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Journal of Petrology | Volume 44 | Number 6 | Pages 995-1029 | 2003
© Oxford University Press 2003
Processes and Conditions During Contact Anatexis, Melt Escape and Restite Formation: the Huntly Gabbro Complex, NE Scotland
1 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF MANCHESTER, MANCHESTER M13 9PL, UK
2 SCHOOL OF EARTH SCIENCES AND GEOGRAPHY, CEESR, KINGSTON UNIVERSITY, PENRHYN ROAD, KINGSTON-UPON-THAMES KT1 2EE, UK
E-mail: Giles.Droop{at}man.ac.uk
RECEIVED DECEMBER 15, 2001; ACCEPTED DECEMBER 2, 2002
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONThe Huntly Gabbro is one of a suite of large, Ordovician, syn-orogenic, mid-crustal, layered, mafic intrusions, emplaced into Proterozoic metaclastic rocks of NE Scotland soon after the thermal peak of static, high-T, low-P regional metamorphism. This gabbro and its associated contact metamorphic rocks illustrate a variety of processes operating during contact anatexis and subsequent melt segregation and extraction. These processes may closely mirror those occurring at much larger scales in the deep crust during high-grade regional metamorphism and the generation of granitic magmas. The emplacement of the Huntly mafic magma resulted in high-grade contact metamorphism and, locally, anatexis of metapelites, leading to the formation of migmatites. The migmatites and country-rock schists were studied to establish the physical conditions of metamorphism and anatexis, the nature of the melting reactions, the compositions of the melts produced, and the extent to which melting was a closed- or open-system process. The country-rock schists immediately to the south of the Huntly Complex contain mineral assemblages characteristic of the regional andalusite zone. Thermobarometry of an andalusite schist yields regional metamorphic conditions of 537 ± 42°C and 0·27 ± 0·12 GPa, consistent with previously published P–T estimates. The contact metamorphic rocks include sillimanite hornfelses, metatexites and diatexites. The metatexites consist of cordierite–K-feldspar hornfels melanosomes and K-feldspar-rich garnetiferous leucosomes. The diatexites consist of schollen of fine-grained granoblastic hornfels and metatexite suspended in igneous-textured matrix rocks composed of abundant sub/euhedral garnet, cordierite, plagioclase and, locally, orthopyroxene, with minor interstitial biotite, K-feldspar and quartz. The hornfels melanosomes and schollen retained their structural integrity during partial melting, but the matrix rocks did not. In the highest-grade diatexites, the assemblage Grt + Opx + Crd + Hc + Pl characterizes both the hornfels schollen and the sub/euhedral minerals of the matrix rocks. Application of phase equilibria to Opx-bearing rocks yields estimated peak-metamorphic conditions of 900 ± 50°C, 0·45 ± 0·1 GPa and aH2O < 0·3. The pressure estimate implies an emplacement depth of
16 ± 3 km. The prograde P–T path of contact-metamorphic rocks had a low, positive d P/d T slope, indicating that the gabbro intrusion increased the lithostatic load on the country rocks by overplating. Pseudomorph textures involving Al-silicates provide strong evidence that the diatexites evolved from andalusite schists via a sillimanite hornfels stage. Mineralogical changes reflect a sequence of dehydration reactions, followed by fluid-absent partial-melting reactions involving biotite breakdown. It was the fluid-absent reactions that generated the sub/euhedral minerals in the diatexites, as peritectic phases. In many of the highest-grade diatexites, quartz-free anhydrous solid assemblages were produced via reactions such as Bt + Sil + Grt = Crd + Hc + Kfs + melt and Bt + Grt + Crd = Opx + Hc + Kfs + melt. Whole-rock major-element geochemical studies indicate that the Opx–Crd hornfelses and diatexite matrices are depleted in Si and K relative to their schist protoliths. Mass-balance calculations indicate that (1) the Opx–Crd hornfels xenoliths represent solid Ca-, Mg-, Fe-, Al-, Na-rich residues left after extraction of 60% melt; (2) the Opx-bearing diatexite matrix rocks are also residual, and represent restite-enriched crystal–liquid mushes left after extraction of 55% melt; (3) the Opx-free diatexite matrix rocks probably represent restite-enriched mushes that retained a higher proportion of residual melt; (4) the anatectic melts were of H2O-undersaturated, peraluminous, low-Ca, potassic granite composition. Melt compositions and proportions were confirmed experimentally by partially melting local metapelite samples at 900 °C and 0·5 GPa, under fluid-absent conditions. The similarity between the compositions of calculated and experimental melts implies that the melts underwent little or no fractional crystallization before their expulsion. At many localities, a large proportion of the melt escaped from the sites of its generation. Segregation of melt from restite was probably achieved through gravity-driven processes. The fugitive granitic melts did not mix with the gabbroic magma to any great extent, although contamination of mafic magma occurred locally, leading to the generation of biotite-bearing gabbros. The fugitive melts probably contributed to the contemporaneous ‘Grampian’ suite of S-type granites. The mid-Ordovician middle crust of NE Scotland was thus a site of crustal differentiation. The results demonstrate that crustal fusion and magma production can occur without significant chemical interaction between the mantle-derived heat source and the crustal melts, and that melt extraction can occur in the absence of regional tectonic deformation. KEY WORDS: migmatite; diatexite; fluid-absent melting; melt segregation; peraluminous granite
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INTRODUCTION |
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The differentiation of the continental crust is widely held to involve the partial melting of mid- to deep-crustal rocks, at high metamorphic grades, with the upward escape of granitic melts, leaving behind high-grade restitic residues (Clemens, 1990; Thompson, 1990; Vielzeuf et al., 1990). Intracrustal heat sources are generally inadequate to power these processes, and major contributions of mantle heat seem to be required (England & Thompson, 1986). Such mantle-derived heat is most easily supplied through intrusion of gabbroic magmas into the crust. Exposed examples of contact anatexis surrounding gabbroic intrusions are therefore of considerable interest. This paper describes a particularly instructive example of this, and explores the nature of the petrogenetic processes involved.
Partial melting phenomena are commonly recognized in the inner parts of the thermal aureoles of large mid- and upper-crustal intrusions, where the wall rocks are of suitable composition (e.g. Ashworth & Chinner, 1978; Pattison & Harte, 1988; Grant & Frost, 1990; Finger & Clemens, 1995; Holness & Clemens, 1999). In the context of crustal differentiation, a key question is the extent to which the anatectic melts migrate away from, or remain within, their sites of generation in such aureoles. In principle, several potential fates can be envisaged for contact-anatectic melts; they can (1) crystallize in situ (e.g. Pattison & Harte, 1988); (2) intrude the chilled margin of the pluton, forming back-veining complexes; (3) invade the magma of the pluton, either to mix with it (if miscible) to produce a hybrid magma (e.g. Greenfield et al., 1996) or mingle with it (if immiscible or inefficiently comminuted) to form enclaves; (4) migrate elsewhere in the inner aureole, either by flowing along fractures or by percolating along grain-boundaries; (5) leave the aureole to intrude cooler country rock at higher crustal levels. In situ crystallization is most likely to occur where the heat-supplying pluton is at relatively low temperature (i.e. a granitoid magma) and generally produces relatively small volumes of anatectic melt by fluid-saturated reactions (e.g. Holness & Clemens, 1999). Such reactions tend to have negative dP/dT slopes, causing near-solidus fluid-saturated melts to solidify immediately if they begin to ascend (Brown & Fyfe, 1970). Conversely, melts are likely to be able to leave the aureole to intrude cooler country rocks only if the heat-supplying pluton is relatively large, hot (i.e. mafic) and produces relatively large volumes of low-viscosity, fluid-undersaturated anatectic melt, by fluid-absent reactions. Such reactions have positive dP/dT slopes, allowing the melts to ascend for appreciable distances without immediately freezing (Brown & Fyfe, 1970; Clemens et al., 1997; Clemens & Droop, 1998).
If it occurs on a large enough scale, the segregation and ascent of anatectic melt from its source region must result in crustal differentiation, with enrichment of the source areas in refractory (restitic) components, and of higher crustal levels in the complementary ‘granitophile’ components. It has been suggested that the intraplating and underplating of the crust by large volumes of mafic magma and the ensuing partial melting of fertile crustal rocks is the major process by which mobile granitoid magmas are produced and the continental crust grows in volume (e.g. Clemens, 1990; Rudnick, 1990; Vielzeuf et al., 1990; Stevens & Clemens, 1993). In a deep-crustal context, petrological and geochemical evidence for granitic melt extraction and restite formation is well documented (e.g. the granulite-facies ‘stronalites’ of the Ivrea Zone, northern Italy; Schnetger, 1994). However, so far, there has been little to suggest that these processes could also operate at mid-crustal levels. In this paper, we examine the thermal effects of a large gabbro intrusion and present evidence that it caused substantial restite formation, at mid-crustal levels. We demonstrate that the country-rock metapelites underwent very high degrees of partial melting and that, locally, a large proportion of the melts segregated from the solid residua, and indeed escaped the source region.
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GEOLOGICAL SETTING |
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The Huntly Gabbro in NE Scotland (Fig. 1) is one of a suite of large, syn-orogenic, layered, mafic intrusions of Middle Ordovician age, locally known as the ‘Newer Gabbros’ (Read, 1919, 1923, 1961). These were emplaced into Dalradian (Late Proterozoic) metaclastic rocks during, or soon after, the thermal peak of the Grampian regional metamorphism (Fettes, 1970).
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The classic Buchan-type regional metamorphism in NE Scotland is characterized by a sequence of biotite, cordierite, andalusite and sillimanite zones in the metapelites (Read, 1923a, 1952). The metamorphism occurred statically under conditions of high temperature and low pressure (Chinner, 1966; Harte & Hudson, 1979; Hudson, 1980, 1985). The regional isograds describe a broad horseshoe pattern around a central area of low-grade rocks (Read, 1952), probably as a result of post-metamorphic folding (Treagus & Roberts, 1981).
