Journal of Petrology

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Journal of Petrology | Volume 40 | Number 4 | Pages 549-573 | 1999
© Oxford University Press 1999

The Petrology of Mullite-bearing Peraluminous Xenoliths: Implications for Contamination Processes in Basaltic Magmas

R. Jeremy Preston^1,*, Tim J. Dempster², Brian R. Bell² and Graeme Rogers³

¹ Department of Geology and Petroleum Geology Meston Building, King'S College University of Aberdeen, Old Aberdeen AB24 3UE, UK
² Division of Earth Sciences, Department of Geography, University of Glasgow Glasgow G12 8QQ, UK
³ Isotope Geosciences Unit, Scottish Universities Research and Reactor Centre East Kilbride, Glasgow G75 0QF, UK

Received September 24, 1997; Revised typescript accepted September 14, 1998

	ABSTRACT

TOP ABSTRACT Introduction Geological Setting Geochemical Techniques The Crustal Xenoliths Contact Metamorphic Effects Geochemistry and Petrogenesis of... Crustal Xenolith Mineralogy and... Origin Of The Aluminous... Magma-Xenolith Interactions in... Implications For Contamination... Final Comments References

 
A suite of high-level inclined sheets ranging in composition from basalt through to rhyolite is intruded into the Palaeogene lava field and underlying Moine Supergroup basement rocks around Loch Scridain, Isle of Mull, Scotland. Many of the sheets are highly xenolithic, containing a wide variety of crustal xenolith types derived from the Moine metasedimentary rocks, along with various gabbroic cumulate xenoliths. The most common xenolith types are almost pure quartzites and a variety of mullite-bearing aluminous buchites, many of the latter having thick crystalline selvages of plagioclase, corundum and aluminous spinel. The plagioclase is highly calcic (up to An₈₇), and adjacent to the host basalt is commonly oscillatory zoned, implying crystallization from a melt. Trapped between plagioclase crystals are pockets of quenched, contaminated basic melt, which contain skeletal plagioclase and clinopyroxene, and preserve evidence of local mixing between the host basalt and the aluminous crustal melts. Sr and Nd isotope values of the buchite cores [e.g. (⁸⁷Sr/⁸⁶Sr)₅₅ = 0.7074–0.7115], plagioclase selvages [e.g. (⁸⁷Sr/⁸⁶Sr)₅₅ = 0.7137–0.7148], and associated trapped melts [e.g. (⁸⁷Sr/⁸⁶Sr)₅₅ = 0.7126–0.7128], imply a complex series of magma–xenolith interactions. The textural characteristics, mineral chemistry and isotope geochemistry of these rims suggest that they have crystallized from a hybrid liquid formed by the complex interaction of the aluminous liquids with basic magmas. Such interaction proceeded via liquid–liquid diffusion, physical mixing of melts and a variety of reactions between the crystallization products of the buchites and the basaltic liquids. These crustal xenoliths preserve a detailed record of mineral–melt reactions within a suite of basaltic sheets, dominated by both the production of granitic melts and the ‘bulk’ melting of Al-rich micaceous lithologies.

KEY WORDS: xenolith; metasedimentary; mullite buchite; plagioclase; isotope geochemistry; crustal contamination; basalt

	Introduction

TOP ABSTRACT Introduction Geological Setting Geochemical Techniques The Crustal Xenoliths Contact Metamorphic Effects Geochemistry and Petrogenesis of... Crustal Xenolith Mineralogy and... Origin Of The Aluminous... Magma-Xenolith Interactions in... Implications For Contamination... Final Comments References

 
Contamination of magmas by crustal lithologies occurs in a variety of magmatic environments. Bowen (1928) was the first to argue that fusible crustal rocks could be assimilated by a basic magma if energy was supplied from the latent heat of the crystallizing magma. Numerous mechanisms for crustal contamination have subsequently been proposed and tested, both experimentally (e.g. Maury & Bizouard, 1974; Watson, 1982; McLeod & Sparks, 1995; Patiño Douce, 1995), and through geochemical studies (e.g. Thompson et al., 1982; Grapes, 1986; Devey & Cox, 1987; Green, 1994). Suggested mechanisms include: bulk assimilation of small amounts of crustal material (e.g. Thompson et al., 1982); assimilation of crustal material with concurrent fractional crystallization (AFC; DePaolo, 1981); assimilation of small-melt fractions derived from the crust, with or without fractional crystallization (e.g. Thompson et al., 1982; Thirlwall & Jones, 1983; Kerr et al., 1994); selective contamination by hydrous fluids, or vapour phase transport of alkalis (e.g. Moorbath & Thompson, 1980; Sinton & Byerly, 1980; Dickin, 1981); and selective cation diffusion and isotopic tracer diffusion between coexisting silicate melts (Baker, 1989, 1991). However, direct evidence of the processes involved is rarely preserved and is difficult to assess from studies of the bulk-rock chemistry of magmatic rocks alone.

Crustal xenoliths within volcanic or rapidly cooled hypabyssal rocks provide an opportunity to study the small-scale interaction between magmas and country rocks, and, although secondary to the purpose of this paper, how these processes may contribute to the large-scale contamination of a suite of magmas. This study concentrates on the geochemical, isotopic and textural variations found in a suite of peraluminous and siliceous upper-crustal xenoliths within basic–silicic minor intrusions in part of the British Tertiary Igneous Province (BTIP). Many of the peraluminous xenoliths preserve a high percentage of fresh glass, and, as such, identification of their protoliths is problematical, but central to the understanding of the processes that occurred during their formation. Although such xenolith types are not usually found within basaltic rocks, those preserved within the Loch Scridain Sill Complex provide an important record of the processes by which the geochemistry of basaltic magmas becomes contaminated by common crustal lithologies.

	Geological Setting

TOP ABSTRACT Introduction Geological Setting Geochemical Techniques The Crustal Xenoliths Contact Metamorphic Effects Geochemistry and Petrogenesis of... Crustal Xenolith Mineralogy and... Origin Of The Aluminous... Magma-Xenolith Interactions in... Implications For Contamination... Final Comments References

 
The central igneous complex on the Isle of Mull is one of a number of intrusive centres in the BTIP, which extends from St Kilda, south along the west coast of Scotland, into Ireland. The basaltic magmatism in the North Atlantic region is believed to have been in response to the impact of a mantle plume at between 65 and 62 Ma (Thirlwall et al., 1994; Morton et al., 1995). Within the BTIP, previous lithospheric stretching in the Mesozoic is thought to have created ‘thinspots’, which later acted as foci for the basaltic magmatism (Thompson & Gibson, 1991). The Loch Scridain Sill Complex (LSSC; Preston et al., 1998) is a suite of high-level, sub-volcanic inclined minor intrusions that invade the Palaeogene (Mussett, 1986; Bell & Jolley, 1997) lava field and underlying Mesozoic sedimentary rocks and Proterozoic (1 Ga) metasedimentary rocks of the Moine Supergroup (Fig. 1). The Moine Supergroup exposed south-west of the Assapol Fault (Fig. 1) comprises garnetiferous schists, which locally contain kyanite and staurolite (the Assapol Group; Holdsworth et al., 1987), along with more psammitic rocks (the Shiaba Group; Holdsworth et al., 1987).

View larger version (32K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 1. Map showing geology of Ross of Mull, and location of the sheets of the Loch Scridain Sill Complex (LSSC). Distribution of xenolithic sheets after Bailey et al., (1924), with number of sheets depicted in suite being reduced for clarity. GGF and MTZ represent possible traces of the Great Glen Fault and the Moine Thrust Zone, respectively. Locations of samples mentioned in the text and figures are shown by the following abbreviations: KB, Kilfinichen Bay; KI, Killunaig; PM, Port Mor; TM, Torr Mor.

 
The basement structure of the Isle of Mull is complicated, and the Moine Supergroup, which crops out in the south-west, is likely to represent a thin crustal slice brought into position along the Moine Thrust Zone (Holdsworth et al., 1987; Potts et al., 1995). The Great Glen Fault also passes through the south-east part of Mull, and Late Proterozoic Dalradian metasedimentary rocks are likely to form the basement just to the south of the island. The Moine rocks are probably underlain by high-grade gneisses, which crop out on Iona (Fig. 1), and which may belong to the Archaean Lewisian Complex. These consist predominantly of granulite and amphibolite facies orthogneisses, although paragneisses are also present. The various crystalline basement rocks are overlain by a thin sequence of Mesozoic (Triassic–Cretaceous) clastic and carbonate sedimentary rocks.

The compositions of the Loch Scridain sheets range from tholeiitic basalt, through tholeiitic andesite and dacite, to rhyolite. Preston et al. (1998) have shown that the basic, intermediate and silicic magmas evolved via separate, although intimately linked, magmatic processes, which will be summarized below.

Many of the sheets are highly xenolithic, with a wide variety of crustal xenolith types preserved in various stages of partial fusion and reaction with their host. The majority of the ‘recognizable’ crustal xenoliths have been derived from the Moine metasedimentary rocks, although some sheets contain sandstone and conglomerate xenoliths from the local Mesozoic succession. The crustal xenoliths have previously been described byThomas, (1922) and Bailey et al. (1924), and their geochemistry has been discussed by Kille (1987). The xenoliths are found as isolated blocks, or in xenolith-rich pods and lenses, situated towards the top surfaces of the sheets, suggesting that they were typically less dense than their host magmas. Most of the xenoliths are 10–60 cm across, although rafts of material several metres across have been found. Contacts between the xenoliths and host basalt are sharp.

