METAMORPHISM AT EXTREME TEMPERATURES DURING OROGENESISThermal-mechanical modelling of collisional orogenic beltssuch as the Himalayas has highlighted the importance ofthe deep crust in the high-temperature deformation thatdrives orogenic systems (Jamieson et al. 2004). High-temperature metamorphic terrains represent the remains ofdeep crustal roots (Harley 1998) and are common not onlyin Precambrian shields but also in Phanerozoic orogenicbelts. They preserve in their mineral assemblages records ofhigh- (>800°C) to ultrahigh-temperature metamorphism(900°C–1100°C) during orogenesis, at pressures equivalentto crustal depths of only 25–50 kilometres (Harley 1998).They also preserve evidence, in the form of diagnostic reac-tion textures between minerals, of a spectrum of post-peakpressure–temperature (P–T) evolutions. These range fromslow cooling (2–4°C/Myr) over several hundred degreeswhile still at deep levels, to maintaining near-constant tem-peratures in excess of 700°C during rapid (1–5 km/Myr)exhumation through several tens of kilometres (Harley1998). Linking these high-temperature P–T evolutions totime and comparing the resultant P–T–time histories withmodel predictions for the evolution of the deep crust inthese ‘hot orogens’ are central to understanding collisionand mountain building and remain major goals in globaltectonics studies.

ZIRCON DATING OF DEEPCRUST IN HOT OROGENSBecause of its ubiquity in crustalrocks and its proven capability inU–Pb geochronology, zircon canprovide a link between P–T andtime in hot orogens. In high-temperature (high-T) terrains, signif-icant parts of the P–T evolutionarypaths lie at temperatures in excessof 800°C. Zircon is in many casesthe only mineral that can provideage information that survivesincreasing temperatures, residenceat the thermal peak, and immedi-ate post-peak metamorphic reac-tions within this critical tempera-ture window. FIGURE 1 depicts ageneric temperature–time evolu-tion through potential zircon-

forming, zircon-consuming and zircon-modifying reactionsand processes that are dependent on rock composition,pressure and fluid–melt–rock interaction.

There are two main reasons why only zircon can providethe age information required to define these thermal histo-ries. First, the temperature window lies above the diffu-sional closure temperatures of many other geochronologi-cal systems – many of which are below 700°C – but does notlie above the diffusional closure temperature of Pb in zircon(Cherniak and Watson 2000). Hence, systems such as U–Pbin rutile, Ar–Ar in micas and even Sm–Nd in garnet in gen-eral record only the later, post-peak cooling history; if cool-ing is slow (e.g. 3°C/Myr in some terrains), the ‘apparent’ages provided by these systems may be too young by tensto hundreds of millions of years. Second, the temperaturewindow corresponds to the conditions in which partialmelting will occur in many rocks that contain hydrousminerals like biotite and hornblende (Sawyer 2001). Zirconcommonly crystallizes in such melts, which makes the min-eral an extremely useful time marker. In many high-T ter-rains, monazite U–Th–Pb dating can be used in conjunctionwith zircon geochronology to constrain the peak to post-peak history. However, only in cases of initial rapid coolingassociated with fast exhumation does the monazite methodrecord ages that reflect the timing of peak conditions.

‘Bracketing’ is the traditional approach to using zircon todate events in high-T terrains (FIG. 1). This method relies onthe ability of zircon formed in one event to survive a secondevent such as high-T metamorphism. For example, usingthis approach a metamorphic episode can be constrained tolie between the age of the youngest pre-metamorphic mag-matic rock and the age of the oldest post-metamorphicintrusion. Clearly, this method works well when the

