1. Introduction
The geological complexity of many Mesozoic–Cenozoic orogens often results from the progressive amalgamation of volcanic arcs and continental fragments during oceanic closure, as well as regionally significant strike-slip displacements that arise either from the oblique nature of a collision or lateral escape during orthogonal convergence (Dewey et al. Reference Dewey, Hempton, Kidd, Saroğlu, Şengör, Coward and Ries1986). Even in well-exposed recent orogens such as the Himalayas, the magnitude of such strike-slip displacements can be difficult to evaluate (e.g. Searle, Weinberg & Dunlap, Reference Searle, Weinberg, Dunlap, Holdsworth, Strachan and Dewey1998; Searle, Reference Searle2006), and such difficulties are compounded in older, Palaeozoic and Precambrian orogens. The Lower Palaeozoic Caledonide orogen in Scotland and Ireland is an excellent ancient example of such complexity. It evolved as a result of the closure of the Iapetus Ocean and the convergence of three continental blocks, Laurentia, Baltica and Avalonia (Soper, Reference Soper, Harris and Fettes1988; Pickering, Bassett & Siveter, Reference Pickering, Bassett and Siveter1988; Soper et al. Reference Soper, Strachan, Holdsworth, Gayer and Greiling1992; Dewey & Strachan, Reference Dewey and Strachan2003). Early arc–continent collisions along the Laurentia and Baltica margins during the Ordovician were followed by final oblique continental collision during Silurian times. Orogen-parallel, sinistral displacements occurred during the final stages of collision along various structures, including the Great Glen Fault in Scotland (Fig. 1).
Figure 1. Sketch map of the southern outcrop of the Moine Supergroup (see inset for location) showing the main geological units, igneous bodies and structures mentioned in the text. Diagonal hatching in inset corresponds to the outcrop of the Moine Supergroup in northern Scotland. Numbers around edge of main map correspond to British National Grid. Abbreviations: A – Ardnish; Agg – Ardgour granite gneiss; CG – Carn Gorm; CS – Central Sutherland; FAgg – Fort Augustus granite gneiss; G – Glenfinnan; GD – Glen Doe; GDS – Glen Dessary syenite; GGF – Great Glen Fault; GM – Glen Moriston; GSM – Glen Scaddle metagabbro; GT – Grampian terrane; K – Knoydart; LE – Loch Eilt; LQ – Loch Quoich; LS – Loch Sunart; MTZ – Moine Thrust Zone; NHT – Northern Highland Terrane; NR – northern Ross-shire; S – Strontian; SB – Sgurr Breac; SBT – Sgurr Beag Thrust; SG – Strontian Granite.
Estimates of displacements along the Great Glen Fault are controversial because of overprinting of the palaeomagnetic record and the lack of any unambiguous correlation across the fault of pre-Devonian markers. An alternative way of evaluating likely displacements across such structures is by comparison of the varying intensity of individual Caledonian events across the orogen as a means of reconstructing original collisional templates. The results of isotopic dating have, for example, already provided indications of a significant contrast in the intensity of Scandian (Silurian) deformational and metamorphic events across the Great Glen Fault (Kinny et al. Reference Kinny, Strachan, Friend, Kocks, Rogers and Paterson2003). In this paper we investigate this further and provide additional evidence that the Great Glen Fault separates crustal blocks with contrasting Caledonian tectonothermal histories.
The Scottish Highland Caledonides comprise a series of fault-bounded terranes derived from the margin of eastern Laurentia. The Great Glen Fault separates the Northern Highland and Grampian terranes (Fig. 1). The Northern Highland terrane is mainly underlain by the early Neoproterozoic Moine Supergroup (Holdsworth, Strachan & Harris, Reference Holdsworth, Strachan, Harris, Gibbons and Harris1994). In contrast, the Grampian terrane is dominated by the late Neoproterozoic to early Palaeozoic Dalradian Supergroup (Harris et al. Reference Harris, Haselock, Kennedy, Mendum, Gibbons and Harris1994). A possible equivalent of the Moine Supergroup, the Dava succession, occurs in the Grampian Terrane (Highton, Hyslop & Noble, Reference Highton, Hyslop and Noble1999; Smith, Robertson & Rollin, Reference Smith, Robertson and Rollin1999), but otherwise there are no obvious lithostratigraphic correlations across the Great Glen Fault.