	Geochemical Techniques

TOP ABSTRACT Introduction Geological Setting Geochemical Techniques The Crustal Xenoliths Contact Metamorphic Effects Geochemistry and Petrogenesis of... Crustal Xenolith Mineralogy and... Origin Of The Aluminous... Magma-Xenolith Interactions in... Implications For Contamination... Final Comments References

 
All mineral analyses were carried out on a Cameca SX-50 electron probe micro-analyser with four wavelength spectrometers, at the University of Glasgow. Typical working conditions were 15 kV and 20 nA, with a beam size of 1–3 µm. Standards comprised a series of pure elements and compound standards supplied by Cameca and Micro Analysis Consultants Ltd.

Whole-rock major and trace element analyses were carried out using a Philips PW 1480 automatic X-ray fluorescence spectrometer at the University of Edinburgh, and using an ARL8410 fully automatic, sequential, wavelength dispersive X-ray fluorescence spectrometer at the Department of Geology and Petroleum Geology, University of Aberdeen. Sample preparation, accuracy and precision of the analyses have been described by Fitton & Dunlop (1985) and by Rice et al. (1995). Whole-rock rare earth element (REE) analyses were carried out at Royal Holloway and Bedford New College by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Details of the ion-exchange separation technique of the REE, together with running procedures, accuracy, and precision of the ICP-AES have been described by Walsh et al. (1981).

Rb–Sr and Sm–Nd isotopic data were determined at the Scottish Universities Research and Reactor Centre (SURRC), East Kilbride. Details of ion-exchange separation techniques have been reported by Barbero et al. (1995). Sr, Nd and Sm isotope analyses were performed on a VG Sector 54–30 thermal ionization mass spectrometer, whereas Rb analyses were carried out using a VG MM30 thermal ionization mass spectrometer. During the course of this study, the JM Nd standard gave ¹⁴³Nd/¹⁴⁴Nd = 0.511500 ± 10 (2 SD), and repeat analyses of NBS 987 gave ⁸⁷Sr/⁸⁶Sr = 0.710236 ± 19 (2 SD). All Sr–Nd isotope data quoted in this paper are initial ratios calculated to an age of 55 Ma (Bell & Jolley, 1997).

	The Crustal Xenoliths

TOP ABSTRACT Introduction Geological Setting Geochemical Techniques The Crustal Xenoliths Contact Metamorphic Effects Geochemistry and Petrogenesis of... Crustal Xenolith Mineralogy and... Origin Of The Aluminous... Magma-Xenolith Interactions in... Implications For Contamination... Final Comments References

 
The crustal xenoliths of the LSSC can be subdivided into two broad groups: (1) siliceous xenoliths; (2) aluminous xenoliths. Each group can be further subdivided on the basis of mineralogy, structure, texture and probable origin. The detailed mineralogy and mineral chemistry of the xenoliths are dealt with in a subsequent section.

Siliceous xenoliths
Highly siliceous xenoliths are ubiquitous within the Loch Scridain sheets. They generally form angular blocks a few tens of centimetres across. As they contain a very high percentage of quartz, the effects of high-temperature metamorphism are limited, and it is possible to match these xenoliths with their source lithologies. The majority of the quartz-rich xenoliths are thought to have been derived from the psammitic lithologies within the Moine Supergroup, which consist largely of polycrystalline quartz. A few sheets also contain sandstone and conglomerate xenoliths derived from the local Mesozoic sedimentary rocks; these typically retain a fine grain size, and as they originally contained a higher proportion of feldspar and mica than the Moine psammites have therefore reacted to a greater extent, producing pockets of silicic partial melts, now quenched to glass.

Aluminous xenoliths
The aluminous xenoliths are the most unusual of the xenolith types, with most being completely glassy (buchites). There are three distinct varieties:

1. mullite buchites, consisting of a mass of mullite (3Al₂O₃.2SiO₂) needles set in a clear glass;
   
2. cordierite buchites, consisting of small crystals of cordierite and mullite needles in a clear glass;
   
3. plagioclase-rimmed mullite buchites, consisting of a core of mullite-rich glass surrounded by a thick rim of coarse-grained plagioclase.
   

The glassy nature of the aluminous xenoliths suggests that they originated by the melting of their protolith, and so identifying their exact origins is difficult. However, the aluminous xenoliths preserve abundant evidence for extensive interaction and reaction with basic magma, and therefore deducing the origins of these xenoliths is important to understanding contamination processes that affected the petrogenesis of the LSSC.

Many different crustal protoliths have been suggested for the aluminous xenoliths of the LSSC, with most theories being based upon trying to match bulk-rock compositions of possible protoliths to those of the xenoliths. These include: aluminous fireclay (Thomas, 1922); aluminous bole (Agrell & Langley, 1958); Jurassic shale (Cameron, 1979; Kille, 1987); and Moine pelitic schist (Cameron, 1976; Thompson et al., 1986). The highly aluminous nature of the glasses, and of the crystalline phases within these xenoliths, is consistent with their derivation from clay-rich sediment, or its metamorphic equivalent (see Thomas, 1922). Unfortunately, the bulk compositions of the xenoliths do not match any of these possible protoliths. However, as we will show, indications of their origin may be obtained from particular aspects of their geochemistry and an examination of the contact metamorphism adjacent to the basic sheets.

	Contact Metamorphic Effects

TOP ABSTRACT Introduction Geological Setting Geochemical Techniques The Crustal Xenoliths Contact Metamorphic Effects Geochemistry and Petrogenesis of... Crustal Xenolith Mineralogy and... Origin Of The Aluminous... Magma-Xenolith Interactions in... Implications For Contamination... Final Comments References

 
Typically, none of the country rocks into which the sheets are emplaced shows signs of metamorphism by the basic magmas, with one notable exception. At Traigh Bhàn na Sgurra (Fig. 1), the Moine schists of the Assapol Group are intruded by a relatively thick (2–6 m), flat-lying basaltic sheet that has numerous xenoliths of mullite buchite ‘ponded’ along the upper contact. Locally, the schists have been strongly metamorphosed and melted in an ‘aureole’ up to 40 cm from the contact with the basaltic sheet (Kille et al., 1986; Preston, 1996). The initial stages of this high-temperature metamorphism involve the breakdown of muscovite and biotite (Brearley, 1986). The sites of original muscovite and biotite crystals are represented by numerous tiny crystals of Fe-rich pleonaste spinel, ilmenite, and new biotite crystals set in K-feldspar-bearing, peraluminous glass. Needles of mullite are also present. The glasses have near-minimum melt granitic compositions (Preston, 1996), similar to the rhyolitic sheets of the LSSC (Preston et al., 1998). At higher degrees of partial melting, pools of melt develop at quartz-feldspar interfaces. Initiation of quartz-feldspar breakdown would have been facilitated by the release of alkalis and volatiles from mica dehydration reactions (Grapes, 1986). The ‘melt’ is typically represented by a microcrystalline mixture of K-feldspar, quartz and minor plagioclase. Edges of original plagioclase take on a ‘streaky’ appearance, which tends to blend in with the surrounding recrystallized melt, and their interiors have a mottled texture. Garnet porphyroblasts are replaced by a mixture of spinel, magnetite–ilmenite and alkali feldspar. Such evidence of high-temperature metamorphism is restricted to localities within the Moine schists where the basic sheets lack a chilled margin. No thermal metamorphic effects are observed adjacent to the chilled margins of the sheet. This evidence suggests that melting of Moine schists occurred in localized zones, probably linked to turbulent flow within the magma conduits (Kille et al., 1986). It also confirms that the country rock adjacent to the sheets may have been significantly modified by the removal of granitic melts.

	Geochemistry and Petrogenesis of the Lssc

TOP ABSTRACT Introduction Geological Setting Geochemical Techniques The Crustal Xenoliths Contact Metamorphic Effects Geochemistry and Petrogenesis of... Crustal Xenolith Mineralogy and... Origin Of The Aluminous... Magma-Xenolith Interactions in... Implications For Contamination... Final Comments References

 
The geochemistry of the sheets has been described in detail by Preston (1996) and Preston et al. (1998), and is summarized here. The major-element characteristics of the suite allow a tripartite subdivision (Fig. 2; after Preston et al., 1998):

View larger version (13K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 2. Plot of wt % TiO2 vs mg-number for the LSSC. Tripartite subdivision after Preston et al. (1998).

 
Group I. Tholeiitic basalts and basaltic andesites are dominated by fractional crystallization processes, but their high incompatible element contents (e.g. Zr = 50–175 ppm; Ce = 10–60 ppm) and variable isotopic ratios (e.g. ⁸⁷Sr/⁸⁶Sr₅₅ = 0.7038–0.7154) require upper-crustal contamination, via AFC processes. Modelling has shown that the contaminant was a high-Nd silicic melt probably derived from the Moine basement rocks (Preston et al., 1998). In addition, the general increase in Ti with degree of fractionation over and above that possible by fractional crystallization alone, and lack of cumulative titanomagnetite, require that certain batches of magma also have interacted with a Ti-enriched contaminant.

Group II. Tholeiitic andesites and porphyritic dacites represent mixtures between silicic (Group III) and basic (Group I) magmas, which evolved further by limited fractional crystallization of plagioclase and pigeonite.

