Western Fiordland Orthogneiss Research Articles

Interplay of Cretaceous transpressional deformation and continental arc magmatism in a long-lived crustal boundary, central Fiordland, New Zealand

Abstract Recovering the time-evolving relationship between arc magmatism and deformation, and the influence of anisotropies (inherited foliations, crustal-scale features, and thermal gradients), is critical for interpreting the location, timing, and geometry of transpressional structures in continental arcs. We investigated these themes of magma-deformation interactions and preexisting anisotropies within a middle- and lower-crustal section of Cretaceous arc crust coinciding with a Paleozoic boundary in central Fiordland, New Zealand. We present new structural mapping and results of Zr-in-titanite thermometry and U-Pb zircon and titanite geochronology from an Early Cretaceous batholith and its host rock. The data reveal how the expression of transpression in the middle and lower crust of a continental magmatic arc evolved during emplacement and crystallization of the ∼2300 km2 lower-crustal Western Fiordland Orthogneiss (WFO) batholith. Two structures within Fiordland’s architecture of transpressional shear zones are identified. The gently dipping Misty shear zone records syn-magmatic oblique-sinistral thrust motion between ca. 123 and ca. 118 Ma, along the lower-crustal WFO Misty Pluton margin. The subhorizontal South Adams Burn thrust records mid-crustal arc-normal shortening between ca. 114 and ca. 111 Ma. Both structures are localized within and reactivate a recently described >10 km-wide Paleozoic crustal boundary, and show that deformation migrated upwards between ca. 118 and ca. 114 Ma. WFO emplacement and crystallization (mainly 118–115 Ma) coincided with elevated (>750 °C) middle- and lower-crustal Zr-in-titanite temperatures and the onset of mid-crustal cooling at 5.9 ± 2.0 °C Ma−1 between ca. 118 and ca. 95 Ma. We suggest that reduced strength contrasts across lower-crustal pluton margins during crystallization caused deformation to migrate upwards into thermally weakened rocks of the mid-crust. The migration was accompanied by partitioning of deformation into domains of arc-normal shortening in Paleozoic metasedimentary rocks and domains that combined shortening and strike-slip deformation in crustal-scale subvertical, transpressional shear zones previously documented in Fiordland. U-Pb titanite dates indicate Carboniferous–Cretaceous (re)crystallization, consistent with reactivation of the inherited boundary. Our results show that spatio-temporal patterns of transpression are influenced by magma emplacement and crystallization and by the thermal structure of a reactivated boundary.

New U–Pb zircon ages for the Glenroy Complex and McKnee Intrusives, southeast Nelson: strengthening links with northern Fiordland and constraining the timing of metamorphism

ABSTRACTThe Glenroy Complex and McKnee Intrusives, southeast Nelson, have previously been correlated with similar rocks in northern Fiordland: the Worsley Pluton of the Western Fiordland Orthogneiss and George Sound Paragneiss, and Mistake Diorite and Darran Leucogabbro, respectively. Two samples of the Glenroy Complex (Woodham Orthogneiss) and one from the McKnee Intrusives yielded laser ablation inductively-coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon ages of 123.7 ± 1.4 and 121.9 ± 1.1 Ma, and 138.8 ± 1.3 Ma, respectively. These new ages fall within the ranges previously published for the Glenroy Complex, McKnee Intrusives and their northern Fiordland counterparts and, thus, provide further evidence for their correlation. An age of 116.3 ± 2.4 Ma was also determined for a post-metamorphic pegmatite–aplite dike, constraining metamorphism of the Woodham Orthogneiss to within 2.0–9.0 Ma following magmatic crystallisation. The time gap between crystallisation and metamorphism is similar to that of the Worsley Pluton (3.7–8.9 Ma), lending additional support for their direct correlation.

Emplacement and Paleozoic and Cretaceous recrystallisation of the Broughton Arm Peridotite in Western Fiordland, New Zealand

ABSTRACTUltramafic rocks are rare on Earth’s surface but can provide important insights into crust and mantle evolution. The recently discovered Broughton Arm Peridotite in the mountains of central Fiordland is exposed as two 75–150 m wide by several hundred metres-long harzburgite-dunite lobes encased sequentially in hornblendite, amphibolite and then quartzofeldspathic gneiss. On the basis of the refractory harzburgitic-dunitic composition and the bulk rock Mg# (86–87) lower than most mantle peridotites, the peridotite is interpreted to have an igneous cumulate rather than an exhumed mantle peridotite origin. The marginal hornblendite and amphibolite and internal pods of edenite-diopside rock may represent metasomatised peridotite. The Broughton Arm Peridotite has been metamorphosed twice. The first event generated an anhydrous assemblage comprising olivine (Mg# = 79.4–92.8), enstatite and Cr-magnetite. This assemblage has been overprinted by an anastomosing weak to pervasive mylonitic foliation of Mg-chlorite, tremolite ± serpentinite ± talc. The age of the anhydrous assemblage is unknown but suspected from regional data to be Early Carboniferous. A titanite 208Pb/238U lower intercept age of 105.6 ± 6.8 Ma in the host Deep Cove Gneiss is correlated with the peridotite hydration-deformation event. The proximity of the Early Cretaceous (≥ 114 Ma) Western Fiordland Orthogneiss to the east, west and north means that the Broughton Arm Peridotite and surrounding rocks may have recrystallised within the extensional Doubtful Sound Shear Zone, which elsewhere is an up to several hundred meter-wide shear zone active at c. 110–100 Ma separating Western Fiordland Orthogneiss from Deep Cove Gneiss. The Broughton Arm Peridotite therefore preserves a history that involves emplacement in the Early Paleozoic, penetrative anhydrous metamorphism in the Early Carboniferous, and hydration and recrystallisation during regional extensional tectonism in the Early Cretaceous.