Both the Moine and Dalradian supergroups were deformed and metamorphosed during an Ordovician (c. 480–465 Ma) Grampian orogenic event that resulted from the collision of the Laurentian margin with an intra-Iapetan volcanic arc (Dewey & Ryan, Reference Dewey and Ryan1990; Dewey & Mange, Reference Dewey, Mange, MacNiociall and Ryan1999; Oliver et al. Reference Oliver, Chen, Buchwaldt and Hegner2000; Strachan et al. Reference Strachan, Smith, Harris, Fettes and Trewin2002; Dewey, Reference Dewey2006). In addition, U–Pb zircon dating of syn-tectonic granites in east Sutherland (Fig. 1) has provided evidence for ductile reworking and associated amphibolite facies metamorphism of the Moine Supergroup during the Silurian (Kinny et al. Reference Kinny, Strachan, Friend, Kocks, Rogers and Paterson2003; Kocks, Strachan & Evans, Reference Kocks, Strachan and Evans2006). This is attributed to the collision of the Northern Highland segment of Laurentia with Baltica and correlated with the Scandian event recognized in Norway (Dallmeyer et al. Reference Dallmeyer, Strachan, Rogers, Watt and Friend2001). The lack of any evidence for a comparable Silurian tectonothermal event in the Grampian terrane has led to the suggestion that at that time it was situated further to the southwest along the Laurentian margin, remote from the Scandian collision (Dewey & Strachan, Reference Dewey and Strachan2003; Kinny et al. Reference Kinny, Strachan, Friend, Kocks, Rogers and Paterson2003). It has been concluded that at least 700 km of subsequent sinistral displacement along the Great Glen Fault would have been necessary to bring the Grampian terrane to its present location relative to the Northern Highland terrane (Dewey & Strachan, Reference Dewey and Strachan2003).
In this paper we review briefly the Caledonian evolution of the Moine rocks of West Inverness-shire (Fig. 1) and present the results of a geochronological study of the Glen Scaddle Metagabbro (Bailey & Maufe, Reference Bailey and Maufe1916; Drever, Reference Drever1940). The structural setting of this intrusion was established by Stoker (1983; M. Stoker, unpub. Ph.D. thesis, Univ. Liverpool, 1980) and confirmed during recent fieldwork by one of us (RAS).
2. Geological setting
The Moine rocks of West Inverness-shire comprise the Morar, Glenfinnan and Loch Eil groups (Fig. 1; Holdsworth, Strachan & Harris, Reference Holdsworth, Strachan, Harris, Gibbons and Harris1994). The Morar Group lies within the Moine Nappe, and the Glenfinnan and Loch Eil groups within the Sgurr Beag Nappe (Powell et al. Reference Powell, Baird, Charnley and Jordan1981; Barr, Holdsworth & Roberts, Reference Barr, Holdsworth and Roberts1986). The upper part of the Sgurr Beag Nappe is dominated by mainly unmigmatized psammites and quartzites of the Loch Eil Group (Stoker, Reference Stoker1983; Strachan, Reference Strachan1985). The base of the Loch Eil Group passes transitionally into the underlying migmatitic pelites and thinly interbanded psammites and semi-pelites of the Glenfinnan Group. A lower limit for deposition of the Glenfinnan and Loch Eil groups is provided by the c. 1000–950 Ma ages of the youngest detrital zircons incorporated within the metasediments (Friend et al. Reference Friend, Strachan, Kinny and Watt2003; Cawood et al. Reference Cawood, Nemchin, Strachan, Kinny and Loewy2004). The West Highland Granitic Gneiss suite, prominent members of which crop out in Ardgour, at Fort Augustus and at Glen Doe (Fig. 1), was emplaced at c. 870 Ma (Friend et al. Reference Friend, Kinny, Rogers, Strachan and Paterson1997; Rogers et al. Reference Rogers, Kinny, Strachan, Friend and Paterson2001). The current consensus is that this probably represents an early rift-related igneous suite, broadly coeval with intrusion of amphibolites and meta-gabbros that record all the tectonothermal events present in their Moine host rocks (Millar, Reference Millar1999; Dalziel & Soper, Reference Dalziel and Soper2001).
