U-Th-Pb zircon and monazite geochronology of Western Province gneissic rocks, central-south Westland, New Zealand

Abstract

Ion-microprobe (Sensitive High Resolution Ion MicroProbe or SHRIMP) zircon and monazite U-Th-Pb geochronology of amphibolite-facies orthogneiss and paragneiss units, mostly from the Fraser and Granite Hill Complexes of Westland, New Zealand, has constrained the ages of protolith rocks and metamorphic overprints. Paragneiss zircon (6 samples, 175 analyses) comprise detrital grains of Paleozoic to Archean age and match patterns from metasedimentary (Ordovician) Greenland Group rocks of the Western Province, which are characteristic of early Paleozoic flysch units around the southwest Pacific margin of Gondwana. Orthogneiss zircon (9 samples, 138 analyses) reflect intense Devonian and Cretaceous magmatism associated with emplacement of the Karamea-Paparoa and Hohonu Batholiths within the Western Province. Both paragneiss and orthogneiss monazite analyses (10 samples, 116 analyses) are dominated by Devonian and Cretaceous ages, reflecting thermal pulses related to regional and localised granitoid intrusions. These results indicate that the Fraser and Granite Hill Complex gneisses are metamorphosed parts of the Western Province Buller Terrane with Karamea-Paparoa and Hohonu Batholith intrusive components.