Group III. Peraluminous rhyolites represent granitic partial melts of upper-crustal lithologies, such as the metasediments of the Moine Supergroup, and are strongly enriched in incompatible elements and have high initial Sr isotope ratios (⁸⁷Sr/⁸⁶Sr₅₅ = 0.7173–0.7204). However, the relatively high Fe contents of these rhyolites suggest that the granitic melts mixed with small quantities of basic magma.

	Crustal Xenolith Mineralogy and Mineral Chemistry

TOP ABSTRACT Introduction Geological Setting Geochemical Techniques The Crustal Xenoliths Contact Metamorphic Effects Geochemistry and Petrogenesis of... Crustal Xenolith Mineralogy and... Origin Of The Aluminous... Magma-Xenolith Interactions in... Implications For Contamination... Final Comments References

 
Siliceous xenoliths
Quartzites and psammites probably derived from the Moine Supergroup dominate the siliceous xenolith suite. The quartzites consist of 100% coarse-grained (up to 5 mm), strained quartz, as irregularly shaped interlocking grains.

A small number of banded quartzo-feldspathic xenoliths also occur. These consist of highly corroded quartz grains that show undulose extinction, along with plagioclase (An₂₈) and minor K-feldspar. These xenoliths may represent disaggregated fragments of original quartz-mica schists, which are common in the Moine of Mull. Where quartz and feldspar were originally in contact, channels of melt have developed, which are now represented by fan spherulites of alkali feldspar (Or₅₈), often nucleated on remnant feldspar grains, along with an Na-rich granitic devitrified glass. The dark bands within these xenoliths consist of minute (1–4 µm) octahedra of hercynite [(Mg_0.51Fe²⁺_0.49)(Al_1.96Fe³⁺_0.04)O₄] and ilmenite, with plates of new Ti-poor biotite in a mixture of microcrystalline K-feldspar (Or₅₈), plagioclase (An₄₂) and granitic glass. These dark bands are interpreted as representing the high-temperature breakdown products of muscovite and biotite. Aggregates of ilmenite-magnetite and spinel may also represent sites of former garnet porphyroblasts. Brearley (1986) found an identical assemblage in partially digested xenoliths from the Traigh Bhàn na Sgurra sheet, and Brearley & Rubie (1990) have synthesized similar assemblages during the experimental disequilibrium breakdown of mixtures of muscovite and quartz.

Small xenoliths of Mesozoic sandstone are relatively common in the LSSC. Compared with their unmodified parent rocks, these xenoliths have lost all of their original feldspar and mica, and quartz grains are considerably corroded with fringes of tabular inverted tridymite. The products of partial fusion are now represented by pools of cryptocrystalline K-feldspar and quartz in association with zoned clinopyroxene.

One feature common to many of the siliceous xenoliths is a coarse-grained clinopyroxene corona at the contact between the xenoliths and host basalt. The heat required to melt and assimilate minerals that would precipitate from the magma only at a late stage (e.g. quartz) is gained by the precipitation of phases with which the magma is currently saturated (Bowen, 1928). During experimental work, Sato (1975) found a complex zonation of reaction products around quartz xenocrysts in basalts and andesites. Around the unfused quartz, alkalis and Al were preferentially concentrated and a zone of glass and Ca-rich pyroxene was formed, followed by a zone composed entirely of Ca-rich pyroxene. Plagioclase rarely appears in these coronas, even if the magma is saturated with this phase and precipitating it elsewhere; the absence of plagioclase is probably controlled by the availability of aluminium. The host basalts and basaltic andesites (Group I) are not olivine bearing (Preston et al., 1998), and so orthopyroxene, a common product of the interaction of silicic xenoliths with olivine-saturated basic magmas, also does not occur in the coronas around the LSSC quartzose xenoliths.

Aluminous xenoliths
Aluminous xenoliths are found in sheets belonging to Groups I and II, throughout the Loch Scridain district, in association with the quartz-rich xenoliths. The mineral textures, and the small-scale geochemical heterogeneities preserved within the aluminous xenoliths, particularly the mullite buchites with plagioclase rims, provide the most useful information concerning magma–xenolith interactions. Fresh quenched glass has been preserved in all types of buchites, although some show evidence of late alteration, which has resulted in crystallization of alkali feldspar. Table 2 lists modal analyses from a number of the aluminous xenoliths.

View this table: [in this window] [in a new window]   Table 2: Mineralogical modes (values are percentages) from a number of mullite buchites, a cordierite buchite, and a plagioclase-rimmed mullite buchite

 
Mullite buchites
The mullite buchites are glassy rocks, which contain rare crystals of corundum, and range in colour from dark grey, through lilac, to almost white. They form irregularly shaped masses, which vary in size from 10 cm across, to large rafts several metres in length. Vesicles filled with fibrous zeolite are common. The mullite buchites consist of 60–75 vol. % Al-rich glass containing a mass of randomly oriented needles of mullite, which typically have dimensions of 50 µm x 2;µm, although some are up to 200 µm in length. The mullites correspond closely to the ideal formula, although most contain appreciable amounts of TiO₂ (0.4–1.4 wt %) and FeO (0.3 wt %) (Table 3). Sillimanite has also been found to coexist with mullite in a few of these xenoliths (Cameron, 1976). Small octahedra (<20 µm) of ilmenite and magnetite are found scattered throughout the glass.

View this table: [in this window] [in a new window]   Table 3: Representative mullite, cordierite and spinel analyses from buchite xenoliths

 
Cordierite (mullite) buchites
The cordierite-bearing buchites are rarer than the mullite buchites. They are black, vitreous rocks, which also form irregularly shaped masses, up to a metre in length. The cordierite buchites (Fig. 3) consist of a dense mat of mullite needles (25 vol. %), along with numerous tiny crystals of cordierite (14 vol. %), and rarer anhedra of tridymite, set in a clear to red–brown glass. The cordierite forms minute (10–20 µm), occasionally twinned, square crystals, which occur either singly or in clusters. The cordierites show little chemical variation [Mg/(Mg + Fe) = 0.41–0.42; Table 3), and no zoning (see Graham et al., 1988). Ragged grains of magnetite (<30 µm) can be found throughout the glass. When compared with the mullite buchites, both glass and whole-rock analyses (Table 1) from the cordierite buchite are enriched in SiO₂ and total Fe, although they are not as Al rich.

View larger version (69K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 3. Photomicrograph of a cordierite buchite from Kilfinichen Bay (KBAX1) showing needles of mullite and cubes of cordierite set in clear brown glass [plane-polarized light (ppl); field of view 0.8 mm x 1.2 mm].

 

View this table: [in this window] [in a new window]   Table 1: Representative whole-rock (WR) XRF (major elements and trace elements except the REE) and ICP-MS (REE) analyses of various buchites together with associated electron microprobe (EM) analyses of the glasses

 
Plagioclase-rimmed mullite buchites
Many of the smaller mullite buchite xenoliths (up to 80 cm diameter) possess a thick crystalline rim, composed mainly of plagioclase with numerous mullite inclusions, along with small inclusions of corundum and spinel. The glasses within these xenoliths have a distinctive, although varied, chemistry, particularly their relatively high K₂O and low FeO concentrations (Table 1). The plagioclase-dominated rim very often forms >80% (by volume) of these xenoliths. It is interesting to note that the plagioclase-rimmed mullite buchites also have low Sr concentrations when compared with unrimmed mullite and cordierite buchites (Fig. 4). This depletion in Sr is thought to result from the precipitation of the plagioclase palisade. The plagioclases are idiomorphic, particularly where in contact with the central mullite buchite, and typically have a lilac coloration and contain numerous small (1–4 mm), dark blue crystals of corundum (sapphire). Spinel is found throughout the rim, but is more concentrated towards the contact with the host basalt, where the plagioclase is vitreous black or dark green. A sketched cross-section of a plagioclase-rimmed mullite buchite is presented in Fig. 5, showing the major textural features.

View larger version (18K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 4. Multi-element plot showing the chemistry of various mullite and cordierite buchites. Also shown is a modelled restite composition, representing the extraction of 30% Group III average rhyolite from an average local Moine pelitic schist. Compositions have been normalized to that of average local Moine pelitic schist calculated from the analyses presented by Preston et al. (1998).

 

View larger version (41K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 5. Sketch showing the major textural relationships within a section through a plagioclase-rimmed mullite buchite xenolith (sample PMFX1) within a Group I basaltic andesite.

 
Mullite
Randomly oriented needles of mullite (50 µm x 2 µm) occur within the glassy cores of the xenoliths; included within plagioclase towards the centre of the xenoliths (Fig. 6); and projecting from the plagioclase into the glassy cores of the xenoliths. This confirms that the mullite was present within the melt at an early stage (see Cameron, 1976), and shows that it has not exsolved from a plagioclase enriched in Si and Al (see Sturt, 1970). Towards the contact with the host basalt, there is a zone within the plagioclase rim in which the mullite inclusions appear to be dissolving, followed by a zone immediately adjacent to the host basalt where all plagioclase is free of mullite inclusions. Also, around included basaltic melt pockets (see below) the plagioclase is completely free of mullite needles.