E L E M E N T S , V O L . 3 , P P . 2 5 – 3 0 FEBRUARY 2007

Simon L. Harley1, Nigel M. Kelly1 and Andreas Möller2

1 Grant Institute of Earth Science, The University of EdinburghEdinburgh EH9 3JW, UKE-mail: simon.harley@ed.ac.uk

2 Institut für Geowissenschaften, Universität PotsdamKarl-Liebknecht-Str. 24, D-14476 Golm, Germany

Zircon Behaviour andthe Thermal Historiesof Mountain Chains

25

Using the U–Pb geochronology of zircon we can understand the growthand collapse of mountain chains, both recent and ancient. In the high-temperature metamorphic rocks that underlie mountain ranges, zircon

may survive from precursor rocks, recrystallize, or grow anew. All thesepossibilities must be considered in the interpretation of zircon ages. Micro-textural characterisation and microanalysis, coupled with considerations ofmineral equilibria and trace element distributions between zircon and neigh-bouring silicate minerals, provide insights into the factors controlling zirconmodification and growth. Zircon ages do not usually correspond to the peakof metamorphism but instead provide information on the history of coolingfrom high temperatures, including the timing and rates of exhumation ofthe deep roots of mountain chains.

KEYWORDS: zircon, ultrahigh-temperature metamorphism,hot orogens, trace elements, U–Pb geochronology

Convoluted zoning in alargely recrystallized

zircon

‘bracket’ is tight – when pre- and post-metamorphic rocksare separated by only a short time interval (a few millionyears; e.g. Möller et al. 2003). However, it is not usefulwhen the bracket covers a long time interval, when thezircon age obtainable from pre-metamorphic rocks is dis-turbed or ambiguous, or when the geological relationshipsthemselves are poorly constrained (e.g. where it is not clearwhether the igneous precursor rocks are pre- or syn-tectonic; Kelly and Harley 2005).

The ‘bracketing’ approach is also limited in that it providesno information on the details of the high-T metamorphismitself. Such features may include the onset of melting anddissolution of zircon; the prograde reaction of zircon andother accessory minerals during growth of major mineralssuch as garnet; the thermal peak; melt segregation andaccumulation, with crystallization of zircon; post-peak sub-solidus mineral reactions; post-melt fluid incursions; andthe relationships between all of these and episodes of defor-mation (FIG. 1). Many of these events and their relatedprocesses occur at temperatures above those at which mostother geochronology minerals are stable or at the very least

able to record age information. Only zircon, with the possi-ble exception of monazite, is able to do this. Reliable ageinformation is crucial for the elucidation of crustal behav-iour during mountain building, and so it is essential tounderstand how zircon behaves and responds to high-Tprocesses. To do this it is necessary to examine ‘metamor-phic’ zircon – i.e. zircon that has grown, been recrystallizedor been modified by the physical and chemical processesthat may occur during metamorphism of the deep crust.

WHAT IS ‘METAMORPHIC’ ZIRCON IN A HIGH-T CONTEXT? The interpretation of zircon age data in high-T terrains iscomplex and problematic because of the highly variableresponse of previously formed zircon to later metamorphicevents (Vavra et al. 1999; Schaltegger et al. 1999; Kelly andHarley 2005). Misunderstandings about what constitutes‘metamorphic’ zircon are related to this complex behaviour.

Several microscale physical and chemical processes canform or modify zircon in the metamorphic environment,producing what is broadly, and somewhat misleadingly,referred to as ‘metamorphic’ zircon. Local-scale processesthat cause zircon growth in the high-T metamorphic regimeinclude subsolidus metamorphic reactions involving Zr-bearing silicates and other accessory minerals (Fraser et al.1997; Degeling et al. 2001; Möller et al. 2003) and partialmelting and melt crystallization (e.g. Roberts and Finger1997; Vavra et al. 1999; Schaltegger et al. 1999; Rubatto2002; Hokada and Harley 2004). Pre-existing zircon can alsobe partially to completely transformed or modified in situ(Pidgeon 1992; Schaltegger et al. 1999; Ashwal et al. 1999;Hoskin and Black 2000), potentially undergoing fine-scalecoupled dissolution–reprecipitation (Vavra et al. 1999; Car-son et al. 2002a; Geisler et al. 2007 this issue) in response tointeractions with fluids and melts. Zircon with initialgrowth zones rich in U and Th may be subject to recovery,

