Fiordland

The Fiordland Block lies in the south western part of South Island and is composed of high-grade, multiply deformed (D1-D4) metasedimentary and plutonic rocks that represent a deeply eroded, Cretaceous magmatic arc.
The Western Fiordland Orthogneiss (WFO) constitutes the majority of the Cretaceous granulite terrane and consists of gabbroic to dioritic orthogneiss, dominantly two-pyroxene-plagioclase-hornblende orthogneiss.

The WFO preserves evidence for mid crustal emplacement (P≈8-10kbar) and tectonic loading to high-grade garnet granulite/eclogite conditions (14-20kbar) followed by exhumation and partial recrystalisation (20 km depth) initiated by 110 Ma (Tulloch & Kimbrough, 1989). Exhumation to upper crustal levels was complete by 90 Ma.

The Breaksea Orthogneiss (116±3.1Ma) is a high-P variant of the WFO exposed at Breaksea Sound. The Breaksea Orthogneiss is a omphacite graulite that contains pods to semi-continuous layers of eclogite and orthopyroxene eclogite. Also exposed in the field area is the Resolution Orthogneiss, which lies below the Resolution Island Shear Zone (RISZ) and is a plagioclase-hornblende ± garnet-clinopyroxene orthogneiss enveloping pods to lenses of eclogite. West of the RISZ, high-strain zones and small-scale shears of Resolution Ortogneiss occur within the Breaksea Orthogneiss. The RISZ is the main controlling structure in the area and most to all fabrics are coplanar to the shallowly dipping RISZ, which separates low-P variants of WFO (12-14kbar) in the east from the Breaksea Orthogneiss (18-20kbar) in the west.

Rocks that record assemblages from both the granuilte and eclogite facies, or display evidence of the garnulite-eclogite facies transition, are extremely rare. These types of rocks are of interest as they represent the deepest parts of Earth’s crust affected by orogenesis and encapsulate processes related to nascent crust and lower crust – upper mantle interaction.

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–O) 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!

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All contents on this website are © Matthew De Paoli (2009-2012)