Geology of the Aberfoyle district: Principal tectonostratigraphical units
|This topic provides a summary of the geology of the Aberfoyle district – covered by the British Geological Survey. 1:50k geological map sheet 38E (Scotland).
Authors: C W Thomas, A M Aitken, E A Pickett, J R Mendum, E K Hyslop, M G Petterson, D Ball, E Burt, B Chacksfield, N Golledge and G Tanner (BGS).
In the revised, largely tectonic stratigraphy presented here, only four units are considered to comprise the Highland Border Complex (s.s.) in the Aberfoyle district. These units include elements of Tanner’s (2007) Garron Point Group, although this lithostratigraphical grouping is not used in this account. The units crop out principally in the Loch Ard and Achray forests, south-west and north-east of Aberfoyle respectively (Figure 5). The revised HBC tectonostratigraphy for the Aberfoyle district is as follows:
|Dounans Limestone Formation||Burn of Mar Sandstone Formation (stratigraphical position equivocal)|
|Corrie Burn Hornblende Schist|
The Corrie Burn Hornblende Schist is the north-westernmost unit, and has a very strongly deformed contact with the Bofrishlie Slate Formation (see also Neoproterozoic to Cambrian). The Lime Hill serpentinite lies to the south-east of the Corrie Burn Hornblende Schist, with isolated occurrences cropping out in Loch Ard Forest. There is general consensus that these mafic and ultramafic units comprise tectonically emplaced elements of an ophiolite (e.g. Tanner, 2007). The Dounans Limestone sits on an erosion surface on the Lime Hill Serpentinite, as seen around Dounans Quarry [NN 532 009] and in steep ground immediately to the south-west. Fabrics and contacts between units dip steeply to the north-west. Because the Dounans Limestone has a sedimentary contact with the serpentinite, and youngs to the south-east, the tectonostratigraphical order is slightly inverted. The tectonic relationships between the Burn of Mar Sandstone Formation and other units within the HBC are obscured by poor exposure and Devonian and Carboniferous cover in the south-west of the district. Consequently, the tectonostratigraphical position of the Burn of Mar Sandstone is highly conjectural.
Corrie Burn Hornblende Schist (DCB)
South-west of Drum of Clashmore [NS 490 977], the Corrie Burn Hornblende Schist (P219927) occupies ground between the Bofrishlie Slate Formation and the carbonated and silicified serpentinite to the south-east. Though the hornblende schist is stripy in appearance, it is clearly of igneous origin over most of its outcrop. However, hornblende schist (s.s) gives way to striped, chlorite-albite schists along the north-west margin of the outcrop. Henderson and Fortey (1982) suggested that these are derived from laminated volcanic sediments, whilst Jehu and Campbell (1917) considered them deformed spilites; they may also be very highly altered hornblende schist.
Petrographical work by Jehu and Campbell (1917) and Henderson and Fortey (1982) showed the hornblende schist to be strongly deformed and, although Jehu and Campbell (1917) recorded igneous textures locally, a weak mineral lineation plunges north-east. The main fabric, defined by acicular green, pargasitic amphibole, has minor folds and a crenulation cleavage (Henderson and Fortey, 1982, p.238). The hornblende schist is also veined by epidote, and calcian almandine is developed locally. Geochemically, the hornblende schist has a ‘within-plate’ alkali-basalt chemistry (Pearce and Cann, 1973; Robertson and Henderson, 1984).
The north-western margin of the Corrie Burn Hornblende Schist is bounded by a tectonic contact with the black pelites of the Bofrishlie Slate Formation, with evidence for very strong deformation and recrystallisation. Figure 5 of plate iv in Jehu and Campbell (1917; fig. 5, plate iv) clearly illustrated mylonitisation of Bofrishlie Slate adjacent to the Hornblende Schist. Henderson and Fortey (1982) showed that the hornblende schist also contains a strong, pervasive fabric, having a ‘marked platiness’ (P219927). On their map of the HBC, Jehu and Campbell (1917, plate vi) recorded thin slivers of ‘metamorphosed black shale and chert’ at the north-west margin of the hornblende schist. They suggested that the slivers ‘exhibit dynamic metamorphism superposed on earlier contact metamorphism’, the contact metamorphism resulting in the growth of porphyroblasts or ‘knots’ of uncertain mineralogy, although one or two apparently contain relict garnet. In addition, they noted that the ‘knots’ are deformed (Jehu and Campbell, 1917, pages 199–200). Henderson and Fortey (1982) indicated that the porphyroblasts have hexagonal form, are made up of muscovite, septachlorite and quartz, and probably represent pinitised cordierite.
The presence of rock of higher metamorphic grade to the south-east of the Bofrishlie Slate, the latter metamorphosed under lowermost greenschist facies conditions, is inconsistent with the general north-westerly direction of increasing grade across the Dalradian outcrop (Chapter 9). The spatial association of amphibolite facies metamorphism with the hornblende schist indicates that the hornblende schist protolith was probably already metamorphosed to amphibolite before emplacement, but may also have still been hot when emplaced. The intense shearing deformation noted along the north-westerly contact of the amphibolite clearly suggests that the Corrie Burn Hornblende Schist was emplaced tectonically, carrying with it slivers of mylonitised slate.
