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

Thermobarometric estimates of early (D-1) and late (D-2) metamorphic recrystallisation of Alpine Schist, Southern Alps of New Zealand, define three polychronous piezothermic arrays: (1) 4.3-7.5 kbar and 260-370 degrees C for chlorite and biotite-albite zone metagreywacke-semischist; (2) 5.2-9.2 kbar and 415-620 degrees C for garnet and oligoclase zone schist, and (3) 5.4-6.6 kbar and 490-540 degrees 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 degrees 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.