Metamorphic evolution of Malaspina Orthogneiss on Resolution Island, Fiordland

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Elliot, Tim

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Malaspina Orthogneiss is part of the larger Early Cretaceous Western Fiordland Orthogniess (WFO) and represents the southernmost pluton of this suite. Malaspina Orthogneiss is composed of a variety of high-pressure metamorphic assemblages that record a two stage crystallisation history. The first stage is represented by two-pyroxene granulites that crystallised at ~9.5 Kbar and 810?C. This was followed by metamorphism at ~18 Kbar and ~1000?C at eclogite facies conditions. This is the second occurrence of eclogite facies conditions recorded in New Zealand, the other being in the nearby Breaksea Orthogneiss. Eclogite facies P-T estimates from the Malaspina Orthogneiss were obtained from omphacite-bearing garnet granulite mineral assemblages, probably because the Malaspina Orthogneiss has a more felsic bulk composition than typical for eclogite and so was able to crystallise plagioclase. The absence of diffusion in garnet and clinopyroxene in the highest-P assemblages, and rutile U-Pb dating and the Zr-in-rutile thermometry, indicate that exhumation began soon after high-pressure metamorphism, and that closure through approximately 600?C occurred at 84 Ma. Vertical exhumation is estimated to be about ~42 km during this time. The occurrence of rocks that record ~18 Kbar (Malaspina and Breaksea Orthogneiss) on either side of the Doubtful Sound Shear Zone indicates that this fault has had minimal movement on it in this area.

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2009Elliot

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http://download.otagogeology.org.nz/temp/Abstracts/2009Elliot.pdf

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Elliot, Tim, “Metamorphic evolution of Malaspina Orthogneiss on Resolution Island, Fiordland,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/503.

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