Geochemistry and eruptive behaviour of the Finca la Nava maar volcano (Campo de Calatrava, south-central Spain)

Abstract

Here we present a detailed investigation into the geochemistry and the excavational/depositional processes involved in the maar-diatreme forming Finca la Nava (FlN) eruption in south-central Spain. Bulk rock compositions of hand-picked juvenile fragments indicate derivation of the FIN magma from a garnet-bearing mantle source, which has subsequently been overprinted in bulk rock samples by incorporation of a combination of spinel-bearing peridotites and upper-crustal lithics (i.e. quartzites and slates). The dominating phenocryst assemblage with clinopyroxene, olivine, amphibole and phlogopite points to the classification of the juvenile magma as being olivine melilititic in composition. Ascent through the lithosphere was rapid as indicated by the calculations of settling rates of mantle peridotites (~0.8 m s−1). The original magma fragmentation level in the conduit was probably relatively shallow carrying mainly juvenile pyroclasts (~60 %) intermixed with accidental crustal lithics (~35 %) and mantle xenoliths (<5 %) to the surface. The shapes of individual pyroclasts are sub-rounded to rounded and with highly variable vesicularities (5–45 %). This fact, in combination with abundant fine-grained material in the beginning of the eruption, indicates that both magmatic and phreatomagmatic fragmentation processes may have played important roles in forming the FIN maar. A relatively constant increase in quartzitic fragments from ~35 to <60 % with increasing stratigraphic height in the FIN deposits further indicates that the crater area successively widened during the eruption, which resulted in an increased recycling of quartzitic fragments. This eruption scenario, with the formation of a diatreme at depth, is also consistent with the absence of layers dipping inwards into the crater area.