Journal of the Geological society, London, Vol. 144, 1987, pp. 281-297, 13 figs. Printed in Northern Ireland

The segregation and emplacement of granitic magmas

S. M. WICKHAM Division of Geological and Planetary Sciences California Institute of Technology, Pasadena, California 91125 USA

Abstract: The segregation of granitic magma from residual crystals at low melt-fraction is strongly dependent on the viscosity of the melt. Theoretical considerations imply that for the typical range of granitic meltviscosities (104Pa S to 10” Pa S) only verylimited separation will bepossible by a compaction mechanism over the typical duration of a crustal melting event (c. 106years). Small-scale segregations (millimetre to metre) of the type observed in migmatite terranes may be generated by compaction (possiblyassisted by -continuousdeformation), or byflow of melt into extensional fractures, but low melt-fraction liquids are unlikely to be extracted to form large (kilometre-size) granitic plutons because of the limited separation efficiency. At higher melt-fractions (>30%) the rapid decrease in strengthand effectiveviscosity during partial meltingallows other segregation processes tooperate. Calculations and experiments indicate that in granitic systems the effective viscosity of partially melted rocks,having a very narrow melt fraction range of 30-50% will fall rapidly to levels at which convective overturn of kilometre-thick zones can occur. Convective motion within anatectic regionsis capable of generating large (kilometre-size) homogeneous, highcrystal- fraction, crustally-derived magma bodies, which are orders of magnitude greater insize than low melt-fraction segregates. Before convective instability is reached, small (centimetre- to metre-sized) pods of granitic liquid may rise buoyantly through, and pond at the top of such partly molten zones; such a process is consistent with the observation that some granulites appear to be residue rocks, chemically depleted in a minimum melt component. The effective viscosity (and hence the suscep- tibility to convection) of a partially melted zone within the crust, is strongly dependent on the water content of the system at a given pressure and temperature, because this controls both the quantity of melt generatedand also the viscosity of the melt. The intrinsic water content of mostcrustal lithologies is incapable of promoting the high percentages of partial melting, or the lowliquid viscosities, required to form large kilometre-sized granitic plutons by convective homogenization, at typical crustal temperatures. This suggests thatthe anatexis involvedin the generation of large crustally-derived magma bodies has in many cases been promoted by an influx of externally derived aqueous fluid. These magma segregation processes are illustrated with respect to the petrogenesis of three different types of granitoid pluton from a Hercynian low-pressure, metamorphic-anatectic terrane in the Pyrenees

Crustal melting is fundamentalto the generation of most used to generate silicic melts from crustal rocks, (e.g. Hoffer graniticplutons (e.g. Tuttle & Bowen 1958; Wyllie 1977), 1978; Winkler 1979; Wickham 1984; Johannes 1985) and in yet there is very little detailed understanding of the many some cases have helped to confirm an anatectic origin for processes involved between the inception of anatexis and graniticsegregations. The heterogeneouscharacter of the emplacement of plutons at high levels in the crust. While migmatites is typically on a scale of 1 cm to 1 m although recognizing that contamination of mantle-derived mafic these are notrigorous limits. Granitic melt segregations magmas with crustalmaterial may be animportant within migmatite terranes are therefore ona small scale, five granite-forming process, this paper will be concerned with or six orders of magnitudesmaller in size than typical the origin of those granites primarily derived by crustal granitic plutons. anatexis (e.g. Allegre & Ben Othman 1980; Hamilton et al. Studies of granitic magma bodies have used petrological, 1980; Vitrac-Michard et al. 1980; Farmer & DePaolo 1983; geochemical and fluid dynamic approaches (e.g. Shaw 1965; Chappell 1984; Frost & O’Nions 1985). There havebeen Hildreth 1981; White & Chappell 1977; Huppert et al. 1982; two main approaches to the study of such melting processes. Marsh 1982; Chappell 1984; Chappell et al. inpress). In One of these is the study of the low melt-fraction rocks contrast to migmatite research, these studies have dealt with (<40% melt) represented by migmatite terranes.The high melt-fraction rocks where physical properties are second involves the study of silicic magmas (i.e. high controlled dominantly by the properties of the liquid part of melt-fractionrocks containing >40% melt) and their the system, in particular the liquid viscosity. Viscosity in differentiation, emplacement and eruption histories. granitic liquids at normal crustal temperatures depends most In petrologicalstudies of migmatites (reviewed in importantly on water content,temperature and bulk Ashworth 1985), mineral phase equilibria can often be used composition but also on pressure (which strongly influences to constrain the physico-chemical conditions within specific water solubility). (In this paper, magma refers to silicate terranes (P, T, aHZ0), (e.g. Tyler & Ashworth 1982; melt containingsuspended crystals; liquid refers to McLellan 1984). Major and trace element geochemical data crystal-free silicate melt.) However, viscosity is complicated can alsoconstrain melt-fractions (e.g. Barr 1985) and by the fact that most bodies of granitic magma contain identify migmatite source rocks (e.g. Brown 1979; Yardley suspended crystals for part or all of their history. In this et al. 1987; Wickham 1987). Melting experiments have been paper particular attention is given to the dramatic change in 281

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effective viscosity at crystal volume fractions in the 50-70% range, where the physical properties of the magma begin to be controlled by theproperties of the solid components ratherthan by the liquid. This pointmarks the divide between low melt-fraction anatectic rocks studied by migmatite petrologists (gross properties dominated by those of the solid phases), and high melt-fraction granitic magma bodies studied by igneous petrologists and fluid dynamicists (gross properties dominated by those of the liquid). The existence of two distinct areas of research also partly reflects the isolation of mostgranitic plutons fromtheir sourcerocks. Conversely, migmatites can seldom be genetically linked to larger plutons, and many only reflect small-scale partial melting processes occurring at relatively high structural levels in the crust. Deep, lower crustal melting zones,where the voluminous melting required to generate largeplutons is more likely tooccur, are rarely exposed and may be hard to identify as such, because most of the magma may have already left the source region.

The physical properties of partially molten rock During crustal anatexis, the mechanical properties of rocks change with increasing proportion of melt, from linearly- elastic to elastic-plastic to viscous (Shaw 1965; Bottinga & Weill 1972; Murase & McBirney 1973; Shaw 1980; Knapp & Norton 1981). There will therefore bea variety offlow behaviour from low to high degrees of partial melting.

High melt-fractions-magmas as dilute suspensions for various values of R. Also shown are experimental A silicate liquid behavesapproximately as an ideal Newtonian fluid, the viscosity of which depends principally measurements of the relative viscosity of increasingly dense suspensions, compiled by Thomas (1965). Figure 1 is thus a on the chemical composition of the melt, and the pressure compilation of experimental data and empirical curves and temperature (Bottinga & Weill 1972). The viscosities of common silicate liquids are well knownfrom experiments fitted to experimental data; thesecurves all suggest little change in the effective viscosity of suspensions for 0.25. (e.g. Shaw 1965; Bottinga & Weill 1972; Murase & < McBirney 1973) and cancalculatedbe easily from This implies that granitic magma bodies withlow crystal compositional data using empirical models (e.g. Shaw 1972). contents (<25%) will have similar viscosities to the same Dueto variablemagmatic water content and to a lesser crystal-free liquids. Furthermore,the viscosityis not extent, temperature, granitic magmas show a wide range in increased by much more than an order of magnitude when = 0.5 (analogous to a half-crystallized magma). viscosity, ranging from 10’ Pa S to 101*Pa S. If a magma contains suspended crystals, it can be treated assuspension,a with certain effective fluid properties Magmas as dense suspensions (Jeffrey & Acrivos 1976; Wildemuth & Williams 1984). In addition to particlevolume fraction, numerous extra Suspensions may be simply modelledas Newtonian fluids factors influence the effective viscosity of dense suspensions, with effective viscosity dependent solely onthe melt resulting in non-Newtonian macroscopic behaviour, (Jeffrey viscosity and the fraction of suspended solids (e.g. Roscoe & Acrivos 1976; McBirney & Murase 1984). Among the 1953; Shaw 1965; Arzi 1978), although many other factors more important factorsrelevant to geological systems are such as the size, shape and distribution of the particles, and graininteractions involving the clumping of particles the type of flow must beconsidered in more rigorous (aggregation), the size distribution of the particles analyses (see McBirney & Murase 1984). For example, (monodisperse or polydisperse), possible anisotropic dis- Roscoe (1953) predicts the viscosity of concentrated tribution of the particles in asuspension, and thixotropic suspensions as follows effects (the time-dependent change in suspension properties resulting from deformation). These complications may lead = (1 R.y-2.5 k - (1) to Bingham-type behaviour where the suspension has a yield PO stress which must be exceeded before motion can start (e.g. where ,us is the viscosity of the suspension, pLois the viscosity Sparks et al. 1977). Polydispersity of particle size results in a of the suspendingliquid, v is the volumefraction of the lower effective viscosity at any particular value of v, but this solid (spherical) particles, R is a constant, and R-’ refers to effect may in part be cancelled by irregularity in grain shape the maximum crystal fraction (vmax)at which the effective which has the opposite effect (Ward & Whitmore 1950). viscosity becomes infinite and the suspension loses mobility. No analytical solutions exist for suspension properties at Thisrelationship and two others suggested by Mooney high particle fractions,and the simple, semi-empirical (1951) and Krieger & Dougherty (1959) are plotted in Fig. 1 equations shown in Fig. 1 are inapplicable to such systems.