Response of zircon to melting and metamorphism in deep arc crust, Fiordland (New Zealand): implications for zircon inheritance in cordilleran granites

The Cretaceous Mount Daniel Complex (MDC) in northern Fiordland, New Zealand was emplaced as a 50 m-thick dyke and sheet complex into an active shear zone at the base of a Cordilleran magmatic arc. It was emplaced below the 20–25 km-thick, 125.3 ± 1.3 Ma old Western Fiordland Orthogneiss (WFO) and is characterized by metre-scale sheets of sodic, low and high Sr/Y diorites and granites. 119.3 ± 1.2 Ma old, pre-MDC lattice dykes and 117.4 ± 3.1 Ma late-MDC lattice dykes constrain the age of the MDC itself. Most dykes were isoclinally folded as they intruded, but crystallised within this deep-crustal, magma-transfer zone as the terrain cooled and was buried from 25 to 50 km (9–14 kbar), based on published P-T estimated from the surrounding country rocks. Zircon grains formed under these magmatic/granulite facies metamorphic conditions were initially characterized by conservatively assigning zircons with oscillatory zoning as igneous and featureless rims as metamorphic, representing 54% of the analysed grains. Further petrological assignment involved additional parameters such as age, morphology, Th/U ratios, REE patterns and Ti-in-zircon temperature estimates. Using this integrative approach, assignment of analysed grains to metamorphic or igneous groupings improved to 98%. A striking feature of the MDC is that only ~ 2% of all igneous zircon grains reflect emplacement, so that the zircon cargo was almost entirely inherited, even in dioritic magmas. Metamorphic zircons of MDC show a cooler temperature range of 740–640 °C, reflects the moderate ambient temperature of the lower crust during MDC emplacement. The MDC also provides a cautionary tale: in the absence of robust field and microstructural relations, the igneous-zoned zircon population at 122.1 ± 1.3 Ma, derived mostly from inherited zircons of the WFO, would be meaningless in terms of actual magmatic emplacement age of MDC, where the latter is further obscured by younger (ca. 114 Ma) metamorphic overgrowths. Thus, our integrative approach provides the opportunity to discriminate between igneous and metamorphic zircon within deep-crustal complexes. Also, without the tight field relations at Mt Daniel, the scatter beyond a statistically coherent group might be ascribed to the presence of “antecrysts”, but it is clear that the WFO solidified before the MDC was emplaced, and these older “igneous” grains are inherited. The bimodal age range of inherited igneous grains, dominated by ~ 125 Ma and 350–320 Ma age clusters, indicate that the adjacent WFO and a Carboniferous metaigneous basement were the main sources of the MDC magmas. Mafic lenses, stretched and highly attenuated into wisps within the MDC and dominated by ~ 124 Ma inherited zircons, are considered to be entrained restitic material from the WFO. A comparison with lower- and upper-crustal, high Sr/Y metaluminous granites elsewhere in Fiordland shows that zircon inheritance is common in the deep crust, near the source region, but generally much less so in coeval, shallow magma chambers (plutons). This is consistent with previous modelling on rapid zircon dissolution rates and high Zr saturation concentrations in metaluminous magmas. Accordingly, unless unusual circumstances exist, such as MDC preservation in the deep crust, low temperatures of magma generation, or rapid emplacement and crystallization at higher structural levels, information on zircon inheritance in upper crustal, Cordilleran plutons is lost during zircon dissolution, along with information on the age, nature and variety of the source material. The observation that dioritic magmas can form at these low temperatures (< 750 °C) also suggests that the petrogenesis of mafic rocks in the arc root might need to be re-assessed.