View larger version (49K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 6. Photomicrograph of plagioclase containing numerous mullite inclusions adjacent to mullite buchite (M). Mullite needles project from the plagioclase into the buchite (arrowed). Where basaltic melts (B) containing quench plagioclase have penetrated the coarse-grained plagioclase along cracks and grain boundaries, plagioclase becomes free of mullite inclusions. (ppl; field of view 2 mm x 3 mm; sample PMFX1).

 
Corundum
Corundum occurs within the cores of these xenoliths, typically as inclusions within mullite-free plagioclase immediately adjacent to the glassy cores and close to, or within basaltic melt pockets (Fig. 5; see below). The corundum occurs as euhedral prisms with a bladed habit, although skeletal forms (Fig. 7) are also common. Analysed corundums contain up 0.47 wt % FeO, 0.29 wt % TiO₂ and 0.25 wt % Cr₂O₃ (Preston, 1996; see Brearley, 1986).

View larger version (58K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 7. Photomicrograph showing skeletal corundum crystal (C) surrounded by, and intergrown with, mullite-free plagioclase (P). Dark areas are melt pockets containing quench plagioclase (QP). From near the buchite core of a plagioclase-rimmed xenolith (PMFX1) (ppl; field of view 2 mm x 3 mm).

 
Spinel
Spinel is concentrated mainly towards the contact with the host basalt, and occurs as small octahedra (<0.5 mm), and larger rounded and irregularly shaped crystals, generally no more than 2 mm in length (see Fig. 5). Skeletal forms are common, with crystals often displaying cores infilled with plagioclase. The spinels vary greatly in colour, from deep olive green, through dark brown to purple, although those adjacent to the basalt are typically opaque. Towards the central parts of these xenoliths, pale brown to yellow spinel replaces corundum (Fig. 8); this appears to be associated with the ingress of basaltic magma into the xenolith, as those parts of the corundum undergoing replacement lie immediately adjacent to invasive basaltic melt pockets. In certain xenoliths, skeletal spinels are armoured by finely crystalline, unzoned plagioclase, suggesting a reaction relationship between the spinel and the liquid from which it crystallized. Pedersen, (1978, 1979) has described similar spinel–corundum and spinel–plagioclase associations in reconstituted shale xenoliths from Disko Island, central West Greenland.

View larger version (61K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 8. Photomicrograph showing the replacement of corundum (C) by darker spinel (S) adjacent to basaltic melt pockets. Plagioclase (P) immediately adjacent to corundum, spinel and dark basaltic melt pockets is free of inclusions of mullite needles (ppl; field of view 2 mm x 3 mm; sample PMFX1).

 
The spinels are usually unzoned, and are Mg-rich pleonastes of a restricted composition, (Mg_0.76–0.71Fe²⁺_0.24–0.29)(Al_1.91–1.97Fe³⁺_0.09–0.03)O₄ (Table 3). However, where the spinel replaces corundum, the spinel closest to the remnant corundum is enriched in Al₂O₃ and FeO. There is a systematic variation in spinel composition within any one xenolith, with those nearer the buchite core being richer in Al and Mg than those nearer the contact with the host basalt (Fig. 9a–d). This variation is independent of proximity to the melt pockets.

View larger version (17K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 9. (a–d) Plots showing variation in spinel composition within plagioclase palisade as a function of distance from the contact with the host basalt. Textural evidence suggests that this compositional gradient was established as a late-stage, solid-state alteration of spinel compositions (see text for details).

 
Melt pockets
Pockets of quenched basaltic melt are trapped between the plagioclase laths (Figs 5 and 10) and within the buchite cores. These consist of microlites of plagioclase and clinopyroxene (<200 µm), along with subordinate Fe–Ti oxides, K-feldspar and glass. The composition of these melt pockets varies according to their position within the xenolith such that those adjacent to the buchite core are enriched in elements such as Al and Si and depleted in Mg and Ca (Fig. 11). The composition of the plagioclase and clinopyroxene, and the amount of K-feldspar present, also vary between different melt pockets according to the position of the melt pocket relative to the outer contact with the basalt. The plagioclase varies from An₆₀ in the melt pockets nearer the basalt contact to An₄₀ in melt pockets nearer the glassy core, and the pyroxene from Wo₃₀En₄₀Fs₃₀ (ferroaugite) to Wo₈En₄₂Fs₅₀ (ferropigeonite) (Preston, 1996).

View larger version (65K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 10. Back-scattered electron image showing the development of a thin Na-rich plagioclase (dark) rim adjacent to basaltic melt pockets (centre). This plagioclase has the same composition as the quench plagioclase in the melt pockets. Bright areas within the melt pockets represent pyroxenes (field of view 400 µm x 600 µm; sample TM1F).

 

View larger version (13K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 11. Plot showing estimated composition of basaltic melt pockets as a function of location within the plagioclase rim. Compositions estimated by scanning electron microscopy (energy dispersive system) analysis of low magnification area of melt pocket (i.e. including feldspars and pyroxenes within the melt).

 
Plagioclase
The bulk of the palisade around the aluminous buchite xenoliths is made of calcic plagioclase, which displays a variety of textures indicative of a complex growth history. Towards the core of the xenoliths the plagioclase forms large (5–8 mm) plates typically elongate perpendicular to the glassy cores. Closer to the host magma contact, the plagioclase is finer grained (1–3 mm), and forms an interlocking mosaic of randomly oriented plates.

The plagioclase varies from An₈₇ to An₆₀ (Table 4), the more calcic plagioclase generally being found closer to the enclosing basalt, and forming rims around spinel and corundum. However, the variation in plagioclase compositions in relation to their position in the rim is not as systematic as that shown by the spinels. The plagioclase closest to the buchite core is also unusually enriched in TiO₂ (up to 0.22 wt %; Table 4). Many crystals in contact with the host basalt display well-developed oscillatory zoning, the extent of which can be of the order of 20 mol % variation in the An content of the plagioclase. This zoning suggests that the plagioclase initially crystallized from a melt, rather than being a restite phase (see Van Bergen & Barton, 1984; Grapes, 1986, 1991). The zoning is less well developed adjacent to the cores of the buchites, although the presence of abundant mullite inclusions suggests that the plagioclase adjacent to the buchite core also crystallized from a melt. The groundmass plagioclase within the host basalt typically has a composition of An_60–50, although occasional small (up to 2 mm) xenocrysts with resorbed crystal outlines have core compositions up to An₈₂ (Preston et al., 1998). Where basaltic melts have infiltrated into the plagioclase rim along cracks and around larger melt pockets, plagioclase free of mullite inclusions is present (Fig. 6). The mullite-free plagioclase is typically slightly more calcic (up to 1–2 mol % An) and, where in contact with melt pockets, may have a narrow sodic overgrowth of a composition similar to that of the quench-feldspar of the basaltic melt (Fig. 10, Table 4).

View this table: [in this window] [in a new window]   Table 4: Representative plagioclase analyses from a plagioclase-rimmed mullite buchite from Port Mor (PMFX1) [NM 435239]

 
The plagioclases in many xenoliths show evidence of extensive recrystallization, particularly close to the basalt contacts. Fine-grained plagioclase (An_82–87) may form rims around spinel and corundum. This plagioclase is probably the product of reaction between the spinel, corundum and aluminous liquid, in response to the influx of basaltic magma into the plagioclase rim. The plagioclase adjacent to the basalt host often has a very distinctive ‘fingerprint’ texture, indicative of reheating after crystallization (Fig. 12) (e.g. Kaczor et al., 1988; Johannes & Holtz, 1992; Philpotts & Asher, 1993).

View larger version (59K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 12. Back-scattered electron image of fingerprint texture within plagioclase. Extensive melting of the plagioclase has occurred with separation of calcic (narrow, bright rims) and sodic (bulk of feldspar) plagioclase, and the development of large pools of potassic melt (dark areas) (field of view 2 mm x 3 mm; sample PM1F).

 
REE and isotope geochemistry of the aluminous xenoliths
Chondrite-normalized REE profiles for an unrimmed mullite buchite, a cordierite buchite and the various components of a plagioclase-rimmed mullite buchite, together with the range of REE profiles displayed by Group I of the LSSC and by local Moine pelitic schists are shown in Fig. 13. As both spinel and corundum are likely to contain negligible amounts of the REE, the profiles for the xenolith rims are considered to be derived predominantly from the plagioclase, with possibly a small contribution from associated basaltic melt pockets. All samples are light REE (LREE) enriched (Table 1). The buchite core of the plagioclase-rimmed xenolith has (Ce/Yb)_N = 1.8, a marked negative Eu anomaly (Eu/Eu* = 0.05), and is notable for the relative enrichment in heavy REE (HREE). The plagioclase adjacent to the buchite core has (Ce/Yb)_N = 12.9, a negative Eu anomaly (Eu/Eux = 0.17), and also has the highest total content of REE. The plagioclase next to the basalt contact is also strongly LREE enriched [(Ce/Yb)_N = 13.5], but has significantly lower total REE contents, with no Euanomaly. The most contaminated Group I basalt is an evolved basaltic andesite with LREE enrichment [(Ce/Yb)_N = 6.5] and a slight negative Eu anomaly (Eu/Eux = 0.75).

View larger version (38K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 13. Chondrite-normalized REE profiles for a mullite buchite (PMAX2), a cordierite buchite (KBAX1) and the components of a plagioclase-rimmed mullite buchite (KIAX2), together with the range of REE profiles for local Moine pelitic schists and basalts and basaltic andesites of Group I (Preston et al., 1998).