The Newer Gabbro intrusions themselves display rhythmic and cryptic layering, and consist mainly of cumulate-textured peridotites (now serpentinized), troctolites, norites, gabbros and, locally, more differentiated monzogabbros (Wadsworth et al., 1966; Clarke & Wadsworth, 1970; Ashcroft & Boyd, 1976; Wadsworth, 1982, 1986, 1988; Munro, 1984). The gabbros post-date at least one phase of regional ductile deformation in the Dalradian country rocks, but were themselves affected by post-magmatic shearing, particularly along their contacts (Ashcroft & Munro, 1978; Boyd & Munro, 1978; Ashcroft et al., 1984). Stewart & Johnson (1960) suggested that the various gabbro bodies represent the dismembered remains of a single lopolith. However, as the details of the modal layering differ from one body to another (Munro, 1984), it is more likely that the intrusions evolved independently by in situ crystal fractionation of separate magma batches (Wadsworth, 1986). Pankhurst (1970) obtained a whole-rock Rb–Sr isochron date of 486 ± 17 Ma for the Newer Gabbros, but more precise U–Pb mineral ages of 468 ± 8 Ma and 470 ± 9 Ma have been obtained by Rogers et al. (1994) and Dempster et al. (2002), respectively.
Many of the Newer Gabbros possess well-developed thermal aureoles (e.g. Stewart, 1946; Gribble, 1966; Allan, 1970; Fettes, 1970; Ashworth, 1975), some of which have locally modified the regional isograd pattern (e.g. Fettes, 1970). Thus, rather than having been entirely responsible for it, the exposed gabbro bodies appear to have locally augmented the regional metamorphism (Pankhurst, 1970). This seems to be a common feature of plutonometamorphism (Powers & Bohlen, 1985; Wickham, 1987; Finger & Clemens, 1995). Pressure estimates of 0·4–0·5 GPa from the thermal aureoles (Ashworth & Chinner, 1978; Droop & Charnley, 1985) indicate that the Newer Gabbros were emplaced at a depth of 15–18 km. The pressure increase recorded by the overprinting of andalusite by kyanite in schists structurally beneath the gabbros (Chinner & Heseltine, 1979; Chinner, 1980) has been attributed to the extra load imposed by the intrusions (Dempster et al., 1995) or to post-intrusive thrusting (Baker, 1987).
Outcrops of xenolithic rocks (diatexitic schollen migmatites), composed of a cordierite-bearing igneous matrix and xenoliths of silica-poor aluminous hornfels, are closely associated with several of the Newer Gabbros, both in their inner aureoles and within the gabbros themselves. Watt (1914) and Read (1923a, 1923b, 1935), who first described these xenolithic complexes, believed that the igneous matrix rocks, which they termed ‘cordierite norites’, had been produced by contamination of mafic magma by assimilation of pelitic wall rocks. Chinner & Schairer (1962), Gribble & O'Hara (1967) and Gribble (1968, 1970) later demonstrated that formation of cordierite norites by bulk assimilation of metapelite by mafic magma could not occur by a process of down-temperature evolution, owing to the existence of the An–Opx–Tri thermal divide in the CMAS system. Instead, they argued that the ‘cordierite norites’ were derived by partial melting of Dalradian metapelites, without significant mixing with the Newer Gabbro magmas, a view supported by their relatively high 87Sr/86Sr initial ratios (0·720–0·730; Pankhurst, 1969).
Granitoid plutons are volumetrically abundant in NE Scotland (Fig. 1). Of these, the ‘Grampian Granites’ (Brown, 1991) are late-tectonic peraluminous granites of Ordovician age (Pankhurst, 1974), which have yielded U–Pb mineral ages of 475 ± 5 Ma (Pidgeon & Aftalion, 1978), 470 ± 1 Ma (Kneller & Aftalion, 1987) and 467 ± 6 Ma (Oliver et al., 2000). The coincidence between these ages and those of the Newer Gabbros indicates that there was a major thermal event in the Buchan area at that time, involving high-T, low-P regional metamorphism, gabbro intrusion, crustal melting and the generation of granite magmas (Rogers et al., 1994).
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THE HUNTLY COMPLEX |
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The Huntly Complex is a poorly exposed, pear-shaped outcrop of gabbros and associated rocks, measuring
10 km x 9 km, intruding broadly north–south-trending Dalradian metasediments of the Argyll and Southern Highland Groups (Fig. 2). The southern contact cuts andalusite-porphyroblastic metapelites of the Boyndie Bay Group and turbiditic metapelites and psammites of the Whitehills Group. The western contact abuts biotite-rich garnet-, staurolite- and sillimanite-bearing schists and gneisses of the Portsoy Group. The mafic igneous rocks crop out in two major areas, one to the west, comprising mainly layered cumulate troctolites, and one in the east–central part, comprising mainly gabbronorites and norites (Munro, 1970; Weedon, 1970). The cumulate layering dips steeply and youngs to the east (Shackleton, 1948), indicating that the western margin of the complex was the original floor of the intrusion. The thermal aureole surrounding the Huntly Complex has not been mapped. This is due partly to the general paucity of exposure and partly (at least around the western margin) to excision of the aureole by faulting (Munro, 1970) along a NNE-trending zone of intense strain (Baker, 1987) known as the ‘Portsoy–Duchray Lineament’ (Harte, 1988). However, closely associated with the mafic igneous rocks are numerous outcrops of migmatite (‘xenolithic complexes’) (Fig. 2), which undoubtedly represent high-grade contact metamorphic rocks. Many of these appear to be arranged around the margins of the complex, particularly on the eastern side, but isolated outcrops of migmatite also occur within the gabbros (Fig. 2) (Munro, 1970). Judging by the trends in strike of the country rock units, it is likely that the migmatites in the southern part of the complex (Huntly–Dunbennan Hill) were derived from schists of the Boyndie Bay and Whitehills Groups. Two kilometre-scale granites also occupy marginal positions in the complex (Fig. 2). The more northerly of these granites (Avochie) is foliated (Munro, 1970) and may belong to the Grampian suite.
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PETROGRAPHY OF METAPELITES |
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In this section we describe the field relations, mineralogy and textures of pelitic country rocks and migmatites from the southern and western parts of the Huntly Complex. Mineral assemblages of representative samples are listed in Table 1.
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Regional metamorphic rocks
The regional metamorphic rocks exposed around the south and west margins of the Huntly Complex span a wide range in mineralogy, texture and grade. Biotite-slates of the lowermost Macduff Slate crop out at St. Michael's Well (British National Grid reference NJ 519359) and Coynachie (NJ 491344), south of the area covered by Fig. 2. These rocks are fine-grained cleaved metasiltstones with millimetre- to centimetre-scale compositional layering (bedding) defined by alternating quartz-rich and quartz-poor layers, and an oblique penetrative cleavage defined by aligned biotite, muscovite and chlorite flakes. The rocks belong to the Buchan biotite zone.
Coarser-grained pelitic mica schists and subordinate semipelites (metagreywackes) and quartzites of the Boyndie Bay Group occur further west, in the vicinity of Clashmach Hill (NJ 498385). The schists contain the assemblage Ms + Bt + Qtz + Pl ± And ± St ± Grt + Ilm. Aligned muscovite flakes measuring 500 µm x 50 µm define the schistosity, whereas more equant, randomly orientated biotite flakes and granoblastic quartz and plagioclase make up the rest of the matrix. Andalusite occurs as sub-rectangular poikiloblasts, up to 6 mm long, containing quartz inclusions. Staurolite forms subhedral micropoikiloblasts up to 0·3 mm long. Rare, tiny (150 µm diameter) subhedral garnets occur in some samples. These rocks belong to the Buchan upper andalusite zone [the staurolite zone of Hudson (1980)].
West of the Huntly Complex, rocks of the Portsoy Group were sampled at Cairnie (NJ 482446). Many samples (e.g. Sin1, Sin101) are medium-grained plagioclase- and quartz-rich garnet–biotite gneisses containing minor staurolite or fibrolitic sillimanite. The rocks possess a biotite ± sillimanite schistosity, parallel to a layering defined by variations in modal biotite proportion. Somewhat finer-grained garnet–mica schists (e.g. Sin2) lack modal layering and contain essential muscovite, which, together with biotite, defines the schistosity. The garnets form anhedral inclusion-free crystals up to 0·4 mm across. These schists and gneisses, which belong to the lower sillimanite zone, appear to belong to a facies series of Barrovian (medium P/T) type.
Non-migmatitic hornfels
A loose block (>1 m diameter) of non-migmatitic cordierite–sillimanite hornfels (Dun1), representing a contact-metamorphosed metapelite that did not undergo partial melting, was found on the eastern side of the River Deveron (NJ 499422) near Dunbennan Hill in the SW part of the complex. The rock contains scattered subhedral garnet porphyroblasts up to 3 mm across and abundant large (8 mm x 2 mm x 2 mm) randomly orientated bundles of sub-parallel prismatic sillimanite crystals, set in a fine-grained granoblastic matrix of biotite, cordierite, plagioclase, quartz and ilmenite (Fig. 3a). Cordierite is locally replaced by retrograde chlorite + andalusite. The sillimanite aggregates are elongated and roughly square in cross-section, suggesting that they are pseudomorphs after andalusite. Similar sillimanite pseudomorphs after andalusite have been described by Vernon (1987).
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Migmatites
The majority of contact-metamorphic rocks in the Huntly Complex are migmatites, and those described below correspond mainly to the ‘cordierite-bearing migmatites’ of Ashworth (1976). The following descriptions, although overlapping in part with those of Ashworth (1976), concentrate on new observations. Migmatite terminology used in this paper is from Ashworth (1985).
Metatexites
Large loose blocks of stromatic metatexite are abundant in the vicinity of Cumrie and Cormalet Hill in the north central part of the Complex (Fig. 2). These rocks typically consist of blue–grey cordierite hornfels melanosomes and veins of garnetiferous leucosome, the latter typically occupying 20–40% of the rock. The hornfels melanosomes are fine grained and locally contain garnet porphyroblasts. The leucosomes are mostly <1 cm thick and tend to form sub-parallel arrays, following the primary lithological layering (bedding), even where the latter is tightly folded. In most metatexites, however, branching, anastomosing and crosscutting veins also occur, indicating the former presence of a melt phase. Leucosome veins have locally coalesced to form irregular granitic patches (>5 cm across), studded with large garnets.
