Conferenza A. Rittmann “ P E R G I O V A N I R I C E R C A T O R I ” Nicolosi (Catania) 7 | 9 giugno 2011 58 Stratigraphy of a Slightly Peralkaline, Rheomorphic Welded Ignimbrite: The Monte Ulmus Unit (SW Sardinia) Maurizio Mulas 1 , Raffaello Cioni 1,2 , Filippo Mundula 1 1 Universit di Cagliari, Dipartimento Scienze Terra, Cagliari, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Italy The Monte Ulmus ignimbrite (15,5 – 0,5 Ma [Morra et al. 1994]) represents one of the last eruptive events of the Oligocene-Miocene volcanic activity in the south-western sector of Sardinia [Cioni et al. 2001]. These volcanic products are referred to the activity of a magmatic arc active between 32.4 and 13.8 Ma [Lecca et al.1997], the results of NNW subduction of oceanic lithosphere [Savelli et al. 2002, and reference therein]. Monte Ulmus unit is a low-aspect-ratio rhyolite to comenditic, high-grade welded ignimbrite with thickness between 8 to 98 m, locally rheomorphic, spread over an area of 300 km 2 (Fig 1a). About 41 stratigraphic sections, sited in San Pietro, SantAntioco and Sulcis-Iglesiente areas, were studied and described. The Monte Ulmus unit drapes a commenditic ignimbrite (Ventrischio Unit) in all its outcroups, while in the northern sector of San Pietro island, it directly rests on some comenditic lava flow units (Nasca and Vinagra units). The Monte Ulmus is overlain by the final products of the Sardinia late Miocene activity, represented by the Paringianu and Serra di Paringianu ignimbrite units and by the last comenditic lava flow event (Monte Tortorisio unit). The Monte Ulmus unit is divided into four main subunits: a basal fall-out deposit followed by three welded pyroclastic flow deposits (named A, B1 and B2) (Fig 1b). Figure 1. a) Total thickness of Monte Ulmus Unit; b) section type sites in the northern sector of San Pietro island. The absence of erosional surfaces and the unique set of columnar jointing which cross-cuts all the subunits of the ignimbrite, suggest a single cooling unit associated to multiple flow units. The coarse, massive, totally argillified ash fall deposit ranges in thickness between 20cm and 2 cm. Upon the fall deposit there is a basal vitrophire,30 cm thick, often strongly argillified. In SantAntioco areas the basal vitrophire, presents oblate eutaxitic white fiammae (10%) with average size of 3 cm and aspect ratio (ratio between mayor and minor axes) (A/R) 2:1-3:1. Quartz and alkali feldspar crystals (lower than 5 vol %) have 2 mm maximum size, while size of the rare rounded lithics clasts is lower than 3 cm. The core of sub-unit A is a 2 to 32m thick, reddish, fine grained, high-grade welded, rheomorphic ignimbrite that crops out mainly in SantAntioco island. Two different types of fiammae are present: blackish (10-15 %), obsidian-like,