Tracing the Caples Terrane through New Zealand using detrital zircon age patterns and radiogenic isotope signatures

Abstract

Rb‐Sr isochron ages and associated initial 87Sr/86Sr ratios of metasedimentary rocks of Caples Group in Southland and Otago, Pelorus Group in Nelson and Marlborough, and their minor North Island correlates in Northland and King Country, reveal the essential continuity of a long Caples Terrane throughout New Zealand. The Rb‐Sr ages, in the range 251–117 Ma, record post‐metamorphic, Early Triassic to Early Cretaceous uplift and cooling, whilst their associated range of initial 87Sr/86Sr ratios, 0.7039–0.7053, allows a discrimination from analogous datasets of neighbouring Torlesse and Waipapa Terranes. Some petrographically and geochemically anomalous Caples Terrane rocks in East Otago, and Otago Schists of the Aspiring Terrane, all with initial 87Sr/86Sr ratios >0.7055, compare best with dataseis of Waipapa Terrane of the North Island. Detrital zircon U‐Pb ages of greywackes from Caples Terrane metasediments have youngest zircon age components from 251 to 215 Ma, and a comparison of dataset patterns from fossiliferous examples suggests that these probably originate from contemporary volcanic sources. Caples volcanism and associated depocentres thus span much of the Triassic. However, their major zircon component, invariably in the range Early Triassic to latest Permian, is most persistent, and constitutes an enormous and enduring zircon source thought to be in the New England Orogen of eastern Australia. Minor, late Paleozoic (295–340 Ma) zircon components are restricted to (Early‐Middle Triassic) Caples Terrane metasediments with older, minimum zircon age components, perhaps originating in nearby primary sources in the northeasternmost Lachlan Fold Belt and southern part of the New England Orogen. More scattered early Paleozoic‐Precambrian zircons are present throughout the Caples Terrane and reflect reworking of zircons from Paleozoic metasediments and plutonic rocks within the New England Orogen (and its immediate hinterland). Caples Terrane metasediments thus represent an offshore, Triassic component of the accretionary prism terranes (Late Carboniferous to Early Cretaceous) of the Eastern Province of New Zealand. Their depocentres were more isolated from the Gondwanaland continental m argin than Torlesse and Waipapa Terrane counterparts, and more influenced by substantial contemporary volcanic centres, perhaps located along the Lord Howe Rise.