Limehill Serpentinite (doULH)
The Limehill Serpentinite is a complex, tectonised unit comprising serpentinised and carbonated ultramafic rocks and red jasper, the latter resulting from silicification of ferroan dolomite (Jehu and Campbell, 1917). Very locally occurring conglomeratic rocks are also assigned to this unit. The components of the Limehill Serpentinite lie to the south-east of the Corrie Burn Hornblende Schist and were almost certainly emplaced tectonically, possibly along with the mafic unit.
In the serpentinite, serpentine minerals are commonly replaced by orange-brown weathering dolomite, magnesite and quartz. Henderson and Robertson (1982) suggested that the protolith was dominated by harzburgite, based on replacement textures and the proportions of replacement minerals. These rocks occupy ground on the south-eastern side of the outcrop of the HBC.
The conglomeratic rocks contain serpentinitic and carbonated clasts, and well-rounded clasts of quarztite in a carbonated, serpentinitic matrix. This lithology is referred to as the ‘Dounans Conglomerate’ by Henderson and Fortey (1982). It is exposed in small outcrops on steep ground immediately beneath Devonian conglomerates about 1.5 km east-north-east of Aberfoyle [NN 532 017] (P643961), and in the Kelty Water [NS 473 964] (Henderson and Fortey, 1982). Sheared serpentinite passes south-eastwards into a rubbly serpentinitic lithology with well-rounded to subrounded, coarse, quartzitic and psammitic clasts, together with grey chert, silicified and carbonated gabbro, serpentinite and limestone (Henderson and Fortey 1982). It is difficult to be certain of their origin, but the conglomeratic rocks are most likely to be sedimentary, indicating weathering of a dominantly serpentinitic and igneous source but also including local sedimentary source rocks (Henderson and Fortey 1982).
Burn of Mar Sandstone Formation (QBM)
The Burn of Mar Sandstone Formation comprises lithic arenite, and is mapped in the Burn of Mar area in the south-west corner of the sheet. It is fault bounded on both margins of its outcrop, is very poorly exposed and has apparently limited extent. To the north lie rocks of the Carboniferous Kinnesswood Formation, and to the south is the Lower Devonian Inchmurrin Conglomerate Formation. It is a medium to coarse-grained, gritty and weakly cleaved, grey, feldspathic and lithic sandstone with thin slaty pelite layers. The key exposure lies 1 km west-south-west of Maol Odhar [NS 4394 9338]. Here, the bedding strikes just west of north and dips very steeply to the east. This contrasts with the east–west strike and southerly steep dip of the bedding in the Inchmurrin Conglomerate Formation immediately to the south, and the variable strike and shallow dip of the rocks in the Kinnesswood Formation to the north. This unit has been assigned to the Highland Border Complex because it contains detrital chromite and chloritised grains, suggesting an ultramafic source in common with other clastic units within the Highland Border Complex. It is similar to other chromite- and chlorite-bearing sedimentary lithologies, informally termed the ‘Loch Lomond Clastics’, that are part of the Highland Border Complex outcrop in the Arrochymore Point area, by Loch Lomond. It is considered that these lithologies were derived in part from an ultramafic source (Henderson and Fortey, 1982).
Dounans Limestone Formation (doLDL)
The Dounans Limestone is a dolomitised and silicified limestone resting unconformably on an eroded serpentinite where exposed in Lime Craig Quarry [NN 53415 01890].
The boundary between the limestone and the serpentinite is poorly exposed, and difficult to discern in outcrop because of the carbonation of the serpentinite. However, examination of outcrop at the edge of Lime Craig Quarry indicates that there are clasts of serpentinite in the basal part of the limestone, showing that the contact is unconformable. Though limited in outcrop, the Dounans Limestone is a very important component of the HBC because of the Arenig (Ordovician) fauna that it contains. Work undertaken during the 1980s by Curry and co-workers at the University of Glasgow (Curry et al., 1982; Curry et al., 1984; Ingham et al., 1985; Curry, 1986) revealed the presence of a sparse, poorly preserved, but diverse, silicified fauna including trilobites, brachiopods, ostracods, gastropods and other remains of clearly organic origin. The trilobites are amongst the most abundant and best preserved (Curry et al. 1982). They have Laurentian affinities and indicate an early Arenig age. Brachipods are less well preserved, but include Archaeorthis sp. and Orthidium sp (Curry et al. 1984).
Age of formation and emplacement of ophiolitic units within the Highland Border Complex The ages of the Corrie Burn Hornblende Schist and the Lime Hill Serpentinite are currently undefined, but can be no younger than early to mid Arenig, as the Dounans Limestone was deposited on eroded serpentinite at about 480 Ma. Dempster and Bluck (1991) dated amphibolite analgous to the Corrie Burn Hornblende Schist from the Highland Border Complex on Bute. Garnet Sm/Nd and amphibole K-Ar analyses yielded ages of 546 ± 42 and 536 ± 11 Ma respectively, indicating an early to mid Cambrian age for the amphibolite facies metamorphism. However, Tanner (2007) inferred an Arenig age for the ophiolitic elements, based on regional considerations. What is clear is that the protolith to the Corrie Burn Hornblende Schist was hot when emplaced against the Bofrishlie Slate Formation, given the evidence for contact metamorphism in the slates. In addition, detailed structural work by Tanner (in press) indicated that emplacement of the ophiolitic rocks very likely predated the end of Grampian D1 deformation.
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