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by heating of the crust,with less attention paid to the residual melt generated by partial crystallization of the underplated basalt intrusions.Here we develop the con-cepts proposed by Annen &Sparks (2002)and consider the full range of possible mechanisms of melt genera-tion in the hot zone,including residual melt from basalt crystallization and partial melting of surrounding crustal rocks (Fig.1).We then consider the evolution
of
Fig.1.Conceptual representation of a hot zone (not to scale).Sills of mantle-derived basaltic magma are injected at a variety of depths,including
(1)the Moho,(2)the lower crust and (3)the Conrad Discontinuity between lower and upper crust.Sills injected at the Moho displace older sills into the mantle,creating a contrast between the petrological Moho (base of sill complex)and seismological Moho (top of sill complex).Sills crystallize from their injection temperature to that of the geotherm,resulting in a wide variety of residual melt fractions at any given time,from near 100%(newly injected sill near Moho)to 0%(old sill injected into lower crust).The fraction of crustal melt varies throughout the hot zone according to the age and proximity of the basalt sills.Melts ascend from the hot zone to shallow storage reservoirs,leaving behind dense refractory cumulates or restites.Residual and crustal melts from different portions of the hot zone may be mixed together prior to ascent or within the shallow reservoir.
507ANNEN et al.DEEP CRUSTAL HOT ZONES
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these melts as they are extracted from their source
rocks and ascend to shallow crustal levels,degassing
and crystallizing en route.The model is developed
primarily for application to the genesis of subduction
zone volcanic and plutonic rocks,and we will refer
collectively to this whole suite of intermediate and
silicic rock types as ‘andesite’,except where a composi-
tional or textural distinction is relevant.However,our
model has general applicability to other tectonic settings,
including continental rift zones where plume-related
basaltic magmas are intruded into the base of the
continental crust.
SOURCES AND MECHANISMS FOR
INTERMEDIATE AND SILICIC
MAGMA GENERATION
There are five currently popular models for the genera-
tion of andesites (sensu lato ),as follows.
Model I.Partial melting of harzburgite in the mantle wedge,fluxed by H 2O-rich fluids or melts liberated from the subducting slab (e.g.Tatsumi,1982;Hirose,1997;Blatter &Carmichael,2001;Carmichael,2002,2004;Parman &Grove,2004).Model II.Crystallization of mantle-derived basalt or basaltic andesite in shallow crustal magma chambers (e.g.Sisson &Grove,1993;Grove et al .,1997;Pichavant et al .,2002b ).Model III.Crystallization of mantle-derived basalt or basaltic andesite in the deep arc crust at or close to the Moho (e.g.Mu ¨ntener et al .,2001;Annen &Sparks,2002;Mortazavi &Sparks,2003;Prouteau &Scaillet,2003).
Model IV.Dehydration partial melting of meta-basalts
(amphibolites)in the lower or middle crust by intrusions of hot,mantle-derived magma (e.g.Smith &Leeman,1987;Petford &Atherton,1996;Jackson et al .,2003).Model V.Mixing between silicic magmas and mantle-derived mafic magmas (e.g.Heiken and Eichelberger,1980).In some cases the silicic component is generated by partial melting of crustal rocks (e.g.Druitt et al .,1999).In this paper we focus on Models III–V,which take place in the middle or lower crust.Models I and II are briefly considered first.Generation of andesite by mantle melting (Model I)has been demonstrated experimentally (Tatsumi,1982;Hirose,1997;Grove et al .,2002,2003;Parman &Grove,2004)and calculated thermodynamic-ally (Carmichael,2002,2004).The andesites produced in this way have elevated MgO contents and high mg-numbers,a requirement for equilibrium with the Mg-rich olivines of mantle harzburgite.Boninite series magmas are widely thought to originate by H 2O-fluxed melting of harzburgite (Falloon &Danyushevsky,2000;Parman &Grove,2004),whereas the generation of ‘high-Mg andesites’may involve reactions between ascending slab-derived silicic melts and mantle peridotite (Yogodzinsky &Kelemen,1998).However,high-Mg andesites and boninites are not the dominant rock types of volcanic arcs;typical arc andesites,with low mg-numbers,could not have been in direct equilibrium with mantle rocks.
