-shaded band represents the optimal
extraction window (a, c, e) and the dominant magma type (in SiO2) produced within that window (b, d, f). In the top panel, estimates of the
volume of cumulate residue required during production of the erupted andesite and rhyolite are given. In the bottom panel, the relative proportions of erupted magma types within the optimal extraction window are given.
1998; Tauhara: Graham & Worthington, 1988), which indicates that these magmas were produced at lower temperatures and under wetter conditions than the andesites from
the Southern TVZ and pre-0·7 Ma Central TVZ.
Crystallization experiments on basalts (e.g. Kawamoto,
1996; Sisson et al., 2005) confirm that liquids of dacitic
composition can be produced at the 50^80% crystal fraction under conditions similar to those for the natural samples (Fig. 7; P ¼ 5^7 kbar and T ¼1000^9008C) at aH2O
near 2 wt % (owing to the crystallization of amphibole
and Fe^Ti oxides; Grove & Donnelly-Nolan, 1986).
We note that the amount of water necessary to stabilize
amphibole and Fe^Ti oxides before the extraction window
(and therefore to drive up the SiO2 content to a dacitic
composition) is only twice that of the dry experiments
of Kawamoto (1996) that are consistent with the conditions under which the pyroxene-bearing andesites were
generated.
Using the same simulation technique as for the dry
basalt case, but changing the phase diagram to account
for the presence of amphibole and Fe^Ti oxides as early
crystallizing phases, we can evaluate the probability of
eruption of different melt compositions from an initially
basaltic magma at 20 km depth using the pressure estimates obtained by amphibole geobarometry (Table 2 and
Fig. 5) with 2 wt % H2O (Fig. 8c and d). At the extraction window, the melt composition is dacitic instead of andesitic as in the deeper^drier case (owing to the slightly
different phase diagram), and we should expect these
dacitic melts to be abundant at the surface. However, the
volume of erupted dacite is volumetrically insignificant in
the Central TVZ, requiring trapping of melts in the crust
and another step of differentiation.
Production of rhyolite in the upper crust
After more than a million years of active magmatism, dacitic melts produced from a lower to mid-crustal mafic
mush zone are likely to be trapped in the upper crust for
the following reasons:
2255
(1) As the continental crust becomes more thermally
mature from the increase in heat flow, the warm
crust will change from brittle to viscoelastic behavior.
These rheological conditions favor growth and storage of silicic magma in the mid- to upper crust rather
than frequent eruptions (Jellinek & DePaolo, 2003).
JOURNAL OF PETROLOGY
VOLUME 52
(2) As wet magmas ascend through the crust, they reach
water saturation, which promotes a viscosity increase
by decompression-driven crystallization (Blundy &
Cashman, 2001; Tamura & Tatsumi, 2002; Brophy,
2009).
Models for rhyolite petrogenesis in the TVZ developed
by Deering et al. (2008, 2011) indicate that cognate,
crystal-rich andesite clasts erupted within crystal-poor
rhyolite during the caldera-forming Matahina eruption
(330 ka) are the cumulate residues formed through
extraction of 50% of the interstitial liquid from an initially dacitic crystalline mush. Although the storage temperature of the cumulates is similar to that of the rhyolite
(8008C), the amphibole phenocrysts also show evidence
of repeated high-T fluxes (temperatures up to around
9508C), similar to the temperature of erupted dacites (e.g.
Carroll, 1998), reflecting periodic increases in the ambient
temperature of the magma during recharge (Deering,
2009). Importantly, these cumulate clasts contain abundant amphibole, a requisite for the production of the TVZ
rhyolites (Deering et al., 2008). Hence, we assume that
dacite is the intermediate progenitor to the rhyolite.
