Plutonic rocks of Western Fiordland, New Zealand: Field relations, geochemistry, correlation, and nomenclature

Abstract

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 hornblendegranulite; 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 SiO 2 . Many aspects of the WFO chemistry (aside from its HiSY character) are similar to that o...

Cordillera Zealandia: A Mesozoic arc flare-up on the palaeo-Pacific Gondwana Margin

“…1). Both provinces are intruded by the Median Batholith (>10,000 km 2 ), which resulted from episodic Cambrian to Early Cretaceous (500–105 Ma) subduction-related plutonism 21, 22, 37 .

Figure 1( A ) Tectonic reconstruction of the Palaeo-Pacific Gondwana Margin at c. 115 Ma (modified from Mortimer, 73 ). The Whitsunday Volcanic Province has been viewed as a correlative to the Cretaceous Flare up in New Zealand 74 , however some features have been interpreted as a large igneous province 75 .

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“…Both the Separation Point Suite and Western Fiordland Orthogneiss are characterised by dominantly high Sr/Y ratios and represent upper and lower crustal components of the flare-up event respectively (Fig. 2)  21–23, 32, 36, 39 .

Figure 2Sr/Y (ppm) over time displays the shift to a garnet rich, plagioclase free source at <c. 130 Ma (Geochemical data from Allibone et al 21, 22 and references therein).

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Evaluating the importance of metamorphism in the foundering of continental crust

“…The arc batholith is dominated by the high Sr/Y c . 126–105 Ma Separation Point Suite and Western Fiordland Orthogneiss (WFO) 22,26–30 , which locally intrude Early Palaeozoic continental metasedimentary rocks and separate them from accreted Permian–Cretaceous volcano-sedimentary terranes 25 . The WFO comprises a series of intermediate to mafic plutons emplaced inboard of the continental margin at various depths (0.9–1.8 GPa) 22 .…”

“…Exposures of the Western Fiordland Orthogneiss (WFO) mostly reflect two major structural levels (1.2 and 1.8 GPa), and examples of the two levels are tectonically juxtaposed 21,22 . The high- P Breaksea Orthogneiss was a composite layered pluton of monzodiorite and gabbro that was patchily deformed and recrystallised at ~850 °C and 1.8 GPa 21,23 .…”

“…The Malaspina Pluton (Fig. 1) presents a shallower equivalent of the Breaksea Orthogneiss 22 . In SW Fiordland, it is largely garnet monzodioritic gneiss with discontinuous layers of garnet pyroxenite, that reflect limited partial melting, patchy deformation and recrystallization at ~750 °C and 1.2 GPa 24 .…”