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All formulas imply that effective viscosity becomes infinite when the solid particles are all in contact, marking the abrupt transition from viscous liquid to elastic solid stress-strain behaviour. As there is at present no wayof accurately calculating the variation in the effective viscosity of dense suspensions with particle fraction, we can merely infer thatat some crystal fraction between 40 and 70%, there will be a dramatic increase by many orders of magnitude in the effective viscosity of a crystallizing granitic magma. The important (andobvious) corollary of this is that during melting of the crust, thestrength of the rocks will fall suddenly as some critical fraction of melt is reached (Arzi 1978; Van der Molen & Paterson 1979). The constraints on the location of this critical pointfor granitic systems are discussed below.

Low melt-fractions There has been very little experimental investigation of low melt-fraction rocks in granitic systems. Because in general the solid grains in such a rock will all be in contact, thegross rheological properties will be dominated by the mechanical properties of the grains, regardless of the viscosity of any Fig. 2. Line drawing of a secondary electron micrograph showing small fraction of intergranular melt. The response to textures observed in partially molten granite at a melt-fraction of deformation will therefore be similar to that of sub-solidus about 24% in the 14-day granite crystallization experiment of rocks, with brittle behaviour at high strain rates and ductile Jurewicz & Watson (1985). Phases are quartz (stipple), feldspar behaviour at low strain rates(Hobbs et al. 1976), and (lined) and melt (unmarked). The grain boundaries were effective shear viscosities of the order of 10" Pa S. As the oversaturated with melt leading to the formation of pools. proportion of partial melt in a rock increases, the strength and effective viscosity decrease (Arzi 1978; Van der Molen & Paterson 1979), falling rapidly when displacive move- of lower hydrostaticpressure (e.g. if extensionalfractures ments can take place in the fluid phase between the grains. develop). Such behaviour will be possible up to the CMF at Following Van der Molen & Paterson this will be called the which suspension-like behaviour ensues. critical melt-fraction (CMF), at which the fraction of solid As the melt-fraction approaches the CMF, the strength material is given by qCMF.For values of 1 qCm,the melt of a partially molten rock decreases rapidly (analogous to will be dispersed throughoutthe rockin intergranular the rapidincrease in effective viscosity as 3 approaches interstices and cracks. qcMF).This was observed by Van der Molen & Paterson The exact distribution, and the ease with which this low (1979), who deformed samples of partially-melted granite, melt-fraction can be extracted from the rock to form a body with melt-fractions up to 25% at800°C (Fig. 3). The of magma is critically dependent on theviscosity of the melt, strength of the samples decreased from about 250 MPa at and on the geometry of the melt with respect to the crystals 5 vol. % melt to about 60 MPa at 15 vol.% melt to less than (Beere 1975; McKenzie 1984). Recent experimental work by 1 MPa at 24% melt. Although an equilibrium melt Jurewicz & Watson (1984, 1985) has revealed that indry distribution may not have been attained at these high strain graniticsystems, thedihedral angle, where melt and two rates (lO-' S-') the results suggest the approach to grains meet is in the range 44-60', implying that the liquid suspension-like behaviouras theCMF is reached (at will form an interconnected intergranular film, theoretically approximately 30-35% melt for this system.) capable of extraction by intergranular flow. If the dihedral angle were greaterthan 60°, the melt would notbe interconnected and extraction would be impossible. Though Critical melt-fraction these experiments involved melts with negligible or very low 'Critical melt-fraction' (CMF) is used to distinguish the water contents, they are probablyrelevant to most low point at which partially melted rocks change from granular melt-fractiongranitic systems. Characteristic equilibrium framework to densesuspension behaviour. Effective shear melt-fractions for granitic liquids shouldbe in the range viscosities for these two states range from values typical of 0-4% melt, (Jurewicz & Watson 1985) andat higher hydrous granitic melts (c. 106 Pa S) to values typical of rocks melt-fractions, the liquid will tendto aggregate in pools (c. 101' Pa S), (see Fig. 3). Uncertainties in the estimation of surrounded by a large number of grains, as shown in Fig. 2. the CMF have been outlined above. Arzi (1978) chooses a This texturehas a directanalogue in the opthalmitic value of 20% f 10% for theCMF (termed by him the migmatitesobserved insome migmatite terranes (e.g. 'rheological critical melt percentage'),but this figure is McLellan 1983), in which the leucocratic granitic component based on little experimental data. A value close to 40% for is distributed patchily throughoutthe rock. At this stage, most geological materials appears to be more reasonable, extraction of excess melt fromthe oversaturated grain based on the randompacking density for spheres (38-39%), boundaries is energetically favoured because it reduces the and the experimental results of Van der Molen & Paterson interfacial free energy of the system.Draining of excess (1979) which suggest that at 20% melt, rock strength is only pooled melt is possible if the liquid can escape into regions about an order of magnitude less than in the unmelted state.

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r (Van der Molen 8

loot

10 -

SC (metres1

Is I- / 0.IL I I I I I 0.02 0 04 0.06 0.08 0.10 4 (meltfraction) Fig. 4. Compaction length for wet granite (7-8 wt.% H,O) calculated from equations (1) and (7) of McKenzie (1985), using a Fig. 3. Diagram showing the rapid decrease in strength of a matrix viscosity of 10" Pa and a mean grain radius of 10-3 m. The partially molten rockwith increasing melt-fraction, at a strain rate melt viscosities (104 and 105 Pa S) cover the range of values calculated for leucogranitemelt from the Trois Seigneurs Massif, of 10-5 S-' and 3 kbar confining pressure (redrawn from Van der Molen & Paterson 1979, fig. 14). The range of maximum differential Pyrenees, at 700-750 "C (see text). stress for low melt-fraction rocksis shown together with the flow stress for suspensionswith a liquid viscosity of 104 Pa S, based on the data of Fig. 1. The critical melt-fraction lies between these two rock at low melt-fraction by compaction.For granitic fields. The typical change in effective viscosityof wet granite, over a similar range of melt-fraction is shown for comparison. systems, the efficiency of separation depends strongly on p, the liquid viscosity, because p vanes more than any other quantity in theequations describing the compaction behaviour. Evenfor the lowest viscosity granitic melts, A similar CMF value is indicated by measurements of separation by compaction is likely to be very limited. In Figs the critical void ratio in soil mechanics (analogous to the 4 and 5, 6, (the compaction length) and to(the compaction CMF), which typically give values inthe range 40-4570 time) are plotted against $J (the porosity) for low granitic void space, although this figure becomes lower if the particle melt viscosities of 104 and 105Pa S and other constants as size distribution is large. Further evidence is provided by listed in the captions(see McKenzie 1984, 1985 for volcanic rocks which sometimescontain 60-65% pheno- definitions of 6,, to and wo). Values of 6, lie mostly crysts (e.g. the Chaodacite lava in Chile) but have never been between 1 and 10 m, but more important, tois of the order observed to contain more than this value. This suggests that of 105 years (McKenzie 1985). Richter & McKenzie (1984) c. 65% represents the limiting crystal-fraction above which a have shown that t,,, the time takento reduce the total magma cannoterupt at the surfacebecause its effective amount of fluid in a layer of thickness, h, by a factor of e is viscosity is too high. Clearly, the CMF will vary in different rock systems withdifferent textures, grain sizes, grain geometries, and macroscopiccompositional inhomo- geneities, and it is therefore fruitless to attempt to specify any exactvalue. Suffice to say that within the range of melt-fractions 30-50%, a hugechange in the effective viscosity of partially molten granitic systems by as many as 14 orders of magnitude is to be expected, implying a radical change in mechanical behaviour.