Using our numerical simulation scheme, an evaluation
of the composition of melts extracted from a dacitic reservoir (10 km depth), indicates that rhyolites are the most
likely liquids to erupt at the surface. The low proportion
of erupted dacite is predicted as magmas will spend little
time in the low-crystallinity, convective, period of the
magma evolution (Fig. 8e and f). On the other hand, the
erupted rhyolite is volumetrically dominant, which is also
predicted as the dacite will spend most of its time in the
50^80% crystal fraction window, in equilibrium with a
rhyolitic melt. Zircon becomes saturated at roughly dacitic
compositions and a long accumulation time is recorded by
the U^Pb and U^Th age spectra in zircon in the TVZ
rhyolites (e.g. Brown & Fletcher, 1999; Charlier et al.,
2005)çthis represents the minimum time for the production and accumulation of large volumes of parental dacitic
magma in the upper crust on the order of 105 years.
Volume estimates
Previously proposed models for the origin of the voluminous rhyolites erupted in the TVZ (46000 km3) that invoke
crystal fractionation or partial melting of various crustal
lithologies have been hampered by the volumetric constraints required to provide enough mass or enough enthalpy, respectively. In our model, the volume of parental
dacite produced in the lower to middle crust can be estimated by making several assumptions within the constraints of our extraction models: (1) the mid- to lower
crust is heavily intruded by gabbro (e.g. Harrison &
White, 2004), which represents the amphibole þ plagioclase basaltic cumulate residues from extraction of dacite
liquid; (2) the entire areal extent of the Central TVZ
NUMBER 11
NOVEMBER 2011
(120 km 40 km) accommodates this 15 km thick gabbroic
cumulate (total of 7·2 104 km3); and (3) a conservative
50 vol. % of the melt is extracted from the basaltic
mush at 50 vol. % crystals. Using these criteria, the estimated volume of dacite intruded into the upper crust is
1·8 104 km3. In turn, and again using a 50 vol. % extraction from a mush at 50 vol. % crystals, an estimated
4·5 103 km3 of rhyolitic melt could be extracted from
the dacite. This is a close match to the estimated volume
of rhyolite erupted in the Central TVZ in the last 0·7 Myr
[4·0 103 km3; combined data and estimates from
Wilson et al. (2009) and Deering et al. (2010)], leading to
an upper crustal plutonic:volcanic ratio of 3:1 [similar
to that suggested by White et al. (2006)], whereas the total
crustal plutonic:volcanic ratio (including the lower crust)
is much higher (10:1).
The key aspect of our model that obviates the volume restrictions is the crystallization of amphibole and Fe^Ti
oxides in the basaltic parent magma in the lower crust;
this mineral assemblage drives a higher volume of the
extracted liquid to a higher SiO2 content (dacite) at a
lower crystallinity compared with the crystallization of
pyroxene and Fe^Ti oxides. Therefore, even using relatively conservative calculations for crystal^liquid separation
as outlined above, the volume constraints required that
are not easily met by previous petrogenetic models for the
generation of rhyolite can be accommodated.
DISCUSSION
Alternative model for rhyolite production
in the TVZ
Many volcanic arcs display gaps in the bulk-rock compositions of the magmatic rocks [see Brophy (1991) for numerous examples]. Following similar reasoning to that of
other studies (e.g. Tamura & Tatsumi, 2002) Price et al.
(2005) suggested that the gap was generated by partial
melting of an andesitic progenitor (i.e. anatexis of previously emplaced igneous rocks through a prolonged thermal
‘pre-conditioning’ of the crust) based on the presence of
rhyolitic glass in andesites from the Southern TVZ.
Although this model is attractive in explaining the compositional gap, there are several reasons that make it unlikely, as follows:
2256
(1) This type of process does not easily explain the presence of two distinct liquid lines of descent (i.e. conditions of crystallization).
(2) Remelting of solidified andesite is energetically costly
as recent numerical models have shown (Dufek &
Bergantz, 2005), especially at shallow crustal levels
such as in the TVZ case.
(3) Huber et al. (2009) showed that a magma would
remain in a highly crystalline mush state for a considerable period of time before fully freezing.
DEERING et al.
ANDESITE TO RHYOLITE VOLCANISM
In addition, as the inception of rhyolitic volcanism
seems to be coincident with an increase in the rate of
extension at 0·9 Ma, this change in the structural state
of the crust would promote mantle upwelling and
higher level intrusions into the crust that would progressively increase, not decrease, the heat flow to this
region, which would promote the maintenance of an
active mush column, rather than allowing it to freeze.
