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Abstract
The Cretaceous was a time of widespread convergent and extensional thermotectonic activity in New Zealand. K‐Ar, Ar‐Ar, and fission track dates thus far obtained from the Haast Schist belt record cooling and exhumation of Jurassic‐Cretaceous age in Otago and late Neogene age in the Southern Alps. In this paper we present results of a pilot U‐Pb and Sm‐Nd geochronological investigation that at last provide ages of local amphibolite facies mineral growth in the Haast Schist, rather than exhumation ages.
We report a U‐Pb age of 71 ± 2 Ma for four monazite fractions from a Mataketake Range garnet biotite greyschist; this result is within error of the U‐Pb age of nearby anatectic granite pegmatite dikes. We also report a four‐point Sm‐Nd isochron age of 100 ± 12 Ma for titanite, garnet, hornblende, and whole rock fractions from a Haast River amphibolite. The two dated rocks are c. 20 km apart, apparently along‐strike from each other and in the same mapped metamorphic zone. Our results support a model of protracted, episodic, spatially heterogeneous Cretaceous mineral growth in the Haast Schist, with the generation of anatectic pegmatites in the latest Cretaceous.
In an appendix we describe our unsuccessful attempts to find and date other high closure temperature minerals by various methods, including U‐Pb dating of titanite in an Otago greyschist and in an Alpine calcmylonite.
