Carboniferous metamorphism and partial melting of the Greenland Group in the Jackson River valley, south Westland

Abstract

The peak conditions and timing of metamorphism have been determined for the early Paleozoic Greenland Group in south Westland, New Zealand. The Greenland Group, which is the most extensive pre-Cenozoic unit west of the Alpine Fault in Westland, is dominated by greenschist facies greywacke and argillite. However, in the Jackson River valley this unit comprises amphibolite facies pelitic and psammitic gneisses. Psammitic horizons are dominated by biotite, plagioclase and quartz. Pelitic horizons have similar mineralogy but also contain sillimanite and K-feldspar, as well as boudinaged K-feldspar-bearing leucosomes. Conventional geothermometry and thermodynamic modelling indicate that peak metamorphism occurred between 660 and 700 °C and 3.5 and 6.0 kbar. The timing of this moderately high-T/low-P event is constrained to be mid-Paleozoic by U-Th-Pb dating of overgrowths on detrital zircon (334 ± 18 Ma) and confirmed to be Carboniferous by a single population of monazite grains (343 ± 3 Ma). Greenland Group metamorphism in the Jackson River valley may represent Carboniferous high crustal heat flow in the Gondwana margin hinterland while crustal thickening was occurring closer to the subduction zone.