In addition to the points raised above for discounting a
petrogenetic model that invokes partial melting of a previously emplaced igneous forerunner of the TVZ rhyolite
(Price et al., 2005), we can use the mineral record to test
this model directly. If crustal melting of igneous forerunners produced the TVZ silicic magmas, we would
also expect a compositional gap in the mineral phases,
even for those that can record continuous solid solution
(e.g. plagioclase, pyroxenes). The two expected mineral
populations would be (1) one in equilibrium with hightemperature basalt, and (2) one in equilibrium with
low-temperature rhyolite. In contrast, if the bulk-rock compositional gap is produced by the discontinuous release of
melts from crystallizing mushes, the compositional ranges
present in minerals are expected to be continuous as the
crystals would precipitate from the whole range of possible
liquids.
A newly compiled mineral composition database (for
plagioclase, pyroxene, and amphibole; see Electronic
Appendices 2^4) from a number of the eruptions from the
Central TVZ (based only on juvenile, amphibole-bearing
clasts of basalt or gabbro to rhyolite; Beresford, 1997;
Brown et al., 1998a; Milner et al., 2003; Schmitz & Smith,
2004; Wilson et al., 2006; Deering et al., 2010) indicates that
the crystals grew from a continuum of melt compositions.
Plagioclase compositions taken from experiments using
basalt and rhyolite as starting materials form two restricted ranges, An80^96 and An0^50, respectively (Fig. 9).
The plagioclases from the natural samples, however, span
the entire range from An0 to An96 (Fig. 9). Although the
An50^80 plagioclase compositions are under-represented,
this is due to the small number of amphibole-bearing
basalt^dacite clasts that have been analyzed relative to
the hundreds of rhyolitic clasts (Beresford, 1997; Milner
et al., 2003). In addition, Beresford and Milner et al. did
not perform detailed analytical traverses on the crystals,
as that was beyond the scope of those studies. However,
core and rim analyses clearly indicate strong normal
zoning in some crystals and record a range for the
melt-bearing gabbros (Brown et al., 1998a; core to rim:
An83^36).
Similar to the plagioclase population, pyroxenes (clinoand orthopyroxene) from the experiments form two
restricted ranges for basaltic and rhyolitic initial compositions (Mg-number 80^90 and 555, respectively; Fig. 9).
The natural pyroxenes lack a compositional gap and instead form a continuous set of compositions from
Mg-number 35 to 90 (Fig. 9).
The samples that primarily fill the compositional gaps
(for plagioclase An50^80; for pyroxene Mg-number 55^80)
come from the andesite mush and variably mingled or
mixed intermediate inclusions from the Matahina eruption
(Deering et al., 2011), the quenched and/or mingled or
mixed mafic inclusions of the Oruanui eruption (Wilson
et al., 2006), and the amphibole-bearing, melt-bearing
gabbro exhumed during the rhyolitic Atiamuri eruption
(Brown et al., 1998a). In particular, the intermediate mush,
located within the mid- to upper crust, is envisaged to be
a major transfer zone where antecrysts are carried from
the lower to the middle crust and might ultimately reach
the surface. These crystals often form glomerocrysts that
include the major phases (plagioclase, amphibole, pyroxene and Fe^Ti oxides). This zone remains at or above the
rheological locking point for viscous intermediate
magmas (450 vol. % crystals) and consequently, is largely
uneruptableça point made obvious by the rarity of samples of this type collected.
In summary, the mineral phases do not record a compositional gap and the complete range of compositions in
the minerals cannot be explained simply by differing conditions (aH2O and/or fO2) in the observed natural basalt
or rhyolite or by mixing between the two contrasting compositions, which often produces thin overgrowths on host
phenocrysts (e.g. Browne et al., 2006). Therefore, based on
this evidence, a model that invokes melting of an andesitic
progenitor to produce a minimum melt (rhyolite) appears
unnecessary.
