Geology of the Aberfoyle district: Pre-caledonian amphibolite sheets (Neo-proterozoic Basic Minor Intrusion Suite)
|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).
The oldest igneous intrusions within the district are a series of cleaved amphibolite/metadolerite sheets of Neoproterozoic age. They crop out around the northern end of Loch Lubnaig, and are particularly common on the northern and eastern slopes of Benvane [NN 535 137], locally forming a dense sill complex. The amphibolite sheets are mostly sills, a few metres or tens of metres thick, occurring predominantly within rocks of the Loch Katrine Volcaniclastic Formation or older units. These intrusions belong to a wider suite of mafic rocks, developed around the Argyll–Southern Highland Group boundary throughout the Dalradian of Scotland, and occurring in the northern half of Ireland. Their emplacement is related to the opening of the Iapetus Ocean at about 600 Ma (e.g. Stephenson and Gould, 1995).
The amphibolites are dark green, commonly massive rocks. Often they have a speckled appearance and preserve relict igneous textures. Hornblende and plagioclase feldspar dominate the mineralogy. The margins of the amphibolite sheets are commonly schistose, whilst the interiors are more isotropic; thin sheets may be schistose throughout. Elsewhere in the southern Highlands similar amphibolites sheets at this level record a complex history of fluid–rock interaction, between the fluids generated by the amphibolites and host rocks and the rocks themselves during prograde and retrograde metamorphism (Skelton et al., 1995; Graham et al., 1997).
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- Skelton, A D L, Graham, C M, and Bickle, M J. 1995. Lithological and Structural Controls of Regional 3-D Fluid Flow patterns during Greenschist Facies Metamorphism of the Dalradian of the SW Scottish Highlands. Journal of Petrology, Vol. 36, 563–586.
- Graham, C M, Skelton, A D L, Bickle, M, and Cole, C. 1997. Lithological, structural and deformation controls on fluid flow during regional metamorphism. 196–226 in Deformation-enhanced fluid transport in the Earth’s crust and mantle. Holness, M B (editor). Mineralogical Society Series, No. 8. (London: Chapman and Hall.)