from sediment to granite: timescales of anatexis in the upper crust

Ž .Chemical Geology 162 2000 155–167www.elsevier.comrlocaterchemgeo

From sediment to granite: timescales of anatexis in the uppercrust

Nigel Harris ), Derek Vance 1, Mike Ayres 2

Department of Earth Sciences, Open UniÕersity, Milton Keynes MK7 6AA, UK

Received 17 March 1998; received in revised form 15 June 1998; accepted 17 June 1998

Abstract

Granite formation is the culmination of a sequence of events initiated by prograde heating of the protolith and followedby formation of a grain-boundary melt, melt segregation into a vein network, ascent of the melt through the network and,finally, crystallisation of the melt. Experimental constraints on the formation of a crustal melt from the incongruent meltingof muscovite combined with geochemical studies of anatexis in the High Himalaya allow the timescales required for each ofthese processes to be assessed. Discordant temperatures determined from monazite and zircon thermometry for Himalayananatectic granites indicate that at least for some intrusives the melt was undersaturated in LREE implying that melts haveprobably been extracted in less than 10 ka. Experimental studies suggest that some Himalayan melts are also undersaturatedin Zr, implying segregation may have occurred within 100 years. Such short timescales confirm that deformation-drivenmechanisms are important in extracting these melts from their source. The transport distances of Himalayan granitic melts of;10 km may be achieved by the ascent of magma through dykes in about 1 day. At such rates even the largest granite couldtheoretically be emplaced in ;10 years. Crystallisation of Himalayan melts involves much longer periods. If emplaced as

Ž .thin sheets ;100 m wide a timescale of )500 years is required compared with )30 ka for single stage intrusion of thelarger laccoliths. For composite sheet complexes magma crystallisation, rather than melt ascent, comprises the rate-determin-ing step on the emplacement of the intrusion. The overall timescales of melt segregation and emplacement for manyorogenic granites are therefore less than 10 ka, and possibly less than 1 ka. In contrast, the timescale required for progradeheating of the protolith is 41 Ma. Since the melt production rate is determined by heat flow into the protolith, and not by

Ž .reaction kinetics for any geologically significant period we conclude that heat flow, determined by both the mechanism ofheating and the thermal diffusivities of crustal rocks, provides the overall rate-determining step of crustal anatexis. q 2000Elsevier Science B.V. All rights reserved.

Keywords: Crustal anatexis; Granite; Himalaya; Trace elements; Diffusion

) Corresponding author. Tel.: q44-1908-655171; fax: q44-1908-655151; E-mail: [email protected]

1 E-mail: [email protected] E-mail: [email protected].

1. Introduction

The rates of melt formation and emplacement formagmas of both felsic and basaltic compositionsremain poorly understood. U-series systematics pro-vide quantitative constraints for melts that are formed

0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 99 00121-7

( )N. Harris et al.rChemical Geology 162 2000 155–167156

and sampled within timescales of less than 350 ka,but while invaluable for the study of recent basalticvolcanism, the technique is clearly inappropriate for

Ž .granitic rocks since i rates of formation and extrac-tion are likely to be much slower due to their high

Ž .viscosities and ii granitic bodies are extremelyunlikely to be exhumed within such a short timescale.Techniques that include the direct isotopic measure-ments of geological events using high-precisionchronometry and methods underpinned by diffu-sional processes are currently yielding preliminaryconstraints on the rates of formation of graniticmelts. Whereas the former approach may be unableto resolve events over timescales of -1 Ma, thelatter suffers from large uncertainties in the appro-priate diffusion coefficients. Moreover, there is ad-ditional uncertainty concerning the nature of the phy-sical process that the diffusional rate-meters aregauging. Thus, despite numerous models for thesegregation and ascent of granitic magmas, there arefew constraints derived from empirical data.

In this paper we review those geochemical charac-teristics of crustally-derived granite melts and theirsource regions that provide some information on therates at which these melts are formed, transportedand emplaced, and assess the uncertainties implicit inthese approaches. This requires an analysis of themechanisms and timescales involved during melting,melt segregation, magma ascent and magma chamberdynamics, each of which is discussed below. Weapply recently-derived diffusional models to rates offormation for the High Himalayan leucogranites,which are among the best-documented of all crustalmelts in terms of their source regions, melt-formingreactions and transport lengthscales.

2. Anatectic granite from the High Himalaya

The combination of high-grade anatecticmigmatites and crustally-derived intrusive granitesfrom the High Himalaya provide a well-documentedlaboratory for examining the timescales of magmaticprocesses during orogenic events. The High Hi-malayan leucogranites form small laccoliths and sheetcomplexes of peraluminous leucogranite intruded into

Žmetasedimentary lithologies 19–22 Ma ago Le Fortet al., 1987; Harris and Massey, 1994; Harrison et

.al., 1997 . Individual laccoliths range in size from3 Ž;3000 km for Manaslu in central Nepal Guillot

. 3and Le Fort, 1995 , to 150 km for Gangotri in theŽ .western Himalaya Scaillet et al., 1995a . Many other

intrusions form sheet complexes comprised of indi-Žvidual granite sheets tens of metres thick Harris,

.1997 . The leucogranites are mostly emplaced intothe northernmost exposures of a sequence of high-

Žgrade metasedimentary rocks the High Himalayan.Crystalline Series within a major detachment zone

that separates these metasediments from the unmeta-morphosed Tethyan sediments to the north.