Mechanisms of melt segregation-low melt-fraction

Compaction In the absence of externally applied stress, a silicate melt can only separate from residual crystals at low melt-fraction

by a compaction process, in which the solid matrix deforms I I 1 I to expel the intergranular liquid. Partially molten material 0.02 0.04 0.06 0.08 0.10 will compact only if the melt is interconnected, and if the 9 (melt fraction) density of the melt differs from that of the matrix. Both of Fig. 5. Compaction time for wet granite (7-8 wt.% H,O) calculated these criteria are satisfied during crustal anatexis (Jurewicz from equations (l), (2), and (3) of McKenzie (1985) using & Watson 1984, 1985). g = 10 ms-*, (p, - pf) = Id kg m-3 and other constants as in Fig. 4. McKenzie (1984) has developed a physical model The melt viscosities are again appropriate for Pyrenean describing the separation of melt from a partially molten leucogranite.

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Mohr envelopes given approximately by 7. ~- h %h th = -_ (2) %(l - $1 - 6, This implies that for h of the order of a few kilometres, th= 109 -years.This is at least two orders of magnitude greater than the likely maximum duration of a metamorphic-anatectic event, (106-10' years). However, if h is small (of the order of a few metres) separation on this limited scale could occur much more quickly (th= 10' years). These calculations indicate that compactionalone can at best only generate small scale (centimetre- to metre-sized) segregations of granitic liquid, and is incapable of generating large kilometre-sizedbodies over reasonable time scales I (McKenzie 1985). l Fig. 7. Fracture orientations relative to principal compressive stress axes in a strongly foliated rock with anisotropic tensile strength (see Extensional fracturing text for discussion). The trend of the fractures (open lenses) relative Fracturing can occur in low melt-fractionrocks, provided to foliation and principal stress axes, is shown beneath each Mohr the strain rate, pore fluid pressure, and the differential stress diagram. are high enough(Jaeger & Cooke 1979; Shaw 1980). Extensional fracturing can occur at any depth in the crust if pore fluid pressure is high. The pore fluid has often been within the material. Thus, between points A and C lies the assumed to be aqueous (e.g. Fyfe et al. 1978; Hubbert & range of failure within which the opening of tensile fractures Rubey 1959), but can equally well be a high viscosity silicate is theoretically possible. In natural rocks the situation will melt (Shaw 1980). During extensional fracturing, large local be more complicated since most high grademetamorphic hydrostatic pressure gradients develop between the fracture rocks and migmatites are mechanically highly anisotropic, andthe surrounding rock, promoting fluid (or melt) flow with lowest tensilestrength in a direction normal tothe towards the fracture, and providing a potential segregation main schistosity (Fyfe et al. 1978). Hence two failure mechanism. envelopes exist, corresponding to failure normal and parallel The stress condition for tensile failure is to schistosity. The orientations of the fractures that may be generated in a schistose rock arethus controlled by the U, - Pf = T (3) magnitude of this difference intensile strength, by the orientation of the schistosity with respect to ul and ug,and where U, is the minimum principal compressive stress, Pf is the pore fluid pressure and T is the tensile strength of the by the differential stressapplied to the system. These rock (Fig. 6). At point A, purely extensional failure occurs, variations are illustrated in Fig. 7 and indicate the important while the circle tangent at point C theoretically involves only textural differences that may result from different fracture compressive shear with no tensilecomponents anywhere orientations. These surely have important implications for the subsequent movement of any melt that might separate into a crack of this type. Duringfailure, Pf dropsto u3 within thefracture, generating a pressure gradient in the rock, the magnitude of which will depend on the fracture spacing. Etheridge et al. (1984) have shown, using equations of Fletcher & Hoffman (1974), that for a fracture half-spacing of 1 m, this gradient will allow migration of aqueous fluid to dilatant sites within years, at permeabilitiesas low as 10-20m2. Order-of- magnitude estimates of advective flow rates of granitic melts to such dilatant sites may be obtained in a similar wayby estimating the velocity for low-Reynolds number advective flow in a porous medium. The average velocity of a fluid particle, 0, is given by

I Normalstress (U" )

-(tensile)-- (comDresslve) where k, is permeability, $J is porosity, p is the fluid viscosity and VP is the pressuregradient. Using the Fig. 6. Mohr diagram showing a family of Mohr circles tangent at permeabilityexpression adopted by McKenzie (1984) and different points along a failure envelope. Extensional fracturing used above in compaction calculations, in which, occurs if a circle (representing a specific stress state) cuts the envelope between A and B. No tensile components act anywhere a2G3 k- within the material for circles tangential to the right of C. Between .$ - low(1 - $y B and C is the transition from fields of dominantly compressive to dominantly tensile failure. where a is the mean radius of the solid particles (lO-, m),

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and taking VP to be 10M Pa m-' (Etheridge et al. 1984) and p tobe 104Pas, 6 can be calculated to vary from 3mmyear-' at 4 = 0.01 to 2m year-' at 4 = 0.2. These rates of fluid migration are high enough for significant segregation of melt to take place over time spans that are very short compared with the average duration of a crustal melting event. Althoughpressure gradients generated by extensionalfracturing are not likely to be long lived, repeated fracturing over a period of time may have the same effect as maintaining a constant, but lower pressure gradient within the system. Extensional fracturing requires Pf to be high, as is likely in partially molten systems where the proportion of melt is higher than the intergranular equilibrium melt-fraction and pooling has begun to occur (Jurewicz & Watson 1985). In Fig. 8. Stress distribution in a series of layers of unequal viscosity granitic systems at low water activity, there is a positive (pq and p,,,) undergoing pure shear, and extending at the samerate, volume change during melting, which will favour high melt parallel to the layering.U, is the same in all layers, while U, varies hydrostaticpressure andpromote extensionalfracturing from higher values in the less competent layers to lower values in (Clemens 1984). Wet melting at pressures less than 15 kbar the more competent layers. A hydrostatic pressure gradient (dotted involves the opposite effect, a decrease in the volume of the line) is set up between the layers. reactants; hydrostaticpressure will in this case depend on the rate at which deformation, or the flow of aqueous fluids or silicate melt intothe low pressure region can accommodate the negative volume change and will probably differing effective shear viscosities. In this two-dimensional not become high until substantial melt has formed, (>5%), the grain boundaries have become saturated, and the melt model, the (vertical) principal compressive stress (azLL)is the same in all layers, but the (horizontal) minimum principal has started to form pools. compressive stress varies because the more competent The efficiency of meltextraction into extensional (au) layers can support a higher differential stress. The fractures depends on the magnitude of the pressure drop, hydrostatic pressure, P,, is given by the permeability of the rock and the time during which the fracture remains open. Althoughonly a little liquid may segregate in a single failure event, repeated fracturing will accumulatelarger amounts of melt, ultimately generating highly differentiatedmigmatites. Although this mechanism and also varies so that any pore fluid will tend to migrate to can explain the small-scale melt extraction,observed in the more competent layers. This effect has been proposed to migmatites, it does not account for kilometre-sized magma accountfor sub-solidus metamorphicdifferentiation, but bodies. At best, the separated liquid might travel along a would also promote movement of interstitial melt (Van der fracturezone and pond where the fracture died out(see Molen 1985b). below). Rocks undergoing crustal anatexis often comprise layers Deformation-induceddilatancy pumping, which was of differing competency, similar to the situation required by observed in the experiments of Van der Molen & Paterson this model. Unlikethe extensionalfracturing scenario, (1979), has to some extenttheopposite effect to hydrostaticpressure gradients set up in this way are segregation.Dilatancy results from deviation from linear maintained for longperiods of time which would favour elastic behaviourduring deformation, resulting in volume segregation. However, the physics of melt movement within increase (Jaeger & Cooke 1979). Van der Molen & Paterson an individual layer arethe same as in the gravitational (1979) observed that this effect caused melt to be drawn into compaction problem. The melt can be expelled from a low cracks within their samples,from a pool of melt onthe competency layer only if the solid matrix can deform to fill outside of the specimen, although they doubted if the effect the space it leaves behind; as outlined above, this process is occurred in nature. However, Ashworth & McLellan (1985) likely to be very sluggish for high viscosity granitic melts have proposed that during deformation of natural anatectic (p = 104- 101'Pa S). Although the compaction analysis is systems at low melt-fractionthis mechanism may disperse really only valid at low stresses(e.g. McKenzie 1984, the melt throughout the rock. equation A15) the same conclusions are still likely to apply in the pure shear scenario of Fig. 8. The situation becomes Segregation during continuous deformation (filter morecomplicated if melt is being generated. If it is pressing) generated within the less competent layers, then thematrix of these layers does not have to deform so much for themelt to Ductile deformation of layers of differing composition can flow towards the more competent layers. On the other hand, theoretically separate melt from low melt-fraction rocks melt generation within themore competent layers may (Robin 1979; Fletcher 1982; Van der Molen 19852, b). This reduce the hydrostaticpressure gradient and inhibit flow. situation is similar to that involved in boudin formation (e.g. The complexities of the situation, prohibit a more detailed Ramberg 1955) and illustrated in Fig. 8, which shows the analysis of segregationphenomena in layered rocks pattern of principal stresses in a layered medium of infinite undergoing active deformation. Suffice to say thatas with lateral extentsubjected topure shear with maximum compaction, this process appears incapable of causing large elongationparallel tothe layers. Alternate layers have scale separation of granitic melt from residual crystals.

