SUMMARY

The Eycott Volcanic Group, at the northern margin of the Lower Palaeozoic inlier in the Lake District, is the smaller of two substantial middle Ordovician (Caradoc) subduction-related volcanic successions that stratigraphically separate marine sedimentary successions of the Skiddaw Group and Windermere Supergroup. Tabular lavas and subordinate sills, in a sequence up to 2400 m thick, mainly comprise porphyritic basalt, basaltic andesite, andesite and dacite, and are locally interbedded with thin units of volcaniclastic sandstone and pyroclastic rocks; these are overlain by 800 m of acid andesitic pyroclastic rocks. The distribution and form of the lava/sill fades association are consistent with emplacement as a lava plateau sequence, remarkably similar to the Birker Fell Formation, the lower part of the Borrowdale Volcanic Group in the central Lake District. The Eycott Volcanic Group rocks are geochemically coherent with characteristics transitional between medium-K, continental-margin tholeiitic, and calc-alkaline andesite suites. Rocks within the suite can be linked by fractionation of an assemblage of plagioclase, pyroxene, Fe–Ti oxide and apatite. A prominent compositional gap between about 58 and 65% SiO2 is attributed to the rapid precipitation and segregation of Fe–Ti oxide. Incompatible element concentrations in the mafic members suggest that magmas were derived possibly from a subcontinental lithosphere source, similar to that of the Borrowdale Volcanic Group. The geochemical differences between these two suites arose through the incorporation of different amounts of the subduction component and different fractionation histories.

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