 
One plagioclase-rimmed mullite buchite (PMFX1, Fig. 5) has been sampled in detail for Sr and Nd isotope data (Table 5; Fig. 14); all Sr and Nd initial ratios have been calculated assuming an age of 55 Ma (Bell & Jolley, 1997). Assuming that the protolith for the aluminous xenoliths is an upper-crustal lithology, such as a Moine schist, any melts derived from this source would be expected to have high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd if in isotopic equilibrium with the source; values from two local Moine schists are about (⁸⁷Sr/⁸⁶Sr)₅₅ = 0.729, and (¹⁴³Nd/¹⁴⁴Nd)₅₅ = 0.5117 (Preston, 1996; Preston et al., 1998). The plagioclase has (⁸⁷Sr/⁸⁶Sr)₅₅ ratios that lie between 0.7136 and 0.7147, which are not consistent with the derivation of the aluminous liquids in equilibrium with the metasediments. However, many field and experimental studies into crustal anatexis have clearly demonstrated that both elemental and isotopic disequilibrium can occur during melting depending upon which phases are involved in the melting process, and upon the rates of heating and melt extraction (e.g. Kaczor et al., 1988; Brearley & Rubie, 1990; Hammouda et al., 1994; Barbero et al., 1995; Knesel & Davidson, 1996). In general, early anatectic melts dominated by the breakdown of high-Rb/Sr phases such as muscovite, biotite and amphibole will have high initial Sr isotope ratios; later melts are likely to be controlled by feldspar and quartz dissolution and will generally have lower initial Sr isotope ratios (Hammouda et al., 1994; Knesel & Davidson, 1996). Thus, Moine schists could plausibly yield aluminous melts with initial Sr isotopic compositions different from those of the bulk rock. Interaction between such melts and basaltic magma would yield a range of initial Sr isotopic compositions, potentially as low as that of the uncontaminated basaltic magma (0.7038; Preston et al., 1998). Table 5 gives isotopic data from buchites, two samples from PMFX1 with a plagioclase rim, and one from an unrimmed mullite buchite. Unfortunately, fresh glass is rarely preserved within PMFX1 and this may account for the variation in the Sr isotope ratios from this buchite; however, the initial Sr isotope ratios of these, and the relatively fresh glass within the unrimmed sample, are much lower than would be expected for material exclusively derived from old crustal rocks, and much lower than the plagioclase that crystallized from these melts. The (⁸⁷Sr/⁸⁶Sr)₅₅ ratios of the basaltic melt pockets (0.7126–0.7129) lie between those of the host basalt (0.7104) and the plagioclase (0.7136–0.7147).

View this table: [in this window] [in a new window]   Table 5: Sr- and Nd-isotopic compositions of a plagioclase-rimmed mullite buchite (PMFX1) and an unrimmed mullite buchite (PL1) from Port Mor [NM 435239]

 

View larger version (23K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 14. (87Sr/86Sr)55 vs (143Nd/144Nd)55 isotope diagram for the components of a plagioclase-rimmed mullite buchite, together with the data arrays for Groups I, II and III of the LSSC and local Moine pelitic schists (after Preston et al., 1998). Initial Sr and Nd isotope ratios calculated assuming an age of 55 Ma (Bell & Jolley, 1997).

 
The Nd isotope ratios of all components from the aluminous xenoliths are relatively similar [(¹⁴³Nd/¹⁴⁴Nd)₅₅ = 0.5118–0.5119], and appear not to have been disturbed by late alteration. Preston & Bell, (1997) and Preston et al., (1998) showed that during the contamination of the LSSC, the Nd-isotope characteristics of the basic magmas were buffered at an early stage at values similar to those from the Moine basement rocks [(¹⁴³Nd/¹⁴⁴Nd)₅₅ 0.5117]. Barbero et al. (1995) showed that disequilibrium melting produced only small changes in ¹⁴³Nd/¹⁴⁴Nd between source and melt. As in the case of Sr isotopes, the direction of change in ¹⁴³Nd/¹⁴⁴Nd will depend on the mineralogy of phases participating in the melting reactions and on the ability of the melt to segregate from the residue. The small variations in (¹⁴³Nd/¹⁴⁴Nd)₅₅ between the aluminous melts and the metasediments is therefore consistent with the derivation of the melt from Moine-like continental crust.

	Origin Of The Aluminous Xenoliths

TOP ABSTRACT Introduction Geological Setting Geochemical Techniques The Crustal Xenoliths Contact Metamorphic Effects Geochemistry and Petrogenesis of... Crustal Xenolith Mineralogy and... Origin Of The Aluminous... Magma-Xenolith Interactions in... Implications For Contamination... Final Comments References

 
Possible protoliths for the mullite buchites
A number of potentially suitable protoliths for the aluminous xenoliths exist in the vicinity of Loch Scridain. Aluminous boles and ‘fireclays’ are present as lateritized ash and claystone layers within the Palaeogene lava field (Bailey et al., 1924; Emeleus et al., 1996). However, such rocks are typically very iron rich (e.g. Kille, 1987; Bell et al., 1996), and total iron is also likely to be enriched in the residue during partial melt extraction processes (e.g. MacRae & Nesbitt, 1980; Grapes, 1986; Graham et al., 1988). If the mullite buchites represent melted residues, their relatively Fe-poor compositions (Table 1) suggest that boles would be unlikely protoliths. Jurassic shales, exposed in south Mull, also provide a possible protolith; however, these were dismissed by Kille (1987) on the basis of their high MgO, Fe₂O₃ and K₂O/Na₂O values. An additional problem with these protoliths is that mullite buchites occur within the sheets that intrude the Moine Supergroup, structurally below the Mesozoic and Palaeogene rocks, whereas Mesozoic sandstone xenoliths do not occur at this low structural level. Dalradian metasediments cannot be ruled out as a source for the aluminous xenoliths. They cannot be excluded in terms of geochemistry, being very similar in composition to the Moine metasediments (e.g. Dickin et al., 1981); however, their structural position in relation to the central igneous complex (Fig. 1) may place doubts on their suitability. The Lewisian gneisses exposed in the Hebridean area are primarily granitic orthogneisses and metabasites, although metasediments are represented in some areas (Fettes et al., 1992). Theoretically, the orthogneisses might be suitable for providing granitic melts for the contamination of the LSSC basic magmas, although they are unlikely to also provide a source for the aluminous melts. Preston et al., (1998) has provided Pb isotope data for a number of basaltic andesite sheets from the LSSC. The ²⁰⁶Pb/²⁰⁴Pb ratios of these sheets are high, ranging from 17.9 to 18.9; Lewisian material has lower ²⁰⁶Pb/²⁰⁴Pb values of between 14 and 17 (Dickin, 1981); Moine metasediments have ²⁰⁶Pb/²⁰⁴Pb values >18 (Thompson et al., 1986; Preston et al., 1998). This, coupled with their high (⁸⁷Sr/⁸⁶Sr)₅₅ (up to 0.7154) and low (¹⁴³Nd/¹⁴⁴Nd)₅₅ (0.5119), shows therefore that the sole recognizable contaminant in the LSSC basic magmas was derived from the Moine schists. This contrasts strongly with the geochemistry of a number of the basal basaltic flows on Mull (the so-called Staffa Magma Type lavas), which show isotopic evidence for having been contaminated with both Lewisian granulite-facies and Moine metasedimentary material (Morrison et al., 1985; Thompson et al., 1986). As the LSSC magmas show no hint of Lewisian involvement in their petrogenesis (Preston et al., 1998), and it is reasonable to assume that the xenoliths represent material that contaminated the basic magmas, we conclude therefore that the Al-rich pelitic schists of the Moine Supergroup represent the protolith for the aluminous xenoliths. These rocks, which preserve the only evidence of in situ high-temperature contact metamorphism and are spatially associated with the highest concentration of buchite xenoliths at Traigh Bhàn na Sgurra, provide rhyolite melts (Group III) for the Loch Scridain suite (Preston et al., 1998), and it seems plausible that other melting reactions may have been responsible for the generation of the aluminous liquids. In summary therefore, the evidence supporting Moine protoliths is listed below:

1. the Moine pelites are suitably Al-rich, containing high modal proportions of muscovite and biotite;
   
2. the LSSC sheets only preserve Sr, Nd and Pb isotopic evidence for contamination with Moine material;
   
3. at Traigh Bhàn na Sgurra there is a great abundance of aluminous xenoliths, which are at the right structural level;
   
4. at Traigh Bhàn na Sgurra the Moine pelites are contact metamorphosed, and here the products of high-temperature alteration contain the assemblage mullite + spinel;
   
5. abundant quartzite and psammite xenoliths can be found throughout the LSSC. These can be traced directly to the Moine;
   
6. the Moine pelites would be suitable for providing the minimum granitic melts which contaminated the basic magmas of the LSSC;
   
7. the Moine pelites and buchites have similar chondrite-normalized REE profiles (except for the Euanomalies).
   

Melting reactions and the formation of peraluminous liquids
Melts with appropriate Al-rich bulk compositions can be produced in two main ways:

1. bulk melting of Al-rich residues after the initial extraction of one or more (broadly) granitic melts from a pelitic protolith;
   
2. disequilibrium melting of more micaceous components within pelitic protoliths.
   

