CONTEMPORANEOUS ECLOGITE AND GRANULITE FACIES ASSEMBLAGES OF THE BREAKSEA ORTHOGNEISS, FIORDLAND, NEW ZEALAND

The Breaksea Orthogneiss, a high-pressure component of the c. 126-116 Ma Western Fiordland Orthogneiss Suite, has distinctive composite gabbroic layering and dyke structures in a dioritic host, mostly transposed into an intense shallowly dipping S₁ foliation. Delicate centimetre- to metre-scale layering and dyking structures are interpreted to reflect mafic sill emplacement and/or cumulate processes. Gabbroic gneiss components preserve eclogite facies S₁ garnet, omphacite and rutile, with or without orthopyroxene, interlayered with dioritic gneiss components that preserve granulite facies assemblages involving S₁ garnet, omphacite, plagioclase, antiperthite, rutile and kyanite. As both eclogite and granulite assemblages reflect peak conditions involving pressures ≈18 kbar and temperatures ≈850 °C, the Breaksea Orthogneiss presents a unique natural example of the eclogite–granulite transition. The facies distinction was controlled by whole rock composition, whereby mafic, gabbroic components recrystallised to eclogite and felsic, dioritic components recrystallised to granulite. Omphacite in both components is partially pseudomorphed by post-S₁ symplectites of sodic–diopside and albitic plagioclase that reflect near-isothermal decompression to pressures ≈14 kbar. The Breaksea Orthogneiss also occurs as pods and layers within the post-S₁/S₂ gabbroic Resolution Orthogneiss, which is distinguished on the basis of it mostly lacking garnet and being homogeneous. Along the northern shore of Resolution Island, hornblende granulite and high-pressure amphibolite facies S₂ assemblages involving garnet, hornblende and clinozoisite are well developed in both the Breaksea and Resolution orthogneisses. A 200 m thick, shallowly north-dipping D₂ shear zone forms a carapace to these orthogneiss units, and separates them from other Cretaceous orthogneiss and Palaeozoic schists that reflect lower grade conditions (pressures ≈12-14 kbar). The Breaksea Orthogneiss extends the thickness of the Cretaceous island arc developed off the New Zealand sector of the Gondwana margin to more than 60 km, close to the thickest of known Andean-style margins.