The phlegraean fields: Magma evolution within a shallow chamber

Abstract

A systematic petrological and chemical study of the volcanic products of the Phlegraean Fields has been accomplished based on the new stratigraphy described by Rosi et al. (this volume). The majority of Phlegraean rocks belong to the “potassic” series of the Roman province. The compositional spectrum ranges from trachybasalts to latites, trachytes, alkali trachytes, and peralkaline phonolitic trachytes. Trachybasalts are extremely rare and there is a sharp compositional gap between them and the latites. The series between latites and the trachytic varieties is complete. Trachytic rocks are much more voluminous than latites. The order of appearance of the main solid phases is: olivine, clinopyroxene, plagioclase, alkali feldspar, biotite, and oxides. Mineral compositions obtained by microprobe analyases are compatible with the evolution of the rock chemistry. However, primitive compositions of plagioclase (An 80) and clinopyroxene (diopside) persist in the cores of phenocrysts, even at the trachytic and alkali trachytic stage. Fractional crystallization within a shallow magma chamber has been the dominant process for the generation of Phlegraean rock series. The volume of the magma chamber is estimated to have been at least 240 km 3 at the moment of the eruption of the Campanian Ignimbrite, nearly 35,000 yr. ago. This event was followed by a large caldera collapse. The depth of the chamber cannot be precisely evaluated. However, its top must have been very shallow, probably at 4–5 km, as suggested by contact metamorphic rocks obtained from deep geothermal wells within the caldera. Volcanological and petrological data favor a model of upward migration of lighter liquids produced mostly by fractionation along the cool walls of the chamber, the deeper part of which is occupied by a convecting trachybasaltic magma. Progressive migration of eruptive vents toward the caldera center and the contemporaneous strong reduction in the volume of the erupted products, suggest that the chamber behaved as a closed system. The volume of magma was progressively reduced by both cooling and extraction to the surface.