Comparison of oceanic island lava flow suites separated by a lateral collapse - how does a major structural change impact on the magma supply system?

Abstract

The massive ~530 ka Cumbre Nueva collapse on the Canary Island of La Palma separates the pre-collapse Cumbre Nueva rift sequence from the post-collapse Bejenado edifice. Such collapses are characteristic of the shield stage of oceanic island growth, and these Canarian volcanoes form an ideal case study to determine how the composition of lavas produced within a magma supply system varies in response to the structural evolution of the island. The Cumbre Nueva rift formed as the result of focused activity along a central rift zone, with the flanks of the edifice becoming progressively more unstable as growth continued. Conversely, the Bejenado volcano erupted entirely within the collapse embayment in a brief, intense period of activity after which volcanism permanently ceased. All flows are composed of alkaline trend lavas, including basanites, tephrites, tephriphonolites and porphyritic ankaramites, with lavas from both edifices showing a tendency towards increased evolution over time. Petrological study (XRF, EPMA and petrography) is being undertaken on representative suites of stratigraphically-sampled lavas from both edifices, with 17 pre-collapse and 8 post-collapse flows currently characterised. Results indicate that, while both suites are similar, there are significant textural differences between the sets. In both cases, the primitive rocks have phenocrysts dominated by clinopyroxene, olivine, and magnetite, while evolved flows exhibit hornblende, apatite, titanite and occasional hauyne (with feldspar phenocrysts mostly absent). These assemblages correspond well to the fractional crystallisation trends inferred from chemical variation diagrams. Incompatible trace element patterns from the pre-collapse volcanism are consistent with variable fractional crystallisation of magmas from a stable mantle source. However, the post-collapse volcanic activity appears to result from more complex processes in the magma supply. While ankaramites are common in both suites, post-collapse flows are notably more glomeroporphyritic, with a much greater abundance of pyroxenite-dominated xenoliths transported within post-collapse flows. Both of these observations indicate post-collapse entrainment and eruption of deep magma-supply system material