Early Palaeozoic magmatism in the English Lake District

Abstract

SUMMARY The Lower Palaeozoic rocks of the Lake District record the rifting of the microcontinental terrane of Avalonia from Gondwana early in the Ordovician, its drift northwards across the Iapetus Ocean and collision with Laurentia and Baltica in the Silurian, and its Acadian deformation in Early Devonian times. Three discrete episodes of magmatism occurred during these events. In late Ordovician (Caradoc) times large volumes of magma were generated in response to subduction of Iapetus oceanic crust beneath Avalonia. Large-scale eruptions formed the basalt–andesite–rhyolite volcanic fields that are now preserved as the Eycott and Borrowdale volcanic groups. Large bodies of mainly granitic magma were emplaced beneath the Borrowdale Volcanic Group as part of the largely Ordovician Lake District batholith; these include, for example, the Eskdale and Ennerdale intrusions. Less than 10 Ma later, a brief episode of silicic volcanism is recorded within the marine sedimentary strata of the lowest part of the overlying Windermere Supergroup. A period of about 40 million years of apparent quiescence then ensued before further granitic masses, the Shap and Skiddaw plutons were emplaced around the margins of the batholith in Early Devonian times. All of these magmatic episodes were accompanied by minor intrusions. The Eycott and Borrowdale volcanic groups are rare examples in the geological record of the products of subaerial volcanism. Preservation is entirely due to subsidence keeping pace with the emplacement of new material. Subsidence resulted both from extensional tectonic processes and, locally, during the movement