New Zealand Journal of Geology and Geophysics abstracts
Rb-Sr age and strontium isotopic characterisation of the Torlesse Supergroup
in Canterbury, New Zealand, and implications for the status of the Rakaia
Terrane
C. J. Adams
Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand
R. Maas
Department of Earth Sciences
La Trobe University
Bundoora, Vic 3083
Australia*
*Present address: Department of Geology, University of Melbourne, Parkville,
Vic 3010, Australia.
Abstract Rb-Sr whole-rock isochron ages from 33 localities
of low-grade metasediments of the Torlesse Supergroup in Canterbury date two
episodes of burial metamorphism. Permian and Triassic metasediments were metamorphosed
in latest Permian to Middle Triassic times (255–235 Ma), and in Late Triassic
to Early Jurassic times (215–195 Ma), respectively. These data also confirm
a consistent age (t), and initial 87Sr/86Sr ratio (i),
relationship in both Permian and Triassic groups from Otago in the South Island
to the Wellington region in the North Island. The (t)-(i) data of all Torlesse
metasediments are more radiogenic than those in other Eastern Province terranes.
In particular, Triassic Torlesse (t)-(i) data confirm a source area dominated
by Late Permian to Early Triassic I-type granitoids, such as the southern-central
part of the New England Fold Belt of northeastern Australia and its hinterland.
Permian Torlesse data are similar but indicate an older and less radiogenic
source of predominantly Early Permian–Carboniferous I-type granodiorites-diorites,
such as those in the northern New England Fold Belt and Cape York Peninsula,
northern Queensland. The metamorphic history of the Permian–Triassic Torlesse
Supergroup and its sediment provenance characteristics are consistent and
distinctive, sufficiently to define the Rakaia Terrane as separate from its
Pahau and Waipapa Terrane neighbours.
Keywords Canterbury; Torlesse Supergroup; Rakaia Terrane;
Permian; Triassic; Jurassic; Rb-Sr; geochronology; strontium isotopes; metamorphic
rocks
G02041; Received 9 August 2002; accepted 9 December 2003; Online publication
date 20 May 2004
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47: 201–217
0028–8306/04/4702–0201 © The Royal Society of New Zealand 2004
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