Evidence from the rhyolitic sheets within the suite, and from the compositions of the partial melts locally produced within the aureoles (and within quartz-rich xenoliths) of the basic sheets, suggests that the production and segregation of granitic melts occurred during the emplacement of the LSSC. Production of aluminous protoliths by the prior removal of LREE-enriched granitic melts leaving garnet in the residue is consistent with the relative HREE enrichment of the buchites. Simple mass balance calculations involving extraction of average Group III rhyolite melts from average Moine pelite show that the chemistry of the buchites does not match that of the modelled restite (Table 6; Fig. 4). Such modelling is complicated by potential exchange between the basaltic host and both the rhyolite and buchite melts, together with plagioclase crystallization from the buchite. The rhyolites have undoubtedly been modified by interaction with basaltic melts, as shown by their high Fe and relatively low K concentrations (Preston et al., 1998). However, as only major element probe analyses are available of in situ xenolithic rhyolitic melts (Preston, 1996), the Group III rhyolites were used for modelling purposes, as full trace element and REE analyses are available. The crystallization of the plagioclase rim could explain the depletion of Sr, Ca and Eu within the buchites relative to the modelled restite, and might also account for relative enrichment of other elements (e.g. Cr, Ti, Zr, Nb) (Fig. 4). Some of the variability, particularly of the more mobile elements (e.g. Ba, Rb) could represent later modification. More detailed modelling based on partition coefficients of trace elements between granitic melts and probable restite phases is impossible because of uncertainties about the mineralogy of the restite. However, the high proportion of muscovite (>50 vol. %) present in many local Moine pelites suggests that, following initial melt extraction, such aluminous phases would remain abundant in the source. Although Al will be concentrated in restites remaining after removal of significant amounts of granitic melt (Table 6), Fe will similarly be strongly enriched, and such enrichment is notably absent within all of the buchites (Table 1). Moreover, the bulk melting of an Al-enriched restite is thermally prohibitive, and given that the thermal regime at the time of sheet emplacement is unlikely to have been extreme, we believe that bulk melting of Al restites is an unlikely scenario. Thus, we suggest that the chemistry of the buchites is most compatible with their production during disequilibrium melting in an environment of rapid heating, at the same time that granitic melts are produced elsewhere in the country rocks.

View this table: [in this window] [in a new window]   Table 6: Modelled aluminous restite composition

 
The distinctive compositions of the various types of buchites may, to a limited extent, be controlled by their differing crystallization histories. However, given the very similar REE profiles of rimmed and unrimmed buchites, it seems plausible that the unrimmed mullite buchites may represent originally plagioclase-mantled buchites that have since lost their plagioclase palisades. Indeed, fragments of plagioclase rim are found as isolated xenoliths. However, there are features such as the K enrichment of the plagioclase-rimmed xenoliths that point to the various buchite melts each being produced from different protoliths. It seems unlikely that mullite crystallized directly from the buchite liquids, because of the extremely high temperatures needed. However, mullite can be produced during disequilibrium melting reactions involving muscovite and quartz breakdown at temperatures between 720 and 760°C (Brearley & Rubie, 1990):

and is produced locally from the breakdown of muscovite in pelitic xenoliths at Traigh Bhàn na Sgurra (Brearley, 1986). It is notable that the glasses within plagioclase-rimmed mullite buchites (e.g. KIFX1; Table 1) have bulk compositions remarkably similar to the composition of muscovite, and the more silica-rich glass analyses of unrimmed buchites (e.g. PL1; Table 1) resemble alkali feldspar compositions. Thus, we conclude that the plagioclase-rimmed mullite buchites are generated by melting reactions dominated by muscovite. The unrimmed mullite buchites typically have higher SiO₂ contents and so may be derived from protoliths with higher proportions of original quartz. Melts with similar compositions to the cordierite buchites have been generated in experiments by the melting of biotite-rich protoliths (Nédélec & Paquet, 1981; Le Breton & Thompson, 1988) and bulk melting of pelites (Vielzeuf & Holloway, 1988), and in this instance both the mullite and cordierite seem to be generated in a melting reaction such as

However, the low K₂O content of the cordierite buchite (both whole-rock and glass compositions) is not typical of melts generated from a biotite-rich protolith. We conclude, therefore, that the alkali (particularly K₂O) content of the buchite glasses may be controlled by both the protolith composition and diffusive exchange with the enclosing basaltic magma (e.g. Watson, 1982; Watson & Jurewicz, 1984). It is also of note that the plagioclase-rimmed mullite buchites have much higher K₂O contents, and it is suggested that the plagioclase rims formed before diffusion could deplete the buchite in alkalis.

	Magma–Xenolith Interactions in the Lssc

TOP ABSTRACT Introduction Geological Setting Geochemical Techniques The Crustal Xenoliths Contact Metamorphic Effects Geochemistry and Petrogenesis of... Crustal Xenolith Mineralogy and... Origin Of The Aluminous... Magma-Xenolith Interactions in... Implications For Contamination... Final Comments References

 
The LSSC is notable, particularly within the BTIP, in that it preserves numerous crustal xenoliths in various stages of reaction. However, what is remarkable is that the suite of sheets are typical tholeiitic basalts and basaltic andesites, and the country rocks are of a typical upper-crustal type. Consequently, the processes seen within the LSSC might be expected to record a common process of magma–crust interaction.

Formation of the mullite and cordierite buchites
As the aluminous phases found within the xenoliths are of a refractory nature, and only a few partially digested xenoliths are found (e.g. Brearley, 1986), heat transfer and melting must have been very efficient. If these aluminous melts are produced by disequilibrium partial melting of pelitic schists, then the processes of melt segregation and accumulation will have a strong control on melt homogenization and hence composition; rapidly segregated and quenched melts are very likely to preserve compositional heterogeneities. The preservation ofbuchite xenoliths with distinctive chemistries suggests a model involving mechanical disintegration of the source and subsequent bulk melting of various lithologies. If country rock suffers extensive melting, its rigidity and mechanical integrity will be greatly reduced, and unstable portions will be dislodged and entrained within the magma (Arzi, 1978). During this process the schists may become broken off from the wall rocks and more micaceous layers may become separated from more quartz-rich material (Fig. 15). The more reactive mica-rich xenoliths will then be subjected to bulk melting, and a variety of different melt compositions will be produced that reflect the local composition of the xenoliths produced during the initial disaggregation of the schists. The melts formed may then be ‘fossilized’ during segregation and quenching. Mullite will also be formed during these melting reactions (Grapes, 1986; Brearley & Rubie, 1990). Similarly, the minute crystals of cordierite will form during the melting reactions in the more Mg- and Fe-rich lithologies. However, the euhedral shapes of the larger spinels and corundums, the oscillatory zoning in the plagioclase within the plagioclase-rimmed xenoliths, and their spatial association with the basaltic melts, suggest that they have crystallized from a liquid, rather than representing restitic phases or phases formed in the melting reactions (see below). The composition of the individual buchites seems to be the critical factor in controlling the subsequent crystallization of these melts, although clearly the precipitation of large volumes of plagioclase will also influence the chemistry of the melts that remain, as exemplified by the enrichment of K₂O in the plagioclase-rimmed buchite.

View larger version (76K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 15. Schematic representation of the environment in which basaltic magma is contaminated and crustal xenoliths are formed within the LSSC.

 
Crystallization of the plagioclase–spinel–corundum rims
The plagioclase palisades formed as reaction rims between two silicate melts, rather than being the products of solid–solid or solid–liquid reactions. Many of the textural features of the plagioclase–spinel–corundum rims described above provide strong evidence that they have crystallized directly from an aluminous liquid, rather than representing restitic phases or phases formed during reactions with the melt. However, certain textural features, together with the REE and Sr–Nd isotope geochemistry of the plagioclase rims, show that their evolution was not one of simple cotectic crystallization, and that their formation is the principal manifestation of interaction between the xenolithic material and the host magma.

Textural and phase relationship evidence
As mullite needles can be seen to project into the mullite buchite from the edges of plagioclases at the core of the plagioclase-rimmed xenoliths (Fig. 6), mullite must have been present at an early stage within the aluminous liquid, and as stated earlier is probably restitic. Coarse-grained plagioclase dominated the crystallization history of the reaction rims and may have continued to do so until the quenching of the xenolith during final emplacement. Although cordierite has been reported from xenoliths with plagioclase rims (Thomas, 1922; Kille, 1987), its precipitation has been avoided in the majority of cases; a silica mineral is also absent from the majority of the buchites. As cordierite and tridymite–cristobalite might be expected to crystallize from aluminous liquids after plagioclase, it is suggested that mixing between the aluminous liquid and small amounts of basaltic magma occurred, effectively changing the liquidus composition. Thus, the crystallization of the rims of these xenoliths is probably dominated by melt compositions that represent mixtures between the basalt host and the aluminous liquids; such mixed liquids are now preserved within the melt pockets. Corundum crystallization occurred towards the central glassy portion of the xenolith, but always spatially associated with basaltic melt pockets. As corundum is surrounded by either mullite-free glass or mullite-free plagioclase, a reaction between the mullite and basaltic liquid is indicated:

The melt pockets associated with corundum are indeed slightly more silicic than their counterparts closer to the host basalt (Preston, 1996). As this reaction would require the input of heat, the mixing of fresh basaltic magma with the aluminous liquids would be a plausible mechanism for driving this reaction. Mixing between the aluminous liquids and batches of basaltic magma occurred within small, partly isolated pockets after most of the plagioclase rim had crystallized. Initially, the resulting Al-rich liquids were capable of crystallizing plagioclase (free of mullite needles), corundum and spinel, and eventually had compositions capable of precipitating plagioclase, clinopyroxene and Fe–Ti oxide. It is suggested that the melt pockets became isolated at a relatively early stage, and therefore the hybrid liquids could evolve along separate crystallization paths.

