Abstract
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, CO2, alkalis, Nb, REE, and Y with loss of Si and H2O. 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.