Major-element abundances and modal mineralogyof the leucogranites indicate minimum-melt compo-

Žsitions Le Fort et al., 1987; Scaillet et al., 1990;.Harris et al., 1995 . The isotope geochemistry of

Ž .these granites Sr, Nd and Pb is strongly indicativeŽof a mature crustal source Gariepy et al., 1985;´

.Deniel et al., 1987 . Trace-element modelling sug-gests that the leucogranites represent low degree

Ž .partial melts F-0.2 of pelitic sources from mus-covite breakdown under vapour-absent conditions at

Ž .temperatures of 700–7508C Harris et al., 1995 .This conclusion is confirmed by experimental studies

Ž .of both leucogranites Scaillet et al., 1995b and theŽ .metapelitic protolith Patino Douce and Harris, 1998 .˜

Two-mica and tourmaline-bearing varieties ofleucogranite have been widely recognised as forming

Ž .distinct intrusive phases Reddy et al., 1993 . Smallbut systematic differences in the initial Sr-isotoperatios between these two melt compositions havebeen ascribed to the modal composition of their

Ž .respective protoliths Guillot and Le Fort, 1995 ,although alternative mechanisms for generating thetwo magma types include fractional crystallisationŽ .Scaillet et al., 1990 and variation in water activity

Ž .during anatexis Harris and Inger, 1992 .

3. Melt generation

Crustal melting is initiated by modifying the phys-ical conditions of the subsolidus assemblage of theprotolith. Melting is caused by the assemblage cross-ing its solidus in PT space through one of threeprocesses: prograde heating, fluid influx or decom-pression. The resultant incipient melts form along the

( )N. Harris et al.rChemical Geology 162 2000 155–167 157

grain boundaries of the reactant phases. Once melt-ing is initiated, the rate of melt production is con-trolled by the kinetics of the melt reaction providedthat the thermal diffusivity of the protolith and therate of heat production in the crust allow the assem-blage to remain above the solidus.

In general, reaction kinetics will proceed rapidlyonce the activation energy of the reaction is ex-ceeded. Numerous experimental studies of naturalrocks confirm that melts with minimum-melt bulkcompositions are generated in a matter of weeksŽe.g., Holtz and Johannes, 1991; Patino Douce and˜

.Johnston, 1991 . Unusually rapid melt production,which may result either from significant temperatureoversteps or from fluid infiltration in rocks that areabove the fluid-present solidus, will result in phasedisequilibrium as observed in experimental studies of

Žincongruent melting of muscovite and quartz Brear-.ley and Rubie, 1990 . Melts formed in this way will

not be of minimum-melt composition but be charac-terised by strongly siliceous and sodic compositionsdue to the rapid dissolution of quartz and the parago-nite component of muscovite.

Once initiated, melting continues until either areactant is consumed, or the temperature of the rockdrops below that of the solidus due to the heatrequired for fusion. If the prograde heating rate issufficiently low that initial melting is buffered byfusional heat loss, then melt production will becontrolled by the thermal diffusivity of the rock, acondition known as thermal buffering.

Different tectonic environments result in stronglydiffering prograde heating rates. For example, advec-

Žtive heating from intrusion of basic magmas Hup-.pert and Sparks, 1988; Davidson et al., 1992 causes

heating rates of the order of 2008CrMa, dependingon the size of, and distance from, the intrusive body.For convergent orogens the most frequently invokedmechanism for heat transfer is conductive heating byinternal heat sources following crustal thickening.Theoretical considerations imply heating rates by

Žthis mechanism of 10–508CrMa England and.Thompson, 1984; Thompson and Connolly, 1995 .

Direct measurements of heating rates by garnetchronometry in regional metamorphic belts have

Žyielded even lower estimates of 5–158CrMa Burtonand O’Nions, 1991; Vance and O’Nions, 1992; Vance

.and Holland, 1993; Christensen et al., 1994 .

In the case of decompression melting resultingfrom a period of rapid exhumation under fluid-absent

Ž .conditions Le Breton and Thompson, 1988 , meltingrates are determined by the relative rates of exhuma-tion and of conductive cooling. Whereas the formerrate is controlled by tectonics, the latter is a functionof the thermal diffusivity of the rock.

