New Zealand Journal of Geology and Geophysics abstracts
Rb-Sr age and strontium isotope characteristics of the Greenland Group,
Buller Terrane, New Zealand, and correlations at the East Gondwanaland margin
C. J. Adams
Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand
Abstract Rb-Sr whole-rock isochron ages of early Paleozoic,
Greenland Group (Buller Terrane) metasediments from 17 localities range from
458 ± 9 to 271 ± 26 Ma, in a pattern similar to, but frequently
20–30 m.y. older than, corresponding K-Ar ages of slates. The ages reflect
an original low-grade regional metamorphism at 450 ± 10 Ma in Late
Ordovician times and subsequent cooling, which has then been pervasively
overprinted by younger thermal events, most probably associated with the
emplacement of Devonian–Carboniferous granitoids. Initial 87Sr/86Sr
ratios (i) at the time of metamorphism (t) increase from c. 0.717 at c. 450
Ma to c. 0.722 at c. 350 Ma—this evolution reflecting a bulk sediment Rb/Sr
ratio of c. 1.0. The (t)-(i) data suggest that the Greenland Group source
area is extensive granitoid (mostly S-type) and older metamorphic complexes,
similar to the Wilson Terrane of the Ross and Adelaide Foldbelts of Antarctica
and Australia. The New Zealand early Paleozoic data follow the (t)-(i) trend
of other late Precambrian to early Paleozoic metagreywacke sequences at the
East Gondwanaland margin, in North Victoria Land and Marie Byrd Land, Antarctica,
and Victoria and New South Wales, Australia. This indicates a large uniform
source, providing well-mixed detritus, and deposition at a passive continental
margin. Preliminary Rb-Sr ages from Cambrian–Ordovician Takaka Terrane metasediments
show no evidence for the Late Ordovician metamorphism recorded in the Buller
Terrane, and reflect only post-Devonian thermal overprints. However, their
(t)-(i) characteristics follow the Greenland Group trend (but along the less
radiogenic part), and indicate a generally similar continent-derived provenance.
Keywords Greenland Group; Buller Terrane; Paleozoic;
turbidite; Rb-Sr; K-Ar; geochronology; metamorphism; strontium isotopes
G02064; Received 17 December 2002; accepted 18 November 2003; Online publication
date 20 May 2004
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47: 189–200
0028–8306/04/4702–0189 © The Royal Society of New Zealand 2004
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