The Eclogite–Granulite Transition: Mafic and Intermediate Assemblages at Breaksea Sound, New Zealand

The Breaksea Orthogneiss, a high-P component of the c. 126–116 Ma Western Fiordland Orthogneiss, has distinctive composite mafic layering and dyke structures in a monzodioritic 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. Mafic components preserve eclogite-facies S1 garnet and omphacite with or without orthopyroxene, interlayered with metadioritic components that preserve granulite-facies assemblages involving S1 garnet, omphacite, plagioclase, K-feldspar and kyanite. As both eclogite and granulite assemblages reflect P ≈ 1·8 GPa and T ≈ 850°C, the Breaksea Orthogneiss presents an unusual natural example of the eclogite–granulite transition. The facies distinction was controlled by whole-rock composition, whereby mafic components recrystallized to eclogite and dioritic components recrystallized to granulite. Omphacite in both components is partially pseudomorphed by symplectites of sodic-diopside and albitic plagioclase that reflect near-isothermal decompression to P ≈ 1·4 GPa. The Breaksea Orthogneiss also occurs as pods and layers in the post-S1 mafic 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-P amphibolite-facies S2 assemblages involving garnet, hornblende and clinozoisite are well developed in both the Breaksea and Resolution orthogneisses. A 200 m thick, shallowly south- to SE-dipping D2 shear zone forms a carapace to these orthogneiss units, and separates them from other Cretaceous orthogneisses and Palaeozoic schists that reflect lower grade conditions (P ≈ 1·2–1·4 GPa). The Breaksea Orthogneiss extends the thickness of the Cretaceous island arc developed off the Gondwana margin to more than 60 km, close to the thickest of known Andean-style margins.