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Surface tension (capillarity) (i.e. low enough for ascent of small 'blobs' of melt, but high Little is known aboutthe possible influence of surface enough for kilometre-sized regions to remain stably tension on meltsegregation in partially molten systems. stratified). It seems more likely that an anatectic zone will Watson (1982) has shown that in the absence of buoyancy either be too viscous to allow either convection orthe effects and differential stress, the total surface energy of a ascent of small bodies of liquid, or that the melt-fraction system with wet grain boundaries will be lower than for dry will be high enough to favourconvection, whichwill grain boundaries if 8, the dihedral angle is below 60". overwhelm buoyant ascent of small bodies. Above 60", dry grainboundaries are favoured(see also Jurewicz & Watson 1985), implying spontaneous segregation of melt (through pooling) at any melt-fraction. However, all Characteristics of granitic magmas segregatedat low existing measurements of 8 in granitic systems have shown melt-fraction 8 S 60" (Jurewicz & Watson 1984, 1985) for which Compositional data from some granulite terranes have been infiltrative dispersal of melt into a dry system would be energetically favoured. used to suggest thatthe rocks differ from lower grade protoliths by near granite minimum-melt compositions, Stevenson & Scott (1986) have shown that assuming instantaneous textural equilibration, the melt flux dueto implying that extraction of magma at low melt-fraction has However,none of the surface tension Aus is given by occurred(e.g. Nesbitt 1980). mechanisms described in this section (except perhaps the last) seem capable of segregating particularly large volumes of magma, of the typically kilometric dimensions of many granitic plutons (e.g. Moore 1963). Compaction mechanisms will only generate very small segregations over geological where k is the permeability, p the liquid viscosity, AE is the time scales, because of the relatively highviscosity of surface energy per unit volume of the two-phase system, f is granitic liquids. The sluggishness of compaction may also the melt-fraction (f<< 1) and X is the direction of melt limit the quantity of melt that can be squeezed out of migration. However,for typical viscosities of granitic incompetent layers during ductile deformation of the matrix. liquids, Aus is likely to be too low tobe important, and Surface tension appears to have negligible influence on melt while 6 < 60", surface tension will not lead to spontaneous segregation in granitic systems. Extensional fracturing may melt segregation. bethe most favourablemechanism, particularly where AVmeltingis positive, but must berepeated many times Ascent and collectionof small volumes of liquid throughout the duration of the melting event in order to 'bleed off a significant quantity of melt.Even so, this Buoyant ascent of small (centimetre-sized) pockets of melt mechanism is only likely to segregate small quantities of is possible if the viscosity the country rock is low enough of granitic liquid (centimetre to metre-scale). If pockets of melt (Fyfe 1970, 1973). For a sphere moving under gravity in a becomelarge enough (c. metre-sized) and if the effective viscous medium, U, the velocity of the sphere is given by viscosity of the 'wall rock' is low enough, they may begin to migrate and coalesce, collecting at the top of the melting v=- 2gr2Ap zone to form larger bodies (Fyfe 1970, 1973). Larger masses 977 of low melt-fraction granite may form if melt draining into fractures is allowed to collect, perhaps if a fracture dies out where g is the gravitational acceleration, r is the radius of upwards into some lower competency ductilely deforming the sphere, Ap is the density contrast between the sphere layer. and the medium and 77 is the viscosity of the medium. This Melts segregatedat low melt-fraction should be implies that for a 10 cm radius drop of liquid to rise 1 km in compositionally easily distinguishable. They are likely to be 106years,the viscosity of the 'country rock' must be less relatively silica-rich, close in composition to minimum melts, than 3 X 10" Pa S. The Fyfe model suggests that a partly and will crystallize to rocks rich in quartz and feldspar and melted zone would have such a low viscosity, and that small poorer in more refractory, mafic phases. Because many centimetre-sized 'blobs' of melt would rise through it and trace elements will be held back in residual phases during collect at itsjunction with overlying sub-solidus rocks, melting (Watson & Harrison 1984; Benard et al. 1985) such where the effective viscosity increasesenormously. Even- liquids will often be geochemically depleted, particularly in tually, enough liquid would accumulate for a large blob to transition metals and rare earth elements. ascend into the upper crust. In summary, low melt-fraction granitesshould be This model fails to explain how 10 cm sized 'blobs' of characterized by small size, and therefore beemplaced fairly liquid might form in the first place, but it remains a plausible close to their site of generation (e.g. migmatite leucosomes). mechanism for liquid extraction at low melt-fraction. The Probableexamples of such lithologies include the small chief criticism is thatat viscosities as low as 10" Pa S bodies of leucogranite encountered in metamorphic- (necessary for liquid ascent to occur), any partially melted anatectic terranes in the Pyrenees (Wickham 1987, see zone greater than 1 km in size would become unstable to below). It should be noted thatthe latest stages of convective motion (see Fig. 9). Convection would interfere crystallization of granitic magma bodies can produce highly with the ascent of discretepockets of liquid, and would fractionated volatile-rich liquids, which because of their low tend to homogenize the wholeregion. Similarly, the very viscosity may be able to segregate at low melt-fraction on a rapid change of viscosity with increasing melt-fraction (Figs small scale to form pegmatites (McKenzie 1985). In many 1 & 3) indicates that there is only a very narrow range of cases these have been emplaced along fractures, analogous values of 9 (or v)with viscosities in the range 109-10" Pa S to the process described earlier in this section.

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- the highest viscosity granitic liquids (p = 10" Pa S) will 10'2 convect if their dimensions are in excess of 1 km. For a 10" - DRY RHYOLITE water saturated magma (p = 104Pa S) convective motion can start in bodies as small as 10 m in thickness. 1010 - The precedingsection has outlined the difficulties associated with extracting large quantities of granitic melt 109- from low melt-fraction rocks. Kilometre-sized granitic 10'- NON CONVECTING plutons are probably generated by an alternative mechan- ism. Consider a compositionally homogeneous region of the v) g 107- crust, several km thick, undergoing partial melting, where X therehas beenlittle or no removal of the melt at low 106- melt-fraction. As the proportion of melt exceeds 30%, the critical melt-fraction is reached, marking the onset of 105- suspension-like behaviour. At this point,the effective

104 - magma viscosity suddenly falls by many orders of magnitude. At 50% melt, the effective viscosity will not be EUCOGRANITE 103- much morethan an order of magnitudein excess of the completely liquid (zero crystal-fraction) state, (see Fig. 1

