只剩骄傲The partially molten crust surrounding the basalt may be older intrusions of related mantle-derived hydrous basalt (or amphibolite)or unrelated metamorphic arc crust.This is Model IV.The volume and composition of the partial melt produced depends on the intrusion rate (heat flux)of the mantle-derived basalts,the prevailing geotherm and the extent to which the melting region is fluxed by H 2O liberated from the crystallizing basalt.Chemically hybrid melts can be formed if the residual melts from basalt crystallization are mixed with crustal partial melts during extraction,ascent and shallow intru-sion;this is Model V.
Models III and IV both involve partially molten hyd-rous basaltic rocks in the lower crust produced,res-pectively,by crystallization and melting.Deep-seated crystallization of hydrous basaltic magmas differs from dehydration melting of the lower crust,as modelled by Raia &Spera (1997),Petford &Gallagher (2001)and Jackson et al .(2003),in one fundamental regard,the availability of H 2O.In dehydration melting the H 2O content of the source rock is strictly limited by the amount of H 2O that can be structurally bound in hyd-rous minerals such as amphibole and mica.For a mafic amphibolite with 40%amphibole,this amounts to $0á8wt %H 2O.Greater quantities of H 2O can be involved only if the heat source efficiently fluxes the source region with H 2O.Although this is likely,no extant models of crustal melting consider this process,largely because it is uncertain whether H 2O passing through a low-porosity source rock triggers melting or is simply carried away along fractures.By contrast,deep-seated crystallization of hydrous arc basalt magmas has no such upper limit on H 2O content.Studies of melt inclusions in primitive arc magmas,together with high-pressure experiments,indicate dissolved H 2O con-tents from almost zero to 10wt %(e.g.Sisson &Layne,1993;Carmichael,2002;Pichavant et al .,2002a ;Grove
509ANNEN et al.DEEP CRUSTAL HOT ZONES
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et al .,2003).The wide range of H 2O contents and bulk compositions of parental arc basalts ensures that crystal-lization of hydrous basalt can generate a wide persity of residual melt compositions,as demonstrated experiment-ally by Sisson et al .(2005).Dehydration melting (Model IV)requires a heat source.In arcs the widespread association of evolved igneous rocks with mantle-derived basalt strongly sug-gests that mafic magmas provide the heat source (Hildreth,1981).However,herein lies a problem:models of heat transfer show that arc basalts emplaced into the base of the crust at temperatures of 1100–1240 C (see Ulmer,2001;Pichavant et al .,2002a )cannot provide enough heat to melt amphibolite lower crust extensively (Petford &Gallagher 2001;Annen &Sparks,2002),because of the high dehydration melting temperature of amphiboles in mafic rocks ($950 C).More fertile upper crustal pelitic protoliths can be melted more effi-ciently,but large amounts of basalt are still needed as a heat source (Annen &Sparks,2002).In addition,silicic rocks in arcs are typically calc-alkaline and metaluminous,which places limits on the amount of pelite that can be melted.The isotopic and geochemical signatures of evolved plutonic and volcanic arc rocks clearly indicate contribution from pelitic crust in some cases (DePaolo et al .,1992),but significant amounts of basalt or meta-basalt (amphibolite)must be involved in their petrogenesis.The problem in arcs is how to generate large volumes of metaluminous,calc-alkaline evolved melts when the proposed amphibolite source is too refractory to undergo significant dehydration melting at plausible temperatures.This paradox can be solved if crystallization of H 2O-bearing mantle-derived basalt is the principal source of the evolved melts.CRYSTALLIZATION OF ANDESITE IN THE SHALLOW CRUST Once generated in the deep crust andesite and dacite residual melts can detach and ascend into the shallow crust.Subduction-related andesites and dacites are com-monly porphyritic,with phenocrysts of plagioclase plus various proportions of hornblende,clinopyroxene,orthopyroxene,biotite and oxides;the exact ferromagne-sian assemblage depends on magma composition,partial pressure of volatiles (especially p H 2O),oxygen fugacity (f O 2)and temperature (e.g.Rutherford et al .,1985;Rutherford &Devine,1988;Blatter &Carmichael,1998,2001;Moore &Carmichael,1998;Scaillet &Evans,1999;Pichavant et al .,2002b ;Izebekov et al .,2004).Invariably the groundmass or matrix glass in porphyritic andesites and dacites is rhyolitic in composition.The phenocryst assemblages commonly have complex textures and zoning patterns,which indicate that magmatic evolution can involve processes such as:repeated mixing of different batches of magma (e.g.Heiken &Eichelberger,1980;Clynne,1999);entrain-ment of old crystals from previously consolidated magma batches (Davidson et al .,1998,2001,2005;Heath et al .,1998;Cooper &Reid,2003;Reagan et al .,2003;Dungan &Davidson,2004)or from assimilation of crustal rocks (Ferrara et al .,1989);convective stirring (Couch et al .,2001);crystal growth induced by degassing (Blundy &Cashman,2001).Whereas some of these phenocrysts grew from the magma in which they are found,others are entrained xenocrysts from earlier magma pulses or from chemically unrelated wall-rocks (e.g.Izebekov et al .,2004;Davidson et al .,2005).Detailed studies of volcano evolution (e.g.Bacon,1983;Bacon &Druitt,1988;Druitt &Bacon,1989;Harford et al .,2002)and con-straints on timescales for crystallization (e.g.Zellmer et al .,1999,2003a ,2003b ;Harford &Sparks,2001)sug-gest that these various processes are the consequence of amalgamation of shallow magma bodies in the upper crust through many episodes of magma ascent from greater depths,sometimes accompanied by eruption.Field and geochronological evidence from calc-alkaline plutonic rocks (‘granites’,sensu lato )also supports their formation by amalgamation of many small intrusions,often of magmas with very similar bulk chemical com-position but subtle textural differences (e.g.John &Blundy,1993)or radiometric ages (e.g.Coleman et al .,2004;Glazner et al .,2004).