2626

Schematic temperature (T)–time (∆t) path for a hypo-thetical high-T terrain formed in the deeper parts of a hot

orogen. The ∆t scale signifies an unquantified time period of the orderof millions of years. (A) indicates the ‘bracketing approach’ to datinghigh-T metamorphism. During heating rocks may undergo partial melting,causing dissolution of finer pre-existing zircon followed by later precip-itation of new zircon on survivor grains (B). Crystallization of high-Tmelts and consequent growth of zircon will generally occur on the post-peak cooling path at different intervals depending on the water contentof the melt (reflecting aH20) or relations with wall rocks (C, D). New zirconmay also grow as a result of Zr-liberating reactions, for example garnet(Grt) breakdown or rutile (Rt) decomposition (E), depending on the P–Tpath. As the rocks cool below the high-T window, ‘residual’ melts willcrystallize some new zircon (F), and liberated fluids may cause selectiverecrystallization of existing zircon (G). Recrystallization of zircondomains can in principle occur at any stage during metamorphism andmay dominate in the near–peak T region.

FIGURE 1

E L E M E N T S FEBRUARY 2007

via diffusion–reaction processes, of domains that have suf-fered radiation damage (Geisler et al. 2007) in order to dis-sipate strain effects (Hartmann et al. 1997; Schaltegger et al.1999; Vavra et al. 1999). Pb loss related to this last processmay yield zircon U–Pb spot ‘ages’ that bear no relation tothe geological events associated with orogenesis.

As several processes are involved in the production of‘metamorphic’ zircon, the interpretation of zircon U–Pbages in high-T terrains in terms of the timing and characterof events requires detailed textural analysis coupled with insitu microanalysis to detect indicative chemical signatures(Vavra et al. 1996, 1999; Schaltegger et al. 1999; Hoskin andBlack 2000; Rubatto 2002; Whitehouse and Platt 2003;Kelly and Harley 2005). The key to evaluating and utilisingzircon U–Pb ages to define P–T–time evolutionary paths indeep crust lies in distinguishing the cause of zircon growthfrom among the processes noted above, in relating growthto the specific reactions that form zircon, and in independ-ently documenting the conditions under which fluidsaccess the rocks to promote late-stage zircon alteration anddissolution–reprecipitation. The criteria applied in makingthese interpretations include the textures of the zircongrains and their relations with other minerals, as well as arange of chemical signatures, such as the Th/U ratio, the Ticontent, and – most importantly – the rare earth element(REE) patterns in zircon, in comparison to those in the hostand neighbouring metamorphic minerals.

ZIRCON TEXTURES AND HIGH-T PROCESSESFIGURE 2 illustrates zircon textures formed by high-Tprocesses. High-T ‘metamorphic’ zircon has been describedas being equant and with a ‘soccerball’ or ‘multi-faceted’habit (FIGS. 2A, B); it reportedly displays little or no oscillatoryzoning but shows planar banding or sector zoning (FIGS. 2B,C, D; e.g. Vavra et al. 1996, 1999; Schaltegger et al. 1999;Hoskin and Black 2000; Kelly and Harley 2005). While thesefeatures are common, it is not always obvious whether theyhave been produced through new zircon growth or byrecrystallization. For example, in some instances, the presenceof a ghost-like relic of oscillatory zoning in otherwiseunzoned zircon rims demonstrates that in situ replacementor recrystallization rather than new growth has occurred(Hoskin and Black 2000) and cautions against a simpleinterpretation of the U–Pb age of such a zircon domain.

