The timing of Dun Mountain Ophiolite emplacement via Rb-Sr isotope dating of metasomatic reactions along the basal Livingstone Fault in New Zealand

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 ⁸⁷Sr/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr composition was essentially frozen at the time of metasomatism, while the ⁸⁷Sr/⁸⁶Sr composition of unaltered schist evolved due to the radioactive decay of ⁸⁷Rb. Back calculating the present day ⁸⁷Sr/⁸⁶Sr composition of the unaltered schist to the frozen ⁸⁷Sr/⁸⁶Sr 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.