COMBINED U-TH-PB MONAZITE AND LU-HF GARNET AGES FROM THE ALPINE SCHIST: IMPLICATIONS FOR THE TIMING AND DURATION OF BARROVIAN METAMORPHISM IN THE SOUTHERN ALPS, NEW ZEALAND

The timescales over which Barrovian-type metamorphic sequences are produced has been a topic of current debate, with recent geochronologic studies demonstrating short timescales of a few million years for the duration of Barrovian metamorphism. The Alpine Schist of the Southern Alps, New Zealand, has been exhumed from depths of up to 40 km, of which 25 km was accommodated by recent (<5 Ma) uplift along the Alpine Fault plate boundary. This has exposed a cross-section through a greenschist- to amphibolite-facies metamorphic field gradient that extends for over 230 km parallel to the Alpine Fault, offering an ideal location to investigate the timescales of Barrovian-type metamorphism. Existing ages for the growth of prograde rock forming and accessory metamorphic minerals in Alpine Schist are scarce, have been obtained using different methods (Sm-Nd and Lu-Hf isochrons, U-Pb TIMS and SHRIMP), and range from 100 – 70 Ma. Due to the lack of data and inconsistency in dating methods, these ages can equally be ascribed to represent either a prolonged (~ 30 My) metamorphic cycle, or, alternatively, multiple episodes of metamorphism. Resolving these competing interpretations requires systematic orogen-scale sampling and the use of a ubiquitous geochronometer. Garnet is a major mineral in greenschist- to amphibolite-facies Alpine Schist, providing the opportunity to carry out campaign-style Lu-Hf geochronology along the entire orogen. In this study we combine LASS (laser ablation split-stream) geochronology of rare matrix monazite with Lu-Hf garnet geochronology on six samples distributed along a 230 km transect parallel to the Alpine Fault. Matrix monazite from an amphibolite-facies quartzofeldspathic schist dated at 71.9 ± 0.4 Ma is significantly younger than previously reported Sm-Nd and Lu-Hf garnet – whole-rock ages. In order to investigate the significance of this ~70 Ma age signature, monazite ages are compared with new Lu-Hf garnet ages which record prograde mineral growth in the same rock sample. These data illustrate the importance of combining multiple geochronometers in assessing the timescales of burial and heating involved in Barrovian-type metamorphism.