Most melanosomes are Opx-free Crd–Kfs hornfelses (e.g. Cor2, Cor6 and Cor11a; Table 1). Cordierite, plagioclase, K-feldspar and ilmenite all occur as equant, anhedral grains and form a typical granoblastic polygonal matrix (Fig. 3b). Randomly orientated, thin, red–brown biotite flakes are partially replaced by fibrolitic sillimanite in some rocks. Quartz, where present, is usually subordinate, interstitial and texturally associated with garnet. In some quartz-free hornfelses, small granules of brown hercynite occur in the matrix. The K-feldspar is commonly microperthitic and locally replaced by myrmekitic quartz–plagioclase intergrowths. Garnet, where present, typically forms euhedral porphyroblasts, up to 5 mm in diameter, containing rounded inclusions of ilmenite, biotite and, in some rocks, hercynite, near its rims. In some hornfelses, however, the garnets have atoll-like or spongy textures and are intergrown with relatively coarse-grained quartz and K-feldspar. Apart from the fact that they appear to be isolated and spheroidal, rather than planar, these quartz–K-feldspar–garnet aggregates closely resemble the leucosomes described by Powell & Downes (1990) from pelitic granulites at Broken Hill and, like them, probably represent localized sites of melting.
The leucosomes are dominated by quartz, K-feldspar and garnet with subordinate biotite, plagioclase and cordierite (e.g. Cor11b: Table 1). The garnet is typically inclusion free, and forms either irregular grains (Fig. 3b) or large (up to 1 cm diameter) euhedral crystals. The K-feldspar forms blocky, subhedral crystals of coarse-grained orthoclase microperthite, to which quartz is locally interstitial.
Garnet is a common solid product of incongruent, biotite-consuming, fluid-absent melting reactions in aluminous rocks (Waters & Whales, 1984; Grant, 1985a, 1985b; Conrad et al., 1988; LeBreton & Thompson, 1988; Vielzeuf & Holloway, 1988; Waters, 1988; Powell & Downes, 1990; Vielzeuf & Montel, 1994; Carrington & Harley, 1995; Patiño Douce & Beard, 1996; Stevens et al., 1997; Pickering & Johnston, 1998). The ubiquity and abundance of garnets in the leucosomes provides strong evidence that this type of reaction was responsible, at least in part, for the migmatization of the Huntly rocks.
Diatexite matrices (Grt tonalites and Crd norites)
Schollen diatexites are common throughout the Huntly Complex (Fig. 2). On a mesoscopic scale, these rocks typically consist of 70–95% of medium- to coarse-grained, igneous textured mobilisate (‘matrix’) with subordinate irregular hornfels or metatexite schollen up to 1 m long. Most of the schollen are pelitic, but other refractory rock types, such as quartzite and calc-silicate, occur locally.
The diatexite matrix rocks have pelitic or semi-pelitic mineralogies but igneous textures. Opx-free varieties [‘granitoids’ of Ashworth (1976)] are well represented in the Cormalet–Cumrie area. The Grt tonalite sample Cor7 (Table 1; Fig. 3c), for example, consists of abundant large (1 cm diameter) euhedral, inclusion-free garnets and sporadic, blocky, euhedral, inclusion-free cordierites in a medium- to coarse-grained matrix of biotite, quartz and euhedral plagioclase. Much of the quartz is clearly interstitial to garnet, cordierite and plagioclase (Fig. 3c). Small patches of fibrolite occur locally, near the cordierite and garnet.
Opx-bearing varieties [‘cordierite norites’ of Read (1923, 1935); ‘noritoids’ of Ashworth (1976)] crop out within the complex at Battlehill Quarry (NJ 539395), and in the River Deveron at Castle Bridge (NJ 533409) and north of Dunbennan Hill (NJ 499422) (Fig. 2). At Battlehill Quarry, such rocks form discordant bodies with sharp contacts against biotite–hornblende gabbro, micronorite and Opx–Crd hornfels. They are typically medium-grained rocks of mafic igneous appearance, rich in prominent, randomly orientated crystals of cordierite, orthopyroxene, garnet and plagioclase. The cordierite is fresh and forms subhedral prismatic crystals with well-developed simple or sector twinning. The orthopyroxene is markedly pleochroic and typically forms elongated subhedral prisms (Fig. 3d), locally enclosing rounded plagioclase inclusions. Garnet, commonly less abundant than Opx and Crd, forms subhedral grains that are either inclusion free or contain scattered ilmenite and biotite inclusions. Plagioclase crystals are subhedral and commonly show normal zoning, with sharply demarcated cores and rims, and locally also oscillatory zoning. In samples from Castle Bridge (e.g. CB1) biotite forms thick, subhedral books up to 1·5 mm across, but in others it appears to be mainly interstitial. In sample BHQ1 it forms large poikilitic patches, up to 8 mm across, enclosing plagioclase and cordierite. Quartz is generally present only in minor amounts and commonly forms small cuspate grains interstitial to garnet, cordierite, orthopyroxene and plagioclase (Fig. 3d). These quartz grains texturally resemble the ‘melt pools’ figured by Sawyer (2001). Samples from Castle Bridge contain modally abundant quartz, in part clearly interstitial to the ferromagnesian minerals. K-feldspar (orthoclase microperthite) is present in only a few cordierite norites and is generally poikilitic or interstitial to plagioclase and ferromagnesian minerals. Dark green hercynite is a constituent of many Crd norites. In samples from Battlehill Quarry hercynite is a minor phase and tends to occur only as small inclusions within cordierite, although in some samples (e.g. BQ38, BQ202) it is also enclosed by orthopyroxene, plagioclase and garnet. Hercynite is modally abundant in many Crd norites from Castle Bridge and Dunbennan Hill, where it forms oblong clusters, measuring up to 5 mm x 2 mm x 2 mm, of 0·01–0·1 mm granules intergrown with fine-grained cordierite. These hercynite clusters locally have square cross-sections and, in one sample (Dun4), are locally cored by corroded aggregates of prismatic sillimanite (Fig. 4a), indicating that they are pseudomorphs after that mineral. This texture also occurs in Crd-rich Grt tonalite sample Fow1. Hercynite has not been found in contact with quartz in any Huntly diatexite matrix rock. Retrograde textures include symplectic intergrowths of biotite and quartz replacing orthopyroxene and fine-grained biotite rimming garnet (Ashworth, 1976).
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The igneous textures of the diatexite mobilisates clearly indicate that they contained a melt phase during crystallization (see also Kenah & Hollister, 1983; Vernon & Collins, 1988; Harte et al., 1991). These rocks resemble cumulates in that the abundant, large, subhedral to euhedral cordierites, orthopyroxenes, garnets and plagioclase cores clearly crystallized early, whereas the plagioclase rims and the modally minor, interstitial or poikilitic quartz, K-feldspar and biotite clearly crystallized later. The question arises as to whether the large crystals precipitated from the melt as phenocrysts or represent the solid products of incongruent melting reactions. We favour the latter interpretation because (1) the temperatures required to completely melt aluminous metapelites are higher, and presumably therefore less readily attained, than those necessary for partial melting, (2) cordierite, garnet and orthopyroxene are known to form during incongruent melting of biotite (e.g. Grant, 1985a, 1985b), and (3) the Opx–Crd hornfelses (see below) contain similar assemblages to some of the cordierite norites but have textures indicative of solid-state equilibration.
Crd–Kfs hornfels schollen
Crd–Kfs hornfels schollen (e.g. Cor8) within garnet granitoids at Cormalet closely resemble hornfels melanosomes in metatexites, both texturally and mineralogically.
Opx–Crd hornfels schollen and xenoliths
Opx–Crd hornfelses occur in the southern and western parts of the Huntly Complex as schollen within ‘cordierite norites’ and as screens in gabbroic rocks. At Battlehill Quarry, sharp contacts are exposed between biotite–hornblende gabbro and screens of Opx–Crd hornfels up to 6 m thick. The attitude of the lithological layering varies dramatically from one screen to the next, indicating that the screens are large xenoliths. A 3 m thick xenolith of Opx–Crd hornfels is in sharp contact with biotite gabbro in a small quarry on the NW side of Battlehill Woods (NJ 511398).
The hornfelses are fine-grained, layered, dark bluish grey cordierite-rich rocks, commonly bearing small (1–2 mm) garnet porphyroblasts. The matrices typically consist mainly of small equant grains of orthopyroxene, plagioclase and twinned cordierite, accompanied by abundant granules of ilmenite and, in many samples, dark green hercynite. The grains have random orientations and a well-developed granoblastic polygonal texture (Fig. 4b), indicating a high degree of textural equilibration. Plagioclase is only rarely zoned. Cordierite tends to have a slightly larger grain size than Opx and Pl and locally forms subhedral porphyroblasts. Plagioclase is absent from samples BQ41 and BQ101, but its place is taken by anorthoclase. Some samples (e.g. BQ17) contain sporadic 2 mm x 1 mm composite plagioclase–K-feldspar intergrowths, which may be exsolved ternary feldspars. Quartz is usually absent. Garnet occurs mainly as subhedral porphyroblasts (Fig. 4b), locally with inclusions of ilmenite and hercynite but, in some samples, also forms tiny euhedral crystals (Fig. 4b) and anhedral rims around ilmenite grains. Hercynite is in contact with cordierite, garnet, orthopyroxene and plagioclase but not quartz. Biotite is rare or absent.
Some hornfelses (e.g. HBHQ5, 9996, 10035, BW202) contain exceptionally high modal amounts of cordierite (>60%), with subordinate plagioclase, orthopyroxene, garnet, ilmenite and hercynite (Fig. 4c), and are referred to here as ‘cordieritites’.
Many Opx–Crd hornfelses possess compositional layering, manifest as variations in both modal mineralogy and mean grain size. Relatively coarse-grained layers tend to be more leucocratic than the finer-grained layers; they lack hercynite, and contain appreciable amounts of interstitial K-feldspar, quartz and biotite. Garnets in such layers are either irregular porphyroblasts or atoll-type grains filled with relatively coarse biotite, K-feldspar and quartz. These layers have textures reminiscent of the leucosomes in the metatexites, but are less clearly demarcated from melanosomes. Distinct leucosomes do, however, exist. A 6 mm thick vein of medium-grained Grt–Crd–Kfs rock cuts the cordieritite of sample 9996. This igneous-textured leucosome consists of abundant, euhedral to subhedral, twinned cordierites (with inclusions of ilmenite and hercynite) and rare garnets enclosed by coarse poikilitic K-feldspar (Fig. 4d). In many respects this rock resembles the diatexite matrix rocks described above.
