The Kakanui Mineral Breccia: Experimental and mineral geochemistry

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Garden, Benjamin P

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Two garnet bearing tuffs are known in North Otago, Kakanui and Alma. The Kakanui Mineral Breccia contain a diverse range of upper mantle derived megacrysts and xenoliths while the Alma contains a limited assemblage of megacrysts and xenoliths. Within the Kakanui Mineral Breccia, three magmas were identified through a detailed EMP and LAM study of the megacryst and garnet pyroxenite suite. Crystallisation pressures for the megacrysts f'ere determined by near liquidus experiments using the nephelinite host composition for the starting material, and geothermometry and geobarometry for the garnet pyroxenite suite (Zack et al (1997). Megacryst crystallisation occurred between 25-15kb and 1200-1150°C with the bulk occurring between 22-17kb. The garnet pyroxenites crystallised between 15-16kb and 930°C. The Kakanui Mineral Breccia underwent a complex petrogenesis involving three different magmas, the nephelinite host (parent to megacrysts suite}, an evolved nephelinite (parent to garnet pyroxenite suite) and an Alkali Olivine Basalt (triggering magma by magma injection). The magma ascent was initiated by the injection of Alkali Olivine Basalt into the megacryst bearing nephelinite. This AOB interacted j with the nephelinite host (mixing causing heterogeneity) and possibly added xenocrysts of higher magnesium garnet compared to the megacryst suite garnet ofthe nephelinite. The nephelinite host intersected a solid garnet pyroxenite pod trapped within lherzolite formed from an evolved nephelinite parent, which reacted with the ; volatile rich host to form metasomatic kaersutite. The origin of the plagioclase bearing garnet pyroxenite is potentially resolved as al'\ j \ extremely evolved garnet pyroxenite (late stage crystallisation). A new megacryst phase from the Kakanui Mineral Breccia is confirmed, zircon, the chemistry is unique to other zircons in the South Island, Hf!Zr ratio suggests crystallisation from a nephelimte or an undersaturated rock. A similar Mineral Breccia at Alrna, is derived from a more evolved magma sourced deeper in the Mantle with unknown relationship to Kakanui.

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ix. 165 p, diagms, 30 cm, +CD-Rom (4 3/4 in.)

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2001Garden

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http://download.otagogeology.org.nz/temp/Abstracts/2001Garden.pdf

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Citation

Garden, Benjamin P, “The Kakanui Mineral Breccia: Experimental and mineral geochemistry,” Otago Geology Theses, accessed April 16, 2021, http://theses.otagogeology.org.nz/items/show/380.

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