lozt and Shaw 1965). Thus, during melting of the crust, there //' will typically be a fall in effective viscosity of between 8 and loll , I I I l0 100 l000 10000 13 orders of magnitude between 20 and 50% melt-fraction, d (metres) assuming that the melt is uniformly distributed. For large, Fig. 9. Magma effective viscosity, p, plotted against d, the depth of kilometre-sized systems generating all but the most viscous a magma chamber (or partially molten zone).The convecting and (dry)granitic liquids, this rapidreduction in effective non-convecting zones are defined for a critical thermal Rayleigh viscosity will quickly reach values where convection can number of 3000 (equation 9) using g = 10 ms-*, cr = 5 X lO-' 'C-', occur, (see Fig. 9). It should bepointed outhere that K = 10-6 m' sK1 and AT = 10 "C and 100 "C. These valuesof AT are convection of the intergranular melt within the solid matrix the boundaries to the stippled zone. The field for biotite granite at low melt-fraction (porous medium convection) will not from the Pyrenees plots well within the convecting field.The low occur for reasonable geological parameters, even with the viscosity of leucogranite magma impliesthat bodies as small as 50 m lowest viscosity graniticmelts (McKenzie 1984, equation will convect. The Rayleigh number, and hencethe boundary 3.9). between the convecting and non-convecting field can also be defined The convection instability field in Fig. 9 assumes that the in terms of the thermal gradient, since AT = Bd. This boundary 8, partially melted region behaves as a Newtonian fluid, which is shown by the dashed lines. is a gross oversimplification. The high concentration of suspended solids and probable layered structure of such a region would bemore likely to have the properties of a Pluton formation at high melt-fraction Bingham fluid with a finite yield strength, ashas been proposed for much lower crystal fraction magmas (Sparks et Convective overturn of partially molten regions al. 1977). Hence, the system could remain stable and not Convection in magma bodies can generally bepredicted convect while having an effective viscosity (p), which exceeded the critical Ra, of c. 103. Bingham-type behaviour given the physical properties of the magma andthe would requirethe critical Rayleigh number beto temperature field. The ratio of the buoyancy force driving convection to the viscous force resisting fluid movement is oversteppedbefore convection started; alternatively exter- given by the dimensionless thermal Rayleigh number, nal forces resulting from deformation or magmatic intrusion intothe partiallymelted region, could trigger convective gcuATd3 overturn. Ra, = - In short, most crustal melting zones generating granitic KTP liquids, morethan a few hundred metres in thickness, where g is the acceleration due togravity, (Y is the coefficient should begin to convect as the fraction of melt in the system of thermal expansion, AT the temperature difference across approaches 50%, the exact pointdepending partly on the the fluid layer of thickness d, K= is the thermal diffusivity distribution and viscosity of the melt, and partly on external and p is the viscosity. Convective motions are initiated when forces which may help to overcome any yield strength Ra, is above c. 103, the exact valuedepending onthe resisting convective flow. The average velocities, W,, during specific boundary conditions (Huppert & Sparks 1984). In thermal convection between parallel plates for Ra, > 106 are geological systems, the most important variables in this given by the expression formulation are p and d. Calculations of the typical Rayleigh numbers of magma chambers indicate that most bodies of magma, including even high viscosity, dry granitic magmas will convect,provided the dimensions of the magma chamberare appropriately large (Shaw 1965; where B is a constant with a value of about 0.44 and all Bartlett 1969). This is illustrated in Fig. 9 where magma other quantities are as in (9) (Sparks et al. 1984). viscosity, p, is plotted against d, the depth of the magma Substituting typical values for graniticplutons into (10) chamber, with other constants given values appropriate for yields velocities of the order of centimetres or millimetres granitic liquids (see caption). The diagram shows that even per second. Such high velocities would tend to smooth out

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compositional andtextural heterogeneitiesand might MUSCOVITE GRANITE 10 kbar promotethe fragmentation and dispersal of refractory residual solid material. During the initial and final stages of convective motion, such material would be subjected to intense deformation. This deformation could generate commonly observed schlieren textures and also mechanically redistribute and recrystallize refractory mafic phases, forminga more homogeneousdistribution throughout the magma body.This implies that plutona containing abundant, homogeneouslydistributed mafic phases, need neverhave been completelymelted. These relatively refractory phases could be solid residues from the original sourcerocks (‘restite’) redistributed by convective motion (cf. Wyborn & Chappell 1986; Chappell et al. in press).

~o~..;=~,~~l,~~~~~~~~t Diapirism Becausegranitic liquid is invariably less dense thanthe 20 .. maxlmumsource-rock water content rocks from which it is derived, anatexis will always lead to .‘l an unstable density distribution within the crust, providing ‘O :‘+water saturation boundary the driving force for a magma to ascend towards the surface. ‘0 2 4 6 8 10 l2 14 l6 Flows resulting from density instabilities asa result of wt % H20 density inversions havebeen widely discussed in the geological literature(e.g. Ramberg 1968, 1970; Marsh & Fig. 10. The percentage melt generated with increasing water Carmichael 1974; Spera 1980; Marsh 1978, 1982); the main content of the system over a range of temperatures, for muscovite application has been tothe study of magma intrusion. granite at 10 kbar, plotted using the data of Piwinskii (1973) and Diapirascent has been modelled using Stoke’sLaw, their extrapolation by Wyllie (1977) into the water-undersaturated (equation 8, e.g. Shaw 1965) and although this relationship field. Contours of effective viscosity have been plotted onto this is limited to non-reactive,isothermal systems, it indicates diagram (see text). The water saturation boundary is shown by the that velocity is controlled most importantly by the size of the dotted line. The limiting viscosity at which a 1 km-thick partially body and the viscosity of the country rock. Note that the melted zone would begin to convect is marked, together with the viscosity of thesphere (or diapir) doesnot enter the typical intrinsic water content of muscovite granite, or a likely formulation. The efficiency of movement depends very crustal source lithology. Very high temperatures are required to strongly onthe ability of the diapir toheat, and hence bring about substantial melting without additional water (in excess soften, the adjacent country rock and reduce drag (Marsh of the intrinsic water content) being added to the system. 1982). This is dependenton the thermal energy of the diapir, which also depends on its size. Thus, size is critical in controlling the height to which a body of magma will rise during emplacement of granitic bodies, layered (gneissose) before it solidifies (Fyfe 1970; Marsh 1982). textures defined by restiticxenoliths rich in mafic If a kilometre-sized region of the crust is being melted, a (refractory) phases should often be observed. Although such density instability may occur on a large scale in which the bodies occur, e.g. in the St Malo migmatite dome (Brun & whole region (including solid and liquid components) rises Martin 1978), the Coomagranodiorite (Flood & Vernon diapirically intothe overlying crust.This mechanism may 1978) and in Donegal (Pitcher & Berger 1972), it is more explain the origin of gneiss domes (Ramberg 1970) and common to find graniticplutons with more homogeneous ‘regional aureole’ S-type granitoids associated with migmat- distributions of mafic phases, implying that convective iteterranes (Flood & Vernon 1978; Soda 1982). The motion has occurred. criteria determining whether diapiric motion will occur are also critical in governing the instability at which convective motion will start.It is probably more common that Importance of water in melting processes convection begins before diapiric rise of magma bodies to Water exertsprofound influence on melting processes in higher crustal levels. The reasons for this are as follows. granitic systems because it controls both phase equilibria (1) The initiation of diapirism involves overcoming some (i.e. thedegree of melting tobe expected at a given sort of yield strength, since neither the partially melted temperature)and melt viscosity. Both thesefactors are region, nor the intruded country rock are ideal Newtonian important to the formulation of the effective viscosity of a fluids. A strongly layeredmigmatite complex will have a partlymelted region, e.g.equation (1). There have been higher yield strength than a homogeneous isotropic partly surprisingly few attemptsto document experimentally the crystalline magma body of the same dimensions and average percentage melting at a given temperature during fusion of density (melt-fraction). crustalmaterials, although this will control the kinematic (2) Larger bodies can rise further than smaller bodies, evolution of partially melted regions. Piwinskii (1973) owing to their greater thermalenergy content and lower rate reports modal glass percentages for 10 kbar water saturated of heat loss. A large homogenized pluton will be more likely melting experiments using various starting materials. These to escape from its site of generation than asmaller body data, and their extrapolation by Wyllie (1977) are used in extracted from a layered anatectic zone. Figs 10 and 11 to construct diagrams showing the amount of (3) If diapiric intrusion without convection is common melt generated at a particular H,O content, for muscovite

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TONALITE 10 kbar normalcrustal temperatures.In contrast, if the water content of either lithology is doubled, extensive melting can then occur atmore typical crustal temperatures (700- 8OO0C), generatingmagmas with relatively low effective viscosity, resulting both from the increased melt-fraction and increased water content of the melt. This implies that the formation of kilometre-sized graniticplutons by large scale anatexis of the continental crust will be more easily achieved in those regions which are infiltrated by externally derived aqueous fluid.