Evaluating the importance of metamorphism in the foundering of continental crust

The metamorphic conditions and mechanisms required to induce foundering in deep arc crust are assessed using an example of representative lower crust in SW New Zealand. Composite plutons of Cretaceous monzodiorite and gabbro were emplaced at ~1.2 and 1.8 GPa are parts of the Western Fiordland Orthogneiss (WFO); examples of the plutons are tectonically juxtaposed along a structure that excised ~25 km of crust. The 1.8 GPa Breaksea Orthogneiss includes suitably dense minor components (e.g. eclogite) capable of foundering at peak conditions. As the eclogite facies boundary has a positive dP/dT, cooling from supra-solidus conditions (T > 950 ºC) at high-P should be accompanied by omphacite and garnet growth. However, a high monzodioritic proportion and inefficient metamorphism in the Breaksea Orthogneiss resulted in its positive buoyancy and preservation. Metamorphic inefficiency and compositional relationships in the 1.2 GPa Malaspina Pluton meant it was never likely to have developed densities sufficiently high to founder. These relationships suggest that the deep arc crust must have primarily involved significant igneous accumulation of garnet–clinopyroxene (in proportions >75%). Crustal dismemberment with or without the development of extensional shear zones is proposed to have induced foundering of excised cumulate material at P > 1.2 GPa.

The tempo of continental arc construction in the Mesozoic Median Batholith, Fiordland, New Zealand

We investigate the temporal record of magmatism in the Fiordland sector of the Median Batholith (New Zealand) with the goal of evaluating models for cyclic and episodic patterns of magmatism and deformation in continental arcs. We compare 20 U-Pb zircon ages from >2300 km2 of Mesozoic lower and middle crust of the Western Fiordland Orthogneiss to existing data from the Median Batholith to: (1) document the tempo of arc construction, (2) estimate rates of magmatic addition at various depths during arc construction, and (3) evaluate the role of cyclical feedbacks between magmatism and deformation during high and low magma addition rate events. Results from the Western Fiordland Orthogneiss indicate that the oldest dates are distributed in northern and southern extremities: the Worsley Pluton (123–121 Ma), eastern McKerr Intrusives (128–120 Ma), and Breaksea Orthogneiss (123 Ma). Dates within the interior of the Western Fiordland Orthogneiss (Misty and Malaspina Plutons, western McKerr Intrusives) primarily range from 118 to 115 Ma and signify a major flux of mafic to intermediate magmatism during which nearly 70% of the arc root was emplaced during a brief, ∼3 m.y., interval. The spatial distribution of dates reveals an inward-focusing, arc-parallel younging of magmatism within the Western Fiordland Orthogneiss during peak magmatic activity. Coupled with existing data from the wider Median Batholith, our data show that Mesozoic construction of the Median Batholith involved at least two high-flux magmatic events: a surge of low-Sr/Y plutonism in the Darran Suite from ca. 147 to 136 Ma, and a terminal surge of high-Sr/Y magmatism in the Separation Point Suite from 128 to 114 Ma, shortly before extensional collapse of the Zealandia Cordillera at 108–106 Ma. Separation Point Suite magmatism occurred at all structural levels, but was concentrated in the lower crust, where nearly 50% of the crust consists of Cretaceous arc-related plutonic rocks. Existing isotopic data suggest that the flare-up of high-Sr/Y magmatism was primarily sourced from the underlying mantle, indicating an externally triggered, dynamic mantle process for triggering the Zealandia high–magma addition rate event, with only limited contributions from upper plate materials.

Complexity of In-situ zircon U–Pb–Hf isotope systematics during arc magma genesis at the roots of a Cretaceous arc, Fiordland, New Zealand

Zircons from seventeen samples of Western Fiordland Orthogneiss (WFO) diorites and three samples of country rock (two schists and one Darran Suite diorite) from the lowermost exposed sections of the Median Batholith, Fiordland, New Zealand, were analysed for in-situ U–Pb and Hf-isotopes. The WFO represents the deeper levels of Early Cretaceous continental arc magmatism on the Pacific margin of Gondwana, marking the final stage of long-lived arc magmatism on the margin spanning the Palaeozoic. The WFO plutons were emplaced at high-P (mid to deep crust at c. 8–12kbar) between 124 and 114Ma. Minor very high-P (c. 18kbar) WFO eclogite and omphacite granulite facies orthogneiss (Breaksea Orthogneiss) are inferred to have crystallised in the base of thickened crust at c. 124Ma. Zircons from the Breaksea Orthogneiss are considered to be variably affected by Pb-loss due to emplacement of the adjacent (Malaspina) Pluton at c. 114Ma. By identifying Pb-loss, magmatic ages were able to be inferred in respect to apparent Pb-loss ages. Hf isotope data for the WFO define an excursion to less radiogenic Hf isotope ratios with time, reflecting increased recycling of an old source component. Peaks at c. 555, 770 and 2480Ma, determine the age spectra of inherited populations of zircons within the WFO. This contrasts with detrital zircon patterns in country rocks of the Takaka terrane, which include peaks at c. 465Ma, and 1250–900Ma that are absent in the WFO inheritance pattern. These results indicate a previously unrecognised Precambrian lower crustal component of New Zealand. Recycling of this lower crust became increasingly important as a source for the final stage or Mesozoic arc magmatism along this segment of the palaeo-Pacific margin of Gondwana.

