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

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 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.

Crystallization of Heterogeneous Pelitic Migmatites: Insights from Thermodynamic Modelling

“…% liquid, as the leucosome changes rheologically from behaving like a liquid to behaving like a solid, passing through the rigid percolation threshold (Vigneresse et al, 1996) or the solid-to-liquid transition (Rosenberg & Handy, 2005). This hypothesis is supported by petrographical and geochemical studies suggesting that some leucosomes represent cumulates after the more evolved liquid was lost (Sawyer, 1987;Ellis & Obata, 1992;Fourcade et al, 1992;Milord et al, 2001;Solar & Brown, 2001;Johnson et al, 2003;Slagstad et al, 2005;Cruciani et al, 2008;Daczko & Halpin, 2009;Morfin et al, 2014;Brown et al, 2016).…”

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

“…Peak metamorphic facies include eclogite, omphacite-, garnet-, two-pyroxeneand hornblende-granulite and amphibolite facies (Oliver 1977;Bradshaw 1989a;Clarke et al 2000;Allibone et al 2009b;Daczko & Haplin 2009;De Paoli et al 2009). Ultramafic rocks at Hawes Head described in this study are surrounded by the Western Fiordland Orthogneiss Misty Pluton, which comprises relatively leucocratic twopyroxene9hornblende, feldspathic diorite, monzodiorite and minor monzonite in the vicinity of Hawes Head.…”

Petrology and metamorphic P–T path of high-pressure mafic granulites from the Jiaobei massif in the Jiao-Liao-Ji Belt, North China Craton

“…With the presence of hornblende, the melt phase was also neglected due to its insignificant proportion (Poli and Schmidt, 2002) and no appropriate mixing model is available for present pseudosection calculation involving melt in mafic rocks. Although the mineral assemblages coexist with melt or fluid in the high-temperature pseudosection fields may be metastable (Daczko and Halpin, 2009), the phase relationships are more or less the same and the mineral-in/out lines may not vary too much observed from the results of experiments, petrologic studies and calculations (Pattison, 2003). 12% of the total Fe was assumed to be present as Fe 2 O 3 set in the data set Sun and McDonough, 1989), and quartz and H 2 O are set to be in excess.…”