U-Pb Zircon Dating of Basement Inliers Within the Moine Supergroup, Scottish Caledonides: Implications of Archaean Protolith Ages
By Friend, C R L Strachan, R A; Kinny, P D
Abstract: Basement gneiss inliers within the Scottish Caledonides have been conventionally correlated with the Archaean Lewisian Gneiss Complex of the Caledonian foreland. Alternatively, the inliers could represent allochthonous terranes accreted to Laurentia before or during the Caledonian orogeny. Secondary ionization mass spectrometry U-Pb zircon dating indicates that the Ribigill, Borgie, Farr and Western Glenelg basement inliers are characterized by late Archaean protolith ages, and a period of isotopic disturbance in the late Palaeoproterozoic. The data are broadly consistent with correlation between the inliers and components of the Lewisian Gneiss Complex of the Caledonian foreland. The c. 2900 Ma protolith ages support correlation of the Borgie and Farr inliers with the Assynt terrane, and a younger, c. 2800 Ma age for the Ribigill inlier supports correlation with the Rhiconich terrane. None of the studied inliers shows a complete match of protolith and early metamorphic histories with any of the Lewisian basement terranes, but differences between the inliers and the foreland are no greater than those recorded within the foreland basement terranes themselves. Therefore, it remains probable that the dated inlier gneisses formed a distal part of the Laurentian margin prior to final telescoping during the Caledonian orogeny.
The complexity of many collisional orogens results, at least in part, from the tectonic assembly of disparate crustal blocks to result in collages of fault-bounded ‘suspect’ or ‘allochthonous’ terranes that have become detached from the cratons whence they originated. One of the main challenges in such a setting involves the identification of single terranes and their possible cratonic sources, and hence an evaluation of the likely displacements along the terrane-bounding faults. In some Phanerozoic orogens the history of terrane accretion and estimates of likely displacements along terrane boundaries can be resolved by palaeomagnetic and palaeontological methods. In older orogens, particularly those characterized by unfossiliferous strata and/or medium- to high- grade metamorphic rocks, identification of terranes and the nature of their original relationship to cratonic foreland rocks is considerably more difficult. Correlation of a terrane with a cratonic foreland requires the identification of common geological features, many of which may have been eroded in older orogens. In such cases, a potential method of analysis involves the characterization of basement rocks in a given terrane, and comparison with the basement rocks of adjacent cratonic forelands. This method may encounter problems as a result of the extensive reworking and resetting of isotopic systems that are common in basement complexes and in orogens, but may be the only tool available for terrane analysis. Despite the potential difficulties, this is the approach adopted in this paper, where U-Pb geochronology has been used to characterize basement inliers within the Early Palaeozoic CaIedonide orogen of NW Scotland, and to assess potential linkages with the Laurentian foreland west of the Moine Thrust Zone (Fig. 1).
Within the Caledonide orogen in NW Scotland (Fig. 1), there are numerous inliers of highly deformed gneissic rocks that differ markedly in their lithological characteristics and structural and metamorphic histories from the adjacent metasedimentary rocks of the Neoproterozoic Moine Supergroup (e.g. Flett 1905; Peach et al. 1910). These inliers have the appearance of crystalline basement now variably overprinted with younger structures, many of which have been interpreted as Caledonian (Ordovician-Silurian) in age (e.g. Rathbone & Harris 1979; Strachan & Holdsworth 1988; Holdsworth 1989). A central problem within the Scottish Caledonides concerns the extent to which the basement inliers and the Moine Supergroup are allochthonous with respect to the Laurentian foreland west of the Moine Thrust Zone (Fig. 1). The foreland comprises the Archaean- Palaeoproterozoic Lewisian Gneiss Complex (e.g. Friend & Kinny 2001; Park et al. 2002; Kinny et al. 2005), which is overlain unconformably by the late Mesoproterozoic to Neoproterozoic Torridonian sedimentary rocks (e.g. Stewart 2002). Mainly on lithological grounds, the basement inliers have been correlated previously with the Lewisian Gneiss Complex (e.g. Johnstone 1975; Rathbone & Harris 1979), and the Moine Supergroup sediments with the Torridonian (e.g. Cheeney & Matthews 1965). If such correlations are correct, they imply that the basement inliers and the Moine rocks evolved as part of Laurentia during the Precambrian. An alternative view is that these rock units form part of an allochthonous terrane that was accreted to the Laurentian margin either prior to or during the Caledonian orogeny (e.g. Bluck et al 1997).
