melting and melt segregation in the aureole of the ... · 40%. the coincidence of the ﬁrst...

Melting and melt segregation in the aureole of the Glenmore Plug,Ardnamurchan

M. B. HOLNESS, K. DANE, R. S IDES, C. RICHARDSON AND M. CADDICKDepartment of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK ([email protected])

ABSTRACT Contact metamorphism caused by the Glenmore plug in Ardnamurchan, a magma conduit active for1 month, resulted in partial melting, with melt now preserved as glass. The pristine nature of much of theaureole provides a natural laboratory in which to investigate the distribution of melt. A simple thermalmodel, based on the first appearance of melt on quartz–feldspar grain boundaries, the first appearance ofquartz paramorphs after tridymite and a plausible magma intrusion temperature, provides a time-scalefor melting. The onset of melting on quartz–feldspar grain boundaries was initially rapid, with an almostconstant further increase in melt rim thickness at an average rate of 0.5–1.0 · 10)9 cm s)1. This rate wasmost probably controlled by the distribution of limited amounts of H2O on the grain boundaries and inthe melt rims. The melt in the inner parts of the aureole formed an interconnected grain-boundary scalenetwork, and there is evidence for only limited melt movement and segregation. Layer-parallelsegregations and cross-cutting veins occur within 0.6 m of the contact, where the melt volume exceeded40%. The coincidence of the first appearance of these signs of the segregation of melt in parts of theaureole that attained the temperature at which melting in the Qtz–Ab–Or system could occur, suggeststhat internally generated overpressure consequent to fluid-absent melting was instrumental in the onsetof melt movement.

Key words: Ardnamurcham; contact metamorphism; Glenmore; melting; melt segregation.

INTRODUCTION

Segregation of melts is ubiquitous in the Earth and isresponsible for transport of heat and mass, leading tochemical differentiation. Given the importance of thisprocess, the rates and mechanisms of melting and meltsegregation have been the subject of intensive researchover the last few decades. The distribution and beha-viour of melt during the earliest stages of anatexis iscritical to our understanding of this problem. Much ofthe research in this area has concentrated on experi-mental studies as it is not always straightforward tointerpret the textural signature of melting in naturalexamples (e.g. Sawyer, 1999, 2001). This interpreta-tional problem is greatest for widespread meltingassociated with regional metamorphism, and is com-pounded by the fact that the distribution of the solid-ified melts remaining in such migmatites reflects a latestage in the process of melting and melt segregation(Sawyer, 2001).

In small contact aureoles the relatively short dur-ation of metamorphism, coupled with the generallystatic nature of the thermal event, means that texturesdeveloped at the metamorphic peak may be well pre-served. Following the previous work on small pyro-metamorphic aureoles surrounding major magmaconduits (e.g. Butler, 1961; Holness, 1999; Holness &Watt, 2002) we report the results of a study of meta-morphism and melting in a small contact aureole inGlenmore, Ardnamurchan. In this aureole, the

textures formed at the metamorphic peak are extra-ordinarily well preserved, with abundant fresh glass inthe country rock. The trade-off for excellent preser-vation is that the solid products of the prograde reac-tions (some of which were probably metastable) aregenerally extremely fine-grained, making problematicthe detailed study required for a rigorous thermaltreatment of the metamorphic history. However,approximate time-scales can be used to place bothtemporal and spatial constraints on the earliest stagesof anatexis, providing an ideal natural laboratory inwhich to study melting and melt transport.

The aureole of the Glenmore plug was first describedby Butler (1961), who undertook a preliminary study ofthe metamorphism, identifying and describing theextensive anatexis there. As part of the current study acollection was made of a more closely spaced set ofsamples. The scope of Butler’s (1961) study has beenextended to include a full description of the aureole, withtemporal constraints placed on its evolution using asimple thermal model. Comparison is then made ofobservational evidence for melt formation and segre-gation in the aureolewith that fromexperimental studiesand studies of other natural examples of anatexis.

GEOLOGICAL SETTING AND FIELDDESCRIPTION

A group of minor mafic intrusions lies to the east of theTertiary Igneous Complex that forms the western end

J. metamorphic Geol., 2005, 23, 29–43 doi:10.1111/j.1525-1314.2005.00560.x

� 2005 Blackwell Publishing Ltd 29

of the Ardnamurchan peninsular (Fig. 1). Butler(1961) identified 14 such intrusions, both dykes andplugs, in the garnet-grade psammitic and pelitic me-tasediments of the Moine Group. The majority of theseminor intrusions have caused melting in their wall-rocks, pointing to their role as magma conduits feedinglava flows. The Glenmore Plug, the smallest of theArdnamurchan intrusions identified by Butler,is unusual in that most of the melt in the country rockis now preserved as pristine glass. The age of the plug isuncertain but, given the freshness of the dolerite andexcellent preservation of aureole textures, it is likely tobe Tertiary (Butler, 1961).

The Glenmore dolerite plug crops out on the hillsideabove the track leading from Glenborrowdale toLochan na Carraige (Fig. 1), at the grid reference(590 637) on OS sheet 390. It forms a marked prom-inence on the hillside, and the blocky, dark-weather-ing, steep-sided outcrop is roughly circular in plan(Fig. 1b). Parts of the plug show well-developedcolumnar jointing aligned horizontally, perpendicularto the steep walls of the intrusion. The margins of theplug are finer grained than the central region. Theoutermost 0.5 m is rich in in-weathering, carbonate-filled, vesicles up to 5 mm in diameter. At the currentlevel of exposure the plug is slightly elongated in thedirection of the strike of the enclosing schists (Fig. 1b).The south-east face is irregular and corrugated, withalternating layers of glass (derived from the countryrock) and fine-grained dolerite oriented with strike137 �, and dipping steeply to the south-west. This isbecause of the formation of foliation-parallel apoph-yses of dolerite in the country rock, consistent with theelongation of the plug being caused by either prefer-ential erosion of particular horizons within the Moine,or by mechanical processes such as hydrofracture and

plucking. No xenoliths of country rock are present inthe dolerite. Field evidence for assimilation of thecountry rock is provided by small ovoid patches offelsic material and large vesicles concentrated in theouter regions of the plug (Butler, 1961).The country rock is dominated by well-bedded, pale

arkose, with scarce, 1 cm thick, pelitic layers. Althoughthe rocks were metamorphosed to garnet-grade priorto contact metamorphism, sedimentary structures,such as cross-bedding, are clearly visible.Three different sets of veins are visible in outcrops of

the country rock. Within 1 m of the contact, the arkosecontains fractures filled with basalt (Fig. 2a). These areup to 2 cm wide, forming either strings of isolated,lensoid sheets, or continuous, parallel-sided sheets.They do not form networks. Within 0.6 m of thecontact are veins filled with silicic glass. They areweakly curved, up to 1 cm thick, spaced as closely as5 cm, and form branching arrays (Fig. 2b). The thirdtype cuts the other two, and forms a network of par-allel-sided sharply bounded quartz- and calcite-filledfractures with two preferred orientations (Fig. 2c).This type occurs within a few metres of the contact.Sample localities are shown in Fig. 1b. Exposure of

the country rock is poor, because of thick peat cover.However, Butler (1961) solved this problem by digginga trench away from the contact. For this study,another trench was dug (the site of his original trenchcan no longer be identified) to collect a closely spaced(c. 20 cm apart) suite of country rock samples, (prefixX, Fig. 1b). The sample suite collected from the newtrench is more closely spaced than that of Butler’s(1961) study, and contains more distal samples thanButler’s suite, which did not extend to the edge of theaureole. The traverse in the trench is at a high angle tocompositional and textural layering in the country

