Timing and conditions of metamorphism in Greenland Group in south Westland, New Zealand

Palmer, Marshall Croft

The peak conditions and timing of metamorphism have been determined for the first time for Greenland Group occurring along the Jackson River in south Westland, South Island, New Zealand. The investigated Greenland Group is composed of pelitic and psammitic gneisses, invaded by granitic orthogneisses. The rocks have been variably mylonitised, retrogressed, and intruded by lamprophyre dikes. The peak mineral paragneiss assemblage (which is most diagnostic of metamorphic grade) is sillimanite, K-feldspar, biotite, calcic plagioclase, quartz, which corresponds to upper amphibolite facies. The rocks experienced muscovite dehydration and partial melting, with pelitic rocks displaying prominent melt segregation textures. Peak conditions of metamorphism were quantitatively constrained through integration of Ti in biotite geothermometry and equilibrium thermodynamic modelling. From this, metamorphism was determined to have occurred at temperatures between 660- 700 oC at pressures between ~3.5 and 5.0 kbar. This result is consistent with the generation of small volumes of partial melt, most of which appears to have remained within the rocks. The timing of metamorphism and partial melting was determined through U-Th-Pb radiogenic isotope dating of zircon and monazite. Monazite grains, which have texturally controlled growth faces, yielded a pooled age of 343 ± 5 Ma. Metamorphic overgrowths on detrital zircon grains, although limited, yield a pooled age of 330 + 30 Ma, which is in agreement with the monazite age. Zircon dating also reveals that the granite orthogneiss protolith was emplaced at 357 Ma, just prior to metamorphism. The granite orthogneiss cannot therefore have formed as a result of partial melting of the currently exposed Greenland Group in which it resides. The calculated metamorphic conditions indicate that the Greenland Group at Jackson River formed under a high temperature, low pressure geothermal gradient. This event is hypothesised to be the result of extension and crustal thinning of the Gondwana margin in the Early Carboniferous. This event occurred contemporaneously with crustal thickening in the Western Province in Fiordland. The amphibolite facies metamorphism in Greenland Group rocks of the Jackson River may therefore be the result of back arc extension while crustal thickening was occurring closer to the inferred subduction zone.

v, 99 pages A4, maps in back pocket

2013Palmer

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