The P-T-t History of the Alpine Schist, New Zealand: Constraining Tectonic Processes During the Late Stages of Gondwana Breakup

Abstract:

Separation of the microcontinent Zealandia from the Marie Byrd Land sector of Antarctica in the Late Cretaceous marked the final stage in the breakup of Gondwana. Two contrasting ideas for the Late Cretaceous rifting of Zealandia from Gondwana have been proposed. One is that subduction at the paleo-Pacific - Gondwana convergent margin ceased after the last pulse of batholith emplacement at ~100 Ma, followed by a rapid transition to extension and seafloor spreading at 82 Ma. The other hypothesis is that convergence continued along Zealandia simultaneously with back-arc extension until ~85 M. This hypothesis is based on recently reported Late Cretaceous ages from the Alpine Schist, a metamorphosed accretionary wedge assemblage. Without accompanying pressure-temperature (P-T) information, the significance of Late Cretaceous ages from the Alpine Schist in terms of the orogenic processes that occurred during cessation of subduction at the paleo-Pacific – Gondwana margin remains unclear.

In this study, Lu-Hf geochronology of Alpine Schist garnet is paired with phase equilibria modelling to elucidate the P-T-t history of the orogen and clarify the mechanisms behind Zealandia-Gondwana rifting. Garnet Lu-Hf dates have been obtained from 9 samples ranging in bulk composition from quartzo-feldspathic schists to mafic amphibolites. Garnet yields Early Cretaceous ages from the southern Alpine Schist, whereas northern Alpine Schist garnet yields Late Cretaceous ages. Garnet textures and major and trace element compositional zoning suggest that an additional, later period of garnet growth or recrystallization may be recorded in the northern samples. P-T-t data from each dated sample is supplemented with thermobarometric analysis from an adjacent sample of different lithology, with the advantage of providing more complete local P-T-t paths. The P-T-t paths define whether garnet grew during increasing P-T (prograde early), decreasing P and increasing T (prograde late), and/or decreasing P-T (retrograde). Our preliminary results indicate that 1) convergence coincided with the initiation of rifting along most of the Zealandia-Gondwana margin and 2) convergence either ceased prior to initiation of extension or did not result in garnet growth at the segment of the margin represented by southern Alpine Schist.