Origin of the change in basalt
crystallization sequence?
To generate the two liquid lines of descent, one that produces a hot andesite and one that produces a cooler dacite
to rhyolite sequence, the composition of the fluid and/or
amount of H2O in the two systems must vary. The cause
of this change in the TVZ remains unclear; however,
we envisage at least two possible processes that could participate in raising the water content or changing the activity of water in the melt in the Central TVZ magmas: (1) a
higher amount of water in the mantle source and/or (2) a
change in the activity of H2O by increasing the H2O/
CO2 ratio in the melt by preferential degassing of CO2 at
depth; CO2 is much less soluble than H2O in magmas
(Lowenstern, 2000; Papale et al., 2006).
A model that invokes an increased amount of water in
the primitive magmas in the TVZ (e.g. focusing of fluid
flux) during the modern extensional period was recently
suggested by Reyners et al. (2007) based on 3D seismic velocity models. Observing swarms of earthquakes beneath
the TVZ, they concluded that fluid movement must occur
in the heavily faulted Central TVZ. This mechanism of
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VOLUME 52
NUMBER 11
NOVEMBER 2011
Fig. 9. Frequency distribution histograms of plagioclase and pyroxene compositions reported as An [Ca/(Ca þ Na þ K)] and Mg-number
[Mg/(Mg þ Fe)], respectively. The condition for selecting the mafic rocks is that they are crystal-rich, amphibole-bearing and have interacted
with the rhyolite directly. The dataset does not include dacitic magmas that have erupted at the surface individually (e.g. Mt. Edgecumbe,
Mangaone, Taupo, Tauhara dacites). Grey-shaded bands represent plagioclase and pyroxene compositions in equilibrium with basalt
(410508C) and dacite^rhyolite (950^7758C and H2O 5^10 wt %) magmas determined from experiments (basalt: Kawamoto, 1996; Mu«ntener
et al., 2001; dacite^rhyolite: Scaillet & Evans, 1999; Costa et al., 2004; Holtz et al., 2005). Data sources: Beresford (1997), Brown et al. (1998a),
Milner et al. (2003), Schmitz & Smith (2004), Wilson et al. (2006) and C. D. Deering (unpublished data).
2258
DEERING et al.
ANDESITE TO RHYOLITE VOLCANISM
transition from andesite to mostly dacite^rhyolite volcanism has also occurred, is less obvious. One possibility is
that new intrusions are emplaced over the previous ones
and eventually lead to a progressive shallowing of the
accumulating crystal mushes. Another possibility might
involve underaccretion (recharge magmas would stop
below previous intrusions) and build, over time, a deeper
mush processing primitive basalts into slightly wetter
more evolved basalt or basaltic andesites at the extraction
window (H2O being incompatible in such magmas) that
would then differentiate to dacite at mid-crustal levels.
CONC LUSIONS
Fig. 10. CO2^H2O vapor^liquid partitioning calculated at crustal
level pressures in the most primitive TVZ basalt (Kakuki) with 2 wt
% H2O and 1wt % CO2 at 10008C using the solubility model of
Papale et al. (2006).
fluid focusing within the volcanic zone would provide an
explanation for wetter magmatism associated with the
strong extensional environment of the Central TVZ. An alternative model is that the amount of water in the source
did not increase much, but that the XH2O increased in
the TVZ primitive magmas, owing to changes in the
H2O/CO2 ratio in the melt by preferential degassing of
CO2 as magmas are trapped at shallower crustal levels.
This process requires that the basaltic magmas are CO2saturated at depth and without good estimates of the concentration of CO2 in the primitive TVZ magmas (at present unavailable), the exact depth at which CO2 bubbles
form cannot be obtained. However, using reasonable contents of CO2 between 0·5 and 1wt % (Fischer & Marty,
2005; Blundy et al., 2010), Wallace (2005) showed that basaltic magmas could be saturated with a gas phase at pressures between 5 and 7 kbar, and that a decrease in
pressure of just 2 kbar (from 7 to 5 kbar) can induce a
30% higher H2O/CO2 ratio in the melt (Fig. 10; Papale
et al., 2006).
