Rb-Sr age and strontium isotopic characterisation of the Torlesse Supergroup in Canterbury, New Zealand, and implications for the status of the Rakaia Terrane

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 Sr-87/Sr-86 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 T-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.