Thermochronology of extensional orogenic collapse in the deep crust of Zealandia

The exhumed Fiordland sector of Zealandia offers a deep-crustal view into the life cycle of a Cordilleran-type orogen from final magmatic construction to extensional orogenic collapse. We integrate U-Pb thermochronologic data from metamorphic zircon and titanite with structural observations from >2000 km 2 of central Fiordland to document the tempo and thermal evolution of the lower crust during the tectonic transition from arc construction and crustal thickening to crustal thinning and extensional collapse. Data reveal that garnet granulite facies metamorphism and partial melting in the lower crust partially overlapped with crustal thickening and batholith construction during emplacement of the Western Fiordland Orthogneiss (WFO) from 118 to 115 Ma. Metamorphic zircons in metasedimentary rocks yield 206 Pb/ 238 U (sensitive high-resolution ion microprobe–reverse geometry) dates of 116.3–112.0 Ma. Titanite laser ablation split stream inductively coupled plasma–mass spectrometry chronology from the same rocks yielded complex results, with relict Paleozoic 206 Pb/ 238 U dates preserved at the margins of the WFO. Within extensional shear zones that developed in the thermal aureole of the WFO, titanite dates range from 116.2 to 107.6 Ma and have zirconium-in-titanite temperatures of ∼900–750 °C. A minor population of metamorphic zircon rims and titanites in the Doubtful Sound region yield younger dates of 105.6–102.3 Ma with corresponding temperatures of 740–730 °C. Many samples record Cretaceous overdispersed dates with 5–10 m.y. ranges. Core-rim traverses and grain maps show complex chemical and temporal variations that cannot easily be attributed to thermally activated volume diffusion or simple core-rim crystallization. We interpret these Cretaceous titanites not as cooling ages, but rather as recording protracted growth and/or crystallization or recrystallization in response to fluid flow, deformation, and/or metamorphic reactions during the transition from garnet granulite to upper amphibolite facies metamorphism. We propose a thermotectonic model that integrates our results with structural observations. Our data reveal a clear tectonic break at 108–106 Ma that marks a change in processes deep within the arc. Prior to this break, arc construction processes dominated and involved (1) emplacement of mafic to intermediate magmas of the Malaspina and Misty plutons from 118 to 115 Ma, (2) contractional deformation at the roof of the Misty pluton in the Caswell Sound fold-thrust belt from 117 to 113 Ma, and (3) eclogite to garnet granulite facies metamorphism and partial melting over >8 m.y. from 116 to 108 Ma. These processes were accompanied by complex patterns of lower crustal flow involving both horizontal and vertical displacements. After this interval, extensional orogenic collapse initiated along upper amphibolite facies shear zones in the Doubtful Sound shear zone at 108–106 Ma. Zircon and titanite growth and/or crystallization or recrystallization at this time clearly link upper amphibolite facies metamorphism to mylonitic fabrics in shear zones. Our observations are significant in that they reveal the persistence of a hot and weak lower crust for ≥15 m.y. following arc magmatism in central Fiordland. We propose that the existence of a thermally weakened lower crust within the Median Batholith was a key factor in controlling the transition from crustal thickening to crustal thinning and extensional orogenic collapse of the Zealandia Cordillera.

The geology, geochronology and affiliation of the Glenroy Complex and adjacent plutonic rocks, southeast Nelson

ABSTRACTThe Glenroy Complex consists of retrogressed monzodioritic granulites (Woodham Orthogneiss) and heterogeneous metasedimentary rocks (Davis Creek Paragneiss). U–Pb zircon dating shows that the Woodham Orthogneiss comprises rocks ranging in age from 121.8 to 126.6 Ma that are correlatives of the geochemically similar Western Fiordland Orthogneiss Worsley Pluton in northern Fiordland. Permian–Jurassic detrital zircon age peaks in the Davis Creek Paragneiss indicate a maximum depositional age of c. 227 Ma, suggesting correlation with George Sound Paragneiss in northern Fiordland and the paleo-Pacific Gondwana margin cover sequence. Metamorphism of the Davis Creek Paragneiss is constrained to 120.6 ± 0.9 Ma by U–Pb dating of low Th/U zircon overgrowths. The McKnee Intrusives were intruded in two discrete episodes at 225.0 ± 2.7 Ma and 137.8 ± 0.7 Ma which, together with their geochemistry, suggests correlation with northern Fiordland Late Triassic and Early Cretaceous Darran Suite plutons. The HiSY Mt Cann Pluton has an age of 113.8 ± 1.0 Ma and is correlated with the Separation Point Granite. The Glenroy Complex and McKnee Intrusives were likely exhumed during at least two stages: initially in the Cretaceous prior to displacement from their northern Fiordland correlatives by the Alpine Fault; and during emplacement as tectonic blocks in the Matakitaki foreland thrust system at the Big Bend.