The first lithologies that are likely to melt duringprograde heating of tectonically thickened crustwill be muscovite-bearing metapelites and meta-greywackes through the incongruent breakdown of

Ž .muscovite Clemens and Vielzeuf, 1987 . The kinet-ics of the muscovite melting reaction, modelled from

Ž .experimental data Rubie and Brearley, 1990 , indi-cate that melting rates are highly sensitive to thetemperature overstep. This results from the nucle-ation energy required for the formation of peritectic

Ž .phases sillimanite and alkali feldspar during theincongruent melt reaction. Assuming a progradeheating rate of 208CrMa, consistent with internalheating in thickened crust, for oversteps of 20–1008Cit takes 1 year or less to achieve the maximum melt

Ž .fraction Fig. 1 , during which time melt productionis kinetically controlled. Thereafter, the melt produc-tion rate is controlled by heat flow in the protolith.Increasing the imposed prograde heating rate willaffect the rate of melting once heat flow is control-

ŽFig. 1. Melt fraction vs. time at varying values of DT tempera-.ture overstep for the muscoviteqquartz melting reaction from

Ž .Rubie and Brearley 1990 . Dashed line marks the boundarybetween kinetic control and heat flow control of melt production.Prograde heating rate s 208Crkm, activation energy s 235kJrmol, latent heat of fusions200 kJrkg. The model assumesthat the protolith is of an appropriate composition to yield aconstant melt fraction at uniform heat flow once reaction is heatflow-controlled.


ling melt production but will not change the periodfor which reaction kinetics determine melt produc-tion. These results imply that for any geologically

Ž .significant period )1 year , heat flow is the princi-pal determinant of the rate of melt production.

3.1. Rates of prograde heating during the Himalayanorogeny

Experimental heating of Himalayan metapelitesunder vapour-absent conditions has resulted in meltpools with homogeneous major-element composi-tions comparable with those of intrusive granites

Žwithin a period of 3 weeks Patino Douce and Harris,˜.1998 . This suggests that equilibrium has been

reached for major elements, at least locally, betweencrystal edges and melt and within the melt itself.This is not predicted to occur while the reaction is

Ž .kinetically controlled Brearley and Rubie, 1990 andthus confirms the conclusions drawn from considera-tions of the reaction kinetics for the muscovite melt-

Ž .ing reaction Fig. 1 . So for geologically significantperiods, the prograde heating rate, a function of boththe heating mechanism and the thermal diffusivity ofthe rocks, will be the critical control on the meltproduction rate.

Although prograde heating in tectonically thick-ened crust is generally derived from internal heatsources, thermal anomalies may result over briefperiods from dissipative heating along major thrust

Ž .zones England et al., 1992 . Furthermore, melting inthe Himalayan orogen may have been triggered or

Ženhanced by decompression Harris and Massey,.1994 such that during exhumation the melt produc-

tion rate will be controlled, in part, by tectonicprocesses. The slope on the fluid-absent solidus for

Ž .Himalayan pelites Fig. 2 indicates that for a 208Coverstep of the muscovite melt reaction, an exhuma-tion of about 5 km is required. Assuming a veryrapid exhumation rate of 5 mmryear, consistent withexhumation rates determined from the western Hi-

Ž .malaya Whittington, 1996 , this overstep will beachieved in about 1 Ma. This is similar to the rate atwhich a 208C overstep will be achieved by progradeheating in tectonically thickened crust. Conse-quently, although we are unaware of experimentalconstraints on the rates of crustal melting duringdecompression, we would expect that reaction kinet-

Ž . ŽFig. 2. Melting curves for 1 wet pelites Le Breton and Thomp-. Ž . Ž . Ž .son, 1988 , 2 muscoviteqalbiteqquartz Peto, 1976 and 3¨

Ž .Himalayan pelite Patino Douce and Harris, 1998 ; dotted line˜indicates melt fraction. Stippled field indicates peak PT conditions

Ž .for Himalayan migmatites Harris and Massey, 1994 . Box ‘E’Žindicates emplacement conditions for leucogranites Guillot et al.,

.1991 . Arrow indicates approximate transport distance for ascend-ing melts.

ics would be the rate-determining step for only avery brief initial stage. Thereafter, the melt produc-tion would be controlled by a combination of ex-humation, which drives the prevailing PT conditionsof the rock further from the solidus into the meltingfield, and the thermal diffusivity of the rock, whichallows the rock to cool towards the solidus. For allbut the most rapid exhumation rates, melt productionwould be controlled by the thermal diffusivity of therock.