Figure 2. Simplified map of the main D3 and D4 folds within the SW Moine (using data from Powell et al. Reference Powell, Baird, Charnley and Jordan1981; Roberts, Smith & Harris, Reference Roberts, Smith and Harris1984; Stoker, Reference Stoker1983; Strachan, Reference Strachan1985). Abbreviations: SBT – Sgurr Beag Thrust; GS – Glenshian Synform; LEA – Loch Eilt Antiform; GDS – Glen Dessary Synform; SMA – Spidean Mialach Antiform; SGF – Sgurr Ghiubhsachain Fold; MCA – Meall na Cuartaige Antiform; BATS – Beinn an Tuim Synform; LES – Loch Eil Synform; SA – Stronchreggan Antiform; CA – Corrlarach Antiform; LCS – Lochan na Cruaich Synform; MFA – Meall an Feidh Antiform; GSS – Glen Scaddle Synform; GGA – Glen Gour Antiform; GGS – Glen Gour Synform.
Figure 3. Generalized geological cross-section across the SW Moine drawn approximately along line XY in Figure 1, showing the westerly increase in the intensity of D4 upright folding to form the Northern Highland Steep Belt (modified from Powell & Glendinning, Reference Powell, Glendinning, Allison, May and Strachan1988). Abbreviations: MT – Moine Thrust; KNT – Knoydart Thrust; SBT – Sgurr Beag Thrust; GS – Glenshian Synform; LEA – Loch Eilt Antiform; GDS – Glen Dessary Synform; SMA – Spidean Mialach Antiform; LQL – Loch Quoich Line; GGF – Great Glen Fault. Note (a) the infold of Loch Eil Group in the core of the Glen Dessary Synform; (b) the local emergence of Glenfinnan Group rocks adjacent to the Great Glen Fault (see Fig. 1).
The Moine rocks of the Loch Eil Group in West Inverness-shire record four main phases of deformation (Strachan, Reference Strachan1985; see also Dalziel, Reference Dalziel1966 and Stoker, Reference Stoker1983). Early minor isoclinal folding (D1) was associated with formation of an S1 schistosity that developed under amphibolite facies conditions. Elsewhere in West Inverness-shire (E. K. Hyslop, unpub. Ph.D. thesis, Univ. Hull, 1992; Rogers et al. Reference Rogers, Hyslop, Strachan, Paterson and Holdsworth1998; Vance, Strachan & Jones, Reference Vance, Strachan and Jones1998; Tanner & Evans, Reference Tanner and Evans2003), a mid-Neoproterozoic (820–730 Ma) age has been demonstrated for early folding, amphibolite-facies metamorphism and segregation of dated syn-metamorphic pegmatites at Ardnish, Sgurr Breac, Knoydart, Loch Eilt and Carn Gorm (Fig. 1). A similar age is assumed for D1 in the Loch Eil Group. A subsequent phase of widespread minor tight to isoclinal folding (D2) resulted in formation of a composite S0/S1/S2 foliation (Strachan, Reference Strachan1985). The foliation carries a stretching and mineral lineation (L2) that trends north–south where unaffected by later deformation. These structures are correlated with the D2 folds and associated fabrics recognized in the northern outcrop of the Loch Eil Group east of Loch Quoich (Fig. 1; Holdsworth & Roberts, Reference Holdsworth and Roberts1984). Titanites aligned within the composite S1/S2 foliation within the Fort Augustus Granite Gneiss (Fig. 1) have yielded a U–Pb age of 470 ± 1 Ma, suggesting that D2 occurred during the Ordovician Grampian orogenic event (Rogers et al. Reference Rogers, Kinny, Strachan, Friend and Paterson2001).
The large-scale structural geology of the southern Loch Eil Group is dominated by major upright D3 and D4 folds (Fig. 2; Strachan, Reference Strachan1985). D3 folds are all upward-facing and plunge towards the Great Glen, with the result that younger rocks come on to the east. Folds are open to close in style, and an S3 mica schistosity (locally with fibrolite) is locally developed axial-planar to D3 folds. The refolding of D3 folds during D4 generated a regional-scale interference pattern (Fig. 2). D4 folds are gentle to open, and vary in trend from north–south in the area north of Loch Eil to NNE–SSW in Ardgour; an axial-planar crenulation fabric (S4) is widespread. Fibrolite, hornblende and biotite all recrystallize around S4 crenulations, and it is therefore concluded that D4 was characterized by amphibolite-faces conditions. D4 folding increases in intensity westwards to form the Northern Highland Steep Belt (Figs 2, 3; Roberts & Harris, Reference Roberts and Harris1983).
