Fiordland

Breaksea Orthogneiss exposures at Breaksea Sound, Fiordland, New Zealand, comprise of interlayered omphacite granulite and eclogite assemblages reflecting pressures of ≈18 kbar and temperatures of ≈850°C. The Breaksea Orthogneiss, a high-P component of the c.126-100 Ma Western Fiordland Orthogneiss (WFO), has distinctive composite layering of gabbroic gneiss, mafic dyke and semi-continuous layer structures in dioritic gneiss, mostly transposed into an intense shallowly-dipping, S2, tectonic fabric. This S-L fabric is coaxial to the Resolution Island Shear Zone (RISZ), which separates the Breaksea Orthogneiss from Cretaceous granulites and Palaeozoic schists that reflect lower pressure conditions. Gabbroic gneiss within mafic lenses and layers in the Breaksea Orthogneiss preserve garnet, omphacite and rutile with or without orthopyroxene, interlayered with dioritic assemblages involving garnet, omphacite, plagioclase, K-feldpsar (anitperthite), rutile and kyanite, on a centimetre to decimetre-scale. Eclogite boudins, lenses and layers to dykes with comparatively enriched magnesian garnet and jadeitic omphacite are also hosted within the Breaksea Orthogneiss.
The P-T evolution of these unusual assemblages can be modelled using calculated P-T-XH2O pseudosections in THERMOCALC using the NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) model system. Quantitative P-T and bulk chemical composition constraints on the development of the omphacite granulite and eclogite mineral assemblages within the Breaksea Orthogniess are assessed with reference to the calculated pseudosections (See Equilibria Modelling).

Click on Keith Klepeis newsletter article.pdf for more insight into why geologists battle the sandflies and rain to gain a glimpse at Fiordland’s rocks!