The reason for the shallowing mafic magma intrusions
with time could be related to rifting in the Central TVZ.
As mentioned above, an acceleration in extension occurred
at 0·9 Ma (Wilson et al., 1995) and has continued to the
present. This is consistent with a recently developed
model whereby mafic magma injections and silicic magmatism have assisted the rifting in the Central TVZ
through time (see Rowland et al., 2010). The result is a positive feedback loop between magmatism and rifting, promoting basaltic injection and rheological trapping of these
magmas higher up in the crust with time.
Why shallower intrusions occur in weakly to strongly
compressive environments such as the Central Andes or
Southern Rocky Mountain volcanic fields, where the
The transition from typical arc-type hot^dry andesitic volcanism to overwhelmingly dominant rhyolite volcanism in
the TVZ is intimately associated with a change from a pyroxene^plagioclase-dominated liquid line of descent to one
that reflects the conspicuous presence of hornblende and
Fe^Ti oxides. As crystal fractionation (þ assimilation) is
considered the main differentiation mechanism in this
region, we use the recently proposed concept of an ‘extraction window’ in crystal mushes (50^80% crystallinity) at
different depths in the crust (Dufek & Bachmann, 2010) to
generate melts of different composition. In the older part
of the Central TVZ (pre-0·7 Ma) and at present in the
southern part, andesitic melts can be produced from a
lower crustal mush by 50^80% crystallization of dry basalts (1wt % H2O, stabilizing mostly pyroxene and
plagioclase). Upon reaching the favorable crystallinity
window for crystal^melt separation to occur, melts can
escape upwards and ascend to the surface. However, for
the last 0·7 Myr, the Central TVZ has seen a shift in the
composition of erupted products. To generate the observed
rhyolitic (and much rarer dacitic) units still within the extraction window, a change in the liquid line of descent is
required. By crystallizing early, low-SiO2 minerals, such
as hornblende and Fe^Ti oxides, basalts can produce a dacitic melt after 50^80% crystallization. Most of these dacitic melts then rise into the upper crust, where they
become trapped and crystallize to 450%, forming an
upper crustal mush where large amounts of rhyolitic melt
are stored that are eventually expelled to the surface.
The change from a dry (pyroxene^plagioclasedominated) to a wetter (hornblende^Fe^Ti oxide dominated) liquid line of descent in the deep crustal mush
zones could be related to an increased amount of water in
the mantle, consistent with the interpretation of 3D seismic
velocity models by Reyners et al. (2007) that indicate fluid
focusing within the rifted portion of the arc. As the switch
from andesite to rhyolite volcanism is synchronous with
the onset of strong extension in the area, this mechanism
would explain why primitive magmas became wetter
rather than drierças would be expected during an extensional period favoring a larger component of
2259
JOURNAL OF PETROLOGY
VOLUME 52
decompression melting. An alternative explanation is that
the early crystallization and conspicuous presence of hornblende (and Fe^Ti oxides) during the last phase of volcanism in the Central TVZ is due to a rift-induced
shallowing of the depth of basaltic intrusions and a related
increase in the XH2O in the melt owing to preferential
degassing of CO2 at depth. The latter model can also be
applied to non-rifted arcs that also show the transition
from andesite to dacite^rhyolite volcanism (Southern
Rocky Mountain volcanic field, Lipman, 2007; central
Andes, Grunder et al., 2008), although the transition in
such arcs seems to take longer (4^6 Myr instead of 1 Myr
for the TVZ) and the shallowing of the basaltic intrusions
must be due to a process other than extension.
AC K N O W L E D G E M E N T S
We thank George Bergantz and Chris Huber for their unfailing support and numerous discussions on the topics
expressed in this paper. J. Davidson, R. Price, and an anonymous reviewer are thanked for thoughtful and constructive reviews that greatly improved this paper.
F U N DI NG
O.B. was supported by NSF-EAR grant 0809828 and J.D.
by NSF-EAR grant 0838200.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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