Vavra et al. (1996) have related the style of zircon growth tothe rate of growth and to roughness criteria. They considerthat equant or ‘soccerball’ zircon grows during progradehigh-T anatexis, at different times in different rock compo-sitions depending on the melting reactions intersected. Dis-solution of very fine zircon into the melt is followed by zircongrowth on surviving cores (Vavra et al. 1999). On the otherhand, Schaltegger et al. (1999) attributed sector-zoned ‘soc-cerball’ zircon to high-T subsolidus growth, whereas planar-zoned overgrowths and acicular grains form during meltcrystallization. ‘Soccerball’ zircon has now been described

27E L E M E N T S FEBRUARY 2007

Zircon formed during high-temperature metamorphismmay exhibit a wide range of morphologies and internal

zoning features. ‘Soccerball’ zircon (A), which may display planar andsector zoning when imaged by scanning electron microscope (B), iscommon in high-T rocks. Similar zoning patterns are also found in zirconknown to have crystallized from partial melt (C), and this new growth

can be extensive, forming on older zircon that has been preservedthrough high-T metamorphic events, such as the core in (D), or asxenocrysts in partial melts (E). Recrystallization may affect only outerdomains (F), propagating as fronts through the zircon (G), or transgressthe grain in a process probably driven by dissolution and regrowthalong a fracture (H). In extreme cases ‘convoluted’ zoning may develop (I).

FIGURE 2

B

A

C

D

E

F

G

H

I

28E L E M E N T S FEBRUARY 2007

from several ultrahigh-temperature terrains and in manycases has been interpreted to have grown from partial melts(e.g. Kelly and Harley 2005).

In situ modification of zircon under high-T conditions mayrange from the complete transgression of older zones bysharp fronts or compositional zones visible under cathodo-luminescence (CL) or backscattered electron imagery, topartial modification in which former oscillatory zones, now‘ghost-like’ or ‘bleached,’ are still visible but blurred (Pid-geon 1992; Vavra et al. 1999; Schaltegger et al. 1999; Hoskinand Black 2000; Corfu et al. 2003). Concepts relevant to theinterpretation of such features are discussed in more detailby Geisler et al. (2007). From the perspective of U–Pbgeochronology, this variability leads to ambiguity: a spreadin zircon U–Pb data along or near a concordia curve mayresult if some memory of the initial zircon composition ispreserved and if the ‘recrystallization’ is only partially effectivein expelling Pb and Th on the microscale (Hoskin and Black2000). This variable modification of zircon, caused by pre-and post-peak high-T reactions, deformation and/or fluidingress, is probably responsible for the 20–100-million-yearscatter in apparent ages recorded in weakly zoned ‘rim’ zircondata from several granulite terrains (e.g. Corfu et al. 1994;Ashwal et al. 1999; Kelly and Harley 2005).

Late-stage fluid ingress has also been implicated in modifyingzircon microtextures in high-T rocks. Vavra et al. (1999)attributed the preferential replacement or recrystallizationof zones with high Th and U contents within zircon and thesurface-controlled replacement of interior domains bylobate transgressive zones (FIG. 2G) to fluid ingress andleaching (FIG. 2H; e.g. Ashwal et al. 1999). It is likely thatfluid-activated dissolution of zircon accompanied by itsreprecipitation is an important process controlling thisbehaviour, as advocated by Geisler et al. (2007).

Th/U – AN UNRELIABLE SIGNATUREOF METAMORPHIC ZIRCONTh/U ratios have become a commonly employed criterionfor distinguishing zircon formation in magmatic, metamor-phic and hydrothermal environments. For example, a Th/Uvalue of less than 0.1 has been widely cited as a discriminantof metamorphic zircon (e.g. Rubatto 2002).

To evaluate this interpretation, it is useful to consider thefactors controlling the Th/U ratio in zircon. The concentra-tions of Th and U in zircon are primarily influenced by factorssuch as element availability within a reaction environmentand partitioning behaviour of Th and U between zircon andco-existing minerals, melts and fluids. For example, theprior or concurrent growth of a mineral such as monazite,in which Th is a major structural constituent, may result inzircon with a low Th concentration and therefore a lowTh/U ratio. In contrast, a zircon crystallizing from a partialmelt prior to the crystallization of monazite may have ahigher Th concentration and therefore a moderate to highTh/U ratio.

