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

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

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