The Eycott Volcanic Group, an Ordovician continental margin andesite suitein the English Lake District

The Eycott Volcanic Group, at the northern margin of the Lower Palaeozoic i nlier in the Lake District, is the smaller of two substantial middle Ordovi cian (Caradoc) subduction-related volcanic successions that stratigraphical ly separate marine sedimentary successions of the Skiddaw Group and Winderm ere Supergroup. Tabular lavas and subordinate sills, in a sequence up to 24 00 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 andesi tic pyroclastic rocks. The distribution and form of the lava/sill facies as sociation are consistent with emplacement as a lava plateau sequence, remar kably similar to the Birker Fell Formation, the lower part of the Borrowdal e Volcanic Group in the central Lake District. The Eycott Volcanic Group ro cks are geochemically coherent with characteristics transitional between me dium-K, continental-margin tholeiitic, and calc-alkaline andesite suites. R ocks within the suite can be linked by fractionation of an assemblage of pl agioclase, 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 maf ic members suggest that magmas were derived possibly from a subcontinental lithosphere source, similar to that of the Borrowdale Volcanic Group. The g eochemical differences between these two suites arose through the incorpora tion of different amounts of the subduction component and different fractio nation histories.