Bottom vitrophiríc fades in rhyolitic pyroclastic flows: a case study from Nuraxi unit (Sulcis, SE Sardinia island, Italy)

The bottom of the Nuraxi unit (Assorgia et a/., 7 990) has a vitrophiric character and the unit shows clear evidences of reomorphic flow during cooling. Both characteristics are present in other acid pyroclastic units of the upper section of the pyroclastic pile of Sulcis mainland, SW Sardinia, and elsewhere in the case of high temperature-deposited pyroclastic flows with high total alkali contents. A detailed mesoand microscopic study of Nuraxi vitrophire shows: 7/ a black and jointed bottom vitrophire (some 40 cm thick) depleted in phenocrysts, with resorbed feldspars; the mesostase reached the sphaerulitic stage of devitrification and shows relicts of an eutaxitic texture. 2/a transitional grey vitrophire, with several degree of ramdom distributed vesiculation, evidence of fair eutaxitic texture and developed vesiculation. 3/a sharp horizontal line of color modification that represents the transit towards a flow banded highly macroscopically vesicled reomorphic pyroclastic deposit (Nuraxi unit). The vitrophire overlies a red paleosoil and a 30 cm-thick crystal-rich deposit. All or most of these characteristics could be preserved and recognized in an ancient deposi