Provenance of Jurassic sandstones in the Rakaia Terrane, Canterbury, New Zealand

Abstract

<p>Detrital zircon U–Pb and muscovite ³⁹Ar/⁴⁰Ar ages from sandstones in the fluviatile to shallow marine mid-Jurassic Clent Hills Group and Wakaepa Formation of mid-Canterbury, South Island, New Zealand, show characteristic patterns that demonstrate a derivation from local Permian (and possibly Triassic) basement rocks of the Rakaia Terrane (Torlesse Composite Terrane). In the Clent Hills and Wakaepa age data sets, Permian to Triassic zircons form the major age groups, 34% and 58% respectively, but early Paleozoic (20% and 10%) and Precambrian (32% and 28%) zircons also form minor groups. The Precambrian zircons are mainly late Mesoproterozoic and late Neoproterozoic. The muscovite ³⁹Ar/⁴⁰Ar age patterns reveal no Precambrian ages and the majority (48%) again fall in the Permian–Triassic age range. We postulate a local sediment source in the predominantly Permian Rakaia Terrane basement during Jurassic time, when the terrane was a deformed and stabilised upland block extending principally to the east. This was thus a convergent plate margin against which contemporary marine sedimentary basins developed and consolidated as the Kaweka Terrane (Torlesse Composite Terrane). The provenance of the Canterbury Jurassic cover rocks contrasts markedly with Jurassic-age cover rocks in the Brook Street Terrane. The latter show an almost unimodal pattern reflecting a volcaniclastic environment well-removed from continental sediment sources.</p