Igneous and metamorphic garnet-clinopyroxene assemblages in eclogite and granulite, Breaksea Orthogneiss, New Zealand: major and rare earth element characteristics

Eclogite and omphacite granulite, interlayered on cm- to decameter-scale, form most of the Cretaceous Breaksea Orthogneiss, which experienced peak conditions of P nearly equal 1.8 GPa and T nearly equal 850 degrees C. It is the highest grade part of the c. 125-115 Ma Western Fiordland Orthogneiss. A gneissic fabric in the host orthogneiss truncates igneous layering in coarsely layered, decametre-scale clinopyroxenite and garnetite inclusions. Field and microstructural relationships, together with rare earth element (REE) characteristics across a broad range of rock types permits the conclusion that most garnet is of igneous origin; geochemical data alone are ambiguous. Igneous diopside persists in coarse-grained, weakly deformed samples. Garnet cores in garnetite, and late-formed garnet rims in garnetite and clinopyroxenite have a range of REE contents interpreted to reflect cumulate processes involving continued grain growth isolated from the parent magma. Clear rims on inclusion- and Ca-Tschemakite-rich diopside in clinopyroxenite of composition distinct to grain cores are interpreted as recrystallization features. Garnet in granulite occurs in three textural settings, the most common Type 1 garnet having REE characteristics identical to garnet in eclogite, but depleted in heavy REE relative to garnet in garnetite. Type 2 garnet in granulite forming metamorphic coronae on omphacite has a pronounced positive Eu anomaly and is depleted in heavy REE compared with Type 1 garnet. Type 3 garnet in granulite migmatite is indistinguishable from Type 1 garnet, consistent with formation through magma injection.