Calc-alkaline lavas from the volcanic complex of Santorini, Aegean Sea, Greece : a petrological, geochemical and stratigraphic study

Abstract

This thesis presents the results of a petrological-geochemical- and stratigraphic study of the calc-alkaline lavas from Santorini, Aegean Sea, Greece. The volcanic complex of Santorini consists of seven eruption centres, of which some have been active contemporaneous. The eruption centres in the northern part of Santorini mainly produced lava flows in contrast with a long-lived eruption centre in the southern part, that mainly produced pyroclastic deposits. The lavas and pyroclastics of Santorini belong to the medium-K- or normal calcalkaline suite and range in composition from high-AI basalt to rhyolite, although the latter is rare. Qualitative petrological- and geochemical evidence shows that the lavas of Santorini are mainly related by a process of fractional crystallization of the observed phenocryst phases, i.e. plagioclase, olivine, clinopyroxene, orthopyroxene, titanomagnetite, ilmenite, and minor apatite and zircon. In the oldest volcanic products of Santorini hornblende, cummingtonite and biotite occur. Calculated major- and trace-element fractionation models confirm the fractional crystallization hypothesis. Apart from fractional crystallization there is strong qualitative- and quantitative evidence that magma mixing occurred in the magma chambers beneath Santorini. Magma mixing took place as the result of the recurrent inflow of new mafic magma into the evolving magma chambers of Santorini. Petrological evidence indicates that these mafic magmas underwent already some fractionation on their way to the surface. No petrological evidence is found for major assimilation of crustal material. The Santorini magmas crystallized in shallow magma chambers which were situated at depths between 3 and 9 km. Pre-eruptive crystallization conditions were estimated using phenocryst-phenocrystand phenocryst-melt equilibria. Pre-eruptive temperatures range from 1150-12000 C (basalt) to 800-8400 C (rhyolite). Pre-eruptive water contents range from 0.5-5 wt%, which indicates that most magmas would be undersaturated (i.e. PH20 < PTotal) in the magma chamber. The lavas of Santorini appear to have crystallized at values of f02 about one or two log units above the Ni-NiO buffer. The eruptive sequences of three volcanic centres have been studied in detail, i.e. two shield volcanoes in the northern part of Santorini and the post-caldera dacites which were erupted after the paroxysmal Minoan eruption of 1390 B.C. The eruptive sequences of the northern shield volcanoes show cyclic compositional variations, while the average erupted composition of the post-caldera lavas has remained constant over a period of 2.200 years. The trace-element data of the Santorini volcanics can be used to discriminate between the lavas produced by the different eruption centres. Incompatible element concentrations are highest in the lavas of the oldest volcanic centres in the north and are lowest in the lavas of the youngest centre (post-caldera lavas). The strongly compatible elements (e.g. Ni, Cr) show an antipathetic relationship. This trend mainly reflects progressive depletion of the upper mantle region during melting. On .the ground of trace-element characteristics it can be concluded that the Minoan eruption tapped the same magma body that earlier had fed one of the northern shield volcanoes