BenHiant

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Fig. 1. (a) Location map for the Glenmore plug in Ardnamurchan. The pale grey area outlines the main region of the TertiaryIgneous Complex, whereas the small dark grey areas show settlements. The dashed line is the road. (b) Detailed map of the plug,showing the location of sampling traverses.

30 M. B . HOLNESS ET AL .

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rock and includes a thin pelitic layer as well as thedominant arkose. Samples were also collected from theplug itself, although smoothly polished outcrop pre-vented a continuous sample traverse.

ANALYTICAL METHODS

Bulk analyses of dolerite were obtained using ICP-MSat the NERC ICP Facility, School of Earth Sciencesand Geography at Kingston University. Micro-analy-sis of mineral and glasses were undertaken using aCameca SX-100 electron microprobe, using energy-dispersive spectrometry at the Department of EarthSciences, University of Cambridge. Spectra were col-lected with a PGT prism 2000 ED detector and thedata reduced with the PGT EXCALIBURPGT EXCALIBUR software. Thenominal beam diameter was 2 lm for mineral analyses,and 10 lm for glasses. The accelerating voltage was15 keV, with a beam current of either 10 nA (for majorelements) or 40 nA (for trace elements).

Back-scatter electron (BSE) images were obtainedusing the JEOL 820 Scanning Electron Microscope inthe Department of Earth Sciences, University ofCambridge, with an accelerating voltage of 20 kV and1 nA beam current. It proved impossible to obtainlarge unpitted areas for BSE imaging as the samplesare highly fractured and do not polish well. TheCambridge JEOL 820 is fitted with the Gatan Mo-noCL4, which was operated in panchromatic mode toobtain cathodoluminesence images. The acceleratingvoltage was 20 kV, with 1 nA beam current, and anacquisition time of 1 min. The majority of CL imageswere collected at a working distance of 16 mm from

the pole-piece, which equates to a sample-CL detectorseparation of c. 0.5 mm.

The underlying causes of variable luminescence inquartz are not properly understood, but are thought toinclude the effects of minor chemical impurities (e.g.Al, Na and OH) that act either directly or indirectly(by causing lattice defects). Quartz recrystallized at lowtemperatures (100–200 �C) is generally weakly lumin-escent, whereas that crystallized at higher temperaturestends to be strongly luminescent. The reasons for thisincrease in luminescence may be related to a higherdefect density (Watt et al., 2000).

Estimates of the modal composition and the volumefraction of melt in the country rocks were made bypoint counting 1000 points per thin section. Measure-ments of melt rim thickness were obtained using aJ. Swift & Son micrometer eye-piece on an opticalmicroscope. Up to 30 individual rim thicknesses weremeasured on each sample. To minimize the difficultiescaused by 2D cross-sectioning of randomly orientedmelt rims only the narrowest rims were measured inorder to obtain a closer approximation to true rimthickness. This procedure necessarily becomes rathersubjective for the samples in which movement of therestitic phases has occurred because the melt rimthickness is changed. In such samples, rims that sur-round grains that appear to be centred within meltpools were measured.

THE PROTOLITH

The quartz-rich metasediments have a weakly definedcompositional layering defined by differences in the

Fig. 2. Photographs of outcrop-scale veins in the country rock. (a) Within 1 m of the contact, the arkose contains dolerite-filledfractures and strings of isolated, lensoid, sheets. (b) Within 0.5 m of the contact are veins filled with silicic glass (arrowed). They are upto 1 cm thick, weakly curved, and form branching arrays. (c) Network of parallel-sided sharply bounded quartz- and calcite-filledfractures with two preferred orientations. These fractures post-date the other two types and occur within a few metres of the contact.

MELT ING AND MELT SEGREGAT ION, GLENMORE 31


amount of biotite and white mica. The mica-rich layersalso contain quantities of fine rounded grains ofaccessory minerals such as titanite and epidote. Rareragged garnet grains occur in some horizons (Fig. 3a,see Table 1 for representative compositions). The endsof many of the larger muscovite grains form irregularsymplectites with quartz, suggestive of retrogradegrowth (perhaps as a replacement for Al2SiO5 phases).Two feldspars are present: albite-rich plagioclase andmicrocline. Abundant signs of low-temperaturedeformation include kinking of mica grains, unduloseextinction in quartz and feldspar, and deformationtwins in plagioclase.

The quartz is weakly luminescent, with an irregulararray of bright points and a tracery of fine, irregularfractures marked by either non-luminescent or weaklyluminescent material (Fig. 4a). This texture is typicalof regional metamorphic quartz unaffected by laterhigh-temperature contact metamorphic events (e.g.Holness & Watt, 2001).

MINERALOGICAL EFFECTS OF THEMETAMORPHISM

The onset of contact metamorphism is marked, 1.65–1.87 m from the contact, by the replacement of mus-covite by a brown, pleochroic, luminescent, fine-grained aggregate (Fig. 3b). Following Brearley (1986)and Holness & Watt (2002) this is interpreted to be anaggregate of biotite, K-feldspar, spinel ± corun-dum ± mullite, with the pronounced pleochroism aconsequence of the preferred orientation of the biotite.This reaction initiates at grain margins and propagatesalong the cleavage planes in the muscovite. It is com-plete 1.39 m from the contact. Closer than c. 1 m fromthe contact, rims of K-feldspar are locally, but rarely,developed around the reacted muscovite grains. Thischanges in the sample collected 0.80 m from the con-tact, in which well-defined K-feldspar rims are presentaround all reacted muscovites, analogous to features

observed very close to other intrusions (e.g. Holness &Watt, 2002). Microcline-twinned K-feldspar is absentin all samples closer than 0.80 m from the contact.Within 1.65 m from the contact, biotite is partially

replaced by a luminescent aggregate of oxide particles(?spinel) concentrated either on the crystal margins, oncleavage planes or on transgranular cracks at highangles to the cleavage, together with lenses of K-feld-spar oriented parallel to the cleavage in the originalmica (Fig. 4b). Details of the fine-grained reactionproducts could not be discerned optically. Thisreplacement is complete at 0.90 m from the contact,with the sites of reacted biotite grains marked byopaque pseudomorphs.The breakdown of both biotite and muscovite is

