Petrology,structure and age of the Rough Point sill,Donegal

The Rough Point sill, which is one of the thickest of a large number of sills intruded into the Appin Group metasediments of the Dalradian sequence in north Donegal, contains a number of internal features including mafic:felsic banding, pegmatite bands and significant variations in grain size. There are also appreciable variations in mineralogy, particularly the proportions of plagioclase and amphibole, and in the major and trace element chemistry. Together these features indicate that the sill is, like the metasediments which it intrudes, inverted, and is therefore older than the Caledonian F₂ structures which caused the inversion. Disruption of the sediments at the sill contact is also earlier than the F₂ event. The timing of the basic magmatic episode together with the geochemistry of the sill invite comparison with the metadolerites of the southwest Highlands of Scotland.