The use of Th/U ratios has been re-evaluated recently in thelight of textural observations and other chemical criteria(e.g. Möller et al. 2003). Although many ‘metamorphic’ zircondomains do have low Th/U values (<0.1; e.g. Schaltegger etal. 1999; Hoskin and Black 2000; Rubatto et al. 2001;Rubatto 2002), there also are numerous cases where meta-morphic zircon may have Th/U values greater than 0.1 orvalues that are highly variable. High Th/U ratios have beenrecorded in recrystallized zircon (e.g. Pidgeon, 1992; Vavraet al., 1999), but also in zircon grown during high-T meta-morphism (Th/U > 0.15 and up to 3.2; Carson et al. 2002b;Kelly and Harley 2005). Möller et al. (2003) showed that

Th/U ratios in recrystallized and newly grown metamorphiczircon were unchanged in comparison to their magmaticzircon precursors (and never less than 0.1), with some sig-nificantly higher. In addition, Schaltegger et al. (1999),Harley et al. (2001) and Hokada and Harley (2004) havereported high Th/U values for zircon from demonstrablysyn-metamorphic, high-T anatectic melts.

Finally, very low Th/U (<0.02) zircon domains may be asso-ciated with late-stage mineral–fluid interactions at temper-atures well below those of the high-T metamorphic event(Vavra et al. 1999; Harley et al. 2001; Carson et al. 2002a).Given this fact and the evidence for variable and high Th/U‘metamorphic’ zircon grains described above, Th/U valuescan only be used with caution and in concert with other,more integrative, chemical criteria to assess the origin of zirconwithin its textural context.

ZIRCON–MINERAL TRACE ELEMENT DISTRIBUTIONS AND DATING OF HIGH-T EVENTSZircon grown or recrystallized contemporaneously withhigh-T garnet exhibits flat heavy rare earth element (HREE)patterns and strong depletion in Eu in chondrite-nor-malised diagrams (see also Rubatto and Hermann 2007 thisissue). Therefore, the most promising chemical methodbeing developed for evaluating the metamorphic characterand timing of zircon growth is the determination of theequilibrium distribution of REE between zircon and coexistingmetamorphic minerals. However, this method must beapplied with some caution, as existing experimental data atappropriate P–T conditions do not enable refinement oftrace element distribution models (van Westrenen et al.1999). The lack of available experimental data is com-pounded by the fact that existing empirical estimates of dis-tribution coefficients [DREE(Zrc/Grt)] for the HREE showorder of magnitude differences among high-T and high-Procks (FIG. 3). While disequilibrium is likely to be a significantfactor in producing these differences in cases wherestrongly zoned or variable zircon and/or garnet have beenused (Schaltegger et al. 1999; Rubatto 2002; Whitehouseand Platt 2003), some of the range in DREE(Zrc/Grt) valuesmay be real, reflecting dependence on either pressure or themajor element composition of garnet (Rubatto 2002).

There is growing confidence that the equilibrium distributionof REE between zircon and Ca-poor Fe–Mg garnet at hightemperature results in DHREE(Zrc/Grt) values near unity, orslightly favouring garnet (Harley et al. 2001; Whitehouseand Platt 2003; Hokada and Harley 2004). The DREE(Zrc/Grt)signatures can be used not only to test for equilibriumbetween dated zircon domains and metamorphic garnetformed in peak high-T assemblages, but also to define tim-ing of zircon growth in relation to garnet and to evaluatethe trace element mobility in post-peak reaction textures.Two examples illustrating the richness of event–time andtemperature–time information on hot orogens that can beobtained from zircon through this approach are consideredin the following paragraphs.

