Evidences for disruption of a crystallizing front in a magma chamber during caldera collapse: an example from the Breccia Museo unit (Campanian Ignimbrite eruption, Italy)

Abstract This work is focussed on juvenile components and some cognate xenoliths of the Breccia Museo (BM) unit. The BM is a coarse-grained proximal unit of the caldera-forming phase of the Ignimbrite Campana eruption, southern Italy. The BM products show some peculiar characteristics that distinguish them from the other Ignimbrite Campana deposits. In particular, different types of pumice fragments constitute the juvenile fraction and their crystal contents are remarkably higher than the other Ignimbrite Campana units. Slightly porphyritic and highly porphyritic trachytic to phonolitic pumices were distinguished in each sample and investigated separately for mineralogy, matrix glass composition, melt and fluid inclusion studies. Most feldspar crystals may have formed at the margins of the magma chamber and the crystal content of both types of pumice fragments can be ascribed to variable entrainment of these crystals (from the solidification front) by the melt. Variably porphyritic (