Petrogenesis and geochemical characterisation of ultramafic cumulate rocks from Hawes Head, Fiordland, New Zealand

Early Cretaceous parts of the western Median Batholith (Western Fiordland Orthogneiss) represent the exposed root of a magmatic arc of dioritic to monzodioritic composition (SiO2=51–55 wt%; Na2O/K2O=3.7–8.8 in this study). We characterise for the first time the field relationships, petrography, mineralogy and geochemistry of ultramafic and mafic cumulates at Hawes Head, the largest exposure of ultramafic rocks in western Fiordland. We distinguish three related rock types at Hawes Head: hornblende peridotite (MgO=21–35 wt%); hornblendite (MgO=15–16 wt%); and pyroxenite (MgO=21 wt%). Petrogenetic relationships between the ultramafic rocks and the surrounding Misty Pluton of the Western Fiordland Orthogneiss are demonstrated by: (i) mutually cross-cutting relationships; (ii) similar mafic phases (e.g. pyroxene and amphibole) with elevated Mg-numbers (e.g. olivine Mg/(Mg+Fe)=0.77–0.82); (iii) fractionation trends in mineral geochemistry; and (iv) shared depleted heavy rare earth element patterns. In addition, the application of solid/liquid partition coefficients indicates that olivine in the ultramafic rocks at Hawes Head crystallised from a magma with Mg/(Mg+Fe)=0.54–0.57. The olivine grains therefore represent a plausible early crystallising phase of the adjacent Western Fiordland Orthogneiss (Mg/(Mg+Fe)=0.51–0.55).

Anti‐clockwise P–T paths in the lower crust: an example from a kyanite‐bearing regional aureole, George Sound, New Zealand

AbstractThe George Sound Paragneiss (GSP) represents a rare Permo‐Triassic unit in Fiordland that occurs as a km‐scale pillar to gabbroic and dioritic gneiss of c. 120 Ma Western Fiordland Orthogneiss (WFO). It is distinguished from Palaeozoic paragneiss common in western Fiordland (Deep Cove Gneiss) by SHRIMP and laser‐ablation U–Pb ages as young as c. 190 Ma and 176Hf/177Lu &gt;0.2828 for detrital zircon grains. The Mesozoic age of the GSP circumvents common ambiguity in the interpretation of Cretaceous v. Palaeozoic metamorphic assemblages in the Deep Cove Gneiss. A shallowly dipping S1 foliation is preserved in the GSP distal to the WFO, cut by 100 m scale migmatite contact zones. All units preserve a steeply dipping S2 foliation. S1 staurolite and sillimanite inclusions in the cores of metapelitic garnet grains distal to the WFO preserve evidence for prograde conditions of T &lt; 650 °C and P &lt; 8 kbar. Contact aureole and S2 assemblages include Mg‐rich, Ca‐poor cores to garnet grains in metapelitic schist that reflect WFO emplacement at ≈760 °C and ≈6.5 kbar. S2 kyanite‐bearing matrix assemblages and Ca‐enriched garnet rims reflect ≈650 °C and ≈11 kbar. Poorly oriented muscovite–biotite intergrowths and rare paragonite reflect post‐S2 high‐P retrogression and cooling. Pseudosection modelling in NCKFMASH defines a high‐P anti‐clockwise P–T history for the GSP involving: (i) mid‐P amphibolite facies conditions; preceding (ii) thermal metamorphism adjacent to the WFO; followed by (iii) burial to high‐P and (iv) high‐P cooling induced by tectonic juxtaposition of cooler country rock.

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

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.

Granulite facies thermal aureoles and metastable amphibolite facies assemblages adjacent to the Western Fiordland Orthogneiss in southwest Fiordland, New Zealand

AbstractIn southwest New Zealand, a suite of felsic diorite intrusions known as the Western Fiordland Orthogneiss (WFO) were emplaced into the mid to deep crust and partially recrystallized to high‐P (12 kbar) granulite facies assemblages. This study focuses on the southern most pluton within the WFO suite (Malaspina Pluton) between Doubtful and Dusky sounds. New mapping shows intrusive contacts between the Malaspina Pluton and adjacent Palaeozoic metasedimentary country rocks with a thermal aureole ∼200–1000 m wide adjacent to the Malaspina Pluton in the surrounding rocks. Thermobarometry on assemblages in the aureole indicates that the Malaspina Pluton intruded the adjacent amphibolite facies rocks while they were at depths of 10–14 kbar. Similar P–T conditions are recorded in high‐P granulite facies assemblages developed locally throughout the Malaspina Pluton. Palaeozoic rocks more than ∼200–1000 m from the Malaspina Pluton retain medium‐P mid‐amphibolite facies assemblages, despite having been subjected to pressures of 10–14 kbar for &gt; 5 Myr. These observations contradict previous interpretations of the WFO Malaspina Pluton as the lower plate of a metamorphic core complex, everywhere separated from the metasedimentary rocks by a regional‐scale extensional shear zone (Doubtful Sound Shear Zone). Slow reaction kinetics, lack of available H2O, lack of widespread penetrative deformation, and cooling of the Malaspina Pluton thermal anomaly within c. 3–4 Myr likely prevented recrystallization of mid amphibolite facies assemblages outside the thermal aureole. If not for the evidence within the thermal aureole, there would be little to suggest that gneissic rocks which underlie several 100 km2 of southwest New Zealand had experienced metamorphic pressures of 10–14 kbar. Similar high‐P metamorphic events may therefore be more common than presently recognized.