Semi-quantitative information on the rate at whichthe Himalayan granite protoliths have been heatedcan be obtained from garnet diffusion profiles from

Ž .pelitic metasediments Ayres and Vance, 1997 . Mnzoning in garnets from Himalayan and Dalradianamphibolite-grade metamorphic rocks, formed undersimilar PT conditions and of similar grainsize, show

Ž .strongly contrasting profiles Fig. 3 . Whereas theDalradian garnet has a flattened profile that hasclearly relaxed from an inferred initial Rayleighgrowth profile, the Himalayan garnet shows an array

Žclose to the initial growth profile Ayres and Vance,.1997 . Such garnet profiles are typical for the Hi-

malayan assemblages and contrast with garnets from


other orogens suggesting that metamorphic rocks inŽthe Himalaya spent considerably less time by a

.factor of 5–10 at, or near, peak metamorphic condi-tions compared to rocks exhumed from an equivalent

Ždepth in the Dalradian orogen Ayres and Vance,.1997 . If appropriate Mn diffusion coefficients are

applied, for the Himalayan garnets, temperatures over7008C were maintained for periods of ;1–2 Ma.Unfortunately, such calculations are subject to largeuncertainties due to extrapolation of experimentaldiffusion data to the low temperatures involved.

Direct calculations on prograde heating rates inthe Himalayan metasediments can be made fromhigh-precision garnet chronometry. Sm–Nd chronol-ogy, in combination with PT histories, suggests thatheating rates in the Himalaya were of the order of15–258CrMa — slightly higher than previously de-

Žtermined for other metamorphic terrains Vance et.al., 1997; Vance and Harris, 1999 . These data can

be combined with formation temperatures ofleucogranites as well as Ar–Ar cooling ages fromthe Zanskar Himalaya to define the thermal history.

Fig. 3. Mn zoning profiles in metamorphic garnets formed atŽ . Ž630–6408C and 700 MPa pressure for a Dalradian garnet from

. Ž . ŽAyres and Vance, 1997 ; b Himalayan garnet Vance, unpub-.lished data . A K of 80 is assumed for calculation of thed

Rayleigh growth profile.

Fig. 4. Simplified temperature–time path for granite protolithfrom High Himalaya Crystalline Series in the Zanskar Himalaya.

Ž .Prograde path heavy line determined from garnet chronometryŽ .Vance and Harris, 1999 , and cooling path from Ar–Ar mus-

Ž .covite ages Vance et al., 1998 . Shaded field indicates periodŽ .above 7008C. T Sr indicates closure temperature for Sr inc

plagioclase. The assymetric shape of the T – t path results fromthe abrupt termination of metamorphic heating by extension.

Ž .From the resulting temperature–time profile Fig. 4 ,it can be seen that the granite protoliths were attemperatures )7008C for a period of ;3–4 Ma.This does not imply that melting was continuousthroughout this period but that during this time melt-ing could occur due to prograde heating or to achange in other variables such as fluid influx orrapid decompression. Once melting is initiated, theheating rate will be slowed down by thermal buffer-ing as has been inferred from the identification of aflat metamorphic gradient from a section of Hi-

Ž .malayan metasediments by Hodges et al. 1988 . Fig.4 also shows that temperatures were in excess of theclosure temperature for Sr in plagioclase for )10Ma, about two-orders of magnitude longer than thetimescale required for Sr-disequilibrium to be pre-

Ž .served during melting Harris and Ayres, 1998 .Thus, the absence of Sr-isotope disequilibrium ob-served between Himalayan granites and source isunsurprising.

4. Melt segregation

Low melt fractions can only escape from theirprotoliths once an interconnecting fluid network ex-

Ž .ists Waff and Bulau, 1979 . Experimental studies of


the dihedral angle between melt and silicates confirmthat such an interconnected grain-boundary fluid will

Žbe present even at low melt fractions Jurewicz and.Watson, 1984; Laporte, 1994 . The liquid-percola-

tion threshold has been defined from rheologicalstudies as that melt fraction below which melt pock-ets formed along grain boundaries are not intercon-nected and thus the melt cannot segregate. Values forthe liquid percolation threshold have been estimated

Ž .at F)0.08 Vigneresse et al., 1996 , and at F)0.05Ž .Dell’Angelo and Tullis, 1988 , where F is the meltfraction by volume. The rheological transition be-tween crystal-supported and fully fluid behaviour isa significant factor in determining the rate of meltproduction in the melt-fraction range 0.05–0.15Ž .Barboza and Bergantz, 1998 . At much higher melt

Ž .fractions buoyant rise of the melt diapirism may bepossible once the melt-escape threshold is reached;

Žthis has been estimated at Fs0.26–0.4 Vigneresse.et al., 1996 .

For melt to escape at melt fractions between thesetwo thresholds, a driving force is required in the

Ž .form of deviatoric stress Sawyer, 1994 . Experimen-Žtal deformation of partially molten granite Rutter

.and Neumann, 1995 identifies shear-enhanced com-paction by porous flow as a process that could drivethe melt from the grain-boundary melt into veins.Major controls on shear-enhanced compaction in-

Žclude the viscosity of the melt closely related to its.water content and temperature and the shear stress

imposed on the protolith.Experimental studies of the role of deviatoric

Žstress on the segregation of granitic melts e.g.,.Rutter and Neumann, 1995 are based on an isotropic

granite source and, perhaps surprisingly, the rate ofsegregation of crustal melts has not been directlymeasured in more realistic metasedimentary pro-toliths. However, well-constrained natural examplesof crustal melting may provide some information onsegregation rates. For example, modelling of thethermal history of anatectic pelites from the aureole

Žof the Ballachulish igneous complex Buntebarth,.1991 indicates that temperatures were only in ex-

Ž .cess of solidus temperatures ;6808C for ;50 ka,providing a maximum constraint for segregationtimes of a granitic melt.