associated with the appearance of brightly luminescentfractures in the adjacent quartz grains. These fracturesfall into two types. The first, Type 1, is not visiblein back-scatter images (Fig. 4c,d), and is very rare inlow-grade samples in which mica breakdown isincomplete. Type 1 fractures are rooted at the sites ofreacted mica grains, and increase in both length andabundance toward the contact, with a sudden increasein abundance at 0.80 m from the contact (in the samplein which the K-feldspar rims become ubiquitous). Thesecond type, Type 2, is much less common, and formsshort tapered cracks rooted at the sites of the reactedmica grains (generally biotite rather than muscovite).Type 2 fractures are visible in back-scatter images andtherefore contain material other than quartz(Fig. 4c,d). Type 2 fractures occur in samples closerthan 0.90 m from the contact, and become both morecommon and longer as grade increases.The onset of extensive melting occurs at 0.79 m from

the contact. Melting is marked optically by the devel-opment of finely crystalline, turbid brown rims ofsolidified melt on both quartz–feldspar and quartz-reacted mica boundaries (Fig. 3c). Both the crystallinematerial solidified from melt and the outer surfaces ofthe quartz grains are strongly luminescent (Fig. 4e,f).

Fig. 3. Optical photomicrographs taken in plane polarized light. (a) Sample Z3, collected 7 m from the contact. Scale bar is 200 lmlong. This sample is unaffected by contact metamorphism and comprises biotite (dark), muscovite (clear), quartz and feldspar (dustedwith inclusions) together with numerous high relief grains of epidote, titanite and zircon. (b) Sample X20, collected 1.62 m fromthe contact. The muscovite is being replaced by a turbid, brown pleochroic aggregate that first appears at the ends of grains andpropagates along the cleavage until the entire grain is replaced. The biotite grains also display partial replacement by oxide particles.Scale bar 200 lm long. (c) Sample X13, collected 0.72 m from the contact. Finely crystalline rims of solidified melt (arrowed) separatequartz (clear) from feldspar (turbid). The muscovite grains (marked ms) are now completely replaced, and the biotite grains (markedbt) are rich in opaque grains, possibly spinel. The boundaries between quartz and reacted mica contain microcrystalline solidifiedmelt. Scale bar is 200 lm long. (d) Sample X12, collected 0.66 m from the contact. The reaction rims separating quartz (centre ofimage) and either feldspar or mica, comprise an inner crystalline portion nucleated on the quartz substrate and an outer glass portion.Scale bar 100 lm. (e) Sample X10, collected 0.56 m from the contact. The melt rims here are completely glassy and separate quartz(note the high crack density) and feldspar, which is beginning to develop a sieve texture. The sites of biotite (now completely opaque)are marked. Scale bar 200 lm long. (f) Sample X3, collected 0.16 m from the contact. The area depicted is comprised of brownglass (with a clear grain of quartz in the top left-hand corner) containing elongate grains of orthopyroxene. Note the bending andbreakage of the longest grain. Scale bar 100 lm long. (g) Sample X4, collected 0.19 m from the contact. This sample contains anirregular vein of invading mafic melt, with diffuse edges. The glass in the country rock contains flow lines marked by colour differencesin the regions close to the infiltrated mafic melt. Scale bar 1 mm long. (h) Sample X6, collected 0.28 m from the contact. Quartz (clear)and brown glass dominate, with an irregular sieve-textured grain of residual feldspar. This contains aggregates of opaque materialafter biotite. The shape of the original biotite grains is clearly discernible within the feldspar, although the continuation of the reactedgrain into the glass is marked by an irregular, dispersed array of spinel grains. Scale bar is 200 lm long.



Rims of solidified melt are ubiquitous on quartz–micaboundaries, but not always present on quartz–feldsparboundaries in samples between 0.79 and 0.72 m fromthe contact. Closer in than this, all quartz–feldsparboundaries show signs of melting.

The solidified melt rims contain quartz paramorphsafter tridymite in samples closer than 0.72 m from thecontact (Fig. 5a). These are evident as fine, plate-like,extensions in optical continuity with their quartz sub-strate. Toward the contact the melt rims become pro-gressively thicker, and lose their crystalline nature.Brown, non-luminescent (apart from the rare devitri-fied areas) glass is present closer than 0.66 m from thecontact, and first appears on the feldspar side of meltrims. The other side of the melt rim is crystalline, anddominated by strongly luminescent quartz that nucle-ated in optical continuity with the substrate (Fig. 3d

& 5b). The melt rims contain small dark needles(?mullite) and shorter, clear stubby prisms (?orthopy-roxene), which are most common near reacted biotitegrains.A sudden change from composite glass + gran-

ophyre melt rims to glass-only rims (containingabundant grains of orthopyroxene) occurs in thesample collected 0.66 m from the contact (Fig. 3e).The transition is associated with a loss of mechanicalcoherency of the reacted biotite grains, and the loss ofthe brown colour of the reacted muscovite grains. Thelatter are now formed of a fine aggregate of high reliefgrains of possibly spinel and mullite. The glassy rimsare porous at the contact with residual quartz, andparamorphs after tridymite apparently grow into porespaces (Fig. 5a,c). There is no porosity in the glassadjacent to residual or recrystallized feldspar. At0.66 m from the contact, the feldspar grains develop asieve texture, indicating internal melting (Fig. 3e),which culminates in the disaggregation and recrystal-lization of restitic feldspar.In the proximal samples, the melt is preserved as

pristine (though locally devitrified), pale brown glass,with clumps of clear, acicular and prismatic orthopy-roxene crystals that nucleated on restitic, rounded(probably corroded), highly fractured quartz. Theorthopyroxene needles are generally straight, althoughsome are strongly curved, with undulose extinction andfracturing (Fig. 3f). The curvature is best developed inthe vicinity of patches of invading mafic melt (Fig. 3g)which appear in samples closer than 0.50 m from thecontact. Restitic, sieve-textured, feldspar is alwayspresent, even at the contact, although much of it hasrecrystallized as clusters of small euhedral grains. Theremains of biotite are present as diffuse concentrationsof opaque grains, best preserved where they are incontact with restitic feldspar, and commonly act asnucleation sites for feldspar growth (Fig. 3h).Within 0.20 m of the contact, the country rock is

dominated by brown glass that contains roundedrestitic quartz (which has surfaces covered with smallorthopyroxene crystals and paramorphs after tridy-mite), sieve-textured and partially disaggregatedfeldspar grains, dispersed clumps of spinel, and scat-tered free-floating orthopyroxene grains.

Table 1. Representative compositions determined by electronmicroprobe, with the ratio of Fe3+ to Fe2 calculated using theprogram AXAX [T. J. B. Holland, pers. comm. (2000)].