Magma segregation and granite petrogenesis in the Trois Seigneurs Massif, Pyrenees

Regional setting In the Pyrenees a large region of Hercynian basement rocks has been uplifted and clearly exposed during the Lower Tertiary. Penetrative deformation and metamorphism of the basement were relatively weak at this time. Consequently, Hercynian (Late Carboniferous) metamorphic and anatectic terranesare well preserved and have not been strongly Fig. 11. The percentage melt generated with increasing water overprinted by later recrystallization. Recent summaries of content for tonalite at 10 kbar, using the data of Piwinskii (1973) the Hercynian geology of the Pyrenees are given by Zwart and Wyllie (1977), and with other featuresas in Fig. 10 (see text for (1979); Soda (1982); Wickham & Oxburgh (1985); discussion). Wickham (1987), and various papers in Banda & Wickham (1986). The Hercynianbasement lithologies largely com- prise Palaeozoica sedimentary sequence, ranging con- granite and tonalite over a range of temperatures. Note that tinuously from Lower Palaeozoic phyllites through Silurian the amount of melt stays constant for all water contents in shalesand Devonian carbonatesto Upper Carboniferous excess of the water saturation boundary. The diagrams are and LowerPermian red beds and volcanics. Within only semi-quantitative because they are based on data in the Cambro-Ordovician pelitic rocks metamorphicsequences water-saturated region, but they give a good indication of are developed showing rapidgradation from low to high the very high temperatures required to generate substantial grade. These andalusite- and sillimanite-bearing metapelitic melt if the water contentof the system is low. sequencesculminate in migmatites and per-aluminous Contours of effective viscosity have beenplotted onto granitoids, implying that widespread crustal anatexis these diagrams, by calculating the viscosity for a particular occurred atrather shallow depths, within the 3-5 kbar temperature at 100% melt from the model of Shaw (1972), pressurerange (Wickham 1984, 1987). The very high and then using Fig. 1 to estimate viscosities at higher crystal thermal gradients implied by such condensedsequences fraction. In the water saturated region the effect of free H,O (80-100 "C/km, Fonteilles 1970; Zwart 1979; Wickham vapour on viscosity is neglected. The viscosity contours 1987) are probablyrelated tothe intrusion of mantle- asymptote towards the 40% melt line at which the viscosity derived mafic material (Soula et al. 1985; Wickham & rises rapidly to that of the solid (c. 101'Pas). In the water Oxburgh 1987) combined with ductileattenuation of the saturated region they are flat because no more melt is being high grade metamorphic rocks (Soula 1982), possibly within produced, and the H,O content of the melt does not vary. continental rift zones (Wickham & Oxburgh 1985, 1986, Although Figs 10 and 11 represent melting of only two 1987). specific lithologies at 10 kbar, the general features are likely In addition to the anatectic granites associated with the to be similar for melting of other rock types at lower migmatites, much larger plutons of kilometric dimensions, pressures. Two limits havebeen marked on the diagrams, composed dominantly of biotite and hornblende granodior- one roughly corresponding to the initial water content of the ite, have invaded the Palaeozoic metasediments throughout lithology being melted,the other to the viscosity limit at the Pyrenees.Radiogenic and stable isotope studies have which convection would start in a kilometre-sized anatectic shown these plutons to be composed dominantly of remelted zone (c. 10' Pa S, see Fig. 9). This latter line lies close to the crustalmaterial (e.g. Vitrac-Michard et al. 1980; Ben c. 40% critical melt-fraction at which viscosity decreases Othman et al. 1984; Wickham & Taylor 1985). However, rapidly. These two limits define the field within which they are geochemically distinct from the smaller per- melting will progress to the limit where convection can occur aluminousgranitoids associated with 'high level' anatexis, without the addition of extra water in excess of the intrinsic and probably result from large-scale melting at structurally water content of the system. The most important feature of deep levels within the Hercynian crust (Wickham 1987). Figs 10 and 11 is the rather high temperature contours that pass through this field. Substantial melting will only occur at these low water contents, if the melting region experiences Metamorphism and anatexisin the Trois Seigneurs temperatures well in excess of 800°C. Although such Massif temperaturesare sometimesreached in the lower crust The Trois Seigneurs Massif exposes many of the main during regional metamorphism, they are much higher than geological features of the Hercynianbasement of the

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sc : -melras the Palaeozoic metasediments by the migmatite zone, and 0 transitional into it (Wickham 1984, 1987). It is commonly chlorite sericite phyllites cordierite-bearing,extremely per-aluminous and contains large numbers of xenoliths of refractorymetasedimentary 500 lithologies (e.g. quartziteand carbonate) andcentimetre- - scale lenses of biotiteand sillimanite, which are

DOlO mineralogically and texturallyidentical to migmatite

IWO \corbonale ~ 0 DO melanosome. The average oxygen isotopic composition of

andalusite schists D I -0 the body is mostly similar tothat of the metasediments 0 ,-, (+11.0 to 11.5%) although a crude trend towards lower 6l80 values is observedin the structurally deeper parts of the IN pluton (Wickham & Taylor 1985). The deep biotite granite was probably derived mostly by partial melting and homogenization of sedimentary material similar to the pelitic schists of the Trois Seigneurs metamorphicsequence (Wickham 1984, 1987). This is suggested both by the geochemical similarities noted above, .*.....*... and by the gradational transition from coherently banded, gneissic pelitic migmatite intothe biotitegranite over a distance of a few metres at the base of the migmatite zone (Figs 12 & 13). At this point coherent melanosome layers, defining a strong fabric,laterally continuous over several metres,and separating pods and lenses of leucogranite, break up into discrete fragments. At the same time, biotite crystals become evenly distributed throughout the rock, and refractory quartziteand metacarbonate layers become dispersed as discrete, xenolithic blocks. The redistribution of biotite from melanosome layers to evenly dispersed individual crystals might result either from Fig. U.Schematic, reconstructed vertical cross-section from lowto the fusion of biotite and subsequent recrystallization from high grade through the metasedimentary and igneous rocks of the silicate melt, or from the mechanical disaggregation of the Trois Seigneurs Massif.The boundary between migmatitic melanosome layers. However,the temperature of the sillimanite gneiss and biotite granite is gradational. The zero point migmatite to biotite granite transition is unlikely to have on the scale arbitrarily marksthe highest structural level mapped in been more than 30°C higher than that represented by the detail by Wickham (1984). This point was located approximately 'migmatite in' (beginning of melting) isograd, because the 7 km below the original Upper Carboniferous surface. migmatite zone exposed today is no more than 300 m thick (see Fig. 12). Since this implies that the transition to biotite graniterepresents a temperature of less than 750°C (Wickham 1984, 1987), it is unlikely that biotite Pyrenees. Here, a typical metamorphic-anatectic sequence is redistribution involved fusion, because atpressures of clearly exposed and plutons relating to both the 'high level' 3-4 kbar, temperatures more than 200 "C above the pelite anatexis (per-aluminous leucogranites and biotite-cordierite solidus are probably necessary for large-scale melting of granite) and to anatexis at deeper levels in the crust (late biotite at high P,,, (Maaloe & Wyllie 1975; Clemens & biotite granodiorite)are associated with the Palaeozoic Wall 1981; Thompson 1982; Wickham 1984, 1987). It seems metasediments and migmatites (Zwart 1979; Wickham 1984 more likely that the biotite granite was never fully melted, 1987). schematicA section throughthe metamorphic and that biotite was redistributed by mechanical disaggrega- sequence is shownin Fig. 12 which also indicates the tion (cf. Chappell et al. in press). structural positions of the three granitoid-types. Note that This process is readily understood if the biotite granite the granodioritesuperimposes acontact aureoleon the were convecting, a situation to be expected given its size, if regional metamorphic assemblages in the local country rock, the melt-fraction were above 50% (see above and Fig. 9). andtherefore appears to have intrudedafter the Materialbalance calculations for the most differentiated metamorphicpeak. On the other hand, the leucogranites migmatites (Fig. 13c) are illustrated in Fig. 15 where and the biotite-cordierite granite are intimately related to compositional data for TroisSeigneurs pelites, leucogranites metamorphism and anatexis of the Palaeozoicmetapelitic and migmatites are plotted on an AI2O3 vs. SiO, variation rocks. Details of migmatite andgranite textures and diagram. Choosing an appropriate melanosome composition morphology are shown in Figs 13 and 14. A detailed account (biotite: sillimanite ratio of 4:l), thedata imply that of the petrogenesis of these granitoids is given by Wickham migmatites that werecompletely differentiated into leuco- (1984, 1987). granite and melanosome (such as illustrated in Fig. 13c and observed in the lower part of the migmatite zone) contained Biotite granitepetrogenesis; convective 55 wt.% melt, and an even higher melt volume percentage (c. 60%). Presumably, the biotite granite also contained at homogenization at high melt-fraction least this quantity of melt. The estimated effective viscosity The deep biotite granite lies at the deepest structural levels range at this melt-fraction is shown in Fig. 9, and indicates exposed in the area, (Fig. U), and is always separated from that a body the size of the biotite granite plots well within

