Genesis of bottom vitrophyre facies in rhyolitic pyroclastic flows: a case study of syneruptive glass welding (Nuraxi unit, Sulcis, SW Sardinia, Italy)

Nuraxi unit is a near a peralkaline lithic-poor glassy pyroclastic rock with a broad outcrop in SW Sardinia. It has a constant vitrophyre at the bottom characterized by dense aggregation of glass lenticles, relative depletion of phenocrysts (respect to the overlying rest of the unit) and widespread cristobalite–anorthoclase spherulite devitrification. There is also local evidence of planar localized shear during cooling. The textural and chemical features of this vitrophyre is that expected for a syn-volcanic formation of the vitrophyre over a load compaction following cooling.