zellmar (2012) petrogenesis of sr rich adakitic rocks at volcanic arcs insights from global...
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Baker et al. 1994; Borg et al. 1997; Streck et al. 2007); in the
central Andes, adakitic rocks have been identified at Parinacota
and Tata Sabaya volcanoes (Davidson et al. 1990; de Silva et al.
1993); in SW Japan, adakitic volcanism occurs in SW Honshu
(Morris 1995; Kimura et al. 2005), and has most recently been
identified as far south as NE Kyushu (Sugimoto et al. 2006); and
in northern Kamchatka, Shiveluch volcano erupts adakitic lavas
(Yogodzinski et al. 2001). Spatial geochemical variations in the
volcanic products in northern Kamchatka have previously been
attributed to increasing contributions from decompression melt-
ing of upwelling asthenospheric mantle towards the north
(Portnyagin et al. 2005).
Figure 4 presents the composition of neovolcanic products
with SiO2 >56 wt% from global volcanic arcs on adakiteADR-type discrimination diagrams. It is noteworthy that a limited
number of adakitic rocks are found in a range of arcs, including
some that do not display excess surface heat flux (see Fig. 4i).
However, adakitic rocks in excess heat flux arcs are characterized
by significant Sr enrichment, typically to .700 ppm. Such highSr contents not only indicate an absence of plagioclase in the
source of these samples, but in addition place some constraints
on the proportion of minerals that may host Y (DY .1), but littleor no Sr (DSr 0). The effect of fractional crystallization andpartial melting of key minerals is reviewed in Figure 4a, using an
average primitive oceanic arc basalt as source composition. The
melt evolution trends demonstrate that strong Sr enrichment
cannot be achieved without a significant proportion of garnet as
part of the phase assemblage, unless extremely low residual melt
fractions are postulated.
Sr-rich adakitic rocks predominantly occur in the Mexican
Volcanic Belt west of c. 1008W, in some Central Andeanvolcanoes north of c. 208S, and in the Californian Cascades southof c. 428N (Fig. 4bd), although they have also been found at anumber of sites in the north of SW Japan and Kamchatka, and
the south of the Philippines and ColombiaEcuador segments. In
Figure 5, it is demonstrated that surface heat flux excess linearly
increases with the arc proportion along which Sr-rich adakitic
compositions are erupted (see the Appendix). Thus, there appears
to be a strong link between excess surface heat flux caused by
the prevalence of higher-temperature, less hydrous magmas, and
the generation of Sr-rich adakitic rocks.
Discussion
Evidence for slab discontinuities
This study quantitatively links Sr-rich adakitic rocks to observed
excess heat fluxes at volcanic arcs (Fig. 5). Excess heat flux in
turn appears to be related to the ascent of higher temperature,
less hydrous magmas (see above). What is the origin of these
higher temperature, less hydrous magmas? It is noteworthy that
in all excess heat flux arcs, with the exception of New Zealand,
where elevated heat flux may be expected from intra-arc rifting
(Bird 2003), there is evidence for slab discontinuities. For
example, beneath western Mexico, rollback of the steeply
dipping (c. 508, Pardo & Suarez 1995) Rivera slab has previouslybeen suggested to have resulted in its separation from the
subhorizontal Cocos plate and lateral influx of asthenospheric
mantle (Ferrari et al. 2001), and slab detachment in the late
Miocene (Calmus et al. 2003) coupled with slow subduction (c.
2 cm a1, Pardo & Suarez 1995) implies that the leading edge ofthe Rivera slab is at a depth of not significantly more than
200 km. In northern Chile, recent seismic studies indicate that
the subducting slab undergoes extension and possibly tearing at
218S (Rietbrock et al. 2006). Beneath the southern Cascades,east of the Mendocino triple junction, the southern edge of the
Juan de Fuca plate is subducted (e.g. Beaudoin et al. 1996). In
SW Japan, slab dip changes in response to a concave seaward
corner of the active margin, and tensional slab rupturing just
north of Kyushu has been observed (Zhao et al. 2002). Slab
detachment and edge subduction has also been identified in
northern Kamchatka (Levin et al. 2002). Discontinuities in the
subducting slab may allow upwelling of anhydrous and hot
asthenospheric mantle from the sub-slab region into the overlying
mantle wedge, potentially coupled with associated decompression
melting. Below, the geochemical effects of this process are
discussed.
Effect of water on phase stability fields
Models for the petrogenesis of adakitic rocks in unusual tectonic
settings, where hot asthenospheric material facilitates the partial
melting of older, cool subducted slabs, have been put forward
previously (e.g. Yogodzinski et al. 2001; Calmus et al. 2003).
