THE SAN-VENANZO MAAR AND TUFF RING, UMBRIA, ITALY - ERUPTIVE BEHAVIOROF A CARBONATITE-MELILITITE VOLCANO

The late Pleistocene San Venanzo maar and nearby Plan di Celle tuff ri ng in the San Venanzo area of Umbria, central Italy, appear to represe nt different aspects of an eruptive cycle accompanied by diatreme form ation. Approximately 6 x 10(6) m(3) of mostly lapilli-sized, juvenile ejecta with lesser amounts of lithics and 1 x 10(6) m(3) of lava were erupted. The stratigraphy indicates intense explosive activity followe d by lava flows and subvolcanic intrusions. The pyroclastic material i ncludes lithic breccia derived from vent and diatreme wall erosion, ro ughly stratified lapilli tuff deposited by concentrated pyroclastic su rge, chaotic scoriaceous pyroclastic flow and inverse graded grain-flo w deposits. The key feature of the pyroclastics is the presence of con centric-shelled lapilli generated by accretion around the lithics duri ng magma ascent in the diatreme conduits. The rock types range from ka lsilite leucite olivine melilitite lavas and subvolcanic intrusions to carbonatite, phonolite and calcitic melilitite pyroclasts. Juvenile e jecta contain essential calcite whose composition and texture indicate a magmatic origin. Pyroclastic carbonatite activity is also indicated by the presence of carbonatite ash beds. The San Venanzo maar-forming event is believed to have been triggered by fluid-rich carbonatite-ph onolite magma. The eruptive centre then moved to the Plan di Celle tuf f ring, where the eruption of degassed olivine melilititic magma and l ate intrusions ended magmatic activity in the area. In both volcanoes the absence of phreatomagmatic features together with the presence of large amounts of primary calcite suggests carbonatite segregation and violent exsolution of CO2 which, flowing through the diatremes, produc ed the peculiar intrusive pyroclastic facies and triggered explosions.