The Caradoc volcanoes of the English Lake District

Abstract

SUMMARY During Late Ordovician (Caradoc) time, the English Lake District was the focus for one of the most intense episodes of magmatism seen during the geological history of the British Isles. There, two thick subaerial volcanic successions aggraded within opposing half-graben. In the northern half-graben, the Eycott Volcanic Group (EVG) comprises basaltic, andesitic and dacitic lavas and sills with subordinate pyroclastic rocks having geochemical affinities that are transitional between medium-K, continental-margin tholeiitic and calc-alkaline suites. In the southern half-graben, in the central Lake District, the Borrowdale Volcanic Group (BVG) comprises more than 6 km of basaltic to rhyolitic lavas, sills and pyroclastic rocks of medium- to high-K calc-alkaline continental margin type. Initial BVG eruptions were phreatomagmatic, but once conduits were opened, the volcanism became dominated by andesite lava effusion from clusters of low-profile volcanoes. The EVG compares to this early phase of BVG volcanism. A switch then occurred to paroxysmal eruptions, which produced many widespread sheets of densely welded silicic ignimbrite. This phase saw the development of major silicic, piecemeal calderas at Scafell and Haweswater. One of the later units, the Lincomb Tarns Formation, which is 150–800 m thick, is the most voluminous ignimbrite preserved within the Lake District. Other large-magnitude silicic eruptions are recorded within depocentres that lay along the current southern margin of the BVG outcrop. During intervals between the major silicic eruptions, andesitic tephra deposits were reworked and re-sedimented by mass-flow and fluvial processes, and were deposited within fluvio-lacustrine basins developed through regional extensional faulting and caldera collapse. Throughout, substantial volumes of magma were intruded contemporaneously into the volcanic piles. Towards the end of the volcanic episode, the granitic Lake District batholith was emplaced beneath the succession. Subsidence during cooling of this mass allowed renewed marine sedimentation across the region.