PETROGENESIS OF PEGMATITES AND THE AGE OF PEAK METAMORPHISM IN THE SOUTHERN ALPS, NEW ZEALAND

Despite its proximity to the Alpine Fault, one of the best-studied strike-slip plate boundary faults in the world, the age of peak metamorphism and mineral growth in the adjacent Haast Schist of New Zealand remains a matter of debate. This study uses a suite of granite pegmatite intrusions exposed in the Haast Schist of the Southern Alps to: 1) determine temporal constraints for peak metamorphism of the Haast Schist and; 2) test previous suggestions that these pegmatites originated via partial melting of the Haast Schist. In an attempt to assess whether the origin of the pegmatites can be tied to peak metamorphism, a petrogenetic study was carried out combining field relationships, whole-rock elemental geochemistry, and Laser Ablation Split Stream U-Th/-Pb geochronology linked to trace-element and isotope analysis on zircon, monazite and xenotime. Pegmatite sills and dikes 0.15 to 30 m in width intrude the Haast Schist within 3 km of the Alpine Fault in the Mataketake Range of the Southern Alps. Sills are concordant with the dominant metamorphic fabric of the Haast Schist which is truncated by the Alpine Fault in this area. Rare folded dikes indicate that both syn- and post-kinematic emplacement occurred. U-Th/-Pb zircon, monazite and xenotime dating of 13 pegmatites indicates that intrusion occurred over a prolonged interval from Late Cretaceous to Late Paleocene (~58Ma to ~80 Ma). In addition to age variation observed between samples, careful analysis of zoned zircon grains reveals protracted zircon growth within individual samples as well as the presence of Paleozoic to Mesozoic inherited cores, the latter suggesting derivation from an older metasedimentary source such as the Haast Schist. This data suggests that a phase of protracted pegmatite intrusion occurred during a time of post-orogenic regional extension and uplift in the South Island of New Zealand. The results provide insights into the tectonic context of peak metamorphism in the Haast Schist and encourage a reassessment of the time scale of Barrovian metamorphism in the South Island of New Zealand.