Our main concern here is to establish under what conditions the common phenocryst assemblages in andesites and granites are formed.Central to this issue are the H 2O contents and temperatures of andesite magmas.The importance of these two variables in inter-preting the phenocryst assemblages and compositions of
andesites has been investigated for over 30years in a
large number of experimental studies at p H 2O ( P tot )of 0á1to !400MPa (Eggler,1972;Green,1972;Eggler &
Burnham,1973;Maksimov et al .,1978;Sekine et al .,
1979;Rutherford et al .,1985;Rutherford &Devine,1988,2003;Luhr,1990;Foden &Green,1992;Sekine &Aramaki,1992;Sisson &Grove,1993;Kawamoto,1996;Grove et al .,1997,2003;Barclay et al .,1998;Blatter &Carmichael,1998,2001;Moore &Carmichael,1998;Cottrell et al .,1999;Martel et al .,1999;Sato et al .,1999;Scaillet &Evans,1999;Pichavant et al .,2002b ;Couch et al .,2003;Prouteau &Scaillet,2003;Barclay &Carmichael,2004;Costa et al .,2004;Izebekov et al .,2004).Although many of these studies are focused on rocks from a specific volcano,some general conclusions can be drawn regarding sub-duction-related andesites and dacites,as follows.(1)Eruption temperatures,as determined by geo-thermometry,are consistently less than low-pressure (<300MPa)andesite liquidus temperatures even under
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H 2O-saturated conditions.In many cases the difference is several tens of degrees and can be as much as 200 C (e.g.Blatter &Carmichael,1998;Barclay &Carmichael,2004).(2)The liquidus phases at low p H 2O often include minerals (e.g.olivine,clinopyroxene)that are absent from the phenocryst assemblage in the natural rocks (e.g.Blatter &Carmichael,1998,2001;Scaillet &Evans,1999;Costa et al .,2004).(3)Although amphibole is a common phenocryst it rarely occurs on the andesite liquidus at p H 2O <400MPa even under oxidizing conditions;where it is stable,amphibole typically appears 100 C below the liquidus (e.g.Rutherford &Devine,1988,2003;Blatter &Carmichael,1998,2001;Moore &Carmichael,1998;Martel et al .,1999;Costa et al .,2004;Izebekov et al .,2004).(4)Plagioclase is stabilized only at low p H 2O and is rarely a true liquidus phase at p H 2O >100–200MPa,even though plagioclase is a ubiquitous phenocryst phase in most andesites (e.g.Eggler,1972;Maksimov et al .,1978;Sekine et al .,1979;Sekine &Aramaki,1992;Blatter &Carmichael,1998,2001;Moore &Carmichael,1998;Martel et al .,1999;Grove et al .,2003).(5)The anorthite (An )content of plagioclase increases with increasing p H 2O (at constant temperature)and increasing temperature (at constant p H 2O).For a given andesite,plagioclase phenocryst rims typically have considerably lower An contents (by up to 30mol %)than the experimentally determined liquidus or near-liquidus plagioclase (e.g.Rutherford et al .,1985;Scaillet &Evans,1999;Rutherford &Devine,2003;Costa et al .,2004).(6)The observed phenocryst assemblage,phase com-positions and crystallinity typically are consistent with H 2O-saturated conditions at pressures of 100–300MPa and at sub-liquidus temperatures consistent with those obtained from mineral thermometry on the natural rocks (e.g.Blatter &Carmichael,1998;Moore &Carmichael,1998;Martel et al .,1999;Costa et al .,2004).The similarity of phase proportions and compositions in both experiments and natural andesites (Fig.3)indic-ates that,to a first approximation,these magmas have undergone near-closed system crystallization from an initial fully molten state to a porphyritic magma under conditions of low-pressure H 2O-saturation.However,very few of the studied andesites contain their full com-plement of experimentally determined liquidus phases under these conditions,suggesting that either magma temperatures were never high enough to form a fully molten andesite liquid at low pressure or that the original liquidus phases were completely eliminated (or re-equilibrated)by reaction with the melt.The interpreta-tion we favour is that andesite liquids,once formed and extracted from the deep crust,typically crystallize under polybaric conditions,at temperatures that do not signi-ficantly exceed their eruption temperature.Thus the ini-tial fully molten state of an andesite is not a consequence of high temperature,but a consequence of high p H 2O.