Direct measurement of segregation rates in naturalrocks requires an observable time-dependent process

that will be initiated once the melt is formed and,ideally, be terminated once the melt has segregated.One potential process is the diffusional transport ofmaterial within a segregating melt. Provided theappropriate diffusion coefficients are known, the dis-solution of selected phases from the protolith into themelt could satisfy the first of these conditions.

The concentrations of LREE and Zr in a graniticmelt formed by anatexis of a metapelitic protolithwill be buffered by the stability of monazite and

Žzircon, respectively Watson and Harrison, 1983;.Montel, 1993 . The key variables in determining

dissolution rates are the size of the zircon, the H O2

content of the melt, and the temperature and heatingŽ .rate of the protolith Watson, 1996 . The rate at

which equilibrium is reached between dissolvingmonazite or zircon and a static melt is limited largelyby the rate at which Zr and LREE can diffuse awayfrom dissolution sites. If melt extraction rates exceedthe rates at which diffusional processes can ho-mogenise the LREE and Zr in the magma, the meltremains undersaturated with respect to these ele-ments. Since the rate at which monazite dissolves is

Žslower than for zircon dissolution for a given melt.structure, water content and temperature then some

rapidly extracted melts might retain saturated Zrabundances, but be undersaturated in LREE.

4.1. Segregation of Himalayan melts

A recent study of Himalayan leucogranite chem-istry has identified inconsistent temperatures derivedfrom monazite and zircon thermometry in some in-

Ž .trusions Ayres et al., 1997 . This observation isinterpreted as evidence for undersaturation in LREEdue to incomplete monazite dissolution prior to meltextraction. Quantitative modelling of the time-depen-dent homogenisation process by this study assumesŽ .i that the melt was a semi-infinite medium which

Ž .behaved as a static fluid and ii that accessory phaseinheritance was minimal. Although there is isotopicevidence for inheritance of zircon and monazite in

Žsome Himalayan granites Copeland et al., 1988;.Harrison et al., 1995 it was argued by Ayres et al.

that the concordant temperatures obtained by the twothermometers in the majority of Himalayan granitesimplied that accessory-phase inheritance does not


contribute significantly to bulk trace-element abun-dances. Based on the observed size and distributionof accessory phases in the protolith and on themeasured trace-element concentrations in the granite,the inference that the melt was saturated in Zr butundersaturated in LREE implies that melts have beenextracted in less than 10 ka, although the propagationof realistic errors through the calculation allow thismaximum constraint to rise to 50 ka.

More recently, an experimental study of a Hi-malayan pelitic metasediment generated melts ofsimilar compositions to Himalayan leucogranites un-der conditions of vapour-absent melting of mus-

Ž .covite Patino Douce and Harris, 1998 . However,˜the solidus temperatures required were elevated by)508C compared to those obtained from the end-

Žmember assemblage muscovite–albite–quartz Fig..2 and were also somewhat higher than those in-

ferred from concordant accessory phase thermometryŽ .650–7508C . For melts formed at pressures )500

Ž .MPa allowing a melt fraction of 0.08 , solidustemperatures )7508C are implied, which is consis-tent both with the temperatures obtained from themigmatites into which the melts are intruded andwith liquidus temperatures for biotite–muscovite Hi-

Ž .malayan leucogranites Scaillet et al., 1995b . Thus,it is possible that accessory phase undersaturationmay have occurred for both zircon and for monazite.In this case the homogenisation time for zircon andmelt, determined by the rate of diffusion of Zr in themelt, provides an additional constraint on the timeavailable for melt segregation.

Three-dimensional modelling of zircon dissolu-tion rates during crustal fusion under a range ofconditions has been undertaken assuming a spherical

Ž .geometry for dissolving crystals Watson, 1996 . Forgranite melts generated at 7508C, a melt water con-tent characteristic of Himalayan leucogranite meltsŽ .6%; Scaillet et al., 1995a,b and an initial undersat-

Žuration in the melt of 90 ppm so the Zr of melt was.-2 ppm prior to zircon dissolution then about 150

years are required for the melt to reach saturationlevels of Zr. The estimate is highly sensitive to water

Žcontent, and increases to ;4 ka for an unrealisti-.cally low melt water content of 3%. Less impor-

tantly, the estimated saturation time decreases ifprograde metamorphism increases the temperatureabove the solidus but, as discussed in the previous

Fig. 5. Degree of undersaturation of Zr vs. time in a meltŽ .H Os6% resulting from anatexis at 7508C, initial zircon radius2

of 15 mm, and initial undersaturation for Zr of 90 ppm. Zrsconcentration of Zr in melt, ZrU szirconium saturation at 7508C.Assumes constant temperature dissolution of spheres calculated

Ž .from Eq. 17 of Watson 1996 . Dashed line indicates undersatura-tion appropriate to Himalayan tourmaline–muscovite granites ifformed at 7508C.

section, it is likely that dTrd t was essentially zerodue to thermal buffering during anatexis.

