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

N. R. Daczko*, J. A. Halpin

*Corresponding author for this work

    Research output: Contribution to journalArticle

    39 Citations (Scopus)

    Abstract

    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 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 <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 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.

    Original languageEnglish
    Pages (from-to)167-185
    Number of pages19
    JournalJournal of Metamorphic Geology
    Volume27
    Issue number2
    DOIs
    Publication statusPublished - 2009

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