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Fig. W. (a) Migmatitic bioite-sillimanite schist from the Trois Seigneursmigmatite zone. (b) Close up of migmatite leucosome (quartz-plagioclase-K feldspar) surroundedby l cm-thick selvedge of biotite and sillimanite (melanosome). (c) Migmatitic biotite-sillimanite gneiss from a structurallylow part of the Trois Seigneurs migmatite zone (seeFig. 12). Rock is now completely differentiated into a leucogranitic (leucosome) component and a mafic biotite-sillimanite (melanosome or restite) component, forming the dark layers crossing the photograph. (a) Outcrop of biotite granite from a structurallydeep level within the pluton. Biotite-sillimanite melanosome xenolithsare common, together with refractory fragments of quartzite and carbonate. the convecting field. It has already been pointed out that convectivemotion in high crystal-fraction magmas will probablylead to deformation and disaggregation of any composltlon non-rigid solid particles. Such motion could have redistrib- uted solid biotite crystals from the mafic melanosome layers pellte = 58% leucogronlte + 42% melanosome intothe more homogeneous distribution observed in the 3 biotite granite. 20

mlgmotltes

leucoqranltes

14

“40fl 50 60 70 80 SIO, (wt %)

Fig. 15. Variation diagram, wt.% A1203vs. SiO, for whole rock samples of migmatite, pelite (stipple) and ten leucogranites from the Trois SeigneursMassif (data from Wickham in press). A model melanosome compositionis also shown, and indicates thatthe completely differentiated migmatitesshown in Fig. 13c represent Fig. 14. Boudinaged leucogranite podswithin a high grade about 58% leucogranite melt to 42% melanosome. Note the metacarbonate unit from a structurallevel just above the migmatite homogeneity of leucogranite compositions(which extends to many zone (see Fig. 12). The outcrop in the photograph is about 10 m other geochemical parameters) and includes samples from bodies high. with a wide range of size.

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Much of the region now occupied by the biotite granite show many of the characteristicsexpected forthe low probably once resembled the present migmatite zone, since melt-fraction segregation mechanisms outlined in a previous it would have had to havegone through a similar low section. For instance,inaddition to their distinctive melt-fractionstage. A critical moment in biotitegranite geochemistry, and with the exception of two anomalous, petrogenesis came when the effective viscosity of this kilometre-sized bodies,they are all fairly small, ranging kilometre-wide migmatitezone became low enoughfor from a few centimetres to a few hundred metres across (see convective overturn of the entire anatectic region to occur, Fig. 13 and Wickham 1987, Figs 3-6). Convection could haveoccurred progressively (from the Inthe generation of the Trois Seigneurs leucogranite bottomup) or catastrophically, with the kilometre-thick pods and migmatite leucosomes, a segregation process has partially melted zone suddenly overturning all at once (see operatedon a small scale,for centimetre-scale leucosome Wickham 1987, fig. 19). Motion was probablynever very lenses aresurrounded by millimetre-thick restite rims vigorous since texturaland compositionalheterogeneities composed of biotite and sillimanite (Fig. 13). Melt has been are preserved throughoutthe body. The thin (200-300m) locally extracted from these thin mafic layers. The efficiency migmatite zone observed today escaped homogenization and of separation would have been strongly dependent on the now separatesthe biotitegranite from the overlying viscosity of the melt, which can be estimated as follows. It is metasediments. most likely that leucogranite magma was close to An external supply of water is required to flux sufficient water-saturated,in part because of the rapid increase to melting for convective homogenization of the biotite granite 40-50vol.% melting over a few hundred metres of the to haveoccurred atthe relatively low temperatures metamorphic section (see Wickham 1984, 1987). Its (700-750°C) required by thismodel (see also Fig. 10). water content has been estimated by infrared spectroscopic Melting to 60% by volumeas observed in the most analysis of glass quenched from 2.5 kbar, water saturated differentiated migmatites (Fig. 13c) and by implication the melting experiments using Trois Seigneurs peliteas a biotite granite,requires over 4 wt.% H20 (of the total startingmaterial (Wickham 1984). This glass is the system) to be dissolved in themelt, because leucogranite experimental analogue and chemical equivalent of leucogra- magma probably contained 7-8 wt.% H20 (see below and nite melt and has a water content close to 7wt.% H20 (S. Wickham 1987). This is substantially greaterthan the M. Wickham & E. M.Stolper unpublished data). This water content of biotite sillimanite schist (the dominant figure is in good agreement with low-beam current electron protolith) of about 1.5-2.0 wt.% H20, particularly since microprobe analyses of the same sample, which gave totals much of this water remains in residual biotite throughout in the range 90-95% (Wickham 1984). melting. Thus, based solely on petrological evidence, a Foran averageleucogranite composition (Wickham considerable excess of H20,above the normal water content 1987), scaled to a water content of 7wt.%, thesedata of pelitic schist, is requiredto account forthe melting indicate a viscosity close to 105PaS (Shaw 1972). Inthe effects. Based onstable isotopeevidence, Wickham & naturalsituation, pressure was higher thanin the Taylor (1985) have proposedthat thiswater was derived experiments (3-4 kbar), so that the watercontent of the from the surface of theearth, and that large volumes of melts would have been slightly higher, implying a viscosity aqueous fluid were available to flow into the melting zone of 104-105 Pa S. At this relatively low value, centimetre-scale during anatexis. separation of leucogranite magma could have occurred by compaction (see above and Figs 4 & 5), or by filter pressing (Fig. 8) over a range of low melt-fractions below the critical Leucogranite petrogenesis; melt segregationat low melt fraction. melt-fraction Alternatively, extensional fracturing (Figs 6 & 7) could Leucogranitepetrogenesis is constrained by the following generate the observedleucogranite segregations with melt observations; these pods and sills are found only within the moving locally into dilatant sites. Fracturing might initially migmatites and the higher grade mica schists, and show a have formed parallel to the main foliation or oblique and size gradation,the smallest occurring at lowest structural been rotatedinto parallelism with it duringsubsequent levels, the larger ones occupying successively higher deformation. The close spatial association of leucosomes positions in the sequence. They show extreme depletions in and melanosomes (Fig. 13) implies that the melt only moved Mg, Fe, Ti, V, Ni, Cr, Zr, CO, Zn, and the light rare earth a few centimetres during this early segregation stage. elements(Wickham 1987). Leucogranites and migmatite Inthe Trois Seigneurs Massif, leucogranites vary leucosomes havevery similar compositions for major and from centimetre-sized migmatiteleucosomes, to composi- traceelements (see Fig. 15) andthere is a continuous tionally identicalbodies several hundred metres across gradationin size and morphology from centimetre-scale are emplaced within sillimanite-bearing mica schist (Fig. leucosomes to 100 m-sized leucogranites. Experiments have 12). Formation of the larger bodies of leucogranite involved shown that biotite-sillimanite schist from the metamorphic the migration of small lenses of liquid, either along fractures sequencecan generate liquids close to leucogranitic or by buoyant ascent to the top of the anatectic zone where composition for all melt-fractions upto and exceeding they ponded (Fyfe 1970, 1973). Eventually,these bodies 4Ovol.%(Wickham 1984). All leucograniteshave similar became large enough to rise into the overlying, sub-solidus oxygen isotopic compositions to mica schists fromthe metamorphic rocks. The many leucogranite pods within the metamorphic sequence (Wickham & Taylor 1985). Trois Seigneurs migmatite zone may be representative of the The leucogranites are interpreted as partial melts of the region where the liquid collected before being removed by Lower Palaeozoic pelitic metasediments, the liquid segrega- intrusion to higher structural levels, or by incorporation into ting from its source rocks at relatively low melt-fraction the deep biotitegranite (see below). Evidently,bodies of (<40% melt),before convective homogenization. of the leucogranite as small as a few metres across were able to biotite graniteoccurred, (Wickham 1987). These bodies move upwards out of the anatectic zone, into high grade