The measured Zr content of ;35 ppm for tour-maline–muscovite leucogranites is less than 40% ofsaturation abundances for Zr at 7508C, equivalent toa reduction in radius of a spherical zircon crystal of;15%. For an initial grain diameter of 30 mmŽ .Ayres et al., 1997 , this is equivalent to a period of

Ž .;40 years Fig. 5 . Because of the uncertainty inmelt water content of leucogranites, this time periodcould rise to ;100 years.

The significance of these timescales for segrega-tion rates depends on the rates of processes thatoccur after the melt has segregated. This is becauseaccessory phase dissolution from the wallrock mayin principle continue as the melt ascends through,and crystallises in, crustal rocks. Hence, accessoryphase dissolution can only provide constraints on thesegregation process if the subsequent magmaticevents can be shown to be extremely rapid.

5. Melt ascent

Once driven into a vein network, the melt maymigrate and ascend through fracture propagationŽ .Clemens and Mawer, 1992 . During fluid-absent


melting, the increase in molar volume will result inthe formation of microcracks provided the volume

Žchange is not accommodated by creep Connolly et.al., 1997 . Such cracks exploit grain boundaries and

form a permeable network. However, whether a dykecan propagate far from a magma reservoir dependson the competition between the rate at which thedyke is propagating and the rate at which crystallisa-

Žtion is occurring within the dyke Rubin, 1995;.Weinberg, 1996 . These authors have argued that

rhyolite dykes will generally be halted by freezingsoon after leaving the source area, thus limiting theability of dykes to transport granites long distances.Such arguments are inconsistent with the commonobservation of feeder dykes associated with granitemagma chambers and may need to be revised in thelight of the decrease in the estimated viscosities of

Žgranitic melts published in recent studies e.g., Hess.and Dingwell, 1996 .

Once in a dyke, laminar flow rates of the meltwill be controlled by the viscosity and the density of

Žthe magma and by the thickness of the dykes Pet-.ford et al., 1993 . Under these conditions it is possi-

ble to estimate extremely rapid ascent rates. For abatholith in the Cordillera Blanca, an ascent rate of;1 cmrs and an emplacement timescale of 350

Ž .years is suggested Petford et al., 1993 . This calcu-lation assumes that a critical dyke width of ;6 mhas been achieved which is needed for the magma ofthe appropriate viscosity not to freeze during ascent.

5.1. Ascent of Himalayan melts

There is abundant field evidence that many, if notall, Himalayan leucogranite melts have been trans-

Žported through feeder dykes Inger and Harris, 1993;.Scaillet et al., 1995a . The lengthscale of melt ascent

through dykes is critical in determining whether ornot melts may freeze. Thermobarometric analysis ofmetapelitic migmatites in the High Himalaya indi-

Žcate pressures of 500–600 MPa Harris and Massey,.1994 , providing a minimum constraint on pressures

of leucogranite melt formation. Melt generation hasŽbeen estimated to occur at 700–1000 MPa Harris

.and Massey, 1994 which compare with emplace-ment pressures of 300–400 MPa estimated fromthermal aureole assemblages of the Manaslu graniteŽ .Guillot et al., 1991 . Transport distances of the melt

Ž .of ;10 km are thus implied Fig. 2 . Such a shortlengthscale confirms that whatever the arguments forthe feasibility of crustal-scale magma transport byfracture propagation, dyke transport provides a real-istic mechanism for migration between vein net-works and emplacement for Himalayan leucogranitemagmas.

The calculated ascent rate is strongly dependenton the viscosity of the magma. The study of ascentrates for magma feeding the Gangotri laccolithŽ .Scaillet et al., 1996 is based on an experimentallydetermined viscosity of 1=104.5 Pa s for Himalayanmelts. Such viscosities are consistent with a reviewof granite melt viscosities for values of melt water

Ž .content of 6% Hess and Dingwell, 1996 . From theŽ .Fig. 4 of Petford 1996 , a critical dyke width of ;3

m is required for flow to occur without freezing. A3-m dyke would permit an average ascent velocity of

Ž9.5 cmrs assuming Newtonian behaviour for the.magma and an ascent time of ;1 day. At such a

rate, the Manaslu leucogranite, the largest knownHimalayan laccolith, could be emplaced in ;10years, assuming it results from a single meltingevent.