However, upwelling of hot asthenospheric mantle will also
facilitate partial melting of previously intruded hydrous arc
basalts in a deep hot zone (Annen et al. 2006). The genesis of
Sr-rich adakitic melts requires garnet to be part of the phase
assemblage unless very low residual melt proportions are
postulated, and indicates that plagioclase is not a residual phase.
Experimental studies have shown that H2O is critical in destabi-
lizing plagioclase (Housh & Luhr 1991) and in controlling the
relative proportions of garnet and amphibole at moderate
pressures (1.2 GPa, Muntener et al. 2001; see Fig. 6). Further-
more, on the basis of melt inclusion data it is well established
that arc basaltic magmas can have high water contents of above
5 wt% (e.g. Sisson & Layne 1993; Roggensack et al. 1997;
Cervantes & Wallace 2003). In the case of high water contents
of 5 wt% H2O, amphibole and pyroxene are the phases critical in
Fig. 5. Surface heat flux excess is derived by taking the horizontal
distance between an arc and the correlation line from Figure 3. In excess
heat flux arcs, heat flux excess correlates with the proportion of each arc
segment in which Sr-rich adakitic compositions are erupted. See the
appendix for details on the calculation of adakitic arc proportions.
G. F. ZELLMER730
-
controlling melt evolution (Fig. 6a). At moderate water contents
of 3.8 wt% H2O, however, garnet becomes stable (Fig. 6b).
Plagioclase becomes a significant part of the phase assemblage
only at even lower water contents (2 wt%; see Muntener et al.
2001). Therefore, the genesis of Sr-rich adakitic rocks will be
favoured at H2O contents slightly lower than that of average arc
magmas. Such conditions may be met in excess heat flux arcs
where the hydrous mantle wedge material is diluted by upwelling
sub-slab asthenospheric mantle. The genesis of Sr-rich adakitic
rocks in excess heat flux arcs is thus consistent with fractional
crystallization of arc basaltic andesites or partial melting of
lower crustal metabasites with moderate H2O contents.
Effect of residual melt fraction
Although Sr-rich adakitic rocks dominantly occur in excess heat
flux arcs, they are also found in Mindanao, southern Philippines
(e.g. Sajona et al. 1993; Castillo et al. 1999; Macpherson et al.
2006; see Fig. 4g), and in Ecuador, where they are erupted at
some arc crest volcanoes (e.g. Bourdon et al. 2003; Samaniego
et al. 2005; Bryant et al. 2006; see Fig. 4h). Both slab melting
and partial melting or fractional crystallization of arc basalts
have been proposed there (see the above references). Here, it
should be noted that, in both cases, (1) the adakitic signature
appears to increase away from the volcanic front, and (2) mafic
samples with less than 54 wt% SiO2 are rare. These observations
are consistent with a decrease in the temperature of solidmelt
equilibrium away from the volcanic front as lithospheric thick-
ness increases (see Macpherson et al. 2006), and relatively low
degrees of melting, probably related to short melting columns in
mantle wedges with lower than usual temperature (see Bryant et
al. 2006). In Ecuador, low wedge temperatures may be due to the
flattening of the subducting slab towards the south (Barazangi &
Isacks 1976), whereas in the southern Philippines they have been
proposed by Dreher et al. (2005) to be due to recent initiation of
subduction of the old and cold Philippine Sea plate. In either
case, if melt fractions are small, significant Sr enrichment may
occur without invoking an expansion of the garnet stability field
through lowered H2O concentrations.
Slab melting v. lower crustal differentiation processes
The data presented here do not allow deduction of the relative
proportions of contributions from the subducted slab and the
mafic lower crust to generate adakitic compositions. Uptake of
partial melts of the subducting slab in upwelling asthenospheric
mantle can in most cases not be precluded, even in arcs with
evidence of significant lower crustal involvement based on
isotopic data (e.g. Davidson et al. 1990; de Silva et al. 1993).
Thermal models incorporating temperature-dependent viscosity
indicate that partial melting of subducted sediment and/or basalt
may occur even if the subducted crust is old (Kelemen et al.
2003), suggesting that slab melting may be more common than
generally accepted. In either case, the observed excess surface
heat flux at arcs erupting Sr-rich adakitic compositions indicates
that high temperatures and moderate H2O contents favour
differentiation from basaltic or eclogitic sources to generate Sr-
rich adakitic melts. High-temperature melts may also facilitate
assimilation of lower crustal materials in general, including
entrainment of ultramafic debris to produce high-magnesium
andesites from dacitic melts, a process suggested by Streck et al.
(2007) to operate at Mount Shasta in the southern Cascades.
