Establishing the P–T path for Alpine Schist, Southern Alps near Hokitika, New Zealand |
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Authors: | J K VRY R POWELL J WILLIAMS |
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Institution: | School of Geography, Environment and Earth Sciences, Victoria University, PO Box 600, Wellington 6040, New Zealand (); School of Earth Sciences, University of Melbourne, Vic 3010, Australia |
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Abstract: | Pressure–temperature pseudosections for ‘greyschist’ (metamorphosed greywacke and argillite) from the Alpine Schist (Haast Schist group) near Hokitika (Southern Alps, New Zealand) are used to gain new insights into its metamorphic history. The rocks were metamorphosed at relatively low‐grade conditions associated with the first appearance and initial growth of garnet in the stability field of albite. The measured and predicted garnet compositional zoning data are used to construct an overall P–T path by combining P–T path results from nearby rocks that have a range of MnO contents. The P–T path obtained is steep from ~380 °C/2.5 kbar up to ~490 °C/8.5 kbar, then recurves sharply with garnet growth continuing during early decompression to ~500 °C/6.5 kbar. Most garnet growth in the study area took place in the stability field of albite, with oligoclase appearing only during decompression, when the peristerite gap was entered. On appearance of oligoclase, there is a marked decrease in the CaO content of garnet. The preservation of mineral assemblages from near‐peak temperature conditions can be understood in terms of the P–T path subsequently becoming tangential to water content contours, during cooling with further decompression. |
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Keywords: | Alpine Schist metamorphism New Zealand P–T path pseudosections |
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