Sample

Garnet Biotite Muscovite

X20A X23A X20A X23A X31A X14A

SiO2 37.13 37.16 35.79 36.42 50.43 49.88

TiO2 0.00 0.00 2.01 1.65 0.86 0.46

Al2O3 19.94 20.22 16.41 17.65 29.41 30.43

Cr2O3 – – – – – –

Fe2O3 2.31 2.43 0.20 – – –

FeO 17.99 20.49 26.02 24.32 3.27 3.94

MnO 8.12 4.94 0.06 0.09 – –

MgO 0.05 0.00 7.16 6.95 0.86 0.43

CaO 14.12 14.78 – – 0.25 0.44

Na2O 0.00 0.00 – – 1.00 1.63

K2O 0.00 0.00 9.41 8.70 9.33 8.45

Total 99.68 100.04 97.07 95.79 95.42 95.57

Si 2.99 2.97 2.77 2.80 3.35 3.31

Ti – – 0.12 0.10 0.04 0.092

Al 1.89 1.91 1.50 1.60 2.31 2.38

Cr – – – – – –

Fe3+ 0.14 0.15 0.01 – – –

Fe2+ 1.21 1.37 0.68 1.57 0.18 0.22

Mn 0.55 0.34 0.00 0.01 – –

Mg 0.01 – 0.83 0.79 0.09 0.04

Ca 1.22 1.27 – – 0.02 0.03

Na – – – – 0.13 0.21

K – – 0.93 0.86 0.79 0.72

Total 8.00 8.00 7.83 7.73 6.91 6.94

Mineral compositions (including the ratio of Fe3+ to Fe2+) were recalculated using the

programAXAX,with garnet recalculated to 12oxygen, and the biotite andmuscovite to 11 oxygen.

Fig. 4. (a) CL image of sample X23, collected 1.87 m from the contact. The original quartz shows a typical signature of regionalmetamorphism while the mica grains are non-luminescent. Original feldspar is highly luminescent. Scale bar 100 lm long. (b) BSEimage of sample X31, collected 1.15 m from the contact. The biotite is partially replaced by fine grains of spinel (bright), and K-feldspar (dark grey) elongated along the biotite cleavage. This reaction is initiated at the margins and along cracks in the biotite. Scalebar is 50 lm long. (c) BSE image of sample X29, collected 0.90 m from the contact. The poor quality of the polish is because ofthe highly cracked nature of these rocks. Note the small melt-filled crack emanating from the end of the reacting biotite grain. Scale baris 50 lm long. (d) CL image of the same area as (c). The melt-filled crack is highly luminescent, pointing to the crystalline nature ofthe solidified melt. Note also the presence of other cracks (arrowed), which are not apparent in BSE mode. These are believed tobe the sites of healed H2O-bearing fractures. The original biotite grain is non-luminescent, whereas the reacted parts are bright. Scalebar is 50 lm long. (e) BSE image of sample X14, collected 0.79 m from the contact. Note the porous nature of the crystalline rim ofsolidified melt. Scale bar is 100 lm long. (f) CL image of the same area shown in (e). Note the non-luminescing un-recrystallized quartz,traversed by bright cracks filled with crystalline solidified melt. The quartz grain is outlined by highly luminescent quartz crystallizedfrom the melt, while the porous regions of the melt rim are dark. Both the feldspar and the reacted mica grains are highly luminescent.Scale bar is 100 lm long.



The melt rims that separate quartz from feldsparand mica have parallel margins in samples further thanc. 0.30 m from the contact. Closer than this, the rimsare commonly asymmetric, with evidence for move-ment of the restitic grains. Closer than 0.20 m some ofthe glasses contain flow lines marked by streaks ofdifferent colour, with which orthopyroxene grains arecommonly aligned. This is most evident close toregions of invading mafic melt (Fig. 3g).In all samples, the compositional layering of the

protolith, defined by alignment of elongate patches ofrestitic material, is well preserved. No evidence wasfound for disruption of the layering, even in samplescollected within 0.10 m of the contact.

DISTRIBUTION OF MELT IN THE AUREOLE

In outcrop, melt is clearly evident as black glass, dis-tributed heterogeneously between the preserved prot-olith layers; some layers contain much more glass thanothers. The glass-poor layers are rich in quartz and wesuggest that the relative paucity of melt in them isbecause of lack of reactants. In thin section most of themelt forms rims that separate the reactant phases. Thisis the geometry found in experimental studies ofmelting of intact rocks (Arzi, 1978; Mehnert et al.,1973), and also of detailed petrographic studies ofother natural examples (Sawyer, 2001).Melt forms elongate pools and veins either parallel

or subparallel to the protolith foliation. These are mostapparent in thin section. Melt also forms veins cuttingthe foliation and compositional layering at high angles(Fig. 2b). Within 0.20 m of the contact these arecharacteristically up to 3 mm wide and spaced c. 5 cmapart.The width of the melt rims separating the two

reacting phases was measured. Given the simplicity ofthe melt-forming reaction only melt rims formedbetween quartz and feldspar were examined. Rimsbecome progressively thicker towards the contact, atan almost constant rate (Fig. 6).

Fig. 5. (a) Scanning electron microscope image of sample X3,collected 0.16 m from the contact, showing the porosity sur-rounding plates of tridymite nucleated on a quartz grain duringcooling. Scale bar is 5 lm long. (b) CL image of sample T4,collected from the contact. The areas with low luminescence areunrecrystallized quartz. Note how they are crossed by brightlyluminescing fractures filled with either solidified (and crystalline)melt or high-temperature quartz grown during the healing ofH2O-filled cracks. The irregular, non-luminescing band withbright boundaries crossing the image is glass, with margins ofhigh-temperature quartz crystallized on restitic quartz duringcooling. Note the oval area with a central bright region (feldspar)an inner non-luminescent zone (glass) and an outer zone ofbright high-temperature quartz. Scale bar is 50 lm long. (c) BSEimage of sample T4, showing the porosity surrounding the fringeof tridymite plates nucleating on restitic quartz. Scale bar is25 lm long.



An examination was also made of the total volumefraction of melt as a function of distance from thecontact. This shows a steady increase toward thecontact, to a maximum of almost 60 vol.%, reached c.0.28 m from the contact (Fig. 7). This flattening-off ofthe graph is associated with an increase in both theamount and the size of restitic quartz grains andaggregates. As melting occurs on the boundaries sep-arating quartz from feldspar and mica, an increase inthe size or proportion of monomineralic quartzdomains will decrease the melt-producing potential ofthe rock. It is this, which is more likely to have causedthe decrease in the volume of melt close to the contact,rather than segregation and expulsion of melt from theinner parts of the aureole. Note that the amount of

restitic feldspar decreases steadily as the contact isapproached. This is because the feldspar is meltinginternally by a quartz-absent reaction (i.e. feldspar ¼melt); the amount of restite is not affected by quartzgrain size.

