Phlogopite-clinopyroxenite nodules from high-K magmas, Roccamonfina Volcano, Italy: evidence for a low-pressure metasomatic origin

Abstract

Dense nodules containing phlogopite (Mg-number > 80), clinopyroxene, and vesicular glass with subordinate olivine and Cr-spinel are found at Roccamonfina Volcano in pyroclastic deposits of widely varying ages and compositions. Significantly, phlogopite has never been observed as a phenocryst in any volcanic rock at Roccamonfina. In the nodules, phlogopite shows a variety of reaction textures in relation to both clinopyroxene and olivine. Each of the major minerals and glass is compositionally homogeneous within a single nodule, and the suite as a whole shows systematic correlations of Mg-number among these phases. Clinopyroxene (En 40–47) and olivine (Fo 81–87) compositions closely match phenocryst compositions from basic lavas at Roccamonfina, but Cr-spinel compositions strongly diverge from those in Roccamonfina volcanic rocks. Sr and Nd isotopic compositions of whole-rock nodules and constituent minerals fall within the field for high-K series (HKS) volcanic rocks, although minor isotopic disequilibrium is noted among minerals of individual nodules. Euhedral crystals are enclosed by interstitial glass, indicating that the glasses represent residual melts and are not products of in-situ partial melting. Of analyzed glasses from five nodules, three are ne-normative and compositionally similar to HKS tephrite magma compositions, one is hy-normative and similar to low-K series (LKS) trachyandesites, and the other has larnite in the norm and is unlike any known volcanic rock. In addition to other more basic host rock types, phlogopite pyroxenites are found within voluminous pyroclastics ranging from leucite tephrites to highly evolved phonolites and trachytes. These rocks have strong depletions in Sr, Ba, and Eu, consistent with extensive fractionation in low-pressure magma chambers. Consequently the phlogopite pyroxenites are also constrained to have formed at low pressures. This interpretation conflicts with several previous hypotheses for the origin of similar phlogopite pyroxenites at other potassic volcanic centers, including metasomatism of mantle protoliths and origins as high-pressure plutonic assemblages. Textures in the Roccamonfina nodules indicate that both clinopyroxene and olivine were consumed by reactions producing phlogopite. We envision formation of these nodules in sub-volcanic magma chambers through open-system reaction of HKS clinopyroxene ± olivine cumulates with a rising potassic HKS melt or vapor phase.