The prevailing view has been that the upright folds and associated metamorphic assemblages formed at a late stage of the Ordovician Grampian orogenic event (e.g. Kelley & Powell, Reference Kelley and Powell1985; Powell & Phillips, Reference Powell, Phillips and Harris1985; Harris, Reference Harris, Hibbard, van Staal and Cawood1995). A lower limit for the formation of the steep belt is provided by the intrusion of the pre-D4 Glen Dessary Syenite (Fig. 1) at 456 ± 5 Ma (U–Pb zircon: van Breemen et al. Reference van Breemen, Aftalion, Pankhurst and Richardson1979; Roberts, Smith & Harris, 1984). An upper limit is apparently provided by a U–Pb monazite age of 450 ± 10 Ma obtained from a pegmatite sampled within the steep belt west of Glenfinnan and thought to pre-date upright folding (van Breemen, Pidgeon & Johnson, Reference van Breemen, Pidgeon and Johnson1974). According to Powell & Phillips (Reference Powell, Phillips and Harris1985), formation of the steep belt was followed by a prolonged period of post-tectonic cooling from c. 453 Ma to c. 405 Ma. The southern part of the steep belt is cut by the Strontian Granite that was intruded at 425 ± 3 Ma (Fig. 1; Rogers & Dunning, Reference Rogers and Dunning1991). Most of the intrusion is unmetamorphosed and dominated by pre-full-crystallization magmatic fabrics (Hutton, Reference Hutton1988).
Figure 4. Composite cross-section across the Glen Scaddle Metagabbro and its Moine country rocks to illustrate the nature of the major D3 folds and the structural setting of the intrusion in the core of the Glen Scaddle Synform (modified from Stoker, Reference Stoker1983). The intrusion carries a moderately to steeply dipping foliation that is axial-planar to the Glen Scaddle Synfom. Note that the Glen Gour Metagabbro to the southwest occupies a lower structural level, in the core of the D3 Glen Gour Antiform. The Moine succession is right-way-up. See Figure 2 for line of section; abbreviations for folds as in Figure 2.
Reappraisal of this interpretation is appropriate because the Grampian event is now thought to have been short-lived (c. 480–465 Ma) and followed by very rapid exhumation (Oliver et al. Reference Oliver, Chen, Buchwaldt and Hegner2000). Furthermore, Kinny et al. (Reference Kinny, Strachan, Friend, Kocks, Rogers and Paterson2003) have demonstrated widespread Silurian ductile deformation and amphibolite-facies metamorphism in the Moines of central and west Sutherland. Upright folds of the steep belt in northern Ross-shire (Fig. 1) deform lineations within the Morar Group that have been correlated with the Silurian fabrics identified in central Sutherland (Kinny et al. Reference Kinny, Strachan, Friend, Kocks, Rogers and Paterson2003). Either regional correlations of structures are in error or the published upper limit on the age of the steep belt (van Breemen, Pidgeon & Johnson, Reference van Breemen, Pidgeon and Johnson1974) is less robust than thought previously. In an effort to place further constraints on the age of upright folding, we now focus on the geology of eastern Ardgour, with particular reference to the structural setting and U–Pb geochronology of the Glen Scaddle Metagabbro (Fig. 1).
3. Geology of the Glen Scaddle Metagabbro
3.a. Structural setting and contact relationships
The Glen Scaddle Metagabbro intrudes the Moine rocks of the Loch Eil Group (Stoker, Reference Stoker1983; Strachan, Reference Strachan1985). In eastern Ardgour these comprise a varied succession of psammites and quartzites which are mostly right-way-up where sedimentary structures are present. The structure of eastern Ardgour is dominated by upright to moderately inclined, tight to close D3 folds (Figs 2, 4). The Glen Scaddle Metagabbro occupies the core of a major D3 fold, the Glen Scaddle Synform, flanked to the northeast by the D3 Meall an Feidh Antiform and to the southwest by the D3 Glen Gour Antiform (Figs 2, 4). A smaller intrusion, the Glen Gour Metagabbro, occupies the core of the Glen Gour Antiform (Fig. 4) and is apparently at a lower structural level than the Glen Scaddle Metagabbro.
