Mineralogy, textures and reactions within pyrometamorphosed xenoliths of Otago Schist


Scanlan, Emma


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Xenoliths of Otago Schist have been pyrometamorphosed upon entrainment within the Miocene basanite at Ram Rock, North Otago. This has resulted in recrystallisation from a greenschist facies mineral assemblage (T < 400°C) to a sanidinite facies assemblage (T > 900°C).
Ram Rock xenoliths were analysed by scanning electron microscope electron dispersal spectrometry to characterise the mineralogy of xenoliths due to the very fine grain-size. The Otago Schist quartz-albite segregations have recrystallized to quartz with orthopyroxene coronas within rhyolitic glass. The micaceous segregations (muscovite, chlorite, garnet) have been completely replaced by olivine, spinel, plagioclase, cordierite, ilmenite, orthopyroxene and an alkali-rich mafic glass. The compositional segregations of the Otago Schist protolith are behaving as isolated blocks during pyrometamorphism, producing separate chemical domains of ultramafic and dacitic composition. Replacement textures indicate that the sanidinite facies minerals crystallised through two mechanisms: as the result of mineral reactions from the former greenschist facies minerals, or they nucleated within a melt and developed quench textures. The presence of quench textures and glass indicates that the xenoliths cooled rapidly to the glass transition temperature, likely through a combination of conductive cooling, minor convective cooling, and high viscosity inhibiting crystal growth.
Metasomatic interaction with the host basanite has enriched the xenolith in 2.3 wt% CaO, 0.6 wt% K2O, and 4.3 wt% SiO2 with minor enrichment of FeO, TiO2, and Na2O within minerals on the basalt-xenolith interface. Inferred mineral reactions give constraints on temperatures reached, with chlorite breakdown occurring at 870°C and muscovite partial melting and complete removal at 900-950°C. Quantitative geothermometry gives temperatures of ~960-1030°C for glass, while the more suitable orthopyroxeneliquid and plagioclase-liquid geothermometers give 940-980°C, allowing more precise constraints on temperatures reached.

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135 pages A4


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Location (WKT, WGS84):

MULTIPOLYGON (((170.497481673411 -45.5623060915417,170.507239729514 -45.5624481285648,170.507911652799 -45.5702505300526,170.497856873926 -45.5698877077138,170.497481673411 -45.5623060915417)))

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Scanlan, Emma, “Mineralogy, textures and reactions within pyrometamorphosed xenoliths of Otago Schist,” Otago Geology Theses, accessed May 28, 2018, http://theses.otagogeology.org.nz/items/show/641.