ABSTRACT

Detrital U-Pb zircon age dating and grain morphometrics were used for provenance and maximum deposition age determination for the Lower Permian Dwyka and Middle to Upper Permian Ecca groups of southern Namibia’s Aranos and Karasburg basins. Thirteen sedimentary samples and one ash fall sample were analysed using laser ablation – single collector – inductively coupled plasma – mass spectrometry (LA-ICP-MS). The results reveal prominent Cambrian to Neoproterozoic (500–750 Ma), a Lower Neoproterozoic to Mesoproterozoic (950–1300 Ma), and Upper Palaeoproterozoic (1775–1950 Ma) detrital zircon age populations. Multiple evidence (e.g. roundness of investigated zircon grains and _ƐHf values) suggest the zircon populations are part of a Gondwana-wide recycling system, where basins act as intermediate reservoirs and eventual dispersers. Classic source to sink dynamics do not apply. A Permian age population (ca. 260–296 Ma) consisting of typically euhedral zircon grains is assumed to be derived directly from protolith rocks. Because no felsic Permian rocks are exposed in the southern African region, these findings suggest a significant input by ash fall from an unknown source, which may be linked to the southern Gondwanan arc volcanism during Permian times. The results reveal a ca. 36 Ma duration of the evolution of the Karasburg Basin and a shorter ca. 30 Ma lasting evolution of the Aranos Basin. The stratigraphic oldest datable unit is an ash bed of the Ganigobis Shale Member of the Zwartbas Fm (Dwyka Group) which gave a maximum deposition age of ca. 296 Ma, whereas the Aussenkjer Fm represents the uppermost Namibian Karoo-aged strata with a maximum age of deposition of ca. 260 Ma.

摘要

碎屑 U-Pb 锆石年龄定年和颗粒形态测量学被用于确定纳米比亚南部阿拉诺斯盆地和卡拉斯堡盆地的下二叠统 Dwyka 和中至上二叠统 Ecca 群的物源和最大沉积年龄。使用激光烧蚀 – 单收集器 – 电感耦合等离子体 – 质谱 (LA-ICP-MS) 分析了 13 个沉积样品和 1 个落灰样品。结果揭示了突出的寒武纪至新元古代 (500–750 Ma)、下新元古代至中元古代 (950–1300 Ma) 和上古元古代 (1775–1950 Ma) 碎屑锆石年龄群。多项证据（例如调查的锆石颗粒的圆度和_ƐHf 值）表明锆石种群是冈瓦纳范围内循环系统的一部分，其中盆地充当中间储层和最终的分散器。经典的源到汇动态不适用。假定由典型的自形锆石颗粒组成的二叠纪年龄群（约 260–296 Ma）直接来自原岩。由于南部非洲地区没有出露二叠纪长英质岩石，这些发现表明来自未知来源的灰烬的重要输入，这可能与二叠纪时期冈瓦纳火山南部的火山活动有关。结果显示约。卡拉斯堡盆地演化的持续时间为 36 Ma，较短的约 Aranos 盆地持续演化 30 Ma。最古老的地层数据单元是 Zwartbas Fm（Dwyka Group）的 Ganigobis 页岩段的灰层，其最大沉积年龄约为 296 Ma，而 Aussenkjer Fm 代表最上层的纳米比亚 Karoo 年龄地层，最大沉积年龄约为 260 马。