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4/3xr3L (where L is the latent heat of crystallization of the leucogranite magma). This is set equal to the heat required to raise the country rock temperature to the temperature of the pod over this vertical distance. Hence, 600

h 50070! 4 - nr3L= CpJcrz/ pz dz 3 0 and it follows that

zoo

This expression has been used to calculate h in terms of r for different values of B (other constants as in caption to Fig. 16). The scatter in the data does not allow definition of a Slze (metres) specific value of p. However, the gradient of 80- Fig. 16. Leucogranite size plotted against distance from the base of 100°C km-' estimated forthe Trois Seigneurs sequence the migmatite zone in a direction normal to the isograds. For (Wickham 1987)is not inconsistent with many of the clarity, the many small bodies within the migmatite zone have been observed leucogranite hla values, implying that the model shown as fields. Calculated distance against sizeis plotted using premises are not unreasonable. equation (12) for various geotherms, and for L = 400 kJ kg-' and Although volumetrically trivial compared with the biotite C, = 1 kJ kg-' K-'. Note that the distance travelled by each pod is granite (<5%), the leucogranite pods emplaced within the really a minimum value because the reference point (the baseof the mica schists managed to separate andrise out of the zone of migmatites) was not fixed over the duration of the melting event melting before the biotitegranite began to convect. and is now located at the highest structural level that it attained. Convection would haveresulted theirin being re- homogenized with their migmatitic protoliths. Two possible explanations are suggested for this earlyseparation behaviour. non-anatectic mica schists (Fig. 12). Even the largest bodies (1) The onset of convection ina body the size of the moved only about 1000 m from the present migmatite zone, biotite granite was estimated to occur at a viscosity of about because they were water saturated and liable to crystallize 109Pa S (see Fig. 9 and above). However, this value assumes rapidly on decompression. They were also rising through a that the partially molten zone behaves as a Newtonian fluid region where thethermal gradient was particularly high, which it clearly is not. Non-Newtonian behaviour requires causing them to cool rapidly with increasing distance from that a finite yield-stress beovercome before convective the melting zone. overturn commences. In this case the region might remain Leucogranites are now mostly concordant with the main stably stratified at effective viscosities perhaps as low as 10' foliation in the countryrock, and may beboudinaged or 106Pas. At these low viscosities, Stokes Law ascent of andtor foliated. Dykes of leucogranite cross-cutting the mica small pockets of liquid would be possible overa realistic schist main schistosity at high angles are very rare in the time scale (see above), and there would perhaps be a long TroisSeigneurs sequence, implying that thesebodies period of time available for leucogranite pods to form and probably intruded as small diapirs, with accompanying ascend before convection began. ductile deformation of the country rock through which they (2)Leucogranite separation and ascent was only moved.Alternatively, thestrong, flatteningdeformation possible at an early stage in the melting process, when d, the may have obliterated clearly intrusive cross-cutting thickness of the anatecticzone was considerably smaller structures. than the present dimensions of the biotite granite. Figure 9 Figure 16 shows the variation of leucogranite size with indicates how strongly the convective instability depends on structural level using the base of the migmatite zone as a this parameter (see equation 9). For instance, if the melting reference (Fig. 12). Since this surface has doubtless changed zonewere only 10m thick, it could remain stableat position during the history of leucograniteintrusion, this viscosities as low as 106 Pas (commensurate with high indicates a minimum distance travelled by the pods during degrees of partial melting, in excess of 50%). Separation of their emplacement.This graph suggests that leucogranite small centimetre-sized pockets of liquid and their ascent and pods tend to travel at least ten timesas far as their own collection at the boundary with an overlying, higher dimensions. To aid interpretation, a simple model for viscosity, non-anatectic region could be readily achieved at leucogranite intrusion has been formulated, starting with the this stage (see above). Convective homogenization to form premise that each spherical pod of leucogranite must heat the biotitegranite could occur when the partially melted up the (theoretically cylindrical) volume of rock through zone had reached larger (? kilometric) dimensions, after the which it passes, to its own temperature, this heat being leucograniteshad been emplaced intothe overlying supplied by latent heat of crystallization. More comprehen- metasediments. Because the thickness of a melting zone is sive investigations of similar problems have been made by governed by the temperature distribution (which is likely to many others (e.g. Marsh 1982). A spherical pod, radius r, is be highly transientduring a melting event),the changing considered to rise a height h through country rock where the thermal.structure of such a region may allow different thermalgradient is B. The heatavailable tothe pod is granite-forming processes tooperate at different stages

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duringprogressive anatexis of the crust(see Wickham & likely to occur by large-scale melting of kilometre-sized Oxburgh 1987). regions of the crust (>50% melt). At such high In conclusion, leucogranites have the typical characteris- melt-fractions, magma viscosity will only be about an order tics expected of low melt-fractionsegregates. Small of magnitudehigher thanthe fully liquid state, and centimetre-scalesegregation was probably promoted by convective homogenization of such regions should occur. compaction and by extensionalfracturing of the anatectic Redistribution of refractory mafic phases by convection may rocks.Centimetre-sized pods may have risen through the generate homogeneous textures despite the fact thatthe anatectic zone to collect into large bodies towards its upper magma was never 100% molten. limit, or may have flowed along fracture zones to a similar (3) Water is the most critical variable controlling the 'collecting zone'. Metre- to hundred metre-sized pods were kinematic evolution of c3stal anatectic systems, since it not largeenough to rise diapirically intothe overlying only reduces the viscosity of granitic liquids by many orders sub-solidus metasediments, thelarger bodies ascending to of magnitude, but also permits large-scale crustal melting to highest structural levels, about a kilometreabove the occur at typical crustal temperatures (i.e. below 900°C). present migmatite zone. Smallerbodies could have The intrinsicwater content of many crustal lithologies is coalesced toform largerbodies at any stage of magma incapable of allowing meltingin excess of 50% atthese movement. temperatures. This suggests thatan external supply of aqueous fluid is necessary topromote anatexis of crustal Generation and emplacementof the late granodiorite lithologies on a scale sufficient to generate kilometre-sized magma bodies. The Trois Seigneurs late granodiorite is a discrete, relatively (4) Inthe Trois Seigneurs Massif, Pyrenees, anatexis homogeneous pluton (Fig. 10) with a sharp, concordant of LowerPalaeozoic metapelite generated per-aluminous contact with the mica schists (Wickham 1984, 1987). It leucogranites and biotite granite, while a larger late contains virtually no xenoliths of local country rock,but granodioritepluton was derived by large-scale melting at many lenticular mafic, amphibole-rich xenoliths, which with deeper structural levels in the lower crust. The mafic mineralsdefine a fabric which is pervasive and volumetrically small leucogranite pods represent liquids systematic throughoutthe pluton. This may represent a segregated from pelitic migmatites at low melt-fraction, magmatic flow texture associated with pluton emplacement while the biotitegranite was probablygenerated by (Wickham 1984). Sm-Nd, Rb-Sr and 0 isotope systematics convective homogenization of the same anatectic pelites at (Wickham 1984; Bickle et al. 1985; Wickham &L Taylor high melt-fraction (>50%). An external supply of water was 1985) suggest thatthe pluton was derivedfrom crustal required to flux the large-scale metasedimentanatexis at material,but its chemical and isotopiccomposition are these relatively low temperatures (700-750 "C). distinct from any other granitoid or metasediment exposed in the Trois Seigneurs Massif. A visiting associateship at the California Institute of Technology, a The granodiorite was probablyderived from a Ca-rich research fellowship atTrinity Hall, Cambridge, and a N.E.R.C. mafic igneous lithology in the Hercynian middle or lower post-doctoral fellowship are gratefully acknowledged. Discussions crust(Wickham 1987), and ascended from this region with Ron Oxburgh,Steve Sparks, Dan McKenzie, Hugh Taylor, with minimal stoping of the local country rock at higher Frank Richter, Phi1 Ihinger, Dave Stevenson and Dana Johnston levels. The eastern parthas a sill-like form (Wickham 1984), have been very helpful.Steve Sparks, Mike Brown and Alan suggesting thatthe magma intrudedand inflated to its Matthews are thanked for constructive reviews of the manuscript. observeddimensions as a laccolithic body within the Stephen Jurewicz is thanked for providing the line drawing shown in strongly foliated schists (see also Soula 1982). The Trois Fig. 2. I am grateful to Ken Coe and Dan McKenzie for inviting me Seigneurs late granodiorite is one of the smallest of its type to present this paper at the 1986 William Smith meeting. This work in thePyrenees; it also occurs atdeeper structural levels was partlysupported by NSF Grant EAR 8313106. Contribution than any other within biotite sillimanite schists. Most of the No. 4333, Publications of the Division of Geological and Planetary Sciences, California Institute of Technology. other bodies of this typeare substantiallylarger (e.g. Maladeta) andare emplaced intoUpper Palaeozoic sediments thatare virtually unaffected by regional References metamorphism. Magma ascent for these plutons may have ALLEGRE,C. J. & BEN OTHMAN, D. 1980.Nd-Sr isotopic relationships in followed a similar patterntheto Trois Seigneurs granitoid rocks and continental crust development: a chemical approach leucogranites, the larger ones rising further because of to orogenesis. Nature, 286, 335-41. Am, A. A. 1978.Critical phenomena in the rheology of partiallymelted greater thermal energy content, and smaller surface area to rocks. Tectonophysics, 44, 173-84. volume ratio. ASHWORTH,J. R. (ed.) 1985. Migmatites. Blackie, Glasgow. - & MCLELLAN,E. L. 1985. Textures. In: ASHWORTH,J. R. (ed.) Migmatites. 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Received 7 May 1986; revised typescript accepted 7 October 1986.

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