Attempts to evaluate Moine-Torridonian linkages have noted that the two sequences have slightly different detrital zircon suites (Rainbird et al. 2001; Friend et al. 2003; see, however, Krabbendam et al. 2008), and moreover the Moine rocks were affected by mid- Neoproterozoic (Knoydartian) erogenic events that are apparently absent on the foreland (e.g. Rogers et al. 1998; Vance et al. 1998; Tanner & Evans 2003). Nevertheless, neither of these differences necessarily precludes a Laurentian origin for the Moine rocks (Cawood et al. 2004). As an alternative method of evaluating potential linkages across the Moine Thrust Zone we provide in this paper new isotopic age data for four basement inliers within the Moine Supergroup. We assess the resultant implications for possible linkages with the Lewisian Gneiss Complex of the Laurentian foreland and
regional tectonic models for this part of the Caledonides.
Regional setting of basement inliers within the Caledonides of NW Scotland
Lithologies and structural setting
Inliers of basement rocks occur widely within the Moine Supergroup, mainly in the Moine and Sgurr Beag nappes, and largely in contact with metasediments assigned to the Morar and Glenfinnan groups. There are three main concentrations of inliers: those in northern Sutherland below the Naver Thrust; those in the Scardroy- Fannich area; and those immediately above the Moine Thrust in the Attadale-Glenelg-south Skye area (Fig. 1). Numerous smaller inliers are also present within other parts of the Moine and Sgurr Beag nappes. The inliers lie consistently at the lowest structural levels of local stratigraphical successions once the effects of thrusting and/or folding are removed. Some inliers apparently lie in the cores of isoclinal folds, for example, Morar (Kennedy 1955; Powell 1966) and Naver (Strachan & Holdsworth 1988), whereas others are allochthonous slices underlain by Caledonian thrusts such as the Sgurr Beag Thrust (e.g. Tanner et al. 1970; Rathbone & Harris 1979) and the Ben Hope Thrust (Holdsworth 1989).
Most inliers are dominated by tonalitic to dioritic hornblenderich gneisses that are commonly highly deformed and variably banded with polyphase pegmatitic material. Thin strips of pelitic metasediment and marble have been recognized in some inliers (e.g. Loch Shin, Eastern Glenelg) and appear to represent an integral part of the basement assemblage. Throughout all of the inliers, the dominant metamorphic assemblages now present are amphibolite facies or lower, as the gneisses have been extensively reworked and retrogressed during Knoydartian (820-730 Ma) and Caledonian (470-420 Ma) erogenic events. Some inliers locally preserve evidence of high-grade, pre-Caledonian metamorphic assemblages. These include the Western Glenelg inlier, where relict granulite-facies mineralogies are preserved (e.g. Barber & May 1975; Sanders 1979), and the Eastern Glenelg inlier, where eclogite- facies rocks are present (e.g. Teall 1891; Alderman 1936; Mercy & O’Hara 1969; Sanders 1979; Sanders et al. 1984). Early high-grade assemblages have also been reported from the Borgie inlier, where garnet- and clinopyroxene-bearing mafic gneisses were retrogressed into amphiboleplagioclase assemblages during the Caledonian orogeny (Moorhouse 1976; Holdsworth et al. 2001).
