Geochemical variation and magmatic cyclicity within an Ordovician continental-arc volcanic field: the lower Borrowdale Volcanic Group, English Lake District

The origin, differentiation and temporal evolution of magmas have been examined from the Ordovician lower Borrowdale Volcanic Group (BVG) — a calc-alkaline, plateau andesite pile of continental arc affinity. The basalt-dacite compositional range can be modelled by POAM-type crystal fractionation of primitive melts, allied with minor crustal assimilation, derived from enriched mantle modified by fluids from the subduction zone. Detailed sampling of sections through 2–3 km of stratigraphy reveals localised and successive highly systematic magmatic evolutionary trends, or cycles. Cycles in which magma becomes less differentiated with time can be viewed as episodes of rapid eruption of a compositionally zoned chamber, possibly due to magma recharge. By contrast, increasing differentiation with time implies reduced recharge and eruption rates whereby fractionation processes dominate. The lack of evidence for compositional change over time represents a balance between recharge, eruption and differentiation processes. Successive cycles within a local sequence can be related in many cases to discrete batches of magma that ascend into sub-volcanic chambers and undergo fractionation. The differences between contemporaneous sequences confirms the current view of the lower BVG as a multicentred volcanic field. It was deposited in a subsiding, extensional volcano-tectonic rift zone, consistent with episodes of rapid upward magma flux, with the eruption locally of primitive (high Mg#, Ni and Cr) basaltic lavas.