The fine-grained rims of plagioclase present around many of the more skeletal spinels also suggest a reaction relationship between spinel and liquid, resulting in the precipitation of highly calcic plagioclase. This relationship may be similar to that seen in the pseudo-ternary system forsterite–diopside–anorthite–silica (Osborn & Tait, 1952). It is possible that those spinels that are not the products of corundum alteration initially crystallized as an Mg- and Al-rich variety, and that the trend (Fig. 9) towards Fe-rich varieties is a product of post-crystallization re-equilibration with the surrounding magma. Evidence for this comes from xenoliths that have plagioclase rims of irregular thicknesses. In such cases, spinel compositions from where the plagioclase rim is at its narrowest are relatively homogeneous, and identical to those nearest the basalt contact where the plagioclase rim is at its broadest. Had the spinel compositions been primary and reflective of spinel crystallization within a diffusion-controlled compositional gradient, a similar spread of compositions would be expected at all positions within the xenolith, irrespective of the thickness of the plagioclase rim. The lack of a similarly coherent trend in plagioclase compositions is also supportive of late-stage modification of spinel compositions, as is the abundant evidence that spinel is easily modified by subsolidus re-equilibration (e.g. Fabriès, 1979; Roeder et al., 1979; Scowen et al., 1991). Moreover, if both rim thickness and mineral compositions were diffusion controlled, then both these factors should be relatively constant, which is clearly not the case. This solid-state modification represents one of the final stages of chemical interaction between the crustal xenolith and the host basalt.

Major and trace element geochemical evidence
The plagioclase rims make up as much as 80 vol. % of the plagioclase rimmed xenoliths. However, the associated buchites are particularly poor in Ca, and it is difficult to explain how they may have crystallized so much calcic plagioclase. An alternative source for the Ca may have been the enclosing magma itself (see Bailey et al., 1924), with the plagioclase growing inwards from the interface with this Ca-rich magma reservoir. Both Ca and Al could be derived through melt diffusion processes either from the basic and aluminous liquids, respectively, or from the reaction of appropriate mineral phases. For example, if the magmas were saturated with clinopyroxene at the time of crystallization of the plagioclase rim, which, judging by the clinopyroxene-bearing cognate xenoliths in the sheets seems likely (Preston & Bell, 1997), a reaction such as the following may have taken place:

The source of the Al may have been directly from the aluminous melts, or, alternatively, from aluminous phases such as mullite. Textural evidence for mullite inclusion dissolution towards the basalt contact within plagioclase-rimmed xenoliths suggests that mullite inclusions may well have been involved in such a reaction, despite their apparent isolation within plagioclase.

The REE are generally incompatible with respect to plagioclase crystallization. The one exception is Eu, which can exist in both divalent and trivalent states. As the oxygen fugacity of the system decreases, more of the Eu will be present as divalent ions and will therefore be progressively partitioned into plagioclase, substituting directly for Ca (Drake & Weill, 1975). If much of the plagioclase from the aluminous xenoliths crystallized from the basalt contact, inwards, then the plagioclase that occurs closest to the basalt contact would be expected to have the lowest total REE contents, and perhaps a positive Eu anomaly. Crystallization of this plagioclase would result in the aluminous liquid becoming progressively enriched in the REE, except Eu. Later-crystallized plagioclase would therefore have higher total REE contents, and might develop a negative Eu anomaly. This is what is seen in the plagioclase-rimmed mullite buchites (Fig. 13). However, the liquid from which the plagioclase precipitates (i.e. the mullite buchite) should always be more enriched in total REE. As Fig. 13 shows, this is not the case. Although the presence of the numerous mullite needles within the buchite would have an approximately 10% diluting effect on the whole-rock REE concentrations of the buchite, the values shown are still far too low. As far as we are aware, no experimental work on the partitioning of the REE within plagioclase in equilibrium with highly unusual peraluminous melts such as these has ever been carried out. It is also probable that the plagioclase did not remain in equilibrium with either the aluminous or basaltic melt for any length of time, as is suggested by the regular and patchy zoning, and the evidence for reaction seen throughout the rims. We are confident, therefore, that although a small contribution to the REE budget would have been made by included basaltic melt pockets within the bulk plagioclase rim sample (Table 1), the overall similarities between these analyses and those made by isotope dilution on small samples (Table 5) confirm that the majority of the REE are indeed held within the plagioclase. The low total REE content of the buchite may also be explained by a melt–melt diffusion process. Lesher (1990) has shown that during diffusion experiments between acid and basic magmas, Nd is preferentially incorporated in the basic melts because of their typically lower Nd concentrations. It is therefore suggested that some of the REE content of the buchites may have been taken up by the basaltic magma that infiltrated the plagioclase rims. This is corroborated by the higher Nd and Sm contents shown by the melt pockets when compared with the enclosing host basalt, and by the substantially higher Nd and Sm contents of the unrimmed buchite (Table 5).

Sr–Nd isotopic evidence
The plagioclases that have been analysed have (⁸⁷Sr/⁸⁶Sr)₅₅ ratios that vary between 0.7136 and 0.7147 (Table 5; Fig. 14), and lie between values typical of Moine metasediments (and equilibrium melts derived therefrom), and uncontaminated basaltic magmas of the LSSC (Fig. 14; Preston et al., 1998). The lower (⁸⁷Sr/⁸⁶Sr)₅₅ ratios of the plagioclase compared with the Moine melts therefore suggest that the aluminous liquid had partly exchanged Sr isotopes with the more primitive basaltic magma before the plagioclase rim started to grow, and potentially much of the Sr incorporated in the plagioclase may also have been derived from the modified basaltic magma. In such a model, the core of buchite would also be expected to have an isotope signature similar to that of the crustal protolith. However, Barbero et al., (1995) and Knesel & Davidson, (1996) have shown that melts with initial Sr isotope ratios distinctly different from those of the protolith can be generated during disequilibrium partial melting events. The possibility remains, therefore, that the Sr isotope ratios of the plagioclase represent the initial values of the melts from which they crystallized. The difference between the initial Sr isotope ratios of the buchite [(⁸⁷Sr/⁸⁶Sr)₅₅ = 0.7074–0.7115] and that of the protolith [(⁸⁷Sr/⁸⁶Sr)₅₅ > 0.728] is much greater than has, to our knowledge, been documented in examples of granitic melts generated by disequilibrium partial melting. Thus, it would appear that some of the buchites may have experienced post-quenching alteration after much of the plagioclase rim had crystallized. The Sr isotope ratios of plagioclase are less likely to be reset in such a manner (e.g. Giletti & Casserly, 1994). The freshness of some glasses, coupled with their low initial ratios, implies that Sr isotopic tracer diffusion (Baker, 1989, 1991) between the Al-rich liquid and relatively uncontaminated basaltic magma is also likely to have occurred. Such diffusion would be approximately an order of magnitude faster than Sr elemental diffusion, making Sr isotopic homogenization of small xenoliths in geologically short periods of time a distinct possibility. For example, Baker, (1991) showed that 80% homogenization of Sr isotopes would occur between molten dacitic enclaves and rhyolitic melt at 900°C, in a system with 6% H₂O and with an enclave radius of 50 cm (values comparable with those in the LSSC xenolith suite) within 100 years. For smaller enclaves (5 cm radius) complete Sr homogenization would occur in 4 years. However, in the case of the LSSC, quenching occurred before complete isotopic equilibration could be achieved. Although Baker's experiments were not conducted with exactly the same compositions as in the buchite xenoliths, and liquid-only systems were used, none the less the results demonstrate that tracer diffusion process can operate on very short timescales during interaction of evolved silicate melts. The (⁸⁷Sr/⁸⁶Sr)₅₅ ratios of the melt pockets (0.7126–0.7129) lie between those of a typical LSSC Group I basalt and the xenolith plagioclase, consistent with a mixture of originally radiogenic aluminous melt and unradiogenic basaltic magma. As the melt pockets contain a high proportion of quench plagioclase, any late-stage isotopic modification is unlikely to have significantly altered their Sr isotope composition to low (⁸⁷Sr/⁸⁶Sr)₅₅ ratios. The complexity of the inferred processes affecting the Sr isotopes in these palisades may imply that the xenoliths were not always surrounded by magma with the same Sr isotopic composition. Such a changing geochemical signature is not unreasonable given the evidence in the LSSC for turbulent magma flow, magma mixing and crustal contamination by diverse lithologies (Preston & Bell, 1997; Preston et al., 1998).

The relatively constant Nd isotope ratios of all components from the aluminous xenoliths appear not to have been disturbed by the xenolith–magma interactions, suggesting that the Nd isotope diffusion rates were not rapid enough to keep pace with the crystallization of the plagioclase rim; Lesher (1990) reported that Sr isotope diffusivities can be of the order of two times greater than Nd isotope diffusivities. Alternatively, the isotopic compositions of the various components were not distinct enough for the inferred processes to produce significant Nd isotopic variations.