Future work: testing the petrogenetic model
Several ways to test the petrogenetic model presented here can
be devised. Geochemically, comparison of volcanic sites within
and away from adakitic segments of excess heat flux arcs in
terms of their primary magma volatile contents and temperatures
will be critical. In detail, H2O concentrations of melt inclusions
within primitive olivines may be compared. The water content in
melt inclusions from adakitic arc segments would be expected to
be lower. For example, preliminary work has shown that olivines
from the Mt. Shasta area in the southern Cascades record water
contents of only up to 3.3 wt% (Sisson & Layne 1993), com-
pared with significantly higher values of well above 5 wt% in arc
segments that do not erupt adakitic compositions (e.g. Sisson &
Layne 1993; Roggensack et al. 1997; Cervantes & Wallace
2003). Future work should be aimed at systematically analysing
melt inclusion water content from samples collected across
excess heat flux arcs. Such work should be combined with
geothermometric studies to estimate primary magma tempera-
tures. These are expected to be elevated in the adakitic sections
of excess heat flux arcs. As outlined above, high-temperature
melts have indeed been found in the southern Cascades and
Fig. 6. Phases in equilibrium with melt of a hydrous basaltic andesite at
1.2 GPa, as determined by experimental work (Muntener et al. 2001,
sample 85-44): (a) 5 wt% H2O; (b) 3.8 wt% H2O. Plagioclase is
suppressed in arc magmas with water contents above 3 wt%. Genesis of
Sr-rich adakitic rocks will also require garnet to be stable, suggesting
moderate H2O contents above 3 wt% but below 5 wt% at pressures
typical for the lowermost crust.
PETROGENESIS OF ADAKITIC ROCKS AT ARCS 731
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western Mexico (e.g. Elkins Tanton et al. 2001; Righter and
Rosas-Elguera 2001), but detailed studies across all excess heat
flux arcs are necessary to confirm if these results are systematic.
Geophysical tests may include seismic tomography to identify
seismically slow bodies in the mantle wedge (see Levin et al.
2002; Lin et al. 2004) that may represent sub-slab asthenospheric
mantle upwelling through discontinuities in the subducting slab;
and studies of seismic anisotropy that map mantle flow, which
has been recognized as toroidal around the slab edge below the
southern Cascades (Zandt & Humphreys 2008) and northern
Kamchatka (Peyton et al. 2001). Detailed studies at the other
excess heat flux arcs are yet to be carried out.
Finally, additional experimental data on the phases in equili-
brium with evolved hydrous arc magmas generated by low-
degree partial melting of eclogitic sources or high-degree crystal
fractionation of mafic arc melts will be required to obtain better
quantitative constraints on the pressures, temperatures and
compositional ranges suitable for the production of adakitic
melts, and potentially to elucidate the relative proportions of slab
melting and lower crustal differentiation processes in the petro-
genesis of adakitic signatures. A first step may be the extension
of existing experimental data using thermodynamic models, as
for example demonstrated by Dufek & Bergantz (2005).
Concluding remarks
Previous work has shown that surface heat flux at volcanic arcs
is largely controlled by advected heat from shallow-level magma
reservoirs (Zellmer 2008). Here, correlations between conver-
gence rates, effusive eruption style and surface heat flux were
used to identify volcanic arcs with excess heat flux and to
elucidate their geochemical signatures. The results can be sum-
marized as follows.
(1) Excess surface heat flux in the Mexican Volcanic Belt,
northern Chile, the Cascades, SW Japan and Kamchatka appears
to be related to the presence of magmas that have higher
temperatures, lower water contents, and possibly in consequence
a somewhat shallower crustal storage depth than magmas in most
other arcs.
(2) Geophysical evidence suggests that slab discontinuities are
present beneath these arc segments. This may facilitate upwelling
and decompression melting of hot, anhydrous, sub-slab astheno-
spheric mantle.
(3) Limited numbers of adakitic rocks are found in a range of
arcs. However, excess heat flux arcs produce a large number of
such samples, and in particular host Sr-rich varieties (with Sr
concentrations typically exceeding 700 ppm). Furthermore, the
arc proportion erupting Sr-rich adakitic rocks is directly propor-
tional to the amount of excess heat flux in these arcs.
The quantitative relationship between the extent of Sr-rich
adakitic signature and the degree of heat flux excess provides
important clues to the petrogenetic processes operating in the
mantle wedge and lower crust at some volcanic arcs. The above
results appear to indicate that the stabilization of garnet, which
besides the absence of plagioclase is required to produce extreme
Sr enrichment, is promoted by the supply of upwelling astheno-
spheric melts with lower water content. The model presented
here has passed preliminary tests, but further work is required to
provide more detailed constraints at specific sites.
