Welded ignimbrites show a large variability of welding intensity and of post-depositional crystallization. According to existing literature, a fine characterization of welded ignimbritic units involves: 1) a classificatiory scheme which includes 5 up to 6 categories of welding intensity, 2) the definition of the style of post depositional crystallization. This classification can result time-consuming and out of the standards required by a general description of ignimbrite units in the case of regional geological survey of volcanic terrains. In the new 1:50,000 geological mapping survey of the San Pietro Island (Sardinia, Italy) performed in the framework of the national CARG Project, a large number of samples of ignimbrite products has been collected. The geology of the area is characterized by Oligo-Miocene volcanic deposits, which testify to an intense explosive activity basically featured by the deposition of many variably welded ignimbritic units and, subordinately, by effusive activity, mainly characterized by lava coulees. All the products derive from the eruption of highly evolved magmas of both calc-alkaline and peralkaline affinity. We present here a simplified scheme for microscopic facies classification of ignimbrite deposits, based on the aspect ratio of the fiammae, the strength of glass shards alignment, and the style of crystallization of the matrix, that can be used for the lithological characterization of ignimbritic units in other mapping projects.
I caratteri tessiturali delle ignimbriti saldate dell’Isola di San Pietro (Sardegna; Italia)
MUNDULA, FILIPPO;
2008-01-01
Abstract
Welded ignimbrites show a large variability of welding intensity and of post-depositional crystallization. According to existing literature, a fine characterization of welded ignimbritic units involves: 1) a classificatiory scheme which includes 5 up to 6 categories of welding intensity, 2) the definition of the style of post depositional crystallization. This classification can result time-consuming and out of the standards required by a general description of ignimbrite units in the case of regional geological survey of volcanic terrains. In the new 1:50,000 geological mapping survey of the San Pietro Island (Sardinia, Italy) performed in the framework of the national CARG Project, a large number of samples of ignimbrite products has been collected. The geology of the area is characterized by Oligo-Miocene volcanic deposits, which testify to an intense explosive activity basically featured by the deposition of many variably welded ignimbritic units and, subordinately, by effusive activity, mainly characterized by lava coulees. All the products derive from the eruption of highly evolved magmas of both calc-alkaline and peralkaline affinity. We present here a simplified scheme for microscopic facies classification of ignimbrite deposits, based on the aspect ratio of the fiammae, the strength of glass shards alignment, and the style of crystallization of the matrix, that can be used for the lithological characterization of ignimbritic units in other mapping projects.
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