Plutonic rocks of the Median Batholith in eastern and central Fiordland, New Zealand: Field relations, geochemistry, correlation, and nomenclature

This paper provides a comprehensive description of all major plutonic rock units in Fiordland between Lakes Poteriteri and Te Anau, and the heads of Doubtful and George Sounds. Plutonic rocks comprise c. 80% of the basement in the area described, the remainder being metase dim entary and metavolcaniclastic rocks. The plutonic rocks, of which c. 50% are granitoids, were emplaced in three phases—at c. 492 Ma, between c. 365 and 318 Ma, and between 168 and 116 Ma. Correlatives of the Devonian Karamea Suite emplaced between c. 375 and 367 Ma, and the Triassic to Early Jurassic part of the Darran Suite emplaced between c. 230 and 168 Ma, are not present in the area described here. The strongly deformed Late Cambrian to Early Ordovician Jaquiery Granitoid Gneiss is one of the oldest plutonic rocks yet discovered in New Zealand and is of similar age to plutonic rocks within the Ross and Delamerian Orogens of Victoria Land and South Australia. Rocks emplaced between c. 365 and 318 Ma include Ridge Suite S‐type granitoids and closely related S/A‐type plutons, Foulwind Suite A‐type mafic and granitoid plutons, Tobin Suite I‐type granitoids, and several unassigned mafic plutons. Rocks emplaced between 168 and 116 Ma include extensive c. 168–128 Ma old calc‐alkaline LoSY gabbros, diorites, and granitoids of the Darran Suite, c. 165–135 Ma old hypersolvus perthitic syenogranites and peralkaline granitoids, c. 125 Ma gneissic diorite similar to the Western Fiordland Orthogneiss, and c. 123–116 Ma old quartz diorites and granitoids of the HiSY Separation Point Suite. Plutons from each suite tend to be concentrated in distinct NNE‐striking parallel belts up to 20 km wide and 110+ km long. These belts are one of the key features which define the regional structural grain of Fiordland basement geology. Their strike remains constant from the Carboniferous through to the Cretaceous. S, S/A, and A‐type plutons of the Carboniferous Ridge and Foulwind Suites are confined to a 125 km long but discontinuous belt in southern and central Fiordland, wholly within the areal extent of early Paleozoic metase dim entary basement. Volumetrically minor Carboniferous Tobin Suite I‐type granitoids are confined to the area east of exposed early Paleozoic metasedimentary basement. Much of eastern Fiordland is underlain by an extensive belt of heterogeneous Darran Suite rocks. Darran Suite rocks extend from Stewart Island to the Darran Mountains of northern Fiordland, forming a belt c. 15 km wide and 300 km long. Correlative Darran Suite rocks also occur further west where they intrude early Paleozoic metasediments, indicating that Jurassic to Early Cretaceous arc‐related plutonism and volcanism occurred inboard of the edge of early Paleozoic basement in some parts of the Median Batholith. Distinctive Jurassic, pink, hypersolvus syenogranite and alkalic granitoids form a narrow discontinuous belt within the wider calcalkaline Darran Suite. Cretaceous Separation Point Suite plutons form two major belts, one in easternmost Fiordland partially covered by Cenozoic sedimentary rocks, and the other stitching inboard and outboard parts of the Median Batholith in central Fiordland.

Metastable persistence of pelitic metamorphic assemblages at the root of a Cretaceous magmatic arc – Fiordland, New Zealand

AbstractFour aluminosilicate‐bearing, amphibolite facies pelitic schists sampled from the root of the long‐lived eastern Gondwana continental magmatic arc now exposed in southwest Fiordland, New Zealand, record remarkably different P–T–t histories. The four samples were collected from within 20 km of each other within the Fanny Bay Group and Deep Cove Gneiss near Dusky Sound. Integrated petrography, mineral chemistry, mineral equilibria modelling and in situ electron microprobe chemical dating of monazite shows that the sample of the Fanny Bay Group south of the Dusky Fault records a Carboniferous history with peak conditions of 4–4.5 kbar at 570–590 °C, while one sample of the Deep Cove Gneiss from Long Island records a Cretaceous history with apparent peak conditions of 7.5 kbar at 650 °C. Two other samples of the Deep Cove Gneiss from Resolution Island record mixed Carboniferous and Cretaceous histories with apparent peak conditions of 7 kbar at 650 °C and 3–7 kbar at 640–720 °C. The metapelitic schists on Resolution Island were intruded by arc magmas including the voluminous high‐P Western Fiordland Orthogneiss, yet they lack mineralogical evidence of the Cretaceous high‐P (&gt;12 kbar) event. Analysis of water isopleths in a model system shows that the amount of water accommodated in the rock mineral assemblage increases with pressure. With the exhaustion of all free water, and without the addition of external water, these rocks persisted metastably within the deep arc during the high‐P event. The emplacement of large volumes of diorite (i.e. the Western Fiordland Orthogneiss) into the root of the Early Cretaceous continental magmatic arc did not lead to regional granulite facies metamorphism of the country rock schists, as large volumes of amphibolite facies rock metamorphosed under medium‐P conditions persisted metastably in the deep arc crust.

