Cumulate-rich xenolith suite in Late Cenozoic basaltic eruptives, Hepburn Lagoon, Newlyn, in relation to the western Victorian lithospheric mantle
Cumulate-textured rocks form 90 vol% of xenoliths erupted with Hy-normative olivine basalt at Hepburn Lagoon, Victoria. The phreatomagmatic host eruption punched through Late Cenozoic (<3 Ma) hawaiites. More than 80% of the xenoliths are pyroxenites, largely websterites, and contain Al- and Ti-enriched augites (Mg# 0.76–0.87) and orthopyroxene (Mg# 0.75–0.79), ±altered olivine and spinel (Mg# 0.52–0.60). Subordinate gabbros, gabbronorites and norites contain sodic to calcic plagioclase (An45−64). The xenoliths include minor Mg-rich metaperidotites (olivine Mg# 0.86–0.90) and metapyroxenites and granulites (± altered garnet or spinel). Thermobarometry for the main cumulates suggests subliquidus equilibration between 1000 and 1100°C at ∼1 ± 0.25 GPa, compatible with observed cumulates in cross-cutting veins in mantle metaperidotites. The re-equilibrated granulites and mantle metaperidotites yield lower temperatures (840–980°C) than the cumulates. Gabbroic banding in some pyroxenites suggests that a common parental mafic melt was involved. A melt composition was reconstructed by combining websterite and gabbronorite analyses in the observed 8:1 pyroxenite/gabbro ratio. This yields a Hy-normative parental basaltic melt (Mg# 0.73–0.76), with oceanic basalt trace-element contents. The overall range in cumulate compositions and in the mantle-normalised incompatible trace-element patterns suggests that some parental melts were Ne-normative. The Hepburn Lagoon xenolith suite is notable for its little-modified, relatively anhydrous pyroxene-rich cumulate assemblages, both in the Victorian context and in intraplate basalt settings elsewhere. It differs markedly from more recrystallised and metasomatised mantle–crust columns represented in xenolith suites to the southwest (Bullenmerri) and southeast (The Anakies). This may reflect its position within the Lachlan Fold Belt, between the older Delamerian (west) and Selwyn Block (east) basements.