Amphibole-plagioclase fractional crystallisation and magma mixing as majordifferentiation processes in the Akrotiri Volcanic Complex, Santorini, Greece

The Pleistocene Akrotiri Volcanic Complex (AVC) builds up the southern part of the island of Thera and comprises the oldest volcanic rocks of the Sant orini volcanic islands with ages probably ranging from 1.6 to 0.5 Ma. The l ower volcanic sequence starts mainly with rhyodacitic magmas, continuing wi th the production of heterogeneous dacites and terminating this first cycle with andesites. The rhyodacitic magmas are results of fractional crystallisation of amphibo le-and plagioclase-rich cumulates from a water-bearing basaltic melt with i sland are characteristics. The andesitic to dacitic magmas, in contrast, ar e mainly products of mechanical mixing (mingling) between rhyodacitic and b asaltic to andesitic melts. These differentiation processes can be inferred from phenocryst and xenocryst chemistry as well as from basic to intermedi ate inclusions, which occur in various proportions in the dacitic and rhyod acitic lavas. The inclusions are interpreted as undercooled and quenched (" pillowed") products, which were formed by injection of vapor-rich hybrid-me lts of basaltic to andesitic composition into cooler dacitic to rhyodacitic magmas. Such injections are seen as trigger mechanism, which started the a scent of silicic magmas from deep-seated crustal magma reservoirs. The upper and younger volcanic sequence of the AVC is composed of high-alum ina basalts and basaltic andesites and erupted after a period of magmatic q uiescence. Complete magma mixing is present in the late Pleistocene Cape Ma vrorachidi eruptive centre, producing cinder, scoriae and lavas of basaltic andesite composition. Textural patterns and mineral chemistry of the large r phenocrysts (xenocrysts) exhibit significant evidence of magma mixing bet ween an evolved andesitic melt and a basaltic melt.