The margins of the Glen Scaddle Metagabbro are broadly parallel to the regional composite S0/S1/S2 foliation within host Moine psammites. Contacts with Moine rocks are generally concordant and sharp [e.g. NN 0050 6920]. Small xenoliths (10–50 cm) of Moine lithologies are locally common within the marginal facies of the intrusion [e.g. NM 9662 7004]. Within a narrow (0.75–1 km) aureole developed within the Moine rocks, regional metamorphic assemblages are overprinted by contact metamorphic minerals (Drever, Reference Drever1940; Ashworth & Chinner, Reference Ashworth and Chinner1978). Quartz–feldspar–biotite rocks that are interpreted as nebulitic migmatites formed from the partial melting and mobilization of Moine lithologies are locally developed on the decametre scale along the northeast contact of the Glen Scaddle body [e.g. NM 9656 7012]. These are characterized by disoriented xenoliths of Moine psammites that apparently resisted melting, as well as zones of complex disharmonic folding.
3.b. Meta-igneous lithologies
The most common rock type is a medium- to coarse-grained, foliated metagabbro. Subordinate meta-igneous rock types range from ultrabasic to acidic in composition. Foliated hornblende peridotite and serpentinite were recorded by Drever (Reference Drever1940) from the margins of the Glen Gour and Glen Scaddle intrusions. Also present within the metagabbro are concordant metre-scale sheets of fine-grained hornblende schist and at least two phases of fine- to medium-grained granitic veins that are mainly developed on the centimetre–decimetre scale but can range up to 2–3 m in thickness. The earliest (meta)granitic veins are foliated and locally associated with hornblende schist; complex textures indicative of mingling between the two parent magmas are sometimes preserved [e.g. NM 9965 6856]. The later granitic veins are generally unfoliated and discordant, locally forming ramifying networks that cross-cut host meta-igneous lithologies.
3.c. Petrology and microfabrics
Relic igneous mineralogies and textures are best preserved in rare areas of low tectonic strain up to several hundred metres wide [e.g. River Scaddle, NM 9814 6818] and it is appropriate to describe these first before outlining the effects of superimposed deformation and metamorphism. In low strain zones, the metagabbro is essentially undeformed and typically consists of hypersthene, clinopyroxene, plagioclase (An35–40) and amphibole with minor quartz, biotite and accessory ilmenite, titanite and apatite. Feldspars are rectangular and randomly oriented. Pyroxene is mostly replaced partially to completely by uralitic amphibole, and this is interpreted to be the result of late stage deuteric alteration within the igneous protolith (Stoker, Reference Stoker1983). Feldspar is commonly highly altered and sericitized. Sub-ophitic textures are common, as are symplectic intergrowths between pyroxene and plagioclase. The effects of apparently static metamorphism are represented by the recrystallization of amphibole into granoblastic aggregates of hornblende, and the growth of radiating mats of biotite. M. Stoker (unpub. Ph.D. thesis, Univ. Liverpool, 1980) recorded a local crude rhythmic layering in metagabbro [e.g. NM 9945 6850] shown by gradation of melanocratic to leucocratic gabbroic types on scales of up to several metres. Low strain areas of metagabbro locally contain metre-scale mafic enclaves [e.g. NM 9852 6827] and are intruded by undeformed granitic sheets.
Areas of low tectonic strain pass transitionally into the foliated metagabbro that forms the bulk of the intrusion. The relic igneous features described above are progressively replaced by a penetrative amphibolite-facies deformation fabric defined by aligned layers of intergrown hornblende and biotite that wrap augen of recrystallized plagioclase (Fig. 5). The mafic enclaves and the undeformed granites present in the low strain zones are interpreted as the protoliths of, respectively, the hornblende schists and the early foliated granitic veins within the foliated metagabbro. Within the hornblende schists, the foliation is defined by a penetrative alignment of fine-grained hornblende and recrystallized feldspar. The early granitic veins are characterized by an intense mylonitic fabric: classic mortar texture is displayed by ribbons of dynamically recrystallized quartz that wrap feldspar augen (Fig. 6). Feldspar porphyroclasts are sometimes fringed by myrmekite which is probably strain-induced (Fig. 6).
Figure 5. Photomicrograph of typical foliated metagabbro (horizontal field of view, 4 mm). F – feldspar, H – hornblende; double-headed arrow indicates orientation of foliation.
