Ordovician intrusions in the English Lake District

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Abstract
Rb–Sr whole rock isochron and K–Ar whole-rock and mineral ages suggest that there were four intrusive episodes in the Lake District in Ordovician times, in addition to the widely recognized end-Silurian event. The Carrock Fell Gabbro has given an age of 468 ± 9 Ma and is probably coeval with the Eycott Volcanic Group. A suite of dioritic lamprophyres, including the Great Cockup ‘Picrite’, has a preferred age of 458±9 Ma and may be related to the main Borrowdale Volcanic Group. The Eskdale Granite was emplaced around 429 Ma, probably during the Caradoc, and the Ennerdale Granophyre, Carrock Fell Granophyres, and the Harestones Felsite are coeval with the Stockdale Rhyolite (421Ma) and of Ashgillian age. Muscovites from various facies of the Eskdale Granite show a wide spread of K–Ar ages from 426 to 388 Ma. The implications of these results for the tectonic and mineralogenic history of the area are discussed and previous research is reviewed in their light.

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