MELT COMPOSITION

We examined variations in glass composition, for bothmica–quartz melt rims and feldspar–quartz melt rims,as a function of position within the aureole. A pre-liminary study of glass composition in the centres ofmelt rims in samples collected between 0.49 and 0.16 mfrom the contact showed that there is no discernibledifference in composition between the central parts ofrims formed by quartz–mica melting and those formedby quartz–feldspar melting. This is consistent with theresults of Butler (1961) who measured the bulk com-position of glass separates. Typically, CIPW normativecompositions are dominated by quartz, albite andorthoclase, with minor amounts of normative corun-dum, anorthite and hypersthene (Table 2).

Despite the compositional uniformity of the bulk ofthe melt rims, the glass composition at their marginsvaries according to the nature of the non-quartz reac-tant phase. Within 10 lm of restitic biotite mica theglass is enriched in Na, K, Ca, Ti and Al, and depletedin Mg compared with that immediately adjacent torestitic feldspar (Table 2).

The compositions of the layer-parallel melt poolsand veins the cut layering at low angles are generallyindistinguishable from those of the glass in the centresof the melt rims. The glass composition from a major,2-mm wide vein, cutting layering at a high angle, at

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in m

elt

Fig. 7. The volume fraction of melt, restitic quartz and restiticfeldspar as a function of position in the aureole, determined frompoint counting. The grey shaded area shows the region of theaureole in which thin section scale invasions of mafic melt arecommon. The first appearance of layer-parallel segregations ofmelt and the onset of asymmetry in the thickness of melt rimsbecause of relative movement of melt and restite are shown bydashed lines. Note the continuous decrease in the abundance ofrestitic feldspar compared with the increase in restitic quartz.This is a consequence of protolith texture and bulk compositionand controls the total volume of melt formed. See text for details.

Table 2. Representative analyses and CIPW norms of glassfrom samples collected 0.49 m (X8G) 0.40 m (X10G), 0.28 m(W16G) and 0.16 m (X3G) from the contact.

Sample W16G X3G X10G X8G

Feldspar Mica Vein Feldspar Mica Feldspar Mica Feldspar Mica Vein

SiO2 77.87 76.53 77.78 73.62 72.25 78.62 78.97 76.15 77.71 75.56

TiO2 0.18 0.49 0.29 0.08 0.39 0.21 0.19 0.26 0.16 0.29

Al2O3 12.34 13.58 12.79 14.26 15.06 11.68 11.74 13.02 12.58 13.47

FeOtot 1.43 0.91 0.93 2.65 2.07 1.31 1.00 1.92 1.39 1.52

MnO 0.11 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.09

MgO 0.21 0.09 0.02 0.70 0.49 0.04 0.02 0.17 0.16 0.15

CaO 0.54 0.68 0.58 0.44 0.62 0.27 0.23 0.43 0.35 0.48

Na2O 3.81 4.12 4.11 3.39 3.63 3.19 3.20 3.39 3.10 3.85

K2O 3.52 3.60 3.51 4.84 5.48 4.67 4.63 4.67 4.52 4.57

Q 37.5 34.6 36.2 29.5 25.2 38.5 39.2 33.6 37.8 31.6

C 1.2 1.6 1.1 2.5 1.9 0.8 1.0 1.5 1.8 1.2

Or 19.6 20.0 19.4 27.1 30.7 26.2 25.9 25.8 25.1 25.7

Ab 30.3 32.8 32.5 27.2 29.2 25.6 25.6 26.8 24.6 31.0

An 2.5 3.2 2.7 2.1 2.9 1.3 1.1 2.0 1.6 2.3

Hy 2.7 1.0 1.2 6.1 4.1 2.0 1.5 3.3 2.6 2.7

Il 0.3 0.9 0.5 0.2 0.7 0.4 0.4 0.5 0.3 0.5

Feldspar and mica denote the original phase reacting with quartz to form the melt, and these

analyses were obtained <15 lm from the residual feldspar or mica.

�Vein� denotes a typical analysis of a cross-cutting melt-filled vein.

FeO and Fe2O3 are grouped together as FeOtot.

All oxide totals normalized to 100 (rounding errors result in a final total which may differ

from 100).



c. 0.40 m from the contact, is given in Table 2, withAl2O3 and SiO2 compositions shown in graphical formin Fig. 8. The vein glass is intermediate in compositionbetween glass in the bulk of the sample analysedimmediately adjacent (within 20 lm) to restitic feld-spar and that of glass immediately adjacent to restiticmaterial after biotite. This shows that the melt segre-gated into the vein is a mixed liquid and was derivedfrom melt formed on quartz–mica and quartz–feldspargrain boundaries.

Many experimental studies of melting involvingfeldspar have shown that melting near the solidus isgenerally out of equilibrium (e.g. Johannes & Holtz,1992). The glass analyses from quartz–feldspar grainboundaries, with Qtz, Ab and Or recalculated to100%, plot on or close to the cotectic in the Qtz–Ab–Or–H2O system (Tuttle & Bowen, 1958) with positionsgenerally consistent with their relative positions in theaureole (Fig. 9). For example, glass analyses fromsample X10, collected 0.56 m from the contact, plot atthe minimum, while those from sample W16 (from0.28 m) plot closer to the field of bulk compositions,consistent with equilibrium.

The compositional differences in the analyses ofglass <15 lm from restitic phases (Fig. 8 and Table 2)demonstrate a diffusional control on melt composi-tion. For example, the majority of the data from X3(0.16 m) fall just within the feldspar phase field withone point on the cotectic. The latter was obtained15 lm from a restitic quartz grain while all the otherswere from the centre or the feldspar side of the meltrim. The homogeneity of the melt composition in muchof the melt rims is most probably because of the highrates of cation diffusion; diffusion of cations withdiffusivities of 10)7–10)9 cm2 s)1 (e.g. Chakraborty,1995) can smooth out initial millimetre scale compo-sitional differences in a few weeks.

