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Unlocking the tectonic history of Zealandia: Insights from garnet and zircon geochronology of the Alpine Schist

Dr Sophie Briggs, University of Otago

The Zealandia portion of the Pacific–Gondwana margin underwent widespread extension, fragmentation, separation and subsidence during the final stages in the breakup of Gondwana. Although these processes shaped the geology of New Zealand, their timing and the timing of subduction cessation in the region remain unclear.

The metamorphic record preserved in the Alpine Schist accretionary wedge complex offers a unique opportunity to investigate the timing of these processes. We dated metamorphic mineral growth in the Alpine Schist using a combination of Lu–Hf garnet geochronology and U-Pb zircon petrochronology. The garnet ages are diachronous, ranging from 97.3 ± 0.3 to 75.4 ± 1.3 Ma. Garnet ages are corroborated by zircon dates acquired using the relatively novel approach of depth profiling using laser-ablation split stream (LASS)-ICP-MS, which enables the analysis of thin and discontinuous metamorphic zircon overgrowths. Zircon rims that crystallized contemporaneously with garnet show trace element signatures consistent with garnet-stable conditions, while the trace element signatures of younger zircon rims indicate formation either in the presence of melt/fluid or during garnet breakdown.

In combination, these results resolve a prolonged history of metamorphism in the Alpine Schist, with diachronous garnet growth from 97 to 75 Ma and zircon rim (re)crystallization from 84 to 50 Ma. The garnet age range provides strong evidence that metamorphism in the Alpine Schist occurred concurrently with the rifting of Zealandia from East Gondwana via opening of the Tasman Sea, a period commonly considered to be dominated by extensional collapse. The prolonged period of zircon rim (re)crystallization overlaps the c. 30 m.y. period of Tasman Sea spreading and water-fluxed melting of the Alpine Schist, and was likely driven by fluid fluxing associated with distributed intraplate deformation between NW and SE Zealandia.

Date Wednesday, 18 July 2018
Time 1:00pm - 2:00pm
Audience Public
Event Category Sciences
Event Type Departmental Seminar
LocationBenson Common Room (Gn9), Geology Building, 360 Leith Street, Dunedin

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