Affinities of the basement inliers and relationships with the Moine Supergroup
When the basement inliers within the Moine Supergroup were first mapped they were interpreted to be sub-Moine, Lewisian rocks because of their lithological similarity to the Lewisian gneisses of the Caledonian foreland (e.g. Flett 1905; Peach et al. 1907, 1910, 1913). This correlation was challenged by Sutton & Watson (1953, 1954), who interpreted the gneissic rocks of central Ross-shire as integral parts of the Moine succession. However, subsequent research in the Glenelg area showed this to be incorrect (Ramsay 1958) and correlation with the Lewisian of the foreland was reinstated and continued in later studies of the Moine nappes (e.g. Sutton & Watson 1962; Barber & May 1975; Rathbone & Harris 1979; Strachan & Holdsworth 1988; Holdsworth 1989; Temperley & Windley 1997). It has been tacitly assumed on the basis of broad lithological similarities that all of the inliers belong to essentially the same portion of the Lewisian (e.g. Sutton & Watson 1962; Watson 1975). However, the use of lithology alone as a basis for correlation of complex gneissic rocks is no longer considered acceptable. This is particularly the case when considering units that (prior to Caledonian thrusting) may have formerly been separated by many tens or even hundreds of kilometres. Furthermore, it has been shown that the Lewisian Complex itself comprises a variety of terranes of differing age and history, some added in the Proterozoic (Kinny et al. 2005). Following the early work that suggested that a tectonized unconformity existed between the Moine and the basement (e.g. Peach et al. 1910, 1913), the present consensus is that the Moine Supergroup was deposited unconformably on the basement inliers (Barr et al. 1988; Holdsworth et al. 1994; Soper et al. 1998). In some places, way-up evidence clearly shows that the Moine sedimentary rocks young away from the basement (e.g. Bailey & Tilley 1952; Kennedy 1955; Ramsay 1958; Moorhouse et al. 1988; Holdsworth 1989), and basal Moine conglomerates are described from several localities, for example, at Glenelg (e.g. Bailey & Tilley 1952; Ramsay 1958) and Attadale (Barber & May 1975). Others, such as at Strathan (Mendum 1976) are more contentious. Some of the reported ‘basal conglomerates’ (e.g. Glen Strathfarrar, Strathan) are now interpreted as being partly of tectonic origin, probably arising from the disruption of quartz veins (Holdsworth et al. 2001; Friend & Strachan, unpubl. data) but this does not necessarily undermine the essentially autochthonous status of the sub-Moine basement. As suggested by Tanner et al. (1970), those inliers occurring along the trace of the Sgurr Beag Thrust in Ross-shire may represent tectonically detached slices of a ‘basement high’, possibly representing rift shoulders that separated the Morar and Glenfinnan- Loch Eil sedimentary basins (see also Soper et al. 1998).
Previous isotopic data
To date, there have been few isotopic age determinations on the basement inliers, their heterogeneity and complex metamorphic history has created difficulties for both whole-rock and mineral isotope studies. Moorbath & Taylor (1974) obtained a Rb-Sr whole- rock isochron age of 2810 +- 120Ma for the Scardroy inlier (Fig. 1), which they claimed to ‘prove beyond any doubt that the rocks of the Scardroy sheet are, indeed, Lewisian’. Moorbath & Taylor (1974) also produced K-Ar dates on hornblendes and biotites from Scardroy in the range 459-420 Ma (with one sample at 535 Ma interpreted to have incompletely outgassed). These results were compared with unpublished data that Moorbath and Taylor had obtained from the Eastern Glenelg inlier, where similar Caledonian K-Ar ages had been obtained, and contrasted with data from the Western Glenelg inlier, where K-Ar ages were in the range 2200-1600 Ma. Miller et al. (1963) produced a 1515 +- 104 Ma K-Ar age date on omphacite from the eclogitic rocks in the Eastern Glenelg inlier. A subsequent study provided Sm-Nd mineral regression ages on the eclogites of 1082 +- 22 and 1010 +- 13 Ma that were interpreted to date a high-grade Grenvillian metamorphic event (Sanders et al. 1984). The existence of this event has been broadly substantiated by U-Pb titanite ages of c. 1000 Ma obtained from the Eastern Glenelg inlier (Brewer et al. 2003).
Zircons separated from the rock samples were mounted in epoxy resin, sectioned, imaged by cathodoluminescence (CL), and dated by sensitive high-resolution ion microprobe (SHRIMP) using the SHRIMP- 11 ion microprobe at the John de Laeter Centre, Curtin University, Perth, Western Australia. Operating conditions for SHRIMP were routine; namely, 25 [mu]m analytical spot size, primary beam current 2-5 nA, mass resolution 5000 (1% valley), and sensitivity for Pb isotopes c. 15 c.p.s. ppm^sup -1^ nA^sup -1^. Correction of isotope ratios for common Pb was based on the measured 204Pb, representing in most cases less than 1% correction to the 206Pb counts (see %com.206Pb, Table 1), with the common Pb composition modelled upon that of Broken Hill ore Pb. Pb/U isotopic ratios were corrected for instrumental inter-element discrimination using the observed co- variation between 206Pb/238U and UO/U (Compston et al. 1984, 1992) determined from interspersed analyses of the Perth standard zircon CZ3 (564 Ma; 206Pb/238U = 0.0914). The uncertainty in Pb/U ratios associated with this correction procedure was in the range 1.5- 2.5%, whereas uncertainties in Pb/Pb ratios were generally lower, being governed principally by counting statistics. Isotope ratios and corresponding ages (calculated using standard decay constants) are listed in Table 1, together with to uncertainties. Unless otherwise stated, all ages discussed in the text are given with 95% confidence limits.
