Magmatic evolution processes as recorded in plagioclase phenocrysts of Nea Kameni rocks (Santorini Volcano, Greece).

Compositional zoning in plagioclase (plg) plays a relevant role in petrogenetic studies due to its wide possible compositional variation and its high sensitivity to chemical and physical conditions. In this work, plagioclase in calcalkaline volcanic rocks from Nea Kameni volcano has been investigated by the combined use of Nomarski Differential Interference Contrast (NDIC), Electron Microprobe (EPM) and Secondary Ion Mass Spectrometry (SIMS) techniques. Despite the diversity of features observed, it is possible to broadly define two major groups of plagioclase. Group-1 plg is generally characterised by a clear poorly zoned core and by variously zoned mantle and rim; resorption surfaces observable in the crystals are generally associated with a non-significant An variation; cores of group-1 plg shows anorthite content in the range An 40-55 % and generally exhibits high Sr, Ba and LREE abundance. Group-2 plg crystals generally display sieved, fritted or patchy cores and irregular zones in the entire crystal or exclusively in the rim and they show always a very Ca-rich core (An > 75%) and a low content of Sr, Ba and LREE. Correspondence between An zoning and TE zoning is not always good. Changes in TE composition of the calculated melt generally correspond to changes in TE abundance in plagioclase but with a few exceptions. The combined EPM-SIMS-NDIC data indicate for the studied plagioclase crystals a complex growth history. The broad range of An and TE abundance seems to suggest that the plg under investigation experienced a broad spectrum of melt composition and/or chemical-physical condition of crystallisation. Evolutionary processes such as mixing and crystal-liquid mixing are proposed to have played an important role in determining many of the Nea Kameni rock characteristics.