MAFIC INCLUSIONS IN PUMICES AND DOMES OF NISYROS VOLCANO, AEGEAN ARC, GREECE

Nisyros, a small strato-volcano at the extreme eastern end of the modern Aegean arc (Greece), is characterized by four major young silicic units. These explosive eruptions were the culmination of a long eruptive history that started with pillow basalts and an andesitic stratocone. The Lower Pumices, Nikia lava flow, Upper Pumices and post caldera domes are all rhyo-dacitic with volumes up to a few km³. The 3.5 km- wide caldera is filled with large silicic domes, of which one spilled over the rim. All four units carry mafic magmatic inclusions of basaltic andesitic composition, most of them with amphibole as an abundant phase. Many show quench rims and evidence of fragmentation after inclusion. The host magmas became partly hybridized with the inclusion magmas, as indicated by abundant xenocrysts from the mafic magmas. Amphibole in the host magmas may be largely a xenocrystic phase derived from the inclusion suites.

Chemical and isotopic data as well as mineral microprobe analyses indicate that each silicic unit carries a characteristic suite of inclusions. The mafic inclusion magmas and silicic units can not be related to each other by processes of simple fractional crystallization. The bulk chemical stratigraphic trend from Lower Pumice to Post Caldera Domes shows increasing MgO and decreasing SiO₂. The matrix glasses in the pumices become more silica rich from Lower Pumice to Upper Pumice, however. Microprobe data of glasses, phenocrysts and xenocrysts will be used to quantify the hybridization processes in the various units.