Geosites Project

The Ben Vuirich Granite is one of a small suite of granitic intrusions formed ~ 600 million years ago, before the main deformation and regional metamorphism of the Dalradian country rocks. Its field relationships reveal evidence for the relative timing of the main structural events that have affected the area.

The Ben Vuirich Granite pluton is a small NE-SW trending, sheet-like body intruding metasedimentary rocks belonging to the middle part of the Dalradian succession. It is a grey medium-grained granite, composed of quartz, plagioclase and potassium feldspar, with minor biotite and muscovite, and conspicuous red garnets. Foliation in the granite dips steeply SE. Its radiometric age of 590 Ma gives a minimum age for the Dalradian. At issue is the timing of granite intrusion relative to the first two phases of deformation of the Dalradian (D1 and D2). At NO991703 the contact metamorphic aureole of the granite is exposed: a spotted hornfels containing andalusite and cordierite with a weak tectonic fabric, suggesting intrusion post-dated D1.
On the lower slopes of Carn Dubh (NO011692), an irregular contact of the granite with quartzose psammite is well exposed, with veins of granite cutting the country rock. The intrusion contains many xenoliths of foliated psammite, ranging in size from tens of centimetres to several metres. Generally the foliation is parallel to the long axis of each xenolith, but in some it is at a high angle and a few show isoclinal folds, also suggesting intrusion was post-D1 but pre-D2. The GCR interpretation is that the folds result from D2 deformation of the foliation in suitably orientated xenoliths. It is also possible that the folds in the xenoliths are of synsedimentary origin.