Disregarding interstitial, leucosome and poikilitic minerals, the characteristic peak-metamorphic mineral assemblage of the majority of Opx–Crd hornfelses is Crd + Opx + Grt + Hc + Pl + Ilm. Fine-grained granoblastic domains clearly indicate crystallization in the solid state, but the local presence of relatively coarse igneous-textured leucosome-like patches suggests the former presence of melt locally.
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MINERAL CHEMISTRY |
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Major-element compositions of minerals in selected samples of regional and contact metamorphic rocks (Tables 2–4) were determined by energy-dispersive spectrometry (EDS) using a converted Geoscan electron microprobe operating at 15 kV, 3 nA specimen current on Co metal, and 40 s count time per analysis. X-ray spectra were processed using Link Systems ZAF4/FLS software. In addition, F and Ba contents in biotites in two samples were determined by wavelength-dispersive spectrometry using a Cameca Camebax electron microprobe with fluorite and celsian standards, respectively. Ferric iron contents in garnets and hercynites were calculated by specifying fixed cation-to-oxygen ratios of, respectively, 16:24 and 3:4 (Droop, 1987).
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Regional metamorphic rocks
Minerals were analysed in two garnet–biotite gneisses (Sin1, Sin2) and one garnet–mica schist (Sin101) from Cairnie, and three mica schists (Bog1, Clas66 and Clas112) from Clashmach Hill.
Garnet
Garnet crystals in the Cairnie rocks are virtually unzoned. Those in Sin 1 and Sin101 have compositions close to Alm70Py23Sps05Grs01Adr01, whereas those of Sin2 cluster around Alm76Py13Sps08Grs03. The garnets in Clas66 are also unzoned, but differ from those at Cairnie in their high Mn and low Mg contents; their compositions are close to Alm57Py08Sps31Grs02Adr02.
Staurolite
Staurolite in Sin101 has an Mg-number value [= 100 Mg/(Fe2+ + Mg)] ranging from 21 to 26 over several grains and contains a small amount of Zn [0·10–0·19 atoms per 48(O)]. The staurolite in Clas66 has Mg-number ranging from 14 to 16 and contains up to 0·19 a.p.f.u. (atoms per formula unit) Zn.
Andalusite
Andalusite in Clas66 contains detectable Fe and Mg impurities (up to 0·01 a.p.f.u. of each).
Biotite
AlVI contents of all analysed regional-metamorphic biotites are closely grouped, mostly between 0·75 and 1·0 atoms per 22(O). Biotites from Cairnie have higher Mg-number values (50–62) and lower Ti contents (0·17–0·25 a.p.f.u.) than those from Clashmach Hill (0·29–0·40 and 0·26–0·40, respectively).
Muscovite
Regional muscovites are all mildly phengitic, with 6·06–6·14 Si and 0·15–0·56 Fe + Mg per 22(O), and contain some Na in their A sites [Na/(Ca + Na + K) = 0·06–0·16 in the Clashmach Hill schists and 0·19–0·26 in Sin2].
Plagioclase
Plagioclases in both Sin1 and Sin2 range from An17 to An23. K contents are below the detection limit. Rims and cores of individual grains have similar compositions. Plagioclases in Clas66 are also uniform, with compositions lying in the range An17–21.
Contact metamorphic rocks
Analyses of coexisting minerals were obtained from the Sil–Crd hornfels (Dun1), a Crd–Kfs hornfels (Cor8), six Opx–Crd hornfelses (Cor10, BQ17, BQ41, BHQ5, Pir1 and Pir4), and six cordierite norites (BHQ1, BQ38, Dun4, Dun12, CasB2i and CasB5). With a few exceptions, minerals in individual rocks are unzoned and show small within-sample variation, assumed to reflect a close approach to chemical equilibrium.
Garnet
Garnets in Sil–Crd and Crd–Kfs hornfelses display slight zonation, with broad cores of uniform composition (approximately Alm73Py17Sps05Grs04Adr01 in Dun1 and Alm79Py16Sps01Grs04 in Cor8) and rims up to 1 mm wide, displaying a smooth increase in Fe and decrease in Mg (Fig. 5a), indicating retrograde diffusional exchange.
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Garnets in Opx–Crd hornfelses are remarkably unzoned (Fig. 5b) and show little within-sample variation. The Mg-number is typically between 24 and 29, with XCa between 0·02 and 0·05.
Garnets in cordierite norites are also typically unzoned (Fig. 5c) and similar in composition to those of Opx–Crd hornfelses. The Mg-number is generally between 26 and 30, Mn contents are low (XMn mostly between 0·004 and 0·02) and XCa ranges from 0·027 to 0·068.
Orthopyroxene
The Mg-number and 
contents of orthopyroxenes from all the Opx–Crd hornfelses are similar, ranging from 40 to 50 and from 0·07 to 0·18, respectively. Cation totals cluster around 4·00 per 6(O), suggesting that Fe3+ contents are very low.
Orthopyroxenes from cordierite norites are generally very similar to those of Opx–Crd hornfelses; Mg-number generally varies from 45 to 52, except in CasB2i, in which it varies from 50 to 58, and is typically between 0·05 and 0·18. 
is low, rarely exceeding 0·01.
Cordierite
The Mg-number values of the two analysed Opx-free hornfelses are similar, ranging from 60 to 68 in Cor8 and from 59 to 71 in Dun1. The cordierites in the Opx–Crd hornfelses are slightly more magnesian, Mg-number varying from 63 to 72 in BQ17, from 61 to 64 in BQ41, and from 61 to 66 in BHQ5 and Pir1. The Mg-number values of cordierites in the cordierite norites overlap substantially with these ranges (62–67 in BHQ1, 72–73 in CasB2i, 63–67 in BQ38 and 71–73 in CasB5). No significant zoning has been detected.
Thermogravimetric determination of total volatile content was carried out for cordierites in two samples: cordierite norite BQ38, and cordieritite 10035. Fifteen milligram aliquots of powdered sample were heated under Ar (to prevent oxidation) and held at 900°C for at least 30 min. The measured mass losses were 1·07% for BQ38 and 0·65% for 10035. H2O and CO2 are known to be able to occupy the channel site in cordierite (e.g. Schreyer & Yoder, 1964; Johannes & Schreyer, 1981; Vry et al., 1990) and are thus the volatile species most likely to have been present. The 1·07% weight loss for BQ38 is equivalent to a maximum XH2O value of 0·36 or a maximum XCO2 value of 0·15. For 10035, the data imply a maximum XH2O value of 0·22 or a maximum XCO2 value of 0·092.
Biotite
Biotites in the Sil–Crd and Crd–Kfs hornfelses have AlVI contents of between 0·67 and 1·46 a.p.f.u. [22(O)]. The Mg-number values range from 44 to 51 in Cor8 and from 54 to 55 in Dun1. Ti contents are relatively low (0·16–0·54 a.p.f.u.). Biotites in contact with garnet generally have higher Mg-number than those remote from garnet, complementing the Fe-rich garnet rims, and confirming that there has been retrograde Fe–Mg exchange.
Biotites in Opx–Crd hornfelses are less aluminous than those of Opx-free hornfelses, with AlVI contents of between 0·24 and 0·35. The Mg-number ranges from 48 to 57, and Ti contents are high (0·57–0·75 a.p.f.u.). Biotites in cordierite norites have similar Mg-number but slightly higher AlVI contents. Again, biotites closest to garnet are richest in Mg. Ba and F were determined in the biotites of two cordierite norites; XF in biotites in BHQ1 range from 0·017 to 0·061, and those in BQ38 from 0·014 to 0·052. Ba contents in both samples are <0·5 wt %.
Hercynite and sillimanite
All analysed spinels are rich in the FeAl2O4 component, with low Cr [0·01–0·05 per 4(O)] and Zn (0·03–0·07 a.p.f.u.), and barely detectable Ti. Calculated Fe3+ contents are <0·12 a.p.f.u. The Mg-number values of hercynites in Opx–Crd hornfelses range from 26 to 27 (BQ17), from 16 to 25 (Pir1) and from 18 to 32 (BHQ5); those of hercynites in cordierite norites are similar, ranging from 22 to 29. Sillimanites in Dun1 and Cor8 are virtually pure, with no significant Fe3+ or Mn3+ substitution.
Feldspars
Three compositional types of feldspar occur in the thermally metamorphosed rocks: plagioclase, K-feldspar and anorthoclase. The plagioclases in the Sil–Crd and Crd–Kfs hornfelses are andesines; those of Cor8 show mild normal zonation from An46 cores to An34 rims. The compositions of small plagioclase grains in Opx–Crd hornfelses Pir1 and Pir4 are tightly clustered between An34 and An38; those of large plagioclases in BQ17 are more variable (An35–40Or02–08) (Fig. 6a). The plagioclases in the cordierite norites are commonly strongly zoned, with distinct An-rich cores and Ab-rich rims. In BQ38, for example, compositions range from An59 to An41 (Fig. 6b) and in Dun12 they range from An74 to An49. Oscillatory zoning occurs within both core and rim regions. Such zoning patterns are normally present only in plagioclases from igneous rocks, rimward enrichment in Na being commonly interpreted as reflecting decreasing temperatures during crystallization. These zoning patterns therefore strongly suggest that the plagioclases in Crd norites crystallized in the presence of melt, with rims crystallizing during cooling.
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K-feldspars in hornfelses Cor8 and BQ17 have compositions Or81–85 and Or87–90, respectively, whereas those in cordierite norite BHQ1 are Or82–88. The composition of the anorthoclase in BQ41 is variable within the thin section, ranging in a linear fashion mainly between Or50Ab43An07 and Or24Ab66An10. No twinning or optical zoning is apparent in either type of feldspar.