599/2: Ribigill inlier
[National Grid Reference NC 580547]
The Ribigill inlier (Fig. 1) occupies an antiformal fold core in the hanging wall of the Ben Hope Thrust (Moorhouse & Moorhouse 1977; Holdsworth 1989; British Geological Survey 1997; Holdsworth et al. 2001). Of the three inliers examined along the north coast, the Ribigill inlier is the structurally lowest and the least far travelled with respect to the Caledonian foreland. The sample analysed was collected from a quarry in the central portion of the inlier that is dominated by banded, quartzofeldspathic gneisses that are broadly granodioritic in composition. The gneissic layering consists of millimetre- to centimetrescale alternations of mafic- and felsic-dominated layers. Some of the latter comprise deformed concordant layers and lenticles of granitic pegmatite, suggesting either that the rocks were invaded by melt or that they underwent a metamorphic event that induced a low degree of partial melting. This melt material was avoided in the sampling. In thin section, the sample comprises a medium-to coarse-grained, granoblastic assemblage of plagioclase and K-feldspar, hornblende, biotite, epidote and quartz.
The separated zircons were euhedral to subhedral prismatic grains. CL imaging revealed regular internal growth zoning in most grains and occasional structural cores (Fig. 2). The analysed zircons produced a range of ^sup 207^Pb/^sup 206^Pb ages, and plot either overlapping concordia or slightly below (Fig. 2). Of the three analyses with highest ^sup 207^Pb/^sup 206^Pb, two (1.1 and 4.1, Table 1) were situated in discrete structural cores, suggesting that they may represent inherited components in the protolith. The combined age of these three analyses is 2822 +- 28 Ma. The main group of analyses record ^sup 207^Pb/^sup 206^Pb ages of between 2760 and 2660 Ma. These probably represent the main protolith population. In addition, three analyses of tips of grains with high U content and low Th/U ratio (3.2, 2.3 and 5.1, Table 1), and appearing dark in CL (Fig. 2), record younger apparent ages, appearing to lie on a discordance line trending towards a c. 1600Ma intercept age, adjacent to analysis 5.1. The formation of these rims could be related to the episode responsible for the granite- pegmatite layers observed in the host rock.
S96/12: Borgte inlier [NC 689573]
The Borgie inlier is the largest on the north coast (Fig. 1) and occupies a structurally higher antiformal fold core between the Ben Hope Thrust and the Naver Thrust (Moorhouse 1976; British Geological Survey 1997; Holdsworth et al. 2001). It contains a range of lithologies. The banded felsic gneisses of the inlier are K- feldspar poor, and variably hornblende- and biotite-bearing; locally they enclose rafts of mafic and ultramafic rocks. Some of the mafic rafts are characterized by garnet-clinopyroxene metamorphic assemblages (Moorhouse 1976; Holdsworth et al. 2001). In thin section, the analysed sample is banded and comprises a coarse, granoblastic assemblage of plagioclase feldspar and quartz with finer-grained, porphyroblastic hornblende and oriented flakes of biotite with minor chlorite. Some mafic-dominated layers comprise aligned biotite and granular epidote. The quartz-rich domains are dominated by annealed ribbon textures, suggesting that the gneiss has undergone high strain.
The separated zircons typically have sub-rounded, modified shapes, and CL imaging revealed that numerous grains have a brightly luminescent mantle over a zoned interior. On a concordia diagram, the analysed points form a coherent discordance array that projects on a shallow trajectory from a late Archaean upper intercept age towards an approximately end-of-Palaeoproterozoic lower intercept age (Fig. 3). The data are interpreted to represent a single disturbed population. The least discordant analyses have ^sup 207^Pb/ ^sup 206^Pb ages of c. 2880 Ma, but there are too few of them to allow a more accurate assessment of the protolith age. The projected lower intercept is c. 1600Ma, suggesting significant isotopic disturbance at about that time.
S99/1: Farr inlier [NC 687614]
The Farr inlier (Fig. 1) occupies the core of an isoclinal fold within the migmatitic Moine rocks of the Naver Nappe (Moorhouse 1979; Moorhouse et al. 1988). It represents the structurally highest inlier examined and is likely to be the furthest travelled relative to the Caledonian foreland. The inlier is dominated by banded, mafic and intermediate hornblendic gneisses that are highly deformed. The sample dated is a coarse-grained gneiss and in thin section comprises sub-equigranular, granoblastic plagioclase feldspar, quartz and hornblende. Minor biotite may be retrogressive and accessory phases include opaques, apatite and zircon.