Figure 6. Photomicrograph of mylonitic granite sheet within the Glen Scaddle Metagabbro (field of view, 4 mm). Q – aggregates of dynamically recrystallized quartz, F – feldspar porphyroclasts, M – myrmekite; double-headed arrow indicates orientation of foliation.
3.d. Structure and age of emplacement
Along the margins of the Glen Scaddle Metagabbro, the foliation described above is broadly parallel to the contact with host Moine rocks. However, within the intrusion the foliation is more uniform in orientation, trending approximately NW–SE and dipping moderately to steeply to the northeast. A moderately plunging mineral and extension lineation is defined locally by alignment of hornblende–biotite aggregates and plagioclase augen [e.g. NM 9965 6856].
The following two observations establish that intrusion post-dated the early fabric-forming events D1 and D2. Firstly, within the aureole around the Glen Scaddle Metagabbro, syn-D1 peak regional metamorphic assemblages are overprinted by contact metamorphic minerals, including sillimanite, biotite, plagioclase, quartz, K-feldspar, cordierite and andalusite which have been reported from various samples of pelite (Ashworth & Chinner, Reference Ashworth and Chinner1978; Stoker, Reference Stoker1983). Secondly, the intrusion locally incorporates xenoliths that contain isoclinal folds that are most probably D2 in age [e.g. NM 9680 6835].
A pre-D3 age of emplacement is consistent with the broad parallelism of the metamorphic foliation in the intrusion to the axial plane of the D3 Glen Scaddle Synform, and the observation that contact metamorphic sillimanite is wrapped by the S3 fabric (Stoker, Reference Stoker1983). Furthermore, the Glen Scaddle Synform is defined within the intrusion by the orientation of primary layering which is folded by NW-trending upright folds that are equated with the D3 folds developed within the host Moine rocks (Stoker, Reference Stoker1983). Local structural complexity within the core of the synform is indicated by the observation that the foliation in the metagabbro and the mylonitic fabric within early granite veins are also locally deformed by tight, steeply plunging D3 folds [e.g. NM 9965 6856]. M. Stoker (unpub. Ph.D. thesis, Univ. Liverpool, 1980) interpreted this complexity as indicating that D3 was locally polyphase.
A pre-D4 age of intrusion is certain because the foliation is deformed and crenulated by NE–SW-trending open folds that are correlated with regional folds of this age within the Moine country rocks. A mesoscopic D4 fold pair deforms the Glen Gour body and the southeasternmost part of the Glen Scaddle Metagabbro (Fig. 2; Stoker, Reference Stoker1983).
3.e. Summary
It is therefore concluded, following Stoker (Reference Stoker1983), that the Glen Scaddle Metagabbro was emplaced after D2, probably as a series of sills that were concordant with the regional S0/S1/S2 foliation within host Moine rocks. This foliation was probably flat-lying after D2. The generalized gradation from ultrabasic rocks at the margins of the Glen Scaddle and Glen Gour intrusions to basic and acidic rocks centrally suggests that they may represent differentiated layered sheet intrusions. The intrusion was then deformed and metamorphosed during D3 and D4. The structural evidence therefore indicates that the Glen Scaddle Metagabbro is an important marker within the regional tectonometamorphic sequence; its age places a lower limit on the timing of regional upright folding and associated amphibolite-facies metamorphism.
4. U–Pb geochronology
4.a. Sample preparation procedure and analytical techniques
The sample analysed was a medium-grained, foliated metagabbro (Fig. 7) collected from the River Scaddle [NN 0004 6869]. A sample of 30 kg was crushed and milled, and the less than 400 μ fraction sieved out. Heavy mineral concentrations were obtained using a Gemini shaking table, followed by a superpanner. A separate, with specific gravity greater than 3.3 gm ml−1, was recovered using Di-iodomethane. The minerals were then separated magnetically using a Frantz LB-1 magnetic separator. The recovered zircons from the non-magnetic ∼ 1.8 Amp fraction were hand-picked under alcohol and abraded. The majority of the zircons in the sample were fragments of quite large grains (about 150 μ length). The fragments were predominantly clear with sharp terminations and edges where visible. A few showed elongate melt inclusions and these were avoided during picking.
