Rb‐Sr age and strontium isotope characteristics of the Greenland Group, Buller Terrane, New Zealand, and correlations at the East Gondwanaland margin

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.