Evidence for Multi-stage Magmatic Evolution during the past 60 kyr at Campi Flegrei (Italy) Deduced from Sr, Nd and Pb Isotope Data

The Campi Flegrei caldera, an active volcanic field in the Campanian INTRODUCTION province, Italy, is a nested structure generated by the Campanian At any stage of their ascent from source to surface, Ignimbrite (37 ka BP) and the Neapolitan Yellow Tuff (12 ka magmas may become contaminated by assimilation of BP) eruptions. Since at least 60 ka BP Campi Flegrei has produced wall rocks. In particular, crustal contamination is conmagmas with variable chemical and Sr isotopic compositions. Sr/ sidered a possible mechanism when mantle-derived Sr ratios increase through time from 0·7068 to 0·7086, with magmas rise through thick sections of continental crust the highest ratios detected in the least-evolved shoshonitic products. and pond at different depths before reaching the surface The origin of this progressive Sr isotopic variability has been (e.g. Leeman, 1983; Hawkesworth & van Calsteren, investigated using new Sr, Nd and Pb isotopic data for volcanic 1984). rocks and entrained xenoliths. The data obtained are combined and Crustal contamination of magmas is favoured in high discussed with previous geochemical and Sr isotope data and used heat flux zones and may occur in a variety of ways to suggest a multi-stage evolution for the magmatic system, mainly including: (1) bulk assimilation of crustal material; (2) involving deeper and shallower crustal magma storage reservoirs. assimilation of partial melts of crustal materials; (3) seThe deeper reservoir is proposed to be a magma chamber periodically lective exchange of specific elements (e.g. De Paolo, 1981; refilled by primitive mafic magmas which subsequently undergo Leeman, 1983; Huppert & Sparks, 1985; Wilson, 1989). contamination by crustal material. The assimilated crustal material Sr, Nd and Pb isotopes are commonly used to model is represented by xenoliths recovered in the shoshonitic pyroclastic crustal contamination processes (e.g. Wilson, 1989, and products. Magma batches originating from the deeper reservoir references therein). However, the nature of the conmigrated towards the surface and fed a shallower complex magmatic taminant is, in some cases, difficult to identify, in parsystem. The deeper chamber was tapped during the eruption of ticular when exposures of basement rocks or occurrences least evolved magmas by regional fault systems. In addition to of entrained xenoliths in the magmas are lacking. crystal–liquid fractionation, open-system processes occurred in the At Campi Flegrei (CF) (Fig. 1) magmas of different shallower system. chemical and Sr isotopic composition have been erupted since at least 60 ka BP (Civetta et al., 1991, 1997; Orsi

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