Figure 7. Hand specimen of foliated metagabbro sampled for isotopic dating in this study.
U and Pb separations followed the procedures of Krogh (Reference Krogh1973) with minor modifications of Corfu & Ayres (Reference Corfu and Ayres1984). Zircon grains were analysed on a VG 354 mass spectrometer at the NERC Isotope Geosciences Laboratory following the procedures of Noble, Tucker & Pharaoh (Reference Noble, Tucker and Pharaoh1993). Chemistry blanks were ∼ 5 pg, and these were monitored in each batch of chemistry. Uranium blanks contained < 0.1 pg U. All results and errors were calculated following the methods of Ludwig (Reference Ludwig1993, Reference Ludwig1994), and the Pb isotope ratios were corrected for initial common Pb in excess of laboratory blank using the model of Stacey & Kramers (Reference Stacey and Kramers1975). Ages were calculated using the decay constants of Jaffey et al. (Reference Jaffey, Flynn, Glendenin, Bentley and Essling1971).
4.b. Results
Three zircon fractions plot either on or close to concordia and give an age of 426 ± 3 Ma (Fig. 8; Table 1). This age is interpreted to date closely igneous crystallization of the Glen Scaddle Metagabbro.
Table 1. U–Pb data for zircon from the Glen Scaddle Metagabbro
(1) All analyses picked from non magnetic at 1.8 Amp fraction of zircons. (2) Sample weights and hence U and Pb concentrations are approximate. (3) Measured ratios are corrected for fractionation and common Pb spike. (4) Corrected for fractionation, spike, laboratory blank and initial common Pb calculated at 427 Ma (Stacey & Kramers, Reference Stacey and Kramers1975). (5) Correlation coefficients of 207Pb/235Pb and 206Pb/ 238U are calculated using procedures and algorithms of (Ludwig, Reference Ludwig1993). Errors for the measured ratios are propagated through data reduction and quoted at the 2 sigma level.
Figure 8. U–Pb concordant ages from the Glen Scaddle metagabbro. The MSWD is of concordance and equivalence at 2σ and takes into account errors on the decay constant. The labelled ellipse is the weighted mean error ellipse of the three data points.
5. Discussion
5.a. Age of polyphase upright folding in the SW Moine
The U–Pb zircon age of 426 ± 3 Ma reported here for the emplacement of the Glen Scaddle Metagabbro suggests that this intrusion is a basic member of the late Caledonian ‘Newer Granite’ suite (Read, Reference Read1961; Stephenson et al. Reference Stephenson, Bevins, Millward, Highton, Parsons, Stone and Wadsworth1999, and references therein). More importantly, in combination with the structural history outlined here, as well as the 425 ± 3 Ma age for the Strontian Granite located only a few kilometres to the south, it implies a very different regional tectonometamorphic history for the Moine Supergroup in West Inverness-shire and Ross-shire to that currently published. Regional-scale D3–D4 upright folding, including the formation of the Northern Highland Steep Belt, must have occurred during a relatively short (Scandian) orogenic event. Western parts of the Strontian Granite are overprinted by locally developed steep, N–S-trending solid-state deformation fabrics (Hutton, Reference Hutton1988), and it is therefore possible that granite emplacement overlapped final stages of D4 folding.
A younger age for regional upright folding than the Ordovician age proposed previously resolves the apparently contradictory structural relationships in Ross-shire, where steep belt folds deform structures assigned to the Scandian event (Kinny et al. Reference Kinny, Strachan, Friend, Kocks, Rogers and Paterson2003). However, a Silurian age for the formation of the Northern Highland Steep Belt is in contradiction to the U–Pb monazite age of 450 ± 10 Ma obtained from a pegmatite west of Glenfinnan that was interpreted to post-date upright folding (van Breemen, Pidgeon & Johnson, Reference van Breemen, Pidgeon and Johnson1974). The exact significance of this age is difficult to evaluate, partly due to the lack of a detailed field description of the dated pegmatite and its structural setting, but also because of the possibility that the dated monazites included an unrecognized inherited component. We consider the new age constraints reported here on the age of regional upright folding more reliable, although the structural setting and age of pegmatites within the steep belt clearly deserves reinvestigation.