Evidence for melt migration enhancing recrystallization of metastable assemblages in mafic lower crust, Fiordland, New Zealand

AbstractA major arc batholith, the Western Fiordland Orthogneiss (WFO) in Fiordland, New Zealand, exhibits irregular, spatially restricted centimetre‐scale recrystallization from two‐pyroxene hornblende granulite to garnet granulite flanking felsic dykes. At Lake Grave, northern Fiordland, the composition and texture of narrow (&lt;10–20 mm across) felsic dykes that cut the orthogneiss are consistent with an igneous origin and injection of melt to form orthogneiss migmatite. New U–Pb geochronology suggests that the injection of dykes and migmatization occurred at c. 115 Ma, during the later stages of arc magmatism. Recrystallization to garnet granulite is promoted by volatile extraction from the host two‐pyroxene hornblende granulite via adjacent dykes and the patchy development of garnet granulite is left as a marker adjacent to the melt migration path. New mineral equilibria modelling suggests that a two‐pyroxene hornblende assemblage is stable at &lt;11 kbar, whereas a garnet granulite assemblage is stable at &gt;12 kbar, suggesting that garnet granulite may have formed with &lt;5 km crustal loading of the batholith. Although the garnet granulite assemblages signify that the WFO experienced high‐P conditions, the very local nature of these textures indicates widespread metastability (&gt;90%) of the two‐pyroxene hornblende granulite assemblages. These results indicate the strongly metastable nature of assemblages in mafic lower arc crust during deep burial and demonstrate that the degree of reaction in the case of Fiordland is related to interaction with migrating melts.

Early Cretaceous dextral transpressional deformation within the Median Batholith, Stewart Island, New Zealand

The character, timing, and significance of deformation within the Median Batholith has been debated since at least 1967, with allochthonous and autochthonous models proposed to account for internal variations in the character of the batholith. Stewart Island provides excellent exposures of intrabatholithic structures, allowing many aspects of the deformation history within the batholith to be analysed, far removed from the effects of later deformation related to the current plate boundary. Median Batholith rocks in northern and central Stewart Island are deformed by three major structures: the Freshwater Fault System, Escarpment Fault, and Gutter Shear Zone. Lineation orientations, Al in hornblende geobarometry, and Ar‐Ar thermochronology indicate up to c. 7 km of NNE‐directed uplift of the hanging wall of the Escarpment Fault between c. 110 and 105 Ma. Unlike the Escarpment Fault, a wide range of mineral elongation lineation orientations, including many oblique to the strike and dip of related foliations, characterise both the Gutter Shear Zone and Freshwater Fault System. Lineation and limited sense of shear data indicate dextral‐reverse movement on both structures during development of their dominant ductile fabrics. Crosscutting and intrusive relationships indicate movement on the Freshwater Fault System after c. 130 Ma and on the Gutter Shear Zone between 120 and 112 Ma. The amount of movement on the Freshwater Fault System and Gutter Shear Zone remains largely unconstrained. However, the 342 ± 24 Ma age of a granite clast in a Paterson Group lithic tuff horizon at Abrahams Bay overlaps that of Carboniferous plutons in the block immediately south of the Freshwater Fault System, implying that the Paterson Group is little displaced from the basement rocks through which it was erupted. The three structures mapped on Stewart Island form part of a narrow transpressional mobile belt active within the Jurassic‐Cretaceous arc on the outboard margin of the Western Province during the later stages of arc development between c. 125 and 105 Ma. Deformation is largely confined to the area outboard of the Paleozoic metasedimentary basement in southern Stewart Island. In central Stewart Island, transpressional deformation began on the broad amphibolite facies Gutter Shear Zone. Subsequent deformation was focused along the narrower Escarpment Fault, which offset the c. 300°C isotherm in the upper crust. All three structures on Stewart Island likely form part of a regional‐scale network of transpressional shear zones and faults that includes the Indecision Creek and Grebe Shear Zones in eastern Fiordland. Transpressional deformation along these structures broadly coincided with loading of the Western Fiordland Orthogneiss in northern Fiordland by c. 6 kbar, indicating that contractional deformation affected all levels of the crust at this time.

