Uplift and exhumation of Alpine Schist, Southern Alps, New Zealand: Thermobarometric constraints

Abstract

Thermobarometric estimates of early (D₁) and late (D₂) metamorphic recrystallisation of Alpine Schist, Southern Alps of New Zealand, define three polychronous piezothermic arrays: (1) 4.3–7.5 kbar and 260–370°C for chlorite and biotite‐albite zone metagreywacke‐semischist; (2) 5.2–9.2 kbar and 415–620°C for garnet and oligoclase zone schist, and (3) 5.4–6.6 kbar and 490–540°C for K‐feldspar zone schist near the Alpine Fault. The metamorphic arrays are used to develop a simple one‐dimensional model to explain metamorphism and uplift of the Alpine Schist in terms of a P‐T‐t(time) loop. Evidence from retrograde albite (together with phengitic mica and K‐feldspar) in oligoclase zone schist implies an early (possibly Mesozoic) schist uplift of 7–8 km corresponding to a temperature and pressure decrease of c. 100°C and 2.0–2.3 kbar. Thermobarometric data from K‐feldspar zone schist suggests a further, possibly Late Cretaceous, uplift of 3–6 km giving a total Mesozoic uplift of the Alpine Schist of between 10 and 14 km. The mylonitic nature of the K‐feldspar zone schists suggests that they may have recrystallised along a mid‐crustal decollement zone that was associated with shear heating during late Cenozoic uplift amounting to 19–25 km along the Alpine Fault. Late Cenozoic uplift of the schist was accompanied by tectonic thinning, especially with respect to garnet and oligoclase zone schists, and may be a function of the high rate of uplift in the central part of the Alps.

Keywords:

• Alpine Schist
• mineral thermobarometry
• retrograde plagioclase
• uplift and exhumation
• P‐T‐t(time) loop