Appendix: Calculating adakitic arc proportions
The following describes the determination of adakitic arc propor-
tions in arcs where Sr-rich adakitic compositions (typically with
Sr .700 ppm) occur. Further details on the definition of each arcsegment have been provided by Zellmer (2008). Here, each
Holocene volcano is assigned a segment length equivalent to half
the distance between its adjacent Holocene eruptive centres.
Exceptions are the Lassen Volcanic Center in the Cascades,
Pinatubo and Paco in the Philippines, and Sangay in Ecuador,
which, as the outermost Holocene volcanoes of their respective
arc segments, are assigned a segment length equivalent to the
full distance to their respective major adjacent Holocene eruptive
centre.
(1) Cascades Volcanic Arc (CAS). Holocene effusive volcan-
ism extends over a total length of c. 1100 km from Mount Baker
in the north to Lassen Volcanic Center in the south. Sr-rich
adakitic rocks are mainly erupted at Lassen and Shasta in the
south, although isolated Sr-rich andesites have also been docu-
mented in the High Cascades of Oregon, where their occurrence
has previously been linked to lower crustal melting above an
unusually hot sub-Cascade mantle (see Conrey et al. 2001, and
references therein). An arc segment length of about 260 km
yields a proportion of 23.7%.
(2) Ecuador and Colombia (ECUCOL), combined here as
they form the continuous arc of the Northern Volcanic Zone of
the Andes. Holocene effusive volcanism extends over a total
length of c. 990 km from Cerro Bravo in the north to Sangay in
the south. Most Sr-rich adakitic rocks are erupted in Ecuador
over an arc segment length of about 130 km between Imabura
and Antisana volcanoes, yielding a proportion of 13.5%.
(3) Kamchatka and northern Kuriles (KAM). Holocene effu-
sive volcanism extends over a total length of c. 1060 km from
Shiveluch in the north to Sinarka in the south. Sr-rich adakitic
rocks are erupted at the northern end of the arc, at Shiveluch,
equivalent to an arc segment length of about 70 km, yielding a
proportion of 6.5%. Here, samples with .550 ppm Sr are definedas Sr-rich, because of the generally lower Sr content of this
segment relative to most other arcs (see Fig. 4f).
(4) Mexican Volcanic Belt (MEX). Holocene effusive volcan-
ism extends over a total length of c. 910 km from Ceboruco in
the west to the Naolinco Volcanic Field in the east. Sr-rich
adakitic rocks are erupted in the western volcanic belt, as far east
as ZitacuaroValle de Bravo, equivalent to an arc segment length
of about 540 km, yielding a proportion of 59.0%.
(5) Northern Chile (NCH). Holocene effusive volcanism
extends over a total length of c. 630 km from Parinacota in the
north to Lascar in the south. Sr-rich adakitic rocks are erupted in
the northern part of the arc, as far south as Tata Sabaya (Bolivia),
equivalent to an arc segment length of about 220 km, yielding a
proportion of 35.3%.
(6) Philippines (PHL). Holocene effusive volcanism extends
over a total length of c. 1000 km from Pinatubo in the north to
Camiguin in the south. Sr-rich adakitic rocks are erupted in the
southern end of the arc, on the Surigao peninsula and at
Camiguin, equivalent to an arc segment length of less than
100 km, yielding a proportion of 8.4%.
(7) SW Japan and northern Ryukyu (SWJ). Holocene effusive
volcanism extends over a total length of c. 520 km from Tsurumi
in the north to Suwanose-jima in the south. Besides occurring at
Abu and Sanbe north of this segment, Sr-rich adakitic rocks are
erupted at Tsurumi and Kuju, equivalent to a segment length of
about 110 km, yielding a proportion of 21.0%. Here, samples
with .500 ppm Sr are defined as Sr-rich, because of thegenerally lower Sr content of this segment relative to most other
arcs (see Fig. 4e).
Sr-rich adakitic rocks in other arcs are rare and are not
representative for their respective arc segments (see Fig. 4i).
G. F. ZELLMER732
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Insightful discussions with C. Annen, J. Davidson, S. de Silva, B.-m.
Jahn, S. Straub and W.-C. Chi shaped the ideas presented here. C.
Hawkesworth and G. Shellnutt are thanked for their detailed comments
on an earlier version of this work. Constructive reviews by J. Garrison
and D. Selles significantly improved the paper, as did the editorial
remarks of D. Pyle. Funding was provided by the National Science
Council of Taiwan (NSC grants 96-2116-M-001-006 and 97-2628-M-
001-027-MY2), and by the Institute of Earth Sciences, Academia Sinica.
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Received 3 June 2008; revised typescript accepted 20 February 2009.
Scientific editing by David Pyle.
G. F. ZELLMER734