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

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.

采自新西兰南岛中坎特伯雷地区河流至浅海相的中侏罗世克兰特山群（Clent Hills Group）与瓦卡埃帕组（Wakaepa Formation）砂岩的碎屑锆石U–Pb及白云母³⁹Ar/⁴⁰Ar定年数据，呈现出特征性的年龄分布模式，指示其物源来自拉基亚地体（Rakaia Terrane，托勒斯复合型地体（Torlesse Composite Terrane））的本地二叠纪（可能包含三叠纪）基底岩石。
在克兰特山群与瓦卡埃帕组的年龄数据集中，二叠纪至三叠纪锆石构成主要年龄组分，占比分别为34%与58%；但早古生代锆石（占比20%与10%）与前寒武纪锆石（占比32%与28%）也构成次要组分。前寒武纪锆石以中元古代晚期及新元古代晚期锆石为主。
白云母³⁹Ar/⁴⁰Ar年龄分布模式则未检出前寒武纪年龄，且大部分（48%）样品年龄仍落在二叠纪—三叠纪区间内。
我们推测，在侏罗纪时期，拉基亚地体作为经变形并趋于稳定的抬升地块，主要向东延展，其主体为二叠纪基底，彼时为本地沉积物提供了物源。该区域属于汇聚板块边缘，同期形成的海相沉积盆地在此背景下发育并固结，形成卡韦卡地体（Kaweka Terrane，托勒斯复合型地体）。
坎特伯雷地区侏罗纪盖层的物源特征，与布鲁克斯街地体（Brook Street Terrane）内的侏罗纪盖层存在显著差异：后者呈现近乎单峰的年龄分布模式，反映其形成于远离大陆物源的火山碎屑沉积环境。