Model II is widely favoured.Basalt and basaltic andesite lavas occur at many arc stratovolcanoes and occasionally contain xenoliths of cumulate origin (e.g.Arculus &Wills,1980).Several experimental studies demonstrate that andesite can be generated by fractional crystallization of H 2O-saturated basalts and basaltic andesites at p H 2O ?P tot of 200–400MPa and temperat-ures of 950–1050 C (Sisson &Grove,1993;Grove et al .,1999,2003;Pichavant et al .,2002b )by crystallizing an assemblage of plagioclase (An 60–90)tclinopyroxene tamphibole toxides ?orthopyroxene ?olivine.One constraint on the origin of andesites is that they typically contain <19%Al 2O 3(Fig.2),indicating that by the time residual melts have attained >57wt %SiO 2they have become saturated in an aluminous phase.In Model
II
0025710d561252d381eb6e31positions of experimentally produced residual melts from
crystallization of hydrous basalts in the lower crust.Squares denote
melt compositions from experiments on a primitive Mount Shasta basaltic andesite,sample 85-44(mg-number 0á71),from Mu ¨ntener et al .(2001)and Grove et al .(2003),at 0á8–1á2GPa,1045–1230 C
and with !2á5wt %added H 2O;filled circles denote experimental
melts from Kawamoto (1996)on a Higushi-Izu high alumina basalt,sample IZ27-2(mg-number 0á60),at 1á0GPa,1000–1150 C with 1wt %added H 2O.The compositions of the two different starting materials are indicated.Symbols that are filled or partially filled denote
glasses in equilibrium with an aluminous phase,as shown in the
legend.All of the IZ27-2glasses are saturated in plagioclase.For reference the compositional field defined by 387published analyses of Cascades andesites is shown.It should be noted that >96%of these andesites contain <19wt %Al 2O 3.
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crystallization of plagioclase serves to limit Al 2O 3enrich-ment in residual melts.The lack of abundant dense com-plementary mafic to ultramafic cumulate rocks in the shallow crust is problematic for Model II unless the asso-ciated mafic cumulates are removed by sinking (Glazner,1994).Model III involves fractional crystallization of similar parental magmas to Model II,but at higher pressure,thereby obviating the problem of the missing mid-or upper-crustal mafic cumulates.Mantle-derived magmas intruded into the deep crust cool and crystallize produ-cing evolved residual melts.The principal difference between high-and low-pressure crystallization of hydrous basalt lies in the nature of the crystallizing assemblage.At higher p H 2O garnet (e.g.Wolf &Wyllie,1994;Rapp,1995)and aluminous amphibole (Grove et al .,2003)are stabilized and can contribute to minimizing Al 2O 3enrichment in residual melts.Conversely,plagioclase stability is reduced and liquidus plagioclase is anorthite-rich,a common finding in arc-related cumulate nodules (e.g.Arculus &Wills,1980).In terms of melt chemistry,it is very hard to distinguish between residual melts pro-duced by crystallization of An -rich plagioclase and pyrox-enes from H 2O-undersaturated basalt at $1á0GPa (Kawamoto,1996)and those produced from H 2O-saturated basalt at 0á2–0á4GPa (e.g.Sisson &Grove,1993;Pichavant et al .,2002b ).The appearance of garnet as the liquidus aluminous phase in andesite and dacite melts at pressures over $1á1GPa (Wolf &Wyllie,1994;Rapp,1995)imparts a distinctive trace element chemistry to residual melts (e.g.high Sr/Y),which provides a clear indication of high-pressure differentiation (e.g.Smith &Leeman,1987;Feeley &Davidson,1994;Feeley &Hacker,1995).In Models II and III,Al 2O 3enrichment in derivative melts is further minimized if the primitive basalt itself has relatively low Al 2O 3.Circumstances for generation of such magmas are inferred in many arcs with a relatively depleted mantle wedge (Grove et al .,2003;Parman &Grove,2004).For example,primitive arc basalts with only 14–15%Al 2O 3have been described for Klyuchevskoy volcano,Kamchatka (Ozerov,2000).When mafic magmas are intruded into the arc crust they transfer heat and volatiles (principally H 2O)into the surrounding crust,which can lead to partial melting of the wall-rocks.The deep crustal hot zone is,therefore,envisaged as a mixture of partially crystallized basalt,partially molten crustal rocks and H 2O liberated from the solidifying basalts (Fig.1).Geophysical evidence is consistent with these concepts.In the Cascades,for example,the release of significant volumes of H 2O from deeply intruded basalts may account for the presence of a highly electrically conductive layer at 10–30km depth (Stanley et al .,1990),and in the central Andes a broad conductive zone (Brasse et al .,2002)is associated with a low-velocity zone at depths of 20–40km (Yuan et al .,2000),interpreted as a laterally extensive region of partial melt,capped by a silicic magma body $1km thick (Chmielowski et al .,1999).Below volcanoes in the Japan arc broadband seismometers have recorded low-frequency tremors and micro-earthquakes at 30–50km depth (Obara,2002;Katsumata &Kamaya,2003).These can be explained by deformation associated with magma intrusions (S.Sachs,personal communication,2003)and their low frequency is consistent with the pres-ence of a fluid phase.Finally,beneath central North Island,New Zealand,a seismically highly reflective layer at 35km depth,interpreted as a body of partially molten rock (Stratford &Stern,2004),suggests that be-neath some arcs the hot zone may be located in the uppermost mantle,rather than within the crust,which is only 16km thick in this region.