TEXTURAL EVIDENCE FOR MELT SEGREGATION

The aureole of the Glenmore plug provides an insightinto the earliest stages of crustal melting, in which meltformation, rather than melt movement, dominates.Given the dominance of reaction site in determiningthe distribution of melt, the actual volume of meltpresent at any point in the aureole is a strong functionof protolith fabric (Fig. 6, and Sawyer, 2001). How-ever, as the rims form an interconnected network evenat very small total volume fractions, there is always thepotential for melt movement. The actual amount ofmelt movement will then be controlled by themechanical behaviour of the partially melted rock,which is a function of the volume fraction of melt andthe time available.There are numerous indicators of melt movement

within the partially melted rocks. Importantly, how-ever, these all indicate only slight relative movementsof melt and restite. These include the breakage oforthopyroxene grains because of forceful juxtapositionwith restitic grains of quartz, the development ofasymmetry of melt rims as a result of relative move-ment of melt and restitic grains, the disaggregation ofpseudomorphs after mica, the disaggregation of restiticfeldspar, the development of veins filled with internallygenerated melt, and the development of flow lines(associated with infiltration of mafic melt) in the glass.The distances from the contact at which these indica-tors appear are shown in Fig. 7. One of the most distalindicators of melt movement is the development of

11.2

11.6

12

12.4

12.8

13.2

71.5 72 72.5 73 73.5 74

SiO2 (wt %)

Al 2

O3

(wt %

)Quartz–mica boundaryQuartz–feldspar boundaryCross-cutting vein

Fig. 8. Composition of a 2-mm wide cross-cutting glass-filledvein compared with the composition of glass both immediatelyadjacent to restitic feldspar and to restitic material after biotite.The vein represents material from both quartz–feldspar andquartz–mica grain boundaries and has a composition interme-diate to them. Ab Or

Qtz

40 60

80

20

20

80

40

60

X10

X3W16

Fig. 9. The CIPW normative compositions of glass fromquartz–feldspar grain boundaries compared with the cotectic inthe H2O-saturated Qz–Ab–Or system at 500 bar (Tuttle &Bowen, 1958). The shaded area shows the region of bulk com-position of the country rock from Butler (1961). The glasscompositions have been recalculated to 100% after omission ofeverything other than Qz, Ab and Or.



elongate (planar in 3D), orthopyroxene-bearing glasspools at low angles to the original compositionallayering. These appear c. 0.5 m from the contact,where the volume fraction of melt exceeds 30–40%.

Where the melt fraction exceeds 42%, pronouncedasymmetry of melt rims is evident, because of relativemovement of the melt and restitic grains. The immo-bile restitic grains are surrounded by 40 mm melt rimsat this point (Fig. 6), and the well-defined onset of thisrelative movement occurs between 0.46 and 0.49 mfrom the contact (39–42 vol.% melt).

The well-developed flow lines defined by colourvariations of the glass appear c. 0.20 cm from thecontact (c. 50 vol.% melt), and probably show regionsof limited advective mixing of infiltrating and inter-nally derived melt. Deformed grains of orthopyroxeneare spatially associated with these regions of meltmixing.

Very minor amounts of melt are found in the Type 2fractures. These have been described in rocks fromsimilar pyrometamorphic settings (Holness & Watt,2002), and are commonly observed in experimentallymelted rocks (e.g. Van der Molen & Paterson, 1979;Connolly et al., 1997). However, these fractures are notobserved in other, larger contact aureoles (e.g. that ofthe Duluth Igneous Complex, Sawyer, 2001). In com-mon with Rosenberg & Riller (2000) and Rosenberg &Handy (2000) we suggest that the difference is primarilyone of heating rate. In small aureoles the rate of meltproduction is rapid, and the fast cooling means thatmelt-filled fractures have little time to heal and areconsequently more likely to be preserved. We do notconsider that these fractures represent a significantpathway formeltmigration (e.g. Holness&Watt, 2002).

Although the lack of exposure and geometry of thesampling traverse precludes direct comparison of pro-tolith composition with that of the partially meltedrock, the presence of melt rims, together with thesmooth variation in melt content with position inthe aureole (Fig. 6), points to minimal loss of melt. Thepartially melted zone preserves an undisrupted proto-lith layering even in samples in which more than halftheir volume was liquid. This is in common with studiesof anatexis in similar settings (e.g. Holness, 1999;Holness & Watt, 2002). The main reason for this mustbe the high melt viscosity, which precludes significantrelative movement of liquid and solid restite driven bygravity alone (McKenzie, 1987), coupled with the es-sentially static nature of the melting; segregation ofthese melts would require some kind of shearingdeformation synchronous with melting (e.g. McLellan,1988; Sawyer, 1994; Brown & Rushmer, 1997).

Given the absence of significant expulsion of melt,what is the mechanism of formation of the abundantveins and layer-parallel melt pools containing meltsourced from the metasediments? Perhaps significantly,the first appearance of these veins and melt poolscoincides with the point in the aureole where fluid-ab-sent melting in the systemQz–Ab–Orwould be expected

to start (Fig. 10). This is consistent with the suggestionof Clemens & Mawer (1992) that the positive volumechange on reaction may provide a force driving meltsegregation. The positive volume change could alsoplausibly create internally generated overpressure lead-ing to fracturing (e.g. Connolly et al., 1997; Holness &Watt, 2002). It is possible that the layer-parallel meltpools may have formed by an opening-up of the folia-tion planes because of melt over-pressure. Although it isdoubtful that the small-scale fractures described byConnolly et al. (1997) and Holness & Watt (2002) willprovide a significant melt flow path (Holness & Watt,2002), an abundance of such layer-parallel melt poolscould result in (highly directional) enhanced meltmobility, given sufficient time.

PRESSSURE AND TEMPERATURE CONDITIONSOF METAMORPHISM

In order to place time constraints, via a simple thermalmodel, on the evolution of the aureole the pressure andtemperature conditions of metamorphism are required.The fine grain-size of reaction products in the aureole ofthe Glenmore plug makes identification and composi-tional determination problematic. Identification ofphases involved in reaction would require TEM (c.f.Brearley, 1986, 1987) but a full TEMstudy is outwith thescope of this project. However, there is sufficient evi-dence to support application of a simple thermal model.

It is important that the reactions used to constrainthe thermal model are not significantly overstepped.This is a similar problem to that encountered by

0 0.5 1.0 2.0 2.51.5

400

500

1200

1100

1000

900

800

700

600

Pyroxene thermometry

Qz-Ab-Or-H2O melting

Quartz–tridymite inversion

Muscovite + quartz reaction

Biotite-out (Vielzuf & Clemens, 1992)


Tem

per

atu

re (

°C) Qz–Ab–Or melting

Fig. 10. The profile of maximum temperature with distance inthe aureole, constrained using the onset of melting on quartz–feldspar grain boundaries, the first appearance of quartz para-morphs after tridymite, the highest temperature obtained frompyroxene thermometry, and the suggested temperature of thebiotite-out reaction of Brearley (1987). The stable muscovite-outreaction is shown for comparison, although it is not clear thatthis reaction actually occurred in the aureole. All possible melt-absent muscovite-out reactions occur at temperatures lower thanthose predicted by the thermal model (shown by the curved line),suggesting that the breakdown of muscovite was significantlyoverstepped. The onset of melting in the H2O-absent Qz–Ab–Orsystem is shown for comparison (see text for details).



Holness (1999) and Holness & Watt (2002) who foundthat internally consistent results could be obtainedfrom the first appearance of paramorphs of tridymiteand the onset of melting in the Qtz–Ab–Or–H2O sys-tem. Using the position in the aureole at which tridy-mite first appears, together with the position of theonset of melting on quartz–feldspar grain boundariesand a plausible magma intrusion temperature, thepressure of metamorphism can be graphically con-strained (Holness & Watt, 2002).