Ilmenite
All analysed ilmenites are close to the FeTiO3 end-member in composition. Fe2+ contents invariably exceed 0·96 atoms per 3(O), the main diluent being Mg.
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P–T–aH2O CONDITIONS OF METAMORPHISM |
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In this section, mineral equilibria are used to estimate the peak P–T conditions of Grampian regional metamorphism in the country rocks of the Huntly Complex, and the peak P–T–aH2O conditions of contact metamorphism in hornfelses and migmatites. The results provide information on (1) the thermal state of the country rocks before intrusion, (2) the crustal level of emplacement of the gabbros, (3) the conditions of migmatization, and (4) the P–T path followed by the rocks.
P–T conditions of Grampian regional metamorphism
Peak-metamorphic conditions of regional rocks to the west and south of the Huntly Complex were estimated by applying version 2.75 (1998) of the program THERMOCALC (Powell & Holland, 1988; Holland & Powell, 1998), in ‘average P–T mode’, to coexisting minerals in schists Clas66 and Sin2. Activity models used were Berman (1990) for grossular, almandine and pyrope components in garnet, Elkins & Grove (1991) for anorthite in plagioclase, Holland & Powell (1990) for muscovite and ferroceladonite in muscovite [with non-ideal parameters for alkali-sites from Chatterjee & Flux (1986)], Holland & Powell (1990) for annite, phlogopite and eastonite in biotite, and ideal ionic mixing for staurolite. Andalusite, quartz and H2O activities were assumed to be unity.
On the basis of five independent equilibria, the best-fit results for andalusite schist Clas66, from the south of the complex, are T = 537 ± 42°C and P = 0·27 ± 0·12 GPa (Fig. 7), with a fit index ( f ) of 0·88, which, passing the 
2 test, indicates that the averaging is reasonable with 95% confidence. These results are consistent with those of Hudson (1985) for andalusite-zone rocks on the Banff coast. The pressure value implies a depth of burial of 10 ± 4 km before gabbro intrusion. Five independent equilibria in sillimanite-zone schist Sin2, from the west of the complex, gave a best fit of 637 ± 31°C and 0·65 ± 0·13 GPa ( f = 0·25). The pressure difference between localities on the south and west of the complex is consistent with the abrupt change in regional metamorphic character across the Portsoy–Duchray Lineament noted by Harte & Hudson (1979) and Beddoe-Stephens (1990).
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P–T conditions of contact metamorphism
Peak contact-metamorphic conditions in the non-migmatitic hornfels and the highest-grade (Opx-bearing) migmatites were estimated using THERMOCALC and independently calibrated thermobarometers. The latter include the Grt–Bt (Bhattacharya et al., 1992), Grt–Crd (Nichols et al., 1992), Grt–Opx (Bhattacharya et al., 1991) Fe,Mg-exchange geothermometers, the Opx(Al)–Grt geothermometer (Harley & Green, 1982) and the Grt–Opx–Pl–Qtz geobarometer (Bhattacharya et al., 1991). For the Grt–Opx–Pl–Qtz geobarometer, end-member activities were calculated using the quaternary mixing model of Berman (1990) for garnet, Elkins & Grove (1991) for plagioclase, and Wood & Banno (1972) for orthopyroxene. Results from most of these calibrations are summarized in Table 5.
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Non-migmatitic hornfels
Application of Fe,Mg-exchange geothermometers to Sil–Crd hornfels sample Dun1 yields temperatures in the vicinity of 600°C. In ‘average P–T mode’, THERMOCALC yields best-fit conditions of T = 628 ± 75°C and P = 0·51 ± 0·08 GPa (f = 0·93) for this rock at an arbitrary aH2O value of 1·0.
High-grade hornfelses and diatexite matrix rocks
Application of Grt–Opx thermometers to Opx-bearing contact rocks yields temperatures mostly in the range 850–950°C (Table 5). Fe,Mg-exchange thermometers involving biotite and cordierite, on the same samples, generally give significantly lower temperatures, reflecting the susceptibility of such exchange equilibria to retrograde re-equilibration (e.g. Frost & Chacko, 1989; Fitzsimons & Harley, 1994). The anorthoclases of sample BQ41 plot mostly between the 900°C and 970°C isotherms on the 5 kbar ternary feldspar solvus of Elkins & Grove (1991), in agreement with the Grt–Opx thermometry.
Application of the Grt–Opx–Pl–Qtz geobarometer to cores of coexisting minerals in Opx–Crd norites yields pressures [at temperatures given by Grt–Opx (Fe,Mg) thermometry] ranging from 0·39 to 0·57 GPa (±>0·1 GPa) and averaging 0·49 GPa, consistent with the results of Ashworth & Chinner (1978) and Droop & Charnley (1985). However, these pressures must be treated with caution because the quartz in these rocks is interstitial and probably crystallized after the garnet, orthopyroxene and plagioclase. As an independent check, P–T conditions were also calculated using equilibria among end-member components in the quartz-absent restitic assemblage Grt + Opx + Crd + Hc + Pl in Crd norites and Opx–Crd hornfelses. The following ideal mixing models were used: for cordierite aFeCrd = (XFe)2 · Xv and aMgCrd = (XMg)2 · Xv, where Xv is the mole fraction of channel-site vacancies, and for hercynite aHc = XFe · (XAl)2 and aSpl = XMg · (XAl)2. The Xv values for Crd norites and Opx–Crd hornfelses were calculated from the maximum H2O contents permitted by the gravimetric data in cordierites from BQ38 (0·36 H2O a.p.f.u.) and 10035 (0·22 H2O a.p.f.u.), respectively. Best-fit results, calculated using THERMOCALC in ‘average P–T mode’, are mostly in the range 900–950°C, 0·4–0·5 GPa ( f < 1·2) (Table 5), in agreement with results of Grt–Opx–Pl–Qtz barometry.
The present thermobarometric results for Opx-bearing rocks confirm earlier pressure estimates (0·45 ± 0·1 GPa) but indicate that peak temperatures were in the region of 900 ± 50°C, higher than previously thought.
H2O activity conditions of contact metamorphism
Having estimated peak P–T conditions in the Opx-bearing rocks, dehydration equilibria can now be used to estimate aH2O conditions (Lamb & Valley, 1988). Crd norite BHQ1 contains biotite, quartz, orthopyroxene and K-feldspar, for which the equilibrium
 | (1) |
0·3. At 830°C, arguably a more realistic temperature for the crystallization of interstitial quartz and K-feldspar and poikilitic biotite around orthopyroxene, lower aH2O values are implied (0·06 and 0·2 for Mg- and Fe-end-member versions, respectively). The data of Clemens & Wall (1981) yield aH2O values of 0·33–0·42 for K-feldspar crystallization at the expense of Opx and melt over a temperature range of 845–820°C at 0·5 GPa in an S-type granitic system. The low volatile contents of cordierites also provide evidence for low values of aH2O in the highest-grade rocks. Application of the H2O–cordierite data of Harley et al. (2002) to BQ38 and 10035 cordierites at 900°C and 0·45 GPa yields maximum aH2O values of 0·27 and 0·13, respectively. The H2O contents of granitic melts under those conditions, calculated using the 900°C, 0·5 GPa cordierite–melt partitioning data of Carrington & Harley (1996), are 4·5 and 2·9 wt %, respectively, significantly lower than those of H2O-saturated eutectic haplogranitic melt (Holtz & Johannes, 1994).
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REACTION HISTORY |
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In this section we use textural evidence and established chemographic relations to reconstruct the mineralogical changes that took place as the Dalradian metapelites, in the vicinity of the Huntly Complex, were thermally metamorphosed and partially melted by the gabbro intrusion.
Subsolidus reactions
The sillimanite pseudomorphs after andalusite in Dun1 provide textural evidence for the conversion of regional andalusite-bearing schists, such as outcrop south of the Huntly Complex, into Sil–Crd hornfels. The simplest set of KFMASH discontinuous reactions by which the dominant regional And + St + Bt + Ms + Pl + Qtz assemblage could have been converted into Grt + Crd + Sil + Bt + Pl + Qtz is as follows:
 | (2) |
 | (3) |
 | (4) |
Reactions (3) and (4) must have occurred in the stated sequence, but the relative timing of reaction (2) is unconstrained.
Partial melting reactions
The assemblage biotite + Al-silicate + quartz, which characterizes many regional schists and the non-migmatitic hornfels, does not appear to occur in equilibrium in the migmatites. The widespread occurrence of garnetiferous leucosomes in the metatexites (Fig. 3b) and of large sub/euhedral garnets and cordierites in the garnet tonalites (Fig. 3c) suggests that this assemblage was destroyed by the incongruent, fluid-absent melting reactions
 | (5) |
 | (6) |
There is no textural or mineralogical evidence for any further melting reactions in the metatexites and garnet tonalites. However, the crystallization of orthopyroxene and hercynite in the cordierite norites and Opx–Crd hornfelses requires that further melting reactions occurred, and indicates that these rocks experienced even higher grades of metamorphism.
In relatively siliceous, low-alumina rocks, sillimanite is likely to be the first mineral to be consumed in reaction (6). Further heating would then result in incongruent, fluid-absent melting of biotite + garnet + quartz (Grant, 1985a, 1985b; Holland et al., 1996):
 | (7) |
In relatively aluminous, silica-poor rocks, quartz may be exhausted before biotite or sillimanite in reaction (6). In these rocks, instead of reaction (7), further heating would result in incongruent melting of biotite + sillimanite + garnet to produce a quartz-free solid product assemblage (Pattison & Tracy, 1991; Srogi et al., 1993):
 | (8) |
On further heating, any biotite remaining after reactions (7) and (8) would ultimately be consumed by reaction with garnet and cordierite (Pattison & Tracy, 1991):
 | (9) |
Independent evidence supporting the occurrence of reactions (2) and (6)–(9) is provided by the fact that the P–T path linking the calculated peak regional-metamorphic conditions with the peak contact-metamorphic conditions crosses the respective equilibrium curves in the expected sequence (Fig. 7). The calculated peak contact-metamorphic conditions lie just up-T of the curve for reaction (9), supporting the view that this was the highest-grade reaction to occur in the migmatites. The low aH2O values calculated for Opx–Crd hornfelses and Crd norites are consistent with the operation of high-T, fluid-absent melting reactions such as (6)–(9). Reactions of this type are strongly implicated in the production of crustally derived granitic magmas (Clemens & Watkins, 2001).
