Andesite petrogenesis in the Ordovician Borrowdale Volcanic Group of the English Lake District by fractionation, assimilation and mixing

Abstract

A suite of composite basalt–andesite dykes in the Ordovician Borrowdale Volcanic Group of the English Lake District records magmatic evolution from basalt to andesite. Although altered, the rocks preserve textures that indicate magma mixing. Primitive basalts were derived from a fertile, subduction-modified mantle source. Evolution by fractional crystallization is suggested by rapid decrease of compatible trace element concentrations and similar incompatible trace element and REE profiles throughout the sample range. Recharge of the evolving magma with fresh batches of basaltic composition is indicated by some samples that have compatible trace element concentrations much higher than expected from their otherwise evolved compositions. Geochemical evidence of magma mixing is provided by linear trends between pairs of incompatible trace elements not at constant ratio. e Nd values indicate addition of a crustal component, but samples do not conform to a simple mixing equation. The combined evidence points to magma evolution by assimilation of wallrock during fractional crystallization (AFC) with periodic recharge. We have modelled an AFC process using a Skiddaw Group metasediment contaminant that can explain trace element and isotopic variation through variable amounts of assimilation, crystal fractionation and recharge, to produce a spectrum of compositions that was sampled periodically by the dykes. There is evidence from other parts of the Borrowdale Volcanic Group that similar processes operated throughout its history.