The dolerite intrusion temperature was constrainedusing the program MELTSMELTS (Ghiorso & Sack, 1995;Asimow & Ghiorso, 1998). Calculations were carriedout for pressures between 100 and 1000 bar, using atypical magma composition (Table 3), with fO2 set atQFM. There is no chilled margin to the plug, so it isdifficult to assess how close to its liquidus (calculatedto be 1390–1540 �C) the magma was on intrusion.However, the presence throughout the current expo-sure of the dolerite of rare, large plagioclase grains,together with hopper-form spinels and rare, large,euhedral olivine grains suggest that these three phases(but not clinopyroxene) were on the liquidus at thetime of emplacement. According to MELTSMELTS modelling,this occurs between 1180 and 1200 �C.

For an intrusion temperature of 1190 �C, the firstappearance of tridymite (at 0.72 m from the contact)and the onset of melting in the Qtz–Ab–Or–H2O system(at 0.79 m) lie on a straight line on a graph of tempera-ture against distance for a pressure of 120 bar (Fig. 10).This graphical method is based on the quartz–tridymiteinversion temperature of 867 �C at 1 bar, with an in-crease of 200 �C kbar)1 (Kennedy et al., 1962; Ostrov-sky, 1966; Grattan-Bellew, 1978), and taking the P–Tconditions of melting in the Qz–Ab–Or–H2O systemfrom Tuttle & Bowen (1958) and Shaw (1963).

In the aureole of the Glenmore Plug there are twodistinct reactions responsible for muscovite break-down. At lower grades, muscovite is associated withType 1 fractures only, and does not develop rims ofK-feldspar. This is plausibly a reaction of the form:

muscoviteþ quartz ¼ Al2SiO5 þK-feldspar

þH2O� biotite:

At 120 bar, and assuming simple endmember reac-tions in KFMASH, this reaction occurs in the regionof 475–500 �C [using the program THERMOCALCTHERMOCALC (Powell& Holland, 1988; Holland & Powell, 1990)].At higher grades, the K-feldspar rim and presence of

both Type 1 and Type 2 fractures are consistent with amelting reaction involving an increase in volume. Thismay be a metastable isochemical breakdown reactionof the type:

muscovite ¼ biotiteþK-feldsparþmullite

þ spinel/corundumþmelt

first suggested by Brearley & Rubie (1990). However, ifmuscovite had reacted completely before this point (i.e.progressive metamorphism) then the melting inferredaround the sites of the reacted muscovite grains wouldhave involved the products of the initial muscovite-outreaction.The precise nature of the biotite breakdown reaction

in the lower temperature regions of the aureole isunclear. A TEM investigation of biotite breakdownunder similar conditions showed that reactioninvolved the growth of magnetite, spinel, K-feldsparand melt, with associated compositional change of theremaining biotite (Brearley, 1987). The suggestedreaction is:

Fe/Al biotite¼Mg/Al biotiteþK-feldspar/melt

þmagnetiteþhercynitic spinelþH2O:

(Brearley, 1987), consistent with our observations.However, at higher grades, the presence of Type 2fractures again points to a melting reaction, beginning0.90 m from the contact. A reaction of the type:

biotiteþ quartzþ plagioclase

¼ orthopyroxene�K-feldsparþmelt

is likely to be stable at low pressures, and from theexperimental work of Vielzuf & Clemens (1992) onfluid-absent melting in the KMASH system, wouldoccur in the region of 800 �C.Other constraints on temperature are given by

pyroxene compositions in the inner part of the aureole.The orthopyroxene thermometer of Brey & Kohler(1994) yields a maximum temperature of 1070 �C forthe sample 0.16 m from the contact. The persistence ofmicrocline to within 10 cm of the position marking theonset of melting in the Ab–Or–Qtz–H2O system dem-onstrates that the microcline to sanidine transition wasprobably significantly overstepped.

THERMAL HISTORY OF THE AUREOLE

In the previous section, plausible temperatures werepresented for the reactions observed in the aureole.Within the constraints of reaction metastability andoverstep these define a trajectory of maximum tem-perature as a function of distance from the contact

Table 3. Representative bulk composition of dolerite, normal-ized to 100.

Oxide Wt%

SiO2 48.40

TiO2 1.55

Al2O3 16.03

FeOtot 11.75

MnO 0.17

MgO 7.74

CaO 10.90

Na2O 2.81

K2O 0.47

P2O5 0.19

LOI 0.75

Total 100.01



(Fig. 10). The temperature constraints within the inneraureole are likely to be more reliable than those furtheraway, as reactions at very high temperatures are lesslikely to be significantly overstepped. Hence one can beconfident that the rocks 0.80 m from the contactreached 900 �C, but it is unlikely that the edge of theaureole was as cool as 500 �C. The muscovite-outreaction is likely to have been significantly overstepped(c.f. Holness & Watt, 2002).

The profile of maximum temperature in the innerparts of the aureole was fit using a simple two-stagefinite difference thermal model. The model is a time-dependent diffusion calculation with a radialgeometry. The domain is divided into two regionscomprising the cylindrical intrusion itself and asurrounding country rock, assumed to be infinite inextent. The initial condition is a fixed temperature forthe intrusion (taken as 1190 �C) and a fixed tempera-ture (30 �C) for the country rock. In the first stage ofthe model, the temperature of the contact is held at theintrusion temperature while heat diffuses into thecountry rock. This reflects the period of time duringwhich magma flowed in the conduit. Subsequently,both regions cool together, simulating the period aftercessation of magma flow in the conduit. The peaktemperature reached at any distance from the centre ofthe intrusion is recorded. Values of critical parameterswere: thermal diffusivity of rock, 10)6 m2 s)1; radiusof plug, 20 m.

This model is a considerable simplification for sev-eral reasons. The first is that magma flow would notcease instantly, but would gradually slow. Another isthat any effects of latent heat of melting and solidifi-cation were ignored. Given that the width of the mel-ted region of the aureole is much less than that of theconduit, the latent heat release during dolerite solidi-fication would have been more important than the heatrequired to melt the aureole. The effects of this sim-plification mean that the duration of magma flow isover-estimated.

Using our simplified model, the best fit in the innerpart of the aureole is achieved if the first stage of thethermal model lasts 30 days. See Fig. 10 for the ther-mal profile.

The aureole of the Glenmore plug is remarkable forthe abundance of pristine glass in the country rock.The diffusive thermal time constant for the intrusionand its aureole is of the order of 2 years. In compar-ison, the aureole of the Traigh Bhan na Sgurra sill, asimilar Tertiary pyrometamorphic setting with abun-dant melting of Moine metasediments (Holness &Humphreys, 2003), had a time constant of severalmonths. The solidified melt in this much short-livedaureole has a microgranophyric texture.

Given the narrowness of the aureole in Glenmore (afew metres) it is unlikely that the presence of glass wascaused by rapid cooling with significant heat loss to thesurface – this was several hundreds of metres above thecurrent level of exposure at the time of metamorphism.

