Geology of the Aberfoyle district: Late-caledonian minor intrusions (North Britain Siluro-Devonian Calc-alkaline 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).
Although not abundant in the region, the most widely distributed minor intrusions belong to the suite of late Caledonian dykes and sheets that intrude Dalradian metasedimentary rocks throughout the Grampian Highlands. These rocks of late Silurian to earliest Devonian age are associated with the widespread subduction-related Late Caledonian calc-alkaline magmatism, that resulted ultimately in the major granite plutons and the Devonian volcanic rocks of the Lorne district and Glen Coe (e.g. Strachan et al., 2002). The minor intrusions can be divided into microdioritic or lamprophyric mafic rocks and felsites.
Microdiorite dykes are uncommon, occurring as dykes and sheets with predominantly northerly to north-easterly trends. They are most numerous in the north-west corner of the district, between grid lines NN44 and NN47, and north of gridline NN15. Typically, they are fine grained and dark grey-green at outcrop. A groundmass of plagioclase feldspar (typically andesine to oligoclase), hornblende, biotite, minor potassium-feldspar and quartz hosts phenocrysts of plagioclase (andesine), with some hornblende and, more rarely, augite and/or biotite. Alteration of the groundmass and phenocrysts is widespread.
Only one lamprophyre intrusion has been recorded in the Aberfoyle district. This crops out in the ground Loch Katrine and Bealach na h-Imriche around [NN 482 105] and has a north-north-west trend.
Felsite dykes and sheets are the most common of the late Caledonian minor intrusions in the district. They are fine-grained quartz- and feldspar-rich microgranitic rocks, with a typically red brick colour, or more buff grey where weathered. The trends of the dykes and sheets vary from northerly to easterly, but are commonly north-easterly in this area. Isolated dykes occur throughout the outcrop of the Dalradian, but they are more numerous in the ground to the north of Loch Katrine. They are unfoliated, but are commonly heavily jointed.
- Strachan, R A, Smith, M, Harris, A L, and Fettes, D J. 2002. The Northern Highland and Grampian Terranes. 81–147 in The Geology of Scotland (Fourth edition). Trewin, N H (editor). (London: The Geological Society.)