Geology of the Aberfoyle district: Regional and crustal-scale context

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 north of the Aberfoyle district, the rocks of the Dalradian Supergroup from part of the Grampian Terrane. To the south of the Highland Boundary Fault Zone, the Devonian and Carboniferous rocks lie within the Midland Valley Terrane. Each of these terrane has distinctive geophysical and geochemical properties that reflect different geological histories.

Studies to date suggest that the basement to much of the Grampian Terrane largely comprises early Proterozoic quartzo-feldspathic, hornblende-bearing meta-igneous gneisses, approximately 1.8–2.0 Ga in age (Trewin and Rollin, 2002, and references therein). Isotopic data derived from putative basement rocks on the Rhinns of Islay (Marcantonio et al., 1988), from Caledonian granites (Halliday, 1984; Stephens and Halliday, 1984), and gneisses from Inishtrahull (Dickin and Bowes, 1991), indicate a protolith derived from juvenile mantle material that is geochemically distinct from the Archaean gneisses of the North West Highlands and Islands. However, the basement beneath the Dalradian is not homogeneous. Most significantly, late Caledonian granitic rocks with high δNd (>–6) appear to be restricted to the southern Highlands south of the ‘Mid Grampian Line’ (Halliday, 1984), where the isotopic evidence indicates the presence of basement less than ~1.0 Ga in age. Thus younger crust appears to be present under the southern Highlands, possibly thrust under the Grampian Terrane (Halliday, 1984), and possibly extending as far north as the Boundary Slide (e.g. Stephenson and Gould, 1995). Furthermore, to account fully for the regional increase in gravity towards the Highland Boundary Fault. Geophysical models require a wedge of higher density basement beneath the southern Highlands Interpretation of seismic data suggests that this could be part of the Midland Valley Terrane basement, as discussed and illustrated by Trewin and Rollin (2002, fig. 1.11, page 23). Although it has been suggested that Highland Border Complex rocks could be thrust under the Dalradian along the Highland border (Bluck, 1984), Halliday et al. (1985) indicated that the underthrust crust cannot comprise rocks of the Highland Border Complex, because the Sm-Nd systematics of the latter is incompatible with those of the granites south of the Mid Grampian Line.

The basement to the Midland Valley Terrane has a markedly different geophysical signature compared to that underlying the Grampian Terrane. LISPB seismic data reveal a high velocity region extending down into the lower crust, from about 8 km beneath the Palaeozoic sedimentary and igneous rocks. As discussed above, it appears that this basement is much younger than that underlying the Grampian Terrane north of the Mid Grampian Line.

Upton et al. (1984) summarised the evidence from xenoliths for the nature of the middle and lower crust under the Midland Valley. Xenolith suites in Late Palaeozoic alkali basalts in the wider Midland Valley and the southernmost southern Highlands suggest that the lower crustal basement is composed dominantly of mafic pyroxene granulites, with relatively common anorthositic varieties and rare garnet granulites. Tonalitic to trodhjemite (leucotonalite) xenoliths are also present, and may reflect more the composition of mid-crustal levels. More quartzo-feldspathic types of metasedimentary origin, containing garnet and sillimanite, are widespread and are also likely to be typical of the rocks at mid-crustal levels.