Glass-bearing felsic nodules from the crystallizing sidewalls of the 1944 Vesuvius magma chamber

In the 1944 Vesuvius eruption, the shallow magma chamber was disrupted duri ng the highly energetic explosive phases. Abundant cognate xenoliths such a s subvolcanic fergusites and cumulates, hornfels, skarns and rare marbles o ccur in tephra deposits. Mineral chemistry, melt inclusions in minerals and glassy matrix compositio ns show that fergusites (highly crystalline rocks made of leucite, clinopyr oxene, plagioclase, olivine, apatite, oxides and glass) do not correspond t o melt compositions but result from combined sidewall accumulation of cryst als, formed from K-tephriphonolitic magma resident in the chamber, and in s itu crystallization of the intercumulus melt. Very low H2O contents in the intercumulus glass are revealed by FTIR and apatite composition. Whole rock compositions are essentially determined by the bulk mineral assemblages. Glass-bearing fergusites constitute the outer shell of the magma chamber co nsisting of a highly Viscous crystal mush with a melt content in the range 20-50 wt.%. The leucite/(clinopyroxene+olivine) modal ratio, varies with th e extraction order of magmas from the chamber, decreasing upwards in the st ratigraphic sequence. This reflects a vertical mineralogical zonation of th e crystal mush. These data contribute to the interpretation of the subvolca nic low-pressure crystallization processes at the magma chamber sidewalls a ffecting alkaline potassic magmas.