COEXISTING Ba-FELDSPAR AND MELILITE IN A MELAFOIDITE LAVA AT MT. VULTURE, ITALY: ROLE OF VOLATILES AND ALKALINE EARTHS IN BRIDGING A PETROLOGICAL INCOMPATIBILITY

Abstract

A Ba-rich alkali feldspar (hyalophane to almost pure celsian) coexists with melilite in the groundmass of the S. Caterina melafoidite lava, Mt. Vulture, Italy. The accompanying phases are a very Al- and Ti-rich clinopyroxene, magnetite, hauyne–sodalite, Ca-rich nepheline, leucite, apatite–britholite and accessory phases (mainly pyrochlore). This mineral assemblage is considered to be the result of the crystallization of a highly silica-undersaturated, alkali-rich magma, in an environment that stabilized S–Cl-rich minerals and inhibited the crystallization of minerals such as phlogopite, owing to the lack of sufficient amounts of volatile species (F, OH) promoting mica crystallization. As a result, the concentration of Ba in the residual liquids, due to its broadly incompatible behavior with respect to the early-crystallized phases, forced crystallization of an increasingly Ba-rich feldspar, along with melilite and volatile-poor feldspathoids such as nepheline and leucite. This apparently incompatible mineral assemblage may occur where interstitial liquid compositions (and thus the eventual crystallizing feldspar) are very rich in Ba or Sr or both, as noted in other rare occurrences worldwide. The final liquid composition crystallized in an assemblage made up of clinopyroxene + feldspathoids + melilite + hyalophane or celsian; it was thus far more undersaturated in silica than a phonolite.