Progressively Fenitised Schist and Carbonatitic Clasts From a Metasomatic Aureole Beneath the Alkalic Dunedin Volcano, Otago, New Zealand

Clasts of albite‐porphyroblastic quartzofeldspathic schist, derived from the Otago Schist basement, occur within the Port Chalmers Breccia, a diatreme at the centre of the Dunedin stratovolcano, New Zealand. Schists have undergone varying degrees of replacement reactions (at temperatures of 300° to >500°C) producing hornfelses, with Ca‐ and K‐enriched albite porphyroblasts coexisting with oligoclase or andesine, biotite and hercynite. Schist quartz is commonly dissolved, being replaced by inter‐ and intra‐crystalline Ca‐ and Ca–Mg carbonates that have mantle stable isotope ratios. Carbonate segregations have marginal albite‐K feldspar coronae with replacement reactions of adjacent schist or hornfels developing accessory allanite, monazite, and Y‐enriched zirconolite, these processes are collectively referred to as fenitisation. Metasomatism occurred under approximately constant volume, oxidising conditions resulting in gain of Ca, CO 2 , alkalis, Nb, REE, and Y with loss of Si and H 2 O. Newly discovered diatreme clasts of calcite‐K feldspar‐Ba muscovite‐Nb rutile are interpreted as fragments of disrupted sub‐volcanic carbonatitic rocks whose associated fluids may have been responsible for the fenitisation, although fluid derivation from subvolcanic nepheline syenite magmas remains a possibility. Hornfelsed and fenitised clasts are derived at depth from a thermal and metasomatic aureole around the major, carbonate‐impregnated conduit of the Dunedin Volcano.