Retrograde reactions
The relatively small modal proportions of interstitial/poikilitic minerals in the cordierite norites and Opx–Crd hornfelses suggests that much of the melt produced by reactions (5)–(9) must have escaped from the rocks; further evidence for this will be provided below. On cooling, the remaining melt crystallized mainly as Bt + Qtz + Kfs + sodic Pl. Localized back-reaction of melt with residual solid phases produced fine-grained retrograde products, e.g. Bt + Qtz by the reverse of reaction (7) and sillimanite by the reverse of reaction (6) [see also Ashworth (1976)]. In Sil–Crd hornfels Dun1, late hydration of cordierite took place by the reaction
 | (10) |
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WHOLE-ROCK GEOCHEMISTRY |
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Some documented anatectic migmatites evidently formed as closed systems (e.g. Dougan, 1981; group 2 of Olsen, 1982), whereas others formed as open systems, recording petrological and/or geochemical evidence of melt loss (e.g. Weber et al., 1985; Barbey et al., 1990; Hansen & Stuk, 1993; Nyman et al., 1995; Hartel & Pattison, 1996; Kriegsman, 2001; Solar & Brown, 2001). For crustal differentiation to occur, open-system behaviour is required, with transport of substantial melt volumes on a scale of kilometres. The question of whether the partial melting of Dalradian metasediments in the Huntly area was a closed- or open-system process has yet to be resolved. The key to this question lies in the geochemical relationships between the derived diatexitic rock-types (hornfelses, Crd norites and Grt tonalites) with respect to the compositions of protoliths, restitic material and melt. In a closed system, the metapelitic hornfels schollen and diatexite matrices should, when combined in their existing proportions, be chemically equivalent to the pelitic/semipelitic protoliths. If the metapelitic schollen represent restitic material, the diatexite matrices should be correspondingly enriched in melt components. In an open system with melt loss, the metapelitic hornfels schollen and diatexite matrices would sum to a composition depleted in melt components with respect to the pelitic/semipelitic protoliths, and the diatexite matrices could themselves be restitic, at least in part (Milord et al., 2001).
In principle, this problem is amenable to analysis using Harker-type bivariate plots and simple mixing calculations (e.g. Dougan, 1979; Olsen, 1982, 1983). On a Harker plot showing bulk-rock concentration of a given oxide vs silica, closed-system behaviour would generate a compositional collinearity between protoliths, hornfelses and diatexite matrices, with protolith compositions straddled by the compositions of the derivative rocks. Mildly open-system behaviour would also generate this type of pattern as long as the diatexite matrices remained enriched in melt components relative to protoliths. Strongly open-system behaviour, involving much melt loss, would also generate a collinearity, but here the hornfels and diatexite matrices would both plot to one side of the protolith field. (Some smearing of the diatexite field into relatively melt-rich and melt-poor compositions might be expected if the proportion of melt loss were highly variable.) At the other end of the lever would be the average composition of the missing melts. In reality, the compositions of successive melt batches generated by reactions (5)–(9) must have changed to some extent, and may have been modified further by crystal fractionation (Milord et al., 2001; Solar & Brown, 2001), but for present purposes such effects are ignored.
Bulk compositions
Major-element concentrations in 72 metasedimentary and mafic igneous rocks were obtained by X-ray fluorescence (XRF) spectrometry. Samples were prepared as pressed boric acid pellets, each containing 2·0 ± 0·05 g of rock powder, and analysed using a Philips PW 1450 X-ray spectrometer at the University of Manchester. Details of the correction procedures used have been described by Brown et al. (1973). Representative analyses are presented in Table 6, and the full dataset has been listed by Dalrymple (1995).
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For purposes of geochemical comparison, the rocks of metasedimentary origin are split into the following geographical groupings: (1) southern complex, including 10 schists from the Whitehills and Boyndie Bay formations, five Opx–Crd hornfelses, and six Crd norites; (2) western complex, including eight schists of the Portsoy formation, and four Crd–Kfs hornfelses and four Grt tonalites from the Cumrie–Cormalet area; (3) samples from any other location. Mafic igneous rocks from the whole complex are subdivided into the following petrographic groups: (1) olivine gabbros (10 samples); (2) biotite gabbros (11 samples); (3) micronorites (two samples). Mean analyses of mafic igneous rocks and metasediments from the southern and western parts of the complex are listed in Table 7.
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Metasediments
The structural trend of country-rock schists to the south of the Huntly Complex, and the petrographic affinities of these rocks with the migmatites within the southern sector of the complex, strongly suggest that the protoliths of the latter were schists of the Whitehills and Boyndie Bay formations. This connection will be assumed in the following geochemical arguments. The identities of the protoliths of the migmatites in the Cumrie–Cormalet area are less certain. These migmatites also occur along strike from in situ schists of the Whitehills and Boyndie Bay formations, but the schists of the Portsoy formation that crop out nearby, across strike to the west (Fig. 2), are also possible candidates.
Data from the southern part of the complex display the following trends (Tables 6 and 7, Fig. 8):
- the bulk compositions of the Opx–Crd hornfelses and Crd norites are similar. (On the basis of the Student's t test, estimated population means for Opx–Crd hornfelses and Crd norites are statistically indistinguishable, at the 95% confidence level, for all 10 oxides listed.)
- The SiO2 and K2O contents of both Opx–Crd hornfelses and Crd norites are significantly lower than those of country-rock schists, whereas FeOT, MgO, CaO and TiO2 contents are higher.
- No oxides have significantly higher concentrations in the Opx–Crd hornfelses than in country-rock schists but lower concentrations in the Crd norites than in the schists. Similarly, no oxides have significantly lower concentrations in the Opx–Crd hornfelses than in country-rock schists but higher concentrations in the Crd norites than in the schists.
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The interpretation of the data for the western part of the complex is less clear-cut. The schists of the Portsoy formation show a much wider range of compositions but are similar, in general, to those of the Whitehills and Boyndie Bay formations, except in having higher MgO and lower CaO and TiO2 contents (Table 7). With respect to either group of schists, the Crd–Kfs hornfelses have somewhat lower SiO2 and higher FeOT and CaO contents, consistent with moderate enrichment in ‘restitic’ components relative to protoliths. The compositions of the Grt tonalites are indistinguishable, within error, from those of either schist group, except in having higher Na2O contents. The fact that the Grt tonalites are not significantly enriched in ‘granitophile’ components relative to schist protoliths (to balance, as it were, the depleted Crd–Kfs hornfelses) implies that the Grt tonalites also formed by an open-system process involving loss of felsic melt, although with smaller proportions of melt loss than in the formation of the Crd norites and Opx–Crd hornfelses.
Gabbros
For many oxides, olivine gabbro compositions are well clustered and plot far from the schist–diatexite matrix–hornfels trend on Harker diagrams. For MgO and CaO (Fig. 8a) olivine gabbros plot consistently above the trend, whereas for TiO2 (Fig. 8b) and FeOT they plot below. The lack of a collinearity between olivine gabbro, Crd norite and schist fields for these oxides adds further weight to the argument that the Crd norites did not form by assimilation of pelitic schist by mafic magma.
Biotite gabbros are similar in composition to olivine gabbros, except in having somewhat higher TiO2, FeOT and K2O, and lower MgO and CaO (Tables 6 and 7). Being rich in biotite, a hydrous mineral, the biotite gabbros must also contain more H2O than the olivine gabbros. However, as the SiO2 contents of the two groups are similar, no collinearity exists between olivine gabbros, biotite gabbros and any possible siliceous ‘fugitive melt’ composition. Thus, the biotite gabbros could not have formed by simple mixing of siliceous melt with basic magma. However, the proximity of the biotite gabbros to contacts with metasediments coupled with their enrichment in the above components suggests that these gabbros have undergone some selective contamination, possibly by diffusion (Watson, 1982).
Mass-balance calculations
Limits on the compositions of the fugitive felsic melts can be obtained by simple linear mixing calculations, for each of the major oxides. The mean compositions of the schists and Opx–Crd hornfelses are taken as the protolith and restite compositions, respectively, for the southern sector. In mixing diagrams (Fig. 9), the melt composition is constrained to lie on the hornfels–protolith mixing lines, somewhere to the SiO2-rich side of the protolith point. All melt components will have positive coefficients, and the closer the melt composition is to that of the protolith, the higher the proportion of extracted melt required to generate the hornfels residue.
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The calculated mixing line for CaO (Fig. 9) descends to zero at
74 wt % SiO2, providing a maximum silica content for the fugitive melt. That of MgO, which is more tightly constrained, descends to zero at a similar value. The composition of the limiting CaO-free melt (‘M’ in Fig. 9) is broadly granitic and yields a minimum melt proportion of 57 wt %. Table 8 shows the compositions of modelled melts that have positive, non-zero values for CaO contents. These calculations have been made for melt proportions of 60% (model melt 1) and 67% (model melt 2). Both of these model melts are peraluminous potassic granites with low normative plagioclase (<10%), high normative corundum (7%), and Mg-number values of 20. The calculated normative C values are similar to those of some of the experimental, alumina-saturated melts reported by Gardien et al. (1995) but somewhat higher than those reported by other workers (e.g. Clemens & Wall, 1981). The minimum proportion of model melt 1 that would have to be extracted from the schist protolith, to generate a Crd norite with the same SiO2 content as the observed mean (52·43 wt %), is 53 wt %.