ZIRCON GROWTH AND THE DYNAMIC HIGH-T MELTING REGIME Partial melting followed by melt segregation, accumulation,transfer and extraction are important processes during high-Tmetamorphism deep within hot orogens. Because zircon issoluble in partial melts, these processes may lead to zirco-nium redistribution during metamorphism through partialdissolution of pre-existing zircon and subsequent zircongrowth during melt crystallization or reaction with wall rocks(Roberts and Finger 1997; Hokada and Harley 2004). Zircon

29E L E M E N T S FEBRUARY 2007

may grow in equilibrium with a high-T melt during the pro-grade melting phase (Vavra et al. 1999; B in FIG. 1) and atvarious times during post-peak cooling and exhumationdepending on the water content of the melt, residencetimes and the character of wall rocks with which transportedmelt comes into contact (Harley 2004; C and D in FIG. 1).

Hokada and Harley (2004) attributed the growth of textu-rally polyphase zircon grains to melt injection followed byequilibration with the wall rock during post-peak melt crys-tallization. This occurred at extreme temperature conditionsof >900°C and over a maximum time interval of 25 millionyears. The key to this interpretation is the zircon and garnettrace element information. Texturally later, sector-zonedand unzoned mantles and rims on zircon grains in crystallizedpartial melts have chondrite-normalised HREE patterns flatterthan those of the earlier zircon grains. Garnets in the wall-rock gneiss have flat HREE patterns and show a remarkabledecrease in Zr close to their rims, coupled with a slightdecrease in overall HREE, but an increase in Yb as comparedto Gd. These chemical changes suggest that the invasivemelt initially precipitated zircon that was out of equilib-rium with the host garnet gneiss, but then equilibrated withthe wall rock and, in doing so, depleted Zr and HREE in thereacting host-rock garnet. Critically, REE data in zirconouter zones and garnet rims conform to the equilibriumDREE(Zrc/Grt) pattern proposed by Harley et al. (2001) andshown in FIGURE 3.

ZIRCON GROWTH AND RECRYSTALLIZATIONALONG P–T–TIME PATHS: REACTION TEXTURESEstablishing the links between zircon growth, recrystalliza-tion and specific P–T conditions along high-temperatureP–T paths requires in situ documentation of zircon zoningfeatures and morphology in relation to the petrographicevidence for subsolidus mineral reactions involving othersilicate phases (e.g. garnet, pyroxene, cordierite, opaqueminerals; Fraser et al. 1997; Degeling et al. 2001; White-house and Platt 2003). Assigning zircon ages to specificreactions, and hence temperature intervals (E in FIG. 1), iswell illustrated in the study of the Rogaland metamorphiccomplex (SW Norway) by Möller et al. (2003).

Möller et al. (2003) established that a ~1000 Ma regionalmetamorphic event in the Rogaland basement was over-printed by ultrahigh-temperature metamorphism associatedwith the emplacement of anorthosite and related igneousintrusions at ~930 Ma. However, their zircon U–Pb age data,obtained from zircon rims and overgrowths, define an age‘smear’ between 950 Ma and 900 Ma along concordia – arange too large to be consistent with a single thermal peakassociated with intrusion of the anorthosite.

In situ microanalysis of Rogaland zircon grains and detailedpetrographic examination of their textural contexts (Mölleret al. 2003; FIG. 4) indicate the presence of two texturallydefined and discrete groups at ~927 Ma and ~908 Ma, thusexplaining the zircon U–Pb ‘smear.’ The older group con-sists of zircon rims intergrown with or included withinminerals grown during the ultrahigh-temperature event(FIG. 4). Texturally later zircon rims that define the youngerage group occur outside these high-T minerals but are over-grown by lower-T coronas and symplectites, thereby con-straining the maximum age of the corona-forming reactionsto be ~908 Ma. Combining the age data with thermobaro-metric constraints on the mineral assemblages leads to anintegrated cooling rate within the aureole of 8 ± 2°C/Myr,from 880°C to 680°C, over some 25 million years. The slowcooling and longevity of this thermal anomaly, deducedfrom detailed zircon information, prompted Möller et al.(2003) to suggest that the UHT event reflects a deeper-seated thermal perturbation, perhaps produced by the con-vective removal of lithosphere.