The separated zircons were typically elongate, subhedral, prismatic grains, with a narrow CL-bright rim evident on most grains, developed over regularly zoned interiors (Fig. 4). Two SHRIMP analyses were undertaken on most grains, one spot in the core and one on the rim. On a concordia diagram, the combined analyses form a broad, essentially continuous discordant array projecting from a late Archaean upper intercept age to an end-of- Palaeoproterozoic lower intercept age (Fig. 4). There is no clear distinction between the age spectra for cores and rims other than that the analyses of rims tend to be the most highly discordant. The four least-discordant analyses of grain cores have a combined age of 2905 +- 24 Ma, which can be considered as a reasonable estimate of the protolith age of this gneiss. The shallow slope of the discordance array suggests isotopic disturbance of the zircons involving Pb loss as early as c. 1600 Ma. S96/41: Western Glenelg inlier, at Dornie [NG 912226]
This sample was taken from one of the high-strain zones within the retrogressed gneisses exposed in road cuttings near Dornie along the shore of Loch Duich (Fig. 1). The sample is blastomylonitic and fine-grained with granoblastic quartz grains pseudomorphing ribbon fabrics. The sample was taken in the hope that some indication of the metamorphism associated with the shearing event might be present.
The zircons from this sample showed typical igneous zonation features under CL imaging, a minority of grains possessing a structural core (Fig. 5). Analyses produced a range of late Archaean to Mesoproterozoic apparent ages. The oldest age obtained (c. 2800 Ma) was recorded from a spot within a structural core (analysis 5.1, Table 1). The main population of least disturbed zircon is represented by five analyses (10.1, 11.1, 12.1, 15.1 and 17.1, Table 1), which record a mean ^sup 207^Pb/^sup 206^Pb age of 2677 +- 16 Ma (Fig. 5). Numerous discordant points trend to lower ^sup 207^Pb/ ^sup 206^Pb ages, following discordant arrays that trend towards different lower intercept ages. The analyses recording the youngest apparent ages include rim analyses (e.g. 14.2 and 16.2) and three analyses of grain 9 (Table 1). These analyses appear to lie on a secondary discordance line projecting from a late Palaeoproterozoic upper intercept to younger apparent ages, implying a complex multistage history of isotopic disturbance.
Comparison with the Lewisian Gneiss Complex of the foreland
The new data show that each of the inliers dated has an Archaean protolith. The wide geographical distribution of the inliers dated suggests that the intervening inliers are of similar age, although this remains to be proven. Amongst the northern inliers, the Ribigill (c. 2760 Ma with c. 2820 Ma inherited components) appears to be younger than the Borgie and Farr inliers (c. 2900 Ma), and the western Glenelg sample appears still younger at c. 2680 Ma (with one c. 2800 Ma core). All three of the northern inliers show similar discordant patterns, reflecting disturbance of protolith grains in latest Palaeoproterozoic times. The zircons from Glenelg show a broadly similar pattern, but with evidence for additional disturbance during younger events. Distinct rims on the Farr and Ribigill inlier zircons appear to have formed in the late Archaean and to have been disturbed in the Proterozoic along with the enclosed cores, whereas some young components in the Glenelg sample may represent components newly grown during Proterozoic events.
Are there any points of similarity between these inliers and the various components of the Lewisian Gneiss Complex of the Caledonian foreland? The age range of the least disturbed protolith zircons within the Ribigill inlier falls within that of the known protoliths of the geographically adjacent Rhiconich terrane on the foreland (2840-2680 Ma). However, the Ribigill gneisses do not appear to contain any examples of the extensive c. 1855 Ma granite sheets that are found throughout the Rhiconich terrane (e.g. Kinny & Friend 1997; Kinny et al. 2005) and the analysed zircons do not appear to show any new growth at this time. On the other hand, the timing of amphibolite-facies metamorphism in the Rhiconich terrane, c. 1740Ma, is broadly consistent with the age of some ancient Pb loss from the Ribigill zircons. The Ribigill inlier therefore displays some similarities to the foreland basement.