6. Magma chamber processes

Within the magma chamber a period of crystal-lisation is involved which may allow interaction

Ž .between melt and wallrock assimilation androrfractional crystallisation. Published estimates of resi-dence times for felsic magma chambers are based onradiogenic isotope systematics. For example, magmachamber residence times can be calculated for rhyo-lite flows by combining Rb–Sr mineral ages anderuption ages. A detailed study of high-silica rhyo-lites from Long Valley, CA, concludes that the bulkof feldspar crystallised in -15 ka and the overall

Žmagma chamber residence time is -360 ka Davies.et al., 1994 . A more recent ion-microprobe study of

younger rhyolites from the same magmatic systemŽ .Reid et al., 1997 harnessed U–Th disequilibrium inmagmatic zircons to infer that the zircons crys-tallised )100 ka before eruption, again constrainingmagma chamber residence times. These studies takentogether suggest that a period of several hundredthousand years may be involved in the crystallisationof large felsic magma chambers.


6.1. Magma chamber dynamics for Himalayan gran-ites

There are few robust empirical estimates of crys-tallisation rates for Himalayan leucogranites. In thecase of the Langtang granite, a detailed isotope studyŽ .Inger and Harris, 1993 concludes that the Sr-iso-

Ž87 86tope ratios of the Langtang granite Srr Srs.0.749"0.007 are characterised by the isotopic

Ž87 86characteristic of the presumed source Srr Srs. Ž87 860.757"0.012 , not the country rock Srr Srs.0.830"0.032 . Hence, there is no detectable ex-

change of Sr between melt and the rocks into whichthe melt has been emplaced. The absence of contam-ination by significant wall-rock Sr in the melt indi-cates crystallisation over a timescale of -100 kaŽ .Harris and Ayres, 1998 . Certainly, the emplace-ment and crystallisation of Himalayan sheet com-plexes into a detachment zone separating sillima-nite-grade migmatites from low-grade carbonates andshales provides strong thermal and rheological con-trasts that would promote rapid freezing.

Thermal modelling suggests that cooling of aleucogranite sheet 2 km thick into rocks at tempera-

Žtures of 5008C will take ;200 ka De Yoreo et al.,.1989; Davidson et al., 1992 . Using these data,

Ž .Scaillet et al. 1996 estimate a time period of 30–100ka for crystallisation of the Gangotri laccolith. Incontrast to the Gangotri study, a detailed field andgeochemical study of the Manaslu laccolith con-cludes that discrete magma batches accreted as ;100

Žm sheets into a cold environment Deniel et al.,.1987 . According to the calculations of De Yoreo et

Ž .al. 1989 , a granite sheet of this thickness requires;500 years to solidify, if intruded into rocks withan initial temperature of 5008C, which rises to 1600years for an initial temperature of 6008C.

7. The rate-determining step of magma formationin the High Himalaya

Arguments for linkage between melt productionrates and granite emplacement rates require an un-derstanding of the rate-determining step. Scaillet et

Ž .al. 1995a infer from field evidence that magmaascent to form the Gangotri laccolith was a continu-ous process and infer that the modelled transport

period of -100 years is a measure of the timescaleof melt production. However, it is also possible thatthe intrusion resulted from individual pulses of meltthat ascend into the magma chamber before crystalli-sation is complete; this scenario would result in anintrusion with the characteristics of continuous em-placement. Since the crystallisation rate is consider-ably slower than the ascent rate, these discrete batchesof melt could have been generated over a period ofthousands of years, as constrained by the crystallisa-tion rate. For sheet complexes, where field relationsindicate that earlier sheets solidify before the intru-

Ž .sion of subsequent sheets Harris, 1997 , the time-Ž .scale of crystallisation )500 years provides a

lower limit on the emplacement rate of the compos-ite body. Indeed, brief discontinuous periods of melt-ing may have occurred over much longer periods as

Ž .allowed by i the temperature–time profile of theŽ . Ž .granite protoliths Fig. 4 and ii the age range of

high precision chronometry from individual intru-Ž .sions Harrison et al., 1995 . Factors that could

trigger specific melting events once the protolithtemperature exceeds the wet solidus include fluidinfiltration and deformation. Recently, monazitechronometry has identified two separate episodes of

Ž .melting separated by 3–4 Ma that contributed tothe Manaslu intrusion, each phase lasting less than 1

Ž .Ma Harrison et al., 1999 . In the absence of robustconstraints on the absolute rates of each of theprocesses involved in forming anatectic melts, it isinstructive to evaluate the overall rate-determiningprocess.

From experimental studies of the Westerly gran-ite, melt extraction has been identified as the rate-de-

Žtermining step for magma extraction Rutter and. 6 7Neumann, 1995 . A melt viscosity of 10 –10 Pa s

is obtained from the Eq. 23 of Rutter and NeumannŽ .1995 for a melt water content of 6% and a solidustemperature of 7508C, appropriate to Himalayanleucogranites. It can be deduced from Point A, Fig.