It is perhaps more plausible that the differencesbetween Glenmore and Traigh Bhan na Sgurra relateto the extent and pervasiveness of late-stage hydro-thermal circulation.

The melt in the aureole of the Traigh Bhan naSgurra sill may also have solidified to a glass, but it ispossible that it devitrified because of the catalytic effectof pervasively infiltrating surface-derived fluids. Thepresence of glass in the Glenmore aureole would thenreflect an absence (or only local presence) of suchfluids. Support for this is provided by the absence oflate-stage hydrothermal veins and the ubiquity of low-temperature chloritisation of biotite at Traigh Bhan naSgurra, suggesting pervasive flow, in contrast to thelack of chloritic replacement of biotite and abundantlate-stage veins (providing a localized, rather thanpervasive, fluid flow network) at Glenmore. This couldbe tested by stable isotopic analysis, but such a study isoutwith the aims of this project.

Interestingly, the well-defined first appearance ofglass in the aureole and the zoned nature of the rims inwhich it first appears, emphasizes the importance ofcooling rate in its formation. In the relatively slowlycooled parts of the aureole the melt crystallized. In thecritical zone sampled by X12 only the quartz-richcomponent of the glass crystallized, while in the rel-atively rapidly cooled inner aureole all the melt solid-ified to glass. If this interpretation is correct, then oursimple thermal model suggests that the critical coolingrate for glass formation was c. 8 �C day)1 (ignoringany possible effects of latent heat of crystallization).

RATES OF MELTING

There are several lines of evidence pointing to theimportance of H2O in controlling melting on quartz–feldspar grain boundaries in the Glenmore aureole.The first is the non-ubiquity of granophyric rims sep-arating quartz and feldspar 0.79 m from the contact.This suggests a limited supply of H2O which is requiredas a reactant during the earliest stages of melting. Thesecond is the persistence of feldspar up to the contactwith the dolerite, despite temperatures sufficient tomelt it (together with co-existing quartz) in the absenceof H2O (Fig. 10), demonstrating a strong kinetic con-trol on the melting reaction.

The thermal profile crosses 960 �C some 0.60 mfrom the contact (Fig. 6). This is where we mightexpect the onset of H2O-absent melting, but the rate ofchange of melt rim thickness does not alter. This sug-gests that H2O may play an important role in con-trolling reaction rate even at temperatures at which it isnot a necessary component for equilibrium melting(e.g. Arzi, 1978), possibly via enhancement of diffusionrates.

The question of how much H2O was present duringmelting is important. The textural evidence for por-osity surrounding the tridymite paramorphs (Fig. 5a,c)suggests that the melt contained significant amounts of



dissolved H2O. Typically the country rock outsidethe aureole contains 25–40 vol.% mica, yielding abulk-rock H2O content of c. 1 wt%. Assuming sat-uration of the melt at 1 wt% (Johannes & Holtz,1996), this would be enough to melt the entire rock, ifall the H2O released by mica breakdown remaineduntil temperatures attained the melting reaction.However, work on partial melting in similar environ-ments suggests that the bulk of H2O released by micabreakdown escapes along fractures and is not allavailable for melting at a later time (e.g. Holness &Watt, 2002).

In the lower-temperature parts of the aureole, thepresence of non-melted quartz–feldspar grain bound-aries near the location of the onset of melting showsthat melting was probably initiated only on grainboundaries containing H2O. How much H2O waspresent on these grain boundaries?

Melt rims occur in X14. This sample was collectedonly 1 cm from X15, in which no melt rims wereobserved. This means that the initial rate of melt rimformation was much higher than subsequently (Fig. 6)with a 7 lm rim forming almost instantly (c.f. theexperimental result of Arzi, 1978). It is probable thatthis initial fast rate is the result of free H2O on thegrain boundaries. Assuming a melt density of2.5 g cm)3 and H2O saturation at 0.7 wt%, thistranslates to c. 0.1 g of H2O m)2 of grain boundarysurface. For comparison this is around 250 times theadsorption density of H2O on a quartz–fluid interface(Holness, 1993). This is a plausible amount of H2O tobe present on grain boundaries in a rapidly heating(and thus cracking) rock-containing dehydrating micasin the shallow crust. Following the arguments of Arzi(1978) we suggest that once this initial supply of H2O isconsumed, the melting rate slows to one controlled bythe rate of supply of more H2O which is required as areactant during melting in the lower temperature partsof the aureole, and possibly as a catalyst at the highesttemperatures.

The thermal model provides an indication of thetime during which any point in the aureole was abovethe solidus. This can be used to determine the averagerate of melting on the quartz–feldspar grain boundar-ies. The average rate of movement of the melting front(taking into account its growth on both margins) iscalculated to be 0.5 · 10)9 cm s)1 in the lower tem-perature parts of the aureole, and 1.0 · 10)9 cm s)1 inthe highest temperature parts of the aureole. This isconsistent with the experimental results of Arzi (1978)who obtained 10)10 to 10)9 cm s)1 for melting underH2O-undersaturated conditions. The rate of growth inthe initial phase of rapid melting cannot be well con-strained given the limitations of our thermal model.

CONCLUSIONS

Contact metamorphism caused by the Glenmore plugin Ardnamurchan, a magma conduit feeding a surface

basalt flow, resulted in melting. The pristine nature ofmuch of the aureole provides a natural laboratory inwhich to investigate the distribution of melt. A simplethermal model is consistent with the magma conduithaving been active for 1 month, and provides a time-scale for comparison with experiments and similarstudies of other small-scale contact aureoles. The onsetof melting on quartz–feldspar grain boundaries wasinitially rapid, with an almost constant further increasein melt rim thickness at an average rate of0.5–1.0 · 10)9 cm s)1. This rate was most probablycontrolled by the distribution of limited amounts ofH2O on the grain boundaries and in the melt rims.The melt in the inner parts of the aureole certainly

formed an interconnected network, although there waslittle movement of melt on the time-scale of meta-morphism (months). Limited movement and segrega-tion of melt is manifest by variations in melt rimthickness because of sinking of restitic grains and, mostsignificantly, by the development of layer-parallel se-gregations and cross-cutting veins within 0.6 m of thecontact where the melt volume exceeded 40% of therock. The coincidence of the first appearance of thesesigns of segregation with the aureole attaining thetemperature at which fluid-absent melting in the Qtz–Ab–Or system could occur suggests that internallygenerated overpressure, consequent to melting, wasinstrumental in the onset of local melt movement.

ACKNOWLEDGEMENTS

We are grateful to M. Carpenter and D. Prior forhelpful discussions during the development of thiswork. S. Reed provided invaluable assistance inobtaining SEM and CL images. Helpful and con-structive comments from E. Sawyer and J. Clemenssignificantly improved the manuscript.

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Received 4 August 2004; revision accepted 18 December 2004.



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