Identifying relic igneous garnet and clinopyroxene in eclogite and granulite, Breaksea Orthogneiss, New Zealand

Eclogite and omphacite granulite occur interlayered on centimetre to decametre scales to form the Cretaceous Breaksea Orthogneiss, with assemblages reflecting peak conditions of P ≈ 1.8GPa and T ≈ 850°C. It is the highest-grade part of the c. 125-115 MaWestern Fiordland Orthogneiss, New Zealand. A gneissic fabric in the host omphacite granulite truncates igneous layering in decametre-scale, coarsely layered clinopyroxenite and garnetite inclusions. Field and microstructural relationships, together with rare earth element (REE) characteristics across all rock types, support the conclusion that most garnet has a common igneous origin; geochemical data alone do not discriminate whether it is igneous or metamorphic. Igneous diopside persists in coarse-grained, weakly deformed samples of clinoyroxenite. Garnet cores in garnetite, late-formed garnet rims in garnetite and delicate garnet films in clinopyroxenite have a range of REE contents interpreted to reflect cumulate processes involving progressive grain growth isolated from the parent magma. Clear rims on inclusion- and Ca-Tschermakite-rich diopside in clinopyroxenite, of a composition that is distinct from grain cores, are interpreted as recrystallization features. Garnet in omphacite granulite occurs in three textural settings, the most commonType 1 garnet having REE characteristics identical to garnet in eclogite, but depleted in heavy REE (HREE) relative to garnet in garnetite. Type 2 garnet in omphacite granulite forms metamorphic coronae on omphacite in contact with plagioclase, has a pronounced positive Eu anomaly and is depleted in HREE compared with Type 1 garnet. Type 3 garnet in omphacite granulite migmatite is indistinguishable from Type 1 garnet, consistent with its formation through magma injection. Cumulate eclogite, pyroxenite and garnetite formed at c. 1.8 GPa, exposing an unusually deep and coherent arc section.18 page(s