THE SIGNIFICANCE OF THE ‘END GAME’: LATE FLUID INGRESSLate-stage fluid ingress, recorded by zircon in transgressiveand lobate textures (Vavra et al. 1999; Harley et al. 2001;Carson et al. 2002a), is likely to be important in resettingzircon U–Pb ages in metamorphic terrains as they areexhumed and cool to temperatures below the high-Twindow (FIG. 1).

Contrasting examples of element distribution data (DREE =distribution coefficient) for the heavy rare earth elements

(Eu to Lu), estimated from empirical studies of zircon (Zrc) and garnet(Grt) in high-T rocks. A zircon that crystallized in equilibrium with garnetwould have D values close to 1. Red circle: Harley et al. 2001; bluesquare: Rubatto 2002; yellow triangle: Whitehouse and Platt 2003; UHT:ultrahigh temperature.

FIGURE 3

A major step forward in age determination using zircon isthe ability to analyse grains in situ within their petrologi-

cal context. This figure shows that by drilling a small disc from a pol-ished thin section, delicate intergrowths and overgrowths of zircon havebeen preserved; these features would normally be lost during the stan-dard method of crushing rocks to obtain grain separates. (A) Transmit-ted light photomicrograph, (B) Secondary electron image, showing ionmicroprobe analysis pits. The larger and deeper pits are analysis sites fortrace elements and the smaller shallow pits are U-Pb analysis sites, anno-tated with concordia ages (C) Backscattered electron image, (D)Cathodoluminescence image.Scale bars are 50 microns. Fsp: feldspar;mag: magnetite; Grt: garnet; Zrc: zircon.

FIGURE 4

A

B

C

D

30E L E M E N T S FEBRUARY 2007

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When a terrain cools down towards the lower limit of thehigh-T window, any melts that have not been extractedfrom it will crystallize (F IN FIG. 1; Sawyer 2001; Harley2004). This will almost inevitably lead, at temperatures inthe range 750–650°C, to the production of fluid that willinteract with and partially hydrate the previously equili-brated high-T mineral assemblages. The quantity of suchfluid will be determined by how much melt has beenretained. Hydration reactions associated with final meltcrystallization and release of water may promote the growthof new zircon rims if zirconium is released through thebreakdown of high-T minerals, e.g. garnet and rutile break-ing down to biotite and sillimanite (G IN FIG. 1). Channelledalong microfractures, the fluids may also attack pre-existingzircon, recrystallizing susceptible zones (FIG. 2H; Vavra et al.1999), leaching trace elements from zircon surfaces (Carsonet al. 2002a) and resetting the U–Pb systematics. Under-standing and evaluating the significance of the processes inwhich melt and fluid affect zircon is central to the inter-pretation of the age of high-T terrains (e.g. Napier Complex:Carson et al. 2002b; Harley 2004; Kelly and Harley 2005).

CONCLUDING REMARKSThe combination of in situ zircon U–Pb geochronologywith detailed microtextural and trace element analysis ofzircon within its petrographic context, coupled with meta-morphic mineral equilibria modelling, is the optimalapproach to defining the thermal histories of high-T terrains.This approach is being refined as more experimental andempirical data become available. New trace element ther-mometers based on zircon, such as Ti in zircon and Zr inother minerals (e.g. Watson et al. 2006) will, when integratedinto the combined in situ approach, undoubtedly lead tomore detailed temperature–time records of deep crustalrocks and so further improve our understanding of thedynamics of hot orogens.

ACKNOWLEDGMENTSWe thank Joe Pyle, Fernando Corfu and Ian Parsons fortheir perceptive and careful reviews. We thank Urs Schalteggerfor kindly providing us with the image used in FIG. 2A. Thiswork was supported by the UK Natural EnvironmentResearch Council. .