Phreato – magmatic Activity of Berican Mountain (Vicenza, Italy): Volcanology Analysis and Petrographic Data of Diatremic Complexes and Environmental Reconstruction: the “Diatrema of Lumignano” Study Case

The Berici Hills are a part of the Southalpine domain only marginally affected by Alpine deformation events. [Castellarin et al., 2006; Dal Piaz, 2001]. The Berican Magmatism is part of the Veneto Volcanic Province, the main volcanic event of the Southern Alps foreland during the Alpine Orogenesis [Coltorti et al, 2000; Macera et al, 2003; Lustrino et al, 2007]. The area is characterized by a volcano-sedimentary succession (Maastrichtian – Langhian age), consisting by platform limestones, marls of basinal sedimentation to passersby, with, Late Paleocene to Late Oligocene in age intercalations of lava flows and volcanoclastic deposits issued by diatremic conduits [Bassi, 2005]. The field data indicates in the Late Eocene a prevalent effusive activity in Lessinian Mountains, an intense explosive activity which are associated to a numerous volcanic craters in Berican Mountain and submarine lava emission (Hyaloclastite and pillow lava products) in Euganean Hills. The environmental condition typical of a lagoon is deduced by paleo - bathymetric data [Bassi, 2005]. These conditions have favoured phenomena of magma-water interactions testify by abundant phreatomagmatic eruptions. Two conjugate faults systems have allowed the formation of several eruptive centres [De Vecchi et al., 1995]. The Berici Hills Diatremes are characterised by elliptical and sub-circular shapes and by a depressed morphology. Three lava groups with distinctive geochemical and petrographical feature and different serial affinity were identified: basanites (Bn), alkaline basalts (AB), olivine basalts (OB), hawaiites (Hw), latibasalts (Lb), andesine basalts (AnB). This composition feature is in agreement with the monogenic and polygenic nature of eruptive centers. The groundmass is mainly composed by prevalent glass in some cases affected of palagonite alteration, with extremely variable devitrification glass/magma mesostasis ratio [Tamas et al., 2003]. It is possible to observe a direct correlation between textural features and distance from the emission crater: in fact, in areas outside the conduit most prevalent are tefra products, while closer to the ducts are observed breccias. The breccias fragments are composed of volcanic bombs and fragments of the volcanic conduit. The distribution and types of volcanic products allow to distinguish three different areas in the Berici Hills: 1) a north-west area, in geological continuity with the South Mountains Lessini, characterised by fissural activities with massive lava flows, mainly constituted by subaerieal activity and tuffs beds resulting from explosive volcanic events from surrounding areas; 2) Mid-South area (Val Liona) dominated by basic submarine magmatism (Hyaloclastic product) due to fissural magmatic activity with intercalations of pyroclastic product from adjacent diatremes; 3) an eastern and central north portion, characterised by many caldera and diatremes where the “Diatreme of Lumignano” is located. On “Diatreme of Lumignano” (diameter of about 300 m) a detailed investigation was carried out that includes: a) the stratigraphic analysis of Eocene formations intruded by the diatreme; b) the study of filling breccia, including tessiturale analysis of the matrix; c) the petrology survey of basaltic breccia and paleontological study of carbonate included. All these volcanic products are typical intra plate magmatism and are similar to magmatism in the Lessinian Mountains to the early basaltic undifferentiated magma of Euganean area [De Vecchi & See, 1995].