The Kolumbo submarine volcanic field (Greece) since its early activity: occurrence of heterogeneous magmas and their evolution in polybaric reservoirs

The Kolumbo submarine volcanic field (NE of Santorini, Greece), including several monogenetic cones and the polygenetic active Kolumbo volcano, represents the major hazard in the region, as demonstrated by the 1650 CE explosive eruption of Kolumbo volcano that caused severe damages and casualties. In this study, we present a detailed petrological study on the devastating 1650 CE eruption products, represented by juvenile pumices with enclaves and fresh lava lithics, allowing for the opportunity to investigate early Kolumbo activity, as well as the VC15 monogenetic cone sited at NE of Kolumbo. Lithics are amphibole-bearing lavas with compositions forming three main clusters, namely crystal-rich andesites, andesites and rhyodacites; juveniles are biotite-bearing rhyolites including mafic enclaves, and VC15 rocks are olivine-bearing andesites. The most primitive magmas (crystal-rich andesites and a juvenile enclave) are not cogenetic, being generated by differentiation of parental magmas deriving from mantle sources variably metasomatised by slab-derived fluids or melts. Trace element and isotope data show that Santorini, Kolumbo and VC15 cone are separate volcanic systems, despite their proximity. Kolumbo mafic/intermediate magmas underwent variable degrees of deep contamination with lower crustal lithologies, in contrast to the neighbouring Santorini where magmas assimilated the upper crust. Our data indicate that the plumbing system for the pre-1650 CE activity of Kolumbo (back to ~ 200–250 ka, at least) is made up of multiple, polybaric (~ 6–18 km), small and variably evolved magma reservoirs. This structure is similar to that proposed for the mafic magma system feeding the 1650 CE rhyolites, suggesting a steady state for the deep plumbing system throughout the history of Kolumbo. The mafic inputs increased with time, arriving at shallow levels during the 1650 CE eruption, and likely continuing in the current state of Kolumbo, as indicated by geophysical data.