Welded ignimbrites of the San Pietro island (Sardinia; Italy): a proposal for a scheme of facies nomenclature based on macroscopic and microscopic features

Welded ignimbrites show a large variability of welding intensity and of post-depositional crystallization. According to existing litera- ture, a complete characterization of welded ignimbritic units involves: 1) a classification scheme which includes up to 5-6 cate- gories of welding intensity; 2) the definition of the style of post depo- sitional crystallization. This classification can be very time-consum- ing and 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 scale 1:50000 (CARG Project), a large number of samples from different ignimbritic units has been collected. The geology of the area is char- acterized by Oligocene-Miocene volcanic deposits, which testify for the intense explosive activity with the deposition of many variably welded ignimbritic units and, subordinately, local effusive activity, mainly characterized by the emplacement of lava coulees. All the products 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 classifi- cation of these ignimbritic deposits. The scheme is based on three main 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 welded ignimbritic units.