Coarse-grained and porphyritic inclusions in phonolitic lavas, Roccamonfina volcano, Italy

Abstract

The inclusions in the 446–213-ka phonolite lavas of the Roccamonfina volcano (Italy) can be classified as coarse-grained and porphyritic types. Both are wide-varying in composition. The coarse-grained inclusions belong to a cumulate clinopyroxenite–gabbro–monzonitic clinopyroxenite–monzonitic gabbro series, and contain some interstitial, vesicular glass. The porphyritic inclusions show non-cumulate structures, and a coarse-grained, crystalline, vesicular matrix. They correspond to tephrites, phonolitic tephrites, trachybasalts, and basaltic trachyandesites. The glass in the coarse-grained inclusions is similar in composition to the porphyritic inclusions, but in part it deviates compositionally from the latter. These glasses are referred to, respectively, as trapped interstitial liquid from which the cumulates crystallized, and as injected, ‘foreign’ (non-comagmatic) melt. The phonolites vary in composition from tephritic phonolites to phonolites and nepheline phonolites, and do not show any traces of vesicularity. Despite the large eruptive time span, and the broad differentiation, they constitute a coherent magmatic group. Mineralogical and chemical similarities between both inclusion categories, and compositional similarity of the trapped glass to the porphyritic types, suggest that both groups of inclusions are cogenetic. By contrast, deviations of many elements of this glass and of the inclusions from the ideal mixing line inclusions–host phonolites rule out any genetic correlation among these rocks. Least-squares mass-balance calculations reinforce this lack of correlation. This conclusion is also supported by the fact that the phonolites are the most evolved derivatives of the basic and middle terms of the Roccamonfina high-K series, whereas the inclusions appear to constitute a potassic suite compositionally intermediate between both the Roccamonfina high-K and low-K series. Mineralogical and petrologic features suggest that the parental magma of the porphyritic types underwent prolonged fractionation in one or more crustal reservoirs at depths exceeding the stability conditions of plagioclase. The magma then migrated upwards to shallower reservoirs. Fractionation and crystal accumulation formed one (or more) layered crystal mush(es) of coarse-grained rock types, whereas the residual magma migrated toward the surface being stored in one (or more) shallower reservoir(s). When the residual melts of both cumulate and porphyritic layers, still incompletely consolidated, disrupted into coherent fragments and crystals, they were brought to the surface by pulses of phonolitic magma coming from below, and acting during a long time span. The residual melts of cumulate and porphyritic types vesiculated during ascent to the surface, quenching during eruption of the host phonolitic bodies.