	Implications For Contamination Processes In Basaltic Magmas

TOP ABSTRACT Introduction Geological Setting Geochemical Techniques The Crustal Xenoliths Contact Metamorphic Effects Geochemistry and Petrogenesis of... Crustal Xenolith Mineralogy and... Origin Of The Aluminous... Magma-Xenolith Interactions in... Implications For Contamination... Final Comments References

 
A variety of crustal melts were produced from the Moine country rocks, which then contaminated the basic magmas of the LSSC. These magmas include granitic melts, which are now represented by the rhyolite sheets (Group III) and which have also mixed with the basic members of the suite (i.e. Group I) to give the Group II intermediate rocks. However, the feature that distinguishes the LSSC from other suites of tholeiitic minor intrusions (e.g. the BTIP regional dyke swarms and cone sheet complexes; dyke swarms associated with the Deccan Traps and Paraná flood basalt provinces; Thompson et al., 1980; Devey & Cox, 1987; Bell et al., 1994; Gibson et al., 1995) is the highly aluminous melts now represented by the buchite xenoliths. There is some evidence from the whole-rock geochemistry of the sheets (Preston et al., 1998) that these too have interacted with some batches of the Group I basaltic magmas, and the preservation of the buchites allows the detail of some of these interactions to be assessed:

1. a variety of aluminous crustal melts were generated during fluid-absent melting, with melt composition depending on the local composition of the original micaceous xenolith, e.g. whether the protolith is dominated by muscovite or biotite.
   
2. Once these melts are formed, the more voluminous batches do not readily mix with the basalt (Fig. 15). Instead, diffusive exchange (e.g. Sr isotopic and REE exchange) between the buchites and basaltic melts occurs, and major element (Ca) exchange promotes plagioclase crystallization at the hybrid interface between some of the buchite xenoliths and their host magmas.
   
3. Further interaction and small-scale mixing of the aluminous melts and their basaltic host takes place via the infiltration of basaltic melt pockets into the plagioclase-dominated palisade.
   
4. The final interaction between the aluminous xenoliths and the basalt is the solid-state re-equilibration of spinel compositions.
   

The tectonic setting and thermal regime associated with the emplacement of the LSSC are not unique or exceptional (e.g. Devey & Cox, 1987; Lightfoot et al., 1990). This suggests that the magmatic processes that have occurred are potentially common. However, the differing initial Sr and Nd isotope signatures displayed by many of the LSSC basic sheets [the most evolved basaltic andesites have (⁸⁷Sr/⁸⁶Sr)₅₅ ratios of the order of 0.7154, and (¹⁴³Nd/¹⁴⁴Nd)₅₅ ratios as low as 0.5118 (Preston et al., 1998)], suggest that they have experienced more magma–crust interaction than basaltic magmas from many other continental flood basalt provinces (see Cox & Hawkesworth, 1985; Petrini et al., 1987). The extreme crustal contamination recorded in the LSSC, and the preservation of highly metamorphosed and melted xenoliths, is probably the consequence of the nature of the magmatic plumbing system. Cox & Hawkesworth (1985) have shown that basaltic plateau lavas may become less contaminated with crustal material up-sequence, as a result of younger lavas using established feeder systems, which were ‘armoured’ with basalt. Alternatively, the earlier magmas may have been contaminated with the easily fusible crustal component, leaving a refractory residue that later magmas found difficult to assimilate (seeKerr et al., 1994). Turbulent magma flow in newly formed sub-volcanic conduits is an ideal process for the continual removal of crustal melts as they form, and for detachment of country rock xenoliths (Fig. 15). However, the LSSC sheets display AFC contamination trends (Preston, 1996; Preston et al., 1998), which suggest that the magmas ponded within the crust. A network of poorly connected sheet-like conduits would provide the largest area of mutual contact between hot basic magma and the country rock, allowing further assimilation while the magma fractionated (e.g. Morrison et al., 1985; Kerr et al., 1994). Although turbulence may have been responsible for the incorporation of large volumes of xenolithic material and the subsequent melting of the micaceous country rock fragments, it appears that it was not sufficient to completely homogenize the resulting mixture of melts.

The preservation of the buchites will also have been enhanced by the crystallization of the plagioclase rims; however, there are many buchites that do not have these protective rims. The presence of such buchites suggests that the aluminous liquids and the basic magmas may have been effectively physically immiscible as a result of viscosity and density differences. However, the LSSC buchites do show extensive chemical interactions with the basic magmas, which were probably controlled by diffusion processes. However, as the volume of aluminous liquid was probably minor in comparison with the large amounts of available basic magma, detecting the signature of xenolith–host magma interactions within the Group I basic rocks is difficult. Small volumes of melted xenoliths might be physically assimilated and larger volumes of aluminous liquid may interact via trace element and isotopic exchange diffusion across a mutual liquid-liquid boundary. Such isotopic exchange proceeds much more quickly than chemical homogenization (Baker, 1989, 1991; Lesher, 1990). In this respect, the buchites are notable for their low (⁸⁷Sr/⁸⁶Sr)₅₅ ratios. This style of diffusion-controlled contamination may explain why the Group I magmas did not evolve further than basaltic andesite, despite having extreme trace element and Sr–Nd isotope signatures.

There have been many experimental studies into the processes of melting and assimilation of xenoliths (e.g. Maury & Bizouard, 1974; Watson, 1982; Beard et al., 1993; McLeod & Sparks, 1995; Patiño Douce, 1995; Knesel & Davidson, 1996). Although these studies highlight the complexities behind the mechanical and chemical breakdown of crustal xenoliths, most have failed to take into account the possibility of extensive but incomplete chemical reaction between the magma and the xenolith during the dissolution process, which, in the case of the LSSC, resulted in the crystallization of an extensive plagioclase reaction rim. However, field studies have highlighted the complex nature of xenolith–magma interactions, and the possible ramifications for magma contamination processes (e.g. Pedersen, 1978; Van Bergen & Barton, 1984; Grapes, 1986; Green, 1994).

Bulk assimilation without concurrent fractionation is thermally prohibitive, as very few magmas arrive at upper-crustal levels with significant superheat (Bowen, 1928). Therefore, AFC-style contamination processes are generally assumed to be the norm (Bowen, 1928; DePaolo, 1981; Petrini et al., 1987). However, what is often uncertain is the composition of the assimilant. Most studies assume the addition of a local crustal lithology in bulk (e.g. Myers & Marsh, 1981; Lightfoot et al., 1990), although the addition of silicic partial melts is also suggested (Kerr et al., 1994; Reiners et al., 1996). Within the LSSC, although the melting of micaceous lithologies coupled with extraction of silicic melts may point to effectively ‘bulk’ contamination of the whole suite of magmas, this bulk assimilation has occurred in discrete stages, resulting in a variety of contamination signatures within the magmatic suite. The LSSC is characterized by a two-component contamination process involving both silicic (minimum melt) compositions and a variety of distinctive aluminous melts derived from micaceous protoliths. The resulting contamination of the basaltic magmas takes place by a variety of processes involving mixing, diffusion, and reaction between the variousliquids, and the various liquids and crystals.

	Final Comments

TOP ABSTRACT Introduction Geological Setting Geochemical Techniques The Crustal Xenoliths Contact Metamorphic Effects Geochemistry and Petrogenesis of... Crustal Xenolith Mineralogy and... Origin Of The Aluminous... Magma-Xenolith Interactions in... Implications For Contamination... Final Comments References

 
The detailed analysis of the intricate behaviour of xenoliths in basaltic magmas in the LSSC highlights the complex nature of melt generation and segregation processes, and chemical interactions during crustal anatexis. Although it is often possible to model individual magmatic processes such as chemical diffusion in the ‘sterile’ environment of the laboratory, in natural systems, where other magmatic processes (e.g. fractional crystallization) may be occurring concurrently, the end products are likely to be substantially more complex than their experimental counterparts. The complexities of magma–xenolith interactions highlighted by the study of the xenolithic LSSC suite imply that the most commonly used models of crustal contamination (AFC, bulk assimilation), although sound in theory, may be oversimplifications when compared with what actually happens. The fact that a particularly normal suite of basaltic magmas has interacted with a particularly normal suite of country rocks in such a complex fashion should breed caution when interpreting the contamination history of other continental basaltic magmas.

	Acknowledgements

 
Thanks go to Malcolm Hole and Clive Rice for helpful and constructive comments on an earlier draft of this paper. J. S. Beard, Tracy Rushmer and Andrew Kerr are thanked for their thoughtful reviews. We are most grateful to Godfrey Fitton and Dodie James, Department of Geology and Geophysics, University of Edinburgh, and Malcolm Hole and Colin Wood of the Department of Geology and Petroleum Geology, University of Aberdeen, for provision of major and trace element data. Nick Walsh of Royal Holloway University, London, kindly made available analytical facilities for the REE. Robert MacDonald provided the technical assistance for the electron probe analysis at the Department of Geology and Applied Geology, University of Glasgow. Anne Kelly and Vincent Gallagher are thanked for their assistance in the radiogenic isotope laboratories at the SURRC. R.J.P. acknowledges with gratitude the receipt of an NERC training award (1992–1995) while at Glasgow University. The isotopic analyses at SURRC were supported by the Scottish Universities.

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^* Corresponding author. Telephone: +44 (0)1224 273467. e-mail: j.preston{at}abdn.ac.uk

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