A simplified scheme for the description of textural features in Welded Ignimbrites: the example of San Pietro Island (Sardinia, Italy)

Filippo Mundula(*), Raffaello Cioni(*), (**) & Roberto Rizzo(***)
(*) Dipartimento di Scienze della Terra, Università degli Studi di Cagliari, Via Trentino, 51 - 09127 Cagliari, Italy. Autore di riferimento: Tel.: +39-070-6757711; Fax: +39-070-282236; E-mail: mundula@unica.it. (**) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Via della Faggiola, 32 - 56122 Pisa, Italy. (***) PROGEMISA Progetto CARG-Sardegna, Via Dolcetta, 19 - Cagliari, Italy.

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 categories of welding intensity; 2) the definition of the style of post depositional crystallization. This classification can be very time-consuming 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 characterized 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 classification 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.

Crystallization styles, facies nomenclature, San Pietro Island, welded ignimbrites, welding intensity.