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

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A series of striking migmatitic structures occur in rectilinear networks through western Fiordland, New Zealand, involving, for the most part, narrow anorthositic dykes that cut hornblende-bearing orthogneiss. Adjacent to the dykes, host rocks show patchy, spatially restricted recrystallization and dehydration on a decimetre-scale to garnet granulite. Although there is general agreement that the migration of silicate melt has formed at least parts of the structures, there is disagreement on the role of silicate melt in dehydrating the host rock. A variety of causal processes have been inferred, including metasomatism due to the ingress of a carbonic, mantle-derived fluid; hornblende-breakdown leading to water release and limited partial melting of host rocks; and dehydration induced by volatile scavenging by a migrating silicate melt. Variability in dyke assemblage, together with the correlation between dehydration structures and host rock silica content, are inconsistent with macroscopic metasomatism, and best match open system behaviour involving volatile scavenging by a migrating trondhjemitic liquid.

Section: Discussionmentioning

confidence: 99%

“…On the basis of U‐Pb SHRIMP data, protoliths to the rocks are calcalkaline plutons dominated by 136–129 Ma magmatic zircon, which experienced high‐grade metamorphism between c . 120 and 100 Ma (Hollis et al. , 2003).…”

Section: Major Physical and Chemical Relationshipsmentioning

confidence: 99%

Roles for fluid and/or melt advection in forming high‐P mafic migmatites, Fiordland, New Zealand

Clarke

Daczko

Klepeis

et al. 2005

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“…The WFO at Lake Grave is alkali‐calcic syeno‐dioritic in composition. SiO 2 content ranges between 53.7 wt% and 54.9 wt%, placing these rocks towards the more mafic end of the spectrum of WFO compositions (Bradshaw, 1990; Hollis et al. , 2003).…”

Section: Whole‐rock Geochemistry and Mineral Chemistrymentioning

confidence: 99%

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

Daczko¹,

Halpin

2009

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A 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 (<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 twopyroxene 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 twopyroxene hornblende assemblage is stable at <11 kbar, whereas a garnet granulite assemblage is stable at >12 kbar, suggesting that garnet granulite may have formed with <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 (>90%) of the twopyroxene 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.

“…Protoliths of these rocks include extensive Mesozoic magmatic‐arc rocks, and Palaeozoic and Mesozoic screens and xenoliths (Tulloch et al. , 2000; Hollis et al. , 2003).…”

Section: Regional Geology and Previous Workmentioning

confidence: 99%

Trace element partitioning during high‐P partial melting and melt‐rock interaction; an example from northern Fiordland, New Zealand

Schröter

Stevenson

Daczko

et al. 2004

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Pods of granulite facies dioritic gneiss in the Pembroke Valley, Milford Sound, New Zealand, preserve peritectic garnet surrounded by trondhjemitic leucosome and vein networks, that are evidence of high-P partial melting. Garnet-bearing trondhjemitic veins extend into host gabbroic gneiss, where they are spatially linked with the recrystallization of comparatively low-P two-pyroxene-hornblende granulite to fine-grained high-P garnet granulite assemblages in garnet reaction zones. New data acquired using a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICPMS) for minerals in various textural settings indicate differences in the partitioning of trace elements in the transition of the two rock types to garnet granulite, mostly due to the presence or absence of clinozoisite. Garnet in the garnet reaction zone (gabbroic gneiss) has a distinct trace element pattern, inherited from reactant gabbroic gneiss hornblende. Peritectic garnet in the dioritic gneiss and garnet in trondhjemitic veins from the Pembroke Granulite have trace element patterns inherited from the melt-producing reaction in the dioritic gneiss. The distinct trace element patterns of garnet link the trondhjemitic veins geochemically to sites of partial melting in the dioritic gneiss.