RECENT SILICIC VOLCANIC DEPOSITS OF NISYROS VOLCANO, AEGEAN ARC, GREECE

Nisyros is a 42km² arc volcano with a central caldera, located on the eastern edge of the Aegean arc, and on the rim of a very large caldera formed after the Kos Plateau Tuff eruption (~160,000 BP). The Yali rhyolitic volcanism (~30,000 BP) postdates the Nisyros caldera formation. The composition of volcanic units on Nisyros ranges from basaltic andesites to rhyolites.

Following the main stratocone building stage, four silicic units of several km³ each were deposited: Lower Pumice, Nikia Lava flow, Upper Pumice, and Post Caldera Domes. These four units consist of two explosive eruptions that each culminated in caldera collapse. Both explosive eruptions were followed by an effusive phase of similar composition. Each depositional unit is rhyo-dacitic and contains mafic magmatic inclusions, which increase in abundance and degree of hybridization with the host magma through the stratigraphic sequence. Textural and compositional evidence suggests that the inclusions were quenched within the silicic magma. The most mafic inclusions are found in the Nikia Lava flow, while the Post Caldera Domes have unusually Sr-rich inclusions (>1000 ppm Sr). Each stratigraphic unit appears to have their own chemical and isotopically distinct inclusion suite. The inclusions range from arc basalt to andesite.

The host rocks and inclusions have been analyzed for major and trace elements (XRF, ICP-MS) as well as Sr-Nd isotopes. Glasses were separated from the pumices and analyzed for major-trace elements and isotopes. Modeling data indicate that the silicic magmas cannot be generated through closed system fractionation from the mafic inclusion magmas. This study addresses the relationships between the four silicic units and the role of their inclusions in generating the compositional variation in the silicic magmas. We compare trace elements (e.g. Sr) and Sr-Nd isotopes in each inclusion suite to determine the compositional and isotopic variability for the mafic inclusion magmas. We also use published data on the Kos volcanic suite to compare with the recent silicic volcanism on Nisyros, to gain broader insights into the magma chamber evolution beneath Nisyros.