Facies vulcaniche e meccanismi di messa in posto delle vulcaniti del Monte Amiata

Mount Amiata is a silicic Quaternary volcano that poses the problem on the genetic interpretation of its products. The major volcanological arguments debated on Amiata, during the last 55 years, is the occurrence of explosive eruptions and the emplacement of pyroclastic flow deposits during its activity. In particular, the lower unit was considered, into the volcanological and petrochemical literature, as either a unique sheet of ignimbrite and rheoignimbrite deposits, or a sequence of lava flows, or, more recently, the collapse of an endogenous mega-dome. We performed field analysis and collected stratigraphic and petrographic data from outcropping deposits and from a deep well drilled on the southern flank of Monte Amiata by Regione Toscana in 2010 that intersected the entire volcano sequence and reached the non-volcanic substratum showing volcanic units older than the outcropping rocks. The reconstructed succession is indicative of a rapid aggradation of effusive products and does not evidence pyroclastic deposits or mega-breccia facies. The previously undivided, extensive basal unit comprises different, single, superposed silicic bodies, in which we distinguish structural and morphological features typical of lava flows, such as basal and top autoclastic breccias, front ramp structures and surficial ogive structures. Both at microscopic and macroscopic scale, the rock texture is not fragmental. Individual flow unit does not show evidence of facies variations from proximal near vent (e.g. co-ignimbrite lithic breccia) to distal areas of deposition that is typical of pyroclastic flow deposits. In the individual flow unit, where is not vertical and/or lateral gradation in welding character from non-welded, to medium grade (with sparse welded zones) to pervasively rheomorphic textures. Individual flow units are of small volume and extension, and show channelized lobe geometry. We suggest that the extensive silicic basal (outcropping) unit of Amiata volcano is a sequence of lava flows with a rapid aggradation. As a whole, the volcanic deposits of Monte Amiata exclusively consisting of a series of acid lava castings, exogenous domes and associated "coulee". Large volumes of lava blocks, dispersed at the periphery of the volcano, and interpreted as blocky lavas in the past, are related to pervasive surface alterations of pressure ridges on top of the major lava flows and to collapse landslides occurred at the margin of the volcanic body in contact with the shaly deposits of the sedimentary substratum.