Chemical and Sr-isotopical evolution of the Phlegraean magmatic system before the Campanian Ignimbrite and the Neapolitan Yellow Tuff eruptions

New geochronological, geochemical, and Sr-isotopic data on volcanics erupte d before the Campanian Ignimbrite (CI, 37 ka) and the Neapolitan Yellow Tuf f (NYT, 12 ka) caldera-forming eruptions at Campi Flegrei (CF) have allowed us to investigate the behavior and temporal evolution of the Phlegraean ma gmatic system. The most prominent feature of the CF magmatic system was the existence of a large, trachytic magma chamber, episodically recharged, whi ch fed eruptions for tens of thousands years before the CI and NYT eruption s. During the pre-CI caldera activity, magmas were episodically erupted fro m vents located outside the present caldera structure. These magmas ranged in composition from trachyte to alkali-trachyte, with Sr-isotope ratios inc reasing through time, and becoming identical to that of the CI magma, at ab out 44 ka ago. This suggests that the Phlegraean magmatic system before the CI eruption was acting as an open system. It was being progressively reple nished by new batches of magma that mixed with the resident less radiogenic , fractionating trachytic magmas and was periodically tapped. The magma cha mber evolution culminated in the catastrophic eruption of the voluminous (1 50 km(3) DRE), chemically and isotopically zoned CI trachytic mag mas, and in the resultant CI caldera formation. Subsequent to the CI eruption, durin g a period of moderate subaereal volcanic activity of about 20 ka duration, magmas predominantly trachytic to alkali-trachytic in composition and isot opically similar to the last emitted CI magma were erupted from vents locat ed inside the CI caldera, The temporal trend shown by Sr-isotope ratios pro vides evidence for a new input of alkali-trachytic magma, at ca. 15 ka, wit h Sr-87/Sr-86 ratio identical to that of the alkali-trachytic magma feeding the first phase of the NYT eruption. These data testify to the arrival in a short time span of a new trachytic to alkaIi-trachytic magma in the syste m, isotopicaIly distinct from the CI magma, that gave rise about 3 ka later to eruption of the NYT (40 km(3) DRE). (C) 1999 Elsevier Science B.V. All rights reserved.