Kelyphitic breakdown of garnets from pyroxenite xenoliths, south-eastern Sicily, Italy

Upper-mantle garnet-pyroxenite xenoliths rarely occur in Miocene basaltic diatremes from the Hyblean Plateau, SE Sicily (Italy). Three representative samples from Valle Guffari tuffbreccia deposits are examined here. A subsolidus origin for garnet may be invoked for two of these samples (one spinel-garnet websterite, UL-c17; one garnet-spinel-orthopyroxene clinopyroxenite, UL-d47). In the third sample (a garnet clinopyroxenite, UL-d21 ), which has mineral compositions similar to those of sample UL-d47, garnet is a cumulus product. Garnet appears partially replaced by an optically unresolvable mineral intergrowth (kelyphite) which, under SEM (EDS), turns out to be composed of Capoor pyroxene, spinel and Ca-plagioclase. Kelyphite bulk compositions, obtained by automatic raster analyses, closely match their relative garnet mineral chemistry. This fact and mass balance calculations suggest a near-isochemical garnet breakdown process, primarily controlled by an increase in temperature and decompression, as a consequence of xenolith entrainment into the host magma and ascent to the surface, although a catalysing effect by fluids was very probable at any stage of the process.