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The results of these mass-balance calculations confirm that the Opx–Crd hornfelses and Crd norites of the southern part of the Huntly Complex are extremely restitic in character and require that large proportions (>
57 wt % and >53 wt %, respectively) of the material that originally constituted the protoliths must have escaped from these rocks as magma. In this respect, the Crd norites resemble the garnet-rich diatexites described by Barbey et al. (1990), although the amounts of melt loss from the former were even higher. In theory, the fugitive melt could have had an SiO2 content anywhere in the range 64–74%, but a value close to the upper end of this spectrum (corresponding to a granite) seems most likely. The reasons for this inference are that (1) this composition would minimize the amount of melt that would have to have been extracted, and (2) this is the kind of melt composition that is produced when metapelites are subjected experimentally to P–T conditions similar to those inferred for the Opx-bearing diatexites. The melt produced by Stevens (1995) from an aluminous metapelite protolith at 0·5 GPa and 900°C is similar to model melt 1, except in having higher normative plagioclase and lower normative C and Hy (Table 8). If the fugitive melts were granitoid (i.e. with 72–75 wt % SiO2) and if, as seems likely on petrographic grounds, many of the Opx–Crd hornfelses were completely or almost completely drained of melt during their formation, then 60% seems a reasonable estimate of the degree of partial melting. The slightly lower proportion of melt loss calculated for the Crd norites is consistent with the petrographic evidence for retention of some melt during the crystallization of these rocks (namely, sub/euhedral growth of restitic phases, interstitial quartz and K-rich phases with small dihedral angles, and igneous-type zoning patterns in plagioclase rims). |
EXPERIMENTS |
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Partial melting experiments were undertaken, on two samples of country-rock schist from the southern margin of the Huntly Complex, to provide independent evidence of the nature and quantity of melt that could have been produced in these protoliths at the inferred P–T conditions of contact metamorphism, and the identities and compositions of coexisting minerals.
Experimental details and melt analysis
The rocks chosen as starting materials were Clas112 and Bog1 (see Table 1 for mineralogy, Table 2 for mineral compositions, and Table 6 for bulk compositions). The mineral modes of the two samples are: for Clas112: 23% Ms, 31% Bt, 24% Qtz, 10% And, 11% Pl, <1% Grt + St + Ilm; for Bog1: 26% Ms, 29% Bt, 35% Qtz, 8% Pl, 2% Tur + Ilm. Bulk H2O contents, determined by thermogravimetry, are 2·62 (Clas112) and 2·97 wt % (Bog1).
Rock powders were prepared by grinding, in an agate ball mill, under acetone, to an average grain size of 1 µm. Aliquots of oven-dried (110°C) powder were sealed into gold capsules and run for 150 h, without added water, in a Holloway-type internally heated argon gas vessel (Ulmer, 1971) at 0·5 GPa and 900°C. A Pd–Ag hydrogen diffusion membrane was used to impose a log fH2 = -1 relative to the quartz–fayalite–magnetite (QFM) buffer at nominal aH2O = 0·35, to prevent loss of H2 through the capsules (Stevens et al., 1997). Pressure was measured by a manganin cell and bridge, believed to be accurate to ±10 MPa. Temperature was controlled to ±1°C and measured by type-K thermocouples, believed to be accurate to ±<5°C.
Run products were observed and analysed by EDS using a Jeol JSM 6400 scanning electron microscope. The detector system and analytical conditions were as for electron microprobe analysis. To prevent alkali loss during analysis of the glasses, the samples were cooled to –193°C by liquid nitrogen on a cryostage. Mean analyses of glasses (quenched melts) and representative analyses of minerals are listed in Tables 8 and 9, respectively.
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Experimental results
Crystalline phases
The product of the Clas112 run (C112) contains abundant small crystals of Opx, Crd, Hc, Ilm and minor Bt, suspended in a matrix of glass (Fig. 10). The Opx forms euhedral prisms up to 50 µm x 15 µm, with numerous inclusions of Ilm and Hc, and has Mg-number of 50–53 and
of 0·18–0·25. Cordierite forms euhedral hexagonal prisms, up to 15 µm long and 10 µm across, with Mg-number ranging from 63 to 74. Biotite forms rare ragged flakes up to 8 µm long, with 0·63–0·74 a.p.f.u. of Ti and Mg-number of 41–46. The biotite in the starting material has only 0·27–0·32 a.p.f.u. of Ti and is also more aluminous and less magnesian (Table 2). These characteristics show that the experimental biotite is a run product and, in that sense, restitic. The hercynite forms small octahedra, mostly 1–2 µm across but occasionally larger, with Mg-number of 20. Ilmenite forms rounded granules <2 µm in diameter. Quartz and feldspar appear to be completely absent. The tight clustering of compositions within C112 confirms a close approach to chemical equilibrium. The Bog1 run product (B1) contains Crd (Mg-number = 64–73), Hc, Ilm and glass. Grain sizes are smaller than in C112 but textures are similar.
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The melt phase
Analysis of images (three low-magnification images per run) indicates that the volume proportion of glass in C112 lies between 55 and 65%; the overall proportion is
60 vol. %. In B1, crystal-rich areas contain a minimum of 55 vol. % glass, whereas more typical areas contain 70 vol. %; the overall proportion is probably at least 65 vol. %. These proportions correspond closely to those reported by Gardien et al. (1995) for experimental melting of two-mica schist at T 950°C.
The glass composition for each run is remarkably uniform, again confirming a close approach to equilibrium. Mean analyses of glasses in C112 and B1 (Table 8) are similar to one another and correspond to peraluminous potassic granite. The peraluminosities of the melts (2·8 and 3·3% normative C for C112 and B1, respectively) compare well with those of other granitic melts produced experimentally in pelitic systems (e.g. Clemens & Wall, 1981; Patiño Douce & Johnston, 1991; Holtz et al., 1992; Carrington & Harley, 1995; Stevens, 1995).
Comparison with natural data
Except for a lack of garnet, the FMAS mineral assemblage of C112 is similar to the non-interstitial (i.e. peak-T) mineral assemblage of the Opx–Crd hornfelses and Crd norites. The assemblage of B1 lacks orthopyroxene and is a less good match. The coexistence of Opx, Hc and melt, and lack of garnet, in C112 is evidence that reaction (9) was crossed during the experiment, and confirms that the equilibrium curve for reaction (9) lies down-temperature of the peak P–T conditions experienced by the highest-grade rocks. The apparent lack of quartz and K-feldspar and paucity of biotite in both runs is consistent with the conclusion that, in the Crd norites, these phases crystallized mainly from interstitial melt during cooling. The apparent absence of plagioclase from the runs is at variance with the deduction that plagioclase is a major constituent of the residual assemblage in both Opx–Crd hornfelses and Crd norites, but the fact that Na concentrations are lower in the glasses than in the corresponding starting rocks implies that some undetected plagioclase was indeed produced in both runs.
Minerals in C112 have compositions similar to those in the high-grade rocks. Measured Mg-number values generally fall within the ranges observed for respective minerals in Opx-bearing rocks, except for biotite, which is slightly less magnesian in C112. Ti and Al contents of C112 biotites are similar to those of Crd norites. Al contents of C112 orthopyroxenes are somewhat higher than their natural counterparts, but no more scattered.
The compositions of experimentally produced melts correspond closely to that of model melt 1 with respect to SiO2, Al2O3, FeO, MgO, K2O and Mg-number, but have slightly higher Na2O and CaO contents and lower TiO2 (Table 8). The match is closer than with the composition of Stevens (1995). Normative Q and C are lower than in model melt 1, normative feldspar components higher. Assuming that volume and mass proportions are not radically different, the amount of melt in the experimental runs (60–65 vol. %) agrees well with the calculated degree of partial melting (60 wt %). Collectively, these results support the findings of the thermobarometry and mass-balance calculations.
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DISCUSSION |
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Our results show that the highest-grade pelitic rocks of the Huntly Complex underwent high degrees of anatexis (
60%), to produce strongly peraluminous, low-Ca, potassic granitic melts, as a result of the mid-crustal intrusion of mafic magma. The close correspondence between the compositions of model and experimental melts implies that little or no fractional crystallization affected the melts before their expulsion. The conclusion that these granitic melts were able to segregate efficiently from the solid products of the melting reactions, but did not mix mechanically with the mafic magma to any great extent, suggests that no great melt pool of felsic magma actually developed above the gabbro intrusion (compare Huppert & Sparks, 1988) and that the melt was rapidly and efficiently evacuated (e.g. Sleep, 1988; Clemens & Mawer, 1992; Harris et al., 2000). This raises three questions: (1) How did the fugitive melts segregate from their restitic solids? (2) Where did they go? (3) What sort of geometric relationships prevented interactions between the felsic and mafic melts?
Melt segregation
Melt segregation mechanisms in migmatites have been discussed by many workers (e.g. McKenzie, 1987; D'Lemos et al., 1992; Brown, 1994; Sawyer, 1994; Brown et al., 1995a, 1995b). Brown et al. (1995b) identified three principal driving forces: (1) forces arising from expansion associated with fluid-absent melting; (2) gravity-induced compaction of solids; (3) deviatoric stress-induced compaction of solids. In the diatexites of the Huntly Complex, expansion forces, although probably important in ‘hydrofracturing’ the rocks and segregation of melts into fractures in the initial stages of melting, are unlikely to have played the dominant role in the expulsion of anatectic melts from restitic solids, because the proportions of melt extracted were much larger than the likely proportional volume increase of fluid-absent melting. We now consider the relevance of the remaining two processes to the Huntly diatexites.
Gravity-driven compaction of solids
This process encompasses two distinct mechanisms, depending on whether the melt proportion exceeds some threshold value, usually referred to as the ‘melt escape threshold’ (MET) (Vigneresse et al., 1996). Estimates of the MET range from 20% to 50% (e.g. van der Molen & Paterson, 1979; Rutter & Neumann, 1995). Below the MET, the solid grains form a contiguous framework, and compaction involves upward percolation of melt by porous flow through a solid matrix of finite bulk viscosity, as modelled by McKenzie (1984, 1985). For this mechanism to operate, the melt must occupy a large enough volume proportion to form an interconnected porosity, i.e. to exceed the ‘liquid percolation threshold’ (LPT). Above the MET, the solid grains are suspended in melt, and compaction occurs in response to the buoyancy forces acting on individual grains (i.e. by gravitational settling or floating).
