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Abstract

Welded ignimbrites show a large variability of welding intensity and of post-depositional crystallization. According to existing literature,a complete characterization of welded ignimbritic units involves: 1) a classification scheme which includes up to 5-6 categoriesof welding intensity; 2) the definition of the style of post depositional crystallization. This classification can be very time-consumingand well beyond the standards required for a general description of ignimbrite units for regional geological mapping purposes.During the geological survey of the San Pietro Island (Sardinia, Italy) in the framework of the new geological map of Italy on a scale1:50000 (CARG Project), a large number of samples from different ignimbritic units has been collected. The geology of the area is characterizedby Oligocene-Miocene volcanic deposits, which testify for the intense explosive activity with the deposition of many variablywelded ignimbritic units and, subordinately, local effusive activity, mainly characterized by the emplacement of lava coulees. All theproducts derive from the eruption of highly evolved magmas of both calc-alkaline and peralkaline affinity.We present here a new simplified scheme for the facies classification of these ignimbritic deposits. The scheme is based on threemain textural features: the aspect ratio of the fiammae, the degree of glass shards alignment and the style of crystallization of the matrix.All these features can be used to describe the ignimbrite units for the scope of their lithological characterization in mapping projects.We suggest that the scheme can have a general interest for the geological mapping of areas dominated by the presence of weldedignimbritic units.