Origin of magmas feeding the Plinian phase of the Campanian Ignimbrite eruption, Phlegrean Fields (Italy): constraints based on matrix-glass and glass-inclusion compositions

Glass inclusions hosted by clinopyroxene and matrix glasses of pumices from the Plinian fall deposit associated with the Campanian Ignimbrite eruption were analysed in order to investigate processes that occurred at various s tages of the magmatic evolution and to characterise the magma-withdrawal dy namics during the eruption. Petrographic, SEM-EDS, and electron microprobe analyses were performed on pumice samples, glass inclusions, and host miner als. Primary non-evolved glass inclusions were found both in homogeneous sa lite and in zoned diopside-salite clinopyroxenes. All the studied glasses a re peralkaline alkali-trachytic in composition. The variation trends of gla ss inclusions and matrix glasses overlap for most elements except for alkal ies and Cl. The least evolved compositions, however, are shown by glass inc lusions found in salitic clinopyroxene at interfaces with diopside from the early erupted Plinian pumices (Lower Fall), whereas the most evolved compo sitions were found in the matrix glasses of the late-erupted Plinian pumice s (Upper Fall). Excluding the most mafic glass inclusions, the observed com positional range is closely comparable to the range for ignimbrite juvenile clast compositions, indicating that the two eruptive phases drained the sa me magmatic reservoir. The compositional evolution of the Campanian Ignimbr ite magmatic system seems to have resulted from different processes occurri ng at different stages: pre-eruptive magma mixing, due to an input of mafic magma into the reservoir, and crystal fractionation starting from differen t hybrid parental magmas to produce the most evolved compositions. These pr ocesses could have generated a vertically and laterally zoned magma chamber with multiple layers of variably hybrid magma extending from the input poi nts of mafic magma, This type of zoning seems to better account for all the complex chemical and mineralogical characteristics of the Campanian Ignimb rite magmatic system, instead of a simple vertically zoned magmatic reservo ir. A syn-eruptive mingling between the least evolved hybrid magmas and the most evolved melts characterised the first phase of Plinian eruption. This uncommon emptying of the magma chamber could be explained by the particula r geometry of the reservoir. (C) 1999 Elsevier Science B.V. All rights rese rved.