Identifying Relic Igneous Garnet and Clinopyroxene in Eclogite and Granulite, Breaksea Orthogneiss, New Zealand

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Abstract

Eclogite and omphacite granulite occur interlayered on centimetre to decametre scales to form the Cretaceous Breaksea Orthogneiss, with assemblages reflecting peak conditions of P ≈1·8 GPa and T ≈850°C. It is the highest-grade part of the c. 125–115 Ma Western 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 common Type 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.

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