5.b. Implications for late-orogenic displacements along the Great Glen Fault
A Silurian age for regional, polyphase upright folding and associated amphibolite-facies metamorphism of the Moine Supergroup is consistent with the current model for Caledonian orogenic activity in the Northern Highland terrane, involving Grampian (470–460 Ma) and Scandian (435–425 Ma) events (Kinny et al. Reference Kinny, Friend, Strachan, Watt and Burns1999, Reference Kinny, Strachan, Friend, Kocks, Rogers and Paterson2003; Friend, Jones & Burns, Reference Friend, Jones and Burns2000; Dallmeyer et al. Reference Dallmeyer, Strachan, Rogers, Watt and Friend2001). The new evidence reported here, in combination with the findings of Kinny et al. (Reference Kinny, Strachan, Friend, Kocks, Rogers and Paterson2003), demonstrates widespread reworking of Grampian structures and metamorphic assemblages during the Scandian event. This culminated in development of the Moine Thrust Zone and emplacement of the Moine rocks onto the Laurentian foreland. The Northern Highland Steep Belt is thought to have formed at the same time and presumably detaches at depth on a ductile thrust zone, probably the Moine Thrust (Barr, Holdsworth & Roberts, Reference Barr, Holdsworth and Roberts1986).
The tectonothermal history detailed above is in marked contrast to that recorded by the Dalradian Supergroup southeast of the Great Glen Fault, where the main phases of folding and associated regional metamorphism are assigned entirely to the Ordovician Grampian orogenic event. The present consensus is that the peak of Grampian metamorphism in the Dalradian rocks occurred at c. 470 Ma and was followed by cooling and uplift relatively soon thereafter at c. 460 Ma (Dempster, Reference Dempster1985; Dempster, Hudson & Rogers, Reference Dempster, Hudson and Rogers1995; Dempster et al. Reference Dempster, Rogers, Tanner, Bluck, Muir, Redwood, Ireland and Paterson2002; Soper, Ryan & Dewey, Reference Soper, Ryan and Dewey1999; Oliver et al. Reference Oliver, Chen, Buchwaldt and Hegner2000; Oliver Reference Oliver2001; Baxter, Ague & DePaulo, Reference Baxter, Ague and DePaulo2002). As yet, there is no evidence that the Dalradian Supergroup was affected by regionally significant Silurian deformation or metamorphism. The conclusions reported here therefore reinforce tectonic models that invoke a minimum of 700 km of late Caledonian (425–390 Ma?) sinistral movement along the Great Glen Fault in order to juxtapose the Grampian and Northern Highland terranes following the Scandian collision between Baltica and the Northern Highland segment of eastern Laurentia (Dewey & Strachan, Reference Dewey and Strachan2003; Kinny et al. Reference Kinny, Strachan, Friend, Kocks, Rogers and Paterson2003).
6. Conclusions
(1) The Glen Scaddle Metagabbro forms a valuable time marker within the Northern Highland Terrane of the Scottish Caledonides because its age places a lower limit on the timing of regional upright folding and associated amphibolite-facies metamorphism.
(2) The intrusion has yielded a U–Pb zircon age of 426 ± 3 Ma that is interpreted to date its igneous crystallization.
(3) Regional-scale D3 and D4 upright folding of the intrusion and its Moine host rocks, including formation of the Northern Highland Steep Belt, occurred during a relatively short Silurian (= Scandian) orogenic event. Final stages of D4 deformation may have overlapped emplacement of the Strontian Granite, dated previously at 425 ± 3 Ma.
(4) The data reported here provide further evidence for widespread Silurian ductile deformation and amphibolite-facies metamorphism northwest of the Great Glen Fault; the apparent absence of this orogenic event within the adjacent Grampian Terrane reinforces arguments that this structure separates crustal blocks with contrasting Caledonian tectonothermal histories.
(5) These differences support tectonic models that invoke a minimum of 700 km late Caledonian sinistral movement along the Great Glen Fault in order to juxtapose the Grampian and Northern Highland terranes following the Scandian collision.
(6) This study therefore shows how comparison of the varying intensities of deformational events across an orogen can be used to reconstruct original collisional templates and constrain likely displacements along late-orogenic transcurrent faults.
Acknowledgements
Isotopic dating was funded by a grant from the Steering Committee of the NERC Isotope Geosciences Laboratory. Martyn Stoker is thanked for discussions about the geology of the Glen Scaddle area; Tony Harris and John Mendum contributed detailed reviews.