Ž .6, that i melt extraction took place over a period ofŽ .;1000 years and ii the required deviatoric stress

during shear-enhanced compaction is )10 MPa.Under these conditions, melt vein-network extractionprovides the rate-determining step. However, condi-tions during the formation of Himalayan leucogran-ites differ from the experimental conditions in two

Ž .important respects: 1 The effective heat source for


Fig. 6. Rate-determinig step for granite melt extraction assumingŽFs0.1 from a protolith 1 km thick after Rutter and Neumann,

.1995 . T – t space divided into shear-enhanced compaction, vein-network extraction and heat flow fields indicating rate-determin-ing step. Dashed line indicates differential stress at 10 MPa and

Ž .dotted lines indicate melt viscosity in Pa s . Point A for meltformed at 7508C, hs106.5 Pa s; point B for melt formed at7508C, hs104.5 Pa s.

prograde heating of the protoliths for Himalayangranites is likely to have been internal radioactivedecay and not advective heating due to the intrusionof mafic magma into the crust. This will greatlyextend the range of conditions over which the rate of

Ž .melt extraction is heat flow-limited. 2 The sourcerock of Himalayan granite is a pelitic metasedimentand not a granite. This provides a pre-existing planarfabric in the protolith which will favour vein forma-tion. Furthermore, during anatexis of a metapelite,melt will initially form around grain boundaries of

Ž .muscovite e.g., Patino Douce and Harris, 1998 . In˜contrast, for a granite protolith, the melt will formaround quartz–feldspar grain boundaries. Since micais considerably less abundant in a pelitic rock than isfeldspar in a granite, the distribution of melt poolswithin the pelitic protolith will be more dispersed,thus retarding the melt segregation process and ex-panding the conditions under which shear-enhancedcompaction provides the rate-determining step.

Perhaps most important of all, viscosities ofleucogranite melt are now believed to be at least twoorders-of-magnitude below values estimated by ear-lier studies. If viscosities of 104.5 Pa s are assumedŽ .from Scaillet et al., 1996 it would drive Himalayanmelts into the field where heat flow provides therate-determining step, with predicted melt-extraction

Ž .times of less than 100 years Point B, Fig. 6 . Ingeneral, it is unlikely that during the formation ofanatectic granites vein-network extraction will everprovide the rate-determining step for melt extraction.

Ž .For Himalayan granites Fig. 7 , it appears thatonce the melt has entered a dyke system, thetimescale for melt ascent will be extremely fastŽ .;10 years . The calculated timescale for accessory

Ž .phase dissolution -10 ka and possibly -100 yearsis therefore an indication of the rate of melt segrega-tion. The conclusion that felsic melts can segregatefrom migmatites at such rapid rates concurs with thestudy of melt formation in the Ballachulish intrusionŽ .Buntebarth, 1991 . In the Himalayan example, meltsegregation may have been accelerated by dilatancypumping resulting from orogenic collapse along the

ŽSouth Tibetan Detachment System Harris et al.,.1995 .

ŽFig. 7. Flow diagram showing stages of melt evolution rectangu-. Ž .lar border linked by processes elliptical border for crustal melts.

Ž . Ž1 Thermal modelling of granite sheet 100 m thick Davidson et. Ž . Ž .al., 1992 ; 2 calculated from Eq. 4 of Petford et al. 1993 ,

Ž .assuming melt transport by fracture flow; 3 from monaziteŽdissolution rate assuming undersaturation in LREE Ayres et al.,

. Ž .1997 ; 4 from zircon dissolution rate assuming undersaturationŽ . Ž .in Zr Watson, 1996 ; 5 from chronological constraints given in

Fig. 4.


Thus it appears that the timescales required formelt segregation, ascent and emplacement are short

Ž .for orogenic granites Fig. 7 . Under these condi-tions, the overall rate of granite formation duringcrustal melting is primarily controlled by heat flowinto the protolith, which is determined by both themechanism of heating and the thermal diffusivitiesof crustal rocks. More precise estimates of the ratesof the processes involved in granite formation re-quire improved precision in appropriate experimentaldiffusional data. Meanwhile, future research avenuesare opening up as technical advances in mass spec-trometry allow high spatial precision for isotopicdeterminations such that disequilibrium features canbe detected over intragrain lengthscales. Unfortu-nately, one consequence of the slow heating rate of

Žthe protolith a characteristic of orogenic granite.formation is that critical prograde information is lost

due to chemical equilibrium between melt and source.It is therefore important to investigate and interpretempirical evidence for chemical disequilibrium innatural examples where melting results from rapid

Ž .heating such as in thermal aureoles , thus preservingprograde information.

Acknowledgements

The award of Royal Society Scientific Investiga-tion grants supported field work in Zanskar, Garhwaland Sikkim Himalaya. We thank Nick Petford forhelpful discussion and Bruno Scaillet and GawenJenkin for perceptive and constructive reviews.

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from sediment to granite: timescales of anatexis in the upper crust

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