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P–T–X(composition) pseudosections constructed for natural monzodioritic to peridotgabbroic rock compositions using Thermocalc in the model system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO) illustrate the dependence of granulite and eclogite assemblages on whole-rock composition at mid-crustal to upper-mantle conditions. Increasing ferric iron content results in a marked contraction of garnet–plagioclase assemblages, and an expansion of orthopyroxene, kyanite, quartz and ilmenite stability across the compositional range of monzodioritic–gabbroic protoliths, and the expansion of plagioclase stability up-pressure in gabbroic–peridotgabbroic compositions. Omphacite granulite defines a transitional stage between garnet granulite and eclogite in monzodioritic to gabbroic compositions. Silicate liquid compositions calculated for a monzodioritic protolith using the haplogranite melt model do not accurately reflect trends in comparable experimental data, and refinement is needed for its application to the modeling of intermediate and mafic equilibria involving more than a few per cent partial melt. Omphacite-bearing granulite mineral equilibria in dioritic protoliths are far less sensitive to changes in whole-rock oxidation state than gabbroic protoliths; a doubling of whole-rock oxygen content displaces the modeled granulite–eclogite transition in gabbroic assemblages by 0·4 GPa up-pressure. Results of low-H2O, high-O equilibria modeling best validate natural assemblages from Breaksea Sound, Fiordland, New Zealand, where co-facial monzodioritic granulite and peridotgabbroic eclogite formed at P≈1·8 GPa and T ≈ 850°C. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

This paper provides a comprehensive description of the plutonic rocks of western Fiordland between Breaksea and Sutherland Sounds. The area is dominated by the Early cretaceous Western Fiordland Orthogneiss (WFO), but also includes smaller bodies of Paleozoic and cretaceous granitoid. Plutonic rocks of western Fiordland intrude metasediments of the Western Province, many of whose age and terrane affinities remain undefined. Paleozoic granitoids in western Fiordland include the Pandora Orthogneiss (c. 500 Ma) and widespread related sills within Paleozoic metasedimentary rocks; the All Round Pluton (c. 340 Ma); the Deas Cove Granite (c. 372 Ma); and possibly the Straight River Granite. The Pandora Orthogneiss is one of the oldest plutons yet found in the Median Batholith. Correlatives include the Jaquiery Granite Gneiss in central Fiordland and orthogneiss in Doubtful Sound. Plutonism of Ross/Delamarian age is therefore widespread in those parts of Fiordland where cambrian or older Western Province metasedimentary rocks form basement. The All Round Pluton and Deas Cove Granite are correlatives of the S-type Ridge and A/I-type Foulwind Suites, respectively. The c. 125-116 Ma WFO includes at least seven major dioritic and monzodioritic plutons in western Fiordland, one in central Fiordland, and one in central Stewart island. Plutons which compose the WFO are distinguished by differences in their age, petrography, structural and metamorphic histories, and geochemistry. The WFO in northern Fiordland and the correlative Walkers Pluton on Stewart island were emplaced in the mid crust (4-9 kbar) at depths comparable with some Separation Point Suite plutons of similar age. WFO plutons in southern Fiordland were emplaced at greater depths (10-18 kbar). WFO plutons have been variably recrystallised to eclogite; omphacite-, garnet-, two-pyroxene-, and hornblende-granulite; and hornblende-amphibolite facies assemblages, reflecting different PTX conditions during metamorphism of each body. Some parts of the WFO remain undeformed and unmetamorphosed. evidence of up to c. 6 kbar loading after emplacement is limited to WFO plutons in northern Fiordland and adjacent country rocks. extensional ductile shear zones previously shown to locally separate the WFO from adjacent rocks are discontinuous later features, commonly localised along earlier intrusive contacts between WFO plutons and metasedimentary country rocks. They do not form a regionally extensive detachment between the upper and lower plates of a metamorphic core complex. The WFO has previously been included in the Separation Point Suite since both units share a high Sr/Y (HiSY) chemistry and were emplaced at broadly the same time. However, the WFO and Separation Point Suite have distinct chemistries. Separation Point Suite rocks generally contain greater Sr, Na, and Al, and have lower Sr/Rb ratios, rare earth element and Y contents, than WFO rocks with comparable amounts of SiO2. Many aspects of the WFO chemistry (aside from its HiSY character) are similar to that of the older Darran Suite rather than the Separation Point Suite. This may reflect a greater amount of partial melting during generation of the SiO2-poor WFO than the SiO2-rich Separation Point Suite. Alternatively it may indicate derivation of the WFO and Separation Point Suite from different sources, albeit at depths greater than those where residual plagioclase is stable. Relatively large variations in the major element chemistry of the Separation Point Suite reflect fractionation and/or accumulation of plagioclase, whereas the more limited variability in the major element chemistry of the WFO reflects minor fractionation and/or accumulation of hornblende and/or clinopyroxene. [ABSTRACT FROM AUTHOR]