The c. 2900 Ma protolith ages obtained from the Farr and Borgie inliers are older than the typical protolith ages of the Rhiconich terrane. However, given that they represent minimum age estimates from disturbed populations, they are a closer match to protolith ages of the Assynt terrane (3030-2960 Ma). Significantly, there is no conclusive evidence in the analysed zircons from any of the inliers for any major disturbance or new zircon growth during either of the high-grade metamorphic events now recognized in the Lewisian Gneiss Complex on the foreland: c. 2730Ma in the Gruinard terrane (Love et al. 2003) and c. 2490Ma in the Assynt terrane (Friend & Kinny 1995). Both events caused U depletion, and in the case of the c. 2490 Ma event it was particularly intense and is very easily recognized analytically in the zircons. Although the metamorphic assemblages of the inliers are largely now amphibolite facies, it might be expected that the zircons would have preserved evidence of this severe U depletion, as found in extensive overgrowth development in both the granulite and retrogressed granulite-facies gneisses of the foreland (e.g. Friend & Kinny 1995; Love et al. 2003). That such effects are absent is strong evidence to suggest that the rocks did not experience either of these events. The Borgie and Fair inliers could, therefore, have been derived from crustal material similar to the Assynt terrane with protolith ages of c. 2900 Ma but that never attained granulite-facies conditions at 2490 Ma. Alternatively, they may have been derived from a different crustal block that underwent a different tectonometamorphic history.
The protolith age of the Western Glenelg inlier (c. 2680 Ma) is younger than the majority of protolith ages obtained from the mainland Lewisian Gneiss Complex as well as from the Outer Hebrides (e.g. Kinny et al. 2005). However, given the high degree of isotopic disturbance to the zircons, the potential affinities of this inlier with the Lewisian foreland terranes cannot be determined with confidence.
In summary, although the zircon age spectra might support correlation of the Borgie and Farr inliers with the Assynt terrane and the Ribigill inlier with the Rhiconich terrane, there is no complete match of both protolith and metamorphic histories of any of the inliers with any of the foreland basement terranes. However, these differences are no greater than those between the terranes that form the Lewisian Gneiss Complex itself. The lack of specific correlation is not unexpected given the likely c. 100km displacements along the Caledonian Moine Thrust Zone (e.g. Elliott & Johnson 1980; Soper & Barber 1982; Butler & Coward 1984). It is noteworthy that the main period of isotopic disturbance to the zircon populations in all four inlier samples was in the late Palaeoproterozoic, broadly corresponding to the timing of episodes of amphibolite-facies reworking in the Lewisian foreland terranes (1740-1670 Ma) (e.g. Friend & Kinny 2001; Kinny et al. 2005).
U-Pb zircon geochronology has shown that the Ribigill, Borgie, Farr and Western Glenelg basement inliers are all characterized by late Archaean protolith ages and a period of isotopic disturbance in the late Palaeoproterozoic. In broad terms, the data are consistent with correlation between the inliers and components of the Lewisian Gneiss Complex of the Caledonian foreland. In particular, the older, c. 2900 Ma protolith ages support correlation of the Borgie and Farr inliers with the Assynt terrane, whereas the younger, c. 2800 Ma age for the Ribigill inlier supports correlation with the Rhiconich terrane. None of the studied inliers shows a complete match of both its protolith and early metamorphic histories with any of the Lewisian basement terranes, but differences between the inliers and the foreland are no greater than those recorded within the foreland basement terranes themselves. It is therefore probable that the dated inlier gneisses formed a distal part of the Laurentian margin prior to final telescoping during the Caledonian orogeny. The results of this study demonstrate the difficulty in assessing potential basement terrane linkages across an orogenic front such as the Moine Thrust when reworking has substantially disturbed isotopic systematics such that pre-thrusting protolith and metamorphic histories cannot be matched with confidence.
We thank I. Burns for assistance in sample collection, and participants in the 2003 Highland Field Workshop for discussions in the field. J. Mendum, M. Whitehouse and M. Krabbendam are thanked for constructive reviews that improved the paper.
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Received 23 August 2007; revised typescript accepted 1 February 2008.
Scientific editing by Maarten Krabbendam
C. R. L. FRIEND1, R. A. STRACHAN2 & P. D. KINNY3 1 45, Stanway Road, Risinghurst, Headington, Oxford OX3 8HU, UK
2 School of Earth & Environmental Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK
3 The Institute for Geoscience Research, Department of Applied Geology, Curtin University of Technology, GPO Box
U1987, Perth, WA 6845, Australia
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