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.
<p>Juxtaposition of oceanic and continental lithosphere along terrane boundary faults is an important tectonic process that can occur during closure of an ocean basin; however, the timing of faulting can be difficult to constrain. Here, we show that a spectacular exposure of the basal fault (Livingstone Fault) to the Dun Mountain Ophiolite in New Zealand may be dated using <sup>87</sup>Sr/<sup>86</sup>Sr isotopes. At this boundary, quartzofeldspathic schist is faulted against the ultramafic base (peridotites and serpentinites) of the ophiolite and has resulted in metasomatic alteration of the schist, driven by the significant geochemical contrast between the contrasting rock types. We show that metasomatic alteration of the schist resulted in near complete removal of Rb due to the loss of mica, an increased modal abundance of metasomatic actinolite and appearance of metasomatic garnet and hedenbergite. Because Rb was removed from the metasomatized schist, its <sup>87</sup>Sr/<sup>86</sup>Sr composition was essentially frozen at the time of metasomatism, while the <sup>87</sup>Sr/<sup>86</sup>Sr composition of unaltered schist evolved due to the radioactive decay of <sup>87</sup>Rb. Back calculating the present day <sup>87</sup>Sr/<sup>86</sup>Sr composition of the unaltered schist to the frozen <sup>87</sup>Sr/<sup>86</sup>Sr composition of the metasomatized schist yields a date of 170 Ma + 5 Ma. This date is broadly consistent with geological reconstructions of the Triassic-Jurassic Zealandia margin and provides a minimum age constraint on the timing of juxtaposition of the Dun Mountain Ophiolite against the crustal rocks and therefore the closure of the vast ocean basin along the eastern margin of Gondwana. Similar metasomatic reactions are described in similar settings elsewhere and so this method may be applied outside of this example.</p>
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