New Zealand Journal of Geology and Geophysics abstracts
U-Pb and Sm-Nd ages from the Alpine Schist, New Zealand
N. Mortimer
Institute of Geological & Nuclear Sciences
Private Bag 1930
Dunedin, New Zealand
A. F. Cooper
Department of Geology
University of Otago
P.O. Box 56
Dunedin, New Zealand
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.
Keywords Alpine Schist; Haast Schist; amphibolite facies;
geochronology; U-Pb dating; Sm-Nd dating; monazite; titanite
G02061; Online publication date 25 February 2004
Received 26 November 2002; accepted 30 October 2003
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47: 21-28
0028-8306/04/4701-0021 © The Royal Society of New Zealand 2004
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