Early Cretaceous extensional exhumation of the lower crust of a magmatic arc: Evidence from the Mount Irene Shear Zone, Fiordland, New Zealand

The Mount Irene Shear Zone (MISZ) in Fiordland, New Zealand, is a low‐angle extensional mylonitic structure that has contributed to exhumation of a deep section of a Cretaceous magmatic arc. The dominantly metasedimentary hanging wall last equilibrated at ∼5.9 kbar, 603°C, whereas the metadioritic footwall, correlated with the Western Fiordland Orthogneiss (WFO), equilibrated at ∼8.5 kbar, 722°C, during retrogression associated with the MISZ. Displacement across the MISZ is also implied from differences in deformational episodes, metamorphic fabrics, and the presence/absence of partial melt products. A late stage synkinematic granitoid dike swarm intrudes the MISZ, but the dikes are invariably truncated and sheared within the basal calc‐silicate mylonites of the hanging wall. U/Pb dates from one such dike demonstrate that juxtaposition of the footwall against the hanging wall was effectively complete by ∼108 Ma. This date makes the MISZ the earliest Cretaceous extensional shear zone yet established in Fiordland and indicates that the switch from a convergent to an extensional tectonic regime on the paleo‐Pacific arc margin of Gondwana occurred between 111 and 108 Ma. Therefore there was only 8 Myr between final intrusion and high‐pressure metamorphism of the WFO and the initiation of extensional exhumation of the lower crust of this magmatic arc.

The regional significance of Cretaceous magmatism and metamorphism in Fiordland, New Zealand, from U–Pb zircon geochronology

AbstractThe western Fiordland Orthogneiss (WFO) is an extensive composite metagabbroic to dioritic arc batholith that was emplaced at c. 20–25 km crustal depth into Palaeozoic and Mesozoic gneiss during collision and accretion of the arc with the Mesozoic Pacific Gondwana margin. Sensitive high‐resolution ion microprobe U–Pb zircon data from central and northern Fiordland indicate that WFO plutons were emplaced throughout the early Cretaceous (123.6 ± 3.0, 121.8 ± 1.7, 120.0 ± 2.6 and 115.6 ± 2.4 Ma). Emplacement of the WFO synchronous with regional deformation and collisional‐style orogenesis is illustrated by (i) coeval ages of a post‐D1 dyke (123.6 ± 3.0 Ma) and its host pluton (121.8 ± 1.7 Ma) at Mt Daniel and (ii) coeval ages of pluton emplacement and metamorphism/deformation of proximal paragneiss in George and Doubtful Sounds. The coincidence emplacement and metamorphic ages indicate that the WFO was regionally significant as a heat source for amphibolite to granulite facies metamorphism. The age spectra of detrital zircon populations were characterized for four paragneiss samples. A paragneiss from Doubtful Sound shows a similar age spectrum to other central Fiordland and Westland paragneiss and SE Australian Ordovician sedimentary rocks, with age peaks at 600–500 and 1100–900 Ma, a smaller peak at c. 1400 Ma, and a minor Archean component. Similarly, one sample of the George Sound paragneiss has a significant Palaeozoic to Archean age spectrum, however zircon populations from the George Sound paragneiss are dominated by Permo‐Triassic components and thus are markedly different from any of those previously studied in Fiordland.

Geochronology and geochemistry of high‐pressure granulites of the Arthur River Complex, Fiordland, New Zealand: Cretaceous magmatism and metamorphism on the palaeo‐Pacific Margin

AbstractThe Arthur River Complex is a suite of gabbroic to dioritic orthogneisses in northern Fiordland, New Zealand. The Arthur River Complex separates rocks of the Median Tectonic Zone, a Mesozoic island arc complex, from Palaeozoic rocks of the palaeo‐Pacific Gondwana margin, and is itself intruded by the Western Fiordland Orthogneiss. New SHRIMP U/Pb single zircon data are presented for magmatic, metamorphic and deformation events in the Arthur River Complex and adjacent rocks from northern Fiordland. The Arthur River Complex orthogneisses and dykes are dominated by magmatic zircon dated at 136–129 Ma. A dioritic orthogneiss that occurs along the eastern margin of the Complex is dated at 154.4 ± 3.6 Ma and predates adjacent plutons of the Median Tectonic Zone. Rims on zircon cores from this sample record a thermal event at c. 120 Ma, attributed to the emplacement of the Western Fiordland Orthogneiss. Migmatitic Palaeozoic orthogneiss from the Arthur River Complex (346 ± 6 Ma) is interpreted as deformed wall rock. Very fine rims (5–20 µm) also indicate a metamorphic age of c. 120–110 Ma. A post‐tectonic pegmatite (81.8 ± 1.8 Ma) may be related to phases of crustal extension associated with the opening of the Tasman Sea. The Arthur River Complex is interpreted as a batholith, emplaced at mid‐crustal levels and then buried to deep crustal levels due to convergence of the Median Tectonic Zone arc and the continental margin.