Gravitational settling can be an efficient segregation mechanism provided there is a sufficiently large density contrast between solids and liquid and the liquid has a sufficiently low viscosity (Shaw, 1965). It has been shown that siliceous (granitic sensu lato) melts, although they have relatively high viscosities, are only one to three orders of magnitude more viscous than typical basalt melts (e.g. Scaillet et al., 1998; Clemens & Petford, 1999). Thus, granitic melt viscosity is sufficiently low to permit settling of relatively dense solids (such as pyroxene, garnet and plagioclase). For this mechanism to operate in diatexites, all the melt produced on heating must remain in contact with the restitic solids until the MET is greatly exceeded. This clearly did not happen in the Huntly Opx–Crd hornfelses, as these rocks evidently retained their integrity during melting, but it could have been important in the diatexite matrices. To test the feasibility of settling in the present case, composition C112 (Table 8) was used to model the physical properties of the anatectic granitic melt, assuming a melt H2O content of 4·5 wt %. The method of Bottinga & Weill (1970) and data of Mo et al. (1982) yield a density of 2240 kg/m3 at 900°C and 0·45 GPa. The calculated viscosity (Hess & Dingwell, 1996) of C112 at 900°C is 16 200 Pa s. Comparison of the densities of the restitic Grt, Opx, Crd and Pl in the diatexites (4080, 3600, 2580 and 2700 kg/m3, respectively) with that of melt indicates that all restitic minerals should settle out of such a melt. Application of Stokes' Law for 2 mm diameter crystals of Grt, Opx, Crd and Pl yields settling velocities of 7·6, 5·6, 1·4 and 1·9 m/a, respectively, at 900°C. At 800°C, the velocities are an order of magnitude lower. These results suggest that restitic cumulates could form in moderate-sized (<1 km diameter) masses of partially molten metapelite on a time-scale that is extremely short in comparison with that of the crystallization of a large gabbro intrusion. However, as the various cumulus minerals would settle at very different rates, dramatic gradations in their relative modes would be expected, unless aggregates of unlike crystals settled together. The absence of compelling field or textural evidence for layering in the Huntly diatexite matrices suggests that settling was not a major segregation mechanism.
Where crystals form a contiguous framework, Stokesian settling cannot operate, and compaction has to occur by a different mechanism. Gravity-driven compaction of a contiguous solid framework requires a relatively impermeable lower boundary, which, in the Huntly Complex, could have been provided by gabbro chilled margins against the screens and pendants of metapelite. In principle, this process might have been responsible for melt segregation from hornfelses and from diatexite matrices. Mecklenburgh (2000) used the equations of McKenzie (1984, 1985) to calculate the time taken to extract granitic melts of varying H2O contents and temperatures by gravity-driven compaction. He showed that, where heat flow is not rate-limiting, extraction of 10% of a melt containing 4·5 wt % H2O at 900°C by porous flow through a solid framework with a grain size of 5 mm would take >1·0 Myr. This is long in comparison with the time taken for a large gabbro intrusion to crystallize. However, Mecklenburgh (2000) also showed that, if an array of 5 cm wide fractures is introduced, the time taken would decrease to <100 a, which would easily be short enough to drain the diatexites of anatectic melt. No field evidence for such fracture arrays has been found in the diatexites.
Deviatoric stress-induced compaction
The existence of a far-field deviatoric stress can, in principle, cause melt segregation in systems with melt fractions between LPT and MET. As deviatoric stress can be transmitted through a contiguous solid framework of mineral grains but not through the intergranular melt, the framework can undergo strain (e.g. by diffusion-accommodated granular flow; Rutter, 1997). Provided that channels (fractures or less compacted regions) exist, the resulting stress gradient can cause expulsion of melt from the intergranular pores into the channels (Rutter & Neumann, 1995; Simakin & Talbot, 2001). Mecklenburgh (2000) used the model of Rutter & Neumann (1995) to assess the feasibility of extracting granitic melt by this mechanism; his results suggest that the time taken to extract 10% of granitic melt containing 4·5 wt % H2O at 900°C, from a solid framework of grain size 5 mm, into a channel network with a 2 m spacing under a constant mean effective stress of 1 MPa is 2 kyr. In the context of the Huntly Complex, where anatexis was not accompanied by tectonism, the only far-field deviatoric stresses that could have acted on bodies of partially melted metapelite would have been those imparted by the weight of overlying lobes of the gabbro intrusion (i.e. 
1 was vertical). Once solidified, individual gabbro lobes may have been rigid enough to sink en masse through their subjacent migmatites. To exert a vertical stress of 1 MPa on its substrate, a slab of gabbro of density 3000 kg/m3 would only need to be 0·5 km thick, if surrounded by partially melted metapelite of density 2800 kg/m3 (30% granitic melt and 70% restite). For this stress to have caused segregation, however, a contiguous solid framework must have extended from the floor to the roof of the partially melted metapelite substrate. One way in which such a framework could have been maintained could be that the melt was extracted continually, as it was produced, thus keeping the melt fraction low throughout the melting history, as advocated by Sawyer (1994).
Fate of the fugitive melts
The anatectic melts of the Huntly Complex were generated under conditions favourable to their migration to higher and cooler levels of the crust. The high temperature of the melts and their H2O-undersaturated compositions are both factors that would have facilitated melt ascent (Brown & Fyfe, 1970; Stevens & Clemens, 1993). The sizes of individual coalesced melt bodies is also a factor. The larger the melt body the larger is its ‘thermal inertia’ during ascent into cold crust. In the absence of good geological control on the 3D shapes of the gabbro and diatexite bodies of the Huntly Complex, neither the total anatectic melt volume nor the volumes of individual melt bodies can be estimated with confidence. Nevertheless, crude estimates of melt volume can be made for individual migmatite bodies assuming that they are equant in 3D and have outcrop areas as in Fig. 2. The most restitic diatexites are those in the south of the complex. Some of these (e.g. NE of the Deveron near Dunbennan Hill) have outcrops 300 m in diameter, yielding a restite volume of 0·015 km3. Assuming a conservative melt fraction of 50%, the local melt volume was also 0·015 km3. The cluster of exposures near the Deveron, north of Huntly, may belong to a single outcrop 1 km across, which would yield a melt volume of 0·5 km3. (The Cumrie–Cormalet outcrop is even larger but has evidently undergone less melt depletion.) Once segregated, such volumes of melt would have been large enough to rise considerable distances through pre-warmed Dalradian country rocks before crystallizing, probably utilizing interconnected fractures, as advocated by Clemens & Mawer (1992) and described by Johnson et al. (2001b).
The equations of Petford et al. (1993, 1994) can be used to estimate the minimum width of fractures required to prevent solidification of the rising magma by freezing. Assuming a magma temperature of 900°C, a magma solidus temperature of 750°C, a far-field temperature of 540°C, a latent heat of fusion of 3 x 105 J/kg, a specific heat capacity of 1200 J/kg/°C, a thermal diffusivity of 8 x 10-7 m2/s, a density difference between country rock and magma of 560 kg/m3, and a magma viscosity of 16 000 Pa s, the critical width of a fracture of height 1 km is estimated to be 0·4 m. This implies that any fractures wider than 0·4 m should have been able to act as magma conduits, and any narrower than this should have frozen to form dykes. For a height of 5 km, the estimated critical width is 0·55 m. Peraluminous aplite and leucogranite sheets, some 1 m thick, which may represent such conduits, occur in the relatively high-grade but unmigmatized parts of the Buchan terrane ( Johnson et al., 2000).
Regarding potential sinks for the fugitive melts, numerous S-type granites occur in the Buchan region east of the Huntly Complex (and at higher structural levels), as both minor (Johnson et al., 2000, 2001b) and major (Brown, 1991) intrusions. Many of these (e.g. Strichen, Aberchirder) belong to the contemporaneous, Grampian suite of peraluminous two-mica granites. The latter have isotopic signatures implying an upper-crustal source (Harmon, 1983). We have shown that the fugitive melts from the Huntly Complex were also peraluminous and granitic, suggesting that they may have contributed to the Grampian suite. Anatectic melts were probably also supplied from the other Newer Gabbro and related mafic complexes (e.g. Johnson et al., 2001b).
A significant implication of this work is that crustal fusion and magma production can occur without significant chemical interaction between the mantle-derived heat source and the crustal melts. This is important because it explains how ‘pure’ S-type magmas might arise.
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CONCLUSIONS |
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- Syn-metamorphic intrusion of mafic magma into warm (550°C) Dalradian metapelites and semi-pelites at mid-crustal depths in the Huntly area, NE Scotland, resulted locally in peak-metamorphic temperatures of 900°C, and in high degrees of partial melting via incongruent, fluid-absent, melting reactions.
- The anatectic melts were of H2O-undersaturated, peraluminous, low-Ca, potassic granite composition. In many places, a large proportion of the melt escaped from the sites of generation. The melts underwent little or no in situ fractional crystallization before escaping.
- Segregation of melts from restites was probably achieved by purely gravitational processes, and appears not to have required regional tectonic deviatoric stresses. The role of transient fracture formation was probably critical to various stages of the segregation process.
- The fugitive granitic melts did not mix significantly with the gabbroic magma, although some selective contamination of the mafic magma may have occurred, leading to the generation of biotite gabbros. The bulk of the melt escaped and probably contributed to the Grampian suite of S-type granites in this region. Thus, crustal fusion and magma production can occur without significant chemical interaction between the mantle-derived heat source and the crustal melts.
- Opx–Crd hornfels xenoliths represent solid Ca-, Mg-, Fe-, Al-, Na-rich residues left after extraction of 60% melt. The Crd norite diatexite matrix rocks are also residual, and represent restite-enriched crystal–liquid mushes left after extraction of 55% melt. Grt tonalites probably also represent restite-enriched mushes, but retaining a higher proportion of residual melt than the Crd norites.
- The mid-Ordovician middle crust of NE Scotland was the site of localized crustal differentiation in response to intrusion of large volumes of mantle-derived mafic magma.
- The prograde P–T path of contact-metamorphic rocks in the SW part of the Huntly Complex had a low positive dP/dT slope, indicating that the gabbro intrusion itself increased the lithostatic load on the country rocks.
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ACKNOWLEDGEMENTS |
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This paper is derived from the Ph.D. thesis of D.J.D., supervised at Manchester by G.T.R.D. and J.D.C. The work was supported by an NERC research studentship, which is gratefully acknowledged. We thank D. Plant, P. Lythgoe, S. Caldwell and C. Davies for technical support. Tracy Rushmer, Marion Holness and an anonymous referee are thanked for their helpful comments.
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